Substance P receptors in the nervous system and possible receptor subtypes

The ability of various related peptides and substance P analogues to compete for the binding of 3H-labelled substance P to rat brain membranes corresponds with their known biological activities, providing a simple model for studies of peptide receptors in the central nervous system. In salivary gland and brain slices substance P and related peptides stimulate the rate of incorporation of phosphatidylinositol, offering an alternative biochemical model for substance P receptor studies. Two types of receptor may be responsible for the actions of substance P on peripheral tissues: the SP-P type, where all tachykinins are approximately equally active, and the SP-E type, where eledoisin and kassinin are more potent than the other tachykinins. Alkyl esters of substance P appear to act as selective SP-P agonists.

The mas oncogene as a neural peptide receptor: expression, regulation and mechanism of action

The human mas oncogene, which renders transfected NIH/3T3 cells tumorigenic, was identified as a subtype of angiotensin receptor by transient expression in Xenopus oocytes and stable expression in the mammalian neuronal cell line, NG115-401L. The mas receptor preferentially recognizes angiotensin III, and is expressed at high levels in brain. The mas/angiotensin receptor functions through the breakdown of inositol lipids and can drive DNA synthesis, unlike another inositol-linked peptide receptor, that for bradykinin. Comparative analysis of several early biochemical events elicited by either angiotensin or bradykinin stimulation of mas-transfected cells has not indicated a specific difference correlated with mitogenic activity. In particular, the inositol lipid kinase, phosphatidylinositol-3-kinase, thought to be involved in the mitogenic mechanism of platelet-derived growth factor receptors, is unaffected by activation of mas. These results have shown that a proto-oncogene encodes a neural peptide receptor, indicating that peptide receptors may be involved in differentiation and proliferation processes, as are other identified proto-oncogenes.

Multiple embolic events in a patient with heparin induced thrombocytopaenic thrombotic syndrome and mural thrombus

This report describes a 55-year-old man admitted for renal colic and found to have an infarcted right kidney, for which he was treated with heparin. Heparinization for this condition was stopped on day 12 when the patient developed pain in his left foot. Heparin-induced thrombocytopaenic thrombotic syndrome was confirmed, and despite treatment with danaparoid and bilateral femoral embolectomy, a left above-knee amputation was required. Echocardiogram showed a sessile mural thrombus in the left ventricle, which was considered a likely embolic source. The preoperative development of acute renal failure delayed recovery and necessitated haemodialysis. Anticoagulation of the patient was complicated; however, warfarin was introduced slowly and with good effect.

Messenger RNA localization and further characterisation of the putative tachykinin receptor NK4 (NK3B)

We have previously shown that a cloned receptor, highly homologous to the NK3 tachykinin peptide receptor, encodes a novel functional tachykinin receptor NK4. Examining sites of receptor mRNA expression by Northern blot we show that NK4 mRNA is expressed in numerous rat tissues, in contrast to the NK3 receptor which has been shown to have a distribution principally in nervous tissues. We have localised the NK4 receptor mRNA in rat brain and spinal cord using in situ hybridisation. NK4 receptor mRNA is widely expressed in neurons in the rat central nervous system, including cerebral cortex, hippocampus, hypothalamus and dorsal horn of the spinal cord. During peripheral inflammation of the hindpaw, NK4 mRNA shows complex patterns of regulation. We have also investigated some pharmacological properties of this receptor expressed ectopically in Xenopus oocytes. We show that the functional antagonism of dynorphin at the NK4 receptor is reversed by the non-specific opioid antagonist naloxone and that tachykinin-evoked responses at the NK4 receptor are inhibited by the non-peptide NK3 receptor antagonist SR142801 in a concentration dependent manner.

HHV8-encoded vMIP-I selectively engages chemokine receptor CCR8. Agonist and antagonist profiles of viral chemokines

Uncertainty regarding viral chemokine function is mirrored by an incomplete knowledge of host chemokine receptor usage by the virally encoded proteins. One such molecule is vMIP-I, a C-C type chemokine of undefined function and binding specificity, encoded by the Kaposi's sarcoma herpesvirus HHV-8. We report here that vMIP-I binds to and induces cytosolic [Ca(2+)] signals in human T cells selectively through CCR8, a CC chemokine receptor associated with Th2 lymphocytes. Furthermore, using a panel of 65 different human, viral, and rodent chemokines, we have established a comprehensive ligand binding "fingerprint" for CCR8. The receptor exhibits marked "high" affinity (K(d) < 15 nM) only for four chemokines, three of them of viral origin: vMIP-I, vMIP-II, vMCC-I, and human I-309. A previously unreported second class of lower affinity ligands includes MCP-3 and possibly two other viral chemokines. vMIP-I and I-309 appear to act as CCR8 agonists: binding to and inducing cytosolic [Ca(2+)] elevation through the receptor. By contrast, vMIP-II and vMCC-I act as potent antagonists: binding without inducing signaling, and blocking the effects of I-309 and vMIP-I. These results suggest a ligand hierarchy for CCR8, identifying vMIP-I as a selective viral chemokine agonist. CCR8 may thus engage a specific subset of chemokines with the potential to regulate each other during viral infection and immune regulation.

Calcium influx factor (CIF) as a diffusible messenger for the activation of capacitative calcium entry in Xenopus oocytes

Acid extracts of thapsigargin-treated Xenopus oocytes revealed Ca2(+)-dependent Cl- currents by microinjection into Xenopus oocytes. These currents were detected in highly purified fractions by carrying out a sequence of purification steps including gel filtration chromatography and high performance thin layer chromatography. The nature of the membrane currents evoked by the highly purified fractions were carried by chloride ions as blockade by the selective chloride channel blocker 1 mM niflumic acid. Injection of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) eradicated the current activities, indicating that the current responses are completely Ca2(+)-dependent. Moreover, the currents were sensitive to the removal of extracellular calcium, indicating the dependence on calcium entry through plasma membrane calcium entry channels. These results elucidate that the highly purified fractions aquired by thapsigargin-stimulated oocytes is an authentic calcium influx factor (CIF). Thus, the detection of increased CIF production from thapsigargin treatment in Xenopus oocytes would give strong support for the existence of CIF as a diffusible messenger for the activation of capacitative calcium entry pathways in Xenopus oocytes.

Calcium influx factor is synthesized by yeast and mammalian cells depleted of organellar calcium stores

Depletion of endoplasmic reticulum Ca2+ stores leads to the entry of extracellular Ca2+ into the cytoplasm, a process termed capacitative or store-operated Ca2+ entry. Partially purified extracts were prepared from the human Jurkat T lymphocyte cell line and yeast in which Ca2+ stores were depleted by chemical and genetic means, respectively. After microinjection into Xenopus laevis oocytes, the extracts elicited a wave of increased cytoplasmic free Ca2+ ([Ca2+]i) that spread from the point of injection across the oocyte. Extracts from cells with replete organellar Ca2+ stores were inactive. The increases depended on extracellular Ca2+, were unaffected by the inositol 1,4,5-trisphosphate (IP3) inhibitor heparin or an anti-IP3 receptor antibody and were unchanged when the endoplasmic reticulum was segregated to the hemisphere opposite the injection site by centrifugation. Confocal microscopy revealed that [Ca2+]i increases were most pronounced at the periphery of the oocyte. The patterns of [Ca2+]i increases were replicated by computer simulations based on a diffusible messenger of about 700 Da that directly activates Ca2+ influx. In addition, ICRAC, a Ca2+ release-activated Ca2+ current monitored in Jurkat cells by whole-cell patch clamp recordings, was more rapidly activated when active extracts were included in the patch pipette than by the inclusion of a Ca2+ chelator or IP3. These data support the existence in yeast and mammalian cells depleted of Ca2+ stores of a functionally conserved diffusible calcium influx factor that directly activates Ca2+ influx.

Double-stranded-RNA-activated protein kinase (PKR) regulates Ca2+ stores in Xenopus oocytes

Expression of the double-stranded-RNA-dependent protein kinase (PKR) in Xenopus oocytes attenuated Ca2+ entry-dependent membrane currents activated by depletion of Ca2+ stores, whereas expression of a dominant-negative PKR mutant had the opposite effect. These results appeared to be due to perturbation of releasable Ca2+ stores, and not actions of PKR on protein synthesis. PKR may thus have novel protein substrates and cellular functions in Ca2+ storage and signalling.

17beta-estradiol stimulates substance P receptor gene expression

The actions of substance P (SP), a widely distributed tachykinin neuropeptide, are mediated by the NK1 receptor, a seven trans-membrane spanning domain cell surface receptor coupled to heterotrimeric G-proteins. SP regulates cellular processes in the CNS, placenta and vasculature including permeability, inflammation, mitogenesis and transformation. Examples of sexual dimorphism in tissue distribution and expression of SP and the SP receptor (SPR) in various organ systems (breast, uterus, brain) suggest the SPR may be under hormonal control. Using Northern blot analysis of SPR mRNA levels, we studied the effects of 17beta-estradiol (E2) on SPR gene expression in AR42J (rat pancreatic acinar) cells which constitutively express high levels of SPR. E2 (100 nM) led to a 2.5-fold increase in SPR mRNA levels (4.7 kb band) which was time- and concentration-dependent. The increase was inhibited by the RNA polymerase inhibitor actinomycin D (5 microg/ml) but not by the translational inhibitor cycloheximide (10 microg/ml). In addition, the antiestrogen tamoxifen (1 microM) blocked the stimulatory effect of E2 on SPR mRNA. Increased SPR mRNA levels in response to E2 were linearly related to increased [3H]SP binding to the SPR. This study has implications for understanding molecular mechanisms of hormonal control of receptor gene expression.

Inducible receptors

While regulation of receptor function is known to occur at many levels (e.g. transcriptional, post-translational), it is generally perceived that a tissue either expresses or does not express a particular receptor in an all-or-none fashion. Many pathological (e.g. tissue injury) and physiological (e.g. angiogenesis) processes have, however, been shown to be associated with the transcriptional induction of specific receptors. Induced receptors are not confined to any particular class, but range from G protein-coupled receptors to receptor tyrosine kinases. The potential implications of de novo receptor expression are profound with respect to potential novel therapeutic targets in specific disease states. Further, this observation may explain unexpected side-effects in the pharmacotherapy of existing disease states. In this article Lucy Donaldson, Michael Hanley and Amparo Villablanca discuss circumstances under which de novo receptor induction has been described, potential mechanisms of induction and the implications for pharmacology.

Diverse molecular provocation of programmed cell death

The experimental study and manipulation of programmed cell death has been greatly assisted by the identification of genetic and pharmacological tools that can either induce or block cell lethality. This review discusses new insights into the molecular sensing of perturbations induced by such tools, as well as the possible consequences of this detection in determining cell survival.

Functional characterization by heterologous expression of a novel cloned tachykinin peptide receptor

An orphan receptor resembling the neurokinin 3 tachykinin receptor (NK3), initially claimed to be an atypical opioid receptor, is shown herein to respond potently to the physiological NK3 receptor ligand, neurokinin B. This 'NK4' receptor did not give functional responses in Xenopus oocytes to opioid agonists. However, NK4 receptor activation was inhibited by nanomolar concentrations of dynorphin. The NK4 receptor is therefore a tachykinin receptor which is functionally antagonized by an endogenous opioid peptide.

Diverse molecular provocation of programmed cell death

The experimental study and manipulation of programmed cell death has been greatly assisted by the identification of genetic and pharmacological tools that can either induce or block cell lethality. This review discusses new insights into the molecular sensing of perturbations induced by such tools, as well as the possible consequences of this detection in determining cell survival.

Regulation of inositol trisphosphate-induced membrane currents in Xenopus oocytes by a Jurkat cell calcium influx factor

The functional interactions of a Jurkat cell-derived calcium influx factor (CIF) with Ins(1,4,5)P3 were examined by microinjection and voltage-clamp recording of current responses in Xenopus oocytes. CIF, which stimulates Ca2+ entry directly on microinjection, was active at dilutions at which it had no direct effect by augmenting both initial rapid Ins(1,4,5)P3-mediated Ca2+ discharge-activated currents and later sustained Ca2+ entry-activated currents. Augmented initial membrane currents were 3-5-fold greater in peak amplitude than currents evoked by injection of the same dose of Ins(1,4,5)P3 alone. The augmented initial response was not decreased by removal of extracellular Ca2+, suggesting that there is potentiation of Ins(1,4,5)P3-mediated discharge from intracellular Ca2+ stores. However, the augmentation of Ins(1,4,5)P3-mediated discharge cannot be due to an enhanced production of endogenous Ins(1,4,5)P3 because maximal Ins(1,4,5)P3-activated currents saturate (approx. 500 nA) with supramaximal levels of Ins(1,4,5)P3 (10-50 microM). Depletion of Ca2+ stores, by pretreatment with thapsigargin or by prior injection with the Ins(1,4,5)P3 receptor antagonist heparin, abolished membrane currents elicited by Ins(1,4,5)P3/CIF co-injection, further suggesting that the Ins(1,4,5)P3 receptor was the target for the initial-current potentiating actions of CIF. In this regard, CIF also induced augmented initial currents with co-injection of either Ins(2,4,5)P3 or Ins(1,3,4,5)P4. The augmentation of Ins(1,4,5)P3-mediated currents by CIF was bell-shaped with regard to Ins(1,4,5)P3 concentration, reminiscent of the regulatory influence of Ca2+ on Ins(1,4,5)P3 responses. Co-injection of Ins(1,4,5)P3 and CIF also augmented (2-3-fold) later current responses arising from sustained Ca2+ entry. The augmented late-current responses were not due to enhanced Ca2+ store depletion because supramaximal levels of Ins(1,4,5)P3 (50 microM) or injection of the poorly metabolized Ins(1,4,5)P3 analogue, Ins(2,4,5)P3, cannot activate the same magnitude of Ca(2+)-entry-dependent currents. These results suggest that CIF at low levels interacts with Ins(1,4,5)P3 to sensitize two pathways of Ca2+ signalling: initial discharge and later Ca2+ entry. Thus under physiological conditions CIF might be more potent as a co-messenger than as a direct Ca2+ entry signal and might provide a novel type of direct feedback regulation between the stores-activated influx pathway and the Ins(1,4,5)P3 receptor. Moreover these results suggest that CIF modulation of the receptor for Ins(1,4,5)P3 may underlie control of both augmentation of discharge and Ca2+ entry, as has been predicted from the conformational coupling model of Ca2+ entry.

Stimulation of Ca(2+)-dependent membrane currents in Xenopus oocytes by microinjection of pyrimidine nucleotide-glucose conjugates

Microinjection, but not extracellular application, of cytidine-5'-diphosphate-D-glucose (CDPG) has been shown to elicit Ca(2+)-dependent currents in Xenopus laevis oocytes. These responses were comparable to those of inositol-1,4,5-trisphosphate (InsP3) in being both rapid and dose dependent. For example, maximal amplitudes of CDPG-induced current were similar (approximately 365 +/- 75 nA at 1 microM CDPG) to those of InsP3. The CDPG currents were insensitive to removal of extracellular Ca2+, indicating the dependence on Ca2+ release from intracellular Ca2+ stores but not on Ca2+ entry through plasma membrane. CDPG-induced currents were reduced or abolished by pretreatment with thapsigargin, by injection of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or by extracellular perfusion of the Cl- channel blocker niflumic acid but were insensitive to injection of the InsP3 antagonist heparin. These results suggest that CDPG induces Ca2+ discharge from intracellular Ca2+ stores via a mechanism distinct from that of InsP3 in Xenopus oocytes. Another pyrimidine nucleotide-glucose derivative, uridine-5'-diphosphate-alpha-D-glucose, also induced Ca(2+)-dependent currents, but the activity was lower than that of CDPG (maximal amplitude, 272 +/- 62 nA). Other nucleotide-glucose compounds (adenosine-5'-diphosphate-D-glucose, guanosine-5'-diphosphate-D-glucose, and thymidine-5'-diphosphate-D-glucose) had no current responses when injected into oocytes. After injection of CDPG, CDPG-induced Ca2+ release appeared to couple to a Ca2+ entry pathway similar to that coupled to InsP3. These results indicate that pyrimidine nucleotide-glucose conjugates may provide novel pharmacological tools for the study of Ca2+ signaling in oocytes.

Chromatographic resolution of an intracellular calcium influx factor from thapsigargin-activated Jurkat cells. Evidence for multiple activities influencing calcium elevation in Xenopus oocytes

Acid extracts of thapsigargin-stimulated Jurkat cells revealed both intracellular and extracellular activities stimulating Ca(2+)-dependent Cl- currents on Xenopus laevis oocytes. Chromatographic fractionation of these extracts on gel filtration separated two active fractions of M(r) approximately 600 and 400. Moreover, the M(r) 600 fraction exhibited both intracellular and extracellular activities. However, the intracellular activity was absent from extracts of unstimulated Jurkat cells, suggesting its production was stimulated by thapsigargin. The further purification of this fraction by high performance thin layer chromatography resolved a single fraction which was active only on microinjection and which required calcium entry for activation of current responses. These results suggest that a single authentic calcium influx factor can be resolved by purification from confounding activities detected in crude acid extracts.

Evaluation of calcium influx factors from stimulated Jurkat T-lymphocytes by microinjection into Xenopus oocytes

Acid extracts of thapsigargin-activated Jurkat cells have been shown to have intracellular activity in inducing a dose-dependent rapid chloride current upon microinjection in Xenopus laevis oocytes. The extracts act by elevation of calcium through calcium entry. The factor(s) responsible for this activity have been termed calcium influx factor (CIF) and have been found to be small, relatively polar molecules (< 1000 daltons) whose activity is abolished by alkaline phosphatase treatment and potentiated by co-injection of okadaic acid (a protein phosphatase inhibitor). CIF is produced in a time-dependent manner following thapsigargin treatment of Jurkat cells, being first elevated above basal levels by 2 min. Intracellular CIF activity is completely absent from NG115-401L neuronal cells, which lack capacitative entry. On this basis, it appears that Jurkat cells, activated by stimuli that deplete internal calcium stores, produce one or more CIF activities acting intracellularly, and Xenopus oocytes may be a powerful tool to purify and characterize CIFs.

Attenuation of agonist-induced desensitization of the rat substance P receptor by microinjection of inositol pentakis-and hexakisphosphates in Xenopus laevis oocytes

Recently, inositol hexakisphosphate (phytic acid) was shown to bind to photoreceptor arrestin and block its interaction with rhodopsin. Such an interaction might predict that inositol polyphosphates could alter G protein-coupled receptor desensitization. To investigate the possible roles of higher inositol polyphosphates on receptor desensitization, we have expressed the rat substance P receptor in Xenopus laevis oocytes. The functional expression of substance P receptor was monitored by voltage-clamp recording of substance P-induced Ca(2+)-dependent Cl- currents. When control oocytes were stimulated with substance P (30 nM), after 10 min of washing the second responses to substance P were approximately 15% of the first responses. Cytosolic injection of inositol pentakisphosphate (100 microM) or inositol hexakisphosphate (100 microM) inhibited the reduction of the second substance P-induced current responses, maintaining the second responses to 57-58% of the initial responses. The protective effects of inositol pentakisphosphate and inositol hexakisphosphate against agonist-induced desensitization were concentration and time dependent and structurally specific, in that inositol hexasulfate and inositol tris- and tetrakisphosphate isomers were inactive. Microinjection of inositol hexakisphosphate did not (a) change the potency of substance P or the sensitivity of the expressed substance P receptor to substance P, (b) inhibit 12-O-tetradecanoylphorbol-13-acetate-induced loss of substance P-induced current responses, or (c) alter the currents elicited by microinjection of inositol-1,4,5-trisphosphate. These results suggest that inositol pentakisphosphate and inositol hexakisphosphate have specific inhibitory effects on the agonist-induced loss of responsiveness of the rat substance P receptor. Moreover, these protective effects of inositol hexakisphosphate against desensitization were also observed with the endogenous lysophosphatidic acid/phosphatidic acid receptor, indicating that this mechanism is not specific to ectopic receptors. These results suggest that inositol pentakisphosphate and inositol hexakisphosphate may be novel pharmacological tools for the study of agonist-induced desensitization.

Attenuation of agonist-induced desensitization of the rat substance P receptor by progressive truncation of the C-terminus

We have investigated the C-terminal tail of the rat substance P receptor (SPR) as a domain essential for agonist-induced desensitization. Four progressively shorter mutants, using premature termination in the C-terminus, were constructed and compared with the unaltered SPR using ectopic expression of wild-type and mutant receptors in Xenopus oocytes. These mutants were designated D16, D47, D70 and D96 with 16, 47, 70 and 96 amino acids residues deleted from the tail, respectively. Wild type SPR, D16 and D47 exhibited normal current responses when challenged with substance P, but D70 and D96 had reduced maximal current responses (70% and 5% of wild type SPR, respectively). D70, however, exhibited substantial resistance to substance P-induced desensitization in that 55%, versus 8% for wild type SPR, of the peak current of the first response was preserved on second challenge with substance P. Therefore, a domain from residues 338 to 360 of the rat SPR, though not necessary for the functional activity of the receptor, plays an essential role in agonist-induced desensitization.

Expression of the MAS proto-oncogene in the retinal pigment epithelium of the rhesus macaque

The MAS proto-oncogene codes for a seven transmembrane protein which has been previously localized to specific regions of the hippocampus and cerebral cortex in the rat central nervous system. Because MAS has biological properties related to the growth and differentiation of cells of neuroectodermal origin, we investigated the distribution of MAS expression in the rhesus macaque retina by in situ hybridization. A 330 base pair (bp) segment of the human MAS sequence was subcloned and used to generate single-stranded cRNA probes for these studies. Our results demonstrated little, if any, positive signal over the neurons of the retina. The use of epi-polarization microscopy, however, revealed a distinct positive labelling of retinal pigment epithelial (RPE) cells. These studies suggest the use of MAS as a possible marker for the retinal pigment epithelium.

Receptor regulation of phosphoinositide 3-hydroxykinase in the NG115-401L-C3 neuronal cell line: stimulation by insulin-like growth factor-I

The activation of phosphoinositide 3-hydroxykinase (P13K) is currently believed to represent the critical regulatory event which leads to the production of a novel intracellular signal. We have examined the control of this pathway by a number of cell-surface receptors in NG115-401L-C3 neuronal cells. Insulin-like growth factor-I stimulated the accumulation of 3-phosphorylated inositol lipids in intact cells and the appearance of P13K in antiphosphotyrosine-antibody-directed immunoprecipitates prepared from lysed cells, suggesting that P13K had been activated by a mechanism involving a protein tyrosine kinase. In contrast, P13K in these cells was not regulated by a variety of G-protein-coupled receptors, nerve growth factor acting via a low affinity receptor, or receptors for transforming growth factor-beta and interleukin-1. The receptor-specificity of P13K activation in these cells places significant constraints on the possible physiological function(s) of this pathway.

Characterization of metal ion-induced [3H]inositol hexakisphosphate binding to rat cerebellar membranes

The binding of [3H]inositol hexakisphosphate ([3H] InsP6) to rat cerebellar membranes has been characterized with the objective of establishing the role, if any, of a membrane protein receptor. In the presence of EDTA, we have previously identified an InsP6-binding site with a capacity of approximately 20 pmol/mg protein (Hawkins, P. T., Reynolds, D. J. M., Poyner, D. R., and Hanley, M. R. (1990) Biochem. Biophys. Res. Commun. 167, 819-827). However, in the presence of 1 mM Mg2+, the capacity of [3H]InsP6 binding to membranes was increased approximately 9-fold. This enhancing effect of Mg2+ was reversed by addition of 10 microM of several cation chelators, suggesting that the increased binding required trace quantities of other metal cations. This is supported by experiments where it was possible to saturate binding by addition of excess membranes, despite not significantly depleting radioligand, pointing to removal of some other factor. Removal of endogenous cations from the binding assay by pretreatment with chelex resin also prevents the Mg(2+)-induced potentiation. Consideration of the specificity of the chelators able to abolish this potentiation suggested involvement of Fe3+ or Al3+. Both these ions (but not several others) were able to increase [3H]InsP6 binding to chelex-pretreated membranes at concentrations of 1 microM. It is possible to demonstrate synergy between Fe3+ and Mg2+ under these conditions. We propose that [3H]InsP6 may interact with membranes through non-protein recognition, possibly via phospholipids, in a manner dependent upon trace metals. The implications of this for InsP6 biology are considered.

Phosphatidic acid and lysophosphatidic acid stimulate receptor-regulated membrane currents in the Xenopus laevis oocyte

External application of dioleoyl-phosphatidic acid and oleoyl-lysophosphatidic acid stimulated Ca(2+)-dependent chloride currents in voltage-clamped Xenopus laevis oocytes. The responses were observed in oocytes from which follicular cells had been removed, indicating they were intrinsic to the oocyte itself. The lipid-induced Ca(2+)-dependent chloride currents were observed in the absence of extracellular calcium, were blocked by intracellular injection of the calcium chelator, bis(O-aminophenoxy)-ethane N,N,N'N'-tetraacetic acid, and could not be elicited by direct intracellular injection of the active lipids. The thresholds for dose-dependent current responses to dioleoyl-phosphatidic acid (100 nM) and for oleoyl-lysophosphatidic acid (10 nM) indicated that the lipid activities on oocytes were potent. With repeated or prolonged administration of either active lipid, responses exhibited desensitization. These results demonstrate that the Xenopus oocyte expresses endogenous functional responses for the mitogenic lipids phosphatidic acid and lysophosphatidic acid and thus provides a powerful model for characterization of the pharmacology and transduction pathways of these responses.

Stimulation of adenylate cyclase by amylin in CHO-K1 cells

The CHO-K1 cell line responds to the peptide amylin by a rapid elevation of cAMP. The related peptide calcitonin gene-related peptide (CGRP) is 100 times less potent at stimulating adenylate cyclase than is amylin. The actions of amylin at this receptor are concentration dependent and not antagonized by the CGRP antagonist CGRP-(8-37). Although these cells have receptors for calcitonin, amylin is unable to take part in any high affinity interaction with these receptors, as assessed by radioligand binding. The CHO-K1 cell line has receptors for amylin that are distinct from those for calcitonin and CGRP.

Pharmacological characterization of a receptor for calcitonin gene-related peptide on rat, L6 myocytes

1 The L6 myocyte cell line expresses high affinity receptors for calcitonin gene-related peptide (CGRP) which are coupled to activation of adenylyl cyclase. The biochemical pharmacology of these receptors has been examined by radioligand binding or adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation. 2 In intact cells at 37 degrees C, human and rat alpha- and beta-CGRP all activated adenylyl cyclase with EC50s of about 1.5 nM. A number of CGRP analogues containing up to five amino acid substitutions showed similar potencies. In membrane binding studies at 22 degrees C in 1 mM Mg2+, the above all bound to a single site with IC50s of 0.1-0.4 nM. 3 The fragment CGRP(8-37) acted as a competitive antagonist of CGRP stimulation of adenylyl cyclase with a calculated Kd of 5 nM. The Kd determined in membrane binding assays was lower (0.5 nM). 4 The N-terminal extended human alpha-CGRP analogue Tyro-CGRP activated adenylyl cyclase and inhibited [125I]-iodohistidyl-CGRP binding less potently than human alpha-CGRP (EC50 for cyclase = 12 nM, IC50 for binding = 4 nM). 5 The pharmacological profile of the L6 CGRP receptor suggests that it most closely resembles sites on skeletal muscle, cardiac myocytes and hepatocytes. The L6 cell line should be a stable homogeneous model system in which to study CGRP mechanisms and pharmacology.

Thapsigargin inhibits the sarcoplasmic or endoplasmic reticulum Ca-ATPase family of calcium pumps

The role of ATP-dependent calcium uptake into intracellular storage compartments is an essential feature of hormonally induced calcium signaling. Thapsigargin, a non-phorboid tumor promoter, increasingly is being used to manipulate calcium stores because it induces a hormone-like elevation of cytosolic calcium. It has been suggested that thapsigargin acts through inhibition of the endoplasmic reticulum calcium pump. We have directly tested the specificity of thapsigargin on all of the known intracellular-type calcium pumps (referred to as the sarcoplasmic or endoplasmic reticulum Ca-ATPase family (SERCA]. Full-length cDNA clones encoding SERCA1, SERCA2a, SERCA2b, and SERCA3 enzymes were expressed in COS cells, and both calcium uptake and calcium-dependent ATPase activity were assayed in microsomes isolated from them. Thapsigargin inhibited all of the SERCA isozymes with equal potency. Furthermore, similar doses of thapsigargin abolished the calcium uptake and ATPase activity of sarcoplasmic reticulum isolated from fast twitch and cardiac muscle but had no influence on either the plasma membrane Ca-ATPase or Na,K-ATPase. The interaction of thapsigargin with the SERCA isoforms is rapid, stoichiometric, and essentially irreversible. These properties demonstrate that thapsigargin interacts with a recognition site found in, and only in, all members of the endoplasmic and sarcoplasmic reticulum calcium pump family.

Regulation of c-fos and c-jun protooncogene expression by the Ca(2+)-ATPase inhibitor thapsigargin

Thapsigargin, a non-phorbol-ester-type tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca(2+)-ATPase. We used this drug to analyze the involvement of Ca2+ and Ca(2+)-ATPases in the control of growth- and transformation-related genes. Here we show that treatment of mouse NIH 3T3 fibroblasts with thapsigargin induced rapid expression of the c-fos and c-jun protooncogenes. Inhibition or depletion of protein kinase C partially diminished the c-fos but not the c-jun response. Furthermore, thapsigargin could synergize with the tumor promoter phorbol 12-myristate 13-acetate to induce c-fos but not c-jun. However, thapsigargin had no effect on basal or phorbol ester-induced protein kinase C activity. Our results indicate that Ca2+ is a potent second messenger that controls expression of growth- and transformation-related genes. Since inhibition of the endoplasmic reticulum Ca(2+)-ATPase results in a strong induction of these genes, our data suggest that this Ca2+ pump may act as a negative regulator of cell growth.

myo-inositol pentakisphosphates. Structure, biological occurrence and phosphorylation to myo-inositol hexakisphosphate

1. Standard and high-performance anion-exchange-chromatographic techniques have been used to purify myo-[3H]inositol pentakisphosphates from various myo-[3H]inositol-prelabelled cells. Slime mould (Dictyostelium discoideum) contained 8 microM-myo-[3H]inositol 1,3,4,5,6-pentakisphosphate, 16 microM-myo-[3H]inositol 1,2,3,4,6-pentakisphosphate and 36 microM-D-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate [calculated intracellular concentrations; Stephens & Irvine (1990) Nature (London) 346, 580-583]; germinating mung-bean (Phaseolus aureus) seedlings contained both D- and L-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate (which was characterized by 31P and two-dimensional proton n.m.r.) and D- and/or L-myo-[3H]inositol 1,2,3,4,5-pentakisphosphate; HL60 cells contained myo-[3H]inositol 1,3,4,5,6-pentakisphosphate (in a 500-fold excess over the other species), myo-[3H]inositol 1,2,3,4,6-pentakisphosphate and D- and/or L-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate; and NG-115-401L-C3 cells contained myo-[3H]inositol 1,3,4,5,6-pentakisphosphate (in a 100-fold excess over the other species), D- and/or L-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate, myo-[3H]inositol 1,2,3,4,6-pentakisphosphate and D- and/or L-myo-[3H]inositol 1,2,3,4,5-pentakisphosphate. 2. Multiple soluble ATP-dependent myo-inositol pentakisphosphate kinase activities have been detected in slime mould, rat brain and germinating mung-bean seedling homogenates. In slime-mould cytosolic fractions, the three myo-inositol pentakisphosphates that were present in intact slime moulds could be phosphorylated to myo-[3H]inositol hexakisphosphate: the relative first-order rate constants for these reactions were, in the order listed above, 1:8:31 respectively (with first-order rate constants in the intact cell of 0.1, 0.8 and 3.1 s-1, assuming a cytosolic protein concentration of 50 mg/ml), and the Km values of the activities for their respective inositol phosphate substrates (in the presence of 5 mM-ATP) were 1.6 microM, 3.8 microM and 1.4 microM. At least two forms of myo-inositol pentakisphosphate kinase activity could be resolved from a slime-mould cytosolic fraction by both pharmacological and chromatographic criteria. Rat brain cytosol and a soluble fraction derived from germinating mung-bean seedlings could phosphorylate myo-inositol D/L-1,2,4,5,6-, D/L-1,2,3,4,5-, 1,2,3,4,6- and 1,3,4,5,6-pentakisphosphates to myo-inositol hexakisphosphate: the relative first-order rate constants were 57:27:77:1 respectively for brain cytosol (with first-order rate constants in the intact cell of 0.0041, 0.0019, 0.0056 and 0.000073 s-1 respectively, assuming a cytosolic protein concentration of 50 mg/ml) and 1:11:12:33 respectively for mung-bean cytosol (with first-order rate constants in a supernatant fraction with a protein concentration of 10 mg/ml of 0.0002, 0.0022, 0.0024 and 0.0066 s-1 respectively).

The role of phospholipases and phospholipid-derived signals in cell activation

The complexity of receptor-regulated breakdown and modification of phospholipids continues to grow. New developments extend our concepts of signalling enzymes and possible messengers.

A monoclonal anti-idiotypic antibody to mu and delta opioid receptors

A mouse monoclonal, anti-idiotypic, anti-opioid receptor antibody (Ab2-AOR) has been generated from monoclonal anti-morphine antibodies (Ab1). Hybridoma culture supernatants were screened by a solid phase radioimmunoassay (RIA), based on their competition with radiolabelled morphine for Ab1. One of the Ab2s that gave a positive RIA also competed at rat brain opioid receptors with tritiated opioid ligands dihydromorphine (DHM), naloxone, etorphine, Tyr-D-Ala-Gly-Phe-D-Leu (DADLE), Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAMGE) and Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE). SDS-PAGE revealed Ab2-AOR to be highly purified after successive affinity and protein A-Sepharose chromatography. Ab2-AOR at concentrations of 10-100 nM competed with both mu- and delta-selective specific ligands for brain opioid receptors. Less than 13 micrograms/ml Ab2-AOR completely inhibited specific opioid radioligand binding to both soluble and membrane-bound opioid receptors. To demonstrate its anti-delta receptor activity further, a double-antibody ELISA procedure was developed that is based on the binding of Ab2-AOR to immobilized NG 108-15 cells (which contain only delta opioid receptors). Dose-dependent, opioid peptide- and opiate alkaloid-competitive binding of Ab2-AOR-containing ascites fluid to NG 108-15 cells was observed. A mu opioid agonist effect was demonstrated for Ab2-AOR, in that it decreased by 70% [3H]thymidine incorporation into DNA of fetal brain cell aggregates. This agonist-like action of Ab2-AOR was blocked by naltrexone. The antibody bound specifically to brain tissue sections and the presence of diprenorphine blocked this interaction. Hence, an Ab2 with mu and delta specificity has been characterized.

Changes in inositol lipids and phosphates after stimulation of the MAS-transfected NG115-401L-C3 cell line by mitogenic and non-mitogenic stimuli

A neuronal cell line (NG115-401L-C3) was stimulated by mitogenic (angiotensin) and non-mitogenic (bradykinin) peptides and examined for the time course of changes in the levels of radiolabelled inositol phosphates and phospholipids. Both peptides stimulated the time-dependent production of Ins(1,4,5)P3 and related metabolites. Bradykinin caused a much larger increase in Ins(1,4,5)P3 than did angiotensin. However, both peptides stimulated similar rises in the levels of Ins(1,3,4)P3 and InsP4. Bradykinin, but not angiotensin, caused a rapid (within 2 s) fall in the levels of PtdIns(4,5)P2 and PtdIns(4)P. Serum pretreatment of the cells caused a 2-3-fold potentiation of both the responses to bradykinin and angiotensin. Although significant levels of PtdIns(3)P were detected in resting cells, neither mitogenic (angiotensin, insulin-like growth factor I, transforming growth factor beta) nor non-mitogenic (bradykinin, nerve growth factor, interleukin-1) receptor activation changed its levels, arguing against regulation of either PtdIns 3-kinase or PtdIns(3)P phosphatase. We conclude that, as judged by the levels of its product. PtdIns(3)P, the enzyme PtdIns 3-kinase is not activated. This questions the significance of this activity in the receptor-mediated initiation of DNA synthesis.

Inositol trisphosphate and thapsigargin discriminate endoplasmic reticulum stores of calcium in rat brain

ATP dependent Ca2+ accumulation into oxalate-loaded rat brain microsomes is potently inhibited by thapsigargin with an IC50 of 2 nM and maximal inhibition at 10 nM. Approximately 15% of the total A23187-releasable microsomal calcium store is insensitive to thapsigargin concentrations up to 100 microM. Inositol-1,4,5-trisphosphate (IP3) maximally inhibits 40% of the net Ca2+ accumulation by whole brain microsomes. Its effects are non-additive with thapsigargin suggesting that the IP3-sensitive Ca2+ pool is a subset of the thapsigargin sensitive Ca2+ pool. Marked regional differences occur in Ca2+ transport rates and sensitivity to both thapsigargin and IP3.

Phosphatidylinositol metabolism in cells transformed by polyomavirus middle T antigen

Associated with the middle T antigen of polyomavirus is a novel phosphatidylinositol (PtdIns) kinase activity which phosphorylates PtdIns at the D-3 position of the inositol ring. We have undertaken an analysis of myo-[3H]inositol-containing compounds in a panel of NIH 3T3 cell lines stably transfected with transforming and nontransforming middle T antigen mutants. All cell lines from which PtdIns 3-kinase activity coprecipitated with middle T antigen exhibited modestly elevated levels of PtdIns(3)P and compounds with predicted PtdIns(3,4)P2 and PtdIns(3,4,5)P3 structures. Complex formation between middle T antigen and PtdIns 3-kinase correlated not with an increase in total inositol phosphate levels but rather with elevated levels of InsP2 and InsP4. A specific increase in the level of an InsP2 species which comigrated in high-pressure liquid chromatography analysis with Ins(3,4)P2 was observed. These results suggest that association of the polyomavirus middle T antigen with PtdIns 3-kinase activates a distinct inositol metabolic pathway.

Effect of capsaicin on the rabbit urinary bladder. What is the function of sensory nerves that contain substance P?

The sensory innervation of the rabbit urinary bladder was studied using local application of the specific sensory neurotoxin capsaicin (8-methyl-N-vanillyl-6-nonenamide). This agent has its major effect by damaging small diameter unmyelinated sensory nerves. The drug produced a 51% reduction in the bladder content of the neuropeptide substance P. It therefore appears that a substantial proportion of the bladder's content of the peptide is to be found in capsaicin-sensitive sensory nerves. Cystometrograms carried out before and after treatment with capsaicin were similar; this suggests that capsaicin-sensitive sensory nerves may not be of importance in the afferent limb of the micturition reflex. In vitro muscle strip studies demonstrated a small reduction in the sensitivity of the detrusor muscle to electrical stimulation of its intramural nerves. It is possible that in vitro intramural nerve stimulation leads to release of neurotransmitters from sensory as well as motor nerves. It is proposed that small diameter sensory nerves in the bladder wall may have a role in the transmission of the sensation of pain and in the triggering of inflammatory reactions rather than forming the afferent limb of the micturition reflex.

Autoradiographic localization of mas proto-oncogene mRNA in adult rat brain using in situ hybridization

The cellular localization and the distribution of the mas proto-oncogene/angiotensin receptor mRNA have been studied in the male rat brain using in situ hybridization with radiolabelled mas cRNA probes. Neuronal cell populations in the forebrain were selectively labelled. A strong specific labelling was demonstrated in the dentate gyrus, the CA3 and CA4 areas of the hippocampus, the olfactory tubercle (medical part), the piriform cortex and the olfactory bulb, while a weak to moderate labelling was present all over the neocortex and especially in the frontal lobe.

Substance P and bombesin elevate cytosolic Ca2+ by different molecular mechanisms in a rat pancreatic acinar cell line

1. Dual-excitation microfluorometry (Fura-2 as indicator) was employed to monitor directly changes in the cytosolic calcium concentration [( Ca2+]i) in single cells. We investigated and compared the effects of stimulation of AR42J rat pancreatic acinar cells by two peptide agonists, substance P and bombesin. 2. Substance P (10(-7) M) and bombesin (10(-8) M) each gave rise to a marked, but transient, elevation in [Ca2+]i. The calcium signals evoked by the two peptides were qualitatively and quantitatively very similar. However, in the absence of extracellular Ca2+ the response to substance P, but not bombesin, was abolished. These results suggest that substance P induces calcium influx across the cell surface membrane but does not release calcium from internal stores. Bombesin in marked contrast releases calcium from intracellular stores in the absence of any detectable calcium influx. 3. Depolarization by high-K+ extracellular solutions evoked a marked, but transient, rise in [Ca2+]i. This elevation in [Ca2+]i was strictly dependent upon the presence of Ca2+ in extracellular media. 4. Nifedipine (5 x 10(-6) M), an antagonist of L-type voltage-dependent Ca2+ channels, blocked the elevations in [Ca2+]i induced by either substance P or high-K+ solutions, but not that evoked by application of bombesin. 5. Patch-clamp, single-channel current recordings from cell-attached patches of membrane confirmed the presence of voltage-dependent calcium channels in the surface membranes of AR42J cells. Whole-cell current recordings demonstrated voltage-dependent inward Ca2+ (Ba2+) currents which were increased in amplitude by substance P and blocked by nifedipine. 6. The protein kinase C (PKC) activators, the phorbol diester, phorbol 1,2-myristate 13-acetate (PMA, 10(-7) M), and cell-permeable diacylglycerol analogues, 1-oleoyl-2-acetyl-sn-glycerol (OAG, 2.5 x 10(-6) M) and sn-2-dioctanoyl glycerol (DiC8, 2.5 x 10(-6) M), mimicked the effect of substance P, but not bombesin, in elevating [Ca2+]i in a manner that was blocked by removal of extracellular Ca2+ or application of nifedipine. 7. The PKC inhibitor, polymyxin B (2.5 x 10(-6) M), applied 2 min prior to stimulation blocked the effects of substance P and PKC activators, but not bombesin, in elevating [Ca2+]i. 8. The calcium signals evoked by substance P and bombesin are achieved by activation of different molecular mechanisms. Substance P, the evidence suggests, activates PKC which in turn stimulates calcium influx by opening voltage-dependent Ca2+ channels in the cell surface membranes.(ABSTRACT TRUNCATED AT 400 WORDS)

Pituitary astrocytes from the neural lobe of rats. A tissue culture and immunohistochemical study

Tissue culture preparations of adult and neonatal rat pituitary neural lobes were examined by use of cell-type specific immunohistochemical markers. Cultures obtained from explanted or dissociated adult tissue or explanted neonatal tissue produced cells immunoreactive for endothelial and fibroblast markers. In contrast, dissociated neonatal tissue produced, in addition, two distinct forms of astrocytic glial cells immunoreactive for glial fibrillary acidic protein, one of which was also immunoreactive for the ganglioside GD3.

Thapsigargin, but not caffeine, blocks the ability of thyrotropin-releasing hormone to release Ca2+ from an intracellular store in GH4C1 pituitary cells

Thapsigargin stimulates an increase of cytosolic free Ca2+ concentration [( Ca2+]c) in, and 45Ca2+ efflux from, a clone of GH4C1 pituitary cells. This increase in [Ca2+]c was followed by a lower sustained elevation of [Ca2+]c, which required the presence of extracellular Ca2+, and was not inhibited by a Ca2(+)-channel blocker, nimodipine. Thapsigargin had no effect on inositol phosphate generation. We used thyrotropin-releasing hormone (TRH) to mobilize Ca2+ from an InsP3-sensitive store. Pretreatment with thapsigargin blocked the ability of TRH to cause a transient increase in both [Ca2+]c and 45Ca2+ efflux. The block of TRH-induced Ca2+ mobilization was not caused by a block at the receptor level, because TRH stimulation of InsP3 was not affected by thapsigargin. Rundown of the TRH-releasable store by Ca2(+)-induced Ca2+ release does not appear to account for the action of thapsigargin on the TRH-induced spike in [Ca2+]c, because BAY K 8644, which causes a sustained rise in [Ca2+]c, did not block Ca2+ release caused by TRH. In addition, caffeine, which releases Ca2+ from intracellular stores in other cell types, caused an increase in [Ca2+]c in GH4C1 cells, but had no effect on a subsequent spike in [Ca2+]c induced by TRH or thapsigargin. TRH caused a substantial decrease in the amount of intracellular Ca2+ released by thapsigargin. We conclude that in GH4C1 cells thapsigargin actively discharges an InsP3-releasable pool of Ca2+ and that this mechanism alone causes the block of the TRH-induced increase in [Ca2+]c.

Thapsigargin, a tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca2(+)-ATPase

Thapsigargin, a tumor-promoting sesquiterpene lactone, discharges intracellular Ca2+ in rat hepatocytes, as it does in many vertebrate cell types. It appears to act intracellularly, as incubation of isolated rat liver microsomes with thapsigargin induces a rapid, dose-dependent release of stored Ca2+. The thapsigargin-releasable pool of microsomal Ca2+ includes the pools sensitive to inositol 1,4,5-trisphosphate and GTP. Thapsigargin pretreatment of microsomes blocks subsequent loading with 45Ca2+, suggesting that its target is the ATP-dependent Ca2+ pump of endoplasmic reticulum. This hypothesis is strongly supported by the demonstration that thapsigargin causes a rapid inhibition of the Ca2(+)-activated ATPase activity of rat liver microsomes, with an identical dose dependence to that seen in whole cell or isolated microsome Ca2+ discharge. The inhibition of the endoplasmic reticulum isoform of the Ca2(+)-ATPase is highly selective, as thapsigargin has little or no effect on the Ca2(+)-ATPases of hepatocyte or erythrocyte plasma membrane or of cardiac or skeletal muscle sarcoplasmic reticulum. These results suggest that thapsigargin increases the concentration of cytosolic free Ca2+ in sensitive cells by an acute and highly specific arrest of the endoplasmic reticulum Ca2+ pump, followed by a rapid Ca2+ leak from at least two pharmacologically distinct Ca2+ stores. The implications of this mechanism of action for the application of thapsigargin in the analysis of Ca2+ homeostasis and possible forms of Ca2+ control are discussed.