Inositol in Disease and Development: Roles of Catabolism via myo-Inositol Oxygenase in Drosophila melanogaster.

Inositol depletion has been associated with diabetes and related complications. Increased inositol catabolism, via myo-inositol oxygenase (MIOX), has been implicated in decreased renal function. This study demonstrates that the fruit fly Drosophila melanogaster catabolizes myo-inositol via MIOX. The levels of mRNA encoding MIOX and MIOX specific activity are increased when fruit flies are grown on a diet with inositol as the sole sugar. Inositol as the sole dietary sugar can support D. melanogaster survival, indicating that there is sufficient catabolism for basic energy requirements, allowing for adaptation to various environments. The elimination of MIOX activity, via a piggyBac WH-element inserted into the MIOX gene, results in developmental defects including pupal lethality and pharate flies without proboscises. In contrast, RNAi strains with reduced levels of mRNA encoding MIOX and reduced MIOX specific activity develop to become phenotypically wild-type-appearing adult flies. myo-Inositol levels in larval tissues are highest in the strain with this most extreme loss of myo-inositol catabolism. Larval tissues from the RNAi strains have inositol levels higher than wild-type larval tissues but lower levels than the piggyBac WH-element insertion strain. myo-Inositol supplementation of the diet further increases the myo-inositol levels in the larval tissues of all the strains, without any noticeable effects on development. Obesity and blood (hemolymph) glucose, two hallmarks of diabetes, were reduced in the RNAi strains and further reduced in the piggyBac WH-element insertion strain. Collectively, these data suggest that moderately increased myo-inositol levels do not cause developmental defects and directly correspond to reduced larval obesity and blood (hemolymph) glucose.

Regulated inositol synthesis is critical for balanced metabolism and development in Drosophila melanogaster.

Myo-inositol is a precursor of the membrane phospholipid, phosphatidylinositol (PI). It is involved in many essential cellular processes including signal transduction, energy metabolism, endoplasmic reticulum stress, and osmoregulation. Inositol is synthesized from glucose-6-phosphate by myo-inositol-3-phosphate synthase (MIPSp). The Drosophila melanogaster Inos gene encodes MIPSp. Abnormalities in myo-inositol metabolism have been implicated in type 2 diabetes, cancer, and neurodegenerative disorders. Obesity and high blood (hemolymph) glucose are two hallmarks of diabetes, which can be induced in Drosophila melanogaster third-instar larvae by high-sucrose diets. This study shows that dietary inositol reduces the obese-like and high-hemolymph glucose phenotypes of third-instar larvae fed high-sucrose diets. Furthermore, this study demonstrates Inos mRNA regulation by dietary inositol; when more inositol is provided there is less Inos mRNA. Third-instar larvae with dysregulated high levels of Inos mRNA and MIPSp show dramatic reductions of the obese-like and high-hemolymph glucose phenotypes. These strains, however, also display developmental defects and pupal lethality. The few individuals that eclose die within two days with striking defects: structural alterations of the wings and legs, and heads lacking proboscises. This study is an exciting extension of the use of Drosophila melanogaster as a model organism for exploring the junction of development and metabolism.

Disruption of INOS, a Gene Encoding myo-Inositol Phosphate Synthase, Causes Male Sterility in Drosophila melanogaster.

Inositol is a precursor for the phospholipid membrane component phosphatidylinositol (PI), involved in signal transduction pathways, endoplasmic reticulum stress, and osmoregulation. Alterations of inositol metabolism have been implicated in human reproductive issues, the therapeutic effects of drugs used to treat epilepsy and bipolar disorder, spinal cord defects, and diseases including diabetes and Alzheimer's. The sole known inositol synthetic enzyme is myo-inositol synthase (MIPS), and the homolog in Drosophilia melanogaster is encoded by the Inos gene. Three identical deletion strains (inosΔDF /CyO) were constructed, confirmed by PCR and sequencing, and homozygotes (inosΔDF /inosΔDF ) were shown to lack the transcript encoding the MIPS enzyme. Without inositol, homozygous inosΔDF deletion fertilized eggs develop only to the first-instar larval stage. When transferred as pupae to food without inositol, however, inosΔDF homozygotes die significantly sooner than wild-type flies. Even with dietary inositol the homozygous inosΔDF males are sterile. An inos allele, with a P-element inserted into the first intron, fails to complement this male sterile phenotype. An additional copy of the Inos gene inserted into another chromosome rescues all the phenotypes. These genetic and phenotypic analyses establish D. melanogaster as an excellent model organism in which to examine the role of inositol synthesis in development and reproduction.

Computational modeling and in silico analysis of differential regulation of myo-inositol catabolic enzymes in Cryptococcus neoformans.

BACKGROUND: Inositol is a key cellular metabolite for many organisms. Cryptococcus neoformans is an opportunistic pathogen which primarily infects the central nervous system, a region of high inositol concentration, of immunocompromised individuals. Through the use of myo-inositol oxygenase C. neoformans can catabolize inositol as a sole carbon source to support growth and viability. RESULTS: Three myo-inositol oxygenase gene sequences were identified in the C. neoformans genome. Differential regulation was suggested by computational analyses of the three gene sequences. This included examination of the upstream regulatory regions, identifying ORE/TonE and UASINO sequences, conserved introns/exons, and in frame termination sequences. Homology modeling of the proteins encoded by these genes revealed key differences in the myo-inositol active site. CONCLUSION: The results suggest there are two functional copies of the myo-inositol oxygenase gene in the C. neoformans genome. The functional genes are differentially expressed in response to environmental inositol concentrations. Both the upstream regulatory regions of the genes and the structure of the specific proteins suggest that MIOX1 would function when inositol concentrations are low, whereas MIOX2 would function when inositol concentrations are high.

Unusual effect of myo-inositol on phospholipid biosynthesis in Cryptococcus neoformans.

Cryptococcus neoformans is an opportunistic fungal pathogen which preferentially localizes to the inositol-rich environment of the central nervous system. One of its distinguishing traits is its capacity to catabolize inositol. Inositol is a precursor for the synthesis of phosphatidylinositol (PI). This study demonstrated that C. neoformans synthesizes inositol. Three inositol-containing sphingolipids were identified in C. neoformans: ceramide-(P-inositol)2mannose, mannose, ceramide-P-inositol-mannose, and ceramide-P-inositol. These inositol-containing sphingolipids are typical of fungi but not higher eukaryotes. The effect of inositol on the membrane lipid composition of C. neoformans was also examined. In contrast to the nonpathogenic yeast Saccharomyces cerevisiae, neither the PI composition nor the synthesis of methylated phospholipids was altered by exogenous inositol. Hence, C. neoformans appears to have a metabolic mechanism for maintaining a steady lipid composition regardless of the inositol in its environment.