Site-specific N-linked glycosylation of receptor guanylyl cyclase C regulates ligand binding, ligand-mediated activation and interaction with vesicular integral membrane protein 36, VIP36

Identification and functional analysis of a novel L-type lectin (NdLTL1) from Neocaridina denticulata sinensis

This study investigates an L-type lectin, NdLTL1, derived from Neocaridina denticulata sinensis, emphasizing its role in immune defense through carbohydrate binding and bacterial agglutination. Bioinformatics analysis identified 179 lectin sequences, leading to subsequent investigations into the structure and function of NdLTL1. The open reading frame (ORF) of NdLTL1 spans 966 bp and encodes a protein consisting of 321 amino acids (36.25 kDa), which features a signal peptide, a transmembrane domain and Lectin_leg-like domain. Three-dimensional modeling revealed three antiparallel β-sheets characteristic of Lectin_leg-like domain, confirming evolutionary links with proteins such as VIP36. Protein-carbohydrate and protein-protein interaction studies showed that NdLTL1 binds to both carbohydrates like N-acetylglucosamine, peptidoglycan, lipopolysaccharides (LPS), and mannose, as well as sorting proteins (COPI/COPII). Gene expression analyses indicated that NdLTL1 exhibits the highest expression levels in cardiac tissues and significant upregulation in gills following exposure to Vibrio parahaemolyticus. Recombinant NdLTL1 expressed in Escherichia coli was shown to bind multiple bacterial strains and exhibit calcium-dependent agglutination properties. Enzyme-linked immunosorbent assay (ELISA) confirmed concentration-dependent carbohydrate binding, particularly rapid for LPS. In vitro experiments suggested that recombinant NdLTL1 may promote bacterial growth under nutrient-limited conditions while potentially triggering immune defenses indirectly.

Rhesus Macaque Killer Cell Ig-like Receptor Domain 0 Glycans Impact Surface Expression and Ligand Specificity

Defining the MHC class I ligands of rhesus macaque killer cell Ig-like receptors (KIRs) is fundamental to NK cell biology in this species as a model for infectious diseases and comparative immunogenetics. Several rhesus macaque KIRs belong to a phylogenetically distinct group with a three-amino acid deletion in domain 0 (D0). This deletion results in polymorphic differences in potential N-linked glycosylation (PNG) sites adjacent to a predicted KIR-MHC class I contact site. Whereas most KIRs have two tandem PNG sites in D0 (N36FTN39FT), the KIRs containing the deletion only have a single site in this region (N36FT). To discern the contribution of glycosylation to KIR expression and ligand recognition, we constructed PNG mutants for six lineage II KIR genes that eliminate or create sites for N-glycan addition at these locations. The impact of these mutations on total and surface expression was determined by immunoblotting and flow cytometry. Ligand engagement was assessed by coincubating reporter cell lines bearing chimeric KIR-CD3ζ receptors with target cells expressing individual MHC class I molecules and were corroborated by staining with KIR IgG-Fc fusion proteins. We found that N36FT is glycosylated in KIR with a single site, and at least one site is glycosylated in KIRs with two tandem sites. In general, for rhesus KIRs with a single D0 glycosylation site, that site contributes to surface expression. For KIRs with two tandem sites, the first site can contribute to ligand specificity. This study establishes that D0 glycosylation of rhesus macaque KIRs modulates surface expression and contributes to ligand specificity.

The evolutionary divergence of receptor guanylyl cyclase C has implications for preclinical models for receptor-directed therapeutics

Mutations in receptor guanylyl cyclase C (GC-C) cause severe gastrointestinal disease, including meconium ileus, early onset acute diarrhea, and pediatric inflammatory bowel disease that continues into adulthood. Agonists of GC-C are US Food and Drug Administration-approved drugs for the treatment of constipation and irritable bowel syndrome. Therapeutic strategies targeting GC-C are tested in preclinical mouse models, assuming that murine GC-C mimics human GC-C in its biochemical properties and downstream signaling events. Here, we reveal important differences in ligand-binding affinity and GC activity between mouse GC-C and human GC-C. We generated a series of chimeric constructs of various domains of human and mouse GC-C to show that the extracellular domain of mouse GC-C contributed to log-orders lower affinity of mouse GC-C for ligands than human GC-C. Further, the Vmax of the murine GC domain was lower than that of human GC-C, and allosteric regulation of the receptor by ATP binding to the intracellular kinase-homology domain also differed. These altered properties are reflected in the high concentrations of ligands required to elicit signaling responses in the mouse gut in preclinical models and the specificity of a GC inhibitor towards human GC-C. Therefore, our studies identify considerations in using the murine model to test molecules for therapeutic purposes that work as either agonists or antagonists of GC-C, and vaccines for the bacterial heat-stable enterotoxin that causes watery diarrhea in humans.

N-Glycosylation can selectively block or foster different receptor-ligand binding modes

While DNA encodes protein structure, glycans provide a complementary layer of information to protein function. As a prime example of the significance of glycans, the ability of the cell surface receptor CD44 to bind its ligand, hyaluronan, is modulated by N-glycosylation. However, the details of this modulation remain unclear. Based on atomistic simulations and NMR, we provide evidence that CD44 has multiple distinct binding sites for hyaluronan, and that N-glycosylation modulates their respective roles. We find that non-glycosylated CD44 favors the canonical sub-micromolar binding site, while glycosylated CD44 binds hyaluronan with an entirely different micromolar binding site. Our findings show (for the first time) how glycosylation can alter receptor affinity by shielding specific regions of the host protein, thereby promoting weaker binding modes. The mechanism revealed in this work emphasizes the importance of glycosylation in protein function and poses a challenge for protein structure determination where glycosylation is usually neglected.

Guanylyl cyclase C as a biomarker for immunotherapies for the treatment of gastrointestinal malignancies

Gastrointestinal cancers encompass a diverse class of tumors arising in the GI tract, including esophagus, stomach, pancreas and colorectum. Collectively, gastrointestinal cancers compose a high fraction of all cancer deaths, highlighting an unmet need for novel and effective therapies. In this context, the transmembrane receptor guanylyl cyclase C (GUCY2C) has emerged as an attractive target for the prevention, detection and treatment of many gastrointestinal tumors. GUCY2C is an intestinally-restricted protein implicated in tumorigenesis that is universally expressed by primary and metastatic colorectal tumors as well as ectopically expressed by esophageal, gastric and pancreatic cancers. This review summarizes the current state of GUCY2C-targeted modalities in the management of gastrointestinal malignancies, with special focus on colorectal cancer, the most incident gastrointestinal malignancy.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

Mutational landscape of receptor guanylyl cyclase C: Functional analysis and disease-related mutations

Guanylyl cyclase C (GC-C) is the receptor for the heat-stable enterotoxin, which causes diarrhea, and the endogenous ligands, guanylin and uroguanylin. GC-C is predominantly expressed in the intestinal epithelium and regulates fluid and ion secretion in the gut. The receptor has a complex domain organization, and in the absence of structural information, mutational analysis provides clues to mechanisms of regulation of this protein. Here, we review the mutational landscape of this receptor that reveals regulatory features critical for its activity. We also summarize the available information on mutations in GC-C that have been reported in humans and contribute to severe gastrointestinal abnormalities. Since GC-C is also expressed in extra-intestinal tissues, it is likely that mutations thus far reported in humans may also affect other organ systems, warranting a close observation of these patients in future.

Development of an enterotoxigenic Escherichia coli vaccine based on the heat-stable toxin

Infection with enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrhea-related illness and death among children under 5 years of age in low– and middle-income countries (LMIC). Recent studies have found that it is the ETEC subtypes that produce the heat-stable enterotoxin (ST), irrespective of whether they also secrete the heat-labile enterotoxin (LT), which contribute most importantly to the disease burden in children from LMIC. Therefore, adding an ST toxoid would importantly complement ongoing ETEC vaccine development efforts. The ST’s potent toxicity, its structural similarity to the endogenous peptides guanylin and uroguanylin, and its poor immunogenicity have all complicated the advancement of ST-based vaccine development. Recent remarkable progress, however, including the unprecedented screening for optimal ST mutants, mapping of cross-reacting ST epitopes and improved ST-carrier coupling strategies (bioconjugation and genetic fusion), enables the rational design of safe, immunogenic, and well-defined ST-based vaccine candidates.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Congenital secretory diarrhoea caused by activating germline mutations in GUCY2C

OBJECTIVE

Congenital sodium diarrhoea (CSD) refers to a form of secretory diarrhoea with intrauterine onset and high faecal losses of sodium without congenital malformations. The molecular basis for CSD remains unknown. We clinically characterised a cohort of infants with CSD and set out to identify disease-causing mutations by genome-wide genetic testing.

DESIGN

We performed whole-exome sequencing and chromosomal microarray analyses in 4 unrelated patients, followed by confirmatory Sanger sequencing of the likely disease-causing mutations in patients and in their family members, followed by functional studies.

RESULTS

We identified novel de novo missense mutations in GUCY2C, the gene encoding receptor guanylate cyclase C (GC-C) in 4 patients with CSD. One patient developed severe, early-onset IBD and chronic arthritis at 4 years of age. GC-C is an intestinal brush border membrane-bound guanylate cyclase, which functions as receptor for guanylin, uroguanylin and Escherichia coli heat-stable enterotoxin. Mutations in GUCY2C were present in different intracellular domains of GC-C, and were activating mutations that enhanced intracellular cyclic guanosine monophosphate accumulation in a ligand-independent and ligand-stimulated manner, following heterologous expression in HEK293T cells.

CONCLUSIONS

Dominant gain-of-function GUCY2C mutations lead to elevated intracellular cyclic guanosine monophosphate levels and could explain the chronic diarrhoea as a result of decreased intestinal sodium and water absorption and increased chloride secretion. Thus, mutations in GUCY2C indicate a role for this receptor in the pathogenesis of sporadic CSD.

Molecular Physiology of Membrane Guanylyl Cyclase Receptors

cGMP controls many cellular functions ranging from growth, viability, and differentiation to contractility, secretion, and ion transport. The mammalian genome encodes seven transmembrane guanylyl cyclases (GCs), GC-A to GC-G, which mainly modulate submembrane cGMP microdomains. These GCs share a unique topology comprising an extracellular domain, a short transmembrane region, and an intracellular COOH-terminal catalytic (cGMP synthesizing) region. GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure/volume and energy balance. GC-B is activated by C-type natriuretic peptide, stimulating endochondral ossification in autocrine way. GC-C mediates the paracrine effects of guanylins on intestinal ion transport and epithelial turnover. GC-E and GC-F are expressed in photoreceptor cells of the retina, and their activation by intracellular Ca(2+)-regulated proteins is essential for vision. Finally, in the rodent system two olfactorial GCs, GC-D and GC-G, are activated by low concentrations of CO2and by peptidergic (guanylins) and nonpeptidergic odorants as well as by coolness, which has implications for social behaviors. In the past years advances in human and mouse genetics as well as the development of sensitive biosensors monitoring the spatiotemporal dynamics of cGMP in living cells have provided novel relevant information about this receptor family. This increased our understanding of the mechanisms of signal transduction, regulation, and (dys)function of the membrane GCs, clarified their relevance for genetic and acquired diseases and, importantly, has revealed novel targets for therapies. The present review aims to illustrate these different features of membrane GCs and the main open questions in this field.

N-linked sugar-regulated protein folding and quality control in the ER

Asparagine-linked glycans (N-glycans) are displayed on the majority of proteins synthesized in the endoplasmic reticulum (ER). Removal of the outermost glucose residue recruits the lectin chaperone malectin possibly involved in a first triage of defective polypeptides. Removal of a second glucose promotes engagement of folding and quality control machineries built around the ER lectin chaperones calnexin (CNX) and calreticulin (CRT) and including oxidoreductases and peptidyl-prolyl isomerases. Deprivation of the last glucose residue dictates the release of N-glycosylated polypeptides from the lectin chaperones. Correctly folded proteins are authorized to leave the ER. Non-native polypeptides are recognized by the ER quality control key player UDP-glucose glycoprotein glucosyltransferase 1 (UGT1), re-glucosylated and re-addressed to the CNX/CRT chaperone binding cycle to provide additional opportunity for the protein to fold in the ER. Failure to attain the native structure determines the selection of the misfolded polypeptides for proteasome-mediated degradation.

Proteomic analyses of courtship pheromones in the redback salamander, Plethodon cinereus

The evolutionary success of plethodontid salamanders for ~100 MY is due partly to the use of courtship pheromones that regulate female receptivity. In ~90 % of plethodontid species, males deliver pheromones by "scratching" a female's dorsum, where pheromones diffuse transdermally into the bloodstream. However, in a single clade, representing ~10 % of Plethodon spp., males apply pheromones to the female's nares for olfactory delivery. Molecular studies have identified three major pheromone families: Plethodontid Receptivity Factor (PRF), Plethodontid Modulating Factor (PMF), and Sodefrin Precursor-like Factor (SPF). SPF and PMF genes are relatively ancient and found in all plethodontid species; however, PRF is found exclusively in the genus Plethodon - which includes species with transdermal, olfactory, and intermediate delivery behaviors. While previous proteomic analyses suggested PRF and PMF are dominant in slapping species and SPF is dominant in non-Plethodon scratching species, it was unclear how protein expression of different pheromone components may vary across delivery modes within Plethodon. Therefore, the aim of this study was to proteomically characterize the pheromones of a key scratching species in this evolutionary transition, Plethodon cinereus. Using mass spectrometry-based techniques, our data support the functional replacement of SPF by PRF in Plethodon spp. and an increase in PMF gene duplication events in both lineage-dependent and delivery-dependent manners. Novel glycosylation was observed on P. cinereus PRFs, which may modulate the metabolism and/or mechanism of action for PRF in scratching species. Cumulatively, these molecular data suggest that the replacement of pheromone components (e.g., SPF by PRF) preceded the evolutionary transition of the functional complex from transdermal to olfactory delivery.

The multiple roles of guanylate cyclase C, a heat stable enterotoxin receptor

PURPOSE OF REVIEW

Guanylate cyclase C (GC-C) is a transmembrane receptor that is expressed primarily on intestinal epithelial cells. Activation of this receptor by its endogenous peptide ligands initiates cyclic guanosine monophosphate-dependent (cGMP) salt and water movement in the intestine. GC-C is targeted by the enterotoxigenic Escherichia coli heat-stable enterotoxin STa, which deregulates this pathway and causes secretory diarrhea. This review discusses current work on the physiological function of GC-C in the intestine.

RECENT FINDINGS

Familial GC-C mutations demonstrate that epithelial cGMP signaling is critical to electrolyte and fluid balance in the neonatal intestine. Chronic deregulation of GC-C activity in early life increases susceptibility to a number of disorders, including obstruction and inflammatory bowel disease. Murine models indicate that GC-C regulates the composition of intestinal commensal microflora and that it suppresses bacterial infection and modulates colonic injury and inflammation. Therapeutic GC-C ligands are used to successfully treat constipation-predominant irritable bowel syndrome and recent studies show that extracellular cGMP is an important mechanism of reducing abdominal pain associated with this disorder.

SUMMARY

Originally identified as a target of E. coli enterotoxin STa, GC-C is an important regulator of physiological salt and water homeostasis and may directly impact a wide range of intestinal disorders.

Cyclic nucleotide signaling in intestinal epithelia: getting to the gut of the matter

The intestine is the primary site of nutrient absorption, fluid-ion secretion, and home to trillions of symbiotic microbiota. The high turnover of the intestinal epithelia also renders it susceptible to neoplastic growth. These diverse processes are carefully regulated by an intricate signaling network. Among the myriad molecules involved in intestinal epithelial cell homeostasis are the second messengers, cyclic AMP (cAMP) and cyclic GMP (cGMP). These cyclic nucleotides are synthesized by nucleotidyl cyclases whose activities are regulated by extrinsic and intrinsic cues. Downstream effectors of cAMP and cGMP include protein kinases, cyclic nucleotide gated ion channels, and transcription factors, which modulate key processes such as ion-balance, immune response, and cell proliferation. The web of interaction involving the major signaling pathways of cAMP and cGMP in the intestinal epithelial cell, and possible cross-talk among the pathways, are highlighted in this review. Deregulation of these pathways occurs during infection by pathogens, intestinal inflammation, and cancer. Thus, an appreciation of the importance of cyclic nucleotide signaling in the intestine furthers our understanding of bowel disease, thereby aiding in the development of therapeutic approaches.