Actin-binding protein contributes to cell volume regulatory ion channel activation in melanoma cells

Should I shrink or should I grow: cell size changes in tissue morphogenesis

Cells change shape, move, divide, and die to sculpt tissues. Common to all these cell behaviours are cell size changes, which have recently emerged as key contributors to tissue morphogenesis. Cells can change their mass-the number of macromolecules they contain-or their volume-the space they encompass. Changes in cell mass and volume occur through different molecular mechanisms and at different timescales, slow for changes in mass and rapid for changes in volume. Therefore, changes in cell mass and cell volume, which are often linked, contribute to the development and shaping of tissues in different ways. Here, we review the molecular mechanisms by which cells can control and alter their size, and we discuss how changes in cell mass and volume contribute to tissue morphogenesis. The role that cell size control plays in developing embryos is only starting to be elucidated. Research on the signals that control cell size will illuminate our understanding of the cellular and molecular mechanisms that drive tissue morphogenesis.

Chilling induces unidirectional solute leak through the locust gut epithelia

Chill-susceptible insects, like the migratory locust, often die when exposed to low temperatures from an accumulation of tissue damage that is unrelated to freezing (chilling injury). Chilling injury is often associated with a loss of ion balance across the gut epithelia. It has recently been suggested that this imbalance is at least partly caused by a cold-induced disruption of epithelial barrier function. Here, we aimed to test this hypothesis in the migratory locust (Locustamigratoria). First, chill tolerance was quantified by exposing locusts to -2°C and recording chill coma recovery time and survival 24 h post-cold exposure. Longer exposure times significantly increased recovery time and caused injury and death. Ion-selective microelectrodes were also used to test for a loss of ion balance in the cold. We found a significant increase of haemolymph K+ and decrease of haemolymph Na+ concentration over time. Next, barrier failure along the gut was tested by monitoring the movement of an epithelial barrier marker (FITC-dextran) across the gut epithelia during exposure to -2°C. We found a significant increase in haemolymph FITC-dextran concentration over time in the cold when assayed in the mucosal to serosal direction. However, when tested in the serosal to mucosal direction, we saw minimal marker movement across the gut epithelia. This suggests that while cold-induced barrier disruption is present, it is apparently unidirectional. It is important to note that these data reveal only the phenomenon itself. The location of this leak as well as the underlying mechanisms remain unclear and require further investigation.

A thrombomodulin-like gene is crucial to the collective migration of epibolic blastomeres during germ layer formation and organogenesis in zebrafish

Background

Thrombomodulin (TM), an integral membrane protein, has long been known for its anticoagulant activity. Recent studies showed that TM displays multifaceted activities, including the involvement in cell adhesion and collective cell migration in vitro. However, whether TM contributes similarly to these biological processes in vivo remains elusive.

Methods

We adapted zebrafish, a prominent animal model for studying molecular/cellular activity, embryonic development, diseases mechanism and drug discovery, to examine how TM functions in modulating cell migration during germ layer formation, a normal and crucial physiological process involving massive cell movement in the very early stages of life. In addition, an in vivo assay was developed to examine the anti-hemostatic activity of TM in zebrafish larva.

Results

We found that zebrafish TM-b, a zebrafish TM-like protein, was expressed mainly in vasculatures and displayed anti-hemostatic activity. Knocking-down TM-b led to malformation of multiple organs, including vessels, heart, blood cells and neural tissues. Delayed epiboly and incoherent movement of yolk syncytial layer were also observed in early TM-b morphants. Whole mount immunostaining revealed the co-localization of TM-b with both actin and microtubules in epibolic blastomeres. Single-cell tracking revealed impeded migration of blastomeres during epiboly in TM-b-deficient embryos.

Conclusion

Our results showed that TM-b is crucial to the collective migration of blastomeres during germ layer formation. The structural and functional compatibility and conservation between zebrafish TM-b and mammalian TM support the properness of using zebrafish as an in vivo platform for studying the biological significance and medical use of TM.

Electronic supplementary material

The online version of this article (10.1186/s12929-019-0549-2) contains supplementary material, which is available to authorized users.

Mechanism for Regulation of Melanoma Cell Death via Activation of Thermo-TRPV4 and TRPV2

Background

Thermo-TRPs (temperature-sensitive transient receptor potential channels) belong to the TRP (transient receptor potential) channel superfamily. Emerging evidence implied that thermo-TRPs have been involved in regulation of cell fate in certain tumors. However, their distribution profiles and roles in melanoma remain incompletely understood.

Methods

Western blot and digital PCR approaches were performed to identify the distribution profiles of six thermo-TRPs. MTT assessment was employed to detect cell viability. Flow cytometry was applied to test cell cycle and apoptosis. Calcium imaging was used to determine the function of channels. Five cell lines, including one normal human primary epidermal melanocytes and two human malignant melanoma (A375, G361) and two human metastatic melanoma (A2058, SK-MEL-3) cell lines, were chosen for this research.

Results

In the present study, six thermo-TRPs including TRPV1/2/3/4, TRPA1, and TRPM8 were examined in human primary melanocytes and melanoma cells. We found that TRPV2/4, TRPA1, and TRPM8 exhibited ectopic distribution both in melanocytes and melanoma cells. Moreover, activation of TRPV2 and TRPV4 could lead to the decline of cell viability for melanoma A2058 and A375 cells. Subsequently, activation of TRPV2 by 2-APB (IC₅₀ = 150 μM) induced cell necrosis in A2058 cells, while activation of TRPV4 by GSK1016790A (IC₅₀ = 10 nM) enhanced apoptosis of A375 cells. Furthermore, TRPV4 mediated cell apoptosis of melanoma via phosphorylation of AKT and was involved in calcium regulation.

Conclusion

Overall, our studies revealed that TRPV4 and TRPV2 mediated melanoma cell death via channel activation and characterized the mechanism of functional TRPV4 ion channel in regulating AKT pathway driven antitumor process. Thus, they may serve as potential biomarkers for the prognosis and are targeted for the therapeutic use in human melanoma.

Astrocyte activation and reactive gliosis-A new target in stroke?

Stroke is an acute insult to the central nervous system (CNS) that triggers a sequence of responses in the acute, subacute as well as later stages, with prominent involvement of astrocytes. Astrocyte activation and reactive gliosis in the acute stage of stroke limit the tissue damage and contribute to the restoration of homeostasis. Astrocytes also control many aspects of neural plasticity that is the basis for functional recovery. Here, we discuss the concept of intermediate filaments (nanofilaments) and the complement system as two handles on the astrocyte responses to injury that both present attractive opportunities for novel treatment strategies modulating astrocyte functions and reactive gliosis.

Effects of Intermediate-Conductance Ca(2+)-Activated K(+) Channels on Human Endometrial Carcinoma Cells

The objective of this study was to investigate the effect of intermediate-conductance Ca(2+)-activated K(+) (KCa3.1) channels on the cell proliferation, cell cycle, apoptosis, migration, and invasion in endometrial cancer (EC) cells. Human EC cell lines HEC-1-A and Ishikawa were cultured in vitro and transfected with recombinant plasmid containing KCa3.1-targeting shRNA. RT-qPCR and Western blot were used to examine the mRNA and protein expression levels of KCa3.1 channels in transfected cells. In addition, the specific inhibitor of KCa3.1, TRAM-34, was used to examine the effect of KCa3.1 blockage on migration capacity and invasiveness of EC cells using transwell assay. Proliferation and apoptotic rates of EC cells transfected with KCa3.1 shRNA or treated with TRAM-34 were analyzed using MTT, BrdU incorporation assay, and flow cytometry. Expression of cell cycle proteins and metalloproteinase-2 (MMP-2) was evaluated by RT-qPCR and Western blotting. TRAM-34 treatment and KCa3.1 silencing using shRNA dramatically suppressed both the mRNA and protein expression of KCa3.1 channels (P < 0.01) compared with control groups. Blockage of KCa3.1 by TRAM-34 treatment and KCa3.1 shRNA transfection exerted inhibitory effect on cell growth of both EC cell lines, as demonstrated by increased cell population at G0-G1 phase and decreased cell population at S phase. However, both the treatments did not result in significant changes in the apoptotic rate (P > 0.05) compared to controls. Protein expressions of cyclin D1, cyclin E, and survivin were significantly decreased in the experimental groups comparing to control. We showed that TRAM-34 treatment led to significantly inhibited migration, invasion, and MMP-2 expression in HEC-1-A and Ishikawa cells, compared with the control group (P < 0.01). Blockage of KCa3.1 channel activity or expression inhibits cell proliferation and cell cycle progression without inducing apoptosis in EC cells. Moreover, TRAM-34 could reduce the ability of EC cells to migrate and invade, which might be related to reduced expression of MMP-2.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

The rate of hypo-osmotic challenge influences regulatory volume decrease (RVD) and mechanical properties of articular chondrocytes

OBJECTIVES

Osteoarthritis (OA) is associated with a gradual reduction in the interstitial osmotic pressure within articular cartilage. The aim of this study was to compare the effects of sudden and gradual hypo-osmotic challenge on chondrocyte morphology and biomechanics.

METHODS

Bovine articular chondrocytes were exposed to a reduction in extracellular osmolality from 327 to 153 mOsmol/kg applied either suddenly (<5 s) or gradually (over 180 min). Temporal changes in cell diameter and the existence of regulatory volume decrease (RVD) were quantified along with changes in cortical actin and chromatin condensation. The cellular viscoelastic mechanical properties were determined by micropipette aspiration.

RESULTS

In response to a sudden hypo-osmotic stress, 66% of chondrocytes exhibited an increase in diameter followed by RVD, whilst 25% showed no RVD. By contrast, cells exposed to gradual hypo-osmotic stress exhibited reduced cell swelling without subsequent RVD. There was an increase in the equilibrium modulus for cells exposed to sudden hypo-osmotic stress. However, gradual hypo-osmotic challenge had no effect on cell mechanical properties. This cell stiffening response to sudden hypo-osmotic challenge was abolished when actin organization was disrupted with cytochalasin D or RVD inhibited with REV5901. Both sudden and gradual hypo-osmotic challenge reduced cortical F-actin distribution and caused chromatin decondensation.

CONCLUSIONS

Sudden hypo-osmotic challenge increases chondrocyte mechanics by activation of RVD and interaction with the actin cytoskeleton. Moreover, the rate of hypo-osmotic challenge is shown to have a profound effect on chondrocyte morphology and biomechanics. This important phenomenon needs to be considered when studying the response of chondrocytes to pathological hypo-osmotic stress.

Tamoxifen inhibits migration of estrogen receptor-negative hepatocellular carcinoma cells by blocking the swelling-activated chloride current

Tamoxifen is a triphenylethylene non-steroidal antiestrogen anticancer agent. It also shows inhibitory effects on metastasis of estrogen receptor (EsR)-independent tumors, but the underlying mechanism is unclear. It was demonstrated in this study that, in EsR-negative and highly metastatic human hepatocellular carcinoma MHCC97H cells, tamoxifen-inhibited cell migration, volume-activated Cl(-) currents (I(Cl,vol)) and regulatory volume decrease (RVD) in a concentration-dependent manner with a similar IC(50). Analysis of the relationships between migration, I(Cl,vol) and RVD showed that cell migration was positively correlated with I(Cl,vol) and RVD. Knockdown of the expression of ClC-3 Cl(-) channel proteins by ClC-3 shRNA or siRNA inhibited I(Cl,vol), and cell migration, and these inhibitory effects could not be increased further by addition of tamoxifen in the medium. The results suggest that knockdown of ClC-3 expression may deplete the effects of tamoxifen; tamoxifen may inhibit cell migration by modulating I(Cl,vol) and cell volume. Moreover, tamoxifen decreased the activity of protein kinase C (PKC) and the effects were reversed by the PKC activator PMA. Activation of PKC by PMA could competitively downregulate the inhibitory effects of tamoxifen on I(Cl,vol). PMA promoted cell migration, and knockdown of ClC-3 expression by ClC-3 siRNA abolished the PMA effect on cell migration. The results suggest that tamoxifen may inhibit I(Cl,vol) by suppressing PKC activation; I(Cl,vol) may be an EsR-independent target for tamoxifen in the anti-metastatic action on cancers, especially on EsR-negative cancers. The finding may have an implication in the clinical use of tamoxifen in the treatments of both EsR-positive and EsR-negative cancers.

Filamin interacts with epithelial sodium channel and inhibits its channel function

Epithelial sodium channel (ENaC) in the kidneys is critical for Na(+) balance, extracellular volume, and blood pressure. Altered ENaC function is associated with respiratory disorders, pseudohypoaldosteronism type 1, and Liddle syndrome. ENaC is known to interact with components of the cytoskeleton, but the functional roles remain largely unclear. Here, we examined the interaction between ENaC and filamins, important actin filament components. We first discovered by yeast two-hybrid screening that the C termini of ENaC α and β subunits bind filamin A, B, and C, and we then confirmed the binding by in vitro biochemical assays. We demonstrated by co-immunoprecipitation that ENaC, either overexpressed in HEK, HeLa, and melanoma A7 cells or natively expressed in LLC-PK1 and IMCD cells, is in the same complex with native filamin. Furthermore, the biotinylation and co-immunoprecipitation combined assays showed the ENaC-filamin interaction on the cell surface. Using Xenopus oocyte expression and two-electrode voltage clamp electrophysiology, we found that co-expression of an ENaC-binding domain of filamin substantially reduces ENaC channel function. Western blot and immunohistochemistry experiments revealed that the filamin A C terminus (FLNAC) modestly reduces the expression of the ENaC α subunit in oocytes and A7 cells. After normalizing the current by plasma membrane expression, we found that FLNAC results in ~50% reduction in the ENaC channel activity. The inhibitory effect of FLNAC was confirmed by lipid bilayer electrophysiology experiments using purified ENaC and FLNAC proteins, which showed that FLNAC substantially reduces ENaC single channel open probability. Taken together, our study demonstrated that filamin reduces ENaC channel function through direct interaction on the cell surface.

Importance of non-selective cation channel TRPV4 interaction with cytoskeleton and their reciprocal regulations in cultured cells

BACKGROUND

TRPV4 and the cellular cytoskeleton have each been reported to influence cellular mechanosensitive processes as well as the development of mechanical hyperalgesia. If and how TRPV4 interacts with the microtubule and actin cytoskeleton at a molecular and functional level is not known.

METHODOLOGY AND PRINCIPAL FINDINGS

We investigated the interaction of TRPV4 with cytoskeletal components biochemically, cell biologically by observing morphological changes of DRG-neurons and DRG-neuron-derived F-11 cells, as well as functionally with calcium imaging. We find that TRPV4 physically interacts with tubulin, actin and neurofilament proteins as well as the nociceptive molecules PKCepsilon and CamKII. The C-terminus of TRPV4 is sufficient for the direct interaction with tubulin and actin, both with their soluble and their polymeric forms. Actin and tubulin compete for binding. The interaction with TRPV4 stabilizes microtubules even under depolymerizing conditions in vitro. Accordingly, in cellular systems TRPV4 colocalizes with actin and microtubules enriched structures at submembranous regions. Both expression and activation of TRPV4 induces striking morphological changes affecting lamellipodial, filopodial, growth cone, and neurite structures in non-neuronal cells, in DRG-neuron derived F11 cells, and also in IB4-positive DRG neurons. The functional interaction of TRPV4 and the cytoskeleton is mutual as Taxol, a microtubule stabilizer, reduces the Ca2+-influx via TRPV4.

CONCLUSIONS AND SIGNIFICANCE

TRPV4 acts as a regulator for both, the microtubule and the actin. In turn, we describe that microtubule dynamics are an important regulator of TRPV4 activity. TRPV4 forms a supra-molecular complex containing cytoskeletal proteins and regulatory kinases. Thereby it can integrate signaling of various intracellular second messengers and signaling cascades, as well as cytoskeletal dynamics. This study points out the existence of cross-talks between non-selective cation channels and cytoskeleton at multiple levels. These cross talks may help us to understand the molecular basis of the Taxol-induced neuropathic pain development commonly observed in cancer patients.

Actin microfilament involved in regulation of pacemaking activity in cultured interstitial cells of Cajal from murine intestine

The present study investigated the effect of actin microfilament structure on pacemaker currents and calcium oscillation in cultured murine intestinal interstitial cells of Cajal (ICCs) by whole-cell patch-clamp technique and calcium imaging technique. Cytochalasin B, a disruptor of actin microfilaments, decreased the amplitude and frequency of pacemaker currents from 491.32 +/- 160.33 pA and 11.73 +/- 0.79 cycles/min to 233.12 +/- 92.00 pA and 10.29 +/- 0.76 cycles/min. Cytochalasin B also decreased the amplitude and frequency of calcium oscillation from 0.32 +/- 0.08 (DeltaF/F0) and 2.75 +/- 0.17 cycles/min to 0.02 +/- 0.01 (DeltaF/F0) and 1.20 +/- 0.08 cycles/min. Phalloidin, a stabilizer of actin microfilaments, increased the amplitude and frequency of pacemaker currents from 751.79 +/- 282.82 pA and 13.93 +/- 1.00 cycles/min to 1234.34 +/- 607.83 pA and 14.68 +/- 1.00 cycles/min. Phalloidin also increased the amplitude and frequency of calcium oscillation from 0.26 +/- 0.01 (DeltaF/F0) and 2.27 +/- 0.18 cycles/min to 0.43 +/- 0.03 (DeltaF/F0) and 2.87 +/- 0.07 cycles/min. 2-Aminoethoxydiphenyl borane (2-APB), an IP(3) receptor blocker, suppressed both pacemaker currents and calcium oscillations. 2-APB also blocked the phalloidin-induced increase in pacemaker currents and calcium oscillation. Ryanodine, an inhibitor of calcium-induced calcium release, did not affect pacemaker current but suppressed calcium oscillations. Ryanodine had no effect on altering phalloidin-induced increases in pacemaker current and calcium oscillation. These results suggest that actin microfilaments regulate pacemaker activity via the IP(3)-induced calcium release signaling pathway.

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.

Mechanical and biochemical modeling of cortical oscillations in spreading cells

Actomyosin-based cortical contractility is a common feature of eukaryotic cells and is involved in cell motility, cell division, and apoptosis. In nonmuscle cells, oscillations in contractility are induced by microtubule depolymerization during cell spreading. We developed an ordinary differential equation model to describe this behavior. The computational model includes 36 parameters. The values for all but two of the model parameters were taken from experimental measurements found in the literature. Using these values, we demonstrate that the model generates oscillatory behavior consistent with current experimental observations. The rhythmic behavior occurs because of the antagonistic effects of calcium-induced contractility and stretch-activated calcium channels. The model makes several experimentally testable predictions: 1), buffering intracellular calcium increases the period and decreases the amplitude of cortical oscillations; 2), increasing the number or activity of stretch activated channels leads to an increase in period and amplitude of cortical oscillations; 3), inhibiting Ca(2+) pump activity increases the period and amplitude of oscillations; and 4), a threshold exists for the calcium concentration below which oscillations cease.

Cell responses regulated by early reorganization of actin cytoskeleton

Microfilaments exist in a dynamic equilibrium between monomeric and polymerized actin and the ratio of monomers to polymeric forms is influenced by a variety of extracellular stimuli. The polymerization, depolymerization and redistribution of actin filaments are modulated by several actin-binding proteins, which are regulated by upstream signalling molecules. Actin cytoskeleton is involved in diverse cellular functions including migration, ion channels activity, secretion, apoptosis and cell survival. In this review we have outlined the role of actin dynamics in representative cell functions induced by the early response to extracellular stimuli.

Intermediate filaments and stress

Before we can explain why so many closely related intermediate filament genes have evolved in vertebrates, while maintaining such dramatically tissue specific expression, we need to understand their function. The best evidence for intermediate filament function comes from observing the consequences of mutation and mis-expression, primarily in human tissues. Mostly these observations suggest that intermediate filaments are important in allowing individual cells, the tissues and whole organs to cope with various types of stress, in health and disease. Exactly how they do this is unclear and many aspects of cell dysfunction have been associated with intermediate filaments to date. In particular, it is still not clear whether the non-mechanical functions now being attributed to intermediate filaments are primary functions of these structural proteins, or secondary consequences of their function to respond to mechanical stress. We discuss selected situations in which responses to stress are clearly influenced by intermediate filaments.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

Ca2+ transient induced by extracellular changes in osmotic pressure in Arabidopsis leaves: differential involvement of cell wall-plasma membrane adhesion

We investigated the mechanism underlying the perception of extracellular changes in osmotic pressure in Vallisneria gigantea Graebner and transgenic Arabidopsis thaliana (L.) Heynh. expressing cytoplasmic aequorin. Hypertonic and hypotonic treatments of A. thaliana leaves each rapidly induced a Ca2+ transient. Both responses were essentially dependent on the presence of extracellular Ca2+ and were sensitive to Gd3+ a potential blocker of stretch-activated Ca2+ channels. Immediately after plasmolysis caused by hypertonic treatment and subsequent deplasmolysis caused by hypotonic treatment, the cells did not respond to a second hypertonic treatment and exhibited an impaired adhesion of the plasma membrane (PM) to the cell wall (CW). Recovery of the responsiveness required about 6 h. By contrast, no refractory phenomenon was observed in response to hypotonic treatment. Pretreatment with cellulase completely inhibited the Ca2+ transient induced by hypertonic treatment, but it did not affect the response to hypotonic treatment. V. gigantea mesophyll cells pretreated with cellulase exhibited an impaired adhesion of the PM to the CW. The leaf cells of multicellular plants can respond to both hypertonic and hypotonic treatments through the stretch-activated Ca2+ channels, whereas cellulase-sensitive adhesion of the PM to the CW is involved only in the response to hypertonic treatment.

Astrocyte activation and reactive gliosis

Astrocytes become activated (reactive) in response to many CNS pathologies, such as stroke, trauma, growth of a tumor, or neurodegenerative disease. The process of astrocyte activation remains rather enigmatic and results in so-called "reactive gliosis," a reaction with specific structural and functional characteristics. In stroke or in CNS trauma, the lesion itself, the ischemic environment, disrupted blood-brain barrier, the inflammatory response, as well as in metabolic, excitotoxic, and in some cases oxidative crises--all affect the extent and quality of reactive gliosis. The fact that astrocytes function as a syncytium of interconnected cells both in health and in disease, rather than as individual cells, adds yet another dimension to this picture. This review focuses on several aspects of astrocyte activation and reactive gliosis and discusses its possible roles in the CNS trauma and ischemia. Particular emphasis is placed on the lessons learnt from mouse genetic models in which the absence of intermediate filament proteins in astrocytes leads to attenuation of reactive gliosis with distinct pathophysiological and clinical consequences.

Importance of cytoskeletal elements in volume regulatory responses of trout hepatocytes

The role of cytoskeletal elements in volume regulation was studied in trout hepatocytes by investigating changes in F-actin distribution during anisotonic exposure and assessing the impact of cytoskeleton disruption on volume regulatory responses. Hypotonic challenge caused a significant decrease in the ratio of cortical to cytoplasmic F-actin, whereas this ratio was unaffected in hypertonic saline. Disruption of microfilaments with cytochalasin B (CB) or cytochalasin D significantly slowed volume recovery following hypo- and hypertonic exposure in both attached and suspended cells. The decrease of net proton release and the intracellular acidification elicited by hypotonicity were unaltered by CB, whereas the increase of proton release in hypertonic saline was dramatically reduced. Because amiloride almost completely blocked the hypertonic increase of proton release and cytoskeleton disruption diminished the associated increase of intracellular pH (pH(i)), we suggest that F-actin disruption affected Na(+)/H(+) exchanger activity. In line with this, pH(i) recovery after an ammonium prepulse was significantly inhibited in CB-treated cells. The increase of cytosolic Na(+) under hypertonic conditions was not diminished but, rather, enhanced by F-actin disruption, presumably due to inhibited Na(+)-K(+)-ATPase activity and stimulated Na(+) channel activity. The elevation of cytosolic Ca(2+) in hypertonic medium was significantly reduced by CB. Altogether, our results indicate that the F-actin network is of crucial importance in the cellular responses to anisotonic conditions, possibly via interaction with the activity of ion transporters and with signalling cascades responsible for their activation. Disruption of microtubules with colchicine had no effect on any of the parameters investigated.

K+ channel inhibition accelerates intestinal epithelial cell wound healing

Restitution is the process by which superficial interruptions in the gastrointestinal mucosa are repaired by the flattening and spreading of epithelial cells surrounding the damage. During this process, mucosal epithelial cells undergo extensive reshaping and cytoskeletal remodeling. K(+) channels, located primarily on the basolateral surface of gut epithelial cells, are central to both actin polymerization, via their control of membrane potential, and cell volume regulation. We questioned whether K(+) channels are involved in restitution using an in vitro model of intestinal epithelium, monolayers of the human colon carcinoma cell line T84. We report that pharmacologic K(+) channel inhibition accelerates wound healing in T84 cell monolayers. Both Ca(++)-dependent and constitutively active channels are involved, as indicated by the sensitivity to clotrimazole, charybdotoxin, and barium. The ability of clotrimazole to accelerate wound resealing was also observed in Caco-2 cell sheets. Pharmacologic stimulation of K(+) channel activity had no effect on the repair rate. Analysis of the resealing process by time lapse and confocal microscopy revealed that K(+) channel inhibitors abolished the initial wound retraction, briefly accelerated the repair rate, and altered the shape of the cell sheet abutting the injury during the early phase of resealing. We hypothesize that K(+) channel inactivation interrupts the coregulation of f-actin polymerization and volume control that is initiated by the healing process.

Astrocyte intermediate filaments in CNS pathologies and regeneration

Astroglial cells are the most abundant cells in the mammalian central nervous system (CNS), yet our knowledge about their function in health and disease has been limited. This review focuses on the recent work addressing the function of intermediate filaments in astroglial cells under severe mechanical or osmotic stress, in hypoxia, and in brain and spinal cord injury. Recent data show that when astrocyte intermediate filaments are genetically ablated in mice, reactive gliosis is attenuated and the course of several CNS pathologies is altered, while the signs of CNS regeneration become more prominent. GFAP is the principal astrocyte intermediate filament protein and dominant mutations in the GFAP gene have been shown to lead to Alexander disease, a fatal neurodegenerative condition in humans.

Sigma Receptors and Cancer

The sigma (sigma) receptor and its agonists have been implicated in a myriad of cellular functions, biological processes and diseases. Whereas the precise molecular mechanism(s) of sigma receptors and their involvement in cancer cell biology have not been elucidated, recent work has started to shed some light on these issues. A molecular model has been proposed for the cloned sigma1 receptor; the precise molecular nature of the sigma2 receptor remains unknown. sigma receptors have been found to be frequently up-regulated in human cancer cells and tissues. sigma2 receptor drugs particularly have been shown to have antiproliferative effects. An interesting possibility is that sigma and/or sigma1 drugs could produce anticancerous effects by modulating ion channels. As well as proliferation, a variety of other metastatic cellular behaviors such as adhesion, motility, and secretion may also be affected. Other mechanisms of sigma receptor action may involve interaction with ankyrin and modulation of intracellular Ca(2+) and sphingolipid levels. Although more research is needed to further define the molecular physiology of sigma receptors, their involvement in the cellular pathophysiology of cancer raises the possibility that sigma drugs could be useful as novel therapeutic agents.

Direct interaction between BKCa potassium channel and microtubule-associated protein 1A

The BKCa channel, a potassium channel that is allosterically activated by voltage and calcium, is expressed in both excitable and non-excitable cells. The channel plays an important role in regulating membrane excitability. The channel activity can be modulated by post-translational modifications such as phosphorylation. Recently, hippocampal BKCa channels were shown to be directly modulated by assembly/disassembly of the submembranous actin cytoskeleton. Here, we report that the BKCa channel physically interacts with the light chain of microtubule associated protein 1A (MAP1A). The light chain was isolated in a yeast two-hybrid screen of a human brain cDNA library. The specificity of the interaction was demonstrated in biochemical experiments utilizing GST fusion protein pulldown assays and reciprocal co-immunoprecipitations from rat brain. Furthermore, utilizing immunofluorescence, the BKCa channel and MAP1A co-localize in the Purkinje cell layer of the cerebellum. These studies identify a novel interaction between the C-terminal tail of the BKCa channel and the light chain of MAP1A, which enables channel association with and modulation by the cytoskeleton.

Contribution of functional voltage-gated Na+ channel expression to cell behaviors involved in the metastatic cascade in rat prostate cancer: I. Lateral motility

Previous work suggested that functional voltage-gated Na(+) channels (VGSCs) are expressed specifically in strongly metastatic cells of rat and human prostate cancer (PCa), thereby raising the possibility that VGSC activity could be involved in cellular behavior(s) related to the metastatic cascade. In the present study, the possible role of VGSCs in the lateral motility of rat PCa cells was investigated in vitro by testing the effect of modulators that either block or enhance VGSC activity. Two rat PCa cell lines of markedly different metastatic ability were used in a comparative approach: the strongly metastatic MAT-LyLu and the weakly metastatic AT-2 cell line, only the former being known to express functional VGSCs. Using both electrophysiological recording and a motility assay, the effects of two VGSC blockers (tetrodotoxin and phenytoin) and four potential openers (veratridine, aconitine, ATX II, and brevetoxin) were monitored on (a) Na(+) channel activity and (b) cell motility over 48 h. Tetrodotoxin (at 1 microM) and phenytoin (at 50 microM) both decreased the motility index of the MAT-LyLu cell line by 47 and 11%, respectively. Veratridine (at 20 microM) and brevetoxin (at 10 nM) had no effect on the motility of either cell line, whilst aconitine (at 100 microM) and ATX II (at 25 pM) significantly increased the motility of the MAT-LyLu cell line by 15 and 9%, respectively. Importantly, at the concentrations used, none of these drugs had effects on the proliferation or viability of either cell line. The results, taken together, would suggest strongly that functional VGSC expression enhances cellular motility of PCa cells. The relevance of these findings to the metastatic process in PCa is discussed.

Loss of the F-actin binding and vesicle-associated protein comitin leads to a phagocytosis defect

Comitin is an F-actin binding and membrane-associated protein from Dictyostelium discoideum, which is present on Golgi and vesicle membranes and changes its localization in response to agents affecting the cytoskeleton. To investigate its in vivo functions we have generated knockout mutants by gene replacement. Based on comitin's in vitro functions we examined properties related to vesicular transport and microfilament function. Whereas cell growth, pinocytosis, secretion, chemotaxis, motility, and development were unaltered, comitin-lacking cells were impaired in the early steps of phagocytosis of Saccharomyces cerevisiae particles and of Escherichia coli, whereas uptake of latex beads was unaffected. Furthermore, the lack of comitin positively affected survival of pathogenic bacteria. Mutant cells also showed an altered response to hyperosmotic shock in comparison to the wild type. The redistribution of comitin during hyperosmotic shock in wild-type cells and its presence on early phagosomes suggest a direct involvement of comitin in these processes.

Osteoclast spreading kinetics are correlated with an oscillatory activation of a calcium-dependent potassium current

Cell movement and spreading involve calcium-dependent processes and ionic channel activation. During bone resorption, osteoclasts alternate between spread, motile and resorptive phases. We investigated whether the electrical membrane properties of osteoclasts were linked to their membrane morphological changes. Rabbit osteoclasts were recorded by time-lapse videomicroscopy performed simultaneously with patch-clamp whole cell and single channel recordings. Original image analysis methods were developed and used to demonstrate for the first time an oscillatory activation of a spontaneous membrane current in osteoclasts, which is directly correlated to the membrane movement rate. This current was identified as a calcium-dependent potassium current (IK(Ca)) that is sensitive to both charybdotoxin and apamin and was generated by a channel with unitary conductance of approximately 25+/-2 pS. Blockade of this current also decreased osteoclast spreading and inhibited bone resorption in vitro, demonstrating a physiological role for this current in osteoclast activity. These results establish for the first time a temporal correlation between lamellipodia formation kinetics and spontaneous peaks of IK(Ca), which are both involved in the control of osteoclast spreading and bone resorption.

The human prostate cancer cell line LNCaP bears functional membrane testosterone receptors that increase PSA secretion and modify actin cytoskeleton

Recent findings have shown that, in addition to the genomic action of steroids, through intracellular receptors, short-time effects could be mediated through binding to membrane sites. In the present study of prostate cancer LNCaP cells, we report that dihydrotestosterone and the non-internalizable analog testosterone-BSA increase rapidly the release of prostate-specific antigen (PSA) in the culture medium. Membrane testosterone binding sites were identified through ligand binding on membrane preparations, flow cytometry, and confocal laser microscopy of the non-internalizable fluorescent analog testosterone-BSA-FITC, on whole cells. Binding on these sites is time- and concentration-dependent and specific for testosterone, presenting a KD of 10.9 nM and a number of 144 sites/mg protein (approximately 13000 sites/cell). Membrane sites differ immunologically for intracellular androgen receptors. The secretion of PSA after membrane testosterone receptor stimulation was inhibited after pretreatment with the actin cytoskeleton disrupting agent cytochalasin B. In addition, membrane testosterone binding modifies the intracellular dynamic equilibrium of monomeric to filamentous actin and remodels profoundly the actin cytoskeleton organization. These results are discussed in the context of a possible involvement of these sites in cancer chemotherapy.

Rho family GTP binding proteins are involved in the regulatory volume decrease process in NIH3T3 mouse fibroblasts

The role of Rho GTPases in the regulatory volume decrease (RVD) process following osmotic cell swelling is controversial and has so far only been investigated for the swelling-activated Cl- efflux. We investigated the involvement of RhoA in the RVD process in NIH3T3 mouse fibroblasts, using wild-type cells and three clones expressing constitutively active RhoA (RhoAV14). RhoAV14 expression resulted in an up to fourfold increase in the rate of RVD, measured by large-angle light scattering. The increase in RVD rate correlated with RhoAV14 expression. RVD in wild-type cells was unaffected by the Rho kinase inhibitor Y-27632 and the phosphatidyl-inositol 3 kinase (PI3K) inhibitor wortmannin. The maximal rates of swelling-activated K+ (86 Rb+ as tracer) and taurine ([3H]taurine as tracer) efflux after a 30 % reduction in extracellular osmolarity were increased about twofold in cells with maximal RhoAV14 expression compared to wild-type cells, but were unaffected by Y-27632. The volume set points for activation of release of both osmolytes appeared to be reduced by RhoAV14 expression. The maximal taurine efflux rate constant was potentiated by the tyrosine phosphatase inhibitor Na(3)VO(4), and inhibited by the tyrosine kinase inhibitor genistein. The magnitude of the swelling-activated Cl- current (I(Cl,swell) ) was higher in RhoAV14 than in wild-type cells after a 7.5 % reduction in extracellular osmolarity, but, in contrast to 86Rb+ and [3H]taurine efflux, similar in both strains after a 30 % reduction in extracellular osmolarity. I(Cl,swell) was inhibited by Y-27632 and strongly potentiated by the myosin light chain kinase inhibitors ML-7 and AV25. It is suggested that RhoA, although not the volume sensor per se, is an important upstream modulator shared by multiple swelling-activated channels on which RhoA exerts its effects via divergent signalling pathways.

Eukaryotic cell locomotion depends on the propagation of self-organized reaction-diffusion waves and oscillations of actin filament assembly

Actin filament (F-actin) assembly kinetics determines the locomotion and shape of crawling eukaryotic cells, but the nature of these kinetics and their determining reactions are unclear. Live BHK21 fibroblasts, mouse melanoma cells, and Dictyostelium amoebae, locomoting on glass and expressing Green Fluorescent Protein-actin fusion proteins, were examined by confocal microscopy. The cells demonstrated three-dimensional bands of F-actin, which propagated throughout the cytoplasm at rates usually ranging between 2 and 5 microm/min in each cell type and produced lamellipodia or pseudopodia at the cell boundary. F-actin's dynamic behavior and supramolecular spatial patterns resembled in detail self-organized chemical waves in dissipative, physico-chemical systems. On this basis, the present observations provide the first evidence of self-organized, and probably autocatalytic, chemical reaction-diffusion waves of reversible actin filament assembly in vertebrate cells and a comprehensive record of wave and locomotory dynamics in vegetative-stage Dictyostelium cells. The intensity and frequency of F-actin wavefronts determine locomotory cell projections and the rotating oscillatory waves, which structure the cell surface. F-actin assembly waves thus provide a fundamental, deterministic, and nonlinear mechanism of cell locomotion and shape, which complements mechanisms based exclusively on stochastic molecular reaction kinetics.

Involvement of actin dephosphorylation in germination of Physarum sclerotium

The plasmodium of Physarum polycephalum grows without cytokinesis and shows an active cytoplasmic streaming under wet and nutritious conditions. It can undergo reversible differentiation into several types of dormancy to survive in adverse environments. Temperature change or osmotic stress leads to cytoplasmic division of the plasmodium into cells containing one or more nuclei: these form a macrocyst, the spherule. Desiccation also induces cell division of the plasmodium followed by formation of a sclerotium, a dormant body resistant to dry stress. More than half of the actin in a sclerotium is phosphorylated at a single site, threonine 203, resulting in loss of its ability to polymerize into actin filaments. In the present study, actin phosphorylation was found in the sclerotium but not in either the plasmodium or in the spherule. This result suggests that phosphorylation of sclerotium actin may be related to the mechanism associated with desiccation resistance rather than morphological changes through cell compartmentalization in the macrocyst formation. Moreover. dephosphorylation of the phosphorylated form of sclerotium actin began within 5 min after addition of water. Dephosphorylation was not affected by sucrose and sorbitol sugars, but was inhibited by ammonium bicarbonate, ammonium phosphate, sodium phosphate, NaCl, and KCl in a dose-dependent manner. On the other hand, in germination of the sclerotium there was measurable sensitivity to both carbohydrates and salts. Actin dephosphorylation seems to be one of the important processes in the early phase of sclerotium germination.

Mechanisms of cell volume regulation and possible nature of the cell volume sensor

In animal organisms, cell volume undergoes dynamic changes in many physiological and pathological processes. To protect themselves against lysis and apoptosis and to maintain an optimal concentration of intracellular enzymes and metabolites, most animal cells actively regulate their volume. In the present review, we shortly summarize the data on ion transport mechanisms involved in regulatory volume decrease (RVD) and regulatory volume increase (RVI) with an emphasis on unresolved aspects of this problem such as: (i) how cells sense their volume changes; (ii) what signals are generated upon cell volume alterations; and (iii) how these signals are transferred to the ion transport systems executing cell volume regulation.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

Ion channels and transporters on the move

Cell migration plays a crucial role in a variety of (patho)physiological processes such as immune defense, wound healing, and formation of tumor metastases. Detailed models have been developed to describe cytoskeletal mechanisms of migration. However, evidence is accumulating that the activity of ion channels and transporters is also required for optimal cell locomotion.

Intracellular pH modulation of ADF/cofilin proteins

The ADF/cofilin (AC) proteins are necessary for the high rates of actin filament turnover seen in vivo. Their regulation is complex enough to underlie the precision in filament dynamics needed by stimulated cells. Disassembly of actin by AC proteins is inhibited in vitro by phosphorylation of ser3 and pH<7.1. This study of Swiss 3T3 cells demonstrates that pH also affects AC behavior in vivo: (1) Wounded cells show pH-dependent AC translocation to alkaline-induced ruffling membrane; (2) The Triton extractable (soluble) ADF from Swiss 3T3 cells decreases from 42+/-4% to 23+/-4% when the intracellular pH (pH(i)) is reduced from 7.4 to 6.6; (3) Covariance and colocalization analyses of immunostained endogenous proteins show that ADF partitions more with monomeric actin and less with polymeric actin when pH(i) increases. However, the distribution of cofilin, a less pH-sensitive AC in vitro, does not change with pH; (4) Only the unphosphorylatable AC mutant (A3), when overexpressed as a GFP chimera, uniquely produces aberrant cellular phenotypes and only if the pH is shifted from 7.1 to 6.6 or 7.4. A mechanism is proposed that explains why AC(A3)-GFP and AC(wt)-GFP chimeras generate different phenotypes in response to pH changes. Phospho-AC levels increase with cell density, and in motile cells, phospho-AC increases with alkalization, suggesting a homeostatic mechanism that compensates for increased AC activity and filament turnover. These results show that the behavior of AC proteins with pH-sensitivity in vitro is affected by pH in vivo.

Actin filament organization is required for proper cAMP-dependent activation of CFTR

Previous studies have indicated a role of the actin cytoskeleton in the regulation of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel. However, the exact molecular nature of this regulation is still largely unknown. In this report human epithelial CFTR was expressed in human melanoma cells genetically devoid of the filamin homologue actin-cross-linking protein ABP-280 [ABP(-)]. cAMP stimulation of ABP(-) cells or cells genetically rescued with ABP-280 cDNA [ABP(+)] was without effect on whole cell Cl(-) currents. In ABP(-) cells expressing CFTR, cAMP was also without effect on Cl(-) conductance. In contrast, cAMP induced a 10-fold increase in the diphenylamine-2-carboxylate (DPC)-sensitive whole cell Cl(-) currents of ABP(+)/CFTR(+) cells. Further, in cells expressing both CFTR and a truncated form of ABP-280 unable to cross-link actin filaments, cAMP was also without effect on CFTR activation. Dialysis of ABP-280 or filamin through the patch pipette, however, resulted in a DPC-inhibitable increase in the whole cell currents of ABP(-)/CFTR(+) cells. At the single-channel level, protein kinase A plus ATP activated single Cl(-) channels only in excised patches from ABP(+)/CFTR(+) cells. Furthermore, filamin alone also induced Cl(-) channel activity in excised patches of ABP(-)/CFTR(+) cells. The present data indicate that an organized actin cytoskeleton is required for cAMP-dependent activation of CFTR.

Role of gelsolin in the actin filament regulation of cardiac L-type calcium channels

The actin cytoskeleton is an important contributor to the modulation of the cell function. However, little is known about the regulatory role of this supermolecular structure in the membrane events that take place in the heart. In this report, the regulation of cardiac myocyte function by actin filament organization was investigated in neonatal mouse cardiac myocytes (NMCM) from both wild-type mice and mice genetically devoid of the actin filament severing protein gelsolin (Gsn-/-). Cardiac L-type calcium channel currents (I(Ca)) were assessed using the whole cell voltage-clamp technique. Addition of the actin filament stabilizer phalloidin to wild-type NMCM increased I(Ca) by 227% over control conditions. The basal I(Ca) of Gsn-/- NMCM was 300% higher than wild-type controls. This increase was completely reversed by intracellular perfusion of the Gsn-/- NMCM with exogenous gelsolin. Further, cytoskeletal disruption of either Gsn-/- or phalloidin-dialyzed wild-type NMCM with cytochalasin D (CD) decreased the enhanced I(Ca) by 84% and 87%, respectively. The data indicate that actin filament stabilization by either a lack of gelsolin or intracellular dialysis with phalloidin increase I(Ca), whereas actin filament disruption with CD or dialysis of Gsn-/- NMCM with gelsolin decrease I(Ca). We conclude that cardiac L-type calcium channel regulation is tightly controlled by actin filament organization. Actin filament rearrangement mediated by gelsolin may contribute to calcium channel inactivation.

Nuclear hourglass technique: an approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Nuclear pore complexes (NPCs) mediate both active transport and passive diffusion across the nuclear envelope (NE). Determination of NE electrical conductance, however, has been confounded by the lack of an appropriate technical approach. The nuclear patch clamp technique is restricted to preparations with electrically closed NPCs, and microelectrode techniques fail to resolve the extremely low input resistance of large oocyte nuclei. To address the problem, we have developed an approach for measuring the NE electrical conductance of Xenopus laevis oocyte nuclei. The method uses a tapered glass tube, which narrows in its middle part to 2/3 of the diameter of the nucleus. The isolated nucleus is sucked into the narrow part of the capillary by gentle fluid movement, while the resulting change in electrical resistance is monitored. NE electrical conductance was unexpectedly large (7.9 +/- 0.34 S/cm(2)). Evaluation of NPC density by atomic force microscopy showed that this conductance corresponded to 3.7 x 10(6) NPCs. In contrast to earlier conclusions drawn from nuclear patch clamp experiments, NPCs were in an electrically "open" state with a mean single NPC electrical conductance of 1.7 +/- 0.07 nS. Enabling or blocking of active NPC transport (accomplished by the addition of cytosolic extracts or gp62-directed antibodies) revealed this large NPC conductance to be independent of the activation state of the transport machinery located in the center of NPCs. We conclude that peripheral channels, which are presumed to reside in the NPC subunits, establish a high ionic permeability that is virtually independent of the active protein transport mechanism.

Modulation of volume-sensitive Cl - channels and cell volume by actin filaments and microtubules in human cervical cancer HT-3 cells

Hypotonicity activates volume-sensitive Cl- currents, which are implicated in the regulatory volume decrease (RVD) responses and transport of taurine in human cervical cancer HT-3 cells. In this study, the role of cytoskeleton in the regulation of volume-sensitive Cl- channels and RVD responses in HT-3 cells was studied. Cells were incubated with various compounds, which depolymerized or polymerized cytoskeletal elements, i.e. actin filaments and microtubules. The hypotonicity-induced changes in Cl- conductance and in cell volume were measured by whole-cell voltage clamping and cell size monitoring, respectively. Our results show that in HT-3 cells hypotonicity activated an outward rectified Cl- current that was abrogated by Cl- channel blockers. Cytochalasin B, an actin-depolymerizing compound, induced a substantial increase in Cl- conductance under isotonic condition and potentiated the expression of Cl- currents in hypotonic stress. Phorbol 12-myristate 13-acetate (PMA) significantly inhibited the cytochalasin B-induced activation of Cl- conductance under isotonic condition. On the other hand, treatment with cytochalasin B significantly prolonged the RVD responses. Phalloidin, a stabilizer of actin polymerization, did not change the basal currents under isotonic condition, but completely abolished the increase in whole-cell Cl- conductance elicited by hypotonicity and retarded the cell volume recovery. Colchicine, a microtubule-assembly inhibitor, had no effect on either basal Cl- conductance or volume-sensitive Cl- current and was unable to inhibit the RVD responses. Taxol, a microtubule-stabilizing compound, did not alter the basal Cl- conductance, but inhibited the activation of volume-sensitive Cl- channels as well as the process of RVD in a dose-dependent manner. These data support the notion that functional integrity of actin filaments and microtubules plays critical roles in maintaining the RVD responses and activation of Cl- channels in human cervical cancer HT-3 cells.

Role of the F-actin cytoskeleton in the RVD and RVI processes in Ehrlich ascites tumor cells

The role of the F-actin cytoskeleton in cell volume regulation was studied in Ehrlich ascites tumor cells, using a quantitative rhodamine-phalloidin assay, confocal laser scanning microscopy, and electronic cell sizing. A hypotonic challenge (160 mOsm) was associated with a decrease in cellular F-actin content at 1 and 3 min and a hypertonic challenge (600 mOsm) with an increase in cellular F-actin content at 1, 3, and 5 min, respectively, compared to isotonic (310 mOsm) control cells. Confocal visualization of F-actin in fixed, intact Ehrlich cells demonstrated that osmotic challenges mainly affect the F-actin in the cortical region of the cells, with no visible changes in F-actin in other cell regions. The possible role of the F-actin cytoskeleton in RVD was studied using 0. 5 microM cytochalasin B (CB), cytochalasin D (CD), or chaetoglobosin C (ChtC), a cytochalasin analog with little or no affinity for F-actin. Recovery of cell volume after hypotonic swelling was slower in cells pretreated for 3 min with 0.5 microM CB, but not in CD- and ChtC-treated cells, compared to osmotically swollen control cells. Moreover, the maximal cell volume after swelling was decreased in CB-treated, but not in CD- or Chtc-treated cells. Following a hypertonic challenge imposed using the RVD/RVI protocol, recovery from cell shrinkage was slower in CB-treated, but not in CD- or Chtc-treated cells, whereas the minimal cell volume after shrinkage was unaltered by either of these treatments. It is concluded that osmotic cell swelling and shrinkage elicit a decrease and an increase in the F-actin content in Ehrlich cells, respectively. The RVD and RVI processes are inhibited by 0.5 microM CB, but not by 0.5 microM CD, which is more specific for actin.

Expression and characterization of Cys374 mutated human beta-actin in two different mammalian cell lines: impaired microfilament organization and stability

Previous studies have demonstrated that addition of glutathione at the penultimate Cys374 residue of actin results in filaments with diminished mechanical stability. In the present work substitutions introducing a negatively charged (Asp and Glu) or a neutral (Ala) amino acid at position 374 of the human beta-actin and tagged at the N-terminus with the flag epitope were studied by transient transfections into Ishikawa human endometrial and opossum kidney cells. Immunofluorescence revealed that microfilaments which incorporated negatively charged mutants were partially to severely disorganized when compared to the almost well-formed actin-Ala374 filaments or the wild type actin filaments. Furthermore, microfilaments containing either negatively charged mutant were more sensitive to the destabilizing action of cytochalasin B. In addition, Triton fractionation resealed a considerable reduction of flag-actin content in the Triton insoluble fraction for cells expressing Asp374 or Glu374 mutant compared to wild type actin. These results demonstrate that negatively charged amino acid residues at the exposed C-terminal tail strongly affect actin microfilament organization and dynamics in vivo.

Regulation of cytoskeletal mechanics and cell growth by myosin light chain phosphorylation

The role of myosin light chain phosphorylation in regulating the mechanical properties of the cytoskeleton was studied in NIH/3T3 fibroblasts expressing a truncated, constitutively active form of smooth muscle myosin light chain kinase (tMK). Cytoskeletal stiffness determined by quantifying the force required to indent the apical surface of adherent cells showed that stiffness was increased twofold in tMK cells compared with control cells expressing the empty plasmid (Neo cells). Cytoskeletal stiffness quantified using magnetic twisting cytometry showed an approximately 1.5-fold increase in stiffness in tMK cells compared with Neo cells. Electronic volume measurements on cells in suspension revealed that tMK cells had a smaller volume and are more resistant to osmotic swelling than Neo cells. tMK cells also have smaller nuclei, and activation of mitogen-activated protein kinase (MAP kinase) and translocation of MAP kinase to the nucleus are slower in tMK cells than in control cells. In tMK cells, there is also less bromodeoxyuridine incorporation, and the doubling time is increased. These data demonstrate that increased myosin light chain phosphorylation correlates with increased cytoskeletal stiffness and suggest that changing the mechanical characteristics of the cytoskeleton alters the intracellular signaling pathways that regulate cell growth and division.

Changes in actin cytoskeleton during volume regulation in C6 glial cells

Changes in actin cytoskeleton in the C6 rat glial cell line were studied during decrease or increase (abrupt or gradual) of extracellular osmolality. Actin cytoskeleton was visualized by confocal microscopy after FITC-phalloidin labeling. G-actin, Triton-soluble F-actin and Triton-insoluble F-actin subfractions were determined by gel electrophoresis and scanning, and by DNase I inhibition assays. In control conditions C6 glial cells exhibited well-defined stress fibers and a relatively smooth cortical network. Extracellular anisosmotic changes induced a rapid actin cytoskeletal reorganization, which further progressed and was not reversed upon cell volume recovery. Hypotonic shock caused membrane ruffling and a shift towards polymerized actin, whereas hypertonicity (abrupt or gradual) led to a distinct morphological appearance of abundant short actin microfilaments with, however, no detectable alteration in actin subfractions. When anisosmotic cell volume regulation was prevented, cytoskeleton reorganization depended on the osmotic change and the experimental protocol, but was not related to the absence of volume readjustment. Therefore, although involvement of cytoskeletal alterations in transduction of volume regulatory responses cannot be excluded, it is likely that the observed changes in actin cytoskeleton in C6 glial cells are linked with, but do not initiate, cell volume regulatory processes.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

Osmotic regulation of intestinal epithelial Na(+)-K(+)-Cl- cotransport: role of Cl- and F-actin

Previous data indicate that adenosine 3',5'-cyclic monophosphate activates the epithelial basolateral Na(+)-K(+)-Cl- cotransporter in microfilament-dependent fashion in part by direct action but also in response to apical Cl- loss (due to cell shrinkage or decreased intracellular Cl-). To further address the actin dependence of Na(+)-K(+)-Cl- cotransport, human epithelial T84 monolayers were exposed to anisotonicity, and isotopic flux analysis was performed. Na(+)-K(+)-Cl- cotransport was activated by hypertonicity induced by added mannitol but not added NaCl. Cotransport was also markedly activated by hypotonic stress, a response that appeared to be due in part to reduction of extracellular Cl- concentration and also to activation of K+ and Cl- efflux pathways. Stabilization of actin with phalloidin blunted cotransporter activation by hypotonicity and abolished hypotonic activation of K+ and Cl- efflux. However, phalloidin did not prevent activation of cotransport by hypertonicity or isosmotic reduction of extracellular Cl-. Conversely, hypertonic but not hypotonic activation was attenuated by the microfilament disassembler cytochalasin D. The results emphasize the complex interrelationship among intracellular Cl- activity, cell volume, and the actin cytoskeleton in the regulation of epithelial Cl- transport.

Microtubule disruption inhibits AVT-stimulated Cl- secretion but not Na+ reabsorption in A6 cells

The effects of microtubule disruption on arginine vasotocin (AVT)-stimulated Na+ and Cl- transport were studied in A6 cells by measuring short-circuit currents (Isc) across cell layers grown in tissue culture on permeable supports. Microtubule disruption inhibited an AVT-stimulated secretory Cl- current but did not prevent activation of amiloride-sensitive Na+ transport. This AVT-stimulated secretory Cl- current was significantly inhibited by glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR). Reverse transcription of RNA isolated from A6 cells followed by polymerase chain reaction (PCR) using primers designed to amplify a portion of the R-domain of CFTR cloned from Xenopus laevis skin and immunocytochemistry demonstrated the presence of CFTR in A6 cells and an apparent recruitment of cytoplasmic CFTR to the apical cell surface after AVT stimulation. In contrast, indirect immunofluorescent labeling of Na+ channels using a polyclonal antibody raised against a biochemically isolated Na+ channel complex from bovine renal medulla labeled the apical plasma membrane but failed to demonstrate intracellular labeling of Na+ channels (except in subconfluent cells) or recruitment of Na+ channels to the apical membrane region after AVT stimulation.