Patronin/Shot Cortical Foci Assemble the Noncentrosomal Microtubule Array that Specifies the Drosophila Anterior-Posterior Axis

摘要

class="head no_bottom_margin" id="sec1title">IntroductionMany differentiated animal cells and all plant cells lack functional centrosomes, yet form highly organized microtubule (MT) arrays that play essential roles in cell polarity, organization, and function (). For example, both Drosophila and rodent hippocampal neurons develop normally without active centrosomes, with the latter extending and even regenerating axons independently of centrosomal MT nucleation (, ). Most Drosophila tissues lack functional centrosomes or microtubule organizing centers (MTOCs) in interphase ().Anterior-posterior axis formation in the Drosophila oocyte provides a well-studied example of the role of noncentrosomal MTs. Although the oocyte contains centrosomes, which cluster near the nucleus, oogenesis proceeds normally in their absence (, , ). Instead, the majority of MTs grow from the anterior/lateral cortex, but not from the posterior, where the plus ends concentrate (, , , ). This noncentrosomal MT array directs the localization of bicoid and oskar mRNAs to the anterior and posterior poles of the oocyte, respectively, to define the main body axis of the embryo (, ). 3D modeling of the oocyte MT cytoskeleton has shown that restricting MT minus ends to the anterior/lateral cortex is sufficient to generate an MT network that can direct the transport of oskar mRNA to the oocyte posterior by kinesin ().The formation of this polarized MT array is under the control of the PAR proteins, which localize in mutually antagonistic anterior and posterior cortical domains (, href="#bib53" rid="bib53" class=" bibr popnode">Shulman et al., 2000). The posterior crescent of the Par-1 kinase transmits this cortical polarity to the MT cytoskeleton by excluding minus ends from the oocyte posterior. It is not known, however, how PAR-1 activity is transduced into the asymmetric organization of MT minus ends, nor how the minus ends associate with the anterior/lateral cortex.The recent discovery of the Patronin family of MT minus-end-binding proteins, consisting of Patronin in Drosophila, CAMSAP1, 2, and 3 in mammals, and PTRN-1 in worms, has begun to reveal how the minus ends of noncentrosomal MTs are organized and maintained (href="#bib1" rid="bib1" class=" bibr popnode">Akhmanova and Steinmetz, 2015, href="#bib6" rid="bib6" class=" bibr popnode">Baines et al., 2009, href="#bib22" rid="bib22" class=" bibr popnode">Goodwin and Vale, 2010, href="#bib39" rid="bib39" class=" bibr popnode">Marcette et al., 2014, href="#bib40" rid="bib40" class=" bibr popnode">Meng et al., 2008, href="#bib47" rid="bib47" class=" bibr popnode">Richardson et al., 2014). The Patronins recognize and stabilize free MT minus ends by protecting them from depolymerization (href="#bib22" rid="bib22" class=" bibr popnode">Goodwin and Vale, 2010, href="#bib24" rid="bib24" class=" bibr popnode">Hendershott and Vale, 2014, href="#bib29" rid="bib29" class=" bibr popnode">Jiang et al., 2014). Patronins appear to play a particularly important role in organizing MTs in differentiated cells. CAMSAP3 localizes to the apical domain in epithelial cells, where it is required for the formation of the apical-basal array of MTs (href="#bib59" rid="bib59" class=" bibr popnode">Tanaka et al., 2012, href="#bib61" rid="bib61" class=" bibr popnode">Toya et al., 2016, href="#bib67" rid="bib67" class=" bibr popnode">Zheng et al., 2013). CAMSAP2 stabilizes neuronal MTs in axon and dendrites, and its knockdown leads to defects in axon specification and dendritic branch formation (href="#bib66" rid="bib66" class=" bibr popnode">Yau et al., 2014). Similarly, Caenorhabditis elegans PTRN-1 is required for normal neurite morphology and axon regeneration (href="#bib14" rid="bib14" class=" bibr popnode">Chuang et al., 2014, href="#bib39" rid="bib39" class=" bibr popnode">Marcette et al., 2014, href="#bib47" rid="bib47" class=" bibr popnode">Richardson et al., 2014). The function of Drosophila Patronin has only been examined in cultured S2 cells, where its depletion leads to a decrease in MT number and an increase in free moving MTs (href="#bib22" rid="bib22" class=" bibr popnode">Goodwin and Vale, 2010).Although it is now clear that the Patronins play an important role in organizing noncentrosomal MTs in differentiated cells, little is known about the regulation of the distribution and activity of the Patronins themselves. Here we show that Patronin is recruited to the anterior/lateral cortex of the Drosophila oocyte by the spectraplakin, Shot, under the control of Par-1. These Shot/Patronin complexes form the cortical noncentrosomal MTOCs that organize the polarized MT network in the oocyte, which specifies the anterior-posterior axis.