Coordinate expression of splice variants of the murine pregnancy-specific glycoprotein (PSG) gene family during placental development

The human and murine pregnancy-specific glycoprotein (PSG) gene families encode a large number of closely related proteins which are abundantly expressed in the fetal trophoblast and secreted into the maternal circulation. Although the presence of a well conserved tripeptide sequence His or Arg-Gly-Asp or Glu or Lys (H/RGD/E/K) similar to the RGD motif found in extracellular matrix proteins hints towards a possible interaction with integrin-type receptors, the function of this group of proteins related to the carcinoembryonic antigen family is still unknown. It is also not clear whether the various members of the PSG family exert the same function. Here we describe the cloning of two splice variants of Cea4 (Cea4a, Cea4b), a murine PSG family member, which lacks the RGD-related consensus motif. Cea4a, like most of the other rodent PSG members, is composed of three immunoglobulin (Ig) variable-like domains (N1-N3) and and one Ig constant-like domain (A). In contrast, Cea4b lacks the N2 domain (N1N3A), demonstrating for the first time that PSG isoforms produced by alternative splicing also exist in mice. The mRNAs coding for Cea4a and Cea4b exhibit the same expression kinetics during placental development as found for two other murine PSGs, Cea2 and Cea3, which contain the RGD-like motif. Expression starts after day 12.5 of embryonic development (E12.5) and maximum steady-state levels are reached around E15.5-E17.5 as determined by RNase protection analyses. At E17.5, PSG transcripts can be detected exclusively in the spongiotrophoblast of the placenta. In addition, PCR analyses revealed that Cea2, Cea3, and Cea4 transcripts are also found in RNA from a pool of embryos (E12-E15) but are absent from a number of adult tissues tested (kidney, lung, testis, ovary, liver, brain, thymus, heart, spleen). These results indicate that the various PSG isoforms exert their function(s) at the same time during placental and embryonic development.