Abstract
In Drosophila, RNAi targeting of either dGyk or dGK can result in two alternative phenotypes: adult glycerol hypersensitivity or larval lethality. Here we compare these two phenotypes at the level of glycerol kinase (GK) phosphorylation activity, dGyk and dGK-RNA expression, and glycerol levels. We found both phenotypes exhibit reduced but similar levels of GK phosphorylation activity. Reduced RNA expression levels of dGyk and dGK corresponded with RNAi progeny that developed into glycerol hypersensitive adult flies. However, quantification of dGyk/dGK expression levels for the larval lethality phenotype revealed unexpected levels possibly due to a compensatory mechanism between dGyk and dGK or RNAi inhibition. The enzymatic role of glycerol kinase converts glycerol to glycerol 3-phosphate. As expected, elevated glycerol levels were observed in larvae that went on to develop into glycerol hypersensitive adults. Interestingly, larvae that died before eclosion revealed extremely low glycerol levels. Further characterization identified a wing phenotype that is enhanced by a dGpdh null mutation, indicating disrupted glycerol metabolism underlies the wing phenotype. In humans, glycerol kinase deficiency (GKD) exhibits a wide range of phenotypic variation with no obvious genotype-phenotype correlations. Additionally, disease severity often does not correlate with GK phosphorylation activity. It is intriguing that both human GKD patients and our GKD Drosophila model show a range of phenotype severity. Additionally, the lack of correlation between GK phosphorylation and dGyk/dGK-RNA expression with phenotypic severity suggests further study including understanding the alternative functions of the GK protein, could provide insights into the complex pathogenic mechanism observed in human GKD patients.
Highlights
Glycerol kinase (GK) is an enzyme that catalyzes the conversion of glycerol to glycerol 3-phosphate in an ATP dependent reaction [1]
We propose that the lack of correlation between RNAi phenotype severity with glycerol kinase phosphorylation activity, dGyk- and dGK-RNA expression levels, and glycerol levels is similar to the complexity observed in glycerol kinase deficiency (GKD) clinical studies
Progeny from these crosses could be divided into two groups: survival to adulthood with no obvious physical phenotype or lethality during larval development
Summary
Glycerol kinase (GK) is an enzyme that catalyzes the conversion of glycerol to glycerol 3-phosphate in an ATP dependent reaction [1] It plays an important role in both human metabolism and development as shown by the symptoms of glycerol kinase deficiency (GKD [MIM 307030]). The severity of GKD patient symptoms does not always correlate with GK phosphorylation activity [2] This suggests the existence of a complex pathogenic mechanism that could involve a role for genetic modifier loci [2,6,7,8,9] or alternative functions of the GK protein [10,11] such as the ATP stimulated translocation of the glucocorticoid receptor [12,13]. We evaluate the potential of a Drosophila GKD model [19] by looking for molecular or metabolic similarities with GKD in humans
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