Mutational analysis of Candida albicans SNF7 reveals genetically separable Rim101 and ESCRT functions and demonstrates divergence in bro1-domain protein interactions

Morphology Changes in Human Fungal Pathogens upon Interaction with the Host

Morphological changes are a very common and effective strategy for pathogens to survive in the mammalian host. During interactions with their host, human pathogenic fungi undergo an array of morphological changes that are tightly associated with virulence. Candida albicans switches between yeast cells and hyphae during infection. Thermally dimorphic pathogens, such as Histoplasma capsulatum and Blastomyces species transform from hyphal growth to yeast cells in response to host stimuli. Coccidioides and Pneumocystis species produce spherules and cysts, respectively, which allow for the production of offspring in a protected environment. Finally, Cryptococcus species suppress hyphal growth and instead produce an array of yeast cells—from large polyploid titan cells to micro cells. While the morphology changes produced by human fungal pathogens are diverse, they all allow for the pathogens to evade, manipulate, and overcome host immune defenses to cause disease. In this review, we summarize the morphology changes in human fungal pathogens—focusing on morphological features, stimuli, and mechanisms of formation in the host.

Extracellular vesicles and infectious diseases: new complexity to an old story

Exosomes and other extracellular microvesicles (ExMVs) have important functions in intercellular communication and regulation. During the course of infection, these vesicles can convey pathogen molecules that serve as antigens or agonists of innate immune receptors to induce host defense and immunity, or that serve as regulators of host defense and mediators of immune evasion. These molecules may include proteins, nucleic acids, lipids, and carbohydrates. Pathogen molecules may be disseminated by incorporation into vesicles that are created and shed by host cells, or they may be incorporated into vesicles shed from microbial cells. Involvement of ExMVs in the induction of immunity and host defense is widespread among many pathogens, whereas their involvement in immune evasion mechanisms is prominent among pathogens that establish chronic infection and is found in some that cause acute infection. Because of their immunogenicity and enrichment of pathogen molecules, exosomes may also have potential in vaccine preparations and as diagnostic markers. Additionally, the ability of exosomes to deliver molecules to recipient cells raises the possibility of their use for drug/therapy delivery. Thus, ExMVs play a major role in the pathogenesis of infection and provide exciting potential for the development of novel diagnostic and therapeutic approaches.

Liaison alcaline: Pals entice non-endosomal ESCRTs to the plasma membrane for pH signaling

The alkaline pH-responsive Pal/Rim signal transduction pathway mediating regulation of gene expression by ambient pH has been extensively studied in Aspergillus nidulans and Saccharomyces cerevisiae. In A. nidulans, PalH, PalI, PalF, PalC, PalA and PalB are required for the proteolytic activation of the executing transcription factor PacC. Although necessary, Pal proteins are insufficient to transmit the signal, which additionally requires ESCRT-I, II and Vps20 with Snf7 in ESCRT-III. Although this initially suggested cooperation between a plasma membrane sensor and an ESCRT-containing Pal complex on endosomes, recent evidence convincingly indicates that pH signaling actually takes place in plasma membrane-associated foci in which Pal proteins and an ESCRT-III polymer scaffold cooperate for pH signaling purposes, representing another non-endosomal role of ESCRT components.

The endosomal sorting complex required for transport machinery influences haem uptake and capsule elaboration in Cryptococcus neoformans

Iron availability is a key determinant of virulence in the pathogenic fungus Cryptococcus neoformans. Previous work revealed that the ESCRT (endosomal sorting complex required for transport) protein Vps23 functions in iron acquisition, capsule formation and virulence. Here, we further characterized the ESCRT machinery to demonstrate that defects in the ESCRT-II and III complexes caused reduced capsule attachment, impaired growth on haem and resistance to non-iron metalloprotoporphyrins. The ESCRT mutants shared several phenotypes with a mutant lacking the pH-response regulator Rim101, and in other fungi, the ESCRT machinery is known to activate Rim101 via proteolytic cleavage. We therefore expressed a truncated and activated version of Rim101 in the ESCRT mutants and found that this allele restored capsule formation but not growth on haem, thus suggesting a Rim101-independent contribution to haem uptake. We also demonstrated that the ESCRT machinery acts downstream of the cAMP/protein kinase A pathway to influence capsule elaboration. Defects in the ESCRT components also attenuated virulence in macrophage survival assays and a mouse model of cryptococcosis to a greater extent than reported for loss of Rim101. Overall, these results indicate that the ESCRT complexes function in capsule elaboration, haem uptake and virulence via Rim101-dependent and independent mechanisms.

The vacuolar-sorting protein Snf7 is required for export of virulence determinants in members of the Cryptococcus neoformans complex

Fungal pathogenesis requires a number of extracellularly released virulence factors. Recent studies demonstrating that most fungal extracellular molecules lack secretory tags suggest that unconventional secretion mechanisms and fungal virulence are strictly connected. Proteins of the endosomal sorting complex required for transport (ESCRT) have been recently associated with polysaccharide export in the yeast-like human pathogen Cryptococcus neoformans. Snf7 is a key ESCRT operator required for unconventional secretion in Eukaryotes. In this study we generated snf7Δ mutant strains of C. neoformans and its sibling species C. gattii. Lack of Snf7 resulted in important alterations in polysaccharide secretion, capsular formation and pigmentation. This phenotype culminated with loss of virulence in an intranasal model of murine infection in both species. Our data support the notion that Snf7 expression regulates virulence in C. neoformans and C. gattii by ablating polysaccharide and melanin traffic. These results are in agreement with the observation that unconventional secretion is essential for cryptococcal pathogenesis and strongly suggest the occurrence of still obscure mechanisms of exportation of non-protein molecules in Eukaryotes.

pH signaling in human fungal pathogens: a new target for antifungal strategies

Fungi are exposed to broadly fluctuating environmental conditions, to which adaptation is crucial for their survival. An ability to respond to a wide pH range, in particular, allows them to cope with rapid changes in their extracellular settings. PacC/Rim signaling elicits the primary pH response in both model and pathogenic fungi and has been studied in multiple fungal species. In the predominant human pathogenic fungi, namely, Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans, this pathway is required for many functions associated with pathogenesis and virulence. Aspects of this pathway are fungus specific and do not exist in mammalian cells. In this review, we highlight recent advances in our understanding of PacC/Rim-mediated functions and discuss the growing interest in this cascade and its factors as potential drug targets for antifungal strategies. We focus on both conserved and distinctive features in model and pathogenic fungi, highlighting the specificities of PacC/Rim signaling in C. albicans, A. fumigatus, and C. neoformans. We consider the role of this pathway in fungal virulence, including modulation of the host immune response. Finally, as now recognized for other signaling cascades, we highlight the role of pH in adaptation to antifungal drug pressure. By acting on the PacC/Rim pathway, it may therefore be possible (i) to ensure fungal specificity and to limit the side effects of drugs, (ii) to ensure broad-spectrum efficacy, (iii) to attenuate fungal virulence, (iv) to obtain additive or synergistic effects with existing antifungal drugs through tolerance inhibition, and (v) to slow the emergence of resistant mutants.

Interaction maps of the Saccharomyces cerevisiae ESCRT-III protein Snf7

The Saccharomyces cerevisiae ESCRT-III protein Snf7 is part of an intricate interaction network at the endosomal membrane. Interaction maps of Snf7 were established by measuring the degree of binding of individual binding partners to putative binding motifs along the Snf7 sequence by glutathione S-transferase (GST) pulldown. For each interaction partner, distinct binding profiles were obtained. The following observations were made. The ESCRT-III subunits Vps20 and Vps24 showed a complementary binding pattern, suggesting a model for the series of events in the ESCRT-III functional cycle. Vps4 bound to individual Snf7 motifs but not to full-length Snf7. This suggests that Vps4 does not bind to the closed conformation of Snf7. We also demonstrate for the first time that the ALIX/Bro1 homologue Rim20 binds to the α6 helix of Snf7. Analysis of a Snf7 α6 deletion mutant showed that the α6 helix is crucial for binding of Bro1 and Rim20 in vivo and is indispensable for the multivesicular body (MVB)-sorting and Rim-signaling functions of Snf7. The Snf7Δα6 protein still appeared to be incorporated into ESCRT-III complexes at the endosomal membrane, but disassembly of the complex seemed to be defective. In summary, our study argues against the view that the ESCRT cycle is governed by single one-to-one interactions between individual components and emphasizes the network character of the ESCRT interactions.

Shuttle vectors for facile gap repair cloning and integration into a neutral locus in Candida albicans

Candida albicans is the most prevalent fungal pathogen of humans. The current techniques used to construct C. albicans strains require integration of exogenous DNA at ectopic locations, which can exert position effects on gene expression that can confound the interpretation of data from critical experiments such as virulence assays. We have identified a large intergenic region, NEUT5L, which facilitates the integration and expression of ectopic genes. To construct and integrate inserts into this novel locus, we re-engineered yeast/bacterial shuttle vectors by incorporating 550 bp of homology to NEUT5L. These vectors allow rapid, facile cloning through in vivo recombination (gap repair) in Saccharomyces cerevisiae and efficient integration of the construct into the NEUT5L locus. Other useful features of these vectors include a choice of three selectable markers (URA3, the recyclable URA3-dpl200 or NAT1), and rare restriction enzyme recognition sites for releasing the insert from the vector prior to transformation into C. albicans, thereby reducing the insert size and preventing integration of non-C. albicans DNA. Importantly, unlike the commonly used RPS1/RP10 locus, integration at NEUT5L has no negative effect on growth rates and allows native-locus expression levels, making it an ideal genomic locus for the integration of exogenous DNA in C. albicans.

The β-arrestin-like protein Rim8 is hyperphosphorylated and complexes with Rim21 and Rim101 to promote adaptation to neutral-alkaline pH

β-Arrestin proteins are critical for G-protein-coupled receptor desensitization and turnover. However, β-arrestins have recently been shown to play direct roles in nonheterotrimeric G-protein signal transduction. The Candida albicans β-arrestin-like protein Rim8 is required for activation of the Rim101 pH-sensing pathway and for pathogenesis. We have found that C. albicans Rim8 is posttranslationally modified by phosphorylation and specific phosphorylation states are associated with activation of the pH-sensing pathway. Rim8 associated with both the receptor Rim21 and the transcription factor Rim101, suggesting that Rim8 bridges the signaling and activation steps of the pathway. Finally, upon activation of the Rim101 transcription factor, C. albicans Rim8 was transcriptionally repressed and Rim8 protein levels were rapidly reduced. Our studies suggest that Rim8 is taken up into multivesicular bodies and degraded within the vacuole. In total, our results reveal a novel mechanism for tightly regulating the activity of a signal transduction pathway. Although the role of β-arrestin proteins in mammalian signal transduction pathways has been demonstrated, relatively little is known about how β-arrestins contribute to signal transduction. Our analyses provide some insights into potential roles.

Rescue of Aspergillus nidulans severely debilitating null mutations in ESCRT-0, I, II and III genes by inactivation of a salt-tolerance pathway allows examination of ESCRT gene roles in pH signalling

The Aspergillus pal pathway hijacks ESCRT proteins into ambient pH signalling complexes. We show that components of ESCRT-0, ESCRT-I, ESCRT-II and ESCRT-III are nearly essential for growth, precluding assessment of null mutants for pH signalling or trafficking. This severely debilitating effect is rescued by loss-of-function mutations in two cation tolerance genes, one of which, sltA, encodes a transcription factor whose inactivation promotes hypervacuolation. Exploiting a conditional expression sltA allele, we demonstrate that deletion of vps27 (ESCRT-0), vps23 (ESCRT-I), vps36 (ESCRT-II), or vps20 or vps32 (both ESCRT-III) leads to numerous small vacuoles, a phenotype also suppressed by SltA downregulation. This situation contrasts with normal vacuoles and vacuole-associated class E compartments seen in Saccharomyces cerevisiae ESCRT null mutants. Exploiting the suppressor phenotype of sltA⁻ mutations, we establish that Vps23, Vps36, Vps20 and Vps32 are essential for pH signalling. Phosphatidylinositol 3-phosphate-recognising protein Vps27 (ESCRT-0) is not, consistent with normal pH signalling in rabB null mutants unable to recruit Vps34 kinase to early endosomes. In contrast to the lack of pH signalling in the absence of Vps20 or Vps32, detectable signalling occurs in the absence of ESCRT-III subunit Vps24. Our data support a model in which certain ESCRT proteins are recruited to the plasma membrane to mediate pH signalling.

Cryptococcal titan cell formation is regulated by G-protein signaling in response to multiple stimuli

The titan cell is a recently described morphological form of the pathogenic fungus Cryptococcus neoformans. Occurring during the earliest stages of lung infection, titan cells are 5 to 10 times larger than the normal yeast-like cells, thereby resisting engulfment by lung phagocytes and favoring the persistence of infection. These enlarged cells exhibit an altered capsule structure, a thickened cell wall, increased ploidy, and resistance to nitrosative and oxidative stresses. We demonstrate that two G-protein-coupled receptors are important for induction of the titan cell phenotype: the Ste3a pheromone receptor (in mating type a cells) and the Gpr5 protein. Both receptors control titan cell formation through elements of the cyclic AMP (cAMP)/protein kinase A (PKA) pathway. This conserved signaling pathway, in turn, mediates its effect on titan cells through the PKA-regulated Rim101 transcription factor. Additional downstream effectors required for titan cell formation include the G(1) cyclin Pcl103, the Rho104 GTPase, and two GTPase-activating proteins, Gap1 and Cnc1560. These observations support developing models in which the PKA signaling pathway coordinately regulates many virulence-associated phenotypes in diverse human pathogens.

The Candida albicans ESCRT pathway makes Rim101-dependent and -independent contributions to pathogenesis

Candida albicans is an opportunistic pathogen that colonizes diverse mucosal niches with distinct environmental characteristics. To adapt to these different sites, C. albicans must activate and attenuate a variety of signal transduction pathways. A mechanism of signal attenuation is through receptor endocytosis and subsequent vacuolar degradation, which requires the endosomal sorting complex required for transport (ESCRT) pathway. This pathway comprises several polyprotein complexes (ESCRT-0, -I, -II, -III, and -DS) that are sequentially recruited to the endosomal membrane. The ESCRT pathway also activates the Rim101 transcription factor, which governs expression of genes required for virulence. Here, we tested the hypothesis that the ESCRT pathway plays a Rim101-independent role(s) in pathogenesis. We generated deletion mutants in each ESCRT complex and determined that ESCRT-I, -II, and -III are required for Rim101 activation but that ESCRT-0 and ESCRT-DS are not. We found that the ESCRT-0 member Vps27 and ESCRT-DS components are required to promote epithelial cell damage and, using a murine model of oral candidiasis, found that the vps27Delta/Delta mutant had a decreased fungal burden compared to that of the wild type. We found that a high-dose inoculum can compensate for fungal burden defects but that mice colonized with the vps27Delta/Delta strain exhibit less morbidity than do mice infected with the wild-type strain. These results demonstrate that the ESCRT pathway has Rim101-independent functions for C. albicans virulence.

Intervention of Bro1 in pH-responsive Rim20 localization in Saccharomyces cerevisiae

Yeast cells contain two Bro1 domain proteins: Bro1, which is required for endosomal trafficking, and Rim20, which is required for the response to the external pH via the Rim101 pathway. Rim20 associates with endosomal structures under alkaline growth conditions, when it promotes activation of Rim101 through proteolytic cleavage. We report here that the pH-dependent localization of Rim20 is contingent on the amount of Bro1 in the cell. Cells that lack Bro1 have increased endosomal Rim20-green fluorescent protein (GFP) under acidic conditions; cells that overexpress Bro1 have reduced endosomal Rim20-GFP under acidic or alkaline conditions. The novel endosomal association of Rim20-GFP in the absence of Bro1 requires ESCRT components including Vps27 but not specific Rim101 pathway components such as Dfg16. Vps27 influences the localization of Bro1 but is not required for RIM101 pathway activation in wild-type cells, thus suggesting that Rim20 enters the Bro1 localization pathway when a vacancy exists. Despite altered localization of Rim20, the lack of Bro1 does not bypass the need for signaling protein Dfg16 to activate Rim101, as evidenced by the expression levels of the Rim101 target genes RIM8 and SMP1. Therefore, endosomal association of Rim20 is not sufficient to promote Rim101 activation.