Neuronal mRNP transport follows an aging Levy walk

A newly transcribed mRNA binds multiple RBPs to form an mRNP, which is subsequently transported into a dendrite. There are multiple microtubule tracks along each dendrite. An mRNP particle is transported bidirectionally by concerted work of motor proteins along the microtubules. Using a genetically engineered mouse that expresses β-actin mRNA labeled with green fluorescent proteins (GFPs), we imaged individual β-actin mRNP particles in hippocampal neurons using wide-field fluorescence microscopy. Because each -actin mRNA is labeled with up to 48 GFPs, individual mRNPs appear as bright spots in fluorescence images. In neurons, mRNP complexes, often called neuronal RNA granules, contain many RBPs and ribosomes and are particularly large with an estimated size of 175-600 nm. The addition of 48 GFPs, each of which is approximately 4 nm in diameter, would not significantly alter the overall size of RNA granules (less than 0.06% difference) or their native dynamics. By tracking these spots in time-lapse images, we monitored the active transport of mRNP particles in live neurons. We selected a region of interest along adendrite, as highlighted in yellow in Fig. 1b, and generated a straightened image of the dendrite. From the straightened time-lapse image stack, we obtained one-dimensional trajectories of individual mRNPs along the curvilinear line of the dendrite they reside in. We collected data from 57 cells in 12 batches of neuron culture and obtained 102 kymographs. The trajectories of 682 mRNP particles were analyzed with a time resolution of t0 = 100 ms and a spatial resolution of 205 nm.

刚转录完成的mRNA结合多个RNA结合蛋白（RNA-binding proteins, RBPs）形成信使核糖核蛋白复合物（mRNP），随后被转运至树突。每个树突上均分布有多条微管轨道。mRNP颗粒可借助分子马达蛋白的协同作用沿微管完成双向运输。
本研究使用表达经绿色荧光蛋白（green fluorescent proteins, GFPs）标记的β-肌动蛋白mRNA的基因工程小鼠，通过宽场荧光显微镜对海马神经元内的单个β-肌动蛋白mRNP颗粒进行成像。由于每个β-肌动蛋白mRNA可被多达48个GFPs标记，单个mRNP在荧光图像中呈现为明亮光斑。
在神经元中，mRNP复合物通常被称为神经元RNA颗粒，其包含多种RBPs与核糖体，尺寸尤为庞大，预估范围为175~600 nm。每个GFP的直径约为4 nm，因此添加48个GFPs不会显著改变RNA颗粒的整体尺寸（差异小于0.06%），也不会影响其天然动力学特性。
通过对延时图像中的这些光斑进行追踪，我们实现了活神经元内mRNP颗粒主动运输过程的实时监测。我们沿树突选取了感兴趣区域（如图1b中黄色高亮部分所示），并生成该树突的拉直图像。从该拉直的延时图像序列中，我们获取了单个mRNP沿其所在树突曲线路径的一维运动轨迹。
本研究从12批神经元培养物的57个细胞中收集数据，共得到102张时空图（kymograph）。我们对682个mRNP颗粒的运动轨迹进行了分析，分析的时间分辨率为t₀=100 ms，空间分辨率为205 nm。