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Integrin-dependent cell-extracellular matrix (ECM) adhesion is a determinant of spindle orientation. However, the signaling pathways that couple integrins to spindle orientation remain elusive. Here, we show that PCTAIRE-1 kinase (PCTK1), a member of the cyclin-dependent kinases (CDKs) whose function is poorly characterized, plays an essential role in this process. PCTK1 regulates spindle orientation in a kinase-dependent manner. Phosphoproteomic analysis together with an RNA interference screen revealed that PCTK1 regulates spindle orientation through phosphorylation of Ser83 on KAP0, a regulatory subunit of protein kinase A (PKA). This phosphorylation is dispensable for KAP0 dimerization and for PKA binding but is necessary for its interaction with myosin X, a regulator of spindle orientation. KAP0 binds to the FERM domain of myosin X and enhances the association of myosin X-FERM with β1 integrin. This interaction between myosin X-FERM and β1 integrin appeared to be crucial for spindle orientation control. We propose that PCTK1-KAP0-myosin X-β1 integrin is a functional module providing a link between ECM and the actin cytoskeleton in the ECM-dependent control of spindle orientation.

Human mutations in KATNB1 (p80) cause severe congenital cortical malformations, which encompass the clinical features of both microcephaly and lissencephaly. Although p80 plays critical roles during brain development, the underlying mechanisms remain predominately unknown. Here, we demonstrate that p80 regulates microtubule (MT) remodeling in combination with NuMA (nuclear mitotic apparatus protein) and cytoplasmic dynein. We show that p80 shuttles between the nucleus and spindle pole in synchrony with the cell cycle. Interestingly, this striking feature is shared with NuMA. Importantly, p80 is essential for aster formation and maintenance in vitro. siRNA-mediated depletion of p80 and/or NuMA induced abnormal mitotic phenotypes in cultured mouse embryonic fibroblasts and aberrant neurogenesis and neuronal migration in the mouse embryonic brain. Importantly, these results were confirmed in p80-mutant harboring patient-derived induced pluripotent stem cells and brain organoids. Taken together, our findings provide valuable insights into the pathogenesis of severe microlissencephaly, in which p80 and NuMA delineate a common pathway for neurogenesis and neuronal migration via MT organization at the centrosome/spindle pole.

Despite theoretical and physical studies implying that cell-extracellular matrix adhesion geometry governs the orientation of the cell division axis, the molecular mechanisms that translate interphase adhesion geometry to the mitotic spindle orientation remain elusive. Here, we show that the cellular edge retraction during mitotic cell rounding correlates with the spindle axis. At the onset of mitotic cell rounding, caveolin-1 is targeted to the retracting cortical region at the proximal end of retraction fibres, where ganglioside GM1-enriched membrane domains with clusters of caveola-like structures are formed in an integrin and RhoA-dependent manner. Furthermore, Gαi1–LGN–NuMA, a well-known regulatory complex of spindle orientation, is targeted to the caveolin-1-enriched cortical region to guide the spindle axis towards the cellular edge retraction. We propose that retraction-induced cortical heterogeneity of caveolin-1 during mitotic cell rounding sets the spindle orientation in the context of adhesion geometry., Studies imply that cell adhesion geometry during interphase dictates the orientation of the cell division axis. Here the authors show that accumulation of caveolin-1 to rapidly retracting regions during cell rounding sets the spindle orientation by recruiting Gαi1-LGN-NuMA to the cortex.

Arp2/3 actin filament nucleating complex drives circumnavigation of cortical actin clusters during mitosis., The actin cytoskeleton undergoes rapid changes in its architecture during mitosis. Here, we demonstrate novel actin assembly dynamics in M phase. An amorphous cluster of actin filaments appears during prometaphase, revolves horizontally along the cell cortex at a constant angular speed, and fuses into the contractile ring after three to four revolutions. Cdk1 activity is required for the formation of this mitotic actin cluster and its revolving movement. Rapid turnover of actin in the filaments takes place everywhere in the cluster and is also required for its cluster rotation during mitosis. Knockdown of Arp3, a component of the actin filament–nucleating Arp2/3 complex, inhibits the formation of the mitotic actin cluster without affecting other actin structures. These results identify Arp2/3 complex as a key factor in the generation of the dynamic actin cluster during mitosis.

SummaryCultured adherent cells divide on the substratum, leading to formation of the cell monolayer. However, how the orientation of this anchorage-dependent cell division is regulated remains unknown. We have previously shown that integrin-dependent adhesion orients the spindle parallel to the substratum, which ensures this anchorage-dependent cell division. Here, we show that phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P3) is essential for this spindle orientation control. In metaphase, PtdIns(3,4,5)P3 is accumulated in the midcortex in an integrin-dependent manner. Inhibition of phosphatidylinositol-3-OH kinase (PI(3)K) reduces the accumulation of PtdIns(3,4,5)P3 and induces spindle misorientation. Introduction of PtdIns(3,4,5)P3 to these cells restores the midcortical accumulation of PtdIns(3,4,5)P3 and proper spindle orientation. PI(3)K inhibition causes dynein-dependent spindle rotations along the z-axis, resulting in spindle misorientation. Moreover, dynactin, a dynein-binding partner, is accumulated in the midcortex in a PtdIns(3,4,5)P3-dependent manner. We propose that PtdIns(3,4,5)P3 directs dynein/dynactin-dependent pulling forces on spindles to the midcortex, and thereby orients the spindle parallel to the substratum.

Endocytic vesicle fusion is inhibited during mitosis, but the molecular pathways that mediate the inhibition remain unclear. Here we uncovered an essential role of Polo-like kinase 1 (Plk1) in this mechanism. Phosphoproteomic analysis revealed that Plk1 phosphorylates the intermediate filament protein vimentin on Ser459, which is dispensable for its filament formation but is necessary for the inhibition of endocytic vesicle fusion in mitosis. Furthermore, this mechanism is required for integrin trafficking toward the cleavage furrow during cytokinesis. Our results thus identify a novel mechanism for fusion inhibition in mitosis and implicate its role in vesicle trafficking after anaphase onset.

Despite the growing evidence for the regulated spindle orientation in mammals, a systematic approach for identifying the responsible genes in mammalian cells has not been established. Here we perform a kinase-targeting RNAi screen in HeLa cells and identify ABL1 as a novel regulator of spindle orientation. Knockdown of ABL1 causes the cortical accumulation of Leu-Gly-Asn repeat-enriched-protein (LGN), an evolutionarily conserved regulator of spindle orientation. This results in the LGN-dependent spindle rotation and spindle misorientation. In vivo inactivation of ABL1 by a pharmacological inhibitor or by ablation of the abl1 gene causes spindle misorientation and LGN mislocalization in mouse epidermis. Furthermore, ABL1 directly phosphorylates NuMA, a binding partner of LGN, on tyrosine 1774. This phosphorylation maintains the cortical localization of NuMA during metaphase, and ensures the LGN/NuMA-dependent spindle orientation control. This study provides a novel approach to identify genes regulating spindle orientation in mammals and uncovers new signalling pathways for this mechanism., A systematic approach for identifying the genes responsible for the regulation of spindle orientation in mammals has been lacking. Now, Matsumura et al. perform a kinase-targeting RNAi screen and identify ABL1, which through the direct phosphorylation of NuMa, is a novel regulator of spindle orientation.

Plk1, an evolutionarily conserved M phase kinase, associates with not only spindle poles but also kinetochores during prometaphase. However, the role of Plk1 at kinetochores has been poorly understood. Here we show that BubR1 mediates the action of Plk1 at kinetochores for proper chromosome alignment. Our results show that BubR1 colocalizes with Plk1 at kinetochores of unaligned chromosomes and physically interacts with Plk1 in prometaphase cells. Down-regulation of Plk1 by small interfering RNA abolished the mobility-shifted, hyperphosphorylated form of BubR1 in the prometaphase-arrested cells. In addition, BubR1 was phosphorylated by Plk1 in vitro at two Plk1 consensus sites in the kinase domain of BubR1. The add-back of either wild-type BubR1 or BubR1 2E, in which the two Plk1 phosphorylation sites were replaced by glutamic acids, but not that of BubR1 2A, an unphosphorylatable mutant, rescued the chromosome alignment defects in BubR1-deficient cells. Moreover, when both Plk1 and BubR1 were down-regulated, the add-back of BubR1 2E, but not that of wild-type BubR1, rescued the chromosome alignment defects. These results taken together suggest that Plk1 facilitates chromosome alignment during prometaphase through BubR1.