高浓度CO2通过调控Ume6蛋白稳定性促进白念珠菌菌丝延长及致病性发挥的分子机制

Candida albicans is the most prevalent fungal pathogen of humans. Candida albicans hyphae can express and secrete many adhensins and virulence factors, which make the pathogen capable of invading human tissues. High CO2 of host microenvironment maintains hyphal development through preventing Ume6 degradation in Candida albicans, however the mechanism is not yet known. Though screening the mutant library of C. albicans, we found that ptc2 mutant (type 2C protein phosphatase) was unable to sustain hyphal elongation in 5% CO2. We then screened the kinase mutant library and found that Ssn3 antagonized the function of Ptc2 in CO2-induced hyphal elongation. Based on these results, we hypothesize that, upon high CO2 induction, Ptc2 and Ssn3 regulate Ume6 protein stability to promote hyphal elongation. In this proposal, we will determine the substrate of Ptc2 through testing the phosphorylation levels of Ssn3 and Ume6 in response to different levels of CO2, and hope to uncover the signaling pathway mediated by high CO2, which prevents Ume6 protein degradation through Ptc2. We will further elucidate the importance of hyphal elongation in response to high CO2 in systemic infection model. Our study will develop new therapies that tackle infectious disease caused by fungal pathogens through blocking morphological transition.

白念珠菌是人体内主要的机会性致病真菌，菌丝形态的白念珠菌会大量表达和释放粘附及毒性因子从而引起组织感染。宿主微环境中高浓度CO2通过调控Ume6蛋白稳定性诱导菌丝延长，但机制尚不清楚。通过对突变体文库的筛选，我们发现蛋白磷酸酶PTC2的缺失阻断了高浓度CO2诱导的菌丝延长，进一步筛选激酶文库发现激酶Ssn3在这一过程中与Ptc2起拮抗作用。据此我们提出假说：磷酸酶Ptc2和激酶Ssn3通过调控Ume6的特异性降解过程介导CO2信号对菌丝延长的激活。本研究将通过检测Ume6和Ssn3在不同CO2浓度下的磷酸化水平确定Ptc2的作用底物，有望阐明高浓度CO2通过Ptc2阻断Ume6蛋白降解过程的分子机制，从而揭示高浓度CO2诱导菌丝的信号通路，并明确该通路在白念珠菌侵入性感染过程中发挥的重要作用。本项目对白念珠菌感受宿主气体微环境机制的鉴定将为阻断致病真菌的形态发生和侵入生长提供新途径。

白念珠菌是人体内主要的机会性致病真菌，其在共生和侵染过程中所面对的CO2浓度是剧烈变化的。对这种不断改变的CO2浓度做出正确应答是白念珠菌致病性发挥的重要因素，研究发现高浓度CO2能够激活菌丝发育。在空气条件下，菌丝激活因子Ume6会快速降解，而在5% CO2（即生理浓度）和低氧条件下，Ume6可以稳定存在并且维持菌丝的延长，但机制尚不清楚。我们发现Ume6蛋白稳定性是由两个平行的E3泛素连接酶所调控的：SCFGrr1负责CO2浓度所调控的Ume6蛋白降解过程；Ubr1负责O2浓度所调控的Ume6蛋白降解过程。通过对突变体文库的筛选，发现蛋白磷酸酶PTC2的缺失阻断了高浓度CO2诱导的菌丝延长，进一步筛选激酶文库发现激酶Ssn3在这一过程中与Ptc2起拮抗作用。进一步的实验发现Ptc2在高浓度CO2作用下可以负调控蛋白激酶Ssn3抑制其活性，从而使Ume6蛋白稳定来促进菌丝发育和侵入生长。本研究阐明了高浓度CO2通过Ptc2阻断Ume6蛋白降解过程的分子机制，从而揭示高浓度CO2诱导菌丝的信号通路。本项目对白念珠菌感受宿主气体微环境机制的鉴定将为阻断致病真菌的形态发生和侵入生长提供新途径。