Deep within the heart lies an intricate nervous system that plays a critical role in its functioning, especially during extreme stress. A research team at Yale University School of Medicine has unveiled this complex system in mice for the first time, challenging the long-held assumption that all nerve cells within the heart are the same. Their findings are detailed in the prestigious journal Cell and accompanied by reports in Nature.
Understanding the “Little Brain” in the Heart
Much like the gut is referred to as the “second brain,” the heart houses its own complex nervous system known as the intrinsic cardiac nervous system. Located in the fat tissue surrounding the heart, this network exchanges signals with the brain and coordinates the heartbeat. However, the rarity of these nerve cells, constituting approximately 0.01% of the total cells in a heart tissue sample, has impeded researchers in deciphering their exact functions for a long time.
Two Nerve Cell Types with Distinct Roles
To identify these elusive cells, researchers genetically modified mice to label all heart nerve cells. They discovered two genetically distinct types. The first type, known as Npy+, serves to stabilize the heart rate during normal circumstances. When these cells were stimulated, the heart rate decreased; their destruction resulted in heart failure and death. Thus, these cells not only moderate a rapid heart rate but are crucial for sustaining the heartbeat itself.
The second type, termed Ddah1+, initially posed a mystery. Whether stimulated or removed, these cells seemed to have no significant impact on the lifespan of the mice. “It was very puzzling,” noted Rui Chang, a co-author of the study.
The Game-Changer Moment
A serendipitous event changed everything. Doctoral student Qian Xu monitored the blood pressure of a mouse lacking Ddah1+ cells when it suddenly died during measurement. The researchers discovered that two-thirds of mice without functioning Ddah1+ cells perished while their blood pressure was measured, unlike the control group. “Just before the animal dies, there is a sudden drop in heart rate, and it never recovers,” Chang explained.
The suspicion arose that stress might be the trigger. Each mouse had been previously restrained in a narrow tube. The team then subjected the animals to various stressors, including prolonged heat exposure. Once again, most of those without Ddah1+ cells did not survive. Conversely, activation of these cells allowed stressed mice to endure.
Implications for Heart Disease
This study illuminates how interconnected the nervous networks of the brain and heart are. Beth Habecker from the Oregon Health and Science University stated, “It’s much more complex than we thought.” While it remains to be seen whether these findings are applicable to humans, the marker genes of the newly discovered mouse nerve cells are also present in human heart nerve cells. “There will undoubtedly be parallels in both systems,” said Kalyanam Shivkumar from the University of California, who reviewed the study.
Chang emphasizes the importance of understanding these cells, especially due to their connections to heart attacks, heart failure, and arrhythmias. As we delve deeper into the mysteries of this “little brain” within the heart, we may uncover new avenues for effective treatments in cardiac disease.
