Expression of the mRNAs and Proteins for the Na+/H+ Exchangers and Their Regulatory Factors in Baboon and Human Placental Syncytiotrophoblast

Abstract

In polarized epithelial cells of several organ systems, e.g. the kidney, a family of Na+/H+ exchangers (e.g. Na+/H+ exchanger-1 and -3) and their regulatory proteins, Na+/H+ exchanger regulatory factor and Na+/H+ exchanger-3 kinase A regulatory protein play a major role in regulating Na+/H+ exchange integral to cellular homeostasis. Because the primate placenta regulates exchange of Na+ and H+ between the mother and fetus critical to fetal-placental homeostasis, the current study determined whether Na+/H+ exchanger-1 and -3 were compartmentalized and associated with expression of Na+/H+ exchanger regulatory factor and Na+/H+ exchanger-3 kinase A regulatory protein in baboon and human syncytiotrophoblast. Using RT-PCR, single 413-bp Na+/H+ exchanger-1 and 190-bp Na+/H+ exchanger-3 products were expressed by baboon and human syncytiotrophoblasts. The 104-kDa Na+/H+ exchanger-1 protein was detected by Western blot in microvillus membranes and to a much lesser extent in the basal membranes of the baboon and human syncytiotrophoblasts. In contrast, the 85-kDa Na+/H+ exchanger-3 protein was detected primarily in membranes contiguous with the basal membranes of the syncytiotrophoblast of both species. Differential localization of Na+/H+ exchanger-1 and -3 was confirmed by immunocytochemistry. The Na+/H+ exchanger-3 regulatory protein, Na+/H+ exchanger-3 kinase A regulatory protein, resided almost exclusively in the basal membranes, whereas Na+/H+ exchanger regulatory factor was localized primarily to the microvillus membranes in the baboon and human syncytiotrophoblast. Collectively, these results are the first to show that the baboon and human term placental syncytiotrophoblast expressed the mRNAs and proteins for Na+/H+ exchanger-1 and -3 and their regulatory factors and that Na+/H+ exchanger-1 and Na+/H+ exchanger regulatory factor resided primarily in the microvillus membranes, whereas Na+/H+ exchanger-3 and Na+/H+ exchanger-3 kinase A regulatory protein were localized to membranes contiguous with the basal membranes and to the basal membranes, respectively. We conclude that a complete Na+/H+ exchange system is present in the baboon and human term placental syncytiotrophoblast and suggest that the primate placenta exhibits polarity with respect to the capacity for regulation of Na+/H+ exchange between the placenta and the maternal and fetal circulations.