脂質磷酸酶PIP5K1A調節內吞蛋白運輸以及維持內膜系統完整性

Abstract

磷脂醯肌醇-4,5-二磷酸（PI(4,5)P2）是一種重要的脂質類的信號分子，在細胞中有許多種功能，其中包括訊號傳遞、分子分泌以及囊泡運輸。磷脂醯肌醇-4,5-二磷酸是由磷脂酰肌醇-5-磷酸酶（PIP5K）磷酸化磷脂醯肌醇4-磷酸（PI(4)P）而成。而磷脂酰肌醇-5-磷酸酶可進一步細分為磷脂酰肌醇-5-磷酸酶1A（PIP5K1A）、磷脂酰肌醇-5-磷酸酶1B（PIP5K1B）和磷脂酰肌醇-5-磷酸酶1C（PIP5K1C）。在先前的研究發現磷脂醯肌醇-4,5-二磷酸可以調節載鐵蛋白受體（TfR）和表皮生長因子受體（EGFR）的內吞運輸。本研究中，我們發現細胞缺失磷脂酰肌醇-5-磷酸酶1A（PIP5K1A）的會抑制內吞作用以及分子運輸到晚期胞內體（Late endosome）或溶酶體（Lysosome）的運輸。此外，缺失磷脂酰肌醇-5-磷酸酶1A（PIP5K1A）的情況下也抑制了載鐵蛋白受體（TfR）回收到細胞膜表面，使其累積在胞內體。除此之外，我們還發現到在缺失磷脂酰肌醇-5-磷酸酶1A（PIP5K1A）的情況下，會使得胞內體成熟的途徑也受到抑制，另外也造成了胞內體重要調節蛋白Rab5活性顯著下調。總括而言，我們的結果表明脂酰肌醇-5-磷酸酶1A（PIP5K1A）不僅在調節內吞運輸途徑中發揮特定作用，還在胞內體的成熟以及空間分佈中有扮演其角色。

Phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] is a key lipid signaling molecule that plays several cellular functions in cells, including signal transduction, secretion, and vesicular trafficking. PI(4,5)P2 is generated from PI(4)P by class 1 PIP5K (Phosphatidylinositol-4-phosphate 5-kinase). Class 1 PIP5K, generating PI(4,5)P2 from PI(4)P, can be further classified as PIP5K1A, PIP5K1B and PIP5K1C. Previous studies demonstrated that PI(4,5)P2 regulates the endocytic transport of transferrin receptor (TfR) and epidermal growth factor receptor (EGFR). Yet, the mechanism that distinguishes PIP5K isoform-dependent endocytic transport has remained elusive. In this study, we examined several intracellular transport pathways for the role of PIP5K during endocytic transport, including clathrin-mediated endocytosis, transferrin recycling pathway, and lysosomal transport. We found that loss of PIP5K1A specifically inhibits cargo internalization to the early endosome, and EGFR transport to the late endosome/lysosome. In addition, knockdown of PIP5K1A inhibited TfR recycling back to the plasma membrane, leading to the accumulation of TfR within the recycling endosome. Moreover, colocalization of the late endosome or recycling endosome with early endosome was dramatically increased, and the early endosome was clustered at the perinuclear region in siPIP5K1A-treated cells, indicating the disorder of the endosomal membrane system. Furthermore, the activity of Rab5, a master early endosome regulator, was downregulated under the PIP5K1A knockdown cells. Together, our results identify the specific role of PIP5K1A not only participates in regulating endocytic transport pathways but also organizing the spatial distribution of endosomes. These advances our understanding of how endosomal membrane systems and transport are tightly interconnected.