Modulation of membrane traffic by mechanical stimuli

Physical Plasma Membrane Perturbation Using Subcellular Optogenetics Drives Integrin-Activated Cell Migration

Cells experience physical deformations to the plasma membrane that can modulate cell behaviors like migration. Understanding the molecular basis for how physical cues affect dynamic cellular responses requires new approaches that can physically perturb the plasma membrane with rapid, reversible, subcellular control. Here we present an optogenetic approach based on light-inducible dimerization that alters plasma membrane properties by recruiting cytosolic proteins at high concentrations to a target site. Surprisingly, this polarized accumulation of proteins in a cell induces directional amoeboid migration in the opposite direction. Consistent with known effects of constraining high concentrations of proteins to a membrane in vitro, there is localized curvature and tension decrease in the plasma membrane. Integrin activity, sensitive to mechanical forces, is activated in this region. Localized mechanical activation of integrin with optogenetics allowed simultaneous imaging of the molecular and cellular response, helping uncover a positive feedback loop comprising SFK- and ERK-dependent RhoA activation, actomyosin contractility, rearward membrane flow, and membrane tension decrease underlying this mode of cell migration.

Adherens Junction Length during Tissue Contraction Is Controlled by the Mechanosensitive Activity of Actomyosin and Junctional Recycling

During epithelial contraction, cells generate forces to constrict their surface and, concurrently, fine-tune the length of their adherens junctions to ensure force transmission. While many studies have focused on understanding force generation, little is known on how junctional length is controlled. Here, we show that, during amnioserosa contraction in Drosophila dorsal closure, adherens junctions reduce their length in coordination with the shrinkage of apical cell area, maintaining a nearly constant junctional straightness. We reveal that junctional straightness and integrity depend on the endocytic machinery and on the mechanosensitive activity of the actomyosin cytoskeleton. On one hand, upon junctional stretch and decrease in E-cadherin density, actomyosin relocalizes from the medial area to the junctions, thus maintaining junctional integrity. On the other hand, when junctions have excess material and ruffles, junction removal is enhanced, and high junctional straightness and tension are restored. These two mechanisms control junctional length and integrity during morphogenesis.

Role of Mechanotransduction and Tension in T Cell Function

T cell migration from blood to, and within lymphoid organs and tissue, as well as, T cell activation rely on complex biochemical signaling events. But T cell migration and activation also take place in distinct mechanical environments and lead to drastic morphological changes and reorganization of the acto-myosin cytoskeleton. In this review we discuss how adhesion proteins and the T cell receptor act as mechanosensors to translate these mechanical contexts into signaling events. We further discuss how cell tension could bring a significant contribution to the regulation of T cell signaling and function.

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.

A Membrane-Bound Biosensor Visualizes Shear Stress-Induced Inhomogeneous Alteration of Cell Membrane Tension

Cell membrane is the first medium from where a cell senses and responds to external stress stimuli. Exploring the tension changes in cell membrane will help us to understand intracellular force transmission. Here, a biosensor (named MSS) based on fluorescence resonance energy transfer is developed to visualize cell membrane tension. Validity of the biosensor is first verified for the detection of cell membrane tension. Results show a shear stress-induced heterogeneous distribution of membrane tension with the biosensor, which is strengthened by the disruption of microfilaments or enhancement of membrane fluidity, but weakened by the reduction of membrane fluidity or disruption of microtubules. These findings suggest that the MSS biosensor is a beneficial tool to visualize the changes and distribution of cell membrane tension. Besides, cell membrane tension does not display obvious polar distribution, indicating that cellular polarity changes do not first occur on the cell membrane during mechanical transmission.

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A FRET-based biosensor (named MSS) is developed to study cell membrane tension

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MSS is a beneficial tool to visualize the distribution of membrane tension

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Membrane tension is inhomogeneous in response to shear stress

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Membrane tension does not display polar distribution during mechanotransduction

Visualizing Tension and Growth in Model Membranes Using Optical Dyes

Cells dynamically regulate their membrane surface area during a variety of processes critical to their survival. Recent studies with model membranes have pointed to a general mechanism for surface area regulation under tension in which cell membranes unfold or take up lipid to accommodate membrane strain. Yet we lack robust methods to simultaneously measure membrane tension and surface area changes in real time. Using lipid vesicles that contain two dyes isolated to spatially distinct parts of the membrane, we introduce, to our knowledge, a new method to monitor the processes of membrane stretching and lipid uptake in model membranes. Laurdan, located within the bilayer membrane, and Förster resonance energy transfer dyes, localized to the membrane exterior, act in concert to report changes in membrane tension and lipid uptake during osmotic stress. We use these dyes to show that membranes under tension take up lipid more quickly and in greater amounts compared to their nontensed counterparts. Finally, we show that this technique is compatible with microscopy, enabling real-time analysis of membrane dynamics on a single vesicle level. Ultimately, the combinatorial use of these probes offers a more complete picture of changing membrane morphology. Our optical method allows us to remotely track changes in membrane tension and surface area with model membranes, offering new opportunities to track morphological changes in artificial and biological membranes and providing new opportunities in fields ranging from mechanobiology to drug delivery.

Nanoparticle localization in blood vessels: dependence on fluid shear stress, flow disturbances, and flow-induced changes in endothelial physiology

Nanoparticles in the bloodstream are subjected to complex fluid forces as they move through the curves and branches of healthy or tumor vasculature. While nanoparticles are known to preferentially accumulate in angiogenic vessels, little is known about the flow conditions in these vessels and how these conditions may influence localization. Here, we report a methodology which combines confocal imaging of nanoparticle-injected transgenic zebrafish embryos, 3D modeling of the vasculature, particle mapping, and computational fluid dynamics, to quantitatively assess the effects of fluid forces on nanoparticle distribution in vivo. Six-fold lower accumulation was found in zebrafish arteries compared to the lower velocity veins. Nanoparticle localization varied inversely with shear stress. Highest accumulation was present in regions of disturbed flow found at branch points and curvatures in the vasculature. To further investigate cell-particle association under flow, human endothelial cells were exposed to nanoparticles under hemodynamic conditions typically found in human vessels. Physiological adaptations of endothelial cells to 20 hours of flow enhanced nanoparticle accumulation in regions of disturbed flow. Overall our results suggest that fluid shear stress magnitude, flow disturbances, and flow-induced changes in endothelial physiology modulate nanoparticle localization in angiogenic vessels.

Modeling Tissue Polarity in Context

Polarity is critical for development and tissue-specific function. However, the acquisition and maintenance of tissue polarity is context dependent. Thus, cell and tissue polarity depend on cell adhesion which is regulated by the cytoskeleton and influenced by the biochemical composition of the extracellular microenvironment and modified by biomechanical cues within the tissue. These biomechanical cues include fluid flow induced shear stresses, cell-density and confinement-mediated compression, and cellular actomyosin tension intrinsic to the tissue or induced in response to morphogens or extracellular matrix stiffness. Here, we discuss how extracellular matrix stiffness and fluid flow influence cell-cell and cell-extracellular matrix adhesion and alter cytoskeletal organization to modulate cell and tissue polarity. We describe model systems that when combined with state of the art molecular screens and high-resolution imaging can be used to investigate how force modulates cell and tissue polarity.

JNK2 silencing and caspase-9 activation by hyperosmotic polymer inhibits tumor progression

c-Jun N-terminal kinase 2 (JNK2) is primarily responsible for the oncogenic transformation of the transcription factor c-Jun. Expression of the proto-oncogene c-Jun progresses the cell cycle from G1 to S phase, but when its expression becomes awry it leads to uncontrolled proliferation and angiogenesis. Delivering a JNK2 siRNA (siJNK2) in tumor tissue was anticipated to reverse the condition with subsequent onset of apoptosis which predominantly requires an efficient delivering system capable of penetrating through the compact tumor mass. In the present study, it was demonstrated that polymannitol-based vector (PMGT) with inherent hyperosmotic properties was able to penetrate through and deliver the siJNK2 in the subcutaneous tumor of xenograft mice. Hyperosmotic activity of polymannitol was shown to account for the enhanced therapeutic delivery both in vitro and in vivo because of the induction of cyclooxygenase-2 (COX-2) which stimulates caveolin-1 for caveolae-mediated endocytosis of the polyplexes. Further suppression of JNK2 and hence c-Jun expression led to the activation of caspase-9 to induce apoptosis and inhibition of tumor growth in xenograft mice model. The study exemplifies PMGT as an efficient vector for delivering therapeutic molecules in compact tumor tissue and suppression of JNK2 introduces a strategy to inhibit tumor progression.

Locally Triggered Release of the Chemokine CCL21 Promotes Dendritic Cell Transmigration across Lymphatic Endothelia

Trafficking cells frequently transmigrate through epithelial and endothelial monolayers. How monolayers cooperate with the penetrating cells to support their transit is poorly understood. We studied dendritic cell (DC) entry into lymphatic capillaries as a model system for transendothelial migration. We find that the chemokine CCL21, which is the decisive guidance cue for intravasation, mainly localizes in the trans-Golgi network and intracellular vesicles of lymphatic endothelial cells. Upon DC transmigration, these Golgi deposits disperse and CCL21 becomes extracellularly enriched at the sites of endothelial cell-cell junctions. When we reconstitute the transmigration process in vitro, we find that secretion of CCL21-positive vesicles is triggered by a DC contact-induced calcium signal, and selective calcium chelation in lymphatic endothelium attenuates transmigration. Altogether, our data demonstrate a chemokine-mediated feedback between DCs and lymphatic endothelium, which facilitates transendothelial migration.

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DC entry into lymphatic capillary induces CCL21 secretion to endothelial junctions

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Chemokine CCL21 secretion is triggered by calcium fluxes

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Direct contact by DC induces calcium signaling in LECs

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Dynamic rather than pre-patterned chemokine CCL21 cues promote DC transmigration

Mechanistic understanding the bioeffects of ultrasound-driven microbubbles to enhance macromolecule delivery

Ultrasound-driven microbubbles can trigger reversible membrane perforation (sonoporation), open interendothelial junctions and stimulate endocytosis, thereby providing a temporary and reversible time-window for the delivery of macromolecules across biological membranes and endothelial barriers. This time-window is related not only to cavitation events, but also to biological regulatory mechanisms. Mechanistic understanding of the interaction between cavitation events and cells and tissues, as well as the subsequent cellular and molecular responses will lead to new design strategies with improved efficacy and minimized side effects. Recent important progress on the spatiotemporal characteristics of sonoporation, cavitation-induced interendothelial gap and endocytosis, and the spatiotemporal bioeffects and the preliminary biological mechanisms in cavitation-enhanced permeability, has been made. On the basis of the summary of this research progress, this Review outlines the underlying bioeffects and the related biological regulatory mechanisms involved in cavitation-enhanced permeability; provides a critical commentary on the future tasks and directions in this field, including developing a standardized methodology to reveal mechanism-based bioeffects in depth, and designing biology-based treatment strategies to improve efficacy and safety. Such mechanistic understanding the bioeffects that contribute to cavitation-enhanced delivery will accelerate the translation of this approach to the clinic.

Detecting order and lateral pressure at biomimetic interfaces using a mechanosensitive second-harmonic-generation probe

A mechanosensitive harmonophore is used to probe the order and lateral pressure in phospholipid monolayers by surface-second harmonic generation.

Turnover and flow of the cell membrane for cell migration

The role of cell membrane dynamics in cell migration is unclear. To examine whether total cell surface area changes are required for cell migration, Dictyostelium cells were flattened by agar-overlay. Scanning electron microscopy demonstrated that flattened migrating cells have no membrane reservoirs such as projections and membrane folds. Similarly, optical sectioning fluorescence microscopy showed that the cell surface area does not change during migration. Interestingly, staining of the cell membrane with a fluorescent lipid analogue demonstrated that the turnover rate of cell membrane is closely related to the cell migration velocity. Next, to clarify the mechanism of cell membrane circulation, local photobleaching was separately performed on the dorsal and ventral cell membranes of rapidly moving cells. The bleached zones on both sides moved rearward relative to the cell. Thus, the cell membrane moves in a fountain-like fashion, accompanied by a high membrane turnover rate and actively contributing to cell migration.

Revisiting Theories of Humidity Transduction: A Focus on Electrophysiological Data

Understanding the mechanism of humidity transduction calls for experimental data and a theory to interpret the data and design new experiments. A comprehensive theory of humidity transduction must start with agreement on what humidity parameters are measured by hygroreceptors and processed by the brain. Hygroreceptors have been found in cuticular sensilla of a broad range of insect species. Their structural features are far from uniform. Nevertheless, these sensilla always contain an antagonistic pair of a moist cell and a dry cell combined with a thermoreceptive cold cell. The strategy behind this arrangement remains unclear. Three main models of humidity transduction have been proposed. Hygroreceptors could operate as mechanical hygrometers, psychrometers or evaporation detectors. Each mode of action measures a different humidity parameter. Mechanical hygrometers measure the relative humidity, psychrometers indicate the wet-bulb temperature, and evaporimeters refer to the saturation deficit of the air. Here we assess the validity of the different functions by testing specific predictions drawn from each of the models. The effect of air temperature on the responses to humidity stimulation rules out the mechanical hygrometer function, but it supports the psychrometer function and highlights the action as evaporation rate detector. We suggest testing the effect of the flow rate of the air stream used for humidity stimulation. As the wind speed strongly affects the power of evaporation, experiments with changing saturation deficit at different flow rates would improve our knowledge on humidity transduction.

Substrate properties modulate cell membrane roughness by way of actin filaments

Cell membrane roughness has been proposed as a sensitive feature to reflect cellular physiological conditions. In order to know whether membrane roughness is associated with the substrate properties, we employed the non-interferometric wide-field optical profilometry (NIWOP) technique to measure the membrane roughness of living mouse embryonic fibroblasts with different conditions of the culture substrate. By controlling the surface density of fibronectin (FN) coated on the substrate, we found that cells exhibited higher membrane roughness as the FN density increased in company with larger focal adhesion (FA) sizes. The examination of membrane roughness was also confirmed with atomic force microscopy. Using reagents altering actin or microtubule cytoskeletons, we provided evidence that the dynamics of actin filaments rather than that of microtubules plays a crucial role for the regulation of membrane roughness. By changing the substrate rigidity, we further demonstrated that the cells seeded on compliant gels exhibited significantly lower membrane roughness and smaller FAs than the cells on rigid substrate. Taken together, our data suggest that the magnitude of membrane roughness is modulated by way of actin dynamics in cells responding to substrate properties.

Linoleic acid: Is this the key that unlocks the quantum brain? Insights linking broken symmetries in molecular biology, mood disorders and personalistic emergentism

In this paper we present a mechanistic model that integrates subneuronal structures, namely ion channels, membrane fatty acids, lipid rafts, G proteins and the cytoskeleton in a dynamic system that is finely tuned in a healthy brain. We also argue that subtle changes in the composition of the membrane's fatty acids may lead to down-stream effects causing dysregulation of the membrane, cytoskeleton and their interface. Such exquisite sensitivity to minor changes is known to occur in physical systems undergoing phase transitions, the simplest and most studied of them is the so-called Ising model, which exhibits a phase transition at a finite temperature between an ordered and disordered state in 2- or 3-dimensional space. We propose this model in the context of neuronal dynamics and further hypothesize that it may involve quantum degrees of freedom dependent upon variation in membrane domains associated with ion channels or microtubules. Finally, we provide a link between these physical characteristics of the dynamical mechanism to psychiatric disorders such as major depression and antidepressant action.

On the determination of elastic moduli of cells by AFM based indentation

The atomic force microscopy (AFM) has been widely used to measure the mechanical properties of biological cells through indentations. In most of existing studies, the cell is supposed to be linear elastic within the small strain regime when analyzing the AFM indentation data. However, in experimental situations, the roles of large deformation and surface tension of cells should be taken into consideration. Here, we use the neo-Hookean model to describe the hyperelastic behavior of cells and investigate the influence of surface tension through finite element simulations. At large deformation, a correction factor, depending on the geometric ratio of indenter radius to cell radius, is introduced to modify the force-indent depth relation of classical Hertzian model. Moreover, when the indent depth is comparable with an intrinsic length defined as the ratio of surface tension to elastic modulus, the surface tension evidently affects the indentation response, indicating an overestimation of elastic modulus by the Hertzian model. The dimensionless-analysis-based theoretical predictions, which include both large deformation and surface tension, are in good agreement with our finite element simulation data. This study provides a novel method to more accurately measure the mechanical properties of biological cells and soft materials in AFM indentation experiments.

Interface Symbiotic Membrane Formation in Root Nodules of Medicago truncatula: the Role of Synaptotagmins MtSyt1, MtSyt2 and MtSyt3

Symbiotic bacteria (rhizobia) are maintained and conditioned to fix atmospheric nitrogen in infected cells of legume root nodules. Rhizobia are confined to the asymmetrical protrusions of plasma membrane (PM): infection threads (IT), cell wall-free unwalled droplets and symbiosomes. These compartments rapidly increase in surface and volume due to the microsymbiont expansion, and remarkably, the membrane resources of the host cells are targeted to interface membrane quite precisely. We hypothesized that the change in the membrane tension around the expanding microsymbionts creates a vector for membrane traffic toward the symbiotic interface. To test this hypothesis, we selected calcium sensors from the group of synaptotagmins: MtSyt1, Medicago truncatula homolog of AtSYT1 from Arabidopsis thaliana known to be involved in membrane repair, and two other homologs expressed in root nodules: MtSyt2 and MtSyt3. Here we show that MtSyt1, MtSyt2, and MtSyt3 are expressed in the expanding cells of the meristem, zone of infection and proximal cell layers of zone of nitrogen fixation (MtSyt1, MtSyt3). All three GFP-tagged proteins delineate the interface membrane of IT and unwalled droplets and create a subcompartments of PM surrounding these structures. The localization of MtSyt1 by EM immunogold labeling has shown the signal on symbiosome membrane and endoplasmic reticulum (ER). To specify the role of synaptotagmins in interface membrane formation, we compared the localization of MtSyt1, MtSyt3 and exocyst subunit EXO70i, involved in the tethering of post-Golgi secretory vesicles and operational in tip growth. The localization of EXO70i in root nodules and arbusculated roots was strictly associated with the tips of IT and the tips of arbuscular fine branches, but the distribution of synaptotagmins on membrane subcompartments was broader and includes lateral parts of IT, the membrane of unwalled droplets as well as the symbiosomes. The double silencing of synaptotagmins caused a delay in rhizobia release and blocks symbiosome maturation confirming the functional role of synaptotagmins.

IN CONCLUSION

synaptotagmin-dependent membrane fusion along with tip-targeted exocytosis is operational in the formation of symbiotic interface.

Oncogenic KRas-induced Increase in Fluid-phase Endocytosis is Dependent on N-WASP and is Required for the Formation of Pancreatic Preneoplastic Lesions

Fluid-phase endocytosis is a homeostatic process with an unknown role in tumor initiation. The driver mutation in pancreatic ductal adenocarcinoma (PDAC) is constitutively active KRasG12D, which induces neoplastic transformation of acinar cells through acinar-to-ductal metaplasia (ADM). We have previously shown that KRasG12D-induced ADM is dependent on RAC1 and EGF receptor (EGFR) by a not fully clarified mechanism. Using three-dimensional mouse and human acinar tissue cultures and genetically engineered mouse models, we provide evidence that (i) KRasG12D leads to EGFR-dependent sustained fluid-phase endocytosis (FPE) during acinar metaplasia; (ii) variations in plasma membrane tension increase FPE and lead to ADM in vitro independently of EGFR; and (iii) that RAC1 regulates ADM formation partially through actin-dependent regulation of FPE. In addition, mice with a pancreas-specific deletion of the Neural-Wiskott-Aldrich syndrome protein (N-WASP), a regulator of F-actin, have reduced FPE and impaired ADM emphasizing the in vivo relevance of our findings. This work defines a new role of FPE as a tumor initiating mechanism.

Mathematical modelling of stretch-induced membrane traffic in bladder umbrella cells

The bladder is a complex organ that is highly adaptive to its mechanical environment. The umbrella cells in the bladder uroepithelium are of particular interest: these cells actively change their surface area through exo- and endocytosis of cytoplasmic vesicles, and likely form a critical component in the mechanosensing process that communicates the sense of 'fullness' to the nervous system. In this paper we develop a first mechanical model for vesicle trafficking in umbrella cells in response to membrane tension during bladder filling. Recent experiments conducted on a disc of uroepithelial tissue motivate our model development. These experiments subject bladder tissue to fixed pressure differences and exhibit counterintuitive area changes. Through analysis of the mathematical model and comparison with experimental data in this setup, we gain an intuitive understanding of the biophysical processes involved and calibrate the vesicle trafficking rate parameters in our model. We then adapt the model to simulate in vivo bladder filling and investigate the potential effect of abnormalities in the vesicle trafficking machinery on bladder pathologies.

Acoustic Cluster Therapy (ACT) enhances the therapeutic efficacy of paclitaxel and Abraxane® for treatment of human prostate adenocarcinoma in mice

Acoustic cluster therapy (ACT) is a novel approach for ultrasound mediated, targeted drug delivery. In the current study, we have investigated ACT in combination with paclitaxel and Abraxane® for treatment of a subcutaneous human prostate adenocarcinoma (PC3) in mice. In combination with paclitaxel (12mg/kg given i.p.), ACT induced a strong increase in therapeutic efficacy; 120days after study start, 42% of the animals were in stable, complete remission vs. 0% for the paclitaxel only group and the median survival was increased by 86%. In combination with Abraxane® (12mg paclitaxel/kg given i.v.), ACT induced a strong increase in the therapeutic efficacy; 60days after study start 100% of the animals were in stable, remission vs. 0% for the Abraxane® only group, 120days after study start 67% of the animals were in stable, complete remission vs. 0% for the Abraxane® only group. For the ACT+Abraxane group 100% of the animals were alive after 120days vs. 0% for the Abraxane® only group. Proof of concept for Acoustic Cluster Therapy has been demonstrated; ACT markedly increases the therapeutic efficacy of both paclitaxel and Abraxane® for treatment of human prostate adenocarcinoma in mice.

Deciphering dynamics of clathrin-mediated endocytosis in a living organism

Current understanding of clathrin-mediated endocytosis (CME) dynamics is based on detection and tracking of fluorescently tagged clathrin coat components within cultured cells. Because of technical limitations inherent to detection and tracking of single fluorescent particles, CME dynamics is not characterized in vivo, so the effects of mechanical cues generated during development of multicellular organisms on formation and dissolution of clathrin-coated structures (CCSs) have not been directly observed. Here, we use growth rates of fluorescence signals obtained from short CCS intensity trace fragments to assess CME dynamics. This methodology does not rely on determining the complete lifespan of individual endocytic assemblies. Therefore, it allows for real-time monitoring of spatiotemporal changes in CME dynamics and is less prone to errors associated with particle detection and tracking. We validate the applicability of this approach to in vivo systems by demonstrating the reduction of CME dynamics during dorsal closure of Drosophila melanogaster embryos.

The Bioeffects Resulting from Prokaryotic Cells and Yeast Being Exposed to an 18 GHz Electromagnetic Field

The mechanisms by which various biological effects are triggered by exposure to an electromagnetic field are not fully understood and have been the subject of debate. Here, the effects of exposing typical representatives of the major microbial taxa to an 18 GHz microwave electromagnetic field (EMF)were studied. It appeared that the EMF exposure induced cell permeabilisation in all of the bacteria and yeast studied, while the cells remained viable (94% throughout the exposure), independent of the differences in cell membrane fatty acid and phospholipid composition. The resulting cell permeabilisation was confirmed by detection of the uptake of propidium iodine and 23 nm fluorescent silica nanospheres using transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM). Upon EMF exposure, the bacterial cell membranes are believed to become permeable through quasi-endocytosis processes. The dosimetry analysis revealed that the EMF threshold level required to induce the uptake of the large (46 nm) nanopsheres was between three and six EMF doses, with a specific absorption rate (SAR) of 3 kW/kg and 5 kW/kg per exposure, respectively, depending on the bacterial taxa being studied. It is suggested that the taxonomic affiliation and lipid composition (e.g. the presence of phosphatidyl-glycerol and/or pentadecanoic fatty acid) may affect the extent of uptake of the large nanospheres (46 nm). Multiple 18 GHz EMF exposures over a one-hour period induced periodic anomalous increases in the cell growth behavior of two Staphylococcus aureus strains, namely ATCC 25923 and CIP 65.8^T.

Mechanotransduction pulls the strings of matrix degradation at invadosome

Degradation of the extracellular matrix is a critical step of tumor cell invasion. Both protease-dependent and -independent mechanisms have been described as alternate processes in cancer cell motility. Interestingly, some effectors of protease-dependent degradation are focalized at invadosomes and are directly coupled with contractile and adhesive machineries composed of multiple mechanosensitive proteins. This review presents recent findings in protease-dependent mechanisms elucidating the ways the force affects extracellular matrix degradation by targeting protease expression and activity at invadosome. The aim is to highlight mechanosensing and mechanotransduction processes to direct the degradative activity at invadosomes, with the focus on membrane tension, proteases and mechanosensitive ion channels.

Mechanical Forces Reshape Differentiation Cues That Guide Cardiomyogenesis

Soluble morphogen gradients have long been studied in the context of heart specification and patterning. However, recent data have begun to challenge the notion that long-standing in vivo observations are driven solely by these gradients alone. Evidence from multiple biological models, from stem cells to ex vivo biophysical assays, now supports a role for mechanical forces in not only modulating cell behavior but also inducing it de novo in a process termed mechanotransduction. Structural proteins that connect the cell to its niche, for example, integrins and cadherins, and that couple to other growth factor receptors, either directly or indirectly, seem to mediate these changes, although specific mechanistic details are still being elucidated. In this review, we summarize how the wingless (Wnt), transforming growth factor-β, and bone morphogenetic protein signaling pathways affect cardiomyogenesis and then highlight the interplay between each pathway and mechanical forces. In addition, we will outline the role of integrins and cadherins during cardiac development. For each, we will describe how the interplay could change multiple processes during cardiomyogenesis, including the specification of undifferentiated cells, the establishment of heart patterns to accomplish tube and chamber formation, or the maturation of myocytes in the fully formed heart.

Membrane Tension Acts Through PLD2 and mTORC2 to Limit Actin Network Assembly During Neutrophil Migration

For efficient polarity and migration, cells need to regulate the magnitude and spatial distribution of actin assembly. This process is coordinated by reciprocal interactions between the actin cytoskeleton and mechanical forces. Actin polymerization-based protrusion increases tension in the plasma membrane, which in turn acts as a long-range inhibitor of actin assembly. These interactions form a negative feedback circuit that limits the magnitude of membrane tension in neutrophils and prevents expansion of the existing front and the formation of secondary fronts. It has been suggested that the plasma membrane directly inhibits actin assembly by serving as a physical barrier that opposes protrusion. Here we show that efficient control of actin polymerization-based protrusion requires an additional mechanosensory feedback cascade that indirectly links membrane tension with actin assembly. Specifically, elevated membrane tension acts through phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin nucleation. In the absence of this pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling suggests roles for both the direct (mechanical) and indirect (biochemical via PLD2 and mTORC2) feedback loops in organizing cell polarity and motility—the indirect loop is better suited to enable competition between fronts, whereas the direct loop helps spatially organize actin nucleation for efficient leading edge formation and cell movement. This circuit is essential for polarity, motility, and the control of membrane tension.

A mechanosensory biochemical cascade involving phospholipase D2 and mTORC2 coordinates physical forces and cytoskeletal rearrangements to allow efficient polarization and migration of neutrophils.

How cells regulate the size and number of their protrusions for efficient polarity and motility is a fundamental question in cell biology. We recently found that immune cells known as neutrophils use physical forces to regulate this process. Actin polymerization-based protrusion stretches the plasma membrane, and this increased membrane tension acts as a long-range inhibitor of actin-based protrusions elsewhere in the cell. Here we investigate how membrane tension limits protrusion. We demonstrate that the magnitude of actin network assembly in neutrophils is determined by a mechanosensory biochemical cascade that converts increases in membrane tension into decreases in protrusion. Specifically, we show that increasing plasma membrane tension acts through a pathway containing the phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin network assembly. Without this negative feedback pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling indicates that this feedback circuit is a favorable topology to enable competition between protrusions during neutrophil polarization. Our work shows how biochemical signals, physical forces, and the cytoskeleton can collaborate to generate large-scale cellular organization.

Mechanics and Activation of Unconventional Myosins

Many types of cellular motility are based on the myosin family of motor proteins ranging from muscle contraction to exo- and endocytosis, cytokinesis, cell locomotion or signal transduction in hearing. At the center of this wide range of motile processes lies the adaptation of the myosins for each specific mechanical task and the ability to coordinate the timing of motor protein mobilization and targeting. In recent years, great progress has been made in developing single molecule technology to characterize the diverse mechanical properties of the unconventional myosins. Here, we discuss the basic mechanisms and mechanical adaptations of unconventional myosins, and emerging principles regulating motor mobilization and targeting.

Multimodal micro, nano, and size conversion ultrasound agents for imaging and therapy

Ultrasound (US) is one of the most commonly used clinical imaging techniques. However, the use of US and US-based intravenous agents extends far beyond imaging. In particular, there has been a surge in the fabrication of multimodality US contrast agents and theranostic US agents for cancer imaging and therapy. The unique interaction of US waves with microscale and nanoscale agents has attracted much attention in the development of contrast agents and drug-delivery vehicles. The dimensions of the agent not only dictate how it behaves in vivo, but also how it interacts with US for imaging and drug delivery. Furthermore, these agents are also unique due to their ability to convert from the nanoscale to the microscale and vice versa, having imaging and therapeutic utility in both dimensions. Here, we review multimodality and multifunctional US-based agents, according to their size, and also highlight recent developments in size conversion US agents. WIREs Nanomed Nanobiotechnol 2016, 8:796-813. doi: 10.1002/wnan.1398 For further resources related to this article, please visit the WIREs website.

Unraveling the role of the Target of Rapamycin signaling in sphingolipid metabolism

Sphingolipids are important bioactive molecules that regulate basic aspects of cellular metabolism and physiology, including cell growth, adhesion, migration, senescence, apoptosis, endocytosis, and autophagy in yeast and higher eukaryotes. Since they have the ability to modulate the activation of several proteins and signaling pathways, variations in the relative levels of different sphingolipid species result in important changes in overall cellular functions and fate. Sphingolipid metabolism and their route of synthesis are highly conserved from yeast to mammalian cells. Studies using the budding yeast Saccharomyces cerevisiae have served in many ways to foster our understanding of sphingolipid dynamics and their role in the regulation of cellular processes. In the past decade, studies in S. cerevisiae have unraveled a functional association between the Target of Rapamycin (TOR) pathway and sphingolipids, showing that both TOR Complex 1 (TORC1) and TOR Complex 2 (TORC2) branches control temporal and spatial aspects of sphingolipid metabolism in response to physiological and environmental cues. In this review, we report recent findings in this emerging and exciting link between the TOR pathway and sphingolipids and implications in human health and disease.

A synaptic F-actin network controls otoferlin-dependent exocytosis in auditory inner hair cells

We show that a cage-shaped F-actin network is essential for maintaining a tight spatial organization of Cav1.3 Ca²⁺ channels at the synaptic ribbons of auditory inner hair cells. This F-actin network is also found to provide mechanosensitivity to the Cav1.3 channels when varying intracellular hydrostatic pressure. Furthermore, this F-actin mesh network attached to the synaptic ribbons directly influences the efficiency of otoferlin-dependent exocytosis and its sensitivity to intracellular hydrostatic pressure, independently of its action on the Cav1.3 channels. We propose a new mechanistic model for vesicle exocytosis in auditory hair cells where the rate of vesicle recruitment to the ribbons is directly controlled by a synaptic F-actin network and changes in intracellular hydrostatic pressure.

DOI:http://dx.doi.org/10.7554/eLife.10988.001

To hear a sound, the pressure produced by sound waves must be converted into an electrical nerve signal. The cells inside the ear that perform this transformation are called hair cells, which are so named because they have hundreds of hair-like structures on their upper surface. Pressure from sound waves causes movements in the inner ear that bend these ‘hairs’. This causes the hair cells to release chemical signals to neighboring nerve cell terminals that ultimately transmit information about the sound to the brain.

The chemical signals are stored inside the hair cells in bubble-like compartments called vesicles. To release the chemicals from the cell, the vesicles merge with the membrane that surrounds the hair cell. Most cells that communicate in this way are limited in how long they can transmit such messages. However, hair cells can continuously fuse vesicles to the membrane even when a sound lasts for a long time. This suggests that the hair cells have a different way of producing vesicles and getting them to the membrane than other cell types.

Inside the hair cells, vesicles are stored in regions called active zones. Each active zone contains a “ribbon” (attached to which are hundreds of vesicles) and also ion channels that allow calcium ions to flow into the cell. (An increase in calcium ion concentration inside the cell is necessary for the vesicle to fuse with the cell membrane and so release its chemical content). Now, Vincent et al. show that in hair cells, a cage-like network made from a protein called actin surrounds each active zone. This network helps to position the calcium ion channels. Treating the hair cells with a compound that disorganized the actin networks speed up the process of vesicle movement, which suggests that the actin network also controls the rate at which vesicles reach the membrane.

Next, it will be important to identify how the actin network interacts with other molecules that help vesicles to release their contents; in particular a protein called otoferlin, which is thought to act as a calcium ion sensor.

DOI:http://dx.doi.org/10.7554/eLife.10988.002

Ezrin is a Major Regulator of Membrane Tension in Epithelial Cells

Plasma membrane tension is responsible for a variety of cellular functions such as motility, cell division, and endocytosis. Since membrane tension is dominated by the attachment of the actin cortex to the inner leaflet of the plasma membrane, we investigated the importance of ezrin, a major cross-linker of the membrane-cytoskeleton interface, for cellular mechanics of confluent MDCK II cells. For this purpose, we carried out ezrin depletion experiments and also enhanced the number of active ezrin molecules at the interface. Mechanical properties were assessed by force indentation experiments followed by membrane tether extraction. PIP₂ micelles were injected into individual living cells to reinforce the linkage between plasma membrane and actin-cortex, while weakening of this connection was reached by ezrin siRNA and administration of the inhibitors neomycin and NSC 668394, respectively. We observed substantial stiffening of cells and an increase in membrane tension after addition of PIP₂ micelles. In contrast, reduction of active ezrin led to a decrease of membrane tension accompanied by loss of excess surface area, increase in cortical tension, remodelling of actin cytoskeleton, and reduction of cell height. The data confirm the importance of the ezrin-mediated connection between plasma membrane and cortex for cellular mechanics and cell morphology.

Mechanosensitive activation of CFTR by increased cell volume and hydrostatic pressure but not shear stress

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl(-) channel that is essential for electrolyte and fluid homeostasis. Preliminary evidence indicates that CFTR is a mechanosensitive channel. In lung epithelia, CFTR is exposed to different mechanical forces such as shear stress (Ss) and membrane distention. The present study questioned whether Ss and/or stretch influence CFTR activity (wild type, ∆F508, G551D). Human CFTR (hCFTR) was heterologously expressed in Xenopus oocytes and the response to the mechanical stimulus and forskolin/IBMX (FI) was measured by two-electrode voltage-clamp experiments. Ss had no influence on hCFTR activity. Injection of an intracellular analogous solution to increase cell volume alone did not affect hCFTR activity. However, hCFTR activity was augmented by injection after pre-stimulation with FI. The response to injection was similar in channels carrying the common mutations ∆F508 and G551D compared to wild type hCFTR. Stretch-induced CFTR activation was further assessed in Ussing chamber measurements using Xenopus lung preparations. Under control conditions increased hydrostatic pressure (HP) decreased the measured ion current including activation of a Cl(-) secretion that was unmasked by the CFTR inhibitor GlyH-101. These data demonstrate activation of CFTR in vitro and in a native pulmonary epithelium in response to mechanical stress. Mechanosensitive regulation of CFTR is highly relevant for pulmonary physiology that relies on ion transport processes facilitated by pulmonary epithelial cells.

Elastic properties of epithelial cells probed by atomic force microscopy

Cellular mechanics plays a crucial role in many biological processes such as cell migration, cell growth, embryogenesis, and oncogenesis. Epithelia respond to environmental cues comprising biochemical and physical stimuli through defined changes in cell elasticity. For instance, cells can differentiate between certain properties such as viscoelasticity or topography of substrates by adapting their own elasticity and shape. A living cell is a complex viscoelastic body that not only exhibits a shell architecture composed of a membrane attached to a cytoskeleton cortex but also generates contractile forces through its actomyosin network. Here we review cellular mechanics of single cells in the context of epithelial cell layers responding to chemical and physical stimuli. This article is part of a Special Issue entitled: Mechanobiology.

18 GHz electromagnetic field induces permeability of Gram-positive cocci

The effect of electromagnetic field (EMF) exposures at the microwave (MW) frequency of 18 GHz, on four cocci, Planococcus maritimus KMM 3738, Staphylococcus aureus CIP 65.8(T), S. aureus ATCC 25923 and S. epidermidis ATCC 14990(T), was investigated. We demonstrate that exposing the bacteria to an EMF induced permeability in the bacterial membranes of all strains studied, as confirmed directly by transmission electron microscopy (TEM), and indirectly via the propidium iodide assay and the uptake of silica nanospheres. The cells remained permeable for at least nine minutes after EMF exposure. It was shown that all strains internalized 23.5 nm nanospheres, whereas the internalization of the 46.3 nm nanospheres differed amongst the bacterial strains (S. epidermidis ATCC 14990(T) ~  0%; Staphylococcus aureus CIP 65.8(T) S. aureus ATCC 25923, ~40%; Planococcus maritimus KMM 3738, ~ 80%). Cell viability experiments indicated that up to 84% of the cells exposed to the EMF remained viable. The morphology of the bacterial cells was not altered, as inferred from the scanning electron micrographs, however traces of leaked cytosolic fluids from the EMF exposed cells could be detected. EMF-induced permeabilization may represent an innovative, alternative cell permeability technique for applications in biomedical engineering, cell drug delivery and gene therapy.

The urothelium of a hibernator: the American black bear

The American black bear undergoes a 3-5 month winter hibernation during which time bears do not eat, drink, defecate, or urinate. During hibernation renal function (GFR) is 16-50% of normal but urine is reabsorbed across the urinary bladder (UB) urothelium thus enabling metabolic recycling of all urinary constituents. To elucidate the mechanism(s) whereby urine is reabsorbed, we examined the UBs of five nonhibernating wild bears using light, electron (EM), and confocal immunofluorescent (IF) microscopy-concentrating on two components of the urothelial permeability barrier - the umbrella cell apical membranes and tight junctions (TJ). Bear UB has the same tissue layers (serosa, muscularis, lamina propria, urothelia) and its urothelia has the same cell layers (basal, intermediate, umbrella cells) as other mammalians. By EM, the bear apical membrane demonstrated a typical mammalian scalloped appearance with hinge and plaque regions - the latter containing an asymmetric trilaminar membrane and, on IF, uroplakins Ia, IIIa, and IIIb. The umbrella cell TJs appeared similar to those in other mammals and also contained TJ proteins occludin and claudin - 4, and not claudin -2. Thus, we were unable to demonstrate urothelial apical membrane or TJ differences between active black bears and other mammals. Expression and localization of UT-B, AQP-1 and -3, and Na(+), K(+)-ATPase on bear urothelial membranes was similar to that of other mammals. Similar studies of urothelia of hibernating bears, including evaluation of the apical membrane lipid bilayer and GAGs layer are warranted to elucidate the mechanism(s) whereby hibernating bears reabsorb their daily urine output and thus ensure successful hibernation.

Effect of hyperoxic and hyperbaric conditions on the adenosinergic pathway and CD26 expression in rat

The nucleoside adenosine acts on the nervous and cardiovascular systems via the A2A receptor (A2AR). In response to oxygen level in tissues, adenosine plasma concentration is regulated in particular via its synthesis by CD73 and via its degradation by adenosine deaminase (ADA). The cell-surface endopeptidase CD26 controls the concentration of vasoactive and antioxidant peptides and hence regulates the oxygen supply to tissues and oxidative stress response. Although overexpression of adenosine, CD73, ADA, A2AR, and CD26 in response to hypoxia is well documented, the effects of hyperoxic and hyperbaric conditions on these elements deserve further consideration. Rats and a murine Chem-3 cell line that expresses A2AR were exposed to 0.21 bar O2, 0.79 bar N2 (terrestrial conditions; normoxia); 1 bar O2 (hyperoxia); 2 bar O2 (hyperbaric hyperoxia); 0.21 bar O2, 1.79 bar N2 (hyperbaria). Adenosine plasma concentration, CD73, ADA, A2AR expression, and CD26 activity were addressed in vivo, and cAMP production was addressed in cellulo. For in vivo conditions, 1) hyperoxia decreased adenosine plasma level and T cell surface CD26 activity, whereas it increased CD73 expression and ADA level; 2) hyperbaric hyperoxia tended to amplify the trend; and 3) hyperbaria alone lacked significant influence on these parameters. In the brain and in cellulo, 1) hyperoxia decreased A2AR expression; 2) hyperbaric hyperoxia amplified the trend; and 3) hyperbaria alone exhibited the strongest effect. We found a similar pattern regarding both A2AR mRNA synthesis in the brain and cAMP production in Chem-3 cells. Thus a high oxygen level tended to downregulate the adenosinergic pathway and CD26 activity. Hyperbaria alone affected only A2AR expression and cAMP production. We discuss how such mechanisms triggered by hyperoxygenation can limit, through vasoconstriction, the oxygen supply to tissues and the production of reactive oxygen species.

In vitro investigation of silica nanoparticle uptake into human endothelial cells under physiological cyclic stretch

BACKGROUND

In general the prediction of the toxicity and therapeutic efficacy of engineered nanoparticles in humans is initially determined using in vitro static cell culture assays. However, such test systems may not be sufficient for testing nanoparticles intended for intravenous application. Once injected, these nanoparticles are caught up in the blood stream in vivo and are therefore in continuous movement. Physical forces such as shear stress and cyclic stretch caused by the pulsatile blood flow are known to change the phenotype of endothelial cells which line the luminal side of the vasculature and thus may be able to affect cell-nanoparticle interactions.

METHODS

In this study we investigated the uptake of amorphous silica nanoparticles in primary endothelial cells (HUVEC) cultured under physiological cyclic stretch conditions (1 Hz, 5% stretch) and compared this to cells in a standard static cell culture system. The toxicity of varying concentrations was assessed using cell viability and cytotoxicity studies. Nanoparticles were also characterized for the induction of an inflammatory response. Changes to cell morphology was evaluated in cells by examining actin and PECAM staining patterns and the amounts of nanoparticles taken up under the different culture conditions by evaluation of intracellular fluorescence. The expression profile of 26 stress-related was determined by microarray analysis.

RESULTS

The results show that cytotoxicity to endothelial cells caused by silica nanoparticles is not significantly altered under stretch compared to static culture conditions. Nevertheless, cells cultured under stretch internalize fewer nanoparticles. The data indicate that the decrease of nanoparticle content in stretched cells was not due to the induction of cell stress, inflammation processes or an enhanced exocytosis but rather a result of decreased endocytosis.

CONCLUSIONS

In conclusion, this study shows that while the toxic impact of silica nanoparticles is not altered by stretch this dynamic model demonstrates altered cellular uptake of nanoparticles under physiologically relevant in vitro cell culture models. In particular for the development of nanoparticles for biomedical applications such improved in vitro cell culture models may play a pivotal role in the reduction of animal experiments and development costs.

Cargo sorting in the endocytic pathway: a key regulator of cell polarity and tissue dynamics

The establishment and maintenance of polarized plasma membrane domains is essential for cellular function and proper development of organisms. Epithelial cells polarize along two fundamental axes, the apicobasal and the planar, both depending on finely regulated protein trafficking mechanisms. Newly synthesized proteins destined for either surface domain are processed along the biosynthetic pathway and segregated into distinct subsets of transport carriers emanating from the trans-Golgi network or endosomes. This exocytic trafficking has been identified as essential for proper epithelial polarization. Accumulating evidence now reveals that endocytosis and endocytic recycling play an equally important role in epithelial polarization and the appropriate localization of key polarity proteins. Here, we review recent work in metazoan systems illuminating the connections between endocytosis, postendocytic trafficking, and cell polarity, both apicobasal and planar, in the formation of differentiated epithelial cells, and how these processes regulate tissue dynamics.

Acoustic behavior of microbubbles and implications for drug delivery

Ultrasound contrast agents are valuable in diagnostic ultrasound imaging, and they increasingly show potential for drug delivery. This review focuses on the acoustic behavior of flexible-coated microbubbles and rigid-coated microcapsules and their contribution to enhanced drug delivery. Phenomena relevant to drug delivery, such as non-spherical oscillations, shear stress, microstreaming, and jetting will be reviewed from both a theoretical and experimental perspective. Further, the two systems for drug delivery, co-administration and the microbubble as drug carrier system, are reviewed in relation to the microbubble behavior. Finally, future prospects are discussed that need to be addressed for ultrasound contrast agents to move from a pre-clinical tool into a clinical setting.

Plasma membrane phosphoinositide balance regulates cell shape during Drosophila embryo morphogenesis

The ratio of different phosphoinositide species coordinates actomyosin contractility and plasma membrane expansion during tissue morphogenesis, thus ensuring proper cell shape.

Remodeling of cell shape during morphogenesis is driven by the coordinated expansion and contraction of specific plasma membrane domains. Loss of this coordination results in abnormal cell shape and embryonic lethality. Here, we show that plasma membrane lipid composition plays a key role in coordinating plasma membrane contraction during expansion. We found that an increase in PI(4,5)P₂ levels caused premature actomyosin contraction, resulting in the formation of shortened cells. Conversely, acute depletion of PI(4,5)P₂ blocked plasma membrane expansion and led to premature actomyosin disassembly. PI(4,5)P₂-mediated contractility is counteracted by PI(3,4,5)P₃ and the zygotic gene bottleneck, which acts by limiting myosin recruitment during plasma membrane expansion. Collectively, these data support a model in which the ratio of PI(4,5)P₂/PI(3,4,5)P₃ coordinates actomyosin contractility and plasma membrane expansion during tissue morphogenesis, thus ensuring proper cell shape.

The effect of substrate elasticity and actomyosin contractility on different forms of endocytosis

Substrate mechanical properties have emerged as potent determinants of cell functions and fate. We here tested the hypothesis that different forms of endocytosis are regulated by the elasticity of the synthetic hydrogels cells are cultured on. Towards this objective, we quantified cell-associated fluorescence of the established endocytosis markers transferrin (Tf) and cholera toxin subunit B (CTb) using a flow-cytometry based protocol, and imaged marker internalization using microscopy techniques. Our results demonstrated that clathrin-mediated endocytosis of Tf following a 10-minute incubation with a fibroblast cell line was lower on the softer substrates studied (5 kPa) compared to those with elasticities of 40 and 85 kPa. This effect was cancelled after 1-hour incubation revealing that intracellular accumulation of Tf at this time point did not depend on substrate elasticity. Lipid-raft mediated endocytosis of CTb, on the other hand, was not affected by substrate elasticity in the studied range of time and substrate elasticity. The use of pharmacologic contractility inhibitors revealed inhibition of endocytosis for both Tf and CTb after a 10-minute incubation and a dissimilar effect after 1 hour depending on the inhibitor type. Further, the internalization of fluorescent NPs, used as model drug delivery systems, showed a dependence on substrate elasticity, while transfection efficiency was unaffected by it. Finally, an independence on substrate elasticity of Tf and CTb association with HeLa cells indicated that there are cell-type differences in this respect. Overall, our results suggest that clathrin-mediated but not lipid-raft mediated endocytosis is potentially influenced by substrate mechanics at the cellular level, while intracellular trafficking and accumulation show a more complex dependence. Our findings are discussed in the context of previous work on how substrate mechanics affect the fundamental process of endocytosis and highlight important considerations for future studies.

Synaptic activity slows vesicular replenishment at excitatory synapses of rat hippocampus

Short-term synaptic depression mainly reflects the depletion of the readily releasable pool (RRP) of quanta. Its dynamics, and especially the replenishment rate of the RRP, are still not well characterized in spite of decades of investigation. Main reason is that the vesicular storage and release system is treated as time-independent. If it is time-dependent all parameters thus estimated become problematic. Indeed the reports about how prolonged stimulation affects the dynamics are contradictory. To study this, we used patterned stimulation on the Schaeffer collateral fiber pathway and model-fitting of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The parameters of a vesicular storage and release model with two pools were estimated by minimizing the squared difference between the ESPC amplitudes and simulated model output. This yields the 'basic' parameters (release coupling, replenishment coupling and RRP size) that underlie the 'derived' and commonly used parameters (fractional release and replenishment rate). The fractional release increases when [Ca(++)]o is raised, whereas the replenishment rate is [Ca(++)]o independent. Fractional release rises because release coupling increases, and the RRP becomes less able to contain quanta. During prolonged stimulation, the fractional release remains generally unaltered, whereas the replenishment rate decreases down to ~10 % of its initial value with a decay time of ~15 s, and this decrease in the replenishment rate significantly contributes to synaptic depression. In conclusion, the fractional release is [Ca(++)]o-dependent and stimulation-independent, whereas the replenishment rate is [Ca(++)]o-independent and stimulation-dependent.