Plekhg5 regulates the unconventional secretion of Sod1 by presynaptic secretory autophagy

Increasing evidence suggests an essential function for autophagy in unconventional protein secretion (UPS). However, despite its relevance for the secretion of aggregate-prone proteins, the mechanisms of secretory autophagy in neurons have remained elusive. Here we show that the lower motoneuron disease-associated protein Plekhg5 drives the UPS of Sod1. Mechanistically, Sod1 is sequestered into the intermembrane space of autophagosomes which fuse with secretory lysosomal-related organelles (LROs). Deletion of Plekhg5 in mice disrupts the secretion of these LROs causing Sod1 accumulation at swollen presynaptic sites. A reduced secretion of toxic ALS-linked SOD1G93A following deletion of Plekhg5 in SOD1G93A mice accelerated disease onset while prolonging survival due to an attenuated microglia activation. Using human isogenic iPSC-derived motoneurons we show an impaired secretion of ALS-linked SOD1D90A, which coincided with a reduced PLEKHG5 expression. Our findings highlight an unexpected pathophysiological mechanism that converges two motoneuron disease-associated proteins into a common pathway. To investigate the impact of Plekhg5 on the phenotype of SOD1-G93A mice, we crossed Plekhg5-deficient mice with SOD1-G93A expressing mice. Total RNA was isolated from spinal cord tissues of 7 months-old mice and we performed gene expression profiling analysis with RNAseq data from wildtype (control) mice, Plekhg5-deficient mice, SOD1-G93A mice, and Plekhg5-deficient SOD1-G93A mice.