The ECM as a coordinator of cardiac function, growth and morphogenesis during heart development

The heart is a complex organ whose shape is tightly linked to function. During embryonic development the heart is initially a linear tube which undergoes rearrangement to generate its final form. This process is called heart morphogenesis, and if it does not occur properly can lead to birth abnormalities known as congenital heart defects.

While the heart is undergoing morphogenesis, two additional events also occur which are crucial for heart development. First, the heart grows in size through the addition of cells to the poles of the heart from a progenitor pool of cells in the surrounding mesoderm called the Second Heart Field (SHF). Although the addition of SHF cells to the heart is an essential part of heart morphogenesis, we understand very little about how these cells become incorporated into the heart.

Second, the heart is already beating during morphogenesis. Our previous work suggests that heart contraction controls how SHF cells are added to the heart, and couples heart growth with morphogenesis.

Cells and tissues do not exist in isolation - the environment around cells is a diverse assortment of protein and sugar molecules called the extracellular matrix (ECM), the composition of which is specific to different tissues. The ECM regulates biochemical signalling to surrounding cells, provides mechanical support and biomechanical cues, and is a key regulator of cell migration, tissue morphogenesis, and organ function.

The ECM is therefore a crucial component of the heart tube and surrounding SHF. We have shown that the ECM component laminin regulates SHF addition, likely by regulating the impact of contractility on SHF cells, providing a new link between heart function and growth. However, the relationship between cardiac function, the ECM, and SHF addition in coupling growth and morphogenesis is unknown.

In this proposal we use live imaging of heart development in zebrafish embryos where heart function is lost or increased to understand the relationship between contractility and the ECM in regulating SHF addition. Zebrafish represent a unique model organism allowing us to define this relationship. The embryos are transparent and develop externally, so we can visualise heart development in a live embryo.

Furthermore, zebrafish can survive the first few days of development without a functioning heart, meaning we can investigate how heart function impacts heart development in a way that is challenging in other organisms.

We will define this relationship via three objectives, all of which will exploit this ability to perform live in vivo imaging of the developing zebrafish heart and SHF. First, we will stop or speed up the embryonic zebrafish heart and assess how contractility affects the ECM itself. We will visualise the amount and distribution of ECM in the heart during morphogenesis, and will label specific ECM components to visualise ECM organisation and composition upon altered contractility.

Second, we will define how loss of the ECM component laminin affects heart tube contractility and SHF cell movement by imaging the beating heart tube and migrating SHF cells over time. Third we will combine the experimental approaches from the first two objectives to define how changes in contractility and/or the ECM together affect the rate of heart growth.

We will generate state-of-the-art 3D reconstructions of the developing heart in the experimental models from objectives 1 and 2, enabling us to define for the first time how contractility and the ECM together shape the heart by mediating growth. Defining how the ECM links heart function, growth, and morphology has many applications. It will help understand the processes driving heart development in humans, as well as how the ECM shapes other tissues in the body during development.

The ECM has also been recently identified as a key driver of heart regeneration, so this work has important implications in advancing our ability to improve tissue regeneration after injury.