Extracellular Adenosine Triphosphate (eATP) and Its Metabolite, Extracellular Adenosine (eAdo), as Opposing "Yin-Yang" Regulators of Nlrp3 Inflammasome in the Trafficking of Hematopoietic Stem/Progenitor Cells

Abstract

Nlrp3 inflammasome plays a pleiotropic role in hematopoietic cells. On the one hand, physiological activation of this intracellular protein complex is crucial to maintaining normal hematopoiesis and the trafficking of hematopoietic stem progenitor cells (HSPCs). On the other hand, its hyperactivation may lead to cell death by pyroptosis, and prolonged activity is associated with sterile inflammation of the BM and, as a consequence, with the HSPCs aging and origination of myelodysplasia and leukemia. Thus, we need to understand better this protein complex's actions to define the boundaries of its safety window and study the transition from being beneficial to being detrimental. As demonstrated, the Nlrp3 inflammasome is expressed and active both in HSPCs and in the non-hematopoietic cells that are constituents of the bone marrow (BM) microenvironment. Importantly, the Nlrp3 inflammasome responds to mediators of purinergic signaling, and while extracellular adenosine triphosphate (eATP) activates this protein complex, its metabolite extracellular adenosine (eAdo) has the opposite effect. In this review, we will discuss and focus on the physiological consequences of the balance between eATP and eAdo in regulating the trafficking of HSPCs in an Nlrp3 inflammasome-dependent manner, as seen during pharmacological mobilization from BM into peripheral blood (PB) and in the reverse mechanism of homing from PB to BM and engraftment. We propose that both mediators of purinergic signaling and the Nlrp3 inflammasome itself may become important therapeutic targets in optimizing the trafficking of HSPCs in clinical settings.

Keywords: NOD-like receptor family pyrin domain-containing protein 3 (Nlrp3) inflammasome; extracellular ATP; purinergic signaling; stem cell homing and engraftment; stem cell mobilization.

Figure 1

The impact of Nrlp3 KO on the mobilization of murine HSPCs. Mononuclear cells (MNCs) were isolated from WT and Nrlp3^–/– mice after 3 days of G-CSF (A) or AMD3100 (B) mobilization. The numbers of WBCs, SKL (Sca-1⁺/c-kit⁺/Lin⁻) cells, and CFU-GM clonogenic progenitors were evaluated in PB. Results from two independent experiments are pooled together. *p ≤ 0.01.

Figure 2

Cell migration-promoting mechanisms at the leading surface/edge and the negative-feedback mechanism at the retracting surface/edge of migrating HSPCs. We propose that in response to BM chemoattractants, HSPCs activate Nox2, which is a membrane lipid raft-associated enzyme and a source of ROS [1]. ROS activates the Nlrp3 inflammasome [2], which releases ATP into the extracellular space surrounding HSPCs [3]. In a positive-feedback mechanism, extracellular ATP (eATP) activates the Nlrp3 inflammasome and membrane lipid raft formation so that cells more robustly respond to BM chemoattractants [4]. In a negative-feedback mechanism, eATP is converted by the cell surface-expressed ectonucleotodases (CD39 and CD73) into extracellular adenosine (eAdo), which via the P1 receptors (A2a, A2b) activates heme oxygenase 1 (HO-1) [5], a negative regulator of the Nlrp3 inflammasome [6].

Figure 3

Opposing positive-negative (“Yin–Yang”) effects of eATP and eAdo on stem cell trafficking. While eATP promotes cell migration by activating the Nlrp3 inflammasome, eAdo-induced intracellular heme oxygenase 1 (HO-1) inhibits the Nlrp3 inflammasome and stem cell trafficking. A similar negative effect may have inducible nitric oxide synthetase (iNOS) (103).

Figure 4

Confocal analysis of membrane lipid rafts in purified murine SKL cells. Representative images of SKL cells sorted from WT BM, stimulated with SDF-1 (50 ng/ml) and LL-37 (2.5 μg/ml); stained with cholera toxin subunit B (a lipid raft marker) conjugated with FITC and rat anti-mouse CXCR4, followed by anti-rat Alexa Fluor 594; and evaluated by confocal microscopy for the formation of membrane lipid rafts. Lipid rafts were formed in SKL cells (upper panel) but not in SKL cells isolated from Nlrp3-KO (middle panel) or cells exposed to apyrase (50 U/ml) lower panel. Representative pictures are shown.