c-Raf promotes angiogenesis during normal growth plate maturation

Area of Science:

• Skeletal biology
• Cell signaling
• Endochondral ossification

Background:

• Extracellular phosphate is crucial for growth plate maturation, inducing hypertrophic chondrocyte apoptosis via Erk1/2 phosphorylation.
• Raf kinases (A-Raf, B-Raf, c-Raf) mediate Mek1/2 and Erk1/2 phosphorylation, but their role in endochondral bone formation is unclear.

Purpose of the Study:

• To investigate the role of c-Raf (Raf1) in chondrocyte-specific regulation of growth plate maturation and endochondral ossification.

Main Methods:

• Generation of chondrocyte-specific c-Raf knockout mice (c-Raf(f/f);ColII-Cre(+)).
• Analysis of growth plate morphology, chondrocyte apoptosis, and signaling pathways (Erk1/2 phosphorylation, VEGF expression) in vivo and in vitro.
• Assessment of vascular invasion and VEGF protein ubiquitylation.

Main Results:

• Loss of c-Raf in chondrocytes caused growth plate expansion and reduced hypertrophic chondrocyte apoptosis, with decreased phospho-Erk1/2.
• In vitro studies showed normal phosphate-induced Erk1/2 phosphorylation in knockout chondrocytes.
• Reduced VEGF immunoreactivity and increased VEGF ubiquitylation were observed in c-Raf deficient chondrocytes, correlating with impaired vascular invasion.

Conclusions:

• Chondrocyte-specific c-Raf is essential for growth plate maturation by regulating VEGF-mediated vascular invasion.
• c-Raf's role in vivo is linked to VEGF regulation rather than direct control of phosphate-induced Erk1/2 phosphorylation in hypertrophic chondrocytes.