Biological activities and antidiabetic potential of novel peptides from frog secretions
The utilisation of natural products as drugs for the treatment of diseases and injuries has developed for thousands of years, but the current pharmaceutical system is predominately occupied by chemical-based drugs, while natural products are rarely applied. Since the widespread of antibiotic resistance and prevalence of chronic and malignant diseases (such as diabetes and cancers), there has been a significant rise of interest in discovering novel drug candidates with potent efficiency in treating these diseases. The amphibian skin, a special morphological and physiological organ, acts as the first line of defence against physical stimuli and lethal invasions. It is a rich source of pharmacological alternatives, some of which possess the potential to be developed in medical applications. In this study, both genomics and proteomics techniques have been used to study the frog skin secretions from the Burmeister’s leaf frog, Phyllomedusa burmeisteri; the hylid frog, Phyllomedusa sauvagei and the Chinese tiger frog, Hoplobatrachus rugulosus. More specifically, the molecular cloning strategy was performed to isolate target mRNAs, establish corresponding cDNA libraries and eventually clone unique peptide precursor-encoding cDNA sequences. Also, the mature peptides were analysed and confirmed via RP-HPLC, MALDI-TOF mass spectrometry and MS/MS fragmentation sequencing. After the peptide primary structure characterisation, synthetic replicates of each peptide were purified before applied for further secondary structural analysis and biofunctional assessments. The secondary structure conformations were determined by circular dichroism, and the functional analysis in this study includes the antimicrobial assay, MTT assay, haemolysis assay, insulinotropic assay and GLP-1 releasing assay. This study reports the isolation, identification and characterisation of one phylloseptin peptide Phylloseptin-PBu, one dermaseptin peptide Dermaseptin-PS1 and four tigerinin peptides (Tigerinin-HR1 to Tigerinin-HR4), they exhibited distinct biological activities. In Chapter 3, the detailed function and correlative mechanisms of novel phylloseptin peptide Phylloseptin-PBu were described. We showed that Phylloseptin-PBu could induce a dose-dependent insulinotropic activity in BRIN-BD11 cells, which is regulated by ATP-sensitive potassium channel depolarisation triggered extracellular calcium influx and GLP-1 receptor-initiated PKA signalling activation. In Chapter 4, the novel Dermaseptin-PS1 was demonstrated to possess anticancer capability in U251MG cells through the induction of intrinsic apoptosis at low concentrations (10-6M), but disrupting the cell membranes at high concentrations (≥10-5 M). In Chapter 5, GLP-1 release activity was observed after the treatment of peptide concentration gradient of four novel tigerinin peptides (Tigerinin-HR1 to Tigerinin-HR4) in STC-1 cells. These discoveries enriched the foundational knowledge of biological peptides from these three-frog species and might contribute to future drug development. This thesis includes six chapters, the Chapter 1 and Chapter 2 are general introduction and general materials and methods, respectively; the Chapter 3 to Chapter 5 are specific experimental chapters; and the Chapter 6 is general discussion chapter.
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