PDGFRa signaling regulates cartilage and fibrous tissue differentiation during synovial joint development
Limb development begins with limb buds that develop into long appendicular bones separated by joint-forming structures called interzones. Mesenchymal progenitor cells of the interzone differentiate into cartilaginous and fibrous tissues of the synovial joint, including articular cartilage, menisci, and ligaments. Platelet-derived growth factor receptor-a (PDGFRa) is expressed by mesenchymal cells of the limb bud, but it is not known to have a role in limb or joint development. By analyzing PDGFRa expression in single-cell RNA sequencing data, immunofluorescence, and lineage tracing, we observed dynamic PDGFRa expression in the mouse limb bud, interzone, and perichondral mesenchyme. To investigate PDGFRa function, we use Prrx1-Cre to cause mouse limb bud mesenchyme to express mutant PDGFRa with a kinase domain point mutation (D842V) that increases receptor signaling. Overall limb development was unaffected. However, the hind limbs were immobile with knee joints fused by cartilage and lacking ligaments and menisci. The mutant perichondrium exhibited ectopic cartilage matrix and the enthesis of the patellar tendon was enlarged. The interzone marker Gdf5 was initially expressed at E12.5 but was downregulated at E13.5 and thereafter, suggesting a defect in maintenance of the interzone. Spatial transcriptomics of the mutant joint identified co-expression of genes for both cartilage (Sox9, Col2a1) and fibrotic tissue (Postn, Col1a1, Col1a2), suggesting fibrocartilage. These data suggest that elevated PDGFRa signaling corrupts joint tissue differentiation by downregulating Gdf5 and redirecting interzone progenitors into a fibrocartilage fate. These results highlight the need to balance chondrogenic and fibrogenic cell fates during joint development, with tight regulation of PDGFRa signaling as a necessity for the development of the mouse knee joint. Overall design: E14.5 knees were sectioned in a coronal orientation and processed for NanoString GeoMX RNA sequencing analysis.