Background/Purpose: Chondrocytes play a pivotal role in osteoarthritis (OA) because they are the only cells in the articular cartilage that are responsible for maintenance of the extracellular matrix (ECM). Articular cartilage intrinsic regenerative capacity is strongly downregulated in humans with age. The use of mesenchymal stromal cells (MSC) is an attractive approach for cartilage repair because MSC are capable of proliferating and differentiating into chondrocytes in vitro. However, without detailed knowledge of how chondrogenic pathways are induced it has not been possible to produce true hyaline cartilage in vitro or in vivo, or to maintain cartilage growth. The present study was undertaken to determine whether follistatin-like protein 1 (FSTL1) regulates chondrogenic differentiation of MSC. We hypothesized that FSTL1 may enhance cell proliferation and anabolic activity in articular chondrocytes and, therefore, play an important role in maintaining cartilage homeostasis.

Methods: To study the role of FSTL1 in chondrogenesis, we made use of FSTL1 knockout (KO) mice generated in our laboratory. MSC were obtained from skulls of E18.5 embryos. Proliferative capacity of MSC was analyzed by flow cytometry using CFSE labeling. Differentiation of MSC into chondrocytes was carried out in a pellet culture system. Gene expression microarray analysis was performed on MSC isolated from FSTL1 KO and wild-type embryos. For signaling study, protein extracts from TGFb-stimulated MSC were analyzed for phospho (p)-SMAD3, p-p38 MAPK, p-AKT by Western blotting.

Results: Homozygous FSTL1 KO mice showed extensive skeletal defects, supporting a role for FSTL1 in chondrogenesis. FSTL1 KO embryos had decreased cellularity in the embryonic vertebral cartilage and FSTL1 KO MSC had reduced proliferative capacity. Microarray analysis of gene expression in FSTL1 KO MSC revealed dysregulation of multiple genes known to be involved in chondrogenesis, including COL1A2, COL2A1, COL10A1, SOX5, SOX9, TGFBR2, CTSK, GREM1, IGFBP4, WISP1. We also demonstrated that FSTL1-deficient MSC displayed defects in TGFb-induced differentiation into chondrocytes, including decreased production of ECM proteoglycans and reduced expression of type II collagen. TGFb-induced phosphorylation of SMAD3, p38 MAPK, and AKT were significantly decreased in FSTL1 KO cells.

Conclusion: These results demonstrate that FSTL1 is a crucial component of the regulatory mechanism controlling chondrocyte proliferation, differentiation, and expression of ECM molecules. Our findings may lead to the development of novel strategies for cartilage repair and provide new disease modifying treatments for OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/follistatin-like-protein-1-is-an-important-regulator-of-chondrogenesis/