Insects are continuous (e.g., Lepidoptera and Diptera larvae) or discontinuous (e.g., predators and hematophagous insects) feeders. Synthesis and secretion of digestive enzymes in continuous feeders seem to be constitutive; that is, these functions occur continuously, whereas in discontinuous feeders they are regulated. It is widely believed (without clear evidence) that putative endocrine cells (Fig. 3I) play a role in regulating midgut events. The presence of food in the midgut is necessary to stimulate synthesis and secretion of digestive enzyme. This was clearly shown in mosquitoes.
Like all animal proteins, digestive enzymes are synthesized in the rough endoplasmic reticulum, processed in the Golgi complex, and packed into secretory vesicles (Fig. 7). There are several mechanisms by which the contents of the secretory vesicles are freed in the mid-gut lumen. During exocytic secretion, secretory vesicles fuse with the midgut cell apical membrane, emptying their contents without any loss of cytoplasm (Fig. 7A) . In contrast, apocrine secretion involves the loss of at least 10% of the apical cytoplasm following the release of secretory vesicles (Fig. 7B). These have previously undergone fusions originating larger vesicles that after release eventually free their contents by solubilization (Fig. 7B) . When the loss of cytoplasm is very small, the secretory mechanism is called microapocrine. Microaprocrine secretion consists of releasing budding double-membrane vesicles (Fig. 7C) or, at least in insect midguts, pinched-off vesicles that may contain a single or several secretory vesicles (Fig. 7D). In both apocrine and microapocrine secretion, the secretory vesicle contents are released by membrane fusion and/or by membrane solubilization due to high pH contents or to the presence of detergents.
FIGURE 7 Models for secretory processes of insect digestive enzymes; (A) exocytic secretion, (B) apocrine secretion, (C) microapocrine secretion with budding vesicles, (D) microapocrine secretion with pinched-off vesicles, and (E) modified exocytic secretion in hemipteran midgut cell. Abbreviations: BSV, budding secretory vesicle; CE, cellular extrusion; DSV, double-membrane secretory vesicle; GC, Golgi complex; M, microvilli; N, nucleus, PMM, perimicrovillar membrane; PSV, pinched-off secretory vesicle; RER, rough endoplasmic reticulum; SV, secretory vesicle.
Secretion by hemipteran midgut cells displays special features. Double-membrane vesicles bud from modified (double-membrane) Golgi structures (Fig. 7E). The double-membrane vesicles move to the cell apex, their outer membranes fuse with the microvillar membrane, and their inner membranes fuse with the perimicrovillar membranes, emptying their contents (Fig. 3H).
Major histocompatibility complex refers to a cluster of genes responsible for immune response, transplantation antigens and proteins of the complement system etc. The MHC contains 3 classes of genes. Class I genes encode glycoprotein expressed on the surface of nearly all nucleated cells. The major function of class I products is the presentation of peptide antigens to cytotoxic cells. Class II MHC genes encode glycoproteins expressed primarily on antigen presenting cells. They present processed antigenic peptides to helper T cells. Class III MHC genes encode various secreted proteins that have immune functions including the synthesis of complements and molecules involved in inflammation.
For Class II associated antigen presentation, extracellular proteins are internalized into endoscopes, where these proteins are proteolytically cleaved by enzymes that function at acidic pH. Newly synthesized class II MHC molecule associated with the Ii are transported from the ER to the endosomal vesicles. Here, the Ii is proteolytically cleaved and a small peptide remnant of the Ii, called CLIP, is removed from the peptide binding cleft of the MHC molecule by the HLA-DM molecules. The peptides that were generated from extracellular proteins then bin to the available cleft of other class II MHC molecule and the trimeric complex i.e class II MHC a and b chains and peptide, moves to and is displayed on the surface of the cell.