Abstract
The cytomatrix at the active zone (CAZ) is a macromolecular complex that facilitates the supply of release-ready synaptic vesicles to support neurotransmitter release at synapses. To reveal the dynamics of this supply process in living synapses, we used super-resolution imaging to track single vesicles at voltage-clamped presynaptic terminals of retinal bipolar neurons, whose CAZ contains a specialized structure—the synaptic ribbon—that supports both fast, transient and slow, sustained modes of transmission. We find that the synaptic ribbon serves a dual function as a conduit for diffusion of synaptic vesicles and a platform for vesicles to fuse distal to the plasma membrane itself, via compound fusion. The combination of these functions allows the ribbon-type CAZ to achieve the continuous transmitter release required by synapses of neurons that carry tonic, graded visual signals in the retina.
DOI:http://dx.doi.org/10.7554/eLife.13245.001
Neurons communicate with one another through junctions known as synapses. When a neuron is activated, it triggers the release of chemicals called neurotransmitters at the synapse, which bind to and activate neighbouring neurons. Neurons involved in vision, sound and balance contain “ribbon” synapses, which are able to release neurotransmitters steadily over longer periods of time than other types of synapse.
Neurotransmitters inside neurons are packaged into small structures called vesicles, which can then fuse with the cell’s surface membrane to release the neurotransmitters from the cell. Unlike other types of synapse, ribbon synapses are able to release these vesicles in a continuous fashion. How vesicles move around at the synapses remains poorly understood because monitoring the vesicles in living cells is technically difficult and previous studies were limited to tracking vesicles in a small part of the synapse. Now, Vaithianathan et al. overcome these technical hurdles to follow the movement of vesicles across whole ribbon synapses in zebrafish eyes.
The experiments use fluorescent proteins to track the movement of the vesicles under a microscope. Vaithianathan et al. find that vesicles at ribbon synapses move very little when the neurons are not active. However, when the neurons are activated, the vesicles that are near the cell membrane fuse with it and release their neurotransmitters. Other vesicles that are further away from the membrane then move to fill in the spaces vacated by the fusing vesicles.
Further experiments show that some of the vesicles that are further away from the membrane can fuse with vesicles that have already released their neurotransmitter but remain in place at the membrane. This process – known as compound fusion – allows neurotransmitters to be released over a longer period of time by providing a path for vesicles to release neurotransmitters without having to approach the membrane first. The next challenge is to develop a computational model using the data from this study to better understand how ribbon synapses work.
DOI:http://dx.doi.org/10.7554/eLife.13245.002
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