Four and a half LIM protein 1: a novel chaperone for atrium-specific Kv1.5 channels with a potential role in atrial arrhythmogenesis

Chronic atrial diseases are associated with electrical, mechanical, and structural changes (remodelling) that result from activation of diverse signal transduction pathways. In atrial fibrillation (AF), the high atrial rate abbreviates the atrial action potential duration primarily by reduced ICa,L and increased inward-rectifier K+ currents like the background current (IK1) and a constitutively active form of the acetylcholine-dependent K+ current (IK,ACh).1,2 There is also evidence for AF-associated reduction in the ultra-rapid delayed-rectifier IKur, although the reported findings have been inconsistent and the underlying molecular mechanisms are less clear.1 Response rate to antiarrhythmic drugs is low with respect to termination of AF or maintenance of sinus rhythm, and many of the drugs lead to adverse effects including ventricular arrhythmias. Given these limitations, an alternative approach is to design drugs that target atrium-selective ion channels like IKur, which plays a role in AF pathophysiology but not in ventricular function. However, our knowledge of the cellular processes underlying normal regulation of IKur are still incomplete. In order to contribute to atrial function in a specific way, the different ion channels have to be in the right place at the right time. This is regulated by multiple cellular processes, such as gene transcription, protein translation, trafficking, and membrane localization. Further fine-tuning is achieved by accessory channel subunits and by post-translational protein modifications. Several findings have implicated accessory KChiP23 and Kvβ14 subunits, and scaffold proteins like PDS975 and cytoskeletal α-actinin6, in regulation of IKur or heterologously expressed K+ currents derived from the principal IKur channel α-subunit Kv1.5 (KCNA5). However, Kv1.5-based K+ currents only partially reproduce the phenotype of native IKur, suggesting involvement of additional regulatory mechanisms. Yang et al.7 provide strong evidence that four and a half LIM protein 1 (FHL1) may represent a novel regulator of human atrial Kv1.5 channels. Specifically, using glutathione- S-transferase (GST) fusion protein and mass spectrometry-based techniques, the authors identified FHL1 as a potential protein partner of human Kv1.5 channels. LIM domain proteins like FHL1 recruit specific proteins, segregate these partners to discrete subcellular locations, and modulate their activities or assemble them into multi-protein complexes.8 Besides being involved in sarcomeric and signalling complexes in the cytosol, LIM domain proteins shuttle between the nucleus and cytosol and interact with transcription factors to regulate gene expression. There is emerging evidence that LIM domain proteins mediate the communication between the nuclear and plasma membrane compartments.8 Therefore, it is not surprising that Yang et al.7 found that FHL1 interacts with human Kv1.5 in the plasma membrane. These authors showed that FHL1 co-immunoprecipitates with Kv1.5 in both human atrium and in HEK293 and COS cells co-transfected with FHL1 and Kv1.5. In addition, immunolabelling of FHL1 and Kv1.5 confirmed their co-localization in the plasma membrane, further supporting potential physical interaction. Subsequent voltage-clamp experiments in COS cells showed that in the presence of FHL1 the amplitude of Kv1.5-derived K+ current was approximately four-fold higher than in controls and that voltage-dependent activation was shifted to more positive potentials. FHL1 also increased the extent and accelerated the speed of slow inactivation and shifted its voltage-dependence to more positive potentials. Taken together, the FHL1-related current phenotype closely resembles that of IKur in atrial myocytes,9 suggesting that FHL1 is a major regulator of atrial IKur. The results reported by Yang et al.7 raise several important issues that require further clarification. They do not provide evidence whether FHL1 itself or FHL1-targeted partners of Kv1.5 mediate the FHL1-associated alterations in Kv1.5 current. For instance, the LIM domain protein Enigma homlogue protein (ENH) acts as an adaptor protein for the formation of a functional protein kinase Cε-ENH-N-type Ca2+ channel complex in neurons.10 Thus, it is possible that targeting protein kinases to Kv1.5 participates in the effects of FHL1. It is furthermore unclear whether FHL1-related alterations in current characteristics involve changes in expression and/or membrane trafficking of Kv1.5. Also, FHL1 is known to serve not only as an adaptor and localizer protein, but also to compete with and change the conformation of targeted proteins. Thus, it is likely that the influence of IKur by FHL1 involves multiple contributing mechanisms. Further work is needed to identify the specific mechanism(s) by which FHL1 modulates IKur function. IKur is an atrium-selective ion current with the potential of being a promising drug target for therapy of atrial arrhythmias without concomitant adverse effects in the ventricles.11 However, experimental data on IKur remodelling in AF patients are unequivocal, showing either unchanged or reduced IKur amplitude.1 These inconsistent findings are assumed to result from variations in expression and post-translational modifications (S-nitrosylation, degradation)12,13 of Kv1.5 and from differences in underlying cardiac diseases and/or concomitant medication.14 The results of Yang et al.7 raise the intriguing possibility that differences in FHL1 expression or function may contribute to the inconsistent findings with IKur. Although the protein levels of FHL1 in patients with AF are currently unknown, a recent study performed in a porcine model of pacing-induced AF suggests that sustained AF is associated with an approximately five-fold and approximately three-fold increase in mRNA and protein levels of FHL1, respectively.15 Although caution is needed when extrapolating these results to the clinical situation, this study suggests that at least in some AF patients increased FHL1 expression may counteract the down-regulation of Kv1.5, providing a plausible explanation for the inconsistent IKur results in AF patients. This hypothesis warrants direct experimental verification in subsequent work. Supported by the German Federal Ministry of Education and Research (BMBF) through the Atrial Fibrillation Competence NETwork (AFNET, grant 01Gi0204), by a grant from the Foundation Leducq and by an European Union grant (NORMACOR, LSHM-CT-2006-018676). Conflict of interest: none declared.