The mammalian zona pellucida is an extracellular matrix comprised of three sulfated glycoproteins (ZP1, ZP2 and ZP3) that surrounds growing oocytes, ovulated eggs and embryos. In mouse, ZP3 is the primary sperm receptor and induces the sperm acrosome reaction; ZP2 is a secondary sperm receptor; and both proteins play a role in the post-fertilization block to polyspermy. The genetic loci of mouse and human Zp-2 and Zp-3 have been characterized. The single copy mouse Zp-2 gene is located on mouse chromosome 7 and contains 18 exons (45-190 bp) spanning 12.1 kb. The human homologue is similarly arrayed. The mouse and human Zp-3 genes are each composed of 8 exons (92-338 bp) that span 8.6 and 18.3 kb of DNA, respectively. Mouse Zp-2 and Zp-3 are transcribed uniquely in oocytes and transcripts accumulate during the narrow two-week growth phase of oogenesis. Deadenylation and degradation of zona transcripts during meiotic maturation is associated with the cessation of zona protein synthesis in ovulated eggs. By comparing the 5' flanking sequences of the four cloned zona genes (human Zp-2 and Zp-3, mouse Zp-2 and Zp-3), conserved elements (712 bp) have been identified within the first 250 bp upstream of the transcription start sites. Reporter genes containing 470 bp of 5' flanking sequence from mouse Zp-3 are expressed after microinjection into growing oocytes. Preliminary studies indicate that mutations of two of the defined DNA elements results in marked diminution of reporter gene activity. Studies are currently underway to identity trans-acting factors that bind to these motifs and modulate oocyte-specific gene expression. The primary amino acid sequence of mouse ZP2 and ZP3 (as well as human ZP3) have been deduced from full length cDNA clones of the cognate transcripts. Epitope libraries have been screened with anti-ZP2 and anti-ZP3 monoclonal antibodies, previously shown to act as effective contraceptive agents. Vaccination with the defined ZP3 epitope can result in long term, reversible contraception. Because the zona genes are conserved from mouse to man, this contraceptive strategy of vaccinating with """"""""self"""""""" zona pellucida may be widely applicable among mammals.
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