UCSF

Abstract

The speed and reliability of computation in neural circuits depends on fast chemical transmission between neurons in the brain. Excitatory transmission in the central nervous relies on the activity-dependent release of glutamate onto ionotropic receptors clustered at the postsynaptic membrane. AMPA receptors mediate the majority of synaptic communication in the mature central nervous system, but NMDA receptors play a critical role under conditions of strong or correlated activity. The precise mechanisms governing the number and subtype of AMPA receptor at synapses are essential for the proper regulation of synaptic strength. Recent work has identified many neuronal proteins involved in the trafficking and scaffolding of AMPA receptors at synaptic sites and has begun to clarify how these proteins interact to support excitatory synaptic transmission, though several fundamental questions remain.

To assess directly the trafficking of AMPA receptors in real time, we used a novel, photo-activatable, irreversible AMPA receptor antagonist. Inactivation of surface receptors reveals that AMPA receptors cycle rapidly between an intracellular domain and the extrasynaptic somatic membrane. The total surface pool at synaptic sites is exchanged with intracellular receptors every 18 hours. Following insertion to the extrasynaptic domain, AMPA receptors can also move laterally within the plasma membrane where they can ultimately stabilize at postsynaptic densities.

Trafficking of AMPA receptors to the neural surface requires isoforms of the protein stargazin. Stargazin controls AMPA receptor trafficking by an interaction involving its intracellular c-terminus. Surprisingly, stargazin also modulates AMPA receptor gating through its first extracellular domain, which acts to stabilize the open state of the AMPA receptor channel. Overexpression of a stargazin with a mutated first extracellular domain reduces charge movement during transmission by accelerating the decay of postsynaptic quantal events. This suggests that stargazin acts as an AMPA receptor auxiliary subunit and is necessary for the normal function of AMPA receptors during ongoing synaptic transmission.

Stargazin recruits AMPA receptors to synapses by its interaction with the postsynaptic scaffolding protein, PSD-95. Through an additional interaction, PSD-95 binds to the transmembrane protein ADAM22, which forms a receptor/ligand complex with the secreted factor Lgi1. Binding of Lgi1 to ADAM22 results in an increase in AMPA-receptor mediated synaptic transmission. This newly identified signaling complex may represent an alternate pathway of AMPA receptor recruitment to synapses.

Long-term potentiation (LTP) at the CA3 to CA1 synapse is thought to rely on the rapid insertion of AMPA receptors into the synaptic membrane following strong NMDA receptor activation. However, LTP does not involve the insertion of calcium permeable AMPA receptors that lack the GluR2 subunit, nor does LTP require ongoing activity to be maintained.

LTP-like processes are believed to play a major role in wiring neural circuitry during development by recruiting AMPA receptors to nascent synapses allowing them to sense glutamate. Although LTP is critically dependent on activation of the NMDA receptor, deletion of the essential NMDA subunit, NR1, in pyramidal neurons results in a net increase in AMPA-receptor mediated transmission. When the deletion occurs embryonically, synapses form in normal number but exhibit increased quantal strength. When NR1 is deleted postnatally in oganotypic culture, synapses exhibit normal quantal strength, but increase dramatically in number. Reintroduction of NR1 to the knockout results in a net loss of synapse number. Thus NMDA receptor activity limits rather than promotes synaptic maturation during brain ontogeny.