Abstract
Changes in glutamatergic synaptic strength in brain are dependent on AMPA-type glutamate receptor (AMPAR) recycling, which is assumed to occur through a single local pathway. In this study, we present evidence that AMPAR recycling occurs through different pathways regulated by synaptic activity. Without synaptic stimulation, most AMPARs recycled in dynamin-independent endosomes containing the GTPase, Arf6. Few AMPARs recycled in dynamin-dependent endosomes labeled by transferrin receptors (TfRs). AMPAR recycling was blocked by alterations in the GTPase, TC10, which co-localized with Arf6 endosomes. TC10 mutants that reduced AMPAR recycling had no effect on increased AMPAR levels with long-term potentiation (LTP) and little effect on decreased AMPAR levels with long-term depression. However, internalized AMPAR levels in TfR-containing recycling endosomes increased after LTP, indicating increased AMPAR recycling through the dynamin-dependent pathway with synaptic plasticity. LTP-induced AMPAR endocytosis is inconsistent with local recycling as a source of increased surface receptors, suggesting AMPARs are trafficked from other sites.
DOI:http://dx.doi.org/10.7554/eLife.06878.001
Cells called neurons transmit information around the brain in the form of electrical signals. At a junction between two neurons—called a synapse—an electrical signal triggers the release of small molecules called neurotransmitters. These molecules travel across the gap between the two neurons and trigger a new electrical signal in the second neuron. Memories can be stored in synapses: high levels of activity can ‘strengthen’ the synapse, which increases the transfer of information between the neurons.
In many synapses, a molecule called glutamate is the neurotransmitter. Proteins called AMPARs, which are found on the surface of the neuron, can detect glutamate and transmit the signal along the second neuron. The strength of synapses is controlled by changes in AMPAR levels through ‘recycling’, where AMPAR proteins are removed from synapses, internalized and later returned to synapses.
It was thought that AMPARs are recycled via just one pathway at synapses. However, the amount of recycling is much higher when the synapses are active and it is not clear how this works. Now, Zheng et al. have used fluorescent tags to track the recycling of AMPARs in synapses from rats under a microscope. The experiments show that when the synapses are not active, most AMPARs are recycled via a pathway marked by a protein called Arf6. However, when the synapses are active, most AMPAR is recycled via a different route marked by so-called ‘transferrin receptor’ proteins.
The experiments also reveal that a protein called TC10 is involved in recycling AMPARs alongside Arf6, but is not required for recycling when the synapses are active and being strengthened. Unexpectedly, AMPAR internalization—via the process involving transferrin receptors—increases during synapse strengthening. This suggests that some of the extra AMPAR proteins sent to the membrane have come from other parts of the neuron away from the synapse.
Zheng et al.'s findings provide evidence that AMPARs are recycled through different routes depending on the activity of the synapse. The next challenge will be to directly test whether AMPARs are transported from other parts of the neuron to the strengthened synapse and to understand how this works.
DOI:http://dx.doi.org/10.7554/eLife.06878.002
Collapse