Emergence of Hematopoietic Precursor Cells from the Aortic Hemogenic Endothelium Involves Distinct Polarity-Dependent Morphodynamics

Core Concepts

The hemogenic endothelium gives rise to hematopoietic precursor cells through an endothelial-to-hematopoietic transition (EHT) process that involves distinct cell emergence types dependent on the establishment of apicobasal polarity.

Abstract

The content describes the heterogeneity in the emergence of hematopoietic precursor cells from the hemogenic endothelium (HE) in the zebrafish embryo. Using live imaging and new transgenic lines, the authors show that two distinct cell types emerge from the HE with radically different morphodynamics.
The first type, termed EHT pol+ cells, maintains apicobasal polarity and an apical/luminal membrane until release, while the second type, EHT pol- cells, emerge through a more dynamic process reminiscent of trans-endothelial migration without clear apicobasal polarity.
The authors provide evidence that the balance between these two emergence types depends on the tuning of apicobasal polarity at the level of the HE, which is sensitive to interference with the transcription factor Runx1. Specifically, expression of a dominant-negative form of Runx1 (dt-Runx1) leads to an accumulation of EHT pol+ cells, suggesting a role for Runx1 in controlling the molecular events that tune apicobasal polarity during the EHT process.
The authors also show that the zebrafish Pard3 isoform, Pard3ba, is sensitive to interference with Runx1 activity. Pard3ba exhibits a heterogeneous spatial distribution along the aortic axis, with its mRNAs preferentially localized in the vicinity of hemogenic/EHT cells in wild-type embryos. This spatial distribution is disrupted in dt-Runx1 mutants, further supporting the idea of a signaling crosstalk between aortic and hemogenic cells that controls cell polarity and its associated features during the EHT process.
Overall, the study highlights critical cellular and dynamic events of the endothelial-to-hematopoietic transition that support emergence complexity, with potential implications for cell fate determination.

Stats

The number of hemogenic cells is significantly increased in dt-Runx1 mutants compared to controls (median values of 25 and 17 cells, respectively).
The number of EHT pol+ cells is significantly increased in dt-Runx1 mutants compared to controls (median values of 11, 9 and 8 cells for 3 mutants out of 7, versus a maximum of 3 cells per embryo for controls).
The number of Pard3ba mRNA spots per aortic segment is significantly increased in dt-Runx1 mutants compared to controls (median values of 167.50 and 65.00 spots, respectively).

Quotes

"Importantly, the fate of this apical-luminal membrane, after the release, may lead to cells potentially endowed with specific functional features."
"Surprisingly, confocal microscopy using our eGFP-podxl2 and mKate2-podxl2 expressing fish lines revealed that HE cells do not appear to be polarized, based on the absence of Podxl2 enrichment at luminal membranes, at the initiation of the EHT time-window and later."
"Interfering with Runx1 function suggests that the balance between the two emergence types depends on tuning apicobasal polarity at the level of the HE."

Key Insights Distilled From

Tuning apicobasal polarity and junctional recycling in the hemogenic endothelium orchestrates the morphodynamic complexity of emerging pre-hematopoietic stem cells

by TORCQ,L., MA... at www.biorxiv.org 09-03-2023

https://www.biorxiv.org/content/10.1101/2023.09.01.555862v2

Deeper Inquiries

How do the distinct morphodynamic characteristics of EHT pol+ and EHT pol- cells impact their downstream fate and function

The distinct morphodynamic characteristics of EHT pol+ and EHT pol- cells have significant implications for their downstream fate and function. EHT pol+ cells, characterized by the maintenance of apicobasal polarity and luminal membrane invagination until release, are likely to give rise to hematopoietic stem cells with specific functional features. The maintenance of apical polarity and luminal membrane integrity in EHT pol+ cells may contribute to their ability to interact with the extracellular environment and potentially influence their differentiation into specific hematopoietic lineages. On the other hand, EHT pol- cells, which do not exhibit luminal membrane invagination, may have a different fate and function. These cells may have altered interactions with the extracellular environment and could potentially differentiate into different hematopoietic cell types compared to EHT pol+ cells. The differences in morphodynamics between these two cell types suggest that apicobasal polarity establishment plays a crucial role in determining their downstream fate and function.

What are the potential counter-arguments to the authors' hypothesis that the control of apicobasal polarity is a key determinant of EHT cell emergence complexity

Counter-arguments to the hypothesis that the control of apicobasal polarity is a key determinant of EHT cell emergence complexity could include the presence of additional regulatory mechanisms or factors that influence the emergence process. While apicobasal polarity is proposed to play a significant role in determining the morphodynamics and fate of EHT cells, other cellular processes such as cell-cell interactions, signaling pathways, or mechanical cues could also contribute to the complexity of EHT cell emergence. Additionally, the heterogeneity observed in EHT cell types may not solely be attributed to apicobasal polarity but could be influenced by a combination of factors that interact in a complex network to regulate hematopoietic stem cell development. Furthermore, the variability in EHT cell emergence could be influenced by environmental factors or stochastic events that are not directly related to apicobasal polarity control.

What are the broader implications of the heterogeneous spatial distribution of Pard3ba mRNAs along the aortic axis, and how might this relate to other developmental processes beyond hematopoiesis

The heterogeneous spatial distribution of Pard3ba mRNAs along the aortic axis has broader implications for developmental processes beyond hematopoiesis. The differential expression of Pard3ba in specific regions of the aorta suggests a potential role in regulating cell polarity and cell-cell interactions in the vascular system. This spatial distribution may indicate a specialized function for Pard3ba in mediating cellular processes within distinct regions of the aorta, such as the hemogenic endothelium. Additionally, the localization of Pard3ba mRNAs near hemogenic/EHT cells implies a potential role in regulating the emergence and differentiation of hematopoietic stem cells. Beyond hematopoiesis, the expression of Pard3ba in specific tissues and cell types may be indicative of its involvement in other developmental processes, such as tissue morphogenesis, organogenesis, or cell migration. The heterogeneous distribution of Pard3ba mRNAs highlights the complexity of regulatory mechanisms that govern cellular behavior and fate during development.