2017-01-23The application of genome-wide CRISPR–Cas9 screening coupled with a fluorescent reporter to interrogate the microRNA pathway reveals that continual transient phosphorylation of Argonaute 2 is required to maintain the global efficiency of microRNA-mediated repression.
2017-01-23Mitochondria are double-membraned organelles with variable shapes influenced by metabolic conditions, developmental stage, and environmental stimuli. Their dynamic morphology is a result of regulated and balanced fusion and fission processes. Fusion is crucial for the health and physiological functions of mitochondria, including complementation of damaged mitochondrial DNAs and the maintenance of membrane potential. Mitofusins are dynamin-related GTPases that are essential for mitochondrial fusion. They are embedded in the mitochondrial outer membrane and thought to fuse adjacent mitochondria via combined oligomerization and GTP hydrolysis. However, the molecular mechanisms of this process remain unknown. Here we present crystal structures of engineered human MFN1 containing the GTPase domain and a helical domain during different stages of GTP hydrolysis. The helical domain is composed of elements from widely dispersed sequence regions of MFN1 and resembles the ‘neck’ of the bacterial dynamin-like protein. The structures reveal unique features of its catalytic machinery and explain how GTP binding induces conformational changes to promote GTPase domain dimerization in the transition state. Disruption of GTPase domain dimerization abolishes the fusogenic activity of MFN1. Moreover, a conserved aspartate residue trigger was found to affect mitochondrial elongation in MFN1, probably through a GTP-loading-dependent domain rearrangement. Thus, we propose a mechanistic model for MFN1-mediated mitochondrial tethering, and our results shed light on the molecular basis of mitochondrial fusion and mitofusin-related human neuromuscular disorders.
2017-01-23Collective oscillatory behaviour is ubiquitous in nature, having a vital role in many biological processes from embryogenesis and organ development to pace-making in neuron networks. Elucidating the mechanisms that give rise to synchronization is essential to the understanding of biological self-organization. Collective oscillations in biological multicellular systems often arise from long-range coupling mediated by diffusive chemicals, by electrochemical mechanisms, or by biomechanical interaction between cells and their physical environment. In these examples, the phase of some oscillatory intracellular degree of freedom is synchronized. Here, in contrast, we report the discovery of a weak synchronization mechanism that does not require long-range coupling or inherent oscillation of individual cells. We find that millions of motile cells in dense bacterial suspensions can self-organize into highly robust collective oscillatory motion, while individual cells move in an erratic manner, without obvious periodic motion but with frequent, abrupt and random directional changes. So erratic are individual trajectories that uncovering the collective oscillations of our micrometre-sized cells requires individual velocities to be averaged over tens or hundreds of micrometres. On such large scales, the oscillations appear to be in phase and the mean position of cells typically describes a regular elliptic trajectory. We found that the phase of the oscillations is organized into a centimetre-scale travelling wave. We present a model of noisy self-propelled particles with strictly local interactions that accounts faithfully for our observations, suggesting that self-organized collective oscillatory motion results from spontaneous chiral and rotational symmetry breaking. These findings reveal a previously unseen type of long-range order in active matter systems (those in which energy is spent locally to produce non-random motion). This mechanism of collective oscillation may inspire new strategies to control the self-organization of active matter and swarming robots.
2017-01-23Sexual reproduction in flowering plants involves double fertilization, the union of two sperm from pollen with two sex cells in the female embryo sac. Modern plant breeders increasingly seek to circumvent this process to produce doubled haploid individuals, which derive from the chromosome-doubled cells of the haploid gametophyte. Doubled haploid production fixes recombinant haploid genomes in inbred lines, shaving years off the breeding process. Costly, genotype-dependent tissue culture methods are used in many crops, while seed-based in vivo doubled haploid systems are rare in nature and difficult to manage in breeding programmes. The multi-billion-dollar maize hybrid seed business, however, is supported by industrial doubled haploid pipelines using intraspecific crosses to in vivo haploid inducer males derived from Stock 6, first reported in 1959 (ref. 5), followed by colchicine treatment. Despite decades of use, the mode of action remains controversial. Here we establish, through fine mapping, genome sequencing, genetic complementation, and gene editing, that haploid induction in maize (Zea mays) is triggered by a frame-shift mutation in MATRILINEAL (MTL), a pollen-specific phospholipase, and that novel edits in MTL lead to a 6.7% haploid induction rate (the percentage of haploid progeny versus total progeny). Wild-type MTL protein localizes exclusively to sperm cytoplasm, and pollen RNA-sequence profiling identifies a suite of pollen-specific genes overexpressed during haploid induction, some of which may mediate the formation of haploid seed. These findings highlight the importance of male gamete cytoplasmic components to reproductive success and male genome transmittance. Given the conservation of MTL in the cereals, this discovery may enable development of in vivo haploid induction systems to accelerate breeding in crop plants.
2017-01-23The development of catalyst-controlled stereoselective olefin metathesis processes has been a pivotal recent advance in chemistry. The incorporation of appropriate ligands within complexes based on molybdenum, tungsten and ruthenium has led to reactivity and selectivity levels that were previously inaccessible. Here we show that molybdenum monoaryloxide chloride complexes furnish higher-energy (Z) isomers of trifluoromethyl-substituted alkenes through cross-metathesis reactions with the commercially available, inexpensive and typically inert Z-1,1,1,4,4,4-hexafluoro-2-butene. Furthermore, otherwise inefficient and non-stereoselective transformations with Z-1,2-dichloroethene and 1,2-dibromoethene can be effected with substantially improved efficiency and Z selectivity. The use of such molybdenum monoaryloxide chloride complexes enables the synthesis of representative biologically active molecules and trifluoromethyl analogues of medicinally relevant compounds. The origins of the activity and selectivity levels observed, which contradict previously proposed principles, are elucidated with the aid of density functional theory calculations.
2017-01-18When some cancer cells delete a tumour-suppressor gene, they also delete nearby genes. It emerges that one of these latter genes has a key metabolic role, revealing a therapeutic opportunity that might be relevant for many tumours.
2017-01-18It emerges that phage viruses, which infect bacteria, use small peptides to communicate with each other. This observation of intercellular communication also reveals how viruses make a key developmental decision.
2017-01-18Materials called hydrogels have potential applications as scaffolds for tissue engineering, but methods are needed to assemble them into complex structures that mimic those found in nature. Just such a method has now been reported.
2017-01-18Interleukin-17 functions as a neuromodulator in the roundworm Caenorhabditis elegans, acting directly on RMG hub interneurons to alter their response properties and contribution to behaviour.
2017-01-18Some phages—viruses that infect bacteria—encode peptides that are secreted from infected cells and that, beyond a certain threshold, stimulate other viruses to switch from the lytic (killing the host cell) to lysogenic (dormant) phase.
2017-01-18The first high-resolution (3.5 Å) structure of a full-length cyclic-nucleotide-gated channel, revealing an unconventional, voltage-insensitive voltage-sensor domain and a unique coupling mechanism between cyclic-nucleotide-binding and pore-opening.
2017-01-18A reactive astrocyte subtype termed A1 is induced after injury or disease of the central nervous system and subsequently promotes the death of neurons and oligodendrocytes.
2017-01-18The genome of pancreatic ductal adenocarcinoma (PDAC) frequently contains deletions of tumour suppressor gene loci, most notably SMAD4, which is homozygously deleted in nearly one-third of cases. As loss of neighbouring housekeeping genes can confer collateral lethality, we sought to determine whether loss of the metabolic gene malic enzyme 2 (ME2) in the SMAD4 locus would create cancer-specific metabolic vulnerability upon targeting of its paralogous isoform ME3. The mitochondrial malic enzymes (ME2 and ME3) are oxidative decarboxylases that catalyse the conversion of malate to pyruvate and are essential for NADPH regeneration and reactive oxygen species homeostasis. Here we show that ME3 depletion selectively kills ME2-null PDAC cells in a manner consistent with an essential function for ME3 in ME2-null cancer cells. Mechanistically, integrated metabolomic and molecular investigation of cells deficient in mitochondrial malic enzymes revealed diminished NADPH production and consequent high levels of reactive oxygen species. These changes activate AMP activated protein kinase (AMPK), which in turn directly suppresses sterol regulatory element-binding protein 1 (SREBP1)-directed transcription of its direct targets including the BCAT2 branched-chain amino acid transaminase 2) gene. BCAT2 catalyses the transfer of the amino group from branched-chain amino acids to α-ketoglutarate (α-KG) thereby regenerating glutamate, which functions in part to support de novo nucleotide synthesis. Thus, mitochondrial malic enzyme deficiency, which results in impaired NADPH production, provides a prime ‘collateral lethality’ therapeutic strategy for the treatment of a substantial fraction of patients diagnosed with this intractable disease.
2017-01-18Discovering ways to control the magnetic state of media with the lowest possible production of heat and at the fastest possible speeds is important in the study of fundamental magnetism, with clear practical potential. In metals, it is possible to switch the magnetization between two stable states (and thus to record magnetic bits) using femtosecond circularly polarized laser pulses. However, the switching mechanisms in these materials are directly related to laser-induced heating close to the Curie temperature. Although several possible routes for achieving all-optical switching in magnetic dielectrics have been discussed, no recording has hitherto been demonstrated. Here we describe ultrafast all-optical photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet. A single linearly polarized femtosecond laser pulse resonantly pumps specific d−d transitions in the cobalt ions, breaking the degeneracy between metastable magnetic states. By changing the polarization of the laser pulse, we deterministically steer the net magnetization in the garnet, thus writing ‘0’ and ‘1’ magnetic bits at will. This mechanism outperforms existing alternatives in terms of the speed of the write–read magnetic recording event (less than 20 picoseconds) and the unprecedentedly low heat load (less than 6 joules per cubic centimetre).
2017-01-16Large interannual variations in the measured growth rate of atmospheric carbon dioxide (CO2) originate primarily from fluctuations in carbon uptake by land ecosystems. It remains uncertain, however, to what extent temperature and water availability control the carbon balance of land ecosystems across spatial and temporal scales. Here we use empirical models based on eddy covariance data and process-based models to investigate the effect of changes in temperature and water availability on gross primary productivity (GPP), terrestrial ecosystem respiration (TER) and net ecosystem exchange (NEE) at local and global scales. We find that water availability is the dominant driver of the local interannual variability in GPP and TER. To a lesser extent this is true also for NEE at the local scale, but when integrated globally, temporal NEE variability is mostly driven by temperature fluctuations. We suggest that this apparent paradox can be explained by two compensatory water effects. Temporal water-driven GPP and TER variations compensate locally, dampening water-driven NEE variability. Spatial water availability anomalies also compensate, leaving a dominant temperature signal in the year-to-year fluctuations of the land carbon sink. These findings help to reconcile seemingly contradictory reports regarding the importance of temperature and water in controlling the interannual variability of the terrestrial carbon balance. Our study indicates that spatial climate covariation drives the global carbon cycle response.
2017-01-16The Southern Ocean houses a diverse and productive community of organisms. Unicellular eukaryotic diatoms are the main primary producers in this environment, where photosynthesis is limited by low concentrations of dissolved iron and large seasonal fluctuations in light, temperature and the extent of sea ice. How diatoms have adapted to this extreme environment is largely unknown. Here we present insights into the genome evolution of a cold-adapted diatom from the Southern Ocean, Fragilariopsis cylindrus, based on a comparison with temperate diatoms. We find that approximately 24.7 per cent of the diploid F. cylindrus genome consists of genetic loci with alleles that are highly divergent (15.1 megabases of the total genome size of 61.1 megabases). These divergent alleles were differentially expressed across environmental conditions, including darkness, low iron, freezing, elevated temperature and increased CO2. Alleles with the largest ratio of non-synonymous to synonymous nucleotide substitutions also show the most pronounced condition-dependent expression, suggesting a correlation between diversifying selection and allelic differentiation. Divergent alleles may be involved in adaptation to environmental fluctuations in the Southern Ocean.
2017-01-16Normal epithelial cells often exert anti-tumour effects against nearby oncogenic cells. In the Drosophila imaginal epithelium, clones of oncogenic cells with loss-of-function mutations in the apico-basal polarity genes scribble or discs large are actively eliminated by cell competition when surrounded by wild-type cells. Although c-Jun N-terminal kinase (JNK) signalling plays a crucial role in this cell elimination, the initial event, which occurs at the interface between normal cells and polarity-deficient cells, has not previously been identified. Here, through a genetic screen in Drosophila, we identify the ligand Sas and the receptor-type tyrosine phosphatase PTP10D as the cell-surface ligand–receptor system that drives tumour-suppressive cell competition. At the interface between the wild-type ‘winner’ and the polarity-deficient ‘loser’ clones, winner cells relocalize Sas to the lateral cell surface, whereas loser cells relocalize PTP10D there. This leads to the trans-activation of Sas–PTP10D signalling in loser cells, which restrains EGFR signalling and thereby enables elevated JNK signalling in loser cells, triggering cell elimination. In the absence of Sas–PTP10D, elevated EGFR signalling in loser cells switches the role of JNK from pro-apoptotic to pro-proliferative by inactivating the Hippo pathway, thereby driving the overgrowth of polarity-deficient cells. These findings uncover the mechanism by which normal epithelial cells recognize oncogenic polarity-deficient neighbours to drive cell competition.
2017-01-11Translation of RNA into proteins is a fundamental process for all cells. Analysis of a mouse model of skin cancer uncovers an atypical RNA-translation program that has a vital role in tumour formation.
2017-01-11The translation of upstream open reading frames in skin tumour models protects some cancer-related mRNAs from global reductions in protein synthesis during the early stages of tumour initiation, suggesting that unconventional translation has a crucial role in tumorigenesis.
2017-01-11Peatlands are carbon-rich ecosystems that cover just three per cent of Earth’s land surface, but store one-third of soil carbon. Peat soils are formed by the build-up of partially decomposed organic matter under waterlogged anoxic conditions. Most peat is found in cool climatic regions where unimpeded decomposition is slower, but deposits are also found under some tropical swamp forests. Here we present field measurements from one of the world’s most extensive regions of swamp forest, the Cuvette Centrale depression in the central Congo Basin. We find extensive peat deposits beneath the swamp forest vegetation (peat defined as material with an organic matter content of at least 65 per cent to a depth of at least 0.3 metres). Radiocarbon dates indicate that peat began accumulating from about 10,600 years ago, coincident with the onset of more humid conditions in central Africa at the beginning of the Holocene. The peatlands occupy large interfluvial basins, and seem to be largely rain-fed and ombrotrophic-like (of low nutrient status) systems. Although the peat layer is relatively shallow (with a maximum depth of 5.9 metres and a median depth of 2.0 metres), by combining in situ and remotely sensed data, we estimate the area of peat to be approximately 145,500 square kilometres (95 per cent confidence interval of 131,900–156,400 square kilometres), making the Cuvette Centrale the most extensive peatland complex in the tropics. This area is more than five times the maximum possible area reported for the Congo Basin in a recent synthesis of pantropical peat extent. We estimate that the peatlands store approximately 30.6 petagrams (30.6 × 1015 grams) of carbon belowground (95 per cent confidence interval of 6.3–46.8 petagrams of carbon)—a quantity that is similar to the above-ground carbon stocks of the tropical forests of the entire Congo Basin. Our result for the Cuvette Centrale increases the best estimate of global tropical peatland carbon stocks by 36 per cent, to 104.7 petagrams of carbon (minimum estimate of 69.6 petagrams of carbon; maximum estimate of 129.8 petagrams of carbon). This stored carbon is vulnerable to land-use change and any future reduction in precipitation.
2017-01-11The critical role in surface reactions and heterogeneous catalysis of metal atoms with low coordination numbers, such as found at atomic steps and surface defects, is firmly established. But despite the growing availability of tools that enable detailed in situ characterization, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resolution, and catalytic conversion can be tracked with a clear chemical signature; however, the combination of the two, which would enable high-spatial-resolution detection of reactions on catalytic surfaces, has rarely been achieved. Single-molecule fluorescence spectroscopy has been used to image and characterize single turnover sites at catalytic surfaces, but is restricted to reactions that generate highly fluorescing product molecules. Herein the chemical conversion of N-heterocyclic carbene molecules attached to catalytic particles is mapped using synchrotron-radiation-based infrared nanospectroscopy with a spatial resolution of 25 nanometres, which enabled particle regions that differ in reactivity to be distinguished. These observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles—which contain metal atoms with low coordination numbers—are more active in catalysing oxidation and reduction of chemically active groups in surface-anchored N-heterocyclic carbene molecules.
2017-01-11Quality control mechanisms intervene appropriately when defective translation events occur, in order to preserve the integrity of protein synthesis. Rescue of ribosomes translating on messenger RNAs that lack stop codons is one of the co-translational quality control pathways. In many bacteria, ArfA recognizes stalled ribosomes and recruits the release factor RF2, which catalyses the termination of protein synthesis. Although an induced-fit mechanism of nonstop mRNA surveillance mediated by ArfA and RF2 has been reported, the molecular interaction between ArfA and RF2 in the ribosome that is responsible for the mechanism is unknown. Here we report an electron cryo-microscopy structure of ArfA and RF2 in complex with the 70S ribosome bound to a nonstop mRNA. The structure, which is consistent with our kinetic and biochemical data, reveals the molecular interactions that enable ArfA to specifically recruit RF2, not RF1, into the ribosome and to enable RF2 to release the truncated protein product in this co-translational quality control pathway. The positively charged C-terminal domain of ArfA anchors in the mRNA entry channel of the ribosome. Furthermore, binding of ArfA and RF2 induces conformational changes in the ribosomal decoding centre that are similar to those seen in other protein-involved decoding processes. Specific interactions between residues in the N-terminal domain of ArfA and RF2 help RF2 to adopt a catalytically competent conformation for peptide release. Our findings provide a framework for understanding recognition of the translational state of the ribosome by new proteins, and expand our knowledge of the decoding potential of the ribosome.
2017-01-09Cell fate perturbations underlie many human diseases, including breast cancer. Unfortunately, the mechanisms by which breast cell fate are regulated are largely unknown. The mammary gland epithelium consists of differentiated luminal epithelial and basal myoepithelial cells, as well as undifferentiated stem cells and more restricted progenitors. Breast cancer originates from this epithelium, but the molecular mechanisms that underlie breast epithelial hierarchy remain ill-defined. Here, we use a high-content confocal image-based short hairpin RNA screen to identify tumour suppressors that regulate breast cell fate in primary human breast epithelial cells. We show that ablation of the large tumour suppressor kinases (LATS) 1 and 2 (refs 5, 6), which are part of the Hippo pathway, promotes the luminal phenotype and increases the number of bipotent and luminal progenitors, the proposed cells-of-origin of most human breast cancers. Mechanistically, we have identified a direct interaction between Hippo and oestrogen receptor-α (ERα) signalling. In the presence of LATS, ERα was targeted for ubiquitination and Ddb1–cullin4-associated-factor 1 (DCAF1)-dependent proteasomal degradation. Absence of LATS stabilized ERα and the Hippo effectors YAP and TAZ (hereafter YAP/TAZ), which together control breast cell fate through intrinsic and paracrine mechanisms. Our findings reveal a non-canonical (that is, YAP/TAZ-independent) effect of LATS in the regulation of human breast cell fate.
2017-01-09Males of many polygynous species compete for access to fertile females without providing them with resources other than sperm and without investing in care for the offspring (male dominance polygyny). In such systems, local competition for access to females is intense and typically only a few males obtain matings, leading to strong sexual selection. Sampling multiple breeding areas could then provide a mechanism for males to increase their chances to reproduce. However, little is known about such sampling behaviour and about the spatial scale at which males compete. Here we show that most males of a migratory, polygynous shorebird, the pectoral sandpiper (Calidris melanotos), that arrived at a known breeding location in northern Alaska subsequently moved through a considerable part of the entire species’ breeding range (up to 13,045 km in a four-week period), sampling as many as 23 additional potential breeding sites. Our data suggest that males do not have a final breeding destination after migration from their wintering quarters, but make nomadic movements that are probably not a consequence of breeding failure. Tenure, the duration of stay at a site, correlated strongly with the number of breeding females at the site, suggesting that decisions to leave are dependent on local mating opportunities. Nomadic movements may allow males to display and sire offspring at multiple sites within a single breeding season. Sexual selection may then favour high-performance males that are able to reduce sleep to compete locally and to fly long distances between breeding sites, leading to a population with unrestricted interbreeding and without local adaptation and speciation.
2016-12-26The lymphangiogenic factor PROX1 transcriptionally upregulates CPT1A, a rate-controlling enzyme in fatty acid β-oxidation, and this co-regulates lymphatic endothelial cell differentiation by epigenetic control of lymphatic gene expression, demonstrating a role for metabolism in developmental biology.
2016-12-21Some CLC proteins are channels that conduct chloride ions passively, whereas others are active co-transporters, a difference that has been hard to understand given their high degree of sequence homology; now, cryo-electron microscopy is used to determine the structure of a mammalian CLC channel, shedding light on this question.
2016-12-21Engineered spin–orbit coupling (SOC) in cold-atom systems can enable the study of new synthetic materials and complex condensed matter phenomena. However, spontaneous emission in alkali-atom spin–orbit-coupled systems is hindered by heating, limiting the observation of many-body effects and motivating research into potential alternatives. Here we demonstrate that spin–orbit-coupled fermions can be engineered to occur naturally in a one-dimensional optical lattice clock. In contrast to previous SOC experiments, here the SOC is both generated and probed using a direct ultra-narrow optical clock transition between two electronic orbital states in 87Sr atoms. We use clock spectroscopy to prepare lattice band populations, internal electronic states and quasi-momenta, and to produce spin–orbit-coupled dynamics. The exceptionally long lifetime of the excited clock state (160 seconds) eliminates decoherence and atom loss from spontaneous emission at all relevant experimental timescales, allowing subsequent momentum- and spin-resolved in situ probing of the SOC band structure and eigenstates. We use these capabilities to study Bloch oscillations, spin–momentum locking and Van Hove singularities in the transition density of states. Our results lay the groundwork for using fermionic optical lattice clocks to probe new phases of matter.
2016-09-21Neutrinos are much lighter than the other constituents of matter. One explanation for this could be that neutrinos are their own antiparticles and belong to a new class of 'Majorana' particle. An experiment sets strong constraints on this scenario.
2016-09-21The human dispersal out of Africa that populated the world was probably paced by climate changes. This is the inference drawn from computer modelling of climate variability during the time of early human migration.
2016-09-14Underactivity of the transcription factor p53 can lead to tumour development. The discovery that the SET protein binds to and inhibits p53 points to a way to unleash the tumour suppressor's activity.
2016-05-25Deadly coral snakes warn predators through striking red-black banding. New data confirm that many harmless snakes have evolved to resemble coral snakes, and suggest that the evolution of this Batesian mimicry is not always a one-way street.