Subunit Regulation of the Human Brain α_1E Calcium Channel

Abstract.

The α₁ subunit coding for the human brain type E calcium channel (Schneider et al., 1994) was expressed in Xenopus oocytes in the absence, and in combination with auxiliary α₂δ and β subunits. α_1E channels directed with the expression of Ba²⁺ whole-cell currents that completely inactivated after a 2-sec membrane pulse. Coexpression of α_1E with α_2bδ shifted the peak current by +10 mV but had no significant effect on whole-cell current inactivation. Coexpression of α_1E with β_2a shifted the peak current relationship by −10 mV, and strongly reduced Ba²⁺ current inactivation. This slower rate of inactivation explains that a sizable fraction (40 ± 10%, n= 8) of the Ba²⁺ current failed to inactivate completely after a 5-sec prepulse. Coinjection with both the cardiac/brain β_2a and the neuronal α_2bδ subunits increased by ≈10-fold whole-cell Ba²⁺ currents although coinjection with either β_2a or α_2bδ alone failed to significantly increase α_1E peak currents. Coexpression with β_2a and α_2bδ yielded Ba²⁺ currents with inactivation kinetics similar to the β_2a induced currents, indicating that the neuronal α_2bδ subunit has little effect on α_1E inactivation kinetics. The subunit specificity of the changes in current properties were analyzed for all four β subunit genes. The slower inactivation was unique to α_1E/β_2a currents. Coexpression with β_1a, β_1b, β₃, and β₄, yielded faster-inactivating Ba²⁺ currents than currents recorded from the α_1E subunit alone. Furthermore, α_1E/α_2bδ/β_1a; α_1E/α_2bδ/β_1b; α_1E/α_2bδ/β₃; α_1E/α_2bδ/β₄ channels elicited whole-cell currents with steady-state inactivation curves shifted in the hyperpolarized direction. The β subunit-induced changes in the properties of α_1E channel were comparable to modulation effects reported for α_1C and α_1A channels with β₃≈β_1b > β_1a≈β₄≫β_2a inducing fastest to slowest rate of whole-cell inactivation.