The guanylyl cyclase family of natriuretic peptide receptors

Purification, characterization, and preliminary serial crystallography diffraction advances structure determination of full-length human particulate guanylyl cyclase A receptor

Particulate Guanylyl Cyclase Receptor A (pGC-A) is a natriuretic peptide membrane receptor, playing a vital role in controlling cardiovascular, renal, and endocrine functions. The extracellular domain interacts with natriuretic peptides and triggers the intracellular guanylyl cyclase domain to convert GTP to cGMP. To effectively develop methods to regulate pGC-A, structural information on the full-length form is needed. However, structural data on the transmembrane and intracellular domains are lacking. This work presents expression and optimization using baculovirus, along with the first purification of functional full-length human pGC-A. In vitro assays revealed the pGC-A tetramer was functional in detergent micelle solution. Based on our purification results and previous findings that dimer formation is required for functionality, we propose a tetramer complex model with two functional subunits. Previous research suggested pGC-A signal transduction is an ATP-dependent, two-step mechanism. Our results show the binding ligand also moderately activates pGC-A, and ATP is not crucial for activation of guanylyl cyclase. Furthermore, crystallization of full-length pGC-A was achieved, toward determination of its structure. Needle-shaped crystals with 3 Å diffraction were observed by serial crystallography. This work paves the road for determination of the full-length pGC-A structure and provides new information on the signal transduction mechanism.

The receptor attributable to C-type natriuretic peptide-induced differentiation of osteoblasts is switched from type B- to type C-natriuretic peptide receptor with aging

C-type natriuretic peptide (CNP) stimulates the differentiation and inhibits the proliferation of osteoblastic lineage cells. In this study, we examined whether the effects of CNP on osteoblastic functions change with aging using calvarial osteoblast-like cells from 25-week-old (young) and 120-week-old (aged) rats. CNP inhibited DNA synthesis and stimulated collagen synthesis and mineralized bone nodule formation. These effects were less pronounced in aged rat cells, suggesting the age-related attenuation of CNP-induced signaling. They were also blocked by the treatment of young rat cells with KT5823, a protein kinase G (PKG) inhibitor, but not by the treatment of aged rat cells with KT5823. CNP stimulated cGMP production in young rat cells, but not in aged rat cells. Natriuretic peptide receptor (NPR)-B, which has a guanylyl cyclase activity domain, and NPR-C, which has no enzyme activity domain, were predominantly expressed in young and aged rat cells, respectively. C-ANF, an NPR-C agonist, mimicked the effects of CNP on the proliferation and differentiation of aged rat cells; these effects were inhibited by the treatment with pertussis toxin (PTX), a Gi protein inhibitor. CNP and C-ANF evoked intracellular levels of inositol-1,4,5-triphosphate and Ca(2+), which are markers for phospholiase C (PLC) activation, in aged rat cells, and the effects of these two peptides were also blocked by the treatment with PTX. From these results, we concluded that CNP acts as a positive regulator of bone formation by osteoblasts and that the signaling pathway for CNP is switched from NPR-B/cGMP/PKG to NPR-C/G(i) protein/PLC with aging.

Adenine nucleotides decrease the apparent Km of endogenous natriuretic peptide receptors for GTP

Natriuretic peptide receptors A (NPR-A) and B (NPR-B) mediate most effects of natriuretic peptides by synthesizing cGMP. ATP increases the activity of these receptors by an unknown mechanism. We recently reported that a nonhydrolyzable form of ATP, adenylyl imidodiphosphate (AMPPNP), stabilizes but is not required for the activation of NPR-A and NPR-B in membranes from highly overexpressing cells. Here, we repeated these studies on receptors expressed in endogenous settings. Kinetic analysis indicated that both AMPPNP and ATP dramatically decrease the apparent K(m) of both receptors for GTP but had little effect on the V(max). The EC(50) for AMPPNP decreased as substrate concentration increased whereas the magnitude of the effect was greater at lower GTP concentrations. ATP increased the activity of a mutant receptor containing glutamates substituted for all known phosphorylation sites similarly to the wild-type receptor, consistent with a phosphorylation independent mechanism. Finally, the putative ATP binding sites were investigated. Mutation of the ATP modulatory domain region had no effect, but mutation of K535A dramatically diminished ANP-dependent cyclase activity in a manner that was unresponsive to ATP. Mutation of the highly conserved 630-KSS to AAA (all alanines) resulted in an expressed receptor that had no detectable guanylyl cyclase activity. We conclude that ATP is not required for the initial activation of NPRs but does increase activity over time by reducing the apparent K(m) for GTP.

Cellular and molecular mechanisms regulating vascular tone. Part 2: regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells

Understanding the physiological mechanisms regulating vascular tone would lead to better circulatory management during general anesthesia. This two-part review provides an overview of current knowledge about the cellular and molecular mechanisms regulating the contractile state of vascular smooth muscle cells (i.e., vascular tone). The first part reviews basic mechanisms controlling the cytosolic Ca2+ concentration in vascular smooth muscle cells, and the Ca2+-dependent regulation of vascular tone. This second part reviews the regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells-including Rho/Rho kinase, protein kinase C, arachidonic acid, Ca2+/calmodulin-dependent protein kinase II, caldesmon, calponin, mitogen-activated protein kinases, tyrosine kinases, cyclic nucleotides, Cl- channels, and K+ channels.

Dendroaspis natriuretic peptide binds to the natriuretic peptide clearance receptor

Dendroaspis natriuretic peptide (DNP) is a newly-described natriuretic peptide which lowers blood pressure via vasodilation. The natriuretic peptide clearance receptor (NPR-C) removes natriuretic peptides from the circulation, but whether DNP interacts with human NPR-C directly is unknown. The purpose of this study was to test the hypothesis that DNP binds to NPR-C. ANP, BNP, CNP, and the NPR-C ligands AP-811 and cANP(4-23) displaced [(125)I]-ANP from NPR-C with pM-to-nM K(i) values. DNP displaced [(125)I]-ANP from NPR-C with nM potency, which represents the first direct demonstration of binding of DNP to human NPR-C. DNP showed high pM affinity for the GC-A receptor and no affinity for GC-B (K(i)>1000 nM). DNP was nearly 10-fold more potent than ANP at stimulating cGMP production in GC-A expressing cells. Blockade of NPR-C might represent a novel therapeutic approach in augmenting the known beneficial actions of DNP in cardiovascular diseases such as hypertension and heart failure.

Recognition and signal transduction mechanism of Escherichia coli heat-stable enterotoxin and its receptor, guanylate cyclase C

Guanylate cyclase C (GC-C), a member of the membrane-bound GC family, consists of an extracellular domain (ECD) and an intracellular domain, which are connected by a single-transmembrane region. GC-C is a receptor protein, i.e. specifically stimulated by the endogenous peptides guanylin, uroguanylin, lymphoguanylin, and the exogenous peptide heat-stable enterotoxin (ST(a)), secreted by pathogenic Escherichia coli and acting on the intestinal brush border membranes. The binding of these peptide ligands to the ECD of GC-C results in the synthesis of cyclic GMP in cells, which, in turn, regulates a variety of intracellular physiologic processes. As the cloning of GC-C, its physiologic functions of each domain have been vigorously investigated. The structural characterization of the ligand-binding domain of the receptor promises to provide important clues for better understanding of the mechanisms of receptor recognition and activation. Recently, structural data for each domain of membrane-bound GCs and related proteins has become available. Coupling information obtained from such work and validation of structure-function relationships of GC-C and its ligands should allow for three-dimensional mapping of their interaction site in detail. Our approach to this issue involved designing photoaffinity-labeling ST(a) analogs, capable of binding covalently to the ligand-binding region of the ECD of GC-C. The photoaffinity-labeling ligand was used to covalently label a soluble form of the recombinant ECD protein. Mass spectrometric analyses of an endoproteinase digest of the ECD revealed that the ligand specifically bound to a narrow region contained in the membrane-proximal subdomain of the ECD of GC-C. These results will enable us to identify the possible binding motifs within the ligand-binding domain by computer modeling. In this review, we summarize the available data on the recognition mechanism between ST(a) and GC-C at the molecular level.

C-type natriuretic peptide and guanylyl cyclase B receptor

Guanylyl cyclases (GC) are widely distributed enzymes that signal via the production of the second messenger cGMP. The particulate guanylyl cyclases share a similar topology: an extracellular ligand binding domain and intracellular regulatory kinase-homology and cyclase catalytic domains. The natriuretic peptide receptors GC-A and -B mediate the effects of a family of peptides, atrial, B- and C-type natriuretic peptide (ANP, BNP and CNP, respectively), with natriuretic, diuretic and vasorelaxant properties. ANP and BNP, through the activation of GC-A, act as endocrine hormones to regulate blood pressure and volume, and inhibit cardiac hypertrophy. CNP, on the other hand, acts in an autocrine/paracrine fashion to induce vasorelaxation and vascular remodeling, and to regulate bone growth through its cognate receptor GC-B. GC-B, like GC-A, is phosphorylated in the basal state, and undergoes both homologous and heterologous desensitization, reflected by dephosphorylation of specific sites in the kinase-homology domain. This review will examine the structure and function of GC-B, and summarize the physiological processes in which this receptor is thought to participate.

Calcium-dependent dephosphorylation mediates the hyperosmotic and lysophosphatidic acid-dependent inhibition of natriuretic peptide receptor-B/guanylyl cyclase-B

C-type natriuretic peptide binding to natriuretic peptide receptor-B (NPR-B) stimulates cGMP synthesis, which regulates vasorelaxation, cell proliferation, and bone growth. Here, we investigated the mechanistic basis for hyperosmotic and lysophosphatidic acid-dependent inhibition of NPR-B. Whole cell cGMP measurements and guanylyl cyclase assays indicated that acute hyperosmolarity decreased NPR-B activity in a reversible, concentration- and time-dependent manner, whereas chronic exposure had no effect. Acute hyperosmolarity elevated intracellular calcium in a concentration-dependent fashion that paralleled NPR-B desensitization. A calcium chelator, but not a protein kinase C inhibitor, blocked both calcium elevations and desensitization. Hyperosmotic medium stimulated NPR-B dephosphorylation, and the receptor was rapidly rephosphorylated and resensitized when the hypertonic media was removed. Lysophosphatidic acid also inhibited NPR-B in a calcium- and phosphorylation-dependent process, consistent with calcium being a universal regulator of NPR-B. The absolute requirement of dephosphorylation in this process was demonstrated by showing that a receptor with glutamates substituted at all known NPR-B phosphorylation sites is unresponsive to hyperosmotic stimuli. This is the first study to measure the phosphorylation state of an endogenous guanylyl cyclase and to link intracellular calcium elevations with its dephosphorylation.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

Lysophosphatidic acid inhibits C-type natriuretic peptide activation of guanylyl cyclase-B

C-type natriuretic peptide (CNP), found in endothelial cells, chondrocytes, and neurons, binds its cognate transmembrane receptor, natriuretic peptide receptor-B (NPR-B/GC-B), and stimulates the synthesis of the intracellular signaling molecule, cGMP. The known physiologic consequences of this binding event are vasorelaxation, inhibition of cell proliferation, and the stimulation of long bone growth. Here we report that 10% fetal bovine serum markedly reduced CNP-dependent cGMP elevations in NIH3T3 fibroblast. The purified serum components platelet-derived growth factor and lysophosphatidic acid (LPA) mimicked the effect of serum on CNP-dependent cGMP elevations, but the latter factor resulted in the most dramatic reductions. The LPA-dependent inhibition was rapid and dose dependent, having t(1/2) and IC(50) values of approximately 5 min and 3.0 micro M LPA, respectively. The decreased cGMP concentrations resulted from reduced CNP-dependent NPR-B guanylyl cyclase activity that did not require losses in receptor protein or activation of protein kinase C, indicating a previously undescribed desensitization pathway. These data suggest that NPR-B is repressed by LPA and that one mechanism by which LPA exerts its effects is through the heterologous desensitization of the CNP/NPR-B/cGMP pathway. We hypothesize that cross-talk between the LPA and CNP signaling pathway maximizes the response of fibroblasts in the wound-healing process.

Roles of cyclic GMP in modulating ovarian functions

The production of a viable oocyte is dependent upon the critical influences of gonadotrophins on follicular development, granulosa cell maturation, ovulation, and luteinization. While the effects of LH and FSH are due in large part to cyclic AMP-dependent signalling mechanisms, it is clear that a number of other factors modulate the actions of gonadotrophins on the ovary via activation of alternative signalling pathways. In this regard, recent studies indicate that the second messenger guanosine 3',5'-cyclic monophosphate (cGMP) mediates a wide range of influences on the ovary. Nitric oxide (NO) is a major regulator of cGMP production via its action on soluble guanylyl cyclase, while natriuretic peptides activate receptors with intrinsic guanylyl cyclase activities. In addition, other factors known to influence ovarian functions are now recognized to act via NO/cGMP pathways. This report will review these previous findings and present new data demonstrating the inhibitory influence of cGMP on cAMP-stimulated LH receptor expression in cultured granulosa cells.

Vasopressin-dependent inhibition of the C-type natriuretic peptide receptor, NPR-B/GC-B, requires elevated intracellular calcium concentrations

Natriuretic peptides bind their cognate cell surface guanylyl cyclase receptors and elevate intracellular cGMP concentrations. In vascular smooth muscle cells, this results in the activation of the type I cGMP-dependent protein kinase and vasorelaxation. In contrast, pressor hormones like arginine-vasopressin, angiotensin II, and endothelin bind serpentine receptors that interact with G(q) and activate phospholipase Cbeta. The products of this enzyme, diacylglycerol and inositol trisphosphate, activate the conventional and novel forms of protein kinase C (PKC) and elevate intracellular calcium concentrations, respectively. The latter response results in vasoconstriction, which opposes the actions of natriuretic peptides. Previous reports have shown that pressor hormones inhibit natriuretic peptide receptors NPR-A or NPR-B in a variety of different cell types. Although the mechanism for this inhibition remains unknown, it has been universally accepted that PKC is an obligatory component of this pathway primarily because pharmacologic activators of PKC mimic the inhibitory effects of these hormones. Here, we show that in A10 vascular smooth muscle cells, neither chronic PKC down-regulation nor specific PKC inhibitors block the AVP-dependent desensitization of NPR-B even though both processes block PKC-dependent desensitization. In contrast, the cell-permeable calcium chelator, BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester), abrogates the AVP-dependent desensitization of NPR-B, and ionomycin, a calcium ionophore, mimics the AVP effect. These data show that the inositol trisphosphate/calcium arm of the phospholipase C pathway mediates the desensitization of a natriuretic peptide receptor in A10 cells. In addition, we report that CNP attenuates AVP-dependent elevations in intracellular calcium concentrations. Together, these data reveal a dominant role for intracellular calcium in the reciprocal regulation of these two important vasoactive signaling systems.

Role of nitric oxide on the vasorelaxant effect of atrial natriuretic peptide on rabbit aorta basal tone

The role of nitric oxide (NO) on the vasorelaxant effect of atrial natriuretic peptide (ANP) on the basal tone of rabbit aortic rings conditioned to angiotensin II (Ang II) was studied. ANP aortic relaxation and nitrite release were measured in the presence and absence of endothelium and a NO-synthase inhibitor. Ang II at 10(-8) M triggered a contractile response, conditioning the vessel to a vasorelaxant effect of ANP (10(-8) M). This effect was significantly enhanced by endothelium removal, NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), and methylene blue (10(-5) M). ANP decrease of basal tone in Ang-II-sensitized aortic rings was improved when a higher concentration of Ang II was used (l0(-6) M). Basal and Ang-II-stimulated nitrite release were measured in stretched (S) and nonstretched (NS) aortic rings. Nitrite release was significantly increased in S rings (p < 0.001). L-NAME (10(-4) M) partially inhibited nitrite release in both basal and Ang-II-stimulated S aortic rings. In NS aortic rings, the NO inhibitor did not inhibit basal nitrite release but blunted the Ang-II-stimulated nitrite level. A significant negative correlation between nitrite release and the ANP vasorelaxant effect on basal tone was dependent on the Ang-II-sensitizing dose. The present results demonstrate that ANP relaxant effects on aortic basal tone are related to NO levels, which are regulated by S- and Ang-II-concentration-dependent NO generation and quenching.

Differential regulation of atrial and brain natriuretic peptides and its implications for the management of left ventricular volume overload

BACKGROUND

In this study, we investigated the possibility that the atrial and brain natriuretic peptide expression in left ventricular volume overload (VOL) is transcriptionally regulated. We further evaluated the diagnostic and/or prognostic potential of this expression for the management of patients with this disorder.

DESIGN

We compared the myocardial mRNA expression and plasma levels of the two peptides in VOL patients using donor hearts and in healthy blood donors as controls.

RESULTS

The atrial natriuretic peptide (ANP) mRNA was elevated by 38% (P < 0.03) in the right atrium and by 53% (P < 0.003) in the left atrium, but was unchanged in the ventricular chambers of the patient group (n = 19) compared with controls (n = 8). Plasma ANP concentration was elevated by 62% (P < 0.001) compared with blood donor controls (n = 79). It increased slightly (by 36%) 2 h following surgery, and remained at 64% higher (P < 0.03 vs. presurgery) for the 5 days following surgery. The brain natriuretic peptide (BNP) mRNA was elevated by approximately one-fold in both the left ventricle (P < 0.02) and right atrium (P < 0.05), by 94% (P < 0.02) in the right ventricle and by 89% (P < 0.05) in the left atrium. Its plasma level in the patients was 3.4-fold (P < 0.00003) higher than in control subjects. It increased significantly by 1.2-fold (P < 0.01) 2 h following surgery, but dropped significantly (P < 0.05 vs. 2 h post surgery) to presurgical levels 5 days following surgery.

CONCLUSION

The results show chamber-specific elevation in both atrial and brain natriuretic peptide expression and differences in their circulating levels in VOL, suggesting that BNP is a potential prognostic indicator in the postsurgical management of these patients.

Natriuretic peptide-dependent lipolysis in fat cells is a primate specificity

We have recently demonstrated that natriuretic peptides (NPs), which are known for regulation of blood pressure via membrane guanylyl cyclase (GC) receptors, are lipolytic in human adipose tissue. In this study, we compared the NP control of lipolysis in adipocytes from humans, nonhuman primates (macaques), rodents (rats, mice, hamsters), and nonrodent mammals (rabbits, dogs). Isolated adipocytes from these species were exposed to increasing concentrations of atrial NP (ANP) or isoproterenol (beta-adrenergic agonist). Although isoproterenol was lipolytic in all of the species, ANP only enhanced lipolysis in human and macaque adipocytes. In primate fat cells, NP-induced lipolysis involved a cGMP-dependent pathway. Binding studies and real-time quantitative PCR assays revealed that rat adipocytes expressed a higher density of NP receptors compared with humans but with a different subtype pattern of expression; type-A GC receptors predominate in human fat cells. This was also confirmed by the weak GC-activity stimulation and the reduced cGMP formation under ANP exposure in rat adipocytes compared with human fat cells. In conclusion, NP-induced lipolysis is a primate specificity, and adipocytes from ANP-nonresponsive species present a predominance of "clearance" receptors and very low expression of "biologically active" receptors.

The atrial natriuretic peptide receptor (NPR-A/GC-A) is dephosphorylated by distinct microcystin-sensitive and magnesium-dependent protein phosphatases

Natriuretic peptide receptor (NPR)-A is the primary signaling receptor for atrial natriuretic peptide and brain natriuretic peptide. Ligand binding to NPR-A rapidly activates its guanylyl cyclase domain, but its rate of cGMP synthesis declines with time. This waning of activity is called homologous desensitization and is mediated in part by receptor dephosphorylation. Here, we characterize two distinct NPR-A phosphatase activities. The serine/threonine protein phosphatase inhibitor, microcystin, inhibited the desensitization of NPR-A in membrane guanylyl cyclase assays in the absence of magnesium. EDTA also inhibited the desensitization, whereas MgCl(2) stimulated the desensitization. Because the effects of microcystin and EDTA were additive, and microcystin did not block the magnesium-dependent desensitization, the targets for these agents appear to be distinct. Incubation of membranes at 37 degrees C stimulated the dephosphorylation of NPR-A, and microcystin blocked the temperature-dependent dephosphorylation. The addition of MgCl(2) or MnCl(2), but not CaCl(2), further stimulated the dephosphorylation of NPR-A, and microcystin failed to inhibit this process. The desensitization required changes in the phosphorylation state of NPR-A because the guanylyl cyclase activity of a receptor variant containing glutamate substitutions at all six phosphorylation sites was unaffected by MgCl(2), EDTA, or microcystin. Together, these data indicate that NPR-A is regulated by two distinct phosphatases, possibly including a member of the protein phosphatase 2C family. Finally, we observed that the desensitization of NPR-A in membranes from mouse kidneys and NIH3T3 cells was increased by prior exposure to atrial natriuretic peptide, suggesting that hormone binding enhances receptor dephosphorylation.

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

Structural insights into the ligand binding domains of membrane bound guanylyl cyclases and natriuretic peptide receptors

Membrane bound guanylyl cyclases are single chain transmembrane receptors that produce the second messenger cGMP by either intra- or extracellular stimuli. This class of type I receptors contain an intracellular catalytic guanylyl cyclase domain, an adjacent kinase-like domain and an extracellular ligand binding domain though some receptors have their ligands yet to be identified. The most studied member is the atrial natriuretic peptide (ANP) receptor, which is involved in blood pressure regulation. Extracellular ANP binding induces a conformational change thereby activating the pre-oligomerized receptor leading to the production of cGMP. The recent crystal structure of the dimerized hormone binding domain of the ANP receptor provides a first three-dimensional view of this domain and can serve as a basis to structurally analyze mutagenesis, cross-linking, and genetic studies of this class of receptors as well as a non-catalytic homolog, the clearance receptor. The fold of the ligand binding domain is that of a bilobal periplasmic binding protein (PBP) very similar to that of the Leu/Ile/Val binding protein, AmiC, multi-domain transmembrane metabotropic glutamate receptors, and several DNA binding proteins such as the lactose repressor. Unlike these structural homologs, the guanylyl cyclase receptors bind much larger molecules at a site seemingly remote from the usual small molecule binding site in periplasmic binding protein folds. Detailed comparisons with these structural homologs offer insights into mechanisms of signal transduction and allosteric regulation, and into the remarkable usage of the periplasmic binding protein fold in multi-domain receptors/proteins.

Detailed analysis of the atrial natriuretic factor receptor hormone-binding domain crystal structure

The X-ray crystal structure of the dimerized atrial natriuretic factor (ANF) receptor hormone-binding domain has provided a first structural view of this anti-hypertensive receptor. The structure reveals a surprising evolutionary link to the periplasmic-binding protein fold family. Furthermore, the presence of a chloride ion in the membrane distal domain and the presence of a second putative effector pocket suggests that the extracellular domain of this receptor is allosterically regulated. The scope of this article is to extensively review the data published on this receptor and to correlate it with the hormone-binding domain structure. In addition, a more detailed description is provided of the important features of this structure including the different binding sites for the ANF hormone, chloride ion, putative effector pocket, glycosylation sites, and dimer interface.Key words: crystal structure, periplasmic-binding protein fold, guanylyl cyclase, hormone receptor.

Porphyrins, porphyrin metabolism and porphyrias. I. Update

The biosynthesis of porphyrins is one of the most conserved parthways known, about the same sequence of reactions taking place in all species. By associating different metals, porphyrins give rise to the "pigments of life": chlorophyll, haem and cobalamin. The unique tetrapyrrolic structure enables it to function in an array of reactions as a single electron carrier and as a catalyst for redox reactions. In this capacity, it constitutes the prosthetic group of enzymes participating in cellular respiration, in conversion reactions involving steroids and lipophilic xenobiotics, in protective mechanisms directed against oxidative stress and in pathways providing central messenger molecules. The formation of haem is accomplished by a sequence of eight dedicated enzymes encoded by different genes, some being active in ubiquitous as well as in erythroid isoforms. Large differences between the participating enzymes with regard to catalytic power, with low capacity steps positioned early in the catalytic chain, constitute a bar against substrate overloading of enzymes processing porphyrins, thus preventing accumulation in the body of these phototoxic compounds under physiological conditions. Most of the haem in the body is produced by the liver and bone marrow, but the mechanisms applied for the control of the synthesis differ between the two organs. The extremely potent hemeprotein enzymes formed in the liver are rapidly turned over in response to current metabolic needs. They have half-lives in the order of minutes or hours and are restored by fast-acting mechanisms for the de novo synthesis, when needed. Uninterrupted and instant availability of the compound is secured by acute deinhibition of the initial enzyme of the synthetic chain, ubiquitous 5-aminolevulinate synthase (ALAS-1), in response to drain of the free cellular haem pool caused by prevailing demands for hemeproteins or by increased catabolism of the compound. In contrast, in the erythroid progenitor cell the haem synthetic machinery is designed for uninterrupted production of huge amounts of haem for combination with globin chains to form hemoglobin at a steady rate. In the erythron the synthesis of the enzymes participating in the formation of haem is under control of erythropoietin, formed under hypoxic conditions. In the absence of iron, to be incorporated in the porphyrin formed in the last step of the synthesis, the mRNA of erythroid 5-aminolevulinate synthase (ALAS-2) is blocked by attachment of an iron-responsive element (IRE) binding cytosolic protein, and transcription of this key enzyme is inhibited. In humans, the genes for each of the haem synthetic enzymes may become the target of mutations that give rise to impaired cellular enzyme activity. Seven of the enzyme deficiencies are associated with accumulation of toxic intermediaries and with disease entities termed porphyrias. The acute porphyrias are characterized by attacks of neuropsychiatric symptoms, which may be due to a toxic surplus of the porphyrin presursor 5-aminolevulinic acid, or a consequence of a deficit of vital hemeproteins resulting from impaired synthesis of haem. In the cutaneous porphyrias, impairment of enzymatic steps where porphyrins are processed gives rise to solar hypersensitivity due to accumulation of phototoxic porphyrins in the skin. Early diagnosis, information to the patient regarding the nature of the illness and counselling aimed at avoidance of triggering factors are cornerstones in the handling of the porphyric diseases. Gene analysis is of incomparable diagnostic reliability in carrier detection, but biochemical methods must be applied in the important task of monitoring porphyric disease activity. In most forms of porphyria the gene carriers run the risk of development of associated diseases in liver or kidneys, a circumstance that prompts application of well-structured surveillance programs.

Inhibitory effect of cyclic guanosine 3',5'-monophosphate (cGMP) on the afferent resting activity in the cephalopod statocyst

The effects of exo- and endogenous cGMP on the resting activity (RA) of afferent crista fibers were studied in isolated preparations of the statocysts of the cuttlefish Sepia officinalis and the squid Sepioteuthis lessoniana. Bath application of the membrane-permeable cGMP analogs 8-bromo-cGMP (B-cGMP) and N(2),2'-o-dibutyryl 3', 5'-cyclic guanosine monophosphate (dB-cGMP), and of the selective inhibitor of cGMP-phosphodiesterase zaprinast (ZAP), caused an inhibition of RA. The inhibitory effects of B-cGMP and dB-cGMP remained when the preparation was pre-treated with: (i) the guanylate cyclase inhibitors 1H-[1,2,4]oxadiazolo[4,3, -a]quinoxalin-1-one (ODQ) or cystamine (CYS); (ii) the adenylate cyclase inhibitors nicotinic acid (NIC-A), 2',3'dideoxyadenosine (DDA), or MDL-12330A (MDL); (iii) the guanylate cyclase inhibitor methylene blue (M-BLU) and the adenylate cyclase inhibitor MDL combined; or (iv) the nitric oxide (NO) synthase inhibitors N(G)-nitric-L-arginine methyl ester HCl (L-NAME) or N(G)-nitro-L-arginine (L-NOARG). These data indicate that cGMP, as an intracellular messenger, has a tonic inhibitory effect on the RA of afferent crista fibers in cephalopod statocysts.