Re-evaluation of Diadenosine Tetraphosphate (ApA) From a Stress Metabolite to Secondary Messenger

Cellular homeostasis requires adaption to environmental stress. In response to various environmental and genotoxic stresses, all cells produce dinucleoside polyphosphates (NpNs), the best studied of which is diadenosine tetraphosphate (ApA). Despite intensive investigation, the precise biological roles of these molecules have remained elusive. However, recent studies have elucidated distinct and specific signaling mechanisms for these nucleotides in prokaryotes and eukaryotes. This review summarizes these key discoveries and describes the mechanisms of ApA and ApN synthesis, the mediators of the cellular responses to increased intracellular levels of these molecules and the hydrolytic mechanisms required to maintain low levels in the absence of stress. The intracellular responses to dinucleotide accumulation are evaluated in the context of the "friend" and "foe" scenarios. The "friend (or alarmone) hypothesis" suggests that ApN act as secondary messengers mediating responses to stress. In contrast, the "foe" hypothesis proposes that ApN and other NpN are produced by non-canonical enzymatic synthesis as a result of physiological and environmental stress in critically damaged cells but do not actively regulate mitigating signaling pathways. In addition, we will discuss potential target proteins, and critically assess new evidence supporting roles for ApN in the regulation of gene expression, immune responses, DNA replication and DNA repair. The recent advances in the field have generated great interest as they have for the first time revealed some of the molecular mechanisms that mediate cellular responses to ApN. Finally, areas for future research are discussed with possible but unproven roles for intracellular ApN to encourage further research into the signaling networks that are regulated by these nucleotides.