Mitochondrial fission, integrity and completion of mitophagy require separable functions of Vps13D in Drosophila neurons

A healthy population of mitochondria, maintained by proper fission, fusion, and degradation, is critical for the long-term survival and function of neurons. Here, our discovery of mitophagy intermediates in fission-impaired Drosophila neurons brings new perspective into the relationship between mitochondrial fission and mitophagy. Neurons lacking either the ataxia disease gene Vps13D or the dynamin related protein Drp1 contain enlarged mitochondria that are engaged with autophagy machinery and also lack matrix components. Reporter assays combined with genetic studies imply that mitophagy both initiates and is completed in Drp1 impaired neurons, but fails to complete in Vps13D impaired neurons, which accumulate compromised mitochondria within stalled mito-phagophores. Our findings imply that in fission-defective neurons, mitophagy becomes induced, and that the lipid channel containing protein Vps13D has separable functions in mitochondrial fission and phagophore elongation. Author summary Neurons are reliant on the maintenance of mitochondrial function to fuel their high-energy demands, and mitochondrial dysfunction is a common pathology associated with neurodegenerative disease. The Vps13 family of proteins are hypothesized to mediate lipid transport between organelles and have been linked to multiple neurological diseases. In Drosophila neurons, we delineate separable functions for the essential family member Vps13D in (1) mitochondrial fission, and (2) mitochondrial degradation via mitophagy, which becomes induced in response to fission defects in neurons. Prior studies in Drosophila intestinal cells have suggested that Vps13D is required for developmental clearance of mitochondria downstream of its role in mitochondrial fission. In contrast, we find that in neurons, rather than blocking mitophagy, impairments in fission lead to an induction in mitophagy. Vps13D depleted neurons initiate but fail to complete mitophagy, leading to the accumulation of stalled, toxic mitophagy intermediates with compromised integrity. These studies establish a new experimental framework for studying mitophagy in neurons in vivo.