![]() However, current models of transport into dendrites rely on plus end-directed motors ( Setou et al., 2004 Hirokawa and Takemura, 2005 Kennedy and Ehlers, 2006 Levy and Holzbaur, 2006). Thus, the simplest model for selective transport from the cell body to dendrites is use of a minus end-directed motor. Where examined, microtubules in vertebrate dendrites have mixed orientation, and in axons they have uniform orientation with all plus ends distal to the cell body. Most neurons have a cell body in which the bulk of proteins are synthesized, dendrites that are specialized to receive signals, and axons that are specialized to send them. Neurons are strikingly polarized and initially seemed that they would be the clearest example of cells in which microtubule orientation formed the basis of directional transport and cell polarity ( Black and Baas, 1989). Many differentiated cells have highly polarized arrays of microtubules that likely play a large role in establishing their specialized architecture and function. As predicted by our map, endosomes travel smoothly between the cell body and axon, but they cannot move directly between the cell body and dendrites. We confirm this prediction, and validate the completeness of our map, by imaging endosome movements in motor neurons. Surprisingly, our map of microtubule orientation predicts that there are no tracks for direct cargo transport between the cell body and dendrites in unipolar neurons. This result suggests that minus-end-out, rather than mixed orientation, microtubules are the signature of the dendritic microtubule cytoskeleton. However, in proximal dendrites of all classes of neuron, ∼90% of dendritic microtubules were oriented with minus ends distal to the cell body. As expected, all axonal microtubules have plus-end-out orientation. Using microtubule plus end dynamics, we mapped microtubule orientation in Drosophila sensory neurons, interneurons, and motor neurons. To determine whether microtubule orientation is a conserved feature of axons and dendrites, we analyzed microtubule orientation in invertebrate neurons. In vertebrate neurons, axons have a uniform arrangement of microtubules with plus ends distal to the cell body (plus-end-out), and dendrites have equal numbers of plus- and minus-end-out microtubules.
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