The effects of organic solvents on the membrane-induced fibrillation of human islet amyloid polypeptide and on the inhibition of the fibrillation

The organic solvent dimethylsulphoxide (DMSO) and 1,1,1,3,3,3-hexafluoro-2-isopropanol (HFIP) have been widely used as a pre-treating agent of amyloid peptides and as a vehicle for water-insoluble inhibitors. These solvents are left in many cases as a trace quantity in bulk and membrane environments with treated amyloid peptides or inhibitors. In the present work, we studied the effects of the two organic solvents on the aggregation behaviors of human islet amyloid polypeptide (hIAPP) and the performances of an all-D-amino-acid inhibitor D-NFGAIL in preventing hIAPP fibrillation both in bulk solution and at phospholipid membrane. We showed that the presence of 1% v/v DMSO or HFIP decreases the rate of fibril formation of hIAPP at the lipid membrane rather than accelerates the fibril formation as what happened in bulk solution. We also showed that the presence of 1% v/v DMSO or HFIP impairs the activity of the inhibitor at the lipid membrane surface dramatically, while it affects the efficiency of the inhibitor in bulk solution slightly. We found that the inhibitor inserts into the lipid membrane more deeply or with more proportion in the presence of the organic solvents than it does in the absence of the organic solvents, which may hinder the binding of the inhibitor to hIAPP at the lipid membrane. Our results suggest that the organic solvents should be used with caution in studying membrane-induced fibrillogenesis of amyloid peptides and in testing amyloid inhibitors under membrane environments to avoid incorrect evaluation to the fibrillation process of amyloid peptides and the activity of inhibitors.     

Atomistic-level study of the interactions between hIAPP protofibrils and membranes: Influence of pH and lipid composition

The pathology of type 2 diabetes mellitus is associated with the aggregation of human islet amyloid polypeptide (hIAPP) and aggregation-mediated membrane disruption. The interactions of hIAPP aggregates with lipid membrane, as well as the effects of pH and lipid composition at the atomic level, remain elusive. Herein, using molecular dynamics simulations, we investigate the interactions of hIAPP protofibrillar oligomers with lipids, and the membrane perturbation that they induce, when they are partially inserted in an anionic dipalmitoyl-phosphatidylglycerol (DPPG) membrane or a mixed dipalmitoyl-phosphatidylcholine (DPPC)/DPPG (7:3) lipid bilayer under acidic/neutral pH conditions. We observed that the tilt angles and insertion depths of the hIAPP protofibril are strongly correlated with the pH and lipid composition. At neutral pH, the tilt angle and insertion depth of hIAPP protofibrils at a DPPG bilayer reach ~52° and ~1.62 nm with respect to the membrane surface, while they become ~77° and ~1.75 nm at a mixed DPPC/DPPG membrane. The calculated tilt angle of hIAPP at DPPG membrane is consistent with a recent chiral sum frequency generation spectroscopic study. The acidic pH induces a smaller tilt angle of ~40° and a shallower insertion depth (~1.24 nm) of hIAPP at the DPPG membrane surface, mainly due to protonation of His18 near the turn region. These differences mainly result from a combination of distinct electrostatic, van der Waals, hydrogen bonding and salt-bridge interactions between hIAPP and lipid bilayers. The hIAPP-membrane interaction energy analysis reveals that besides charged residues K1, R11 and H18, aromatic residues Phe15 and Phe23 also exhibit strong interactions with lipid bilayers, revealing the crucial role of aromatic residues in stabilizing the membrane-bound hIAPP protofibrils. hIAPP-membrane interactions disturb the lipid ordering and the local bilayer thickness around the peptides. Our results provide atomic-level information of membrane interaction of hIAPP protofibrils, revealing pH-dependent and membrane-modulated hIAPP aggregation at the early stage.     

Molecular basis of the anchoring and stabilization of human islet amyloid polypeptide in lipid hydroperoxidized bilayers

The molecular structure of membrane lipids is formed by mono- or polyunsaturations on their aliphatic tails that make them susceptible to oxidation, facilitating the incorporation of hydroperoxide (R-OOH) functional groups. Such groups promote changes in both composition and complexity of the membrane significantly modifying its physicochemical properties. Human Langerhans islets amyloid polypeptide (hIAPP) is the main component of amyloid deposits found in the pancreas of patients with type-2 diabetes (T2D). hIAPP in the presence of membranes with oxidized lipid species accelerates the formation of amyloid fibrils or the formation of intermediate oligomeric structures. However, the molecular bases at the initial stage of the anchoring and stabilization of the hIAPP in a hydroperoxidized membrane are not yet well understood. To shed some light on this matter, in this contribution, three bilayer models were modeled: neutral (POPC), anionic (POPS), and oxidized (POPC_OOH), and full atom Molecular Dynamics (MD) simulations were performed. Our results show that the POPC_OOH bilayer increases the helicity in hIAPP when compared to POPC or POPS bilayer. The modification in the secondary structure covers the residues of the so-called amyloidogenic core of the hIAPP. Overall, the hydroperoxidation of the neutral lipids modifies both the anchoring and the stabilization of the peptide hIAPP by reducing the random conformations of the peptide and increasing of hydrogen bond population with the hydroperoxidized lipids.     

Lipid interaction and membrane perturbation of human islet amyloid polypeptide monomer and dimer by molecular dynamics simulations

The aggregation of human islet amyloid polypeptide (hIAPP or amylin) is associated with the pathogenesis of type 2 diabetes mellitus. Increasing evidence suggests that the interaction of hIAPP with β-cell membranes plays a crucial role in cytotoxicity. However, the hIAPP-lipid interaction and subsequent membrane perturbation is not well understood at atomic level. In this study, as a first step to gain insight into the mechanism of hIAPP-induced cytotoxicity, we have investigated the detailed interactions of hIAPP monomer and dimer with anionic palmitoyloleolyophosphatidylglycerol (POPG) bilayer using all-atom molecular dynamics (MD) simulations. Multiple MD simulations have been performed by employing the initial configurations where the N-terminal region of hIAPP is pre-inserted in POPG bilayer. Our simulations show that electrostatic interaction between hIAPP and POPG bilayer plays a major role in peptide-lipid interaction. In particular, the N-terminal positively-charged residues Lys1 and Arg11 make a dominant contribution to the interaction. During peptide-lipid interaction process, peptide dimerization occurs mostly through the C-terminal 20-37 region containing the amyloidogenic 20-29-residue segment. Membrane-bound hIAPP dimers display a pronounced ability of membrane perturbation than monomers. The higher bilayer perturbation propensity of hIAPP dimer likely results from the cooperativity of the peptide-peptide interaction (or peptide aggregation). This study provides insight into the hIAPP-membrane interaction and the molecular mechanism of membrane disruption by hIAPP oligomers.     

Macromolecular Crowding as a Suppressor of Human IAPP Fibril Formation and Cytotoxicity

The biological cell is known to exhibit a highly crowded milieu, which significantly influences protein aggregation and association processes. As several cell degenerative diseases are related to the self-association and fibrillation of amyloidogenic peptides, understanding of the impact of macromolecular crowding on these processes is of high biomedical importance. It is further of particular relevance as most in vitro studies on amyloid aggregation have been performed in diluted solution which does not reflect the complexity of their cellular surrounding. The study presented here focuses on the self-association of the type-2 diabetes mellitus related human islet amyloid polypeptide (hIAPP) in various crowded environments including network-forming macromolecular crowding reagents and protein crowders. It was possible to identify two competing processes: a crowder concentration and type dependent stabilization of globular off-pathway species and a – consequently - retarded or even inhibited hIAPP fibrillation reaction. The cause of these crowding effects was revealed to be mainly excluded volume in the polymeric crowders, whereas non-specific interactions seem to be most dominant in protein crowded environments. Specific hIAPP cytotoxicity assays on pancreatic β-cells reveal non-toxicity for the stabilized globular species, in contrast to the high cytotoxicity imposed by the normal fibrillation pathway. From these findings it can be concluded that cellular crowding is able to effectively stabilize the monomeric conformation of hIAPP, hence enabling the conduction of its normal physiological function and prevent this highly amyloidogenic peptide from cytotoxic aggregation and fibrillation.     

How type II diabetes-related islet amyloid polypeptide damages lipid bilayers

A leading hypothesis for the decimation of insulin-producing β-cells in type 2 diabetes attributes the cause to islet amyloid polypeptide (IAPP) for its deleterious effects on the cell membranes. This idea has produced extensive investigations on human IAPP (hIAPP) and its interactions with lipid bilayers. However, it is still difficult to correlate the peptide-lipid interactions with its effects on islet cells in culture. The hIAPP fibrils have been shown to interact with lipids and damage lipid bilayers, but appear to have no effect on islet cells in culture. Thus, a modified amyloid hypothesis assumes that the toxicity is caused by hIAPP oligomers, which are not preamyloid fibrils or protofibrils. However, so far such oligomers have not been isolated or identified. The hIAPP monomers also bind to lipid bilayers, but the mode of interaction is not clear. Here, we performed two types of experiments that, to our knowledge, have not been done before. We used x-ray diffraction, in conjunction with circular dichroism measurement, to reveal the location of the peptide bound to a lipid bilayer. We also investigated the effects of hIAPP on giant unilamellar vesicles at various peptide concentrations. We obtained the following qualitative results. Monomeric hIAPP binds within the headgroup region and expands the membrane area of a lipid bilayer. At low concentrations, such binding causes no leakage or damage to the lipid bilayer. At high concentrations, the bound peptides transform to β-aggregates. The aggregates exit the headgroup region and bind to the surface of lipid bilayers. The damage by the surface bound β-aggregates depends on the aggregation size. The initial aggregation extracts lipid molecules, which probably causes ion permeation, but no molecular leakage. However, the initial β-aggregates serve as the seed for larger fibrils, in the manner of the Jarrett-Lansbury seeded-polymerization model, that eventually disintegrate lipid bilayers by electrostatic and hydrophobic interactions.     

Interfacial effect on the ability of peptide-modified gold nanoclusters to inhibit hIAPP fibrillation and cytotoxicity

Deposit of amyloid peptides in the cells is related to various amyloidosis diseases. A variety of nanomaterials have been developed to resist amyloid deposit. Most of the research on the inhibition of nanomaterials against amyloid aggregation are undertaken in solution, while the membranes that may mediate fibrillar aggregation and affect interaction of inhibitors with amyloid peptides in biotic environment are little taken into account. In this study, we synthesized three kinds of gold nanoclusters modified with cysteine (C@AuNCs), glutathione (GSH@AuNCs) and a peptide derived from the core region of hIAPP fibrillation (C-HL-8P@AuNCs), and investigated their inhibitory activities against hIAPP fibrillation in the absence and presence of lipid vesicles (POPC/POPG 4:1 LUVs) by the experiments of ThT fluorescence kinetics, AFM and CD. We also explored the inhibitions of hIAPP-induced membrane damage and cytotoxicity by peptide@AuNCs using fluorescent dye leakage and cell viability assays. Our study revealed that the inhibitory efficiency of these peptide@AuNCs against hIAPP fibrillation follows C-HL-8P@AuNCs≅GSH@AuNCs>C@AuNCs in lipid-free solution and C-HL-8P@AuNCs≫GSH@AuNCs>C@AuNCs in lipid membrane environment. Compared with the results obtained in lipid-free solution, the inhibitions of hIAPP fibrillation observed in lipid membrane environment were more associated with the inhibitions of hIAPP-induced damages of lipid vesicles and INS-1 cells (C-HL-8P@AuNCs≫GSH@AuNCs>C@AuNCs). An additional hydrophobic interaction with the homologous core region of hIAPP, which is only provided by C-HL-8P@AuNCs and largely suppressed in lipid-free solution, enhanced in the membrane environment and therefore made C-HL-8P@AuNCs much more powerful than GSH@AuNCs and C@AuNCs in the inhibitions of hIAPP fibrillation and cytotoxicity.     

Effect of membrane characteristics on phase separation and domain formation in cholesterol-lipid mixtures

We examine, using an analytical mean-field model, the distribution of cholesterol in a lipid bilayer. The model accounts for the perturbation of lipid packing induced by the embedded cholesterol, in a manner similar to that of transmembrane proteins. We find that the membrane-induced interactions between embedded cholesterol molecules vary as a function of the cholesterol content. Thus, the effective lipid-cholesterol interaction is concentration-dependent. Moreover, it transitions from repulsive to attractive to repulsive as the cholesterol content increases. As the concentration of cholesterol in the bilayer exceeds a critical value, phase separation occurs. The coexistence between cholesterol-rich and cholesterol-poor domains is universal for any bilayer parameters, although the composition of the cholesterol-rich phase varies as a function of the lipid properties. Although we do not assume specific cholesterol-lipid interactions or the formation of a lipid-cholesterol cluster, we find that the composition of the cholesterol-rich domains is constant, independent of the cholesterol content in the bilayer.     

Probing ion channel activity of human islet amyloid polypeptide (amylin)

Interactions of human islet amyloid polypeptide (hIAPP or amylin) with the cell membrane are correlated with the dysfunction and death of pancreatic islet β-cells in type II diabetes. Formation of receptor-independent channels by hIAPP in the membrane is regarded as one of the membrane-damaging mechanisms that induce ion homeostasis and toxicity in islet β-cells. Here, we investigate the dynamic structure, ion conductivity, and membrane interactions of hIAPP channels in the DOPC bilayer using molecular modeling and molecular dynamics simulations. We use the NMR-derived β-strand-turn-β-strand motif as a building block to computationally construct a series of annular-like hIAPP structures with different sizes and topologies. In the simulated lipid environments, the channels lose their initial continuous β-sheet network and break into oligomeric subunits, which are still loosely associated to form heterogeneous channel conformations. The channels' shapes, morphologies and dimensions are compatible with the doughnut-like images obtained by atomic force microscopy, and with those of modeled channels for Aβ, the β(2)-microglobulin-derived K3 peptides, and the β-hairpin-based channels of antimicrobial peptide PG-1. Further, all channels induce directional permeability of multiple ions across the bilayers from the lower to the upper leaflet. This similarity suggests that loosely-associated β-structure motifs can be a general feature of toxic, unregulated channels. In the absence of experimental high-resolution atomic structures of hIAPP channels in the membrane, this study represents a first attempt to delineate some of the main structural features of the hIAPP channels, for a better understanding of the origin of amyloid toxicity and the development of pharmaceutical agents.     

Blockade of islet amyloid polypeptide fibrillation and cytotoxicity by the secretory chaperones 7B2 and proSAAS

The deposition of fibrillated human islet β-cell peptide islet amyloid polypeptide (hIAPP) into amyloid plaques is characteristic of the pathogenesis of islet cell death during type 2 diabetes. We investigated the effects of the neuroendocrine secretory proteins 7B2 and proSAAS on hIAPP fibrillation in vitro and on cytotoxicity. In vitro, 21-kDa 7B2 and proSAAS blocked hIAPP fibrillation. Structure–function studies showed that a central region within 21-kDa 7B2 is important in this effect and revealed the importance of the N-terminal region of proSAAS. Both chaperones blocked the cytotoxic effects of exogenous hIAPP on Rin5f cells; 7B2 generated by overexpression was also effective. ProSAAS and 7B2 may perform a chaperone role as secretory anti-aggregants in normal islet cell function and in type 2 diabetes.     

Lipid tail chain asymmetry and the strength of membrane-induced interactions between membrane proteins

Many lipids are composed of asymmetric tail chains that differ by their molecular weight (MW) and/or degree of saturation. Previous studies found that membrane moduli vary with the degree of lipid tail asymmetry. However, to date little is known regarding the effect (if any) of tail asymmetry on the membrane-induced interactions between embedded proteins. In this paper we use a self-consistent field model to examine the effect of lipid tail asymmetry on membrane proteins. We first examine the case where the overall tail length (sum of both chains) is held constant, which implies that the membrane thickness remains constant as well, independent of tail asymmetry. We find that, in these systems, the membrane area stretch and bending moduli decrease with increasing chain asymmetry, thereby reducing the magnitude of the membrane-induced barrier to protein aggregation. Since in symmetric lipid bilayers the energy barrier is typically of order ∼ 1-2 times the thermal energy kT, the asymmetry-induced reduction in barrier height may increase the probability of protein aggregation significantly. In systems where one tail chain is held constant, increasing asymmetry involves changes in the bilayer thickness which are found to dominate any effect arising from the asymmetry.     

Impact of Ca2+ on membrane catalyzed IAPP amyloid formation and IAPP induced vesicle leakage

Human islet amyloid polypeptide (hIAPP, also known as amylin) is a 37 amino acid pancreatic polypeptide hormone that plays a role in regulating glucose levels, but forms pancreatic amyloid in type-2 diabetes. The process of amyloid formation by hIAPP contributes to β-cell death in the disease. Multiple mechanisms of hIAPP induced toxicity of β-cells have been proposed including disruption of cellular membranes. However, the nature of hIAPP membrane interactions and the effect of ions and other molecules on hIAPP membrane interactions are not fully understood. Many studies have used model membranes with a high content of anionic lipids, often POPS, however the concentration of anionic lipids in the β-cell plasma membrane is low. Here we study the concentration dependent effect of Ca²⁺ (0 to 50 mM) on hIAPP membrane interactions using large unilamellar vesicles (LUVs) with anionic lipid content ranging from 0 to 50 mol%. We find that Ca²⁺ does not effectively inhibit hIAPP amyloid formation and hIAPP induced membrane leakage from binary LUVs with a low percentage of POPS, but has a greater effect on LUVs with a high percentage of POPS. Mg²⁺ had very similar effects, and the effects of Ca²⁺ and Mg²⁺ can be largely rationalized by the neutralization of POPS charge. The implications for hIAPP-membrane interactions are discussed.     

A hIAPP-derived all-d-amino-acid inhibits hIAPP fibrillation efficiently at membrane surface by targeting α-helical oligomeric intermediates

A variety of peptides and peptide derivatives have been constructed using the “β-sheet core segment” of amyloid proteins as inhibitors of amyloidogenic fibrillation. A novel all-d-amino-acid from hIAPP β-sheet core segment (hIAPP 22–27) is demonstrated to inhibit hIAPP fibril formation efficiently both at the phospholipid membrane and in bulk solution. The inhibitor terminates hIAPP aggregation to the α-helical oligomeric intermediates at the membrane surface, whereas it stops the aggregation at the stage of β-sheet oligomeric intermediates in bulk solution. This is the first evidence that the inhibition mechanism of the inhibitor at membrane surface is significantly different from that in bulk solution.     

Conformational preferences of the amylin nucleation site in SDS micelles: an NMR study

Human islet amyloid polypeptide (hIAPP), or amylin, is a 37 amino acid hormone secreted by pancreatic beta-cells. hIAPP constitutes approximately 90% of the amyloid deposits found in type II diabetic patients. It has been shown that the central region of the peptide (hIAPP(20-29)) constitutes the nucleation site for the amyloidogenic process with F23 playing a key role in the formation of the beta-pleated structures. In addition, it has been proposed that an important stage in the cytotoxicity of hIAPP is its interaction with the beta-cell membranes. As a first step toward the characterization of the interaction of hIAPP with cell membranes, we determined conformational preferences of hIAPP(20-29) in membrane-mimicking environments. We found that upon interacting with negatively charged micelles, the dominant conformation of hIAPP(20-29) is a distorted type I beta-turn centered on residues F23 and G24, with F23, A25, and I26 forming a small hydrophobic cluster that may facilitate the interaction of this peptide with the membrane bilayer. Moreover, we were able to elucidate the topological orientation of the peptide that is absorbed on the micelle surface, with the hydrophobic cluster oriented toward the hydrocarbon region of the micelles and both N- and C-termini exposed to the solvent.     

Amyloidogenic Propensities and Conformational Properties of ProIAPP and IAPP in the Presence of Lipid Bilayer Membranes

Human islet amyloid polypeptide (hIAPP), which is considered the primary culprit for β-cell loss in type 2 diabetes mellitus patients, is synthesized in β-cells of the pancreas from its precursor pro-islet amyloid polypeptide (proIAPP), which may be important in early intracellular amyloid formation as well. We compare the amyloidogenic propensities and conformational properties of proIAPP and hIAPP in the presence of negatively charged lipid membranes, which have been discussed as loci of initiation of the fibrillation reaction. Circular dichroism studies verify the initial secondary structures of proIAPP and hIAPP to be predominantly unordered with small amounts of ordered secondary structure elements, and exhibit minor differences between these two peptides only. Using attenuated total reflection-Fourier transform infrared spectroscopy and thioflavin T fluorescence spectroscopy, as well as atomic force microscopy, we show that in the presence of negatively charged membranes, proIAPP exhibits a much higher amyloidogenic propensity than in bulk solvent. Compared to hIAPP, it is still much less amyloidogenic, however. Although differences in the secondary structures of the aggregated species of hIAPP and proIAPP at the lipid interface are small, they are reflected in morphological changes. Unlike hIAPP, proIAPP forms essentially oligomeric-like structures at the lipid interface. Besides the interaction with anionic membranes [1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) + x1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)]], interaction with zwitterionic homogeneous (DOPC) and heterogeneous (1,2-dipalmitoyl-sn-glycero-3-phosphocholine:DOPC:cholesterol 1:2:1 model raft mixture) membranes has also been studied. Both peptides do not aggregate significantly at DOPC bilayers. In the presence of the model raft membrane, hIAPP aggregates markedly as well. Conversely, proIAPP clusters into less ordered structures and to a minor extent at raft membranes only. The addition of proIAPP to hIAPP retards the hIAPP fibrillation process also in the presence of negatively charged lipid bilayers. In excess proIAPP, increased aggregation levels are finally observed, however, which could be attributed to seed-induced cofibrillation of proIAPP.     

Liposome Damage and Modeling of Fragments of Human Islet Amyloid Polypeptide (IAPP) Support a Two-Step Model of Membrane Destruction

A key factor in the development of Type II diabetes is the loss of insulin-producing beta-cells. Human islet amyloid polypeptide (hIAPP) is believed to play a crucial role in this process by forming small aggregates that disrupt the cellular membrane. During Type II diabetes mellitus, human IAPP (hIAPP) fibrillizes to form amyloid deposits. However, the role of various regions of the 37 amino acid peptide in the process of membrane disruption has yet to be fully elucidated. Therefore, several fragments (10–19, 20–29, 10–29, 1–19) of hIAPP were synthesized and compared to full length hIAPP for their effects on model PC/PS bilayers. These fragments were also modeled using density functional methods and analyzed by circular dichroism, to determine possible correlations between activity and available conformations and charge distribution. Results from dye leakage and Thioflavin T fluorescence assays confirmed that the hIAPP fragments disrupt the lipid bilayer to varying extents, despite their inability to form amyloid fibrils. The longer and more positively charged fragments were most active in the assay (1–19 > 10–29 > 10–19 > 20–29), though none rivaled the activity of the native full length peptide. This may reflect their relative abilities to interact with the negatively charged membrane. Data support a two-step model for membrane disruption: insertion by the N-terminus followed by fibrillization mediated by the middle to C-terminal region.     

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.

Structured summary

pIAPP and pIAPPbind: shown by molecular sieving (view interactions 1, 2)hIAPP and hIAPPbind: shown by molecular sieving (view interactions 1, 2)     

Lipid tail chain asymmetry and the strength of membrane-induced interactions between membrane proteins

Many lipids are composed of asymmetric tail chains that differ by their molecular weight (MW) and/or degree of saturation. Previous studies found that membrane moduli vary with the degree of lipid tail asymmetry. However, to date little is known regarding the effect (if any) of tail asymmetry on the membrane-induced interactions between embedded proteins. In this paper we use a self-consistent field model to examine the effect of lipid tail asymmetry on membrane proteins. We first examine the case where the overall tail length (sum of both chains) is held constant, which implies that the membrane thickness remains constant as well, independent of tail asymmetry. We find that, in these systems, the membrane area stretch and bending moduli decrease with increasing chain asymmetry, thereby reducing the magnitude of the membrane-induced barrier to protein aggregation. Since in symmetric lipid bilayers the energy barrier is typically of order approximately 1-2 times the thermal energy kT, the asymmetry-induced reduction in barrier height may increase the probability of protein aggregation significantly. In systems where one tail chain is held constant, increasing asymmetry involves changes in the bilayer thickness which are found to dominate any effect arising from the asymmetry.     

The N-terminal fragment of human islet amyloid polypeptide is non-fibrillogenic in the presence of membranes and does not cause leakage of bilayers of physiologically relevant lipid composition

Human islet amyloid polypeptide (hIAPP) forms amyloid fibrils in pancreatic islets of patients with type 2 diabetes mellitus (DM2). The formation of hIAPP fibrils has been shown to cause membrane damage which most likely is responsible for the death of pancreatic islet β-cells during the pathogenesis of DM2. Previous studies have shown that the N-terminal part of hIAPP, hIAPP_1-19, plays a major role in the initial interaction of hIAPP with lipid membranes. However, the exact role of this N-terminal part of hIAPP in causing membrane damage is unknown. Here we investigate the structure and aggregation properties of hIAPP_1-19 in relation to membrane damage in vitro by using membranes of the zwitterionic lipid phosphatidylcholine (PC), the anionic lipid phosphatidylserine (PS) and mixtures of these lipids to mimic membranes of islet cells. Our data reveal that hIAPP_1-19 is weakly fibrillogenic in solution and not fibrillogenic in the presence of membranes, where it adopts a secondary structure that is dependent on lipid composition and stable in time. Furthermore, hIAPP_1-19 is not able to induce leakage in membranes of PC/PS or PC bilayers, indicating that the membrane interaction of the N-terminal fragment by itself is not responsible for membrane leakage under physiologically relevant conditions. In bilayers of the anionic lipid PS, the peptide does induce membrane damage, but this leakage is not correlated to fibril formation, as it is for mature hIAPP. Hence, membrane permeabilization by the N-terminal fragment of hIAPP in anionic lipids is most likely an aspecific process, occurring via a mechanism that is not relevant for hIAPP-induced membrane damage in vivo.     

Recent computational studies of membrane interaction and disruption of human islet amyloid polypeptide: Monomers,oligomers and protofibrils

The amyloid deposits of human islet amyloid polypeptide (hIAPP) are found in type 2 diabetes patients. hIAPP monomer is intrinsically disordered in solution, whereas it can form amyloid fibrils both in vivo and in vitro. Extensive evidence suggests that hIAPP causes the disruption of cellular membrane, and further induces cytotoxicity and the death of islet β-cells in pancreas. The presence of membrane also accelerates the hIAPP fibril formation. hIAPP oligomers and protofibrils in the early stage of aggregation were reported to be the most cytotoxic, disrupting the membrane integrity and giving rise to the pathological process. The detailed molecular mechanisms of hIAPP-membrane interactions and membrane disruption are complex and remain mostly unknown. Here in this review, we focus on recent computational studies that investigated the interactions of full length and fragmentary hIAPP monomers, oligomers and protofibrils with anionic, zwitterionic and mixed anionic-zwitterionic lipid bilayers. We mainly discuss the binding orientation of monomers at membrane surface, the conformational ensemble and the oligomerization of hIAPP inside membranes, the effect of lipid composition on hIAPP oligomers/protofibrils-membrane interactions, and the hIAPP-induced membrane perturbation. This review provides mechanistic insights into the interactions between hIAPP and lipid bilayers with different lipid composition at an atomistic level, which is helpful to understand the hIAPP cytotoxicity mediated by membrane. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.