Stabilized 5' Cap Analogue for Optochemical Activation of mRNA Translation.

The 5' cap is a distinguishing feature of transcripts made by polymerase II and characterized by an N7-methylated guanosine (m7G) linked to the first transcribed nucleotide by a 5'-5' triphosphate bridge. It stabilizes eukaryotic mRNAs and plays a crucial role in translation initiation. Its importance in mRNA processing, translation, and turnover makes the 5' cap a privileged structure for engineering by non-natural modifications. A photocleavable group at the 5' cap of guanosine was recently used to mute translation of exogenous mRNAs. Its removal by light enabled direct control of protein production at the posttranscriptional level. Modifications in the triphosphate bridge impede degradation by specific decapping enzymes and maintain translation. Here, we combined 5' cap modifications at different positions and investigated how they impact 5' cap-dependent processes in distinct manners. We synthesized 5' cap analogues with a photocleavable group at the N2-position of m7G in addition to a medronate in the triphosphate bridge to obtain a photoactivatable 5' cap analogue featuring a methylene group between the ß and γ phosphates. The resulting Medronate-FlashCap transiently or permanently impeded distinct crucial interactions of the 5' cap required for translation and degradation. We show that the Medronate-FlashCap is compatible with in vitro transcription to generate muted mRNA and that light can be used to activate translation in cells. After light-induced removal of the photocleavable group, the Medronate-FlashCap remained stable against degradation by the decapping enzyme DcpS. The additional methylene group renders the 5' cap resistant to DcpS, while maintaining the interaction with cap-binding proteins.

MePMe-seq: antibody-free simultaneous m6A and m5C mapping in mRNA by metabolic propargyl labeling and sequencing.

Internal modifications of mRNA have emerged as widespread and versatile regulatory mechanism to control gene expression at the post-transcriptional level. Most of these modifications are methyl groups, making S-adenosyl-L-methionine (SAM) a central metabolic hub. Here we show that metabolic labeling with a clickable metabolic precursor of SAM, propargyl-selenohomocysteine (PSH), enables detection and identification of various methylation sites. Propargylated A, C, and G nucleosides form at detectable amounts via intracellular generation of the corresponding SAM analogue. Integration into next generation sequencing enables mapping of N6-methyladenosine (m6A) and 5-methylcytidine (m5C) sites in mRNA with single nucleotide precision (MePMe-seq). Analysis of the termination profiles can be used to distinguish m6A from 2'-O-methyladenosine (Am) and N1-methyladenosine (m1A) sites. MePMe-seq overcomes the problems of antibodies for enrichment and sequence-motifs for evaluation, which was limiting previous methodologies. Metabolic labeling via clickable SAM facilitates the joint evaluation of methylation sites in RNA and potentially DNA and proteins.

Light-Activated Translation of Different mRNAs in Cells via Wavelength-Dependent Photouncaging.

The 5' cap is a hallmark of eukaryotic mRNA involved in the initiation of translation. Its modification with a single photo-cleavable group can bring translation of mRNA under the control of light. However, UV irradiation causes cell stress and downregulation of translation. Furthermore, complex processes often involve timed expression of more than one gene. The approach would thus greatly benefit from the ability to photo-cleave by blue light and to control more than one mRNA at a time. We report the synthesis of a 5' cap modified with a 7-(diethylamino)coumarin (CouCap) and adapted conditions for in vitro transcription. Translation of the resulting CouCap-mRNA is muted in vitro and in mammalian cells, and can be initiated by irradiation with 450 nm. The native cap is restored and no non-natural residues nor sequence alterations remain in the mRNA. Multiplexing for two different mRNAs was achieved by combining cap analogs with coumarin- and ortho-nitrobenzyl-based photo-cleavable groups.

Photocaged 5' cap analogues for optical control of mRNA translation in cells.

The translation of messenger RNA (mRNA) is a fundamental process in gene expression, and control of translation is important to regulate protein synthesis in cells. The primary hallmark of eukaryotic mRNAs is their 5' cap, whose molecular contacts to the eukaryotic translation initiation factor eIF4E govern the initiation of translation. Here we report 5' cap analogues with photo-cleavable groups (FlashCaps) that prohibit binding to eIF4E and resist cleavage by decapping enzymes. These compounds are compatible with the general and efficient production of mRNAs by in vitro transcription. In FlashCap-mRNAs, the single photocaging group abrogates translation in vitro and in mammalian cells without increasing immunogenicity. Irradiation restores the native cap, triggering efficient translation. FlashCaps overcome the problem of remaining sequence or structure changes in mRNA after irradiation that limited previous designs. Together, these results demonstrate that FlashCaps offer a route to regulate the expression of any given mRNA and to dose mRNA therapeutics with spatio-temporal control.

Quantification of mRNA cap-modifications by means of LC-QqQ-MS.

Enzymatic modification of the 5'-cap is a versatile approach to modulate the properties of mRNAs. Transfer of methyl groups from S-adenosyl-l-methionine (AdoMet) or functional moieties from non-natural analogs by methyltransferases (MTases) allows for site-specific modifications at the cap. These modifications have been used to tune translation or control it in a temporal manner and even influence immunogenicity of mRNA. For quantification of the MTase-mediated cap modification, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) provides the required sensitivity and accuracy. Here, we describe the complete workflow starting from in vitro transcription to produce mRNAs, via their enzymatic modification at the cap with natural or non-natural moieties to the quantification of these cap-modifications by LC-QqQ-MS.

Acyclic nucleoside phosphonates with adenine nucleobase inhibit Trypanosoma brucei adenine phosphoribosyltransferase in vitro.

All medically important unicellular protozoans cannot synthesize purines de novo and they entirely rely on the purine salvage pathway (PSP) for their nucleotide generation. Therefore, purine derivatives have been considered as a promising source of anti-parasitic compounds since they can act as inhibitors of the PSP enzymes or as toxic products upon their activation inside of the cell. Here, we characterized a Trypanosoma brucei enzyme involved in the salvage of adenine, the adenine phosphoribosyl transferase (APRT). We showed that its two isoforms (APRT1 and APRT2) localize partly in the cytosol and partly in the glycosomes of the bloodstream form (BSF) of the parasite. RNAi silencing of both APRT enzymes showed no major effect on the growth of BSF parasites unless grown in artificial medium with adenine as sole purine source. To add into the portfolio of inhibitors for various PSP enzymes, we designed three types of acyclic nucleotide analogs as potential APRT inhibitors. Out of fifteen inhibitors, four compounds inhibited the activity of the recombinant APRT1 with Ki in single µM values. The ANP phosphoramidate membrane-permeable prodrugs showed pronounced anti-trypanosomal activity in a cell-based assay, despite the fact that APRT enzymes are dispensable for T. brucei growth in vitro. While this suggests that the tested ANP prodrugs exert their toxicity by other means in T. brucei, the newly designed inhibitors can be further improved and explored to identify their actual target(s).

Helicobacter pylori Xanthine-Guanine-Hypoxanthine Phosphoribosyltransferase-A Putative Target for Drug Discovery against Gastrointestinal Tract Infections.

Helicobacter pylori (Hp) is a human pathogen that lives in the gastric mucosa of approximately 50% of the world's population causing gastritis, peptic ulcers, and gastric cancer. An increase in resistance to current drugs has sparked the search for new Hp drug targets and therapeutics. One target is the disruption of nucleic acid production, which can be achieved by impeding the synthesis of 6-oxopurine nucleoside monophosphates, the precursors of DNA and RNA. These metabolites are synthesized by Hp xanthine-guanine-hypoxanthine phosphoribosyltransferase (XGHPRT). Here, nucleoside phosphonates have been evaluated, which inhibit the activity of this enzyme with Ki values as low as 200 nM. The prodrugs of these compounds arrest the growth of Hp at a concentration of 50 µM in cell-based assays. The kinetic properties of HpXGHPRT have been determined together with its X-ray crystal structure in the absence and presence of 9-[(N-3-phosphonopropyl)-aminomethyl-9-deazahypoxanthine, providing a basis for new antibiotic development.

A Benzophenone-Based Photocaging Strategy for the N7 Position of Guanosine.

Selective modification of nucleobases with photolabile caging groups enables the study and control of processes and interactions of nucleic acids. Numerous positions on nucleobases have been targeted, but all involve formal substitution of a hydrogen atom with a photocaging group. Nature, however, also uses ring-nitrogen methylation, such as m7 G and m1 A, to change the electronic structure and properties of RNA and control biomolecular interactions essential for translation and turnover. We report that aryl ketones such as benzophenone and α-hydroxyalkyl ketone are photolabile caging groups if installed at the N7 position of guanosine or the N1 position of adenosine. Common photocaging groups derived from the ortho-nitrobenzyl moiety were not suitable. Both chemical and enzymatic methods for site-specific modification of N7G in nucleosides, dinucleotides, and RNA were developed, thereby opening the door to studying the molecular interactions of m7 G and m1 A with spatiotemporal control.

Flubendazole and mebendazole impair migration and epithelial to mesenchymal transition in oral cell lines.

Benzimidazole anthelmintics flubendazole and mebendazole are microtubule-targeting drugs that showed considerable anti-cancer activity in different preclinical models. In this study, the effects of flubendazole and mebendazole on proliferation, migration and cadherin switching were studied in a panel of oral cell lines in vitro. Both compounds reduced the viability of the PE/CA-PJ15 and H376 oral squamous carcinoma cells and of the premalignant oral keratinocytes DOK with the IC50 values in the range of 0.19-0.26 µM. Normal oral keratinocytes and normal gingival fibroblasts were less sensitive to the treatment. Flubendazole and mebendazole also reduced the migration of the PE/CA-PJ15 cell in concentrations that had no anti-migratory effects on the normal gingival fibroblasts. Levels of the focal adhesion kinase FAK, Rho-A and Rac1 GTPases and the Rho guanine nucleotide exchange factor GEF-H1 were decreased in both PE/CA-PJ15 cells and gingival fibroblasts following treatment. Both drugs also interfered with cadherin switching in the model of TGF-ß-induced epithelial to mesenchymal transition (EMT) in the DOK cell line. Levels of N-cadherin were reduced in the TGF-ß induced cells co-treated with flubendazol and mebendazole in very low concentration (50 nM). These results suggest direct effects of both benzimidazoles on selected processes of EMT in oral cell lines such as cadherin switching as well as cellular migration.

Evaluation of the Trypanosoma brucei 6-oxopurine salvage pathway as a potential target for drug discovery.

Due to toxicity and compliance issues and the emergence of resistance to current medications new drugs for the treatment of Human African Trypanosomiasis are needed. A potential approach to developing novel anti-trypanosomal drugs is by inhibition of the 6-oxopurine salvage pathways which synthesise the nucleoside monophosphates required for DNA/RNA production. This is in view of the fact that trypanosomes lack the machinery for de novo synthesis of the purine ring. To provide validation for this approach as a drug target, we have RNAi silenced the three 6-oxopurine phosphoribosyltransferase (PRTase) isoforms in the infectious stage of Trypanosoma brucei demonstrating that the combined activity of these enzymes is critical for the parasites' viability. Furthermore, we have determined crystal structures of two of these isoforms in complex with several acyclic nucleoside phosphonates (ANPs), a class of compound previously shown to inhibit 6-oxopurine PRTases from several species including Plasmodium falciparum. The most potent of these compounds have Ki values as low as 60 nM, and IC50 values in cell based assays as low as 4 µM. This data provides a solid platform for further investigations into the use of this pathway as a target for anti-trypanosomal drug discovery.

Synthesis and Evaluation of Asymmetric Acyclic Nucleoside Bisphosphonates as Inhibitors of Plasmodium falciparum and Human Hypoxanthine-Guanine-(Xanthine) Phosphoribosyltransferase.

Acyclic nucleoside bisphosphonates (ANbPs) have previously been shown to be good inhibitors of human hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and Plasmodium falciparum (Pf) hypoxanthine-guanine-xanthine phosphoribosyltransferase (PfHGXPRT). On the basis of this scaffold, a new series of ANbPs was synthesized. One of these new ANbPs, [3-(guanine-9-yl)-2-((2-phosphonoethoxy)methyl)propoxy]methylphosphonic acid, exhibited Ki values of 6 and 70 nM for human HGPRT and Pf HGXPRT, respectively. These low Ki values were achieved by inserting an extra carbon atom in the linker connecting the N9 atom of guanine to one of the phosphonate groups. The crystal structure of this ANbP in complex with human HGPRT was determined at 2.0 Å resolution and shows that it fills three key pockets in the active site. The most potent phosphoramidate prodrugs of these compounds have IC50 values in the low micromolar range in Pf lines and low toxicity in human A549 cells, demonstrating that these ANbPs are excellent antimalarial drug leads.

Synthesis and evaluation of symmetric acyclic nucleoside bisphosphonates as inhibitors of the Plasmodium falciparum, Plasmodium vivax and human 6-oxopurine phosphoribosyltransferases and the antimalarial activity of their prodrugs.

Two new series of symmetric acyclic nucleoside bisphosphonates (ANbPs) have been synthesised as potential inhibitors of the Plasmodium falciparum (Pf) and vivax (Pv) 6-oxopurine phosphoribosyltransferases. The structural variability between these symmetric ANbPs lies in the number of atoms in the two acyclic linkers connecting the N9 atom of the purine base to each of two phosphonate groups and the branching point of the acyclic moiety relative to the purine base, which occurs at either the alpha or beta positions. Within each series, six different 6-oxopurine bases have been attached. In general, the ANbPs with either guanine or hypoxanthine have lower Ki values than for those containing either the 8-bromo or 7-deaza 6-oxopurine bases. The lowest Ki values obtained for the two parasite enzymes were 0.1µM (Pf) and 0.2µM (Pv) for this series of compounds. Two phosphoramidate prodrugs of these inhibitors exhibited antimalarial activity against Pf in infected erythrocyte cell culture with IC50 values of 0.8 and 1.5µM. These two compounds exhibited low cytotoxicity in human A549 cells having CC50 values of >300µM resulting in an excellent selectivity index.

First Crystal Structures of Mycobacterium tuberculosis 6-Oxopurine Phosphoribosyltransferase: Complexes with GMP and Pyrophosphate and with Acyclic Nucleoside Phosphonates Whose Prodrugs Have Antituberculosis Activity.

Human tuberculosis is a chronic infectious disease affecting millions of lives. Because of emerging resistance to current medications, new therapeutic drugs are needed. One potential new target is hypoxanthine-guanine phosphoribosyltransferase (MtHGPRT), a key enzyme of the purine salvage pathway. Here, newly synthesized acyclic nucleoside phosphonates (ANPs) have been shown to be competitive inhibitors of MtHGPRT with Ki values as low as 0.69 µM. Prodrugs of these compounds arrest the growth of a virulent strain of M. tuberculosis with MIC50 values as low as 4.5 µM and possess low cytotoxicity in mammalian cells (CC50 values as high as >300 µM). In addition, the first crystal structures of MtHGPRT (2.03-2.76 Å resolution) have been determined, three of these in complex with novel ANPs and one with GMP and pyrophosphate. These data provide a solid foundation for the further development of ANPs as selective inhibitors of MtHGPRT and as antituberculosis agents.

Inhibition of the Escherichia coli 6-oxopurine phosphoribosyltransferases by nucleoside phosphonates: potential for new antibacterial agents.

Escherichia coli (Ec) cells possess two purine salvage enzymes: xanthine-guanine phosphoribosyltransferase (XGPRT) and hypoxanthine phosphoribosyltransferase (HPRT). EcXGPRT shares a common structural feature with other members of this family, a flexible loop that closes over the active site during catalysis. The replacement of six of these amino acids by alanine has no effect on the Km for the two substrates. However, the Ki for the nucleoside monophosphate increases by 27-fold, and the kcat is reduced by ∼200-fold. Nucleoside phosphonates (NP) are good inhibitors of EcXGPRT and EcHPRT, with Ki values as low as 10 nM. In the absence of the flexible loop, these values increase by 5- to 30-fold, indicating the importance of the loop for high-affinity inhibition. Crystal structures of two NPs in complex with EcXGPRT explain the tight binding. Prodrugs of NPs with low Ki values for EcXGPRT or EcHPRT exhibit IC50 values between 5 and 23 µM against Mycobacterium tuberculosis in cell-based assays, suggesting that these compounds are therapeutic leads against pathogenic bacteria.

Acyclic nucleoside phosphonates containing a second phosphonate group are potent inhibitors of 6-oxopurine phosphoribosyltransferases and have antimalarial activity.

Acyclic nucleoside phosphonates (ANPs) that contain a 6-oxopurine base are good inhibitors of the Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) 6-oxopurine phosphoribosyltransferases (PRTs). Chemical modifications based on the crystal structure of 2-(phosphonoethoxy)ethylguanine (PEEG) in complex with human HGPRT have led to the design of new ANPs. These novel compounds contain a second phosphonate group attached to the ANP scaffold. {[(2-[(Guanine-9H-yl)methyl]propane-1,3-diyl)bis(oxy)]bis(methylene)}diphosphonic acid (compound 17) exhibited a Ki value of 30 nM for human HGPRT and 70 nM for Pf HGXPRT. The crystal structure of this compound in complex with human HGPRT shows that it fills or partially fills three critical locations in the active site: the binding sites of the purine base, the 5'-phosphate group, and pyrophosphate. This is the first HG(X)PRT inhibitor that has been able to achieve this result. Prodrugs have been synthesized resulting in IC50 values as low as 3.8 µM for Pf grown in cell culture, up to 25-fold lower compared to the parent compounds.