65 research outputs found
Resistin: A reappraisal
Abstract From a biological point of view, aging can be considered a progressive inability of an organism to react to stress, maintain homeostasis, and survive unfavourable changes during post-maturational life. The expression of several adipokines changes during aging and for some changes, a role in the onset of chronic disease and frailty has been proposed. Among adipokines, resistin was shown in recent studies to play a key role in aging. Resistin is a small secreted protein that regulates glucose metabolism in mammalians. High resistin levels induce insulin resistance and exert proinflammatory effects. Consistently, resistin has been shown to play a pivotal role in various metabolic, inflammatory, and autoimmune diseases. Herein, the role of resistin as a molecular link between aging and age-related conditions was reviewed and the clinical implications of this knowledge discussed
vitamin C, aging and Alzheimer's disease
Accumulating evidence in mice models of accelerated senescence indicates a rescuing role of ascorbic acid in premature aging. Supplementation of ascorbic acid appeared to halt cell growth, oxidative stress, telomere attrition, disorganization of chromatin, and excessive secretion of inflammatory factors, and extend lifespan. Interestingly, ascorbic acid (AA) was also found to positively modulate inflamm-aging and immunosenescence, two hallmarks of biological aging. Moreover, ascorbic acid has been shown to epigenetically regulate genome integrity and stability, indicating a key role of targeted nutrition in healthy aging. Growing in vivo evidence supports the role of ascorbic acid in ameliorating factors linked to Alzheimerâs disease (AD) pathogenesis, although evidence in humans yielded equivocal results. The neuroprotective role of ascorbic acid not only relies on the general free radical trapping, but also on the suppression of pro-inflammatory genes, mitigating neuroinflammation, on the chelation of iron, copper, and zinc, and on the suppression of amyloid-beta peptide (Aβ) fibrillogenesis. Epidemiological evidence linking diet, one of the most important modifiable lifestyle factors, and risk of Alzheimer's disease is rapidly increasing. Thus, dietary interventions, as a way to epigenetically modulate the human genome, may play a role in the prevention of AD. The present review is aimed at providing an up to date overview of the main biological mechanisms that are associated with ascorbic acid supplementation/bioavailability in the process of aging and Alzheimerâs disease. In addition, we will address new fields of research and future directions.</jats:p
Electronic and phononic states of the Holstein-Hubbard dimer of variable length
We consider a model Hamiltonian for a dimer including all the electronic one-
and two-body terms consistent with a single orbital per site, a free Einstein
phonon term, and an electron-phonon coupling of the Holstein type. The bare
electronic interaction parameters were evaluated in terms of Wannier functions
built from Gaussian atomic orbitals. An effective polaronic Hamiltonian was
obtained by an unrestricted displaced-oscillator transformation, followed by
evaluation of the phononic terms over a squeezed-phonon variational wave
function. For the cases of quarter-filled and half-filled orbital, and over a
range of dimer length values, the ground state was identified by simultaneously
and independently optimizing the orbital shape, the phonon displacement and the
squeezing effect strength. As the dimer length varies, we generally find
discontinuous changes of both electronic and phononic states, accompanied by an
appreciable renormalization of the effective electronic interactions across the
transitions, due to the equilibrium shape of the wave functions strongly
depending on the phononic regime and on the type of ground state.Comment: 11 pages, RevTeX, 10 PostScript figures; to appear in Phys. Rev.
Exact ground states for the four-electron problem in a two-dimensional finite Hubbard square system
We present exact explicit analytical results describing the exact ground
state of four electrons in a two dimensional square Hubbard cluster containing
16 sites taken with periodic boundary conditions. The presented procedure,
which works for arbitrary even particle number and lattice sites, is based on
explicitly given symmetry adapted base vectors constructed in r-space. The
Hamiltonian acting on these states generates a closed system of 85 linear
equations providing by its minimum eigenvalue the exact ground state of the
system. The presented results, described with the aim to generate further
creative developments, not only show how the ground state can be exactly
obtained and what kind of contributions enter in its construction, but
emphasize further characteristics of the spectrum. On this line i) possible
explications are found regarding why weak coupling expansions often provide a
good approximation for the Hubbard model at intermediate couplings, or ii)
explicitly given low lying energy states of the kinetic energy, avoiding double
occupancy, suggest new roots for pairing mechanism attracting decrease in the
kinetic energy, as emphasized by kinetic energy driven superconductivity
theories.Comment: 37 pages, 18 figure
Electronic states, Mott localization, electron-lattice coupling, and dimerization for correlated one-dimensional systems. II
We discuss physical properties of strongly correlated electron states for a
linear chain obtained with the help of the recently proposed new method
combining the exact diagonalization in the Fock space with an ab initio
readjustment of the single-particle orbitals in the correlated state. The
method extends the current discussion of the correlated states since the
properties are obtained with varying lattice spacing. The finite system of N
atoms evolves with the increasing interatomic distance from a Fermi-liquid-like
state into the Mott insulator. The criteria of the localization are discussed
in detail since the results are already convergent for N>=8. During this
process the Fermi-Dirac distribution gets smeared out, the effective band mass
increases by ~50%, and the spin-spin correlation functions reduce to those for
the Heisenberg antiferromagnet. Values of the microscopic parameters such as
the hopping and the kinetic-exchange integrals, as well as the magnitude of
both intra- and inter-atomic Coulomb and exchange interactions are calculated.
We also determine the values of various local electron-lattice couplings and
show that they are comparable to the kinetic exchange contribution in the
strong-correlation limit. The magnitudes of the dimerization and the zero-point
motion are also discussed. Our results provide a canonical example of a
tractable strongly correlated system with a precise, first-principle
description as a function of interatomic distance of a model system involving
all hopping integrals, all pair-site interactions, and the exact one-band
Wannier functions.Comment: 18 pages, REVTEX, submitted to Phys. Rev.
The boson-fermion model with on-site Coulomb repulsion between fermions
The boson-fermion model, describing a mixture of itinerant electrons
hybridizing with tightly bound electron pairs represented as hard-core bosons,
is here generalized with the inclusion of a term describing on-site Coulomb
repulsion between fermions with opposite spins. Within the general framework of
the Dynamical Mean-Field Theory, it is shown that around the symmetric limit of
the model this interaction strongly competes with the local boson-fermion
exchange mechanism, smoothly driving the system from a pseudogap phase with
poor conducting properties to a metallic regime characterized by a substantial
reduction of the fermionic density. On the other hand, if one starts from
correlated fermions described in terms of the one-band Hubbard model, the
introduction in the half-filled insulating phase of a coupling with hard-core
bosons leads to the disappearance of the correlation gap, with a consequent
smooth crossover to a metallic state.Comment: 7 pages, 6 included figures, to appear in Phys. Rev.
Isotope Effect in the Presence of Magnetic and Nonmagnetic Impurities
The effect of impurities on the isotope coefficient is studied theoretically
in the framework of Abrikosov-Gor'kov approach generalized to account for both
potential and spin-flip scattering in anisotropic superconductors. An
expression for the isotope coefficient as a function of the critical
temperature is obtained for a superconductor with an arbitrary contribution of
spin-flip processes to the total scattering rate and an arbitrary degree of
anisotropy of the superconducting order parameter, ranging from isotropic
s-wave to d-wave and including anisotropic s-wave and mixed (s+d)-wave as
particular cases. It is found that both magnetic and nonmagnetic impurities
enhance the isotope coefficient, the enhancement due to magnetic impurities
being generally greater than that due to nonmagnetic impurities. From the
analysis of the experimental results on La-Sr-Cu-M-O high temperature
superconductor, it is concluded that the symmetry of the pairing state in this
system differs from a pure d-wave.Comment: 4 pages, 3 figure
Quantum Monte Carlo and variational approaches to the Holstein model
Based on the canonical Lang-Firsov transformation of the Hamiltonian we
develop a very efficient quantum Monte Carlo algorithm for the Holstein model
with one electron. Separation of the fermionic degrees of freedom by a
reweighting of the probability distribution leads to a dramatic reduction in
computational effort. A principal component representation of the phonon
degrees of freedom allows to sample completely uncorrelated phonon
configurations. The combination of these elements enables us to perform
efficient simulations for a wide range of temperature, phonon frequency and
electron-phonon coupling on clusters large enough to avoid finite-size effects.
The algorithm is tested in one dimension and the data are compared with
exact-diagonalization results and with existing work. Moreover, the ideas
presented here can also be applied to the many-electron case. In the
one-electron case considered here, the physics of the Holstein model can be
described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte
Marine mammal hotspots across the circumpolar Arctic
Aim: Identify hotspots and areas of high species richness for Arctic marine mammals. Location: Circumpolar Arctic. Methods: A total of 2115 biologging devices were deployed on marine mammals from 13 species in the Arctic from 2005 to 2019. Getis-Ord Gi* hotspots were calculated based on the number of individuals in grid cells for each species and for phyloge-netic groups (nine pinnipeds, three cetaceans, all species) and areas with high spe-cies richness were identified for summer (Jun-Nov), winter (Dec-May) and the entire year. Seasonal habitat differences among speciesâ hotspots were investigated using Principal Component Analysis. Results: Hotspots and areas with high species richness occurred within the Arctic continental-shelf seas and within the marginal ice zone, particularly in the âArctic gatewaysâ of the north Atlantic and Pacific oceans. Summer hotspots were generally found further north than winter hotspots, but there were exceptions to this pattern, including bowhead whales in the Greenland-Barents Seas and species with coastal distributions in Svalbard, Norway and East Greenland. Areas with high species rich-ness generally overlapped high-density hotspots. Large regional and seasonal dif-ferences in habitat features of hotspots were found among species but also within species from different regions. Gap analysis (discrepancy between hotspots and IUCN ranges) identified species and regions where more research is required. Main conclusions: This study identified important areas (and habitat types) for Arctic marine mammals using available biotelemetry data. The results herein serve as a benchmark to measure future distributional shifts. Expanded monitoring and teleme-try studies are needed on Arctic species to understand the impacts of climate change and concomitant ecosystem changes (synergistic effects of multiple stressors). While efforts should be made to fill knowledge gaps, including regional gaps and more com-plete sex and age coverage, hotspots identified herein can inform management ef-forts to mitigate the impacts of human activities and ecological changes, including creation of protected areas
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