Molecular Nature of Anomalous L-Type Calcium Channels in Mouse Cerebellar Granule Cells

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Abstract
Single-channel analysis revealed the existence of neuronal L-type Ca2+ channels (LTCCs) with fundamentally different gating properties; in addition to LTCCs resembling cardiac channels, LTCCs with anomalous gating were identified in a variety of neurons, including cerebellar granule cells. Anomalous LTCC gating is mainly characterized by long reopenings after repolarization following strong depolarizations or trains of action potentials. To elucidate the unknown molecular nature of anomalous LTCCs, we performed single-channel patch-clamp recordings from cerebellar granule cells of wild-type, Cav1.3−/− and Cav1.2DHP−/− [containing a mutation in the Cav1.2 α1 subunit that eliminates dihydropyridine (DHP) sensitivity] mice. Quantitative reverse transcription-PCR revealed that Cav1.2 accounts for 89% and Cav1.3 for 11% of the LTCC transcripts in wild-type cerebellar granule cells, whereas Cav1.1 and Cav1.4 are expressed at insignificant levels. Anomalous LTCCs were observed in neurons of Cav1.3−/− mice with a frequency not different from wild type. In the presence of the DHP agonist (+)-(S)-202-791, the typical prepulse-induced reopenings of anomalous LTCCs after repolarization were shorter in Cav1.2DHP−/− neurons than in Cav1.3−/− neurons. Reopenings in Cav1.2DHP−/− neurons in the presence of the DHP agonist were similar to those in wild-type neurons in the absence of the agonist. These data show that Cav1.2α1 subunits are the pore-forming subunits of anomalous LTCCs in mouse cerebellar granule cells. Given the evidence that Cav1.2 channels are specifically involved in sustained Ras-MAPK (mitogen-activated protein kinase)-dependent cAMP response element-binding protein phosphorylation and LTCC-dependent hippocampal long-term potentiation (LTP) (Moosmang et al., 2005), we discuss the hypothesis that anomalous rather than cardiac-type Cav1.2 channels are specifically involved in LTCC-dependent and gene transcription-dependent LTP.