TRPM2-mediated rise in mitochondrial Zn2+ promotes palmitate-induced mitochondrial fission and pancreatic β-cell death in rodents

Rise in plasma free fatty acids (FFAs) represents a major risk factor for obesity-induced type 2 diabetes. Saturated FFAs cause a progressive decline in insulin secretion by promoting pancreatic β-cell death through increased production of reactive oxygen species (ROS). Recent studies have demonstrated that palmitate (a C16-FFA)-induced rise in ROS causes β-cell death by triggering mitochondrial fragmentation, but the underlying mechanisms are unclear. Using the INS1-832/13 β-cell line, here we demonstrate that palmitate generates the ROS required for mitochondrial fission by activating NOX (NADPH oxidase)-2. More importantly, we show that chemical inhibition, RNAi-mediated silencing and knockout of ROS-sensitive TRPM (transient receptor potential melastatin)-2 channels prevent palmitate-induced mitochondrial fission. Although TRPM2 activation affects the intracellular dynamics of Ca2+ and Zn2+, chelation of Zn2+ alone was sufficient to prevent mitochondrial fission. Consistent with the role of Zn2+, palmitate caused a rise in mitochondrial Zn2+, leading to Zn2+-dependent mitochondrial recruitment of Drp-1 (a protein that catalyses mitochondrial fission) and loss of mitochondrial membrane potential. In agreement with the previous reports, Ca2+ caused Drp-1 recruitment, but it failed to induce mitochondrial fission in the absence of Zn2+. These results indicate a novel role for Zn2+ in mitochondrial dynamics. Inhibition or knockout of TRPM2 channels in mouse islets and RNAi-mediated silencing of TRPM2 expression in human islets prevented FFA/cytokine-induced β-cell death, findings that are consistent with the role of abnormal mitochondrial fission in cell death. To conclude, our results reveal a novel, potentially druggable signalling pathway for FFA-induced β-cell death. The cascade involves NOX-2-dependent production of ROS, activation of TRPM2 channels, rise in mitochondrial Zn2+, Drp-1 recruitment and abnormal mitochondrial fission

Generation and characterization of pancreatic ß-Cell specific Prohibitin-2 knockout mice

Prohibitin-2 (Phb-2) controls mitochondrial dynamics and apoptosis. In pancreatic ß-cells, mitochondria play a pivotal role in the control of insulin secretion. In this work, we investigated the role of Prohibitin 2 (Phb2) and mitochondrial dynamics in ß-cell physiology by generating pancreatic ß-cell specific Prohibitin 2 knockout (ß-Phb2-/-) mouse model. Our results show that Phb2 depletion impaired primarily mitochondrial function, insulin secretion and ß-cell survival, eventually leading to diabetes in this mouse model as early as 6 weeks of age. In ß-cells, loss of Phb2 altered stability of L-Opa1 (a mitochondrial protein involved in membrane fusion) and induced mitochondrial fragmentation. Our study shows that prohibitin complex is indispensable for ß-cell maintenance in-vivo. It also demonstrates that regulatory cleavage of Opa1, as well as mitochondrial fusion and stability of mitochondrial genome in ß-cells is controlled by prohibitins. Thus, we have established ß-Phb2-/- mouse as a novel genetic model of mitochondrial diabetes

Mitochondrial dysfunction in pancreatic β cells

In pancreatic β cells, mitochondria play a central role in coupling glucose metabolism to insulin exocytosis, thereby ensuring strict control of glucose-stimulated insulin secretion. Defects in mitochondrial function impair this metabolic coupling, and ultimately promote apoptosis and β cell death. Various factors have been identified that may contribute to mitochondrial dysfunction. In this review we address the emerging concept of complex links between these factors. We also discuss the role of the mitochondrial genome and mutations associated with diabetes, the effect of oxidative stress and reactive oxygen species, the sensitivity of mitochondria to lipotoxicity, and the adaptive dynamics of mitochondrial morphology. Better comprehension of the molecular mechanisms contributing to mitochondrial dysfunction will help drive the development of effective therapeutic approaches

Metabolomics identifies a biomarker revealing in vivo loss of functional ß-cell mass before diabetes onset

Identification of pre-diabetic individuals with decreased functional ß-cell mass is essential for the prevention of diabetes. However, in vivo detection of early asymptomatic ß-cell defect remains unsuccessful. Metabolomics emerged as a powerful tool in providing read-outs of early disease states before clinical manifestation. We aimed at identifying novel plasma biomarkers for loss of functional ß-cell mass in the asymptomatic pre-diabetic stage. Non-targeted and targeted metabolomics were applied on both lean ß-Phb2-/- mice (ß-cell-specific prohibitin-2 knockout) and obese db/db mice (leptin receptor mutant), two distinct mouse models requiring neither chemical nor diet treatments to induce spontaneous decline of functional ß-cell mass promoting progressive diabetes development. Non-targeted metabolomics on ß-Phb2-/- mice identified 48 and 82 significantly affected metabolites in liver and plasma, respectively. Machine learning analysis pointed to deoxyhexose sugars consistently reduced at the asymptomatic pre-diabetic stage, including in db/db mice, showing strong correlation with the gradual loss of ß-cells. Further targeted metabolomics by GC-MS uncovered the identity of the deoxyhexose with 1,5-anhydroglucitol displaying the most significant changes. In conclusion, this study identified 1,5-anhydroglucitol associated with the loss of functional ß-cell mass and uncovered metabolic similarities between liver and plasma, providing insights into the systemic effects caused by early decline in ß-cells

Loss of prohibitin induces mitochondrial damages altering ss-cell function and survival and responsible for gradual diabetes development.

Prohibitins are highly conserved proteins mainly implicated in the maintenance of mitochondrial function and architecture. Their dysfunctions are associated with aging, cancer, obesity, and inflammation. However, their possible role in pancreatic β-cells remains unknown. The current study documents the expression of prohibitins in human and rodent islets and their key role for β-cell function and survival. Ablation of Phb2 in mouse β-cells sequentially resulted in impairment of mitochondrial function and insulin secretion, loss of β-cells, progressive alteration of glucose homeostasis, and, ultimately, severe diabetes. Remarkably, these events progressed over a 3-week period of time after weaning. Defective insulin supply in β-Phb2(-/-) mice was contributed by both β-cell dysfunction and apoptosis, temporarily compensated by increased β-cell proliferation. At the molecular level, we observed that deletion of Phb2 caused mitochondrial abnormalities, including reduction of mitochondrial DNA copy number and respiratory chain complex IV levels, altered mitochondrial activity, cleavage of L-optic atrophy 1, and mitochondrial fragmentation. Overall, our data demonstrate that Phb2 is essential for metabolic activation of mitochondria and, as a consequence, for function and survival of β-cells