Modulation of membrane traffic by mechanical stimuli

Fluid mechanics based classification of the respiratory efficiency of several nasal cavities

The flow in the human nasal cavity is of great importance to understand rhinologic pathologies like impaired respiration or heating capabilities, a diminished sense of taste and smell, and the presence of dry mucous membranes. To numerically analyze this flow problem a highly efficient and scalable Thermal Lattice-BGK (TLBGK) solver is used, which is very well suited for flows in intricate geometries. The generation of the computational mesh is completely automatic and highly parallelized such that it can be executed efficiently on High Performance Computers (HPCs). An evaluation of the functionality of nasal cavities is based on an analysis of pressure drop, secondary flow structures, wall-shear stress distributions, and temperature variations from the nostrils to the pharynx. The results of the flow fields of three completely different nasal cavities allow their classification into ability groups and support the a priori decision process on surgical interventions.

Mechanical forces and feedbacks in cell motility

Cell movement is driven by a self-organized assembly of numerous actin polymers and accessory proteins surrounded by a flexible membrane. While the identity of the molecular components involved is largely known, we are still far from understanding how this enormous ensemble of molecules self-organizes into a dynamic motile cell. A great deal of work in the field has focused on the role of biochemical signaling in establishing and maintaining cellular organization. More recently, mechanical forces and feedbacks have emerged as equally important contributors to the large-scale organization of motile cells. Here we review recent progress in the field, focusing on processes related to the actin cytoskeleton and its interplay with the cell membrane.

Membrane tension in rapidly moving cells is determined by cytoskeletal forces

BACKGROUND

Membrane tension plays an essential role in cell motility. The load imposed by the tensed membrane restrains actin polymerization, promotes rear retraction, and influences membrane transport. Moreover, membrane tension is crucial for large-scale coordination of cell boundary dynamics. Despite its importance, little is known about how membrane tension is set and regulated in cells. The prevailing hypothesis is that membrane tension is largely controlled by membrane-cytoskeleton adhesion and/or changes in membrane area.

RESULTS

In this work, we measure the apparent membrane tension in rapidly moving fish epithelial keratocytes under normal and perturbed conditions with a tether-pulling assay. We find that enlargement of the cell surface area by fusion with giant unilamellar vesicles (GUVs) has only minor effects on membrane tension and on cell movement. However, modulation of the cytoskeletal forces has a substantial influence on tension: reduction of the actin-pushing forces along the cell's leading edge leads to a significant decrease in membrane tension, whereas increase of the strength of adhesion and/or decrease of myosin-induced contraction leads to higher tension.

CONCLUSIONS

We find that the membrane tension in rapidly moving keratocytes is primarily determined by a mechanical force balance between the cell membrane and cytoskeletal forces. Our results highlight the role of membrane tension as a global mechanical regulator of cell behavior.

Mechanisms of the ultrasound-mediated intracellular delivery of liposomes and dextrans

The mechanism involved in the ultrasoundenhanced intracellular delivery of fluorescein-isothiocyanate (FITC)-dextran (molecular weight 4 to 2000 kDa) and liposomes containing doxorubicin (Dox) was studied using HeLa cells and an ultrasound transducer at 300 kHz, varying the acoustic power. The cellular uptake and cell viability were measured using flow cytometry and confocal microscopy. The role of endocytosis was investigated by inhibiting clathrin- and caveolae-mediated endocytosis, as well as macropinocytosis. Microbubbles were found to be required during ultrasound treatment to obtain enhanced cellular uptake. The percentage of cells internalizing Dox and dextran increased with increasing mechanical index. Confocal images and flow cytometric analysis indicated that the liposomes were disrupted extracellularly and that released Dox was taken up by the cells. The percentage of cells internalizing dextran was independent of the molecular weight of dextrans, but the amount of the small 4-kDa dextran molecules internalized per cell was higher than for the other dextrans. The inhibition of endocytosis during ultrasound exposure resulted in a significant decrease in cellular uptake of dextrans. Therefore, the improved uptake of Dox and dextrans may be a result of both sonoporation and endocytosis.

Membrane tension homeostasis of epithelial cells through surface area regulation in response to osmotic stress

Osmotic stress poses one of the most fundamental challenges to living cells. Particularly, the largely inextensible plasma membrane of eukaryotic cells easily ruptures under in-plane tension calling for sophisticated strategies to readily respond to osmotic stress. We describe how epithelial cells react and adapt mechanically to the exposure to hypotonic and hypertonic solutions in the context of a confluent monolayer. Site-specific indentation experiments in conjunction with tether pulling on individual cells have been carried out with an atomic force microscope to reveal spatio-temporal changes in membrane tension and surface area. We found that cells compensate for an increase in lateral tension due to hypoosmotic stress by sacrificing excess of membrane area stored in protrusions and invaginations such as microvilli and caveolae. At mild hypotonic conditions lateral tension increases partly compensated by surface are regulation, i.e. the cell sacrifices some of its membrane reservoirs. A loss of membrane-actin contacts occurs upon exposure to stronger hypotonic solutions giving rise to a drop in lateral tension. Tension release recovers on longer time scales by an increasing endocytosis, which efficiently removes excess membrane from the apical side to restore the initial pre-stress. Hypertonic solutions lead to shrinkage of cells and collapse of the apical membrane onto the cortex. Exposure to distilled water leads to stiffening of cells due to removal of excess surface area and tension increase due to elevated osmotic pressure across the plasma membrane.

Impact of diabetes mellitus on bladder uroepithelial cells

Diabetic bladder dysfunction (DBD), a prevalent complication of diabetes mellitus (DM), is characterized by a broad spectrum of symptoms including urinary urgency, frequency, and incontinence. As DBD is commonly diagnosed late, it is important to understand the chronic impact of DM on bladder tissues. While changes in bladder smooth muscle and innervation have been reported in diabetic patients, the impact of DM on the specialized epithelial lining of the urinary bladder, the urothelium (UT), is largely unknown. Quantitative polymerase chain reaction analysis and electron microscopy were used to evaluate UT gene expression and cell morphology 3, 9, and 20 wk following streptozotocin (STZ) induction of DM in female Sprague-Dawley rats compared with age-matched control tissue. Desquamation of superficial (umbrella) cells was noted at 9 wk DM, indicating a possible breach in barrier function. One causative factor may be metabolic burden due to chronic hyperglycemia, suggested by upregulation of the polyol pathway and glucose transport genes in DM UT. While superficial UT repopulation occurred by 20 wk DM, the phenotype was different, with significant upregulation of receptors associated with UT mechanosensation (transient receptor potential vanilloid subfamily member 1; TRPV1) and UT autocrine/paracrine signaling (acetylcholine receptors AChR-M2 and -M3, purinergic receptors P2X(2) and P2X(3)). Compromised barrier function and alterations in UT mechanosensitivity and cell signaling could contribute to bladder instability, hyperactivity, and altered bladder sensation by modulating activity of afferent nerve endings, which appose the urothelium. Our results show that DM impacts urothelial homeostasis and may contribute to the underlying mechanisms of DBD.

Persistent symmetry frustration in pollen tubes

Pollen tubes are extremely rapidly growing plant cells whose morphogenesis is determined by spatial gradients in the biochemical composition of the cell wall. We investigate the hypothesis (MP) that the distribution of the local mechanical properties of the wall, corresponding to the change of the radial symmetry along the axial direction, may lead to growth oscillations in pollen tubes. We claim that the experimentally observed oscillations originate from the symmetry change at the transition zone, where both intervening symmetries (cylindrical and spherical) meet. The characteristic oscillations between resonating symmetries at a given (constant) turgor pressure and a gradient of wall material constants may be identified with the observed growth-cycles in pollen tubes.

Use the force: membrane tension as an organizer of cell shape and motility

Many cell phenomena that involve shape changes are affected by the intrinsic deformability of the plasma membrane (PM). Far from being a passive participant, the PM is now known to physically, as well as biochemically, influence cell processes ranging from vesicle trafficking to actin assembly. Here we review current understanding of how changes in PM tension regulate cell shape and movement, as well as how cells sense PM tension.

Dietary protein affects urea transport across rat urothelia

Recent evidence suggests that regulated solute transport occurs across mammalian lower urinary tract epithelia (urothelia). To study the effects of dietary protein on net urothelial transport of urea, creatinine, and water, we used an in vivo rat bladder model designed to mimic physiological conditions. We placed groups of rats on 3-wk diets differing only by protein content (40, 18, 6, and 2%) and instilled 0.3 ml of collected urine in the isolated bladder of anesthetized rats. After 1 h dwell, retrieved urine volumes were unchanged, but mean urea nitrogen (UN) and creatinine concentrations fell 17 and 4%, respectively, indicating transurothelial urea and creatinine reabsorption. The fall in UN (but not creatinine) concentration was greatest in high protein (40%) rats, 584 mg/dl, and progressively less in rats receiving lower protein content: 18% diet, 224 mg/dl; 6% diet, 135 mg/dl; and 2% diet, 87 mg/dl. The quantity of urea reabsorbed was directly related to a urine factor, likely the concentration of urea in the instilled urine. In contrast, the percentage of instilled urea reabsorbed was greater in the two dietary groups receiving the lowest protein (26 and 23%) than in those receiving higher protein (11 and 9%), suggesting the possibility that a bladder/urothelial factor, also affected by dietary protein, may have altered bladder permeability. These findings demonstrate significant regulated urea transport across the urothelium, resulting in alteration of urine excreted by the kidneys, and add to the growing evidence that the lower urinary tract may play an unappreciated role in mammalian solute homeostasis.

Mechanical feedback between membrane tension and dynamics

The plasma membrane represents a physical inelastic barrier with a given area that adheres to the underlying cytoskeleton. The tension in the membrane physically affects cell functions and recent studies have highlighted that this physical signal orchestrates complex aspects of trafficking and motility. Despite its undeniable importance, little is known about the mechanisms by which membrane tension regulates cell functions or stimulates signals. The maintenance of membrane tension is also a matter of debate, particularly the nature of the membrane reservoir and trafficking pathways that buffer tension. In this review we discuss the importance of membrane area and of tension as a master integrator of cell functions, particularly for membrane traffic.

A subunit of eukaryotic translation initiation factor 2α-phosphatase (CreP/PPP1R15B) regulates membrane traffic

The constitutive reverter of eIF2α phosphorylation (CReP)/PPP1r15B targets the catalytic subunit of protein phosphatase 1 (PP1c) to phosphorylated eIF2α (p-eIF2α) to promote its dephosphorylation and translation initiation. Here, we report a novel role and mode of action of CReP. We found that CReP regulates uptake of the pore-forming Staphylococcus aureus α-toxin by epithelial cells. This function was independent of PP1c and translation, although p-eIF2α was involved. The latter accumulated at sites of toxin attack and appeared conjointly with α-toxin in early endosomes. CReP localized to membranes, interacted with phosphomimetic eIF2α, and, upon overexpression, induced and decorated a population of intracellular vesicles, characterized by accumulation of N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine (N-Rh-PE), a lipid marker of exosomes and intralumenal vesicles of multivesicular bodies. By truncation analysis, we delineated the CReP vesicle induction/association region, which comprises an amphipathic α-helix and is distinct from the PP1c interaction domain. CReP was also required for exocytosis from erythroleukemia cells and thus appears to play a broader role in membrane traffic. In summary, the mammalian traffic machinery co-opts p-eIF2α and CReP, regulators of translation initiation.

On the role of the difference in surface tensions involved in the allosteric regulation of NHE-1 induced by low to mild osmotic pressure, membrane tension and lipid asymmetry

The sodium-proton exchanger 1 (NHE-1) is a membrane transporter that exchanges Na(+) for H(+) ion across the membrane of eukaryotic cells. It is cooperatively activated by intracellular protons, and this allosteric regulation is modulated by the biophysical properties of the plasma membrane and related lipid environment. Consequently, NHE-1 is a mechanosensitive transporter that responds to osmotic pressure, and changes in membrane composition. The purpose of this study was to develop the relationship between membrane surface tension, and the allosteric balance of a mechanosensitive transporter such as NHE-1. In eukaryotes, the asymmetric composition of membrane leaflets results in a difference in surface tensions that is involved in the creation of a reservoir of intracellular vesicles and membrane buds contributing to buffer mechanical constraints. Therefore, we took this phenomenon into account in this study and developed a set of relations between the mean surface tension, membrane asymmetry, fluid phase endocytosis and the allosteric equilibrium constant of the transporter. We then used the experimental data published on the effects of osmotic pressure and membrane modification on the NHE-1 allosteric constant to fit these equations. We show here that NHE-1 mechanosensitivity is more based on its high sensitivity towards the asymmetry between the bilayer leaflets compared to mean global membrane tension. This compliance to membrane asymmetry is physiologically relevant as with their slower transport rates than ion channels, transporters cannot respond as high pressure-high conductance fast-gating emergency valves.

Selective stimulation of caveolae-mediated endocytosis by an osmotic polymannitol-based gene transporter

Controlling the cellular uptake mechanism and consequent intracellular route of polyplexes is important to improve the transfection efficiency of the non-viral gene delivery. Here, we report a new non-viral vector, polymannitol-based gene transporter (PMT), generated by crosslinking low molecular weight polyethylenimine with mannitol diacrylate, which has low cytotoxicity and good transfection efficiency. Interestingly, the uptake pathway of PMT/DNA complexes was shifted into caveolae-mediated endocytosis, avoiding lysosomal degradation. The mechanism of increased caveolae-mediated endocytosis of PMT/DNA complexes was found to be correlated with mechanosensing signal transduction by the hyperosmotic polymannitol part. Our results suggested that PMT, polymannitol-based gene transporter, is a safe and efficient gene delivery system with a well-modulated uptake pathway and intracellular route for gene therapy.

Use of ultrasound for non-invasive assessment of flow-mediated dilation

The pathological complications of atherosclerosis, namely heart attacks and strokes, remain the leading cause of mortality in the Western world. Preceding atherosclerosis is endothelial dysfunction. There is therefore interest in the application of non-invasive clinical tools to assess endothelial function. The flow-mediated dilation (FMD) test is the standard tool used to assess endothelial function. Reduced FMD is an early marker of atherosclerosis and has been noted for its capacity to predict future cardiovascular disease events. This review discusses the measurement of endothelial function using ultrasound, with a focus on the FMD technique.

Active reinforcement of externally imposed folding in amphibians embryonic tissues

Although the folding of epithelial layers is one of the most common morphogenetic events, the underlying mechanisms of this process are still poorly understood. We aimed to determine whether an artificial bending of an embryonic cell sheet, which normally remains flat, is reinforced and stabilized by intrinsic cell transformations. We observed both reinforcement and stabilization in double explants of blastocoel roof tissue from Xenopus early gastrula embryos. The reinforcement of artificial bending occurred over the course of a few hours and was driven by the gradual apical constriction and radial elongation of previously compressed cells situated at the bending arch of the concave layer of explant. Apical constriction was associated with actomyosin contraction and endocytosis-mediated engulfing of the apical cell membranes. Cooperative apical constrictions of the concave layer of cells produced a tensile force that extended over the entire surface of the explant and correlated with apical contraction of the concave side cells. In the explants taken from the anterior regions of the embryo, this reinforcement was more stable and the bending better expressed than in those taken from suprablastoporal areas. The morphogenetic role of cell responses to the bending force is discussed.

The importance of velocity acceleration to flow-mediated dilation

The validity of the flow-mediated dilation test has been questioned due to the lack of normalization to the primary stimulus, shear stress. Shear stress can be calculated using Poiseuille's law. However, little attention has been given to the most appropriate blood velocity parameter(s) for calculating shear stress. The pulsatile nature of blood flow exposes the endothelial cells to two distinct shear stimuli during the cardiac cycle: a large rate of change in shear at the onset of flow (velocity acceleration), followed by a steady component. The parameter typically entered into the Poiseuille's law equation to determine shear stress is time-averaged blood velocity, with no regard for flow pulsatility. This paper will discuss (1) the limitations of using Posieuille's law to estimate shear stress and (2) the importance of the velocity profile-with emphasis on velocity acceleration-to endothelial function and vascular tone.

Ablation of ceramide synthase 2 strongly affects biophysical properties of membranes

Little is known about the effects of altering sphingolipid (SL) acyl chain structure and composition on the biophysical properties of biological membranes. We explored the biophysical consequences of depleting very long acyl chain (VLC) SLs in membranes prepared from lipid fractions isolated from a ceramide synthase 2 (CerS2)-null mouse, which is unable to synthesize C22-C24 ceramides. We demonstrate that ablation of CerS2 has different effects on liver and brain, causing a significant alteration in the fluidity of the membrane and affecting the type and/or extent of the phases present in the membrane. These changes are a consequence of the depletion of VLC and unsaturated SLs, which occurs to a different extent in liver and brain. In addition, ablation of CerS2 causes changes in intrinsic membrane curvature, leading to strong morphological alterations that promote vesicle adhesion, membrane fusion, and tubule formation. Together, these results show that depletion of VLC-SLs strongly affects membrane biophysical properties, which may compromise cellular processes that critically depend on membrane structure, such as trafficking and sorting.

Mechanophysical stimulations of mucin secretion in cultures of nasal epithelial cells

Nasal epithelial cells secret mucins and are exposed in vivo to airflow-induced mechanophysical stresses, including wall shear stress (WSS), temperature, and humidity. In this work, human nasal epithelial cells cultured under air-liquid interface conditions were subjected to fields of airflow-induced oscillatory WSS at different temperature and humidity conditions. Changes in mucin secretion due to WSS were measured and the role of the cytoskeleton in mucin secretion was explored. Mucin secretion significantly increased in response to WSS in a magnitude-dependent manner with respect to static cultures and independently of the airflow temperature and humidity. In static cultures, mucin secretion decreased at high humidity with or without elevation of the temperature with respect to cultures at a comfortable climate. In cultures exposed to WSS, mucin secretion increased at high temperature with respect to cultures at comfortable climate conditions. The polymerization of actin microfilaments was shown to increase mucin secretion under WSS, whereas the dynamics of microtubule polymerization did not affect secretion. In conclusion, the data in this study show that mucin secretion is sensitive to oscillatory WSS as well as high temperature and humidity conditions.

Therapeutic ultrasound promotes plasmid DNA uptake by clathrin-mediated endocytosis

BACKGROUND

Ultrasound (US) has been widely used to improve the efficiency of nonviral vector transfection. The mechanism of plasmid uptake is usually attributed to sonoporation, although there is not clear evidence for this attribution. Based on our previous results, we hypothesized that other mechanisms, such as endocytosis, could be involved in this process.

METHODS

NIH3T3 cells were transfected with plasmid vector pEGFP-N3 (4.7 kb) using a therapeutic US without microbubbles. Bioeffects such as calcium influx, reactive oxygen species (ROS) generation and membrane potential alterations were accessed with fluorescent dyes in real-time by confocal microscopy after US insonation. Localization of labeled plasmid DNA in cells was also monitored with endocytosis markers using an immunofluorescence assay.

RESULTS

US at 2 W/cm(2) with a duty-cycle of 20% for 30 s resulted in approximately 40% transfection efficiency but, at 1 W/cm(2) , resulted in a very low level of transfection. Both the production of ROS and calcium influx were augmented during the insonation, although they were stopped soon after turning off US, with the exception of calcium influx with 1 W/cm(2) . US also changed the cell membrane potential to the hyperpolarization state, which returned to the normal state soon after insonation. Labeled plasmids DNA could be co-localized with clathrin-mediated endocytosis marker but not with caveolin-1.

CONCLUSIONS

The present data indicate that plasmid DNA uptake promoted by US should occur via clathrin-mediated endocytosis.

Constitutively active ezrin increases membrane tension, slows migration, and impedes endothelial transmigration of lymphocytes in vivo in mice

ERM (ezrin, radixin moesin) proteins in lymphocytes link cortical actin to plasma membrane, which is regulated in part by ERM protein phosphorylation. To assess whether phosphorylation of ERM proteins regulates lymphocyte migration and membrane tension, we generated transgenic mice whose T-lymphocytes express low levels of ezrin phosphomimetic protein (T567E). In these mice, T-cell number in lymph nodes was reduced by 27%. Lymphocyte migration rate in vitro and in vivo in lymph nodes decreased by 18% to 47%. Lymphocyte membrane tension increased by 71%. Investigations of other possible underlying mechanisms revealed impaired chemokine-induced shape change/lamellipod extension and increased integrin-mediated adhesion. Notably, lymphocyte homing to lymph nodes was decreased by 30%. Unlike most described homing defects, there was not impaired rolling or sticking to lymph node vascular endothelium but rather decreased migration across that endothelium. Moreover, decreased numbers of transgenic T cells in efferent lymph suggested defective egress. These studies confirm the critical role of ERM dephosphorylation in regulating lymphocyte migration and transmigration. Of particular note, they identify phospho-ERM as the first described regulator of lymphocyte membrane tension, whose increase probably contributes to the multiple defects observed in the ezrin T567E transgenic mice.

Albumin handling by renal tubular epithelial cells in a microfluidic bioreactor

Epithelial cells in the proximal tubule of the kidney reclaim and metabolize protein from the glomerular filtrate. Proteinuria, an overabundance of protein in the urine, affects tubular cell function and is a major factor in the progression of chronic kidney disease. By developing experimental systems to study tubular protein handling in a setting that simulates some of the environmental conditions of the kidney tubule in vivo, we can better understand how microenviromental conditions affect cellular protein handling to determine if these conditions are relevant in disease. To this end, we used two in vitro microfluidic models to evaluate albumin handling by renal proximal tubule cells. For the first system, cells were grown in a microfluidic channel and perfused with physiological levels of shear stress to evaluate the effect of mechanical stress on protein uptake. In the second system, a porous membrane was used to separate an apical and basolateral compartment to evaluate the fate of protein following cellular metabolism. Opossum kidney (OK) epithelial cells were exposed to fluorescently labeled albumin, and cellular uptake was determined by measuring the fluorescence of cell lysates. Confocal fluorescence microscopy was used to compare uptake in cells grown under flow and static conditions. Albumin processed by the cells was examined by size exclusion chromatography (SEC) and SDS-PAGE. Results showed that cellular uptake and/or degradation was significantly increased in cells exposed to flow compared to static conditions. This was confirmed by confocal microscopy. Size exclusion chromatography and SDS-PAGE showed that albumin was broken down into small molecular weight fragments and excreted by the cells. No trace of intact albumin was detectable by either SEC or SDS-PAGE. These results indicate that fluid shear stress is an important factor mediating cellular protein handling, and the microfluidic bioreactor provides a novel tool to investigate this process.

The role of the membrane confinement in the surface area regulation of cells

We propose a new in vitro system to study the mechanics of surface area regulation in cells, which takes into an account the spatial confinement of the cell membrane. By coupling a lipid bilayer to the strain-controlled deformation of an elastic sheet, we show that upon straining the supported lipid bilayer expands its surface area by absorbing adherent lipid vesicles and upon compression decreases its area by expelling lipid tubes out of its plane. The processes are reversible and closely resemble in vivo observations on shrinking cells. Our results suggest that the mechanics of the area regulation in cells is controlled primarily by the membrane tension and the effects of the membrane confinement.

Magnetically enhanced nucleic acid delivery. Ten years of magnetofection-progress and prospects

Nucleic acids carry the building plans of living systems. As such, they can be exploited to make cells produce a desired protein, or to shut down the expression of endogenous genes or even to repair defective genes. Hence, nucleic acids are unique substances for research and therapy. To exploit their potential, they need to be delivered into cells which can be a challenging task in many respects. During the last decade, nanomagnetic methods for delivering and targeting nucleic acids have been developed, methods which are often referred to as magnetofection. In this review we summarize the progress and achievements in this field of research. We discuss magnetic formulations of vectors for nucleic acid delivery and their characterization, mechanisms of magnetofection, and the application of magnetofection in viral and nonviral nucleic acid delivery in cell culture and in animal models. We summarize results that have been obtained with using magnetofection in basic research and in preclinical animal models. Finally, we describe some of our recent work and end with some conclusions and perspectives.

L-type calcium channel activity in osteoblast cells is regulated by the actin cytoskeleton independent of protein trafficking

Voltage-dependent L-type calcium channels (VDCC) play important roles in many cellular processes. The interaction of the actin cytoskeleton with the channel in nonexcitable cells is less well understood. We performed whole-cell patch-clamp surface biotinylation and calcium imaging on different osteoblast cells to determine channel kinetics, amplitude, surface abundance, and intracellular calcium, respectively. Patch-clamp studies showed that actin polymerization by phalloidin increased the peak current density of I (Ca), whereas actin depolymerization by cytochalasin D (CD) significantly decreased the current amplitude. This result is consistent with calcium imaging, which showed that CD significantly decreased Bay K8644-induced intracellular calcium increase. Surface biotinylation studies showed that CD is not able to affect the surface expression of the pore-forming subunit α(1C). Interestingly, application of CD caused a significantly negative shift in the steady-state inactivation kinetics of I (Ca). There were decreases in the voltage at half-maximal inactivation that changed in a dose-dependent manner. CD also reduced the effect of activated vitamin D(3) (1α,25-D3) on VDCC and intracellular calcium. We conclude that in osteoblasts the actin cytoskeleton affects α(1C) by altering the channel kinetic properties, instead of changing the surface expression, and it is able to regulate 1α,25-D3 signaling through VDCC. Our study provides a new insight into calcium regulation in osteoblasts, which are essential in many physiological functions of this cell.

Cell motility: the integrating role of the plasma membrane

The plasma membrane is of central importance in the motility process. It defines the boundary separating the intracellular and extracellular environments, and mediates the interactions between a motile cell and its environment. Furthermore, the membrane serves as a dynamic platform for localization of various components which actively participate in all aspects of the motility process, including force generation, adhesion, signaling, and regulation. Membrane transport between internal membranes and the plasma membrane, and in particular polarized membrane transport, facilitates continuous reorganization of the plasma membrane and is thought to be involved in maintaining polarity and recycling of essential components in some motile cell types. Beyond its biochemical composition, the mechanical characteristics of the plasma membrane and, in particular, membrane tension are of central importance in cell motility; membrane tension affects the rates of all the processes which involve membrane deformation including edge extension, endocytosis, and exocytosis. Most importantly, the mechanical characteristics of the membrane and its biochemical composition are tightly intertwined; membrane tension and local curvature are largely determined by the biochemical composition of the membrane and the biochemical reactions taking place; at the same time, curvature and tension affect the localization of components and reaction rates. This review focuses on this dynamic interplay and the feedbacks between the biochemical and biophysical characteristics of the membrane and their effects on cell movement. New insight on these will be crucial for understanding the motility process.

Temporary increase in plasma membrane tension coordinates the activation of exocytosis and contraction during cell spreading

Cell migration and spreading involve the coordination of membrane trafficking, actomyosin contraction, and modifications to plasma membrane tension and area. The biochemical or biophysical basis for this coordination is however unknown. In this study, we show that during cell spreading, lamellipodia protrusion flattens plasma membrane folds and blebs and, once the plasma membrane area is depleted, there is a temporary increase in membrane tension by over twofold that is followed by activation of exocytosis and myosin contraction. Further, an artificial increase in plasma membrane tension stopped lamellipodia protrusion and activated an exocytotic burst. Subsequent decrease in tension restored spreading with activation of contraction. Conversely, blebbistatin inhibition of actomyosin contraction resulted in an even greater increase in plasma membrane tension and exocytosis activation. This spatiotemporal synchronization indicates that membrane tension is the signal that coordinates membrane trafficking, actomyosin contraction, and plasma membrane area change. We suggest that cells use plasma membrane tension as a global physical parameter to control cell motility.

Preparation of solvent-free, pore-spanning lipid bilayers: modeling the low tension of plasma membranes

Plasma membrane tension, produced by the underlying cytoskeleton, governs many dynamic processes such as fusion, blebbing, exo- and endocytosis, cell migration, and adhesion. Here, a new protocol is introduced to model this intricate and often overlooked aspect of the plasma membrane. Lipid bilayers spanning pores of 600 nm radius were prepared by adsorption and spreading of giant unilamellar vesicles (GUVs) on moderately hydrophilic porous substrates prepared by gold-coating and subsequent self-assembly of a mercaptoethanol monolayer. Rupture of GUVs formed tens of micrometer sized pore-spanning membrane patches displaying low tension of σ ≤ 3.5 mN m(-1) and lateral diffusion constants of about 8 μm(2) s(-1). Site-specific force indentation experiments were performed to determine membrane tension as a function of lipid composition: for pure DOPC bilayers, a tension of 1.018 ± 0.014 mN m(-1) was measured, which was increased by the addition of cholesterol to 3.50 ± 0.15 mN m(-1). Compared to DOPC, POPC bilayers displayed a larger tension of 2.00 ± 0.09 mN m(-1). Addition and subsequent partitioning of 2-propanol was shown to significantly reduce the membrane tension as a function of its concentration.

Mechanics of surface area regulation in cells examined with confined lipid membranes

Cells are wrapped in inelastic membranes, yet they can sustain large mechanical strains by regulating their area. The area regulation in cells is achieved either by membrane folding or by membrane exo- and endocytosis. These processes involve complex morphological transformations of the cell membrane, i.e., invagination, vesicle fusion, and fission, whose precise mechanisms are still under debate. Here we provide mechanistic insights into the area regulation of cell membranes, based on the previously neglected role of membrane confinement, as well as on the strain-induced membrane tension. Commonly, the membranes of mammalian and plant cells are not isolated, but rather they are adhered to an extracellular matrix, the cytoskeleton, and to other cell membranes. Using a lipid bilayer, coupled to an elastic sheet, we are able to demonstrate that, upon straining, the confined membrane is able to regulate passively its area. In particular, by stretching the elastic support, the bilayer laterally expands without rupture by fusing adhered lipid vesicles; upon compression, lipid tubes grow out of the membrane plane, thus reducing its area. These transformations are reversible, as we show using cycles of expansion and compression, and closely reproduce membrane processes found in cells during area regulation. Moreover, we demonstrate a new mechanism for the formation of lipid tubes in cells, which is driven by the membrane lateral compression and may therefore explain the various membrane tubules observed in shrinking cells.

The role of mechanical forces in plant morphogenesis

The shape of an organism relies on a complex network of genetic regulations and on the homeostasis and distribution of growth factors. In parallel to the molecular control of growth, shape changes also involve major changes in structure, which by definition depend on the laws of mechanics. Thus, to understand morphogenesis, scientists have turned to interdisciplinary approaches associating biology and physics to investigate the contribution of mechanical forces in morphogenesis, sometimes re-examining theoretical concepts that were laid out by early physiologists. Major advances in the field have notably been possible thanks to the development of computer simulations and live quantitative imaging protocols in recent years. Here, we present the mechanical basis of shape changes in plants, focusing our discussion on undifferentiated tissues. How can growth be translated into a quantified geometrical output? What is the mechanical basis of cell and tissue growth? What is the contribution of mechanical forces in patterning?

Dynamics and regulation of endocytotic fission pores: role of calcium and dynamin

Although endocytosis involves the fission pore, a transient structure that produces the scission between vesicle and plasma membranes, the dimensions and dynamics of fission pores remain unclear. Here we report that the pore resistance changes proceed in three distinct phases: an initial phase where the resistance increases at 31.7 ± 2.9 GΩ/second, a slower linear phase with an overall slope of 11.7 ± 1.9 GΩ/second and a final increase in resistance more steeply (1189 ± 136 GΩ/second). The kinetics of these changes was calcium dependent. These sequential stages of the fission pore may be interpreted in terms of pore geometry as changes, first in pore diameter and then in pore length, according to which, before fission, the pore diameter consistently decreased to a value near 4 nm, whereas the pore length ranged between 20 and 300 nm. Dynamin, a mechanochemical GTPase, plays an important role in accelerating the fission event, preferentially in endocytotic vesicles of regular size, by increasing the rates of pore closure during the first and second phases of the fission pore, but hardly affected larger and longer-lived endocytotic events. These results suggest that fission pores are dynamic structures that form thin and long membrane necks regulated by intracellular calcium. Between calcium mediators, dynamin functions as a catalyst to increase the speed of single vesicle endocytosis.

Dynamical organization of the cytoskeletal cortex probed by micropipette aspiration

Bleb-based cell motility proceeds by the successive inflation and retraction of large spherical membrane protrusions ("blebs") coupled with substrate adhesion. In addition to their role in motility, cellular blebs constitute a remarkable illustration of the dynamical interactions between the cytoskeletal cortex and the plasma membrane. Here we study the bleb-based motions of Entamoeba histolytica in the constrained geometry of a micropipette. We construct a generic theoretical model that combines the polymerization of an actin cortex underneath the plasma membrane with the myosin-generated contractile stress in the cortex and the stress-induced failure of membrane-cortex adhesion. One major parameter dictating the cell response to micropipette suction is the stationary cortex thickness, controlled by actin polymerization and depolymerization. The other relevant physical parameters can be combined into two characteristic cortex thicknesses for which the myosin stress (i) balances the suction pressure and (ii) provokes membrane-cortex unbinding. We propose a general phase diagram for cell motions inside a micropipette by comparing these three thicknesses. In particular, we theoretically predict and experimentally verify the existence of saltatory and oscillatory motions for a well-defined range of micropipette suction pressures.

Enhancement of magnetic nanoparticle-mediated gene transfer to astrocytes by 'magnetofection': effects of static and oscillating fields

AIMS

To assess the feasibility of using magnetic nanoparticles (MNPs) to transfect astrocytes derived for transplantation and determine if transfection efficacy can be enhanced by static and oscillating magnetic fields.

METHODS

Astrocytes were transfected using MNPs functionalized with a plasmid encoding a reporter protein. Transfection efficacies were compared following application of static fields and a novel, oscillating array system at a range of frequencies. The transplantation potential of transfected cells was tested in organotypic cerebellar slice cultures.

RESULTS

Rat astrocytes can be efficiently transfected using MNPs with applied static/oscillating fields; the latter effect is frequency dependent. Transfected astrocytes could survive and differentiate following introduction into 3D neural tissue arrays.

CONCLUSION

MNP vectors can safely and effectively transfect rodent astrocytes and could form the basis of a 'multifunctional nanoplatform' for neural cell transplantation.

Single-channel properties of a stretch-sensitive chloride channel in the human mast cell line HMC-1

A stretch-activated (SA) Cl(-) channel in the plasma membrane of the human mast cell line HMC-1 was identified in outside-out patch-clamp experiments. SA currents, induced by pressure applied to the pipette, exhibited voltage dependence with strong outward rectification (55.1 pS at +100 mV and an about tenfold lower conductance at -100 mV). The probability of the SA channel being open (P (o)) also showed steep outward rectification and pressure dependence. The open-time distribution was fitted with three components with time constants of tau(1o) = 755.1 ms, tau(2o) = 166.4 ms, and tau(3o) = 16.5 ms at +60 mV. The closed-time distribution also required three components with time constants of tau(1c) = 661.6 ms, tau(2c) = 253.2 ms, and tau(3c) = 5.6 ms at +60 mV. Lowering extracellular Cl(-) concentration reduced the conductance, shifted the reversal potential toward chloride reversal potential, and decreased the P (o) at positive potentials. The SA Cl(-) currents were reversibly blocked by the chloride channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) but not by (Z)-1-(p-dimethylaminoethoxyphenyl)-1,2-diphenyl-1-butene (tamoxifen). Furthermore, in HMC-1 cells swelling due to osmotic stress, DIDS could inhibit the increase in intracellular [Ca(2+)] and degranulation. We conclude that in the HMC-1 cell line, the SA outward currents are mediated by Cl(-) influx. The SA Cl(-) channel might contribute to mast cell degranulation caused by mechanical stimuli or accelerate membrane fusion during the degranulation process.

Beyond neurons: Involvement of urothelial and glial cells in bladder function

AIM

The urothelium, or epithelial lining of the lower urinary tract (LUT), is likely to play an important role in bladder function by actively communicating with bladder nerves, smooth muscle, and cells of the immune and inflammatory systems. Recent evidence supports the importance of non-neuronal cells that may extend to both the peripheral and central processes of the neurons that transmit normal and nociceptive signals from the urinary bladder. Using cats diagnosed with a naturally occurring syndrome termed feline interstitial cystitis (FIC), we investigated whether changes in physiologic parameters occur within 3 cell types associated with sensory transduction in the urinary bladder: 1) the urothelium, 2) identified bladder dorsal root ganglion (DRG) neurons and 3) grey matter astrocytes in the lumbosacral (S1) spinal cord. As estrogen fluctuations may modulate the severity of many chronic pelvic pain syndromes, we also examined whether 17beta-estradiol (E2) alters cell signaling in rat urothelial cells.

RESULTS

We have identified an increase in nerve growth factor (NGF) and substance P (SP) in urothelium from FIC cats over that seen in urothelium from unaffected (control) bladders. The elevated NGF expression by FIC urothelium is a possible cause for the increased cell body size of DRG neurons from cats with FIC, reported in this study. At the level of the spinal cord, astrocytic GFAP immuno-intensity was significantly elevated and there was evidence for co-expression of the primitive intermediate filament, nestin (both indicative of a reactive state) in regions of the FIC S1 cord (superficial and deep dorsal horn, central canal and laminae V-VIl) that receive input from pelvic afferents. Finally, we find that E2 triggers an estrus-modifiable activation of p38 MAPK in rat urothelial cells. There were cyclic variations with E2-mediated elevation of p38 MAPK at both diestrus and estrus, and inhibition of p38 MAPK in proestrous urothelial cells.

CONCLUSION

Though urothelial cells are often viewed as bystanders in the processing of visceral sensation, these and other findings support the view that these cells function as primary transducers of some physical and chemical stimuli. In addition, the pronounced activation of spinal cord astrocytes in an animal model for bladder pain syndrome (BPS) may play an important role in the pain syndrome and open up new potential approaches for drug intervention.

Spatially resolved shear distribution in microfluidic chip for studying force transduction mechanisms in cells

Fluid shear stress has profound effects on cell physiology. Here we present a versatile microfluidic method capable of generating variable magnitudes, gradients, and different modes of shear flow, to study sensory and force transduction mechanisms in cells. The chip allows cell culture under spatially resolved shear flow conditions as well as study of cell response to shear flow in real-time. Using this chip, we studied the effects of chronic shear stress on cellular functions of Madin-Darby Canine Kidney (MDCK), renal epithelial cells. We show that shear stress causes reorganization of actin cytoskeleton, which suppresses flow-induced Ca(2+) response.

Boning up on Wolff's Law: mechanical regulation of the cells that make and maintain bone

Bone tissue forms and is remodeled in response to the mechanical forces that it experiences, a phenomenon described by Wolff's Law. Mechanically induced formation and adaptation of bone tissue is mediated by bone cells that sense and respond to local mechanical cues. In this review, the forces experienced by bone cells, the mechanotransduction pathways involved, and the responses elicited are considered. Particular attention is given to two cell types that have emerged as key players in bone mechanobiology: osteocytes, the putative primary mechanosensors in intact bone; and osteoprogenitors, the cells responsible for bone formation and recently implicated in ectopic calcification of cardiovascular tissues. Mechanoregulation of bone involves a complex interplay between these cells, their microenvironments, and other cell types. Thus, dissection of the role of mechanics in regulating bone cell fate and function, and translation of that knowledge to improved therapies, requires identification of relevant cues, multifactorial experimental approaches, and advanced model systems that mimic the mechanobiological environment.

Actomyosin contractility controls cell surface area of oligodendrocytes

BACKGROUND

To form myelin oligodendrocytes expand and wrap their plasma membrane multiple times around an axon. How is this expansion controlled?

RESULTS

Here we show that cell surface area depends on actomyosin contractility and is regulated by physical properties of the supporting matrix. Moreover, we find that chondroitin sulfate proteoglycans (CSPG), molecules associated with non-permissive growth properties within the central nervous system (CNS), block cell surface spreading. Most importantly, the inhibitory effects of CSPG on plasma membrane extension were completely prevented by treatment with inhibitors of actomyosin contractility and by RNAi mediated knockdown of myosin II. In addition, we found that reductions of plasma membrane area were accompanied by changes in the rate of fluid-phase endocytosis.

CONCLUSION

In summary, our results establish a novel connection between endocytosis, cell surface extension and actomyosin contractility. These findings open up new possibilities of how to promote the morphological differentiation of oligodendrocytes in a non-permissive growth environment. See related minireview by Bauer and ffrench-Constant: http://www.jbiol.com/content/8/8/78.

On a Path to Unfolding the Biological Mechanisms of Orthodontic Tooth Movement

Orthodontic forces deform the extracellular matrix and activate cells of the paradental tissues, facilitating tooth movement. Discoveries in mechanobiology have illuminated sequential cellular and molecular events, such as signal generation and transduction, cytoskeletal re-organization, gene expression, differentiation, proliferation, synthesis and secretion of specific products, and apoptosis. Orthodontists work in a unique biological environment, wherein applied forces engender remodeling of both mineralized and non-mineralized paradental tissues, including the associated blood vessels and neural elements. This review aims at identifying events that affect the sequence, timing, and significance of factors that determine the nature of the biological response of each paradental tissue to orthodontic force. The results of this literature review emphasize the fact that mechanoresponses and inflammation are both essential for achieving tooth movement clinically. If both are working in concert, orthodontists might be able to accelerate or decelerate tooth movement by adding adjuvant methods, whether physical, chemical, or surgical.

Reactive oxygen species mediate shear stress-induced fluid-phase endocytosis in vascular endothelial cells

To elucidate the role of shear stress in fluid-phase endocytosis of vascular endothelial cells (EC), we used a rotating-disk shearing apparatus to investigate the effects of shear stress on the uptake of lucifer yellow (LY) by cultured bovine aortic endothelial cells (BAEC). Exposure of EC to shear stress (area-mean value of 10 dynes/cm2) caused an increase in LY uptake that was abrogated by the antioxidant, N-acetyl-L-cysteine (NAC), the NADPH oxidase inhibitor, acetovanillone, and two inhibitors of protein kinase C (PKC), calphostin C and GF109203X. These results suggest that fluid-phase endocytosis is regulated by both reactive oxygen species (ROS) and PKC. Shear stress increased both ROS production and PKC activity in EC, and the increase in ROS was unaffected by calphostin C or GF109203X, whereas the activation of PKC was reduced by NAC and acetovanillone. We conclude that shear stress-induced increase in fluid-phase endocytosis is mediated via ROS generation followed by PKC activation in EC.

Control of cell membrane tension by myosin-I

All cell functions that involve membrane deformation or a change in cell shape (e.g., endocytosis, exocytosis, cell motility, and cytokinesis) are regulated by membrane tension. While molecular contacts between the plasma membrane and the underlying actin cytoskeleton are known to make significant contributions to membrane tension, little is known about the molecules that mediate these interactions. We used an optical trap to directly probe the molecular determinants of membrane tension in isolated organelles and in living cells. Here, we show that class I myosins, a family of membrane-binding, actin-based motor proteins, mediate membrane/cytoskeleton adhesion and thus, make major contributions to membrane tension. These studies show that class I myosins directly control the mechanical properties of the cell membrane; they also position these motor proteins as master regulators of cellular events involving membrane deformation.

Estimation of hydrodynamic shear stresses developed on human osteoblasts cultured on Ti-6Al-4V and strained by four point bending. Effects of mechanical loading to specific gene expression

The aim of the present investigation was to study the effects of mechanical strain on the orthopedic biomaterial Ti-6Al-4V-osteoblast interface, using an in vitro model. Homogeneous strain was applied to Human Bone Marrow derived Osteoblasts (HBMDOs) cultured on Ti-6Al-4V, at levels which are considered physiological, by a four-point bending mechanostimulatory system. A simple model for the estimation of maximum hydrodynamic shear stresses developed on cell culture layer and induced by nutrient medium flow during mechanical loading, as a function of the geometry of the culture plate and the load characteristics, is proposed. Shear stresses were lower than those which can elicit cell response. Mechanical loading was found that contributes to the regulation of osteoblast differentiation by influencing the expression of the osteoblast-specific transcription factor Cbfa1, both at the mRNA and protein level, and also the osteocalcin expression, whereas osteopontin gene expression was unaffected by mechanical loading at all experimental conditions.

Ultrasound and microbubble-targeted delivery of macromolecules is regulated by induction of endocytosis and pore formation

Contrast microbubbles in combination with ultrasound (US) are promising vehicles for local drug and gene delivery. However, the exact mechanisms behind intracellular delivery of therapeutic compounds remain to be resolved. We hypothesized that endocytosis and pore formation are involved during US and microbubble targeted delivery (UMTD) of therapeutic compounds. Therefore, primary endothelial cells were subjected to UMTD of fluorescent dextrans (4.4 to 500 kDa) using 1 MHz pulsed US with 0.22-MPa peak-negative pressure, during 30 seconds. Fluorescence microscopy showed homogeneous distribution of 4.4- and 70-kDa dextrans through the cytosol, and localization of 155- and 500-kDa dextrans in distinct vesicles after UMTD. After ATP depletion, reduced uptake of 4.4-kDa dextran and no uptake of 500-kDa dextran was observed after UMTD. Independently inhibiting clathrin- and caveolae-mediated endocytosis, as well as macropinocytosis significantly decreased intracellular delivery of 4.4- to 500-kDa dextrans. Furthermore, 3D fluorescence microscopy demonstrated dextran vesicles (500 kDa) to colocalize with caveolin-1 and especially clathrin. Finally, after UMTD of dextran (500 kDa) into rat femoral artery endothelium in vivo, dextran molecules were again localized in vesicles that partially colocalized with caveolin-1 and clathrin. Together, these data indicated uptake of molecules via endocytosis after UMTD. In addition to triggering endocytosis, UMTD also evoked transient pore formation, as demonstrated by the influx of calcium ions and cellular release of preloaded dextrans after US and microbubble exposure. In conclusion, these data demonstrate that endocytosis is a key mechanism in UMTD besides transient pore formation, with the contribution of endocytosis being dependent on molecular size.

Enhanced caveolae-mediated endocytosis by diagnostic ultrasound in vitro

The modulation of cellular endothelial permeability is a desirable goal for targeted delivery of labels and therapeutic macromolecules; the underlying mechanisms, however, remains poorly understood. Here, we hypothesize that a higher endothelial permeability may result as an outcome of selective enhancement of caveolar endocytosis by ultrasound (US), in the frequency and intensity range of current clinical diagnostic use. To assess the role of free radicals in this phenomenon, we exposed confluent human endothelial cells to pulsed diagnostic US for 30 min, with a mechanical index (MI) of 0.5 and 1.2, using a 1.6-MHz cardiac US scan, and endothelial cells not exposed to US were used as control. Here we show that pulsed diagnostic US with a MI of 1.2 (high mechanical index ultrasound [HMIUS]) were able to selectively enhance endothelial caveolar internalization of recombinant glutathione-S-transferase (GST)-Tat11-EGFP fusion protein (26 +/- 1 vs. 11.6 +/- 1 A.U, p < 0.001 vs. control), without disruption of plasma membrane integrity. Moreover, pulsed diagnostic US with a MI of 0.5 (low mechanical index ultrasound) did not increase caveolar endocytosis compared with control (11.4 +/- 1.2 vs. 11.6 +/- 1). Free-radical generation inhibitors, such as catalase and superoxide dismutase, reduced the HMIUS-induced caveolar internalization by a 49.29% factor; finally, HMIUS-induced caveolar endocytosis was found to be associated with a significant increase in the phosphorylation of tyr-14-caveolin1, ser1177-eNOS and Thr202/Tyr204-ERK(1/2) compared with control. These findings show how HMIUS irradiation of human endothelial cells cause a selective enhancement of caveolar-dependent permeability, partially mediated by free radicals generation, inducing a marked increase of phosphorylation of caveolar-related proteins. Thus, the use of diagnostic US could potentially be used as an adjuvant to drive caveolar traffic of extracellular peptides by using a higher level of US energy.