Increased expression of matrix metalloproteinase-1 in osteocytes precedes bone resorption as stimulated by disuse: evidence for autoregulation of the cell's mechanical environment?

Osteocytic Pericellular Matrix (PCM): Accelerated Degradation under In Vivo Loading and Unloading Conditions Using a Novel Imaging Approach

The proteoglycan-containing pericellular matrix (PCM) controls both the biophysical and biochemical microenvironment of osteocytes, which are the most abundant cells embedded and dispersed in bones. As a molecular sieve, osteocytic PCMs not only regulate mass transport to and from osteocytes but also act as sensors of external mechanical environments. The turnover of osteocytic PCM remains largely unknown due to technical challenges. Here, we report a novel imaging technique based on metabolic labeling and “click-chemistry,” which labels de novo PCM as “halos” surrounding osteocytes in vitro and in vivo. We then tested the method and showed different labeling patterns in young vs. old bones. Further “pulse-chase” experiments revealed dramatic difference in the “half-life” of PCM of cultured osteocytes (~70 h) and that of osteocytes in vivo (~75 d). When mice were subjected to either 3-week hindlimb unloading or 7-week tibial loading (5.1 N, 4 Hz, 3 d/week), PCM half-life was shortened (~20 d) and degradation accelerated. Matrix metallopeptidase MMP-14 was elevated in mechanically loaded osteocytes, which may contribute to PCM degradation. This study provides a detailed procedure that enables semi-quantitative study of the osteocytic PCM remodeling in vivo and in vitro.

A Systematic Review of Animal Models of Disuse-Induced Bone Loss

OBJECTIVE

Several different animal models are used to study disuse-induced bone loss. This systematic review aims to give a comprehensive overview of the animal models of disuse-induced bone loss and provide a detailed narrative synthesis of each unique animal model.

METHODS

PubMed and Embase were systematically searched for animal models of disuse from inception to November 30, 2019. In addition, Google Scholar and personal file archives were searched for relevant publications not indexed in PubMed or Embase. Two reviewers independently reviewed titles and abstracts for full-text inclusion. Data were extracted using a predefined extraction scheme to ensure standardization.

RESULTS

1964 titles and abstracts were screened of which 653 full-text articles were included. The most common animal species used to model disuse were rats (59%) and mice (30%). Males (53%) where used in the majority of the studies and genetically modified animals accounted for 7%. Twelve different methods to induce disuse were identified. The most frequently used methods were hindlimb unloading (44%), neurectomy (15%), bandages and orthoses (15%), and botulinum toxin (9%). The median time of disuse was 21 days (quartiles: 14 days, 36 days) and the median number of animals per group subjected to disuse was 10 (quartiles: 7, 14). Random group allocation was reported in 43% of the studies. Fewer than 5% of the studies justified the number of animals per group by a sample size calculation to ensure adequate statistical power.

CONCLUSION

Multiple animal models of disuse-induced bone loss exist, and several species of animals have successfully been studied. The complexity of disuse-induced bone loss warrants rigid research study designs. This systematic review emphasized the need for standardization of animal disuse research and reporting.

MMP-14 single-nucleotide polymorphisms are related to steroid-induced osteonecrosis of the femoral head in the population of northern China

Background

Osteonecrosis of the femoral head (ONFH) is a refractory disease which frequently occurs in young and middle‐aged people. Recent studies indicated that MMP‐14 played an important role in the development of chondrocytes, metabolism of osteoblasts as well as fate decision of hypertrophic chondrocytes. The aim of this study was to investigate the association between polymorphisms of MMP‐14 and steroid‐induced osteonecrosis of the femoral head in the Chinese population.

Methods

We selected 7 SNPs (rs3751488, rs1003349, rs1042703, rs2236302, rs1042704, rs2236303, and rs2236304) on gene MMP‐14. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using the chi‐squared test, genetic model analysis, haplotype analysis, and stratification analysis.

Results

We discovered that the genotype “G/G” of rs2236302 was associated with ONFH risk in the MMP‐14 in the codominant model (OR = 8.62, 95% CI = 1.07–69.46, p = 0.038) and recessive model (OR = 8.86, 95% CI = 1.10–71.31, p = 0.013).

Conclusions

We have confirmed that the susceptive SNPs (rs2236302) of MMP‐14 from the MMPs/TIMPs system exhibit a significant association with increased risk of steroid‐induced ONFH in the population of northern China.

MMP-3 and MMP-8 single-nucleotide polymorphisms are related to alcohol-induced osteonecrosis of the femoral head in Chinese males

Our study investigated the association between MMP-3 and MMP-8 single-nucleotide polymorphisms (SNPs) and alcohol-induced osteonecrosis of the femoral head (ONFH) in 695 Chinese males (299 cases and 396 control subjects). The minor allele of MMP-3 rs650108 was associated with a 0.78-fold decrease in alcohol-induced ONFH risk in the allelic model (95% CI = 0.63-0.97, P = 0.026). In the genetic model adjusted for age, rs650108 was associated with decreased risk of alcohol-induced ONFH in the dominant model (OR = 0.68, 95% CI = 0.49-0.95, P = 0.022) and log-additive model (OR = 0.78, 95% CI = 0.63-0.98, P = 0.030); MMP-8 rs11225394 was associated with increased risk in the codominant model (OR = 1.72, 95% CI = 1.15-2.58, P= 0.010), dominant model (OR = 1.67, 95% CI = 1.12-2.48, P = 0.012), over-dominant model (OR = 1.73, 95% CI = 1.16-2.59, P = 0.007) and log-additive model (OR = 1.57, 95% CI= 1.07-2.32, P = 0.022); and MMP-8 rs2012390 was associated with decreased risk in the dominant model (OR = 0.72, 95% CI = 0.53-0.97, P = 0.032) and log-additive model (OR = 0.77, 95% CI = 0.60-0.98, P = 0.035). Haplotype analysis showed that the CGATATGT sequence mediated decreased alcohol-induced ONFH risk (OR = 0.75, 95% CI = 0.57-0.97, P = 0.029). Therefore, among Chinese males, MMP-3 rs650108 and MMP-8 rs2012390 decrease alcohol-induced ONFH risk and MMP-8 rs11225394 increases it. Further study is needed to validate our conclusion.

Matrix Metalloproteinases in Bone Resorption, Remodeling, and Repair

Matrix metalloproteinases (MMPs) are the major protease family responsible for the cleavage of the matrisome (global composition of the extracellular matrix (ECM) proteome) and proteins unrelated to the ECM, generating bioactive molecules. These proteins drive ECM remodeling, in association with tissue-specific and cell-anchored inhibitors (TIMPs and RECK, respectively). In the bone, the ECM mediates cell adhesion, mechanotransduction, nucleation of mineralization, and the immobilization of growth factors to protect them from damage or degradation. Since the first description of an MMP in bone tissue, many other MMPs have been identified, as well as their inhibitors. Numerous functions have been assigned to these proteins, including osteoblast/osteocyte differentiation, bone formation, solubilization of the osteoid during bone resorption, osteoclast recruitment and migration, and as a coupling factor in bone remodeling under physiological conditions. In turn, a number of pathologies, associated with imbalanced bone remodeling, arise mainly from MMP overexpression and abnormalities of the ECM, leading to bone osteolysis or bone formation. In this review, we will discuss the functions of MMPs and their inhibitors in bone cells, during bone remodeling, pathological bone resorption (osteoporosis and bone metastasis), bone repair/regeneration, and emergent roles in bone bioengineering.

Association between genetic polymorphisms of MMP8 and the risk of steroid-induced osteonecrosis of the femoral head in the population of northern China

BACKGROUND

Steroid-induced osteonecrosis of the femoral head (ONFH) is the most common clinical nontraumatic ONFH. Once ONFH occurs, it seriously reduces patients' quality of life. The matrix metalloproteinase/tissue inhibitor of metalloproteinase (MMP/TIMP) system was found to play a significant role in the development of ONFH. The aim of this study was to identify the associations between 7 genes selected from the MMP/TIMP system and steroid-induced ONFH.

METHODS

We genotyped 34 single-nucleotide polymorphisms (SNPs) of 7 genes selected from the MMP/TIMP system in a case-control study with 285 cases of steroid-induced ONFH and 308 healthy controls. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using the chi-squared test, genetic model analysis, haplotype analysis, and stratification analysis.

RESULTS

We found that the minor alleles of rs1940475 and rs11225395 in MMP8 were associated with a 1.32-fold increased risk of steroid-induced ONFH in the allelic model analysis (P = 0.021 and 0.022, respectively). In the genetic model analysis, we found that rs3740938, rs2012390, rs1940475, and rs11225395 were associated with an increased risk of steroid-induced ONFH. In further stratification analysis, rs3740938 and rs2012390 displayed a significantly increased risk of steroid-induced ONFH in females under the dominant (rs3740938, OR = 2.69, 95% CI: 1.50-4.83, P = 0.001; rs2012390, OR = 2.30, 95% CI: 1.31-4.03, P = 0.012) and additive (rs3740938, OR = 2.02, 95% CI: 1.24-3.29, P = 0.010; rs2012390, OR = 1.77, 95% CI: 1.12-2.80, P = 0.047) models. In addition, haplotype "AGTCA" of MMP8 was found to be associated with a 1.40-fold increased risk of steroid-induced ONFH (95% CI: 1.04-1.88, P = 0.025).

CONCLUSION

Our results verify that genetic variants of MMP8 contribute to steroid-induced ONFH susceptibility in the population of northern China. In addition, we found that gender differences might interact with MMP8 polymorphisms to contribute to the overall susceptibility to steroid-induced ONFH.

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.

Loss of homeostatic strain alters mechanostat "set point" of tendon cells in vitro

Tendon cells respond to mechanical loads. The character (anabolic or catabolic) and sensitivity of this response is determined by the mechanostat set point of the cell, which is governed by the cytoskeleton and its interaction with the extracellular matrix. To determine if loss of cytoskeletal tension following stress deprivation decreases the mechanoresponsiveness of tendon cells, we cultured rat tail tendons under stress-deprived conditions for 48 hours and then cyclically loaded them for 24 hours at 1%, 3%, or 6% strain at 0.17 Hz. Stress deprivation upregulated MMP-13 mRNA expression and caused progressive loss of cell-matrix contact compared to fresh controls. The application of 1% strain to fresh tendons for 24 hours inhibited MMP-13 mRNA expression compared to stress-deprived tendons over the same period. However, when tendons were stress-deprived for 48 hours and then subjected to the same loading regime, the inhibition of MMP-13 mRNA expression was decreased. In stress-deprived tendons, it was necessary to increase the strain magnitude to 3% to achieve the same level of MMP-13 mRNA inhibition seen in fresh tendons exercised at 1% strain. The data suggest loss of cytoskeletal tension alters the mechanostat set point and decreases the mechanoresponsiveness of tendon cells.

Novel explant model to study mechanotransduction and cell-cell communication

To understand in situ behavior of osteocytes, we characterized a model of osteocytes in their native bone matrix and demonstrated real-time biologic activity of osteocytes while bending the bone matrix. Using 43 male Sprague-Dawley rats, dumbbell-shaped explants were harvested from stainless steel femoral implants after 6-12 weeks and incubated in culture medium or fixed. Sixteen specimens were used to determine bone volume density (BV/TV), volumetric bone mineral density (BMD) and histology for different implantation periods. Osteocyte viability was evaluated by L-lactate dehydrogenase (LDH) activity in 12 cultured explants. Confocal microscopy was used to assess tracer diffusion in three explants and changes in osteocyte pH of a mechanically loaded explant. From 6 to 12 weeks, explant BV/TV and volumetric BMD trended up 92.5% and 101%, respectively. They were significantly and highly correlated. Tissues were uniformly intramembranous and all bone cell types were present. Explants maintained LDH activity through culture day 8. Diffusion at 200 microM was limited to 1,209 Da. Explants appeared capable of reproducing complex bone biology. This model may be useful in understanding osteocyte mechanotransduction in the context of a physiologically relevant bone matrix.

Influence of mechanical and biological signals on gene expression in human MG-63 cells: evidence for a complex interplay between hydrostatic compression and vitamin D3 or TGF-beta1 on MMP-1 and MMP-3 mRNA levels

Biological mediators can influence the activity and differentiation of bone cells. 1,25-dihydroxy-vitamin D3 (1,25-(OH)2D3) is known to induce differentiation of precursors into mature osteoblasts, and transforming growth factor-beta1 (TGF-beta1) can modulate the activity of bone cells leading to alterations in proliferation and gene expression patterns. Bone-derived cells were loaded via intermittent cyclic hydrostatic pressure (icHP) on cells under basal conditions and in the presence of 1,25-(OH)2D3 or TGF-beta1. Evaluating the effects of loading on the cells allowed for a comparison to be made between responsiveness to biomechanical and biochemical stimuli and their potential interplay. The effects of icHP on mRNA levels for the specific genes involved in bone remodelling and differentiation were measured in MG-63 cells using reverse transcription-polymerase chain reaction (RT-PCR). The mRNA levels for matrix metalloproteinase-1 and -3 (MMP-1 and MMP-3) were significantly, and uniquely, increased (p < 0.001) in cells exposed to icHP under serum-free conditions for 4-12 h. However, mRNA levels for MMP-3, but not MMP-1, were significantly enhanced in cells subjected to static hydrostatic pressure (HP). Treatment of cells with 1,25-(OH)2D3 resulted in increased (p < 0.001) mRNA levels for osteocalcin and decreased (p < 0.001) mRNA levels for both MMP-1 and MMP-3. In cells exposed to icHP and 1,25-(OH)2D3, the mRNA levels for both MMP-1 and MMP-3 were elevated (p < 0.001) compared with hormone alone, but not to the same degree (p < 0.01) as cells subjected to icHP alone. Addition of TGF-beta1 to cells led to increases in cell proliferation and expression of collagen I, as well as decreases in expression of osteocalcin and MMP-1 and MMP-3. Exposure of cells to icHP and TGF-beta1 again led to unique and significant increases in expression of MMP-1 and MMP-3. No changes in mRNA levels for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or any of the other 9 genes assessed, including those for MMP-2 and MMP-13, were detected under any of the conditions described. Therefore, icHP can induce alterations in mRNA levels for a specific subset of genes in both premature and mature osteoblasts. Such stimuli can modulate the impact of potent biological mediators in defining patterns of gene expression by bone cells and potentially modify function in vivo.

In vitro alterations in cytoskeletal tensional homeostasis control gene expression in tendon cells

An in vitro collagen gel system was used to determine the effect of alterations in cytoskeletal tensional homeostasis on gene expression in tendon cells. Collagen gel matrices, seeded with rat tail tendon cells, underwent cytochalasin D and gel contraction treatments designed to alter the internal cytoskeletal homeostasis of the cells. Gels were examined for cytoskeletal organization using a rhodamine phalloidin stain for actin. The effect of altered cytoskeletal organization on mRNA expression of a catabolic (interstitial collagenase) and anabolic (alpha1(I) collagen) gene was examined using northern blot analysis. Tendon cells in adhered gels demonstrated a highly organized cytoskeleton and showed evidence of alpha1(I) collagen mRNA expression but no evidence of collagenase mRNA expression. Treatment of the attached gel with cytochalasin D disrupted the cytoskeletal organization and resulted in the up-regulation of collagenase mRNA and the inhibition of alpha1(I) collagen mRNA expression. Release of the gels resulted in a cell mediated gel contraction, an immediate loss of cytoskeletal organization, and an mRNA expression pattern similar to that seen with cytochalasin D treatment. Isometric contraction of the gel on itself or around a 3-point traction device resulted in an mRNA expression pattern similar to the adhered gel. Gene expression in the contracted gels could be reversed through chemical cytoskeletal disruption or removal of the traction device which permitted further gel contraction. The results of the study suggest that tendon cells can establish an internal cytoskeletal tension through interactions with their local extracellular environment. Alterations in this tension appear to control the expression of both catabolic and anabolic genes in a reciprocal manner.

Upregulation of osteopontin by osteocytes deprived of mechanical loading or oxygen

The pathway(s) by which disuse is transduced into locally mediated osteoclastic resorption remain unknown. We found that both acute disuse (in vivo) and direct hypoxia (in vitro) induced rapid upregulation of OPN expression by osteocytes. Within the context of OPN's role in osteoclast migration and attachment, hypoxia-induced osteocyte OPN expression may serve to mediate disuse-induced bone resorption.

INTRODUCTION

We have recently reported that disuse induces osteocyte hypoxia. Because hypoxia upregulates osteopontin (OPN) in nonconnective tissue cells, we hypothesized that both disuse and hypoxia would rapidly elevate expression of OPN by osteocytes.

MATERIALS AND METHODS

The response of osteocytes to 24 h of disuse was explored by isolating the left ulna diaphysis of adult male turkeys from loading (n = 5). Cortical osteocytes staining positive for OPN were determined using immunohistochemistry and confocal microscopy. In vitro experiments were performed to determine if OPN expression was altered in MLO-Y4 osteocytes by direct hypoxia (3, 6, 24, and 48 h) or hypoxia (3 and 24 h) followed by 24 h of reoxygenation. A final in vitro experiment explored the potential of protein kinase C (PKC) to regulate hypoxia-induced osteocyte OPN mRNA alterations.

RESULTS

We found that 24 h of disuse significantly elevated osteocyte OPN expression in vivo (145% versus intact bones; p = 0.02). We confirmed this finding in vitro, by observing rapid and significant upregulation of OPN protein expression after 24 and 48 h of hypoxia. Whereas 24 h of reoxygenation after 3 h of hypoxia restored normal osteocyte OPN expression levels, 24 h of reoxygenation after 24 h of hypoxia did not mitigate elevated osteocyte OPN expression. Finally, preliminary inhibitor studies suggested that PKC serves as a potent upstream regulator of hypoxia-induced osteocyte OPN expression.

CONCLUSIONS

Given the documented roles of OPN as a mediator of environmental stress (e.g., hypoxia), an osteoclast chemotaxant, and a modulator of osteoclastic attachment to bone, we speculate that hypoxia-induced osteocyte OPN expression may serve to mediate disuse-induced osteoclastic resorption. Furthermore, it seems that a brief window of time exists in which reoxygenation (as might be achieved by reloading bone) can serve to inhibit this pathway.

Immunolocalization of matrix metalloproteinase-13 on bone surface under osteoclasts in rat tibia

Matrix metalloproteinase (MMP)-13 (an interstitial collagenase also called collagenase 3) is involved in degradation of extracellular matrix in various tissues. Using immunohistochemistry and Western blotting, we investigated localization of MMP-13 in rat tibia, to clarify the role of MMP-13 in bone resorption. MMP-13 reactivity was mainly seen on bone surfaces under osteoclasts, and in some osteocytes and their lacunae near osteoclasts. However, immunoreactivity was not seen in chondrocytes or osteoclasts. MMP-13 was also localized on cement lines in the epiphysis. In the growth plate erosion zone, perivascular cells showed MMP-13 reactivity. Immunoelectron microscopy revealed that MMP-13 was localized on the bone surfaces, under the ruffled borders and some clear zones of osteoclasts. Gold-labeled MMP-13 was closely associated with collagen fibrils. Gold labeling was also detected in Golgi apparatus of osteocytes adjacent to osteoclasts and bone lining cells. Western blotting showed that MMP-13 was mainly associated with mineralized bone matrix. These findings suggest that MMP-13 synthesized and secreted by osteoblast-lineage cells is localized under the ruffled borders of osteoclasts. MMP-13 may play an important role in degradation of type I collagen in bone matrix, acting in concert with cathepsin K and MMP-9 produced by osteoclasts. MMP-13 in perivascular cells may be involved in removal of cartilage matrix proteins such as type II collagen and aggrecan.

Altered fracture repair in the absence of MMP9

The regeneration of adult skeletal tissues requires the timely recruitment of skeletal progenitor cells to an injury site, the differentiation of these cells into bone or cartilage, and the re-establishment of a vascular network to maintain cell viability. Disturbances in any of these cellular events can have a detrimental effect on the process of skeletal repair. Although fracture repair has been compared with fetal skeletal development, the extent to which the reparative process actually recapitulates the fetal program remains uncertain. Here, we provide the first genetic evidence that matrix metalloproteinase 9 (MMP9) regulates crucial events during adult fracture repair. We demonstrate that MMP9 mediates vascular invasion of the hypertrophic cartilage callus, and that Mmp9(-/-) mice have non-unions and delayed unions of their fractures caused by persistent cartilage at the injury site. This MMP9- dependent delay in skeletal healing is not due to a lack of vascular endothelial growth factor (VEGF) or VEGF receptor expression, but may instead be due to the lack of VEGF bioavailability in the mutant because recombinant VEGF can rescue Mmp9(-/-) non-unions. We also found that Mmp9(-/-) mice generate a large cartilage callus even when fractured bones are stabilized, which implicates MMP9 in the regulation of chondrogenic and osteogenic cell differentiation during early stages of repair. In conclusion, the resemblance between Mmp9(-/-) fetal skeletal defects and those that emerge during Mmp9(-/-) adult repair offer the strongest evidence to date that similar mechanisms are employed to achieve bone formation, regardless of age.

The type I collagen fragments ICTP and CTX reveal distinct enzymatic pathways of bone collagen degradation

Bone resorption may generate collagen fragments such as ICTP and CTX, which can be quantified in serum and/or urine by using specific immunoassays, and which are used as clinical markers. However, the relative abundance of ICTP and CTX varies according to the type of bone pathology, suggesting that these two fragments are generated through distinct collagenolytic pathways. In this study, we analyzed the release of ICTP and CTX from bone collagen by the proteinases reported to play a role in the solubilization of bone matrix. Cathepsin K released large amounts of CTX, but did not allow a detectable release of ICTP. Conversely, the matrix metalloproteinases (MMPs) MMP-2, -9, -13, or -14 released ICTP, but did not allow a detectable release of CTX. Next we analyzed the release of ICTP and CTX from bone explants cultured in the presence of well-established inhibitors of these proteinases and of matrix solubilization. An inhibitor of cysteine proteinases including cathepsin K, inhibited the release of CTX, but not the release of ICTP. MMP inhibitors inhibited the release of ICTP, but also that of CTX, in agreement with the putative role of MMPs in the initiation of bone resorption in addition to matrix solubilization. Similarly the treatment of mice bearing bone metastasis with an MMP inhibitor led to a significant reduction of serum ICTP and CTX, and osteolytic lesions. We conclude that the generation of ICTP and CTX depends on different collagenolytic pathways. This finding may explain why these two markers may discriminate between different bone pathologies.

Mechanical strain, induced noninvasively in the high-frequency domain, is anabolic to cancellous bone, but not cortical bone

Departing from the premise that it is the large-amplitude signals inherent to intense functional activity that define bone morphology, we propose that it is the far lower magnitude, high-frequency mechanical signals that continually barrage the skeleton during longer term activities such as standing, which regulate skeletal architecture. To examine this hypothesis, we proposed that brief exposure to slight elevations in these endogenous mechanical signals would suffice to increase bone mass in those bones subject to the stimulus. This was tested by exposing the hind limbs of adult female sheep (n = 9) to 20 min/day of low-level (0.3g), high-frequency (30 Hz) mechanical signals, sufficient to induce a peak of approximately 5 microstrain (micro epsilon) in the tibia. Following euthanasia, peripheral quantitative computed tomography (pQCT) was used to segregate the cortical shell from the trabecular envelope of the proximal femur, revealing a 34.2% increase in bone density in the experimental animals as compared with controls (p = 0.01). Histomorphometric examination of the femur supported these density measurements, with bone volume per total volume increasing by 32% (p = 0.04). This density increase was achieved by two separate strategies: trabecular spacing decreased by 36.1% (p = 0.02), whereas trabecular number increased by 45.6% (p = 0.01), indicating the formation of cancellous bone de novo. There were no significant differences in the radii of animals subject to the stimulus, indicating that the adaptive response was local rather than systemic. The anabolic potential of the signal was evident only in trabecular bone, and there were no differences, as measured by any assay, in the cortical bone. These data suggest that subtle mechanical signals generated during predominant activities such as posture may be potent determinants of skeletal morphology. Given that these strain levels are three orders of magnitude below strains that can damage bone tissue, we believe that a noninvasive stimulus based on this sensitivity has potential for treating skeletal complications such as osteoporosis.

Inhibition of osteopenia by low magnitude, high-frequency mechanical stimuli

The identification of anabolic agents for the treatment of metabolic bone disease is a highly prized, and elusive, goal. In searching for the osteogenic (bone-producing) constituents within mechanical stimuli, it was determined that high frequency (10-100 Hz) and low magnitude (<10 microstrain) stimuli were capable of augmenting bone mass and morphology, thereby benefiting both bone quantity and quality. Using animal models, it is shown that these mechanical signals can double bone-formation rates, inhibit disuse osteoporosis and increase the strength of trabecular bone by 25%. Considering that the magnitude of these mechanical signals are several orders of magnitude below those which cause damage to the bone tissue, it is proposed that this modality could be useful in the treatment of metabolic bone diseases.

Selected Contribution: Osteocytes upregulate HIF-1alpha in response to acute disuse and oxygen deprivation

Loss of mechanical loading, or disuse, rapidly precipitates locally mediated bone resorption. However, the pathway by which this process is initiated and mediated is poorly understood. In this study, we used a complementary in vivo and in vitro approach to determine whether disuse-induced osteocyte hypoxia resulted in upregulation of the hypoxia-dependent transcription factor HIF-1alpha. We found that acute disuse (1-5 days) resulted in a significant increase in the percentage of osteocytes staining positive for HIF-1alpha vs. normal bone (30.9 +/- 6.1 vs. 14.1 +/- 3.8%) and that this response was uniform around the cortex. In addition, we found that acute oxygen deprivation (4-12 h of 2% O2) resulted in a 2.1- to 3.7-fold upregulation of HIF-1alpha protein expression in MLO-Y4 osteocyte-like cells compared with cells cultured in parallel under normal oxygen conditions. Given known HIF-1alpha targets genes, we suggest that osteocyte hypoxia and subsequent upregulation of hypoxia-dependent pathways may serve to initiate and mediate disuse-induced bone resorption.

Estimation of bone matrix apparent stiffness variation caused by osteocyte lacunar size and density

The role of osteocyte lacunar size and density on the apparent stiffness of bone matrix was predicted using a mechanical model from the literature. Lacunar size and lacunar density for different bones from different gender and age groups were used to predict the range of matrix apparent stiffness values for human cortical and cancellous tissue. The results suggest that bone matrix apparent stiffness depends on tissue type (cortical versus cancellous), age, and gender, the magnitudes of the effects being significant but small in all cases. Males had a higher predicted matrix apparent stiffness than females for vertebral cancellous bone (p< I0(-7)) and the difference increased with age (p =0.0007). In contrast, matrix apparent stiffness was not different between males and females forfemoral cortical bone and increased with age in both males (p < 0.0001) and females (p < 0.0364). Osteocyte lacunar density and size may cause significant gender and age-related variations in bone matrix apparent stiffness. The magnitude of variations in matrix apparent stiffness was small within the physiological range of lacunar size and density for healthy bone, whereas the variations can be profound in certain pathological cases. It was proposed that the mechanical effects of osteocyte density be uncoupled from their biological effects by controlling lacunar size in normal bone.