- Elongated Cells Drive Morphogenesis in a Surface-Wrapped Finite-Element Model of Germband Retraction. [Journal Article]
- BJBiophys J 2019 Jun 05
- During Drosophila embryogenesis, the germband first extends to curl around the posterior end of the embryo and then retracts back; however, retraction is not simply the reversal of extension. At a ti…
During Drosophila embryogenesis, the germband first extends to curl around the posterior end of the embryo and then retracts back; however, retraction is not simply the reversal of extension. At a tissue level, extension is coincident with ventral furrow formation, and at a cellular level, extension occurs via convergent cell neighbor exchanges in the germband, whereas retraction involves only changes in cell shape. To understand how cell shapes, tissue organization, and cellular forces drive germband retraction, we investigate this process using a whole-embryo, surface-wrapped cellular finite-element model. This model represents two key epithelial tissues-amnioserosa and germband-as adjacent sheets of two-dimensional cellular finite elements that are wrapped around an ellipsoidal three-dimensional approximation of an embryo. The model reproduces the detailed kinematics of in vivo retraction by fitting just one free model parameter, the tension along germband cell interfaces; all other cellular forces are constrained to follow ratios inferred from experimental observations. With no additional parameter adjustments, the model also reproduces quantitative assessments of mechanical stress using laser dissection and failures of retraction when amnioserosa cells are removed via mutations or microsurgery. Surprisingly, retraction in the model is robust to changes in cellular force values but is critically dependent on starting from a configuration with highly elongated amnioserosa cells. Their extreme cellular elongation is established during the prior process of germband extension and is then used to drive retraction. The amnioserosa is the one tissue whose cellular morphogenesis is reversed from germband extension to retraction, and this reversal coordinates the forces needed to retract the germband back to its pre-extension position and shape. In this case, cellular force strengths are less important than the carefully established cell shapes that direct them. VIDEO ABSTRACT .
- Emotional expressions associated with therapeutic inertia in multiple sclerosis care. [Journal Article]
- MSMult Scler Relat Disord 2019 Jun 04; 34:17-28
- CONCLUSIONS: TI was observed in half of participants in at least one case-scenario. Our data suggest that facial metrics (e.g. brow furrow, nose wrinkle) and emotional expressions (e.g. disgust) are associated with physicians' choices and partially mediate the effect of aversion to ambiguity on TI.
- The Exocyst complex and Rab5 are required for abscission by localizing ESCRT III subunits to the cytokinetic bridge. [Journal Article]
- JCJ Cell Sci 2019 Jun 20
- Cytokinesis is the final step of cell division following chromosome segregation that generates two daughter cells. The conserved Exocyst complex is required for scission of the intercellular cytokine…
Cytokinesis is the final step of cell division following chromosome segregation that generates two daughter cells. The conserved Exocyst complex is required for scission of the intercellular cytokinetic bridge, although the molecular mechanisms it employs in this process are unclear. We identify and validate the early endocytic GTPase Rab5 as an interactor of the Exocyst complex. Rab5 localizes in the cytokinetic bridge and on the midbody ring in a manner similar to the Exocyst complex. Depletion of Rab5 led to delayed abscission. Caenorhabditis elegans orthologs of both Exocyst complex subunits and Rab5 localize along the cleavage furrow and are required for cytokinesis in early embryos. Cytokinetic cells depleted of either Rab5 or the Exocyst subunits Exoc3 and Exoc4 showed impaired deposition of the ESCRT III subunits CHMP2B and/or CHMP4B near the midbody ring. The study reveals an evolutionarily conserved role for the early endocytic marker Rab5 in cytokinetic abscission. In addition, it uncovers a key requirement of the Exocyst and Rab5 for the delivery of components of the membrane severing ESCRT III machinery to complete cytokinesis.
- Increasing ergosterol levels delays formin-dependent assembly of F-actin cables and disrupts division plane positioning. [Journal Article]
- JCJ Cell Sci 2019 Jun 19
- In most eukaryotes, cytokinesis is mediated by the constriction of a contractile acto-myosin ring (CR) which promotes the ingression of the cleavage furrow. Many components of the CR interact with pl…
In most eukaryotes, cytokinesis is mediated by the constriction of a contractile acto-myosin ring (CR) which promotes the ingression of the cleavage furrow. Many components of the CR interact with plasma membrane lipids suggesting that lipids may regulate CR assembly and function. Although there is clear evidence that phospho-inositides play an important role in cytokinesis, much less is known about the role of sterols in this process. Here we studied how sterols influence division plane positioning and CR assembly in fission yeast. We show that increasing ergosterol levels on the plasma membrane blocks the assembly of F-actin cables from cytokinetic precursor nodes, preventing their compaction into a ring. Abnormal F-actin cables form after a delay, leading to randomly placed septa. Since the formin Cdc12 was detected on cytokinetic precursors and the phenotype can be partially rescued by inhibiting the Arp2/3 complex, which competes with formins for F-actin nucleation, we propose that ergosterol may inhibit formin dependent assembly of F-actin cables from cytokinetic precursors.
- A Septin Double Ring Controls the Spatiotemporal Organization of the ESCRT Machinery in Cytokinetic Abscission. [Journal Article]
- CBCurr Biol 2019 Jun 04
- Abscission is the terminal step of mitosis that physically separates two daughter cells [1, 2]. Abscission requires the endocytic sorting complex required for transport (ESCRT), a molecular machinery…
Abscission is the terminal step of mitosis that physically separates two daughter cells [1, 2]. Abscission requires the endocytic sorting complex required for transport (ESCRT), a molecular machinery of multiple subcomplexes (ESCRT-I/II/III) that promotes membrane remodeling and scission [3-5]. Recruitment of ESCRT-I/II complexes to the midbody of telophase cells initiates ESCRT-III assembly into two rings, which subsequently expand into helices and spirals that narrow down to the incipient site of abscission [6-8]. ESCRT-III assembly is highly dynamic and spatiotemporally ordered, but the underlying mechanisms are poorly understood. Here, we report that, after cleavage furrow closure, septins form a membrane-bound double ring that controls the organization and function of ESCRT-III. The septin double ring demarcates the sites of ESCRT-III assembly into rings and disassembles before ESCRT-III rings expand into helices and spirals. We show that septin 9 (SEPT9) depletion, which abrogates abscission, impairs recruitment of VPS25 (ESCRT-II) and CHMP6 (ESCRT-III). Strikingly, ESCRT-III subunits (CHMP4B and CHMP2A/B) accumulate to the midbody, but they are highly disorganized, failing to form symmetric rings and to expand laterally into the cone-shaped helices and spirals of abscission. We found that SEPT9 interacts directly with the ubiquitin E2 variant (UEV) domain of ESCRT-I protein TSG101 through two N-terminal PTAP motifs, which are required for the recruitment of VPS25 and CHMP6, and the spatial organization of ESCRT-III (CHMP4B and CHMP2B) into functional rings. These results reveal that septins function in the ESCRT-I-ESCRT-II-CHMP6 pathway of ESCRT-III assembly and provide a framework for the spatiotemporal control of the ESCRT machinery of cytokinetic abscission.
- Microhair on the adaxial leaf surface of salt secreting halophytic Oryza coarctata Roxb. show distinct morphotypes: Isolation for molecular and functional analysis. [Journal Article]
- PSPlant Sci 2019; 285:248-257
- Halophytic Oryza coarctata is a good model system to examine mechanisms of salinity tolerance in rice. O. coarctata leaves show the presence of microhairs in adaxial leaf surface furrows that secrete…
Halophytic Oryza coarctata is a good model system to examine mechanisms of salinity tolerance in rice. O. coarctata leaves show the presence of microhairs in adaxial leaf surface furrows that secrete salt under salinity. However, detailed molecular and physiological studies of O. coarctata microhairs are limited due to their relative inaccessibility. This work presents a detailed characterization of O. coarctata leaf features. O. coarctata has two types of microhairs on the adaxial leaf surface: longer microhairs (three morphotypes) lining epidermal furrow walls and shorter microhairs (reported first time) arising from bulliform cells. Microhair morphotypes include (i) finger-like, tubular structures, (ii) tubular hairs with bilobed and flattened heads and (iii) bi-or trifurcated hairs. The unicellular nature of microhairs was confirmed by propidium iodide (PI) staining. An efficient method for the isolation and enrichment of O. coarctata microhairs is presented (yield averaging ˜2 × 105/g leaf tissue). The robustness of the microhair isolation procedure was confirmed by subsequent viability staining (PI), total RNA isolation and RT-PCR amplification of O. coarctata trichome-specific WUSCHEL-related homeobox 3B (OcWox3B) and transporter gene-specific cDNA sequences. The present microhair isolation work from O. coarctata paves the way for examining genes involved in ion secretion in this halophytic wild rice model.
- Disassembly of Actin and Keratin Networks by Aurora B Kinase at the Midplane of Cleaving Xenopus laevis Eggs. [Journal Article]
- CBCurr Biol 2019 Jun 17; 29(12):1999-2008.e4
- The large length scale of Xenopus laevis eggs facilitates observation of bulk cytoplasm dynamics far from the cortex during cytokinesis. The first furrow ingresses through the egg midplane, which is …
The large length scale of Xenopus laevis eggs facilitates observation of bulk cytoplasm dynamics far from the cortex during cytokinesis. The first furrow ingresses through the egg midplane, which is demarcated by chromosomal passenger complex (CPC) localized on microtubule bundles at the boundary between asters. Using an extract system, we found that local kinase activity of the Aurora B kinase (AURKB) subunit of the CPC caused disassembly of F-actin and keratin between asters and local softening of the cytoplasm as assayed by flow patterns. Beads coated with active CPC mimicked aster boundaries and caused AURKB-dependent disassembly of F-actin and keratin that propagated ∼40 μm without microtubules and much farther with microtubules present. Consistent with extract observations, we observed disassembly of the keratin network between asters in zygotes fixed before and during 1st cytokinesis. We propose that active CPC at aster boundaries locally reduces cytoplasmic stiffness by disassembling actin and keratin networks. Possible functions of this local disassembly include helping sister centrosomes move apart after mitosis, preparing a soft path for furrow ingression, and releasing G-actin from internal networks to build cortical networks that support furrow ingression.
- Septin clearance from the division site triggers cytokinesis in budding yeast. [Comment]
- MCMicrob Cell 2019 May 22; 6(6):295-298
- In many eukaryotic cells cytokinesis involves a contractile actomyosin ring (CAR) that drives cleavage furrow ingression. What triggers CAR constriction at a precise time of the cell cycle and how co…
In many eukaryotic cells cytokinesis involves a contractile actomyosin ring (CAR) that drives cleavage furrow ingression. What triggers CAR constriction at a precise time of the cell cycle and how constriction is coupled to chromosome segregation are fundamental questions. In the budding yeast Saccharomyces cerevisiae, CAR assembly strictly requires a rigid septin collar that forms at the bud neck early during the cell cycle. At the time of cytokinesis, a sudden remodelling of the septin collar occurs, leading to its splitting into two separate rings that sandwich the CAR. We have shown that septin displacement during splitting is an essential prerequisite for CAR constriction [Tamborrini et al., Nat Commun. 9(1):4308]. Thus, cytokinesis in budding yeast is a two-step mechanism: during the first step, the septin collar organizes the assembly of the cytokinetic machinery at the right place while restraining CAR-driven membrane ingression; during the second step, a confined eviction of septins from the division site during septin ring splitting triggers CAR constriction. Our data further indicate that septin ring splitting is prompted by the Mitotic Exit Network (MEN), and in particular by its downstream phosphatase Cdc14, independently of its mitotic exit function. Surprisingly, MEN signalling at spindle pole bodies (SPBs) is critical for septin ring splitting and cytokinesis. Ubiquitination of the MEN anchor at SPBs by the Dma1/2 ubiquitin ligase attenuates MEN signalling and could have a decisive role in coupling cytokinesis to chromosome and organelle segregation. Altogether, our data emphasize the importance of septin ring splitting, which has been mysterious so far, and highlight a novel mechanism to prevent CAR constriction and cytokinesis in unpropitious conditions.
- Factors other than hTau overexpression that contribute to tauopathy-like phenotype in rTg4510 mice. [Journal Article]
- NCNat Commun 2019 06 06; 10(1):2479
- The tauopathy-like phenotype observed in the rTg4510 mouse line, in which human tauP301L expression specifically within the forebrain can be temporally controlled, has largely been attributed to high…
The tauopathy-like phenotype observed in the rTg4510 mouse line, in which human tauP301L expression specifically within the forebrain can be temporally controlled, has largely been attributed to high overexpression of mutant human tau in the forebrain region. Unexpectedly, we found that in a different mouse line with a targeted-insertion of the same transgene driven by the same tetracycline-TransActivator (tTA) allele, but with even higher overexpression of tauP301L than rTg4510, atrophy and tau histopathology are delayed, and a different behavioral profile is observed. This suggests that it is not overexpression of mutant human tau alone that contributes to the phenotype in rTg4510 mice. Furthermore we show that the tauopathy-like phenotype seen in rTg4510 requires a ~70-copy tau-transgene insertion in a 244 kb deletion in Fgf14, a ~7-copy tTA-transgene insertion in a 508 kb deletion that disrupts another five genes, in addition to high transgene overexpression. We propose that these additional effects need to be accounted for in any studies using rTg4510.
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- Fission yeast TRP channel Pkd2p localizes to the cleavage furrow and regulates cell separation during cytokinesis. [Journal Article]
- MBMol Biol Cell 2019 May 22; :mbcE18040270
- Force plays a central role in separating daughter cells during cytokinesis, the last stage of cell division. However, the mechanism of force-sensing during cytokinesis remains unknown. Here we discov…
Force plays a central role in separating daughter cells during cytokinesis, the last stage of cell division. However, the mechanism of force-sensing during cytokinesis remains unknown. Here we discovered that Pkd2p, a putative force-sensing TRP channel, localizes to the cleavage furrow during cytokinesis of the fission yeast, Schizosaccharomyces pombe. Pkd2p, whose human homologues are associated with Autosomal Polycystic Kidney Disease, is an essential protein whose localization depends on the contractile ring and the secretory pathway. We identified and characterized a novel pkd2 mutant pkd2-81KD. The pkd2 mutant cells show signs of osmotic stress, including temporary shrinking, paused turnover of the cytoskeletal structures and hyper-activated MAPK signaling. During cytokinesis, although the contractile ring constricts more rapidly in the pkd2 mutant than the wild-type cells (50% higher), the cell separation in the mutant is slower and often incomplete. These cytokinesis defects are also consistent with mis-regulated turgor pressure. Lastly, the pkd2 mutant exhibits strong genetic interactions with two mutants of the SIN pathway, a signaling cascade essential for cytokinesis. We propose that Pkd2p modulates osmotic homeostasis and is potentially a novel regulator of cytokinesis. Movie S1 Movie S1 Movie S1: Pkd2p-GFP localized to the cleavage furrow. Time-lapse microscopy of cells expressing Pkd2p-GFP (green) and Rlc1p-tdTomato (red). Pkd2p localized to the cleavage furrow following the contractile ring assembly. Movie S2 Movie S2 Movie S2: Depletion of Pkd2p by the pkd2-81KD mutation. Time-lapse microscopy of pkd2::P81nmt1-pkd2-GFP cells in YE5s. Left: bright-field images. Right: fluorescence images. Pkd2p-GFP fluorescence was greatly reduced because of the pkd2-81KD mutation and many mutant cells shrunk temporarily, visible in the bright-field video. Movie S3 Movie S3 Movie S3: Temporary shrinking of pkd2-81KD cells. Time-lapse microscopy of pkd2-81KD cells in YE5s. Many mutant cells deflated temporarily, visible in the bright-field video. Movie S4 Movie S4 Movie S4: A "wavy" contractile ring in a deflated pkd2-81KD cell. Time-lapse microscopy of a pkd2-81KD mutant cell expressing Rlc1p-tdTomato. Temporary shrinking of the cell converted the circular ring into a "wavy band". Movie S5 Movie S5 Movie S5: The microtubule turnover in wild-type cells. Time-lapse microscopy of wild-type cells expressing Atb2p-mCherry, a marker for microtubules. Left: bright-field images. Right: fluorescence images. The interphase microtubule bundles of these wild-type cells underwent continuous turnover. Movie S6 Movie S6 Movie S6: Deflation of the pkd2 mutant cells stopped the microtubule turnover. Time-lapse microscopy of pkd2-81KD cells expressing Atb2p-mCherry. Left: bright-field images. Right: fluorescence images. Compared to the wild-type cells (Movie S5), turnover of the interphase microtubule bundles, stopped temporarily in the deflate mutant cells (arrows). Movie S7 Movie S7 Movie S7: Deflation of the pkd2 mutant cells triggered nuclear localization of Sty1p. Time-lapse microscopy of pkd2-81KD cells expressing Sty1p-GFP. Left: bright-field images. Right: fluorescence images. Nuclear localization of Sty1p-GFP increased significantly in those deflated mutant cells. Movie S8 Movie S8 Movie S8: The contractile ring assembly, maturation and closure in the wild-type cells. Time-lapse fluorescence microscopy of wild-type cells expressing Rlc1p-tdTomato (Red) and Sad1-GFP (Green). Movie S9 Movie S9 Movie S9: The contractile ring assembly, maturation and closure in pkd2-81KD cells. Time-lapse fluorescence microscopy of pkd2-81KD cells expressing Rlc1p-tdTomato (red) and Sad1-GFP (green). The mutant cells assembled and matured their rings more slowly but their ring closure was more rapid, compared to the wild-type cells (Movie S8). In the mutant cells, the contractile ring placement was not altered by the pkd2 mutation and the rings didn't slide either.