Molecular architecture of secretin receptors: the specific covalent labelling of a 51 kDa peptide after cross-linking of [125I]iodosecretin to intact rat pancreatic acini

Secretin: Should we revisit its metabolic outcomes?

Metabolic pathologies such as Type 2 Diabetes have become a major health problem for worldwide populations. Unfortunately, efforts to cure and especially to prevent these significant global problems have so far been met with disappointment. Recently, the involvement of the gut-derived hormonal dysregulation in the development of obesity-related disturbances has been intensively studied. For instance, studies of gut-derived peptides such as peptide YY 3-36, glucagon-like peptide-1, oxyntomodulin and, more recently, ghrelin have significantly improved our understanding of mechanisms underlying weight and metabolic regulation. Even though early reports of the existence of secretin, the first peptide hormone to be described, date back as far as 1825, so much and yet so little is still known about its physiological role in mammals, including humans. However, recent years have provided a better understanding of how the release of secretin is regulated by enteral secretagogues. On the other hand, most basic questions about its role in the post-prandial regulation of metabolic functions in normal and pathophysiological conditions remain to be elucidated. The present work intends to review the physiology of secretin along with its central and peripheral outcomes on metabolic functions.

Role of N-linked glycosylation on the function and expression of the human secretin receptor

Secretin is a 27-amino acid long peptide hormone that regulates pancreatic water, bicarbonate, enzymes, and potassium ion secretion. The human secretin receptor (hSR) is a glycoprotein consisting of 440 amino acids, of which there are 5 putative N-linked glycosylation sites at positions Asn72, Asn100, Asn106, Asn128 (N-terminal ectodomain), and Asn291 (second exoloop). Through functional analysis of the hSR-transfected cells cultured in the presence of various glycosylation inhibitors, it was found that tunicamycin and castanospermine were able to significantly reduce the secretin-stimulated cAMP response. On the other hand, the effects of other inhibitors, swainsonine and deoxymannojirimycin, were much lower, suggesting that the high mannose-type carbohydrate side-chain is essential to the expression of a fully functional hSR. The role of individual N-linked glycosylation sites was studied by mutation analysis (Asn to Leu or Ser to Ala) coupled to measurements of cAMP accumulation and extracellular acidification rate. The ED50 values of the wild-type receptor in these two assay systems were 0.25 and 0.11 nM, respectively, and mutation at position 100, 106, or 291 did not affect either the ED50 values or the maximal responses in the two assays. However, the Asn72Leu and Ser74Ala mutations reduced the maximal responses and increased the ED50 values in both assays, suggesting that this site is a true glycosylation signal. This hypothesis was further supported by competitive binding studies, the same mutants were found to be defective in binding with [125I]secretin. To evaluate whether the change in receptor function of the mutants is caused by the change in the process of presenting the receptor to the cell surface, the mutants and the wild-type receptor were tagged with a c-Myc epitope at the C-termini. Using an anti-c-Myc monoclonal antibody and confocal microscopy, all of the mutant receptors were found to be expressed and delivered to the plasma membrane.

Cellular distribution of secretin receptor expression in rat pancreas

Secretin is an important regulator of pancreatic function, but the molecular basis of its actions is not well understood. We have, therefore, used in situ autoradiography, photoaffinity labeling, and RNase protection assays with healthy rat pancreas, dispersed acinar cells, and pancreas depleted of acinar cells to explore the cellular distribution and molecular identity of high-affinity secretin receptors in this complex organ. The autoradiographic examination of 125I-labeled [Tyr10]rat secretin-27 binding to normal pancreas demonstrated saturable and specific high-affinity binding sites on both acinar and duct cells, with a uniform lobular distribution, but with no binding above background over islets or vascular structures. Photoaffinity labeling demonstrated that the ductular binding site in acinar cell-depleted copper-deficient rat pancreas represented the same glycoprotein with a molecular weight of 50,000-62,000 that was present on acinar cells. RNase protection assays confirmed the molecular identity of the secretin receptors expressed on these distinct cells. The apparent absence or extreme low density of similar secretin receptors on islets and pancreatic vascular structures suggests that the pharmacological effects of secretin on those cells may either be indirect or mediated by another secretin family receptor that recognizes this hormone with lower affinity.

Intrinsic photoaffinity labeling of native and recombinant rat pancreatic secretin receptors

BACKGROUND

Structural characterization of pancreatic secretin receptors has been limited by difficulties in generating suitable radioligands and obtaining sufficient substrate. The aims of this study were to design, synthesize, and characterize high affinity radiolabeled analogues of secretin suitable for "intrinsic" photoaffinity labeling and to clone, express, and characterize the recombinant rat pancreatic secretin receptor.

METHODS

The ability of synthetic analogues to stimulate amylase secretion by pancreatic acini was studied. Receptor complementary DNA (cDNA) was cloned by screening a rat pancreatic library with a probe based on the sequence of a neural cell secretin-binding protein. Competition binding and affinity labeling were performed with membranes prepared from rat pancreas and transfected cells.

RESULTS

Two probes were fully efficacious secretagogues, which bound in a specific, high-affinity, rapid, and temperature-dependent manner. Only ([125I]Tyr10, pNO2-Phe22) rat secretin 27 covalently labeled a 50,000-62,000-molecular weight pancreatic membrane protein, with labeling inhibited in a concentration-dependent manner by secretin but not vasoactive intestinal polypeptide. Hybridization screening yielded a full-length cDNA identical to the neural clone. Photoaffinity labeling of this recombinant receptor identified a 57,000-62,000-molecular weight protein with specificity similar to that of native pancreas. Both native and recombinant receptors migrated at a molecular weight of 42,000 after endoglycosidase F deglycosylation.

CONCLUSIONS

This study provides evidence for the molecular identity of the pancreatic secretin receptor and presents a novel probe important in structural characterization of its agonist-binding domain.