Protein and Peptide Letters - Volume 11, Issue 3, 2004

A number of human neurodegenerative (NG) diseases are known to involve the release of unfoldedpeptides and proteins, which tend to aggregate and in the process harm neuronal cells. These diseases and theassociated aggregating proteins include Alzheimer' disease (βamyloid), Parkinson' disease (αsynuclein), CJD(PrPSc), BRI dementias (Abri and Adan), Huntington' Disease (Hungtin), spinocerebellar ataxias (ataxins), andCADASIL (Notch-3 receptor). Our understanding of the pathogenic mechanisms of these NG diseases has been derived from geneticanalyses of those subsets of NG patients showing clear familial inheritance. An examination of the sequences ofthe identified genes and reading frames has identified those proteins in parentheses above; There is considerableevidence that aggregated forms of the proteins themselves, or fragments of the proteins released by proteolysis,deposit in the brain, either intra-or extraneuronally. Indeed, sequence similarities and structural similaritiessuggest common mechanisms of aggregation of the amyloids. There is accumulating evidence that the initialsoluble aggregated forms of these peptides or proteins are toxic to neurones both in vitro, in laboratory condition,and in vivo, explaining at least in part the loss of functional neurones and ensuing cognitive or neuromusculardegeneration that give rise to abnormalities in patients. Much research has been focussed on the chemical andcellular mechanisms of aggregation and toxicity, with the aim of devising suitable therapeutics to halt theneurodegenerative process. In this special volume, invited authors elucidate structural features of the aggregationmechanisms of several amyloid systems, and relate these to cellular properties and clinical symptoms.

Familial British dementia, a rare autosomal dominant neurodegenerative disorder, shares features with Alzheimer's disease, including amyloid plaque deposits, neurofibrillary tangles, neuronal loss,progressive dementia, but clinically presents with additional physical defects [1,2]. A mutation in the termination codon of the BRI gene produces a BRI precursor protein 11 amino acids longer than the wild-type protein [3,4]. Mutant and wild-type precursor proteins both may undergo furin cleavage between residues 243 and 244, producing a peptide of 34 amino acids in the case of ABri and 23 amino acids long in the case of the wild type peptide. The ABri 4kDa peptide is the main component of the amyloid deposits found in familial British dementia brains. A decamer duplication in the 3' region of the BRI gene originates the peptide Adan that is associated with dementia in Familial Danish dementia (FDD), similar to BDD clinically, but with additional hearing and eyesight loss [5] . The resulting reading frame is extended to 277 amino acid residues, and cleavage by furin releases a peptide of 34 residues, which is identical to Abri and WT in its N-terminal 22-residues, but contains a distinct C-terminal 10 residues composed of mainly hydrophobic residues. Here we demonstrate that C-terminal extensions of Abri and Adan are required to elongate initially-formed dimers to neurotoxic soluble oligomers and fibrils. In contrast, the shorter wild-type peptide does not aggregate under the same conditions and is not toxic. Conformational analyses indicate triple-β-sheet structures. Soluble nonfibrillar oligomers of oxidised ABri and reduced Adan were observed in solution (pH7.4) of peptides prior to the appearance of mature fibrils.

Extracellular fibrous amyloid deposits or intracellular inclusion bodies containing abnormal protein fibrils characterize many different neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), dementia with Lewy bodies, multiple system atrophy, Huntington's disease, and the transmissible ‘prion’; dementias. There is strong evidence from genetic, transgenic mouse and biochemical studies to support the idea that the accumulation of protein aggregates in the brain plays a seminal role in the pathogenesis of these diseases. How monomeric proteins ultimately convert to highly polymeric deposits is unknown. However, studies employing, synthetic, cell-derived and purified recombinant proteins suggest that amyloid proteins first come together to form soluble low n-oligomers. Further association of these oligomers results in higher molecular weight assemblies including so-called ‘protofibrils’ and ‘ADDLs’ and these eventually exceed solubility limits until, finally, they are deposited as amyloid fibrils. With particular reference to AD and PD, we review recent evidence that soluble oligomers are the principal pathogenic species that drive neuronal dysfunction.

Lesions known as Lewy bodies (LBs) and Lewy neurites (LNs) characterise brains of Parkinson's disease (PD) patients. Intracellular aggregation of α-synuclein (α-syn) appears to play a key role in the generation of LBs and LNs. Such aggregation in the presence of redox metals may initiate Fenton reaction-mediated generation of reactive oxygen species (ROS). ROS thus generated may result in cytotoxic mechanisms such as the induction of DNA single-strand breaks.

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders caused by proteins with expanded polyQ regions. Although the pathological mechanisms of these diseases have not yet been elucidated, the processes of protein misfolding and aggregation seem to be a direct cause of neurodegeneration. Detailed structural information on polyQ proteins is therefore essential in order to understand the mechanisms underlying pathogenesis and to design therapeutic strategies. In the past decade, several studies have investigated the structural properties of polyQ proteins and the molecular basis of aggregation and fibre formation. The results obtained in these studies are reviewed here.

Transmissible spongiform encepahalopathies (TSEs) are fatal diseases that damage the central nervous system. TSEs are unique in that they may be inherited, infectious or spontaneous. The central pathogenic agent is thought to be a conformationally distinct form (PrPSc) of the endogenous prion protein(PrPc), which is high in beta-sheet content and is resistant to proteases; infectivity is thought to involve formation of PrPSc via imprinting of abnormal conformation on the normal form of the protein (PrPc) by seeds of PrPSc. A number of compounds found to inhibit the conversion of PrPc to PrPSc have been proposed as therapeutics to halt TSEs.

β-amyloid, the 39-43 amino acid peptide fragment originating from amyloid precursor protein, is today, generally accepted as the biological entity responsible for causing the debilitating human disorder Alzheimer's disease. Understanding the exact biological effects of β-amyloid in vitro and in vivo is clearly important to provide therapeutic strategies for the disease. Recent in vitro studies have focused on the production of reactive oxygen species by aggregating β- amyloid, but the cellular effects of β-amyloid induced reactive oxygen species production have not been fully elucidated.

α-Synuclein is a major component of Lewy bodies in Parkinson's disease and is found associated with several other forms of dementia. As with other neurodegenerative diseases, the ability of α-synuclein to aggregate and form fibrillar deposits seems central to its pathology. We have defined a sequence within the NAC region of α-synuclein that is necessary for aggregation. Exploitation of chemically modified analogues of this peptide may produce inhibitors of aggregation.