Nuclear Species Research Articles

Cytonuclear discordance and the species status of Myotis myotis and Myotis blythii (Chiroptera)

We used both highly variable mitochondrial and nuclear loci to investigate the large mouse‐eared bat species complex in the Western Palaearctic to clarify their systematic position. Although mitochondrial lineages show no species segregation and some haplotypes are shared between Myotis myotis and Myotis blythii sensu lato, Bayesian clustering methods based on multilocus genotypes indicate highly concordant nuclear and morphological species assignment. These multilocus, nuclear analyses detected only a single putative F1 hybrid in the extensive areas of sympatry sampled, thus confirming the biological species status of M. myotis and M. blythii s.l. We propose that the strong cytonuclear discordance in these species complex results from a combination of prior spatial isolation of the two species in different glacial refugia, followed by a succession of mitochondrial introgression events that occurred during the eastward and westward expansions of M. myotis and of M. blythii, respectively. The nuclear markers further indicate the presence of a notable genetic discontinuity within M. myotis that broadly separates populations into an eastern and a western component with an overlap zone in the Balkans. This eastern and western discontinuity is also apparent in the mitochondrial lineages with the D haplogroup largely confined to samples found in Thrace and Asia Minor. None of these genetic discontinuities correspond to the distribution of the two commonly recognized M. myotis subspecies (myotis and macrocephalicus). We also show that distinct morphological subspecies within M. blythii (oxygnathus, omari, risorius and lesviacus) in Europe and the near‐East do not correlate with significant evolutionarily units, whether identified by mitochondrial or nuclear data and thus only represent local morphological variants with little taxonomic relevance.

Toxicity evaluation of ethanol treatment during in vitro maturation of porcine oocytes and subsequent embryonic development following parthenogenetic activation and in vitro fertilization.

Ethanol is frequently used as a solvent in several techniques for in vitro production (IVP). It is also used for the parthenogenetic activation (PA) of oocytes. Although a number of studies have suggested that ethanol has detrimental effects on fibroblasts and neuronal cells, little attention has been paid to the effects of ethanol on porcine oocytes. Thus, the aim of this study was to evaluate the effects of the addition of ethanol to in vitro maturation (IVM) medium. We investigated the effects of ethanol (0, 1 and 3%) on the following parameters: nuclear maturation, intracellular glutathione (GSH) and reactive oxygen species (ROS) levels, and subsequent embryonic development following PA and in vitro fertilization (IVF). After 44 h of IVM, the 3% group showed a significant (P<0.05) decrease in nuclear maturation (34.0%) compared with the control group (70.3%). The 1 and 3% groups exhibited a significant (P<0.05) decrease in GSH levels and an increase in ROS levels compared with the control group. Compared with the control group, the 3% group had significantly (P<0.05) lower cleavage rates following PA (51.6 vs. 86.9%) and IVF (53.2 vs. 70.6%), as well as lower blastocyst formation rates and decreased total cell numbers following PA (11.3% and 31.8 vs. 53.6% and 65.4, respectively) and IVF (4.1% and 22.0 vs. 36.1% and 70.3, respectively). We evaluated the mRNA expression levels of DNA repair‑related and apoptosis‑related genes in the cumulus oocyte complexes (COCs). The 1% ethanol group showed significantly (P<0.05) higher mRNA expression levels of poly(ADP‑ribose) polymerase‑1 (PARP‑1), Bax, Bak and caspase‑3, and the 3% ethanol group had significantly (P<0.05) increased PARP‑1, Bax and caspase‑3 mRNA expression levels compared with the control group. Our results suggest that treatment with >1% ethanol during IVM exerts a toxic effect on the developmental potential of PA and IVF porcine embryos by decreasing the intracellular GSH level, thereby increasing the intracellular ROS level and upregulating the expression of apoptosis‑related genes.

The crystal structure of mono- and di-nuclear copper(I) complexes with substituted triphenylphosphine ligands

Stable mono- and bi-nuclear halide copper(I) complexes with several tri-tolylphosphines as ligands have been synthesized and characterized. Their molecular structures have been determined by X-ray diffraction. Two of these compounds, [CuBr{P(p-tolyl)3}3] and [CuBr{P(m-tolyl)3}3], are distorted tetrahedral mono nuclear species. The binuclear complex [CuCl{P(o-tolyl)3}]2 has the copper atoms linked through μ-chloro bridges in a trigonal planar coordination and they were obtained in two different crystal systems, monoclinic and triclinic. The complex [{(p-tolyl)3P}3Cu2Cl2] is also binuclear but shows two different coordination modes for each copper center: one is a distorted tetrahedral and the other is trigonal planar. In this binuclear complex the 3-coordinate metal is located in an almost planar structure and the Cu2Cl2 moiety lies in a perfect plane. The Cu–P and Cu–X, (X=Cl, Br) distances are dependent on the structural environment. Although the P–Cu–P and P–Cu–X angles depart from the ideal tetrahedral geometry the orthogonal planes containing Cu and two additional atoms are nearly perpendicular. NMR results suggest that bromine compounds retain in solution their structures established in solid. However the chlorine compounds exhibit a dynamic behavior indicating the presence of more than one species in solution.

Low-temperature dynamics of matrix isolated methane molecules in fullerite C60: The heat capacity, isotope effects

The heat capacity of the interstitial solid solution (CH4)0.4C60 has been investigated in the temperature interval 1.4–120 K. The contribution of CH4 molecules to the heat capacity of the solution has been separated. The contributions of CH4 and CD4 molecules to the heat capacity of the solutions (CH4)0.40C60 and (CD4)0.40C60 have been compared. It is found that above 90 K the character of the rotational motion of CH4 and CD4 molecules changes from libration to hindered rotation. In the interval 14–35 K the heat capacities of CH4 and CD4 molecules are satisfactorily described by contributions of the translational and libration vibrations, as well as the tunnel rotation for the equilibrium distribution of the nuclear spin species. The isotope effect is due to mainly, the difference in the frequencies of local translational and libration vibrations of molecules CH4 and CD4. The contribution of the tunnel rotation of the CH4 and CD4 molecules to the heat capacity is dominant below 8 K. The isotopic effect is caused by the difference between both the conversion rates and the rotational spectra of the nuclear spin species of CH4 and CD4 molecules. The conversion rate of CH4 molecules is several times lower than that of CD4 ones. Weak features observed in the curves of temperature dependencies of the heat capacity of CH4 and CD4 molecules near 6 and 8 K, respectively, are most likely a manifestation of first-order polyamorphic phase transitions in the orientational glasses of these solutions.

Single and double resonance spectroscopy of methanol embedded in superfluid helium nanodroplets.

Methanol is one of the simplest molecules that undergo torsional oscillations, and so it has been extensively studied in the gas phase by various spectroscopic techniques. At 300 K, a large number of rotational, torsional, and vibrational energy levels is populated, and this makes for a rather complicated spectrum, which is still not fully understood. It is expected that in going from 300 K to 0.4 K (the temperature of helium nanodroplets) the population distribution of methanol will mainly collapse into two states; the JK = 00 state for the A1 nuclear spin symmetry species (with ICH3 = 3/2), and the JK = 1-1 state for the E species (ICH3 = 1/2). This results in a simplified spectrum that consists of narrow a-type (ΔK = 0) lines and broader b- and c-type (ΔK = ±1) lines. We have recorded the rotovibrational spectrum of CH3OH in the OH stretching, CH3 stretching and bending, CH3 rocking, and CO stretching regions, and have firmly assigned five bands (v1, v2, v3, v7, and v8), and tentatively assigned five others (v9, 2v4, v4 + v10, 2v10, and v4 + v5). To our knowledge, the transitions we have assigned within the v4 + v10, 2v10, and v4 + v5 bands have not yet been assigned in the gas phase, and we hope that considering the very small "matrix" shift in helium nanodroplets (<1 cm(-1) for most subband origins of CH3OH), those made here can aid in their gas phase identification. Microwave-infrared double resonance spectroscopy was used to confirm the initially tentative a-type infrared assignments in the OH stretching (v1) band of A1 species methanol, in addition to revealing "warm" b-type lines. From a rotovibrational analysis, the B rotational constant is found to be reduced quite significantly (56%) with respect to the gas phase, and the torsional tunneling splittings are relatively unaffected and are at most reduced by 16%. While most rovibrational peaks are Lorentzian shaped, and those which are significantly perturbed by vibrational coupling in the gas phase are additionally broadened, the narrowest ΔJ = +1 peaks are asymmetric, and a skew-type analysis suggests that the response time of the helium solvent upon excitation is of the order of 1 ns.

Supplementation of zinc on oocyte in vitro maturation improves preimplatation embryonic development in pigs

We investigated the effects of zinc supplementation during the IVM of porcine oocytes. Nuclear maturation, intracellular glutathione (GSH) and reactive oxygen species (ROS) levels, subsequent embryonic development, and gene expression were evaluated. Zinc concentrations in porcine plasma and follicular fluid were 0.82 and 0.84 μg/mL, respectively. Zinc was not detected in IVM medium. After treatment with various zinc concentrations (0.0, 0.4, 0.8, 1.2, and 1.6 μg/mL), no significant difference in IVM was observed among groups (85.7%, 88.7%, 90.4%, 90.3%, and 87.2%, respectively). The effects of different zinc concentrations on porcine oocyte intracellular GSH and ROS levels were examined in mature oocytes. Intracellular GSH levels were significantly higher in the 0.8-, 1.2-, and 1.6-μg/mL groups than in the control (P < 0.05). Intracellular ROS levels of oocytes matured with 0.8, 1.2, and 1.6 μg/mL were reduced significantly (P < 0.05) compared with the control and 0.4-μg/mL groups. The developmental competence of oocytes matured with different zinc concentrations was evaluated after parthenogenetic activation (PA) and in vitro fertilization (IVF). Oocytes treated with zinc during IVM showed no significant difference in cleavage rate after PA. Oocytes treated with 0.8 and 1.2 μg/mL zinc during IVM had significantly higher blastocyst formation rates after PA (41.5% and 41.1%, respectively) than the control (27.2%). IVF embryos showed similar results. The blastocyst formation rate was significantly higher (28.2%) in the 0.8-μg/mL group. TNFAIP2 and Bax were decreased in zinc-treated cumulus cells. Increased POU5F1 and decreased Bax transcript levels were observed in zinc-treated oocytes. POU5F1 and Bcl-2 transcript levels were significantly higher in zinc-treated IVF blastocysts. These results indicate that treatment with adequate zinc concentrations during IVM improved the developmental potential of porcine embryos by regulating the intracellular GSH concentration, the ROS level, and transcription factor expression.

Misperceived invasion: the Lusitanian slug (Arion lusitanicus auct. non-Mabille or Arion vulgaris Moquin-Tandon 1855) is native to Central Europe.

The Lusitanian slug, presumed to be native to south-west Europe, was ranked among the 100 worst invading species in Central Europe. However, from the very beginning of its recognition in the presumed invasion area, there was little evidence that the species was actually anthropogenically introduced. We investigated the invasive status of the species by comparing specific predictions on the population genetic structure in the invasion area with the pattern actually found. In a DNA-taxonomy approach, the species could not be found in its presumed native range. Using statistical phylogeographic techniques on a mitochondrial (COI) and nuclear (ZF) marker and species distribution modelling, we could show that the species is with very high probability not an invasor, but native to Central Europe. The study underlines the value of statistical phylogeography in rigorously testing hypotheses on the dynamics of biological invasions.

S-matrix approach to the equation of state of dilute nuclear matter

Based on the general analysis of the grand canonical partition function in the S-matrix framework, a method is presented to calculate the equation of state of dilute warm nuclear matter. The result is a model-independent virial series for the pressure and density that systematically includes contributions from all the ground and excited states of all the stable nuclear species and their scattering channels. The multiplicity distribution of these species to keep the matter in statistical equilibrium is found out and then the pressure, incompressibility and the symmetry energy of the system are evaluated. The calculated symmetry energy coefficients are found to be in fair agreement with the recent experimental data.

Essential ingredients in core-collapse supernovae

Carrying 1044 joules of kinetic energy and a rich mix of newly synthesized atomic nuclei, core-collapse supernovae are the preeminent foundries of the nuclear species which make up our solar system and ourselves. Signaling the inevitable death of a massive star, and the birth of a neutron star or black hole, core-collapse supernovae combine physics over a wide range in spatial scales, from kilometer-sized hydrodynamic motions (eventually growing to gigameter scale) down to femtometer-scale nuclear reactions. We will discuss our emerging understanding of the convectively-unstable, neutrino-driven explosion mechanism, based on increasingly realistic neutrino radiation hydrodynamic simulations that include progressively better nuclear and particle physics. Multi-dimensional models with spectral neutrino transport from several research groups, which slowly develop successful explosions for a range of progenitors, have recently motivated changes in our understanding of the neutrino reheating mechanism. In a similar fashion, improvements in nuclear physics, most notably explorations of weak interactions on nuclei and the nuclear equation of state, continue to refine our understanding of the births of neutron stars and the supernovae that result. Recent progress on both the macroscopic and microscopic effects that affect core-collapse supernovae are discussed.

Resource utilization of wood decomposers: mycelium nuclear phases and host tree species affect wood decomposition by Dacrymycetes

Throughout evolution, wood-decaying fungi have adapted to different woody plants, resulting in wide species diversity. Dacrymycetes, which are brown-rot fungi and include host-recurrent species, are useful for studying fungal adaptation to host trees. When estimating the decay abilities of basidiomycetes, the nuclear phases of the mycelium should be considered, since dikaryons are thought to be more efficient wood-decayers than monokaryons; however, the difference in their physiological performances remains largely untested. In this study, we verified the decay capabilities of dikaryotic and monokaryotic mycelia and tested the hypothesis that the host tree-recurrence of wood-decaying fungi results from their resource utilization in each host wood. The mass loss caused by eight dacrymycetous species from wood of four tree species was investigated in pure cultures. The decomposition ability of dikaryons was greater than that of monokaryons in these experiments. Dikaryotization is expected to raise certain physiological parameters, such as the quantity and variety of wood-decomposing enzymes, thus enhancing the decomposition rate of wood decomposers. The high decomposition ability of dikaryons suggests their superiority over monokaryons to survive under natural conditions. All dacrymycetous strains caused high mass loss from Pinus wood, the main host tree of Dacrymycetes. However, most of the individual tested strains did not cause the greatest mass loss from the wood of their original host group. This result suggested that host-recurrence can be partly explained by resource utilization, but it is likely that other micro-organisms and abiotic factors also affect host-recurrence in the field environment.

Development of gamma de-excitation model for prediction of prompt gamma-rays and isomer production based on energy-dependent level structure treatment

A new theoretical model to simulate gamma de-excitation of excited nuclei, EBITEM (ENSDF-Based Isomeric Transition and isomEr production Model), is developed based on the Evaluated Nuclear Structure Data File (ENSDF), supplementary evaluated data tables, and theories. In the model, reaction products after nucleon evaporation were de-excited by using theoretical calculations if the excitation energy was higher than 3000keV and the mass number was greater than 40amu. Otherwise, the nuclei were de-excited based on the scheme provided in the ENSDF. Thus by tracking nuclear de-excitation, production of prompt gamma-rays and isomers was simulated.The model is applicable for neutron capture products and spallation products of 1071 nuclear species from Li to Bk. Except for some of the light nuclei with discrete level structure, simulated isomer production and prompt gamma-ray spectra agree generally within 40% and a factor of 3, respectively.

Sensitivity studies for the weak r process: neutron capture rates

Rapid neutron capture nucleosynthesis involves thousands of nuclear species far from stability, whose nuclear properties need to be understood in order to accurately predict nucleosynthetic outcomes. Recently sensitivity studies have provided a deeper understanding of how the r process proceeds and have identified pieces of nuclear data of interest for further experimental or theoretical study. A key result of these studies has been to point out the importance of individual neutron capture rates in setting the final r-process abundance pattern for a ‘main’ (A ∼ 130 peak and above) r process. Here we examine neutron capture in the context of a ‘weak’ r process that forms primarily the A ∼ 80 r-process abundance peak. We identify the astrophysical conditions required to produce this peak region through weak r-processing and point out the neutron capture rates that most strongly influence the final abundance pattern.

Sensitivity studies for the main r process: β-decay rates

The pattern of isotopic abundances produced in rapid neutron capture, or r-process, nucleosynthesis is sensitive to the nuclear physics properties of thousands of unstable neutron-rich nuclear species that participate in the process. It has long been recognized that the some of the most influential pieces of nuclear data for r-process simulations are β-decay lifetimes. In light of experimental advances that have pushed measurement capabilities closer to the classic r-process path, we revisit the role of individual β-decay rates in the r process. We perform β-decay rate sensitivity studies for a main (A &amp;gt; 120) r process in a range of potential astrophysical scenarios. We study the influence of individual rates during (n, γ)-(γ, n) equilibrium and during the post-equilibrium phase where material moves back toward stability. We confirm the widely accepted view that the most important lifetimes are those of nuclei along the r-process path for each astrophysical scenario considered. However, we find in addition that individual β-decay rates continue to shape the final abundance pattern through the post-equilibrium phase, for as long as neutron capture competes with β decay. Many of the lifetimes important for this phase of the r process are within current or near future experimental reach.

Self-quenching of nuclear spin dynamics in the central spin problem

We consider, in the framework of the central spin $s=1/2$ model, driven dynamics of two electrons in a double quantum dot subject to hyperfine interaction with nuclear spins and spin-orbit coupling. The nuclear subsystem dynamically evolves in response to Landau-Zener singlet-triplet transitions of the electronic subsystem controlled by external gate voltages. Without noise and spin-orbit coupling, subsequent Landau-Zener transitions die out after about ${10}^{4}$ sweeps, the system self-quenches, and nuclear spins reach one of the numerous glassy dark states. We present an analytical model that captures this phenomenon. We also account for the multi-nuclear-specie content of the dots and numerically determine the evolution of around ${10}^{7}$ nuclear spins in up to $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ Landau-Zener transitions. Without spin-orbit coupling, self-quenching is robust and sets in for arbitrary ratios of the nuclear spin precession times and the waiting time between Landau-Zener sweeps as well as under moderate noise. In the presence of spin-orbit coupling of a moderate magnitude, and when the waiting time is in resonance with the precession time of one of the nuclear species, the dynamical evolution of nuclear polarization results in stroboscopic screening of spin-orbit coupling. However, small deviations from the resonance or strong spin-orbit coupling destroy this screening. We suggest that the success of the feedback loop technique for building nuclear gradients is based on the effect of spin-orbit coupling.

Heteronuclear Cross‐Relaxation Effects in the NMR Spectroscopy of Hyperpolarized Targets

Dissolution dynamic nuclear polarization (DNP) enables high-sensitivity solution-phase NMR experiments on long-lived nuclear spin species such as (15)N and (13)C. This report explores certain features arising in solution-state (1)H NMR upon polarizing low-γ nuclear species. Following solid-state hyperpolarization of both (13)C and (1)H, solution-phase (1)H NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring (13)C nuclei appeared larger than the typically dominant (12)C-bonded (1)H resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two- and three-spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from (13)C→(1)H polarization-transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross-relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the (13)C-bound (1)H NMR satellite resonances. The dynamics of these novel cross-relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced-sensitivity (1)H NMR traces was explored.

Nuclear Nox4-Derived Reactive Oxygen Species in Myelodysplastic Syndromes

A role for intracellular ROS production has been recently implicated in the pathogenesis and progression of a wide variety of neoplasias. ROS sources, such as NAD(P)H oxidase (Nox) complexes, are frequently activated in AML (acute myeloid leukemia) blasts and strongly contribute to their proliferation, survival, and drug resistance. Myelodysplastic syndromes (MDS) comprise a heterogeneous group of disorders characterized by ineffective hematopoiesis, with an increased propensity to develop AML. The molecular basis for MDS progression is unknown, but a key element in MDS disease progression is the genomic instability. NADPH oxidases are now recognized to have specific subcellular localizations, this targeting to specific compartments for localized ROS production. Local Nox-dependent ROS production in the nucleus may contribute to the regulation of redox-dependent cell growth, differentiation, senescence, DNA damage, and apoptosis. We observed that Nox1, 2, and 4 isoforms and p22phox and Rac1 subunits are expressed in MDS/AML cell lines and MDS samples, also in the nuclear fractions. Interestingly, Nox4 interacts with ERK and Akt1 within nuclear speckle domain, suggesting that Nox4 could be involved in regulating gene expression and splicing factor activity. These data contribute to the elucidation of the molecular mechanisms used by nuclear ROS to drive MDS evolution to AML.

Influence of RNA extraction methods and library selection schemes on RNA-seq data.

BackgroundGene expression analysis by RNA sequencing is now widely used in a number of applications surveying the whole transcriptomes of cells and tissues. The recent introduction of ribosomal RNA depletion protocols, such as RiboZero, has extended the view of the polyadenylated transcriptome to the poly(A)- fraction of the RNA. However, substantial amounts of intronic transcriptional activity has been reported in RiboZero protocols, raising issues regarding their potential nuclear origin and the impact on the actual sequence depth in exonic regions.ResultsUsing HEK293 human cells as source material, we assessed here the impact of the two commonly used RNA extraction methods and of the library construction protocols (rRNA depletion versus mRNA) on 1) the relative abundance of intronic reads and 2) on the estimation of gene expression values. We benchmarked the rRNA depletion-based sequencing with a specific analysis of the cytoplasmic and nuclear transcriptome fractions, suggesting that the large majority of the intronic reads correspond to unprocessed nuclear transcripts rather than to independent transcriptional units. We show that Qiagen or TRIzol extraction methods retain differentially nuclear RNA species, and that consequently, rRNA depletion-based RNA sequencing protocols are particularly sensitive to the extraction methods.ConclusionsWe could show that the combination of Trizol-based RNA extraction with rRNA depletion sequencing protocols led to the largest fraction of intronic reads, after the sequencing of the nuclear transcriptome. We discuss here the impact of the various strategies on gene expression and alternative splicing estimation measures. Further, we propose guidelines and a double selection strategy for minimizing the expression biases, without loss of information.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-675) contains supplementary material, which is available to authorized users.

無機-有機複合型強酸性陽イオン交換体による汚染水からの核種分離に関する研究

無機-有機複合型強酸性陽イオン交換体による汚染水からの核種分離に関する研究

The carp–goldfish nucleocytoplasmic hybrid has mitochondria from the carp as the nuclear donor species

It is widely accepted that mitochondria and its DNA (mtDNA) exhibit strict maternal inheritance, with sperm contributing no or non-detectable mitochondria to the next generation. In fish, nuclear transfer (NT) through the combination of a donor nucleus and an enucleated oocyte can produce fertile nucleocytoplasmic hybrids (NCHs) even between different genera and subfamilies. One of the best studied fish NCHs is CyCa produced by transplanting the nuclei plus cytoplasm from the common carp (Cyprinus carpio var. wuyuanensis) into the oocytes of the wild goldfish (Carassius auratus), which has been propagated by self-mating for three generations. These NCH fish thus provide a unique model to study the origin of mitochondria. Here we report the complete mtDNA sequence of the CyCa hybrid and its parental species carp and goldfish as nuclear donor and cytoplasm host, respectively. Interestingly, the mtDNA of NCH fish CyCa is 99.69% identical to the nuclear donor species carp, and 89.25% identical to the oocyte host species goldfish. Furthermore, an amino acid sequence comparison of 13 mitochondrial proteins reveals that CyCa is 99.68% identical to the carp and 87.68% identical to the goldfish. On an mtDNA-based phylogenetic tree, CyCa is clustered with the carp but separated from the goldfish. A real-time PCR analysis revealed the presence of carp mtDNA but the absence of goldfish mtDNA. These results demonstrate – for the first time to our knowledge – that the mtDNA of a NCH such as CyCa fish may originate from its nuclear donor rather than its oocyte host.

Strong neutrino cooling by cycles of electron capture and β− decay in neutron star crusts

The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities, neutrino cooling and heat transport from the interior. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope. Here we report that cycles of electron capture and its inverse, β(-) decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This 'Urca' mechanism has been studied in the context of white dwarfs and type Ia supernovae, but hitherto was not considered in neutron stars, because previous models computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface.