Ian V. J. Murray

β-Synuclein gene alterations in dementia with Lewy bodies

Objective: To determine whether mutations in the genes for α-synuclein or β-synuclein are responsible for dementia with Lewy bodies (DLB), a disorder closely related to Parkinson disease (PD). Methods: The authors ascertained 33 sporadic cases of DLB and 10 kindreds segregating DLB. DNA samples from the 43 index cases were screened for alterations in the genes for α-synuclein and β-synuclein, as α-synuclein alterations cause PD and β-synuclein may modulate α-synuclein aggregation and neurotoxicity. Results: Two amino acid alterations were identified in unrelated DLB index cases: a valine to methionine substitution at codon 70 (V70M) and a proline to histidine substitution at codon 123 (P123H), both in the β-synuclein gene. These amino acid substitutions occur at conserved residues in highly conserved regions of the β-synuclein protein. Screening of at least 660 chromosomes from control subjects matched to the patients’ population groups failed to identify another V70M or P123H allele. Cosegregation analysis of an extended pedigree segregating the P123H β-synuclein alteration suggested that it is a dominant trait with reduced penetrance or a risk factor polymorphism. Histopathology and immunohistochemistry analysis of index case brain sections revealed widespread Lewy body pathology and α-synuclein aggregation without evidence of β-synuclein aggregation. Conclusion: Mutations in the β-synuclein gene may predispose to DLB.

Oxidative Stress and Cell Membranes in the Pathogenesis of Alzheimer's Disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimers disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimers disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Age-dependent synuclein pathology following traumatic brain injury in mice

Synucleins (Syn), a family of synaptic proteins, includes alpha-Syn, which plays a pivotal role in Parkinsons disease and related neurodegenerative diseases (synucleinopathies) by forming distinct brain pathologies (Lewy bodies and neurites). Since traumatic brain injury (TBI) is a poorly understood risk factor for Parkinsons disease, we examined the effects of TBI in the young and aged mouse brain on alpha-, beta-, and gamma-Syn. Immunohistochemical analysis showed that brains from sham-injured young and aged mice had normal alpha- and beta-Syn immunoreactivity (IR) in neuropil of cortex, striatum, and hippocampus with little or no gamma-Syn IR. At 1 week post TBI, the aged mouse brain showed a transient increase of alpha- and beta-Syn IR in the neuropil as well as an induction of gamma-Syn IR in subcortical axons. This was associated with strong labeling of striatal axon bundles by antibodies to altered or nitrated epitopes in alpha-Syn as well as by antibodies to inducible nitric oxide synthase. However, these TBI-induced changes disappeared by 16 weeks post TBI, and altered Syn IR was not seen in young mice subjected to TBI nor in alpha-Syn knockout mice while Western blots confirmed that TBI induced transient alterations of alpha-Syn in the mouse brains. This model of age-dependent TBI-induced transient alterations in alpha-Syn provides an opportunity to examine possible links between TBI and mechanisms of disease in synucleinopathies.

Membrane-mediated Amyloidogenesis and the Promotion of Oxidative Lipid Damage by Amyloid β Proteins

Evidence of oxidative stress and the accumulation of fibrillar amyloid β proteins (Aβ) in senile plaques throughout the cerebral cortex are consistent features in the pathology of Alzheimer disease. To define a mechanistic link between these two processes, various aspects of the relationship between oxidative lipid membrane damage and amyloidogenesis were characterized by chemical and physical techniques. Earlier studies of this relationship demonstrated that oxidatively damaged synthetic lipid membranes promoted amyloidogenesis. The studies reported herein specify that 4-hydroxy-2-nonenal (HNE) is produced in both synthetic lipids and human brain lipid extracts by oxidative lipid damage and that it can account for accelerated amyloidogenesis. Aβ promotes the copper-mediated generation of HNE from polyunsaturated lipids, and in turn, HNE covalently modifies the histidine side chains of Aβ. HNE-modified Aβ have an increased affinity for lipid membranes and an increased tendency to aggregate into amyloid fibrils. Thus, the prooxidant activity of Aβ leads to its own covalent modification and to accelerated amyloidogenesis. These results illustrate how lipid membranes may be involved in templating the pathological misfolding of Aβ, and they suggest a possible chemical mechanism linking oxidative stress with amyloid formation.

Promotion of Amyloid β Protein Misfolding and Fibrillogenesis by a Lipid Oxidation Product

Oxidatively damaged lipid membranes are known to promote the aggregation of amyloid beta proteins and fibril formation. Oxidative damage typically produces 4-hydroxy-2-nonenal when lipid membranes contain omega-6 polyunsaturated fatty acyl chains, and this compound is known to modify the three His residues in Abeta proteins by Michael addition. In this report, the ability of 4-hydroxy-2-nonenal to reproduce the previously observed amyloidogenic effects of oxidative lipid damage on amyloid beta proteins is demonstrated and the mechanism by which it exerts these effects is examined. Results indicate that 4-hydroxy-2-nonenal modifies the three His residues in amyloid beta proteins, which increases their membrane affinity and causes them to adopt a conformation on membranes that is similar to their conformation in a mature amyloid fibril. As a consequence, fibril formation is accelerated at relatively low protein concentrations, and the ability to seed the formation of fibrils by unmodified amyloid beta proteins is enhanced. These in vitro findings linking oxidative stress to amyloid fibril formation may be significant to the in vivo mechanism by which oxidative stress is linked to the formation of amyloid plaques in Alzheimers disease.

Synucleinopathies: a pathological and molecular review

Abstract Synucleins are a family of small, presynaptic neuronal proteins comprised of α-, β-, γ-synucleins and synoretin, of which only α-synuclein aggregates have been associated with several neurological diseases. The normal neuronal function of synucleins are presently unknown, but several activities such as lipid vesicle binding, inhibition of phospholipase D2 and protein kinase C, dopamine uptake and as a chaperone have been ascribed to α-synuclein. The role of synuclein in the etiology of neuropathology has developed from several observations. Pathologically, synuclein was identified as the major component of Lewy bodies (LBs), the hallmark inclusions of Parkinsons disease (PD), and a fragment thereof was isolated from amyloid plaques of a different neurological disease, Alzheimers disease (AD). Biochemically, recombinant α-synuclein was shown to be able to form fibrils which recapitulated the ultrastructural features of α-synuclein isolated from patients with dementia with LBs (DLB), PD and multiple system atrophy (MSA). Additionally, the identification of mutations within the synuclein gene, albeit in rare cases of familial PD, demonstrated an unequivocal link between synuclein pathology and neurodegenerative disease. The common involvement of α-synuclein in a spectrum of diseases such as PD, DLB, MSA and the LB variant of AD has led to the classification of these diseases under the umbrella term of synucleinopathies.

Amyloid Plaque-Associated Oxidative Degradation of Uniformly Radiolabeled Arachidonic Acid

Oxidative stress is a frequently observed feature of Alzheimers disease, but its pathological significance is not understood. To explore the relationship between oxidative stress and amyloid plaques, uniformly radiolabeled arachidonate was introduced into transgenic mouse models of Alzheimers disease via intracerebroventricular injection. Uniform labeling with carbon-14 is used here for the first time, and made possible meaningful quantification of arachidonate oxidative degradation products. The injected arachidonate entered a fatty acid pool that was subject to oxidative degradation in both transgenic and wild-type animals. However, the extent of its degradation was markedly greater in the hippocampus of transgenic animals where amyloid plaques were abundant. In human Alzheimers brain, plaque-associated proteins were post-translationally modified by hydroxynonenal, a well-known oxidative degradation product of arachidonate. These results suggest that several recurring themes in Alzheimers pathogenesis, amyloid β proteins, transition metal ions, oxidative stress, and apolipoprotein isoforms, may be involved in a common mechanism that has the potential to explain both neuronal loss and fibril formation in this disease.