Studies on cytosolic guanylate cyclase from human placenta

New views on endothelial dysfunction in gestational hypertension and potential therapy targets

The placenta has vital roles in metabolite exchange, fetal growth, and pre-eclampsia (PE). In this review, we discuss the pathogenesis of hypertension in pregnancy, focusing on four major theories to explain PE, discussing endothelial roles in those theories. We focus in particular on the roles of nitric oxide (NO) and prostacyclin (PGI₂) in placental endothelium, and propose new hypotheses for the influence and mechanisms of endothelial NO and PGI₂ signaling pathways in PE.

Native human nitric oxide sensitive guanylyl cyclase: purification and characterization

The only published report of the purification of native human soluble guanylyl cyclase (sGC) used placenta as starting material. This enzyme preparation showed low fold-activation by NO and a maximal absorption of the prosthetic heme-group at 417nm indicative of a prosthetic heme-group in a hexa-coordinate state. These data are in contrast to what has subsequently been found for the recombinant human enzymes. Apart from this placental enzyme preparation, a native functional human NO-sensitive sGC has not been successfully purified. The aim of the current study was to purify and characterize native human sGC from another source, to see whether the discrepancies between native and recombinant sGC seen for placenta are a general phenomenon. We chose human platelets as starting material since the properties of this enzyme are directly relevant for the development of innovative antiplatelet and antianginal drugs. Our results indicate that the native platelet enzyme exists as a highly NO-sensitive, heterodimeric enzyme with an alpha(1) and beta(1) subunit. In contrast to the native human placental enzyme and in accordance with the human recombinant enzymes, the native human platelet enzyme contains a ferrous, penta-coordinate heme group. To our knowledge this is the first report of the successful purification and characterization of the native human nitric oxide sensitive alpha(1)/beta(1) isoform of sGC which is widely expressed in the cardiovascular system and is an important target of innovative drugs.

Organic phosphates as a new class of soluble guanylate cyclase inhibitors

This study aimed to examine effects of varied organic phosphates on activities of soluble guanylate cyclase (sGC). The enzyme was purified from bovine lung. Physiologically relevant concentrations of ATP, 2,3-bisphosphoglyceric acid and inositol hexakisphosphate inhibited its enzyme activities under steady-state conditions as well as those determined under stimulation with S-nitroso-N-acetylpenicillamine, a nitric oxide donor, carbon monoxide or YC-1. Lineweaver-Burk plot analyses revealed that these three organic phosphates act as competitive inhibitors. Other organic phosphates such as cardiolipin and sphingomyelin but not inorganic phosphates exhibited such inhibitory actions. These results suggest that organic phosphates serve as inhibitors for sGC-dependent signaling events.

Dominant negative mutants of guanylyl cyclase: probes for global functions and intramolecular mechanisms

Dominant negative mutants are unique tools to define functions of a protein, not only within complex cellular and organismal contexts, but also mechanistically within a protein. Guanylyl cyclases are amenable to studies with dominant negative mutants, with their own sets of opportunities for insight and pitfalls to overcome. Membrane and soluble forms of guanylyl cyclase represent self-contained signal transduction modules that recognize, transduce, and amplify an external signal to give a carefully controlled response. Beginning with recognition of peptide hormones versus nitric oxide, membrane and soluble guanylyl cyclases are considerably different, except that their catalytic domains are closely related. Studies on these catalytic domains and their counterparts in adenylyl cyclases have raised an integral question of whether one or two domains form a catalytic site, which remains unresolved. Regardless of which model is correct, guanylyl cyclases appear to require an oligomeric state to function properly. The inferred relationship between protein-protein interaction and function is the basis for developing dominant negative mutants, which can be designed without prior structural information. Soluble guanylyl cyclases exist in a heterodimeric state, whereas membrane guanylyl cyclases are homodimeric, or possibly higher-order oligomers. These properties dictate that dominant negative mutants of membrane and soluble guanylyl cyclases be approached in fundamentally different ways, with regard to their design, their functional consequences, and their limitations. Using dominant negative mutants as specific inhibitors in complex systems, such as transgenic animals, represents a significant advance, and continuing improvements are just an inkling of the extraordinary potential of this approach. For example, the function of a protein can be obscured because it is expressed in multiple cell types; by restricting its pattern of expression, a cell-specific promoter, coupled to a dominant negative mutant, can pinpoint this function. As more sophisticated methods are developed, dominant negative mutants will provide additional opportunities to unveil new regulatory mechanisms, new signaling pathways, or even new therapeutic approaches.

Tissue distribution of the human soluble guanylate cyclases

Soluble guanylate cyclase (sGC) is an important component of the NO signaling pathway. Human sGC isoforms alpha(1), alpha(2), and beta(1) show differential mRNA tissue distributions. alpha(1) and beta(1) are expressed in most tissues; however, the alpha(2) isoform shows a more restricted expression pattern with high levels in brain, placenta, spleen, and uterus only. Both alpha subunits exist as multiple transcripts whereas beta(1) exists as a single message. This study reports for the first time the tissue distribution of human sGC message and demonstrates that sGC isoforms are nonuniformly expressed which may be useful if the enzyme is to be exploited as a therapeutic target.

Nitric oxide regulation of gene transcription via soluble guanylate cyclase and type I cGMP-dependent protein kinase

Nitric oxide (NO) regulates the expression of multiple genes but in most cases its precise mechanism of action is unclear. We used baby hamster kidney (BHK) cells, which have very low soluble guanylate cyclase and cGMP-dependent protein kinase (G-kinase) activity, and CS-54 arterial smooth muscle cells, which express these two enzymes, to study NO regulation of the human fos promoter. The NO-releasing agent Deta-NONOate (ethanamine-2,2'-(hydroxynitrosohydrazone)bis-) had no effect on a chloramphenicol acetyltransferase (CAT) reporter gene under control of the fos promoter in BHK cells transfected with an empty vector or in cells transfected with a G-kinase Ibeta expression vector. In BHK cells transfected with expression vectors for guanylate cyclase, Deta-NONOate markedly increased the intracellular cGMP concentration and caused a small (2-fold) increase in CAT activity; the increased CAT activity appeared to be from cGMP activation of cAMP-dependent protein kinase. In BHK cells co-transfected with guanylate cyclase and G-kinase expression vectors, CAT activity was increased 5-fold in the absence of Deta-NONOate and 7-fold in the presence of Deta-NONOate. Stimulation of CAT activity in the absence of Deta-NONOate appeared to be largely from endogenous NO since we found that: (i) BHK cells produced high amounts of NO; (ii) CAT activity was partially inhibited by a NO synthase inhibitor; and (iii) the inhibition by the NO synthase inhibitor was reversed by exogenous NO. In CS-54 cells, we found that NO increased fos promoter activity and that the increase was prevented by a guanylate cyclase inhibitor. In summary, we found that NO activates the fos promoter by a guanylate cyclase- and G-kinase-dependent mechanism.

Presence of two enzymes, different from the F1F0-ATPase, hydrolyzing nucleotides in human term placental mitochondria

The hydrolysis of ATP, ADP or GTP was characterized in mitochondria and submitochondrial particles since a tightly-bound ATPase associated with the inner mitochondrial membrane from the human placenta has been described. Submitochondrial particles, which are basically inner membranes, were used to define the location of this enzyme. Mitochondria treated with trypsin and specific inhibitors were also used. The oxygen consumption stimulated by ATP or ADP was 100% inhibited in intact mitochondria by low concentrations of oligomycin (0.5 microgram/mg) or venturicidine (0.1 microgram/mg), while the hydrolysis of ATP or ADP was insensitive to higher concentrations of these inhibitors but it was inhibited by vanadate. Oligomycin or venturicidine showed a different inhibition pattern in intact mitochondria in relation to the hydrolysis of ATP, ADP or GTP. When submitochondrial particles were isolated from mitochondria incubated with oligomycin or venturicidine, no further inhibition of the nucleotide hydrolysis was observed, contrasting with the partial inhibition observed in the control. By incubating the placental mitochondria with trypsin, a large fraction of the hydrolysis of nucleotides was eliminated. In submitochondrial particles obtained from mitochondria treated with trypsin or trypsin plus oligomycin, the hydrolysis of ATP was 100% sensitive to oligomycin at low concentrations, resembling the oxygen consumption; however, this preparation still showed some ADP hydrolysis. Native gel electrophoresis showed two bands hydrolyzing ADP, suggesting at least two enzymes involved in the hydrolysis of nucleotides, besides the F1F0-ATPase. It is concluded that human placental mitochondria possesses ADPase and ATP-diphosphohydrolase activities (247).

Functional properties of a naturally occurring isoform of soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC), the target enzyme of the signalling molecule NO, contains one prosthetic haem group and consists of an alpha and a beta subunit. So far, only the alpha1beta1 heterodimer has been shown to exist in different cells and tissues, and most biochemical studies of sGC have been performed with the alpha1 beta1 heterodimer. Here we demonstrate for the first time the natural occurrence of the alpha2 subunit on the protein level. The alpha2 subunit co-precipitated with the beta1 subunit from human placenta, showing the existence of the alpha2 beta1 isoform in vivo. The new enzyme was expressed in and purified from cells from the Spodoptera frugiperda ovary cell line Sf 9. Spectral analysis showed that the alpha2 beta1 heterodimer contains a prosthetic haem group revealing the same characteristics as the haem in the alpha1 beta1 form. The kinetic properties of both isoforms and sensitivity towards NO were indistinguishable. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of sGC, abolished NO-stimulated activity of both heterodimers. The new NO-independent activator, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), increased the maximal NO-stimulated activity of the new isoform, caused a leftward-shift in the NO concentration-response curve and turned CO into an effective activator, as it did for the alpha1 beta1 heterodimer (200-fold activation). In summary, the differences in primary structure of both alpha subunits are contrasted by their functional similarity. Further studies will be needed to elucidate the physiological purpose of the new isoform.

Synergistic activation of soluble guanylate cyclase by YC-1 and carbon monoxide: implications for the role of cleavage of the iron-histidine bond during activation by nitric oxide

BACKGROUND

Nitric oxide (.NO) is used in biology as both an intercellular signaling agent and a cytotoxic agent. In signaling, submicromolar quantities of .NO stimulate the soluble isoform of guanylate cyclase (sGC) in the receptor cell. .NO increases the Vmax of this heterodimeric hemoprotein up to 400-fold by interacting with the heme moiety of sGC to form a 5-coordinate complex. Carbon monoxide (CO) binds to the heme to form a 6-coordinate complex, but only activates the enzyme 5-fold, YC-1 is a recently discovered compound that relaxes vascular smooth muscle by stimulating sGC.

RESULTS

In the presence of YC-1, CO activates sGC to the same specific activity as attained with .NO. YC-1 did not affect the NO-stimulated activity. The on-rate (kon) and off-rate (koff) of CO for binding to sGC in the presence of YC-1 were determined by stopped-flow spectrophotometry. Neither the kon nor the koff varied from values previously obtained in the absence of YC-1, indicating that YC-1 has no effect on the affinity of CO for the heme. In the presence of YC-1, the visible spectrum of the sGC-CO complex has a Soret peak at 423 nm, indicating the complex is 6-coordinate.

CONCLUSIONS

YC-1 has no effect on the affinity of CO for the heme of sGC. In the presence of YC-1, maximal activation of sGC by CO is achieved by formation of a 6-coordinate complex between CO and the heme indicating that cleavage of the Fe-His bond is not required for maximal activation of sGC.

Regulation of human erythrocyte Na+/H+ exchange by soluble and particulate guanylate cyclase

Guanylate cyclase activity in human erythrocytes is investigated by evaluating the intracellular guanosine 3',5'-cyclic monophosphate (cGMP) content in the presence of various agents that exert specific effects on soluble or particulate guanylate cyclase. The increase in the intraerythrocyte cGMP content by the soluble guanylate cyclase activators nitroprusside and NaNO2 suggests the presence of this enzyme in human erythrocytes. The effects of four different atrial natriuretic peptide (ANP) fragments on the intraerythrocyte cGMP content is also studied. ANP II and ANP III increase the intraerythrocyte cGMP content, whereas ANP I and des-Ser5,des-Ser6-ANP III are ineffective. Thus our data show that human erythrocytes possess particulate guanylate cyclase together with the soluble enzyme. The ANP fragments ANP II and ANP III also activate the erythrocyte Na+/H+ exchange. Nitroprusside, M & B 22948 (an inhibitor of cGMP phosphodiesterase), and the cGMP analogues dibutyryl cGMP and 8-bromoguanosine 3',5'-cyclic monophosphate also increase the erythrocyte Na+/H+ exchange rate. The latter data also suggest that the erythrocyte Na+/H+ exchange is regulated by cGMP.

Characterization of the F1F0-ATPase and the tightly-bound ATPase activities in submitochondrial particles from human term placenta

In a previous study we demonstrated the existence of a tightly-bound ATPase in the human placental mitochondria (Martínez et al., 1993). The current study characterizes the ATP hydrolysis produced by the F1F0-ATPase and the tightly-bound ATPase in submitochondrial particles from the human term placenta. Both enzymes were not differentiated by pH. Inhibitors were necessary to distinguish the activity of each enzyme. The kinetic of the total ATP hydrolysis fitted into a model of two enzymes. During the characterization, it was observed that the tightly-bound ATPase activity was partially inhibited by vanadate and Mg2+, whereas the F1F0-ATPase was totally inhibited by Mg2+. Different nucleotides were hydrolyzed by the tightly-bound ATPase; the F1F0-ATPase hydrolyzed exclusively ATP. Glucose-6-phosphate, p-nitrophenylphosphate, or pyrophosphate were not hydrolyzed by the F1F0-ATPase, although some hydrolysis was observed with the tightly-bound ATPase. It is concluded that the tightly-bound ATPase activity corresponded to a 5'-nucelotidase, and that the human placental mitochondria could participate in the metabolism of nucleotides.

Control of vascular resistance in the maternal and feto-placental arterial beds

This review aims to provide a comprehensive summary of the mechanisms involved in the physiological adaptation of the vasculature to pregnancy. Profound changes occur both systemically and in discrete circulations in the mother, but it is debatable which factors are responsible. Similarly, whilst the feto-placental circulation must be substantially controlled by humoral mechanisms, the exact role of each potential contributor is not known. In view of the hitherto unappreciated and very important role of the endothelium-derived vasodilator, nitric oxide, in the control of peripheral vascular resistance, considerable emphasis will be placed on the many recent investigations in this area.