DARPP-32 is a robust integrator of dopamine and glutamate signals

Abstract

Integration of neurotransmitter and neuromodulator signals in the striatum plays a central role in the functions and dysfunctions of the basal ganglia. DARPP-32 is a key actor of this integration in the GABAergic medium-size spiny neurons, in particular in response to dopamine and glutamate. When phosphorylated by cAMP-dependent protein kinase (PKA), DARPP-32 inhibits protein phosphatase-1 (PP1), whereas when phosphorylated by cyclin-dependent kinase 5 (CDK5) it inhibits PKA. DARPP-32 is also regulated by casein kinases and by several protein phosphatases. These complex and intricate regulations make simple predictions of DARPP-32 dynamic behaviour virtually impossible. We used detailed quantitative modelling of the regulation of DARPP-32 phosphorylation to improve our understanding of its function. The models included all the combinations of the three best-characterized phosphorylation sites of DARPP-32, their regulation by kinases and phosphatases, and the regulation of those enzymes by cAMP and Ca 2þ signals. Dynamic simulations allowed us to observe the temporal relationships between cAMP and Ca 2þ signals. We confirmed that the proposed regulation of protein phosphatase-2A (PP2A) by calcium can account for the observed decrease of Threonine 75 phosphorylation upon glutamate receptor activation. DARPP-32 is not simply a switch between PP1-inhibiting and PKA-inhibiting states. Sensitivity analysis showed that CDK5 activity is a major regulator of the response, as previously suggested. Conversely, the strength of the regulation of PP2A by PKA or by calcium had little effect on the PP1-inhibiting function of DARPP-32 in these conditions. The simulations showed that DARPP-32 is not only a robust signal integrator, but that its response also depends on the delay between cAMP and calcium signals affecting the response to the latter. This integration did not depend on the concentration of DARPP-32, while the absolute effect on PP1 varied linearly. In silico mutants showed that Ser137 phosphorylation affects the influence of the delay between dopamine and glutamate, and that constitutive phosphorylation in Ser137 transforms DARPP-32 in a quasi-irreversible switch. This work is a first attempt to better understand the complex interactions between cAMP and Ca 2þ regulation of DARPP-32. Progressive inclusion of additional components should lead to a realistic model of signalling networks underlying the function of striatal neurons.

Recruitment of FoxP2‐expressing neurons to area X varies during song development

“…In zebra finch Area X slices, dopamine can influence the excitatory inputs to spiny neurons (Ding et al, 2003) and also modulates their excitability directly (Ding and Perkel, 2002). Consistent with this, the spiny neurons express DARPP32, which is a key integrator of neurotransmitter and neuromodulator signals, particularly dopamine and glutamate and involved in several aspects of neural plasticity related to learning and memory (Haesler et al, 2004;Thomas and Huganir, 2004;Fernandez et al, 2006). Furthermore, spiny neurons in Area X show long term potentiation starting around D50 in juvenile birds which could underlie sensory-motor learning and song maintenance (Ding and Perkel, 2004).…”

Influence of Phasic and Tonic Dopamine Release on Receptor Activation

“…However, the pathways activated in postsynaptic striatal medium spiny neurons (MSNs) are complex (Fernandez et al, 2006;Surmeier et al, 2007), and the postsynaptic effect of dopamine may deviate from the prediction of Equation 12. Therefore, we found it worthwhile to consider the effect of burst firing in an alternative scenario in which the postsynaptic cascade is activated only if a critical fraction of receptors are occupied.…”

A Protein Kinase A–Independent Pathway Controlling Aquaporin 2 Trafficking as a Possible Cause for the Syndrome of Inappropriate Antidiuresis Associated with Polycystic Kidney Disease 1 Haploinsufficiency

“…contained two functional and highly conserved Ca 2+ -binding EF-hand motifs 22 modulating its activity, 23 the intracellular free calcium concentration was measured. Incubation with R-roscovitine significantly reduced the concentration of free calcium compared with unstimulated cells (R=193.767.12 nM, n=178 cells versus CTR=66.7662.06 nM, n=236 cells, P,0.001) (Figure 8), which might account for the decreased activity of the calcium-regulated PP2A.…”