Endothelial cell biology of Endoglin in hereditary hemorrhagic telangiectasia.

PURPOSE OF REVIEW: Mutations in the Endoglin (Eng) gene, an auxiliary receptor in the transforming growth factor beta (TGFß)-superfamily signaling pathway, are responsible for the human vascular disorder hereditary hemorrhagic telangiectasia (HHT) type 1, characterized in part by blood vessel enlargement. A growing body of work has uncovered an autonomous role for Eng in endothelial cells. We will highlight the influence of Eng on distinct cellular behaviors, such as migration and shape control, which are ultimately important for the assignment of proper blood vessel diameters. RECENT FINDINGS: How endothelial cells establish hierarchically ordered blood vessel trees is one of the outstanding questions in vascular biology. Mutations in components of the TGFß-superfamily of signaling molecules disrupt this patterning and cause arteriovenous malformations (AVMs). Eng is a TGFß coreceptor enhancing signaling through the type I receptor Alk1. Recent studies identified bone morphogenetic proteins (BMPs) 9 and 10 as the primary ligands for Alk1/Eng. Importantly, Eng potentiated Alk1 pathway activation downstream of hemodynamic forces. New results furthermore revealed how Eng affects endothelial cell migration and cell shape control in response to these forces, thereby providing new avenues for our understanding of AVM cause. SUMMARY: We will discuss the interplay of Eng and hemodynamic forces, such as shear stress, in relation to Alk1 receptor activation. We will furthermore detail how this signaling pathway influences endothelial cell behaviors important for the establishment of hierarchically ordered blood vessel trees. Finally, we will provide an outlook how these insights might help in developing new therapies for the treatment of HHT.

Bnip3lb-driven mitophagy sustains expansion of the embryonic hematopoietic stem cell pool.

Embryonic hematopoietic stem and progenitor cells (HSPCs) have the unique ability to undergo rapid proliferation while maintaining multipotency, a clinically-valuable quality which currently cannot be replicated in vitro. Here, we show that embryonic HSPCs achieve this state by precise spatio-temporal regulation of reactive oxygen species (ROS) via Bnip3lb-associated developmentally-programmed mitophagy, a distinct autophagic regulatory mechanism from that of adult HSPCs. While ROS drives HSPC specification in the dorsal aorta, scRNAseq and live-imaging of Tg(ubi:mitoQC) zebrafish indicate that mitophagy initiates as HSPCs undergo endothelial-to-hematopoietic transition and colonize the caudal hematopoietic tissue (CHT). Knockdown of bnip3lb reduced mitophagy and HSPC numbers in the CHT by promoting myeloid-biased differentiation and apoptosis, which was rescued by anti-oxidant exposure. Conversely, induction of mitophagy enhanced both embryonic HSPC and lymphoid progenitor numbers. Significantly, mitophagy activation improved ex vivo functional capacity of hematopoietic progenitors derived from human-induced pluripotent stem cells (hiPSCs), enhancing serial-replating hematopoietic colony forming potential. HIGHLIGHTS: ROS promotes HSPC formation in the dorsal aorta but negatively affects maintenance thereafter.HSPCs colonizing secondary niches control ROS levels via Bnip3lb-directed mitophagy.Mitophagy protects nascent HSPCs from ROS-associated apoptosis and maintains multipotency.Induction of mitophagy enhances long-term hematopoietic potential of iPSC-derived HSPCs.

CellComm infers cellular crosstalk that drives haematopoietic stem and progenitor cell development.

Intercellular communication orchestrates a multitude of physiologic and pathologic conditions. Algorithms to infer cell-cell communication and predict downstream signalling and regulatory networks are needed to illuminate mechanisms of stem cell differentiation and tissue development. Here, to fill this gap, we developed and applied CellComm to investigate how the aorta-gonad-mesonephros microenvironment dictates haematopoietic stem and progenitor cell emergence. We identified key microenvironmental signals and transcriptional networks that regulate haematopoietic development, including Stat3, Nr0b2, Ybx1 and App, and confirmed their roles using zebrafish, mouse and human models. Notably, CellComm revealed extensive crosstalk among signalling pathways and convergence on common transcriptional regulators, indicating a resilient developmental programme that ensures dynamic adaptation to changes in the embryonic environment. Our work provides an algorithm and data resource for the scientific community.

Making Blood from the Vessel: Extrinsic and Environmental Cues Guiding the Endothelial-to-Hematopoietic Transition.

It is increasingly recognized that specialized subsets of endothelial cells carry out unique functions in specific organs and regions of the vascular tree. Perhaps the most striking example of this specialization is the ability to contribute to the generation of the blood system, in which a distinct population of "hemogenic" endothelial cells in the embryo transforms irreversibly into hematopoietic stem and progenitor cells that produce circulating erythroid, myeloid and lymphoid cells for the lifetime of an animal. This review will focus on recent advances made in the zebrafish model organism uncovering the extrinsic and environmental factors that facilitate hemogenic commitment and the process of endothelial-to-hematopoietic transition that produces blood stem cells. We highlight in particular biomechanical influences of hemodynamic forces and the extracellular matrix, metabolic and sterile inflammatory cues present during this developmental stage, and outline new avenues opened by transcriptomic-based approaches to decipher cell-cell communication mechanisms as examples of key signals in the embryonic niche that regulate hematopoiesis.

YAP Regulates Hematopoietic Stem Cell Formation in Response to the Biomechanical Forces of Blood Flow.

Hematopoietic stem and progenitor cells (HSPCs), first specified from hemogenic endothelium (HE) in the ventral dorsal aorta (VDA), support lifelong hematopoiesis. Their de novo production promises significant therapeutic value; however, current in vitro approaches cannot efficiently generate multipotent long-lived HSPCs. Presuming this reflects a lack of extrinsic cues normally impacting the VDA, we devised a human dorsal aorta-on-a-chip platform that identified Yes-activated protein (YAP) as a cyclic stretch-induced regulator of HSPC formation. In the zebrafish VDA, inducible Yap overexpression significantly increased runx1 expression in vivo and the number of CD41+ HSPCs downstream of HE specification. Endogenous Yap activation by lats1/2 knockdown or Rho-GTPase stimulation mimicked Yap overexpression and induced HSPCs in embryos lacking blood flow. Notably, in static human induced pluripotent stem cell (iPSC)-derived HE culture, compound-mediated YAP activation enhanced RUNX1 levels and hematopoietic colony-forming potential. Together, our findings reveal a potent impact of hemodynamic Rho-YAP mechanotransduction on HE fate, relevant to de novo human HSPC production.

Genetic dissection of endothelial transcriptional activity of zebrafish aryl hydrocarbon receptors (AHRs).

The aryl hydrocarbon receptor (AHR) is a basic helix-loop-helix transcription factor conserved across phyla from flies to humans. Activated by a number of endogenous ligands and environmental toxins, studies on AHR function and gene regulation have largely focused on a toxicological perspective relating to aromatic hydrocarbons generated by human activities and the often-deleterious effects of exposure on vertebrates mediated by AHR activation. A growing body of work has highlighted the importance of AHR in physiologic processes, including immune cell differentiation and vascular patterning. Here we dissect the contribution of the 3 zebrafish AHRs, ahr1a, ahr1b and ahr2, to endothelial cyp1a1/b1 gene regulation under physiologic conditions and upon exposure to the AHR ligand Beta-naphthoflavone. We show that in fish multiple AHRs are functional in the vasculature, with vessel-specific differences in the ability of ahr1b to compensate for the loss of ahr2 to maintain AHR signaling. We further provide evidence that AHR can regulate the expression of the chemokine receptor cxcr4a in endothelial cells, a regulatory mechanism that may provide insight into AHR function in the endothelium.

Endoglin controls blood vessel diameter through endothelial cell shape changes in response to haemodynamic cues.

The hierarchical organization of properly sized blood vessels ensures the correct distribution of blood to all organs of the body, and is controlled via haemodynamic cues. In current concepts, an endothelium-dependent shear stress set point causes blood vessel enlargement in response to higher flow rates, while lower flow would lead to blood vessel narrowing, thereby establishing homeostasis. We show that during zebrafish embryonic development increases in flow, after an initial expansion of blood vessel diameters, eventually lead to vessel contraction. This is mediated via endothelial cell shape changes. We identify the transforming growth factor beta co-receptor endoglin as an important player in this process. Endoglin mutant cells and blood vessels continue to enlarge in response to flow increases, thus exacerbating pre-existing embryonic arterial-venous shunts. Together, our data suggest that cell shape changes in response to biophysical cues act as an underlying principle allowing for the ordered patterning of tubular organs.