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

Abstract

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 α, β, and γ subunits were determined in meiotically incompetent oocytes, fully-grown competent oocytes, metaphase II-arrested eggs, one-, two-, and eight-cell embryos, 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 heterotrimeric G protein mRNA transcripts were present in oocytes, eggs, or preimplantation embryos: Gαq family (q, 11, and 14), Gα12 family (12 and 13), Gαi family (i1, i2, i3, t2, z, and s), β subunits 1, 2, 4, and 5, and γ subunits 2, 3, 5, and 7. A recently described large molecular weight G protein, Gαh (Nakaoka et al., 1994: Science 264:1593–1596), was also present. Gα15, Gαt1, Gαolf, GαoA, Gβ3, Gγ1, and Gγ8 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. © 1996 Wiley-Liss, Inc.