The homeodomain-interacting protein kinase HPK-1 preserves protein homeostasis and longevity through master regulatory control of the HSF-1 chaperone network and TORC1-restricted autophagy in <i>Caenorhabditis elegans</i>

An extensive proteostatic network comprised of molecular chaperones and protein clearance mechanisms functions collectively to preserve the integrity and resiliency of the proteome. The efficacy of this network deteriorates during aging, coinciding with many clinical manifestations, including protein aggregation diseases of the nervous system. A decline in proteostasis can be delayed through the activation of cytoprotective transcriptional responses, which are sensitive to environmental stress and internal metabolic and physiological cues. The homeodomain-interacting protein kinase (hipk) family members are conserved transcriptional co-factors that have been implicated in both genotoxic and metabolic stress responses from yeast to mammals. We demonstrate that constitutive expression of the sole Caenorhabditis elegans Hipk homolog, hpk-1, is sufficient to delay aging, preserve proteostasis, and promote stress resistance, while loss of hpk-1 is deleterious to these phenotypes. We show that HPK-1 preserves proteostasis and extends longevity through distinct but complementary genetic pathways defined by the heat shock transcription factor (HSF-1), and the target of rapamycin complex 1 (TORC1). We demonstrate that HPK-1 antagonizes sumoylation of HSF-1, a post-translational modification associated with reduced transcriptional activity in mammals. We show that inhibition of sumoylation by RNAi enhances HSF-1-dependent transcriptional induction of chaperones in response to heat shock. We find that hpk-1 is required for HSF-1 to induce molecular chaperones after thermal stress and enhances hormetic extension of longevity. We also show that HPK-1 is required in conjunction with HSF-1 for maintenance of proteostasis in the absence of thermal stress, protecting against the formation of polyglutamine (Q35::YFP) protein aggregates and associated locomotory toxicity. These functions of HPK-1/HSF-1 undergo rapid down-regulation once animals reach reproductive maturity. We show that HPK-1 fortifies proteostasis and extends longevity by an additional independent mechanism: induction of autophagy. HPK-1 is necessary for induction of autophagosome formation and autophagy gene expression in response to dietary restriction (DR) or inactivation of TORC1. The autophagy-stimulating transcription factors pha-4/FoxA and mxl-2/Mlx, but not hlh-30/TFEB or the nuclear hormone receptor nhr-62, are necessary for extended longevity resulting from HPK-1 overexpression. HPK-1 expression is itself induced by transcriptional mechanisms after nutritional stress, and post-transcriptional mechanisms in response to thermal stress. Collectively our results position HPK-1 at a central regulatory node upstream of the greater proteostatic network, acting at the transcriptional level by promoting protein folding via chaperone expression, and protein turnover via expression of autophagy genes. HPK-1 therefore provides a promising intervention point for pharmacological agents targeting the protein homeostasis system as a means of preserving robust longevity.由分子伴侣（molecular chaperones）与蛋白质清除机制共同构成的广泛蛋白质稳态网络，协同发挥功能以维持蛋白质组（proteome）的完整性与弹性。该网络的效能会随衰老进程逐渐衰退，并伴随诸多临床表征，包括神经系统的蛋白质聚集疾病。蛋白质稳态的衰退可通过激活细胞保护性转录应答得以延缓，这类应答对环境应激以及内部代谢与生理信号敏感。同源结构域相互作用蛋白激酶（homeodomain-interacting protein kinase, HIPK）家族成员是一类保守的转录辅因子，从酵母到哺乳动物均参与了遗传毒性与代谢应激应答过程。我们的研究证实，单独组成型表达秀丽隐杆线虫（Caenorhabditis elegans）唯一的HIPK同源物hpk-1，即可延缓衰老、维持蛋白质稳态并增强应激抗性；而hpk-1功能缺失则会对这些表型产生不利影响。研究表明，HPK-1通过两条既独立又互补的遗传通路维持蛋白质稳态并延长寿命，这两条通路分别由热休克转录因子（heat shock transcription factor, HSF-1）以及雷帕霉素靶蛋白复合物1（target of rapamycin complex 1, TORC1）介导。我们证实，HPK-1可拮抗HSF-1的SUMO化修饰（sumoylation）——这类翻译后修饰在哺乳动物中与转录活性降低相关。研究显示，通过RNA干扰抑制SUMO化修饰，可增强HSF-1依赖的、热休克诱导的分子伴侣基因转录激活。我们发现，热应激后HSF-1诱导分子伴侣表达的过程依赖hpk-1，且HPK-1可增强毒物兴奋效应介导的寿命延长。我们还证实，在无热应激的条件下，HPK-1需与HSF-1协同维持蛋白质稳态，以抵御多聚谷氨酰胺（polyglutamine, Q35::YFP）蛋白聚集及其引发的运动毒性。HPK-1/HSF-1的这些功能会在动物达到生殖成熟后快速下调。我们的研究还显示，HPK-1可通过另一独立机制强化蛋白质稳态并延长寿命：诱导自噬（autophagy）。HPK-1是营养限制（dietary restriction, DR）或TORC1失活诱导自噬体（autophagosome）形成以及自噬相关基因表达所必需的。自噬刺激型转录因子pha-4/FoxA与mxl-2/Mlx，而非hlh-30/TFEB或核激素受体nhr-62，是HPK-1过表达介导的寿命延长所必需的。HPK-1的表达本身会在营养应激后通过转录机制被诱导，而在热应激后则通过转录后机制被调控。综上，我们的研究结果将HPK-1定位为上游核心调控节点，主导整个蛋白质稳态网络：它通过转录层面促进分子伴侣基因表达以辅助蛋白质折叠，并通过上调自噬相关基因表达以促进蛋白质周转。因此，HPK-1为以蛋白质稳态系统为靶点的药物干预提供了极具前景的切入点，有望助力维持健康长寿。