Search

Osteocytes are dynamic, bone matrix-remodeling cells that form an intricate network of interconnected projections through the bone matrix, called the lacunar-canalicular system. Osteocytes are the dominant mechanosensory cells in bone and their mechanosensory and mechanotransductive functions follow their morphological form. During osteocytogenesis and development of the osteocyte lacunar-canalicular network, osteocytes must dramatically remodel both their cytoskeleton and their extracellular matrix. In this review, we summarize our current understanding of the mechanisms that govern osteocyte differentiation, cytoskeletal morphogenesis, mechanotransduction, and matrix remodeling. We postulate that the physiologic activation of matrix remodeling in adult osteocytes, known as perilacunar/canalicular remodeling (PLR) represents a re-activation of the developmental program by which the osteocyte network is first established. While much of osteocyte biology remains unclear, new tools and approaches make the present moment a particularly fruitful and exciting time to study the development of these remarkable cells.

Bone is a dynamic tissue that can adapt its architecture in response to mechanical signals under the control of osteocytes, which sense mechanical deformation of the mineralized bone. However, cells in the marrow are also mechanosensitive and may contribute to load-induced bone adaptation, as marrow is subjected to mechanical stress during bone deformation. We investigated the contribution of mechanotransduction in marrow cells to trabecular bone formation by applying low magnitude mechanical stimulation (LMMS) to porcine vertebral trabecular bone explants in an in situ bioreactor. The bone formation rate was higher in stimulated explants compared to unloaded controls which represent a disuse condition (CNT). However, sclerostin protein expression in osteocytes was not different between groups, nor was expression of osteocytic mechanoregulatory genes SOST, IGF-1, CTGF, and Cyr61, suggesting the mechanoregulatory program of osteocytes was unaffected by the loading regime. In contrast, c-Fos, a gene indicative of mechanical stimulation, was upregulated in the marrow cells of mechanically stimulated explants, while the level of activated c-Jun decreased by 25%. The activator protein 1 (AP-1) transcription factor is a heterodimer of c-Fos and c-Jun, which led us to investigate the expression of the downstream target gene cyclin-D1, a gene associated with cell cycle progression and osteogenesis. Cyclin-D1 gene expression in the stimulated marrow was approximately double that of the controls. The level of phosphorylated PYK2, a purported inhibitor of osteoblast differentiation, also decreased in marrow cells from stimulated explants. Taken together, mechanotransduction in marrow cells induced trabecular bone formation independent of osteocyte signaling. Identifying the specific cells and signaling pathways involved, and verifying them with inhibition of specific signaling molecules, could lead to potential therapeutic targets for diseases characterized by bone loss.

article Although severe extremity trauma is often inclusive of skeletal and vascular damage in combination, segmen- tal bone defect repair with concomitant vascular injury has yet to be experimentally investigated. To this end, we developed a novel rat composite limb injury model by combining a critically-sized segmental bone defect with surgically-induced hind limb ischemia (HLI). Unilateral 8 mm femoral defects were created alone (BD) or in combination with HLI (BD + HLI), and all defects were treated with rhBMP-2 via a hybrid biomaterial delivery system. Based on reported clinical and experimental observations on the importance of vascular net- works in bone repair, we hypothesized that HLI would impair bone regeneration. Interestingly, the BD + HLI group displayed improved radiographic bridging, and quantitative micro-CT analysis revealed enhanced bone regeneration as early as week 4 (p b 0.01) that was sustained through week 12 (p b 0.001) and confirmed histologically. This effect was observed in two independent studies and at two different doses of rhBMP-2. Micro-CT angiography was used to quantitatively evaluate vascular networks at week 12 in both the thigh and the regenerated bone defect. No differences were found between groups in total blood vessel volume in the thigh, but clear differences in morphology were present as the BD + HLI group possessed a more interconnected network of smaller diameter vessels (p b 0.001). Accordingly, while the overall thigh vessel volume was comparable between groups, the contributions to vessel volume based on vessel diameter dif- fered significantly. Despite this evidence of a robust neovascular response in the thigh of the BD + HLI group, differences were not observed between groups for bone defect blood vessel volume or morphology. In total, our results demonstrate that a transient ischemic insult and the subsequent recovery response to HLI significantly enhanced BMP-2-mediated segmental bone defect repair, providing additional complexity to the relationship between vascular tissue networks and bone healing. Ultimately, a better understanding of the coupling mechanisms may reveal important new strategies for promoting bone healing in challenging clinical scenarios.

Osteogenic growth factors that promote endogenous repair mechanisms hold considerable potential for repairing challenging bone defects. The local delivery of one such growth factor, bone morphogenetic protein (BMP), has been successfully translated to clinical practice for spinal fusion and bone fractures. However, improvements are needed in the spatial and temporal control of BMP delivery to avoid the currently used supraphysiologic doses and the concomitant adverse effects. We have recently introduced a hybrid protein delivery system comprised of two parts: a perforated nanofibrous mesh that spatially confines the defect region and a functionalized alginate hydrogel that provides temporal growth factor release kinetics. Using this unique spatiotemporal delivery system, we previously demonstrated BMP-mediated functional restoration of challenging 8 mm femoral defects in a rat model. In this study, we compared the efficacy of the hybrid system in repairing segmental bone defects to that of the current clinical standard, collagen sponge, at the same dose of recombinant human BMP-2. In addition, we investigated the specific role of the nanofibrous mesh tube on bone regeneration. Our results indicate that the hybrid delivery system significantly increased bone regeneration and improved biomechanical function compared to collagen sponge delivery. Furthermore, we observed that presence of the nanofiber mesh tube was essential to promote maximal mineralized matrix synthesis, prevent extra-anatomical mineralization, and guide an integrated pattern of bone formation. Together, these results suggest that spatiotemporal strategies for osteogenic protein delivery may enhance clinical outcomes by improving localized protein retention.