The sliver of brain called striatum plays a key role in integrating dopaminergic inputs from the midbrain and glutamatergic inputs from the thalamus and neocortex, a job that enables a person to process motor- and reward-related behaviors properly. Malfunction of the striatum results in the loss of dopaminergic neurons and an imbalance in neurotransmission—hallmarks of Parkinson’s disease (PD), drug addiction, and schizophrenia. Cyclin-dependent kinase 5 (CDK5) is a protein that regulates postsynaptic dopamine signaling by phosphorylating the postsynaptic protein DARPP-32, and subsequently inhibiting protein kinase A (PKA). When it comes to neurodegeneration, CDK5 is better known for its suspected role in Alzheimer’s disease, but recently has been implicated in a mouse model of Parkinson’s, as well (Smith et al., 2003). In the February 9 PNAS, Karima Chergui and Per Svenningsson, working with Paul Greengard at The Rockefeller University, New York, characterize the role of CDK5 in regulating presynaptic dopaminergic and glutamatergic transmission in the striatum. They propose that CDK5 inhibitors be developed as a novel treatment for disorders characterized by dopamine deficiency.
In the past, studies about CDK5’s role in neurodegeneration concentrated mostly on Alzheimer’s disease and stroke (for example, see ARF related news story, ARF CDK story, ARF Orlando CDK story, ARF calpain story; Liu et al., 2004; Smith et al., 2004). This work made attractive the idea of using a selective CDK5 inhibitor that competes with ATP binding on the kinase. Indirubins are one such new class of inhibitors that are derived from an active ingredient in traditional Chinese medicine (Polychronopoulos et al.). However, CDK5 inhibition could be harmful, too. Just last month, Li-Huei Tsai’s lab reported that CDK5 inhibition blocks LTP induction and NMDA-evoked currents in the hippocampus. Apparently, CDK5 phosphorylates the postsynaptic density protein 95 (PSD-95), and the resulting clustering of NMDA receptors might help determine the efficacy of synaptic transmission (Morabito et al., 2004).
Focusing their attention on the interplay between dopamine and glutamate in the striatum, Chergui and Svenningsson first used in situ hybridization to determine the distribution of CDK5 mRNA in the substantia nigra and neocortex, two main striatal inputs that might modulate dopamine release. Then, electrophysiology experiments in mouse brain slices revealed that, like cocaine, the CDK5 inhibitor roscovitine increased evoked dopamine release in nigrostriatal fibers. In fact, roscovitine augmented the effects of cocaine. This suggested to the authors that the effect of CDK5 inhibitors might be both presynaptic and postsynaptic. Looking to the striatal projection neurons and the neocortex, the researchers measured NMDA and AMPA excitatory postsynaptic currents (EPSC) in brain slices in which glutamatergic fibers were excited. When they added roscovitine or butyrolactone I (another CDK5 inhibitor) to the solution, they observed increased NMDA-EPSC amplitude and no effect on the AMPA-EPSC, leading them to conclude that CDK5 regulates the striatal release of dopamine but not glutamate.
Seeing that phosphorylation levels increased after incubating brain slices with CDK5 inhibitor, the researchers then examined the regulation of phosphorylation of key proteins. They infused mouse brain slices with roscovitine and amphetamine, a dopamine re-uptake inhibitor, and then observed increased phosphorylation of the NMDA receptor subunit NR1, and DARPP-32, a postsynaptic protein enriched in the striatum. They were able to weaken this effect with a D1 receptor antagonist, by damaging dopaminergic innervation, or by knocking out the D1 receptor, and these results convinced the authors that the CDK5 inhibitors exert a presynaptic action. In an effort to separate the presynaptic and postsynaptic actions, the researchers mutated the CDK5 site of DARPP-32, a change that handicapped roscovitine’s ability to increase phosphorylation of NR1 and DARPP-32. What’s more, addition of a D1 receptor antagonist potentiated this effect. This reduction in DARPP-32 phosphorylation indicated a postsynaptic action of CDK5, while the compounding effect caused by the D1 antagonist indicated a presynaptic regulation of dopamine release, according to the authors.
The researchers conclude that CDK5 regulates dopamine release, and that its activation of dopamine D1 receptors indirectly modulates the function of NMDA receptors. Furthermore, they say that this latter presynaptic action cooperates with a postsynaptic mechanism involving DARPP-32 to modulate glutamatergic transmission in the striatum.—Erene Mina
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