The Nlrp3 Inflammasome Orchestrates Mobilization of Bone Marrow-Residing Stem Cells into Peripheral Blood

Abstract

Mobilization of stem cells from bone marrow (BM) into peripheral blood (PB) in response to tissue or organ injury, infections, strenuous exercise, or mobilization-inducing drugs is as we postulated result of a “sterile inflammation” in the BM microenvironment that triggers activation of the Complement Cascade (ComC). Therefore, we became interested in the role of the Nlrp3 inflammasome in this process and show for the first time that its activation in ATP-dependent manner orchestrates BM egress of hematopoietic stem/progenitor cells (HSPCs) as well as other stem cells, including mesenchymal stroma cells (MSCs), endothelial progenitor cells (EPCs), and very small embryonic-like stem cells (VSELs). To explain this extracellular ATP is a potent activator of the Nrlp3 inflammasome, which leads to the release of interleukin 1β and interleukin 18, as well as several danger-associated molecular pattern molecules (DAMPs) that activate the mannan-binding lectin (MBL) pathway of the ComC, from cells of the innate immunity network. In support of this mechanism, we demonstrate that the Nlrp3 inflammasome become activated in innate immunity cells by granulocyte colony stimulating factor (G-CSF) and AMD3100 in an ATP-dependent manner. Moreover, administration of the Nlrp3 inflammasome activator nigericin induces mobilization in mice, and the opposite effect is obtained by administration of an Nlrp3 inhibitor (MCC950) to mice mobilized by G-CSF or AMD3100. In summary, our results further support the crucial role of innate immunity, BM sterile inflammation, and novel role of the ATP–Nlrp3–ComC axis in the egress of stem cells into PB.

Novel Evidence that Purinergic Signaling - Nlrp3 Inflammasome Axis Regulates Circadian Rhythm of Hematopoietic Stem/Progenitor Cells Circulation in Peripheral Blood

“…Additional plasma was collected for colorimetric assays. For SKL, CFU-GM, VSEL, MSC, and EPC analysis, blood was collected from the vena cava (with a 25gauge needle and 1-ml syringe containing 250 U heparin) [15,17].…”

“…Cultures were incubated for 7 days (37°C, 95% humidity, and 5% CO2), at which time they were scored under an inverted microscope for the number of colonies. For evaluation of the number of circulating CFU-GM colonies the following formula was used: (number of white blood cells (WBCs) × number of CFU-GM colonies)/number of WBCs plated = number of CFU-GM per μl of PB [15,17].…”

“…Recent evidence from our laboratories indicates that an important role in triggering stress-related or pharmacological mobilization of HSPCs is played by extracellular adenosine triphosphate (ATP), a crucial mediator of the purinergic signaling network [15][16][17]. ATP is released from activated or stressed cells, as seen during hypoxia or after administration of pro-mobilizing agents, and activates the Nlrp3 inflammasome protein complex in innate immunity cells by engaging the P2X7 purinergic receptor on the cell surface [17][18][19][20][21].…”

“…Supporting a crucial role for the Nlrp3 inflammasome complex in the mobilization of HSPCs, inhibition of this complex by the small-molecule inhibitor MCC950 impairs mobilization in response to administration of the pro-mobilizing cytokine granulocyte colony-stimulating factor (G-CSF) or the CXCR4 receptor antagonist AMD3100 [17]. Based on these observations, we hypothesized that deep-sleep hypoxia first releases ATP, which subsequently triggers the Nlrp3 inflammasome and in turn mobilizes HSPCs into PB in a circadian rhythm-dependent manner.…”

“…To address this intriguing question we measured the diurnal level of ATP in PB, evaluated changes in expression of mRNAs for Nlrp3 inflammasome-related genes, inhibited secretion of ATP by employing the pannexin 1 channel-blocking peptide 10Panx [21], and exposed mice to a small-molecule inhibitor of the Nlrp3 inflammasome, MCC950 [17,22,23]. Here for the first time we provide evidence that extracellular ATP-mediated purinergic signaling drives circadian release of HSPCs into PB in an Nlrp3 inflammasome-dependent manner.…”