Identification of anterior cruciate ligament fibroblasts and their contribution for knee osteoarthritis progression by single-cell analyses

Abstract

Abstract Objective A better Understanding of the key regulatory cells in the anterior cruciate ligament (ACL), and their role and regulatory mechanisms in knee osteoarthritis (KOA) progression can facilitate the development of targeted treatment strategies for KOA. Methods The relationship between ACL degeneration and KOA was first explored using human ACL specimens and mouse models. Next, single-cell RNA sequencing (scRNA-seq) and single-cell detection of transposase accessible and chromatin sequencing (scATAC-seq) data were integrated to reveal the transcriptional and epigenomic landscape of ACL in normal and osteoarthritis (OA) states. Results Six cell populations were identified in the human ACL, among which were inflammation-associated fibroblasts (IAFs). Degeneration of the ACL during OA mechanically alters the knee joint homeostasis and influences the microenvironment by regulating inflammatory- and osteogenic-related factors, thereby contributing to the progression of KOA. Specifically, a IAF subpopulation identified in OA ACL was found to enhance the transcription and secretion of EGER via SOX5 upregulation, with consequent activation of the EGER–EGFR signaling pathway. These molecular events led to the upregulation of downstream inflammatory and osteogenic factors, and the downregulation of the extracellular matrix-associated factor, thereby leading to knee osteoid formation, cartilage degeneration, and OA progression. Conclusions In summary, this study identifies a novel subpopulation of fibroblasts in the ACL, which confirms the importance of the ACL in knee joint homeostasis and disease. Additionally, the specific mechanism by which these IAFs regulate KOA progression was uncovered, which provides new foundation for the development of targeted treatments for KOA.