Heart muscle cells fuse when the heart grows and regenerates

The mammalian heart has very limited regenerative ability. Studying organisms capable of repairing their cardiac muscle after injury may help us to understand how to improve regeneration of the human heart. We found that in zebrafish individual cardiac muscle cells can transiently fuse with each other, allowing exchange of cell contents in response to high demand of cell proliferation during development and regeneration of the heart. Our data suggest that transient cell fusion in the heart might be an important mechanism regulating proliferation of cardiac cells. The discovery of this cellular event opens up new research avenues to investigate the potential of cell fusion as a new therapeutic strategy to repair the injured heart.

HFSP Long-Term Fellow Suphansa Sawamiphak and colleagues
authored on Thu, 01 February 2018

While generally regarded as discrete units, some cells can give up their individuality by merging their plasma membranes, typically, giving rise to multinucleated cells. Under physiological conditions, skeletal muscles as well as bone-resorbing osteoclasts are among the examples of multinucleated cells found in vertebrates. Function and homeostasis of these cell types are thought to depend on the formation of these enlarged multinucleated syncytia. Cell fusion is also associated with several pathological conditions. In response to chronic inflammation or virus infection, for example, macrophages can fuse with each other to form giant cells. Blood lineage cells seems to be especially prone to undergoing fusion. Following bone marrow transplantation, fusion between circulating blood cells and other cell types sporadically occurs in different tissues. It has also been proposed that fusion with blood cells could confer metastatic phenotype to cancer cells. An intriguing possibility is that hybrid cells derived from fusion events could develop emerging properties not present in one or both of the fusion partners. For example, quiescent cells could become proliferative after undergoing cell fusion. The ability of cell fusion to induce cell fate reprogramming or reversal of differentiated state could profoundly benefit regenerative medicine. However, we still know little about the significance of cell fusion in tissue regeneration, identities of cells undergoing fusion, molecular mechanisms mediating fusion events, and phenotypic changes of fusion-derived cells. Addressing these questions is a prerequisite for developing medical applications of cell fusion.

Figure: Cardiomyocytes undergo membrane fusion in the developing heart of genetically-modified zebrafish carrying the new reporter transgene fluoresce (green), allowing reliable detection of fusion events in living organisms. All cardiomyocytes in the ventricles are labeled with a membrane-localized fluorescent protein (red).   

Cardiac muscle cells (cardiomyocytes) of the adult mammalian heart are generally considered post-mitotic. The heart copes with acute loss of cardiomyocytes caused by ischemic injury by forming non-contractile fibrotic scars, which severely impair cardiac function. Several studies showed that following an infarct, some cardiomyocytes could fuse with circulating cells. These fusion events are, however, sporadic. Fusion-derived hybrid cells detected in these studies were too few to allow examination of their roles in cardiac repair. The study of cell fusion has been hampered by the lack of efficient tools to track and manipulate cells undergoing fusion. The most common technique used to investigate cell fusion in vivo has been transplantation of genetically labeled cell types into unmarked or differentially labeled hosts, a time-consuming and inefficient method. To understand the significance of cell fusion in cardiac regeneration, we developed new genetic tools in the teleost zebrafish, whose cardiomyocytes, unlike mammalian ones, maintain self-renewal capacity through adulthood. We generated novel transgenic zebrafish lines that enable, for the first time, ubiquitous detection of fusion-derived cells and tracing of their behaviors in vivo. Using these tools, we have uncovered previously unrecognized fusion events that occur between adjacent cardiomyocytes in response to high proliferative demands of the heart during larval development as well as regeneration. Live imaging of embryonic hearts showed that, unlike typical cell fusion events, homotypic cardiomyocyte fusion occurs only transiently and does not give rise to multinucleation. Intriguingly, in the developing heart lacking a molecular mediator of cell fusion, cardiomyocyte proliferation decreases and cardiac contractile function is compromised. In the adult heart, when cardiomyocytes turnover rate is low during steady state, we observed a substantial reduction of fusion events. In contrast, acute death of cardiomyocytes induced by cardiac lesions drives large numbers of cardiomyocytes, particularly those locating adjacent to the lesion, to undergo fusion.

The strong correlation between the occurrence of fusion and proliferation rate suggests the intriguing possibility that transient membrane fusion induces quiescent cardiomyocytes to enter the cell cycle. Fusion-driven cardiomyocyte cycling would not only explain the rarity of fusion-derived cardiomyocytes observed in the non-regenerative mouse heart, but also imply that boosting transient fusion competency could revert the post-mitotic state of mature mammalian cardiomyocytes. The mechanistic link between membrane fusion and cardiomyocyte mitotic activity is currently under investigation in our laboratory. Identification of molecular regulators of cardiomyocyte fusion in the regenerative heart might open new therapeutic avenues for repairing damaged human hearts by exploiting the potential of membrane fusion for restoring self-renewal ability to cardiomyocytes.

The support I received from the Human Frontier Science Program Long-Term Fellowship has been instrumental in allowing me to carry out this project, particularly by encouraging me to take new challenges, including the use of zebrafish for cardiac research. Although I had worked with several model organisms before, joining the zebrafish research community as a postdoctoral fellow in Prof. Didier Stainier’s group has been an eye-opening experience. Encouragement through the HFSP-LTF to engage multidisciplinary approaches in life science allowed me to investigate development and regenerative mechanisms of the cardiac tissue as an integrative process involving distinct organ systems, a less explored area in cardiac biology. The interactions between cardiac and immune cells, in particular, have been the focus of my newly established research group at the Max Delbrueck Center for Molecular Medicine in Berlin.   


Transient cardiomyocyte fusion regulates cardiac development in zebrafish. Suphansa Sawamiphak, Zacharias Kontarakis, Alessandro Filosa, Sven Reischauer & Didier Y. R. Stainier. Nature Communications 2017 Nov 15;8(1):1525. doi: 10.1038/s41467-017-01555-8.

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