Guanylyl Cyclase C Regulation And Pathophysiology

Abstract

The survival of the any living organism depends on its availability to communicate, and a breakdown of cellular signaling can have dire consequences such as uncontrolled cellular proliferations or even cell death. Environmental cues or ligands are perceived by cognate receptors, expressed primarily on the cell surface, and transmitted to the interior of the cell to elicit a response. This universal phenomenon is termed as signal transduction. During this process, second messengers such as cyclic nucleotides, cAMP and cGMP, are produced which serve to amplify the signal. Cyclic GMP is emerging as a universal second messenger, and is found in both prokaryotes and eukaryotes. It is synthesized from GTP by the action of guanylyl cyclases. Vertebrate guanylyl cyclases are of two forms, soluble and membrane-associated. Soluble guanylyl cyclases are heterodimeric enzymes which are activated by nitic oxide. Membrane-associated guanylyl cyclases on the other hand are homodimeric enzymes that act as receptors for divers polypeptide ligands. In mammals, there are seven isoforms of receptors guanylyl cyclase, GC-A through GC-G. These recptors have a highly conseved modular domin organization with an N-terminal extracellular domain, a single transmembrane domain and a C- terminal intra cellular regions. The intracellular region contains a juxtamembrane domain followed by a protein-kinase domain, a linker region and a catalytic guanylyl cyclase domain. The coordinated actions of these domains ensure fine tuned-regulations of the catalytic domain. Guanylyl cyclase-C (GC-C) is a member of the membrane-bound guanylyl cyclases. GC-C is predominantely present in the intestine, on the apical surface of epithelial cells, but has also been detected in the rat epididymis. In the intestine it serves as the guanylin, uroguanylin and lymphoguanylin which are the endogenous peptide ligands, while heat- stable entrotoxins (ST) peptides secreted by enterotoxigenic E.coil, are exogenic ligands. Activation of GC-C by these ligands results in an increase in intracellular cGMP levels, which then activates cGMP-dependent protein kinase and cross-activates protein kinase A. In turn, these activated kinases phosphorylate and active the cystic fibrosis transmembrane conductance regulator (CFTR), resulting in chloride and water secretion into the intestine lumen, thus regulating salt and water homeostasis in the intestine. ST peptide has a greater affinity for GC-C than the endogenous ligands and activation of GC-C by ST results in masiive fluid and ion efflux from the intestine cells from which manifests as Travelers’ Diarrhea. The GC-C mediated cGMP signal transduction pathway also maintains intestinal crypt-villus axis homeostatis by exerting a cytostatic effect on the epithelial cells, there by regulating their turn over. Over the years multiple mechanisms of regulation of GC-C activity has been identified including allosteric controlled by various domains in the receptor and phosphorylation-mediated regulation of guanylyl cyclase activity. The current study describes aspects of the regulation of GC-C by gycosylation, and also reports the molecular phenotypes of a naturally occurring mutation in GC-C causes sever diarrhea in affected individuals. GC-C is expressed as a differentially glycosylated protein (130k Da and 145kDa). While both forms bind with equal affinity, only one the 145 kDa form is activated by its ligands. It is this higher glycosylated form which is selectively downregulated in the process of decensitization of GC-C in colomn carcinoma cells (Caco2). Give the critical role gycosylation plays in protein folding, trafficking, receptor activity and mediating protein inter actions, its influence on GC-C was analysed.