ALK1/Endoglin Signaling Restricts Vein Cell Size Increases in Response to Hemodynamic Cues

Academic Background

The normal development and function of the vascular system rely on precise regulation of blood vessel diameter. Hemodynamic cues, such as fluid shear stress (FSS), are considered critical factors in regulating vessel diameter. According to the shear stress set point theory, increased blood flow leads to vessel dilation, while decreased blood flow results in vessel constriction. However, abnormalities in vessel diameter regulation can lead to congenital arteriovenous malformations (AVMs), particularly in patients with hereditary hemorrhagic telangiectasia (HHT). HHT is a genetic disorder caused by mutations in genes such as ALK1 or Endoglin, characterized by vascular malformations and a tendency for bleeding.

Although the shear stress set point theory has been widely validated in arteries, the mechanisms underlying venous responses to changes in blood flow remain unclear. This study aims to use zebrafish embryos to reveal the role of ALK1/Endoglin signaling in regulating venous endothelial cell (EC) size and to explore its mechanism in AVM formation.

Source of the Paper

This paper was co-authored by Zeenat Diwan, Jia Kang, Emma Tsz Too, and Arndt F. Siekmann from the Department of Cell and Developmental Biology at the Perelman School of Medicine, University of Pennsylvania. The paper was published on October 20, 2024, in the journal Angiogenesis, with the DOI 10.1007/s10456-024-09955-3.

Research Process and Results

1. Morphological Differences Between Arterial and Venous Endothelial Cells in Zebrafish Embryos

The study began by using time-lapse imaging to observe the development of the major artery (dorsal aorta, DA) and the major vein (posterior cardinal vein, PCV) in zebrafish embryos from 24 hours post-fertilization (hpf) to 72 hpf. The research found significant differences in the morphology and size of arterial and venous endothelial cells. Arterial endothelial cells significantly enlarged in response to increased blood flow, while venous endothelial cells remained relatively stable. This difference was closely related to changes in vessel diameter, with arterial diameter increases primarily dependent on endothelial cell enlargement, while venous diameter changes were minimal.

2. Effects of Blood Flow Changes on Arterial and Venous Diameters

To validate the shear stress set point theory, the study reduced blood flow in zebrafish embryos through Tricaine treatment and observed changes in vessel diameter and endothelial cell morphology. The results showed that reduced blood flow led to a significant decrease in arterial endothelial cell size and vessel diameter, while venous endothelial cells exhibited smaller changes. This indicates that arterial responses to blood flow changes align with the shear stress set point theory, while veins exhibit different response mechanisms.

3. Role of Endoglin and ALK1 in Regulating Venous Endothelial Cell Size

The study found that Endoglin and ALK1 signaling play a crucial role in venous endothelial cells, restricting their size expansion in response to increased blood flow. Through chimeric embryo experiments, researchers discovered that venous endothelial cells with Endoglin or ALK1 mutations significantly enlarged under normal hemodynamic conditions, while arterial endothelial cells showed no obvious phenotype. This suggests that Endoglin and ALK1 autonomously regulate venous endothelial cell size, preventing excessive expansion in response to increased blood flow.

4. Mechanism of AVM Formation in Endoglin Mutants

Further research revealed that Endoglin mutations lead to increased venous diameter, subsequently triggering AVMs. The increase in venous diameter results in higher blood flow, causing arterial endothelial cells to further expand in response to the increased flow, thereby exacerbating AVM formation. This finding highlights the critical role of Endoglin and ALK1 signaling in maintaining normal vascular development.

Conclusions and Significance

This study reveals the crucial role of ALK1/Endoglin signaling in regulating venous endothelial cell size and elucidates its mechanism in AVM formation. The results demonstrate that venous endothelial cells restrict their size expansion in response to increased blood flow through ALK1/Endoglin signaling, thereby preventing excessive vessel dilation. This discovery not only deepens our understanding of vascular development and hemodynamic regulation but also provides new insights for the treatment of vascular diseases such as HHT.

Research Highlights

1. Key Finding: The study is the first to reveal the critical role of ALK1/Endoglin signaling in regulating venous endothelial cell size and its mechanism in AVM formation.
2. Methodological Innovation: Through chimeric embryo experiments, researchers successfully distinguished the different roles of Endoglin and ALK1 in arterial and venous endothelial cells, uncovering their cell-autonomous regulatory mechanisms.
3. Application Value: The findings provide new therapeutic targets for vascular diseases such as HHT, offering significant clinical value.

Additional Valuable Information

The study also found that venous endothelial cell size is inversely correlated with vessel diameter, while arterial endothelial cell size is positively correlated with vessel diameter. This finding provides a new perspective for further research on vascular development and hemodynamic regulation. Additionally, the study emphasizes the necessity of distinguishing primary from secondary effects in disease models, offering important methodological guidance for future research.

By using zebrafish embryos, this study reveals the critical role of ALK1/Endoglin signaling in regulating venous endothelial cell size, providing new insights into vascular development and hemodynamic regulation, and offering potential therapeutic targets for related diseases.