Spectraplakin cooperates with noncentrosomal microtubule regulators to orient dendritic microtubules in Drosophila

The differential microtubule organization in axons and dendrites underlies neurite-specific transport and morphogenetic processes like developmental neurite pruning, but how it arises is incompletely understood. While axonal "plus end-out" microtubules just grow from the soma, the origin of the dendritic "plus end-in" microtubules is more difficult to explain. A transient microtubule organizing center (MTOC) was recently detected at dendrite tips in C. elegans, but the players and generalizability are unclear. In a survey for factors involved in dendrite pruning of Drosophila sensory neurons, we found the spectraplakin Short stop (Shot), an actin-microtubule crosslinker known to bundle axonal microtubules. In dendrites, however, Shot is required for proper "plus end-in" microtubule organization. During early dendrite development, Shot localizes to dendrite tips in an actin-dependent manner, providing a potential mechanism for dendrite tip recognition. Our data show that Shot cooperates with a functional MTOC containing the small GTPase Rab11. Localized MTOC activity may commonly underlie dendritic microtubule organization.