46 research outputs found
Inhomogeneous scalar field solutions and inflation
We present new exact cosmological inhomogeneous solutions for gravity coupled
to a scalar field in a general framework specified by the parameter .
The equations of motion (and consequently the solutions) in this framework
correspond either to low-energy string theory or Weyl integrable spacetime
according to the sign of . We show that different inflationary
behaviours are possible, as suggested by the study of the violation of the
strong energy condition. Finally, by the analysis of certain curvature scalars
we found that some of the solutions may be nonsingular.Comment: LaTex file, 14 page
Dust-filled axially symmetric universes with a cosmological constant
Following the recent recognition of a positive value for the vacuum energy
density and the realization that a simple Kantowski-Sachs model might fit the
classical tests of cosmology, we study the qualitative behavior of three
anisotropic and homogeneous models: Kantowski-Sachs, Bianchi type-I and Bianchi
type-III universes, with dust and a cosmological constant, in order to find out
which are physically permitted. We find that these models undergo
isotropization up to the point that the observations will not be able to
distinguish between them and the standard model, except for the Kantowski-Sachs
model
with smaller than some critical value
. Even if one imposes that the Universe should be nearly
isotropic since the last scattering epoch (), meaning that the
Universe should have approximately the same Hubble parameter in all directions
(considering the COBE 4-Year data), there is still a large range for the matter
density parameter compatible with Kantowsky-Sachs and Bianchi type-III if
, for a very small . The
Bianchi type-I model becomes exactly isotropic owing to our restrictions and we
have in this case. Of course, all these models
approach locally an exponential expanding state provided the cosmological
constant .Comment: 12 pages, 9 figures, 1 table. Published in Physical Review D 1
Evolution of the Bianchi I, the Bianchi III and the Kantowski-Sachs Universe: Isotropization and Inflation
We study the Einstein-Klein-Gordon equations for a convex positive potential
in a Bianchi I, a Bianchi III and a Kantowski-Sachs universe. After analysing
the inherent properties of the system of differential equations, the study of
the asymptotic behaviors of the solutions and their stability is done for an
exponential potential. The results are compared with those of Burd and Barrow.
In contrast with their results, we show that for the BI case isotropy can be
reached without inflation and we find new critical points which lead to new
exact solutions. On the other hand we recover the result of Burd and Barrow
that if inflation occurs then isotropy is always reached. The numerical
integration is also done and all the asymptotical behaviors are confirmed.Comment: 22 pages, 12 figures, Self-consistent Latex2e File. To be published
in Phys. Rev.
Scalar fields in an anisotropic closed universe
We study in this article a class of homogeneous, but anisotropic cosmological
models in which shear viscosity is included. Within the matter content we
consider a component (the quintessence component) determined by the barotropic
equations of state, , with . We establish conditions
under which a closed axisymmetrical cosmological model may look flat al low
redshift.Comment: 6 pages, Latex, 2 figures, accepted in Phys. Rev.
Superconductivity Mediated by Nematic Fluctuations in Tetragonal
Nematic phases, where electrons in a solid spontaneously break rotational
symmetry while preserving the translational symmetry, exist in several families
of unconventional superconductors [1, 2]. Although superconductivity mediated
by nematic fluctuations is well established theoretically [3-7], it has yet to
be unambiguously identified experimentally [8, 9]. A major challenge is that
nematicity is often intertwined with other degrees of freedom, such as
magnetism and charge order. The FeSeS family of iron based
superconductors provides a unique opportunity to explore this concept, as it
features an isolated nematic phase that can be suppressed by sulfur
substitution at a quantum critical point (QCP) near , where nematic
fluctuations are the largest [10-12]. Here, we performed scanning tunneling
spectroscopy measurements to visualize Boguliubov quasiparticle interference
patterns, from which we determined the momentum structure of the
superconducting gap near the Brillouin zone point of
FeSeS. The results reveal an anisotropic, near nodal gap with
minima that are rotated with respect to the Fe-Fe direction,
characteristic of a nematic pairing interaction, contrary to the usual
isotropic gaps due to spin mediated pairing in other tetragonal Fe-based
superconductors. The results are also in contrast with pristine FeSe, where the
pairing is mediated by spin fluctuations and the gap minima are aligned with
the Fe-Fe direction. Therefore, the measured gap structure demonstrates not
only a fundamental change of the pairing mechanism across the phase diagram of
FeSeS, but it also indicates the existence of superconductivity
mediated by nematic fluctuations in FeSeS
Bianchi Type I Cosmologies in Arbitrary Dimensional Dilaton Gravities
We study the low energy string effective action with an exponential type
dilaton potential and vanishing torsion in a Bianchi type I space-time
geometry. In the Einstein and string frames the general solution of the
gravitational field equations can be expressed in an exact parametric form.
Depending on the values of some parameters the obtained cosmological models can
be generically divided into three classes, leading to both singular and
nonsingular behaviors. The effect of the potential on the time evolution of the
mean anisotropy parameter is also considered in detail, and it is shown that a
Bianchi type I Universe isotropizes only in the presence of a dilaton field
potential or a central deficit charge.Comment: REVTEX, 10 pages, 8 figure
Complementation of diverse HIV-1 Env defects through cooperative subunit interactions: a general property of the functional trimer
<p>Abstract</p> <p>Background</p> <p>The HIV-1 Env glycoprotein mediates virus entry by catalyzing direct fusion between the virion membrane and the target cell plasma membrane. Env is composed of two subunits: gp120, which binds to CD4 and the coreceptor, and gp41, which is triggered upon coreceptor binding to promote the membrane fusion reaction. Env on the surface of infected cells is a trimer consisting of three gp120/gp41 homo-dimeric protomers. An emerging question concerns cooperative interactions between the protomers in the trimer, and possible implications for Env function.</p> <p>Results</p> <p>We extended studies on cooperative subunit interactions within the HIV-1 Env trimer, using analysis of functional complementation between coexpressed inactive variants harboring different functional deficiencies. In assays of Env-mediated cell fusion, complementation was observed between variants with a wide range of defects in both the gp120 and gp41 subunits. The former included gp120 subunits mutated in the CD4 binding site or incapable of coreceptor interaction due either to mismatched specificity or V3 loop mutation. Defective gp41 variants included point mutations at different residues within the fusion peptide or heptad repeat regions, as well as constructs with modifications or deletions of the membrane proximal tryptophan-rich region or the transmembrane domain. Complementation required the defective variants to be coexpressed in the same cell. The observed complementation activities were highly dependent on the assay system. The most robust activities were obtained with a vaccinia virus-based expression and reporter gene activation assay for cell fusion. In an alternative system involving Env expression from integrated provirus, complementation was detected in cell fusion assays, but not in virus particle entry assays.</p> <p>Conclusion</p> <p>Our results indicate that Env function does not require every subunit in the trimer to be competent for all essential activities. Through cross-talk between subunits, the functional determinants on one defective protomer can cooperatively interact to trigger the functional determinants on an adjacent protomer(s) harboring a different defect, leading to fusion. Cooperative subunit interaction is a general feature of the Env trimer, based on complementation activities observed for a highly diverse range of functional defects.</p
Prediction of HIV-1 virus-host protein interactions using virus and host sequence motifs
<p>Abstract</p> <p>Background</p> <p>Host protein-protein interaction networks are altered by invading virus proteins, which create new interactions, and modify or destroy others. The resulting network topology favors excessive amounts of virus production in a stressed host cell network. Short linear peptide motifs common to both virus and host provide the basis for host network modification.</p> <p>Methods</p> <p>We focused our host-pathogen study on the binding and competing interactions of HIV-1 and human proteins. We showed that peptide motifs conserved across 70% of HIV-1 subtype B and C samples occurred in similar positions on HIV-1 proteins, and we documented protein domains that interact with these conserved motifs. We predicted which human proteins may be targeted by HIV-1 by taking pairs of human proteins that may interact via a motif conserved in HIV-1 and the corresponding interacting protein domain.</p> <p>Results</p> <p>Our predictions were enriched with host proteins known to interact with HIV-1 proteins ENV, NEF, and TAT (p-value < 4.26E-21). Cellular pathways statistically enriched for our predictions include the T cell receptor signaling, natural killer cell mediated cytotoxicity, cell cycle, and apoptosis pathways. Gene Ontology molecular function level 5 categories enriched with both predicted and confirmed HIV-1 targeted proteins included categories associated with phosphorylation events and adenyl ribonucleotide binding.</p> <p>Conclusion</p> <p>A list of host proteins highly enriched with those targeted by HIV-1 proteins can be obtained by searching for host protein motifs along virus protein sequences. The resulting set of host proteins predicted to be targeted by virus proteins will become more accurate with better annotations of motifs and domains. Nevertheless, our study validates the role of linear binding motifs shared by virus and host proteins as an important part of the crosstalk between virus and host.</p
Rab7A Is Required for Efficient Production of Infectious HIV-1
Retroviruses take advantage of cellular trafficking machineries to assemble and release new infectious particles. Rab proteins regulate specific steps in intracellular membrane trafficking by recruiting tethering, docking and fusion factors, as well as the actin- and microtubule-based motor proteins that facilitate vesicle traffic. Using virological tests and RNA interference targeting Rab proteins, we demonstrate that the late endosome-associated Rab7A is required for HIV-1 propagation. Analysis of the late steps of the HIV infection cycle shows that Rab7A regulates Env processing, the incorporation of mature Env glycoproteins into viral particles and HIV-1 infectivity. We also show that siRNA-mediated Rab7A depletion induces a BST2/Tetherin phenotype on HIV-1 release. BST2/Tetherin is a restriction factor that impedes HIV-1 release by tethering mature virus particles to the plasma membrane. Our results suggest that Rab7A contributes to the mechanism by which Vpu counteracts the restriction factor BST2/Tetherin and rescues HIV-1 release. Altogether, our results highlight new roles for a major regulator of the late endocytic pathway, Rab7A, in the late stages of the HIV-1 replication cycle
HIV-1 assembly in macrophages
The molecular mechanisms involved in the assembly of newly synthesized Human Immunodeficiency Virus (HIV) particles are poorly understood. Most of the work on HIV-1 assembly has been performed in T cells in which viral particle budding and assembly take place at the plasma membrane. In contrast, few studies have been performed on macrophages, the other major target of HIV-1. Infected macrophages represent a viral reservoir and probably play a key role in HIV-1 physiopathology. Indeed macrophages retain infectious particles for long periods of time, keeping them protected from anti-viral immune response or drug treatments. Here, we present an overview of what is known about HIV-1 assembly in macrophages as compared to T lymphocytes or cell lines