Primary cilia mediate mechanotransduction through control of ATP-induced Ca2+ signaling in compressed chondrocytes

Up-regulated HIF-2α contributes to the Osteoarthritis development through mediating the primary cilia loss

BACKGROUNDS

Up-regulated HIF-2α (hypoxia induced factor 2) had been demonstrated to contribute to Osteoarthritis (OA) development via inducing the expression of matrix-degrading enzymes. However, the HIF-2α also could promote primary cilia loss through HIF-2α/AURKA (Aurora kinase A)/NEDD9 pathway. And the primary cilia dysfunction is another characteristic of the OA. Thus, we investigated here whether the HIF-2α also contributes the OA development through mediating the primary cilia loss.

METHODS

The primary chondrocytes were isolated from the experimental OA mice induced by destabilization of the medial meniscus (DMM). Chondrocytes were cultured under normoxia (21% O₂) or hypoxia (2% O₂) conditions. The HIF-1α and HIF-2α expressions were assessed by western blot. The cilia formation was counted by immuno-staining the acetylated tubulin. The contribution of HIF-1α or HIF-2α to the primary cilia loss was assessed by knocking-down the HIF-1α or HIF-2α individually. The HIF-2α/AURKA/NEDD9 pathway was validated through over-expressing or knocking-down specific components of the pathway and then counting the primary cilia number. Finally, the pathway was further confirmed in the OA mice.

RESULTS

Hypoxia could induce the expression of both HIF-1α and HIF-2α, and also reduce the number of primary cilia on the chondrocytes isolated from the experimental OA mice. Knocking-down or over-expressing HIF-1α or HIF-2α individually showed that the HIF-2α could induce the primary cilia reduction rather than the HIF-1α. Manipulating the HIF-2α expression could positively affect the AURKA and NEDD9 expression. Manipulating the AURKA and NEDD9 expressions could reverse the function of HIF-2α on primary cilia. In the mice, knocking-down both AURKA and NEDD9 could alleviate the OA development significantly.

CONCLUSION

Up-regulated HIF-2α contributes to the Osteoarthritis development through mediating the primary cilia loss, which might be developed as therapeutic targets for OA treatment.

Mechanical loading inhibits cartilage inflammatory signalling via an HDAC6 and IFT-dependent mechanism regulating primary cilia elongation

OBJECTIVE

Physiological mechanical loading reduces inflammatory signalling in numerous cell types including articular chondrocytes however the mechanism responsible remains unclear. This study investigates the role of chondrocyte primary cilia and associated intraflagellar transport (IFT) in the mechanical regulation of interleukin-1β (IL-1β) signalling.

DESIGN

Isolated chondrocytes and cartilage explants were subjected to cyclic mechanical loading in the presence and absence of the cytokine IL-1β. Nitric oxide (NO) and prostaglandin E₂ (PGE₂) release were used to monitor IL-1β signalling whilst Sulphated glycosaminoglycan (sGAG) release provided measurement of cartilage degradation. Measurements were made of HDAC6 activity and tubulin polymerisation and acetylation. Effects on primary cilia were monitored by confocal and super resolution microscopy. Involvement of IFT was analysed using ORPK cells with hypomorphic mutation of IFT88.

RESULTS

Mechanical loading suppressed NO and PGE₂ release and prevented cartilage degradation. Loading activated HDAC6 and disrupted tubulin acetylation and cilia elongation induced by IL-1β. HDAC6 inhibition with tubacin blocked the anti-inflammatory effects of loading and restored tubulin acetylation and cilia elongation. Hypomorphic mutation of IFT88 reduced IL-1β signalling and abolished the anti-inflammatory effects of loading indicating the mechanism is IFT-dependent. Loading reduced the pool of non-polymerised tubulin which was replicated by taxol which also mimicked the anti-inflammatory effects of mechanical loading and prevented cilia elongation.

CONCLUSIONS

This study reveals that mechanical loading suppresses inflammatory signalling, partially dependent on IFT, by activation of HDAC6 and post transcriptional modulation of tubulin.

Cartilage biomechanics: A key factor for osteoarthritis regenerative medicine

Osteoarthritis (OA) is a joint disorder that is highly extended in the global population. Several researches and therapeutic strategies have been probed on OA but without satisfactory long-term results in joint replacement. Recent evidences show how the cartilage biomechanics plays a crucial role in tissue development. This review describes how physics alters cartilage and its extracellular matrix (ECM); and its role in OA development. The ECM of the articular cartilage (AC) is widely involved in cartilage turnover processes being crucial in regeneration and joint diseases. We also review the importance of physicochemical pathways following the external forces in AC. Moreover, new techniques probed in cartilage tissue engineering for biomechanical stimulation are reviewed. The final objective of these novel approaches is to create a cellular implant that maintains all the biochemical and biomechanical properties of the original tissue for long-term replacements in patients with OA.

Physiological dynamic compression regulates central energy metabolism in primary human chondrocytes

Chondrocytes use the pathways of central metabolism to synthesize molecular building blocks and energy for cartilage homeostasis. An interesting feature of the in vivo chondrocyte environment is the cyclical loading generated in various activities (e.g., walking). However, it is unknown whether central metabolism is altered by mechanical loading. We hypothesized that physiological dynamic compression alters central metabolism in chondrocytes to promote production of amino acid precursors for matrix synthesis. We measured the expression of central metabolites (e.g., glucose, its derivatives, and relevant co-factors) for primary human osteoarthritic chondrocytes in response to 0-30 minutes of compression. To analyze the data, we used principal components analysis and ANOVA-simultaneous components analysis, as well as metabolic flux analysis. Compression-induced metabolic responses consistent with our hypothesis. Additionally, these data show that chondrocyte samples from different patient donors exhibit different sensitivity to compression. Most importantly, we find that grade IV osteoarthritic chondrocytes are capable of synthesizing non-essential amino acids and precursors in response to mechanical loading. These results suggest that further advances in metabolic engineering of chondrocyte mechanotransduction may yield novel translational strategies for cartilage repair.

Primary cilia and autophagy interaction is involved in mechanical stress mediated cartilage development via ERK/mTOR axis

AIMS

Biomechanical reactivity is a special property of chondrocytes and mechanical stress can affect the development of cartilage. Primary cilia have been proved a cellular sensory which can detect physical and chemical stimuli extracellular and initiate multiple signaling transduction. Autophagy is an important environmental adaptive mechanism for cells maintenance of homeostasis. The aims of this study were to detect whether there is an interaction between primary cilia and autophagy in the regulation of mechanical stress-mediated cartilage development and to explore the underlying mechanism.

MAIN METHODS

In this study, chondrocytes were treated with cyclic tensile strain (CTS) by the four-point bending system. Chondrocytes viability, proliferation and differentiation capacities were analyzed by western blot and live/dead assays after CTS of different intensities. Meanwhile, primary cilia incidence and length changes, and autophagy expression were detected by immunofluorescence staining. The primary cilia and autophagy interaction regulation and the underlying mechanism were detected by immunofluorescence double staining and western blot.

KEY FINDINGS

Mechanical stress could affect chondrocytes proliferation, phenotype and viability in an intensity dependent manner. The incidence and length of primary cilia as well as autophagy expression could be regulated by CTS. The integrity of primary cilia structure is vital for mechanical stress regulated ERK/mTOR signaling transduction and autophagy expression in chondrocyte.

SIGNIFICANCE

These findings indicate that mechanical stress could affect the interaction between primary cilia and autophagy and help to reveal the underlying mechanism of stress regulated cartilage development.

Chasing Chimeras - The elusive stable chondrogenic phenotype

The choice of the best-suited cell population for the regeneration of damaged or diseased cartilage depends on the effectiveness of culture conditions (e.g. media supplements, three-dimensional scaffolds, mechanical stimulation, oxygen tension, co-culture systems) to induce stable chondrogenic phenotype. Herein, advances and shortfalls in in vitro, preclinical and clinical setting of various in vitro microenvironment modulators on maintaining chondrocyte phenotype or directing stem cells towards chondrogenic lineage are critically discussed. Chondrocytes possess low isolation efficiency, limited proliferative potential and rapid phenotypic drift in culture. Mesenchymal stem cells are relatively readily available, possess high proliferation potential, exhibit great chondrogenic differentiation capacity, but they tend to acquire a hypertrophic phenotype when exposed to chondrogenic stimuli. Embryonic and induced pluripotent stem cells, despite their promising in vitro and preclinical data, are still under-investigated. Although a stable chondrogenic phenotype remains elusive, recent advances in in vitro microenvironment modulators are likely to develop clinically- and commercially-relevant therapies in the years to come.

Mesenchymal stem cell mechanotransduction is cAMP dependent and regulated by adenylyl cyclase 6 and the primary cilium

Mechanical loading is a potent stimulus of bone adaptation, requiring the replenishment of the osteoblast from a progenitor population. One such progenitor is the mesenchymal stem cell (MSC), which undergoes osteogenic differentiation in response to oscillatory fluid shear. Yet, the mechanism mediating stem cell mechanotransduction, and thus the potential to target this therapeutically, is poorly understood. In this study, we demonstrate that MSCs utilise cAMP as a second messenger in mechanotransduction, which is required for flow-mediated increases in osteogenic gene expression. Furthermore, we demonstrate that this mechanosignalling is dependent on the primary cilium and the ciliary localised adenylyl cyclase 6. Finally, we also demonstrate that this mechanotransduction mechanism can be targeted therapeutically to enhance cAMP signalling and early osteogenic signalling, mimicking the beneficial effect of physical loading. Our findings therefore demonstrate a novel mechanism of MSC mechanotransduction that can be targeted therapeutically, demonstrating a potential mechanotherapeutic for bone-loss diseases such as osteoporosis.This article has an associated First Person interview with the first author of the paper.

New functions for the proprioceptive system in skeletal biology

Muscle spindles and Golgi tendon organs (GTOs) are two types of sensory receptors that respond to changes in length or tension of skeletal muscles. These mechanosensors have long been known to participate in both proprioception and stretch reflex. Here, we present recent findings implicating these organs in maintenance of spine alignment as well as in realignment of fractured bones. These discoveries have been made in several mouse lines lacking functional mechanosensors in part or completely. In both studies, the absence of functional spindles and GTOs produced a more severe phenotype than that of spindles alone. Interestingly, the spinal curve phenotype, which appeared during peripubertal development, bears resemblance to the human condition adolescent idiopathic scoliosis. This similarity may contribute to the study of the disease by offering both an animal model and a clue as to its aetiology. Moreover, it raises the possibility that impaired proprioceptive signalling may be involved in the aetiology of other conditions. Overall, these new findings expand considerably the scope of involvement of proprioception in musculoskeletal development and function.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

Ciliary signalling in cancer

Although tumours initiate from oncogenic changes in a cancer cell, subsequent tumour progression and therapeutic response depend on interactions between the cancer cells and the tumour microenvironment (TME). The primary monocilium, or cilium, provides a spatially localized platform for signalling by Hedgehog, Notch, WNT and some receptor tyrosine kinase pathways and mechanosensation. Changes in ciliation of cancer cells and/or cells of the TME during tumour development enforce asymmetric intercellular signalling in the TME. Growing evidence indicates that some oncogenic signalling pathways as well as some targeted anticancer therapies induce ciliation, while others repress it. The links between the genomic profile of cancer cells, drug treatment and ciliary signalling in the TME likely affect tumour growth and therapeutic response.

Mechanoadaptation: articular cartilage through thick and thin

The articular cartilage is exquisitely sensitive to mechanical load. Its structure is largely defined by the mechanical environment and destruction in osteoarthritis is the pathophysiological consequence of abnormal mechanics. It is often overlooked that disuse of joints causes profound loss of volume in the articular cartilage, a clinical observation first described in polio patients and stroke victims. Through the 1980s, the results of studies exploiting experimental joint immobilisation supported this. Importantly, this substantial body of work was also the first to describe metabolic changes that resulted in decreased synthesis of matrix molecules, especially sulfated proteoglycans. The molecular mechanisms that underlie disuse atrophy are poorly understood despite the identification of multiple mechanosensing mechanisms in cartilage. Moreover, there has been a tendency to equate cartilage loss with osteoarthritic degeneration. Here, we review the historic literature and clarify the structural, metabolic and clinical features that clearly distinguish cartilage loss due to disuse atrophy and those due to osteoarthritis. We speculate on the molecular sensing pathways in cartilage that may be responsible for cartilage mechanoadaptation.

Mechanical loading induces primary cilia disassembly in tendon cells via TGFβ and HDAC6

This study used isolated human tenocytes to test the hypothesis that cyclic mechanical strain directly stimulates primary cilia disassembly, and to elucidate the mechanisms involved. Cells were seeded onto flexible membranes and strained at 0–3%; 1 Hz, for up to 24 hours. Cilia length and prevalence progressively reduced with increasing strain duration but showed full recovery within 2 hours of strain removal. The response to loading was not influenced by actin organisation as seen in other cell types. However, the loading response could be recreated by treatment with TGFβ. Furthermore, treatment with the HDAC6 inhibitor Tubacin, or a TGFβ receptor inhibitor both prevented strain induced cilia disassembly. These data are the first to describe primary cilia expression in isolated tenocytes, showing that mechanical strain regulates cilia expression independent of changes in tendon extracellular matrix. Furthermore, we show that cilia disassembly is mediated by the activation of TGFβ receptors leading to activation of HDAC6. Previous studies have shown that cilia are required for TGFβ signalling and that tendon mechanosignalling is mediated by TGFβ. The present study therefore suggests a novel feedback mechanism whereby cilia disassembly inhibits prolonged TGFβ activation in response to continuous cyclic loading.

Cilia protein IFT88 regulates extracellular protease activity by optimizing LRP-1-mediated endocytosis

Matrix protease activity is fundamental to developmental tissue patterning and remains influential in adult homeostasis. In cartilage, the principal matrix proteoglycan is aggrecan, the protease-mediated catabolism of which defines arthritis; however, the pathophysiologic mechanisms that drive aberrant aggrecanolytic activity remain unclear. Human ciliopathies exhibit altered matrix, which has been proposed to be the result of dysregulated hedgehog signaling that is tuned within the primary cilium. Here, we report that disruption of intraflagellar transport protein 88 (IFT88), a core ciliary trafficking protein, increases chondrocyte aggrecanase activity in vitro. We find that the receptor for protease endocytosis in chondrocytes, LDL receptor-related protein 1 (LRP-1), is unevenly distributed over the cell membrane, often concentrated at the site of cilia assembly. Hypomorphic mutation of IFT88 disturbs this apparent hot spot for protease uptake, increases receptor shedding, and results in a reduced rate of protease clearance from the extracellular space. We propose that IFT88 and/or the cilium regulates the extracellular remodeling of matrix-independently of Hedgehog regulation-by enabling rapid LRP-1-mediated endocytosis of proteases, potentially by supporting the creation of a ciliary pocket. This result highlights new roles for the cilium's machinery in matrix turnover and LRP-1 function, with potential relevance in a range of diseases.-Coveney, C. R., Collins, I., Mc Fie, M., Chanalaris, A., Yamamoto, K., Wann, A. K. T. Cilia protein IFT88 regulates extracellular protease activity by optimizing LRP-1-mediated endocytosis.

Sending mixed signals: Cilia-dependent signaling during development and disease

Molecular signals are the guiding force of development, imparting direction upon cells to divide, migrate, differentiate, etc. The mechanisms by which a cell can receive and transduce these signals into measurable actions remains a 'black box' in developmental biology. Primary cilia are ubiquitous, microtubule-based organelles that dynamically extend from a cell to receive and process molecular and mechanical signaling cues. In the last decade, this organelle has become increasingly intriguing to the research community due to its ability to act as a cellular antenna, receive and transduce molecular stimuli, and initiate a cellular response. In this review, we discuss the structure of primary cilia, emphasizing how the ciliary components contribute to the transduction of signaling pathways. Furthermore, we address how the cilium integrates these signals and conveys them into cellular processes such as proliferation, migration and tissue patterning. Gaining a deeper understanding of the mechanisms used by primary cilia to receive and integrate molecular signals is essential, as it opens the door for the identification of therapeutic targets within the cilium that could alleviate pathological conditions brought on by aberrant molecular signaling.

The primary cilium as a signaling nexus for growth plate function and subsequent skeletal development

The primary cilium is a solitary, antenna-like sensory organelle with many important roles in cartilage and bone development, maintenance, and function. The primary cilium's potential role as a signaling nexus in the growth plate makes it an attractive therapeutic target for diseases and disorders associated with bone development and maintenance. Many signaling pathways that are mediated by the cilium-such as Hh, Wnt, Ihh/PTHrP, TGFβ, BMP, FGF, and Notch-are also known to influence endochondral ossification, primarily by directing growth plate formation and chondrocyte behavior. Although a few studies have demonstrated that these signaling pathways can be directly tied to the primary cilium, many pathways have yet to be evaluated in context of the cilium. This review serves to bridge this knowledge gap in the literature, as well as discuss the cilium's importance in the growth plate's ability to sense and respond to chemical and mechanical stimuli. Furthermore, we explore the importance of using the appropriate mechanism to study the cilium in vivo and suggest IFT88 deletion is the best available technique. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:533-545, 2018.

Calcium channels in primary cilia

PURPOSE OF REVIEW

Primary cilia have become important organelles implicated in embryonic development, organogenesis, health, and diseases. Although many studies in cell biology have focused on changes in ciliary length or ciliogenesis, the most common readout for evaluating ciliary function is intracellular calcium.

RECENT FINDINGS

Recent tools have allowed us to examine intracellular calcium in more precise locations, that is, the cilioplasm and cytoplasm. Advances in calcium imaging have also allowed us to identify which cilia respond to particular stimuli. Furthermore, direct electrophysiological measurement of ionic currents within a cilium has provided a wealth of information for understanding the sensory roles of primary cilia.

SUMMARY

Calcium imaging and direct measurement of calcium currents demonstrate that primary cilia are sensory organelles that house several types of functional calcium channels. Although intracellular calcium now allows a functional readout for primary cilia, discussions on the relative contributions of the several channel types have just begun. Perhaps, all of these calcium channels are required and necessary to differentiate stimuli in different microenvironments.

Primary cilia: Cell and molecular mechanosensors directing whole tissue function

Primary cilia are immotile, microtubule-based organelles extending from the surface of nearly every mammalian cell. Mechanical stimulation causes deflection of the primary cilium, initiating downstream signaling cascades to the rest of the cell. The cilium forms a unique subcellular microdomain, and defects in ciliary protein composition or physical structure have been associated with a myriad of human pathologies. In this review, we discuss the importance of ciliary mechanotransduction at the cell and tissue level, and how furthering our molecular understanding of primary cilia mechanobiology may lead to therapeutic strategies to treat human diseases.

Basic fibroblast growth factor increases IFT88 expression in chondrocytes

Intraflagellar transport protein 88 (IFT88) is protein crucial for the assembly and maintenance of primary cilia in chondrocytes. Primary cilia regulate mechanical and chemical signals in chondrocytes; however, the effects of cytokines on IFT88 expression and cilia formation and maintenance remain to be elucidated. Therefore, the role of basic fibroblast growth factor (bFGF) on IFT88 expression were examined in the ATDC5 murine chondrocytic line, in order to investigate the signaling pathways involved in this process. bFGF treatment upregulated IFT88 expression in a dose- and time-dependent manner in ATDC5 cells. The effects of bFGF on IFT88 protein expression were suppressed in the presence of the extracellular signal-regulated protein kinase (ERK) inhibitor PD0325901 and the FGF receptor inhibitor BGJ398. In addition, treatment with IFT88-trageting small interfering (si)RNA downregulated the protein expression of IFT88 and ERK, thus suggesting that the ERK signaling pathway may be involved in the regulation of IFT88 expression in ATDC5 cells. bFGF treatment increased the number of ciliated ATDC5 cells and primary cultured chondrocytes. Downregulation of IFT88 expression by PD0325901, BGJ398, or IFT88-targeting siRNA was revealed to reduce the number of ciliated cells. bFGF also upregulated the mRNA and protein expression of IFT88 in primary cultured chondrocytes. In conclusion, the findings of the present study suggested that bFGF may enhance the expression of IFT88, and promote primary cilia development, through the mitogen-activated protein kinase/ERK-mediated pathway in chondrocytes.

Reduced primary cilia length and altered Arl13b expression are associated with deregulated chondrocyte Hedgehog signaling in alkaptonuria

Alkaptonuria (AKU) is a rare inherited disease resulting from a deficiency of the enzyme homogentisate 1,2-dioxygenase which leads to the accumulation of homogentisic acid (HGA). AKU is characterized by severe cartilage degeneration, similar to that observed in osteoarthritis. Previous studies suggest that AKU is associated with alterations in cytoskeletal organization which could modulate primary cilia structure/function. This study investigated whether AKU is associated with changes in chondrocyte primary cilia and associated Hedgehog signaling which mediates cartilage degradation in osteoarthritis. Human articular chondrocytes were obtained from healthy and AKU donors. Additionally, healthy chondrocytes were treated with HGA to replicate AKU pathology (+HGA). Diseased cells exhibited shorter cilia with length reductions of 36% and 16% in AKU and +HGA chondrocytes respectively, when compared to healthy controls. Both AKU and +HGA chondrocytes demonstrated disruption of the usual cilia length regulation by actin contractility. Furthermore, the proportion of cilia with axoneme breaks and bulbous tips was increased in AKU chondrocytes consistent with defective regulation of ciliary trafficking. Distribution of the Hedgehog-related protein Arl13b along the ciliary axoneme was altered such that its localization was increased at the distal tip in AKU and +HGA chondrocytes. These changes in cilia structure/trafficking in AKU and +HGA chondrocytes were associated with a complete inability to activate Hedgehog signaling in response to exogenous ligand. Thus, we suggest that altered responsiveness to Hedgehog, as a consequence of cilia dysfunction, may be a contributing factor in the development of arthropathy highlighting the cilium as a novel target in AKU.

The Proprioceptive System Regulates Morphologic Restoration of Fractured Bones

Successful fracture repair requires restoration of bone morphology and mechanical integrity. Recent evidence shows that fractured bones of neonatal mice undergo spontaneous realignment, dubbed "natural reduction." Here, we show that natural reduction is regulated by the proprioceptive system and improves with age. Comparison among mice of different ages revealed, surprisingly, that 3-month-old mice exhibited more rapid and effective natural reduction than newborns. Fractured bones of null mutants for transcription factor Runx3, lacking functional proprioceptors, failed to realign properly. Blocking Runx3 expression in the peripheral nervous system, but not in limb mesenchyme, recapitulated the null phenotype, as did inactivation of muscles flanking the fracture site. Egr3 knockout mice, which lack muscle spindles but not Golgi tendon organs, displayed a less severe phenotype, suggesting that both receptor types, as well as muscle contraction, are required for this regulatory mechanism. These findings uncover a physiological role for proprioception in non-autonomous regulation of skeletal integrity.

Persistent mTORC1 signaling in cell senescence results from defects in amino acid and growth factor sensing

Mammalian target of rapamycin complex 1 (mTORC1) and cell senescence are intimately linked to each other and to organismal aging. Inhibition of mTORC1 is the best-known intervention to extend lifespan, and recent evidence suggests that clearance of senescent cells can also improve health and lifespan. Enhanced mTORC1 activity drives characteristic phenotypes of senescence, although the underlying mechanisms responsible for increased activity are not well understood. We have identified that in human fibroblasts rendered senescent by stress, replicative exhaustion, or oncogene activation, mTORC1 is constitutively active and resistant to serum and amino acid starvation. This is driven in part by depolarization of senescent cell plasma membrane, which leads to primary cilia defects and a resultant failure to inhibit growth factor signaling. Further, increased autophagy and high levels of intracellular amino acids may act to support mTORC1 activity in starvation conditions. Interventions to correct these phenotypes restore sensitivity to the mTORC1 signaling pathway and cause death, indicating that persistent signaling supports senescent cell survival.

A novel role for primary cilia in airway remodeling

Primary cilia (PC) are solitary cellular organelles that play critical roles in development, homeostasis, and disease pathogenesis by modulating key signaling pathways such as Sonic Hedgehog and calcium flux. The antenna-like shape of PC enables them also to facilitate sensing of extracellular and mechanical stimuli into the cell, and a critical role for PC has been described for mesenchymal cells such as chondrocytes. However, nothing is known about the role of PC in airway smooth muscle cells (ASMCs) in the context of airway remodeling. We hypothesized that PC on ASMCs mediate cell contraction and are thus integral in the remodeling process. We found that PC are expressed on ASMCs in asthmatic lungs. Using pharmacological and genetic methods, we demonstrated that PC are necessary for ASMC contraction in a collagen gel three-dimensional model both in the absence of external stimulus and in response to the extracellular component hyaluronan. Mechanistically, we demonstrate that the effect of PC on ASMC contraction is, to a small extent, due to their effect on Sonic Hedgehog signaling and, to a larger extent, due to their effect on calcium influx and membrane depolarization. In conclusion, PC are necessary for the development of airway remodeling by mediating calcium flux and Sonic Hedgehog signaling.

Identification of Elongated Primary Cilia with Impaired Mechanotransduction in Idiopathic Scoliosis Patients

The primary cilium is an outward projecting antenna-like organelle with an important role in bone mechanotransduction. The capacity to sense mechanical stimuli can affect important cellular and molecular aspects of bone tissue. Idiopathic scoliosis (IS) is a complex pediatric disease of unknown cause, defined by abnormal spinal curvatures. We demonstrate significant elongation of primary cilia in IS patient bone cells. In response to mechanical stimulation, these IS cells differentially express osteogenic factors, mechanosensitive genes, and signaling genes. Considering that numerous ciliary genes are associated with a scoliosis phenotype, among ciliopathies and knockout animal models, we expected IS patients to have an accumulation of rare variants in ciliary genes. Instead, our SKAT-O analysis of whole exomes showed an enrichment among IS patients for rare variants in genes with a role in cellular mechanotransduction. Our data indicates defective cilia in IS bone cells, which may be linked to heterogeneous gene variants pertaining to cellular mechanotransduction.

Zonal variation in primary cilia elongation correlates with localized biomechanical degradation in stress deprived tendon

Tenocytes express primary cilia, which elongate when tendon is maintained in the absence of biomechanical load. Previous work indicates differences in the morphology and metabolism of the tenocytes in the tendon fascicular matrix (FM) and the inter-fascicular matrix (IFM). This study tests the hypothesis that primary cilia in these two regions respond differently to stress deprivation and that this is associated with differences in the biomechanical degradation of the extracellular matrix. Rat tail tendon fascicles were examined over a 7-day period of either stress deprivation or static load. Seven days of stress deprivation induced cilia elongation in both regions. However, elongation was greater in the IFM compared to the FM. Stress deprivation also induced a loss of biomechanical integrity, primarily in the IFM. Static loading reduced both the biomechanical degradation and cilia elongation. The different responses to stress deprivation in the two tendon regions are likely to be important for the aetiology of tendinopathy. Furthermore, these data suggest that primary cilia elongate in response to biomechanical degradation rather than simply the removal of load. This response to degradation is likely to have important consequences for cilia signalling in tendon and as well as in other connective tissues. © 2016 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 34:2146-2153, 2016.

Changes in the osteochondral unit during osteoarthritis: structure, function and cartilage-bone crosstalk

In diarthrodial joints, the articular cartilage, calcified cartilage, and subchondral cortical and trabecular bone form a biocomposite - referred to as the osteochondral unit - that is uniquely adapted to the transfer of load. During the evolution of the osteoarthritic process the compositions, functional properties, and structures of these tissues undergo marked alterations. Although pathological processes might selectively target a single joint tissue, ultimately all of the components of the osteochondral unit will be affected because of their intimate association, and thus the biological and physical crosstalk among them is of great importance. The development of targeted therapies against the osteoarthritic processes in cartilage or bone will, therefore, require an understanding of the state of these joint tissues at the time of the intervention. Importantly, these interventions will not be successful unless they are applied at the early stages of disease before considerable structural and functional alterations occur in the osteochondral unit. This Review describes the changes that occur in bone and cartilage during the osteoarthritic process, and highlights strategies for how this knowledge could be applied to develop new therapeutic interventions for osteoarthritis.

Cartilage-Specific Knockout of the Mechanosensory Ion Channel TRPV4 Decreases Age-Related Osteoarthritis

Osteoarthritis (OA) is a progressive degenerative disease of articular cartilage and surrounding tissues, and is associated with both advanced age and joint injury. Biomechanical factors play a critical role in the onset and progression of OA, yet the mechanisms through which physiologic or pathologic mechanical signals are transduced into a cellular response are not well understood. Defining the role of mechanosensory pathways in cartilage during OA pathogenesis may yield novel strategies or targets for the treatment of OA. The transient receptor potential vanilloid 4 (TRPV4) ion channel transduces mechanical loading of articular cartilage via the generation of intracellular calcium ion transients. Using tissue-specific, inducible Trpv4 gene-targeted mice, we demonstrate that loss of TRPV4-mediated cartilage mechanotransduction in adulthood reduces the severity of aging-associated OA. However, loss of chondrocyte TRPV4 did not prevent OA development following destabilization of the medial meniscus (DMM). These results highlight potentially distinct roles of TRPV4-mediated cartilage mechanotransduction in age-related and post-traumatic OA, and point to a novel disease-modifying strategy to therapeutically target the TRPV4-mediated mechanotransduction pathway for the treatment of aging-associated OA.

Cell-based tissue engineering strategies used in the clinical repair of articular cartilage

One of the most important issues facing cartilage tissue engineering is the inability to move technologies into the clinic. Despite the multitude of current research in the field, it is known that 90% of new drugs that advance past animal studies fail clinical trials. The objective of this review is to provide readers with an understanding of the scientific details of tissue engineered cartilage products that have demonstrated a certain level of efficacy in humans, so that newer technologies may be developed upon this foundation. Compared to existing treatments, such as microfracture or autologous chondrocyte implantation, a tissue engineered product can potentially provide more consistent clinical results in forming hyaline repair tissue and in filling the entirety of the defect. The various tissue engineering strategies (e.g., cell expansion, scaffold material, media formulations, biomimetic stimuli, etc.) used in forming these products, as collected from published literature, company websites, and relevant patents, are critically discussed. The authors note that many details about these products remain proprietary, not all information is made public, and that advancements to the products are continuously made. Nevertheless, by understanding the design and production processes of these emerging technologies, one can gain tremendous insight into how to best use them and also how to design the next generation of tissue engineered cartilage products.

Mechanotransduction: Relevance to Physical Therapist Practice-Understanding Our Ability to Affect Genetic Expression Through Mechanical Forces

Mechanotransduction, the mechanism by which mechanical perturbation influences genetic expression and cellular behavior, is an area of molecular biology undergoing rapid exploration and discovery. Cells are sensitive to forces such as shear, tension, and compression, and they respond accordingly through cellular proliferation, migration, tissue repair, altered metabolism, and even stem cell differentiation and maturation. The study of how cells sense and respond to mechanical stimulation is under robust expansion, with new scientific methods and technologies at our disposal. The application of these technologies to physical therapist practice may hold answers to some of our age-old questions while creating new avenues for our profession to optimize movement for societal health. Embracing this science as foundational to our profession will allow us to be valuable scientific collaborators with distinctive knowledge of the effects of loading. These partnerships will be key to augmenting the clinical utility of emerging therapies such as regenerative medicine, tissue engineering, and gene therapy. Collaboration with other scientific disciplines in these endeavors, along with the inclusion and application of these discoveries in our academic programs, will enhance the understanding of the impact of our practice on biologic and genetic processes. A basic understanding of mechanotransduction and its relevance to physical therapist practice is warranted to begin the conversation.

Strain-induced mechanotransduction through primary cilia, extracellular ATP, purinergic calcium signaling, and ERK1/2 transactivates CITED2 and downregulates MMP-1 and MMP-13 gene expression in chondrocytes

OBJECTIVE

To determine the strain-induced signaling pathways involved in regulating the transactivation of the transcription regulator Cbp/p300 Interacting Transactivator with ED-rich tail 2 (CITED2) and downstream targets in chondrocytes.

METHODS

Primary human chondrocytes or C28/I2 chondrocytic cells were subjected to various strain regimes. C57BL/6 mice were subjected to treadmill running. Loss-of-function was carried out using siRNA or inhibitors specific for targeted molecules. mRNA levels were assayed by RT-qPCR, and proteins by western blotting, immunofluorescence, and/or immunohistochemical staining. CITED2 promoter activity was assayed in chondrocytes using wild-type or mutant constructs.

RESULTS

Cyclic strain at 5%, 1 Hz induced CITED2 expression and suppressed expression of matrix metalloproteinase (MMP)-1 and -13 at the messenger RNA (mRNA) and protein levels in human chondrocytes. Abolishing primary cilia through knockdown of intraflagellar transport protein (IFT88) attenuated CITED2 gene expression and decreased protein levels. Similar effects were observed with inhibitors of extracellular adenosine triphosphate (ATP) or P2 purinergic receptors, or antagonists of Ca(2+) signaling. Knockdown of IFT88 in articular chondrocytes in vivo diminished treadmill induced-CITED2 expression and upregulated MMPs. Knockdown of hypoxia-inducible factor (HIF)1α, specificity protein 1 (Sp1), or deletion of the shear stress response element (SSRE) in the CITED2 promoter limited cyclic strain-induced transactivation of CITED2. However, the strain induced-transactivation of CITED2 was abolished only on knockdown of HIF1α, Sp1, and SSRE or by loss-of-function of IFT88 or extracellular-signal-regulated kinases (ERK)1/2.

CONCLUSIONS

CITED2 transactivation is a critical event in signaling generated by strain and transduced by primary cilia, extracellular ATP, P2 purinergic receptors, and Ca(2+) signaling. Strain-induced CITED2 transactivation requires HIF1α, Sp1, and an intact SSRE and leads to the downregulation of MMPs such as MMP-1 and MMP-13.

Cell-based tissue engineering strategies used in the clinical repair of articular cartilage

One of the most important issues facing cartilage tissue engineering is the inability to move technologies into the clinic. Despite the multitude of current research in the field, it is known that 90% of new drugs that advance past animal studies fail clinical trials. The objective of this review is to provide readers with an understanding of the scientific details of tissue engineered cartilage products that have demonstrated a certain level of efficacy in humans, so that newer technologies may be developed upon this foundation. Compared to existing treatments, such as microfracture or autologous chondrocyte implantation, a tissue engineered product can potentially provide more consistent clinical results in forming hyaline repair tissue and in filling the entirety of the defect. The various tissue engineering strategies (e.g., cell expansion, scaffold material, media formulations, biomimetic stimuli, etc.) used in forming these products, as collected from published literature, company websites, and relevant patents, are critically discussed. The authors note that many details about these products remain proprietary, not all information is made public, and that advancements to the products are continuously made. Nevertheless, by understanding the design and production processes of these emerging technologies, one can gain tremendous insight into how to best use them and also how to design the next generation of tissue engineered cartilage products.

IFT88 influences chondrocyte actin organization and biomechanics

OBJECTIVES

Primary cilia are microtubule based organelles which control a variety of signalling pathways important in cartilage development, health and disease. This study examines the role of the intraflagellar transport (IFT) protein, IFT88, in regulating fundamental actin organisation and mechanics in articular chondrocytes.

METHODS

The study used an established chondrocyte cell line with and without hypomorphic mutation of IFT88 (IFT88(orpk)). Confocal microscopy was used to quantify F-actin and myosin IIB organisation. Viscoelastic cell and actin cortex mechanics were determined using micropipette aspiration with actin dynamics visualised in live cells transfected with LifeACT-GFP.

RESULTS

IFT88(orpk) cells exhibited a significant increase in acto-myosin stress fibre organisation relative to wild-type (WT) cells in monolayer and an altered response to cytochalasin D. Rounded IFT88(orpk) cells cultured in suspension exhibited reduced cortical actin expression with reduced cellular equilibrium modulus. Micropipette aspiration resulted in reduced membrane bleb formation in IFT88(orpk) cells. Following membrane blebbing, IFT88(orpk) cells exhibited slower reformation of the actin cortex. IFT88(orpk) cells showed increased actin deformability and reduced cortical tension confirming that IFT regulates actin cortex mechanics. The reduced cortical tension is also consistent with the reduced bleb formation.

CONCLUSIONS

This study demonstrates for the first time that the ciliary protein IFT88 regulates fundamental actin organisation and the stiffness of the actin cortex leading to alterations in cell deformation, mechanical properties and blebbing in an IFT88 chondrocyte cell line. This adds to the growing understanding of the role of primary cilia and IFT in regulating cartilage biology.

Differentiation of mesenchymal stem cells for cartilage tissue engineering: Individual and synergetic effects of three-dimensional environment and mechanical loading

Chondrogenesis of dedifferentiated chondrocytes and mesenchymal stem cells is influenced not only by soluble molecules like growth factors, but also by the cell environment itself. The latter is achieved through both mechanical cues - which act as stimulation factor and influences nutrient transport - and adhesion to extracellular matrix cues - which determine cell shape. Although the effects of soluble molecules and cell environment have been intensively addressed, few observations and conclusions about the interaction between the two have been achieved. In this work, we review the state of the art on the single effects between mechanical and biochemical cues, as well as on the combination of the two. Furthermore, we provide a discussion on the techniques currently used to determine the mechanical properties of materials and tissues generated in vitro, their limitations and the future research needs to properly address the identified problems.

STATEMENT OF SIGNIFICANCE

The importance of biomechanical cues in chondrogenesis is well known. This paper reviews the existing literature on the effect of mechanical stimulation on chondrogenic differentiation of mesenchymal stem cells in order to regenerate hyaline cartilage. Contradictory results found with respect to the effect of different modes of external loading can be explained by the different properties of the scaffolding system that holds the cells, which determine cell adhesion and morphology and spatial distribution of cells, as well as the stress transmission to the cells. Thus, this review seeks to provide an insight into the interplay between external loading program and scaffold properties during chondrogenic differentiation. The review of the literature reveals an important gap in the knowledge in this field and encourages new experimental studies. The main issue is that in each of the few cases in which the interplay is investigated, just two groups of scaffolds are compared, leaving intermediate adhesion conditions out of study. The authors propose broader studies implementing new high-throughput techniques for mechanical characterization of tissue engineering constructs and the inclusion of fatigue analysis as support methodology to more exhaustive mechanical characterization.

Mechanobiology of TGFβ signaling in the skeleton

Physical and biochemical cues play fundamental roles in the skeleton at both the tissue and cellular levels. The precise coordination of these cues is essential for skeletal development and homeostasis, and disruption of this coordination can drive disease progression. The growth factor TGFβ is involved in both the regulation of and cellular response to the physical microenvironment. It is essential to summarize the current findings regarding the mechanisms by which skeletal cells integrate physical and biochemical cues so that we can identify and address remaining gaps that could ultimately improve skeletal health. In this review, we describe the role of TGFβ in mechanobiological signaling in bone and cartilage at the tissue and cellular levels. We provide detail on how static and dynamic physical cues at the macro-level are transmitted to the micro-level, ultimately leading to regulation at each level of the TGFβ pathway and to cell differentiation. The continued integration of engineering and biological approaches is needed to answer many remaining questions, such as the mechanisms by which cells generate a coordinated response to physical and biochemical cues. We propose one such mechanism, through which the combination of TGFβ and an optimal physical microenvironment leads to synergistic induction of downstream TGFβ signaling.

Differential effects of LifeAct-GFP and actin-GFP on cell mechanics assessed using micropipette aspiration

The actin cytoskeleton forms a dynamic structure involved in many fundamental cellular processes including the control of cell morphology, migration and biomechanics. Recently LifeAct-GFP (green fluorescent protein) has been proposed for visualising actin structure and dynamics in live cells as an alternative to actin-GFP which has been shown to affect cell mechanics. Here we compare the two approaches in terms of their effect on cellular mechanical behaviour. Human mesenchymal stem cells (hMSCs) were analysed using micropipette aspiration and the effective cellular equilibrium and instantaneous moduli calculated using the standard linear solid model. We show that LifeAct-GFP provides clearer visualisation of F-actin organisation and dynamics. Furthermore, LifeAct-GFP does not alter effective cellular mechanical properties whereas actin-GFP expression causes an increase in the cell modulus. Interestingly, LifeAct-GFP expression did produce a small (~10%) increase in the percentage of cells exhibiting aspiration-induced membrane bleb formation, whilst actin-GFP expression reduced blebbing. Further studies examined the influence of LifeAct-GFP in other cell types, namely chondrogenically differentiated hMSCs and murine chondrocytes. LifeAct-GFP also had no effect on the moduli of these non-blebbing cells for which mechanical properties are largely dependent on the actin cortex. In conclusion we show that LifeAct-GFP enables clearer visualisation of actin organisation and dynamics without disruption of the biomechanical properties of either the whole cell or the actin cortex. Thus the study provides new evidence supporting the use of LifeAct-GFP rather than actin-GFP for live cell microscopy and the study of cellular mechanobiology.

Adult Stem Cell Responses to Nanostimuli

Adult or mesenchymal stem cells (MSCs) have been found in different tissues in the body, residing in stem cell microenvironments called "stem cell niches". They play different roles but their main activity is to maintain tissue homeostasis and repair throughout the lifetime of an organism. Their ability to differentiate into different cell types makes them an ideal tool to study tissue development and to use them in cell-based therapies. This differentiation process is subject to both internal and external forces at the nanoscale level and this response of stem cells to nanostimuli is the focus of this review.

Physiological mechanisms and therapeutic potential of bone mechanosensing

Skeletal loading is an important physiological regulator of bone mass. Theoretically, mechanical forces or administration of drugs that activate bone mechanosensors would be a novel treatment for osteoporotic disorders, particularly age-related osteoporosis and other bone loss caused by skeletal unloading. Uncertainty regarding the identity of the molecular targets that sense and transduce mechanical forces in bone, however, has limited the therapeutic exploitation of mechanosesning pathways to control bone mass. Recently, two evolutionally conserved mechanosensing pathways have been shown to function as "physical environment" sensors in cells of the osteoblasts lineage. Indeed, polycystin-1 (Pkd1, or PC1) and polycystin-2 (Pkd2, or PC2' or TRPP2), which form a flow sensing receptor channel complex, and TAZ (transcriptional coactivator with PDZ-binding motif, or WWTR1), which responds to the extracellular matrix microenvironment act in concert to reciprocally regulate osteoblastogenesis and adipogenesis through co-activating Runx2 and a co-repressing PPARγ activities. Interactions of polycystins and TAZ with other putative mechanosensing mechanism, such as primary cilia, integrins and hemichannels, may create multifaceted mechanosensing networks in bone. Moreover, modulation of polycystins and TAZ interactions identify novel molecular targets to develop small molecules that mimic the effects of mechanical loading on bone.

The primary cilium functions as a mechanical and calcium signaling nexus

BACKGROUND

The primary cilium is an antenna-like, nonmotile structure that extends from the surface of most mammalian cell types and is critical for chemosensing and mechanosensing in a variety of tissues including cartilage, bone, and kidney. Flow-induced intracellular calcium ion (Ca(2+)) increases in kidney epithelia depend on primary cilia and primary cilium-localized Ca(2+)-permeable channels polycystin-2 (PC2) and transient receptor potential vanilloid 4 (TRPV4). While primary cilia have been implicated in osteocyte mechanotransduction, the molecular mechanism that mediates this process is not fully understood. We directed a fluorescence resonance energy transfer (FRET)-based Ca(2+) biosensor to the cilium by fusing the biosensor sequence to the sequence of the primary cilium-specific protein Arl13b. Using this tool, we investigated the role of several Ca(2+)-permeable channels that may mediate flow-induced Ca(2+) entry: PC2, TRPV4, and PIEZO1.

RESULTS

Here, we report the first measurements of Ca(2+) signaling within osteocyte primary cilia using a FRET-based biosensor fused to ARL13B. We show that fluid flow induces Ca(2+) increases in osteocyte primary cilia which depend on both intracellular Ca(2+) release and extracellular Ca(2+) entry. Using siRNA-mediated knockdowns, we demonstrate that TRPV4, but not PC2 or PIEZO1, mediates flow-induced ciliary Ca(2+) increases and loading-induced Cox-2 mRNA increases, an osteogenic response.

CONCLUSIONS

In this study, we show that the primary cilium forms a Ca(2+) microdomain dependent on Ca(2+) entry through TRPV4. These results demonstrate that the mechanism of mechanotransduction mediated by primary cilia varies in different tissue contexts. Additionally, we anticipate that this work is a starting point for more studies investigating the role of TRPV4 in mechanotransduction.

The mechanobiology of articular cartilage: bearing the burden of osteoarthritis

Articular cartilage injuries and degenerative joint diseases are responsible for progressive pain and disability in millions of people worldwide, yet there is currently no treatment available to restore full joint functionality. As the tissue functions under mechanical load, an understanding of the physiologic or pathologic effects of biomechanical factors on cartilage physiology is of particular interest. Here, we highlight studies that have measured cartilage deformation at scales ranging from the macroscale to the microscale, as well as the responses of the resident cartilage cells, chondrocytes, to mechanical loading using in vitro and in vivo approaches. From these studies, it is clear that there exists a complex interplay among mechanical, inflammatory, and biochemical factors that can either support or inhibit cartilage matrix homeostasis under normal or pathologic conditions. Understanding these interactions is an important step toward developing tissue engineering approaches and therapeutic interventions for cartilage pathologies, such as osteoarthritis.

Dynamic 3D culture: models of chondrogenesis and endochondral ossification

The formation of cartilage from stem cells during development is a complex process which is regulated by both local growth factors and biomechanical cues, and results in the differentiation of chondrocytes into a range of subtypes in specific regions of the tissue. In fetal development cartilage also acts as a precursor scaffold for many bones, and mineralization of this cartilaginous bone precursor occurs through the process of endochondral ossification. In the endochondral formation of bones during fetal development the interplay between cell signalling, growth factors, and biomechanics regulates the formation of load bearing bone, in addition to the joint capsule containing articular cartilage and synovium, generating complex, functional joints from a single precursor anlagen. These joint tissues are subsequently prone to degeneration in adult life and have poor regenerative capabilities, and so understanding how they are created during development may provide useful insights into therapies for diseases, such as osteoarthritis, and restoring bone and cartilage lost in adulthood. Of particular interest is how these tissues regenerate in the mechanically dynamic environment of a living joint, and so experiments performed using 3D models of cartilage development and endochondral ossification are proving insightful. In this review, we discuss some of the interesting models of cartilage development, such as the chick femur which can be observed in ovo, or isolated at a specific developmental stage and cultured organotypically in vitro. Biomaterial and hydrogel-based strategies which have emerged from regenerative medicine are also covered, allowing researchers to make informed choices on the characteristics of the materials used for both original research and clinical translation. In all of these models, we illustrate the essential importance of mechanical forces and mechanotransduction as a regulator of cell behavior and ultimate structural function in cartilage.

Physicochemical and biomechanical stimuli in cell-based articular cartilage repair

Articular cartilage is a unique load-bearing connective tissue with a low intrinsic capacity for repair and regeneration. Its avascularity makes it relatively hypoxic and its unique extracellular matrix is enriched with cations, which increases the interstitial fluid osmolarity. Several physicochemical and biomechanical stimuli are reported to influence chondrocyte metabolism and may be utilized for regenerative medical approaches. In this review article, we summarize the most relevant stimuli and describe how ion channels may contribute to cartilage homeostasis, with special emphasis on intracellular signaling pathways. We specifically focus on the role of calcium signaling as an essential mechanotransduction component and highlight the role of phosphatase signaling in this context.

The growth plate's response to load is partially mediated by mechano-sensing via the chondrocytic primary cilium

Mechanical load plays a significant role in bone and growth-plate development. Chondrocytes sense and respond to mechanical stimulation; however, the mechanisms by which those signals exert their effects are not fully understood. The primary cilium has been identified as a mechano-sensor in several cell types, including renal epithelial cells and endothelium, and accumulating evidence connects it to mechano-transduction in chondrocytes. In the growth plate, the primary cilium is involved in several regulatory pathways, such as the non-canonical Wnt and Indian Hedgehog. Moreover, it mediates cell shape, orientation, growth, and differentiation in the growth plate. In this work, we show that mechanical load enhances ciliogenesis in the growth plate. This leads to alterations in the expression and localization of key members of the Ihh-PTHrP loop resulting in decreased proliferation and an abnormal switch from proliferation to differentiation, together with abnormal chondrocyte morphology and organization. Moreover, we use the chondrogenic cell line ATDC5, a model for growth-plate chondrocytes, to understand the mechanisms mediating the participation of the primary cilium, and in particular KIF3A, in the cell's response to mechanical stimulation. We show that this key component of the cilium mediates gene expression in response to mechanical stimulation.

Function and regulation of primary cilia and intraflagellar transport proteins in the skeleton

Primary cilia are microtubule-based organelles that project from the cell surface to enable transduction of various developmental signaling pathways. The process of intraflagellar transport (IFT) is crucial for the building and maintenance of primary cilia. Ciliary dysfunction has been found in a range of disorders called ciliopathies, some of which display severe skeletal dysplasias. In recent years, interest has grown in uncovering the function of primary cilia/IFT proteins in bone development, mechanotransduction, and cellular regulation. We summarize recent advances in understanding the function of cilia and IFT proteins in the regulation of cell differentiation in osteoblasts, osteocytes, chondrocytes, and mesenchymal stem cells (MSCs). We also discuss the mechanosensory function of cilia and IFT proteins in bone cells, cilia orientation, and other functions of cilia in chondrocytes.

Digoxin and adenosine triphosphate enhance the functional properties of tissue-engineered cartilage

Toward developing engineered cartilage for the treatment of cartilage defects, achieving relevant functional properties before implantation remains a significant challenge. Various chemical and mechanical stimuli have been used to enhance the functional properties of engineered musculoskeletal tissues. Recently, Ca(2+)-modulating agents have been used to enhance matrix synthesis and biomechanical properties of engineered cartilage. The objective of this study was to determine whether other known Ca(2+) modulators, digoxin and adenosine triphosphate (ATP), can be employed as novel stimuli to increase collagen synthesis and functional properties of engineered cartilage. Neocartilage constructs were formed by scaffold-free self-assembling of primary bovine articular chondrocytes. Digoxin, ATP, or both agents were added to the culture medium for 1 h/day on days 10-14. After 4 weeks of culture, neocartilage properties were assessed for gross morphology, biochemical composition, and biomechanical properties. Digoxin and ATP were found to increase neocartilage collagen content by 52-110% over untreated controls, while maintaining proteoglycan content near native tissue values. Furthermore, digoxin and ATP increased the tensile modulus by 280% and 180%, respectively, while the application of both agents increased the modulus by 380%. The trends in tensile properties were found to correlate with the amount of collagen cross-linking. Live Ca(2+) imaging experiments revealed that both digoxin and ATP were able to increase Ca(2+) oscillations in monolayer-cultured chondrocytes. This study provides a novel approach toward directing neocartilage maturation and enhancing its functional properties using novel Ca(2+) modulators.

Mechanobiology of Ciliogenesis

Cilia are force-generating and -sensing organelles that serve as mechanical interfaces between the cell and the extracellular environment. Cilia are present in tissues that adaptively respond to mechanical loading and fluid flow, and defects in ciliary function can lead to diseases affecting these tissues. As might be expected for a mechanical interface, the formation of cilia is, itself, regulated by mechanical forces, and these links between mechanics and ciliary formation are providing new entry points for dissecting the regulatory pathways of ciliogenesis.

Synergy between Piezo1 and Piezo2 channels confers high-strain mechanosensitivity to articular cartilage

Diarthrodial joints are essential for load bearing and locomotion. Physiologically, articular cartilage sustains millions of cycles of mechanical loading. Chondrocytes, the cells in cartilage, regulate their metabolic activities in response to mechanical loading. Pathological mechanical stress can lead to maladaptive cellular responses and subsequent cartilage degeneration. We sought to deconstruct chondrocyte mechanotransduction by identifying mechanosensitive ion channels functioning at injurious levels of strain. We detected robust expression of the recently identified mechanosensitive channels, PIEZO1 and PIEZO2. Combined directed expression of Piezo1 and -2 sustained potentiated mechanically induced Ca(2+) signals and electrical currents compared with single-Piezo expression. In primary articular chondrocytes, mechanically evoked Ca(2+) transients produced by atomic force microscopy were inhibited by GsMTx4, a PIEZO-blocking peptide, and by Piezo1- or Piezo2-specific siRNA. We complemented the cellular approach with an explant-cartilage injury model. GsMTx4 reduced chondrocyte death after mechanical injury, suggesting a possible therapy for reducing cartilage injury and posttraumatic osteoarthritis by attenuating Piezo-mediated cartilage mechanotransduction of injurious strains.