The role of HCN channels for the synchronization process in the sinoatrial node of the heart

起搏器通道是超极化激活的环核苷酸门控 (HCN) 阳离子通道的成员，被认为是心脏窦房结 (SAN) 中产生心跳的重要马达。在人类和小鼠中发现了四种通道亚型 HCN1-HCN4。 HCN4 代表主要同工型并在整个 SAN 中表达。 HCN 通道通过超极化打开。此外，通道的激活受到环磷酸腺苷 (cAMP) 的调节。在这种情况下，交感神经系统激活期间发生的细胞内 cAMP 浓度增加会导致 HCN 通道活性增加。在之前的提案中，我们解决了第二信使 cAMP 如何通过影响 HCN4 通道来调节心跳的问题。为了解决这个问题，我们培育了 cAMP 不再与 HCN4 结合的敲入小鼠。通过研究 SAN 中的单个起搏细胞，我们观察到 SAN 细胞不仅可以采用众所周知的细胞激发自发动作电位并驱动心跳的模式（激发模式），还可以采用细胞保持静止长达 1 分钟的非激发模式。我们有证据表明，处于非放电模式的起搏细胞在 SAN 网络中充当“制动器”，抑制邻近处于放电模式的起搏细胞的活动。我们的假设是，在 SAN 网络中，处于非放电模式的细胞数量是通过 HCN4 的 cAMP 依赖性调节来调节的。因此，通过HCN4的cAMP依赖性调节，可以根据情况调整窦房结内抑制的精确剂量。这样，心率就可以得到有效的稳定。此外，我们假设 HCN1 和 HCN2 的 cAMP 依赖性调节可能与 HCN4 的 cAMP 依赖性调节具有相似的重要性。在本后续提案中，将在体外和体内研究这些假设。总而言之，我们期望本提案中获得的发现能够提供有关起搏机制的重要信息，并了解 HCN 通道功能障碍如何导致病态窦房结综合征。

Pacemaker channels are members of the hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels and are considered essential motors for the generation of the heart beat in the sinoatrial node (SAN) of the heart. Four channel subtypes, HCN1-HCN4 are found in humans and mice. HCN4 represents the major isoform and is expressed throughout the SAN. HCN channels are opened by hyperpolarization. In addition, activation of the channels is regulated by cyclic adenosine monophosphate (cAMP). In this context, an increase in intracellular cAMP concentration, as it occurs during sympathetic nervous system activation, leads to an increase in HCN channel activity. In the previous proposal, we addressed the question how the secondary messenger cAMP regulates heartbeat via an effect on HCN4 channels. To address this question, we generated knock-in mice in which cAMP can no longer bind to HCN4. By studying single pacemaker cells from the SAN, we observed that SAN cells can not only adopt the well-known mode in which the cells fire spontaneous action potentials and drive the heartbeat (firing mode), but can also adopt a non-firing mode in which the cells remain quiescent for a period of up to 1 minute. We have evidence that pacemaker cells in non-firing mode act as "brakes" in the SAN network, inhibiting the activity of neighboring pacemaker cells in firing mode. Our hypothesis is that in the network of the SAN, the number of cells in non-firing mode is adjusted by the cAMP-dependent regulation of HCN4. Accordingly, the precise dose for inhibition within the sinus node could be adjusted according to the situation via the cAMP-dependent regulation of HCN4. Thus, heart rate could be effectively stabilized. Furthermore, we hypothesize that cAMP-dependent regulation of HCN1 and HCN2 may be of similar importance as cAMP-dependent regulation of HCN4. These hypotheses will be investigated in vitro and in vivo in this follow-up proposal. Taken together, we expect the findings obtained in this proposal to provide important information about the pacing mechanism as well as to understand how dysfunction of HCN channels leads to the sick sinus syndrome.