MOLECULAR PHYSIOLOGY OF SODIUM-GLUCOSE COTRANSPORTERS

The development of a novel technique to detect the activity of membrane transport proteins that have heretofore eluded biochemical characterization has led to the cloning of the cDNA for the classical intestinal Na+-glucose cotransporter. This breakthrough has greatly extended our insights into Na+- coupled transport processes and revealed the existence of a new family of Na+-coupled transporter genes. An important member of this family is the low-affinity Na+-glucose cotransporter SGLT2, the major cotransporter responsible for renal glucose reabsorption. The identification, cloning and localization of these two Na+-glucose cotransporters, along with members of the facilitative glucose transporter family, have provided a satisfying picture of how tissue- and cell-specific expression of the glucose transporter isoforms subserve function. From studies on the dietary dependence of Na+-glucose cotransporter expression, the regulation of expression in sheep intestine and dysregulation in experimental diabetes, there is accumulating evidence that regulation occurs primarily at the translational or posttranslational levels. Probes and/or antibodies to detect the SGLT1 and SGLT2 transcripts and proteins now permit a systematic analysis of the expression levels of these important genes in both normal physiology and pathological conditions. It is anticipated that in the future the molecular defect(s) in renal glycosuria will be understood, that analysis of the promoters will provide clues as to how these genes are regulated, that additional Na+-glucose cotransporters may be identified, and that functions other than the classically defined absorption role will be delineated for Na+-glucose cotransporters expressed in tissues such as lung and liver. An important goal in the future will also be to develop an in-depth knowledge of the molecular architecture of Na+-glucose cotransporters. The availability of the SGLT1 and SGLT2 clones enables us to analyze the steps in the Na+- coupled transport cycle. The high expression level achievable in Xenopus oocytes will permit a wide range of functional, electrophysiological, and biochemical studies to elucidate the mechanism of Na+-glucose cotransport. These studies will provide important information for ion-coupled organic solute transporters that play crucial roles not only in absorptive tissues such as intestine and kidney but also in the central nervous system to provide reuptake of released neurotransmitters. Genetic engineering of cloned transporters in combination with Xenopus oocyte expression and electrophysiology provide a powerful approach to define how these proteins link uphill solute transport to the electrochemical ion gradients. These approaches will not solve all of the problems, and structural information will also be required. However, the hydrophobicity and low abundance of the transporters make isolation of Na+-glucose cotransporters from tissues for atomic analysis a challenge. So far, expression systems such as E. coli, yeast, and insect cells have not yielded sufficient quantities for structural studies (149). Innovative approaches will need to be developed to address the three-dimensional structure of Na+-glucose cotransporters.