Smcr8 Collaborates With the ALS Linked Gene C9orf72 to Inhibit Autoimmunity and to Regulate Lysosome Exocytosis

Abstract

The (GGGGCC)n hexanucleotide repeat expansion in the first intron of C9ORF72 is the most common genetic cause of ALS\FTLD. This mutation causes reduction of C9ORF72 expression, suggesting insufficient expression of the gene product could contribute to the disease progression. However, the function of C9ORF72 remains elusive, although roles in the immune system and autophagy have been recently proposed. We found that C9ORF72 both interacted with and stabilized SMCR8, enabling further interactions to occur with WDR41 and the autophagy initiation complex composed of ULK1, FIP200 and ATG13. However suppressing the expression of C9ORF72 or SMCR8 did not result in remarkable change in autophagy flux, indicating that the primary function of the protein complex was not to modulate bulk macroautophagy. We then generated Smcr8 loss of function mutant mice using CRSPR/Cas9 gene editing in the zygote. The homozygote Smcr8 mutant mice developed phenotypes that were characteristic of the C9orf72 loss of function mutant animals including autoimmunity, indicating collaborative in vivo functions of the two proteins. In addition, loss of Smcr8 resulted in systemic reduction of intracellular lysosomal protein expression in macrophages. This was accompanied by increased cell surface Lamp1 expression on mutant macrophages, indicating more active lysosome exocytosis in the absence of Smcr8. Such phenotype was reproducibly seen in the C9orf72 mutant animals. Therefore our findings proposed a role of the C9orf72-Smcr8 protein complex in negatively regulating lysosome exocytosis, a process that is essential in regulating immune cell functions and could be a possible link between the cell biological and in vivo functions of the C9orf72-Smcr8 protein complex.