The Mysteries of Cell Rounding: AQUAPORINs and Hematopoietic Transition

Endothelial-to-hematopoietic transition (EHT) is a fundamental transdifferentiation event that involves the transformation of hemogenic endothelial cells (HECs) into definitive HSCs. The research sheds light on the various external signaling cues and transcriptional regulatory networks that govern HEC specification and hematopoietic cell emergence. One of the key factors in this process is the runt family transcription factor Runx1, which is essential for EHT. The Runx1 is essential for EHT, and its expression distinguishes HECs from non-HECs. Moreover, ectopic RUNX1 expression in non-HECs is sufficient to convert them into HECs. However, the conversion efficiency is highly dependent on the developmental timing of the ectopic expression and other additional factors modulate the competency for RUNX1 to initiate EHT.

In a new study published in the peer-reviewed Journal Development, led by Dr. Yuki Sato and a team of researchers from Kyushu University in collaboration with Okayama University, discussed the complexity of hematopoietic cell development. Specifically, their study focused on the phenomenon of EHT, which is pivotal in the formation of HSCs during embryogenesis. This process has long fascinated scientists and clinicians alike, and their finding provided novel insights into the underlying mechanisms.

A notable observation made by the researchers is the involvement of vacuoles in the cell rounding process, a crucial step during EHT. It is known that Runx1-deficient embryos lack vacuole-like organelles with specific structures in their prospective HECs. These cells also display irregular cell flattening, further suggesting the significance of vacuoles in the cell rounding process. While vacuoles are well-known for their role in regulating cell morphology and size in plant cells in response to osmotic pressure, their role in animal development, particularly in EHT, was less understood. The study also highlighted the presence of Aquaporin (AQP) family proteins, which are responsible for water transport in cell and vacuole membranes. These AQPs, particularly AQP1, are implicated in various cellular processes, such as cell migration, tumor invasion, and epithelial-to-mesenchymal transition (EMT).

The authors found that AQP1 is localized in the plasma and vacuole membranes of endothelial cells during the EHT process. This localization was found in HECs and hematopoietic cells, indicating a role for AQP1 throughout this developmental stage. Additionally, AQP1 expression decreased after EHT completion, suggesting that its role is temporally regulated. The researchers used AQP1 overexpression experiments and observed that it led to increased vacuole size and significant cell rounding. This finding suggests that AQP1 plays a critical role in promoting cell rounding by facilitating water permeation into vacuoles.

Moreover, they demonstrated that AQP1 overexpression induces ectopic cell rounding and detachment from the endothelium. This is a remarkable finding as it suggests that excess AQP1 expression can drive cellular responses similar to those observed during EHT, even in non-HEC endothelial cells. The authors successfully provided evidence of these ectopically rounded cells entering the circulation, further emphasizing the profound impact of AQP1 on cell behavior during development. The study extends beyond avian embryos, as the in vitro experiments involving quail embryo-derived presomitic mesoderm reaffirm the role of AQPs in vacuole formation and cell rounding. It also shows that AQP1 expression is associated with an upregulation of genes related to HEC and hematopoietic cell specification.

One interesting findings of the study is the redundancy of AQP channels. While AQP1 was the focus of their study, the presence of AQP5, AQP8, and AQP9 in the endothelium adds complexity to the picture. The authors showed that multiple AQP knockouts in HECs result in the failure of morphological EHT, highlighting the collective importance of AQP activity in the process. This finding challenges the idea that EHT is solely dependent on a single AQP channel. Dr. Yuki Sato and colleagues conducted comprehensive analyses to investigate Runx1’s role in avian embryos. While the role of Runx1 has been extensively studied in the context of EHT in mouse embryos, its function in avian embryos had remained less explored. This study indicates that Runx1 may not be essential for cell morphological changes during EHT in avian embryos, providing some clues into species-specific variations in hematopoietic development. The authors concluded with a proposed model of AQP-mediated transcellular water transport. This model suggests that AQP channels facilitate the movement of water across cells, playing a crucial role in EHT and the regulation of cell morphology. Taken together, Yuki Sato et al study provides a number of significant findings, first it advances our knowledge on hematopoietic development. Secondly, their findings reveal the mechanisms of EHT, highlighting the critical role of AQP1 and the redundancy of AQP channels in the process. Moreover, the findings will pave they way for future investigations into the therapeutic applications of AQPs in hematopoietic disorders and other developmental processes.