Modulation of periodic cold receptor activity by ouabain

The effect of ouabain on the periodic discharge pattern of feline cold receptors was studied in order to substantiate a possible contribution of Na/K pump activity to signal transduction. Afferent activity was recorded from the cold fibres of an isolated preparation of the tongue. The periodic pattern consisted of beating activity and of grouped discharges and was characterized by two parameters, the oscillation frequency and the number of impulses initiated per cycle. Ouabain (10(-7)-10(-6)M) induced in all receptors excitatory responses, consisting of a short vigorous increase of activity followed by inhibition. Thus the receptors never stabilized to or maintained a new static level of activity. The ouabain-induced responses occurred repeatedly in several receptors and were produced by remarkable stereotyped modifications of both the oscillation frequency and the number of impulses per cycle. The oscillation frequency attained peak values which increased monotonically with higher static temperatures and which were considerably greater than peak control values. The data indicate that an electrogenic Na/K pump contributes to the transducer process of cold receptors and that inhibition of this pump evidently gives rise to a depolarizing imbalance of the membrane potential, accelerating the oscillation frequency to a maximum value. Thus the oscillation frequency seems to be controlled by temperature and by membrane potential in cold receptors.

An essential signaling role for the m3G cap in the transport of U1 snRNP to the nucleus

The major small nuclear ribonucleoprotein particles (snRNPs) U1, U2, U4 + U6, and U5 have to be transported from the cytoplasm, where they are synthesized, to the nucleus, where they splice pre-messenger RNAs. Since the free core snRNP proteins in the cytoplasm do not enter the nucleus on their own, the nuclear location signal must either reside on the snRNA or be created as a result of snRNA-protein interaction. Here the involvement by the 5'-terminal cap of snRNA molecules in the nucleo-cytoplasmic transport of UsnRNPs has been studied by microinjection of synthetic U1 RNA molecules into frog oocytes; the U1 RNA bore either the normal cap (m3G) or a chemical derivative. Antibodies in the cytoplasm against the m3G cap inhibited the nuclear uptake of U1 snRNP. U1 RNA that was uncapped or contained an unnatural ApppG cap did not enter the nucleus, even though it carried a normal complement of protein molecules. When the ribose ring of the m3G cap was oxidized with periodate, nuclear transport of U1 snRNPs was severely inhibited. Finally, microinjection of m3G cap alone (but not m7G cap) into oocytes severely inhibited the transport of U1 snRNPs to the nucleus. These data suggest that one step in the nuclear uptake of U1 snRNPs involves the m3G cap structure.

Signal transduction of the human granulocyte-macrophage colony-stimulating factor and interleukin-3 receptors involves tyrosine phosphorylation of a common set of cytoplasmic proteins

Human granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) exert multiple effects on the proliferation, differentiation, and function of myeloid lineage cells through their interaction with specific cell-surface receptors. There is a considerable degree of overlap in the biological effects of these two growth factors, but little is known about the mechanisms of postreceptor signal transduction. We have investigated the effects of GM-CSF and IL-3 on protein tyrosine-kinase activity in a human cell line, MO7E, which proliferates in response to either factor. Tyrosine-kinase activity was detected using immunoblotting with a monoclonal antibody (MoAb) specific for phosphotyrosine. GM-CSF and IL-3 were found to induce a nearly identical pattern of protein tyrosine phosphorylation using both one- and two-dimensional gel electrophoresis. Tyrosine phosphorylation of two cytosolic proteins in particular was increased more than 10-fold, a 93-Kd protein (pp93) and a 70-Kd protein (pp70). Tyrosine phosphorylation of pp93 and pp70 was observed within 1 minute, reached a maximum at 5 to 15 minutes, and gradually decreased thereafter. Other proteins of 150, 125, 63, 55, 42, and 36 Kd were also phosphorylated on tyrosine in response to both GM-CSF and IL-3, although to a lesser degree. Tyrosine phosphorylation was dependent on the concentration of GM-CSF over the range of 0.1 to 10 ng/mL and on IL-3 over the range of 1 to 30 ng/mL. Stimulation of MO7E cells with 12-0-tetradecanoyl-phorbol-13-acetate (TPA) or cytokines such as G-CSF, M-CSF, interleukin-1 (IL-1), interleukin-4 (IL-4), interleukin-6 (IL-6), interferon gamma, tumor necrosis factor (TNF), or transforming growth factor-beta (TGF-beta) did not induce tyrosine phosphorylation of pp93 or pp70, suggesting that these two phosphoproteins are specific for GM-CSF-or IL-3-induced activation. The extent and duration of phosphorylation of all the substrates were increased by pretreatment of cells with vanadate, an inhibitor of protein-tyrosine phosphatases. Importantly, culture of MO7E cells with vanadate (up to 10 mumol/L) resulted in a dose-dependent increase in GM-CSF-or IL-3-induced proliferation of up to 1.8-fold. These results suggest that tyrosine phosphorylation may be important for GM-CSF and IL-3 receptor-mediated signal transduction and that cell proliferation may be, at least partially, regulated by a balance between CSF-induced protein-tyrosine kinase activity and protein-tyrosine phosphatase activity.

Possible mechanisms of abnormal norepinephrine sensitivity and reactivity of resistance vessels and the development of hypertension in spontaneously hypertensive rats. A hypothesis

This study examined structural and functional changes of mesenteric resistance vessels in early, developing, and established stages of hypertension development in spontaneously hypertensive rats in an attempt to identify possible mechanisms of the development and maintenance of hypertension. Our results suggest that the development of hypertension in spontaneously hypertensive rats may be caused by genetic structural and functional abnormalities of resistance vessels. Both abnormalities may be caused by hyperreactivity to norepinephrine through an altered signal transduction process, including the regulation of protein kinase C in smooth muscle cells of resistance vessels in spontaneously hypertensive rats.

Autonomy in tumor cell proliferation

Autonomous replication of tumor cells seems to be an essential factor in the definition of the malignant tumor itself, although tumor cell proliferation is, in general, controlled by the host response including immunological reactions and microenvironment. The cause of the autonomy can hypothetically be classified into four categories as follow: (a) auto- and paracrine growth stimulation; (b) growth factor receptor abnormalities; (c) abnormal signal transduction; (d) self-incitement of 'initiator-replicon' system in DNA replication. These intracellular mechanisms may play important roles in the autonomy as shown in autocrine growth factors from the data obtained in protein-free cell culture. Hypothetically, negative regulation systems on the initiator-replicon may play roles of cell replication in multicellular organisms. Oncogene products and growth factors may affect this regulation system.

Epidermal growth factor receptor regulation and function

The epidermal growth factor (EGF) receptor is a transmembrane, cell-surface glycoprotein that mediates the mitogenic action of a family of ligands, including EGF and transforming growth factor alpha (TGF alpha). Perturbation of this signal transduction pathway by exposure to excess ligand, by overproduction of the normal EGF receptor, or by the presence of specific mutated forms of this receptor can result in dramatic alterations in cellular phenotype, including malignant transformation. Overstimulation of normal cells is avoided by precise control of the synthesis and degradation of EGF receptors. Regulation occurs at multiple levels, including transcriptional control. A number of DNA-binding proteins have now been identified which positively and negatively modulate EGF receptor gene transcription.

Regulation of intracellular calcium in epithelial cells

The role of [Ca2+]i as a second messenger in non-excitable cells has been appreciated for almost 3 decades. The advent of fluorescent Ca2+ indicators has allowed the monitoring of Ca2+ signalling in suspensions of these cells. Agonist mediated changes in [Ca2+]i usually show an initial Ca2+ transient followed by a maintained increase. The former has been shown to be due to Ca2+ release from one or more intracellular stores, the latter due to activation of receptor operated Ca2+ entry (ROCE). More recently it has been recognized that many cells show distinct maintained oscillatory behavior when examined by single cell optical methods. It is proposed here that these oscillations are the consequence of IP3 and Ca2+ stimulation of Ca2+ release and ligand activation of ROCE followed by Ca2+ inhibition of Ca2+ and ROCE as Ca2+ pumps are activated. These oscillations allow more exact regulation of a pump/leak controlled second messenger such as [Ca2+]i.

Insulin stimulates protein synthesis and phospholipid signaling systems but does not regulate glucose uptake in the inner ear

High-affinity insulin receptors exist in the organ of Corti (Kd = 1.1 +/- 0.5 nM) and in the lateral wall (stria vascularis and ligamentum spirale; Kd = 1.1 +/- 0.4 nM) of the inner ear of the guinea pig as determined by the binding of radiolabeled porcine or bovine insulin in vitro. Carrier-mediated transport of glucose (defined as the cytochalasin B-sensitive part of total uptake) was measured in vitro with 2-deoxy-D-glucose as the substrate. Its Km was 188 microM in the lateral wall (r = 0.99 and 0.94, respectively). Neither the Km nor the rates of transport (0.20 +/- 0.10 pmol/micrograms protein/hr in the organ of Corti, and 0.56 +/- 0.34 pmol/micrograms protein/hr in the lateral was) were affected by insulin. In contrast, 0.1 mM ouabain decreased deoxyglucose uptake in the organ or Corti by 37% and in the lateral wall tissues by 45% indicating the presence of an active, Na(+)-dependent transporter in these tissues. Insulin influenced both protein and lipid metabolism in the inner ear. Proteins and lipids were labeled in situ by perfusion of the perilymphatic space of the cochlea with [3]-leucine or [32P]-orthophosphate and [3H]-glycerol, respectively. Thirty nM insulin stimulated the incorporation of [3H]-leucine into protein of the organ of Corti from 39 to 56 pmol/mg protein but was ineffective in the tissues of the lateral wall. In the organ of Corti, [32P]-orthophosphate was incorporated into the phosphoinositides and phosphatidate, and 30 nM insulin increased this incorporation by 101 to 149%.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmission of acoustic or vibratory signals from a contracting muscle to relatively distant tissues

In 20 normal subjects, acoustic or vibratory signals generated during sustained voluntary isometric contraction of the biceps brachii muscle were recorded directly over the muscle belly. The signals were also recorded simultaneously at a distance from the contracting biceps muscle over the posterior upper arm near the triceps muscle. Frequency amplitude spectra (FAS) of the recordings were determined. The amount of transmission of the most dominant vibratory signal from the biceps muscle to the posterior arm was estimated from the amplitude ratio of similar frequency peaks in the FAS obtained from recordings in these two separate regions. The mean amplitude ratio was 0.46. This finding suggests that when several groups of muscles are contracting simultaneously, there may be significant transmissions of acoustic signals from one muscle to the other. These transmissions could potentially interfere with the selective recording of sounds produced by a specific muscle.

Structural requirements for signal transduction of the insulin receptor

Structural requirements for signal processing by human placental insulin receptors have been examined. Insulin binding has been found to change the physico-chemical properties of (alpha beta)2 receptors solubilized with Triton X-100, indicating a marked alteration of the form, i.e. size and shape, of the molecular complex. (a) The Stokes radius decreases from about 9.5 nm to 7.9 nm, as determined by PAGE with Triton X-100 in the buffer (Triton X-100/PAGE), and from 9.1 nm to 8.7 nm, as assessed by gel filtration. (b) The sedimentation coefficient s20,w rises from 10.1 S to 11.4 S. Upon dissociation of the receptor-hormone complex, the alterations are reversed. After autophosphorylation of hormone-bound (alpha beta)2-insulin receptors, phosphate incorporation was found for 7.9-nm receptor forms when receptor-insulin complexes were crosslinked with disuccinimide suberate prior to Triton X-100/PAGE. However, phosphate incorporation was demonstrated for the 9.5-nm receptor forms when receptor-insulin complexes were not prevented from dissociation. This strongly indicates that the (alpha beta)2 receptor is autophosphorylated after assuming its 7.9-nm form upon insulin binding. Moreover, the insulin-dependent structural alterations are not affected by autophosphorylation. In contrast to (alpha beta)2 receptors, the diffusion and the sedimentation behaviour of alpha beta receptors, which carry a dormant tyrosine kinase even in the hormone-laden state, has been found to be insensitive to insulin binding. Different molecular properties of alpha beta and (alpha beta)2 receptors have also been detected by hormone binding studies. Insulin binding to (alpha beta)2 and alpha beta receptors differs markedly with respect to pH, ionic strength, and temperature. This might indicate that the structure of the hormone binding domain of alpha beta receptor changes on association into the (alpha beta)2 species. Alternatively, distinct hormone-induced conformational alterations at the molecular level of alpha beta and (alpha beta)2 receptor species may lead to the different binding properties. Our data demonstrate that the (alpha beta)2-insulin receptor undergoes extended conformational alterations upon insulin binding. This capacity for structural changes coincides with the hormone-inducable enhancement of tyrosine autophosphorylation of the 7.9-nm insulin-bound receptor form. In contrast, alpha beta receptors appear to be locked in an inactive nonconvertable state. Thus, interaction between two alpha beta receptor units is required to allow extended conformational alterations, which are assumed to be the triggering event for augmented auto-phosphorylation.

Rhodopsin-stimulated activation-deactivation cycle of transducin: kinetics of the intrinsic fluorescence response of the alpha subunit

The intrinsic tryptophan fluorescence of the alpha subunit of transducin (alpha T) has been shown to be sensitive to the binding of guanine nucleotides, with the fluorescence being enhanced by as much as 2-fold upon the binding of GTP or nonhydrolyzable GTP analogues [cf. Phillips and Cerione (1988) J. Biol. Chem. 263, 15498-15505]. In this work, we have used these fluorescence changes to analyze the kinetics for the activation (GTP binding)-deactivation (GTPase) cycle of transducin in a well-defined reconstituted phospholipid vesicle system containing purified rhodopsin and the alpha T and beta gamma T subunits of the retinal GTP-binding protein. Both the rate and the extent of the GTP-induced fluorescence enhancement are dependent on [rhodopsin], while only the rate (and not the extent) of the GTP gamma S-induced enhancement is dependent on the levels of rhodopsin. Comparisons of the fluorescence enhancements elicited by GTP gamma S and GTP indicate that the GTP gamma S-induced enhancements directly reflect the GTP gamma S-binding event while the GTP-induced enhancements represent a composite of the GTP-binding and GTP hydrolysis events. At high [rhodopsin], the rates for GTP binding and GTPase are sufficiently different such that the GTP-induced enhancement essentially reflects GTP binding. A fluorescence decay, which always follows the GTP-induced enhancement, directly reflects the GTP hydrolytic event. The rate of the fluorescence decay matches the rate of [32P]Pi production due to [gamma-32P]GTP hydrolysis, and the decay is immediately reversed by rechallenging with GTP. The GTP-induced fluorescence changes (i.e., the enhancement and ensuing decay) could be fit to a simple model describing the activation-deactivation cycle of transducin. The results of this modeling suggest the following points: (1) the dependency of the activation-deactivation cycle on [rhodopsin] can be described by a simple dose response profile; (2) the rate of the rhodopsin-stimulated activation of multiple alpha T(GDP) molecules is dependent on [rhodopsin] and when [alpha T] greater than [rhodopsin], the activation of the total alpha T pool may be limited by the rate of dissociation of rhodopsin from the activated alpha T(GTP) species; and (3) under conditions of optimal rhodopsin-alpha T coupling (i.e., high [rhodopsin]), the cycle is limited by GTP hydrolysis with the rate of Pi release, or any ensuing conformational change, being at least as fast as the hydrolytic event.

Transcription factor levels in medullary thyroid carcinoma cells differentiated by Harvey ras oncogene: c-jun is increased

In the TT cell line of human medullary thyroid carcinoma, the viral Harvey ras (v-rasH) oncogene induces differentiation, marked by morphological changes, diminution of growth, and increased expression of the calcitonin gene. Here, we show that the transcriptional factor c-jun is increased during v-rasH induced differentiation of TT cells both at the mRNA and functional protein levels. In contrast, nuclear proteins with binding activities related to AP2, AP3, NF1/CTF, and Sp1 were unchanged in v-rasH differentiated TT cells.

Shear stress increases inositol trisphosphate levels in human endothelial cells

To elucidate some of the early mechanisms underlying the response of primary human endothelial cells to the initiation of flow, we investigated the changes in inositol lipid metabolism in cells exposed to arterial and venous levels of shear stress. We used a radioimmunoassay specific for inositol-1,4,5-trisphosphate (Ins1,4,5P3) to demonstrate that initiation of an arterial shear stress caused a rapid rise in Ins1,4,5P3 levels which peaked after approximately 30 seconds of flow (2.1 +/- 0.2 fold stimulation) and remained elevated for at least 6 minutes after the initiation of flow. This increased Ins1,4,5P3 concentration is similar in magnitude to the increase caused by 10 microM histamine (2.8 +/- 0.3 fold stimulation). Thus these cells may detect the presence of mechanical stress by a signal transduction pathway involving inositol lipid metabolism.

Effect of static stretching on elastin production by porcine aortic smooth muscle cells

An in vitro model was developed to assay the effects of static stretching on soluble elastin (tropoelastin, TE) synthesis by porcine aorta smooth muscle cells (SMC). Culture dishes containing SMC adherent to the deformable bottoms were placed over hard convex templates. Stress was applied by placing a weight on the dish covers. Measurement of TE was accomplished by a specific ELISA assay. With this model we demonstrated reproducible stimulation of TE synthesis by stretched SMC. Significant results (161.4% of control; p = less than 0.003 by Student's t-test) were obtained by stretching passage 2 SMC for 3 days with a medium change after the first 18-24 hours, use of 5% newborn calf serum in cultures during stretching, and 50-g weight. DNA content in stretched cultures did not increase over control values. Thus, stretching alone did not cause hyperplasia or hyperploidy in these SMC and, in the absence of other vascular cell elements, was sufficient to increase production of this extracellular matrix protein. Transduction of mechanical force into elastin gene expression by SMC may contribute to the development of thickened arterial tunica media characteristic of hypertensive vessels.

Endothelial injury during extracorporeal circulation. Role of the complement system

This study attempted to elucidate in vitro, endothelial cell (EC) injury induced by complement activation in extracorporeal devices. The in vitro injury model using cultured bovine ECs showed the following: 1) a dose-dependent transmembrane injury, determined by monitoring intracellular calcium mobilization, occurred spontaneously upon addition of complement-activated serum. 2) The respiratory rate of adherent ECs, monitored by an oxygen electrode, was reduced with increases in activated complement serum levels, and increased further with time. 3) Whole cell activity, measured by the rate of cellular proliferation, was reduced with an increase in activated serum complement levels. 4) These cellular responses were almost completely absent in the presence of C3 antiserum, strongly indicating that activated C3 fragments are responsible for EC injury. Thus, complement activation during extracorporeal circulation may injure endothelium at the transmembranous, mitochondrial, and/or proliferative levels, depending upon contact time and concentration of activated complement.

Membrane-delimited stimulation of heart cell calcium current by beta-adrenergic signal-transducing Gs protein

A severalfold increase in calcium current (ICa) is a signal feature of the maximal beta-adrenergic response of the heart. It is generally ascribed to enhanced adenosine 3',5'-cyclic monophosphate (cAMP)-dependent phosphorylation of calcium (Ca) channels after beta-receptor activation of the guanosine nucleotide-binding (G) protein Gs, and Gs activation of the adenylyl cyclase cascade. We blocked phosphorylation pathways in guinea pig cardiomyocytes to unmask other possible ICa-stimulatory modes. In blocked cells, ICa increased by approximately 50% during 1) beta-receptor activation of Gs, 2) intracellular activation of Gs, and 3) intracellular application of preactivated Gs, We conclude that fast, membrane-delimited Gs modulation participates in the physiological regulation of cardiac ICa.

Pheromone response in yeast

In response to peptide pheromones, yeast cells prepare themselves for mating; changes include arrest of the cell cycle and induction of transcription. Proteins involved in this signal transduction pathway include the pheromone receptors, subunits of a G protein, protein kinases and DNA-binding proteins. Understanding of this pathway has been facilitated by yeast genetics, which has allowed the genes encoding all of these proteins to be identified and characterized.

Divalent metal ion binding to the CheY protein and its significance to phosphotransfer in bacterial chemotaxis

Signal transduction in bacterial chemotaxis involves transfer of a phosphoryl group between the cytoplasmic proteins CheA and CheY. In addition to the established metal ion requirement for autophosphorylation of CheA, divalent magnesium ions are necessary for the transfer of phosphate from CheA to CheY. The work described here demonstrates via fluorescence studies that CheY contains a magnesium ion binding site. This site is a strong candidate for the metal ion site required to facilitate phosphotransfer from phospho-CheA to CheY. The diminished magnesium ion interaction with CheY mutant D13N and the lack of metal ion binding to D57N along with significant reduction in phosphotransfer to these two mutants are in direct contrast to the behavior of wild-type CheY. This supports the hypothesis that the acidic pocket formed by Asp13 and Asp57 is essential to metal binding and phosphotransfer activity. Metal ion is also required for the dephosphorylation reaction, raising the possibility that the phosphotransfer and hydrolysis reactions occur by a common metal-phosphoprotein transition-state intermediate. The highly conserved nature of the proposed metal ion binding site and site of phosphorylation within the large family of phosphorylated regulatory proteins that are homologous to CheY supports the hypothesis that all these proteins function by a similar catalytic mechanism.

Lymphotoxin induces secretion of granule proteins from adherent neutrophils: possible role of intracellular free calcium

Lymphotoxin (LT) can activate human neutrophils. Using a hemolytic plaque assay to detect secretion of lactoferrin and myeloperoxidase (MPO) from single adherent neutrophils, we showed that LT induced secretion from both primary and secondary granules. Incubation of cells with cytochalasin B was required for MPO secretion, and it enhanced lactoferrin secretion. Pertussis toxin, which blocks a G-protein in the plasma membrane, inhibited LT-induced exocytosis of MPO, but not of lactoferrin. Incubation with LT did not induce any detectable changes of the cytoplasmic free [Ca2+] in neutrophils. On the other hand, secretion of granule proteins from adherent neutrophils in response to LT was blocked by loading neutrophils with quin-2 in order to increase the intracellular calcium buffering capacity. This was achieved at a concentration of quin-2, at which the secretion induced by the phorbol ester PMA and the chemotactic peptide FMLP was unaffected. Trifluoroperazine (TFP), a dual protein kinase C and calmodulin inhibitor, significantly inhibited the LT-mediated secretion of lactoferrin from adherent granulocytes. The PMA effect was unaltered by TFP under these conditions, suggesting that the inhibitory effect was on a calcium-calmodulin dependent step. The secretion induced by TNF and GM-CSF was also blocked by buffering changes in the intracellular [Ca2+] and inhibited to a similar extent by TFP. Our results suggest that calmodulin and minute changes in the cytoplasmic free [Ca2+] may be involved in a common signal transduction pathway engaged in activation of adherent neutrophils by several cytokines.

Gravity-induced changes in intracellular potentials in elongating cortical cells of mung bean roots

Gravity-induced changes in intracellular potentials in primary roots of 2-day-old mung bean (Vigna mungo L. cv. black matpe) seedlings were investigated using glass microelectrodes held by 3-dimensional hydraulic micro-drives. The electrodes were inserted into outer cortical cells within the elongation zone. Intracellular potentials, angle of root orientation with respect to gravity, and position within the root of the impaled cortical cell were measured simultaneously. Gravistimulation caused intracellular potential changes in cortical cells of the elongation zone. When the roots were oriented vertically, the intracellular potentials of the outer cortical cells (2 mm behind the root apex) were approximately - 115 mV. When the roots were placed horizontally cortical cells on the upper side hyperpolarized to - 154 mV within 30 s while cortical cells on the lower side depolarized to about - 62 mV. This electrical asymmetry did not occur in cells of the maturation zone. Because attempts to insert the electrode into cells of the root cap were unsuccessful, these cells were not measured. The hyperpolarization of cortical cells on the upper side was greatly reduced upon application of N,N'-dicyclohexylcarbodiimide (DCCD), an inhibitor of respiratory energy coupling. When stimulated roots were returned to the vertical, the degree of hyperpolarization of cortical cells on the previous upper side decreased within 30 s and approached that of cortical cells in non-stimulated roots. This cycle of hyperpolarization/loss of hyperpolarization was repeatable at least ten times by alternately turning the root from the vertical to the horizontal and back again. The very short (<30 s) lag period of these electrical changes indicates that they may result from stimulus-perception and transduction within the elongation zone rather than from transmission of a signal from the root cap.

Differentiation to a neuronal phenotype in bovine chromaffin cells is repressed by protein kinase C and is not dependent on c-fos oncoproteins

We investigated the intracellular signals underlying the neurotrophic response of adult bovine chromaffin cells to histamine and basic fibroblast growth factor (bFGF). Histamine produced significant neurite outgrowth within 48 hr, whereas the response to bFGF developed after 1 week. H7, a protein kinase C (PKC) inhibitor potentiated both the histamine and the bFGF responses, while another PKC antagonist, staurosporine, induced a rapid and efficient differentiation response when applied alone. These observations suggest that basal PKC activity is required for stabilization of the endocrine phenotype in these cells. They contrast with findings on NGF induction of neurite outgrowth in PC12 cells where PKC promotes differentiation, apparently by activating the fos/jun complex. Thus, we examined the role of c-fos in our model. Both histamine and bFGF induced c-fos gene expression transiently. To determine whether increased levels of c-fos oncoprotein were essential to the differentiation process, we used a hybrid arrest approach employing an innovative transfection technique applicable to primary culture systems. Transfection with plasmid pSVsof, producing antisense c-fos mRNA, reduced c-fos oncoprotein levels but did not diminish histamine-induced neurite outgrowth. We infer that histamine-induced differentiation in bovine chromaffin cells is independent of increased levels of c-fos oncoprotein.

On the relation between photo- and gravitropically induced spatial memory in maize coleoptiles

The interaction of photo- and gravitropic stimulation was studied by analysing the curvature of maize (Zea mays L.) coleoptiles subjected to rotation on horizontal clinostats. Gravitropic curvature in different directions with respect to the stimulation plane was found to be transient. This instability was caused by an increasing deviation of response direction from the stimulation plane towards the caryopsis. The bending angle as such, however, increased steadily. This reorientation of the gravitropic response towards the caryopsis is thought to be caused by the clinostat-elicited nastic curvature found in maize coleoptiles. In contrast, the response to phototropic stimulation was stable, in both, orientation and curving. Although stimulation by gravity was not capable of inducing a stable tropistic response, it could inhibit the response to opposing phototropic stimulation, if the counterstimulation was given more than 90 min after the onset of gravistimulation. For shorter time intervals the influence of the phototropic stimulus obscured the response to the first, gravitropic stimulation. For time intervals exceeding 90 min, however, the phototropic effects disappeared and the response was identical to that for gravity stimulation alone. This gravity-induced inhibition of the phototropic response was confined to the plane of gravity stimulation, because a phototropic stimulation in the perpendicular direction remained unaffected, irrespective of the time interval between the stimulations. This concerned not only the stable phototropic curving, but also the capacity of the phototropic induction to elicit a stable directional memory as described earlier (P. Nick and F. Schafer, 1988b, Planta 175, 380-388). This was tested by a second blue-light pulse opposing the first. It is suggested that gravity, too, can induce a directional memory differing from the blue-light elicited memory. The mechanisms mediating gravi- and phototropic directional memories are thought to branch off the respective tropistic signal chain at a stage where photo- and gravitropic transduction are still separate.

Stretch-activated channels in the basolateral membrane of single proximal cells of frog kidney

Epithelial cells are capable of regulating their volume in response to osmotic swelling or shrinkage. In the present paper a channel is described which may be involved in such a volume-regulatory response. Channels were studied in cell-attached patches of the basolateral membrane of cells isolated from frog kidneys using the patch-clamp technique. The open probability of the channels is increased by the application of negative pressure to the rear of the patch pipette or by bathing the cells in hypotonic fluid. In addition, the channels are voltage-sensitive, such that depolarisation increases the open probability. The channels have a conductance of 25 pS with amphibian Ringer as the pipette solution and appear not to discriminate between potassium and sodium. Replacement of chloride by gluconate as the dominant anion in the pipette solution did not affect the current/voltage relationship, suggesting that the channels are cation-non-selective. Inward currents are observed at the resting membrane potential with either potassium or sodium as the dominant cation in the pipette solution: this obviates the channels serving a role as the route for solute exit from the cell during a volume-regulatory decrease response and suggests that they may act as the transduction mechanism sensing changes in cell volume.

Downregulation of the early genomic growth factor response in neu oncogene-transformed cells

Peptide growth factor-induced signal transduction leads to a long-term adjustment of the genetic programs of responding cells. A point mutation in the transmembrane domain of the neu receptor has been found to activate its tyrosine kinase and oncogenic potential. Our previous studies show that ligand stimulation of a chimeric epidermal growth factor receptor-neu proto-oncogene (EGF-R/neu) induces the neu tyrosine kinase and leads to the programmed activation of cell growth-regulated genes. We have now studied the effect of the neu oncoprotein on the genomic growth factor response in cells expressing the EGF-regulated neu tyrosine kinase. Expression of the neu oncogene in these cells inhibited 75-90% of the EGF-stimulated mRNA induction of the immediate early serum response genes, such as junB encoding a transcription factor, N10 encoding a putative nuclear hormone binding receptor for an as yet undefined ligand, and B10, the protein product of which is still unknown. The relative lack of mRNA induction was not due to a loss of the chimeric EGF-R/neu receptors from the cell surface. Also, the neu oncogene decreased serum- and tumor promoter induction of these genes. Our results suggest that the neu oncogene is capable of deregulating mRNA responses to extracellular signalling, similar to the effects of the c-Ha-ras oncogene. Knowledge of the mechanisms responsible for these changes in gene regulation will help to define oncogenic transformation of cells in molecular terms.

Effects of spinal cord temperature on the generation and transmission of temperature signals in the goat

A series of 38 experiments were performed in five conscious goats at air temperatures of +20 degrees C or +30 degrees C to see whether a temperature dependence of spinal cord signal transmission affects the relationships between body temperature and metabolic rate (MR) or respiratory evaporative heat loss (REHL). Prior to the experiments the animals received peridural thermodes to clamp the spinal cord temperature by perfusion temperatures of 31 degrees C, 38 degrees C or 43 degrees C (45 degrees C), carotid loops to clamp the brain temperature at 39 degrees C or 39.5 degrees C, and arteriovenous shunts to alter the trunk temperature and to determine thresholds and slopes of MR and REHL over trunk temperature. The trunk temperature thresholds, at which MR and REHL increased, were inversely related to the spinal cord temperature, thereby confirming previous observations on the generation of specific spinal temperature signals. The slopes at which MR rose below the threshold, increased with decreasing spinal cord temperature. The slopes of REHL over trunk temperature were independent of spinal cord temperature. Both observations are at variance with previously observed temperature effects on hypothalamic signal transmission and imply that temperature-dependent signal transmission at the spinal level cannot account for nonlinear interaction of various body temperatures in the control of MR and REHL.

Activation of platelets induced by mAb P256 specific for glycoprotein IIb-IIIa. Possible evidence for a role for IIb-IIIa in membrane signal transduction

Monoclonal antibody P256, which is specific for glycoprotein IIb-IIIa complex, was found to induce aggregation of normal platelets in plasma. The mechanism of platelet activation induced by this monoclonal antibody was thoroughly studied. The divalent binding to the IIb-IIIa molecule was necessary for triggering aggregation since Fab' fragments did not induce aggregation as did IgG and F(ab')2 fragments; however, F(ab')2 did not induce the release as did the whole IgG. P256-induced aggregation was accompanied by release of all three granule constituents, namely dense granules, alpha-granules and lysosomes, with parallel kinetics showing half-maximum release 50 s after addition of P256. Thromboxane synthesis was initiated at the same time. Using 32P-prelabeled platelets, no variation in level of [32P]phosphatidylinositol 4,5-bisphosphate could be detected in the first minute after P256 addition, indicating no activation of the calcium-independent phospholipase C specific for polyphosphoinositol phospholipid. P256 induced a calcium mobilization as measured by Indo-1 fluorescence of about the third of that measured in the presence of a thrombin concentration giving the same intensity of aggregation. P256 induced phosphorylation of the myosin light chain p20 and of the main substrate of protein kinase C, p43. Addition of aspirin inhibited almost totally calcium mobilization and partially aggregation, release and protein phosphorylations. By contrast, in the absence of external calcium, although no aggregation could occur, the release reaction was only partially reduced. In this activation, the glycoprotein IIb-IIIa complex thus appears to play a role in modulating platelet response, not only via calcium fluxes but also in activating protein kinase C responsible for p43 phosphorylation.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

Muscarinic receptors in airways: recent developments

Recently there have been important advances in our understanding of muscarinic receptors in airways that have important implications for understanding airway control and for future therapy of airway diseases. The transduction mechanisms involved in muscarinic receptor activation are now better understood. Receptor-linked phosphoinositide hydrolysis leads to release of calcium ions from intracellular stores, resulting in contraction of airway smooth muscle. At least five subtypes of muscarinic receptor have now been cloned, although only three subtypes can be distinguished pharmacologically. M1 receptors are facilitatory to neurotransmission in airway parasympathetic ganglion cells and have also been identified in airway submucosal glands and on the alveolar walls of human lung. M2 receptors are located on postganglionic nerves and function as powerful feedback inhibitory receptors (autoreceptors) that are likely to be involved in modulation of reflex bronchoconstriction. These receptors may be dysfunctional in asthmatic airways. M3 receptors are present on airway smooth muscle and submucosal glands and mediate the classical muscarinic effects in airways. Molecular biology techniques should now allow further study of the factors that regulate transcription and expression of muscarinic receptors in airways.

The vasopressin receptor system in the neonatal pituitary gland: evidence for reduced binding capacity and signal transmission

The present study was aimed at evaluating the capacity of anterior pituitary cells from neonatal rats to bind arginine vasopressin (AVP) and show AVP-receptor-mediated signal transmission. We found that in cultures of pituitary cells of 10-day-old pups, in contrast to cultures of cells of adults, AVP was unable to trigger sustained adrenocorticotropin (ACTH) secretion and, in addition, was also less potent in synergizing with the effect of corticotropin-releasing factor (CRF) on both ACTH output and cyclic AMP formation. Binding studies revealed the existence of a much lower number of AVP receptor sites in membranes of neonatal pituitary gland than in those of adult tissue (32.3 +/- 9.0 and 137.6 +/- 6.2 fmol/mg protein, respectively), although the binding of agonists and the apparent molecular weight (Mr about 120,000) of the receptors were similar. Activation by phorbol ester PMA of protein kinase C, a messenger involved in AVP action, resulted in a dose-related enhancement of ACTH secretion that was 2-3 times smaller for immature corticotrophs than for mature ones. Importantly, PMA treatment allowed AVP to significantly stimulate ACTH secretion from neonatal cells, while it failed to similarly affect AVP-evoked hormone output from adult tissue. Our results indicate that pituitary corticotrophs of rat pups fail to properly transduce AVP-receptor-mediated signalling and, thereby, suggest an explanation for the postnatal 'stress nonresponsive period'.

Role of nuclear protein kinase C in the mitogenic response to platelet-derived growth factor

We have assessed the involvement of nuclear envelope protein phosphorylation in the mitogenic response to platelet-derived growth factor (PDGF) in NIH/3T3 fibroblasts. We find that stimulation of quiescent NIH/3T3 cells with PDGF or with the mitogenic protein kinase C (PKC) activators phorbol 12-myristate 13-acetate (PMA) or bryostatin 1 (bryo) leads to rapid, dose-dependent phosphorylation of several nuclear envelope polypeptides. The predominant nuclear envelope targets for mitogen-induced phosphorylation are immunologically identified as the nuclear envelope lamins. All three lamin species (A, B and C) are phosphorylated in response to PMA or bryo, while lamins A and C are preferentially phosphorylated in response to PDGF. Phosphopeptide mapping and phosphoamino acid analysis indicate that similar serine sites on the lamins are phosphorylated in response to PDGF, PMA and bryo. Both mitogenicity and lamina phosphorylation induced by these mitogens can be inhibited by the selective PKC inhibitor staurosporine at 2 nM. Treatment of quiescent NIH/3T3 cells with PDGF, PMA or bryo leads to rapid translocation of PKC to the nuclear envelope. These data indicate that rapid nuclear events, including translocation of cytosolic PKC to the nuclear membrane and lamina phosphorylation, may play a role in the transduction of the mitogenic signals of PDGF from the cytoplasm to the nucleus in NIH/3T3 fibroblasts.

Signal transduction mechanisms of Ca2+ mobilizing hormones: the case of gonadotropin-releasing hormone

Multiple (at least seven) steps are involved in GnRH-induced gonadotropin secretion and gonadotropin gene expression. After binding to specific receptors located exclusively on pituitary gonadotrophs, GnRH stimulates a rapid phosphodiesteric hydrolysis of phosphoinositides for which no rise in [Ca2+]i is required. Activation of PLC is most likely mediated by a pertussis toxin-insensitive GTP-binding protein (Gp). In its activated state (Gp-GTP) the binding affinity of GnRH to is receptor is reduced. Rapid formation of IP3 will enhance Ca2+ release from intracellular sources most likely via a specific IP3 receptor. The transient Ca2+ rise might be responsible for a burst phase of LH release lasting for about 100 sec, which is not dependent on extracellular Ca2+. The backbone moiety of the phosphoinositides, DG, and the elevated [Ca2+]i are most likely responsible for translocation of PKC subspecies from the cytosol to the membrane. The most likely candidates are alpha- and beta II-PKC. The activated PKC subspecies phosphorylate substrate proteins which activate secretory reactions and participate in gonadotropin gene expression. In parallel Ca2(+)-influx via nifedipine-sensitive and insensitive channels further elevates [Ca2+]i, which participates in the sustained phase of gonadotropin secretion in concert with the activated PKCs. GnRH also triggers the release of AA and the formation of lipoxygenase and/or epoxygenase products of the fatty acid which are also involved in the process of the exocytosis. We predict that the continuous supply of DG and AA needed for GnRH action is also provided via activated PLD which will also supply phosphatidic acid, the role of which is as yet unclear. The interaction of the various second messengers involved in GnRH action (IP3, Ca2+, DG, AA) and their relative roles in gonadotropin secretion and gonadotropin gene expression await further investigation. In several aspects GnRH action on gonadotropin secretion is unique when compared to other Ca2(+)-mobilizing ligands: 1) At physiological concentrations GnRH up-regulates its own receptors whereas most ligands down-regulate the respective receptor; 2) PKC up-regulates GnRH receptors whereas in most cases PKC down-regulates the ligand receptor; 3) GnRH stimulation of PLC activity is most likely mediated by Gp whereas some Ca2(+)-mobilizing ligands operate via Gi; 4) Activated PKC does not exert negative feedback upon GnRH-induced inositol phosphate production as is the case with several other peptides; 5) Activated PKC might be responsible for Ca2+ influx whereas in several other systems PKC is inhibitory to Ca2+ influx.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium influx regulates antibody-induced glycoprotein movements within the Chlamydomonas flagellar membrane

The Chlamydomonas flagellar surface exhibits a number of dynamic membrane phenomena associated with whole-cell gliding locomotion and the early events in fertilization. Crosslinking of a specific population of flagellar surface-exposed glycoproteins with the lectin concanavalin A or an anti-carbohydrate mouse monoclonal antibody, designated FMG-1, results in a characteristic pattern of glycoprotein redistribution within the plane of the flagellar membrane. Recent evidence suggests that flagellar membrane glycoprotein movements are associated with both whole-cell gliding motility and the early events in mating. It is of interest to determine the transmembrane signaling pathway whereby crosslinking of the external domains of flagellar glycoproteins activates the intraflagellar machinery responsible for translocation of flagellar membrane glycoproteins. The redistribution of flagellar membrane glycoproteins requires micromolar levels of free calcium in the medium; lowering the free calcium concentration to 10(−7) M results in complete but reversible inhibition of redistribution. Redistribution is maximal in the presence of 20 microM free calcium in the medium. Redistribution is inhibited in the presence of 20 microM free calcium by the calmodulin antagonists trifluoperazine, W-7 and calmidazolium, the calcium channel blockers diltiazem, methoxyverapamil (D-600) and barium chloride, and the local anesthetics, lidocaine and procaine. The actions of all of these agents can be interpreted in terms of a requirement for calcium in the signaling mechanism associated with flagellar glycoprotein redistribution. In particular, the requirement for micromolar calcium in the external medium and the effects of specific calcium channel blockers suggest that flagellar membrane glycoprotein crosslinking may induce an increase in calcium influx, which may be the initial trigger for activating the flagellar machinery responsible for active movement of flagellar membrane glycoproteins.

Protein kinase C and tumor promoters

The complexity of PKC has made it difficult to define the precise biochemical processes associated with the various PKC-related cellular responses observed. In the past year, we have seen progress in complementary approaches that are helping to solve the puzzles. These include purification and characterization of individual isozymes, expression of normal and mutant PKCs, immunolocalization, and identification of specific activators and inhibitors. All of these will be useful in identifying the primary targets of PKC phosphorylation and their function.

Signal transmission through the dark-adapted retina of the toad (Bufo marinus). Gain, convergence, and signal/noise

Responses to light were recorded from rods, horizontal cells, and ganglion cells in dark-adapted toad eyecups. Sensitivity was defined as response amplitude per isomerization per rod for dim flashes covering the excitatory receptive field centers. Both sensitivity and spatial summation were found to increase by one order of magnitude between rods and horizontal cells, and by two orders of magnitude between rods and ganglion cells. Recordings from two hyperpolarizing bipolar cells showed a 20 times response increase between rods and bipolars. At absolute threshold for ganglion cells (Copenhagen, D.R., K. Donner, and T. Reuter. 1987. J. Physiol. 393:667-680) the dim flashes produce 10-50-microV responses in the rods. The cumulative gain exhibited at each subsequent synaptic transfer from the rods to the ganglion cells serves to boost these small amplitude signals to the level required for initiation of action potentials in the ganglion cells. The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signal-to-noise ratio. Thus, at absolute threshold for ganglion cells, the convergence typically increases the maximal signal-to-noise ratio from 0.6 in rods to 4.6 in ganglion cells.