Abstract
Amyloid precursor protein, the parent molecule of the neurotoxic amyloid beta protein associated with Alzheimer’s disease, can also liberate the secreted amyloid precursor protein alpha (sAPPα). This molecule facilitates long term potentiation (LTP) and spatial memory through mechanisms that are not yet fully elucidated, but is known to both modulate gene expression in a time-dependent manner, and induce local protein synthesis in isolated synapses. Protein synthesis is a requirement for the persistence of memory and LTP. Furthermore, regulation of the AMPA-subtype of glutamate receptors (AMPARs) is critical to LTP, and insertion of receptors containing the GluA1 and GluA2 subunits underlies synaptic potentiation. We aimed to investigate whether application of sAPPα (1 nM, 2 h) to cultured primary hippocampal neurons would modulate protein synthesis, including the AMPAR subunits GluA1 and GluA2.
Using fluorescent non-canonical amino acid tagging with proximity ligation assays (FUNCAT-PLA) to label newly synthesized proteins, sAPPα was shown to enhance protein synthesis in cultured hippocampal neurons. Interestingly, a significant increase was observed in the synthesis of GluA1, but not the GluA2 AMPAR subunit. Furthermore, while the expression of new GluA1 was increased in the dendrites, de novo synthesis of GluA2 was decreased. Together, these findings suggest the synthesis of GluA1-containing AMPARs. FUNCAT-PLA labelling of new proteins on the neuronal surface revealed that fewer newly made proteins were expressed on the cell surface in response to sAPPα, and the surface expression of de novo-synthesized GluA1 was unaffected. However, increased expression of pre-existing GluA1 protein, in complex with GluA2 in heteromers, was observed on the neuronal surface indicating increased trafficking of existing AMPARs in response to sAPPα.
Together, the findings suggest application of sAPPα to hippocampal neurons stimulates the synthesis of proteins including GluA1, but not GluA2. This specific increase suggests the rapid production of GluA1-containing receptors, that are not expressed on the neuronal surface by the completion of the 2 h experimental window. However, an increase in existing membrane-bound GluA1 protein is observed on the dendrites following incubation. The results provide a novel mechanism through which sAPPα enhances synaptic strength, priming the postsynaptic terminal for subsequent stimulation, and facilitating LTP.