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Am J Physiol Cell Physiol 274: C1215-C1225, 1998;
0363-6143/98 $5.00
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Vol. 274, Issue 5, C1215-C1225, May 1998

Human Na+-myo-inositol cotransporter gene: alternate splicing generates diverse transcripts

Francesca Porcellati1, Tommy Hlaing2, Masaki Togawa1, Martin J. Stevens1, Dennis D. Larkin1, Yoshiyuki Hosaka1, Thomas W. Glover3, Douglas N. Henry4, Douglas A. Greene1, and Paul D. Killen2

Departments of 1 Internal Medicine, 2 Pathology, 3 Human Genetics, and 4 Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor, Michigan 48109

Na+-myo-inositol cotransport activity generally maintains millimolar intracellular concentrations of myo-inositol and specifically promotes transepithelial myo-inositol transport in kidney, intestine, retina, and choroid plexus. Glucose-induced, tissue-specific myo-inositol depletion and impaired Na+-myo-inositol cotransport activity are implicated in the pathogenesis of diabetic complications, a process modeled in vitro in cultured human retinal pigment epithelium (RPE) cells. To explore this process at the molecular level, a human RPE cDNA library was screened with a canine Na+-dependent myo-inositol cotransporter (SMIT) cDNA. Overlapping cDNAs spanning 3569 nt were cloned. The resulting cDNA sequence contained a 2154-nt open reading frame, 97% identical to the canine SMIT amino acid sequence. Genomic clones containing SMIT exons suggested that the cDNA is derived from at least five exons. Hypertonic stress induced a time-dependent increase, initially in a 16-kb transcript and subsequently in 11.5-, 9.8-, 8.5-, 3.8-, and ~1.2-kb SMIT transcripts, that was ascribed to alternate exon splicing using exon-specific probes and direct cDNA sequencing. The human SMIT gene is a complex multiexon transcriptional unit that by alternate exon splicing generates multiple SMIT transcripts that accumulate differentially in response to hypertonic stress.

human retinal pigment epithelial cells; hypertonic stress; exon splicing; osmoregulation; diabetes mellitus


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