Fine structure of islets of Langerhans in insular amyloidosis

Cross-sectional imaging of the pancreas in diabetes

Diabetes mellitus presents a global health challenge characterized by dysregulated glucose metabolism and insulin resistance. Pancreas dysfunction contributes to the development and progression of diabetes. Cross-sectional imaging modalities have provided new insight into the structural and functional alterations of the pancreas in individuals with diabetes. This review summarizes MRI and CT studies that characterize pancreas alterations in both type 1 and type 2 diabetes and discusses future applications of these techniques.

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

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

Molecular and Biochemical Pathways Encompassing Diabetes Mellitus and Dementia

Diabetes mellitus is a major metabolic disorder that has now emerged as an epidemic, and it affects the brain through an array of pathways. Diabetes mellitus patients can develop pathological changes in the brain, which eventually take the shape of mild cognitive impairment progressing to Alzheimer's Disease. A number of preclinical and clinical studies demonstrate this fact, and it comes out to be those molecular pathways such as amyloidogenesis, oxidative stress, inflammation, and impaired insulin signaling are identical in diabetes mellitus and dementia. However, the critical player involved in the vicious cycle of diabetes mellitus and dementia is insulin, whose signaling, when impaired in diabetes mellitus (both type 1 and 2), leads to a decline in cognition, although other pathways are also essential contributors. Moreover, it is not only that diabetes mellitus patients indicate cognitive decline at a later stage; many Alzheimer's Disease patients also reflect symptoms of diabetes mellitus, thus creating a vicious cycle inculcating a web of complex molecular mechanisms and hence categorizing Alzheimer's Disease as 'brain diabetes'. Thus, it is practical to suggest that anti-diabetic drugs are beneficial in Alzheimer's Disease; but only smaller trials, not the larger ones, have showcased positive outcomes mainly because of the late onset of therapy. Therefore, it is extremely important to develop more of such molecules that target insulin in dementia patients along with such methods that diagnose impaired insulin signaling and the associated cognitive decline so that early therapy may be initiated and the progression of the disease be prevented.

β cell aging and age-related diabetes

Type 2 diabetes is characterized by insulin resistance and loss of β cell mass and function. Aging is considered as a major risk factor for development of type 2 diabetes. However, the roles of pancreatic β cell senescence and systemic aging in the pathogenesis of type 2 diabetes in elderly people remain poorly understood. In this review, we aimed to discuss the current findings and viewpoints focusing on β cell aging and the development of type 2 diabetes.

Proteostasis of Islet Amyloid Polypeptide: A Molecular Perspective of Risk Factors and Protective Strategies for Type II Diabetes

The possible link between hIAPP accumulation and β-cell death in diabetic patients has inspired numerous studies focusing on amyloid structures and aggregation pathways of this hormone. Recent studies have reported on the importance of early oligomeric intermediates, the many roles of their interactions with lipid membrane, pH, insulin, and zinc on the mechanism of aggregation of hIAPP. The challenges posed by the transient nature of amyloid oligomers, their structural heterogeneity, and the complex nature of their interaction with lipid membranes have resulted in the development of a wide range of biophysical and chemical approaches to characterize the aggregation process. While the cellular processes and factors activating hIAPP-mediated cytotoxicity are still not clear, it has recently been suggested that its impaired turnover and cellular processing by proteasome and autophagy may contribute significantly toward toxic hIAPP accumulation and, eventually, β-cell death. Therefore, studies focusing on the restoration of hIAPP proteostasis may represent a promising arena for the design of effective therapies. In this review we discuss the current knowledge of the structures and pathology associated with hIAPP self-assembly and point out the opportunities for therapy that a detailed biochemical, biophysical, and cellular understanding of its aggregation may unveil.

Beta cell dysfunction in diabetes: the islet microenvironment as an unusual suspect

Cells in different tissues, including endocrine cells in the pancreas, live in complex microenvironments that are rich in cellular and acellular components. Intricate interactions with their microenvironment dictate most cellular properties, such as their function, structure and size, and maintain tissue homeostasis. Pancreatic islets are populated by endocrine, vascular and immune cells that are immersed in the extracellular matrix. While the intrinsic properties of beta cells have been vastly investigated, our understanding of their interactions with their surroundings has only recently begun to unveil. Here, we review current research on the interplay between the islet cellular and acellular components, and the role these components play in beta cell physiology and pathophysiology. Although beta cell failure is a key pathomechanism in diabetes, its causes are far from being fully elucidated. We, thus, propose deleterious alterations of the islet niche as potential underlying mechanisms contributing to beta cell failure. In sum, this review emphasises that the function of the pancreatic islet depends on all of its components. Graphical abstract.

Molecular recognition of human islet amyloid polypeptide assembly by selective oligomerization of thioflavin T

Selective oligomerization is a common phenomenon existing widely in the formation of intricate biological structures in nature. The precise design of drug molecules with an oligomerization state that specifically recognizes its receptor, however, remains substantially challenging. Here, we used scanning tunneling microscopy (STM) to identify the oligomerization states of an amyloid probe thioflavin T (ThT) on hIAPP_8-37 assembly to be exclusively even numbers. We demonstrate that both adhesive interactions between ThT and the protein substrate and cohesive interactions among ThT molecules govern the oligomerization state of the bounded ThT. Specifically, the work of the cohesive interaction between two head/tail ThTs is determined to be 6.4 k _B T, around 50% larger than that of the cohesive interaction between two side-by-side ThTs (4.2 k _B T). Overall, our STM imaging and theoretical understanding at the single-molecule level provide valuable insights into the design of drug compounds using the selective oligomerization of molecular probes to recognize protein self-assembly.

Acidic Environment Significantly Alters Aggregation Pathway of Human Islet Amyloid Polypeptide at Negative Lipid Membrane

The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) at cell membrane has a close relationship with the development of type 2 diabetes (T2DM). This aggregation process is susceptible to various physiologically related factors, and systematic studies on condition-mediated hIAPP aggregation are therefore essential for a thorough understanding of the pathology of T2DM. In this study, we combined surface-sensitive amide I and amide II spectral signals from the protein backbone, generated simultaneously in a highly sensitive femtosecond broad-band sum frequency generation vibrational spectroscopy system, to examine the effect of environmental pH on the dynamical structural changes of hIAPP at membrane surface in situ and in real time. Such a combination can directly discriminate the formation of β-hairpin-like monomer and oligomer/fibril at the membrane surface. It is evident that, in an acidic milieu, hIAPP slows down its conformational evolution and alters its aggregation pathway, leading to the formation of off-pathway oligomers. When matured hIAPP aggregates are exposed to basic subphase, partial conversion from β-sheet oligomers into ordered β-sheet fibrillar structures is observed. When exposed to acidic environment, however, hIAPP fibrils partially converse into more loosely patterned β-sheet oligomeric structures.

Membranes as modulators of amyloid protein misfolding and target of toxicity

Abnormal protein aggregation is a hallmark of various human diseases. α-Synuclein, a protein implicated in Parkinson's disease, is found in aggregated form within Lewy bodies that are characteristically observed in the brains of PD patients. Similarly, deposits of aggregated human islet amyloid polypeptide (IAPP) are found in the pancreatic islets in individuals with type 2 diabetes mellitus. Significant number of studies have focused on how monomeric, disaggregated proteins transition into various amyloid structures leading to identification of a vast number of aggregation promoting molecules and processes over the years. Inasmuch as these factors likely enhance the formation of toxic, misfolded species, they might act as risk factors in disease. Cellular membranes, and particularly certain lipids, are considered to be among the major players for aggregation of α-synuclein and IAPP, and membranes might also be the target of toxicity. Past studies have utilized an array of biophysical tools, both in vitro and in vivo, to expound the membrane-mediated aggregation. Here, we focus on membrane interaction of α-synuclein and IAPP, and how various kinds of membranes catalyze or modulate the aggregation of these proteins and how, in turn, these proteins disrupt membrane integrity, both in vitro and in vivo. The membrane interaction and subsequent aggregation has been briefly contrasted to aggregation of α-synuclein and IAPP in solution. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Membrane-mediated amyloid deposition of human islet amyloid polypeptide

Amyloid deposition of human islet amyloid polypeptide (hIAPP) within the islet of Langerhans is closely associated with type II diabetes mellitus. Accumulating evidence indicates that the membrane-mediated aggregation and subsequent deposition of hIAPP are linked to the dysfunction and death of insulin-producing pancreatic β-cells, but the molecular process of hIAPP deposition is poorly understood. In this review, I focus on recent in vitro studies utilizing model membranes to observe the membrane-mediated aggregation/deposition of hIAPP. Membrane surfaces can serve as templates for both hIAPP adsorption and aggregation. Using high-sensitivity surface analyzing/imaging techniques that can characterize the processes of hIAPP aggregation and deposition at the membrane surface, these studies provide valuable insights into the mechanism of membrane damage caused by amyloid deposition of the peptide.

Noncerebral Amyloidoses: Aspects on Seeding, Cross-Seeding, and Transmission

More than 30 proteins form amyloid in humans, most of them outside of the brain. Deposition of amyloid in extracerebral tissues is very common and seems inevitable for an aging person. Most deposits are localized, small, and probably without consequence, but in some instances, they are associated with diseases such as type 2 diabetes. Other extracerebral amyloidoses are systemic, with life-threatening effects on the heart, kidneys, and other organs. Here, we review how amyloid may spread through seeding and whether transmission of amyloid diseases may occur between humans. We also discuss whether cross-seeding is important in the development of amyloidosis, focusing specifically on the amyloid proteins AA, transthyretin, and islet amyloid polypeptide (IAPP).

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.

Cellular mechanism of fibril formation from serum amyloid A1 protein

Serum amyloid A1 (SAA1) is an apolipoprotein that binds to the high-density lipoprotein (HDL) fraction of the serum and constitutes the fibril precursor protein in systemic AA amyloidosis. We here show that HDL binding blocks fibril formation from soluble SAA1 protein, whereas internalization into mononuclear phagocytes leads to the formation of amyloid. SAA1 aggregation in the cell model disturbs the integrity of vesicular membranes and leads to lysosomal leakage and apoptotic death. The formed amyloid becomes deposited outside the cell where it can seed the fibrillation of extracellular SAA1. Our data imply that cells are transiently required in the amyloidogenic cascade and promote the initial nucleation of the deposits. This mechanism reconciles previous evidence for the extracellular location of deposits and amyloid precursor protein with observations the cells are crucial for the formation of amyloid.

The Effects of Lipid Membranes, Crowding and Osmolytes on the Aggregation, and Fibrillation Propensity of Human IAPP

Type 2 diabetes mellitus (T2DM) is an age-related and metabolic disease. Its development is hallmarked, among others, by the dysfunction and degeneration of β-cells of the pancreatic islets of Langerhans. The major pathological characteristic thereby is the formation of extracellular amyloid deposits consisting of the islet amyloid polypeptide (IAPP). The process of human IAPP (hIAPP) self-association, and the intermediate structures formed as well as the interaction of hIAPP with membrane systems seem to be, at least to a major extent, responsible for the cytotoxicity. Here we present a summary and comparison of the amyloidogenic propensities of hIAPP in bulk solution and in the presence of various neutral and charged lipid bilayer systems as well as biological membranes. We also discuss the cellular effects of macromolecular crowding and osmolytes on the aggregation pathway of hIAPP. Understanding the influence of different cellular factors on hIAPP aggregation will provide more insight into the onset of T2DM and help to develop novel therapeutic strategies.

On the Environmental Factors Affecting the Structural and Cytotoxic Properties of IAPP Peptides

Pancreatic islets in type 2 diabetes mellitus (T2DM) patients are characterized by reduced β-cells mass and diffuse extracellular amyloidosis. Amyloid deposition involves the islet amyloid polypeptide (IAPP), a neuropancreatic hormone cosecreted with insulin by β-cells. IAPP is physiologically involved in glucose homeostasis, but it may turn toxic to β-cells owing to its tendency to misfold giving rise to oligomers and fibrils. The process by which the unfolded IAPP starts to self-assemble and the overall factors promoting this conversion are poorly understood. Other open questions are related to the nature of the IAPP toxic species and how exactly β-cells die. Over the last decades, there has been growing consensus about the notion that early molecular assemblies, notably small hIAPP oligomers, are the culprit of β-cells decline. Numerous environmental factors might affect the conformational, aggregation, and cytotoxic properties of IAPP. Herein we review recent progress in the field, focusing on the influences that membranes, pH, and metal ions may have on the conformational conversion and cytotoxicity of full-length IAPP as well as peptide fragments thereof. Current theories proposed for the mechanisms of toxicity will be also summarized together with an outline of the underlying molecular links between IAPP and amyloid beta (Aβ) misfolding.

Apoptosis induced by islet amyloid polypeptide soluble oligomers is neutralized by diabetes-associated specific antibodies

Soluble oligomeric assemblies of amyloidal proteins appear to act as major pathological agents in several degenerative disorders. Isolation and characterization of these oligomers is a pivotal step towards determination of their pathological relevance. Here we describe the isolation of Type 2 diabetes-associated islet amyloid polypeptide soluble cytotoxic oligomers; these oligomers induced apoptosis in cultured pancreatic cells, permeated model lipid vesicles and interacted with cell membranes following complete internalization. Moreover, antibodies which specifically recognized these assemblies, but not monomers or amyloid fibrils, were exclusively identified in diabetic patients and were shown to neutralize the apoptotic effect induced by these oligomers. Our findings support the notion that human IAPP peptide can form highly toxic oligomers. The presence of antibodies identified in the serum of diabetic patients confirms the pathological relevance of the oligomers. In addition, the newly identified structural epitopes may also provide new mechanistic insights and a molecular target for future therapy.

Islet amyloid polypeptide in pancreatic islets from type 2 diabetic subjects

AIMS/HYPOTHESIS

Islet amyloid polypeptide (IAPP) is a chief constituent of amyloid deposits in pancreatic islets, characteristic histopathology for type 2 diabetes. The goal of this study was to analyze islet cell composition in diabetic islets for the process of transforming water-soluble IAPP in β-cells to water-insoluble amyloid deposits by Immunocytochemical staining using different dilutions of anti-IAPP antibody. IAPP in β-cell granules may initiate β-cell necrosis through apoptosis to form interstitial amyloid deposits in type 2 diabetic islets.

RESULTS

Control islets revealed twice as much β-cells as α-cells whereas 15 of 18 type 2 diabetic cases (83%) revealed α- cells as major cells in larger islets. Diabetic islets consisted of more larger islets with more σ-cells than β-cells, which contribute to hyperglucagonemia. In control islets, percentage of IAPP-positive cells against β-cells was 40-50% whereas percentage for type 2 diabetic islets was about 25%. Amyloid deposits in diabetic islets were not readily immunostained for IAPP using 1: 800 diluted antibody, however, 1: 400 and 1: 200 diluted solutions provided stronger immunostaining in early stages of islet amyloidogenesis after treating the deparaffinized sections with formic acid.

METHODS

Using commercially available rabbit antihuman IAPP antibody, immunocytochemical staining was performed on 18 cases of pancreatic tissues from type 2 diabetic subjects by systematically immunostaining for insulin, glucagon, somatostatin (SRIF) and IAPP compared with controls. Sizes of islets were measured by 1 cm scale, mounted in 10X eye piece.

CONCLUSIONS/INTERPRETATION

α cells were major islet cells in majority of diabetic pancreas (83%) and all diabetic islets contained less IAPP-positive cells than controls, indicating that IAPP deficiency in pancreatic islets is responsible for decreased IAPP in blood. In diabetic islets, water-soluble IAPP disappeared in β-cell granules, which transformed to water-insoluble amyloid deposits. Amyloid deposits were not readily immunostained using IAPP 1: 800 diluted antibody but were stronger immunostained for IAPP in early stages of amyloid deposited islets using less diluted solutions after formic acid treatment. In early islet amyloidogenesis, dying β-cell cytoplasm was adjacently located to fine amyloid fibrils, supporting that IAPP in secretary granules from dying β cells served as nidus for islet β-sheet formation.

Islet amyloid polypeptide: aggregation and fibrillogenesis in vitro and its inhibition

The development of type 2 diabetes mellitus is associated with the dysfunction of b-cells which is correlated to the formation of deposits consisting of the islet amyloid polypeptide (IAPP). The process of human IAPP (hIAPP) self-association, the intermediate structures formed as well as the interaction of hIAPP with membrane systems seem to be responsible for the cytotoxicity. For monomeric hIAPP, a natively random coil conformation with transient a-helical parts could be determined in bulk solution, which rapidly converts to an amyloid structure consisting of cross b-sheets. By comparing the amyloidogenic propensities of hIAPP in the bulk and in the presence of various neutral and charged lipid bilayer systems as well as biological membranes, an enhancing effect of anionic and heterogeneous membranes to hIAPP fibril formation has been found. We also discuss the cross-interaction of hIAPP with other amyloidogenic peptides (e.g., insulin and Ab) and present first small-molecule inhibitors of the fibrillation process of hIAPP.

Amyloid in the islets of Langerhans: thoughts and some historical aspects

Deposition of amyloid, derived from the polypeptide hormone islet amyloid polypeptide (IAPP; 'amylin') is the single most typical islet alteration in type 2 diabetes. Islet amyloid was described as hyalinization already in 1901, but not until 1986 was it understood that it is a polymerization product of a novel β-cell regulatory product. The subject of this focused review deals with the pathogenesis and importance of the islet amyloid itself, not with the biological effect of the polypeptide. Similar to the situation in Alzheimer's disease, it has been argued that the amyloid may not be of importance since there is no strict correlation between the degree of islet amyloid infiltration and the disease. However, it is hardly discussable that the amyloid is important in subjects where islets have been destroyed by pronounced islet amyloid deposits. Even when there is less islet amyloid the deposits are widely spread, and β-cells show ultrastructural signs of cell membrane destruction. It is suggested that type 2 diabetes is heterogeneous and that in one major subtype aggregation of IAPP into amyloid fibrils is determining the progressive loss of β-cells. Interestingly, development of islet amyloid may be an important event in the loss of β-cell function after islet transplantation into type 1 diabetic subjects.

Gut microbiota in health and disease

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

Differences between amyloid toxicity in alpha and beta cells in human and mouse islets and the role of caspase-3

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by decreased beta cell mass and islet amyloid formation. Islet amyloid formed by aggregation of human islet amyloid polypeptide (hIAPP) is associated with beta cell apoptosis. We used human and transgenic mouse islets in culture to examine whether deletion of caspase-3 protects islets from apoptosis induced by endogenously produced and exogenously applied hIAPP and compared hIAPP toxicity in islet alpha and beta cells.

METHODS

Human and wild-type or caspase-3 knockout mouse islet cells were treated with hIAPP. Rat insulinoma INS-1 cells were similarly cultured with hIAPP and the amyloid inhibitor Congo Red or caspase-3 inhibitor. Human and hIAPP-expressing caspase-3 knockout mouse islets were cultured to form amyloid fibrils and assessed for beta and alpha cell apoptosis, beta cell function and caspase-3 activation.

RESULTS

hIAPP-treated INS-1 cells had increased caspase-3 activation and apoptosis, both of which were reduced by inhibitors of amyloid or caspase-3. Similarly, hIAPP-treated human and mouse islet beta cells had elevated active caspase-3- and TUNEL-positive cells, whereas mouse islet cells lacking caspase-3 had markedly lower beta cell but comparable alpha cell apoptosis. During culture, human islets that formed amyloid had higher active caspase-3- and TUNEL-positive beta cells than those without detectable amyloid. Finally, cultured hIAPP-expressing mouse islets lacking caspase-3 had markedly lower beta cell apoptosis than those expressing caspase-3, associated with an increase in islet beta cell/alpha cell ratio, insulin content and glucose response.

CONCLUSIONS/INTERPRETATION

Prevention of caspase-3 activation protects islet beta cells from apoptosis induced by fibrillogenesis of endogenously secreted and exogenously applied hIAPP. Islet beta cells are more susceptible to hIAPP toxicity than alpha cells cultured under the same conditions.

Effect of lipid type on the binding of lipid vesicles to islet amyloid polypeptide amyloid fibrils

Amyloid deposits, composed primarily of the 37-residue islet amyloid polypeptide (IAPP), are observed near pancreatic beta-cells of type II diabetics, with their presence strongly correlating with a loss of beta-cell mass and decreased pancreatic function. Although beta-cell membranes have been implicated as the likely target of amyloidogenic IAPP toxicity, interactions between membranes and IAPP in the fibrillar state have not been well characterized. In this study, turbidity measurements were conducted to provide a detailed description of the binding reaction between IAPP fibrils and lipid vesicles made from phosphatidylcholine. The kinetic data representing the rate and extent of the fibril-vesicle binding reaction are described well by an empirical double-exponential equation. The extent of binding was found to increase with increasing amyloid fibril concentration. Modification of the vesicle composition significantly altered the observed binding reaction kinetics, with the change quantified using the parameters obtained from the double-exponential fitting analysis. When the vesicles contained a significant amount of the lipid phosphatidylglycerol, substantial sedimentation of the vesicles under gravity was detected following the initial binding reaction. To rationalize the observed kinetic binding data, we developed a mesoscopic simulation model based on a hard particle representation of the species involved. In light of the observed data and simulation predictions, the potential roles of IAPP amyloid fibrils in membrane binding are discussed.

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.

Cholesterol regulates assembly of human islet amyloid polypeptide on model membranes

Amylin, a 37-aa pancreatic hormone, is the major constituent of islet amyloid, a hallmark of type II diabetes mellitus. Recent studies have revealed a pivotal role of anionic phospholipids in membrane-catalyzed amylin fibrillogenesis and aggregation. However, cholesterol, an integral component of eukaryotic cell membranes, also could have a role. In this study, we have examined the effect of cholesterol on amylin polymerization both on planar membranes and in solution. Using time-lapse atomic force microscopy, we have studied the dynamics and macromolecular organization of amylin on anionic and neutral planar membranes that lack or include cholesterol. On cholesterol-depleted planar membranes, amylin formed highly symmetrical tetrameric and pentameric pore-like supramolecular structures composed of 25- to 35-nm intermediate-sized globular structures or oligomers. Conversely, on membranes incorporating cholesterol, amylin formed highly compact approximately 200- to 500-nm protein clusters that constituted seeds or nuclei for continuing amylin binding and aggregation. However, cholesterol inhibited amylin nucleation with a 7-fold decrease in the number of amylin particles. Consequently, cholesterol-containing membranes accumulated significantly less amyloid with some membrane areas completely free of amyloid particles. The inhibitory effect of cholesterol on amylin aggregation in solution was also demonstrated as a 16-fold decrease in the aggregation rate. Consistent with this, circular dichroism spectroscopy revealed a stable, soluble random-coil conformation for amylin in the presence of cholesterol that could explain the inhibitory effect of cholesterol on amylin polymerization in solution and on membranes. The modulatory effect of cholesterol was largely independent of membrane charge or phospholipids, suggesting a novel cholesterol-regulated amylin polymerization process.

A model of amyloid's role in disease based on fibril fracture

Although the correlative evidence relating the presence of amyloid fibrils and certain disease states (e.g. Alzheimer's disease and Type 2 Diabetes) is overwhelming, a direct causative role for amyloid has proved harder to establish. Current thinking links a narrow region of the aggregate protein size distribution, the so called 'early aggregate' domain to cellular toxicity. A troubling feature of this theory however is that the nucleated reaction mechanism by which amyloid formation is believed to occur results in a very low number concentration of early aggregates which are rapidly extended to form amyloid fibrils. This situation means that the concentration of early aggregates is very low at the time when they are supposedly at their most toxic. Here we adopt a novel explicit simulation strategy to examine a kinetic regime involving nucleated growth combined with fibril fragmentation under which this situation can be reversed so as to produce a high number concentration of small on-pathway toxic aggregates. Dependent upon the rate of fragmentation, the time scale for generation of toxic early aggregates may be coupled, uncoupled or disassociated from the time scale for the appearance of amyloid fibrils. Furthermore the model presents itself as a biochemical 'switch' transitioning between modes of amyloid induced cell death dependent upon either specific amyloid toxicity or non-specific solid mass induced tissue damage.

Membrane permeabilization by Islet Amyloid Polypeptide

Membrane permeabilization by Islet Amyloid Polypeptide (IAPP) is suggested to be the main mechanism for IAPP-induced cytotoxicity and death of insulin-producing beta-cells in type 2 diabetes mellitus (T2DM). The insoluble fibrillar IAPP deposits (amyloid) present in the pancreas of most T2DM patients are not the primary suspects responsible for permeabilization of beta-cell membranes. Instead, soluble IAPP oligomers are thought to be cytotoxic by forming membrane channels or by inducing bilayer disorder. In addition, the elongation of IAPP fibrils at the membrane, but not the fibrils themselves, could cause membrane disruption. Recent reports substantiate the formation of an alpha-helical, membrane-bound IAPP monomer as possible intermediate on the aggregation pathway. Here, the structures and membrane interactions of various IAPP species will be reviewed, and the proposed hypotheses for IAPP-induced membrane permeabilization and cytotoxicity will be discussed.

Amyloid deposition in transplanted human pancreatic islets: a conceivable cause of their long-term failure

Following the encouraging report of the Edmonton group, there was a rejuvenation of the islet transplantation field. After that, more pessimistic views spread when long-term results of the clinical outcome were published. A progressive loss of the β-cell function meant that almost all patients were back on insulin therapy after 5 years. More than 10 years ago, we demonstrated that amyloid deposits rapidly formed in human islets and in mouse islets transgenic for human IAPP when grafted into nude mice. It is, therefore, conceivable to consider amyloid formation as one potential candidate for the long-term failure. The present paper reviews attempts in our laboratories to elucidate the dynamics of and mechanisms behind the formation of amyloid in transplanted islets with special emphasis on the impact of long-term hyperglycemia.

Calcium elevation in mouse pancreatic beta cells evoked by extracellular human islet amyloid polypeptide involves activation of the mechanosensitive ion channel TRPV4

AIMS/HYPOTHESIS

To investigate the mechanism by which human islet amyloid polypeptide (hIAPP) fibril formation results in calcium influx across the plasma membrane of pancreatic beta cells, and its association with apoptosis.

METHODS

Cytoplasmic intracellular calcium concentrations ([Ca(2+)](i)) were monitored for 2 h as the 340/380 nm fluorescence ratio in fura-2 loaded cells of the MIN6 mouse pancreatic beta cell line. Cell morphology was evaluated by transmission electron microscopy, and viability by FACS.

RESULTS

hIAPP (10 micromol/l) increased [Ca(2+)](i) in 21% of MIN6 cells in standard buffer, and in 8% of cells in Na(+)-free buffer. Transient receptor potential (TRP) channel inhibitors (gadolinium and ruthenium red) prevented the [Ca(2+)](i) rise under both conditions, whilst nifedipine was only effective in the presence of Na(+). hIAPP increased apoptosis in both insulinoma cells and islets in primary culture, and cell viability was partially rescued by ruthenium red (p < 0.001). By RT-PCR, we detected expression of the mechanosensitive TRP cation channel subfamily V member 4 (Trpv4) in MIN6 cells and mouse pancreas. Small interference RNA against Trpv4 prevented hIAPP-induced [Ca(2+)](i) rises, decreased hIAPP-triggered expression of the endoplasmic reticulum (ER) stress response, and reduced hIAPP-triggered cell death by 50% (p < 0.05).

CONCLUSIONS/INTERPRETATION

Alterations in [Ca(2+)](i) play a key role in hIAPP-induced beta cell cytotoxicity. By electron microscopy, we detected extracellular hIAPP aggregates adjacent to irregular invaginated regions of the plasma membrane. We propose that TRPV4 channels may sense physical changes in the plasma membrane induced by hIAPP aggregation, enabling Ca(2+) entry, membrane depolarisation and activation of L-type Ca(2+) channels. Decreasing the activity of TRPV4 prevented hIAPP-induced [Ca(2+)](i) changes, reduced hIAPP-triggered ER stress and improved cell viability.

Insulin resistance and amyloidogenesis as common molecular foundation for type 2 diabetes and Alzheimer's disease

Characterized as a peripheral metabolic disorder and a degenerative disease of the central nervous system respectively, it is now widely recognized that type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and amyloidogenesis. Several recent studies suggest that this is not an epiphenomenon, but rather these two diseases disrupt common molecular pathways and each disease compounds the progression of the other. For instance, in AD the accumulation of the amyloid-beta peptide (Abeta), which characterizes the disease and is thought to participate in the neurodegenerative process, may also induce neuronal insulin resistance. Conversely, disrupting normal glucose metabolism in transgenic animal models of AD that over-express the human amyloid precursor protein (hAPP) promotes amyloid-peptide aggregation and accelerates the disease progression. Studying these processes at a cellular level suggests that insulin resistance and Abeta aggregation may not only be the consequence of excitotoxicity, aberrant Ca(2+) signals, and proinflammatory cytokines such as TNF-alpha, but may also promote these pathological effectors. At the molecular level, insulin resistance and Abeta disrupt common signal transduction cascades including the insulin receptor family/PI3 kinase/Akt/GSK3 pathway. Thus both disease processes contribute to overlapping pathology, thereby compounding disease symptoms and progression.

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.

Recent insights in islet amyloid polypeptide-induced membrane disruption and its role in beta-cell death in type 2 diabetes mellitus

The presence of fibrillar protein deposits (amyloid) of human islet amyloid polypeptide (hIAPP) in the pancreatic islets of Langerhans is thought to be related to death of the insulin-producing islet β-cells in type 2 diabetes mellitus (DM2). The mechanism of hIAPP-induced β-cell death is not understood. However, there is growing evidence that hIAPP-induced disruption of β-cell membranes is the cause of hIAPP cytotoxicity. Amyloid cytotoxicity by membrane damage has not only been suggested for hIAPP, but also for peptides and proteins related to other misfolding diseases, like Alzheimer's disease, Parkinson's disease, and prion diseases. Here we review the interaction of hIAPP with membranes, and discuss recent progress in the field, with a focus on hIAPP structure and on the proposed mechanisms of hIAPP-induced membrane damage in relation to β-cell death in DM2.

Islet amyloid in type 2 diabetes, and the toxic oligomer hypothesis

Type 2 diabetes (T2DM) is characterized by insulin resistance, defective insulin secretion, loss of beta-cell mass with increased beta-cell apoptosis and islet amyloid. The islet amyloid is derived from islet amyloid polypeptide (IAPP, amylin), a protein coexpressed and cosecreted with insulin by pancreatic beta-cells. In common with other amyloidogenic proteins, IAPP has the propensity to form membrane permeant toxic oligomers. Accumulating evidence suggests that these toxic oligomers, rather than the extracellular amyloid form of these proteins, are responsible for loss of neurons in neurodegenerative diseases. In this review we discuss emerging evidence to suggest that formation of intracellular IAPP oligomers may contribute to beta-cell loss in T2DM. The accumulated evidence permits the amyloid hypothesis originally developed for neurodegenerative diseases to be reformulated as the toxic oligomer hypothesis. However, as in neurodegenerative diseases, it remains unclear exactly why amyloidogenic proteins form oligomers in vivo, what their exact structure is, and to what extent these oligomers play a primary or secondary role in the cytotoxicity in what are now often called unfolded protein diseases.

Membrane damage by human islet amyloid polypeptide through fibril growth at the membrane

Fibrillar protein deposits (amyloid) in the pancreatic islets of Langerhans are thought to be involved in death of the insulin-producing islet beta cells in type 2 diabetes mellitus. It has been suggested that the mechanism of this beta cell death involves membrane disruption by human islet amyloid polypeptide (hIAPP), the major constituent of islet amyloid. However, the molecular mechanism of hIAPP-induced membrane disruption is not known. Here, we propose a hypothesis that growth of hIAPP fibrils at the membrane causes membrane damage. We studied the kinetics of hIAPP-induced membrane damage in relation to hIAPP fibril growth and found that the kinetic profile of hIAPP-induced membrane damage is characterized by a lag phase and a sigmoidal transition, which matches the kinetic profile of hIAPP fibril growth. The observation that seeding accelerates membrane damage supports the hypothesis. In addition, variables that are well known to affect hIAPP fibril formation, i.e., the presence of a fibril formation inhibitor, hIAPP concentration, and lipid composition, were found to have the same effect on hIAPP-induced membrane damage. Furthermore, electron microscopy analysis showed that hIAPP fibrils line the surface of distorted phospholipid vesicles, in agreement with the notion that hIAPP fibril growth at the membrane and membrane damage are physically connected. Together, these observations point toward a mechanism in which growth of hIAPP fibrils, rather than a particular hIAPP species, is responsible for the observed membrane damage. This hypothesis provides an additional mechanism next to the previously proposed role of oligomers as the main cytotoxic species of amyloidogenic proteins.

Newer therapeutic options for children with diabetes mellitus: theoretical and practical considerations

Recent studies in adult patients with type 1 diabetes mellitus (T1DM) and T2DM have examined the potential utility, benefits, and side effects of agents that augment insulin secretion after oral ingestion of nutrients in comparison with intravenous nutrient delivery, the so-called incretins. Two families of incretin-like substances are now approved for use in adults. Glucagon-like peptide-1 (GLP-1) or agents that bind to its receptor (exenatide, Byetta) or agents that inhibit its destruction [dipeptidyl peptidase-IV (DPP-IV) inhibitors, Vildagliptin] improve insulin secretion, delay gastric emptying, and suppress glucagon secretion while decreasing food intake without increasing hypoglycemia. Pramlintide, a synthetic amylin analog, also decreases glucagon secretion and delays gastric emptying, improves hemoglobin A1c (HbA1C), and facilitates weight reduction without causing hypoglycemia. We review the historical discovery of these agents, their physiology [corrected] and their current applications. Remarkably, only one or two studies have been reported in children. Pediatricians caring for children with T1DM and T2DM should become familiar with these agents and investigate their applicability, as they seem likely to enhance our therapeutic armamentarium to treat children with diabetes mellitus.

Thiol reducing compounds prevent human amylin-evoked cytotoxicity

Human amylin (hA) is a small fibrillogenic protein that is the major constituent of pancreatic islet amyloid, which occurs in most subjects with type-2 diabetes mellitus (T2Dm). There is growing evidence that hA toxicity towards islet beta-cells is responsible for their gradual loss of function in T2Dm. Preventing hA-mediated cytotoxicity has been proposed as a route to halt the progression of this disease, although this has not yet been demonstrated in vivo. The aim of our studies, in which we show that a small number of hA-treated cells exhibit intracellular accumulation of reactive oxygen species (ROS), was to evaluate the role of oxidative stress in the mechanism of hA-mediated cytotoxicity. Here we report that catalase and n-propyl gallate, antioxidants that are thought to act mainly as free radical scavengers, afford RINm5F cells only limited protection against hA-mediated toxicity. By contrast, the thiol antioxidants, N-acetyl-L-cysteine (NAC), GSH and dithiothreitol, which not only react with ROS, but also modulate the cellular redox potential by increasing intracellular levels of GSH and/or by acting as thiol reducing agents, afford almost complete protection and inhibit the progression of hA-evoked apoptosis. We also show that hA treatment is not associated with changes in intracellular GSH levels and that inhibition of GSH biosynthesis has no effect on either hA-mediated cytotoxicity or NAC-mediated protection. These results indicate that, in addition to the induction of oxidative stress, hA appears to mediate cytotoxicity through signalling pathways that are sensitive to the actions of thiol antioxidants.

Aberrant processing of human proislet amyloid polypeptide results in increased amyloid formation

The amyloid present in the islets of Langerhans in type 2 diabetes is polymerized islet amyloid polypeptide (IAPP). The precursor protein proIAPP is posttranslationally modified, a process involving the removal of NH2- and COOH-terminal flanking peptides. This step is performed by the prohormone convertases PC2 and PC1/3. PC2 processes proIAPP preferably at the NH2-terminal processing site, and PC1/3 processes proIAPP exclusively at the COOH-terminal site. Little is known regarding the exact circumstances leading to islet amyloid formation. In this study, we have examined the possible significance of aberrant processing of proIAPP on amyloid formation in several in vitro cellular systems. In our studies, human (h)-proIAPP was transfected into beta-TC-6 cells expressing both prohormone convertases and in which proIAPP is processed into IAPP. Additionally, h-proIAPP was transfected into three different pituitary-derived cell lines with different prohormone convertase profiles: AtT-20 cells (deficient in PC2), GH3 cells (deficient in PC1/3), and GH4C1 cells (deficient in both convertases). We followed the processing of h-proIAPP with antibodies specific for the respective cleavage sites and stained the cells with Congo red to verify the accumulation of amyloid. Incomplete processing of h-proIAPP that occurs in AtT-20 and GH4C1 cells resulted in the formation of intracellular amyloid. No amyloid developed in beta-TC-6 and GH3 cells lines with full processing of proIAPP. An intracellular increase in proIAPP and/or its metabolic products may thus promote intracellular amyloid formation, thereby causing cell death. When extracellularly exposed, this amyloid might act as template for continuing amyloid formation from processed IAPP released from the surrounding beta-cells.

Formation of amyloid in human pancreatic islets transplanted to the liver and spleen of nude mice

In previous studies we have shown that apparently normal human islets, transplanted under the renal capsule of nude mice, frequently and rapidly develop amyloid deposits derived from the beta-cell hormone islet amyloid polypeptide (IAPP). In the present study, we show for the first time that human islets, transplanted into the liver or spleen of nude mice, also develop islet amyloid rapidly. Ultrastructural studies of such islets showed that the first aggregation of IAPP takes place within the beta-cells and that extracellular deposits show up later in the amyloid formation process. We also found that the amount of amyloid formed in human islet grafts placed under the kidney capsule increased with extended (26 weeks) observation time. Moreover, prolonged in vitro culture (14 days) prior to the implantation under the renal capsule seemed to enhance the formation of amyloid in the grafted islets. Since aggregated IAPP has been shown to be toxic to beta-cells, the finding of amyloid deposits in transplanted islets offers a possible explanation to the frequent loss of function of islets transplanted into diabetic patients.

Amyloidogenicity and cytotoxicity of islet amyloid polypeptide

Insoluble amyloid formation by islet amyloid polypeptide (IAPP) in the islets of Langerhans of the pancreas is a major pathophysiological feature of noninsulin dependent diabetes mellitus (NIDDM) or type II diabetes. Because in vivo formed amyloid colocalizes with areas of cell degeneration and IAPP amyloid aggregates are cytotoxic per se, the process of IAPP amyloid formation has been strongly associated with the progressive pancreatic cell degeneration and thus much of the pathology of type II diabetes. IAPP is a pancreatic polypeptide of 37 residues that, in its soluble form, is believed to play a role as a regulator of glucose homeostasis. The molecular cause and mechanism of the conversion of soluble IAPP into insoluble amyloid aggregates in vivo and its role in disease progress still remain to be clarified. Nevertheless, in the past few years significant progress has been made in understanding the amyloidogenesis pathway of IAPP in vitro and gaining insight into the structural and conformational "requirements" of IAPP amyloidogenesis and related cytotoxic effects. Importantly, several of the studies have revealed significant similarities of the above features of IAPP to other amyloidogenic polypeptides such as the beta-amyloid polypeptide Abeta. This suggests that, at the molecular level, amyloidogenesis, and possibly related cell degeneration and disease pathogenesis by completely different polypeptide sequences, may obey to common structural and conformational "rules" and follow similar molecular pathways. This review describes studies on the structural and conformational features of IAPP amyloid formation and cytotoxicity, and the application of the obtained knowledge for the understanding of the molecular mechanism of the IAPP amyloidogenesis pathway and the related cytotoxicity.

Enhanced in vitro production of amyloid-like fibrils from mutant (S20G) islet amyloid polypeptide

Islet amyloid polypeptide (IAPP, "amylin") is the amyloid-fibril-forming polypeptide in the islets of Langerhans associated with type 2 diabetes mellitus. A missense mutation in the IAPP gene associated with early-onset type 2 diabetes has been identified in the Japanese population. This mutation results in a glycine for serine substitution at position 20 of the mature IAPP molecule. Whether or not formation of islet amyloid with resulting destruction of islet tissue is the cause of this diabetes is yet not known. The present in vitro study was performed in order to investigate any influence of the amino acid substitution on the fibril formation capacity. Synthetic full-length wild type (IAPPwt) and mutant (IAPPS20G) as well as corresponding truncated peptides (position 18-29) were dissolved in dimethylsulfoxide (DMSO) or in 10% acetic acid at a concentration of 10 mg/mL and their fibril forming capacity was checked by Congo red staining, electron microscopy, a Congo red affinity assay and Thioflavine Tfluorometric assay. It was found that full-length and truncated IAPPS20G both formed more amyloid-like fibrils and did this faster compared to IAPPwt. The fibril morphology differed slightly between the preparations.

CONCLUSION

The amino acid substitution (S20G) is situated close to the region of the IAPP molecule implicated in the IAPP fibrillogenesis. The significantly increased formation of amyloid-like fibrils by IAPPS20G is highly interesting and may be associated with an increased islet amyloid formation in vivo and of fundamental importance in the pathogenesis of this specific form of diabetes.

Islet amyloid and type 2 diabetes: from molecular misfolding to islet pathophysiology

Islet amyloid polypeptide (IAPP, amylin) is secreted from pancreatic islet beta-cells and converted to amyloid deposits in type 2 diabetes. Conversion from soluble monomer, IAPP 1-37, to beta-sheet fibrils involves changes in the molecular conformation, cellular biochemistry and diabetes-related factors. In addition to the recognised amyloidogenic region, human IAPP (hIAPP) 20-29, the peptides human or rat IAPP 30-37 and 8-20, assume beta-conformation and form fibrils. These three amyloidogenic regions of hIAPP can be modelled as a folding intermediate with an intramolecular beta-sheet. A hypothesis is proposed for co-secretion of proIAPP with proinsulin in diabetes and formation of a 'nidus' adjacent to islet capillaries for subsequent accumulation of secreted IAPP to form the deposit. Although intracellular fibrils have been identified in experimental systems, extracellular deposition predominates in animal models and man. Extensive fibril accumulations replace islet cells. The molecular species of IAPP that is cytotoxic remains controversial. However, since fibrils form invaginations in cell membranes, small non-toxic IAPP fibrillar or amorphous accumulations could affect beta-cell stimulus-secretion coupling. The level of production of hIAPP is important but not a primary factor in islet amyloidosis; there is little evidence for inappropriate IAPP hypersecretion in type 2 diabetes and amyloid formation is generated in transgenic mice overexpressing the gene for human IAPP only against a background of obesity. Animal models of islet amyloidosis suggest that diabetes is induced by the deposits whereas in man, fibril formation appears to result from diabetes-associated islet dysfunction. Islet secretory failure results from progressive amyloidosis which provides a target for new therapeutic interventions.

Pituitary adenylate cyclase-activating polypeptide and islet amyloid polypeptide in primary sensory neurons: functional implications from plasticity in expression on nerve injury and inflammation

Primary sensory neurons serve a dual role as afferent neurons, conveying sensory information from the periphery to the central nervous system, and as efferent effectors mediating, e.g., neurogenic inflammation. Neuropeptides are crucial for both these mechanisms in primary sensory neurons. In afferent functions, they act as messengers and modulators in addition to a principal transmitter; by release from peripheral terminals, they induce an efferent response, "neurogenic inflammation," which comprises vasodilatation, plasma extravasation, and recruitment of immune cells. In this article, we introduce two novel members of the sensory neuropeptide family: pituitary adenylate cyclase-activating polypeptide (PACAP) and islet amyloid polypeptide (IAPP). Whereas PACAP, a vasoactive intestinal polypeptide-resembling peptide, predominantly occurs in neuronal elements, IAPP, which is structurally related to calcitonin gene-related peptide, is most widely known as a pancreatic beta-cell peptide; as such, it has been recognized as a constituent of amyloid deposits in type 2 diabetes. In primary sensory neurons, under normal conditions, both peptides are predominantly expressed in small-sized nerve cell bodies, suggesting a role in nociception. On axotomy, the expression of PACAP is rapidly induced, whereas that of IAPP is reduced. Such a regulation of PACAP suggests that it serves a protective role during nerve injury, but that of IAPP may indicate that it is an excitatory messenger under normal conditions. In contrast, in localized adjuvant-induced inflammation, expression of both peptides is rapidly induced. For IAPP, studies in IAPP-deficient mice support the notion that IAPP is a pronociceptive peptide, because these mutant mice display a reduced nociceptive response when challenged with formalin.

Differences in amyloid deposition in islets of transgenic mice expressing human islet amyloid polypeptide versus human islets implanted into nude mice

Islet amyloid polypeptide (IAPP)-derived amyloid is frequently deposited in the islets of Langerhans in patients with chronic non-insulin-dependent diabetes mellitus (NIDDM). When human islets were implanted under the renal capsule in nude mice, amyloid occurred in 73% of the grafts within 2 weeks. In this study, we compare the deposition of amyloid in islets from a transgenic mouse strain expressing human IAPP (hIAPP) and in normal human islets after implantation in nude mice. The implantations were performed as follows: (1) nondiabetic recipients were given islets from transgenic mice alone, (2) human islets were implanted in the upper pole of the kidney and islets from transgenic mice were implanted in the lower pole of the kidney, (3) grafts containing a mixture of human and transgenic islets were implanted, and (4) transgenic islets and islets from nontransgenic littermates were implanted in therapeutic numbers into recipients made diabetic by a single injection of alloxan prior to implantation. The implants were removed after various periods from 4 days to 8 weeks. The implants were either fixed in Formalin, stained for amyloid, and viewed in polarized light, or processed for immunoelectron microscopy and studied after immunolabeling with specific antibodies against IAPP. We found that the course of amyloid deposition differed significantly between human islets and hIAPP-expressing mouse islets. In human islets, amyloid was mainly deposited intracellularly and only small amounts of amyloid were found extracellularly. In contrast, in islets from transgenic mice, amyloid was exclusively deposited extracellularly and deposition in this site was preceded by an aggregation of immunoreactive material along the basement membrane. These findings point to separate mechanisms for amyloid formation in these two models.

Macrophages and pancreatic islet amyloidosis

Islet amyloid formed from islet amyloid polypeptide (IAPP, amylin) is found in spontaneously diabetic monkeys and cats. Islet amyloidosis is progressive, apparently irreversible and is associated with destruction of insulin-secreting cells. The role of macrophages in the destruction and removal of islet amyloid is unknown. Therefore, the presence and morphology of macrophages were determined by electron and quantitative light microscopy in islets of diabetic and nondiabetic man and monkeys and in transgenic mice expressing the gene for human IAPP. Tissue macrophages were present in all pancreatic sections and tissue distribution was similar in exocrine and endocrine areas. There was no difference in macrophage density in amyloidotic and amyloid-free islets in monkeys and man. Macrophage density was similar in islets of transgenic mice expressing human IAPP which do not contain amyloid in vivo but in which fibrils are formed in vitro following islet isolation compared to islets from mice expressing rat IAPP which is not amyloidogenic. IAPP amyloid fibrils were visible by electron microscopy in lysosomes of pancreatic macrophages in man, monkeys and human IAPP transgenic mice. Thus, human IAPP is internalised but inefficiently degraded by tissue macrophages. Diabetes-associated amyloidosis is not associated with visible recruitment of macrophages for removal of amyloid or islet debris.

Spontaneous diabetes mellitus in transgenic mice expressing human islet amyloid polypeptide

The islet in non-insulin-dependent diabetes mellitus (NIDDM) is characterized by loss of beta cells and large local deposits of amyloid derived from the 37-amino acid protein, islet amyloid polypeptide (IAPP). We have hypothesized that IAPP amyloid forms intracellularly causing beta-cell destruction under conditions of high rates of expression. To test this we developed a homozygous transgenic mouse model with high rates of expression of human IAPP. Male transgenic mice spontaneously developed diabetes mellitus by 8 weeks of age, which was associated with selective beta-cell death and impaired insulin secretion. Small intra- and extracellular amorphous IAPP aggregates were present in islets of transgenic mice during the development of diabetes mellitus. However, IAPP derived amyloid deposits were found in only a minority of islets at approximately 20 weeks of age, notably after development of diabetes mellitus in male transgenic mice. Approximately 20% of female transgenic mice spontaneously developed diabetes mellitus at 30+ weeks of age, when beta-cell degeneration and both amorphous and amyloid deposits of IAPP were present. We conclude that overexpression of human IAPP causes beta-cell death, impaired insulin secretion, and diabetes mellitus. Large deposits of IAPP derived amyloid do not appear to be important in this cytotoxicity, but early, small amorphous intra- and extracellular aggregates of human IAPP were consistently present at the time of beta-cell death and therefore may be the most cytotoxic form of IAPP.

Pore formation by the cytotoxic islet amyloid peptide amylin

Amylin is a 37-amino acid cytotoxic constituent of amyloid deposits found in the islets of Langerhans of patients with type II diabetes. Extracellular accumulation of this peptide results in damage to insulin-producing beta cell membranes and cell death. We report here that at cytotoxic concentrations, amylin forms voltage-dependent, relatively nonselective, ion-permeable channels in planar phospholipid bilayer membranes. Channel formation is dependent upon lipid membrane composition, ionic strength, and membrane potential. At 1-10 microM, cytotoxic human amylin dramatically increases the conductance of lipid bilayer membranes, while non-cytotoxic rat amylin does not. We suggest that channel formation may be the mechanism of cytotoxicity of human amylin.