Biology subjects

Michanski, S.

Publications and source records attributed to Michanski, S..

7 recordsLinked to original sources

bioRxiv2025

Adaptor protein complex 2 (AP2) participates in biogenesis and homeostasis of myelin sheaths in the central nervous system

Myelination of CNS axons requires oligodendrocytes to undergo extensive morphological changes by producing large amounts of myelin membrane with defined protein composition and structure. The formation of myelin sheaths thus involves efficient trafficking and sorting of future myelin constituents via vesicles that fuse with prospective myelin membranes by exocytotic mechanisms. However, the functional relevance of other trafficking steps in oligodendocytes for myelin biogenesis is largely unknown. Here, we followed the hypothesis that developmental myelination involves endocytic mechanisms. In this model, Golgi-derived vesicles fuse with the oligodendroglial plasma membrane, from which myelin constituents are retrieved by endocytosis into endosomal/lysosomal organelles before their final integration into the growing sheath. Considering that adaptor protein complex-2 subunit-{micro} (AP2M) facilitates AP2-dependent endocytosis, we recombined the Ap2m-gene in myelin-forming oligodendrocytes, causing both hypomyelination and specific changes in the myelin proteome. Most strikingly, lysosomal membrane proteins accumulate in the abaxonal (outermost) myelin layer, identifying this membrane as an active site for retrieving constituents from myelin sheaths. These data demonstrate that the AP2 complex serves a critical function in developmental myelination in vivo. Unexpectedly, we also observed pathological myelin outfoldings indicative of focal hypermyelination. Consistent with the hypothesis that this phenotype reflects impaired maintenance rather than biogenesis of myelin sheaths, recombination of the Ap2m-gene in oligodendrocytes of adult mice caused late-onset progressive focal hypermyelination. These results indicate that, in addition to astrocytic and microglial phagocytosis, oligodendrocytes cell-autonomously contribute to maintaining the structure of healthy myelin sheaths via AP2-dependent mechanisms.

neuroscience↗

bioRxiv2025

Gating of hair cell Ca2+ channels governs the activity of cochlear neurons

Our sense of hearing processes sound intensities spanning six orders of magnitude. In the ear, postsynaptic spiral ganglion neurons (SGNs) tile this intensity range with their firing rate codes. Presynaptic inner hair cells (IHCs) vary Ca2+-influx among their active zones (AZs) diversifying glutamate release and likely contributing to SGN firing diversity. Here we show that low-voltage activation of IHC-Ca2+-influx of mice, modeling the human CaV1.3A749G mutation, increases spontaneous SGN-firing and lowers sound threshold. Altered synaptic morphology in CaV1.3A749G/A749G mice already at ambient sound levels of standard mouse husbandry indicates a risk for noise-induced alterations in CaV1.3A749G patients. We conclude that heterogeneous voltage-dependence of CaV1.3 activation among IHC-AZs contributes to the diversity of SGN firing for sound intensity coding and synaptic vulnerability.

neuroscience↗

bioRxiv2024

SynapseNet: Deep Learning for Automatic Synapse Reconstruction

Electron microscopy is an important technique for the study of synaptic morphology and its relation to synaptic function. The data analysis for this task requires the segmentation of the relevant synaptic structures, such as synaptic vesicles, active zones, mitochondria, presynaptic densities, synaptic ribbons, and synaptic compartments. Previous studies were predominantly based on manual segmentation, which is very time-consuming and prevented the systematic analysis of large datasets. Here, we introduce SynapseNet, a tool for the automatic segmentation and analysis of synapses in electron micrographs. It can reliably segment synaptic vesicles and other synaptic structures in a wide range of electron microscopy approaches, thanks to a large annotated dataset, which we assembled, and domain adaptation functionality we developed. We demonstrated its capability for (semi-)automatic biological analysis in two applications and made it available as an easy-to-use tool to enable novel data-driven insights into synapse organization and function.

bioinformatics↗

bioRxiv2022

Age-dependent structural reorganization of utricular ribbon synapses

In mammals, spatial orientation is synaptically-encoded by sensory hair cells of the vestibular labyrinth. Vestibular hair cells (VHCs) harbor synaptic ribbons at their presynaptic active zones (AZs), which play a critical role in molecular scaffolding and facilitate synaptic release and vesicular replenishment. With advancing age, the prevalence of vestibular deficits increases; yet, a direct link to the functional decline of VHC ribbon synapses remains to be demonstrated. To address this issue, we investigated the effects of aging on the ultrastructure of the ribbon-type AZs in murine utricles using various electron microscopic techniques and combined them with confocal and super-resolution light microscopy as well as metabolic imaging up to one year of age. In older animals, we detected predominantly in type I VHCs the formation of floating ribbon clusters. Our findings suggest that VHC ribbon-type AZs undergo dramatic structural alterations upon aging.

cell biology↗

bioRxiv2022

Piccolino regulates the architecture of the ribbon at cochlear inner hair cell synapses

Cochlear inner hair cells (IHCs) form specialized ribbon synapses with spiral ganglion neurons that tireless-ly transmit sound information at high rates over long time periods with extreme temporal precision. This functional specialization is essential for precise sound encoding and is attributed to a distinct molecular machinery with unique players or splice variants compared to conventional neuronal synapses. Among these is the active zone (AZ) scaffold protein piccolo/aczonin, which is represented by its short splice variant piccolino at cochlear and retinal ribbon synapses. While the function of piccolo at synapses of the central nervous system has been intensively investigated, the role of piccolino at IHC synapses remains unclear. In this study, we characterized the structure and function of IHC-synapses in piccolo gene-trap mutant rats (Pclogt/gt). We found a mild hearing deficit with elevated thresholds and reduced amplitudes of auditory brainstem responses. Ca2+ channel distribution and ribbon morphology were altered in apical IHCs, while their presynaptic function seemed unchanged. We conclude that piccolino contributes to the AZ organization in IHCs and is essential for normal synaptic transmission.

neuroscience↗

bioRxiv2022

Optogenetics and electron tomography for structure-function analysis of cochlear ribbon synapses

Ribbon synapses of cochlear inner hair cells (IHCs) are specialized to indefatigably transmit sound information at high rates. To understand the underlying mechanisms, structure-function analysis of the active zone (AZ) of these synapses is essential. Previous electron microscopy studies of synaptic vesicle (SV) dynamics at the IHC AZ used potassium stimulation, which limited the temporal resolution to minutes. Here, we established optogenetic IHC stimulation followed by quick freezing within milliseconds and electron tomography to study the ultrastructure of functional synapse states with good temporal resolution. We characterized optogenetic IHC stimulation by patch-clamp recordings from IHCs and postsynaptic boutons revealing robust IHC depolarization and transmitter release. Ultrastructurally, the number of docked SVs increased and distances to the presynaptic density decreased upon short (17-25 ms) and long (48-76 ms) light stimulation paradigms. We did not observe enlarged SVs or other morphological correlates of homotypic fusion events. Our results suggest a rapid replenishment of docked SVs at IHC ribbon synapses and argue against synchronized multiquantal release under our experimental conditions.

neuroscience↗

bioRxiv2019

Macromolecular and electrical coupling between inner hair cells in the rodent cochlea

Inner hair cells (IHCs) are the primary receptors for hearing. They are housed in the cochlea and convey sound information to the brain via synapses with the auditory nerve. IHCs have been thought to be electrically and metabolically independent from each other. We report that, upon developmental maturation, 30% of the IHCs are electrochemically coupled in mini-syncytia. This coupling permits transfer of fluorescently-labeled metabolites and macromolecular tracers. The membrane capacitance, Ca2+-current, and resting current increase with the number of dye-coupled IHCs. Dual voltage-clamp experiments substantiate low resistance electrical coupling. Pharmacology and tracer permeability rule out coupling by gap junctions and purinoceptors. 3D-electron-microscopy indicates instead that IHCs are coupled by membrane fusion sites. Consequently, depolarization of one IHC triggers presynaptic Ca2+-influx at active zones in the entire mini-syncytium. Based on our findings and modeling, we propose that IHC-mini-syncytia enhance sensitivity and reliability of cochlear sound encoding.

neuroscience↗