Review: formation and properties of amyloid-like fibrils derived from alpha-synuclein and related proteins.

Synucleins are small proteins that are highly expressed in brain tissue and are localised at presynaptic terminals in neurons. alpha-Synuclein has been identified as a component of intracellular fibrillar protein deposits in several neurodegenerative diseases, and two mutant forms of alpha-synuclein have been associated with autosomal-dominant Parkinson's Disease. A fragment of alpha-synuclein has also been identified as the non-Abeta component of Alzheimer's Disease amyloid. In this review we describe some structural properties of alpha-synuclein and the two mutant forms, as well as alpha-synuclein fragments, with particular emphasis on their ability to form beta-sheet on ageing and aggregate to form amyloid-like fibrils. Differences in the rates of aggregation and morphologies of the fibrils formed by alpha-synuclein and the two mutant proteins are highlighted. Interactions between alpha-synuclein and other proteins, especially those that are components of amyloid or Lewy bodies, are considered. The toxicity of alpha-synuclein and related peptides towards neurons is also discussing in relation to the aetiology of neurodegenerative diseases.

BACE1 Polymorphisms Do Not Influence Platelet Membrane β-secretase Activity or Genetic Susceptibility for Alzheimer’s Disease in the Northern Irish Population

ß-Site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is a biological and positional candidate gene for Alzheimer’s disease (AD). BACE1 is a protease that catalyses APP cleavage at the ß-secretase site. We evaluated all common and putatively functional polymorphisms in the genomic region encompassing BACE1 for an association with AD, and for functional effects on platelet ß-secretase activity. Tag SNPs (n = 10) derived from phase II of the International HapMap Project, and a nonsynonymous variant, were successfully genotyped in 901 Caucasian individuals from Northern Ireland using Sequenom iPLEX and TaqMan technologies. APOE genotyping was performed by PCR-RFLP. Platelet membrane ß-secretase activity was assayed in a subset of individuals (n = 311). Hardy–Weinberg equilibrium was observed for all variants. Evidence for an association with AD was observed with multi-marker haplotype analyses (P = 0.01), and with rs676134 when stratified for APOE genotype (P = 0.02), however adjusting for multiple testing negated the evidence for association of this variant with AD. ?2 analysis of genotype and allele frequencies in cases versus controls for individual SNPs revealed no evidence for association (5% level). No genetic factors were observed that significantly influenced platelet membrane ß-secretase activity. We have selected an appropriate subset of variants suitable for comprehensive genetic investigation of the BACE1 gene. Our results suggest that common BACE1 polymorphisms and putatively functional variants have no significant influence on genetic susceptibility to AD, or platelet ß-secretase activity, in this Caucasian Northern Irish population.

Impairments of hippocampal synaptic plasticity induced by aggregated beta-amyloid (25-35) are dependent on stimulation-protocol and genetic background

The aggregation of beta-amyloid to plaques in the brain is one of the hallmarks of Alzheimer disease (AD). Numerous studies have tried to elucidate to what degree amyloid peptides play a role in the neurodegenerative developments seen in AD. While most studies report an effect of amyloid on neural activity and cognitive abilities of rodents, there have been many inconsistencies in the results. This study investigated to what degree the different genetic backgrounds affect the outcome of beta-amyloid fragment (25-35) on synaptic plasticity in vivo in the rat hippocampus. Two strains, Wistar and Lister hooded rats, were tested. In addition, the effects of a strong (600 stimuli) and a weak stimulation protocol (100 stimuli) on impairments of LTP were analysed. Furthermore, since the state of amyloid aggregation appears to play a role in the induction of toxic processes, it was tested by dual polarisation interferometry to what degree and at what speed beta-amyloid (25-35) can aggregate in vitro. It was found that 100 nmol beta-amyloid (25-35) injected icv did impair LTP in Wistar rats when using the weak but not the strong stimulation protocol (P <0.001). One-hundred nano mole of the reverse sequence amyloid (35-25) had no effect. LTP in Lister Hooded rats was not impaired by amyloid at any stimulation protocol. The aggregation studies showed that amyloid (25-35) aggregated within hours, while amyloid (35-25) did not. These results show that the genetic background and the stimulation protocol are important variables that greatly influence the experimental outcome. The fact that amyloid (25-35) aggregated quickly and showed neurophysiological effects, while amyloid (35-25) did not aggregate and did not show any effects indicates that the state of aggregation plays an important role in the physiological effects.

A novel reciprocal and biphasic relationship between membrane cholesterol and beta-secretase activity in SH-SY5Y cells and in human platelets.

Research into the cause of Alzheimer's disease (AD) has identified strong connections to cholesterol. Cholesterol and cholesterol esters can modulate amyloid precursor protein (APP) processing, thus altering production of the A beta peptides that deposit in cortical amyloid plaques. Processing depends on the encounter between APP and cellular secretases, and is thus subject to the influence of cholesterol-dependent factors including protein trafficking, and distribution between membrane subdomains. We have directly investigated endogenous membrane beta-secretase activity in the presence of a range of membrane cholesterol levels in SH-SY5Y human neuroblastoma cells and human platelets. Membrane cholesterol significantly influenced membrane beta-secretase activity in a biphasic manner, with positive correlations at higher membrane cholesterol levels, and negative correlations at lower membrane cholesterol levels. Platelets from individuals with AD or mild cognitive impairment (n = 172) were significantly more likely to lie within the negative correlation zone than control platelets (n = 171). Pharmacological inhibition of SH-SY5Y beta-secretase activity resulted in increased membrane cholesterol levels. Our findings are consistent with the existence of a homeostatic feedback loop between membrane cholesterol level and membrane beta-secretase activity, and suggest that this regulatory mechanism is disrupted in platelets from individuals with cognitive impairment.