Examination of Poly in an insulin resistance type 2 diabetes model in Drosophila melanogaster
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The protein Poly was first discovered in Drosophila melanogaster, during a screening for third chromosome lethal mutations. Drosophila poly mutant larvae exhibit a slower rate of development. However, they reach the third instar larval stage and remain at that stage for 21 days before they die without reaching pupation. This phenotype is attributed to developmental impairment of the imaginal discs, therefore suggesting defects in cell growth and/or proliferation. During that stage, the mutant larvae develop melanotic masses. Poly is conserved and its homolog, Elp6, is one of the small subunits of the Elongator Complex, a complex involved in many cellular functions including transcription and translation. Drosophila larvae mutated at the Elp3 gene, the gene encoding the catalytic subunit of the Elongator complex, develop melanotic masses, a phenotype very similar to that of poly. The Heck laboratory published that Poly is a positive mediator of the Insulin Receptor/TOR (InR/TOR) pathway, which leads to protein, glycogen and fatty acid synthesis, regulates cell growth and apoptosis. It was shown that Poly interacts with InR, at least in some cases, therefore promoting cell growth and metabolism (Bolukbasi et al., 2012). The Drosophila genome shares 60% similarity to the human, with 77% of the genes attributed to a human disease having a Drosophila counterpart (Chien et al., 2002). In 2011, an intriguing study by Musselman et al. reported that feeding wild type Drosophila larvae with excessive amounts of sucrose led to the development of an insulin resistance phenotype similar to that of Type 2 Diabetes (T2D), thus rendering Drosophila an easily accessible T2D model. The phenotype included impaired metabolism, slower rate of development, and excessive accumulation of triglycerides (TAG) in the larval fat body. In my thesis research, I examined the involvement of Poly in insulin resistance - T2D using Drosophila as a model. The understanding of the connection between the protein and the disease came upon the discovery of a new form of Poly, Poly14. Poly14 is enriched in the Drosophila fat body, the equivalent of the human fat tissue and liver and its protein levels are significantly decreased when larvae are fed a high sucrose diet, compared to other types of diets – potentially linking the protein to the onset of T2D. Poly mRNA levels were also lower. To examine whether the overexpression of poly might be able to rescue the insulin resistance phenotype, two new Drosophila transgenes were generated with the ability to express the gene in a tissue of interest. In these two new transgenes, Poly is tagged with tRFP (Red Fluorescence Protein) at the N’- (UAS_N’RFPpoly) or the C’-terminus (UAS_polyC’RFP). Overexpression of Poly rescued the insulin resistance phenotype, therefore implicating Poly as a possible important regulator in the development of the insulin resistance phenotype. All of the above findings suggest a vital role of Poly in metabolism and the development of the insulin resistance/diabetic phenotype in Drosophila, providing us the opportunity for new tools in this very medically-relevant field of research.