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

Chondroprotective role of the osmotically sensitive ion channel transient receptor potential vanilloid 4: age- and sex-dependent progression of osteoarthritis in Trpv4-deficient mice

OBJECTIVE

Mechanical loading significantly influences the physiology and pathology of articular cartilage, although the mechanisms of mechanical signal transduction are not fully understood. Transient receptor potential vanilloid 4 (TRPV4) is a Ca(++)-permeable ion channel that is highly expressed by articular chondrocytes and can be gated by osmotic and mechanical stimuli. The goal of this study was to determine the role of Trpv4 in the structure of the mouse knee joint and to determine whether Trpv4(-/-) mice exhibit altered Ca(++) signaling in response to osmotic challenge.

METHODS

Knee joints of Trpv4(-/-) mice were examined histologically and by microfocal computed tomography for osteoarthritic changes and bone structure at ages 4, 6, 9, and 12 months. Fluorescence imaging was used to quantify chondrocytic Ca(++) signaling within intact femoral cartilage in response to osmotic stimuli.

RESULTS

Deletion of Trpv4 resulted in severe osteoarthritic changes, including cartilage fibrillation, eburnation, and loss of proteoglycans, that were dependent on age and male sex. Subchondral bone volume and calcified meniscal volume were greatly increased, again in male mice. Chondrocytes from Trpv4(+/+) mice demonstrated significant Ca(++) responses to hypo-osmotic stress but not to hyperosmotic stress. The response to hypo-osmotic stress or to the TRPV4 agonist 4α-phorbol 12,13-didecanoate was eliminated in Trpv4(-/-) mice.

CONCLUSION

Deletion of Trpv4 leads to a lack of osmotically induced Ca(++) signaling in articular chondrocytes, accompanied by progressive, sex-dependent increases in bone density and osteoarthritic joint degeneration. These findings suggest a critical role for TRPV4-mediated Ca(++) signaling in the maintenance of joint health and normal skeletal structure.

The transcription factor Znf219 regulates chondrocyte differentiation by assembling a transcription factory with Sox9

Sox9 is an essential transcription factor for chondrogenesis by regulating the expression of chondrogenic genes. However, its regulatory mechanism is not fully understood. To address this, we attempted to identify the transcriptional partners of Sox9 by screening the cDNA library of the chondrogenic cell line ATDC5 using the collagen 2α1 (Col2α1) gene promoter fused to a luciferase reporter gene. One of the positive clones encoded the Znf219 gene. Whole mount in situ hybridization experiments indicated that Znf219 mRNA was specifically expressed in the developing limb buds where Col2α1 and Sox9 were strongly expressed. Znf219 markedly enhanced the transcriptional activity of Sox9 on the Col2a1 gene promoter. In addition, Znf219 is physically associated with Sox9 and is colocalized with Sox9 in the nucleus. We also found that overexpression of Znf219 profoundly increased Sox9-induced mRNA expression of Col2a1, aggrecan and Col11a2. Consistently, knockdown of Znf219 decreased the Sox9-induced mRNA expression of these genes. Furthermore, a dominant-negative mutant Znf219 inhibited Bmp2-induced chondrocyte differentiation. Our results suggest that Znf219 plays an important role in the regulation of chondrocyte differentiation as a transcriptional partner of Sox9.

The emerging chondrocyte channelome

Chondrocytes are the resident cells of articular cartilage and are responsible for synthesizing a range of collagenous and non-collagenous extracellular matrix macromolecules. Whilst chondrocytes exist at low densities in the tissue (1-10% of the total tissue volume in mature cartilage) they are extremely active cells and are capable of responding to a range of mechanical and biochemical stimuli. These responses are necessary for the maintenance of viable cartilage and may be compromised in inflammatory diseases such as arthritis. Although chondrocytes are non-excitable cells their plasma membrane contains a rich complement of ion channels. This diverse channelome appears to be as complex as one might expect to find in excitable cells although, in the case of chondrocytes, their functions are far less well understood. The ion channels so far identified in chondrocytes include potassium channels (K(ATP), BK, K(v), and SK), sodium channels (epithelial sodium channels, voltage activated sodium channels), transient receptor potential calcium or non-selective cation channels and chloride channels. In this review we describe this emerging channelome and discuss the possible functions of a range of chondrocyte ion channels.

The role of transient receptor potential cation channels in Ca2+ signaling

The 28 mammalian members of the super-family of transient receptor potential (TRP) channels are cation channels, mostly permeable to both monovalent and divalent cations, and can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are widely expressed in a large number of different tissues and cell types, and their biological roles appear to be equally diverse. In general, considered as polymodal cell sensors, they play a much more diverse role than anticipated. Functionally, TRP channels, when activated, cause cell depolarization, which may trigger a plethora of voltage-dependent ion channels. Upon stimulation, Ca2+ permeable TRP channels generate changes in the intracellular Ca2+ concentration, [Ca2+]i, by Ca2+ entry via the plasma membrane. However, more and more evidence is arising that TRP channels are also located in intracellular organelles and serve as intracellular Ca2+ release channels. This review focuses on three major tasks of TRP channels: (1) the function of TRP channels as Ca2+ entry channels; (2) the electrogenic actions of TRPs; and (3) TRPs as Ca2+ release channels in intracellular organelles.

Dominant TRPV4 mutations in nonlethal and lethal metatropic dysplasia

Metatropic dysplasia is a clinical heterogeneous skeletal dysplasia characterized by short extremities, a short trunk with progressive kyphoscoliosis, and craniofacial abnormalities that include a prominent forehead, midface hypoplasia, and a squared-off jaw. Dominant mutations in the gene encoding TRPV4, a calcium permeable ion channel, were identified all 10 of a series of metatropic dysplasia cases, ranging in severity from mild to perinatal lethal. These data demonstrate that the lethal form of the disorder is dominantly inherited and suggest locus homogeneity in the disease. Electrophysiological studies demonstrated that the mutations activate the channel, indicating that the mechanism of disease may result from increased calcium in chondrocytes. Histological studies in two cases of lethal metatropic dysplasia revealed markedly disrupted endochondral ossification, with reduced numbers of hypertrophic chondrocytes and presence of islands of cartilage within the zone of primary mineralization. These data suggest that altered chondrocyte differentiation in the growth plate leads to the clinical findings in metatropic dysplasia.

Regulation of BMP-dependent chondrogenesis in early limb mesenchyme by TGFbeta signals

In the developing axial skeleton, sequential sonic hedgehog (SHH) and bone morphogenetic protein (BMP) signals are required for specification of a chondrogenic fate in presomitic tissue. A similar paradigm is thought to operate in the limb, but the signals involved are unclear. To investigate the nature of these signals, we examined BMP action in mesenchymal populations derived from the early murine limb bud (approximately embryonic day 10.5). These populations exhibited a graded response to BMPs, in which early limb mesenchymal cells (from the distal hind limb) displayed an anti-chondrogenic response, whereas BMPs promoted chondrogenesis in more mature cell populations (from the proximal fore limb). Under these conditions, multiple Gata genes were induced by BMPs and the extent of induction correlated with BMP anti-chondrogenic activity. A screen of limb-bud-expressed ligands revealed that prior short-term exposure to transforming growth factor beta1 (TGFbeta1) ameliorated the anti-chondrogenic response to BMP. Furthermore, brief activation of the TGFbeta pathway was found to be necessary for subsequent induction of chondrogenesis by BMPs. Our findings indicate that, similar to axial skeletogenesis, induction of chondrogenesis in the appendicular skeleton is a two-step process. However, the programs differ in the transient signals driving chondrogenic responsiveness to BMPs, with SHH operating in the former and TGFbeta activation in the latter.

Transient receptor potential vanilloid 4: The sixth sense of the musculoskeletal system?

The critical discovery in the past two decades of the transient receptor potential (TRP) superfamily of ion channels has revealed the potential mechanisms by which cells sense diverse stimuli beyond the prototypical "five senses," identifying ion channels that are gated by heat, cold, mechanical loading, osmolarity, and other physical and chemical stimuli. TRP vanilloid 4 (TRPV4) is a Ca(2+)-permeable nonselective cation channel that appears to play a mechanosensory or osmosensory role in several musculoskeletal tissues. In articular cartilage, TRPV4 exhibits osmotic sensitivity, controlling cellular volume recovery, and other physiologic responses to osmotic stress. TRPV4 is expressed in both osteoblasts and osteoclasts, and the absence of TRPV4 prevents disuse-induced bone loss. TRPV4 activation promotes chondrogenesis by inducing SOX9 transcription, whereas a TRPV4 gain-of-function mutation leads to a developmental skeletal dysplasia, suggesting a critical role for TRPV4 in skeletal development. These studies provide mounting evidence for a regulatory role for the sensory channel TRPV4 in control of musculoskeletal tissues.

Cellular functions of transient receptor potential channels

Transient Receptor Potential channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by increasing cellular Ca(2+). In this review we focus on the roles of these channels in: (i) cell death (ii) proliferation and differentiation and (iii) transmitter release. Cell death: Ca(2+) influx participates in apoptotic and necrotic cell death. The Ca(2+) permeability and high sensitivity of part of these channels to oxidative/metabolic stress make them important participants in cell death. Several examples are given. Transient Receptor Potential Melastatin 2 is activated by H(2)O(2), inducing cell death through an increase in cellular Ca(2+) and activation of Poly ADP-Ribose Polymerase. Exposure of cultured cortical neurons to oxygen-glucose deprivation, in vitro, causes cell death via cation influx, mediated by Transient Receptor Potential Melastatin 7. Metabolic stress constitutively activates the Ca(2+) permeable Transient Receptor Potential channels of Drosophila photoreceptor in the dark, potentially leading to retinal degeneration. Similar sensitivity to metabolic stress characterizes several mammalian Transient Receptor Potential Canonical channels. Proliferation and differentiation: The rise in cytosolic Ca(2+) induces cell growth, differentiation and proliferation via activation of several transcription factors. Activating a variety of store operated and Transient Receptor Potential channels cause a rise in cytosolic Ca(2+), making these channels components involved in proliferation and differentiation. Transmitter release: Transient Receptor Potential Melastatin 7 channels reside in synaptic vesicles and regulate neurotransmitter release by a mechanism that is not entirely clear. All the above features of Transient Receptor Potential channels make them crucial components in important, sometimes conflicting, cellular processes that still need to be explored.

Functional characterization of TRPV4 as an osmotically sensitive ion channel in porcine articular chondrocytes

OBJECTIVE

Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+)-permeable channel that can be gated by tonicity (osmolarity) and mechanical stimuli. Chondrocytes, the cells in cartilage, respond to their osmotic and mechanical environments; however, the molecular basis of this signal transduction is not fully understood. This study was undertaken to demonstrate the presence and functionality of TRPV4 in chondrocytes.

METHODS

TRPV4 protein expression was measured by immunolabeling and Western blotting. In response to TRPV4 agonist/antagonists, osmotic stress, and interleukin-1 (IL-1), changes in Ca(2+) signaling, cell volume, and prostaglandin E(2) (PGE(2)) production were measured in porcine chondrocytes using fluorescence microscopy, light microscopy, or immunoassay, respectively.

RESULTS

TRPV4 was expressed abundantly at the RNA and protein levels. Exposure to 4alpha-phorbol 12,13-didecanoate (4alphaPDD), a TRPV4 activator, caused Ca(2+) signaling in chondrocytes, which was blocked by the selective TRPV4 antagonist, GSK205. Blocking TRPV4 diminished the chondrocytes' response to hypo-osmotic stress, reducing the fraction of Ca(2+) responsive cells, the regulatory volume decrease, and PGE(2) production. Ca(2+) signaling was inhibited by removal of extracellular Ca(2+) or depletion of intracellular stores. Specific activation of TRPV4 restored the defective regulatory volume decrease caused by IL-1. Chemical disruption of the primary cilium eliminated Ca(2+) signaling in response to either 4alphaPDD or hypo-osmotic stress.

CONCLUSION

Our findings indicate that TRPV4 is present in articular chondrocytes, and chondrocyte response to hypo-osmotic stress is mediated by this channel, which involves both an extracellular Ca(2+) and intracellular Ca(2+) release. TRPV4 may also be involved in modulating the production or influence of proinflammatory molecules in response to osmotic stress.

The vanilloid transient receptor potential channel TRPV4: from structure to disease

The Transient Receptor Potential Vanilloid 4 channel, TRPV4, is a Ca(2+) and Mg(2+) permeable non-selective cation channel involved in many different cellular functions. It is activated by a variety of physical and chemical stimuli, including heat, mechano-stimuli, endogenous substances such as arachidonic acid and its cytochrome P450-derived metabolites (epoxyeicosatrienoic acids), endocannabinoids (anandamide and 2-arachidonoylglycerol), as well as synthetic alpha-phorbol derivatives. Recently, TRPV4 has been characterized as an important player modulating osteoclast differentiation in bone remodelling and as a urothelial mechanosensor that controls normal voiding. Several TRPV4 gain-of-function mutations are shown to cause autosomal-dominant bone dysplasias such as brachyolmia and Koszlowski disease. In this review we comprehensively describe the structural, biophysical and (patho)physiological properties of the TRPV4 channel and we summarize the current knowledge about the role of TRPV4 in the pathogenesis of several diseases.

Regulation of endoplasmic reticulum stress response by a BBF2H7-mediated Sec23a pathway is essential for chondrogenesis

Many tissues have a specific signal transduction system for endoplasmic reticulum (ER) dysfunction; however, the mechanisms underlying the ER stress response in cartilage remain unclear. BBF2H7 (BBF2 human homologue on chromosome 7), an ER-resident basic leucine zipper transcription factor, is activated in response to ER stress and is highly expressed in chondrocytes. In this study, we generated Bbf2h7(-/-) mice to assess the in vivo function of BBF2H7. The mice showed severe chondrodysplasia and died by suffocation shortly after birth because of an immature chest cavity. The cartilage showed a lack of typical columnar structure in the proliferating zone and a decrease in the size of the hypertrophic zone, resulting in a significant reduction of extracellular matrix proteins. Interestingly, proliferating chondrocytes showed abnormally expanded ER, containing aggregated type II collagen (Col2) and cartilage oligomeric matrix protein (COMP). We identified Sec23a, which encodes a coat protein complex II component responsible for protein transport from the ER to the Golgi, as a target of BBF2H7, which directly bound to a CRE-like sequence in the promoter region of Sec23a to activate its transcription. When Sec23a was introduced to Bbf2h7(-/-) chondrocytes, the impaired transport and secretion of cartilage matrix proteins was totally restored, indicating that by activating protein secretion the BBF2H7-Sec23a pathway has a crucial role in chondrogenesis. Our findings provide a new link by which ER stress is converted to signalling for the activation of ER-to-Golgi trafficking.

Control of stem cell fate by physical interactions with the extracellular matrix

A diverse array of environmental factors contributes to the overall control of stem cell activity. In particular, new data continue to mount on the influence of the extracellular matrix (ECM) on stem cell fate through physical interactions with cells, such as the control of cell geometry, ECM geometry/topography at the nanoscale, ECM mechanical properties, and the transmission of mechanical or other biophysical factors to the cell. Here, we review some of the physical processes by which cues from the ECM can influence stem cell fate, with particular relevance to the use of stem cells in tissue engineering and regenerative medicine.

Paraspeckle protein p54nrb links Sox9-mediated transcription with RNA processing during chondrogenesis in mice

The Sox9 transcription factor plays an essential role in promoting chondrogenesis and regulating expression of chondrocyte extracellular-matrix genes. To identify genes that interact with Sox9 in promoting chondrocyte differentiation, we screened a cDNA library generated from the murine chondrogenic ATDC5 cell line to identify activators of the collagen, type II, alpha 1 (Col2a1) promoter. Here we have shown that paraspeckle regulatory protein 54-kDa nuclear RNA-binding protein (p54nrb) is an essential link between Sox9-regulated transcription and maturation of Sox9-target gene mRNA. We found that p54nrb physically interacted with Sox9 and enhanced Sox9-dependent transcriptional activation of the Col2a1 promoter. In ATDC5 cells, p54nrb colocalized with Sox9 protein in nuclear paraspeckle bodies, and knockdown of p54(nrb) suppressed Sox9-dependent Col2a1 expression and promoter activity. We generated a p54nrb mutant construct lacking RNA recognition motifs, and overexpression of mutant p54nrb in ATDC5 cells markedly altered the appearance of paraspeckle bodies and inhibited the maturation of Col2a1 mRNA. The mutant p54nrb inhibited chondrocyte differentiation of mesenchymal cells and mouse metatarsal explants. Furthermore, transgenic mice expressing the mutant p54nrb in the chondrocyte lineage exhibited dwarfism associated with impairment of chondrogenesis. These data suggest that p54nrb plays an important role in the regulation of Sox9 function and the formation of paraspeckle bodies during chondrogenesis.

Bril: a novel bone-specific modulator of mineralization

In the course of attempting to define the bone "secretome" using a signal-trap screening approach, we identified a gene encoding a small membrane protein novel to osteoblasts. Although previously identified in silico as ifitm5, no localization or functional studies had been undertaken on this gene. We characterized the expression patterns and localization of this gene in vitro and in vivo and assessed its role in matrix mineralization in vitro. The bone specificity and shown role in mineralization led us to rename the gene bone restricted ifitm-like protein (Bril). Bril encodes a 14.8-kDa 134 amino acid protein with two transmembrane domains. Northern blot analysis showed bone-specific expression with no expression in other embryonic or adult tissues. In situ hybridization and immunohistochemistry in mouse embryos showed expression localized on the developing bone. Screening of cell lines showed Bril expression to be highest in osteoblasts, associated with the onset of matrix maturation/mineralization, suggesting a role in bone formation. Functional evidence of a role in mineralization was shown by adenovirus-mediated Bril overexpression and lentivirus-mediated Bril shRNA knockdown in vitro. Elevated Bril resulted in dose-dependent increases in mineralization in UMR106 and rat primary osteoblasts. Conversely, knockdown of Bril in MC3T3 osteoblasts resulted in reduced mineralization. Thus, we identified Bril as a novel osteoblast protein and showed a role in mineralization, possibly identifying a new regulatory pathway in bone formation.

Gain-of-function mutations in TRPV4 cause autosomal dominant brachyolmia

The brachyolmias constitute a clinically and genetically heterogeneous group of skeletal dysplasias characterized by a short trunk, scoliosis and mild short stature. Here, we identify a locus for an autosomal dominant form of brachyolmia on chromosome 12q24.1-12q24.2. Among the genes in the genetic interval, we selected TRPV4, which encodes a calcium permeable cation channel of the transient receptor potential (TRP) vanilloid family, as a candidate gene because of its cartilage-selective gene expression pattern. In two families with the phenotype, we identified point mutations in TRPV4 that encoded R616Q and V620I substitutions, respectively. Patch clamp studies of transfected HEK cells showed that both mutations resulted in a dramatic gain of function characterized by increased constitutive activity and elevated channel activation by either mechano-stimulation or agonist stimulation by arachidonic acid or the TRPV4-specific agonist 4alpha-phorbol 12,13-didecanoate (4alphaPDD). This study thus defines a previously unknown mechanism, activation of a calcium-permeable TRP ion channel, in skeletal dysplasia pathogenesis.

Control of chondrogenesis by the transcription factor Sox9

Cell-fate determination of pluripotent cells, cell proliferation, differentiation, and maturation, as well as the maintenance of stem cells, are essential cellular events during organogenesis. Previous reports show that some distinct cell-specific transcription factors are the master genes that control cell lineage commitment and the subsequent cell proliferation and differentiation. Some of these transcription factors generate hierarchical regulation of expression and act in concert to fulfill their roles. This review discusses the molecular properties and mechanisms of Sry-related high-mobility-group box (Sox) transcription factor, Sox9, in chondrogenesis.