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

Area of Science:

• Biomedical Engineering
• Cell Biology
• Orthopedics

Background:

• Ectopic endochondral ossification in tendons/ligaments results from mechanical overload or inflammation.
• Tendon stem/progenitor cells (TSPCs) are crucial for tissue repair, with some expressing proteoglycan 4 (PRG4).
• Mechanisms linking TSPCs to ectopic ossification remain unclear.

Purpose of the Study:

• To investigate the characteristics of Prg4-positive (Prg4+) cells.
• To identify the role of R-spondin 2 (RSPO2) in TSPCs and ectopic ossification.
• To explore RSPO2's therapeutic potential in tendon and ligament disorders.

Main Methods:

• Characterization of Prg4+ TSPCs and identification of RSPO2 expression.
• Overexpression of RSPO2 in a mouse Achilles tendon puncture model.
• Analysis of RSPO2 levels in human patients with ligament ossification and spondylosis.
• Assessment of RSPO2's effect on chondrogenic differentiation of human ligament cells.
• Investigating RSPO2 induction by inflammatory and mechanical stimuli via nuclear factor κB (NF-κB).

Main Results:

• A distinct Prg4+ TSPC cluster expressing R-spondin 2 (RSPO2) was identified.
• RSPO2 overexpression suppressed ectopic ossification by inhibiting chondrogenic differentiation in a mouse model.
• Lower RSPO2 levels were observed in patients with ossification of the posterior longitudinal ligament compared to spondylosis patients.
• RSPO2 protein inhibited chondrogenic differentiation of human ligament cells.
• RSPO2 induction was mediated by inflammatory stimulation and mechanical loading via NF-κB.

Conclusions:

• RSPO2 is a key factor in a distinct TSPC subset and plays an inhibitory role in ectopic ossification.
• RSPO2 functions by suppressing chondrogenic differentiation, offering a potential therapeutic target.
• RSPO2 expression is dysregulated in patients with ligament ossification.
• RSPO2 induction by pathological stimuli suggests its role in maintaining tendon/ligament homeostasis.