Pannexin 1 channels mediate ‘find-me’ signal release and membrane permeability during apoptosis

Abstract

Apoptotic cells were shown recently to discharge the nucleotides ATP and UTP to act as 'find-me' signals for phagocytes that engulf dying cells before they release potentially harmful cellular contents. This paper shows that the release of ATP and UTP is through the plasma membrane channel pannexin 1, which is specifically opened by caspase activity. The discovery of a role for pannexin 1 in phagocyte chemoattraction could have implications for human diseases that arise from defective clearance of dying cells. Apoptotic cells discharge ATP and UTP, which act as 'find-me' signals for phagocytes that in turn engulf dying cells before potentially harmful cellular contents are released. These authors show that the release of ATP and UTP is exclusively by means of the plasma membrane channel pannexin 1, which is opened specifically by caspase activity. Apoptotic cells release ‘find-me’ signals at the earliest stages of death to recruit phagocytes1. The nucleotides ATP and UTP represent one class of find-me signals2, but their mechanism of release is not known. Here, we identify the plasma membrane channel pannexin 1 (PANX1) as a mediator of find-me signal/nucleotide release from apoptotic cells. Pharmacological inhibition and siRNA-mediated knockdown of PANX1 led to decreased nucleotide release and monocyte recruitment by apoptotic cells. Conversely, PANX1 overexpression enhanced nucleotide release from apoptotic cells and phagocyte recruitment. Patch-clamp recordings showed that PANX1 was basally inactive, and that induction of PANX1 currents occurred only during apoptosis. Mechanistically, PANX1 itself was a target of effector caspases (caspases 3 and 7), and a specific caspase-cleavage site within PANX1 was essential for PANX1 function during apoptosis. Expression of truncated PANX1 (at the putative caspase cleavage site) resulted in a constitutively open channel. PANX1 was also important for the ‘selective’ plasma membrane permeability of early apoptotic cells to specific dyes3. Collectively, these data identify PANX1 as a plasma membrane channel mediating the regulated release of find-me signals and selective plasma membrane permeability during apoptosis, and a new mechanism of PANX1 activation by caspases.