Prolactin functions as a survival factor during zebrafish embryogenesis

Prolactin is a multifaceted hormone that is capable of modulating hundreds of physiological processes in adult vertebrates. However, the physiological functions of prolactin in embryonic development are still controversial. One of these biological functions of prolactin is to promote survival of the cells. Almost all studies on the anti-apoptotic action of prolactin have been focused on a variety of mammalian cell lines and tissues, while no study has been reported on prolactin's anti-apoptotic role in the embryo. In order to determine whether prolactin acts as a survival factor for embryonic cells during development, prolactin protein was knocked-down in zebrafish embryos by microinjection of prolactin-morpholino antisense (PRL-MO). Significant increase in the number of apoptotic cells was observed in embryos treated with PRL-MO compared to control embryos injected with control morpholino or non-injected controls. The number of apoptotic cells increased more significantly between 15 and 35 h post-fertilization in the PRL-MO treated group than that of the control. Interestingly, apoptotic cells were restricted to the central nervous system, particularly in the eyes and brain. Apoptosis of these cells was further demonstrated using the comet assay to detect DNA damage, a hallmark of apoptosis. It was found that the level of DNA damage was dose-dependent on the concentration of PRL-MO injected and consistent with higher levels of nick ends detected by the TUNEL assay in PRL-MO embryos. Further examination of apoptotic genes indicated the transcript of caspase-8, a representative caspase gene of the extrinsic pathway, was much higher in prolactin knockdown embryos than the non-injected control. Together, these results suggest that prolactin acts as a survival factor during zebrafish embryogenesis.