PI(3)P regulates mitochondrial dynamics through FGD-dependent actin organization

Area of Science:

• Cell Biology
• Mitochondrial Biology
• Molecular Mechanisms

Background:

• Mitochondria form dynamic networks crucial for cellular homeostasis.
• The precise mechanisms governing mitochondrial network formation are not fully understood.
• Mitochondrial dynamics involve fission, fusion, branching, and elongation.

Purpose of the Study:

• To elucidate the molecular mechanisms regulating mitochondrial network formation.
• To identify novel factors involved in maintaining mitochondrial homeostasis and dynamics.
• To investigate the role of phosphoinositides in mitochondrial organization.

Main Methods:

• Genetic screening in Caenorhabditis elegans to identify mutations affecting mitochondrial morphology.
• Biochemical assays to determine protein-lipid interactions (EXC-5 and PI(3)P).
• Analysis of protein localization at endosome-mitochondrion contacts.
• Guanine nucleotide exchange factor (GEF) activity assays for CDC-42 activation.
• Microscopy to assess mitochondrial and actin network integrity.
• Functional rescue experiments using constitutively active CDC-42.

Main Results:

• Mutations in EXC-5 disrupt mitochondrial network formation, leading to spherical, unconnected mitochondria.
• EXC-5 binds to phosphoinositide 3-phosphate (PI(3)P) on endosomes and is recruited to endosome-mitochondrion contacts.
• EXC-5 acts as a GEF for CDC-42, and its loss, or loss of PI(3)P production by VPS-34, disrupts mitochondrial and actin networks.
• Defective mitochondrial fission, branching, and elongation were observed in exc-5 and vps-34 mutants.
• Overexpression of activated CDC-42 partially rescued defective mitochondrial networks in an actin-dependent manner.

Conclusions:

• A novel PI(3)P-dependent signaling axis (PI(3)P-EXC-5-CDC-42) regulates mitochondrial dynamics.
• This pathway operates at endosome-mitochondrion contacts to coordinate actin organization for mitochondrial network maintenance.
• The findings reveal a critical link between phosphoinositide metabolism, small GTPase signaling, and mitochondrial morphology.