Functional insights into the creatine transporter.

Creatine and phosphocreatine provide an intracellular, high-energy phosphate buffering system, essential to maintain ATP levels in tissues with high energy demands. A specific plasma membrane creatine transporter (CRT) is required for the cellular uptake of creatine. This transporter is related to the gamma-aminobutyric acid (GAT) and norepinephrine (NET) transporters and is part of a large gene family of Na(+) - and Cl(-) -dependent neurotransmitter transporters, now known as solute carrier family 6 (SLC6). CRT is essential for normal brain function as mutations in the CRT gene (SLC6A8) result in X-linked mental retardation, associated with the almost complete lack of creatine in the brain, severe speech and language delay, epilepsy, and autistic behaviour. Insight into the structure and function of the CRT has come from studies of creatine transport by tissues and cells, in vitro studies of CRT mutations, identification of mutations associated with CRT deficiency, and from the recent high resolution structure of a prokaryotic homologue of the SLC6 transporters. CRT antibodies have been developed enabling the localization of creatine uptake sites in the brain, retina, muscle and other tissues. These tools in conjunction with the use of appropriate cell models should allow further progress in our knowledge on the regulation and cellular trafficking of the CRT. Development of suitable mouse models may allow improved understanding of the importance of the CRT for normal brain function and how the transporter is regulated in vivo.