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Phospholipase C-zeta (PLCζ) is a strong candidate of the mammalian sperm factor that causes IP3 receptor-mediated repetitive Ca2+ release in the egg at fertilization and triggers egg activation. Evidence suggests that the sperm factor is driven into the egg cytoplasm upon sperm-egg fusion and then into the male and female pronuclei (PN) formed after egg activation. Ca 2+ oscillations cease upon PN formation 5-6 h after fertilization, possibly because the sperm factor in the egg cytoplasm is removed by sequestration into the PN. The nuclear translocation of PLCζ was continuously observed, when RNA encoding PLCζ fused with a fluorescent protein 'Venus' was injected into unfertilized mouse eggs and induced Ca2+ oscillations and egg activation. PLCζ accumulated in the (female) PN was seen between 5 and 15 h after RNA injection, and was lost upon breakdown of the nuclear envelop at ∼16 h just before cell division. After the first cleavage, PLCζ was accumulated again into the nuclei of two blastomeres. This was observed as well in the 4-cell, 8-cell, 16-cell embryo and the morula after fertilization, when PLCζ-Venus RNA was injected into a blastomere of the 2-cell or 4-cell embryo in which the nucleus was clearly seen in the bright field. Thus, the ability of nuclear accumulation of PLCζ is preserved during early embryonic development. The nuclear translocation of PLCζ might regulate cell cycle-dependent Ca2+ oscillations in mouse embryos. [Jpn J Physiol 55 Suppl:S74 (2005)]
