Guanylyl cyclase C (GCC) is a transmembrane receptor implicated in the regulation of intestinal fluid homeostasis. Ligand binding to the luminal receptor domain, activates the intracellular guanylyl cyclase that catalyzes the formation of the second messenger guanosine-3$\sp\prime5\sp\prime$-monophosphate (cGMP). Accumulation of cGMP in the epithelium initiates a cascade of events that culminate in chloride flux, establishment of an osmotic gradient and movement of water into the intestinal lumen. Many pathogenic organisms have evolved specific toxins to induce diarrhea in their hosts to magnify their distribution into the environment. One of these secretory compounds is an E. coli heat-stable enterotoxin (STa) that binds GCC and activates its signalling cascade culminating in secretory diarrhea. Therefore, molecular mechanisms regulating GCC are potential targets for treatment of STa-induced secretory diarrhea.
Kinetic analysis of GCC in native and recombinant systems determined that 2-substituted adenine nucleotides (2-ATP) interact with an intracellular target to inhibit guanylyl cyclase activity by a non-competitive, guanosine-5$\sp\prime$-triphosphate (GTP)-dependent mechanism. High concentrations of GTP, in the absence of 2-ATP, could also inhibit GCC suggesting GTP acts downstream of 2-ATP.
This regulatory pathway was applied to prevent the STa signalling mechanism in intact intestinal epithelial cells and identify a target to prevent intestinal diarrhea. Following conversion to 2-ATP, 2-substituted nucleosides activated the inhibitory pathway and prevented STa-induced cGMP synthesis and electrolyte flux at the guanylyl cyclase step. These studies identify a molecular pathway regulating the receptor guanylyl cyclases, characterize its mechanism and apply it to prevent the molecular process responsible for secretory diarrhea.
