Potential function for the Huntingtin protein as a scaffold for selective autophagy

Abstract

Although dominant gain-of-function triplet repeat expansions in the Huntingtin (HTT) gene are the underlying cause of Huntington disease (HD), understanding the normal functions of nonmutant HTT protein has remained a challenge. We report here findings that suggest that HTT plays a significant role in selective autophagy. Loss of HTT function in Drosophila disrupts starvationinduced autophagy in larvae and conditional knockout of HTT in the mouse CNS causes characteristic cellular hallmarks of disrupted autophagy, including an accumulation of striatal p62/SQSTM1 over time. We observe that specific domains of HTT have structural similarities to yeast Atg proteins that function in selective autophagy, and in particular that the C-terminal domain of HTT shares structural similarity to yeast Atg11, an autophagic scaffold protein. To explore possible functional similarity between HTT and Atg11, we investigated whether the C-terminal domain of HTT interacts with mammalian counterparts of yeast Atg11-interacting proteins. Strikingly, this domain of HTT coimmunoprecipitates with several key Atg11 interactors, including the Atg1/Unc-51-like autophagy activating kinase 1 kinase complex, autophagic receptor proteins, and mammalian Atg8 homologs. Mutation of a phylogenetically conserved WXXL domain in a C-terminal HTT fragment reduces coprecipitation with mammalian Atg8 homolog GABARAPL1, suggesting a direct interaction. Collectively, these data support a possible central role for HTT as an Atg11-like scaffold protein. These findings have relevance to both mechanisms of disease pathogenesis and to therapeutic intervention strategies that reduce levels of both mutant and normal HTT.rodegenerative disorder caused by an expansion of a CAG trinucleotide repeat encoding a polyglutamine (polyQ) tract near the N terminus of the 350-kD Huntingtin protein (HTT) (1). Identifying the normal biological function of the HTT protein is important in the effort to design and implement effective therapeutic interventions for HD, but has proved challenging.In the mouse, loss of HTT leads to lethality during gastrulation at embryonic day 7 (2-4). Conditional inactivation of HTT in the mouse forebrain at postnatal or late embryonic stages causes a progressive neurodegenerative phenotype associated with neuronal degeneration, motor phenotypes, and early mortality (5). Loss of HTT in mouse cells reduces primary cilia formation, and deletion of HTT in ependymal cells leads to alteration of the cilia layer, suggesting a role for HTT in ciliogenesis (6). Mutant HTT expression and HTT knockdown have also been found to impair axonal trafficking of vesicles, mitochondria, and autophagosomes in neurons in vitro and in vivo (7-9). A clear molecular mechanism to relate these findings to the function of the HTT protein, however, has not yet emerged.In contrast to the embryonic lethality observed in the mouse, Drosophila lacking the endogenous Htt gene develop normally. However, adult Drosophila HTT loss-of-function (LOF) flies show an accelerated neurodege...

Primary cilia and autophagic dysfunction in Huntington’s disease

“…1,3,8 Important recent findings also describe a potential function as a scaffold for multiple autophagyassociated proteins in selective autophagy. 11 Consistent with these putative functions, HTT contains multiple leucinerich HEAT repeats that are involved in protein-protein interactions. 12,13 The exact nature of the polyQ-HTT toxic gain of function remains elusive, but several possibilities have been identified.…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…Similar to Atg11, ALFY (autophagy-linked FYVE protein) is a scaffolding protein implicated in aggrephagy that links cargo to the autophagic machinery (Isakson et al, 2013). Moreover, Huntingtin (HTT) has been proposed to serve as adaptor for any type of selective autophagy, because the domain of HTT shares structure similarities and binding activity with the yeast Atg11 protein and interacts with autophagic effector proteins (Ochaba et al, 2014). It is tempting to speculate that ALFY or HTT are part of the RPC mediating HIF-2α-induced pexophagy and thus, functioning as scaffold protein(s).…”

Commentary on interactions between neurotropic pathogens, neuroinflammatory pathways, and autophagic neural cell death

“…Recent studies have demonstrated an intrinsic connection between selective autophagy impairment and neurodegenerative diseases, including Alzheimer's disease (AD) [4][5][6][7] , Parkinson's disease (PD) [8][9][10] , Huntington's disease (HD) [11][12][13][14] , amyotrophic lateral sclerosis (ALS) [15][16][17] , and multiple sclerosis (MS) [1,[18][19][20][21][22][23] .…”

“…HD is caused by expansion of a CAG trinucleotide repeat in the first exon of the huntingtin gene that encodes the mutant huntingtin: this protein is highly expressed in neurons and participates in many cellular functions, including vesicle and organelle transport and autophagy [13,27] . Mutant huntingtin interferes with the correct autophagic function; therapeutic strategies that improve the clearance of this mutant protein by autophagy reduce neuronal toxicity [14] .…”