Giant multilevel cation channels formed by Alzheimer disease amyloid beta-protein [A beta P-(1-40)] in bilayer membranes.
We have recently shown that the Alzheimer disease 40-residue amyloid beta-protein [A beta P-(1-40)] can form cation-selective channels when incorporated into planar lipid bilayers by fusion of liposomes containing the peptide. Since A beta P-(1-40) comprises portions of the putative extracellular and membrane-spanning domains of the amyloid precursor protein (APP751), we suggested that the channel-forming property could be the underlying cause of amyloid neurotoxicity. The peptide has been proposed to occur in vivo in both membrane-bound and soluble forms, and we now report that soluble A beta P-(1-40) can also form similar channels in solvent-free lipid bilayers formed at the tip of a patch pipet, as well as in the planar lipid bilayer system. As in the case of liposome-mediated incorporation, the amyloid channel activity in the patch pipet exhibits multiple conductance levels between 40 and 400 pS, cation selectivity, and sensitivity to tromethamine (Tris). Further studies with A beta P channels incorporated into planar lipid bilayers from the liposome complex have also revealed that the channel activity can express spontaneous transitions to a much higher range of conductances between 400 and 4000 pS. Under these conditions, the amyloid channel continues to be cation selective. Amyloid channels were insensitive to nitrendipine at either conductance range. We calculate that if such channels were expressed in cells, the ensuing ion fluxes down their electrochemical potential gradients would be homeostatically dissipative. We therefore interpret these data as providing further support for the concept that cell death in Alzheimer disease may be due to amyloid ion-channel activity.