DARPP-32 in the striatum : multiple regulation and physiological role

Abstract

Protein phosphorylation is an important mechanism involved in intracellular signal transduction. The activity of many proteins is affected by their state of phosphorylation, which in turn is determined by the combined activities of protein kinases and protein phosphatases. DARPP-32 (dopamine and cAMP regulated phosphoprotein of 32 kDa) is a neuronal phosphoprotein expressed at particular high levels in the projection neurons of the striatum. This brain region is the major receiving component of the basal ganglia, and integrates inputs from several neurotransmitters, e.g. glutamate from the cortex and dopamine from the substantia nigra. When DARPP-32 is phosphorylated on the threonyl residue, Thr34 it is a potent inhibitor of protein phosphatase- 1 (PP- 1). Conversely, when DARPP-32 is phosphorylated on the threonyl residue, Thr75, it is an inhibitor of PKA. The DARPP-32/PP- 1 /PKA cascade is important in regulating the state of phosphorylation, and thereby the activity, of several effector proteins, e.g. Ca2+ and Na+ channels.

This study shows that, in rat striatal slices, activation of dopamine D, receptors and adenosine A 2A receptors stimulates the phosphorylation of DARPP-32 on Thr34 through activation of the cAMP/PKA pathway. Conversely, activation of dopamine D2 receptors inhibits both A 2A and D, receptor-mediated phosphorylation of DARPP-32 on Thr 34. Dopamine D 2 receptors interact with adenosine A 2A receptors at the level of the same striatal projection neurons whereas the effect of a D 2 receptor agonist on D, stimulated DARPP-32 phosphorylation appears to require cell-cell interaction. Opioid receptors are also involved in the regulationof the state of phosphorylation of DARPP32. Activation of mu-opioid receptors selectively counteracts D, receptor stimu ate DARPP-32 phosphorylation on Thr34, whereas activation of delta-opioid receptors reduces A 2A receptor stimulated phosphorylation.

In vivo, blockade of D 2 receptors increases the amount of phospho[Thr34]DARPP- 32, and this effect is prevented by concomitant blockade of D, and A 2A receptors. Thus, under normal conditions, the state of phosphorylation of DARPP-32 at Thr34 is determined by tonic activation of adenosine and dopamine receptors. In contrast, phosphorylation of DARPP-32 on Thr75 is induced by blocking the adenosine A 2A receptor using a specific A 2A receptor antagonist or the psychostimulant drug caffeine. This effect appears to require a constitutive level of CDK5 activity.

DARPP-32 is important for the behavioural effects of adenosine, since the motor stimulant effects of caffeine or a specific adenosine A 2A antagonist are significantly reduced in mice lacking the gene coding for DARPP-32. The motor depressant effect of an adenosine A 2A receptor agonist is also markedly reduced in DARPP-32 knock-out mice.

Taken together these results show that under normal conditions, the phosphorylation, and activity, of DARPP-32 is regulated through the interaction of dopamine and adenosine and that DARPP-32 is implicated in the modulation of voluntary movements exerted by adenosine agonists and antagonists.