Evidence for adenylate cyclase as a scaffold protein for Ras2-Ira interaction in Saccharomyces cerevisie

To each its own: Mechanisms of cross-talk between GPI biosynthesis and cAMP-PKA signaling in Candida albicans versus Saccharomyces cerevisiae

Candida albicans is an opportunistic fungal pathogen that can switch between yeast and hyphal morphologies depending on the environmental cues it receives. The switch to hyphal form is crucial for the establishment of invasive infections. The hyphal form is also characterized by the cell surface expression of hyphae-specific proteins, many of which are GPI-anchored and important determinants of its virulence. The coordination between hyphal morphogenesis and the expression of GPI-anchored proteins is made possible by an interesting cross-talk between GPI biosynthesis and the cAMP-PKA signaling cascade in the fungus; a parallel interaction is not found in its human host. On the other hand, in the nonpathogenic yeast, Saccharomyces cerevisiae, GPI biosynthesis is shut down when filamentation is activated and vice versa. This too is achieved by a cross-talk between GPI biosynthesis and cAMP-PKA signaling. How are diametrically opposite effects obtained from the cross-talk between two reasonably well-conserved pathways present ubiquitously across eukarya? This Review attempts to provide a model to explain these differences. In order to do so, it first provides an overview of the two pathways for the interested reader, highlighting the similarities and differences that are observed in C. albicans versus the well-studied S. cerevisiae model, before going on to explain how the different mechanisms of regulation are effected. While commonalities enable the development of generalized theories, it is hoped that a more nuanced approach, that takes into consideration species-specific differences, will enable organism-specific understanding of these processes and contribute to the development of targeted therapies.

A mathematical model captures the role of adenyl cyclase Cyr1 and guanidine exchange factor Ira2 in creating a growth-to-hyphal bistable switch in Candida albicans

Recent biochemical experiments have indicated that in Candida albicans, a commensal fungal pathogen, the Ras signaling pathway plays a significant role in the yeast-to-hyphal transition; specifically, two enzymes in this pathway, Adenyl Cyclase Cyr1 and GTPase activating protein Ira2, facilitate this transition, in the presence of energy sensor ATP. However, the precise mechanism by which protein interactions between Ira2 and Cyr1 and the energy sensor ATP result in the yeast-to-hyphal transition and create a switch-like process are unknown. We propose a new set of biochemical reaction steps that captures all the essential interactions between Ira2, Cyr1, and ATP in the Ras pathway. With the help of chemical reaction network theory, we demonstrate that this set of biochemical reaction steps results in bistability. Further, bifurcation analysis of the differential equations based on this set of reaction steps supports the existence of a bistable switch, and this switch may act as a checkpoint mechanism for the promotion of growth-to-hyphal transition in C. albicans.

Carbon Catabolite Repression in Filamentous Fungi

Carbon Catabolite Repression (CCR) has fascinated scientists and researchers around the globe for the past few decades. This important mechanism allows preferential utilization of an energy-efficient and readily available carbon source over relatively less easily accessible carbon sources. This mechanism helps microorganisms to obtain maximum amount of glucose in order to keep pace with their metabolism. Microorganisms assimilate glucose and highly favorable sugars before switching to less-favored sources of carbon such as organic acids and alcohols. In CCR of filamentous fungi, CreA acts as a transcription factor, which is regulated to some extent by ubiquitination. CreD-HulA ubiquitination ligase complex helps in CreA ubiquitination, while CreB-CreC deubiquitination (DUB) complex removes ubiquitin from CreA, which causes its activation. CCR of fungi also involves some very crucial elements such as Hexokinases, cAMP, Protein Kinase (PKA), Ras proteins, G protein-coupled receptor (GPCR), Adenylate cyclase, RcoA and SnfA. Thorough study of molecular mechanism of CCR is important for understanding growth, conidiation, virulence and survival of filamentous fungi. This review is a comprehensive revision of the regulation of CCR in filamentous fungi as well as an updated summary of key regulators, regulation of different CCR-dependent mechanisms and its impact on various physical characteristics of filamentous fungi.

Mitochondrial Activity and Cyr1 Are Key Regulators of Ras1 Activation of C. albicans Virulence Pathways

Candida albicans is both a major fungal pathogen and a member of the commensal human microflora. The morphological switch from yeast to hyphal growth is associated with disease and many environmental factors are known to influence the yeast-to-hyphae switch. The Ras1-Cyr1-PKA pathway is a major regulator of C. albicans morphogenesis as well as biofilm formation and white-opaque switching. Previous studies have shown that hyphal growth is strongly repressed by mitochondrial inhibitors. Here, we show that mitochondrial inhibitors strongly decreased Ras1 GTP-binding and activity in C. albicans and similar effects were observed in other Candida species. Consistent with there being a connection between respiratory activity and GTP-Ras1 binding, mutants lacking complex I or complex IV grew as yeast in hypha-inducing conditions, had lower levels of GTP-Ras1, and Ras1 GTP-binding was unaffected by respiratory inhibitors. Mitochondria-perturbing agents decreased intracellular ATP concentrations and metabolomics analyses of cells grown with different respiratory inhibitors found consistent perturbation of pyruvate metabolism and the TCA cycle, changes in redox state, increased catabolism of lipids, and decreased sterol content which suggested increased AMP kinase activity. Biochemical and genetic experiments provide strong evidence for a model in which the activation of Ras1 is controlled by ATP levels in an AMP kinase independent manner. The Ras1 GTPase activating protein, Ira2, but not the Ras1 guanine nucleotide exchange factor, Cdc25, was required for the reduction of Ras1-GTP in response to inhibitor-mediated reduction of ATP levels. Furthermore, Cyr1, a well-characterized Ras1 effector, participated in the control of Ras1-GTP binding in response to decreased mitochondrial activity suggesting a revised model for Ras1 and Cyr1 signaling in which Cyr1 and Ras1 influence each other and, together with Ira2, seem to form a master-regulatory complex necessary to integrate different environmental and intracellular signals, including metabolic status, to decide the fate of cellular morphology.

Candida albicans is a successful fungal commensal and pathogen of humans. It is a polymorphic organism and the ability to switch from yeast to hyphal growth is associated with the commensal-to-pathogen switch. Previous research identified the Ras1-cAMP-protein kinase A pathway as a key regulator of hyphal growth. Here, we report that mitochondrial activity plays a key role in Ras1 activation, as respiratory inhibition decreased Ras1 activity and Ras1-dependent filamentation. We found that intracellular ATP modulates Ras1 activity through a pathway involving the GTPase activating protein Ira2 and the adenylate cyclase Cyr1. Based on our data the canonical Ras1 signaling model in C. albicans needs to be restructured in such a way that Cyr1 is no longer placed downstream of Ras1 but rather in a major signaling node with Ras1 and Ira2. Our studies suggest that the energy status of the cell is the most important signal involved in the decision of C. albicans to undergo the yeast-to-hyphae switch or express genes associated with the hyphal morphology as low intracellular ATP or associated cues override several hypha-inducing signals. Future studies will show if this knowledge can be used to develop therapies that would favor benign host-Candida interactions by promoting low Ras1 activity.

A RASopathy gene commonly mutated in cancer: the neurofibromatosis type 1 tumour suppressor

Neurofibromatosis type 1 (NF1) is a common genetic disorder that predisposes affected individuals to tumours. The NF1 gene encodes a RAS GTPase-activating protein called neurofibromin and is one of several genes that (when mutant) affect RAS-MAPK signalling, causing related diseases collectively known as RASopathies. Several RASopathies, beyond NF1, are cancer predisposition syndromes. Somatic NF1 mutations also occur in 5-10% of human sporadic cancers and may contribute to resistance to therapy. To highlight areas for investigation in RASopathies and sporadic tumours with NF1 mutations, we summarize current knowledge of NF1 disease, the NF1 gene and neurofibromin, neurofibromin signalling pathways and recent developments in NF1 therapeutics.

Transmembrane signaling in Saccharomyces cerevisiae as a model for signaling in metazoans: state of the art after 25 years

In the very first article that appeared in Cellular Signalling, published in its inaugural issue in October 1989, we reviewed signal transduction pathways in Saccharomyces cerevisiae. Although this yeast was already a powerful model organism for the study of cellular processes, it was not yet a valuable instrument for the investigation of signaling cascades. In 1989, therefore, we discussed only two pathways, the Ras/cAMP and the mating (Fus3) signaling cascades. The pivotal findings concerning those pathways undoubtedly contributed to the realization that yeast is a relevant model for understanding signal transduction in higher eukaryotes. Consequently, the last 25 years have witnessed the discovery of many signal transduction pathways in S. cerevisiae, including the high osmotic glycerol (Hog1), Stl2/Mpk1 and Smk1 mitogen-activated protein (MAP) kinase pathways, the TOR, AMPK/Snf1, SPS, PLC1 and Pkr/Gcn2 cascades, and systems that sense and respond to various types of stress. For many cascades, orthologous pathways were identified in mammals following their discovery in yeast. Here we review advances in the understanding of signaling in S. cerevisiae over the last 25 years. When all pathways are analyzed together, some prominent themes emerge. First, wiring of signaling cascades may not be identical in all S. cerevisiae strains, but is probably specific to each genetic background. This situation complicates attempts to decipher and generalize these webs of reactions. Secondly, the Ras/cAMP and the TOR cascades are pivotal pathways that affect all processes of the life of the yeast cell, whereas the yeast MAP kinase pathways are not essential. Yeast cells deficient in all MAP kinases proliferate normally. Another theme is the existence of central molecular hubs, either as single proteins (e.g., Msn2/4, Flo11) or as multisubunit complexes (e.g., TORC1/2), which are controlled by numerous pathways and in turn determine the fate of the cell. It is also apparent that lipid signaling is less developed in yeast than in higher eukaryotes. Finally, feedback regulatory mechanisms seem to be at least as important and powerful as the pathways themselves. In the final chapter of this essay we dare to imagine the essence of our next review on signaling in yeast, to be published on the 50th anniversary of Cellular Signalling in 2039.