A new enzymatic assay for guanosine 3':5'-cyclic monophosphate and its application to the ductus deferens of the rat

Semicircular canal morphogenesis in the zebrafish inner ear requires the function of gpr126 (lauscher), an adhesion class G protein-coupled receptor gene

Morphogenesis of the semicircular canal ducts in the vertebrate inner ear is a dramatic example of epithelial remodelling in the embryo, and failure of normal canal development results in vestibular dysfunction. In zebrafish and Xenopus, semicircular canal ducts develop when projections of epithelium, driven by extracellular matrix production, push into the otic vesicle and fuse to form pillars. We show that in the zebrafish, extracellular matrix gene expression is high during projection outgrowth and then rapidly downregulated after fusion. Enzymatic disruption of hyaluronan in the projections leads to their collapse and a failure to form pillars: as a result, the ears swell. We have cloned a zebrafish mutant, lauscher (lau), identified by its swollen ear phenotype. The primary defect in the ear is abnormal projection outgrowth and a failure of fusion to form the semicircular canal pillars. Otic expression of extracellular matrix components is highly disrupted: several genes fail to become downregulated and remain expressed at abnormally high levels into late larval stages. The lau mutations disrupt gpr126, an adhesion class G protein-coupled receptor gene. Expression of gpr126 is similar to that of sox10, an ear and neural crest marker, and is partially dependent on sox10 activity. Fusion of canal projections and downregulation of otic versican expression in a hypomorphic lau allele can be restored by cAMP agonists. We propose that Gpr126 acts through a cAMP-mediated pathway to control the outgrowth and adhesion of canal projections in the zebrafish ear via the regulation of extracellular matrix gene expression.

Biochemical Detection of cGMP From Past to Present: An Overview

Cyclic guanosine monophosphate (cGMP), generated via the guanylate cyclase (GC)-catalyzed conversion from GTP, is unequivocally recognized as crucial second messenger, intimately involved in the regulation of a broad range of physiological processes such as long term potentiation, blood pressure regulation, or platelet aggregation (for review: Hobbs 2000). Since its first identification in rat urine by Ashman and co-workers (1963), various approaches have been conceived and established to quantify cGMP in biological samples, or to detect cGMP as the reaction product of enzymatic assays, allowing the determination of kinetic parameters. These approaches have evolved from laborious handling of small numbers of samples with average sensitivity to highly developed biochemical detection assays allowing the processing of very large numbers of samples. The present article focuses upon the history of biochemical cGMP detection from the pioneering work of the early years to the actual state-of-the-art approaches for the detection of this important biological messenger.

A study of cyclic nucleotides as second messengers after interleukin 2 stimulation of human T lymphocytes

Interleukin 2 (IL-2) was shown to induce a small but significant increase in the level of cGMP after 20 min stimulation and a subsequent fall after 1 h in activated T lymphocytes. No change in the level of cAMP was observed. Addition of the cyclic nucleotide analogues dbcAMP or dbcGMP did not stimulate DNA synthesis. On the contrary, IL-2-induced [3H]thymidine incorporation was inhibited by these drugs. Further, the phosphodiesterase inhibitor theophylline inhibited proliferation of activated T lymphocytes. Our results indicate that neither cAMP nor cGMP act as 'second messengers' for IL-2 but support the theory that cAMP is a negative regulator of cell proliferation.

The role of cyclic GMP in cells with the properties of smooth muscle cultured from the rat myometrium

Cells growing in culture with previously described properties of rat uterine smooth muscle accumulated 45Ca2+ from the medium. Ca2+ uptake by these cells was stimulated by the addition to the medium of 8-bromo-cGMP but not by 8-bromo-cAMP. Ca2+ uptake was also stimulated by carbachol and by the nitro-vasodilator nitroprusside. Although cholinergic agonists have been shown previously to stimulate contraction but not cGMP synthesis in the rat myometrium, both carbachol and nitroprusside stimulated cGMP production by the cultured cells. These results suggested the cells had cholinergic receptor-mediated functions that reflected some neurotransmitter-sensitive properties of uterine smooth muscle in situ. When determined by a specific radioligand binding assay, subcellular fractions of the cultured cells bound muscarinic cholinergic agonists and antagonists with affinities expected of the muscarinic receptor. The cells were also sensitive to the beta-adrenergic catecholamine agonist isoproterenol, which stimulated cAMP production but not Ca2+ uptake. Carbachol failed to inhibit isoproterenol-dependent cAMP production, which is an important property of the cholinergic receptor in uterine smooth muscle in situ. These results suggest some but not all acetylcholine-sensitive properties of uterine smooth muscle may be retained in cell culture.

Cyclic nucleotide levels in human breast cancer and in rat mammary tissues during tumor development

The levels of cyclic adenosine 3':5'-monophosphate (cAMP) and cyclic guanosine 3':5'-monophosphate (cGMP) were studied in dimethylbenz(a)anthracene (DMBA)-induced mammary tumors of Sprague-Dawley rats and in human breast cancer. In the rat carcinomas, these levels were significantly lower than in non-malignant tissues when calculated on the basis of DNA content, but higher (cAMP) or equal (cGMP) when calculated on the basis of weight. In human breast cancer the cyclic nucleotide levels were higher than in non-malignant tissues according to both methods of calculation. No correlation was found in human carcinomas between the cyclic nucleotide levels and mitotic index, nuclear grade, tumor size, or lymph node involvement. The rat tumors were subclassified according to state of differentiation, mitotic index, and state of development. Not all the sub-groups had cAMP levels different from normal values. Differences in cAMP levels between the sub-groups could not be correlated with tumor growth rates and/or mitotic index. Thus, cyclic nucleotides may not be useful in prognosis of breast cancer.

Mitogenic effect of prostaglandin E1 in Swiss 3T3 cells: role of cyclic AMP

Addition of prostaglandin E1 (PGE1) to quiescent cultures of Swiss 3T3 cells rapidly elevates the intracellular levels of cAMP and increases the activity of adenylate cyclase in particulate fractions of these cells. In the presence of insulin, PGE1 stimulates the reinitiation of DNA synthesis. Both effects (increase in cellular cAMP and stimulation of DNA synthesis) are markedly potentiated by 1-methyl-3-isobutyl xanthine (IBMX) or by 4-(3-butoxy-4 methoxy benzyl)-2-imidazolidine (Ro 20-1724), both of which are potent inhibitors of cyclic nucleotide phosphodiesterase activity. In the presence of 50 microM IBMX, PGE1 caused a dose-dependent increase in cAMP levels and in [3H]thymidine incorporation into acid-insoluble material at concentrations (5-50 ng/ml) that are orders of magnitude lower than those used in previous studies (50 micrograms/ml) to demonstrate growth-inhibitory effects. Thus, the inhibitory effects produced by adding high concentrations of PGE1 on the initiation of DNA synthesis in Swiss 3T3 cells are not mediated by cAMP and should be regarded as nonspecific. In contrast, the mitogenic activity of PGE1 parallels its ability to increase the intracellular levels of cAMP. The findings support the proposition that a sustained increase in the level of this cyclic nucleotide acts as a mitogenic signal for confluent and quiescent Swiss 3T3 cells.

Synergistic stimulation of DNA synthesis by cyclic AMP derivatives and growth factors in mouse 3T3 cells

Addition of the cAMP derivatives butcAMP or 8BrcAMP to quiescent cultures of Swiss 3T3 causes synergistic stimulation of DNAk synthesis with insulin, phorbol esters, vasopressin, epidermal growth factor, or fetal bovine serum (2-5%). In the presence of insulin, 8BrcAMP, and butcAMP stimulate [3H]-thymidine incorporation into acid-precipitable material in a dose-dependent manner. The effect of these agents is specific since 8Br5'AMP, 5'AMP, butyrate, or 8BrcGMP fail to stimulate DNA synthesis under identical experimental conditions. Furthermore, the mitogenic effects of the cAMP derivatives were markedly potentiated by 1-methyl-3-isobutyl xanthine and 4-(3-butoxy-4-methoxy benzyl)-2-imidazolidine, both of which are potent inhibitors of cyclic nucleotide phosphodiesterase activity. The growth-promoting effects of the cAMP derivatives were demonstrated by [3H]-thymidine incorporation (either by scintillation counting or by autoradiography), by flow cytofluorometric analysis, and by increase in cell number. When quiescent Swiss 3T3 cells were exposed to butcAMP and insulin, DNA synthesis began after a lag of 17h. The result of sequential additions of cAMP derivatives and insulin to quiescent 3T3 cells suggest that these agents must act simultaneously in G0/G1 to stimulate entry into DNA synthesis in these cells. The findings support the proposition that an increase in cellular levels of cAMP (but not cGMP) act sas a mitogenic stimulus for confluent and quiescent Swiss 3T3 cells.

Cyclic AMP stimulation of Na-K pump activity in quiescent swiss 3T3 cells

Recently, we have found that an increase in the intracellular level of cAMP acts as a mitogenic signal for Swiss 3T3 cells (Rozengurt et al., Proc. Natl. Acad, Sci. USA, 78:4392, 1981). The results presented in this paper demonstrate that addition of cAMP-elevating agents to confluent and quiescent cultures of Swiss 3T# causes a marked increase in the rate of 86Rb+ uptake but has no effect on the rate of cation efflux. The stimulation of ion uptake is mediated by the Na-K pump as shown by the ouabain sensitivity of the 86Rb+ fluxes. The increase in Na-K pump activity occurs whether cAMP is generated endogenously by stimulation of adenylate cyclase activity by cholera toxin, adenosine agonists, or PGE1 or added exogenously as 8BrcAMP. The stimulatory effect of these compounds on 86Rb+ uptake is potentiated by inhibitors of cyclic nucleotide phosphodiesterase activity. Cholera toxin stimulates the Na-K pump in a dose-dependent manner; half-maximal effect is achieved at 0.7 ng/ml. The stimulation of ouabain-sensitive 86Rb+ uptake by cAMP-elevating agents reaches a maximum after 2-3 h of incubation. This contrasts with the rapid (within minutes) stimulation of the Na-K pump caused by serum and other mitogenic agents. Further, cAMP-elevating agents fail to increase Na+ influx into 3T3 cells whereas serum causes a marked increase in Na+ influx, under identical experimental conditions. These findings suggest that the stimulation of Na-K pump activity caused by increased cAMP levels contrasts mechanistically with the rapid control of pump activity by serum which is primarily mediated by increased Na+ entry into the cells.

Cyclic adenosine-3',5'-monophosphate stimulates islet B cell replication in neonatal rat pancreatic monolayer cultures

A possible role for cyclic adenosine 3',5'-monophosphate (cAMP) in islet B cell replication was examined in neonatal rat pancreatic monolayer cultures. Islet cells deteriorated and insulin release decreased during 12 d of culture in medium with 5.6 mM glucose, whereas the cells survived and insulin release increased during culture in medium with 5.6 mM glucose plus the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 0.1 mM), or in medium with 16.7 mM glucose with or without IBMX. IBMX also increased the mitotic index and stimulated dose-dependent increases in [(3)H]thymidine incorporation in nuclei of islet B cells in aldehydethionine stained radioautographs; maximal stimulation of B cell replication occurred with addition of 0.1 mM IBMX to 5.6 mM glucose (+170%, P < 0.001), and this increase was similar to that observed with 16.7 mM glucose (+185%, P < 0.001). Also, 8-bromo-adenosine-3',5-monophosphate, but not 8-bromo-guanosine-3',5'-monophosphate produced dose-dependent increases in islet B cell replication in medium with 5.6 mM glucose. Measurement of cAMP levels in the cultures revealed dissociations between effects on B cell replication and insulin release. Thus, addition of 0.1 mM IBMX, or 0.1 nM cholera toxin, to 5.6 mM glucose produced slightly greater increases in cAMP levels and B cell replication than did 16.7 mM glucose, whereas insulin release was increased significantly more with 16.7 mM glucose. Also, addition of 0.1 mM IBMX, or 0.1 nM cholera toxin, to 16.7 mM glucose stimulated further increases in cAMP levels and insulin release in the cultures, but no further increases in B cell replication. We conclude that (a) cAMP stimulates islet B cell replication, (b) cAMP may mediate the effects of glucose on B cell replication, and (c) mechanisms regulating B cell replication may be more sensitive to cAMP and/or different from those regulating insulin secretion.

Effect of acetylcholine on the norepinephrine-induced positive chronotropy and increase in cyclic nucleotides of isolated rabbit sinoatrial node

Transmural stimulation of, or application of nicotine to, the isolated rabbit sinoatrial (SA) node resulted in initial negative and late positive chronotropy. Simultaneous application of acetylcholine and norepinephrine produced a similar biphasic chronotropic effect. These procedures produced an initial increase in cyclic guanosine 3':5'-monophosphate (cyclic GMP) and a delayed elevation in cyclic adenosine 3':5'-monophosphate (cyclic AMP). The initial and late effects on rate and nucleotide levels were inhibited by pretreatment with atropine and propranolol, respectively. Pretreatment with atropine shortened the time of maximum increase in cyclic AMP level and heart rate from 3 to 1 minute after the simultaneous application of acetylcholine and norepinephrine and enhanced the positive chronotropic effect. Physostigmine prolonged the duration of the increase in cyclic GMP and negative chronotropic effect after the simultaneous application. These results suggest that when acetylocholine and norepinephrine are present simultaneously in the SA node region, the former interacts predominantly with muscarinic receptors and stimulates the cyclic GMP system, which in effect delays the cyclic AMP elevation and reduces the positive chronotropic effect of norepinephrine. However, these effects of acetylcholine cannot be explained solely on the basis of changes in the cyclic GMP level, because sodium nitroprusside produced a marked elevation of the cyclic GMP levels without decreasing the heart rate and did not affect the norepinephrine-induced increase in pacemaker rate and cyclic AMP. Sodium nitroprusside may affect cyclic GMP pools other than those susceptible to acetylcholine. These cyclic GMP pools may not exert chronotropic effects.

Glucose activation of liver glycogen synthase. Insulin-mediated restoration of glucose effect in diabetic rats is blocked by protein synthesis inhibitor

The loss of glucose regulation of glycogen synthase in perfused livers from diabetic rats was associated with a substantial reduction in synthase phosphatase activity. Treatment of diabetic rats with insulin alone resulted in total restoration of the glucose effect and synthase phosphatase activity, while simultaneous treatment with cycloheximide severely reduced the hormonal effect. Although treatment of normal rats with cycloheximide had no effect on glucose activation of synthase, it did result in severe depletion of liver glycogen, increased liver glycogen phosphorylase activity, and elevation of liver adenosine 3',5'-monophosphate (cyclic AMP), but without elevation of liver protein kinase activity. Simultaneous treatment of alloxan-diabetic rats with insulin and cycloheximide resulted in reduction of total liver glycogen, increased phosphorylase activity, a reduction in the ability of insulin to lower hepatic cyclic AMP, and a further reduction of protein kinase activity. In summary, the effect of insulin treatment of diabetic rats to restore glucose regulation of hepatic glycogen synthase probably involves synthesis of new protein, and the data remain consistent with the hypothesis that the defect may be due to a diabetes-related deficiency in a specific synthase phosphatase and/or alteration of the synthase molecule itself.

Epinephrine-induced elevation of guanosine 3':5'-cyclic monophosphate in isolated fat cells of rat

The effects of epinephrine (as low as 0.1 muM) on guanosine 3':5'-cyclic monophosphate (cGMP) and adenosine 3':5'-cyclic monophosphate (cAMP) in isolated fat cells were examined. Epinephrine increased both cGMP and cAMP levels, with the elevation of cAMP preceding the rise of cGMP. Maximal elevation was obtained with 1 muM epinephrine for each nucleotide. The increase in content of cGMP and cAMP due to epinephrine was completely blocked by a beta-adrenergic antagonist (5 muM propranolol). Phentolamine (10-100 muM), an alpha-adrenergic antagonist, enhanced the response to epinephrine resulting in elevation of cAMP levels, whereas a high concentration (100 muM) of phentolamine suppressed the elevation of cGMP. The ability of epinephrine to increase cGMP and cAMP levels was markedly diminished by "feedback regulator" partially purified from the incubation mixtures of isolated fat cells exposed to epinephrine [Ho, R.J. & Sutherland, E. W. (1971) J. Biol. Chem. 246, 6822-6827], whereas an increase in cGMP, but not cAMP, levels was observed in isolated fat cells incubated with "feedback regulator" alone (without epinephrine). These observations suggest the possibility that the epinephrine-induced elevation of cGMP levels in isolated fat cells might be mediated by an increase in formation of intracellular "feedback regulator" due to an elevation of cAMP by epinephrine.

Phosphorylated proteins as physiological effectors

A variety of neurotransmitters, hormones, and other regulatory agents affect the phosphorylation of specific proteins in their target tissues. The types of stimuli that share this common effect on protein phosphorylation include numerous substances that do not act through cyclic AMP. These and other observations suggest that many different classes of regulatory substances achieve certain of their biological effects by altering the phosphorylation of specific proteins.

Cyclic nucleotide metabolism in solid tumor tissues

The examination of the regulation of the system of 3'-5' cyclic nucleotide monophosphates has only begun in cancer tissues. In human cancers, these studies are notably non-existent. However, in animal cancers, especially the Morris hepatomas, enough data has been gathered that, while risky, certain trends seem to begin to appear. Cyclic AMP is constant or lowered, while cyclic GMP is elevated in the fast growing hepatomas. Regulation of adenylate cyclase by protein hormones is reduced, while regulation by epinephrine may be increased. Binding of glucagon is decreased in the fast growing hepatomas. Guanylate cyclase, while being predominantly cytoplasmic in the normal liver, is predominantly membrane bound in the tumors. The liver enzyme is also readily stimulated by several chemical carcinogens. The cyclic GMP phosphodiesterases are decreased in these tumors; while the cAMP phosphodiesterases are increased. Although the cyclic nucleotide dependent protein kinases (histone as substrate) are altered in the hepatomas, observations of unique cyclic nucleotide binding proteins or cAMP independent protein kinases in cancer tissues may be of even greater significance for the development of or the maintenance of the neoplastic state of cells.

Catecholamine-stimulated cyclic GMP accumulation in the rat pineal: apparent presynaptic site of action

Guanosine 3':5'-cyclic monophosphate (cGMP) increased 7-fold in rat pineal glands incubated in the presence of l-norepinephrine. This response consisted of two components-one was stereospecific and inhibited by alpha-adrenergic antagonists while the other was not stereospecific and not readily inhibited by antagonists. Although l-isoproterenol was more potent than l-norepinephrine it had less intrinsic activity and its action was not stereospecifc. The increase in cGMP caused by these catecholamines, unlike that of adenosine 3':5'-cyclic monophosphate (cAMP), was dependent upon extracellular calcium. Ouabain and high levels of potassium produced a marked, calcium-dependent increase in pineal cGMP, without affecting cAMP. There was no effect of cholinergic agonists on cGMP. Surgical denervation markedly reduced the cGMP response to stimulation by l-norepinephrine, potassium, or ouabain. This was in contrast to the enhanced response of cAMP in denervated glands. The nonspecific increase in cGMP caused by l-isoproterenol, however, was not affected by denervation. These data demonstrate the existence of a calcium-dependent presynaptic mechanism for the generation of cGMP which may be mediated by an alpha-adrenergic-like receptor. In addition, the mechanisms regulating pineal cGMP appear to be physiologically distinct from those regulating cAMP.

Effects of concanavalin A and a succinylated derivative on lymphocyte proliferation and cyclic nucleotide levels

To better define cell surface-related changes involved in lymphocyte activation, we studied native concanavalin A (Con A) and succinylated concanavalin A (Suc-Con A) for their effects on proliferation and cyclic nucleotide levels of human peripheral blood lymphocytes. At optimal mitogenic concentrations, the two forms of Con A induce equivalent proliferation; however, the mitogenic activity of Con A progressively decreases above 50 mug/ml. In contrast, the mitogenic activity of Suc-Con A is not decreased even at 250 mug/ml.

Possible role for cyclic nucleotides and phosphorylated membrane proteins in postsynaptic actions of neurotransmitters

The postsynaptic actions of some neurotransmitters may be mediated through cyclic nucleotides and cyclic nucleotide-dependent phosphorylation of specific membrane proteins in postsynaptic cells. In addition to providing a molecular basis for the actions of several neurotransmitters and of certain drugs affecting behaviour, the model suggests a mechanism by which neurotransmitter signals may be converted into electrophysiological responses in postsynaptic cells.

Isoprinosine augmentation of phytohemagglutinin-induced lymphocyte proliferation

The need for agents designed to modify immune response in the treatment of patients with viral infection, immunodeficiency, or cancer prompted the present study on the mechanisms of action of isoprinosine, a compound developed for antiviral use and whose therapeutic activity may involve the immune system. The effect of isoprinosine on in vitro proliferation of human peripheral blood lymphocytes stimulated by phytohemagglutinin (PHA) and on lymphocyte levels of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate was analyzed. Over a concentration range from 0.2 to 250 mug/ml, isoprinosine augmented PHA-induced proliferation; maximal stimulation was observed between 25 to 50 mug/ml. Isoprinosine in the absence of PHA had no effect on proliferation. The relative lack of effect of isoprinosine during a 90-min exposure and the lack of effect on lymphocyte cyclic nucleotide levels indicate that isoprinosine potentiates the PHA response by a mechanism different than a number of hormonal agents and such immunopotentiators as levamisole, polyadenylic-acid, and endotoxin. Further evaluation of isoprinosine as an immunopotentiator is indicated.

Changes in cyclic nucleotide metabolism in aorta and heart of neurogenically hypertensive rats: possible trigger mechanism of hypertension

Changes in cyclic nucleotide metabolism similar to those characteristic of the chronic forms of hypertension were observed in an acute neurogenic form of hypertension in rats produced by electrolytic lesions of the nucleus tractus solitarii. These changes that were evident 2 hr after the lesions were made included decreased cyclic AMP levels in the heart, increased cGMP:cAMP ratio, cAMP phosphodiesterase (3':5'-cAMP 5'-nucleotidohydrolase, EC 3.1.4.17) and guanylyl cyclase (GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2) activities in the aorta and decreased snesitivity of adenylyl cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) in both the aorta and heart to stimulation by the beta-adrenergic stimulant isoproterenol. These changes appear to depend on catecholamine release and are not due to mechanical distortion secondary to the increased arterial pressure. These studies provide biochemical support to the concept that the sympathetic nervous system may play a critical role in the initiation of the hypertensive syndrome and that chronic hypertension could result from the fixation of the biochemical effects of increased sympathetic activity.