Early loading-related changes in the activity of glucose 6-phosphate dehydrogenase and alkaline phosphatase in osteocytes and periosteal osteoblasts in rat fibulae in vivo

The potential role of mechanotransduction in the management of pediatric calvarial bone flap repair

Pediatric patients suffering traumatic brain injuries may require a decompressive craniectomy to accommodate brain swelling by removing a portion of the skull. Once the brain swelling subsides, the preserved calvarial bone flap is ideally replaced as an autograft during a cranioplasty to restore protection of the brain, as it can reintegrate and grow with the patient during immature skeletal development. However, pediatric patients exhibit a high prevalence of calvarial bone flap resorption post-cranioplasty, causing functional and cosmetic morbidity. This review examines possible solutions for mitigating pediatric calvarial bone flap resorption by delineating methods of stimulating mechanosensitive cell populations with mechanical forces. Mechanotransduction plays a critical role in three main cell types involved with calvarial bone repair, including mesenchymal stem cells, osteoblasts, and dural cells, through mechanisms that could be exploited to promote osteogenesis. In particular, physiologically relevant mechanical forces, including substrate deformation, external forces, and ultrasound, can be used as tools to stimulate bone repair in both in vitro and in vivo systems. Ultimately, combating pediatric calvarial flap resorption may require a combinatorial approach using both cell therapy and bioengineering strategies.

The Osteocyte: From "Prisoner" to "Orchestrator"

Osteocytes are the most abundant bone cells, entrapped inside the mineralized bone matrix. They derive from osteoblasts through a complex series of morpho-functional modifications; such modifications not only concern the cell shape (from prismatic to dendritic) and location (along the vascular bone surfaces or enclosed inside the lacuno-canalicular cavities, respectively) but also their role in bone processes (secretion/mineralization of preosseous matrix and/or regulation of bone remodeling). Osteocytes are connected with each other by means of different types of junctions, among which the gap junctions enable osteocytes inside the matrix to act in a neuronal-like manner, as a functional syncytium together with the cells placed on the vascular bone surfaces (osteoblasts or bone lining cells), the stromal cells and the endothelial cells, i.e., the bone basic cellular system (BBCS). Within the BBCS, osteocytes can communicate in two ways: by means of volume transmission and wiring transmission, depending on the type of signals (metabolic or mechanical, respectively) received and/or to be forwarded. The capability of osteocytes in maintaining skeletal and mineral homeostasis is due to the fact that it acts as a mechano-sensor, able to transduce mechanical strains into biological signals and to trigger/modulate the bone remodeling, also because of the relevant role of sclerostin secreted by osteocytes, thus regulating different bone cell signaling pathways. The authors want to emphasize that the present review is centered on the morphological aspects of the osteocytes that clearly explain their functional implications and their role as bone orchestrators.

Metabolic regulation of skeletal cell fate and function in physiology and disease

The skeleton is diverse in its functions, which include mechanical support, movement, blood cell production, mineral storage and endocrine regulation. This multifaceted role is achieved through an interplay of osteoblasts, chondrocytes, bone marrow adipocytes and stromal cells, all generated from skeletal stem cells. Emerging evidence shows the importance of cellular metabolism in the molecular control of the skeletal system. The different skeletal cell types not only have distinct metabolic demands relating to their particular functions but also are affected by microenvironmental constraints. Specific metabolites control skeletal stem cell maintenance, direct lineage allocation and mediate cellular communication. Here, we discuss recent findings on the roles of cellular metabolism in determining skeletal stem cell fate, coordinating osteoblast and chondrocyte function, and organizing stromal support of haematopoiesis. We also consider metabolic dysregulation in skeletal ageing and degenerative diseases, and provide an outlook on how the field may evolve in the coming years.

Loss of Adenylyl Cyclase 6 in Leptin Receptor-Expressing Stromal Cells Attenuates Loading-Induced Endosteal Bone Formation

Bone marrow stromal/stem cells represent a quiescent cell population that replenish the osteoblast bone‐forming cell pool with age and in response to injury, maintaining bone mass and repair. A potent mediator of stromal/stem cell differentiation in vitro and bone formation in vivo is physical loading, yet it still remains unclear whether loading‐induced bone formation requires the osteogenic differentiation of these resident stromal/stem cells. Therefore, in this study, we utilized the leptin receptor (LepR) to identify and trace the contribution of bone marrow stromal cells to mechanoadaptation of bone in vivo. Twelve‐week‐old Lepr‐cre;tdTomato mice were subjected to compressive tibia loading with an 11 N peak load for 40 cycles, every other day for 2 weeks. Histological analysis revealed that Lepr‐cre;tdTomato⁺ cells arise perinatally around blood vessels and populate bone surfaces as lining cells or osteoblasts before a percentage undergo osteocytogenesis. Lepr‐cre;tdTomato⁺ stromal cells within the marrow increase in abundance with age, but not following the application of tibial compressive loading. Mechanical loading induces an increase in bone mass and bone formation parameters, yet does not evoke an increase in Lepr‐cre;tdTomato⁺ osteoblasts or osteocytes. To investigate whether adenylyl cyclase‐6 (AC6) in LepR cells contributes to this mechanoadaptive response, Lepr‐cre;tdTomato mice were further crossed with AC6^fl/fl mice to generate a LepR⁺ cell‐specific knockout of AC6. These Lepr‐cre;tdTomato;AC6^fl/fl animals have an attenuated response to compressive tibia loading, characterized by a deficient load‐induced osteogenic response on the endosteal bone surface. This, therefore, shows that Lepr‐cre;tdTomato⁺ cells contribute to short‐term bone mechanoadaptation. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

WISP-2 expression induced by Teriparatide treatment affects in vitro osteoblast differentiation and improves in vivo osteogenesis

The Osteocyte, recognized as a major orchestrator of osteoblast and osteoclast activity, is the most important key player during bone remodeling processes. Imbalances occurring during bone remodeling, caused by hormone perturbations or by mechanical loading alterations, can induce bone pathologies such as osteoporosis. Recently, the active fraction of parathormone, PTH (1-34) or Teriparatide (TPTD), was chosen as election treatment for osteoporosis. The effect of such therapy is dependent on the temporal manner of administration. The molecular reasons why the type of administration regimen is so critical for the fate of bone remodeling are numerous and not yet well known. Our study attempts to analyze diverse signaling pathways directly activated in osteocytes upon TPTD treatment. By means of gene array analysis, we found many molecules upregulated or downregulated in osteocytes. Later, we paid attention to Wisp-2, a protein involved in the Wnt pathway, that is secreted by MLO-Y4 cells and increases upon TPTD treatment and that is able to positively influence the early phases of osteogenic differentiation. We also confirmed the pro osteogenic property of Wisp-2 during mesenchymal stem cell differentiation into the preliminary osteoblast phenotype. The same results were confirmed with an in vivo approach confirming a remarkable Wisp-2 expression in metaphyseal trabecular bone. These results highlighted the anabolic roles unrolled by osteocytes in controlling the action of neighboring cells, suggesting that the perturbation of certain signaling cascades, such as the Wnt pathway, is crucial for the positive regulation of bone formation.

The response of anosteocytic bone to controlled loading

The bones of the skeleton of most advanced teleost fish do not contain osteocytes. Considering the pivotal role assigned to osteocytes in the process of modeling and remodeling (the adaptation of external and internal bone structure and morphology to external loads and the repair of areas with micro-damage accumulation, respectively) it is unclear how, and even whether, their skeleton can undergo modeling and remodeling. Here, we report on the results of a study of controlled loading of the anosteocytic opercula of tilapia (Oreochromis aureus). Using a variety of microscopy techniques we show that the bone of the anosteocytic tilapia actively adapts to applied loads, despite the complete absence of osteocytes. We show that in the directly loaded area, the response involves a combination of bone resorption and bone deposition; we interpret these results and the structure of the resultant bone tissue to mean that both modeling and remodeling are taking place in response to load. We further show that adjacent to the loaded area, new bone is deposited in an organized, layered manner, typical of a modeling process. The material stiffness of the newly deposited bone is higher than that of the bone which was present prior to loading. The absence of osteocytes requires another candidate cell for mechanosensing and coordinating the modeling process, with osteoblasts seeming the most likely candidates.

ClC-3 chloride channel functions as a mechanically sensitive channel in osteoblasts

Mechanical stimulation usually causes the volume changes of osteoblasts. Whether these volume changes could be sensed by the ClC-3 chloride channel, a volume-sensitive ion channel, and further promote the osteodifferentiation in osteoblasts has not been determined. In this study, we applied persistent static compression on MC3T3-E1 cells to detect the expression changes of ClC-3, osteogenic markers, as well as some molecules related with signaling transduction pathway. We tested the key role of ClC-3 in transferring the mechanical signal to osteoinduction by ClC-3 overexpressing and siRNA technique. We found that ClC-3 level was up-regulated by mechanical stimulation in MC3T3-E1 cells. Mechanical force also up-regulated the mRNA level of osteogenic markers such as alkaline phosphatase (Alp), bone sialoprotein (Bsp), and osteocalcin (Oc), which could be blocked or strengthened by Clcn3 siRNA or overexpressing, and Alp expression was more sensitive to the changes of ClC-3 level. We also found that runt-related transcription factor 2 (Runx2), transforming growth factor beta 1 (TGF-β1), and Wnt pathway might be involved in ClC-3 mediated mechanical transduction in osteoblasts. The data from the current study suggest that the ClC-3 chloride channel acts as a mechanically sensitive channel to regulate osteodifferentiation in osteoblasts.

Four-point bending protocols to study the effects of dynamic strain in osteoblastic cells in vitro

Strain engendered within bone tissue by mechanical loading of the skeleton is a major influence on the processes of bone modeling and remodeling and so a critical determinant of bone mass and architecture. The cells best placed to respond to strain in bone tissue are the resident osteocytes and osteoblasts. To address the mechanisms of strain-related responses in osteoblast-like cells, our group uses both in vivo and in vitro approaches, including a system of four-point bending of the substrate on which cells are cultured. A range of cell lines can be studied using this system but we routinely compare their responses to those in primary cultures of osteoblast-like cells derived from explants of mouse long bones. These cells show a range of well-characterized responses to physiological levels of strain, including increased proliferation, which in vivo is a feature of the osteogenic response.

A new method to investigate how mechanical loading of osteocytes controls osteoblasts

Mechanical loading, a potent stimulator of bone formation, is governed by osteocyte regulation of osteoblasts. We developed a three-dimensional (3D) in vitro co-culture system to investigate the effect of loading on osteocyte-osteoblast interactions. MLO-Y4 cells were embedded in type I collagen gels and MC3T3-E1(14) or MG63 cells layered on top. Ethidium homodimer staining of 3D co-cultures showed 100% osteoblasts and 86% osteocytes were viable after 7 days. Microscopy revealed osteoblasts and osteocytes maintain their respective ovoid/pyriform and dendritic morphologies in 3D co-cultures. Reverse-transcriptase quantitative polymerase chain reaction (RT-qPCR) of messenger ribonucleic acid (mRNA) extracted separately from osteoblasts and osteocytes, showed that podoplanin (E11), osteocalcin, and runt-related transcription factor 2 mRNAs were expressed in both cell types. Type I collagen (Col1a1) mRNA expression was higher in osteoblasts (P < 0.001), whereas, alkaline phosphatase mRNA was higher in osteocytes (P = 0.001). Immunohistochemistry revealed osteoblasts and osteocytes express E11, type I pro-collagen, and connexin 43 proteins. In preliminary experiments to assess osteogenic responses, co-cultures were treated with human recombinant bone morphogenetic protein 2 (BMP-2) or mechanical loading using a custom built loading device. BMP-2 treatment significantly increased osteoblast Col1a1 mRNA synthesis (P = 0.031) in MLO-Y4/MG63 co-cultures after 5 days treatment. A 16-well silicone plate, loaded (5 min, 10 Hz, 2.5 N) to induce 4000-4500 με cyclic compression within gels increased prostaglandin E2 (PGE2) release 0.5 h post-load in MLO-Y4 cells pre-cultured in 3D collagen gels for 48, 72 h, or 7 days. Mechanical loading of 3D co-cultures increased type I pro-collagen release 1 and 5 days later. These methods reveal a new osteocyte-osteoblast co-culture model that may be useful for investigating mechanically induced osteocyte control of osteoblast bone formation.

Single bout short duration fluid shear stress induces osteogenic differentiation of MC3T3-E1 cells via integrin β1 and BMP2 signaling cross-talk

Fluid shear stress plays an important role in bone osteogenic differentiation. It is traditionally believed that pulsed and continuous stress load is more favorable for fracture recovery and bone homeostasis. However, according to our clinical practice, we notice that one single stress load is also sufficient to trigger osteogenic differentiation. In the present study, we subject osteoblast MC3T3-E1 cells to single bout short duration fluid shear stress by using a parallel plate flow system. The results show that 1 hour of fluid shear stress at 12 dyn/cm² promotes terminal osteogenic differentiation, including rearrangement of F-actin stress fiber, up-regulation of osteogenic genes expression, elevation of alkaline phosphatase activity, secretion of type I collagen and osteoid nodule formation. Moreover, collaboration of BMP2 and integrin β1 pathways plays a significant role in such differentiation processes. Our findings provide further experimental evidence to support the notion that single bout short duration fluid shear stress can promote osteogenic differentiation.

Expression of endothelial nitric oxide synthase protein is not necessary for mechanical strain-induced nitric oxide production by cultured osteoblasts

Regulation of nitric oxide (NO) production is considered essential in mechanical load-related osteogenesis. We examined whether osteoblast endothelial NO synthase (eNOS)-derived NO production was regulated by HSP90. We found that HSP90 is essential for strain-related NO release but appears to be independent of eNOS in cultured osteoblasts.

INTRODUCTION

NO is a key regulator of bone mass, and its production by bone cells is regarded as essential in mechanical strain-related osteogenesis. We sought to identify whether bone cell NO production relied upon eNOS, considered to be the predominant NOS isoform in bone, and whether this was regulated by an HSP90-dependent mechanism.

METHODS

Using primary rat long bone-derived osteoblasts, the ROS 17/2.8 cell line and primary mouse osteoblasts, derived from wild-type and eNOS-deficient (eNOS(-/-)) mice, we examined by immunoblotting the expression of eNOS using a range of well-characterised antibodies and extraction methods, measured NOS activity by monitoring the conversion of radiolabelled L-arginine to citrulline and examined the production of NO by bone cells subjected to mechanical strain application under various conditions.

RESULTS

Our studies have revealed that eNOS protein and activity were both undetectable in osteoblast-like cells, that mechanical strain-induced NO production was retained in bone cells from eNOS-deficient mice, but that this strain-related induction of NO production was, however, dependent upon HSP90.

CONCLUSIONS

Together, our studies indicate that HSP90 activity is essential for strain-related NO release by cultured osteoblasts and that this is highly likely to be achieved by an eNOS-independent mechanism.

Osteocyte signaling in bone

Osteocytes, the cells residing within the bone matrix and comprising 90% to 95% of the all bone cells, have long been considered quiescent bystander cells compared to the osteoblasts and osteoclasts whose activities cause bone gain and loss, and whose dysfunction lead to growth defects and osteoporosis. However, recent studies show that osteocytes play a crucial, central role in regulating the dynamic nature of bone in all its diverse functions. Osteocytes are now known to be the principal sensors for mechanical loading of bone. They produce the soluble factors that regulate the onset of both bone formation and resorption. Osteocytes regulate local mineral deposition and chemistry at the bone matrix level, and they also function as endocrine cells producing factors that target distant organs such as the kidney to regulate phosphate transport. Osteocytes appear to be the major local orchestrator of many of bone's functions.

Load/strain distribution between ulna and radius in the mouse forearm compression loading model

Finite element analysis (FEA) of the mouse forearm compression loading model is used to relate strain distributions with downstream changes in bone formation and responses of bone cells. The objective of this study was to develop two FEA models - the first one with the traditional ulna only and the second one in which both the ulna and radius are included, in order to examine the effect of the inclusion of the radius on the strain distributions in the ulna. The entire mouse forearm was scanned using microCT and images were converted into FEA tetrahedral meshes using a suite of software programs. The performance of both linear and quadratic tetrahedral elements and coarse and fine meshes were studied. A load of 2N was applied to the ulna/radius model and a 1.3N load (based on previous investigations of load sharing between the ulna and radius in rats) was applied to the ulna only model for subsequent simulations. The results showed differences in the cross sectional strain distributions and magnitude within the ulna for the combined ulna/radius model versus the ulna only model. The maximal strain in the combined model occurred about 4mm toward the distal end from the ulna mid-shaft in both models. Results from the FEA model simulations were also compared to experimentally determined strain values. We conclude that inclusion of the radius in FE models to predict strains during in vivo forearm loading increases the magnitude of the estimated ulna strains compared to those predicted from a model of the ulna alone but the distribution was similar. This has important ramifications for future studies to understand strain thresholds needed to activate bone cell responses to mechanical loading.

Leptin receptor (Lepr) is a negative modulator of bone mechanosensitivity and genetic variations in Lepr may contribute to the differential osteogenic response to mechanical stimulation in the C57BL/6J and C3H/HeJ pair of mouse strains

This study investigated the role of leptin receptor (Lepr) signaling in determining the bone mechanosensitivity and also evaluated whether differences in the Lepr signaling may contribute to the differential osteogenic response of the C57BL/6J (B6) and C3H/HeJ (C3H) pair of mouse strains to mechanical stimuli. This study shows that a loading strain of ∼2,500 με, which was insufficient to produce a bone formation response in B6 mice, significantly increased bone formation parameters in leptin-deficient ob(-)/ob(-) mice and that a loading strain of ∼3,000 με also yielded greater osteogenic responses in Lepr-deficient db(-)/db(-) mice than in wild-type littermates. In vitro, a 30-min steady shear stress increased [(3)H]thymidine incorporation and Erk1/2 phosphorylation in ob(-)/ob(-) osteoblasts and db(-)/db(-) osteoblasts much greater than those in corresponding wild-type osteoblasts. The siRNA-mediated suppression of Lepr expression in B6 osteoblasts enhanced (but in osteoblasts of C3H (the mouse strain with poor bone mechanosensitivity) restored) their anabolic responses to shear stress. The Lepr signaling (leptin-induced Jak2/Stat3 phosphorylation) in C3H osteoblasts was higher than that in B6 osteoblasts. One of the three single nucleotide polymorphisms in the C3H Lepr coding region yielded an I359V substitution near the leptin binding region, suggesting that genetic variation of Lepr may contribute to a dysfunctional Lepr signaling in C3H osteoblasts. In conclusion, Lepr signaling is a negative modulator of bone mechanosensitivity. Genetic variations in Lepr, which result in a dysfunctional Lepr signaling in C3H mice, may contribute to the poor osteogenic response to loading in C3H mice.

Quantification of Lacunar-Canalicular Interstitial Fluid Flow Through Computational Modeling of Fluorescence Recovery After Photobleaching

Skeletal adaptation to mechanical loading has been widely hypothesized to involve the stimulation of osteocytes by interstitial fluid flow (IFF). However, direct investigation of this hypothesis has been difficult due in large part to the inability to directly measure IFF velocities within the lacunar-canalicular system. Measurements of fluorescence recovery after photobleaching (FRAP) within individual lacunae could be used to quantify lacunar-canalicular IFF when combined with mathematical modeling. In this study, we used a computational transport model to characterize the relationship between flow frequency (0.5-10 Hz), peak flow velocity (0-300 μm/s), tracer diffusion coefficient (100-300 μm(2)/s), and transport enhancement (i.e., (k/k(0)) - 1, where k and k(0) are the transport rates in the presence/absence of flow) during lacunar FRAP investigations. We show that this relationship is well described by a simple power law with frequency-dependent coefficients, and is relatively insensitive to variations in lacunar geometry. Using this power law relationship, we estimated peak IFF velocities in hindlimb mice subjected to intramedullary pressurization using values of k and k(0) previously obtained from ex vivo lacunar FRAP investigations. Together, our findings suggest that skeletal adaptation in hindlimb suspended mice subjected to dynamic intramedullary pressure occurred in the presence of IFF at levels associated with physiological loading.

Would increased interstitial fluid flow through in situ mechanical stimulation enhance bone remodeling?

Bone accommodates to changes in its functional environment ensuring that sufficient skeletal mass is appropriately positioned to withstand the mechanical loads that result from functional activities. Increasing physical activity will result in increased bone mass, while the removal of functional loading would result in bone loss. Bone is a composite material made up of a collagen-hydroxyapatite matrix and a complex network of lacunae-canaliculi channels occupied by osteocyte and osteoblast processes, immersed in interstitial fluid. There are strong indications that changes in interstitial fluid flow velocity or pressure are the means by which an external load signal is communicated to the cell. In vitro studies indicate that shear stress, induced by interstitial fluid flow, is a potent bone cell behavior regulator. One of the forms of altering interstitial fluid flow is through the mechanical deformation of skeletal tissue in response to applied loads. Other methods include increased intramedullary pressure, negative-pressure tissue regeneration, or external mechanical stimulation. Analysis of these methods poses the question of process effectiveness. The efficacy of each method theoretically will depend on the mechanical efficiency of transmitting an external load and converting it into changes in interstitial fluid flow. In this paper, we combine recent knowledge on the effect of the bone's interstitial fluid flow, different fluid patterns, the role of gap junctions, and the concept of mechanical effectiveness of different methods that influence interstitial fluid flow within bone, and we hypothesize that the efficiency of bone remodeling can be improved if a small mechanical percussion device could be placed directly in contact with the bone, thus inducing local interstitial fluid flow variations. Enhancement of bone repair and remodeling through controlled interstitial fluid flow possesses many clinical applications. Further investigations and in vivo experiments are required. Practical methods and clinical apparatuses need to be conceived and developed to validate and facilitate the clinical use of this technique.

Genetic selection for fast growth generates bone architecture characterised by enhanced periosteal expansion and limited consolidation of the cortices but a diminution in the early responses to mechanical loading

Bone strength is, in part, dependent on a mechanical input that regulates the (re)modelling of skeletal elements to an appropriate size and architecture to resist fracture during habitual use. The rate of longitudinal bone growth in juveniles can also affect fracture incidence in adulthood, suggesting an influence of growth rate on later bone quality. We have compared the effects of fast and slow growth on bone strength and architecture in the tibiotarsi of embryonic and juvenile birds. The loading-related biochemical responses (intracellular G6PD activity and NO release) to mechanical load were also determined. Further, we have analysed the proliferation and differentiation characteristics of primary tibiotarsal osteoblasts from fast and slow-growing strains. We found that bones from chicks with divergent growth rates display equal resistance to applied loads, but weight-correction revealed that the bones from juvenile fast growth birds are weaker, with reduced stiffness and lower resistance to fracture. Primary osteoblasts from slow-growing juvenile birds proliferated more rapidly and had lower alkaline phosphatase activity. Bones from fast-growing embryonic chicks display rapid radial expansion and incomplete osteonal infilling but, importantly, lack mechanical responsiveness. These findings are further evidence that the ability to respond to mechanical inputs is crucial to adapt skeletal architecture to generate a functionally appropriate bone structure and that fast embryonic and juvenile growth rates may predispose bone to particular architectures with increased fragility in the adult.

On a Path to Unfolding the Biological Mechanisms of Orthodontic Tooth Movement

Orthodontic forces deform the extracellular matrix and activate cells of the paradental tissues, facilitating tooth movement. Discoveries in mechanobiology have illuminated sequential cellular and molecular events, such as signal generation and transduction, cytoskeletal re-organization, gene expression, differentiation, proliferation, synthesis and secretion of specific products, and apoptosis. Orthodontists work in a unique biological environment, wherein applied forces engender remodeling of both mineralized and non-mineralized paradental tissues, including the associated blood vessels and neural elements. This review aims at identifying events that affect the sequence, timing, and significance of factors that determine the nature of the biological response of each paradental tissue to orthodontic force. The results of this literature review emphasize the fact that mechanoresponses and inflammation are both essential for achieving tooth movement clinically. If both are working in concert, orthodontists might be able to accelerate or decelerate tooth movement by adding adjuvant methods, whether physical, chemical, or surgical.

Role of protein-tyrosine phosphatases in regulation of osteoclastic activity

Osteoclasts, the primary cell type mediating bone resorption, are multinucleated, giant cells derived from hematopoietic cells of monocyte-macrophage lineage. Osteoclast activity is, in a large part, regulated by protein-tyrosine phosphorylation. While information about functional roles of several protein-tyrosine kinases (PTK), including c-Src, in osteoclastic resorption has been accumulated, little is known about the roles of protein-tyrosine phosphatases (PTPs) in regulation of osteoclast activity. Recent evidence implicates important regulatory roles for four PTPs (SHP-1, cyt-PTP-epsilon, PTP-PEST, and PTPoc) in osteoclasts. Cyt-PTP-epsilon, PTP-PEST, and PTP-oc are positive regulators of osteoclast activity, while SHP-1 is a negative regulator. Of these PTPs in osteoclasts, only PTP-oc is a positive regulator of c-Src PTK through dephosphorylation of the inhibitory phosphotyrosine-527 residue. Although some information about mechanisms of action of these PTPs to regulate osteoclast activity is reviewed in this article, much additional work is required to provide more comprehensive details about their functions in osteoclasts.

From streaming-potentials to shear stress: 25 years of bone cell mechanotransduction

Mechanical loads are vital regulators of skeletal mass and architecture as evidenced by the increase in bone formation following the addition of exogenous loads and loss of bone mass following their removal. While our understanding of the molecular mechanisms by which bone cells perceive changes in their mechanical environment has increased rapidly in recent years, much remains to be learned. Here, we outline the effects of interstitial fluid flow, a potent biophysical signal induced by the deformation of skeletal tissue in response to applied loads, on bone cell behavior. We focus on the molecular mechanisms by which bone cells are hypothesized to perceive interstitial fluid flow, the cell signaling cascades activated by fluid flow, and the use of this signal in tissue engineering protocols.

Fluid and Solute Transport in Bone: Flow-Induced Mechanotransduction

Much recent evidence suggests that bone cells sense their mechanical environment via interstitial fluid flow. In this review, we summarize theoretical and experimental approaches to quantify fluid and solute transport in bone, starting with the early investigations of fluid shear stress applied to bone cells. The pathways of bone interstitial fluid and solute movement are high-lighted based on recent theoretical models, as well as a new generation of tracer experiments that have clarified and refined the structure and function of the osteocyte pericellular matrix. Then we trace how the fluid-flow models for mechanotransduction have evolved as new ultrastructural features of the osteocyte lacunar-canalicular porosity have been identified and how more recent in vitro fluid-flow and cell-stretch experiments have helped elucidate at the molecular level the possible pathways for cellular excitation in bone.

Identifying the cellular basis for reimplantation failure in repair of the rotator cuff

We examined cultured osteoblasts derived from paired samples from the greater tuberosity and acromion from eight patients with large chronic tears of the rotator cuff. We found that osteoblasts from the tuberosity had no apparent response to mechanical stimulation, whereas those derived from the acromion showed an increase in alkaline phosphatase activity and nitric oxide release which is normally a response of bone cells to mechanical strain. By contrast, we found that cells from both regions were able to respond to dexamethasone, a well-established promoter of osteoblastic differentiation, with the expected increase in alkaline phosphatase activity. Our findings indicate that the failure of repair of the rotator cuff may be due, at least in part, to a compromised capacity for mechanoadaptation within the greater tuberosity. It remains to be seen whether this apparent decrease in the sensitivity of bone cells to mechanical stimulation is the specific consequence of the reduced load-bearing history of the greater tuberosity in these patients.

Mechanical force-induced midpalatal suture remodeling in mice

Mechanical stress is an important epigenetic factor for regulating skeletal remodeling, and application of force can lead to remodeling of both bone and cartilage. Chondrocytes, osteoblasts and osteoclasts all participate and interact with each other in this remodeling process. To study cellular responses to mechanical stimuli in a system that can be genetically manipulated, we used mouse midpalatal suture expansion in vivo. Six-week-old male C57BL/6 mice were subjected to palatal suture expansion by opening loops with an initial force of 0.56 N for the periods of 1, 3, 5, 7, 14 or 28 days. Periosteal cells in expanding sutures showed increased proliferation, with Ki67-positive cells representing 1.8+/-0.1% to 4.5+/-0.4% of total suture cells in control groups and 12.0+/-2.6% to 19.9+/-1.2% in experimental/expansion groups (p<0.05). Starting at day 1, cells expressing alkaline phosphatase and type I collagen were seen. New cartilage and bone formation was observed at the oral edges of the palatal bones at day 7; at the nasal edges only bone formation without cartilage appeared to occur. An increase in osteoclast numbers suggested increased bone remodeling, ranging from 60 to 160% throughout the experimental period. Decreased Saffranin O staining after day 3 suggested decreased proteoglycan content in the secondary cartilage. Micro-CT showed a significant increase in maxillary width at days 14 and 28 (from 2334+/-4 microm to 2485+/-3 microm at day 14 and from 2383+/-5 microm to 2574+/-7 microm at day 28, p<0.001). The suture width was increased at days 14 and 28, except in the oral third region at day 28 (from 48+/-5 microm to 36+/-4 microm, p<0.05). Bone volume/total volume was significantly reduced at days 14 and 28 (50.2+/-0.7% vs. 68.0+/-3.7% and 56.5+/-1.0% vs. 60.9+/-1.3%, respectively, p<0.05), indicative of increased bone marrow space. These findings demonstrate that expansion forces across the midpalatal suture promote bone resorption through activation of osteoclasts and bone and cartilage formation via increased proliferation and differentiation of periosteal cells. Mouse midpalatal suture expansion would be useful in further studies of the ability of mineralized tissues to respond to mechanical stimulation.

The response of bone, articular cartilage and tendon to exercise in the horse

Horses can gallop within hours of birth, and may begin training for athletic competition while still growing. This review cites studies on the effects of exercise on bone, tendon and articular cartilage, as detected by clinical and research imaging techniques, tissue biochemical analysis and microscopy of various kinds. For bone, alterations in bone mineral content, mineral density and the morphology of the mineralized tissue are the most common end-points. Apparent bone density increases slightly after athletic training in the cortex, but substantially in the major load paths of the epiphyses and cuboidal bones, despite the lower material density of the new bone, which is deposited subperiosteally and on internal surfaces without prior osteoclastic resorption. With training of greater intensity, adaptive change is supervened by patho-anatomical change in the form of microdamage and frank lesions. In tendon, collagen fibril diameter distribution changes significantly during growth, but not after early training. The exact amount and type of protracted training that does cause reduction in mass average diameter (an early sign of progressive microdamage) have not been defined. Training is associated with an increase in the cross-sectional area of some tendons, possibly owing to slightly greater water content of non-collagenous or newly synthesized matrix. Early training may be associated with greater thickness of hyaline but not calcified articular cartilage, at least in some sites. The age at which adaptation of cartilage to biomechanical influences can occur may thus extend beyond very early life. However, cartilage appears to be the most susceptible of the three tissues to pathological alteration. The effect of training exercise on the anatomical or patho-anatomical features of connective tissue structures is affected by the timing, type and amount of natural or imposed exercise during growth and development which precedes the training.

Age of donor alters the effect of cyclic hydrostatic pressure on production by human macrophages and osteoblasts of sRANKL, OPG and RANK

Background

Cyclic hydrostatic pressure within bone has been proposed both as a stimulus of aseptic implant loosening and associated bone resorption and of bone formation. We showed previously that cyclical hydrostatic pressure influenced macrophage synthesis of several factors linked to osteoclastogenesis. The osteoprotegerin/soluble receptor activator of NF-kappa β ligand /receptor activator of NF-kappa β (OPG/ RANKL/ RANK) triumvirate has been implicated in control of bone resorption under various circumstances. We studied whether cyclical pressure might affect bone turnover via effects on OPG/ sRANKL/ RANK.

Methods

In this study, cultures of human osteoblasts or macrophages (supplemented with osteoclastogenic factors) or co-cultures of macrophages and osteoblasts (from the same donor), were subjected to cyclic hydrostatic pressure. Secretion of OPG and sRANKL was assayed in the culture media and the cells were stained for RANK and osteoclast markers. Data were analysed by nonparametric statistics.

Results

In co-cultures of macrophages and osteoblasts, pressure modulated secretion of sRANKL or OPG in a variable manner. Examination of the OPG:sRANKL ratio in co cultures without pressurisation showed that the ratio was greater in donors <70 years at the time of operation (p < 0.05 Mann Whitney U) than it was in patients >70 years. However, with pressure the difference in the OPG:sRANKL ratios between young and old donors was not significant. It was striking that in some patients the OPG:sRANKL ratio increased with pressure whereas in some it decreased. The tendency was for the ratio to decrease with pressure in patients younger than 70 years, and increase in patients ≥ 70 years (Fishers exact p < 0.01).

Cultures of osteoblasts alone showed a significant increase in both sRANKL and OPG with pressure, and again there was a decrease in the ratio of OPG:RANKL. Secretion of sRANKL by cultures of macrophages alone was not modulated by pressure. Only sRANKL was assayed in this study, but transmembrane RANKL may also be important in this system. Macrophages subjected to pressure (both alone and in co-culture) stained more strongly for RANK on immunohistochemstry than non-pressurized controls and 1,25-dihydroxyvitamin D₃ (1,25 D₃) further increased this. Immunocytochemical staining also demonstrated that more cells in pressurized co-cultures exhibited osteoclast markers (tartrate-resistant acid phosphatase, vitronectin receptor and multinuclearity) than did unpressurized controls.

Conclusion

These data show that in co-cultures of osteoblasts and macrophages the ratio of OPG : sRANKL was decreased by pressure in younger patients but increased in older patients. As falls in this ratio promote bone resorption, this finding may be important in explaining the relatively high incidence of osteolysis around orthopaedic implants in young patients. The finding that secretion of OPG and sRANKL by osteoblasts in monoculture was sensitive to hydrostatic pressure, and that hydrostatic pressure stimulated the differentiation of macrophages into cells exhibiting osteoclast markers indicates that both osteoblasts and preosteoclasts are sensitive to cyclic pressure. However, the effects of pressure on cocultures were not simply additive and coculture appears useful to examine the interaction of these cell types.

These findings have implications for future therapies for aseptic loosening and for the development of tests to predict the development of this condition.

Analysis of avian bone response to mechanical loading—Part One: Distribution of bone fluid shear stress induced by bending and axial loading

Mechanical loading-induced signals are hypothesized to be transmitted and integrated by a bone-connected cellular network (CCN) before reaching the bone surfaces where adaptation occurs. Our objective is to establish a computational model to explore how bone cells transmit the signals through intercellular communication. In this first part of the study the bone fluid shear stress acting on every bone cell in a CCN is acquired as the excitation signal for the computational model. Bending and axial loading-induced fluid shear stress is computed in transverse sections of avian long bones for two adaptation experiments (Gross et al. in J Bone Miner Res 12:982-988, 1997 and Judex et al. in J Bone Miner Res 12:1737-1745, 1997). The computed fluid shear stress is found to be correlated with the radial strain gradient but not with bone formation. These results suggest that the radial strain gradient is the driving force for bone fluid flow in the radially distributed lacunar-canalicular system and that bone formation is not linearly related to the loading-induced local stimulus.

Problems in quantifying bone response to exercise in horses: a review

Detecting changes in bone during growth, training, rest from competition (spelling), and disease in horses requires imaging techniques that have a high level of accuracy and precision. Currently, most imaging techniques used in horses do not possess such characteristics and are more suitable for detecting end-stage disease than subtle changes. Some are incapable of detecting changes in bone size. Non-planar techniques should be used for estimating or determining bone strength. This review outlines available imaging techniques and shows why cross-sectional procedures are required to accurately estimate bone strength. If these can be validated and improved for use in the standing horse, serial examination can be used to show the magnitude of changes in bone strength that occur during training or varying management regimens. Such knowledge can then be used to improve training and spelling regimens for equine athletes, hopefully resulting in a reduction in racing or athletic injury.

Osteoblast-like cells from estrogen receptor alpha knockout mice have deficient responses to mechanical strain

INTRODUCTION

In vivo, bones' osteogenic response to mechanical loading involves proliferation of surface osteoblasts. This response is replicated in vitro and involves ERK-mediated activation of the estrogen receptor (ER) alpha and upregulation of estrogen response element activity. This proliferative response can be blocked by selective estrogen receptor modulators and increased by transfection of additional ERalpha.

MATERIALS AND METHODS

We have now investigated the mechanisms of ER involvement in osteoblast-like cells' early responses to strain by comparing the responses of primary cultures of these cells derived from homozygous ERalpha knockout (ERKO) mice (ERalpha-/-) with those from their wildtype (ERalpha+/+) and heterozygous (ERalpha+/-) littermates and from ER/beta knockout (BERKO) mice (ERbeta+/+, ERbeta+/-, and ERbeta-/-).

RESULTS

Whereas ERalpha+/+, ERalpha+/-, ERbeta+/+, and ERbeta-/- cells proliferate in response to a single 10-minute period of cyclic strain, ERalpha-/- cells do not. Transfection of fully functional, but not mutant, ERalpha rescues the proliferative response to strain in these cells. The strain-related response of ERalpha-/- cells is also deficient in that they show no increased activity of an AP-I driven reporter vector and no strain-related increases in NO production. Their strain-related increase in prostacyclin production is retained. They proliferate in response to fibroblast growth factor-2 but not insulin-like growth factor (IGF)-I or IGF-II, showing the importance of ERalpha in the IGF axis and the ability of ERalpha-/- cells to proliferate normally in response to a mitogenic stimulus that does not require functional ERalpha.

CONCLUSIONS

These data indicate ERalpha's obligatory involvement in a number of early responses to mechanical strain in osteoblast-like cells, including those that result in proliferation. They support the hypothesis that reduction in ERalpha expression or activity after estrogen withdrawal results in a less osteogenic response to loading. This could be important in the etiology of postmenopausal osteoporosis.

The influence on gene-expression profiling of osteoblasts behavior following treatment with the ionic products of sintered β-dicalcium pyrophosphate dissolution

Sintered dicalcium pyrophosphate (SDCP) is biocompatible to bone tissue both in the in vivo and in vitro model. However, the molecular mechanisms that mediated these processes have yet to be identified. In this study, we investigated the influence of SDCP ions on in vitro osteoblasts behavior. The powder of sintered beta-dicalcium pyrophosphate (SDCP) was dissolved by HCl and then diluted into different concentration of solutions by culture medium used in the osteoblast cell culture. The effects of various concentration of SDCP on bone cell activities were evaluated by using MTT assay. For the differentiation of osteoblasts, alkaline phosphatase (AP) staining, von Kossa stain for mineralized nodules and bone markers messenger ribonucleic acid (mRNA) isolation and identification were performed at 3h, days 1, 3, 7 and 14. In the presence of 10(-8)M SDCP for 14 days, the osteoblasts population was still significantly higher than that of control. In the qualitative analysis for the formation of AP staining colonies and mineralization nodules formation were not affected by SDCP ions. When osteoblasts cultured in the presence of 10(-8)M SDCP ions, the osteocalcin mRNA expression was up-regulated; while the collagen, osteonectin and osteopontin mRNA expression were down-regulated. In this study, we demonstrated that the elevated concentration of calcium and pyrophosphate ions can activate genes of the bone cells. This study will contribute to a better understanding of cell/biomaterial interactions and mechanisms that SDCP affect the bone cells.

Osteocyte dendrogenesis in static and dynamic bone formation: An ultrastructural study

The present ultrastructural investigation into osteocyte dendrogenesis represents a continuation of a previous study (Ferretti et al., Anat. Embryol., 2002; 206:21-29), in which we pointed out that, during intramembranous ossification, the well-known dynamic bone formation (DBF), performed by migrating osteoblast laminae, is preceded by static bone formation (SBF), in which cords of stationary osteoblasts transform into osteocytes in the same site where they differentiated. The research was carried out on the perichondral center of ossification surrounding the mid shaft level of various long bones of chick embryos and newborn rabbits. Transmission electron microscope observations showed that the formation of osteocyte dendrites is quite different in the two types of osteogenesis, mainly depending on whether or not osteoblast movement occurs. In DBF, osteoblasts transform into small ovoidal/ellipsoidal osteocytes and their dendrites form in an asynchronous and asymmetrical manner in concomitance with, and depending on, the advancing mineralizing surface and the receding osteogenic laminae. In SBF, stationary osteoblasts give rise to big globous osteocytes, located inside confluent lacunae, with short and symmetrical dendrites that can radiate simultaneously all around their cell body because they are completely surrounded by unmineralized matrix. Contacts and gap junctions were observed between all osteocytes (both SBF- and DBF-derived) and between osteocytes and osteoblasts. Finally, a continuous osteocyte network extends throughout the bone, regardless of its static or dynamic origin. This network has the characteristic of a functional syncytium, potentially capable of modulating, by wiring transmission, the cells of the osteogenic lineage covering the bone surfaces.

Uniaxial cyclic stretch induces osteogenic differentiation and synthesis of bone morphogenetic proteins of spinal ligament cells derived from patients with ossification of the posterior longitudinal ligaments

Ossification of the posterior longitudinal ligament of the spine (OPLL) is characterized by ectopic bone formation in the spinal ligaments. Mechanical stress, which acts on the posterior ligaments, is thought to be an important factor in the progression of OPLL. To elucidate this mechanism, we investigated the effects of in vitro sinusoidal cyclic stretch (120% peak to peak, at 1 Hz) on cultured spinal ligament cells derived from OPLL and non-OPLL patients. The mRNA expressions of alkaline phosphatase (ALP), osteopontin, bone morphogenetic protein (BMP)-2, BMP-4, and BMP receptors as well as ALP activity in cell layers and production of BMPs into the conditioned medium were significantly increased by cyclic stretch in OPLL cells, whereas no change was observed in non-OPLL cells. A stretch-activated Ca(2+) channel blocker, Gd(3+), the voltage-dependent L-type Ca(2+) channel blockers diltiazem and nifedipine, and Ca(2+)-free medium suppressed stretch-induced ALP activity, which suggests a role of Ca(2+) influx in the signal transduction of mechanical stress to the osteogenic response of OPLL cells. Our study provides first evidences that mechanical stress plays a key role in the progression of OPLL through the induction of osteogenic differentiation in spinal ligament cells and the promotion of the autocrine/paracrine mechanism of BMPs in this lesion.

Acromial spur formation in patients with rotator cuff tears

In this study we analyzed the acromial spurs of 15 patients with impingement syndrome undergoing open rotator cuff repair. Mineral apposition analysis and quantitative cytochemical techniques for glucose-6-phosphate dehydrogenase (G6PD) activity (pentose phosphate pathway), alkaline phosphatase (ALP) activity (osteoblast activity), and tartrate-resistant acid phosphatase (TRAP) activity (osteoclast phenotype) were used to examine the distribution and level of activity of selected marker enzymes within the acromial spur insertion into the coracoacromial ligament in order to establish whether local behavior of bone cells is consistent with the proposed secondary development of the acromial spur. Our results indicate that G6PD and ALP activity was higher in osteoblasts on the inferior surface compared with the superior surface of the acromial spur in all patients (P <.001). This area correlated to the most intense area of mineral apposition shown by dual tetracycline labeling. TRAP activity revealed a heterogeneous distribution within the samples. A greater G6PD activity per cell (mean increase of 87%) was seen at the tip compared with that in post- and pre-tip zones within the coronal plane (P <.0002). The qualitative and quantitative enzyme analyses show that the acromial insertion of the coracoacromial ligament is actively involved in bone turnover. The spatial distribution patterns of metabolically active bone-forming osteoblastic cells compared with a heterogeneous distribution of TRAP-positive osteoclasts provide evidence of bone remodeling consistent with the morphologic contours of the acromial enthesis. The sites of oxytetracycline labeling appear to correlate with the sites of high ALP and G6PD activity, which supports the concept of spur formation being a secondary phenomenon in the presence of established rotator cuff tears.

The effect of Gu-Sui-Bu (Drynaria fortunei J. Sm) on bone cell activities

In the traditional Chinese medicine, Gu-Sui-Bu [Drynaria fortunei (kunze) J. Sm] has been reported as a good enhancer for bone healing. In this experiment, we investigate the biochemical effects of this traditional Chinese medicine on the bone cells culture. Different concentrations of crude extract of Gu-Sui-Bu were added to rat bone cells culture. The mitochondria activity of the bone cells after exposure was determined by colorimetric assay. Biochemical markers such as alkaline phosphatase (ALP), acid phosphatase (ACP) titer, prostaglandin E2 (PGE2) titer and the expression of both osteopontin and osteonectin mRNA were evaluated. The effect on the osteoclasts differentiation was evaluated by tartrate-resistant acid phosphatase (TRAP) stain. The most effective concentration of Gu-Sui-Bu on bone cells was 1 mg/ml. The addition of 1 mg/ml Gu-Sui-Bu to bone cells culture for 7 days can statistically increase the intracellular ALP amount; while the ACP and PGE2 amount in culture medium were significantly increased. In Northern blot analysis, the expression of both osteopontin and osteonectin mRNA were down-regulated after adding Gu-Sui-Bu into bone cells culture. The formation of multi-nucleated osteoclasts was more active than that of the control group, but no giant osteoclasts formation was observed. In this study, we demonstrated that Gu-Sui-Bu has potential effects on the bone cells culture. One of the major effects of Gu-Sui-Bu on the bone cells is probably mediated by its effect on the osteclasts activities. Continued and advanced study on the alterations in gene expression of bone cells by Chinese medicines will provide a basis for understanding the observed bone cell responses to various pharmacological interventions.

Postmenopausal osteoporosis as a failure of bone's adaptation to functional loading: a hypothesis

There is substantial evidence that bones' ability to withstand functional loading without damage depends on the processes of bone modeling and remodeling, which are responsible for establishing and maintaining bone architecture, being influenced by a feedback mechanism related to the control of functional strains. It is probably useful to consider the diminished ability to maintain bone strength in postmenopausal osteoporosis as a failure of this mechanism. Acceptance of this approach would not only increase understanding of the etiology of postmenopausal osteoporosis but also significantly influence the ways in which it is investigated and treated. This would not mean that the many other factors affecting bone mass and bone cell activity will be ignored, but rather these factors will be put in perspective. Research to prevent or treat osteoporosis could be directed usefully to understanding how osteoblasts, lining cells, and osteocytes respond to mechanically derived information and how these responses are converted into stimuli controlling structurally appropriate modeling and remodeling. Evidence suggesting that early strain-related responses of bone cells in males and females involve the estrogen receptor (ER) could explain decreased effectiveness of this pathway when ER levels are low.

Temporal pattern of stimulation of osteoblast-associated genes during mechanically-induced osteogenesis in vivo: early responses of osteocalcin and type I collagen

Mechanical loading is an essential environmental factor in skeletal homeostasis, but the response of osteoblast-associated genes to mechanical osteogenic signal is largely unknown. This study uses our recently characterized in vivo osteoinductive model to analyze the sequence of stimulation and the time course of expression of osteoblast-associated genes in mechanically loaded mouse periodontium. Temporal pattern of regulation of osteocalcin (OC), alkaline phosphatase (ALP), and type I collagen (collagen I) was determined during mechanically-induced osteoblast differentiation in vivo, using a mouse tooth movement model earlier shown to induce bone formation and cell-specific regulation of genes in osteoblasts. The expression of target genes was determined after 1, 2, 3, 4, and 6 days of orthodontic movement of the mouse first molar. mRNA levels were measured in the layer of osteoblasts adjacent to the alveolar bone surface, using in situ hybridization and a relative quantitative video image analysis of cell-specific hybridization intensity, with non-osseous mesenchymal periodontal cells as an internal standard. After 24 hours of loading, the level of OC in osteoblasts slightly decreased, followed by a remarkable 4.6-fold cell-specific stimulation between 1 and 2 days of treatment. The high level expression of OC was maintained throughout the treatment with a peak 7-fold stimulation at day 4. The expression of collagen I gene was not significantly affected after 1 day, but it was stimulated 3-fold at day 2, and maintained at a similar level through day 6. The ALP gene, which we previously found to be mechanically stimulated during the first 24 hours, remained enhanced from 1.8- to 2.2-fold throughout the 6 days of treatment. Thus, in an intact alveolar bone compartment, mechanical loading resulted in a defined temporal sequence of induction of osteoblast-associated genes. Stimulation of OC 48 h after the onset of loading (and 24 h prior to deposition of osteoid) temporally coincided with that of collagen I, and was preceded for 24 h by an enhancement of ALP. Identification of OC as a mechanically responsive gene induced in functionally active osteoblasts in this study is consistent with its potential role in limiting the rate of mechanically-induced bone modeling. Furthermore, these results show that temporal progression of mechanically-induced osteoblast phenotype in this in vivo model occurs very rapidly. This suggests that physiologically relevant mechanical osteoinductive signal in vivo is targeting a population of committed osteoblast precursor cells that are capable of rapidly responding by entering a differentiation pathway and initiating an anabolic skeletal adaptation process.

The effect of exercise on systemic and bone concentrations of growth factors in rats

Exercise can prevent bone loss and increase bone density. Growth factors such as insulin-like growth factor-I (IGF-I) and transforming growth factor beta (TGFbeta) are thought to be involved in the local response to mechanical loading, resulting in bone remodelling. We tested the effect of additional weight bearing during exercise on the systemic response of IGF-I and local bone response of IGF-I and TGFbeta. Thirty-four female Wistar rats (aged 3 months, weight 226.9 +/- 20.2 g) were randomly divided in four groups: group 1 baseline controls; group 2 sedentary controls; group 3 ran 15 min a day on a motor-driven exercise belt; group 4 ran 15 min a day with a backpack containing 40 g. The animals ran 5 days a week, for 6 weeks, with an average velocity of 16.6 +/- 4.4 m/min, and a slope of 5 degrees uphill. The serum growth hormone (GH) concentration was significantly higher in the running rats (group 3, P = 0.009) than in the sedentary controls (group 2). The IGF-I and IGF binding protein 3 (IGFBP3) levels in serum and the IGF-I levels in liver were similar in all groups. In the tibia no significant differences were observed in IGF-I, IGFBP3 and TGFbeta concentration. In the humerus, the IGF-I concentration was lower in the running rats (group 3) than in the sedentary controls (P = 0.04), but it was higher in the rats that ran with additional weight than in those without (P = 0.009). The TGFbeta concentration in the humerus was lower in both group 3 (P = 0.001) and 4 (P = 0.03) than in the sedentary controls. The effects in bone caused by mechanical stimulation cannot be explained by changes in serum IGF-I and IGF-I produced in the liver. The concentrations of IGF-I and TGFbeta in bone appeared to be modulated by running exercise.

Osteoporose por desuso: aplicação na reabilitação do lesado medular

A osteoporose é uma doença óssea metabólica muito freqüente em pacientes que sofreram lesão medular. Seu aparecimento pode prejudicar os tratamentos de reabilitação destes pacientes, devido à possibilidade de ocorrência de fraturas em seus ossos osteoporóticos. A osteoporose em lesados medulares está relacionada com o desuso causado pela paralisia, a qual provoca diminuição da tensão mecânica sobre os ossos, e consequentemente, diminuição do estímulo à formação de osso com aumento desproporcional da reabsorção óssea, tornando o osso mais frágil. Assim, tratamentos alternativos não farmacológicos, baseados no princípio biomecânico do osso, estão sendo estudados, os quais incluem a análise da sustentação de peso causada pela estimulação elétrica neuro-muscular (EENM), e o ultra-som de baixa intensidade. Este artigo propõe explicar a importância do estímulo mecânico sobre os ossos e as conseqüências de sua ausência, com ênfase nos pacientes lesados medulares. Além de mostrar tratamentos alternativos que têm sido estudados.

Calcium-channel activation and matrix protein upregulation in bone cells in response to mechanical strain

Femur-derived osteoblasts cultured from rat femora were loaded with Fluo-3 using the AM ester. A quantifiable stretch was applied and [Ca(2+)]i levels monitored by analysis of fluorescent images obtained using an inverted microscope and laser scanning confocal imaging system. Application of a single pulse of tensile strain via an expandable membrane resulted in immediate increase in [Ca(2+)]i in a proportion of the cells, followed by a slow and steady decrease to prestimulation levels. Application of parathyroid hormone (10(-6) M) prior to mechanical stimulation potentiated the load-induced elevation of [Ca(2+)]i. Mechanically stimulating osteoblasts in Ca(2+)-free media or in the presence of either nifedipine (10 microM; L-type Ca(2+)-channel blocker) or thapsigargin (1 microM; depletes intracellular Ca(2+) stores) reduced strain-induced increases in [Ca(2+) ]i. Furthermore, strain-induced increases in [Ca(2+)]i were enhanced in the presence of Bayer K 8644 (500 nm), an agonist of L-type calcium channels. The effects of mechanical strain with and without inhibitors and agonists are described on the total cell population and on single cell responses. Application of strain and strain in the presence of the calcium-channel agonist Bay K 8644 to periosteal-derived osteoblasts increased levels of the extracellular matrix proteins osteopontin and osteocalcin within 24 h postload. This mechanically induced increase in osteopontin and osteocalcin was inhibited by the addition of the calcium-channel antagonist, nifedipine. Our results suggest an important role for L-type calcium channels and a thapsigargin-sensitive component in early mechanical strain transduction pathways in osteoblasts.

Mechanical strain stimulates nitric oxide production by rapid activation of endothelial nitric oxide synthase in osteocytes

Previous studies have indicated that physiological levels of dynamic mechanical strain produce rapid increases in nitric oxide (NO) release from rat ulna explants and primary cultures of osteoblast-like cells and embryonic chick osteocytes derived from long bones. To establish the mechanism by which loading-induced NO production may be regulated, we have examined: nitric oxide synthase (NOS) isoform mRNA and protein expression, the effect of mechanical loading in vivo on NOS mRNA expression, and the effect of mechanical strain on NO production by bone cells in culture. Using Northern blot analyses, in situ hybridization, and immunocytochemistry we have established that the predominant NOS isoform expressed in rat long bone periosteal osteoblasts and in a distinct population of cortical bone osteocytes is the endothelial form of NOS (eNOS), with little or no expression of the inducible NOS or neuronal NOS isoforms. In contrast, in non-load-bearing calvariae there are no detectable levels of eNOS in osteocytes and little in osteoblasts. Consistent with these observations, ulnar explants release NO rapidly in response to loading in vitro, presumably through the activation of eNOS, whereas calvarial explants do not. The relative contribution of different bone cells to these rapid increases in strain-induced NO release was established by assessment of medium nitrite (stable NO metabolite) concentration, which showed that purified populations of osteocytes produce significantly greater quantities of NO per cell in response to mechanical strain than osteoblast-like cells derived from the same bones. Using Northern blot hybridization, we have also shown that neither a single nor five consecutive daily periods of in vivo mechanical loading produced any significant effect on different NOS isoform mRNA expression in rat ulnae. In conclusion, our results indicate that eNOS is the prevailing isoform expressed by cells of the osteoblast/osteocyte lineage and that strain produces increases in the activity of eNOS without apparently altering the levels of eNOS mRNA.

Bone's early responses to mechanical loading differ in distinct genetic strains of chick: selection for enhanced growth reduces skeletal adaptability

Bone's functional competence is established and maintained, at least partly, by mechanisms involving appropriate adaptation to mechanical loading. These appear to fail in chickens selectively bred either for maximum egg (Egg-type) or meat (Meat-type) production, which show high rates of fracture and skeletal abnormality, respectively. By measuring several early strain-induced responses in cultured embryonic tibiotarsi from commercially bred (Egg-type and Meat-type) and wild-type (Wild-type) chicks, we have investigated the possibility that these skeletal failures are the product of a compromised ability to respond appropriately to loading-induced mechanical strain. Axial loads engendering peak dynamic (1 Hz) longitudinal strains of between -1300 microepsilon and -1500 microepsilon (for 10 minutes) in vitro in tibiotarsi from the three types of 18-day-old chicks increased periosteal osteoblast glucose 6-phosphate dehydrogenase (G6PD) activity in both Wild-type (26%, p < 0.01) and Egg-type (49%, p < 0.001) chicks in situ, while Meat-type chicks did not show any significant changes (11%). Load-induced increases in medium nitrite accumulation (stable nitric oxide [NO] metabolite) were produced in Egg-type and Wild-type tibiotarsi (82 +/- 12%, p < 0.01; 39 +/- 8%, p < 0.01), respectively. In contrast, loading produced no change in NO release from Meat-type chick tibiotarsi. These changes in NO release correlated with load-related increases in G6PD activity (R2 = 0.98, p < 0.05) in the different chick types. Wild-type and Meat-type tibiotarsal periosteal osteoblasts responded in a biphasic manner to exogenous prostacyclin (PGI2), with maximal stimulation of G6PD activity at 10(-7) M and 10(-6) M PGI2. However, Egg-type chick osteoblasts showed smaller, progressive increases up to 10(-5) M PGI2. These results indicate that early phases of the adaptive response to loading differ in different genetic strains of embryonic chick; that skeletal abnormalities which develop in genetically selected, high growth rate chicks may reflect a compromised ability to respond to load; and that load-induced increases in osteoblastic G6PD activity appear to be closely associated with increased rates of NO release. It is probable that similar genetically related differences in bones' responsiveness to mechanical loading occur in other species.

Role of osteopontin in bone remodeling caused by mechanical stress

Changes in the number and proportion of osteopontin mRNA (Opn) expressing osteocytes and osteoclasts caused by the mechanical stress applied during experimental tooth movement were examined in the present study. Opn expression was detected in the osteocytes on the pressure side at the early stage, and gradually spread to those on the tension side and also to the osteoblasts and bone-lining cells in the alveolar bone. Only 3.3% of the osteocytes located on the pressure side expressed Opn in the interradicular septum of control rats; in contrast, the value was increased to 87.5% at 48 h after the initiation of tooth movement. These results indicate that these cells responded to mechanical stress loaded on the bone with expression of the osteopontin gene. Following the increased expression of Opn in these cells, a 17-fold greater number of osteoclasts compared with the control and numerous resorption pits were observed on the pressure side of the alveolar bone. Injection of arginine-glycine-aspartic acid-serine peptide but not that of arginine-glycine-glutamic acid-serine peptide strongly inhibited the increase in the number of osteoclasts. Furthermore, an in vitro migration assay demonstrated the chemotactic activity of osteopontin (OPN) on the precursor of osteoclasts. Our study strongly suggests that OPN is an important factor triggering bone remodeling caused by mechanical stress.

Mechanotransduction pathways in bone: calcium fluxes and the role of voltage-operated calcium channels

Changes in strain distribution across the vertebrate skeleton induce modelling and remodelling of bone structure. This relationship, like many in biomedical science, has been recognised since the 1800s, but it is only the recent development of in vivo and in vitro models that is allowing detailed investigation of the cellular mechanisms involved. A number of secondary messenger pathways have been implicated in load transduction by bone cells, and many of these pathways are similar to those proposed for other load-responsive cell types. It appears that load transduction involves interaction between several messenger pathways, rather than one specific switch. Interaction between these pathways may result in a cascade of responses that promote and maintain bone cell activity in remodelling of bone. The paper outlines research on the early rapid signals for load transduction and, in particular, activation of membrane channels in osteoblasts. The involvement of calcium channels in the immediate load response and the modulation of intracellular calcium as an early signal are discussed. These membrane channels present a possible target for manipulation in the engineering of bone tissue repair.

Time course of osteoblast appearance after in vivo mechanical loading

The time course of the bone cellular response to mechanical loading is important in the design of optimal exercise prescriptions. This study examined the time course of periosteal cellular changes in the rat tibia following a single exposure of mechanical loading in four-point bending. The right tibiae of adult female Sprague Dawley rats (n = 48, 346 +/- 29 g) were loaded at 40 N (2000 mu epsilon) for 36 cycles at 2 Hz. Right loaded (L) and left nonloaded (NL) tibiae were collected on days 1, 2, 3, 4, 6, and 9 after loading. Cross sections from the loaded region were examined for periosteal differences in bone lining cell surface length, osteoblast surface length, and both alkaline phosphatase-positive cell surface length and width in the cellular layer. A single loading session increased osteoblast surface length as early as day 2, with a peak in expression on day 3. Nine days after a single loading session osteoblast surface length was not different from nonloaded control levels. Alkaline phosphatase width in the cellular periosteum was elevated by day 2 and remained elevated through day 9. This study shows the transient increase in osteoblast surface following a single loading session. It provides fundamental information regarding the timing of osteoblast appearance and the longevity of the response following mechanical stimulation.

Morphometric investigation on osteocytes in human auditory ossicles

An osteocyte lacunae differential count (1-lacunae with live osteocytes, 2-lacunae with degenerating osteocytes, 3-empty lacunae) was carried out on ear ossicles and clavicles from cadavers as well as on stapes removed by stapedotomy. The distance of the three types of lacunae from the vascular source was also determined by a computer-assisted light microscope. Results showed that the delayed fixation of bone from cadavers does not significantly interfere with osteocyte preservation, at least with the scope of this investigation. The results of osteocyte differential count show that the number of empty lacunae and lacunae with degenerating osteocytes: (a) is significantly higher in ear ossicles than in clavicles, (b) increases with age, (c) is higher in stapes than in incuses and mallei, (d) increases with the distance from the vascular sources in both ear ossicles and clavicles. Additionally it appeared that the process of osteocyte degeneration in ear ossicles is very rapid and widespread, over 40% of the cells being dead within the 2nd year of age. In the light of the recent literature and personal findings, which ascribe to osteocytes the function of mechanical detectors, and considering that bone remodeling occasionally occurs in ear ossicles, it is postulated that osteocyte death in these bones could be a programmed phenomenon (apoptosis?), due to which they lose the ability to react to strains and stresses and achieve the structural stability they need to perform their peculiar stereotyped function.