Aging, Osteocytes, and Mechanotransduction

Aging, Osteo-Sarcopenia, and Musculoskeletal Mechano-Transduction

The decline in the mass and function of bone and muscle is an inevitable consequence of healthy aging with early onset and accelerated decline in those with chronic disease. Termed osteo-sarcopenia, this condition predisposes the decreased activity, falls, low-energy fractures, and increased risk of co-morbid disease that leads to musculoskeletal frailty. The biology of osteo-sarcopenia is most understood in the context of systemic neuro-endocrine and immune/inflammatory alterations that drive inflammation, oxidative stress, reduced autophagy, and cellular senescence in the bone and muscle. Here we integrate these concepts to our growing understanding of how bone and muscle senses, responds and adapts to mechanical load. We propose that age-related alterations in cytoskeletal mechanics alter load-sensing and mechano-transduction in bone osteocytes and muscle fibers which underscores osteo-sarcopenia. Lastly, we examine the evidence for exercise as an effective countermeasure to osteo-sarcopenia.

Osteocyte calcium signaling - A potential translator of mechanical load to mechanobiology

Osteocytes are embedded dendritic bone cells; by virtue of their position in bone tissue, ability to coordinate bone building osteoblasts and resorbing osteoclasts, and sensitivity to tissue level mechanical loading, they serve as the resident bone mechanosensor. The mechanisms osteocytes use to change mechanical loading into biological signals that drive tissue level changes has been well studied over the last 30 years, however the ways loading parameters are encoded at the cellular level are still not fully understood. Calcium signaling is a first messenger signal exhibited by osteocytes in response to mechanical forces. A body of work interrogating the mechanisms of osteocyte calcium signaling exists and is presently expanding, presenting the opportunity to better understand the relationship between calcium signaling characteristics and tuned osteocyte responses to tissue level strain features (e.g. magnitude, duration, frequency). This review covers the history of osteocyte load induced calcium signaling and highlights potential cellular mechanisms used by osteocytes to turn details about loading parameters into biological events.

Induction of somatopause in adult mice compromises bone morphology and exacerbates bone loss during aging

Somatopause refers to the gradual declines in growth hormone (GH) and insulin‐like growth factor‐1 throughout aging. To define how induced somatopause affects skeletal integrity, we used an inducible GH receptor knockout (iGHRKO) mouse model. Somatopause, induced globally at 6 months of age, resulted in significantly more slender bones in both male and female iGHRKO mice. In males, induced somatopause was associated with progressive expansion of the marrow cavity leading to significant thinning of the cortices, which compromised bone strength. We report progressive declines in osteocyte lacunar number, and increases in lacunar volume, in iGHRKO males, and reductions in lacunar number accompanied by ~20% loss of overall canalicular connectivity in iGHRKO females by 30 months of age. Induced somatopause did not affect mineral/matrix ratio assessed by Raman microspectroscopy. We found significant increases in bone marrow adiposity and high levels of sclerostin, a negative regulator of bone formation in iGHRKO mice. Surprisingly, however, despite compromised bone morphology, osteocyte senescence was reduced in the iGHRKO mice. In this study, we avoided the confounded effects of constitutive deficiency in the GH/IGF‐1 axis on the skeleton during growth, and specifically dissected its effects on the aging skeleton. We show here, for the first time, that induced somatopause compromises bone morphology and the bone marrow environment.

Biologic and pathologic aspects of osteocytes in the setting of medication-related osteonecrosis of the jaw (MRONJ)

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe, debilitating condition affecting patients with cancer and patients with osteoporosis who have been treated with powerful antiresorptives (pARs) or angiogenesis inhibitors (AgIs). Oral risk factors associated with the development of MRONJ include tooth extraction and inflammatory dental disease (e.g., periodontitis, periapical infection). In bone tissues, osteocytes play a bidirectional role in which they not only act as the "receiver" of systemic signals from blood vessels, such as hormones and drugs, or local signals from the mineralized matrix as it is deformed, but they also play a critical role as "transmitter" of signals to the cells that execute bone modeling and remodeling (osteoclasts, osteoblasts and lining cells). When the survival capacity of osteocytes is overwhelmed, they can die. Osteocyte death has been associated with several pathological conditions. Whereas the causes and mechanisms of osteocyte death have been studied in conditions like osteonecrosis of the femoral head (ONFH), few studies of the causes and mechanisms of osteocyte death have been done in MRONJ. The three forms of cell death that affect most of the different cells in the body (apoptosis, autophagy, and necrosis) have been recognized in osteocytes. Notably, necroptosis, a form of regulated cell death with "a necrotic cell death phenotype," has also been identified as a form of cell death in osteocytes under certain pathologic conditions. Improving the understanding of osteocyte death in MRONJ may be critical for preventing disease and developing treatment approaches. In this review, we intend to provide insight into the biology of osteocytes, cell death, in general, and osteocyte death, in particular, and discuss hypothetical mechanisms involved in osteocyte death associated with MRONJ.

Tunnels in the rock: Dynamics of osteocyte morphogenesis

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.

TRPV6 is a potential regulator of bone resorption in bone loss induced by estrogen deficiency

The precise effect of estrogen (E2) on osteoclast function is still poorly understood. The aim of this study was to investigate the potential role of transient receptor potential vanilloid 6 (TRPV6) in E2-mediated osteoclast function and to characterize the relevant underlying mechanisms. Here, we found that Trpv6 is drastically decreased in ovariectomy operation animals and the administration of E2 results in an increased expression of Trpv6 in osteoclasts. In contrast, Trpv6 depletion significantly blocked the inhibitory effects of E2 on bone resorption activity, and silencing Trpv6 alleviated E2-induced osteoclast apoptosis. In addition, we found that E2 regulates the transcription of Trpv6 through ERα, by interacting with C/EBPβ and NF-κB. Chip assay analysis indicated that C/EBPβ regulates Trpv6 transcription by binding to Trpv6 promoter fragments −1,866 nt to −1,761 nt and −2,685 nt to −2,580 nt, whereas NF-κB binds to the −953 nt to −851 nt region. We conclude that TRPV6 has a significant effect on E2-mediated osteoclast function.

•

E2 induces Trpv6 expression in osteoclasts

•

TRPV6 was involved in the effect of E2-mediated osteoclast function

•

E2 regulates the transcription of Trpv6 through Erα in osteoclasts

Control of osteocyte dendrite formation by Sp7 and its target gene osteocrin

Some osteoblasts embed within bone matrix, change shape, and become dendrite-bearing osteocytes. The circuitry that drives dendrite formation during "osteocytogenesis" is poorly understood. Here we show that deletion of Sp7 in osteoblasts and osteocytes causes defects in osteocyte dendrites. Profiling of Sp7 target genes and binding sites reveals unexpected repurposing of this transcription factor to drive dendrite formation. Osteocrin is a Sp7 target gene that promotes osteocyte dendrite formation and rescues defects in Sp7-deficient mice. Single-cell RNA-sequencing demonstrates defects in osteocyte maturation in the absence of Sp7. Sp7-dependent osteocyte gene networks are associated with human skeletal diseases. Moreover, humans with a SP7R316C mutation show defective osteocyte morphology. Sp7-dependent genes that mark osteocytes are enriched in neurons, highlighting shared features between osteocytic and neuronal connectivity. These findings reveal a role for Sp7 and its target gene Osteocrin in osteocytogenesis, revealing that pathways that control osteocyte development influence human bone diseases.

Maternal bone adaptation to mechanical loading during pregnancy, lactation, and post-weaning recovery

The maternal skeleton undergoes dramatic bone loss during pregnancy and lactation, and substantial bone recovery post-weaning. The structural adaptations of maternal bone during reproduction and lactation exert a better protection of the mechanical integrity at the critical load-bearing sites, suggesting the importance of physiological load-bearing in regulating reproduction-induced skeletal alterations. Although it is suggested that physical exercise during pregnancy and breastfeeding improves women's physical and psychological well-being, its effects on maternal bone health remain unclear. Therefore, the objective of this study was to investigate the maternal bone adaptations to external mechanical loading during pregnancy, lactation, and post-weaning recovery. By utilizing an in vivo dynamic tibial loading protocol in a rat model, we demonstrated improved maternal cortical bone structure in response to dynamic loading at tibial midshaft, regardless of reproductive status. Notably, despite the minimal loading responses detected in the trabecular bone in virgins, rat bone during lactation experienced enhanced mechano-responsiveness in both trabecular and cortical bone compartments when compared to rats at other reproductive stages or age-matched virgins. Furthermore, our study showed that the lactation-induced elevation in osteocyte peri-lacunar/canalicular remodeling (PLR) activities led to enlarged osteocyte lacunae. This may result in alterations in interstitial fluid flow-mediated mechanical stimulation on osteocytes and an elevation in solute transport through the lacunar-canalicular system (LCS) during high-frequency dynamic loading, thus enhancing mechano-responsiveness of maternal bone during lactation. Taken together, findings from this study provide important insights into the relationship between reproduction- and lactation-induced skeletal changes and external mechanical loading, emphasizing the importance of weight-bearing exercise on maternal bone health during reproduction and postpartum.

Does Provisional Minifragment Fixation Prevent Compression With Dynamic Compression Plating? A Biomechanical Analysis

OBJECTIVES

To compare the compressive force generated by a 3.5-mm compression plate with and without provisional fixation using a 2.0-mm minifragment plate.

METHODS

Fourth generation composite large humeral sawbones underwent transection and were divided into 2 groups. The first group underwent fixation with a 3.5-mm compression plate; the second group underwent provisional fixation with a 2.0-mm plate followed by definitive fixation using a 3.5-mm plate. Using a load cell, the compressive force generated was measured after insertion of each of 2 eccentrical placed screws and the total compression recorded.

RESULTS

There was no difference in the force generated after each successive compression screw (P = 0.59 and 0.58, respectively). Likewise, there was no significant difference in the total compression generated when the preload was accounted for (P = 0.93).

CONCLUSION

Provisional minifragment fixation does not have any adverse effect on the forces generated during compression plating. These findings suggest that provisional minifragment plates do not need to be removed before definitive fixation.

The endothelium-bone axis in development, homeostasis and bone and joint disease

Blood vessels form a versatile transport network that is best known for its critical roles in processes such as tissue oxygenation, metabolism and immune surveillance. The vasculature also provides local, often organ-specific, molecular signals that control the behaviour of other cell types in their vicinity during development, homeostasis and regeneration, and also in disease processes. In the skeletal system, the local vasculature is actively involved in both bone formation and resorption. In addition, blood vessels participate in inflammatory processes and contribute to the pathogenesis of diseases that affect the joints, such as rheumatoid arthritis and osteoarthritis. This Review summarizes the current understanding of the architecture, angiogenic growth and functional properties of the bone vasculature. The effects of ageing and pathological conditions, including arthritis and osteoporosis, are also discussed.

Safety and efficacy of bone marrow derived-mesenchymal stem cells transplantation in patients with amyotrophic lateral sclerosis

Stem cell-based treatments have emerged as potentially effective approaches to delay the progression of amyotrophic lateral sclerosis (ALS). This study was designed as a single-center, prospective, and open-label study without a placebo control group to assess the safety and efficacy of concurrent intrathecal (IT) and intravenous (IV) administration of autologous bone marrow-derived mesenchymal stem cells (BM-MSCs) in patients with ALS. Autologous BM-MSCs were isolated and expanded under standard conditions. Fifteen patients were neurologically examined before BM-MSCs transplantation (1 × 10 ⁶ cells/kg BW) to evaluate the rate of pre-treatment disease progression. To assess the safety and efficacy, patients were examined at 1, 3, and 6 months following the treatment with BM-MSCs. Adverse reactions were assessed, and the clinical outcome was determined by the evaluation of the ALS functional rating scale-revised (ALSFRS-R) and forced vital capacity (FVC). No serious adverse reaction was observed after combined IT and IV administration of BM-MSCs. The mean ALSFRS-R and FVC values remained stable during the first 3 months of the treatment. However, a significant reduction in ALSFRS-R and FVC levels was observed in these patients 6 months after BM-MSCs administration. Our study revealed that the concurrent IT and IV application of BM-MSCs in patients with ALS is a safe procedure. Furthermore, our data indicate a temporary delay in the progression of ALS after a single combined IT and IV administration of BM-MSCs. Further studies are required to explore if the repeated applications of BM-MSCs could prolong survival and delay the progression of ALS.

3D assessment of intervertebral disc degeneration in zebrafish identifies changes in bone density that prime disc disease

Back pain is a common condition with a high social impact and represents a global health burden. Intervertebral disc disease (IVDD) is one of the major causes of back pain; no therapeutics are currently available to reverse this disease. The impact of bone mineral density (BMD) on IVDD has been controversial, with some studies suggesting osteoporosis as causative for IVDD and others suggesting it as protective for IVDD. Functional studies to evaluate the influence of genetic components of BMD in IVDD could highlight opportunities for drug development and repurposing. By taking a holistic 3D approach, we established an aging zebrafish model for spontaneous IVDD. Increased BMD in aging, detected by automated computational analysis, is caused by bone deformities at the endplates. However, aged zebrafish spines showed changes in bone morphology, microstructure, mineral heterogeneity, and increased fragility that resembled osteoporosis. Elements of the discs recapitulated IVDD symptoms found in humans: the intervertebral ligament (equivalent to the annulus fibrosus) showed disorganized collagen fibers and herniation, while the disc center (nucleus pulposus equivalent) showed dehydration and cellular abnormalities. We manipulated BMD in young zebrafish by mutating sp7 and cathepsin K, leading to low and high BMD, respectively. Remarkably, we detected IVDD in both groups, demonstrating that low BMD does not protect against IVDD, and we found a strong correlation between high BMD and IVDD. Deep learning was applied to high-resolution synchrotron µCT image data to analyze osteocyte 3D lacunar distribution and morphology, revealing a role of sp7 in controlling the osteocyte lacunar 3D profile. Our findings suggest potential avenues through which bone quality can be targeted to identify beneficial therapeutics for IVDD.

Next‑generation sequencing of miRNAs and lncRNAs from rat femur and tibia under mechanical stress

Exercise intervention has become one of the most effective methods to prevent and treat osteoporosis, which is a common age‑related disease and seriously affects the health and quality of life of the elderly. However, the molecular mechanism remains to be elucidated. The present study demonstrated the exercise‑induced promotion of osteogenic differentiation and inhibition of adipogenic differentiation in femur and tibia by establishing an animal exercise model using a treadmill exercise system. MicroRNA (miRNA/miR) and long non‑coding (lnc)RNA sequencing analyses identified 16 upregulated and two downregulated miRNAs in the exercise group, as well as 44 upregulated lncRNAs and 39 downregulated lncRNAs in the exercise group. There was increased expression of miR‑9942 and miR‑7704 in both the femur and tibia and an upregulation of miR‑30d, miR‑5100 and miR‑1260 in the femur of animals from the exercise group. In addition, four of the five most downregulated lncRNAs, including lncRNA MSTRG.2625, lncRNA MSTRG.1557, lncRNA MSTRG.691 and lncRNA MSTRG.7497, were demonstrated to be suppressed in both the femur and tibia after treadmill exercise. The results of the present study provided a valuable resource for further exploring the molecular mechanisms underlying the regulation of osteoporosis by exercise.

Physiological Loading-Induced Interstitial Fluid Dynamics in Osteon of Osteogenesis Imperfecta Bone

Osteogenesis imperfecta (OI), also known as "brittle bone disease," is a genetic bone disorder. OI bones experience frequent fractures. Surgical procedures are usually followed by clinicians in the management of OI. It has been observed physical activity is equally beneficial in reducing OI bone fractures in both children and adults as mechanical stimulation improves bone mass and strength. Loading-induced mechanical strain and interstitial fluid flow stimulate bone remodeling activities. Several studies have characterized strain environment in OI bones, whereas very few studies attempted to characterize the interstitial fluid flow. OI significantly affects bone micro-architecture. Thus, this study anticipates that canalicular fluid flow reduces in OI bone in comparison to the healthy bone in response to physiological loading due to altered poromechanical properties. This work attempts to understand the canalicular fluid distribution in single osteon models of OI and healthy bone. A poromechanical model of osteon is developed to compute pore-pressure and interstitial fluid flow as a function of gait loading pattern reported for OI and healthy subjects. Fluid distribution patterns are compared at different time-points of the stance phase of the gait cycle. It is observed that fluid flow significantly reduces in OI bone. Additionally, flow is more static than dynamic in OI osteon in comparison to healthy subjects. This work attempts to identify the plausible explanation behind the diminished mechanotransduction capability of OI bone. This work may further be extended for designing better biomechanical therapies to enhance the fluid flow in order to improve osteogenic activities in OI bone.

Innovative Biomaterials for the Treatment of Bone Cancer

Bone cancer is a demanding challenge for contemporary medicine due to its high frequency of presentation and significant heterogeneity of malignant lesions developing within the bone. To date, available treatments are rarely curative and are primarily aimed at prolonging patients' survival and ameliorating their quality of life. Furthermore, both pharmacological and surgical therapies are aggravated by a consistent burden of adverse events and subsequent disability due to the loss of healthy bone structural and functional properties. Therefore, great research efforts are being made to develop innovative biomaterials able to selectively inhibit bone cancer progression while reducing the loss of bone structural properties secondary to local tissue invasion. In this review, we describe the state of the art of innovative biomaterials for the treatment of bone cancer. Along with physiological bone remodeling, the development of bone metastasis and osteosarcoma will be depicted. Subsequently, recent advances on nanocarrier-based drug delivery systems, as well as the application of novel, multifunctional biomaterials for the treatment of bone cancer will be discussed. Eventually, actual limitations and promising future perspectives regarding the employment of such approaches in the clinical scenario will be debated.

The role of inhibition of osteocyte apoptosis in mediating orthodontic tooth movement and periodontal remodeling: a pilot study

BACKGROUND

Orthodontic tooth movement (OTM) has been shown to induce osteocyte apoptosis in alveolar bone shortly after force application. However, how osteocyte apoptosis affects orthodontic tooth movement is unknown. The goal of this study was to assess the effect of inhibition of osteocyte apoptosis on osteoclastogenesis, changes in the alveolar bone density, and the magnitude of OTM using a bisphosphonate analog (IG9402), a drug that affects osteocyte and osteoblast apoptosis but does not affect osteoclasts.

MATERIAL AND METHODS

Two sets of experiments were performed. Experiment 1 was used to specifically evaluate the effect of IG9402 on osteocyte apoptosis in the alveolar bone during 24 h of OTM. For this experiment, twelve mice were divided into two groups: group 1, saline administration + OTM_24-h (n=6), and group 2, IG9402 administration + OTM_24-h (n=6). The contralateral unloaded sides served as the control. The goal of experiment 2 was to evaluate the role of osteocyte apoptosis on OTM magnitude and osteoclastogenesis 10 days after OTM. Twenty mice were divided into 4 groups: group 1, saline administration without OTM (n=5); group 2, IG9402 administration without OTM (n=5); group 3, saline + OTM_10-day (n=6); and group 4, IG9402 + OTM_10-day (n=4). For both experiments, tooth movement was achieved using Ultra Light (25g) Sentalloy Closed Coil Springs attached between the first maxillary molar and the central incisor. Linear measurements of tooth movement and alveolar bone density (BVF) were assessed by MicroCT analysis. Cell death (or apoptosis) was assessed by terminal dUTP nick-end labeling (TUNEL) assay, while osteoclast and macrophage formation were assessed by tartrate-resistant acid phosphatase (TRAP) staining and F4/80+ immunostaining.

RESULTS

We found that IG9402 significantly blocked osteocyte apoptosis in alveolar bone (AB) at 24 h of OTM. At 10 days, IG9402 prevented OTM-induced loss of alveolar bone density and changed the morphology and quality of osteoclasts and macrophages, but did not significantly affect the amount of tooth movement.

CONCLUSION

Our study demonstrates that osteocyte apoptosis may play a significant role in osteoclast and macrophage formation during OTM, but does not seem to play a role in the magnitude of orthodontic tooth movement.

Update on the effects of microgravity on the musculoskeletal system

With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

Effects of advanced glycation end products on osteocytes mechanosensitivity

Osteocytes are extremely sensitive to mechanical loading and govern bone remodeling process. Advanced glycation end products (AGEs) have the capacity to induce osteocyte apoptosis. In order to investigate the effects of AGEs on the mechanosensitivity of osteocytes, the osteocytic-like cells (MLO-Y4) were treated with low (50 μg/ml) and high (400 μg/ml) concentrations of AGEs for 1day and exposed to 15 dyne/cm² of fluid shear stress. Then the F-actin cytoskeleton, prostaglandin E2(PGE2), Nitric oxide (NO), the Wnt/β-catenin signaling pathway activity mRNA expressions were detected for osteocytes mechanical response changes; osteocalcin (OCN) and receptor activator of nuclear factor-kappa B ligand (RANKL)/osteoprotegerin (OPG) were detected for the regulation on bone remodeling function of osteocytes. The results showed that AGEs accumulation inhibited the sense of osteocytes to external mechincal loading, promoted shear-induced NO and PGE2 release, suppressed the mechanosensitivity of Wnt/β-catenin signaling pathway, and furthermore promoted OCN and RANKL/OPG mRNA expressions. These indicated AGEs had an adverse impact on the mechanosensitivity of osteocytes, and led to a negative effect on their regulation of bone remodeling process under mechanical stimulation. This work provides a new perspective to interpret the alteration mechanism of osteocytes mechanosensitivity and provides a novel clue for exploring the mechanism of osteoporosis.

Osteocyte Dysfunction in Joint Homeostasis and Osteoarthritis

Structural disturbances of the subchondral bone are a hallmark of osteoarthritis (OA), including sclerotic changes, cystic lesions, and osteophyte formation. Osteocytes act as mechanosensory units for the micro-cracks in response to mechanical loading. Once stimulated, osteocytes initiate the reparative process by recruiting bone-resorbing cells and bone-forming cells to maintain bone homeostasis. Osteocyte-expressed sclerostin is known as a negative regulator of bone formation through Wnt signaling and the RANKL pathway. In this review, we will summarize current understandings of osteocytes at the crossroad of allometry and mechanobiology to exploit the relationship between osteocyte morphology and function in the context of joint aging and osteoarthritis. We also aimed to summarize the osteocyte dysfunction and its link with structural and functional disturbances of the osteoarthritic subchondral bone at the molecular level. Compared with normal bones, the osteoarthritic subchondral bone is characterized by a higher bone volume fraction, a larger trabecular bone number in the load-bearing region, and an increase in thickness of pre-existing trabeculae. This may relate to the aberrant expressions of sclerostin, periostin, dentin matrix protein 1, matrix extracellular phosphoglycoprotein, insulin-like growth factor 1, and transforming growth factor-beta, among others. The number of osteocyte lacunae embedded in OA bone is also significantly higher, yet the volume of individual lacuna is relatively smaller, which could suggest abnormal metabolism in association with allometry. The remarkably lower percentage of sclerostin-positive osteocytes, together with clustering of Runx-2 positive pre-osteoblasts, may suggest altered regulation of osteoblast differentiation and osteoblast-osteocyte transformation affected by both signaling molecules and the extracellular matrix. Aberrant osteocyte morphology and function, along with anomalies in molecular signaling mechanisms, might explain in part, if not all, the pre-osteoblast clustering and the uncoupled bone remodeling in OA subchondral bone.

The role of biomineralization in disorders of skeletal development and tooth formation

The major mineralized tissues are bone and teeth, which share several mechanisms governing their development and mineralization. This crossover includes the hormones that regulate circulating calcium and phosphate concentrations, and the genes that regulate the differentiation and transdifferentiation of cells. In developing endochondral bone and in developing teeth, parathyroid hormone-related protein (PTHrP) acts in chondrocytes to delay terminal differentiation, thereby increasing the pool of precursor cells. Chondrocytes and (in specific circumstances) pre-odontoblasts can also transdifferentiate into osteoblasts. Moreover, bone and teeth share outcomes when affected by systemic disorders of mineral homeostasis or of the extracellular matrix, and by adverse effects of treatments such as bisphosphonates and fluoride. Unlike bone, teeth have more permanent effects from systemic disorders because they are not remodelled after they are formed. This Review discusses the normal processes of bone and tooth development, followed by disorders that have effects on both bone and teeth, versus disorders that have effects in one without affecting the other. The takeaway message is that bone specialists should know when to screen for dental disorders, just as dental specialists should recognize when a tooth disorder should raise suspicions about a possible underlying bone disorder.

Duration-Dependent Increase of Human Bone Matrix Mineralization in Long-Term Bisphosphonate Users with Atypical Femur Fracture

Bisphosphonates (BPs) are the most widely used drugs for the treatment of osteoporosis but prolonged use of BPs might increase the risk of atypical femur fracture (AFF). There are only a few studies that address the bone material quality in patients on long-term BP treatment with or without AFFs. We analyzed 52 trans-iliac bone biopsies from patients on long-term BP therapy with (n = 26) and without (n = 26) AFF. At the microscopic level, the degree of mineralization of bone (DMB) was assessed on whole bone by X-ray digitized microradiography while microhardness by Vickers microindentation, and bone matrix characteristics by Fourier transform infrared microspectroscopy (FTIRM) (mineral/organic ratio, mineral maturity and crystallinity, and collagen maturity) were measured at random focal areas. The AFF patients were treated longer than non-AFF patients (9.7 ± 3.3 years versus 7.9 ± 2.7 years). As expected, bone remodeling was low in both groups, without difference between them. The AFF group had significantly higher DMB in cortical bone (+2.9%, p = .001), which remained so after adjusting for treatment duration (p = .007), and showed a trend in cancellous bone (+1.6%, p = .05). Consistent with higher DMB, heterogeneity index (HI) was lower in the AFF than in the non-AFF group, illustrating lower heterogeneity of mineralization in the AFF group. A significant positive correlation between the duration of treatment and DMB in cortical bone was found in AFF, and not in the non-AFF group. Microhardness and bone matrix characteristics were similar between groups. We conclude that the AFF group had a duration-dependent increase in DMB leading to a significantly higher DMB than the non-AFF. Because BPs have high affinity to bone mineral and lining the walls of the osteocyte lacunae, the accumulation of matrix-bound BPs in AFF could lead to inhibition of the osteocyte cytoskeleton blunting their response to mechanical strains, a hypothesis to be further investigated. © 2021 American Society for Bone and Mineral Research (ASBMR).

Enhancement of osteoporotic fracture healing by vibration treatment: The role of osteocytes

The prevalence of osteoporotic fracture is high due to global aging problem. Delayed and impaired healing in osteoporotic fractures increase the socioeconomic burden significantly. Through intensive animal and clinical research in recent years, the pathogenesis of osteoporotic fracture healing is unveiled, including decreased inflammatory response, reduced mesenchymal stem cells and deteriorated angiogenesis, etc. The enhancement of osteoporotic fracture healing is important in shortening hospitalization, thus reducing related complications. Mechanical stimulation is currently the most well-accepted approach for rehabilitation of osteoporotic fracture patients. Some new interventions providing mechanical signals were explored extensively in recent years, including vibration treatment, and osteoporotic fracture healing was found to respond very well to these signals. Vibration treatment could accelerate osteoporotic fracture healing with improved callus formation, mineralization and remodeling. However, the mechanism of how osteoporotic fracture bones sense mechanical signals and relay to bone formation remains unanswered. Osteocytes are the most abundant cells in bone tissues. Cumulative evidence confirm that osteocyte is a type of mechanosensory cell and shows altered morphology and reduced cell density during aging. Meanwhile, osteocytes serve as endocrine cells to regulate bone and mineral homeostasis. However, the contribution of osteocytes in osteoporotic fracture healing is largely unknown. A recent in vivo study was conducted to examine the morphological and functional changes of osteocytes after vibration treatment in an osteoporotic metaphyseal fracture rat model. The findings demonstrated that vibration treatment induced significant outgrowth of canaliculi and altered expression of various proteins (E11, DMP1, FGF23 and sclerostin), particularly osteocyte-specific dentin matrix protein 1 (DMP1) which was greatly increased. DMP1 may play a major role in relaying mechanical signals to bone formation, which may require further experiments to consolidate. Most importantly, vibration treatment significantly increased the mineralization and accelerated the osteoporotic fracture healing in metaphyseal fracture model. In summary, osteocyte is the major cell type to sense mechanical signals and facilitate downstream healing in osteoporotic fracture bone. Vibration treatment has good potential to be translated for clinical application to benefit osteoporotic fracture patients, while randomized controlled trials are required to validate its efficacy.

Impaired 1,25 dihydroxyvitamin D3 action and hypophosphatemia underlie the altered lacuno-canalicular remodeling observed in the Hyp mouse model of XLH

Osteocytes remodel the perilacunar matrix and canaliculi. X-linked hypophosphatemia (XLH) is characterized by elevated serum levels of fibroblast growth factor 23 (FGF23), leading to decreased 1,25 dihydroxyvitamin D₃ (1,25D) production and hypophosphatemia. Bones from mice with XLH (Hyp) have enlarged osteocyte lacunae, enhanced osteocyte expression of genes of bone remodeling, and impaired canalicular structure. The altered lacuno-canalicular (LCN) phenotype is improved with 1,25D or anti-FGF23 antibody treatment, pointing to roles for 1,25D and/or phosphate in regulating this process. To address whether impaired 1,25D action results in LCN alterations, the LCN phenotype was characterized in mice lacking the vitamin D receptor (VDR) in osteocytes (VDR^f/f;DMP1Cre+). Mice lacking the sodium phosphate transporter NPT2a (NPT2aKO) have hypophosphatemia and high serum 1,25D levels, therefore the LCN phenotype was characterized in these mice to determine if increased 1,25D compensates for hypophosphatemia in regulating LCN remodeling. Unlike Hyp mice, neither VDR^f/f;DMP1Cre+ nor NPT2aKO mice have dramatic alterations in cortical microarchitecture, allowing for dissecting 1,25D and phosphate specific effects on LCN remodeling in tibial cortices. Histomorphometric analyses demonstrate that, like Hyp mice, tibiae and calvariae in VDR^f/f;DMP1Cre+ and NPT2aKO mice have enlarged osteocyte lacunae (tibiae: 0.15±0.02μm²(VDR^f/f;DMP1Cre-) vs 0.19±0.02μm²(VDR^f/f;DMP1Cre+), 0.12±0.02μm²(WT) vs 0.18±0.0μm²(NPT2aKO), calvariae: 0.09±0.02μm²(VDR^f/f;DMP1Cre-) vs 0.11±0.02μm²(VDR^f/f;DMP1Cre+), 0.08±0.02μm²(WT) vs 0.13±0.02μm²(NPT2aKO), p<0.05 all comparisons) and increased immunoreactivity of bone resorption marker Cathepsin K (Ctsk). The osteocyte enriched RNA isolated from tibiae in VDR^f/f;DMP1Cre+ and NPT2aKO mice have enhanced expression of matrix resorption genes that are classically expressed by osteoclasts (Ctsk, Acp5, Atp6v0d2, Nhedc2). Treatment of Ocy454 osteocytes with 1,25D or phosphate inhibits the expression of these genes. Like Hyp mice, VDR^f/f;DMP1Cre+ and NPT2aKO mice have impaired canalicular organization in tibia and calvaria. These studies demonstrate that hypophosphatemia and osteocyte-specific 1,25D actions regulate LCN remodeling. Impaired 1,25D action and low phosphate levels contribute to the abnormal LCN phenotype observed in XLH.

Stress fiber strain recognition by the LIM protein testin is cryptic and mediated by RhoA

The actin cytoskeleton is a key regulator of mechanical processes in cells. The family of LIM domain proteins have recently emerged as important mechanoresponsive cytoskeletal elements capable of sensing strain in the actin cytoskeleton. The mechanisms regulating this mechanosensitive behavior, however, remain poorly understood. Here we show that the LIM domain protein testin is peculiar in that despite the full-length protein primarily appearing diffuse in the cytoplasm, the C-terminal LIM domains alone recognize focal adhesions and strained actin, while the N-terminal domains alone recognize stress fibers. Phosphorylation mutations in the dimerization regions of testin, however, reveal its mechanosensitivity and cause it to relocate to focal adhesions and sites of strain in the actin cytoskeleton. Finally, we demonstrate that activated RhoA causes testin to adorn stress fibers and become mechanosensitive. Together, our data show that testin’s mechanoresponse is regulated in cells and provide new insights into LIM domain protein recognition of the actin cytoskeleton’s mechanical state.

The role of osteocytes-specific molecular mechanism in regulation of mechanotransduction - A systematic review

Background

Osteocytes, composing over 90% of bone cells, are well known for their mechanosensing abilities. Aged osteocytes with impaired morphology and function are less efficient in mechanotransduction which will disrupt bone turnover leading to osteoporosis. The aim of this systematic review is to delineate the mechanotransduction mechanism at different stages in order to explore potential target for therapeutic drugs.

Methods

A systematic literature search was performed in PubMed and Web of Science. Original animal, cell and clinical studies with available English full-text were included. Information was extracted from the included studies for review.

Results

The 26 studies included in this review provided evidence that mechanical loading are sensed by osteocytes via various sensing proteins and transduced to different signaling molecules which later initiate various biochemical responses. Studies have shown that osteocyte plasma membrane and cytoskeletons are emerging key players in initiating mechanotransduction. Bone regulating genes expressions are altered in response to load sensed by osteocytes, but the genes involved different signaling pathways and the spatiotemporal expression pattern had made mechanotransduction mechanism complicated. Most of the included studies described the important role of osteocytes in pathways that regulate mechanosensing and bone remodeling.

Conclusions

This systematic review provides an up-to-date insight to different steps of mechanotransduction. A better understanding of the mechanotransduction mechanism is beneficial in search of new potential treatment for osteoporotic patients. By delineating the unique morphology of osteocytes and their interconnected signaling network new targets can be discovered for drug development.

Translational potential of this article

This systematic review provides an up-to-date sequential overview and highlights the different osteocyte-related pathways and signaling molecules during mechanotransduction. This allows a better understanding of mechanotransduction for future development of new therapeutic interventions to treat patients with impaired mechanosensitivity.

Relationships between the Bone Expression of Alzheimer's Disease-Related Genes, Bone Remodelling Genes and Cortical Bone Structure in Neck of Femur Fracture

Neck of femur (NOF) fracture is a prevalent fracture type amongst the ageing and osteoporotic populations, commonly requiring total hip replacement (THR) surgery. Increased fracture risk has also been associated with Alzheimer's disease (AD) in the aged. Here, we sought to identify possible relationships between the pathologies of osteoporosis and dementia by analysing bone expression of neurotropic or dementia-related genes in patients undergoing THR surgery for NOF fracture. Femoral bone samples from 66 NOF patients were examined for expression of the neurotropic genes amyloid precursor protein (APP), APP-like protein-2 (APLP2), Beta-Secretase Cleaving Enzyme-1 (BACE1) and nerve growth factor (NGF). Relationships were examined between the expression of these and of bone regulatory genes, systemic factors and bone structural parameters ascertained from plain radiographs. We found strong relative levels of expression and positive correlations between APP, APLP2, BACE1 and NGF levels in NOF bone. Significant correlations were found between APP, APLP2, BACE1 mRNA levels and bone remodelling genes TRAP, RANKL, and the RANKL:OPG mRNA ratio, indicative of potential functional relationships at the time of fracture. Analysis of the whole cohort, as well as non-dementia (n = 53) and dementia (n = 13) subgroups, revealed structural relationships between APP and APLP2 mRNA expression and lateral femoral cortical thickness. These findings suggest that osteoporosis and AD may share common molecular pathways of disease progression, perhaps explaining the common risk factors associated with these diseases. The observation of a potential pathologic role for AD-related genes in bone may also provide alternative treatment strategies for osteoporosis and fracture prevention.

Increased Osteocyte Lacunae Density in the Hypermineralized Bone Matrix of Children with Osteogenesis Imperfecta Type I

Osteocytes are terminally differentiated osteoblasts embedded within the bone matrix and key orchestrators of bone metabolism. However, they are generally not characterized by conventional bone histomorphometry because of their location and the limited resolution of light microscopy. OI is characterized by disturbed bone homeostasis, matrix abnormalities and elevated bone matrix mineralization density. To gain further insights into osteocyte characteristics and bone metabolism in OI, we evaluated 2D osteocyte lacunae sections (OLS) based on quantitative backscattered electron imaging in transiliac bone biopsy samples from children with OI type I (n = 19) and age-matched controls (n = 24). The OLS characteristics were related to previously obtained, re-visited histomorphometric parameters. Moreover, we present pediatric bone mineralization density distribution reference data in OI type I (n = 19) and controls (n = 50) obtained with a field emission scanning electron microscope. Compared to controls, OI has highly increased OLS density in cortical and trabecular bone (+50.66%, +61.73%; both p < 0.001), whereas OLS area is slightly decreased in trabecular bone (−10.28%; p = 0.015). Correlation analyses show a low to moderate, positive association of OLS density with surface-based bone formation parameters and negative association with indices of osteoblast function. In conclusion, hyperosteocytosis of the hypermineralized OI bone matrix associates with abnormal bone cell metabolism and might further impact the mechanical competence of the bone tissue.

Sensing morphogenesis of bone cells under microfluidic shear stress by holographic microscopy and automatic aberration compensation with deep learning

We present sensing time-lapse morphogenesis of living bone cells under micro-fluidic shear stress (FSS) by digital holographic (DH) microscopy. To remove the effect of aberrations on quantitative measurements, we propose a numerical and automatic method to compensate for aberrations based on a convolutional neural network (CNN). For the first time, the aberration compensation issue is considered as a regression task where optimal coefficients for constructing the phase aberration map act as responses corresponding to the input aberrated phase image. We adopted tens of thousands of living cells' phase images reconstructed from digital holograms for training the CNN. The experiments demonstrate that, based on the trained network, phase aberrations can be totally removed in real-time without any hypothesis of object and aberration phase, knowledge of the setup's physical parameters, and the operation of selecting background regions; hence, the morphogenesis of the bone cells under FSS is accurately detected and quantitatively analyzed. The results show that the proposed method could provide a highly efficient and versatile way to investigate the effects of micro-FSS on living biological cells in microfluidic lab-on-chip platforms thanks to the combination of phase-contrast label-free microcopy with artificial intelligence.

Disparate bone anabolic cues activate bone formation by regulating the rapid lysosomal degradation of sclerostin protein

The downregulation of sclerostin in osteocytes mediates bone formation in response to mechanical cues and parathyroid hormone (PTH). To date, the regulation of sclerostin has been attributed exclusively to the transcriptional downregulation of the Sost gene hours after stimulation. Using mouse models and rodent cell lines, we describe the rapid, minute-scale post-translational degradation of sclerostin protein by the lysosome following mechanical load and PTH. We present a model, integrating both new and established mechanically and hormonally activated effectors into the regulated degradation of sclerostin by lysosomes. Using a mouse forelimb mechanical loading model, we find transient inhibition of lysosomal degradation or the upstream mechano-signaling pathway controlling sclerostin abundance impairs subsequent load-induced bone formation by preventing sclerostin degradation. We also link dysfunctional lysosomes to aberrant sclerostin regulation using human Gaucher disease iPSCs. These results reveal how bone anabolic cues post-translationally regulate sclerostin abundance in osteocytes to regulate bone formation.

Kindlin-2 mediates mechanotransduction in bone by regulating expression of Sclerostin in osteocytes

Osteocytes act as mechanosensors in bone; however, the underlying mechanism remains poorly understood. Here we report that deleting Kindlin-2 in osteocytes causes severe osteopenia and mechanical property defects in weight-bearing long bones, but not in non-weight-bearing calvariae. Kindlin-2 loss in osteocytes impairs skeletal responses to mechanical stimulation in long bones. Control and cKO mice display similar bone loss induced by unloading. However, unlike control mice, cKO mice fail to restore lost bone after reloading. Osteocyte Kindlin-2 deletion impairs focal adhesion (FA) formation, cytoskeleton organization and cell orientation in vitro and in bone. Fluid shear stress dose-dependently increases Kindlin-2 expression and decreases that of Sclerostin by downregulating Smad2/3 in osteocytes; this latter response is abolished by Kindlin-2 ablation. Kindlin-2-deficient osteocytes express abundant Sclerostin, contributing to bone loss in cKO mice. Collectively, we demonstrate an indispensable novel role of Kindlin-2 in maintaining skeletal responses to mechanical stimulation by inhibiting Sclerostin expression during osteocyte mechanotransduction.

Osteon: Structure, Turnover, and Regeneration

Bone is composed of dense and solid cortical bone and honeycomb-like trabecular bone. Although cortical bone provides the majority of mechanical strength for a bone, there are few studies focusing on cortical bone repair or regeneration. Osteons (the Haversian system) form structural and functional units of cortical bone. In recent years, emerging evidences have shown that the osteon structure (including osteocytes, lamellae, lacunocanalicular network, and Haversian canals) plays critical roles in bone mechanics and turnover. Therefore, reconstruction of the osteon structure is crucial for cortical bone regeneration. This article provides a systematic summary of recent advances in osteons, including the structure, function, turnover, and regenerative strategies. First, the hierarchical structure of osteons is illustrated and the critical functions of osteons in bone dynamics are introduced. Next, the modeling and remodeling processes of osteons at a cellular level and the turnover of osteons in response to mechanical loading and aging are emphasized. Furthermore, several bioengineering approaches that were recently developed to recapitulate the osteon structure are highlighted. Impact statement This review provides a comprehensive summary of recent advances in osteons, especially the roles in bone formation, remodeling, and regeneration. Besides introducing the hierarchical structure and critical functions of osteons, we elucidate the modeling and remodeling of osteons at a cellular level. Specifically, we highlight the bioengineering approaches that were recently developed to mimic the hierarchical structure of osteons. We expect that this review will provide informative insights and attract increasing attentions in orthopedic community, shedding light on cortical bone regeneration in the future.

Lacunar-canalicular bone remodeling: Impacts on bone quality and tools for assessment

Osteocytes can resorb as well as replace bone adjacent to the expansive lacunar-canalicular system (LCS). Suppressed LCS remodeling decreases bone fracture toughness, but it is unclear how altered LCS remodeling impacts bone quality. The first goal of this review is to assess how LCS remodeling impacts LCS morphology as well as the composition and mechanical properties of surrounding bone tissue. The second goal is to compare tools available for the assessment of bone quality at length-scales that are physiologically-relevant to LCS remodeling. We find that changes to LCS morphology occur in response to a variety of physiological conditions and diseases and can be classified in two general phenotypes. In the 'aging phenotype', seen in aging and in some disuse models, the LCS is truncated and osteocytes apoptosis is increased. In the 'osteocytic osteolysis' phenotype, which is adaptive in some physiological settings and possibly maladaptive in others, the LCS enlarges and osteocytes generally maintain viability. Bone composition and mechanical properties vary near the osteocyte and change with at least some conditions that alter LCS morphology. However, few studies have evaluated bone composition and mechanical properties close to the LCS and so the impacts of LCS remodeling phenotypes on bone tissue quality are still undetermined. We summarize the current understanding of how LCS remodeling impacts LCS morphology, tissue-scale bone composition and mechanical properties, and whole-bone material properties. Tools are compared for assessing tissue-scale bone properties, as well as the resolution, advantages, and limitations of these techniques.

Effects of whole-body vibration on bone properties in aged rats

OBJECTIVE

This study aimed to explore optimal conditions of whole-body vibration (WBV) for improving bone properties in aged rats.

METHODS

Eighty-week-old rats were divided into baseline control (BC), age-matched control (CON) and experimental groups, which underwent WBV (0.5 g) at various frequencies (15, 30, 45, 60 or 90 Hz) or WBV (45 Hz) with various magnitudes (0.3, 0.5, 0.7 or 1.0 g) for 7 weeks. After interventions, femur bone size, bone mechanical strength and circulating bone formation/resorption markers were measured, and trabecular bone microstructure (TBMS) and cortical bone geometry (CBG) of femurs were analyzed by micro-CT.

RESULTS

Several TBMS parameters and trabecular bone mineral content were significantly lower in the 15 Hz WBV (0.5 g) group than in the CON group, suggesting damage to trabecular bone. On the other hand, although frequency/magnitude of WBV did not influence any CBG parameters, the 0.7 g and 1.0 g WBV (45 Hz) group showed an increase in tissue mineral density of cortical bone compared with the BC and CON groups, suggesting the possibility of improving cortical bone properties.

CONCLUSION

Based on these findings, it should be noted that WBV conditions are carefully considered when applied to elderly people.

The roles of osteocytes in alveolar bone destruction in periodontitis

Periodontitis, a bacterium-induced inflammatory disease that is characterized by alveolar bone loss, is highly prevalent worldwide. Elucidating the underlying mechanisms of alveolar bone loss in periodontitis is crucial for understanding its pathogenesis. Classically, bone cells, such as osteoclasts, osteoblasts and bone marrow stromal cells, are thought to dominate the development of bone destruction in periodontitis. Recently, osteocytes, the cells embedded in the mineral matrix, have gained attention. This review demonstrates the key contributing role of osteocytes in periodontitis, especially in alveolar bone loss. Osteocytes not only initiate physiological bone remodeling but also assist in inflammation-related changes in bone remodeling. The latest evidence suggests that osteocytes are involved in regulating bone anabolism and catabolism in the progression of periodontitis. The altered secretion of receptor activator of NF-κB ligand (RANKL), sclerostin and Dickkopf-related protein 1 (DKK1) by osteocytes affects the balance of bone resorption and formation and promotes bone loss. In addition, the accumulation of prematurely senescent and apoptotic osteocytes observed in alveolar bone may exacerbate local destruction. Based on their communication with the bloodstream, it is noteworthy that osteocytes may participate in the interaction between local periodontitis lesions and systemic diseases. Overall, further investigations of osteocytes may provide vital insights that improve our understanding of the pathophysiology of periodontitis.

Crosstalk between the COX2-PGE2-EP4 signaling pathway and primary cilia in osteoblasts after mechanical stimulation

Primary cilia have been found to function as mechanosensors in low-magnitude high-frequency vibration (LMHFV)-induced osteogenesis. The PGE2 also regulates bone homeostasis and mechanical osteogenesis through its receptor EP4 signaling, but its involvement in LMHFV-induced or in primary cilia-induced osteogenesis has not been investigated. We hypothesized that LMHFV stimulates osteoblast (OB) differentiation by activating the COX2-PGE2-EP pathway in a manner dependent on primary cilia and that primary cilia are also affected by the PGE2 pathway. In this study, through western blot analysis, RNA interference, enzyme-linked immunosorbent assay, real-time quantitative polymerase chain reaction, and cytochemical staining, we observed that COX2, mPGES-1, and PGE2 levels were markedly elevated in cells treated with LMHFV and were greatly decreased in LMHFV-treated cells following IFT88 silencing. EP4 expression was significantly increased in OBs following LMHFV treatment, but IFT88 silencing significantly blocked this increase. EP4 localized to the bases of primary cilia. LMHFV reduced the length and abundance of primary cilia, but the cells could self-repair their primary cilia after mechanical damage. EP4 antagonism significantly blocked the LMHFV-induced increase in IFT88 expression and blocked the recovery of primary cilia length and the proportion of cells with primary cilia. In addition, COX2 or EP4 antagonism disrupted LMHFV-induced osteogenesis. These results demonstrate the integration of and crosstalk between primary cilia and the COX2-PGE2-EP4 signaling pathway under mechanical stimulation.

Embedding cells within nanoscale, rapidly mineralizing hydrogels: A new paradigm to engineer cell-laden bone-like tissue

Bone mineralization is a highly specific and dynamic nanoscale process that has been studied extensively from a structural, chemical, and biological standpoint. Bone tissue, therefore, may be defined by the interplay of its intricately mineralized matrix and the cells that regulate its biological function. However, the far majority of engineered bone model systems and bone replacement materials have been unable to replicate this key characteristic of bone tissue; that is, the ability of cells to be gradually and rapidly embedded in a three-dimensional (3D) heavily calcified matrix material. Here we review the characteristics that define the bone matrix from a nanostructural perspective. We then revisit the benefits and challenges of existing model systems and engineered bone replacement materials, and discuss recent efforts to replicate the biological, cellular, mechanical, and materials characteristics of bone tissue on the nano- to microscale. We pay particular attention to a recently proposed method developed by our group, which seeks to replicate key aspects of the entrapment of bone cells within a mineralized matrix with precisions down to the level of individual nano-crystallites, inclusive of the bone vasculature, and osteogenic differentiation process. In summary, this paper discusses existing and emerging evidence pointing towards future developments bridging the gap between the fields of biomineralization, structural biology, stem cells, and tissue engineering, which we believe will hold the key to engineer truly functional bone-like tissue in the laboratory.

Loss and gain of bone in spondyloarthritis: what drives these opposing clinical features?

The breadth of bone lesion types seen in spondyloarthritis is unprecedented in medicine and includes increased bone turnover, bone loss and fragility, osteitis, osteolysis and erosion, osteosclerosis, osteoproliferation of soft tissues adjacent to bone and spinal skeletal structure weakness. Remarkably, these effects can be present simultaneously in the same patient. The search for a potential unifying cause of effects on the skeleton necessarily focuses on inflammation arising from the dysregulation of immune response to microorganisms, particularly dysregulation of T_H17 lymphocytes, and the dysbiosis of established gut and other microbiota. The compelling notion that a common antecedent pathological mechanism affects existing bone and tissues with bone-forming potential (entheses), simultaneously with variable effect in the former but bone-forming in the latter, drives basic research forward and focuses our awareness on the effects on these bone mechanisms of the increasing portfolio of targeted immunotherapies used in the clinic.

LncRNA, Important Player in Bone Development and Disease

BACKGROUND

Bone is an important tissue and its normal function requires tight coordination of transcriptional networks and signaling pathways, and many of these networks/ pathways are dysregulated in pathological conditions affecting cartilage and bones. Long non-coding RNA (lncRNA) refers to a class of RNAs with a length of more than 200 nucleotides, lack of protein-coding potential, and exhibiting a wide range of biological functions. Although studies on lcnRNAs are still in their infancy, they have emerged as critical players in bone biology and bone diseases. The functions and exact mechanism of bone-related lncRNAs have not been fully classified yet.

OBJECTIVE

The objective of this article is to summarize the current literature on lncRNAs on the basis of their role in bone biology and diseases, focusing on their emerging molecular mechanism, pathological implications and therapeutic potential.

DISCUSSION

A number of lncRNAs have been identified and shown to play important roles in multiple bone cells and bone disease. The function and mechanism of bone-related lncRNA remain to be elucidated.

CONCLUSION

At present, majority of knowledge is limited to cellular levels and less is known on how lncRNAs could potentially control the development and homeostasis of bone. In the present review, we highlight some lncRNAs in the field of bone biology and bone disease. We also delineate some lncRNAs that might have deep impacts on understanding bone diseases and providing new therapeutic strategies to treat these diseases.

Osteokines and Bone Markers at Rest and following Plyometric Exercise in Pre- and Postmenopausal Women

The effect of plyometric exercise on bone biomarkers has been studied in pediatric and young adult populations in order to better understand how exercise influences bone homeostasis. However, there are no such data in postmenopausal women, a group characterized by an uncoupling of the bone resorption-formation cycle. This study examined the serum concentrations of sclerostin, dickkopf-1 (DKK1), c-terminal crosslinking telopeptides of type I collagen (CTXI), and procollagen type I amino-terminal propeptide (PINP) at rest and following a single bout of plyometric exercise in 20 premenopausal (23.1 ± 2.3 years) and 20 postmenopausal women (57.9 ± 4.3 years). The exercise consisted of 128 jumps, organized into 5 circuit stations. Blood samples were obtained prior to and 5 min, 1 h, and 24 h postexercise. At rest, postmenopausal women had significantly higher sclerostin and CTXI, but lower DKK1 than premenopausal women. Sclerostin increased 5 min postexercise only in the premenopausal group. DKK1 decreased 24 h postexercise in the premenopausal women while it decreased 1 h postexercise in the postmenopausal women. In both groups, CTXI did not change across time and PINP decreased 5 min and 1 h postexercise (p < 0.05). The PINP/CTXI ratio decreased 5 min and 1 h postexercise then significantly increased 24 h postexercise only in premenopausal women. These results indicate that although plyometric exercise is effective in eliciting osteoanabolic effects in younger women; such an effect is not evident in postmenopausal women.

A 3D, Dynamically Loaded Hydrogel Model of the Osteochondral Unit to Study Osteocyte Mechanobiology

Osteocytes are mechanosensitive cells that orchestrate signaling in bone and cartilage across the osteochondral unit. The mechanisms by which osteocytes regulate osteochondral homeostasis and degeneration in response to mechanical cues remain unclear. This study introduces a novel 3D hydrogel bilayer composite designed to support osteocyte differentiation and bone matrix deposition in a bone-like layer and to recapitulate key aspects of the osteochondral unit's complex loading environment. The bilayer hydrogel is fabricated with a soft cartilage-like layer overlaying a stiff bone-like layer. The bone-like layer contains a stiff 3D-printed hydrogel structure infilled with a soft, degradable, cellular hydrogel. The IDG-SW3 cells embedded within the soft hydrogel mature into osteocytes and produce a mineralized collagen matrix. Under dynamic compressive strains, near-physiological levels of strain are achieved in the bone layer (≤ 0.08%), while the cartilage layer bears the majority of the strains (>99%). Under loading, the model induces an osteocyte response, measured by prostaglandin E2, that is frequency, but not strain, dependent: a finding attributed to altered fluid flow within the composite. Overall, this new hydrogel platform provides a novel approach to study osteocyte mechanobiology in vitro in an osteochondral tissue-mimetic environment.

Osteocyte apoptosis: the roles and key molecular mechanisms in resorption-related bone diseases

Vital osteocytes have been well known to function as an important orchestrator in the preservation of robustness and fidelity of the bone remodeling process. Nevertheless, some key pathological factors, such as sex steroid deficiency and excess glucocorticoids, and so on, are implicated in inducing a bulk of apoptotic osteocytes, subsequently resulting in resorption-related bone loss. As much, osteocyte apoptosis, under homeostatic conditions, is in an optimal state of balance tightly controlled by pro- and anti-apoptotic mechanism pathways. Importantly, there exist many essential signaling proteins in the process of osteocyte apoptosis, which has a crucial role in maintaining a homeostatic environment. While increasing in vitro and in vivo studies have established, in part, key signaling pathways and cross-talk mechanism on osteocyte apoptosis, intrinsic and complex mechanism underlying osteocyte apoptosis occurs in various states of pathologies remains ill-defined. In this review, we discuss not only essential pro- and anti-apoptotic signaling pathways and key biomarkers involved in these key mechanisms under different pathological agents, but also the pivotal role of apoptotic osteocytes in osteoclastogenesis-triggered bone loss, hopefully shedding new light on the attractive and proper actions of pharmacotherapeutics of targeting apoptosis and ensuing resorption-related bone diseases such as osteoporosis and fragility fractures.

Using confocal imaging approaches to understand the structure and function of osteocytes and the lacunocanalicular network

Although overlooked in the past, osteocytes have come to the forefront of skeletal biology and are now recognized as a key cell type that integrates hormonal, mechanical and other signals to control bone mass through regulation of both osteoblast and osteoclast activity. With the surge of recent interest in osteocytes as bone regulatory cells and the discovery that they also function as endocrine regulators of phosphate homeostasis, there has been renewed interest in understanding the structure and function of these unique and relatively inaccessible cells. Osteocytes are embedded within the mineralized bone matrix and are housed within a complex lacunocanalicular system which connects them with the circulation and with other organ systems. This has presented unique challenges for imaging these cells. This review summarizes recent advances in confocal imaging approaches for visualizing osteocytes and their lacunocanalicular networks in both living and fixed bone specimens and discusses how computational approaches can be combined with live and fixed cell imaging techniques to generate quantitative outputs and predictive models. The integration of advanced imaging with computational approaches promises to lead to a more in depth understanding of the structure and function of osteocyte networks and the lacunocanalicular system in the healthy and aging state as well as in pathological conditions in bone.

Dehydroepiandrosterone and age-related musculoskeletal diseases: Connections and therapeutic implications

Musculoskeletal disorders related to ageing are one of the most common causes of mortality and morbidity among elderly individuals worldwide. The typical constitutive components of the musculoskeletal system, including bone, muscle, and joints, gradually undergo a process of tissue loss and degeneration as a result of life-long mechanical and biological stress, ultimately leading to the onset of a series of age-related musculoskeletal diseases, including osteoporosis (OP), sarcopenia, and osteoarthritis (OA). Dehydroepiandrosterone (DHEA), a precursor of androgen secreted mainly by the adrenal gland, has attracted much attention as a marker for senescence due to its unique age-related changes. This pre-hormone has been publicly regarded as an "antidote for ageing" because of its favourable effect against a wide range of age-related diseases, such as Alzheimer disease, cardiovascular diseases, immunosenescence and skin senescence, though its effect on age-related musculoskeletal diseases has been explored to a lesser extent. In the present review, we summarized the action of DHEA against OP, sarcopenia and OA. Extensive detailed descriptions of the pathogenesis of each of these musculoskeletal disorders are beyond the scope of this review; instead, we aim to highlight the association of changes in DHEA with the processes of OP, sarcopenia and OA. A special focus will also be placed on the overlapping pathogeneses among these three diseases, and the molecular mechanisms underlying the action of DHEA against these diseases are discussed or postulated.

Bone Muscle Crosstalk in Spinal Cord Injuries: Pathophysiology and Implications for Patients' Quality of Life

PURPOSE OF REVIEW

The goal of this review is to provide a comprehensive overview of (i) bone and muscle tissue modifications pathophysiology in spinal cord injury (SCI), (ii) experimental data on the physiopathological mechanisms underpinning these modifications and their similarities with the aging process, and (iii) potential clinical implications in the management of the disabling sequelae of SCI.

RECENT FINDINGS

Several studies attempted to describe the biology underpinning the links between bone and muscle tissues in the setting of highly disabling conditions, such as osteoporosis, sarcopenia, and neurodegenerative disorders, although these bidirectional connections remain still unclear. SCI could be considered an in vivo paradigmatic model of the bone muscle interactions in unloading conditions that might be expanded in the field of neurodegenerative disorders or cancer studies. Future studies should take into consideration the newer insights into bone muscle crosstalk in order to develop multitargeted and therapeutic interventions.

Three-dimensional microstructures of the intracortical canals in the animal model of osteoporosis

Osteoporosis is a major disease in aged women, increasing the risk for fractures accompanied by changes in the microarchitecture. The aim of this study was to investigate the three-dimensional (3D) histomorphology of femur diaphysis in the animal model for postmenopausal osteoporosis. The cortical bone of femur diaphysis of the rat was serially sectioned at a thickness of 5 mm and evaluated age-associated changes of the intracortical (osteonal) canal networks three-dimensionally. Cortical microstructures of 10-month old rats were not affected by ovariectomy. Intracortical canal networks were radial toward endosteal aspect and frequently interconnected across the neighboring canals with short arciform and irregular canals reminiscent for resorption spaces in ovarectomized 16-month old rats, contrary to intact canals in 16-month old control rat. Increased proportion of the periosteal circumference lamella and deformed endosteal regions with rare cortical canals hampered reconstructive histomorphology in ovarectomized rats of 26 month age. We have shown that 3D reconstruction of rat femur of the aged model over 16-month old is suitable methods that evaluate and microstructural change of the intracortical canals and cortical bone porosity by estrogen depletion.

Local stimulation of osteocytes using a magnetically actuated oscillating beam

Mechanical loading on bone tissue is an important physiological stimulus that plays a key role in bone growth, fracture repair, and treatment of bone diseases. Osteocytes (bone cells embedded in bone matrix) are well accepted as the sensor cells to mechanical loading and play a critical role in regulating the bone structure in response to mechanical loading. To understand the response of osteocytes to differential mechanical stimulation in physiologically relevant arrangements, there is a need for a platform which can locally stimulate bone cells with different levels of fluid shear stress. In this study, we developed a device aiming to achieve non-contact local mechanical stimulation of osteocytes with a magnetically actuated beam that generates the fluid shear stresses encountered in vivo. The stimulating beam was made from a composite of magnetic powder and polymer, where a magnetic field was used to precisely oscillate the beam in the horizontal plane. The beam is placed above a cell-seeded surface with an estimated gap height of 5 μm. Finite element simulations were performed to quantify the shear stress values and to generate a shear stress map in the region of interest. Osteocytes were seeded on the device and were stimulated while their intracellular calcium responses were quantified and correlated with their position and local shear stress value. We observed that cells closer to the oscillating beam respond earlier compared to cells further away from the local shear stress gradient generated by the oscillating beam. We have demonstrated the capability of our device to mimic the propagation of calcium signalling to osteocytes outside of the stimulatory region. This device will allow for future studies of osteocyte network signalling with a physiologically accurate localized shear stress gradient.

Comparative profiles of DNA methylation and differential gene expression in osteocytic areas from aged and young mice

Altered DNA methylation upon ageing may result in many age-related diseases such as osteoporosis. However, the changes in DNA methylation that occur in cortical bones, the major osteocytic areas, remain unknown. In our study, we extracted total DNA and RNA from the cortical bones of 6-month-old and 24-month-old mice and systematically analysed the differentially methylated regions (DMRs), differentially methylated promoters (DMPs) and differentially expressed genes (DEGs) between the mouse groups. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the DMR-related genes revealed that they were mainly associated with metabolic signalling pathways, including glycolysis, fatty acid and amino acid metabolism. Other genes with DMRs were related to signalling pathways that regulate the growth and development of cells, including the PI3K-AKT, Ras and Rap1 signalling pathways. The gene expression profiles indicated that the DEGs were mainly involved in metabolic pathways and the PI3K-AKT signalling pathway, and the profiles were verified through real-time quantitative PCR (RT-qPCR). Due to the pivotal roles of the affected genes in maintaining bone homeostasis, we suspect that these changes may be key factors in age-related bone loss, either together or individually. Our study may provide a novel perspective for understanding the osteocyte and its relationship with osteoporosis during ageing. SIGNIFICANCE OF THE STUDY: Our study identified age-related changes in gene expressions in osteocytic areas through whole-genome bisulfite sequencing (WGBS) and RNA-seq, providing new theoretical foundations for the targeted treatment of senile osteoporosis.

Molecular mechanosensors in osteocytes

Osteocytes, the most abundant and long-lived cells in bone, are the master regulators of bone remodeling. In addition to their functions in endocrine regulation and calcium and phosphate metabolism, osteocytes are the major responsive cells in force adaptation due to mechanical stimulation. Mechanically induced bone formation and adaptation, disuse-induced bone loss and skeletal fragility are mediated by osteocytes, which sense local mechanical cues and respond to these cues in both direct and indirect ways. The mechanotransduction process in osteocytes is a complex but exquisite regulatory process between cells and their environment, between neighboring cells, and between different functional mechanosensors in individual cells. Over the past two decades, great efforts have focused on finding various mechanosensors in osteocytes that transmit extracellular mechanical signals into osteocytes and regulate responsive gene expression. The osteocyte cytoskeleton, dendritic processes, Integrin-based focal adhesions, connexin-based intercellular junctions, primary cilium, ion channels, and extracellular matrix are the major mechanosensors in osteocytes reported so far with evidence from both in vitro and in vitro studies. This review aims to give a systematic introduction to osteocyte mechanobiology, provide details of osteocyte mechanosensors, and discuss the roles of osteocyte mechanosensitive signaling pathways in the regulation of bone homeostasis.

Molecular Basis of Bone Aging

A decline in bone mass leading to an increased fracture risk is a common feature of age-related bone changes. The mechanisms underlying bone senescence are very complex and implicate systemic and local factors and are the result of the combination of several changes occurring at the cellular, tissue and structural levels; they include alterations of bone cell differentiation and activity, oxidative stress, genetic damage and the altered responses of bone cells to various biological signals and to mechanical loading. The molecular mechanisms responsible for these changes remain greatly unclear and many data derived from in vitro or animal studies appear to be conflicting and heterogeneous, probably due to the different experimental approaches; nevertheless, understanding the main physio-pathological processes that cause bone senescence is essential for the development of new potential therapeutic options for treating age-related bone loss. This article reviews the current knowledge concerning the molecular mechanisms underlying the pathogenesis of age-related bone changes.

Human skeletal physiology and factors affecting its modeling and remodeling

Human skeleton is a living tissue that performs structural and metabolic functions. It is not only the largest storehouse for calcium and other essential ions but also a depot for toxic chemicals faced by human body throughout life. Skeletal modeling starts at conception and then throughout life undergoes constant remodeling to adopt its shape and strength according to human needs. With the passage of time, like other tissues in the body, bones also bear the brunt of life and in this life long process loses its strength and vitality. Multiple genetic and environmental factors play an integral part in its formation, strength, and decline.