Neuron maintenance and survival require late endocytic transport from distal processes to the soma where lysosomes are predominantly localized. Here, we report a role for Snapin in attaching dynein to late endosomes through its intermediate chain (DIC). Snapin (−/−) neurons exhibit aberrant accumulation of immature lysosomes, clustering and impaired retrograde transport of late endosomes along processes, reduced lysosomal proteolysis due to impaired delivery of internalized proteins and hydrolase precursors from late endosomes to lysosomes, and impaired clearance of autolysosomes, combined with reduced neuron viability and neurodegeneration. The phenotypes are rescued by expressing the snapin transgene, but not the DIC-binding defective Snapin-L99K mutant. Snapin overexpression in wild-type neurons enhances late endocytic transport and lysosomal function, while expressing the mutant defective in Snapin-DIC coupling shows a dominant-negative effect. Altogether, our study highlights new mechanistic insights into how Snapin-DIC coordinates retrograde transport and late endosomal-lysosomal trafficking critical for autophagy-lysosomal function, thus neuronal homeostasis.

Reduced axonal mitochondrial transport has been observed in major neurodegenerative diseases, including fALS patients and SOD1 G93A mice. However, it is unclear whether this defect plays a critical role in axonal degeneration or simply reflects sequelae of general transport alteration. Using genetic mouse models combined with time-lapse imaging of live neurons, we previously discovered that axon-targeted syntaphilin (SNPH) acts as a docking receptor specific for axonal mitochondria. Deletion of the snph gene in mice results in a substantially higher proportion of axonal mitochondria in the mobile state without any effect on the transport of other axonal organelles. Here we address whether increased (rescued) axonal mitochondrial mobility changes the disease course by crossing fALSlinked transgenic SOD1G93A and snph ؊/؊ knock-out mice. We found that a 2-fold increase in axonal mitochondrial mobility in SOD1 G93A /snph ؊/؊ mice did not affect the onset of ALS-like symptoms. Both SOD1 G93A and SOD1 G93A /snph ؊/؊ mice exhibit similar weight loss, deterioration in motor function and motor neuron loss, significant gliosis, and a lifespan of 152-154 days. Thus, for the first time, our study provides genetic and pathological evidence that the impairment of mitochondrial transport seen in SOD1 G93A mice plays a minimal role in the rapid-onset of fALS-linked pathology.

Synopsis The autophagy–lysosomal pathway is an intracellular degradation process essential for maintaining neuronal homoeostasis. Defects in this pathway have been directly linked to a growing number of neurodegenerative disorders. We recently revealed that Snapin plays a critical role in co-ordinating dynein-driven retrograde transport and late endosomal–lysosomal trafficking, thus maintaining efficient autophagy–lysosomal function. Deleting snapin in neurons impairs lysosomal proteolysis and reduces the clearance of autolysosomes. The role of the autophagy–lysosomal system in neuronal development is, however, largely uncharacterized. Here, we report that snapin deficiency leads to developmental defects in the central nervous system. Embryonic snapin−/− mouse brain showed reduced cortical plates and intermediate zone cell density, increased apoptotic death in the cortex and third ventricle, enhanced membrane-bound LC3-II staining associated with autophagic vacuoles and an accumulation of polyubiquitinated proteins in the cortex and hippocampus. Thus our results provide in vivo evidence for the essential role of late endocytic transport and autophagy–lysosomal function in maintaining neuronal survival and development of the mammalian central nervous system. In addition, our study supports the existence of a functional interplay between the autophagy–lysosome and ubiquitin–proteasome systems in the protein quality-control process.