The role of calcium-permeable AMPA receptors and arc in secreted amyloid precursor protein alpha-mediated plasticity

Abstract

The orchestrated regulation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-subtype of glutamate receptors by neuronal activity and neuromodulators is critical to the expression of both long-term potentiation (LTP) and memory. In particular, GluA1-containing, Ca2+-permeable AMPAR (CP-AMPAR) comprise a unique role in these processes due to their transient, activity-regulated expression at the synapse. Importantly, many of the mechanisms which govern these processes are negatively affected in neurodegenerative disorders such as Alzheimer’s disease, suggesting that understanding the mode of action of neuromodulatory molecules may reveal much needed novel therapeutic interventions. Secreted amyloid precursor protein-alpha (sAPPα), a metabolite of the parent amyloid precursor protein (APP) has been previously shown to enhance hippocampal LTP and facilitate memory formation. Accordingly, we hypothesised that sAPPα may act via modulation of AMPAR synthesis and cell surface expression.

Using primary hippocampal neurons grown in culture, we found that sAPPα (1 nM) differentially regulates the expression of cell surface GluA1-, GluA2-, and GluA3-containing AMPAR. Interestingly, using fluorescent non-canonical amino acid tagging with proximity ligation assay (FUNCAT-PLA), we found that short-term sAPPα treatments (1 nM, 30 min) rapidly enhanced the cell surface expression of newly synthesised extrasynaptic GluA1-, but not GluA2-containing AMPAR, while long-term treatments of sAPPα (1 nM, 120 min) increased levels of pre-existing GluA1/2-containing heteromers at the cell surface, indicating a dynamic regulation of distinct AMPARs following treatment. Moreover, using electrophysiology in area CA1 of acute hippocampal slices, we provide evidence that the expression of CP-AMPAR is important in the induction of sAPPα-enhanced LTP. Using immunocytochemistry and siRNA knockdown, we provide evidence that internalization of CP-AMPARs may be governed, at least in part by sAPPα-driven expression of the activity-regulated cytoskeletal-associated protein (Arc). Further, we show that Arc expression is not induced by the related APP metabolite sAPPβ, but is dependent on synergistic activation of N-Methyl-D-Aspartate and α7-nicotinic acetylcholine receptors, as well as downstream activation of CaMKII, MAPK, and PKG.

Together, these findings suggest that application of sAPPα to hippocampal neurons engages a cascade of mechanisms which enhance the synthesis and expression of AMPAR and Arc protein, in the regulation of synaptic strength and the expression of hippocampal LTP. These experiments expand upon our current knowledge underlying mechanisms of synaptic plasticity in hippocampal neurons.