Lysosomes in the cessation of vitellogenin secretion by the mosquito fat body; selective degradation of Golgi complexes and secretory granules.

A massive and selective degradation of Golgi complexes, secretory granules, and RER is the mechanism responsible for the rapid termination of Vg secretion by trophocytes of the mosquito fat body. These cells are involved in an intensive synthesis of a glycoprotein, vitellogenin (Vg), which is accumulated by developing oocytes as yolk protein. Previously, assays for lysosomal enzymes have demonstrated that the cessation of Vg synthesis is characterized by a sharp increase in lysosomal activity; and fluorescent microscopy has shown that, during this intense lysosomal activity, Vg concentrates in lysosomes. In this report, electron microscopy combined with cytochemistry for lysosomal enzymes and localization of Vg with colloidal gold immunocytochemistry has shown that this lysosomal activity is directed towards selective degradation of Vg and organelles associated with its synthesis and secretion. Three organelles undergo lysosomal breakdown: the Golgi complex, Vg-containing secretory granules, and RER. The degradation of Golgi complexes occurs in two steps similar to that for RER: first, the organelle is sequestered by double isolation membranes, and the resulting pre-lysosome then fuses with a primary or secondary lysosome. In contrast, mature Vg-containing secretory granules fuse with lysosomes directly. This combination of crino- and autophagy is a specific, highly intense, and precisely timed event.