Amyloids here, amyloids there…What’s wrong with them?

Abstract: Amyloid formation is inherent property of proteins which under certain circumstances can become a pathologic feature of a group of diseases called amyloidosis. There are about 30 known human amyloidosis and more than 27 identified proteins involved in these pathologies.  Besides these proteins, there are a growing number of proteins non-related to diseases shown to form amyloid-like structures in vitro, which make them excellent tools for studying amyloid formation mechanisms, physicochemical properties of different amyloid species and the nature of their influence on tissues and cells.  It is important to understand the mechanisms by which amyloids interact with different types of cells, as the leading hypothesis in amyloid field suggests that amyloids and especially their intermediate states are the main harmful, toxic species causing tissue and cell degeneration.Using de-novo synthesized protein albebetin as a model of amyloidogenic protein, we demonstrated that it forms amyloid-like structures under physiological conditions (pH 7 and 37°C). During aggregation it forms 2 different types of intermediate oligomers — cross-b sheet containing and lacking β-sheet oligomers. Only the former induces cellular toxicity in a dose dependent manner. Further aggregation leads to the formation of fully mature amyloid-like fibrils, which are not toxic to the cells during studied period of incubation.Another model protein in our studies was hen egg white lysozyme, which readily forms amyloid under denaturing conditions (pH 2,2 and 57°C). In contrast to albebetin and many other proteins reported in the literature, we showed that both oligomers and mature fibrils from hen lysozyme affect cell viability. Targeting different mechanisms involved in cellular death, we revealed that oligomers induce slow and apoptotic-like cell death, while mature fibrils cause rapid and mainly necrotic-like cellular death.   One of the important aspects of amyloid studies is to develop measures for inhibiting or re-directing the process of amyloid formation to abolish or neutralize toxic amyloid species. Among the agents having inhibitory or modulatory properties small, phenol containing molecules are widely studied. We investigated the effect of the novel nootropic drug noopept on amyloid formation process of α-synuclein, as this drug is a small dipeptide containing a phenol ring. We showed that noopept is able to modulate amyloid formation process by accelerating it to rapid conversion of α-synuclein into fully mature fibrils, thus eliminating the stage of population of toxic oligomeric species.  Using wide range of cytotoxicity assays we showed that amyloid-like fibrils formed in the presence of noopept have no cytotoxic properties.  As this medicine is becoming popular and freely available in some countries as a cognitive enhancer, neuroprotective and nootropic agent, further detailed investigations and clinical trials are needed to assess the safety and benefit of noopept in particular for the patients with amyloid related neurodegenerative diseases (such as Parkinson’s or Alzheimer’s diseases).    While in vitro models are useful to study some specific aspects of protein aggregation, their properties and effects on cell viability, it is very difficult or practically impossible to create an absolutely accurate model of in vivo situation. Therefore, it is important to turn to in vivo/ex vivo studies to relate the knowledge accumulated from in vitro studies to the real situation in the body.Using human brain hippocampus tissues from individuals with Alzheimer’s disease, we found that besides well-known and widely accepted main pathological hallmark — Ab peptide deposition, S100A9 and S100A8 pro-inflammatory calcium-binding proteins are also localized in the plaques and in surrounding tissues and very explicitly co-localized with Ab. Moreover, we found the presence of S100A9 within the neuronal cells, which has not been reported before and can be an important clue for understanding the mechanisms of neurodegeneration. In vitro cytotoxicity studies showed that S100A9 protein can efficiently induce cytotoxicity when added exogenously to the neuronal cell culture. These findings suggest that S100A8 and S100A9 proteins play an important role in Alzheimer’s pathology, and potentially can be candidates for the amyloid plaque formation and neurodegeneration. Whether they are associated with inflammatory processes underlying the early onset of disease or produced and accumulated as a consequence of A-beta induced pathology remain to be clarified.We found that Alzheimer’s disease is not the only pathology associated with A-beta and S100A9 deposition in a form of plaques. Immunohistochemical studies of an aortic valve surgically removed from a patient with aortic stenosis revealed plaque-like structures positively stained with A-beta and S100A9 proteins. These areas are also positively stained with fibril-specific antibodies as well as with Congo red, which also shows very distinct apple-green birefringence under the polarized light. Besides, there is intracellular localization and co-localization of both proteins in interstitial cells throughout the whole fibrous tissue of the valve. The presented case report is the first finding suggesting inflammatory protein S100A9 as well as A-beta peptide as potential candidates for amyloid formation in aortic stenosis valves.  We suggest that there is a specific interaction between A-beta and S100A9 during amyloid formation, which can be involved in amyloid-associated pathology in various tissues and organs in the body, which can potentially be caused by inflammatory processes, particularly by its chronic, long lasting forms.