Synthesis of dinucleoside tetraphosphates in transfected cells by a firefly luciferase reporter gene

Re-evaluation of Diadenosine Tetraphosphate (Ap4A) From a Stress Metabolite to Bona Fide Secondary Messenger

Cellular homeostasis requires adaption to environmental stress. In response to various environmental and genotoxic stresses, all cells produce dinucleoside polyphosphates (Np_nNs), the best studied of which is diadenosine tetraphosphate (Ap₄A). 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 Ap₄A and Ap₄N 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 Ap_nN act as bona fide secondary messengers mediating responses to stress. In contrast, the "foe" hypothesis proposes that Ap_nN and other Np_nN 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 Ap_nN 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 Ap_nN. Finally, areas for future research are discussed with possible but unproven roles for intracellular Ap_nN to encourage further research into the signaling networks that are regulated by these nucleotides.

Ap4A Regulates Directional Mobility and Antigen Presentation in Dendritic Cells

The significance of intracellular Ap₄A levels over immune activity of dendritic cells (DCs) has been studied in Nudt2^fl/fl/CD11c-cre mice. The transgenic mice have been generated by crossing floxed NUDT2 gene mice with DC marker CD11c recombinase (cre) mice. The DCs derived from these mice have higher levels of Ap₄A (≈30-fold) compared with those derived from Nudt2^+/+ mice. Interestingly, the elevated Ap₄A in DCs has led them to possess higher motility and lower directional variability. In addition, the DCs are able to enhance immune protection indicated by the higher cross-presentation of antigen and priming of CD8⁺ OT-I T cells. Overall, the study denotes prominent impact of Ap₄A over the functionality of DCs. The Nudt2^fl/fl/CD11c-cre mice could serve as a useful tool to study the influence of Ap₄A in the critical immune mechanisms of DCs.

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DCs of Nudt2^fl/fl/CD11c-cre mice exhibit low directional variability and high motility

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DCs elevate proliferation of OVA-specific T cell receptor transgenic CD8⁺ T cells

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The escalation of Ap₄A levels in DCs could enhance their immune protective activity

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Mice can serve as useful functional tool to study the role of Ap₄A in various cells

Enzymatic synthesis of mono and dinucleoside polyphosphates

BACKGROUND

Mono and dinucleoside polyphosphates (p(n)Ns and Np(n)Ns) exist in living organisms and induce diverse biological effects through interaction with intracellular and cytoplasmic membrane proteins. The source of these compounds is associated with secondary activities of a diverse group of enzymes.

SCOPE OF REVIEW

Here we discuss the mechanisms that can promote their synthesis at a molecular level. Although all the enzymes described in this review are able to catalyse the in vitro synthesis of Np(n)Ns (and/or p(n)N), it is not clear which ones are responsible for their in vivo accumulation.

MAJOR CONCLUSIONS

Despite the large amount of knowledge already available, important questions remain to be answered and a more complete understanding of p(n)Ns and Np(n)Ns synthesis mechanisms is required. With the possible exception of (GTP:GTP guanylyltransferase of Artemia), all enzymes able to catalyse the synthesis of p(n)Ns and Np(n)Ns are unspecific and the factors that can promote their synthesis relative to the canonical enzyme activities are unclear.

GENERAL SIGNIFICANCE

The fact that p(n)Ns and Np(n)Ns syntheses are promiscuous activities of housekeeping enzymes does not reduce its physiological or pathological importance. Here we resume the current knowledge regarding their enzymatic synthesis and point the open questions on the field.

Diadenosine tetraphosphate hydrolase is part of the transcriptional regulation network in immunologically activated mast cells

We previously discovered that microphthalmia transcription factor (MITF) and upstream stimulatory factor 2 (USF2) each forms a complex with its inhibitor histidine triad nucleotide-binding 1 (Hint-1) and with lysyl-tRNA synthetase (LysRS). Moreover, we showed that the dinucleotide diadenosine tetraphosphate (Ap(4)A), previously shown to be synthesized by LysRS, binds to Hint-1, and as a result the transcription factors are released from their suppression. Thus, transcriptional activity is regulated by Ap(4)A, suggesting that Ap(4)A is a second messenger in this context. For Ap(4)A to be unambiguously established as a second messenger, several criteria have to be fulfilled, including the presence of a metabolizing enzyme. Since several enzymes are able to hydrolyze Ap(4)A, we provided here evidence that the "Nudix" type 2 gene product, Ap(4)A hydrolase, is responsible for Ap(4)A degradation following the immunological activation of mast cells. The knockdown of Ap(4)A hydrolase modulated Ap(4)A accumulation, resulting in changes in the expression of MITF and USF2 target genes. Moreover, our observations demonstrated that the involvement of Ap(4)A hydrolase in gene regulation is not a phenomenon exclusive to mast cells but can also be found in cardiac cells activated with the beta-agonist isoproterenol. Thus, we have provided concrete evidence establishing Ap(4)A as a second messenger in the regulation of gene expression.

Firefly luminescence: a historical perspective and recent developments

Significant advances have occurred regarding our knowledge of firefly luciferase mechanisms. Although most of this progress was an outcome of molecular biology and structural studies, important achievements have also occurred on its fundamental chemistry. Those developments are here summarized and presented in a historical perspective.

Correlation of intracellular diadenosine triphosphate (Ap3A) with apoptosis in Fhit-positive HEK293 cells

The pro-apoptotic Fhit tumor suppressor protein binds and hydrolyses diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A) in vitro. We have measured the level of both these nucleotides in Fhit-positive HEK293 cells exposed to various apoptosis inducers. Cold shock, anti-Fas, cadmium ions and etoposide all increased the basal level of Ap4A of 0.500pmol/10(6)cells by about 50%. However, the corresponding increases in Ap3A from a basal 0.079pmol/10(6)cells correlated closely with the degree of apoptosis produced, up to a maximum of 0.510pmol/10(6)cells with etoposide. These results support the view that Ap3A is the in vivo Fhit ligand and that an inhibition of Fhit activity is a key element in Fhit-mediated apoptosis.