Unexpected similarity of intestinal sugar absorption by SGLT1 and apical GLUT2 in an insect (Aphidius ervi,Hymenoptera) and mammals

Abstract

Sugars are critical substrates for insect metabolism, but little is known about the transporters and epithelial routes that ensure their constant supply from dietary resources. We have characterized glucose and fructose uptakes across the apical and basolateral membranes of the isolated larval midgut of the aphid parasitoid Aphidius ervi. The uptake of radiolabeled glucose at the basal side of the epithelium was almost suppressed by 200 microM cytochalasin B, uninhibited by phlorizin, and showed the following decreasing rank of specificity for the tested substrates: glucose > glucosamine > fructose, with no recognition of galactose. These functional properties well agree with the expression of GLUT2-like transporters in this membrane. When the apical surface of the epithelium was also exposed to the labeled medium, a cation-dependent glucose uptake, inhibited by 10 microM phlorizin and by an excess of galactose, was detected suggesting the presence in the apical membrane of a cation-dependent cotransporter. Radiolabeled fructose uptakes were only partially inhibited by cytochalasin B. SGLT1-like and GLUT5-like transporters were detected in the apical membranes of the epithelial cell by immunocytochemical experiments. These results, along with the presence of GLUT2-like transporters both in the apical and basolateral cell membranes of the midgut, as we recently demonstrated, allow us to conclude that the model for sugar transepithelial transport in A. ervi midgut appears to be unexpectedly similar to that recently proposed for sugar intestinal absorption in mammals.