SOX9 determines RUNX2 transactivity by directing intracellular degradation

Area of Science:

• Cell Biology
• Developmental Biology
• Biochemistry

Background:

• Mesenchymal stem cell (MSC) differentiation into bone (osteogenesis) and cartilage (chondrogenesis) is regulated by complex signaling networks.
• RUNX2 and SOX9 are key transcription factors orchestrating osteogenesis and chondrogenesis, respectively, with overlapping expression during development.

Purpose of the Study:

• To investigate the functional interaction between RUNX2 and SOX9 in regulating MSC differentiation.
• To elucidate the molecular mechanisms by which SOX9 affects RUNX2 activity and stability.

Main Methods:

• Co-immunoprecipitation to assess physical interaction between RUNX2 and SOX9.
• Western blotting to analyze protein levels and degradation pathways.
• Reporter assays to measure transactivation inhibition.
• Immunofluorescence microscopy to track protein localization.

Main Results:

• RUNX2 and SOX9 physically interact within cells, leading to mutual transactivation inhibition.
• SOX9 induces dose-dependent degradation of RUNX2 via a proteasome-independent, phosphorylation-dependent pathway.
• SOX9 targets RUNX2 for lysosomal degradation and also promotes lysosomal breakdown of β-catenin, a Wnt signaling mediator.

Conclusions:

• SOX9-mediated RUNX2 degradation represents a novel regulatory mechanism impacting osteochondrogenesis.
• The interaction between SOX9 and RUNX2, involving lysosomal pathways, offers insights into broader signaling networks controlling mesenchymal fate.