Molecular cloning, characterization and expression analysis of cathepsin O in silkworm Bombyx mori related to bacterial response.

Cathepsins are the main members of the cysteine family and play important roles in immune response in vertebrates. The Cathepsin O of Bombyx mori (BmCathepsin O) was cloned from the hemocytes by the rapid amplification of cDNA ends (RACE). The genomic DNA was 6131bp long with a total of six exons and five introns. Its pre-mRNA was spliced to generate two spliceosomes. By comparisons with other reported cathepsins O, it was concluded that the identity between them ranged from 29 to 39%. Expression analysis indicated that BmCathepsin O was specific-expressed in hemocytes, and highly expressed at the 4th molting and metamorphosis stages. Immunofluorescence assay and qRT-PCR showed that BmCathepsin O was expressed in granulocytes and plasmatocytes. Interestingly, BmCathepsin O was significantly up-regulated after stimulated by 20-hydroxyecdysone (20-E) in vivo, which suggested that BmCathepsin O may be regulated by 20E. Moreover, activation of BmCathepsin O was also observed in hemocytes challenged by Escherichia coli, indicating its potential involvement in the innate immune system of silkworm, B. mori. In summary, our studies provide a new insight into the functional features of Cathepsin O.