Protocol for in vivo and ex vivo assessments of glucose-stimulated insulin secretion in mouse islet β cells

Inhibition of dynamin-related protein 1-filamin interaction improves systemic glucose metabolism

BACKGROUND AND PURPOSE

Maintaining mitochondrial quality is attracting attention as a new strategy to treat diabetes and diabetic complications. We previously reported that mitochondrial hyperfission by forming a protein complex between dynamin-related protein (Drp) 1 and filamin, mediates chronic heart failure and cilnidipine, initially developed as an L/N-type Ca2+ channel blocker, improves heart failure by inhibiting Drp1-filamin protein complex. We investigated whether cilnidipine improves hyperglycaemia of various diabetic mice models.

EXPERIMENTAL APPROACH

Retrospective analysis focusing on haemoglobin A1c (HbA1c) was performed in hypertensive and hyperglycaemic patients taking cilnidipine and amlodipine. After developing diabetic mice by streptozotocin (STZ) treatment, an osmotic pump including drug was implanted intraperitoneally, followed by weekly measurements of blood glucose levels. Mitochondrial morphology was analysed by electron microscopy. A Ca2+ channel-insensitive cilnidipine derivative (1,4-dihydropyridine [DHP]) was synthesized and its pharmacological effect was evaluated using obese (ob/ob) mice fed with high-fat diet (HFD).

KEY RESULTS

In patients, cilnidipine was superior to amlodipine in HbA1c lowering effect. Cilnidipine treatment improved systemic hyperglycaemia and mitochondrial morphological abnormalities in STZ-exposed mice, without lowering blood pressure. Cilnidipine failed to improve hyperglycaemia of ob/ob mice, with suppressing insulin secretion. 1,4-DHP improved hyperglycaemia and mitochondria abnormality in ob/ob mice fed HFD. 1,4-DHP and cilnidipine improved basal oxygen consumption rate of HepG2 cells cultured under 25 mM glucose.

CONCLUSION AND IMPLICATIONS

Inhibition of Drp1-filamin protein complex formation becomes a new strategy for type 2 diabetes treatment.

β-Cell miRNA-503-5p Induced by Hypomethylation and Inflammation Promotes Insulin Resistance and β-Cell Decompensation

Chronic inflammation promotes pancreatic β-cell decompensation to insulin resistance because of local accumulation of supraphysiologic interleukin 1β (IL-1β) levels. However, the underlying molecular mechanisms remain elusive. We show that miR-503-5p is exclusively upregulated in islets from humans with type 2 diabetes and diabetic rodents because of its promoter hypomethylation and increased local IL-1β levels. β-Cell-specific miR-503 transgenic mice display mild or severe diabetes in a time- and expression-dependent manner. By contrast, deletion of the miR-503 cluster protects mice from high-fat diet-induced insulin resistance and glucose intolerance. Mechanistically, miR-503-5p represses c-Jun N-terminal kinase-interacting protein 2 (JIP2) translation to activate mitogen-activated protein kinase signaling cascades, thus inhibiting glucose-stimulated insulin secretion (GSIS) and compensatory β-cell proliferation. In addition, β-cell miR-503-5p is packaged in nanovesicles to dampen insulin signaling transduction in liver and adipose tissues by targeting insulin receptors. Notably, specifically blocking the miR-503 cluster in β-cells effectively remits aging-associated diabetes through recovery of GSIS capacity and insulin sensitivity. Our findings demonstrate that β-cell miR-503-5p is required for the development of insulin resistance and β-cell decompensation, providing a potential therapeutic target against diabetes.

ARTICLE HIGHLIGHTS

Promoter hypomethylation during natural aging permits miR-503-5p overexpression in islets under inflammation conditions, conserving from rodents to humans. Impaired β-cells release nanovesicular miR-503-5p to accumulate in liver and adipose tissue, leading to their insulin resistance via the miR-503-5p/insulin receptor/phosphorylated AKT axis. Accumulated miR-503-5p in β-cells impairs glucose-stimulated insulin secretion via the JIP2-coordinated mitogen-activated protein kinase signaling cascades. Specific blockage of β-cell miR-503-5p improves β-cell function and glucose tolerance in aging mice.

Excessive iron inhibits insulin secretion via perturbing transcriptional regulation of SYT7 by OGG1

Although iron overload is closely related to the occurrence of type 2 diabetes mellitus (T2DM), the specific mechanism is unclear. Here, we found that excessive iron inhibited the secretion of insulin (INS) and impaired islet β cell function through downregulating Synaptotagmin 7 (SYT7) in iron overload model in vivo and in vitro. Our results further demonstrated that 8-oxoguanine DNA glycosylase (OGG1), a key protein in the DNA base excision repair, was an upstream regulator of SYT7. Interestingly, such regulation could be suppressed by excessive iron. Ogg1-null mice, iron overload mice and db/db mice exhibit reduced INS secretion, weakened β cell function and subsequently impaired glucose tolerance. Notably, SYT7 overexpression could rescue these phenotypes. Our data revealed an intrinsic mechanism by which excessive iron inhibits INS secretion through perturbing the transcriptional regulation of SYT7 by OGG1, which suggested that SYT7 was a potential target in clinical therapy for T2DM.

Perfluorooctanoic acid promotes pancreatic β cell dysfunction and apoptosis through ER stress and the ATF4/CHOP/TRIB3 pathway

Perfluorooctanoic acid (PFOA), a widely used chemical substance, causes an increased risk of human type 2 diabetes (T2D), but its underlying mechanism is not well elucidated. The aim of the present study was to investigate whether PFOA regulates the functions of pancreatic β cells, which are specialized for the biosynthesis and secretion of insulin. The treatment of the mouse pancreatic β cell line (MIN6 cells) with PFOA caused a time- and dose-dependent inhibition of cell viability in CCK-8 assays. Annexin V/PI and TUNEL staining results confirmed that exposure to a high PFOA dose (500 μM) promoted apoptosis of β cells, while a low dose (300 μM) had no effects on β cell survival. PFOA treatment, even at a low dose, diminished glucose-stimulated insulin secretion (GSIS) in both primary islet perfusion and MIN6 cell experiments. RNA-sequencing data showed significantly increased expression of endoplasmic reticulum (ER) stress-associated genes, with tribbles homolog 3 (Trib3) ranking first among the altered genes. The activation of ER stress pathways was verified by qRT-PCR assays, and the ATF4/CHOP/TRIB3 pathway contributed to PFOA-induced β cell damage. The inhibition of TRIB3 expression significantly protected MIN6 cells from PFOA-induced GSIS defects and apoptosis by ameliorating ER stress. These findings reveal a link between ER stress and PFOA-induced β cell defects, opening up a new set of questions about the pathogenesis of T2D due to environmental chemicals.

A protocol for studying glucose homeostasis and islet function in mice

Glucose tolerance test and glucose stimulated insulin secretion are important measures to determine glucose homeostasis and islet function and assess diabetes. Here, we provide a protocol where glucose tolerance test is used to study glucose homeostasis and insulin secretion in vivo, followed by islet isolation and glucose stimulated insulin secretion to determine islet function ex vivo. This protocol enables evaluation of glucose homeostasis and islets in mice which can also be applied to rat, beta cell lines and human studies. For complete details on the use and execution of this profile, please refer to Al Rijjal et al. (2021).