Aurora B-dependent polarization of the cortical actomyosin network during mitotic exit.

The isotropic metaphase actin cortex progressively polarizes as the anaphase spindle elongates during mitotic exit. This involves the loss of actomyosin cortex from opposing cell poles and the accumulation of an actomyosin belt at the cell centre. Although these spatially distinct cortical remodelling events are coordinated in time, here we show that they are independent of each other. Thus, actomyosin is lost from opposing poles in anaphase cells that lack an actomyosin ring owing to centralspindlin depletion. In examining potential regulators of this process, we identify a role for Aurora B kinase in actin clearance at cell poles. Upon combining Aurora B inhibition with centralspindlin depletion, cells exiting mitosis fail to change shape and remain completely spherical. Additionally, we demonstrate a requirement for Aurora B in the clearance of cortical actin close to anaphase chromatin in cells exiting mitosis with a bipolar spindle and in monopolar cells forced to divide while flat. Altogether, these data suggest a novel role for Aurora B activity in facilitating DNA-mediated polar relaxation at anaphase, polarization of the actomyosin cortex, and cell division

CD40 Signaling Is Impaired in L. major–infected Macrophages and Is Rescued by a p38MAPK Activator Establishing a Host-protective Memory T Cell Response

Leishmania, a protozoan parasite, lives and multiplies as amastigote within macrophages. It is proposed that the macrophage expressed CD40 interacts with CD40 ligand on T cells to induce IFN-γ, a Th1-type cytokine that restricts the amastigote growth. Here, we demonstrate that CD40 cross-linking early after infection resulted in inducible nitric oxide synthetase type-2 (iNOS2) induction and iNOS2-dependent amastigote elimination. Although CD40 expression remained unaltered on L. major–infected macrophages, delay in the treatment of macrophages or of mice with anti-CD40 antibody resulted in significant reduction in iNOS2 expression and leishmanicidal function suggesting impaired CD40 signaling in Leishmania infection. The inhibition of CD40-induced iNOS2 expression by SB203580, a p38-mitogen activated protein kinase (p38MAPK)-specific inhibitor, and the reversal of the inhibition by anisomycin, a p38MAPK activator, suggested a crucial role of p38MAPK in CD40 signaling. Indeed, the CD40-induced p38MAPK phosphorylation, iNOS2 expression and anti-leishmanial function were impaired in Leishmania-infected macrophages but were restored by anisomycin. Anisomycin's effects were reversed by SB203580 emphasizing the role of p38MAPK in CD40-induced iNOS2-dependent leishmanicidal function. Anisomycin administration in L. major–infected BALB/c mice resulted in significant reduction in the parasite load and established a host-protective Th1-type memory response. Also implicated in these findings is a scientific rationale to define novel anti-parasite drug targets and to bypass the problem of drug resistance

Local actin nucleation tunes centrosomal microtubule nucleation during passage through mitosis

Cells going through mitosis undergo precisely timed changes in cell shape and organisation, which serve to ensure the fair partitioning of cellular components into the two daughter cells. These structural changes are driven by changes in actin filament and microtubule dynamics and organisation. While most evidence suggests that the two cytoskeletal systems are remodelled in parallel during mitosis, recent work in interphase cells has implicated the centrosome in both microtubule and actin nucleation, suggesting the potential for regulatory crosstalk between the two systems. Here, by using both in vitro and in vivo assays to study centrosomal actin nucleation as cells pass through mitosis, we show that mitotic exit is accompanied by a burst in cytoplasmic actin filament formation that depends on WASH and the Arp2/3 complex. This leads to the accumulation of actin around centrosomes as cells enter anaphase and to a corresponding reduction in the density of centrosomal microtubules. Taken together, these data suggest that the mitotic regulation of centrosomal WASH and the Arp2/3 complex controls local actin nucleation, which may function to tune the levels of centrosomal microtubules during passage through mitosis

Coupling changes in cell shape to chromosome segregation

Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell–substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance

The Role Of Crumbs Proteins In Mammalian Gastrulation

Crumbs, a transmembrane protein with a short cytoplasmic domain and very large extracellular domain, was identified as an apical polarity determinant in Drosophila that is required for epithelial integrity. In mammals the Crumbs family consists of three members, Crumbs1, Crumbs2 and Crumbs3. Here I will show that mammalian Crumbs family is not essential for establishing epithelial polarity in the early mouse embryo and that the earliest function of the Crumbs family in mammals is the regulation of gastrulation by Crumbs2. Mutations in human CRUMBS1 have been associated with Retinitis pigmentosa and retinal degeneration. Most mutations lie in the extracellular domain of Crumbs, whose function still remains unclear. Here I show that the extracellular domain of Crumbs2 is O-glucosylated by an enzyme Protein O-glucosyltransferse 1, which regulates its membrane localization. The apical membrane localization of Crumbs2 is essential for its function in mammalian gastrulation. The gastrulation phenotype in Crumbs2 mutants results in the accumulation of cells at the primitive streak. Although Crumbs proteins are expected to be required to maintain the apical domain of the cell, mammalian Crumbs2 is required for cells to lose apical adherens junctions and to facilitate cell delamination during gastrulation epithelial-to-mesenchymal transition. The requirement of Crumbs2 in epithelial organization and cell delamination at the primitive streak is locally non-cell autonomous, as Crumbs2 mutant cells can delaminate, migrate away and be incorporated into all lineages when surrounded by wild-type cells. Further, we find that Crumbs2 is required in the epiblast adjacent to the primitive streak to maintain epithelial integrity, but only when neighboring cells are undergoing cell rearrangements such as the gastrulation EMT, as the local disorganization of cells in the epiblast does not occur in the absence of the primitive streak. Thus I propose that Crumbs2 is part of a global system that maintains epiblast integrity while cells delaminate during the EMT in mammalian gastrulation