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

Methods for monitoring protein-membrane binding. Comparison based on the interactions between amyloidogenic protein human cystatin C and phospholipid liposomes

A cell membrane is an essential cellular component providing protection against the outer environment. It is also a host for proteins and carbohydrates responsible for, e.g. transporter, receptor, or enzymatic functions. In parallel, the membrane may also be implicated in pathological processes leading, e.g. to the oligomerization of amyloid-forming proteins, a hallmark of i.a. Alzheimer's disease. The increasing need for detailed information on mechanisms driving the amyloid formation and the potential role of cell membranes in the process proves the research on protein-membrane interactions biologically relevant. Considering the potential and limitations of the relatively well established and newly developed methods, this study focused on selecting methods that allow a broad and comprehensive description of interactions between amyloidogenic protein human cystatin C and lipid bilayers. In the first step, dot-blot and ELISA tests were selected as techniques allowing fast screening for protein-ligand interactions. Next, surface plasmon resonance, spectral shift, biolayer interferometry, and switchSENSE® technology were used to determine kinetic parameters and binding constants for interactions between human cystatin C and the selected lipid bilayers. Based on the obtained results we have proposed the most promising candidates for monitoring of interactions and determining affinity between amyloidogenic proteins and membrane mimetics.

The ASC inflammasome adapter governs SAA-derived protein aggregation in inflammatory amyloidosis

Extracellularly released molecular inflammasome assemblies -ASC specks- cross-seed Aβ amyloid in Alzheimer's disease. Here we show that ASC governs the extent of inflammation-induced amyloid A (AA) amyloidosis, a systemic disease caused by the aggregation and peripheral deposition of the acute-phase reactant serum amyloid A (SAA) in chronic inflammatory conditions. Using super-resolution microscopy, we found that ASC colocalized tightly with SAA in human AA amyloidosis. Recombinant ASC specks accelerated SAA fibril formation and mass spectrometry after limited proteolysis showed that ASC interacts with SAA via its pyrin domain (PYD). In a murine model of inflammatory AA amyloidosis, splenic amyloid load was conspicuously decreased in Pycard-/- mice which lack ASC. Treatment with anti-ASCPYD antibodies decreased amyloid loads in wild-type mice suffering from AA amyloidosis. The prevalence of natural anti-ASC IgG (-logEC50 ≥ 2) in 19,334 hospital patients was <0.01%, suggesting that anti-ASC antibody treatment modalities would not be confounded by natural autoimmunity. These findings expand the role played by ASC and IL-1 independent inflammasome employments to extraneural proteinopathies and suggest that anti-ASC immunotherapy may contribute to resolving such diseases.

Monomer-Dimer Equilibrium of Human Cystatin C During Internalization Into Cancer Cells

Human cystatin C (hCC) is a physiologically important protein that serves as intra- and extracellular cysteine proteinase inhibitor in homeostasis. However, in pathological states it dimerizes and further oligomerizes accumulating into a toxic amyloid. HCC forms an active monomer in the extracellular space and becomes an inactive dimer when internalized in cellular organelles. However, hCC cell penetration and its oligomeric state during this process are not well understood. To determine if and how the oligomeric state influences hCC transmembrane migration, we investigated the internalization of the hCC wild type protein as well as three different mutants, which exclusively exist in the monomeric or multimeric state into HeLa cells via confocal fluorescence microscopy. Our results showed that the preferred pathway was endocytosis and that the oligomeric state did not significantly influence the internalization because both monomeric and dimeric hCC migrated into HeLa cells. Considering the differences of the active monomeric and the passive dimeric states of hCC, our findings contribute to a better understanding of the intra and extra cellular functions of hCC and their interaction with cysteine proteases.

DPPA as a Potential Cell Membrane Component Responsible for Binding Amyloidogenic Protein Human Cystatin C

A phospholipid bilayer is a typical structure that serves crucial functions in various cells and organelles. However, it is not unusual for it to take part in pathological processes. The cell membrane may be a binding target for amyloid-forming proteins, becoming a factor modulating the oligomerization process leading to amyloid deposition-a hallmark of amyloidogenic diseases-e.g., Alzheimer's disease. The information on the mechanisms governing the oligomerization influenced by the protein-membrane interactions is scarce. Therefore, our study aims to describe the interactions between DPPA, a cell membrane mimetic, and amyloidogenic protein human cystatin C. Circular dichroism spectroscopy and differential scanning calorimetry were used to monitor (i) the secondary structure of the human cystatin C and (ii) the phase transition temperature of the DPPA, during the protein-membrane interactions. NMR techniques were used to determine the protein fragments responsible for the interactions, and molecular dynamics simulations were applied to provide a molecular structure representing the interaction. The obtained data indicate that the protein interacts with DPPA, submerging itself into the bilayer via the AS region. Additionally, the interaction increases the content of α-helix within the protein's secondary structure and stabilizes the whole molecule against denaturation.

Monitoring the interactions between POPG phospholipid bilayer and amyloid-forming protein human cystatin C. Does the bilayer influence the oligomeric state and structure of the protein?

A biological membrane is a structure characteristic for various cells and organelles present in almost all living organisms. Even though, it is one of the most common structures in organisms, where it serves crucial functions, a phospholipid bilayer may also take part in pathological processes leading to severe diseases. Research indicates that biological membranes have a profound impact on the pathological processes of oligomerization of amyloid-forming proteins. These processes are a hallmark of amyloid diseases, a group of pathological states involving, e.g., Parkinson's or Alzheimer's disease. Even though amyloidogenic diseases reap the harvest in modern societies, especially in elderly patients, the mechanisms governing the amyloid deposition are not clearly described. Therefore, the presented study focuses on the description of interactions between a model biological membrane (POPG) and one of amyloid forming proteins - human cystatin C. For the purpose of the study molecular dynamics simulations were applied to confirm interactions between the protein and POPG membrane. Next the NMR techniques were used to verify how the data obtained in solution compared to MD simulations and determine fragments of the protein responsible for interactions with POPG. Finally, circular dichroism was used to monitor the changes in secondary structure of the protein and size exclusion chromatography was used to monitor its oligomerization process. Obtained data indicates that the protein interacts with POPG submerging itself into the bilayer with the AS region. However, the presence of POPG bilayer does not significantly affect the structure or oligomerization process of human cystatin C.

Phospholipid-functionalized gold electrode for cellular membrane interface studies - interactions between DMPC bilayer and human cystatin C

This work describes the electrochemical studies on the interactions between V57G mutant of human cystatin C (hCC V57G) and membrane bilayer immobilized on the surface of a gold electrode. The electrode was modified with 6-mercaptohexan-1-ol (MCH) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). DMPC was used as a membrane mimetic for monitoring electrochemical changes resulting from the interactions between the functionalized electrode surface and human cystatin C. The interactions between the modified electrode and hCC V57G were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in a phosphate buffered saline (PBS) containing Fe(CN)63-/4- as a redox probe. The electrochemical measurements confirm that fabricated electrode is sensitive to hCC V57G at the concentration of 1 × 10-14 M. The incubation studies carried out at higher concentrations resulted in insignificant changes observed in cyclic voltammetry and electrochemical impedance spectroscopy measurements. The calculated values of surface coverage θR confirm that the electrode is equally covered at higher concentrations of hCC V57G. Measurements of wettability and surface free energy made it possible to determine the influence of individual structural elements of the modified gold electrode on its properties, and thus allowed to understand the nature of the interactions. Contact angle values confirmed the results obtained during electrochemical measurements, indicating the sensitivity of the electrode towards hCC V57G at the concentration of 1 × 10-14 M. In addition, the XPS spectra confirmed the successful anchoring of hCC V57G to the DMPC-functionalized surface.

The Amyloid Assembly of the Bacterial Hfq Is Lipid-Driven and Lipid-Specific

Under specific conditions, some proteins can self-assemble into fibrillar structures called amyloids. Initially, these proteins were associated with neurodegenerative diseases in eucaryotes. Nevertheless, they have now been identified in the three domains of life. In bacteria, they are involved in diverse biological processes and are usually useful for the cell. For this reason, they are classified as "functional amyloids". In this work, we focus our analysis on a bacterial functional amyloid called Hfq. Hfq is a pleiotropic regulator that mediates several aspects of genetic expression, mainly via the use of small noncoding RNAs. Our previous work showed that Hfq amyloid-fibrils interact with membranes. This interaction influences Hfq amyloid structure formation and stability, but the specifics of the lipid on the dynamics of this process is unknown. Here, we show, using spectroscopic methods, how lipids specifically drive and modulate Hfq amyloid assembly or, conversely, its disassembly. The reported effects are discussed in light of the consequences for bacterial cell life.

Investigation of serum amyloid a within animal species focusing on the 1-25 amino acid region

AA amyloidosis, characterized by the misfolding of serum amyloid A (SAA) protein, is the most common amyloid protein disorder across multiple species. SAA is a positive-acute phase protein synthesized by the liver in response to inflammation or stress, and it normally associates with high-density lipoprotein at its N-terminus. In this study, we focused on the 1-25 amino acid (aa) region of the complete 104 aa SAA sequence to examine the aggregation propensity of AA amyloid. A library comprising eight peptides from different species was assembled for analysis. To access the aggregation propensity of each peptide region, a bioinformatic study was conducted using the algorithm TANGO. Congo red (CR) binding assays, Thioflavin T (ThT) assays, and transmission electron microscopy (TEM) were utilized to evaluate whether the synthesized peptides formed amyloid-like fibrils. All synthetic SAA 1-25 congeners resulted in amyloid-like fibrils formation (per CR and/or ThT staining and TEM detection) at the exception of the ferret SAA1-25 fragment, which generated plaque-like materials by TEM. Ten residues were preserved among SAA 1-25 congeners resulting in amyloid-like fibrils, i.e. F6, E9, A10, G13, D16, M17, A20, Y21, D23, and M24. Amino acid residues highlighted by this study may have a role in increasing the propensity for amyloid-like fibril formation. This study put an emphasis on region 1-25 in the mechanism of SAA1 misfolding.

Unraveling Membrane Perturbations Caused by the Bacterial Riboregulator Hfq

Hfq is a pleiotropic regulator that mediates several aspects of bacterial RNA metabolism. The protein notably regulates translation efficiency and RNA decay in Gram-negative bacteria, usually via its interaction with small regulatory RNAs. Previously, we showed that the Hfq C-terminal region forms an amyloid-like structure and that these fibrils interact with membranes. The immediate consequence of this interaction is a disruption of the membrane, but the effect on Hfq structure was unknown. To investigate details of the mechanism of interaction, the present work uses different in vitro biophysical approaches. We show that the Hfq C-terminal region influences membrane integrity and, conversely, that the membrane specifically affects the amyloid assembly. The reported effect of this bacterial master regulator on membrane integrity is discussed in light of the possible consequence on small regulatory RNA-based regulation.

On the Structural Diversity and Individuality of Polymorphic Amyloid Protein Assemblies

The prediction of highly ordered three-dimensional structures of amyloid protein fibrils from the amino acid sequences of their monomeric self-assembly precursors constitutes a challenging and unresolved aspect of the classical protein folding problem. Because of the polymorphic nature of amyloid assembly whereby polypeptide chains of identical amino acid sequences under identical conditions are capable of self-assembly into a spectrum of different fibril structures, the prediction of amyloid structures from an amino acid sequence requires a detailed and holistic understanding of its assembly free energy landscape. The full extent of the structure space accessible to the cross-β molecular architecture of amyloid must also be resolved. Here, we review the current understanding of the diversity and the individuality of amyloid structures, and how the polymorphic landscape of amyloid links to biology and disease phenotypes. We present a comprehensive review of structural models of amyloid fibrils derived by cryo-EM, ssNMR and AFM to date, and discuss the challenges ahead for resolving the structural basis and the biological consequences of polymorphic amyloid assemblies.

DMPC Phospholipid Bilayer as a Potential Interface for Human Cystatin C Oligomerization: Analysis of Protein-Liposome Interactions Using NMR Spectroscopy

Studies revolving around mechanisms responsible for the development of amyloid-based diseases lay the foundations for the recognition of molecular targets of future to-be-developed treatments. However, the vast number of peptides and proteins known to be responsible for fibril formation, combined with their complexity and complexity of their interactions with various cellular components, renders this task extremely difficult and time-consuming. One of these proteins, human cystatin C (hCC), is a well-known and studied cysteine-protease inhibitor. While being a monomer in physiological conditions, under the necessary stimulus—usually a mutation, it tends to form fibrils, which later participate in the disease development. This process can potentially be regulated (in several ways) by many cellular components and it is being hypothesized that the cell membrane might play a key role in the oligomerization pathway. Studies involving cell membranes pose several difficulties; therefore, an alternative in the form of membrane mimetics is a very attractive solution. Here, we would like to present the first study on hCC oligomerization under the influence of phospholipid liposomes, acting as a membrane mimetic. The protein–mimetic interactions are studied utilizing circular dichroism, nuclear magnetic resonance, and size exclusion chromatography.

Amyloid Proteins and Peripheral Neuropathy

Painful peripheral neuropathy affects millions of people worldwide. Peripheral neuropathy develops in patients with various diseases, including rare familial or acquired amyloid polyneuropathies, as well as some common diseases, including type 2 diabetes mellitus and several chronic inflammatory diseases. Intriguingly, these diseases share a histopathological feature-deposits of amyloid-forming proteins in tissues. Amyloid-forming proteins may cause tissue dysregulation and damage, including damage to nerves, and may be a common cause of neuropathy in these, and potentially other, diseases. Here, we will discuss how amyloid proteins contribute to peripheral neuropathy by reviewing the current understanding of pathogenic mechanisms in known inherited and acquired (usually rare) amyloid neuropathies. In addition, we will discuss the potential role of amyloid proteins in peripheral neuropathy in some common diseases, which are not (yet) considered as amyloid neuropathies. We conclude that there are many similarities in the molecular and cell biological defects caused by aggregation of the various amyloid proteins in these different diseases and propose a common pathogenic pathway for "peripheral amyloid neuropathies".

Amyloid Signature Proteins in Feline Amyloidosis

In human amyloidoses, amyloid signature proteins (ASPs), such as serum amyloid P component (SAP) and apolipoprotein E (ApoE), are deposited in tissues together with amyloid fibrils and are implicated in the pathogenesis of amyloidosis. Few reports describe ASPs in animals. In this study, we examined feline amyloidosis and performed immunohistochemical and proteomic analyses of SAP, ApoE, apolipoprotein A-I (ApoAI) and apolipoprotein A-IV (ApoAIV). Ten cases of systemic amyloidosis, three cases of amyloid-producing odontogenic tumour and three cases of islet amyloidosis were used for immunohistochemistry (IHC) and/or proteomic analyses. IHC showed that ApoE was present in amyloid deposits in all samples. ApoAI and ApoAIV differed in the degree of co-deposition with amyloid depending on the type of amyloid and the affected organ. SAP was negative in all amyloid deposits. Proteomic analysis showed that ApoE was present in all samples, but ApoAI and ApoAIV were detected only in some samples and SAP was not detected in any samples. The observation that ApoE was detected in all types of amyloid suggests the involvement of ApoE in the development of feline amyloidosis. ASPs in feline amyloidosis are significantly different from those in human amyloidosis, suggesting that the involvement of ASPs in the pathological condition differs between animal species.

Deciphering the role of premicellar and micellar concentrations of sodium dodecyl benzenesulfonate surfactant in insulin fibrillation at pH 2.0

Amyloid fibril formation by proteins and their deposition in cells and tissues are associated with several amyloid-based disorders. Understanding the mechanism of amyloid fibril formation is thus of the utmost importance for the designing ligands that could prevent or inhibit the fibrillation process and help to treat of such disorders. We describe the stimulatory effect of sodium dodecyl benzenesulfonate (SDBS) on insulin amyloid fibrillation at pH 2.0 and the characterization of SDBS-induced insulin aggregation using spectroscopy and microscopy. We found that SDBS induced amyloid-like aggregates of insulin at sub-micellar (0.1-1.2 mM), but not post-micellar (≥2.0 mM) concentrations. The amyloid fibrillation of insulin induced by SDBS was kinetically rapid and escaped the lag phase. Far-UV CD findings suggested that the α-helical content of insulin transformed into cross-β structure and mixed α and β structures when incubated with sub-micellar and post-micellar SDBS concentrations, respectively. The overall results indicated that low, but not high SDBS concentrations induce amyloid-like insulin aggregates and fibrils.

Serum Amyloid A-Mediated Inflammasome Activation of Microglial Cells in Cerebral Ischemia

Serum amyloid A (SAA) proteins are acute-phase reactant associated with high-density lipoprotein (HDL) particles and increase in the plasma 1000-fold during inflammation. Recent studies have implicated SAAs in innate immunity and various disorders; however, the precise mechanism eludes us. Previous studies have shown SAAs are elevated following stroke and cerebral ischemia, and our studies demonstrated that SAA-deficient mice reduce inflammation and infarct volumes in a mouse stroke model. Our studies demonstrate that SAA increases the cytokine interleukin-1β (IL-1β), which is mediated by Nod-like receptor protein 3 (NLRP3) inflammasome, cathepsin B, and caspase-1 and may play a role in the pathogenesis of neurological disorders. SAA induced the expression of NLRP3, which mediated IL-1β induction in murine BV-2 cells and both sex primary mouse microglial cells, in a dose- and time-dependent fashion. Inhibition or KO of the NLRP3 in microglia prevented the increase in IL-1β. N-acetyl-l-cysteine and mito-TEMPO blocked the induction of IL-1β by inhibiting ROS with SAA treatment. In addition, inhibition of cathepsin B with different drugs or microglia from CatB-deficient mice attenuated inflammasome activation. Our studies suggest that the impact of SAA on inflammasome stimulation is mediated in part by the receptor for advanced glycation endproducts and Toll-like receptor proteins 2 and 4. SAA induced inflammatory cytokines and an M1 phenotype in the microglial cells while downregulating anti-inflammation M2 phenotype. These studies suggest that brain injury to can elicit a systemic inflammatory response mediated through SAA that contributes to the pathological outcomes.SIGNIFICANCE STATEMENT In the present study, serum amyloid A can induce that activation of the inflammasome in microglial cells and give rise to IL-1β release, which can further inflammation in the brain following neurological diseases. The also presents a novel target for therapeutic approaches in stroke.