Non-Vesicular Release of Alarmin Prothymosin α Complex Associated with Annexin-2 Flop-Out

Nuclear protein prothymosin & alpha; (ProT & alpha;) is a unique member of damage-associated molecular patterns (DAMPs)/alarmins. ProT & alpha; prevents neuronal necrosis by causing a cell death mode switch in serum-starving or ischemic/reperfusion models in vitro and in vivo. Underlying receptor mechanisms include Toll-like receptor 4 (TLR4) and G(i)-coupled receptor. Recent studies have revealed that the mode of the fatal stress-induced extracellular release of nuclear ProT & alpha; from cortical neurons in primary cultures, astrocytes and C6 glioma cells has two steps: ATP loss-induced nuclear release and the Ca2+-mediated formation of a multiple protein complex and its extracellular release. Under the serum-starving condition, ProT & alpha; is diffused from the nucleus throughout the cell due to the ATP loss-induced impairment of importin & alpha;-mediated nuclear transport. Subsequent mechanisms are all Ca2+-dependent. They include the formation of a protein complex with ProT & alpha;, S100A13, p40 Syt-1 and Annexin A2 (ANXA2); the fusion of the protein complex to the plasma membrane via p40 Syt-1-Stx-1 interaction; and TMEM16F scramblase-mediated ANXA2 flop-out. Subsequently, the protein complex is extracellularly released, leaving ANXA2 on the outer cell surface. The ANXA2 is then flipped in by a force of ATP8A2 activity, and the non-vesicular release of protein complex is repeated. Thus, the ANXA2 flop-out could play key roles in a new type of non-vesicular and non-classical release for DAMPs/alarmins, which is distinct from the modes conducted via gasdermin D or mixed-lineage kinase domain-like pseudokinase pores.

Automated summary

Nuclear protein prothymosin α (ProTα) is a member of DAMPs/alarmins, which prevents neuronal necrosis by causing a cell death mode switch. Its extracellular release is a Ca2+-dependent process involving the formation of a protein complex and subsequent release. This non-vesicular and non-classical release mediated by ANXA2 flop-out could play key roles in DAMPs/alarmins release.