The intracellular dissipation of cytosolic calcium following glucose re-addition to carbohydrate depleted Saccharomyces cerevisiae

The yeast Gdt1 protein mediates the exchange of H+ for Ca2+ and Mn2+ influencing the Golgi pH

The GDT1 family is broadly spread and highly conserved among living organisms. GDT1 members have functions in key processes like glycosylation in humans and yeasts, and photosynthesis in plants. These functions are mediated by their ability to transport ions. While transport of Ca²⁺ or Mn²⁺ is well established for several GDT1 members, their transport mechanism is poorly understood. Here, we demonstrate that H⁺ ions are transported in exchange for Ca²⁺ and Mn²⁺ cations by the Golgi-localized yeast Gdt1 protein. We performed direct transport measurement across a biological membrane by expressing Gdt1p in Lactococcus lactis bacterial cells and by recording either the extracellular pH or the intracellular pH during the application of Ca²⁺, Mn²⁺ or H⁺ gradients. Besides, in vivo cytosolic and Golgi pH measurements were performed in Saccharomyces cerevisiae with genetically encoded pH probes targeted to those subcellular compartments. These data point out that the flow of H⁺ ions carried by Gdt1p could be reversed according to the physiological conditions. Together, our experiments unravel the influence of the relative concentration gradients for Gdt1p-mediated H⁺ transport and pave the way to decipher the regulatory mechanisms driving the activity of GDT1 orthologs in various biological contexts.

Investigating the Antifungal Mechanism of Action of Polygodial by Phenotypic Screening in Saccharomyces cerevisiae

Polygodial is a "hot" peppery-tasting sesquiterpenoid that was first described for its anti-feedant activity against African armyworms. Using the haploid deletion mutant library of Saccharomyces cerevisiae, a genome-wide mutant screen was performed to shed more light on polygodial's antifungal mechanism of action. We identified 66 deletion strains that were hypersensitive and 47 that were highly resistant to polygodial treatment. Among the hypersensitive strains, an enrichment was found for genes required for vacuolar acidification, amino acid biosynthesis, nucleosome mobilization, the transcription mediator complex, autophagy and vesicular trafficking, while the resistant strains were enriched for genes encoding cytoskeleton-binding proteins, ribosomal proteins, mitochondrial matrix proteins, components of the heme activator protein (HAP) complex, and known regulators of the target of rapamycin complex 1 (TORC1) signaling. WE confirm that polygodial triggers a dose-dependent vacuolar alkalinization and that it increases Ca²⁺ influx and inhibits glucose-induced Ca²⁺ signaling. Moreover, we provide evidence suggesting that TORC1 signaling and its protective agent ubiquitin play a central role in polygodial resistance, suggesting that they can be targeted by polygodial either directly or via altered Ca²⁺ homeostasis.

Ca(2+) homeostasis in the budding yeast Saccharomyces cerevisiae: Impact of ER/Golgi Ca(2+) storage

Yeast has proven to be a powerful tool to elucidate the molecular aspects of several biological processes in higher eukaryotes. As in mammalian cells, yeast intracellular Ca(2+) signalling is crucial for a myriad of biological processes. Yeast cells also bear homologs of the major components of the Ca(2+) signalling toolkit in mammalian cells, including channels, co-transporters and pumps. Using yeast single- and multiple-gene deletion strains of various plasma membrane and organellar Ca(2+) transporters, combined with manipulations to estimate intracellular Ca(2+) storage, we evaluated the contribution of individual transport systems to intracellular Ca(2+) homeostasis. Yeast strains lacking Pmr1 and/or Cod1, two ion pumps implicated in ER/Golgi Ca(2+) homeostasis, displayed a fragmented vacuolar phenotype and showed increased vacuolar Ca(2+) uptake and Ca(2+) influx across the plasma membrane. In the pmr1Δ strain, these effects were insensitive to calcineurin activity, independent of Cch1/Mid1 Ca(2+) channels and Pmc1 but required Vcx1. By contrast, in the cod1Δ strain increased vacuolar Ca(2+) uptake was not affected by Vcx1 deletion but was largely dependent on Pmc1 activity. Our analysis further corroborates the distinct roles of Vcx1 and Pmc1 in vacuolar Ca(2+) uptake and point to the existence of not-yet identified Ca(2+) influx pathways.

The Ca2+/Mn2+ ion-pump PMR1 links elevation of cytosolic Ca(2+) levels to α-synuclein toxicity in Parkinson's disease models

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons, which arises from a yet elusive concurrence between genetic and environmental factors. The protein α-synuclein (αSyn), the principle toxic effector in PD, has been shown to interfere with neuronal Ca(2+) fluxes, arguing for an involvement of deregulated Ca(2+) homeostasis in this neuronal demise. Here, we identify the Golgi-resident Ca(2+)/Mn(2+) ATPase PMR1 (plasma membrane-related Ca(2+)-ATPase 1) as a phylogenetically conserved mediator of αSyn-driven changes in Ca(2+) homeostasis and cytotoxicity. Expression of αSyn in yeast resulted in elevated cytosolic Ca(2+) levels and increased cell death, both of which could be inhibited by deletion of PMR1. Accordingly, absence of PMR1 prevented αSyn-induced loss of dopaminergic neurons in nematodes and flies. In addition, αSyn failed to compromise locomotion and survival of flies when PMR1 was absent. In conclusion, the αSyn-driven rise of cytosolic Ca(2+) levels is pivotal for its cytotoxicity and requires PMR1.

Acidic calcium stores of Saccharomyces cerevisiae

Fungi and animals constitute sister kingdoms in the eukaryotic domain of life. The major classes of transporters, channels, sensors, and effectors that move and respond to calcium ions were already highly networked in the common ancestor of fungi and animals. Since that time, some key components of the network have been moved, altered, relocalized, lost, or duplicated in the fungal and animal lineages and at the same time some of the regulatory circuitry has been dramatically rewired. Today the calcium transport and signaling networks in fungi provide a fresh perspective on the scene that has emerged from studies of the network in animal cells. This review provides an overview of calcium signaling networks in fungi, particularly the model yeast Saccharomyces cerevisiae, with special attention to the dominant roles of acidic calcium stores in fungal cell physiology.

Cellular stress induces cytoplasmic RNA granules in fission yeast

Severe stress causes plant and animal cells to form large cytoplasmic granules containing RNA and proteins. Here, we demonstrate the existence of stress-induced cytoplasmic RNA granules in Schizosaccharomyces pombe. Homologs to several known protein components of mammalian processing bodies and stress granules are found in fission yeast RNA granules. In contrast to mammalian cells, poly(A)-binding protein (Pabp) colocalizes in stress-induced granules with decapping protein. After glucose deprivation, protein kinase A (PKA) is required for accumulation of Pabp-positive granules and translational down-regulation. This is the first demonstration of a role for PKA in RNA granule formation. In mammals, the translation initiation protein eIF2α is a key regulator of formation of granules containing poly(A)-binding protein. In S. pombe, nonphosphorylatable eIF2α does not block but delays granule formation and subsequent clearance after exposure to hyperosmosis. At least two separate pathways in S. pombe appear to regulate stress-induced granules: pka1 mutants are fully proficient to form granules after hyperosmotic shock; conversely, eIF2α does not affect granule formation in glucose starvation. Further, we demonstrate a Pka1-dependent link between calcium perturbation and RNA granules, which has not been described earlier in any organism.

Vacuolar cation/H+ antiporters of Saccharomyces cerevisiae

We previously demonstrated that Saccharomyces cerevisiae vnx1Δ mutant strains displayed an almost total loss of Na(+) and K(+)/H(+) antiporter activity in a vacuole-enriched fraction. However, using different in vitro transport conditions, we were able to reveal additional K(+)/H(+) antiporter activity. By disrupting genes encoding transporters potentially involved in the vnx1 mutant strain, we determined that Vcx1p is responsible for this activity. This result was further confirmed by complementation of the vnx1Δvcx1Δ nhx1Δ triple mutant with Vcx1p and its inactivated mutant Vcx1p-H303A. Like the Ca(2+)/H(+) antiporter activity catalyzed by Vcx1p, the K(+)/H(+) antiporter activity was strongly inhibited by Cd(2+) and to a lesser extend by Zn(2+). Unlike as previously observed for NHX1 or VNX1, VCX1 overexpression only marginally improved the growth of yeast strain AXT3 in the presence of high concentrations of K(+) and had no effect on hygromycin sensitivity. Subcellular localization showed that Vcx1p and Vnx1p are targeted to the vacuolar membrane, whereas Nhx1p is targeted to prevacuoles. The relative importance of Nhx1p, Vnx1p, and Vcx1p in the vacuolar accumulation of monovalent cations will be discussed.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Phenotypic heterogeneity can enhance rare-cell survival in 'stress-sensitive' yeast populations

Individual cells within isogenic microbial cultures exhibit phenotypic heterogeneity, an issue that is attracting intense interest. Heterogeneity could confer benefits, in generating variant subpopulations that may be better equipped to persist during perturbation. We tested this hypothesis by comparing the survival of wild-type Saccharomyces cerevisiae with that of mutants which are considered stress-sensitive but which, we demonstrate, also have increased heterogeneity. The mutants (e.g. vma3, ctr1, sod1) exhibited the anticipated sensitivities to intermediate doses of nickel, copper, alkaline pH, menadione or paraquat. However, enhanced heterogeneity meant that the resistances of individual mutant cells spanned a broad range, and at high stress occasional-cell survival in most of these populations overtook that of the wild type. Green fluorescent protein (GFP) reporter studies showed that this heterogeneity-dependent advantage was not related to perturbation of buffered gene expression. Deletion strain screens combined with other approaches revealed that vacuolar alkalinization resulting from loss of Vma-dependent vacuolar H(+)-ATPase activity was not the cause of vma mutants' net stress sensitivities. An alternative Vma-dependent resistance mechanism was found to suppress an influence of variable vacuolar pH on the metal resistances of individual wild-type cells. In addition to revealing new mechanisms of heterogeneity generation, the results demonstrate experimentally a benefit under adverse conditions that arises specifically from heterogeneity, and in populations conventionally considered to be disadvantaged.

Hailey-Hailey disease as an orthodisease of PMR1 deficiency in Saccharomyces cerevisiae

The term orthodisease has recently been introduced to define human disorders in which the pathogenic gene has orthologs in model organism genomes. Here, we describe Hailey-Hailey disease (HHD), a blistering skin disorder caused by haploinsufficiency of ATP2C1 as an orthodisease from a Saccharomyces cerevisiae perspective. ATP2C1 encodes the human secretory pathway Ca(2+)/Mn(2+) ATPase hSPCA1 and is orthologous to the PMR1 gene in S. cerevisiae. hSPCA1 fully complements PMR1 deficiency in yeast and pmr1DeltaS. cerevisiae has proved to be a valuable tool to screen ATP2C1 mutations and address potential pathogenic/pharmacologic mechanisms in HHD. Consequently, this human skin disorder is an ideal example of an orthodisease.

Current awareness on yeast

In order to keep subscribers up‐to‐date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly‐published material on yeasts. Each bibliography is divided into 10 sections. 1 Books, Reviews & Symposia; 2 General; 3 Biochemistry; 4 Biotechnology; 5 Cell Biology; 6 Gene Expression; 7 Genetics; 8 Physiology; 9 Medical Mycology; 10 Recombinant DNA Technology. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted. (4 weeks journals ‐ search completed 10th. Nov. 2004)