[Amyloid of islets of Langerhans and its relation to diabetes mellitus (author's transl)]

Islet amyloid polypeptide aggregation exerts cytotoxic and proinflammatory effects on the islet vasculature in mice

AIMS/HYPOTHESIS

The islet vasculature, including its constituent islet endothelial cells, is a key contributor to the microenvironment necessary for normal beta cell health and function. In type 2 diabetes, islet amyloid polypeptide (IAPP) aggregates, forming amyloid deposits that accumulate between beta cells and islet capillaries. This process is known to be toxic to beta cells but its impact on the islet vasculature has not previously been studied. Here, we report the first characterisation of the effects of IAPP aggregation on islet endothelial cells/capillaries using cell-based and animal models.

METHODS

Primary and immortalised islet endothelial cells were treated with amyloidogenic human IAPP (hIAPP) alone or in the presence of the amyloid blocker Congo Red or the Toll-like receptor (TLR) 2/4 antagonist OxPAPc. Cell viability was determined0 along with mRNA and protein levels of inflammatory markers. Islet capillary abundance, morphology and pericyte coverage were determined in pancreases from transgenic mice with beta cell expression of hIAPP using conventional and confocal microscopy.

RESULTS

Aggregated hIAPP decreased endothelial cell viability in immortalised and primary islet endothelial cells (by 78% and 60%, respectively) and significantly increased expression of inflammatory markers Il6, Vcam1 and Edn1 mRNA relative to vehicle treatment in both cell types (p<0.05; n=4). Both cytotoxicity and the proinflammatory response were ameliorated by Congo Red (p<0.05; n=4); whereas TLR2/4-inhibition blocked inflammatory gene expression (p<0.05; n=6) without improving viability. Islets from high-fat-diet-fed amyloid-laden hIAPP transgenic mice also exhibited significantly increased expression of most markers of endothelial inflammation (p<0.05; n=5) along with decreased capillary density compared with non-transgenic littermates fed the same diet (p<0.01). Moreover, a 16% increase in capillary diameter was observed in amyloid-adjacent capillaries (p<0.01), accompanied by a doubling in pericyte structures positive for neuron-glial antigen 2 (p<0.001).

CONCLUSIONS/INTERPRETATION

Islet endothelial cells are susceptible to hIAPP-induced cytotoxicity and exhibit a TLR2/4-dependent proinflammatory response to aggregated hIAPP. Additionally, we observed amyloid-selective effects that decreased islet capillary density, accompanied by increased capillary diameter and increased pericyte number. Together, these data demonstrate that the islet vasculature is a target of the cytotoxic and proinflammatory effects of aggregated hIAPP that likely contribute to the detrimental effects of hIAPP aggregation on beta cell function and survival in type 2 diabetes.

Lipid accelerating the fibril of islet amyloid polypeptide aggravated the pancreatic islet injury in vitro and in vivo

Background

The fibrillation of islet amyloid polypeptide (IAPP) triggered the amyloid deposition, then enhanced the loss of the pancreatic islet mass. However, it is not clear what factor is the determinant in development of the fibril formation. The aim of this study is to investigate the effects of lipid on IAPP fibril and its injury on pancreatic islet.

Methods

The fibril form of human IAPP (hIAPP) was tested using thioflavin-T fluorescence assay and transmission electron microscope technology after incubated with palmitate for 5 h at 25 °C. The cytotoxicity of fibril hIAPP was evaluated in INS-1 cells through analyzing the leakage of cell membrane and cell apoptosis. Type 2 diabetes mellitus (T2DM) animal model was induced with low dose streptozotocin combined the high-fat diet feeding for two months in rats. Plasma biochemistry parameters were measured before sacrificed. Pancreatic islet was isolated to evaluate their function.

Results

The results showed that co-incubation of hIAPP and palmitate induced more fibril form. Fibril hIAPP induced cell lesions including cell membrane leakage and cell apoptosis accompanied insulin mRNA decrease in INS-1 cell lines. In vivo, Plasma glucose, triglyceride, rIAPP and insulin increased in T2DM rats compared with the control group. In addition, IAPP and insulin mRNA increased in pancreatic islet of T2DM rats. Furthermore, T2DM induced the reduction of insulin receptor expression and cleaved caspase-3 overexpression in pancreatic islet.

Conclusions

Results in vivo and in vitro suggested that lipid and IAPP plays a synergistic effect on pancreatic islet cell damage, which implicated in enhancing the IAPP expression and accelerating the fibril formation of IAPP.

Human Islets Have Fewer Blood Vessels than Mouse Islets and the Density of Islet Vascular Structures Is Increased in Type 2 Diabetes

Human and rodent islets differ substantially in several features, including architecture, cell composition, gene expression and some aspects of insulin secretion. Mouse pancreatic islets are highly vascularized with interactions between islet endothelial and endocrine cells being important for islet cell differentiation and function. To determine whether human islets have a similar high degree of vascularization and whether this is altered with diabetes, we examined the vascularization of islets from normal human subjects, subjects with type 2 diabetes (T2D), and normal mice. Using an integrated morphometry approach to quantify intra-islet capillary density in human and mouse pancreatic sections, we found that human islets have five-fold fewer vessels per islet area than mouse islets. Islets in pancreatic sections from T2D subjects showed capillary thickening, some capillary fragmentation and had increased vessel density as compared with non-diabetic controls. These changes in islet vasculature in T2D islets appeared to be associated with amyloid deposition, which was noted in islets from 8/9 T2D subjects (and occupied 14% ± 4% of islet area), especially around the intra-islet capillaries. The physiological implications of the differences in the angioarchitecture of mouse and human islets are not known. Islet vascular changes in T2D may exacerbate β cell/islet dysfunction and β cell loss.

Complete characterization of the mutation landscape reveals the effect on amylin stability and amyloidogenicity

Type-II diabetes is believed to be partially aggravated by the emergence of toxic amylin protein deposits in the extracellular space of the pancreas β-cells. Amylin, the regulatory hormone that is co-secreted with insulin, has been observed to misfold into toxic structures. Pramlintide, an FDA approved injectable amylin analog mutated at positions 25, 28, and 29 was therefore developed to create a more stable, soluble, less-aggregating, and equipotent peptide that is used as an adjunctive therapy for diabetes. However, because Pramlintide is not ideal, researchers have been exploring other amylin analogs as therapeutic replacements. In this work, we assist the finding of optimal analogs by computationally revealing the mutational landscape of amylin. We computed the structure energies of all possible single-point mutations and studied the effect they have on amylin stability and amyloidogenicity. Each of the 37 amylin residues was mutated in silico into the 19 canonical amino acids and an energy function computing the Lennard-Jones, Coulomb and solvation energy was used to analyze changes in stability. The mutation landscape identified amylin's conserved stable regions, residues that can be tweaked to further stabilize structure, regions that are susceptible to mutations, and mutations that are amyloidogenic. We used the single-point mutational landscape data to generate estimations for higher-order multiple-point mutational landscapes and discovered millions of three-point mutations that are more stable and less amyloidogenic than Pramlintide. The landscapes provided an explanation for the effect of the S20G and Q10R mutations on the onset of diabetes of the Chinese and Maori populations, respectively.

Non-selective ion channel activity of polymorphic human islet amyloid polypeptide (amylin) double channels

Fundamental understanding of ion channel formation by amyloid peptides, which is strongly linked to cell toxicity, is very critical for (pre)clinical treatment of neurodegenerative diseases. Here, we combine atomistic simulations and experiments to demonstrate a broad range of conformational states of hIAPP double channels in lipid membranes. All individual channels display high selectivity for Cl(-) ions over cations, but the co-existence of polymorphic double channels of different conformations and orientations with different populations determines the non-ionic selectivity nature of the channels, which is different from the typical amyloid-β channels that exhibit Ca(2+) selective ion-permeable characteristics. This work provides a more complete physicochemical mechanism of amyloid-channel-induced toxicity.

Acidic pH retards the fibrillization of human Islet Amyloid Polypeptide due to electrostatic repulsion of histidines

The human Islet Amyloid Polypeptide (hIAPP) is the major constituent of amyloid deposits in pancreatic islets of type-II diabetes. IAPP is secreted together with insulin from the acidic secretory granules at a low pH of approximately 5.5 to the extracellular environment at a neutral pH. The increased accumulation of extracellular hIAPP in diabetes indicates that changes in pH may promote amyloid formation. To gain insights and underlying mechanisms of the pH effect on hIAPP fibrillogenesis, all-atom molecular dynamics simulations in explicit solvent model were performed to study the structural properties of five hIAPP protofibrillar oligomers, under acidic and neutral pH, respectively. In consistent with experimental findings, simulation results show that acidic pH is not conducive to the structural stability of these oligomers. This provides a direct evidence for a recent experiment [L. Khemtemourian, E. Domenech, J. P. F. Doux, M. C. Koorengevel, and J. A. Killian, J. Am. Chem. Soc. 133, 15598 (2011)], which suggests that acidic pH inhibits the fibril formation of hIAPP. In addition, a complementary coarse-grained simulation shows the repulsive electrostatic interactions among charged His18 residues slow down the dimerization process of hIAPP by twofold. Besides, our all-atom simulations reveal acidic pH mainly affects the local structure around residue His18 by destroying the surrounding hydrogen-bonding network, due to the repulsive interactions between protonated interchain His18 residues at acidic pH. It is also disclosed that the local interactions nearby His18 operating between adjacent β-strands trigger the structural transition, which gives hints to the experimental findings that the rate of hIAPP fibril formation and the morphologies of the fibrillar structures are strongly pH-dependent.

Degradation of islet amyloid polypeptide by neprilysin

AIMS/HYPOTHESIS

A progressive loss of pancreatic beta cell function, a decrease in beta cell mass and accumulation of islet amyloid is characteristic of type 2 diabetes mellitus. The main constituent of islet amyloid is islet amyloid polypeptide (IAPP). In this study, we examined the ability of the peptidase neprilysin to cleave IAPP and prevent human IAPP-induced pancreatic beta cell toxicity.

METHODS

Neprilysin and a catalytically compromised neprilysin mutant were tested for their ability to inhibit human IAPP fibrillisation and human IAPP-induced pancreatic beta cell cytotoxicity. Degradation of human IAPP by neprilysin was followed by HPLC, and the degradation products were identified by MS.

RESULTS

Neprilysin prevented IAPP fibrillisation by cleaving IAPP at Arg(11)-Leu(12), Leu(12)-Ala(13), Asn(14)-Phe(15), Phe(15)-Leu(16), Asn(22)-Phe(23) and Ala(25)-Ile(26). It also appears to prevent human IAPP fibrillisation through a non-catalytic interaction. Neprilysin protected against beta cell cytotoxicity induced by exogenously added or endogenously produced human IAPP.

CONCLUSIONS/INTERPRETATION

The data presented support a potential therapeutic role for neprilysin in preventing type 2 diabetes mellitus. This study supports the hypothesis that extracellular human IAPP contributes to human IAPP-induced beta cell cytotoxicity. Whether human IAPP exerts its cytotoxic effect through a totally extracellular mechanism or through a cellular reuptake mechanism is unclear at this time.

Type 2 diabetes and the aging pancreatic beta cell

The incidence of and susceptibility to Type 2 diabetes increases with age, but the underlying mechanism(s) within beta cells that contribute to this increased susceptibility have not been fully elucidated. Here we review how aging affects the proliferative and regenerative capacity of beta cells and how this impacts beta cell mass. In addition we review changes that occur in beta cell function with age. Although we focus on the different rodent models that have provided insight into the characteristics of the aging beta cell, the limited knowledge from non-rodent models is also reviewed. Further studies are needed in order to identify potential beta cell targets for preventing or slowing the progression of diabetes that occurs with age.

Toxic oligomers and islet beta cell death: guilty by association or convicted by circumstantial evidence?

Type 2 diabetes is a progressive disease characterised by islet amyloid deposits in the majority of patients. Amyloid formation is considered a significant factor in deterioration of islet function and reduction in beta cell mass, and involves aggregation of monomers of the normally soluble beta cell peptide, human islet amyloid polypeptide (hIAPP) into oligomers, fibrils and, ultimately, mature amyloid deposits. Despite extensive in vitro studies, the process of hIAPP aggregation in vivo is poorly understood, though it is widely reported to promote cytotoxicity. Recently, studies have suggested that only the early stages of fibril assembly, and in particular small hIAPP oligomers, are responsible for beta cell cytotoxicity. This challenges the prior concept that newly formed fibrils and/or mature fibrillar amyloid are cytotoxic. Herein, evidence both for and against the toxic hIAPP oligomer hypothesis is presented; from this, it is apparent that what exactly causes beta cell death when hIAPP aggregates remains debatable. Moreover, substantially more work with more specific reagents and techniques than are currently available will be required to identify conclusively the toxic species resulting from hIAPP aggregation. Keeping an open mind on the nature of the cytotoxic insult has implications for therapeutic developments and clinical care in type 2 diabetes.

Amyloidogenesis of natively unfolded proteins

Aggregation and subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. The accumulated data support the model where protein fibrillogenesis proceeds via the formation of a relatively unfolded amyloidogenic conformation, which shares many structural properties with the pre-molten globule state, a partially folded intermediate first found during the equilibrium and kinetic (un)folding studies of several globular proteins and later described as one of the structural forms of natively unfolded proteins. The flexibility of this structural form is essential for the conformational rearrangements driving the formation of the core cross-beta structure of the amyloid fibril. Obviously, molecular mechanisms describing amyloidogenesis of ordered and natively unfolded proteins are different. For ordered protein to fibrillate, its unique and rigid structure has to be destabilized and partially unfolded. On the other hand, fibrillogenesis of a natively unfolded protein involves the formation of partially folded conformation; i.e., partial folding rather than unfolding. In this review recent findings are surveyed to illustrate some unique features of the natively unfolded proteins amyloidogenesis.

Islet amyloid polypeptide gene promoter polymorphisms are not associated with Type 2 diabetes or with the severity of islet amyloidosis

The over-expression of the islet amyloid polypeptide (IAPP) gene could be a causal factor for islet amyloidosis and beta-cell destruction in Type 2 diabetes (T2DM). An IAPP gene promoter polymorphism, IAPP-132G to A, has been associated with T2DM in Spain. To investigate this polymorphism in other cohorts and in relation to therapy, DNA from 425 T2DM and 279 unrelated, non-diabetic UK subjects (ND) and 102 T2DM and 80 ND Finnish subjects was examined. The relationship of amyloid severity (percent amyloid/islet) to prevalence (number of islets affected) and the association of IAPP-132G/A with amyloid was determined in post-mortem pancreas from 38 T2DM subjects. The -132G/A was not associated with T2DM in the UK cohorts (4.5% T2DM; 3.2% ND) or associated with requirement for insulin therapy by 6 years. The mutation was and undetected in the Finnish samples but a new variant, -166T/C, was identified in 2 Finnish T2DM subjects. -132G/A was found in 2/38 diabetic, amyloid-containing and 3/19 ND, amyloid-free subjects. The islet amyloid severity was linearly correlated with the prevalence in T2DM. The IAPP-132G/A promoter polymorphism is not associated with T2DM, a requirement for insulin therapy or with the degree of islet amyloidosis in cohorts from the UK or Finland.

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.

Apolipoprotein E genotype, islet amyloid deposition and severity of Type 2 diabetes

Islet amyloid is found in 90% of patients with Type 2 (non-insulin-dependent) diabetes at post-mortem. More extensive amyloidosis is associated with decreased islet function and requirement for insulin therapy. Severity of cerebral amyloidosis in Alzheimer's disease (AD) is increased in subjects with the apolipoprotein E (ApoE) epsilon 4 allele. To determine if ApoE genotype was associated with severity of islet amyloidosis and diabetes, samples were genotyped from 32 specimens of post-mortem pancreas and from patients classified by disease progression. DNA was extracted from blood samples from Caucasian patients diagnosed with Type 2 diabetes, at age >40 years, classified according to disease progression: group 1 on oral therapy for at least 10 years from diagnosis, (n=147) and group 2, requiring insulin within 6 years from diagnosis, (n=187). ApoE genotype was determined by restriction-fragment length polymorphism analysis. DNA in pancreatic extracts (23 diabetic; 9 non-diabetic subjects) showed no association of ApoE polymorphisms with either degree of islet amyloidosis or disease severity. The distributions of ApoE epsilon 2, epsilon 3 and epsilon 4 were similar in both clinical patient groups and in the non-diabetic group and unrelated to progression of disease. It is unlikely that the common polymorphisms for the ApoE gene are linked to amyloid formation or progression of islet dysfunction in Type 2 diabetes.

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.

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.

Identification of a novel human islet amyloid polypeptide beta-sheet domain and factors influencing fibrillogenesis

Human islet amyloid polypeptide (hIAPP) accumulates as pancreatic amyloid in type 2 diabetes and readily forms fibrils in vitro. Investigations into the mechanism of hIAPP fibril formation have focused largely on residues 20 to 29, which are considered to comprise a primary amyloidogenic domain. In rodents, proline substitutions within this region and the subsequent beta-sheet disruption, prevents fibril formation. An additional amyloidogenic fragment within the C-terminal sequence, residues 30 to 37, has been identified recently. We have extended these observations by examining a series of overlapping peptide fragments from the human and rodent sequences. Using protein spectroscopy (CD/FTIR), electron microscopy and X-ray diffraction, a previously unrecognised amyloidogenic domain was localised within residues 8 to 20. Synthetic peptides corresponding to this region exhibited a transition from random coil to beta-sheet conformation and assembled into fibrils having a typical amyloid-like morphology. The comparable rat 8-20 sequence, which contains a single His18Arg substitution, was also capable of assembling into amyloid-like fibrils. Examination of peptide fragments corresponding to residues 1 to 13 revealed that the immediate N-terminal region is likely to have only a modulating influence on fibril formation or conformational conversion. The contributions of charged residues as they relate to the amyloid-forming 8-20 sequence were also investigated using IAPP fragments and by assessing the effects of pH and counterions. The identification of these principal amyloidogenic sequences and the effects of associated factors provide details on the IAPP aggregation pathway and structure of the peptide in its fibrillar state.

A feline model of experimentally induced islet amyloidosis

The study of the pathogenesis of islet amyloidosis and its relationship to the development and progression of type 2 diabetes mellitus has been hampered by the lack of an experimentally inducible animal model. The domestic cat, by virtue of the fact that it is one of the few species that spontaneously develop a form of diabetes mellitus that closely resembles human type 2 diabetes, including the formation of amyloid deposits derived from islet amyloid polypeptide (IAPP), was considered to be an excellent candidate species in which to attempt to develop a nontransgenic animal model for this disease process. To develop the model, 8 healthy domestic cats were given a 50% pancreatectomy, which was followed by treatment with growth hormone and dexamethasone. Once a stable diabetic state was established, cats were randomly assigned to groups treated with either glipizide or insulin at doses appropriate to control hyperglycemia. Cats were maintained on this treatment regimen for 18 months and then euthanized. Based on light microscopic examination of Congo red-stained sections of pancreas, all cats were negative for the presence of islet amyloid at the time of pancreatectomy. At the end of the study all 4 glipizide-treated cats had islet amyloid deposits, whereas only 1 of 4 insulin-treated cats had detectable amyloid. In addition, the glipizide treated cats had threefold higher basal and fivefold higher glucose-stimulated plasma IAPP concentrations than insulin-treated cats, suggesting an association between elevated IAPP secretion and islet amyloidosis. Blood-glycosylated hemoglobin concentrations were not significantly different between the two treatment groups. This study documents for the first time an inducible model of islet amyloidosis in a nontransgenic animal.

Parallel changes of proinsulin and islet amyloid polypeptide in glucose intolerance

Elevated proinsulin secretion and islet amyloid deposition are both features of Type 2 diabetes but their relationship to beta-cell dysfunction is unknown. To determine if islet amyloid polypeptide (IAPP) secretion is disproportionate with other beta-cell products at any stage of glucose intolerance, 116 subjects were studied. Non-diabetic subjects with equivalent body mass index (BMI) were assigned to three groups, (i) normal fasting glucose, fpg<5.5 mmol l(-1); (ii) intermediate fasting glucose, fpg> or =5.5<6.15 mmol l(-1); (iii) impaired fasting glucose (IFG), fpg> or =6.1<7.0 mmol l(-1). Diabetic subjects were divided according to therapy (9 diet, 19 tablet, and 11 insulin). IAPP, C-peptide and proinsulin were measured fasting and at the end of a 1-h glucose infusion. Fasting C-peptide, IAPP and proinsulin were significantly elevated in the IFG group compared with the other non-diabetic groups (P<0.02); fasting IAPP/C-peptide and proinsulin/C-peptide were 1-2% in all non-diabetic groups. Fasting and 1-h proinsulin and proinsulin/C-peptide were higher in diabetic compared with non-diabetic subjects (P<0.01). IAPP and IAPP/C-peptide in diabetic groups were similar to that in non-diabetic subjects but reduced in the insulin-treated group (P<0.01). Proinsulin was disproportionately increased compared with C-peptide and IAPP in Type 2 diabetes particularly in severe beta-cell failure implying more than one concurrent beta-cell pathology.

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.

A comparative immunohistochemical study of pancreatic islets in laboratory animals (rats, dogs, minipigs, nonhuman primates)

The aim of the present study was to distinguish and describe the patterns of distribution of pancreatic islets within the pancreas of four species of laboratory animals, including rats, dogs, minipigs and monkeys, and furthermore, to identify immunohistochemically various islet cell types and characterize their content. Histopathological examinations were performed on sections stained with hematoxylin and eosin (H&E) and immunostained using rabbit polyclonal antibodies (pAb) against insulin, glucagon, pancreatic polypeptide (PP), somatostatin, chromogranin A, keratin, bombesin and gastrin, or mouse monoclonal antibodies (mAb) against synaptophysin, Leu-7 and proliferating cell nuclear antigen (PCNA) in three-step rabbit immunoperoxidase (PAP) and streptavidin/peroxidase (StreptABC/HRP) reactions. Positive immunohistochemical reactions were observed in the pancreatic islets of all animal species with all antibodies, except with anti-bombesin and anti-gastrin antibodies. Our results revealed that: 1) there is species specific regional arrangement of islets in the pancreas, 2) each species presents a characteristic distribution of cells producing different hormones. 3) immunoreactivity with immunohistochemical markers varies between species and/or age. The present comparative immunohistochemical study could be helpful for answering questions which are important for understanding some of the intricate mechanisms that govern the integrated function of the endocrine pancreas.

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.

Islet amyloid in type 2 (non-insulin-dependent) diabetes

Amyloid deposits are found in pancreatic islets of 90% of type 2 (non-insulin-dependent) diabetic subjects at postmortem. Islet amyloid is formed from islet amyloid polypeptide (IAPP). IAPP is a 37 amino acid peptide which is a normal constituent of beta cells and is co-secreted with insulin in animals and in man. The causative factors for fibrillogenesis of IAPP are unclear, but could be related to the sequence of IAPP and abnormal production of the peptide. The lack of islet amyloid in rodent models of diabetes is due to proline substitutions in the amyloidogenic region of IAPP. Amyloid fibrils are deposited between beta cells and islet capillaries: fibrils in invaginations of the plasma membrane may interfere with membrane signalling and insulin release. Amyloid fibrils are formed within 2 days in culture in islets isolated from transgenic mice expressing the gene for human IAPP, but not in vivo. Overexpression and decreased clearance of human IAPP from islet spaces may be important factors. Progressive deposition of IAPP fibrils combined with the associated reduction in the insulin-secreting beta cells is likely to contribute to deterioration of islet function in the course of type 2 diabetes.

Pancreatic islet amyloid formation in patients with noninsulin-dependent diabetes mellitus. Implication for therapeutic strategy

Islet amyloid polypeptide (IAPP or amylin) is the main component of pancreatic islet amyloid found in the vast majority of patients with noninsulin-dependent (Type-2) diabetes mellitus (NIDDM). IAPP may also act as a hormone that antagonizes the effects of insulin on peripheral tissues, but the results with IAPP overproducing transgenic mice and other recent findings indicate that IAPP overproduction is unlikely to induce peripheral insulin resistance in NIDDM. However, IAPP may contribute to the progression of NIDDM by impairing beta-cell function via islet amyloid formation. This may be initiated by locally elevated IAPP concentrations, induced by insulin-resistance-associated beta-cell hyperactivity. In order to improve therapeutic results, we propose strategies to inhibit IAPP production and islet amyloid formation during the pathogenesis of NIDDM.

Intra- and extracellular amyloid fibrils are formed in cultured pancreatic islets of transgenic mice expressing human islet amyloid polypeptide

Islet amyloid polypeptide (IAPP) is the constituent peptide of amyloid deposits found in the islets of non-insulin-dependent diabetic patients. Formation of islet amyloid is associated with a progressive destruction of insulin-producing beta cells. Factors responsible for the conversion of IAPP into insoluble amyloid fibrils are unknown. Both the amino acid sequence of human IAPP (hIAPP) and hypersecretion of hIAPP have been implicated as factors for amyloid fibril formation in man. We have generated transgenic mice using rat insulin promoter-hIAPP or rat IAPP (rIAPP) gene constructs. No fibrillar islet amyloid was detectable in vivo in these normoglycemic mice, although small amorphous perivascular accumulations of IAPP were observed in hIAPP mice only. To determine the effects of glucose on IAPP secretion and fibrillogenesis, pancreatic islets from transgenic and control mice were examined in vitro. Islet IAPP secretion and content were increased in transgenic islets compared with control islets. IAPP-immunoreactive fibrils were formed at both intra- and extracellular sites in isolated hIAPP islets cultured with glucose at 11.1 and 28 mM for only 7 days. At 28 mM glucose, fibrils were present in deep invaginations of beta cells as observed in non-insulin-dependent diabetic patients. No fibrils were present at low glucose concentrations in hIAPP islets or at any glucose concentration in rIAPP or control islets. Thus, glucose-induced expression and secretion of hIAPP in transgenic mouse islets can lead to formation of amyloid fibrils similar to that found in non-insulin-dependent diabetes mellitus.

Frequency of pancreatic amyloid deposition in cats from south-eastern Queensland

Stereological procedures were used to estimate the amount of amyloid deposition in the pancreatic islets of 83 cats from random sources in south-eastern Queensland. Most had only minor deposits of less than 20% of islet volume (median 9%), but deposits equal to more than 50% of the islet volume were found in 10% of the cats. Amyloid deposition in pancreatic islets was correlated with the age of the cat. Although similar observations have been made previously in cats from the USA, the frequency of amyloid deposition was higher in this population of cats from south-eastern Queensland.

Diabetes mellitus in Macaca mulatta monkeys is characterised by islet amyloidosis and reduction in beta-cell population

Diabetes mellitus in Macaca mulatta rhesus monkeys is preceded by phases of obesity and hyperinsulinaemia and is similar to Type 2 (non-insulin-dependent) diabetes mellitus in man. To relate the progression of the disease to quantitative changes in islet morphology, post-mortem pancreatic tissue from 26 monkeys was examined. Four groups of animals were studied: group I--young, lean and normal (n = 3); group II--older (> 10 years), lean and obese, normoglycaemic (n = 9); group III--normoglycaemic and hyperinsulinaemic (n = 6); group IV--diabetic (n = 8). Areas of islet amyloid, beta cells and islets were measured on stained histological sections. Islet size was larger in animals from groups III (p < 0.01) and IV (p < 0.0001) compared to groups I and II. The mean beta-cell area per islet in micron 2 was increased in group III (p < 0.05) and reduced in group IV (p < 0.001) compared to groups I and II. Mean beta-cell area per islet correlated with fasting plasma insulin (r = 0.76, p < 0.001) suggesting that hyper- and hypoinsulinaemia are related to the beta-cell population. Amyloid was absent in group I but small deposits were present in three of nine (group II) and in four of six (group III) animals, occupying between 0.03-45% of the islet space. Amyloid was present in eight of eight diabetic animals (group IV) occupying between 37-81% of the islet area. Every islet was affected in seven of eight diabetic monkeys. There was no correlation of degree of amyloidosis with age, body weight, body fat proportion or fasting insulin. Islet amyloid appears to precede the development of overt diabetes in Macaca mulatta and is likely to be a factor in the destruction of islet cells and onset of hyperglycaemia.

Immunohistology of islet amyloid polypeptide in diabetes mellitus: semi-quantitative studies in a post-mortem series

Immunoreactivity for islet amyloid polypeptide (IAPP) in the islets of Langerhans of non-insulin-dependent diabetic patients and non-diabetic patients of a non-selected post-mortem series was studied with a new polyclonal IAPP antibody. Out of 133 patients examined, 124 exhibited immunoreactivity for IAPP. Immunoreactivity was localized intra- and extracellularly and was limited to the islets of Langerhans. No extracellular immunoreactivity was observed in amyloid-negative cases. Co-localization of insulin and IAPP in the same islet-cells was verified by double staining with monoclonal insulin and polyclonal IAPP antibodies. Of 100 patients with non-insulin-dependent diabetes mellitus (NIDDM) and islet amyloid, 98 exhibited IAPP-positive deposits and 71 exhibited intracellular immunoreactivity. Evaluation of intracellular immunoreactivity and degree of islet amyloid deposition in cases of overt NIDDM revealed an inverse relationship, in that intracellular IAPP immunoreactivity were reduced in patients with developing islet amyloid deposition. Our data are consistent with the hypothesis of primary beta-cell dysfunction leading to amyloid formation, with subsequent disturbance of beta-cell homeostasis.

Non-linkage of the islet amyloid polypeptide gene with type 2 (non-insulin-dependent) diabetes mellitus

Type 2 (non-insulin-dependent) diabetes is associated with the deposition of islet amyloid. The major formative peptide, islet amyloid polypeptide, has recently been characterised and an abnormality of the structure or expression of this gene is a possible candidate for the inherited component of Type 2 diabetes. A restriction fragment length polymorphism of the gene has been identified with Pvu II. To study the relationship between the islet amyloid polypeptide gene and Type 2 diabetes, two distinct genetic approaches have been undertaken. Firstly, non-linkage has been demonstrated in four pedigrees, with four normoglycaemic first degree relatives having an allele associated with diabetes in other family members, and one affected relative not having the putatively associated allele. The LOD score taking age-related penetrance into account was -1.68, making linkage unlikely (p = 0.02). Secondly, in a population-based restriction fragment length polymorphism survey, no linkage disequilibrium of the alleles was found between a population of unrelated Caucasian subjects with Type 2 diabetes and a normal population. A mutation in or near the islet amyloid polypeptide gene is thus unlikely to be a common cause of Type 2 diabetes.

Islet amyloid polypeptide in diabetic and non-diabetic Pima Indians

Islet amyloid may have a pathological role in the development of Type 2 (non-insulin-dependent) diabetes mellitus. The prevalence of islet amyloid has been investigated on post-mortem pancreatic tissue from both diabetic and non-diabetic Pima Indian subjects who had previously been assessed by oral glucose tolerance tests. Islets were examined for amyloid deposits and for cellular immunoreactivity to pancreatic hormones and islet amyloid polypeptide, the constituent peptide of islet amyloid. Twenty of 26 diabetic subjects (77%) had islet amyloid, compared with one of 14 non-diabetic subjects (7%). Twelve of the diabetic subjects (46%) had amyloid in more than 10% of their islets, whereas only 4% of islets were affected in a single non-diabetic subject. Positive immunoreactivity for islet amyloid peptide was present in the islet amyloid and in islet cells in 54% of the diabetic and 50% of the non-diabetic subjects. Islet amyloid in diabetic Pima Indians may indicate a primary Beta-cell defect which interacts with insulin resistance to produce diabetes, or may develop as a result of Beta-cell dysfunction induced by insulin resistance and hyperglycaemia.

Purification and characterization of a peptide from amyloid-rich pancreases of type 2 diabetic patients

Deposition of amyloid in pancreatic islets is a common feature in human type 2 diabetic subjects but because of its insolubility and low tissue concentrations, the structure of its monomer has not been determined. We describe a peptide, of calculated molecular mass 3905 Da, that was a major protein component of amyloid-rich pancreatic extracts of three type 2 diabetic patients. After collagenase treatment, an extract containing 20-50% amyloid was solubilized by sonication into 70% formic acid and the peptide was purified by gel filtration followed by reverse-phase high-performance liquid chromatography. We term this peptide diabetes-associated peptide, as it was not detected in extracts of pancreas from any of six normal subjects. Diabetes-associated peptide contains 37 amino acids and is 46% identical to the sequences of rat and human calcitonin gene-related peptide, indicating that these peptides are related in evolution. Sequence identities with conserved residues of the insulin A chain were also seen in a 16-residue segment. On extraction, the islet amyloid is particulate and insoluble like the core particles of Alzheimer disease. Their monomers have similar molecular masses, each having a hydropathic region that can probably form beta-pleated sheets. The accumulation of amyloid, including diabetes-associated peptide, in islets may impair islet function in type 2 diabetes mellitus.

Islet amyloid formed from diabetes-associated peptide may be pathogenic in type-2 diabetes

Pancreatic islet amyloid deposits were found in 22 of 24 type-2 diabetic subjects (aged 48-68 years) and were not present in 10 age-matched controls. A novel peptide, 37 aminoacids long, termed diabetes-associated peptide (DAP), has been identified in amyloid-containing pancreatic extracts from 3 type-2 diabetic patients but not in extracts from 6 non-diabetic subjects. DAP has major homology with calcitonin-gene related peptide (CGRP) and the islet amyloid of all 22 diabetics showed CGRP immunoreactivity. The immunoreactivity was inhibited by preabsorption of three different CGRP antisera either with CGRP carboxyterminal peptide 28-37 or with extracted DAP. Both diabetic and non-diabetic subjects had CGRP/DAP immunoreactivity in islet B-cells. Electron microscopy of islets containing amyloid indicated fibrillar amyloid between the endocrine cells and capillaries, usually penetrating into deep invaginations of the plasma membrane of the B-cells. These results suggest that islet amyloid contains DAP, which may originate from B-cells. Accumulation of amyloid in islets is likely to impair islet function and may be a causal factor in the development of type-2 diabetes.

Longitudinal studies on the development of diabetes in individual Macaca nigra

Development of spontaneous diabetes has been monitored in individual Macaca nigra. In this study, pancreatic biopsies were taken, islets were assessed morphologically, and results were related to the metabolic/clinical status. A biopsy or autopsy sample was obtained 4 to 10 years later, and the islet morphological state was again related to the metabolic/clinical status. Metabolic deterioration was correlated to the islet lesion, in which there was gradual loss of islet secretory cells and concurrent amyloid deposition. As nondiabetic monkeys with 0 to 3% islet amyloid progressed up to 20 to 40% amyloid, the insulin secretion and glucose clearance were both decreased (p less than or equal to 0.01), and the glucose and glucagon levels increased (p = 0.05). Impaired monkeys progressed to overt diabetes when islet amyloid exceeded 50 to 60%. Diabetic monkeys developed hyperglycaemia, along with impaired insulin secretion and glucose clearance (p less than 0.01). Loss of islet cells results in metabolic deterioration. The lesion precedes development of overt diabetes in Macaca nigra.

Diabetes mellitus: relationships of nonhuman primates and other animal models to human forms of diabetes

Results from studies with M. nigra allow some conclusions and predictions about the etiology and development of diabetes relative to the islet lesion in monkeys and human beings. Some factor or factors must initiate the lesion; whether this is genetic, environmental, or a combination of both is not known. Amyloid is not the initiating factor to the islet lesion, but appears later as there is deterioration of cells. Sufficient evidence does not yet exist to choose from among the alternatives regarding the source of amyloid. With gradual deterioration of cells and replacement by amyloid, secretion of insulin is impaired and concentrations of glucagon increase. Sufficient circulating insulin is probably chronically available to the cells in this moderately impaired state, so that an acute decrease in delta IRI in response to glucose in an iv-administered GTT does not cause significant impairment in glucose clearance. The increase in circulating glucagon is probably due to a loss of controls on alpha-cell secretion or synthesis of glucagon. Fasting glucose levels increase but remain within the nondiabetic range. Eventually there is sufficient accretion of amyloid, usually greater than 50%, so that substantial beta-cell loss occurs and the monkey can no longer maintain fasting normoglycemia. The monkey then is hyperglycemic and hypoinsulinemic. Only at this time are the impairments detectable by the usual diagnostic clinical criterion of hyperglycemia. The ICAs arise in response to secretory cell deterioration and are present until there no longer are sufficient cells to elicit an immune response. Results from M. nigra can give insight into a similar condition that probably exists in a subpopulation of older diabetic humans. Humans probably pass through stages similar to those discerned in monkeys. Nondiabetic humans with sufficient beta cells to sustain adequate secretion of insulin, but with moderate amyloid infiltration, probably would be in a category equivalent to BD monkeys; since these people are not overtly hyperglycemic, they are not clinically recognizable as diabetic and would be classified retrospectively as nondiabetic. Continued loss of cells with concomitant amyloid deposition would eventually lead to hyperglycemia; if examined at autopsy, these people would have visible islet amyloid as well as a retrospective diagnosis of diabetes. Older type II diabetic humans with ICA usually proceed to insulin therapy more rapidly than do those who are ICA negative (Irvine et al., 1977; Del Prete et al., 1977; Gray et al., 1980).(ABSTRACT TRUNCATED AT 400 WORDS)