Glycerol Hypersensitivity in a Drosophila Model for Glycerol Kinase Deficiency Is Affected by Mutations in Eye Pigmentation Genes

Abstract

Glycerol kinase plays a critical role in metabolism by converting glycerol to glycerol 3-phosphate in an ATP dependent reaction. In humans, glycerol kinase deficiency results in a wide range of phenotypic variability; patients can have severe metabolic and CNS abnormalities, while others possess hyperglycerolemia and glyceroluria with no other apparent phenotype. In an effort to help understand the pathogenic mechanisms underlying the phenotypic variation, we have created a Drosophila model for glycerol kinase deficiency by RNAi targeting of dGyk (CG18374) and dGK (CG7995). As expected, RNAi flies have reduced glycerol kinase RNA expression, reduced phosphorylation activity and elevated glycerol levels. Further investigation revealed these flies to be hypersensitive to fly food supplemented with glycerol. Due to the hygroscopic nature of glycerol, we predict glycerol hypersensitivity is a result of greater susceptibility to desiccation, suggesting glycerol kinase to play an important role in desiccation resistance in insects. To evaluate a role for genetic modifier loci in determining severity of the glycerol hypersensitivity observed in knockdown flies, we performed a preliminary screen of lethal transposon insertion mutant flies using a glycerol hypersensitive survivorship assay. We demonstrate that this type of screen can identify both enhancer and suppressor genetic loci of glycerol hypersensitivity. Furthermore, we found that the glycerol hypersensitivity phenotype can be enhanced or suppressed by null mutations in eye pigmentation genes. Taken together, our data suggest proteins encoded by eye pigmentation genes play an important role in desiccation resistance and that eye pigmentation genes are strong modifiers of the glycerol hypersensitive phenotype identified in our Drosophila model for glycerol kinase deficiency.

Disruption of Glycerol Metabolism by RNAi Targeting of Genes Encoding Glycerol Kinase Results in a Range of Phenotype Severity in Drosophila

“…Progeny from these crosses could be divided into two groups: survival to adulthood with no obvious physical phenotype (named dGyk -IR-sur and dGK -IR-sur) or lethality during larval development (named dGyk -IR-let and dGK -IR-let). Adult flies were subsequently found to be hypersensitive to glycerol [19]. Results were confirmed in at least 2 fly lines for each phenotype and in alternative gene regions for RNAi targeting [19].…”

“…Each RNAi fly line (9 and 10 each for dGyk -IR and 10× dGK -IR, respectively) was crossed to the Tub -GAL4 driver fly line and the progeny examined for physical phenotypes [19]. Progeny from these crosses could be divided into two groups: survival to adulthood with no obvious physical phenotype (named dGyk -IR-sur and dGK -IR-sur) or lethality during larval development (named dGyk -IR-let and dGK -IR-let).…”

“…Although the mouse model for GKD displays neonatal death [14], [15], study of this mouse model has revealed a role for glycerol kinase in apoptosis [16] in addition to altered expression of gene networks involved in lipid metabolism, carbohydrate metabolism, and insulin signaling [17], [18]. Here, we evaluate the potential of a Drosophila GKD model [19] by looking for molecular or metabolic similarities with GKD in humans.…”

“…RNAi targeting of the Drosophila glycerol kinase genes dGyk (CG18374) or dGK (CG7995) results in two alternative phenotypes: larval lethality or glycerol hypersensitive adult flies [19]. Previously, the analysis of 3 rd instar larvae that developed into glycerol hypersensitive adults revealed successful targeting of dGyk and dGK that correlated with reduced glycerol kinase phosphorylation activity and elevated glycerol levels.…”

“…Previously, the analysis of 3 rd instar larvae that developed into glycerol hypersensitive adults revealed successful targeting of dGyk and dGK that correlated with reduced glycerol kinase phosphorylation activity and elevated glycerol levels. Glycerol hypersensitive flies die rapidly when placed on a food source supplemented with glycerol, and sensitivity is enhanced by null mutations in eye pigmentation genes [19]. The glycerol hypersensitive phenotype suggests the flies are unable to tolerate the strong hydrophilic properties of glycerol in the food media.…”