After birth, mammalian hearts usually never develop new cardiomyocytes, the heart muscle cells. As a result, adult heart attack victims’ dead tissue is not replaced with fresh cardiomyocytes. In its place, scar tissue makes it harder for the heart to pump blood, which can lead to heart failure. Direct cardiac reprogramming of heart fibroblast cells into cardiomyocytes is a promising method for remuscularizing the wounded heart. In an article that was published in Circulation, researchers from the University of Alabama in Birmingham named TBX20 as the essential missing transcription factor in current cocktails for direct cardiac reprogramming of human fibroblasts.
According to their findings, adding TBX20 to the reprogramming mixture MGT 133 aided in cardiac reprograming and activated genes related to cardiac contractility, maturation, and ventricular placement of the heart muscle cell. They found that TBX20 works with the MGT reprogramming factors at the enhancers of cardiac genes to turn on the target genes strongly.
Zhou said that the unrecognized function of TBX20 as a crucial regulator of direct human cardiac reprogramming is highlighted by our findings. The study says that for this method to be used in therapy, direct cardiac reprogramming from human fibroblasts must be made more effective and better.
Zhou says that it will be important to study TBX20 in vivo for direct reprogramming in the future.
The effectiveness of the current cocktails for direct reprogramming of human fibroblasts is poor, and not enough functional cardiomyocytes are produced. When the UAB researchers evaluated cardiomyocytes produced from fibroblasts with a current cocktail to function cardiomyocytes, they found that TBX20 was the most underexpressed component. As evidenced by the activation of cardiac genes involved in sarcomere formation, ion channels, and heart contractions, the addition of TBX20 facilitated cardiac reprogramming. The smallest functional unit of striated muscle is called a sarcomere.
The initial target for testing in vivo reprogramming will be the ventricular fibroblasts of the human heart. By showing that TBX20 was needed to turn these fibroblasts into induced-cardiomyocytes, Zhou and his colleagues made it more likely that TBX20 could be used as a therapy.
In the induced-cardiomyocytes, TBX20 largely activated genes at the late stage of reprogramming, improving calcium flux, contractility, and mitochondrial function. The energy for heart muscle contractions comes from mitochondria. TBX20 seemed to help the mitochondria in the induced cardiomyocytes change to a way of breathing like an adult cardiomyocyte.
The UAB scientists discovered that TBX20 was coupled to and triggered cardiac gene enhancers mechanically. An enhancer is a brief DNA regulatory region that interacts with activator proteins to start or boost the transcription of a certain gene target. The process of transcription entails the creation of messenger RNA from a DNA gene that codes for a protein. Enhancers frequently work independently of their gene targets.
Zhou and colleagues discovered that the three MGT cocktail components were necessary for the TBX20 enhancer binding to assist in activating cardiac genes. TBX20 on its own wasn’t enough to change the fate of cardiac cells; the lack of any one of the three MGT factors made reprogramming much less effective.
Our findings support the notion that direct cardiac reprogramming is a synergistically regulated process mediated by a number of variables, and that TBX20 is required for the promotion of direct reprogramming and cardiac gene activation.
Following TBX20 reprogramming, single-cell RNA sequencing profiling revealed two significant clusters of cells. Induced cardiomyocytes were fully reprogrammed in one group but not in the other. The group of reprogrammed cardiomyocytes displayed ventricular cardiomyocyte-like gene expression profiles.
Zhou and Lu are both assistant professors at UAB. They work in the division of hematology and oncology in the departments of biomedical engineering and medicine, respectively.
