Arabinose- and xylose-modified analogs of 2',3'-cGAMP act as STING agonists.

Stimulator of interferon genes (STING) agonists are promising candidates for vaccine adjuvants and antitumor immune stimulants. The most potent natural agonist of STING, 2',3'-cyclic GMP-AMP (2',3'-cGAMP), is subject to nuclease-mediated inherent metabolic instability, thereby placing limits on its clinical efficacy. Here, we report on a new class of chemically synthesized sugar-modified analogs of 2',3'-cGAMP containing arabinose and xylose sugar derivatives that bind mouse and human STING alleles with high affinity. The co-crystal structures demonstrate that such analogs act as 2',3'-cGAMP mimetics that induce the "closed" conformation of human STING. These analogs show significant resistance to hydrolysis mediated by ENPP1 and increased stability in human serum, while retaining similar potency as 2',3'-cGAMP at inducing IFN-ß secretion from human THP1 cells. The arabinose- and xylose-modified 2',3'-cGAMP analogs open a new strategy for overcoming the inherent nuclease-mediated vulnerability of natural ribose cyclic nucleotides, with the additional benefit of high translational potential as cancer therapeutics and vaccine adjuvants.

Starting and stopping RNA polymerase III transcription on single-stranded DNA oligonucleotides.

Circularized single-stranded DNA oligonucleotides, or coligos, show promise as promoter-independent RNA polymerase III (Pol III) transcription templates for generating small RNA in human cells. Using a modified small RNA-seq method, we studied the sequence and secondary structure characteristics that determine Pol III initiation and termination on six coligo templates. The coligos each consisted of an imperfectly base-paired stem flanked by one larger and one smaller loop and were unrelated in sequence. Small RNA-seq data from Pol III coligo transcripts revealed a strong preference for initiating transcription within a 5-nucleotide (nt) window spanning the stem-larger loop junction (loop size 11-24 nt). Transcription in all cases proceeded into the stem rather than into the larger loop, indicating the junction is a site-specific, secondary structure-based Pol III transcription initiator. On average, 81% of sequencing reads showed initiation within this 5 nt junction region, with a template start site nucleotide preference of C > T >> A > G, and a requirement for a template purine at Tss-1. Termination was less precise than initiation and occurred in the larger loop at the same end of the stem where transcription initiated. Termination efficiency was on average 82% and was distributed among the first 11 single-stranded larger loop nt following the stem. The size heterogeneity of Pol III coligo transcripts is thus mainly due to 3' end heterogeneity, whereas the RNA 5' ends were more predictable and homogeneous. Transcription termination did not require an oligo dA template sequence, indicating that termination in this context may be mechanistically different than Pol III's normal gene-context termination. A stepwise model for coligo transcription by Pol III is proposed.

Human cGAS catalytic domain has an additional DNA-binding interface that enhances enzymatic activity and liquid-phase condensation.

The cyclic GMP-AMP synthase (cGAS)-cGAMP-STING pathway plays a key role in innate immunity, with cGAS sensing both pathogenic and mislocalized DNA in the cytoplasm. Human cGAS (h-cGAS) constitutes an important drug target for control of antiinflammatory responses that can contribute to the onset of autoimmune diseases. Recent studies have established that the positively charged N-terminal segment of cGAS contributes to enhancement of cGAS enzymatic activity as a result of DNA-induced liquid-phase condensation. We have identified an additional cGASCD-DNA interface (labeled site-C; CD, catalytic domain) in the crystal structure of a human SRY.cGASCD-DNA complex, with mutations along this basic site-C cGAS interface disrupting liquid-phase condensation, as monitored by cGAMP formation, gel shift, spin-down, and turbidity assays, as well as time-lapse imaging of liquid droplet formation. We expand on an earlier ladder model of cGAS dimers bound to a pair of parallel-aligned DNAs to propose a multivalent interaction-mediated cluster model to account for DNA-mediated condensation involving both the N-terminal domain of cGAS and the site-C cGAS-DNA interface. We also report the crystal structure of the h-cGASCD-DNA complex containing a triple mutant that disrupts the site-C interface, with this complex serving as a future platform for guiding cGAS inhibitor development at the DNA-bound h-cGAS level. Finally, we solved the structure of RU.521 bound in two alternate alignments to apo h-cGASCD, thereby occupying more of the catalytic pocket and providing insights into further optimization of active-site-binding inhibitors.

Development of human cGAS-specific small-molecule inhibitors for repression of dsDNA-triggered interferon expression.

Cyclic GMP-AMP synthase (cGAS) is the primary sensor for aberrant intracellular dsDNA producing the cyclic dinucleotide cGAMP, a second messenger initiating cytokine production in subsets of myeloid lineage cell types. Therefore, inhibition of the enzyme cGAS may act anti-inflammatory. Here we report the discovery of human-cGAS-specific small-molecule inhibitors by high-throughput screening and the targeted medicinal chemistry optimization for two molecular scaffolds. Lead compounds from one scaffold co-crystallize with human cGAS and occupy the ATP- and GTP-binding active site. The specificity and potency of these drug candidates is further documented in human myeloid cells including primary macrophages. These novel cGAS inhibitors with cell-based activity will serve as probes into cGAS-dependent innate immune pathways and warrant future pharmacological studies for treatment of cGAS-dependent inflammatory diseases.

Small RNA-seq: The RNA 5'-end adapter ligation problem and how to circumvent it.

The preparation of small RNA cDNA sequencing libraries depends on the unbiased ligation of adapters to the RNA ends. Small RNA with 5' recessed ends are poor substrates for enzymatic adapter ligation, but this 5' adapter ligation problem can go undetected if the library preparation steps are not monitored. Here we illustrate the severity of the 5' RNA end ligation problem using several pre-miRNA-like hairpins that allow us to expand the definition of the problem to include 5' ends close to a hairpin stem, whether recessed or in a short extension. The ribosome profiling method can avoid a difficult 5' adapter ligation, but the enzyme typically used to circularize the cDNA has been reported to be biased, calling into question the benefit of this workaround. Using the TS2126 RNA ligase 1 (a.k.a. CircLigase) as the circularizing enzyme, we devised a bias test for the circularization of first strand cDNA. All possible dinucleotides were circle-ligated with similar efficiency. To re-linearize the first strand cDNA in the ribosome profiling approach, we introduce an improved method wherein a single ribonucleotide is placed between the sequencing primer binding sites in the reverse transcriptase primer, which later serves as the point of re-linearization by RNase A. We incorporate this step into the ribosomal profiling method and describe a complete improved library preparation method, Coligo-seq, for the sequencing of small RNA with secondary structure close to the 5' end. This method accepts a variety of 5' modified RNA, including 5' monophosphorylated RNA, as demonstrated by the construction of a HeLa cell microRNA cDNA library.

Publisher Correction: Small molecule inhibition of cGAS reduces interferon expression in primary macrophages from autoimmune mice.

The previously published version of this Article contained errors in Fig. 6. In panel h the units of the x axis were incorrectly given as mM and should have been given as µM. Also, the IC50s for RU.365, RU.332 and RU.521 within panel h were incorrectly given as mM and should have been given as µM. These errors have been corrected in both the PDF and HTML versions of the Article.

Small molecule inhibition of cGAS reduces interferon expression in primary macrophages from autoimmune mice.

Cyclic GMP-AMP synthase is essential for innate immunity against infection and cellular damage, serving as a sensor of DNA from pathogens or mislocalized self-DNA. Upon binding double-stranded DNA, cyclic GMP-AMP synthase synthesizes a cyclic dinucleotide that initiates an inflammatory cellular response. Mouse studies that recapitulate causative mutations in the autoimmune disease Aicardi-Goutières syndrome demonstrate that ablating the cyclic GMP-AMP synthase gene abolishes the deleterious phenotype. Here, we report the discovery of a class of cyclic GMP-AMP synthase inhibitors identified by a high-throughput screen. These compounds possess defined structure-activity relationships and we present crystal structures of cyclic GMP-AMP synthase, double-stranded DNA, and inhibitors within the enzymatic active site. We find that a chemically improved member, RU.521, is active and selective in cellular assays of cyclic GMP-AMP synthase-mediated signaling and reduces constitutive expression of interferon in macrophages from a mouse model of Aicardi-Goutières syndrome. RU.521 will be useful toward understanding the biological roles of cyclic GMP-AMP synthase and can serve as a molecular scaffold for development of future autoimmune therapies.Upon DNA binding cyclic GMP-AMP synthase (cGAS) produces a cyclic dinucleotide, which leads to the upregulation of inflammatory genes. Here the authors develop small molecule cGAS inhibitors, functionally characterize them and present the inhibitor and DNA bound cGAS crystal structures, which will facilitate drug development.

RNA polymerase III initiation on coligo DNA templates containing loops of variable sequence, size and nucleotide chemistry.

Circularized oligonucleotides, or coligos, were previously found to serve as RNA polymerase III (Pol III) templates in vitro and in human tissue culture cells. Here we randomized the 12-nucleotide larger loop (L-loop) of a well characterized coligo and found unexpectedly that in vitro transcription by FLAG-Pol III was not significantly affected. This observation allowed us to test the variable of coligo L-loop size separately from the variable of its sequence. Transcription efficiency increased with L-loop size from 3 to 12 nucleotides of randomized sequence, and the smallest loop forced initiation to move into the stem region. To test further the need for any specific sequence we compared seven nucleotide L-loops composed of random, abasic and abasic-acyclic nucleotides, and all supported transcription by Pol III. Transcription of a series of coligos containing twelve contiguous randomized nucleotides placed at different locations within the coligo structure provided further evidence that the stem-loop junction structure is important for precise initiation. Nearly the same transcript pattern was formed in vitro by Pol III from yeast and human cells. Overall, these experiments support structure, rather than L-loop sequence, as the major determinant of coligo transcription initiation by Pol III.

Adenylylation of small RNA sequencing adapters using the TS2126 RNA ligase I.

Many high-throughput small RNA next-generation sequencing protocols use 5' preadenylylated DNA oligonucleotide adapters during cDNA library preparation. Preadenylylation of the DNA adapter's 5' end frees from ATP-dependence the ligation of the adapter to RNA collections, thereby avoiding ATP-dependent side reactions. However, preadenylylation of the DNA adapters can be costly and difficult. The currently available method for chemical adenylylation of DNA adapters is inefficient and uses techniques not typically practiced in laboratories profiling cellular RNA expression. An alternative enzymatic method using a commercial RNA ligase was recently introduced, but this enzyme works best as a stoichiometric adenylylating reagent rather than a catalyst and can therefore prove costly when several variant adapters are needed or during scale-up or high-throughput adenylylation procedures. Here, we describe a simple, scalable, and highly efficient method for the 5' adenylylation of DNA oligonucleotides using the thermostable RNA ligase 1 from bacteriophage TS2126. Adapters with 3' blocking groups are adenylylated at >95% yield at catalytic enzyme-to-adapter ratios and need not be gel purified before ligation to RNA acceptors. Experimental conditions are also reported that enable DNA adapters with free 3' ends to be 5' adenylylated at >90% efficiency.

New insights into the promoterless transcription of DNA coligo templates by RNA polymerase III.

Chemically synthesized DNA can carry small RNA sequence information but converting that information into small RNA is generally thought to require large double-stranded promoters in the context of plasmids, viruses and genes. We previously found evidence that circularized oligodeoxynucleotides (coligos) containing certain sequences and secondary structures can template the synthesis of small RNA by RNA polymerase III in vitro and in human cells. By using immunoprecipitated RNA polymerase III we now report corroborating evidence that this enzyme is the sole polymerase responsible for coligo transcription. The immobilized polymerase enabled experiments showing that coligo transcripts can be formed through transcription termination without subsequent 3' end trimming. To better define the determinants of productive transcription, a structure-activity relationship study was performed using over 20 new coligos. The results show that unpaired nucleotides in the coligo stem facilitate circumtranscription, but also that internal loops and bulges should be kept small to avoid secondary transcription initiation sites. A polymerase termination sequence embedded in the double-stranded region of a hairpin-encoding coligo stem can antagonize transcription. Using lessons learned from new and old coligos, we demonstrate how to convert poorly transcribed coligos into productive templates. Our findings support the possibility that coligos may prove useful as chemically synthesized vectors for the ectopic expression of small RNA in human cells.

Circularized synthetic oligodeoxynucleotides serve as promoterless RNA polymerase III templates for small RNA generation in human cells.

Synthetic RNA formulations and viral vectors are the two main approaches for delivering small therapeutic RNA to human cells. Here we report findings supporting an alternative strategy in which an endogenous human RNA polymerase (RNAP) is harnessed to make RNA hairpin-containing small RNA from synthetic single-stranded DNA oligonucleotides. We report that circularizing a DNA template strand encoding a pre-microRNA hairpin mimic can trigger its circumtranscription by human RNAP III in vitro and in human cells. Sequence and secondary structure preferences that appear to promote productive transcription are described. The circular topology of the template is required for productive transcription, at least in part, to stabilize the template against exonucleases. In contrast to bacteriophage and Escherichia coli RNAPs, human RNAPs do not carry out rolling circle transcription on circularized templates. While transfected DNA circles distribute between the nucleus and cytosol, their transcripts are found mainly in the cytosol. Circularized oligonucleotides are synthetic, free of the hazards of viral vectors and maintain small RNA information in a stable form that RNAP III can access in a cellular context with, in some cases, near promoter-like precision and biologically relevant efficiency.