SCAMP5 Links Endoplasmic Reticulum Stress to the Accumulation of Expanded Polyglutamine Protein Aggregates via Endocytosis Inhibition

Accumulation of expanded polyglutamine proteins is considered to be a major pathogenic biomarker of Huntington disease. We isolated SCAMP5 as a novel regulator of cellular accumulation of expanded polyglutamine track protein using cell-based aggregation assays. Ectopic expression of SCAMP5 augments the formation of ubiquitin-positive and detergent-resistant aggregates of mutant huntingtin (mtHTT). Expression of SCAMP5 is markedly increased in the striatum of Huntington disease patients and is induced in cultured striatal neurons by endoplasmic reticulum (ER) stress or by mtHTT. The increase of SCAMP5 impairs endocytosis, which in turn enhances mtHTT aggregation. On the contrary, down-regulation of SCAMP5 alleviates ER stress-induced mtHTT aggregation and endocytosis inhibition. Moreover, stereotactic injection into the striatum and intraperitoneal injection of tunicamycin significantly increase mtHTT aggregation in the striatum of R6/2 mice and in the cortex of N171-82Q mice, respectively. Taken together, these results suggest that exposure to ER stress increases SCAMP5 in the striatum, which positively regulates mtHTT aggregation via the endocytosis pathway.The expansion of CAG repeats (usually beyond a critical threshold of ～37 glutamine repeats) encoding polyglutamine (polyQ)³ causes, to date, nine late-onset progressive neurodegenerative disorders (1, 2). Expanded polyQ-containing huntingtin is the main aggregate component in the affected neurons (3). Also, molecular chaperones, such as Hsp70, Hsp40/HDJ1 (dHDJ1), and chaperonin TRiC, perturb the aggregation of polyQ track protein and reduce polyQ track cytotoxicity in yeast and cell lines (4–6) and in Drosophila and mouse models (4, 7). Thus, it seems that HD pathology is closely correlated with the accumulation of insoluble aggregates of mutant huntingtin (mtHTT) containing expanded polyQ (2, 3, 8, 9).Endoplasmic reticulum (ER) stress is crucial in many biological responses and is generated by various signals, such as unfolded protein response, aberrant calcium regulation, oxidative stress, and inflammation (10, 11). ER stress response is generally considered an adaptive reaction of cells to environmental stress, serving as a survival signal (10). On the other hand, increasing evidence also strengthens the importance of ER stress in human diseases. A malfunction or excess of ER stress response caused by aging, genetic mutations, and environmental insults is implicated in human diseases, such as Alzheimer disease, Parkinson disease, diabetes mellitus, and inflammation (12–16). mtHTT also induces ER stress at the early stage of HD, and pathogenic ER stress from an aging or stressful environment is severe at the late stage of HD (17–19). However, the molecular event linking the aggregation of polyQ track protein to ER stress response is unknown.The ubiquitin/proteasome pathway, a major protein degradation system, is altered or impaired in the cell culture model of HD (20–22). On the contrary, autophagy employing lysosomal degradation has been recently considered as a major clearance pathway of insoluble aggregates of polyQ track protein. Thus, inhibition of autophagy has been suggested to modulate the aggregate formation of mtHTT and to affect the toxicity of polyglutamine expansions in fly and mouse models of HD (23–25). However, a key molecule controlling the aggregation and clearance of polyQ track proteins needs to be identified.To further our understanding of the regulation of polyQ track protein aggregation, we screened human full-length cDNAs and isolated SCAMP5 (secretory carrier membrane protein 5) as a modulator of polyQ track protein aggregation. SCAMP5 is up-regulated by mtHTT and ER stress and functions to inhibit endocytosis to increase mtHTT aggregation.