Mitochondrial Calcium Signaling in Pancreatic β-Cell

Ca2+ signaling and metabolic stress-induced pancreatic β-cell failure

Early in the development of Type 2 diabetes (T2D), metabolic stress brought on by insulin resistance and nutrient overload causes β-cell hyperstimulation. Herein we summarize recent studies that have explored the premise that an increase in the intracellular Ca2+ concentration ([Ca2+]i), brought on by persistent metabolic stimulation of β-cells, causes β-cell dysfunction and failure by adversely affecting β-cell function, structure, and identity. This mini-review builds on several recent reviews that also describe how excess [Ca2+]i impairs β-cell function.

STIM1 regulates pancreatic β-cell behaviour: A modelling study

Pancreatic β-cells are equipped with the molecular machinery allowing them to respond to high glucose levels in the form of electrical activity and Ca2+ oscillations. These oscillations drive insulin secretion. Two key ionic mechanisms involved in this response are the Store-Operated Current and the current through ATP-dependent K+ channels. Both currents have been shown to be regulated by the protein STIM1, but this dual regulation by STIM1 has not been studied before. In this paper, we use mathematical modelling to gain insight into the role of STIM1 in the β-cell response. We extended a previous β-cell model to include the dynamics of STIM1 and described the dependence of the ATP-dependent K+ current on STIM1. Our simulations suggest that the total concentration of STIM1 modifies the bursting frequency, the burst duration and the intracellular Ca2+ levels. These results are in good agreement with experimental reports, and the contribution of the studied currents to electrical activity and Ca2+ dynamics is discussed. The model predicts that in the absence of STIM1 the excitability of the plasma membrane increases and that the glucose threshold for electrical activity is shifted to lower concentrations. These computational predictions may be related to impaired insulin secretion under conditions of reduced STIM1 in the diabetic state.

Implications of MCU complex in metabolic diseases

Metabolic diseases are considered the primary culprit for physical and mental health of individuals. Although the diagnosis of these diseases is relatively easy, more effective and convenient potent drugs are still being explored. Ca2+ across the inner mitochondrial membrane is a vital intracellular messenger that regulates energy metabolism and cellular Ca2+ homeostasis and is involved in cell death. Mitochondria rely on a selective mitochondrial Ca2+ unidirectional transport complex (MCU complex) in their inner membrane for Ca2+ uptake. We found that the channel contains several subunits and undergoes dramatic transformations in various pathological processes, especially in metabolic diseases. In this way, we believe that the MCU complex becomes a target with significant potential for these diseases. However, there is no review linking the two factors, thus hindering the possibility of new drug production. Here, we highlight the connection between MCU complex-related Ca2+ transport and the pathophysiology of metabolic diseases, adding understanding and insight at the molecular level to provide new insights for targeting MCU to reverse metabolism-related diseases.

Impact of Ca2+ on membrane catalyzed IAPP amyloid formation and IAPP induced vesicle leakage

Human islet amyloid polypeptide (hIAPP, also known as amylin) is a 37 amino acid pancreatic polypeptide hormone that plays a role in regulating glucose levels, but forms pancreatic amyloid in type-2 diabetes. The process of amyloid formation by hIAPP contributes to β-cell death in the disease. Multiple mechanisms of hIAPP induced toxicity of β-cells have been proposed including disruption of cellular membranes. However, the nature of hIAPP membrane interactions and the effect of ions and other molecules on hIAPP membrane interactions are not fully understood. Many studies have used model membranes with a high content of anionic lipids, often POPS, however the concentration of anionic lipids in the β-cell plasma membrane is low. Here we study the concentration dependent effect of Ca2+ (0 to 50 mM) on hIAPP membrane interactions using large unilamellar vesicles (LUVs) with anionic lipid content ranging from 0 to 50 mol%. We find that Ca2+ does not effectively inhibit hIAPP amyloid formation and hIAPP induced membrane leakage from binary LUVs with a low percentage of POPS, but has a greater effect on LUVs with a high percentage of POPS. Mg2+ had very similar effects, and the effects of Ca2+ and Mg2+ can be largely rationalized by the neutralization of POPS charge. The implications for hIAPP-membrane interactions are discussed.

The mitochondrial calcium uniporter (MCU) activates mitochondrial respiration and enhances mobility by regulating mitochondrial redox state

Regulation of mitochondrial redox balance is emerging as a key event for cell signaling in both physiological and pathological conditions. However, the link between the mitochondrial redox state and the modulation of these conditions remains poorly defined. Here, we discovered that activation of the evolutionary conserved mitochondrial calcium uniporter (MCU) modulates mitochondrial redox state. By using mitochondria-targeted redox and calcium sensors and genetic MCU-ablated models, we provide evidence of the causality between MCU activation and net reduction of mitochondrial (but not cytosolic) redox state. Redox modulation of redox-sensitive groups via MCU stimulation is required for maintaining respiratory capacity in primary human myotubes and C. elegans, and boosts mobility in worms. The same benefits are obtained bypassing MCU via direct pharmacological reduction of mitochondrial proteins. Collectively, our results demonstrate that MCU regulates mitochondria redox balance and that this process is required to promote the MCU-dependent effects on mitochondrial respiration and mobility.

Ca2+-Mediated Signaling Pathways: A Promising Target for the Successful Generation of Mature and Functional Stem Cell-Derived Pancreatic Beta Cells In Vitro

Diabetes mellitus is a chronic disease affecting over 500 million adults globally and is mainly categorized as type 1 diabetes mellitus (T1DM), where pancreatic beta cells are destroyed, and type 2 diabetes mellitus (T2DM), characterized by beta cell dysfunction. This review highlights the importance of the divalent cation calcium (Ca²⁺) and its associated signaling pathways in the proper functioning of beta cells and underlines the effects of Ca²⁺ dysfunction on beta cell function and its implications for the onset of diabetes. Great interest and promise are held by human pluripotent stem cell (hPSC) technology to generate functional pancreatic beta cells from diabetic patient-derived stem cells to replace the dysfunctional cells, thereby compensating for insulin deficiency and reducing the comorbidities of the disease and its associated financial and social burden on the patient and society. Beta-like cells generated by most current differentiation protocols have blunted functionality compared to their adult human counterparts. The Ca²⁺ dynamics in stem cell-derived beta-like cells and adult beta cells are summarized in this review, revealing the importance of proper Ca²⁺ homeostasis in beta-cell function. Consequently, the importance of targeting Ca²⁺ function in differentiation protocols is suggested to improve current strategies to use hPSCs to generate mature and functional beta-like cells with a comparable glucose-stimulated insulin secretion (GSIS) profile to adult beta cells.

Formulation strategies to provide oxygen-release to contrast local hypoxia for transplanted islets

Islet transplantation refers to the transfusion of healthy islet cells into the diabetic recipients and reconstruction of their endogenous insulin secretion to achieve insulin independence. It is a minimally invasive surgery that holds renewed prospect as a therapeutic method for type 1 diabetes mellitus. However, poor oxygenation in the early post-transplantation period is considered as one of the major causes of islet loss and dysfunction. Due to the metabolism chacteristics, islets required a high supply of oxygen for cell survival while a hypoxia environment would lead to severe islet loss and graft failure. Emerging strategies have been proposed, including providing external oxygen and speeding up revascularization. From the perspective of formulation science, it is feasible and practical to protect transplanted islets by oxygen-release before revascularization as opposed to local hypoxia. In this study, we review the potential formulation strategies that could provide oxygen-release by either delivering external oxygen or triggering localized oxygen generation for transplanted islets.

Small Molecule-mediated Insulin Hypersecretion Induces Transient ER Stress Response and Loss of Beta Cell Function

Pancreatic islet beta cells require a fine-tuned endoplasmic reticulum (ER) stress response for normal function; abnormal ER stress contributes to diabetes pathogenesis. Here, we identified a small molecule, SW016789, with time-dependent effects on beta cell ER stress and function. Acute treatment with SW016789 potentiated nutrient-induced calcium influx and insulin secretion, while chronic exposure to SW016789 transiently induced ER stress and shut down secretory function in a reversible manner. Distinct from the effects of thapsigargin, SW016789 did not affect beta cell viability or apoptosis, potentially due to a rapid induction of adaptive genes, weak signaling through the eIF2α kinase PERK, and lack of oxidative stress gene Txnip induction. We determined that SW016789 acted upstream of voltage-dependent calcium channels (VDCCs) and potentiated nutrient- but not KCl-stimulated calcium influx. Measurements of metabolomics, oxygen consumption rate, and G protein-coupled receptor signaling did not explain the potentiating effects of SW016789. In chemical cotreatment experiments, we discovered synergy between SW016789 and activators of protein kinase C and VDCCs, suggesting involvement of these pathways in the mechanism of action. Finally, chronically elevated calcium influx was required for the inhibitory impact of SW016789, as blockade of VDCCs protected human islets and MIN6 beta cells from hypersecretion-induced dysfunction. We conclude that beta cells undergoing this type of pharmacological hypersecretion have the capacity to suppress their function to mitigate ER stress and avoid apoptosis. These results have the potential to uncover beta cell ER stress mitigation factors and add support to beta cell rest strategies to preserve function.

Ca2+ Sensors Assemble: Function of the MCU Complex in the Pancreatic Beta Cell

The Mitochondrial Calcium Uniporter Complex (MCU Complex) is essential for β-cell function due to its role in sustaining insulin secretion. The MCU complex regulates mitochondrial Ca²⁺ influx, which is necessary for increased ATP production following cellular glucose uptake, keeps the cell membrane K⁺ channels closed following initial insulin release, and ultimately results in sustained insulin granule exocytosis. Dysfunction in Ca²⁺ regulation results in an inability to sustain insulin secretion. This review defines the functions, structure, and mutations associated with the MCU complex members mitochondrial calcium uniporter protein (MCU), essential MCU regulator (EMRE), mitochondrial calcium uptake 1 (MICU1), mitochondrial calcium uptake 2 (MICU2), and mitochondrial calcium uptake 3 (MICU3) in the pancreatic β-cell. This review provides a framework for further evaluation of the MCU complex in β-cell function and insulin secretion.

Analysis of 55 patients with multiple endocrine neoplasia type 1-associated insulinoma from a single center in China

Objective

To investigate the clinical characteristics of patients with multiple endocrine neoplasia type 1 (MEN1)-related insulinoma and their relationship with specific biochemical changes and to summarize the features of treatment options for the Chinese population with this disease and the impact on long-term prognosis.

Methods

“MEN1” and “insulinoma” were used when searching the Peking Union Medical College Hospital (PUMCH) medical record retrieval system to obtain clinical information about patients. We identified patients diagnosed with MEN1-associated insulinoma based on endocrinological, radiological, and pathological examinations, and subsequently analyzed their clinical data.

Results

A total of 55 patients with MEN1-associated insulinoma were included, including 29 (52.7%) men and 26 (47.3%) women. The parathyroid gland was the most commonly affected (78.2%), followed by the pituitary gland (69.1%) and adrenal gland (16.4%). Insulinoma was the first manifestation of MEN1 in at least 23.6% (13/55) of patients. Nineteen (34.5%) patients presented with initial symptoms of hypoglycemia before the age of 22 years. Among the 24 Patients with high serum calcium (Ca) had significantly lower serum insulin levels than those with normal serum Ca levels (p < 0.001) during hypoglycemic episodes. However, serum C-peptide level at 0.5 h and serum insulin level at 1 h was higher in patients with hypercalcemia than in patients with normal serum Ca levels in the oral glucose tolerance test (OGTT), although the differences were not statistically significant. Multifocal pancreatic neuroendocrine tumors (pNETs) were present in 38 (69.1%) patients; most of them (55.6%, 20/36) underwent multiple enucleations, and 45% (9/20) had a postoperative recurrence. Five patients (10%) who underwent distal pancreatectomy developed pancreatic insufficiency after an average of seven years. patients who underwent genetic testing, 23 (95.8%) were positive for MEN1 mutation, with mutations most commonly found in exons 2 (21.7%) and 3 (13%).

Conclusions

In our study, the rates of postoperative recurrence and long-term complications in patients with MEN1 with multifocal pNETs were significantly different from those in other international centers and might be related to the choice of surgical method. In addition, elevated serum Ca levels in patients with primary hyperparathyroidism may affect insulin secretion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02370-1.

Endoplasmic Reticulum (ER) Stress and Its Role in Pancreatic β-Cell Dysfunction and Senescence in Type 2 Diabetes

An increased life span and accompanying nutritional affluency have led to a rapid increase in diseases associated with aging, such as obesity and type 2 diabetes, imposing a tremendous economic and health burden on society. Pancreatic β-cells are crucial for controlling glucose homeostasis by properly producing and secreting the glucose-lowering hormone insulin, and the dysfunction of β-cells determines the outcomes for both type 1 and type 2 diabetes. As the native structure of insulin is formed within the endoplasmic reticulum (ER), ER homeostasis should be appropriately maintained to allow for the proper metabolic homeostasis and functioning of β-cells. Recent studies have found that cellular senescence is critically linked with cellular stresses, including ER stress, oxidative stress, and mitochondrial stress. These studies implied that β-cell senescence is caused by ER stress and other cellular stresses and contributes to β-cells’ dysfunction and the impairment of glucose homeostasis. This review documents and discusses the current understanding of cellular senescence, β-cell function, ER stress, its associated signaling mechanism (unfolded protein response), and the effect of ER stress on β-cell senescence and dysfunction.