During her PhD, Silja Burkhard from the Bakkers group studied the development of cardiac pacemaker cells in the zebrafish. She defended her thesis “Keeping the Rhythm: Cardiac Pacemaker Cell Development” on November 28, 2017.

Cardiac conduction
The heart is responsible for continuously pumping blood through the body during your entire life. The contraction of the heart is initiated by the pacemaker cells, specialized heart cells, which are located in the sinus node and regulate heart rhythm and heart rate. They are also able to adapt the heart rate, for instance during sports.


Zebrafish pacemaker
Together with colleagues, Burkhard discovered that the zebrafish also has pacemaker cells, and that these are marked by the expression of a gene called Islet-1. In the zebrafish heart, Islet-1 is also expressed in the pacemaker cells, which the researchers found to be located in a ring around the inflow of the heart, contrary to humans in which these cells are located in the sinus node (see figure).

Zebrafish in which the Islet-1 gene is mutated display a very low heart rate. The heart stops beating for periods of time and eventually these zebrafish do not survive. This showed Burkhard and her colleagues that Islet-1 is not only a marker of pacemaker cells, but also has an important role in the development of these cells.

To find out more about the pacemaker cells in the zebrafish, Burkhard used tomo-seq, a technique that was recently developed in the Hubrecht Institute. Using this technique, she cut the embryonic zebrafish heart in very thin slices (see figure) and analyzed the gene expression in each of these slices. This way, she was able to investigate the expression of genes at any specific place in the heart.

Wnt signaling
Using tomo-seq Burkhard found that the cells pacemaker cells that express Islet-1, also specifically express other genes. Among these are genes involved in the Wnt signaling pathway. It turned out that Wnt signaling is required for the influence of the autonomous nervous system on the heart rate.

CAID syndrome
In collaboration with researchers in Canada, Burkhard also studied the CAID (Chronic Atrial and Intestinal Dysrhythmia) syndrome. Patients with this syndrome are known to have defective pacemaker cells, both in the heart and in the intestine. These patients have a very slow and irregular heartbeat and their intestines are much less peristaltic than in healthy persons. It turned out that a mutation in the SGOL1 gene was causing this syndrome. Burkhard could show some of the symptoms of these patients in SGOL1-inhibited zebrafish: they have a lower heart rate too.

Biological pacemaker
What can we learn from the research on the cardiac conduction system? “More knowledge on the molecular mechanisms of the sinus node and its development might help the development of a biological pacemaker in the future”, Burkhard says. “If we learn how pacemaker cells develop during embryogenesis, we might be able to generate these cells in the lab and use them to treat sinus node dysfunction.”

Silja Burkhard, PhD after her thesis defense.