The SQSTM1 L341V Variant Associated With Sporadic ALS Promotes the Accumulation of Enlarged Ubiquitin‐Positive SQSTM1

ABSTRACT

SQSTM1 is one of the causative genes of neurodegenerative disorders, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The SQSTM1 protein regulates the degradation of polyubiquitinated proteins and autophagosome formation through its interaction with microtubule‐associated protein light chain 3 (MAP1LC3/LC3). However, the molecular mechanisms by which SQSTM1‐LC3 binding regulates the autophagy‐endolysosomal system (APELS) remain unclear. To elucidate the spatiotemporal role of SQSTM1, we transiently expressed wild‐type SQSTM1 or missense mutants carrying mutations in the LC3‐interacting region (LIR), fused with the photoconvertible fluorescent protein Dendra2. Live‐cell fluorescence imaging and co‐localization analyses with markers of the APELS were then performed. Particle analysis of photoconverted or non‐photoconverted SQSTM1‐positive structures in live cells revealed that the pathogenic L341V variant formed larger structures than the wild‐type. Co‐localization analyses further showed that both the L341V and artificial LIR3A mutants accumulated in large ubiquitin‐positive structures, likely due to impaired localization to autophagosomes. These results suggest that mutations within the LIR differentially affect autophagosome formation and cargo degradation within APELS‐related compartments, highlighting the importance of SQSTM1 structural integrity in ALS/FTD pathogenesis.