Microtubules Have Opposite Orientation in Axons and Dendrites of Drosophila Neurons

Abstract

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. To determine whether microtubule orientation is a conserved feature of axons and dendrites, we analyzed microtubule orientation in invertebrate neurons. Using microtubule plus end dynamics, we mapped microtubule orientation in Drosophila sensory neurons, interneurons, and motor neurons. As expected, all axonal microtubules have plus-end-out orientation. However, in proximal dendrites of all classes of neuron, ϳ90% of dendritic microtubules were oriented with minus ends distal to the cell body. This result suggests that minus-end-out, rather than mixed orientation, microtubules are the signature of the dendritic microtubule cytoskeleton. 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. We confirm this prediction, and validate the completeness of our map, by imaging endosome movements in motor neurons. 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. INTRODUCTIONMany differentiated cells have highly polarized arrays of microtubules that likely play a large role in establishing their specialized architecture and function. 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). 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. 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. Thus, the simplest model for selective transport from the cell body to dendrites is use of a minus end-directed motor. However, current models of transport into dendrites rely on plus enddirected motors (Setou et al., 2004;Hirokawa and Takemura, 2005;Kennedy and Ehlers, 2006;Levy and Holzbaur, 2006). These models raise the question: are minus-end-out microtubules important for directional transport or neuronal polarity?Axonal microtubule orientation has been examined in a variety of neurons, all with the same result: Ͼ95% of plus ends are oriented away from the cell body (plus-end-out). Original studies on axonal microtubule orientation relied on decoration of microtubules with exogenous tubulin, which forms curved hooks on the sides of existing microtubules, and analysis by electron microscopy. The direction of hook curvature indicates microtubule polarity. This method was used to determine axonal microtubule orientation in many different types of vertebrate neurons (Burton and Paige, 1981;Heidemann et al., 1981;Baas et al., 1987Baas et al...

Microtubule Organization and Microtubule-Associated Proteins (MAPs)

“…In Drosophila and C. elegans neurons, MTs in axons are arranged with their plus ends distal to the cell body, as in vertebrates. However, in dendrites most MTs are arranged with their minus ends distal to the cell body, although some mixed MTs have also been observed (Stone et al 2008;Maniar et al 2011). It is thought that differences in the MT cytoskeleton in axons and dendrites can facilitate polarized cargo trafficking (Kapitein and Hoogenraad 2011;Rolls 2011).…”

Modeling transport of a pulse of radiolabeled organelles in a Drosophila unipolar motor neuron

“…Understanding transport of organelles in unipolar neurons is challenging. Indeed, as noted in Rolls [13], it is not clear how dendrites receive any cargo, as it was shown in Stone et al [7], no microtubules travel from the primary neurite into the dendrite. The purpose of this paper is to investigate this situation theoretically and test the hypothesis that when the dendritedestined organelles enter the axon, they detach from kinesin and reattach to dynein motors.…”

“…This is different from mammalian dendrites where MTs have mixed polarity orientation, with approximately 50% having their plus-ends directed outward and 50% having their minus-ends directed outward (Baas et al [9], Stepanova et al [10]). According to Stone et al [7], this suggests that minus-end-out rather than mixed MT polarity orientation may be a signature of the dendritic cytoskeleton. There are also some interesting studies on the ability of Drosophila neurons to regenerate after trauma, such as after complete axon removal (Stone et al [11]).…”

“…This research has already brought some interesting insights. Recently, Rolls et al [6] and Stone et al [7] (see also Baas and Lin [8]) investigated microtubule orientation in Drosophila neurons and reported that in axons all MTs have plus-end-out orientation while in dendrites more than 90% of MTs have minus-end-out orientation. This is different from mammalian dendrites where MTs have mixed polarity orientation, with approximately 50% having their plus-ends directed outward and 50% having their minus-ends directed outward (Baas et al [9], Stepanova et al [10]).…”

“…In such neurons, the neuron body (soma) gives rise to a single process (a primary neurite) that after some distance bifurcates into one axonal and one dendritic arbor, see Fig. 1a (Tuthill [5], Stone et al [7]). The purpose of an axon is to transmit signals while the purpose of a dendrite is to receive signals [12].…”