Journal club: heart development

Main messages
The three major cell types of the heart develop and mature at different time points during heart development.The precise development depends on the localization of a cell within the heart. A mutation in the gene NKX2.5 can disturb the developmental process in mice. This probably explains the heart defect of both newborn people and mice that have this mutation in their DNA.

Background and aim
At birth, our hearts are fully functional pumps, pumping oxygen-poor blood to the lungs and oxygen-rich blood from the lungs to the rest of the body. The blood flows into the heart in the two atria, which can be visualized as chambers. Upon contraction of the walls of these chambers, the blood is pushed into the next chambers: the ventricles. From there, the blood is pumped out of the heart to the rest of the body and lungs. To be able to perform it’s pumping function, the heart contains muscle cells, allowing the heart to contract, endothelial cells, lining the blood vessels and heart chambers like a kind of inside skin, and fibroblasts, making the support framework of the heart, the so-called the ‘extracellular matrix’. This complex structure needs to develop during only nine months of pregnancy. In their paper, DeLaughter and colleagues studied the expression of genes in the heart of embryonic and newborn mice to learn more about how a heart and its different cell types develop. Ultimately, this will help us understand how defects in certain genes result in heart defects in newborn babies.

Method
The researchers determined the expression of genes using RNA sequencing. They determined gene expression at different time points during development: at embryonic day 9.5, 11.5, 14.5 and 18.5, on the day of birth and at 3, 7 and 21 days after birth. Interestingly, they did not study gene expression in the whole heart at once, but in single cells. As explained here, different cell types perform different functions because different genes are expressed in each of them. By studying gene expression in separate cells, the researchers could determine the cell type and maturity of each of the analyzed cells.

Results & discussion
The ratio of the different cell types changes over time
The researchers discovered that the proportion of endothelial cells was stable over development. In contrast, the number of fibroblasts increased from none at all at embryonic day 9.5 to a little less than half of all cells at birth. This increase was at the expense of the cardiomyocytes. The decrease in the percentage of cardiomyocytes is possibly due to the decrease in cell division of these cells: more than half of the cardiomyocytes expressed genes associated with cell division during the start of embryogenesis, but none expressed them 21 days after birth.

Cells in the different heart chambers and different development time points can be distinguished based on different gene expression patterns
Interestingly, gene expression of the cardiomyocytes from the atria was clearly different from that of the cardiomyocytes from the ventricles. Not only that, even the cells from the left and the right ventricle could be distinguished based on gene expression! The difference between the ventricles became less clear over time, indicating that the chambers first establish their own characteristics, after which all of the cardiomyocytes mature.
The samples obtained at different time points were also clearly different from one another. Interestingly, comparing gene expression data from human hearts to this mice data, showed that human gene expression in the heart shortly after birth corresponded to the mice data obtained at 1-3 days after birth.

A mutation causing heart defects in humans delays maturation of the heart cells
Finally, the researchers looked at mice with a defect in one of their genes: gene NKX2.5. Both humans and mice with a defect in this gene, so with a so-called mutation, develop hearts in which the four chambers are not fully separated. Heart cells of mice with this mutation showed gene expression differences compared to mice with healthy hearts. These differences indicated that the mutant hearts had much fewer cardiomyocyte cells that had reached maturity at birth. Interestingly, while NKX2.5 is expressed mostly in the cardiomycytes, the maturation of endothelial cells was also affected.

Conclusion
DeLaughter and his colleagues have given us a better idea how heart cells develop and mature in an embryo. They also showed how this information can be used to understand the cause of heart abnormalities present at birth.

DeLaughter DM, Bick AG, Wakimoto H, McKean D, Gorham JM, Kathiriya IS, Hinson JT, Hornsy J, Gray J, Pu W, Bruneau BG, Seidman JG, Seidman CE (2016) Single-cell resokution of temporatl gene expression during heart development. Developmental cell  39: 480-490.
DOI: http://dx.doi.org/10.1016/j.devcel.2016.10.001

Disclaimer: blog posts in the category ‘journal club’ are not intended to cover the whole paper discussed. Instead, I discuss the parts that I think are most interesting for a general public. I try my utmost to prevent any mistakes in these blogs, I apologize in advance for any mistakes that I make anyway.

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