The chromosomal passenger complex and centralspindlin independently contribute to contractile ring assembly

In contrast to their sequential roles in midzone assembly, the CPC and centralspindlin act through independent mechanisms to regulate contractile ring assembly

Signaling from the asters and spindle midzone is required to promote cytokinesis in the early C. elegans embryo

Cytokinesis is the final step of the cell division that physically separates a single cell into two daughter cells following chromosome segregation. In order to insure that each daughter cell receives the proper genetic complement, cytokinesis must be both spatially and temporally coupled to chromosome segregation. Cytokinesis is accomplished by formation and constriction of a contractile ring made up of actin, myosin, anillin, and the septins. One of the critical questions in the field is how does the cell maintain precise spatial and temporal control over the assembly and constriction of the contractile ring? Although the precise molecular signals are still being debated, it is known that signals from the anaphase spindle are critical for furrow formation and ingression. Using the early C. elegans embryo I have found that integration of signals from astral microtubules and the spindle midzone is critical for the formation of a single furrow during cytokinesis. During early anaphase, astral microtubules provide an inhibitory signal to prevent accumulation of contractile ring proteins in the anterior and posterior of the cell, leading to their enrichment in the equatorial region. Proper separation of the asters is important for this early signal, since delaying or preventing aster separation disrupts the equatorial enrichment of anillin and myosin and delays furrow formation. Following this initial signal from the astral microtubules, a positive signal from the midzone - mediated by Centralspindlin and the Chromosomal Passenger Complex (CPC) - drives furrow ingression and completion. Although the initial patterning of contractility and the timing of furrow formation are unaffected, depletion of Centralspindlin or the CPC leads to a decrease in the rate of ingression and failure of cytokinesis. My work has also shown that Centralspindlin and the CPC are involved in distinct pathways to promote furrow ingression. The key role of the Centralspindlin complex is to inactivate the small GTPase, Rac, whereas the role of the CPC is likely through promoting contractile ring disassembly through regulation of anillin and/or septi

Integrins Regulate Apical Constriction via Microtubule Stabilization in the Drosophila Eye Disc Epithelium

During morphogenesis, extracellular signals trigger actomyosin contractility in subpopulations of cells to coordinate changes in cell shape. To illuminate the link between signaling-mediated tissue patterning and cytoskeletal remodeling, we study the progression of the morphogenetic furrow (MF), the wave of apical constriction that traverses the Drosophila eye imaginal disc preceding photoreceptor neurogenesis. Apical constriction depends on actomyosin contractility downstream of the Hedgehog (Hh) and bone morphogenetic protein (BMP) pathways. We identify a role for integrin adhesion receptors in MF progression. We show that Hh and BMP regulate integrin expression, the loss of which disrupts apical constriction and slows furrow progression; conversely, elevated integrins accelerate furrow progression. We present evidence that integrins regulate MF progression by promoting microtubule stabilization, since reducing microtubule stability rescues integrin-mediated furrow acceleration. Thus, integrins act as a genetic link between tissue-level signaling events and morphological change at the cellular level, leading to morphogenesis and neurogenesis in the eye

The midbody ring scaffolds the abscission machinery in the absence of midbody microtubules.

Abscission completes cytokinesis to form the two daughter cells. Although abscission could be organized from the inside out by the microtubule-based midbody or from the outside in by the contractile ring-derived midbody ring, it is assumed that midbody microtubules scaffold the abscission machinery. In this paper, we assess the contribution of midbody microtubules versus the midbody ring in the Caenorhabditis elegans embryo. We show that abscission occurs in two stages. First, the cytoplasm in the daughter cells becomes isolated, coincident with formation of the intercellular bridge; proper progression through this stage required the septins (a midbody ring component) but not the membrane-remodeling endosomal sorting complex required for transport (ESCRT) machinery. Second, the midbody and midbody ring are released into a specific daughter cell during the subsequent cell division; this stage required the septins and the ESCRT machinery. Surprisingly, midbody microtubules were dispensable for both stages. These results delineate distinct steps during abscission and highlight the central role of the midbody ring, rather than midbody microtubules, in their execution

Dephosphorylation of the Ndc80 Tail Stabilizes Kinetochore-Microtubule Attachments via the Ska Complex

During cell division, genome inheritance is orchestrated by microtubule attachments formed at kinetochores of mitotic chromosomes. The primary microtubule coupler at the kinetochore, the Ndc80 complex, is regulated by Aurora kinase phosphorylation of its N-terminal tail. Dephosphorylation is proposed to stabilize kinetochore-microtubule attachments by strengthening electrostatic interactions of the tail with the microtubule lattice. Here, we show that removal of the Ndc80 tail, which compromises in vitro microtubule binding, has no effect on kinetochore-microtubule attachments in the Caenorhabditis elegans embryo. Despite this, preventing Aurora phosphorylation of the tail results in prematurely stable attachments that restrain spindle elongation. This premature stabilization requires the conserved microtubule-binding Ska complex, which enriches at attachment sites prior to anaphase onset to dampen chromosome motion. We propose that Ndc80-tail dephosphorylation promotes stabilization of kinetochore-microtubule attachments via the Ska complex and that this mechanism ensures accurate segregation by constraining chromosome motion following biorientation on the spindle

The midbody ring scaffolds the abscission machinery in the absence of midbody microtubules

Abscission completes cytokinesis to form the two daughter cells. Although abscission could be organized from the inside out by the microtubule-based midbody or from the outside in by the contractile ring–derived midbody ring, it is assumed that midbody microtubules scaffold the abscission machinery. In this paper, we assess the contribution of midbody microtubules versus the midbody ring in the Caenorhabditis elegans embryo. We show that abscission occurs in two stages. First, the cytoplasm in the daughter cells becomes isolated, coincident with formation of the intercellular bridge; proper progression through this stage required the septins (a midbody ring component) but not the membrane-remodeling endosomal sorting complex required for transport (ESCRT) machinery. Second, the midbody and midbody ring are released into a specific daughter cell during the subsequent cell division; this stage required the septins and the ESCRT machinery. Surprisingly, midbody microtubules were dispensable for both stages. These results delineate distinct steps during abscission and highlight the central role of the midbody ring, rather than midbody microtubules, in their execution