Polarity sorting of axonal microtubules: a computational study.

We present a computational model to test a "polarity sorting" mechanism for microtubule (MT) organization in developing axons. We simulate the motor-based axonal transport of short MTs to test the hypothesis that immobilized cytoplasmic dynein motors transport short MTs with their plus ends leading, so "mal-oriented" MTs with minus-end-out are transported toward the cell body while "correctly" oriented MTs are transported in the anterograde direction away from the soma. We find that dynein-based transport of short MTs can explain the predominately plus-end-out polarity pattern of axonal MTs but that transient attachments of plus-end-directed motor proteins and nonmotile cross-linker proteins are needed to explain the frequent pauses and occasional reversals observed in live-cell imaging of MT transport. Static cross-linkers increase the likelihood of a stalled "tug-of-war" between retrograde and anterograde forces on the MT, providing an explanation for the frequent pauses of short MTs and the immobility of longer MTs. We predict that inhibition of the proposed static cross-linker will produce disordered transport of short MTs and increased mobility of longer MTs. We also predict that acute inhibition of cytoplasmic dynein will disrupt the polarity sorting of MTs by increasing the likelihood of "incorrect" sorting of MTs by plus-end-directed motors.

Cytoplasmic Dynein Transports Axonal Microtubules in a Polarity-Sorting Manner.

Axonal microtubules are predominantly organized into a plus-end-out pattern. Here, we tested both experimentally and with computational modeling whether a motor-based polarity-sorting mechanism can explain this microtubule pattern. The posited mechanism centers on cytoplasmic dynein transporting plus-end-out and minus-end-out microtubules into and out of the axon, respectively. When cytoplasmic dynein was acutely inhibited, the bi-directional transport of microtubules in the axon was disrupted in both directions, after which minus-end-out microtubules accumulated in the axon over time. Computational modeling revealed that dynein-mediated transport of microtubules can establish and preserve a predominantly plus-end-out microtubule pattern as per the details of the experimental findings, but only if a kinesin motor and a static cross-linker protein are also at play. Consistent with the predictions of the model, partial depletion of TRIM46, a protein that cross-links axonal microtubules in a manner that influences their polarity orientation, leads to an increase in microtubule transport.