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

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.甘油激酶（glycerol kinase）通过ATP依赖反应将甘油转化为甘油3-磷酸（glycerol 3-phosphate），在代谢过程中发挥关键作用。在人体中，甘油激酶缺乏症（glycerol kinase deficiency）可引发广泛的表型异质性；患者可能出现严重的代谢异常与中枢神经系统（Central Nervous System, CNS）异常，而部分患者仅表现为高甘油血症与甘油尿症，无其他明显表型。为阐明该表型差异背后的致病机制，我们通过RNA干扰（RNA interference, RNAi）靶向dGyk（CG18374）与dGK（CG7995），构建了甘油激酶缺乏症的果蝇（Drosophila）模型。实验结果符合预期：RNAi敲低果蝇的甘油激酶RNA表达水平降低、磷酸化活性下降，且甘油水平升高。进一步研究发现，这些果蝇对添加了甘油的果蝇培养基表现出超敏反应。鉴于甘油具有吸湿性，我们推测甘油超敏反应源于其对干燥的易感性升高，这表明甘油激酶在昆虫的抗干燥（desiccation resistance）能力中发挥重要作用。为评估遗传修饰位点对敲低果蝇中甘油超敏反应严重程度的调控作用，我们利用甘油超敏存活实验，对致死性转座子插入突变果蝇开展了初步筛选。我们证实，此类筛选可识别甘油超敏反应的增强子与抑制子遗传位点。此外，我们发现眼色素沉着基因的无效突变可增强或抑制该甘油超敏表型。综上，我们的研究数据表明，眼色素沉着基因编码的蛋白在抗干燥能力中发挥重要作用，且这类基因可有效修饰我们构建的甘油激酶缺乏症果蝇模型中出现的甘油超敏表型。