Traffic jams during vesicle cycling lead to synaptic depression

Endocytosis sustains release at photoreceptor ribbon synapses by restoring fusion competence

Endocytosis is an essential process at sites of synaptic release. Not only are synaptic vesicles recycled by endocytosis, but the removal of proteins and lipids by endocytosis is needed to restore release site function at active zones after vesicle fusion. Synaptic exocytosis from vertebrate photoreceptors involves synaptic ribbons that serve to cluster vesicles near the presynaptic membrane. In this study, we hypothesize that this clustering increases the likelihood that exocytosis at one ribbon release site may disrupt release at an adjacent site and therefore that endocytosis may be particularly important for restoring release site competence at photoreceptor ribbon synapses. To test this, we combined optical and electrophysiological techniques in salamander rods. Pharmacological inhibition of dynamin-dependent endocytosis rapidly inhibits release from synaptic ribbons and slows recovery of ribbon-mediated release from paired pulse synaptic depression. Inhibiting endocytosis impairs the ability of second-order horizontal cells to follow rod light responses at frequencies as low as 2 Hz. Inhibition of endocytosis also increases lateral membrane mobility of individual Ca2+ channels, showing that it changes release site structure. Visualization of single synaptic vesicles by total internal reflection fluorescence microscopy reveals that inhibition of endocytosis reduces the likelihood of fusion among vesicles docked near ribbons and increases the likelihood that they will retreat from the membrane without fusion. Vesicle advance toward the membrane is also reduced, but the number of membrane-associated vesicles is not. Endocytosis therefore appears to be more important for restoring later steps in vesicle fusion than for restoring docking. Unlike conventional synapses in which endocytic restoration of release sites is evident only at high frequencies, endocytosis is needed to maintain release from rod ribbon synapses even at modest frequencies.

How to maintain active zone integrity during high-frequency transmission

In the central nervous system, the frequency at which reliable synaptic transmission can be maintained varies strongly between different types of synapses. Several pre- and postsynaptic processes must interact to enable high-frequency synaptic transmission. One of the mechanistically most challenging issues arises during repetitive neurotransmitter release, when synaptic vesicles fuse in rapid sequence with the presynaptic plasma membrane within the active zone (AZ), potentially interfering with the structural integrity of the AZ itself. Here we summarize potential mechanisms that help to maintain AZ integrity, including arrangement and mobility of release sites, calcium channel mobility, as well as release site clearance via lateral diffusion of vesicular proteins and via endocytotic membrane retrieval. We discuss how different types of synapses use these strategies to maintain high-frequency synaptic transmission.

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

Bassoon speeds vesicle reloading at a central excitatory synapse

Sustained rate-coded signals encode many types of sensory modalities. Some sensory synapses possess specialized ribbon structures, which tether vesicles, to enable high-frequency signaling. However, central synapses lack these structures, yet some can maintain signaling over a wide bandwidth. To analyze the underlying molecular mechanisms, we investigated the function of the active zone core component Bassoon in cerebellar mossy fiber to granule cell synapses. We show that short-term synaptic depression is enhanced in Bassoon knockout mice during sustained high-frequency trains but basal synaptic transmission is unaffected. Fluctuation and quantal analysis as well as quantification with constrained short-term plasticity models revealed that the vesicle reloading rate was halved in the absence of Bassoon. Thus, our data show that the cytomatrix protein Bassoon speeds the reloading of vesicles to release sites at a central excitatory synapse.