Investigating the effects of inositol depletion using a simple eukaryotic organism, Dictyostelium discoideum

Inositol, produced in vivo by the biosynthetic enzyme inositol-3phosphate synthase (Ino1), is an essential component of inositol phosphates and phosphoinositides that are involved in important cell signalling pathways. Dysregulation of inositol signalling has been implicated in various diseases, including bipolar disorder, Alzheimer’s disease, and diabetes. Research relating to inositol regulation in cells has primarily focused on the molecular and cellular effects of inositol depletion without considering the effects of altered Ino1 levels. This PhD study employed a simple eukaryote, Dictyostelium discoideum, to investigate the distinct effects of the loss of Ino1 and inositol depletion. Ablation of ino1 in this model produced an inositol auxotrophy phenotype and affected development, with the former being only partially restored by an exogenous inositol supply. The removal of exogenous inositol from the ino1 mutant resulted in a 56% decrease in intracellular inositol levels within 12 hours, a reduction in cytokinesis and substrate adhesion, and an accumulation of autophagosomes. Inositol depletion also triggered a decrease in phosphoinositide levels. Intriguingly, the absence of the Ino1 protein and inositol depletion cause distinct metabolic changes, with the greatest changes seen following Ino1 loss. These data suggest a role for the Ino1 protein beyond inositol biosynthesis. To investigate this role, an immunoprecipitation approach was used, where an Ino1 binding partner, Q54IX5, was identified. Q54IX5 is likely to function as a macromolecular adaptor protein. Thus, this study has identified distinct cellular and metabolic effects for both inositol depletion and loss of Ino1, with implications for our understanding of human diseases. List of Tables and Figures Figure 1.1 The chemical structures of inositol, inositol phosphate and phosphoinositide pg. 17 Figure 1.2 Inositol uptake, de novo biosynthesis, and recycling pg. 18 Figure 1.3 The structure of valproic acid (VPA) pg. 20 Figure 1.4 Dihydroxyacetone phosphate (DHAP) in glycolysis metabolic pathway pg. 23 Figure 1.5 SUMO pathway regulation of inositol production pg. 24 Figure 1.6 Dictyostelium discoideum development pg. 27 Figure 1.7 Regulation of chemotaxis in Dictyostelium pg. 29 Figure 1.8 Regulation of actin binding proteins by PIP2 pg. 31 Figure 1.9 Activation of actin filament assembly by PIP3 pg. 32 Figure 1.10 Cytokinesis in Dictyostelium pg. 34 Figure 1.11 Autophagosome formation in an eukaryotic cell pg. 35 Figure 2.1 Schematic of the ino1 gene knock-out and screening procedure pg. 49 Figure 3.1 Inositol signalling pg. 58 Figure 3.2 Domain positioning in the Dictyostelium Ino1 monomer pg. 61 Table 3.1 Homology search results (BLAST analysis) identifying proteins related to Dictyostelium Ino1 (EC 5.5.1.4) in other species pg. 62 Figure 3.3 Phylogenetic analysis of Ino1 protein across different biological kingdoms pg. 64 Figure 3.4 Conservation of the Ino1 catalytic region pg. 65 Figure 3.5 Homology between human Ino1 isoforms and Dictyostelium Ino1 pg. 66 Figure 3.6 Homology between human and Dictyostelium Ino1 proteins pg. 67 Figure 3.7 Conservation of the Ino1 protein between Dictyostelium and humans pg. 68 Figure 4.1 Homologous recombination pg. 74 Figure 4.2 Construction and restriction digest analysis of the ino1 knock-out vector pg. 76 Figure 4.3 PCR screening analysis of the ino1 cells pg. 77 Figure 4.4 RTPCR analysis of the ino1 gene transcription in the wild-type and the ino1 cells pg. 79 Figure 4.5 Overexpressing Dictyostelium ino1 gene tagged with RFP in the ino1 mutant pg. 80 Figure 4.6 Localisation of GFP-dIno1 in ino1 cells pg. 81 Figure 4.7 Localisation of the human GFP-Ino1 and Ino1-RFP in ino1 cells pg. 82 Figure 4.8 Growth of the ino1 mutant in liquid medium pg. 83 Figure 4.9 Growth of ino1 cells on bacteria pg. 84 Figure 4.10 Ino1 ability to phagocytose in shaking suspension pg. 85 Table 4.1 Percentage of bacteria consumed over time by wild-type and ino1 strains pg. 86 Figure 4.11 The effect of ino1 ablation on Dictyostelium development pg. 87 Figure 5.1 The effect of inositol depletion on intracellular inositol concentration pg. 95 Figure 5.2 The effect of inositol depletion on cell-substrate adhesion in ino1 cells pg. 97 Figure 5.3 Inositol depletion leads to a cytokinesis defect pg. 98 Figure 5.4 The effect of inositol depletion on the accumulation of autophagosomes pg. 100 Figure 5.5 The effect of inositol depletion and Ino1 loss on chemotaxis pg. 102 Table 5.1 Average velocity, aspect and persistence of ino1 cells during chemotaxis towards cAMP pg. 103 Figure 5.6 Comparison of the metabolic profiles of wild-type and ino1 mutant cells during inositol depletion pg. 105 Figure 5.7 Comparative analysis of the metabolic profile of the ino1 mutant under different inositol treatment conditions pg. 108 Figure 5.8 Levels of metabolites in the wild-type and ino1-cells grown under different inositol conditions pg. 109 Figure 5.9 Inositol depletion affects the level of phosphoinositol and its phosphate derivatives pg. 112 Figure 6.1 Monitoring Ino1 levels via ino1 transcription in response to inositol pg. 126 Figure 6.2 Creating a catalytically-inactive Ino1 pg. 127 Figure 6.3 Growth of Dictyostelium cells expressing catalytically-inactive Ino1 pg. 129 Figure 6.4 Creating Ino1 phosphomutations pg. 131 Figure 6.5 Development of the ino1 mutants pg. 133 Figure 6.6 Immunoprecipitation of the Ino1 protein pg. 136 Figure 6.7 Ino1 protein interaction analysis pg. 137 Figure 7.1 Effects of inositol depletion on Dictyostelium ino1 cells pg. 152 Figure 7.2 Effects of the Ino1 loss pg. 154 Table S1 The list of oligonucleotides used for cloning and screening procedures pg. 160 Table S2 The list of putative Ino1 interacting proteins pg. 162