Ruhang Tang

Obesity is a risk factor for osteoarthritis (OA), and leptin is among the adipokines implicated in obesity-induced OA. However, the specific role of leptin in OA severity and pain is not known. Using lipodystrophic (LD) mice, we show that fat-secreted factors are required for knee OA development, implicating a fat-cartilage cross-talk. Fat pad implantation or systemic leptin restoration in LD mice reintroduced structural OA and pain, whereas implantation of leptin-deficient fat pad did not change OA susceptibility. Isochronic parabiosis and spatial transcriptomics confirmed that a fat-joint cross-talk likely occurred via soluble mediators. Global unsupervised multiomics of conditioned media from fat implants revealed that leptin exerts a regulatory effect on adipsin (or complement factor D), the activity of which modulates the contrastive OA structural and pain phenotype. These findings suggest that adipokines influence OA pathogenesis, providing conclusive evidence of a fat-joint cross-talk and implicating OA as a systemic disease of adipose tissue.

Osteoarthritis (OA) is characterized by articular cartilage degeneration, leading to pain and loss of joint function. Recent studies have demonstrated that omega‐3 (ω3) polyunsaturated fatty acid (PUFA) supplementation can decrease injury‐induced OA progression in mice fed a high‐fat diet. Furthermore, PUFAs have been shown to influence the mechanical properties of chondrocyte membranes, suggesting that alterations in mechanosensitive ion channel signaling could contribute to the mechanism by which ω3 PUFAs decreased OA pathogenesis. Here, we hypothesized that PUFAs may alter mechanical signaling through PIEZO1 (activated by changes in membrane tension) and TRPV4 (activated by physiologic mechano‐osmotic signals), as these mechanosensitive cation channels have been shown to influence OA progression. Our results demonstrated that PUFAs reduced chondrocyte sensitivity to single‐cell mechanical compression and to pharmacologic agonists of PIEZO1 and TRPV4, with ω3 PUFAs having the most significant effects overall. We also found that supplementation with ω6 PUFA linoleic acid (LA) altered the biophysical properties of chondrocytes, as evidenced by increased intracellular lipid droplet formation and more rapid membrane rupture in response to hypo‐osmotic shock, suggesting that LA increases chondrocyte membrane susceptibility to damage. Our findings underscore the differential impacts of specific PUFAs on chondrocyte signaling and membrane properties and provide important considerations in the development of nutritional interventions to prevent or treat OA.

Obesity is one of the primary risk factors for osteoarthritis (OA), acting through cross talk among altered biomechanics, metabolism, adipokines, and dietary free fatty acid (FA) composition. Obesity and aging have been linked to cellular senescence in various tissues, resulting in increased local and systemic inflammation and immune dysfunction. We hypothesized that obesity and joint injury lead to cellular senescence that is typically associated with increased OA severity or with aging and that the ratio of omega-6 (ω-6) to omega-3 (ω-3) FAs regulates these pathologic effects. Mice were placed on an ω-6-rich high-fat diet or a lean control diet and underwent destabilization of the medial meniscus to induce OA. Obesity and joint injury significantly increased cellular senescence in subcutaneous and visceral fat as well as joint tissues such as synovium and cartilage. Using adeno-associated virus (AAV) gene therapy for fat-1 , a fatty acid desaturase that converts ω-6 to ω-3 FAs, decreasing the serum ω-6:ω-3 FA ratio had a strong senomorphic and therapeutic effect, mitigating metabolic dysfunction, cellular senescence, and joint degeneration. In vitro coculture of bone marrow-derived macrophages and chondrocytes from control and AAV8- fat1- treated mice were used to examine the roles of various FA mediators in regulating chondrocyte senescence. Our results suggest that obesity and joint injury result in a premature “aging” of the joint as measured by senescence markers, and these changes can be ameliorated by altering FA composition using fat-1 gene therapy. These findings support the potential for fat-1 gene therapy to treat obesity- and/or injury-induced OA clinically.

This study aimed to compare the efficacy and safety of oral and transdermal estradiol in alleviating menopausal symptoms. A total of 257 recently menopausal women were randomized into two groups. The t-E₂ group received transdermal estradiol (2.5 g per day) (n = 128) and the o-E₂V group received oral estradiol valerate (2 mg per day) (n = 129) for 24 weeks; both groups received micronized progesterone (200 mg per day). The primary outcome measure is the change in the modified Kupperman Menopausal Index (KMI) after 24 weeks of treatment. Menopausal symptoms were recorded at screening and at 4, 12 and 24 weeks using both the KMI and the Menopause Rating Scale (MRS). Significant amelioration was observed by KMI and MRS scores for both groups after treatment (p < 0.001). The mean KMI scores showed no difference between the two groups. The mean MRS scores were similar between the two groups at baseline and after 4 weeks of treatment. The results showed statistical differences after 12 weeks and 24 weeks of treatment (p = 0.005 and p = 0.011). Both the after-treatment scores minus the baseline scores of KMI and MRS and the incidence of adverse effects showed no difference between the two groups. This study shows that both transdermal and oral estradiol are effective in relieving menopausal symptoms, with little difference in treatment efficacy and safety. ChiCTR2300073146.

Adipose tissue is an active endocrine organ that can signal bidirectionally to many tissues and organ systems in the body. With obesity, adipose tissue is a source of low-level inflammation that contributes to various co-morbidities and damage to downstream effector tissues. The ability to synthesize genetically engineered adipose tissue could have critical applications in studying adipokine signaling and the use of adipose tissue for novel therapeutic strategies. This study aimed to develop a method for non-viral adipogenic differentiation of genome-edited murine induced pluripotent stem cells (iPSCs) and to test the ability of such cells to engraft in mice in vivo . Designer adipocytes were created from iPSCs, which can be readily genetically engineered using CRISPR-Cas9 to knock out or insert individual genes of interest. As a model system for adipocyte-based drug delivery, an existing iPSC cell line that transcribes interleukin 1 receptor antagonist under the endogenous macrophage chemoattractant protein-1 promoter was tested for adipogenic capabilities under these same differentiation conditions. To understand the role of various adipocyte subtypes and their impact on health and disease, an efficient method was devised for inducing browning and whitening of IPSC-derived adipocytes in culture. Finally, to study the downstream effects of designer adipocytes in vivo , we transplanted the designer adipocytes into fat-free lipodystrophic mice as a model system for studying adipose signaling in different models of disease or repair. This novel translational tissue engineering and regenerative medicine platform provides an innovative approach to studying the role of adipose interorgan communication in various conditions.

Osteoarthritis is a chronic disease that can be initiated by altered joint loading or injury of the cartilage. The mechanically sensitive PIEZO ion channels have been shown to transduce injurious levels of biomechanical strain in articular chondrocytes and mediate cell death. However, the mechanisms of channel gating in response to high cellular deformation and the strain thresholds for activating PIEZO channels remain unclear. We coupled studies of single-cell compression using atomic force microscopy (AFM) with finite element modeling (FEM) to identify the biophysical mechanisms of PIEZO-mediated calcium (Ca ²⁺ ) signaling in chondrocytes. We showed that PIEZO1 and PIEZO2 are needed for initiating Ca ²⁺ signaling at moderately high levels of cellular deformation, but at the highest strains, PIEZO1 functions independently of PIEZO2. Biophysical factors that increase apparent chondrocyte membrane tension, including hypoosmotic prestrain, high compression magnitudes, and low deformation rates, also increased PIEZO1-driven Ca ²⁺ signaling. Combined AFM/FEM studies showed that 50% of chondrocytes exhibit Ca ²⁺ signaling at 80 to 85% nominal cell compression, corresponding to a threshold of apparent membrane finite principal strain of E = 1.31, which represents a membrane stretch ratio (λ) of 1.9. Both intracellular and extracellular Ca ²⁺ are necessary for the PIEZO1-mediated Ca ²⁺ signaling response to compression. Our results suggest that PIEZO1-induced signaling drives chondrocyte mechanical injury due to high membrane tension, and this threshold can be altered by factors that influence membrane prestress, such as cartilage hypoosmolarity, secondary to proteoglycan loss. These findings suggest that modulating PIEZO1 activation or downstream signaling may offer avenues for the prevention or treatment of osteoarthritis.

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. 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 upregulated several anterior HOX genes and downregulated antioxidant genes CAT and GSTA1 throughout chondrogenesis. BMP4 treatment upregulated 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.

Significance: Obesity is a primary risk factor for osteoarthritis, involving complex interactions among the metabolic, biomechanical, and inflammatory factors caused by increased adiposity. Using a mouse model of lipodystrophy, we demonstrate that fat-free mice on either a chow or high-fat diet are protected from cartilage damage, despite demonstrating many factors previously associated with osteoarthritis. Furthermore, implantation of a small fat pad reverses cartilage protection in lipodystrophic mice, indicating a direct relationship between adipose tissue and cartilage damage, independent of body weight. This study provides evidence of adipose–cartilage signaling, indicating that tissues outside the joint can influence cartilage health. These findings support the notion that osteoarthritis involves a systemic component, providing insight into the development of new therapeutic targets for osteoarthritis.

Osteoarthritis (OA), the leading cause of pain and disability worldwide, disproportionally affects obese individuals. The mechanisms by which adipose tissue leads to the onset and progression of OA are unclear due to the complex interactions between the metabolic, biomechanical, and inflammatory factors that accompany obesity. We used a murine model of lipodystrophy (LD) to examine the direct contribution of adipose tissue to OA. Knee joints of LD mice were protected from spontaneous or post-traumatic OA, on either a chow and high fat diet, despite similar body weight and the presence of systemic inflammation. These findings indicate that adipose tissue itself plays a critical role in the pathophysiology of OA. Susceptibility to post-traumatic OA was reintroduced into LD mice using implantation of adipose tissue derived from wildtype animals or mouse embryonic fibroblasts that undergo spontaneous adipogenesis, implicating paracrine signaling from fat, rather than body weight, as a critical mediator of joint degeneration.