热休克蛋白Hsp90alfa在细胞初级纤毛以及视杆细胞中的作用

Primary cilia are cellular membrane protrusions that can sense and mediate the extracellular stimuli and are considered as sensory organelle. Many extracellular signals, such as growth factors, morphogens and even mechanic force, are sensed by cilia through the membrane receptors or cilia bending. And the signals are passed into the intracellular molecules through intraflagellar transportation. Many intracellular signal molecules are recruited to cilia or the ciliary basal body. Thus, primary cilia organize the membrane receptor and its downstream signal molecules into a signaling apparatus. The organization of primary cilia facilitates the interactions between the receptor and signal molecules. .Heat shock protein Hsp90alfa is a molecular chaperone and is identified to be an important chaperone at the base of cilia. It helps to recruit intracellular signal molecules into ciliary base and stabilizes those recruited signal molecules. At ciliary base, Hsp90alfa and the recruited signal molecules form a chaperone-signal molecule complex. Within this complex, Hsp90alfa provides a chaperone platform to stabilize the signal molecules and to facilitate their interactions. Thus, Hsp90alfa plays as an important mediator in cilia mediated membrane receptor signal transduction. For example, during cilia mediated IGF-1 receptor signaling Hsp90alfa helps to recruit IRS-1, the direct downstream substrate of IGF-1 receptor kinase, into the basal body from which IRS-1 is transported alone the ciliary axoneme, phosphorylated by the receptor kinase in axonemal membrane and transported back to the Hsp90alfa platform at the base to interact with other signal molecules. .Hsp90alfa has been identified in all the cilia examined, including the specialized cilia formed rod cells in retina, the sperm flagellum and the motile cilia of tracheal epithelial cells. On the other hand, Hsp90 has been gaining the momentum to be an important molecular target for the development of anti-cancer drugs in the past 20 years. About 20 different inhibitors of Hsp90 chaperone activity have been under clinical trial for their cancer therapeutic efficacy. It is essential to evaluate the effect of Hsp90 inhibitor on its cilia-related functions. As cilia are ubiquitously present in most type of cells, the understanding of the function of Hsp90 in cilia will help to develop anti-cancer Hsp90 inhibitor that focus on cancer cells rather than affect many ciliary cells.

细胞初级纤毛是细胞膜表面感应外界信号的重要感应细胞器。许多外界信号，如生长因子（growth factors）、形态发生素（morphogens）和机械张力都是通过位于细胞膜纤毛中的受体或纤毛本身的弯曲作用于细胞，从而激活细胞内的信号通络。纤毛在感应信号时，一方面接受外界配体的作用，另一方面募集细胞内的信号分子，通过纤毛的组织，促进信号分子的相互作用。热休克蛋白质Hsp90a是纤毛基部参与信号募集以及稳定的重要分子伴侣蛋白质，它在纤毛基部形成的信号分子-分子伴侣复合体起到了稳定信号分子蛋白质以及促进它们相互作用的功能。研究发现Hsp90a普遍存在于所有的纤毛中，比如纤毛特化形成的视杆细胞以及精子尾巴。作为一个重要的分子伴侣蛋白质，Hsp90是许多抗肿瘤药物的靶蛋白，有近20种Hsp90的抑制剂作为抗肿瘤药物在临床实验中。初步研究表明，Hsp90抑制剂对于纤毛的功能具有显著的抑制作用。

Hsp90是Hsp90分子中只在视网膜、睾丸以及大脑中表达的一个诱导表达亚型，相比于在所有组织中组成性表达的Hsp90亚型对Hsp90的认识还十分有限。在发现Hsp90特异性地与细胞纤毛结合的基础上，我们对Hsp90在视网膜感光细胞与精子这两种纤毛细胞中的功能进行了研究，发现在小鼠中缺失Hsp90或者Hsp90活性被抑制以后视网膜发生退化而睾丸大幅度地缩小，并且两者都失去了功能，小鼠最终失明而小鼠的睾丸也不能形成精子。进一步研究表明，Hsp90缺失导致小鼠失明是因为小鼠视网膜退化，其中的感光细胞发生凋亡。免疫荧光显微镜和电镜结果表明缺失Hsp90的感光细胞呈现出外段膨大和膜盘扭曲，细胞内的高尔基体也呈现出碎片化，大量的视紫蛋白质被阻滞在感光细胞的内段，无法被运输到外段的膜盘上。这一感光细胞内囊泡运输障碍使得感光细胞的外段得不到足够的蛋白质与膜脂的补充，在感光细胞的快速交替过程中难以维持外段所必要的物质输送，进而导致细胞发生畸形和凋亡。通过蛋白质组学发现Hsp90在感光细胞中与微管结合蛋白质MAP1B相互作用，通过稳定微管蛋白质的乙酰化水平而稳定微管骨架。缺失Hsp90蛋白质导致MAP1B降解，微管蛋白发生去乙酰化，微管骨架稳定性下降，进而影响了高尔基体的完整性，引起高尔基体碎片化。Hsp90缺失导致高尔基体碎片化可以被过表达MAP1B或去乙酰化酶抑制剂tricostatin A所逆转，证实了微管结合蛋白质与其调控的微管乙酰化是Hsp90作用的下游蛋白质和细胞活动事件，最终导致了高尔基体的碎片化。在小鼠的精子发育过程中，Hsp90参与了精子发育过程中的尾巴鞭毛的生成，在成熟精子中参与了精子的运动，Hsp90抑制剂显著抑制精子的游动速度，并且抑制了精子使卵细胞授精的能力。