Intracellular signaling through biochemical pathways allows cells to integrate and process information. The interactions between the components of the signaling pathways can give non-intuitive responses to inputs. In spiny projection neurons of the striatum, the cAMP dependent protein kinase (PKA) cascade is modulated by the phosphoprotein DARPP-32, which can either potentiate or attenuate the effect of PKA, depending on the site of phosphorylation. We have made a kinetic model of the bimolecular and enzyme reactions of the PKA cascade. Rate constants for the specific reactions are taken from experimental data. The model includes DARPP-32 phosphorylation at threonine (Thr) 34 and 75 by PKA and cdk5, respectively, and dephosphorylated by protein phosphatase (PP) 2B and 2A, respectively. The upstream regulation of these enzymes includes the G-protein cycle, Ca2+ influx, activation of Ca2+/calmodulin, cAMP formation and cdk5 activation. PhosphoThr34-DARPP-32 inhibits PP1, whereas PhosphoThr75-DARPP-32 inhibits PKA. To evaluate the role of DARPP-32 in the PKA cascade, we measure the phosphorylation of a hypothetical substrate that is phosphorylated by PKA and dephosphorylated by PP1.
This model shows that DARPP-32 slows down the activity of the PKA cascade, making the increase of phosphorylation of the substrate following cAMP stimulation longe lasting compared to a cascade where DARPP-32 is omitted. These results fit well with experimental data and have important implications for temporal summation of inputs.