HUMAN NA-MYO-INOSITOL COTRANSPORTER GENE - ALTERNATE SPLICING GENERATES DIVERSE TRANSCRIPTS()

Na+-myo-inositol cotransport activity generally maintains millimolar i ntracellular concentrations of myo-inositol and specifically promotes transepithelial myoinositol transport in kidney, intestine, retina, an d choroid plexus. Glucose-induced, tissue-specific myo-inositol deplet ion and impaired Na+-myo-inositol cotransport activity are implicated in the pathogenesis of diabetic complications, a process modeled in vi tro in cultured human retinal pigment epithelium (RPE) cells. To explo re this process at the molecular level, a human RPE cDNA library was s creened with a canine Na+-dependent myo-inositol cotransporter (SMIT) cDNA. Overlapping cDNAs spanning 3569 nt were cloned. The resulting cD NA sequence contained a 2154-nt open reading frame, 97% identical to t he canine SMIT amino acid sequence. Genomic clones containing SMIT exo ns suggested that the cDNA is derived from at least five exons. Hypert onic stress induced a time-dependent increase, initially in a 16-kb tr anscript and subsequently in 11.5-, 9.8-, 8.5-, 3.8-, and similar to 1 .2-kb SMIT transcripts, that was ascribed to alternate exon splicing u sing exon-specific probes and direct cDNA sequencing. The human SMIT g ene is a complex multiexon transcriptional unit that by alternate exon splicing generates multiple SMIT transcripts that accumulate differen tially in response to hypertonic stress.