Central role of Nrg1 in the niche-specific coordination of morphogenesis stress responses and metabolism in the fungal pathogen Candida albicans

这种真菌，白色念珠菌在医学上很重要，因为它会引起一系列人类感染，其中一些可能是致命的。它经常引起口腔感染，特别是在婴儿、老年人和艾滋病患者中。此外，大多数女性在一生中至少遭受一次“鹅口疮”感染。念珠菌还会在免疫防御明显减弱的重症监护病人（例如移植病人和癌症病人）中引起危及生命的“全身性”感染。对于“是什么使念珠菌成为如此好的病原体”这个问题，人们进行了大量的研究。许多研究小组都把注意力集中在毒力因素上。这些已被定义为直接与人类宿主相互作用以促进感染（即增强真菌致病性）的真菌因子。这些毒力因素包括这种真菌在酵母样生长形式和菌丝（菌丝）生长形式之间可逆切换的能力，这被认为有助于真菌侵入组织。念珠菌根据特定的环境因素（包括血清或葡萄糖）转变为菌丝生长。这个开关是由一个复杂的生物电路控制的：一些调节蛋白在这些环境触发下激活菌丝发育，而另一些在没有这些触发时抑制菌丝发育。我们已经详细研究了一个关键的抑制因子——Nrg1。我们已经证明Nrg1是一种调节蛋白，在没有诱导菌丝的环境触发条件下阻止菌丝生长。在这些研究过程中，我们发现Nrg1除了调节菌丝发育外，还控制胁迫保护和碳代谢相关基因的表达。这是非常重要的，因为这些过程对念珠菌的致病性至关重要。(人体免疫细胞通过将微生物入侵者暴露在氧化化学物质中来杀死它们。念珠菌必须能够保护自己免受这些氧化应激，如果它要成功地抵消这些宿主的防御并建立感染。此外，念珠菌必须吸收宿主的营养才能生长并感染宿主。这种养分的吸收依赖于碳代谢。)因此，调控因子Nrg1似乎在协调宿主生存所需的一系列重要细胞功能（菌丝发育、应激保护和代谢）方面发挥着至关重要的作用。Nrg1是如何做到的呢？这个项目的主要目标就是解决这个问题。我们已经证明Nrg1蛋白被化学修饰（磷酸化），磷酸化水平在酵母到菌丝的转换过程中发生变化。我们的假设是，准确的化学修饰对控制Nrg1活性至关重要，因此对念珠菌协调菌丝发育、应激保护和代谢的能力至关重要。在这个项目中，我们将使用最新实验工具的强大组合来验证这一假设。我们将确定Nrg1上的哪些位点被修饰，确定哪些酶催化这些修饰，并确定破坏这些修饰对念珠菌致病性的影响。这些研究将极大地促进我们对念珠菌致病性如何在分子水平上调控的理解。

The fungus, Candida albicans is medically important because it causes a range of infections in humans, some of which can be lethal. It causes frequent infections of the mouth, especially in young babies, the aged and in AIDS patients. Also, most women suffer at least one 'thrush' infection during their lifetime. Candida also causes life-threatening 'systemic' infections in intensive care patients whose immunological defences are significantly weakened (e.g. in transplant and cancer patients). Considerable research efforts have been devoted to the question 'What makes Candida such a good pathogen?'. Many research groups have focussed their attention on virulence factors. These have been defined as fungal factors that interact directly with the human host to promote infection (i.e. enhance fungal pathogenicity). These virulence factors include the ability of this fungus to switch reversibly between a yeast-like growth form and a hyphal (mycelial) growth form, which is believed to help the fungus invade tissue. Candida switches to hyphal growth in response to specific environmental triggers, which include serum or glucose. This switch is controlled by a complex biological circuitry: some regulatory proteins activate hyphal development in response to these environmental triggers, whilst others repress hyphal development in the absence of these triggers. We have studied a key repressor in some detail - Nrg1. We have shown that Nrg1 is a regulatory protein that prevents hyphal growth in the absence of hypha-inducing environmental triggers. In the course of these studies we have found that, in addition to regulating hyphal development, Nrg1 controls the expression of genes involved in stress protection and carbon metabolism. This is highly significant because these processes are essential for the pathogenicity of Candida. (Human immune cells kill microbial invaders by exposing them to oxidizing chemicals. Candida must be able to protect itself from these oxidative stresses if it is to counteract these host defences successfully and establish an infection. Also, Candida must assimilate nutrients from the host to be able to grow and infect its host. This nutrient assimilation depends upon carbon metabolism.) Hence the regulator Nrg1 seems to be playing a vital role by coordinating a range of important cellular functions that are required for survival in the host (hyphal development, stress protection and metabolism). How does Nrg1 do this? The main objective of this project is to address this question. We have shown that the Nrg1 protein is chemically modified (phosphorylated) and that the level of phosphorylation changes during the yeast-to-hyphal switch. Our hypothesis is that accurate chemical modification is vital for the control of Nrg1 activity, and hence central to the ability of Candida to coordinate its hyphal development, stress protection and metabolism. In this project we will test this hypothesis using a powerful combination of the latest experimental tools. We will define which sites on Nrg1 are modified, identify which enzymes catalyse these modifications, and establish the effects of disrupting these modifications upon the pathogenicity of Candida. These studies will dramatically advance our understanding of how Candida pathogenicity is regulated at the molecular level.