Shapes of Discoid Intracellular Compartments with Small Relative Volumes

A prominent feature of many intracellular compartments is a large membrane surface area relative to their luminal volume, i.e., the small relative volume. In this study we present a theoretical analysis of discoid membrane compartments with a small relative volume and then compare the theoretical results to quantitative morphological assessment of fusiform vesicles in urinary bladder umbrella cells. Specifically, we employ three established extensions of the standard approach to lipid membrane shape calculation and determine the shapes that could be expected according to three scenarios of membrane shaping: membrane adhesion in the central discoid part, curvature driven lateral segregation of membrane constituents, and existence of stiffer membrane regions, e.g., support by protein scaffolds. The main characteristics of each scenario are analyzed. The results indicate that even though all three scenarios can lead to similar shapes, there are values of model parameters that yield qualitatively distinctive shapes. Consequently, a distinctive shape of an intracellular compartment may reveal its membrane shaping mechanism and the membrane structure. The observed shapes of fusiform vesicles fall into two qualitatively different classes, yet they are all consistent with the theoretical results and the current understanding of their structure and function

Membrane localization of Piezo1 in the context of its role in the regulation of red blood cell volume

Piezo1 is a membrane nonspecific cation channel involved in red blood cells (RBCs) in the regulation of their volume. Recently, it was shown that it is distributed on the RBC membrane in a nonuniform manner. Here it is shown that it is possible to interpret the lateral distribution of Piezo1 molecules on RBC membrane by the curvature dependent Piezo1—bilayer interaction which is the consequence of the mismatch between the intrinsic principal curvatures of the Piezo1 trimer and the principal curvatures of the membrane at Piezo1\u27s location but without its presence. This result supports the previously proposed model for the role of Piezo1 in the regulation of RBC volume

Mechanical and molecular basis for the symmetrical division of the fission yeast nuclear envelope

International audienceIn fission yeast Schizosaccharomyces pombe, the nuclear envelope remains intact throughout mitosis and undergoes a series of symmetrical morphological changes when the spindle pole bodies (SPBs), embedded in the nuclear envelope, are pushed apart by elongating spindle microtubules. These symmetrical membrane shape transformations do not correspond to the shape behavior of an analogous system based on lipid vesicles. Here we report that the symmetry of the dividing fission yeast nucleus is ensured by SPB–chromosome attachments, as loss of kinetochore clustering in the vicinity of SPBs results in the formation of abnormal asymmetric shapes with long membrane tethers. We integrated these findings in a biophysical model, which explains the symmetry of the nuclear shapes on the basis of forces exerted by chromosomes clustered at SPBs on the extending nuclear envelope. Based on this analysis we conclude that the fission yeast nuclear envelope exhibits the same mechanical properties as simple lipid vesicles, but interactions with other cellular components, such as chromosomes, influence the nuclear shape during mitosis, allowing the formation of otherwise energetically unfavorable symmetrical dumbbell structures upon spindle elongation. The model allows us to explain the appearance of abnormal asymmetric shapes in fission yeast mutants with mis-segregated chromosomes as well as with altered nuclear membrane composition

Equilibrium Shapes of Erythrocytes in Rouleau Formation

A well known physiological property of erythrocytes is that they can aggregate and form a rouleau. We present a theoretical analysis of erythrocyte shapes in a long rouleau composed of cells with identical sizes. The study is based on the area difference elasticity model of lipid membranes, and takes into consideration the adhesion of curved axisymmetric membranes. The analysis predicts that the erythrocytes in the rouleau can have either a discoid or a cup-like shape. These shapes are analogous to the discoid and stomatocyte shapes of free erythrocytes. The transitions between the discoid and cup-like shapes in the rouleau are characterized. The occurrence of these transitions depends on three model parameters: the cell relative volume, the preferred difference between the areas of the membrane bilayer leaflets, and the strength of the adhesion between the membranes. The cup-like shapes are favored at small relative volumes and small preferred area differences, and the discoid shapes are favored at large values of these parameters. Increased adhesion strength enlarges the contact area between the cells, flattens the cells, and consequently promotes the discoid shapes

The pore-forming action of polyenes

The incidence of resistant fungal pathogens has been increasing, especially in immuno-compromised people. As such, considerable research has been focused on discovering anti-fungal agents with new mechanisms of action and on optimizing the use of existing agents. In this context, interest in the polyene group of anti-fungals has recently been renewed, since they are known to be effective against a broad spectrum of fungal pathogens that only rarely develop a resistance to them. In the past 10 years considerable efforts have been made to improve their efficacy and, simultaneously, to reduce their toxicity. Knowledge about the basic mechanisms of their action will be of crucial importance to further optimizing their use. The mechanisms of polyene action at the membrane level are reviewed here, focusing primarily on their pore-forming activity and on the resulting osmotic responses of artificial lipid vesicles and different eukaryotic cells

Osmotic Effects Induced by Pore-Forming Agent Nystatin: From Lipid Vesicles to the Cell

<div><p>The responses of Chinese hamster ovary epithelial cells, caused by the pore-forming agent nystatin, were investigated using brightfield and fluorescence microscopy. Different phenomena, i.e., the detachment of cells, the formation of blebs, the occurrence of “cell-vesicles” and cell ruptures, were observed. These phenomena were compared to those discovered in giant lipid vesicles. A theoretical model, based on the osmotic effects that occur due to the size-discriminating nystatin transmembrane pores in lipid vesicles, was extended with a term that considers the conservation of the electric charge density in order to describe the cell’s behavior. The increase of the cellular volume was predicted and correlated with the observed phenomena.</p></div

Organization of the actin in blebs.

<p>In “living” cells the actin is organized in small (a) and bigger blebs (b), as depicted by fluorescent signal. Some representative blebs are indicated by arrows.</p

Organization of fluorescent actin structures in “living” cells as seen using confocal microscopy after the addition of 300 μmol/L of nystatin solution.

<p>Top view (in the middle) and side views (bottom and right) of the nystatin treated cells along the thin lines are presented at different time points as indicated (first row). As a control, the actin structures after the “methanol only” treatment at 3% volume fraction are shown (second row). The step size was equal to 0.5 μm, while the number of images in the stack was 86.</p

Characteristic behavior of GUVs at different nystatin concentrations.

<p>The GUVs show different behavior due to the pore-formation process in vesicles with smaller glucose molecules outside and larger sucrose molecules inside, as seen using phase-contrast microscopy [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165098#pone.0165098.ref026" target="_blank">26</a>]. Development of typical membrane protrusions (indicated by arrows) due to the nystatin binding to the outer membrane monolayer at low nystatin concentrations (a). Loss of vesicle contrast due to transient tension pores at intermediate nystatin concentrations (b). Slow (c) and fast (d) vesicle ruptures at high nystatin concentrations: GUVs before the rupture (left), during the rupture (middle, tension pore indicated by arrow), and immediately after the rupture (right) are shown. The times after the GUV transfer into the nystatin solution are depicted on the images. The bars represent 20 μm. For more detailed descriptions see Refs. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165098#pone.0165098.ref026" target="_blank">26</a>] and [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165098#pone.0165098.ref029" target="_blank">29</a>].</p

Slow (upper row) and fast (lower row) “cell-vesicle” ruptures as observed by the brightfield microscopy at a 400 μmol/L nystatin concentration.

<p>The left-hand-side images represent the “cell-vesicles” right before the rupture, the middle ones during the rupture and the right ones after the rupture. The times after the nystatin addition are indicated on the images. The white bars represent 20 μm.</p