Functional characterization of sugar transport in the intestinal epithelium of an insect (Aphidius ervi, Hymenoptera)

Aphidius ervi is an entomophagous wasp, widely used in biological control because adult females lay the eggs in the haemocoel of several aphid species. After hatching, A. ervi larvae develop in the living host feeding on its haemolymph and, just before metamorphosis, kill the host by devouring its internal tissues and pupating inside the body wall, from which the adult will finally emerge. Like many other parasitoids, A. ervi finely redirects host metabolism in order to optimize haemolymph nutrient contents to support larval specific needs. In aphid haemolymph, sugar titre is extremely high, reaching a concentration of 129 and 60 mM for fructose and glucose respectively. The peculiar sugar-rich environment in which A. ervi larvae develop, suggestive of a critical role of these nutrients for the insect survival, stimulated us to identify the transporters involved in glucose and fructose intestinal absorption, especially considering that information on sugar transport in insect midgut is scanty. The study performed in larval A. ervi intestine, isolated and incubated in vitro, allowed us to detect, with functional, immunohistochemical and immunoblotting approaches, the transport proteins responsible for glucose and fructose uptake and to characterise their basic properties. The results unequivocally show that the cellular model for sugar transport in the larval stage of this insect is surprisingly similar to that conventionally described for mammals (i.e. SGLT1-like and GLUT5-like transporters on the apical membrane of the intestinal epithelial cell, and GLUT2-like transporters on the basolateral one), with the expression, apparently constitutive, of GLUT2 transporters also in the apical membrane, in agreement with the model for the transcellular absorption of hexoses recently proposed (Kellett, J. Physiol. 531, 585, 2001)