3/24/2023 0 Comments Neurons dendrite![]() ![]() Both hook labeling and microtubule plus end-binding protein dynamics have been used to analyze microtubule orientation in dendrites in cultured rodent interneurons. In these dendrites, approximately equal numbers of microtubules had plus and minus ends distal to the cell body throughout the length of the dendrite ( Burton, 1988). The hook method has been used to analyze dendritic microtubule orientation in one type of neuron with branched dendrites in vivo: frog mitral cells, which are interneurons. However, dendrites are generally much more difficult to study, and their microtubule organization has been examined much less than that of axons. Similarly, mixed orientation of microtubules has been considered a signature of dendrites ( Alberts et al., 2002). Uniform plus-end-out microtubule orientation thus seems to be a universal and evolutionarily conserved signature of axons. Additionally, second harmonic generation microscopy has confirmed axonal microtubules in vivo and in vitro have uniform microtubule orientation ( Dombeck et al., 2003). Using both assays, in sensory and central neurons, in organisms ranging from the invertebrate Aplysia to mammals, >95% of axonal microtubules have been found to be plus-end-out. More recently, the direction of movement of proteins that bind to growing microtubule plus ends was used to analyze axonal microtubule orientation in cultured mouse hippocampal and Purkinje neurons ( Stepanova et al., 2003) and cultured Aplysia neurons ( Erez et al., 2007). 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., 1987, 1988 Troutt and Burnside, 1988). The direction of hook curvature indicates microtubule polarity. 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. These models raise the question: are minus-end-out microtubules important for directional transport or neuronal polarity?Īxonal microtubule orientation has been examined in a variety of neurons, all with the same result: >95% of plus ends are oriented away from the cell body (plus-end-out). 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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