Identification of a sodium independent nucleoside transporter in rat brain

Nearly all mammalian cells utilize externally supplied nucleosides that are carried across their plasma membranes via integral membrane proteins. In a previous study, a sodium independent nucleoside transporter (hENT1) was cloned and characterized from human placental tissue (Griffiths et al, Nature Medicine, 3:89, 1997). The objective of our study was to determine if the rat species expresses this transporter and if it is present in rat brain tissue. Degenerate primers were designed from the human amino acid sequence and used in a polymerase chain reaction (PCR) with a rat brain cDNA library as template. Electrophoretic analysis demonstrated the formation of an expected 587 bp PCR product that upon sequencing showed sequence homology to hENT1. Through sequence analysis and primer walking, a 1518 bp sequence of a rat brain cDNA was eventually obtained. This sequence is 81% identical to the human hENT1 on the nucleotide level and is 78% identical with respect to the amino acid sequence. Based on its high degree of sequence identity to hENT1 this cDNA is believed to be the rat sodium independent nucleoside transporter (rENT1). Total RNA was isolated from rat pup brain, adult brain, adult heart, and adult liver. Northern blot analysis of these samples using a 734 bp PCR generated probe indicates an approximate 2.4 Kb mRNA present in all four tissues. These results demonstrate the partial sequencing of a rat cDNA that is similar in sequence to the human sodium independent nucleoside transporter. They also indicate that rENT1 is expressed in suckling and adult rat brain as well as other rat tissues.