Reverse engineering the L-type Ca²⁺ channel α_1c subunit in adult cardiac myocytes using novel adenoviral vectors

The α_1c subunit of the cardiac L-type Ca²⁺ channel, which contains the channel pore, voltage- and Ca²⁺-dependent gating structures, and drug binding sites, has been well studied in heterologous expression systems, but many aspects of L-type Ca²⁺ channel behavior in intact cardiomyocytes remain poorly characterized. Here, we develop adenoviral constructs with E1, E3 and fiber gene deletions, to allow incorporation of full-length α_1c gene cassettes into the adenovirus backbone. Wild-type (α_1c-wt) and mutant (α_1c-D-) Ca²⁺ channel adenoviruses were constructed. The α_1c-D- contained four point substitutions at amino acid residues known to be critical for dihydropyridine binding. Both α_1c-wt and α_1c-D- expressed robustly in A549 cells (peak L-type Ca²⁺ current (I_CaL) at 0 mV: α_1c-wt -9.94 ± 1.00 pA/pF, n = 9; α _1c-D- -10.30 pA/pF, n = 12). I_CaL carried by α_1c-D- was markedly less sensitive to nitrendipine (IC ₅₀ 17.1 μM) than α_1c-wt (IC₅₀ 88 nM); a feature exploited to discriminate between engineered and native currents in transduced guinea-pig myocytes. 10 μM nitrendipine blocked only 51 ± 5% (n = 9) of I_CaL in α_1c-D--expressing myocytes, in comparison to 86 ± 8% (n = 9) of I_CaL in control myocytes. Moreover, in 20 μM nitrendipine, calcium transients could still be evoked in α_1c-D--transduced cells, but were largely blocked in control myocytes, indicating that the engineered channels were coupled to sarcoplasmic reticular Ca²⁺ release. These α_1c adenoviruses provide an unprecedented tool for structure-function studies of cardiac excitation-contraction coupling and L-type Ca²⁺ channel regulation in the native myocyte background.