It seemed unlikely that most microtubules could be nucleated at t

It seemed unlikely that most microtubules could be nucleated at the centrosome of a neuron’s cell body and

still reach the periphery of the dendritic arbor. A few recent studies have shown that, in fact, acentrosomal nucleation occurs in neurons. Stiess et al. (2010) discovered that axon growth can still occur after the centrosome located in the cell body has been ablated, and that very few microtubules emanate from the centrosome in mature neurons. Nguyen et al. (2011) examined microtubule organization in neurons without a functional centrosome and found that microtubules are organized independently of the centrosome. These recent findings have raised three possibilities for new microtubule nucleation in neurons: (1) microtubules are formed at the centrosome, cleaved, and then transported to the proper compartment, (2) microtubules are severed in the periphery, which could provide a scaffold for nucleation/polymerization, and (3) microtubules are nucleated at unknown acentrosomal sites (reviewed by Kuijpers and Hoogenraad, 2011). In this issue of Neuron, Ori-McKenney et al. (2012) provide significant new insights into our understanding of the location of microtubule nucleation in neurons by visualizing acentrosomal

MT nucleation in the dendrites of Drosophila da neurons. This is a class of large neurons present in the peripheral nervous system of the larva that has become a model system for the study of dendritic morphogenesis (reviewed Jan and Jan, 2010). Their results reveal for the first time

that Golgi outpost-associated acentrosomal MT nucleation plays a key role in dendritic morphogenesis. Using time-lapse microscopy enough of a genetically-encoded probe for microtubule plus-end (EB1-EGFP), Ori-McKenney et al. (2012) began their study by examining microtubule nucleation events in primary, intermediate and terminal branches of the highly branched class IV da neurons. They confirm previous results showing that in Drosophila neurons, primary dendrites contain mostly minus-end distal MTs, while intermediate branches have a mixed orientation of MTs. Interestingly, terminal branches are composed mostly of plus-end distal MTs. After analyzing the dynamics of EB1-EGFP comets in these different branch types, the authors realized that most anterogradely and retrogradely translocating comets initiate within the branch, at branch points or at the distal end, but not from the cell body. This observation reminded the authors of previous work performed in their lab showing that Golgi outposts in Drosophila are present along the dendrites, at dendritic branch points, and at the distal tips ( Ye et al., 2007), a property also found in mammalian neurons ( Horton et al., 2005).

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