Effect of cell swelling on membrane and cytoplasmic distribution of pICln

An ICln homolog contributes to osmotic and low-nitrate tolerance by enhancing nitrate accumulation in Arabidopsis

Nitrate (NO₃^- ) is a source of plant nutrients and osmolytes, but its delivery machineries under osmotic and low-nutrient stress remain largely unknown. Here, we report that AtICln, an Arabidopsis homolog of the nucleotide-sensitive chloride-conductance regulatory protein family (ICln), is involved in response to osmotic and low-NO₃^- stress. The gene AtICln, encoding plasma membrane-anchored proteins, was upregulated by various osmotic stresses, and its disruption impaired plant tolerance to osmotic stress. Compared with the wild type, the aticln mutant retained lower anions, particularly NO₃^- , and its growth retardation was not rescued by NO₃^- supply under osmotic stress. Interestingly, this mutant also displayed growth defects under low-NO₃ stress, which were accompanied by decreases in NO₃^- accumulation, suggesting that AtICln may facilitate the NO₃^- accumulation under NO₃^- deficiency. Moreover, the low-NO₃^- hypersensitive phenotype of aticln mutant was overridden by the overexpression of NRT1.1, an important NO₃^- transporter in Arabidopsis low-NO₃^- responses. Further genetic analysis in the plants with altered activity of AtICln and NRT1.1 indicated that AtICln and NRT1.1 play a compensatory role in maintaining NO₃^- homeostasis under low-NO₃^- environments. These results suggest that AtICln is involved in cellular NO₃^- accumulation and thus determines osmotic adjustment and low-NO₃^- tolerance in plants.

Identification of Methylosome Components as Negative Regulators of Plant Immunity Using Chemical Genetics

Nucleotide-binding leucine-rich repeat (NLR) proteins serve as immune receptors in both plants and animals. To identify components required for NLR-mediated immunity, we designed and carried out a chemical genetics screen to search for small molecules that can alter immune responses in Arabidopsis thaliana. From 13 600 compounds, we identified Ro 8-4304 that was able to specifically suppress the severe autoimmune phenotypes of chs3-2D (chilling sensitive 3, 2D), including the arrested growth morphology and heightened PR (Pathogenesis Related) gene expression. Further, six Ro 8-4304 insensitive mutants were uncovered from the Ro 8-4304-insensitive mutant (rim) screen using a mutagenized chs3-2D population. Positional cloning revealed that rim1 encodes an allele of AtICln (I, currents; Cl, chloride; n, nucleotide). Genetic and biochemical analysis demonstrated that AtICln is in the same protein complex with the methylosome components small nuclear ribonucleoprotein D3b (SmD3b) and protein arginine methyltransferase 5 (PRMT5), which are required for the biogenesis of small nuclear ribonucleoproteins (snRNPs) involved in mRNA splicing. Double mutant analysis revealed that SmD3b is also involved in the sensitivity to Ro 8-4304, and the prmt5-1 chs3-2D double mutant is lethal. Loss of AtICln, SmD3b, or PRMT5 function results in enhanced disease resistance against the virulent oomycete pathogen Hyaloperonospora arabidopsidis Noco2, suggesting that mRNA splicing plays a previously unknown negative role in plant immunity. The successful implementation of a high-throughput chemical genetic screen and the identification of a small-molecule compound affecting plant immunity indicate that chemical genetics is a powerful tool to study whole-organism plant defense pathways.

Electrolyte and Fluid Transport in Mesothelial Cells

Mesothelial cells are specialized epithelial cells, which line the pleural, pericardial, and peritoneal cavities. Accumulating evidence suggests that the monolayer of mesothelial cells is permeable to electrolyte and fluid, and thereby govern both fluid secretion and re-absorption in the serosal cavities. Disorders in these salt and fluid transport systems may be fundamental in the pathogenesis of pleural effusion, pericardial effusion, and ascites. In this review, we discuss the location, physiological function, and regulation of active transport (Na(+)-K(+)-ATPase) systems, cation and anion channels (Na(+), K(+), Cl(-), and Ca(2+) channels), antiport (exchangers) systems, and symport (co-transporters) systems, and water channels (aquaporins). These secretive and absorptive pathways across mesothelial monolayer cells for electrolytes and fluid may provide pivotal therapeutical targets for novel clinical intervention in edematous diseases of serous cavities.

The ICln interactome

The many different functional phenotypes described in mammalian cells can only be explained by an intense interaction of the underlying proteins, substantiated by the fact that the number of independently expressed proteins in living cells seems not to exceed 25 K, a number way too small to explain the >250 K different phenotypes on a one-protein-one-function base. Therefore, the study of the interactome of the different proteins is of utmost importance. Here, we describe the present knowledge of the ICln interactome. ICln is a protein, we cloned and whose function was reported to be as divers as (i) ion permeation, (ii) cytoskeletal organization, and (iii) RNA processing. The role of ICln in these different functional modules can be described best as being a 'connector hub' with 'date hub' function.

The symmetrical dimethylarginine post-translational modification of the SmD3 protein is not required for snRNP assembly and nuclear transport

The SmB, SmD1, and SmD3 proteins have the rare symmetrical dimethylarginine post-translational modification in their C-termini. In this report, we investigate the function of this modification in the assembly and intracellular transport of the SmD3 protein. We show that the elimination of this methylation in the SmD3 protein, by mutating the modified arginines to leucines, does not interfere with the assembly and the nuclear transport of the transiently expressed SmD3 variant. This suggests this modification is not essential for maturation of the SmD3 protein.

Structure and pharmacology of swelling-sensitive chloride channels, I(Cl,swell)

Since several years, the interest for chloride channels and more particularly for the enigmatic swelling-activated chloride channel (I(Cl,swell)) is increasing. Despite its well-characterized electrophysiological properties, the I(Cl,swell) structure and pharmacology are not totally elucidated. These channels are involved in a variety of cell functions, such as cardiac rhythm, cell proliferation and differentiation, cell volume regulation and cell death through apoptosis. This review will consider different aspects regarding structure, electrophysiological properties, pharmacology, modulation and functions of these swelling-activated chloride channels.

ICln, a Novel Integrin αIIbβ3-Associated Protein, Functionally Regulates Platelet Activation

A critical role for the conserved alpha-integrin cytoplasmic motif, KVGFFKR, is recognized in the regulation of activation of the platelet integrin alpha(IIb)beta(3). To understand the molecular mechanisms of this regulation, we sought to determine the nature of the protein interactions with this cytoplasmic motif. We used a tagged synthetic peptide, biotin-KVGFFKR, to probe a high density protein expression array (37,200 recombinant human proteins) for high affinity interactions. A number of potential integrin-binding proteins were identified. One such protein, a chloride channel regulatory protein, ICln, was characterized further because its affinity for the integrin peptide was highest as was its expression in platelets. We verified the presence of ICln in human platelets by PCR, Western blots, immunohistochemistry, and its co-association with alpha(IIb)beta(3) by surface plasmon resonance. The affinity of this interaction was 82.2 +/- 24.4 nm in a cell free assay. ICln co-immunoprecipitates with alpha(IIb)beta(3) in platelet lysates demonstrating that this interaction is physiologically relevant. Furthermore, immobilized KVGFFKR peptides, but not control KAAAAAR peptides, specifically extract ICln from platelet lysates. Acyclovir (100 microm to 5 mm), a pharmacological inhibitor of the ICln chloride channel, specifically inhibits integrin activation (PAC-1 expression) and platelet aggregation without affecting CD62 P expression confirming a specific role for ICln in integrin activation. In parallel, a cell-permeable peptide corresponding to the potential integrin-recognition domain on ICln (AKFEEE, 10-100 microm) also inhibits platelet function. Thus, we have identified, verified, and characterized a novel functional interaction between the platelet integrin and ICln, in the platelet membrane.

Volume-regulated Cl- channels in human pleural mesothelioma cells

Anion channels in human mesothelial and mesothelioma cell lines were characterized by patch-clamp and biomolecular approaches. We found an outwardly rectifying anionic current which was inactivated at positive voltages and inhibited by extracellular adenosine 5'-triphosphate (ATP). Mesothelial and mesothelioma cells behaved differently concerning current inactivation properties. Inactivation is more pronounced and has a steeper onset in mesothelial cells. Different reversal potentials, in asymmetrical Cl(-) solutions, that could be attributed to a different selectivity of the channel, have been observed in the two cell lines. Mesothelioma cell single-channel analysis indicates that the number of the same active anion channel (3-4 pS) increased under hypoosmotic conditions. Immunocytochemistry experiments showed the presence of ICln protein in the cytosol and in the plasma membrane. Western blot analysis revealed an increase of ICln in the membrane under hypotonic conditions, an event possibly related to the activation of Cl(-) channels.

Cell volume regulation and swelling-activated chloride channels

Maintenance of a constant volume is essential for normal cell function. Following cell swelling, as a consequence of reduction of extracellular osmolarity or increase of intracellular content of osmolytes, animal cells are able to restore their original volume by activation of potassium and chloride conductances. The loss of these ions, followed passively by water, is responsible for the homeostatic response called regulatory volume decrease (RVD). Activation of a chloride conductance upon cell swelling is a key step in RVD. Several proteins have been proposed as candidates for this chloride conductance. The status of the field is reviewed, with particular emphasis on ClC-3, a member of the ClC family which has been recently proposed as the chloride channel involved in cell volume regulation.

Cell swelling stimulates cytosol to membrane transposition of ICln

ICln is a multifunctional protein that is essential for cell volume regulation. It can be found in the cytosol and is associated with the cell membrane. Besides its role in the splicing process, ICln is critically involved in the generation of ion currents activated during regulatory volume decrease after cell swelling (RVDC). If reconstituted in artificial bilayers, ICln can form ion channels with biophysical properties related to RVDC. We investigated (i) the cytosol versus cell membrane distribution of ICln in rat kidney tubules, NIH 3T3 fibroblasts, Madin-Darby canine kidney (MDCK) cells, and LLC-PK1 epithelial cells, (ii) fluorescence resonance energy transfer (FRET) in living fibroblasts between fluorescently tagged ICln and fluorochromes in the cell membrane, and (iii) possible functional consequences of an enhanced ICln presence at the cell membrane. We demonstrate that ICln distribution in rat kidneys depends on the parenchymal localization and functional state of the tubules and that cell swelling causes ICln redistribution from the cytosol to the cell membrane in NIH 3T3 fibroblasts and LLC-PK1 cells. The addition of purified ICln protein to the extracellular solution or overexpression of farnesylated ICln leads to an increased anion permeability in NIH 3T3 fibroblasts. The swelling-induced redistribution of ICln correlates to altered kinetics of RVDC in NIH 3T3 fibroblasts, LLC-PK1 cells, and MDCK cells. In these cells, RVDC develops more rapidly, and in MDCK cells the rate of swelling-induced depolarization is accelerated if cells are swollen for a second time. This coincides with an enhanced ICln association with the cell membrane.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Osmoregulation and cell volume regulation in the preimplantation embryo

The early preimplantation mammalian embryo possesses mechanisms that regulate intracellular osmolarity and cell volume. While transport of osmotically active inorganic ions might play a role in this process in embryos, the major mechanisms that have been identified and studied are those that employ organic osmolytes. Organic osmolytes provide a substantial portion of intracellular osmotic support in embryos and are required for their development under in vivo conditions. The main osmolytes that have been identified in cleavage stage embryos are accumulated via two transport systems of the neurotransmitter transporter family active in early preimplantation embryos--the glycine transport system (GLY) and the beta-amino acid transport system (system beta). While system beta has been established to have a similar role in many other cells, this is a novel function for the GLY transport system. The intracellular concentration of organic osmolytes such as glycine in early preimplantation embryos is regulated by tonicity, allowing the embryo to regulate its volume against shrinkage and to control its internal osmolarity. In addition, the cells of the embryo can regulate against an increase in volume via controlled release of osmolytes from the cytoplasm. This is mediated by a swelling-activated anion channel that is also highly permeable to a range of organic osmolytes, and which closely resembles similar channels found in many other cell types (VSOAC channels). Together, these mechanisms appear to regulate cell volume in the egg through the early cleavage stages of embryogenesis, after which there are indications that the mechanisms of osmoregulation change.

Diverse effects of cytochalasin B on priming and triggering the respiratory burst activity in human neutrophils and monocytes

Cytochalasin B, despite its potent enhancing effect on superoxide (O2-) release triggered by N-formyl-methionyl-leucyl-phenylalanine (FMLP) and many other agonists, significantly inhibited O2- release triggered by interleukin 8 (IL-8) and platelet-activating factor in human neutrophils. Cytochalasin B also enhanced changes in membrane potential stimulated by FMLP but inhibited those stimulated by IL-8. Using IL-8 as a triggering agonist, we found that the priming effect of tumor necrosis factor (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) on O2- release was slightly but significantly potentiated by cytochalasin B. O2- release triggered by TNF and GM-CSF was completely abolished by cytochalasin B. In contrast to these diverse effects of cytochalasin B on O2- release, changes in cytoplasmic pH stimulated by FMLP, IL-8, TNF, and GM-CSF were not or were only minimally affected by cytochalasin B. Unlike human neutrophils, human monocytes stimulated by FMLP showed inhibition of O2- release and changes in membrane potential in response to cytochalasin B, and the priming effect of TNF and GM-CSF on O2- release in human monocytes was completely abolished by cytochalasin B. These findings indicate the diverse effects of cytochalasin B on phagocytes and suggest distinct regulatory mechanisms according to the functions, agonists, and cell types.

Matrix-dependent mechanism of neutrophil-mediated release and activation of matrix metalloproteinase 9 in myocardial ischemia/reperfusion

BACKGROUND

A key component of reperfusion of myocardial infarction is an immediate inflammatory response, which enhances tissue repair. Matrix turnover is crucial to tissue repair, and matrix metalloproteinases (MMPs) are key enzymes involved in matrix degradation. The hypothesis tested is that one inflammation-based effector of tissue repair is the secretion and activation of MMP-9 by infiltrating neutrophils.

METHODS AND RESULTS

Cardiac lymph and tissue were assayed for atent and active MMP-2 and MMP-9 by zymography and immunochemistry. Dual-labeling immunofluorescence determined the cellular source of MMP-9 protein. Isolated canine neutrophils were incubated with preischemic and postischemic cardiac lymph in the presence and absence of collagen-fibronectin pads, and the supernatants were assayed for latent and active MMP-9. MMP-9 increased during the first hours of reperfusion in both lymph supernatants and myocardial extracts, and this increase was of neutrophil origin. MMP-9 in the cardiac lymph remained latent but was activatable. In contrast, MMP-9 in the myocardium was in both latent and active forms. In situ zymography demonstrated that activated MMP-9 surrounded the infiltrated neutrophils. When postischemic cardiac lymph was incubated with neutrophils in vitro, MMP-9 secretion and activation occurred only in the presence of a collagen-fibronectin substrate; preischemic cardiac lymph did not induce significant secretion or activation.

CONCLUSIONS

Infiltrating neutrophils are an early source of MMP-9 after reperfusion, and a portion of MMP-9 in the myocardium is active. Infiltrating neutrophils may localize MMP-9 activation by secreting MMP-9 and as a source of activating proteases.

ICln is essential for cellular and early embryonic viability

pICln is a 26-kDa protein that is ubiquitously expressed and highly conserved from Xenopus laevis to Homo sapiens. The physiological functions of pICln remain to be established. To address this question, we disrupted the ICln gene in embryonic stem cells. We found that murine embryos lacking ICln die early in gestation (between stages E3.5 and E7.5). Furthermore, we found that ICln is essential for embryonic stem cell viability. Previously, we showed that pICln interacts directly with a homolog of a yeast protein that binds a PAK-like kinase and participates in the regulation of cell morphology and cell cycling. pICln also forms a complex with several core spliceosomal proteins, and this interaction may play a role in the regulation of spliceosomal biogenesis. Collectively, these data strongly suggest that pICln participates in critical cellular pathways, including regulation of the cell cycle and RNA processing.

The role of ClC-3 in volume-activated chloride currents and volume regulation in bovine epithelial cells demonstrated by antisense inhibition

1. A chloride current with mild outward rectification was induced in the native bovine non-pigmented ciliary epithelial (NPCE) cells by a 23 % hypotonic solution. The current showed no or little inactivation at depolarized steps. 2. ATP blocked 88 and 61 % of the outward and inward components of the volume-activated chloride current (ICl,vol) with an IC50 of 5.3 and 9.6 mM, respectively. 3. The volume-activated chloride current was decreased and the activation of the current was delayed by inhibiting endogenous ClC-3 expression using a ClC-3 antisense oligonucleotide. The inhibition of the current as a function of antisense concentration was asymptotic with a maximum about 60 %. The remaining current was probably not derived from ClC-3 and was inhibited by ATP. 4. ClC-3 expression in the bovine NPCE cells was verified by immunofluorescence studies. ClC-3 immunofluorescence was distributed throughout the cells but with the predominant location within the nucleus. The expression of ClC-3 protein was diminished by the ClC-3 antisense oligonucleotide with the greatest diminution occurring in the nuclear region. 5. The size of the volume-activated chloride current was positively correlated with the ClC-3 immunofluorescence level. 6. Regulatory volume decrease of the NPCE cells was reduced by ClC-3 antisense oligonucleotide. 7. We conclude that endogenous ClC-3 is associated with the volume-activated chloride current and is involved in cell volume regulation, but that it can only contribute towards a proportion of the current in NPCE cells. 8. The nuclear predominance of ClC-3 immunofluorescence in NPCE cells, the absence of basal activity of chloride current and the marked pharmacological differences between IClC-3 and ICl,vol argue against ClC-3 being the only, or even the main, volume-activated chloride channel in NPCE cells.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Relationships between the actin cytoskeleton and cell volume regulation

The actin cytoskeleton mediates a variety of essential biological functions in cells, including division, shape changes, and movement. A number of studies have suggested that the abundant submembranous actin cytoskeleton present in the cortex of many cell types is involved in the regulation of cell volume. This relationship is supported by numerous works which document the changes in the structural organization of the actin cytoskeleton which accompany cell volume changes and the F-actin-dependence of the regulatory volume responses. In addition, other studies demonstrate structural and functional relationships between the actin cytoskeleton and the membrane transporters known to be involved in cell volume homeostasis. This review provides a summary of the current level of knowledge in this area and discusses the mechanisms which may underlie the linkage between the actin cytoskeleton and cell volume regulation.

Association of intrinsic pICln with volume-activated Cl- current and volume regulation in a native epithelial cell

We investigated the relationship between pICln, the volume-activated Cl- current, and volume regulation in native bovine nonpigmented ciliary epithelial (NPCE) cells. Immunofluorescence studies demonstrated the presence of pICln protein in the NPCE cells. Exposure to hypotonic solution activated a Cl- current and induced regulatory volume decrease (RVD) in freshly isolated bovine NPCE cells. Three antisense oligonucleotides complementary to human pICln mRNA were used in the experiments. The antisense oligonucleotides were taken up by the cells in a dose-dependent manner. The antisense oligonucleotides, designed to be complementary to the initiation codon region of the human pICln mRNA, "knocked down" the pICln protein immunofluorescence, delayed the activation of volume-activated Cl- current, diminished the value of the current, and reduced the ability of the cells to volume regulate. We conclude that pICln is involved in the activation pathway of the volume-activated Cl- current and RVD following hypotonic swelling.

Molecular physiology of renal chloride channels

Chloride channels are present in all cells of the kidney. Physiological studies have revealed a bewildering variety of kidney chloride channels, but only in the past few years has molecular information on some of these channels emerged. This review will focus on cloned chloride channels expressed in renal cells.