Interaction of Gila monster venom with secretin receptors in rat pancreatic membranes

Signaling mechanisms of secretin receptor

Secretin, a 27-amino acid gastrointestinal peptide, was initially discovered based on its activities in stimulating pancreatic juice. In the past 20 years, secretin was demonstrated to exhibit pleiotropic functions in many different tissues and more importantly, its role as a neuropeptide was substantiated. To carry out its activities in the central nervous system and in peripheral organs, secretin interacts specifically with one known receptor. Secretin receptor, a member of guanine nucleotide-binding protein (G protein)-coupled receptor (GPCR) in the secretin/VIP/glucagon subfamily, possesses the characteristics of GPCR with seven conserved transmembrane domains, a relatively large amino-terminal extracellular domain and an intracellular carboxyl terminus. The structural features and signal transduction pathways of the secretin receptor in various tissues are reviewed in this article.

Characterization of the VIP receptor from SUP T1 lymphoblasts

The SUP T1 lymphoblasts express an original subtype of VIP receptors characterized by a high affinity for the VIP analogue from lizard venom named helodermin, a preference for the neuropeptide PACAP-38 over PACAP-27 and VIP, and an extremely low affinity for secretin. The molecular cloning of that receptor revealed its identity with the VIP2 receptor subtype first cloned in rat and mouse tissues. The access to selective probes permits the detection of the mRNA coding for the VIP2 receptor by Northern blot, reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. These highly selective and sensitive techniques identify the cell types that are equipped to synthesize the receptor but do not prove that the receptor is indeed efficiently expressed at the cell surface. VIP2 mRNA was detected in selected areas of the brain different from that expressing the classical VIP1 receptor, in pituitary, in pineal, in pancreatic islets, in testes and ovary. It was also detected in the stomach, in the thymus and in spleen and in T lymphoblastic cell lines. A systematic screening of the immunocompetent cells must still be performed.

Isolation and characterization of helothermine, a novel toxin from Heloderma horridum horridum (Mexican beaded lizard) venom

A protein toxic to mice was isolated from the venom of the Mexican beaded lizard Heloderma horridum horridum by a combination of gel filtration (Sephadex G-75) and ion exchange chromatography (both diethylaminoethyl-cellulose [DE-cellulose] and carboxymethyl-cellulose [CM-cellulose]). The purified polypeptide component has an apparent mol. wt of 25,500 and is composed of approximately 220 amino acid residues. It has an isoelectric point (pI) of 6.8 and its N-terminal amino acid sequence was shown to be: Glu-Ala-Ser-Pro-Lys-Leu-Pro-Gly-Leu-Met-Thr-Ser-Asn-Pro-Asp-Gln-Gln-Thr- Glu-Ile. The sequence has no significant similarity with any other protein previously reported in the literature. Enzymatic activities such as phospholipase, hyaluronidase and proteinase, commonly present in venoms, could not be demonstrated in this protein. Patch-clamp experiments conducted with excitable membranes show no effects on Na+, K+ or Ca2+ ion channels. Among the constant physiological effects observed in mice injected with this toxin are lethargy, partial paralysis of rear limbs and lowering of body temperature, suggesting that it might be a hypothermic toxin. We propose calling this toxin Helothermine.

Helodermin-like peptides in thyroid C cells: stimulation of thyroid hormone secretion and suppression of calcium incorporation into bone

Helodermin is a vasoactive intestinal peptide-like peptide in the salivary gland venom of the lizard Heloderma suspectum. Helodermin-like immunofluorescence was observed in the parafollicular (C) cells in several mammals and in the C cell homologues of the chicken ultimobranchial gland. Thus, helodermin-like peptides coexist with calcitonin. The results of radioimmunoassay agreed with the immunocytochemical findings. HPLC of rat thyroid extracts revealed one major peak of helodermin-like immunoreactivity, which eluted in a position close to that of lizard helodermin. Helodermin stimulated basal thyroid hormone secretion and colloid droplet formation in conscious mice. The effect of large doses of helodermin was quite long-lasting and the maximal response occurred after 2-6 hr. In addition, helodermin suppressed the incorporation of calcium into bone in conscious rats. The findings suggest that helodermin-like peptides in C cells may be involved in the local regulation of thyroid hormone secretion and in the maintenance of calcium homeostasis.

Chemical, immunological and biological properties of peptides like vasoactive-intestinal-peptide and peptide-histidine-isoleucinamide extracted from the venom of two lizards (Heloderma horridum and Heloderma suspectum)

Having previously isolated helodermin, the major peptide like vasoactive-intestinal-peptide and peptide-histidine-isoleucinamide, from the venom of the lizard Heloderma suspectum, we decided on a systematic exploration of all (VIP-PHI)-like peptides present in the venom of another lizard of the Helodermatidae family: Heloderma horridum. Six (VIP-PHI)-like peptides (PHH1 to 6) were purified to homogeneity from the venom of the lizard H. horridum with PHH3 and PHH4 representing two minor forms. All peptides cross-reacted in radioimmunoassays for helodermin and PHI but not for VIP. They yielded four fragments (T1 to T4) after trypsin digestion. T1, T2 and T3 showed the same retention time by reverse-phase HPLC and the same amino acid composition; the differences were confined to T4, the C-terminal sequence. PHH5 and PHH6 were found to be identical to synthetic helospectins I and II respectively. PHH1 and PHH3 probably resulted from a secondary modification of PHH5, while PHH2 and PHH4 derived from PHH6. Thus, the VIP-like peptides, previously called helospectins, are in fact typical of H. horridum venom. We confirmed that helodermin is the major (VIP-PHI)-like peptide of the venom of H. suspectum and observed its absence in H. horridum venom. Also, we found that positions 8 and 9 of helodermin are occupied by two Glu residues instead of two Gln as previously published. Helospectin-like material was also present in H. suspectum venom but in very small amount. In both venoms all VIP-like peptides were equally potent and efficient when tested for (a) their ability to occupy VIP as well as secretin receptors in rat pancreatic membranes and VIP receptors in rat liver membranes, and (b) the ensuing activation of adenylate cyclase in both membrane preparations.

Helodermin has a VIP-like effect upon canine blood flow

The effect of helodermin on vascular physiology was studied in anesthetized dogs using a synthetic replicate of helodermin and helodermin related peptides. Intraarterial infusion of helodermin caused a dose-dependent increase in femoral blood flow. Helodermin was 16 times less potent than VIP and 5 times more potent than PHM (human PHI). The helodermin effect lasted significantly longer; the half-life of the helodermin effect was 6.5 times longer than VIP. Synthetic helodermin (Hd) N-terminal fragment Hd(1-27)NH2 retained substantial activity similar to the full helodermin molecule but the prolonged effect was lost. Hd(7-35) and Hd(22-35) were inactive in this system. Intravenous injection of synthetic helodermin produced prolonged systemic hypotension and tachycardia; and, similar to VIP, it increased the common carotid arterial blood flow while those of the superior mesenteric and femoral arteries were decreased. The results demonstrate the VIP-like vasodilating activity and cardiovascular effects of helodermin in anesthetized dogs.

The receptors of the VIP family peptides (VIP, secretin, GRF, PHI, PHM, GIP, glucagon and oxyntomodulin). Specificities and identity

A model is proposed for the receptors of the VIP family peptides including a ligand and a cellular domain. Specificities of the receptors are due to different ligand binding sites. Three subgroups of the family can be distinguished accordingly: glucagon and oxyntomodulin; GIP; VIP, secretin r and hGRF, PHI and PHM. In the same species, the expression of these different sites is cell-specific resulting in a stoichiometry of the ligand-receptor interaction which is compatible with physiological regulation of cell function. Specificities of the interaction as studied by native and synthetic analogs is supported both by restricted sequences of amino acids (such as that including the N-terminal histidine residue), and membrane-induced configuration of the ligand. Identity of the receptors is related to their interactions with subunits of the adenylate cyclase system. Arguments are put forward indicating that the alpha subunit of the guanyl regulatory protein is a reasonable candidate for directly transducing to the adenylyl cyclase the information contained in the activated ligand-binding site subunits. Evidence of functional and molecular heterogeneity of the recognizing site and of the alpha subunits leads to the supposition that some types of specific complementarity is retained at this level of interaction, further enhancing the possibility of species and cell differences. On the other hand, the identities found in other sequences of the alpha and ras oncogene products extend to the receptor of the VIP family peptides a pattern of organization which is similar to that recently described for the insulin family of receptors. The role of ligand specific receptor mediated regulation in homologous or heterologous desensitization is reviewed in brief for the peptides of the VIP family as well as the appearance of the specific receptor during the ontogenesis or the cell differentiation. The co-distribution of plasma membrane receptors from other families further adds to the cell specificity resulting for each differentiated cell in unique patterns of recognizing site. Some examples of receptor-receptor interaction are given, indicating that the integration of the different signals by cells might occur at an early step through the transmembranair domain of the receptor.

Presence of helodermin-like peptides of the VIP-secretin family in mammalian salivary glands and saliva

Helodermin is a biologically active peptide isolated from the venom of the Gila monster lizard (Heloderma suspectum) whose structure is related to that of vasoactive intestinal peptide and secretin. Using a specific radioimmunoassay based on antisera prepared by immunizing rabbits with natural helodermin, we demonstrated the presence of helodermin-like material in mammalian salivary glands, including parotid, submaxillary and sublingual glands from rat and dog, and parotid and submaxillary glands from man. All helodermin-like materials had an apparent molecular mass of 4-12 kDa. Dog saliva, collected after pilocarpine stimulation, revealed similar immunoreactivity with a major component around 6 kDa.

Immunoreactive helodermin-like peptides in rat: a new class of mammalian neuropeptides related to secretin and VIP

Helodermin is a peptide from the venom of the lizard Heloderma suspectum (Gila Monster) showing a high degree of sequence similarity with VIP, PHI and secretin in its N-terminal moiety. The present data support the presence of peptide(s) closely related to helodermin in the brain, gut and salivary glands of rat. In our radioimmunoassays, we routinely used one of the three specific antisera obtained from rabbits that were immunized against lizard helodermin coupled to bovine serum albumin with carbodiimide. Heat- and acid-stable immunoreactive helodermin-like material was more abundant in striatum, hippocampus and anterior pituitary than in cerebral cortex and hypothalamus. High levels of helodermin-like material were also present in salivary glands, duodenum and jejunum. When submitted to gel permeation chromatography on a TSK-G 2000 SW column, the apparent molecular radius of most of the immunoreactive material ranged from 6 to 12 KDa.

Primary structure of helodermin, a VIP-secretin-like peptide isolated from Gila monster venom

The complete amino acid sequence of helodermin isolated from the venom of Gila monster was elucidated. The peptide was shown to be a basic pentatriacontapeptide amide: His-Ser-Asp-Ala-Ile-Phe-Thr-Gln-Gln-Tyr-Ser-Lys-Leu-Leu-Ala-Lys-Leu-Ala- Leu-Gln-Lys- Tyr-Leu-Ala-Ser-Ile-Leu-Gly-Ser-Arg-Thr-Ser-Pro-Pro-Pro-NH2. A high degree of sequence similarities to secretin/VIP/PHI/(PHM)/GRF from mammal and bird was observed over the entire N-terminal 1-27 sequence. In particular, the amino acid residues in positions 3, 6 and 7 were found to be common to 9 peptides of the family. Another interesting feature of the structure of helodermin was its C-terminal -Pro-Pro-Pro-NH2 sequence. Isolation of helodermin was the first demonstration of the existence of a secretin/VIP-related peptide in an animal that is neither mammal nor bird.