The role of nicotinamide riboside metabolism in pancreatic beta cells

Angélique Satoko Cercillieux
EPFL, 2022
EPFL thesis

Abstract

Pancreatic beta cells play a major role in the regulation of glucose homeostasis by secreting insulin in response to elevated blood glucose levels. A gradual deterioration of functional beta cell mass and the chronic elevation of blood glucose levels are central to development of type 2 diabetes. While the etiologies of Type 2 diabetes and age-related metabolic complications are complex, they are often associated with a decline of intracellular nicotinamide adenine dinucleotide (NAD+). NAD+ levels are maintained through the balance between its synthesis from NAD+ precursors and its degradation by several families of enzymes, including the sirtuin family of protein deacetylases, the ADP-ribosyl transferase enzymes (ARTDs or PARPs) and cADP ribose hydrolases (CD38). NAD+ declines are mostly associated with increased NAD+ consumption, which can be counteracted by dietary supplementation of NAD+ precursors. However, our knowledge on how different tissues utilize and respond to different NAD+ precursors for NAD+ synthesis is still incomplete. This, in turn, limits our ability to provide therapeutic benefits in clinical settings. In this project, we aim to understand the relevance of a particular NAD+ precursor, nicotinamide riboside (NR), in beta cell physiology and its potential use to treat beta cell dysfunction. To do so, we have used mice that are deficient for nicotinamide riboside kinase 1 (NRK1), the first and rate-limiting enzyme for the transformation of NR into NAD+. We demonstrate that NRK1 whole body knockout mice exhibit impaired insulin secretion after a meal or upon glucose challenge when fed a high fat diet. However, NRK1 deficiency in beta cell was not the causality for beta cell failure as differences observed in the NRK1 whole body KO mice were not phenocopied in NRK1 beta cell specific KO mice. Aged NRK1 whole body KO mice also exhibited dysfunctional beta cells and significant loss of insulin content and beta cell mass. Furthermore, NRK1 deficiency exacerbates the decline of NAD+ bioavailability and mitochondria function with age, even promoting fibrosis in multiple tissues. Together, our data suggests that endogenous NR metabolism is critical in maintaining whole body NAD+ homeostasis and to protect against mitochondria dysfunction and stress-related cell responses in pancreatic beta cells.

Official source

https://infoscience.epfl.ch/record/293805?ln=en

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Angélique Satoko Cercillieux
EPFL, 2022
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/293805?ln=en

About this result

Related concepts (21)

Related concepts

Related publications (13)

Related publications

The role of nicotinamide riboside kinases in mammalian NAD⁺ metabolism

Joanna Ratajczak

NAD+ has emerged as a central metabolic node and an important co-substrate for the activity of various enzymes, including the protein deacetylase SIRT1. Different strategies to increase NAD+ bioavailability have been shown to boost SIRT1 activity, which results in protection against metabolic disease, neurodegenerative disorders and certain types of cancer. Nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) have recently been discovered as new NAD+ precursors. Recent studies have shown that dietary supplementation with NR or NMN prevented against high-fat diet-induced body weight gain. However, the physiological role of NR in mammalian metabolism remains largely unexplored. In this project, we focused on the first step of NR metabolism â the phosphorylation of NR by nicotinamide riboside kinases (NRK1 and NRK2). We have investigated the role of NRKs in the metabolism of NAD+ precursors and mammalian physiology in vivo. The results obtained indicate that NRKs are the rate-limiting and essential step for NR- and NMN-driven NAD+ synthesis in hepatocytes and that NMN has to be extracellularly converted to NR for its use as an NAD+ precursor. Accordingly, animals lacking NRK1 have defects in NR/NMN utilization. We demonstrated that effective NR utilization is key for the maintenance of metabolic homeostasis as NRK1 whole-body KO mice display numerous alterations in glucose and lipid management. Using NRK1 liver-specific KO mice, we demonstrate that NRK1 is required to maintain hepatic NAD+ levels and mitochondrial function upon high-fat feeding. Consequently, NRK1LKO mice are sensitized to diet-induced glucose intolerance and insulin resistance. Finally, we show that both isoforms of NRK are redundant in mediating NR/NMN effects in muscle. Hence, this work identifies NRKs as a novel valuable target in the fight against metabolic disease and indicates that the regulation of NRK will enable us to find novel ways of enhancing NAD+ availability. Moreover, it has unveiled a critical role of NRK1 for the metabolism of diverse forms of Vitamin B3 and highlights how circulating NR is key to preserve metabolic health upon dietary challenges.

EPFL2017

Role of metabolism and mitochondria during glucose sensing and glucose-induced dysfunction in human pancreatic beta-cells

Isabelle Chareyron

Glucose sensing and regulation of insulin secretion are the two main functions performed by the pancreatic beta-cell. Together these processes contribute to the tight control of blood glucose in our body. Compromising the ability of the beta-cells to properly secrete insulin can lead to the development of Type 2 diabetes. Mitochondria play an important role in beta-cell function, by integrating nutrients signals and modulating insulin secretion. In this thesis, we aimed at better understanding the role of metabolism and mitochondria during glucose sensing and glucose-induced dysfunction in human pancreatic beta-cells. To that end, we designed three studies looking at different aspects of beta-cell mitochondrial function. In the first one, we developed an in vitro model of glucose overload in human islet. We report an over activation of the mitochondria when returned to basal conditions after a 4 day culture in elevated glucose. Additionally, these mitochondria only poorly responded to nutrient stimulation. Impaired metabolism secretion coupling and insulin secretion indicated a partial loss of beta-cell function, despite the increased mitochondrial activity. After 4 days of elevated glucose treatment, unusually high levels of glycerol-3-phosphate and pyruvate, even in sub-stimulatory extracellular glucose concentrations, were observed. We propose that the accumulation of these metabolites renders beta-cells blind to subsequent glucose stimulation. After removing the glucose stress for 24 hours the mitochondrial energy metabolism was fully restored and metabolism-secretion coupling was normalized. In the second approach, we measured for the first time the matrix pH in primary human pancreatic beta-cells. We showed that the mitochondrial pH in beta-cell is more acidic than in other cell types. Furthermore, in response to glucose, the beta-cell mitochondrial matrix slightly acidifies, possibly due to uptake and metabolism of pyruvate. Lastly, we observed that adenoviruses used to deliver the genetically encoded pH sensor can have an intrinsic impact on mitochondrial pH and calcium signaling. It must be used at low infection units per cell in the study of primary beta-cells. In the third project, we tried to modify the mitochondrial energy metabolism in beta-cells by inhibiting mitochondrial protein synthesis. We used different antibiotics to achieve this, in particular chloramphenicol, known to interfere with the activity of the mitochondrial ribosome. Surprisingly, we observe that when mitochondrial protein synthesis is inhibited, the respiration of insulin-secreting cells (INS-1E cells, human islets) is not affected. Expression of mtDNA encoded respiratory chain subunits is reduced and a mitochondrial unfolded protein response initiated without reducing respiratory rates. This raises the possibility that limited activation of unfolded protein response has a positive effect on the cells. This doctoral thesis provides novel insights into various aspects of mitochondrial behavior in pancreatic beta-cells, both during physiological and pathophysiological conditions. Additional work is needed to better understand the molecular mechanisms underlying the mitochondrial adaptation to nutrient overload and the associated beta-cell dysfunction leading to the accelerated impairment of glucose homeostasis during the development of Type 2 diabetes.

EPFL2019

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Aims/hypothesis In islets from individuals with type 2 diabetes and in islets exposed to chronic elevated glucose, mitochondrial energy metabolism is impaired. Here, we studied early metabolic changes and mitochondrial adaptations in human beta cells during chronic glucose stress. Methods Respiration and cytosolic ATP changes were measured in human islet cell clusters after culture for 4 days in 11.1 mmol/l glucose. Metabolomics was applied to analyse intracellular metabolite changes as a result of glucose stress conditions. Alterations in beta cell function were followed using insulin secretion assays or cytosolic calcium signalling after expression of the calcium probe YC3.6 specifically in beta cells of islet clusters. Results At early stages of glucose stress, mitochondrial energy metabolism was augmented in contrast to the previously described mitochondrial dysfunction in beta cells from islets of diabetic donors. Following chronic glucose stress, mitochondrial respiration increased (by 52.4%,p < 0.001) and, as a consequence, the cytosolic ATP/ADP ratio in resting human pancreatic islet cells was elevated (by 27.8%,p < 0.05). Because of mitochondrial overactivation in the resting state, nutrient-induced beta cell activation was reduced. In addition, chronic glucose stress caused metabolic adaptations that resulted in the accumulation of intermediates of the glycolytic pathway, the pentose phosphate pathway and the TCA cycle; the most strongly augmented metabolite was glycerol 3-phosphate. The changes in metabolites observed are likely to be due to the inability of mitochondria to cope with continuous nutrient oversupply. To protect beta cells from chronic glucose stress, we inhibited mitochondrial pyruvate transport. Metabolite concentrations were partially normalised and the mitochondrial respiratory response to nutrients was markedly improved. Furthermore, stimulus-secretion coupling as assessed by cytosolic calcium signalling, was restored. Conclusion/interpretation We propose that metabolic changes and associated mitochondrial overactivation are early adaptations to glucose stress, and may reflect what happens as a result of poor blood glucose control. Inhibition of mitochondrial pyruvate transport reduces mitochondrial nutrient overload and allows beta cells to recover from chronic glucose stress.

2020