Argonaute (Ago) proteins associate with miRNAs as well as numerous additional proteins in RNA-induced silencing complexes (RISCs) to direct translational repression of target mRNAs. We are investigating how the induction of synaptic plasticity affects this process, by studying the dynamics of RISC protein-protein interactions, how they are regulated (phosphorylation, direct binding of Ca2+, etc.) and their impact on synaptic structure and function. In a current project, we have found that the interaction between Ago2 and key RISC components including GW182 are increased by Ago2 phosphorylation in response to the induction of LTD. This mechanism leads to increased translational repression of key proteins involved in synaptic structure, and hence causes spine shrinkage (see diagram; Rajgor et al., EMBO Journal 2018).
During LTD induction, NMDAR stimulation activates signalling pathways that phosphorylate Ago2, causing increased binding to GW182 and a consequent increase in translational repression. This reduces the expression of proteins involved in regulating the actin cytoskeleton, causing a decrease in spine size. ORF=open reading frame; rib=ribosome.
We have also demonstrated that PICK1 inhibits miRNA activity by binding to Ago2 on endosomal compartments in dendrites (Antoniou et al., 2014, EMBO Reports). This interaction is disrupted by Ca2+ions, and therefore NMDAR-dependent LTD causes a dissociation of Ago2 from PICK1, increasing miRNA activity and hence translational repression (Rajgor et al., 2017, J.Biol.Chem).
We are also very interested in how these mechanisms of RISC regulation are affected by pathologies such as Alzheimer's and ischaemia.
We have recently (March 2019) secured an Alzheimer's Research UK (ARUK) grant to study the role of RISC regulation in dendritic spine loss and synaptic dysfunction in mouse models of AD.