Deficiency of Rbpj Leads to Defective Stress-Induced Hematopoietic Stem Cell Functions and Hif Mediated Activation of Non-canonical Notch Signaling Pathways.

Ram Lakhan,Chozha V Rathinam

doi:10.3389/fcell.2020.622190

Abstract

Deregulated notch signaling has been associated with human pathobiology. However, functions of notch pathways in hematopoiesis remain incompletely understood. Here, we ablated canonical notch pathways, through genetic deletion of Rbpj, in hematopoietic stem cells (HSCs). Our data identified that loss of canonical notch results in normal adult HSC pool, at steady state conditions. However, HSC maintenance and functions in response to radiation-, chemotherapy-, and cytokine- induced stress were compromised in the absence of canonical notch. Rbpj deficient HSCs exhibit decreased proliferation rates and elevated expression of p57Kip2. Surprisingly, loss of Rbpj resulted in upregulation of key notch target genes and augmented binding of Hes1 to p57 and Gata2 promoters. Further molecular analyses identified an increase in notch activity, elevated expression and nuclear translocation of Hif proteins, and augmented binding of Hif1α to Hes1 promoter in the absence of Rbpj. These studies, for the first time, identify a previously unknown role for non-canonical notch signaling and establish a functional link between Hif and Notch pathways in hematopoiesis.

Highlights

Notch signaling pathway is highly conserved and plays a vital role in development and adulthood
In vitro studies based on overexpression of Notch receptors and treatment of hematopoietic stem cells (HSCs) with notch ligands have provided a compelling evidence on the positive role of notch in HSCs (Pui et al, 1999; Varnum-Finney et al, 2000; Stier et al, 2002; Duncan et al, 2005; Rathinam et al, 2006; Francis et al, 2017)
Even though there appears to be an obvious disagreement regarding the functions of notch in HSCs, these differences are likely multifactorial and possible explanations for these discrepancies could be as follows; (1) In view of the fact that are multiple notch receptors and their ligands in the mammalian system, it remains possible that deletion of an individual notch receptor/ligand can be compensated by the other notch receptors

Summary

Introduction

Notch signaling pathway is highly conserved and plays a vital role in development and adulthood. There are essentially four classes of molecules; Notch receptors, ligands, positive, and negative regulators, and transcription factors, that control notch signaling network (Pajcini et al, 2011; Lobry et al, 2014). Rbpj Regulates Stress-Induced Hematopoiesis heterodimerizes with the DNA binding transcription factorrecombination signal binding protein for immunoglobulin k J region (Rbpj, known as CSL/CBF-1) and recruits additional co-factors, including mastermind proteins (MAML 1–3), to activate target genes, such as members of the Hairy enhancer of split (Hes) and Hairy related (Hey or Hrt) gene families (Gordon et al, 2008; Kopan and Ilagan, 2009; Radtke et al, 2010; Pajcini et al, 2011; Lobry et al, 2014). Functions of notch in the maintenance of adult HSCs remain controversial

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