CONTRIBUTIONS OF NA+ H+ EXCHANGER ISOFORMS TO PREIMPLANTATION DEVELOPMENT OF THE MOUSE/

Previous work provided evidence of Na+/H+ exchanger activity in the ap ical domain of mouse trophectodermal plasma membranes that provides a route for entry of extracellular Na+ (Manejwala et at., 1989). This ac tivity was hypothesized to contribute to the trans-trophectodermal Na flux that is required for blastocoel expansion. In the present work, we have used reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry to identify members of the Na+/H+ exchanger (N HE) family that are likely to participate in this process. When cDNA p reparations from ovulated oocytes and several stages of preimplantatio n development were tested with PCR primers specific for the NHE-1, -2, -3, and -4 isoforms of the exchanger, only amplicons representing the NHE-1 and NHE-3 isoforms were detected. The identity of these amplico ns was confirmed by direct sequencing. NHE-1 mRNA is present in oocyte s and in all preimplantation stages, increasing threefold on a per emb ryo basis between the 4-cell and blastocyst stages. NHE-3 mRNA, on the other hand, was only detected in oocytes. Immunocytochemical analysis of blastocysts revealed that NHE-1 is localized in the basolateral do main of the trophectoderm, whereas NHE-3 is localized in the apical do main, a situation like that in epithelia of adult organs. We conclude that NHE-3, an oogenetic product that persists into the blastocyst sta ge, is the Na+/H+ exchanger isoform most likely to be involved in blas tocoel expansion. (C) 1998 Wiley-Liss, Inc.