β Ca2+/CaM-dependent kinase type II triggers upregulation of GluA1 to coordinate adaptation to synaptic inactivity in hippocampal neurons

Prolonged AMPA-receptor blockade in hippocampal neuron cultures leads to both an increased expression of GluA1 postsynaptically and an increase in vesicle pool size and turnover rate presynaptically, adaptive changes that extend beyond simple synaptic scaling. As a molecular correlate, expression of the beta Ca2+/CaM-dependent kinase type II (beta CaMKII) is increased in response to synaptic inactivity. Here we set out to clarify the role of beta CaMKII in the various manifestations of adaptation. Knockdown of beta CaMKII by lentiviral-mediated expression of shRNA prevented the synaptic inactivity-induced increase in GluA1, as did treatment with the CaM kinase inhibitor KN-93, but not the inactive analog KN-92. These results demonstrate that, spurred by AMPA-receptor blockade, up-regulation of beta CaMKII promotes increased GluA1 expression. Indeed, transfection of beta CaMKII, but not a kinase-dead mutant, increased GluA1 expression on dendrites and elevated vesicle turnover (Syt-Ab uptake), mimicking the effect of synaptic inactivity on both sides of the synapse. In cells with elevated beta CaMKII, relief of synaptic-activity blockade uncovered an increase in the frequency of miniature excitatory postsynaptic currents that could be rapidly and fully suppressed by PhTx blockade of GluA1 receptors. This increased mini frequency involved a genuine presynaptic enhancement, not merely an increased abundance of synapses. This finding suggests that Ca2+ flux through GluA1 receptors may trigger the acute release of a retrograde messenger. Taken together, our results indicate that synaptic inactivity-induced increases in beta CaMKII expression set in motion a series of events that culminate in coordinated pre- and postsynaptic adaptations in synaptic transmission.