Selective autophagy and Huntingtin: learning from disease

Autophagy, the process responsible for degradation and recycling of intracellular components in lysosomes, is essential for both the maintenance of cellular energetics and in quality control of proteins and organelles.1 Failure of such functions is detrimental in non-dividing differentiated cells such as neurons, explaining why autophagy malfunctioning has been associated with severe neurodegenerative diseases such as Alzheimer, Parkinson and Huntington's disease. The challenge is now to discriminate between pathological conditions in which the defect in autophagy is primary and those in which failure originates from the toxic effect of pathogenic proteins over the autophagy machinery. Surprisingly, we have recently found that in the case of Huntington's disease (HD), autophagy malfunctioning occurs through a combination of these 2 possibilities, as we have identified a key role in autophagy of huntingtin (HTT), the protein mutated in this devastating disorder.2 This study represents an interesting example of how, by learning about disease conditions, we can gain a better understanding of fundamental cellular processes such as, in this case, selective autophagy. HD is a genetic neurodegenerative disorder linked to a single mutation in the HTT gene that generates a prone-to-aggregate HTT protein with an abnormally large number of glutamine repeats (polyQ) in its N-terminus.3 Although neurological symptoms, due to neuronal loss, are the most noticeable ones, patients also develop systemic deficiencies, as HTT is ubiquitously expressed. Early studies demonstrating changes in intracellular protein degradation and alterations in the endolysosomal system4 pointed toward a possible malfunctioning of autophagy in HD. Our early work demonstrated that cells from HD animal models and from patients contain abnormally looking autophagosomes that seem to lack content inside.5 Proteomic analysis revealed that these apparently “empty autophagosomes” contain cytosolic proteins but fail to sequester organelles in a specific manner. Because we found mutant HTT decorating the inner part of autophagosomes, we hypothesized that the extended polyQ track was somehow compromising recognition of cargo by the forming autophagosome. Consequently, we proposed that the autophagic failure in HD was secondary to mutant HTT toxicity on this system.5 However, our recent work has made us realize that, while mutant HTT is still behind the autophagic defect in HD, the reason for autophagy malfunctioning is not just a mere non-specific toxic effect of the polyQ on the cargo recognition machinery. On the contrary, it raises a provocative possibility that the extended polyQ interferes with a previously unknown physiological function of HTT in selective autophagy.2 The key finding in support of this conceptual change was the fact that depletion of wild-type HTT in flies and mammalian cells was sufficient to reproduce a very similar autophagic phenotype to the one observed in HD but now in the absence of the pathogenic protein. Complementation studies, also in the fly, helped us to identify functional interactions between HTT and 2 important components of the autophagic machinery, SQSTM1/p626 and ULK1, involved in cargo recognition and autophagy initiation, respectively. We found that the appearance of “empty” autophagosomes in HTT-defective cells relates to the function of HTT as a scaffold protein in the interaction of SQSTM1/p62 with lysine 63 (K63)-ubiquitin chains in the cargo and with LC3, the major constituent of forming autophagosomes.2 In the absence of HTT, this binding is compromised preventing recruitment of LC3 to selective cargo. However, the role of HTT extends beyond this stabilizing function, as HTT also contributes to bring to the cargo ULK1, a kinase required for initiation of the autophagic process. Interestingly, this HTT-mediated recruitment of ULK1 to the site of autophagosome formation answers a long standing question in the autophagy field, that was, how is it possible to activate autophagy in response to non-nutrient related stressors? ULK1 is normally maintained inactive through its phosphorylation and direct sequestration by mTORC1.7 During starvation, the lack of nutrients renders mTORC1 inactive with the subsequent release of ULK1 to contribute to autophagosome formation. However, activation of autophagy by stressors, such as proteotoxicity or organelle damage, was puzzling, as those conditions do not affect mTORC1 activity and ULK1 should remain sequestered by this kinase complex. We have identified that ULK1 interacts with both mTOR and HTT but in exclusive complexes and that HTT and mTOR compete each other out for binding with ULK1. Thus, upon stress conditions that require activation of selective autophagy, HTT will compete ULK1 out of the inhibitory effect of mTORC1 and bring it to the specific cargo, thanks to its dual interaction with cargo receptors (Fig.1). Figure 1. Novel function of HTT in selective autophagy. Left: In response to starvation “in bulk” autophagy is initiated through inactivation of mTORC1 and subsequent release of the ULK1 kinase complex toward sites of autophagosome biogenesis. ... This novel function of HTT in selective autophagy identifies HTT itself as an interesting therapeutic target for the modulation of autophagy in the fight against neurodegeneration.