Frog skin host-defence peptides and their synthetic analogues with therapeutic potential for type 2 diabetes

Abstract

Amphibian skin secretions are a rich source of biomolecules (peptides, alkaloids and biogenic amines) which play a significant role in protecting the host from predators microbial attack. Peptides from frog skin secretions have shown antimicrobial activity against a wide range of microorganisms. Also, these peptides have been shown to possess anticancer, immunomodulatory and insulinotropic activities. In this thesis, peptides from skin secretions of frogs belonging to the family of Alytidae, Hylidae, Pipidae, and Ranidae were examined for insulinotropic and antidiabetic activities. Frenatin 2D and its synthetic analogues from Discoglossus sardus, [A14K] and [S4K] analogues of PGLa-AM1 and CPF-AM1 respectively from Xenopus amieti, temporin peptides from Rana temporaria and esculentin-1 from Rana esculenta demonstrated concentration-dependent insulinotropic activities in rat clonal pancreatic beta cells (BRIN-BD11) and human-derived pancreatic beta cells (1.1B4). Insulinotropic activities of the esculentin-1 peptides were associated with an increase in membrane potential and intracellular calcium, whereas frenatin 2D and temporin peptides had no effect on these parameters. In BRIN-BD11 cells, frenatin 2D peptides produced a significant increase in cAMP production and its insulin-releasing activity was abolished in PKA downregulated cells. In addition to their insulinotropic activities, these peptides protected BRIN-BD11 cells against cytokine-induced apoptosis as well as stimulated proliferation of beta-cell. Frenatin 2D and its synthetic analogues [D1W] and [G7W], temporin G and esculentin (1-21)1c improved blood glucose and increased insulin concentration in lean mice.
In genetically obese-diabetic mice (db/db), frenatin 2D, [A14K] PGLa-AM1 and [S4K] CPF-AM1 improved blood glucose, insulin sensistivity, insulin secretory responses of islets to glucose and established insulin secretagogues, lipid profile and both kidney and liver function. The gradual demise of beta cells and a decrease of circulating insulin in db/db mice was delayed significantly by these peptides. Also, genes involved in both insulin signalling and secretion were improved.
In conclusion, this thesis highlights the potential of frog skin peptides belonging to the family of Alytidae, Hylidae, Pipidae and Ranidae for further development into therapeutic agents for type 2 diabetes. 

Date of Award	Apr 2019
Original language	English
Supervisor	Yasser Abdel-Wahab (Supervisor), Peter Flatt (Supervisor) & John Michael Conlon (Supervisor)