title: Expression and 1,4-dihydropyridine-binding properties of brain L-type calcium channel isoforms.
abstract: The L-type calcium channel (LTCC) isoforms Ca(v)1.2 and Ca(v)1.3 display similar 1,4-dihydropyridine (DHP) binding properties and are both expressed in mammalian brain. Recent work implicates Ca(v)1.3 channels as interesting drug targets, but no isoform-selective modulators exist. It is also unknown to what extent Ca(v)1.1 and Ca(v)1.4 contribute to L-type-specific DHP binding activity in brain. To address this question and to determine whether DHPs can discriminate between Ca(v)1.2 and Ca(v)1.3 binding pockets, we combined radioreceptor assays and quantitative polymerase chain reaction (qPCR). We bred double mutants (Ca(v)-DM) from mice expressing mutant Ca(v)1.2 channels [Ca(v)1.2DHP(-/-)] lacking high affinity for DHPs and from Ca(v)1.3 knockouts [Ca(v)1.3(-/-)]. (+)-[(3)H]isradipine binding to Ca(v)1.2DHP(-/-) and Ca(v)-DM brains was reduced to 15.1 and 4.4% of wild type, respectively, indicating that Ca(v)1.3 accounts for 10.7% of brain LTCCs. qPCR revealed that Ca(v)1.1 and Ca(v)1.4 alpha(1) subunits comprised 0.08% of the LTCC transcripts in mouse whole brain, suggesting that they cannot account for the residual binding. Instead, this could be explained by low-affinity binding (127-fold K(d) increase) to the mutated Ca(v)1.2 channels. Inhibition of (+)-[(3)H]isradipine binding to Ca(v)1.2DHP(-/-) (predominantly Ca(v)1.3) and wild-type (predominantly Ca(v)1.2) brain membranes by unlabeled DHPs revealed a 3- to 4-fold selectivity of nitrendipine and nifedipine for the Ca(v)1.2 binding pocket, a finding further confirmed with heterologously expressed channels. This suggests that small differences in their binding pockets may allow development of isoform-selective modulators for LTCCs and that, because of their very low expression, Ca(v)1.1 and Ca(v)1.4 are unlikely to serve as drug targets to treat CNS diseases.
authors.affiliation: Department of Pharmacology and Toxicology, Institute of Pharmacy, Center for Molecular Biosciences Innsbruck, University of Innsbruck, Innsbruck, Austria.
authors.forename: Martina J
authors.initials: MJ
authors.lastname: Sinnegger-Brauns
authors.forename: Irene G
authors.initials: IG
authors.lastname: Huber
authors.forename: Alexandra
authors.initials: A
authors.lastname: Koschak
authors.forename: Claudia
authors.initials: C
authors.lastname: Wild
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Obermair
authors.forename: Ursula
authors.initials: U
authors.lastname: Einzinger
authors.forename: Jean-Charles
authors.initials: JC
authors.lastname: Hoda
authors.forename: Simone B
authors.initials: SB
authors.lastname: Sartori
authors.forename: Jörg
authors.initials: J
authors.lastname: Striessnig
mesh_terms: D020816:Amino Acid Motifs / Q000502:physiology; D000818:Animals; D001921:Brain / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000235:genetics / Q000378:metabolism*; D004095:Dihydropyridines / Q000378:metabolism*; D005260:Female; D015870:Gene Expression; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism*
chemical_list: D020746:Calcium Channels, L-Type; D004095:Dihydropyridines; D020033:Protein Isoforms; C038806:1,4-dihydropyridine