Activity drives local CaMKII synthesis and subcellular localization via autophosphorylation-dependent pathways
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Strong repeated stimulation of a neuron triggers an increase in local protein synthesis along with a change in localization of proteins, allowing the neuron to rapidly adjust its proteome in response to activity. One of these proteins, calcium/calmodulin-dependent protein kinase II (CaMKII), is involved in mediating structural and functional changes after activity. While postsynaptic CaMKII translation has been studied, very little is known about presynaptic synthesis, including its mechanisms and the functional consequences. We utilized the Drosophila larval neuromuscular junction (NMJ) as a model to study the molecular requirements for activity-dependent synthesis along with the localization of CaMKII protein. Presynaptic-specific tagging of endogenous CaMKII demonstrates that spaced stimulation rapidly increases presynaptic CaMKII through local translation of pre-existing mRNA, independently of somatic factors. This activity-dependent synthesis requires the distal 3′ untranslated region (3′UTR) of the CaMKII mRNA, which is also necessary for steady-state synaptic accumulation. Additionally, we show that activity-dependent synthesis requires CaMKII T287 autophosphorylation-induced activation of the PI3K/Akt/mTor pathway. While activity-dependent redistribution of CaMKII has been previously identified, very little is known about how local synthesis and translocation may interact to define different pools of protein that may have distinct functions. We demonstrate that CaMKII with a T287D phosphomimetic mutant localizes to the synaptic membrane and pulse-chase experiments show that the localization of newly-synthesized CaMKII differs from pre-existing CaMKII, indicating neuronal activity generates spatially distinct, and likely functionally distinct, CaMKII populations which may underlie long-lasting plasticity.
Significance Statement
Sustained synaptic changes after periods of neuronal activity require precise regulation of local protein synthesis. However, the mechanisms that control presynaptic synthesis are poorly understood. We demonstrate that spaced stimulation induces 3’UTR-dependent local translation of CaMKII, a key regulator of learning and memory. Additionally, synthesis of CaMKII requires phosphorylation of CaMKII itself, and activation of the PI3K/Akt/mTor pathway. Newly-synthesized and pre-existing pools of CaMKII occupy distinct regions of the bouton, suggesting that new and old CaMKII pools may have specialized functions. These findings illustrate how neuronal activity induces local protein signaling to adjust both the quantity and spatial distribution of CaMKII protein, which has lasting effects on synaptic structure and function.
