Control of AMPA receptor trafficking during hippocampal long-term depression (LTD): the role of the p38 MAPK-MK2-LIMK1-cofilin1 signalling cascade

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Learning and memory are processes associated with long-lasting change in information flow between nerve cells in the brain (synapse transmission). Decreased information flow, known as long-term depression (LTD), is induced in the hippocampus by the activation of group I-metabotropic glutamate receptors (GpI-mGluR), which leads through a cascade of events to the loss of AMPA receptor (AMPAR) from the synapse. Therefore, it is very important to know the identity and functional role of the proteins that are involved in the cascade of events initiated by the activation of GpI-mGluR located on the surface of the nerve cell leading to the loss (internalisation) of AMPAR also situated on the surface of the nerve cell. This process involves several steps and different proteins. p38 mitogen-activated protein kinase (p38 MAPK) is known to be involved in the induction of GpI-mGluR-LTD. A kinase is an enzyme which function is to add phosphate to another protein (substrate). The added phosphate activates or inhibits the function of the substrate. During GpI-mGluR-LTD, the substrates for p38 MAPK are not yet known. The aim of this proposal is to test the hypothesis that MK2, which is a direct substrate of p38 MAPK, is the missing link between mGluR-mediated p38 MAPK activation and the internalisation of the AMPAR during mGluR-LTD using a broad range of techniques. Pilot data have already found cofilin1 as a potential target of p38 MAPK during GpI-mGluR-LTD. Furthermore, increased levels in p38 MAPK and MK2 phosphorylation was also observed in mGluR-LTD. To confirm the biological function of p38 MAPK-MK2-LIMK1-cofilin1 signalling cascade, we will use primary cultures from MK2/3 and MK2 knockout mice combined with molecular biology techniques, immunocytochemistry, confocal imaging and electrophysiological recordings. First, we will use neurons lacking MK2/3 and induce mGluR-LTD to address the internalisation of AMPAR. We will also replace the phosphorylated amino acids in the cofilin1 and LIMK1 to inactive amino-acids and determine whether the replacement affects the function of the endogenous protein. For example, if the endogenous protein is involved in the internalisation of AMPAR, will the manipulated protein block the internalisation of AMPAR. AMPAR has a very important role in neuronal function and therefore in brain function. Loss of brain function, which can be caused by several factors including normal ageing, has been associated with several neurological disorders such as Alzheimer's disease. Thus, understanding the mechanisms regulating internalisation of AMPAR will provide insight into both normal and abnormal synaptic mechanisms and may also identify novel targets to slow cognitive decline in ageing and in neurodegeneration.