Expression of the Receptor Guanylyl Cyclase C and Its Ligands in Reproductive Tissues of the Rat: A Potential Role for a Novel Signaling Pathway in the Epididymis1

Abstract

Guanylyl cyclase C (GC-C) is a membrane-associated form of guanylyl cyclase and serves as the receptor for the heat-stable enterotoxin (ST) peptide and endogenous ligands guanylin, uroguanylin, and lymphoguanylin. The major site of expression of GC-C is the intestinal epithelial cell, although GC-C is also expressed in extraintestinal tissue such as the kidney, airway epithelium, perinatal liver, stomach, brain, and adrenal glands. Binding of ligands to GC-C leads to accumulation of intracellular cGMP, the activation of protein kinases G and A, and phosphorylation of the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel that regulates salt and water secretion. We examined the expression of GC-C and its ligands in various tissues of the reproductive tract of the rat. Using reverse transcriptase and the polymerase chain reaction, we demonstrated the presence of GC-C, uroguanylin, and guanylin mRNA in both male and female reproductive organs. Western blot analysis using a monoclonal antibody to GC-C revealed the presence of differentially glycosylated forms of GC-C in the caput and cauda epididymis. Exogenous addition of uroguanylin to minced epididymal tissue resulted in cGMP accumulation, suggesting an autocrine or endocrine activation of GC-C in this tissue. Immunohistochemical analyses demonstrated expression of GC-C in the tubular epithelial cells of both the caput epididymis and cauda epididymis. Our results suggest that the GC-C signaling pathway could converge on CFTR in the epididymis and perhaps control fluid and ion balance for optimal sperm maturation and storage in this tissue.

The rat kidney contains high levels of prouroguanylin (the uroguanylin precursor) but does not express GC-C (the enteric uroguanylin receptor)

“…This biochemical evidence for enteric production of Ugn has since been corroborated and extended by numerous Northern blot studies showing high and relatively selective expression of preproUgn mRNA in the intestine (2, 11, 19, 45, 49, 55-57, 67, 88). However, several of these Northern blot studies have also noted the presence of very low levels of preproUgn mRNA in the kidney, an observation that has been consistently confirmed by more sensitive RT-PCR assays (5,13,36,50). Here, we show that these low levels of renal mRNA are associated with an unexpectedly high abundance of proUgn polypeptide.…”

Characterization of Immunological Cross-Reactivity between Enterotoxigenic Escherichia coli Heat-Stable Toxin and Human Guanylin and Uroguanylin

“…Several studies support the view postulated by Forte (16) that uroguanylin participates in an endocrine axis connecting the gastrointestinal tract with the kidneys in the regulation of body sodium balance (23,25,26). In addition, the GC-C receptor is expressed in the epididymis (27) and brain (28). Since guanylin, uroguanylin, and their cognate receptor GC-C are present in several body compartments, potential autoimmune reactions that interfere with GC-C-mediated signaling may have serious consequences.…”

Uroguanylin, an Intestinal Natriuretic Peptide, Is Delivered to the Kidney as an Unprocessed Propeptide

“…Somewhat surprisingly, this fundamental principle has not yet been established. Although Northern blot studies have shown that the small intestine is by far the major site for pre-proUGn mRNA expression (10,22,23), detectable amounts of pre-proUGn mRNA and/or immunoreactive UGn-like polypeptides have been reported in a variety of other tissues, including the kidney itself, as well as the colon, heart, stomach, lung, pancreas, brain, testis, ovary, oviduct, epididymis, prostate, parotid gland, submandibular gland, nasal mucosa, and gall bladder (10,(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32). One caveat to these studies is that the methods used have either been indirect (RNA based) or nonquantitative (immunocytochemical), and, where antibodies have been used, they have, for the most part, been unable to discriminate between UGn and proUGn.…”