Expression, purification, and characterization of the intra-cellular domain of the ANP receptor

Research progress of cGAS-STING signaling pathway in intestinal diseases

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signal has drawn much consideration due to its sensitivity to DNA in innate immune mechanisms. Activation of the cGAS-STIN signaling pathway induces the production of interferon and inflammatory cytokines, resulting in immune responses, or inflammatory diseases. The intestinal tract is a vital organ for the body's nutrition absorption, recent studies have had various points of view on the job of cGAS-STING pathway in various intestinal sicknesses. Therefore, understanding its role and mechanism in the intestinal environment can help to develop new strategies for the treatment of intestinal diseases. This article examines the mechanism of the cGAS-STING pathway and its function in inflammatory bowel disease, intestinal cancer, and long-injury ischemia-reperfusion, lists the current medications that target it for the treatment of intestinal diseases, and discusses the impact of intestinal flora on this signaling pathway, to offer a theoretical and scientific foundation for upcoming targeted therapies for intestinal disorders via the cGAS-STING pathway.

The pseudokinase domain in receptor guanylyl cyclases

Cyclic GMP is produced by enzymes called guanylyl cyclases, of which the membrane-associated forms contain an intracellular pseudokinase domain that allosterically regulates the C-terminal guanylyl cyclase domain. Ligand binding to the extracellular domain of these single transmembrane-spanning domain receptors elicits an increase in cGMP levels in the cell. The pseudokinase domain (or kinase-homology domain) in these receptors appears to be critical for ligand-mediated activation. While the pseudokinase domain does not possess kinase activity, biochemical evidence indicates that the domain can bind ATP and thereby allosterically regulate the catalytic activity of these receptors. The pseudokinase domain also appears to be the site of interaction of regulatory proteins, as seen in the retinal guanylyl cyclases that are involved in visual signal transduction. In the absence of structural information on the pseudokinase-guanylyl cyclase domain organization of any member of this family of receptors, biochemical evidence has provided clues to the physical interaction of the pseudokinase and guanylyl cyclase domain. An α-helical linker region between the pseudokinase domain and the guanylyl cyclase domain regulates the basal activity of these receptors in the absence of a stimulatory ligand and is important for stabilizing the structure of the pseudokinase domain that can bind ATP. Here, we present an overview of salient features of ATP-mediated regulation of receptor guanylyl cyclases and describe biochemical approaches that allow a clearer understanding of the intricate interplay between the pseudokinase domain and catalytic domain in these proteins.

The pseudokinase domains of guanylyl cyclase-A and -B allosterically increase the affinity of their catalytic domains for substrate

Natriuretic peptides regulate multiple physiologic systems by activating transmembrane receptors containing intracellular guanylyl cyclase domains, such as GC-A and GC-B, also known as Npr1 and Npr2, respectively. Both enzymes contain an intracellular, phosphorylated pseudokinase domain (PKD) critical for activation of the C-terminal cGMP-synthesizing guanylyl cyclase domain. Because ATP allosterically activates GC-A and GC-B, we investigated how ATP binding to the PKD influenced guanylyl cyclase activity. Molecular modeling indicated that all the residues of the ATP-binding site of the prototypical kinase PKA, except the catalytic aspartate, are conserved in the PKDs of GC-A and GC-B. Kinase-inactivating alanine substitutions for the invariant lysine in subdomain II or the aspartate in the DYG-loop of GC-A and GC-B failed to decrease enzyme phosphate content, consistent with the PKDs lacking kinase activity. In contrast, both mutations reduced enzyme activation by blocking the ability of ATP to decrease the Michaelis constant without affecting peptide-dependent activation. The analogous lysine-to-alanine substitution in a glutamate-substituted phosphomimetic mutant form of GC-B also reduced enzyme activity, consistent with ATP stimulating guanylyl cyclase activity through an allosteric, phosphorylation-independent mechanism. Mutations designed to rigidify the conserved regulatory or catalytic spines within the PKDs increased guanylyl cyclase activity, increased sensitivity to natriuretic peptide, or reduced the Michaelis constant in the absence of ATP, consistent with ATP binding stabilizing the PKD in a conformation analogous to that of catalytically active kinases. We conclude that allosteric mechanisms evolutionarily conserved in the PKDs promote the catalytic activation of transmembrane guanylyl cyclases.

Structural and functional characterization of receptor kinases with nucleotide cyclase activity

There has been an increase in the identification and characterization of plant receptor kinases possessing nucleotide cyclase activity. This has necessitated the development of robust methodologies for the structural and functional characterization of this biologically important family of proteins. Here we outline some of the techniques that can be effectively used in the characterization of this bifunctional family of proteins.

Guanylyl cyclases A and B are asymmetric dimers that are allosterically activated by ATP binding to the catalytic domain

It is not known how natriuretic peptides and adenosine triphosphate (ATP) activate guanylyl cyclase A (GC-A) and GC-B, which generate the second messenger cyclic guanosine monophosphate. We determined that natriuretic peptides increased the maximum rate of these enzymes >10-fold in a positive cooperative manner in the absence of ATP. In the absence of natriuretic peptides, ATP shifted substrate-velocity profiles from cooperative to linear but did not increase the affinity of GCs for the substrate guanosine triphosphate (GTP) since the Michaelis constant was unchanged. However, in the presence of natriuretic peptides, ATP competed with GTP for binding to an allosteric site, which enhanced the activation of GCs by decreasing the Michaelis constant. Thus, natriuretic peptide binding was required for communication of the allosteric activation signal to the catalytic site. The ability of ATP to activate GCs decreased and enzyme potency (a measure of sensitivity to stimulation) increased with increasing GTP concentrations. Point mutations in the purine-binding site of the catalytic domain abolished GC activity but not allosteric activation. Coexpression of inactive mutants produced half the activity expected for symmetric catalytic dimers. 2'-Deoxy-ATP and 2'-deoxy-GTP were poor allosteric activators, but 2'-deoxy-GTP was an effective substrate, consistent with distinct binding requirements for the allosteric and catalytic sites. We conclude that membrane GC domains are asymmetric homodimers with distinct and reciprocally regulated catalytic and allosteric sites that bind to GTP and ATP, respectively. These data define a new membrane GC activation model and provide evidence of a previously unidentified GC drug interaction site.

Regulation and therapeutic targeting of peptide-activated receptor guanylyl cyclases

Cyclic GMP is a ubiquitous second messenger that regulates a wide array of physiologic processes such as blood pressure, long bone growth, intestinal fluid secretion, phototransduction and lipolysis. Soluble and single-membrane-spanning enzymes called guanylyl cyclases (GC) synthesize cGMP. In humans, the latter group consists of GC-A, GC-B, GC-C, GC-E and GC-F, which are also known as NPR-A, NPR-B, StaR, Ret1-GC and Ret2-GC, respectively. Membrane GCs are activated by peptide ligands such as atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), C-type natriuretic peptide (CNP), guanylin, uroguanylin, heat stable enterotoxin and GC-activating proteins. Nesiritide and carperitide are clinically approved peptide-based drugs that activate GC-A. CD-NP is an experimental heart failure drug that primarily activates GC-B but also activates GC-A at high concentrations and is resistant to degradation. Inactivating mutations in GC-B cause acromesomelic dysplasia type Maroteaux dwarfism and chromosomal mutations that increase CNP concentrations are associated with Marfanoid-like skeletal overgrowth. Pump-based CNP infusions increase skeletal growth in a mouse model of the most common type of human dwarfism, which supports CNP/GC-B-based therapies for short stature diseases. Linaclotide is a peptide activator of GC-C that stimulates intestinal motility and is in late-stage clinical trials for the treatment of chronic constipation. This review discusses the discovery of cGMP, guanylyl cyclases, the general characteristics and therapeutic applications of GC-A, GC-B and GC-C, and emphasizes the regulation of transmembrane guanylyl cyclases by phosphorylation and ATP.

Ca2+ signaling by plant Arabidopsis thaliana Pep peptides depends on AtPepR1, a receptor with guanylyl cyclase activity, and cGMP-activated Ca2+ channels

A family of peptide signaling molecules (AtPeps) and their plasma membrane receptor AtPepR1 are known to act in pathogen-defense signaling cascades in plants. Little is currently known about the molecular mechanisms that link these signaling peptides and their receptor, a leucine-rich repeat receptor-like kinase, to downstream pathogen-defense responses. We identify some cellular activities of these molecules that provide the context for a model for their action in signaling cascades. AtPeps activate plasma membrane inwardly conducting Ca(2+) permeable channels in mesophyll cells, resulting in cytosolic Ca(2+) elevation. This activity is dependent on their receptor as well as a cyclic nucleotide-gated channel (CNGC2). We also show that the leucine-rich repeat receptor-like kinase receptor AtPepR1 has guanylyl cyclase activity, generating cGMP from GTP, and that cGMP can activate CNGC2-dependent cytosolic Ca(2+) elevation. AtPep-dependent expression of pathogen-defense genes (PDF1.2, MPK3, and WRKY33) is mediated by the Ca(2+) signaling pathway associated with AtPep peptides and their receptor. The work presented here indicates that extracellular AtPeps, which can act as danger-associated molecular patterns, signal by interaction with their receptor, AtPepR1, a plasma membrane protein that can generate cGMP. Downstream from AtPep and AtPepR1 in a signaling cascade, the cGMP-activated channel CNGC2 is involved in AtPep- and AtPepR1-dependent inward Ca(2+) conductance and resulting cytosolic Ca(2+) elevation. The signaling cascade initiated by AtPeps leads to expression of pathogen-defense genes in a Ca(2+)-dependent manner.

Neuronal differentiation of NG108-15 cells has impact on nitric oxide- and membrane (natriuretic peptide receptor-A) cyclic GMP-generating proteins

Cyclic GMP (cGMP), produced in response to either nitric oxide (NO) or certain peptides, controls important neuronal functions. NG108-15 cells were used to characterize the expression of NO- and cGMP-generating proteins and to identify potential alterations associated with neuronal differentiation (neurite outgrowth). We find that these cells contain exclusively neuronal NO synthase (nNOS) isoforms as well as both NO- (soluble guanylyl cyclase, sGC) and natriuretic peptide- (natriuretic peptide receptor-A, NPR-A) responsive cGMP-producing enzymes. The sGC beta(1) subunit (unlike protein phosphatase 2A subunits) is highly membrane-associated. Membrane concentrations of NPR-A and nNOS, but not sGC beta(1) protein are up-regulated with neuronal differentiation. Intriguingly, the rate of hormone-induced cGMP production by NPR-A is significantly diminished in differentiated cells. These findings support roles for NPR-A, the common receptor of atrial (ANP) and B-type (BNP) natriuretic peptide in mature neurons and provide evidence for pronounced changes in neuronal submembrane cGMP signalling during neuronal differentiation.

Homologous desensitization of guanylyl cyclase A, the receptor for atrial natriuretic peptide, is associated with a complex phosphorylation pattern

Atrial natriuretic peptide (ANP), via its guanylyl cyclase A (GC-A) receptor and intracellular guanosine 3',5'-cyclic monophosphate production, is critically involved in the regulation of blood pressure. In patients with chronic heart failure, the plasma levels of ANP are increased, but the cardiovascular actions are severely blunted, indicating a receptor or postreceptor defect. Studies on metabolically labelled GC-A-overexpressing cells have indicated that GC-A is extensively phosphorylated, and that ANP-induced homologous desensitization of GC-A correlates with receptor dephosphorylation, a mechanism which might contribute to a loss of function in vivo. In this study, tandem MS analysis of the GC-A receptor, expressed in the human embryonic kidney cell line HEK293, revealed unambiguously that the intracellular domain of the receptor is phosphorylated at multiple residues: Ser487, Ser497, Thr500, Ser502, Ser506, Ser510 and Thr513. MS quantification based on multiple reaction monitoring demonstrated that ANP-provoked desensitization was accompanied by a complex pattern of receptor phosphorylation and dephosphorylation. The population of completely phosphorylated GC-A was diminished. However, intriguingly, the phosphorylation of GC-A at Ser487 was selectively enhanced after exposure to ANP. The functional relevance of this observation was analysed by site-directed mutagenesis. The substitution of Ser487 by glutamate (which mimics phosphorylation) blunted the activation of the GC-A receptor by ANP, but prevented further desensitization. Our data corroborate previous studies suggesting that the responsiveness of GC-A to ANP is regulated by phosphorylation. However, in addition to the dephosphorylation of the previously postulated sites (Ser497, Thr500, Ser502, Ser506, Ser510), homologous desensitization seems to involve the phosphorylation of GC-A at Ser487, a newly identified site of phosphorylation. The identification and further characterization of the specific mechanisms involved in the downregulation of GC-A responsiveness to ANP may have important pathophysiological implications.