Monotreme glucagon-like peptide-1 in venom and gut: one gene – two very different functions

Abstract

The importance of Glucagon like peptide 1 (GLP-1) for metabolic control and insulin release sparked the evolution of genes mimicking GLP-1 action in venomous species (e.g. Exendin-4 in Heloderma suspectum (gila monster)). We discovered that platypus and echidna express a single GLP-1 peptide in both intestine and venom. Specific changes in GLP-1 of monotreme mammals result in resistance to DPP-4 cleavage which is also observed in the GLP-1 like Exendin-4 expressed in Heloderma venom. Remarkably we discovered that monotremes evolved an alternative mechanism to degrade GLP-1. We also show that monotreme GLP-1 stimulates insulin release in cultured rodent islets, but surprisingly shows low receptor affinity and bias toward Erk signaling. We propose that these changes in monotreme GLP-1 are the result of conflicting function of this peptide in metabolic control and venom. This evolutionary path is fundamentally different from the generally accepted idea that conflicting functions in a single gene favour duplication and diversification, as is the case for Exendin-4 in gila monster. This provides novel insight into the remarkably different metabolic control mechanism and venom function in monotremes and an unique example of how different selective pressures act upon a single gene in the absence of gene duplication.

Variation in the Evolution and Sequences of Proglucagon and the Receptors for Proglucagon-Derived Peptides in Mammals

“…To allow comparison among the three receptors, this analysis was limited to the 25 ligand interacting (see Supplementary Tables 10-12 for sites) and 17 G-protein interacting sites that were shared by the GCGR and GLP1R sequences (64,65). For Gcgr, a larger number of species were found to have substitutions at ligand-interacting (64) sites than at G protein-interacting sites (22) (Supplementary Tables 10 and 13). For G-protein interacting sites, very few (0-2) substitutions were accepted in most species, with only two species (both from Afrotheria, E. edwardii (Cape elephant shrew) and O. afer (aardvark)) displaying three substitutions (Supplementary Tables 10 and 13).…”

“…A physiological role for glicentin has not clearly been identified (12), but see (13), while OXM is involved in the regulation of gastric acid release and other intestinal secretions and has roles in food intake and energy expenditure (10,11). Miniglucagon, a C-terminal secondary proteolytically processed form of glucagon (Glucagon [19][20][21][22][23][24][25][26][27][28][29], is antagonistic to many of the roles of glucagon (14). Other less studied peptides produced by the proteolytic processing of Gcg, including glicentin-related pancreatic polypeptide (GRPP), major proglucagon fragment (MPGF), and intervening peptide-1 and -2 (IP-1 and IP2), may also have roles in human physiology (1)(2)(3)(4)10).…”

“…Jawless fish (e.g., the lamprey Petromyzon marinus ) ( 18 ) and teleost fish (e.g., zebrafish ( Danio rerio ) and anglerfish ( Lophius americanus )) ( 16 , 19 , 20 ) are a pair of lineages that have duplicated Gcg genes, where in both lineages one of the duplicated genes has lost an exon that encodes a glucagon-like peptide (the GLP-1 encoding exon in lamprey and the GLP-2 encoding exon in teleost fish). In addition to these changes in the structure of the Gcg gene in diverse vertebrates, changes affecting the coding sequence that potentially alter hormone function have been identified ( 15 , 21 , 22 ). The glucagon hormones produced by New World rodents (i.e., suborder Hystricomorpha, which includes the guinea pig, Cavia porcellus ) have reduced biological activity and have sequences with increased numbers of amino acid substitutions ( 21 ).…”

“…The glucagon hormones produced by New World rodents (i.e., suborder Hystricomorpha, which includes the guinea pig, Cavia porcellus ) have reduced biological activity and have sequences with increased numbers of amino acid substitutions ( 21 ). The Gcg gene of the platypus ( Ornithorhynchus anatinus ), a member of the earliest diverging lineage of mammals (order Monotremata), encodes a GLP-1 7-37 sequence that has accumulated many amino acid substitutions ( 15 ) and was found to be a component of the venom produced by this species ( 22 ). Accelerated evolution of the GLP-2 sequence was inferred to have occurred on the early mammalian lineage, however the physiological consequence of this change remains unknown ( 15 ).…”

Insulin Release Mechanism Modulated by Toxins Isolated from Animal Venoms: From Basic Research to Drug Development Prospects

“…In addition to exendin-4, other Glp-1 analogues from Ornithorhynchus anatinus (platypus) and Tachyglossus aculeatus (echidna) could be good candidates for T2D [292]. These peptides show interesting biophysical characteristics, including resistance to DPP-4 cleavage in a similar way to exendin-4, at concentration of 100 nM, equivalent stimulation of the insulin secretion process and relative bias toward pERK1/2, which is involved in the activation of mitogenic signaling pathways, unlike human GLP-1 and exendin-4 with relative bias for cAMP and intracellular calcium mobilization [244]. These signaling differences generated by monotreme peptides could be another option for the development of new GLP-1 as anti-diabetic agents, such as several peptides and their analogs isolated from skin of frog, especially, the insulinotropic peptide “FSIP” isolated from the skin secretion of the frog Agalychnis litodryas , which is in phase 3 of clinical trials [293].…”