Berns Manon M M, Yildiz Mirza, Winkelmann Stefanie, Walter Alexander M
Department of Neuroscience, University of Copenhagen, Copenhagen, Denmark.
Zuse Institute Berlin, Berlin, Germany.
J Physiol. 2025 Jan 14. doi: 10.1113/JP286651.
Synaptic vesicle (SV) trafficking toward the plasma membrane (PM) and subsequent SV maturation are essential for neurotransmitter release. These processes, including SV docking and priming, are co-ordinated by various proteins, such as SNAREs, Munc13 and synaptotagmin (Syt), which connect (tether) the SV to the PM. Here, we investigated how tethers of varying lengths mediate SV docking using a simplified mathematical model. The heights of the three tether types, as estimated from the structures of the SNARE complex, Munc13 and Syt, defined the SV-PM distance ranges for tether formation. Geometric considerations linked SV-PM distances to the probability and rate of tether formation. We assumed that SV tethering constrains SV motility and that multiple tethers are associated by independent interactions. The model predicted that forming multiple tethers favours shorter SV-PM distances. Although tethers acted independently in the model, their geometrical properties often caused their sequential assembly, from longer ones (Munc13/Syt), which accelerated SV movement towards the PM, to shorter ones (SNAREs), which stabilized PM-proximal SVs. Modifying tether lengths or numbers affected SV trafficking. The independent implementation of tethering proteins enabled their selective removal to mimic gene knockout (KO) situations. This showed that simulated SV-PM distance distributions qualitatively aligned with published electron microscopy studies upon removal of SNARE and Syt tethers, whereas Munc13 KO data were best approximated when assuming additional disruption of SNARE tethers. Thus, although salient features of SV docking can be accounted for by independent tethering alone, our results suggest that functional tether interactions not yet featured in our model are crucial for biological function. KEY POINTS: A mathematical model describing the role of synaptic protein tethers to localize transmitter-containing vesicles is developed based on geometrical considerations and structural information of synaptotagmin, Munc13 and SNARE proteins. Vesicle movement, along with tether association and dissociation, are modelled as stochastic processes, with tethers functioning independently of each other. Multiple tethers cooperate to recruit vesicles to the plasma membrane and keep them there: Munc13 and Syt as the longer tethers accelerate the movement towards the membrane, whereas short SNARE tethers stabilize them there. Model predictions for situations in which individual tethers are removed agree with the results from experimental studies upon gene knockout. Changing tether length or copy numbers affects vesicle trafficking and steady-state distributions.
突触小泡（SV）向质膜（PM）的运输以及随后的SV成熟对于神经递质释放至关重要。这些过程，包括SV对接和引发，由各种蛋白质协调，如SNARE蛋白、Munc13和突触结合蛋白（Syt），它们将SV连接（拴系）到PM上。在这里，我们使用简化的数学模型研究了不同长度的拴系如何介导SV对接。根据SNARE复合体、Munc13和Syt的结构估计的三种拴系类型的高度，定义了拴系形成的SV-PM距离范围。几何因素将SV-PM距离与拴系形成的概率和速率联系起来。我们假设SV拴系限制了SV的运动性，并且多个拴系通过独立相互作用相关联。该模型预测，形成多个拴系有利于缩短SV-PM距离。尽管拴系在模型中独立起作用，但其几何特性常常导致它们按顺序组装，从较长的拴系（Munc13/Syt）开始，其加速SV向PM的移动，到较短的拴系（SNARE蛋白），其稳定靠近PM的SV。改变拴系长度或数量会影响SV运输。拴系蛋白的独立实现使得能够选择性去除它们以模拟基因敲除（KO）情况。这表明，在去除SNARE和Syt拴系后，模拟的SV-PM距离分布在质量上与已发表的电子显微镜研究结果一致，而当假设SNARE拴系存在额外破坏时，Munc13 KO数据得到最佳近似。因此，尽管SV对接的显著特征可以仅由独立拴系来解释，但我们的结果表明，我们模型中尚未体现的功能性拴系相互作用对于生物学功能至关重要。要点：基于突触结合蛋白、Munc13和SNARE蛋白的几何因素和结构信息，开发了一个描述突触蛋白拴系在定位含递质小泡中作用的数学模型。小泡运动以及拴系的结合和解离被建模为随机过程，拴系彼此独立起作用。多个拴系协同作用将小泡招募到质膜并使其保持在那里：Munc13和Syt作为较长的拴系加速向膜的移动，而短的SNARE拴系使其在那里稳定。对去除单个拴系情况的模型预测与基因敲除实验研究结果一致。改变拴系长度或拷贝数会影响小泡运输和稳态分布。
