Probing complex RNA structures by mechanical force

RNA secondary structures of increasing complexity are probed combining single molecule stretching experiments and stochastic unfolding/refolding simulations. We find that force-induced unfolding pathways cannot usually be interpretated by solely invoking successive openings of native helices. Indeed, typical force-extension responses of complex RNA molecules are largely shaped by stretching-induced, long-lived intermediates including non-native helices. This is first shown for a set of generic structural motifs found in larger RNA structures, and then for Escherichia coli's 1540-base long 16S ribosomal RNA, which exhibits a surprisingly well-structured and reproducible unfolding pathway under mechanical stretching. Using out-of-equilibrium stochastic simulations, we demonstrate that these experimental results reflect the slow relaxation of RNA structural rearrangements. Hence, micromanipulations of single RNA molecules probe both their native structures and long-lived intermediates, so-called "kinetic traps", thereby capturing -at the single molecular level- the hallmark of RNA folding/unfolding dynamics.Comment: 9 pages, 9 figure

Prediction and statistics of pseudoknots in RNA structures using exactly clustered stochastic simulations

Ab initio RNA secondary structure predictions have long dismissed helices interior to loops, so-called pseudoknots, despite their structural importance. Here, we report that many pseudoknots can be predicted through long time scales RNA folding simulations, which follow the stochastic closing and opening of individual RNA helices. The numerical efficacy of these stochastic simulations relies on an O(n^2) clustering algorithm which computes time averages over a continously updated set of n reference structures. Applying this exact stochastic clustering approach, we typically obtain a 5- to 100-fold simulation speed-up for RNA sequences up to 400 bases, while the effective acceleration can be as high as 100,000-fold for short multistable molecules (<150 bases). We performed extensive folding statistics on random and natural RNA sequences, and found that pseudoknots are unevenly distributed amongst RNAstructures and account for up to 30% of base pairs in G+C rich RNA sequences (Online RNA folding kinetics server including pseudoknots : http://kinefold.u-strasbg.fr/ ).Comment: 6 pages, 5 figure

Entanglement, elasticity and viscous relaxation of actin solutions

We have investigated the viscosity and the plateau modulus of actin solutions with a magnetically driven rotating disc rheometer. For entangled solutions we observed a scaling of the plateau modulus versus concentration with a power of 7/5. The measured terminal relaxation time increases with a power 3/2 as a function of polymer length. We interpret the entanglement transition and the scaling of the plateau modulus in terms of the tube model for semiflexible polymers.Comment: 5 pages, 4 figures, published versio

Polymer Induced Bundling of F-actin and the Depletion Force

The inert polymer polyethylene glycol (PEG) induces a "bundling" phenomenon in F-actin solutions when its concentration exceeds a critical onset value C_o. Over a limited range of PEG molecular weight and ionic strength, C_o can be expressed as a function of these two variables. The process is reversible, but hysteresis is also observed in the dissolution of the bundles, with ionic strength having a large influence. Additional actin filaments are able to join previously formed bundles. Little, if any, polymer is associated with the bundle structure. Continuum estimates of the Asakura-Oosawa depletion force, Coulomb repulsion, and van der Waals potential are combined for a partial explanation of the bundling effect and hysteresis. Conjectures are presented concerning the apparent limit in bundle size

Field Theory of the RNA Freezing Transition

Folding of RNA is subject to a competition between entropy, relevant at high temperatures, and the random, or random looking, sequence, determining the low- temperature phase. It is known from numerical simulations that for random as well as biological sequences, high- and low-temperature phases are different, e.g. the exponent rho describing the pairing probability between two bases is rho = 3/2 in the high-temperature phase, and approximatively 4/3 in the low-temperature (glass) phase. Here, we present, for random sequences, a field theory of the phase transition separating high- and low-temperature phases. We establish the existence of the latter by showing that the underlying theory is renormalizable to all orders in perturbation theory. We test this result via an explicit 2-loop calculation, which yields rho approximatively 1.36 at the transition, as well as diverse other critical exponents, including the response to an applied external force (denaturation transition).Comment: 96 pages, 188 figures. v2: minor correction

Statistical mechanics of secondary structures formed by random RNA sequences

The formation of secondary structures by a random RNA sequence is studied as a model system for the sequence-structure problem omnipresent in biopolymers. Several toy energy models are introduced to allow detailed analytical and numerical studies. First, a two-replica calculation is performed. By mapping the two-replica problem to the denaturation of a single homogeneous RNA in 6-dimensional embedding space, we show that sequence disorder is perturbatively irrelevant, i.e., an RNA molecule with weak sequence disorder is in a molten phase where many secondary structures with comparable total energy coexist. A numerical study of various models at high temperature reproduces behaviors characteristic of the molten phase. On the other hand, a scaling argument based on the extremal statistics of rare regions can be constructed to show that the low temperature phase is unstable to sequence disorder. We performed a detailed numerical study of the low temperature phase using the droplet theory as a guide, and characterized the statistics of large-scale, low-energy excitations of the secondary structures from the ground state structure. We find the excitation energy to grow very slowly (i.e., logarithmically) with the length scale of the excitation, suggesting the existence of a marginal glass phase. The transition between the low temperature glass phase and the high temperature molten phase is also characterized numerically. It is revealed by a change in the coefficient of the logarithmic excitation energy, from being disorder dominated to entropy dominated.Comment: 24 pages, 16 figure

Two phase I studies of BI 836880, a vascular endothelial growth factor/angiopoietin-2 inhibitor, administered once every 3 weeks or once weekly in patients with advanced solid tumors

BACKGROUND: BI 836880 is a humanized bispecific nanobody® that inhibits vascular endothelial growth factor and angiopoietin-2. Here, we report results from two phase I, nonrandomized, dose-escalation studies (NCT02674152 and NCT02689505; funded by Boehringer Ingelheim) evaluating BI 836880 in patients with confirmed locally advanced or metastatic solid tumors, refractory to standard therapy, or for which standard therapy was ineffective. PATIENTS AND METHODS: Patients aged ≥18 years, with an Eastern Cooperative Oncology Group performance status of 0-2 and adequate organ function received escalating intravenous doses of BI 836880 once every 3 weeks (Q3W; Study 1336.1) or once weekly (QW; Study 1336.6). Primary objectives were maximum tolerated dose (MTD) and recommended phase II dose of BI 836880, based on dose-limiting toxicities (DLTs) during the first cycle. RESULTS: Patients received one of five dosages of 40-1000 mg Q3W (29 patients) or 40-240 mg QW (24 patients). One DLT occurred with Q3W treatment [Grade (G) 3 pulmonary embolism (1000 mg)]. Five DLTs occurred in four patients treated QW [G2 proteinuria (120 mg); G3 hypertension (180 mg); G3 proteinuria and G3 hypertension (240 mg); and G4 respiratory distress (240 mg)]. All patients experienced adverse events, most commonly hypertension with Q3W treatment (89.7%; G3 41.4%), and asthenia with QW treatment (62.5%). Two patients treated Q3W (both 1000 mg) and three patients treated QW (120 mg, 2 patients; 180 mg, 1 patient) experienced partial response. CONCLUSIONS: The MTD of BI 836880 was 720 mg Q3W and 180 mg QW. BI 836880 was generally manageable and demonstrated preliminary efficacy. CLINICAL TRIAL REGISTRATION: ClinicalTrials.govNCT02674152; https://clinicaltrials.gov/ct2/show/NCT02674152 and NCT02689505; https://clinicaltrials.gov/ct2/show/NCT0268950

Force-Velocity Measurements of a Few Growing Actin Filaments

The authors propose a new mechanism for actin-based force generation based on results using chains of actin-grafted magnetic colloids

Individual Actin Filaments in a Microfluidic Flow Reveal the Mechanism of ATP Hydrolysis and Give Insight Into the Properties of Profilin

A novel microfluidic approach allows the analysis of the dynamics of individual actin filaments, revealing both their local ADP/ADP-Pi-actin composition and that Pi release is a random mechanism

Transat—A Method for Detecting the Conserved Helices of Functional RNA Structures, Including Transient, Pseudo-Knotted and Alternative Structures

The prediction of functional RNA structures has attracted increased interest, as it allows us to study the potential functional roles of many genes. RNA structure prediction methods, however, assume that there is a unique functional RNA structure and also do not predict functional features required for in vivo folding. In order to understand how functional RNA structures form in vivo, we require sophisticated experiments or reliable prediction methods. So far, there exist only a few, experimentally validated transient RNA structures. On the computational side, there exist several computer programs which aim to predict the co-transcriptional folding pathway in vivo, but these make a range of simplifying assumptions and do not capture all features known to influence RNA folding in vivo. We want to investigate if evolutionarily related RNA genes fold in a similar way in vivo. To this end, we have developed a new computational method, Transat, which detects conserved helices of high statistical significance. We introduce the method, present a comprehensive performance evaluation and show that Transat is able to predict the structural features of known reference structures including pseudo-knotted ones as well as those of known alternative structural configurations. Transat can also identify unstructured sub-sequences bound by other molecules and provides evidence for new helices which may define folding pathways, supporting the notion that homologous RNA sequence not only assume a similar reference RNA structure, but also fold similarly. Finally, we show that the structural features predicted by Transat differ from those assuming thermodynamic equilibrium. Unlike the existing methods for predicting folding pathways, our method works in a comparative way. This has the disadvantage of not being able to predict features as function of time, but has the considerable advantage of highlighting conserved features and of not requiring a detailed knowledge of the cellular environment