Acute and chronic effects of incretin signalling on islet cell energy metabolism

Abstract

Gut incretin hormones GLP-1 and GIP exert an acute glucose-dependent insulinotropic effect, mediated predominantly via the cAMP signalling in pancreatic islet β-cells. The targets of cAMP are believed to amplify the secretion of insulin at the late stages of vesicle fusion, however, their impact on processing the glucose signal that triggers the early stages of the metabolic sensing, such as the increase in glycolytic flux, elevation of cytosolic ATP or Ca2+ influx, is less clear. Similarly, mysterious are the mechanisms of β-cell function adjustments, induced by chronic incretin signal, alongside its global cardioprotective and adipocytic roles. This thesis examines the fundamental role of incretins in the cAMP-dependent regulation of energy metabolism in pancreatic islet β-cells on the acute and chronic timescales. To that end, the impact of the incretin signalling on the key metabolic steps in mouse and human islet β-cells was uncovered by real-time imaging of the energy fluxes (glycolysis, ATP production) and intracellular signals (cytosolic Ca2+ and cAMP) using a palette of genetically encoded sensors. This approach was complemented by probing the incretin impact on islet cell gene expression (via q-PCR), cell biology (immunocytochemistry) and in vitro insulin secretion (radioimmunoassay).

To demonstrate an upregulation of the metabolic fluxes by GLP-1 and GIP agonism on the acute timescale, consistent with the expression of the PfkFbp2 and PfkFbp3 isoforms of PFK2 in pancreatic islets. On the chronic timescale, however, the two peptides had opposing effects, with GLP-1 attenuating and GIP enhancing the energy fluxes. The GLP-1 functional profile closely matched that of glucagon, while GIP, dual GLP-1-GIP agonist tirzepatide and broad-spectrum phosphodiesterase inhibitor IBMX exerted pro-metabolic effects on both timescales. Notably, the effect of tirzepatide showed a higher dependence on the GIP-mediated component of the xv dual agonist. Furthermore, the incretin-induced cAMP signalling significantly impacted glycolytic flux in other islet cell populations, such as pancreatic α-cells. Finally, IR imaging of key glucose metabolites demonstrated two distinct subpopulations of islet cells, favouring glycolytic or mitochondrial metabolism, respectively.

Together, these findings underline the role of energy metabolism in mediating the regulatory effects of G protein receptor-coupled signalling. The metabolic paradigm is likely to be of the key importance for shaping the long-term effects of the incretin peptide signals, such as those observed under diabetes and obesity therapy with synthetic long-living polypeptides.

Date of Award	Jun 2024
Original language	English
Supervisor	Andrei Tarasov (Supervisor), Patrick Rorsman (Supervisor) & Peter Flatt (Supervisor)