Metabolic and molecular characterization of novel insulinotropic peptides from skin secretions of frogs of the Pipidae family for the treatment of type 2 diabetes

Abstract

One of the natural defenses of amphibians is the production of molecules such as peptides, alkaloids and biogenic amines in their granular glands. Characterization of peptides isolated from the skin secretions of diverse amphibian species have shown that they possess multifunctional activities including antimicrobial and insulinotropic actions. In this thesis, the insulinotropic activities of peptides characterized from the frogs of the family Pipidae was investigated.

Peptides isolated from Hymenochirus boettgeri (hymenochirin-1B) and Xenopus amieti (CPF-AM1 and PGLa-AM1) together with their synthetic analogues stimulated insulin release from the clonal pancreatic cell line, BRIN-BD11 cells and isolated mouse islets in a dose dependent manner. Insulinotropic effects of the peptides revealed that amino acid substitution enhanced the insulin-releasing potency of the peptides. Insulinotropic effects of peptides from Hymenochirus boettgeri did not involve membrane depolarization and increase of intracellular Ca2+, while peptides from Xenopus amieti did. In the presence of [P5K] analogue of hymenochirin-1B and [A14K] analogue of PGLa-AM1 beta cell cAMP production was increased and downregulation of PKA pathway resulted in the abolishment of insulin-releasing activities of the peptides. [S4K] analogue of CPF-AM1 had no effect on cAMP production. Acute effects of [D9k] and [P5K] analogues of hymenochirin-1B, CPF-AM1, [S4K]CPF-AM1, [A14K]PGLaAM1 and [A20K]PGLa-AM1 from X. amieti on glucose tolerance revealed significant reduction in glycaemic excursion associated with increased insulin secretion in normal and diet-induced insulin resistant NIH Swiss mice.

Administration of [P5K]hymenochirin-1B, [S4K]CPF-AM1 and [A14K]PGLa-AM1 (75nmol/kg body weight) for 28 days to high fat fed mice significantly improved glucose tolerance, insulin sensitivity, lipid profile, islet morphology and secretory function of islets in response to glucose and secretagogues. A significant reduction in plasma and pancreatic glucagon level was also observed. [P5K]hymenochirin-1B, [S4K]CPF-AM1 and [A14K]PGLa-AM1 treatment also significantly up-regulated the expression of insulin signaling genes in the skeletal muscle of high fat fed mice.

These results suggest that peptides isolated from the skin secretions of frogs of the Pipidae family and their analogues may be useful as future antidiabetic drugs.

Thesis is embargoed until 31st December 2017

Date of Award	Dec 2015
Original language	English
Supervisor	Yasser Abdel-Wahab (Supervisor) & Peter Flatt (Supervisor)