Effects of long-acting analogues of lamprey GLP-1 and paddlefish glucagon on alpha- to beta-cell transdifferentiation in an insulin-deficient transgenic mouse model

Alpha-cells and therapy of diabetes: Inhibition, antagonism or death?

Absolute or relative hyperglucagonaemia is a characteristic of both Type 1 and Type 2 diabetes, resulting in fasting hyperglycaemia due in part to increased hepatic glucose production and lack of postprandial suppression of circulating glucagon concentrations. Consequently, therapeutics that target glucagon secretion or biological action may be effective antidiabetic agents. In this regard, specific glucagon receptor (GCGR) antagonists have been developed that exhibit impressive glucose-lowering actions, but unfortunately may cause off-target adverse effects in humans. Further to this, several currently approved antidiabetic agents, including GLP-1 mimetics, DPP-4 inhibitors, metformin, sulphonylureas and pramlintide likely exert part of their glucose homeostatic actions through direct or indirect inhibition of GCGR signalling. In addition to agents that inhibit the release of glucagon, compounds that enhance the transdifferentiation of glucagon secreting alpha-cells towards an insulin positive beta-cell phenotype could also help curb excess glucagon secretion in diabetes. Use of alpha-cell toxins represents another possible strategy to address hyperglucagonaemia in diabetes. In that respect, research from the 1920 s with diguanides such as synthalin A demonstrated effective glucose-lowering with alpha-cell ablation in both animal models and humans with diabetes. However, further clinical use of synthalin A was curtailed due its adverse effects and the increased availability of insulin. Overall, these observations with therapeutics that directly target alpha-cells, or GCGR signaling, highlight a largely untapped potential for diabetes therapy that merits further detailed consideration.

Dual-agonist incretin peptides from fish with potential for obesity-related Type 2 diabetes therapy - A review

The long-acting glucagon-like peptide-1 receptor (GLP1R) agonist, semaglutide and the unimolecular glucose-dependent insulinotropic polypeptide receptor (GIPR)/GLP1R dual-agonist, tirzepatide have been successfully introduced as therapeutic options for patients with Type-2 diabetes (T2DM) and obesity. Proglucagon-derived peptides from phylogenetically ancient fish act as naturally occurring dual agonists at the GLP1R and the glucagon receptor (GCGR) with lamprey GLP-1 and paddlefish glucagon being the most potent and effective in stimulating insulin release from BRIN-BD11 clonal β-cells. These peptides were also the most effective in lowering blood glucose and elevating plasma insulin concentrations when administered intraperitoneally to overnight-fasted mice together with a glucose load. Zebrafish GIP acts as a dual agonist at the GIPR and GLP1R receptors. Studies with the high fat-fed mouse, an animal model with obesity, impaired glucose-tolerance and insulin-resistance, have shown that twice-daily administration of the long-acting analogs [D-Ala²]palmitoyl-lamprey GLP-1 and [D-Ser²]palmitoyl-paddlefish glucagon over 21 days improves glucose tolerance and insulin sensitivity. This was associated with β-cell proliferation, protection of β-cells against apoptosis, decreased pancreatic glucagon content, improved lipid profile, reduced food intake and selective alteration in the expression of genes involved in β-cell stimulus-secretion coupling. In insulin-deficient Glu^CreERT2;ROSA26-eYFP transgenic mice, the peptides promoted an increase in β-cell mass with positive effects on transdifferentiation of glucagon-producing to insulin-producing cells. Naturally occurring fish dual agonist peptides, particularly lamprey GLP-1 and paddlefish glucagon, provide templates for development into therapeutic agents for obesity-related T2DM.