4 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 transmission, which has been implicated in the pathogenesis of schizophrenia.
Glutamate is the major excitatory neurotransmitter in the brain, and it has been proposed that disruption in glutamate signaling may underlie many of the symptoms of schizophrenia, including the so-called positive (delusions and hallucinations), negative (such as apathy or flattened affect), and cognitive symptoms (see related SRF current hypothesis penned by Bita Moghaddam). Much of the evidence for this hypothesis centers on the NMDA-type glutamate receptor. NMDA antagonists mimic most symptoms of the illness, for example, and postmortem analysis of brain tissue reveals that not all is well with NMDA receptors in schizophrenia patients—some NMDA receptor subunits are elevated, while levels of NMDA partner proteins, such as postsynaptic density 95 (PSD95), are decreased (see Clinton et al., 2004). But the glutamate role may be more complex because there are hints that AMPA receptors (AMPARs) also contribute to schizophrenia symptoms, both independently or via effects on NMDARs. In patients, levels of some AMPAR subunits are lower in the hippocampus (see Eastwood et al., 1997), an area of the brain involved in memory and cognition, which are both compromised in the illness. Studies also suggest that ampakines, or compounds that positively modulate AMPARs, can improve memory in humans (see Ingvar et al., 1997), and such drugs are being tested in clinical trials to see if they can improve the cognitive symptoms associated with schizophrenia. The development of such therapies might benefit from a better understanding of the dynamics of glutamate receptor trafficking.
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, 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. Whether that might help with schizophrenia or not remains to be seen, but it has been demonstrated that mood-altering drugs can modulate AMPAR trafficking (see SRF related news story).—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.
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