442 Biochemical Society Transactions (2008) Volume 36, part 3 Endosome positioning during cytokinesis Guillaume Montagnac*† 1 and Philippe Chavrier*† *Institut Curie, Centre de Recherche, Paris F-75248 France, and †CNRS, UMR144, Membrane and cytoskeleton dynamics, 26 rue d’Ulm, 75248 Paris Cedex 05, France Abstract In mammalian cells, completion of cytokinesis relies on targeted delivery of recycling membranes to the midbody. At this step of mitosis, recycling endosomes are organized as clusters located at the mitotic spindle poles as well as at both sides of the midbody. However, the mechanism that controls endosome positioning during cytokinesis is not known. Here, we discuss the possible mechanisms that drive the formation of endosomal clusters and the importance of this process for the targeted delivery of recycling membranes to the midbody. Introduction During mitosis of animal cells, the mitotic MT (microtubule) spindle drives the separation of chromosomes while the plasma membrane ingresses between the two nascent daughter cells. Additional to the mitotic spindle is the central spindle (or midzone MTs), a set of antiparallel MTs that become bundled between the separating chromosomes as the cleavage furrow continues to ingress [1]. This results in the formation of a narrow cytoplasmic bridge filled with MTs of the central spindle that are oriented with their fast growing plus-ends embedded in a central electron-dense matrix called the midbody. Completion of cytokinesis is the final step of mitosis, resulting in cleavage of the bridge in a process called abscission, which allows for proper separation of daughter cells. Recent work focused on the role of membrane trafficking during cytokinesis and in particular it has been demonstrated that membrane insertion at the midbody is required for abscission of the bridge [2]. The probable scenario is that this intercellular channel is first plugged with delivered vesicles which then fuse together and with the plasma membrane, thus facilitating abscission. The origin of membranes that are trafficked toward the midbody of mammalian cells is still not clear. While the necessity of post-Golgi trafficking for the completion of cytokinesis is a matter of debate [1,3], it is well documented that endocytic recycling is required for abscission [1,2]. During cytokinesis, recycling endosomes cluster around the two poles of the mitotic spindle as well as at both extremities of the central spindle [4–6]. Interestingly, the Golgi displays the very same distribution at this step of mitosis [7]. Recycling vesicles targeted to the midbody are likely to be transported from these endosomal clusters. In this regard, clusters proximal to the central spindle are ideally located because they face MT bundles that are penetrating into the bridge and because these bundles could provide Key words: cytokinesis, endosome, membrane recycling, microtubule. Abbreviations used: CLASP, cytoplasmic linker protein-associated protein; MT, microtubule. 1 To whom correspondence should be addressed (email guillaume.montagnac@curie.fr). tracks for the transport of vesicles toward the midbody. Thus it is essential to understand the mechanism of central spindle cluster formation as well as the importance of this process for the efficiency of membrane delivery to the midbody. Clustering endosomes: finding MT minus ends During interphase, clustering of various organelles, including Golgi cisternae, late endosomes/lysosomes as well as recycling endosomes in a juxtanuclear region close to the centrosome, relies on the MT network and on the activity of MT-dependent motors [8]. The minus end-directed motor dynein is particularly important in this regard as inhibition of its function results in both Golgi and perinuclear recycling endosome scattering throughout the cytoplasm [9]. The centrosome is the main MT-organizing centre as it has the ability to nucleate MTs and to anchor MT minus-ends [10]. MT nucleation depends on γ -tubulin, an essential component of centrosome, which provides a template for the assembly of α- and β -tubulin heterodimers [10]. Before mitosis, the centrosome duplicates to allow the formation of a bipolar mitotic spindle with the two centrosomes located at each spindle pole [11]. It is therefore not surprising to find endosome accumulation at the spindle poles as most of MT minus ends localize there. The fact that endosomes also cluster at both ends of the central spindle is more puzzling. A subset of MTs of the cen- tral spindle emanates from the two mitotic spindle poles. The central spindle is also composed of shorter MTs decorated with γ -tubulin at their extremities [12], suggesting that minus ends also exist at the edges of the central spindle. Yet, how these minus ends are generated and stabilized at this location is not known. One hypothesis is that some mitotic spindle MTs are released from the poles to form a subset of mid- zone MTs, as was observed in Drosophila [13]. Another pos- sibility is the de novo formation of MTs at the central spindle. Indeed, new MTs could be nucleated from the surface of pre- existing mitotic spindle MTs in a process requiring γ -tubulin C  The Authors Journal compilation C  2008 Biochemical Society Biochem. Soc. Trans. (2008) 36, 442–443; doi:10.1042/BST0360442 Biochemical Society Transactions www.biochemsoctrans.org