Phospholipase C Zeta 1 Supports Post-Fertilization Embryo Development beyond Oocyte Activation: Evidence from Sperm mRNA Analysis and Functional Studies

Phospholipase C zeta 1 (PLCZ1), the principal sperm-derived factor, initiates intracellular calcium (Ca²⁺) oscillations at fertilization; however, its contribution to post-fertilization embryonic development remains unclear. This study investigated whether PLCZ1 protein and PLCZ1 mRNA levels influence embryo quality beyond their canonical role in oocyte activation (OA) in human assisted reproductive technology (ART) cycles and functionally examined this hypothesis using a mouse embryo model. Sperm PLCZ1 protein expression assessed in 48 infertile couples undergoing in vitro fertilization/intracytoplasmic sperm injection significantly correlated with clinical fertilization rates. Sperm PLCZ1 mRNA levels were significantly higher in samples from patients producing good quality embryos (Grade A or B) than in those from patients yielding poor quality embryos (Grade C and D, p < 0.001), independent of paternal age. Functionally, microinjection of human PLCZ1 (hPLCZ1) cRNA into mouse metaphase II oocytes induced repetitive Ca²⁺ oscillations, thus confirming biological activity. Injecting hPLCZ1 cRNA into mouse pronuclear-stage embryos following parthenogenetic activation significantly enhanced blastocyst development compared with that in diethyl pyrocarbonate-treated water-injected controls (p = 0.0101), achieving rates comparable to those of diploid embryos generated using cytochalasin B treatment. These findings are consistent with a dual-component model in which sperm-derived PLCZ1 protein initiates OA at fertilization, while sperm-derived PLCZ1 mRNA is translationally competent within the oocyte and may contribute to sustained post-fertilization PLCZ1 activity and preimplantation developmental competence. Because the functional data were generated with exogenous cRNA rather than by manipulating the endogenous sperm transcript, this model remains a hypothesis that will require loss-of-function validation in future work. Despite limitations such as the moderate clinical sample size and use of mRNA as a surrogate for PLCZ1 protein in clinical samples, sperm PLCZ1 mRNA quantification represents a candidate, hypothesis-generating biomarker for embryo quality that warrants prospective validation.