The metabolic consequences of altered glucose transporter expression in transgenic mice.

Glucose transporters are a family of membrane proteins which mediate glucose uptake across the cell membrane. The facilitative glucose transporter proteins are products of unique genes and are expressed in a tissue-specific manner. They are very similar structurally, containing 12 putative membrane spanning domains. Functionally they vary in their affinity for glucose and sensitivity to hormones such as insulin. Glucose homeostasis depends mainly on controlled changes in glucose transport in insulin-responsive tissues such as skeletal muscle and adipose cells where both glucose transporter 1 and glucose transporter 4 are expressed. Glucose transporter 4 is the major glucose transporter in these tissues and translocates from an intracellular vesicle to the cell membrane in response to insulin. Alterations of the level of expression of these glucose transporters should result in changes in insulin sensitivity and modification of whole-body metabolism. To test these hypotheses transgenic mouse models have been generated which overexpress glucose transporters in specific tissues or in the whole body. Glucose transporter 1 and glucose transporter 4 have been overexpressed specifically in skeletal muscle and glucose transporter 4 specifically in adipose tissue. Mice have also been made which overexpress glucose transporter 4 in the whole body. Using homologous recombination technology to disrupt the glucose transporter 4 gene, a "knockout" mouse has been created which expresses no glucose transporter 4. The metabolic consequences of these genetic manipulations on the level of expression of glucose transporters in the mouse are reviewed. The future applications of transgenic mouse technology in creating models which mimic human diseases are also discussed.