Amyloid inhibitors enhance survival of cultured human islets

A human antibody against pathologic IAPP aggregates protects beta cells in type 2 diabetes models

In patients with type 2 diabetes, pancreatic beta cells progressively degenerate and gradually lose their ability to produce insulin and regulate blood glucose. Beta cell dysfunction and loss is associated with an accumulation of aggregated forms of islet amyloid polypeptide (IAPP) consisting of soluble prefibrillar IAPP oligomers as well as insoluble IAPP fibrils in pancreatic islets. Here, we describe a human monoclonal antibody selectively targeting IAPP oligomers and neutralizing IAPP aggregate toxicity by preventing membrane disruption and apoptosis in vitro. Antibody treatment in male rats and mice transgenic for human IAPP, and human islet-engrafted mouse models of type 2 diabetes triggers clearance of IAPP oligomers resulting in beta cell protection and improved glucose control. These results provide new evidence for the pathological role of IAPP oligomers and suggest that antibody-mediated removal of IAPP oligomers could be a pharmaceutical strategy to support beta cell function in type 2 diabetes.

Crosstalk between phytochemicals and inflammatory signaling pathways

Novel bioactive constituents from natural sources are actively being investigated. The phytochemicals in these phenolic compounds are believed to have a variety of beneficial effects on human health. Several phenolic compounds have been found in plants. The antioxidant potential of phenols has been discussed in numerous studies along with their anti-inflammatory effects on pro-inflammatory cytokine, inducible cyclooxygenase-2, and nitric oxide synthase. Through current study, an attempt is made to outline and highlight a wide variety of inflammation-associated signaling pathways that have been modified by several natural compounds. These signaling pathways include nuclear factor-kappa B (NF-кB), activator protein (AP)-1, protein tyrosine kinases (PTKs), mitogen-activated protein kinases (MAPKs), nuclear factor erythroid 2-related factor 2 (Nrf2) transcription factors, tyrosine phosphatidylinositol 3-kinase (PI3K)/AKT, and the ubiquitin-proteasome system. In light of the influence of natural substances on signaling pathways, their impact on the production of inflammatory mediator is highlighted in this review.

Type I Diabetes Pathoetiology and Pathophysiology: Roles of the Gut Microbiome, Pancreatic Cellular Interactions, and the 'Bystander' Activation of Memory CD8+ T Cells

Type 1 diabetes mellitus (T1DM) arises from the failure of pancreatic β-cells to produce adequate insulin, usually as a consequence of extensive pancreatic β-cell destruction. T1DM is classed as an immune-mediated condition. However, the processes that drive pancreatic β-cell apoptosis remain to be determined, resulting in a failure to prevent ongoing cellular destruction. Alteration in mitochondrial function is clearly the major pathophysiological process underpinning pancreatic β-cell loss in T1DM. As with many medical conditions, there is a growing interest in T1DM as to the role of the gut microbiome, including the interactions of gut bacteria with Candida albicans fungal infection. Gut dysbiosis and gut permeability are intimately associated with raised levels of circulating lipopolysaccharide and suppressed butyrate levels, which can act to dysregulate immune responses and systemic mitochondrial function. This manuscript reviews broad bodies of data on T1DM pathophysiology, highlighting the importance of alterations in the mitochondrial melatonergic pathway of pancreatic β-cells in driving mitochondrial dysfunction. The suppression of mitochondrial melatonin makes pancreatic β-cells susceptible to oxidative stress and dysfunctional mitophagy, partly mediated by the loss of melatonin's induction of PTEN-induced kinase 1 (PINK1), thereby suppressing mitophagy and increasing autoimmune associated major histocompatibility complex (MHC)-1. The immediate precursor to melatonin, N-acetylserotonin (NAS), is a brain-derived neurotrophic factor (BDNF) mimic, via the activation of the BDNF receptor, TrkB. As both the full-length and truncated TrkB play powerful roles in pancreatic β-cell function and survival, NAS is another important aspect of the melatonergic pathway relevant to pancreatic β-cell destruction in T1DM. The incorporation of the mitochondrial melatonergic pathway in T1DM pathophysiology integrates wide bodies of previously disparate data on pancreatic intercellular processes. The suppression of Akkermansia muciniphila, Lactobacillus johnsonii, butyrate, and the shikimate pathway-including by bacteriophages-contributes to not only pancreatic β-cell apoptosis, but also to the bystander activation of CD8⁺ T cells, which increases their effector function and prevents their deselection in the thymus. The gut microbiome is therefore a significant determinant of the mitochondrial dysfunction driving pancreatic β-cell loss as well as 'autoimmune' effects derived from cytotoxic CD8⁺ T cells. This has significant future research and treatment implications.

SORLA mediates endocytic uptake of proIAPP and protects against islet amyloid deposition

Objective

Sorting-related receptor with type A repeats (SORLA) is a neuronal sorting receptor that prevents accumulation of amyloid-beta peptides, the main constituent of senile plaques in Alzheimer disease. Recent transcriptomic studies show that SORLA transcripts are also found in beta cells of pancreatic islets, yet the role of SORLA in islets is unknown. Based on its protective role in reducing the amyloid burden in the brain, we hypothesized that SORLA has a similar function in the pancreas via regulation of amyloid formation from islet amyloid polypeptide (IAPP).

Methods

We generated human IAPP transgenic mice lacking SORLA (hIAPP:SORLA KO) to assess the consequences of receptor deficiency for islet histopathology and function in vivo. Using both primary islet cells and cell lines, we further investigated the molecular mechanisms whereby SORLA controls the cellular metabolism and accumulation of IAPP.

Results

Loss of SORLA activity in hIAPP:SORLA KO resulted in a significant increase in islet amyloid deposits and associated islet cell death compared to hIAPP:SORLA WT animals. Aggravated islet amyloid deposition was observed in mice fed a normal chow diet, not requiring high-fat diet feeding typically needed to induce islet amyloidosis in mouse models. In vitro studies showed that SORLA binds to and mediates the endocytic uptake of proIAPP, but not mature IAPP, delivering the propeptide to an endolysosomal fate.

Conclusions

SORLA functions as a proIAPP-specific clearance receptor, protecting against islet amyloid deposition and associated cell death caused by IAPP.

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SORLA is an endocytic receptor for amyloidogenic peptides expressed in islet beta cells.

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SORLA mediates cellular clearance of proIAPP.

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Loss of SORLA activity in mouse models causes spontaneous islet amyloid deposition.

Human islet amyloid polypeptide: A therapeutic target for the management of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) and other metabolic disorders are often silent and go unnoticed in patients because of the lack of suitable prognostic and diagnostic markers. The current therapeutic regimens available for managing T2DM do not reverse diabetes; instead, they delay the progression of diabetes. Their efficacy (in principle) may be significantly improved if implemented at earlier stages. The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) or amylin has been associated with a gradual decrease in pancreatic β-cell function and mass in patients with T2DM. Hence, hIAPP has been recognized as a therapeutic target for managing T2DM. This review summarizes hIAPP's role in mediating dysfunction and apoptosis in pancreatic β-cells via induction of endoplasmic reticulum stress, oxidative stress, mitochondrial dysfunction, inflammatory cytokine secretion, autophagy blockade, etc. Furthermore, it explores the possibility of using intermediates of the hIAPP aggregation pathway as potential drug targets for T2DM management. Finally, the effects of common antidiabetic molecules and repurposed drugs; other hIAPP mimetics and peptides; small organic molecules and natural compounds; nanoparticles, nanobodies, and quantum dots; metals and metal complexes; and chaperones that have demonstrated potential to inhibit and/or reverse hIAPP aggregation and can, therefore, be further developed for managing T2DM have been discussed.

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Misfolded species of hIAPP form toxic oligomers in pancreatic β-cells.

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hIAPP amyloids has been detected in the pancreas of about 90% subjects with T2DM.

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Inhibitors of hIAPP aggregation can help manage T2DM.

Molecular Mechanisms of Amylin Turnover, Misfolding and Toxicity in the Pancreas

Amyloidosis is a common pathological event in which proteins self-assemble into misfolded soluble and insoluble molecular forms, oligomers and fibrils that are often toxic to cells. Notably, aggregation-prone human islet amyloid polypeptide (hIAPP), or amylin, is a pancreatic hormone linked to islet β-cells demise in diabetics. The unifying mechanism by which amyloid proteins, including hIAPP, aggregate and kill cells is still matter of debate. The pathology of type-2 diabetes mellitus (T2DM) is characterized by extracellular and intracellular accumulation of toxic hIAPP species, soluble oligomers and insoluble fibrils in pancreatic human islets, eventually leading to loss of β-cell mass. This review focuses on molecular, biochemical and cell-biology studies exploring molecular mechanisms of hIAPP synthesis, trafficking and degradation in the pancreas. In addition to hIAPP turnover, the dynamics and the mechanisms of IAPP–membrane interactions; hIAPP aggregation and toxicity in vitro and in situ; and the regulatory role of diabetic factors, such as lipids and cholesterol, in these processes are also discussed.

4-Phenylbutyrate (PBA) treatment reduces hyperglycemia and islet amyloid in a mouse model of type 2 diabetes and obesity

Amyloid deposits in pancreatic islets, mainly formed by human islet amyloid polypeptide (hIAPP) aggregation, have been associated with loss of β-cell mass and function, and are a pathological hallmark of type 2 diabetes (T2D). Treatment with chaperones has been associated with a decrease in endoplasmic reticulum stress leading to improved glucose metabolism. The aim of this work was to investigate whether the chemical chaperone 4-phenylbutyrate (PBA) prevents glucose metabolism abnormalities and amyloid deposition in obese agouti viable yellow (A^vy) mice that overexpress hIAPP in β cells (A^vy hIAPP mice), which exhibit overt diabetes. Oral PBA treatment started at 8 weeks of age, when A^vy hIAPP mice already presented fasting hyperglycemia, glucose intolerance, and impaired insulin secretion. PBA treatment strongly reduced the severe hyperglycemia observed in obese A^vy hIAPP mice in fasting and fed conditions throughout the study. This effect was paralleled by a decrease in hyperinsulinemia. Importantly, PBA treatment reduced the prevalence and the severity of islet amyloid deposition in A^vy hIAPP mice. Collectively, these results show that PBA treatment elicits a marked reduction of hyperglycemia and reduces amyloid deposits in obese and diabetic mice, highlighting the potential of chaperones for T2D treatment.

Type 2 Diabetes, Obesity, and Cancer Share Some Common and Critical Pathways

Diabetes and cancer are among the most frequent and complex diseases. Epidemiological evidence showed that the patients suffering from diabetes are significantly at higher risk for a number of cancer types. There are a number of evidence that support the hypothesis that these diseases are interlinked, and obesity may aggravate the risk(s) of type 2 diabetes and cancer. Multi-level unwanted alterations such as (epi-)genetic alterations, changes at the transcriptional level, and altered signaling pathways (receptor, cytoplasmic, and nuclear level) are the major source which promotes a number of complex diseases and such heterogeneous level of complexities are considered as the major barrier in the development of therapeutic agents. With so many known challenges, it is critical to understand the relationships and the commonly shared causes between type 2 diabetes and cancer, which is difficult to unravel and understand. Furthermore, the real complexity arises from contended corroborations that specific drug(s) (individually or in combination) during the treatment of type 2 diabetes may increase or decrease the cancer risk or affect cancer prognosis. In this review article, we have presented the recent and most updated evidence from the studies where the origin, biological background, the correlation between them have been presented or proved. Furthermore, we have summarized the methodological challenges and tasks that are frequently encountered. We have also outlined the physiological links between type 2 diabetes and cancers. Finally, we have presented and summarized the outline of the hallmarks for both these diseases, diabetes and cancer.

Targeting Human Islet Amyloid Polypeptide Aggregation and Toxicity in Type 2 Diabetes: An Overview of Peptide-Based Inhibitors

Type 2 diabetes (T2D) is a chronic metabolic disease characterized by insulin resistance and a progressive loss of pancreatic islet β-cell mass, which leads to insufficient secretion of insulin and hyperglycemia. Emerging evidence suggests that toxic oligomers and fibrils of human islet amyloid polypeptide (hIAPP) contribute to the death of β-cells and lead to T2D pathogenesis. These observations have opened new avenues for the development of islet amyloid therapies for the treatment of T2D. The peptide-based inhibitors are of great value as therapeutic agents against hIAPP aggregation in T2D owing to their biocompatibility, feasibility of synthesis and modification, high specificity, low toxicity, proteolytic stability (modified peptides), and weak immunogenicity as well as the large size of involved interfaces during self-aggregation of hIAPP. An understanding of what has been done and achieved will provide key insights into T2D pathology and assist in the discovery of more potent drug candidates for the treatment of T2D. In this article, we review various peptide-based inhibitors of hIAPP aggregation, including those derived from the hIAPP sequence and those not based on the sequence, consisting of both natural as well as unnatural amino acids and their derivatives. The present review will be beneficial in advancing the field of peptide medicine for the treatment of T2D.

Heterologous Expression of Immature Forms of Human Islet Amyloid Polypeptide in Yeast Triggers Intracellular Aggregation and Cytotoxicity

Diabetes is a major public health issue that has attained alarming levels worldwide. Pancreatic aggregates of human islet amyloid polypeptide (IAPP) represent a major histopathological hallmark of type 2 diabetes. IAPP is expressed in β-cells as pre-pro-IAPP (ppIAPP) that is first processed to pro-IAPP (pIAPP) and finally to its mature form (matIAPP), being released upon glucose stimulation together with insulin. Impairment and overload of the IAPP processing machinery seem to be associated with the accumulation of immature IAPP species and the formation of toxic intracellular oligomers, which have been associated with β-cell dyshomeostasis and apoptosis. Nevertheless, the pathological importance of these immature IAPP forms for the assembly and cytotoxicity of these oligomers is not completely understood. Here, we describe the generation and characterization of unprecedented Saccharomyces cerevisiae models recapitulating IAPP intracellular oligomerization. Expression of green fluorescent protein (GFP) fusions of human ppIAPP, pIAPP, and matIAPP proved to be toxic in yeast cells at different extents, with ppIAPP exerting the most deleterious effect on yeast growth and cell viability. Although expression of all IAPP constructs induced the formation of intracellular aggregates in yeast cells, our data point out the accumulation of insoluble oligomeric species enriched in immature ppIAPP as the trigger of the high toxicity mediated by this construct in cells expressing ppIAPP-GFP. In addition, MS/MS analysis indicated that oligomeric species found in the ppIAPP-GFP lysates contain the N-terminal sequence of the propeptide fused to GFP. These models represent powerful tools for future research focused on the relevance of immature forms in IAPP-induced toxicity. Furthermore, they are extremely useful in high-throughput screenings for genetic and chemical modulators of IAPP aggregation.

Influence of human amylin on the membrane stability of rat primary hippocampal neurons

The two most common aging-related diseases, Alzheimer's disease and type 2 diabetes mellitus, are associated with accumulation of amyloid proteins (β-amyloid and amylin, respectively). This amylin aggregation is reportedly cytotoxic to neurons. We found that aggregation of human amylin (hAmylin) induced neuronal apoptosis without obvious microglial infiltration in vivo. High concentrations of hAmylin irreversibly aggregated on the surface of the neuronal plasma membrane. Long-term incubation with hAmylin induced morphological changes in neurons. Moreover, hAmylin permeabilized the neuronal membrane within 1 min in a manner similar to Triton X-100, allowing impermeable fluorescent antibodies to enter the neurons and stain intracellular antigens. hAmylin also permeabilized the cell membrane of astrocytes, though more slowly. Under scanning electron microscopy, we observed that hAmylin destroyed the integrity of the cell membranes of both neurons and astrocytes. Additionally, it increased intracellular reactive oxygen species generation and reduced the mitochondrial membrane potential. Thus, by aggregating on the surface of neurons, hAmylin impaired the cell membrane integrity, induced reactive oxygen species production, reduced the mitochondrial membrane potential, and ultimately induced neuronal apoptosis.

Use of the PET ligand florbetapir for in vivo imaging of pancreatic islet amyloid deposits in hIAPP transgenic mice

AIMS/HYPOTHESIS

Islet amyloid deposits contribute to beta cell dysfunction and death in most individuals with type 2 diabetes but non-invasive methods to determine the presence of these pathological protein aggregates are currently not available. Therefore, we examined whether florbetapir, a radiopharmaceutical agent used for detection of amyloid-β deposits in the brain, also allows identification of islet amyloid in the pancreas.

METHODS

Saturation binding assays were used to determine the affinity of florbetapir for human islet amyloid polypeptide (hIAPP) aggregates in vitro. Islet amyloid-prone transgenic mice that express hIAPP in their beta cells and amyloid-free non-transgenic control mice were used to examine the ability of florbetapir to detect islet amyloid deposits in vitro, in vivo and ex vivo. Mice or mouse pancreases were subjected to autoradiographic, histochemical and/or positron emission tomography (PET) analyses to assess the utility of florbetapir in identifying islet amyloid.

RESULTS

In vitro, florbetapir bound synthetic hIAPP fibrils with a dissociation constant of 7.9 nmol/l. Additionally, florbetapir bound preferentially to amyloid-containing hIAPP transgenic vs amyloid-free non-transgenic mouse pancreas sections in vitro, as determined by autoradiography (16,475 ± 5581 vs 5762 ± 575 density/unit area, p < 0.05). In hIAPP transgenic and non-transgenic mice fed a high-fat diet for 1 year, intravenous administration of florbetapir followed by PET scanning showed that the florbetapir signal was significantly higher in amyloid-laden hIAPP transgenic vs amyloid-free non-transgenic pancreases in vivo during the first 5 min of the scan (36.83 ± 2.22 vs 29.34 ± 2.03 standardised uptake value × min, p < 0.05). Following PET, pancreases were excised and florbetapir uptake was determined ex vivo by γ counting. Pancreatic uptake of florbetapir was significantly correlated with the degree of islet amyloid deposition, the latter assessed by histochemistry (r = 0.74, p < 0.001).

CONCLUSIONS/INTERPRETATION

Florbetapir binds to islet amyloid deposits in a specific and quantitative manner. In the future, florbetapir may be useful as a non-invasive tool to identify islet amyloid deposits in humans.

In Vivo Mitigation of Amyloidogenesis through Functional-Pathogenic Double-Protein Coronae

Amyloid diseases are global epidemics with no cure available. Herein, we report a first demonstration of in vivo mitigation of amyloidogenesis using biomimetic nanotechnology. Specifically, the amyloid fragments (b_a) of β-lactoglobulin, a whey protein, were deposited onto the surfaces of carbon nanotubes (b_aCNT), which subsequently sequestered human islet amyloid polypeptide (IAPP) through functional-pathogenic double-protein coronae. Conformational changes at the b_a-IAPP interface were studied by Fourier transform infrared, circular dichroism, and X-ray scattering spectroscopies. b_aCNT eliminated the toxic IAPP species from zebrafish embryos, as evidenced by the assays of embryonic development, cell morphology, hatching, and survival as well as suppression of oxidative stress. In addition to IAPP, b_aCNT also displayed high potency against the toxicity of amyloid-β, thereby demonstrating the broad applicability of this biomimetic nanotechnology and the use of an embryonic zebrafish model for the high-throughput screening of a range of amyloidogenesis and their inhibitors in vivo.

Accelerated pre-senile systemic amyloidosis in PACAP knockout mice - a protective role of PACAP in age-related degenerative processes

Dysregulation of neuropeptides may play an important role in aging‐induced impairments. Among them, pituitary adenylate cyclase‐activating polypeptide (PACAP) is a potent cytoprotective peptide that provides an endogenous control against a variety of tissue‐damaging stimuli. We hypothesized that the progressive decline of PACAP throughout life and the well‐known general cytoprotective effects of PACAP lead to age‐related pathophysiological changes in PACAP deficiency, supported by the increased vulnerability to various stressors of animals partially or totally lacking PACAP. Using young and aging CD1 PACAP knockout (KO) and wild type (WT) mice, we demonstrated pre‐senile amyloidosis in young PACAP KO animals and showed that senile amyloidosis appeared accelerated, more generalized, more severe, and affected more individuals. Histopathology showed age‐related systemic amyloidosis with mainly kidney, spleen, liver, skin, thyroid, intestinal, tracheal, and esophageal involvement. Mass spectrometry‐based proteomic analysis, reconfirmed with immunohistochemistry, revealed that apolipoprotein‐AIV was the main amyloid protein in the deposits together with several accompanying proteins. Although the local amyloidogenic protein expression was disturbed in KO animals, no difference was found in laboratory lipid parameters, suggesting a complex pathway leading to increased age‐related degeneration with amyloid deposits in the absence of PACAP. In spite of no marked inflammatory histological changes or blood test parameters, we detected a disturbed cytokine profile that possibly creates a pro‐inflammatory milieu favoring amyloid deposition. In summary, here we describe accelerated systemic senile amyloidosis in PACAP gene‐deficient mice, which might indicate an early aging phenomenon in this mouse strain. Thus, PACAP KO mice could serve as a model of accelerated aging with human relevance. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Regulated Cell Death Seen through the Lens of Islet Transplantation

Clinical islet transplantation effectively restores euglycemia and corrects glycosylated hemoglobin in labile type 1 diabetes mellitus (T1DM). Despite marked improvements in islet transplantation outcomes, acute islet cell death remains a substantial obstacle that compromises long-term engraftment outcomes. Multiple organ donors are routinely required to achieve insulin independence. Therapeutic agents that ameliorate cell death and/or control injury-related inflammatory cascades offer potential to improve islet transplant success. Apoptotic cell death has been identified as a major contributor to cellular demise and therapeutic strategies that subvert initiation and consequences of apoptotic cell death have shown promise in pre-clinical models. Indeed, in numerous pathologies and diseases apoptosis has been the most extensively described form of regulated cell death. However, recent identification of novel, alternative regulated cell death pathways in other disease states and solid organ transplantation suggest that these additional pathways may also have substantial relevance in islet transplantation. These regulated, non-apoptotic cell death pathways exhibit distinct biochemical characteristics but have yet to be fully characterized within islet transplantation. We review herein the various regulated cell death pathways and highlight their relative potential contributions to islet viability, engraftment failure and islet dysfunction.

IAPP/amylin and β-cell failure: implication of the risk factors of type 2 diabetes

In type 2 diabetes (T2D), the most significant pathological change in pancreatic islets is amyloid deposits, of which a major component is islet amyloid polypeptide (IAPP), also called amylin. IAPP is expressed in β-cells and co-secreted with insulin. Together with the inhibitory effects of synthetic human IAPP (hIAPP) on insulin secretion, our studies, using hIAPP transgenic mice, in which glucose-stimulated insulin secretion was moderately reduced without amyloid deposit, and hIAPP gene-transfected β-cell lines, in which insulin secretion was markedly impaired without amyloid, predicted that soluble hIAPP-related molecules would exert cytotoxicity on β-cells. Human IAPP is one of the most aggregation-prone peptides that interact with cell membranes. While it is widely reported that soluble hIAPP oligomers promote cytotoxicity, this is still a hypothesis since the mechanisms are not yet fully defined. Several hIAPP transgenic mouse models did not develop diabetes; however, in models with backgrounds characterized for diabetic phenotypes, β-cell function and glucose tolerance did worsen, compared to those in non-transgenic models with similar backgrounds. Together with these findings, many studies on metabolic and molecular disorders induced by risk factors of T2D suggest that in T2D subjects, toxic IAPP oligomers accumulate in β-cells, impair their function, and reduce mass through disruption of cell membranes, resulting in β-cell failure. IAPP might be central to β-cell failure in T2D. Anti-amyloid aggregation therapeutics will be developed to create treatments with more durable and beneficial effects on β-cell function.

Development of proteolytically stable N-methylated peptide inhibitors of aggregation of the amylin peptide implicated in type 2 diabetes

Islet amyloid polypeptide, also known as amylin, is the main component of the amyloid deposits present in approximately 90% of people with type 2 diabetes mellitus (T2DM). In this disease, amylin aggregates into multimeric β-pleated sheet structures which cause damage to pancreatic islet β-cells. Inhibitors of early-stage amylin aggregation could therefore provide a disease-modifying treatment for T2DM. In this study, overlapping peptides were designed to target the 'binding' region (RLANFLVHSS, residues 11-20) of human amylin, and their effects on amyloid fibril formation were determined by thioflavin-T assay. The first generation peptides showed less than 50% inhibition of aggregation, but a second generation peptide (H₂N-RGANFLVHGR-CONH₂) showed strong inhibitory effects on amylin aggregation, and this was confirmed by negative stain electron microscopy. Cytotoxicity studies revealed that this peptide protected human pancreatic 1.4E7 (ECACC 10070102) insulin-secreting cells from the toxic effects of human amylin. Unlike the retro-inverso version of this peptide, which stimulated aggregation, two N-methylated peptides (H₂N-RGAmNFmLVmHGR-CONH₂ and H₂N-RGANmFLmVHmR-CONH₂) gave very clear dose-dependent inhibition of fibril formation. These two peptides were also stable against a range of different proteolytic enzymes, and in human plasma. These N-methylated peptides could provide a novel treatment for slowing progression of T2DM.

Apoptosis Repressor With Caspase Recruitment Domain Ameliorates Amyloid-Induced β-Cell Apoptosis and JNK Pathway Activation

Islet amyloid is present in more than 90% of individuals with type 2 diabetes, where it contributes to β-cell apoptosis and insufficient insulin secretion. Apoptosis repressor with caspase recruitment domain (ARC) binds and inactivates components of the intrinsic and extrinsic apoptosis pathways and was recently found to be expressed in islet β-cells. Using a human islet amyloid polypeptide transgenic mouse model of islet amyloidosis, we show ARC knockdown increases amyloid-induced β-cell apoptosis and loss, while ARC overexpression decreases amyloid-induced apoptosis, thus preserving β-cells. These effects occurred in the absence of changes in islet amyloid deposition, indicating ARC acts downstream of amyloid formation. Because islet amyloid increases c-Jun N-terminal kinase (JNK) pathway activation, we investigated whether ARC affects JNK signaling in amyloid-forming islets. We found ARC knockdown enhances JNK pathway activation, whereas ARC overexpression reduces JNK, c-Jun phosphorylation, and c-Jun target gene expression (Jun and Tnf). Immunoprecipitation of ARC from mouse islet lysates showed ARC binds JNK, suggesting interaction between JNK and ARC decreases amyloid-induced JNK phosphorylation and downstream signaling. These data indicate that ARC overexpression diminishes amyloid-induced JNK pathway activation and apoptosis in the β-cell, a strategy that may reduce β-cell loss in type 2 diabetes.

The β-cell assassin: IAPP cytotoxicity

Islet amyloid polypeptide (IAPP) forms cytotoxic oligomers and amyloid fibrils in islets in type 2 diabetes (T2DM). The causal factors for amyloid formation are largely unknown. Mechanisms of molecular folding and assembly of human IAPP (hIAPP) into β-sheets, oligomers and fibrils have been assessed by detailed biophysical studies of hIAPP and non-fibrillogenic, rodent IAPP (rIAPP); cytotoxicity is associated with the early phases (oligomers/multimers) of fibrillogenesis. Interaction with synthetic membranes promotes β-sheet assembly possibly via a transient α-helical molecular conformation. Cellular hIAPP cytotoxicity can be activated from intracellular or extracellular sites. In transgenic rodents overexpressing hIAPP, intracellular pro-apoptotic signals can be generated at different points in β-cell protein synthesis. Increased cellular trafficking of proIAPP, failure of the unfolded protein response (UPR) or excess trafficking of misfolded peptide via the degradation pathways can induce apoptosis; these data indicate that defects in intracellular handling of hIAPP can induce cytotoxicity. However, there is no evidence for IAPP overexpression in T2DM. Extracellular amyloidosis is directly related to the degree of β-cell apoptosis in islets in T2DM. IAPP fragments, fibrils and multimers interact with membranes causing disruption in vivo and in vitro These findings support a role for extracellular IAPP in β-sheet conformation in cytotoxicity. Inhibitors of fibrillogenesis are useful tools to determine the aberrant mechanisms that result in hIAPP molecular refolding and islet amyloidosis. However, currently, their role as therapeutic agents remains uncertain.

Anakinra Protects Against Serum Deprivation-Induced Inflammation and Functional Derangement in Islets Isolated From Nonhuman Primates

We investigated whether serum deprivation induces islet amyloid polypeptide (IAPP) oligomer accumulation and/or a proinflammatory response and, if so, whether the addition of interleukin (IL)-1 receptor antagonist to the culture medium can relieve the proinflammatory response during serum-deprived culture of nonhuman primate (NHP) islets. After culture in medium with and without Ana under serum-deprived culture conditions, IAPP oligomer/amyloid accumulation, in vitro viability, islet function, cytokine secretion, and posttransplantation outcome in streptozotocin-induced diabetic nude mice were determined in islets isolated from heterozygote human IAPP transgenic (hIAPP^+/- ) mice and/or NHP islets. Serum deprivation induced accumulation of IAPP oligomer, but not amyloid, in NHP islets. Anakinra (Ana) protected islets from the serum deprivation-induced impairment of in vitro viability and glucose-stimulated insulin secretion and attenuated serum deprivation-induced caspase-1 activation, transcription, and secretion of IL-1β, IL-6, and tumor necrosis factor-α in hIAPP^+/- mice and NHP islets. Supplementation of medium with Ana during serum-deprived culture also improved posttransplantation in vivo outcomes of NHP islets. In conclusion, serum deprivation induced accumulation of IAPP oligomers and proinflammatory responses in cultured isolated islets. Supplementation of the culture medium with Ana attenuated the functional impairment and proinflammatory responses induced by serum deprivation in ex vivo culture of NHP islets.

Effects of Serine 129 Phosphorylation on α-Synuclein Aggregation, Membrane Association, and Internalization

Although trace levels of phosphorylated α-synuclein (α-syn) are detectable in normal brains, nearly all α-syn accumulated within Lewy bodies in Parkinson disease brains is phosphorylated on serine 129 (Ser-129). The role of the phosphoserine residue and its effects on α-syn structure, function, and intracellular accumulation are poorly understood. Here, co-expression of α-syn and polo-like kinase 2 (PLK2), a kinase that targets Ser-129, was used to generate phosphorylated α-syn for biophysical and biological characterization. Misfolding and fibril formation of phosphorylated α-syn isoforms were detected earlier, although the fibrils remained phosphatase- and protease-sensitive. Membrane binding of α-syn monomers was differentially affected by phosphorylation depending on the Parkinson disease-linked mutation. WT α-syn binding to presynaptic membranes was not affected by phosphorylation, whereas A30P α-syn binding was greatly increased, and A53T α-syn was slightly lower, implicating distal effects of the carboxyl- on amino-terminal membrane binding. Endocytic vesicle-mediated internalization of pre-formed fibrils into non-neuronal cells and dopaminergic neurons matched the efficacy of α-syn membrane binding. Finally, the disruption of internalized vesicle membranes was enhanced by the phosphorylated α-syn isoforms, a potential means for misfolded extracellular or lumenal α-syn to access cytosolic α-syn. Our results suggest that the threshold for vesicle permeabilization is evident even at low levels of α-syn internalization and are relevant to therapeutic strategies to reduce intercellular propagation of α-syn misfolding.

Matrix Metalloproteinase-9 Protects Islets from Amyloid-induced Toxicity

Deposition of human islet amyloid polypeptide (hIAPP, also known as amylin) as islet amyloid is a characteristic feature of the pancreas in type 2 diabetes, contributing to increased β-cell apoptosis and reduced β-cell mass. Matrix metalloproteinase-9 (MMP-9) is active in islets and cleaves hIAPP. We investigated whether hIAPP fragments arising from MMP-9 cleavage retain the potential to aggregate and cause toxicity, and whether overexpressing MMP-9 in amyloid-prone islets reduces amyloid burden and the resulting β-cell toxicity. Synthetic hIAPP was incubated with MMP-9 and the major hIAPP fragments observed by MS comprised residues 1-15, 1-25, 16-37, 16-25, and 26-37. The fragments 1-15, 1-25, and 26-37 did not form amyloid fibrils in vitro and they were not cytotoxic when incubated with β cells. Mixtures of these fragments with full-length hIAPP did not modulate the kinetics of fibril formation by full-length hIAPP. In contrast, the 16-37 fragment formed fibrils more rapidly than full-length hIAPP but was less cytotoxic. Co-incubation of MMP-9 and fragment 16-37 ablated amyloidogenicity, suggesting that MMP-9 cleaves hIAPP 16-37 into non-amyloidogenic fragments. Consistent with MMP-9 cleavage resulting in largely non-amyloidogenic degradation products, adenoviral overexpression of MMP-9 in amyloid-prone islets reduced amyloid deposition and β-cell apoptosis. These findings suggest that increasing islet MMP-9 activity might be a strategy to limit β-cell loss in type 2 diabetes.

Islet amyloid inhibitors improve glucose homeostasis in a transgenic mouse model of type 2 diabetes

Increasing evidence points to the cytotoxicity of islet amyloid polypeptide (IAPP) aggregates as a major contributor to the loss of β-cell mass in type 2 diabetes. Prevention of IAPP formation represents a potential treatment to increase β-cell survival and function. The IAPP inhibitory peptide, D-ANFLVH, has been previously shown to prevent islet amyloid accumulation in cultured human islets. To assess its activity in vivo, D-ANFLVH was administered by intraperitoneal injection into a human IAPP transgenic mouse model, which replicates type 2 diabetes islet amyloid pathology. The peptide was a potent inhibitor of islet amyloid deposition, resulting in reduced islet cell apoptosis and preservation of β-cell area leading to improved glucose tolerance. These findings provide support for a key role of islet amyloid in β-cell survival and validate the application of anti-amyloid compounds as therapeutic strategies to maintain normal insulin secretion in patients with type 2 diabetes.

Assay development and multivariate scoring for high-content discovery of chemoprotectants of endoplasmic-reticulum-stress-mediated amylin-induced cytotoxicity in pancreatic beta cells

The underlying pathogenesis of type-II diabetes mellitus is in the dysfunction and selective loss of pancreatic islet β-cells, which ultimately leads to underproduction of endogenous insulin. Amylin, a 37-amino-acid human hormone that is cosecreted with insulin, helps regulate gastric emptying and maintain blood glucose homeostasis through improved postprandial satiety. It is hypothesized that amylin protofibrils cause selective loss of pancreatic β-cells in a manner similar to amyloid β aggregation in Alzheimer's disease. β-Cell death occurs in vitro when isolated human or rodent β-cells are exposed to micromolar concentrations of amylin, but the exact mechanism of selective β-cell loss in vivo remains unknown. Therefore, pursuing small-molecule drug discovery for chemoprotectants of amylin-induced β-cell toxicity is a viable phenotypic target that can lead to potential pharmacotherapies for the preservation of β-cell mass, delaying insulin dependence and allowing additional opportunities for lifestyle intervention. Additionally, chronic endoplasmic reticulum (ER) stress induced by chronic hyperglycemia and hyperlipidemia is a potentiating factor of amylin-induced β-cell loss. Herein, we describe a high-content/high-throughput screening (HTS) assay for the discovery of small molecules that are chemoprotective of amylin-induced, ER-stress-potentiated β-cell loss. We also put forth a general method for construction of a robust well-level multivariate scoring system using partial least squares regression analysis to improve high-content assay performance and to streamline the association of complex high-content data into HTS activity databases where univariate responses are typical.

Amyloid formation in human islets is enhanced by heparin and inhibited by heparinase

Islet transplantation is a promising therapy for patients with diabetes, but its long-term success is limited by many factors, including the formation of islet amyloid deposits. Heparin is employed in clinical islet transplantation to reduce clotting but also promotes fibrillization of amyloidogenic proteins. We hypothesized that heparin treatment of islets during pre-transplant culture may enhance amyloid formation leading to beta cell loss and graft dysfunction. Heparin promoted the fibrillization of human islet amyloid polypeptide (IAPP) and enhanced its toxicity to INS-1 beta cells. Heparin increased amyloid deposition in cultured human islets, but surprisingly decreased islet cell apoptosis. Treatment of human islets with heparin prior to transplantation increased the likelihood of graft failure. Removal of islet heparan sulfate glycosaminoglycans, which localize with islet amyloid deposits in type 2 diabetes, by heparinase treatment decreased amyloid deposition and protected against islet cell death. These findings raise the possibility that pretransplant treatment of human islets with heparin could potentiate IAPP aggregation and amyloid formation and may be detrimental to subsequent graft function.

De novo design of self-assembled hexapeptides as β-amyloid (Aβ) peptide inhibitors

The ability of peptides to construct specific secondary structures provides a useful function for biomaterial design that cannot be achieved with traditional organic molecules and polymers. Inhibition of amyloid formation is a promising therapeutic approach for the treatment of neurodegenerative diseases. Existing peptide-based inhibitors are mainly derived from original amyloid sequences, which have very limited sequence diversity and activity. It is highly desirable to explore other peptide-based inhibitors that are not directly derived from amyloid sequences. Here, we develop a hybrid high-throughput computational method to efficiently screen and design hexapeptide inhibitors against amyloid-β (Aβ) aggregation and toxicity from the first principle. Computationally screened/designed inhibitors are then validated for their inhibition activity using biophysical experiments. We propose and demonstrate a proof-of-concept of the "like-interacts-like" design principle that the self-assembling peptides are able to interact strongly with conformationally similar motifs of Aβ peptides and to competitively reduce Aβ-Aβ interactions, thus preventing Aβ aggregation and Aβ-induced toxicity. Such a de novo design can also be generally applicable to design new peptide inhibitors against other amyloid diseases, beyond traditional peptide inhibitors with homologous sequences to parent amyloid peptides.

Human IAPP amyloidogenic properties and pancreatic β-cell death

A hallmark of type 2 diabetes mellitus (T2DM) is the presence of extracellular amyloid deposits in the islets of Langerhans. These deposits are formed by the human islet amyloid polypeptide, hIAPP (or amylin), which is a hormone costored and cosecreted with insulin. Under normal conditions, the hormone remains in solution but, in the pancreas of T2DM individuals, it undergoes misfolding giving rise to oligomers and cross-β amyloid fibrils. Accumulating evidence suggests that the amyloid deposits that accompany type 2 diabetes mellitus are not just a trivial epiphenomenon derived from the disease progression. Rather, hIAPP aggregation induces processes that impair the functionality and viability of β-cells and may lead to apoptosis. The present review article aims to summarize a few aspects of the current knowledge of this amyloidogenic polypeptide. In the first place, the physicochemical properties which condition its propensity to misfold and form aggregates. Secondly, how these properties confer hIAPP the capacity to interfere with some signaling of the pancreatic β-cell, interact with membranes, form channels or affect natural ion channels, including calcium channels. Finally, how misfolded hIAPP cytotoxicity results in apoptosis. A number of pathophysiological changes of the T2DM islet can be related to the amyloidogenic properties of hIAPP. However, in a certain way, the in vivo aggregation of the polypeptide also reflects a failure of chaperones and, in general, of cellular proteostasis, supporting the view that T2DM may also be considered as a conformational disorder.

Molecular and cytotoxic properties of hIAPP17-29 and rIAPP17-29 fragments: a comparative study with the respective full-length parent polypeptides

The human islet polypeptide (hIAPP) or amylin is a 37-residue peptide hormone secreted by β-cells of the islet of Langerhans in the pancreas. Unlike the rat variant of IAPP (rIAPP), human amylin is highly amyloidogenic and is found as amyloid deposits in nearly 95% of patients afflicted with type 2 diabetes mellitus (T2DM). Human and rat IAPP have nearly identical primary sequence differing at only six positions which are encompassed within the 17-29 aminoacid region. Using Circular Dichroism (CD), Dynamic Light Scattering (DLS) and ThT-fluorescence (Th-T), we examined the aggregation properties of both full-length hIAPP1-37 and the related peptide fragment hIAPP17-29. For the sake of comparison, similar experiments were carried out on the respective rat variants rIAPP1-37 and rIAPP17-29. These studies were conducted at physiological pH in buffered solution not containing fluorinated co-solvents as well as in the presence of model membranes (LUV). In addition, the cytotoxic activity of the investigated peptides was determined toward different pancreatic β-cell lines. All the peptide studied in this work resulted cytotoxic despite β-sheet structure being observed, in vitro, for the hIAPP1-37 only. This suggests that β-sheet conformational transition that generally precedes the fibril formation, is not a prerequisite for toxicity towards β-cells. Interestingly, confocal microscopy indicated that the IAPP peptides can enter the cell and might exert their toxic action at an intracellular level.

Cross-sequence interactions between human and rat islet amyloid polypeptides

Human islet amyloid polypeptide (hIAPP) can assemble into toxic oligomers and fibrils, which are associated with cell degeneration and the pathogenesis of type 2 diabetes. Cross-interaction of hIAPP with rat IAPP (rIAPP)--a non-amyloidogenic peptide with high sequence similarity to hIAPP--might influence the aggregation and toxicity of hIAPP. However, the exact role of rIAPP in hIAPP aggregation and toxicity still remains unclear. In this work, we investigated the effect of cross-sequence interactions between full-length hIAPP(1-37) and rIAPP(1-37) on hybrid amyloid structures, aggregation kinetics, and cell toxicity using combined computational and experimental approaches. Experimental results indicate a contrasting role of rIAPP in hIAPP aggregation, in which rIAPP initially inhibits the early aggregation and nuclei formation of hIAPP, but hIAPP seeds can also recruit both hIAPP and rIAPP to form more hybrid fibrils, thus promoting amyloid fibrillation ultimately. The coincubation of hIAPP and rIAPP also decreases cell viability, presumably due to the formation of more toxic hybrid oligomers at the prolonged lag phase. Comparative MD simulations confirm that the cross-sequence interactions between hIAPP and rIAPP stabilize β-sheet structure and thus likely promote their fibrillization. This work provides valuable insights into a critical role of cross-amyloid interactions in protein aggregation.

Stress and the inflammatory process: a major cause of pancreatic cell death in type 2 diabetes

Type 2 diabetes (T2D) is a complex metabolic disorder characterized by hyperglycemia in the context of insulin resistance, which precedes insulin deficiency as a result of β-cell failure. Accumulating evidence indicates that β-cell loss in T2D results as a response to the combination of oxidative stress and endoplasmic reticulum (ER) stress. Failure of the ER's adaptive capacity and further activation of the unfolded protein response may trigger macroautophagy (hereafter referred as autophagy) as a process of self-protection and inflammation. Many studies have shown that inflammation plays a very important role in the pathogenesis of T2D. Inflammatory mechanisms and cytokine production activated by stress via the inflammasome may further alter the normal structure of β-cells by inducing pancreatic islet cell apoptosis. Thus, the combination of oxidative and ER stress, together with autophagy insufficiency and inflammation, may contribute to β-cell death or dysfunction in T2D. Therapeutic approaches aimed at ameliorating stress and inflammation may therefore prove to be promising targets for the development of new diabetes treatment methods. Here, we discuss different mechanisms involved in stress and inflammation, and the role of antioxidants, endogenous and chemical chaperones, and autophagic pathways, which may shift the tendency from ER stress and apoptosis toward cell survival. Strategies targeting cell survival can be essential for relieving ER stress and reestablishing homeostasis, which may diminish inflammation and prevent pancreatic β-cell death associated with T2D.

Full length amylin oligomer aggregation: insights from molecular dynamics simulations and implications for design of aggregation inhibitors

Amyloid oligomers are considered to play essential roles in the pathogenesis of amyloid-related degenerative diseases including type 2 diabetes. Using an explicit solvent all atomic MD simulation, we explored the stability, conformational dynamics and association force of different single-layer models of the full-length wild-type and glycine mutants of amylin (pentamer) obtained from a recent high resolution fibril model. The RMSF profile shows enhanced flexibility in the disorder (Lys1-Cys7) and turn region (Ser19-Gly23), along with smallest fluctuation at the residues (Asn14-Phe15-Leu16-Val17-His18) of β1 region and (Ala25-Ile26-Leu27-Ser28-Ser29) of the β2 region. We obtained a significant difference in backbone RMSD between the wild-type and the mutants, indicating that mutations affected the stability of the peptide. The RMSD and RMSF profiles indicate the edge and loop residues are the primary contributors to the overall conformational changes. The degree of structural similarity between the oligomers in the simulation and the fibril conformation is proposed as the possible explanation for experimentally observed shortening of the nucleation lag phase of amylin with oligomer seeding. On the basis of structure-stability findings, the β1 and β2 portions are optimal target for further anti-amyloid drug design. The MM-PBSA binding energy calculation reveals the binding of amylin: amylin strands in single layer is dominated by contributions from van der Waals interactions. The non-polar solvation term is also found to be favorable. While the electrostatic interactions and polar solvation energy was found to be favorable for the interaction for the larger aggregate and unfavorable for the smaller aggregates. A per-residue decomposition of the binding free energy has been performed to identify the residues contributing most to the self-association free energy. Residues found in the β-sheet regions were found to be key residue making the largest favorable contributions to the single-layer association. The result from our simulation could be used in rational design of new amylinomimetic agent, amylin aggregation inhibitors and amylin-specific biomarkers.

Distinct internalization pathways of human amylin monomers and its cytotoxic oligomers in pancreatic cells

Toxic human amylin oligomers and aggregates are implicated in the pathogenesis of type 2 diabetes mellitus (TTDM). Although recent studies have shown that pancreatic cells can recycle amylin monomers and toxic oligomers, the exact uptake mechanism and trafficking routes of these molecular forms and their significance for amylin toxicity are yet to be determined. Using pancreatic rat insulinoma (RIN-m5F) beta (β)-cells and human islets as model systems we show that monomers and oligomers cross the plasma membrane (PM) through both endocytotic and non-endocytotic (translocation) mechanisms, the predominance of which is dependent on amylin concentrations and incubation times. At low (≤ 100 nM) concentrations, internalization of amylin monomers in pancreatic cells is completely blocked by the selective amylin-receptor (AM-R) antagonist, AC-187, indicating an AM-R dependent mechanism. In contrast at cytotoxic (µM) concentrations monomers initially (1 hour) enter pancreatic cells by two distinct mechanisms: translocation and macropinocytosis. However, during the late stage (24 hours) monomers internalize by a clathrin-dependent but AM-R and macropinocytotic independent pathway. Like monomers a small fraction of the oligomers initially enter cells by a non-endocytotic mechanism. In contrast a majority of the oligomers at both early (1 hour) and late times (24 hours) traffic with a fluid-phase marker, dextran, to the same endocytotic compartments, the uptake of which is blocked by potent macropinocytotic inhibitors. This led to a significant increase in extra-cellular PM accumulation, in turn potentiating amylin toxicity in pancreatic cells. Our studies suggest that macropinocytosis is a major but not the only clearance mechanism for both amylin's molecular forms, thereby serving a cyto-protective role in these cells.

Conformationally restricted short peptides inhibit human islet amyloid polypeptide (hIAPP) fibrillization

Inhibition of human islet amyloid polypeptide (hIAPP) fibrillisation by peptides incorporating a helicogenic amino acid, dehydrophenylalanine: implications for Type-2 diabetes.

hIAPP fibrillization implicated in Type 2 diabetes pathology involves formation of oligomers toxic to insulin producing pancreatic β-cells. We report design, synthesis, 3D structure and functional characterization of dehydrophenylalanine (ΔF) containing peptides which inhibit hIAPP fibrillization. The inhibitor protects β-cells from hIAPP induced toxicity.

Amylin uncovered: a review on the polypeptide responsible for type II diabetes

Amylin is primarily responsible for classifying type II diabetes as an amyloid (protein misfolding) disease as it has great potential to aggregate into toxic nanoparticles, thereby resulting in loss of pancreatic β-cells. Although type II diabetes is on the increase each year, possibly due to bad eating habits of modern society, research on the culprit for this disease is still in its early days. In addition, unlike the culprit for Alzheimer's disease, amyloid β-peptide, amylin has failed to receive attention worthy of being featured in an abundance of review articles. Thus, the aim of this paper is to shine the spotlight on amylin in an attempt to put it onto the top of researchers' to-do list since the secondary complications of type II diabetes have far-reaching and severe consequences on public health both in developing and fully developed countries alike. This paper will cover characteristics of the amylin aggregates, mechanisms of toxicity, and a particular focus on inhibitors of toxicity and techniques used to assess these inhibitors.

Metabolic stress, IAPP and islet amyloid

Amyloid forms within pancreatic islets in type 2 diabetes from aggregates of the β-cell peptide islet amyloid polypeptide (IAPP). These aggregates are toxic to β-cells, inducing β-cell death and dysfunction, as well as inciting islet inflammation. The β-cell is subject to a number of other stressors, including insulin resistance and hyperglycaemia, that may contribute to amyloid formation by increasing IAPP production by the β-cell. β-Cell dysfunction, evident as impaired glucose-stimulated insulin secretion and defective prohormone processing and exacerbated by metabolic stress, is also a likely prerequisite for islet amyloid formation to occur in type 2 diabetes. Islet transplants in patients with type 1 diabetes face similar stressors, and are subject to rapid amyloid formation and impaired proinsulin processing associated with progressive loss of β-cell function and mass. Declining β-cell mass is predicted to increase metabolic demand on remaining β-cells, promoting a feed-forward cycle of β-cell decline.

Experimental inhibition of peptide fibrillogenesis by synthetic peptides, carbohydrates and drugs

Peptide fibrillogenesis generally begins by the transformation of normally soluble proteins into elongated aggregates which are called as amyloid. These fibrils mainly consist of ß-sheets. They share certain common characteristics such as a cross-ß x-ray diffraction pattern, association with other common proteins and typical staining by the dye Congo Red. The individual form of the deposit consists of a disease-specific peptide/protein. The disease-specific protein serves as the basis for the classification of the amyloids. The association of fibril-forming peptides/proteins with diseases makes them primary disease-targets. Understanding the molecular interactions involved in the fibril formation becomes the foremost requirement to characterize the target. Interference with these interactions of ß-sheets in vitro prevents and sometimes reverses the fibril assembly. A small molecule capable of interfering with the formation of fibril could have therapeutic applications in these diseases. This anti-aggregation approach appears to be a viable treatment option. A search for such a molecule is pursued actively world over. All types of compounds and approaches to slow down or prevent the aggregation process have been described in the literature. These efforts are reviewed in this chapter.

β-cell loss and β-cell apoptosis in human type 2 diabetes are related to islet amyloid deposition

Amyloid deposition and reduced β-cell mass are pathological hallmarks of the pancreatic islet in type 2 diabetes; however, whether the extent of amyloid deposition is associated with decreased β-cell mass is debated. We investigated the possible relationship and, for the first time, determined whether increased islet amyloid and/or decreased β-cell area quantified on histological sections is correlated with increased β-cell apoptosis. Formalin-fixed, paraffin-embedded human pancreas sections from subjects with (n = 29) and without (n = 39) diabetes were obtained at autopsy (64 ± 2 and 70 ± 4 islets/subject, respectively). Amyloid and β cells were visualized by thioflavin S and insulin immunolabeling. Apoptotic β cells were detected by colabeling for insulin and by TUNEL. Diabetes was associated with increased amyloid deposition, decreased β-cell area, and increased β-cell apoptosis, as expected. There was a strong inverse correlation between β-cell area and amyloid deposition (r = -0.42, P < 0.001). β-Cell area was selectively reduced in individual amyloid-containing islets from diabetic subjects, compared with control subjects, but amyloid-free islets had β-cell area equivalent to islets from control subjects. Increased amyloid deposition was associated with β-cell apoptosis (r = 0.56, P < 0.01). Thus, islet amyloid is associated with decreased β-cell area and increased β-cell apoptosis, suggesting that islet amyloid deposition contributes to the decreased β-cell mass that characterizes type 2 diabetes.

Clustering and internalization of toxic amylin oligomers in pancreatic cells require plasma membrane cholesterol

Self-assembly of the human pancreatic hormone amylin into toxic oligomers and aggregates is linked to dysfunction of islet β-cells and pathogenesis of type 2 diabetes mellitus. Recent evidence suggests that cholesterol, an essential component of eukaryotic cells membranes, controls amylin aggregation on model membranes. However, the pathophysiological consequence of cholesterol-regulated amylin polymerization on membranes and biochemical mechanisms that protect β-cells from amylin toxicity are poorly understood. Here, we report that plasma membrane (PM) cholesterol plays a key role in molecular recognition, sorting, and internalization of toxic amylin oligomers but not monomers in pancreatic rat insulinoma and human islet cells. Depletion of PM cholesterol or the disruption of the cytoskeleton network inhibits internalization of amylin oligomers, which in turn enhances extracellular oligomer accumulation and potentiates amylin toxicity. Confocal microscopy reveals an increased nucleation of amylin oligomers across the plasma membrane in cholesterol-depleted cells, with a 2-fold increase in cell surface coverage and a 3-fold increase in their number on the PM. Biochemical studies confirm accumulation of amylin oligomers in the medium after depletion of PM cholesterol. Replenishment of PM cholesterol from intracellular cholesterol stores or by the addition of water-soluble cholesterol restores amylin oligomer clustering at the PM and internalization, which consequently diminishes cell surface coverage and toxicity of amylin oligomers. In contrast to oligomers, amylin monomers followed clathrin-dependent endocytosis, which is not sensitive to cholesterol depletion. Our studies identify an actin-mediated and cholesterol-dependent mechanism for selective uptake and clearance of amylin oligomers, impairment of which greatly potentiates amylin toxicity.

IL-1 blockade attenuates islet amyloid polypeptide-induced proinflammatory cytokine release and pancreatic islet graft dysfunction

Islets from patients with type 2 diabetes exhibit β cell dysfunction, amyloid deposition, macrophage infiltration, and increased expression of proinflammatory cytokines and chemokines. We sought to determine whether human islet amyloid polypeptide (hIAPP), the main component of islet amyloid, might contribute to islet inflammation by recruiting and activating macrophages. Early aggregates of hIAPP, but not nonamyloidogenic rodent islet amyloid polypeptide, caused release of CCL2 and CXCL1 by islets and induced secretion of TNF-α, IL-1α, IL-1β, CCL2, CCL3, CXCL1, CXCL2, and CXCL10 by C57BL/6 bone marrow-derived macrophages. hIAPP-induced TNF-α secretion was markedly diminished in MyD88-, but not TLR2- or TLR4-deficient macrophages, and in cells treated with the IL-1R antagonist (IL-1Ra) anakinra. To determine the significance of IL-1 signaling in hIAPP-induced pancreatic islet dysfunction, islets from wild-type or hIAPP-expressing transgenic mice were transplanted into diabetic NOD/SCID recipients implanted with mini-osmotic pumps containing IL-1Ra (50 mg/kg/d) or saline. IL-1Ra significantly improved the impairment in glucose tolerance observed in recipients of transgenic grafts 8 wk following transplantation. Islet grafts expressing hIAPP contained amyloid deposits in close association with F4/80-expressing macrophages. Transgenic grafts contained 50% more macrophages than wild-type grafts, an effect that was inhibited by IL-1Ra. Our results suggest that hIAPP-induced islet chemokine secretion promotes macrophage recruitment and that IL-1R/MyD88, but not TLR2 or TLR4 signaling is required for maximal macrophage responsiveness to prefibrillar hIAPP. These data raise the possibility that islet amyloid-induced inflammation contributes to β cell dysfunction in type 2 diabetes and islet transplantation.

Unique example of amyloid aggregates stabilized by main chain H-bond instead of the steric zipper: molecular dynamics study of the amyloidogenic segment of amylin wild-type and mutants

Most proteins do not aggregate while in their native functional states. However, they may be disturbed from their native conformation by certain change in the environment, and form unwanted oligomeric or polymeric aggregates. Recent experimental data demonstrate that soluble oligomers of amyloidogenic proteins are responsible for amyloidosis and its cytotoxicity. Human islet amyloid polypeptide (IAPP or amylin) is a 37-residue hormone found as fibrillar deposits in pancreatic extracts of nearly all type II diabetics. In this study we performed in silico mutation analysis to examine the stability of the double layer five strand aggregates formed by heptapeptide NNFGAIL segment from amyline peptide. This segment is one of the shortest fragments that can form amyloid fibrils similar to those formed by the full length peptide. The mutants obtained by single glycine replacement were also studied to investigate the specificity of the dry self-complementary interface between the neighboring β-sheet layers. The molecular dynamics simulations of the aggregates run for 20 ns at 330 K, the degree of the aggregate disassembly was investigated using several geometry analysis tools: the root mean square deviations of the C(α) atoms, root mean square fluctuations per residue, twist angles, interstrand distances, fraction of the secondary structure elements, and number of H-bonds. The analysis shows that most mutations make the aggregates unstable, and their stabilities were dependent to a large extent on the position of replaced residues. Our mutational simulations are in agreement with the pervious experimental observations. We also used free binding energy calculations to determine the role of different components: nonpolar effects, electrostatics and entropy in binding. Nonpolar effects remained consistently more favorable in wild type and mutants reinforcing the importance of hydrophobic effects in protein-protein binding. While entropy systematically opposed binding in all cases, there was no clear trend in the entropy difference between wildtype and glycine mutants. Free energy decomposition shows residues situated at the interface were found to make favorable contributions to the peptide-peptide association. The study of the wild type and mutants in an explicit solvent could provide valuable insight into the future computer guided design efforts for the amyloid aggregation inhibitor.

Islet amyloid polypeptide acts on glucose- stimulated beta cells to reduce voltage-gated calcium channel activation, intracellular Ca(2+) concentration, and insulin secretion

OBJECTIVE

the mechanism by which islet amyloid polypeptide (IAPP) inhibits insulin release is unclear. We hypothesized that reduced voltage-gated calcium channel activity and intracellular Ca(2+) concentration might contribute to IAPP-mediated inhibition of glucose-stimulated insulin release.

RESEARCH DESIGN AND METHODS

rat islet beta cells were cultured and treated with various extracellular concentrations of IAPP, and insulin release was stimulated via addition of glucose. Activation voltage, high voltage-gated calcium channel currents, intracellular Ca(2+) concentration, and insulin secretion were detected by patch clamp electrophysiology, fluorescent digital imaging microscopy using calcium-sensitive fluorescent dye, and radioimmunoassay, respectively.

RESULTS

high voltage-gated calcium channel currents, intracellular Ca(2+) concentration, and insulin secretion increased in a dose-dependent manner when rat beta cells were exposed to glucose. After short-term IAPP treatment (5 or 10 µM), these parameters decreased significantly in glucose-stimulated beta cells. However, no significant changes were observed with lower doses of IAPP.

CONCLUSIONS

glucose-stimulated islet beta-cell high voltage-gated calcium channels were activated in conjunction with insulin secretion, while high extracellular concentrations of IAPP inhibited beta-cell high voltage-gated calcium channel activation and insulin secretion in a dose-dependent manner.