Physiological mechanisms and therapeutic potential of bone mechanosensing

Runx2 and Polycystins in Bone Mechanotransduction: Challenges for Therapeutic Opportunities

Bone mechanotransduction is a critical process during skeletal development in embryogenesis and organogenesis. At the same time, the type and level of mechanical loading regulates bone remodeling throughout the adult life. The aberrant mechanosensing of bone cells has been implicated in the development and progression of bone loss disorders, but also in the bone-specific aspect of other clinical entities, such as the tumorigenesis of solid organs. Novel treatment options have come into sight that exploit the mechanosensitivity of osteoblasts, osteocytes, and chondrocytes to achieve efficient bone regeneration. In this regard, runt-related transcription factor 2 (Runx2) has emerged as a chief skeletal-specific molecule of differentiation, which is prominent to induction by mechanical stimuli. Polycystins represent a family of mechanosensitive proteins that interact with Runx2 in mechano-induced signaling cascades and foster the regulation of alternative effectors of mechanotransuction. In the present narrative review, we employed a PubMed search to extract the literature concerning Runx2, polycystins, and their association from 2000 to March 2024. The keywords stated below were used for the article search. We discuss recent advances regarding the implication of Runx2 and polycystins in bone remodeling and regeneration and elaborate on the targeting strategies that may potentially be applied for the treatment of patients with bone loss diseases.

IFT80 promotes early bone healing of tooth sockets through the activation of TAZ/RUNX2 pathway

Intraflagellar transport (IFT) proteins have been reported to regulate cell growth and differentiation as the essential functional component of primary cilia. The effects of IFT80 on early bone healing of extraction sockets have not been well studied. To investigate whether deletion of Ift80 in alveolar bone-derived mesenchymal stem cells (aBMSCs) affected socket bone healing, we generated a mouse model of specific knockout of Ift80 in Prx1 mesenchymal lineage cells (Prx1Cre ;IFT80f/f ). Our results demonstrated that deletion of IFT80 in Prx1 lineage cells decreased the trabecular bone volume, ALP-positive osteoblastic activity, TRAP-positive osteoclastic activity, and OSX-/COL I-/OCN-positive areas in tooth extraction sockets of Prx1Cre ; IFT80f/f mice compared with IFT80f/f littermates. Furthermore, aBMSCs from Prx1Cre ; IFT80f/f mice showed significantly decreased osteogenic markers and downregulated migration and proliferation capacity. Importantly, the overexpression of TAZ recovered significantly the expressions of osteogenic markers and migration capacity of aBMSCs. Lastly, the local administration of lentivirus for TAZ enhanced the expression of RUNX2 and OSX and promoted early bone healing of extraction sockets from Prx1Cre ; IFT80f/f mice. Thus, IFT80 promotes osteogenesis and early bone healing of tooth sockets through the activation of TAZ/RUNX2 pathway.

Genetic interactions between polycystin-1 and Wwtr1 in osteoblasts define a novel mechanosensing mechanism regulating bone formation in mice

Molecular mechanisms transducing physical forces in the bone microenvironment to regulate bone mass are poorly understood. Here, we used mouse genetics, mechanical loading, and pharmacological approaches to test the possibility that polycystin-1 and Wwtr1 have interdependent mechanosensing functions in osteoblasts. We created and compared the skeletal phenotypes of control Pkd1flox/+;Wwtr1flox/+, Pkd1Oc-cKO, Wwtr1Oc-cKO, and Pkd1/Wwtr1Oc-cKO mice to investigate genetic interactions. Consistent with an interaction between polycystins and Wwtr1 in bone in vivo, Pkd1/Wwtr1Oc-cKO mice exhibited greater reductions of BMD and periosteal MAR than either Wwtr1Oc-cKO or Pkd1Oc-cKO mice. Micro-CT 3D image analysis indicated that the reduction in bone mass was due to greater loss in both trabecular bone volume and cortical bone thickness in Pkd1/Wwtr1Oc-cKO mice compared to either Pkd1Oc-cKO or Wwtr1Oc-cKO mice. Pkd1/Wwtr1Oc-cKO mice also displayed additive reductions in mechanosensing and osteogenic gene expression profiles in bone compared to Pkd1Oc-cKO or Wwtr1Oc-cKO mice. Moreover, we found that Pkd1/Wwtr1Oc-cKO mice exhibited impaired responses to tibia mechanical loading in vivo and attenuation of load-induced mechanosensing gene expression compared to control mice. Finally, control mice treated with a small molecule mechanomimetic, MS2 that activates the polycystin complex resulted in marked increases in femoral BMD and periosteal MAR compared to vehicle control. In contrast, Pkd1/Wwtr1Oc-cKO mice were resistant to the anabolic effects of MS2. These findings suggest that PC1 and Wwtr1 form an anabolic mechanotransduction signaling complex that mediates mechanical loading responses and serves as a potential novel therapeutic target for treating osteoporosis.

Distinct differences between calvarial and long bone osteocytes in cell morphologies, gene expression and aging responses

Osteocytes are the terminally differentiated bone cells resulted from bone formation. Although there are two distinct processes of bone formation, intramembranous and endochondral ossifications contributing to the formation of calvarial and long bones, it is not clear whether the distinct pathways determine the differences between calvaria and femoral cortical bone derived osteocytes. In the present study, we employed confocal structured illumination microscopy and mRNA-sequencing analysis to characterize the morphologic and transcriptomic expression of osteocytes from murine calvaria and mid-shaft femoral cortical bone. Structured illumination microscopy and geometric modelling showed round shaped and irregularly scattered calvarial osteocytes compared to spindle shaped and orderly arrayed cortical osteocytes. mRNA-sequencing analysis indicated different transcriptomic profiles between calvarial and cortical osteocytes and provided evidence that mechanical response of osteocytes may contribute to geometrical differences. Furthermore, transcriptomic analysis showed that these two groups of osteocytes come from distinct pathways with 121 ossification-related genes differentially expressed. Analysis of correlation between ossification and osteocyte geometries via a Venn diagram showed that several genes related to ossification, cytoskeleton organization and dendrite development were differentially expressed between calvarial and cortical osteocytes. Finally, we demonstrated that aging disrupted the organization of dendrites and cortical osteocytes but had no significant effects on calvarial osteocytes. Together, we conclude that calvarial and cortical osteocytes are different in various aspects, which is probably the consequence of their distinct pathways of ossification.

Pathophysiological mechanism of acute bone loss after fracture

BACKGROUND

Acute bone loss after fracture is associated with various effects on the complete recovery process and a risk of secondary fractures among patients. Studies have reported similarities in pathophysiological mechanisms involved in acute bone loss after fractures and osteoporosis. However, given the silence nature of bone loss and bone metabolism complexities, the actual underlying pathophysiological mechanisms have yet to be fully elucidated.

AIM OF REVIEW

To elaborate the latest findings in basic research with a focus on acute bone loss after fracture. To briefly highlight potential therapeutic targets and current representative drugs. To arouse researchers' attention and discussion on acute bone loss after fracture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Bone loss after fracture is associated with immobilization, mechanical unloading, blood supply damage, sympathetic nerve regulation, and crosstalk between musculoskeletals among other factors. Current treatment strategies rely on regulation of osteoblasts and osteoclasts, therefore, there is a need to elucidate on the underlying mechanisms of acute bone loss after fractures to inform the development of efficacious and safe drugs. In addition, attention should be paid towards ensuring long-term skeletal health.

Effects of running a marathon on sclerostin and parathyroid hormone concentration in males aged over 50

The aim of our study was to verify whether running a marathon (32nd Wroclaw Marathon) was associated with changes in sclerostin and intact PTH (iPTH) concentration in middle-aged males. We enrolled 33 males who completed the marathon race. Blood samples were taken 60 minutes before (V1), immediately after (V2), and 7 days after the run (V3). The mean serum sclerostin concentration was 42.4 ± 10.8 pmol/L at V1, increased to 62.9 ± 12.6 pmol/L at V2 (t= -11.206; p < 0.001) and returned to baseline in V3 (t = 8.344; p < 0.001, V3 vs. V2). A similar trend was recorded for iPTH (t= -7.440; p < 0.001, for V2 vs. V1; t = 6.229; p < 0.001, for V3 vs. V2), at V3, iPTH levels remained significantly higher than V1 (t= -2.759; p = 0.010). The results of our study suggest that, in middle-aged males, running a marathon affects skeletal metabolism by activating two counteracting mechanisms, although temporarily overlapping: first, by a sudden inhibition of bone formation, through induction sclerostin expression and, secondly, by a long-lasting induction of PTH, which also guarantees the maintenance of adequate circulating levels of calcium. The net effect would be the maintenance of adequately high levels of circulating calcium to be used for neuromuscular activity and muscle contraction.

Piezo1-mediated mechanotransduction promotes entheseal pathological new bone formation in ankylosing spondylitis

OBJECTIVE

The aim of this study was to identify the role of Piezo1-mediated mechanotransduction in entheseal pathological new bone formation and to explore the underlying molecular mechanism.

METHODS

Spinal ligament tissues were collected from 14 patients with ankylosing spondylitis (AS) and 14 non-AS controls and bulk RNA sequencing was conducted. Collagen antibody-induced arthritis models were established to observe pathological new bone formation. Pharmacological inhibition and genetic ablation of Piezo1 was performed in animal models to identify the essential role of Piezo1. Entheseal osteo-chondral lineage cells were collected and in vitro cell culture system was established to study the role and underlying mechanism of Piezo1 in regulation of chondrogenesis, osteogenesis and its own expression.

RESULTS

Piezo1 was aberrantly upregulated in ligaments and entheseal tissues from patients with AS and animal models. Pharmaceutical and genetic inhibition of Piezo1 attenuated while activation of Piezo1 promoted pathological new bone formation. Mechanistically, activation of CaMKII (Calcium/calmodulin dependent protein kinase II) signalling was found essential for Piezo1-mediated mechanotransduction. In addition, Piezo1 was upregulated by AS-associated inflammatory cytokines.

CONCLUSION

Piezo1-mediated mechanotransduction promotes entheseal pathological new bone formation through CaMKII signalling in AS.

Fin ray branching is defined by TRAP+ osteolytic tubules in zebrafish

The shaping of bone structures relies on various cell types and signaling pathways. Here, we use the zebrafish bifurcating fin rays during regeneration to investigate bone patterning. We found that the regenerating fin rays form via two mineralization fronts that undergo an osteoblast-dependent fusion/stitching until the branchpoint, and that bifurcation is not simply the splitting of one unit into two. We identified tartrate-resistant acid phosphatase-positive osteolytic tubular structures at the branchpoints, hereafter named osteolytic tubules (OLTs). Chemical inhibition of their bone-resorbing activity strongly impairs ray bifurcation, indicating that OLTs counteract the stitching process. Furthermore, by testing different osteoactive compounds, we show that the position of the branchpoint depends on the balance between bone mineralization and resorption activities. Overall, these findings provide a unique perspective on fin ray formation and bifurcation, and reveal a key role for OLTs in defining the proximo-distal position of the branchpoint.

Associations of the Geriatric Nutritional Risk Index With Femur Bone Mineral Density and Osteoporosis in American Postmenopausal Women: Data From the National Health and Nutrition Examination Survey

Background

The geriatric nutritional risk index (GNRI) has been used as a significant tool to access the nutritional status of the elderly. However, the relationship between the GNRI and femur bone mineral density (BMD) and the risk of osteoporosis remains unclear in American postmenopausal women.

Objectives

We aimed to explore associations between the GNRI with femur BMD and the risk of osteoporosis in American postmenopausal women.

Methods

We merged the continuous National Health and Nutrition Examination Survey (NHANES) 2005–2006, 2007–2008, 2009–2010, 2013–2014, and 2017–2018 to ensure a large and representative sample, including 3,152 participants. The linear relationship between the GNRI and femur BMD was assessed via a weighted multivariate linear regression model. The odds ratios (ORs) and 95% confidence intervals (95% CIs) for the association between the GNRI and the risk of osteoporosis were assessed by a weighted logistic regression model. Moreover, the nonlinear relationship was also characterized by smooth curve fitting (SCF) and a weighted generalized additive model (GAM).

Results

After adjusting for all covariates, the weighted multivariable linear regression models demonstrated that the GNRI was positively correlated with femur BMD. The weighted logistic regression models demonstrated that each unit of increased GNRI value was associated with a decreased risk of osteoporosis of 4.13%. When categorizing GNRI based on quartiles, ORs between the risk of osteoporosis and the GNRI across quintiles 2, 3, and 4 compared with quintile 1 were 0.5565 (95% CI: 0.4791, 0.6463; P &lt; 0.000001), 0.5580 (95% CI: 0.4600, 0.6769; P &lt; 0.000001), and 0.3475 (95% CI: 0.2681, 0.4505; P &lt; 0.000001). The trends similar to the above were also observed in SCF and GAM.

Conclusion

This study indicated that nutritional status, represented by the GNRI, was positively associated with femur BMD and negatively associated with the risk of osteoporosis in American postmenopausal women. The GNRI may be a good tool to identify American postmenopausal women who need further bone health nutritional support.

Mechanical regulation of bone remodeling

Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.

Regulation of bone health through physical exercise: Mechanisms and types

Osteoporosis, characterized by bone mineral density reduction, bone mass loss, increased bone fragility, and propensity to fractures, is a common disease in older individuals and one of the most serious health problems worldwide. The imbalance between osteoblasts and osteoclasts results in the predominance of bone resorption and decreased bone formation. In recent years, it has been found that regular and proper exercise not only helps prevent the occurrence of osteoporosis but also adds benefits to osteoporosis therapy; accordingly, bone homeostasis is closely associated with mechanical stress and the intricate crosstalk between osteoblasts and osteoclasts. In this review, we summarize the mechanisms of exercise on osteoporosis and provide new proposals for the prevention and treatment of osteoporosis.

Mechanical loading activates the YAP/TAZ pathway and chemokine expression in the MLO-Y4 osteocyte-like cell line

Osteocytes are mechanosensitive cells that control bone remodeling in response to mechanical loading. To date, specific signaling pathways modulated by mechanical loading in osteocytes are not well understood. Yes associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), the main effectors of the Hippo pathway, are reported to play a role in mechanotransduction and during osteoblastogenesis. Here, we hypothesized that YAP/TAZ signaling mediates osteocyte mechanosensing to target genes of the bone remodeling process. We aimed to investigate the contribution of YAP/TAZ in modulating the gene expression in an osteocyte-like cell line MLO-Y4. We developed a 3D osteocyte compression culture model from an MLO-Y4 osteocyte cell line embedded in concentrated collagen hydrogel. 3D-mechanical loading led to the increased expression of mechanosensitive genes and a subset of chemokines, including M-csf, Cxcl1, Cxcl2, Cxcl3, Cxcl9, and Cxcl10. The transcription regulators YAP and TAZ translocated to the nucleus and upregulated their target genes and proteins. RNAseq analysis revealed that YAP/TAZ knockdown mediated the regulation of several genes including osteocyte dendrite formation. Use of YAP/TAZ knockdown partially blunted the increase in M-csf and Cxcl3 levels in response to MLO-Y4 compression. These findings demonstrate that YAP/TAZ signaling is required for osteocyte-like cell mechano-transduction, regulates the gene expression profiles and controls chemokine expression.

The mechanosensory and mechanotransductive processes mediated by ion channels and the impact on bone metabolism: A systematic review

Mechanical environments were associated with alterations in bone metabolism. Ion channels present on bone cells are indispensable for bone metabolism and can be directly or indirectly activated by mechanical stimulation. This review aimed to discuss the literature reporting the mechanical regulatory effects of ion channels on bone cells and bone tissue. An electronic search was conducted in PubMed, Embase and Web of Science. Studies about mechanically induced alteration of bone cells and bone tissue by ion channels were included. Ion channels including TRP family channels, Ca²⁺ release-activated Ca²⁺ channels (CRACs), Piezo1/2 channels, purinergic receptors, NMDA receptors, voltage-sensitive calcium channels (VSCCs), TREK2 potassium channels, calcium- and voltage-dependent big conductance potassium (BK_Ca) channels, small conductance, calcium-activated potassium (SK_Ca) channels and epithelial sodium channels (ENaCs) present on bone cells and bone tissue participate in the mechanical regulation of bone development in addition to contributing to direct or indirect mechanotransduction such as altered membrane potential and ionic flux. Physiological (beneficial) mechanical stimulation could induce the anabolism of bone cells and bone tissue through ion channels, but abnormal (harmful) mechanical stimulation could also induce the catabolism of bone cells and bone tissue through ion channels. Functional expression of ion channels is vital for the mechanotransduction of bone cells. Mechanical activation (opening) of ion channels triggers ion influx and induces the activation of intracellular modulators that can influence bone metabolism. Therefore, mechanosensitive ion channels provide new insights into therapeutic targets for the treatment of bone-related diseases such as osteopenia and aseptic implant loosening.

Cilia kinases in skeletal development and homeostasis

Primary cilia are dynamic compartments that regulate multiple aspects of cellular signaling. The production, maintenance, and function of cilia involve more than 1000 genes in mammals, and their mutations disrupt the ciliary signaling which manifests in a plethora of pathological conditions-the ciliopathies. Skeletal ciliopathies are genetic disorders affecting the development and homeostasis of the skeleton, and encompass a broad spectrum of pathologies ranging from isolated polydactyly to lethal syndromic dysplasias. The recent advances in forward genetics allowed for the identification of novel regulators of skeletogenesis, and revealed a growing list of ciliary proteins that are critical for signaling pathways implicated in bone physiology. Among these, a group of protein kinases involved in cilia assembly, maintenance, signaling, and disassembly has emerged. In this review, we summarize the functions of cilia kinases in skeletal development and disease, and discuss the available and upcoming treatment options.

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.

Piezo Channels: Awesome Mechanosensitive Structures in Cellular Mechanotransduction and Their Role in Bone

Piezo channels are mechanosensitive ion channels located in the cell membrane and function as key cellular mechanotransducers for converting mechanical stimuli into electrochemical signals. Emerged as key molecular detectors of mechanical forces, Piezo channels' functions in bone have attracted more and more attention. Here, we summarize the current knowledge of Piezo channels and review the research advances of Piezo channels' function in bone by highlighting Piezo1's role in bone cells, including osteocyte, bone marrow mesenchymal stem cell (BM-MSC), osteoblast, osteoclast, and chondrocyte. Moreover, the role of Piezo channels in bone diseases is summarized.

The effects of locomotion on bone marrow mesenchymal stem cell fate: insight into mechanical regulation and bone formation

Bone marrow mesenchymal stem cells (BMSCs) refer to a heterogeneous population of cells with the capacity for self-renewal. BMSCs have multi-directional differentiation potential and can differentiate into chondrocytes, osteoblasts, and adipocytes under specific microenvironment or mechanical regulation. The activities of BMSCs are closely related to bone quality. Previous studies have shown that BMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. Thus, a goal of this review is to discuss how these ubiquious signals arising from mechanical stimulation are perceived by BMSCs and then how the cells respond to them. Studies in recent years reported a significant effect of locomotion on the migration, proliferation and differentiation of BMSCs, thus, contributing to our bone mass. This regulation is realized by the various intersecting signaling pathways including RhoA/Rock, IFG, BMP and Wnt signalling. The mechanoresponse of BMSCs also provides guidance for maintaining bone health by taking appropriate exercises. This review will summarize the regulatory effects of locomotion/mechanical loading on BMSCs activities. Besides, a number of signalling pathways govern MSC fate towards osteogenic or adipocytic differentiation will be discussed. The understanding of mechanoresponse of BMSCs makes the foundation for translational medicine.

Bone-to-Brain: A Round Trip in the Adaptation to Mechanical Stimuli

Besides the classical ones (support/protection, hematopoiesis, storage for calcium, and phosphate) multiple roles emerged for bone tissue, definitively making it an organ. Particularly, the endocrine function, and in more general terms, the capability to sense and integrate different stimuli and to send signals to other tissues, has highlighted the importance of bone in homeostasis. Bone is highly innervated and hosts all nervous system branches; bone cells are sensitive to most of neurotransmitters, neuropeptides, and neurohormones that directly affect their metabolic activity and sensitivity to mechanical stimuli. Indeed, bone is the principal mechanosensitive organ. Thanks to the mechanosensing resident cells, and particularly osteocytes, mechanical stimulation induces metabolic responses in bone forming (osteoblasts) and bone resorbing (osteoclasts) cells that allow the adaptation of the affected bony segment to the changing environment. Once stimulated, bone cells express and secrete, or liberate from the entrapping matrix, several mediators (osteokines) that induce responses on distant targets. Brain is a target of some of these mediator [e.g., osteocalcin, lipocalin2, sclerostin, Dickkopf-related protein 1 (Dkk1), and fibroblast growth factor 23], as most of them can cross the blood-brain barrier. For others, a role in brain has been hypothesized, but not yet demonstrated. As exercise effectively modifies the release and the circulating levels of these osteokines, it has been hypothesized that some of the beneficial effects of exercise on brain functions may be associated to such a bone-to-brain communication. This hypothesis hides an interesting clinical clue: may well-addressed physical activities support the treatment of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases?

Role of the polycystins as mechanosensors of extracellular stiffness

Polycystin-1 (PC-1) is a transmembrane protein, encoded by the PKD1 gene, mutated in autosomal dominant polycystic kidney disease (ADPKD). This common genetic disorder, characterized by cyst formation in both kidneys, ultimately leading to renal failure, is still waiting for a definitive treatment. The overall function of PC-1 and the molecular mechanism responsible for cyst formation are slowly coming to light, but they are both still intensively studied. In particular, PC-1 has been proposed to act as a mechanosensor, although the precise signal that activates the mechanical properties of this protein has been long debated and questioned. In this review, we report studies and evidence of PC-1 function as a mechanosensor, starting from the peculiarity of its structure, through the long journey that progressively shed new light on the potential initiating events of cystogenesis, concluding with the description of PC-1 recently shown ability to sense the mechanical stimuli provided by the stiffness of the extracellular environment. These new findings have potentially important implications for the understanding of ADPKD pathophysiology and potentially for designing new therapies.NEW & NOTEWORTHY Polycystin-1 has recently emerged as a possible receptor able to sense extracellular stiffness and to negatively control the cellular actomyosin contraction machinery. Here, we revisit a large body of literature on autosomal dominant polycystic kidney disease providing a new possible mechanistic view on the topic.

Co-culture with Endothelial Progenitor Cells promotes the Osteogenesis of Bone Mesenchymal Stem Cells via the VEGF-YAP axis in high-glucose environments

Patients with type 2 diabetes mellitus (T2DM) have a high risk of fracture and experience poor bone healing. In recent years, bone mesenchymal stem cells (BMSCs) and endothelial progenitor cells (EPCs) have become the most commonly used cells in cell therapy and tissue engineering. In this study, we found that high glucose levels had a negative effect on the differentiation of BMSCs and EPCs. Considering that EPCs-BMSCs sheets can provide endothelial cells and osteoblastic cells, we transplanted cell sheets into T2DM rats with bilateral skull defects. The outcomes of the in vivo study revealed that EPCs-BMSCs sheets promoted ossification, which was verified by micro-CT and immunohistochemistry (IHC) analyses. Furthermore, we detected the VEGF content in the culture supernatant using an enzyme-linked immunosorbent assay (ELISA). The results showed that the BMSCs co-cultured with EPCs presented a higher level of VEGF than other cells. To assess the differentiation and migration of BMSCs exposed to VEGF, ALP staining, scratch assay and qRT-PCR analysis were performed. In addition, we used immunofluorescence and western blotting analysis to further explore the related mechanisms. The results showed that cells cultured with VEGF had a stronger actin cytoskeleton and a greater amount of nuclear and total YAP than cells cultured without VEGF. Taken together, our results indicate that co-culture with EPCs could promote the osteogenesis of BMSCs partially via VEGF. Furthermore, YAP and F-actin play important roles in this process.

Osteoporosis: Mechanism, Molecular Target and Current Status on Drug Development

CDATA[Osteoporosis is a pathological loss of bone mass due to an imbalance in bone remodeling where osteoclast-mediated bone resorption exceeds osteoblast-mediated bone formation resulting in skeletal fragility and fractures. Anti-resorptive agents, such as bisphosphonates and SERMs, and anabolic drugs that stimulate bone formation, including PTH analogues and sclerostin inhibitors, are current treatments for osteoporosis. Despite their efficacy, severe side effects and loss of potency may limit the long term usage of a single drug. Sequential and combinational use of current drugs, such as switching from an anabolic to an anti-resorptive agent, may provide an alternative approach. Moreover, there are novel drugs being developed against emerging new targets such as Cathepsin K and 17β-HSD2 that may have less side effects. This review will summarize the molecular mechanisms of osteoporosis, current drugs for osteoporosis treatment, and new drug development strategies.

The regulation of RANKL by mechanical force

Receptor activator of nuclear factor-κB ligand (RANKL) is a key mediator of osteoclast differentiation and bone resorption. Osteoblast-lineage cells including osteoblasts and osteocytes express RANKL, which is regulated by several different factors, including hormones, cytokines, and mechanical forces. In vivo and in vitro analyses have demonstrated that various types of mechanosensing proteins on the cell membrane (i.e. mechanosensors) and intracellular mechanosignaling proteins play essential roles in the differentiation and functions of osteoblasts, osteoclasts, and osteocytes via soluble factors, such as sclerostin, Wnt ligands, and RANKL. This section provides an overview of the in vivo and in vitro evidence for the regulation of RANKL expression by mechanosensing and mechanotransduction.

Exosomal miR-365a-5p derived from HUC-MSCs regulates osteogenesis in GIONFH through the Hippo signaling pathway

The pathogenesis of glucocorticoid (GC)-induced osteonecrosis of the femoral head (GIONFH) is still disputed, and abnormal bone metabolism caused by GCs may be an important factor. In vitro, Cell Counting Kit-8 (CCK-8) and 5-ethynyl-2′-deoxyuridine (EdU) staining were used to evaluate cellular proliferation, and western blotting was used to investigate osteogenesis. In vivo, we used micro-computed tomography (micro-CT), H&E staining, Masson staining, and immunohistochemistry (IHC) analysis to evaluate the impact of exosomes. In addition, the mechanism by which exosomes regulate osteogenesis through the miR-365a-5p/Hippo signaling pathway was investigated using RNA sequencing (RNA-seq), luciferase reporter assays, fluorescence in situ hybridization (FISH), and western blotting. The results of western blotting verified that the relevant genes in osteogenesis, including BMP2, Sp7, and Runx2, were upregulated. RNA-seq and qPCR of the exosome and Dex-treated exosome groups showed that miR-365a-5p was upregulated in the exosome group. Furthermore, we verified that miR-365a-5p promoted osteogenesis by targeting SAV1. Additional in vivo experiments revealed that exosomes prevented GIONFH in a rat model, as shown by micro-CT scanning and histological and IHC analysis. We concluded that exosomal miR-365a-5p was effective in promoting osteogenesis and preventing the development of GIONFH via activation of the Hippo signaling pathway in rats.

Pharmacological Regulation of Primary Cilium Formation Affects the Mechanosensitivity of Osteocytes

Primary cilia are responsible for sensing mechanical loading in osteocytes. However, the underlying working mechanism of cilia remains elusive. An osteocyte model is necessary to reveal the role of cilia. Furthermore, the osteocyte model should be with upregulated or downregulated primary cilium expression. Herein, we used a pharmacological method to regulate the cilium formation of osteocytes. After screening, some pharmacological agents can regulate the cilium formation of osteocytes. We performed a CCK-8 assay to analyze the optimal working conditions of the drugs for MLO-Y4 cells. The agents include chloral hydrate (CH), Gd³⁺, Li⁺, and rapamycin. The expression of cilia affects the cellular functions, including mechanosensitivity, of osteocytes. Results showed that CH downregulated the cilium formation and ciliogenesis of osteocytes. In addition, Gd³⁺, Li⁺, and rapamycin upregulated the cilium expression of osteocytes. Moreover, the cilium expression positively correlated with the mechanosensitivity of osteocytes. This work reveals the role of primary cilia in the mechanosensing of osteocytes.

Bone's Response to Mechanical Loading in Aging and Osteoporosis: Molecular Mechanisms

Mechanotransduction is pivotal in the maintenance of homeostasis in different tissues and involves multiple cell signaling pathways. In bone, mechanical stimuli regulate the balance between bone formation and resorption; osteocytes play a central role in this regulation. Dysfunctions in mechanotransduction signaling or in osteocytes response lead to an imbalance in bone homeostasis. This alteration is very relevant in some conditions such as osteoporosis and aging. Both are characterized by increased bone weakness due to different causes, for example, the increase of osteocyte apoptosis that cause an alteration of fluid space, or the alteration of molecular pathways. There are intertwined yet very different mechanisms involved among the cell-intrinsic effects of aging on bone, the cell-intrinsic and tissue-level effects of estrogen/androgen withdrawal on bone, and the effects of reduced mechanical loading on bone, which are all involved to some degree in how aged bone fails to respond properly to stress/strain compared to younger bone. This review aims at clarifying how the cellular and molecular pathways regulated and induced in bone by mechanical stimulation are altered with aging and in osteoporosis, to highlight new possible targets for antiresorptive or anabolic bone therapeutic approaches.

The kynurenine connection: how exercise shifts muscle tryptophan metabolism and affects energy homeostasis, the immune system, and the brain

Tryptophan catabolism through the kynurenine pathway generates a variety of bioactive metabolites. Physical exercise can modulate kynurenine pathway metabolism in skeletal muscle and thus change the concentrations of select compounds in peripheral tissues and in the central nervous system. Here we review recent advances in our understanding of how exercise alters tryptophan-kynurenine metabolism in muscle and its subsequent local and distal effects. We propose that the effects of kynurenine pathway metabolites on skeletal muscle, adipose tissue, immune system, and the brain suggest that some of these compounds could qualify as exercise-induced myokines. Indeed, some of the more recently discovered biological activities for kynurenines include many of the best-known benefits of exercise: improved energy homeostasis, promotion of an anti-inflammatory environment, and neuroprotection. Finally, by considering the tissue expression of the different membrane and cytosolic receptors for kynurenines, we discuss known and potential biological activities for these tryptophan metabolites.

A distinct bone phenotype in ADPKD patients with end-stage renal disease

Autosomal dominant polycystic kidney disease (ADPKD) is among the most common hereditary nephropathies. Low bone turnover osteopenia has been reported in mice with conditional deletion of the PKD1 and PKD2 genes in osteoblasts, and preliminary clinical data also suggest suppressed bone turnover in patients with ADPKD. The present study compared the bone phenotype between patients with end stage renal disease (ESRD) due to ADPKD and controls with ESRD due to other causes. Laboratory parameters of bone mineral metabolism (fibroblast growth factor 23 and sclerostin), bone turnover markers (bone alkaline phosphatase, tartrate-resistant acid phosphatase 5b) and bone mineral density (BMD, by dual energy x-ray absorptiometry, DXA) were assessed in 518 patients with ESRD, including 99 with ADPKD. Bone histomorphometry data were available in 71 patients, including 10 with ADPKD. Circulating levels of bone alkaline phosphatase were significantly lower in patients with ADPKD (17.4 vs 22.6 ng/mL), as were histomorphometric parameters of bone formation. Associations between ADPKD and parameters of bone formation persisted after adjustment for classical determinants including parathyroid hormone, age, and sex. BMD was higher in skeletal sites rich in cortical bone in patients with ADPKD compared to non-ADPKD patients (Z-score midshaft radius -0.04 vs -0.14; femoral neck -0.72 vs -1.02). Circulating sclerostin levels were significantly higher in ADPKD patients (2.20 vs 1.84 ng/L). In conclusion, patients with ESRD due to ADPKD present a distinct bone and mineral phenotype, characterized by suppressed bone turnover, better preserved cortical BMD, and high sclerostin levels.

Verification of osteoblast differentiation on airborne-particle abrasion, large-grit, acid-etched surface of titanium implants regulated by yes-associated protein and transcriptional coactivator with PDZ-binding motif

Although airborne-particle abrasion, large-grit, acid-etched (SLA) surface technology can promote implant osseointegration; its mechanism remains unclear. By preparing the SLA titanium (Ti) plate (SLA Ti) and Polished Ti plate (Polished Ti), this experiment investigates the expression and distribution of the Yes-associated protein (YAP) and transcriptional coactivator with the PDZ-binding motif (TAZ) in MC3T3-E1 cells. In addition, gene YAP and TAZ silencing on the SLA Ti was conducted to observe changes in the osteoblast differentiation markers, runt-related transcription factor-2 (Runx2) and bone sialoprotein (BSP). The results demonstrated that SLA Ti surface microtopography could induce YAP/TAZ's transfer from the cytoplasm to the nuclei of MC3T3-E1 cells. The expression of YAP/TAZ increased in terms of mRNA and protein. After silencing the YAP/TAZ genes, Runx2 and BSP decreased, suggesting that YAP/TAZ plays an important regulatory role in this process. Meanwhile, the results also showed that SLA microtopography enhanced the expression of integrins α1, α2, and β1. After silencing the integrin α1, α2, and β1 genes, YAP and TAZ decreased in terms of mRNA and protein. Therefore, this experiment was the first to confirm that SLA surface microtopography facilitates osteoblast differentiation by regulating YAP/TAZ and confirms that the process can be related to integrins α1, α2, and β1.

Polycystins and Mechanotransduction in Human Disease

Alterations in the process of mechanotransduction have been implicated in the pathogenesis of several diseases such as genetic diseases, osteoporosis, cardiovascular anomalies, and cancer. Several studies over the past twenty years have demonstrated that polycystins (polycystin-1, PC1; and polycystin-2, PC2) respond to changes of extracellular mechanical cues, and mediate pathogenic mechanotransduction and cyst formation in kidney cells. However, recent reports reveal the emergence of polycystins as key proteins that facilitate the transduction of mechano-induced signals in various clinical entities besides polycystic kidney disease, such as cancer, cardiovascular defects, bone loss, and deformations, as well as inflammatory processes like psoriasis. Herewith, we discuss data from recent studies that establish this role with potential clinical utility.

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

The role of transient receptor potential polycystin channels in bone diseases

Transient receptor potential (TRP) channels are cation channels which act as molecular sensors that enable cells to detect and respond to a plethora of mechanical and environmental cues. TRPs are involved in various physiological processes, such as mechanosensation, non-inception and thermosensation, while mutations in genes encoding them can lead to pathological conditions, called "channelopathies". The subfamily of transient receptor potential polycystins (TRPPs), Polycystin 1 (PC1, TRPP1) and Polycystin 2 (PC2, TRPP2), act as mechanoreceptors, sensing external mechanical forces, including strain, stretch and fluid shear stress, triggering a cascade of signaling pathways involved in osteoblastogenesis and ultimately bone formation. Both in vitro studies and research on animal models have already identified their implications in bone homeostasis. However, uncertainty veiling the role of polycystins (PCs) in bone disease urges studies to elucidate further their role in this field. Mutations in TRPPs have been related to autosomal polycystic kidney disease (ADKPD) and research groups try to identify their role beyond their well-established contribution in kidney disease. Such an elucidation would be beneficial for identifying signaling pathways where polycystins are involved in bone diseases related to exertion of mechanical forces such as osteoporosis, osteopenia and craniosynostosis. A better understanding of the implications of TRPPs in bone diseases would possibly lay the cornerstone for effective therapeutic schemes.

Key developments that impacted the field of mechanobiology and mechanotransduction

Advances in mechanobiology have evolved through insights from multiple disciplines including structural engineering, biomechanics, vascular biology, and orthopaedics. In this paper, we reviewed the impact of key reports related to the study of applied loads on tissues and cells and the resulting signal transduction pathways. We addressed how technology has helped advance the burgeoning field of mechanobiology (over 33,600 publications from 1970 to 2016). We analyzed the impact of critical ideas and then determined how these concepts influenced the mechanobiology field by looking at the citation frequency of these reports as well as tracking how the overall number of citations within the field changed over time. These data allowed us to understand how a key publication, idea, or technology guided or enabled the field. Initial observations of how forces acted on bone and soft tissues stimulated the development of computational solutions defining how forces affect tissue modeling and remodeling. Enabling technologies, such as cell and tissue stretching, compression, and shear stress devices, allowed more researchers to explore how deformation and fluid flow affect cells. Observation of the cell as a tensegrity structure and advanced methods to study genetic regulation in cells further advanced knowledge of specific mechanisms of mechanotransduction. The future of the field will involve developing gene and drug therapies to simulate or augment beneficial load regimens in patients and in mechanically conditioning organs for implantation. Here, we addressed a history of the field, but we limited our discussions to advances in musculoskeletal mechanobiology, primarily in bone, tendon, and ligament tissues. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:605-619, 2018.

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.

Increased YAP Activation Is Associated With Hepatic Cyst Epithelial Cell Proliferation in ARPKD/CHF

Autosomal recessive polycystic kidney disease/congenital hepatic fibrosis (ARPKD/CHF) is a rare but fatal genetic disease characterized by progressive cyst development in the kidneys and liver. Liver cysts arise from aberrantly proliferative cholangiocytes accompanied by pericystic fibrosis and inflammation. Yes-associated protein (YAP), the downstream effector of the Hippo signaling pathway, is implicated in human hepatic malignancies such as hepatocellular carcinoma, cholangiocarcinoma, and hepatoblastoma, but its role in hepatic cystogenesis in ARPKD/CHF is unknown. We studied the role of the YAP in hepatic cyst development using polycystic kidney (PCK) rats, an orthologous model of ARPKD, and in human ARPKD/CHF patients. The liver cyst wall epithelial cells (CWECs) in PCK rats were highly proliferative and exhibited expression of YAP. There was increased expression of YAP target genes, Ccnd1 (cyclin D1) and Ctgf (connective tissue growth factor), in PCK rat livers. Extensive expression of YAP and its target genes was also detected in human ARPKD/CHF liver samples. Finally, pharmacological inhibition of YAP activity with verteporfin and short hairpin (sh) RNA-mediated knockdown of YAP expression in isolated liver CWECs significantly reduced their proliferation. These data indicate that increased YAP activity, possibly through dysregulation of the Hippo signaling pathway, is associated with hepatic cyst growth in ARPKD/CHF.

Polycystin-1 interacts with TAZ to stimulate osteoblastogenesis and inhibit adipogenesis

The molecular mechanisms that transduce the osteoblast response to physical forces in the bone microenvironment are poorly understood. Here, we used genetic and pharmacological experiments to determine whether the polycystins PC1 and PC2 (encoded by Pkd1 and Pkd2) and the transcriptional coactivator TAZ form a mechanosensing complex in osteoblasts. Compound-heterozygous mice lacking 1 copy of Pkd1 and Taz exhibited additive decrements in bone mass, impaired osteoblast-mediated bone formation, and enhanced bone marrow fat accumulation. Bone marrow stromal cells and osteoblasts derived from these mice showed impaired osteoblastogenesis and enhanced adipogenesis. Increased extracellular matrix stiffness and application of mechanical stretch to multipotent mesenchymal cells stimulated the nuclear translocation of the PC1 C-terminal tail/TAZ (PC1-CTT/TAZ) complex, leading to increased runt-related transcription factor 2-mediated (Runx2-mediated) osteogenic and decreased PPARγ-dependent adipogenic gene expression. Using structure-based virtual screening, we identified a compound predicted to bind to PC2 in the PC1:PC2 C-terminal tail region with helix:helix interaction. This molecule stimulated polycystin- and TAZ-dependent osteoblastogenesis and inhibited adipogenesis. Thus, we show that polycystins and TAZ integrate at the molecular level to reciprocally regulate osteoblast and adipocyte differentiation, indicating that the polycystins/TAZ complex may be a potential therapeutic target to increase bone mass.

Mechanical microenvironment regulation of age-related diseases involving degeneration of human skeletal and cardiovascular systems

Age-related diseases involving degeneration of human skeletal and cardiovascular systems are now critical problems worldwide. The current review focuses on a common pathophysiological association between primary osteoporosis and vascular calcification, and reviews the mechanical response of bone cells and vascular cells to mechanical stress, as well as the coordination mechanism for intercellular signaling. With aging, calcium is lost from bones but deposited in the cardiovascular system. Bone metabolism-related molecules, such as alkaline phosphatase, matrix Gla protein, osteocalcin, osteopontin, and collagen type I; inflammatory cytokines, such as interleukin-1, -6, and tumor necrosis factor; and lipid metabolism related molecules, such as oxidized low density lipoprotein; mediate signaling in primary osteoporosis and vascular calcification. The mechanical microenvironment is a common pathophysiological basis for primary osteoporosis and vascular calcification. Mobilization of calcium from bone to vessel determines the regression rate, which could be controlled using a mechanical microenvironment. We highlight several issues: (1) linked features between primary osteoporosis and vascular calcification, and detailed changes of the mechanical microenvironment in degenerative bone or blood vessels, (2) signaling coordination mechanism between bone and vascular wall cells, and (3) calcium translocation mechanism. The degree to which these issues can be solved will help develop prevention and treatment strategies for age-related regression.

Osteocyte Mechanobiology

PURPOSE OF REVIEW

Over the past decades, osteocytes have emerged as mechano-sensors of bone and master regulators of bone homeostasis. This article summarizes latest research and progress made in understanding osteocyte mechanobiology and critically reviews tools currently available to study these cells.

RECENT FINDINGS

Whereas increased mechanical forces promote bone formation, decrease loading is always associated with bone loss and skeletal fragility. Recent studies identified cilia, integrins, calcium channels, and G-protein coupled receptors as important sensors of mechanical forces and Ca²⁺ and cAMP signaling as key effectors. Among transcripts regulated by mechanical forces, sclerostin and RANKL have emerged as potential therapeutic targets for disuse-induced bone loss. In this paper, we review the mechanisms by which osteocytes perceive and transduce mechanical cues and the models available to study mechano-transduction. Future directions of the field are also discussed.

Early anti-inflammatory intervention ameliorates axial disease in the proteoglycan-induced spondylitis mouse model of ankylosing spondylitis

Background

Ankylosing spondylitis (AS) is characterised by immune-mediated arthritis and osteoproliferation, ultimately leading to joint ankylosis. Whether inflammation is necessary for osteoproliferation is controversial, fuelled by the unclear efficacy of anti-inflammatory treatments on radiographic progression. In proteoglycan-induced spondylitis (PGISp), a mouse model of AS, inflammation is the prerequisite for osteoproliferation as osteoproliferation was only observed following inflammation-driven intervertebral disc (IVD) destruction. We hypothesised that early intervention with a potent anti-inflammatory therapy would protect IVD integrity and consequently alter disease progression.

Methods

PGISp mice received vehicle or a combination of etanercept (ETN) plus prednisolone (PRD) therapy for 2 or 6 weeks initiated at an early disease stage. Peripheral arthritis was scored longitudinally. Spinal disease was assessed using a semi-quantitative histological scoring regimen including inflammation, joint destruction and excessive tissue formation.

Results

ETN + PRD therapy significantly delayed the onset of peripheral arthritis. IVD integrity was significantly protected when treatment was commenced in early disease. Six-weeks of treatment resulted in trends towards reductions in intervertebral joint damage and excessive tissue formation. IVD score distribution was dichotomized, likely reflecting the extent of axial disease at initiation of therapy. In the sub-group of mice with high IVD destruction scores, ETN + PRD treatment significantly reduced IVD destruction severity, inflammation and bone erosion and reduced cartilage damage and excessive tissue formation.

Conclusions

Early intervention with anti-inflammatory treatment not only improved inflammatory symptoms but also ameliorated structural damage of spine in PGISp mice. This preclinical observation suggests that early anti-inflammatory intervention may slow radiographic progression in AS patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-017-1600-7) contains supplementary material, which is available to authorized users.

Sclerostin─A Debutant on the Autosomal Dominant Polycystic Kidney Disease Scene?

Introduction

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disease originating from a mutation in genes encoding polycystin 1 and 2. Recent evidence suggests that these polycystins mediate mechanosensation not only in the primary cilium of kidney cells but also in bone cells. The Wnt/β-catenin signaling pathway plays a central role in mechanotransduction in osteocytes. Mechanical unloading causes the upregulation of the Wnt inhibitor sclerostin. We tested the hypothesis that ADPKD associates with higher circulating sclerostin levels.

Methods

In this observational, cross-sectional study, circulating levels of sclerostin and other laboratory parameters of mineral and bone disease, including intact parathyroid hormone (PTH), calcium, phosphate, magnesium, 25(OH) D-vitamin, 1,25 (OH)₂ D-vitamin, and bone specific alkaline phosphatase (BALP) were assessed in 100 patients with end-stage renal disease recruited from an ongoing longitudinal cohort study in Stockholm, Sweden.

Results

Patients with ADPKD had higher sclerostin levels and lower BALP levels as compared to patients with other primary renal disease. In multivariate analysis, ADPKD associated with circulating sclerostin levels, independent of the established determinants including age, gender, body mass index, diabetes, phosphate, PTH, and 1,25 (OH)₂ D-vitamin.

Discussion

Circulating sclerostin levels are increased in ADPKD, possibly reflecting impaired mechanosensation. The clinical relevance of this finding, especially with regard to bone health, remains to be investigated. Our finding draws attention to the etiology of kidney disease as an important, yet neglected, confounder of the association between renal failure and mineral and bone disease.

Primary Cilia and Intraflagellar Transport Proteins in Bone and Cartilage

Primary cilia, present on most mammalian cells, function as a sensor to sense the environment change and transduce signaling. Loss of primary cilia causes a group of human pleiotropic syndromes called Ciliopathies. Some of the ciliopathies display skeletal dysplasias, implying the important role of primary cilia in skeletal development and homeostasis. Emerging evidence has shown that loss or malfunction of primary cilia or ciliary proteins in bone and cartilage is associated with developmental and function defects. Intraflagellar transport (IFT) proteins are essential for cilia formation and/or function. In this review, we discuss the role of primary cilia and IFT proteins in the development of bone and cartilage, as well as the differentiation and mechanotransduction of mesenchymal stem cells, osteoblasts, osteocytes, and chondrocytes. We also include the role of primary cilia in tooth development and highlight the current advance of primary cilia and IFT proteins in the pathogenesis of cartilage diseases, including osteoarthritis, osteosarcoma, and chondrosarcoma.