The inositol pyrophosphate metabolism of Dictyostelium discoideum does not regulate inorganic polyphosphate (polyP) synthesis.

Yann Desfougères,Filipy Borghi,Paloma Portela-Torres,Danye Qiu,Robert K Harmel,Adolfo Saiardi,Thomas M Livermore,Henning J Jessen,Dorothea Fiedler

doi:10.1016/j.jbior.2021.100835

Abstract

Initial studies on the inositol phosphates metabolism were enabled by the social amoeba Dictyostelium discoideum. The abundant amount of inositol hexakisphosphate (IP6 also known as Phytic acid) present in the amoeba allowed the discovery of the more polar inositol pyrophosphates, IP7 and IP8, possessing one or two high energy phosphoanhydride bonds, respectively. Considering the contemporary growing interest in inositol pyrophosphates, it is surprising that in recent years D. discoideum, has contributed little to our understanding of their metabolism and function. This work fulfils this lacuna, by analysing the ip6k, ppip5k and ip6k-ppip5K amoeba null strains using PAGE, 13C-NMR and CE-MS analysis. Our study reveals an inositol pyrophosphate metabolism more complex than previously thought. The amoeba Ip6k synthesizes the 4/6-IP7 in contrast to the 5-IP7 isomer synthesized by the mammalian homologue. The amoeba Ppip5k synthesizes the same 1/3-IP7 as the mammalian enzyme. In D. discoideum, the ip6k strain possesses residual amounts of IP7. The residual IP7 is also present in the ip6k-ppip5K strain, while the ppip5k single mutant shows a decrease in both IP7 and IP8 levels. This phenotype is in contrast to the increase in IP7 observable in the yeast vip1Δ strain. The presence of IP8 in ppip5k and the presence of IP7 in ip6k-ppip5K indicate the existence of an additional inositol pyrophosphate synthesizing enzyme. Additionally, we investigated the existence of a metabolic relationship between inositol pyrophosphate synthesis and inorganic polyphosphate (polyP) metabolism as observed in yeast. These studies reveal that contrary to the yeast, Ip6k and Ppip5k do not control polyP cellular level in amoeba.

Highlights

The social amoeba Dictyostelium discoideum was one of the primary experimental models to study inositol phosphate metabolism and signalling in the 1980s (Europe-Finner et al, 1991)
The screening of D. discoideum genome revealed the presence of seven inositol phosphate kinases (Table 3)
As opposed to the four kinases found in yeast (Laha et al, 2021; Saiardi et al, 2018; Tsui and York, 2010), the seven D. discoideum kinases pointed towards a higher complexity of inositol phosphate synthesis in the amoeba, more similar to mammalian cells

Summary

Introduction

The social amoeba Dictyostelium discoideum was one of the primary experimental models to study inositol phosphate metabolism and signalling in the 1980s (Europe-Finner et al, 1991). By the middle of the 90s, it was discovered that phospholipase C in D. discoideum is not required to produce IP3 nor to control calcium signalling (Van Dijken et al, 1995, 1997). Perhaps for these reasons, the interest of inositol scientists in this organism faded over the years. The previously characterized ip6k amoeba (Luo et al, 2003) possess a biochemical phenotype, the absence of inositol pyrophosphate, similar to the mutant of the homologous yeast Saccharomyces cerevisiae gene Kcs (kcs1Δ) (Saiardi et al, 2000)

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Results

Conclusion