A role for the lysosomal protease cathepsin B in zebrafish follicular apoptosis

This study presents evidence that cathepsin B, a lysosomal protease, may be involved in the regulation of apoptosis during serum-starvation in teleost follicles. Zebrafish vitellogenic follicles were isolated, incubated under serum-free conditions and homogenized. The follicle extracts demonstrated caspase-3-like activity using the fluorogenic substrate DEVD-AMC, indicating the onset of apoptosis. Cathepsin B activity as measured using the fluorogenic cathepsin B substrate, Z-Arg-Arg-AMC was elevated within the first 6 h of incubation in serum-free media and coincided with the onset of apoptosis. This increase in cathepsin B activity was sensitive to the cathepsin B inhibitor, CA-074-ME. Furthermore, adding CA-074-ME to the follicle incubation blocked caspase-3-like activation, suggesting that cathepsin B activity is a positive regulator of the apoptotic cascade during serum-starvation. Interestingly, the increase in cathepsin-B-like activity was not preceded by an increase in cathepsin B mRNA transcription, suggesting that regulation of this enzyme is at a level other than of the gene. These results suggest a regulatory role for cathepsin B during follicular apoptosis in zebrafish ovarian follicles.