Molecular Dynamics of Lysozyme Amyloid Polymorphs Studied by Incoherent Neutron Scattering

Sub-nanosecond dynamics of phospholipid membranes interacting with polymorphic amyloid fibrils observed by elastic incoherent neutron scattering

Amyloidosis such as Alzheimer's or Parkinson's disease is characterized by deposition of amyloid fibrils in the brain or various internal organs. The onset of amyloidosis is related to the strength of cytotoxicity caused by toxic amyloid species. In addition, amyloid fibrils show a polymorphism, i.e., some types of fibrils are more cytotoxic than others. It is thus important to elucidate the molecular mechanism of cytotoxicity, which is ultimately caused by interactions between amyloid fibrils and cell membranes. In this study, modulation of molecular dynamics of phospholipid membranes induced by the binding of amyloid polymorphic fibrils with different levels of cytotoxicity was studied by elastic incoherent neutron scattering in a temperature range between 280 K and 310 K. The amyloid fibrils were formed by a model system of hen egg white lysozyme at pH 2.7 or 6.0 and phospholipid vesicles were formed by DMPG or DMPC. The elastic incoherent neutron scattering curves were analyzed in terms of the mean square positional fluctuations (MSPF) of atomic motions, including its distribution, as a function of temperature, which is related to molecular flexibility. The major findings are: (1) Both more and less cytotoxic fibrils decreased the molecular flexibility of DMPG. (2) While less cytotoxic fibrils decreased the molecular flexibility of DMPC, more cytotoxic fibrils increased it. (3) Close to the physiological body temperature, more cytotoxic fibrils caused larger MSPFs of both phospholipids with an enhanced motional heterogeneity. These results imply that enhanced dynamics of phospholipids is associated with the stronger cytotoxicity.

Phospholipid-induced secondary structural changes of lysozyme polymorphic amyloid fibrils studied using vacuum-ultraviolet circular dichroism

The hallmark of amyloidosis, such as Alzheimer's disease and Parkinson's disease, is the deposition of amyloid fibrils in various internal organs. The onset of the disease is related to the strength of cytotoxicity caused by toxic amyloid species. Furthermore, amyloid fibrils show polymorphism, where some types of fibrils are cytotoxic while others are not. It is thus essential to understand the molecular mechanism of cytotoxicity, part of which is caused by the interaction between amyloid polymorphic fibrils and cell membranes. Here, using amyloid polymorphs of hen egg white lysozyme, which is associated with hereditary systemic amyloidosis, showing different levels of cytotoxicity and liposomes of DMPC and DMPG, changes in the secondary structure of the polymorphs and the structural state of phospholipid membranes caused by the interaction were investigated using vacuum-ultraviolet circular dichroism (VUVCD) and Laurdan fluorescence measurements, respectively. Analysis has shown that the more cytotoxic polymorph increases the antiparallel β-sheet content and causes more disorder in the membrane structure while the other less cytotoxic polymorph shows the opposite structural changes and causes less structural disorder in the membrane. These results suggest a close correlation between the structural properties of amyloid fibrils and the degree of structural disorder of phospholipid membranes, both of which are involved in the fundamental process leading to amyloid cytotoxicity.

Amyloid fibril cytotoxicity and associated disorders

Misfolded proteins assemble into fibril structures that are called amyloids. Unlike usually folded proteins, misfolded fibrils are insoluble and deposit extracellularly or intracellularly. Misfolded proteins interrupt the function and structure of cells and cause amyloid disease. There is increasing evidence that the most pernicious species are oligomers. Misfolded proteins disrupt cell function and cause cytotoxicity by calcium imbalance, mitochondrial dysfunction, and intracellular reactive oxygen species. Despite profound impacts on health, social, and economic factors, amyloid diseases remain untreatable. To develop new therapeutics and to understand the pathological manifestations of amyloidosis, research into the origin and pathology of amyloidosis is urgently needed. This chapter describes the basic concept of amyloid disease and the function of atypical amyloid deposits in them.

Relaxation dynamics measure the aggregation propensity of amyloid-β and its mutants

Atomistic molecular dynamics simulations are employed to investigate the global and segmental relaxation dynamics of the amyloid-β protein and its causative and protective mutants. Amyloid-β exhibits significant global/local dynamics that span a broad range of length and time scales due to its intrinsically disordered nature. The relaxation dynamics of the amyloid-β protein and its mutants is quantitatively correlated with its experimentally measured aggregation propensity. The protective mutant has slower relaxation dynamics, whereas the causative mutants exhibit faster global dynamics compared with that of the wild-type amyloid-β. The local dynamics of the amyloid-β protein or its mutants is governed by a complex interplay of the charge, hydrophobicity, and change in the molecular mass of the mutated residue.

Sub-Nanosecond Dynamics of Pathologically Relevant Bio-Macromolecules Observed by Incoherent Neutron Scattering

Incoherent neutron scattering (iNS) is one of the most powerful techniques to study the dynamical behavior of bio-macromolecules such as proteins and lipid molecules or whole cells. This technique has widely been used to elucidate the fundamental aspects of molecular motions that manifest in the bio-macromolecules in relation to their intrinsic molecular properties and biological functions. Furthermore, in the last decade, iNS studies focusing on a possible relationship between molecular dynamics and biological malfunctions, i.e., human diseases and disorders, have gained importance. In this review, we summarize recent iNS studies on pathologically relevant proteins and lipids and discuss how the findings are of importance to elucidate the molecular mechanisms of human diseases and disorders that each study targets. Since some diseases such as amyloidosis have become more relevant in the aging society, research in this field will continue to develop further and be more important in the current increasing trend for longevity worldwide.