Esculentin-2CHa(1-30) and its analogues: stability and mechanisms of insulinotropic action

Genetically engineered long-acting Esculentin-2CHa(1-30) fusion protein with potential applicability for the treatment of NAFLD

Esculentin-2CHa(1-30) (‟ESC") has been reported as a potent anti-diabetic peptide with little toxicity. However, its very short plasma residence time severely limits the therapeutic efficacy. To address this issue, we genetically engineered a fusion protein of tandem trimeric ESC with an albumin binding domain (ABD) and a fusion partner, SUMO (named ‟SUMO-3×ESC-ABD"). The SUMO-3×ESC-ABD, successfully produced from E. coli, showed low cellular and hemolytic toxicity while displaying potent activities for the amelioration of hyperglycemia as well as non-alcoholic fatty liver disease (NAFLD) in vitro. In animal studies, the estimated plasma half-life of SUMO-3×ESC-ABD was markedly longer (427-fold) than that of the ESC peptide. In virtue of the extended plasma residence, the SUMO-3×ESC-ABD could produce significant anti-hyperglycemic effects that lasted for >2 days, while both the ESC or ESC-ABD peptides elicited little effects. Further, twice-weekly treatment for 10 weeks, the SUMO-3×ESC-ABD displayed significant improvement in blood glucose control with a reduction in body weight. Most importantly, a significant improvement in the conditions of NAFLD was observed in the SUMO-3×ESC-ABD-treated mice. Along the systemic effects (by improved glucose tolerance and body weight reduction), direct inhibition of the hepatocyte lipid uptake was suggested as the major mechanism of the anti-NAFLD effects. Overall, this study demonstrated the utility of the long-acting SUMO-3×ESC-ABD as a potent drug candidate for the treatment of NAFLD.

Venom Peptides, Polyphenols and Alkaloids: Are They the Next Antidiabetics That Will Preserve β-Cell Mass and Function in Type 2 Diabetes?

Improvement of insulin secretion by pancreatic β-cells and preservation of their mass are the current challenges that future antidiabetic drugs should meet for achieving efficient and long-term glycemic control in patients with type 2 diabetes (T2D). The successful development of glucagon-like peptide 1 (GLP-1) analogues, derived from the saliva of a lizard from the Helodermatidae family, has provided the proof of concept that antidiabetic drugs directly targeting pancreatic β-cells can emerge from venomous animals. The literature reporting on the antidiabetic effects of medicinal plants suggests that they contain some promising active substances such as polyphenols and alkaloids, which could be active as insulin secretagogues and β-cell protectors. In this review, we discuss the potential of several polyphenols, alkaloids and venom peptides from snake, frogs, scorpions and cone snails. These molecules could contribute to the development of new efficient antidiabetic medicines targeting β-cells, which would tackle the progression of the disease.

Truncated analog Brevinin2-CE-N26V5K: Revelation the Augmentation of Antimicrobial Activity

Brevinin2-CE (B2CE), a natural peptide containing 37 amino acids, was first isolated from the skin secretions of the Chinese forest frog Rana chensinensis. B2CE shows good antibacterial activity. In this study, a series of B2CE analogs with differences in cationicity, α-helicity, hydrophobicity and amphipathic properties were designed through chain-length deletion and amino acid substitution. The most potent, nontoxic analog, B2CE-N26V5K, was identified by examination of its antibacterial activity, hemolytic activity, and stability under physiological conditions. The increased cationicity, hydrophobicity and more obvious hydrophilic and hydrophobic surface of B2CE-N26-N16WA18KG23K did not improve the antibacterial activity but increased the hemolytic activity of this modified peptide. The helicity might promote antibacterial activity for brevinin-2 peptides, as the 15-aa analogs with lower helicity show decreased potency against different test bacteria (approximately 2- to 72-fold) compared to B2CE-N26V5K. Additionally, the results indicated that the "Rana box" does not affect the antimicrobial activity of brevinin-2 peptides, as B2CE, B2CE-nonDS and B2CE-C31-37 S have similar strong inhibitory effects on both gram-positive and gram-negative bacteria. However, the "Rana box" does affect the hemolytic activity, as the HC₅₀ values of the 3 peptides range from 25 ~ 130 µM. Furthermore, B2CE-N26V5K caused obvious morphological alterations of the bacterial surfaces, as shown by atomic force microscopy. Additionally, B2CE-N26V5K exhibited strong membrane-disrupting activity when examined using the LIVE/DEAD Bac Light Bacterial Viability Kit. Thus, the antibacterial effect of B2CE-N26V5K on gram-negative and gram-positive bacteria may be caused by cell membrane attack. In conclusion, the excellent candidate B2CE-N26V5K was obtained and has application prospects as a novel anti-infective agent.

Beneficial actions of esculentin-2CHa(GA30) on high sucrose-induced oxidative stress in Drosophila melanogaster

Hyperglycaemia-induced oxidative stress plays a critical role in the development of diabetes and its complications. This study investigated actions of esculentin-2CHa(GA30) on high sucrose-induced oxidative stress in adult Drosophila melanogaster. Adult flies were exposed to diets containing graded concentrations of sucrose in the presence or absence of esculentin-2CHa(GA30) (5.0-10 μmol/kg diet) for 7 days. Effects of high sucrose diet and/or esculentin-2CHa(GA30) on survival and longevity of flies, and markers of oxidative stress, antioxidant status and glucose were assessed. High-sucrose diet (15-30%) and esculentin-2CHa(GA30) (5-10 μmol/kg diet) enhanced the percentage of surviving flies by 33.5%-46.2% (P < 0.01) and 7.4%-26.9% (P < 0.01) respectively. Concentration-dependent reduction in total thiol (19.3-51.3%, P < 0.01), reduced glutathione (22.6-54.9%, P < 0.05-0.01), catalase activity (36.8-57.3%, P < 0.05-0.01) and elevated glucose concentration (1.8-2.9-fold, P < 0.001) were observed in high sucrose-fed flies. Esculentin-2CHa(GA30) alone did not affect levels of total thiol, reduced glutathione, glucose and catalase activity. Improved survival (1.2-1.3-fold, P < 0.05-0.01) and longevity (1.3-fold) were observed in flies treated with the peptide (5.0 and 7.5 μmol/kg diet). Feeding on sucrose and esculentin-2CHa(1-30) (5.0 and 7.0 μmol/kg diet) for 7 days increased total thiol (2 - 3-fold, P < 0.001) and reduced glutathione (1.6-1.8-fold, P < 0.05) levels. Reduced catalase activity (21.4-36.4%, P < 0.01) and reduced glucose level (38.6-49.4%, P < 0.01) were observed in peptide-treated flies. Esculentin-2CHa(1-30) inhibited sucrose-induced generation of hydrogen peroxide (7.5-13.7%, P < 0.05) and nitric oxide (22.3-42.9%, P < 0.01) in adult flies. Overall, findings from this study offered further insights into the anti-oxidative properties of esculentin-2CHa(GA30).

Antimicrobial Peptides and Proteins: From Nature's Reservoir to the Laboratory and Beyond

Rapid rise of antimicrobial resistance against conventional antimicrobials, resurgence of multidrug resistant microbes and the slowdown in the development of new classes of antimicrobials, necessitates the urgent development of alternate classes of therapeutic molecules. Antimicrobial peptides (AMPs) are small proteins present in different lifeforms in nature that provide defense against microbial infections. They have been effective components of the host defense system for a very long time. The fact that the development of resistance by the microbes against the AMPs is relatively slower or delayed compared to that against the conventional antibiotics, makes them prospective alternative therapeutics of the future. Several thousands of AMPs have been isolated from various natural sources like microorganisms, plants, insects, crustaceans, animals, humans, etc. to date. However, only a few of them have been translated commercially to the market so far. This is because of some inherent drawbacks of the naturally obtained AMPs like 1) short half-life owing to the susceptibility to protease degradation, 2) inactivity at physiological salt concentrations, 3) cytotoxicity to host cells, 4) lack of appropriate strategies for sustained and targeted delivery of the AMPs. This has led to a surge of interest in the development of synthetic AMPs which would retain or improve the antimicrobial potency along with circumventing the disadvantages of the natural analogs. The development of synthetic AMPs is inspired by natural designs and sequences and strengthened by the fusion with various synthetic elements. Generation of the synthetic designs are based on various strategies like sequence truncation, mutation, cyclization and introduction of unnatural amino acids and synthons. In this review, we have described some of the AMPs isolated from the vast repertoire of natural sources, and subsequently described the various synthetic designs that have been developed based on the templates of natural AMPs or from de novo design to make commercially viable therapeutics of the future. This review entails the journey of the AMPs from their natural sources to the laboratory.

Beneficial actions of the [A14K] analog of the frog skin peptide PGLa-AM1 in mice with obesity and degenerative diabetes: A mechanistic study

The antidiabetic actions of [A14K]PGLa-AM1, an analog of peptide glycine-leucine-amide-AM1 isolated from skin secretions of the octoploid frog Xenopus amieti, were investigated in genetically diabetic-obese db/db mice. Twice daily administration of [A14K]PGLa-AM1 (75 nmol/kg body weight) for 28 days significantly (P < 0.05) decreased circulating blood glucose and HbA1c and increased plasma insulin concentrations leading to improvements in glucose tolerance. The elevated levels of triglycerides, LDL and cholesterol associated with the db/db phenotype were significantly reduced by peptide administration. Elevated plasma alanine transaminase, aspartic acid transaminase, and alkaline phosphatase activities and creatinine concentrations were also significantly decreased. Peptide treatment increased pancreatic insulin content and improved the responses of isolated islets to established insulin secretagogues. No significant changes in islet β-cell and α-cell areas were observed in [A14K]PGLa-AM1 treated mice but the loss of large and medium-size islets was prevented. Peptide administration resulted in a significant (P < 0.01) increase in islet expression of the gene encoding Pdx-1, a major transcription factor in islet cells determining β-cell survival and function, resulting in increased expression of genes involved with insulin secretion (Abcc8, Kcnj11, Slc2a2, Cacn1c) together with the genes encoding the incretin receptors Glp1r and Gipr. In addition, the elevated expression of insulin signalling genes (Slc2a4, Insr, Irs1, Akt1, Pik3ca, Ppm1b) in skeletal muscle associated with the db/db phenotype was downregulated by peptide treatment These data suggest that the anti-diabetic properties of [A14K]PGLa-AM1 are mediated by molecular changes that enhance both the secretion and action of insulin.

Drug Delivery Strategies for Enhancing the Therapeutic Efficacy of Toxin-Derived Anti-Diabetic Peptides

Toxin peptides derived from the skin secretions of amphibians possess unique hypoglycemic activities. Many of these peptides share cationic and amphipathic structural similarities and appear to possess cell-penetrating abilities. The mechanism of their insulinotropic action is yet not elucidated, but they have shown great potential in regulating the blood glucose levels in animal models. Therefore, they have emerged as potential drug candidates as therapeutics for type 2 diabetes. Despite their anti-diabetic activity, there remain pharmaceutical challenges to be addressed for their clinical applications. Here, we present an overview of recent studies related to the toxin-derived anti-diabetic peptides derived from the skin secretions of amphibians. In the latter part, we introduce the bottleneck challenges for their delivery in vivo and general drug delivery strategies that may be applicable to extend their blood circulation time. We focus our research on the strategies that have been successfully applied to improve the plasma half-life of exendin-4, a clinically available toxin-derived anti-diabetic peptide drug.

Enhanced therapeutic index of an antimicrobial peptide in mice by increasing safety and activity against multidrug-resistant bacteria

The rising prevalence of antibiotic resistance underscores the urgent need for novel antimicrobial agents. Antimicrobial peptides (AMPs) are potentially effective therapeutics that disrupt bacterial membranes regardless of resistance to traditional antibiotics. We have developed engineered cationic AMPs (eCAPs) with broad activity against multidrug-resistant (MDR) bacteria, but stability remains an important concern. Therefore, we sought to enhance the clinical utility of eCAP WLBU2 in biological matrices relevant to respiratory infection. A designed substitution of d-Val for l-Val resulted in increased resistance to protease enzymatic degradation. We observed multiple gains of functions such as higher activity against bacteria in biofilm mode of growth, significantly lower toxicity to erythrocytes and white blood cells compared to WLBU2, with increased safety in mice. Direct airway delivery revealed a therapeutic index of >140 for the selected enantiomer compared to that of <35 for WLBU2. The data warrant clinical exploration by aerosolized delivery to mitigate MDR-related respiratory infection.

Antimicrobial Peptides: Amphibian Host Defense Peptides

Antimicrobial Peptides (AMPs) are one of the most common components of the innate immune system that protect multicellular organisms against microbial invasion. The vast majority of AMPs are isolated from the frog skin. Anuran (frogs and toads) skin contains abundant AMPs that can be developed therapeutically. Such peptides are a unique but diverse group of molecules. In general, more than 50% of the amino acid residues form the hydrophobic part of the molecule. Normally, there are no conserved structural motifs responsible for activity, although the vast majority of the AMPs are cationic due to the presence of multiple lysine residues; this cationicity has a close relationship with antibacterial activity. Notably, recent evidence suggests that synthesis of AMPs in frog skin may confer an advantage on a particular species, although they are not essential for survival. Frog skin AMPs exert potent activity against antibiotic-resistant bacteria, protozoa, yeasts, and fungi by permeating and destroying the plasma membrane and inactivating intracellular targets. Importantly, since they do not bind to a specific receptor, AMPs are less likely to induce resistance mechanisms. Currently, the best known amphibian AMPs are esculentins, brevinins, ranacyclins, ranatuerins, nigrocin-2, magainins, dermaseptins, bombinins, temporins, and japonicins-1 and -2, and palustrin-2. This review focuses on these frog skin AMPs and the mechanisms underlying their antimicrobial activity. We hope that this review will provide further information that will facilitate further study of AMPs and cast new light on novel and safer microbicides.

Immunomodulatory, insulinotropic, and cytotoxic activities of phylloseptins and plasticin-TR from the Trinidanian leaf frog Phyllomedusa trinitatis

The aim of the study was to determine the in vitro immunomodulatory, cytotoxic, and insulin-releasing activities of seven phylloseptin-TR peptides and plasticin-TR, first isolated from the frog Phyllomedusa trinitatis. The most cationic peptides, phylloseptin-1.1TR and phylloseptin-3.1TR, showed greatest cytotoxic potency against A549, MDA-MB231, and HT-29 human tumor-derived cells and against mouse erythrocytes. Phylloseptin-4TR was the most hydrophobic and the most effective peptide at inhibiting production of the proinflammatory cytokines TNF-α and IL-1β by mouse peritoneal cells but was without effect on production of the antiinflammatory cytokine IL-10. Phylloseptin-2.1TR and phylloseptin-3.3TR were the most effective at stimulating the production of IL-10. The noncytotoxic peptide, plasticin-TR, inhibited production of TNF-α and IL-1β but was without effect on IL-10 production. The results of CD spectroscopy suggest that the different properties of plasticin-TR compared with the immunostimulatory activities of the previously characterized plasticin-L1 from Leptodactylus laticeps may arise from greater ability of plasticin-TR to oligomerize and adopt a stable helical conformation in a membrane-mimetic environment. All peptides stimulated release of insulin from BRIN-BD11 rat clonal β cells with phylloseptin-3.2TR being the most potent and effective and phylloseptin-2.1TR the least effective suggesting that insulinotropic potency correlates inversely with helicity. The study has provided insight into structure-activity relationships among the phylloseptins. The combination of immunomodulatory and insulinotropic activities together with low cytotoxicity suggests that phylloseptin-3.3TR and plasticin-TR may represent templates for the development of agents for use in antiinflammatory and type 2 diabetes therapies.

Peptides from frog skin with potential for development into agents for Type 2 diabetes therapy

Several frog skin peptides, first identified as result of their antimicrobial or immunomodulatory activities, have subsequently been shown to stimulate insulin release both in vitro and in vivo and so show potential for development into incretin-based drugs for treatment of patients with Type 2 diabetes mellitus. However, their therapeutic potential as anti-diabetic agents is not confined to this activity as certain frog skin-derived peptides, such as magainin-AM2 and CPF-SE1 and analogs of hymenochirin-1B, tigerinin-1R, and esculentin-2CHa, have been shown to increase insulin sensitivity, promote β-cell proliferation, suppress pancreatic and circulating glucagon concentrations, improve the lipid profile, and selectively alter expression of genes involved in insulin secretion and action in mice with diet-induced obesity, insulin resistance and impaired glucose tolerance. This review assesses the therapeutic possibilities of peptides from frogs belonging to the Pipidae, Dicroglossidae, and Ranidae families, focusing upon work that has been carried out since 2014.

The Potential of Frog Skin-Derived Peptides for Development into Therapeutically-Valuable Immunomodulatory Agents

The aim of this article is to review the immunoregulatory actions of frog skin-derived peptides in order to assess their potential as candidates for immunomodulatory or anti-inflammatory therapy. Frog skin peptides with demonstrable immunomodulatory properties have been isolated from skin secretions of a range of species belonging to the families Alytidae, Ascaphidae, Discoglossidae, Leptodactylidae, Pipidae and Ranidae. Their effects upon production of inflammatory and immunoregulatory cytokines by target cells have been evaluated ex vivo and effects upon cytokine expression and immune cell activity have been studied in vivo by flow cytometry after injection into mice. The naturally-occurring peptides and/or their synthetic analogues show complex and variable actions on the production of proinflammatory (TNF-α, IL-1β, IL-12, IL-23, IL-8, IFN-γ and IL-17), pleiotropic (IL-4 and IL-6) and immunosuppressive (IL-10 and TGF-β) cytokines by peripheral and spleen cells, peritoneal cells and/or isolated macrophages. The effects of frenatin 2.1S include enhancement of the activation state and homing capacity of Th1-type lymphocytes and NK cells in the mouse peritoneal cavity, as well as the promotion of their tumoricidal capacities. Overall, the diverse effects of frog skin-derived peptides on the immune system indicate their potential for development into therapeutic agents.