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.