Calnexin Regulates Apoptosis Induced by Inositol Starvation in Fission Yeast

Abstract

Inositol is a precursor of numerous phospholipids and signalling molecules essential for the cell. Schizosaccharomyces pombe is naturally auxotroph for inositol as its genome does not have a homologue of the INO1 gene encoding inositol-1-phosphate synthase, the enzyme responsible for inositol biosynthesis. In this work, we demonstrate that inositol starvation in S. pombe causes cell death with apoptotic features. This apoptotic death is dependent on the metacaspase Pca1p and is affected by the UPR transducer Ire1p. Previously, we demonstrated that calnexin is involved in apoptosis induced by ER stress. Here, we show that cells expressing a lumenal version of calnexin exhibit a 2-fold increase in the levels of apoptosis provoked by inositol starvation. This increase is reversed by co-expression of a calnexin mutant spanning the transmembrane domain and C-terminal cytosolic tail. Coherently, calnexin is physiologically cleaved at the end of its lumenal domain, under normal growth conditions when cells approach stationary phase. This cleavage suggests that the two naturally produced calnexin fragments are needed to continue growth into stationary phase and to prevent cell death. Collectively, our observations indicate that calnexin takes part in at least two apoptotic pathways in S. pombe, and suggest that the cleavage of calnexin has regulatory roles in apoptotic processes involving calnexin.

Metacaspases

“…Since then, a pro-cell death role for Yca1 has been demonstrated in budding yeast cells in response to viral toxins and different types of abiotic and metabolic defect-associated stresses and chronological aging ( Table 2). 27 A similar requirement for a metacaspase has been reported in the fission yeast, albeit for a more limited number of cell death conditions, including lipotoxic stress 18 and inositol starvation 19 ( Table 2). Further in the fungi kingdom it has been established that both PaMca1 and PaMca2 are required for senescence-associated cell death in P. anserina, 23 a single Candida albicans metacaspase, CaMCA1, mediates oxidative stress-induced cell death in this pathogenic yeast, 28 whereas one of the two metacaspases, CasA, promotes ER stressassociated cell death in A. nidulans 25 (Table 2).…”

“…18,19 LmjMCA, a single metacaspase of Leishmania major, is involved in cell cycle regulation 20 and, when overexpressed, stimulates oxidative stress-induced cell death. 21 Further studies using corresponding deletion and inactivation mutants are required to determine whether the single metacaspases present in L. major and fission yeast are multifunctional proteins comparable to Yca1.…”

Apoptotic Cell Death in the Fission YeastSchizosaccharomyces pombeInduced by Valproic Acid and Its Extreme Susceptibility to pH Change

“…In recent years, it has been found that a number of unicellular organisms can also undergo apoptotic cell death. [1][2][3][4][5] In budding yeast Saccharomyces cerevisiae, apoptotic cell death was induced in response to several stimuli 2) in a manner dependent on 4) or independent of 5) metacaspase, the yeast homolog of metazoan caspases, while in fission yeast Schizosaccharomyces pombe, an evolutionarily distant relative of S. cerevisiae and a valuable tool for studying the basic cellular processes of higher eukaryotes, apoptotic cell death has been observed for relatively few stimuli, 6,7) and those stimuli are not those of S. cerevisiae. Considering the usefulness of comparative studies of S. cerevisiae and S. pombe for elucidating basic cellular mechanisms, to uncover a common agent inducing apoptotic death in both S. cerevisiae and S. pombe cells and identifying the mechanisms of cell death induced by the reagent are of critical importance.…”

Metacaspases are caspases. Doubt no more

“…2 We thus had revealed that one metacaspase, YCA1, was involved in the same process as caspases, namely programmed cell death (PCD). Indeed, many groups subsequently unfolded the crucial contribution of metacaspases to cell death execution upon various stresses in yeast and other fungi, [3][4][5][6] as well as in plants. 7 Bozhkov, Zhivotovsky and colleagues 8 could even demonstrate that the plant metacaspase mcII-Pa shapes the embryo of Norway spruce (Picea abies) during development, presumably through its implication in developmental cell death.…”