Targeted Deletion of Kv4.2 Eliminates I _to,f and Results in Electrical and Molecular Remodeling, With No Evidence of Ventricular Hypertrophy or Myocardial Dysfunction

Abstract

Abstract-Previous studies have demonstrated a role for voltage-gated K ϩ (Kv) channel ␣ subunits of the Kv4 subfamily in the generation of rapidly inactivating/recovering cardiac transient outward K ϩ current, I to,f , channels. Biochemical studies suggest that mouse ventricular I to,f channels reflect the heteromeric assembly of Kv4.2 and Kv4.3 with the accessory subunits, KChIP2 and Kv␤1, and that Kv4.2 is the primary determinant of regional differences in (mouse ventricular) I to,f densities. Interestingly, the phenotypic consequences of manipulating I to,f expression in different mouse models are distinct. In the experiments here, the effects of the targeted deletion of Kv4. 1 Considerable progress has been made in characterizing the properties of myocardial Kv channels and in defining the roles of individual Kv channel pore-forming (␣) and accessory (␤) subunits in the generation of these channels. 1 In adult mouse ventricles, for example, multiple Kv currents are coexpressed. 2-10 All available evidence suggests that ␣ subunits of the Kv4 subfamily underlie fast inactivating and recovering cardiac transient outward, I to,f , channels. 1 Biochemical studies suggest that mouse ventricular I to,f channels reflect the heteromeric assembly of Kv4.2 and Kv4.3. 10 In large mammals, however, Kv4.2 is not expressed, and I to,f channels are thought to reflect Kv4.3 homotetramers. 11,12 Multiple splice variants of Kv4.3 have been identified, 12 although the role(s) of these variants in the generation of I to,f channels is unclear.The accessory subunit, KChIP2, 13 coimmunoprecipitates with Kv4.2 and Kv4.3 from adult mouse ventricles, 10 and it has been reported that I to,f is eliminated in ventricular myocytes isolated from mice in which the KChIP2 locus was disrupted. 14 In canine ventricles, KChIP2 message 15,16 and protein 17 expression parallel variations in I to,f densities, suggesting that KChIP2 is the primary determinant of I to,f gradients. 15,17 In rodent ventricles, however, KChIP2 is uniformly expressed, and regional differences in I to,f densities are correlated with heterogeneities in Kv4.2 expression. 10,18 Interestingly, the phenotypic consequences of manipulating I to,f expression in vivo are distinct. 4,7,14,19 Recordings from ventricular myocytes isolated from transgenic mice expressing a pore mutant of Kv4.2, Kv4.2W362F, that functions as a dominant negative (Kv4.2DN), for example, revealed that I to,f is eliminated. 4 Materials and MethodsAnimals were handled in accordance with the NIH Guide for the Care and Use of Laboratory Animals; all protocols were approved by the Washington University Animal Studies Committee. The generation of the Kv4.2 Ϫ/Ϫ mice and the methods/protocols used in the present study are detailed in the online data supplement available at http://circres.ahajournals.org. Results Targeted Disruption of the KCND2 (Kv4.2) LocusIn the targeting construct used to generate Kv4.2 Ϫ/Ϫ mice ( Figure 1A), described in the expanded Materials and Methods section in the online data supp...

Mutant Analysis of the Shal (Kv4) Voltage-gated Fast Transient K+ Channel in Caenorhabditis elegans

“…Repolarization to potentials negative to Ϫ50 mV is typically required to restore channel availability. A-type currents have a widespread distribution and are abundantly expressed in neurons and cardiac and smooth muscle cells, where they are thought to play many important physiological roles (2,(7)(8)(9)(10)(11).…”

“…I A has also been demonstrated to limit the back propagation of action potentials into the dendritic arbor (14), to impact long term potentiation in CA1 hippocampal neurons (15), to affect neuronal excitability in the visual cortex (16), and to modulate compartmentalization of membrane excitability in distal dendritic spines (17). Fast transient K ϩ currents in cardiac muscles contribute both to myocyte excitability (18) and to the fast repolarization phase of the cardiac action potential (2,8,19,20). Finally, the pharmacological block of channels that carry fast transient currents in gastrointestinal smooth muscle in mice supports the conclusion that the window currents carried by these channels contribute to the resting membrane potential and cellular excitability of smooth muscle (21,22).…”

Left Ventricular Hypertrophy

“…In contrast with the global increases in transcript expression with hypertrophy, transcript expression of the I to,f channel ␣ subunits Kv4.2 (KCND2) and Kv4.3 (KCND3) 19,24 was not significantly different in TAC and sham LV, and expression of the I to,f channel accessory subunit KChIP2 (KCNIP2) 24 was actually slightly lower in TAC LV ( Figure 5C). The expression of transcripts encoding I K1 channel ␣ subunits Kir2.1 (KCNJ2) and Kir2.2 (KCNJ12), 23 and of the K2P channel subunit TASK1 (KCNK3), which has been suggested to underlie I ss in rat cardiomyocytes, 26 as well as TASK2 (KCNK5), was unaffected in TAC LV ( Figure 5C).…”

“…19 Biotinylation of isolated LV myocytes was used to examine cell surface protein expression. Protein quantification was performed with the BCA Protein Assay Kit (Pierce).…”

“…14,19,[22][23][24] Because accumulating evidence suggests that cardiac ion channels function as components of macromolecular complexes, 25 TaqMan low-density arrays 16 were exploited to allow quantitative determinations of multiple transcripts simultaneously. The amount of total RNA isolated from TAC LV was significantly (PϽ0.01) higher (1.7-fold on average) than from sham LV ( Figure 5A).…”