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

Mitochondria form highly complex and dynamic networks to maintain their homeostasis. However, the underlying mechanisms remain elusive. Here we report a PI(3)P-dependent mechanism that regulates the mitochondrial dynamics required for formation of mitochondrial networks. Using genetic screening, we reveal that mutations of Caenorhabditis elegans EXC-5/FGD lead to formation of spherical and unconnected mitochondria. EXC-5 binds to endosomal PI(3)P generated by the PI 3-kinase VPS-34 and is recruited to endosome-mitochondrion contacts, where it acts as the guanine nucleotide exchange factor to activate the CDC-42 GTPase. Loss of exc-5 or vps-34 similarly disrupts mitochondrial and actin networks as well as mitochondrial recruitment of DRP-1, leading to failure of mitochondrial fission, branching, and elongation. In contrast, expression of constitutively activated CDC-42 ameliorates the defective mitochondrial networks in an actin-dependent manner. Together, these findings suggest a PI(3)P-EXC-5-CDC-42 axis that acts at endosome-mitochondrion contacts to regulate actin organization for maintenance of mitochondrial dynamics and networks.