Extracellular pH modulation by Candida albicans in vitro and in vivo

描述（由申请人提供）：白色念珠菌是最重要的人类相关真菌，作为微生物群的组成部分和多器官系统的病原体存在。C.白色念珠菌引起的感染范围从容易治疗的粘膜感染（鹅口疮、阴道炎）到难治性和经常致命的全身感染。传播 感染通常是先天免疫缺陷的结果，这激发了对C.白色念珠菌-吞噬细胞相互作用。巨噬细胞的吞噬作用在真菌中诱导动态和复杂的反应，包括急剧的代谢转变为致炎性生长模式，我们已经证明，这种转变的许多方面是完全毒力所必需的。在体外对该系统进行建模，揭示了这种生物体作为这些代谢变化的副产品从根本上改变细胞外pH的先前未知的能力。氨基酸被预测在宿主中是丰富的，并且它们用于满足细胞的碳需求导致细胞外pH的急剧上升，这是由氨以及可能其他碱性含氮化合物的输出驱动的。我们提出了一个模型，其中氨来自氨基酸的氨基或侧链胺基团作为代谢副产物。虽然这是大多数生物体中氨基酸催化剂的潜在结果，但念珠菌属（C.特别是白色念珠菌）显示出比先前描述的更强的pH变化。了解这一过程是我长期感兴趣的一部分，我想知道这种生物体如何适应基本的代谢功能，以支持它成为一种成功的寄生虫和病原体。遗传分析已经鉴定出非碱化突变体，其富含氨基酸输入和分解代谢功能。最严重的缺陷突变体是在STP 2中，编码调节氨基酸通透酶表达的转录。缺乏STP 2的突变体在与宿主细胞接触期间受损：它们发芽不良，并且比野生型对照更容易被巨噬细胞杀死，对巨噬细胞造成的损伤更小。此外，在酸性吞噬溶酶体中发现stp 2 Δ细胞，杀死的细胞也是如此。相比之下，野生型细胞占据更中性的吞噬溶酶体，表明它们具有改变stp 2 Δ细胞中缺乏的细胞内pH的机制。其他研究表明，stp 2 Δ突变体在念珠菌病的苍蝇模型中是减毒的;另外两种非碱化突变体，编码氨基酸渗透酶的ER伴侣的csh 3 Δ和编码氨基酸分解代谢酶尿素酰胺解酶的dur 1，2Δ，在小鼠中是无毒的。因此，有强有力的初步证据表明，调节碱化的因素也会改变宿主-病原体的相互作用，尽管两者之间的确切联系尚未确定。我们最近发现了第二个碱化机制，在生长过程中的TCA循环的二羧酸中间体，并将此机制的特点。本申请旨在验证关键宿主生态位（包括吞噬溶酶体）的碱化可促进C.白念珠菌通过抑制宿主免疫反应。我们将通过测试我们的代谢适应模型来实现这一点，这些代谢适应使这种现象在C中更加活跃。白念珠菌中比其他真菌，包括确定来源的氨（和其他潜在的基本代谢产物）释放。我们还将确定是否碱化发生在吞噬细胞内，如果这是诱导菌丝形态发生的关键信号。最后，我们将使用活细胞成像来解剖C。白念珠菌在巨噬细胞中的作用，并确定碱化和氨释放在调节内吞成熟中的作用。

DESCRIPTION (provided by applicant): Candida albicans is the most important human-associated fungus, existing as both an integral component of the microbiota and as a pathogen of multiple organ systems. C. albicans causes infections ranging from easily treatable mucosal infections (thrush, vaginitis) to refractory and frequently fatal systemic infections. Disseminated infections are often a result of defects in innate immunity and this has motivated studies of C. albicans-phagocyte interactions. Phagocytosis by macrophages induces a dynamic and complex response in the fungus, including a dramatic metabolic shift to a gluconeogenic mode of growth, and we have shown that many aspects of this shift are required for full virulence. Modeling this system in vitro has uncovered a previously unknown ability of this organism to radically change extracellular pH as a byproduct of these metabolic changes. Amino acids are predicted to be plentiful in the host, and their use to satisfy cellular carbon requirements result in a dramatic rise in extracellular pH, driven by the export of ammonia, and perhaps other basic nitrogenous compounds. We propose a model in which this ammonia derives from the amino or side-chain amine groups of the amino acids as a metabolic byproduct. While this is a potential consequence of amino acid catabolism in most organisms, Candida species (C. albicans in particular) show a far more robust pH change than has been previously described. Understanding this process is part of my long-standing interest in how this organism has adapted basic metabolic functions to support it as a successful commensal and pathogen. Genetic analysis has identified non-alkalinizing mutants, which are enriched for amino acid import and catabolic functions. The most severely deficient mutant is in STP2, encoding a transcription that regulates amino acid permease expression. Mutants lacking STP2 are impaired during contact with host cells: they germinate poorly and are both more susceptible to killing by macrophages and cause less damage to macrophages than wild-type controls. Moreover, stp2Δ cells are found in acidic phagolysosomes, as are killed cells. In contrast, wild-type cells occupy a more neutral phagolysosome, suggesting they have a mechanism to alter intracellular pH lacking in stp2Δ cells. Others have shown that stp2Δ mutants are attenuated in a fly model of candidiasis; two other non-alkalinizing mutant, csh3Δ, encoding an ER chaperone for amino acid permeases, and dur1,2Δ, encoding the amino acid catabolic enzyme urea amidolyase, are avirulent in mice. Thus, there is strong preliminary evidence that factors that regulate alkalinization also alter the host-pathogen interaction, though the precise connection between the two has not been firmly established. We have recently discovered a second alkalinization mechanism during growth on dicarboxylic acid intermediates of the TCA cycle, and will characterize this mechanism as well. This application proposes to test the hypothesis that alkalinization of key host niches, including the phagolysosome, promotes virulence of C. albicans by inhibiting the host immune response. We will do so by testing our model for the metabolic adaptations that make this phenomenon much more vigorous in C. albicans than in other fungi, including determining the source of the ammonia (and other potential basic metabolites) released. We will also determine whether alkalinization occurs within the phagocyte and if this is the key signal that induces hyphal morphogenesis. Finally, we will use live cell imaging to dissect the intracellular trafficking of C. albicans in the macrophage and determine the role of alkalinization and ammonia release in modulating endocytic maturation.