Heterogeneous amyloid-formed ion channels as a common cytotoxic mechanism: implications for therapeutic strategies against amyloidosis.

The amyloidoses consist of human and animal chronic, progressive, and sometimes fatal diseases that are characterized by the deposition of insoluble proteinaceous amyloid fibrils in various tissues. Despite the biochemical diversity of amyloids, they share certain properties. The amphipathic and the charged nature of many amyloid-forming peptides point to their intrinsic ability to form diverse beta-sheet-based aggregates and channel types in negatively charged membranes. We hypothesize that the formation of heterogeneous channels represents a common cytotoxic mechanism that accentuates the changes in the signal transduction that underlie amyloid-induced cell malfunction. One group of amyloid-forming peptides that could mediate their action via the formation of heterogeneous channels includes the extensively examined prions and amyloid beta protein that are associated with conformational neurodegenerative diseases. The aim of this study is to examine heterogeneous channels formed in bilayers with amyloid-forming peptides as a common mechanism of malfunction of signal transduction. The observed amyloid-formed channel types include the following. (1) Natriuretic peptides: (i) 68-pS H2O2- and Ba2+-sensitive channel with fast kinetics. The fast channel had three modes (spike mode, burst mode, and open mode), which differ in their kinetics but not in their conductance properties; (ii) a 273-pS inactivating large conductance channel; and (iii) a 160-pS transiently activated channel. (2) Prions: (i) a 140-pS GSSG- and TEA-sensitive channel with fast kinetics; (ii) a 41-pS dithiothreitol (DTT)-sensitive channel with slow kinetics; (iii) a 900 to 1444-pS large channel. (3) Amyloid beta protein: (i) a 17 to 63-pS AbetaP[1-40]-formed "bursting" fast cation channel, (ii) the AbetaP[1-40]-formed "spiky" fast cation channel with a similar kinetics to the "bursting" fast channel except for the absence of the long intraburst closures, (iii) 275-pS AbetaP[1-40]-formed medium conductance channel, and (iv) 589- to 704-pS AbetaP[1-40]-formed inactivating large conductance channel. This heterogeneity is one of the most common features of these charged cytotoxic amyloid-formed channels, reflecting these channels' ability to modify multiple cellular functions. Although the diversity of these aggregated-peptide-formed channels may indicate that a stochastic mechanism governs their formation, the fact that certain channel types are often observed point to preferential channel protein conformations. In addition, the fact that other amyloids have similar structural properties (e.g. hydrophobicity, charged residues, and beta-structural linkages, suggests that, despite the intrinsic ability to form diverse conformations, certain conformations and, hence, certain channel types could be a common pathologic conformation among these amyloid-forming peptides. It is concluded that conformation-based channel diversity is an important mechanism for enhancing the toxicity of amyloid-forming peptides. The cytotoxic nature of these self-associated beta-based protein channels suggests that under normal physiological conditions cells employ well-evolved protective mechanisms against seeding and/or propagation of channel-forming peptides; for example, (a) compartmentalization of these peptides as membrane bound in internal vesicles and/or (b) degradation of these peptides by enzymes. The pharmacological diversity of the amyloid-forming channels implies that multiple therapeutic interventions may be necessary for blocking and reversing heterogeneous channel formations and preventing their associated diseases.