Notch Signaling in HSC Emergence: When, Why and How

Abstract

The hematopoietic stem cell (HSC) sustains blood homeostasis throughout life in vertebrates. During embryonic development, HSCs emerge from the aorta-gonads and mesonephros (AGM) region along with hematopoietic progenitors within hematopoietic clusters which are found in the dorsal aorta, the main arterial vessel. Notch signaling, which is essential for arterial specification of the aorta, is also crucial in hematopoietic development and HSC activity. In this review, we will present and discuss the evidence that we have for Notch activity in hematopoietic cell fate specification and the crosstalk with the endothelial and arterial lineage. The core hematopoietic program is conserved across vertebrates and here we review studies conducted using different models of vertebrate hematopoiesis, including zebrafish, mouse and in vitro differentiated Embryonic stem cells. To fulfill the goal of engineering HSCs in vitro, we need to understand the molecular processes that modulate Notch signaling during HSC emergence in a temporal and spatial context. Here, we review relevant contributions from different model systems that are required to specify precursors of HSC and HSC activity through Notch interactions at different stages of development.

Keywords: AGM; HSC; Notch signaling; developmental hematopoiesis.

Figure 1

Schematic representation of Notch signaling molecule interaction, their localization within the cellular compartment and their life cycle. (A) Notch receptors undergo S1 cleavage in the Golgi vesicle. (B) Notch receptors can be directly degraded (without ligand interaction) via NEDD4/ITCH from the cellular surface or processed to Notch Intracellular Domain (NICD) in the absence of a Notch ligand by Deltex. (C) In the canonical Notch receptor processing, Notch receptor is cleaved upon contact with a ligand by ADAM family members at the c-terminal (intracellular) end that leads to exposure of the S3 cleavage site. (D) The ligand sending cell endocytoses the extracellular domain of the Notch receptor (NECD) that it “pulled off” when interacting with the receptor. (E) The NICD is then further processed by the γ-secretase complex to an activated NICD (cleavage at Val1744) within endosomes/lysosomes that can either be targeted for degradation or translocate to the nucleus for target gene modulation. Ub: ubiquitination, Co-R: undefined co-repressor, Co-A: undefined co-activator, MAM: MAML. Created with BioRender.com (accessed on 15 January 2022).

Figure 2

Scheme of lateral inhibition (A) and lateral induction (B). (A) Within an equipotent cell population with fluctuating Notch activity, a stochastic up-regulation of Notch activation can induce the expression of the Notch downstream transcriptional repressors that in turn silence the expression of ligand transcription. The remaining receptors on the surface of this cell can now act as signal senders for neighboring cells and induce a different fate to its own. Over time, a salt and pepper pattern emerges. A cell with high levels of ligand (sender) is positioned surrounded by receptor expressing cells (receivers). This mechanism of cell fate determination is termed lateral inhibition. (B) Lateral induction is the term used for sequential induction of Notch activity within adjacent cells. In this scenario, Notch activity induces further transcriptional activation of Notch receptors and ligands. The cell stays activated (through the newly synthesized receptor) but further activates the adjacent through the ligand. This cycle is repeated over time to establish a group of cells with the identical cell fate. Arrow depicts direction of activation. Created with BioRender.com (accessed on 15 January 2022).

Figure 3

Concepts of hemogenic endothelium specification. Hematopoietic potential is highlighted in green. Early mesodermal vascular progenitors might contain hemangioblasts, a bipotential cell with endothelial and hematopoietic potential already, or hematopoietic potential is acquired secondary after commitment to a vascular arterial fate. Top: early vascular progenitor cells have a rare population of cells that are hemogenic. bottom: hemogenic endothelial cells are specified after arterialization of the endothelium. HE: hemogenic endothelium. Green: vascular cell with hematopoietic properties. Created with BioRender.com (accessed on 15 January 2022).

Figure 4

A comparison of zebrafish and mouse embryonic aortic hematopoiesis. Depicted are Notch signaling molecules that act during hemogenic endothelium specification and intra-aortic cluster formation. In mouse EHT, the hematopoietic cells accumulate as clusters and have low(er) Notch activity (left). During zebrafish hematopoiesis, cells undergoing EHT leave the aorta individually and do not accumulate as clusters. Green: cells with hematopoietic properties. HE: hemogenic endothelium. Created with BioRender.com (accessed on 15 January 2022).