Macrophages and pancreatic islet amyloidosis

Protein aggregation: A detrimental symptom or an adaptation mechanism?

Protein quality control mechanisms oversee numerous aspects of protein lifetime. From the point of protein synthesis, protein homeostasis machineries take part in folding, solubilization, and/or degradation of impaired proteins. Some proteins follow an alternative path upon loss of their solubility, thus are secluded from the cytosol and form protein aggregates. Protein aggregates differ in their function and composition, rendering protein aggregation a complex phenomenon that continues to receive plenty of attention in the scientific and medical communities. Traditionally, protein aggregates have been associated with aging and a large spectrum of protein folding diseases, such as neurodegenerative diseases, type 2 diabetes, or cataract. However, a body of evidence suggests that they may act as an adaptive mechanism to overcome transient stressful conditions, serving as a sink for the removal of misfolded proteins from the cytosol or storage compartments for machineries required upon stress release. In this review, we present examples and evidence elaborating different possible roles of protein aggregation and discuss their potential roles in stress survival, aging, and disease, as well as possible anti-aggregation interventions.

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.

Formation of amyloid in encapsulated human pancreatic and human stem cell-generated beta cell implants

Detection of amyloid in intraportal islet implants of type 1 diabetes patients has been proposed as cause in their functional decline. The present study uses cultured adult human islets devoid of amyloid to examine conditions of its formation. After intraportal injection in patients, amyloid deposits <15 µm diameter were identified in 5%-12% of beta cell containing aggregates, 3-76 months posttransplant. Such deposits also formed in glucose-controlling islet implants in the kidney of diabetic mice but not in failing implants. Alginate-encapsulated islets formed amyloid during culture when functional, and in all intraperitoneal implants that corrected diabetes in mice, exhibiting larger sizes than in functioning nonencapsulated implants. After intraperitoneal injection in a patient, retrieved single capsules presented amyloid near living beta cells, whereas no amyloid occurred in clustered capsules with dead cells. Amyloid was also demonstrated in functional human stem cell-generated beta cell implants in subcutaneous devices of mice. Deposits up to 35 µm diameter were localized in beta cell-enriched regions and related to an elevated IAPP over insulin ratio in the newly generated beta cells. Amyloid in device-encapsulated human stem cell-generated beta cell implants marks the formation of a functional beta cell mass but also an imbalance between its activated state and its microenvironment.

Immune dysfunction in developmental programming of type 2 diabetes mellitus

Intrauterine growth restriction (IUGR) is a common complication of pregnancy and increases the risk of the offspring developing type 2 diabetes mellitus (T2DM) later in life. Alterations in the immune system are implicated in the pathogenesis of IUGR-induced T2DM. The development of the fetal immune system is a delicate balance as it must remain tolerant of maternal antigens whilst also preparing for the post-birth environment. In addition, the fetal immune system is susceptible to an altered intrauterine milieu caused by maternal and placental inflammatory mediators or secondary to nutrient and oxygen deprivation. Pancreatic-resident macrophages populate the pancreas during fetal development, and their phenotype is dynamic through the neonatal period. Furthermore, macrophages in the islets are instrumental in islet development as they influence β-cell proliferation and islet neogenesis. In addition, cytokines, derived from β-cells and macrophages, are important to islet homeostasis in the fetus and adult and, when perturbed, can cause islet dysfunction. Several activated immune pathways have been identified in the islets of people who experienced IUGR, with alternations in the levels of IL-1β and IL-4 as well as changes in TGFβ signalling. Leptin levels are also altered. Immunomodulation has shown therapeutic benefit in T2DM and might be particularly useful in IUGR-induced T2DM.

Human islet amyloid polypeptide (hIAPP) - a curse in type II diabetes mellitus: insights from structure and toxicity studies

The human islet amyloid polypeptide (hIAPP) or amylin, a neuroendocrine peptide hormone, is known to misfold and form amyloidogenic aggregates that have been observed in the pancreas of 90% subjects with Type 2 Diabetes Mellitus (T2DM). Under normal physiological conditions, hIAPP is co-stored and co-secreted with insulin; however, under chronic hyperglycemic conditions associated with T2DM, the overexpression of hIAPP occurs that has been associated with the formation of amyloid deposits; as well as the death and dysfunction of pancreatic β-islets in T2DM. Hitherto, various biophysical and structural studies have shown that during this process of aggregation, the peptide conformation changes from random structure to helix, then to β-sheet, subsequently to cross β-sheets, which finally form left-handed helical aggregates. The intermediates, formed during this process, have been shown to induce higher cytotoxicity in the β-cells by inducing cell membrane disruption, endoplasmic reticulum stress, mitochondrial dysfunction, oxidative stress, islet inflammation, and DNA damage. As a result, several research groups have attempted to target both hIAPP aggregation phenomenon and the destabilization of preformed fibrils as a therapeutic intervention for T2DM management. In this review, we have summarized structural aspects of various forms of hIAPP viz. monomer, oligomers, proto-filaments, and fibrils of hIAPP. Subsequently, cellular toxicity caused by toxic conformations of hIAPP has been elaborated upon. Finally, the need for performing structural and toxicity studies in vivo to fill in the gap between the structural and cellular aspects has been discussed.

IAPP and type 1 diabetes: implications for immunity, metabolism and islet transplants

Islet amyloid polypeptide (IAPP), the main component of islet amyloid in type 2 diabetes and islet transplants, is now recognized as a contributor to beta cell dysfunction. Increasingly, evidence warrants its investigation in type 1 diabetes owing to both its immunomodulatory and metabolic actions. Autoreactive T cells to IAPP-derived epitopes have been described in humans, suggesting that IAPP is an islet autoantigen in type 1 diabetes. In addition, although aggregates of IAPP have not been implicated in type 1 diabetes, they are potent pro-inflammatory stimuli to innate immune cells, and thus, could influence autoimmunity. IAPP aggregates also occur rapidly in transplanted islets and likely contribute to islet transplant failure in type 1 diabetes through sterile inflammation. In addition, since type 1 diabetes is a disease of both insulin and IAPP deficiency, clinical trials have examined the potential benefits of IAPP replacement in type 1 diabetes with the injectable IAPP analogue, pramlintide. Pramlintide limits postprandial hyperglycemia by delaying gastric emptying and suppressing hyperglucagonemia, underlining the possible role of IAPP in postprandial glucose metabolism. Here, we review IAPP in the context of type 1 diabetes: from its potential involvement in type 1 diabetes pathogenesis, through its role in glucose metabolism and use of IAPP analogues as therapeutics, to its potential role in clinical islet transplant failure and considerations in this regard for future beta cell replacement strategies.

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.

Effects of bone marrow on the microenvironment of the human pancreatic islet: A Protein Profile Approach

Stem cells are a new therapeutic modality that may support the viability and function of human organs and tissue. Our previous studies have revealed that human allogeneic bone marrow (BM) sustains pancreatic β cell function and survival. This paper examines whether BM creates a microenvironment that supports human pancreatic islets in vitro by evaluating 107 proteins in culture media from BM, islet, and islet/bone marrow (IB) with mass spectrometry. Proteins were considered up- or down-regulated if p-values < 0.05 and fold change was greater than 2 fold I VS. IB. In addition, proteins identified that were uniquely found in islets co-cultured with bone marrow, but not in islets or bone marrow. A 95% protein probability was used as a threshold. Twenty three proteins were upregulated, and sixteen proteins were downregulated. The function of each protein is listed based on the protein database, which include structural proteins (9 upregulated, 4 downregulated); anti-protease and anti-endopeptidase enzymes (8 upregulated); cation binding proteins (6 up-regulated). Six proteins were uniquely identified in islet co-cultured with bone marrow. Three are anti-proteases or anti-endopeptidases, and 1 is a structural protein. These findings suggest that BM, by changing culture media proteins, may be one of mechanisms to maintain human islet function and survival.

Islet inflammation in type 2 diabetes and physiology

The finding of islet inflammation in type 2 diabetes (T2D) and its involvement in β cell dysfunction has further highlighted the significance of inflammation in metabolic diseases. The number of intra-islet macrophages is increased in T2D, and these cells are the main source of proinflammatory cytokines within islets. Multiple human studies of T2D have shown that targeting islet inflammation has the potential to be an effective therapeutic strategy. In this Review we provide an overview of the cellular and molecular mechanisms by which islet inflammation develops and causes β cell dysfunction. We also emphasize the regulation and roles of macrophage polarity shift within islets in the context of T2D pathology and β cell health, which may have broad translational implications for therapeutics aimed at improving islet function.

Neuroendocrine hormone amylin in diabetes

The neuroendocrine hormone amylin, also known as islet amyloid polypeptide, is co-localized, co-packaged and co-secreted with insulin from adult pancreatic islet β cells to maintain glucose homeostasis. Specifically, amylin reduces secretion of nutrient-stimulated glucagon, regulates blood pressure with an effect on renin-angiotensin system, and delays gastric emptying. The physiological actions of human amylin attribute to the conformational α-helix monomers whereas the misfolding instable oligomers may be detrimental to the islet β cells and further transform to β-sheet fibrils as amyloid deposits. No direct evidence proves that the amylin fibrils in amyloid deposits cause diabetes. Here we also have performed a systematic review of human amylin gene changes and reported the S20G mutation is minor in the development of diabetes. In addition to the metabolic effects, human amylin may modulate autoimmunity and innate inflammation through regulatory T cells to impact on both human type 1 and type 2 diabetes.

Dynamic pathology of islet endocrine cells in type 2 diabetes: β-Cell growth, death, regeneration and their clinical implications

Diabetes is defined as a disease of hyperglycemic metabolic disorder caused by impaired insulin action or low insulin secretion, resulting in the occurrence of vascular complications. Based on this definition, diabetes therapy has long been oriented to correct hyperglycemia against the specific complications of diabetes. This definition has posed some difficulties, however, in understanding of the pathophysiology of this complicated disease and as such in the establishment of an effective treatment. With continuing efforts to explore the structural basis for diabetes onset and methodological development of immunohistochemistry, progressive decline of β-cells is now established as a salient feature of type 2 diabetes. Accordingly, diabetes therapy has now turned out to protect β-cells concurrently with the correction of hyperglycemia. Together with this effort, exploration of the means to regenerate β-cells or to supply new β-cells by, for example, induced pluripotential stem cells, are vigorously made with the search for the mechanism of β-cell decline in diabetes. In the present review, we describe the advances in the islet pathology in type 2 diabetes with special reference to the dynamic alterations of islet endocrine cells in the milieu of maturation, obesity, aging and ethnic differences. The effect of amyloid deposition is also discussed. We hope it will help with understanding the pathophysiology of diabetes, and suggest the future direction of diabetes treatment.

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.

Islet amyloid with macrophage migration correlates with augmented β-cell deficits in type 2 diabetic patients

AIMS

Islet amyloid is a hallmark in type 2 diabetic subjects, but its implication in clinical features and development of islet pathology is still unclear.

METHODS

From 118 autopsy cases with type 2 diabetes, 26 cases with islet amyloid deposition (DA+) were selected. Twenty diabetic subjects without obvious amyloid deposition (DA-) matched for the age and diabetes duration and 20 non-diabetic subjects (ND) served for comparison. We examined the severity of amyloid deposition and its relationships with population of endocrine cells, expression of cell damage markers or macrophage infiltration. Correlation of clinical profile with islet pathology was also sought on the subset of the investigated patients.

RESULTS

β-Cell volume density was nearly 40% less in DA+ and 20% less in DA- when compared to ND. Severity of amyloid deposition correlated with reduced volume densities of β-cell and α-cell, and increased body mass index (BMI), but not with duration of diabetes, age or HbA1c. Amyloid-rich islets contained an increased number of macrophages mixed with β-cells with oxidative stress-related DNA damage, characterized by γH2AX expression, and suppressed (pro)insulin mRNA expression.

CONCLUSIONS

In Japanese type 2 diabetic patients, islet amyloid was more common with severe β-cell loss and high BMI, associated with macrophage infiltration.

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.

Resident macrophages mediate islet amyloid polypeptide-induced islet IL-1β production and β-cell dysfunction

Islet amyloid polypeptide (IAPP) aggregates to form amyloid fibrils in patients with type 2 diabetes and acts as a potent stimulus for interleukin (IL)-1β secretion by bone marrow-derived macrophages. We sought to determine the contribution of resident islet macrophages to IAPP-induced inflammation and β-cell dysfunction. In cultured islets, macrophages (F4/80(+)CD11b(+)CD11c(+) cells) were required for IAPP-induced mRNA expression of the proinflammatory cytokines IL-1β, tumor necrosis factor-α, and IL-6 and the anti-inflammatory cytokines IL-10 and IL-1 receptor antagonist. Moreover, IAPP-induced IL-1β synthesis and caspase-1 activation were detected in macrophages but not other islet cell types. Transgenic mice with β-cell human IAPP (hIAPP) expression had impaired glucose tolerance, elevated islet Il1b mRNA, and decreased Il10 and Il1rn expression following high-fat feeding. Islet macrophages were the major source of these transcripts and expressed increased cell surface Ly6C and CD11c in hIAPP transgenic mice. Clodronate liposome-mediated depletion of islet macrophages improved glucose tolerance and blocked proinflammatory gene expression in hIAPP-expressing mice, despite increasing the amount of islet amyloid. These data provide the first evidence that IAPP aggregates skew resident islet macrophages toward a proinflammatory phenotype and suggest a mechanism by which anti-inflammatory therapies may protect β-cells from IAPP-induced islet dysfunction.

Revascularization of transplanted pancreatic islets and role of the transplantation site

Since the initial reporting of the successful reversal of hyperglycemia through the transplantation of pancreatic islets, significant research efforts have been conducted in elucidating the process of revascularization and the influence of engraftment site on graft function and survival. During the isolation process the intrinsic islet vascular networks are destroyed, leading to impaired revascularization after transplant. As a result, in some cases a significant quantity of the beta cell mass transplanted dies acutely following the infusion into the portal vein, the most clinically used site of engraftment. Subsequently, despite the majority of patients achieving insulin independence after transplant, a proportion of them recommence small, supplemental exogenous insulin over time. Herein, this review considers the process of islet revascularization after transplant, its limiting factors, and potential strategies to improve this critical step. Furthermore, we provide a characterization of alternative transplant sites, analyzing the historical evolution and their role towards advancing transplant outcomes in both the experimental and clinical settings.

CD36 coordinates NLRP3 inflammasome activation by facilitating intracellular nucleation of soluble ligands into particulate ligands in sterile inflammation

Particulate ligands including cholesterol crystals and amyloid fibrils induce NLRP3-dependent production of interleukin-1β (IL-1β) in atherosclerosis, Alzheimer's disease and diabetes. Soluble endogenous ligands including oxidized-LDL, amyloid-β and amylin peptides accumulate in these diseases. Here we identify a CD36-mediated endocytic pathway that coordinates the intracellular conversion of these soluble ligands to crystals or fibrils, resulting in lysosomal disruption and NLRP3-inflammasome activation. Consequently, macrophages lacking CD36 failed to elicit IL-1β production in response to these ligands and targeting CD36 in atherosclerotic mice reduced serum IL-1β and plaque cholesterol crystal accumulation. Collectively, these findings highlight the importance of CD36 in the accrual and nucleation of NLRP3 ligands from within the macrophage and position CD36 as a central regulator of inflammasome activation in sterile inflammation.

Extensive amyloid formation in transplanted microencapsulated mouse and human islets

OBJECTIVE

Deposition of cell toxic islet amyloid is a frequent finding in type 2 diabetes and also in transplanted human islets, where it is a possible explanation for their long-term failure. One suggested reason for amyloid in transplanted islets is that their low vascular density results in a disturbed local clearance of islet amyloid polypeptide (IAPP). To test this hypothesis we analysed accumulation of amyloid in microencapsulated islets, which exemplify a non-vascularised islet graft.

METHODS

Isolated islets from human or transgenic mice expressing human IAPP were microencapsulated in alginate and cultured in vitro or transplanted under the kidney capsule of normoglycemic nude mice. The degree of amyloid was determined after Congo red staining and subcellular alterations were analysed with electron microscopy.

RESULTS

Insulin and IAPP secretion from transgenic mouse islets were markedly increased during stimulation with glucose after one week of culture, but encapsulated islets in general released less insulin. Amyloid was detected after both one and three weeks of culture in the transgenic mouse islets and the encapsulated islets were most affected. After transplantation, electron microscopy displayed both intra- and extracellular amyloid in microencapsulated as well as in non-encapsulated human and transgenic mouse islet grafts. However, amyloid was more frequent in the encapsulated grafts.

CONCLUSION

Micro-encapsulation of pancreatic islets might serve as an important tool for studies of amyloid formation under enhanced circumstances.

Characterization of the response of primary cells relevant to dialysis-related amyloidosis to β2-microglobulin monomer and fibrils

The formation of insoluble amyloid fibrils is associated with an array of devastating human diseases. Dialysis-related amyloidosis (DRA) is a severe complication of hemodialysis that results in the progressive destruction of the bones and joints. Elevated concentrations of β₂-microglobulin (β₂m) in the serum of subjects on hemodialysis promote the formation of amyloid fibrils in the osteoarticular tissues, but the cellular basis for the destruction of these tissues in DRA is poorly understood. In this study we performed a systematic analysis of the interaction of monomeric and fibrillar β₂m with primary human cells of the types present in the synovial joints of subjects with DRA. Building upon observations that macrophages infiltrate β₂m amyloid deposits in vivo we demonstrate that monocytes, the precursors of macrophages, cannot degrade β₂m fibrils, and that both monomeric β₂m and fibrillar β₂m are cytotoxic to these cells. β₂m fibrils also impair the formation of bone resorbing osteoclasts from monocytes and reduce the viability of osteoblasts, the cell type that produces bone. As a consequence, we predict that β₂m amyloid will disrupt the remodelling of the bone, which is critical for the maintenance of this tissue. Moreover, we show that β₂m fibrils reduce the viability of chondrocytes, rationalizing the loss of cartilage in DRA. Together, our observations demonstrate that β₂m cytotoxicity has multiple cellular targets in the osteoarticular tissues and is likely to be a key factor in the bone and joint destruction characteristic of DRA.

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.

NLRP3 inflammasomes link inflammation and metabolic disease

A strong link between inflammation and metabolism is becoming increasingly evident. A number of recent landmark studies have implicated the activation of the NLRP3 inflammasome, an interleukin-1β family cytokine-activating protein complex, in a variety of metabolic diseases including obesity, atherosclerosis and type 2 diabetes. Here, we review these new developments and discuss their implications for a better understanding of inflammation in metabolic disease, and the prospects of targeting the NLRP3 inflammasome for therapeutic intervention.

Activation of the NLRP3 inflammasome by islet amyloid polypeptide provides a mechanism for enhanced IL-1β in type 2 diabetes

Interleukin 1β (IL-1β) is an important inflammatory mediator of type 2 diabetes. Here we show that oligomers of islet amyloid polypeptide (IAPP), a protein that forms amyloid deposits in the pancreas during type 2 diabetes, triggered the NLRP3 inflammasome and generated mature IL-1β. One therapy for type 2 diabetes, glyburide, suppressed IAPP-mediated IL-1β production in vitro. Processing of IL-1β initiated by IAPP first required priming, a process that involved glucose metabolism and was facilitated by minimally oxidized low-density lipoprotein. Finally, mice transgenic for human IAPP had more IL-1β in pancreatic islets, which localized together with amyloid and macrophages. Our findings identify previously unknown mechanisms in the pathogenesis of type 2 diabetes and treatment of pathology caused by IAPP.

Microscopic anatomy of the human islet of Langerhans

Human islets of Langerhans are complex micro-organs responsible for maintaining glucose homeostasis. Islets contain five different endocrine cell types, which react to changes in plasma nutrient levels with the release of a carefully balanced mixture of islet hormones into the portal vein. Each endocrine cell type is characterized by its own typical secretory granule morphology, different peptide hormone content, and specific endocrine, paracrine, and neuronal interactions. During development, a cascade of transcription factors determines the formation of the endocrine pancreas and its constituting islet cell types. Differences in ontogeny between the ventrally derived head section and the dorsally derived head, body, and tail section are responsible for differences in innervation, blood supply, and endocrine composition. Islet cells show a close topographical relationship to the islet vasculature, and are supplied with a five to tenfold higher blood flow than the exocrine compartment. Islet microanatomy is disturbed in patients with type 1 diabetes, with a marked reduction in beta-cell content and the presence of inflammatory infiltrates. Histopathological lesions in type 2 diabetes are less pathognomonic with a more limited reduction in beta-cell content and occasional deposition of amyloid in the islet interstitial space.

Human islet amyloid polypeptide transgenic mice: in vivo and ex vivo models for the role of hIAPP in type 2 diabetes mellitus

Human islet amyloid polypeptide (hIAPP), a pancreatic islet protein of 37 amino acids, is the main component of islet amyloid, seen at autopsy in patients with type 2 diabetes mellitus (DM2). To investigate the roles of hIAPP and islet amyloid in DM2, we generated transgenic mice expressing hIAPP in their islet beta cells. In this study, we found that after a long-term, high-fat diet challenge islet amyloid was observed in only 4 of 19 hIAPP transgenic mice. hIAPP transgenic females exhibited severe glucose intolerance, which was associated with a downregulation of GLUT-2 mRNA expression. In isolated islets from hIAPP males cultured for 3 weeks on high-glucose medium, the percentage of amyloid containing islets increased from 5.5% to 70%. This ex vivo system will allow a more rapid, convenient, and specific study of factors influencing islet amyloidosis as well as of therapeutic strategies to interfere with this pathological process.

Complement activation by islet amyloid polypeptide (IAPP) and alpha-synuclein 112

Complement can damage host tissue when overactivated. Evidence of complement self damage exists for Alzheimer disease (AD), age-related macular degeneration, type 1 diabetes mellitus (T1DM), and Parkinson disease (PD). Known complement activators include Abeta, found in AD, and IgG found in T1DM. We compared their complement activating ability in vitro with those of islet amyloid polypeptide (IAPP), which aggregates in the pancreas of T2DM, and alpha-synuclein (alpha-Syn), which aggregates in PD. We found that IAPP and the alternatively spliced alpha-Syn 112 form, but not full-length alpha-Syn 140, activated complement in vitro. Complement activation may contribute to death of insulin-secreting cells in T2DM or to neuronal death in Parkinson disease (PD) and related synucleinopathies where alpha-Syn 112 occurs. This suggests the possibility of anti-inflammatory treatment in these pathologies. It also suggests that blockers of complement activation may be an appropriate therapeutic target for a range of age-related degenerative diseases.

An immune origin of type 2 diabetes?

Subclinical, low-grade systemic inflammation has been observed in patients with type 2 diabetes and in those at increased risk of the disease. This may be more than an epiphenomenon. Alleles of genes encoding immune/inflammatory mediators are associated with the disease, and the two major environmental factors the contribute to the risk of type 2 diabetes-diet and physical activity-have a direct impact on levels of systemic immune mediators. In animal models, targeting of immune genes enhanced or suppressed the development of obesity or diabetes. Obesity is associated with the infiltration and proinflammatory activity of macrophages in adipose tissue, and immune mediators may be important regulators of insulin resistance, mitochondrial function, ectopic lipid storage and beta cell dysfunction or death. Intervention studies targeting these pathways would help to determine the contribution of an activated innate immune system to the development of type 2 diabetes.

Islet amyloid: a complication of islet dysfunction or an aetiological factor in Type 2 diabetes?

The role of islet amyloidosis in the onset and progression of Type 2 diabetes remains obscure. Islet amyloid polypeptide is a 37 amino-acid, beta-cell peptide which is co-stored and co-released with insulin. Human islet amyloid polypeptide refolds to a beta-conformation and oligomerises to form insoluble fibrils; proline substitutions in rodent islet amyloid polypeptide prevent this molecular transition. Pro-islet amyloid polypeptide (67 amino acids in man) is processed in secretory granules. Refolding of islet amyloid polypeptide may be prevented by intragranular heterodimer formation with insulin (but not proinsulin). Diabetes-associated abnormal proinsulin processing could contribute to de-stabilisation of granular islet amyloid polypeptide. Increased pro-islet amyloid polypeptide secretion as a consequence of islet dysfunction could promote fibrillogenesis; the propeptide forms fibrils and binds to basement membrane glycosamino-glycans. Islet amyloid polypeptide gene polymorphisms are not universally associated with Type 2 diabetes. Transgenic mice expressing human islet amyloid polypeptide gene have increased islet amyloid polypeptide concentrations but develop islet amyloid only against a background of obesity and/or high fat diet. In transgenic mice, obese monkeys and cats, initially small perivascular deposits progressively increase to occupy 80% islet mass; the severity of amyloidosis in animal models is related to the onset of hyperglycaemia, suggesting that islet amyloid and the associated destruction of islet cells cause diabetes. In human diabetes, islet amyloid can affect less than 1% or up to 80% of islets indicating that islet amyloidosis largely results from diabetes-related pathologies and is not an aetiological factor for hyperglycaemia. However, the associated progressive beta-cell destruction leads to severe islet dysfunction and insulin requirement.

Amyloid beta and amylin fibrils induce increases in proinflammatory cytokine and chemokine production by THP-1 cells and murine microglia

Activated microglia surrounding amyloid beta-containing senile plaques synthesize interleukin-1, an inflammatory cytokine that has been postulated to contribute to Alzheimer's disease pathology. Studies have demonstrated that amyloid beta treatment causes increased cytokine release in microglia and related cell cultures. The present work evaluates the specificity of this cellular response by comparing the effects of amyloid beta to that of amylin, another amyloidotic peptide. Both lipopolysaccharide-treated THP-1 monocytes and mouse microglia showed significant increases in mature interleukin-1beta release 48 h following amyloid beta or human amylin treatment, whereas nonfibrillar rat amylin had no effect on interleukin-1beta production by THP-1 cells. Lipopolysaccharide-stimulated THP-1 cells treated with amyloid beta or amylin also showed increased release of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-6, as well as the chemokines interleukin-8 and macrophage inflammatory protein-1alpha and -1beta. THP-1 cells incubated with fibrillar amyloid beta or amylin in the absence of lipopolysaccharide also showed significant increases of both interleukin-1beta and tumor necrosis factor-alpha mRNA. Furthermore, treatment of THP-1 cells with amyloid fibrils resulted in an elevated expression of the immediate-early genes c-fos and junB. These studies provide further evidence that fibrillar amyloid peptides can induce signal transduction pathways that initiate an inflammatory response that is likely to contribute to Alzheimer's disease pathology.

Fibrillar islet amyloid polypeptide differentially affects oxidative mechanisms and lipoprotein uptake in correlation with cytotoxicity in two insulin-producing cell lines

We reported recently that fibrillar human islet amyloid polypeptide (IAPP) is cytotoxic to RIN5mF cells but not to HIT-T15 cells, both being insulin-producing cell lines. In the present study, we explored the basis for this difference by studying oxidative stress responses and low density lipoprotein (LDL) binding and uptake. In RINm5F but not in HIT-T15 cells, plasma membrane NADPH oxidase activity and intracellular lipid peroxidation increased by challenge with IAPP fibrils for 24 h (10 microM), whereas glutathione peroxidase activity was not changed. Furthermore, although both cell lines express (125)I-LDL binding sites, IAPP fibrils increased (125)I-LDL binding and uptake only in RINm5F cells and not in HIT-T15 cells. The cytotoxic action of IAPP fibrils in RINm5F cells is therefore paralleled by increased oxidative responses and LDL uptake, suggesting that cytotoxic mechanisms of IAPP fibrils in insulin-producing cells involve changes in pathways of cellular oxidative stress systems and lipid homeostasis.