Yeast inositol mono- and trisphosphate levels are modulated by inositol monophosphatase activity and nutrients

Crosstalk between Ras and inositol phosphate signaling revealed by lithium action on inositol monophosphatase in Schizophyllum commune

Mushroom forming basidiomycete Schizophyllum commune has been used as a tractable model organism to study fungal sexual development. Ras signaling activation via G-protein-coupled receptors (GPCRs) has been postulated to play a significant role in the mating and development of S. commune. In this study, a crosstalk between Ras signaling and inositol phosphate signaling by inositol monophosphatase (IMPase) is revealed. Constitutively active Ras1 leads to the repression of IMPase transcription and lithium action on IMPase activity is compensated by the induction of IMPase at transcriptome level. Astonishingly, in S. commune lithium induces a considerable shift to inositol phosphate metabolism leading to a massive increase in the level of higher phosphorylated inositol species up to the inositol pyrophosphates. The lithium induced metabolic changes are not observable in a constitutively active Ras1 mutant. In addition to that, proteome profile helps us to elucidate an overview of lithium action to the broad aspect of fungal metabolism and cellular signaling. Taken together, these findings imply a crosstalk between Ras and inositol phosphate signaling.

Involvement of endoplasmic reticulum stress in a novel Classic Galactosemia model

Inherited deficiency of galactose-1-phosphate uridyltransferase (GALT) activity in humans leads to a potentially lethal disorder called Classic Galactosemia. It is well known that patients often accumulate high levels of galactose metabolites such as galactose-1-phosphate (gal-1-p) in their tissues. However, specific targets of gal-1-p and other accumulated metabolites remain uncertain. In this study, we developed a new model system to study this toxicity using primary fibroblasts derived from galactosemic patients. GALT activity was reconstituted in these primary cells through lentivirus-mediated gene transfer. Gene expression profiling showed that GALT-deficient cells, but not normal cells, responded to galactose challenge by activating a set of genes characteristic of endoplasmic reticulum (ER) stress. Western blot analysis showed that the master regulator of ER stress, BiP, was up-regulated at least threefold in these cells upon galactose challenge. We also found that treatment of these cells with galactose, but not glucose or hexose-free media reduced Ca2+ mobilization in response to activation of Gq-coupled receptors. To explore whether the muted Ca2+ mobilization is related to reduced inositol turnover, we discovered that gal-1-p competitively inhibited human inositol monophosphatase (hIMPase1). We hypothesize that galactose intoxication under GALT-deficiency resulted from accumulation of toxic galactose metabolite products, which led to the accumulation of unfolded proteins, altered calcium homeostasis, and subsequently ER stress.

The Medicago truncatula DMI1 protein modulates cytosolic calcium signaling

In addition to establishing symbiotic relationships with arbuscular mycorrhizal fungi, legumes also enter into a nitrogen-fixing symbiosis with rhizobial bacteria that results in the formation of root nodules. Several genes involved in the development of both arbuscular mycorrhiza and legume nodulation have been cloned in model legumes. Among them, Medicago truncatula DMI1 (DOESN'T MAKE INFECTIONS1) is required for the generation of nucleus-associated calcium spikes in response to the rhizobial signaling molecule Nod factor. DMI1 encodes a membrane protein with striking similarities to the Methanobacterium thermoautotrophicum potassium channel (MthK). The cytosolic C terminus of DMI1 contains a RCK (regulator of the conductance of K(+)) domain that in MthK acts as a calcium-regulated gating ring controlling the activity of the channel. Here we show that a dmi1 mutant lacking the entire C terminus acts as a dominant-negative allele interfering with the formation of nitrogen-fixing nodules and abolishing the induction of calcium spikes by the G-protein agonist Mastoparan. Using both the full-length DMI1 and this dominant-negative mutant protein we show that DMI1 increases the sensitivity of a sodium- and lithium-hypersensitive yeast (Saccharomyces cerevisiae) mutant toward those ions and that the C-terminal domain plays a central role in regulating this response. We also show that DMI1 greatly reduces the release of calcium from internal stores in yeast, while the dominant-negative allele appears to have the opposite effect. This work suggests that DMI1 is not directly responsible for Nod factor-induced calcium changes, but does have the capacity to regulate calcium channels in both yeast and plants.

An Ins(1,4,5)P3 receptor in Paramecium is associated with the osmoregulatory system

In the ciliate Paramecium, a variety of well characterized processes are regulated by Ca2+, e.g. exocytosis, endocytosis and ciliary beat. Therefore, among protozoa, Paramecium is considered a model organism for Ca2+ signaling, although the molecular identity of the channels responsible for the Ca2+ signals remains largely unknown. We have cloned - for the first time in a protozoan - the full sequence of the gene encoding a putative inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3) receptor from Paramecium tetraurelia cells showing molecular characteristics of higher eukaryotic cells. The homologously expressed Ins(1,4,5)P3-binding domain binds [3H]Ins(1,4,5)P3, whereas antibodies unexpectedly localize this protein to the osmoregulatory system. The level of Ins(1,4,5)P3-receptor expression was reduced, as shown on a transcriptional level and by immuno-staining, by decreasing the concentration of extracellular Ca2+ (Paramecium cells rapidly adjust their Ca2+ level to that in the outside medium). Fluorochromes reveal spontaneous fluctuations in cytosolic Ca2+ levels along the osmoregulatory system and these signals change upon activation of caged Ins(1,4,5)P3. Considering the ongoing expulsion of substantial amounts of Ca2+ by the osmoregulatory system, we propose here that Ins(1,4,5)P3 receptors serve a new function, i.e. a latent, graded reflux of Ca2+ to fine-tune [Ca2+] homeostasis.

Role of N-terminal hydrophobic region in modulating the subcellular localization and enzyme activity of the bisphosphate nucleotidase from Debaryomyces hansenii

3', 5'-Bisphosphate nucleotidase is a ubiquitous enzyme that converts 3'-phosphoadenosine-5'-phosphate to adenosine-5'-phosphate and inorganic phosphate. These enzymes are highly sensitive to sodium and lithium and, thus, perform a crucial rate-limiting metabolic step during salt stress in yeast. Recently, we have identified a bisphosphate nucleotidase gene (DHAL2) from the halotolerant yeast Debaryomyces hansenii. One of the unique features of Dhal2p is that it contains an N-terminal 54-amino-acid-residue hydrophobic extension. In this study, we have shown that Dhal2p exists as a cytosolic as well as a membrane-bound form and that salt stress markedly influences the accumulation of the latter form in the cell. We have demonstrated that the N-terminal hydrophobic region was necessary for the synthesis of the membrane-bound isoform. It appeared that an alternative translation initiation was the major mechanism for the synthesis of these two forms. Moreover, the two forms exhibit significant differences in their substrate specificity. Unlike the cytosolic form, the membrane-bound form showed very high activity against inositol-1,4-bisphosphate. Thus, the present study for the first time reports the existence of multiple forms of a bisphosphate nucleotidase in any organism.

Evidence for inositol triphosphate as a second messenger for glucose-induced calcium signalling in budding yeast

The Saccharomyces cerevisiae phospholipase C Plc1 is involved in cytosolic transient glucose-induced calcium increase, which also requires the Gpr1/Gpa2 receptor/G protein complex and glucose hexokinases. Differing from mammalian cells, this increase in cytosolic calcium concentration is mainly due to an influx from the external medium. No inositol triphosphate receptor homologue has been identified in the S. cerevisiae genome; and, therefore, the transduction mechanism from Plc1 activation to calcium flux generation still has to be identified. Inositol triphosphate (IP(3)) in yeast is rapidly transformed into IP(4) and IP(5) by a dual kinase, Arg82. Then another kinase, Ipk1, phosphorylates the IP(5) into IP(6). In mutant cells that do not express either of these kinases, the glucose-induced calcium signal was not only detectable but was even wider than in the wild-type strain. IP(3) accumulation upon glucose addition was completely absent in the plc1Delta strain and was amplified both by deletion of either ARG82 or IPK1 genes and by overexpression of PLC1. These results taken together suggest that Plc1p activation by glucose, leading to cleavage of PIP(2) and generation of IP(3), seems to be sufficient for raising the calcium level in the cytosol. This is the first indication for a physiological role of IP(3) signalling in S. cerevisiae. Many aspects about the signal transduction mechanism and the final effectors require further study.