A new alternatively spliced transcript of the mouse connexin32 gene is expressed in embryonic stem cells, oocytes, and liver.
The rodent gap junction protein connexin32 (Cx32) is highly expressed in hepatocytes, less abundantly in Schwann cells and oligodendrocytes, and at low levels in the early mouse embryo. In both hepatocytes and Schwann cells, Cx32 expression is directed by alternative promoter regions (P1 and P2) which activate differently spliced transcript isoforms. Here we describe a third Cx32 transcript isoform expressed in embryonic cells, oocytes, and liver. Using competitive polymerase chain reaction, we have found that this new Cx32 transcript containing exon 1A is 200-fold less abundant in liver than the Cx32 isoform with exon 1. In mouse oocytes, the exon 1A-containing Cx32 transcript is exclusively expressed. Immunoblot analyses revealed no Cx32 protein expression in embryonic stem cells, whereas it has previously been demonstrated in oocytes. When the putative Cx32 promoter region upstream of exon 1A was cloned before the lacZ reporter gene, transient transfection yielded weak expression in embryonic stem cells. Our results suggest that the exon 1A-containing Cx32 isoform is likely to be inherited as an oogenetic product but not translated during early embryogenesis.