Receptor Guanylyl Cyclase C and Cyclic GMP in Health and Disease: Perspectives and Therapeutic Opportunities

Abstract

Receptor Guanylyl Cyclase C (GC-C) was initially characterized as an important regulator of intestinal fluid and ion homeostasis. Recent findings demonstrate that GC-C is also causally linked to intestinal inflammation, dysbiosis, and tumorigenesis. These advances have been fueled in part by identifying mutations or changes in gene expression in GC-C or its ligands, that disrupt the delicate balance of intracellular cGMP levels and are associated with a wide range of clinical phenotypes. In this review, we highlight aspects of the current knowledge of the GC-C signaling pathway in homeostasis and disease, emphasizing recent advances in the field. The review summarizes extra gastrointestinal functions for GC-C signaling, such as appetite control, energy expenditure, visceral nociception, and behavioral processes. Recent research has expanded the homeostatic role of GC-C and implicated it in regulating the ion-microbiome-immune axis, which acts as a mechanistic driver in inflammatory bowel disease. The development of transgenic and knockout mouse models allowed for in-depth studies of GC-C and its relationship to whole-animal physiology. A deeper understanding of the various aspects of GC-C biology and their relationships with pathologies such as inflammatory bowel disease, colorectal cancer, and obesity can be leveraged to devise novel therapeutics.

Keywords: cGMP (cyclic GMP); colorectal cancer type; guanylyl cyclase C; guanylyl cyclase C agonists; intestine.

Figure 1

Canonical and emerging roles of GC-C in intestinal homeostasis. The conventional roles of GC-C in the regulation of intestinal fluid ion homeostasis and associated human pathologies are well established. Homozygous and compound heterozygous loss of function mutations in GC-C cause meconium ileus due to decreased fluid and ion secretion. Gain of function mutations in GC-C cause congenital secretory diarrhea due to increased fluid and ion secretion. The emerging roles of the GC-C/cGMP signaling axis in the pathogenesis of several human diseases, most notably colorectal cancer and inflammatory bowel disease, are becoming evident. Loss of GC-C/cGMP signaling because of prominent downregulation of guanylin and uroguanylin is associated with tumorigenesis. Gain of GC-C/cGMP signaling upregulates interferon-stimulated genes and STAT1 activation in intestinal tissue, leading to chronic inflammation and inflammatory bowel disease. Impaired gut barrier integrity and dysbiosis of the microbiome associated with loss (or gain) of GC-C mediated signaling may be linked to inflammatory bowel disease (or colorectal cancer) are shown in dotted lines. The figure was prepared using Biorender.

Figure 2

Schematic representation of the domain architecture of GC-C. GC-C is predicted to be a homodimeric multidomain protein that includes an extracellular domain that binds peptide ligands, a transmembrane domain, a juxta-membrane domain, a kinase homology domain that binds ATP, a linker region, a guanylyl cyclase domain that forms a head-to-tail dimer, that converts GTP to cGMP, and a C-terminal domain. The domain boundaries of human GC-C are shown in the linear schematic on the right of the domain architecture, with a single letter amino acid code at each position. Numbers in brackets represent the number of amino acids within the predicted domain boundaries. The figure was prepared using Biorender.

Figure 3

The GC-C/cGMP signaling axis and fluid-ion homeostasis in the intestine. Binding of ligands (heat-stable enterotoxin/ST) produced by enterotoxigenic E.coli and the endogenous hormones guanylin and uroguanylin) to GC-C catalyzes the formation of cGMP from GTP. Increased intracellular cGMP levels result in the activation of cGMP-dependent protein kinase II (PKGII) and the inhibition of cAMP-specific phosphodiesterase (PDE3), which in turn leads to the cross-activation of cAMP-dependent protein kinase (PKA). Reduced intestinal sodium absorption is caused by PKGII-mediated inhibitory phosphorylation of NHE3. PKGII and PKA phosphorylated and activate the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, increasing intestinal chloride and water secretion. Elevated intracellular cGMP increases duodenal bicarbonate secretion via CFTR and perhaps additional unknown mechanisms. Increased cGMP activates cyclic nucleotide-gated ion channels (CNG), promoting Ca²⁺-influx, which recruits calcium-sensing G-protein coupled receptors (CaR) to the plasma membrane. PDE5, a cGMP-dependent phosphodiesterase, and PDE10 hydrolyze cGMP to 5’ GMP to attenuate GC-C signaling. Pharmacological GC-C agonists (e.g., linaclotide) and cGMP-specific phosphodiesterase PDE5 antagonists (e.g., sildenafil citrate) increase intracellular cGMP levels suggesting, that they may have a synergistic antiproliferative effect and reduce the likelihood of resistance to both drugs in colorectal cancer. The figure was prepared using Biorender.

Figure 4

Signaling pathways of the GC-C/cGMP axis that regulate cellular proliferation. Ligand-mediated activation of GC-C increases intracellular cGMP. Cyclic GMP production activates PKGII and p38 MAPK resulting in phosphorylation of the Sp1 transcription factor. Sp1 upregulates the expression of p21 and mediates cytostasis. PKGII-mediated signaling opposes pro-survival and pro-proliferative phenotypes mediated by the β-catenin/TCF and Akt pathways. Increased GMP activates cyclic nucleotide-gated ion channels (CNG), promoting Ca²⁺-influx that mediates cytostasis and recruiting calcium-sensing G-protein coupled receptors (CaR) to the plasma membrane. TGs denotes target genes. The figure was prepared using Biorender.

Figure 5

Summary of the multiple biological functions of GC-C/cGMP signaling in health and disease. The illustration depicts the role of GC-C/cGMP signaling in the intestine and extraintestinal tissues, with key functions highlighted. The figure was prepared using Biorender.