The CART receptors: Background and recent advances

Previous evidence obtained from several behavioral and biochemical studies suggested the existence of multiple CART receptors. However, identification of CART receptor binding has been largely unsuccessful until recently. The first evidence of CART signaling properties came from a study demonstrating that CART 55–102 inhibited voltage-dependent intracellular calcium signaling. More recent studies showed CART-induced dose- and time-dependent activation of extracellular signal-regulated kinase (ERK) 1 and 2 in AtT20 cell line. The activation of ERK was blocked by pertussis toxin but not genisten suggesting the involvement of Gi/o linked cascade in CART's signaling properties in AtT20 cells. Shortly after these findings, the evidence of CART 61–102 specific binding was obtained from the same cell line. This study demonstrated that [125I]-CART 61–102 was displaced only by active CART peptide but not by inactive CART fragments or several other unrelated peptides or drugs. The [125I]-CART 61–102 binding was saturable and it had a high affinity for a single site in AtT20 cells. The binding was also dependent on time, pH, temperature and protein concentration. The average (±S.E.M.) Bmax and Kd values were 101.4 ± 8.8 fmol/mg protein and 21.9 ± 8.0 pM, respectively. These data indicate the existence of specific CART receptor binding in AtT20 cells where CART signaling has been demonstrated. The identification of a receptor clone in these cells may help us elucidate CART receptors in other tissues. Because CART is implicated with several physiological functions including feeding, drug reward and stress, identification of a CART receptor would provide a novel target for the development of pharmacological tools and drugs for obesity and other disorders.