Diarthrodial joints are essential for load bearing and locomotion. Physiologically, articular cartilage sustains millions of cycles of mechanical loading. Chondrocytes, the cells in cartilage, regulate their metabolic activities in response to mechanical loading. Pathological mechanical stress can lead to maladaptive cellular responses and subsequent cartilage degeneration. We sought to deconstruct chondrocyte mechanotransduction by identifying mechanosensitive ion channels functioning at injurious levels of strain. We detected robust expression of the recently identified mechanosensitive channels, PIEZO1 and PIEZO2. Combined directed expression of Piezo1 and -2 sustained potentiated mechanically induced Ca 2+ signals and electrical currents compared with single-Piezo expression. In primary articular chondrocytes, mechanically evoked Ca 2+ transients produced by atomic force microscopy were inhibited by GsMTx4, a PIEZO-blocking peptide, and by Piezo1-or Piezo2-specific siRNA. We complemented the cellular approach with an explant-cartilage injury model. GsMTx4 reduced chondrocyte death after mechanical injury, suggesting a possible therapy for reducing cartilage injury and posttraumatic osteoarthritis by attenuating Piezo-mediated cartilage mechanotransduction of injurious strains.A rticular cartilage is a hydrated connective tissue that supports loads and minimizes friction in the diarthrodial joints. It has a highly differentiated extracellular matrix (ECM) composed primarily of type II collagen, the large aggregating proteoglycan, aggrecan, and water. Chondrocytes are the only cells in cartilage and are responsible for maintaining and remodeling cartilage through a homeostatic balance of anabolic and catabolic activities. Under normal physiologic conditions, chondrocytes are exposed to millions of cycles of mechanical loading per year (1). These mechanical signals play an important role in regulating chondrocyte anabolic and biosynthetic activity, as evidenced by cartilage atrophy following periods of disuse or immobilization (2-7). However, under abnormal loading conditions (e.g., due to obesity, trauma, or joint instability), mechanical factors play a critical role in the onset and progression of osteoarthritis (1). Such "injurious" loading has been modeled in vitro using explant culture systems that replicate many of the early cellular and molecular events characteristic of osteoarthritis (8). Osteoarthritis is a painful and debilitating disease of weight-bearing joints that affects over 26 million people in the United States (9) with posttraumatic arthritis being responsible for ∼12% of the incidence of osteoarthritis (10).Despite the critical importance of mechanical loading in health and disease of synovial joints, the mechanisms of mechanotransduction of chondrocytes are not fully understood and are likely to differ under physiologic and pathologic conditions (11)(12)(13)(14). Although many different mechanisms have been shown to be involved in chondrocyte mechanotransduction (13,(15)(16)(17), recent st...

Objective Mechanical factors play a critical role in the physiology and pathology of articular cartilage, although the mechanisms of mechanical signal transduction are not fully understood. We examined the hypothesis that type VI collagen is necessary for mechanotransduction in articular cartilage, by determining the effects of type VI collagen knockout on the activation of the mechano-osmosensitive calcium-permeable channel, transient receptor potential vanilloid 4 (TRPV4), osmotically-induced chondrocyte swelling, and pericellular matrix (PCM) mechanical properties. Methods Confocal laser scanning microscopy was used to image TRPV4-mediated calcium signaling and osmotically-induced cell swelling in intact femora from 2 and 9 month old wild type (WT) and type VI collagen deficient (Col6a1−/−) mice. Immunofluorescence-guided atomic force microscopy was used to map PCM mechanical properties based on the presence of perlecan. Results Hypo-osmotic stress induced TRPV4-mediated calcium signaling was increased in Col6a1−/− mice relative to WT controls at 2 months. Col6a1−/− mice exhibited significantly increased osmotically-induced cell swelling and decreased PCM moduli relative to WT controls at both ages. Conclusion In contrast to our original hypothesis, type VI collagen was not required for TRPV4-mediated Ca2+ signaling; however, knockout of type VI collagen altered the mechanical properties of the PCM, which in turn increased the extent of cell swelling and osmotically-induced TRPV4 signaling in an age-dependent manner. These findings emphasize the role of the PCM as a transducer of mechanical and physicochemical signals, and suggest that alterations in PCM properties, as may occur with aging or osteoarthritis, can influence mechanotransduction via TRPV4 or other ion channels.

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

Mutation or loss of collagen VI has been linked to a variety of musculoskeletal abnormalities, particularly muscular dystrophies, tissue ossification and/or fibrosis, and hip osteoarthritis. However, the role of collagen VI in bone and cartilage structure and function in the knee is unknown. In this study, we examined the role of collagen VI in the morphology and physical properties of bone and cartilage in the knee joint of Col6a1−/− mice by micro-computed tomography (microCT), histology, atomic force microscopy (AFM), and scanning microphotolysis (SCAMP). Col6a1−/− mice showed significant differences in trabecular bone structure, with lower bone volume, connectivity density, trabecular number, and trabecular thickness but higher structure model index and trabecular separation compared to Col6a1+/+ mice. Subchondral bone thickness and mineral content increased significantly with age in Col6a1+/+ mice, but not in Col6a1−/− mice. Col6a1−/− mice had lower cartilage degradation scores, but developed early, severe osteophytes compared to Col6a1+/+mice. In both groups, cartilage roughness increased with age, but neither the frictional coefficient nor compressive modulus of the cartilage changed with age or genotype, as measured by AFM. Cartilage diffusivity, measured via SCAMP, varied minimally with age or genotype. The absence of type VI collagen has profound effects on knee joint structure and morphometry, yet minimal influences on the physical properties of the cartilage. Together with previous studies showing accelerated hip osteoarthritis in Col6a1−/− mice, these findings suggest different roles for collagen VI at different sites in the body, consistent with clinical data.

Background Tumor-induced osteomalacia is a paraneoplastic syndrome resulting in renal phosphate wasting and decreased bone mineralization. Phosphaturic mesenchymal tumors represent a rare etiology of tumor-induced osteomalacia. Nonspecific symptoms of fatigue, bone pain, and musculoskeletal weakness make the diagnosis elusive and lead to a delay in surgical treatment. Questions/purposes In this case series, the following three questions were asked: (1) How do the clinical presentation and features of phosphaturic mesenchymal tumors delay the diagnosis? (2) What is the clinical course after surgical treatment of phosphaturic mesenchymal tumors? (3) How frequently do phosphaturic mesenchymal tumors recur and are there factors associated with recurrence?Methods This study retrospectively reviewed the cases of five adults diagnosed and treated for phosphaturic mesenchymal tumors. Patients were identified through an internal orthopaedic oncology database with clinical, surgical, and histologic data obtained through a systematic chart review. Results Five patients presented with a long-standing history of osteomalacia, generalized fatigue, pain, and weakness before the diagnosis was reached at an average of 7.2 years (range, 2-12 years) after initial symptom onset. The diagnosis appeared to be delayed owing to the cryptic medical presentation, difficulty in locating tumor by imaging, and confirming histologic appearance. Two patients treated with wide surgical resection did not experience recurrence compared with three patients who did show recurrent signs and symptoms after marginal excision. A postoperative increase in fibroblast-derived growth factor-23 was associated with recurrent disease. Conclusions Although uncommon, the diagnosis of phosphaturic mesenchymal tumor should be considered in any patient who presents with hypophosphaturic osteomalacia and no other physiologic cause. Definitive treatment is early, wide surgical resection. Level of Evidence Level IV, therapeutic study. See the Instructions for Authors for a complete description of levels of evidence.