Hypertonicity triggers RhoA-dependent assembly of myosin-containing striated polygonal actin networks in endothelial cells

Volumetric Compression Shifts Rho GTPase Balance and Induces Mechanobiological Cell State Transition

During development and disease progression, cells are subject to osmotic and mechanical stresses that modulate cell volume, which fundamentally influences cell homeostasis and has been linked to a variety of cellular functions. It is not well understood how the mechanobiological state of cells is programmed by the interplay of intracellular organization and complex extracellular mechanics when stimulated by cell volume modulation. Here, by controlling cell volume via osmotic pressure, we evaluate physical phenotypes (including cell shape, morphodynamics, traction force, and extracellular matrix (ECM) remodeling) and molecular signaling (YAP), and we uncover fundamental transitions in active biophysical states. We demonstrate that volumetric compression shifts the ratiometric balance of Rho GTPase activities, thereby altering mechanosensing and cytoskeletal organization in a reversible manner. Specifically, volumetric compression controls cell spreading, adhesion formation, and YAP nuclear translocation, while maintaining cell contractile activity. Furthermore, we show that on physiologically relevant fibrillar collagen I matrices, which are highly non-elastic, cells exhibit additional modes of cell volume-dependent mechanosensing that are not observable on elastic substrates. Notably, volumetric compression regulates the dynamics of cell-ECM interactions and irreversible ECM remodeling via Rac-directed protrusion dynamics, at both the single-cell level and the multicellular level. Our findings support that cell volume is a master biophysical regulator and reveal its roles in cell mechanical state transition, cell-ECM interactions, and biophysical tissue programming.

The uptake of metal-organic frameworks: a journey into the cell

The application of metal-organic frameworks (MOFs) in drug delivery has advanced rapidly over the past decade, showing huge progress in the development of novel systems. Although a large number of versatile MOFs that can carry and release multiple compounds have been designed and tested, one of the main limitations to their translation to the clinic is the limited biological understanding of their interaction with cells and the way they penetrate them. This is a crucial aspect of drug delivery, as MOFs need to be able not only to enter into cells but also to release their cargo in the correct intracellular location. While small molecules can enter cells by passive diffusion, nanoparticles (NPs) usually require an energy-dependent process known as endocytosis. Importantly, the fate of NPs after being taken up by cells is dependent on the endocytic pathways they enter through. However, no general guidelines for MOF particle internalization have been established due to the inherent complexity of endocytosis as a mechanism, with several factors affecting cellular uptake, namely NP size and surface chemistry. In this review, we cover recent advances regarding the understanding of the mechanisms of uptake of nano-sized MOFs (nanoMOFs)s, their journey inside the cell, and the importance of biological context in their final fate. We examine critically the impact of MOF physicochemical properties on intracellular trafficking and successful cargo delivery. Finally, we highlight key unanswered questions on the topic and discuss the future of the field and the next steps for nanoMOFs as drug delivery systems.

Lung Cancer Induces NK Cell Contractility and Cytotoxicity Through Transcription Factor Nuclear Localization

Actomyosin-mediated cellular contractility is highly conserved for mechanotransduction and signalling. While this phenomenon has been observed in adherent cell models, whether/how contractile forces regulate the function of suspension cells like natural killer (NK) cells during cancer surveillance, is unknown. Here, we demonstrated in coculture settings that the evolutionarily conserved NK cell transcription factor, Eomes, undergoes nuclear shuttling during lung cancer cell surveillance. Biophysical and biochemical analyses revealed mechanistic enhancement of NK cell actomyosin-mediated contractility, which is associated with nuclear flattening, thus enabling nuclear entry of Eomes associated with enhanced NK cytotoxicity. We found that NK cells responded to the presumed immunosuppressive TGFβ in the NK-lung cancer coculture medium to sustain its intracellular contractility through myosin light chain phosphorylation, thereby promoting Eomes nuclear localization. Therefore, our results demonstrate that lung cancer cells provoke NK cell contractility as an early phase activation mechanism and that Eomes is a plausible mechano-responsive protein for increased NK cytotoxicity. There is scope for strategic application of actomyosin-mediated contractility modulating drugs ex vivo, to reinvigorate NK cells prior to adoptive cancer immunotherapy in vivo (177 words).

Investigation of the pathway dependent endocytosis of gold nanoparticles by surface-enhanced Raman scattering

Endocytosis is a critical mechanism providing not only internalization of biomacromolecular structures but also communication with the environment where cells reside. Due to being the first step at the interaction interface, the route of cellular uptake has a major role governing the intracellular destinations and behaviors of molecular and non-molecular species including nanoparticles. To this end, various methods employing variety of techniques are investigated. In this study, surface-enhanced Raman spectroscopy (SERS) based approach for the investigation of endocytosis of gold nanoparticles (AuNPs) is reported. Internalization pathways of AuNPs were examined by flow cytometry via specific inhibitors for each endocytosis pathway type using three model cell lines Beas-2b, A549 and PNT1A. Macropinocytosis was blocked by cytochalasin D (CytoD), clathrin mediated endocytosis (CME) by sucrose (Scr), and caveolae mediated endocytosis (CE) by filipin (Fil). The results showed that cell type dependent AuNPs internalization affects not only the response of the cells to the inhibitors but also the obtained SERS spectra. SERS spectra of PNT1A cells treated with inhibitors was influenced most. The inhibition of each endocytosis pathway significantly affected the SERS spectral pattern and the spectral changes in different endocytosis pathways were clearly discriminated from each other. This means that SERS can significantly contribute to the investigation of different endosomal pathways from single living cells without any disruption of the cells or labeling.

High salt medium activates RhoA/ROCK and downregulates eNOS expression via the upregulation of ADMA

Endothelial dysfunction has an important role in the development and progression of salt-sensitive hypertension. Asymmetric dimethylarginine (ADMA), which is an endogenous inhibitor of nitric oxide synthase (NOS), has been demonstrated to be involved in the pathophysiological processes of endothelial dysfunction and salt‑sensitive hypertension. However, it is currently unclear how high salt intake may induce these processes. The present study investigated the effects of high salt medium on ADMA, endothelial NOS (eNOS) and the Ras homolog gene family, member A (RhoA)/Rho-associated protein kinase (ROCK) pathway in the EA.hy926 umbilical vein cell line. The results demonstrated that high salt medium significantly increased the concentration of ADMA, the expression of protein arginine methyltransferase 1 (PRMT‑1) and RhoA, and the activity of ROCK, and downregulated the expression of eNOS. Knockdown of PRMT-1 with small interfering RNA (siRNA) significantly abrogated the aforementioned effects. These results indicated that ADMA has a key role in high salt‑mediated activation of the RhoA/ROCK pathway and inhibition of eNOS biosynthesis. siRNA‑PRMT‑1 may be considered a novel remedy for the treatment of endothelial dysfunction.

Search for inhibitors of endocytosis: Intended specificity and unintended consequences

We discuss here the variety of approaches that have been taken to inhibit different forms of endocytosis. Typically, both non-specific and specific chemical inhibitors of endocytosis are tried in order to “classify” entry of a new plasma membrane protein into one of the various types of endocytosis. This classification can be confirmed through genetic approaches of protein depletion or overexpression of mutants of known endocytosis machinery components. Although some new compounds have been designed to be selective in biochemical assays, we caution investigators to be alert to the unintended consequences that sometimes arise when these compounds are applied to intact cells.

Hyperosmotic stress regulates the distribution and stability of myocardin-related transcription factor, a key modulator of the cytoskeleton

Hyperosmotic stress initiates several adaptive responses, including the remodeling of the cytoskeleton. Besides maintaining structural integrity, the cytoskeleton has emerged as an important regulator of gene transcription. Myocardin-related transcription factor (MRTF), an actin-regulated coactivator of serum response factor, is a major link between the actin skeleton and transcriptional control. We therefore investigated whether MRTF is regulated by hyperosmotic stress. Here we show that hypertonicity induces robust, rapid, and transient translocation of MRTF from the cytosol to the nucleus in kidney tubular cells. We found that the hyperosmolarity-triggered MRTF translocation is mediated by the RhoA/Rho kinase (ROK) pathway. Moreover, the Rho guanine nucleotide exchange factor GEF-H1 is activated by hyperosmotic stress, and it is a key contributor to the ensuing RhoA activation and MRTF translocation, since siRNA-mediated GEF-H1 downregulation suppresses these responses. While the osmotically induced RhoA activation promotes nuclear MRTF accumulation, the concomitant activation of p38 MAP kinase mitigates this effect. Moderate hyperosmotic stress (600 mosM) drives MRTF-dependent transcription through the cis-element CArG box. Silencing or pharmacological inhibition of MRTF prevents the osmotic stimulation of CArG-dependent transcription and renders the cells susceptible to osmotic shock-induced structural damage. Interestingly, strong hyperosmolarity promotes proteasomal degradation of MRTF, concomitant with apoptosis. Thus, MRTF is an osmosensitive and osmoprotective transcription factor, whose intracellular distribution is regulated by the GEF-H1/RhoA/ROK and p38 pathways. However, strong osmotic stress destabilizes MRTF, concomitant with apoptosis, implying that hyperosmotically induced cell death takes precedence over epithelial-myofibroblast transition, a potential consequence of MRTF-mediated phenotypic reprogramming.

Gap junctions favor normal rat kidney epithelial cell adaptation to chronic hypertonicity

Upon hypertonic stress most often resulting from high salinity, cells need to balance their osmotic pressure by accumulating neutral osmolytes called compatible osmolytes like betaine, myo-inositol, and taurine. However, the massive uptake of compatible osmolytes is a slow process compared with other defense mechanisms related to oxidative or heat stress. This is especially critical for cycling cells as they have to double their volume while keeping a hospitable intracellular environment for the molecular machineries. Here we propose that clustered cells can accelerate the supply of compatible osmolytes to cycling cells via the transit, mediated by gap junctions, of compatible osmolytes from arrested to cycling cells. Both experimental results in epithelial normal rat kidney cells and theoretical estimations show that gap junctions indeed play a key role in cell adaptation to chronic hypertonicity. These results can provide basis for a better understanding of the functions of gap junctions in osmoregulation not only for the kidney but also for many other epithelia. In addition to this, we suggest that cancer cells that do not communicate via gap junctions poorly cope with hypertonic environments thus explaining the rare occurrence of cancer coming from the kidney medulla.

Rear polarization of the microtubule-organizing center in neointimal smooth muscle cells depends on PKCα, ARPC5, and RHAMM

Directed migration of smooth muscle cells (SMCs) from the media to the intima in arteries occurs during atherosclerotic plaque formation and during restenosis after angioplasty or stent application. The polarized orientation of the microtubule-organizing center (MTOC) is a key determinant of this process, and we therefore investigated factors that regulate MTOC polarity in vascular SMCs. SMCs migrating in vivo from the medial to the intimal layer of the rat carotid artery following balloon catheter injury were rear polarized, with the MTOC located posterior of the nucleus. In tissue culture, migrating neointimal cells maintained rear polarization, whereas medial cells were front polarized. Using phosphoproteomic screening and mass spectrometry, we identified ARPC5 and RHAMM as protein kinase C (PKC)-phosphorylated proteins associated with rear polarization of the MTOC in neointimal SMCs. RNA silencing of ARPC5 and RHAMM, PKC inhibition, and transfection with a mutated nonphosphorylatable ARPC5 showed that these proteins regulate rear polarization by organizing the actin and microtubule cytoskeletons in neointimal SMCs. Both ARPC5 and RHAMM, in addition to PKC, were required for migration of neointimal SMCs.

Inhibition of suicidal erythrocyte death by blebbistatin

Blebbistatin, a myosin II inhibitor, interferes with myosin-actin interaction and microtubule assembly. By influencing cytoskeletal dynamics blebbistatin counteracts apoptosis of several types of nucleated cells. Even though lacking nuclei and mitochondria, erythrocytes may undergo suicidal cell death or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include energy depletion and osmotic shock, which enhance cytosolic Ca(2+) activity with subsequent Ca(2+)-sensitive cell shrinkage and cell membrane scrambling. The present study explored the effect of blebbistatin on eryptosis. Cell membrane scrambling was estimated from binding of annexin V to phosphatidylserine at the erythrocyte surface, cell volume from forward scatter in fluorescence-activated cell sorting analysis and cytosolic Ca(2+) concentration from Fluo3 fluorescence. Exposure to blebbistatin on its own (1-50 μM) did not significantly modify cytosolic Ca(2+) concentration, forward scatter, or annexin V binding. Glucose depletion (48 h) was followed by a significant increase of Fluo3 fluorescence and annexin V binding, effects significantly blunted by blebbistatin (Fluo3 fluorescence ≥ 25 μM, annexin V binding ≥ 10 μM). Osmotic shock (addition of 550 mM sucrose) again significantly increased Fluo3 fluorescence and annexin binding, effects again significantly blunted by blebbistatin (Fluo3 fluorescence ≥ 25 μM, annexin V binding ≥ 25 μM). The present observations disclose a novel effect of blebbistatin, i.e., an influence on Ca(2+) entry and suicidal erythrocyte death following energy depletion and osmotic shock.

Seawater immersion depresses the migration of vascular endothelial cell and inhibits angiogenesis

AIM: To investigate the effects of seawater immersion on vascular endothelial cell migration and angiogenesis.

METHODS: A scratch wound model was established using ECV304 cells. Cells were divided into control group (immersed in normal saline) and experimental group (immersed in seawater). Cell migration was examined using the scratch wound healing recovery assay. A nude mouse xenograft model was developed by subcutaneous inoculation of esophageal squamous cells (ECO156) in nude mice. Tumors were also divided into control group (immersed in normal saline) and experimental group (immersed in seawater). Tumor volume and weight were measured, and tumor growth curves were plotted. Neovascular vessels surrounding the tumor were observed, and microvessel density was detected by immunohistochemical staining.

RESULTS: After seawater immersion, ECV304 cells lost their normal morphology, and showed obvious dehydration with the prolongation of immersion duration. Seventeen hours after wounding, the scratch wound in the control group had almost completely healed, while that in the experiment group was still obvious, suggesting that the healing of scratch wound in the experiment group was delayed significantly and the migration of cells was depressed. The microvessel density in the experiment group was significantly lower than that in the control group (1.20 ± 0.44 vs 3.20 ± 0.83, P < 0.05). Compared with the control group, tumors in the experiment group grew slowly and had significantly smaller size (P < 0.05).

CONCLUSION: Seawater immersion may depress the migration of vascular endothelial cells and inhibit angiogenesis.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Hyperosmotic stress induces Rho/Rho kinase/LIM kinase-mediated cofilin phosphorylation in tubular cells: key role in the osmotically triggered F-actin response

Hyperosmotic stress induces cytoskeleton reorganization and a net increase in cellular F-actin, but the underlying mechanisms are incompletely understood. Whereas de novo F-actin polymerization likely contributes to the actin response, the role of F-actin severing is unknown. To address this problem, we investigated whether hyperosmolarity regulates cofilin, a key actin-severing protein, the activity of which is inhibited by phosphorylation. Since the small GTPases Rho and Rac are sensitive to cell volume changes and can regulate cofilin phosphorylation, we also asked whether they might link osmostress to cofilin. Here we show that hyperosmolarity induced rapid, sustained, and reversible phosphorylation of cofilin in kidney tubular (LLC-PK1 and Madin-Darby canine kidney) cells. Hyperosmolarity-provoked cofilin phosphorylation was mediated by the Rho/Rho kinase (ROCK)/LIM kinase (LIMK) but not the Rac/PAK/LIMK pathway, because 1) dominant negative (DN) Rho and DN-ROCK but not DN-Rac and DN-PAK inhibited cofilin phosphorylation; 2) constitutively active (CA) Rho and CA-ROCK but not CA-Rac and CA-PAK induced cofilin phosphorylation; 3) hyperosmolarity induced LIMK-2 phosphorylation, and 4) inhibition of ROCK by Y-27632 suppressed the hypertonicity-triggered LIMK-2 and cofilin phosphorylation.We thenexamined whether cofilin and its phosphorylation play a role in the hypertonicity-triggered F-actin changes. Downregulation of cofilin by small interfering RNA increased the resting F-actin level and eliminated any further rise upon hypertonic treatment. Inhibition of cofilin phosphorylation by Y-27632 prevented the hyperosmolarity-provoked F-actin increase. Taken together, cofilin is necessary for maintaining the osmotic responsiveness of the cytoskeleton in tubular cells, and the Rho/ROCK/LIMK-mediated cofilin phosphorylation is a key mechanism in the hyperosmotic stress-induced F-actin increase.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Screening of senescence-associated genes with specific DNA array reveals the role of IGFBP-3 in premature senescence of human diploid fibroblasts

Repeated exposures to sublethal concentrations of tert-butylhydroperoxide and ethanol trigger premature senescence of WI-38 human diploid fibroblasts. We found 16 replicative senescence-related genes with similar alterations in expression level in replicative senescence and two models of stress-induced premature senescence. Among these genes was IGFBP-3. Using a siRNA approach, we showed that IGFBP-3 regulates the appearance of several biomarkers of senescence after repeated exposures of WI-38 fibroblasts to tert-butylhydroperoxide and ethanol.

Pharmacological inhibition of endocytic pathways: is it specific enough to be useful?

Eukaryotic cells constantly form and internalize plasma membrane vesicles in a process known as endocytosis. Endocytosis serves a variety of housekeeping and specialized cellular functions, and it can be mediated by distinct molecular pathways. Among them, internalization via clathrin-coated pits, lipid raft/caveolae-mediated endocytosis and macropinocytosis/phagocytosis are the most extensively characterized. The major endocytic pathways are usually distinguished on the basis of their differential sensitivity to pharmacological/chemical inhibitors, although the possibility of nonspecific effects of such inhibitors is frequently overlooked. This review provides a critical evaluation of the selectivity of the most widely used pharmacological inhibitors of clathrin-mediated, lipid raft/caveolae-mediated endocytosis and macropinocytosis/phagocytosis. The mechanisms of actions of these agents are described with special emphasis on their reported side effects on the alternative internalization modes and the actin cytoskeleton. The most and the least-selective inhibitors of each major endocytic pathway are highlighted.

Vasodilatation of afferent arterioles and paradoxical increase of renal vascular resistance by furosemide in mice

Loop diuretics like furosemide have been shown to cause renal vasodilatation in dogs and humans, an effect thought to result from both a direct vascular dilator effect and from inhibition of tubuloglomerular feedback. In isolated perfused afferent arterioles preconstricted with angiotensin II or N(G)-nitro-L-arginine methyl ester, furosemide caused a dose-dependent increase of vascular diameter, but it was without effect in vessels from NKCC1-/- mice suggesting that inhibition of NKCC1 mediates dilatation in afferent arterioles. In the intact kidney, however, furosemide (2 mg/kg iv) caused a 50.5 +/- 3% reduction of total renal blood flow (RBF) and a 27% reduction of superficial blood flow (SBF) accompanied by a marked and immediate increase of tubular pressure and volume. At 10 mg/kg, furosemide reduced RBF by 60.4 +/- 2%. Similarly, NKCC1-/- mice responded to furosemide with a 45.4% decrease of RBF and a 29% decrease of SBF. Decreases in RBF and SBF and increases of tubular pressure by furosemide were ameliorated by renal decapsulation. In addition, pretreatment with candesartan (2 mg/kg) or indomethacin (5 mg/kg) attenuated the reduction of RBF and peak urine flows caused by furosemide. Our data indicate that furosemide, despite its direct vasodilator potential in isolated afferent arterioles, causes a marked increase in flow resistance of the vascular bed of the intact mouse kidney. We suggest that generation of angiotensin II and/or a vasoconstrictor prostaglandin combined with compression of peritubular capillaries by the expanding tubular compartment are responsible for the reduction of RBF in vivo.