4.2. Metabolic characterization of the midgut epithelium
Starvation is a potent stress that, in the insect midgut, determines a series of events such as mobilization of stored metabolites (Satake et al., 2000), misregulation of metabolic enzyme activity (Ban, 1974), shortening of microvilli (Li et al., 2009), and activation of compensatory mechanisms such as autophagy (Khoa and Takeda, 2012). Since the functional activity of the midgut epithelium was partially reduced during the molting period, as discussed above, we investigated how metabolic activity within midgut cells is modified, in order to evaluate how they cope with starvation. To this aim we first analyzed the occurrence of autophagy in the epithelium. In fact, when the cell is subjected to nutrient deprivation, this cellular self-eating process can be activated to break down part of its reserves in order to stay alive until the situation improves (He and Klionsky, 2009). In insects, as well as in other animal models, autophagy has been shown to be rapidly induced when the organism undergoes structural remodeling, such as during metamorphosis, or when cells need to generate intracellular nutrients and energy, e.g., during starvation (Tettamanti et al., 2007b; Malagoli et al., 2010; Romanelli et al., 2014). For example, in Drosophila melanogaster, a few hours of nutrient starvation induces autophagy in larval organs (Scott et al., 2004)
