Apical Contraction Research Articles

Defining a PARticular Pathway of Neural Tube Closure

Mammalian neurulation is completed when the dorsolateral neural folds bend inwards, their tips make adhesive contacts across the midline, and the epithelia remodel to create a closed neural tube. Two recent papers (one by Camerer et al. in this issue of Developmental Cell) demonstrate a vital role for protease-activated G protein-coupled receptor signaling in these late closure events, opening up new avenues for exploring the molecular basis of mammalian neural tube morphogenesis.

A neutron diffraction study of half-metallic ferromagnet [formula omitted] nanorods

Temperature dependent micro-structural and magnetic properties of CrO 2 nanorods have been studied by neutron diffraction from 3.5 K to 450 K. Rietveld analysis considering the anisotropic shape of nanorods suggested that each CrO 2 nanorod probably consisted of a few microscopically oriented grains. Around the ferromagnetic transition temperature (∼395K), a dominant lattice distortion in the tetragonal phase was not a change along the c axis, but a contraction along the ab axes perpendicular to the lengthwise direction of the nanorods, which was caused by an apical bond contraction in CrO 6 octahedra. The obtained structural and magnetic data of CrO 2 nanorods were compared with the temperature dependent resistivity of the epitaxial CrO 2/TiO 2 film, and strong couplings between structural, magnetic, and electric properties in the half-metallic ferromagnet CrO 2 were observed.

Mechanical control of global cell behaviour during dorsal closure in Drosophila.

Halfway through embryonic development, the epidermis of Drosophila exhibits a gap at the dorsal side covered by an extraembryonic epithelium, the amnioserosa (AS). Dorsal closure (DC) is the process whereby interactions between the two epithelia establish epidermal continuity. Although genetic and biomechanical analysis have identified the AS as a force-generating tissue, we do not know how individual cell behaviours are transformed into tissue movements. To approach this question we have applied a novel image-analysis method to measure strain rates in local domains of cells and performed a kinematic analysis of DC. Our study reveals spatial and temporal differences in the rate of apical constriction of AS cells. We find a slow phase of DC, during which apical contraction of cells at the posterior end predominates, and a subsequent fast phase, during which all the cells engage in the contraction, which correlates with the zippering process. There is a radial gradient of AS apical contraction, with marginal cells contracting earlier than more centrally located cells. We have applied this analysis to the study of mutant situations and associated a particular genotype with quantitative and reproducible changes in the rate of cell contraction and hence in the overall rate of the process. Our mutant analysis reveals the contribution of mechanical elements to the rate and pattern of DC.

Chaos Begets Order: Asynchronous Cell Contractions Drive Epithelial Morphogenesis

Apical cell contraction triggers tissue folding and invagination in epithelia. During Drosophila gastrulation, ventral furrow formation was thought to be driven by smooth, purse-string-like constriction of an acto-myosin belt underlying adherens junctions. Now Martin et al. report in Nature that ventral furrow formation is triggered by asynchronous pulsed contractions of the apical acto-myosin cortex in individual cells.

Abstract 811: Natural History and Expanded Clinical Profile of Stress (Tako-Tsubo) Cardiomyopathy in United States Women

Stress cardiomyopathy (SC) is a newly reported condition of older women, triggered by emotionally and physically stressful events, characterized by acute heart failure with a distinctive angiographic profile, and regarded as a reversible process. To date, extended follow-up of SC patients is largely unavailable. We have assembled a substantial consecutive group of patients with SC to assess short and long-term clinical consequences of this condition. Between 2001–2008, we prospectively identified and followed (mean 2.0 years, range 0–6.7) 113 consecutive women with SC at a tertiary care United States hospital. Patients were female, aged 32–92 years (mean 68±13), and 16 (14%) were < 55 years. In 105 (93%) a triggering stressful event (emotional in 50, acute illness in 55) was identified; however, in 8 patients (7%) no such event preceded SC. The ECG showed ST-segment elevation in 58(51%) patients and ejection fraction (EF) was 31±11%. Troponin was elevated in 109 (96%); peak troponin (T) was 0.64±0.76 ng/ml. Of the 113 patients, 110(97%) survived the acute event: 3 patients (3%) died in-hospital (cardiogenic shock in 2; subarachnoid hemorrhage in 1). Other complications included: cardiac arrest 2 (2%), hypotension requiring inotropic drug and/or intra-aortic balloon pump in 22 (19%), pulmonary or cerebral embolism in 3 (3%), left ventricular (LV) or right ventricular thrombus in 7 (6%) and LV outflow obstruction in 13 (12%). CMR findings included absent delayed hyperenhancement (gadolinium) in 82/83 (99%), normal LV apical contraction in 46%, RV akinesia in 22%, and pleural effusions due to heart failure in 46%. At follow-up, EF returned to normal in all patients, but one or more SC events recurred in 7 (6%) patients (complicated by non-fatal cardiac arrest in 1), of whom 3 were taking beta-blockers. Post-hospital death occurred in 15 (13%) patients of which 14 were noncardiac and 1 of unknown cause. Among this large cohort of women, some SC events occurred atypically either without a stress trigger or in younger premenopausal patients. SC also led to death in the acute phase or to later non-fatal recurrences of SC or cardiac arrest in about 10%. Therefore, the clinical profile of SC is much broader than previously regarded.

Near-drowning syndrome: a possible trigger of tako-tsubo cardiomyopathy

We report a case of transient tako-tsubo cardiomyopathy characterized by an unusual trigger in a woman victim of near-drowning syndrome. After 24 h, electrocardiogram changes and a typical echocardiographic pattern of apical ballooning with a mild increase of serum troponin level induced the suspicion of tako-tsubo cardiomyopathy despite the absence of chest pain. Left ventriculography confirmed the apical ballooning, and coronary angiography revealed normal coronary arteries. Electrocardiogram changes and apical contraction abnormalities were reversed within 1 month. In conclusion, we hypothesize that hypoxemia related to near-drowning syndrome could have induced transient myocardial dysfunction mediated by a sympathetic nerve activation.

Takotsubo cardiomyopathy — A new variant and widening disease spectrum

Classic takotsubo cardiomyopathy had left ventricular apical ballooning. Variants with mid-ventricular ballooning without involvement of the apex have been described. We describe a new variant of takotsubo cardiomyopathy where apical contraction was preserved while the rest of the ventricle was dyskinetic.

Modeling Ischemia-Induced Dyssynchronous Myocardial Contraction

Left ventricular (LV) contraction dyssynchrony is not easily quantified. We previously described a model for quantifying LV dyssynchrony that referenced regional amplitude and phase angles to global LV systole using esmolol-induced regional dyskinesis. We tested the hypothesis that our sine wave model and phase angle analysis of regional dyssynchrony in a canine model could also assess dyssynchrony of contraction during regional ischemia. Hence we compared intracoronary esmolol and matched regional ischemia in 10 anesthetized open-chest dogs. Regional and total LV volumes (conductance catheter), piezoelectric crystal shortening, and LV pressures were measured before, during, and after esmolol-induced apical dyskinesis and matched regional ischemia. We defined regional phase angle of contraction (alpha) as the relative distance, measured in degrees, that regional minimal volume differed from global end-systole. We also compared maximal stroke volume (SV), observed effective SV (that portion of regional SV contributing to total SV for each treatment), and calculated effective SV (total regional SV x cosine alpha). Dobutamine infusion increased homogeneity of regional alpha relative to baseline. Both esmolol and ischemia significantly delayed (P < 0.05) apical contraction as quantified by increased alpha (12.4 degrees +/- 28.1 degrees to 27.4 degrees +/- 30.4 degrees and 54.2 degrees +/- 32.6 degrees , respectively) (mean +/- sd) and decreased regional effective SV (4.7 +/- 2.5 mL to 3.6 +/- 2.2 mL and 4 +/- 2.5 mL, respectively) relative to baseline. Our study indicates that intracoronary esmolol and ischemia induced qualitatively similar mechanical effects on myocardial function and that a sine wave model to estimate regional effective SV is a sensitive method to detect and quantify regional dyssynchrony induced by ischemia. Potentially, phase angle and regional amplitude analyses may prove to be effective measures to identify and quantify the beneficial effects of resynchronization therapies on myocardial function.

Nonmuscle Myosin II Generates Forces that Transmit Tension and Drive Contraction in Multiple Tissues during Dorsal Closure

The morphogenic movements that characterize embryonic development require the precise temporal and spatial control of cell-shape changes. Drosophila dorsal closure is a well-established model for epithelial sheet morphogenesis, and mutations in more than 60 genes cause defects in closure. Closure requires that four forces, derived from distinct tissues, be precisely balanced. The proteins responsible for generating each of the forces have not been determined. We document dorsal closure in living embryos to show that mutations in nonmuscle myosin II (encoded by zipper; zip/MyoII) disrupt the integrity of multiple tissues during closure. We demonstrate that MyoII localization is distinct from, but overlaps, F-actin in the supracellular purse string, whereas in the amnioserosa and lateral epidermis each has similar, cortical distributions. In zip/MyoII mutant embryos, we restore MyoII function either ubiquitously or specifically in the leading edge, amnioserosa, or lateral epidermis and find that zip/MyoII function in any one tissue can rescue closure. Using a novel, transgenic mosaic approach, we establish that contractility of the supracellular purse string in leading-edge cells requires zip/MyoII-generated forces; that zip/MyoII function is responsible for the apical contraction of amnioserosa cells; that zip/MyoII is important for zipping; and that defects in zip/MyoII contractility cause the misalignment of the lateral-epidermal sheets during seam formation. We establish that zip/MyoII is responsible for generating the forces that drive cell-shape changes in each of the force-generating tissues that contribute to closure. This highly conserved contractile protein likely drives cell-sheet movements throughout phylogeny.

Structural study of Pr 0.8Na 0.2MnO 3 at high pressure

The crystal and magnetic structures of the orthoperovskite Pr 0.8Na 0.2MnO 3 with non-integral mean valence (Mn 3+/Mn 4+) have been studied by means of powder neutron diffraction up to high-quasi-hydrostatic pressure of 4.6 GPa. At normal conditions, the system undergoes a charge ordering transition at T CO=215 K, followed by an antiferromagnetic arrangement of the pseudo-CE type at T N=175 K. Under high external pressures, a microscopically separated state is formed, consisting of a mixture of the A-type AFM phase ( T N∼155 K) and metastable FM regions ( T C∼175 K). A possible reason for the appearance of the A-type AFM phase is the compression anisotropy which leads to marked apical contraction of MnO 6 octahedra under high pressure and creates favorable conditions for appearance of the d( x 2− y 2) orbital polarization, prerequisite for the A-type AFM order. It is argued that the pressure-induced structural changes in Pr 0.8Na 0.2MnO 3 are accompanied by an insulator–metal transition.

Embryonic wound healing by apical contraction and ingression in Xenopus laevis.

We have characterized excisional wounds in the animal cap of early embryos of the frog Xenopus laevis and found that these wounds close accompanied by three distinct processes: (1) the assembly of an actin purse-string in the epithelial cells at the wound margin, (2) contraction and ingression of exposed deep cells, and (3) protrusive activity of epithelial cells at the margin. Microsurgical manipulation allowing fine control over the area and depth of the wound combined with videomicroscopy and confocal analysis enabled us to describe the kinematics and challenge the mechanics of the closing wound. Full closure typically occurs only when the deep, mesenchymal cell-layer of the ectoderm is left intact; in contrast, when deep cells are removed along with the superficial, epithelial cell-layer of the ectoderm, wounds do not close. Actin localizes to the superficial epithelial cell-layer at the wound margin immediately after wounding and forms a contiguous "purse-string" in those cells within 15 min. However, manipulation and closure kinematics of shaped wounds and microsurgical cuts made through the purse-string rule out a major force-generating role for the purse-string. Further analysis of the cell behaviors within the wound show that deep, mesenchymal cells contract their apical surfaces and ingress from the exposed surface. High resolution time-lapse sequences of cells at the leading edge of the wound show that these cells undergo protrusive activity only during the final phases of wound closure as the ectoderm reseals. We propose that assembly of the actin purse-string works to organize and maintain the epithelial sheet at the wound margin, that contraction and ingression of deep cells pulls the wound margins together, and that protrusive activity of epithelial cells at the wound margin reseals the ectoderm and re-establishes tissue integrity during wound healing in the Xenopus embryonic ectoderm.

Apical spectrin is essential for epithelial morphogenesis but not apicobasal polarity in Drosophila.

Changes in cell shape and position drive morphogenesis in epithelia and depend on the polarized nature of its constituent cells. The spectrin-based membrane skeleton is thought to be a key player in the establishment and/or maintenance of cell shape and polarity. We report that apical βHeavy-spectrin (βH), a terminal web protein that is also associated with the zonula adherens, is essential for normal epithelial morphogenesis of the Drosophila follicle cell epithelium during oogenesis. Elimination of βH by the karst mutation prevents apical constriction of the follicle cells during mid-oogenesis, and is accompanied by a gross breakup of the zonula adherens. We also report that the integrity of the migratory border cell cluster, a group of anterior follicle cells that delaminates from the follicle epithelium, is disrupted.Elimination of βH prevents the stable recruitment of α-spectrin to the apical domain, but does not result in a loss of apicobasal polarity, as would be predicted from current models describing the role of spectrin in the establishment of cell polarity. These results demonstrate a direct role for apical (αβH)2-spectrin in epithelial morphogenesis driven by apical contraction, and suggest that apical and basolateral spectrin do not play identical roles in the generation of apicobasal polarity.

NO modulates the apicolateral cytoskeleton of isolated hepatocytes by a PKC-dependent, cGMP-independent mechanism.

Nitric oxide (NO) induces smooth muscle relaxation. We examined whether NO or its mediator of this action, guanosine 3',5'-cyclic monophosphate (cGMP), similarly induces relaxation of the apicolateral cytoskeleton in hepatocytes. Apical (canalicular) contractions were observed in isolated rat hepatocyte couplets by videomicroscopy, tight junction permeability was determined in the couplets by paracellular penetration of Texas red-dextran, and cytosolic Ca2+ (Ca2+i) was measured in isolated hepatocytes by fluorescence imaging. Unexpectedly, the NO donor sodium nitroprusside potentiated rather than inhibited apical contraction, in a cGMP-independent manner. This action of nitroprusside was blocked by hemoglobin or by inhibition of protein kinase C (PKC). Nitroprusside and 3-morpholinosydnonimine, another NO donor, each increased the permeability of hepatocyte tight junctions, a known effect of PKC in this cell type, and induced translocation of that kinase to the plasma membrane, as determined by immunocytochemistry. Neither nitroprusside nor dibutyryl cGMP changed the amplitude or frequency of Ca2+i signals in hepatocytes. Exogenous NO thus modulates the apicolateral cytoskeleton of hepatocytes via PKC activation rather than via cGMP or Ca2+i. These observations suggest a new role for NO: to activate PKC.

Secretion mode of the harderian gland of rats after stimulation by cholinergic secretagogues.

We studied the morphological changes in rat Harderian glands 30 min after injection of cholinergic secretagogues. In controls, the glands exhibited a tubuloalveolar structure with relatively wide lumina, in which some osmiophilic dense droplets exocytosed from glandular cells were observed. Also two types of glandular cells (type A cells and type B cells sometimes showing exocytotic figures of lipid-secretory vacuoles) and myoepithelial cells were recognized. After injection of carbamylcholine chloride (subcutaneously, 0.1 mg/kg body weight), which has both nicotinic and muscarinic actions, many of the alveolar lumina dilated and contained a small number of osmiophilic droplets. Exocytotic figures in both types of cells and a pronounced decrease in the number of vacuoles in the glandular cells were observed. However, there was no evidence of apocrine or holocrine secretion. The injection of the higher dose of carbamylcholine (1.0 mg/kg) caused fusion of secretory vacuoles in the apical cytoplasm and contraction of myoepithelial cells. Most alveoli showed no clear lumina; their centers were jammed with cytoplasmic fragments and accumulated secretory products. Massive discharge of cytoplasmic fragments containing some secretory vacuoles was often observed. This may be classified as apocrine secretion. Bethanechol chloride (subcutaneous injection, 1.0 mg/kg), a muscarinic agonist, stimulated the Harderian-gland secretion, and enhanced exocytosis was observed. The discharge from the glandular cells, following injection of various doses of carbamylcholine, were almost inhibited by atropine sulfate, a muscarinic antagonist. The present results suggested that the cholinergic systems regulate the secretion of rat Harderian-gland cells which have muscarinic receptors.

Drosophila gastrulation: analysis of cell shape changes in living embryos by three-dimensional fluorescence microscopy

The first event of Drosophila gastrulation is the formation of the ventral furrow. This process, which leads to the invagination of the mesoderm, is a classical example of epithelial folding. To understand better the cellular changes and dynamics of furrow formation, we examined living Drosophila embryos using three-dimensional time-lapse microscopy. By injecting fluorescent markers that visualize cell outlines and nuclei, we monitored changes in cell shapes and nuclear positions. We find that the ventral furrow invaginates in two phases. During the first 'preparatory' phase, many prospective furrow cells in apparently random positions gradually begin to change shape, but the curvature of the epithelium hardly changes. In the second phase, when a critical number of cells have begun to change shape, the furrow suddenly invaginates. Our results suggest that furrow formation does not result from an ordered wave of cell shape changes, contrary to a model for epithelial invagination in which a wave of apical contractions causes invagination. Instead, it appears that cells change their shape independently, in a stochastic manner, and the sum of these individual changes alters the curvature of the whole epithelium.

Computer modelling of neural tube defects

Neurulation, the curling of the neuroepithelium to form the neural tube, is an essential component of the development of animal embryos. Defects of neural tube formation, which occur with an overall frequency of one in 500 human births, are the cause of severe and distressing congenital abnormalities. However, despite the fact that there is increasing information from animal experiments about the mechanisms which effect neural tube formation, much less is known about the fundamental causes of neural tube defects (NTD). The use of computer models provides one way of gaining clues about the ways in which neurulation may be compromised. Here we employ one computer model to examine the robustness of different cellular mechanisms which are thought to contribute to neurulation. The model, modified from that of Odell et al (Odell, G.M., Oster, G., Alberch, P. and Burnside, B., (1981)) mimics neurulation by laterally propagating a wave of apical contraction along an active zone within a ring of cells. We link the results to experimental evidence gained from studies of embryos in which neurulation has been perturbed. The results indicate that alteration of one of the properties of non-neural tissue can delay or inhibit neurulation, supporting the idea, gained from observation of embryos bearing genes which predispose to NTD, that the tissue underlying the neuroepithelium may contribute to the elevation of the neural folds. The results also show that reduction of the contractile properties of a small proportion of the neuroepithelial cell population may have a profound effect on overall tissue profiling. The results suggest that the elevation of the neural folds, and hence successful neurulation, may be vulnerable to relatively minor deficiencies in cell properties.

Organelle distribution in chick neuroepithelial cells: effects of colchicine and cytochalasin B.

The intracellular distribution of mitochondria, cytoplasmic inclusions and rough endoplasmic reticulum cisternae of chick neuroepithelial cells was investigated at neurulation stages 6, 8, 10 and 12. These neuroepithelial cells were subdivided into three zones: apical, median and basal and the distribution percentages of distribution of these organelles were obtained. Mitochondrial distribution was related to the energy supply that mitochondria provide for apical microfilament contraction. Cytoplasmic inclusions were distributed preferentially in the apical zone of the neuroepithelial cells during the four stages. Rough endoplasmic reticulum cisternae were homogeneously distributed in the three zones at stages 10 and 12, but at stages 6 and 8 there are more elevated percentages of rough endoplasmic reticulum in the apical zones than in the other zones. Experimental treatments with colchicine and cytochalasin B does not modify the patterns of mitochondria and rough endoplasmic reticulum cisternae but alters the distribution of cytoplasmic inclusions. Finally, there is a correlation in the normal neurulating neuroepithelial cells between the distributions of mitochondria and rough endoplasmic reticulum distribution and between the distributions of mitochondria and cytoplasmic inclusions distribution. This relationship is retained in the treated neuroepithelial cells.

The behaviour and function of bottle cells during gastrulation of Xenopus laevis.

The behaviour of bottle cells in normal and microsurgically altered gastrulae and in cultured explants of Xenopus laevis was analysed, using time-lapse micrography, scanning electron microscopy (SEM) and cell tracing with fluorescein dextran amine (FDA). The results shed new light on the function of bottle cells. Bottle cells forming in vivo show a predominantly animal-vegetal apical contraction and a concurrent apical-basal elongation, whereas those forming in cultured explants show uniform apical contraction and remain rotund. Bottle cells forming in embryos with fewer subblastoporal cells contract more uniformly than those in normal embryos and release of normal bottle cells from supra- and subblastoporal cells results in immediate loss of the bottle shape. These results, and an analysis of the effects of bottle cell formation on the shapes and movements of surrounding tissues, show that unique shape of bottle cells and their probable function in development are not intrinsic properties but result from a modulation of the effect of a uniform and intrinsic apical contraction by the geometric and mechanical properties of the surrounding tissue. Mechanical simulations of bottle cell formation, using the finite element method, suggest how the site of bottle cell formation and the thickness and stiffness of adjacent tissues might change the effects of their formation. These results and FDA marking of prospective bottle cells and the adjacent deep mesodermal cells suggest that bottle cells function during their formation to initiate the involution of the prospective mesodermal mantle. Later they respread to deepen the archenteron and to form its peripheral wall.

The cytoskeletal mechanics of brain morphogenesis

There is a functional device in embryonic ectodermal cells that we propose causes them to differentiate into either neuroepithelial or epidermal tissue during the process called primary neural induction. We call this apparatus the "cell state splitter." Its main components are the apical microfilament ring and the coplanar apical mat of microtubules, which exert forces in opposite radial directions. We analyze the mechanical interaction between these cytoskeletal components and show that they are in an unstable mechanical equilibrium. The role of the cell state splitter is thus to create a mechanical instability corresponding to the embryonic state of "competence" in an otherwise mechanically stable cell. When the equilibrium of the cell state splitter is disturbed so as to produce a slight contraction of the apical end, apical contraction continues and the distinctive columnar neuroepithelial cells are produced. A slight expansion from the equilibrium state, on the other hand, results in flattened epidermal cells. The calculated forces are consistent with the known constitutive and force-generating properties and morphology of microfilaments and microtubules, and with free tubulin concentrations. There are no free parameters in the analysis. The first cells to assume the neuroepithelial state lie over the notochord. Propagation of the neuroepithelial state (homoiogenetic induction) then proceeds via stretch-induced constriction of the apical microfilament rings, until a hemisphere is covered, at which point the high rate of change of the meridional stress component necessary for further propagation vanishes. The remaining cells are stretched somewhat by this process and become epidermis. A sharp boundary between the tissues is thus formed (explaining "compartmentalization" and the binary nature of differentiation in general). Normal induction apparently involves setup of the cell state splitters in all of the ectoderm cells, perhaps synchronously timed by global embryo tension. The initial transition of cells from the ectodermal to the neuroepithelial state begins at the notoplate, where cell attachments to the notochord may both cause basal actin deposition and significantly reduce the stress induced in the ectoderm by the global tension, biasing the notoplate cell state splitters toward the neuroepithelial state. Introduction of an organizer or other solid substrate (artificial inducer) elsewhere, to which ectodermal cells can adhere, may likewise have both of these effects. Differentiation to either epidermis or neuroepithelium is thus a mechanical event followed by the synthesis of specific proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

A new, simplified and accurate method for determining ejection fraction with two-dimensional echocardiography.

A new method to determine left ventricular (LV) ejection fraction (EF) with wide-angle, two-dimensional echocardiography (2-D echo) has been developed using the parasternal long-axis, apical four-chamber and apical long-axis views. End-diastolic and end-systolic measurements of LV short axes at the base and mid-LV cavity in the parasternal long-axis view and at the upper, middle and lower thirds of the cavity in the apical views are made, from which an averaged minor axis at end-diastolic and at end-systole is calculated. Fractional shortening of the LV long axis (delta L) is estimated from apical contraction. Satisfactory 2-D echoes were obtained in 55 of 58 nonselected patients (all three views in 32 patients, two views in 22 and one view in one); 42 of 55 patients had coronary artery disease. EF by 2-D echo was compared with EF by gated cardiac blood pool imaging in all patients (r = 0.927, SEE = 6.7%) and to EF by single-plane cineangiography (angio) in 35 of 55 patients (r = 0.913, SEE = 7.4%). LV dyssynergy was frequently present and involved the apex in 29 of 55 patients. Using angio as the standard for evaluating wall motion at the apex, 2-D echo was 100% sensitive and specific in detecting abnormal apical wall motion. We conclude that EF can be determined accurately with 2-D echo in a large group of patients with and without dyssynergy by a simple method that eliminates the need for planimetry or computer assistance.