Red blood cell flickering activity locally controlled by holographic optical tweezers.

Red blood cells possess a singular mechanobiology, enabling efficient navigation through capillaries smaller than their own size. Their plasma membrane exhibits non-equilibrium shape fluctuation, often reported as enhanced flickering activity. Such active membrane motion is propelled by motor proteins that mediate interactions between the spectrin skeleton and the lipid bilayer. However, modulating the flickering in living red blood cells without permanently altering their mechanical properties represents a significant challenge. In this study, we developed holographic optical tweezers to generate a force field distributed along the equatorial membrane contour of individual red blood cells. In free-standing red blood cells, we observed heterogeneous flickering activity, attributed to localized membrane kickers. By employing holographic optical forces, these active kickers can be selectively halted under minimal invasion. Our findings shed light on the dynamics of membrane flickering and established a manipulation tool that could open new avenues for investigating mechanotransduction processes in living cells.

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.

Safety and Immunogenicity of a Newcastle Disease Virus Vector-Based SARS-CoV-2 Vaccine Candidate, AVX/COVID-12-HEXAPRO (Patria), in Pigs.

Vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were developed in record time and show excellent efficacy and effectiveness against coronavirus disease 2019 (COVID-19). However, currently approved vaccines cannot meet the global demand. In addition, none of the currently used vaccines is administered intranasally to potentially induce mucosal immunity. Here, we tested the safety and immunogenicity of a second-generation SARS-CoV-2 vaccine that includes a stabilized spike antigen and can be administered intranasally. The vaccine is based on a live Newcastle disease virus vector expressing a SARS-CoV-2 spike protein stabilized in a prefusion conformation with six beneficial proline substitutions (AVX/COVID-12-HEXAPRO; Patria). Immunogenicity testing in the pig model showed that both intranasal and intramuscular application of the vaccine as well as a combination of the two induced strong serum neutralizing antibody responses. Furthermore, substantial reactivity to B.1.1.7, B.1.351, and P.1 spike variants was detected. Finally, no adverse reactions were found in the experimental animals at any dose level or delivery route. These results indicate that the experimental vaccine AVX/COVID-12-HEXAPRO (Patria) is safe and highly immunogenic in the pig model. IMPORTANCE Several highly efficacious vaccines for SARS-CoV-2 have been developed and are used in the population. However, the current production capacity cannot meet the global demand. Therefore, additional vaccines-especially ones that can be produced locally and at low cost-are urgently needed. This work describes preclinical testing of a SARS-CoV-2 vaccine candidate which meets these criteria.

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.

Multiple particle tracking analysis in isolated nuclei reveals the mechanical phenotype of leukemia cells.

The nucleus is fundamentally composed by lamina and nuclear membranes that enclose the chromatin, nucleoskeletal components and suspending nucleoplasm. The functional connections of this network integrate external stimuli into cell signals, including physical forces to mechanical responses of the nucleus. Canonically, the morphological characteristics of the nucleus, as shape and size, have served for pathologists to stratify and diagnose cancer patients; however, novel biophysical techniques must exploit physical parameters to improve cancer diagnosis. By using multiple particle tracking (MPT) technique on chromatin granules, we designed a SURF (Speeded Up Robust Features)-based algorithm to study the mechanical properties of isolated nuclei and in living cells. We have determined the apparent shear stiffness, viscosity and optical density of the nucleus, and how the chromatin structure influences on these biophysical values. Moreover, we used our MPT-SURF analysis to study the apparent mechanical properties of isolated nuclei from patients of acute lymphoblastic leukemia. We found that leukemia cells exhibited mechanical differences compared to normal lymphocytes. Interestingly, isolated nuclei from high-risk leukemia cells showed increased viscosity than their counterparts from normal lymphocytes, whilst nuclei from relapsed-patient's cells presented higher density than those from normal lymphocytes or standard- and high-risk leukemia cells. Taken together, here we presented how MPT-SURF analysis of nuclear chromatin granules defines nuclear mechanical phenotypic features, which might be clinically relevant.

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 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.

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.

Injectable Hybrid Hydrogels, with Cell-Responsive Degradation, for Tumor Resection.

Biocompatible soft materials have recently found applications in interventional endoscopy to facilitate resection of mucosal tumors. When neoplastic lesions are in organs that can be easily damaged by perforation, such as stomach, intestine, and esophagus, the formation of a submucosal fluid cushion (SFC) is needed to lift the tumor from the underlying muscle during the resection of neoplasias. Such procedure is called endoscopic submucosal dissection (ESD). We describe an injectable, biodegradable, hybrid hydrogel able to form a SFC and to facilitate ESD. The hydrogel, based on polyamidoamines, contains breakable silica nanocapsules covalently bound to its network and able to release biomolecules. To promote degradation, the hydrogel is composed of cleavable disulfide moieties that are reduced by the cells through secretion of glutathione. The same stimulus triggers the breaking of the silica nanocapsules; therefore, the entire hybrid material can be completely degraded and its decomposition depends entirely on the presence of cells. Interestingly, the hydrogel precursor solution showed rapid gelation when injected in vivo and afforded a long-lasting high mucosal elevation, keeping the cushion volume constant during the dissection. This novel material can provide a solution to ESD limitations and promote healing of tissues after surgery.

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.

Nonequilibrium fluctuations of lipid membranes by the rotating motor protein F1F0-ATP synthase.

ATP synthase is a rotating membrane protein that synthesizes ATP through proton-pumping activity across the membrane. To unveil the mechanical impact of this molecular active pump on the bending properties of its lipid environment, we have functionally reconstituted the ATP synthase in giant unilamellar vesicles and tracked the membrane fluctuations by means of flickering spectroscopy. We find that ATP synthase rotates at a frequency of about 20 Hz, promoting large nonequilibrium deformations at discrete hot spots in lipid vesicles and thus inducing an overall membrane softening. The enhanced nonequilibrium fluctuations are compatible with an accumulation of active proteins at highly curved membrane sites through a curvature-protein coupling mechanism that supports the emergence of collective effects of rotating ATP synthases in lipid membranes.