Gene expression and lysosomal content of silkworm peritracheal athrocytes

To examine the function of silkworm Bombyx mori L. athrocytes (nephrocytes), we constructed cDNAs of larval peritracheal athrocytes that were anatomically isolated from surrounding tissues. Larval expression levels of genes encoding hemolymph proteins, such as arylphorin, the 30 K proteins, and lysozyme, were lower in peritracheal athrocytes than in the fat body, whereas genes involved in protein degradation were highly expressed in athrocytes. Real time RT-PCR revealed that a member of the Hsp40/Dnaj protein family, DjA2 (also known as Rdj2, Dj3, Dnj3, Cpr3, and Hirip4), an endocytic gene, was highly expressed in the peritracheal athrocytes compared to the fat body. Homologs of the Drosophila ATG1, ATG5, ATG6, and ATG8 genes had high expression levels in the peritracheal athrocytes. Observations using laser confocal microscopy with lysosomal fluorescent probes showed that silkworm athrocytes, including pericardial cells, suboesophageal body, and peritracheal athrocytes, were rich in lysosomes, in contrast to other tissues. Peritracheal athrocytes had lysotracker-positive spots at all times from the fourth larval molt to the pupa. Of these, molting larval and pupal peritracheal athrocytes had larger spots. Starvation for 24 h induced greater lysotracker staining, but the number of spots decreased. Silkworm peritracheal athrocytes are lysosome-rich tissues and may function in the degradation of proteins.