P2X7 is an archaic scavenger receptor recognizing apoptotic neuroblasts in early human neurogenesis

Abstract

The expression and function of P2X7 receptors in adult CNS have been widely studied, however, the roles of these purinergic receptors in human neural development has largely focused on the effects of receptor activation. Previous studies of embryonic and adult rodent neural precursors suggest adenosine triphosphate (ATP), the physiological agonist for P2X receptors, can act as a potent modifier of proliferation, migration and differentiation, mediated via intracellular calcium ([Ca²⁺]_i) signaling. The P2X7 receptor has a ubiquitous distribution in the body but is most abundant on macrophages and microglia where its activation by ATP leads to secretion of proinflammatory cytokines. However, extracellular ATP concentrations in the CNS are usually at sub-micromolar levels suggesting that ATP-induced activation of the P2X7 receptor will not occur under physiological circumstances in the CNS. Another possible role for P2X7 receptors has been suggested by recent work on macrophages and neural precursor cells. In these studies the P2X7 receptor was shown to act as a scavenger receptor i.e. a receptor present on a phagocytotic cell which detects molecules present on the surface of apoptotic cells and facilitates phagocytosis of the apoptotic cell. In a recent study of human neural precursor cells (hNPCs) and neuroblasts isolated from human fetal telencephalons at 16-19 WG, our group showed that both P2X7R^high/DCX^low hNPCs and P2X7R^high/DCX^high neuroblasts were capable of phagocytic engulfment of a range of targets including latex beads, apoptotic ReN cells and apoptotic neuroblasts. We found that these neuroblasts and their precursor cells expressed functional P2X7 receptors on their cell surface. Although expression of P2X7 is widespread in the cells of the neuroblast, it is those DCX⁺ neuroblasts with the highest expression of P2X7 which are actively phagocytic towards an autologous apoptotic neighbour or other phagocytic targets, including latex beads and apoptotic ReNcells. Pre-incubation of P2X7^high neuroblasts with ATP or oxidized ATP inhibited phagocytosis of targets by these cells. Moreover siRNA knockdown of P2X7R also inhibited phagocytosis of the apoptotic targets. This review considers this major new role for the P2X7 receptor in early human neurogenesis.