Hydrolysis of the non-canonical cyclic nucleotide cUMP by PDE9A: kinetics and binding mode

Cyclic nucleotides - the rise of a family

Cyclic nucleotides 3',5'-cAMP and 3',5'-cGMP are now established signaling components of the plant cell while their 2',3' positional isomers are increasingly recognized as such. 3',5'-cAMP/cGMP is generated by adenylate cyclases (ACs) or guanylate cyclases (GCs) from ATP or GTP, respectively, whereas 2',3'-cAMP/cGMP is produced through the hydrolysis of double-stranded DNA or RNA by synthetases. Recent evidence suggests that the cyclic nucleotide generating and inactivating enzymes moonlight in proteins with diverse domain architecture operating as molecular tuners to enable dynamic and compartmentalized regulation of cellular signals. Further characterization of such moonlighting enzymes and extending the studies to noncanonical cyclic nucleotides promises new insights into the complex regulatory networks that underlie plant development and responses, thus offering exciting opportunities for crop improvement.

Pharmacological potential of cyclic nucleotide signaling in immunity

Cyclic nucleotides are important signaling molecules that play many critical physiological roles including controlling cell fate and development, regulation of metabolic processes, and responding to changes in the environment. Cyclic nucleotides are also pivotal regulators in immune signaling, orchestrating intricate processes that maintain homeostasis and defend against pathogenic threats. This review provides a comprehensive examination of the pharmacological potential of cyclic nucleotide signaling pathways within the realm of immunity. Beginning with an overview of the fundamental roles of cAMP and cGMP as ubiquitous second messengers, this review delves into the complexities of their involvement in immune responses. Special attention is given to the challenges associated with modulating these signaling pathways for therapeutic purposes, emphasizing the necessity for achieving cell-type specificity to avert unintended consequences. A major focus of the review is on the recent paradigm-shifting discoveries regarding specialized cyclic nucleotide signals in the innate immune system, notably the cGAS-STING pathway. The significance of cyclic dinucleotides, exemplified by 2'3'-cGAMP, in controlling immune responses against pathogens and cancer, is explored. The evolutionarily conserved nature of cyclic dinucleotides as antiviral agents, spanning across diverse organisms, underscores their potential as targets for innovative immunotherapies. Findings from the last several years have revealed a striking diversity of novel bacterial cyclic nucleotide second messengers which are involved in antiviral responses. Knowledge of the existence and precise identity of these molecules coupled with accurate descriptions of their associated immune defense pathways will be essential to the future development of novel antibacterial therapeutic strategies. The insights presented herein may help researchers navigate the evolving landscape of immunopharmacology as it pertains to cyclic nucleotides and point toward new avenues or lines of thinking about development of therapeutics against the pathways they regulate.

cCMP and cUMP phosphodiesterases in viral infections

In bacteria, cCMP and cUMP have a key role in defense against infection with bacterial viruses. Bacteriophages encode phosphodiesterases (PDEs; 'nucleases'; Apyc1), which cleave cCMP/cUMP, counteracting this defense. We propose that PDEs are of broader biological relevance, including cCMP/cUMP-cleaving PDEs of eukaryotic viruses, which may constitute new drug targets.

Apoptotic and anti-proliferative effect of guanosine and guanosine derivatives in HuT-78 T lymphoma cells

The effects of 100 μM of 3',5'-cGMP, cAMP, cCMP, and cUMP as well as of the corresponding membrane-permeant acetoxymethyl esters on anti-CD3-antibody (OKT3)-induced IL-2 production of HuT-78 cutaneous T cell lymphoma (Sézary lymphoma) cells were analyzed. Only 3',5'-cGMP significantly reduced IL-2 production. Flow cytometric analysis of apoptotic (propidium iodide/annexin V staining) and anti-proliferative (CFSE staining) effects revealed that 3',5'-cGMP concentrations > 50 μM strongly inhibited proliferation and promoted apoptosis of HuT-78 cells (cultured in the presence of αCD3 antibody). Similar effects were observed for the positional isomer 2',3'-cGMP and for 2',-GMP, 3'-GMP, 5'-GMP, and guanosine. By contrast, guanosine and guanosine-derived nucleotides had no cytotoxic effect on peripheral blood mononuclear cells (PBMCs) or acute lymphocytic leukemia (ALL) xenograft cells. The anti-proliferative and apoptotic effects of guanosine and guanosine-derived compounds on HuT-78 cells were completely eliminated by the nucleoside transport inhibitor NBMPR (S-(4-Nitrobenzyl)-6-thioinosine). By contrast, the ecto-phosphodiesterase inhibitor DPSPX (1,3-dipropyl-8-sulfophenylxanthine) and the CD73 ecto-5'-nucleotidase inhibitor AMP-CP (adenosine 5'-(α,β-methylene)diphosphate) were not protective. We hypothesize that HuT-78 cells metabolize guanosine-derived nucleotides to guanosine by yet unknown mechanisms. Guanosine then enters the cells by an NBMPR-sensitive nucleoside transporter and exerts cytotoxic effects. This transporter may be ENT1 because NBMPR counteracted guanosine cytotoxicity in HuT-78 cells with nanomolar efficacy (IC50 of 25-30 nM). Future studies should further clarify the mechanism of the observed effects and address the question, whether guanosine or guanosine-derived nucleotides may serve as adjuvants in the therapy of cancers that express appropriate nucleoside transporters and are sensitive to established nucleoside-derived cytostatic drugs.