Identification of a post-translationally myristoylated autophagy-inducing domain released by caspase cleavage of Huntingtin

Abstract

Huntington disease (HD) is a debilitating neurodegenerative disease characterized by the loss of motor control and cognitive ability that ultimately leads to death. It is caused by the expansion of a polyglutamine tract in the huntingtin (HTT) protein, which leads to aggregation of the protein and eventually cellular death. Both the wild-type and mutant form of the protein are highly regulated by post-translational modifications including proteolysis, palmitoylation and phosphorylation. We now demonstrate the existence of a new post-translational modification of HTT: the addition of the 14 carbon fatty acid myristate to a glycine residue exposed on a caspase-3-cleaved fragment (post-translational myristoylation) and that myristoylation of this fragment is altered in a physiologically relevant model of mutant HTT. Myristoylated HTT553-585-EGFP, but not its non-myristoylated variant, initially localized to the ER, induced the formation of autophagosomes and accumulated in abnormally large autophagolysosomal/lysosomal structures in a variety of cell types, including neuronal cell lines under nutrient-rich conditions. Our results suggest that accumulation of myristoylated HTT553-586 in cells may alter the rate of production of autophagosomes and/or their clearance through the heterotypic autophagosomal/lysosomal fusion process. Overall, our novel observations establish a role for the post-translational myristoylation of a caspase-3-cleaved fragment of HTT, highly similar to the Barkor/ATG14L autophagosome-targeting sequence domain thought to sense, maintain and/or promote membrane curvature in the regulation of autophagy. Abnormal processing or production of this myristoylated HTT fragment might be involved in the pathophysiology of HD.

Myristoylation confers noncanonical AMPK functions in autophagy selectivity and mitochondrial surveillance

“…It is currently unclear how N-myristoylation mediates AMPK association with damaged mitochondria. Since protein lipidation is one of the mechanisms of sensing membrane curvature and stress-induced membrane curvature is important for the recruitment of autophagy machinery to form autophagosomes 61,62 , a plausible hypothesis is that membrane depolarization and the subsequent membrane curvature may allow the integration of the myristic acyl-chain into damage sites. Indeed, CCCP-induced mitochondrial fragmentation is characterized by the formation of circular mitochondria ( Supplementary Fig.…”

Autophagosome dynamics in neurodegeneration at a glance

“…Myristoylation of a caspase-3-cleaved fragment of huntingtin promotes membrane curvature during autophagosome formation (Martin et al, 2014), while ubiquilin 2, an ALSassociated protein that binds to LC3, promotes autophagosome formation (Rothenberg et al, 2010). With recent advances in our understanding of neuronal autophagy, it will be crucial to examine the role of localized autophagosome formation in neurodegeneration.…”

Regulation of Autophagy in Health and Disease

“…Alteration in some of this modification was shown to affect the rate of autophagosome formation in neuronal cell lines under basal conditions [127]. In line with this, autophagic clearance of mutant Htt correlated with its acetylation at K444 [128].…”