Imaging of AMPA-type glutamate receptors in hippocampal neurons

Synaptic plasticity is the ability of the connections between nerve cells, called synapses, to change in strength. The presence of a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors (AMPAR) at the synapse controls the strength of excitatory transmission and their number can be modulated. Controlled surface insertion of AMPA-type glutamate receptors and their lateral diffusion into synapses are proposed to be major mechanisms for the regulation of synaptic plasticity. Here we applied the recently established Acyl-Carrier-Protein-labeling technique in combination with lentiviral expression in hippocampal neurons to label individually ACP-tagged AMPA receptor subunits at the surface of neurons. This technique uses a carrier protein from the fatty acid synthesis and a mutant therof as a tag, which were fused by genetic engineering to the AMPAR subunits. The labeling procedure is specific for proteins, AMPARs in our case, which are present at the plasma membrane of the cell. We show that this technique allows to differentially label two subunits of the receptor. Moreover, these subunits are integrated into heteromeric receptors together with endogenous subunits, and these labeled receptors are targeted to functional synapses. The diffusion coefficient of labeled GluR2 at the surface of living neurons is significantly higher in GluR2/GluR3-infected neurons compared to GluR1/GluR2-infected neurons suggesting a higher mobility of GluR2/3 receptors compared to GluR1/2 receptors. Finally, in sequential labeling experiments we detected newly inserted receptor subunits colocalized with presynaptic markers. The GluR1 subunit displayed a faster kinetics of insertion compare to the other subunits. Internalized GluR2 receptors have been shown to accumulate in perinuclear late endosomes in HEK 293T cells. Moreover, we expressed and labeled surface tagged-GluR2 receptors in hippocampal slices, using similar labeling technique as ACP. High-resolution images, sub-difraction limited, of hippocampal neurons stained for GluR2 were acquired with an experimental scanning near-field optical microscopy (SNOM). Together our results strongly indicate that receptor insertion is faster for GluR1 and that insertion occurs at or in close vicinity of synapses in mature hippocampal neuron cultures.