Osteoarthritis is a painful joint disease characterized by progressive degeneration of the articular cartilage as well as associated changes to the subchondral bone, synovium, and surrounding joint tissues. While the effects of osteoarthritis on the cartilage extracellular matrix (ECM) have been well recognized, it is now becoming apparent that in many cases, the onset of the disease may be initially reflected in the matrix region immediately surrounding the chondrocytes, termed the pericellular matrix (PCM). Growing evidence suggests that the PCM - which along with the enclosed chondrocytes are termed the "chondron" - acts as a critical transducer or "filter" of biochemical and biomechanical signals for the chondrocyte, serving to help regulate the homeostatic balance of chondrocyte metabolic activity in response to environmental signals. Indeed, it appears that alterations in PCM properties and cell-matrix interactions, secondary to genetic, epigenetic, metabolic, or biomechanical stimuli, could in fact serve as initiating or progressive factors for osteoarthritis. Here, we discuss recent advances in the understanding of the role of the PCM, with an emphasis on the reciprocity of changes that occur in this matrix region with disease, as well as how alterations in PCM properties could serve as a driver of ECM-based diseases such as osteoarthritis. Further study of the structure, function, and composition of the PCM in normal and diseased conditions may provide new insights into the understanding of the pathogenesis of osteoarthritis, and presumably new therapeutic approaches for this disease.

The differentiation of human induced pluripotent stem cells (hiPSCs) to prescribed cell fates enables the engineering of patient-specific tissue types, such as hyaline cartilage, for applications in regenerative medicine, disease modeling, and drug screening. In many cases, however, these differentiation approaches are poorly controlled and generate heterogeneous cell populations. Here we demonstrate cartilaginous matrix production in three unique hiPSC lines using a robust and reproducible differentiation protocol. To purify chondroprogenitors produced by this protocol, we engineered a COL2A1-GFP knock-in reporter hiPSC line by CRISPR-Cas9 genome editing. Purified chondroprogenitors demonstrated an improved chondrogenic capacity compared to unselected populations. The ability to enrich for chrondroprogenitors and generate homogenous matrix without contaminating cell types will be essential for regenerative and disease modeling applications.

The therapeutic application of human induced pluripotent stem cells (hiPSCs) for cartilage regeneration is largely hindered by the low yield of chondrocytes accompanied by unpredictable and heterogeneous off-target differentiation of cells during chondrogenesis. Here, we combine bulk RNA sequencing, single cell RNA sequencing, and bioinformatic analyses, including weighted gene co-expression analysis (WGCNA), to investigate the gene regulatory networks regulating hiPSC differentiation under chondrogenic conditions. We identify specific WNTs and MITF as hub genes governing the generation of off-target differentiation into neural cells and melanocytes during hiPSC chondrogenesis. With heterocellular signaling models, we further show that WNT signaling produced by off-target cells is responsible for inducing chondrocyte hypertrophy. By targeting WNTs and MITF, we eliminate these cell lineages, significantly enhancing the yield and homogeneity of hiPSC-derived chondrocytes. Collectively, our findings identify the trajectories and molecular mechanisms governing cell fate decision in hiPSC chondrogenesis, as well as dynamic transcriptome profiles orchestrating chondrocyte proliferation and differentiation.