β-SNAP activity in the outer segment growth period is critical for preventing BNip1-dependent apoptosis in zebrafish photoreceptors

BNip1, which functions as a t-SNARE component of the syntaxin18 complex, is localized on the ER membrane and regulates retrograde transport from Golgi to the ER. BNip1 also has a BH3 domain, which generally releases pro-apoptotic proteins from Bcl2-mediated inhibition. Previously we reported that retinal photoreceptors undergo BNip1-dependent apoptosis in zebrafish β-snap1 mutants. Here, we investigated physiological roles of BNip1-dependent photoreceptor apoptosis. First, we examined the spatio-temporal profile of photoreceptor apoptosis in β-snap1 mutants, and found that apoptosis occurs only during a small developmental window, 2–4 days-post-fertilization (dpf), in which an apical photoreceptive membrane structure, called the outer segment (OS), grows rapidly. Transient expression of β-SNAP1 during this OS growing period prevents photoreceptor apoptosis in β-snap1 mutants, enabling cone to survive until at least 21 dpf. These observations suggest that BNip1-mediated apoptosis is linked to excessive activation of vesicular transport associated with rapid growth of the OS. Consistently, knockdown of Ift88 and Kif3b, which inhibits protein transport to the OS, rescued photoreceptor apoptosis in β-snap1 mutants. Treatment with rapamycin, which inhibits protein synthesis via the mTOR pathway, also rescued photoreceptor apoptosis in β-snap1 mutants. These data suggest that BNip1 performs risk assessment to detect excessive vesicular transport in photoreceptors.