The Brown Foundation Institute of Molecular Medicine, McGovern Medical School at the University of Texas Health Science Center at Houston (UTHealth), Houston, Texas, United States of America.
Department of Translational and Discovery Research, IRBM SpA, Pomezia (RM), Italy.
PLoS Genet. 2022 Jul 19;18(7):e1010302. doi: 10.1371/journal.pgen.1010302. eCollection 2022 Jul.
Perturbation of huntingtin (HTT)'s physiological function is one postulated pathogenic factor in Huntington's disease (HD). However, little is known how HTT is regulated in vivo. In a proteomic study, we isolated a novel ~40kDa protein as a strong binding partner of Drosophila HTT and demonstrated it was the functional ortholog of HAP40, an HTT associated protein shown recently to modulate HTT's conformation but with unclear physiological and pathologic roles. We showed that in both flies and human cells, HAP40 maintained conserved physical and functional interactions with HTT. Additionally, loss of HAP40 resulted in similar phenotypes as HTT knockout. More strikingly, HAP40 strongly affected HTT's stability, as depletion of HAP40 significantly reduced the levels of endogenous HTT protein while HAP40 overexpression markedly extended its half-life. Conversely, in the absence of HTT, the majority of HAP40 protein were degraded, likely through the proteasome. Further, the affinity between HTT and HAP40 was not significantly affected by polyglutamine expansion in HTT, and contrary to an early report, there were no abnormal accumulations of endogenous HAP40 protein in HD cells from mouse HD models or human patients. Lastly, when tested in Drosophila models of HD, HAP40 partially modulated the neurodegeneration induced by full-length mutant HTT while showed no apparent effect on the toxicity of mutant HTT exon 1 fragment. Together, our study uncovers a conserved mechanism governing the stability and in vivo functions of HTT and demonstrates that HAP40 is a central and positive regulator of endogenous HTT. Further, our results support that mutant HTT is toxic regardless of the presence of its partner HAP40, and implicate HAP40 as a potential modulator of HD pathogenesis through its multiplex effect on HTT's function, stability and the potency of mutant HTT's toxicity.
亨廷顿病（HD）的一个假设致病因素是亨廷顿蛋白（HTT）的生理功能受到干扰。然而，目前尚不清楚 HTT 在体内是如何被调控的。在一项蛋白质组学研究中，我们分离到一种新型的~40kDa 蛋白，它是果蝇 HTT 的强结合伴侣，并证明它是 HAP40 的功能同源物，最近有研究表明 HAP40 可以调节 HTT 的构象，但对其生理和病理作用尚不清楚。我们发现，在果蝇和人类细胞中，HAP40 与 HTT 保持着保守的物理和功能相互作用。此外，HAP40 的缺失导致与 HTT 敲除相似的表型。更引人注目的是，HAP40 强烈影响 HTT 的稳定性，因为 HAP40 的缺失显著降低了内源性 HTT 蛋白的水平，而 HAP40 的过表达则显著延长了其半衰期。相反，在没有 HTT 的情况下，大部分 HAP40 蛋白被降解，可能是通过蛋白酶体。此外，HTT 中的多聚谷氨酰胺扩展对 HTT 与 HAP40 之间的亲和力没有显著影响，与早期的一份报告相反，在来自小鼠 HD 模型或人类患者的 HD 细胞中，没有内源性 HAP40 蛋白的异常积累。最后，当在 HD 的果蝇模型中进行测试时，HAP40 部分调节全长突变 HTT 诱导的神经退行性变，而对突变 HTT 外显子 1 片段的毒性没有明显影响。总的来说，我们的研究揭示了一个保守的机制来调节 HTT 的稳定性和体内功能，并表明 HAP40 是内源性 HTT 的一个核心和积极的调节因子。此外，我们的结果支持无论其伴侣 HAP40 存在与否，突变 HTT 都是有毒的，并暗示 HAP40 通过其对 HTT 功能、稳定性和突变 HTT 毒性的效力的多重影响，是 HD 发病机制的一个潜在调节剂。
