Calcium-activated calpain-2 is a mediator of beta cell dysfunction and apoptosis in type 2 diabetes

Dysfunctional β-cell longevity in diabetes relies on energy conservation and positive epistasis

Long-lived PFKFB3-expressing β-cells are dysfunctional partly because of prevailing glycolysis that compromises metabolic coupling of insulin secretion. Their accumulation in type 2 diabetes (T2D) appears to be related to the loss of apoptotic competency of cell fitness competition that maintains islet function by favoring constant selection of healthy "winner" cells. To investigate how PFKFB3 can disguise the competitive traits of dysfunctional "loser" β-cells, we analyzed the overlap between human β-cells with bona fide "loser signature" across diabetes pathologies using the HPAP scRNA-seq and spatial transcriptomics of PFKFB3-positive β-cells from nPOD T2D pancreata. The overlapping transcriptional profile of "loser" β-cells was represented by down-regulated ribosomal biosynthesis and genes encoding for mitochondrial respiration. PFKFB3-positive "loser" β-cells had the reduced expression of HLA class I and II genes. Gene-gene interaction analysis revealed that PFKFB3 rs1983890 can interact with the anti-apoptotic gene MAIP1 implicating positive epistasis as a mechanism for prolonged survival of "loser" β-cells in T2D. Inhibition of PFKFB3 resulted in the clearance of dysfunctional "loser" β-cells leading to restored glucose tolerance in the mouse model of T2D.

p20BAP31 promotes cell apoptosis via interaction with GRP78 and activating the PERK pathway in colorectal cancer

Colorectal cancer (CRC) is the second most deadly cancer worldwide. Although various treatments for CRC have made progress, they have limitations. Therefore, the search for new effective molecular targets is important for the treatment of CRC. p20BAP31 induces apoptosis through diverse pathways and exhibits greater sensitivity in CRC. Therefore, a comprehensive exploration of the molecular functions of p20BAP31 is important for its application in anti-tumor therapy. In this study, we showed that exogenous p20BAP31 was still located in the ER and significantly activated the unfolded protein response (UPR) through the PERK pathway. The activation of the PERK pathway is prominent in p20BAP31-induced reactive oxygen species (ROS) accumulation and apoptosis. We found, for the first time, that p20BAP31 leads to ER stress and markedly attenuates tumor cell growth in vivo. Importantly, mechanistic investigations indicated that p20BAP31 competitively binds to GRP78 from PERK and causes hyperactivation of the UPR. Furthermore, p20BAP31 upregulates the expression of GRP78 by promoting HSF1 nuclear translocation and enhancing its binding to the GRP78 promoter. These findings reveal p20BAP31 as a regulator of ER stress and a potential target for tumor therapy, and elucidate the underlying mechanism by which p20BAP31 mediates signal transduction between ER and mitochondria.

Calpain inhibition decreases oxidative stress via mitochondrial regulation in a swine model of chronic myocardial ischemia

INTRODUCTION

Calpain overexpression is implicated in mitochondrial damage leading to tissue oxidative stress and myocardial ischemic injury. The aim of this study was to determine the effects of calpain inhibition (CI) on mitochondrial impairment and oxidative stress in a swine model of chronic myocardial ischemia and metabolic syndrome.

METHODS

Yorkshire swine were fed a high-fat diet for 4 weeks to induce metabolic syndrome then underwent placement of an ameroid constrictor to the left circumflex artery. Three weeks later, animals received: no drug (control, "CON"; n= 7); a low-dose calpain inhibitor (0.12 mg/kg; "LCI", n= 7); or high-dose calpain inhibitor (0.25 mg/kg; "HCI", n=7). Treatment continued for 5 weeks, followed by tissue harvest. Cardiac tissue was assayed for protein carbonyl content, as well as antioxidant and mitochondrial protein expression. Reactive oxygen species (ROS) and mitochondrial respiration was measured in H9c2 cells following exposure to normoxia or hypoxia (1%) for 24 h with or without CI.

RESULTS

In ischemic myocardial tissue, CI was associated with decreased total oxidative stress compared to control. CI was also associated with increased expression of mitochondrial proteins superoxide dismutase 1, SDHA, and pyruvate dehydrogenase compared to control. 100 nM of calpain inhibitor decreased ROS levels and respiration in both normoxic and hypoxic H9c2 cardiomyoblasts.

CONCLUSIONS

In the setting of metabolic syndrome, CI improves oxidative stress in chronically ischemic myocardial tissue. Decreased oxidative stress may be via modulation of mitochondrial proteins involved in free radical scavenging and production.

Efficacy of IAPP suppression in mouse and human islets by GLP-1 analogue conjugated antisense oligonucleotide

Insulin resistance is the major risk factor for Type 2 diabetes (T2D). In vulnerable individuals, insulin resistance induces a progressive loss of insulin secretion with islet pathology revealing a partial deficit of beta cells and islet amyloid derived from islet amyloid polypeptide (IAPP). IAPP is co-expressed and secreted with insulin by beta cells, expression of both proteins being upregulated in response to insulin resistance. If IAPP expression exceeds the threshold for clearance of misfolded proteins, beta cell failure occurs exacerbated by the action of IAPP toxicity to compromise the autophagy lysosomal pathway. We postulated that suppression of IAPP expression by an IAPP antisense oligonucleotide delivered to beta cells by the GLP-1 agonist exenatide (eGLP1-IAPP-ASO) is a potential disease modifying therapy for T2D. While eGLP1-IAPP-ASO suppressed mouse IAPP and transgenic human IAPP expression in mouse islets, it had no discernable effects on IAPP expression in human islets under the conditions studied. Suppression of transgenic human IAPP expression in mouse islets attenuated disruption of the autophagy lysosomal pathway in beta cells, supporting the potential of this strategy.

β-cyclodextrin based nano gene delivery using pharmaceutical applications to treat Wolfram syndrome

Wolfram syndrome is a rare multisystem autosomal recessive neurodegenerative disorder that affects the brain and central nervous system. Currently, there is no cure or treatment for Wolfram syndrome. Therefore, new techniques are needed to target the loss of the WFS1 gene. Gene therapy approach to introduce a functional gene using a viral or a non-viral vector could be a treatment strategy for Wolfram syndrome 1 (WS1). Viral vectors have therapeutic benefits and greater efficiency; however, they pose a high health risk. Recently pharmaceutical therapeutic research has developed cell-penetrating non-viral nano molecules that could be used as vectors for gene delivery. Among nonviral vectors, the unique properties of β-cyclodextrin suggest that it can be a promising safe vector for gene delivery.

Linking hIAPP misfolding and aggregation with type 2 diabetes mellitus: a structural perspective

There are over 40 identified human disorders that involve certain proteins folding incorrectly, accumulating in the body causing damage to cells and organs and causing disease. Type 2 Diabetes Mellitus (T2DM) is one of these protein misfolding disorders (PMDs) and involves human islet amyloid polypeptide (hIAPP) misfolding and accumulating in parts of the body, primarily in the pancreas, causing damage to islet cells and affecting glucose regulation. In this review, we have summarised our current understanding of what causes hIAPP to misfold, what conformations are found in different parts of the body with a particular focus on what is known about the structure of hIAPP and how this links to T2DM. Understanding the molecular basis behind these misfolding events is essential for understanding the role of hIAPP to develop better therapeutics since type 2 diabetes currently affects over 4.9 million people in the United Kingdom alone and is predicted to increase as our population ages.

β-cell-specific deletion of PFKFB3 restores cell fitness competition and physiological replication under diabetogenic stress

HIF1α and PFKFB3 play a critical role in the survival of damaged β-cells in type–2 diabetes while rendering β-cells non-responsive to glucose stimulation. To discriminate the role of PFKFB3 from HIF1α in vivo, we generated mice with conditional β-cell specific disruption of the Pfkfb3 gene on a human islet pancreatic polypeptide (hIAPP^+/−) background and a high-fat diet (HFD) [PFKFB3^βKO + diabetogenic stress (DS)]. PFKFB3 disruption in β-cells under DS led to selective purging of hIAPP-damaged β-cells and the disappearance of insulin- and glucagon positive bihormonal cells. PFKFB3 disruption induced a three-fold increase in β-cell replication as evidenced by minichromosome maintenance 2 protein (MCM2) expression. Unlike high-, lower DS or switch to restricted chow diet abolished HIF1α levels and reversed glucose intolerance of PFKFB3^βKO DS mice. Our data suggest that replication and functional recovery of β-cells under DS depend on β-cell competitive and selective purification of HIF1α and PFKFB3-positive β-cells.

β-cell specific deletion of PFKFB3 results in removal of bihormonal cells and increase in β-cell replication, suggesting that this could lead to β-cell replenishment in type–2 diabetes.

Research progress on the mechanism of beta-cell apoptosis in type 2 diabetes mellitus

Type 2 diabetes mellitus(T2DM) is regarded as one of the most severe chronic metabolic diseases worldwide, which poses a great threat to human safety and health. The main feature of T2DM is the deterioration of pancreatic beta-cell function. More and more studies have shown that the decline of pancreatic beta-cell function in T2DM can be attributable to beta-cell apoptosis, but the exact mechanisms of beta-cell apoptosis in T2DM are not yet fully clarified. Therefore, in this review, we will focus on the current status and progress of research on the mechanism of pancreatic beta-cell apoptosis in T2DM, to provide new ideas for T2DM treatment strategies.

Islet amyloid toxicity: From genesis to counteracting mechanisms

Islet amyloid polypeptide (IAPP or amylin) is a hormone co-secreted with insulin by pancreatic β-cells and is the major component of islet amyloid. Islet amyloid is found in the pancreas of patients with type 2 diabetes (T2D) and may be involved in β-cell dysfunction and death, observed in this disease. Thus, investigating the aspects related to amyloid formation is relevant to the development of strategies towards β-cell protection. In this sense, IAPP misprocessing, IAPP overproduction, and disturbances in intra- and extracellular environments seem to be decisive for IAPP to form islet amyloid. Islet amyloid toxicity in β-cells may be triggered in intra- and/or extracellular sites by membrane damage, endoplasmic reticulum stress, autophagy disruption, mitochondrial dysfunction, inflammation, and apoptosis. Importantly, different approaches have been suggested to prevent islet amyloid cytotoxicity, from inhibition of IAPP aggregation to attenuation of cell death mechanisms. Such approaches have improved β-cell function and prevented the development of hyperglycemia in animals. Therefore, counteracting islet amyloid may be a promising therapy for T2D treatment.

Genetically encoded cell-death indicators (GEDI) to detect an early irreversible commitment to neurodegeneration

Cell death is a critical process that occurs normally in health and disease. However, its study is limited due to available technologies that only detect very late stages in the process or specific death mechanisms. Here, we report the development of a family of fluorescent biosensors called genetically encoded death indicators (GEDIs). GEDIs specifically detect an intracellular Ca²⁺ level that cells achieve early in the cell death process and that marks a stage at which cells are irreversibly committed to die. The time-resolved nature of a GEDI delineates a binary demarcation of cell life and death in real time, reformulating the definition of cell death. We demonstrate that GEDIs acutely and accurately report death of rodent and human neurons in vitro, and show that GEDIs enable an automated imaging platform for single cell detection of neuronal death in vivo in zebrafish larvae. With a quantitative pseudo-ratiometric signal, GEDIs facilitate high-throughput analysis of cell death in time-lapse imaging analysis, providing the necessary resolution and scale to identify early factors leading to cell death in studies of neurodegeneration.

Cell death is a critical process in health and disease, yet available markers record later stages of cell death once a cell has already begun to decompose. Here the authors show the use of a genetically encoded calcium indicator that demarcates an irreversible stage of cell death earlier than previously possible.

MicroRNA-137-3p Protects PC12 Cells Against Oxidative Stress by Downregulation of Calpain-2 and nNOS

The imbalance between excess reactive oxygen species (ROS) generation and insufficient antioxidant defenses contribute to a range of neurodegenerative diseases. High ROS levels damage cellular macromolecules such as DNA, proteins and lipids, leading to neuron vulnerability and eventual death. However, the underlying molecular mechanism of the ROS regulation is not fully elucidated. Recently, an increasing number of studies suggest that microRNAs (miRNAs) emerge as the targets in regulating oxidative stress. We recently reported the neuroprotective effect of miR-137-3p for brachial plexus avulsion-induced motoneuron death. The present study is sought to investigate whether miR-137-3p also could protect PC12 cells against hydrogen peroxide (H2O2) induced neurotoxicity. By using cell viability assay, ROS assay, gene and protein expression assay, we found that PC-12 cells exposed to H2O2 exhibited decreased cell viability, increased expression levels of calpain-2 and neuronal nitric oxide synthase (nNOS), whereas a decreased miR-137-3p expression. Importantly, restoring the miR-137-3p levels in H2O2 exposure robustly inhibited the elevated nNOS, calpain-2 and ROS expression levels, which subsequently improved the cell viability. Furthermore, the suppressive effect of miR-137-3p on the elevated ROS level under oxidative stress was considerably blunted when we mutated the binding site of calpain-2 targted by miR-137-3p, suggesting the critical role of calpain-2 involving the neuroprotective effect of miR-137-3p. Collectively, these findings highlight the neuroprotective role of miR-137-3p through down-regulating calpain and NOS activity, suggesting its potential role for combating oxidative stress insults in the neurodegenerative diseases.

Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

Amyloid-Mediated Mechanisms of Membrane Disruption

Protein aggregation and amyloid formation are pathogenic events underlying the development of an increasingly large number of human diseases named “proteinopathies”. Abnormal accumulation in affected tissues of amyloid β (Aβ) peptide, islet amyloid polypeptide (IAPP), and the prion protein, to mention a few, are involved in the occurrence of Alzheimer’s (AD), type 2 diabetes mellitus (T2DM) and prion diseases, respectively. Many reports suggest that the toxic properties of amyloid aggregates are correlated with their ability to damage cell membranes. However, the molecular mechanisms causing toxic amyloid/membrane interactions are still far to be completely elucidated. This review aims at describing the mutual relationships linking abnormal protein conformational transition and self-assembly into amyloid aggregates with membrane damage. A cross-correlated analysis of all these closely intertwined factors is thought to provide valuable insights for a comprehensive molecular description of amyloid diseases and, in turn, the design of effective therapies.

The human islet amyloid polypeptide in protein misfolding disorders: Mechanisms of aggregation and interaction with biomembranes

Human islet amyloid polypeptide (hIAPP), otherwise known as amylin, is a 37-residue peptide hormone which is reported to be a common factor in protein misfolding disorders such as type-2 diabetes mellitus, Alzheimer's disease and Parkinson's disease, due to deposition of insoluble hIAPP amyloid in the pancreas and brain. Multiple studies point to the importance of the peptide's interaction with biological membranes and the cytotoxicity of hIAPP species. Here, we discuss the aggregation pathways of hIAPP amyloid fibril formation and focus on the complex interplay between membrane-mediated assembly of hIAPP and the associated mechanisms of membrane damage caused by the peptide species. Mitochondrial membranes, which are unique in their lipid composition, are proposed as prime targets for the early intracellular formation of hIAPP toxic entities. We suggest that future studies should include more physiologically-relevant and in-cell studies to allow a more accurate model of in vivo interactions. Finally, we underscore an urgent need for developing effective therapeutic strategies aimed at hindering hIAPP-phospholipid interactions.

The adaptor protein APPL2 controls glucose-stimulated insulin secretion via F-actin remodeling in pancreatic β-cells

Filamentous actin (F-actin) cytoskeletal remodeling is critical for glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells, and its dysregulation causes type 2 diabetes. The adaptor protein APPL1 promotes first-phase GSIS by up-regulating soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein expression. However, whether APPL2 (a close homology of APPL1 with the same domain organization) plays a role in β-cell functions is unknown. Here, we show that APPL2 enhances GSIS by promoting F-actin remodeling via the small GTPase Rac1 in pancreatic β-cells. β-cell specific abrogation of APPL2 impaired GSIS, leading to glucose intolerance in mice. APPL2 deficiency largely abolished glucose-induced first- and second-phase insulin secretion in pancreatic islets. Real-time live-cell imaging and phalloidin staining revealed that APPL2 deficiency abolished glucose-induced F-actin depolymerization in pancreatic islets. Likewise, knockdown of APPL2 expression impaired glucose-stimulated F-actin depolymerization and subsequent insulin secretion in INS-1E cells, which were attributable to the impairment of Ras-related C3 botulinum toxin substrate 1 (Rac1) activation. Treatment with the F-actin depolymerization chemical compounds or overexpression of gelsolin (a F-actin remodeling protein) rescued APPL2 deficiency-induced defective GSIS. In addition, APPL2 interacted with Rac GTPase activating protein 1 (RacGAP1) in a glucose-dependent manner via the bin/amphiphysin/rvs-pleckstrin homology (BAR-PH) domain of APPL2 in INS-1E cells and HEK293 cells. Concomitant knockdown of RacGAP1 expression reverted APPL2 deficiency-induced defective GSIS, F-actin remodeling, and Rac1 activation in INS-1E cells. Our data indicate that APPL2 interacts with RacGAP1 and suppresses its negative action on Rac1 activity and F-actin depolymerization thereby enhancing GSIS in pancreatic β-cells.

KCNH6 protects pancreatic β-cells from endoplasmic reticulum stress and apoptosis

Adult patients with dysfunction in human ether-a-go-go 2 (hERG2) protein, encoded by KCNH6, present with hypoinsulinemia and hyperglycemia. However, the mechanism of KCNH6 action in glucose disorders has not been clearly defined. Previous studies identified that sustained endoplasmic reticulum (ER) stress-mediated apoptosis of pancreatic β-cells and directly contributed to diabetes. In the present study, we showed that Kcnh6 knockout (KO) mice had impaired glucose tolerance mediated by high ER stress levels, and showed increased apoptosis and elevated intracellular calcium levels in pancreatic β-cells. In contrast, KCNH6 overexpression in islets isolated from C57BL/6J mice attenuated ER stress induced by thapsigargin or palmitic acid. This effect contributed to better preservation of β-cells, as reflected in increased β cell survival and enhanced glucose-stimulated insulin secretion. These results were further corroborated by studies evaluating KCNH6 overexpression in KO islets. Similarly, induction of Kcnh6 in KO mice by lentivirus injection improved glucose tolerance by reducing pancreatic ER stress and apoptosis. Our data provide new insights into how Kcnh6 deficiency causes ER calcium depletion and β cell dysfunction.

Calpain inhibitor and ibudilast rescue β cell functions in a cellular model of Wolfram syndrome

Wolfram syndrome is a rare multisystem disease characterized by childhood-onset diabetes mellitus and progressive neurodegeneration. Most cases are attributed to pathogenic variants in a single gene, Wolfram syndrome 1 (WFS1). There currently is no disease-modifying treatment for Wolfram syndrome, as the molecular consequences of the loss of WFS1 remain elusive. Because diabetes mellitus is the first diagnosed symptom of Wolfram syndrome, we aimed to further examine the functions of WFS1 in pancreatic β cells in the context of hyperglycemia. Knockout (KO) of WFS1 in rat insulinoma (INS1) cells impaired calcium homeostasis and protein kinase B/Akt signaling and, subsequently, decreased cell viability and glucose-stimulated insulin secretion. Targeting calcium homeostasis with reexpression of WFS1, overexpression of WFS1's interacting partner neuronal calcium sensor-1 (NCS1), or treatment with calpain inhibitor and ibudilast reversed deficits observed in WFS1-KO cells. Collectively, our findings provide insight into the disease mechanism of Wolfram syndrome and highlight new targets and drug candidates to facilitate the development of a treatment for this disorder and similar diseases.

Diet-Induced Obesity Mice Execute Pulmonary Cell Apoptosis via Death Receptor and ER-Stress Pathways after E. coli Infection

Obesity has developed into a considerable health problem in the whole world. Escherichia coli (E. coli) can cause nosocomial pneumonia and induce cell apoptosis during injury and infection. Normal (lean) and diet-induced obesity mice (DIO, fed with high-fat diet) were chosen to perform nasal instillation with E. coli to establish a nonfatal acute pneumonia model. At 0 h, 12 h, 24 h, and 72 h postinfection, lung tissues were obtained to measure cell apoptosis. As shown in this study, both lean and DIO mice exhibited histopathological lesions of acute pneumonia and increased cell apoptosis in the lung infected with E. coli. Interestingly, the relative mRNA and protein expressions associated with either endoplasmic reticulum stress or death receptor apoptotic pathway were all dramatically increased in the DIO mice after infection, while only significant upregulation of death receptor apoptotic pathway in the lean mice at 72 h. These results indicated that the DIO mice executed excess cell apoptosis in the nonfatal acute pneumonia induced by E. coli infection through endoplasmic reticulum stress and death receptor apoptotic pathway.

ER stress increases store-operated Ca2+ entry (SOCE) and augments basal insulin secretion in pancreatic beta cells

Type 2 diabetes mellitus (T2DM) is characterized by impaired glucose-stimulated insulin secretion and increased peripheral insulin resistance. Unremitting endoplasmic reticulum (ER) stress can lead to beta-cell apoptosis and has been linked to type 2 diabetes. Although many studies have attempted to link ER stress and T2DM, the specific effects of ER stress on beta-cell function remain incompletely understood. To determine the interrelationship between ER stress and beta-cell function, here we treated insulin-secreting INS-1(832/13) cells or isolated mouse islets with the ER stress-inducer tunicamycin (TM). TM induced ER stress as expected, as evidenced by activation of the unfolded protein response. Beta cells treated with TM also exhibited concomitant alterations in their electrical activity and cytosolic free Ca2+ oscillations. As ER stress is known to reduce ER Ca2+ levels, we tested the hypothesis that the observed increase in Ca2+ oscillations occurred because of reduced ER Ca2+ levels and, in turn, increased store-operated Ca2+ entry. TM-induced cytosolic Ca2+ and membrane electrical oscillations were acutely inhibited by YM58483, which blocks store-operated Ca2+ channels. Significantly, TM-treated cells secreted increased insulin under conditions normally associated with only minimal release, e.g. 5 mm glucose, and YM58483 blocked this secretion. Taken together, these results support a critical role for ER Ca2+ depletion-activated Ca2+ current in mediating Ca2+-induced insulin secretion in response to ER stress.

Islet Amyloid Polypeptide: A Partner in Crime With Aβ in the Pathology of Alzheimer's Disease

Diabetes affects hundreds of millions of patients worldwide. Despite the advances in understanding the disease and therapeutic options, it remains a leading cause of death and of comorbidities globally. Islet amyloid polypeptide (IAPP), or amylin, is a hormone produced by pancreatic β-cells. It contributes to the maintenance of glucose physiological levels namely by inhibiting insulin and glucagon secretion as well as controlling adiposity and satiation. IAPP is a highly amyloidogenic polypeptide forming intracellular aggregates and amyloid structures that are associated with β-cell death. Data also suggest the relevance of unprocessed IAPP forms as seeding for amyloid buildup. Besides the known consequences of hyperamylinemia in the pancreas, evidence has also pointed out that IAPP has a pathological role in cognitive function. More specifically, IAPP was shown to impair the blood–brain barrier; it was also seen to interact and co-deposit with amyloid beta peptide (Aß), and possibly with Tau, within the brain of Alzheimer's disease (AD) patients, thereby contributing to diabetes-associated dementia. In fact, it has been suggested that AD results from a metabolic dysfunction in the brain, leading to its proposed designation as type 3 diabetes. Here, we have first provided a brief perspective on the IAPP amyloidogenic process and its role in diabetes and AD. We have then discussed the potential interventions for modulating IAPP proteotoxicity that can be explored for therapeutics. Finally, we have proposed the concept of a “diabetes brain phenotype” hypothesis in AD, which may help design future IAPP-centered drug developmentstrategies against AD.

Insulin: The Friend and the Foe in the Development of Type 2 Diabetes Mellitus

Insulin, a hormone produced by pancreatic β-cells, has a primary function of maintaining glucose homeostasis. Deficiencies in β-cell insulin secretion result in the development of type 1 and type 2 diabetes, metabolic disorders characterized by high levels of blood glucose. Type 2 diabetes mellitus (T2DM) is characterized by the presence of peripheral insulin resistance in tissues such as skeletal muscle, adipose tissue and liver and develops when β-cells fail to compensate for the peripheral insulin resistance. Insulin resistance triggers a rise in insulin demand and leads to β-cell compensation by increasing both β-cell mass and insulin secretion and leads to the development of hyperinsulinemia. In a vicious cycle, hyperinsulinemia exacerbates the metabolic dysregulations that lead to β-cell failure and the development of T2DM. Insulin and IGF-1 signaling pathways play critical roles in maintaining the differentiated phenotype of β-cells. The autocrine actions of secreted insulin on β-cells is still controversial; work by us and others has shown positive and negative actions by insulin on β-cells. We discuss findings that support the concept of an autocrine action of secreted insulin on β-cells. The hypothesis of whether, during the development of T2DM, secreted insulin initially acts as a friend and contributes to β-cell compensation and then, at a later stage, becomes a foe and contributes to β-cell decompensation will be discussed.

IAPP toxicity activates HIF1α/PFKFB3 signaling delaying β-cell loss at the expense of β-cell function

The islet in type 2 diabetes (T2D) is characterized by amyloid deposits derived from islet amyloid polypeptide (IAPP), a protein co-expressed with insulin by β-cells. In common with amyloidogenic proteins implicated in neurodegeneration, human IAPP (hIAPP) forms membrane permeant toxic oligomers implicated in misfolded protein stress. Here, we establish that hIAPP misfolded protein stress activates HIF1α/PFKFB3 signaling, this increases glycolysis disengaged from oxidative phosphorylation with mitochondrial fragmentation and perinuclear clustering, considered a protective posture against increased cytosolic Ca²⁺ characteristic of toxic oligomer stress. In contrast to tissues with the capacity to regenerate, β-cells in adult humans are minimally replicative, and therefore fail to execute the second pro-regenerative phase of the HIF1α/PFKFB3 injury pathway. Instead, β-cells in T2D remain trapped in the pro-survival first phase of the HIF1α injury repair response with metabolism and the mitochondrial network adapted to slow the rate of cell attrition at the expense of β-cell function.

Type 2 diabetes is associated with islet amyloid deposits derived from islet amyloid polypeptide (IAPP) expressed by β-cells. Here the authors show that IAPP misfolded protein stress induces the hypoxia inducible factor 1 alpha injury repair pathway and activates survival metabolic changes mediated by PFKFB3.

Proteomic insight into the pathogenesis of CAPN5-vitreoretinopathy

CAPN5 Neovascular Inflammatory Vitreoretinopathy (CAPN5-NIV; OMIM 193235) is a poorly-understood rare, progressive inflammatory intraocular disease with limited therapeutic options. To profile disease effector proteins in CAPN5-NIV patient vitreous, liquid vitreous biopsies were collected from two groups: eyes from control subjects (n = 4) with idiopathic macular holes (IMH) and eyes from test subjects (n = 12) with different stages of CAPN5-NIV. Samples were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Protein expression changes were evaluated by principal component analysis, 1-way ANOVA (significant p-value < 0.05), hierarchical clustering, gene ontology, and pathway representation. There were 216 differentially-expressed proteins (between CAPN5-NIV and control vitreous), including those unique to and abundant in each clinical stage. Gene ontology analysis revealed decreased synaptic signaling proteins in CAPN5-NIV vitreous compared to controls. Pathway analysis revealed that inflammatory mediators of the acute phase response and the complement cascade were highly-represented. The CAPN5-NIV vitreous proteome displayed characteristic enrichment of proteins and pathways previously-associated with non-infectious posterior uveitis, rhegmatogenous retinal detachment (RRD), age-related macular degeneration (AMD), proliferative diabetic retinopathy (PDR), and proliferative vitreoretinopathy (PVR). This study expands our knowledge of affected molecular pathways in CAPN5-NIV using unbiased, shotgun proteomic analysis rather than targeted detection platforms. The high-levels and representation of acute phase response proteins suggests a functional role for the innate immune system in CAPN5-NIV pathogenesis.

Selective deletion of endothelial cell calpain in mice reduces diabetic cardiomyopathy by improving angiogenesis

AIMS/HYPOTHESIS

The role of non-cardiomyocytes in diabetic cardiomyopathy has not been fully addressed. This study investigated whether endothelial cell calpain plays a role in myocardial endothelial injury and microvascular rarefaction in diabetes, thereby contributing to diabetic cardiomyopathy.

METHODS

Endothelial cell-specific Capns1-knockout (KO) mice were generated. Conditions mimicking prediabetes and type 1 and type 2 diabetes were induced in these KO mice and their wild-type littermates. Myocardial function and coronary flow reserve were assessed by echocardiography. Histological analyses were performed to determine capillary density, cardiomyocyte size and fibrosis in the heart. Isolated aortas were assayed for neovascularisation. Cultured cardiac microvascular endothelial cells were stimulated with high palmitate. Angiogenesis and apoptosis were analysed.

RESULTS

Endothelial cell-specific deletion of Capns1 disrupted calpain 1 and calpain 2 in endothelial cells, reduced cardiac fibrosis and hypertrophy, and alleviated myocardial dysfunction in mouse models of diabetes without significantly affecting systemic metabolic variables. These protective effects of calpain disruption in endothelial cells were associated with an increase in myocardial capillary density (wild-type vs Capns1-KO 3646.14 ± 423.51 vs 4708.7 ± 417.93 capillary number/high-power field in prediabetes, 2999.36 ± 854.77 vs 4579.22 ± 672.56 capillary number/high-power field in type 2 diabetes and 2364.87 ± 249.57 vs 3014.63 ± 215.46 capillary number/high-power field in type 1 diabetes) and coronary flow reserve. Ex vivo analysis of neovascularisation revealed more endothelial cell sprouts from aortic rings of prediabetic and diabetic Capns1-KO mice compared with their wild-type littermates. In cultured cardiac microvascular endothelial cells, inhibition of calpain improved angiogenesis and prevented apoptosis under metabolic stress. Mechanistically, deletion of Capns1 elevated the protein levels of β-catenin in endothelial cells of Capns1-KO mice and constitutive activity of calpain 2 suppressed β-catenin protein expression in cultured endothelial cells. Upregulation of β-catenin promoted angiogenesis and inhibited apoptosis whereas knockdown of β-catenin offset the protective effects of calpain inhibition in endothelial cells under metabolic stress.

CONCLUSIONS/INTERPRETATION

These results delineate a primary role of calpain in inducing cardiac endothelial cell injury and impairing neovascularisation via suppression of β-catenin, thereby promoting diabetic cardiomyopathy, and indicate that calpain is a promising therapeutic target to prevent diabetic cardiac complications.

Redox Regulation of Calpains: Consequences on Vascular Function

SIGNIFICANCE

Calpains (CAPNs) are a family of calcium-activated cysteine proteases. The ubiquitous isoforms CAPN1 and CAPN2 have been involved in the maintenance of vascular integrity, but uncontrolled CAPN activation plays a role in the pathogenesis of vascular diseases. Recent Advances: It is well accepted that chronic and acute overproduction of reactive oxygen species (ROS) is associated with the development of vascular diseases. There is increasing evidence that ROS can also affect the CAPN activity, suggesting CAPN as a potential link between oxidative stress and vascular disease.

CRITICAL ISSUES

The physiopathological relevance of ROS in regulating the CAPN activity is not fully understood but seems to involve direct effects on CAPNs, redox modifications of CAPN substrates, as well as indirect effect on CAPNs via changes in Ca²⁺ levels. Finally, CAPNs can also stimulate ROS production; however, data showing in which context ROS are the causes or the consequences of CAPN activation are missing.

FUTURE DIRECTIONS

Detailed characterization of the molecular mechanisms underlying the regulation of the different members of the CAPN system by specific ROS would help understanding the pathophysiological role of CAPN in the modulation of the vascular function. Moreover, given that CAPNs have been found in different cellular compartments such as mitochondria and nucleus as well as in the extracellular space, identification of new CAPN targets as well as their functional consequences would add new insights in the function of these enigmatic proteases.

Calpain Inhibition Restores Autophagy and Prevents Mitochondrial Fragmentation in a Human iPSC Model of Diabetic Endotheliopathy

The relationship between diabetes and endothelial dysfunction remains unclear, particularly the association with pathological activation of calpain, an intracellular cysteine protease. Here, we used human induced pluripotent stem cells-derived endothelial cells (iPSC-ECs) to investigate the effects of diabetes on vascular health. Our results indicate that iPSC-ECs exposed to hyperglycemia had impaired autophagy, increased mitochondria fragmentation, and was associated with increased calpain activity. In addition, hyperglycemic iPSC-ECs had increased susceptibility to cell death when subjected to a secondary insult-simulated ischemia-reperfusion injury (sIRI). Importantly, calpain inhibition restored autophagy and reduced mitochondrial fragmentation, concurrent with maintenance of ATP production, normalized reactive oxygen species levels and reduced susceptibility to sIRI. Using a human iPSC model of diabetic endotheliopathy, we demonstrated that restoration of autophagy and prevention of mitochondrial fragmentation via calpain inhibition improves vascular integrity. Our human iPSC-EC model thus represents a valuable platform to explore biological mechanisms and new treatments for diabetes-induced endothelial dysfunction.

MiR-451a suppressing BAP31 can inhibit proliferation and increase apoptosis through inducing ER stress in colorectal cancer

The global morbidity and mortality of colorectal cancer (CRC) are ranked the third among gastrointestinal tumors in the world. MiR-451a is associated with several types of cancer, including CRC. However, the roles and mechanisms of miR-451a in CRC have not been elucidated. BAP31 is a predicted target gene of miR-451a in our suppression subtractive hybridization library. Its relationship with miR-451a and function in CRC are unclear. We hypothesized that miR-451a could induce apoptosis through suppressing BAP31 in CRC. Immunohistochemistry and real-time PCR were used to measure BAP31 expressions in CRC tissues and pericarcinous tissues from 57 CRC patients and CRC cell lines. Dual-luciferase reporter assay was used to detect the binding of miR-451a to BAP31. The expression of BAP31 protein in CRC tissues was significantly higher than that in pericarcinous tissues, which was correlated with distant metastasis and advanced clinical stages of CRC patients. The expression of BAP31 was higher in HCT116, HT29, SW620, and DLD cells than that in the normal colonic epithelial cell line NCM460. The expression of BAP31 was absolutely down-regulated when over-expressing miR-451a in HCT116 and SW620 cells compared with control cells. Mir-451a inhibited the expression of BAP31 by binding to its 5'-UTR. Over-expressing miR-451a or silencing BAP31 suppressed the proliferation and apoptosis of CRC cells by increasing the expressions of endoplasmic reticulum stress (ERS)-associated proteins, including GRP78/BIP, BAX, and PERK/elF2α/ATF4/CHOP, which resulted in increased ERS, cytoplasmic calcium ion flowing, and apoptosis of CRC cells. These changes resulting from over-expressing miR-451a were reversed by over-expressing BAP31 with mutated miR-451a-binding sites. Over-expressing miR-451a or silencing BAP31 inhibited tumor growth by inducing ERS. The present study demonstrated that miR-451a can inhibit proliferation and increase apoptosis through inducing ERS by binding to the 5'-UTR of BAP31 in CRC.

Killing Two Angry Birds with One Stone: Autophagy Activation by Inhibiting Calpains in Neurodegenerative Diseases and Beyond

Proteolytic machineries execute vital cellular functions and their disturbances are implicated in diverse medical conditions, including neurodegenerative diseases. Interestingly, calpains, a class of Ca²⁺-dependent regulatory proteases, can modulate the degradational system of autophagy by cleaving proteins involved in this pathway. Moreover, both machineries are common players in many molecular pathomechanisms and have been targeted individually or together, as a therapeutic strategy in experimental setups. In this review, we briefly introduce calpains and autophagy, with their roles in health and disease, and focus on their direct pathologically relevant interplay in neurodegeneration and beyond. The modulation of calpain activity may comprise a promising treatment approach to attenuate the deregulation of these two essential mechanisms.

In Vivo Monitoring of Calpain Activity by Forster Resonance Energy Transfer

Calpains are a 15-member class of calcium-activated nonlysosomal neutral proteases. They are involved in many cellular processes and are highly upregulated in pathological conditions. Some are ubiquitously expressed (CAPN1, CAPN2, CAPN4, CAPN5, CAPN7, and CAPN10), but others are thought to be localized in specific tissues. The monitoring of in vivo calpain activity is required for physiological, pathological, and therapeutic evaluations. This past decade, a tool for monitoring calpain activity in such conditions was developed using Forster resonance energy transfer (FRET). Studies showed that the level of calpain activity correlates with a decrease in FRET between the two fluorescent proteins. This chapter describes the methodologies from the design of the construct to the imaging procedure and analysis to evaluate ubiquitous calpain activity in vivo.

Calpain-10 drives podocyte apoptosis and renal injury in diabetic nephropathy

BACKGROUND

Diabetic nephropathy (DN) is a progressive microvascular complication of diabetes mellitus (DM), driven largely by podocyte apoptosis. The cysteine protease Calpain 10 is known to augment apoptosis and necrosis, and is a potential therapeutic target in DN.

METHODS

Type 2 diabetes was induced in SD rats by high-fat diet (HFD) feeding and streptozotocin (STZ) injections, and simulated in vitro by culturing conditionally immortalized mouse podocytes in hyperlipidemic (PA, 100 μM) conditions. The rate of apoptosis in the renal tissues and cultured podocytes was determined by TUNEL assay. The expression of Calpain 10 and its biological effects were assayed by real-time PCR, Western blotting, immunofluorescence and electron microscopy.

RESULTS

Calpain 10 was up-regulated in the kidneys of DN rats, as well as immortalized mouse podocytes. High levels of Calpain 10 was associated with renal dysfunction and tissue destruction, and podocyte injury and apoptosis. Knockdown of Calpain 10 protected podocytes by decreasing apoptosis rate, and upregulated nephrin.

CONCLUSION

Calpain 10 is a pro-apoptotic factor in DN, and can be targeted for treating glomerular diseases.

Prolonged Exposure to Insulin Inactivates Akt and Erk1/2 and Increases Pancreatic Islet and INS1E β-Cell Apoptosis

Chronic hyperinsulinemia, in vivo, increases the resistance of peripheral tissues to insulin by desensitizing insulin signaling. Insulin, in a heterologous manner, can also cause IGF-1 resistance. The aim of the current study was to investigate whether insulin-mediated insulin and IGF-1 resistance develops in pancreatic β-cells and whether this resistance results in β-cell decompensation. Chronic exposure of rat islets or INS1E β-cells to increasing concentrations of insulin decreased Akt^S473 phosphorylation in response to subsequent acute stimulation with 10 nM insulin or IGF-1. Prolonged exposure to high insulin levels not only inhibited Akt^S473 phosphorylation, but it also resulted in a significant inhibition of the phosphorylation of P70S6 kinase and Erk_1/2 phosphorylation in response to the acute stimulation by glucose, insulin, or IGF-1. Decreased activation of Akt, P70S6K, and Erk_1/2 was associated with decreased insulin receptor substrate 2 tyrosine phosphorylation and insulin receptor β-subunit abundance; neither IGF receptor β-subunit content nor its phosphorylation were affected. These signaling impairments were associated with decreased SERCA2 expression, perturbed plasma membrane calcium current and intracellular calcium handling, increased endoplasmic reticulum stress markers such as eIF2α^S51 phosphorylation and Bip (GRP78) expression, and increased islet and β-cell apoptosis. We demonstrate that prolonged exposure to high insulin levels induces not only insulin resistance, but in a heterologous manner causes resistance to IGF-1 in rat islets and insulinoma cells resulting in decreased cell survival. These findings suggest the possibility that chronic exposure to hyperinsulinemia may negatively affect β-cell mass by increasing β-cell apoptosis.

Targeting protein misfolding to protect pancreatic beta-cells in type 2 diabetes

The islet in type 2 diabetes is characterized by beta-cell dysfunction and deficit, increased beta-cell apoptosis and amyloid deposits that derived from islet amyloid polypeptide (IAPP). In species such as humans that are vulnerable to developing type 2 diabetes, IAPP has the propensity to form toxic oligomers that contribute to beta-cell dysfunction and apoptosis, defining type 2 diabetes as a protein misfolding disorder. In this report, we review mechanisms known to contribute to protein misfolding and formation of toxic oligomers, and the deleterious consequences of these oligomers on beta-cell function and survival. Finally, we will consider approaches to prevent protein misfolding and formation of toxic oligomers as potential novel therapeutic targets for type 2 diabetes and other protein misfolding diseases.

The receptor for advanced glycation endproducts is a mediator of toxicity by IAPP and other proteotoxic aggregates: Establishing and exploiting common ground for novel amyloidosis therapies

Proteotoxicity plays a key role in many devastating human disorders, including Alzheimer's, Huntington's and Parkinson's diseases; type 2 diabetes; systemic amyloidosis; and cardiac dysfunction, to name a few. The cellular mechanisms of proteotoxicity in these disorders have been the focus of considerable research, but their role in prevalent and morbid disorders, such as diabetes, is less appreciated. There is a large body of literature on the impact of glucotoxicity and lipotoxicity on insulin-producing pancreatic β-cells, and there is increasing recognition that proteotoxicty plays a key role. Pancreatic islet amyloidosis by the hormone IAPP, the production of advanced glycation endproducts (AGE), and insulin misprocessing into cytotoxic aggregates are all sources of β-cell proteotoxicity in diabetes. AGE, produced by the reaction of reducing sugars with proteins and lipids are ligands for the receptor for AGE (RAGE), as are the toxic pre-fibrillar aggregates of IAPP produced during amyloid formation. The mechanisms of amyloid formation by IAPP in vivo or in vitro are not well understood, and the cellular mechanisms of IAPP-induced β-cell death are not fully defined. Here, we review recent findings that illuminate the factors and mechanisms involved in β-cell proteotoxicity in diabetes. Together, these new insights have far-reaching implications for the establishment of unifying mechanisms by which pathological amyloidoses imbue their injurious effects in vivo.

A novel de novo CAPN5 mutation in a patient with inflammatory vitreoretinopathy, hearing loss, and developmental delay

Mutations that activate the protease calpain-5 (CAPN5) cause a nonsyndromic adult-onset autoinflammatory eye disease characterized by uveitis, altered synaptic signaling, retinal degeneration, neovascularization, and intraocular fibrosis. We describe a pediatric patient with severe inflammatory vitreoretinopathy accompanied by hearing loss and developmental delay associated with a novel, de novo CAPN5 missense mutation (c.865C>T, p.Arg289Trp) that shows greater hyperactivation of the calpain protease, indicating a genotype–phenotype correlation that links mutation severity to proteolytic activity and the possibility of earlier onset syndromic disease with auditory and neurological abnormalities.

Wolfram Syndrome: A Case Report and Review of Clinical Manifestations, Genetics Pathophysiology, and Potential Therapies

Background

Classical Wolfram syndrome (WS) is a rare autosomal recessive disorder caused by mutations in WFS1, a gene implicated in endoplasmic reticulum (ER) and mitochondrial function. WS is characterized by insulin-requiring diabetes mellitus and optic atrophy. A constellation of other features contributes to the acronym DIDMOAD (Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, and Deafness). This review seeks to raise awareness of this rare form of diabetes so that individuals with WS are identified and provided with appropriate care.

Case

We describe a woman without risk factors for gestational or type 2 diabetes who presented with gestational diabetes (GDM) at the age of 39 years during her first and only pregnancy. Although she had optic atrophy since the age of 10 years, WS was not considered as her diagnosis until she presented with GDM. Biallelic mutations in WFS1 were identified, supporting a diagnosis of classical WS.

Conclusions

The distinct natural history, complications, and differences in management reinforce the importance of distinguishing WS from other forms of diabetes. Recent advances in the genetics and pathophysiology of WS have led to promising new therapeutic considerations that may preserve β-cell function and slow progressive neurological decline. Insight into the pathophysiology of WS may also inform strategies for β-cell preservation for individuals with type 1 and 2 diabetes.

Amyloid growth and membrane damage: Current themes and emerging perspectives from theory and experiments on Aβ and hIAPP

Alzheimer's Disease (AD) and Type 2 diabetes mellitus (T2DM) are two incurable diseases both hallmarked by an abnormal deposition of the amyloidogenic peptides Aβ and Islet Amyloid Polypeptide (IAPP) in affected tissues. Epidemiological data demonstrate that patients suffering from diabetes are at high risk of developing AD, thus making the search for factors common to the two pathologies of special interest for the design of new therapies. Accumulating evidence suggests that the toxic properties of both Aβ or IAPP are ascribable to their ability to damage the cell membrane. However, the molecular details describing Aβ or IAPP interaction with membranes are poorly understood. This review focuses on biophysical and in silico studies addressing these topics. Effects of calcium, cholesterol and membrane lipid composition in driving aberrant Aβ or IAPP interaction with the membrane will be specifically considered. The cross correlation of all these factors appears to be a key issue not only to shed light in the countless and often controversial reports relative to this area but also to gain valuable insights into the central events leading to membrane damage caused by amyloidogenic peptides. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Unfolding Novel Mechanisms of Polyphenol Flavonoids for Better Glycaemic Control: Targeting Pancreatic Islet Amyloid Polypeptide (IAPP)

Type 2 diabetes (T2D) is characterised by hyperglycaemia resulting from defective insulin secretion, insulin resistance, or both. The impact of over-nutrition and reduced physical activity, evidenced by the exponential rise in obesity and the prevalence of T2D, strongly supports the implementation of lifestyle modification programs. Accordingly, an increased consumption of fruits and plant-derived foods has been advocated, as their intake is inversely correlated with T2D prevalence; this has been attributed, in part, to their contained polyphenolic compounds. Over the last decade, a body of work has focussed on establishing the mechanisms by which polyphenolic compounds exert beneficial effects to limit carbohydrate digestion, enhance insulin-mediated glucose uptake, down-regulate hepatic gluconeogenesis and decrease oxidative stress; the latter anti-oxidative property being the most documented. Novel effects on the inhibition of glucocorticoid action and the suppression of amylin misfolding and aggregation have been identified more recently. Amyloid fibrils form from spontaneously misfolded amylin, depositing in islet cells to elicit apoptosis, beta cell degeneration and decrease insulin secretion, with amyloidosis affecting up to 80% of pancreatic islet cells in T2D. Therefore, intervening with polyphenolic compounds offers a novel approach to suppressing risk or progression to T2D. This review gives an update on the emerging mechanisms related to dietary polyphenol intake for the maintenance of glycaemic control and the prevention of T2D.

Targeting Cellular Calcium Homeostasis to Prevent Cytokine-Mediated Beta Cell Death

Pro-inflammatory cytokines are important mediators of islet inflammation, leading to beta cell death in type 1 diabetes. Although alterations in both endoplasmic reticulum (ER) and cytosolic free calcium levels are known to play a role in cytokine-mediated beta cell death, there are currently no treatments targeting cellular calcium homeostasis to combat type 1 diabetes. Here we show that modulation of cellular calcium homeostasis can mitigate cytokine- and ER stress-mediated beta cell death. The calcium modulating compounds, dantrolene and sitagliptin, both prevent cytokine and ER stress-induced activation of the pro-apoptotic calcium-dependent enzyme, calpain, and partly suppress beta cell death in INS1E cells and human primary islets. These agents are also able to restore cytokine-mediated suppression of functional ER calcium release. In addition, sitagliptin preserves function of the ER calcium pump, sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA), and decreases levels of the pro-apoptotic protein thioredoxin-interacting protein (TXNIP). Supporting the role of TXNIP in cytokine-mediated cell death, knock down of TXNIP in INS1-E cells prevents cytokine-mediated beta cell death. Our findings demonstrate that modulation of dynamic cellular calcium homeostasis and TXNIP suppression present viable pharmacologic targets to prevent cytokine-mediated beta cell loss in diabetes.

Promoting Glucose Transporter-4 Vesicle Trafficking along Cytoskeletal Tracks: PAK-Ing Them Out

Glucose is the principal cellular energy source in humans and maintenance of glucose homeostasis is critical for survival. Glucose uptake into peripheral skeletal muscle and adipose tissues requires the trafficking of vesicles containing glucose transporter-4 (GLUT4) from the intracellular storage compartments to the cell surface. Trafficking of GLUT4 storage vesicles is initiated via the canonical insulin signaling cascade in skeletal muscle and fat cells, as well as via exercise-induced contraction in muscle cells. Recent studies have elucidated steps in the signaling cascades that involve remodeling of the cytoskeleton, a process that underpins the mechanical movement of GLUT4 vesicles. This review is focused upon an alternate phosphoinositide-3 kinase-dependent pathway involving Ras-related C3 botulinum toxin substrate 1 signaling through the p21-activated kinase p21-activated kinase 1 and showcases related signaling events that co-regulate both the depolymerization and re-polymerization of filamentous actin. These new insights provide an enriched understanding into the process of glucose transport and yield potential new targets for interventions aimed to improve insulin sensitivity and remediate insulin resistance, pre-diabetes, and the progression to type 2 diabetes.

The Molecular Physiopathogenesis of Islet Amyloidosis

Human islet amyloid polypeptide or amylin (hA) is a 37-amino acid peptide hormone produced and co-secreted with insulin by pancreatic β-cells. Under physiological conditions, hA regulates a broad range of biological processes including insulin release and slowing of gastric emptying, thereby maintaining glucose homeostasis. However, under the pathological conditions associated with type 2 diabetes mellitus (T2DM), hA undergoes a conformational transition from soluble random coil monomers to alpha-helical oligomers and insoluble β-sheet amyloid fibrils or amyloid plaques. There is a positive correlation between hA oligomerization/aggregation, hA toxicity, and diabetes progression. Because the homeostatic balance between hA synthesis, release, and uptake is lost in diabetics and hA aggregation is a hallmark of T2DM, this chapter focuses on the biophysical and cell biology studies investigating molecular mechanisms of hA uptake, trafficking, and degradation in pancreatic cells and its relevance to h's toxicity. We will also discuss the regulatory role of endocytosis and proteolytic pathways in clearance of toxic hA species. Finally, we will discuss potential pharmacological approaches for specific targeting of hA trafficking pathways and toxicity in islet β-cells as potential new avenues toward treatments of T2DM patients.

Cell-specific increased expression of calpastatin prevents diabetes induced by islet amyloid polypeptide toxicity

The islet in type 2 diabetes (T2D) shares many features of the brain in protein misfolding diseases. There is a deficit of β cells with islet amyloid derived from islet amyloid polypeptide (IAPP), a protein coexpressed with insulin. Small intracellular membrane-permeant oligomers, the most toxic form of IAPP, are more frequent in β cells of patients with T2D and rodents expressing human IAPP. β Cells in T2D, and affected cells in neurodegenerative diseases, share a comparable pattern of molecular pathology, including endoplasmic reticulum stress, mitochondrial dysfunction, attenuation of autophagy, and calpain hyperactivation. While this adverse functional cascade in response to toxic oligomers is well described, the sequence of events and how best to intervene is unknown. We hypothesized that calpain hyperactivation is a proximal event and tested this in vivo by β cell-specific suppression of calpain hyperactivation with calpastatin overexpression in human IAPP transgenic mice. β Cell-specific calpastatin overexpression was remarkably protective against β cell dysfunction and loss and diabetes onset. The critical autophagy/lysosomal pathway for β cell viability was protected with calpain suppression, consistent with findings in models of neurodegenerative diseases. We conclude that suppression of calpain hyperactivation is a potentially beneficial disease-modifying strategy for protein misfolding diseases, including T2D.

Proteasome regulates turnover of toxic human amylin in pancreatic cells

Toxic human amylin (hA) oligomers and aggregates are implicated in the pathogenesis of type 2 diabetes mellitus (T2DM). Although recent studies demonstrated a causal connection between hA uptake and toxicity in pancreatic cells, the mechanism of amylin's clearance following its internalization and its relationship to toxicity is yet to be determined, and hence was investigated here. Using pancreatic rat insulinoma β-cells and human islets as model systems, we show that hA, following its internalization, first accumulates in the cytosol followed by its translocation into nucleus, and to a lesser extent lysosomes, keeping the net cytosolic amylin content low. An increase in hA accumulation in the nucleus of pancreatic cells correlated with its cytotoxicity, suggesting that its excessive accumulation in the nucleus is detrimental. hA interacted with 20S core and 19S lid subunits of the β-cell proteasomal complex, as suggested by immunoprecipitation and confocal microscopy studies, which subsequently resulted in a decrease in the proteasome's proteolytic activity in these cells. In vitro binding and activity assays confirmed an intrinsic and potent ability of amylin to interact with the 20S core complex thereby modulating its proteolytic activity. Interestingly, less toxic and aggregation incapable rat amylin (rA) showed a comparable inhibitory effect on proteasome activity and protein ubiquitination, decoupling amylin aggregation/ toxicity and amylin-induced protein stress. In agreement with these studies, inhibition of proteasomal proteolytic activity significantly increased intracellular amylin content and toxicity. Taken together, our results suggest a pivotal role of proteasomes in amylin's turnover and detoxification in pancreatic cells.

Monogenic Diabetes: What It Teaches Us on the Common Forms of Type 1 and Type 2 Diabetes

To date, more than 30 genes have been linked to monogenic diabetes. Candidate gene and genome-wide association studies have identified > 50 susceptibility loci for common type 1 diabetes (T1D) and approximately 100 susceptibility loci for type 2 diabetes (T2D). About 1-5% of all cases of diabetes result from single-gene mutations and are called monogenic diabetes. Here, we review the pathophysiological basis of the role of monogenic diabetes genes that have also been found to be associated with common T1D and/or T2D. Variants of approximately one-third of monogenic diabetes genes are associated with T2D, but not T1D. Two of the T2D-associated monogenic diabetes genes-potassium inward-rectifying channel, subfamily J, member 11 (KCNJ11), which controls glucose-stimulated insulin secretion in the β-cell; and peroxisome proliferator-activated receptor γ (PPARG), which impacts multiple tissue targets in relation to inflammation and insulin sensitivity-have been developed as major antidiabetic drug targets. Another monogenic diabetes gene, the preproinsulin gene (INS), is unique in that INS mutations can cause hyperinsulinemia, hyperproinsulinemia, neonatal diabetes mellitus, one type of maturity-onset diabetes of the young (MODY10), and autoantibody-negative T1D. Dominant heterozygous INS mutations are the second most common cause of permanent neonatal diabetes. Moreover, INS gene variants are strongly associated with common T1D (type 1a), but inconsistently with T2D. Variants of the monogenic diabetes gene Gli-similar 3 (GLIS3) are associated with both T1D and T2D. GLIS3 is a key transcription factor in insulin production and β-cell differentiation during embryonic development, which perturbation forms the basis of monogenic diabetes as well as its association with T1D. GLIS3 is also required for compensatory β-cell proliferation in adults; impairment of this function predisposes to T2D. Thus, monogenic forms of diabetes are invaluable "human models" that have contributed to our understanding of the pathophysiological basis of common T1D and T2D.

Role of Calpain in Pathogenesis of Human Disease Processes

Calpains are a 15-member class of calcium activated nonlysosomal neutral proteases which are involved in a broad range of cellular function. Calpains are usually localized to the cytosol and within mitochondria. Calpastatin is an endogenous protein that specifically binds to and inhibits calpain. Overactivation of calpain has been implicated in a number of disease processes of the brain, eyes, heart, lungs, pancreas, kidneys, vascular system and skeletal muscle. Therefore, calpain may serve as a potential therapeutic target for a wide variety of disease processes. This review briefly outlines the current literature regarding the involvement of calpain overactivation in the pathogenesis of almost every organ in the body.

Molecular Structure, Membrane Interactions, and Toxicity of the Islet Amyloid Polypeptide in Type 2 Diabetes Mellitus

Human islet amyloid polypeptide (hIAPP) is the major component of the amyloid deposits found in the pancreatic islets of patients with type 2 diabetes mellitus (T2DM). Mature hIAPP, a 37-aa peptide, is natively unfolded in its monomeric state but forms islet amyloid in T2DM. In common with other misfolded and aggregated proteins, amyloid formation involves aggregation of monomers of hIAPP into oligomers, fibrils, and ultimately mature amyloid deposits. hIAPP is coproduced and stored with insulin by the pancreatic islet β-cells and is released in response to the stimuli that lead to insulin secretion. Accumulating evidence suggests that hIAPP amyloid deposits that accompany T2DM are not just an insignificant phenomenon derived from the disease progression but that hIAPP aggregation induces processes that impair the functionality and the viability of β-cells. In this review, we particularly focus on hIAPP structure, hIAPP aggregation, and hIAPP-membrane interactions. We will also discuss recent findings on the mechanism of hIAPP-membrane damage and on hIAPP-induced cell death. Finally, the development of successful antiamyloidogenic agents that prevent hIAPP fibril formation will be examined.

Islet Amyloid Polypeptide: Structure, Function, and Pathophysiology

The hormone islet amyloid polypeptide (IAPP, or amylin) plays a role in glucose homeostasis but aggregates to form islet amyloid in type-2 diabetes. Islet amyloid formation contributes to β-cell dysfunction and death in the disease and to the failure of islet transplants. Recent work suggests a role for IAPP aggregation in cardiovascular complications of type-2 diabetes and hints at a possible role in type-1 diabetes. The mechanisms of IAPP amyloid formation in vivo or in vitro are not understood and the mechanisms of IAPP induced β-cell death are not fully defined. Activation of the inflammasome, defects in autophagy, ER stress, generation of reactive oxygen species, membrane disruption, and receptor mediated mechanisms have all been proposed to play a role. Open questions in the field include the relative importance of the various mechanisms of β-cell death, the relevance of reductionist biophysical studies to the situation in vivo, the molecular mechanism of amyloid formation in vitro and in vivo, the factors which trigger amyloid formation in type-2 diabetes, the potential role of IAPP in type-1 diabetes, the development of clinically relevant inhibitors of islet amyloidosis toxicity, and the design of soluble, bioactive variants of IAPP for use as adjuncts to insulin therapy.

Type 2 diabetes as a protein misfolding disease

Type 2 diabetes (T2D) is a highly prevalent and chronic metabolic disorder. Recent evidence suggests that formation of toxic aggregates of the islet amyloid polypeptide (IAPP) might contribute to β-cell dysfunction and disease. However, the mechanism of protein aggregation and associated toxicity remains unclear. Misfolding, aggregation, and accumulation of diverse proteins in various organs is the hallmark of the group of protein misfolding disorders (PMDs), including highly prevalent illnesses affecting the central nervous system (CNS) such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this review we discuss the current understanding of the mechanisms implicated in the formation of protein aggregates in the endocrine pancreas and associated toxicity in the light of the long-standing knowledge from neurodegenerative disorders associated with protein misfolding.

Role of Cholesterol and Phospholipids in Amylin Misfolding, Aggregation and Etiology of Islet Amyloidosis

Amyloidosis is a biological event in which proteins undergo structural transitions from soluble monomers and oligomers to insoluble fibrillar aggregates that are often toxic to cells. Exactly how amyloid proteins, such as the pancreatic hormone amylin, aggregate and kill cells is still unclear. Islet amyloid polypeptide, or amylin, is a recently discovered hormone that is stored and co-released with insulin from pancreatic islet β-cells. The pathology of type 2 diabetes mellitus (T2DM) is characterized by an excessive extracellular and intracellular accumulation of toxic amylin species, soluble oligomers and insoluble fibrils, in islets, eventually leading to β-cell loss. Obesity and elevated serum cholesterol levels are additional risk factors implicated in the development of T2DM. Because the homeostatic balance between cholesterol synthesis and uptake is lost in diabetics, and amylin aggregation is a hallmark of T2DM, this chapter focuses on the biophysical and cell biology studies exploring molecular mechanisms by which cholesterol and phospholipids modulate secondary structure, folding and aggregation of human amylin and other amyloid proteins on membranes and in cells. Amylin turnover and toxicity in pancreatic cells and the regulatory role of cholesterol in these processes are also discussed.

A calcium-dependent protease as a potential therapeutic target for Wolfram syndrome

Wolfram syndrome is a genetic disorder characterized by diabetes and neurodegeneration and considered as an endoplasmic reticulum (ER) disease. Despite the underlying importance of ER dysfunction in Wolfram syndrome and the identification of two causative genes, Wolfram syndrome 1 (WFS1) and Wolfram syndrome 2 (WFS2), a molecular mechanism linking the ER to death of neurons and β cells has not been elucidated. Here we implicate calpain 2 in the mechanism of cell death in Wolfram syndrome. Calpain 2 is negatively regulated by WFS2, and elevated activation of calpain 2 by WFS2-knockdown correlates with cell death. Calpain activation is also induced by high cytosolic calcium mediated by the loss of function of WFS1. Calpain hyperactivation is observed in the WFS1 knockout mouse as well as in neural progenitor cells derived from induced pluripotent stem (iPS) cells of Wolfram syndrome patients. A small-scale small-molecule screen targeting ER calcium homeostasis reveals that dantrolene can prevent cell death in neural progenitor cells derived from Wolfram syndrome iPS cells. Our results demonstrate that calpain and the pathway leading its activation provides potential therapeutic targets for Wolfram syndrome and other ER diseases.