No abstract

Synaptic vesicle docking, priming, and fusion at active zones are orchestrated by a complex molecular machinery. We employed hippocampal organotypic slice cultures from mice lacking key presynaptic proteins, cryofixation, and three-dimensional electron tomography to study the mechanism of synaptic vesicle docking in the same experimental setting, with high precision, and in a near-native state. We dissected previously indistinguishable, sequential steps in synaptic vesicle active zone recruitment (tethering) and membrane attachment (docking) and found that vesicle docking requires Munc13/CAPS family priming proteins and all three neuronal SNAREs, but not Synaptotagmin-1 or Complexins. Our data indicate that membrane-attached vesicles comprise the readily releasable pool of fusion-competent vesicles and that synaptic vesicle docking, priming, and trans-SNARE complex assembly are the respective morphological, functional, and molecular manifestations of the same process, which operates downstream of vesicle tethering by active zone components.

Ribbon synapses of inner hair cells (IHCs) mediate high rates of synchronous exocytosis to indefatigably track the stimulating sound with sub-millisecond precision. The sophisticated molecular machinery of the inner hair cell active zone realizes this impressive performance by enabling a large number of synaptic voltage-gated CaV1.3 Ca2+-channels, their tight coupling to synaptic vesicles (SVs) and fast replenishment of fusion competent SVs. Here we studied the role of RIM-binding protein 2 (RIM-BP2)—a multidomain cytomatrix protein known to directly interact with Rab3 interacting molecules (RIMs), bassoon and CaV1.3—that is present at the inner hair cell active zones. We combined confocal and stimulated emission depletion (STED) immunofluorescence microscopy, electron tomography, patch-clamp and confocal Ca2+-imaging, as well as auditory systems physiology to explore the morphological and functional effects of genetic RIM-BP2 disruption in constitutive RIM-BP2 knockout mice. We found that RIM-BP2 (1) positively regulates the number of synaptic CaV1.3 channels and thereby facilitates synaptic vesicle release and (2) supports fast synaptic vesicle recruitment after readily releasable pool (RRP) depletion. However, Ca2+-influx—exocytosis coupling seemed unaltered for readily releasable SVs. Recordings of auditory brainstem responses (ABR) and of single auditory nerve fiber firing showed that RIM-BP2 disruption results in a mild deficit of synaptic sound encoding.

The afferent synapses between inner hair cells (IHC) and spiral ganglion neurons are specialized to faithfully encode sound with sub-millisecond precision over prolonged periods of time. Here, we studied the role of Rab3 interacting molecule-binding proteins (RIM-BP) 1 and 2 – multidomain proteins of the active zone known to directly interact with RIMs, Bassoon and CaV1.3 – in IHC presynaptic function and hearing. Recordings of auditory brainstem responses and otoacoustic emissions revealed that genetic disruption of RIM-BPs 1 and 2 in mice (RIM-BP1/2–/–) causes a synaptopathic hearing impairment exceeding that found in mice lacking RIM-BP2 (RIM-BP2–/–). Patch-clamp recordings from RIM-BP1/2–/– IHCs indicated a subtle impairment of exocytosis from the readily releasable pool of synaptic vesicles that had not been observed in RIM-BP2–/– IHCs. In contrast, the reduction of Ca2+-influx and sustained exocytosis was similar to that in RIMBP2–/– IHCs. We conclude that both RIM-BPs are required for normal sound encoding at the IHC synapse, whereby RIM-BP2 seems to take the leading role.

Sound encoding is mediated by

No abstract

To achieve accurate encoding of sounds, inner hair cell (IHC) ribbon-type synapses are highly specialized to release synaptic vesicles (SVs) with high rates and temporal precision.A sophisticated, by far not yet fully disentangled unconventional molecular machinery at the synapse active zone (AZ) realizes this impressive performance. It regulates the number of synaptic CaV1.3 Ca 2+ -channels, their tight coupling to SVs, and fast re-supply of SVs for sustained rates of exocytosis. Sound-evoked glutamate release from an IHC synapse is sensed by the postsynaptic spiral ganglion neurons (SGNs). Each SGN is innervated by only one ribbon-type synapse. Even though one might expect similar response characteristics from SGNs innervating the same IHC, this is surprisingly not the case.Postsynaptic spike responses of SGNs differ remarkably, which is likely used as a presynaptic mechanism to encode sounds of varying intensity. Recently, a positive correlation between different SGN response types and presynaptic synapse properties has been found. However, a functional link between heterogeneous presynaptic properties and postsynaptic SGN spike response diversity remains to be demonstrated. To draw a clearer picture of the synaptic transmission mechanism in IHCs, it is key to characterize the molecular components. Furthermore, it is critical to understand the coding strategies of sensory IHCs that specialize and fine-tune the synapses to mediate sound coding. In this work, I addressed these questions in two different approaches. (1) First, the molecular physiology of synaptic transmission at the IHC ribbon synapse was investigated by examining the role of RIM-binding protein 2 (RIM-BP2), a multidomain cytomatrix protein acting as molecular hub between Ca 2+ -channels and vesicular release sites. A multidisciplinary approach including confocal and STED immunofluorescence microscopy, electron microscopy, patch-clamp, and confocal Ca 2+ -imaging, as well as auditory systems physiology was utilized to explore the morphological and physiological effects of genetic RIM-BP2 disruption in constitutive RIM-BP2 knockout mice. I found evidence that RIM-BP2 positively regulates the number of synaptic CaV1.3 Ca 2+ -channels and thereby facilitates SV release and enhances fast SV recruitment after RRP depletion. Furthermore, recordings of auditory brainstem responses (ABRs) and of single auditory nerve fibers (ANFs) showed a mild deficit of sound encoding. (2) Second, an experimental setup for voltage imaging in SGNs was established, to simultaneously monitor multiple SGN responses innervating the same IHC and thereby create a system to understand the synaptic coding strategies of IHCs. The genetically encoded voltage indicators (GEVIs) QuasAr2 and 3 were specifically targeted to SGNs, however only QuasAr3 elicited fluorescence responses in SGN boutons adjacent to an IHC. Thus, henceforth QuasAr3 might be a suitable tool to probe the presynaptic mechanism of postsynaptic response diversity.