P2X7 Receptors Regulate Phagocytosis and Proliferation in Adult Hippocampal and SVZ Neural Progenitor Cells: Implications for Inflammation in Neurogenesis

Abstract

Identifying the signaling mechanisms that regulate adult neurogenesis is essential to understanding how the brain may respond to neuro-inflammatory events. P2X7 receptors can regulate pro-inflammatory responses, and in addition to their role as cation channels they can trigger cell death and mediate phagocytosis. How P2X7 receptors may regulate adult neurogenesis is currently unclear. Here, neural progenitor cells (NPCs) derived from adult murine hippocampal subgranular (SGZ) and cerebral subventricular (SVZ) zones were utilized to characterize the roles of P2X7 in adult neurogenesis, and assess the effects of high extracellular ATP, characteristic of inflammation, on NPCs. Immunocytochemistry found NPCs in vivo and in vitro expressed P2X7, and the activity of P2X7 in culture was demonstrated using calcium influx and pore formation assays. Live cell and confocal microscopy, in conjunction with flow cytometry, revealed P2X7 NPCs were able to phagocytose fluorescent beads, and this was inhibited by ATP, indicative of P2X7 involvement. Furthermore, P2X7 receptors were activated with ATP or BzATP, and 5-ethynyl-2'-deoxyuridine (EdU) used to observe a dose-dependent decrease in NPC proliferation. A role for P2X7 in decreased NPC proliferation was confirmed using chemical inhibition and NPCs from P2X7 mice. Together, these data present three distinct roles for P2X7 during adult neurogenesis, depending on extracellular ATP concentrations: (a) P2X7 receptors can form transmembrane pores leading to cell death, (b) P2X7 receptors can regulate rates of proliferation, likely via calcium signaling, and (c) P2X7 can function as scavenger receptors in the absence of ATP, allowing NPCs to phagocytose apoptotic NPCs during neurogenesis. Stem Cells 2018;36:1764-1777.

P2X7 receptor activity landscape in rat and human glioma cell lines

“…While P2X7-dependent calcium increase is not surprising in case of C6 cells (Wei et al, 2008), to our knowledge there are no reports specifically describing presence or absence of P2X7-initiated calcium influx in the two human glioma cell lines. However, P2X7-mediated calcium signals were observed in human astrocytes, murine neural progenitor cells and rat oligodendrocyte precursor cells and all these cell types are top candidates to be the putative cells of origin for glioma (Alloisio et al, 2006;Narcisse et al, 2005;Jiang & Uhrbom, 2012;Wang et al, 2009;Leeson et al, 2018). The detailed nature of the observed signal is however not exactly clear since there are strong proofs for its metabotropic character (Supłat-Wypych et al, 2010) as well as suggestions of active ATP release through P2X7 receptors (Brandao-Burch et al, 2012).…”

P2X7 as a scavenger receptor for innate phagocytosis in the brain

“…Emerging evidence, both functional and genetic, now points to an important scavenger role of an archaic purinergic receptor, P2X7, within the CNS in which P2X7 was found expressed in brain macrophages, microglia, astrocytes, oligodendrocytes, new born rat and human embryonic brain neurones but not adult cortical neurones (Collo et al, 1997;Sim et al, 2004;Hamilton et al, 2009;Lovelace et al, 2015;Yamamoto et al, 2013). A recent study using wild-type and P2X7 À/À C57BL/6 mice demonstrated the expression of the P2X7 receptor on neural progenitor cells isolated from hippocampal subgranular and cerebral subventricular zones (Leeson et al, 2018). Most published studies of P2X7 receptors have focused on the pro-inflammatory 'pore' function of this receptor, but the requirement for high concentrations (>100 μM, but usually 1-5 mM depending on ambient divalent cations) of extracellular ATP for pore opening would be rarely achieved in physiological conditions, suggesting that this receptor has an alternative non-inflammatory function in the CNS.…”

Calcium Channels in Adult Brain Neural Stem Cells and in Glioblastoma Stem Cells

“…Extracellular nucleotides mediate their effects through metabotropic G-protein-coupled P2Y purinergic receptors and ionotropic purinergic P2X receptors, both of which are expressed in the V-SVZ and induce Ca 2+ transients (Stafford et al, 2007;Messemer et al, 2013). Specifically, the P2X7R subtype that has been detected in ependymal (E) is also expressed in transient amplifying progenitors (C cells) where it plays a dual role: in the absence of the ligand, P2X7R functions as a scavenger receptor involved in phagocytosis and following its activation by ATP, the P2X7R reduces proliferation of C cells (Genzen et al, 2009;Messemer et al, 2013;Leeson et al, 2018). It should be mentioned here that although these data appear contradictory with the fact that increased levels of extracellular nucleotides favor cell proliferation, ATP also acts on other receptors among which the metabotropic P2Y1R receptors that foster expansion of the C cell population (Suyama et al, 2012).…”