Tafazzin Deficiency Reduces Basal Insulin Secretion and Mitochondrial Function in Pancreatic Islets From Male Mice

Recent Developments in Islet Biology: A Review With Patient Perspectives

Navigating the coronavirus disease-2019 (COVID-19, now COVID) pandemic has required resilience and creativity worldwide. Despite early challenges to productivity, more than 2,000 peer-reviewed articles on islet biology were published in 2021. Herein, we highlight noteworthy advances in islet research between January 2021 and April 2022, focussing on 5 areas. First, we discuss new insights into the role of glucokinase, mitogen-activated protein kinase-kinase/extracellular signal-regulated kinase and mitochondrial function on insulin secretion from the pancreatic β cell, provided by new genetically modified mouse models and live imaging. We then discuss a new connection between lipid handling and improved insulin secretion in the context of glucotoxicity, focussing on fatty acid-binding protein 4 and fetuin-A. Advances in high-throughput "omic" analysis evolved to where one can generate more finely tuned genetic and molecular profiles within broad classifications of type 1 diabetes and type 2 diabetes. Next, we highlight breakthroughs in diabetes treatment using stem cell-derived β cells and innovative strategies to improve islet survival posttransplantation. Last, we update our understanding of the impact of severe acute respiratory syndrome-coronavirus-2 infection on pancreatic islet function and discuss current evidence regarding proposed links between COVID and new-onset diabetes. We address these breakthroughs in 2 settings: one for a scientific audience and the other for the public, particularly those living with or affected by diabetes. Bridging biomedical research in diabetes to the community living with or affected by diabetes, our partners living with type 1 diabetes or type 2 diabetes also provide their perspectives on these latest advances in islet biology.

Phenotypic Characterization of Male Tafazzin-Knockout Mice at 3, 6, and 12 Months of Age

Barth syndrome (BTHS) is an X-linked mitochondrial disease caused by mutations in the gene encoding for tafazzin (TAZ), a key enzyme in the remodeling of cardiolipin. Mice with a germline deficiency in Taz have been generated (Taz-KO) but not yet fully characterized. We performed physiological assessments of 3-, 6-, and 12-month-old male Taz-KO mice, including measures of perinatal survival, growth, lifespan, gross anatomy, whole-body energy and substrate metabolism, glucose homeostasis, and exercise capacity. Taz-KO mice displayed reduced viability, with lower-than-expected numbers of mice recorded at 4 weeks of age, and a shortened lifespan due to disease progression. At all ages, Taz-KO mice had lower body weights compared with wild-type (Wt) littermates despite similar absolute food intakes. This finding was attributed to reduced adiposity and diminutive organs and tissues, including heart and skeletal muscles. Although there were no differences in basal levels of locomotion between age-matched genotypes, indirect calorimetry studies showed higher energy expenditure measures and respiratory exchange ratios in Taz-KO mice. At the youngest age, Taz-KO mice had comparable glucose tolerance and insulin action to Wt mice, but while these measures indicated metabolic impairments in Wt mice with advancing age that were likely associated with increasing adiposity, Taz-KO mice were protected. Comparisons across the three age-cohorts revealed a significant and more severe deterioration of exercise capacity in Taz-KO mice than in their Wt littermate controls. The Taz-KO mouse model faithfully recapitulates important aspects of BTHS, and thus provides an important new tool to investigate pathophysiological mechanisms and potential therapies.

Stabilization protects islet integrity during respirometry in the Oroboros Oxygraph-2K analyzer

Metabolic dysfunction of β-cells has been implicated as a contributor to diabetes pathogenesis, and efforts are ongoing to optimize analytical techniques that evaluate islet metabolism. High-resolution respirometry offers sensitive measurements of the respiratory effects of metabolic substrates and customizable manipulation of electron transport chain components, though the delicate nature of islets can pose challenges to conventional analyses. An affordable and reliable option for respirometry is the Oroboros Oxygraph-2 K system, which utilizes a stir bar to circulate reagents around cells. While this technique may be suitable for individual cells or mitochondria, the continual force exerted by the stir bar can have damaging effects on islet integrity. Herein, we demonstrate the protective benefits of a novel 3D-printed islet stabilization device and highlight the destructive effects of conventional Oxygraph analysis on islet integrity. Islet containment did not inhibit cellular responses to metabolic modulatory drugs, as indicated by robust fluctuations in oxygen consumption rates. The average size of wild-type mouse islets was significantly reduced following a standard Mito Stress Test within Oxygraph chambers, with a clear disruption in islet morphology and viability. Alternatively, containment of the islets within the interior chamber of the islet stabilization device yielded preservation of both islet morphology and increased cell viability/survival after respirometry analysis. Collectively, our study introduces a new and easily accessible tool to improve conventional Oxygraph respirometry of pancreatic islets by preserving natural islet structure and function throughout metabolic analysis.

Never-in-Mitosis A-Related Kinase 8 (NEK8) Regulates Adipogenesis, Glucose Homeostasis, and Obesity

Background

Adipogenesis is a complex biological process and the leading main cause of obesity. We evaluated the role of never-in-mitosis A-related kinase 8 (NEK8) in adipocyte development and insulin sensitivity in the present study.

Methods

NEK8 expression was manipulated using a specific shRNA or the NEK8-full-length expressing recombinant plasmids. The interaction between NEK8 and Tafazzin (TAZ, an oncogenic transcriptional regulator) was examined by Co-immunoprecipitation (Co-IP) and confocal immunofluorescence staining. Western blot assay was performed to determine the protein expression. The in vivo role of NEK8 was explored in a mouse model of high-fat diet- (HFD-) induced insulin resistance.

Results

During adipogenesis, the expression of NEK8 was elevated while TAZ was downregulated. Overexpression of NEK8 promoted lipid accumulation and expression of markers for adipocyte differentiation. Mechanically, NEK8 interacted with TAZ and suppressed its expression in adipocytes. Functionally, lentiviral-mediated NEK8 inhibition ameliorates HFD-induced insulin resistance in adipocytes.

Conclusion

These findings suggest that NEK8 plays a critical role in adipocyte proliferation, providing novel insight into the link between NEK8 and type 2 diabetes- (T2DM-) related obesity.

Cardiolipin Regulates Mitochondrial Ultrastructure and Function in Mammalian Cells

Cardiolipin (CL) is a unique, tetra-acylated diphosphatidylglycerol lipid that mainly localizes in the inner mitochondria membrane (IMM) in mammalian cells and plays a central role in regulating mitochondrial architecture and functioning. A deficiency of CL biosynthesis and remodeling perturbs mitochondrial functioning and ultrastructure. Clinical and experimental studies on human patients and animal models have also provided compelling evidence that an abnormal CL content, acyl chain composition, localization, and level of oxidation may be directly linked to multiple diseases, including cardiomyopathy, neuronal dysfunction, immune cell defects, and metabolic disorders. The central role of CL in regulating the pathogenesis and progression of these diseases has attracted increasing attention in recent years. In this review, we focus on the advances in our understanding of the physiological roles of CL biosynthesis and remodeling from human patients and mouse models, and we provide an overview of the potential mechanism by which CL regulates the mitochondrial architecture and functioning.

Myocardial disturbances of intermediary metabolism in Barth syndrome

Barth Syndrome (BTHS) is a rare X-linked mitochondrial disorder due to mutations in the gene TAFAZZIN, which leads to immature cardiolipin (CL) remodeling and is characterized by the development of cardiomyopathy. The immature CL remodeling in BTHS results in electron transport chain respiratory defects and destabilization of supercomplexes, thereby impairing ATP production. Thus, BTHS-related cardiomyopathy appears to share metabolic characteristics of the failing heart being an "engine out of fuel." As CL associates with numerous mitochondrial enzymes involved in ATP production, BTHS is also characterized by several defects in intermediary energy metabolism. Herein we will describe the primary disturbances in intermediary energy metabolism relating to the heart's major fuel sources, fatty acids, carbohydrates, ketones, and amino acids. In addition, we will interrogate whether these disturbances represent potential metabolic targets for alleviating BTHS-related cardiomyopathy.

Pulsatile Basal Insulin Secretion Is Driven by Glycolytic Oscillations

In fasted and fed states, blood insulin levels are oscillatory. While this phenomenon is well studied at high glucose levels, comparatively little is known about its origin under basal conditions. We propose a possible mechanism for basal insulin oscillations based on oscillations in glycolysis, demonstrated using an established mathematical model. At high glucose, this is superseded by a calcium-dependent mechanism.

Misaligned feeding schedule elicits divergent circadian reorganizations in endo- and exocrine pancreas clocks

Misaligned feeding may lead to pancreatic insufficiency, however, whether and how it affects circadian clock in the exocrine pancreas is not known. We exposed rats to a reversed restricted feeding regimen (rRF) for 10 or 20 days and analyzed locomotor activity, daily profiles of hormone levels (insulin, glucagon, and corticosterone) in plasma, and clock gene expression in the liver and endocrine and exocrine pancreas. In addition, we monitored responses of the exocrine pancreatic clock in organotypic explants of mPer2Luc mice in real time to acetylcholine, insulin, and glucocorticoids. rRF phase-reversed the clock in the endocrine pancreas, similar to the clock in the liver, but completely abolished clock gene rhythmicity and significantly downregulated the expression of Cpb1 and Cel in the exocrine pancreas. rRF desynchronized the rhythms of plasma insulin and corticosterone. Daily profiles of their receptor expression differed in the two parts of the pancreas and responded differently to rRF. Additionally, the pancreatic exocrine clock responded differently to treatments with insulin and the glucocorticoid analog dexamethasone in vitro. Mathematical simulation confirmed that the long-term misalignment between these two hormonal signals, as occurred under rRF, may lead to dampening of the exocrine pancreatic clock. In summary, our data suggest that misaligned meals impair the clock in the exocrine part of the pancreas by uncoupling insulin and corticosterone rhythms. These findings suggest a new mechanism by which adverse dietary habits, often associated with shift work in humans, may impair the clock in the exocrine pancreas and potentially contribute to exocrine pancreatic insufficiency.

Targeting pancreatic beta cell death in type 2 diabetes by polyphenols

Diabetes is a very complex disease which is characterized by the appearance of insulin resistance that is primarily compensated by an increase in pancreatic beta cell mass, generating hyperinsulinemia. After time, pancreatic beta cells die by apoptosis appearing in the second phase of the disease, and characterized by hypoinsulinemia. There are multiple conditions that can alter pancreatic beta cell homeostasis and viability, being the most relevant ones; ER stress, cytotoxicity by amylin, mTORC1 hyperactivity, oxidative stress, mitochondrial dysfunction, inflammation and alterations in autophagy/mitophagy flux. In addition, the possible effects that different polyphenols could exert in the modulation of these mechanisms and regulating pancreatic beta cell viability are analyzed. It is necessary a profound analysis and understanding of all the possible mechanisms involved in the control and maintenance of pancreatic beta cell viability to develop more accurate and target treatments for controlling beta cell homeostasis and preventing or even reversing type 2 diabetes mellitus.

Tafazzin Deficiency Reduces Basal Insulin Secretion and Mitochondrial Function in Pancreatic Islets From Male Mice

Tafazzin (TAZ) is a cardiolipin (CL) biosynthetic enzyme important for maintaining mitochondrial function. TAZ affects both the species and content of CL in the inner mitochondrial membrane, which are essential for normal cellular respiration. In pancreatic β cells, mitochondrial function is closely associated with insulin secretion. However, the role of TAZ and CL in the secretion of insulin from pancreatic islets remains unknown. Male 4-month-old doxycycline-inducible TAZ knock-down (KD) mice and wild-type littermate controls were used. Immunohistochemistry was used to assess β-cell morphology in whole pancreas sections, whereas ex vivo insulin secretion, CL content, RNA-sequencing analysis, and mitochondrial oxygen consumption were measured from isolated islet preparations. Ex vivo insulin secretion under nonstimulatory low-glucose concentrations was reduced ~52% from islets isolated from TAZ KD mice. Mitochondrial oxygen consumption under low-glucose conditions was also reduced ~58% in islets from TAZ KD animals. TAZ deficiency in pancreatic islets was associated with significant alteration in CL molecular species and elevated polyunsaturated fatty acid CL content. In addition, RNA-sequencing of isolated islets showed that TAZ KD increased expression of extracellular matrix genes, which are linked to pancreatic fibrosis, activated stellate cells, and impaired β-cell function. These data indicate a novel role for TAZ in regulating pancreatic islet function, particularly under low-glucose conditions.