RSPO2 defines a distinct undifferentiated progenitor in the tendon/ligament and suppresses ectopic ossification

Abstract

Ectopic endochondral ossification in the tendon/ligament is caused by repetitive mechanical overload or inflammation. Tendon stem/progenitor cells (TSPCs) contribute to tissue repair, and some express lubricin [proteoglycan 4 (PRG4)]. However, the mechanisms of ectopic ossification and association of TSPCs are not yet known. Here, we investigated the characteristics of Prg4-positive ( + ) cells and identified that R-spondin 2 (RSPO2), a WNT activator, is specifically expressed in a distinct Prg4 + TSPC cluster. The Rspo2 + cluster was characterized as mostly undifferentiated, and RSPO2 overexpression suppressed ectopic ossification in a mouse Achilles tendon puncture model via chondrogenic differentiation suppression. RSPO2 expression levels in patients with ossification of the posterior longitudinal ligament were lower than those in spondylosis patients, and RSPO2 protein suppressed chondrogenic differentiation of human ligament cells. RSPO2 was induced by inflammatory stimulation and mechanical loading via nuclear factor κB. Rspo2 + cells may contribute to tendon/ligament homeostasis under pathogenic conditions.

Sorafenib inhibits ossification of the posterior longitudinal ligament by blocking LOXL2-mediated vascularization

“…Ligament cells, isolated using the explant method, have been established as playing a pivotal role in ligament ossification, a finding confirmed in prior studies. 7 , 10 In another transcriptomic study (GSE69787) focusing on ligament cells isolated from OPLL and non-OPLL patients, we observed upregulation of LOXL2 in OPLL-derived ligament cells (Fig. S4c ).…”

“…PRG4 + cells are considered stem cells in heterotopic ossification. 7 Furthermore, we conducted staining on patient tissue sections and identified PRG4 + cells. In non-ossified tissues, we observed regions with a high density of PRG4 + cells alongside areas lacking PRG4 + cells.…”

“… 6 During the initiation and progression of OPLL, the disorder is thought to develop through endochondral ossification at the histological level. 7 , 8 The avascular ligamentous tissue is progressively transformed and replaced by highly vascularised bone tissue. Numerous genes 9 and non-coding RNAs (ncRNAs) 10 that regulate the osteogenic differentiation of ligament cells have been identified; however, the formation of ossification encompasses various other biological processes beyond osteogenic differentiation.…”

A single-cell transcriptional atlas reveals resident progenitor cell niche functions in TMJ disc development and injury

“…Therefore, we defined these 2 FB clusters as chondrogenesis related fibroblast 1 and 2, and other 4 FB clusters as non-chondrogenic fibroblasts, with additional definition according to their DEGs. For instance, FB2 was defined as proliferation related non-chondrogenic FB cluster for its high expression of Mki67 and Top2a 43 , 44 , FB3 was defined as apoptosis related non-chondrogenic FB cluster for increased percentage of mitochondria 45 , and FB4 was defined as chondrogenesis inhibition related FB cluster for specific expression of Tspan15 , Procr and Rspo2 42 (Fig. 2g–i ).…”

“…However, there was no separated cell type defined as chondrocyte cluster in the UMAP. To locate the chondrocytelike cells in our single cell data, we used previous published marker genes of chondrogenic differentiation or cartilage formation to identify the chondrocyte-like cells in our 4 main cell types [40][41][42] . We found that expressions of both chondrogenic differentiation markers such as Sox5, Sox6, Sox9 and chondrocyte metabolism markers such as Chad, Comp, Col11a1 were enriched in FBs cluster (Fig.…”