Phase Analysis of Metabolic Oscillations and Membrane Potential in Pancreatic Islet β -Cells

Abstract

Metabolism in islet β-cells displays oscillations that can trigger pulses of electrical activity and insulin secretion. There has been a decades-long debate among islet biologists about whether metabolic oscillations are intrinsic or occur in response to oscillations in intracellular Ca(2+) that result from bursting electrical activity. In this article, the dynamics of oscillatory metabolism were investigated using five different optical reporters. Reporter activity was measured simultaneously with membrane potential bursting to determine the phase relationships between the metabolic oscillations and electrical activity. Our experimental findings suggest that Ca(2+) entry into β-cells stimulates the rate of mitochondrial metabolism, accounting for the depletion of glycolytic intermediates during each oscillatory burst. We also performed Ca(2+) clamp tests in which we clamped membrane potential with the KATP channel-opener diazoxide and KCl to fix Ca(2+) at an elevated level. These tests confirm that metabolic oscillations do not require Ca(2+) oscillations, but show that Ca(2+) plays a larger role in shaping metabolic oscillations than previously suspected. A dynamical picture of the mechanisms of oscillations emerged that requires the restructuring of contemporary mathematical β-cell models, including our own dual oscillator model. In the companion article, we modified our model to account for these new data.

Decreased Consumption of Branched-Chain Amino Acids Improves Metabolic Health

“…We isolated pancreatic islets from mice on the Control, Low BCAA and Low AA diets, and examined ex vivo glucose stimulated insulin secretion as well as the metabolic and Ca 2+ oscillations that drive secretion (Merrins et al, 2016; Merrins et al, 2013; Truchan et al, 2015). Islets isolated from the mice fed Low BCAA and Low AA diets secreted significantly less insulin per islet than islets from the mice fed Control diet (Figure 3A), while the islet insulin content was also decreased by both diets, significantly in the case of the Low AA diet (Figure 3B).…”

“…Islets isolated from the mice fed Low BCAA and Low AA diets secreted significantly less insulin per islet than islets from the mice fed Control diet (Figure 3A), while the islet insulin content was also decreased by both diets, significantly in the case of the Low AA diet (Figure 3B). By comparison with the Control diet, Low BCAA and Low AA diets reduced the plateau fraction (a measure of the time the islet spends in the active state (Merrins et al, 2016; Nunemaker et al, 2006)) of both ATP/ADP and Ca 2+ oscillations (Figure 3C and 3D), reflecting decreased metabolic flux, a highly advantageous state as unchecked β cell metabolic workload can lead to β cell failure over time (Porat et al, 2011). Ca 2+ oscillation amplitude was increased in these cases (Figure 3C and 3E), an efficiency which explains how the islets maintained adequate secretion on the Low BCAA and Low AA diets without the need for a higher metabolic rate.…”

Pyruvate kinase controls signal strength in the insulin secretory pathway

“…Transition between the 2 states is toggled by the large amount of ATP hydrolysis associated with membrane depolarization, pumps, and vesicle fusion (Nicholls, 2016;Affourtit et al, 2018). A key advantage of allosteric PK recruitment is the ability to reinforce metabolic oscillations in response to metabolic regulators like FBP (Merrins et al, 2013(Merrins et al, , 2016. Consequently, the apex of PK activity occurs just prior to membrane depolarization, at the nadir of OxPhos, which resumes following depolarization-initiated ATP hydrolysis.…”

“…Glucose carbons that transit through PC generate 40% of cytosolic PEP through the cataplerotic mitochondrial PEP carboxykinase (PEPCK-M) reaction (Stark et al, 2009). This 'PEP cycle' has been linked to insulin secretion (Jesinkey et al, 2019;Stark et al, 2009) and provides a mechanism distinct from OxPhos for cytosolic ATP/ADP generation via pyruvate kinase (PK), which is allosterically activated by fructose 1,6-bisphosphate (FBP) prior to membrane depolarization (Merrins et al, 2013(Merrins et al, , 2016.…”

Glucose Oscillations Can Activate an Endogenous Oscillator in Pancreatic Islets

“…This could be true as long as the oscillator has a similar bifurcation structure. However, we are unaware of what this alternate endogenous oscillator could be, while there is now ample direct evidence for a glycolytic oscillator in islet β -cells [26–28]. …”