Expression and transient nuclear translocation of protein convertase 1 (PC1) during mouse preimplantation embryonic development

Abstract

Preimplantation embryos express a number of hormones, neuropeptides and membrane receptors known to derive from proteolytic activation of their precursors by proprotein convertases (PCs). The goal of this study was to determine the pattern of expression of the neuroendocrine protein convertase 1 (PC1) in mouse preimplantation embryos. Previous data have shown that PC1 transcripts are detectable by reverse transcription-polymerase chain reaction in unfertilized and fertilized eggs as well as at all stages of preimplantation embryonic development (Croissandeau and Mbikay, unpublished data). In this study, we show by immunoblotting that the zymogen and mature forms of PC1 are present at these stages. Using immunofluorescence laser confocal microscopy, we have examined its subcellular location: PC1-specific staining was observed throughout the cytoplasm of unfertilized eggs. After fertilization, surprisingly, the staining was transiently concentrated in pronuclei. It relocated to the cytoplasm at post-zygotic stages and was particularly strong at junctions between blastomeres. The nuclear translocation of PC1 in fertilized eggs is probably mediated by its prodomain. Indeed, when transfected in human colon carcinoma LoVo cells, a mutant proPC1 incapable of cleaving off its prodomain accumulated in the nucleus. Furthermore, when N-terminally fused to a green fluorescent protein, this domain was able to direct the reporter protein to the nucleus of these cells. Collectively, these data suggest that PC1 is a potential convertase for precursor proteins in preimplantation embryos. They also raise the possibility of a nuclear function for this enzyme during zygote formation.