Shelterin Protects Chromosome Ends by Compacting Telomeric Chromatin.

Telomeres, repetitive DNA sequences at chromosome ends, are shielded against the DNA damage response (DDR) by the shelterin complex. To understand how shelterin protects telomere ends, we investigated the structural organization of telomeric chromatin in human cells using super-resolution microscopy. We found that telomeres form compact globular structures through a complex network of interactions between shelterin subunits and telomeric DNA, but not by DNA methylation, histone deacetylation, or histone trimethylation at telomeres and subtelomeric regions. Mutations that abrogate shelterin assembly or removal of individual subunits from telomeres cause up to a 10-fold increase in telomere volume. Decompacted telomeres accumulate DDR signals and become more accessible to telomere-associated proteins. Recompaction of telomeric chromatin using an orthogonal method displaces DDR signals from telomeres. These results reveal the chromatin remodeling activity of shelterin and demonstrate that shelterin-mediated compaction of telomeric chromatin provides robust protection of chromosome ends against the DDR machinery.

Nanometer-resolution tracking of single cargo reveals dynein motor mechanisms.

Cytoplasmic dynein is essential for intracellular transport. Despite extensive in vitro characterizations, how the dynein motors transport vesicles by processive steps in live cells remains unclear. To dissect the molecular mechanisms of dynein, we develop optical probes that enable long-term single-particle tracking in live cells with high spatiotemporal resolution. We find that the number of active dynein motors transporting cargo switches stochastically between one and five dynein motors during long-range transport in neuronal axons. Our very bright optical probes allow the observation of individual molecular steps. Strikingly, these measurements reveal that the dwell times between steps are controlled by two temperature-dependent rate constants in which two ATP molecules are hydrolyzed sequentially during each dynein step. Thus, our observations uncover a previously unknown chemomechanical cycle of dynein-mediated cargo transport in living cells.

Single-particle spectroscopy for functional nanomaterials.

Tremendous progress in nanotechnology has enabled advances in the use of luminescent nanomaterials in imaging, sensing and photonic devices. This translational process relies on controlling the photophysical properties of the building block, that is, single luminescent nanoparticles. In this Review, we highlight the importance of single-particle spectroscopy in revealing the diverse optical properties and functionalities of nanomaterials, and compare it with ensemble fluorescence spectroscopy. The information provided by this technique has guided materials science in tailoring the synthesis of nanomaterials to achieve optical uniformity and to develop novel applications. We discuss the opportunities and challenges that arise from pushing the resolution limit, integrating measurement and manipulation modalities, and establishing the relationship between the structure and functionality of single nanoparticles.

Trends of extreme US weather events in the changing climate.

Trends in extreme 100-y events of temperature and rainfall amounts in the continental United States are estimated, to see effects of climate change. This is a nontrivial statistical problem because climate change effects have to be extracted from "noisy" weather data within a limited time range. We use nonparametric Bayesian methods to estimate the trends of extreme events that have occurred between 1979 and 2019, based on data for temperature and rainfall. We focus on 100-y events for each month in [Formula: see text] geographical areas looking at hourly temperature and 5-d cumulative rainfall. Distribution tail models are constructed using extreme value theory (EVT) and data on 33-y events. This work shows it is possible to aggregate data from spatial points in diverse climate zones for a given month and fit an EVT model with the same parameters. This surprising result means there are enough extreme event data to see the trends in the 41-y record for each calendar month. The yearly trends of the risk of a 100-y high-temperature event show an average 2.1-fold increase over the last 41 y of data across all months, with a 2.6-fold increase for the months of July through October. The risk of high rainfall extremes increases in December and January 1.4-fold, but declines by 22% for the spring and summer months.

Single-molecule studies of the neuronal SNARE fusion machinery.

SNAREs are essential components of the machinery for Ca(2+)-triggered fusion of synaptic vesicles with the plasma membrane, resulting in neurotransmitter release into the synaptic cleft. Although much is known about their biophysical and structural properties and their interactions with accessory proteins such as the Ca(2+) sensor synaptotagmin, their precise role in membrane fusion remains an enigma. Ensemble studies of liposomes with reconstituted SNAREs have demonstrated that SNAREs and accessory proteins can trigger lipid mixing/fusion, but the inability to study individual fusion events has precluded molecular insights into the fusion process. Thus, this field is ripe for studies with single-molecule methodology. In this review, we discuss applications of single-molecule approaches to observe reconstituted SNAREs, their complexes, associated proteins, and their effect on biological membranes. Some of the findings are provocative, such as the possibility of parallel and antiparallel SNARE complexes or of vesicle docking with only syntaxin and synaptobrevin, but have been confirmed by other experiments.

Knowledge and attitude regarding organ donation among medical students in Hong Kong: a cross-sectional study.

OBJECTIVES: The rate of organ donation in Hong Kong is among the lowest in developed regions. Since medical students will play an important role in counselling patients for organ donation and identifying potential donors in the future, their knowledge, attitudes and action for organ donation are important. This study aims to understand knowledge, attitudes and actions with regard to organ donation among medical students and investigate the factors determining the knowledge and attitudes. DESIGN: A cross-sectional study. SETTING AND PARTICIPANTS: Medical students in Hong Kong were invited to complete a questionnaire. 377 medical students participated in the study. METHODS: The questionnaire assessed their attitudes, knowledge, action of organ donation, belief and perception on organ donation, and other factors. Linear regression analyses and logistic regression were performed to analyse the effect of the variables on knowledge, attitudes and action for organ donation. RESULTS: Almost all medical students (99.5%) held a positive attitude towards organ donation, but only 28.1% have signed up as organ donors. Determinants of knowledge of organ donation included belief in preservation of intact body after death (ß = -0.14, 95% CI = -0.24 to -0.04) and perceived confidence and competence of organ donation discussion (ß = -0.12, 95% CI = -0.22 to -0.02). Predictors of organ donor registration status included knowledge of organ donation (OR=1.03, 95% CI=1.00 to 1.06), perceived convenience of organ donation registration (OR=3.75, 95% CI=1.62 to 8.71), commitment to organ donation (OR=3.81, 95% CI=2.01 to 7.21) and exposure to organ donation (OR=4.28, 95% CI=2.37 to 7.74). CONCLUSIONS: Knowledge is positively associated with organ donation action. The above determinants of organ donation could be emphasised in medical education.

Direct electrochemical generation of supercooled sulfur microdroplets well below their melting temperature.

Supercooled liquid sulfur microdroplets were directly generated from polysulfide electrochemical oxidation on various metal-containing electrodes. The sulfur droplets remain liquid at 155 °C below sulfur's melting point (Tm = 115 °C), with fractional supercooling change (Tm - Tsc)/Tm larger than 0.40. In operando light microscopy captured the rapid merging and shape relaxation of sulfur droplets, indicating their liquid nature. Micropatterned electrode and electrochemical current allow precise control of the location and size of supercooled microdroplets, respectively. Using this platform, we initiated and observed the rapid solidification of supercooled sulfur microdroplets upon crystalline sulfur touching, which confirms supercooled sulfur's metastability at room temperature. In addition, the formation of liquid sulfur in electrochemical cell enriches lithium-sulfur-electrolyte phase diagram and potentially may create new opportunities for high-energy Li-S batteries.

Three-Dimensional Analysis of Particle Distribution on Filter Layers inside N95 Respirators by Deep Learning.

The global COVID-19 pandemic has changed many aspects of daily lives. Wearing personal protective equipment, especially respirators (face masks), has become common for both the public and medical professionals, proving to be effective in preventing spread of the virus. Nevertheless, a detailed understanding of respirator filtration-layer internal structures and their physical configurations is lacking. Here, we report three-dimensional (3D) internal analysis of N95 filtration layers via X-ray tomography. Using deep learning methods, we uncover how the distribution and diameters of fibers within these layers directly affect contaminant particle filtration. The average porosity of the filter layers is found to be 89.1%. Contaminants are more efficiently captured by denser fiber regions, with fibers <1.8 µm in diameter being particularly effective, presumably because of the stronger electric field gradient on smaller diameter fibers. This study provides critical information for further development of N95-type respirators that combine high efficiency with good breathability.

Reversibly Photoswitching Upconversion Nanoparticles for Super-Sensitive Photoacoustic Molecular Imaging.

Photoacoustic (PA) imaging uses light excitation to generate the acoustic signal for detection and improves tissue penetration depth and spatial resolution in the clinically relevant depth of living subjects. However, strong background signals from blood and pigments have significantly compromised the sensitivity of PA imaging with exogenous contrast agents. Here we report a nanoparticle-based probe design that uses light to reversibly modulate the PA emission to enable photoacoustic photoswitching imaging (PAPSI) in living mice. Such a nanoprobe is built with upconverting nanocrystals and photoswitchable small molecules and can be switched on by NIR light through upconversion to UV energy. Reversibly photoswitching of the nanoprobe reliably removed strong tissue background, increased the contrast-to-noise ratio, and thus improved imaging sensitivity. We have shown that PAPSI can image 0.05 nM of the nanoprobe in hemoglobin solutions and 104 labeled cancer cells after implantation in living mice using a commercial PA imager.

Household Materials Selection for Homemade Cloth Face Coverings and Their Filtration Efficiency Enhancement with Triboelectric Charging.

The COVID-19 pandemic is currently causing a severe disruption and shortage in the global supply chain of necessary personal protective equipment (e.g., N95 respirators). The U.S. CDC has recommended use of household cloth by the general public to make cloth face coverings as a method of source control. We evaluated the filtration properties of natural and synthetic materials using a modified procedure for N95 respirator approval. Common fabrics of cotton, polyester, nylon, and silk had filtration efficiency of 5-25%, polypropylene spunbond had filtration efficiency 6-10%, and paper-based products had filtration efficiency of 10-20%. An advantage of polypropylene spunbond is that it can be simply triboelectrically charged to enhance the filtration efficiency (from 6 to >10%) without any increase in pressure (stable overnight and in humid environments). Using the filtration quality factor, fabric microstructure, and charging ability, we are able to provide an assessment of suggested fabric materials for homemade facial coverings.

Transcription initiation by human RNA polymerase II visualized at single-molecule resolution.

Forty years of classical biochemical analysis have identified the molecular players involved in initiation of transcription by eukaryotic RNA polymerase II (Pol II) and largely assigned their functions. However, a dynamic picture of Pol II transcription initiation and an understanding of the mechanisms of its regulation have remained elusive due in part to inherent limitations of conventional ensemble biochemistry. Here we have begun to dissect promoter-specific transcription initiation directed by a reconstituted human Pol II system at single-molecule resolution using fluorescence video-microscopy. We detected several stochastic rounds of human Pol II transcription from individual DNA templates, observed attenuation of transcription by promoter mutations, observed enhancement of transcription by activator Sp1, and correlated the transcription signals with real-time interactions of holo-TFIID molecules at individual DNA templates. This integrated single-molecule methodology should be applicable to studying other complex biological processes.

Sub-20 nm Core-Shell-Shell Nanoparticles for Bright Upconversion and Enhanced Förster Resonant Energy Transfer.

Upconverting nanoparticles provide valuable benefits as optical probes for bioimaging and Förster resonant energy transfer (FRET) due to their high signal-to-noise ratio, photostability, and biocompatibility; yet, making nanoparticles small yields a significant decay in brightness due to increased surface quenching. Approaches to improve the brightness of UCNPs exist but often require increased nanoparticle size. Here we present a unique core-shell-shell nanoparticle architecture for small (sub-20 nm), bright upconversion with several key features: (1) maximal sensitizer concentration in the core for high near-infrared absorption, (2) efficient energy transfer between core and interior shell for strong emission, and (3) emitter localization near the nanoparticle surface for efficient FRET. This architecture consists of ß-NaYbF4 (core) @NaY0.8-xErxGd0.2F4 (interior shell) @NaY0.8Gd0.2F4 (exterior shell), where sensitizer and emitter ions are partitioned into core and interior shell, respectively. Emitter concentration is varied (x = 1, 2, 5, 10, 20, 50, and 80%) to investigate influence on single particle brightness, upconversion quantum yield, decay lifetimes, and FRET coupling. We compare these seven samples with the field-standard core-shell architecture of ß-NaY0.58Gd0.2Yb0.2Er0.02F4 (core) @NaY0.8Gd0.2F4 (shell), with sensitizer and emitter ions codoped in the core. At a single particle level, the core-shell-shell design was up to 2-fold brighter than the standard core-shell design. Further, by coupling a fluorescent dye to the surface of the two different architectures, we demonstrated up to 8-fold improved emission enhancement with the core-shell-shell compared to the core-shell design. We show how, given proper consideration for emitter concentration, we can design a unique nanoparticle architecture to yield comparable or improved brightness and FRET coupling within a small volume.

Strongly Cavity-Enhanced Spontaneous Emission from Silicon-Vacancy Centers in Diamond.

Quantum emitters are an integral component for a broad range of quantum technologies, including quantum communication, quantum repeaters, and linear optical quantum computation. Solid-state color centers are promising candidates for scalable quantum optics due to their long coherence time and small inhomogeneous broadening. However, once excited, color centers often decay through phonon-assisted processes, limiting the efficiency of single-photon generation and photon-mediated entanglement generation. Herein, we demonstrate strong enhancement of spontaneous emission rate of a single silicon-vacancy center in diamond embedded within a monolithic optical cavity, reaching a regime in which the excited-state lifetime is dominated by spontaneous emission into the cavity mode. We observe 10-fold lifetime reduction and 42-fold enhancement in emission intensity when the cavity is tuned into resonance with the optical transition of a single silicon-vacancy center, corresponding to 90% of the excited-state energy decay occurring through spontaneous emission into the cavity mode. We also demonstrate the largest coupling strength (g/2π = 4.9 ± 0.3 GHz) and cooperativity (C = 1.4) to date for color-center-based cavity quantum electrodynamics systems, bringing the system closer to the strong coupling regime.

Cavity-Enhanced Raman Emission from a Single Color Center in a Solid.

We demonstrate cavity-enhanced Raman emission from a single atomic defect in a solid. Our platform is a single silicon-vacancy center in diamond coupled with a monolithic diamond photonic crystal cavity. The cavity enables an unprecedented frequency tuning range of the Raman emission (100 GHz) that significantly exceeds the spectral inhomogeneity of silicon-vacancy centers in diamond nanostructures. We also show that the cavity selectively suppresses the phonon-induced spontaneous emission that degrades the efficiency of Raman photon generation. Our results pave the way towards photon-mediated many-body interactions between solid-state quantum emitters in a nanophotonic platform.

Ruthenium-Catalyzed Ring-Opening Addition of Anilides to 7-Azabenzonorbornadienes: A Diastereoselective Route to Hydronaphthylamines.

The ruthenium(II)-catalyzed direct ring-opening reaction of 7-azabenzonorbornadienes with anilides via C-H activation to access hydronaphthylamines has been developed. The transformation proceeds with a high stereoselectivity to give cis-configuration products under redox-neutral conditions.

Opportunities and challenges for a sustainable energy future.

Access to clean, affordable and reliable energy has been a cornerstone of the world's increasing prosperity and economic growth since the beginning of the industrial revolution. Our use of energy in the twenty-first century must also be sustainable. Solar and water-based energy generation, and engineering of microbes to produce biofuels are a few examples of the alternatives. This Perspective puts these opportunities into a larger context by relating them to a number of aspects in the transportation and electricity generation sectors. It also provides a snapshot of the current energy landscape and discusses several research and development opportunities and pathways that could lead to a prosperous, sustainable and secure energy future for the world.

Ras-GTP dimers activate the Mitogen-Activated Protein Kinase (MAPK) pathway.

Rat sarcoma (Ras) GTPases regulate cell proliferation and survival through effector pathways including Raf-MAPK, and are the most frequently mutated genes in human cancer. Although it is well established that Ras activity requires binding to both GTP and the membrane, details of how Ras operates on the cell membrane to activate its effectors remain elusive. Efforts to target mutant Ras in human cancers to therapeutic benefit have also been largely unsuccessful. Here we show that Ras-GTP forms dimers to activate MAPK. We used quantitative photoactivated localization microscopy (PALM) to analyze the nanoscale spatial organization of PAmCherry1-tagged KRas 4B (hereafter referred to KRas) on the cell membrane under various signaling conditions. We found that at endogenous expression levels KRas forms dimers, and KRas(G12D), a mutant that constitutively binds GTP, activates MAPK. Overexpression of KRas leads to formation of higher order Ras nanoclusters. Conversely, at lower expression levels, KRas(G12D) is monomeric and activates MAPK only when artificially dimerized. Moreover, dimerization and signaling of KRas are both dependent on an intact CAAX (C, cysteine; A, aliphatic; X, any amino acid) motif that is also known to mediate membrane localization. These results reveal a new, dimerization-dependent signaling mechanism of Ras, and suggest Ras dimers as a potential therapeutic target in mutant Ras-driven tumors.

Vertical-Substrate MPCVD Epitaxial Nanodiamond Growth.

Color center-containing nanodiamonds have many applications in quantum technologies and biology. Diamondoids, molecular-sized diamonds have been used as seeds in chemical vapor deposition (CVD) growth. However, optimizing growth conditions to produce high crystal quality nanodiamonds with color centers requires varying growth conditions that often leads to ad-hoc and time-consuming, one-at-a-time testing of reaction conditions. In order to rapidly explore parameter space, we developed a microwave plasma CVD technique using a vertical, rather than horizontally oriented stage-substrate geometry. With this configuration, temperature, plasma density, and atomic hydrogen density vary continuously along the vertical axis of the substrate. This variation allowed rapid identification of growth parameters that yield single crystal diamonds down to 10 nm in size and 75 nm diameter optically active center silicon-vacancy (Si-V) nanoparticles. Furthermore, this method may provide a means of incorporating a wide variety of dopants in nanodiamonds without ion irradiation damage.

The path towards sustainable energy.

Civilization continues to be transformed by our ability to harness energy beyond human and animal power. A series of industrial and agricultural revolutions have allowed an increasing fraction of the world population to heat and light their homes, fertilize and irrigate their crops, connect to one another and travel around the world. All of this progress is fuelled by our ability to find, extract and use energy with ever increasing dexterity. Research in materials science is contributing to progress towards a sustainable future based on clean energy generation, transmission and distribution, the storage of electrical and chemical energy, energy efficiency, and better energy management systems.