A model for molecular mechanisms of synaptic competition for a finite resource

Ca2+/calmodulin-dependent protein kinase II (CaMKII) undergoes Ca2+/calmodulindependent autophosphorylation of threonine-286/287 (Thr286/287). Extremely high concentration of CaMKII in the postsynaptic spine indicates that increase in the Ca2+ concentration in the spine induced by synaptic activation results in Thr286/287 autophosphorylation of this enzyme. It has recently been shown that the Kd value of CaMKII for Ca2+/calmodulin (Ca2+/CaM) drastically decreases after Thr286/287 autophosphorylation. Therefore, Ca2+/CaM associated with CaMKII becomes tightly bound to this kinase after Thr286/287 autophosphorylation. This has been called `Ca2+/CaM trappingʹ. We discussed the functional significance of Ca2+/CaM trapping in the neuronal system by a mathematicalmodelling approach. We considered neighbouring synapses formed on the same dendrite and different increase in the Ca2+ concentration in each spine. CaMKII undergoing Thr286/287 autophosphorylation in each spine is eager to recruit nearby calmodulin in the dendrite for Ca2 +/CaM trapping. Since the amount of calmodulin is limited, recruiting calmodulin to each spine causes competition among synapses for this finite resource. The results of our computer simulation show that this competition leads to `winner-take-allʹ: almost all calmodulin is taken by a few synapses to which relatively large increases in the Ca2+ concentration are assigned. These results suggest a novel form of synaptic encoding of information.