827 research outputs found
How the other half lives: CRISPR-Cas's influence on bacteriophages
CRISPR-Cas is a genetic adaptive immune system unique to prokaryotic cells
used to combat phage and plasmid threats. The host cell adapts by incorporating
DNA sequences from invading phages or plasmids into its CRISPR locus as
spacers. These spacers are expressed as mobile surveillance RNAs that direct
CRISPR-associated (Cas) proteins to protect against subsequent attack by the
same phages or plasmids. The threat from mobile genetic elements inevitably
shapes the CRISPR loci of archaea and bacteria, and simultaneously the
CRISPR-Cas immune system drives evolution of these invaders. Here we highlight
our recent work, as well as that of others, that seeks to understand phage
mechanisms of CRISPR-Cas evasion and conditions for population coexistence of
phages with CRISPR-protected prokaryotes.Comment: 24 pages, 8 figure
The importance of the weak: Interaction modifiers in artificial spin ices
The modification of geometry and interactions in two-dimensional magnetic
nanosystems has enabled a range of studies addressing the magnetic order,
collective low-energy dynamics, and emergent magnetic properties, in e.g.
artificial spin ice structures. The common denominator of all these
investigations is the use of Ising-like mesospins as building blocks, in the
form of elongated magnetic islands. Here we introduce a new approach: single
interaction modifiers, using slave-mesospins in the form of discs, within which
the mesospin is free to rotate in the disc plane. We show that by placing these
on the vertices of square artificial spin ice arrays and varying their
diameter, it is possible to tailor the strength and the ratio of the
interaction energies. We demonstrate the existence of degenerate ice-rule
obeying states in square artificial spin ice structures, enabling the
exploration of thermal dynamics in a spin liquid manifold. Furthermore, we even
observe the emergence of flux lattices on larger length-scales, when the energy
landscape of the vertices is reversed. The work highlights the potential of a
design strategy for two-dimensional magnetic nano-architectures, through which
mixed dimensionality of mesospins can be used to promote thermally emergent
mesoscale magnetic states.Comment: 17 pages, including methods, 4 figures. Supplementary information
contains 16 pages and 15 figure
The phases of deuterium at extreme densities
We consider deuterium compressed to higher than atomic, but lower than
nuclear densities. At such densities deuterium is a superconducting quantum
liquid. Generically, two superconducting phases compete, a "ferromagnetic" and
a "nematic" one. We provide a power counting argument suggesting that the
dominant interactions in the deuteron liquid are perturbative (but screened)
Coulomb interactions. At very high densities the ground state is determined by
very small nuclear interaction effects that probably favor the ferromagnetic
phase. At lower densities the symmetry of the theory is effectively enhanced to
SU(3), and the quantum liquid enters a novel phase, neither ferromagnetic nor
nematic. Our results can serve as a starting point for investigations of the
phase dynamics of deuteron liquids, as well as exploration of the stability and
dynamics of the rich variety of topological objects that may occur in phases of
the deuteron quantum liquid, which range from Alice strings to spin skyrmions
to Z_2 vortices.Comment: 9 pages, 6 figures; v2: fixed typo
Difference in symptom severity between early and late grass pollen season in patients with seasonal allergic rhinitis
<p>Abstract</p> <p>Background</p> <p>For the development of forecasts for seasonal allergic rhinitis symptoms, it is essential to understand the relationship between grass pollen concentrations and the symptoms of grass pollen allergic patients.</p> <p>Objective</p> <p>The aim of this study was to delineate this relationship between seasonal allergic rhinitis symptoms and grass pollen concentrations in the Netherlands.</p> <p>Methods</p> <p>Grass pollen allergic patients (n = 80 [2007] - 84 [2008]) were enrolled into the study. They were asked to enter their seasonal allergic rhinitis symptoms (runny nose, sneezing, blocked nose, post nasal drip, and eye symptoms) daily on a scale from 0 to 3 to the study centre either by short message service (SMS) or by internet from May-July 2007 and April-July 2008. Daily pollen counts were used to define the early and the late grass pollen season as the period 'before and during' respectively 'after' the first grass pollen peak (more than 150 pollen/m<sup>3</sup>).</p> <p>Results</p> <p>At similar grass pollen concentrations, the daily mean of the individual maximum symptom scores reported in the early season were higher as compared to that reported in the late season [differences of -0.41 (2007) and -0.30 (2008)]. This difference could not be explained by medication use by the patients nor by co-sensitization to birch.</p> <p>Conclusions</p> <p>We conclude that seasonal allergic rhinitis symptoms at similar grass pollen concentrations are more severe in the early flowering season as compared to those in the late flowering season. This finding is not only relevant for development of forecasts for seasonal allergic rhinitis symptoms but also for understanding symptom development and planning and analysis of clinical studies.</p
Stationary Black Holes: Uniqueness and Beyond
The spectrum of known black-hole solutions to the stationary Einstein
equations has been steadily increasing, sometimes in unexpected ways. In
particular, it has turned out that not all black-hole-equilibrium
configurations are characterized by their mass, angular momentum and global
charges. Moreover, the high degree of symmetry displayed by vacuum and
electro-vacuum black-hole spacetimes ceases to exist in self-gravitating
non-linear field theories. This text aims to review some developments in the
subject and to discuss them in light of the uniqueness theorem for the
Einstein-Maxwell system.Comment: Major update of the original version by Markus Heusler from 1998.
Piotr T. Chru\'sciel and Jo\~ao Lopes Costa succeeded to this review's
authorship. Significantly restructured and updated all sections; changes are
too numerous to be usefully described here. The number of references
increased from 186 to 32
Theorems on existence and global dynamics for the Einstein equations
This article is a guide to theorems on existence and global dynamics of
solutions of the Einstein equations. It draws attention to open questions in
the field. The local-in-time Cauchy problem, which is relatively well
understood, is surveyed. Global results for solutions with various types of
symmetry are discussed. A selection of results from Newtonian theory and
special relativity that offer useful comparisons is presented. Treatments of
global results in the case of small data and results on constructing spacetimes
with prescribed singularity structure or late-time asymptotics are given. A
conjectural picture of the asymptotic behaviour of general cosmological
solutions of the Einstein equations is built up. Some miscellaneous topics
connected with the main theme are collected in a separate section.Comment: Submitted to Living Reviews in Relativity, major update of Living
Rev. Rel. 5 (2002)
Quantum Gravity in 2+1 Dimensions: The Case of a Closed Universe
In three spacetime dimensions, general relativity drastically simplifies,
becoming a ``topological'' theory with no propagating local degrees of freedom.
Nevertheless, many of the difficult conceptual problems of quantizing gravity
are still present. In this review, I summarize the rather large body of work
that has gone towards quantizing (2+1)-dimensional vacuum gravity in the
setting of a spatially closed universe.Comment: 61 pages, draft of review for Living Reviews; comments, criticisms,
additions, missing references welcome; v2: minor changes, added reference
Febrile seizures and mechanisms of epileptogenesis: insights from an animal model.
Temporal lobe epilepsy (TLE) is the most prevalent type of human epilepsy, yet the causes for its development, and the processes involved, are not known. Most individuals with TLE do not have a family history, suggesting that this limbic epilepsy is a consequence of acquired rather than genetic causes. Among suspected etiologies, febrile seizures have frequently been cited. This is due to the fact that retrospective analyses of adults with TLE have demonstrated a high prevalence (20-->60%) of a history of prolonged febrile seizures during early childhood, suggesting an etiological role for these seizures in the development of TLE. Specifically, neuronal damage induced by febrile seizures has been suggested as a mechanism for the development of mesial temporal sclerosis, the pathological hallmark of TLE. However, the statistical correlation between febrile seizures and TLE does not necessarily indicate a causal relationship. For example, preexisting (genetic or acquired) 'causes' that result independently in febrile seizures and in TLE would also result in tight statistical correlation. For obvious reasons, complex febrile seizures cannot be induced in the human, and studies of their mechanisms and of their consequences on brain molecules and circuits are severely limited. Therefore, an animal model was designed to study these seizures. The model reproduces the fundamental key elements of the human condition: the age specificity, the physiological temperatures seen in fevers of children, the length of the seizures and their lack of immediate morbidity. Neuroanatomical, molecular and functional methods have been used in this model to determine the consequences of prolonged febrile seizures on the survival and integrity of neurons, and on hyperexcitability in the hippocampal-limbic network. Experimental prolonged febrile seizures did not lead to death of any of the seizure-vulnerable populations in hippocampus, and the rate of neurogenesis was also unchanged. Neuronal function was altered sufficiently to promote synaptic reorganization of granule cells, and transient and long-term alterations in the expression of specific genes were observed. The contribution of these consequences of febrile seizures to the epileptogenic process is discussed
Increased Mast Cell Density and Airway Responses to Allergic and Non-Allergic Stimuli in a Sheep Model of Chronic Asthma
BACKGROUND: Increased mast cell (MC) density and changes in their distribution in airway tissues is thought to contribute significantly to the pathophysiology of asthma. However, the time sequence for these changes and how they impact small airway function in asthma is not fully understood. The aim of the current study was to characterise temporal changes in airway MC density and correlate these changes with functional airway responses in sheep chronically challenged with house dust mite (HDM) allergen. METHODOLOGY/PRINCIPAL FINDINGS: MC density was examined on lung tissue from four spatially separate lung segments of allergic sheep which received weekly challenges with HDM allergen for 0, 8, 16 or 24 weeks. Lung tissue was collected from each segment 7 days following the final challenge. The density of tryptase-positive and chymase-positive MCs (MC(T) and MC(TC) respectively) was assessed by morphometric analysis of airway sections immunohistochemically stained with antibodies against MC tryptase and chymase. MC(T) and MC(TC) density was increased in small bronchi following 24 weeks of HDM challenges compared with controls (P<0.05). The MC(TC)/MC(T) ratio was significantly increased in HDM challenged sheep compared to controls (P<0.05). MC(T) and MC(TC) density was inversely correlated with allergen-induced increases in peripheral airway resistance after 24 weeks of allergen exposure (P<0.05). MC(T) density was also negatively correlated with airway responsiveness after 24 challenges (P<0.01). CONCLUSIONS: MC(T) and MC(TC) density in the small airways correlates with better lung function in this sheep model of chronic asthma. Whether this finding indicates that under some conditions mast cells have protective activities in asthma, or that other explanations are to be considered requires further investigation
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