Solubilization of guanylyl cyclase from bovine rod outer segments and effects of lowering Ca2+ and nitro compounds

Multilimbed membrane guanylate cyclase signaling system, evolutionary ladder

One monumental discovery in the field of cell biology is the establishment of the membrane guanylate cyclase signal transduction system. Decoding its fundamental, molecular, biochemical, and genetic features revolutionized the processes of developing therapies for diseases of endocrinology, cardio-vasculature, and sensory neurons; lastly, it has started to leave its imprints with the atmospheric carbon dioxide. The membrane guanylate cyclase does so via its multi-limbed structure. The inter-netted limbs throughout the central, sympathetic, and parasympathetic systems perform these functions. They generate their common second messenger, cyclic GMP to affect the physiology. This review describes an historical account of their sequential evolutionary development, their structural components and their mechanisms of interaction. The foundational principles were laid down by the discovery of its first limb, the ACTH modulated signaling pathway (the companion monograph). It challenged two general existing dogmas at the time. First, there was the question of the existence of a membrane guanylate cyclase independent from a soluble form that was heme-regulated. Second, the sole known cyclic AMP three-component-transduction system was modulated by GTP-binding proteins, so there was the question of whether a one-component transduction system could exclusively modulate cyclic GMP in response to the polypeptide hormone, ACTH. The present review moves past the first question and narrates the evolution and complexity of the cyclic GMP signaling pathway. Besides ACTH, there are at least five additional limbs. Each embodies a unique modular design to perform a specific physiological function; exemplified by ATP binding and phosphorylation, Ca²⁺-sensor proteins that either increase or decrease cyclic GMP synthesis, co-expression of antithetical Ca²⁺ sensors, GCAP1 and S100B, and modulation by atmospheric carbon dioxide and temperature. The complexity provided by these various manners of operation enables membrane guanylate cyclase to conduct diverse functions, exemplified by the control over cardiovasculature, sensory neurons and, endocrine systems.

Integrative Signaling Networks of Membrane Guanylate Cyclases: Biochemistry and Physiology

This monograph presents a historical perspective of cornerstone developments on the biochemistry and physiology of mammalian membrane guanylate cyclases (MGCs), highlighting contributions made by the authors and their collaborators. Upon resolution of early contentious studies, cyclic GMP emerged alongside cyclic AMP, as an important intracellular second messenger for hormonal signaling. However, the two signaling pathways differ in significant ways. In the cyclic AMP pathway, hormone binding to a G protein coupled receptor leads to stimulation or inhibition of an adenylate cyclase, whereas the cyclic GMP pathway dispenses with intermediaries; hormone binds to an MGC to affect its activity. Although the cyclic GMP pathway is direct, it is by no means simple. The modular design of the molecule incorporates regulation by ATP binding and phosphorylation. MGCs can form complexes with Ca²⁺-sensing subunits that either increase or decrease cyclic GMP synthesis, depending on subunit identity. In some systems, co-expression of two Ca²⁺ sensors, GCAP1 and S100B with ROS-GC1 confers bimodal signaling marked by increases in cyclic GMP synthesis when intracellular Ca²⁺ concentration rises or falls. Some MGCs monitor or are modulated by carbon dioxide via its conversion to bicarbonate. One MGC even functions as a thermosensor as well as a chemosensor; activity reaches a maximum with a mild drop in temperature. The complexity afforded by these multiple limbs of operation enables MGC networks to perform transductions traditionally reserved for G protein coupled receptors and Transient Receptor Potential (TRP) ion channels and to serve a diverse array of functions, including control over cardiac vasculature, smooth muscle relaxation, blood pressure regulation, cellular growth, sensory transductions, neural plasticity and memory.

What is next in nitric oxide research? From cardiovascular system to cancer biology

The broad role of nitric oxide (NO) and cyclic GMP in biochemistry and biology as important messenger molecules is evident from the numerous publications in this research field. NO and cGMP have been known as components of the key signaling pathway in regulating numerous processes such as vascular dilation, blood pressure, neurotransmission, cardiovascular function, and renal function. In spite of almost 150,000 publications with nitric oxide and cyclic GMP, there are few publications regarding the effects of these messenger molecules on gene regulation, cell differentiation and cell proliferation. Our research data with embryonic stem cells and several cancer cell lines suggest that nitric oxide, its receptor soluble guanylyl cyclase (sGC) and sGC's product cyclic GMP can regulate the processes of proliferation and differentiation. Furthermore, we have found that undifferentiated stem cells and some malignant tumors such as human glioma have decreased levels of sGC and translocation of the sGCβ1 subunit to the nucleus. We propose that sGC and cyclic GMP function as tumor suppressors. An understanding of the mechanisms of the translocation of the sGCβ1 subunit into the nucleus and the possible regulation of gene expression of NO and/or cyclic CMP could lead to novel and innovative approaches to cancer therapy and stem cell proliferation and differentiation.

Membrane guanylate cyclase, a multimodal transduction machine: history, present, and future directions

A sequel to these authors' earlier comprehensive reviews which covered the field of mammalian membrane guanylate cyclase (MGC) from its origin to the year 2010, this article contains 13 sections. The first is historical and covers MGC from the year 1963–1987, summarizing its colorful developmental stages from its passionate pursuit to its consolidation. The second deals with the establishment of its biochemical identity. MGC becomes the transducer of a hormonal signal and founder of the peptide hormone receptor family, and creates the notion that hormone signal transduction is its sole physiological function. The third defines its expansion. The discovery of ROS-GC subfamily is made and it links ROS-GC with the physiology of phototransduction. Sections ROS-GC, a Ca²⁺-Modulated Two Component Transduction System to Migration Patterns and Translations of the GCAP Signals Into Production of Cyclic GMP are Different cover its biochemistry and physiology. The noteworthy events are that augmented by GCAPs, ROS-GC proves to be a transducer of the free Ca²⁺ signals generated within neurons; ROS-GC becomes a two-component transduction system and establishes itself as a source of cyclic GMP, the second messenger of phototransduction. Section ROS-GC1 Gene Linked Retinal Dystrophies demonstrates how this knowledge begins to be translated into the diagnosis and providing the molecular definition of retinal dystrophies. Section Controlled By Low and High Levels of [Ca²⁺]_i, ROS-GC1 is a Bimodal Transduction Switch discusses a striking property of ROS-GC where it becomes a “[Ca²⁺]_i bimodal switch” and transcends its signaling role in other neural processes. In this course, discovery of the first CD-GCAP (Ca²⁺-dependent guanylate cyclase activator), the S100B protein, is made. It extends the role of the ROS-GC transduction system beyond the phototransduction to the signaling processes in the synapse region between photoreceptor and cone ON-bipolar cells; in section Ca²⁺-Modulated Neurocalcin δ ROS-GC1 Transduction System Exists in the Inner Plexiform Layer (IPL) of the Retinal Neurons, discovery of another CD-GCAP, NCδ, is made and its linkage with signaling of the inner plexiform layer neurons is established. Section ROS-GC Linkage With Other Than Vision-Linked Neurons discusses linkage of the ROS-GC transduction system with other sensory transduction processes: Pineal gland, Olfaction and Gustation. In the next, section Evolution of a General Ca²⁺-Interlocked ROS-GC Signal Transduction Concept in Sensory and Sensory-Linked Neurons, a theoretical concept is proposed where “Ca²⁺-interlocked ROS-GC signal transduction” machinery becomes a common signaling component of the sensory and sensory-linked neurons. Closure to the review is brought by the conclusion and future directions.

ROS-GC interlocked Ca(2+)-sensor S100B protein signaling in cone photoreceptors: review

Photoreceptor rod outer segment membrane guanylate cyclase (ROS-GC) is central to visual transduction; it generates cyclic GMP, the second messenger of the photon signal. Photoexcited rhodopsin initiates a biochemical cascade that leads to a drop in the intracellular level of cyclic GMP and closure of cyclic nucleotide gated ion channels. Recovery of the photoresponse requires resynthesis of cyclic GMP, typically by a pair of ROS-GCs, 1 and 2. In rods, ROS-GCs exist as complexes with guanylate cyclase activating proteins (GCAPs), which are Ca(2+)-sensing elements. There is a light-induced fall in intracellular Ca(2+). As Ca(2+) dissociates from GCAPs in the 20-200 nM range, ROS-GC activity rises to quicken the photoresponse recovery. GCAPs then progressively turn down ROS-GC activity as Ca(2+) and cyclic GMP levels return to baseline. To date, GCAPs mediate the only known mechanism of ROS-GC regulation in the photoreceptors. However, in mammalian cone outer segments, cone synapses and ON bipolar cells, another Ca(2+) sensor protein, S100B, complexes with ROS-GC1 and senses the Ca(2+) signal with a K1/2 of 400 nM. Unlike GCAPs, S100B stimulates ROS-GC activity when Ca(2+) is bound. Thus, the ROS-GC system in cones functions as a Ca(2+) bimodal switch; with rising intracellular Ca(2+), its activity is first turned down by GCAPs and then turned up by S100B. This presentation provides a historical perspective on the role of S100B in the photoreceptors, offers a pictorial model for the "bimodal" operation of the ROS-GC switch and projects future tasks that are needed to understand its operation. Some accounts of this review have been adopted from the original publications of these authors.

Membrane guanylate cyclase is a beautiful signal transduction machine: overview

This article is a sequel to the four earlier comprehensive reviews which covered the field of membrane guanylate cyclase from its origin to the year 2002 (Sharma in Mol Cell Biochem 230:3-30, 2002) and then to the year 2004 (Duda et al. in Peptides 26:969-984, 2005); and of the Ca(2+)-modulated membrane guanylate cyclase to the year 1997 (Pugh et al. in Biosci Rep 17:429-473, 1997) and then to 2004 (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). This article contains three parts. The first part is "Historical"; it is brief, general, and freely borrowed from the earlier reviews, covering the field from its origin to the year 2004 (Sharma in Mol Cell Biochem, 230:3-30, 2002; Duda et al. in Peptides 26:969-984, 2005). The second part focuses on the "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily". It is divided into two sections. Section "Historical" and covers the area from its inception to the year 2004. It is also freely borrowed from an earlier review (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). Section "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily" covers the area from the year 2004 to May 2009. The objective is to focus on the chronological development, recognize major contributions of the original investigators, correct misplaced facts, and project on the future trend of the field of mammalian membrane guanylate cyclase. The third portion covers the present status and concludes with future directions in the field.

Sodium nitroprusside alters dark voltage and light responses in isolated retinal rods during whole-cell recording

Dark voltage and light responses of isolated retinal rods of Rana esculenta were investigated by employing the whole-cell patch-clamp technique. When the recording pipette was filled with a medium devoid of nucleotides, a spontaneous hyperpolarization of the dark voltage partly due to a diffusional loss of cGMP and its precursor GTP and a retardation in the recovery of the light responses was observed. The larger part of the retardation of the light responses was prevented by 1 mM ATP. Addition of GTP attenuated the hyperpolarization, but did not abolish it completely. When the nitric-oxide-releasing substance sodium nitroprusside plus GTP was applied, the tendency of hyperpolarization disappeared and a stable dark voltage or even a slight depolarization was measured during the whole-cell recording period. Similar results were also obtained when GTP was given in combination with either EGTA or IBMX which are both known to interfere with the cGMP regulating enzymes in retinal rods. In addition to its effects on the dark voltage, an acceleration of the recovery phase of the light responses by sodium nitroprusside was also observed. Our observations strongly suggest that sodium nitroprusside activates guanylate cyclase in photoreceptors, as it does in other tissues, but we cannot exclude with certainty an effect on the phosphodiesterase.

SOUL in mouse eyes is a new hexameric heme-binding protein with characteristic optical absorption, resonance Raman spectral, and heme-binding properties

SOUL is specifically expressed in the retina and pineal gland and displays more than 40% sequence homology with p22HBP, a heme protein ubiquitously expressed in numerous tissues. SOUL was purified as a dimer in the absence of heme from the Escherichia coli expression system but displayed a hexameric structure upon heme binding. Heme-bound SOUL displayed optical absorption and resonance Raman spectra typical of 6-coordinate low-spin heme protein, with one heme per monomeric unit for both the Fe(III) and Fe(II) complexes. Spectral data additionally suggest that one of the axial ligands of the Fe(III) heme complex is His. Mutation of His42 (the only His of SOUL) to Ala resulted in loss of heme binding, confirming that this residue is an axial ligand of SOUL. The K(d) value of heme for SOUL was estimated as 4.8 x 10(-9) M from the association and dissociation rate constants, suggesting high binding affinity. On the other hand, p22HBP was obtained as a monomer containing one heme per subunit, with a K(d) value of 2.1 x 10(-11) M. Spectra of heme-bound p22HBP were different from those of SOUL but similar to those of heme-bound bovine serum albumin in which heme bound to a hydrophobic cavity with no specific axial ligand coordination. Therefore, the heme-binding properties and coordination structure of SOUL are distinct from those of p22HBP, despite high sequence homology. The physiological role of the new heme-binding protein, SOUL, is further discussed in this report.

NADPH diaphorase activity in the rabbit retina is modulated by glutamatergic pathways

NADPH diaphorase activity in the rabbit retina is modulated by the state of visual adaptation. In this study, we tested possible glutamatergic control of this phenomenon. Rabbits were injected intravitreally with agonists and antagonists of glutamate. After adaptation (3 hours) to either room light or darkness, the rabbits were killed and the retinae were prepared for NADPH diaphorase histochemistry. Kainic acid significantly reduced the number of NADPH diaphorase amacrine cells but augmented NADPH diaphorase activity in horizontal cells in both light- and dark-adapted animals. 6,7-Dinitroquinoxaline-2,3(1H,4H)-dione exerted no effect on amacrine cells but eliminated NADPH diaphorase activity in horizontal cells. 2-Amino-4-phosphono butyric acid did not affect NADPH diaphorase activity in horizontal cells but reduced the degree of staining in the neuronal processes of amacrine cells. MK-801 and N-methyl-D-aspartic acid (NMDA) had no effect on NADPH diaphorase activity in horizontal cells. However, MK-801 reduced staining in the neuronal processes of amacrine cells but not in their cell bodies. NMDA effects were expressed in a significant reduction in the number and size of amacrine cells that were NADPH diaphorase positive. These results indicate that activation of NADPH diaphorase in horizontal cells by darkness is mediated by the activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid (KA)-type glutamate receptors. The ON pathway in the retina is probably involved in modulation of NADPH diaphorase in the neuronal processes of amacrine cells. Amacrine cells that are NADPH diaphorase positive contain NMDA-type and AMPA/KA-type receptors and are highly susceptible to NMDA and kainic acid toxicity.

Steric effect and effect of metal coordination on the reactivity of nitric oxide with cysteine-containing proteins under anaerobic conditions

A comparison is made between the reactivity of nitric oxide (NO) with cysteine, bovine serum albumin (BSA) and metallothionein-1 (MT1) at pH 7 under strictly anaerobic conditions. The rate of reaction of NO with these amino acid/proteins was found to be of the order: cysteine > BSA >> MT1, in clear disparity with the size of the reactants. The difference in the reaction rates is attributed to steric effects due to the high molecular size in the case of BSA and to effects of metal coordination proper as well as to steric effects associated with the closed dual shell-like structure resulting from the tight coordination of the thiolate groups with Zn2+ in MT1. The mechanisms of the reaction of NO with cysteine, BSA and MT and its possible implication for the rate of the respective reactions are discussed.

Nitric oxide-mediated cGMP synthesis in Helix neural ganglia

The central nervous system of the mollusc Helix pomatia was stimulated with NO donors sodium nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP) or hydroxylamine, in the presence of a phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (IBMX). Radioimmunoassay revealed that all of the three NO donors significantly increased cGMP levels by 22-27-fold above basal levels. Compared with controls, strong cGMP immunoreactivity was observed in axons and cytoplasm of the stimulated neurons. About 80% of cGMP-immunoreactive neurons colocalized with NADPH-diaphorase activity. Some glial cells and giant neurons were not stained by NADPH-diaphorase histochemistry but were cGMP-immunoreactive. The results suggest the existence of a NO/cGMP pathway and indicate NO as an intra- and intercellular signaling molecule in the Helix central nervous system.

Nitric oxide: a review of its role in retinal function and disease

Nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide from L-arginine, exists in three major isoforms, neuronal, endothelial, and immunologic. Neuronal and endothelial isoforms are constitutively expressed, and require calcium for activation. Both of these isoforms can be induced (i.e., new protein synthesis occurs) under appropriate conditions. The immunologic isoform is not constitutively expressed, and requires induction usually by immunologic activation; calcium is not necessary for its activation. Neuronal and immunologic NOS have been detected in the retina. Neuronal NOS may be responsible for producing nitric oxide in photoreceptors and bipolar cells. Nitric oxide stimulates guanylate cyclase of photoreceptor rod cells and increases calcium channel currents. In the retina of cats, NOS inhibition impairs phototransduction as assessed by the electroretinogram. Inducible nitric oxide synthase, found in Müller cells and in retinal pigment epithelium, may be involved in normal phagocytosis of the retinal outer segment, in infectious and ischemic processes, and in the pathogenesis of diabetic retinopathy. Nitric oxide contributes to basal tone in the retinal circulation. To date, findings are conflicting with respect to its role in retinal autoregulation. During glucose and oxygen deprivation, nitric oxide may increase blood flow and prevent platelet aggregation, but it may also mediate the toxic effects of excitatory amino acid release. This reactive, short-lived gas is involved in diverse processes within the retina, and its significance continues to be actively studied.

Distribution of guanylate cyclase within photoreceptor outer segments

Guanylate cyclases play an essential role in the recovery of vertebrate photoreceptor cells after light activation. Here, we have investigated how one such guanylate cyclase, RetGC-1, is distributed within light- and dark-adapted rod photoreceptor cells. Guanylate cyclase activity partitioned with the photoreceptor outer segment (OS) cytoskeleton in a light-sensitive manner. RetGC-1 was found to bind actin filaments in actin blot overlays, suggesting a mechanism for its association with the OS cytoskeleton. In retinal sections, this enzyme was immunodetected only in the OSs, where it appeared to be distributed throughout the disk membranes.

Nitric oxide synthase in tiger salamander retina

Previous studies have indicated that nitric oxide, a labile freely diffusible biological messenger synthesized by nitric oxide synthase, may modulate light transduction and signal transmission in the retina. In the present work, the large size of retinal cells in tiger salamander (Ambystoma tigrinum) allowed the utilization of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and nitric oxide synthase immunocytochemistry to delineate the cell-specific intracellular localization of nitric oxide synthase. NADPH-diaphorase activity was highly concentrated in the outer retina, in rod and cone inner segment ellipsoids, and between and adjacent to the photoreceptor cell bodies in the outer nuclear layer. Examination of enzymatically isolated retinal cells indicated that outer nuclear layer NADPH-diaphorase activity was localized to the distal processes of the retinal glial (Müller) cells and to putative bipolar cell Landolt clubs. Less intense NADPH-diaphorase activity was seen in the photoreceptor inner segment myoid region, in a small number of inner nuclear layer cells, in cap-like configurations at the distal poles of cells in the ganglion cell layer and surrounding ganglion cell layer somata, and in punctate form within both plexiform layers, the pigment epithelium, and the optic nerve. Nitric oxide synthase-like immunoreactivity was similarly localized, but was also concentrated along a thin sublamina centered within the inner plexiform layer. The potential for nitric oxide generation at multiple retinal sites suggests that this molecule may play a number of roles in the processing of visual information in the retina.

Regulation of bovine rod outer segment membrane guanylate cyclase by ATP, phosphodiesterase and metal ions

In vertebrate retina, rod outer segment is the site of visual transduction. The inward cationic current in the dark-adapted outer segment is regulated by cyclic GMP. A light flash on the outer segment activates a cyclic GMP phosphodiesterase resulting in rapid hydrolysis of the cyclic nucleotide which in turn causes a decrease in the dark current. Restoration of the dark current requires inactivation of the phosphodiesterase and synthesis of cyclic GMP. The latter is accomplished by the enzyme guanylate cyclase which catalyzes the formation of cyclic GMP from GTP. Therefore, factors regulating the cyclase activity play a critical role in visual transduction. But regulation of the cyclase by some of these factors--phosphodiesterase, ATP, the soluble proteins and metal cofactors (Mg and Mn)--is controversial. The availability of different types of cyclase preparations, dark-adapted rod outer segments with fully inhibited phosphodiesterase activity, partially purified cyclase without PDE contamination, cloned rod outer segment cyclase free of other rod outer segment proteins, permitted us to address these controversial issues. The results show that ATP inhibits the basal cyclase activity but enhances the stimulation of the enzyme by soluble activator, that cyclase can be activated in the dark at low calcium concentrations under conditions where phosphodiesterase activity is fully suppressed, and that greater activity is observed with manganese as cofactor than magnesium. These results provide a better understanding of the controls on cyclase activity in rod outer segments and suggest how regulation of this cyclase by ATP differs from that of other known membrane guanylate cyclases.

Inhibition of nitric oxide synthase alters light responses and dark voltage of amphibian photoreceptors

We studied the effects of competitive inhibitors of nitric oxide synthase (L-NMMA and L-NNA) on dark voltage and flash responses of retinal rods of the frog. Substances were applied intracellularly via whole-cell patch-clamp electrodes while the membrane voltage was recorded simultaneously. During recording the exchange of substances by diffusion between cytosol and pipette medium affects the cell's function. Under control conditions this exchange is reflected by a slow hyperpolarization of the dark voltage with time and a prolongated flash response recovery, which is mainly due to a loss of nucleotides. Application of L-NMMA and L-NNA accelerated the spontaneous hyperpolarization of the membrane voltage during the course of an experiment, while the recovery of the flash responses was slowed down. The effects observed upon intracellular application of NO-synthase inhibitors were opposite to those observed previously upon application of sodium nitroprusside. Sodium nitroprusside was much less effective when the intracellular calcium level was decreased by application of EGTA at the same time. It is reasonable to assume that the observed effects are linked to nitric oxide synthase and to a NO-dependent soluble guanylate cyclase. The results suggest that the activity of NO-synthase in photoreceptor cells has an influence on concentration and metabolic flux of cGMP in photoreceptors, which may be of relevance for flash response recovery and adaptation processes. It is likely that the regulation of the soluble guanylate cyclase requires a physiological level of calcium.

Bovine retinal rod guanyl cyclase represents a new N-glycosylated subtype of membrane-bound guanyl cyclases

The molecular properties of retinal rod guanyl cyclase were investigated. Peptides were derived from a 112-kDa protein previously identified as the particulate bovine retinal rod guanyl cyclase. The peptides showed 100% identity to the deduced amino acid sequence of the cloned human retina-specific membrane guanyl cyclase, whereas identity to the members of the natriuretic peptide receptor guanyl cyclases was 14-59%. The 112-kDa protein was further purified by a new approach using wheat-germ agglutinin chromatography. This indicated N-linked glycosylation in retinal rod guanyl cyclase. N-glycosylation was unexpected from the sequence of the human retina-specific membrane guanyl cyclase, although it is a common property of natriuretic peptide receptors. Therefore, we further analyzed the carbohydrate composition of bovine retinal rod guanyl cyclase by lectin binding using the lectins Galanthus nivalis agglutinin, Sambucus nigra agglutinin, Maackia amurensis agglutinin, Ricinus communis agglutinin, Datura stramonium agglutinin, peanut agglutinin and by chromatography of the purified enzyme using concanavalin-A-Sepharose. The oligosaccharide side chains were of the high-mannose type or hybrid type, probably with mannose, N-acetylglucosamine and sialic acid as terminal sugars. Enzymic deglycosylation by N-glycosidase F was achieved after proteolytic digestion with endoproteinase Glu-C. Lectins neither influenced the basal nor the stimulated guanyl-cyclase activity at low calcium concentrations. Our results indicate that the particulate rod guanyl cyclase represents an unusual new subtype of membrane-bound guanyl cyclases.

Cross talk among cyclic AMP, cyclic GMP, and Ca(2+)-dependent intracellular signalling mechanisms in brain capillary endothelial cells

C-type natriuretic peptide and sodium nitroprusside, a nitric oxide donor molecule, induced large increases in cyclic GMP formation in cultured rat brain capillary endothelial cells. Isoproterenol, a potent agonist of adenylate cyclase, potentiated the actions of C-type natriuretic peptide and of sodium nitroprusside. These actions were not observed in the presence of isobutylmethylxanthine and were mimicked by forskolin. Endothelin-1 had no action on basal cyclic GMP levels. It reduced cyclic GMP formation induced by C-type natriuretic peptide and sodium nitroprusside by about 50%. These actions involved an ETA receptor subtype and a Ca(2+)-dependent and protein kinase C-independent mechanism. Finally, increasing cyclic GMP slightly prolonged intracellular Ca2+ transients induced by endothelin-1. The results suggest the presence of extensive cross talk among cyclic AMP, cyclic GMP, and Ca(2+)-dependent mechanisms in endothelial cells of brain microvessels. The relevance of the results to the regulation of the blood-brain barrier permeability is discussed.

Regulation of cytosolic guanylyl cyclase by nitric oxide: the NO-cyclic GMP signal transduction system

The understanding of guanylyl cyclase regulation by nitrovasodilators has provided a great deal of information explaining the mechanisms of action of these cardiovascular drugs that have been in clinical use for the past century. The biochemical characterization of guanylyl cyclases and their regulation by NO have also permitted us, and others, to understand the mechanism of action of endothelium-dependent vasodilators and, subsequently, the roles for the nitric oxide-cyclic GMP signal transduction system in numerous cells and tissues. The potential importance of this signal transduction cascade is probably not fully appreciated since numerous additional studies obviously need to be performed. Also, as in many areas of science, serendipitous experiments and observations have added critical data to our present understanding in this field.

Inducible nitric oxide synthase in the central nervous system

There is an accumulation of evidence indicating that induction of the calcium-independent isoform of nitric oxide synthase (iNOS) in glial cells can contribute to nitric oxide-mediated neural-cell damage. Elucidation of iNOS inducing signals and mechanisms regulating its augmentation and suppression may have implications for our understanding of basic processes underlying some forms of central nervous system disease.

NADPH-diaphorase (nitric oxide synthase)-reactive amacrine cells of rabbit retina: putative target cells and stimulation by light

In the mammalian retina there are two populations of nitric oxide synthase-containing amacrine cells that stain with the nicotinamide adenine dinucleotide phosphate-diaphorase reaction. To determine the response of these neurons to light, immunoreactivity to Fos proteins was used as a marker of synaptic activation. Fos immunoreactivity is absent in dark-adapted retinas, but 70% of large, Type I nicotinamide adenine dinucleotide phosphate-diaphorase-reactive amacrine cells and 5-10% of the smaller but more numerous Type II nicotinamide adenine dinucleotide phosphate-diaphorase-reactive amacrine cells contain Fos proteins after light stimulation. To localize putative cellular targets of nitric oxide in the retina, retinas were stained immunocytochemically for cyclic GMP after the local administration of the nitric oxide donors sodium nitroprusside and S-nitroso-N-acetylpenicillamine. Both compounds induce strong cyclic GMP immunoreactivity in ON cone bipolar cells. The data suggest that the light-induced inward current in ON cone bipolar cells is enhanced by a nitric oxide-cyclic GMP pathway and that the major source of nitric oxide is the nicotinamide adenine dinucleotide phosphate-diaphorase-reactive amacrine cells in the rabbit retina.

A 56 kDa binding protein for Escherichia coli heat-stable enterotoxin isolated from the cytoskeleton of rat intestinal membrane does not possess guanylate cyclase activity

Proteins binding Escherichia coli heat-stable enterotoxin were isolated from the cytoskeleton of intestinal membranes using an affinity matrix of biotinylated ST immobilized on monomeric avidin-agarose. ST binding proteins were purified 343-fold using this affinity technique, with 7% of the initial binding activity recovered in these preparations. ST binding proteins isolated by affinity chromatography possessed a native and subunit molecular mass of 56 kDa. These preparations exhibited both high- and low-affinity binding sites for ST. Guanylate cyclase in extracts of the intestinal membrane cytoskeleton was completely recovered in fractions which did not associate with the affinity matrix. In addition, ST binding proteins isolated by affinity chromatography were devoid of guanylate cyclase activity. These data, taken together with those obtained previously with crude and partially purified receptors, suggest that ST binds to different proteins in intestinal membranes, some of which do not possess guanylate cyclase activity.

Solubilization and characterization of functionally coupled Escherichia coli heat-stable toxin receptors and particulate guanylate cyclase associated with the cytoskeleton compartment of intestinal membranes

1. Particulate guanylate cyclase and receptors for E. coli heat-stable enterotoxin were solubilized from the rat intestinal cytoskeletal compartment using Lubrol-PX and KCl. 2. Thirty to forty percent of the ST receptor and guanylate cyclase activities were extracted from the lipid layer with Lubrol-PX alone. 2. Seventy percent of the remaining activities were solubilized from the cytoskeleton with Lubrol-PX and KCl. 3. Guanylate cyclase solubilized from either compartment exhibited similar reaction kinetics. 4. Both high- and low-affinity classes of ST receptors were solubilized from the lipid and cytoskeleton compartments. 5. In the presence of ATP gamma S, ST selectively activated the guanylate cyclase solubilized from the cytoskeleton compared to that solubilized from the lipid bilayer. 6. Crosslinking experiments demonstrated a preferential solubilization of the 130 kDa receptor subunit from the cytoskeleton and the 56 kDa subunit from the lipid bilayer. 7. Development of a procedure to solubilize ST receptors and guanylate cyclase from the intestinal membrane cytoskeleton will permit purification and further detailed studies of the coupling of these activities.

L-arginine and nicotinamide adenine dinucleotide phosphate alter dark voltage and accelerate light response recovery in isolated retinal rods of the frog (Rana temporaria)

Effects of intracellularly applied L-arginine and nicotinamide adenine dinucleotide phosphate (NADPH) on the dark voltage and light responses of retinal rods were studied by means of the whole-cell patch-clamp technique. In this mode an exchange of substances by diffusion between cytosol and pipette medium occurs (Pflügers Arch., 411 (1988) 204-211). In retinal rods a loss of nucleotides is reflected by a hyperpolarization of the dark voltage and by a prolongation of the light responses (Vis. Neurosci., 2 (1989) 101-108). Intracellular application of L-arginine prevented the prolongation of the light responses and NADPH accelerated the light response recovery and in addition depolarized the photoreceptor cells. The effects were similar to those observed before upon application of the nitric oxide (NO)-releasing substance sodium nitroprusside (Vis. Neurosci., 9 (1992) 205-209). It is therefore assumed that the observed effects are linked to the NO-synthase and to an activation of a guanylate cyclase by NO. It is concluded that the level of NADPH in photoreceptor cells may affect the metabolic flux of guanosine 3',5'-cyclic monophosphate (cGMP).

Mono ADP-ribosylation of transducin catalyzed by rod outer segment extract

Transducin is the retinal rod outer segment (ROS)-specific G protein coupling the photoexcited rhodopsin to cyclic GMP-phosphodiesterase. The alpha subunit of transducin is known to be ADP-ribosylated by bacterial toxins. We investigated the possibility that transducin is modified in vitro by an endogenous ADP-ribosyltransferase activity. By using either ROS, cytosolic extract of ROS or purified transducin in the presence of [alpha-32P]nicotinamide adenine dinucleotide (NAD+), the alpha and beta subunits of transducin were found to be radiolabeled. The labeling was decreased by snake venom phosphodiesterase I (PDE I). The modification was shown to be mono ADP-ribosylation by analyses on thin layer chromatography of the PDE I-hydrolyzed products which revealed only 5'AMP residues. In addition we report that sodium nitroprusside activates the ADP-ribosylation of transducin.

Nitroprusside-sensitive and insensitive guanylate cyclases in retinal rod outer segments

Rod outer segments of retina contain guanylate cyclase activity both in the cytosol and membrane fractions. Though the activity in the cytosol is a small fraction of the total activity, it is highly activated by nitroprusside, a nitric oxide generating agent. The membrane guanylate cyclase on the other hand is unaffected by nitroprusside both before and after solubilization. The effects of nitroprusside or nitric oxide on photoreceptor function should therefore be mediated by the cytosolic and not the membrane guanylate cyclase.

Signal transduction enzymes of vertebrate photoreceptors

A flash of light initiates a cascade of biochemical reactions inside vertebrate photoreceptor cells, culminating in hydrolysis of intracellular cyclic GMP and hyperpolarization of the cell. The cell recovers by shutting down this cascade and resynthesizing cGMP. Many of the reactions responsible for the excitation and recovery phases of the photoresponse have been identified. Here I review some characteristics of the proteins that participate in these reactions.

Studies on cytosolic guanylate cyclase from human placenta

We have purified the soluble form of guanylate cyclase from human placenta greater than 2400-fold. The enzyme shared several characteristics with the enzyme purified from other sources including molecular mass and subunit composition, activation by divalent cations, inhibition by ATP and Michaelis constants. The enzyme, however, had a lower absorption maximum in the Soret region (417 +/- 1 nm) than the enzyme from other sources and was activated only one-fifth as much by nitric oxide as the bovine lung enzyme. It appears that the heme prosthetic group in the human placental enzyme may be hexa-coordinate and in the bovine lung enzyme the heme group may be penta-coordinate.

Purification of guanylyl cyclase from rod outer segments

The particulate form of guanylyl cyclase from bovine rod outer segments has been solubilized and purified to near homogeneity by a combination of liquid chromatography and native gel electrophoresis. The procedure enriches enzyme activity 6700-fold from rod outer segment extracts to a final specific activity of 17.5 mumol/min per mg (when assayed with Mn-GTP as substrate). Purified preparations of guanylyl cyclase contain a single glycoprotein with an apparent molecular mass of 60,000 Da and a native isoelectric point of 7.6. Although crude or partially purified enzyme activity is modulated by sub-micromolar concentrations of Ca2+, the fully purified enzyme is insensitive to this cation. However, the purified enzyme remains sensitive to nitrovasodilators, being stimulated over 10-fold by sodium nitroprusside. These data suggest that retinal rods contain a unique isoform of guanylyl cyclase.

Synthesis of nitric oxide in the bovine retina

In the absence of light, high concentrations of cGMP open ion channels in the plasma membranes of rod outer segments. The source of stimulation of retinal guanylate cyclase is not known. Nitric oxide is a potent stimulator of guanylate cyclase in other cell systems. The present data demonstrate that nitric oxide synthase, an enzyme responsible for the production of nitric oxide, is present in retina, and specifically in the rod outer segments. This enzyme uses L-arginine as a substrate and is NADPH- and calcium- dependent. L-arginine-derived nitric oxide may be a source of activation of guanylate cyclase in the retina.