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Ferrata Storti Foundation
Hematopoiesis
Membrane protein CAR promotes hematopoietic regeneration upon stress
Guojin Wu and Cheng Cheng Zhang
Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX, USA
Haematologica 2021 Volume 106(8):2180-2190
ABSTRACT
Adult hematopoietic stem cells (HSC) are quiescent most of the time, and how HSC switch from quiescence to proliferation fol- lowing hematopoietic stress is unclear. Here we demonstrate that upon stress the coxsackievirus and adenovirus receptor CAR (also known as CXADR) is upregulated in HSC and critical for HSC entry into the cell cycle. Wild-type HSC were detected with more rapid repopula- tion ability than the CAR knockout counterparts. After fluorouracil treat- ment, CAR knockout HSC had lower levels of Notch1 expression and elevated protein level of Numb, a Notch antagonist. The Notch signaling inhibitor DAPT, dominant negative form of MAML (a transcriptional coactivator of Notch), or dominant negative mutant of LNX2 (an E3 lig- ase that acts on Numb and binds to CAR), all were capable of abrogating the function of CAR in HSC. We conclude that CAR activates Notch1 signaling by downregulating Numb protein expression to facilitate entry of quiescent HSC into the cell cycle during regeneration.
Introduction
In adults stem cells with self-renewal and differentiation capabilities are required for tissue homeostasis and regeneration. Quiescence protects stem cells from exhaustion. Hematopoietic stem cells (HSC) are largely quiescent during nor- mal hematopoiesis，1 and adult hematopoiesis is sustained primarily by “short- term” HSC (ST-HSC).1 During injury or inflammation, quiescent HSC enter the cell cycle to accelerate hematopoietic flux,1-3 and cycling HSC return to quiescence after the injury is repaired or inflammation is resolved. Multiple signaling path- ways are known to be important for regulation of cell fates and regeneration of HSC.4-10 The molecular mechanisms that regulate HSC to transit from quiescence to proliferation during regeneration remain largely unknown.
The coxsackievirus and adenovirus receptor (CAR, also known as CXADR) was first reported to mediate viral attachment and infection11 and later was demon- strated to be a tight junction protein.12 CAR expression is required for normal atri- oventricular conduction and cardiac function.13 Its constitutive expression in vari- ous cancerous and normal tissues has also been reported.14,15 CAR was also report- ed to be critical for survival of oral squamous cell carcinomas.16 Interestingly, CAR expression increases during tissue regeneration,17 suggesting that it plays an impor- tant role in repairing injury. Here we demonstrate that CAR expression is tran- siently increased in HSC during fluorouracil (5-FU)-induced hematopoietic injury and bone marrow (BM) transplantation and supports HSC regeneration. CAR does not alter HSC self-renewal but rather induces quiescent HSC to enter cell cycle. Mechanistic studies indicated that CAR activates Notch1 signaling in stressed HSC by degrading the Notch inhibitor Numb.
Methods
Mice
CARloxP/loxP mice and UBC-Cre-ERT2 mice were purchased from Jackson Laboratory. Mice were maintained at the University of Texas Southwestern Medical Center animal facility. All animal experiments were performed with the approval of The University of Texas Southwestern Committee on Animal Care.
Correspondence:
CHENGCHENG ZHANG
Alec.Zhang@UTSouthwestern.edu
Received: November 28, 2019. Accepted: June 22, 2020. Pre-published: June 25, 2020.
https://doi.org/10.3324/haematol.2019.243998
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