Pay attention to membrane tension: Mechanobiology of the cell surface

The cell surface is a mechanobiological unit that encompasses the plasma membrane, its interacting proteins, and the complex underlying cytoskeleton. Recently, attention has been directed to the mechanics of the plasma membrane, and in particular membrane tension, which has been linked to diverse cellular processes such as cell migration and membrane trafficking. However, how tension across the plasma membrane is regulated and propagated is still not completely understood. Here, we review recent efforts to study the interplay between membrane tension and the cytoskeletal machinery and how they control cell form and function. We focus on factors that have been proposed to affect the propagation of membrane tension and as such could determine whether it can act as a global or local regulator of cell behavior. Finally, we discuss the limitations of the available tool kit as new approaches that reveal its dynamics in cells are needed to decipher how membrane tension regulates diverse cellular processes.

Autoinhibition of Munc18-1 modulates synaptobrevin binding and helps to enable Munc13-dependent regulation of membrane fusion

Munc18-1 orchestrates SNARE complex assembly together with Munc13-1 to mediate neurotransmitter release. Munc18-1 binds to synaptobrevin, but the relevance of this interaction and its relation to Munc13 function are unclear. NMR experiments now show that Munc18-1 binds specifically and non-specifically to synaptobrevin. Specific binding is inhibited by a L348R mutation in Munc18-1 and enhanced by a D326K mutation designed to disrupt the ‘furled conformation’ of a Munc18-1 loop. Correspondingly, the activity of Munc18-1 in reconstitution assays that require Munc18-1 and Munc13-1 for membrane fusion is stimulated by the D326K mutation and inhibited by the L348R mutation. Moreover, the D326K mutation allows Munc13-1-independent fusion and leads to a gain-of-function in rescue experiments in Caenorhabditis elegans unc-18 nulls. Together with previous studies, our data support a model whereby Munc18-1 acts as a template for SNARE complex assembly, and autoinhibition of synaptobrevin binding contributes to enabling regulation of neurotransmitter release by Munc13-1.

DOI:http://dx.doi.org/10.7554/eLife.24278.001

Nerve cells communicate with other nerve cells by releasing small molecules called neurotransmitters. The neurotransmitters are first packaged inside bubble-like structures called vesicles, which fuse with the membrane of the nerve cell when it is stimulated. Once the vesicle and membrane have fused, the neurotransmitters are released outside the nerve cell and are detected when they bind to proteins on the surface of other nearby nerve cells.

A machinery of different proteins controls membrane fusion. Amongst these proteins are five called Munc18-1, Munc13-1, syntaxin-1, synaptobrevin and SNAP-25. The last three form a tight bundle called SNARE complex that brings the vesicle and cell membrane together and is essential for the two to fuse. Munc18-1 and Munc13-1 orchestrate the assembly of the SNARE complex. Previous studies suggested that Munc18-1 binds to synaptobrevin, providing a template to bring syntaxin-1 and synaptobrevin together and thereby helping the SNARE complex to form. However, the importance of the interaction between Munc18-1 and synaptobrevin was not clearly established, and it was not known how Munc13-1 is involved.

Sitarska, Xu et al. have now measured how mutated versions of Munc18-1 bind to synaptobrevin and tested how the mutations affect membrane fusion. A mutation in Munc18-1 that increased binding to synaptobrevin increased membrane fusion too, while a mutation that decreased binding had the opposite effect and reduced fusion. The results support the idea that Munc18-1 provides a template for the SNARE complex to form. One mutation stimulated Munc18-1 so that Munc13-1 was no longer needed for fusion when the mutant Munc18-1 was tested in fusion assays with artificial membranes. This mutation was designed to perturb the structure of a region of Munc18-1 protein that normally inhibits the binding of synaptobrevin.

These results suggest that by adopting a state where it cannot bind synaptobrevin, Munc18-1 can only be stimulated to form the SNARE complex and trigger release of neurotransmitter when Munc13-1 is present. This provides a way for Munc13-1, which is regulated by many factors, to fine-tune the release of neurotransmitter. Future work will test whether these proteins work in the same way in living animals. This will help us understand how communication between neurons is finely controlled to enable the brain to carry out its many different tasks.

DOI:http://dx.doi.org/10.7554/eLife.24278.002

Cell confinement reveals a branched-actin independent circuit for neutrophil polarity

Migratory cells use distinct motility modes to navigate different microenvironments, but it is unclear whether these modes rely on the same core set of polarity components. To investigate this, we disrupted actin-related protein 2/3 (Arp2/3) and the WASP-family verprolin homologous protein (WAVE) complex, which assemble branched actin networks that are essential for neutrophil polarity and motility in standard adherent conditions. Surprisingly, confinement rescues polarity and movement of neutrophils lacking these components, revealing a processive bleb-based protrusion program that is mechanistically distinct from the branched actin-based protrusion program but shares some of the same core components and underlying molecular logic. We further find that the restriction of protrusion growth to one site does not always respond to membrane tension directly, as previously thought, but may rely on closely linked properties such as local membrane curvature. Our work reveals a hidden circuit for neutrophil polarity and indicates that cells have distinct molecular mechanisms for polarization that dominate in different microenvironments.

Cells display a high degree of plasticity in migration, but how polarity is organized in different microenvironments has remained unclear. This study uses mechanical perturbations to reveal that migration using actin-rich or bleb-based protrusions are both organized around Rac GTPase.

Hydroxyurea has the capacity to induce damage to human erythrocytes which can be modified by radical scavengers

The treatment of human erythrocytes with hydroxyurea [HU] results in the azide-dependent changes in osmotic fragility and in increased methemoglobin formation. Similar changes were induced by H2O2 treatment. However when H2O2 in the presence of azide stimulated malondialdehyde production, in the HU-treated cells no malondialdehyde was detectable. When subjected to an oxidant stress [sodium ascorbate] HU-treated erythrocytes were more fragile and revealed changes in the absorption spectrum of the TBA-reactive material in comparison with the cells treated with ascorbate alone. Partial protection by radical scavengers against certain HU-induced changes can be achieved. The results indicate that HU can damage erythrocytes and suggest the radical origin of these effects.

Evaluation of the effect of experimental cow endometritis on bactericidal capability of phagocytizing cells isolated from the blood and uterine lumen

Studies were undertaken to assess the bactericidal activity of phagocytes isolated from blood and the uterine lumen of clinically healthy cows after ovulation, and from cows in which endometritis was induced experimentally. Experiments were carried out on 28 clinically healthy cows of the black and white lowland breed. Animals were aged 5 years and were used between the 2nd and 8th day after spontaneous ovulation. Cows were divided into four groups. Group I comprised animals in which cell-mediated type immune reaction was induced in the left uterine horn by intrauterine challenge with tuberculin. Cows in this group were initially vaccinated with M. bovis via the intrauterine route. In group II, Arthus type immune reaction was induced by challenging immunized animals with C. fetus ssp. veneralis through intrauterine instillation. The non-specific inflammatory process was initiated in the uterus of animals in group III by one instillation of lipopolysaccharide from S. abortus equi. Animals in group IV were set as control and received a phosphate buffered saline instillation into the uterine lumen. The cells from the left uterine horn were washed out 6 h after induction. Neutrophils were isolated from blood samples collected from all animals within the same time. The bacterial activity of cells from the uterine lumen and blood was assessed with the nitro-blue tetrazolium reduction test. Results are presented as increase in optical density resulting from a constant number of phagocytizing cells (delta OD/10(6) cells). Induction of cell-mediated immune reaction or Arthus type immune reaction in the uterus significantly boosts the intracellular capability of uterine cells to kill bacteria through the oxidation system. Experimentally induced non-specific endometritis weakens the bactericidal activity of uterine phagocytes, while peripheral blood phagocytes efficiently kill the engulfed bacteria.

Sensing their plasma membrane curvature allows migrating cells to circumvent obstacles

To navigate through diverse tissues, migrating cells must balance persistent self-propelled motion with adaptive behaviors to circumvent obstacles. We identify a curvature-sensing mechanism underlying obstacle evasion in immune-like cells. Specifically, we propose that actin polymerization at the advancing edge of migrating cells is inhibited by the curvature-sensitive BAR domain protein Snx33 in regions with inward plasma membrane curvature. The genetic perturbation of this machinery reduces the cells' capacity to evade obstructions combined with faster and more persistent cell migration in obstacle-free environments. Our results show how cells can read out their surface topography and utilize actin and plasma membrane biophysics to interpret their environment, allowing them to adaptively decide if they should move ahead or turn away. On the basis of our findings, we propose that the natural diversity of BAR domain proteins may allow cells to tune their curvature sensing machinery to match the shape characteristics in their environment.

The effect of organic chlorine compounds and their metabolites present in human milk on newborn mice

The effect of pesticides and their metabolites (DDE, DDT, DDD, alpha, beta and gamma-HCH and PCBs) isolated from human milk on the blood and liver morphology of the mouse were studied. Mouse neonates were fed an extract of the organochlorine compounds in linseed oil for a period of 6 weeks. The lowest dose used in the experiment equalled that which a human infant can receive with its mother's milk, calculated per gram of body weight. Doses 10 and 100 times higher were also used. At the end of the experiment, tissue samples for electron microscopy and blood samples for haemotological examination were taken. Haematological examinations, in mice receiving pesticides, revealed a significant rise in the number of Heinz bodies in erythrocytes and an increased number of lymphocytes. These changes were related to the concentrations of the organic chlorine compounds that the animals were given. Electronmicroscopy demonstrated that organochlorine pesticides at doses equal to that might be received by infants caused proliferation of the smooth endoplasmic reticulum in hepatocytes without any clear damage to other subcellular structures. Doses 10 and 100 times higher resulted in more extensive proliferation of the smooth endoplasmic reticulum, reduction of the rough elements of the endoplasmic reticulum and damage to mitochondria. The latest changes were associated with an increased number of prominent Kupffer cells and the appearance of immigratory cells with traits characteristic of lymphocytes and monocytes.

Amelioration of hydroxyurea-induced suppression of phagocytosis in human granulocytes by free radical scavengers

The exposure of human circulatory white cells in vitro to 0.1-1-10 mol/l hydroxyurea (HU) for 20 h induced a progressive dose-dependent suppression of the phagocytic activity of granulocytes. The suppressing effect of 20 h exposure to 1 mol/l HU was used to examine the protection afforded by free radical scavengers against HU-induced cytotoxicity. It has been found that, in the suitable concentration of the protecting agent, a substantial protective effect of sodium benzoate, acetylosalicylic acid, alpha-tocopherol, ascorbic acid, catalase, peroxidase or superoxide dismutase can be achieved.

Mechanism of unbalanced growth-induced cell damage. I. A probable role for hydrolytic enzymes synthesis

This study examines the relationship between cell progress into the state of unbalanced growth, hydrolytic enzyme activities and cell survival during the exposure of L5178Y cells to hydroxyurea (HU), excess thymidine (dThR), hydroxyurea with excess of four deoxyribonucleosides (dNR) or excess dTHR with deoxycytidine (dCR). Cell progress into the state of unbalanced growth was measured as cell size, protein/DNA ratio and protein content per cell. Activities of two lysosomal (acid phosphatase, beta-N-acetylglucosaminidase) and one cytoplasmic non-lysosomal (LDH) enzymes were determined. It has been found that in cells arrested by HU or excess dThR, a progressive cell volume increase with protein/DNA imbalance is correlated with a progressive increase in lysosomal and non-lysosomal hydrolase activities in the cells and in the medium and with a marked lethal effect. Cell volume increase, enhancement of enzyme activities and cell killing could be prevented in HU-arrested cells by concomitant addition of excess dNR (deoxyadenosine, deoxyguanosine, thymidine, deoxycytidine) leading to equal inhibition of DNA and protein synthesis. Control-like values of all parameters were achieved also in cells in which the dThR-inhibiting effect was reversed by dCR addition. It is suggested that a common pathway in the mode of action of the chemotherapeutic agents inducing cell killing through the state of unbalanced growth can be the over-production, abnormal accumulation and progressive leakage of numerous hydrolytic enzymes through the cell membranes, leading in consequence to 'lytic' cell death.

Phagocytic activity of polymorphonuclear leukocytes lavaged from the lungs of horses with clinically diagnosed chronic pulmonary disease

The aim of this study was to compare phagocytic activity of polymorphonuclear cells (PMNs) from the bronchoalveolar lavage of clinically healthy horses and those with severe chronic bronchiolitis. Research was carried out on 28 horses. Chronic inflammation of the lower airways was diagnosed in nine horses. Cells from the respiratory tract were lavaged according to accepted methods. For comparison, PMNs were isolated from peripheral blood of all investigated horses. The phagocytic activity of PMNs was determined in relation to two standard strains of Staphylococcus aureus, Staph, aureus Smith which was phagocytized after previous opsonization, and Staph, aureus 305, phagocytized without opsonization. From the investigations, it is shown that the PMNs present in the terminal airways of horses with severe chronic bronchiolitis are characterized by decreased phagocytic activity in relation to opsonized Staphylococcus aureus Smith and increased activity in relation to non-opsonized Staphylococcus aureus 305, as compared to the PMNs lavaged from the terminal airways of clinically healthy horses. No changes in the phagocytic activity of the peripheral blood PMNs were observed between clinically diseased horses and healthy horses.

Load adaptation by endocytic actin networks

Clathrin-mediated endocytosis (CME) robustness under elevated membrane tension is maintained by actin assembly-mediated force generation. However, whether more actin assembles at endocytic sites in response to increased load has not previously been investigated. Here actin network ultrastructure at CME sites was examined under low and high membrane tension. Actin and N-WASP spatial organization indicate that actin polymerization initiates at the base of clathrin-coated pits and that the network then grows away from the plasma membrane. Actin network height at individual CME sites was not coupled to coat shape, raising the possibility that local differences in mechanical load feed back on assembly. By manipulating membrane tension and Arp2/3 complex activity, we tested the hypothesis that actin assembly at CME sites increases in response to elevated load. Indeed, in response to elevated membrane tension, actin grew higher, resulting in greater coverage of the clathrin coat, and CME slowed. When membrane tension was elevated and the Arp2/3 complex was inhibited, shallow clathrin-coated pits accumulated, indicating that this adaptive mechanism is especially crucial for coat curvature generation. We propose that actin assembly increases in response to increased load to ensure CME robustness over a range of plasma membrane tensions.

Effect of organochlorine pesticides on the phagocytic activity of bovine milk cells

The effect of a complex of organochlorine compounds on the phagocytic activity of macrophages and neutrophils in milk was assessed in vitro using a mixture of DDT, DDE, DDD, alpha, beta and gamma-HCH and HCB and PCBs in concentrations occurring in milk and in concentrations being multiples of the concentrations in milk. Standard strains 305 and Smith of S. aureus were used for phagocytosis assessment. A suppressing effect of these compounds was observed on the per cent of cells phagocytizing both these strains, with a significant decrease in the number of bacteria of both strains phagocytized by macrophages, and the number of S. aureus strain 305 bacteria phagocytized by neutrophils. The concentrations of the mixture of organochlorine compounds used in the experiment had no significant effect on the number of Smith strain bacteria phagocytized by neutrophils.

Performance of Language-Coordinated Collective Systems: A Study of Wine Recognition and Description

Most of our perceptions of and engagements with the world are shaped by our immersion in social interactions, cultural traditions, tools and linguistic categories. In this study we experimentally investigate the impact of two types of language-based coordination on the recognition and description of complex sensory stimuli: that of red wine. Participants were asked to taste, remember and successively recognize samples of wines within a larger set in a two-by-two experimental design: (1) either individually or in pairs, and (2) with or without the support of a sommelier card—a cultural linguistic tool designed for wine description. Both effectiveness of recognition and the kinds of errors in the four conditions were analyzed. While our experimental manipulations did not impact recognition accuracy, bias-variance decomposition of error revealed non-trivial differences in how participants solved the task. Pairs generally displayed reduced bias and increased variance compared to individuals, however the variance dropped significantly when they used the sommelier card. The effect of sommelier card reducing the variance was observed only in pairs, individuals did not seem to benefit from the cultural linguistic tool. Analysis of descriptions generated with the aid of sommelier cards shows that pairs were more coherent and discriminative than individuals. The findings are discussed in terms of global properties and dynamics of collective systems when constrained by different types of cultural practices.

The effect of persistent chlorinated hydrocarbons on the secretion of estradiol and progesterone by bovine granulosa cells in vitro

The role of persistent chlorinated hydrocarbons (PCH) in the reproductive disorders in ruminants is not well documented. In the present study we have examined the effect of these compounds and their metabolites on the secretion of estradiol (E2) and progesterone (P4) by bovine granulosa cells in vitro. Granulosa cells were isolated from large follicles (> or = 8 mm diameter) by gently washing the internal follicle wall. Aliquots of approximately 4 X 10(5) viable granulosa cells in 0.5 ml medium were cultured at 37 degrees C in an atmosphere of 5% CO2 and 95% air. Granulosa cells were cultured for 96h in a medium containing different concentrations (10(-1)-10(-4) ng/ml) of a PCH combination. Estradiol and progesterone were measured in unextracted granulosa cell culture medium by Enzyme Immunoassay (EIA). The exposure of granulosa cells to a combination of these organochlorine compounds in vitro results in a slight decrease of estradiol secretion only at the highest studied concentration of the PCH combination. However, the secretion of progesterone by these cells was seriously decreased, even by concentrations found in ovaries from animals kept under natural environmental conditions. The in vitro culture system of granulosa cells from preovulatory follicles may be useful in screening toxic effects of pesticides in animal reproduction.

Effect of a protein-free haemodialysate on tissue respiration and healing of burns

Investigations were carried out in three groups of rats (10 rats per group) sustaining partial skin thickness burns covering 2.2 per cent of body surface area. Thirty minutes after burning animals received: Group I, 1 ml Solcoseryl, IP; Group II, 1 ml normal saline, IP; Group III, no treatment (controls). Two hours after burning tissue respiration and the tissue concentrations of potassium, magnesium, calcium and sodium ions were measured in biopsies from the burn wounds. It was found that early post-burn administration of Solcoseryl significantly inhibited the secondary destructive processes and accelerated the regeneration of damaged tissue.

Antioxidant parameters of horses according to age, sex, breed and environment

Antioxidant defences interact to form an integrated system. There is no comprehensive and uniform view on issues concerning the antioxidant status in horses. Therefore, the aim of the present study was to estimate antioxidant parameters in horses of different age, sex and breed as well as environment and relationship between different antioxidants. Parameters of selected antioxidants including superoxide dismutase (SOD), glutathione peroxidase (GSH-px), total antioxidant status (TAS), ceruloplasmin (CP), bilirubin, uric acid, zinc, copper and selenium were determined in blood of 80 clinically healthy horses. Antioxidant parameters significantly varied between horses form different environments or different breed and sex. Age of horses had no significant effect on antioxidant parameters. Significant correlation coefficients were observed between enzymatic and nonenzimatic antioxidants. The nature of the relationship between the antioxidant system in horses with respect to environmental factors is rather complex and to date only a part of system is known.

In vitro assessment of the influence of different leucocyte count lavaged from normal and inflamed cow uterus on the mouse embryo development

The aim of this study was to evaluate the effect of different leucocyte count from normal and inflamed uterus in excess of PMNs on the development of mouse embryos during the morula or blastocyst stages in vitro. Results showed that cultivation of blastocysts in the presence of leucocytes washed from the uterus, after mediation of specific or nonspecific endometritis at a concentration of 1 x 10(4), led to significant inhibition of embryo development. No similar observations were made with blastocysts cultivated in the presence of leucocytes obtained from the normal uterus at 2-4 days after spontaneous ovulation.

[Free amino acids in leukemic leukocytes]

Using the method of ion-exchange chromatography in automatic analyser the level of free amino acids was determined in peripheral blood leucocytes of 14 healthy blood donors and 36 patients with various types of leukaemia. In comparison with granulocytes the concentration of free amino acids in lymphocytes was reduced. The concentration of free amino acids in lymphocytes of chronic lymphatic leukemia was not essentially changed. The leucocytes in chronic myeloid leukemia showed a rise in the concentration of most free amino acids. In the blast cells of patients with acute myeloid the concentration of free amino acids was reduced in relation to DNA contents while in relation to proteins this concentration was raised.

[Effect of gelatin and dextran infusion on the reticuloendothelial system]

The purpose of this study was evaluation of the phagocytic ability of peripheral blood leucocytes and reticuloendothelial system of the lungs, liver and spleen after one infusion of plasma-substituting preparations: Fluigel and dextran 70 in volumes of 20 ml/kg of body weight. The experiments were done in rabbits. The response of the reticuloendothelial system was assessed on the basis of its ability of eliminating foreign erythrocytes labelled with 51Cr from the circulating blood and radioactivity accumulation in the liver and spleen. It was found that infusion of Fluigel and dextran produced transient changes in the function of the reticuloendothelial system with increased number of granulocytes and a fall of their radioactivity 3 and 6 hours after infusion, reduced ability of pulmonary 51Cr accumulation and increased storage in the spleen observed 24 hours after infusion.