Link to Pubmed [PMID] – 38809590
Link to DOI – 10.7554/eLife.91429
Elife 2024 May; 12():
Hematopoietic stem cells emerge in the embryo from an aortic-derived tissue called the hemogenic endothelium (HE). The HE appears to give birth to cells of different nature and fate but the molecular principles underlying this complexity are largely unknown. Here we show, in the zebrafish embryo, that two cell types emerge from the aortic floor with radically different morphodynamics. With the support of live imaging, we bring evidence suggesting that the mechanics underlying the two emergence types rely, or not, on apicobasal polarity establishment. While the first type is characterized by reinforcement of apicobasal polarity and maintenance of the apical/luminal membrane until release, the second type emerges via a dynamic process reminiscent of trans-endothelial migration. Interfering with Runx1 function suggests that the balance between the two emergence types depends on tuning apicobasal polarity at the level of the HE. In support of this and unexpectedly, we show that Pard3ba – one of the four Pard3 proteins expressed in the zebrafish – is sensitive to interference with Runx1 activity, in aortic endothelial cells. This supports the idea of a signaling cross talk controlling cell polarity and its associated features, between aortic and hemogenic cells. In addition, using new transgenic fish lines that express Junctional Adhesion Molecules and functional interference, we bring evidence for the essential role of ArhGEF11/PDZ-RhoGEF in controlling the HE-endothelial cell dynamic interface, including cell-cell intercalation, which is ultimately required for emergence completion. Overall, we highlight critical cellular and dynamic events of the endothelial-to-hematopoietic transition that support emergence complexity, with a potential impact on cell fate.In mammals and other animals with backbones, the cells that will make up blood and immune cells are generated during a very narrow timeframe in embryonic development. These cells, called hematopoietic stem cells and progenitors (or HSPCs for short), emerge from tissue known as hemogenic endothelium that makes up the floor of early blood vessels. For HPSCs to eventually specialise into different types of blood and immune cells, they require diverse migratory and homing properties that, ultimately, will determine the specific type of functions they exert. An important question for scientists studying the development of different blood and immune cell types is when this commitment to functional diversity is established. It could, for example, arise due to cells in the hemogenic endothelium having different origins. Alternatively, the signals that generate hemogenic endothelium cells could be responsible. It is also possible that both explanations are true, and that having different mechanisms involved ensures diversity in populations of HSPCs. To investigate differences between the HSPCs emerging from the hemogenic endothelium, Torcq et al. studied zebrafish embryos that had been modified so that one of the proteins involved in sensing cell polarity – where the top and bottom of the cell are located – was fluorescent. Live imaging of the embryos showed that two types of cells, with striking differences in morphology, emerge from the hemogenic tissue. In addition, one cell type displays the same polarity as the other vessel cells, whereas the other does not. Torcq et al. also present evidence suggesting that the signals responsible for controlling this cell polarity are provided by surrounding blood vessel cells, supporting the idea of an interplay between the different cell types. The finding that two different cell types emerge from the hemogenic endothelium, reveals a potential new source of diversity in HSPCs. Ultimately, this is expected to contribute to their functional complexity, resulting in both long-term stem cells that retain their full regenerative potential into adulthood and more specialized blood and immune cells.