Ribbon synapses

The ribbon synapse is a type of neuronal synapse which is characterized by unique mechanisms of multivesicular release and calcium channel positioning which promote rapid neurotransmitter release and signal transmission. Ribbon synapses undergo an ongoing cycle of exocytosis and endocytosis in response to graded changes of membrane potential.

These unique features specialize the ribbon synapse to enable extremely fast, precise and sustained neurotransmissions, which are critical for the perception of complex senses such as vision and hearing. Ribbon synapses are found in retinal photoreceptor cells, vestibular organ receptors, cochlear hair cells and retinal bipolar cells.

The synaptic ribbon is a unique structure at the synapse's active zone. It hovers several nanometers above the pre-synaptic membrane and tethers 100 or more synaptic vesicles. Each pre-synaptic cell can have from 10 to 100 ribbons tethered to it raising the total count to 1000-10000 vesicles.

Function
Features of the ribbon synapse enable it to process information extremely quickly. Bipolar neurons present a good model for how ribbon synapses function.

In photoreceptor and bipolar cells information is transferred via the release of the neurotransmitter glutamate at the ribbon synapse. Conventional neurons encode information by changes in the rate of action potentials, but for complex senses like vision, this is not sufficient. Ribbon synapses enable neurons to transmit light signals over a dynamic range of several orders of magnitude in intensity. This is achieved by encoding changes in tonic rate of transmitter release which requires the release of several hundred to several thousand synaptic vesicles per second.

To accomplish this level of performance, the sensory neurons of the eye maintain large pools of fast releasable vesicles that are equipped with ribbon synapses. This enables the cell to exocytose hundreds of vesicles per second, greatly exceeding the rate of normal neurons without the specialized ribbon synapse.

The current hypothesis of calcium-dependent exocytosis at retinal ribbon synapses suggests that the ribbon accommodates a reservoir of primed releasable vesicles. The vesicles that are in closest contact with the presynaptic plasma membrane at the ribbon base constitute the small, rapidly releasable pool of vesicles, whereas the remaining vesicles tethered to the ribbon constitute the large, readily (slower) releasable pool. These regularly aligned rows of synaptic vesicles tethered to either side of the ribbon along with the expression of the kinesin motor protein KIF3A at retinal ribbon synapses can move vesicles like a conveyor belt to the docking/release site at the ribbon base.

Microscopic structure
The photoreceptor ribbon synapse is around 30 nm in thickness. It sticks out into the cytoplasm around 200-1000 nm and anchors along its base to the arciform density which is an electron dense structure that is anchored to the presynaptic membrane. The arciform density is located within the synaptic ridge, a small evagination of the presynaptic membrane. Hair cells lack an arciform density so the anchor of this ribbon is considered to be invisible by electron microscope. The ribbon’s surface has small particles that are around 5 nm wide where the synaptic vesicles tether densely via fine protein filaments. There are multiple filaments per vesicle. There are also voltage gated L-type calcium channels on the docking sites of the ribbon synapse which trigger neurotransmitter release. Specifically, ribbon synapses contain specialized organelles called synaptic ribbons, which are large presynaptic structures associated in the active zone. They are thought to fine tune the synaptic vesicle cycle. Synaptic ribbons are in close proximity to synaptic vesicles, which, in turn, are close to the presynaptic neurotransmitter release site via the ribbon.

Postsynaptic structures differ for cochlear cells and photoreceptor cells. Hair cells allow for one vesicle release for one action potential propagation. The hair cells supply one vesicle release onto the postsynaptic bouton, which is enough to create an action potential in auditory afferent cells. Photoreceptors allow one vesicle release for many action potential propagation. The rod terminal and cone ribbon synapse of the photoreceptors have horizontal synaptic spines expressing AMPA receptors with additional bipolar dendrites exhibiting the mGluR6 receptors. These structures allow for the binding of multiple molecules of glutamate, allowing for the propagation of many action potentials.

Molecular structure
Different protein components of the synaptic ribbon have been identified. Several proteins of the synaptic ribbon have also been found to be associated with conventional synapses. RIM (Rab3-interacting proteins) is a GTPase expressed on synaptic vesicles that is important in priming synaptic vesicles. Immunostaining has revealed the presence of KIF3A, a component of the kinesin II motor complex whose function is still unknown. The presynaptic cytomatrix proteins Bassoon and Piccolo are both expressed at photoreceptor ribbons, but Piccolo is only expressed at retinal bipolar synaptic ribbons. Bassoon is responsible for attaching itself to the base of the synaptic ribbons and subsequently anchoring the synaptic ribbons. The function of Piccolo is unknown. Also important is the filaments that tether the vesicles to the ribbon synapse. These are shed during high rates of exocytosis. The only unique protein associated with the synaptic ribbon is RIBEYE. It is found to be a part of all vertebrate synaptic ribbons in ribbon synapses and is the central portion of ribbon synapses.

Structural plasticity
In correspondence to its activity, ribbon synapses can have synaptic ribbons that vary in size. In mouse photoreceptor synapses when the neurotransmitter release rate is high and exocytosis is high, the synaptic ribbons are long. When neurotransmitter release rate is low and exocytosis is low, the synaptic ribbons are short. This has been identified with RIBEYE with the current hypothesis being that synaptic ribbons can enlarge by the addition of more RIBEYE subunit. RIBEYE interactions are required to form a scaffold formation protein of the synaptic ribbon.

Exocytosis
During exocytosis at the bipolar ribbon synapse, vesicles are seen to pause at the membrane and then upon opening of the calcium channels to promptly release their contents within milliseconds. Like most exocytosis, Ca2+ regulates the release of vesicles from the presynaptic membrane. Different types of ribbon synapses have different dependence on Ca2+ releases. The hair cell ribbon synapses exhibit a steep dependence on Ca2+ concentration, while the photoreceptor synapses is less steeply dependent on Ca2+ and is stimulated by much lower levels of free Ca2+. Exocytosis in the ribbons synapse shows that the vesicle fully collapses into the plasma membrane. This means that the synaptic vesicle fuses with the presynaptic membrane and releases its contents into the synapse.

The bipolar cell active zone of the ribbon synapse can release neurotransmitter continuously for hundreds of milliseconds during strong stimulation. This release of neurotransmitters occurs in two kinetically distinct phases: a small fast pool where about twenty percent of the total is released in about 1 millisecond, and a large sustained pool where the remaining components are released over hundreds of milliseconds. The existence of correspondence between the pool of tethered vesicles and the pool for sustained release in the rods and bipolar cells of the ribbon reveals that the ribbon may serve as a platform where the vesicles can be primed to allow sustained release of neurotransmitters. This large size of the sustained large component is what separates the ribbon synapse active zones from those of conventional neurons where sustained release is small in comparison. Once the presynaptic vesicles have been depleted, the bipolar cell’s releasable pool requires several seconds to refill with the help of ATP hydrolysis.

Endocytosis
A high amount of endocytosis is necessary due to the large amount of exocytosis during continued neurotransmitter release in ribbon synapses. Synaptic vesicles need to be recycled for further transmission to occur. These vesicles are directly recycled and because of their mobility, quickly replenish the neurotransmitters required for continued release. In cone photoreceptors, the fused membrane is recycled into the synaptic vesicle without pooling of the membrane into the endosomes. Bipolar cells rely on a different mechanism. It involves taking a large portion of the membrane which is endocytosed and gives rise to synaptic vesicles. This mechanism is conserved in hair cells as well.

Loss of hearing and sight in mice
Research has shown that abnormal expression of otoferlin, a ribbon synapse associated protein, is responsible for the impairment in exocytosis of ribbon-bound vesicles in auditory inner hair cells. Otoferlin has displays similar functional characteristics as synaptotagmin in auditory inner hair cells, and impaired hearing has been shown to be associated with incorrect expression of otoferlin in and mice.

In studies of retinal genetic coding of laboratory mice, several mutated ribbon synapse associated voltage-gated L-type calcium channel auxiliary subunits were shown to be associated with dysfunctional rod and cone activity and information transmission. Mice were shown to express significantly reduced scotopic vision, and further research has shown the dysregulation of calcium homeostasis may have a significant role in rod photoreceptor degradation and death.

Human implications
Much of the genetic information associated with the proteins observed in laboratory mice are shared with humans. The protein otoferlin is observed phenotypically in human auditory inner hair cells, and abnormal expression has been linked with deafness. In humans, cochlear implants have shown to reduce the debilitating effects of abnormal otoferlin expression by surpassing the synapse associated with the auditory inner hair cells. The genetic code for retinal subunits associated with impaired scotopic vision and rod photoreceptor degradation are conserved at approximately 93% between mice and humans. Further research into the abnormal functioning of these mechanisms could open the door to therapeutic techniques to relieve auditory and visual impairments.

Other areas of investigation
Several recent studies have provided evidence that loss-of-function mutations in pre-synaptic proteins of the photoreceptor cells ribbon synapse can cause X-linked congenital stationary night blindness (CSNB) through mutations in the CACNA1F gene, which codes for the αF1-subunit of the L-type calcium channel Cav1.4. The gene is expressed at the active zone of photoreceptor ribbon synapses. The mutation is characterized by a significant reduction in both night and variable perturbation of daylight vision. The mutations in CACNA1F and Cav1.4 have also been observed to co-localize with CaBP4, a photoreceptor-specific calcium-binding protein. CaBP4 has been theorized to modulate the activity of the Cav1.4 channel. It has been theorized to be associated with the proper establishment and maintenance of photoreceptor ribbon synapses. While no evidence has been published, the association between CaBP4 and Cav1.4 is an area of continued research.

There has been a significant amount of research into the pre-synaptic cytomatrix protein Bassoon, which is a multi-domain scaffolding protein universally expressed at synapses in the central nervous system. Mutations in Bassoon have been shown to result in decreased synaptic transmission. However, the underlying mechanisms behind this observed phenomenon are not fully understood and are currently being investigated. It has been observed that in the retina of Bassoon-mutant mice, photoreceptor ribbon synapses are not anchored to pre-synaptic active zones during photoreceptor synaptogenesis. The photoreceptor ribbon synapses are observed to be free floating in the cytoplasm of the photoreceptor terminals. These observations have led to the conclusion that Bassoon plays a critical role in the formation of the photoreceptor ribbon synapse.