IκBα controls dormancy induction in Hematopoietic stem cell development via retinoic acid

Abstract

Recent findings are challenging the classical hematopoietic model in which long-term hematopoietic stem cells (LT-HSC) are the base of the hematopoietic system. Clonal dynamics analysis of the hematopoietic system indicate that LT-HSC are not the main contributors of normal hemapoiesis in physiological conditions and the hematopoietic system is mainly maintained by multipotent progenitors (MPPs, hereafter HPC) and LT-HSCs are mostly in a non-active state. The first HSCs emerge from the aorta-gonad and mesonephros (AGM) region along with hematopoietic progenitors (HPC) within hematopoietic clusters. Molecular pathways that determine the HSC fate instead of HPC are still unknown, although inflammatory signaling, including NF-ΚB has been implicated in the development of HSCs. Here, we identify a chromatin binding function for IΚBα (also known as the inhibitor of NF-ΚB) that is Polycomb repression complex 2 (PRC2)- dependent and specifically determines dormant vs proliferating HSCs from the onset of their emergence in the AGM. We find a specific reduction of LT-HSCs in the IΚBα knockout new-born pups. This defect is manifested at the FL stage already, and traceable to the first emerging HSCs in the E11.5 AGM, without affecting the general HPC population. IΚBα-deficient LT-HSCs express dormancy signature genes, are less proliferative and can robustly respond to activation stimuli such asin vitroculture and serial transplantation. At the molecular level, we find decreased PRC2-dependent H3K27me3 at the promoters of several retinoic acid signaling elements in the IΚBα- deficient aortic endothelium and E14.5 FL LT-HSCs. Additionally, IΚBα binding itself is found in the promoters of retinoic acid receptors rarα in the AGM, and rarγ in the LT-HSC of FL. Overall, we demonstrate that the retinoic acid pathway is over-activated in the hematopoietic clusters of IΚBα-deficient AGMs leading to premature dormancy of LT- HSCs that persists in the FL LT-HSCs.

Chromatin enhancer activity of IκBα mediates the exit from naïve pluripotency

“…While IκBα is commonly known for its role as an inhibitor of the NF-κB signaling pathway (Zhang et al 2017), our laboratory has previously furnished compelling evidence of its alternative role through its interaction with histones and other chromatin components (Aguilera et al 2004; Mulero et al 2013; Marruecos et al 2021). We have demonstrated that chromatin-bound IκBα plays a crucial role in regulating skin and intestinal stem cells (Mulero et al 2013; Marruecos et al 2020), hematopoietic stem cell development (Thambyrajah et al 2022), and it influences the regenerative capacity of these tissues, and in some cases, their susceptibility to neoplastic transformation. Disentangling the chromatin-related function of IκBα from its canonical role as an inhibitor of NF-κB has been challenging, making it difficult to determine its truly biological significance.…”

“…We have previously identified an alternative nuclear function of IκBα involving the interaction with histone deacetylases (HDACs), core elements of the Polycomb Repressor Complex 2 (PRC2) and histones (Aguilera et al 2004; 4 Mulero et al 2013;Marruecos et al 2021). This nuclear IκBα function is crucial for tissue stem cell homeostasis and the proper differentiation of epidermal, intestinal and hematopoietic cells (Mulero et al 2013;Marruecos et al 2020Marruecos et al , 2021Thambyrajah et al 2022).…”

“…We previously have identified an alternative nuclear function of IκBα involving the interaction with different transcriptional regulators, histone deacetylases (HDACs), core elements of the Polycomb Repressor Complex 2 (PRC2) as well as histones H2A and H4 (Aguilera et al 2004;Mulero et al 2013;Marruecos et al 2021). This nuclear IκBα function is crucial to efficiently silence sets of developmental genes and ensure a proper cell fate specification of somatic stem cells in the epidermis, intestine and hematopoietic stem cells (Mulero et al 2013;Marruecos et al 2020Marruecos et al , 2021Thambyrajah et al 2022). We have now investigated the role of IκBα in the pluripotency of mESCs by using deletion mutants and characterizing their capacity to self-renew and differentiate.…”