Transient Calcium and Dopamine Increase PKA Activity and DARPP-32 Phosphorylation
Figure 6
Role of the PKA–PP2A–phosphoThr75 Feedback Loop on PKAc and phosphoThr34
(A) Eight high-amplitude, brief, dopamine pulses, 20 s apart, lead to a larger buildup of free PKAc (A1) or Thr34 (A2) compared with control (black) if the reaction rates are set to zero in the PKA phosphorylation of PP2A only (purple; reaction (i) in (E) opened), in the phosphoThr75 inhibition of PKA only (light blue; reaction (iii) in (E) opened), or in both reactions (orange). Thus, both reactions (i) and (iii) in the PKA–PP2A–phosphoThr75 loop work as a sink for the PKAc signal.
(B) High-amplitude, fast calcium inputs lead to a decreased level of free PKAc (B1) and Thr34 (B2) if reactions (i) and (iii) are prevented. Here the PKA–PP2A–phosphoThr75 loop behaves as a positive feedback loop on the PKAc concentration.
(C) Paired fast calcium and dopamine elevations enhance PKAc (C1) and phosphoThr34 levels (C2) when the PKA–PP2A–phosphoThr75 loop is present (black lines), but decrease PKAc and phosphoThr34 when reactions (i) and (iii) are eliminated (orange lines).
(D) Activation of PP2Ac by binding to calcium is required for the stimulatory effect of calcium on PKAc and phosphoThr34. Binding of calcium to PP2A shifts equilibrium of reaction (i) to the substrates, thus PKAc dissociates from PP2A, and shifts the equilibrium of reaction (iii) to the substrates, thus PKAc dissociates from phosphoThr75. If calcium activation of PP2A is prevented, dopamine alone (green lines) results in more PKAc (D1) and phosphoThr34 (D2) than paired calcium and dopamine (red lines). Also, calcium alone (blue lines) produces no significant response in PKAc and even causes a decrease in phosphoThr34.