A mouse oocyte-specific protein that binds to a region of mZP3 promoter responsible for oocyte-specific mZP3 gene expression

Zona Pellucida Genes and Proteins: Essential Players in Mammalian Oogenesis and Fertility

All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.

Zona pellucida glycoproteins

All mammalian eggs are surrounded by a relatively thick extracellular coat, the zona pellucida, that plays vital roles during oogenesis, fertilization, and preimplantation development. The mouse zona pellucida consists of three glycoproteins that are synthesized solely by growing oocytes and assemble into long fibrils that constitute a matrix. Zona pellucida glycoproteins are responsible for species-restricted binding of sperm to unfertilized eggs, inducing sperm to undergo acrosomal exocytosis, and preventing sperm from binding to fertilized eggs. Many features of mammalian and non-mammalian egg coat polypeptides have been conserved during several hundred million years of evolution.

Reconstitution of enhancer function in paternal pronuclei of one-cell mouse embryos

How chromatin-mediated transcription regulates the beginning of mammalian development is currently unknown. Factors responsible for promoter repression and enhancer-mediated relief of this repression are not present in the paternal pronuclei of one-cell mouse embryos but are present in the zygotic nuclei of two-cell embryos. Here we show that coinjection of purified histones and a plasmid-encoded reporter gene into the paternal pronuclei of one-cell embryos at a specific histone-DNA concentration could recreate the behavior observed in two-cell embryos: acquisition of promoter repression and subsequent relief of this repression either by functional enhancers or by histone deacetylase inhibitors. Furthermore, the extent of enhancer-mediated stimulation in one-cell embryos depended on the acetylation status of the injected histones, on the treatment of embryos with a histone deacetylase inhibitor, and on the developmentally regulated appearance of enhancer-specific coactivator activity. The coinjected plasmids in one-cell embryos also exhibited chromatin assembly, as determined by a supercoiling assay. Thus, injection of histones into one-cell embryos faithfully reproduced the chromatin-mediated transcription observed in two-cell embryos. These results suggest that the need for enhancers to stimulate promoters through relief of chromatin-mediated repression occurs once the parental genomes are organized into chromatin. Furthermore, we present a model mammalian system in which the role of individual histones, and particular domains within the histones that are targeted in enhancer function, can be examined using purified mutant histones.

Transcription and masking of mRNA in germ cells: involvement of Y-box proteins

Gametogenesis is directed by various specialized genetic mechanisms which, to a considerable extent, apply to the production of both eggs and sperm and have been conserved across a wide spectrum of eukaryotic organisms. Two key aspects which are discussed here are: germ-cell-specific gene transcription; and translational repression (masking) of mRNA accumulated in oocytes and spermatocytes/spermatids. Together, these two processes conspire to deliver often large amounts of essential proteins at the appropriate stages of development. It is perhaps not surprising that recent evidence points to a functional link between transcription activation and translation repression, both processes being determined in the nucleus and involving common components. One set of components which has been studied recently are members of the Y-box family of regulatory proteins. Most information of the involvement of Y-box proteins in germ cell development comes from studies on amphibian oocytes and mammalian spermatids. In these cells, Y-box proteins have been detected as major components of both maternal and paternal mRNP particles and have been shown to be instrumental in the masking process. Y-box proteins are also implicated in the regulation of several germ-cell-specific genes. Possible connections between these processes are discussed.

Targeting gene expression to the vascular wall in transgenic mice using the murine preproendothelin-1 promoter

To develop a system for overexpressing genes in the vascular wall, we created transgenic mice using the reporter gene luciferase and the murine preproendothelin-1 promoter. In vitro analysis suggested that the murine 5'-flanking region contained endothelial-specific elements in a 5.9-kb fragment. Five transgenic mice colonies established from independent founders all exhibited the highest level of luciferase activity in the aorta with up to 8,540 light units per microgram of protein. Immunohistochemistry with anti-luciferase antisera revealed high levels of expression in the endothelial cells of both large and small arteries and lower levels of expression in veins and capillaries. Significant expression was also seen in arterial smooth muscle cells and in select epithelial surfaces which is consistent with the known distribution of endothelin-1 in mammals. The further demonstrate the targeting capability of this system, we overexpressed the lipid-peroxidating enzyme, human 15-lipoxygenase, in the vessel wall of transgenic mice. As with luciferase, expression of active enzyme and immunohistochemical localization in vascular cells were documented in transgenic animals. Hence, this new system can be used to direct expression of molecules to the vascular wall for the purpose of examining the biological significance of either overexpression or inhibition of select proteins.