G protein gene expression during mouse oocyte growth and maturation, and preimplantation embryo development

Fertilization in mammals initiates "egg activation," a series of events leading to embryo development. The signal transduction events that occur as a result of sperm-egg interactions and that initiate egg activation may be analogous to a ligand-receptor-effector pathway, but the details of this signaling pathway are poorly understood. Several lines of evidence support a role for guanine nucleotide-binding regulatory proteins (G proteins) in mammalian egg activation. Prior to initiating studies to examine further the role of specific G proteins in sperm- induced mouse egg activation, we needed to define the complement of G proteins expressed in the egg. Using a reverse transcription-polymerase chain reaction (RT-PCR) assay, the relative levels of mRNAs encoding specific G protein a, p, and y subunits were determined in meiotically incompetent oocytes, fully-grown competent oocytes, metaphase Il-arrested eggs, one-, two-, and eight-cell em- bryos, and blastocysts. mRNA transcripts representing all of the heterotrimeric G protein families were present at all of the stages examined, and all underwent significant changes in their patterns of expression. The following het- erotrimeric G protein mRNA transcripts were present in oocytes, eggs, or preimplantation embryos: Gaq family (9, 11, and 141, Gal2 family (12 and 131, Gai family (il, i2, i3, t2, z, and s), p subunits 1,2,4, and 5, and y subunits 2, 3, 5, and 7. A recently described large molecular weight G protein, Gah (Nakaoka et al., 1994: Science 264:1593- 1596), was also present. Ga15, Gatl, Gaolf, GaoA, Gp3, Gyl , and Gy8 mRNA transcripts were not detected using this method. The most common pattern of expression observed was a maturation-associated decrease followed by an increase after the two-cell stage. Some transcripts, however, were expressed at low levels until the eight-cell to blastocyst stages, whereas others were expressed at high levels in the oocyte but following maturation declined and remained at a low level throughout preimplantation development. embryo from the two haploid gametes. Events compris- ing egg activation include elevation of intracellular cal- cium, cortical granule exocytosis, resumption of the cell cycle, and recruitment of maternal mRNAs. The signal transduction events that occur as a result of sperm-egg plasma membrane binding and fusion and that initiate mouse egg activation may be analogous to ligand-recep- tor-effector pathways in somatic cells. The details of this signaling pathway, however, are poorly understood. Several lines of evidence support the hypothesis that G proteins play a role in mammalian fertilization. For exam- ple, mouse eggs expressing the human ml muscarinic re- ceptor, a G protein-coupled receptor, undergo the full complement of sperm-induced egg activation events when stimulated by acetylcholine (Williams et al., 1992; Mooreet al., 1993). Thisresponse is presumablymediated by the stimulation of an endogenous egg G proteinb) and is pertussis toxin (PT)-insensitive. Although mouse eggs possess Gaq (Williams et al., 1992), which is a PT-insensi- tive G protein, microinjection of inhibitory antibodies to Gaq does not inhibit sperm-induced egg activation (Moore et al., 1994). This lack ofan inhibitory effect could be due to either insufficient inhibition ofendogenous Gaq