Biology subjects

Sorensen, J. B.

Publications and source records attributed to Sorensen, J. B..

7 recordsLinked to original sources

bioRxiv2024

CryoVesNet: A Dedicated Framework for Synaptic Vesicle Segmentation in Cryo Electron Tomograms

Cryo-electron Tomography (Cryo-ET) has the potential to reveal cell structure down to atomic resolution. Nevertheless, cellular cryo-ET data is often highly complex, and visualization, as well as quantification, of subcellular structures require image segmentation. Due to a relatively high level of noise and anisotropic resolution in cryo-ET data, automatic segmentation based on classical computer vision approaches usually does not perform satisfactorily. For this reason, cryo-ET researchers have mostly performed manual segmentation. Communication between neurons relies on neurotransmitter-filled synaptic vesicle (SV) exocytosis. Recruitment of SVs to the plasma membrane is an important means of regulating exocytosis and is influenced by interactions between SVs. Cryo-ET study of the spatial organization of SVs and of their interconnections allows a better understanding of the mechanisms of exocytosis regulation. Extremely accurate SV segmentation is a prerequisite to obtaining a faithful representation of SVs state of connectivity. Hundreds to thousands of SVs are present in a typical synapse, and their time-consuming manual segmentation is a bottleneck in this analysis. Several attempts to automate vesicle segmentation by classical computer vision or machine learning algorithms have not yielded robust results. We addressed this problem by designing a workflow consisting of a U-Net convolutional segmentation network followed by post-processing steps. This combination yields highly accurate results. Furthermore, we provide an interactive tool for accurately segmenting spherical vesicles in a fraction of the time required by available manual segmentation methods. This tool can be used to segment vesicles that were missed by the fully automatic procedure or to quickly segment a handful of vesicles while bypassing the fully automatic procedure. Our pipeline can in principle be used to segment any spherical vesicle in any cell type as well as extracellular vesicles.

neuroscience↗

bioRxiv2023

SNAP25 disease mutations change the energy landscape for synaptic exocytosis due to aberrant SNARE interactions

SNAP25 is one of three neuronal SNAREs driving synaptic vesicle exocytosis. We studied three mutations in SNAP25 that cause epileptic encephalopathy: V48F, and D166Y in the Synaptotagmin-1 (Syt1) binding interface, and I67N, which destabilizes the SNARE-complex. All three mutations reduced Syt1-dependent vesicle docking to SNARE-carrying liposomes and Ca2+-stimulated membrane fusion in vitro and in neurons. The V48F and D166Y mutants (with potency D166Y > V48F) led to reduced Readily Releasable Pool (RRP) size, due to increased spontaneous (mEPSC) release and decreased priming rates. These mutations lowered the energy barrier for fusion and increased the release probability, which are gain-of-function features not found in Syt1 knockout (KO) neurons; normalized mEPSC release rates were higher (potency D166Y>V48F) than in the Syt1 KO. These mutations (potency D166Y > V48F) increased spontaneous association to partner SNAREs, resulting in unregulated membrane fusion. In contrast, the I67N mutant decreased mEPSC frequency and evoked EPSC amplitudes due to an increase in the apparent height of the energy barrier for fusion, whereas the RRP size was unaffected. This could be partly compensated by positive charges lowering the energy barrier. Overall, pathogenic mutations in SNAP25 cause complex changes in the energy landscape for priming and fusion.

neuroscience↗

bioRxiv2023

Microcircuit failure in STXBP1 encephalopathy leads to hyperexcitability

De novo mutations in Stxbp1 are among the most prevalent causes of neurodevelopmental disorders, and lead to haploinsufficiency, cortical hyperexcitability, epilepsy and other symptoms. Given that Munc18-1, the protein encoded by Stxbp1, is essential for both excitatory and inhibitory synaptic transmission, it is currently not understood why mutations cause hyperexcitability. We discovered that overall inhibition in canonical feedforward microcircuits is defective in a validated mouse model for Stxbp1 haploinsufficiency. However, unexpectedly, we found that inhibitory synapses were largely unaffected. Instead, excitatory synapses failed to recruit inhibitory interneurons. Modelling experiments confirmed that defects in the recruitment of inhibitory neurons in microcircuits cause hyperexcitation. Ampakines, compounds that enhance excitatory synapses, restored interneuron recruitment and prevented hyperexcitability. These findings identify deficits in excitatory synapses in microcircuits as a key underlying mechanism for cortical hyperexcitability in Stxbp1 disorder and identify compounds enhancing excitation as a direction for therapy design. Highlights- Neocortical microcircuits fail in Stxbp1 haploinsufficiency mouse models (Stxbp1hap) - Microcircuit impairments leads to cortical hyperexcitability due to a lack of inhibition. - Inhibitory synapses are not severely affected in Stxbp1hap, instead, excitatory synapses fail to recruit interneurons. - AMPAkines rescue microcircuit failure in Stxbp1hap

neuroscience↗

bioRxiv2022

Phorbolester-activated Munc13-1 and ubMunc13-2 exert opposing effects on dense-core vesicle secretion

Munc13 proteins are priming factors for SNARE-dependent exocytosis, which are activated by diacylglycerol (DAG)-binding to their C1-domain. Several Munc13 paralogs exist, but their differential roles are not well understood. We studied the interdependence of phorbolesters (DAG mimics) with Munc13-1 and ubMunc13-2 in mouse adrenal chromaffin cells. Although expression of either Munc13-1 or ubMunc13-2 stimulated secretion, phorbolester was only stimulatory for secretion when ubMunc13-2 expression dominated, but inhibitory when Munc13-1 dominated. Accordingly, phorbolester stimulated secretion in wildtype cells, or cells overexpressing ubMunc13-2, but inhibited secretion in Munc13-2/Unc13b knockout (KO) cells or in cells overexpressing Munc13-1. Phorbolester was more stimulatory in the Munc13-1/Unc13a KO than in WT littermates, showing that endogenous Munc13-1 limits the effects of phorbolester. Imaging showed that ubMunc13-2, but not Munc13-1, traffics to the plasma membrane with a time-course matching Ca2+-dependent secretion, and trafficking is independent of synaptotagmin-7 (syt7). However, in the absence of syt7, phorbolester became inhibitory for both Munc13-1 and ubMunc13-2 driven secretion, indicating that stimulatory phorbolester x Munc13-2 interaction depends obligatorily on functional pairing with syt7. Overall, DAG/phorbolester, ubMunc13-2 and syt7 form a stimulatory triad for dense-core vesicle priming.

neuroscience↗

bioRxiv2022

Morphofunctional changes at the active zone during synaptic vesicle exocytosis

The fusion of synaptic vesicles (SVs) with the plasma membrane (PM) proceeds through intermediate steps that remain poorly resolved. Additionally, the effect of persistent high or low exocytosis activity on intermediate steps remains unknown. Through time-resolved cryo-electron tomography, we ordered events into a sequence. Following stimulation, proximal tethered SVs rapidly form additional tethers with the PM. Simultaneously, fusion initiation occurs by membrane curvature ( buckling) of the SV and PM. It is followed by the formation of a fusion pore, and the collapse of SV membrane. At this time, membrane-proximal, but not membrane-distal, vesicles lose their interconnections, allowing them to move towards the PM. Two mutations of SNAP-25 that arrests or disinhibit spontaneous release, respectively, both caused a loss of interconnectors, while the disinhibiting mutant also caused a loss of membrane proximal multiple-tethered SVs. Overall, tether formation and connector dissolution is triggered by stimulation and respond to the spontaneous fusion rate. These morphological observations likely correspond to the transition of SVs from one functional pool to another.

neuroscience↗

bioRxiv2021

Allosteric stabilization of calcium and lipid binding engages three synaptotagmins in fast exocytosis

Synaptic communication relies on the fusion of synaptic vesicles with the plasma membrane, which leads to neurotransmitter release. This exocytosis is triggered by brief and local elevations of intracellular Ca2+ with remarkably high sensitivity. How this is molecularly achieved is unknown. While synaptotagmins confer the Ca2+ sensitivity of neurotransmitter exocytosis, biochemical measurements reported Ca2+ affinities too low to account for synaptic function. However, synaptotagmins Ca2+ affinity increases upon binding the plasma membrane phospholipid PI(4,5)P2 and, vice versa, Ca2+-binding increases synaptotagmins PI(4,5)P2 affinity, indicating a stabilization of the Ca2+/PI(4,5)P2 dual-bound syt. We here devise a molecular exocytosis model based on this positive allosteric stabilization and the assumptions that (1.) synaptotagmin Ca2+/PI(4,5)P2 dual binding lowers the energy barrier for vesicle fusion and that (2.) the effect of multiple synaptotagmins on the energy barrier is additive. The model, which relies on biochemically measured Ca2+/PI(4,5)P2 affinities and protein copy numbers, reproduced the steep Ca2+ dependency of neurotransmitter release. Our results indicate that each synaptotagmin dual binding Ca2+/PI(4,5)P2 lowers the energy barrier for vesicle fusion by 4.85 kBT and that allosteric stabilization of this state enables the synchronized engagement of three synaptotagmins for fast exocytosis. Furthermore, we show that mutations altering synaptotagmins allosteric properties may show dominant-negative effects, even though synaptotagmins act independently on the energy barrier, and that dynamic changes of local PI(4,5)P2 (e.g. upon vesicle movement) dramatically impact synaptic responses. We conclude that allosterically stabilized Ca2+/PI(4,5)P2 dual binding enables synaptotagmins to exert their coordinated function in neurotransmission.

neuroscience↗

bioRxiv2020

Synaptotagmin-7 places dense-core vesicles at the cell membrane to promote Munc13-2- and Ca2+-dependent priming

The functional consequences of the co-expression of synaptotagmin-1 and synaptotagmin-7 are unclear. We show that when present separately, synaptotagmin-1 and synaptotagmin-7 act as standalone fast and slow Ca2+-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-7 stimulates Ca2+-dependent vesicle priming and inhibits depriming. The priming effect of Synaptotagmin-7 extends to the Readily Releasable Pool, whose fusion is executed by synaptotagmin-1, indicating synergistic action of the two Ca2+-sensors, although they are only partially colocalized. Synaptotagmin-7 promotes ubMunc13-2-dependent priming and the absence of synaptotagmin-7 renders phorbolesters less effective in stimulating priming, although synaptotagmin-7 independent priming is also observed. Morphologically, synaptotagmin-7 places vesicles in close membrane apposition (< 6 nm); in its absence vesicles accumulate out of reach of the fusion complex (20-40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca2+-dependent priming and sets the stage for fast and slow exocytosis triggering.

neuroscience↗