Resolving the intricate binding of neomycin B to multiple binding motifs of a neomycin-sensing riboswitch aptamer by native top-down mass spectrometry and NMR spectroscopy.

Understanding small molecule binding to RNA can be complicated by an intricate interplay between binding stoichiometry, multiple binding motifs, different occupancies of different binding motifs, and changes in the structure of the RNA under study. Here, we use native top-down mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy to experimentally resolve these factors and gain a better understanding of the interactions between neomycin B and the 40 nt aptamer domain of a neomycin-sensing riboswitch engineered in yeast. Data from collisionally activated dissociation of the 1:1, 1:2 and 1:3 RNA-neomycin B complexes identified a third binding motif C of the riboswitch in addition to the two motifs A and B found in our previous study, and provided occupancies of the different binding motifs for each complex stoichiometry. Binding of a fourth neomycin B molecule was unspecific according to both MS and NMR data. Intriguingly, all major changes in the aptamer structure can be induced by the binding of the first neomycin B molecule regardless of whether it binds to motif A or B as evidenced by stoichiometry-resolved MS data together with titration data from 1H NMR spectroscopy in the imino proton region. Specific binding of the second and third neomycin B molecules further stabilizes the riboswitch aptamer, thereby allowing for a gradual response to increasing concentrations of neomycin B, which likely leads to a fine-tuning of the cellular regulatory mechanism.

The PR-10 Protein Pru p 1 is an Endonuclease that Preferentially Cleaves Single-Stranded RNA.

Pathogenesis-related class 10 (PR-10) proteins play a crucial role in plant defense by acting as ribonucleases. The specific mechanism of action and substrate specificity of these proteins have remained largely unexplored so far. In this study, we elucidate the enzymatic activity of Pru p 1, a PR-10 protein from peach. We demonstrate that this protein catalyzes the endonucleolytic backbone cleavage of RNA substrates into short oligonucleotides. Initial cleavage products, identified through kinetic analysis, can bind again, priming them for further degradation. NMR binding site mapping reveals that the large internal cavity of Pru p 1, which is characteristic for PR-10 proteins, serves as an anchoring site for single-stranded ribonucleotide chains. We propose a structure-based mechanistic model that accounts for the observed cleavage patterns and the inhibitory effect of zeatin, a nucleoside analog, on the ribonuclease activity of Pru p 1.

Native Top-Down Mass Spectrometry Uncovers Two Distinct Binding Motifs of a Functional Neomycin-Sensing Riboswitch Aptamer.

Understanding how ligands bind to ribonucleic acids (RNA) is important for understanding RNA recognition in biological processes and drug development. Here, we have studied neomycin B binding to neomycin-sensing riboswitch aptamer constructs by native top-down mass spectrometry (MS) using electrospray ionization (ESI) and collisionally activated dissociation (CAD). Our MS data for a 27 nt aptamer construct reveal the binding site and ligand interactions, in excellent agreement with the structure derived from nuclear magnetic resonance (NMR) studies. Strikingly, for an extended 40 nt aptamer construct, which represents the sequence with the highest regulatory factor for riboswitch function, we identified two binding motifs for neomycin B binding, one corresponding to the bulge-loop motif of the 27 nt construct and the other one in the minor groove of the lower stem, which according to the MS data are equally populated. By replacing a noncanonical with a canonical base pair in the lower stem of the 40 nt aptamer, we can reduce binding to the minor groove motif from ∼50 to ∼30%. Conversely, the introduction of a CUG/CUG motif in the lower stem shifts the binding equilibrium in favor of minor groove binding. The MS data reveal site-specific and stoichiometry-resolved information on aminoglycoside binding to RNA that is not directly accessible by other methods and underscore the role of noncanonical base pairs in RNA recognition by aminoglycosides.

Investigating the Intramolecular Competition of Different RNA Binding Motifs for Neomycin B by Native Top-Down Mass Spectrometry.

The ongoing search for small molecule drugs that target ribonucleic acids (RNA) is complicated by a limited understanding of the principles that govern RNA-small molecule interactions. Here we have used stoichiometry-resolved native top-down mass spectrometry (MS) to study the binding of neomycin B to small model hairpin RNAs, an unstructured RNA, and a viral RNA construct. For 15-22 nt model RNAs with hairpin structure, we found that neomycin B binding to hairpin loops relies on interactions with both the nucleobases and the 2'-OH groups, and that a simple 5' or 3' overhang can introduce an additional binding motif. For a 47 nt RNA construct derived from stem IA of the human immunodeficiency virus 1 (HIV-1) rev response element (RRE) RNA, native top-down MS identified four different binding motifs, of which the purine-rich internal loop showed the highest affinity for neomycin B. Stoichiometry-resolved binding site mapping by native top-down MS allows for a new perspective on binding specificity, and has the potential to reveal unexpected principles of small molecule binding to RNA.

Ultra-Accurate Correlation between Precursor and Fragment Ions in Two-Dimensional Mass Spectrometry: Acetylated vs Trimethylated Histone Peptides.

Two-dimensional mass spectrometry (2D MS) is a method for tandem mass spectrometry in which precursor and fragment ions are correlated by manipulating ion radii rather than by ion isolation. A 2D mass spectrum contains the fragmentation patterns of all analytes in a sample, acquired in parallel. We report ultrahigh-resolution narrowband 2D mass spectra of a mixture of two histone peptides with the same sequence, one of which carries an acetylation and the other a trimethylation (m/z 0.006 difference). We reduced the distance between data points in the precursor ion dimension and compared the accuracy of the precursor-fragment correlation with the resolving power. We manage to perform label-free quantification on the histone peptide mixture and show that precursor and fragment ions can be accurately correlated even though the precursor ions are not resolved. Finally, we show that increasing the resolution of a 2D mass spectrum in the precursor ion dimension too far can lead to a decline in the signal-to-noise ratio.

Weight-gain independent effect of mirtazapine on fasting plasma lipids in healthy men.

Treatment with mirtazapine, a widely prescribed antidepressant, has been linked to weight gain and dyslipidemia. Whether dyslipidemia occurs secondary to increased appetite due to antidepressant treatment, or due to direct pharmacological effects of mirtazapine is unknown. The aim of this analysis is to complement our previously published results of the effect of mirtazapine on metabolism and energy substrate partitioning from a proof-of-concept, open-label clinical study (ClinicalTrials.gov NCT00878540) in 12 healthy males (20-25 years). We report the effect of a seven-day administration of mirtazapine 30 mg per day on weight and lipid metabolism in healthy men under highly standardized conditions with respect to diet, physical activity and day-night-rhythm and under continuous clinical observation. After a 7-day administration of mirtazapine 30 mg, we observed a statistically significant increase in triglyceride levels (mean change + 4.4 mg/dl; 95% CI [- 11.4; 2.6]; p = 0.044) as well as TG/HDL-C ratio (mean change + 0.2; 95% CI [- 0.4; 0.1]; p = 0.019) and a decrease in HDL-cholesterol (mean change - 4.3 mg/dl; 95% CI [2.1; 6.5]; p = 0.004), LDL-cholesterol (mean change - 8.7 mg/dl; 95% CI [3.8; 13.5]; p = 0.008), total cholesterol (mean change - 12.3 mg/dl; 95% CI [5.4; 19.1]; p = 0.005), and non-HDL-C (mean change - 8.0 mg/dl; 95% CI [1.9; 14.0]; p = 0.023). Notably, weight (mean change - 0.6 kg; 95% CI [0.4; 0.8]; p = 0.002) and BMI (mean change - 0.2; 95% CI [0.1; 0.2]; p = 0.002) significantly decreased. No change in waist circumference (mean change - 0.4 cm; 95% CI [- 2.1; 2.9]; p = 0.838) or waist-to-hip-ratio (mean change 0.0; 95% CI [- 0.0; 0.0]; p = 0.814) was observed. This is the first study showing unfavorable changes in lipid metabolism under mirtazapine in healthy individuals despite highly standardized conditions including dietary restriction, and despite the observation of a decrease of weight. Our findings support the hypothesis that mirtazapine has direct pharmacological effects on lipid metabolism. ClinicalTrials.gov: NCT00878540.

A natural riboswitch scaffold with self-methylation activity.

Methylation is a prevalent post-transcriptional modification encountered in coding and non-coding RNA. For RNA methylation, cells use methyltransferases and small organic substances as methyl-group donors, such as S-adenosylmethionine (SAM). SAM and other nucleotide-derived cofactors are viewed as evolutionary leftovers from an RNA world, in which riboswitches have regulated, and ribozymes have catalyzed essential metabolic reactions. Here, we disclose the thus far unrecognized direct link between a present-day riboswitch and its inherent reactivity for site-specific methylation. The key is O6-methyl pre-queuosine (m6preQ1), a potentially prebiotic nucleobase which is recognized by the native aptamer of a preQ1 class I riboswitch. Upon binding, the transfer of the ligand's methyl group to a specific cytidine occurs, installing 3-methylcytidine (m3C) in the RNA pocket under release of pre-queuosine (preQ1). Our finding suggests that nucleic acid-mediated methylation is an ancient mechanism that has offered an early path for RNA epigenetics prior to the evolution of protein methyltransferases. Furthermore, our findings may pave the way for the development of riboswitch-descending methylation tools based on rational design as a powerful alternative to in vitro selection approaches.

Native mass spectrometry reveals the initial binding events of HIV-1 rev to RRE stem II RNA.

Nuclear export complexes composed of rev response element (RRE) ribonucleic acid (RNA) and multiple molecules of rev protein are promising targets for the development of therapeutic strategies against human immunodeficiency virus type 1 (HIV-1), but their assembly remains poorly understood. Using native mass spectrometry, we show here that rev initially binds to the upper stem of RRE IIB, from where it is relayed to binding sites that allow for rev dimerization. The newly discovered binding region implies initial rev recognition by nucleotides that are not part of the internal loop of RRE stem IIB RNA, which was previously identified as the preferred binding region. Our study highlights the unique capability of native mass spectrometry to separately study the binding interfaces of RNA/protein complexes of different stoichiometry, and provides a detailed understanding of the mechanism of RRE/rev association with implications for the rational design of potential drugs against HIV-1 infection.

Effect of mirtazapine on metabolism and energy substrate partitioning in healthy men.

BACKGROUND: Weight gain and metabolic changes during treatment with antidepressant drugs have emerged as an important concern, particularly in long-term treatment. It is still a matter of ongoing debate whether weight gain and metabolic perturbations with antidepressant use are the consequence of increased appetite and weight gain, respectively, or represents direct pharmacological effects of the drug on metabolism. METHODS: We therefore conducted a proof-of-concept, open-label clinical trial, hypothesizing that in exceptionally healthy men no change of metabolic parameters would occur under mirtazapine, when environmental factors such as nutrition, sleep, and physical exercise were controlled and kept constant. Over a 3-week preparation phase, 10 healthy, young men were attuned to a standardized diet adjusted to their individual caloric need, to a regular sleep/wake cycle and moderate exercise. Continuing this protocol, we administered 30 mg mirtazapine daily for 7 days. RESULTS: While no significant weight gain or changes in resting energy expenditure were observed under these conditions, hunger and appetite for sweets increased with mirtazapine, accompanied by a shift in energy substrate partitioning towards carbohydrate substrate preference as assessed by indirect calorimetry. Furthermore, with mirtazapine, insulin and C-peptide release increased in response to a standardized meal. CONCLUSION: Our findings provide important insights into weight-independent metabolic changes associated with mirtazapine and allow a better understanding of the long-term metabolic effects observed in patients treated with antidepressant drugs. CLINICALTRIALS: gov NCT00878540. FUNDING: Nothing to declare.