Conditional Deletion of Activating Rearranged During Transfection Receptor Tyrosine Kinase Leads to Impairment of Photoreceptor Ribbon Synapses and Disrupted Visual Function in Mice

Purpose: The rearranged during transfection (RET) receptor tyrosine kinase plays a key role in transducing signals related to cell growth and differentiation. Ret mutant mice show abnormal retinal activity and abnormal levels and morphology of bipolar cells, yet die on the 21st day after birth as a result of renal underdevelopment. To extend the observation period, we generated the Ret conditional knockout Chx10-Cre;C-Retlx/lx mouse model and analyzed the retinal function and morphological changes in mature and aging Chx10-Cre;C-Retlx/lx mice.Methods: Retina-specific depletion of Ret was achieved using mice with floxed alleles of the Ret gene with CHX10-driven Cre recombinase; floxed mice without Cre expression were used as controls. Retinal function was examined using electroretinography (ERG), and 2-, 4-, 12-, and 24-month-old mice were analyzed by hematoxylin staining and immunohistochemistry to evaluate retinal morphological alterations. The ultrastructure of photoreceptor synapses was evaluated using electron microscopy.Results: The results of the ERG testing showed that b-wave amplitudes were reduced in Chx10-Cre;C-Retlx/lx mice, whereas a-waves were not affected. A histopathological analysis revealed a thinner and disorganized outer plexiform layer at the ages of 12 and 24 months in Chx10-Cre;C-Retlx/lx mice. Moreover, the data provided by immunohistochemistry showed defects in the synapses of photoreceptor cells. This result was confirmed at the ultrastructural level, thus supporting the participation of Ret in the morphological changes of the synaptic ribbon.Conclusion: Our results provide evidence of the role of Ret in maintaining the function of the retina, which was essential for preserving the structure of the synaptic ribbon and supporting the integrity of the outer plexiform layer.

The mammalian rod synaptic ribbon is essential for Cav channel facilitation and ultrafast synaptic vesicle fusion

Rod photoreceptors (PRs) use ribbon synapses to transmit visual information. To signal 'no light detected' they release glutamate continually to activate post-synaptic receptors. When light is detected glutamate release pauses. How a rod's individual ribbon enables this process was studied here by recording evoked changes in whole-cell membrane capacitance from wild type and ribbonless (Ribeye-ko) mice. Wild type rods filled with high (10 mM) or low (0.5 mM) concentrations of the Ca2+-buffer EGTA created a readily releasable pool (RRP) of 87 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ < 0.4 msec. The lower concentration of EGTA accelerated Cav channel opening and facilitated release kinetics. In contrast, ribbonless rods created a much smaller RRP of 22 SVs, and they lacked Cav channel facilitation; however, Ca2+ channel-release coupling remained tight. These release deficits caused a sharp attenuation of rod-driven light responses. We conclude that the synaptic ribbon facilitates Ca2+-influx and establishes a large RRP of SVs.

Early Changes in Exo- and Endocytosis in the EAE Mouse Model of Multiple Sclerosis Correlate with Decreased Synaptic Ribbon Size and Reduced Ribbon-Associated Vesicle Pools in Rod Photoreceptor Synapses

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system that finally leads to demyelination. Demyelinating optic neuritis is a frequent symptom in MS. Recent studies also revealed synapse dysfunctions in MS patients and MS mouse models. We previously reported alterations of photoreceptor ribbon synapses in the experimental auto-immune encephalomyelitis (EAE) mouse model of MS. In the present study, we found that the previously observed decreased imunosignals of photoreceptor ribbons in early EAE resulted from a decrease in synaptic ribbon size, whereas the number/density of ribbons in photoreceptor synapses remained unchanged. Smaller photoreceptor ribbons are associated with fewer docked and ribbon-associated vesicles. At a functional level, depolarization-evoked exocytosis as monitored by optical recording was diminished even as early as on day 7 after EAE induction. Moreover compensatory, post-depolarization endocytosis was decreased. Decreased post-depolarization endocytosis in early EAE correlated with diminished synaptic enrichment of dynamin3. In contrast, basal endocytosis in photoreceptor synapses of resting non-depolarized retinal slices was increased in early EAE. Increased basal endocytosis correlated with increased de-phosphorylation of dynamin1. Thus, multiple endocytic pathways in photoreceptor synapse are differentially affected in early EAE and likely contribute to the observed synapse pathology in early EAE.

Noise-Induced Hearing Loss: Updates on Molecular Targets and Potential Interventions

Noise overexposure leads to hair cell loss, synaptic ribbon reduction, and auditory nerve deterioration, resulting in transient or permanent hearing loss depending on the exposure severity. Oxidative stress, inflammation, calcium overload, glutamate excitotoxicity, and energy metabolism disturbance are the main contributors to noise-induced hearing loss (NIHL) up to now. Gene variations are also identified as NIHL related. Glucocorticoid is the only approved medication for NIHL treatment. New pharmaceuticals targeting oxidative stress, inflammation, or noise-induced neuropathy are emerging, highlighted by the nanoparticle-based drug delivery system. Given the complexity of the pathogenesis behind NIHL, deeper and more comprehensive studies still need to be fulfilled.

Resolving the molecular architecture of the photoreceptor active zone by MINFLUX nanoscopy

Cells assemble macromolecular complexes into scaffoldings that serve as substrates for catalytic processes. Years of molecular neurobiology indicate that neurotransmission depends on such optimization strategies, yet the molecular topography of the presynaptic Active Zone (AZ) where transmitter is released upon synaptic vesicle (SV) fusion remains to be visualized. Therefore, we implemented MINFLUX optical nanoscopy to resolve the AZ of rod photoreceptors. To facilitate MINFLUX nanoscopy of the AZ, we developed and verified an immobilization technique, we name Heat Assisted Rapid Dehydration (HARD). Here fresh retinal slices are directly stamped onto glass coverslips yielding a single layer of rod AZs. These AZs exhibited excellent labeling efficiency and minimal signal redundancy in the Z-direction. Our data indicate that the SV release site is a molecular complex of bassoon-Rab3-binding molecule 2 (RIM2)-ubMunc13-2-CAST. The complexes are serially duplicated longitudinally, and reflected in register along the axis of symmetry of the synaptic ribbon.

The mammalian rod synaptic ribbon is essential for Cav channel facilitation and ultrafast fusion of the readily releasable pool of vesicles

Rod photoreceptors (PRs) use ribbon synapses to transmit visual information. To signal ‘no light detected’ they release glutamate continually to activate post-synaptic receptors, and when light is detected glutamate release pauses. How a rod’s individual ribbon enables this process was studied here by recording evoked changes in whole-cell membrane capacitance from wild type and ribbonless (RIBEYE-ko) rods. Wild type rods created a readily releasable pool (RRP) of 92 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ < 0.4 msec. Lowering intracellular Ca2+-buffering accelerated Cav channel opening and facilitated release kinetics, but RRP size was unaltered. In contrast, ribbonless rods created an RRP of 24 SVs, and lacked Cav channel facilitation; however, Ca2+ channel-release coupling remained tight. The release deficits caused a sharp attenuation of rod-driven light responses measured from RIBEYE-ko mice. We conclude that the synaptic ribbon facilitates Ca2+-influx and establishes a large RRP of SVs.

Direct Observation Of Vesicle Transport On The Synaptic Ribbon Provides Evidence That Vesicles Are Mobilized And Prepared Rapidly For Release

SUMMARYSynaptic ribbons are thought to provide vesicles for continuous synaptic transmission in some retinal non-spiking neurons, yet recent studies indicate that genetic removal of the ribbon has little effect on vesicle release kinetics. To investigate vesicle replenishment at synaptic ribbons, we imaged synaptic vesicles and ribbons in retinal bipolar cells with TIRF microscopy during stimulation with trains of 30-ms depolarizations. Analysis of vesicles released by the stimuli revealed that the vast majority of releasable vesicles reside within 300 nm of the ribbon center. A single 30-ms step to 0 mV was sufficient to deplete the most membrane-proximal vesicle pool, while triggering rapid stepwise movements of distal vesicles along the ribbon and toward the plasma membrane.Replenishment only becomes rate-limiting for recovery from paired-pulse depression for interstimulus intervals shorter than 250 ms. For longer interstimulus intervals, vesicle movement down the ribbon is fast enough to replenish released vesicles, but newly arrived vesicles are not release-ready. Notably, vesicle re-supply is 40-to 50-fold faster than previously measured in non-ribbon conventional synapses, whereas vesicle maturation rate is comparable. Moreover, in contrast to conventional synapses, vesicles docked at the base of the ribbon release with high fidelity. Lastly, our data show that with multiple stimuli, the delay in vesicle departure increases. Our results support a role for ribbons in the rapid supply and efficient preparation of vesicles for release, provide direct measurements of vesicle movement down the synaptic ribbon and suggest that multiple factors contribute to paired-pulse depression.