FUNCTIONAL KNOCKOUT OF THE TRANSIENT OUTWARD CURRENT, LONG-QT SYNDROME, AND CARDIAC REMODELING IN MICE EXPRESSING A DOMINANT-NEGATIVE KV4 ALPHA-SUBUNIT

A novel in vivo experimental strategy, involving cell type-specific ex pression of a dominant-negative K+ channel pore-forming alpha subunit, was developed and exploited to probe the molecular identity of the ca rdiac transient outward K+ current (I-to). A point mutation (W to F) w as introduced at position 362 in the pore region of Kv4.2 to produce a nonconducting mutant (Kv4.2W362F) subunit. Coexpression of Kv4.2W362F with Kv4.2 (or Kv4.3) attenuates the wild-type currents, and the effe ct is subfamily specific, ie, Kv4.2W362F does not affect heterologousl y expressed Kv1.4 currents. With the use of the alpha-myosin heavy cha in promoter to direct cardiac-specific expression, several lines of Kv 4.2W362F transgenic mice were generated. Electrophysiological recordin gs reveal that I-to is selectively eliminated in ventricular myocytes isolated from transgenic mice expressing Kv4.3W362F, thereby demonstra ting directly that the Kv 4 subfamily underlies I-to in the mammalian heart. Functional knockout of I-to leads to marked increases in action potential durations in ventricular myocytes and to prolongation of th e QT interval in surface ECG recordings. In addition, a novel rapidly activating and inactivating K+ current, which is not detectable in myo cytes from nontransgenic littermates, is evident in Kv4.2W362F-express ing ventricular cells. Importantly, these results demonstrate that ele ctrical remodeling occurs in the heart when the expression of endogeno us K+ channels is altered.