The aggregation potential of human amylin determines its cytotoxicity towards islet beta-cells

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.

Atomic structures of IAPP (amylin) fusions suggest a mechanism for fibrillation and the role of insulin in the process

Islet Amyloid Polypeptide (IAPP or amylin) is a peptide hormone produced and stored in the beta-islet cells of the pancreas along with insulin. IAPP readily forms amyloid fibrils in vitro, and the deposition of fibrillar IAPP has been correlated with the pathology of type II diabetes. The mechanism of the conversion that IAPP undergoes from soluble to fibrillar forms has been unclear. By chaperoning IAPP through fusion to maltose binding protein, we find that IAPP can adopt a alpha-helical structure at residues 8-18 and 22-27 and that molecules of IAPP dimerize. Mutational analysis suggests that this dimerization is on the pathway to fibrillation. The structure suggests how IAPP may heterodimerize with insulin, which we confirmed by protein crosslinking. Taken together, these experiments suggest the helical dimerization of IAPP accelerates fibril formation and that insulin impedes fibrillation by blocking the IAPP dimerization interface.

Association of highly compact type II diabetes related islet amyloid polypeptide intermediate species at physiological temperature revealed by diffusion NMR spectroscopy

Self-association of human islet amyloid polypeptide (hIAPP) is correlated with the development of type II diabetes by the disruption of cellular homeostasis in islet cells through the formation of membrane-active oligomers. The toxic species of hIAPP responsible for membrane damage has not been identified. In this study, we show by pulsed field gradient NMR spectroscopy that the monomeric form of the toxic, amyloidogenic human variant of IAPP (hIAPP) adopts a temperature dependent compact folded conformation that is absent in both the nontoxic and nonamyloidogenic rat variant of IAPP and absent in hIAPP at low temperatures, suggesting this compact form of monomeric hIAPP may be linked to its later aggregation and cytotoxicity. In addition to the monomeric form of hIAPP, a large oligomeric species greater than 100 nm in diameter is also present but does not trigger the nucleation-dependent aggregation of IAPP at 4 degrees C, indicating the large oligomeric species may be an off-pathway intermediate that has been predicted by kinetic models of IAPP fiber formation. Furthermore, analysis of the polydispersity of the calculated diffusion values indicates small oligomeric species of hIAPP are absent in agreement with a recent ultracentrifugation study. The absence of small oligomeric species in solution suggests the formation of small, well-defined ion channels by hIAPP may proceed by aggregation of monomeric IAPP on the membrane, rather than by the insertion of preformed structured oligomers from the solution state as has been proposed for other amyloidogenic proteins.

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.

Two-dimensional IR spectroscopy and isotope labeling defines the pathway of amyloid formation with residue-specific resolution

There is considerable interest in uncovering the pathway of amyloid formation because the toxic properties of amyloid likely stems from prefibril intermediates and not the fully formed fibrils. Using a recently invented method of collecting 2-dimensional infrared spectra and site-specific isotope labeling, we have measured the development of secondary structures for 6 residues during the aggregation process of the 37-residue polypeptide associated with type 2 diabetes, the human islet amyloid polypeptide (hIAPP). By monitoring the kinetics at 6 different labeled sites, we find that the peptides initially develop well-ordered structure in the region of the chain that is close to the ordered loop of the fibrils, followed by formation of the 2 parallel beta-sheets with the N-terminal beta-sheet likely forming before the C-terminal sheet. This experimental approach provides a detailed view of the aggregation pathway of hIAPP fibril formation as well as a general methodology for studying other amyloid forming proteins without the use of structure-perturbing labels.

Induction of negative curvature as a mechanism of cell toxicity by amyloidogenic peptides: the case of islet amyloid polypeptide

The death of insulin-producing beta-cells is a key step in the pathogenesis of type 2 diabetes. The amyloidogenic peptide Islet Amyloid Polypeptide (IAPP, also known as amylin) has been shown to disrupt beta-cell membranes leading to beta-cell death. Despite the strong evidence linking IAPP to the destruction of beta-cell membrane integrity and cell death, the mechanism of IAPP toxicity is poorly understood. In particular, the effect of IAPP on the bilayer structure has largely been uncharacterized. In this study, we have determined the effect of the amyloidogenic and toxic hIAPP(1-37) peptide and the nontoxic and nonamyloidogenic rIAPP(1-37) peptide on membranes by a combination of DSC and solid-state NMR spectroscopy. We also characterized the toxic but largely nonamyloidogenic rIAPP(1-19) and hIAPP(1-19) fragments. DSC shows that both amyloidogenic (hIAPP(1-37)) and largely nonamyloidogenic (hIAPP(1-19) and rIAPP(1-19)) toxic versions of the peptide strongly favor the formation of negative curvature in lipid bilayers, while the nontoxic full-length rat IAPP(1-37) peptide does not. This result was confirmed by solid-state NMR spectroscopy which shows that in bicelles composed of regions of high curvature and low curvature, nontoxic rIAPP(1-37) binds to the regions of low curvature while toxic rIAPP(1-19) binds to regions of high curvature. Similarly, solid-state NMR spectroscopy shows that the toxic rIAPP(1-19) peptide significantly disrupts the lipid bilayer structure, whereas the nontoxic rIAPP(1-37) does not have a significant effect. These results indicate IAPP may induce the formation of pores by the induction of excess membrane curvature and can be used to guide the design of compounds that can prevent the cell-toxicity of IAPP. This mechanism may be important to understand the toxicity of other amyloidogenic proteins. Our solid-state NMR results also demonstrate the possibility of using bicelles to measure the affinity of biomolecules for negatively or positively curved regions of the membrane, which we believe will be useful in a variety of biochemical and biophysical investigations related to the cell membrane.

Oxidative stress is induced by islet amyloid formation and time-dependently mediates amyloid-induced beta cell apoptosis

AIMS/HYPOTHESIS

Islet amyloid in type 2 diabetes contributes to loss of beta cell mass and function. Since islets are susceptible to oxidative stress-induced toxicity, we sought to determine whether islet amyloid formation is associated with induction of oxidative stress.

METHODS

Human islet amyloid polypeptide transgenic and non-transgenic mouse islets were cultured for 48 or 144 h with or without the antioxidant N-acetyl-L: -cysteine (NAC) or the amyloid inhibitor Congo Red. Amyloid deposition, reactive oxygen species (ROS) production, beta cell apoptosis, and insulin secretion, content and mRNA were measured.

RESULTS

After 48 h, amyloid deposition was associated with increased ROS levels and increased beta cell apoptosis, but no change in insulin secretion, content or mRNA levels. Antioxidant treatment prevented the rise in ROS, but did not prevent amyloid formation or beta cell apoptosis. In contrast, inhibition of amyloid formation prevented the induction of oxidative stress and beta cell apoptosis. After 144 h, amyloid deposition was further increased and was associated with increased ROS levels, increased beta cell apoptosis and decreased insulin content. At this time-point, antioxidant treatment and inhibition of amyloid formation were effective in reducing ROS levels, amyloid formation and beta cell apoptosis. Inhibition of amyloid formation also increased insulin content.

CONCLUSIONS/INTERPRETATION

Islet amyloid formation induces oxidative stress, which in the short term does not mediate beta cell apoptosis, but in the longer term may feed back to further exacerbate amyloid formation and contribute to beta cell apoptosis.

A single mutation in the nonamyloidogenic region of islet amyloid polypeptide greatly reduces toxicity

Islet amyloid polypeptide (IAPP or amylin) is a 37-residue peptide secreted with insulin by beta-cells in the islets of Langerhans. The aggregation of the peptide into either amyloid fibers or small soluble oligomers has been implicated in the death of beta-cells during type 2 diabetes through disruption of the cellular membrane. The actual form of the peptide responsible for beta-cell death has been a subject of controversy. Previous research has indicated that the N-terminal region of the peptide (residues 1-19) is primarily responsible for the membrane-disrupting effect of the hIAPP peptide and induces membrane disruption to a similar extent as the full-length peptide without forming amyloid fibers when bound to the membrane. The rat version of the peptide, which is both noncytotoxic and nonamyloidogenic, differs from the human peptide by only one amino acid residue: Arg18 in the rat version while His18 in the human version. To elucidate the effect of this difference, we have measured in this study the effects of the rat and human versions of IAPP(1-19) on islet cells and model membranes. Fluorescence microscopy shows a rapid increase in intracellular calcium levels of islet cells after the addition of hIAPP(1-19), indicating disruption of the cellular membrane, while the rat version of the IAPP(1-19) peptide is significantly less effective. Circular dichroism experiments and dye leakage assays on model liposomes show that rIAPP(1-19) is deficient in binding to and disrupting lipid membranes at low but not at high peptide to lipid ratios, indicating that the ability of rIAPP(1-19) to form small aggregates necessary for membrane binding and disruption is significantly less than hIAPP(1-19). At pH 6.0, where H18 is likely to be protonated, hIAPP(1-19) resembles rIAPP(1-19) in its ability to cause membrane disruption. Differential scanning calorimetry suggests a different mode of binding to the membrane for rIAPP(1-19) compared to hIAPP(1-19). Human IAPP(1-19) has a minimal effect on the phase transition of lipid vesicles, suggesting a membrane orientation of the peptide in which the mobility of the acyl chains of the membrane is relatively unaffected. Rat IAPP(1-19), however, has a strong effect on the phase transition of lipid vesicles at low concentrations, suggesting that the peptide does not easily insert into the membrane after binding to the surface. Our results indicate that the modulation of the peptide orientation in the membrane by His18 plays a key role in the toxicity of nonamyloidogenic forms of hIAPP.

Amyloid oligomers in diabetic and nondiabetic human pancreas

The amyloid hypothesis of type 2 diabetes mellitus postulates that elevated levels of normally expressed monomeric proteins of human islet amyloid polypeptide (hIAPP) trigger oligomerization that independently causes fibril formation and disease progression. The aim of this study was to demonstrate the existence of amyloid oligomers in human pancreatic islets. Human pancreas tissues were obtained at autopsy of 8 nondiabetic control subjects (mean age = 75.8 +/- 11.7 years, 4 males), 8 type 2 diabetic cases without islet amyloid (mean age = 78.8 +/- 8.5 years, 4 males), and 8 type 2 diabetic patients with islet amyloid (mean age = 73.7 +/- 14.2 years, 4 males). Several markers for insulin, IAPP, amyloid fibrils (thioflavin T), and apoptosis (cleaved caspase-3) were used in combination with an oligomer-specific antibody. Two distinct forms of oligomers were found in pancreatic islets. Small spherical puncta were found in approximately 3% to 20% of the islet cells of nondiabetic subjects, and large curvilinear structures as extracellular oligomers were identified frequently in diabetic islets. Large oligomers were spatially localized adjacent to amyloid fibrils and were associated with apoptosis. This report demonstrates the presence of 2 morphologic classes of amyloid oligomers in human pancreatic islets. The observations warrant function studies to investigate the clinical implications of the amyloid oligomerization in the pathogenesis of type 2 diabetes.

Amyloid formation results in recurrence of hyperglycaemia following transplantation of human IAPP transgenic mouse islets

AIMS/HYPOTHESIS

Islet transplantation is a potential cure for diabetes; however, rates of graft failure remain high. The aim of the present study was to determine whether amyloid deposition is associated with reduced beta cell volume in islet grafts and the recurrence of hyperglycaemia following islet transplantation.

METHODS

We transplanted a streptozotocin-induced mouse model of diabetes with 100 islets from human IAPP (which encodes islet amyloid polypeptide) transgenic mice that have the propensity to form islet amyloid (n = 8-12) or from non-transgenic mice that do not develop amyloid (n = 6-10) in sets of studies that lasted 1 or 6 weeks.

RESULTS

Plasma glucose levels before and for 1 week after transplantation were similar in mice that received transgenic or non-transgenic islets, and at that time amyloid was detected in all transgenic grafts and, as expected, in none of the non-transgenic grafts. However, over the 6 weeks following transplantation, plasma glucose levels increased in transgenic but remained stable in non-transgenic islet graft recipients (p < 0.05). At 6 weeks, amyloid was present in 92% of the transgenic grafts and in none of the non-transgenic grafts. Beta cell volume was reduced by 30% (p < 0.05), beta cell apoptosis was twofold higher (p < 0.05), and beta cell replication was reduced by 50% (p < 0.001) in transgenic vs non-transgenic grafts. In summary, amyloid deposition in islet grafts occurs prior to the recurrence of hyperglycaemia and its accumulation over time is associated with beta cell loss.

CONCLUSIONS/INTERPRETATION

Islet amyloid formation may explain, in part, the non-immune loss of beta cells and recurrence of hyperglycaemia following clinical islet transplantation.

Small interfering RNA-mediated suppression of proislet amyloid polypeptide expression inhibits islet amyloid formation and enhances survival of human islets in culture

OBJECTIVE

Islet amyloid, formed by aggregation of the beta-cell peptide islet amyloid polypeptide (IAPP; amylin), is a pathological characteristic of pancreatic islets in type 2 diabetes. Toxic IAPP aggregates likely contribute to the progressive loss of beta-cells in this disease. We used cultured human islets as an ex vivo model of amyloid formation to investigate whether suppression of proIAPP expression would inhibit islet amyloid formation and enhance beta-cell survival and function.

RESEARCH DESIGN AND METHODS

Islets from cadaveric organ donors were transduced with a recombinant adenovirus expressing a short interfering RNA (siRNA) designed to suppress human proIAPP (Ad-hProIAPP-siRNA), cultured for 10 days, and then assessed for the presence of islet amyloid, beta-cell apoptosis, and beta-cell function.

RESULTS

Thioflavine S-positive amyloid deposits were clearly present after 10 days of culture. Transduction with Ad-hProIAPP-siRNA reduced proIAPP expression by 75% compared with nontransduced islets as assessed by Western blot analysis of islet lysates 4 days after transduction. siRNA-mediated inhibition of IAPP expression decreased islet amyloid area by 63% compared with nontransduced cultured islets. Cell death assessed by transferase-mediated dUTP nick-end labeling staining was decreased by 50% in transduced cultured human islets, associated with a significant increase in islet insulin content (control, 100 +/- 4 vs. +Ad-siRNA, 153 +/- 22%, P < 0.01) and glucose-stimulated insulin secretion (control, 222 +/- 33 vs. +Ad-siRNA, 285 +/- 21 percent basal, P < 0.05).

CONCLUSIONS

These findings demonstrate that inhibition of IAPP synthesis prevents amyloid formation and beta-cell death in cultured human islets. Inhibitors of IAPP synthesis may have therapeutic value in type 2 diabetes.

Spontaneous diabetes in hemizygous human amylin transgenic mice that developed neither islet amyloid nor peripheral insulin resistance

OBJECTIVES

We sought to 1) Determine whether soluble-misfolded amylin or insoluble-fibrillar amylin may cause or result from diabetes in human amylin transgenic mice and 2) determine the role, if any, that insulin resistance might play in these processes.

RESEARCH DESIGN AND METHODS

We characterized the phenotypes of independent transgenic mouse lines that display pancreas-specific expression of human amylin or a nonaggregating homolog, [(25,28,29)Pro]human amylin, in an FVB/n background.

RESULTS

Diabetes occurred in hemizygous human amylin transgenic mice from 6 weeks after birth. Glucose tolerance was impaired during the mid- and end-diabetic phases, in which progressive beta-cell loss paralleled decreasing pancreatic and plasma insulin and amylin. Peripheral insulin resistance was absent because glucose uptake rates were equivalent in isolated soleus muscles from transgenic and control animals. Even in advanced diabetes, islets lacked amyloid deposits. In islets from nontransgenic mice, glucagon and somatostatin cells were present mainly at the periphery and insulin cells were mainly in the core; in contrast, all three cell types were distributed throughout the islet in transgenic animals. [(25,28,29)Pro]human amylin transgenic mice developed neither beta-cell degeneration nor glucose intolerance.

CONCLUSIONS

Overexpression of fibrillogenic human amylin in these human amylin transgenic mice caused beta-cell degeneration and diabetes through mechanisms independent from both peripheral insulin resistance and islet amyloid. These findings are consistent with beta-cell death evoked by misfolded but soluble cytotoxic species, such as those formed by human amylin in vitro.

Higher islet amyloid load in men than in women with type 2 diabetes mellitus

OBJECTIVES

Type 2 diabetes mellitus is characterized histopathologically by islet amyloid deposits formed from islet amyloid polypeptide. The aim of this study was to investigate sex difference in islet amyloid of type 2 diabetic patients.

METHODS

Pancreas specimens were collected from 235 autopsies with type 2 diabetes mellitus. Islet amyloid was identified with Congo red stain. The load of islet amyloid deposits was assessed by prevalence (percentage of cases with islet amyloid deposits), frequency (percentage of islets containing amyloid deposits), and severity (percentage of islet area occupied by amyloid deposits).

RESULTS

Women (n = 80) and men (n = 155) had similar age of death, duration of diabetes, body mass index, and hemoglobin (Hb)A1c level. Islet amyloid was found in 30.0% of the women and in 44.5% of the men (P = 0.035). None of 9 women younger than 50 years had islet amyloid. Frequency of amyloid-affected islets was 31.5% +/- 13.1% in women and 41.1% +/- 14.3% in men (P = 0.008). Severity of amyloid-affected islet area was 29.0% +/- 12.5% in women and 38.5% +/- 14.6% in men (P = 0.007).

CONCLUSIONS

Sex is a determinant of the development of islet amyloid in type 2 diabetes mellitus. This sex difference in islet amyloid may be related to a potential benefit of female sex hormones.

Calcium-activated membrane interaction of the islet amyloid polypeptide: implications in the pathogenesis of type II diabetes mellitus

The role played by Ca(2+) ions in the interaction of the human islet amyloid polypeptide (hIAPP) with model membranes has been investigated by differential scanning calorimetry (DSC) and circular dichroism (CD) experiments. In particular, the interaction of hIAPP and its rat isoform (rIAPP) with zwitterionic dipalmitoyl-phosphatidylcholine (DPPC), negatively charged dipalmitoyl-phosphatidylserine (DPPS) vesicles and with a 3:1 mixtures of them, has been studied in the presence of Ca(2+) ions. The experiments have evidenced that amorphous, soluble hIAPP assemblies interact with the hydrophobic core of DPPC bilayers. Conversely, the presence of Ca(2+) ions is necessary to activate a preferential interaction of hIAPP with the hydrophobic core of DPPS membranes. These findings support the hypothesis that an impaired cellular homeostasis of Ca(2+) ions may promote the insertion of hIAPP into the hydrophobic core of carrier vesicles which is thought to contribute to an eventual intracellular accumulation of beta-sheet rich hIAPP aggregates.

Atomic structure of the cross-beta spine of islet amyloid polypeptide (amylin)

Human islet amyloid polypeptide (IAPP or amylin) is a 37-residue hormone found as fibrillar deposits in pancreatic extracts of nearly all type II diabetics. Although the cellular toxicity of IAPP has been established, the structure of the fibrillar form found in these deposits is unknown. Here we have crystallized two segments from IAPP, which themselves form amyloid-like fibrils. The atomic structures of these two segments, NNFGAIL and SSTNVG, were determined, and form the basis of a model for the most commonly observed, full-length IAPP polymorph.

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.

Amyloid fiber formation and membrane disruption are separate processes localized in two distinct regions of IAPP, the type-2-diabetes-related peptide

Aggregation of Islet Amyloid Polypeptide (IAPP) has been implicated in the development of type II diabetes. Because IAPP is a highly amyloidogenic peptide, it has been suggested that the formation of IAPP amyloid fibers causes disruption of the cellular membrane and is responsible for the death of beta-cells during type II diabetes. Previous studies have shown that the N-terminal 1-19 region, rather than the amyloidogenic 20-29 region, is primarily responsible for the interaction of the IAPP peptide with membranes. Liposome leakage experiments presented in this study confirm that the pathological membrane disrupting activity of the full-length hIAPP is also shared by hIAPP 1-19. The hIAPP 1-19 fragment at a low concentration of peptide induces membrane disruption to a near identical extent as the full-length peptide. At higher peptide concentrations, the hIAPP 1-19 fragment induces a greater extent of membrane disruption than the full-length peptide. Similar to the full-length peptide, hIAPP 1-19 exhibits a random coil conformation in solution and adopts an alpha-helical conformation upon binding to lipid membranes. However, unlike the full-length peptide, the hIAPP 1-19 fragment did not form amyloid fibers when incubated with POPG vesicles. These results indicate that membrane disruption can occur independently from amyloid formation in IAPP, and the sequences responsible for amyloid formation and membrane disruption are located in different regions of the peptide.

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.

Fas-associated death receptor signaling evoked by human amylin in islet beta-cells

OBJECTIVE

Aggregation of human amylin (hA) into beta-sheet-containing oligomers is linked to islet beta-cell dysfunction and the pathogenesis of type 2 diabetes. Here, we investigated possible contributions of Fas-associated death-receptor signaling to the mechanism of hA-evoked beta-cell apoptosis.

RESEARCH DESIGN AND METHODS

We measured responses to hA in isolated mouse islets and two insulinoma cell lines, wherein we measured Fas/Fas ligand (FasL) and Fas-associated death domain (FADD) expression by quantitative RT-PCR, Western blotting, and immunofluorescence staining. We used two anti-Fas/FasL blocking antibodies and the Fas/FasL antagonist Kp7-6 to probe roles of Fas interactions in the regulation of apoptosis in hA-treated beta-cells and measured Kp7-6-mediated effects on beta-sheet formation and aggregation using circular dichroism and thioflavin-T binding.

RESULTS

hA treatment stimulated Fas and FADD expression in beta-cells. Both blocking antibodies suppressed hA-evoked apoptosis but did not modify its aggregation. Therefore, Fas receptor interactions played a critical role in induction of this pathway. Interestingly, hA-evoked beta-cell apoptosis was suppressed and rescued by Kp7-6, which also impaired hA beta-sheet formation.

CONCLUSIONS

This is the first report linking hA-evoked induction and activation of Fas and FADD to beta-cell apoptosis. We have identified a Fas/FasL antagonist, Kp7-6, as a potent inhibitor of hA aggregation and related beta-cell death. These results also support an interaction between hA and Fas on the surface of apoptotic beta-cells. Increased expression and activation of Fas in beta-cells could constitute a molecular event common to the pathogenesis of both type 1 and type 2 diabetes, although the mode of pathway activation may differ between these common forms of diabetes.

Sedimentation studies on human amylin fail to detect low-molecular-weight oligomers

Sedimentation velocity experiments show that only monomers coexist with amyloid fibrils of human islet amyloid-polypeptide. No oligomers containing <100 monomers could be detected, suggesting that the putative toxic oligomers are much larger than those found for the Alzheimer's peptide, Abeta(1-42).

Induction of endoplasmic reticulum stress-induced beta-cell apoptosis and accumulation of polyubiquitinated proteins by human islet amyloid polypeptide

The islet in type 2 diabetes is characterized by an approximately 60% beta-cell deficit, increased beta-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). Human IAPP (hIAPP) but not rodent IAPP (rIAPP) forms toxic oligomers and amyloid fibrils in an aqueous environment. We previously reported that overexpression of hIAPP in transgenic rats triggered endoplasmic reticulum (ER) stress-induced apoptosis in beta-cells. In the present study, we sought to establish whether the cytotoxic effects of hIAPP depend on its propensity to oligomerize, rather than as a consequence of protein overexpression. To accomplish this, we established a novel homozygous mouse model overexpressing rIAPP at a comparable expression rate and, on the same background, as a homozygous transgenic hIAPP mouse model previously reported to develop diabetes associated with beta-cell loss. We report that by 10 wk of age hIAPP mice develop diabetes with a deficit in beta-cell mass due to increased beta-cell apoptosis. The rIAPP transgenic mice counterparts do not develop diabetes or have decreased beta-cell mass. Both rIAPP and hIAPP transgenic mice have increased expression of BiP, but only hIAPP transgenic mice have elevated ER stress markers (X-box-binding protein-1, nuclear localized CCAAT/enhancer binding-protein homologous protein, active caspase-12, and accumulation of ubiquitinated proteins). These findings indicate that the beta-cell toxic effects of hIAPP depend on the propensity of IAPP to aggregate, but not on the consequence of protein overexpression.

Copper(II) inhibits the formation of amylin amyloid in vitro

The amyloidogenic peptide amylin is found associated with pancreatic islet beta-cells and is implicated in the aetiology of type-2 diabetes mellitus. We have used fluorimetry and transmission electron microscopy to investigate in vitro the influence of Al(III), Fe(III), Zn(II) and Cu(II) on amylin amyloid formation under near-physiological conditions. Cu(II) at 10.0 microM inhibited amylin of 0.4 and 2.0 microM from forming amyloid fibrils while the same concentration of either Al(III) or Zn(II) promoted the formation of beta-pleated sheet structures. If amylin amyloid is cytotoxic to beta-cells then Cu(II) should protect against the degeneration of the islets in type-2 diabetes mellitus.

Fibrillogenic amylin evokes the apoptosis of human mesangial cells

OBJECTIVE

To explore the apoptotic role of amylin on human mesangial cell (MC).

MATERIALS AND METHODS

Primarily cultured human MCs were applied and treated with fresh amylin preparation. Human MCs were identified by the morphology and immunofluorescence staining. The apoptotic cells were determined by ultrastructure changes, TUNEL, and DNA fragmentation analysis. Propidium iodide staining and flow cytometry was employed for quantitative measurement of apoptosis.

RESULTS

Under the light and transmission electronic microscopy (TEM), the human MCs with condensed chromatin, plasma shrinkage, marginated nuclear chromatin or apoptotic body were observed in amylin-treated MCs. Positive TUNEL staining, hypolipoid DNA peak, and typical DNA "ladder" pattern were also detected in amylin-treated MCs. Quantitative analysis of the apoptotic MCs showed that human amylin induced an increase of the percentage of apoptotic cells in a dose-dependent manner. Amylin nano-scale fibrils (5-18nm) in diameter were detected in the cultured solution using negative staining under the TEM. Compared to the control, no significant changes of lactate dehydrogenase release were observed in amylin-treated MCs (P>0.05).

CONCLUSIONS

Fibrillogenic amylin evokes the apoptosis of human MCs in vitro, which may explain the mechanism of the hypocellular mesangial damage and progressive glomerulosclerosis of the patients with diabetic nephropathy.

Membrane fragmentation by an amyloidogenic fragment of human Islet Amyloid Polypeptide detected by solid-state NMR spectroscopy of membrane nanotubes

A key factor in the development of Type II diabetes is the loss of insulin producing pancreatic beta-cells. The amyloidogenic human Islet Amyloid Polypeptide (hIAPP also known as human amylin) is believed to play a crucial role in this biological process. Previous studies have shown that hIAPP forms small aggregates that kill beta-cells by disrupting the cellular membrane. In this study, we report membrane fragmentation by hIAPP using solid-state NMR experiments on nanotube arrays of anodic aluminum oxide containing aligned phospholipid membranes. In a narrow concentration range of hIAPP, an isotropic (31)P chemical shift signal indicative of the peptide-induced membrane fragmentation was detected. Solid-state NMR results suggest that membrane fragmentation is related to peptide aggregation as the presence of Congo Red, an inhibitor of amyloid formation, prevented membrane fragmentation and the non-amyloidogenic rat-IAPP did not cause membrane fragmentation. The disappearance of membrane fragmentation at higher concentrations of hIAPP suggests an alternate kinetic pathway to fibril formation in which membrane fragmentation is inhibited.

Copper-mediated formation of hydrogen peroxide from the amylin peptide: a novel mechanism for degeneration of islet cells in type-2 diabetes mellitus?

Amyloid deposits derived from the amylin peptide accumulate within pancreatic islet beta-cells in most cases of type-2 diabetes mellitus (T2Dm). Human amylin 'oligomers' are toxic to these cells. Using two different experimental techniques, we found that H(2)O(2) was generated during the aggregation of human amylin into amyloid fibrils. This process was greatly stimulated by Cu(II) ions, and human amylin was retained on a copper affinity column. In contrast, rodent amylin, which is not toxic, failed to generate any H(2)O(2) and did not interact with copper. We conclude that the formation of H(2)O(2) from amylin could contribute to the progressive degeneration of islet cells in T2Dm.

Cytotoxicity of Insulin within its Self-assembly and Amyloidogenic Pathways

Solvational perturbations were employed to selectively tune the aggregational preferences of insulin at 60 degrees C in vitro in purely aqueous acidic solution and in the presence of the model co-solvent ethanol (EtOH) (at 40%(w/w)). Dynamic light scattering (DLS), thioflavin T (ThT)-fluorescence, Fourier transform infrared (FTIR) and atomic force microscopy (AFM) techniques were employed to characterize these pathways biophysically with respect to the pre-aggregational assembly of the protein, the aggregation kinetics, and finally the aggregate secondary structure and morphology. Using cell viability assays, the results were subsequently correlated with the cytotoxicity of the insulin species that form in the two distinct aggregation pathways. In the cosolvent-free solution, predominantly dimeric insulin self-assembles via the well-known amyloidogenic pathway, yielding exclusively fibrillar aggregates, whereas in the solution containing EtOH, the aggregation of predominantly monomeric insulin proceeds via a pathway that leads to exclusively non-fibrillar, amorphous aggregates. Initially present native insulin assemblies as well as partially unfolded monomeric species and low molecular mass oligomeric aggregates could be ruled out as direct and major cytotoxic species. Apart from the slower overall aggregation kinetics under amorphous aggregate promoting conditions, which is due to the chaotropic nature of high EtOH concentrations, however, both pathways were unexpectedly found to evoke insulin aggregates that were cytotoxic to cultured rat insulinoma cells. The observed kinetics of the decrease of cell viabilities correlated well with the results of the DLS, ThT, FTIR and AFM studies, revealing that the formation of cytotoxic species correlated well with the formation of large-sized, beta-sheet-rich assemblies (>500 nm) of both fibrillar and amorphous nature. These results suggest that large-sized, beta-sheet-rich insulin assemblies of both fibrillar and amorphous nature are toxic to pancreatic beta-cells. In the light of the ongoing discussion about putative cytotoxic effects of prefibrillar and fibrillar amyloid aggregates, our results support the hypothesis that, in the case of insulin, factors other than the specific secondary or quarternary structural features of the various different aggregates may define their cytotoxic properties. Two such factors might be the aggregate size and the aggregate propensity to expose hydrophobic surfaces to a polar environment.

Amylin deposition in the kidney of patients with diabetic nephropathy

Amylin (islet amyloid peptide) plays a critical role in islet amyloidosis and in the development of beta-cell dysfunction in patients with diabetes; however, the involvement of amylin in renal amyloidosis has not been studied. For this reason, we surveyed 149 patients with biopsy-proven diabetic nephropathy (DN). The results were compared to 95 renal disease control patients, which included membranoproliferative glomerulonephritis, light-chain deposition, IgA nephropathy, and obesity-related glomerulopathy (ORG). Seventy-two of the 149 patients with DN showed amylin deposition in their renal tissue. Amylin was mainly distributed in the expanded mesangial area, Kimmelstiel-Wilson nodules, Bowman's capsule, and in blood vessels. The frequencies of mesangial proliferation, glomerular nodule lesions, and glomerular sclerosis were higher in DN patients with amylin deposits. Furthermore, the tubular interstitial lesions were more severe in these patients. Of the 95 disease-control patients, four with ORG were positive for renal amylin deposits. Our study has found renal amylin deposition in patients with DN and that the deposition was associated with disease severity. We suggest that strict metabolic control and reversing insulin resistance in patients with diabetes may blunt the process of amylin deposition in the kidney and possibly protect renal function in these patients.

Amyloid induced suicidal erythrocyte death

Amyloid peptides are known to induce apoptosis in a wide variety of cells. Erythrocytes may similarly undergo suicidal death or eryptosis, which is characterized by scrambling of the cell membrane with subsequent exposure of phosphatidylserine (PS) at the cell surface. Eryptosis is triggered by increase of cytosolic Ca(2+) activity and by activation of acid sphingomyelinase with subsequent formation of ceramide. Triggers of eryptosis include energy depletion and isosmotic cell shrinkage (replacement of extracellular Cl(-) by impermeable gluconate for 24 h). The present study explored whether amyloid peptide Abeta (1-42) could trigger eryptosis and to possibly identify underlying mechanisms. Erythrocytes from healthy volunteers were exposed to amyloid and PS-exposure (annexin V binding), cell volume (forward scatter), cytosolic Ca(2+) activity (Fluo3 fluorescence) and ceramide formation (anti-ceramide antibody) were determined by FACS analysis. Exposure of erythrocytes to the amyloid peptide Abeta (1-42) (> or = 0.5 microM) for 24 h significantly triggered annexin V binding, an effect mimicked to a lesser extent by the amyloid peptide Abeta (1-40) (1 microM). Abeta (1-42) (> or = 1.0 microM) further significantly decreased forward scatter of erythrocytes. The effect of Abeta (1-42) (> or = 0.5 microM) on erythrocyte annexin V binding was paralleled by formation of ceramide but not by significant increase of cytosolic Ca(2+) activity. The presence of Abeta (1-42) further significantly enhanced the eryptosis following Cl(-) depletion but not of glucose depletion for 24 hours. The present observations disclose a novel action of Abeta (1-42), which may well contribute to the pathophysiological effects of amyloid peptides, such as vascular complications in Alzheimer's disease.