Eccentric localization of osteocytes expressing enzymatic activities, protein, and mRNA signals for type 5 tartrate-resistant acid phosphatase (TRAP)

Pathophysiologic Spine Adaptations and Countermeasures for Prolonged Spaceflight

Low back pain due to spaceflight is a common complaint of returning astronauts. Alterations in musculoskeletal anatomy during spaceflight and the effects of microgravity (μg) have been well-studied; however, the mechanisms behind these changes remain unclear. The National Aeronautics and Space Administration has released the Human Research Roadmap to guide investigators in developing effective countermeasure strategies for the Artemis Program, as well as commercial low-orbit spaceflight. Based on the Human Research Roadmap, the existing literature was examined to determine the current understanding of the effects of microgravity on the musculoskeletal components of the spinal column. In addition, countermeasure strategies will be required to mitigate these effects for long-duration spaceflight. Current pharmacologic and nonpharmacologic countermeasure strategies are suboptimal, as evidenced by continued muscle and bone loss, alterations in muscle phenotype, and bone metabolism. However, studies incorporating the use of ultrasound, beta-blockers, and other pharmacologic agents have shown some promise. Understanding these mechanisms will not only benefit space technology but likely lead to a return on investment for the management of Earth-bound diseases.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

High resolution imaging in bone tissue research-review

This review article focuses on imaging of bone tissue to understand skeletal health with regards to bone quality. Skeletal fragility fractures are due to bone diseases such as osteoporosis which result in low bone mass and bone mineral density (BMD) leading to high risk of fragility fractures. Recent advances in imaging and analysis technologies have highly benefitted the field of biological sciences. In particular, their application in skeletal health has been of significant importance in understanding bone mechanical behavior (structure and properties) at the tissue level. While synchrotron based microCT technique has remained the gold standard for non-destructive evaluation of structure in material and biological sciences, several lab based microCT systems have been developed to provide high resolution imaging of specimens with greater access, and ease of use in laboratory settings. Lab based microCT scanners are widely used in the bone field as a standard tool to evaluate three-dimensional (3D) morphologies of bone structure at image resolutions appropriate for bone samples from small animals to bone biopsy specimens from humans. Both synchrotron and standard lab based microCT systems provide high resolution imaging ex vivo for a small sized specimen. A few X-ray based systems are also commercially available for in vivo scanning at relatively low image resolutions. Synchrotron-based CT microscopy is being used for various ultra-high-resolution image analyses using complex 3D software. However, the synchrotron-based CT technology is in high demand, allows only limited numbers of specimens, expensive, requires complex additional instrumentation, and is not easily available to researchers as it requires access to a synchrotron source which is always limited. Therefore, desktop laboratory scanners (microXCT, Zeiss/Xradia, Scanco, SkyScan. etc.), mimicking the synchrotron based CT technology or image resolution, have been developed to solve the accessibility issues. These lab based scanners have helped both material science, and the bone field to investigate bone tissue morphologies at submicron mage resolutions. Considerable progress has been made in both in vivo and ex vivo imaging towards providing high resolution images of bone tissue. Both clinical and research imaging technologies will continue to improve and help understand osteoporosis and other related skeletal issues in order to develop targeted treatments for bone fragility. This review summarizes the high resolution imaging work in bone research.

Osteocyte dysfunction promotes osteoarthritis through MMP13-dependent suppression of subchondral bone homeostasis

Osteoarthritis (OA), long considered a primary disorder of articular cartilage, is commonly associated with subchondral bone sclerosis. However, the cellular mechanisms responsible for changes to subchondral bone in OA, and the extent to which these changes are drivers of or a secondary reaction to cartilage degeneration, remain unclear. In knee joints from human patients with end-stage OA, we found evidence of profound defects in osteocyte function. Suppression of osteocyte perilacunar/canalicular remodeling (PLR) was most severe in the medial compartment of OA subchondral bone, with lower protease expression, diminished canalicular networks, and disorganized and hypermineralized extracellular matrix. As a step toward evaluating the causality of PLR suppression in OA, we ablated the PLR enzyme MMP13 in osteocytes while leaving chondrocytic MMP13 intact, using Cre recombinase driven by the 9.6-kb DMP1 promoter. Not only did osteocytic MMP13 deficiency suppress PLR in cortical and subchondral bone, but it also compromised cartilage. Even in the absence of injury, osteocytic MMP13 deficiency was sufficient to reduce cartilage proteoglycan content, change chondrocyte production of collagen II, aggrecan, and MMP13, and increase the incidence of cartilage lesions, consistent with early OA. Thus, in humans and mice, defects in PLR coincide with cartilage defects. Osteocyte-derived MMP13 emerges as a critical regulator of cartilage homeostasis, likely via its effects on PLR. Together, these findings implicate osteocytes in bone-cartilage crosstalk in the joint and suggest a causal role for suppressed perilacunar/canalicular remodeling in osteoarthritis.

Osteocytes Acidify Their Microenvironment in Response to PTHrP In Vitro and in Lactating Mice In Vivo

Osteocytes appear to mobilize calcium within minutes in response to PTH injections; we have previously shown that osteocytes remove their perilacunar matrix during lactation through activation of the PTH type 1 receptor. Mechanisms utilized by osteocytes to mobilize calcium are unknown but we hypothesized that the molecular components may be similar to those used by osteoclasts. Here we show, using IDG-SW3 cells that ATP6V0D2, an essential component of vacuolar ATPase in osteoclasts, and other genes associated with osteoclastic bone resorption, increase with osteoblast to osteocyte differentiation. Furthermore, PTHrP increases ATP6V0D2 expression and induces proton generation by primary osteocytes, which is blocked by bafilomycin, a vacuolar ATPase inhibitor. These in vitro proton measurements raised the question of osteocyte viability in an acidic environment. Interestingly, osteocytes, showed enhanced viability at pH as low as 5 compared to osteoblasts and fibroblasts in vitro. To study in vivo acidification by osteocytes, virgin and lactating CD1 mice on a low calcium diet were injected with the pH indicator dye, acridine orange, and their osteocyte lacuno-canalicular system imaged by confocal microscopy. Lower pH was observed in lactating compared to virgin animals. In addition, a novel transgenic mouse line with a topaz variant of green fluorescent protein (GFPtpz)-tagged collagen α2(I) chain was used. Instead of the expected reduction in GFP-fluorescence only in the perilacunar matrix, reduced fluorescence was observed in the entire bone matrix of lactating mice. Based on our experiments showing quenching of GFP in vitro, we propose that the observed reduction in GFP fluorescence in lactating mice is due to quenching of GFP by the acidic pH generated by osteocytes. Together these findings provide novel mechanistic insight into how osteocytes remove calcium from their perilacunar/pericanalicular matrices through active acidification of their microenvironment and show that osteocytes, like osteoclasts, are resistant to the negative effects of acid on viability. © 2017 American Society for Bone and Mineral Research.

The Role of the Osteocyte in Bone and Nonbone Disease

When normal physiologic functions go awry, disorders and disease occur. This is universal; even for the osteocyte, a cell embedded within the mineralized matrix of bone. It was once thought that this cell was simply a placeholder in bone. Within the last decade, the number of studies of osteocytes has increased dramatically, leading to the discovery of novel functions of these cells. With the discovery of novel physiologic functions came the discoveries of how these cells can also be responsible for not only bone diseases and disorders, but also those of the kidney, heart, and potentially muscle.

Synchronous disruption of anisotropic arrangement of the osteocyte network and collagen/apatite in melanoma bone metastasis

Cancer metastasis to bones increases the risk of fragility fracture by altering bone metabolism and disrupting bone structure. Osteocytes, which organize a dense network that is closely linked with the circumambient matrix, play a key role in regulation of bone microstructure and material properties. The aim of this study was to elucidate the influence of cancer metastasis on the organization of the osteocyte network and collagen/biological apatite (BAp) microstructure in the context of osteocyte/matrix coupling. Using a mouse model intracardially injected with B16F10 melanoma cells or vehicle, the geometric and metabolic changes to osteocytes were analyzed by nano-computed tomography (nano-CT) and histology, and the alignment of collagen fibrils and BAp was analyzed by birefringence measurement and microbeam-X-ray diffraction, respectively. The material properties of bones were further analyzed with nanoindentation method. These experiments revealed that the osteocyte network was markedly disorganized in cancer-bearing bone tissues. The osteocytes showed a variety of residing states in the lacunae; some lacunae were osteolytic while some were replete with immature matrix, suggesting significant disruption in osteocyte/matrix coupling. Collagen/BAp microstructure was also disorganized in cancer-bearing bones as observed by significant decreases in the preferential alignment of both collagen fibrils and BAp; the latter was further shown to be significantly correlated with Young's modulus. The present study revealed that the disruption in the arrangement of the osteocyte network and collagen/BAp microstructure and the deterioration of mechanical function occurred synchronously during cancer bone metastasis.

Fixation stability dictates the differentiation pathway of periosteal progenitor cells in fracture repair

This study compared fracture repair stabilized by intramedullary pin (IMP) or external fixation (EF) in GFP reporter mice. A modified IMP was used as control while EF utilized six needles inserted transversely through the tibia and into a segment of a syringe barrel. X-rays taken at days 0, 14, and 35 showed that IMP resulted in significant three-dimensional deformity with a large callus while EF showed minimal deformity and callus formation. Cryohistological analysis of IMP at day 14 confirmed a large ColX-RFPchry+ callus surrounded by woven bone (Col3.6-GFPcyan) and TRAP+ osteoclasts with mature bone (hOC-GFPtpz) at the base. By day 35, cartilaginous components had been resorbed and an outer cortical shell (OCS) showed evidence of inward modeling. In contrast, the EF at day 14 showed no evidence of cartilage formation. Instead, periosteal-derived osteoblasts (Col3.6-GFPcyan) entered the fracture cleft and formed woven bone that spanned the marrow space. By day 35, mature bone had formed that was contiguous with the opposing cortical bone. Fracture site stability greatly affects the cellular response during repair and must be considered in the preclinical models that test therapies for improving fracture healing.

Tartrate-resistant acid phosphatase (TRAP) co-localizes with receptor activator of NF-KB ligand (RANKL) and osteoprotegerin (OPG) in lysosomal-associated membrane protein 1 (LAMP1)-positive vesicles in rat osteoblasts and osteocytes

Tartrate-resistant acid phosphatase (TRAP) is well known as an osteoclast marker; however, a recent study from our group demonstrated enhanced number of TRAP + osteocytes as well as enhanced levels of TRAP located to intracellular vesicles in osteoblasts and osteocytes in experimental osteoporosis in rats. Such vesicles were especially abundant in osteoblasts and osteocytes in cancellous bone as well as close to bone surface and intracortical remodeling sites. To further investigate TRAP in osteoblasts and osteocytes, long bones from young, growing rats were examined. Immunofluorescence confocal microscopy displayed co-localization of TRAP with receptor activator of NF-KB ligand (RANKL) and osteoprotegerin (OPG) in hypertrophic chondrocytes and diaphyseal osteocytes with Pearson's correlation coefficient ≥0.8. Transmission electron microscopy showed co-localization of TRAP and RANKL in lysosomal-associated membrane protein 1 (LAMP1) + vesicles in osteoblasts and osteocytes supporting the results obtained by confocal microscopy. Recent in vitro data have demonstrated OPG as a traffic regulator for RANKL to LAMP1 + secretory lysosomes in osteoblasts and osteocytes, which seem to serve as temporary storage compartments for RANKL. Our in situ observations indicate that TRAP is located to RANKL-/OPG-positive secretory lysosomes in osteoblasts and osteocytes, which may have implications for osteocyte regulation of osteoclastogenesis.

Increased tartrate-resistant Acid phosphatase expression in osteoblasts and osteocytes in experimental osteoporosis in rats

Tartrate-resistant acid phosphatase (TRAP) is known as an osteoclast marker, but osteoblasts and osteocytes in the vicinity of bone remodeling sites also express TRAP. Cell culture studies suggest that osteoblasts endocytose osteoclastic TRAP for inactivation. To evaluate whether changes in osteoclast activity could alter TRAP expression in osteoblasts and/or osteocytes in vivo, we studied the ovariectomized and vitamin D-deficient rat (Ovx-D) and rats healing from rickets. Bone sections were analyzed for TRAP gene expression by in situ hybridization, TRAP protein by immunogold labeling, and TRAP enzyme activity using the fluorescent substrate ELF97. Osteoblasts and osteocytes close to intracortical remodeling sites and bone surfaces demonstrated TRAP, most prominently in cancellous bone and osteocytes. Intracellular TRAP was located to electron-dense vesicles with similar morphology in both cell types. Ovx-D increased osteoclast activity (p < 0.001) and ELF97⁺ osteocytes (p < 0.05) in cancellous bone, but no corresponding increase was observed in the osteocyte lacunar area. The level of TRAP⁺ vesicles in cortical osteoblasts (p < 0.01) in Ovx-D rats was also increased. Enhanced osteoclast activity was noted in healing rickets after 72 h (p < 0.05), but no differences in TRAP expression were detected in osteoblasts or osteocytes. Thus, increased osteoclast activity does not affect TRAP expression in osteoblasts and osteocytes, favoring the notion that increased TRAP in these cells is rather due to increased synthesis. Although the role of TRAP in osteoblasts and osteocytes remains elusive, we speculate that the function is related to the capability of the enzyme to regulate the phosphorylation of proteins known to be expressed by these cells.

Evidence for the role of connexin 43-mediated intercellular communication in the process of intracortical bone resorption via osteocytic osteolysis

Background

Connexin 43 (Cx43) is the predominant gap junction protein in bone. Mice with a bone-specific deletion of Cx43 (cKO) have an osteopenic cortical phenotype. In a recent study, we demonstrated that cKO mice are resistant to bone loss induced by hindlimb suspension (HLS), an animal model of skeletal unloading. This protective effect occurred primarily as a result of lower osteoclast-mediated bone resorption. Interestingly, we also documented a significant increase in cortical osteocyte apoptosis and reduced sclerostin production. In the present study, we investigated whether osteocytic osteolysis – bone resorption by osteocytes within lacunae – is induced by HLS and the potential effect of Cx43 deficiency on this process during unloading.

Methods

6-month-old male Cx43 cKO or wild-type (WT) mice were subjected to three weeks of HLS (Suspended) or normal loading conditions (Control) (n = 5/group). Lacunar morphology and tartrate-resistant acid phosphatase (TRACP) staining were assessed on sections of femur cortical bone. Experimental groups were compared via two-way ANOVA.

Results

Empty lacunae were 26% larger in cKO-Control vs. WT-Control (p < 0.05), while there was no difference in the size of empty lacunae between Control and Suspended within WT or cKO (p > 0.05). Similarly, there was a trend (p = 0.06) for 11% larger lacunae containing viable osteocytes for cKO-Control vs. WT-Control, with no apparent effect of loading condition. There was no difference in the proportion of TRACP + cells between WT-Control and cKO-Control (p > 0.05); however, WT-Suspended mice had 246% more TRACP + osteocytes compared WT-Control mice (p < 0.05). There was no difference in TRACP staining between cKO-Control and cKO-Suspended (p > 0.05).

Conclusions

Prior to undergoing apoptosis, osteocytes in cKO mice may be actively resorbing their respective lacunae via the process of osteocytic osteolysis. Interestingly, the proportion of TRACP + osteocytes increased dramatically following unloading of WT mice, an effect that was not observed in cKO mice subjected to HLS. The results of the present study provide initial evidence that osteocytic osteolysis is occurring in cortical bone in response to mechanical unloading. Furthermore, Cx43 deficiency appears to protect against osteocytic osteolysis in a manner similar to the inhibition of unloading-induced osteoclast activation that we have documented previously.

Osteolysis and pain due to experimental bone metastases are improved by treatment with rapamycin

In advanced breast cancer, bone metastases occur in 70 % of patients. Managing the devastating pain associated with the disease is difficult. Rapamycin is an immunomodulatory drug that targets the mammalian target of rapamycin pathway. Rapamycin has been shown to decrease osteolysis associated with metastatic breast cancer in pre-clinical models and to reduce pain in inflammatory and neuropathic models. The aim of this study was to evaluate the effectiveness of rapamycin in reducing pain associated with experimental osteolytic metastases. Bone cancer was induced by intra-tibial injections of murine mammary carcinoma cells (4T1) in immunocompetent BALB/c mice and treated intraperitoneally for up to 5 weeks with vehicle, rapamycin or pamidronate (a bisphosphonate currently used to reduce bone loss in bone cancer patients). The control group received intra-tibial injection with saline (sham) and was treated with vehicle intraperitoneally. Cancer-induced osteolysis was observed histologically and radiographically 2-3 weeks following cancer inoculation and gradually increased with time. Measures of evoked nociceptive behaviors including sensitivity to mechanical, thermal, and cold stimuli and spontaneous nociceptive behaviors (limping, guarding) were evaluated. Significant hypersensitivity to sensory stimuli developed in cancer-bearing mice compared to sham 3 weeks following inoculation. Rapamycin decreased or delayed the development of cancer-induced mechanical, heat, and cold hypersensitivity, while pamidronate reduced heat and cold hypersensitivity. Both rapamycin and pamidronate had a partial protective effect on the spontaneous nociceptive behaviors, limping and guarding. Our data suggest that rapamycin may have efficacy in the management of pain associated with metastatic breast cancer.

The osteocyte: an endocrine cell ... and more

Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney and potentially other organs. In fact, until the last few years, osteocytes were perceived by many as passive, metabolically inactive cells. However, exciting recent discoveries have shown that osteocytes encased within mineralized bone matrix are actually multifunctional cells with many key regulatory roles in bone and mineral homeostasis. In addition to serving as endocrine cells and regulators of phosphate homeostasis, these cells control bone remodeling through regulation of both osteoclasts and osteoblasts, are mechanosensory cells that coordinate adaptive responses of the skeleton to mechanical loading, and also serve as a manager of the bone's reservoir of calcium. Osteocytes must survive for decades within the bone matrix, making them one of the longest lived cells in the body. Viability and survival are therefore extremely important to ensure optimal function of the osteocyte network. As we continue to search for new therapeutics, in addition to the osteoclast and the osteoblast, the osteocyte should be considered in new strategies to prevent and treat bone disease.

The ability of H1 or H2 receptor antagonists or their combination in counteracting the glucocorticoid-induced alveolar bone loss in rats

BACKGROUND

The aim of the present study was to compare between three possible osteoporotic treatments in prevention of glucocorticoid-induced alveolar bone loss.

METHODS

Fifty adult female Wistar rats with an average weight 150-200 g were randomized into five groups: group I (control) was intraperitoneally injected with saline. The other experimental groups (II & III, IV & V) were intraperitoneally injected with 200 µg/100 g body weight dexamethasone. The experimental groups III, IV and V received intraperitoneal injection of 10 mg/kg/day pheniramine maleate (H1 receptor antagonist), ranitidine hydrochloride (H2 receptor antagonist) and concomitant doses of both H1 & H2 receptor antagonists respectively. After 30 days, the rats have been sacrificed. The mandibles were examined histologically, histochemically and histomorphometrically. The bone mineral density was measured using dual-energy X-ray absorptiometry (DEXA).

RESULTS

Histopathologically the glucocorticoid group showed wide medullary cavities with wide osteocytic lacunae. These marrow cavities were reduced in the prophylactic groups (III, IV) but increased in group V. Bone histomorphometric analysis revealed improvement in static bone parameters in groups III and IV and deterioration in group V in comparison to group II. The DEXA revealed significant reduction in the bone mineral density in all experimental groups compared to the control group.

CONCLUSIONS

In a rat model, the administration of H1 or H2 receptor antagonists separately could minimize the alveolar bone loss caused by the administration of glucocorticoids while concomitant administration of both H1 and H2 receptor antagonists deteriorated the bone condition.

Microgravity induces pelvic bone loss through osteoclastic activity, osteocytic osteolysis, and osteoblastic cell cycle inhibition by CDKN1a/p21

Bone is a dynamically remodeled tissue that requires gravity-mediated mechanical stimulation for maintenance of mineral content and structure. Homeostasis in bone occurs through a balance in the activities and signaling of osteoclasts, osteoblasts, and osteocytes, as well as proliferation and differentiation of their stem cell progenitors. Microgravity and unloading are known to cause osteoclast-mediated bone resorption; however, we hypothesize that osteocytic osteolysis, and cell cycle arrest during osteogenesis may also contribute to bone loss in space. To test this possibility, we exposed 16-week-old female C57BL/6J mice (n = 8) to microgravity for 15-days on the STS-131 space shuttle mission. Analysis of the pelvis by µCT shows decreases in bone volume fraction (BV/TV) of 6.29%, and bone thickness of 11.91%. TRAP-positive osteoclast-covered trabecular bone surfaces also increased in microgravity by 170% (p = 0.004), indicating osteoclastic bone degeneration. High-resolution X-ray nanoCT studies revealed signs of lacunar osteolysis, including increases in cross-sectional area (+17%, p = 0.022), perimeter (+14%, p = 0.008), and canalicular diameter (+6%, p = 0.037). Expression of matrix metalloproteinases (MMP) 1, 3, and 10 in bone, as measured by RT-qPCR, was also up-regulated in microgravity (+12.94, +2.98 and +16.85 fold respectively, p<0.01), with MMP10 localized to osteocytes, and consistent with induction of osteocytic osteolysis. Furthermore, expression of CDKN1a/p21 in bone increased 3.31 fold (p<0.01), and was localized to osteoblasts, possibly inhibiting the cell cycle during tissue regeneration as well as conferring apoptosis resistance to these cells. Finally the apoptosis inducer Trp53 was down-regulated by −1.54 fold (p<0.01), possibly associated with the quiescent survival-promoting function of CDKN1a/p21. In conclusion, our findings identify the pelvic and femoral region of the mouse skeleton as an active site of rapid bone loss in microgravity, and indicate that this loss is not limited to osteoclastic degradation. Therefore, this study offers new evidence for microgravity-induced osteocytic osteolysis, and CDKN1a/p21-mediated osteogenic cell cycle arrest.

Histomorphometric study and three-dimensional reconstruction of the osteocyte lacuno-canalicular network one hour after applying tensile and compressive forces

The occurrence of very early morphological changes in the osteocyte lacuno-canalicular network following application of tensile and/or compressive forces remains unknown to date. Thus, the aim of this study was to perform a morphological and morphometric evaluation of the changes in the three-dimensional structure of the lacuno-canalicular network and the osteocyte network of alveolar bone that take place very early after applying tensile and compressive forces in vivo, conducting static histomorphometry on bright-field microscopy and confocal laser scanning microscopy images. Our results showed that both the tensile and compressive forces induced early changes in osteocytes and their lacunae, which manifested as an increase in lacunar volume and changes in lacunar shape and orientation. An increase in canalicular width and a decrease in the width and an increase in the length of cytoplasmic processes were also observed. The morphological changes in the lacuno-canalicular and osteocyte networks that occur in vivo very early after application of tensile and compressive forces would be an indication of an increase in permeability within the system. Thus, both compressive and tensile forces would cause fluid displacement very soon after being applied; the latter would in turn rapidly activate alveolar bone osteocytes, enhancing transmission of the signals to the entire osteocyte network and the effector cells located at the bone surface.

Osteocyte regulation of bone mineral: a little give and take

Osteocytes actively participate in almost every phase of mineral handling by bone. They regulate the mineralisation of osteoid during bone formation, and they are also a major RANKL-producing cell. Osteocytes are thus able to liberate bone mineral by regulating osteoclast differentiation and activity in response to a range of stimuli, including bone matrix damage, bone disuse and mechanical unloading, oestrogen deficiency, high-dose glucocorticoid and chemotherapeutic agents. At least some of these activities may be regulated by the osteocyte-secreted product, sclerostin. There is also mounting evidence that in addition to regulating phosphate homeostasis systemically, osteocytes contribute directly to calcium homeostasis in the mature skeleton. Osteocyte cell death and the local loss of control of bone mineralisation may be the cause of focal hypermineralisation of bone and osteopetrosis, as seen in aging and pathology. The sheer number of osteocytes in bone means that "a little give and take" in terms of regulation of bone mineral content translates into a powerful whole organism effect.

TGF β-1 administration during ex vivo expansion of human articular chondrocytes in a serum-free medium redirects the cell phenotype toward hypertrophy

Cell-based cartilage resurfacing requires ex vivo expansion of autologous articular chondrocytes. Defined culture conditions minimize expansion-dependent phenotypic alterations but maintenance of the cells' differentiation potential must be carefully assessed. Transforming growth factor β-1 (TGF β-1) positively regulates the expression of several cartilage proteins, but its therapeutic application in damaged cartilage is controversial. Thus we evaluated the phenotypic outcomes of cultured human articular chondrocytes exposed to TGF β-1 during monolayer expansion in a serum-free medium. After five doublings cells were transferred to micromass cultures to assess their chondrogenic differentiation, or replated in osteogenic medium. Immunocytostainings of micromasses of TGF-expanded cells showed loss of aggrecan and type II collagen. Positivity was evidenced for RAGE, IHH, type X collagen and for apoptotic cells, paralleling a reduction of BCL-2 levels, suggesting hypertrophic differentiation. TGF β-1-exposed cells also evidenced increased mRNA levels for bone sialoprotein, osteopontin, matrix metalloproteinase-13, TIMP-3, VEGF and SMAD7, enhanced alkaline phosphatase activity and pyrophosphate availability. Conversely, SMAD3 mRNA and protein contents were reduced. After osteogenic induction, only TGF-expanded cells strongly mineralized and impaired p38 kinase activity, a contributor of chondrocytes' differentiation. To evaluate possible endochondral ossification progression, we seeded the chondrocytes on hydroxyapatite scaffolds, subsequently implanted in an in vivo ectopic setting, but cells failed to reach overt ossification; nonetheless, constructs seeded with TGF-exposed cells displayed blood vessels of the host vascular supply with enlarged diameters, suggestive of vascular remodeling, as in bone growth. Thus TGF-exposure during articular chondrocytes expansion induces a phenotype switch to hypertrophy, an undesirable effect for cells possibly intended for tissue-engineered cartilage repair.

Methotrexate chemotherapy promotes osteoclast formation in the long bone of rats via increased pro-inflammatory cytokines and enhanced NF-κB activation

Cancer chemotherapy with methotrexate (MTX) is known to cause bone loss. However, the underlying mechanisms remain unclear. This study investigated the potential role of MTX-induced pro-inflammatory cytokines and activation of NF-κB in the associated osteoclastogenesis in rats. MTX (0.75 mg/kg per day) was administered for 5 days, and bone and bone marrow specimens were collected on days 6, 9, and 14. Compared with a normal control, MTX increased the density of osteoclasts within the metaphyseal bone and the osteoclast formation potential of marrow cells on day 9. RT-PCR analysis of mRNA expression for pro-osteoclastogenic cytokines in the metaphysis indicated that, although the receptor activator of NF-κB ligand/osteoprotegerin axis was unaffected, expression of tumor necrosis factor (TNF)-α, IL-1, and IL-6 increased on day 9. Enzyme-linked immunosorbent assay analysis of plasma showed increased levels of TNF-α on day 6 and of IL-6 on day 14. Plasma from treated rats induced osteoclast formation from normal bone marrow cells, which was attenuated by a TNF-α-neutralizing antibody. Indicative of a role for NF-κB signaling, plasma on day 6 increased NF-κB activation in RAW(264.7) cells, and plasma-induced osteoclastogenesis was abolished in the presence of the NF-κB inhibitor, parthenolide. Our results demonstrate mechanisms for MTX-induced osteoclastogenesis and show that MTX induces osteoclast differentiation by generating a pro-osteoclastogenic environment in both bone and the circulation, specifically with increased TNF-α levels and activation of NF-κB.

Demonstration of osteocytic perilacunar/canalicular remodeling in mice during lactation

Osteoclasts are thought to be solely responsible for the removal of bone matrix. However, we show here that osteocytes can also remove bone matrix by reversibly remodeling their perilacunar/canalicular matrix during the reproductive cycle. In contrast, no osteocytic remodeling was observed with experimental unloading despite similar degrees of bone loss. Gene array analysis of osteocytes from lactating animals revealed an elevation of genes known to be utilized by osteoclasts to remove bone, including tartrate-resistant acid phosphatase (TRAP) and cathepsin K, that returned to virgin levels upon weaning. Infusion of parathyroid hormone-related peptide (PTHrP), known to be elevated during lactation, induced TRAP activity and cathepsin K expression in osteocytes concurrent with osteocytic remodeling. Conversely, animals lacking the parathyroid hormone type 1 receptor (PTHR1) in osteocytes failed to express TRAP or cathepsin K or to remodel their osteocyte perilacunar matrix during lactation. These studies show that osteocytes remove mineralized matrix through molecular mechanisms similar to those utilized by osteoclasts.

Osteocytes: central conductors of bone biology in normal and pathological conditions

Osteocytes are the most abundant and longest-living cells in the adult skeleton. For a long time, osteocytes were considered static and inactive cells, but in recent years, it has been suggested that they represent the key responder to various stimuli that regulate bone formation and remodelling as well as one of the key endocrine regulators of bone metabolism. Osteocytes respond to mechanical stimuli by producing and secreting several signalling molecules, such as nitric oxide and prostaglandin E(2) , that initiate local bone remodelling. Moreover, they can control bone formation by modulating the WNT signalling pathway, an essential regulator of cell fate and commitment, as they represent the main source of sclerostin, a negative regulator of bone formation. Osteocytes can also act as an endocrine organ by releasing fibroblast growth factor 23 and several other proteins (DMP-1, MEPE, PHEX) that regulate phosphate metabolism. It has been demonstrated that various bone diseases are associated with osteocyte abnormalities, although it is not clear if these changes are the direct cause of the pathology or if they are secondary to the pathological changes in the bone microenvironment. Thus, a better understanding of these cells could offer exciting opportunities for new advances in the prevention and management of different bone diseases.

Changes in intracortical microporosities induced by pharmaceutical treatment of osteoporosis as detected by high resolution micro-CT

Bone's microporosities play important biologic and mechanical roles. Here, we quantified 3D changes in cortical osteocyte-lacunae and other small porosities induced by estrogen withdrawal and two different osteoporosis treatments. Unlike 2D measurements, these data collected via synchrotron radiation-based μCT describe the size and 3D spatial distribution of a large number of porous structures. Six-month old female Sprague-Dawley rats were separated into four groups of age-matched controls, untreated OVX, OVX treated with PTH, and OVX treated with Alendronate (ALN). Intracortical microporosity of the medial quadrant of the femoral diaphysis was quantified at endosteal, intracortical, and periosteal regions of the samples, allowing the quantification of osteocyte lacunae that were formed primarily before versus after the start of treatment. Across the overall thickness of the medial cortex, lacunar volume fraction (Lc.V/TV) was significantly lower in ALN treated rats compared to PTH. In the endosteal region, average osteocyte lacunar volume (<Lc.V>) of untreated OVX rats was significantly lower than in age-matched controls, indicating a decrease in osteocyte lacunar size in bone formed on the endosteal surface after estrogen withdrawal. The effect of treatment (OVX, ALN, PTH) on the number of lacunae per tissue volume (Lc.N/TV) was dependent on the specific location within the cortex (endosteal, intracortical, periosteal). In both the endosteal and intracortical regions, Lc.N/TV was significantly lower in ALN than in untreated OVX, suggesting a site-specific effect in osteocyte lacuna density with ALN treatment. There also were a significantly greater number of small pores (5-100 μm(3) in volume) in the endosteal region for PTH compared to ALN. The mechanical impact of this altered microporosity structure is unknown, but might serve to enhance, rather than deteriorate bone strength with PTH treatment, as smaller osteocyte lacunae may be better able to absorb shear forces than larger lacunae. Together, these data demonstrate that current treatments of osteoporosis can alter the number, size, and distribution of microporosities in cortical rat lamellar bone.

Transgenic overexpression of tartrate-resistant acid phosphatase is associated with induction of osteoblast gene expression and increased cortical bone mineral content and density

Bone remodeling is a central event in the maintenance of skeletal tissue, and involves cycles of resorption followed by the formation of bone tissue. The activity of osteoclasts and osteoblasts during these cycles is tightly regulated by systemic and local factors coupling the action of these cells. Tartrate-resistant acid phosphatase (TRAP) is predominantly expressed in bone by osteoclasts but has also been detected in osteoblasts and osteocytes. Moreover, TRAP can stimulate the differentiation of mesenchymal lineage cells, i.e. progenitors of osteoblasts and adipocytes. In order to further explore the effects of TRAP on bone turnover, the structural and molecular phenotypes of osteoclasts and osteoblasts were assessed in TRAP-overexpressing transgenic mice. Transgenic mice of both sexes display increased cortical bone mineral content and density, which cannot be accounted for by decreased bone resorption since osteoclast numbers and resorptive activity do not differ from wild-type mice. Examination of the osteoblast phenotype revealed that markers of bone formation, i.e. procollagen type I N-terminal propeptides, and osteoblast lineage markers as well as the TRAP 1B mRNA transcript are increased in TRAP-overexpressing mice. Expression of the osteoclast-selective TRAP 1C mRNA is not increased in TRAP transgenic mice. Elevated expression of TRAP mRNA and protein were detected in osteoblasts, osteocytes and in the bone matrix of TRAP transgenic mice, suggesting that TRAP overexpression in osteoblast lineage cells is associated with increased cortical bone mineral content and density. The data presented here support the hypothesis that TRAP overexpression in the osteoblastic cell lineage stimulates the differentiation and/or activation of these cells.

Expression and proteolytic processing of mammalian purple acid phosphatase in CHO-K1 cells

Rat recombinant purple acid phosphatase (PAP) stably expressed in fibroblast-like CHO-K1 cells was purified and characterized with respect to post-translational modifications such as N-glycosylation and proteolytic processing in order to elucidate subcellular and molecular pathways for proteolytic activation. In these cells, proteolytically processed PAP was more abundant than the monomeric form. PAP-transfected CHO-K1 cells were expressing active cathepsin K intracellularly, which was partially co-localized with PAP. However, neither cathepsin K nor trypsin digestion of the purified monomeric PAP in vitro did result in a two-subunit form with kinetic and electrophoretic properties resembling the endogenous cellular two-subunit form. Treatment of PAP-transfected CHO-K1 cells with the cysteine proteinase inhibitor E-64 suggested that only a minor fraction of secreted PAP is processed intracellularly by cysteine proteinases. These data do not support a dominant or critical role for cathepsins or trypsin-like serine proteinases in the proteolytic activation of PAP in CHO-K1 cells, implicating yet unidentified proteinases in the proteolytic processing of both intracellular and secreted PAP in this cell line.

Increased tartrate-resistant acid phosphatase (TRAP) expression in malignant breast, ovarian and melanoma tissue: an investigational study

Background

Tartrate-resistant acid phosphatase (TRAP) is a metalloprotein enzyme that belongs to the acid phosphatases and is known to be expressed by osteoclasts. It has already been investigated as a marker of bone metastases in cancer patients. In this study, which examined the value of serum TRAP concentrations as a marker of bone disease in breast cancer patients, we observed high concentrations of TRAP even in patients without bone metastases. To elucidate this phenomenon, we examined the expression of TRAP in breast cancer cells and the cells of several other malignancies.

Methods

TRAP concentrations in the serum of tumor patients were determined by ELISA. The expression of TRAP in breast, ovarian, and cervical cancer and malignant melanoma was analyzed by immunohistochemistry. RT-PCR and immunocytology were used to evaluate TRAP expression in cultured tumor cells.

Results

A marked increase in serum TRAP concentrations was observed in patients with breast and ovarian cancer, regardless of the presence or absence of bone disease. TRAP expression was found in breast and ovarian cancers and malignant melanoma, while cervical cancer showed only minimal expression of TRAP. Expression of TRAP was absent in benign tissue or was much less marked than in the corresponding malignant tissue. TRAP expression was also demonstrated in cultured primary cancer cells and in commercially available cell lines.

Conclusion

Overexpression of TRAP was detected in the cells of various different tumors. TRAP might be useful as a marker of progression of malignant disease. It could also be a potential target for future cancer therapies.

Endocytosis of tartrate-resistant acid phosphatase by osteoblast-like cells is followed by inactivation of the enzyme

Tartrate-resistant acid phosphatase (TRACP) is generally used as a marker of osteoclasts. Yet, other bone-associated cells, such as osteoblasts and osteocytes, may also express activity of this enzyme. Osteoblasts containing TRACP activity are seen particularly in the vicinity of areas of bone resorption, suggesting that osteoclasts somehow induce TRACP activity in osteoblasts. In a recent study, we found that osteoblast-like cells appeared to have the capacity to endocytose TRACP released by osteoclast precursors. In the present study, we investigated the endocytosis of TRACP in more detail as well as the fate of the endocytosed enzyme. We found that incubation of osteoblast-like cells with TRACP-coated beads resulted in attachment of a high number of beads to the cells. After culturing osteoblast-like cells with medium conditioned by blood monocytes that contain TRACP, activity of the enzyme was found in the cells. Following replacement of the medium by normal medium that did not contain TRACP, a decrease in the level of TRACP activity in osteoblast-like cells occurred. Our data strongly suggest that osteoblast-like cells recognize TRACP released by osteoclast precursors and that upon endocytosis inactivation of the enzyme occurs. We propose that uptake of the enzyme is important for the control of enzyme activity, thereby preventing degradation of matrix constituents.