The islet amyloid polypeptide (amylin) gene S20G mutation in Chinese subjects: evidence for associations with type 2 diabetes and cholesterol levels

Understanding the structural dynamics of human islet amyloid polypeptide: Advancements in and applications of ion-mobility mass spectrometry

Human islet amyloid polypeptide (hIAPP) forms amyloid deposits that contribute to β-cell death in pancreatic islets and are considered a hallmark of Type II diabetes Mellitus (T2DM). Evidence suggests that the early oligomers of hIAPP formed during the aggregation process are the primary pathological agent in islet amyloid induced β-cell death. The self-assembly mechanism of hIAPP, however, remains elusive, largely due to limitations in conventional biophysical techniques for probing the distribution or capturing detailed structures of the early, structurally dynamic oligomers. The advent of Ion-mobility Mass Spectrometry (IM-MS) has enabled the characterisation of hIAPP early oligomers in the gas phase, paving the way towards a deeper understanding of the oligomerisation mechanism and the correlation of structural information with the cytotoxicity of the oligomers. The sensitivity and the rapid structural characterisation provided by IM-MS also show promise in screening hIAPP inhibitors, categorising their modes of inhibition through "spectral fingerprints". This review delves into the application of IM-MS to the dissection of the complex steps of hIAPP oligomerisation, examining the inhibitory influence of metal ions, and exploring the characterisation of hetero-oligomerisation with different hIAPP variants. We highlight the potential of IM-MS as a tool for the high-throughput screening of hIAPP inhibitors, and for providing insights into their modes of action. Finally, we discuss advances afforded by recent advancements in tandem IM-MS and the combination of gas phase spectroscopy with IM-MS, which promise to deliver a more sensitive and higher-resolution structural portrait of hIAPP oligomers. Such information may help facilitate a new era of targeted therapeutic strategies for islet amyloidosis in T2DM.

Protein aggregation: A detrimental symptom or an adaptation mechanism?

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

Investigating the aggregation perspective of Dengue virus proteome

Dengue viruses are human pathogens that are transmitted through mosquitoes. Apart from the typical symptoms associated with viral fevers, DENV infections are known to cause several neurological complications such as meningitis, encephalitis, intracranial haemorrhage, retinopathies along with the more severe, and sometimes fatal, vascular leakage and dengue shock syndrome. This study was designed to investigate, in detail, the predicted viral protein aggregation prone regions among all serotypes. Further, in order to understand the cross-talk between viral protein aggregation and aggregation of cellular proteins, cross-seeding experiments between the DENV NS1 (1-30), corresponding to the β-roll domain and the diabetes hallmark protein, amylin, were performed. Various techniques such as fluorescence spectroscopy, circular dichroism, atomic force microscopy and immunoblotting have been employed for this. We observe that the DENV proteomes have many predicted APRs and the NS1 (1-30) of DENV1-3, 2K and capsid anchor of DENV2 and DENV4 are capable of forming amyloids, in vitro. Further, the DENV NS1 (1-30), aggregates are also able to cross-seed and enhance amylin aggregation and vice-versa. This knowledge may lead to an opportunity for designing suitable inhibitors of protein aggregation that may be beneficial for viral infections and comorbidities.

Understanding the mechanism of amylin aggregation: From identifying crucial segments to tracing dominant sequential events to modeling potential aggregation suppressors

One of the most abundant, prevailing, and life-threatening human diseases that are currently baffling the scientific community is type 2 diabetes (T2D). The self-association of human amylin has been implicated in the pathogenesis of T2D, though with an inconclusive understanding of the mechanism. Hence, we focused on the characterization of the conformational ensembles of all the species that are believed to define the structural polymorphism of the aggregation process - the functional monomeric, the initially self-associated oligomeric, and the structured protofibril - by employing near-equilibrium, non-equilibrium, and equilibrium atomistic simulations on the sporadic, two familial variants (S20G and G33R), and their proline-substituted forms (S20P and G33P). The dynamic near-equilibrium assays hint toward - the abundance of helical conformation in the monomeric state, the retainment of the helicity in the initial self-associated oligomeric phase pointing toward the existence of the helix-helix association mechanism, the difference in preference of specific segments to have definite secondary structural features, the phase-dependent variability in the dominance of specific segments and mutation sites, and the simultaneous presence of generic and unique features among various sequences. Furthermore, the non-equilibrium pulling assays exemplify a generic sequential unzipping mechanism of the protofibrils, however, the sequence-dependent uniqueness comes from the difference in location and magnitude of the control of a specific terminus. Importantly, the equilibrium thermodynamic assays efficiently rank order the potential of aggregability among sequences and consequently suggests the probability of designing effective aggregation suppressors against sporadic and familial amylin variants incorporating proline as the mutation.

Analysis of Sheep and Goat IAPP Provides Insight into IAPP Amyloidogenicity and Cytotoxicity

Human islet amyloid polypeptide (hIAPP) plays a role in glucose regulation but forms pancreatic amyloid deposits in type 2 diabetes, and that process contributes to β-cell dysfunction. Not all species develop diabetes, and not all secrete an IAPP that is amyloidogenic in vitro under normal conditions, a perfect correlation currently exists between both. Studies of IAPPs from such organisms can provide clues about the high amyloidogenicity of hIAPP and can inform the design of soluble analogues of hIAPP. Sheep and goat IAPP are among the most divergent from hIAPP, with 13 and 11 substitutions, respectively, including an unusual Tyr to His substitution at the C-terminus. The properties of sheep and goat IAPP were examined in solution and in the presence of anionic vesicles, resulting in no observed amyloid formation, even at increased concentrations. Furthermore, both peptides are considerably less toxic to cultured β-cells than hIAPP. The effect of the Y37H replacements was studied in the context of hIAPP, as was a Y37R substitution. Buffer- and salt-dependent effects were observed. There was little impact on the time to form amyloid in phosphate-buffered saline; however, a significant deceleration was observed in Tris buffer, and amyloid formation was slower in the absence of added salt. The Y37H substitution had little impact on toxicity, while the Y37R replacement led to a 30% decrease in toxicity compared with that of hIAPP. The implications for the amyloidogenicity of hIAPP and the design of soluble analogues of the human peptide are discussed.

Pathophysiology, phenotypes and management of type 2 diabetes mellitus in Indian and Chinese populations

Nearly half of all adults with type 2 diabetes mellitus (T2DM) live in India and China. These populations have an underlying predisposition to deficient insulin secretion, which has a key role in the pathogenesis of T2DM. Indian and Chinese people might be more susceptible to hepatic or skeletal muscle insulin resistance, respectively, than other populations, resulting in specific forms of insulin deficiency. Cluster-based phenotypic analyses demonstrate a higher frequency of severe insulin-deficient diabetes mellitus and younger ages at diagnosis, lower β-cell function, lower insulin resistance and lower BMI among Indian and Chinese people compared with European people. Individuals diagnosed earliest in life have the most aggressive course of disease and the highest risk of complications. These characteristics might contribute to distinctive responses to glucose-lowering medications. Incretin-based agents are particularly effective for lowering glucose levels in these populations; they enhance incretin-augmented insulin secretion and suppress glucagon secretion. Sodium-glucose cotransporter 2 inhibitors might also lower blood levels of glucose especially effectively among Asian people, while α-glucosidase inhibitors are better tolerated in east Asian populations versus other populations. Further research is needed to better characterize and address the pathophysiology and phenotypes of T2DM in Indian and Chinese populations, and to further develop individualized treatment strategies.

Sequence-dependent aggregation-prone conformations of islet amyloid polypeptide

Amyloid proteins, which aggregate to form highly ordered structures, play a crucial role in various disease pathologies. Despite many previous studies on amyloid fibrils, which are an end product of protein aggregation, the structural characteristics of amyloid proteins in the early stage of aggregation and their related aggregation mechanism still remain elusive. The role of the amino acid sequence in the aggregation-prone structures of amyloid proteins at such a stage is not understood. Here, we have studied the sequence-dependent structural characteristics of islet amyloid polypeptide based on atomistic simulations and spectroscopic experiments. We show that the amino acid sequence determines non-bonded interactions that play a leading role in the formation of aggregation-prone conformations. Specifically, a single point mutation critically changes the population of aggregation-prone conformations, resulting in a change of the aggregation mechanism. Our simulation results were supported by experimental results suggesting that mutation affects the kinetics of aggregation and the structural characteristics of amyloid aggregates. Our study provides an insight into the role of sequence-dependent aggregation-prone conformations in the underlying mechanisms of amyloid aggregation.

Structural effects driven by rare point mutations in amylin hormone, the type II diabetes-associated peptide

BACKGROUND

Amylin is a 37-amino-acid peptide hormone co-secreted with insulin, which participates in glucose homeostasis. This hormone is able to aggregate in a β-sheet conformation and deposit in islet amyloids, a hallmark in type II diabetes. Since amylin is a gene-encoded hormone, this peptide has variants caused by point mutations that can impact its functions.

METHODS

Here, we analyzed the structural effects caused by S20G and G33R point mutations which, according to the 1000 Genomes Project, have frequency in East Asian and European populations, respectively. The analyses were performed by means of aggrescan server, SNP functional effect predictors, and molecular dynamics.

RESULTS

We found that both mutations have aggregation potential and cause changes in the monomeric forms when compared with wild-type amylin. Furthermore, comparative analyses with pramlintide, an amylin drug analogue, allowed us to infer that second α-helix maintenance may be related to the aggregation potential.

CONCLUSIONS

The S20G mutation has been described as pathologically related, which is in agreement with our findings. In addition, our data suggest that the G33R mutation might have a deleterious effect. The data presented here also provide new therapy opportunities, whether for creating more effective drugs for diabetes or implementing specific treatment for patients with these mutations.

GENERAL SIGNIFICANCE

Our data could help to better understand the impact of mutations on the wild-type amylin sequence, as a starting point for the evaluation and characterization of other variations. Moreover, these findings could improve the health of patients with type II diabetes.

Differential effects of serine side chain interactions in amyloid formation by islet amyloid polypeptide

Islet amyloid polypeptide (IAPP), a 37 residue polypeptide, is the main protein component of islet amyloid deposits produced in the pancreas in Type 2 diabetes. Human IAPP contains five serine residues at positions 19, 20, 28, 29, and 34. Models of the IAPP amyloid fibril indicate a structure composed of two closely aligned columns of IAPP monomers with each monomer contributing to two intermolecular β-strands. Ser 19 and Ser 20 are in the partially ordered β-turn region, which links the two strands, whereas Ser 28, Ser 29, and Ser 34 are in the core region of the amyloid fibril. Ser 29 is involved in contacts between the two columns of monomers and is the part of the steric zipper interface. We undertook a study of individual serine substitutions with the hydrophobic isostere 2-aminobutyric acid (2-Abu) to examine the site-specific role of serine side chains in IAPP amyloid formation. All five variants formed amyloid. The Ser 19 to 2-Abu mutant accelerates amyloid formation by a factor of 3 to 4, while the Ser 29 to 2-Abu mutation modestly slows the rate of amyloid formation. 2-Abu replacements at the other sites had even smaller effects. The data demonstrate that the cross-column interactions made by residue 29 are not essential for amyloid formation and also show that cross-strand networks of hydrogen-bonded Ser side chains, so called Ser-ladders, are not required for IAPP amyloid formation. The effect of the Ser 19 to 2-Abu mutant suggests that residues in this region are important for amyloid formation by IAPP.

Cryo-EM structure and inhibitor design of human IAPP (amylin) fibrils

Human islet amyloid polypeptide (hIAPP) functions as a glucose-regulating hormone but deposits as amyloid fibrils in more than 90% of patients with type II diabetes (T2D). Here we report the cryo-EM structure of recombinant full-length hIAPP fibrils. The fibril is composed of two symmetrically related protofilaments with ordered residues 14-37. Our hIAPP fibril structure (i) supports the previous hypothesis that residues 20-29 constitute the core of the hIAPP amyloid; (ii) suggests a molecular mechanism for the action of the hIAPP hereditary mutation S20G; (iii) explains why the six residue substitutions in rodent IAPP prevent aggregation; and (iv) suggests regions responsible for the observed hIAPP cross-seeding with β-amyloid. Furthermore, we performed structure-based inhibitor design to generate potential hIAPP aggregation inhibitors. Four of the designed peptides delay hIAPP aggregation in vitro, providing a starting point for the development of T2D therapeutics and proof of concept that the capping strategy can be used on full-length cryo-EM fibril structures.

Inhibition of Human Amylin Aggregation and Cellular Toxicity by Lipoic Acid and Ascorbic Acid

More than 30 human degenerative diseases result from protein aggregation such as Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM). Islet amyloid deposits, a hallmark in T2DM, are found in pancreatic islets of more than 90% of T2DM patients. An association between amylin aggregation and reduction in β-cell mass was also established by post-mortem studies. A strategy in preventing protein aggregation-related disorders is to inhibit the protein aggregation and associated toxicity. In this study, we demonstrated that two inhibitors, lipoic acid and ascorbic acid, significantly inhibited amylin aggregation. Compared to amylin (15 μM) as 100%, lipoic acid and ascorbic acid reduced amylin fibril formation to 42.1 ± 17.2% and 42.9 ± 12.8%, respectively, which is confirmed by fluorescence and TEM images. In cell viability tests, both inhibitors protected RIN-m5f β-cells from the toxicity of amylin aggregates. At 10:1 molar ratio of lipoic acid to amylin, lipoic acid with amylin increased the cell viability to 70.3%, whereas only 42.8% RIN-m5f β-cells survived in amylin aggregates. For ascorbic acid, an equimolar ratio achieved the highest cell viability of 63.3% as compared to 42.8% with amylin aggregates only. Docking results showed that lipoic acid and ascorbic acid physically interact with amylin amyloidogenic region (residues Ser20-Ser29) via hydrophobic interactions; hence reducing aggregation levels. Therefore, lipoic acid and ascorbic acid prevented amylin aggregation via hydrophobic interactions, which resulted in the prevention of cell toxicity in vitro.

Common fibrillar spines of amyloid-β and human islet amyloid polypeptide revealed by microelectron diffraction and structure-based inhibitors

Amyloid-β (Aβ) and human islet amyloid polypeptide (hIAPP) aggregate to form amyloid fibrils that deposit in tissues and are associated with Alzheimer's disease (AD) and type II diabetes (T2D), respectively. Individuals with T2D have an increased risk of developing AD, and conversely, AD patients have an increased risk of developing T2D. Evidence suggests that this link between AD and T2D might originate from a structural similarity between aggregates of Aβ and hIAPP. Using the cryoEM method microelectron diffraction, we determined the atomic structures of 11-residue segments from both Aβ and hIAPP, termed Aβ(24-34) WT and hIAPP(19-29) S20G, with 64% sequence similarity. We observed a high degree of structural similarity between their backbone atoms (0.96-Å root mean square deviation). Moreover, fibrils of these segments induced amyloid formation through self- and cross-seeding. Furthermore, inhibitors designed for one segment showed cross-efficacy for full-length Aβ and hIAPP and reduced cytotoxicity of both proteins, although by apparently blocking different cytotoxic mechanisms. The similarity of the atomic structures of Aβ(24-34) WT and hIAPP(19-29) S20G offers a molecular model for cross-seeding between Aβ and hIAPP.

The β-cell assassin: IAPP cytotoxicity

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

Prion-Like Protein Aggregates and Type 2 Diabetes

Type 2 diabetes (T2D) is a highly prevalent metabolic disease characterized by chronic insulin resistance and β-cell dysfunction and loss, leading to impaired insulin release and hyperglycemia. Although the mechanism responsible for β-cell dysfunction and death is not completely understood, recent findings suggest that the accumulation of misfolded aggregates of the islet amyloid polypeptide (IAPP) in the islets of Langerhans may play an important role in pancreatic damage. Misfolding and aggregation of diverse proteins and their accumulation as amyloid in different organs is the hallmark feature in a group of chronic, degenerative diseases termed protein misfolding disorders (PMDs). PMDs include highly prevalent human illnesses such as Alzheimer's and Parkinson's disease, as well as more than 25 rarer disorders. Among them, prion diseases are unique because the pathology can be transmitted by a proteinaceous infectious agent, termed a prion, which induces disease by propagating protein misfolding and aggregation. This phenomenon has a striking resemblance to the process of protein misfolding and aggregation in all of the PMDs, suggesting that misfolded aggregates have an intrinsic potential to be transmissible. Indeed, recent studies have shown that the pathological hallmarks of various PMDs can be induced in vivo under experimental conditions by inoculating tissue extracts containing protein aggregates into animal models. In this review, we describe our current understanding of the molecular mechanism underlying the prion-like transmission of protein aggregates and its possible role in T2D.

Atomic structures of fibrillar segments of hIAPP suggest tightly mated β-sheets are important for cytotoxicity

hIAPP fibrils are associated with Type-II Diabetes, but the link of hIAPP structure to islet cell death remains elusive. Here we observe that hIAPP fibrils are cytotoxic to cultured pancreatic β-cells, leading us to determine the structure and cytotoxicity of protein segments composing the amyloid spine of hIAPP. Using the cryoEM method MicroED, we discover that one segment, 19-29 S20G, forms pairs of β-sheets mated by a dry interface that share structural features with and are similarly cytotoxic to full-length hIAPP fibrils. In contrast, a second segment, 15-25 WT, forms non-toxic labile β-sheets. These segments possess different structures and cytotoxic effects, however, both can seed full-length hIAPP, and cause hIAPP to take on the cytotoxic and structural features of that segment. These results suggest that protein segment structures represent polymorphs of their parent protein and that segment 19-29 S20G may serve as a model for the toxic spine of hIAPP.

Neuroendocrine hormone amylin in diabetes

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

Type 2 diabetes as a protein misfolding disease

Type 2 diabetes (T2D) is a highly prevalent and chronic metabolic disorder. Recent evidence suggests that formation of toxic aggregates of the islet amyloid polypeptide (IAPP) might contribute to β-cell dysfunction and disease. However, the mechanism of protein aggregation and associated toxicity remains unclear. Misfolding, aggregation, and accumulation of diverse proteins in various organs is the hallmark of the group of protein misfolding disorders (PMDs), including highly prevalent illnesses affecting the central nervous system (CNS) such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this review we discuss the current understanding of the mechanisms implicated in the formation of protein aggregates in the endocrine pancreas and associated toxicity in the light of the long-standing knowledge from neurodegenerative disorders associated with protein misfolding.

Computational re-engineering of Amylin sequence with reduced amyloidogenic potential

BACKGROUND

The aggregation of amyloid proteins into fibrils is associated with neurodegenerative diseases such as Alzheimer's and Type II Diabetes. Different methods have explored ways to impede and inhibit amyloid aggregation. Most attempts in the literature involve applying stress to the environment around amyloids. Varying pH levels, modifying temperature, applying pressure through protein crowding and ligand docking are classical examples of these methods. However, environmental stress usually affects molecular pathways and protein functions in the cell and is challenging to construct in vivo. In this paper, we explore destabilizing amyloid proteins through the manipulation of genetic code to create beneficial substitute molecules for patients with certain deficiencies.

RESULTS

To unravel sequence mutations that destabilize amyloid fibrils yet simultaneously conserve native fold, we analyze the structural landscape of amyloid proteins and search for potential areas that could be exploited to weaken aggregation. Our tool, FibrilMutant, analyzes these regions and studies the effect of amino acid point mutations on nucleation and aggregation. This multiple objective approach impedes aggregation without stressing the cellular environment. We identified six main regions in amyloid proteins that contribute to structural stability and generated amino acid mutations to destabilize those regions. Full length fibrils were built from the mutated amyloid monomers and a dipolar-solvent model capturing the effect of dipole-dipole interactions between water and very large molecular systems to assess their aqueous stability was used to generate energy plots.

CONCLUSION

Our results are in agreement with experimental studies and suggest novel targeted single point mutations in the Amylin protein, potentially creating a better therapeutic agent than the currently administered Pramlintide drug for diabetes patients.

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.

Molecular simulations indicate marked differences in the structure of amylin mutants, correlated with known aggregation propensity

Human islet amyloid polypeptide (hIAPP), a 37-residue protein cosecreted with insulin by β-cells in the pancreas, is known to form amyloid fibrils in type II diabetes patients. This fibril formation is also associated with β-cell death. However, rat IAPP (rIAPP) does not aggregate into fibrils, nor is it associated with β-cell toxicity. Determining solution properties of hIAPP experimentally is difficult because it aggregates quickly. Our study uses molecular dynamics simulation to explore and compare in-solution characteristics of hIAPP and rIAPP, as well as two single-point hIAPP mutants, hIAPP I26P and hIAPP S20G, which exhibit observed differences from hIAPP in aggregation propensities. We find that all four polypeptide monomers sample structured states in solution. More importantly, differences in the helicity over residues 7-16 may play an important role in early aggregation, although this region is outside of commonly assumed amyloidogenic region 20-29. The long-range contacts, though unexpected of IDPs, cause variation in sampled conformations among four polypeptides within same amino acid sequence. Our results also yield evidence that previously determined structures bound to micelles are also transiently sampled in the solution state. In particular, similarities found in region 8-17 together with the helical differences that we observe in region 7-16 lead us to suggest that the region 7-16 is potentially responsible for amyloidogenic behavior of amylin peptides. Our results also provide support for the proposed mechanism of fibril formation based on experimentally observed transient helices in amyloidogenic peptides.

Conformations of islet amyloid polypeptide monomers in a membrane environment: implications for fibril formation

The amyloid fibrils formed by islet amyloid polypeptide (IAPP) are associated with type II diabetes. One of the proposed mechanisms of the toxicity of IAPP is that it causes membrane damage. The fatal mutation of S20G human IAPP was reported to lead to early onset of type II diabetes and high tendency of amyloid formation in vitro. Characterizing the structural features of the S20G mutant in its monomeric state is experimentally difficult because of its unusually fast aggregation rate. Computational work complements experimental studies. We performed a series of molecular dynamics simulations of the monomeric state of human variants in the membrane. Our simulations are validated by extensive comparisons with experimental data. We find that a helical disruption at His18 is common to both human variants. An L-shaped motif of S20G mutant is observed in one of the conformational families. This motif that bends at His18 resembles the overall topology of IAPP fibrils. The conformational preorganization into the fibril-like topology provides a possible explanation for the fast aggregation rate of S20G IAPP.

Sensitivity of amyloid formation by human islet amyloid polypeptide to mutations at residue 20

Islet amyloid polypeptide (IAPP, amylin) is responsible for amyloid formation in type 2 diabetes and in islet cell transplants. The only known natural mutation found in mature human IAPP is a Ser20-to-Gly missense mutation, found with small frequency in Chinese and Japanese populations. The mutation appears to be associated with increased risk of early-onset type 2 diabetes. Early measurements in the presence of organic co-solvents showed that S20G-IAPP formed amyloid more quickly than the wild type. We confirm that the mutant accelerates amyloid formation under a range of conditions including in the absence of co-solvents. Ser20 adopts a normal backbone geometry, and the side chain makes no steric clashes in models of IAPP amyloid fibers, suggesting that the increased rate of amyloid formation by the mutant does not result from the relief of steric incompatibility in the fiber state. Transmission electronic microscopy, circular dichroism, and seeding studies were used to probe the structure of the resulting fibers. The S20G-IAPP peptide is toxic to cultured rat INS-1 (transformed rat insulinoma-1) β-cells. The sensitivity of amyloid formation to the identity of residue 20 was exploited to design a variant that is much slower to aggregate and that inhibits amyloid formation by wild-type IAPP. An S20K mutant forms amyloid with an 18-fold longer lag phase in homogeneous solution. Thioflavin T binding assays, together with experiments using a p-cyanophenylalanine (p-cyanoPhe) variant of human IAPP, show that the designed S20K mutant inhibits amyloid formation by human IAPP. The experiments illustrate how p-cyanoPhe can be exploited to monitor amyloid formation even in the presence of other amyloidogenic proteins.

Drosophila melanogaster as a model system for studies of islet amyloid polypeptide aggregation

Background

Recent research supports that aggregation of islet amyloid polypeptide (IAPP) leads to cell death and this makes islet amyloid a plausible cause for the reduction of beta cell mass, demonstrated in patients with type 2 diabetes. IAPP is produced by the beta cells as a prohormone, and proIAPP is processed into IAPP by the prohormone convertases PC1/3 and PC2 in the secretory granules. Little is known about the pathogenesis for islet amyloid and which intracellular mechanisms are involved in amyloidogenesis and induction of cell death.

Methodology/Principal Findings

We have established expression of human proIAPP (hproIAPP), human IAPP (hIAPP) and the non-amyloidogenic mouse IAPP (mIAPP) in Drosophila melanogaster, and compared survival of flies with the expression driven to different cell populations. Only flies expressing hproIAPP in neurons driven by the Gal4 driver elav^C155,Gal4 showed a reduction in lifespan whereas neither expression of hIAPP or mIAPP influenced survival. Both hIAPP and hproIAPP expression caused formation of aggregates in CNS and fat body region, and these aggregates were both stained by the dyes Congo red and pFTAA, both known to detect amyloid. Also, the morphology of the highly organized protein granules that developed in the fat body of the head in hIAPP and hproIAPP expressing flies was characterized, and determined to consist of 15.8 nm thick pentagonal rod-like structures.

Conclusions/Significance

These findings point to a potential for Drosophila melanogaster to serve as a model system for studies of hproIAPP and hIAPP expression with subsequent aggregation and developed pathology.

Porcine islet amyloid polypeptide fragments are refractory to amyloid formation

Of 10 variation sites between sequences of amyloid-resistant porcine islet amyloid polypeptide (pIAPP) and amyloid-prone human IAPP (hIAPP), seven locate within residues 17-29, the most amyloidogenic fragment within hIAPP. To investigate how these variations affect amyloidogenicity, 26 IAPP(17-29) or IAPP(20-29) variants were synthesized and their secondary structures, amyloidogenicity, oligomerization and cytotoxicity were studied. Our results indicated that pIAPP fragments are refractory to amyloid formation and significantly less cytotoxic compared with hIAPP fragments. A novel stable dimer was observed in pIAPP(20-29) solution, whereas hIAPP(20-29) exists mostly as monomers and trimers. Among all human to porcine substitutions, S20R caused the most prolonged lag time and significantly attenuated cytotoxicity. The different oligomerization and amyloidogenic properties of hIAPP and pIAPP fragments are discussed.

Gut microbiota in health and disease

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

The parallel superpleated beta-structure as a model for amyloid fibrils of human amylin

Human amylin is a 37 amino acid residue peptide hormone whose fibrillogenesis has been correlated with type 2 diabetes. These fibrils are rope-like bundles of several 5nm diameter protofilaments. Here, we propose, as a model for the protofilament, a variant of the parallel superpleated beta-structure previously derived for amyloid filaments of the yeast prion Ure2p. In the amylin model, individual polypeptides from residues 9 to 37 have a planar S-shaped fold with three beta-strands. These serpentines are stacked in register, with a 0.47 nm axial rise and a small rotational twist per step, generating an array of three parallel beta-sheets in cross-beta conformation. The interior, the two "bays" sandwiched between adjacent sheets, are occupied by non-polar and by polar/uncharged residues that are predicted to form H-bonded ladders, similar to those found in beta-helical proteins. The N-terminal peptide containing a disulfide bond occupies an extraneous peripheral position in the protofilament. The left-handed twist of the beta-sheets is shown to underlie left-handed coiling of amylin protofilaments in fibrils. The model is consistent with current biophysical, biochemical and genetic data and, in particular, affords a plausible explanation for why rodent amylin does not form fibrils.

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.

S20G mutation of the amylin gene is associated with a lower body mass index in Korean type 2 diabetic patients

Islet amyloid deposition, which is mainly composed of amylin, is a characteristic pathological finding in patients with type 2 diabetes mellitus. A missense mutation in amylin at amino acid 20 from Serine to Glycine (S20G) has been shown to be associated with type 2 diabetes. In this study, the frequency and clinical characteristics of the S20G mutation in Korean was examined with 364 unrelated type 2 diabetic and 151 non-diabetic subjects. The S20G mutation was found in seven out of 364 diabetic patients (1.92%) and in three out of 151 non-diabetic subjects (1.99%). The body mass index (BMI) of the patients with the S20G mutation was lower than those with the wild type (21.2+/-1.8 vs. 24.3+/-3.0 kg/m(2); P<0.01). The prediabetic maximum BMI was also lower in patients with the S20G mutation (22.4+/-2.3 vs. 26.4+/-3.2 kg/m(2); P<0.01). Patients with the S20G mutation had a higher HbA(1c) level than those with the wild type (9.3+/-1.4 vs. 7.7+/-1.3%; P<0.01). In summary, the frequency of the S20G mutation in the amylin gene is 1.92% in unrelated Korean type 2 diabetic patients and this mutation is associated with a lower BMI and a higher HbA(1c) level.

Lessons learned from young-onset diabetes in China

The prevalence of young-onset diabetes is rapidly rising in China. Young-onset diabetes is etiologically and phenotypically heterogeneous. Thirty percent to 50% of these patients have insulin secretory failure owing to autoimmune or monogenic or other yet to be identified forms of diabetes. Others have a strong family history of diabetes and exhibit features of the metabolic syndrome. Management of these young patients poses major diagnostic and therapeutic challenges, which require a multidisciplinary and holistic approach to ensure that these subjects are identified early and managed appropriately. Understanding the molecular basis of diabetes in these subjects may also eventually lead to improvement in diagnosis, classification, and treatment.

Pathogenesis of feline diabetes mellitus

The common form of spontaneous diabetes mellitus that occurs in domestic cats bears close resemblance clinically and pathologically to human type 2 diabetes mellitus (T2DM). For example, the typical diabetic cat is obese and middle-aged, and has low but detectable circulating insulin levels. However, the most striking similarity is the occurrence of islet amyloidosis (IA) in nearly all diabetic cats and in over 90% of humans with T2DM. IA in both humans and cats is derived from islet amyloid polypeptide (IAPP, or amylin) which is a hormone produced and secreted along with insulin by the pancreatic beta cells. Since all cats and humans normally produce IAPP, additional factors must be invoked in order to explain the development of IA. Several lines of evidence support the concept that IA is caused by chronically increased stimulus for beta cells to secrete IAPP (and insulin). For example, peripheral insulin resistance such as in chronic obesity results in increased IAPP and insulin secretion. A recent study, in which diabetes mellitus was induced in cats, demonstrated that IAPP hypersecretion was induced by treatment with a sulfonylurea drug and resulted in 4/4 cats in this group developing IA. In contrast, cats treated with insulin had low IAPP secretion and minimal IA developed in 1/4 cats. Several human-IAPP transgenic mouse models, in which there is IAPP overexpression, also support the notion that prolonged high expression of IAPP leads to IA. In vitro models of IAPP overexpression also support this mechanism for IA formation and by demonstrating an association between IA formation and beta cell toxicity, suggest a linkage between IA formation and loss of beta cells in T2DM. A recent study has indicated that intermediate-sized IAPP-derived amyloid fibrils can disrupt cell membranes and therefore, may be involved in the destruction of beta cells. Striking parallels between the pathogenesis of IA and beta-amyloid plaque formation in Alzheimer's disease suggest possible parallel pathogenetic mechanisms of cell death and provide potential avenues for future studies into the pathogenesis of IA.

Amylin-induced cytotoxicity is associated with activation of caspase-3 and MAP kinases

Nanomolar concentrations of human amylin promote death of RINm5F cells in a time- and concentrationdependent manner. Morphological changes of chromatin integrity suggest that cells are predominantly undergoing apoptosis. Human amylin induces significant activation of caspase-3 and strong and sustained phosphorylation of stress-activated protein kinases, c-Jun N-terminal kinase (JNK) and p38, that precedes cell death. Extracellular signal-regulated kinase (ERK) activation was not concomitant with JNK and/or p38 activation. Activation of caspase-3 and mitogen-activated protein kinases (MAPKs) was detected by Western blot analysis. Addition of the MEK1 inhibitor PD 98059 had no effect on amylin-induced apoptosis, suggesting that ERK activation does not play a role in this apoptotic scenario. A correlative inhibition of JNK activation by the immunosuppressive drug FK506, as well as a selective inhibition of p38 MAPK activation by SB 203580, significantly suppressed procaspase-3 processing and the extent of amylin-induced cell death. Moreover, simultaneous pretreatment with both FK506 and SB 203580, or with the caspase-3 inhibitor Ac-DEVD-CHO alone, almost completely abolished procaspase-3 processing and cell death. Thus, our results suggest that amylin-induced apoptosis proceeds through sustained activation of JNK and p38 MAPK followed by caspase-3 activation.

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.

Diabetes mellitus--a special medical challenge from a Chinese perspective

As we enter the new millennium, Asia is being hit by an epidemic of diabetes and its related diseases. The rising prevalence of young onset diabetes which is closely associated with obesity and genetic factors as well as the increased propensity to develop kidney disease are special challenges in the management of Chinese diabetic patients. Although diabetic patients have earlier mortality and increased risks for micro and macrovascular complications, there is strong evidence that these devastating complications can be largely prevented by patient education, periodic assessments and use of appropriate therapeutic agents to optimize metabolic control and improve cardiovascular risk factors. However, a multidisciplinary approach is often required to deliver these complex disease management protocols. Hence, it is not surprising that large scale studies often revealed substandard diabetes management in both the hospital and community settings. This is often due to a combination of factors such as non-adherence to recommended guidelines both by patients and doctors as well as the 'non-urgent' and 'silent nature' of diabetes and its complications. To minimize the impacts of diabetes on quality of life, society productivity and utilization of health care resources, concerted efforts between health care professionals and public bodies are urgently needed to increase awareness, improve standards of care and develop better diagnostics and treatment modalities.