Cell blebbing and membrane area homeostasis in spreading and retracting cells

The Plasma Membrane and Mechanoregulation in Cells

Cells inhabit a mechanical microenvironment that they continuously sense and adapt to. The plasma membrane (PM), serving as the boundary of the cell, plays a pivotal role in this process of adaptation. In this Review, we begin by examining well-studied processes where mechanoregulation proves significant. Specifically, we highlight examples from the immune system and stem cells, besides discussing processes involving fibroblasts and other cell types. Subsequently, we discuss the common molecular players that facilitate the sensing of the mechanical signal and transform it into a chemical response covering integrins YAP/TAZ and Piezo. We then review how this understanding of molecular elements is leveraged in drug discovery and tissue engineering alongside a discussion of the methodologies used to measure mechanical properties. Focusing on the processes of endocytosis, we discuss how cells may respond to altered membrane mechanics using endo- and exocytosis. Through the process of depleting/adding the membrane area, these could also impact membrane mechanics. We compare pathways from studies illustrating the involvement of endocytosis in mechanoregulation, including clathrin-mediated endocytosis (CME) and the CLIC/GEEC (CG) pathway as central examples. Lastly, we review studies on cell-cell fusion during myogenesis, the mechanical integrity of muscle fibers, and the reported and anticipated roles of various molecular players and processes like endocytosis, thereby emphasizing the significance of mechanoregulation at the PM.

Mechano-regulation by clathrin pit-formation and passive cholesterol-dependent tubules during de-adhesion

Adherent cells ensure membrane homeostasis during de-adhesion by various mechanisms, including endocytosis. Although mechano-chemical feedbacks involved in this process have been studied, the step-by-step build-up and resolution of the mechanical changes by endocytosis are poorly understood. To investigate this, we studied the de-adhesion of HeLa cells using a combination of interference reflection microscopy, optical trapping and fluorescence experiments. We found that de-adhesion enhanced membrane height fluctuations of the basal membrane in the presence of an intact cortex. A reduction in the tether force was also noted at the apical side. However, membrane fluctuations reveal phases of an initial drop in effective tension followed by saturation. The area fractions of early (Rab5-labelled) and recycling (Rab4-labelled) endosomes, as well as transferrin-labelled pits close to the basal plasma membrane, also transiently increased. On blocking dynamin-dependent scission of endocytic pits, the regulation of fluctuations was not blocked, but knocking down AP2-dependent pit formation stopped the tension recovery. Interestingly, the regulation could not be suppressed by ATP or cholesterol depletion individually but was arrested by depleting both. The data strongly supports Clathrin and AP2-dependent pit-formation to be central to the reduction in fluctuations confirmed by super-resolution microscopy. Furthermore, we propose that cholesterol-dependent pits spontaneously regulate tension under ATP-depleted conditions.

YAP Signaling Regulates the Cellular Uptake and Therapeutic Effect of Nanoparticles

Interactions between living cells and nanoparticles are extensively studied to enhance the delivery of therapeutics. Nanoparticles size, shape, stiffness, and surface charge are regarded as the main features able to control the fate of cell-nanoparticle interactions. However, the clinical translation of nanotherapies has so far been limited, and there is a need to better understand the biology of cell-nanoparticle interactions. This study investigates the role of cellular mechanosensitive components in cell-nanoparticle interactions. It is demonstrated that the genetic and pharmacologic inhibition of yes-associated protein (YAP), a key component of cancer cell mechanosensing apparatus and Hippo pathway effector, improves nanoparticle internalization in triple-negative breast cancer cells regardless of nanoparticle properties or substrate characteristics. This process occurs through YAP-dependent regulation of endocytic pathways, cell mechanics, and membrane organization. Hence, the study proposes targeting YAP may sensitize triple-negative breast cancer cells to chemotherapy and increase the selectivity of nanotherapy.

Blebs promote cell survival by assembling oncogenic signalling hubs

Most human cells require anchorage for survival. Cell-substrate adhesion activates diverse signalling pathways, without which cells undergo anoikis-a form of programmed cell death1. Acquisition of anoikis resistance is a pivotal step in cancer disease progression, as metastasizing cells often lose firm attachment to surrounding tissue2,3. In these poorly attached states, cells adopt rounded morphologies and form small hemispherical plasma membrane protrusions called blebs4-11. Bleb function has been thoroughly investigated in the context of amoeboid migration, but it has been examined far less in other scenarios12. Here we show by three-dimensional imaging and manipulation of cell morphological states that blebbing triggers the formation of plasma membrane-proximal signalling hubs that confer anoikis resistance. Specifically, in melanoma cells, blebbing generates plasma membrane contours that recruit curvature-sensing septin proteins as scaffolds for constitutively active mutant NRAS and effectors. These signalling hubs activate ERK and PI3K-well-established promoters of pro-survival pathways. Inhibition of blebs or septins has little effect on the survival of well-adhered cells, but in detached cells it causes NRAS mislocalization, reduced MAPK and PI3K activity, and ultimately, death. This unveils a morphological requirement for mutant NRAS to operate as an effective oncoprotein. Furthermore, whereas some BRAF-mutated melanoma cells do not rely on this survival pathway in a basal state, inhibition of BRAF and MEK strongly sensitizes them to both bleb and septin inhibition. Moreover, fibroblasts engineered to sustain blebbing acquire the same anoikis resistance as cancer cells even without harbouring oncogenic mutations. Thus, blebs are potent signalling organelles capable of integrating myriad cellular information flows into concerted cellular responses, in this case granting robust anoikis resistance.

The emerging role of microtubules in invasion plasticity

The ability of cells to switch between different invasive modes during metastasis, also known as invasion plasticity, is an important characteristic of tumor cells that makes them able to resist treatment targeted to a particular invasion mode. Due to the rapid changes in cell morphology during the transition between mesenchymal and amoeboid invasion, it is evident that this process requires remodeling of the cytoskeleton. Although the role of the actin cytoskeleton in cell invasion and plasticity is already quite well described, the contribution of microtubules is not yet fully clarified. It is not easy to infer whether destabilization of microtubules leads to higher invasiveness or the opposite since the complex microtubular network acts differently in diverse invasive modes. While mesenchymal migration typically requires microtubules at the leading edge of migrating cells to stabilize protrusions and form adhesive structures, amoeboid invasion is possible even in the absence of long, stable microtubules, albeit there are also cases of amoeboid cells where microtubules contribute to effective migration. Moreover, complex crosstalk of microtubules with other cytoskeletal networks participates in invasion regulation. Altogether, microtubules play an important role in tumor cell plasticity and can be therefore targeted to affect not only cell proliferation but also invasive properties of migrating cells.

Volume regulation in adhered cells: Roles of surface tension and cell swelling

The volume of adhered cells has been shown experimentally to decrease during spreading. This effect can be understood from the pump-leak model, which we have extended to include mechano-sensitive ion transporters. We identify a novel effect that has important consequences on cellular volume loss: cells that are swollen due to a modulation of ion transport rates are more susceptible to volume loss in response to a tension increase. This effect explains in a plausible manner the discrepancies between three recent, independent experiments on adhered cells, between which both the magnitude of the volume change and its dynamics varied substantially. We suggest that starved and synchronized cells in two of the experiments were in a swollen state and, consequently, exhibited a large volume loss at steady state. Nonswollen cells, for which there is a very small steady-state volume decrease, are still predicted to transiently lose volume during spreading due to a relaxing viscoelastic tension that is large compared with the steady-state tension. We elucidate the roles of cell swelling and surface tension in cellular volume regulation and discuss their possible microscopic origins.

Cell-surface contacts determine volume and mechanical properties of human embryonic kidney 293 T cells

Alterations in the organization of the cytoskeleton precede the escape of adherent cells from the framework of cell-cell and cell-matrix interactions into suspension. With cytoskeletal dynamics being linked to cell mechanical properties, many studies elucidated this relationship under either native adherent or suspended conditions. In contrast, tethered cells that mimic the transition between both states have not been the focus of recent research. Using human embryonic kidney 293 T cells we investigated all three conditions in the light of alterations in cellular shape, volume, as well as mechanical properties and relate these findings to the level, structure, and intracellular localization of filamentous actin (F-actin). For cells adhered to a substrate, our data shows that seeding density affects cell size but does not alter their elastic properties. Removing surface contacts leads to cell stiffening that is accompanied by changes in cell shape, and a reduction in cellular volume but no alterations in F-actin density. Instead, we observe changes in the organization of F-actin indicated by the appearance of blebs in the semi-adherent state. In summary, our work reveals an interplay between molecular and mechanical alterations when cells detach from a surface that is mainly dominated by cell morphology.

Poly(ε-Caprolactone) Substrates with Micro/Nanohierarchical Patterned Structures for Cell Culture

A simple, efficient and controllable one-step template method is proposed to fabricate poly(ε-caprolactone) substrates with micro/nanohierarchical patterned structures. Two kinds of geometric patterns with and without nanowires, i.e., hexagonal and strip with controllable island size and spacing are designed and fabricated. Furthermore, the influence of geometric patterns, island size, island spacing, and patterned nanowires (pNW) on the growth behavior of MG-63 cells is studied in terms of cell density, distribution, proliferation, morphogenesis, and cellular alignment. It is found that MG-63 cells prefer to adhere and grow on the substrate with smaller island size or spacing. Moreover, unlike the hexagonal structure, the strip structure can guide cellular alignment on its surface. In addition, the microisland structures and the pNW play different roles in promoting cell proliferation, distribution, and morphogenesis. It is concluded that the growth behavior of MG-63 cells can be well controlled by precisely adjusting the micro/nanostructure of the substrate surface. A simple and effective method is provided here for the regulation of cell growth behavior.

Heterologously secreted MbxA from Moraxella bovis induces a membrane blebbing response of the human host cell

Many proteins of the Repeats in Toxins (RTX) protein family are toxins of Gram-negative pathogens including hemolysin A (HlyA) of uropathogenic E. coli. RTX proteins are secreted via Type I secretion systems (T1SS) and adopt their native conformation in the Ca²⁺-rich extracellular environment. Here we employed the E. coli HlyA T1SS as a heterologous surrogate system for the RTX toxin MbxA from the bovine pathogen Moraxella bovis. In E. coli the HlyA system successfully activates the heterologous MbxA substrate by acylation and secretes the precursor proMbxA and active MbxA allowing purification of both species in quantities sufficient for a variety of investigations. The activating E. coli acyltransferase HlyC recognizes the acylation sites in MbxA, but unexpectedly in a different acylation pattern as for its endogenous substrate HlyA. HlyC-activated MbxA shows host species-independent activity including a so-far unknown toxicity against human lymphocytes and epithelial cells. Using live-cell imaging, we show an immediate MbxA-mediated permeabilization and a rapidly developing blebbing of the plasma membrane in epithelial cells, which is associated with immediate cell death.

Nanoparticle entry into cells; the cell biology weak link

Nanoparticles (NP) are attractive options for the therapeutic delivery of active pharmaceutical drugs, proteins and nucleic acids into cells, tissues and organs. Research into the development and application of NP most often starts with a diverse group of scientists, including chemists, bioengineers and material and pharmaceutical scientists, who design, fabricate and characterize NP in vitro (Stage 1). The next step (Stage 2) generally investigates cell toxicity as well as the processes by which NP bind, are internalized and deliver their cargo to appropriate model tissue culture cells. Subsequently, in Stage 3, selected NP are tested in animal systems, mostly mouse. Whereas the chemistry-based development and analysis in Stage 1 is increasingly sophisticated, the investigations in Stage 2 are not what could be regarded as 'state-of-the-art' for the cell biology field and the quality of research into NP interactions with cells is often sub-standard. In this review we describe our current understanding of the mechanisms by which particles gain entry into mammalian cells via endocytosis. We summarize the most important areas for concern, highlight some of the most common mis-conceptions, and identify areas where NP scientists could engage with trained cell biologists. Our survey of the different mechanisms of uptake into cells makes us suspect that claims for roles for caveolae, as well as macropinocytosis, in NP uptake into cells have been exaggerated, whereas phagocytosis has been under-appreciated.

Vibropolyfection: coupling polymer-mediated gene delivery to mechanical stimulation to enhance transfection of adherent cells

Background

With the success of recent non-viral gene delivery-based COVID-19 vaccines, nanovectors have gained some public acceptance and come to the forefront of advanced therapies. Unfortunately, the relatively low ability of the vectors to overcome cellular barriers adversely affects their effectiveness. Scientists have thus been striving to develop ever more effective gene delivery vectors, but the results are still far from satisfactory. Therefore, developing novel strategies is probably the only way forward to bring about genuine change. Herein, we devise a brand-new gene delivery strategy to boost dramatically the transfection efficiency of two gold standard nucleic acid (NA)/polymer nanoparticles (polyplexes) in vitro.

Results

We conceived a device to generate milli-to-nanoscale vibrational cues as a function of the frequency set, and deliver vertical uniaxial displacements to adherent cells in culture. A short-lived high-frequency vibrational load (t = 5 min, f = 1,000 Hz) caused abrupt and extensive plasmalemma outgrowths but was safe for cells as neither cell proliferation rate nor viability was affected. Cells took about 1 hr to revert to quasi-naïve morphology through plasma membrane remodeling. In turn, this eventually triggered the mechano-activated clathrin-mediated endocytic pathway and made cells more apt to internalize polyplexes, resulting in transfection efficiencies increased from 10-to-100-fold. Noteworthy, these results were obtained transfecting three cell lines and hard-to-transfect primary cells.

Conclusions

In this work, we focus on a new technology to enhance the intracellular delivery of NAs and improve the transfection efficiency of non-viral vectors through priming adherent cells with a short vibrational stimulation. This study paves the way for capitalizing on physical cell stimulation(s) to significantly raise the effectiveness of gene delivery vectors in vitro and ex vivo.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01571-x.

Membrane area gain and loss during cytokinesis

In cytokinesis of animal cells, the cell is symmetrically divided into two. Since the cell's volume is conserved, the projected area has to increase to allow for the change of shape. Here we aim to predict how membrane gain and loss adapt during cytokinesis. We work with a kinetic model in which membrane turnover depends on membrane tension and cell shape. We apply this model to a series of calculated vesicle shapes as a proxy for the shape of dividing cells. We find that the ratio of kinetic turnover parameters changes nonmonotonically with cell shape, determined by the dependence of exocytosis and endocytosis on membrane curvature. Our results imply that controlling membrane turnover will be crucial for the successful division of artificial cells.

Apoptotic Features in Non-Apoptotic Processes

Apoptosis is the most thoroughly studied type of regulated cell death. Certain events, such as externalization of phosphatidylserine (PS) into the outer leaflet of plasma membrane, mitochondrial outer membrane permeabilization, caspase cascade activation, DNA fragmentation and blebbing, are widely considered to be hallmarks of apoptosis as well as being traditionally viewed as irreversible. This review shows that under particular circumstances these events can also participate in physiological processes not associated with initiation of apoptosis, such as cell differentiation, division, and motility, as well as non-apoptotic types of cell death. Moreover, these events may often be reversible. This review focuses on three processes: phosphatidylserine externalization, blebbing, and activation of apoptotic caspases. Mitochondrial outer membrane permeabilization and DNA fragmentation are not discussed.

Ferroptosis induces membrane blebbing in placental trophoblasts

Ferroptosis is a regulated, non-apoptotic form of cell death, characterized by hydroxy-peroxidation of discrete phospholipid hydroperoxides, particularly hydroperoxyl (Hp) forms of arachidonoyl- and adrenoyl-phosphatidylethanolamine, with a downstream cascade of oxidative damage to membrane lipids, proteins and DNA, culminating in cell death. We recently showed that human trophoblasts are particularly sensitive to ferroptosis caused by depletion or inhibition of glutathione peroxidase 4 (GPX4) or the lipase PLA2G6. Here, we show that trophoblastic ferroptosis is accompanied by a dramatic change in the trophoblast plasma membrane, with macro-blebbing and vesiculation. Immunofluorescence revealed that ferroptotic cell-derived blebs stained positive for F-actin, but negative for cytoplasmic organelle markers. Transfer of conditioned medium that contained detached macrovesicles or co-culture of wild-type target cells with blebbing cells did not stimulate ferroptosis in target cells. Molecular modeling showed that the presence of Hp-phosphatidylethanolamine in the cell membrane promoted its cell ability to be stretched. Together, our data establish that membrane macro-blebbing is characteristic of trophoblast ferroptosis and can serve as a useful marker of this process. Whether or not these blebs are physiologically functional remains to be established.

Ferroptosis, trophoblast lipotoxic damage, and adverse pregnancy outcome

Programmed cell death is a central process in the control of tissue development, organismal physiology, and disease. Ferroptosis is a recently identified form of programmed cell death that is uniquely defined by redox-active iron-dependent hydroxy-peroxidation of polyunsaturated fatty acid (PUFA)-containing phospholipids and a loss of lipid peroxidation repair capacity. This distinctive form of lipotoxic cell death has been recently implicated in multiple human diseases, spanning ischemia-reperfusion heart injury, brain damage, acute kidney injury, cancer, and asthma. Intriguingly, settings that have been associated with ferroptosis are linked to placental physiology and trophoblast injury. Such circumstances include hypoxia-reperfusion during placental development, physiological uterine contractions or pathological changes in placental bed perfusion, the abundance of trophoblastic iron, evidence for lipotoxicity during the pathophysiology of major placental disorders such as preeclampsia, fetal growth restriction, and preterm birth, and reduced glutathione peroxidation capacity and lipid peroxidation repair during placental injury. We recently interrogated placental ferroptosis in placental dysfunction in human and mouse pregnancy, dissected its relevance to placental injury, and validated the role of glutathione peroxidase-4 in guarding placental trophoblasts against ferroptotic injury. We also uncovered a role for the phospholipase PLA2G6 (PNPLA9) in attenuating trophoblast ferroptosis. Here, we summarize current data on trophoblast ferroptosis, and the role of several proteins and microRNAs as regulators of this process. Our text offers insights into new opportunities for regulating ferroptosis as a means for protecting placental trophoblasts against lipotoxic injury.

Adhesion G protein-coupled receptor VLGR1/ADGRV1 regulates cell spreading and migration by mechanosensing at focal adhesions

VLGR1 (very large G protein-coupled receptor-1) is by far the largest adhesion G protein-coupled receptor in humans. Homozygous pathologic variants of VLGR1 cause hereditary deaf blindness in Usher syndrome 2C and haploinsufficiency of VLGR1 is associated with epilepsy. However, its molecular function remains elusive. Herein, we used affinity proteomics to identify many components of focal adhesions (FAs) in the VLGR1 interactome. VLGR1 is localized in FAs and assembles in FA protein complexes in situ. Depletion or loss of VLGR1 decreases the number and length of FAs in hTERT-RPE1 cells and in astrocytes of Vlgr1 mutant mice. VLGR1 depletion reduces cell spread and migration kinetics as well as the response to mechanical stretch characterizing VLGR1 as a metabotropic mechanosensor in FAs. Our data reveal a critical role of VLGR1 in the FA function and enlighten potential pathomechanisms in diseases related to VLGR1.

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VLGR1 is an integral part of focal adhesions and crucial for their assembly

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Absence of VLGR1 from focal adhesions alters cell spreading and cell migration

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VLGR1 is a metabotropic mechanosensor in focal adhesions

Plasma Membrane Blebbing Dynamics Involved in the Reversibly Perforated Cell by Ultrasound-Driven Microbubbles

The perforation of plasma membrane by ultrasound-driven microbubbles (i.e., sonoporation) provides a temporary window for transporting macromolecules into the cytoplasm that is promising with respect to drug delivery and gene therapy. To improve the efficacy of delivery while ensuring biosafety, membrane resealing and cell recovery are required to help sonoporated cells defy membrane injury and regain their normal function. Blebs are found to accompany the recovery of sonoporated cells. However, the spatiotemporal characteristics of blebs and the underlying mechanisms remain unclear. With a customized platform for ultrasound exposure and 2-D/3-D live single-cell imaging, localized membrane perforation was induced with ultrasound-driven microbubbles, and the cellular responses were monitored using multiple fluorescent probes. The results indicated that localized blebs undergoing four phases (nucleation, expansion, pausing and retraction) on a time scale of tens of seconds to minutes were specifically involved in the reversibly sonoporated cells. The blebs spatially correlated with the membrane perforation site and temporally lagged (about tens of seconds to minutes) the resealing of perforated membrane. Their diameter (about several microns) and lifetime (about tens of seconds to minutes) positively correlated with the degree of sonoporation. Further studies revealed that intracellular calcium transients might be an upstream signal for triggering blebbing nucleation; exocytotic lysosomes not only contributed to resealing of the perforated membrane, but also to the increasing bleb volume during expansion; and actin components accumulation facilitated bleb retraction. These results provide new insight into the short-term strategies that the sonoporated cell employs to recover on membrane perforation and to remodel membrane structure and a biophysical foundation for sonoporation-based therapy.

An acute exposure to perfluorooctanoic acid causes non-reversible plasma membrane injury in HeLa cells

Health and environmental risks regarding perfluorooctanoic acid, a well-known perfluorinated compound, are still a subject of great concern. Ubiquitous exposure and disparity of results make it difficult to determine the underlying mechanism of action, especially at the cellular level. This study proposes an experimental design to assess the reversibility of adverse effects after a one-time exposure to the compound, in comparison with other more conventional timings. Complementary endpoints including total protein content, neutral red uptake and MTT reduction tests along with division rates and microscopic observations were evaluated in HeLa cells. In addition, PFOA quantification inside the cells was performed. The cellular effects exerted after 24 h exposure to perfluorooctanoic acid are non-reversible after a 48 h recovery period. In addition, we describe for the first time the induction of plasma membrane blebbing and the activation of membrane repair mechanisms after recovery from non-cytotoxic treatments with the compound. This experimental design has provided relevant information regarding the toxicity of this perfluorinated compound, relating all the adverse effects detected to its interaction with the plasma membrane.

Cellular Blebs and Membrane Invaginations Are Coupled through Membrane Tension Buffering

Bleb-type cellular protrusions play key roles in a range of biological processes. It was recently found that bleb growth is facilitated by a local supply of membrane from tubular invaginations, but the interplay between the expanding bleb and the membrane tubes remains poorly understood. On the one hand, the membrane area stored in tubes may serve as a reservoir for bleb expansion. On the other hand, the sequestering of excess membrane in stabilized invaginations may effectively increase the cell membrane tension, which suppresses spontaneous protrusions. Here, we investigate this duality through physical modeling and in vivo experiments. In agreement with observations, our model describes the transition into a tube-flattening mode of bleb expansion while also predicting that the blebbing rate is impaired by elevating the concentration of the curved membrane proteins that form the tubes. We show both theoretically and experimentally that the stabilizing effect of tubes could be counterbalanced by the cortical myosin contractility. Our results largely suggest that proteins able to induce membrane tubulation, such as those containing N-BAR domains, can buffer the effective membrane tension-a master regulator of all cell deformations.

Spoiled for Choice: Diverse Endocytic Pathways Function at the Cell Surface

Endocytosis has long been identified as a key cellular process involved in bringing in nutrients, in clearing cellular debris in tissue, in the regulation of signaling, and in maintaining cell membrane compositional homeostasis. While clathrin-mediated endocytosis has been most extensively studied, a number of clathrin-independent endocytic pathways are continuing to be delineated. Here we provide a current survey of the different types of endocytic pathways available at the cell surface and discuss a new classification and plausible molecular mechanisms for some of the less characterized pathways. Along with an evolutionary perspective of the origins of some of these pathways, we provide an appreciation of the distinct roles that these pathways play in various aspects of cellular physiology, including the control of signaling and membrane tension.

Artificial Cell Membranes Interfaced with Optical Tweezers: A Versatile Microfluidics Platform for Nanomanipulation and Mechanical Characterization

Cell lipid membranes are the site of vital biological processes, such as motility, trafficking, and sensing, many of which involve mechanical forces. Elucidating the interplay between such bioprocesses and mechanical forces requires the use of tools that apply and measure piconewton-level forces, e.g., optical tweezers. Here, we introduce the combination of optical tweezers with free-standing lipid bilayers, which are fully accessible on both sides of the membrane. In the vicinity of the lipid bilayer, optical trapping would normally be impossible due to optical distortions caused by pockets of the solvent trapped within the membrane. We solve this by drastically reducing the size of these pockets via tuning of the solvent and flow cell material. In the resulting flow cells, lipid nanotubes are straightforwardly pushed or pulled and reach lengths above half a millimeter. Moreover, the controlled pushing of a lipid nanotube with an optically trapped bead provides an accurate and direct measurement of important mechanical properties. In particular, we measure the membrane tension of a free-standing membrane composed of a mixture of dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) to be 4.6 × 10^-6 N/m. We demonstrate the potential of the platform for biophysical studies by inserting the cell-penetrating trans-activator of transcription (TAT) peptide in the lipid membrane. The interactions between the TAT peptide and the membrane are found to decrease the value of the membrane tension to 2.1 × 10^-6 N/m. This method is also fully compatible with electrophysiological measurements and presents new possibilities for the study of membrane mechanics and the creation of artificial lipid tube networks of great importance in intra- and intercellular communication.

Modulating matrix-multicellular response using polysucrose-blended with poly-L-lactide or polydioxanone in electrospun scaffolds for skin tissue regeneration

Polysucrose (PSuc) is hydrophilic, has excellent biocompatibility with cells as a density gradient and is resistant to enzymes. Its use in electrospun mats for tissue engineering applications has not been investigated due to its amorphous nature. For spinnability and robustness, polysucrose was blended with poly-L-lactide (PLLA) and polydioxanone (PDX) respectively and electrospun into nanofibrous mats. Interaction with cells was assessed using L929 mouse fibroblasts and HaCaT keratinocytes separately and in co-culture. Effect of parameters such as porosity, fiber diameter, surface wettability and mechanical properties of mats on cell-scaffold interactions was studied. Depending on nature and composition of mats, fibroblasts showed dendritic, spindle or round cell morphologies along with the formation of lamellipodia, filopodia, fibrillar or fiber-like projections of 100 nm and 200-300 nm in diameter respectively from the periphery or center of cells. Granular extracellular matrix was formed on both PLLA-PSuc and PDX-PSuc 50-50 seeded with keratinocytes. Growth of keratinocytes was enhanced in co-culture with fibroblasts with the formation of a skin-like layer. Both cells showed the ability to form multilayer structures. The mats maintained their physical integrity during the period of study. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3275-3291, 2018.

Interaction and fusion dynamics between cellular blebs

Membrane blebbing, as a mechanism for cells to regulate their internal pressure and membrane tension, is believed to play important roles in processes such as cell migration, spreading and apoptosis. However, the fundamental question of how different blebs interact with each other during their life cycles remains largely unclear. Here, we report a combined theoretical and experimental investigation to examine how the growth and retraction of a cellular bleb are influenced by neighboring blebs as well as the fusion dynamics between them. Specifically, a boundary integral model was developed to describe the shape evolution of cell membrane during the blebbing/retracting process. We showed that a drop in the intracellular pressure will be induced by the formation of a bleb whose retraction then restores the pressure level. Consequently, the volume that a second bleb can reach was predicted to heavily depend on its initial weakened size and the time lag with respect to the first bleb, all in quantitative agreement with our experimental observations. In addition, it was found that as the strength of membrane-cortex adhesion increases, the possible coalescence of two neighboring blebs changes from smooth fusion to abrupt coalescence and eventually to no fusion at all. Phase diagrams summarizing the dependence of such transition on key physical factors, such as the intracellular pressure and bleb separation, were also obtained.

Mechanochemical feedback control of dynamin independent endocytosis modulates membrane tension in adherent cells

Plasma membrane tension regulates many key cellular processes. It is modulated by, and can modulate, membrane trafficking. However, the cellular pathway(s) involved in this interplay is poorly understood. Here we find that, among a number of endocytic processes operating simultaneously at the cell surface, a dynamin independent pathway, the CLIC/GEEC (CG) pathway, is rapidly and specifically upregulated upon a sudden reduction of tension. Moreover, inhibition (activation) of the CG pathway results in lower (higher) membrane tension. However, alteration in membrane tension does not directly modulate CG endocytosis. This requires vinculin, a mechano-transducer recruited to focal adhesion in adherent cells. Vinculin acts by controlling the levels of a key regulator of the CG pathway, GBF1, at the plasma membrane. Thus, the CG pathway directly regulates membrane tension and is in turn controlled via a mechano-chemical feedback inhibition, potentially leading to homeostatic regulation of membrane tension in adherent cells.

Plasma membrane tension is an important factor that regulates many key cellular processes. Here authors show that a specific dynamin-independent endocytic pathway is modulated by changes in tension via the mechano-transducer vinculin.

Dissecting the biochemical architecture and morphological release pathways of the human platelet extracellular vesiculome

Platelet extracellular vesicles (PEVs) have emerged as potential mediators in intercellular communication. PEVs exhibit several activities with pathophysiological importance and may serve as diagnostic biomarkers. Here, imaging and analytical techniques were employed to unveil morphological pathways of the release, structure, composition, and surface properties of PEVs derived from human platelets (PLTs) activated with the thrombin receptor activating peptide (TRAP). Based on extensive electron microscopy analysis, we propose four morphological pathways for PEVs release from TRAP-activated PLTs: (1) plasma membrane budding, (2) extrusion of multivesicular α-granules and cytoplasmic vacuoles, (3) plasma membrane blistering and (4) "pearling" of PLT pseudopodia. The PLT extracellular vesiculome encompasses ectosomes, exosomes, free mitochondria, mitochondria-containing vesicles, "podiasomes" and PLT "ghosts". Interestingly, a flow cytometry showed a population of TOM20+LC3+ PEVs, likely products of platelet mitophagy. We found that lipidomic and proteomic profiles were different between the small PEV (S-PEVs; mean diameter 103 nm) and the large vesicle (L-PEVs; mean diameter 350 nm) fractions separated by differential centrifugation. In addition, the majority of PEVs released by activated PLTs was composed of S-PEVs which have markedly higher thrombin generation activity per unit of PEV surface area compared to L-PEVs, and contribute approximately 60% of the PLT vesiculome procoagulant potency.

Cytotoxic Swelling of Sick Excitable Cells - Impaired Ion Homeostasis and Membrane Tension Homeostasis in Muscle and Neuron

When they become simultaneously leaky to both Na⁺ and Cl^-, excitable cells are vulnerable to potentially lethal cytotoxic swelling. Swelling ensues in spite of an isosmotic milieu because the entering ions add osmolytes to the cytoplasm's high concentration of impermeant anionic osmolytes. An influx of osmotically-obliged water is unavoidable. A cell that cannot stanch at least one the leaks will succumb to death by Donnan effect. "Sick excitable cells" are those injured through ischemia, trauma, inflammation, hyperactivity, genetically-impaired membrane skeletons and other insults, all of which foster bleb-damage to regions of the plasma membrane. Nav channels resident in damaged membrane exhibit left-shifted kinetics; the corresponding Nav window conductance constitutes a Na⁺-leak. In cortical neurons, sustained depolarization to ∼-20mV elicits a sustained lethal gCl. Underlying V_rest in skeletal muscle is a constitutively active gCl; not surprisingly therefore, dystrophic muscle fibers, which are prone to bleb damage and which exhibit Nav-leak and Na⁺-overload, are prone to cytotoxic swelling. To restore viability in cytotoxically swelling neurons and muscle, the imperative of fully functional ion homeostasis is well-recognized. However, as emphasized here, in a healthy excitable cell, fully functional membrane tension homeostasis is also imperative. ATPase-pumps keep plasma membrane batteries charged, and ATPase-motor proteins maintain membrane tone. In sick excitable cells, neither condition prevails.

Analysis of adhesion kinetics of cancer cells on inflamed endothelium using a microfluidic platform

Metastasis is the ultimate cause of death among the vast majority of cancer patients. This process is comprised of multiple steps, including the migration of circulating cancer cells across microvasculature. This trans-endothelial migration involves the adhesion and eventual penetration of cancer cells to the vasculature of the target organ. Many of these mechanisms remain poorly understood due to poor control of pathophysiological conditions in tumor models. In this work, a microfluidic device was developed to support the culture and observation of engineered microvasculature with systematic control of the environmental characteristics. This device was then used to study the adhesion of circulating cancer cells to an endothelium under varying conditions to delineate the effects of hemodynamics and inflammations. The resulting understanding will help to establish a quantitative and biophysical mechanism of interactions between cancer cells and endothelium.

Riding the cell jamming boundary: Geometry, topology, and phase of human corneal endothelium

It is important to assess the viability of eye-banked corneas prior to transplantation due to inherent senescence and known loss of endothelial cells during surgical manipulation. Corneal endothelial cells have a complex basal and paracellular shape making them challenging to accurately measure, particularly in oedematous ex vivo tissue. This study used calibrated centroidal Voronoi Diagrams to segment cells in images of these human corneas, in order to characterize endothelial geometry, topology, and phase. Hexagonal cells dominated the endothelia, with most comprised of five different pleomorphs exhibiting self-similar topological coarsening through most of the endothelial cell density range. There was a linear relationship between cell size and shape, though cells with greater than six sides were present in larger proportions than cells with less. Hexagonal cell regularity was stable and largely independent of density. Cell and tissue phase was also examined, using the cell shape index relative to the recently discovered 'cell jamming' phase transition boundary. Images showed fluid endothelia with a range of shape indices spanning the boundary, independent of density but dependent on hexagonal regularity. The cells showed a bimodal distribution centred at the boundary, with the largest proportion of cells on the fluid side. A shoulder at the boundary suggested phase switching via shape transformation across the energy barrier, with cells either side having distinctly different size and shape characteristics. Regular hexagonal cells were closest to the boundary. This study showed the corneal endothelium acts as a glassy viscous foam characterized by well-established physical laws. Endothelial cell death transiently and locally increases cell fluidity, which is subsequently arrested by jamming of the pleomorphically diverse cell collective, via rearrangement and shape change of a small proportion of cells, which become locked in place by their neighbours thereby maintaining structural equilibrium with little energy expenditure.

Specific ADAM10 inhibitors localize in exosome-like vesicles released by Hodgkin lymphoma and stromal cells and prevent sheddase activity carried to bystander cells

Shedding of ADAM10 substrates, like TNFα, MICA or CD30, is reported to affect both anti-tumor immune response and antibody-drug-conjugate (ADC)-based immunotherapy. Soluble forms of these molecules and ADAM10 can be carried and spread in the microenvironment by exosomes released by tumor cells. We reported new ADAM10 inhibitors able to prevent MICA shedding in Hodgkin lymphoma (HL), leading to recognition of HL cells by cytotoxic lymphocytes.

In this paper, we show that the mature bioactive form of ADAM10 is released in exosome-like vesicles (ExoV) by HL cells and lymph node mesenchymal stromal cells (MSC). We demonstrate that ADAM10 inhibitors are released in ExoV by MSC or HL cells, endocytosed by bystander cells and localized in the endolysosomal compartment in HL MSC. ExoV released by HL cells can enhance MICA shedding by MSC, while ExoV from MSC induce TNFα or CD30 shedding by HL cells. Of note, ADAM10 sheddase activity carried by ExoV is prevented with the ADAM10 inhibitors LT4 and CAM29, pretreating either the ExoV-producing or the ExoV-receiving cells. In particular, both inhibitors reduce CD30 shedding maintaining the anti-tumor effects of the ADC Brentuximab-Vedotin or the anti-CD30 Iratumumab on HL cells.

Thus, spreading of ADAM10 activity due to ExoV can result in the release of cytokines, like TNFα, a lymphoma growth factor, or soluble molecules, like sMICA or sCD30, that potentially interfere with host immune surveillance or immunotherapy. ADAM10 blockers can interfere with this process, allowing the development of anti-lymphoma immune response and/or efficient ADC-based or human antibody-based immunotherapy.

Bleb Expansion in Migrating Cells Depends on Supply of Membrane from Cell Surface Invaginations

Cell migration is essential for morphogenesis, organ formation, and homeostasis, with relevance for clinical conditions. The migration of primordial germ cells (PGCs) is a useful model for studying this process in the context of the developing embryo. Zebrafish PGC migration depends on the formation of cellular protrusions in form of blebs, a type of protrusion found in various cell types. Here we report on the mechanisms allowing the inflation of the membrane during bleb formation. We show that the rapid expansion of the protrusion depends on membrane invaginations that are localized preferentially at the cell front. The formation of these invaginations requires the function of Cdc42, and their unfolding allows bleb inflation and dynamic cell-shape changes performed by migrating cells. Inhibiting the formation and release of the invaginations strongly interfered with bleb formation, cell motility, and the ability of the cells to reach their target.

A combined experimental and theoretical investigation on cellular blebbing

Although accumulating evidence has demonstrated the important role of membrane blebbing in various cellular processes, the fundamental question of how the initiation/evolution of blebs are influenced by physical factors like membrane-cortex interactions and intracellular pressure remains unclear. Here, we report a combined modeling and experimental study to address this outstanding issue. Specifically, boundary integral method was used to track the motion of membrane (in 3D) during blebbing while possible rupture of the bilayer-cortex adhesion has also been taken into account. We showed that, for a given differential pressure across the cell membrane, the size of the weakened cortex must be over a critical value for blebbing to occur and the steady-state volume of a bleb is proportional to its initial growth rate, all in good agreement with recent experiments. The predicted shape evolution of blebs also matches well with our observations. Finally, a blebbing map, summarizing the essential physics involved, was obtained which exhibits three distinct regimes: no bleb formation corresponding to a low intracellular pressure or a small weakened cortex region; bleb formed with a fixed width when the disrupted cortex zone is very large; and a growing bleb resulted from progressive membrane-cortex detachment under intermediate weakened cortex size.

Membrane tension: A challenging but universal physical parameter in cell biology

The plasma membrane separates the interior of cells from the outside environment. The membrane tension, defined as the force per unit length acting on a cross-section of membrane, regulates many vital biological processes. In this review, we summarize the first historical findings and the latest advances, showing membrane tension as an important physical parameter in cell biology. We also discuss how this parameter must be better integrated and we propose experimental approaches for key unanswered questions.

Non-equilibrium Membrane Homeostasis in Expanding Cellular Domains

Many cell behaviors involve cell-shape transformations that impose considerable changes in the cell's surface area, requiring a constant adaptation of the cell's plasma membrane area to prevent cell lysis. Here, we theoretically describe the interplay between the plasma membrane dynamics and a physically connected cell cortex or wall, accounting for spatial variations in membrane recycling and tension. In-plane membrane net flows result naturally from these dynamics and, in the presence of an expanding cell cortex or wall, regions of converging or diverging flow patterns emerge. These flow patterns can potentially explain the spatial localization/segregation of membrane proteins in processes such as cell polarization. We also identify the relevant parameters that control membrane homeostasis and derive the range of parameters for which homeostatic states exist.

NKCC1 Regulates Migration Ability of Glioblastoma Cells by Modulation of Actin Dynamics and Interacting with Cofilin

Glioblastoma (GBM) is the most aggressive primary brain tumor in adults. The mechanisms that confer GBM cells their invasive behavior are poorly understood. The electroneutral Na⁺-K⁺-2Cl^- co-transporter 1 (NKCC1) is an important cell volume regulator that participates in cell migration. We have shown that inhibition of NKCC1 in GBM cells leads to decreased cell migration, in vitro and in vivo. We now report on the role of NKCC1 on cytoskeletal dynamics. We show that GBM cells display a significant decrease in F-actin content upon NKCC1 knockdown (NKCC1-KD). To determine the potential actin-regulatory mechanisms affected by NKCC1 inhibition, we studied NKCC1 protein interactions. We found that NKCC1 interacts with the actin-regulating protein Cofilin-1 and can regulate its membrane localization. Finally, we analyzed whether NKCC1 could regulate the activity of the small Rho-GTPases RhoA and Rac1. We observed that the active forms of RhoA and Rac1 were decreased in NKCC1-KD cells. In summary, we report that NKCC1 regulates GBM cell migration by modulating the cytoskeleton through multiple targets including F-actin regulation through Cofilin-1 and RhoGTPase activity. Due to its essential role in cell migration NKCC1 may serve as a specific therapeutic target to decrease cell invasion in patients with primary brain cancer.

Inhibition of the PI3K/AKT/mTOR pathway activates autophagy and compensatory Ras/Raf/MEK/ERK signalling in prostate cancer

The PI3K/AKT/mTOR pathway is frequently activated in advanced prostate cancer, due to loss of the tumour suppressor PTEN, and is an important axis for drug development. We have assessed the molecular and functional consequences of pathway blockade by inhibiting AKT and mTOR kinases either in combination or as individual drug treatments. In established prostate cancer cell lines, a decrease in cell viability and in phospho-biomarker expression was observed. Although apoptosis was not induced, a G1 growth arrest was observed in PTEN null LNCaP cells, but not in BPH1 or PC3 cells. In contrast, when the AKT inhibitor AZD7328 was applied to patient-derived prostate cultures that retained expression of PTEN, activation of a compensatory Ras/MEK/ERK pathway was observed. Moreover, whilst autophagy was induced following treatment with AZD7328, cell viability was less affected in the patient-derived cultures than in cell lines. Surprisingly, treatment with a combination of both AZD7328 and two separate MEK1/2 inhibitors further enhanced phosphorylation of ERK1/2 in primary prostate cultures. However, it also induced irreversible growth arrest and senescence.

Ex vivo treatment of a patient-derived xenograft (PDX) of prostate cancer with a combination of AZD7328 and the mTOR inhibitor KU-0063794, significantly reduced tumour frequency upon re-engraftment of tumour cells.

The results demonstrate that single agent targeting of the PI3K/AKT/mTOR pathway triggers activation of the Ras/MEK/ERK compensatory pathway in near-patient samples. Therefore, blockade of one pathway is insufficient to treat prostate cancer in man.

CCM2 and PAK4 act downstream of atrial natriuretic peptide signaling to promote cell spreading

Atrial natriuretic peptide (ANP) is a cardiac hormone released by the atrium in response to stretching forces. Via its receptor, guanylyl cyclase-A (GC-A), ANP maintains cardiovascular homeostasis by exerting diuretic, natriuretic, and hypotensive effects mediated, in part, by endothelial cells. Both in vivo and in vitro, ANP enhances endothelial barrier function by reducing RhoA activity and reorganizing the actin cytoskeleton. We established mouse endothelial cells that stably express GC-A and used them to analyze the molecular mechanisms responsible for actin reorganization. Stimulation by ANP resulted in phosphorylation of myosin light chain (MLC) and promotion of cell spreading. p21-activated kinase 4 (PAK4) and cerebral cavernous malformations 2 (CCM2), a scaffold protein involved in a cerebrovascular disease, were required for the phosphorylation of MLC and promotion of cell spreading by ANP. Finally, in addition to the GC domain, the kinase homology domain of GC-A was also required for ANP/GC-A signaling. Our results indicate that CCM2 and PAK4 are important downstream mediators of ANP/GC-A signaling involved in cell spreading, an important initial step in the enhancement of endothelial barrier function.

HAART induces cell death in a cervical cancer cell line, HCS-2: A Scanning Electron Microscopy study

Apoptosis is a tightly programmed cell suicide which occurs in multiple physiologic and pathological conditions where it plays an important role in tissue development and homeostasis by eliminating unwanted and damaged cells. Appropriate apoptosis signalling is crucial in maintaining the fine balance between cell death and cell survival in cancer. In response to death stimuli the morphology of the cell undergoes unique changes. The aim of this study was to examine and compare the changes in the cell surface morphology using scanning electron microscopy in HCS-2 cells, following 24 hour treatment with components of highly active antiretroviral therapy (HAART) at their clinical plasma concentrations. The cells were fixed in 2.5% Glutaraldehyde and post-fixed in 1% osmium tetroxide. The cells were then dehydrated through a graded series of alcohol and treated with hexamethyl-disilazane, then coated with a double layer of carbon. The cells were viewed under a Zeiss Ultra FEG Scanning Electron Microscope and a one way ANOVA and Tukey Kramer Post Hoc test was conducted based on the scoring of surface morphology of the cells using JMP 11 statistical software. The drugs used in this study induced morphological features which are known to be characteristic of apoptotic cell death. The drug combinations (ATP and LPV/r) were seemingly more effective than individual treatments in inducing cell death because morphological features observed were more advanced than those observed in individual treatments. However, LPV/r was more potent than ATP. In conclusion, HAART showed anticancer properties by inducing cell death through apoptosis.

New View on Endothelial Cell Migration

Objective—

Cell–matrix interactions are crucial for regulating cellular activities, such as migration. This is of special importance for morphogenic processes, such as angiogenesis (the development of new blood vessels). Most of our understanding of cell migration relies on 2-dimensional (2D) experiments. However, the awareness that 3D settings might elicit different results has increased. Knowledge about endothelial cell (EC) behavior in 3D environments and the influence of matrix composition on EC migration, in particular, is still limited.

Approach and Results—

We characterize the migration of single ECs through 2 structurally different hydrogels: spongy Matrigel and fibrillar collagen I. Our observations reveal an elongated migration phenotype in Matrigel and a rounded phenotype with pronounced cell blebs (blebs >2 µm) in collagen I, which have not previously been described in ECs. Directed migration seems to depend on Rac1 and Cdc42 in collagen, but not in Matrigel (shown using appropriate pharmacological inhibitors). By applying anti-integrin antibodies and supplementing laminin in collagen gels, we identify laminin as the main determinant of the elongated phenotype. Laminin seems to induce a morphological switch between modes of migration. As an in situ proof of principle, we performed live imaging of EC migration during vascular growth in a murine retina in the absence and presence of anti-integrin antibodies.

Conclusions—

We show that, surprisingly, ECs can evade the pharmacological inhibition of central signaling pathways involved in migration (contractility, small GTPases, and proteolysis) by shifting gears between modes of migration. This finding indicates an unexpected contextual plasticity of EC behavior.

Physical view on migration modes

Cellular motility is essential for many processes such as embryonic development, wound healing processes, tissue assembly and regeneration, immune cell trafficing and diseases such as cancer. The migration efficiency and the migratory potential depend on the type of migration mode. The previously established migration modes such as epithelial (non-migratory) and mesenchymal (migratory) as well as amoeboid (squeezing motility) relay mainly on phenomenological criteria such as cell morphology and molecular biological criteria such as gene expression. However, the physical view on the migration modes is still not well understood. As the process of malignant cancer progression such as metastasis depends on the migration of single cancer cells and their migration mode, this review focuses on the different migration strategies and discusses which mechanical prerequisites are necessary to perform a special migration mode through a 3-dimensional microenvironment. In particular, this review discusses how cells can distinguish and finally switch between the migration modes and what impact do the physical properties of cells and their microenvironment have on the transition between the novel migration modes such as blebbing and protrusive motility.

Bleb Nucleation through Membrane Peeling

We study the nucleation of blebs, i.e., protrusions arising from a local detachment of the membrane from the cortex of a cell. Based on a simple model of elastic linkers with force-dependent kinetics, we show that bleb nucleation is governed by membrane peeling. By this mechanism, the growth or shrinkage of a detached membrane patch is completely determined by the linker kinetics, regardless of the energetic cost of the detachment. We predict the critical nucleation radius for membrane peeling and the corresponding effective energy barrier. These may be typically smaller than those predicted by classical nucleation theory, implying a much faster nucleation. We also perform simulations of a continuum stochastic model of membrane-cortex adhesion to obtain the statistics of bleb nucleation times as a function of the stress on the membrane. The determinant role of membrane peeling changes our understanding of bleb nucleation and opens new directions in the study of blebs.

Involvement of Membrane Blebbing in Immunological Disorders and Cancer

Cellular blebbing is a unique form of dynamic protrusion emanating from the plasma membrane which can be either apoptotic or nonapoptotic in nature. Blebs have been observed in a wide variety of cell types and in response to multiple mechanical and chemical stimuli. They have been linked to various physiological and pathological processes including tumor motility and invasion, as well as to various immunological disorders. They can form and retract extremely rapidly in seconds or minutes, or slowly over hours or days. This review focuses on recent evidence regarding the role of blebbing in cell locomotion with particular emphasis on its role in tumor metastasis, indicating the role of specific causative molecules. The phenomenon of blebbing has been observed in endocrine-resistant breast cancer cells in response to brief exposure to extracellular alkaline pH, which leads to enhanced invasive capacity. Genetic or pharmacological targeting of cellular blebs could serve as a potential therapeutic option to control tumor metastasis.

Force fluctuations in three-dimensional suspended fibroblasts

Cells are sensitive to mechanical cues from their environment and at the same time generate and transmit forces to their surroundings. To test quantitatively forces generated by cells not attached to a substrate, we used a dual optical trap to suspend 3T3 fibroblasts between two fibronectin-coated beads. In this simple geometry, we measured both the cells' elastic properties and the force fluctuations they generate with high bandwidth. Cell stiffness decreased substantially with both myosin inhibition by blebbistatin and serum-starvation, but not with microtubule depolymerization by nocodazole. We show that cortical forces generated by non-muscle myosin II deform the cell from its rounded shape in the frequency regime from 0.1 to 10 Hz. The amplitudes of these forces were strongly reduced by blebbistatin and serum starvation, but were unaffected by depolymerization of microtubules. Force fluctuations show a spectrum that is characteristic for an elastic network activated by random sustained stresses with abrupt transitions.

Membrane tension and cytoskeleton organization in cell motility

Cell membrane shape changes are important for many aspects of normal biological function, such as tissue development, wound healing and cell division and motility. Various disease states are associated with deregulation of how cells move and change shape, including notably tumor initiation and cancer cell metastasis. Cell motility is powered, in large part, by the controlled assembly and disassembly of the actin cytoskeleton. Much of this dynamic happens in close proximity to the plasma membrane due to the fact that actin assembly factors are membrane-bound, and thus actin filaments are generally oriented such that their growth occurs against or near the membrane. For a long time, the membrane was viewed as a relatively passive scaffold for signaling. However, results from the last five years show that this is not the whole picture, and that the dynamics of the actin cytoskeleton are intimately linked to the mechanics of the cell membrane. In this review, we summarize recent findings concerning the role of plasma membrane mechanics in cell cytoskeleton dynamics and architecture, showing that the cell membrane is not just an envelope or a barrier for actin assembly, but is a master regulator controlling cytoskeleton dynamics and cell polarity.

Loss of Cell Surface CD47 Clustering Formation and Binding Avidity to SIRPα Facilitate Apoptotic Cell Clearance by Macrophages

CD47, a self recognition marker expressed on tissue cells, interacts with immunoreceptor SIRPα expressed on the surface of macrophages to initiate inhibitory signaling that prevents macrophage phagocytosis of healthy host cells. Previous studies suggested that cells may lose surface CD47 during aging or apoptosis to enable phagocytic clearance. In the current study, we demonstrate that the level of cell surface CD47 is not decreased, but the distribution pattern of CD47 is altered, during apoptosis. On nonapoptotic cells, CD47 molecules are clustered in lipid rafts forming punctates on the surface, whereas on apoptotic cells, CD47 molecules are diffused on the cell surface following the disassembly of lipid rafts. We show that clustering of CD47 in lipid rafts provides a high binding avidity for cell surface CD47 to ligate macrophage SIRPα, which also presents as clusters, and elicits SIRPα-mediated inhibitory signaling that prevents phagocytosis. In contrast, dispersed CD47 on the apoptotic cell surface is associated with a significant reduction in the binding avidity to SIRPα and a failure to trigger SIRPα signal transduction. Disruption of plasma membrane lipid rafts with methyl-β-cyclodextrin diffuses CD47 clusters, leading to a decrease in the cell binding avidity to SIRPα and a concomitant increase in cells being engulfed by macrophages. Taken together, our study reveals that CD47 normally is clustered in lipid rafts on nonapoptotic cells but is diffused in the plasma membrane when apoptosis occurs; this transformation of CD47 greatly reduces the strength of CD47-SIRPα engagement, resulting in the phagocytosis of apoptotic cells.

Site of fluorescent label modifies interaction of melittin with live cells and model membranes

The mechanism of membrane disruption by melittin (MLT) of giant unilamellar vesicles (GUVs) and live cells was studied using fluorescence microscopy and two fluorescent synthetic analogues of MLT. The N-terminus of one of these was acylated with thiopropionic acid to enable labeling with maleimido-AlexaFluor 430 to study the interaction of MLT with live cells. It was compared with a second analogue labeled at P14C. The results indicated that the fluorescent peptides adhered to the membrane bilayer of phosphatidylcholine GUVs and inserted into the plasma membrane of HeLa cells. Fluorescence and light microscopy revealed changes in cell morphology after exposure to MLT peptides and showed bleb formation in the plasma membrane of HeLa cells. However, the membrane disruptive effect was dependent upon the location of the fluorescent label on the peptide and was greater when MLT was labeled at the N-terminus. Proline at position 14 appeared to be important for antimicrobial activity, hemolysis and cytotoxicity, but not essential for cell membrane disruption.

Cell blebbing upon addition of cryoprotectants: a self-protection mechanism

In this work, the mechanism of cell bleb formation upon the addition of cryoprotectants (CPAs) was investigated, and the role of cell blebs in protecting cells was determined. The results show that after adding CPAs, the hyperosmotic stress results in the breakage of the cortical cytoskeleton and the detachment of the cell membrane from the cortical cytoskeleton, causing the formation of cell blebs. Multiple blebs decrease the intracellular hydrostatic pressure induced by the extracellular hyperosmotic shock and alleviate the osmotic damage to cells, which reduces the cell mortality rate. In the presence of a low concentration of CPAs, cell blebs can effectively protect cells. In contrast, in the presence of a high concentration of CPAs, the protective effect is limited because of severe disruption in the cortical cytoskeleton. To determine the relationship between blebs and the mortality rate of cells, we defined a bleb index and found that the bleb index of 0.065 can be regarded as a reference value for the safe addition of DMSO to HeLa cells. The bleb index can also explain why the stepwise addition of CPAs is better than the single-step addition of CPAs. Moreover, the mechanism of the autophagy of cells induced by the hyperosmotic stress was studied, and the protective effect associated with the autophagy was compared with the effect of the blebbing. The findings reported here elucidate a self-protection mechanism of cells experiencing the hyperosmotic stress in the presence of CPAs, and they provide significant evidence for cell tolerance in the field of cryopreservation.

Compressive stress induces dephosphorylation of the myosin regulatory light chain via RhoA phosphorylation by the adenylyl cyclase/protein kinase A signaling pathway

Mechanical stress that arises due to deformation of the extracellular matrix (ECM) either stretches or compresses cells. The cellular response to stretching has been actively studied. For example, stretching induces phosphorylation of the myosin regulatory light chain (MRLC) via the RhoA/RhoA-associated protein kinase (ROCK) pathway, resulting in increased cellular tension. In contrast, the effects of compressive stress on cellular functions are not fully resolved. The mechanisms for sensing and differentially responding to stretching and compressive stress are not known. To address these questions, we investigated whether phosphorylation levels of MRLC were affected by compressive stress. Contrary to the response in stretching cells, MRLC was dephosphorylated 5 min after cells were subjected to compressive stress. Compressive loading induced activation of myosin phosphatase mediated via the dephosphorylation of myosin phosphatase targeting subunit 1 (Thr853). Because myosin phosphatase targeting subunit 1 (Thr853) is phosphorylated only by ROCK, compressive loading may have induced inactivation of ROCK. However, GTP-bound RhoA (active form) increased in response to compressive stress. The compression-induced activation of RhoA and inactivation of its effector ROCK are contradictory. This inconsistency was due to phosphorylation of RhoA (Ser188) that reduced affinity of RhoA to ROCK. Treatment with the inhibitor of protein kinase A that phosphorylates RhoA (Ser188) induced suppression of compression-stimulated MRLC dephosphorylation. Incidentally, stretching induced phosphorylation of MRLC, but did not affect phosphorylation levels of RhoA (Ser188). Together, our results suggest that RhoA phosphorylation is an important process for MRLC dephosphorylation by compressive loading, and for distinguishing between stretching and compressing cells.