Displacement currents associated with the insertion of Alzheimer disease amyloid beta-peptide into planar bilayer membranes

The role of endogenous amyloid beta-peptides as causal factors of neurodege nerative diseases is largely unknown. We have previously reported that inte ractions between Alzheimer's disease A beta P[1-40] peptide in solution and planar bilayer membranes made from anionic phospholipids lead to the forma tion of cation-selective channels. We now find and report here that the spo ntaneous insertion of free A beta P[1-40] across the bilayer can be detecte d as an increase in bilayer capacity. To this end we recorded the displacem ent currents across planar bilayers (50 mM KCI on both sides) in response t o sudden displacements of the membrane potential, from -300 to 300 mV in 20 -mV increments. To monitor the A beta P[1-40]-specific displacement current s, we added A beta P[1-40] (1-5 mu M) to the solution on either side of the membrane and noted that the direction of the displacement current depended on the side with A beta P[1-40], The size of the A beta P[1-40]-specific c harge displaced during a pulse was always equal to the charge returning to the original configuration after the pulse, suggesting that the dipole mole cules are confined to the membrane. As a rule, the steady-state distributio n of the A beta P[1-40]-specific charges within the bilayer could be fit by a Boltzmann distribution. The potential at which the charges were found to be equally distributed (V-o) were similar to -135 mV (peptide added to the solution in the compartment electrically connected to earth) and 135 mV (p eptide added to the solution connected to the input of the amplifier). The A beta P[1-40]-specific transfer of charge reached a maximum value (Q(max)) when the electrical potential of the side containing the amyloid beta-prot ein was taken to either -300 or 300 mV. For a circular membrane of 25-mu m radius (similar to 2000 mu m(2)), the total A beta P[1-40]-specific charge Q(max) was estimated as 55 fC, corresponding to some 170 e.c./mu m(2). Rega rdless of the side selected for the addition of A beta P[1-40], at V-o the charge displaced underwent an e-fold change for a similar to 27-mV change i n potential. The effective valence (a) of the A beta P[1-40] dipole (i.e., the actual valence Z multiplied by the fraction of the electric field chi a cting on the dipole) varied from 1 to 2 electronic charges. We also tested, with negative results, the amyloid peptide with the reverse sequence (A be ta P[40-1]). These data demonstrate that A beta P[1-40] molecules can span the low dielectric domain of the bilayer, exposing charged residues (D-1, E -3, R-5, H-6, D-7, E-11, H-13, and H-14) to the electric field. Thus the A beta P[1-40] molecules in solution must spontaneously acquire suitable conf ormations (beta-pleated sheet) allowing specific interactions with charged phospholipids. Interestingly, the domain from residues 676 to 704 in the AP P(751) is homologous with the consensus sequence for lipid binding found in other membrane proteins regulated by anionic phospholipids.