Irisin Mediates Effects on Bone and Fat via αV Integrin Receptors.

Irisin is secreted by muscle, increases with exercise, and mediates certain favorable effects of physical activity. In particular, irisin has been shown to have beneficial effects in adipose tissues, brain, and bone. However, the skeletal response to exercise is less clear, and the receptor for irisin has not been identified. Here we show that irisin binds to proteins of the αV class of integrins, and biophysical studies identify interacting surfaces between irisin and αV/ß5 integrin. Chemical inhibition of the αV integrins blocks signaling and function by irisin in osteocytes and fat cells. Irisin increases both osteocytic survival and production of sclerostin, a local modulator of bone remodeling. Genetic ablation of FNDC5 (or irisin) completely blocks osteocytic osteolysis induced by ovariectomy, preventing bone loss and supporting an important role of irisin in skeletal remodeling. Identification of the irisin receptor should greatly facilitate our understanding of irisin's function in exercise and human health.

Colony stimulating factor-1 producing endothelial cells and mesenchymal stromal cells maintain monocytes within a perivascular bone marrow niche.

Macrophage colony stimulating factor-1 (CSF-1) plays a critical role in maintaining myeloid lineage cells. However, congenital global deficiency of CSF-1 (Csf1op/op) causes severe musculoskeletal defects that may indirectly affect hematopoiesis. Indeed, we show here that osteolineage-derived Csf1 prevented developmental abnormalities but had no effect on monopoiesis in adulthood. However, ubiquitous deletion of Csf1 conditionally in adulthood decreased monocyte survival, differentiation, and migration, independent of its effects on bone development. Bone histology revealed that monocytes reside near sinusoidal endothelial cells (ECs) and leptin receptor (Lepr)-expressing perivascular mesenchymal stromal cells (MSCs). Targeted deletion of Csf1 from sinusoidal ECs selectively reduced Ly6C- monocytes, whereas combined depletion of Csf1 from ECs and MSCs further decreased Ly6Chi cells. Moreover, EC-derived CSF-1 facilitated recovery of Ly6C- monocytes and protected mice from weight loss following induction of polymicrobial sepsis. Thus, monocytes are supported by distinct cellular sources of CSF-1 within a perivascular BM niche.

Osteoblast-lineage calcium/calmodulin-dependent kinase 2 delta and gamma regulates bone mass and quality.

Bone regulates its mass and quality in response to diverse mechanical, hormonal, and local signals. The bone anabolic or catabolic responses to these signals are often received by osteocytes, which then coordinate the activity of osteoblasts and osteoclasts on bone surfaces. We previously established that calcium/calmodulin-dependent kinase 2 (CaMKII) is required for osteocytes to respond to some bone anabolic cues in vitro. However, a role for CaMKII in bone physiology in vivo is largely undescribed. Here, we show that conditional codeletion of the most abundant isoforms of CaMKII (delta and gamma) in mature osteoblasts and osteocytes [Ocn-cre:Camk2d/Camk2g double-knockout (dCKO)] caused severe osteopenia in both cortical and trabecular compartments by 8 wk of age. In addition to having less bone mass, dCKO bones are of worse quality, with significant deficits in mechanical properties, and a propensity to fracture. This striking skeletal phenotype is multifactorial, including diminished osteoblast activity, increased osteoclast activity, and altered phosphate homeostasis both systemically and locally. These dCKO mice exhibited decreased circulating phosphate (hypophosphatemia) and increased expression of the phosphate-regulating hormone fibroblast growth factor 23. Additionally, dCKO mice expressed less bone-derived tissue nonspecific alkaline phosphatase protein than control mice. Consistent with altered phosphate homeostasis, we observed that dCKO bones were hypo-mineralized with prominent osteoid seams, analogous to the phenotypes of mice with hypophosphatemia. Altogether, these data reveal a fundamental role for osteocyte CaMKIIδ and CaMKIIγ in the maintenance of bone mass and bone quality and link osteoblast/osteocyte CaMKII to phosphate homeostasis.

Exploring neuronal mechanisms of osteosarcopenia in older adults.

Until recently, research on the pathogenesis and treatment of osteoporosis and sarcopenia has primarily focused on local and systemic humoral mechanisms, often overlooking neuronal mechanisms. However, there is a growing body of literature on the neuronal regulation of bone and skeletal muscle structure and function, which may provide insights into the pathogenesis of osteosarcopenia. This review aims to integrate these neuronal regulatory mechanisms to form a comprehensive understanding and inspire future research that could uncover novel strategies for preventing and treating osteosarcopenia. Specifically, the review explores the functional adaptation of weight-bearing bone to mechanical loading throughout evolutionary development, from Wolff's law and Frost's mechanostat theory to the mosaic hypothesis, which emphasizes neuronal regulation. The recently introduced bone osteoregulation reflex points to the importance of the osteocytic mechanoreceptive network as a receptor in this neuronal regulation mechanism. Finally, the review focuses on the bone myoregulation reflex, which is known as a mechanism by which bone loading regulates muscle functions neuronally. Considering the ageing-related regressive changes in the nerve fibres that provide both structural and functional regulation in bone and skeletal muscle tissue and the bone and muscle tissues they innervate, it is suggested that neuronal mechanisms might play a central role in explaining osteosarcopenia in older adults.

Erythropoietin regulates osteoclast formation via up-regulating PPARγ expression.

Erythropoietin (EPO), expressed in red blood progenitor cells, primarily regulates erythropoiesis by binding to its receptor. Besides anemia, recent studies have identified new therapeutic indications for EPO that are not connected to red blood cell formation. Elevated EPO levels harm bone homeostasis in adult organisms and are associated with increased osteoclast; however, the underlying molecular mechanisms remain unclear. This study demonstrated that EPO enhanced osteoclast differentiation and bone resorption in vitro. We showed that EPO promoted osteoclast formation by up-regulating PPARγ expression through activating the Jak2/ERK signaling pathway. Consistently, PPARγ antagonists rescued the hyperactivation of osteoclasts due to EPO, while PPARγ agonists reversed the EMP9-mediated decrease in osteoclast differentiation. Further, exposing female mice to EPO for two months led to a decrease in bone mass and increased osteoclast numbers. The present results suggested that EPO promotes osteoclastogenesis by regulating the Jak2/ERK/ PPARγ signaling pathway. From a clinical perspective, the risk of compromised bone health should be considered when using EPO to treat anemia in post-operative patients with intertrochanteric fractures of the femur, as it could significantly impact the patient's recovery and quality of life.

Nitric oxide contributes to rapid sclerostin protein loss following mechanical load.

In response to mechanical loading of bone, osteocytes produce nitric oxide (NO•) and decrease sclerostin protein expression, leading to an increase in bone mass. However, it is unclear whether NO• production and sclerostin protein loss are mechanistically linked, and, if so, the nature of their hierarchical relationship within an established mechano-transduction pathway. Prior work showed that following fluid-shear stress (FSS), osteocytes produce NOX2-derived reactive oxygen species, inducing calcium (Ca2+) influx. Increased intracellular Ca2+ results in calcium-calmodulin dependent protein kinase II (CaMKII) activation, which regulates the lysosomal degradation of sclerostin protein. Here, we extend our discoveries, identifying NO• as a regulator of sclerostin degradation downstream of mechano-activated CaMKII. Pharmacological inhibition of nitric oxide synthase (NOS) activity in Ocy454 osteocyte-like cells prevented FSS-induced sclerostin protein loss. Conversely, short-term treatment with a NO• donor in Ocy454 cells or isolated murine long bones was sufficient to induce the rapid decrease in sclerostin protein abundance, independent of changes in Sost gene expression. Ocy454 cells express all three NOS genes, and transfection with siRNAs targeting eNOS/Nos3 was sufficient to prevent FSS-induced loss of sclerostin protein, while siRNAs targeting iNOS/Nos2 mildly blunted the loss of sclerostin but did not reach statistical significance. Similarly, siRNAs targeting both eNOS/Nos3 and iNOS/Nos2 prevented FSS-induced NO• production. Together, these data show iNOS/Nos2 and eNOS/Nos3 are the primary producers of FSS-dependent NO•, and that NO• is necessary and sufficient for sclerostin protein control. Further, selective inhibition of elements within this sclerostin-controlling mechano-transduction pathway indicated that NO• production occurs downstream of CaMKII activation. Targeting Camk2d and Camk2g with siRNA in Ocy454 cells prevented NO• production following FSS, indicating that CaMKII is needed for NO• production. However, NO• donation (1min) resulted in a significant increase in CaMKII activation, suggesting that NO• may have the ability to tune CaMKII response. Together, these data support that CaMKII is necessary for, and may be modulated by NO•, and that the interaction of these two signals is involved in the control of sclerostin protein abundance, consistent with a role in bone anabolic responses.

Alterations in compositional and cellular properties of the subchondral bone are linked to cartilage degeneration in hip osteoarthritis.

OBJECTIVE: The subchondral bone is an emerging regulator of osteoarthritis (OA). However, knowledge of how specific subchondral alterations relate to cartilage degeneration remains incomplete. METHOD: Femoral heads were obtained from 44 patients with primary OA during total hip arthroplasty and from 30 non-OA controls during autopsy. A multiscale assessment of the central subchondral bone region comprising histomorphometry, quantitative backscattered electron imaging, nanoindentation, and osteocyte lacunocanalicular network characterization was employed. RESULTS: In hip OA, thickening of the subchondral bone coincided with a higher number of osteoblasts (controls: 3.7 ± 4.5 mm-1, OA: 16.4 ± 10.2 mm-1, age-adjusted mean difference 10.5 mm-1 [95% CI 4.7 to 16.4], p < 0.001) but a similar number of osteoclasts compared to controls (p = 0.150). Furthermore, higher matrix mineralization heterogeneity (CaWidth, controls: 2.8 ± 0.2 wt%, OA: 3.1 ± 0.3 wt%, age-adjusted mean difference 0.2 wt% [95% CI 0.1 to 0.4], p = 0.011) and lower tissue hardness (controls: 0.69 ± 0.06 GPa, OA: 0.67 ± 0.06 GPa, age-adjusted mean difference -0.05 GPa [95% CI -0.09 to -0.01], p = 0.032) were detected. While no evidence of altered osteocytic perilacunar/canalicular remodeling in terms of fewer osteocyte canaliculi was found in OA, specimens with advanced cartilage degeneration showed a higher number of osteocyte canaliculi and larger lacunocanalicular network area compared to those with low-grade cartilage degeneration. Multiple linear regression models indicated that several subchondral bone properties, especially osteoblast and osteocyte parameters, were closely related to cartilage degeneration (R2 adjusted = 0.561, p < 0.001). CONCLUSION: Subchondral bone properties in OA are affected at the compositional, mechanical, and cellular levels. Based on their strong interaction with cartilage degeneration, targeting osteoblasts/osteocytes may be a promising therapeutic OA approach. DATA AND MATERIALS AVAILABILITY: All data are available in the main text or the supplementary materials.

Osteocyte Sptbn1 Deficiency Alters Cell Survival and Mechanotransduction Following Formation of Plasma Membrane Disruptions (PMD) from Mechanical Loading.

We and others have shown that application of high-level mechanical loading promotes the formation of transient plasma membrane disruptions (PMD) which initiate mechanotransduction. We hypothesized that increasing osteocyte cell membrane fragility, by disrupting the cytoskeleton-associated protein ß2-spectrin (Sptbn1), could alter osteocytic responses and bone adaptation to loading in a PMD-related fashion. In MLO-Y4 cells, treatment with the spectrin-disrupting agent diamide or knockdown of Sptbn1 via siRNA increased the number of PMD formed by fluid shear stress. Primary osteocytes from an osteocyte-targeted DMP1-Cre Sptbn1 conditional knockout (CKO) model mimicked trends seen with diamide and siRNA treatment and suggested the creation of larger PMD, which repaired more slowly, for a given level of stimulus. Post-wounding cell survival was impaired in all three models, and calcium signaling responses from the wounded osteocyte were mildly altered in Sptbn1 CKO cultures. Although Sptbn1 CKO mice did not demonstrate an altered skeletal phenotype as compared to WT littermates under baseline conditions, they showed a blunted increase in cortical thickness when subjected to an osteogenic tibial loading protocol as well as evidence of increased osteocyte death (increased lacunar vacancy) in the loaded limb after 2 weeks of loading. The impaired post-wounding cell viability and impaired bone adaptation seen with Sptbn1 disruption support the existence of an important role for Sptbn1, and PMD formation, in osteocyte mechanotransduction and bone adaptation to mechanical loading.

Subchondral Bone Osteocyte Lacunae Morphology in End-Stage Osteoarthritis of the Human Tibial Plateau.

Subchondral bone remodeling, mediated by osteocytes within the lacuno-canalicular network, plays a crucial role in osteoarthritis (OA) progression. Following cell death, lacunae preserve integrity, offering insights into bone remodeling mechanisms. Limited and controversial data on osteocyte lacuna morphology in OA result from small sample sizes and two-dimensional (2D) techniques that have been used thus far. This study aimed to quantify three-dimensional (3D) osteocyte lacunar characteristics at well-defined tibial plateau locations, known to be differently affected by OA. Specifically, 11 tibial plateaus were obtained from end-stage knee-OA patients with varus deformity. Each plateau provided one sample from the less affected lateral compartment and two samples from the medial compartment, at minimum and maximum bone volume fraction (BV/TV) locations. High-resolution desktop micro-computed tomography (micro-CT) at 0.7 µm voxel resolution imaged the 33 samples. Lacuna number density (Lc.N/BV) and lacuna volume density (Lc.TV/BV) were significantly lower (p < 0.02) in samples from the medial side with maximum BV/TV compared to lateral side samples. In the medial compartment at maximum local BV/TV, mean lacuna volume (Lc.V), total lacuna volume (Lc.TV), and Lc.TV/BV were significantly (p < 0.001) lower than in the region with minimum BV/TV. Lc.N/BV was also significantly lower (p < 0.02) at the maximum local BV/TV location compared to the region with minimum BV/TV. Our findings suggest that subchondral bone lacunae adapt to the changing loads in end-stage OA.

Bone development by Hedgehog and Wnt signaling, Runx2, and Sp7.

Hedgehog and canonical Wnt signaling pathways and the transcription factors Runx2 and Sp7 are essential for osteoblast differentiation. Ihh is necessary for the commitment of perichondrial mesenchymal cells to Runx2+ osteoprogenitors and for the formation of the bone collar and primary spongiosa. Runx2 is needed for osteoblast differentiation during both endochondral and intramembranous ossification. It regulates the commitment of mesenchymal cells to osteoblast-lineage cells and their proliferation by inducing the expression of Hedgehog, Fgf, Wnt, Pthlh signaling pathway genes, and Dlx5. The Runx2-induced expression of Fgfr2 and Fgfr3 is important for the proliferation of osteoblast-lineage cells. Runx2 induces Sp7 expression and Runx2+ osteoprogenitors become Runx2+Sp7+ preosteoblasts. Runx2, Sp7, and canonical Wnt signaling induce the differentiation of preosteoblasts into osteoblasts. Canonical Wnt signaling, but not Sp7, enhances the proliferation of osteoblast-lineage cells. In mature osteoblasts, Runx2 plays an important role in the expression of major bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and Bglap/Bglap2. The canonical Wnt signaling pathway is also crucial for bone formation by mature osteoblasts. Sp7 is needed for osteocytes to acquire a sufficient number of processes and a reduction in these processes results in osteocyte apoptosis and cortical porosity.

Mechanically strained osteocyte-derived exosomes contained miR-3110-5p and miR-3058-3p and promoted osteoblastic differentiation.

BACKGROUND: Osteocytes are critical mechanosensory cells in bone, and mechanically stimulated osteocytes produce exosomes that can induce osteogenesis. MicroRNAs (miRNAs) are important constituents of exosomes, and some miRNAs in osteocytes regulate osteogenic differentiation; previous studies have indicated that some differentially expressed miRNAs in mechanically strained osteocytes likely influence osteoblastic differentiation. Therefore, screening and selection of miRNAs that regulate osteogenic differentiation in exosomes of mechanically stimulated osteocytes are important. RESULTS: A mechanical tensile strain of 2500 µÎµ at 0.5 Hz 1 h per day for 3 days, elevated prostaglandin E2 (PGE2) and insulin-like growth factor-1 (IGF-1) levels and nitric oxide synthase (NOS) activity of MLO-Y4 osteocytes, and promoted osteogenic differentiation of MC3T3-E1 osteoblasts. Fourteen miRNAs differentially expressed only in MLO-Y4 osteocytes which were stimulated with mechanical tensile strain, were screened, and the miRNAs related to osteogenesis were identified. Four differentially expressed miRNAs (miR-1930-3p, miR-3110-5p, miR-3090-3p, and miR-3058-3p) were found only in mechanically strained osteocytes, and the four miRNAs, eight targeted mRNAs which were differentially expressed only in mechanically strained osteoblasts, were also identified. In addition, the mechanically strained osteocyte-derived exosomes promoted the osteoblastic differentiation of MC3T3-E1 cells in vitro, the exosomes were internalized by osteoblasts, and the up-regulated miR-3110-5p and miR-3058-3p in mechanically strained osteocytes, were both increased in the exosomes, which was verified via reverse transcription quantitative polymerase chain reaction (RT-qPCR). CONCLUSIONS: In osteocytes, a mechanical tensile strain of 2500 µÎµ at 0.5 Hz induced the fourteen differentially expressed miRNAs which probably were in exosomes of osteocytes and involved in osteogenesis. The mechanically strained osteocyte-derived exosomes which contained increased miR-3110-5p and miR-3058-3p (two of the 14 miRNAs), promoted osteoblastic differentiation.

Regulatory Effect of Osteocytes on Extramedullary and Bone Marrow Adipose Tissue Development and Function.

PURPOSE OF REVIEW: This review summarizes evidence on osteocyte support of extramedullary and bone marrow adipocyte development and discusses the role of endogenous osteocyte activities of nuclear receptors peroxisome proliferator-activated receptor gamma (PPARG) and alpha (PPARA) in this support. RECENT FINDINGS: PPARG and PPARA proteins, key regulators of glucose and fatty acid metabolism, are highly expressed in osteocytes. They play significant roles in the regulation of osteocyte secretome and osteocyte bioenergetics; both activities contributing to the levels of systemic energy metabolism in part through an effect on metabolic function of extramedullary and bone marrow adipocytes. The PPARs-controlled osteocyte endocrine/paracrine activities, including sclerostin expression, directly regulate adipocyte function, while the PPARs-controlled osteocyte fuel utilization and oxidative phosphorylation contribute to the skeletal demands for glucose and fatty acids, whose availability is under the control of adipocytes. Bone is an inherent element of systemic energy metabolism with PPAR nuclear receptors regulating osteocyte-adipocyte metabolic axes.

The Role of Osteocytes in Pre-metastatic Niche Formation.

PURPOSE OF REVIEW: The formation of a pre-metastatic niche (PMN), in which primary cancer cells prime the distant site to be favorable to their engraftment and survival, may help explain the strong osteotropism observed in multiple cancers, such as breast and prostate. PMN formation, which includes extracellular matrix remodeling, increased angiogenesis and vascular permeability, enhanced bone marrow-derived cell recruitment and immune suppression, has mostly been described in soft tissues. In this review, we summarize current literature of PMN formation in bone. We also present evidence of a potential role for osteocytes to be the primary mediators of PMN development. RECENT FINDINGS: Osteocytes regulate the bone microenvironment in myriad ways beyond canonical bone tissue remodeling, including changes that contribute to PMN formation. Perilacunar tissue remodeling, which has been observed in both bone and non-bone metastatic cancers, is a potential mechanism by which osteocyte-cancer cell signaling stimulates changes to the bone microenvironment. Osteocytes also protect against endothelial permeability, including that induced by cancer cells, in a loading-mediated process. Finally, osteocytes are potent regulators of cells within the bone marrow, including progenitors and immune cells, and might be involved in this aspect of PMN formation. Osteocytes should be examined for their role in PMN formation.

Osteocytes and Paget's Disease of Bone.

PURPOSE OF REVIEW: To describe the contributions of osteocytes to the lesions in Paget's disease, which are characterized by locally overactive bone resorption and formation. RECENT FINDINGS: Osteocytes, the most abundant cells in bone, are altered in Paget's disease lesions, displaying increased size, decreased canalicular length, incomplete differentiation, and less sclerostin expression compared to controls in both patients and mouse models. Pagetic lesions show increased senescent osteocytes that express RANK ligand, which drives osteoclastic bone resorption. Abnormal osteoclasts in Paget's disease secrete abundant IGF1, which enhances osteocyte senescence, contributing to lesion formation. Recent data suggest that osteocytes contribute to lesion formation in Paget's disease by responding to high local IGF1 released from abnormal osteoclasts. Here we describe the characteristics of osteocytes in Paget's disease and their role in bone lesion formation based on recent results with mouse models and supported by patient data.

2D vs. 3D Evaluation of Osteocyte Lacunae - Methodological Approaches, Recommended Parameters, and Challenges: A Narrative Review by the European Calcified Tissue Society (ECTS).

PURPOSE OF REVIEW: Quantification of the morphology of osteocyte lacunae has become a powerful tool to investigate bone metabolism, pathologies and aging. This review will provide a brief overview of 2D and 3D imaging methods for the determination of lacunar shape, orientation, density, and volume. Deviations between 2D-based and 3D-based lacunar volume estimations are often not sufficiently addressed and may give rise to contradictory findings. Thus, the systematic error arising from 2D-based estimations of lacunar volume will be discussed, and an alternative calculation proposed. Further, standardized morphological parameters and best practices for sampling and segmentation are suggested. RECENT FINDINGS: We quantified the errors in reported estimation methods of lacunar volume based on 2D cross-sections, which increase with variations in lacunar orientation and histological cutting plane. The estimations of lacunar volume based on common practice in 2D imaging methods resulted in an underestimation of lacunar volume of up to 85% compared to actual lacunar volume in an artificial dataset. For a representative estimation of lacunar size and morphology based on 2D images, at least 400 lacunae should be assessed per sample.

Vitality of autologous retromolar bone grafts for alveolar ridge augmentation after a 3-months healing period: A prospective histomorphometrical analysis.

OBJECTIVES: The incorporation of retromolar bone grafts used for alveolar ridge augmentation is not well understood. This prospective observational study aims to supply histomorphometrical data from bone graft biopsies taken at the time of retrieval and after a 3-month healing period using patient-matched biopsies. MATERIALS AND METHODS: In 17 patients, trephine biopsies of the graft were acquired at the time of graft retrieval and after a 3-month healing period. The biopsies were compared histomorphometrically regarding the number of osteocytes, appearance of osteocyte lacunae, quantity, surface area, and activity of the Haversian canals. RESULTS: All grafts appeared clinically stable after screw removal and 17 implants were placed. Histomorphometric analysis revealed no significant difference in the number of osteocytes (p = .413), osteocyte lacunae (p = .611), the ratio of filled/empty osteocyte lacunae (p = .467) and active Haversian canals (p = .495) between the biopsies retrieved after a 3-months healing period with those at the time of grafting. The only significant difference was noted in the mean surface area of the Haversian canals (p = .002). Specifically, the grafts post 3-month healing showed a significantly larger mean area (0.069 mm2) compared to the time of grafting (0.029 mm2). CONCLUSION: This study demonstrates, compared to other data, a high rate of vital structures in retromolar bone block grafts after 3 months of healing, exhibiting the same histological features in comparison to the biopsies from the native alveolar ridge. Standard histomorphometrical parameters, e.g., the amount of filled or empty osteocyte lacunae for the description of the vitality of the graft need to be reappraised.

Medication-related osteonecrosis of the jaw (MRONJ): a review of pathogenesis hypothesis and therapy strategies.

Medication-related osteonecrosis of the jaw (MRONJ), a severe side effect caused by antiresorptive antiangiogenic medication, particularly bisphosphonates (BPs), has become a challenging disease with serious and profound effects on the physical and mental health of patients. Although it occurs with high frequency and is harmful, the exact mechanism of MRONJ remains unknown, and systematic and targeted approaches are still lacking. Maxillofacial surgeons focus on the etiology of osteonecrosis in the mandible and maxilla as well as the appropriate oral interventions for high-risk patients. Adequate nursing care and pharmacotherapy management are also crucial. This review provides a current overview of the clinicopathologic feature and research of MRONJ caused by BPs, with an emphasis on the potential mechanisms and current therapy and prevention strategies of the disease. We are of the opinion that an in-depth comprehension of the mechanisms underlying MRONJ will facilitate the development of more precise and efficacious therapeutic approaches, resulting in enhanced clinical outcomes for patients.

Disrupted osteocyte connectivity and pericellular fluid flow in bone with aging and defective TGF-ß signaling.

Skeletal fragility in the elderly does not simply result from a loss of bone mass. However, the mechanisms underlying the concurrent decline in bone mass, quality, and mechanosensitivity with age remain unclear. The important role of osteocytes in these processes and the age-related degeneration of the intricate lacunocanalicular network (LCN) in which osteocytes reside point to a primary role for osteocytes in bone aging. Since LCN complexity severely limits experimental dissection of these mechanisms in vivo, we used two in silico approaches to test the hypothesis that LCN degeneration, due to aging or an osteocyte-intrinsic defect in transforming growth factor beta (TGF-ß) signaling (TßRIIocy-/-), is sufficient to compromise essential osteocyte responsibilities of mass transport and exposure to mechanical stimuli. Using reconstructed confocal images of bone with fluorescently labeled osteocytes, we found that osteocytes from aged and TßRIIocy-/- mice had 33 to 45% fewer, and more tortuous, canaliculi. Connectomic network analysis revealed that diminished canalicular density is sufficient to impair diffusion even with intact osteocyte numbers and overall LCN architecture. Computational fluid dynamics predicts that the corresponding drop in shear stress experienced by aged or TßRIIocy-/- osteocytes is highly sensitive to canalicular surface area but not tortuosity. Simulated expansion of the osteocyte pericellular space to mimic osteocyte perilacunar/canalicular remodeling restored predicted shear stress for aged osteocytes to young levels. Overall, these models show how loss of LCN volume through LCN pruning may lead to impaired fluid dynamics and osteocyte exposure to mechanostimulation. Furthermore, osteocytes emerge as targets of age-related therapeutic efforts to restore bone health and function.

Runx2 Regulates Galnt3 and Fgf23 Expressions and Galnt3 Decelerates Osteoid Mineralization by Stabilizing Fgf23.

Runx2 (runt related transcription factor 2) is an essential transcription factor for osteoblast proliferation and differentiation. Uridine diphosphate (UDP)-N-acetylgalactosamine (GalNAc): polypeptide GalNAc-transferase 3 (Galnt3) prevents proteolytic processing of fibroblast growth factor 23 (Fgf23), which is a hormone that regulates the serum level of phosphorus. Runx2 and Galnt3 were expressed in osteoblasts and osteocytes, and Fgf23 expression was restricted to osteocytes in bone. Overexpression and knock-down of Runx2 upregulated and downregulated, respectively, the expressions of Galnt3 and Fgf23, and Runx2 directly regulated the transcriptional activity of Galnt3 in reporter assays. The expressions of Galnt3 and Fgf23 in osteoblast-specific Runx2 knockout (Runx2fl/flCre) mice were about half those in Runx2fl/fl mice. However, the serum levels of phosphorus and intact Fgf23 in Runx2fl/flCre mice were similar to those in Runx2fl/fl mice. The trabecular bone volume was increased during aging in both male and female Galnt3-/- mice, but the osteoid was reduced. The markers for bone formation and resorption in Galnt3-/- mice were similar to the control in both sexes. Galnt3-/- mice exhibited hyperphosphatemia and hypercalcemia, and the intact Fgf23 was about 40% that of wild-type mice. These findings indicated that Runx2 regulates the expressions of Galnt3 and Fgf23 and that Galnt3 decelerates the mineralization of osteoid by stabilizing Fgf23.

Local E-rhBMP-2/ß-TCP Application Rescues Osteocyte Dendritic Integrity and Reduces Microstructural Damage in Alveolar Bone Post-Extraction in MRONJ-like Mouse Model.

The pathology of medication-related osteonecrosis of the jaw (MRONJ), often associated with antiresorptive therapy, is still not fully understood. Osteocyte networks are known to play a critical role in maintaining bone homeostasis and repair, but the exact condition of these networks in MRONJ is unknown. On the other hand, the local application of E-coli-derived Recombinant Human Bone Morphogenetic Protein 2/ß-Tricalcium phosphate (E-rhBMP-2/ß-TCP) has been shown to promote bone regeneration and mitigate osteonecrosis in MRONJ-like mouse models, indicating its potential therapeutic application for the treatment of MRONJ. However, the detailed effect of BMP-2 treatment on restoring bone integrity, including its osteocyte network, in an MRONJ condition remains unclear. Therefore, in the present study, by applying a scanning electron microscope (SEM) analysis and a 3D osteocyte network reconstruction workflow on the alveolar bone surrounding the tooth extraction socket of an MRONJ-like mouse model, we examined the effectiveness of BMP-2/ß-TCP therapy on the alleviation of MRONJ-related bone necrosis with a particular focus on the osteocyte network and alveolar bone microstructure (microcrack accumulation). The 3D osteocyte dendritic analysis showed a significant decrease in osteocyte dendritic parameters along with a delay in bone remodeling in the MRONJ group compared to the healthy counterpart. The SEM analysis also revealed a notable increase in the number of microcracks in the alveolar bone surface in the MRONJ group compared to the healthy group. In contrast, all of those parameters were restored in the E-rhBMP-2/ß-TCP-treated group to levels that were almost similar to those in the healthy group. In summary, our study reveals that MRONJ induces osteocyte network degradation and microcrack accumulation, while application of E-rhBMP-2/ß-TCP can restore a compromised osteocyte network and abrogate microcrack accumulation in MRONJ.