Diversity of amyloid β protein fragment [1-40]-formed channels

1. The lipid bilayer technique was used to characterize the biophysical and pharmacological properties of several ion channels formed by incorporating amyloid beta protein fragment (AβP) 1-40 into lipid membranes. Based on the conductance, kinetics, selectivity, and pharmacological properties, the following AβP[1-40]-formed ion channels have been identified: (i) The AβP[1-40]-formed "bursting" fast cation channel was characterized by (a) a single channel conductance of 63 pS (250/50 mM KCl cis/trans) at +140 mV, 17 pS (250/50 mM KCl cis/trans) at -160 mV, and the nonlinear current-voltage relationship drawn to a third-order polynomial, (b) selectivity sequence P_K > P_Na > P_Li = 1.0:0.60:0.47, (c) P_o of 0.22 at 0 mV and 0.55 at +120 mV, and (d) Zn²⁺-induced reduction in current amplitude, a typical property of a slow block mechanism. (ii) The AβP[1-40]-formed "spiky" fast cation channel was characterized by (a) a similar kinetics to the "bursting" fast channel with exception for the absence of the long intraburst closures, (b) single channel conductance of 63 pS (250/50 KCl) at +140 mV 17 pS (250/50 KCl) at -160 mV, the current-voltage relationship nonlinear drawn to a third-order polynomial fit, and (c) selectivity sequence P_Rb > P_K > P_Cs > P_Na > P_Li = 1.3:1.0:0.46:0.40:0.27. (iii) The AβP[1-40]-formed medium conductance channel was charcterized by (a) 275 pS (250/50 mM KCl cis/trans) at +140 mV and 19 pS (250/50 mM KCl cis/trans) at -160 mV and (b) inactivation at V_ms more negative than -120 and more positive than +120 mV. (iv) The AβP[1-40]-formed inactivating large conductance channel was characterized by (a) fast and slow modes of opening to seven multilevel conductances ranging between 0-589 pS (in 250/50 mM KCl) at +140 mV and 0-704 pS (in 250/50 mM KCl) at -160 mV, (b) The fast mode which had a conductance of <250 pS was voltage dependent. The inactivation was described by a bell-shaped curve with a peak lag time of 7.2 s at +36 mV. The slow mode which had a conductance of >250 pS was also voltage dependent. The inactivation was described by a bell-shaped curve with a peak lag time of 7.0 s at -76 mV, (c) the value of P_K/P_choline for the fast mode was 3.9 and selectivity sequence P_K > P_Cs > P_Na > P_Li = 1.0:0.94:0.87:0.59. The value of P_K/P_choline for the slow mode was 2.7 and selectivity sequence P_K > P_Na > P_Li > P_Cs = 1.0:0.59:0.49:0.21, and (d) asymmetric blockade with 10 mM Zn²⁺-induced reduction in the large conductance state of the slow mode mediated via slow block mechanism. The fast mode of the large conductance channel was not affected by 10 mM Zn²⁺. 2. It has been suggested that, although the "bursting" fast channel, the "spiky" fast channel and the inactivating medium conductance channel are distinct, it is possible that they are intermediate configurations of yet another configuration underlying the inactivating large conductance channel. It is proposed that this heterogeneity is one of the most common features of these positively-charged cytotoxic amyloid-formed channels reflecting these channels ability to modify multiple cellular functions. 3. Furthermore, the formation of β-sheet based oligomers could be an important common step in the formation of cytotoxic amyloid channels.