Direct imaging of lateral movements of AMPA receptors inside synapses (0704.3858v1)

Abstract: Trafficking of AMPA receptors in and out of synapses is crucial for synaptic plasticity. Previous studies have focused on the role of endo/exocytosis processes or that of lateral diffusion of extra-synaptic receptors. We have now directly imaged AMPAR movements inside and outside synapses of live neurons using single-molecule fluorescence microscopy. Inside individual synapses, we found immobile and mobile receptors, which display restricted diffusion. Extra-synaptic receptors display free diffusion. Receptors could also exchange between these membrane compartments through lateral diffusion. Glutamate application increased both receptor mobility inside synapses and the fraction of mobile receptors present in a juxtasynaptic region. Block of inhibitory transmission to favor excitatory synaptic activity induced a transient increase in the fraction of mobile receptors and a decrease in the proportion of juxtasynaptic receptors. Altogether, our data show that rapid exchange of receptors between a synaptic and extra-synaptic localization occurs through regulation of receptor diffusion inside synapses.

• The paper reveals a bifurcation in AMPA receptor mobility, distinguishing between restricted and dynamic receptor populations within synapses.
• The paper finds that glutamate exposure and inhibitory blockade modulate receptor diffusion by increasing mobility inside synapses while reducing juxtasynaptic accumulation.
• The paper shows that temperature manipulation impacts receptor endocytosis and immobilization, underscoring its role in regulating synaptic receptor dynamics.

AMPA Receptor Lateral Movements and Synaptic Regulation

The research presented addresses the dynamics of AMPA receptor (AMPAR) trafficking within neuronal synapses, a crucial component in understanding synaptic plasticity and neurotransmission. This paper employs single-molecule fluorescence microscopy to elucidate the behavior of GluR2-containing AMPA receptors in live neurons, shedding light on the balance between receptor stabilization and mobility across synaptic and extrasynaptic sites.

Overview and Context

AMPARs are integral for fast excitatory neurotransmission and synaptic plasticity in the central nervous system, primarily through their localization within the postsynaptic density (PSD). This research extends previous findings on receptor trafficking mechanisms, transitioning from endocytosis/exocytosis models to include lateral diffusion processes of receptors across synaptic membranes. The paper highlights the influence of ligand binding and intracellular signaling pathways on receptor mobility, providing insights into the modulation of synaptic efficacy.

Key Findings

1. Receptor Mobility in Synapses: The paper reveals a bifurcation in receptor mobility within synapses: a set of AMPARs exhibits restricted diffusion, while others remain largely immobile. Extrasynaptic receptors display greater freedom, diffusing more freely.
2. Role of Glutamate and Synaptic Activity: Receptor dynamics were notably affected by bath application of glutamate, which increased mobility inside synapses and enhanced the population of receptors in the juxtasynaptic region. Conversely, blocking inhibitory synaptic transmission with Bic/Gly resulted in a transient rise in mobile synaptic receptors and a reduced population of receptors in the juxtasynaptic region after an extended duration.
3. Temperature and Endocytosis: Experiments manipulating temperature established a correlation between receptor endocytosis and immobilization. Specifically, lowered temperatures decreased the internalization of receptors, effectively increasing their mobility post-synaptically.

Implications

From a theoretical perspective, the findings underscore the significance of lateral diffusion in the regulation of synaptic receptor populations, which could revise models of synaptic plasticity to incorporate diffusion dynamics effectually. Practically, understanding the modulation of receptor mobility by glutamate and synaptic activity can inform therapeutic strategies aimed at neurological diseases where synaptic functionality is impacted, such as Alzheimer's and other neurodegenerative disorders.

The insights from this paper also have broader implications in the field of receptor dynamics, suggesting potential parallels in the trafficking mechanisms of other receptor types, such as NMDA receptors, as lateral diffusion might play a role in their synaptic transport and localization as well.

Future Directions

This paper opens avenues for more in-depth analyses regarding the molecular components facilitating receptor diffusion and stabilization within synaptic domains. Future research may focus on identifying specific interactions between AMPARs and synaptic scaffolding proteins, such as PSD-95, or studying the role of specific molecular signals (e.g., intracellular calcium changes) in modulating receptor movement and synaptic plasticity over shorter timescales.

Furthermore, investigating the differential diffusion and localization patterns of AMPA receptor subunits could yield further insight into receptor-specific pathways in synaptic adjustment and the contributions of various subunit compositions to overall receptor dynamics.

This comprehensive paper provides an elaborate picture of AMPA receptor dynamics, reinforcing the complexity of synaptic function regulation and setting a foundation for the expansion of research into synaptic trafficking and its implications in neurophysiology.