Regulation and physiological role of tyrosine hydroxylase phosphorylation in the striatum

Sammanfattning: Tyrosine hydroxylase (TH) catalyses the conversion of tyrosine to DOPA and is the rate- limiting enzyme in the biosynthesis of catecholamines, a group of neurotransmitters involved in various brain functions. Short-term regulation of TH activity is achieved by phosphorylation of the protein's regulatory domain at three seryl residues in positions 19, 31 and 40. This thesis focuses on the investigation of the molecular mechanisms involved in the regulation of the state of phosphorylation and activity of TH in the striatum, a region of the brain implicated in motor control and associative learning. Dopamine is the most abundant catecholamine in the brain and plays a critical role in striatal function, as dramatically shown by the severe motor impairment caused by degeneration of cloparninergic nigrostriatal neurons, occurring in Parkinson's disease. In slices of rat striatum, activation of glutamate NMDA receptors inhibits dopamine synthesis by reducing cAMP levels and decreasing cAMP-dependent protein kinase catalysed phosphorylation of TH at Ser4O. Synthesis of striatal dopamine is controlled by dopamine D2 autoreceptors located on nigrostriatal nerve terminals. It is demonstrated that such a control specifically involves the short isoform (D2S) of the dopamine D2 receptor. In addition, evidence is provided indicating that activation of D2S receptors reduces TH activity via inhibition of cAMP production and decrease in TH phosphorylation at Ser40. This effect is likely to result in reduced dopamine synthesis, which may ultimately affect neurotransmitter release. Finally, it is shown that high K+- induced depolarization increases TH activity via stimulation of extracellular signal- activated protein kinases I and 2 (ERK1/2), two mitogen -activated protein kinases (MAPKs). This effect is accompanied by E RK 1/2-depen dent phosphorylation of TH at Ser31 and Ser4O, but not at Ser19. These results demonstrate that, in the brain, the MAPK cascade is involved in short-term regulation of presynaptic cloparninergic transmission.