In cells, the curved clathrin structures in vesicle budding are well characterized, while the flat ones remain poorly understood. Here, we reconstitute the flat assembly of ESCRT-0 protein HRS and clathrin onto lipid membranes in vitro. HRS forms gel-like protein condensates at micromolar concentrations in solutions. These condensates spread as a two-dimensional layer on negatively charged membranes and, together with clathrin, form multilayered coats. Importantly, the two-dimensional condensates spontaneously form only on membranes at HRS concentrations below 50 nM, its cytoplasmic concentration. Correlative cryo-electron tomography of HRS-labelled endosomes in cells reveals a multilayered structure containing a flat clathrin layer 16 nm away from the membrane, consistent with our in vitro findings. Cholesterol enhances HRS recruitment to the membrane both in cells and in supported bilayers. Furthermore, cholesterol promotes the phase separation of HRS onto membranes, which in turn concentrates cholesterol underneath. This positive feedback promotes the formation of HRS-clathrin microdomains that sorts reconstituted ubiquitinated cargoes. Altogether, our results show that the distinct architecture of ESCRT-0 is assembled by the two-dimensional phase-separation of HRS which drives the assembly of flat clathrin coats.

The emergence of cell compartmentalization depends on membrane fission to create the endomembrane compartments. In eukaryotes, membrane fission is commonly executed by ESCRT-III, a protein complex conserved in all domains of life. However, whether membrane fission was an ancestral ESCRT-III activity predating eukaryogenesis remains unknown. Here we show that ESCRT-IIIA from Asgard Heimdallarchaeota, the closest archaeal relatives of eukaryotes, performs membrane fission through an N-terminal amphipathic helix, which we term Hofund. In eukaryotes, Hofund is fragmented across ESCRT-IIIA paralogs, and disrupting these regions causes severe fission defect in yeast. Remarkably, Heimdallarchaeota Hofund restores fission when fused to defective eukaryotic paralogs. These findings suggest that ESCRT-III-mediated fission arose before eukaryogenesis and later diversified to support the regulatory complexity of eukaryotic compartmentalization. ### Competing Interest Statement The authors have declared no competing interest.

Bioelectronics is an evolving field focused on creating efficient interfaces between electronic systems and biological environments, where electrodes are crucial for detecting, recording, and stimulating signals. While traditional fabrication techniques offer high precision, they are costly and lack scalability. Vacuum soft lithography offers a scalable, time-efficient alternative for manufacturing microstructured electrodes. When combined with poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS), a conducting polymer known for its biocompatibility and stability, this approach presents a promising solution for advancing bioelectronic applications. This study demonstrates the successful application of vacuum soft lithography for fabricating PEDOT:PSS-based microstructured electrodes. These PEDOT:PSS-based microstructured electrodes are fabricated and characterized based on their morphology, mechanics, and electrochemical properties. The PEDOT:PSS mixture containing 2% (3-glycidyloxypropyl)trimethoxysilane (GOPS) demonstrates optimal properties, achieving an excellent balance of reproducibility, conductivity, and mechanical integrity, with in vitro cytocompatibility confirmed through testing with the MDA-MB-231 breast cancer cell line. This approach offers significant advantages, including reduced costs and production times, making it a viable alternative to conventional methods. Future research will explore its use with other conducting materials for broader bioelectronic applications.

Translation of photophysical properties of fluorescent sensors from solution to solid-gas environments via functionalized surfaces constitutes a challenge in chemistry. In this work, we report on the chemical synthesis, barium capture ability and photophysical properties of two families of monocolor and bicolor fluorescent sensors. These sensors were prepared to capture barium cations that can be produced in neutrinoless double beta decay of Xe-136. These sensors incorporate crown ether units, two different fluorophores, aliphatic spacers of different lengths, and a silatrane linker that forms covalent bonds with indium tin oxide (ITO) surfaces. Both species shared excellent Ba2+ binding abilities. Fluorescent monocolor indicators (FMIs), based on naphthyl fluorophores, showed an off-on character in solution controlled by photoinduced electron transfer. Fluorescent bicolor indicators (FBIs), based on benzo[a]imidazo[5,1,2-cd] fluorophores, exhibited a significant change in their emission spectra on going from the free to the barium-bound state. Both FMIs and FBIs showed similar photophysics in solution and on ITO. However, their performance on ITO was found to be attenuated, but not fully extinguished, with respect to the values obtained in solution, both in terms of intensity and selectivity between the free and Ba2+-bound states. Despite this issue, improved performance of the FBIs based on confocal microscopy of the directly attached molecules was observed. These selective FMI and FBI chemosensors installed on tailor-made functionalized surfaces are promising tools to capture the barium cations produced in the double beta decay of Xe-136. The identification of this capture would boost the sensitivity of the experiments searching for the Xe-136-based neutrinoless double beta decay, as backgrounds would be almost totally suppressed.

Tension propagates in lipid bilayers over hundreds of microns within milliseconds, seemingly precluding the formation of tension gradients. Nevertheless, plasma membrane tension gradients have been reported in migrating cells and along growing axons. Here, we show that the mechanosensitive, fluorescent membrane probe Flipper-TR visualizes membrane tension gradients in artificial and cellular membranes. Images of tension gradients allow their quantitative characterization, showing that they are long-ranged and linear in all migratory adherent cells. Using this tool, we unexpectedly reveal that tension gradients also exist in non-migrating adherent cells while they are absent in non-adherent migrating cells. This suggests that actomyosin forces can generate tension gradients even in non-moving cells, but that adhesion to a substrate is needed to sustain these gradients. Treatment of cells with drugs perturbing actomyosin show that branched actin increases tension, creating gradients. Furthermore, specific adhesion mediated by clathrin plaques colocalizes with regions of low tension, and chemical disruption of clathrin plaques strongly affect tension gradients. Altogether, our results show that the combined action of actomyosin and adhesion forces create tension gradients in the plasma membrane of adherent cells, even the ones not migrating.

The precise mechanisms underlying the cellular response to static electric cues remain unclear, limiting the design and development of biomaterials that utilize this parameter to enhance specific biological behaviours. To gather information on this matter we have explored the interaction of collagen type-I, the most abundant mammalian extracellular protein, with poly(vinylidene fluoride) (PVDF), an electroactive polymer with great potential for tissue engineering applications. Our results reveal significant differences in collagen affinity, conformation, and interaction strength depending on the electric charge of the PVDF surface, which subsequently affects the behaviour of mesenchymal stem cells seeded on them. These findings highlight the importance of surface charge in the establishment of the material-protein interface and ultimately in the biological response to the material. STATEMENT OF SIGNIFICANCE: The development of new tissue engineering strategies relies heavily on the understanding of how biomaterials interact with biological tissues. Although several factors drive this process and their driving principles have been identified, the relevance and mechanism by which the surface potential influences cell behaviour is still unknown. In our study, we investigate the interaction between collagen, the most abundant component of the extracellular matrix, and poly(vinylidene fluoride) with varying surface charges. Our findings reveal substantial variations in the binding forces, structure and adhesion of collagen on the different surfaces, which collectively explain the differential cellular responses. By exposing these differences, our research fills a critical knowledge gap and paves the way for innovations in material design for advanced tissue regeneration strategies.

The ESCRT-III-like protein Vipp1 couples filament polymerization with membrane remodeling. It assembles planar sheets as well as 3D rings and helical polymers, all implicated in mitigating plastid-associated membrane stress. The architecture of Vipp1 planar sheets and helical polymers remains unknown, as do the geometric changes required to transition between polymeric forms. Here we show how cyanobacterial Vipp1 assembles into morphologically-related sheets and spirals on membranes in vitro. The spirals converge to form a central ring similar to those described in membrane budding. Cryo-EM structures of helical filaments reveal a close geometric relationship between Vipp1 helical and planar lattices. Moreover, the helical structures reveal how filaments twist—a process required for Vipp1, and likely other ESCRT-III filaments, to transition between planar and 3D architectures. Overall, our results provide a molecular model for Vipp1 ring biogenesis and a mechanism for Vipp1 membrane stabilization and repair, with implications for other ESCRT-III systems. Using AFM and cryo-EM, the authors investigate how ESCRT-III-like protein Vipp1 transitions between polymorphic forms of planar sheets and helical filaments, providing insights into membrane-repair processes.

Measuring forces within living cells remains a technical challenge. In this Tutorial, we cover the development of hydrophobic mechanosensing fluorescent probes called Flippers, whose fluorescence lifetime depends on lipid packing. Flipper probes can therefore be used as reporters for membrane tension via the measurement of changes in their fluorescence lifetime. We describe the technical optimization of the probe for imaging and provide working examples for their characterizations in a variety of biological and in vitro systems. We further provide a guideline to measure biophysical parameters of cellular membranes by fluorescence lifetime imaging microscopy using Flipper probes, providing evidence that flippers can report long range forces in cells, tissues and organs.

Tension propagates in lipid bilayers over hundreds of microns within milliseconds, precluding the formation of tension gradients. Nevertheless, plasma membrane tension gradients have been evidenced in migrating cells and along axons. Here, using a fluorescent membrane tension probe, we show that membrane tension gradients exist in all adherent cells, whether they migrate or not. Non-adhering cells do not display tension gradients. We further show that branched actin increases tension, while membrane-to-cortex attachments facilitate its propagation. Tension is the lowest at the edge of adhesion sites and highest at protrusions, setting the boundaries of the tension gradients. By providing a quantitative and mechanistic basis behind the organization of membrane tension gradients, our work explains how they are actively sustained in adherent cells. ### Competing Interest Statement The authors have declared no competing interest.

Summary ESCRT-III family proteins build dynamic filaments that remodel membrane. Although the transition of planar filaments into 3D membrane budding structures is fundamental for their function, the geometric changes in polymer architecture driving the transition remain obscure. Here we show how bacterial Vipp1 polymerises into dynamic planar sheets and spirals on membrane. The spirals converge to form a central ring like those known to bud membrane. To probe how Vipp1 morphs between polymers, we determine the architecture of multiple helical filaments. As well as describing filament constriction and membrane tubulation, the geometric relationship between helical and planar lattices enables Vipp1 sheets and spirals to be modelled. Moreover, the helical structures show filaments twisting – a process needed for Vipp1 to transition between planar and 3D architectures. Given the structural conservation between Vipp1 and ESCRT-III, our results may represent the broad changes in geometry required for some ESCRT-III filaments to switch between 2D and 3D forms.

HYPOTHESIS:Response of lipid bilayers to external mechanical stimuli is an active area of research with implications for fundamental and synthetic cell biology. Developing novel tools for systematically imposing mechanical strains and non-invasively mapping out interfacial (membrane) stress distributions on lipid bilayers can accelerate research in this field.EXPERIMENTS:We report a miniature platform to manipulate model cell membranes in the form of droplet interface bilayers (DIBs), and non-invasively measure spatio-temporally resolved interfacial stresses using two photon fluorescence lifetime imaging of an interfacially active molecular flipper (Flipper-TR). We established the effectiveness of the developed framework by investigating interfacial stresses accompanying three key processes associated with DIBs: thin film drainage between lipid monolayer coated droplets, bilayer formation, and bilayer separation.FINDINGS:The measurements revealed fundamental aspects of DIBs including the existence of a radially decaying interfacial stress distribution post bilayer formation, and the simultaneous build up and decay of stress respectively at the bilayer corner and center during bilayer separation. Finally, utilizing interfacial rheology measurements and MD simulations, we also reveal that the tested molecular flipper is sensitive to membrane fluidity that changes with interfacial stress - expanding the scientific understanding of how molecular flippers sense stress.

Measuring forces within living cells remains a technical challenge. We developed hydrophobic mechanosensing fluorescent probes called Flippers, whose fluorescence lifetime depends on lipid packing and can report on membrane tension. Here, we describe technical optimization of the probe imaging, and diverse characterizations in various biological and in vitro systems. We provide a guideline to measure biophysical parameters of cellular membranes by FLIM microscopy with Flipper probes, providing evidences that flippers can report long range forces in cells, tissues and organs i .

α-synuclein aggregation is present in Parkinson's disease and other neuropathologies. Among the assemblies that populate the amyloid formation process, oligomers and short fibrils are the most cytotoxic. The human Hsc70-based disaggregase system can resolve α-synuclein fibrils, but its ability to target other toxic assemblies has not been studied. Here, we show that this chaperone system preferentially disaggregates toxic oligomers and short fibrils, while its activity against large, less toxic amyloids is severely impaired. Biochemical and kinetic characterization of the disassembly process reveals that this behavior is the result of an all-or-none abrupt solubilization of individual aggregates. High-speed atomic force microscopy explicitly shows that disassembly starts with the destabilization of the tips and rapidly progresses to completion through protofilament unzipping and depolymerization without accumulation of harmful oligomeric intermediates. Our data provide molecular insights into the selective processing of toxic amyloids, which is critical to identify potential therapeutic targets against increasingly prevalent neurodegenerative disorders.

This article describes four fluorescent membrane tension probes that have been designed, synthesized, evaluated, commercialized and applied to current biology challenges in the context of the NCCR Chemical Biology. Their names are Flipper-TR®, ER Flipper-TR®, Lyso Flipper-TR®, and Mito Flipper-TR®. They are available from Spirochrome.

Envelope glycoproteins from genetically-divergent virus families comprise fusion peptides (FPs) that have been posited to insert and perturb the membranes of target cells upon activation of the virus-cell fusion reaction. Conserved sequences rich in aromatic residues juxtaposed to the external leaflet of the virion-wrapping membranes are also frequently found in viral fusion glycoproteins. These membrane-proximal external regions (MPERs) have been implicated in the promotion of the viral membrane restructuring event required for fusion to proceed, hence, proposed to comprise supplementary FPs. However, it remains unknown whether the structure-function relationships governing canonical FPs also operate in the mirroring MPER sequences. Here, we combine infrared spectroscopy-based approaches with cryo-electron microscopy to analyze the alternating conformations adopted, and perturbations generated in membranes by CpreTM, a peptide derived from the MPER of the HIV-1 Env glycoprotein. Altogether, our structural and morphological data support a cholesterol-dependent conformational plasticity for this HIV-1 sequence, which could assist cell-virus fusion by destabilizing the viral membrane at the initial stages of the process.

During mitochondrial fission, key molecular and cellular factors assemble on the outer mitochondrial membrane, where they coordinate to generate constriction. Constriction sites can eventually divide, or reverse upon disassembly of the machinery. However, a role for membrane tension in mitochondrial fission, although speculated, has remained undefined. We captured the dynamics of constricting mitochondria in mammalian cells using live-cell structured illumination microscopy (SIM). By analyzing the diameters of tubules that emerge from mitochondria and implementing a fluorescence lifetime-based mitochondrial membrane tension sensor, we discovered that mitochondria are indeed under tension. Under perturbations that reduce mitochondrial tension, constrictions initiate at the same rate, but are less likely to divide. We propose a model based on our estimates of mitochondrial membrane tension and bending energy in living cells which accounts for the observed probability distribution for mitochondrial constrictions to divide.

The increase in speed of the high-speed atomic force microscopy (HS-AFM) compared to that of the conventional AFM made possible the first-ever visualisation at the molecular-level of the activity of an antimicrobial peptide on a membrane. We investigated the medically prescribed but poorly understood lipopeptide Daptomycin under infection-like conditions (37 °C, bacterial lipid composition and antibiotic concentrations). We confirmed so far hypothetical models: Dap oligomerization and the existence of half pores. Moreover, we detected unknown molecular mechanisms: new mechanisms to form toroidal pores or to resist Dap action, and to unprecedently quantify the energy profile of interacting oligomers. Finally, the biological and medical relevance of the findings was ensured by a multi-scale multi-nativeness—from the molecule to the cell—correlation of molecular-level information from living bacteria ( Bacillus subtilis strains) to liquid-suspended vesicles and supported-membranes using electron and optical microscopies and the lipid tension probe FliptR, where we found that the cells with a healthier state of their cell wall show smaller membrane deformations.

We report the design, synthesis, and evaluation of fluorescent flipper probes for single-molecule super-resolution imaging of membrane tension in living cells. Reversible switching from bright-state ketones to dark-state hydrates, hemiacetals, and hemithioacetals is demonstrated for twisted and planarized mechanophores in solution and membranes. Broadband femtosecond fluorescence up-conversion spectroscopy evinces ultrafast chalcogen-bonding cascade switching in the excited state in solution. According to fluorescence lifetime imaging microscopy, the new flippers image membrane tension in live cells with record red shifts and photostability. Single-molecule localization microscopy with the new tension probes resolves membranes well below the diffraction limit.

How junctional proteins regulate the mechanics of the plasma membrane and how actin and myosin isoforms are selectively localized at epithelial cell-cell junctions is poorly understood. Here we show by atomic force indentation microscopy, immunofluorescence analysis and FLIM membrane tension imaging that the tight junction (TJ) protein cingulin maintains apical surface stiffness and TJ membrane tortuosity and down-regulates apico-lateral membrane tension in MDCK cells. KO of cingulin in MDCK, mCCD and Eph4 cells results in a decrease in the juxta-membrane accumulation of labeling for cytoplasmic myosin-2B (NM2B), γ-actin, phalloidin and ARHGEF18, but no detectable effect on myosin-2A (NM2A) and β-actin. Loss of paracingulin leads to weaker mechanical phenotypes in MDCK cells, correlating with no detectable effect on the junctional accumulation of myosins and actins. Cingulin and paracingulin form biomolecular condensates, bind to the ZU5 domain of ZO-1, and are recruited as clients into ZO-1 condensates in a ZU5-dependent manner. Cingulin binding to ZO-1 promotes the unfolding of ZO-1, as determined by interaction with DbpA in cells lacking ZO-2 and in vitro. Cingulin promotes the accumulation of a pool of ZO-1 at the TJ and is required in a ZU5-dependent manner for the recruitment of phalloidin-labelled actin filaments into ZO-1 condensates, suggesting that ZU5-cingulin interaction promotes ZO-1 interaction with actin filaments. Our results indicate that cingulin tethers the juxta-membrane and apical branched γ-actin-NM2B network to TJ to modulate ZO-1 conformation and the TJ assembly of a pool of ZO-1 and fine-tune the distribution of forces to apical and TJ membranes.

Membrane shape is a key feature of many cellular processes, including cell differentiation, division, migration, and trafficking.The development of nanostructured surfaces allowing for the in situ manipulation of membranes in living cells is crucial to understand these processes, but this requires complicated and limited-access technologies.Here, we investigate the self-organization of cellular membranes by using a customizable and bench top method allowing to engineer 1D SiO2 nanopillar arrays of defined sizes and shapes on high-performance glass compatible with advanced microscopies.As a result of this original combination, we provide a mapping of the morphology-induced modulation of the cell membrane mechanics, dynamics and steady-state organization of key protein complexes implicated in cellular trafficking and signal transduction.