Diversity of amyloid beta 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 (A betaP) 1-40 into lipid membranes. Based o n the conductance, kinetics, selectivity, and pharmacological properties, t he following A betaP [1-40]-formed ion channels have been identified: (i) The A betaP[1-40]-formed "bursting" fast cation channel was characteriz ed by (a) a single channel conductance of 63 pS (250/50 mM KCl cis/trans) a t +140 mV, 17 pS (250/50 mM KC/ cis/trans) at -160 mV, and the nonlinear cu rrent-voltage relationship drawn to a third-order polynomial, (b) selectivi ty 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 A betaP[1-40]-formed "spiky" fast cation channel was characterized by (a) a similar kinetics to the "bursting" fast channel with exception fo r the absence of the long intraburst closures, (b) single channel conductan ce of 63 pS (250/50 KCI) at +140 mV 17 pS (250/50 KCI) at -160 mV, the curr ent-voltage relationship nonlinear drawn to a third-order polynomial fit, a nd (c) selectivity sequence P-Rb>P-K>P-Cs>P-Na>P-Li = 1.3:1.0:0.46:0.40:027 . (iii) The A betaP[1-40]-formed medium conductance channel was characterized by (a) 275 pS (250/50 mM KCL cis/trans) at +140 mV and 19 pS (250/50 mM KC l cis/trans) at -160 mV and (b) inactivation at V(m)s more negative than -1 20 and more positive than + 120 mV. (iv) The A betaP[1-40] -formed inactivating large conductance channel was c haracterized by (a) fast and slow modes of opening to seven multilevel cond uctances ranging between 0-589 pS (in 250/50 mM KCl) at +140 mV and 0-704 p S (in 250/50 mM KCI) at -160 mV, (b)The fast mode which had a conductance o f <250 pS was voltage dependent. The inactivation was described by a bell-s haped curve with a peak lag time of 7.2 s at +36 mV. The slow mode which ha d 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 se quence P-K>F-Cs>P-Na>P-Li = 1.0:0.94:0.87:0.59. The value of P-K/P-choline for the blow 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 reduct ion 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 "s piky" fast channel and the inactivating medium conductance channel are dist inct, it is possible that they are intermediate configurations of yet anoth er configuration underlying the inactivating large conductance channel. It is proposed that this heterogeneity is one of the most common features of t hese positively-charged cytotoxic amyloid-formed channels reflecting these channels ability to modify multiple cellular functions. 3. Furthermore, the formation of beta -sheet based oligomers could be an im portant common step in the formation of cytotoxic amyloid channels.