Calorie Restriction modulates beta cell IP 3 R activity to regulate Ca 2+ homeostasis and cell network connectivity
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Calorie restriction (CR) promotes beta cell longevity by regulating cell identity, organelle and protein homeostasis, and metabolism pathways. CR beta cells have higher cAMP levels and mitochondria with an elevated potential to generate ATP. However, CR beta cells have reduced insulin secretion due to increased peripheral insulin sensitivity. How CR impacts beta cell Ca 2+ homeostasis to regulate beta cell insulin release remains unknown. We investigated this question using acute pancreatic tissue slices prepared from ad-libitum (AL) or CR mice loaded with a low affinity Ca 2+ indicator and recorded cytosolic Ca 2+ gradients with fast confocal imaging. We exposed these slices to increasing glucose concentrations and applied our semi-automatic analysis pipeline to detect thousands of individual beta cells followed by identification of individual Ca 2+ spiking events. We observed that CR beta cells have fast short-amplitude Ca 2+ oscillations that correlate with largely disconnected beta cell networks across the islet. Using acetylcholine stimulation, we found that faster IP 3 R-driven Ca 2+ oscillations linked to higher cytosolic cAMP levels protect beta cells against acute depletion of ER Ca 2+ stress. Therefore, this study provides new mechanistic insight into adaptation of beta cell and of beta cell networks to CR interventions.
Article highlights
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Beta cells from calorie restricted (CR) mice have decreased insulin release, however the mechanisms underlying this adaptive response remain unknown.
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CR beta cells have elevated basal cytosolic cAMP ([cAMP] cyt ) compared to beta cells in control ad libitum fed (AL) mice, and they operate with faster and shorter cytosolic Ca 2+ oscillations.
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While AL beta cells form interconnected activity networks, CR beta cells are largely disconnected and fire more independently of each other.
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Islets of CR mice can sustain prolonged activity during ER stressing conditions due to elevated IP 3 R activity and improved Ca 2+ homeostasis.
Why did we undertake this study?
We have previously shown that calorie restriction (CR) promotes beta cell longevity by enhancing beta cell identity and organelle homeostasis mechanisms. This long-lived phenotype correlated with the onset of enhanced peripheral insulin sensitivity and reduced beta cell insulin release in vivo despite higher cAMP levels and increased potential for mitochondrial ATP generation. However, the mechanisms underlying the reduced cell insulin release phenotype of CR beta cells remains unknown. Therefore, we investigated the underlying Ca 2+ homeostasis mechanisms regulating insulin release in AL and CR beta cells.
What is the specific question(s) we wanted to answer?
We were interested in determining what are the cell Ca 2+ activity patterns during basal and glucose-stimulated conditions in AL and CR beta cells. In addition, we also investigated how CR beta cells respond to epinephrine inhibition and supra-stimulatory concentrations of acetylcholine (ACh), which drive acute beta cell stress by disrupting normal cAMP and ER Ca 2+ signaling, respectively. Finally, we investigate whether CR beta cells formed more interconnected beta cell networks driven by changes in Ca 2+ activity patterns.
What did we find?
We found that CR beta cells are more active with significantly higher rates of Ca 2+ oscillation at basal and high glucose concentrations. In fact, CR beta cells have shorter inter-Ca 2+ event intervals that are more resistant to depletion of cAMP by epinephrine application. In contrast, stimulation of IP 3 R activity (to force depletion of ER Ca 2+ stores) by supraphysiological ACh concentrations revealed that CR beta cells were able to sustain a prolonged Ca 2+ activity versus AL beta cells. Surprisingly, this enhanced beta cell activity profile reduced beta cell activity network connectivity.
What are the implications of our findings?
Our work demonstrates that CR beta cells have higher baseline and glucose-stimulated Ca 2+ activity due to higher cAMP levels. These cells also have dominant IP 3 R activity that grants improved ER Ca 2+ homeostasis and significantly reduces beta cell network connectivity to tone down insulin secretion. These studies provide a mechanistic understanding of how beta cells adapt to CR and to CR-associated enhanced insulin sensitivity.
