The hyperpolarization-activated channel HCN4 is required for the generation of pacemaker action potentials in the embryonic heart

Abstract

Hyperpolarization-activated, cyclic nucleotide-gated cation currents, termed I f or Ih, are generated by four members of the hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channel family. These currents have been proposed to contribute to several functions including pacemaker activity in heart and brain, control of resting potential, and neuronal plasticity. Transcripts of the HCN4 isoform have been found in cardiomyocytes and neurons, but the physiological role of this channel is unknown. Here we show that HCN4 is essential for the proper function of the developing cardiac conduction system. In wild-type embryos, HCN4 is highly expressed in the cardiac region where the early sinoatrial node develops. Mice lacking HCN4 channels globally, as well as mice with a selective deletion of HCN4 in cardiomyocytes, died between embryonic days 9.5 and 11.5. On average, I f in cardiomyocytes from mutant embryos is reduced by 85%. Hearts from HCN4-deficient embryos contracted significantly slower compared with wild type and could not be stimulated by cAMP. In both wild-type and HCN4 ؊/؊ mice, cardiac cells with ''primitive'' pacemaker action potentials could be found. However, cardiac cells with ''mature'' pacemaker potentials, observed in wild-type embryos starting at day 9.0, were not detected in HCN4-deficient embryos. Thus, HCN4 channels are essential for the proper generation of pacemaker potentials in the emerging sinoatrial node. Four genes encoding hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels have been identified and functionally expressed. All four HCN channels carry an inward current with the typical features of a current termed I h in the brain and I f in the heart (1-4). These currents have been implicated in a wide range of physiological functions including pacemaking activity of spontaneously firing brain and heart cells, control of resting membrane potential, response to sour taste, neuronal plasticity, and dendritic integration (reviewed in refs. 5-7).HCN4 is the predominant HCN transcript in the adult sinoatrial node (8-11). I f and HCN transcripts have also been identified in mouse embryonic hearts with HCN4 being the prevalent type at early stages (12). Little is known about the specific contributions of the individual HCN isoforms to the function of the heart. One of the main, but still controversially discussed hypotheses is that I f is one of the major currents contributing to the spontaneous diastolic depolarization of pacemaker cells and thereby to sinus node rhythm (5, 13). Furthermore, the ␤-adrenergic up-regulation of sinus node rhythm has been attributed to the binding of cAMP to HCN channels resulting in an enhanced I f . However, it has been shown that diastolic depolarization is generated by multiple ionic currents with complex interactions (reviewed in ref. 14). The importance of I f has also been questioned because activation thresholds of I f vary and conflicting results were obtained with I f blockers (15-18).Direct evidence demonstrating the...

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