Contraction of heart muscle cells (cardiomyocytes) is initiated at tiny elements called dyads, where Ca2+ is released. Here, there are several Ca2+-transporting channels nearby each other that allow this process to occur. Specifically, L-type Ca2+ channels and Na+-Ca2+ exchangers in membrane tubules (t-tubules) are located across a narrow gap from Ca2+ release channels called Ryanodine Receptors in the Sarcoplasmic Reticulum. Evidence from small rodents indicates that dyads are assembled gradually in the postnatal developing heart. But how does this process occur in large mammals? And what are the signals that control this process? Is the gradually increasing workload placed on the developing heart important?
To address these questions, Ornella Manfra and colleagues formed a new collaboration with the Thornburg lab at Oregon Hospital Science and University (https://www.ohsu.edu/knight-cardiovascular-institute/center-developmental-health). Their study examined the developing sheep heart, and the effects of workload-altering interventions. By employing advanced imaging methods, Ornella observed that tubule growth and dyadic assembly occur already in the fetal stage of sheep development. This process parallels progressive increases in fetal systolic blood pressure, and includes gradual alignment of L-type Ca2+ channels and Na+-Ca2+ exchangers with Ryanodine Receptors (see Figure 1).
She further observed that experimentally increasing fetal systolic blood pressure accelerated t-tubule growth, while reducing blood pressure did the opposite. Thus, Ornella’s work suggests that naturally augmenting blood pressure and workload during normal fetal development is an important signal that assembles dyads, thereby strengthening the contraction of cardiomyocytes and whole heart. A schematic summary of these findings is provided in Figure 2.
Why is this work important? Ornella and her colleagues aim to better understand prenatal cardiac health, and her data indicate that abnormal changes in fetal blood pressure can have detrimental consequences for how cardiomyocytes are assembled and function. These workload-dependent changes in fetal heart maturation and differentiation may also affect health after birth and long-term cardiovascular function. Furthermore, since cardiomyocyte structure and function are known to be degraded in diseases like heart failure, Ornella’s data suggest that these structures may be repaired by optimizing the heart’s workload. Finally, Ornella’s results have implications for ongoing work aimed at maturing human stem cells into well-differentiated cardiomyocytes suitable for cardiac transplantation and repair.
Figure 1. Time course of t-tubule and cardiac dyad formation during development. A, Super-resolution Airyscan images of t-tubules (wheat germ agglutinin labeling, green) in the left ventricle of fetal (gestational ages 93, 125, and 135 days) and neonatal (8-9 days) sheep (Scale bars = 10 μm). B and C, Super-resolution images of isolated cardiomyocytes labeled with antibodies against Ryanodine Receptors (RyR), L-type Ca2+ channels (LTCC) or Na+-Ca2+ exchangers (NCX). Zoom-ins of the boxed regions are shown in the lower panels. Areas where the proteins are present nearby each other (colocalized) are shown white. Scale bars = 10 in full images, 2 μm in zoom-ins. DAPI labeling (blue) was used to visualize the nuclei.
Figure 2. Schematic overview of the study’s findings. In the sheep heart, t-tubule growth begins in the fetal stage of development. Even at this early stage, growing t-tubules contain both L-type Ca2+ channels (LTCCs) and Na+-Ca2+ exchangers (NCX), which are nearby Ryanodine Receptors (RyRs) in dyadic junctions with the sarcoplasmic reticulum (SR). Progressive dyadic maturation is linked to increasing expression of two important dyadic regulators, Junctophilin-2 and BIN1. This gradual structural organization continues after birth. T-tubule development is highly workload sensitive, and driven by naturally increasing blood pressure in the fetal period.
The Journal of Physiology