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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Dominant-negative effects of human P/Q-type Ca²⁺ channel mutations associated with episodic ataxia type 2

¹School of Medicine, Fu Jen Catholic University, Hsin-Chuang, Taipei County; and ²Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan

Submitted 25 May 2005 ; accepted in final form 17 November 2005

Episodic ataxia type 2 (EA2) is an inherited autosomal dominant disorder related to cerebellar dysfunction and is associated with mutations in the pore-forming _1A-subunits of human P/Q-type Ca²⁺ channels (Cav2.1 channels). The majority of EA2 mutations result in significant loss-of-function phenotypes. Whether EA2 mutants may display dominant-negative effects in human, however, remains controversial. To address this issue, five EA2 mutants in the long isoform of human _1A-subunits were expressed in Xenopus oocytes to explore their potential dominant-negative effects. Upon coexpressing the cRNA of _1A-WT with each _1A-mutant in molar ratios ranging from 1:1 to 1:10, the amplitude of Ba²⁺ currents through wild-type (WT)-Cav2.1 channels decreased significantly as the relative molar ratio of _1A-mutants increased, suggesting the presence of an _1A-mutant-specific suppression effect. When we coexpressed _1A-WT with proteins not known to interact with Cav2.1 channels, we observed no significant suppression effects. Furthermore, increasing the amount of auxiliary subunits resulted in partial reversal of the suppression effects in nonsense but not missense EA2 mutants. On the other hand, when we repeated the same coinjection experiments of _1A-WT and mutant using a splice variant of _1A-subunit that contained a considerably shorter COOH terminus (i.e., the short isoform), no significant dominant-negative effects were noted until we enhanced the relative molar ratio to 1:10. Altogether, these results indicate that for human WT-Cav2.1 channels comprising the long-_1A-subunit isoform, both missense and nonsense EA2 mutants indeed display prominent dominant-negative effects.

channelopathy; voltage clamp; Xenopus oocytes; cerebellum; splice variants

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