Current Medicinal Chemistry - Volume 9, Issue 19, 2002

Amyloid beta (Aβ) protein is the key component of amyloid plaques in Alzheimer's disease brain whereas stefin B is an intracellular cysteine proteinase inhibitor, broadly distributed in different tissue and recently reported to form amyloid fibrils in vitro. By reducing the pH to 4.6, the native conformation of both polypeptides are changed into less ordered metastable intermediates that are stabilized by formation of the more stable fibrils. In Aβ, the Glu at position 11 was found to be responsible for the conformational change at pH 4.6. Metal ions, including copper and zinc, could also induce conformational changes of Aβ at neutral pH. The acid modified Aβ conformer exhibited protease K resistance, preferential internalization and accumulation in the human glial cells. In stefin B, reducing the pH to pH 3.3 results in another intermediate of the moltenglobule type which also leads to amyloid fibril formation. Multiple sequence alignment revealed distinct similarities of Aβ (1-42) peptide, stefin B (13 to 61 residues) and prion fragment (90 to 144 residues).

The process of amyloid fibrils formation is a common mechanism of a large number of unrelated infectious, genetic and spontaneous diseases. A partial list includes the bovine spongiform encephalopathy (BSE), Alzheimer's diseases, Type II diabetes, Creutzfeldt-Jakob disease, and various unrelated amyloidosis diseases. In spite of its significant clinical importance, the mechanism of fibrillization is not fully understood. This review discusses the recent advancements in the mechanistic studies of amyloid formation by the use peptide fragments and analogues of amyloid-forming proteins and polypeptides. The use of short peptide shed much light of the mechanism of amyloid fibrillization. Recent studies clearly prove that very short peptide fragments (as short as pentapeptides) can form well-ordered amyloidal structures. Therefore, the molecular recognition and self-assembly process that lead to the formation of order structures is being mediated by small structural elements. Analysis of short amyloid-related fragment by the use of an alaninescan and sequence analysis of a variety of unrelated peptide and protein fragments suggest that aromatic interaction may play a central role in the process of amyloid formation. Inhibitors that are based on the short aromatic elements already demonstrated clear potency in arresting the process of amyloid fibrils formation. Taken together, the recent advancement in the mechanistic understanding of the process of amyloid fibrils formation has a major importance in the development of inhibitors of fibrillization that may serve as future therapeutic means to treat amyloid diseases.

More and more evidence shows that Alzheimer's and prion-related diseases belong to the family of conformational diseases characterized by protein self-association and tissue deposition as amyloid fibrils. Regardless of the nature of the protein constituent, all forms of amyloid are stable assemblies based on noncovalent interactions between subunits of crossed β-sheet structure. Understanding the mechanism and molecular details of the pathological conformational conversion of amyloidogenic proteins may be of importance to the development of approaches towards prevention and treatment of such diseases. We previously found that monoclonal antibodies (mAbs) interact at strategic sites where protein unfolding is initiated, thereby stabilizing the protein and preventing further precipitation. Indeed, site-directed mAbs raised against the N-terminal region of Alzheimer's β-peptide (AβP) disaggregate AβP fibrils, restore peptide solubility and prevent its neurotoxic effects. Similarly, selected mAbs raised against the human prion peptide 106-126 modulate conformational changes occurring in the prion peptide exposed to aggregating conditions, preventing its aggregation and related neurotoxicity on cultivated neural-like cells. All these data and related procedures bring more attention to the immunological concept in the treatment of conformational diseases, and the recent performance of such antibodies in transgenic mice, as a model for human diseases, suggests the development of vaccination approaches against such diseases.

The accumulation of proteinaceous deposits has been recognised to occur in several neurodegenerative conditions including Prion diseases, Alzheimer's disease, Parkinson's disease, and Huntington's disease. Over the last two decades interest in these conditions has increased markedly, fuelled partially by an increasing prevalence of these diseases in the Western world. Evidence indicates that anomalous protein misfolding and aggregation, with an accompanying “toxic gain of function” is central to the neuropathogenesis of these diseases. An increased understanding of the similarities and differences in the production, aggregation and accumulation of the respective proteins involved in these diseases, and the associated mechanisms of neurodegeneration, should aid in the development of new therapeutic agents to treat this group of related disorders.

Main neuropathological hallmarks of Alzheimer's disease (AD) and other neurodegenerative disorders are the deposition of neurofibrillary tangles consisting of abnormally phosphorylated protein tau and of senile plaques largely containing insoluble ß-amyloid peptides (Aß), containing up to 43 amino acid residues derived from the ß-amyloid precursor protein. Such Aß-sheets become visible by using suitable histochemical methods.Molecular simulation showed that the central, α-helical, lipophilic, antigenic folding domain of the Aßpeptide loop is a promising molecular target of ß-sheet breakers that thus prevent the polymerization of Aßinto aggregates. It seems that di- and tetramers of Aß-peptides have a ß-barrel- like structure.In the present review, an optimized neural network analysis was applied to recognize possible structureactivity relationships of peptidomimetic ß-sheet breakers. The anti-aggregatory potency of ß-sheet breakers largely depends upon their total, electrostatic, and hydration energy as derived from their geometry-optimized conformations using the hybrid Gasteiger-molecular mechanics approach.Moreover, we also summarize peptide misfolding in several disorders with distinct clinical symptoms, including prion diseases and a broad variety of systemic amyloidoses, as the common pathogenic step driving these disorders. In particular, conversion of nontoxic α-helix / random-coils to ß-sheet conformation was recognized as being critical in producing highly pathogenic peptide assemblies. Whereas conventional pharmacotherapy of AD is mainly focused on restoring cholinergic activity and diminishing inflammatory responses as a consequence of amyloid accumulation, we here survey potential approaches aimed at preventing or reserving the transition of neurotoxic peptide species from α- helical / random coil to ß-sheet conformation and thus abrogating their effects in a broad variety of disorders.