Insulin-Like Peptides Regulate Feeding Preference and Metabolism in Drosophila

Abstract

Fruit flies have eight identified Drosophila insulin-like peptides (DILPs) that are involved in the regulation of carbohydrate concentrations in hemolymph as well as in accumulation of storage metabolites. In the present study, we investigated diet-dependent roles of DILPs encoded by the genes dilp1-5, and dilp7 in the regulation of insect appetite, food choice, accumulation of triglycerides, glycogen, glucose, and trehalose in fruit fly bodies and carbohydrates in hemolymph. We have found that the wild type and the mutant lines demonstrate compensatory feeding for carbohydrates. However, mutants on dilp2,3, dilp3, dilp5, and dilp7 showed higher consumption of proteins on high yeast diets. To evaluate metabolic differences between studied lines on different diets we applied response surface methodology. High nutrient diets led to a moderate increase in concentration of glucose in hemolymph of the wild type flies. Mutations on dilp genes changed this pattern. We have revealed that the dilp2 mutation led to a drop in glycogen levels independently on diet, lack of dilp3 led to dramatic increase in circulating trehalose and glycogen levels, especially at low protein consumption. Lack of dilp5 led to decreased levels of glycogen and triglycerides on all diets, whereas knockout on dilp7 caused increase in glycogen levels and simultaneous decrease in triglyceride levels at low protein consumption. Fruit fly appetite was influenced by dilp3 and dilp7 genes. Our data contribute to the understanding of Drosophila as a model for further studies of metabolic diseases and may serve as a guide for uncovering the evolution of metabolic regulatory pathways.

Keywords: Drosophila insulin-like peptides; capillary feeding; dietary response surface; geometric framework; macronutrient balance; nutrient intake trajectories.

Figure 1

Consumption of carbohydrates by wild type and DILP-deficient fruit flies. (A) Volumes of sucrose solution ingested. (B) Amounts of carbohydrate consumed. Tubes with tested fruit flies were supplied by pair of capillaries: one filled with solution of sucrose, and the other filled with solution of yeast autolysate. It was allowed flies to choose between capillaries for eating either yeast autolysate (mainly, source proteins and essential micronutrients such as vitamins, nitrogen bases, etc.) or sucrose (a source of carbohydrates). The combinations of 3, 6, and 12% sucrose, and 3, 6, and 12% of yeast autolysate were used. Data are n = 3 replicates (each containing 3–8 flies) for each diet and fly line combination.

Figure 2

Consumption of protein-rich food source by control and DILP-deficient fruit flies. (A) Volumes of yeast autolysate solution ingested. (B) Amounts of protein consumed. Data are n = 3 replicates (each containing 3–8 flies) for each diet and fly line combination.

Figure 3

Nutrient intake trajectories resulting from volumes ingested of the two food solutions (sucrose and yeast) for all fruit fly lines tested. Data are n = 3 replicates (each containing 3–8 flies) for each diet and fly line combination. Panels (A–I) correspond to dietary treatments 3S-3Y, 3S-6Y, 3S-12Y, 6S-3Y, 6S-6Y, 6S-12Y, 12S-3Y, 12S-6Y, and 12S-12Y.

Figure 4

dILPs deficiency changes the diet-dependent pattern of glucose concentration in fruit fly hemolymph. (A) Dietary response surfaces depicting dependence of concentration of glucose in fly hemolymph (mM) on concentration of yeast and sucrose in the diet. There were tested nine combinations of sucrose (3, 6, and 12%) and yeast (3, 6, and 12%). In each case, the flies of tested lines were able to choose between of sucrose- or yeast-containing medium. All five surfaces are placed under one scale. (B) The remarkable differences between hemolymph glucose values in flies of each line kept on different diets. Red-colored boxes designate statistically significant increase (indicated by the asterisk; p < 0.05, Tukey's test with Bonferroni correction) whereas blue-colored boxes designate statistically significant decrease as compared to the values for flies consumed diet designated by gray color box. (C) The remarkable differences between hemolymph glucose values in flies of different lines on particular diets. (D) Dietary responses surface depicting dependence of hemolymph glucose in wild type and dilp mutant lines on amounts of protein and carbohydrate consumed. Each surface has own scale shown by contour lines. Data are n = 3–6 replicates for each diet and fly line combination.

Figure 5

The effect of mutations on dilp genes on the diet-dependent pattern of trehalose concentration in fruit fly hemolymph. (A) Dietary response surfaces representing dependence of concentration of trehalose in fly hemolymph (mM) on concentrations of yeast and sucrose in the diet. There were tested nine combinations of sucrose (3, 6, and 12%) and yeast (3, 6, and 12%). Axes' titles are the same as in Figure 4. In each case, the flies of tested lines were able to choose between of sucrose- or yeast-containing medium. All five surfaces are placed under one scale. (B) The remarkable differences between hemolymph trehalose values in flies of each line kept on different diets. Red-colored boxes designate statistically significant increase (indicated by the asterisk; p < 0.05, Tukey's test with Bonferroni correction) whereas blue-colored boxes designate statistically significant decrease as compared to the values for flies diet treatment 3S-3Y designated by gray color box. (C) Dietary response surfaces showing dependence of hemolymph glucose in wild type and dilp mutant lines on amounts of protein and carbohydrate consumed. Each surface has own scale shown by contour lines. Data are n = 3–6 replicates for each diet and fly line combination.

Figure 6

The effect of dilp mutations on the diet-dependent pattern of glycogen accumulation in fruit fly body. (A) Dietary response surfaces depicting dependence of glycogen content in the body of individuals of wild type (w¹¹¹⁸) and dilp mutant lines on concentrations of yeast and sucrose in the diet. Axes' titles are the same as in Figure 4. (B) The remarkable differences in glycogen content between investigated fruit fly lines. Red-colored boxes designate statistically significant increase (indicated by the asterisk; p < 0.05, Tukey's test with Bonferroni correction) whereas blue-colored boxes designate statistically significant decrease as compared to the values for the wild type flies. (C) Dietary response surfaces showing dependence of glycogen content in wild type and dilp mutant lines on amounts of protein and carbohydrate consumed. Each surface has own scale shown by contour lines. Data are n = 3–6 replicates for each diet and fly line combination.

Figure 7

The effect of dilp mutations on the diet-dependent pattern of triacylglyceride (TAG) accumulation in fruit fly body. (A) Dietary response surfaces depicting dependence of TAG content in the body of individuals of wild type (w¹¹¹⁸) and dilp mutant lines on concentrations of yeast and sucrose in the diet. Axes' titles are the same as in Figure 4. (B) The remarkable differences in TAG content between investigated fruit fly lines. (C) Dietary response surfaces showing dependence of TAG content in wild type and dilp mutant lines on amounts of protein and carbohydrate consumed. Each surface has own scale shown by contour lines. Data are n = 3–6 replicates for each diet and fly line combination.