Phosphoregulation of Argonaute function in synaptic plasticity and memory

Regulation of local protein synthesis is a critical aspect of synaptic plasticity and memory formation. Dendritic protein synthesis can be rapidly downregulated by microRNAs in conjunction with the RNA-induced silencing complex (RISC), of which Argonaute-2 (Ago2) is the core component. We previously demonstrated that NMDAR stimulation in vitro induces Ago2 phosphorylation at S387, which downregulates Limk1 expression via miR-134 as a result of increased binding of phosphorylated Ago2 to Limk1 mRNA. To explore the role of Ago2 S387 phosphorylation in vivo, we generated an Ago2S387A/S387A (phospho-null) transgenic mouse line. Western blotting indicates that Limk1 is upregulated in Ago2S387A/S387A mice compared to WTs. Furthermore, other key regulators of spine morphology or synaptic function are affected, suggesting that Ago2 S387 phosphorylation regulates the synthesis of a specific population of genes involved in synaptic function and/or plasticity. Using Golgi-Cox staining, we find that dendritic spines on CA1 pyramidal neurons are smaller in Ago2S387A/S387A mice compared to WTs. Field EPSPs recorded after inducing long-term depression in the hippocampal CA1 region and perirhinal cortex of acute brain slices suggest that blocking S387 phosphorylation confers brain region-specific effects on plasticity. Running mice through a series of behavioural tasks to assess novel object recognition memory and object location memory yielded behavioural phenotypes consistent with the electrophysiology results, indicating hippocampal-specific memory and plasticity deficits in Ago2S387A/S387A mice. Together, these results suggest that Ago2 S387 phosphorylation is a critical signalling event in modulating the synthesis of a specific population of synaptic proteins required for hippocampal memory formation.