Joint biomechanical functions rely on the integrity of cartilage extracellular matrix. Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints. It is not as hard and rigid as bone, but it is much stiffer and much less flexible than muscle. Understanding the molecular activities that govern cartilage matrix assembly is critical for developing effective cartilage regeneration strategies.

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance that is rich in proteoglycan and elastin fibers. Cartilage does not contain blood vessels (it is avascular) or nerves (it is aneural). Nutrition is supplied to the chondrocytes by diffusion. The compression of the articular cartilage or flexion of the elastic cartilage generates fluid flow, which assists diffusion of nutrients to the chondrocytes. Compared to other connective tissues, cartilage has a very slow turnover of its extracellular matrix and does not repair.

This study elucidated the role of decorin, a small leucine rich proteoglycan, in the structure and biomechanical functions of cartilage. In decorin-null cartilage, we discovered a substantial reduction of aggrecan content, the major proteoglycan of cartilage matrix, and mild changes in collagen fibril structure. This loss of aggrecan resulted in significantly impaired biomechanical properties of cartilage, including decreased modulus, elevated hydraulic permeability and reduced energy dissipation capabilities. At the cellular level, we found that decorin functions to increase the retention of aggrecan in the neo-matrix of chondrocytes, rather than to directly influence the biosynthesis of aggrecan. At the molecular level, we demonstrated that decorin significantly increases the adhesion between aggrecan and aggrecan molecules, and between aggrecan molecules and collagen II fibrils. We hypothesize that decorin plays a crucial structural role in mediating the matrix integrity and biomechanical functions of cartilage by providing physical linkages to increase the adhesion and assembly of aggrecan molecules at the nanoscale.

To delineate the activities of decorin and biglycan in the progression of post-traumatic osteoarthritis (PTOA), PTOA murine model was studied. Three-month-old inducible biglycan (Bgn^iKO) and decorin/biglycan compound (Dcn/Bgn^iKO) knockout mice were subjected to the destabilization of the medial meniscus (DMM) surgery to induce PTOA. The OA phenotype was evaluated by assessing joint morphology and sulfated glycosaminoglycan (sGAG) staining via histology, surface collagen fibril nanostructure and calcium content via scanning electron microscopy, tissue modulus via atomic force microscopy-nanoindentation, as well as subchondral bone structure and meniscus ossification via micro-computed tomography. Outcomes were compared with previous findings in the inducible decorin (Dcn^iKO) knockout mice. In the DMM model, Bgn^iKO mice developed similar degree of OA as the control, which is in contrast to the more severe OA phenotype observed in Dcn^iKO mice. The compound Dcn/Bgn^iKO mice exhibited similar OA phenotype as Dcn^iKO mice, including aggravated loss of sGAGs, salient surface fibrillation and formation of osteophyte. Also, Dcn/Bgn^iKO showed further cartilage thinning, resulting in the exposure of underlying calcified tissues and aberrantly high surface modulus. In the subchondral bone, although Dcn^iKO mice did not show appreciable changes, both Bgn^iKO and Dcn/Bgn^iKO mice developed altered subchondral trabecular bone structure for both Sham and DMM groups. Those findings suggest that in PTOA, decorin plays a more crucial role than biglycan in regulating cartilage degeneration, while biglycan is more important in regulating subchondral bone structure. The two have limited synergistic activities in this process.

Additionally, we conducted an in-depth review of the recent technical advances of atomic force microscopy (AFM)-based nanomechanical tests, and their contribution to a better understanding and diagnosis of osteoarthritis (OA), as well as the repair of tissues undergoing degeneration during OA progression. We first summarize a range of technical approaches for AFM-based nanoindentation, including considerations in both experimental design and data analysis. We then provide a more detailed description of two recently developed modes of AFMnanoindentation, a high bandwidth nanorheometer system for studying poroviscoelasticity, and an immunofluorescence-guided nanomechanical mapping technique for delineating the pericellular matrix (PCM) and territorial/interterritorial matrix (T/IT-ECM) of surrounding cells in connective tissues. Next, we summarize recent applications of these approaches to three aspects of jointrelated healthcare and disease:​ cartilage aging and OA, developmental biology and OA pathogenesis in murine models, and nanomechanics of the meniscus. These studies were performed over a hierarchy of length scales, from the molecular, cellular to the whole tissue level. The advances described here have contributed greatly to advancing the fundamental knowledge base for improved understanding, detection and treatment of OA.