Abscission is the least understood step of cytokinesis. It consists of the final cut of the intercellular bridge connecting the sister cells at the end of mitosis, and is thought to involve membrane trafficking as well as lipid and cytoskeleton remodelling. We previously identified the Rab35 GTPase as a regulator of a fast recycling endocytic pathway that is essential for post-furrowing cytokinesis stages. Here, we report that the phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) 5-phosphatase OCRL, which is mutated in Lowe syndrome patients, is an effector of the Rab35 GTPase in cytokinesis abscission. GTP-bound (active) Rab35 directly interacts with OCRL and controls its localization at the intercellular bridge. Depletion of Rab35 or OCRL inhibits cytokinesis abscission and is associated with local abnormal PtdIns(4,5)P2 and F-actin accumulation in the intercellular bridge. These division defects are also found in cell lines derived from Lowe patients and can be corrected by the addition of low doses of F-actin depolymerization drugs. Our data demonstrate that PtdIns(4,5)P2 hydrolysis is important for normal cytokinesis abscission to locally remodel the F-actin cytoskeleton in the intercellular bridge. They also reveal an unexpected role for the phosphatase OCRL in cell division and shed new light on the pleiotropic phenotypes associated with Lowe disease.

Cytokinesis bridge instability leads to binucleated cells that can promote tumorigenesis in vivo. Membrane trafficking is crucial for animal cell cytokinesis, and several endocytic pathways regulated by distinct GTPases (Rab11, Rab21, Rab35, ARF6, RalA/B) contribute to the postfurrowing steps of cytokinesis. However, little is known about how these pathways are coordinated for successful cytokinesis. The Rab35 GTPase controls a fast endocytic recycling pathway and must be activated for SEPTIN cytoskeleton localization at the intercellular bridge, and thus for completion of cytokinesis. Here, we report that the ARF6 GTPase negatively regulates Rab35 activation and hence the Rab35 pathway. Human cells expressing a constitutively activated, GTP-bound ARF6 mutant display identical endocytic recycling and cytokinesis defects as those observed upon overexpression of the inactivated, GDP-bound Rab35 mutant. As a molecular mechanism, we identified the Rab35 GAP EPI64B as an effector of ARF6 in negatively regulating Rab35 activation. Unexpectedly, this regulation takes place at clathrin-coated pits, and activated ARF6 reduces Rab35 loading into the endocytic pathway. Thus, an effector of an ARF protein is a GAP for a downstream Rab protein, and we propose that this hierarchical ARF/Rab GTPase cascade controls the proper activation of a common endocytic pathway essential for cytokinesis.

Dlg1 recruits LGN to the cortex of cells in the chick neuroepithelium and may provide instructive cues that drive planar spindle orientation.

ERM activation by SLK kinase promotes polarized association at the mitotic cortex of LGN and NuMA, a necessary step in proper spindle orientation.

No abstract