Добірка наукової літератури з теми "N6-Methyl Adenosine"

MutT homologue 1 (MTH1) removes oxidized nucleotides from the nucleotide pool and thereby prevents their incorporation into the genome and thereby reduces genotoxicity. We previously reported that MTH1 is an efficient catalyst of O6-methyl-dGTP hydrolysis suggesting that MTH1 may also sanitize the nucleotide pool from other methylated nucleotides. We here show that MTH1 efficiently catalyzes the hydrolysis of N6-methyl-dATP to N6-methyl-dAMP and further report that N6-methylation of dATP drastically increases the MTH1 activity. We also observed MTH1 activity with N6-methyl-ATP, albeit at a lower level. We show that N6-methyl-dATP is incorporated into DNA in vivo, as indicated by increased N6-methyl-dA DNA levels in embryos developed from MTH1 knock-out zebrafish eggs microinjected with N6-methyl-dATP compared with noninjected embryos. N6-methyl-dATP activity is present in MTH1 homologues from distantly related vertebrates, suggesting evolutionary conservation and indicating that this activity is important. Of note, N6-methyl-dATP activity is unique to MTH1 among related NUDIX hydrolases. Moreover, we present the structure of N6-methyl-dAMP–bound human MTH1, revealing that the N6-methyl group is accommodated within a hydrophobic active-site subpocket explaining why N6-methyl-dATP is a good MTH1 substrate. N6-methylation of DNA and RNA has been reported to have epigenetic roles and to affect mRNA metabolism. We propose that MTH1 acts in concert with adenosine deaminase-like protein isoform 1 (ADAL1) to prevent incorporation of N6-methyl-(d)ATP into DNA and RNA. This would hinder potential dysregulation of epigenetic control and RNA metabolism via conversion of N6-methyl-(d)ATP to N6-methyl-(d)AMP, followed by ADAL1-catalyzed deamination producing (d)IMP that can enter the nucleotide salvage pathway.

The contribution of adenosine receptors was evaluated in vascular relaxation in experimental hypothyroidism. Hypothyroid aortic rings contracted less than normal controls with noradrenaline, phenylephrine, and KCl; the difference was maintained after incubation with 1,3-dipropyl-8-p-sulfophenylxanthine (an A1 and A2 adenosine receptor blocker). The vascular relaxation induced by acetylcholine or carbachol was similar in normal and hypothyroid aortic rings. However, adenosine, N6-cyclopentyladenosine (an A1 adenosine receptor analogue), and 5'-N-ethylcarbox amidoadenosine (an A2 and A3 adenosine analogue) induced vasodilation that was larger in hypothyroid than in normal aortas. Nω-nitro-L-arginine methyl ester shifted the dose-response curves of adenosine, N6-cyclopentyladenosine, or 5'-N-ethylcarboxamidoadenosine to the right in both normal and hypothyroid vessels. The blocker 1,3-dipropyl-8-p-sulfophenylxanthine significantly reduced adenosine-induced relaxation in the hypothyroid but not in the normal aortic vessels. These results suggest that in hypothyroid aortas, a larger adenosine-mediated vasodilation is observed probably due to an increase in receptor number or sensitivity.Key words: adenosine receptors, nitric oxide, hypothyroidism, smooth muscle, rat aorta.

A series of 1′-(6-aminopurin-9-yl)-1′-deoxy-N-methyl-β-d-ribofuranuronamides that were characterised by 2-dialkylamino-7-methyloxazolo[4,5-b]pyridin-5-ylmethyl substituents on N6 of interest for screening as selective adenosine A₃ receptor agonists, have been synthesised.

Adenosine mediates vascular smooth muscle relaxation in the pulmonary circulation. The A2 receptor has been suggested to mediate adenosine-induced vasodilation (AIV). In this study, the effect(s) of selective adenosine agonist and antagonist on the hypoxic pressor response (HPR) was assessed in the isolated blood-perfused rat lung. Adenosine (0.075-7.5 mM) infusion (0.125 ml/min) into the pulmonary artery dose dependently attenuated the HPR. AIV was mimicked by 10 microM 5'-(N-ethylcarboxamido)adenosine (NECA), a nonselective adenosine agonist. Adenosine- and NECA-induced vasodilation were attenuated by 67 microM 8-(p-sulfophenyl)theophylline. In contrast, NECA-induced vasodilation was not attenuated by the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (1 microM). At 10 microM, a minimal vasodilatory effect was seen with the nonselective adenosine agonists CV-1808 and N6-(2-phenylisopropyl)adenosine (R-PIA) compared with NECA. The highly selective A2a agonist 2-[p-(2-carboxyethyl)phenyl amino]-5'-N-ethyl carboxamido adenosine (CGS-21680C, 10 microM) and A1 agonist 2-chloro-N6-cyclopentyladenosine (CCPA, 10 microM) had no vasodilatory effect. Neither the K+ channel blockers tetraethylammonium chloride (10 mM) and glibenclamide (100 microM) nor the NO synthase inhibitor N omega-nitro-L-arginine methyl ester attenuated NECA-induced vasodilation. These findings suggest that AIV is mediated via the A2b receptor and that AIV occurs via an NO-independent mechanism.

Cytokinins, classical phytohormones, affect all stages of plant ontogenesis, but their application in agriculture is limited because of the lack of appropriate ligands, including those specific for individual cytokinin receptors. In this work, a series of chiral N6-benzyladenine derivatives were studied as potential cytokinins or anticytokinins. All compounds contained a methyl group at the α-carbon atom of the benzyl moiety, making them R- or S-enantiomers. Four pairs of chiral nucleobases and corresponding ribonucleosides containing various substituents at the C2 position of adenine heterocycle were synthesized. A nucleophilic substitution reaction by secondary optically active amines was used. A strong influence of the chirality of studied compounds on their interaction with individual cytokinin receptors of Arabidopsis thaliana was uncovered in in vivo and in vitro assays. The AHK2 and CRE1/AHK4 receptors were shown to have low affinity for the studied S-nucleobases while the AHK3 receptor exhibited significant affinity for most of them. Thereby, three synthetic AHK3-specific cytokinins were discovered: N6-((S)-α-methylbenzyl)adenine (S-MBA), 2-fluoro,N6-((S)-α-methylbenzyl)adenine (S-FMBA) and 2-chloro,N6-((S)-α-methylbenzyl)adenine (S-CMBA). Interaction patterns between individual receptors and specific enantiomers were rationalized by structure analysis and molecular docking. Two other S-enantiomers (N6-((S)-α-methylbenzyl)adenosine, 2-amino,N6-((S)-α-methylbenzyl)adenosine) were found to exhibit receptor-specific and chirality-dependent anticytokinin properties.

Epigenetic modifications on individual bases in DNA and RNA can encode inheritable genetic information beyond the canonical bases. Among the nucleic acid modifications, DNA N6-methadenine (6mA) and RNA N6-methyladenosine (m6A) have recently been well-studied due to the technological development of detection strategies and the functional identification of modification enzymes. The current findings demonstrate a wide spectrum of 6mA and m6A distributions from prokaryotes to eukaryotes and critical roles in multiple cellular processes. It is interesting that the processes of modification in which the methyl group is added to adenine and adenosine are the same, but the outcomes of these modifications in terms of their physiological impacts in organisms are quite different. In this review, we summarize the latest progress in the study of enzymes involved in the 6mA and m6A methylation machinery, including methyltransferases and demethylases, and their functions in various biological pathways. In particular, we focus on the mechanisms by which 6mA and m6A regulate the expression of target genes, and we highlight the future challenges in epigenetic regulation.

A high-affinity iodinated agonist radioligand for the A2 adenosine receptor has been synthesized to facilitate studies of the A2 adenosine receptor binding subunit. The radioligand 125I-labeled PAPA-APEC (125I-labeled 2-[4-(2-[2-[(4- aminophenyl)methylcarbonylamino]ethylaminocarbonyl]- ethyl)phenyl]ethylamino-5'-N-ethylcarboxamidoadenosine) was synthesized and found to bind to the A2 adenosine receptor in bovine striatal membranes with high affinity (Kd = 1.5 nM) and A2 receptor selectivity. Competitive binding studies reveal the appropriate A2 receptor pharmacologic potency order with 5'-N-ethylcarboxamidoadenosine (NECA) greater than (-)-N6-[(R)-1-methyl- 2-phenylethyl]adenosine (R-PIA) greater than (+)-N6-[(S)-1-methyl-2- phenylethyl]adenosine (S-PIA). Adenylate cyclase assays, in human platelet membranes, demonstrate a dose-dependent stimulation of cAMP production. PAPA-APEC (1 microM) produces a 43% increase in cAMP production, which is essentially the same degree of increase produced by 5'-N- ethylcarboxamidoadenosine (the prototypic A2 receptor agonist). These findings combined with the observed guanine nucleotide-mediated decrease in binding suggest that PAPA-APEC is a full A2 agonist. The A2 receptor binding subunit was identified by photoaffinity-crosslinking studies using 125I-labeled PAPA-APEC and the heterobifunctional crosslinking agent N-succinimidyl 6-(4'-azido-2'-nitrophenylamino)hexanoate (SANPAH). After covalent incorporation, a single specifically radiolabeled protein with an apparent molecular mass of 45 kDa was observed on NaDodSO4/PAGE/autoradiography. Incorporation of 125I-labeled PAPA-APEC into this polypeptide is blocked by agonists and antagonists with the expected potency for A2 receptors (see above) and is decreased in the presence of 10(-4) M guanosine 5'-[beta, gamma-imido]triphosphate. Photoaffinity crosslinking of the A1 adenosine receptor binding subunit with 125I-labeled 8-[4-[2-(4- aminophenylacetylamino)ethyl]carbonylmethyloxyphenyl]-1,3-di propylxanthine (PAPAXAC) (an A1 selective photoaffinity probe) in the same tissue reveals a 38-kDa peptide that exhibits the appropriate A1 receptor pharmacology. 125I-labeled PAPA-APEC, therefore, has identified the A2 receptor binding subunit as a 45-kDa protein that is unique and distinct from the A1 binding subunit.

The major roadblocks in treatment for GBM are resistance to therapy and recurrence of GBM cells. Regardless of various treatment strategies, the average survival of GBM patients is poor and incidence of recurrence remains high. The presence of GSCs, a dynamic cellular system within GBM contributes to chemo/radio resistance and recurrence. The plasticity of GSCs is supported by reversible biological processes including DNA methylation, histone modifications and RNA modifications. m6A is a reversible mRNA methylation which regulates various steps of RNA processing. In this thesis, we attempted to elucidate the METTL3- mediated m6A as a molecular mechanism behind the dynamic nature of GSCs. It comprises of two parts: Part 1- The landscape of METTL3-dependent m6A-epitranscriptome in GSCs and the functional orchestration of m6A targets Part 2- Essential role of METTL3 mediated m6A modification in glioma stem-like cells maintenance and radioresistance In first part, we demonstrate levels of m6A modified RNAs and METTL3 are maintained high in GSCs and they are attenuated during serum induced differentiation. Other members involved in m6A modification are either down regulated or unregulated in GSCs which confirms the dependency of GSCs in METTL3 for creating m6A marks. In addition, previous reports provide support that solely METTL3 carry catalytic domain for SAMbinding. Based on these facts, we elucidate the METTL3-dependent global m6A modification in GSCs and the impact of METTL3 targets in coordinating various functions. By comprehensive analysis of m6A-RIP seq and whole transcriptome post METTL3 silencing in GSCs, we identified the direct and indirect targets for METTL3-mediated m6A modification. The genes which preserve stem cell properties and aid in tumorigenesis were predominantly inhibited by METTL3 silencing in GSCs suggesting a universal oncogenic role of METTL3 in GBM. Large subset of genes was down regulated after METTL3 silencing suggesting a global destabilization of transcripts. The enrichment of m6A peaks near stop codon and 3’UTR indicates functional importance of METTL3 in RNA stabilization and translation termination. In addition to protein coding genes, METTL3 fine-tunes the expression of non coding RNAs which includes lncRNAs, anti-sense RNAs etc. Interestingly, we identified m6A peaks which encompass miRNA target sites and we hypothesize that m6A modification may scrutinize the binding affinity of miRNAs. We further determined the inter-play between chromatin remodeling with m6A epitranscriptome. Genes which carry active chromatin marks and transcription factor binding are further stabilized by METLT3- dependent m6A modification. The altered secondary/tertiary structure induced by m6A may act as loading site for various RBPs and achieve various RNA processing functions. Our analysis deduced RBPs HuR and QKI preferentially bind to m6A marked RNAs and helps in enhancing the expression of m6A modified targets. Together, this study provides a panoramic view on global m6A modification mediated by METTL3 in GSCs. In second part, we examined the crucial role of METTL3 in glioma stem cell physiology. Inhibition of METTL3 hinder the neurosphere formation and stem cell properties of GSCs. Anti-METTL3 RIP studies combined with m6A RIP-seq results identified SOX2 as a key m6A mediator of METTL3 and the m6A marks created by METTL3 sustains SOX2 transcript stability. The exogenous over expression of 3’UTR-less SOX2 significantly alleviated the inhibition of neurosphere formation observed in METTL3 silenced GSCs. METTL3 interaction and m6A modification in vivo required intact three METTL3/m6A sites present in the SOX2- 3’UTR. Further, we found that HuR recruitment to m6A modified RNA is essential for SOX2 mRNA stabilization by METTL3 and at global level HuR-RNA interaction prefers the m6A modified transcripts. METTL3 silenced GSCs showed enhanced sensitivity to γ-irradiation due to reduced DNA repair. Exogenous overexpression of 3’UTRless SOX2 in METTL3 silenced GSCs rescues efficiency of DNA repair and specifically homologous recombination repair. It also resulted in the significant rescue of neurosphere formation from METTL3 silencing induced radiosensitivity. GBM tumors have elevated METTL3 transcripts and silencing METTL3 in GSCs inhibited tumor growth and prolonged mice survival. METTL3 transcript levels predicted poor survival in GBMs which are enriched for GSC-specific signature. Thus our study reports the importance of m6A modification in GSCs and uncovers METTL3 as a potential molecular target in GBM therapy