7 March 2007. Synapses, those miniscule gaps across which neurons signal one another, are far from static. Molecules are constantly moving in and out of both pre- and postsynaptic membranes in a process called trafficking. So how do neurons ensure that essential neurotransmitter receptors are retained in the synapse? A paper in the March 1 Neuron shows that in the case of glutamatergic synapses, a protein called Stargazin seems to exert some pull. Daniel Choquet and colleagues of the University of Bordeaux, France, report that Stargazin regulates trafficking of AMPA-type glutamate receptors, trapping and stabilizing them at the postsynaptic density, exactly where neurotransmitters are received. The finding adds to our basic understanding of glutamatergic receptor dynamics, and may be particularly relevant for Alzheimer disease, which is characterized by a gradual loss of synaptic contacts.

Glutamate is the major excitatory neurotransmitter in the brain. It transmits signals by binding to post-synaptic receptors, including the NMDA- and AMPA-type receptors that are implicated in the toxicity of Amyloidβ (Aβ). Aβ promotes an influx of calcium through NMDA receptors that compromises synaptic integrity (see ARF related news story) and it can induce ablation of AMPA receptors at the cell surface, leading to synaptic depression (see ARF related news story). Understanding receptor biology could help scientists find new treatments for AD and other neurologic disorders.

AMPAR trafficking in and out of the postsynaptic density is a dynamic process. Receptor complexes must be synthesized and delivered to the correct location in the cell membrane, and this must be balanced against receptor uptake, or endocytosis, and simple diffusion of the receptors along the surface of the cell membrane and out of the postsynaptic space. It was this simple diffusion process that Choquet and colleagues set out to study.

To image receptor diffusion, first author Cecile Bats and colleagues used highly fluorescent nanoparticles called quantum dots. Because quantum dots have extremely sharp light emission characteristics, several can be used simultaneously to track different molecules without the risk of interference. In addition, quantum dots have the added advantage that they do not bleach when irradiated—this means that a single dot can be tracked over a long period of time, a huge plus when studying molecular trafficking.

Bats and colleagues coupled dots to antibodies that recognize various proteins thought to influence AMPA receptor trafficking and monitored their motion. They found that diffusion of AMPA receptors was dramatically reduced when they came into contact with PSD95, a major postsynaptic protein. Next they looked at the role of Stargazin, so called because a mutation in the protein can cause a form of epilepsy in mice that makes the animals freeze. Stargazin is needed for clustering of AMPA receptors (see Chen et al., 2000), and it also binds to PSD95, another molecule that has been linked to Aβ toxicity (see ARF related news story), but whether it regulates diffusion of AMPARs was unclear. Bats and colleagues found that if the PDZ domain of Stargazin is mutated, then AMPA receptor diffusion was dramatically increased—the PDZ domain is how Stargazin interacts with many proteins, including PSD95. To test if the latter is also important for regulating AMPAR diffusion, the researchers used a mutated form of Stargazin that only reacts with PSD95. That Stargazin retarded the diffusion of AMPA receptors, indicating that Stargazin, PSD95, and the receptor complex form a unit that is restricted from moving out of the postsynaptic space.

The findings are “consistent with the idea that Stargazin is a constitutive AMPAR auxiliary subunit that binds AMPARs early in the synthetic pathway and is required for AMPAR trafficking to the surface,” write the authors. Interestingly, modulation of Stargazin activity by phosphorylation was implicated in both long-term potentiation and long-term depression, which regulate synaptic strength and are essential for remodeling synaptic circuits involved in learning and memory (see Tomita et al., 2005), suggesting that Stargazin, or upstream signals, might be potential drug targets.—Tom Fagan.

Editor’s note: To read more about the complex, and confusing, mechanisms that control AMPARs at the synaptic membrane, see Ziff, 2007, in the same issue of Neuron.

References:
Bats C, Groc L, Choquet D. The interaction between Stargazin and PSD-95 regulates AMPA receptor surface trafficking. Neuron. 2007 Mar 1;53:719-734. Abstract

Ziff EB. TARPs and the AMPA receptor trafficking paradox. Neuron. 2007 Mar 1;53:627-633. Abstract