Podocalyxin and ciliary neurotrophic factor receptor are novel components of the surfaceome of chondrogenic cells

Background
Osteoarthritis is a degenerative joint disease characterized by progressive loss of articular cartilage and limited capacity for intrinsic repair. A major barrier to developing effective regenerative strategies is the incomplete understanding of the molecular mechanisms regulating chondrogenesis and cartilage maintenance. Cell surface proteins are key mediators of extracellular communication, adhesion, and signaling, yet the chondrogenic surfaceome remains incompletely mapped, with prior studies focusing primarily on mature or cytokine-activated chondrocytes. The aim of this study was to provide a temporal profile of the surfaceome during in vitro chondrogenic differentiation and to identify novel membrane proteins with potential roles in cartilage biology.

Methods
We applied a sialoglycoprotein-targeted glycocapture strategy to selectively enrich plasma membrane proteins from chick embryonic limb bud-derived micromass cultures undergoing chondrogenesis. Enriched samples underwent high-resolution shotgun proteomic analysis, and differentially expressed candidates were validated by western blotting, immunocytochemistry, and transient gene silencing. Functional effects on extracellular matrix gene regulation were assessed by quantitative RT-PCR and matrix histochemistry.

Results
This approach generated the temporal surfaceome map of chondrogenic progenitors. Among identified candidates, two proteins not previously linked to chondrogenesis, podocalyxin (PODXL) and ciliary neurotrophic factor receptor (CNTFR), were detected at the plasma membrane and confirmed at the protein and transcript levels. Both proteins exhibited time-dependent downregulation during differentiation. Targeted knockdown revealed differential regulation of the fibrocartilage marker COL1A1 expression, indicating non-redundant roles in cell-matrix signaling and survival pathways. Single-cell transcriptomic meta-analysis confirmed expression of both proteins in discrete human articular chondrocyte subpopulations.

Conclusions
This study expands the molecular framework of chondrogenesis, identifying PODXL and CNTFR as novel, temporally regulated surfaceome components with distinct roles in extracellular matrix signaling. These findings complement prior proteomic analyses of cytokine-activated mature articular chondrocytes and suggest new candidates for developmental cartilage biomarkers and therapeutic targets for osteoarthritis. Our results provide a resource for future cross-species surfaceome studies and highlight key pathways for further investigation into cartilage lineage specification and matrix adaptation.