Calcium response in single osteocytes to locally applied mechanical stimulus: differences in cell process and cell body

Computational modeling based on confocal imaging predicts changes in osteocyte and dendrite shear stress due to canalicular loss with aging

Exercise and physical activity exert mechanical loading on the bones which induces bone formation. However, the relationship between the osteocyte lacunar-canalicular morphology and mechanical stress experienced locally by osteocytes transducing signals for bone formation is not fully understood. In this study, we used computational modeling to predict the effect of canalicular density, the number of fluid inlets, and load direction on fluid flow shear stress (FFSS) and bone strains and how these might change following the microstructural deterioration of the lacunar-canalicular network that occurs with aging. Four distinct computational models were initially generated of osteocytes with either ten or eighteen dendrites using a fluid-structure interaction method with idealized geometries. Next, a young and a simulated aged osteocyte were developed from confocal images after FITC staining of the femur of a 4-month-old C57BL/6 mouse to estimate FFSS using a computational fluid dynamics approach. The models predicted higher fluid velocities in the canaliculi versus the lacunae. Comparison of idealized models with five versus one fluid inlet indicated that with four more inlets, one-half of the dendrites experienced FFSS greater than 0.8 Pa, which has been associated with osteogenic responses. Confocal image-based models of real osteocytes indicated a six times higher ratio of canalicular to lacunar surface area in the young osteocyte model than the simulated aged model and the average FFSS in the young model (FFSS = 0.46 Pa) was three times greater than the aged model (FFSS = 0.15 Pa). Interestingly, the surface area with FFSS values above 0.8 Pa was 23 times greater in the young versus the simulated aged model. These findings may explain the impaired mechano-responsiveness of osteocytes with aging.

Osteocytes and Primary Cilia

PURPOSE OF REVIEW

The purpose of this review is to provide a background on osteocytes and the primary cilium, discussing the role it plays in osteocyte mechanosensing.

RECENT FINDINGS

Osteocytes are thought to be the primary mechanosensing cells in bone tissue, regulating bone adaptation in response to exercise, with the primary cilium suggested to be a key mechanosensing mechanism in bone. More recent work has suggested that, rather than being direct mechanosensors themselves, primary cilia in bone may instead form a key chemo-signalling nexus for processing mechanoregulated signalling pathways. Recent evidence suggests that pharmacologically induced lengthening of the primary cilium in osteocytes may potentiate greater mechanotransduction, rather than greater mechanosensing. While more research is required to delineate the specific osteocyte mechanobiological molecular mechanisms governed by the primary cilium, it is clear from the literature that the primary cilium has significant potential as a therapeutic target to treat mechanoregulated bone diseases, such as osteoporosis.

Multi-scale numerical simulation on mechano-transduction of osteocytes in different gravity fields

A three-dimensional model for osteocyte was established to research the mechanisms of mechano-transduction and amplification of primary cilium and osteocyte process in every gravity field. The results showed that significant stress concentration was observed in the area of physical connection between TES and the osteocyte process, where the fluid shear stress (FSS) was around two orders of magnitude higher than that in other areas. Due to the significant amplification effect of the TES structure on mechanical stimulation, making osteocyte process the "optimal mechanical receptor". In microgravity, the mechanical signal conduction ability of the osteocyte decreased significantly.. HighlightsAt the micro-nano scale, a 3D model for single bone lacunae-osteocyte system is established.The stress amplification mechanism of the transverse element is verified.Compared with the primary cilium, osteocyte process is the 'optimal mechanical receptor'.In microgravity, the mechanical signal conduction ability of osteocyte system decreased.

The myosin and RhoGAP MYO9B influences osteocyte dendrite growth and responses to mechanical stimuli

Introduction: Myosin IXB (MYO9B) is an unconventional myosin with RhoGAP activity and thus is a regulator of actin cytoskeletal organization. MYO9B was previously shown to be necessary for skeletal growth and health and to play a role in actin-based functions of both osteoblasts and osteoclasts. However, its role in responses to mechanical stimulation of bone cells has not yet been described. Therefore, experiments were undertaken to determine the role of MYO9B in bone cell responses to mechanical stress both in vitro and in vivo. Methods: MYO9B expression was knocked down in osteoblast and osteocyte cell lines using RNA interference and the resulting cells were subjected to mechanical stresses including cyclic tensile strain, fluid shear stress, and plating on different substrates (no substrate vs. monomeric or polymerized collagen type I). Osteocytic cells were also subjected to MYO9B regulation through Slit-Robo signaling. Further, wild-type or Myo9b -/- mice were subjected to a regimen of whole-body vibration (WBV) and changes in bone quality were assessed by micro-CT. Results: Unlike control cells, MYO9B-deficient osteoblastic cells subjected to uniaxial cyclic tensile strain were unable to orient their actin stress fibers perpendicular to the strain. Osteocytic cells in which MYO9B was knocked down exhibited elongated dendrites but were unable to respond normally to treatments that increase dendrite length such as fluid shear stress and Slit-Robo signaling. Osteocytic responses to mechanical stimuli were also found to be dependent on the polymerization state of collagen type I substrates. Wild-type mice responded to WBV with increased bone tissue mineral density values while Myo9b -/- mice responded with bone loss. Discussion: These results demonstrate that MYO9B plays a key role in mechanical stress-induced responses of bone cells in vitro and in vivo.

SOST gene suppression stimulates osteocyte Wnt/β-catenin signaling to prevent bone resorption and attenuates particle-induced osteolysis

The most common cause for prosthetic revision surgery is wear particle-induced periprosthetic osteolysis, which leads to aseptic loosening of the prosthesis. Both SOST gene and its synthetic protein, sclerostin, are hallmarks of osteocytes. According to our previous findings, blocking SOST induces bone formation and protects against bone loss and deformation caused by titanium (Ti) particles by activating the Wnt/β-catenin cascade. Although SOST has been shown to influence osteoblasts, its ability to control wear-particle-induced osteolysis via targeting osteoclasts remains unclear. Mice were subjected to development of a cranial osteolysis model. Micro CT, HE staining, and TRAP staining were performed to evaluate bone loss in the mouse model. Bone marrow-derived monocyte-macrophages (BMMs) made from the C57BL/6 mice were exposed to the medium of MLO-Y4 (co-cultured with Ti particles) to transform them into osteoclasts. Bioinformatics methods were used to predict and validate the interaction among SOST, Wnt/β-catenin, RANKL/OPG, TNF-α, and IL-6. Local bone density and bone volume improved after SOST inhibition, both the number of lysis pores and the rate of skull erosion decreased. Histological research showed that β-catenin and OPG expression were markedly increased after SOST inhibition, whereas TRAP and RANKL levels were markedly decreased. In-vitro, Ti particle treatment elevated the expression of sclerostin, suppressed the expression of β-catenin, and increased the RANKL/OPG ratio in the MLO-Y4 cell line. TNF-α and IL-6 also elevated after treatment with Ti particles. The expression levels of NFATc1, CTSK, and TRAP in osteoclasts were significantly increased, and the number of positive cells for TRAP staining was increased. Additionally, the volume of bone resorption increased at the same time. In contrast, when SOST expression was inhibited in the MLO-Y4 cell line, these effects produced by Ti particles were reversed. All the results strongly show that SOST inhibition triggered the osteocyte Wnt/β-catenin signaling cascade and prevented wear particle-induced osteoclastogenesis, which might reduce periprosthetic osteolysis. KEY MESSAGES: SOST is a molecular regulator in maintaining bone homeostasis. SOST plays in regulating bone homeostasis through the Wnt/β-catenin signaling pathway. SOST gene suppression stimulates osteocyte Wnt/β-catenin signaling to prevent bone resorption and attenuates particle-induced osteolysis.

The Effect of IFT80 Deficiency in Osteocytes on Orthodontic Loading-Induced and Physiologic Bone Remodeling: In Vivo Study

Osteocytes are the main mechanosensory cells during orthodontic and physiologic bone remodeling. However, the question of how osteocytes transmit mechanical stimuli to biological responses remains largely unanswered. Intraflagellar transport (IFT) proteins are important for the formation and function of cilia, which are proposed to be mechanical sensors in osteocytes. In particular, IFT80 is highly expressed in mouse skulls and essential for ciliogenesis. This study aims to investigate the short- and long-term effects of IFT80 deletion in osteocytes on orthodontic bone remodeling and physiological bone remodeling in response to masticatory force. We examined 10-week-old experimental DMP1 CRE+.IFT80f/f and littermate control DMP1 CRE-.IFT80f/f mice. After 5 and 12 days of orthodontic force loading, the orthodontic tooth movement distance and bone parameters were evaluated using microCT. Osteoclast formation was assessed using TRAP-stained paraffin sections. The expression of sclerostin and RANKL was examined using immunofluorescence stain. We found that the deletion of IFT80 in osteocytes did not significantly impact either orthodontic or physiologic bone remodeling, as demonstrated by similar OTM distances, osteoclast numbers, bone volume fractions (bone volume/total volume), bone mineral densities, and the expressions of sclerostin and RANKL. Our findings suggest that there are other possible mechanosensory systems in osteocytes and anatomic limitations to cilia deflection in osteocytes in vivo.

A morphometric analysis of the osteocyte canaliculus using applied automatic semantic segmentation by machine learning

INTRODUCTION

Osteocytes play a role as mechanosensory cells by sensing flow-induced mechanical stimuli applied on their cell processes. High-resolution imaging of osteocyte processes and the canalicular wall are necessary for the analysis of this mechanosensing mechanism. Focused ion beam-scanning electron microscopy (FIB-SEM) enabled the visualization of the structure at the nanometer scale with thousands of serial-section SEM images. We applied machine learning for the automatic semantic segmentation of osteocyte processes and canalicular wall and performed a morphometric analysis using three-dimensionally reconstructed images.

MATERIALS AND METHODS

Six-week-old-mice femur were used. Osteocyte processes and canaliculi were observed at a resolution of 2 nm/voxel in a 4 × 4 μm region with 2000 serial-section SEM images. Machine learning was used for automatic semantic segmentation of the osteocyte processes and canaliculi from serial-section SEM images. The results of semantic segmentation were evaluated using the dice similarity coefficient (DSC). The segmented data were reconstructed to create three-dimensional images and a morphological analysis was performed.

RESULTS

The DSC was > 83%. Using the segmented data, a three-dimensional image of approximately 3.5 μm in length was reconstructed. The morphometric analysis revealed that the median osteocyte process diameter was 73.8 ± 18.0 nm, and the median pericellular fluid space around the osteocyte process was 40.0 ± 17.5 nm.

CONCLUSION

We used machine learning for the semantic segmentation of osteocyte processes and canalicular wall for the first time, and performed a morphological analysis using three-dimensionally reconstructed images.

Symmetry breaking and effects of nutrient walkway in time-dependent bone remodeling incorporating poroelasticity

Bone is an extraordinary biological material that continuously adapts its hierarchical microstructure to respond to static and dynamic loads for offering optimal mechanical features, in terms of stiffness and toughness, across different scales, from the sub-microscopic constituents within osteons—where the cyclic activity of osteoblasts, osteoclasts, and osteocytes redesigns shape and percentage of mineral crystals and collagen fibers—up to the macroscopic level, with growth and remodeling processes that modify the architecture of both compact and porous bone districts. Despite the intrinsic complexity of the bone mechanobiology, involving coupling phenomena of micro-damage, nutrients supply driven by fluid flowing throughout hierarchical networks, and cells turnover, successful models and numerical algorithms have been presented in the literature to predict, at the macroscale, how bone remodels under mechanical stimuli, a fundamental issue in many medical applications such as optimization of femur prostheses and diagnosis of the risk fracture. Within this framework, one of the most classical strategies employed in the studies is the so-called Stanford’s law, which allows uploading the effect of the time-dependent load-induced stress stimulus into a biomechanical model to guess the bone structure evolution. In the present work, we generalize this approach by introducing the bone poroelasticity, thus incorporating in the model the role of the fluid content that, by driving nutrients and contributing to the removal of wastes of bone tissue cells, synergistically interacts with the classical stress fields to change homeostasis states, local saturation conditions, and reorients the bone density rate, in this way affecting growth and remodeling. Through two paradigmatic example applications, i.e. a cylindrical slice with internal prescribed displacements idealizing a tract of femoral diaphysis pushed out by the pressure exerted by a femur prosthesis and a bone element in a form of a bent beam, it is highlighted that the present model is capable to catch more realistically both the transition between spongy and cortical regions and the expected non-symmetrical evolution of bone tissue density in the medium–long term, unpredictable with the standard approach. A real study case of a femur is also considered at the end in order to show the effectiveness of the proposed remodeling algorithm.

Finite Element Models of Osteocytes and Their Load-Induced Activation

PURPOSE OF REVIEW

Osteocytes are the conductors of bone adaptation and remodelling. Buried inside the calcified matrix, they sense mechanical cues and signal osteoclasts in case of low activity, and osteoblasts when stresses are high. How do osteocytes detect mechanical stress? What physical signal do they perceive? Finite element analysis is a useful tool to address these questions as it allows calculating stresses, strains and fluid flow where they cannot be measured. The purpose of this review is to evaluate the capabilities and challenges of finite element models of bone, in particular the osteocytes and load-induced activation mechanisms.

RECENT FINDINGS

High-resolution imaging and increased computational power allow ever more detailed modelling of osteocytes, either in isolation or embedded within the mineralised matrix. Over the years, homogeneous models of bone and osteocytes got replaced by heterogeneous and microstructural models, including, e.g. the lacuno-canalicular network and the cytoskeleton. The lacuno-canalicular network induces strain amplifications and the osteocyte protrusions seem to be stimulated much more than the cell body, both by strain and fluid flow. More realistic cell geometries, like minute constrictions of the canaliculi, increase this effect. Microstructural osteocyte models describe the transduction of external stimuli to the nucleus. Supracellular multiscale models (e.g. of a tunnelling osteon) allow to study differential loading of osteocytes and to distinguish between strain and fluid flow as the pivotal stimulatory cue. In the future, the finite element models may be enhanced by including chemical transport and intercellular communication between osteocytes, osteoclasts and osteoblasts.

Computational framework for analyzing flow-induced strain on osteocyte as modulated by microenvironment

Osteocytes buried in bone matrix are major mechanosensory cells that regulate bone remodeling in response to interstitial fluid flow in a lacuno-canalicular porosity. To gain an understanding of the mechanism of osteocyte mechanosensing, it is important to be able to evaluate the local strain on the osteocyte process membrane induced by the interstitial fluid flow. The microenvironment of the osteocytes, including the pericellular matrix (PCM) and canalicular ultrastructure, is a key modulator of the flow-induced strain on the osteocyte process membrane because it produces heterogeneous flow patterns in the pericellular space. To investigate the effect of changes in the microenvironment of osteocytes on the flow-induced strain, we developed a novel computational framework for analyzing the fluid-structure interaction. Computer simulations based on the proposed framework enabled evaluation of the spatial distribution of flow-induced strain on the osteocyte process membrane according to changes in the PCM density and canalicular curvature. The simulation results reveal that a decrease in PCM density and an increase in canalicular curvature, each of which is associated with aging and bone disease, have the notable effect of enhancing local flow-induced strain on the osteocyte process membrane. We believe that the proposed computational framework is a promising framework for investigating cell-specific mechanical stimuli and that it has the potential to accelerate the mechanobiological study of osteocytes by providing a deeper understanding of their mechanical environment in living bone tissue.

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.

Control of osteoblast arrangement by osteocyte mechanoresponse through prostaglandin E2 signaling under oscillatory fluid flow stimuli

Anisotropic collagen/apatite microstructure is a prominent determinant of bone tissue functionalization; in particular, bone matrix modulates its anisotropic microstructure depending on the surrounding mechanical condition. Although mechanotransduction in bones is governed by osteocyte function, the precise mechanisms linking mechanical stimuli and anisotropic formation of collagen/apatite microstructure are poorly understood. Here we developed a novel anisotropic mechano-coculture system which enables the understanding of the biological mechanisms regulating the oriented bone matrix formation, which is constructed by aligned osteoblasts. The developed model provides bone-mimetic coculture platform that enables simultaneous control of mechanical condition and osteoblast-osteocyte communication with an anisotropic culture scaffold. The engineered coculture device helps in understanding the relationship between osteocyte mechanoresponses and osteoblast arrangement, which is a significant contributor to anisotropic organization of bone tissue. Our study showed that osteocyte responses to oscillatory flow stimuli regulated osteoblast arrangement through soluble molecular interactions. Importantly, we found that prostaglandin E2 is a novel determinant for oriented collagen/apatite organization of bone matrix, through controlling osteoblast arrangement.

Quantitative Evaluation of Osteocyte Morphology and Bone Anisotropic Extracellular Matrix in Rat Femur

Osteocytes are believed to play a crucial role in mechanosensation and mechanotransduction which are important for maintenance of mechanical integrity of bone. Recent investigations have revealed that the preferential orientation of bone extracellular matrix (ECM) mainly composed of collagen fibers and apatite crystallites is one of the important determinants of bone mechanical integrity. However, the relationship between osteocytes and ECM orientation remains unclear. In this study, the association between ECM orientation and anisotropy in the osteocyte lacuno-canalicular system, which is thought to be optimized along with the mechanical stimuli, was investigated using male rat femur. The degree of ECM orientation along the femur longitudinal axis was significantly and positively correlated with the anisotropic features of the osteocyte lacunae and canaliculi. At the femur middiaphysis, there are the osteocytes with lacunae that highly aligned along the bone long axis (principal stress direction) and canaliculi that preferentially extended perpendicular to the bone long axis, and the highest degree of apatite c-axis orientation along the bone long axis was shown. Based on these data, we propose a model in which osteocytes can change their lacuno-canalicular architecture depending on the mechanical environment so that they can become more susceptible to mechanical stimuli via fluid flow in the canalicular channel.

High-resolution image-based simulation reveals membrane strain concentration on osteocyte processes caused by tethering elements

Osteocytes are vital for regulating bone remodeling by sensing the flow-induced mechanical stimuli applied to their cell processes. In this mechanosensing mechanism, tethering elements (TEs) connecting the osteocyte process with the canalicular wall potentially amplify the strain on the osteocyte processes. The ultrastructure of the osteocyte processes and canaliculi can be visualized at a nanometer scale using high-resolution imaging via ultra-high voltage electron microscopy (UHVEM). Moreover, the irregular shapes of the osteocyte processes and the canaliculi, including the TEs in the canalicular space, should considerably influence the mechanical stimuli applied to the osteocytes. This study aims to characterize the roles of the ultrastructure of osteocyte processes and canaliculi in the mechanism of osteocyte mechanosensing. Thus, we constructed a high-resolution image-based model of an osteocyte process and a canaliculus using UHVEM tomography and investigated the distribution and magnitude of flow-induced local strain on the osteocyte process by performing fluid–structure interaction simulation. The analysis results reveal that local strain concentration in the osteocyte process was induced by a small number of TEs with high tension, which were inclined depending on the irregular shapes of osteocyte processes and canaliculi. Therefore, this study could provide meaningful insights into the effect of ultrastructure of osteocyte processes and canaliculi on the osteocyte mechanosensing mechanism.

Alterations in osteocyte lacunar morphology affect local bone tissue strains

Osteocytes are capable of remodeling their perilacunar bone matrix, which causes considerable variations in the shape and size of their lacunae. If these variations in lacunar morphology cause changes in the mechanical environment of the osteocytes, in particular local strains, they would subsequently affect bone mechanotransduction, since osteocytes are likely able to directly sense these strains. The purpose of this study is to quantify the effect of alterations in osteocyte lacunar morphology on peri-lacunar bone tissue strains. To this end, we related the actual lacunar shape in fibulae of six young-adult (5-month) and six old (23-month) mice, quantified by high-resolution micro-computed tomography, to microscopic strains, analyzed by micro-finite element modeling. We showed that peak effective strain increased by 12.6% in osteocyte cell bodies (OCYs), 9.6% in pericellular matrix (PCM), and 5.3% in extra cellular matrix (ECM) as the lacunae volume increased from 100-200 μm³ to 500-600 μm³. Lacunae with a larger deviation (>8°) in orientation from the longitudinal axis of the bone are exposed to 8% higher strains in OCYs, 6.5% in PCM, 4.2% in ECM than lacunae with a deviation in orientation below 8°. Moreover, increased lacuna sphericity from 0 to 0.5 to 0.7-1 led to 25%, 23%, and 13% decrease in maximum effective strains in OCYs, PCM, and ECM, respectively. We further showed that due to the presence of smaller and more round lacunae in old mice, local bone tissue strains are on average 5% lower in the vicinity of lacunae and their osteocytes of old mice compared to young. Understanding how changes in lacunar morphology affect the micromechanical environment of osteocytes presents a first step in unraveling their potential role in impaired bone mechanoresponsiveness with e.g. aging.

Disrupted osteocyte connectivity and pericellular fluid flow in bone with aging and defective TGF-β 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-β) signaling (TβRIIocy-/-), 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 TβRIIocy-/- 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 TβRIIocy-/- 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.

Changes in the intra- and peri-cellular sclerostin distribution in lacuno-canalicular system induced by mechanical unloading

INTRODUCTION

Mechanical stimuli regulate Sclerostin (Scl), a negative regulator of bone formation, expression in osteocytes. However, the detailed Scl distribution in osteocytes in response to mechanical unloading remains unclear.

MATERIALS AND METHODS

Twelve-week-old male rats were used. The sciatic and femoral nerves on the right side were excised as mechanical unloading treatment. A sham operation was performed on the left side. One week after neurotrauma, the bone density of the femora was evaluated by peripheral quantitative computed tomography, and immunofluorescence was performed in coronal sections of the femoral diaphysis. The mean fluorescence intensity and fluorescent profile of Scl from the marrow to the periosteal side were analyzed to estimate the Scl expression and determine to which side (marrow or periosteal) the Scl prefers to distribute in response to mechanical unloading. The most sensitive region indicated by the immunofluorescence results was further investigated by transmission electron microscopy (TEM) with immunogold staining to show the Scl expression changes in different subcellular structures.

RESULTS

In femur distal metaphysis, neurotrauma-induced mechanical unloading significantly decreased the bone density, made the distribution of Scl closer to the marrow on the anterior and medial side, and increased the Scl expression only on the lateral side. TEM findings showed that only the expression of Scl in canaliculi was increased by mechanical unloading.

CONCLUSIONS

Our results showed that even short-term mechanical unloading is enough to decrease bone density, and mechanical unloading not only regulated the Scl expression but also changed the Scl distribution in both the osteocyte network and subcellular structures.

Large magnitude of force leads to NO-mediated cell shrinkage in single osteocytes implying an initial apoptotic response

Damage accumulation in the bone under continuous daily loading causes local mechanical overloading known to induce osteocyte apoptosis, which promotes bone resorption to repair bone damage. However, only a few studies have investigated the mechanism of apoptosis in mechanically overloaded osteocytes. As mechanically stimulated osteocytes produce nitric oxide (NO), which triggers apoptosis in various cell types, we aimed to elucidate the mechanism underlying apoptosis in mechanically overloaded osteocytes, focusing on intracellular NO. To investigate the effects of force magnitude on apoptosis and intracellular NO production, we isolated osteocytes from DMP1-EGFP mice and subjected them to quantitative local forces via fibronectin-coated micro beads targeting integrin on the cell surface using a magnetic tweezer. Cell shrinkage was microscopically examined, and intracellular NO production was visualized using DAR-4 M. Mechanical stimulation revealed relationships between force magnitude, apoptosis, and intracellular NO production. The application of a smaller force resulted in no significant cell shrinkage or intracellular NO production; however, a larger force caused a rapid increase in intracellular NO production followed by cell shrinkage. Besides, intracellular NOS (NO synthase) inhibition and NO donation revealed the pro-apoptotic roles of NO in osteocytes. L-NAME (NOS inhibitor)-treated cells displayed no significant shrinkage under a larger force, whereas SNP (NO donor)-treated cells showed cell shrinkage and Annexin V fluorescence, indicating apoptosis. Collectively, our study demonstrates that larger force leads to NO production-mediated osteocyte shrinkage, implying an initial apoptotic response and highlighting the importance of NO production in bone damage.

The Osteocyte: New Insights

Osteocytes are an ancient cell, appearing in fossilized skeletal remains of early fish and dinosaurs. Despite its relative high abundance, even in the context of nonskeletal cells, the osteocyte is perhaps among the least studied cells in all of vertebrate biology. Osteocytes are cells embedded in bone, able to modify their surrounding extracellular matrix via specialized molecular remodeling mechanisms that are independent of the bone forming osteoblasts and bone-resorbing osteoclasts. Osteocytes communicate with osteoclasts and osteoblasts via distinct signaling molecules that include the RankL/OPG axis and the Sost/Dkk1/Wnt axis, among others. Osteocytes also extend their influence beyond the local bone environment by functioning as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function. These cells are also finely tuned sensors of mechanical stimulation to coordinate with effector cells to adjust bone mass, size, and shape to conform to mechanical demands.

Lateral static overload on immediately restored implants decreases the osteocyte index in peri-implant bone: a secondary analysis of a pre-clinical study in dogs

OBJECTIVES

This animal study was conducted to evaluate the osteocyte index in the peri-implant bone around immediately restored implants under static lateral overload.

MATERIAL AND METHODS

Seven mongrel dogs received three implants on each side of the mandible. Forty-two implants were distributed into three groups (14 implants per group); each animal received two implants connected to a 4.5-mm opened expansion device (experimental group); in the other mandible side, two implants were connected into an expansion device without activation (control group); one implant each side of the mandible was left submerged (unload group). After 4 months under daily mechanical and chemical plaque control, the animals were euthanized; dental implants and surrounding bone were removed and processed to obtain thin ground sections. Histomorphometry was used to evaluate the osteocyte index in the peri-implant bone contact to implant.

RESULTS

A higher, statistically significant mean number of osteocytes × 10^-5 μm² (54.74 ± 23.91) was found in the control group compared with the test group (22.57 ± 22.55) (p = 0.0221). The correlation between percentage of bone-implant contact and osteocyte index for submerged implants was not statistically significant (p = 0.2667), whereas the value for immediately loaded implants was statistically significant (p = 0.0480).

CONCLUSION

The lower number of osteocytes in the peri-implant bone around overloaded implants could be related to the need for functional adaptation of the bone tissue to overloading and to the hypothesized involvement of the osteocytes in the maintenance of the bone matrix in the control group.

CLINICAL RELEVANCE

Osteocytes play a pivotal role in bone adaptation to mechanical loading, and the osteocyte network has been regarded as being the main mechanosensory mechanism.

Is There a Governing Role of Osteocytes in Bone Tissue Regeneration?

PURPOSE OF REVIEW

Bone regeneration plays an important role in contemporary clinical treatment. Bone tissue engineering should result in successful bone regeneration to restore congenital or acquired bone defects in the human skeleton. Osteocytes are thought to have a governing role in bone remodeling by regulating osteoclast and osteoblast activity, and thus bone loss and formation. In this review, we address the so far largely unknown role osteocytes may play in bone tissue regeneration.

RECENT FINDINGS

Osteocytes release biochemical signaling molecules involved in bone remodeling such as prostaglandins, nitric oxide, Wnts, and insulin-like growth factor-1 (IGF-1). Treatment of mesenchymal stem cells in bone tissue engineering with prostaglandins (e.g., PGE₂, PGI₂, PGF_2α), nitric oxide, IGF-1, or Wnts (e.g., Wnt3a) improves osteogenesis. This review provides an overview of the functions of osteocytes in bone tissue, their interaction with other bone cells, and their role in bone remodeling. We postulate that osteocytes may have a pivotal role in bone regeneration as well, and consequently that the bone regeneration process may be improved effectively and rapidly if osteocytes are optimally used and stimulated.

The mTORC2 Component Rictor Is Required for Load-Induced Bone Formation in Late-Stage Skeletal Cells

Bone relies on mechanical cues to build and maintain tissue composition and architecture. Our understanding of bone cell mechanotransduction continues to evolve, with a few key signaling pathways emerging as vital. Wnt/β‐catenin, for example, is essential for proper anabolic response to mechanical stimulation. One key complex that regulates β‐catenin activity is the mammalian target of rapamycin complex 2 (mTORc2). mTORc2 is critical for actin cytoskeletal reorganization, an indispensable component in mechanotransduction in certain cell types. In this study, we probed the impact of the mTORc2 signaling pathway in osteocyte mechanotransduction by conditionally deleting the mTORc2 subunit Rictor in Dmp1‐expressing cells of C57BL/6 mice. Conditional deletion of the Rictor was achieved using the Dmp1–Cre driver to recombine Rictor floxed alleles. Rictor mutants exhibited a decrease in skeletal properties, as measured by DXA, μCT, and mechanical testing, compared with Cre‐negative floxed littermate controls. in vivo axial tibia loading conducted in adult mice revealed a deficiency in the osteogenic response to loading among Rictor mutants. Histological measurements of osteocyte morphology indicated fewer, shorter cell processes in Rictor mutants, which might explain the compromised response to mechanical stimulation. In summary, inhibition of the mTORc2 pathway in late osteoblasts/osteocytes leads to decreased bone mass and mechanically induced bone formation. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Mechanotransduction in osteogenesis

Bone is one of the most highly adaptive tissues in the body, possessing the capability to alter its morphology and function in response to stimuli in its surrounding environment. The ability of bone to sense and convert external mechanical stimuli into a biochemical response, which ultimately alters the phenotype and function of the cell, is described as mechanotransduction. This review aims to describe the fundamental physiology and biomechanisms that occur to induce osteogenic adaptation of a cell following application of a physical stimulus. Considerable developments have been made in recent years in our understanding of how cells orchestrate this complex interplay of processes, and have become the focus of research in osteogenesis. We will discuss current areas of preclinical and clinical research exploring the harnessing of mechanotransductive properties of cells and applying them therapeutically, both in the context of fracture healing and de novo bone formation in situations such as nonunion.

Cite this article: Bone Joint Res 2019;9(1):1–14.

Periostin Mediates Oestrogen-Induced Osteogenic Differentiation of Bone Marrow Stromal Cells in Ovariectomised Rats

Osteoporosis is a metabolic disease that results in the progressive loss of bone mass, which, in postmenopausal women, is related to oestrogen deficiency. Periostin (POSTN) plays a key role in the early stages of bone formation. However, whether POSTN participates in oestradiol regulation of osteogenic differentiation of bone marrow stromal cells (BMSCs) from ovariectomised (OVX) rats remains unclear. In vivo, using microcomputed tomography (micro-CT), immunohistochemistry, and dynamic analysis of femurs, we found that 17β-E2 promotes bone formation and POSTN expression at the endosteal surface. In vitro, 17β-E2 upregulated POSTN expression in OVX-BMSCs. POSTN overexpression activated the Wnt/β-catenin signalling pathway and enhanced osteogenic differentiation of OVX-BMSCs. Furthermore, knockdown of Postn blocks the involvement of 17β-E2 in the osteogenic differentiation of OVX-BMSCs. Collectively, our study indicated the role of POSTN in the osteogenesis and stemness of OVX-BMSCs and proves that 17β-E2 reduces osteoporosis and promotes osteogenesis through the POSTN-Wnt/β-catenin pathway. POSTN could, therefore, be a novel target gene for anti-osteoporosis therapies.

Phenomenon of osteocyte lacunar mineralization: indicator of former osteocyte death and a novel marker of impaired bone quality?

An increasing number of patients worldwide suffer from bone fractures that occur after low intensity trauma. Such fragility fractures are usually associated with advanced age and osteoporosis but also with long-term immobilization, corticosteroid therapy, diabetes mellitus, and other endocrine disorders. It is important to understand the skeletal origins of increased bone fragility in these conditions for preventive and therapeutic strategies to combat one of the most common health problems of the aged population. This review summarizes current knowledge pertaining to the phenomenon of micropetrosis (osteocyte lacunar mineralization). As an indicator of former osteocyte death, micropetrosis is more common in aged bone and osteoporotic bone. Considering that the number of mineralized osteocyte lacunae per bone area can distinguish healthy, untreated osteoporotic and bisphosphonate-treated osteoporotic patients, it could be regarded as a novel structural marker of impaired bone quality. Further research is needed to clarify the mechanism of lacunar mineralization and to explore whether it could be an additional target for preventing or treating bone fragility related to aging and various endocrine diseases.

Can we enhance osteoporotic metaphyseal fracture healing through enhancing ultrastructural and functional changes of osteocytes in cortical bone with low-magnitude high-frequency vibration?

Fragility fractures are related to the loss of bone integrity and deteriorated morphology of osteocytes. Our previous studies have reported that low-magnitude high-frequency vibration (LMHFV) promoted osteoporotic fracture healing. As osteocytes are known for mechanosensing and initiating bone repair, we hypothesized that LMHFV could enhance osteoporotic fracture healing through enhancing morphological changes in the osteocyte lacuna-canalicular network (LCN) and mineralization. A metaphyseal fracture model was established in female Sprague-Dawley rats to investigate changes in osteocytes and healing outcomes from early to late phase post-fracture. Our results showed that the LCN exhibited an exuberant outgrowth of canaliculi in the osteoporotic fractured bone at day 14 after LMHFV. LMHFV upregulated the E11, dentin matrix protein 1 (DMP1), and fibroblast growth factor 23 (FGF23), but downregulated sclerostin (Sost) in osteocytes. Moreover, LMHFV promoted mineralization with significant enhancements of Ca/P ratio, mineral apposition rate (MAR), mineralizing surface (MS/BS), and bone mineral density (BMD) in the osteoporotic group. Consistently, better healing was confirmed by microarchitecture and mechanical properties, whereas the enhancement in osteoporotic group was comparable or even greater than the normal group. This is the first report to reveal the enhancement effect of LMHFV on the osteocytes' morphology and functions in osteoporotic fracture healing.

Network architecture strongly influences the fluid flow pattern through the lacunocanalicular network in human osteons

A popular hypothesis explains the mechanosensitivity of bone due to osteocytes sensing the load-induced flow of interstitial fluid squeezed through the lacunocanalicular network (LCN). However, the way in which the intricate structure of the LCN influences fluid flow through the network is largely unexplored. We therefore aimed to quantify fluid flow through real LCNs from human osteons using a combination of experimental and computational techniques. Bone samples were stained with rhodamine to image the LCN with 3D confocal microscopy. Image analysis was then performed to convert image stacks into mathematical network structures, in order to estimate the intrinsic permeability of the osteons as well as the load-induced fluid flow using hydraulic circuit theory. Fluid flow was studied in both ordinary osteons with a rather homogeneous LCN as well as a frequent subtype of osteons-so-called osteon-in-osteons-which are characterized by a ring-like zone of low network connectivity between the inner and the outer parts of these osteons. We analyzed 8 ordinary osteons and 9 osteon-in-osteons from the femur midshaft of a 57-year-old woman without any known disease. While the intrinsic permeability was 2.7 times smaller in osteon-in-osteons compared to ordinary osteons, the load-induced fluid velocity was 2.3 times higher. This increased fluid velocity in osteon-in-osteons can be explained by the longer path length, needed to cross the osteon from the cement line to the Haversian canal, including more fluid-filled lacunae and canaliculi. This explanation was corroborated by the observation that a purely structural parameter-the mean path length to the Haversian canal-is an excellent predictor for the average fluid flow velocity. We conclude that osteon-in-osteons may be particularly significant contributors to the mechanosensitivity of cortical bone, due to the higher fluid flow in this type of osteons.

Functional Adaptation of the Fibrocartilage and Bony Trabeculae at the Attachment Sites of the Anterior Cruciate Ligament

The direct insertion of an enthesis is composed of uncalcified fibrocartilage (FC) and calcified FC, believed to function as buffers for multidirectional forces applied to the ligament. This study was performed to investigate the relationship between the FC thickness and bony trabecular orientation of the anterior cruciate ligament (ACL) on both the femoral and tibial sides. Six cadavers were examined (age at death: 73-92 years). Both femoral and tibial insertions of the ACL were harvested and used to make 0.5 mm interval semi-serial sections. Microdigital images were taken and used to measure the maximum thickness of both the uncalcified FC and Calcified FC. Two-dimensional discrete Fourier analysis was also performed to determine the orientation angle and orientation intensity of bony trabeculae directly under the FC. The mean thicknesses of the uncalcified FC at the femoral and tibial insertions were 0.98 mm and 0.49 mm, respectively. The mean thicknesses of the calcified FC were 0.47 mm and 0.38 mm, respectively. There was a significant difference in the uncalcified FC thickness, but not in the calcified FC thickness. The bony trabeculae showed a prominent orientation parallel to the ACL fiber on both sides, but it was more intense on the tibial side than on the femoral side. The trabecular orientation was prominent in the proximal-posterior part of the femoral side and in the anteromedial part of the tibial side, suggesting that mechanical stress is greater in these parts. The dominant bony trabecular angle was 69.0° on the femoral side and 59.8° on the tibial side when the tidemark was set at 0°. These findings suggest that the femoral side receives multidirectional stresses, while relatively unidirectional stress is loaded on the tibial side. Furthermore, stress is considered to be greater in the proximal-posterior part of the femoral insertion and in the anteromedial part of the tibial insertion. At the time of ACL reconstruction, attention should be paid to assign a robust portion of the graft to the posterior part of the femoral insertion and anteromedial part of the tibial insertion. Clin. Anat., 33:988-996, 2020. © 2019 Wiley Periodicals, Inc.

Vibration of osteoblastic cells using a novel motion-control platform does not acutely alter cytosolic calcium, but desensitizes subsequent responses to extracellular ATP

Low-magnitude high-frequency mechanical vibration induces biological responses in many tissues. Like many cell types, osteoblasts respond rapidly to certain forms of mechanostimulation, such as fluid shear, with transient elevation in the concentration of cytosolic free calcium ([Ca²⁺ ]_i ). However, it is not known whether vibration of osteoblastic cells also induces acute elevation in [Ca²⁺ ]_i . To address this question, we built a platform for vibrating live cells that is compatible with microscopy and microspectrofluorometry, enabling us to observe immediate responses of cells to low-magnitude high-frequency vibrations. The horizontal vibration system was mounted on an inverted microscope, and its mechanical performance was evaluated using optical tracking and accelerometry. The platform was driven by a sinusoidal signal at 20-500 Hz, producing peak accelerations from 0.1 to 1 g. Accelerometer-derived displacements matched those observed optically within 10%. We then used this system to investigate the effect of acceleration on [Ca²⁺ ]_i in rodent osteoblastic cells. Cells were loaded with fura-2, and [Ca²⁺ ]_i was monitored using microspectrofluorometry and fluorescence ratio imaging. No acute changes in [Ca²⁺ ]_i or cell morphology were detected in response to vibration over the range of frequencies and accelerations studied. However, vibration did attenuate Ca²⁺ transients generated subsequently by extracellular ATP, which activates P2 purinoceptors and has been implicated in mechanical signaling in bone. In summary, we developed and validated a motion-control system capable of precisely delivering vibrations to live cells during real-time microscopy. Vibration did not elicit acute elevation of [Ca²⁺ ]_i , but did desensitize responses to later stimulation with ATP.

Spontaneous cellular vibratory motions of osteocytes are regulated by ATP and spectrin network

Vibration at high frequency has been demonstrated to be anabolic for bone and embedded osteocytes. The response of osteocytes to vibration is frequency-dependent, but the mechanism remains unclear. Our previous computational study using an osteocyte finite element model has predicted a resonance effect involving in the frequency-dependent response of osteocytes to vibration. However, the cellular spontaneous vibratory motion of osteocytes has not been confirmed. In the present study, the cellular vibratory motions (CVM) of osteocytes were recorded by a custom-built digital holographic microscopy and quantitatively analyzed. The roles of ATP and spectrin network in the CVM of osteocytes were studied. Results showed the MLO-Y4 osteocytes displayed dynamic vibratory motions with an amplitude of ~80 nm, which is relied both on the ATP content and spectrin network. Spectrum analysis showed several frequency peaks in CVM of MLO-Y4 osteocytes at 30 Hz, 39 Hz, 83 Hz and 89 Hz. These peak frequencies are close to the commonly used effective frequencies in animal training and in-vitro cell experiments, and show a correlation with the computational predictions of the osteocyte finite element model. These results implicate that osteocytes are dynamic and the cellular dynamic motion is involved in the cellular mechanotransduction of vibration.

Mechanobiological modulation of in situ and in vivo osteocyte calcium oscillation by acoustic radiation force

The biological effect of ultrasound on bone regeneration has been well documented, yet the underlying mechanotransduction mechanism is largely unknown. In relation to the mechanobiological modulation of the cytoskeleton and Ca²⁺ influx by short-term focused acoustic radiation force (FARF), the current study aimed to visualize and quantify Ca²⁺ oscillations in real-time of in situ and in vivo osteocytes in response to focused low-intensity pulsed ultrasound (FLIPUS). For in situ studies, fresh mice calvaria were subjected to FLIPUS stimulation at 0.05, 0.2, 0.3, and 0.7 W. For the in vivo study, 3-month-old C57BL/6J Ai38/Dmp1-Cre mice were subjected to FLIPUS at 0.15, 1, and 1.5 W. As observed via real-time confocal imaging, in situ FLIPUS led to more than 80% of cells exhibiting Ca²⁺ oscillations at 0.3-0.7 W and led to a higher number of Ca²⁺ spikes with larger values at >0.3 W. In vivo FLIPUS at 1-1.5 W led to more than 90% of cells exhibiting Ca²⁺ oscillations. Higher FLIPUS energies led to larger Ca²⁺ spike magnitudes. In conclusion, this study provided a pilot study of both in situ and in vivo osteocytic Ca²⁺ oscillations under noninvasive FARF, which aids further exploration of the mechanosensing mechanism of the controlled bone cell motility response to the stimulus.

Cell Condensation Triggers the Differentiation of Osteoblast Precursor Cells to Osteocyte-Like Cells

Though the three-dimensional (3D) in vitro culture system has received attention as a powerful tool for conducting biological research, in vitro bone formation and osteocyte differentiation studies have mostly been based on results obtained using two-dimensional (2D) culture systems. Here, we introduced a rotatory culture system to fabricate 3D spheroids, using mouse osteoblast precursor cells. These spheroids, incubated for 2 days without chemical induction by osteogenic supplements, exhibited notably up-regulated osteocyte marker levels; osteoblast marker levels were down-regulated, as compared to those of the conventional 2D monolayer model. The cell condensation achieved with the 3D spheroid structure triggered a greater level of differentiation of osteoblast precursor cells into osteocyte-like cells than that observed during chemical induction. Our study might imply that osteoblasts proliferate and become condensed at the targeted bone remodeling site, because of which osteoblasts achieved the capability to differentiate into osteocytes in vivo.

Talin is required to increase stiffness of focal molecular complex in its early formation process

For cellular adaptation in mechanical environments, it is important to consider transmission of forces from the outside to the inside of cells via a focal molecular complex. The focal molecular complex, which consists of integrin, talin, vinculin and actin, is known to form in response to a force applied via the extra-cellular matrix (ECM). In the early formation process of the complex, the complex-actin connection is reinforced. These structural changes of the nascent complex result in an increase in its mechanical integrity and overall stiffness, possibly leading to the maturation of the nascent complex by enhancing force transmission. In this study, we hypothesized that the complex component talin is a crucial factor in increasing the stiffness of the nascent complex. To test the hypothesis, we used atomic force microscopy (AFM) to measure the stiffness of the nascent complex using a probe coated with fibronectin. Stiffness measurements were conducted for intact and talin knocked-down cells. Our results demonstrated that talin was required to increase the stiffness of the nascent complex, which could be caused by the reinforced connection between the complex and actin filaments mediated by talin.

Canalicular Junctions in the Osteocyte Lacuno-Canalicular Network of Cortical Bone

The osteocyte lacuno-canalicular network (LCN) is essential for bone remodeling because osteocytes regulate cell recruitment. This has been proposed to occur through liquid-flow-induced shear forces in the canaliculi. Models of the LCN have thus far assumed that it contains canaliculi connecting the osteocyte lacunae. However, here, we reveal that enlarged spaces occur at places where several canaliculi cross; we name these spaces canalicular junctions. We characterize them in detail within mice cortical bone using synchrotron nanotomography at two length scales, with 50 and 130 nm voxel size, and show that canalicular junctions occur at a density similar to that of osteocyte lacunae and that canalicular junctions tend to cluster. Through confocal laser scanning microscopy, we show that canalicular junctions are widespread as we have observed them in cortical bone from several species, even though the number density of the canalicular junctions was not universal. Fluid flow simulations of a simple model system with and without a canalicular junction clearly show that liquid mass transport and flow velocities are altered by the presence of canalicular junctions. We suggest that these canalicular junctions may play an important role in osteocyte communication and possibly also in canalicular fluid flow. Therefore, we believe that they constitute an important component in the bone osteocyte network.

Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling

Intracellular calcium ([Ca²⁺]_i) oscillation is a fundamental signaling response of cartilage cells under mechanical loading or osmotic stress. Chondrocytes are usually considered as nonexcitable cells with no spontaneous [Ca²⁺]_i signaling. This study proved that chondrocytes can exhibit robust spontaneous [Ca²⁺]_i signaling without explicit external stimuli. The intensity of [Ca²⁺]_i peaks from individual chondrocytes maintain a consistent spatiotemporal pattern, acting as a unique "fingerprint" for each cell. Statistical analysis revealed lognormal distributions of the temporal parameters of [Ca²⁺]_i peaks, as well as strong linear correlations between their means and sds. Based on these statistical findings, we hypothesized that the spontaneous [Ca²⁺]_i peaks may result from an autocatalytic process and that [Ca²⁺]_i oscillation is controlled by a threshold-regulating mechanism. To test these 2 mechanisms, we established a multistage biophysical model by assuming the spontaneous [Ca²⁺]_i signaling of chondrocytes as a combination of deterministic and stochastic processes. The theoretical model successfully explained the lognormal distribution of the temporal parameters and the fingerprint feature of [Ca²⁺]_i peaks. In addition, by using antagonists for 10 pathways, we revealed that the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes requires the presence of extracellular Ca²⁺, and that the PLC-inositol 1,4,5-trisphosphate pathway, which controls the release of calcium from the endoplasmic reticulum, can affect the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes. The purinoceptors and transient receptor potential vanilloid 4 channels on the plasma membrane also play key roles in the spontaneous [Ca²⁺]_i signaling of chondrocytes. In contrast, blocking the T-type or L-type voltage-gated calcium channel promoted the spontaneous calcium signaling. This study represents a systematic effort to understand the features and initiation mechanisms of spontaneous [Ca²⁺]_i signaling in chondrocytes, which are critical for chondrocyte mechanobiology.-Zhou, Y., Lv, M., Li, T., Zhang, T., Duncan, R., Wang, L., Lu, X. L. Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling.

Analysis of Ca2+ response of osteocyte network by three-dimensional time-lapse imaging in living bone

Osteocytes form a three-dimensional (3D) cellular network within the mineralized bone matrix. The cellular network has important roles in mechanosensation and mechanotransduction related to bone homeostasis. We visualized the embedded osteocyte network in chick calvariae and observed the flow-induced Ca²⁺ signaling in osteocytes using 3D time-lapse imaging. In response to the flow, intracellular Ca²⁺ ([Ca²⁺]_i) significantly increased in developmentally mature osteocytes in comparison with young osteocytes in the bone matrix. To investigate the differences in response between young and developmentally mature osteocytes in detail, we evaluated the expression of osteocyte-related genes using the osteocyte-like cell line MLO-Y4, which was 3D-cultured within type I collagen gels. We found that the c-Fos, Cx43, Panx3, Col1a1, and OCN mRNA levels significantly increased on day 15 in comparison with day 7. These findings indicate that developmentally mature osteocytes are more responsive to mechanical stress than young osteocytes and have important functions in bone formation and remodeling.

Mechanical loading disrupts osteocyte plasma membranes which initiates mechanosensation events in bone

Osteocytes sense loading in bone, but their mechanosensation mechanisms remain poorly understood. Plasma membrane disruptions (PMD) develop with loading under physiological conditions in many cell types (e.g., myocytes, endothelial cells). These PMD foster molecular flux across cell membranes that promotes tissue adaptation, but this mechanosensation mechanism had not been explored in osteocytes. Our goal was to investigate whether PMD occur and initiate consequent mechanotransduction in osteocytes during physiological loading. We found that osteocytes experience PMD during in vitro (fluid flow) and in vivo (treadmill exercise) mechanical loading, in proportion to the level of stress experienced. In fluid flow studies, osteocyte PMD preferentially formed with rapid as compared to gradual application of loading. In treadmill studies, osteocyte PMD increased with loading in weight bearing locations (tibia), but this trend was not seen in non-weight bearing locations (skull). PMD initiated osteocyte mechanotransduction including calcium signaling and expression of c-fos, and repair rates of these PMD could be enhanced or inhibited pharmacologically to alter downstream mechanotransduction and osteocyte survival. PMD may represent a novel mechanosensation pathway in bone and a target for modifying skeletal adaptation signaling in osteocytes. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:653-662, 2018.

Quantitative observations on cytoskeleton changes of osteocytes at different cell parts using digital holographic microscopy

Cytoskeletons such as F-actin have different distributions in different cell parts and they are the cause of different degrees of cell collapse when the F-actin is disrupted. It is challenging to use conventional methods such as fluorescence microscopy and atomic force microscopy to conduct real-time and three-dimensional observations on the dynamic processes at different cell parts due to the slow measuring speed and the need for live-cell staining. In this study, the morphological variations of different bone cell parts caused by F-actin disruption are dynamically measured by using digital holographic microscopy (DHM). We separately analyze local parameters (cell height and cell width) and global parameters (cell projected area and cell volume) of cells to address variations of specific cell areas and quantify the changing process of the whole cell. We found significant differences in temporal variations of both local and global cell parameters between the cell body and cell process, which is consistent with the qualitative observation by fluorescence staining. Our study not only validates the unique ability of DHM to simultaneously investigate the dynamic process at different cell parts, but also provides sufficient experimental bases for exploring the mechanism for F-actin disruption.

Disruption of collagen/apatite alignment impairs bone mechanical function in osteoblastic metastasis induced by prostate cancer

Prostate cancer (PCa) frequently metastasizes to the bone, generally inducing osteoblastic alterations that increase bone brittleness. Although there is growing interest in the management of the physical capability of patients with bone metastasis, the mechanism underlying the impairment of bone mechanical function remains unclear. The alignment of both collagen fibrils and biological apatite (BAp) c-axis, together with bone mineral density, is one of the strongest contributors to bone mechanical function. In this study, we analyzed the bone microstructure of the mouse femurs with and without PCa cell inoculation. Histological assessment revealed that the bone-forming pattern in the PCa-bearing bone was non-directional, resulting in a spongious structure, whereas that in the control bone was unidirectional and layer-by-layer, resulting in a compact lamellar structure. The degree of preferential alignment of collagen fibrils and BAp, which was evaluated by quantitative polarized microscopy and microbeam X-ray diffraction, respectively, were significantly lower in the PCa-bearing bone than in the control bone. Material parameters including Young's modulus and toughness, measured by the three-point bending test, were simultaneously decreased in the PCa-bearing bone. Specifically, there was a significant positive correlation between the degree of BAp c-axis orientation and Young's modulus. In conclusion, the impairment of mechanical function in the PCa-bearing bone is attributable to disruption of the anisotropic microstructure of bone in multiple phases. This is the first report demonstrating that cancer bone metastasis induces disruption of the collagen/BAp alignment in long bones, thereby impairing their mechanical function.

Spatial heterogeneity in the canalicular density of the osteocyte network in human osteons

Osteocytes interconnect with each other forming an intricate cell network within the mineralized bone matrix. One important function of the osteocyte network is the mechano-regulation of bone remodeling, where a possible mechanism includes the fluid flow through the porosity housing the cell network - the osteocyte lacuno-canalicular network (OLCN). In our study the OLCN in human osteons was three-dimensionally imaged with the aim to obtain a quantitative description of the canalicular density and spatial variations of this quantity within osteons. The topology of the OLCN was determined by first staining the bone samples with rhodamine, then imaging the OLCN with confocal laser scanning microscopy and finally using image analysis to obtain a skeletonized version of the network for further analysis. In total 49 osteons were studied from the femoral cortical bone of four different middle-aged healthy women. The mean canalicular density given as length of the canaliculi in a unit volume was 0.074 ± 0.015 μm/μm³ (corresponding to 74 km/cm³). No correlation was found between the canalicular density and neither the size of the osteon nor the volume fraction occupied by osteocyte lacunae. Within osteons the canalicular density varied substantially with larger regions without any network. On average the canalicular density decreases when moving from the Haversian canal outwards towards the cement line. We hypothesize that a decrease in accessible canaliculi with tissue age as a result of micropetrosis can reduce the local mechanosensitivity of the bone. Systematic future studies on age- and disease-related changes on the topology of the OLCN have to demonstrate the diagnostic potential of the presented characterization method.

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accurate three-dimensional representation of the canalicular network topology using confocal microscopy and image analysis

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quantitative network description with an average canalicular density of 74 km per cubic centimeter in healthy human osteons

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substantial variability of the canalicular density within osteons with 10% of the bone having a density twice the average

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in osteons large regions lacking an accessible network were found

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radial decrease of the canalicular density from the Haversian canal towards the cement line

Implications of exercise-induced adipo-myokines in bone metabolism

Physical inactivity has been recognized, by the World Health Organization as the fourth cause of death (5.5 % worldwide). On the contrary, physical activity (PA) has been associated with improved quality of life and decreased risk of several diseases (i.e., stroke, hypertension, myocardial infarction, obesity, malignancies). Bone turnover is profoundly affected from PA both directly (load degree is the key determinant for BMD) and indirectly through the activation of several endocrine axes. Several molecules, secreted by muscle (myokines) and adipose tissues (adipokines) in response to exercise, are involved in the fine regulation of bone metabolism in response to the energy availability. Furthermore, bone regulates energy metabolism by communicating its energetic needs thanks to osteocalcin which acts on pancreatic β-cells and adipocytes. The beneficial effects of exercise on bone metabolism depends on the intermittent exposure to myokines (i.e., irisin, IL-6, LIF, IGF-I) which, instead, act as inflammatory/pro-resorptive mediators when chronically elevated; on the other hand, the reduction in the circulating levels of adipokines (i.e., leptin, visfatin, adiponectin, resistin) sustains these effects as well as improves the whole-body metabolic status. The aim of this review is to highlight the newest findings about the exercise-dependent regulation of these molecules and their role in the fine regulation of bone metabolism.

A new perspective on mechanisms governing skeletal complications in type 1 diabetes

This review focuses on bone mechanobiology in type 1 diabetes (T1D), an area of research on diabetes-associated skeletal complications that is still in its infancy. We first provide a brief overview of the deleterious effects of diabetes on the skeleton and of the knowledge gained from studies with rodent models of T1D. Second, we discuss two specific hallmarks of T1D, low insulin and high glucose, and address the extent to which they affect skeletal health. Third, we highlight the mechanosensitive nature of bone tissue and the importance of mechanical loading for bone health. We also summarize recent advances in bone mechanobiology that implicate osteocytes as the mechanosensors and major regulatory cells in the bone. Finally, we discuss recent evidence indicating that the diabetic bone is "deaf" to mechanical loading and that osteocytes are central players in mechanisms that lead to bone loss in T1D.

From mechanical stimulus to bone formation: A review

Bone is a remarkable tissue that can respond to external stimuli. The importance of mechanical forces on the mass and structural development of bone has long been accepted. This adaptation behaviour is very complex and involves multidisciplinary concepts, and significant progress has recently been made in understanding this process. In this review, we describe the state of the art studies in this area and highlight current insights while simultaneously clarifying some basic yet essential topics related to the origin of mechanical stimulus in bone, the biomechanisms associated with mechanotransduction, the nature of physiological bone stimuli and the test systems most commonly used to study the mechanical stimulation of bone.

Experimental studies of bone mechanoadaptation: bridging in vitro and in vivo studies with multiscale systems

Despite advancements in technology and science over the last century, the mechanisms underlying Wolff's law-bone structure adaptation in response to physical stimuli-remain poorly understood, limiting the ability to effectively treat and prevent skeletal diseases. A challenge to overcome in the study of the underlying mechanisms of this principle is the multiscale nature of mechanoadaptation. While there exist in silico systems that are capable of studying across these scales, experimental studies are typically limited to interpretation at a single dimension or time point. For instance, studies of single-cell responses to defined physical stimuli offer only a limited prediction of the whole bone response, while overlapping pathways or compensatory mechanisms complicate the ability to isolate critical targets in a whole animal model. Thus, there exists a need to develop experimental systems capable of bridging traditional experimental approaches and informing existing multiscale theoretical models. The purpose of this article is to review the process of mechanoadaptation and inherent challenges in studying its underlying mechanisms, discuss the limitations of traditional experimental systems in capturing the many facets of this process and highlight three multiscale experimental systems which bridge traditional approaches and cover relatively understudied time and length scales in bone adaptation.

Altered mechanical environment of bone cells in an animal model of short- and long-term osteoporosis

Alterations in bone tissue composition during osteoporosis likely disrupt the mechanical environment of bone cells and may thereby initiate a mechanobiological response. It has proved challenging to characterize the mechanical environment of bone cells in vivo, and the mechanical environment of osteoporotic bone cells is not known. The objective of this research is to characterize the local mechanical environment of osteocytes and osteoblasts from healthy and osteoporotic bone in a rat model of osteoporosis. Using a custom-designed micromechanical loading device, we apply strains representative of a range of physical activity (up to 3000 με) to fluorescently stained femur samples from normal and ovariectomized rats. Confocal imaging was simultaneously performed, and digital image correlation techniques were applied to characterize cellular strains. In healthy bone tissue, osteocytes experience higher maximum strains (31,028 ± 4213 με) than osteoblasts (24,921 ± 3,832 με), whereas a larger proportion of the osteoblast experiences strains >10,000 με. Most interestingly, we show that osteoporotic bone cells experience similar or higher maximum strains than healthy bone cells after short durations of estrogen deficiency (5 weeks), and exceeded the osteogenic strain threshold (10,000 με) in a similar or significantly larger proportion of the cell (osteoblast, 12.68% vs. 13.68%; osteocyte, 15.74% vs. 5.37%). However, in long-term estrogen deficiency (34 weeks), there was no significant difference between bone cells in healthy and osteoporotic bone. These results suggest that the mechanical environment of bone cells is altered during early-stage osteoporosis, and that mechanobiological responses act to restore the mechanical environment of the bone tissue after it has been perturbed by ovariectomy.