Stem Cell Biology

Early in development, embryonic stem cells (ESCs) are formed from the inner cell mass of the blastocyst. ESCs are unique for their ability to proliferate indefinitely, also known as self-renewal, and for their pluripotency – the ability to give rise to any of the three primary germ layers and their eventual cell types.

During the process of gastrulation, ESCs disappear as embryonic tissues organize into the ectoderm, endoderm and mesoderm with each of the germ layers ultimately giving rise to a distinct set of tissues and organs. The ectoderm primarily gives rise to the nervous system and epithelial tissues, the endoderm to the gut and associated organs and the mesoderm to muscle and other organs including the heart.

Prior to gastrulation, vertebrate ESCs maintain their pluripotency thanks to a finely tuned balance of signaling pathways that culminate in steady state expression of three main transcription factors: NANOG, POU5F1/OCT4 and SOX2 (1). All three form a positive auto feedback loop, binding to their own and each other’s promoters, helping to maintain their expression and the cell’s ESC identity.

The three factors promote ESC pluripotency by upregulating genes that activate pluripotency and self-renewal mechanisms, including FGF4 and KLF4. At the same time, they repress genes that promote differentiation, either directly as in the case of HOXB1, HAND1, MEIS1 and others, or indirectly through regulation of expression of components of chromatin remodeling and histone-modifying complexes.

Multiple upstream signaling pathways contribute to regulation of NANOG, POU5F1/OCT4 and SOX2 (2, 3). In mouse ESCs, the LIF/STAT3 pathway plays a prominent role in maintaining self-renewal and pluripotency along with the BMP4/SMAD pathway. In human ESCs, the LIF/STAT3 pathway plays a much less significant role while the BMP4/SMAD pathway can, paradoxically, induce differentiation. In human ESCs it is the TFG-β /Activin A/Nodal, FGF and IGF pathways that play the most prominent role.

As development progresses, due to a shift in balance to WNT and other signaling pathways, activity of the pluripotent transcription factors decreases while the activity of lineage-specific transcription factors increases. As this shift takes place, one of the first organs to begin to develop is the heart.

In mesoderm progenitor cells, T-box family transcription factors and MESP1 lead the shift to differentiation of the first heart field and second heart field. Through the activation of lineage-specific GATA and other transcription factors, these early progenitors go on to differentiate into more specific, yet still intermediate, cell types. Some of the most important include cardiac conduction cells, cardiac muscle cells, endothelial cells and postnatal cardiac progenitor cells. Further changes in expression of lineage-specific transcription factors in these progenitors and their progeny ultimately leads to formation of the different cells, tissue and chambers of the heart.

Heart development is only one example. This process of successive decreases in pluripotency and differentiation into increasingly refined lineages is a consistent feature of development that occurs across all organs. Understanding the complex interactions that regulate the shift from pluripotency to lineage specification can help inform development of new treatments focused on cardiac and other organ regeneration.

References

1. Young, RA. Control of the embryonic stem cell state. Cell. 2011;144(6):940–54.
2. Varzideh F, Gambardella J, Kansakar U, Jankauskas SS, Santulli G. Molecular mechanisms underlying pluripotency and self-renewal of embryonic stem cells. Int J Mol Sci. 2023;24(9):8386.
3. Schnerch A, Cerdan C, Bhatia M. Distinguishing between mouse and human pluripotent stem cell regulation: the best laid plans of mice and men. Stem Cells. 2010;28(3):419–30.
4. Bruneau BG. Signaling and transcriptional networks in heart development and regeneration. Cold Spring Harb Perspect Biol. 2013;5(3): a008292.