Jagged2-Notch axis: keeping stem cells in asymmetric balance Free

In this issue of Blood, Matteini et al¹ demonstrate that age-associated loss of Jagged2 (J2) expression by bone marrow endothelial cells (BMECs) impairs hematopoietic stem cell (HSC) polarity, asymmetric cell division (ACD), and regenerative potential by promoting cis inhibition of Notch signaling during aging.

This study elegantly integrates decades of observations, proposing J2/Notch interaction as a key regulator of HSC divisional symmetry and aging and offering insights that may inform strategies to restore regenerative function in aged hematopoiesis.

By combining high-resolution 3-dimensional multicolor imaging to capture dividing HSCs in fixed bone samples with advanced genetic models, the authors dissect the cross talk between intrinsic and extrinsic mechanisms regulating HSC function during aging, with a focus on Notch signaling dynamics and the role of the bone marrow vascular niche.

In the first set of experiments, Matteini et al used the Hes1p-2dEGFP reporter mouse to visualize rapid Notch activation-deactivation cycles in HSCs, alongside VEGFR3Cre-driven inducible deletion of J2 in sinusoidal endothelium. Their results reveal that Notch signaling is heterogeneously activated within the HSC pool: most Notch-active HSCs localize near J2⁺ sinusoids and exhibit higher levels of H4K16ac, a polarity-associated epigenetic mark previously linked to regenerative potential.² Exploring deeper, the authors observed that sinusoidal endothelial J2 deletion in mice not only caused displacement of HSCs away from the sinusoidal niches but also led to a doubling in HSC numbers, while reducing polarity (H4K16ac) and increasing dividing pairs. These observations prompted investigation into whether J2/Notch disruption perturbs ACD. ACD is recognized as a fundamental mechanism that balances self-renewal and differentiation, thereby maintaining a stable stem cell pool across diverse stem cell systems and organisms. However, documenting ACD within the hematopoietic system has proven technically challenging due to the rarity of HSCs and the difficulty in tracking daughter cell fates over time. Only recently, thanks to advances in live-cell imaging and genetic lineage tracing, has ACD been directly demonstrated in HSCs.³ Nonetheless, current approaches are still challenging and remain largely limited to in vitro live imaging or snapshot analyses of dividing HSCs in fixed tissues.

Given these constraints, the authors’ effort to interrogate ACD within the context of Notch signaling disruption is particularly valuable. They present compelling evidence that, although HSC divisions are infrequent and predominantly asymmetric in the wild-type setting, the frequency of H4K16ac-symmetric divisions increases markedly in the absence of sinusoidal J2. Their findings are especially notable considering the well-established role of Notch in regulating ACD in other cellular systems.³ These data reinforce the importance of endothelial-derived Notch cues in maintaining HSC polarity and division asymmetry but also highlight a functional link between niche-derived J2 signaling and stem cell fate decisions. Notably, this work provides a powerful framework for studying HSC ACD in distinct mouse genetic and physiological contexts, expanding the experimental tool kit to dissect how microenvironmental signals shape stem cell behavior in vivo.

To explore whether altered ACD influences HSC fate, the authors used an elegant approach by using the Fgd5ZsGreen-TdTomato model, enabling simultaneous HSC identification (ZsGreen⁺) and lineage tracing of progeny (TdTomato⁺). In transplantation assays, endothelial J2 deletion increased the clustering and frequency of myeloid-biased HSCs while reducing production of committed progenitors and mature blood cells. These data align with earlier findings that Notch activation inhibits myeloid differentiation in murine and human CD34⁺ cells in vitro^4,5 and enhances murine HSC self-renewal in vivo.⁶ 

Collectively, this study builds on and advances the field, beginning with the first demonstration that human BMECs express J2 and preserve the multipotentiality of CD34⁺ progenitors via cell-cell contact and Notch signaling in vitro.⁷ This early work provided foundational evidence for a J2-Notch axis, later confirmed in vivo, in which endothelial J2 activates Notch in HSCs and promotes hematopoietic recovery after myelosuppression.⁸ In this study, Matteini et al add spatial and functional resolution, linking Notch signal intensity, ACD, and regenerative potential, while revealing heterogeneous Notch activity across the HSC pool, which may underlie population-level fluctuations and functional diversity.

An intriguing open question is how cell-cycle dynamics intersect with Notch-regulated ACD and cell fate. Although Notch generally enhances cell cycling in adult stem/progenitor cells, its precise role in balancing self-renewal and differentiation in HSCs remains unclear. Notch may prime HSCs for stress-induced proliferation or modulate fate through altered kinetics, for instance, by shortening G1 and reducing the fraction of cells responsive to differentiation cues, as postulated by Pardee and reported in some studies.^5,9,10 

A critical aspect of this work is its link to aging, highlighted by the discovery of a switch from J2-mediated transactivation to J2 cis inhibition in aged HSCs. A hallmark of aging tissues is endothelial dysfunction that undermines stem cell homeostasis; in the bone marrow, this manifests as arteriolar regression and loss of type H endothelial cells, diminishing Jagged ligands availability. In response to decreased Notch receptor engagement, Matteini et al found that HSCs upregulate the J2 ligand themselves. Although this can enhance Notch transactivation in some cells, in others, such as, in this case, HSCs, it results in Notch cis inhibition, consistent with the paradigm proposed by Thambyrajah et al during embryonic hematopoietic development and discussed by Matteini et al.

Together, these findings suggest that loss of sinusoidal J2 mimics bone marrow aging, leading to HSC displacement from sinusoids, increased J2-driven Notch cis inactivation, enhanced symmetric divisions, myeloid bias, and, ultimately, loss of regenerative capacity.

In conclusion, Matteini et al identify endothelial J2 as a key regulator of HSC polarity and fate through both cell-extrinsic and cell-intrinsic Notch signaling. They show that aging shifts J2’s role from activating to inhibitory, linking niche remodeling and altered Notch signaling to stem cell decline. Although it remains unclear whether the same mechanisms apply to human HSCs, these findings offer a new framework for understanding HSC aging and suggest potential strategies to rejuvenate aged hematopoietic systems by restoring endothelial-HSC interactions that support balanced self-renewal.

Conflict-of-interest disclosure: The author declares no competing financial interests.