In contrast, dendritic microtubules have mixed polarity, and so any motor protein could carry cargo into dendrites. In axons, all microtubules have their plus ends directed away from the cell body (plus-end-out), and thus only plus end-directed kinesins can carry cargo into axons. In primary cultures of mammalian neurons, microtubules in axons and dendrites have different arrangements. This long-distance transport is primarily microtubule-based, and so the arrangement of microtubules has the potential to control directional trafficking and neuronal polarity. Most neuronal protein synthesis takes place in the cell body, and so proteins must be moved to their place of function after synthesis. In order to establish and maintain functionally distinct compartments, different proteins must accumulate in axons and dendrites. Many neurons are specialized to receive information through dendrites and send signals through axons. Dendrite identity is established before the mature microtubule arrangement is attained, during the period of mixed microtubule polarity. Over time they mature to uniform microtubule polarity. We conclude that developing Drosophila dendrites initially have mixed microtubule polarity. Dendritic markers also localized correctly in this case. To confirm that dendrite character could be acquired in Drosophila while microtubules were mixed, we genetically disrupted uniform dendritic microtubule organization. An axonal marker was also quite well excluded from dendrites throughout development, although this was perhaps more efficient in mature neurons. In all cases the markers took on their characteristic distribution while dendrites had mixed polarity. To determine whether features characteristic of dendrites were localized before uniform polarity was attained, we analyzed dendritic markers as dendrites developed. Over a period of several days this mixed microtubule array gradually matured to a minus-end-out array. As dendrites initially extended, they contained mixed polarity microtubules, like mammalian neurons developing in culture. To determine whether Drosophila and mammalian dendrites have a common microtubule organization during development, we analyzed microtubule polarity in Drosophila dendritic arborization neuron dendrites at different stages of outgrowth from the cell body in vivo. This microtubule arrangement is different from that of cultured mammalian neurons, which have mixed polarity microtubules in dendrites. Drosophila neurons have dendrites that contain minus-end-out microtubules.
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