Passive coupling of membrane tension and cell volume during active response of cells to osmosis

Intracellular pressure controls the propagation of tension in crumpled cell membranes

Propagation of membrane tension mediates mechanical signal transduction along surfaces of live cells and sets the time scale of mechanical equilibration of cell membranes. Recent studies in several cell types and under different conditions revealed a strikingly wide variation range of the tension propagation speeds including extremely low ones. The latter suggests a possibility of long-living inhomogeneities of membrane tension crucially affecting mechano-sensitive membrane processes. Here, we propose, analyze theoretically, and support experimentally a mechanism of tension propagation in membranes crumpled by the contractile cortical cytoskeleton. The tension spreading is mediated by the membrane flow between the crumples. We predict the pace of the tension propagation to be controlled by the intra-cellular pressure and the degree of the membrane crumpling. We provide experimental support for the suggested mechanism by monitoring the rate of tension propagation in cells exposed to external media of different osmolarities.

Cytoskeletal activation of NHE1 regulates mechanosensitive cell volume adaptation and proliferation

Mammalian cells rapidly respond to environmental changes by altering transmembrane water and ion fluxes, changing cell volume. Contractile forces generated by actomyosin have been proposed to mechanically regulate cell volume. However, our findings reveal a different mechanism in adherent cells, where elevated actomyosin activity increases cell volume in normal-like cells (NIH 3T3 and others) through interaction with the sodium-hydrogen exchanger isoform 1 (NHE1). This leads to a slow secondary volume increase (SVI) following the initial regulatory volume decrease during hypotonic shock. The active cell response is further confirmed by intracellular alkalinization during mechanical stretch. Moreover, cytoskeletal activation of NHE1 during SVI deforms the nucleus, causing immediate transcriptomic changes and ERK-dependent growth inhibition. Notably, SVI and its associated changes are absent in many cancer cell lines or cells on compliant substrates with reduced actomyosin activity. Thus, actomyosin acts as a sensory element rather than a force generator during adaptation to environmental challenges.

Confinement-sensitive volume regulation dynamics via high-speed nuclear morphological measurements

Diverse tissues in vivo present varying degrees of confinement, constriction, and compression to migrating cells in both homeostasis and disease. The nucleus in particular is subjected to external forces by the physical environment during confined migration. While many systems have been developed to induce nuclear deformation and analyze resultant functional changes, much remains unclear about dynamic volume regulation in confinement due to limitations in time resolution and difficulty imaging in PDMS-based microfluidic chips. Standard volumetric measurement relies on confocal microscopy, which suffers from high phototoxicity, slow speed, limited throughput, and artifacts in fast-moving cells. To address this, we developed a form of double fluorescence exclusion microscopy, designed to function at the interface of microchannel-based PDMS sidewalls, that can track cellular and nuclear volume dynamics during confined migration. By verifying the vertical symmetry of nuclei in confinement, we obtained computational estimates of nuclear surface area. We then tracked nuclear volume and surface area under physiological confinement at a time resolution exceeding 30 frames per minute. We find that during self-induced entrance into confinement, the cell rapidly expands its surface area until a threshold is reached, followed by a rapid decrease in nuclear volume. We next used osmotic shock as a tool to alter nuclear volume in confinement, and found that the nuclear response to hypo-osmotic shock in confinement does not follow classical scaling laws, suggesting that the limited expansion potential of the nuclear envelope might be a constraining factor in nuclear volume regulation in confining environments in vivo.

Cells Prioritize the Regulation of Cell Mass Density

A cell's global physical state is characterized by its volume and dry mass. The ratio of cell mass to volume is the cell mass density (CMD), which is also a measure of macromolecular crowding and concentrations of all proteins. Using the Fluorescence eXclusion method (FXm) and Quantitative Phase Microscopy (QPM), we investigate CMD dynamics after exposure to sudden media osmolarity change. We find that while the cell volume and mass exhibit complex behavior after osmotic shock, CMD follows a straightforward monotonic recovery in 48 hours. The recovery is cell-cycle independent and relies on a coordinated adjustment of protein synthesis and volume growth rates. Surprisingly, we find that the protein synthesis rate decreases when CMD increases. This result is explained by CMD-dependent nucleoplasm-cytoplasm transport, which serves as negative regulatory feedback on CMD. The Na+/H+ exchanger NHE plays a role in regulating CMD by affecting both protein synthesis and volume change. Taken together, we reveal that cells possess a robust control system that actively regulates CMD during environmental change.

Small Molecule Modulator of the mTORC2 Pathway Discovered from a DEL Library Designed to Bind to Pleckstrin Homology Domains

Pleckstrin homology (PH) domains are structural motifs critical for cellular processes, such as signal transduction and cytoskeletal organization. Due to their involvement in various diseases, PH domains are promising therapeutic targets, yet their highly charged and hydrophobic binding sites are not ideal for traditional small drugs. In this study, we designed a DNA-encoded library (DEL) mimicking phospholipids to identify novel modulators targeting PH domains with uncharted chemical properties. Screening against several PH domains led to the discovery of 2DII, a small molecule that selectively binds to mSin1PH. This compound can modulate mTORC2 activity by impairing mTORC2's membrane interactions, resulting in reduced AKT1 phosphorylation. A micromapping via Dexter energy transfer based on 2DII bearing an iridium catalyst (2DII-Ir), along with a biotin-diazirine small molecule was used for target identification by proteomics, which confirmed mSin1 as the primary intracellular target of 2DII, demonstrating its potential for selective mTORC2 pathway modulation. These findings introduce a novel strategy for targeting PH domains and provide a foundation for the development of therapeutic interventions that modulate PH-domain-dependent signaling pathways.

Harnessing osmotic shock for enhanced intracellular delivery of (nano)cargos

Efficient intracellular delivery of exogenous (nano)materials is critical for both research and therapeutic applications. The physicochemical properties of the cargo play a crucial role in determining internalization efficacy. Consequently, significant research efforts are focused on developing innovative and effective methodologies to optimize (nano)material delivery. In this study, we utilized osmotic shock to enhance (nano)cargos internalization. We examined the effects of hypotonic/hypertonic shock on both primary and cell lines, assessing parameters such as cell viability, cell volume, membrane tension changes, and particle uptake. Our results indicate that short-lived osmotic shock does not harm cells. Hypotonic shock induced temporary shape changes lasting up to 5 min, followed by a 15-minute recovery period. Importantly, hypotonic shock increased the uptake of 100-nm and 500-nm particles by ∼ 3- and ∼ 5-fold, respectively, compared to isotonic conditions. In contrast, the hypertonic shock did not impact cell behavior or particle uptake. Notably, the internalization mechanisms triggered by osmotic shock operate independently of active endocytic pathways, making hypotonic stimulation particularly beneficial for hard-to-treat cells. When primary fibroblasts derived from amyotrophic lateral sclerosis (ALS)-patients were exposed to hypotonic shock in the presence of the therapeutic cargo icerguastat, there was an increased expression of miR-106b-5p compared to isotonic conditions. In conclusion, osmotic shock presents a promising strategy for improving drug delivery within cells and, potentially, in tissues such as muscles or skin, where localized drug administration is preferred.

Hypo-osmolarity promotes naive pluripotency by reshaping cytoskeleton and increasing chromatin accessibility

INTRODUCTION

Cell fate determination and transition are of paramount importance in biology and medicine. Naive pluripotency could be achieved by reprogramming differentiated cells. However, the mechanism is less clear. Osmolarity is an essential physical factor that acts on living cells, especially for pluripotent cells, but its significance in cell fate transition remains unexplored.

OBJECTIVES

To investigate the role of osmolarity in cell fate transition and its underlying mechanism.

METHODS

Flow cytometry, quantitative real-time PCR, teratoma and chimeric mice assays were performed to assess reprogramming efficiency and characterize iPSCs. TEM, immunofluorescence staining, western blot, chemical treatment and genetic modification were utilized to evaluate cell morphology, signaling pathways, cytoskeleton and nuclear structure. Multiomic sequencings were applied to unveil the transcriptome, histone markers and chromatin accessibility of EpiSCs in hypo-osmotic condition.

RESULTS

In hypo-osmotic condition, the reprogramming efficiency of hypo-osmotic EpiSCs increased over 60-fold than that of iso-osmotic cells (1100 vs 18 colonies per 3 × 105 cells), whereas no colony formed in hyper-osmotic cells. As expected, the converted cells displayed naive pluripotency. The hypo-osmotic EpiSCs exhibited larger cell size, nuclear area and less heterochromatin; ATAC-seq and ChIP-seq confirmed the increased accessibility of naive pluripotent gene loci with more H3K27ac. Mechanistically, hypo-osmolarity activated PI3K-AKT-SP1 signaling in EpiSCs, which reshaped cytoskeleton and nucleoskeleton, resulting in genome reorganization and pluripotent gene expression. In contrast, hypo-osmolarity delayed the ESCs' exit from naive pluripotency. Moreover, in MEFs reprograming, hypo-osmolarity promoted the conversion to naive pluripotency.

CONCLUSION

Hypo-osmolarity promotes cell fate transition by remodeling cytoskeleton, nucleoskeleton and genome via PI3K-AKT-SP1 pathway.

Mechanochemical control systems regulating animal cell size

Cell size regulation arises from physical manifestations of cell proliferation and metabolic pathways. On one hand, coordination between these systems yields a constant cell size over generations to maintain cell size homeostasis. However, active regulation of cell size is crucial to physiology and to establish broad variation of cell sizes within an individual organism, and is accomplished via physical and biochemical pathways modulated by myriad intrinsic and extrinsic cues. In this review, we explore recent data elucidating the mechanobiological regulation of the volume of animal cells and its coordination with metabolic and proliferative pathways.

Membrane Tension Regulation is Required for Wound Repair

Disruptions of the eukaryotic plasma membrane due to chemical and mechanical challenges are frequent and detrimental and thus need to be repaired to maintain proper cell function and avoid cell death. However, the cellular mechanisms involved in wound resealing and restoration of homeostasis are diverse and contended. Here, it is shown that clathrin-mediated endocytosis is induced at later stages of plasma membrane wound repair following the actual resealing of the wound. This compensatory endocytosis occurs near the wound, predominantly at sites of previous early endosome exocytosis which is required in the initial stage of membrane resealing, suggesting a spatio-temporal co-ordination of exo- and endocytosis during wound repair. Using cytoskeletal alterations and modulations of membrane tension and membrane area, membrane tension is identified as a major regulator of the wounding-associated exo- and endocytic events that mediate efficient wound repair. Thus, membrane tension changes are a universal trigger for plasma membrane wound repair modulating the exocytosis of early endosomes required for resealing and subsequent clathrin-mediated endocytosis acting at later stages to restore cell homeostasis and function.

Tutorial: fluorescence lifetime microscopy of membrane mechanosensitive Flipper probes

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.

How sterols affect protoplasts plasma membrane water permeability and their volume under osmotic shock

Protoplasts isolated from Arabidopsis leaves were used to study the initial stages of the plant cell response to osmotic stress. The role of sterols in these processes was investigated by their extraction from the protoplast plasma membrane in the presence of the oligosaccharide - methyl-β-cyclodextrin (MβCD). Depletion of membrane sterols caused by MβCD treatment did not alter protoplast volume under isosmotic conditions; however, volumes changed significantly when protoplasts were exposed to osmotic stress. Estimation of the plasma membrane water permeability coefficient (Pos), calculated from the initial rate of protoplast osmotic shrinkage, showed that control suspension is characterized by a high dispersion of the Pos values. However, Pos became more homogeneous after plasma membrane sterol depletion. Protoplasts were stained with FM 1-43 to assess how sterol extraction affects vesicular transport under osmotic shock. In order to determine the protoplast non-osmotic volume (Vb) steady-state volumes at different external osmolarities were fitted with linear dependences of the Boyle-van't Hoff (BVH) plot. It was found that sterol extraction is accompanied by a change in the slope of the BVH plot and a decrease in the apparent Vb. Several possible mechanisms behind the change in the protoplast volume and plasma membrane Pos regulation by sterols under osmotic stress are discussed.

Decisive role of mDia-family formins in cell cortex function of highly adherent cells

Cortical formins, pivotal for the assembly of linear actin filaments beneath the membrane, exert only minor effects on unconfined cell migration of weakly and moderately adherent cells. However, their impact on migration and mechanostability of highly adherent cells remains poorly understood. Here, we demonstrate that loss of cortical actin filaments generated by the formins mDia1 and mDia3 drastically compromises cell migration and mechanics in highly adherent fibroblasts. Biophysical analysis of the mechanical properties of the mutant cells revealed a markedly softened cell cortex in the poorly adherent state. Unexpectedly, in the highly adherent state, associated with a hyperstretched morphology with exaggerated focal adhesions and prominent high-strain stress fibers, they exhibited even higher cortical tension compared to control. Notably, misguidance of intracellular forces, frequently accompanied by stress-fiber rupture, culminated in the formation of tension- and contractility-induced macroapertures, which was instantly followed by excessive lamellipodial protrusion at the periphery, providing critical insights into mechanotransduction of mechanically stressed and highly adherent cells.

The Role of the Swollen State in Cell Proliferation

Cell swelling is known to be involved in various stages of the growth of plant cells and microorganisms but in mammalian cells how crucial a swollen state is for determining the fate of the cellular proliferation remains unclear. Recent evidence has increased our understanding of how the loss of the cell surface interactions with the extracellular matrix at early mitosis decreases the membrane tension triggering curvature changes in the plasma membrane and the activation of the sodium/hydrogen (Na +/H +) exchanger (NHE1) that drives osmotic swelling. Such a swollen state is temporary, but it is critical to alter essential membrane biophysical parameters that are required to activate Ca2 + channels and modulate the opening of K + channels involved in setting the membrane potential. A decreased membrane potential across the mitotic cell membrane enhances the clustering of Ras proteins involved in the Ca2 + and cytoskeleton-driven events that lead to cell rounding. Changes in the external mechanical and osmotic forces also have an impact on the lipid composition of the plasma membrane during mitosis.

Mechanical characterization of freestanding lipid bilayers with temperature-controlled phase

Coexistence of lipid domains in cell membranes is associated with vital biological processes. Here, we investigate two such membranes: a multi-component membrane composed of DOPC and DPPC lipids with gel and fluid separated domains, and a single component membrane composed of PMPC lipids forming ripples. We characterize their mechanical properties below their melting point, where ordered and disordered regions coexist, and above their melting point, where they are in fluid phase. To conduct these inquiries, we create lipid bilayers in a microfluidic chip interfaced with a heating system and optical tweezers. The chip features a bubble trap and enables high-throughput formation of planar bilayers. Optical tweezers experiments reveal interfacial hydrodynamics (fluid-slip) and elastic properties (membrane tension and bending rigidity) at various temperatures. For PMPC bilayers, we demonstrate a higher fluid slip at the interface in the fluid-phase compared to the ripple phase, while for the DOPC:DPPC mixture, similar fluid slip is measured below and above the transition point. Membrane tension for both compositions increases after thermal fluidization. Bending rigidity is also measured using the forces required to extend a lipid nanotube pushed out of the freestanding membranes. This novel temperature-controlled microfluidic platform opens numerous possibilities for thermomechanical studies on freestanding planar membranes.

Intercellular fluid dynamics in tissue morphogenesis

During embryonic development, cells shape our body, which is mostly made up of water. It is often forgotten that some of this water is found in intercellular fluid, which, for example, immerses the cells of developing embryos. Intercellular fluid contributes to the properties of tissues and influences cell behaviour, thereby participating in tissue morphogenesis. While our understanding of the role of cells in shaping tissues advances, the exploration of the contribution of intercellular fluid dynamics is just beginning. In this review, we delve into the intricate mechanisms employed by cells to control fluid movements both across and within sealed tissue compartments. These mechanisms encompass sealing by tight junctions and controlled leakage, osmotic pumping, hydraulic fracturing of cell adhesion, cell and tissue contractions, as well as beating cilia. We illustrate key concepts by drawing extensively from the early mouse embryo, which successively forms multiple lumens that play essential roles in its development. Finally, we detail experimental approaches and emerging techniques that allow for the quantitative characterization and the manipulation of intercellular fluids in vivo, as well as theoretical frameworks that are crucial for comprehending their dynamics.

High-frequency MHz-order vibration enables cell membrane remodeling and lipid microdomain manipulation

We elucidate the mechanism underpinning a recently discovered phenomenon in which cells respond to MHz-order mechanostimuli. Deformations induced along the plasma membrane under these external mechanical cues are observed to decrease the membrane tension, which, in turn, drives transient and reversible remodeling of its lipid structure. In particular, the increase and consequent coalescence of ordered lipid microdomains leads to closer proximity to mechanosensitive ion channels-Piezo1, in particular-that, due to crowding, results in their activation to mobilize influx of calcium (Ca2+) ions into the cell. It is the modulation of this second messenger that is responsible for the downstream signaling and cell fates that ensue. In addition, we show that such spatiotemporal control over the membrane microdomains in cells-without necessitating biochemical factors-facilitates aggregation and association of intrinsically disordered tau proteins in neuroblastoma cells, and their transformation to pathological conditions implicated in neurodegenerative diseases, thereby paving the way for the development of therapeutic intervention strategies.

Single-Cell Hypertrophy Promotes Contractile Function of Cultured Human Airway Smooth Muscle Cells via Piezo1 and YAP Auto-Regulation

Severe asthma is characterized by increased cell volume (hypertrophy) and enhanced contractile function (hyperresponsiveness) of the airway smooth muscle cells (ASMCs). The causative relationship and underlying regulatory mechanisms between them, however, have remained unclear. Here, we manipulated the single-cell volume of in vitro cultured human ASMCs to increase from 2.7 to 5.2 and 8.2 × 103 μm3 as a simulated ASMC hypertrophy by culturing the cells on micropatterned rectangular substrates with a width of 25 μm and length from 50 to 100 and 200 μm, respectively. We found that as the cell volume increased, ASMCs exhibited a pro-contractile function with increased mRNA expression of contractile proteins, increased cell stiffness and traction force, and enhanced response to contractile stimulation. We also uncovered a concomitant increase in membrane tension and Piezo1 mRNA expression with increasing cell volume. Perhaps more importantly, we found that the enhanced contractile function due to cell volume increase was largely attenuated when membrane tension and Piezo1 mRNA expression were downregulated, and an auto-regulatory loop between Piezo1 and YAP mRNA expression was also involved in perpetuating the contractile function. These findings, thus, provide convincing evidence of a direct link between hypertrophy and enhanced contractile function of ASMCs that was mediated via Piezo1 mRNA expression, which may be specifically targeted as a novel therapeutic strategy to treat pulmonary diseases associated with ASMC hypertrophy such as severe asthma.

Exploring the role of macromolecular crowding and TNFR1 in cell volume control

The excessive cosolute densities in the intracellular fluid create a physicochemical condition called macromolecular crowding (MMC). Intracellular MMC entropically maintains the biochemical thermodynamic equilibria by favoring associative reactions while hindering transport processes. Rapid cell volume shrinkage during extracellular hypertonicity elevates the MMC and disrupts the equilibria, potentially ushering cell death. Consequently, cells actively counter the hypertonic stress through regulatory volume increase (RVI) and restore the MMC homeostasis. Here, we establish fluorescence anisotropy of EGFP as a reliable tool for studying cellular MMC and explore the spatiotemporal dynamics of MMC during cell volume instabilities under multiple conditions. Our studies reveal that the actin cytoskeleton enforces spatially varying MMC levels inside adhered cells. Within cell populations, MMC is uncorrelated with nuclear DNA content but anti-correlated with the cell spread area. Although different cell lines have statistically similar MMC distributions, their responses to extracellular hypertonicity vary. The intensity of the extracellular hypertonicity determines a cell's ability for RVI, which correlates with nuclear factor kappa beta (NFkB) activation. Pharmacological inhibition and knockdown experiments reveal that tumor necrosis factor receptor 1 (TNFR1) initiates the hypertonicity-induced NFkB signaling and RVI. At severe hypertonicities, the elevated MMC amplifies cytoplasmic microviscosity and hinders receptor interacting protein kinase 1 (RIPK1) recruitment at the TNFR1 complex, incapacitating the TNFR1-NFkB signaling and consequently, RVI. Together, our studies unveil the involvement of TNFR1-NFkB signaling in modulating RVI and demonstrate the pivotal role of MMC in determining cellular osmoadaptability.

Cell swelling enhances ligand-driven β-adrenergic signaling

G protein-coupled receptors' conformational landscape can be affected by their local, microscopic interactions within the cell plasma membrane. We employ here a pleiotropic stimulus, namely osmotic swelling, to alter the cortical environment within intact cells and monitor the response in terms of receptor function and downstream signaling. We observe that in osmotically swollen cells the β2-adrenergic receptor, a prototypical GPCR, favors an active conformation, resulting in cAMP transient responses to adrenergic stimulation that have increased amplitude. The results are validated in primary cell types such as adult cardiomyocytes, a model system where swelling occurs upon ischemia-reperfusion injury. Our results suggest that receptors' function is finely modulated by their biophysical context, and specifically that osmotic swelling acts as a potentiator of downstream signaling, not only for the β2-adrenergic receptor, but also for other receptors, hinting at a more general regulatory mechanism.

Injectable PEG Hydrogels with Tissue-Like Viscoelasticity Formed through Reversible Alendronate-Calcium Phosphate Crosslinking for Cell-Material Interactions

Synthetic hydrogels provide controllable 3D environments, which can be used to study fundamental biological phenomena. The growing body of evidence that cell behavior depends upon hydrogel stress relaxation creates a high demand for hydrogels with tissue-like viscoelastic properties. Here, a unique platform of synthetic polyethylene glycol (PEG) hydrogels in which star-shaped PEG molecules are conjugated with alendronate and/or RGD peptides, attaining modifiable degradability as well as flexible cell adhesion, is created. Novel reversible ionic interactions between alendronate and calcium phosphate nanoparticles, leading to versatile viscoelastic properties with varying initial elastic modulus and stress relaxation time, are identified. This new crosslinking mechanism provides shear-thinning properties resulting in differential cellular responses between cancer cells and stem cells. The novel hydrogel system is an improved design to the other ionic crosslink platforms and opens new avenues for the development of pathologically relevant cancer models, as well as minimally invasive approaches for cell delivery for potential regenerative therapies.

Nanoscale intracellular ultrastructures affected by osmotic pressure using small-angle X-ray scattering

Although intracellular ultrastructures have typically been studied using microscopic techniques, it is difficult to observe ultrastructures at the submicron scale of living cells due to spatial resolution (fluorescence microscopy) or high vacuum environment (electron microscopy). We investigate the nanometer scale intracellular ultrastructures of living CHO cells in various osmolality using small-angle X-ray scattering (SAXS), and especially the structures of ribosomes, DNA double helix, and plasma membranes in-cell environment are observed. Ribosomes expand and contract in response to osmotic pressure, and the inter-ribosomal correlation occurs under isotonic and hyperosmolality. The DNA double helix is not dependent on the osmotic pressure. Under high osmotic pressure, the plasma membrane folds into form a multilamellar structure with a periodic length of about 6 nm. We also study the ultrastructural changes caused by formaldehyde fixation, freezing and heating.

Cell size regulates human endoderm specification through actomyosin-dependent AMOT-YAP signaling

Cell size is a crucial physical property that significantly impacts cellular physiology and function. However, the influence of cell size on stem cell specification remains largely unknown. Here, we investigated the dynamic changes in cell size during the differentiation of human pluripotent stem cells into definitive endoderm (DE). Interestingly, cell size exhibited a gradual decrease as DE differentiation progressed with higher stiffness. Furthermore, the application of hypertonic pressure or chemical to accelerate the reduction in cell size significantly and specifically enhanced DE differentiation. By functionally intervening in mechanosensitive elements, we have identified actomyosin activity as a crucial mediator of both DE differentiation and cell size reduction. Mechanistically, the reduction in cell size induces actomyosin-dependent angiomotin (AMOT) nuclear translocation, which suppresses Yes-associated protein (YAP) activity and thus facilitates DE differentiation. Together, our study has established a novel connection between cell size diminution and DE differentiation, which is mediated by AMOT nuclear translocation. Additionally, our findings suggest that the application of osmotic pressure can effectively promote human endodermal lineage differentiation.

Follow the flow: Actin and membrane act as an integrated system to globally coordinate cell shape and movement

Migratory cells are polarized with protrusive fronts and contractile rears. This spatial organization necessitates long-range coordination of the signals that organize protrusions and contractions. Cells leverage reciprocal interactions of short-range biochemical signals and long-range mechanical forces for this integration. The interface between the plasma membrane and actin cortex is where this communication occurs. Here, we review how the membrane and cortex form an integrated system for long-range coordination of cell polarity. We highlight the role of membrane-to-cortex-attachment proteins as regulators of tension transmission across the cell and discuss the interplay between actin-membrane and polarity signaling complexes. Rather than presenting an exhaustive list of recent findings, we focus on important gaps in our current understanding.

HiViPore: a highly viable in-flow compression for a one-step cell mechanoporation in microfluidics to induce a free delivery of nano- macro-cargoes

BACKGROUND

Among mechanoporation techniques for intracellular delivery, microfluidic approaches succeed in high delivery efficiency and throughput. However, especially the entry of large cargoes (e.g. DNA origami, mRNAs, organic/inorganic nanoparticles) is currently impaired since it requires large cell membrane pores with the need to apply multi-step processes and high forces, dramatically reducing cell viability.

RESULTS

Here, HiViPore presents as a microfluidic viscoelastic contactless compression for one-step cell mechanoporation to produce large pores while preserving high cell viability. Inducing an increase of curvature at the equatorial region of cells, formation of a pore with a size of ~ 1 μm is obtained. The poration is coupled to an increase of membrane tension, measured as a raised fluorescence lifetime of 12% of a planarizable push-pull fluorescent probe (Flipper-TR) labelling the cell plasma membrane. Importantly, the local disruptions of cell membrane are transient and non-invasive, with a complete recovery of cell integrity and functions in ~ 10 min. As result, HiViPore guarantees cell viability as high as ~ 90%. In such conditions, an endocytic-free diffusion of large nanoparticles is obtained with typical size up to 500 nm and with a delivery efficiency up to 12 times higher than not-treated cells.

CONCLUSIONS

The proposed one-step contactless mechanoporation results in an efficient and safe approach for advancing intracellular delivery strategies. In detail, HiViPore solves the issues of low cell viability when multiple steps of poration are required to obtain large pores across the cell plasma membrane. Moreover, the compression uses a versatile, low-cost, biocompatible viscoelastic fluid, thus also optimizing the operational costs. With HiViPore, we aim to propose an easy-to-use microfluidic device to a wide range of users, involved in biomedical research, imaging techniques and nanotechnology for intracellular delivery applications in cell engineering.

Unveiling the Intricate Connection: Cell Volume as a Key Regulator of Mechanotransduction

The volumes of living cells undergo dynamic changes to maintain the cells' structural and functional integrity in many physiological processes. Minor fluctuations in cell volume can serve as intrinsic signals that play a crucial role in cell fate determination during mechanotransduction. In this review, we discuss the variability of cell volume and its role in vivo, along with an overview of the mechanisms governing cell volume regulation. Additionally, we provide insights into the current approaches used to control cell volume in vitro. Furthermore, we summarize the biological implications of cell volume regulation and discuss recent advances in understanding the fundamental relationship between cell volume and mechanotransduction. Finally, we delve into the potential underlying mechanisms, including intracellular macromolecular crowding and cellular mechanics, that govern the global regulation of cell fate in response to changes in cell volume. By exploring the intricate interplay between cell volume and mechanotransduction, we underscore the importance of considering cell volume as a fundamental signaling cue to unravel the basic principles of mechanotransduction. Additionally, we propose future research directions that can extend our current understanding of cell volume in mechanotransduction. Overall, this review highlights the significance of considering cell volume as a fundamental signal in understanding the basic principles in mechanotransduction and points out the possibility of controlling cell volume to control cell fate, mitigate disease-related damage, and facilitate the healing of damaged tissues.

Cell Membrane Tension Gradients, Membrane Flows, and Cellular Processes

Cell membrane tension affects and is affected by many fundamental cellular processes, yet it is poorly understood. Recent experiments show that membrane tension can propagate at vastly different speeds in different cell types, reflecting physiological adaptations. Here we briefly review the current knowledge about membrane tension gradients, membrane flows, and their physiological context.

Microtubule-associated protein MAP7 promotes tubulin posttranslational modifications and cargo transport to enable osmotic adaptation

Cells remodel their cytoskeletal networks to adapt to their environment. Here, we analyze the mechanisms utilized by the cell to tailor its microtubule landscape in response to changes in osmolarity that alter macromolecular crowding. By integrating live-cell imaging, ex vivo enzymatic assays, and in vitro reconstitution, we probe the impact of cytoplasmic density on microtubule-associated proteins (MAPs) and tubulin posttranslational modifications (PTMs). We find that human epithelial cells respond to fluctuations in cytoplasmic density by modulating microtubule acetylation, detyrosination, or MAP7 association without differentially affecting polyglutamylation, tyrosination, or MAP4 association. These MAP-PTM combinations alter intracellular cargo transport, enabling the cell to respond to osmotic challenges. We further dissect the molecular mechanisms governing tubulin PTM specification and find that MAP7 promotes acetylation and inhibits detyrosination. Our data identify MAP7 in modulating the tubulin code, resulting in microtubule cytoskeleton remodeling and alteration of intracellular transport as an integrated mechanism of cellular adaptation.

Comparing the Mechanical Properties of Rice Cells and Protoplasts under PEG6000 Drought Stress Using Double Resonator Piezoelectric Cytometry

Plant cells' ability to withstand abiotic stress is strongly linked to modifications in their mechanical characteristics. Nevertheless, the lack of a workable method for consistently tracking plant cells' mechanical properties severely restricts our comprehension of the mechanical alterations in plant cells under stress. In this study, we used the Double Resonator Piezoelectric Cytometry (DRPC) method to dynamically and non-invasively track changes in the surface stress (ΔS) generated and viscoelasticity (storage modulus G' and loss modulus G″) of protoplasts and suspension cells of rice under a drought stress of 5-25% PEG6000. The findings demonstrate that rice suspension cells and protoplasts react mechanically differently to 5-15% PEG6000 stress, implying distinct resistance mechanisms. However, neither of them can withstand 25% PEG6000 stress; they respond mechanically similarly to 25% PEG6000 stress. The results of DRPC are further corroborated by the morphological alterations of rice cells and protoplasts observed under an optical microscope. To sum up, the DRPC technique functions as a precise cellular mechanical sensor and offers novel research tools for the evaluation of plant cell adversity and differentiating between the mechanical reactions of cells and protoplasts under abiotic stress.

Efficacy of different bioactive glass S53P4 formulations in biofilm eradication and the impact of pH and osmotic pressure

AIM

The challenging properties of biofilm-associated infections and the rise of multidrug-resistant bacteria are prompting the exploration of alternative treatment options. This study investigates the efficacy of different bioactive glass (BAG) formulations - alone or combined with vancomycin - to eradicate biofilm. Further, we study the influence of BAG on pH and osmotic pressure as important factors limiting bacterial growth.

METHOD

Different BAG S53P4 formulations were used for this study, including (a) powder (<45 μm), (b) granules (500-800 µm), (c) a cone-shaped scaffold and (d) two putty formulations containing granules with no powder (putty A) or with additional powder (putty B) bound together by a synthetic binder. Inert glass beads (1.0-1.3 mm) were included as control. All formulations were tested in a concentration of 1750 mg/ml in Müller-Hinton-Broth against methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant Staphylococcus epidermidis (MRSE). Vancomycin was tested at the minimum-inhibitory concentration for each strain. Changes in pH and osmolality over time were assessed at 0 h, 24 h, 72 h and 168 h.

RESULTS

All tested BAG formulations showed antibiofilm activity against MRSA and MRSE. Powder and putty B were the most effective formulations suppressing biofilm leading to its complete eradication after up to 168 h of co-incubation, followed by granules, scaffold and putty A. In general, MRSE appeared to be more susceptible to bioactive glass compared to MRSA. The addition of vancomycin had no substantial impact on biofilm eradication. We observed a positive correlation between a higher pH and higher antibiofilm activity.

CONCLUSIONS

BAG S53P4 has demonstrated efficient biofilm antibiofilm activity against MRSA and MRSE, especially in powder-containing formulations, resulting in complete eradication of biofilm. Our data indicate neither remarkable increase nor decrease in antimicrobial efficacy with addition of vancomycin. Moreover, high pH appears to have a direct antimicrobial impact; the role of high osmolality needs further investigation.

Dynamic mechanisms for membrane skeleton transitions

The plasma membrane and the underlying skeleton form a protective barrier for eukaryotic cells. The molecules forming this complex composite material constantly rearrange under mechanical stress to confer this protective capacity. One of those molecules, spectrin, is ubiquitous in the membrane skeleton and primarily located proximal to the inner leaflet of the plasma membrane and engages in protein-lipid interactions via a set of membrane-anchoring domains. Spectrin is linked by short actin filaments and its conformation varies in different types of cells. In this work, we developed a generalized network model for the membrane skeleton integrated with myosin contractility and membrane mechanics to investigate the response of the spectrin meshwork to mechanical loading. We observed that the force generated by membrane bending is important to maintain a smooth skeletal structure. This suggests that the membrane is not just supported by the skeleton, but has an active contribution to the stability of the cell structure. We found that spectrin and myosin turnover are necessary for the transition between stress and rest states in the skeleton. Our model reveals that the actin-spectrin meshwork dynamics are balanced by the membrane forces with area constraint and volume restriction promoting the stability of the membrane skeleton. Furthermore, we showed that cell attachment to the substrate promotes shape stabilization. Thus, our proposed model gives insight into the shared mechanisms of the membrane skeleton associated with myosin and membrane that can be tested in different types of cells.

Pip5k1γ regulates axon formation by limiting Rap1 activity

During their differentiation, neurons establish a highly polarized morphology by forming axons and dendrites. Cortical and hippocampal neurons initially extend several short neurites that all have the potential to become an axon. One of these neurites is then selected as the axon by a combination of positive and negative feedback signals that promote axon formation and prevent the remaining neurites from developing into axons. Here, we show that Pip5k1γ is required for the formation of a single axon as a negative feedback signal that regulates C3G and Rap1 through the generation of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2). Impairing the function of Pip5k1γ results in a hyper-activation of the Fyn/C3G/Rap1 pathway, which induces the formation of supernumerary axons. Application of a hyper-osmotic shock to modulate membrane tension has a similar effect, increasing Rap1 activity and inducing the formation of supernumerary axons. In both cases, the induction of supernumerary axons can be reverted by expressing constitutively active Pip5k. Our results show that PI(4,5)P2-dependent membrane properties limit the activity of C3G and Rap1 to ensure the extension of a single axon.

Measuring single-cell density with high throughput enables dynamic profiling of immune cell and drug response from patient samples

Cell density, the ratio of cell mass to volume, is an indicator of molecular crowding and therefore a fundamental determinant of cell state and function. However, existing density measurements lack the precision or throughput to quantify subtle differences in cell states, particularly in primary samples. Here we present an approach for measuring the density of 30,000 single cells per hour with a precision of 0.03% (0.0003 g/mL) by integrating fluorescence exclusion microscopy with a suspended microchannel resonator. Applying this approach to human lymphocytes, we discovered that cell density and its variation decrease as cells transition from quiescence to a proliferative state, suggesting that the level of molecular crowding decreases and becomes more regulated upon entry into the cell cycle. Using a pancreatic cancer patient-derived xenograft model, we found that the ex vivo density response of primary tumor cells to drug treatment can predict in vivo tumor growth response. Our method reveals unexpected behavior in molecular crowding during cell state transitions and suggests density as a new biomarker for functional precision medicine.

Optogenetically controlled inflammasome activation demonstrates two phases of cell swelling during pyroptosis

Inflammasomes are multiprotein platforms that control caspase-1 activation, which process the inactive precursor forms of the inflammatory cytokines IL-1β and IL-18, leading to an inflammatory type of programmed cell death called pyroptosis. Studying inflammasome-driven processes, such as pyroptosis-induced cell swelling, under controlled conditions remains challenging because the signals that activate pyroptosis also stimulate other signaling pathways. We designed an optogenetic approach using a photo-oligomerizable inflammasome core adapter protein, apoptosis-associated speck-like containing a caspase recruitment domain (ASC), to temporally and quantitatively manipulate inflammasome activation. We demonstrated that inducing the light-sensitive oligomerization of ASC was sufficient to recapitulate the classical features of inflammasomes within minutes. This system showed that there were two phases of cell swelling during pyroptosis. This approach offers avenues for biophysical investigations into the intricate nature of cellular volume control and plasma membrane rupture during cell death.

Salmonella exploits membrane reservoirs for invasion of host cells

Salmonella utilizes a type 3 secretion system to translocate virulence proteins (effectors) into host cells during infection1. The effectors modulate host cell machinery to drive uptake of the bacteria into vacuoles, where they can establish an intracellular replicative niche. A remarkable feature of Salmonella invasion is the formation of actin-rich protuberances (ruffles) on the host cell surface that contribute to bacterial uptake. However, the membrane source for ruffle formation and how these bacteria regulate membrane mobilization within host cells remains unclear. Here, we show that Salmonella exploits membrane reservoirs for the generation of invasion ruffles. The reservoirs are pre-existing tubular compartments associated with the plasma membrane (PM) and are formed through the activity of RAB10 GTPase. Under normal growth conditions, membrane reservoirs contribute to PM homeostasis and are preloaded with the exocyst subunit EXOC2. During Salmonella invasion, the bacterial effectors SipC, SopE2, and SopB recruit exocyst subunits from membrane reservoirs and other cellular compartments, thereby allowing exocyst complex assembly and membrane delivery required for bacterial uptake. Our findings reveal an important role for RAB10 in the establishment of membrane reservoirs and the mechanisms by which Salmonella can exploit these compartments during host cell invasion.

Geometry-Sensitive Protrusion Growth Directs Confined Cell Migration

The migratory dynamics of cells can be influenced by the complex microenvironment through which they move. It remains unclear how the motility machinery of confined cells responds and adapts to their microenvironment. Here, we propose a biophysical mechanism for a geometry-dependent coupling between cellular protrusions and the nucleus that leads to directed migration. We apply our model to geometry-guided cell migration to obtain insights into the origin of directed migration on asymmetric adhesive micropatterns and the polarization enhancement of cells observed under strong confinement. Remarkably, for cells that can choose between channels of different size, our model predicts an intricate dependence for cellular decision making as a function of the two channel widths, which we confirm experimentally.

Molecular mechanism of GPCR spatial organization at the plasma membrane

G-protein-coupled receptors (GPCRs) mediate many critical physiological processes. Their spatial organization in plasma membrane (PM) domains is believed to encode signaling specificity and efficiency. However, the existence of domains and, crucially, the mechanism of formation of such putative domains remain elusive. Here, live-cell imaging (corrected for topography-induced imaging artifacts) conclusively established the existence of PM domains for GPCRs. Paradoxically, energetic coupling to extremely shallow PM curvature (<1 µm-1) emerged as the dominant, necessary and sufficient molecular mechanism of GPCR spatiotemporal organization. Experiments with different GPCRs, H-Ras, Piezo1 and epidermal growth factor receptor, suggest that the mechanism is general, yet protein specific, and can be regulated by ligands. These findings delineate a new spatiomechanical molecular mechanism that can transduce to domain-based signaling any mechanical or chemical stimulus that affects the morphology of the PM and suggest innovative therapeutic strategies targeting cellular shape.

Understanding the cell: Future views of structural biology

Determining the structure and mechanisms of all individual functional modules of cells at high molecular detail has often been seen as equal to understanding how cells work. Recent technical advances have led to a flush of high-resolution structures of various macromolecular machines, but despite this wealth of detailed information, our understanding of cellular function remains incomplete. Here, we discuss present-day limitations of structural biology and highlight novel technologies that may enable us to analyze molecular functions directly inside cells. We predict that the progression toward structural cell biology will involve a shift toward conceptualizing a 4D virtual reality of cells using digital twins. These will capture cellular segments in a highly enriched molecular detail, include dynamic changes, and facilitate simulations of molecular processes, leading to novel and experimentally testable predictions. Transferring biological questions into algorithms that learn from the existing wealth of data and explore novel solutions may ultimately unveil how cells work.

To squeeze or not: Regulation of cell size by mechanical forces in development and human diseases

Physical constraints, such as compression, shear stress, stretching and tension play major roles during development and tissue homeostasis. Mechanics directly impact physiology, and their alteration is also recognized as having an active role in driving human diseases. Recently, growing evidence has accumulated on how mechanical forces are translated into a wide panel of biological responses, including metabolism and changes in cell morphology. The aim of this review is to summarize and discuss our knowledge on the impact of mechanical forces on cell size regulation. Other biological consequences of mechanical forces will not be covered by this review. Moreover, wherever possible, we also discuss mechanosensors and molecular and cellular signaling pathways upstream of cell size regulation. We finally highlight the relevance of mechanical forces acting on cell size in physiology and human diseases.

Subphthalocyanine-flipper dyads for selective membrane staining

The design, synthesis and evaluation of a subphthalocyanine-flipper (SubPc-Flipper) amphiphilic dyad is reported. This dyad combines two fluorophores that function in the visible region (420-800 nm) for the simultaneous sensing of both ordered and disordered lipidic membranes. The flipper probes part of the dyad possesses mechanosensitivity, long fluorescence lifetimes (τ = 3.5-5 ns) and selective staining of ordered membranes. On the other hand, subphthalocyanines (SubPc) are short-lifetime (τ = 1-2.5 ns) fluorophores that are insensitive to membrane tension. As a result of a Förster Resonance Energy Transfer (FRET) process, the dyad not only retains the mechanosensitivity of flippers but also demonstrates high selectivity and emission in different kinds of lipidic membranes. The dyad exhibits high emission and sensitivity to membrane tension (Δτ = 3.5 ns) when tested in giant unilamellar vesicles (GUVs) with different membrane orders. Overall, the results of this study represent a significant advancement in the applications of flippers and dyads in mechanobiology.

Characterization of Cancer Cell Mechanics by Measuring Active Deformation Behavior

Active deformation behavior reflects cell structural dynamics adapting to varying environmental constraints during malignancy progression. In most cases, cell mechanics is characterized by modeling using static equilibrium systems, which fails to comprehend cell deformation behavior leading to inaccuracies in distinguishing cancer cells from normal cells. Here, a method is introduced to measure the active deformation behavior of cancer cells using atomic force microscopy (AFM) and the newly developed deformation behavior cytometry (DBC). During the measurement, cells are deformed and allows a long timescale relaxation (≈5 s). Two parameters are derived to represent deformation behavior: apparent Poisson's ratio for adherent cells, which is measured with AFM and refers to the ratio of the lateral strain to the longitudinal strain of the cell, and shape recovery for suspended cells, which is measured with DBC. Active deformation behavior defines cancer cell mechanics better than traditional mechanical parameters (e.g., stiffness, diffusion, and viscosity). Additionally, aquaporins are essential for promoting the deformation behavior, while the actin cytoskeleton acts as a downstream effector. Therefore, the potential application of the cancer cell active deformation behavior as a biomechanical marker or therapeutic target in cancer treatment should be evaluated.

Cortical tension promotes Kibra degradation via Par-1

The Hippo pathway is an evolutionarily conserved regulator of tissue growth. Multiple Hippo signaling components are regulated via proteolytic degradation. However, how these degradation mechanisms are themselves modulated remains unexplored. Kibra is a key upstream pathway activator that promotes its own ubiquitin-mediated degradation upon assembling a Hippo signaling complex. Here, we demonstrate that Hippo complex-dependent Kibra degradation is modulated by cortical tension. Using classical genetic, osmotic, and pharmacological manipulations of myosin activity and cortical tension, we show that increasing cortical tension leads to Kibra degradation, whereas decreasing cortical tension increases Kibra abundance. Our study also implicates Par-1 in regulating Kib abundance downstream of cortical tension. We demonstrate that Par-1 promotes ubiquitin-mediated Kib degradation in a Hippo complex-dependent manner and is required for tension-induced Kib degradation. Collectively, our results reveal a previously unknown molecular mechanism by which cortical tension affects Hippo signaling and provide novel insights into the role of mechanical forces in growth control.

Cell wall dynamics: novel tools and research questions

Years ago, a classic textbook would define plant cell walls based on passive features. For instance, a sort of plant exoskeleton of invariable polysaccharide composition, and probably painted in green. However, currently, this view has been expanded to consider plant cell walls as active, heterogeneous, and dynamic structures with a high degree of complexity. However, what do we mean when we refer to a cell wall as a dynamic structure? How can we investigate the different implications of this dynamism? While the first question has been the subject of several recent publications, defining the ideal strategies and tools needed to address the second question has proven to be challenging due to the myriad of techniques available. In this review, we will describe the capacities of several methodologies to study cell wall composition, structure, and other aspects developed or optimized in recent years. Keeping in mind cell wall dynamism and plasticity, the advantages of performing long-term non-invasive live-imaging methods will be emphasized. We specifically focus on techniques developed for Arabidopsis thaliana primary cell walls, but the techniques could be applied to both secondary cell walls and other plant species. We believe this toolset will help researchers in expanding knowledge of these dynamic/evolving structures.

NINJ1 is activated by cell swelling to regulate plasma membrane permeabilization during regulated necrosis

Plasma membrane permeabilization (PMP) is a defining feature of regulated necrosis. It allows the extracellular release of damage-associated molecular patterns (DAMPs) that trigger sterile inflammation. The pore forming molecules MLKL and GSDMs drive PMP in necroptosis and pyroptosis, respectively, but the process of PMP remains unclear in many other forms of regulated necrosis. Here, we identified NINJ1 as a crucial regulator of PMP and consequent DAMP release during ferroptosis, parthanatos, H2O2-induced necrosis and secondary necrosis. Importantly, the membrane-permeabilizing function of NINJ1 takes place after the metabolic death of the cells and is independent of the pore-forming molecules MLKL, GSDMD and GSDME. During ferroptosis, NINJ1 acts downstream of lipid peroxidation, which suggested a role for reactive oxygen species (ROS) in NINJ1 activation. Reactive oxygen species were however neither sufficient nor required to trigger NINJ1-dependent PMP. Instead, we found that NINJ1 oligomerization is induced by the swelling of the cell and that its permeabilizing potential still requires an addition, and yet to be discovered, activation mechanism.

A palisade-shaped membrane reservoir is required for rapid ring cell inflation in Drechslerella dactyloides

Fusion of individual vesicles carrying membrane-building materials with the plasma membrane (PM) enables gradual cell expansion and shape change. Constricting ring (CR) cells of carnivorous fungi triple in size within 0.1-1 s to capture passing nematodes. Here, we investigated how a carnivorous fungus, Drechslerella dactyloides, executes rapid and irreversible PM expansion during CR inflation. During CR maturation, vesicles carrying membrane-building materials accumulate and fuse, forming a structure named the Palisade-shaped Membrane-building Structure (PMS) around the rumen side of ring cells. After CR inflation, the PMS disappears, with partially inflated cells displaying wavy PM and fully inflated cells exhibiting smooth PM, suggesting that the PMS serves as the reservoir for membrane-building materials to enable rapid and extensive PM expansion. The DdSnc1, a v-SNARE protein, accumulates at the inner side of ring cells and is necessary for PMS formation and CR inflation. This study elucidates the unique cellular mechanisms underpinning rapid CR inflation.

In situ quantification of osmotic pressure within living embryonic tissues

Mechanics is known to play a fundamental role in many cellular and developmental processes. Beyond active forces and material properties, osmotic pressure is believed to control essential cell and tissue characteristics. However, it remains very challenging to perform in situ and in vivo measurements of osmotic pressure. Here we introduce double emulsion droplet sensors that enable local measurements of osmotic pressure intra- and extra-cellularly within 3D multicellular systems, including living tissues. After generating and calibrating the sensors, we measure the osmotic pressure in blastomeres of early zebrafish embryos as well as in the interstitial fluid between the cells of the blastula by monitoring the size of droplets previously inserted in the embryo. Our results show a balance between intracellular and interstitial osmotic pressures, with values of approximately 0.7 MPa, but a large pressure imbalance between the inside and outside of the embryo. The ability to measure osmotic pressure in 3D multicellular systems, including developing embryos and organoids, will help improve our understanding of its role in fundamental biological processes.

Morphology as indicator of adaptive changes of model tissues in osmotically and chemically changing environments

We investigate the formation and maintenance of the homeostatic state in the case of 2D epithelial tissues following an induction of hyperosmotic conditions, using media enriched with 80 to 320 mOsm of mannitol, NaCl, and urea. We characterise the changes in the tissue immediately after the osmotic shock, and follow it until the new homeostatic state is formed. We characterise changes in cooperative motility and proliferation pressure in the tissue upon treatment with the help of a theoretical model based on the delayed Fisher-Kolmogorov formalism, where the delay in density evolution is induced by the the finite time of the cell division. Finally we explore the adaptation of the homeostatic tissue to highly elevated osmotic conditions by evaluating the morphology and topology of cells after 20 days in incubation. We find that hyperosmotic environments together with changes in the extracellular matrix induce different mechanical states in viable tissues, where only some remain functional. The perspective is a relation between tissue topology and function, which could be explored beyond the scope of this manuscript. Experimental investigation of morphological effect of change of osmotic conditions on long-term tissue morphology and topology Effect of osmotic changes on transient tissue growth behaviour Analysis of recovery process of tissues post-osmotic-shock Toxicity limits of osmolytes in mid- to long-term tissue evolution Tissue adaptation to physiological changes in environment Long-term tissue stabilisation under altered osmotic conditions.

Macromolecular condensation buffers intracellular water potential

Optimum protein function and biochemical activity critically depends on water availability because solvent thermodynamics drive protein folding and macromolecular interactions1. Reciprocally, macromolecules restrict the movement of 'structured' water molecules within their hydration layers, reducing the available 'free' bulk solvent and therefore the total thermodynamic potential energy of water, or water potential. Here, within concentrated macromolecular solutions such as the cytosol, we found that modest changes in temperature greatly affect the water potential, and are counteracted by opposing changes in osmotic strength. This duality of temperature and osmotic strength enables simple manipulations of solvent thermodynamics to prevent cell death after extreme cold or heat shock. Physiologically, cells must sustain their activity against fluctuating temperature, pressure and osmotic strength, which impact water availability within seconds. Yet, established mechanisms of water homeostasis act over much slower timescales2,3; we therefore postulated the existence of a rapid compensatory response. We find that this function is performed by water potential-driven changes in macromolecular assembly, particularly biomolecular condensation of intrinsically disordered proteins. The formation and dissolution of biomolecular condensates liberates and captures free water, respectively, quickly counteracting thermal or osmotic perturbations of water potential, which is consequently robustly buffered in the cytoplasm. Our results indicate that biomolecular condensation constitutes an intrinsic biophysical feedback response that rapidly compensates for intracellular osmotic and thermal fluctuations. We suggest that preserving water availability within the concentrated cytosol is an overlooked evolutionary driver of protein (dis)order and function.

Dynamic response of the cell traction force to osmotic shock

Osmotic pressure is vital to many physiological activities, such as cell proliferation, wound healing and disease treatment. However, how cells interact with the extracellular matrix (ECM) when subjected to osmotic shock remains unclear. Here, we visualize the mechanical interactions between cells and the ECM during osmotic shock by quantifying the dynamic evolution of the cell traction force. We show that both hypertonic and hypotonic shocks induce continuous and large changes in cell traction force. Moreover, the traction force varies with cell volume: the traction force increases as cells shrink and decreases as cells swell. However, the direction of the traction force is independent of cell volume changes and is always toward the center of the cell-substrate interface. Furthermore, we reveal a mechanical mechanism in which the change in cortical tension caused by osmotic shock leads to the variation in traction force, which suggests a simple method for measuring changes in cell cortical tension. These findings provide new insights into the mechanical force response of cells to the external environment and may provide a deeper understanding of how the ECM regulates cell structure and function. Traction force exerted by cells under hypertonic and hypotonic shocks. Scale bar, 200 Pa. Color bar, Pa. The black arrows represent the tangential traction forces.

Problems of human spermatozoa cryopreservation: research methods, solutions

Cryopreservation of male gametes is one of the most important methods of assisted reproductive technologies, which allows preserving gametes for research or further use. However, the fertilizing ability of spermatozoa after cryopreservation decreases by 30-70%, which makes it urgent to search for new substances with cryoprotective properties. The review considers the main causes of cell damage during cryopreservation. The relevance of methods for assessing the formation of crystals and the physicochemical properties of cryoprotective media depending on various compositions is discussed. The problem of stabilization of the spermatozoa membrane during cryopreservation is considered. A possible solution to the problem of membrane integrity may consist in modification of the basic cryoprotective media with yolk emulsion or development of methods for saturation of the membrane phospholipid layer with cholesterol.

Compression drives diverse transcriptomic and phenotypic adaptations in melanoma

Physical forces are prominent during tumor progression. However, it is still unclear how they impact and drive the diverse phenotypes found in cancer. Here, we apply an integrative approach to investigate the impact of compression on melanoma cells. We apply bioinformatics to screen for the most significant compression-induced transcriptomic changes and investigate phenotypic responses. We show that compression-induced transcriptomic changes are associated with both improvement and worsening of patient prognoses. Phenotypically, volumetric compression inhibits cell proliferation and cell migration. It also induces organelle stress and intracellular oxidative stress and increases pigmentation in malignant melanoma cells and normal human melanocytes. Finally, cells that have undergone compression become more resistant to cisplatin treatment. Our findings indicate that volumetric compression is a double-edged sword for melanoma progression and drives tumor evolution.