A heartbeat arises through the rhythmic contraction and relaxation of the heart muscle. The contraction of the heart muscle is based on its electrical activity. This electrical activity is determined by the complex interaction of different ion channels in the heart muscles. The molecular identity of these channels and their importance for the control of the heart rhythm is a controversial topic among scientists. Together with researchers from the University Hospital of Münster, we were able to show that, in mice, the hyperpolarization-activated and cyclic nucleotide-gated channel (HCN4) plays a crucial role in the regulation of the heartbeat during embryonic development, while it only appears to play a subordinate role in the adult animal.
HCN channels are involved in the rhythmic activity of cells in both the heart and the brain. For this reason they are known as pacemaker channels. The activity of the channel is regulated directly by the messenger substance cAMP: Heart cells increase the production of this messenger if the heart needs to beat faster, e.g. during exercise or stress. It was unclear up to now, however, whether cAMP triggers the acceleration of the heartbeat via the pacemaker channel or via other signaling pathways.
We succeeded in answering this question with the help of genetically modified mice in which the molecular appearance of the pacemaker channel was altered so that it could no longer bind cAMP. Surprisingly, the mice that carried the mutations on both chromosomes died early in their embryonic development. Prior to their deaths, their hearts beat far more slowly than those of wild-type mice. The heart frequency did not increase when more cAMP was produced.
Therefore, at this stage of development, the pacemaker channel is the key target protein through which cAMP accelerates the heartbeat during exercise or stress. The observation that the basal rhythm of the heartbeat only guarantees the organism’s blood supply in the presence of cAMP was completely unexpected. The regulation of the HCN4 channel activity by cAMP is therefore more significant than originally assumed.
Figure 1: Mouse embryo on day 9.5 of embryonic development (E9.5). At his point in its development, the heart is located outside of the embryo. The atrium, ventricle, and sinus venosus are indicated in red in this schematic diagram.
Mice carrying the mutation on only one chromosome develop normally. Although their heartbeat is slower during embryonic development than that of normal animals, it is faster than that of animals with the mutation on both chromosomes. The heart rhythm of adult animals was monitored using tiny telemetric transmitters. Surprisingly, the heart frequencies of the adult mice carrying the mutation on only one chromosome no longer differed from those of their “normal” siblings. The regulation of the heart rhythm was also normal, even under stress conditions. However, a condition known as sinus node block was occasionally observed among these animals. In these cases, a single heartbeat was missed and after that the heart continued to beat normally (Figure 2).
This means that different pacemakers play an important role during different stages in the development of the mouse. The HCN4 channel is crucial to the regulation of the heart rhythm during embryonic development and ensures the survival of the organism in this way. Its significance appears to decline in adulthood, at least in the case of mature mice. In humans, the relative importance of HCN4 for pacing the heart may be different.
Figure 2: Electrocardiogram (ECG) following a stress test. Unlike normal mice (HCN4+/+), sinus node blocks arise in animals which only carry the mutation on one chromosome (HCN4+/R669Q).
Harzheim, D., Pfeiffer, K.H., Fabritz, L., Kremmer, E., Buch, T., Waisman, A., Kirchhof, P., Kaupp, U.B., and Seifert R. (2008) "Cardiac pacemaker function of HCN4 channels in mice is confined to embryonic development and requires cyclic AMP"Embo J. 27, 692-703