The effect of in vivo chondrocyte depletion on the structural and functional properties of murine articular cartilage

Abstract

Purpose: Articular cartilage is an intricate and remarkable tissue found within synovial joints. It is essential for providing low-friction and load bearing during movement, resulting in pain-free mobility. Chondrocytes are the sole cell population present within the articular cartilage and play a critical role in tissue homeostasis. As the cellular building blocks of cartilage, they direct the synthesis and maintenance of this tissues proteoglycan-rich collagenous extracellular matrix (ECM), which in turn confers the mechanical properties required for cartilage to withstand shear and compressive loadings generated about 1000's of times per day. The cartilage ECM is enriched for negatively charged proteoglycans, specifically aggrecan. The abundance of these large aggregating proteoglycans results in increased tissue hydration and in the context of a collagen network (mainly type II); confers unique strength and shape to the articular cartilage, providing structural support and load resistance. Interestingly, previous research examining the effect of ablating superficial chondrocytes on the integrity of articular cartilage in mice knee joints surprisingly observed that chondrocyte depletion did not result in cartilage degradation. Expanding on this finding, the current study depleted chondrocytes within all 3 zones of articular cartilage (superficial, transitional, deep) to investigate how its compositional and structural integrity was affected. Furthermore, it was investigated if and how articular chondrocyte depletion relates to changes in cartilage biomechanical properties.Methods: Acan-CreER T2 and Gt(Rosa) DTA mice were bred to obtain Acan-CreER T2þ/-: Gt(Rosa) DTAþ/offspring, hereafter referred to as ACAN mice. Through tamoxifen treatment, diphtheria toxin (DTA) expression was induced in aggrecan expressing cells (chondrocytes) leading to cell death. Knee joints were harvested 5 and 9 weeks post-DTA induction and samples were analyzed histologically and biomechanically. Safranin-O/Fast Green staining was performed, and tissue structure was compared to wild-type (WT) control mice Acan-CreER T2-/-: Gt(Rosa) DTAþ/-. Biomechanical testing using a 3-axis mechanical tester (Biomomentum) was carried out on the articular surface of the femoral condyles to obtain their spatial distribution of stiffness. Using automated mapping and a pre-defined set of standardized positions and boundaries, stiffness and thickness measurements were quantified and compared to the WT controls. Long-term histological assessment of knee joints was also carried out 4 and 7 months post-DTA induction. Results: Histological analysis of ACAN mice knee joints demonstrated a regional lack of proteoglycan staining and decreased chondrocyte numbers when compared to the WT control ( Figure 1). Differences in cartilage structure between the WT and ACAN mice were evident by 5weeks post-DTA induction. By 9 weeks post-DTA induction, the results suggest a partial rescue of phenotype (proteoglycan staining, chondrocyte numbers) within the articular carti...

Loss of Grem1-lineage chondrogenic progenitor cells causes osteoarthritis

“…3k, o, p ). These data are consistent with previous Acan- and Prg4- chondrocyte ablation experiments reported to generate mild or insignificant OA 35 , 36 . Given these differences, we note that OA is not just a disease of mature chondrocyte loss, but is likely also a failure of local progenitor regeneration.…”

“…Previous studies noted that ablation of chondrocytes marked by Acan or Prg4 results in mild OA that resolves over time, or no OA 35 , 36 . To further discriminate the role of Acan+ chondrocytes in comparison to Grem1 -lineage CPs in OA, we compared the OA phenotype caused by ablation of each cell population using similar genetic models and induction regimens.…”