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 Ca2+ signals. Dynamic simulations allowed us to observe the temporal relationships between cAMP and Ca2+ 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 Ca2+ regulation of DARPP-32. Progressive inclusion of additional components should lead to a realistic model of signalling networks underlying the function of striatal neurons.

A mathematical tool for exploring the dynamics of biological networks

“…A more detailed picture of the allowed states of phosphorylation of DARPP-32 (the ''DARPP-32 subnetwork'') is shown in Because much is understood about the composition of the DARPP-32 signaling cascade, it is a natural setting for computational studies. Recently, for example, two groups have simulated aspects of the network to study the effect of transient Ca 2ϩ and dopamine (or cAMP) signals on DARPP-32 phosphorylation (21,22). In both cases, the authors found interesting, time-dependent features of the pathway.…”

Systems Biology and Addiction

“…Phospholipase C activates Calmodulin that elevates Calcium. Also D2 receptor triggered processes converge on DARPP32 (dopamine and cAMP regulated phosphoprotein 32) out by the studies [23,44] . The computer simulation of the model shows, that a brief dopamine pulse is transformed into a saw-tooth like signal on the level of proteinkinase A (PKA;…”

MicroRNAs in Schizophrenia: Implications for Synaptic Plasticity and Dopamine–Glutamate Interaction at the Postsynaptic Density. New Avenues for Antipsychotic Treatment Under a Theranostic Perspective

“…Indeed, PSD proteins such as DARPP-32 and the Ca(2+)/calmodulin kinase type II (CaMKII) are pivotal biochemical bridges between dopamine and glutamate signaling pathways [147,148]. Moreover, molecules such as protein kinase B (Akt), glycogen synthase kinase-3β (GSK-3β), and extracellular signal-regulated kinase (Erk) are involved in both dopamine and glutamate signaling cascades at the post-receptor level and interact with PSD molecules [2,[149][150][151][152][153].…”