Notch signaling controls chondrocyte hypertrophy via indirect regulation of Sox9

Area of Science:

• Skeletal Biology
• Developmental Biology
• Molecular Genetics

Background:

• Notch signaling is crucial for chondrocyte hypertrophy and maturation during endochondral ossification.
• The precise molecular mechanisms by which Notch signaling regulates SOX9 transcription in chondrocytes remain unclear.
• Understanding this regulation is key to elucidating Notch's role in cartilage development and hypertrophy.

Purpose of the Study:

• To provide conclusive genetic evidence linking RBPjk-dependent Notch signaling to SOX9 expression and chondrocyte hypertrophy.
• To investigate the role of SOX9 in mediating Notch signaling's control over chondrocyte maturation.
• To elucidate the molecular mechanisms of Notch-mediated Sox9 gene regulation in chondrogenic cells.

Main Methods:

• Generation and analysis of tissue-specific Rbpjk mutant and Rbpjk/Sox9 haploinsufficient mouse embryos.
• Examination of SOX9 expression and chondrocyte hypertrophy during cartilage development.
• In vitro molecular analyses of the Sox9 promoter and gene expression in chondrogenic cells.

Main Results:

• Notch signaling regulates the onset of chondrocyte maturation in a SOX9-dependent manner.
• Notch-mediated regulation of terminal chondrocyte maturation appears independent of SOX9.
• Notch signaling directly induces Sox9 expression acutely but suppresses it with prolonged signaling requiring secondary effectors.

Conclusions:

• RBPjk-dependent Notch signaling plays a critical, SOX9-dependent role in initiating chondrocyte maturation.
• Notch signaling exhibits differential regulation of chondrocyte maturation stages, with SOX9 being critical for the onset but not terminal maturation.
• The temporal dynamics of Notch signaling influence Sox9 transcription through direct and indirect mechanisms, impacting chondrocyte hypertrophy.