bluuebunny/crossref_metadata_2025 huggingface dataset:
Expression of Q227L-Gα
s
inhibits intimal vessel wall hyperplasia after balloon injury
Authors: Holness Wilfred , Santore Tara A. , Brown George P. , Fallon John T. , Taubman Mark B. , Iyengar Ravi
Published: July 2002

Interaction between signaling pathways regulates many cellular
functions, including proliferation. The Gα
s
/cAMP
pathway is known to inhibit signal flow from receptor tyrosine kinases
to mitogen-activated protein kinase (MAPK)-1,2 and, thus, inhibit
proliferation. Elevation of cAMP or adenovirus-directed expression of
mutant (Q227L)–Gα
s
(α
s
*) inhibits the
proliferation of rat vascular smooth muscle cells (VSMCs) in culture.
Platelet-derived growth factor (PDGF) stimulated MAPK activation and
DNA synthesis was also blocked by expression of α
s
*.
However, it is not known whether such mechanisms are operative
in vivo
. Proliferation of vascular smooth muscle cells
in vivo
was induced by balloon injury of carotid
arteries in the rat. Recombinant adenovirus encoding β-galactosidase
(β-gal) or α
s
* was applied to arterial segments injured
by the balloon catheters. The α
s
*-treated vessels showed
decreased phospho-MAPK staining in the intima as compared with
β-gal-treated vessels. Application of α
s
*, but not
β-gal containing adenovirus, inhibited formation of neointima by
50%. No change was observed in total vessel diameter or in the media
or adventitia. These results suggest that the interaction between the
Gα
s
and MAPK pathways can regulate proliferation
in vivo
and that targeted expression of activated
Gα
s
may have therapeutic potential for the treatment of
vascular pathophysiologies that arise from intimal hyperplasia.
The Jevonian Revolution in International Trade Theory
Authors: Aldrich John
Published: July 2002

There was no Jevonian revolution in international trade theory. The new ideas of marginal analysis entering British economics in the late nineteenth century seem to have left the field untouched. According to Schumpeter (1954, p. 609), Marshall “did not do more than polish and develop Mill's meaning … Edgeworth's famous restatement added many interesting details but also did not go beyond Mill in fundamentals.”
A Three-dimensional Collagen Lattice Induces Protein Kinase C-ζ Activity: Role in α2 Integrin and Collagenase mRNA Expression
Authors: Xu Jiahua , Clark Richard A.F.
Published: July 2002

A three-dimensional collagen lattice can provide skin fibroblasts with a cell culture environment that simulates normal dermis. Such a collagen matrix environment regulates interstitial collagenase (type I metalloproteinase [MMP-1], collagenase-1) and collagen receptor α2 subunit mRNA expression in both unstimulated or platelet-derived growth factor–stimulated dermal fibroblasts (Xu, J., and R.A.F. Clark. 1996. J. Cell Biol. 132:239–249). Here we report that the collagen gel can signal protein kinase C (PKC)-ζ activation in human dermal fibroblasts. An in vitro kinase assay demonstrated that autophosphorylation of PKC-ζ immunoprecipitates was markedly increased by a collagen matrix. In contrast, no alteration in PKC-ζ protein levels or intracellular location was observed. DNA binding activity of nuclear factor κB (NF-κB), a downstream regulatory target of PKC-ζ, was also increased by fibroblasts grown in collagen gel. The composition of the NF-κB/Rel complexes that contained p50, was not changed. The potential role of PKC-ζ in collagen gel–induced mRNA expression of collagen receptor α2 subunit and human fibroblast MMP-1 was assessed by the following evidence. Increased levels of α2 and MMP-1 mRNA in collagen gel–stimulated fibroblasts were abrogated by bisindolylmaleimide GF 109203X and calphostin C, chemical inhibitors for PKC, but retained when cells were depleted of 12-myristate 13-acetate (PMA)–inducible PKC isoforms by 24 h of pretreatment with phorbol PMA. Antisense oligonucleotides complementary to the 5′ end of PKC-ζ mRNA sequences significantly reduced the collagen lattice–stimulated α2 and MMP-1 mRNA levels. Taken together, these data indicate that PKC-ζ, a PKC isoform not inducible by PMA or diacylglycerol, is a component of collagen matrix stimulatory pathway for α2 and MMP-1 mRNA expression. Thus, a three-dimensional collagen lattice maintains the dermal fibroblast phenotype, in part, through the activation of PKC-ζ.
Temporal Phases in Apoptosis Defined by the Actions of Src Homology 2 Domains, Ceramide, Bcl-2, Interleukin-1β Converting Enzyme Family Proteases, and a Dense Membrane Fraction
Authors: Farschon David M. , Couture Clément , Mustelin Tomas , Newmeyer Donald D.
Published: July 2002

We have begun to explore the mechanisms of apoptosis using a cell-free system based on extracts from Xenopus eggs. Nuclei assembled or placed in these extracts undergo the morphological changes typical of apoptosis and eventually disintegrate. We used this system to investigate the potential involvement in apoptosis of proteins containing Src homology 2 (SH2) domains, which are known to interact with specific tyrosine-phosphorylated ligands. SH2 domains from a number of signaling proteins, including Lck, Src, and Abl, inhibited apoptosis when present at concentrations of 10–100 nM. The inhibition was dependent on specific interaction with endogenous tyrosine-phosphorylated ligands. A synthetic peptide ligand for Src family SH2 domains also inhibited apoptosis in a phosphotyrosine-dependent manner. Kinetic analysis defined three phases in the apoptotic process occurring in this cell-free system. SH2 domains and ceramide act throughout the first 60–90 min of the process (the “initiation” phase). Next, Bcl-2, interleukin-1β converting enzyme family(CPP32-like) proteases, and the heavy membrane fraction act in a period occurring ∼90–120 min after the start of incubation (the “sentencing” phase). In the final phase (“execution”), the process of active nuclear destruction ensues.
Mutational Analysis of Mdm1p Function in Nuclear and Mitochondrial Inheritance
Authors: Fisk Harold A. , Yaffe Michael P.
Published: July 2002

Nuclear and mitochondrial transmission to daughter buds of Saccharomyces cerevisiae depends on Mdm1p, an intermediate filament-like protein localized to numerous punctate structures distributed throughout the yeast cell cytoplasm. These structures disappear and organelle inheritance is disrupted when mdm1 mutant cells are incubated at the restrictive temperature. To characterize further the function of Mdm1p, new mutant mdm1 alleles that confer temperature-sensitive growth and defects in organelle inheritance but produce stable Mdm1p structures were isolated. Microscopic analysis of the new mdm1 mutants revealed three phenotypic classes: Class I mutants showed defects in both mitochondrial and nuclear transmission; Class II alleles displayed defective mitochondrial inheritance but had no effect on nuclear movement; and Class III mutants showed aberrant nuclear inheritance but normal mitochondrial distribution. Class I and II mutants also exhibited altered mitochondrial morphology, possessing primarily small, round mitochondria instead of the extended tubular structures found in wild-type cells. Mutant mdm1 alleles affecting nuclear transmission were of two types: Class Ia and IIIa mutants were deficient for nuclear movement into daughter buds, while Class Ib and IIIb mutants displayed a complete transfer of all nuclear DNA into buds. The mutations defining all three allelic classes mapped to two distinct domains within the Mdm1p protein. Genetic crosses of yeast strains containing different mdm1 alleles revealed complex genetic interactions including intragenic suppression, synthetic phenotypes, and intragenic complementation. These results support a model of Mdm1p function in which a network comprised of multimeric assemblies of the protein mediates two distinct cellular processes.
A Transgenic Myogenic Cell Line Lacking Ryanodine Receptor Protein for Homologous Expression Studies: Reconstitution of Ry1R Protein and Function
Authors: Moore R.A. , Nguyen H. , Galceran J. , Pessah I.N. , Allen P.D.
Published: July 2002

Abstract. CCS embryonic stem (ES) cells possessing two mutant alleles (ry1r−/ry1r−) for the skeletal muscle ryanodine receptor (RyR) have been produced and injected subcutaneously into severely compromised immunodeficient mice to produce teratocarcinomas in which Ry1R expression is absent. Several primary fibroblast cell lines were isolated and subcloned from one of these tumors that contain the knockout mutation in both alleles and exhibit a doubling time of 18–24 h, are not contact growth inhibited, do not exhibit drastic morphological change upon serum reduction, and possess the normal complement of chromosomes. Four of these fibroblast clones were infected with a retrovirus containing the cDNA encoding myoD and a puromycin selection marker. Several (1–2 μg/ml) puromycin-resistant subclones from each initial cell line were expanded and examined for their ability to express myoD and to form multinucleated myotubes that express desmin and myosin upon removal of mitogens. One of these clones (1B5 cells) was selected on this basis for further study. These cells, upon withdrawal of mitogens for 5–7 d, were shown by Western blot analysis to express key triadic proteins, including skeletal triadin, calsequestrin, FK506-binding protein, 12 kD, sarco(endo)plasmic reticulum calcium–ATPase1, and dihydropyridine receptors. Neither RyR isoform protein, Ry1R (skeletal), Ry2R (cardiac), nor Ry3R (brain), were detected in differentiated 1B5 cells. Measurements of intracellular Ca2+ by ratio fluorescence imaging of fura-2–loaded cells revealed that differentiated 1B5 cells exhibited no responses to K+ (40 mM) depolarization, ryanodine (50–500 μM), or caffeine (20–100 mM). Transient transfection of the 1B5 cells with the full-length rabbit Ry1R cDNA restored the expected responses to K+ depolarization, caffeine, and ryanodine. Depolarization-induced Ca2+ release was independent of extracellular Ca2+, consistent with skeletal-type excitation–contraction coupling. Wild-type Ry1R expressed in 1B5 cells were reconstituted into bilayer lipid membranes and found to be indistinguishable from channels reconstituted from rabbit sarcoplasmic reticulum with respect to unitary conductance, open dwell times, and responses to ryanodine and ruthenium red. The 1B5 cell line provides a powerful and easily managed homologous expression system in which to study how Ry1R structure relates to function.
Kinetic Control of Multiple Forms of Ca2+ Spikes by Inositol Trisphosphate in Pancreatic Acinar Cells
Authors: Ito Koichi , Miyashita Yasushi , Kasai Haruo
Published: July 2002

The mechanisms of agonist-induced Ca2+ spikes have been investigated using a caged inositol 1,4,5-trisphosphate (IP3) and a low-affinity Ca2+ indicator, BTC, in pancreatic acinar cells. Rapid photolysis of caged IP3 was able to reproduce acetylcholine (ACh)-induced three forms of Ca2+ spikes: local Ca2+ spikes and submicromolar (<1 μM) and micromolar (1–15 μM) global Ca2+ spikes (Ca2+ waves). These observations indicate that subcellular gradients of IP3 sensitivity underlie all forms of ACh-induced Ca2+ spikes, and that the amplitude and extent of Ca2+ spikes are determined by the concentration of IP3. IP3-induced local Ca2+ spikes exhibited similar time courses to those generated by ACh, supporting a role for Ca2+-induced Ca2+ release in local Ca2+ spikes. In contrast, IP3- induced global Ca2+ spikes were consistently faster than those evoked with ACh at all concentrations of IP3 and ACh, suggesting that production of IP3 via phospholipase C was slow and limited the spread of the Ca2+ spikes. Indeed, gradual photolysis of caged IP3 reproduced ACh-induced slow Ca2+ spikes. Thus, local and global Ca2+ spikes involve distinct mechanisms, and the kinetics of global Ca2+ spikes depends on that of IP3 production particularly in those cells such as acinar cells where heterogeneity in IP3 sensitivity plays critical role.
Yes-Associated Protein 65 Localizes P62c-Yes to the Apical Compartment of Airway Epithelia by Association with Ebp50
Authors: Mohler Peter J. , Kreda Silvia M. , Boucher Richard C. , Sudol Marius , Stutts M. Jackson , Milgram Sharon L.
Published: July 2002

We recently showed that the COOH terminus of the cystic fibrosis transmembrane conductance regulator associates with the submembranous scaffolding protein EBP50 (ERM-binding phosphoprotein 50 kD; also called Na+/H+ exchanger regulatory factor). Since EBP50 associates with ezrin, this interaction links the cystic fibrosis transmembrane conductance regulator (CFTR) to the cortical actin cytoskeleton. EBP50 has two PDZ domains, and CFTR binds with high affinity to the first PDZ domain. Here, we report that Yes-associated protein 65 (YAP65) binds with high affinity to the second EBP50 PDZ domain. YAP65 is concentrated at the apical membrane in airway epithelia and interacts with EBP50 in cells. The COOH terminus of YAP65 is necessary and sufficient to mediate association with EBP50. The EBP50–YAP65 interaction is involved in the compartmentalization of YAP65 at the apical membrane since mutant YAP65 proteins lacking the EBP50 interaction motif are mislocalized when expressed in airway epithelial cells. In addition, we show that the nonreceptor tyrosine kinase c-Yes is contained within EBP50 protein complexes by association with YAP65. Subapical EBP50 protein complexes, containing the nonreceptor tyrosine kinase c-Yes, may regulate apical signal transduction pathways leading to changes in ion transport, cytoskeletal organization, or gene expression in epithelial cells.
Intranuclear Anchoring of Repetitive DNA Sequences
Authors: Weipoltshammer Klara , Schöfer Christian , Almeder Marlene , Philimonenko Vlada V. , Frei Klemens , Wachtler Franz , Hozák Pavel
Published: July 2002

Centromeres, telomeres, and ribosomal gene clusters consist of repetitive DNA sequences. To assess their contributions to the spatial organization of the interphase genome, their interactions with the nucleoskeleton were examined in quiescent and activated human lymphocytes. The nucleoskeletons were prepared using “physiological” conditions. The resulting structures were probed for specific DNA sequences of centromeres, telomeres, and ribosomal genes by in situ hybridization; the electroeluted DNA fractions were examined by blot hybridization. In both nonstimulated and stimulated lymphocytes, centromeric alpha-satellite repeats were almost exclusively found in the eluted fraction, while telomeric sequences remained attached to the nucleoskeleton. Ribosomal genes showed a transcription-dependent attachment pattern: in unstimulated lymphocytes, transcriptionally inactive ribosomal genes located outside the nucleolus were eluted completely. When comparing transcription unit and intergenic spacer, significantly more of the intergenic spacer was removed. In activated lymphocytes, considerable but similar amounts of both rDNA fragments were eluted. The results demonstrate that: (a) the various repetitive DNA sequences differ significantly in their intranuclear anchoring, (b) telomeric rather than centromeric DNA sequences form stable attachments to the nucleoskeleton, and (c) different attachment mechanisms might be responsible for the interaction of ribosomal genes with the nucleoskeleton.
Influenza M2 Proton Channel Activity Selectively Inhibits Trans-Golgi Network Release of Apical Membrane and Secreted Proteins in Polarized Madin-Darby Canine Kidney Cells
Authors: Henkel Jennifer R. , Gibson Gregory A. , Poland Paul A. , Ellis Mark A. , Hughey Rebecca P. , Weisz Ora A.
Published: July 2002

The function of acidification in protein sorting along the biosynthetic pathway has been difficult to elucidate, in part because reagents used to alter organelle pH affect all acidified compartments and are poorly reversible. We have used a novel approach to examine the role of acidification in protein sorting in polarized Madin-Darby canine kidney (MDCK) cells. We expressed the influenza virus M2 protein, an acid-activated ion channel that equilibrates lumenal and cytosolic pH, in polarized MDCK cells and examined the consequences on the targeting and delivery of apical and basolateral proteins. M2 activity affects the pH of only a subset of acidified organelles, and its activity can be rapidly reversed using ion channel blockers (Henkel, J.R., G. Apodaca, Y. Altschuler, S. Hardy, and O.A. Weisz. 1998. Mol. Biol. Cell. 8:2477–2490; Henkel, J.R., J.L. Popovich, G.A. Gibson, S.C. Watkins, and O.A. Weisz. 1999. J. Biol. Chem. 274:9854–9860). M2 expression significantly decreased the kinetics of cell surface delivery of the apical membrane protein influenza hemagglutinin, but not of the basolaterally delivered polymeric immunoglobulin receptor. Similarly, the kinetics of apical secretion of a soluble form of γ-glutamyltranspeptidase were reduced with no effect on the basolaterally secreted fraction. Interestingly, M2 activity had no effect on the rate of secretion of a nonglycosylated protein (human growth hormone [hGH]) that was secreted equally from both surfaces. However, M2 slowed apical secretion of a glycosylated mutant of hGH that was secreted predominantly apically. Our results suggest a role for acidic trans-Golgi network pH in signal-mediated loading of apical cargo into forming vesicles.
Toc64, a New Component of the Protein Translocon of Chloroplasts
Authors: Sohrt Karen , Soll Jürgen
Published: July 2002

A subunit of the preprotein translocon of the outer envelope of chloroplasts (Toc complex) of 64 kD is described, Toc64. Toc64 copurifies on sucrose density gradients with the isolated Toc complex. Furthermore, it can be cross-linked in intact chloroplasts to a high molecular weight complex containing both Toc and Tic subunits and a precursor protein. The 0 Å cross-linker CuCl2 yields the reversible formation of disulfide bridge(s) between Toc64 and the established Toc complex subunits in purified outer envelope membranes. Toc64 contains three tetratricopeptide repeat motifs that are exposed at the chloroplast cytosol interface. We propose that Toc64 functions early in preprotein translocation, maybe as a docking protein for cytosolic cofactors of the protein import into chloroplasts.
Phosphorylated Pleckstrin Induces Cell Spreading via an Integrin-Dependent Pathway
Authors: Roll Richard L. , Bauman Eve Marie , Bennett Joel S. , Abrams Charles S.
Published: July 2002

Pleckstrin is a 40-kD phosphoprotein containing NH2- and COOH-terminal pleckstrin homology (PH) domains separated by a disheveled-egl 10-pleckstrin (DEP) domain. After platelet activation, pleckstrin is rapidly phosphorylated by protein kinase C. We reported previously that expressed phosphorylated pleckstrin induces cytoskeletal reorganization and localizes in microvilli along with glycoproteins, such as integrins. Given the role of integrins in cytoskeletal organization and cell spreading, we investigated whether signaling from pleckstrin cooperated with signaling pathways involving the platelet integrin, αIIbβ3. Pleckstrin induced cell spreading in both transformed (COS-1 & CHO) and nontransformed (REF52) cell lines, and this spreading was regulated by pleckstrin phosphorylation. In REF52 cells, pleckstrin-induced spreading was matrix dependent, as evidenced by spreading of these cells on fibrinogen but not on fibronectin. Coexpression with αIIbβ3 did not enhance pleckstrin-mediated cell spreading in either REF52 or CHO cells. However, coexpression of the inactive variant αIIbβ3 Ser753Pro, or β3 Ser753Pro alone, completely blocked pleckstrin-induced spreading. This implies that αIIbβ3 Ser753Pro functions as a competitive inhibitor by blocking the effects of an endogenous receptor that is used in the signaling pathway involved in pleckstrin-induced cell spreading. Expression of a chimeric protein composed of the extracellular and transmembrane portion of Tac fused to the cytoplasmic tail of β3 completely blocked pleckstrin-mediated spreading, whereas chimeras containing the cytoplasmic tail of β3 Ser753Pro or αIIb had no effect. This suggests that the association of an unknown signaling protein with the cytoplasmic tail of an endogenous integrin β-chain is also required for pleckstrin-induced spreading. Thus, expressed phosphorylated pleckstrin promotes cell spreading that is both matrix and integrin dependent. To our knowledge, this is the first example of a mutated integrin functioning as a dominant negative inhibitor.
Apical Membrane Localization of the Adenomatous Polyposis Coli Tumor Suppressor Protein and Subcellular Distribution of the β-Catenin Destruction Complex in Polarized Epithelial Cells
Authors: Reinacher-Schick Anke , Gumbiner Barry M.
Published: July 2002

The adenomatous polyposis coli (APC) protein is implicated in the majority of hereditary and sporadic colon cancers. APC is known to function as a tumor suppressor through downregulation of β-catenin as part of a high molecular weight complex known as the β-catenin destruction complex. The molecular composition of the intact complex and its site of action in the cell are still not well understood. Reports on the subcellular localization of APC in various cell systems have differed significantly and have been consistent with an association with a cytosolic complex, with microtubules, with the nucleus, or with the cortical actin cytoskeleton. To better understand the role of APC and the destruction complex in colorectal cancer, we have begun to characterize and isolate these complexes from confluent polarized human colon epithelial cell monolayers and other epithelial cell types. Subcellular fractionation and immunofluorescence microscopy reveal that a predominant fraction of APC associates tightly with the apical plasma membrane in a variety of epithelial cell types. This apical membrane association is not dependent on the mutational status of either APC or β-catenin. An additional pool of APC is cytosolic and fractionates into two distinct high molecular weight complexes, 20S and 60S in size. Only the 20S fraction contains an appreciable portion of the cellular axin and small but detectable amounts of glycogen synthase kinase 3β and β-catenin. Therefore, it is likely to correspond to the previously characterized β-catenin destruction complex. Dishevelled is almost entirely cytosolic, but does not significantly cofractionate with the 20S complex. The disproportionate amount of APC in the apical membrane and the lack of other destruction complex components in the 60S fraction of APC raise questions about whether these pools of APC take part in the degradation of β-catenin, or alternatively, whether they could be involved in other functions of the protein that still must be determined.
Evidence that the entire Golgi apparatus cycles in interphase HeLa cells
Authors: Miles Suzanne , McManus Heather , Forsten Kimberly E. , Storrie Brian
Published: July 2002

We tested whether the entire Golgi apparatus is a dynamic structure in interphase mammalian cells by assessing the response of 12 different Golgi region proteins to an endoplasmic reticulum (ER) exit block. The proteins chosen spanned the Golgi apparatus and included both Golgi glycosyltransferases and putative matrix proteins. Protein exit from ER was blocked either by microinjection of a GTP-restricted Sar1p mutant protein in the presence of a protein synthesis inhibitor, or by plasmid-encoded expression of the same dominant negative Sar1p. All Golgi region proteins examined lost juxtanuclear Golgi apparatus–like distribution as scored by conventional and confocal fluorescence microscopy in response to an ER exit block, albeit with a differential dependence on Sar1p concentration. Redistribution of GalNAcT2 was more sensitive to low Sar1pdn concentrations than giantin or GM130. Redistribution was most rapid for p27, COPI, and p115. Giantin, GM130, and GalNAcT2 relocated with approximately equal kinetics. Distinct ER accumulation could be demonstrated for all integral membrane proteins. ER-accumulated Golgi region proteins were functional. Photobleaching experiments indicated that Golgi-to-ER protein cycling occurred in the absence of any ER exit block. We conclude that the entire Golgi apparatus is a dynamic structure and suggest that most, if not all, Golgi region–integral membrane proteins cycle through ER in interphase cells.
Heteroclitic Immunization Induces Tumor Immunity
Authors: Dyall Ruben , Bowne Wilbur B. , Weber Lawrence W. , LeMaoult Joel , Szabo Paul , Moroi Yoichi , Piskun Gregory , Lewis Jonathan J. , Houghton Alan N. , Nikolić-Žugić Janko
Published: July 2002

In tumor transplantation models in mice, cytotoxic T lymphocytes (CTLs) are typically the primary effector cells. CTLs recognize major histocompatibility complex (MHC) class I–associated peptides expressed by tumors, leading to tumor rejection. Peptides presented by cancer cells can originate from viral proteins, normal self-proteins regulated during differentiation, or altered proteins derived from genetic alterations. However, many tumor peptides recognized by CTLs are poor immunogens, unable to induce activation and differentiation of effector CTLs. We used MHC binding motifs and the knowledge of class I:peptide:TCR structure to design heteroclitic CTL vaccines that exploit the expression of poorly immunogenic tumor peptides. The in vivo potency of this approach was demonstrated using viral and self-(differentiation) antigens as models. First, a synthetic variant of a viral antigen was expressed as a tumor antigen, and heteroclitic immunization with peptides and DNA was used to protect against tumor challenge and elicit regression of 3-d tumors. Second, a peptide from a relevant self-antigen of the tyrosinase family expressed by melanoma cells was used to design a heteroclitic peptide vaccine that successfully induced tumor protection. These results establish the in vivo applicability of heteroclitic immunization against tumors, including immunity to poorly immunogenic self-proteins.
Acceleration of Sodium-Calcium Exchange Activity during ATP-induced Calcium Release in Transfected Chinese Hamster Ovary Cells
Authors: Vázquez Melissa , Fang Yu , Reeves John P.
Published: July 2002

The P2U purinergic agonist ATP (0.3 mM) elicited an increase in [Ca2+]i due to Ca2+ release from intracellular stores in transfected Chinese hamster ovary cells that express the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells). The following observations indicate that ATP-evoked Ca2+ release was accompanied by a Ca2+- dependent regulatory activation of Na+/Ca2+ exchange activity: Addition of extracellular Ca2+ (0.7 mM) 0–1 min after ATP evoked a dramatic rise in [Ca2+]i in Na+-free media (Li+ substitution) compared to Na+-containing media; no differences between Na+- and Li+-based media were observed with vector-transfected cells. In the presence of physiological concentrations of extracellular Na+ and Ca2+, the ATP-evoked rise in [Ca2+]i declined more rapidly in CK1.4 cells compared to control cells, but then attained a long-lived plateau of elevated [Ca2+]i which eventually came to exceed the declining [Ca2+]i values in control cells. ATP elicited a transient acceleration of exchange-mediated Ba2+ influx, consistent with regulatory activation of the Na+/Ca2+ exchanger. The acceleration of Ba2+ influx was not observed in vector-transfected control cells, or in CK1.4 cells in the absence of intracellular Na+ or when the Ca2+ content of the intracellular stores had been reduced by prior treatment with ionomycin. The protein kinase C activator phorbol 12-myristate 13-acetate attenuated the exchange-mediated rise in [Ca2+]i under Na+-free conditions, but did not inhibit the ATP-evoked stimulation of Ba2+ influx. The effects of PMA are therefore not due to inhibition of exchange activity, but probably reflect the influence of protein kinase C on other Ca2+ homeostatic mechanisms. We conclude that exchange activity is accelerated during ATP-evoked Ca2+ release from intracellular stores through regulatory activation by increased [Ca2+]i. In the absence of extracellular Ca2+, the stimulation of exchange activity is short-lived and follows the time course of the [Ca2+]i transient; in the presence of extracellular Ca2+, we suggest that the exchanger remains activated for a longer period of time, thereby stabilizing and prolonging the plateau phase of store-dependent Ca2+ entry.
Slow Inactivation Does Not Affect Movement of the Fast Inactivation Gate in Voltage-gated Na+ Channels
Authors: Vedantham Vasanth , Cannon Stephen C.
Published: July 2002

Voltage-gated Na+ channels exhibit two forms of inactivation, one form (fast inactivation) takes effect on the order of milliseconds and the other (slow inactivation) on the order of seconds to minutes. While previous studies have suggested that fast and slow inactivation are structurally independent gating processes, little is known about the relationship between the two. In this study, we probed this relationship by examining the effects of slow inactivation on a conformational marker for fast inactivation, the accessibility of a site on the Na+ channel III–IV linker that is believed to form a part of the fast inactivation particle. When cysteine was substituted for phenylalanine at position 1304 in the rat skeletal muscle sodium channel (μl), application of [2-(trimethylammonium)ethyl]methanethiosulfonate (MTS-ET) to the cytoplasmic face of inside-out patches from Xenopus oocytes injected with F1304C RNA dramatically disrupted fast inactivation and displayed voltage-dependent reaction kinetics that closely paralleled the steady state availability (h∞•) curve. Based on this observation, the accessibility of cys1304 was used as a conformational marker to probe the position of the fast inactivation gate during the development of and the recovery from slow inactivation. We found that burial of cys1304 is not altered by the onset of slow inactivation, and that recovery of accessibility of cys1304 is not slowed after long (2–10 s) depolarizations. These results suggest that (a) fast and slow inactivation are structurally distinct processes that are not tightly coupled, (b) fast and slow inactivation are not mutually exclusive processes (i.e., sodium channels may be fast- and slow-inactivated simultaneously), and (c) after long depolarizations, recovery from fast inactivation precedes recovery from slow inactivation.
Role of the S4 in Cooperativity of Voltage-dependent Potassium Channel Activation
Authors: Smith-Maxwell Catherine J. , Ledwell Jennifer L. , Aldrich Richard W.
Published: July 2002

Charged residues in the S4 transmembrane segment of voltage-gated cation channels play a key role in opening channels in response to changes in voltage across the cell membrane. However, the molecular mechanism of channel activation is not well understood. To learn more about the role of the S4 in channel gating, we constructed chimeras in which S4 segments from several divergent potassium channels, Shab, Shal, Shaw, and Kv3.2, were inserted into a Shaker potassium channel background. These S4 donor channels have distinctly different voltage-dependent gating properties and S4 amino acid sequences. None of the S4 chimeras have the gating behavior of their respective S4 donor channels. The conductance–voltage relations of all S4 chimeras are shifted to more positive voltages and the slopes are decreased. There is no consistent correlation between the nominal charge content of the S4 and the slope of the conductance–voltage relation, suggesting that the mutations introduced by the S4 chimeras may alter cooperative interactions in the gating process. We compared the gating behavior of the Shaw S4 chimera with its parent channels, Shaker and Shaw, in detail. The Shaw S4 substitution alters activation gating profoundly without introducing obvious changes in other channel functions. Analysis of the voltage-dependent gating kinetics suggests that the dominant effect of the Shaw S4 substitution is to alter a single cooperative transition late in the activation pathway, making it rate limiting. This interpretation is supported further by studies of channels assembled from tandem heterodimer constructs with both Shaker and Shaw S4 subunits. Activation gating in the heterodimer channels can be predicted from the properties of the homotetrameric channels only if it is assumed that the mutations alter a cooperative transition in the activation pathway rather than independent transitions.
Monovalent Permeability, Rectification, and Ionic Block of Store-operated Calcium Channels in Jurkat T Lymphocytes
Authors: Kerschbaum Hubert H. , Cahalan Michael D.
Published: July 2002

We used whole-cell recording to characterize ion permeation, rectification, and block of monovalent current through calcium release-activated calcium (CRAC) channels in Jurkat T lymphocytes. Under physiological conditions, CRAC channels exhibit a high degree of selectivity for Ca2+, but can be induced to carry a slowly declining Na+ current when external divalent ions are reduced to micromolar levels. Using a series of organic cations as probes of varying size, we measured reversal potentials and calculated permeability ratios relative to Na+, PX/PNa, in order to estimate the diameter of the conducting pore. Ammonium (NH4+) exhibited the highest relative permeability (PNH4/PNa = 1.37). The largest permeant ion, tetramethylammonium with a diameter of 0.55 nm, had PTMA/PNa of 0.09. N-methyl-d-glucamine (0.50 × 0.64 × 1.20 nm) was not measurably permeant. In addition to carrying monovalent current, NH4+ reduced the slow decline of monovalent current (“inactivation”) upon lowering [Ca2+]o. This kinetic effect of extracellular NH4+ can be accounted for by an increase in intracellular pH (pHi), since raising intracellular pH above 8 reduced the extent of inactivation. In addition, decreasing pHi reduced monovalent and divalent current amplitudes through CRAC channels with a pKa of 6.8. In several channel types, Mg2+ has been shown to produce rectification by a voltage-dependent block mechanism. Mg2+ removal from the pipette solution permitted large outward monovalent currents to flow through CRAC channels while also increasing the channel's relative Cs+ conductance and eliminating the inactivation of monovalent current. Boltzmann fits indicate that intracellular Mg2+ contributes to inward rectification by blocking in a voltage-dependent manner, with a zδ product of 1.88. Ca2+ block from the outside was also found to be voltage dependent with zδ of 1.62. These experiments indicate that the CRAC channel, like voltage-gated Ca2+ channels, achieves selectivity for Ca2+ by selective binding in a large pore with current–voltage characteristics shaped by internal Mg2+.
Functional Differences in Ionic Regulation between Alternatively Spliced Isoforms of the Na+-Ca2+ Exchanger from Drosophila melanogaster
Authors: Omelchenko Alexander , Dyck Christopher , Hnatowich Mark , Buchko John , Nicoll Debora A. , Philipson Kenneth D. , Hryshko Larry V.
Published: July 2002

Ion transport and regulation were studied in two, alternatively spliced isoforms of the Na+-Ca2+ exchanger from Drosophila melanogaster. These exchangers, designated CALX1.1 and CALX1.2, differ by five amino acids in a region where alternative splicing also occurs in the mammalian Na+-Ca2+ exchanger, NCX1. The CALX isoforms were expressed in Xenopus laevis oocytes and characterized electrophysiologically using the giant, excised patch clamp technique. Outward Na+-Ca2+ exchange currents, where pipette Ca2+o exchanges for bath Na+i, were examined in all cases. Although the isoforms exhibited similar transport properties with respect to their Na+i affinities and current–voltage relationships, significant differences were observed in their Na+i- and Ca2+i-dependent regulatory properties. Both isoforms underwent Na+i-dependent inactivation, apparent as a time-dependent decrease in outward exchange current upon Na+i application. We observed a two- to threefold difference in recovery rates from this inactive state and the extent of Na+i-dependent inactivation was approximately twofold greater for CALX1.2 as compared with CALX1.1. Both isoforms showed regulation of Na+-Ca2+ exchange activity by Ca2+i, but their responses to regulatory Ca2+i differed markedly. For both isoforms, the application of cytoplasmic Ca2+i led to a decrease in outward exchange currents. This negative regulation by Ca2+i is unique to Na+-Ca2+ exchangers from Drosophila, and contrasts to the positive regulation produced by cytoplasmic Ca2+ for all other characterized Na+-Ca2+ exchangers. For CALX1.1, Ca2+i inhibited peak and steady state currents almost equally, with the extent of inhibition being ≈80%. In comparison, the effects of regulatory Ca2+i occurred with much higher affinity for CALX1.2, but the extent of these effects was greatly reduced (≈20–40% inhibition). For both exchangers, the effects of regulatory Ca2+i occurred by a direct mechanism and indirectly through effects on Na+i-induced inactivation. Our results show that regulatory Ca2+i decreases Na+i-induced inactivation of CALX1.2, whereas it stabilizes the Na+i-induced inactive state of CALX1.1. These effects of Ca2+i produce striking differences in regulation between CALX isoforms. Our findings indicate that alternative splicing may play a significant role in tailoring the regulatory profile of CALX isoforms and, possibly, other Na+-Ca2+ exchange proteins.
Cyclic Nucleotide–Gated Channels
Authors: Becchetti Andrea , Gamel Katia , Torre Vincent
Published: July 2002

In voltage- and cyclic nucleotide–gated ion channels, the amino-acid loop that connects the S5 and S6 transmembrane domains, is a major component of the channel pore. It determines ion selectivity and participates in gating. In the α subunit of cyclic nucleotide–gated channels from bovine rod, the pore loop is formed by the residues R345–S371, here called R1-S27. These 24 residues were mutated one by one into a cysteine. Mutant channels were expressed in Xenopus laevis oocytes and currents were recorded from excised membrane patches. The accessibility of the substituted cysteines from both sides of the plasma membrane was tested with the thiol-specific reagents 2-aminoethyl methanethiosulfonate (MTSEA) and [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET). Residues V4C, T20C, and P22C were accessible to MTSET only from the external side of the plasma membrane, and to MTSEA from both sides of the plasma membrane. The effect of MTSEA applied to the inner side of T20C and P22C was prevented by adding 10 mM cysteine to the external side of the plasma membrane. W9C was accessible to MTSET from the internal side only. L7C residue was accessible to internal MTSET, but the inhibition was partial, ∼50% when the MTS compound was applied in the absence of cGMP and 25% when it was applied in the presence of cGMP, suggesting that this residue is not located inside the pore lumen and that it changes its position during gating. Currents from T15C and T16C mutants were rapidly potentiated by intracellular MTSET. In T16C, a slower partial inhibition took place after the initial potentiation. Current from I17C progressively decayed in inside-out patches. The rundown was accelerated by inwardly applied MTSET. The accessibility results of MTSET indicate a well-defined topology of the channel pore in which residues between L7 and I17 are inwardly accessible, residue G18 and E19 form the narrowest section of the pore, and T20, P21, P22 and V4 are outwardly accessible.
Role of Leaflet Asymmetry in the Permeability of Model Biological Membranes to Protons, Solutes, and Gases
Authors: Hill Warren G. , Rivers Rickey L. , Zeidel Mark L.
Published: July 2002

Bilayer asymmetry in the apical membrane may be important to the barrier function exhibited by epithelia in the stomach, kidney, and bladder. Previously, we showed that reduced fluidity of a single bilayer leaflet reduced water permeability of the bilayer, and in this study we examine the effect of bilayer asymmetry on permeation of nonelectrolytes, gases, and protons. Bilayer asymmetry was induced in dipalmitoylphosphatidylcholine liposomes by rigidifying the outer leaflet with the rare earth metal, praseodymium (Pr3+). Rigidification was demonstrated by fluorescence anisotropy over a range of temperatures from 24 to 50°C. Pr3+-treatment reduced membrane fluidity at temperatures above 40°C (the phase-transition temperature). Increased fluidity exhibited by dipalmitoylphosphatidylcholine liposomes at 40°C occurred at temperatures 1–3°C higher in Pr3+-treated liposomes, and for both control and Pr3+-treated liposomes permeability coefficients were approximately two orders of magnitude higher at 48° than at 24°C. Reduced fluidity of one leaflet correlated with significantly reduced permeabilities to urea, glycerol, formamide, acetamide, and NH3. Proton permeability of dipalmitoylphosphatidylcholine liposomes was only fourfold higher at 48° than at 24°C, indicating a weak dependence on membrane fluidity, and this increase was abolished by Pr3+. CO2 permeability was unaffected by temperature. We conclude: (a) that decreasing membrane fluidity in a single leaflet is sufficient to reduce overall membrane permeability to solutes and NH3, suggesting that leaflets in a bilayer offer independent resistances to permeation, (b) bilayer asymmetry is a mechanism by which barrier epithelia can reduce permeability, and (c) CO2 permeation through membranes occurs by a mechanism that is not dependent on fluidity.
The Molecular Basis for Na-Dependent Phosphate Transport in Human Erythrocytes and K562 Cells
Authors: Timmer Richard T. , Gunn Robert B.
Published: July 2002

The kinetics of sodium-stimulated phosphate flux and phosphate-stimulated sodium flux in human red cells have been previously described (Shoemaker, D.G., C.A. Bender, and R.B. Gunn. 1988. J. Gen. Physiol. 92:449–474). However, despite the identification of multiple isoforms in three gene families (Timmer, R.T., and R.B. Gunn. 1998. Am. J. Physiol. Cell Physiol. 274:C757–C769), the molecular basis for the sodium-phosphate cotransporter in erythrocytes is unknown. Most cells express multiple isoforms, thus disallowing explication of isoform-specific kinetics and function. We have found that erythrocyte membranes express one dominant isoform, hBNP-1, to which the kinetics can thus be ascribed. In addition, because the erythrocyte Na-PO4 cotransporter can also mediate Li-PO4 cotransport, it has been suggested that this transporter functions as the erythrocyte Na–Li exchanger whose activity is systematically altered in patients with bipolar disease and patients with essential hypertension. To determine the molecular basis for the sodium-phosphate cotransporter, we reasoned that if the kinetics of phosphate transport in a nucleated erythroid-like cell paralleled those of the Na-activated pathway in anucleated erythrocytes and yet were distinct from those known for other Na-PO4 cotransporters, then the expressed genes may be the same in both cell types. In this study, we show that the kinetics of sodium phosphate cotransport were similar in anuclear human erythrocytes and K562 cells, a human erythroleukemic cell line. Although the erythrocyte fluxes were 750-fold smaller, the half-activation concentrations for phosphate and sodium and the relative cation specificities for activation of 32PO4 influx were similar. Na-activation curves for both cell types showed cooperativity consistent with the reported stoichiometry of more than one Na cotransported per PO4. In K562 cells, external lithium activation of phosphate influx was also cooperative. Inhibition by arsenate, KI = 2.6–2.7 mM, and relative inhibition by amiloride, amiloride analogs, phosphonoformate, and phloretin were similar. These characteristics were different from those reported for hNaPi-3 and hPiT-1 in other systems. PCR analysis of sodium-phosphate cotransporter isoforms in K562 cells demonstrated the presence of mRNAs for hPiT-1, hPiT-2, and hBNP-1. The mRNAs for hNaPi-10 and hNaPi-3, the other two known isoforms, were absent. Western analysis of erythrocytes and K562 cells with isoform-specific antibodies detected the presence of only hBNP-1, an isoform expressed in brain neurons and glia. The similarities in the kinetics and the expression of only hBNP-1 protein in the two cell types is strong evidence that hBNP-1 is the erythrocyte and K562 cell sodium-phosphate cotransporter.
Regulation of Exocytosis by Protein Kinases and Ca2+ in Pancreatic Duct Epithelial Cells
Authors: Koh Duk-Su , Moody Mark W. , Nguyen Toan D. , Hille Bertil
Published: July 2002

We asked if the mechanisms of exocytosis and its regulation in epithelial cells share features with those in excitable cells. Cultured dog pancreatic duct epithelial cells were loaded with an oxidizable neurotransmitter, dopamine or serotonin, and the subsequent release of these exogenous molecules during exocytosis was detected by carbon-fiber amperometry. Loaded cells displayed spontaneous exocytosis that may represent constitutive membrane transport. The quantal amperometric events induced by fusion of single vesicles had a rapid onset and decay, resembling those in adrenal chromaffin cells and serotonin-secreting leech neurons. Quantal events were frequently preceded by a “foot,” assumed to be leak of transmitters through a transient fusion pore, suggesting that those cell types share a common fusion mechanism. As in neurons and endocrine cells, exocytosis in the epithelial cells could be evoked by elevating cytoplasmic Ca2+ using ionomycin. Unlike in neurons, hyperosmotic solutions decreased exocytosis in the epithelial cells, and giant amperometric events composed of many concurrent quantal events were observed occasionally. Agents known to increase intracellular cAMP in the cells, such as forskolin, epinephrine, vasoactive intestinal peptide, or 8-Br-cAMP, increased the rate of exocytosis. The forskolin effect was inhibited by the Rp-isomer of cAMPS, a specific antagonist of protein kinase A, whereas the Sp-isomer, a specific agonist of PKA, evoked exocytosis. Thus, PKA is a downstream effector of cAMP. Finally, activation of protein kinase C by phorbol-12-myristate-13-acetate also increased exocytosis. The PMA effect was not mimicked by the inactive analogue, 4α-phorbol-12,13-didecanoate, and it was blocked by the PKC antagonist, bisindolylmaleimide I. Elevation of intracellular Ca2+ was not needed for the actions of forskolin or PMA. In summary, exocytosis in epithelial cells can be stimulated directly by Ca2+, PKA, or PKC, and is mediated by physical mechanisms similar to those in neurons and endocrine cells.
On Iwasawa 𝜆₃-invariants of cyclic cubic fields of prime conductor
Authors: Fukuda Takashi , Komatsu Keiichi
Published: July 2002

For certain cyclic cubic fields

k
k

, we verified that Iwasawa invariants

λ
3

(
k
)

\lambda _3(k)

vanished by calculating units of abelian number field of degree 27. Our method is based on the explicit representation of a system of cyclotomic units of those fields.