The MAPK Erk5 is necessary for proper skeletogenesis involving a Smurf-Smad-Sox9 molecular axis

Area of Science:

• Cellular signaling
• Developmental biology
• Molecular genetics

Background:

• Erk5 (extracellular signal-regulated kinase 5) is a member of the mitogen-activated protein kinase (MAPK) family.
• Erk5, phosphorylated by Mek5, regulates diverse signaling pathways in various cell types.
• Sox9 is a critical transcription factor essential for skeletogenesis.

Purpose of the Study:

• To investigate the role of Erk5 in mammalian skeletal development.
• To elucidate the molecular mechanisms by which Erk5 influences skeletogenesis.
• To identify the downstream targets and signaling pathways regulated by Erk5 in mesenchymal cells.

Main Methods:

• Erk5 inactivation in mesenchymal cells of mice.
• Analysis of skeletal abnormalities in Erk5-deficient mice.
• Biochemical assays to determine Erk5 phosphorylation and activation of Smurf2.
• Assessment of Smad protein degradation and Smad1 phosphorylation.
• Analysis of Sox9 expression levels.
• Genetic rescue experiments by removing one Sox9 allele.

Main Results:

• Erk5 inactivation in mesenchymal cells leads to skeletal development abnormalities.
• Erk5 directly phosphorylates and activates Smurf2 (a ubiquitin E3 ligase) at Thr249.
• Activated Smurf2 promotes proteasomal degradation of Smad proteins.
• Erk5 signaling phosphorylates Smad1 at Ser206, enhancing its degradation by Smurf1.
• Smads transcriptionally activate Sox9 expression in mesenchymal cells.
• Partial rescue of skeletal abnormalities was observed in mice with one Sox9 allele removed.

Conclusions:

• The Mek5-Erk5-Smurf-Smad-Sox9 signaling axis is vital for mammalian skeletogenesis.
• Erk5 plays a critical role in regulating skeletal development through the modulation of Smad and Sox9 activity.
• Understanding this pathway provides insights into developmental disorders affecting bone formation.