CLONING AND EXPRESSION OF A CDNA-ENCODING A BRAIN-SPECIFIC NA+-DEPENDENT INORGANIC-PHOSPHATE COTRANSPORTER

We have isolated a brain-specific cDNA that encodes a Na+-dependent inorganic phosphate (P-i) cotransporter (BNPI). The nucleotide sequence of BNPI predicts a protein of 560 amino acids with 6-8 putative transmembrane-spanning segments that is approximate to 32% identical to the rabbit kidney Na+-dependent P-i cotransporter. Expression of BNPI mRNA in Xenopus oocytes results in Na+-dependent P-i transport similar to that reported for the recombinantly expressed or native kidney Na+-dependent cotransporter. RNA blot analysis reveals that BNPI mRNA is expressed predominantly (if not exclusively) in brain, and in situ hybridization histochemistry reveals BNPI transcripts in neurons of the cerebral cortex, hippocampus, and cerebellum. Furthermore, we have confirmed the presence of saturable Na+-dependent P-i cotransport in cultured cerebellar granule cells. Together, these data demonstrate the presence of a specific neuronal Na+-dependent transport system for P-i in brain.