Linking Nanoscale Dynamics of AMPA Receptor Organization to Plasticity of Excitatory Synapses and Learning

Abstract

The spatiotemporal organization of neurotransmitter receptors in the postsynaptic membrane is a fundamental determinant of synaptic transmission and thus of information processing by the brain. The ionotropic AMPA subtype of glutamate receptors (AMPARs) mediate fast excitatory synaptic transmission in the CNS. The number of AMPARs located en face presynaptic glutamate release sites sets the efficacy of synaptic transmission. Understanding how this number is set and regulated has been the topic of intense research in the last two decades. We showed that AMPARs are not stable in the synapse as initially thought. They continuously enter and exit the postsynaptic density by lateral diffusion, and they exchange between the neuronal surface and intracellular compartments by endocytosis and exocytosis at extrasynaptic sites. Regulation of these various trafficking pathways has emerged as a key mechanism for activity-dependent plasticity of synaptic transmission, a process important for learning and memory. I here present my view of these findings. In particular, the advent of super-resolution microscopy and single-molecule tracking has helped to uncover the intricacy of AMPARs' dynamic organization at the nanoscale. In addition, AMPAR surface diffusion is highly regulated by a variety of factors, including neuronal activity, stress hormones, and neurodegeneration, suggesting that AMPAR diffusion-trapping may play a central role in synapse function. Using innovative tools to understand further the link between receptor dynamics and synapse plasticity is now unveiling new molecular mechanisms of learning. Modifying AMPAR dynamics may emerge as a new target to correct synapse dysfunction in the diseased brain.

Keywords: AMPA receptors; Long Term Potentiation; Synaptic plasticity; neurodegenerative diseases; receptor trafficking; super resolution imaging.

Figure 1.

Single-molecule tracking of AMPARs in the plasma membrane. A, Because of diffraction, photons emitted by a single dye distribute according to a point-spread function that can be approximated by a Gaussian with FWHM approximately half the wavelength of the emitted light (i.e., 250–300 nm). This Gaussian can be fitted to localize the individual molecules with an SE (σ) (uncertainty) equal S divided by N^0.5 where S is the SD of the Gaussian and N is the number of photons captured from the fluorescent molecule. B, individual AMPARs can be tracked with 20–50 nm accuracy by detecting over time single fluorophores bound to the receptors. Fluorophores can be either fluorescent proteins genetically fused to the receptors or organic dyes covalently bound to ligands of the receptors. These are usually full or the antigen-binding Fab fragments of antibodies. Combining small monovalent ligands and organic dyes provides the highest accuracy and allows tracking endogenous receptors. Using this approach, the stochastic nature of receptor movement in the plane of the membrane as well as their continuous alternation between stabilized and diffusive states are revealed. C, Tracks of GluA2-containing AMPAR movements at the surface of a dendritic spine of a cultured rat hippocampal neuron. D, Distribution of AMPAR diffusion coefficients in spines. Approximately 30 to 50 % of the receptors are mobile with a D > 0.01 μm²/s.

Figure 2.

Model of AMPAR trafficking in and out the postsynaptic density. (1) Newly synthetized receptors are transported intracellularly in vesicles by molecular motors on microtubules. (2) Vesicles are exocytosed, largely in the dendritic shaft. (3) Once at the cell surface, receptors move randomly by Brownian diffusion and (4) can be reversibly stabilized by diffusion trapping at the PSD through interaction with scaffold proteins. (5) Diffusing receptors are internalized at extrasynaptic endocytic zones by clathrin-dependent endocytosis. (6) Endocytosed receptors can be recycled back by exocytosis.

Figure 3.

AMPAR trafficking during the first steps of LTP and LTD. The molecular mechanisms of synaptic plasticity have been best worked out at the synapses between Schaeffer collaterals and CA1 pyramidal cells in the hippocampus (top). Stimulation (Stim.) of Schaeffer collaterals induces rapid plasticity of synaptic transmission that can endure for hours to days (bottom left). High-frequency stimulation (top schemes) induces within seconds translocation of CaMKII (pink) to spines, and subsequent phosphorylation of the γ2 and γ8 AMPAR auxiliary subunits. This leads to their increased binding to PSD95 resulting in accumulation of AMPARs at the PSD through a process of diffusion-trapping (1). In parallel, intracellular receptors, whether newly synthetized or from recycling compartments, are exocytosed within minutes of the stimulation, largely in the dendritic shaft, but also in the spine (2). These exocytosed receptors then replenish the extrasynaptic pool and further diffuse to the synapse. Conversely, low-frequency stimulation (bottom schemes) induces AMPAR escape from the PSD (1). These escaped receptors then diffuse and get trapped at endocytic zones in the spine or in the dendritic shaft where they enter the endocytic pathway (2). These first steps lead to LTP and LTD, respectively.