Protein degradation in silkworm peritracheal athrocytes and its physiological role in metamorphosis

The major functions of silkworm peritracheal athrocytes (nephrocytes) include endocytosis. Although athrocytes are also believed to function in protein degradation, there is limited experimental evidence for this. In this study, we detected the uptake and degradation of foreign proteins in peritracheal athrocytes by immunohistochemical, Western blot, and ex vivo analyses. IgG-FITC was detected in the athrocytes of silkworm larvae following injection, and LysoTracker analysis showed endosomal and lysosomal colocalizations. Athrocytes from larvae injected with IgG were incubated in Grace's medium for 2 days before being analyzed for the degradation of IgG by Western blotting. The level of incorporated IgG decreased and degradation products appeared following ex vivo culture. The highest level of IgG incorporation and degradation in the athrocytes was observed at the early pupal stage. The athrocytes also incorporated arylphorin, a major larval haemolymph protein and storage protein in silkworms. At the early pupal stage, arylphorin was actively degraded in the athrocytes. These results indicate that, in cooperation with the fat body, peritracheal athrocytes may function in the digestion of arylphorin during silkworm metamorphosis.