Variance sum rule for entropy production

Entropy production is the hallmark of nonequilibrium physics, quantifying irreversibility, dissipation, and the efficiency of energy transduction processes. Despite many efforts, its measurement at the nanoscale remains challenging. We introduce a variance sum rule (VSR) for displacement and force variances that permits us to measure the entropy production rate σ in nonequilibrium steady states. We first illustrate it for directly measurable forces, such as an active Brownian particle in an optical trap. We then apply the VSR to flickering experiments in human red blood cells. We find that σ is spatially heterogeneous with a finite correlation length, and its average value agrees with calorimetry measurements. The VSR paves the way to derive σ using force spectroscopy and time-resolved imaging in living and active matter.

Mechanical stress confers nuclear and functional changes in derived leukemia cells from persistent confined migration

Nuclear deformability plays a critical role in cell migration. During this process, the remodeling of internal components of the nucleus has a direct impact on DNA damage and cell behavior; however, how persistent migration promotes nuclear changes leading to phenotypical and functional consequences remains poorly understood. Here, we described that the persistent migration through physical barriers was sufficient to promote permanent modifications in migratory-altered cells. We found that derived cells from confined migration showed changes in lamin B1 localization, cell morphology and transcription. Further analysis confirmed that migratory-altered cells showed functional differences in DNA repair, cell response to chemotherapy and cell migration in vivo homing experiments. Experimental modulation of actin polymerization affected the redistribution of lamin B1, and the basal levels of DNA damage in migratory-altered cells. Finally, since major nuclear changes were present in migratory-altered cells, we applied a multidisciplinary biochemical and biophysical approach to identify that confined conditions promoted a different biomechanical response of the nucleus in migratory-altered cells. Our observations suggest that mechanical compression during persistent cell migration has a role in stable nuclear and genomic alterations that might handle the genetic instability and cellular heterogeneity in aging diseases and cancer.

Inhomogeneous Canham-Helfrich Abscission in Catenoid Necks under Critical Membrane Mosaicity

The mechanical effects of membrane compositional inhomogeneities are analyzed in a process analogous to neck formation in cellular membranes. We cast on the Canham-Helfrich model of fluid membranes with both the spontaneous curvature and the surface tension being non-homogeneous functions along the cell membrane. The inhomogeneous distribution of necking forces is determined by the equilibrium mechanical equations and the boundary conditions as considered in the axisymmetric setting compatible with the necking process. To establish the role played by mechanical inhomogeneity, we focus on the catenoid, a surface of zero mean curvature. Analytic solutions are shown to exist for the spontaneous curvature and the constrictive forces in terms of the border radii. Our theoretical analysis shows that the inhomogeneous distribution of spontaneous curvature in a mosaic-like neck constrictional forces potentially contributes to the membrane scission under minimized work in living cells.

Rheology of Pseudomonas fluorescens biofilms: From experiments to predictive DPD mesoscopic modeling

Bacterial biofilms mechanically behave as viscoelastic media consisting of micron-sized bacteria cross-linked to a self-produced network of extracellular polymeric substances (EPSs) embedded in water. Structural principles for numerical modeling aim at describing mesoscopic viscoelasticity without losing details on the underlying interactions existing in wide regimes of deformation under hydrodynamic stress. Here, we approach the computational challenge to model bacterial biofilms for predictive mechanics in silico under variable stress conditions. Up-to-date models are not entirely satisfactory due to the plethora of parameters required to make them functioning under the effects of stress. As guided by the structural depiction gained in a previous work with Pseudomonas fluorescens [Jara et al., Front. Microbiol. 11, 588884 (2021)], we propose a mechanical modeling by means of Dissipative Particle Dynamics (DPD), which captures the essentials of topological and compositional interactions between bacterial particles and cross-linked EPS-embedding under imposed shear. The P. fluorescens biofilms have been modeled under mechanical stress mimicking shear stresses as undergone in vitro. The predictive capacity for mechanical features in DPD-simulated biofilms has been investigated by varying the externally imposed field of shear strain at variable amplitude and frequency. The parametric map of essential biofilm ingredients has been explored by making the rheological responses to emerge among conservative mesoscopic interactions and frictional dissipation in the underlying microscale. The proposed coarse grained DPD simulation qualitatively catches the rheology of the P. fluorescens biofilm over several decades of dynamic scaling.

Erythroid SLC7A5/SLC3A2 amino acid carrier controls red blood cell size and maturation

Inhibition of the heterodimeric amino acid carrier SLC7A5/SLC3A2 (LAT1/CD98) has been widely studied in tumor biology but its role in physiological conditions remains largely unknown. Here we show that the SLC7A5/SLC3A2 heterodimer is constitutively present at different stages of erythroid differentiation but absent in mature erythrocytes. Administration of erythropoietin (EPO) further induces SLC7A5/SLC3A2 expression in circulating reticulocytes, as it also occurs in anemic conditions. Although Slc7a5 gene inactivation in the erythrocyte lineage does not compromise the total number of circulating red blood cells (RBCs), their size and hemoglobin content are significantly reduced accompanied by a diminished erythroblast mTORC1 activity. Furthermore circulating Slc7a5-deficient reticulocytes are characterized by lower transferrin receptor (CD71) expression as well as mitochondrial activity, suggesting a premature transition to mature RBCs. These data reveal that SLC7A5/SLC3A2 ensures adequate maturation of reticulocytes as well as the proper size and hemoglobin content of circulating RBCs.

Lipid nanoparticles for antisense oligonucleotide gene interference into brain border-associated macrophages

A colloidal synthesis’ proof-of-concept based on the Bligh–Dyer emulsion inversion method was designed for integrating into lipid nanoparticles (LNPs) cell-permeating DNA antisense oligonucleotides (ASOs), also known as GapmeRs (GRs), for mRNA interference. The GR@LNPs were formulated to target brain border-associated macrophages (BAMs) as a central nervous system (CNS) therapy platform for silencing neuroinflammation-related genes. We specifically aim at inhibiting the expression of the gene encoding for lipocalin-type prostaglandin D synthase (L-PGDS), an anti-inflammatory enzyme expressed in BAMs, whose level of expression is altered in neuropsychopathologies such as depression and schizophrenia. The GR@LNPs are expected to demonstrate a bio-orthogonal genetic activity reacting with L-PGDS gene transcripts inside the living system without interfering with other genetic or biochemical circuitries. To facilitate selective BAM phagocytosis and avoid subsidiary absorption by other cells, they were functionalized with a mannosylated lipid as a specific MAN ligand for the mannose receptor presented by the macrophage surface. The GR@LNPs showed a high GR-packing density in a compact multilamellar configuration as structurally characterized by light scattering, zeta potential, and transmission electronic microscopy. As a preliminary biological evaluation of the mannosylated GR@LNP nanovectors into specifically targeted BAMs, we detected in vivo gene interference after brain delivery by intracerebroventricular injection (ICV) in Wistar rats subjected to gene therapy protocol. The results pave the way towards novel gene therapy platforms for advanced treatment of neuroinflammation-related pathologies with ASO@LNP nanovectors.

Membrane rigidity regulates E. coli proliferation rates

Combining single cell experiments, population dynamics and theoretical methods of membrane mechanics, we put forward that the rate of cell proliferation in E. coli colonies can be regulated by modifiers of the mechanical properties of the bacterial membrane. Bacterial proliferation was modelled as mediated by cell division through a membrane constriction divisome based on FtsZ, a mechanically competent protein at elastic interaction against membrane rigidity. Using membrane fluctuation spectroscopy in the single cells, we revealed either membrane stiffening when considering hydrophobic long chain fatty substances, or membrane softening if short-chained hydrophilic molecules are used. Membrane stiffeners caused hindered growth under normal division in the microbial cultures, as expected for membrane rigidification. Membrane softeners, however, altered regular cell division causing persistent microbes that abnormally grow as long filamentous cells proliferating apparently faster. We invoke the concept of effective growth rate under the assumption of a heterogeneous population structure composed by distinguishable individuals with different FtsZ-content leading the possible forms of cell proliferation, from regular division in two normal daughters to continuous growing filamentation and budding. The results settle altogether into a master plot that captures a universal scaling between membrane rigidity and the divisional instability mediated by FtsZ at the onset of membrane constriction.

Moulding hydrodynamic 2D-crystals upon parametric Faraday waves in shear-functionalized water surfaces

Faraday waves, or surface waves oscillating at half of the natural frequency when a liquid is vertically vibrated, are archetypes of ordering transitions on liquid surfaces. Although unbounded Faraday waves patterns sustained upon bulk frictional stresses have been reported in highly viscous fluids, the role of surface rigidity has not been investigated so far. Here, we demonstrate that dynamically frozen Faraday waves-that we call 2D-hydrodynamic crystals-do appear as ordered patterns of nonlinear gravity-capillary modes in water surfaces functionalized with soluble (bio)surfactants endowing in-plane shear stiffness. The phase coherence in conjunction with the increased surface rigidity bears the Faraday waves ordering transition, upon which the hydrodynamic crystals were reversibly molded under parametric control of their degree of order, unit cell size and symmetry. The hydrodynamic crystals here discovered could be exploited in touchless strategies of soft matter and biological scaffolding ameliorated under external control of Faraday waves coherence.

Mechanical conditions for stable symmetric cell constriction

Cell constriction is a decisive step for division in many cells. However, its physical pathway remains poorly understood, calling for a quantitative analysis of the forces required in different cytokinetic scenarios. Using a model cell composed by a flexible membrane (actin cortex and cell membrane) that encloses the cytoplasm, we study the mechanical conditions necessary for stable symmetric constriction under radial equatorial forces using analytical and numerical methods. We deduce that stable symmetric constriction requires positive effective spontaneous curvature, while spontaneous constriction requires a spontaneous curvature higher than the characteristic inverse cell size. Surface tension reduction (for example by actin cortex growth and membrane trafficking) increases the stability and spontaneity of cellular constriction. A reduction of external pressure also increases stability and spontaneity. Cells with prolate lobes (elongated cells) require lower stabilization forces than oblate-shaped cells (discocytes). We also show that the stability and spontaneity of symmetric constriction increase as constriction progresses. Our quantitative results settle the physical requirements for stable cytokinesis, defining a quantitative framework to analyze the mechanical role of the different constriction machinery and cytokinetic pathways found in real cells, so contributing to a deeper quantitative understanding of the physical mechanism of the cell division process.

Membrane stress and torque induced by Frank's nematic textures: A geometric perspective using surface-based constraints

An elastic membrane with embedded nematic molecules is considered as a model of anisotropic fluid membrane with internal ordering. By considering the geometric coupling between director field and membrane curvature, the nematic texture is shown to induce anisotropic stresses additional to Canham-Helfrich elasticity. Building upon differential geometry, analytical expressions are found for the membrane stress and torque induced by splaying, twisting, and bending of the nematic director as described by the Frank energy of liquid crystals. The forces induced by prototypical nematic textures are visualized on the sphere and on cylindrical surfaces.

Membrane curvature induces cardiolipin sorting

Cardiolipin is a cone-shaped lipid predominantly localized in curved membrane sites of bacteria and in the mitochondrial cristae. This specific localization has been argued to be geometry-driven, since the CL's conical shape relaxes curvature frustration. Although previous evidence suggests a coupling between CL concentration and membrane shape in vivo, no precise experimental data are available for curvature-based CL sorting in vitro. Here, we test this hypothesis in experiments that isolate the effects of membrane curvature in lipid-bilayer nanotubes. CL sorting is observed with increasing tube curvature, reaching a maximum at optimal CL concentrations, a fact compatible with self-associative clustering. Observations are compatible with a model of membrane elasticity including van der Waals entropy, from which a negative intrinsic curvature of -1.1 nm-1 is predicted for CL. The results contribute to understanding the physicochemical interplay between membrane curvature and composition, providing key insights into mitochondrial and bacterial membrane organization and dynamics.

Microfluidic fabrication of vesicles with hybrid lipid/nanoparticle bilayer membranes

Hybrid lipid/nanoparticle membranes are suitable model systems both to study the complex interactions between nanoparticles and biological membranes, and to demonstrate technological concepts in cellular sensing and drug delivery. Unfortunately, embedding nanoparticles into the bilayer membrane of lipid vesicles is challenging due to the poor control over the vesicle fabrication process of conventional methodologies and the fragility of the modified lipid bilayer assembly. Here, the utility of water-in-oil-in-water double emulsion drops with ultrathin oil shells as templates to form vesicles with hybrid lipid/nanoparticle membranes is reported. Moreover, upon bilayer formation, which occurs through dewetting of the oil solvent from the double emulsion drops, a phase separation is observed in the vesicle membrane, with solid-like nanoparticle-rich microdomains segregated into a continuous fluid-like nanoparticle-poor phase. This phase coexistence evidences the complex nature of the interactions between nanoparticles and lipid membranes. In this context, this microfluidic-assisted fabrication strategy may play a crucial role in thoroughly understanding such interactions given the uniform membrane properties of the resulting productions. Furthermore, the high encapsulation efficiency of both the vesicle membrane and core endows these vesicles with great potential for sensing applications and drug delivery.

A gradient-based, GPU-accelerated, high-precision contour-segmentation algorithm with application to cell membrane fluctuation spectroscopy

We present a novel intensity-gradient based algorithm specifically designed for nanometer-segmentation of cell membrane contours obtained with high-resolution optical microscopy combined with high-velocity digital imaging. The algorithm relies on the image oversampling performance and computational power of graphical processing units (GPUs). Both, synthetic and experimental data are used to quantify the sub-pixel precision of the algorithm, whose analytic performance results comparatively higher than in previous methods. Results from the synthetic data indicate that the spatial precision of the presented algorithm is only limited by the signal-to-noise ratio (SNR) of the contour image. We emphasize on the application of the new algorithm to membrane fluctuations (flickering) in eukaryotic cells, bacteria and giant vesicle models. The method shows promising applicability in several fields of cellular biology and medical imaging for nanometer-precise boundary-determination and mechanical fingerprinting of cellular membranes in optical microscopy images. Our implementation of this high-precision flicker spectroscopy contour tracking algorithm (HiPFSTA) is provided as open-source at www.github.com/michaelmell/hipfsta.

Supramolecular zippers elicit interbilayer adhesion of membranes producing cell death

Background

The fluorescent dye 10-N-nonyl acridine orange (NAO) is widely used as a mitochondrial marker. NAO was reported to have cytotoxic effects in cultured eukaryotic cells when incubated at high concentrations. Although the biochemical response of NAO-induced toxicity has been well identified, the underlying molecular mechanism has not yet been explored in detail.

Methods

We use optical techniques, including fluorescence confocal microscopy and lifetime imaging microscopy (FLIM) both in model membranes built up as giant unilamellar vesicles (GUVs) and cultured cells. These experiments are complemented with computational studies to unravel the molecular mechanism that makes NAO cytotoxic.

Results

We have obtained direct evidence that NAO promotes strong membrane adhesion of negatively charged vesicles. The attractive forces are derived from van der Waals interactions between anti-parallel H-dimers of NAO molecules from opposing bilayers. Semi-empirical calculations have confirmed the supramolecular scenario by which anti-parallel NAO molecules form a zipper of bonds at the contact region. The membrane remodeling effect of NAO, as well as the formation of H-dimers, was also confirmed in cultured fibroblasts, as shown by the ultrastructure alteration of the mitochondrial cristae.

Conclusions

We conclude that membrane adhesion induced by NAO stacking accounts for the supramolecular basis of its cytotoxicity.

General significance

Mitochondria are a potential target for cancer and gene therapies. The alteration of the mitochondrial structure by membrane remodeling agents able to form supramolecular assemblies via adhesion properties could be envisaged as a new therapeutic strategy.

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NAO promotes interbilayer adhesion of negatively charged lipid vesicles.

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Membrane adhesion derives from the self-assembly of NAO into antiparallel H-dimers.

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The adhesion strength promoted by antiparallel H-aggregates is 10⁻⁶ J/m².

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The formation of NAO H-aggregates produces cell death in fibroblasts.

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The molecular mechanism of NAO cytotoxicity relies on the adhesion ability of H-dimers.

WDR5 modulates cell motility and morphology and controls nuclear changes induced by a 3D environment

Cell migration through extracellular matrices requires nuclear deformation, which depends on nuclear stiffness. In turn, chromatin structure contributes to nuclear stiffness, but the mechanosensing pathways regulating chromatin during cell migration remain unclear. Here, we demonstrate that WD repeat domain 5 (WDR5), an essential component of H3K4 methyltransferase complexes, regulates cell polarity, nuclear deformability, and migration of lymphocytes in vitro and in vivo, independent of transcriptional activity, suggesting nongenomic functions for WDR5. Similarly, depletion of RbBP5 (another H3K4 methyltransferase subunit) promotes similar defects. We reveal that a 3D environment increases the H3K4 methylation dependent on WDR5 and results in a globally less compacted chromatin conformation. Further, using atomic force microscopy, nuclear particle tracking, and nuclear swelling experiments, we detect changes in nuclear mechanics that accompany the epigenetic changes induced in 3D conditions. Indeed, nuclei from cells in 3D environments were softer, and thereby more deformable, compared with cells in suspension or cultured in 2D conditions, again dependent on WDR5. Dissecting the underlying mechanism, we determined that actomyosin contractility, through the phosphorylation of myosin by MLCK (myosin light chain kinase), controls the interaction of WDR5 with other components of the methyltransferase complex, which in turn up-regulates H3K4 methylation activation in 3D conditions. Taken together, our findings reveal a nongenomic function for WDR5 in regulating H3K4 methylation induced by 3D environments, physical properties of the nucleus, cell polarity, and cell migratory capacity.

Bovine anaplasmosis and tick-borne pathogens in cattle of the Galapagos Islands

A cross-sectional study was conducted to determine the species of Anaplasma spp. and estimate its prevalence in cattle of the three main cattle-producing Galapagos Islands (Santa Cruz, San Cristóbal and Isabela) using indirect PCR assays, genetic sequencing and ELISA. Ticks were also collected from cattle and scanned for 47 tick-borne pathogens in a 48 × 48 real-time PCR chip. A mixed effects logistic regression was performed to identify potential risk factors explaining Anaplasma infection in cattle. A. phagocytophilum was not detected in any of the tested animals. Genetic sequencing allowed detection of A. platys-like strains in 11 (36.7%) of the 30 Anaplasma spp.-positive samples analysed. A. marginale was widespread in the three islands with a global between-herd prevalence of 100% [89; 100]_{95% CI} and a median within-herd prevalence of 93%. A significant association was found between A. marginale infection and age with higher odds of being positive for adults (OR = 3.3 [1.2; 9.9]_{95% Bootstrap CI} ). All collected ticks were identified as Rhipicephalus microplus. A. marginale, Babesia bigemina, Borrelia theileri and Francisella-like endosymbiont were detected in tick pools. These results show that the Galapagos Islands are endemic for A. marginale.

Aescin Incorporation and Nanodomain Formation in DMPC Model Membranes

The saponin aescin from the horse chestnut tree is a natural surfactant well-known to self-assemble as oriented-aggregates at fluid interfaces. Using model membranes in the form of lipid vesicles and Langmuir monolayers, we study the mixing properties of aescin with the phase-segregating phospholipid 1,2-dimyristoyl-sn-glycero-phosphocholine (DMPC). The binary membranes are experimentally studied on different length scales ranging from the lipid headgroup area to the macroscopic scale using small-angle X-ray scattering (SAXS), photon correlation spectroscopy (PCS), and differential scanning calorimetry (DSC) with binary bilayer vesicles and Langmuir tensiometry (LT) with lipid monolayers spread on the surface of aescin solutions. The binary interaction was found to strongly depend on aescin concentration in two well differentiated concentration regimes. Below 7 mol %, the results reveal phase segregation of nanometer-sized aescin-rich domains in an aescin-poor continuous bilayer. Above this concentration, aescin-aescin interactions dominate, which inhibit vesicle formation but lead to the formation of new membrane aggregates of smaller sizes. From LT studies in monolayers, the interaction of aescin with DMPC was shown to be stronger in the condensed phase than in the liquid expanded phase. Furthermore, a destructuring role was revealed for aescin on phospholipid membranes, similar to the fluidizing effect of cholesterol and nonsteroidal anti-inflammatory drugs (NSAIDs) on lipid bilayers.

Dissipative dynamics of fluid lipid membranes enriched in cholesterol

Cholesterol is an intriguing component of fluid lipid membranes: It makes them stiffer but also more fluid. Despite the enormous biological significance of this complex dynamical behavior, which blends aspects of membrane elasticity with viscous friction, their mechanical bases remain however poorly understood. Here, we show that the incorporation of physiologically relevant contents of cholesterol in model fluid membranes produces a fourfold increase in the membrane bending modulus. However, the increase in the compression rigidity that we measure is only twofold; this indicates that cholesterol increases coupling between the two membrane leaflets. In addition, we show that although cholesterol makes each membrane leaflet more fluid, it increases the friction between the membrane leaflets. This dissipative dynamics causes opposite but advantageous effects over different membrane motions: It allows the membrane to rearrange quickly in the lateral dimension, and to simultaneously dissipate out-of-plane stresses through friction between the two membrane leaflets. Moreover, our results provide a clear correlation between coupling and friction of membrane leaflets. Furthermore, we show that these rigid membranes are optimal to resist slow deformations with minimum energy dissipation; their optimized stability might be exploited to design soft technological microsystems with an encoded mechanics, vesicles or capsules for instance, useful beyond classical applications as model biophysical systems.

The enzymatic sphingomyelin to ceramide conversion increases the shear membrane viscosity at the air-water interface

Whereas most of lipids have viscous properties and they do not have significant elastic features, ceramides behave as very rigid solid assemblies, displaying viscoelastic behaviour at physiological temperatures. The present review addresses the surface rheology of lipid binary mixtures made of sphingomyelin and ceramide. However, ceramide is formed by the enzymatic cleavage of sphingomyelin in cell plasma membranes. The consequences of the enzymatically-driven ceramide formation involve mechanical alterations of the embedding membrane. Here, an increase on surface shear viscosity was evidenced upon enzymatic incubation of sphingomyelin monolayers. The overall rheological data are discussed in terms of the current knowledge of the thermotropic behaviour of ceramide-containing model membranes.

Permeability modes in fluctuating lipid membranes with DNA-translocating pores

Membrane pores can significantly alter not only the permeation dynamics of biological membranes but also their elasticity. Large membrane pores able to transport macromolecular contents represent an interesting model to test theoretical predictions that assign active-like (non-equilibrium) behavior to the permeability contributions to the enhanced membrane fluctuations existing in permeable membranes [Maneville et al. Phys. Rev. Lett. 82, 4356 (1999)]. Such high-amplitude active contributions arise from the forced transport of solvent and solutes through the open pores, which becomes even dominant at large permeability. In this paper, we present a detailed experimental analysis of the active shape fluctuations that appear in highly permeable lipid vesicles with large macromolecular pores inserted in the lipid membrane, which are a consequence of transport permeability events occurred in an osmotic gradient. The experimental results are found in quantitative agreement with theory, showing a remarkable dependence with the density of membrane pores and giving account of mechanical compliances and permeability rates that are compatible with the large size of the membrane pore considered. The presence of individual permeation events has been detected in the fluctuation time-series, from which a stochastic distribution of the permeation events compatible with a shot-noise has been deduced. The non-equilibrium character of the membrane fluctuations in a permeation field, even if the membrane pores are mere passive transporters, is clearly demonstrated. Finally, a bio-nano-technology outlook of the proposed synthetic concept is given on the context of prospective uses as active membrane DNA-pores exploitable in gen-delivery applications based on lipid vesicles.

Surface hydrodynamics of viscoelastic fluids and soft solids: Surfing bulk rheology on capillary and Rayleigh waves

From the recent advent of the new soft-micro technologies, the hydrodynamic theory of surface modes propagating on viscoelastic bodies has reinvigorated this field of technology with interesting predictions and new possible applications, so recovering its scientific interest very limited at birth to the academic scope. Today, a myriad of soft small objects, deformable meso- and micro-structures, and macroscopically viscoelastic bodies fabricated from colloids and polymers are already available in the materials catalogue. Thus, one can envisage a constellation of new soft objects fabricated by-design with a functional dynamics based on the mechanical interplay of the viscoelastic material with the medium through their interfaces. In this review, we recapitulate the field from its birth and theoretical foundation in the latest 1980s up today, through its flourishing in the 90s from the prediction of extraordinary Rayleigh modes in coexistence with ordinary capillary waves on the surface of viscoelastic fluids, a fact first confirmed in experiments by Dominique Langevin and me with soft gels [Monroy and Langevin, Phys. Rev. Lett. 81, 3167 (1998)]. With this observational discovery at sight, we not only settled the theory previously formulated a few years before, but mainly opened a new field of applications with soft materials where the mechanical interplay between surface and bulk motions matters. Also, new unpublished results from surface wave experiments performed with soft colloids are reported in this contribution, in which the analytic methods of wave surfing synthetized together with the concept of coexisting capillary-shear modes are claimed as an integrated tool to insightfully scrutinize the bulk rheology of soft solids and viscoelastic fluids. This dedicatory to the figure of Dominique Langevin includes an appraisal of the relevant theoretical aspects of the surface hydrodynamics of viscoelastic fluids, and the coverage of the most important experimental results obtained during the three decades of research on this field.

Modeling the Mechanics of Cell Division: Influence of Spontaneous Membrane Curvature, Surface Tension, and Osmotic Pressure

Many cell division processes have been conserved throughout evolution and are being revealed by studies on model organisms such as bacteria, yeasts, and protozoa. Cellular membrane constriction is one of these processes, observed almost universally during cell division. It happens similarly in all organisms through a mechanical pathway synchronized with the sequence of cytokinetic events in the cell interior. Arguably, such a mechanical process is mastered by the coordinated action of a constriction machinery fueled by biochemical energy in conjunction with the passive mechanics of the cellular membrane. Independently of the details of the constriction engine, the membrane component responds against deformation by minimizing the elastic energy at every constriction state following a pathway still unknown. In this paper, we address a theoretical study of the mechanics of membrane constriction in a simplified model that describes a homogeneous membrane vesicle in the regime where mechanical work due to osmotic pressure, surface tension, and bending energy are comparable. We develop a general method to find approximate analytical expressions for the main descriptors of a symmetrically constricted vesicle. Analytical solutions are obtained by combining a perturbative expansion for small deformations with a variational approach that was previously demonstrated valid at the reference state of an initially spherical vesicle at isotonic conditions. The analytic approximate results are compared with the exact solution obtained from numerical computations, getting a good agreement for all the computed quantities (energy, area, volume, constriction force). We analyze the effects of the spontaneous curvature, the surface tension and the osmotic pressure in these quantities, focusing especially on the constriction force. The more favorable conditions for vesicle constriction are determined, obtaining that smaller constriction forces are required for positive spontaneous curvatures, low or negative membrane tension and hypertonic media. Conditions for spontaneous constriction at a given constriction force are also determined. The implications of these results for biological cell division are discussed. This work contributes to a better quantitative understanding of the mechanical pathway of cellular division, and could assist the design of artificial divisomes in vesicle-based self-actuated microsystems obtained from synthetic biology approaches.

Nanomechanical properties of composite protein networks of erythroid membranes at lipid surfaces

Erythrocyte membranes have been particularly useful as a model for studies of membrane structure and mechanics. Native erythroid membranes can be electroformed as giant unilamellar vesicles (eGUVs). In the presence of ATP, the erythroid membrane proteins of eGUVs rearrange into protein networks at the microscale. Here, we present a detailed nanomechanical study of individual protein microfilaments forming the protein networks of eGUVs when spread on supporting surfaces. Using Peak Force tapping Atomic Force Microscopy (PF-AFM) in liquid environment we have obtained the mechanical maps of the composite lipid-protein networks supported on solid surface. In the absence of ATP, the protein pool was characterized by a Young's Modulus E_pool≈5-15MPa whereas the complex filaments were found softer after protein supramolecular rearrangement; E_fil≈0.4MPa. The observed protein softening and reassembling could be relevant for understanding the mechanisms of cytoskeleton reorganization found in pathological erythrocytes or erythrocytes that are affected by biological agents.

Nanocomposite Hydrogels as Platform for Cells Growth, Proliferation, and Chemotaxis

The challenge of mimicking the extracellular matrix with artificial scaffolds that are able to reduce immunoresponse is still unmet. Recent findings have shown that mesenchymal stem cells (MSC) infiltrating into the implanted scaffold have effects on the implant integration by improving the healing process. Toward this aim, a novel polyamidoamine-based nanocomposite hydrogel is synthesized, cross-linked with porous nanomaterials (i.e., mesoporous silica nanoparticles), able to release chemokine proteins. A comprehensive viscoelasticity study confirms that the hydrogel provides optimal structural support for MSC infiltration and proliferation. The efficiency of this hydrogel, containing the chemoattractant stromal cell-derived factor 1α (SDF-1α), in promoting MSC migration in vitro is demonstrated. Finally, subcutaneous implantation of SDF-1α-releasing hydrogels in mice results in a modulation of the inflammatory reaction. Overall, the proposed SDF-1α-nanocomposite hydrogel proves to have potential for applications in tissue engineering.

Susceptibility and possible resistance mechanisms in the palm species Phoenix dactylifera, Chamaerops humilis and Washingtonia filifera against Rhynchophorus ferrugineus (Olivier, 1790) (Coleoptera: Curculionidae)

Rhynchophorus ferrugineus, known as the Red Palm Weevil (RPW), is reported as a pest of up to 40 palm species. However, the susceptibility degree and the defense mechanisms of these species against this weevil are still poorly known. In Europe, the RPW is a major pest of Phoenix canariensis while other palm species, including the congeneric Phoenix dactylifera, seem to be less suitable hosts for this insect. The aim of our study was to compare the defensive response of P. dactylifera, Chamaerops humilis and Washingtonia filifera against R. ferrugineus and try to define the mechanisms of resistance that characterize these species. Bioassays were carried out to evaluate the mortality induced on RPW larvae by extracts from the leaf rachis of the studied palm species. Tests at semi-field scale were also conducted, based either on forced palm infestation, with larvae of RPW, or on natural infestation, with adult females. Rachis extracts from C. humilis and W. filifera caused 100% larval mortality after 2 days of exposure, while extracts of P. dactylifera did not impair larval survival. Independently of the effect of the leaf extracts, the weevils were unable to naturally infest the three palm species, although larval survival was high after forced infestation of the plants. We concluded that the observed lack of infestation of P. dactylifera by RPW is due to factors other than antibiosis. In W. filifera and C. humilis, although the presence of antixenosis mechanisms cannot be excluded, resistance to R. ferrugineus seems to rely on the presence of antibiosis compounds.

Direct Cytoskeleton Forces Cause Membrane Softening in Red Blood Cells

Erythrocytes are flexible cells specialized in the systemic transport of oxygen in vertebrates. This physiological function is connected to their outstanding ability to deform in passing through narrow capillaries. In recent years, there has been an influx of experimental evidence of enhanced cell-shape fluctuations related to metabolically driven activity of the erythroid membrane skeleton. However, no direct observation of the active cytoskeleton forces has yet been reported to our knowledge. Here, we show experimental evidence of the presence of temporally correlated forces superposed over the thermal fluctuations of the erythrocyte membrane. These forces are ATP-dependent and drive enhanced flickering motions in human erythrocytes. Theoretical analyses provide support for a direct force exerted on the membrane by the cytoskeleton nodes as pulses of well-defined average duration. In addition, such metabolically regulated active forces cause global membrane softening, a mechanical attribute related to the functional erythroid deformability.

Thermomechanical transitions of egg-ceramide monolayers

Ceramides have unique biophysical properties. Their high melting temperature and their ability to form lateral domains have converted ceramides into the paradigm of rigid lipids. Here, using shear surface rheology of egg-ceramide Langmuir monolayers, a solid to fluid transition was evidenced as a vanishing shear rigidity at lower temperatures than the lipid melting temperature. Such a mechanical transition, which depends on the lipid lateral pressure, was found in a broad range temperature (40-50 °C). The solid to fluid transition was correlated to a LC to LC+LE phase transition, as confirmed by BAM experiments. Interestingly, together with the softening transition, a supercooling process compatible with a glassy behavior was found upon freezing. A new phase scenario is then depicted that broadens the mechanical behavior of natural ceramides. The phase diversity of ceramides might have important implications in their physiological roles.

Analytical results for cell constriction dominated by bending energy

Analytical expressions are obtained for the main magnitudes of a symmetrically constricted vesicle. These equations provide an easy and compact way to predict minimal requirements for successful constriction and its main magnitudes. Thus, they can be useful for the design of synthetic divisomes and give good predictions for magnitudes including constriction energy, length of the constriction zone, volume and area of the vesicle, and the stability coefficient for symmetric constriction. The analytical expressions are derived combining a perturbative expansion in the Lagrangian for small deformations with a cosine ansatz in the constriction region. Already the simple fourth-order (or sixth-order) approximation provides a good approximation to the values of the main physical magnitudes during constriction, as we show through comparison with numerical results. Results are for vesicles with negligible effects from spontaneous curvature, surface tension, and pressure differences. This is the case when membrane components generating spontaneous curvature are scarce, membrane trafficking is present with low energetic cost, and the external medium is isotonic.

Fluctuation dynamics of bilayer vesicles with intermonolayer sliding: experiment and theory

The presence of coupled modes of membrane motion in closed shells is extensively predicted by theory. The bilayer structure inherent to lipid vesicles is suitable to support hybrid modes of curvature motion coupling membrane bending with the local reorganization of the bilayer material through relaxation of the dilatational stresses. Previous experiments evidenced the existence of such hybrid modes facilitating membrane bending at high curvatures in lipid vesicles [Rodríguez-García, R., Arriaga, L.R., Mell, M., Moleiro, L.H., López-Montero, I., Monroy, F., 2009. Phys. Rev. Lett. 102, 128201.]. For lipid bilayers that are able to undergo intermonolayer sliding, the experimental fluctuation spectra are found compatible with a bimodal schema. The usual tension/bending fluctuations couple with the hybrid modes in a mechanical interplay, which becomes progressively efficient with increasing vesicle radius, to saturate at infinity radius into the behavior expected for a flat membrane. Grounded on the theory of closed shells, we propose an approximated expression of the bimodal spectrum, which predicts the observed dependencies on the vesicle radius. The dynamical features obtained from the autocorrelation functions of the vesicle fluctuations are found in quantitative agreement with the proposed theory.

Ultrathin shell double emulsion templated giant unilamellar lipid vesicles with controlled microdomain formation

A microfluidic approach is reported for the high-throughput, continuous production of giant unilamellar vesicles (GUVs) using water-in-oil-in-water double emulsion drops as templates. Importantly, these emulsion drops have ultrathin shells; this minimizes the amount of residual solvent that remains trapped within the GUV membrane, overcoming a major limitation of typical microfluidic approaches for GUV fabrication. This approach enables the formation of microdomains, characterized by different lipid compositions and structures within the GUV membranes. This work therefore demonstrates a straightforward and versatile approach to GUV fabrication with precise control over the GUV size, lipid composition and the formation of microdomains within the GUV membrane.

Mechanics of constriction during cell division: a variational approach

During symmetric division cells undergo large constriction deformations at a stable midcell site. Using a variational approach, we investigate the mechanical route for symmetric constriction by computing the bending energy of deformed vesicles with rotational symmetry. Forces required for constriction are explicitly computed at constant area and constant volume, and their values are found to be determined by cell size and bending modulus. For cell-sized vesicles, considering typical bending modulus of [Formula: see text], we calculate constriction forces in the range [Formula: see text]. The instability of symmetrical constriction is shown and quantified with a characteristic coefficient of the order of [Formula: see text], thus evidencing that cells need a robust mechanism to stabilize constriction at midcell.

Morphometric image analysis of giant vesicles: a new tool for quantitative thermodynamics studies of phase separation in lipid membranes

We have developed a strategy to determine lengths and orientations of tie lines in the coexistence region of liquid-ordered and liquid-disordered phases of cholesterol containing ternary lipid mixtures. The method combines confocal-fluorescence-microscopy image stacks of giant unilamellar vesicles (GUVs), a dedicated 3D-image analysis, and a quantitative analysis based in equilibrium thermodynamic considerations. This approach was tested in GUVs composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-palmitoyl-sn-glycero-3-phosphocholine/cholesterol. In general, our results show a reasonable agreement with previously reported data obtained by other methods. For example, our computed tie lines were found to be nonhorizontal, indicating a difference in cholesterol content in the coexisting phases. This new, to our knowledge, analytical strategy offers a way to further exploit fluorescence-microscopy experiments in GUVs, particularly retrieving quantitative data for the construction of three lipid-component-phase diagrams containing cholesterol.

Bending stiffness of biological membranes: what can be measured by neutron spin echo?

Large vesicles obtained by the extrusion method represent adequate membrane models to probe membrane dynamics with neutron radiation. Particularly, the shape fluctuations around the spherical average topology can be recorded by neutron spin echo (NSE). In this paper we report on the applicable theories describing the scattering contributions from bending-dominated shape fluctuations in diluted vesicle dispersions, with a focus on the relative relevance of the master translational mode with respect to the internal fluctuations. Different vesicle systems, including bilayer and non-bilayer membranes, have been scrutinized. We describe the practical ranges where the exact theory of bending fluctuations is applicable to obtain the values of the bending modulus from experiments, and we discuss about the possible internal modes that could be alternatively contributing to shape fluctuations.

Shear and compression rheology of Langmuir monolayers of natural ceramides: solid character and plasticity

The present work addresses the fundamental question of membrane elasticity of ceramide layers with a special focus on the plastic regime. The compression and shear viscoelasticity of egg-ceramide Langmuir monolayers were investigated using oscillatory surface rheology in the linear regime and beyond. High compression and shear moduli were measured at room temperature-a clear signature for a solid behavior. At deformations larger than one per mill, egg-ceramide monolayers display plastic features characterized by a decrease of the storage modulus followed by a viscous regime typical of fluid lipids. This behavior is accompanied by a marked decrease of the loss modulus with increasing stress above a yield point. The results permit to univocally classify ceramide monolayers as 2D solids able to undergo plastic deformations, at the difference of typical fluid lipid monolayers. These unusual features are likely to have consequences in the mechanical behavior of ceramide-rich emplacements in biological membranes.

Membrane reconstitution of FtsZ-ZipA complex inside giant spherical vesicles made of E. coli lipids: large membrane dilation and analysis of membrane plasticity

During the division process of Escherichia coli, the globular protein FtsZ is early recruited at the constriction site. The Z-ring, based on FtsZ filaments associated to the inner cell membrane, has been postulated to exert constriction forces. Membrane anchoring is mediated by ZipA, an essential transmembrane protein able to specifically bind FtsZ. In this work, an artificial complex of FtsZ-ZipA has been reconstituted at the inner side of spherical giant unilamellar vesicles made of E. coli lipids. Under these conditions, FtsZ polymerization, triggered when a caged GTP analogue is UV-irradiated, was followed by up to 40% vesicle inflation. The homogeneous membrane dilation was accompanied by the visualization of discrete FtsZ assemblies at the membrane. Complementary rheological data revealed enhanced elasticity under lateral dilation. This explains why vesicles can undergo large dilations in the regime of mechanical stability. A mechanical role for FtsZ polymers as promoters of membrane softening and plasticization is hypothesized.

Efficient orthogonal integration of the bacteriophage ϕ29 DNA-portal connector protein in engineered lipid bilayers

The portal connector of bacteriophage viruses constitutes a robust molecular machine for DNA translocation. In this paper we propose an optimized reconstitution method for efficient orthogonal integration of native viral connectors into lipid bilayers, particularly of giant unilamellar vesicles. Our nanoengineering plan considers the hydrophilic connector protein of the bacteriophage virus ϕ29 integrated into a specifically engineered bilayer made of "hydrophylized" lipids. From the precise knowledge of the connector structure, the membrane chemistry was designed by tuning reactivity in the bilayer using specific functional lipids. We show details on the reconstitution methods and experimental evidence about the integration of the portal protein in the engineered membrane. The proposed route provides an efficient method for orthogonal integration of native viral connectors into lipid bilayers in conditions adequate for functional DNA translocation. This concept could be potentially exploited in advanced nanotechnological realizations, particularly for the integration of these powerful machines into giant lipid vesicles with the aim of building a cargo-device useful for gene delivery applications.

Artificial Spectrin Shells Reconstituted on Giant Vesicles

In the experimental approach to a synthetic minimal cell, the membrane compartment is a main component. Lipid vesicles represent the natural host for the artificial reconstruction of a cytomimetic membrane skeleton able to support mechanical function. Using the membrane component of human erythroid cells, we have reconstructed a membrane shell composed of a spectrin skeleton and fed by ATP. The structural and mechanical analysis reveals this spectrin skeleton as topological network supporting mechanical rigidity. Such an artificial shell would define a membrane compartment mechanically stable under physiological conditions.

Solid character of membrane ceramides: a surface rheology study of their mixtures with sphingomyelin

The compression and shear viscoelasticities of egg-ceramide and its mixtures with sphingomyelin were investigated using oscillatory surface rheology performed on Langmuir monolayers. We found high values for the compression and shear moduli for ceramide, compatible with a solid-state membrane, and extremely high surface viscosities when compared to typical fluid lipids. A fluidlike rheological behavior was found for sphingomyelin. Lateral mobilities, measured from particle tracking experiments, were correlated with the monolayer viscosities through the usual hydrodynamic relationships. In conclusion, ceramide increases the solid character of sphingomyelin-based membranes and decreases their fluidity, thus drastically decreasing the lateral mobilities of embedded objects. This mechanical behavior may involve important physiological consequences in biological membranes containing ceramides.

Active membrane viscoelasticity by the bacterial FtsZ-division protein

At the early stages of the division process in Escherichia coli, the protein FtsZ forms a septal ring at the midcell. This Z-ring causes membrane constriction during bacterial division. The Z-ring associates to the lipid membrane through several membrane proteins, ZipA among them. Here, a simplified FtsZ-ZipA model was reconstituted onto Langmuir monolayers based in E. coli polar lipid extract. Brewster angle and atomic force microscopy have revealed membrane FtsZ-polymerization upon GTP hydrolysis. The compression viscoelasticity of these monolayers has been also investigated. The presence of protein induced softening and fluidization with respect to the bare lipid membrane. An active mechanism, based on the internal forces stressed by FtsZ filaments and transduced to the lipid membrane by ZipA, was suggested to underlie the observed behavior.

Amorphous freezing in two dimensions: from soft coils to rigid particles

The topic of the gel transition in two dimensions is revisited by considering data on the shear elasticity of Langmuir monolayers of different spherical objects. Amorphous freezing can be associated to structural percolation in a lattice able to resist shear stresses. The shear modulus and its dependence on the packing fraction are found to strongly depend on the details of the interaction potential and largely differ from expectations for entropic networks. This behaviour can be interpreted in terms of more elaborated percolation theories including central forces and bond-bending forces.