Enhanced cartilage formation of chondrocytes in viscoelastic ECMs under mechanical loading

Project Summary Moderate exercise is often recommended to patients for maintaining joint health, due to the strong scientific evidence showing that mechanical loading enhances cartilage formation from the chondrocytes in the joint tissue. Despite this understanding of joint physiology, a detailed mechanism by which cartilage formation enhancement occurs in chondrocytes under mechanical loading remains unclear. Seminal works have demonstrated the crucial role of the mechanosensitive ion channel TRPV4 in sensing physiological mechanical loads and expressing anabolic genes for matrix production. However, how TRPV4 is activated under such small mechanical loading environments remains unclear, especially as TRPV4 has been demonstrated to be insensitive to direct deformations of the cell membrane. Here, we present a hypothesis that TRPV4 activation and enhanced cartilage formation arises in chondrocytes due to the cell volume expansion facilitated by the accelerated dynamic remodeling of the constituents (i.e., viscoelasticity) of the surrounding extracellular matrix (ECM) under mechanical loading. This hypothesis is supported by emerging evidence that cells can sense the mechanical confinement by the ECM, wherein a dynamically remodeling ECM facilitates cellular volume expansion and thus activates TRPV4, and that small-strain mechanical loading can accelerate the dynamic remodeling of gels and biopolymer networks. We will explore this hypothesis through the following specific aims, by 1) exploring how anabolic loading conditions are influenced by the viscoelastic properties of the ECM, 2) establishing the biophysical consequence of anabolic loading on the viscoelastic ECM and the chondrocytes embedded within, and 3) exploring the ramifications of these findings on healthy and osteoarthritic (OA) tissues. The pursuit of these specific aims is innovative because it connects recently established physics of hydrogels to important biological consequences in vivo, which will help us address important health questions in other biological contexts in the future. Altogether, we will establish a detailed biophysical and biochemical understanding of enhanced cartilage formation in chondrocytes under mechanical loading. These results will be medically significant as they will advance our understanding of cartilage homeostasis in both healthy and OA patients, improve tissue engineering strategies for the treatment of cartilage defects, and unravel important insights into cell-ECM mechanotransduction overall. The project will be carried out at Stanford University, a leading institute for medical research, and tackled by a diverse team of experts, including the trainee who is an expert in the mechanics and viscoelasticity of soft materials, the sponsor Dr. Ovijit Chaudhuri who is an expert in cell-ECM mechanotransduction, the co-sponsor Dr. Marc Levenston who is an expert in cartilage mechanics, and collaborator Dr. Nidhi Bhutani who is an expert in cartilage disease and regeneration. The proposed research and training plan will prepare the trainee for a career as an independent researcher at the intersection of soft mechanics, mechanobiology, and biomaterials.