Accelerates Amyloid Formation Research Articles

Exploring the Potential Long-term Impact of SARS-CoV-2 on Protein Misfolding and Amyloid-related Conditions

The long-term impact of the COVID-19 pandemic concerns risk to human health, particularly its potential association with protein misfolding and amyloidosis. This review article explores the causality relationship between SARS-CoV-2 infection, and protein misfolding, leading to amyloid-related conditions. It delves into the mechanisms by which viral proteins may accelerate amyloid formation, exacerbating post-infection complications, including neurological sequelae. Drawing from interdisciplinary research and clinical observations, the potential links between COVID-19, vaccination, and amyloidosis, emphasize the importance of understanding the longterm effect of post-COVID symptoms. This review examines the potential role of COVID-19-related proteins in the formation of amyloid in other related proteins of amyloidosis.

Protein G-quadruplex interactions and their effects on phase transitions and protein aggregation.

The SERF family of proteins were originally discovered for their ability to accelerate amyloid formation. Znf706 is an uncharacterized protein whose N-terminus is homologous to SERF proteins. We show here that human Znf706 can promote protein aggregation and amyloid formation. Unexpectedly, Znf706 specifically interacts with stable, non-canonical nucleic acid structures known as G-quadruplexes. G-quadruplexes can affect gene regulation and suppress protein aggregation; however, it is unknown if and how these two activities are linked. We find Znf706 binds preferentially to parallel G-quadruplexes with low micromolar affinity, primarily using its N-terminus, and upon interaction, its dynamics are constrained. G-quadruplex binding suppresses Znf706's ability to promote protein aggregation. Znf706 in conjunction with G-quadruplexes therefore may play a role in regulating protein folding. RNAseq analysis shows that Znf706 depletion specifically impacts the mRNA abundance of genes that are predicted to contain high G-quadruplex density. Our studies give insight into how proteins and G-quadruplexes interact, and how these interactions affect both partners and lead to the modulation of protein aggregation and cellular mRNA levels. These observations suggest that the SERF family of proteins, in conjunction with G-quadruplexes, may have a broader role in regulating protein folding and gene expression than previously appreciated.

Control of amyloidogenic protein aggregation by ultrasonic irradiation

Protein aggregation causes intractable diseases such as Alzheimer’s disease. In these diseases, crystal-like protein aggregates, so-called amyloid fibrils, form in vivo and induce severe malfunction of biological tissues. Amyloid fibril formation comprises primary nucleation from a supersaturated solution of protein monomers and subsequent fibril growth. Within them, primary nucleation possesses an extremely high energy barrier, resulting in a long induction time for nucleation, typically over a decade in vivo and several days even in a test tube. Our previous study (1) revealed that ultrasonic irradiation to supersaturated protein solution drastically accelerates amyloid formation because of the effects of ultrasonic cavitation, which is applicable to clinical diagnosis (2). However, the degree of acceleration has not been controlled yet. Furthermore, the reproducibility of the induction time for nucleation highly depends on ultrasonic-irradiation conditions and the degree of supersaturation of initial protein solutions. Herein, we systematically investigate the optimized condition to accurately control amyloid formation using an originally developed sonoreactor (3) and discuss the underlying mechanism governing ultrasonic induction of amyloid formation. (1) K. Nakajima et al., Ultrason. Sonochem. 36, 206-211 (2017). (2) K. Nakajima et al., Nat. Commun. 13, 5689 (2022). (3) K. Nakajima et al., Ultrason. Sonochem. 73 105508 (2022).

Relationship between Changes in the Protein Folding Pathway and the Process of Amyloid Formation: The Case of Bovine Carbonic Anhydrase II

Many proteins form amyloid fibrils only under conditions when the probability of transition from a native (structured, densely packed) to an intermediate (labile, destabilized) state is increased. It implies the assumption that some structural intermediates are more convenient for amyloid formation than the others. Hence, if a mutation affects the protein folding pathway, one should expect that this mutation could affect the rate of amyloid formation as well. In the current work, we have compared the effects of amino acid substitutions of bovine carbonic anhydrase II on its unfolding pathway and on its ability to form amyloids at acidic pH and an elevated temperature. Wild-type protein and four mutant forms (L78A, L139A, I208A, and M239A) were studied. We analyzed the change of the protein unfolding pathway by the time-resolved fluorescence technique and the process of amyloid formation by thioflavin T fluorescence assay and electron microscopy. It was revealed that I208A substitution accelerates amyloid formation and affects the structure of the late (molten globule-like)-intermediate state of carbonic anhydrase, whereas the other mutations slow down the growth of amyloids and have either no effect on the unfolding pathway (L78A, L139A) or alter the conformational states arising at the early unfolding stage (M239A).

Differential Effects of Aromatic Residues on Amyloid Formation and Cytotoxicity of Human IAPP.

Islet amyloid polypeptide (IAPP) is a 37-residue polypeptide hormone secreted by the pancreatic β-cells. IAPP plays a role in glycemic regulation, but in the pre-type-2 diabetic state, it aggregates to form an islet amyloid. The process of islet amyloid formation contributes to β-cell dysfunction and disease progression. The features of the IAPP sequence that modulate amyloid formation are still not understood. Human IAPP contains three aromatic residues, F15, F23, and Y37. F15 and Y37 are highly conserved, while F23 is more commonly a Leu or Ile in other species. The role of the aromatic residues in modulating the time course of amyloid formation and the cytotoxicity was examined using aromatic to Leu mutations. All three single and double mutants and the triple mutant were studied. F23 plays a dominant role in both amyloid formation and toxicity. An F15L mutant accelerated amyloid formation, a Y37L mutant had little effect, while an F23L replacement slowed amyloid formation by a factor of 2.6. Double mutants, which contained an F23L replacement, had a larger effect than those that did not, and there are non-additive effects between pairs of aromatic residues. F23 also plays a key role in toxicity. Single or multiple mutants that contain the F23L replacement were noticeably less toxic than the wild-type or mutants which did not include the F23L substitution. In contrast, the F15L mutant was more toxic than the wild-type one. The implications for IAPP amyloid formation and for the design of non-aggregating analogues of IAPP are discussed.

SERF, a family of tiny highly conserved, highly charged proteins with enigmatic functions.

Amyloid formation is a misfolding process that has been linked to age-related diseases, including Alzheimer's and Huntington's. Understanding how cellular factors affect this process in vivo is vital in realizing the dream of controlling this insidious process that robs so many people of their humanity. SERF (small EDRK-rich factor) was initially isolated as a factor that accelerated polyglutamine amyloid formation in a C. elegans model. SERF knockouts inhibit amyloid formation of a number of proteins that include huntingtin, α-synuclein and β-amyloid which are associated with Huntington's, Parkinson's and Alzheimer's disease, respectively, and purified SERF protein speeds their amyloid formation in vitro. SERF proteins are highly conserved, highly charged and conformationally dynamic proteins that form a fuzzy complex with amyloid precursors. They appear to act by specifically accelerating the primary step of amyloid nucleation. Brain-specific SERF knockout mice, though viable, appear to be more prone to deposition of amyloids, and show modified fibril morphology. Whole-body knockouts are perinatally lethal due to an apparently unrelated developmental issue. Recently, it was found that SERF binds RNA and is localized to nucleic acid-rich membraneless compartments. SERF-related sequences are commonly found fused to zinc finger sequences. These results point towards a nucleic acid-binding function. How this function relates to their ability to accelerate amyloid formation is currently obscure. In this review, we discuss the possible biological functions of SERF family proteins in the context of their structural fuzziness, modulation of amyloid pathway, nucleic acid binding and their fusion to folded proteins.

Backbone resonance assignments and dynamics of S. cerevisiae SERF.

Abnormal protein aggregation and precipitation are associated with the perturbation of cellular function and underlie a variety of neurodegenerative diseases. S. cerevisiae SERF (ScSERF), a homolog of modifier of aggregation-4 (MOAG-4) and small EDRK-rich factor protein (SERF1a) is highly conserved and discovered as an enhancer of amyloid formation of Aβ40 and α-synuclein both in vitro and in vivo. However, the detailed molecular mechanism whereby ScSERF and its homologs accelerate amyloid formation is not well known yet. Herein, we present the 1H, 15N and 13C NMR assignments of the 68 amino acids long ScSERF. Although ScSERF displays a very high degree of disorder, secondary chemical shifts of Cα, Cβ, 15N{1H}-NOE values and the residue-specific secondary structure propensity (SSP) scores indicate the segment spanning residues 36E-65K has a strong helical propensity. This work sets the stage for further detailed structural and dynamic investigations of ScSERF and the molecular mechanism it utilizes in accelerating amyloid formation.

Fibulin-4 Accelerates Amyloid Formation by Binding with a Keratin 5 Peptide Fragment

Keratins are the major amyloid fibril component in localized cutaneous amyloidosis. We analyzed the amyloid components in the skin of patients with localized cutaneous amyloidosis by immunohistochemical staining using antisera against extracellular matrix proteins and keratin 5 (K5). Fibulin-4 and K5 colocalized in the amyloid deposits. Using 14 synthetic peptides, we screened for amyloidogenic sequences in the C-terminal region of K5, including the α-helical rod domain and the tail domain. Two peptides stained with thioflavin T possessed a β-sheet structure and formed amyloid-like fibrils. Among the amyloidogenic peptides, a peptide KT5-6 (YQELMNTKLALDVEIATYRKLLEGE) derived from the α-helical rod domain of K5 specifically bound to fibulin-4. In addition, amyloid formation of KT5-6 was accelerated by fibulin-4. These results suggest that degraded fragments of K5 containing the KT5-6 sequence form amyloid fibrils with fibulin-4. The data further suggest that degraded fragments of K5 and fibulin-4 have the potential to initiate cutaneous amyloidosis.

Interactions between SARS-CoV-2 N-Protein and α-Synuclein Accelerate Amyloid Formation.

First cases that point at a correlation between SARS-CoV-2 infections and the development of Parkinson’s disease (PD) have been reported. Currently, it is unclear if there is also a direct causal link between these diseases. To obtain first insights into a possible molecular relation between viral infections and the aggregation of α-synuclein protein into amyloid fibrils characteristic for PD, we investigated the effect of the presence of SARS-CoV-2 proteins on α-synuclein aggregation. We show, in test tube experiments, that SARS-CoV-2 spike protein (S-protein) has no effect on α-synuclein aggregation, while SARS-CoV-2 nucleocapsid protein (N-protein) considerably speeds up the aggregation process. We observe the formation of multiprotein complexes and eventually amyloid fibrils. Microinjection of N-protein in SH-SY5Y cells disturbed the α-synuclein proteostasis and increased cell death. Our results point toward direct interactions between the N-protein of SARS-CoV-2 and α-synuclein as molecular basis for the observed correlation between SARS-CoV-2 infections and Parkinsonism.

Amyloid particles facilitate surface-catalyzed cross-seeding by acting as promiscuous nanoparticles

Amyloid seeds are nanometer-sized protein particles that accelerate amyloid assembly as well as propagate and transmit the amyloid protein conformation associated with a wide range of protein misfolding diseases. However, seeded amyloid growth through templated elongation at fibril ends cannot explain the full range of molecular behaviors observed during cross-seeded formation of amyloid by heterologous seeds. Here, we demonstrate that amyloid seeds can accelerate amyloid formation via a surface catalysis mechanism without propagating the specific amyloid conformation associated with the seeds. This type of seeding mechanism is demonstrated through quantitative characterization of the cross-seeded assembly reactions involving two nonhomologous and unrelated proteins: the human Aβ42 peptide and the yeast prion-forming protein Sup35NM. Our results demonstrate experimental approaches to differentiate seeding by templated elongation from nontemplated amyloid seeding and rationalize the molecular mechanism of the cross-seeding phenomenon as a manifestation of the aberrant surface activities presented by amyloid seeds as nanoparticles.

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.

Analysis of Baboon IAPP Provides Insight into Amyloidogenicity and Cytotoxicity of Human IAPP

The polypeptide hormone islet amyloid polypeptide (IAPP) forms islet amyloid in type 2 diabetes, a process which contributes to pancreatic β-cell dysfunction and death. Not all species form islet amyloid, and the ability to do so correlates with the primary sequence. Humans form islet amyloid, but baboon IAPP has not been studied. The baboon peptide differs from human IAPP at three positions containing K1I, H18R, and A25T substitutions. The K1I substitution is a rare example of a replacement in the N-terminal region of amylin. The effect of this mutation on amyloid formation has not been studied, but it reduces the net charge, and amyloid prediction programs suggest that it should increase amyloidogenicity. The A25T replacement involves a nonconservative substitution in a region of IAPP that is believed to be important for aggregation, but the effects of this replacement have not been examined. The H18R point mutant has been previously shown to reduce aggregation in vitro. Baboon amylin forms amyloid on the same timescale as human amylin in vitro and exhibits similar toxicity toward cultured β-cells. The K1I replacement in human amylin slightly reduces toxicity, whereas the A25T substitution accelerates amyloid formation and enhances toxicity. Photochemical cross-linking reveals that the baboon amylin, like human amylin, forms low-order oligomers in the lag phase of amyloid formation. Ion-mobility mass spectrometry reveals broadly similar gas phase collisional cross sections for human and baboon amylin monomers and dimers, with some differences in the arrival time distributions. Preamyloid oligomers formed by baboon amylin, but not baboon amylin fibers, are toxic to cultured β-cells. The toxicity of baboon oligomers and lack of significantly detectable toxicity with exogenously added amyloid fibers is consistent with the hypothesis that preamyloid oligomers are the most toxic species produced during IAPP amyloid formation.

Amyloidogenicity and cytotoxicity of des-Lys-1 human amylin provides insight into amylin self-assembly and highlights the difficulties of defining amyloidogenicity.

The polypeptide amylin is responsible for islet amyloid in type 2 diabetes, a process which contributes to β-cell death in the disease. The role of the N-terminal region of amylin in amyloid formation is relatively unexplored, although removal of the disulfide bridged loop between Cys-2 and Cys-7 accelerates amyloid formation. We examine the des Lys-1 variant of human amylin (h-amylin), a variant which is likely produced in vivo. Lys-1 is a region of high charge density in the h-amylin amyloid fiber. The des Lys-1 polypeptide forms amyloid on the same time scale as wild-type amylin in phosphate buffered saline, but does so more rapidly in Tris. The des Lys-1 variant is somewhat less toxic to cultured INS cells than wild type. The implications for the in vitro mechanism of amyloid formation and for comparative analysis of amyloidogenicity are discussed.

Amyloid Formation under Complicated Conditions in Which β2-Microglobulin Coexists with Its Proteolytic Fragments.

Amyloid formation in vivo occurs under complicated conditions in which various amyloidogenic and non-amyloidogenic components coexist, often under crowding. Controversy surrounds the role of additional components under complicated conditions. They have been suggested to accelerate amyloid formation because molecular crowding or interactions with additives increase effective concentrations and, thus, break the supersaturation of amyloidogenic proteins. On the other hand, cellular crowding conditions with various heterogeneous components may retard or prevent amyloid formation because they impede homologous amyloidogenic associations. To elucidate the roles of these additional components, we examined the amyloid formation of β2-microglobulin (β2m), a protein responsible for dialysis-related amyloidosis, with a simplified model system in which intact β2m and its proteolytic peptides coexist. Among the nine proteolytic peptides of β2m produced in vitro with lysyl endopeptidase, the 22-residue K3 peptide is highly amyloidogenic. The amyloid formation of the K3 peptide, which occurred with a lag time of 1 h at pH 2 and 37 °C, was significantly retarded by the coexistence of β2m or a mixture of the proteolytic digests. To identify the sites of inhibitory interactions, we performed paramagnetic relaxation enhancement measurements using spin-labeled K3 and uniformly 15N-labeled β2m with nuclear magnetic resonance detection. The results revealed that K3 interacted weakly with a broad cluster of the hydrophobic residues of β2m, which accommodated the residues located in some distant sequence, leading to competitive inhibition. The results showed that relatively weak and broad interactions formed a nonproductive complex, implying a role for heterogeneous interactions under complicated conditions.

Lipid membranes accelerate amyloid formation in the mouse model of AA amyloidosis

Introduction: AA amyloidosis develops as a result of prolonged inflammation and is characterized by deposits of N-terminal proteolytic fragments of the acute phase reactant serum amyloid A (SAA). Macrophages are usually found adjacent to amyloid, suggesting their involvement in the formation and/or degradation of the amyloid fibrils. Furthermore, accumulating evidence suggests that lipid membranes accelerate the fibrillation of different amyloid proteins.Methods: Using an experimental mouse model of AA amyloidosis, we compared the amyloidogenic effect of liposomes and/or amyloid-enhancing factor (AEF). Inflammation was induced by subcutaneous injection of silver nitrate followed by intravenous injection of liposomes and/or AEF to accelerate amyloid formation.Results: We showed that liposomes accelerate amyloid formation in inflamed mice, but the amyloidogenic effect of liposomes was weaker compared with AEF. Regardless of the induction method, amyloid deposits were mainly found in the marginal zones of the spleen and coincided with the depletion of marginal zone macrophages, while red pulp macrophages and metallophilic marginal zone macrophages proved insensitive to amyloid deposition.Conclusions: We conclude that increased intracellular lipid content facilitates AA amyloid fibril formation and show that the mouse model of AA amyloidosis is a suitable system for further mechanistic studies.

Analysis of the Role of the Conserved Disulfide in Amyloid Formation by Human Islet Amyloid Polypeptide in Homogeneous and Heterogeneous Environments.

Human islet amyloid polypeptide (hIAPP) is a hormone secreted from β-cells in the Islets of Langerhans in response to the same stimuli that lead to insulin secretion. hIAPP plays an adaptive role in glucose homeostasis but misfolds to form insoluble, fibrillar aggregates in type II diabetes that are associated with the disease. Along the misfolding pathway, hIAPP forms species that are toxic to β-cells, resulting in reduced β-cell mass. hIAPP contains a strictly conserved disulfide bond between residues 2 and 7, which forms a small loop at the N-terminus of the molecule. The loop is located outside of the cross β-core in all models of the hIAPP amyloid fibrils. Mutations in this region are rare, and the disulfide loop plays a role in receptor binding; however, the contribution of this region to the aggregation of hIAPP is not well understood. We define the role of the disulfide by analyzing a collection of analogues that remove the disulfide, by mutation of Cys to Ser, by reduction and modification of the Cys residues, or by deletion of the first seven residues. The cytotoxic properties of hIAPP are retained in the Cys to Ser disulfide-free mutant. Removal of the disulfide bond accelerates amyloid formation in all constructs, both in solution and in the presence of model membranes. Removal of the disulfide weakens the ability of hIAPP to induce leakage of vesicles consisting of POPS and POPC. Smaller effects are observed with vesicles that contain 40 mol % cholesterol, although N-terminal truncation still reduces the extent of leakage.

Stabilization and structural analysis of a membrane-associated hIAPP aggregation intermediate.

Membrane-assisted amyloid formation is implicated in human diseases, and many of the aggregating species accelerate amyloid formation and induce cell death. While structures of membrane-associated intermediates would provide tremendous insights into the pathology and aid in the design of compounds to potentially treat the diseases, it has not been feasible to overcome the challenges posed by the cell membrane. Here, we use NMR experimental constraints to solve the structure of a type-2 diabetes related human islet amyloid polypeptide intermediate stabilized in nanodiscs. ROSETTA and MD simulations resulted in a unique β-strand structure distinct from the conventional amyloid β-hairpin and revealed that the nucleating NFGAIL region remains flexible and accessible within this isolated intermediate, suggesting a mechanism by which membrane-associated aggregation may be propagated. The ability of nanodiscs to trap amyloid intermediates as demonstrated could become one of the most powerful approaches to dissect the complicated misfolding pathways of protein aggregation.

Seed-Induced Heterogeneous Cross-Seeding Self-Assembly of Human and Rat Islet Polypeptides.

Amyloid peptides can misfold and aggregate into amyloid oligomers and fibrils containing conformationally similar β-sheet structures, which are linked to the pathological hallmark of many neurodegenerative diseases. These β-sheet-rich amyloid aggregates provide common structural motifs to accelerate amyloid formation by acting as seeds. However, little is known about how one amyloid peptide aggregation will affect another one (namely, cross-seeding). In this work, we studied the cross-seeding possibility and efficiency between rat islet amyloid polypeptide (rIAPP) and human islet amyloid polypeptide (hIAPP) solution with preformed aggregates at different aggregation phases, using a combination of different biophysical techniques. hIAPP is a well-known peptide hormone that forms amyloid fibrils and induces cytotoxicity to β-cells in type 2 diabetes, whereas rIAPP is a nonaggregating and nontoxic peptide. Experimental results showed that all different preformed hIAPP aggregates can cross-seed rIAPP to promote the final fibril formation but exhibit different cross-seeding efficiencies. Evidently, hIAPP seeds preformed at a growth phase show the strongest cross-seeding potential to rIAPP, which accelerates the conformational transition from random structures to β-sheet and the aggregation process at the fibrillization stage. Homoseeding of hIAPP is more efficient in initiating and promoting aggregation than cross-seeding of hIAPP and rIAPP. Moreover, the cross-seeding of rIAPP with hIAPP at the lag phase also reduced cell viability, probably because of the formation of more toxic hybrid oligomers at the prolonged lag phase. The cross-seeding effects in this work may add new insights into the mechanistic understanding of the aggregation and coaggregation of amyloid peptides linked to different neurodegenerative diseases.

Islet Amyloid Polypeptide Membrane Interactions: Effects of Membrane Composition.

Amyloid formation by islet amyloid polypeptide (IAPP) contributes to β-cell dysfunction in type 2 diabetes. Perturbation of the β-cell membrane may contribute to IAPP-induced toxicity. We examine the effects of lipid composition, salt, and buffer on IAPP amyloid formation and on the ability of IAPP to induce leakage of model membranes. Even low levels of anionic lipids promote amyloid formation and membrane permeabilization. Increasing the percentage of the anionic lipids, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS) or 1,2-dioleoyl-sn-glycero-3-phospho(1'-rac-glycerol), enhances the rate of amyloid formation and increases the level of membrane permeabilization. The choice of zwitterionic lipid has no noticeable effect on membrane-catalyzed amyloid formation but in most cases affects leakage, which tends to decrease in the following order: 1,2-dioleoyl-sn-glycero-3-phosphocholine > 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine > sphingomyelin. Uncharged lipids that increase the level of membrane order weaken the ability of IAPP to induce leakage. Leakage is due predominately to pore formation rather than complete disruption of the vesicles under the conditions used in these studies. Cholesterol at or below physiological levels significantly reduces the rate of vesicle-catalyzed IAPP amyloid formation and decreases the susceptibility to IAPP-induced leakage. The effects of cholesterol on amyloid formation are masked by 25 mol % POPS. Overall, there is a strong inverse correlation between the time to form amyloid and the extent of vesicle leakage. NaCl reduces the rate of membrane-catalyzed amyloid formation by anionic vesicles, but accelerates amyloid formation in solution. The implications for IAPP membrane interactions are discussed, as is the possibility that the loss of phosphatidylserine asymmetry enhances IAPP amyloid formation and membrane damage in vivo via a positive feedback loop.

Evolutionary Adaptation and Amyloid Formation: Does the Reduced Amyloidogenicity and Cytotoxicity of Ursine Amylin Contribute to the Metabolic Adaption of Bears and Polar Bears?

Much of our knowledge of diabetes is derived from studies of rodent models. An alternative approach explores evolutionary solutions to physiological stress by studying organisms that face challenging metabolic environments. Polar bears eat an enormously lipid-rich diet without deleterious metabolic consequences. In contrast, transgenic rodents expressing the human neuropancreatic polypeptide hormone amylin develop hyperglycemia and extensive pancreatic islet amyloid when fed a high fat diet. The process of islet amyloid formation by human amylin contributes to β-cell dysfunction and loss of β-cell mass in type-2 diabetes. We show that ursine amylin is considerably less amyloidogenic and less toxic to β-cells than human amylin, consistent with the hypothesis that part of the adaptation of bears to metabolic challenges might include protection from islet amyloidosis-induced β-cell toxicity. Ursine and human amylin differ at four locations: H18R, S20G, F23L, and S29P. These are interesting from a biophysical perspective since the S20G mutation accelerates amyloid formation but the H18R slows it. An H18RS20G double mutant of human amylin behaves similarly to the H18R mutant, indicating that the substitution at position 18 dominates the S20G replacement. These data suggest one possible mechanism underpinning the protection of bears against metabolic challenges and provide insight into the design of soluble analogs of human amylin.