Differentiation between Oppositely Oriented Microtubules Controls Polarized Neuronal Transport

Abstract

Microtubules are essential for polarized transport in neurons, but how their organization guides motor proteins to axons or dendrites is unclear. Because different motors recognize distinct microtubule properties, we used optical nanoscopy to examine the relationship between microtubule orientations, stability, and modifications. Nanometric tracking of motors to super-resolve microtubules and determine their polarity revealed that in dendrites, stable and acetylated microtubules are mostly oriented minus-end out, while dynamic and tyrosinated microtubules are oriented oppositely. In addition, microtubules with similar orientations and modifications form bundles that bias transport. Importantly, because the plus-end-directed Kinesin-1 selectively interacts with acetylated microtubules, this organization guides this motor out of dendrites and into axons. In contrast, Kinesin-3 prefers tyrosinated microtubules and can enter both axons and dendrites. This separation of distinct microtubule subsets into oppositely oriented bundles constitutes a key architectural principle of the neuronal microtubule cytoskeleton that enables polarized sorting by different motor proteins.

Keywords: axon; dendrites; kinesin; microtubule orientation; microtubule polarity; microtubules; motor proteins; neuronal polarity; neurons; polarized transport.

Figure 1

motor-PAINT: Super-Resolution Imaging of Microtubules and Their Orientation (A) Assay: after extraction and fixation, purified and fluorescently labeled motors are added and map out the microtubule array by unidirectional runs. See also Figures S1 and S2. (B) Super-resolved image of an extracted U2OS cell obtained by subpixel localization of thousands of motor binding events. (C) Left: super-resolution reconstruction of the same cell with all microtubule segments colored according to their absolute orientation. Legend arrows point in the direction of the plus end. Right: zooms showing free microtubule plus and minus end. See also Figure S2. (D) Orientation mapping of a centrosomal microtubule array obtained after nocodazole washout in a COS7 cell. (E) Motor-based super-resolution reconstruction of microtubules in dendrites and axons of cultured rat hippocampal neurons (DIV16–DIV17). Top images are based on all binding events (>42,405 events per image). Bottom images are color coded for absolute orientation. Track interpolation was used for all run-based images. (F) Quantification of inward- and outward-moving kinesins in 5-μm-long proximal, middle, and distal dendritic segments, reflecting the number minus-end-outward- and -inward-oriented microtubules, respectively. (G) Percentage of minus-end-out-oriented microtubules in proximal, middle, and distal dendritic segments, based on the graphs shown in (F). (H) Average percentage of minus-end-out-oriented microtubules in proximal, middle, and distal dendritic segments (mean ± SEM, n = 7 segments from 7 neurons for every category). Scale bars, 5 μm (B), 1 μm (D and E).

Figure 2

Dendritic Microtubule Arrays Spatially Segregate by Orientation and Modification (A) Three examples of dendrites demonstrating bundles of preferred polarity (left-right). Motor-based super-resolution reconstructions based on all binding events (top), inward runs (middle right), outward runs (middle left), or runs of both directions (bottom). Track interpolation was used for the run-based images. (B) Intensity profiles for inward- and outward-pointing microtubules along the lines indicated in (A). See also Figure S3. (C) Ratio between outward and inward runs or inverse for regions marked with 1 or 2 in (A). Mean ± SEM. (D) STED image from the soma of a DIV2 neuron immunostained for tyrosinated and acetylated MTs (top) and the individual tyrosinated (bottom left) and acetylated channel (bottom right). (E) Zooms from DIV2 and DIV7 neurites highlighting spatial segregation between tyrosinated and acetylated MTs. Corresponding intensity profiles along the indicated line is shown next to the image on the right. See also Figure S4. Scale bars, 1 μm (A and E), 5 μm (D).

Figure 3

Minus-End-Out-Oriented Microtubules Are More Stable and More Acetylated (A and B) Overview and zoom of DIV9 neurons immunostained for tyrosinated and acetylated microtubules in control conditions (A) or following 2.5 hr incubation with 4 μM nocodazole (B). (C and D) motor-PAINT performed on a dendritic segment in control conditions (C) or after nocodazole treatment (D). (E) Percentage of minus-end-out-oriented microtubules in dendritic segments in control and nocodazole-treated neurons. Mean ± SD, control: n = 7, nocodazole: n = 16 acquired in 3 independent experiments. t test: ^∗∗∗p < 0.001. (F) Correlative reconstructed images of minus-end in (left), minus-end out (middle), and acetylated microtubules (right) of a dendritic segment. See also Figure S5. (G) Intensity profiles measured for both microtubule orientations and the acetylated tubulin channel along the line indicated in (F). Scale bars, 5 μm (A and B), 1 μm (C, D, and F).

Figure 4

Kinesin-1 Prefers Stable, Acetylated Microtubules in Neuronal Axon and Dendrites (A and B) STED image of DIV4 polarized neuronal soma (A) or neurites (B) after 1 day expression of GFP-kif5a rigor stained for GFP and acetylated-tubulin or GFP and tyrosinated-tubulin. See also Figure S6. (C) Cartoon illustrating the existing and new model for dendritic microtubule organization. Arrowheads on microtubules depict plus ends. Kinesin-1 and Kinesin-3 preferentially move over stable/acetylated and dynamic/tyrosinated microtubules, respectively. (D) The new model can explain the selective entry of Kinesin-1 into axons. Arrows depict bias in transport directionality (see also Figure S7). Scale bars, 2 μm (A and B).