Emerging regulators of endosomal dynamics during mitosis

Abstract

Homotypic and heterotypic fusions characterize the endosomal system and ensure proper cargo sorting and exchange in interphase. However, during mitosis, endosomal fusion was found to be inhibited.1 For many years, this finding lead to an underestimation of the importance of endocytic transport during mitosis, and therefore to a limited focus on endosomal dynamics during mitosis and cytokinesis. In the last few years, with new knowledge, the interest in the endosomal trafficking for cytokinesis regulation has increased. ESCRT proteins, Rab, and Arf GTPases, together with the cytoskeleton and their interactors are all found to be involved in the regulation of the endocytic membrane transport during cytokinesis.2 However, little is known regarding the regulatory mechanisms responsible for the arrest of endosomal fusion during the early stages of mitosis. Toyoshima’s group has now identified Polo-like kinase 1 (Plk1) as an inhibitor of endosomal fusion during mitosis.3 Plk1 is a master regulator of cell division known to play important and different roles at several steps of mitosis and cytokinesis, being responsible for phosphorylation of more than 100 sites on distinct spindle proteins.4 By nano-scale liquid chromatography-tandem mass spectrometry on purified early endosomal fraction of HeLa cells arrested in M phase, Ikawa and coworkers were able to identify Ser459 on vimentin as the target of Plk1 responsible for the inhibition of endosomal fusion during mitosis.3 Vimentin, like other IF proteins, is phosphorylated at different sites during mitosis, and its site-specific phosphorylation is important for filament disassembly and efficient segregation of IFs into daughter cells.5 Interestingly, Ser459 of vimentin, in contrast to other known phosphorylation sites, does not affect vimentin assembly.3 Therefore, the phosphorylation by Plk1 on that site seems to be specific only for endosomal fusion regulation. The small GTPase Rab5 and its interactor EEA1 control the fusion of early endosomes in interphase, and it has been suggested that an increase of GAP activity toward Rab5 in mitosis may be one of the mechanisms that inhibits fusion.6 However, Ikawa et al. do not detect any influence of Plk1 on Rab5 activity, suggesting that the Ser459-phosphorylated form of vimentin recruits another specific protein required for the inhibition of endocytic fusion.3 An earlier report showed that EEA1 is only weakly associated with early endosomes during mitosis, and the tight binding (long-lived fraction) is gone.7 A reduced binding of the tethering protein EEA1 during mitosis could then abrogate homotypic fusion of endosomes, but it remains to be seen whether the specific phosphorylation of vimentin is part of this process. The results from Toyoshima’s group may suggest a novel and unconventional mechanism for endosomal fusion regulation. Alternatively, the phosphorylation of Ser459 on vimentin could be important in order to control the spatial distribution of endosomes and, therefore, to prevent them from making contact for fusion. Conversely, when phosphorylation is inhibited, endosomes would be free to move, meet, and fuse through known mechanisms regulated by tethering and fusion molecules such as Rab5-EEA1 and SNAREs. It is interesting to note that a recent work identifies Rab7a as a vimentin interactor.8 As purified endosomes with bound vimentin are positive for both Rab5 and Rab7a, but vimentin does not seem to directly affect Rab5,3 it would be interesting to investigate in the future whether Rab7a has a role in the vimentin control of endosomal fusion during mitosis. In agreement with the role of vimentin in regulation of endosome dynamics during mitosis, Ikawa et al. revealed that Rab21-regulated integrin trafficking to the cleavage furrow during later phases of mitosis is also dependent on the phosphorylation site Ser459 of vimentin by Plk1.3 In conclusion, the authors propose a model where Plk1 phosphorylation of Ser459 of vimentin inhibits endosomal fusion to keep cargos in separate endosomes and therefore specifically promotes the transport of Rab21-positive endosomes associated with integrin to the cleavage furrow (Fig.1). Figure 1. During mitosis, Plk1 phosphorylates vimentin on Ser459. This causes a block of endosomal fusion but not depolymerization of vimentin filaments. The block of endosomal fusion may be mediated by the recruitment of an unknown protein. In ... The importance of endosomal transport and fusion to ensure a fast and targeted delivery of lipids and regulatory molecules during cell division is increasingly recognized, but the mechanisms controlling these processes are still unclear. As several distinct subpopulations of endosomes like Rab11/FIP3 endosomes, Rab35 endosomes, and PI3P endosomes has recently been shown to play essential roles in cytokinesis,2 it will be important to investigate whether the Plk1–vimentin axis also regulates the fusion ability and transport of such endosomes to the cleavage furrow.