Phosphorylation serotonin receptors

The majority of the studies examining the phosphorylation of serotonin receptors have emphasized its role on receptor desensitization. A general mechanism of G protein-coupled receptor desensitization involves phosphorylation of the intracellular domains of the receptor by second-messenger kinases such as protein kinase A (PKA) or C (PKC) and specific G protein-coupled receptor kinases, that lead to the binding of arrestins to the receptor and G protein uncoupling (149,150). The mechanisms have been best elucidated for the P-adreneigic receptors, where desensitization is mediated by both PARK and PKA (151). However, mechanisms underlying desensitization of serotonin... 

Regarding posttranslational modifications, palmitoylation, glycosylation, and phosphorylation have been found in 5-HT receptors each of these modifications has distinct roles in regulation of receptor functions. It is well known that phosphorylation, the modification of the serotonin receptors that has been studied most widely, plays an important role in receptor desensitization. 

Key Words Guanosine-5 -0-(3-thio)triphosphate phosphorylation serotonin (5-hydroxytryptamine) G protein kinase receptor calmodulin phospholipase. 

Activation of serotonin receptors in canine femoral VSMC evoked tyrosine phosphorylation of a group of substrates similar to those that were tyrosine phosphorylated during stimulation of a -adrenergic receptors with phenylephrine (Fig. 8A). Preincubation of the cells with 110 xM genistein suppressed tyrosine phosphorylation that was evoked by stimulation of either serotonin or adrenergic receptors (Fig. 8A, lanes 4 and 5). The same concentration of genistein that inhibited receptor-activated increases in [Ca +Jj (Fig. 5) also inhibited receptor-activated increases in protein tyrosine phosphorylation. In contrast, preincubation of the VSMC with 110 xM diadzein, a structural analog... 

FIGURE 9 Tyrosine phosphorylation of rasGAP occurs before the transient increase in [Ca +Jj and declines before Ca + declines during activation of a,-adrenergic receptors with phenylephrine. All experiments were performed with canine femoral VSMC. Each hatched bar represents the mean values for Ca + determination in 64 cells, whereas the open bar represents mean values for tyrosine phosphorylation of rasGAP in four experiments. Maximal phosphorylation as determined by densitometric analysis was taken to be 100% and all other points were expressed as percentage of maximum. Similar results were obtained during the transient increase in [Ca +li evoked by stimulation of serotonin receptors (not shown). 

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

Backstrom JR, et al. Deletion of the serotonin 5-HT2C receptor PDZ recognition motif prevents receptor phosphorylation and delays resensitization of receptor responses. J Biol Chem 2000 275(31) 23,620-23,626. 

Westphal RS, Backstrom JR, Sanders-Bush E. Increased basal phosphorylation of the constitutively active serotonin 2C receptor accompanies agonist-mediated desensitization. Mol Pharmacol 1995 48(2) 200-205. 

Turner JH, Gelasco AK, Raymond JR. Calmodulin interacts with the third intracellular loop of the serotonin 5-hydroxytryptamine1A receptor at two distinct sites putative role in receptor phosphorylation by protein kinase C. J Biol Chem 2004 279 17,027-17,037. 

Parker LL, Backstrom JR, Sanders-Bush E, Shieh BH. Agonist-induced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1. J Biol Chem 2003 278 21,576-21,583. 

Synaptic transmission between pairs of neurons in Aplysia (a marine snail) is enhanced by serotonin, a neurotransmitter that is released by adjacent intemeurons. Serotonin binds to a 7TM receptor to trigger an adenylate cyclase cascade. The rise in cAMP level activates PKA, which facilitates the closing of potassium channels by phosphorylating them. Closure of potassium channels increases the excitability of the target cell. 

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