Isoforms of amino acid transporters in placental syncytiotrophoblast: plasma membrane localization and potential role in maternal/fetal transport.

Many cell proteins exist as isoforms arising either from gene duplication or alternate RNA splicing. There is growing evidence that isoforms with different, but closely related, functional characteristics are often directed to discrete cellular locations. Thus, specialized functions may be carried out by proteins of similar evolutionary origin in different membrane compartments. In polarized epithelial cells, this mechanism allows the cell to control amino acid transport independently at each of its specialized apical and basolateral plasma membrane domains. Investigations of isoform localization in these membranes have generally been performed in epithelia other than the placental trophoblast.This review of placental amino acid transporter isoforms first provides an overview of their properties and preliminary plasma membrane localization. We then discuss studies suggesting various roles of isoform localization in trophoblast function. To provide insights into the molecular basis of this localization in trophoblast, we present a review of current knowledge of plasma membrane protein localization as derived from investigations with a widely used epithelial model cell line. Finally, we discuss a potential approach using cultured trophoblast-derived cells for studies of transporter isoform localization and function. We hope that this review will stimulate investigation of the properties of trophoblast transporter isoforms, their membrane localization and their contribution to the cellular mechanism of maternal-fetal nutrient transport.