An inherited developmental disorder reveals functions for WHAMM and the actin nucleation machinery in early stages of autophagy

Abstract

Human cells rely on the actin cytoskeleton to properly organize, shape, and move their membrane‐bound organelles. However, the nucleation factors responsible for assembling actin filaments have not been well characterized in relation to human disease. Nephrocerebellar syndrome (NCS) is a recently‐described developmental disorder in which patients encode truncated mutants of the WD40‐repeat protein WDR73 and the actin nucleation factor WHAMM. Here we show that cells from NCS patients exhibit cytoskeletal irregularities, possess enlarged ER‐Golgi intermediate compartments, and have severe defects in the cytoplasmic degradation process of autophagy. Re‐introduction of wild type WHAMM into patient cells restored autophagosome biogenesis, whereas depletion or genetic inactivation of WHAMM in healthy human cell lines impaired both the lipidation of the autophagosomal protein LC3 and the clearance of ubiquitinated protein aggregates. Wild type WHAMM localized to subdomains of nascent autophagosomes, and proper WHAMM function involved binding to the phospholipid PI(3)P, the small G‐protein Rab1, and actin. These results reveal a cytoskeletal mechanism for controlling autophagosome remodeling that is perturbed in cells from NCS patients.