Differential β3 and β1 Integrin Expression in Bone Marrow and Cortical Bone of Estrogen Deficient Rats

Osteocyte β3 integrin promotes bone mass accrual and force-induced bone formation in mice

Background

Cell culture studies demonstrate the importance of β3 integrin in osteocyte mechanotransduction. However, the in vivo roles of osteocyte β3 integrin in the regulation of bone homeostasis and mechanotransduction are poorly defined.

Materials and methods

To study the in vivo role of osteocyte β3 integrin in bone, we utilized the 10-kb Dmp1 (dentin matrix acidic phosphoprotein 1)-Cre to delete β3 integrin expression in osteocyte in mice. Micro-computerized tomography (μCT), bone histomorphometry and in vitro cell culture experiments were performed to determine the effects of osteocyte β3 integrin loss on bone mass accrual and biomechanical properties. In addition, in vivo tibial loading model was applied to study the possible involvement of osteocyte β3 integrin in the mediation of bone mechanotransduction.

Results

Deletion of β3 integrin in osteocytes resulted in a low bone mass and impaired biomechanical properties in load-bearing long bones in adult mice. The loss of β3 integrin led to abnormal cell morphology with reduced number and length of dentritic processes in osteocytes. Furthermore, osteocyte β3 integrin loss did not impact the osteoclast formation, but significantly reduced the osteoblast-mediated bone formation rate and reduced the osteogenic differentiation of the bone marrow stromal cells in the bone microenvironment. In addition, mechanical loading failed to accelerate the anabolic bone formation in mutant mice.

Conclusions

Our studies demonstrate the essential roles of osteocyte β3 integrin in regulating bone mass and mechanotransduction.

Obacunone inhibits RANKL/M-CSF-mediated osteoclastogenesis by suppressing integrin- FAK-Src signaling

Disrupted osteoblastogenesis or aberrant activation of osteoclastogenesis usually results in the break of bone homeostasis thus causing bone-associated diseases like osteoporosis. Obacunone, as a natural compound present in citrus fruits, has been demonstrated for various biological activities including anti-cancer and anti-inflammatory properties. However, the role of obacunone in regulating osteoclastogenesis has not been elucidated so far. Here, using in vitro cell models of RANKL (Receptor activator of nuclear factor-kB ligand) and M-CSF (Macrophage-colony-stimulating factor)-induced osteoclastogenesis, we showed that obacunone inhibited osteoclast differentiation in RAW264.7 cells and bone marrow macrophages (BMMs), as evidenced by obacunone dose-dependent reduction in numbers of osteoclasts and downregulated expressions of osteoclastogenesis-associated key genes. The anti-osteoclastic properties of obacunone were associated with downregulated expressions of Integrin α1 and attenuated activation of Focal adhesion kinase (FAK) and Steroid receptor coactivator (Src) signaling. Functional Integrin α1 blockade or FAK-Src inhibition suppressed RANKL/M-CSF-induced osteoclastogenesis, while Integrin α1 overexpression or FAK/Src activation partially attenuated obacunone's effects on suppressing RANKL/M-CSF-induced osteoclast differentiation. Furthermore, in vivo administration of obacunone displayed super therapeutic effects in attenuating ovariectomy-induced bone loss in mice, as indicated by decreases in serum biomarkers of bone turnover, restoring of femur fracture maximum force, and reversing of the worsened bone-related parameters in ovariectomized animals. Taken together, these findings demonstrate that obacunone has pharmacological activities to suppress osteoclast differentiation through modulating the Integrin-FAK-Src pathway, and suggest that obacunone is a therapeutic candidate for the treatment and prevention of bone diseases such as osteoporosis.

Osteocytes and Estrogen Deficiency

PURPOSE OF REVIEW

Postmenopausal osteoporosis reduces circulating estrogen levels, which leads to osteoclast resorption, bone loss, and fracture. This review addresses emerging evidence that osteoporosis is not simply a disease of bone loss but that mechanosensitive osteocytes that regulate both osteoclasts and osteoblasts are also impacted by estrogen deficiency.

RECENT FINDINGS

At the onset of estrogen deficiency, the osteocyte mechanical environment is altered, which coincides with temporal changes in bone tissue composition. The osteocyte microenvironment is also altered, apoptosis is more prevalent, and hypermineralization occurs. The mechanobiological responses of osteocytes are impaired under estrogen deficiency, which exacerbates osteocyte paracrine regulation of osteoclasts. Recent research reveals changes in osteocytes during estrogen deficiency that may play a critical role in the etiology of the disease. A paradigm change for osteoporosis therapy requires an advanced understanding of such changes to establish the efficacy of osteocyte-targeted therapies to inhibit resorption and secondary mineralization.

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

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

Scl-Ab reverts pro-osteoclastogenic signalling and resorption in estrogen deficient osteocytes

Background

Neutralising antibodies to sclerostin (Scl-Ab) have shown significant potential to induce bone formation and decrease bone resorption, increase strength and substantially reduce fracture risk in animal studies and clinical trials. Mechanical loading negatively regulates sclerostin expression, and sclerostin has been shown to induce RANKL synthesis in osteocytes. However, how Scl-Ab governs osteocyte regulation of osteoclast differentiation and function is not fully understood. We have recently discovered that osteoblasts and osteocytes alter osteoclastogenic signalling (RANKL/OPG) during estrogen-deficiency, and that osteoblast-induced osteoclastogenesis and resorption are exacerbated. However, it is not known whether estrogen deficient osteocytes exacerbate osteoclastogenesis. The aims of this study were to (1) establish whether osteocytes induce osteoclastogenesis and bone resorption during estrogen deficiency in vitro (2) investigate whether the sclerostin antibody can revert osteocyte-mediated osteoclastogenesis and resorption by attenuating RANKL/OPG expression.

Results

Using conditioned media and co-culture experiments we found increased osteocyte-induced osteoclastogenesis and bone resorption in estrogen deficient conditions. This is the first study to report that administration of Scl-Ab has the ability to revert osteocyte-mediated osteoclastogenesis and resorption by decreasing RANKL/OPG ratio expression and increasing WISP1 expression in estrogen deficient osteocytes.

Conclusions

This study provides an enhanced understanding of the biological changes underpinning decreases in bone resorption following Scl-Ab treatment observed in vivo by revealing that Scl-Ab can reduce pro-osteoclastogenic cell signalling between osteocytes and osteoclasts.

A Novel 3D Osteoblast and Osteocyte Model Revealing Changes in Mineralization and Pro-osteoclastogenic Paracrine Signaling During Estrogen Deficiency

Recent in vitro studies have revealed that the mechanobiological responses of osteoblasts and osteocytes are fundamentally impaired during estrogen deficiency. However, these two-dimensional (2D) cell culture studies do not account for in vivo biophysical cues. Thus, the objectives of this study are to (1) develop a three-dimensional (3D) osteoblast and osteocyte model integrated into a bioreactor and (2) apply this model to investigate whether estrogen deficiency leads to changes in osteoblast to osteocyte transition, mechanosensation, mineralization, and paracrine signaling associated with bone resorption by osteoclasts. MC3T3-E1s were expanded in media supplemented with estrogen (17β-estradiol). These cells were encapsulated in gelatin-mtgase before culture in (1) continued estrogen (E) or (2) no further estrogen supplementation. Constructs were placed in gas permeable and water impermeable cell culture bags and maintained at 5% CO₂ and 37°C. These bags were either mechanically stimulated in a custom hydrostatic pressure (HP) bioreactor or maintained under static conditions (control). We report that osteocyte differentiation, characterized by the presence of dendrites and staining for osteocyte marker dentin matrix acidic phosphoprotein 1 (DMP1), was significantly greater under estrogen withdrawal (EW) compared to under continuous estrogen treatment (day 21). Mineralization [bone sialoprotein (BSP), osteopontin (OPN), alkaline phosphatase (ALP), calcium] and gene expression associated with paracrine signaling for osteoclastogenesis [receptor activator of nuclear factor kappa-β ligand (RANKL)/osteoprotegerin OPG ratio] were significantly increased in estrogen deficient and mechanically stimulated cells. Interestingly, BSP and DMP-1 were also increased at day 1 and day 21, respectively, which play a role in regulation of biomineralization. Furthermore, the increase in pro-osteoclastogenic signaling may be explained by altered mechanoresponsiveness of osteoblasts or osteocytes during EW. These findings highlight the impact of estrogen deficiency on bone cell function and provide a novel in vitro model to investigate the mechanisms underpinning changes in bone cells after estrogen deficiency.

Alterations in osteocyte mediated osteoclastogenesis during estrogen deficiency and under ROCK-II inhibition: An in vitro study using a novel postmenopausal multicellular niche model

This study sought to derive an enhanced understanding of the complex intracellular interactions that drive bone loss in postmenopausal osteoporosis. We applied an in-vitro multicellular niche to recapitulate cell-cell signalling between osteocytes, osteoblasts and osteoclasts to investigate (1) how estrogen-deficient and mechanically loaded osteocytes regulate osteoclastogenesis and (2) whether ROCK-II inhibition affects these mechanobiological responses. We report that mechanically stimulated and estrogen-deficient osteocytes upregulated RANKL/OPG and M-CSF gene expression, when compared to those treated with 10 nM estradiol. Osteoclast precursors (RAW 264.7) cultured within this niche underwent significant reduction in osteoclastogenic gene expression (CTSK), and there was an increasing trend in the area covered by TRAP⁺ osteoclasts (24% vs. 19.4%, p = 0.06). Most interestingly, upon treatment with the ROCK-II inhibitor, RANKL/OPG and M-CSF gene expression by estrogen-deficient osteocytes were downregulated. Yet, this inhibition of the pro-osteoclastogenic factors by osteocytes did not ultimately reduce the differentiation of osteoclast precursors. Indeed, TRAP and CTSK gene expressions in osteoclast precursors were upregulated, and there was an increased trend for osteoclast area (30.4% vs. 24%, p = 0.07), which may have been influenced by static osteoblasts (MC3T3-E1) that were included in the niche. We conclude that ROCK-II inhibition can attenuate bone loss driven by osteocytes during estrogen deficiency.

Integrin-associated molecules and signalling cross talking in osteoclast cytoskeleton regulation

In the ageing skeleton, the balance of bone reconstruction could commonly be broken by the increasing of bone resorption and decreasing of bone formation. Consequently, the bone resorption gradually occupies a dominant status. During this imbalance process, osteoclast is unique cell linage act the bone resorptive biological activity, which is a highly differentiated ultimate cell derived from monocyte/macrophage. The erosive function of osteoclasts is that they have to adhere the bone matrix and migrate along it, in which adhesive cytoskeleton recombination of osteoclast is essential. In that, the podosome is a membrane binding microdomain organelle, based on dynamic actin, which forms a cytoskeleton superstructure connected with the plasma membrane. Otherwise, as the main adhesive protein, integrin regulates the formation of podosome and cytoskeleton, which collaborates with the various molecules including: c-Cbl, p130^Cas , c-Src and Pyk2, through several signalling cascades cross talking, including: M-CSF and RANKL. In our current study, we discuss the role of integrin and associated molecules in osteoclastogenesis cytoskeletal, especially podosomes, regulation and relevant signalling cascades cross talking.

Integrins in Osteocyte Biology and Mechanotransduction

PURPOSE OF REVIEW

Osteocytes are the main mechanosensitive cells in bone. Integrin-based adhesions have been shown to facilitate mechanotransduction, and therefore play an important role in load-induced bone formation. This review outlines the role of integrins in osteocyte function (cell adhesion, signalling, and mechanotransduction) and possible role in disease.

RECENT FINDINGS

Both β₁ and β₃ integrins subunits have been shown to be required for osteocyte mechanotransduction. Antagonism of these integrin subunits in osteocytes resulted in impaired responses to fluid shear stress. Various disease states (osteoporosis, osteoarthritis, bone metastases) have been shown to result in altered integrin expression and function. Osteocyte integrins are required for normal cell function, with dysregulation of integrins seen in disease. Understanding the mechanism of faulty integrins in disease may aid in the creation of novel therapeutic approaches.

Estrogen deficiency impairs integrin αvβ3-mediated mechanosensation by osteocytes and alters osteoclastogenic paracrine signalling

The integrin α_vβ₃ has been shown to play an important role in osteocyte mechanotransduction. It has been reported that there are fewer β₃ integrin-containing cells in osteoporotic bone cells. Osteocytes cultured in vitro under estrogen deficient conditions demonstrate altered mechanotransduction. However, it is unknown whether the altered mechanotransduction in estrogen deficient osteocytes is directly associated with defective α_vβ₃ expression or signalling. The objective of this study is to investigate the role of estrogen deficiency for regulating MLO-Y4 cell morphology, α_vβ₃ expression, focal adhesion formation and mechanotransduction by osteocytes. Here, we report that estrogen withdrawal leads to a smaller focal adhesion area and reduced α_vβ₃ localisation at focal adhesion sites, resulting in an increased Rankl/Opg ratio and defective Cox-2 responses to oscillatory fluid flow. Interestingly, α_vβ₃ antagonism had a similar effect on focal adhesion assembly, Rankl/Opg ratio, and Cox-2 responses to oscillatory fluid flow. Taken together, our results provide the first evidence for a relationship between estrogen withdrawal and defective α_vβ₃-mediated signalling. Specifically, this study implicates estrogen withdrawal as a putative mechanism responsible for altered α_vβ₃ expression and resultant changes in downstream signalling in osteocytes during post-menopausal osteoporosis, which might provide an important, but previously unidentified, contribution to the bone loss cascade.

Participation of integrin β3 in osteoblast differentiation induced by titanium with nano or microtopography

The major role of integrins is to mediate cell adhesion but some of them are involved in the osteoblasts-titanium (Ti) interactions. In this study, we investigated the participation of integrins in osteoblast differentiation induced by Ti with nanotopography (Ti-Nano) and with microtopography (Ti-Micro). By using a PCR array, we observed that, compared with Ti-Micro, Ti-Nano upregulated the expression of five integrins in mesenchymal stem cells, including integrin β3, which increases osteoblast differentiation. Silencing integrin β3, using CRISPR-Cas9, in MC3T3-E1 cells significantly reduced the osteoblast differentiation induced by Ti-Nano in contrast to the effect on T-Micro. Concomitantly, integrin β3 silencing downregulated the expression of integrin αv, the parent chain that combines with other integrins and several components of the Wnt/β-catenin and BMP/Smad signaling pathways, all involved in osteoblast differentiation, only in cells cultured on Ti-Nano. Taken together, our results showed the key role of integrin β3 in the osteogenic potential of Ti-Nano but not of Ti-Micro. Additionally, we propose a novel mechanism to explain the higher osteoblast differentiation induced by Ti-Nano that involves an intricate regulatory network triggered by integrin β3 upregulation, which activates the Wnt and BMP signal transductions. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1303-1313, 2019.