Expression and subunit interaction of voltage-dependent Ca²⁺ channels in PC12 cells

Nerve growth factor (NGF)-induced differentiation in PC12 cells is accompanied by changes in the expression of voltage-dependent Ca²⁺ channels. Ca²⁺ channels are multimeric complexes composed of at least three subunits (α₁, β, and α₂δ) and are involved in neuronal migration, gene expression, and neurotransmitter release. Although attempts have been undertaken to elucidate NGF regulation of Ca²⁺ channel expression, the changes in subunit composition of these channels during differentiation still remain uncertain. In the present study, patch-clamp recordings show that in addition to the previously documented L-type and N-type Ca²⁺ currents, undifferentiated PC12 cells also express an ω-agatoxin-IVA-sensitive (P/Q- type) component. In addition, the corresponding mRNA encoding the pore- forming α₁ subunits for these channels (C, B, and A, respectively) was detected. Likewise, mRNA for three distinct auxiliary β subunits (1, 2, 3) were also found, β₃ protein being dominantly expressed. Immunoprecipitation experiments show that the N-type Ca²⁺ channel is associated with either a β₂ or β₃ subunit and that NGF increases the channel expression without affecting its β subunit association. These results (1) indicate that the diversity of Ca²⁺ currents in PC12 cells arise from the expression of three distinct α₁ and three different subunit genes; (2) support a model for heterogenous β subunit association of the N-type Ca²⁺ channel in a single cell type; and (3) suggest that the regulation of the N-type Ca²⁺ channel during NGF-mediated differentiation involves an increase in the number of functional channels with no apparent changes in subunit composition.