Differential targeting of the L-type Ca2+ channel α1C (Cav1.2) to synaptic and extrasynaptic compartments in hippocampal neurons

In central nervous system neurons L-type Ca2+ Channels are involved in developmental processes, the integration and conduction of postsynaptic electric activity and synaptic plasticity. However, little is known about the channel isoforms underlying each of these functions or about the exact localization and targeting properties of the major L-type channel isoform alpha(1C) (Ca(v)1.2) in neurons. We addressed these questions using high-resolution immunofluorescence analysis of the endogenous alpha(1C) and epitope-tagged recombinant channel isoforms expressed in mouse hippocampal neurons. Endogenous alpha(1C) and surface-expressed hemagglutinin (HA)tagged alpha(1C)-HA were localized in small clusters distributed between the axon initial segment and the apical branches of the dendritic tree. The average cluster size was estimated to be eight channels per alpha(1C)-HA cluster. Analysis of the subcellular localization of alpha(1C)-HA clusters relative to known synaptic markers suggested the existence of two distinct populations of alpha(1C) clusters, extrasynaptic and synaptic, the latter associated with glutamatergic synapses in dendritic spines. Both glutamatergic and GABAergic neurons expressed alpha(1C) in the soma and dendrites. In contrast to the N-type channel GFP-alpha(1B), GFP-alpha(1C) was excluded from distal axons and nerve terminals of mature neurons. In developing neurons, however, alpha(1C) and alpha(1C)-HA were robustly expressed in the growth cone, indicating that specific targeting properties of neuronal compartments change during differentiation. Synaptic and extrasynaptic localizations of alpha(1C) correspond to putative roles of L-type Ca2+ currents in synaptic modulation and in the propagation of dendritic Ca2+ spikes, respectively The transiently expressed alpha(1C) in the growth cone may be involved in neurite extension and axonal pathfinding.