Estrogen depletion on In vivo osteocyte calcium signaling responses to mechanical loading

Microstructural adaptation of bone in response to mechanical stimuli is diminished with estrogen deprivation. Here we tested in vivo whether ovariectomy (OVX) alters the acute response of osteocytes, the principal mechanosensory cells of bone, to mechanical loading in mice. We also used super resolution microscopy (Structured Illumination microscopy or SIM) in conjunction with immunohistochemistry to assess changes in the number and organization of "osteocyte mechanosomes" - complexes of Panx1 channels, P2X7 receptors and CaV3 voltage-gated Ca²⁺ channels clustered around α_vβ₃ integrin foci on osteocyte processes. Third metatarsals bones of mice expressing an osteocyte-targeted genetically encoded Ca²⁺ indicator (DMP1-GCaMP3) were cyclically loaded in vivo to strains from 250 to 3000 με and osteocyte intracellular Ca²⁺ signaling responses were assessed in mid-diaphyses using multiphoton microscopy. The number of Ca²⁺ signaling osteocytes in control mice increase monotonically with applied strain magnitude for the physiological range of strains. The relationship between the number of Ca²⁺ signaling osteocytes and loading was unchanged at 2 days post-OVX. However, it was altered markedly at 28 days post-OVX. At loads up to 1000 με, there was a dramatic reduction in number of responding (i.e. Ca²⁺ signaling) osteocytes; however, at higher strains the numbers of Ca²⁺ signaling osteocytes were similar to control mice. OVX significantly altered the abundance, make-up and organization of osteocyte mechanosome complexes on dendritic processes. Numbers of α_vβ₃ foci also staining with either Panx 1, P2X7R or CaV3 declined by nearly half after OVX, pointing to a loss of osteocyte mechanosomes on the dendritic processes with estrogen depletion. At the same time, the areas of the remaining foci that stained for α_vβ₃ and channel proteins increased significantly, a redistribution of mechanosome components suggesting a potential compensatory response. These results demonstrate that the deleterious effects of estrogen depletion on skeletal mechanical adaptation appear at the level of mechanosensation; osteocytes lose the ability to sense small (physiological) mechanical stimuli. This decline may result at least partly from changes in the structure and organization of osteocyte mechanosomes, which contribute to the distinctive sensitivity of osteocytes (particularly their dendritic processes) to mechanical stimulation.

A Bisphosphonate With a Low Hydroxyapatite Binding Affinity Prevents Bone Loss in Mice After Ovariectomy and Reverses Rapidly With Treatment Cessation

Bisphosphonates (BPs) are a mainstay of osteoporosis treatment; however, concerns about bone health based on oversuppression of remodeling remain. Long‐term bone remodeling suppression adversely affects bone material properties with microdamage accumulation and reduced fracture toughness in animals and increases in matrix mineralization and atypical femur fractures in patients. Although a “drug holiday” from BPs to restore remodeling and improve bone quality seems reasonable, clinical BPs have long functional half‐lives because of their high hydroxyapatite (HAP) binding affinities. This places a practical limit on the reversibility and effectiveness of a drug holiday. BPs with low HAP affinity and strong osteoclast inhibition potentially offer an alternative approach; their antiresorptive effect should reverse rapidly when dosing is discontinued. This study tested this concept using NE‐58025, a BP with low HAP affinity and moderate osteoclast inhibition potential. Young adult female C57Bl/6 mice were ovariectomized (OVX) and treated with NE‐58025, risedronate, or PBS vehicle for 3 months to test effectiveness in preventing long‐term bone loss. Bone microarchitecture, histomorphometry, and whole‐bone mechanical properties were assessed. To test reversibility, OVX mice were similarly treated for 3 months, treatment was stopped, and bone was assessed up to 3 months post‐treatment. NE‐58025 and RIS inhibited long‐term OVX‐induced bone loss, but NE‐58025 antiresorptive effects were more pronounced. Withdrawing NE‐58025 treatment led to the rapid onset of trabecular resorption with a 200% increase in osteoclast surface and bone loss within 1 month. Cessation of risedronate treatment did not lead to increases in resorption indices or bone loss. These results show that NE‐58025 prevents OVX‐induced bone loss, and its effects reverse quickly following cessation treatment in vivo. Low‐HAP affinity BPs may have use as reversible, antiresorptive agents with a rapid on/off profile, which may be useful for maintaining bone health with long‐term BP treatment. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Apoptotic Osteocytes Induce RANKL Production in Bystanders via Purinergic Signaling and Activation of Pannexin Channels

Localized apoptosis of osteocytes, the tissue-resident cells within bone, occurs with fatigue microdamage and activates bone resorption. Osteoclasts appear to target and remove dying osteocytes, resorbing damaged bone matrix as well. Osteocyte apoptosis similarly activates bone resorption with estrogen loss and in disuse. Apoptotic osteocytes trigger viable neighbor (ie, bystander) osteocytes to produce RANKL, the cytokine required for osteoclast activation. Signals from apoptotic osteocytes that trigger this bystander RANKL expression remain obscure. Studying signaling among osteocytes has been hampered by lack of in vitro systems that model the limited communication among osteocytes in vivo (ie, via gap junctions on cell processes and/or paracrine signals through thin pericellular fluid spaces around osteocytes). Here, we used a novel multiscale fluidic device (the Macro-micro-nano, or Mμn) that reproduces these key anatomical features. Osteocytes in discrete compartments of the device communicate only via these limited pathways, which allows assessment of their roles in triggering osteocytes RANKL expression. Apoptosis of MLOY-4 osteocytes in the Mμn device caused increased osteocyte RANKL expression in the neighboring compartment, consistent with in vivo findings. This RANKL upregulation in bystander osteocytes was prevented by blocking Pannexin 1 channels as well as its ATP receptor. ATP alone caused comparable RANKL upregulation in bystander osteocytes. Finally, blocking Connexin 43 gap junctions did not abolish osteocyte RANKL upregulation, but did alter the distribution of RANKL expressing bystander osteocytes. These findings point to extracellular ATP, released from apoptotic osteocytes via Panx1 channels, as a major signal for triggering bystander osteocyte RANKL expression and activating bone remodeling. © 2020 American Society for Bone and Mineral Research.

CITED2 mediates the cross-talk between mechanical loading and IL-4 to promote chondroprotection

Osteoarthritis (OA) pathogenesis is mediated largely through the actions of proteolytic enzymes such as matrix metalloproteinase (MMP) 13. The transcriptional regulator CITED2, which suppresses the expression of MMP13 in chondrocytes, is induced by interleukin (IL)-4 in T cells and macrophages, and by moderate mechanical loading in chondrocytes. We tested the hypothesis that CITED2 mediates cross-talk between IL-4 signaling and mechanical loading-induced pathways that result in chondroprotection, at least in part, by downregulating MMP13. IL-4 induced CITED2 gene expression in human chondrocytes in a dose- and time-dependent manner through JAK/STAT signaling. Mechanical loading combined with IL-4 resulted in additive effects on inducing CITED2 expression and downregulating of MMP13 in human chondrocytes in vitro. In vivo, IL-4 gene knockout (KO) mice exhibited reduced basal levels of CITED2 expression in chondrocytes. While moderate treadmill running induced CITED2 expression and reduced MMP13 expression in wild-type mice, these effects were blunted (for CITED2) or abolished (for MMP13) in chondrocytes of IL-4 gene KO mice. Moreover, intra-articular injections of mouse recombinant IL-4 combined with regular cage activity mitigated post-traumatic OA to a greater degree compared to immobilized mice treated with IL-4 alone. These data suggest that using moderate loading to enhance IL-4 may be a potential therapeutic strategy for chondroprotection in OA.

CITED2 mediates the mechanical loading-induced suppression of adipokines in the infrapatellar fat pad

Adipokines secreted from the infrapatellar fat pad (IPFP), such as adipsin and adiponectin, have been implicated in osteoarthritis pathogenesis. CITED2, a mechanosensitive transcriptional regulator with chondroprotective activity, may modulate their expression. Cited2 haploinsufficient mice (Cited2^+/- ) on a high-fat diet (HFD) exhibited increased body weight and increased IPFP area compared to wild-type (WT) mice on an HFD. While an exercise regimen of moderate treadmill running induced the expression of CITED2, as well as PGC-1α, and reduced the expression of adipsin and adiponectin in the IPFP of WT mice on an HFD, Cited2 haploinsufficiency abolished the loading-induced expression of PGC-1α and loading-induced suppression of adipsin and adiponectin. Furthermore, knocking down or knocking out CITED2 in adipose stem cells (ASCs)/preadipocytes derived from the IPFP in vitro led to the increased expression of adipsin and adiponectin and reduced PGC-1α, and abolished the loading-induced suppression of adipsin and adiponectin and loading-induced expression of PGC-1α. Overexpression of PGC-1α in these ASC/preadipocytes reversed the effects caused by CITED2 deficiency. The current data suggest that CITED2 is a critical regulator in physiologic loading-induced chondroprotection in the context of an HFD and PGC-1α is required for the inhibitory effects of CITED2 on the expression of adipokines such as adipsin and adiponectin in the IPFP.

Potential role for a specialized β3 integrin-based structure on osteocyte processes in bone mechanosensation

Osteocyte processes are an order of magnitude more sensitive to mechanical loading than their cell bodies. The mechanisms underlying this remarkable mechanosensitivity are not clear, but may be related to the infrequent αV β3 integrin sites where the osteocyte cell processes attach to canalicular walls. These sites develop dramatically elevated strains during load-induced fluid flow in the lacunar-canalicular system and were recently shown to be primary sites for osteocyte-like MLO-Y4 cell mechanotransduction. These αV β3 integrin sites lack typical integrin transduction mechanisms. Rather, stimulation at these sites alters Ca2+ signaling, ATP release and membrane potential. In the current studies, we tested the hypothesis that in authentic osteocytes in situ, key membrane proteins implicated in osteocyte mechanotransduction are preferentially localized at or near to β3 integrin-foci. We analyzed these spatial relationships in mouse bone osteocytes using immunohistochemistry combined with Structured Illumination Super Resolution Microscopy, a method that permits structural resolution at near electron microscopy levels in tissue sections. We discovered that the purinergic channel pannexin1, the ATP-gated purinergic receptor P2 × 7R and the low voltage transiently opened T-type calcium channel CaV3.2-1 all reside in close proximity to β3 integrin attachment foci on osteocyte processes, suggesting a specialized mechanotransduction complex at these sites. We further confirmed this observation on isolated osteocytes in culture using STochasitc Optical Resonance Microscopy. These findings identify a possible structural basis for the unique mechanosensation and transduction capabilities of the osteocyte process. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:642-652, 2018.

Lactation-Induced Changes in the Volume of Osteocyte Lacunar-Canalicular Space Alter Mechanical Properties in Cortical Bone Tissue

Osteocytes can remove and remodel small amounts of their surrounding bone matrix through osteocytic osteolysis, which results in increased volume occupied by lacunar and canalicular space (LCS). It is well established that cortical bone stiffness and strength are strongly and inversely correlated with vascular porosity, but whether changes in LCS volume caused by osteocytic osteolysis are large enough to affect bone mechanical properties is not known. In the current studies we tested the hypotheses that (1) lactation and postlactation recovery in mice alter the elastic modulus of bone tissue, and (2) such local changes in mechanical properties are related predominantly to alterations in lacunar and canalicular volume rather than bone matrix composition. Mechanical testing was performed using microindentation to measure modulus in regions containing solely osteocytes and no vascular porosity. Lactation caused a significant (∼13%) reduction in bone tissue-level elastic modulus (p < 0.001). After 1 week postweaning (recovery), bone modulus levels returned to control levels and did not change further after 4 weeks of recovery. LCS porosity tracked inversely with changes in cortical bone modulus. Lacunar and canalicular void space increased 7% and 15% with lactation, respectively (p < 0.05), then returned to control levels at 1 week after weaning. Neither bone mineralization (assessed by high-resolution backscattered scanning electron microscopy) nor mineral/matrix ratio or crystallinity (assessed by Raman microspectroscopy) changed with lactation. Thus, changes in bone mechanical properties induced by lactation and recovery appear to depend predominantly on changes in osteocyte LCS dimensions. Moreover, this study demonstrates that tissue-level cortical bone mechanical properties are rapidly and reversibly modulated by osteocytes in response to physiological challenge. These data point to a hitherto unappreciated role for osteocytes in modulating and maintaining local bone mechanical properties. © 2016 American Society for Bone and Mineral Research.

Regional differences in oxidative metabolism and mitochondrial activity among cortical bone osteocytes

Metabolic oxidative stress has been implicated as a cause of osteocyte apoptosis, an essential step in triggering bone remodeling. However, little is known about the oxidative behavior of osteocytes in vivo. We assessed the redox status and distribution of total and active mitochondria in osteocytes of mouse metatarsal cortical bone in situ. Multiphoton microscopy (MPM) was used to measure fluorescence of reduced pyridine nucleotides (NADH) under normoxic conditions and acutely following extreme (postmortem) hypoxic stress. Under non-hypoxic conditions, osteocytes exhibited no detectable fluorescence, indicating rapid NADH re-oxidation. With hypoxia, NADH levels peaked and returned to near baseline levels over 3h. Cells near the periosteal surface reached maximum NADH levels twice as rapidly as osteocytes near the mid-cortex, due to the time required to initiate NADH accumulation; once started, NADH accumulation followed a similar exponential relationship at all sites. Osteocytes near periosteal and endosteal bone surfaces also had higher mitochondrial content than those in mid-cortex based on immunohistochemical staining for mitochondrial ATPase-5A (Complex V ATPase). The content of active mitochondria, assessed in situ using the potentiometric dye TMRM, was also high in osteocytes near periosteum, but low in osteocytes near endocortical surfaces, similar to levels in mid-cortex. These results demonstrate that cortical osteocytes maintain normal oxidative status utilizing mainly aerobic (mitochondrial) pathways but respond to hypoxic stress differently depending on their location in the cortex, a difference linked to mitochondrial content. An apparently high proportion of poorly functional mitochondria in osteocytes near endocortical surfaces, where increased apoptosis mainly occurs in response to bone remodeling stimuli, further suggest that regional differences in oxidative function may in part determine osteocyte susceptibility to undergo apoptosis in response to stimuli that trigger bone remodeling.

Osteocyte Apoptosis Caused by Hindlimb Unloading is Required to Trigger Osteocyte RANKL Production and Subsequent Resorption of Cortical and Trabecular Bone in Mice Femurs

Osteocyte apoptosis is essential to activate bone remodeling in response to fatigue microdamage and estrogen withdrawal, such that apoptosis inhibition in vivo prevents the onset of osteoclastic resorption. Osteocyte apoptosis has also been spatially linked to bone resorption owing to disuse, but whether apoptosis plays a similar controlling role is unclear. We, therefore, 1) evaluated the spatial and temporal effects of disuse from hindlimb unloading (HLU) on osteocyte apoptosis, receptor activator of NF-κB ligand (RANKL) expression, bone resorption, and loss in mouse femora, and 2) tested whether osteocyte apoptosis was required to activate osteoclastic activity in cortical and trabecular bone by treating animals subjected to HLU with the pan-caspase apoptosis inhibitor, QVD (quinolyl-valyl-O-methylaspartyl-[-2,6-difluorophenoxy]-methylketone). Immunohistochemistry was used to identify apoptotic and RANKL-producing osteocytes in femoral diaphysis and distal trabecular bone, and µCT was used to determine the extent of trabecular bone loss owing to HLU. In both cortical and trabecular bone, 5 days of HLU increased osteocyte apoptosis significantly (3- and 4-fold, respectively, p < 0.05 versus Ctrl). At day 14, the apoptotic osteocyte number in femoral cortices declined to near control levels but remained elevated in trabeculae (3-fold versus Ctrl, p < 0.05). The number of osteocytes producing RANKL in both bone compartments was also significantly increased at day 5 of HLU (>1.5-fold versus Ctrl, p < 0.05) and further increased by day 14. Increases in osteocyte apoptosis and RANKL production preceded increases in bone resorption at both endocortical and trabecular surfaces. QVD completely inhibited not only the HLU-triggered increases in osteocyte apoptosis but also RANKL production and activation of bone resorption at both sites. Finally, µCT studies revealed that apoptosis inhibition completely prevented the trabecular bone loss caused by HLU. Together these data indicate that osteocyte apoptosis plays a central and controlling role in triggering osteocyte RANKL production and the activation of new resorption leading to bone loss in disuse. © 2016 American Society for Bone and Mineral Research.

Pannexin-1 and P2X7-Receptor Are Required for Apoptotic Osteocytes in Fatigued Bone to Trigger RANKL Production in Neighboring Bystander Osteocytes

Osteocyte apoptosis is required to induce intracortical bone remodeling after microdamage in animal models, but how apoptotic osteocytes signal neighboring "bystander" cells to initiate the remodeling process is unknown. Apoptosis has been shown to open pannexin-1 (Panx1) channels to release adenosine diphosphate (ATP) as a "find-me" signal for phagocytic cells. To address whether apoptotic osteocytes use this signaling mechanism, we adapted the rat ulnar fatigue-loading model to reproducibly introduce microdamage into mouse cortical bone and measured subsequent changes in osteocyte apoptosis, receptor activator of NF-κB ligand (RANKL) expression and osteoclastic bone resorption in wild-type (WT; C57Bl/6) mice and in mice genetically deficient in Panx1 (Panx1KO). Mouse ulnar loading produced linear microcracks comparable in number and location to the rat model. WT mice showed increased osteocyte apoptosis and RANKL expression at microdamage sites at 3 days after loading and increased intracortical remodeling and endocortical tunneling at day 14. With fatigue, Panx1KO mice exhibited levels of microdamage and osteocyte apoptosis identical to WT mice. However, they did not upregulate RANKL in bystander osteocytes or initiate resorption. Panx1 interacts with P2X7 R in ATP release; thus, we examined P2X7 R-deficient mice and WT mice treated with P2X7 R antagonist Brilliant Blue G (BBG) to test the possible role of ATP as a find-me signal. P2X7 RKO mice failed to upregulate RANKL in osteocytes or induce resorption despite normally elevated osteocyte apoptosis after fatigue loading. Similarly, treatment of fatigued C57Bl/6 mice with BBG mimicked behavior of both Panx1KO and P2X7 RKO mice; BBG had no effect on osteocyte apoptosis in fatigued bone but completely prevented increases in bystander osteocyte RANKL expression and attenuated activation of resorption by more than 50%. These results indicate that activation of Panx1 and P2X7 R are required for apoptotic osteocytes in fatigued bone to trigger RANKL production in neighboring bystander osteocytes and implicate ATP as an essential signal mediating this process.

The Role of Scleraxis in Fate Determination of Mesenchymal Stem Cells for Tenocyte Differentiation

Mesenchymal stem cells (MSCs) are pluripotent cells that primarily differentiate into osteocytes, chondrocytes, and adipocytes. Recent studies indicate that MSCs can also be induced to generate tenocyte-like cells; moreover, MSCs have been suggested to have great therapeutic potential for tendon pathologies. Yet the precise molecular cascades governing tenogenic differentiation of MSCs remain unclear. We demonstrate scleraxis, a transcription factor critically involved in embryonic tendon development and formation, plays a pivotal role in the fate determination of MSC towards tenocyte differentiation. Using murine C3H10T1/2 pluripotent stem cells as a model system, we show scleraxis is extensively expressed in the early phase of bone morphogenetic protein (BMP)-12-triggered tenocytic differentiation. Once induced, scleraxis directly transactivates tendon lineage-related genes such as tenomodulin and suppresses osteogenic, chondrogenic, and adipogenic capabilities, thus committing C3H10T1/2 cells to differentiate into the specific tenocyte-like lineage, while eliminating plasticity for other lineages. We also reveal that mechanical loading-mediated tenocytic differentiation follows a similar pathway and that BMP-12 and cyclic uniaxial strain act in an additive fashion to augment the maximal response by activating signal transducer Smad8. These results provide critical insights into the determination of multipotent stem cells to the tenocyte lineage induced by both chemical and physical signals.

Green tea polyphenol treatment is chondroprotective, anti-inflammatory and palliative in a mouse post-traumatic osteoarthritis model

Introduction

Epigallocatechin 3-gallate (EGCG), a polyphenol present in green tea, was shown to exert chondroprotective effects in vitro. In this study, we used a posttraumatic osteoarthritis (OA) mouse model to test whether EGCG could slow the progression of OA and relieve OA-associated pain.

Methods

C57BL/6 mice were subjected to surgical destabilization of the medial meniscus (DMM) or sham surgery. EGCG (25 mg/kg) or vehicle control was administered daily for 4 or 8 weeks by intraperitoneal injection starting on the day of surgery. OA severity was evaluated using Safranin O staining and Osteoarthritis Research Society International (OARSI) scores, as well as by immunohistochemical analysis to detect cleaved aggrecan and type II collagen and expression of proteolytic enzymes matrix metalloproteinase 13 (MMP-13) and A disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5). Real-time PCR was performed to characterize the expression of genes critical for articular cartilage homeostasis. During the course of the experiments, tactile sensitivity testing (von Frey test) and open-field assays were used to evaluate pain behaviors associated with OA, and expression of pain expression markers and inflammatory cytokines in the dorsal root ganglion (DRG) was determined by real-time PCR.

Results

Four and eight weeks after DMM surgery, the cartilage in EGCG-treated mice exhibited less Safranin O loss and cartilage erosion, as well as lower OARSI scores compared to vehicle-treated controls, which was associated with reduced staining for aggrecan and type II collagen cleavage epitopes, and reduced staining for MMP-13 and ADAMTS5 in the articular cartilage. Articular cartilage in the EGCG-treated mice also exhibited reduced levels of Mmp1, Mmp3, Mmp8, Mmp13,Adamts5, interleukin 1 beta (Il1b) and tumor necrosis factor alpha (Tnfa) mRNA and elevated gene expression of the MMP regulator Cbp/p300 interacting transactivator 2 (Cited2). Compared to vehicle controls, mice treated with EGCG exhibited reduced OA-associated pain, as indicated by higher locomotor behavior (that is, distance traveled). Moreover, expression of the chemokine receptor Ccr2 and proinflammatory cytokines Il1b and Tnfa in the DRG were significantly reduced to levels similar to those of sham-operated animals.

Conclusions

This study provides the first evidence in an OA animal model that EGCG significantly slows OA disease progression and exerts a palliative effect.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-014-0508-y) contains supplementary material, which is available to authorized users.

Osteocyte apoptosis is required for production of osteoclastogenic signals following bone fatigue in vivo

Osteocyte apoptosis is spatially, temporally and functionally linked to the removal and replacement of microdamage in the bone. Recently we showed that microdamage elicits distinct responses in two populations of osteocytes near the injury site. Osteocytes directly adjacent to microdamage undergo apoptosis, whereas there is a second group of osteocytes located adjacent to the apoptotic population that upregulate expression of osteoclastogenic signaling molecules. In this study we used the pan-caspase inhibitor QVD to test the hypothesis that osteocyte apoptosis is an obligatory step in the production of key osteoclastogenic signals by in situ osteocytes in fatigue-damaged bone. We found, based on real-time PCR and immunohistochemistry assays, that expression of the apoptosis marker caspase-3 as well osteoclastogenic proteins RANKL and VEGF were increased following fatigue, while expression of the RANKL antagonist OPG decreased. However, when apoptosis was inhibited using QVD, these changes in gene expression were completely blocked. This dependence on apoptosis for neighboring non-apoptotic cells to produce signals that promote tissue remodeling also occurs in response to focal ischemic injury in the brain and heart, indicating that osteoclastic bone remodeling follows a common paradigm for localized tissue repair.

Osteocyte differentiation is regulated by extracellular matrix stiffness and intercellular separation

Osteocytes are terminally differentiated bone cells, derived from osteoblasts, which are vital for the regulation of bone formation and resorption. ECM stiffness and cell seeding density have been shown to regulate osteoblast differentiation, but the precise cues that initiate osteoblast-osteocyte differentiation are not yet understood. In this study, we cultured MC3T3-E1 cells on (A) substrates of different chemical compositions and stiffnesses, as well as, (B) substrates of identical chemical composition but different stiffnesses. The effect of cell separation was investigated by seeding cells at different densities on each substrate. Cells were evaluated for morphology, alkaline phosphatase (ALP), matrix mineralisation, osteoblast specific genes (Type 1 collagen, Osteoblast specific factor (OSF-2)), and osteocyte specific proteins (dentin matrix protein 1 (DMP-1), sclerostin (Sost)). We found that osteocyte differentiation (confirmed by dendritic morphology, mineralisation, reduced ALP, Col type 1 and OSF-2 and increased DMP-1 and Sost expression) was significantly increased on soft collagen based substrates, at low seeding densities compared to cells on stiffer substrates or those plated at high seeding density. We propose that the physical nature of the ECM and the necessity for cells to establish a communication network contribute substantially to a concerted shift toward an osteocyte-like phenotype by osteoblasts in vitro.

Activation of resorption in fatigue-loaded bone involves both apoptosis and active pro-osteoclastogenic signaling by distinct osteocyte populations

Osteocyte apoptosis is required to initiate osteoclastic bone resorption following fatigue-induced microdamage in vivo; however, it is unclear whether apoptotic osteocytes also produce the signals that induce osteoclast differentiation. We determined the spatial and temporal patterns of osteocyte apoptosis and expression of pro-osteoclastogenic signaling molecules in vivo. Ulnae from female Sprague-Dawley rats (16-18weeks old) were cyclically loaded to a single fatigue level, and tissues were analyzed 3 and 7days later (prior to the first appearance of osteoclasts). Expression of genes associated with osteoclastogenesis (RANKL, OPG, VEGF) and apoptosis (caspase-3) were assessed by qPCR using RNA isolated from 6mm segments of ulnar mid-diaphysis, with confirmation and spatial localization of gene expression performed by immunohistochemistry. A novel double staining immunohistochemistry method permitted simultaneous localization of apoptotic osteocytes and osteocytes expressing pro-osteoclastogenic signals relative to microdamage sites. Osteocyte staining for caspase-3 and osteoclast regulatory signals exhibited different spatial distributions, with apoptotic (caspase 3-positive) cells highest in the damage region and declining to control levels within several hundred microns of the microdamage focus. Cells expressing RANKL or VEGF peaked between 100 and 300μm from the damage site, then returned to control levels beyond this distance. Conversely, osteocytes in non-fatigued control bones expressed OPG. However, OPG staining was reduced markedly in osteocytes immediately surrounding microdamage. These results demonstrate that while osteocyte apoptosis triggers the bone remodeling response to microdamage, the neighboring non-apoptotic osteocytes are the major source of pro-osteoclastogenic signals. Moreover, both the apoptotic and osteoclast-signaling osteocyte populations are localized in a spatially and temporally restricted pattern consistent with the targeted nature of this remodeling response.

Migration of connective tissue-derived cells is mediated by ultra-low concentration gradient fields of EGF

The directed migration of cells towards chemical stimuli incorporates simultaneous changes in both the concentration of a chemotactic agent and its concentration gradient, each of which may influence cell migratory response. In this study, we utilized a microfluidic system to examine the interactions between epidermal growth factor (EGF) concentration and EGF gradient in stimulating the chemotaxis of connective tissue-derived fibroblast cells. Cells seeded within microfluidic devices were exposed to concentration gradients established by EGF concentrations that matched or exceeded those required for maximum chemotactic responses seen in transfilter migration assays. The migration of individual cells within the device was measured optically after steady-state gradients had been experimentally established. Results illustrate that motility was maximal at EGF concentration gradients between .01- and 0.1-ng/(mL.mm) for all concentrations used. In contrast, the number of motile cells continually increased with increasing gradient steepness for all concentrations examined. Microfluidics-based experiments exposed cells to minute changes in EGF concentration and gradient that were in line with the acute EGFR phosphorylation measured. Correlation of experimental data with established mathematical models illustrated that the fibroblasts studied exhibit an unreported chemosensitivity to minute changes in EGF concentration, similar to that reported for highly motile cells, such as macrophages. Our results demonstrate that shallow chemotactic gradients, while previously unexplored, are necessary to induce the rate of directed cellular migration and the number of motile cells in the connective tissue-derived cells examined.

Activation of bone remodeling after fatigue: differential response to linear microcracks and diffuse damage

Recent experiments point to two predominant forms of fatigue microdamage in bone: linear microcracks (tens to a few hundred microns in length) and "diffuse damage" (patches of diffuse stain uptake in fatigued bone comprised of clusters of sublamellar-sized cracks). The physiological relevance of diffuse damage in activating bone remodeling is not known. In this study microdamage amount and type were varied to assess whether linear or diffuse microdamage has similar effects on the activation of intracortical resorption. Activation of resorption was correlated to the number of linear microcracks (Cr.Dn) in the bone (R(2)=0.60, p<0.01). In contrast, there was no activation of resorption in response to diffuse microdamage alone. Furthermore, there was no significant change in osteocyte viability in response to diffuse microdamage, suggesting that osteocyte apoptosis, which is known to activate remodeling at typical linear microcracks in bone, does not result from sublamellar damage. These findings indicate that inability of diffuse microdamage to activate resorption may be due to lack of a focal injury response. Finally, we found that duration of loading does not affect the remodeling response. In conclusion, our data indicate that osteocytes activate resorption in response to linear microcracks but not diffuse microdamage, perhaps due to lack of a focal injury-induced apoptotic response.

Tendon-derived stem/progenitor cell aging: defective self-renewal and altered fate

Aging is a major risk factor for tendon injury and impaired tendon healing, but the basis for these relationships remains poorly understood. Here we show that rat tendon- derived stem ⁄ progenitor cells (TSPCs) differ in both self-renewal and differentiation capability with age. The frequency of TSPCs in tendon tissues of aged animals is markedly reduced based on colony formation assays. Proliferation rate is decreased, cell cycle progression is delayed and cell fate patterns are also altered in aged TSPCs. In particular, expression of tendon lineage marker genes is reduced while adipocytic differentiation increased. Cited2, a multi-stimuli responsive transactivator involved in cell growth and senescence, is also downregulated in aged TSPCs while CD44, a matrix assembling and organizing protein implicated in tendon healing, is upregulated, suggesting that these genes participate in the control of TSPC function.

Physiological loading of joints prevents cartilage degradation through CITED2

Both overuse and disuse of joints up-regulate matrix metalloproteinases (MMPs) in articular cartilage and cause tissue degradation; however, moderate (physiological) loading maintains cartilage integrity. Here, we test whether CBP/p300-interacting transactivator with ED-rich tail 2 (CITED2), a mechanosensitive transcriptional coregulator, mediates this chondroprotective effect of moderate mechanical loading. In vivo, hind-limb immobilization of Sprague-Dawley rats up-regulates MMP-1 and causes rapid, histologically detectable articular cartilage degradation. One hour of daily passive joint motion prevents these changes and up-regulates articular cartilage CITED2. In vitro, moderate (2.5 MPa, 1 Hz) intermittent hydrostatic pressure (IHP) treatment suppresses basal MMP-1 expression and up-regulates CITED2 in human chondrocytes, whereas high IHP (10 MPa) down-regulates CITED2 and increases MMP-1. Competitive binding and transcription assays demonstrate that CITED2 suppresses MMP-1 expression by competing with MMP transactivator, Ets-1 for its coactivator p300. Furthermore, CITED2 up-regulation in vitro requires the p38δ isoform, which is specifically phosphorylated by moderate IHP. Together, these studies identify a novel regulatory pathway involving CITED2 and p38δ, which may be critical for the maintenance of articular cartilage integrity under normal physical activity levels.

Elevated serum levels of IGF-1 are sufficient to establish normal body size and skeletal properties even in the absence of tissue IGF-1

Use of recombinant insulin-like growth factor 1 (IGF-1) as a treatment for primary IGF-1 deficiency in children has become increasingly common. When untreated, primary IGF-1 deficiency may lead to a range of metabolic disorders, including lipid abnormalities, insulin resistance, and decreased bone density. To date, results of this therapy are considered encouraging; however, our understanding of the role played by IGF-1 during development remains limited. Studies on long-term treatment with recombinant IGF-1 in both children and animals are few. Here, we used two novel transgenic mouse strains to test the long-term effects of elevated circulating IGF-1 on body size and skeletal development. Overexpression of the rat igf1 transgene in livers of mice with otherwise normal IGF-1 expression (HIT mice) resulted in approximately threefold increases in serum IGF-1 levels throughout growth, as well as greater body mass and enhanced skeletal size, architecture, and mechanical properties. When the igf1 transgene was overexpressed in livers of igf1 null mice (KO-HIT), the comparably elevated serum IGF-1 failed to overcome growth and skeletal deficiencies during neonatal and early postnatal growth. However, between 4 and 16 weeks of age, increased serum IGF-1 fully compensated for the absence of locally produced IGF-1 because body weights and lengths of KO-HIT mice became comparable with controls. Furthermore, micro-computed tomography (microCT) analysis revealed that early deficits in skeletal structure of KO-HIT mice were restored to control levels by adulthood. Our data indicate that in the absence of tissue igf1 gene expression, maintaining long-term elevations in serum IGF-1 is sufficient to establish normal body size, body composition, and both skeletal architecture and mechanical function.

Osteocyte apoptosis and control of bone resorption following ovariectomy in mice

INTRODUCTION

Osteocyte apoptosis has been linked to bone resorption resulting from estrogen depletion and other resorptive stimuli; however, precise spatial and temporal relationships between the two events have not been clearly established. The purpose of this study was to characterize the patterns of osteocyte apoptosis in relation to bone resorption following ovariectomy to test whether osteocyte apoptosis occurs preferentially in areas known to activate resorption. Moreover, we report that osteocyte apoptosis is necessary to initiate endocortical remodeling in response to estrogen withdrawal.

MATERIALS AND METHODS

Adult female C57BL/6J mice (17 weeks old) underwent either bilateral ovariectomy (OVX), or sham surgery (SHAM) and were euthanized on days 3, 7, 14, or 21 days after OVX. Diaphyseal cross-sections were stained by immunohistochemistry for activated caspase-3 as a marker of apoptosis. The percentages of caspase-positive stained osteocytes (Casp+Ot.) were measured along major and minor anatomical axes around the femoral diaphysis to evaluate the distribution of osteocyte apoptosis after estrogen loss; resorption surface was measured at the adjacent endocortical regions. In a second study to test whether osteocyte apoptosis plays a regulatory role in the initiation of bone resorption, a group of OVX mice received the pan-caspase inhibitor, QVDOPh, to inhibit osteocyte apoptosis. Remaining experimental and sham groups received either QVD or Vehicle.

RESULTS

OVX increased osteocyte apoptosis in a non-uniform distribution throughout the femoral diaphyses. Increases in Casp+osteocytes were predominantly located in the posterior diaphyseal cortex. Here, the number of apoptotic osteocytes 4- to 7-fold higher than sham controls (p<0.005) by day 3 post-OVX and remained elevated. Increases in resorption post-OVX also occurred along the posterior endocortical surface overlying the region of osteocyte apoptosis, but these increases occurred only at 14 and 21 days post-OVX (p<0.002) well after the increases in osteocyte apoptosis. Treatment with QVD in OVX animals suppressed osteocyte apoptosis, with levels in QVD-treated samples equivalent to baseline. Moreover, the increases in osteoclastic resorption normally observed after estrogen loss did not occur in OVX mice treated with QVD.

CONCLUSIONS

The results of this study demonstrate that osteocyte apoptosis following estrogen loss occur regionally, rather than uniformly throughout the cortex. We also showed that estrogen loss increased osteocyte apoptosis. Apoptotic osteocytes were overwhelmingly localized within the posterior cortical region, the location where endocortical resorption was subsequently activated in ovariectomized mice. Finally, the increases in osteoclastic resorption normally observed after estrogen withdrawal did not occur in the absence of osteocyte apoptosis indicating that this apoptosis is necessary to activate endocortical remodeling following estrogen loss.

Matrix metalloproteinase-3 in articular cartilage is upregulated by joint immobilization and suppressed by passive joint motion

Both underloading and overloading of joints can lead to articular cartilage degradation, a process mediated in part by matrix metalloproteinases (MMPs). Here we examine the effects of reduced loading of rat hindlimbs on articular cartilage expression of MMP-3, which not only digests matrix components but also activates other proteolytic enzymes. We show that hindlimb immobilization resulted in elevated MMP-3 mRNA expression at 6h that was sustained throughout the 21day immobilization period. MMP-3 upregulation was higher in the medial condyle than the lateral, and was greatest in the superficial cartilage zone, followed by middle and deep zones. These areas also showed decreases in safranin O staining, consistent with reduced cartilage proteoglycan content, as early as 7days after immobilization. One hour of daily moderate mechanical loading, applied as passive joint motion, reduced the MMP-3 and ADAMTS-5 increases that resulted from immobilization, and also prevented changes in safranin O staining. Intra-articular injections of an MMP-3 inhibitor, N-isobutyl-N-(4-methoxyphenylsulfonyl)-glycylhydroxamic acid (NNGH), dampened the catabolic effects of a 7day immobilization period, indicating a likely requirement for MMP-3 in the regulation of proteoglycan levels through ADAMTS-5. These results suggest that biomechanical forces have the potential to combat cartilage destruction and can be critical in developing effective therapeutic strategies.

Identification of CITED2 as a negative regulator of fracture healing

The transcription regulator CITED2 (CBP/p300-Interacting-Transactivator-with-ED-rich-tail-2) is known to suppress genes mediating angiogenesis and extracellular matrix (ECM) remodeling. However, it is unclear whether CITED2 has a role in controlling skeletal repair or remodeling. We tested the hypothesis that CITED2 functions in bone fracture healing by suppressing the expression of genes critical to ECM remodeling, angiogenesis and osteogenesis, importantly the matrix metalloproteinases (MMPs). Three hours following mandibular osteotomy or sham surgery of adult rats, osteotomy fronts were harvested and the expression of CITED2 and genes associated with fracture healing was ascertained by quantitative PCR. In parallel, gain-of-function studies examined the effect of overexpressing CITED2 on the expression and activity of several MMPs. In the fractured mandible, CITED2 expression was inversely related to the expression of MMP-2, -3, -9, -13, VEGF, HIF-1alpha, M-CSF, RANK-L, and OPG. Consistent with this, the over-expression of CITED2 in osteoblasts inhibited the expression and activity of MMP-2, -3, -9, and -13. Taken together, the studies suggest that CITED2 is a critical upstream regulator of fracture healing. The suppression of CITED2 early after fracture may allow an optimal initiation of the healing response.

Attachment of osteocyte cell processes to the bone matrix

In order for osteocytes to perceive mechanical information and regulate bone remodeling accordingly they must be anchored to their extracellular matrix (ECM). To date the nature of this attachment is not understood. Osteocytes are embedded in mineralized bone matrix, but maintain a pericellular space (50-80 nm) to facilitate fluid flow and transport of metabolites. This provides a spatial limit for their attachment to bone matrix. Integrins are cell adhesion proteins that may play a role in osteocyte attachment. However, integrin attachments require proximity between the ECM, cell membrane, and cytoskeleton, which conflicts with the osteocytes requirement for a pericellular fluid space. In this study, we hypothesize that the challenge for osteocytes to attach to surrounding bone matrix, while also maintaining fluid-filled pericellular space, requires different "engineering" solutions than in other tissues that are not similarly constrained. Using novel rapid fixation techniques, to improve cell membrane and matrix protein preservation, and transmission electron microscopy, the attachment of osteocyte processes to their canalicular boundaries are quantified. We report that the canalicular wall is wave-like with periodic conical protrusions extending into the pericellular space. By immunohistochemistry we identify that the integrin alphavbeta3 may play a role in attachment at these complexes; a punctate pattern of staining of beta3 along the canalicular wall was consistent with observations of periodic protrusions extending into the pericellular space. We propose that during osteocyte attachment the pericellular space is periodically interrupted by underlying collagen fibrils that attach directly to the cell process membrane via integrin-attachments.

Serum complexes of insulin-like growth factor-1 modulate skeletal integrity and carbohydrate metabolism

Serum insulin-like growth factor (IGF) -1 is secreted mainly by the liver and circulates bound to IGF-binding proteins (IGFBPs), either as binary complexes or ternary complexes with IGFBP-3 or IGFBP-5 and an acid-labile subunit (ALS). The purpose of this study was to genetically dissect the role of IGF-1 circulatory complexes in somatic growth, skeletal integrity, and metabolism. Phenotypic comparisons of controls and four mouse lines with genetic IGF-1 deficits-liver-specific IGF-1 deficiency (LID), ALS knockout (ALSKO), IGFBP-3 (BP3) knockout, and a triply deficient LID/ALSKO/BP3 line-produced several novel findings. 1) All deficient strains had decreased serum IGF-1 levels, but this neither predicted growth potential or skeletal integrity nor defined growth hormone secretion or metabolic abnormalities. 2) IGF-1 deficiency affected development of both cortical and trabecular bone differently, effects apparently dependent on the presence of different circulating IGF-1 complexes. 3) IGFBP-3 deficiency resulted in increased linear growth. In summary, each IGF-1 complex constituent appears to play a distinct role in determining skeletal phenotype, with different effects on cortical and trabecular bone compartments.

Loss of MMP-2 disrupts skeletal and craniofacial development and results in decreased bone mineralization, joint erosion and defects in osteoblast and osteoclast growth

The 'vanishing bone' or inherited osteolysis/arthritis syndromes represent a heterogeneous group of skeletal disorders characterized by mineralization defects of affected bones and joints. Differing in anatomical distribution, severity and associated syndromic features, gene identification in each 'vanishing bone' disorder should provide unique insights into genetic/molecular pathways contributing to the overall control of skeletal growth and development. We previously described and then demonstrated that the novel autosomal recessive osteolysis/arthritis syndrome, multicentric osteolysis with arthritis (MOA) (MIM #605156), was caused by inactivating mutations in the MMP2 gene [Al Aqeel, A., Al Sewairi, W., Edress, B., Gorlin, R.J., Desnick, R.J. and Martignetti, J.A. (2000) Inherited multicentric osteolysis with arthritis: A variant resembling Torg syndrome in a Saudi family. Am. J. Med. Genet., 93, 11-18.]. These in vivo results were counterintuitive and unexpected since previous in vitro studies suggested that MMP-2 overexpression and increased activity, not deficiency, would result in the bone and joint features of MOA. The apparent lack of a murine model [Itoh, T., Ikeda, T., Gomi, H., Nakao, S., Suzuki, T. and Itohara, S. (1997) Unaltered secretion of beta-amyloid precursor protein in gelatinase A (matrix metalloproteinase 2)-deficient mice. J. Biol. Chem., 272, 22389-22392.] has hindered studies on disease pathogenesis and, more fundamentally, in addressing the paradox of how functional loss of a single proteolytic enzyme results in an apparent increase in bone loss. Here, we report that Mmp2-/- mice display attenuated features of human MOA including progressive loss of bone mineral density, articular cartilage destruction and abnormal long bone and craniofacial development. Moreover, these changes are associated with markedly and developmentally restricted decreases in osteoblast and osteoclast numbers in vivo. Mmp2-/- mice have approximately 50% fewer osteoblasts and osteoclasts than control littermates at 4 days of life but these differences have nearly resolved by 4 weeks of age. In addition, despite normal cell numbers in vivo at 8 weeks of life, Mmp2-/- bone marrow cells are unable to effectively support osteoblast and osteoclast growth and differentiation in culture. Targeted inhibition of MMP-2 using siRNA in human SaOS2 and murine MC3T3 osteoblast cell lines resulted in decreased cell proliferation rates. Taken together, our findings suggest that MMP-2 plays a direct role in early skeletal development and bone cell growth and proliferation. Thus, Mmp2-/- mice provide a valuable biological resource for studying the pathophysiological mechanisms underlying the human disease and defining the in vivo physiological role of MMP-2.

Mice lacking cathepsin K maintain bone remodeling but develop bone fragility despite high bone mass

Bone microstructural and biomechanical properties were analyzed in mice genetically lacking cathepsin K (CatK). CatK deficiency (CatK(-/-)) produced mild osteopetrosis, elevated numbers of osteoclasts, regions of disorganized bone microstructure, and increased bone fragility, showing how chronic alteration of enzyme activity during skeletal development dramatically affects bone organization and function.

INTRODUCTION

Mouse models of CatK deficiency recapitulate the osteopetrosis of human pyknodysostosis and allow study of clinically relevant issues: how inhibition of this enzyme activity affects bone integrity structurally and biomechanically. To address these questions, we generated CatK-deficient mice by targeted disruption of the Ctsk gene and compared their bone structural and mechanical properties with wildtype (WT) controls.

MATERIALS AND METHODS

Standard histomorphometric and biomechanical analyses were performed on femora from C57BL/6J male and female CatK(-/-), CatK(+/-), and WT mice.

RESULTS

CatK(-/-) femora exhibited the mild metaphyseal osteopetrosis, a greater cortical bone area and thickness, normal bone strength, but a high degree of brittleness (nearly 50-70% decrease in postyield displacement versus WT) and a 30-40% reduction in the work-to-failure. In cancellous bone, osteoclast numbers and resorption surface were increased markedly (approximately 150% and 50%, respectively), despite the overall decrease in net bone resorption for CatK-deficient mice. Bone formation indices were altered in CatK(-/-) mice as well, with significant increases in mineral appositional rate, but not in bone formation surface; these data suggest difference in osteoblast work but not in their recruitment in CatK deficiency. CatK-deficient cortical bones had large areas of woven bone and intracortical resorption spaces within the disorganized tissue. Bone phenotype in CatK(-/-) was similar in males and females.

CONCLUSIONS

Genetic CatK deficiency in mice results not only in the impairment of osteoclast function and osteopetrosis, but also altered osteoblast function, defective tissue organization, and very brittle bones. Whether this bone fragility in CatK deficiency results entirely from indirect effects of suppressed bone turnover because of impaired osteoclast function or perhaps represents a previously unappreciated more direct role for CatK in bone formation remains to be established.

High-dose risedronate treatment partially preserves cancellous bone mass and microarchitecture during long-term disuse

Disuse induces rapid and severe bone loss in larger mammals as a result of greatly elevated osteoclastic resorption. In this study, we tested whether risedronate (RIS), a potent inhibitor of osteoclastic activity, would effectively prevent cancellous bone loss in female beagles (5-7 years old, N = 28) subjected to single forelimb immobilization (IM) for 12 months. Age-matched, non-IM dogs served as controls (Con). Half the animals from each group received RIS 1 mg/kg p.o. daily (Con + RIS, IM + RIS). Remaining dogs received sterile water (Con, IM). Histomorphometry showed that IM caused a dramatic reduction in cancellous bone mass (-71%) of distal 2nd metacarpals, characterized by marked decreases in trabecular width (-51%) and number (-41%), and 4-fold increases in the indices of bone resorption (eroded surface, osteoclast number, and surface). Bone formation indices (calcein-labeled surface, osteoid surface, and bone formation rate) were also significantly higher in IM than in controls. Activation frequency in IM increased about 4-fold beyond control level. RIS treatment reduced, but did not abolish cancellous bone loss due to immobilization. IM animals treated with RIS lost nearly 50% of cancellous bone mass, while trabecular width and number were reduced by 31% and 25%, respectively. In both RIS-treated control and IM animals, overall bone formation parameters (mineralized bone surface fraction and bone formation rate) remained roughly at intact control levels; however, mineral apposition rate relative to intact control was reduced 40% in RIS-treated control and 86% in RIS-treated IM animals. These results indicate that high-dose RIS treatment might suppress osteoblastic function, especially under long-term disuse. Interestingly, bone resorption parameters in RIS-treated IM animals reached levels even higher than in vehicle-treated IM animals; values for eroded surface, osteoclast number, and surface were 84%, 53%, and 83% above vehicle-treated IM values, respectively. Our data indicate that risedronate treatment is partially effective in preventing cancellous bone loss during long-term disuse. Moreover, our results suggest that bisphosphonates can impair the ability of mature osteoclasts to resorb bone, but cannot overcome the strong stimulus for osteoclast recruitment caused by long-term disuse.

In situ measurement of solute transport in the bone lacunar-canalicular system

Solute transport through the bone lacunar-canalicular system is believed to be essential for osteocyte survival and function but has proved difficult to measure. We report an approach that permits direct measurement of real-time solute movement in intact bones. By using fluorescence recovery after photobleaching, the movement of a vitally injected fluorescent dye (sodium fluorescein) among individual osteocytic lacunae was visualized in situ beneath the periosteal surface of mouse cortical bone at depths up to 50 microm with laser scanning confocal microscopy. Transport was analyzed by using a two-compartment mathematical model of solute diffusion that accounted for the characteristic anatomical features of the lacunar-canalicular system. The diffusion coefficient of fluorescein (376 Da) was determined to be 3.3 +/- 0.6 x 10(-6) cm2/sec, which is 62% of its diffusion coefficient in water and is similar to diffusion coefficients measured for comparably sized molecules in cartilage. The diffusion of fluorescein in bone is also consistent with the presence of an osteocyte pericellular matrix whose structure resembles that proposed for the endothelial glycocalyx [Squire, J. M., Chew, M., Nneji, G., Neal, C., Barry, J. & Michel, C. (2001) J. Struct. Biol. 136, 239-255]. To our knowledge, this is the first instance where the dynamics of molecular movement has been measured directly in the bone lacunar-canalicular system. This in situ imaging approach should also facilitate the analysis of convection-based transport mechanisms in bones of living animals.

In situ measurement of solute transport in the bone lacunar-canalicular system

Solute transport through the bone lacunar-canalicular system is believed to be essential for osteocyte survival and function but has proved difficult to measure. We report an approach that permits direct measurement of real-time solute movement in intact bones. By using fluorescence recovery after photobleaching, the movement of a vitally injected fluorescent dye (sodium fluorescein) among individual osteocytic lacunae was visualized in situ beneath the periosteal surface of mouse cortical bone at depths up to 50 microm with laser scanning confocal microscopy. Transport was analyzed by using a two-compartment mathematical model of solute diffusion that accounted for the characteristic anatomical features of the lacunar-canalicular system. The diffusion coefficient of fluorescein (376 Da) was determined to be 3.3 +/- 0.6 x 10(-6) cm2/sec, which is 62% of its diffusion coefficient in water and is similar to diffusion coefficients measured for comparably sized molecules in cartilage. The diffusion of fluorescein in bone is also consistent with the presence of an osteocyte pericellular matrix whose structure resembles that proposed for the endothelial glycocalyx [Squire, J. M., Chew, M., Nneji, G., Neal, C., Barry, J. & Michel, C. (2001) J. Struct. Biol. 136, 239-255]. To our knowledge, this is the first instance where the dynamics of molecular movement has been measured directly in the bone lacunar-canalicular system. This in situ imaging approach should also facilitate the analysis of convection-based transport mechanisms in bones of living animals.

Osteocyte density in woven bone

Woven bone forms rapidly during tissue growth, following injury and in response to certain anabolic stimuli. Functional differences between woven and lamellar bone may be due, in part, to differences in osteocyte density (cells per unit tissue). Woven bone has been estimated to contain four to eight times more osteocytes than lamellar bone, although primary data to support this assertion are limited. Given recent findings implicating osteocytes as regulators of bone remodeling, bone formation and bone volume, such large differences in osteocyte density between woven and lamellar bone may have important consequences. In this study, we compared the density of osteocyte lacunae (lacunae/mm(2) tissue) in rat lamellar bone with that in woven bone formed under several different circumstances. We found that the lacunar density of lamellar cortical bone in the rat (834+/-83 cells/mm2, mean+/-SD) did not differ significantly from that of periosteal woven bone formed via intramembranous osteogenesis, either in response to mechanical loading (921+/-204 cells/mm2) or in the periosteal buttressing region of the fracture callus (1138+/-168 cells/mm2). In contrast, lacunar density of endochondrally derived woven bone in the center (gap) region of fracture callus was nearly 100% greater (1875+/-270 cells/mm2) than in lamellar cortical bone while lacunar density of primary spongiosa of the growth plate was 40% greater (1674+/-228 cells/mm2) than that in lamellar cancellous bone (1189+/-164). These findings demonstrate that lacunar density in woven bone varies depending on skeletal site and developmental history and appears to be elevated in endochondrally derived woven bone adjacent to marrow space. Given the considerable evidence supporting osteocytes as local initiators of bone remodeling, we suggest that woven bone with increased lacunar density may undergo remodeling at an accelerated rate.

A single percutaneous injection of recombinant human bone morphogenetic protein-2 accelerates fracture repair

BACKGROUND

Recombinant human bone morphogenetic protein-2 (rhBMP-2), surgically implanted with a matrix material, has been shown to induce bone formation and enhance fracture repair. The purpose of this investigation was to test the hypothesis that a single, local, percutaneous injection of rhBMP-2 would accelerate fracture-healing in a standard rat femoral fracture model.

METHODS

Fractures were created, following intramedullary pinning, in the femora of 144 male Sprague-Dawley rats. The animals were divided into three groups of forty-eight each. Six hours after the fracture, one group received an injection of 80 micro g of rhBMP-2 in 25 micro L of buffer vehicle, one received an injection of 25 micro L of buffer vehicle alone, and one did not receive an injection. Twelve animals from each of these three groups were killed at one, two, three, and four weeks after treatment, and the femora were harvested for torsional biomechanical testing. An additional cohort of seventy-two animals, in which a fracture was also created, was divided into the same three treatment groups; six animals from each of these groups was killed at one, two, three, and four weeks; and the femora were processed for qualitative histological analysis.

RESULTS

Torsional biomechanical testing indicated that the stiffness of the rhBMP-2-treated fractures was twice that of both control groups at the two, three, and four-week time-points. The strength of the rhBMP-2-treated fractures was 34% greater than that of the buffer-treated controls (p = 0.03) at three weeks and, at four weeks, was 60% and 77% greater than that of the buffer-treated controls and that of the untreated controls, respectively (p < 0.005). At four weeks, the stiffness and strength of the rhBMP-2-treated fractures were equal to those of the intact contralateral femora, whereas the buffer-treated and untreated fractures were significantly weaker than the intact femora. At two and three weeks, large areas of bone formation, typically spanning the fracture, were observed histologically in the rhBMP-2-treated sites. In contrast, the control fractures exhibited primarily soft cartilaginous callus at these time-points. By four weeks, remodeling of the hard callus and recorticalization were observed in the rhBMP-2-treated fracture sites, whereas cartilage and/or soft tissue was still present in the control fracture sites.

CONCLUSIONS

These data demonstrate that a single, local, percutaneous injection of rhBMP-2 accelerates fracture repair in this rat femoral fracture model. This effect appears to result from a combination of the induction of bone formation at the fracture site and acceleration of the rate at which the fracture callus matures.

Hierarchical relationship between bone traits and mechanical properties in inbred mice

Osteoporotic fracture incidence and underlying risk factors like low peak bone mass are heritable, but the genetic basis of osteoporosis remains poorly understood. Based on beam theory, stating that mechanical properties of a structure depend on both the amount and quality of the constituent materials, we investigated the relationship between whole bone mechanical properties and a set of morphological and compositional traits in femurs of eight inbred mouse strains. K-means cluster analysis revealed that individual femora could be classified reliably according to genotype based on the combination of bone area (tissue amount), moment of inertia (tissue distribution), and ash content (tissue quality). This trait combination explained 66-88% of the inter-strain variability in four whole-bone mechanical properties that describe all aspects of the failure process, including measures of brittleness. Stiffness and maximum load were functionally associated with cortical area, while measures of brittleness were associated with ash content. In contrast, work-to-failure was not directly related to a single trait but depended on a combination of trait magnitudes. From these findings, which were entirely consistent with established mechanical theory, we developed a hierarchical paradigm relating the mechanical properties that define bone fragility with readily measurable phenotypic traits that exhibit strong heritability. This paradigm will help guide the search for genes that underlie fracture susceptibility and osteoporosis. Moreover, because the traits we examined are measurable with non-invasive means, this approach may also prove directly applicable to osteoporosis risk assessment.

Spatial distribution of Bax and Bcl-2 in osteocytes after bone fatigue: complementary roles in bone remodeling regulation?

Osteocyte apoptosis appears to play a key role in the mechanism by which osteoclastic resorption activity targets bone for removal, because osteocyte apoptosis occurs in highly specific association with microdamage and subsequent remodeling after fatigue. However, beyond terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-biotin nick end labeling (TUNEL) assay, little is known about the mechanisms controlling osteocyte apoptosis in vivo. In the current studies, expression of Bax, a proapoptotic gene product, and Bcl-2, an antiapoptotic gene product, was determined in osteocytes of fatigued rat bone using immunocytochemical staining and compared with TUNEL staining patterns. Bax and Bcl-2 were evident in osteocytes by 6 h after loading. Moreover, Bax and Bcl-2 in osteocytes were expressed differently as a function of distance from microdamage sites. The peak of Bax expression and TUNEL+ staining in osteocytes was observed immediately at the microcrack locus, which is where bone resorption occurs in this system; in contrast, Bcl-2 expression, the antiapoptotic signal, reached its greatest level at some distance (1-2 mm) from microcracks. These data suggest that near sites of microinjury in bone, those osteocytes that do not undergo apoptosis are prevented from doing so by active protection mechanisms. Moreover, the zone of apoptotic osteocytes around microcracks was effectively "walled in" by a surrounding halo of surviving osteocytes actively expressing Bc1-2. Thus, the expression pattern of apoptosis-inhibiting gene products by osteocytes surrounding the apoptotic osteocyte at microdamage sites also may provide important signals in the guidance of resorption processes that occur in association with osteocyte apoptosis after fatigue.

Estrogen regulation of growth and alkaline phosphatase expression by cultured human bone marrow stromal cells

Estrogen has been reported to regulate the growth and differentiation of cultured murine osteoprogenitor cells in bone marrow stroma. This study tested the ability of 17beta-estradiol (E2) to regulate growth and expression of alkaline phosphatase (ALP), an osteoblastic differentiation marker, in strains of normal human bone marrow stromal cells derived from different donors. In eight strains examined, E2 at 1 and 10 nM produced at most modest effectxs on growth and ALP activity. Growth inhibition, seen in 4 of the 8 strains, was more common than stimulation (2 of the 8 strains); the greatest observed E2 effect was an inhibition of ca. 50%. E2 altered ALP activity less dramatically than cell growth. Differences from control in total ALP per culture were seen in only two strains: one was a reduction, one an increase. Colony forming assays were used to determine if E2 changed the proportion of ALP-expressing cells in marrow stromal cell cultures. In contrast to growth experiments, ALP expression under colony forming conditions (200 cells per 35 mm-diameter well) was dependent on the type of serum supplementation used. Under permissive conditions using medium supplemented with 10% charcoal-treated fetal bovine serum, 10 nM E2 increased the number of ALP-positive colonies (cfu-ap) but not the total number of colonies formed (cfu-f). When cells cultured in the presence or absence of 10 nM E2 were replated at colony forming densities, significantly higher proportions of cfu-ap were found in 2 of 6 strains examined, while pretreatment with E2 affected the number of cfu-f in only 1 of the 6 strains. Similar results were obtained when colony formation was carried out in the presence of dexamethasone and ascorbate, although these agents themselves increased the formation of both cfu-f and cfu-ap. These results show that the direct effects of E2 on human marrow stromal cells are small and vary depending on the cell strain and on the experimental conditions; however, the E2 actions observed in this study were consistent with reports that E2 exerts direct actions on osteoblasts and osteoblast progenitor cells that favor rather than suppress their phenotypic expression.

Prevention of fracture healing in rats by an inhibitor of angiogenesis

Angiogenesis is considered essential to fracture healing, but its role in the healing process remains poorly understood. Angiogenesis inhibitors, which block new blood vessel formation by specifically targeting vascular cells, are currently under development for use in cancer chemotherapy, and are potentially powerful tools for defining the consequences of angiogenic impairment on fracture healing. In this study, we directly tested the effects of the angiogenesis inhibitor TNP-470 on the healing of closed femoral fractures in an established rat model system. Beginning 1 day after fracture, animals received either angiogenesis inhibitor at a therapeutically effective antitumor dose, or a weight-adjusted amount of carrier vehicle. The progress of fracture healing was assessed at weekly intervals for 21 days by radiography and histology; functional assessment was carried out at day 24 by biomechanical testing. By all three criteria, treatment with the angiogenesis inhibitor completely prevented fracture healing. Formation of both callus and periosteal woven bone were suppressed, indicating that both the intramembranous and endochondral pathways of osteogenesis were affected. The resulting tissue resembled "atrophic nonunions" often seen clinically in cases of failed fracture healing, but rarely achieved in animal models. These results show that angiogenesis is essential to very early stages of fracture healing, and suggest this model system may be useful for understanding the mechanisms underlying fracture nonunions due to vascular impairment. Finally, the data raise the possibility that impairment of fracture healing may be an adverse effect of clinical treatments with antiangiogenic drugs.

Effects of diabetes and steroids on fracture healing

Fracture healing is largely controlled by local regulatory interactions among cells and tissues near the site of injury; however, many systemic hormones including insulin, the glucocorticoids, and the gonadal steroids also can influence the course of tissue repair, particularly in the case of pathologic hormone excess or deficiency. Using well-defined animal models, recent studies have established that deficiencies in insulin and estrogen impair fracture healing, but data from this type of experiment are limited. Still, the similarities between morphogenetic events in fracture healing and those found in normal bone development and remodeling suggest that testable predictions can be made concerning hormonal effects on the progress of fracture healing. One concept that has received some direct experimental support in fracture healing model studies is that systemic hormones exert pleiotropic effects on callus tissue by regulating the expression and activity of local growth factors. Further verification of this and other predicted hormone effects should increase our understanding of the fundamental mechanisms underlying fracture repair, and may aid development of means to improve fracture healing in states of altered endocrine function.

Detection of c-fos expression in benign and malignant musculoskeletal lesions

The proto-oncogene c-fos has been implicated in the development of both benign and malignant lesions of bone. Although c-fos expression in such lesions has been well studied in transgenic mouse models, less is known about its role in human musculoskeletal pathology. To clarify this relationship, we used in situ hybridization to localize c-fos m-RNA transcripts in 26 fibrous lesions (eight cases of extra-abdominal fibromatosis and six cases each of fibrous dysplasia, fibrosarcoma, and malignant fibrous histiocytoma of bone) as well as six chondrosarcomas and eight conventional high grade osteosarcomas. We found detectable levels of c-fos expression in tissues from each type of lesion tested. Moreover, all fibrous lesions consistently demonstrated high levels of expression in a majority of cells in each lesion. Chondrosarcomas and osteosarcomas exhibited more heterogeneity in c-fos expression than fibrous tissues. Three of six chondrosarcomas showed moderate expression of c-fos while only one of six was considered high. Similarly, only three of eight osteosarcomas had high expression of c-fos. These findings indicate that the expression of c-fos may be important in the development of a broad range of fibrous lesions as well as in bone and cartilaginous tumors. Additionally, this is the first report, to our knowledge, of detectable c-fos m-RNA in human chondrosarcoma.

Steroid regulation of human placental integrins: suppression of alpha2 integrin expression in cytotrophoblasts by glucocorticoids

Maintenance of uterine-placental attachment during human pregnancy may depend at least partly on adhesive interactions between cytotrophoblasts and their extracellular matrix (ECM). Such interactions are often mediated by integrins, signal-transducing heterodimeric transmembrane glycoproteins. We previously showed that glucocorticoid (GC) suppressed the expression of collagen and laminin in human placenta; here we show that GC also modulates the expression by human cytotrophoblasts of the integrin subunits alpha2 and beta1, components of a known receptor for these ECM ligands. Cytotrophoblasts were isolated from human term placentas, cultured up to 4 days in the presence of 0-1000 nM dexamethasone (DEX), and assayed for 1) integrin messenger RNA (mRNA) levels by Northern hybridization, 2) integrin subunit synthesis after [35S]methionine labeling, or 3) cell surface integrin levels after 125I labeling by lactoperoxidase. In four independent experiments, 100 nM DEX reduced mRNA levels for integrin alpha2 to 6+/-1% of the control value. This effect was similar between 1-4 days of treatment and was dose dependent between 1-1000 nM DEX. Cortisol treatment (100 nM) inhibited levels of integrin alpha2 mRNA, but 100 nM testosterone, estradiol, and progesterone were less effective, suggesting that this response was specific to GC. In immunoprecipitation studies, treatment of cytotrophoblasts with 100 nM DEX for 2 days reduced the rates of synthesis of the alpha2 integrin subunit as well as its expression on the cell surface to 1-10% of control levels. DEX effects on the beta1 integrin subunit were less dramatic. DEX reduced beta1 mRNA levels to only 69+/-8% of control levels, a smaller reduction compared with effects on alpha2 integrin mRNA. DEX inhibited beta1 protein synthesis and cell surface expression to 60-70% of control levels. In all experiments, DEX had no effect on total protein synthesis. Thus, our results demonstrate that GC treatment specifically and markedly down-regulates expression of alpha2 integrin subunit by human cytotrophoblasts. This finding is consistent with the concept that uterine-placental adherence across gestation may be regulated by coordinate effects on ECM ligands and cellular adhesion receptors.

Parathyroid hormone enhances fracture healing. A preliminary report

This investigation tested the hypothesis that daily parenterally administered parathyroid hormone (1-34) improves fracture healing. Twenty, 3-month-old, male Sprague Dawley rats weighing approximately 400 g each, underwent the production of closed, unilateral mid-diaphyseal femoral fractures. Animals were divided into two groups of 10; the animals received either a daily subcutaneous injection of delivery vehicle (0.9% saline) or 80 micrograms/kg parathyroid hormone. On Day 21 after fracture the animals were euthanized, the femurs were removed and subjected to biomechanical testing, bone densitometry (dual energy x-ray absorptiometry, peripheral quantitative computed tomography), and histologic examination. Treatment with parathyroid hormone resulted in statistically significant increases in callus area and strength. Histologic examination of the calluses showed an increase in the amount of new bone formed. No differences were observed in the weights of the animals or the sizes of the bones. Values obtained using dual energy x-ray absorptiometry and peripheral quantitative computed tomography indicate an increase in density in the parathyroid hormone treated fractures consistent with the histologic appearance and the findings of increased strength, although these bone density changes did not achieve statistical significance. These results suggest that parenterally administered parathyroid hormone (1-34) may enhance or accelerate normal fracture healing and support the concept that this hormone be tested clinically as a systemic treatment for fractures that are slow to heal.

Spatial and temporal distribution of CD44 and osteopontin in fracture callus

The multifunctional adhesion molecule CD44 is a major cell-surface receptor for hyaluronic acid (HUA). Recent data suggest that it may also bind the ubiquitous bone-matrix protein, osteopontin (OPN). Because OPN has been shown to be a potentially important protein in bone remodelling, we investigated the hypothesis that OPN interactions with the CD44 receptor on bone cells participate in the regulation of the healing of fractures. We examined the spatial and temporal patterns of expression of OPN and CD44 in healing fractures of rat femora by in situ hybridisation and immunohistochemistry. We also localised HUA in the fracture callus using biotinylated HUA-binding protein. OPN was expressed in remodelling areas of the hard callus and was found in osteocytes, osteoclasts and osteoprogenitor cells, but not in cuboidal osteoblasts which were otherwise shown to express osteocalcin. The OPN signal in osteocytes was not uniformly distributed, but was restricted to specific regions near sites where OPN mRNA-positive osteoclasts were attached to bone surfaces. In the remodelling callus, intense immunostaining for CD44 was detected in osteocyte lacunae, along canaliculi, and on the basolateral plasma membrane of osteoclasts, but not in the cuboidal osteoblasts. HUA staining was detected in fibrous tissues but little was observed in areas of hard callus where bone remodelling was progressing. Our findings suggest that OPN, rather than HUA, is the major ligand for CD44 on bone cells in the remodelling phase of healing of fractures. They also raise the possibility that such interactions may be involved in the communication of osteocytes with each other and with osteoclasts on bone surfaces. The interactions between CD44 and OPN may have important clinical implications in the repair of skeletal tissues.

Bone and cartilage formation in an experimental model of distraction osteogenesis

OBJECTIVES

(a) To develop a reliable and reproducible system for distraction osteogenesis in the rat to establish a model for future investigations of bone repair and regeneration. (b) To describe and characterize the histological events in distraction osteogenesis in the rat and to determine whether cartilage development is a normal component of the process.

STUDY DESIGN

Species-specific, longitudinal time study.

METHODS

Twenty rats underwent production of a middiaphyseal femoral osteotomy and application of a monolateral external fixator specifically designed for distraction. Animals were divided into five groups based on the time and extent of lengthening.

RESULTS

During distraction, gap tissue showed collagen bundles and fibroblasts that were oriented longitudinally to the direction of the distraction force. Woven bone appeared to be laid down on these collagen scaffolds, and the newly formed vascular sinuses appeared to be the sites from which bone formation was initiated within the distraction gap. All groups undergoing active distraction showed intramembranous ossification in the distraction gap and endochondral ossification peripherally. However, when distraction was discontinued, endochondral ossification was observed in the gap.

CONCLUSION

Distraction produces an environment in the distraction gap that suppresses the formation of cartilage. The formation of cartilage by injured periosteum, however, is obligatory and does not appear to be influenced by distraction. Bone formation within the distraction gap occurs where angiogenesis develops.

Spatial and temporal expression of fibril-forming minor collagen genes (types V and XI) during fracture healing

Skeletal development involves the coordinated participation of several types of collagen, including both major and minor fibrillar collagens. Although much is known about the major fibrillar collagens, such as types I and II, less is known about the minor fibrillar collagens, and their role in the repair and regeneration of bone has not been extensively studied. To clarify the role of minor fibrillar collagens in fracture repair, we examined the spatial and temporal expression of mRNAs for pro-alpha 2(V) collagen and pro-alpha 1(XI) collagen in healing fractures in the rat by in situ hybridization and compared their patterns of expression with those of mRNAs for pro-alpha 1(I) collagen, pro-alpha 1(II) collagen, and osteocalcin. A strong signal for pro-alpha 2(V) was detected in the periosteal osteoprogenitor cells, whereas osteocalcin mRNA was strongly expressed only in the deep layers of the hard callus. The distribution of the pro-alpha 2(V) signal was correlated with that of pro-alpha 1(I) but was mutually exclusive of that of pro-alpha 1(II). The expression of pro-alpha 1(XI) mRNA was synchronously regulated with that of pro-alpha 1(II) during chondrogenesis in the soft callus. In the hard callus, pro-alpha 1(XI) signal was found in osteoblastic cells at the site of intramembranous and endochondral ossification. These cells simultaneously expressed pro-alpha 2(V), although they were negative for pro-alpha 1(II). These findings suggest that the alpha 2(V) collagen chain participates in the formation of the noncartilaginous fibrillar network in the hard callus and preferentially contributes to the initial stage of the intramembranous bone formation. Recent reports have revealed that type-XI collagen, which had been classified as a cartilage-type collagen, is not necessarily specific for cartilage. The present results advanced this recognition and demonstrated a coexpression of alpha 1(XI) mRNA and alpha 2(V) mRNA in the noncartilaginous tissues in the fracture callus; this suggests the presence of tissue-specific and stage-specific heterotrimers consisting of alpha 1(XI) and alpha 2(V) collagen chains and the association of such hybrid trimers with the major fibrillar collagens in the process of fracture healing.

Stimulation of systemic bone formation induced by experimental blood loss

Direct physical injury to bone marrow is associated with a systemic osteogenic response. However, blood loss, a condition that stimulates hemopoietic stem cells, also may activate osteoprogenitor cells in the bone marrow. To determine if bleeding induces a systemic osteogenic response, the mineral appositional rates and osteoblast numbers were determined in the bones of rats that were subjected to controlled cardiac bleeding and compared with those of rats subjected to ablation of their tibial bone marrow. In addition, a study of the kinetics of the osteogenic responses during the first 10 days after operative treatment was performed by quantitating the serum levels of biochemical indices known to be associated with systemic bone formation. The results showed that animals that sustained acute blood loss (1% or 3% body weight) or injury to their tibial bone marrow had statistically significant increases in mineral appositional rate, osteoblast number, and serum levels of osteogenic growth peptide. The kinetics studies showed that osteogenic growth peptide levels peaked on the tenth postoperative day and declined sharply thereafter. An enhancement of serum osteocalcin activity occurred only on the second postoperative day, was increased in all experimental groups when compared with untreated control animals, but immediately declined to baseline levels. Alkaline phosphatase activities increased in the experimental groups, peaking on Day 10 after tibial bone marrow ablation and on Day 12 in the group that underwent bleeding. These findings suggest that bleeding alone, independent of any skeletal trauma, may evoke a systemic osteogenic response. This response is similar in its timing and magnitude to that which has been shown to follow direct physical injury to bone marrow. The observation that systemic bone formation follows bone marrow activation induced by two different stimuli suggests that these responses may be mediated by common regulatory mechanisms. The ability to trigger or control these responses may form the basis for future therapeutic strategies to enhance bone formation.

The expression of cytokine activity by fracture callus

Cytokines, a group of proteins known to regulate hemopoietic and immune functions, are also involved in inflammation, angiogenesis, and bone and cartilage metabolism. Since all of these processes occur following bone injury, or are known to contribute to wound repair mechanisms, this investigation sought to test the hypothesis that cytokines are involved in fracture healing. Two sets of 60 male Sprague-Dawley rats underwent the production of standard closed femoral fractures. The animals were then euthanized in groups of 15 on days 3, 7, 14, and 21 postfracture. A separate control group was also used for the harvesting of intact unfractured bone. At the time of euthanasia, calluses or bone specimens were explanted to organ culture and treated with either media alone or media containing the inducing agents lipopolysaccharide or concanavalin A. A titration of conditioned medium from these cultures was then added to factor-dependent clonal cell lines that are known to be specifically responsive to interleukin-1, interleukin-6, granulocyte-macrophage colony stimulating factor or macrophage-colony stimulating factor. To confirm the identities of each of these cytokines, neutralizing antibody studies were performed. The results showed that interleukin-1 is expressed at very low constitutive levels throughout the period of fracture healing but can be induced to high activities in the early inflammatory phase (day 3). Granulocyte-macrophage colony stimulating factor showed no constitutive activity but could also be induced to high activities with lipopolysaccharide. The ability of these two cytokines to be induced declined progressively as fracture healing proceeded. Interleukin-6 showed high constitutive activity early in the healing process (day 3), and treatment with inducing agent did not increase the activity of this cytokine at this timepoint. Lipopolysaccharide did increase interleukin-6 activity in day 7 and 14 fracture calluses. Although macrophage-colony stimulating factor is thought to be involved in a variety of metabolic bone conditions, it could not be detected or induced from any of the callus samples. Moreover, none of the samples of unfractured bone showed constitutive or inducible activities for any of these cytokines. A separate experiment in which calluses and samples of unfractured bone from similar cultures were examined histologically and tested for DNA or protein synthesis at two timepoints in the culture period (days 1 and 4) showed that tissue viability was maintained. Thus the inability to detect macrophage colony-stimulating factor in fracture callus or any cytokine activity in unfractured bones was not due to cell death.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure-activity studies of the s-echistatin inhibition of bone resorption

Synthetic Arg-Gly-Asp (RGD)-containing peptides were examined in bone resorption or attachment and detachment assays with isolated mammalian osteoclasts in an effort to elucidate the mechanistic and structural basis for the inhibition of bone resorption by s-echistatin. Bone resorption was the process most sensitive to inhibition by s-echistatin, with IC50 = 0.3 nM; inhibition of attachment to bone or detachment (lamellipodial retraction) was 30- to 70-fold less sensitive, with IC50 = 10 or 20 nM, respectively. Single amino acid substitutions within the 49-residue sequence of s-echistatin showed that although the efficacy of s-echistatin is dependent on the Arg24-Gly25-Asp26 sequence, additional residues, including Asp27, Met28, and Cys39, are also critical for potent inhibition of the resorbing activity of isolated rat osteoclasts. Because of the identification of the av beta 3 as the primary integrin on rat osteoclasts interacting the RGD peptides (Helfrich et al.), we examined the possibility of modeling bone resorption with other beta 3-mediated processes. Specifically, av beta 3 endothelial cell (human or rat) attachment to vitronectin and aIIb beta 3 platelet aggregation were compared with bone resorption for sensitivity to s-echistatin analogs, linear RGD peptides, and cyclic RGD peptides. Essentially no similarity in sensitivity to RGD peptides were observed between bone resorption, platelet aggregation, or endothelial cell attachment. Because rat osteoclasts and human giant cell tumors (osteoclastomas) shared similar sensitivity to s-echistatin and rat and human endothelial cells showed a similar sensitivity profile to RGD peptides, the dissimilarity of bone resorption to other beta 3-mediated processes cannot be explained in terms of species differences.(ABSTRACT TRUNCATED AT 250 WORDS)