Ribbon Synapses In Retina Research Articles

SMAD4 Defect Causes Auditory Neuropathy Via Specialized Disruption of Cochlear Ribbon Synapses in Mice.

More than 100 genes have been associated with deafness. However, SMAD4 is rarely considered a contributor to deafness in humans, except for its well-defined role in cell differentiation and regeneration. Here, we report that a SMAD4 defect in mice can cause auditory neuropathy, which was defined as a mysterious hearing and speech perception disorder in human for which the genetic background remains unclear. Our study showed that a SMAD4 defect induces failed formation of cochlear ribbon synapse during the earlier stage of auditory development in mice. Further investigation found that there are nearly normal morphology of outer hair cells (OHCs) and post-synapse spiral ganglion nerves (SGNs) in SMAD4 conditional knockout mice (cKO); however, a preserved distortion product of otoacoustic emission (DPOAE) and cochlear microphonic (CM) still can be evoked in cKO mice. Moreover, a partial restoration of hearing detected by electric auditory brainstem response (eABR) has been obtained in the cKO mice using electrode stimuli toward auditory nerves. Additionally, the ribbon synapses in retina are not affected by this SMAD4 defect. Thus, our findings suggest that this SMAD4 defect causes auditory neuropathy via specialized disruption of cochlear ribbon synapses.

Three-Dimensional Visualization of Whole Synapses by Stem Tomography

Scanning transmission electron microscope (STEM) tomography enables determination of 3D ultrastructure from 1 or 2-micrometer thick sections of cells and tissues. These sections are considerably thicker than can be analyzed by conventional electron tomography, where resolution is limited by chromatic aberration due to multiple inelastic scattering. In STEM tomography a probe of small angular convergence gives a large depth of field throughout the thickness of the specimen while maintaining a probe diameter of approximately 2 nm; and the use of an on-axis bright-field detector reduces the effects of beam broadening and thus improves the spatial resolution compared to that attainable by STEM dark-field tomography. We have found that STEM tomography is ideal for visualizing entire synapses in the nervous system, and for making quantitative measurements on the numbers, sizes and shapes of synaptic components. We applied the technique to study the architecture of ribbon synapses in retina, and the structure of postsynaptic densities in brain cortex. For the first time, it was possible to determine a full 3D architecture of ribbon synapses in mammalian (rat) rod bipolar cells, in which regular docked and tethered vesicles, as well as larger (possibly pre-fused) vesicles were visualized. Quantitative analysis revealed a readily releasable pool of vesicles, which correlates structurally with previous physiological data. We have also applied STEM tomography to reconstruct entire spine postsynaptic densities in mouse hippocampus, both in control preparations as well as in preparations, where RNAi knockdown eliminates specific PSD scaffolding proteins to illustrate their key role in organizing the PSD. STEM tomography of thick sections thus provides a novel approach for correlating the nanoscale structure of synapses with function.This work was supported by the Intramural Research Programs of NIBIB and NINDS, NIH.

CSPalpha-deficiency causes massive and rapid photoreceptor degeneration.

Cysteine string protein (CSP) alpha is an abundant synaptic vesicle protein that contains a DNA-J domain characteristic of Hsp40-type cochaperones. Previous studies showed that deletion of CSPalpha in mice leads to massive lethal neurodegeneration but did not clarify how the neurodegeneration affects specific subpopulations of neurons. Here, we analyzed the effects of the CSPalpha deficiency on tonically active ribbon synapses of the retina and the inner ear. We show that CSPalpha-deficient photoreceptor terminals undergo dramatic and rapidly progressive neurodegeneration that starts before eye opening and initially does not affect other retinal synapses. These changes are associated with progressive blindness. In contrast, ribbon synapses of auditory hair cells did not exhibit presynaptic impairments in CSPalpha-deficient mice. Hair cells, but not photoreceptor cells or central neurons, express CSPbeta, thereby accounting for the lack of a hair-cell phenotype in CSPalpha knockout mice. Our data demonstrate that tonically active ribbon synapses in retina are particularly sensitive to the deletion of CSPalpha and that expression of at least one CSP isoform is essential to protect such tonically active synapses from neurodegeneration.

Differential expression of the presynaptic cytomatrix protein bassoon among ribbon synapses in the mammalian retina.

Bassoon is a 420-kDa presynaptic protein which is highly concentrated at the active zones of nerve terminals of conventional synapses, both excitatory glutamatergic and inhibitory GABAergic, in rat brain. It is thought to be involved in the organization of the cytomatrix at the site of neurotransmitter release. In the retina, there are two structurally and functionally distinct types of synapses: ribbon and conventional synapses. Antibodies against bassoon were applied to sections of rat and rabbit retina. Strong punctate immunofluorescence was found in the outer and inner plexiform layers. Using pre- and post-embedding immunostaining and electron microscopy, bassoon was localized in the outer plexiform layer at ribbon synapses formed by rods and cones but was absent from basal synaptic contacts formed by cones. In the inner plexiform layer a different picture emerged. As in the brain, bassoon was found at conventional inhibitory GABAergic synapses, made by amacrine cells, but it was absent from the bipolar cell ribbon synapses. These data demonstrate differences in the molecular composition of the presynaptic apparatuses of outer and inner plexiform layer ribbon synapses. Thus, differential equipment with cytomatrix proteins may account for the functional differences observed between the two types of ribbon synapses in the retina.