Multifunction of the ER P-Type Calcium Pump Spf1 During Hyphal Development in Candida albicans

Candida albicans is one of the most important fungal pathogens. Hyphal development is required for the virulence of this pathogen. Our previous study has revealed that Spf1, an ER P-type calcium pump, plays an important role in hyphal development. However, the detailed mechanisms by which this protein functions in this process remain to be investigated. In this study, we found that loss of Spf1 led to decreased growth biomass under the hypha-inducing condition, suggesting a role of this protein in maintaining hyphal growth rate. Actin staining further revealed that the spf1Δ/Δ mutant showed attenuated tip-localization of actin patches and the defect in transport of both the chitin synthase Chs3 and the hypha-related factor Hwp1, implying that Spf1 functions in polarized growth of the hyphae by regulating actin organization and consequent polarized transport of morphogenesis-associated factors. Moreover, deletion of SPF1 led to abnormal vacuolar morphology under the hypha-inducing condition, which may also contribute to the defect of hyphal development in the spf1Δ/Δ mutant. This study revealed the pleiotropic role of Spf1-regulated calcium homeostasis in controlling hyphal development in C. albicans.

Interaction among the vacuole, the mitochondria, and the oxidative stress response is governed by the transient receptor potential channel in Candida albicans

Candida albicans is one of the most important opportunistic pathogens, causing both mucosal candidiasis and life-threatening systemic infections. To survive in the host immune defense system, this pathogen uses an elaborate signaling network to recognize and respond to oxidative stress, which is essential for its pathogenicity. However, the exact mechanisms that this fungus employs to integrate the oxidative stress response (OSR) with functions of various organelles remain uncharacterized. Our previous work implicated a connection between the calcium signaling system and the OSR. In this study, we find that the vacuolar transient receptor potential (TRP) channel Yvc1, one of the calcium signaling members, plays a critical role in cell tolerance to oxidative stress. We further provide evidence that this channel is required not only for activation of Cap1-related transcription of OSR genes but also for maintaining the stability of both the mitochondria and the vacuole in a potassium- and calcium-dependent manner. Element assays reveal that this TRP channel affects calcium influx and potassium transport from the vacuole to the mitochondria. Therefore, the TRP channel governs the novel interaction among the OSR, the vacuole, and the mitochondria by mediating ion transport in this pathogen under oxidative stress.

A novel role of the vacuolar calcium channel Yvc1 in stress response, morphogenesis and pathogenicity of Candida albicans

C. albicans is a common opportunistic pathogen, causing both superficial and life-threatening systemic infections. Calcium signaling is an intriguing aspect in its physiology, attributing to the roles in stress response and morphogenesis. Until recently, little was known about the mechanisms by which the calcium signaling-associated elements affect its pathogenicity. In this study, we found that Yvc1, a member of the transient receptor potential (TRP) family, localized on the vacuolar membrane. The yvc1Δ/Δ mutant displayed decreased ability of stress response, morphogenesis and attenuated virulence. The Spitzenkörper required for polarized growth were not detected in the hyphal tip of this mutant, suggesting a key role of Yvc1 in hyphal polarized growth and re-orientation to host signals. This study demonstrates, for the first time, that the putative vacuolar calcium channel Yvc1 plays an important role in C. albicans infection and survival in host tissues, which is associated with its pleiotropic effects in several fungal physiological processes, including stress response, morphogenesis, and polarized growth.

Identification of vacuole defects in fungi

Fungal vacuoles are involved in a diverse range of cellular functions, participating in cellular homeostasis, degradation of intracellular components, and storage of ions and molecules. In recent years there has been a significant increase in the number of studies linking these organelles with the regulation of growth and control of cellular morphology, particularly in those fungal species able to undergo yeast-hypha morphogenetic transitions. This has contributed to the refinement of previously published protocols and the development of new techniques, particularly in the area of live-cell imaging of membrane trafficking events and vacuolar dynamics. The current review outlines recent advances in the imaging of fungal vacuoles and assays for characterization of trafficking pathways, and other physiological activities of this important cell organelle.

The endocytic adaptor proteins of pathogenic fungi: charting new and familiar pathways

Intracellular transport is an essential biological process that is highly conserved throughout the eukaryotic organisms. In fungi, adaptor proteins implicated in the endocytic cycle of endocytosis and exocytosis were found to be important for growth, differentiation, and/or virulence. For example, Saccharomyces cerevisiae Pan1 is an endocytic protein that regulates membrane trafficking, the actin cytoskeleton, and signaling. In Cryptococcus neoformans, a multi-modular endocytic protein, Cin1, was recently found to have pleiotropic functions in morphogenesis, endocytosis, exocytosis, and virulence. Interestingly, Cin1 is homologous to human intersectin ITSN1, but homologs of Cin1/ITSN1 were not found in ascomycetous S. cerevisiae and Candida albicans, or zygomycetous fungi. Moreover, an Eps15 protein homologous to S. cerevisiae Pan1/Ede1 and additional relevant protein homologs were identified in C. neoformans, suggesting the existence of either a distinct endocytic pathway mediated by Cin1 or pathways by either Cin1 or/and Pan1/Ede1 homologs. Whether and how the Cin1-mediated endocytic pathway represents a unique role in pathogenesis or reflects a redundancy of a transport apparatus remains an open and challenging question. This review discusses recent findings of endocytic adaptor proteins from pathogenic fungi and provides a perspective for novel endocytic machinery operating in C. neoformans. An understanding of intracellular trafficking mechanisms as they relate to pathogenesis will likely reveal the identity of novel antifungal targets.