Functional gene screening system identified TRPV4 as a regulator of chondrogenic differentiation

Tuning local matrix compliance accelerates mesenchymal stem cell chondrogenesis in 3D sliding hydrogels

The mechanical properties of the extracellular matrix critically regulate stem cell differentiation in 3D. Alginate hydrogels with tunable bulk stiffness and viscoelasticity can modulate differentiation in 3D through mechanotransduction. Such enhanced differentiation is correlated with changes in the local matrix compliance- the extent of matrix deformation under applied load. However, the causal effect of local matrix compliance changes without altering bulk hydrogel mechanics on stem cell differentiation remains unclear. To address this, we report sliding hydrogel (SG) designs with tunable local matrix compliance obtained by varying the molecular mobility of the hydrogel network without changing bulk mechanics. Atomic force microscopy showed increasing SG mobility allowed cells to increasingly deform local niches with lesser forces, indicating higher local matrix compliance. Increasing SG mobility accelerates MSC chondrogenesis in a mobility-dependent manner and is independent of exogenous adhesive ligands or cell volume expansion. The enhanced chondrogenesis in SG is accompanied by enhanced cytoskeletal organization and TRPV4 expression, and blocking these elements abolished the effect. In conclusion, this study establishes a causal link between local matrix compliance and stem cell differentiation and establishes it as a crucial hydrogel design parameter. Furthermore, it offers novel SG designs to probe the role of local matrix compliance in various biological processes.

Skeletal dysplasia-causing mutations in TRPV4 alter the chondrocyte transcriptomic response to mechanical loading

Transient receptor potential vanilloid 4 (TRPV4) is a mechanosensitive ion channel highly expressed in chondrocytes that supports cartilage development and homeostasis. Mutations in the channel can cause skeletal dysplasias, including the gain-of-function mutations V620I and T89I, which lead to brachyolmia and metatropic dysplasia, respectively. These mutations suppress hypertrophic differentiation, but the mechanisms by which they alter chondrocyte response to mechanical load remain to be elucidated. To determine the effect of these mutations on chondrocyte mechanotransduction, tissue-engineered cartilage was derived from differentiated clustered regularly interspaced short palindromic repeats (CRISPR)-edited human-induced pluripotent stem cells (hiPSCs) harboring the moderate V620I or severe T89I TRPV4 mutations. Wild-type and mutant tissue-engineered hiPSC-derived cartilage contructs were subjected to compressive mechanical loading at physiological levels, and transcriptomic signatures were assessed by RNA-sequencing. Our results demonstrate that the V620I and T89I mutations diminish the mechanoresponsiveness of chondrocytes, as evidenced by reduced gene expression downstream of TRPV4 activation, including those involved in endochondral ossification. Changes in the expression of genes involved in extracellular matrix production and organization were found to contribute toward the phenotype in V620I mutant chondrocytes, whereas dysregulated retinoic acid signaling was linked to T89I, and disrupted proliferation was common to both. Our findings suggest that dysfunctional mechanotransduction due to V620I and T89I mutations in TRPV4 contribute to the developmental phenotypes, supporting TRPV4 modulation as a potential pharmacologic target.NEW & NOTEWORTHY Gain-of-function mutations in TRPV4, a mechano- and osmosensitive ion channel, are linked to skeletal dysplasias, but their effects on chondrocyte mechanotransduction remain unknown. Using human iPSCs harboring skeletal dysplasia-causing mutations, we developed and mechanically loaded tissue-engineered cartilage. Our findings show that V620I and T89I mutations reduce chondrocyte mechanoresponsiveness, evidenced by decreased gene expression downstream of TRPV4 activation, providing insight into TRPV4-related skeletal disorders and potential pharmacological targets.

Matrix metalloproteinases accelerate pericellular matrix breakdown and disrupt mechanotransduction in osteoarthritis

The pericellular matrix (PCM) is a specialized, narrow matrix surrounding each chondrocyte in articular cartilage, together constituting the chondron - the fundamental metabolic and functional unit of cartilage. The PCM plays a vital role in mediating biomechanical and biochemical signals essential for chondrocyte function. In osteoarthritis (OA), a chronic joint disorder characterized by progressive cartilage degradation, the PCM is one of the earliest sites of catabolic degradation, primarily driven by matrix metalloproteinases (MMPs). This study aims to investigate the functional relationship between PCM degradation and chondrocyte mechanosignaling, with an emphasis on MMP-driven changes in mechanotransduction in osteoarthritic cartilage. Human chondrons (N = 64) were incubated with MMP-2, MMP-3, and MMP-7, and structural changes were assessed histologically by evaluating perlecan and collagen type VI. Cellular elasticity was measured using atomic force microscopy (AFM), and mechanically evoked intracellular Ca2+ transients were assessed via AFM single-cell indentations (500 nN). All three MMPs induced pronounced catabolic effects on the PCM structure, showing distinct impacts on collagen type VI and perlecan, as well as on the biomechanical properties (p < 0.001). MMP-driven alterations in PCM integrity significantly reduced the Ca2+ transients of chondrons in response to mechanical stimuli (p < 0.001). While TRPV4 activation was elevated in intact chondrons, PIEZO channels were involved in mechanotransduction in both healthy and MMP-treated chondrons. In osteoarthritic stages, the mechanotransduction dynamics shifted significantly towards PIEZO channels. This study elucidates the interplay between MMP-mediated PCM degradation, structural-functional dynamics, and chondrocyte mechanotransduction, underscoring the critical role of the PCM in maintaining normal chondrocyte functionality and mechanosensing. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) is a prevalent degenerative joint disease affecting millions worldwide. Central to its pathology is the degradation of the pericellular matrix (PCM) by matrix metalloproteinases (MMPs), which disrupts chondrocyte mechanotransduction, altering cellular responses to mechanical stimuli. This study explores the impact of MMP-2, MMP-3, and MMP-7 on PCM structure and chondrocyte mechanosensing. Our results reveal that MMP-induced degradation significantly compromises PCM structural integrity, leading to altered mechanotransduction dynamics in chondrocytes. Degradation specifically redirects the primary function of ion channels from TRPV4 to PIEZO channels in cells impacted by MMPs. This highlights the interplay between MMP-mediated PCM degradation, chondrocyte mechanotransduction and as thus structural-functional dynamics, underscoring the critical role of the PCM in maintaining normal chondrocyte functionality and mechanosensing.

Serinc5 Regulates Sequential Chondrocyte Differentiation by Inhibiting Sox9 Function in Pre-Hypertrophic Chondrocytes

The growth plate is the primary site of longitudinal bone growth with chondrocytes playing a pivotal role in endochondral bone development. Chondrocytes undergo a series of differentiation steps, resulting in the formation of a unique hierarchical columnar structure comprising round, proliferating, pre-hypertrophic, and hypertrophic chondrocytes. Pre-hypertrophic chondrocytes, which exist in the transitional stage between proliferating and hypertrophic stages, are a critical cell population in the growth plate. However, the molecular basis of pre-hypertrophic chondrocytes remains largely undefined. Here, we employed scRNA-seq analysis on fluorescently labeled growth plate chondrocytes for their molecular characterization. Serine incorporator 5 (Serinc5) was identified as a marker gene for pre-hypertrophic chondrocytes. Histological analysis revealed that Serinc5 is specifically expressed in pre-hypertrophic chondrocytes, overlapping with Indian hedgehog (Ihh). Serinc5 represses cell proliferation and Col2a1 and Acan expression by inhibiting the transcriptional activity of Sox9 in primary chondrocytes. Chromatin profiling using ChIP-seq and ATAC-seq revealed an active enhancer of Serinc5 located in intron 1, with its chromatin status progressively activated during chondrocyte differentiation. Collectively, our findings suggest that Serinc5 regulates sequential chondrocyte differentiation from proliferation to hypertrophy by inhibiting Sox9 function in pre-hypertrophic chondrocytes, providing novel insights into the mechanisms underlying chondrocyte differentiation in growth plates.

Roles for TRPV4 in disease: A discussion of possible mechanisms

The transient receptor potential vanilloid 4 (TRPV4) ion channel is a ubiquitously expressed Ca2+-permeable ion channel that controls intracellular calcium ([Ca2+]i) homeostasis in various types of cells. The physiological roles for TRPV4 are tissue specific and the mechanisms behind this specificity remain mostly unclarified. It is noteworthy that mutations in the TRPV4 channel have been associated to a broad spectrum of congenital diseases, with most of these mutations mainly resulting in gain-of-function. Mutations have been identified in human patients showing a variety of phenotypes and symptoms, mostly related to skeletal and neuromuscular disorders. Since TRPV4 is so widely expressed throughout the body, it comes as no surprise that the literature is growing in evidence linking this protein to malfunction in systems other than the skeletal and neuromuscular. In this review, we summarize the expression patterns of TRPV4 in several tissues and highlight findings of recent studies that address critical structural and functional features of this channel, particularly focusing on its interactions and signaling pathways related to Ca2+ entry. Moreover, we discuss the roles of TRPV4 mutations in some diseases and pinpoint some of the mechanisms underlying pathological states where TRPV4's malfunction is prominent.

Mechanical stimulation promotes fibrochondrocyte proliferation by activating the TRPV4 signaling pathway during tendon-bone insertion healing: CCN2 plays an important regulatory role

Background

We previously confirmed that mechanical stimulation is an important factor in the repair of tendon-bone insertion (TBI) injuries and that mechanoreceptors such as transient receptor potential ion-channel subfamily V member 4 (TRPV4; also known as transient receptor potential vanilloid 4) are key to transforming mechanical stimulation into intracellular biochemical signals. This study aims to elucidate the mechanism of mechanical stimulation regulating TRPV4.

Methods

Immunohistochemical staining and western blotting were used to evaluate cartilage repair at the TBI after injury. The RNA expression and protein expression of mechanoreceptors and key pathway molecules regulating cartilage proliferation were analyzed. TBI samples were collected for transcriptome sequencing to detect gene expression. Calcium-ion imaging and flow cytometry were used to evaluate the function of TPRV4 and cellular communication network factor 2 (CCN2) after the administration of siRNA, recombinant adenovirus and agonists.

Results

We found that treadmill training improved the quality of TBI healing and enhanced fibrochondrocyte proliferation. The transcriptome sequencing results suggested that the elevated expression of the mechanistically stimulated regulator CCN2 and the exogenous administration of recombinant human CCN2 significantly promoted TRPV4 protein expression and fibrochondrocyte proliferation. In vitro, under mechanical stimulation conditions, small interfering RNA (siRNA)-CCN2 not only inhibited the proliferation of primary fibrochondrocytes but also suppressed TRPV4 protein expression and activity. Subsequently, primary fibrochondrocytes were treated with the TRPV4 agonist GSK1016790A and the recombinant adenovirus TRPV4 (Ad-TRPV4), and GSK1016790A partially reversed the inhibitory effect of siRNA-CCN2. The phosphoinositide 3-kinase/ protein kinase B (PI3K/AKT) signaling pathway participated in the above process.

Conclusions

Mechanical stimulation promoted fibrochondrocyte proliferation and TBI healing by activating TRPV4 channels and the PI3K/AKT signaling pathway, and CCN2 may be a key regulatory protein in maintaining TRPV4 activation.

Effect of dynamic loading on calcium signaling in In-Situ chondrocytes

Chondrocytes respond to mechanical stimuli by increasing their intracellular calcium concentration. The response depends on the cellular environment. Previous studies have investigated chondrocytes under slow strain rates or cells embedded in hydrogels, but the response of chondrocytes in their native environment under physiologically relevant cyclic loads and dynamic hydrostatic pressure has not been studied. This study investigated the calcium signaling response of in-situ chondrocytes under physiological cyclic compressive loads and hydrostatic pressure with varying frequency and load rates. Bovine cartilage explants were stained with a fluorescent calcium indicator dye and subjected to physiologically relevant cyclic loads using a custom-built loading device secured on a confocal/multiphoton microscope. Calcium fluorescence intensities of the cells were tracked and analyzed. Loading groups were compared using one-way ANOVA followed by a post-hoc test with Tukey correction (α = 0.05). The percentage of cells signaling increased in all compressive loading conditions compared to the no-load baseline. The percentage of cells responding under 1 Hz load was significantly greater than the slow ramp and 0.1 Hz group (p < 0.05). The number of compression cycles had no effect on the calcium signaling response (p > 0.05). The width and time between consecutive peaks were not different between different loading conditions (p > 0.05). Calcium signaling of in-situ chondrocytes did not increase under dynamic hydrostatic pressure of magnitudes up to 0.2 MPa at frequencies of 0.5 Hz and 0.05 Hz (p > 0.05). In conclusion, in-situ chondrocytes respond to physiological compressive loads in a strain rate-dependent manner with an increased number of responsive cells and unaltered temporal characteristics.

Ion channels in osteoarthritis: emerging roles and potential targets

Osteoarthritis (OA) is a highly prevalent joint disease that causes substantial disability, yet effective approaches to disease prevention or to the delay of OA progression are lacking. Emerging evidence has pinpointed ion channels as pivotal mediators in OA pathogenesis and as promising targets for disease-modifying treatments. Preclinical studies have assessed the potential of a variety of ion channel modulators to modify disease pathways involved in cartilage degeneration, synovial inflammation, bone hyperplasia and pain, and to provide symptomatic relief in models of OA. Some of these modulators are currently being evaluated in clinical trials. This review explores the structures and functions of ion channels, including transient receptor potential channels, Piezo channels, voltage-gated sodium channels, voltage-dependent calcium channels, potassium channels, acid-sensing ion channels, chloride channels and the ATP-dependent P2XR channels in the osteoarthritic joint. The discussion spans channel-targeting drug discovery and potential clinical applications, emphasizing opportunities for further research, and underscoring the growing clinical impact of ion channel biology in OA.

The chondrocyte "mechanome": Activation of the mechanosensitive ion channels TRPV4 and PIEZO1 drives unique transcriptional signatures

The mechanosensitive ion channels Transient Receptor Potential Vanilloid 4 (TRPV4) and PIEZO1 transduce physiologic and supraphysiologic magnitudes of mechanical signals in the chondrocyte, respectively. TRPV4 activation promotes chondrogenesis, while PIEZO1 activation by supraphysiologic deformations drives cell death. The mechanisms by which activation of these channels discretely drives changes in gene expression to alter cell behavior remain to be determined. To date, no studies have contrasted the transcriptomic response to activation of these channels nor has any published data attempted to correlate these transcriptomes to alterations in cellular function. This study used RNA sequencing to comprehensively investigate the transcriptomes associated with activation of TRPV4 or PIEZO1, revealing that TRPV4 and PIEZO drive distinct transcriptomes and also exhibit unique co-regulated clusters of genes. Notably, activation of PIEZO1 through supraphysiologic deformation induced a transient inflammatory profile that overlapped with the interleukin (IL)-1-responsive transcriptome and contained genes associated with cartilage degradation and osteoarthritis progression. However, both TRPV4 and PIEZO1 were also shown to elicit anabolic effects. PIEZO1 expression promoted a pro-chondrogenic transcriptome under unloaded conditions, and daily treatment with PIEZO1 agonist Yoda1 significantly increased sulfated glycosaminoglycan deposition in vitro. These findings emphasize the presence of a broad "mechanome" with distinct effects of TRPV4 and PIEZO1 activation in chondrocytes, suggesting complex roles for PIEZO1 in both the physiologic and pathologic responses of chondrocytes. The identification of transcriptomic profiles unique to or shared by PIEZO1 and TRPV4 (distinct from IL-1-induced inflammation) could inform future therapeutic designs targeting these channels for the management and treatment of osteoarthritis.

Mechanical overload induces TMJ disc degeneration via TRPV4 activation

OBJECTIVE

The temporomandibular joint (TMJ) disc cushions intraarticular stress during mandibular movements. While mechanical overloading is related to cartilage degeneration, the pathogenesis of TMJ disc degeneration is unclear. Here, we determined the regulatory role of mechanoinductive transient receptor potential vanilloid 4 (TRPV4) in mechanical overload-induced TMJ disc degeneration.

METHODS

We explored the effect of mechanical overload on the TMJ discs in a rat occlusal interference model in vivo, and by applying sustained compressive force in vitro. TRPV4 inhibition was delivered by small interfering RNA or GSK2193874; TRPV4 activation was delivered by GSK1016790A. The protective effect of TRPV4 inhibition was validated in the rat occlusal interference model.

RESULTS

Occlusal interference induced TMJ disc degeneration with enhanced extracellular matrix degradation in vivo and mechanical overload promoted inflammatory responses in the TMJ disc cells via Ca2+ influx with significantly upregulated TRPV4. TRPV4 inhibition reversed mechanical overload-induced inflammatory responses; TRPV4 activation simulated mechanical overload-induced inflammatory responses. Moreover, TRPV4 inhibition alleviated TMJ disc degeneration in the rat occlusal interference model.

CONCLUSION

Our findings suggest TRPV4 plays a pivotal role in the pathogenesis of mechanical overload-induced TMJ disc degeneration and may be a promising target for the treatment of degenerative changes of the TMJ disc.

Mechanosensitive TRPV4 channel guides maturation and organization of the bilayered mammary epithelium

Biophysical cues from the cell microenvironment are detected by mechanosensitive components at the cell surface. Such machineries convert physical information into biochemical signaling cascades within cells, subsequently leading to various cellular responses in a stimulus-dependent manner. At the surface of extracellular environment and cell cytoplasm exist several ion channel families that are activated by mechanical signals to direct intracellular events. One of such channel is formed by transient receptor potential cation channel subfamily V member, TRPV4 that is known to act as a mechanosensor in wide variaty of tissues and control ion-influx in a spatio-temporal way. Here we report that TRPV4 is prominently expressed in the stem/progenitor cell populations of the mammary epithelium and seems important for the lineage-specific differentiation, consequently affecting mechanical features of the mature mammary epithelium. This was evident by the lack of several markers for mature myoepithelial and luminal epithelial cells in TRPV4-depleted cell lines. Interestingly, TRPV4 expression is controlled in a tension-dependent manner and it also impacts differentation process dependently on the stiffness of the microenvironment. Furthermore, such cells in a 3D compartment were disabled to maintain normal mammosphere structures and displayed abnormal lumen formation, size of the structures and disrupted cellular junctions. Mechanosensitive TRPV4 channel therefore act as critical player in the homeostasis of normal mammary epithelium through sensing the physical environment and guiding accordingly differentiation and structural organization of the bilayered mammary epithelium.

The interplay between biochemical mediators and mechanotransduction in chondrocytes: Unravelling the differential responses in primary knee osteoarthritis

In primary or idiopathic osteoarthritis (OA), it is unclear which factors trigger the shift of articular chondrocyte activity from pro-anabolic to pro-catabolic. In fact, there is a controversy about the aetiology of primary OA, either mechanical or inflammatory. Chondrocytes are mechanosensitive cells, that integrate mechanical stimuli into cellular responses in a process known as mechanotransduction. Mechanotransduction occurs thanks to the activation of mechanosensors, a set of specialized proteins that convert physical cues into intracellular signalling cascades. Moderate levels of mechanical loads maintain normal tissue function and have anti-inflammatory effects. In contrast, mechanical over- or under-loading might lead to cartilage destruction and increased expression of pro-inflammatory cytokines. Simultaneously, mechanotransduction processes can regulate and be regulated by pro- and anti-inflammatory soluble mediators, both local (cells of the same joint, i.e., the chondrocytes themselves, infiltrating macrophages, fibroblasts or osteoclasts) and systemic (from other tissues, e.g., adipokines). Thus, the complex process of mechanotransduction might be altered in OA, so that cartilage-preserving chondrocytes adopt a different sensitivity to mechanical signals, and mechanic stimuli positively transduced in the healthy cartilage may become deleterious under OA conditions. This review aims to provide an overview of how the biochemical exposome of chondrocytes can alter important mechanotransduction processes in these cells. Four principal mechanosensors, i.e., integrins, Ca2+ channels, primary cilium and Wnt signalling (canonical and non-canonical) were targeted. For each of these mechanosensors, a brief summary of the response to mechanical loads under healthy or OA conditions is followed by a concise overview of published works that focus on the further regulation of the mechanotransduction pathways by biochemical factors. In conclusion, this paper discusses and explores how biological mediators influence the differential behaviour of chondrocytes under mechanical loads in healthy and primary OA.

Osteochondrodysplasia and the c.1024G>T variant of TRPV4 gene in Scottish Fold cats: genetic and radiographic evaluation

OBJECTIVES

The objectives of this study were to investigate the c.1024G>T SNP in the TRPV4 gene in Scottish Straight and Fold cats, and to evaluate the pattern of skeletal phenotype and the evolution of radiological signs of Scottish Fold osteochondrodysplasia (SFOCD) over time in heterozygous subjects.

METHODS

DNA was obtained from blood samples of 17 cats (Scottish Fold: n = 12; Scottish Straight: n = 5) and subsequently genotyped by sequencing in a 249 bp region of the TRPV4 gene (exon 6), including the known c.1024G>T causative mutation for osteochondrodysplasia. Orthopaedic and radiographic analyses were performed on animals carrying the mutant allele.

RESULTS

Genotyping by sequencing confirmed that all and only the Scottish Fold cats carried the mutant allele in a heterozygous asset. Furthermore, two other exon variants, already described in the literature as silent variants, were found in some of the sampled cats. Comparative orthogonal radiographic views of the shoulder, elbow, carpus, hip, stifle and tarsus were obtained. A mediolateral projection of the thoracic and lumbar column was also performed. Three out of four cats were clinically and radiographically examined again 1.5 years later.

CONCLUSIONS AND RELEVANCE

Although the presence of the mutant allele in all the tested Scottish Fold cats was confirmed, only 1/12 showed clinical signs of SFOCD. Furthermore, no cats in the 1.5-year follow-up showed skeletal changes. Although significant, the c.1024G>T mutation in the TRPV4 gene, supposedly, is not the only cause or risk of developing SFOCD.

An estimation of osteochondrodysplasia prevalence in Australian Scottish Fold cats: a retrospective study using VetCompass Data

BACKGROUND

All Scottish Fold cats are believed to be affected by osteochondrodysplasia, a painful degenerative joint disorder. This retrospective study aimed to estimate the prevalence of osteochondrodysplasia in Scottish Fold and Scottish Straight cats in Australian veterinary clinics using electronic patient records (EPRs), collected between 1992 and 2018.

RESULTS

Consultation events (34,926) in EPRs from veterinary clinics located in New South Wales, Queensland, and Victoria, were collected from 1,131 Scottish Fold and 117 Scottish Shorthair cats. A clinical diagnosis of osteochondrodysplasia was made in 12/1,131 Scottish Fold cats. Additionally, 69 cats were identified with suspected osteochondrodysplasia. Of these, 64 were Scottish Fold and 5 were Scottish Shorthair cats. Male and female cats were equally represented. However, a significant difference was observed for the age clinical signs were first recorded in the EPRs. Cats diagnosed clinically with osteochondrodysplasia were significantly younger (p < 0.0001) compared to cats identified as suspected SFOCD cases.

CONCLUSIONS

Findings from this study suggest a relatively low prevalence of clinically diagnosed Scottish Fold osteochondrodysplasia (SFOCD) in the studied Australian Scottish Fold population, with cats generally diagnosed with SFOCD at less than 30 months of age. Further evidence is required to accurately assess the clinical relevance of SFOCD in the Scottish Fold population.

Inflammatory and Metabolic Signaling Interfaces of the Hypertrophic and Senescent Chondrocyte Phenotypes Associated with Osteoarthritis

Osteoarthritis (OA) is a complex disease of whole joints with progressive cartilage matrix degradation and chondrocyte transformation. The inflammatory features of OA are reflected in increased synovial levels of IL-1β, IL-6 and VEGF, higher levels of TLR-4 binding plasma proteins and increased expression of IL-15, IL-18, IL-10 and Cox2, in cartilage. Chondrocytes in OA undergo hypertrophic and senescent transition; in these states, the expression of Sox-9, Acan and Col2a1 is suppressed, whereas the expression of RunX2, HIF-2α and MMP-13 is significantly increased. NF-kB, which triggers many pro-inflammatory cytokines, works with BMP, Wnt and HIF-2α to link hypertrophy and inflammation. Altered carbohydrate metabolism and the upregulation of GLUT-1 contribute to the formation of end-glycation products that trigger inflammation via the RAGE pathway. In addition, a glycolytic shift, increased rates of oxidative phosphorylation and mitochondrial dysfunction generate reactive oxygen species with deleterious effects. An important surveyor mechanism, the YAP/TAZ signaling system, controls chondrocyte differentiation, inhibits ageing by protecting the nuclear envelope and suppressing NF-kB, MMP-13 and aggrecanases. The inflammatory microenvironment and synthesis of key matrix components are also controlled by SIRT1 and mTORc. Senescent chondrocytes represent the functional end stage of hypertrophic differentiation and characteristically upregulate p16 and p21, but also a variety of inflammatory cytokines, chemokines and metalloproteinases, developing the senescence-associated secretory phenotype. Senolysis with dendrobin, miR29b-5p and other agents has been shown to be efficient under experimental conditions, and appears to be a promising tool for the treatment of OA, as it restores COL2A1 and aggrecan synthesis, suppressing NF-kB and destructive metalloproteinases.

Extracellular matrix-induced signaling pathways in mesenchymal stem/stromal cells

The extracellular matrix (ECM) is a crucial component of the stem cell microenvironment, or stem-cell niches, and contributes to the regulation of cell behavior and fate. Accumulating evidence indicates that different types of stem cells possess a large variety of molecules responsible for interactions with the ECM, mediating specific epigenetic rearrangements and corresponding changes in transcriptome profile. Signals from the ECM are crucial at all stages of ontogenesis, including embryonic and postnatal development, as well as tissue renewal and repair. The ECM could regulate stem cell transition from a quiescent state to readiness to perceive the signals of differentiation induction (competence) and the transition between different stages of differentiation (commitment). Currently, to unveil the complex networks of cellular signaling from the ECM, multiple approaches including screening methods, the analysis of the cell matrixome, and the creation of predictive networks of protein-protein interactions based on experimental data are used. In this review, we consider the existing evidence regarded the contribution of ECM-induced intracellular signaling pathways into the regulation of stem cell differentiation focusing on mesenchymal stem/stromal cells (MSCs) as well-studied type of postnatal stem cells totally depended on signals from ECM. Furthermore, we propose a system biology-based approach for the prediction of ECM-mediated signal transduction pathways in target cells. Video Abstract.

Biomimetic Hyaluronic Acid-Based Brush Polymers Modulate Chondrocyte Homeostasis via ROS/Ca2+/TRPV4

Bionic mimics using natural cartilage matrix molecules can modulate the corresponding metabolic activity by improving the microenvironment of chondrocytes. A bionic brush polymer, HA/PX, has been found to reverse the loss of cartilage extracellular matrix (ECM) and has promising applications in the clinical treatment of osteoarthritis (OA). However, the unknown bioremediation mechanism of HA/PX severely hinders its clinical translation. In OA, the massive loss of the ECM may be attributed to a decrease in transient receptor potential vanilloid 4 (TRPV4) activity, which affects reactive oxygen species (ROS) clearance and [Ca2+]i signaling, initiating downstream catabolic pathways. In this study, we investigated the bioremediation mechanism of HA/PX in a model of interleukin 1β (IL-1β)-induced inflammation. Through TRPV4, HA/PX reduced ROS accumulation in chondrocytes and enhanced [Ca2+]i signaling, reflecting a short-term protection capacity for chondrocytes. In addition, HA/PX balanced the metabolic homeostasis of chondrocytes via TRPV4, including promoting the secretion of type II collagen (Col-II) and aggrecan, the major components of the ECM, and reducing the expression of matrix metal-degrading enzyme (MMP-13), exerting long-term protective effects on chondrocytes. Molecular dynamics (MD) simulations showed that HA/PX could act as a TRPV4 activator. Our results suggest that HA/PX can regulate chondrocyte homeostasis via ROS/Ca2+/TRPV4, thereby improving cartilage regeneration. Because the ECM is a prevalent feature of various cell types, HA/PX holds promising potential for improving regeneration and disease modification for not only cartilage-related healthcare but many other tissues and diseases.

Human Osteoarthritic Chondrocytes Express Nineteen Different TRP-Genes-TRPA1 and TRPM8 as Potential Drug Targets

Transient receptor potential (TRP) ion channels are expressed in neuronal and some non-neuronal cells and are involved particularly in pain and thermosensation. We previously showed that TRPA1 is functionally expressed in human osteoarthritic (OA) chondrocytes and mediates inflammation, cartilage degradation, and pain in monosodium-iodoacetate-induced experimental OA. In the present study, we explored the expression of TRP-channels in primary human OA chondrocytes and investigated whether drugs used in the treatment of OA, ibuprofen and glucocorticoids, have effects on TRP-channel expression. OA cartilage was obtained from knee replacement surgery and chondrocytes were isolated with enzyme digestion. NGS analysis showed the expression of 19 TRP-genes in OA chondrocytes, with TRPM7, TRPV4, TRPC1, and TRPM8 having the highest counts in unstimulated cells. These results were verified with RT-PCR in samples from a different group of patients. Interleukin-1β (IL-1β) significantly increased TRPA1 expression, while TRPM8 and TRPC1 expression was decreased, and TRPM7 and TRPV4 expression remained unaffected. Furthermore, dexamethasone attenuated the effect of IL-1β on TRPA1 and TRPM8 expression. The TRPM8 and TRPA1 agonist menthol increased the expression of the cartilage-degrading enzymes MMP-1, MMP-3, and MMP-13 and the inflammatory factors iNOS and IL-6 in OA chondrocytes. In conclusion, human OA chondrocytes express 19 different TRP-genes, of which the significant TRPM8 expression is a novel finding. Dexamethasone attenuated IL-1β-induced TRPA1 expression. Interestingly, the TRPM8 and TRPA1 agonist menthol increased MMP expression. These results support the concept of TRPA1 and TRMP8 as potential novel drug targets in arthritis.

TRPV4 expression in the renal tubule is necessary for maintaining whole body K+ homeostasis

The Ca2+-permeable transient receptor potential vanilloid type 4 (TRPV4) channel serves as the sensor of tubular flow, thus being well suited to govern mechanosensitive K+ transport in the distal renal tubule. Here, we directly tested whether the TRPV4 function is significant in affecting K+ balance. We used balance metabolic cage experiments and systemic measurements with different K+ feeding regimens [high (5% K+), regular (0.9% K+), and low (<0.01% K+)] in newly created transgenic mice with selective TRPV4 deletion in the renal tubule (TRPV4fl/fl-Pax8Cre) and their littermate controls (TRPV4fl/fl). Deletion was verified by the absence of TRPV4 protein expression and lack of TRPV4-dependent Ca2+ influx. There were no differences in plasma electrolytes, urinary volume, and K+ levels at baseline. In contrast, plasma K+ levels were significantly elevated in TRPV4fl/fl-Pax8Cre mice on high K+ intake. K+-loaded knockout mice exhibited lower urinary K+ levels than TRPV4fl/fl mice, which was accompanied by higher aldosterone levels by day 7. Moreover, TRPV4fl/fl-Pax8Cre mice had more efficient renal K+ conservation and higher plasma K+ levels in the state of dietary K+ deficiency. H+-K+-ATPase levels were significantly increased in TRPV4fl/fl-Pax8Cre mice on a regular diet and especially on a low-K+ diet, pointing to augmented K+ reabsorption in the collecting duct. Consistently, we found a significantly faster intracellular pH recovery after intracellular acidification, as an index of H+-K+-ATPase activity, in split-opened collecting ducts from TRPV4fl/fl-Pax8Cre mice. In summary, our results demonstrate an indispensable prokaliuretic role of TRPV4 in the renal tubule in controlling K+ balance and urinary K+ excretion during variations in dietary K+ intake. NEW & NOTEWORTHY The mechanoactivated transient receptor potential vanilloid type 4 (TRPV4) channel is expressed in distal tubule segments, where it controls flow-dependent K+ transport. Global TRPV4 deficiency causes impaired adaptation to variations in dietary K+ intake. Here, we demonstrate that renal tubule-specific TRPV4 deletion is sufficient to recapitulate the phenotype by causing antikaliuresis and higher plasma K+ levels in both states of K+ load and deficiency.

Alterations in the microenvironment and the effects produced of TRPV5 in osteoporosis

The pathogenesis of osteoporosis involves multiple factors, among which alterations in the bone microenvironment play a crucial role in disrupting normal bone metabolic balance. Transient receptor potential vanilloid 5 (TRPV5), a member of the TRPV family, is an essential determinant of the bone microenvironment, acting at multiple levels to influence its properties. TRPV5 exerts a pivotal influence on bone through the regulation of calcium reabsorption and transportation while also responding to steroid hormones and agonists. Although the metabolic consequences of osteoporosis, such as loss of bone calcium, reduced mineralization capacity, and active osteoclasts, have received significant attention, this review focuses on the changes in the osteoporotic microenvironment and the specific effects of TRPV5 at various levels.

Spatiotemporal Characterization of Human Early Intervertebral Disc Formation at Single-Cell Resolution

The intervertebral disc (IVD) acts as a fibrocartilaginous joint to anchor adjacent vertebrae. Although several studies have demonstrated the cellular heterogeneity of adult mature IVDs, a single-cell transcriptomic atlas mapping early IVD formation is still lacking. Here, the authors generate a spatiotemporal and single cell-based transcriptomic atlas of human IVD formation at the embryonic stage and a comparative mouse transcript landscape. They identify two novel human notochord (NC)/nucleus pulposus (NP) clusters, SRY-box transcription factor 10 (SOX10)⁺ and cathepsin K (CTSK)⁺ , that are distributed in the early and late stages of IVD formation and they are validated by lineage tracing experiments in mice. Matrisome NC/NP clusters, T-box transcription factor T (TBXT)⁺ and CTSK⁺ , are responsible for the extracellular matrix homeostasis. The IVD atlas suggests that a subcluster of the vertebral chondrocyte subcluster might give rise to an inner annulus fibrosus of chondrogenic origin, while the fibroblastic outer annulus fibrosus preferentially expresseds transgelin and fibromodulin . Through analyzing intercellular crosstalk, the authors further find that notochordal secreted phosphoprotein 1 (SPP1) is a novel cue in the IVD microenvironment, and it is associated with IVD development and degeneration. In conclusion, the single-cell transcriptomic atlas will be leveraged to develop preventative and regenerative strategies for IVD degeneration.

Ca2+-Activated K+ Channels in Progenitor Cells of Musculoskeletal Tissues: A Narrative Review

Musculoskeletal disorders represent one of the main causes of disability worldwide, and their prevalence is predicted to increase in the coming decades. Stem cell therapy may be a promising option for the treatment of some of the musculoskeletal diseases. Although significant progress has been made in musculoskeletal stem cell research, osteoarthritis, the most-common musculoskeletal disorder, still lacks curative treatment. To fine-tune stem-cell-based therapy, it is necessary to focus on the underlying biological mechanisms. Ion channels and the bioelectric signals they generate control the proliferation, differentiation, and migration of musculoskeletal progenitor cells. Calcium- and voltage-activated potassium (KCa) channels are key players in cell physiology in cells of the musculoskeletal system. This review article focused on the big conductance (BK) KCa channels. The regulatory function of BK channels requires interactions with diverse sets of proteins that have different functions in tissue-resident stem cells. In this narrative review article, we discuss the main ion channels of musculoskeletal stem cells, with a focus on calcium-dependent potassium channels, especially on the large conductance BK channel. We review their expression and function in progenitor cell proliferation, differentiation, and migration and highlight gaps in current knowledge on their involvement in musculoskeletal diseases.

Comparison of genome-wide DNA methylation patterns between antler precartilage and cartilage

Deer antlers are the only mammalian organs that can fully regenerate after being lost and provide a valuable model for cartilage development. As one of the best-studied epigenetic mechanisms, DNA methylation is known to engage in organ and tissue development. This study aimed to investigate the role of DNA methylation in antler chondrogenesis by comparing whole-genome DNA methylation between precartilage and cartilage. Quantitative reverse transcription PCR (RT-qPCR) showed significant differences in the expression levels of DNA methyltransferase genes (DNMT1, DNMT3A, and DNMT3B) between precartilage and cartilage. Subsequently, we obtained DNA methylation profiles of antler precartilage and cartilage tissues by whole-genome bisulfite sequencing. Although sequencing data indicated that overall methylation levels at CpG and non-CpG sites were similar between precartilage and cartilage, 140,784 differentially methylated regions (DMRs, P < 0.05) and 3,941 DMR-related genes were identified. Gene ontology (GO) analysis of DMR-related genes demonstrated some significantly enriched GO terms (P < 0.05) related to chondrogenesis, including insulin receptor binding, collage trimer, integrin binding, and extracellular matrix structural constituent. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of DMR-related genes uncovered that the PI3K/AKT, cortisol synthesis and secretion, glycosaminoglycan biosynthesis-keratan sulfate, Hippo, and NF-κB signaling pathways might play a pivotal role in the transition of precartilage to cartilage. Moreover, we found that 25 DMR-related genes, including CD44, IGF1, ITGAV, ITGB1, RUNX1, COL2A1, COMP, and TAGLN, were most likely involved in antler chondrogenesis. In conclusion, this study revealed the genome-wide DNA methylation patterns of antler precartilage and cartilage, which may contribute to understanding the epigenetic regulation of antler chondrogenesis.

Skeletal dysplasia-causing TRPV4 mutations suppress the hypertrophic differentiation of human iPSC-derived chondrocytes

Mutations in the TRPV4 ion channel can lead to a range of skeletal dysplasias. However, the mechanisms by which TRPV4 mutations lead to distinct disease severity remain unknown. Here, we use CRISPR-Cas9-edited human-induced pluripotent stem cells (hiPSCs) harboring either the mild V620I or lethal T89I mutations to elucidate the differential effects on channel function and chondrogenic differentiation. We found that hiPSC-derived chondrocytes with the V620I mutation exhibited increased basal currents through TRPV4. However, both mutations showed more rapid calcium signaling with a reduced overall magnitude in response to TRPV4 agonist GSK1016790A compared to wildtype (WT). There were no differences in overall cartilaginous matrix production, but the V620I mutation resulted in reduced mechanical properties of cartilage matrix later in chondrogenesis. mRNA sequencing revealed that both mutations up-regulated several anterior HOX genes and down-regulated antioxidant genes CAT and GSTA1 throughout chondrogenesis. BMP4 treatment up-regulated several essential hypertrophic genes in WT chondrocytes; however, this hypertrophic maturation response was inhibited in mutant chondrocytes. These results indicate that the TRPV4 mutations alter BMP signaling in chondrocytes and prevent proper chondrocyte hypertrophy, as a potential mechanism for dysfunctional skeletal development. Our findings provide potential therapeutic targets for developing treatments for TRPV4-mediated skeletal dysplasias.

Transcriptomic analyses of joint tissues during osteoarthritis development in a rat model reveal dysregulated mechanotransduction and extracellular matrix pathways

OBJECTIVE

Transcriptomic changes in joint tissues during the development of osteoarthritis (OA) are of interest for the discovery of biomarkers and mechanisms of disease. The objective of this study was to use the rat medial meniscus transection (MMT) model to discover stage and tissue-specific transcriptomic changes.

DESIGN

Sham or MMT surgeries were performed in mature rats. Cartilage, menisci and synovium were scored for histopathological changes at 2, 4 and 6 weeks post-surgery and processed for RNA-sequencing. Differentially expressed genes (DEG) were used to identify pathways and mechanisms. Published transcriptomic datasets from animal models and human OA were used to confirm and extend present findings.

RESULTS

The total number of DEGs was already high at 2 weeks (723 in meniscus), followed by cartilage (259) and synovium (42) and declined to varying degrees in meniscus and synovium but increased in cartilage at 6 weeks. The most upregulated genes included tenascins. The 'response to mechanical stimulus' and extracellular matrix-related pathways were enriched in both cartilage and meniscus. Pathways that were enriched in synovium at 4 weeks indicate processes related to synovial hyperplasia and fibrosis. Synovium also showed upregulation of IL-11 and several MMPs. The mechanical stimulus pathway included upregulation of the mechanoreceptors PIEZO1, PIEZO2 and TRPV4 and nerve growth factor. Analysis of data from prior RNA-sequencing studies of animal models and human OA support these findings.

CONCLUSION

These results indicate several shared pathways that are affected during OA in cartilage and meniscus and support the role of mechanotransduction and other pathways in OA pathogenesis.

The role of mechanosensor Piezo1 in bone homeostasis and mechanobiology

Bones and articular cartilage are important load-bearing tissues. The fluid flow inside the bone cells and cell interaction with the extracellular matrix serve as the mechanical cues for bones and joints. Piezo1 is an ion channel found on the cell surface of many cell types, including osteocytes and chondrocytes. It is activated in response to mechanical stimulation, which subsequently mediates a variety of signaling pathways in osteoblasts, osteocytes, and chondrocytes. Piezo1 activation in osteoblastic cells positively regulates osteogenesis, while its activation in joints mediates cartilage degradation. This review focuses on the most recent research on Piezo1 in bone development and regeneration.

Backbone NMR assignments of the extensive human and chicken TRPV4 N-terminal intrinsically disordered regions as important players in ion channel regulation

Transient receptor potential (TRP) channels are important pharmacological targets due to their ability to act as sensory transducers on the organismic and cellular level, as polymodal signal integrators and because of their role in numerous diseases. However, a detailed molecular understanding of the structural dynamics of TRP channels and their integration into larger cellular signalling networks remains challenging, in part due to the systematic absence of highly dynamic regions pivotal for channel regulation from available structures. In human TRP vanilloid 4 (TRPV4), a ubiquitously expressed homotetrameric cation channel involved in temperature, osmo- and mechano-sensation and in a multitude of (patho)physiological processes, the intrinsically disordered N-terminus encompasses 150 amino acids and thus represents > 17% of the entire channel sequence. Its deletion renders the channel significantly less excitable to agonists supporting a crucial role in TRPV4 activation and regulation. For a structural understanding and a comparison of its properties across species, we determined the NMR backbone assignments of the human and chicken TRPV4 N-terminal IDRs.

Local mechanical stimuli correlate with tissue growth in axolotl salamander joint morphogenesis

Movement-induced forces are critical to correct joint formation, but it is unclear how cells sense and respond to these mechanical cues. To study the role of mechanical stimuli in the shaping of the joint, we combined experiments on regenerating axolotl (Ambystoma mexicanum) forelimbs with a poroelastic model of bone rudiment growth. Animals either regrew forelimbs normally (control) or were injected with a transient receptor potential vanilloid 4 (TRPV4) agonist during joint morphogenesis. We quantified growth and shape in regrown humeri from whole-mount light sheet fluorescence images of the regenerated limbs. Results revealed significant differences in morphology and cell proliferation between groups, indicating that TRPV4 desensitization has an effect on joint shape. To link TRPV4 desensitization with impaired mechanosensitivity, we developed a finite element model of a regenerating humerus. Local tissue growth was the sum of a biological contribution proportional to chondrocyte density, which was constant, and a mechanical contribution proportional to fluid pressure. Computational predictions of growth agreed with experimental outcomes of joint shape, suggesting that interstitial pressure driven from cyclic mechanical stimuli promotes local tissue growth. Predictive computational models informed by experimental findings allow us to explore potential physical mechanisms involved in tissue growth to advance our understanding of the mechanobiology of joint morphogenesis.

The Role of Mechanically-Activated Ion Channels Piezo1, Piezo2, and TRPV4 in Chondrocyte Mechanotransduction and Mechano-Therapeutics for Osteoarthritis

Mechanical factors play critical roles in the pathogenesis of joint disorders like osteoarthritis (OA), a prevalent progressive degenerative joint disease that causes debilitating pain. Chondrocytes in the cartilage are responsible for extracellular matrix (ECM) turnover, and mechanical stimuli heavily influence cartilage maintenance, degeneration, and regeneration via mechanotransduction of chondrocytes. Thus, understanding the disease-associated mechanotransduction mechanisms can shed light on developing effective therapeutic strategies for OA through targeting mechanotransducers to halt progressive cartilage degeneration. Mechanosensitive Ca2+-permeating channels are robustly expressed in primary articular chondrocytes and trigger force-dependent cartilage remodeling and injury responses. This review discusses the current understanding of the roles of Piezo1, Piezo2, and TRPV4 mechanosensitive ion channels in cartilage health and disease with a highlight on the potential mechanotheraputic strategies to target these channels and prevent cartilage degeneration associated with OA.

Temperature evolution following joint loading promotes chondrogenesis by synergistic cues via calcium signaling

During loading of viscoelastic tissues, part of the mechanical energy is transformed into heat that can locally increase the tissue temperature, a phenomenon known as self-heating. In the framework of mechanobiology, it has been accepted that cells react and adapt to mechanical stimuli. However, the cellular effect of temperature increase as a by-product of loading has been widely neglected. In this work, we focused on cartilage self-heating to present a 'thermo-mechanobiological' paradigm, and demonstrate how the coupling of a biomimetic temperature evolution and mechanical loading could influence cell behavior. We thereby developed a customized in vitro system allowing to recapitulate pertinent in vivo physical cues and determined the cells chondrogenic response to thermal and/or mechanical stimuli. Cellular mechanisms of action and potential signaling pathways of thermo-mechanotransduction process were also investigated. We found that co-existence of thermo-mechanical cues had a superior effect on chondrogenic gene expression compared to either signal alone. Specifically, the expression of Sox9 was significantly upregulated by application of the physiological thermo-mechanical stimulus. Multimodal transient receptor potential vanilloid 4 (TRPV4) channels were identified as key mediators of thermo-mechanotransduction process, which becomes ineffective without external calcium sources. We also observed that the isolated temperature evolution, as a by-product of loading, is a contributing factor to the cell response and this could be considered as important as the conventional mechanical loading. Providing an optimal thermo-mechanical environment by synergy of heat and loading portrays new opportunity for development of novel treatments for cartilage regeneration and can furthermore signal key elements for emerging cell-based therapies.

TRPV4 and PIEZO Channels Mediate the Mechanosensing of Chondrocytes to the Biomechanical Microenvironment

Articular cartilage and their chondrocytes are physiologically submitted to diverse types of mechanical cues. Chondrocytes produce and maintain the cartilage by sensing and responding to changing mechanical loads. TRPV4 and PIEZOs, activated by mechanical cues, are important mechanosensing molecules of chondrocytes and have pivotal roles in articular cartilage during health and disease. The objective of this review is to introduce the recent progress indicating that the mechanosensitive ion channels, TRPV4 and PIEZOs, are involved in the chondrocyte sensing of mechanical and inflammatory cues. We present a focus on the important role of TRPV4 and PIEZOs in the mechanotransduction regulating diverse chondrocyte functions in the biomechanical microenvironment. The review synthesizes the most recent advances in our understanding of how mechanical stimuli affect various cellular behaviors and functions through differentially activating TRPV4 and PIEZO ion channels in chondrocyte. Advances in understanding the complex roles of TRPV4/PIEZO-mediated mechanosignaling mechanisms have the potential to recapitulate physiological biomechanical microenvironments and design cell-instructive biomaterials for cartilage tissue engineering.

Distribution and Assembly of TRP Ion Channels

In the last several decades, a large family of ion channels have been identified and studied intensively as cellular sensors for diverse physical and/or chemical stimuli. Named transient receptor potential (TRP) channels, they play critical roles in various aspects of cellular physiology. A large number of human hereditary diseases are found to be linked to TRP channel mutations, and their dysregulations lead to acute or chronical health problems. As TRP channels are named and categorized mostly based on sequence homology rather than functional similarities, they exhibit substantial functional diversity. Rapid advances in TRP channel study have been made in recent years and reported in a vast body of literature; a summary of the latest advancements becomes necessary. This chapter offers an overview of current understandings of TRP channel distribution and subunit assembly.

The anabolic effect of inorganic polyphosphate on chondrocytes is mediated by calcium signalling

Inorganic polyphosphates (polyP) are polymers composed of phosphate residues linked by energy-rich phosphoanhydride bonds. As polyP can bind calcium, the hypothesis of this study is that polyP enters chondrocytes and exerts its anabolic effect by calcium influx through calcium channels. PolyP treatment of cartilage tissue formed in 3D culture by bovine chondrocytes showed an increase in proteoglycan accumulation but only when calcium was also present at a concentration of 1.5 mM. This anabolic effect could be prevented by treatment with either ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid or the calcium channel inhibitors gadolinium and nifedipine. Calcium and polyP cotreatment of chondrocytes in monolayer culture resulted in calcium oscillations that were polyP chain length specific and were inhibited by gadolinium and nifedipine. The calcium influx resulted in increased gene expression of sox9, collagen type II, and aggrecan which was prevented by treatment with either calphostin, an inhibitor of protein kinase C, and W7, an inhibitor of calmodulin; suggesting activation of the protein kinase C-calmodulin pathway. Tracing studies using  4',6-diamidino-2-phenylindole, Mitotracker Red, and/or Fura-AM staining showed that polyP was detected in the nucleus, mitochondria, and intracellular vacuoles suggesting that polyP may also enter the cell. PolyP colocalizes with calcium in mitochondria. This study demonstrates that polyP requires the influx of calcium to regulate chondrocyte matrix production, likely via activating calcium signaling. These findings identify the mechanism regulating the anabolic effect of polyP in chondrocytes which will help in its clinical translation into a therapeutic agent for cartilage repair.

Pthlha and mechanical force control early patterning of growth zones in the zebrafish craniofacial skeleton

Secreted signals in patterning systems often induce repressive signals that shape their distributions in space and time. In developing growth plates (GPs) of endochondral long bones, Parathyroid hormone-like hormone (Pthlh) inhibits Indian hedgehog (Ihh) to form a negative-feedback loop that controls GP progression and bone size. Whether similar systems operate in other bones and how they arise during embryogenesis remain unclear. We show that Pthlha expression in the zebrafish craniofacial skeleton precedes chondrocyte differentiation and restricts where cells undergo hypertrophy, thereby initiating a future GP. Loss of Pthlha leads to an expansion of cells expressing a novel early marker of the hypertrophic zone (HZ), entpd5a, and later HZ markers, such as ihha, whereas local Pthlha misexpression induces ectopic entpd5a expression. Formation of this early pre-HZ correlates with onset of muscle contraction and requires mechanical force; paralysis leads to loss of entpd5a and ihha expression in the pre-HZ, mislocalized pthlha expression and no subsequent ossification. These results suggest that local Pthlh sources combined with force determine HZ locations, establishing the negative-feedback loop that later maintains GPs.

INTRACELLULAR CALCIUM AND SODIUM MODULATION OF SELF-ASSEMBLED NEOCARTILAGE USING COSTAL CHONDROCYTES

Ion signaling via Ca²⁺ and Na⁺ plays a key role in mechanotransduction and encourages a chondrogenic phenotype and tissue maturation. Here, we propose that the pleiotropic effects of Ca²⁺ and Na⁺ modulation can be used to induce maturation and improvement of neocartilage derived from re-differentiated expanded chondrocytes from minipig rib cartilage. Three ion modulators were employed: 1) 4α-phorbol-12,13-didecanoate (4-αPDD), an agonist of the Ca²⁺-permeable transient receptor potential vanilloid 4 (TRPV4), 2) ouabain, an inhibitor of the Na⁺/K⁺ pump, and 3) ionomycin, a Ca²⁺ ionophore. These ion modulators were used individually or in combination. While no beneficial effects were observed when using combinations of the ion modulators, single treatment of constructs with the three ion modulators resulted in multiple effects in structure-function relationships. The most significant findings were related to ionomycin. Treatment of neocartilage with ionomycin produced 61% and 115% increases in glycosaminoglycan and pyridinoline crosslink content, respectively, compared to the control. Moreover, treatment with this Ca²⁺ ionophore resulted in a 45% increase of the aggregate modulus, and a 63% increase in the tensile Young's modulus, resulting in aggregate and Young's moduli of 567 kPa and 8.43 MPa, respectively. These results support the use of ion modulation to develop biomimetic neocartilage using expanded re-differentiated costal chondrocytes.

Insulin-like growth factor-1 regulates the mechanosensitivity of chondrocytes by modulating TRPV4

Both mechanical and biochemical stimulation are required for maintaining the integrity of articular cartilage. However, chondrocytes respond differently to mechanical stimuli in osteoarthritic cartilage when biochemical signaling pathways, such as Insulin-like Growth Factor-1 (IGF-1), are altered. The Transient Receptor Potential Vanilloid 4 (TRPV4) channel is central to chondrocyte mechanotransduction and regulation of cartilage homeostasis. Here, we propose that changes in IGF-1 can modulate TRPV4 channel activity. We demonstrate that physiologic levels of IGF-1 suppress hypotonic-induced TRPV4 currents and intracellular calcium flux by increasing apparent cell stiffness that correlates with actin stress fiber formation. Disruption of F-actin following IGF-1 treatment results in the return of the intracellular calcium response to hypotonic swelling. Using point mutations of the TRPV4 channel at the microtubule-associated protein 7 (MAP-7) site shows that regulation of TRPV4 by actin is mediated via the interaction of actin with the MAP-7 domain of TRPV4. We further highlight that ATP release, a down-stream response to mechanical stimulation in chondrocytes, is mediated by TRPV4 during hypotonic challenge. This response is significantly abrogated with IGF-1 treatment. As chondrocyte mechanosensitivity is greatly altered during osteoarthritis progression, IGF-1 presents as a promising candidate for prevention and treatment of articular cartilage damage.

Neuromedin B promotes chondrocyte differentiation of mesenchymal stromal cells via calcineurin and calcium signaling

Background

Articular cartilage is a complex tissue with poor healing capacities. Current approaches for cartilage repair based on mesenchymal stromal cells (MSCs) are often disappointing because of the lack of relevant differentiation factors that could drive MSC differentiation towards a stable mature chondrocyte phenotype.

Results

We used a large-scale transcriptomic approach to identify genes that are modulated at early stages of chondrogenic differentiation using the reference cartilage micropellet model. We identified several modulated genes and selected neuromedin B (NMB) as one of the early and transiently modulated genes. We found that the timely regulated increase of NMB was specific for chondrogenesis and not observed during osteogenesis or adipogenesis. Furthermore, NMB expression levels correlated with the differentiation capacity of MSCs and its inhibition resulted in impaired chondrogenic differentiation indicating that NMB is required for chondrogenesis. We further showed that NMB activated the calcineurin activity through a Ca²⁺-dependent signaling pathway.

Conclusion

NMB is a newly described chondroinductive bioactive factor that upregulates the key chondrogenic transcription factor Sox9 through the modulation of Ca²⁺ signaling pathway and calcineurin activity.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00695-1.

Transient receptor potential vanilloid 4 as a regulator of induced pluripotent stem cell chondrogenesis

Transient receptor potential vanilloid 4 (TRPV4) is a polymodal calcium-permeable cation channel that is highly expressed in cartilage and is sensitive to a variety of extracellular stimuli. The expression of this channel has been associated with the process of chondrogenesis in adult stem cells as well as several cell lines. Here, we used a chondrogenic reporter (Col2a1-GFP) in murine induced pluripotent stem cells (iPSCs) to examine the hypothesis that TRPV4 serves as both a marker and a regulator of chondrogenesis. Over 21 days of chondrogenesis, iPSCs showed significant increases in Trpv4 expression along with the standard chondrogenic gene markers Sox9, Acan, and Col2a1, particularly in the green fluorescent protein positive (GFP+) chondroprogenitor subpopulation. Increased gene expression for Trpv4 was also reflected by the presence of TRPV4 protein and functional Ca²⁺ signaling. Daily activation of TRPV4 using the specific agonist GSK1016790A resulted in significant increases in cartilaginous matrix production. An improved understanding of the role of TRPV4 in chondrogenesis may provide new insights into the development of new therapeutic approaches for diseases of cartilage, such as osteoarthritis, or channelopathies and hereditary disorders that affect cartilage during development. Harnessing the role of TRPV4 in chondrogenesis may also provide a novel approach for accelerating stem cell differentiation in functional tissue engineering of cartilage replacements for joint repair.

Dynamic loading enhances chondrogenesis of human chondrocytes within a biodegradable resilient hydrogel

Hyaline cartilage in the knee joint is a soft tissue that is both stiff and elastic, which raises unique challenges in developing scaffolds for the repair of cartilage injury. In this study, we mixed poly-d,l-lactic acid/polyethylene glycol/poly-d,l-lactic acid (PEG-PDLLA-DA) with polycaprolactone-poly(ethylene glycol)-polycaprolactone (PEG-PCL-DA) with the aim to create a cartilage-like hydrogel. Results indicated that the hydrogel made from PEG-PDLLA-DA/PEG-PCL-DA (50/50) was biodegradable and resilient, able to bear compressive loads with strains up to 50%. Human chondrocytes maintained high viability after being seeded in the hydrogel and underwent robust chondrogenesis upon stimulation. The application of dynamic compressive loading further promoted the generation of cartilage matrix and increased the compressive moduli of engineered cartilage tissues. Then engineered cartilage tissues, with or without being stimulated by dynamic loading, were implanted subcutaneously in mice, and results showed that the cartilage matrices and chondrocyte phenotypes were well preserved. Lastly, we conducted the mechanistic study to understand how dynamic loading influenced chondrogenesis. Specifically, the levels p-Erk and p38 kinases were found to remarkably increase on day 1 upon dynamic compressive loading, decrease on day 3, and then slightly elevate on day 7. In comparison, the expression of YAP and RhoA peaked on day 3 after mechanical loading. Levels of PIEZO1 and TRPV4 protein increased with the extension of dynamic loading culture time. Taken together, this newly developed resilient hydrogel represents a robust scaffold for cartilage regeneration. Moreover, based on the time their levels reach the peak, three groups of proteins are identified in mediating chondrocyte response to dynamic loading, which has not been previously reported.

Regulation of TGFβ Signalling by TRPV4 in Chondrocytes

The growth factor TGFβ and the mechanosensitive calcium-permeable cation channel TRPV4 are both important for the development and maintenance of many tissues. Although TRPV4 and TGFβ both affect core cellular functions, how their signals are integrated is unknown. Here we show that pharmacological activation of TRPV4 significantly increased the canonical response to TGFβ stimulation in chondrocytes. Critically, this increase was only observed when TRPV4 was activated after, but not before TGFβ stimulation. The increase was prevented by pharmacological TRPV4 inhibition or knockdown and is calcium/CamKII dependent. RNA-seq analysis after TRPV4 activation showed enrichment for the TGFβ signalling pathway and identified JUN and SP1 as key transcription factors involved in this response. TRPV4 modulation of TGFβ signalling represents an important pathway linking mechanical signalling to tissue development and homeostasis.

Dental Pulp-Derived Mesenchymal Stem Cells for Modeling Genetic Disorders

A subpopulation of mesenchymal stem cells, developmentally derived from multipotent neural crest cells that form multiple facial tissues, resides within the dental pulp of human teeth. These stem cells show high proliferative capacity in vitro and are multipotent, including adipogenic, myogenic, osteogenic, chondrogenic, and neurogenic potential. Teeth containing viable cells are harvested via minimally invasive procedures, based on various clinical diagnoses, but then usually discarded as medical waste, indicating the relatively low ethical considerations to reuse these cells for medical applications. Previous studies have demonstrated that stem cells derived from healthy subjects are an excellent source for cell-based medicine, tissue regeneration, and bioengineering. Furthermore, stem cells donated by patients affected by genetic disorders can serve as in vitro models of disease-specific genetic variants, indicating additional applications of these stem cells with high plasticity. This review discusses the benefits, limitations, and perspectives of patient-derived dental pulp stem cells as alternatives that may complement other excellent, yet incomplete stem cell models, such as induced pluripotent stem cells, together with our recent data.

Role of mitochondria in mediating chondrocyte response to mechanical stimuli

As the most common form of arthritis, osteoarthritis (OA) has become a major cause of severe joint pain, physical disability, and quality of life impairment in the affected population. To date, precise pathogenesis of OA has not been fully clarified, which leads to significant obstacles in developing efficacious treatments such as failures in finding disease-modifying OA drugs (DMOADs) in the last decades. Given that diarthrodial joints primarily display the weight-bearing and movement-supporting function, it is not surprising that mechanical stress represents one of the major risk factors for OA. However, the inner connection between mechanical stress and OA onset/progression has yet to be explored. Mitochondrion, a widespread organelle involved in complex biological regulation processes such as adenosine triphosphate (ATP) synthesis and cellular metabolism, is believed to have a controlling role in the survival and function implement of chondrocytes, the singular cell type within cartilage. Mitochondrial dysfunction has also been observed in osteoarthritic chondrocytes. In this review, we systemically summarize mitochondrial alterations in chondrocytes during OA progression and discuss our recent progress in understanding the potential role of mitochondria in mediating mechanical stress-associated osteoarthritic alterations of chondrocytes. In particular, we propose the potential signaling pathways that may regulate this process, which provide new views and therapeutic targets for the prevention and treatment of mechanical stress-associated OA.

Structural determinants of TRPV4 inhibition and identification of new antagonists with antiviral activity

Background and Purpose

The transient receptor potential vanilloid 4 (TRPV4) cation channel participates in multiple physiological processes and is also at the core of different diseases, making this channel an interesting pharmacological target with therapeutic potential. However, little is known about the structural elements governing its inhibition.

Experimental Approach

We have now combined in silico drug discovery and molecular dynamics simulation based on Xenopus tropicalis xTRPV4 structure with functional studies measuring cell Ca²⁺ influx mediated by human TRPV4 channel to characterize the binding site of known TRPV4 inhibitors and to identify novel small molecule channel modulators.

Key Results

We have found that the inhibitor HC067047 binds to a pocket conformed by residues from S2–S3 linker (xTRPV4‐D542), S4 (xTRPV4‐M583 and Y587 and S5 (xTRPV4‐D609 and F613). This pocket was also used for structure‐based virtual screening in the search of novel channel modulators. Forty potential hits were selected based on the lower docking scores (from ~250,000 compounds) and their effect upon TRPV4 functionally tested. Three were further analysed for stability using molecular dynamics simulation and functionally tested on TRPV4 channels carrying mutations in the binding pocket. Compound NSC151066, shown to require residue xTRPV4‐M583 for its inhibitory effect, presented an IC₅₀ of 145 nM and demonstrated to be an effective antiviral against Zika virus with a potency similar to HC067047.

Conclusion and Implications

Together, we propose structural insights into the inhibition of TRPV4 and how this information can be used for the design of novel channel modulators.

LINKED ARTICLES

This article is part of a themed issue on Structure Guided Pharmacology of Membrane Proteins (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.14/issuetoc

TRPV4 activity regulates nuclear Ca2+ and transcriptional functions of β-catenin in a renal epithelial cell model

TRPV4 is a nonselective cationic channel responsive to several physical and chemical stimuli. Defects in TRPV4 channel function result in human diseases, such as skeletal dysplasias, arthropathies, and peripheral neuropathies. Nonetheless, little is known about the role of TRPV4 in other cellular functions, such as nuclear Ca²⁺ homeostasis or Ca²⁺ -regulated transcription. Here, we confirmed the presence of the full-length TRPV4 channel in the nuclei of nonpolarized Madin-Darby canine kidney cells. Confocal Ca²⁺ imaging showed that activation of the channel increases cytoplasmic and nuclear Ca²⁺ leading to translocation of TRPV4 out of the nucleus together with β-catenin, a transcriptional regulator in the Wnt signaling pathway fundamental in embryogenesis, organogenesis, and cellular homeostasis. TRPV4 inhibits β-catenin transcriptional activity through a direct interaction dependent upon channel activity. This interaction also occurs in undifferentiated osteoblastoma and neuroblastoma cell models. Our results suggest a mechanism in which TRPV4 may regulate differentiation in several cellular contexts.

Roles of CCN2 as a mechano-sensing regulator of chondrocyte differentiation

Cellular communication network factor 2 (CCN2) is a cysteine-rich secreted matricellular protein that regulates various cellular functions including cell differentiation. CCN2 is highly expressed under several types of mechanical stress, such as stretch, compression, and shear stress, in mesenchymal cells including chondrocytes, osteoblasts, and fibroblasts. In particular, CCN2 not only promotes cell proliferation and differentiation of various cells but also regulates the stability of mRNA of TRPV4, a mechanosensitive ion channel in chondrocytes. Of note, CCN2 behaves like a biomarker to sense suitable mechanical stress, because CCN2 expression is down-regulated when chondrocytes are subjected to excessive mechanical stress. These findings suggest that CCN2 is a mechano-sensing regulator. CCN2 expression is regulated by the activation of various mechano-sensing signaling pathways, e.g., mechanosensitive ion channels, integrin-focal adhesion-actin dynamics, Rho GTPase family members, Hippo-YAP signaling, and G protein-coupled receptors. This review summarizes the characterization of mechanoreceptors involved in CCN2 gene regulation and discusses the role of CCN2 as a mechano-sensing regulator of mesenchymal cell differentiation, with particular focus on chondrocytes.

An immortalized human adipose-derived stem cell line with highly enhanced chondrogenic properties

Human adipose-derived stem cells (ASCs) are a commonly used cell type for cartilage tissue engineering. However, donor-to-donor variability, cell heterogeneity, inconsistent chondrogenic potential, and limited expansion potential can hinder the use of these cells for modeling chondrogenesis, in vitro screening of drugs and treatments for joint diseases, or translational applications for tissue engineered cartilage repair. The goal of this study was to create an immortalized ASC line that showed enhanced and consistent chondrogenic potential for applications in cartilage tissue engineering as well as to provide a platform for investigation of biological and mechanobiological pathways involved in cartilage homeostasis and disease. Starting with the ASC52telo cell line, a hTERT-immortalized ASC line, we used lentivirus to overexpress SOX9, a master regulator of chondrogenesis, and screened several clonal populations of SOX9 overexpressing cells to form a new stable cell line with high chondrogenic potential. One clonal line, named ASC52telo-SOX9, displayed increased GAG and type II collagen synthesis and was found to be responsive to both mechanical and inflammatory stimuli in a manner similar to native chondrocytes. The development of a clonal line such as ASC52telo-SOX9 has the potential to be a powerful tool for studying cartilage homeostasis and disease mechanisms in vitro, and potentially as a platform for in vitro drug screening for diseases that affect articular cartilage. Our findings provide an approach for the development of other immortalized cell lines with improved chondrogenic capabilities in ASCs or other adult stem cells.