2,533 research outputs found
L-selectin mediated leukocyte tethering in shear flow is controlled by multiple contacts and cytoskeletal anchorage facilitating fast rebinding events
L-selectin mediated tethers result in leukocyte rolling only above a
threshold in shear. Here we present biophysical modeling based on recently
published data from flow chamber experiments (Dwir et al., J. Cell Biol. 163:
649-659, 2003) which supports the interpretation that L-selectin mediated
tethers below the shear threshold correspond to single L-selectin carbohydrate
bonds dissociating on the time scale of milliseconds, whereas L-selectin
mediated tethers above the shear threshold are stabilized by multiple bonds and
fast rebinding of broken bonds, resulting in tether lifetimes on the timescale
of seconds. Our calculations for cluster dissociation suggest that
the single molecule rebinding rate is of the order of Hz. A similar
estimate results if increased tether dissociation for tail-truncated L-selectin
mutants above the shear threshold is modeled as diffusive escape of single
receptors from the rebinding region due to increased mobility. Using computer
simulations, we show that our model yields first order dissociation kinetics
and exponential dependence of tether dissociation rates on shear stress. Our
results suggest that multiple contacts, cytoskeletal anchorage of L-selectin
and local rebinding of ligand play important roles in L-selectin tether
stabilization and progression of tethers into persistent rolling on endothelial
surfaces.Comment: 9 pages, Revtex, 4 Postscript figures include
Impacts of China’s Foreign Direct Investment in Sub-Saharan Africa
This paper examines the impacts of foreign direct investment, as well as China’s FDI on GDP growth of Sub-Saharan African countries from a macroeconomic perspective. By using the data from 44 Sub-Saharan African countries from 2003-2010, our GMM results show that neither China’s FDI nor FDI net inflow in SSA has significant effect on economic growth of SSA countries. The possible explanations about the insignificant results include crowding out effect of China’s FDI on domestic investment, the declining in outward FDI in traditional sectors and rising in service sector which ignored in the current model, and the types of sectors in which Chinese FDI has been concentrated. We also test other economic growth determinants of SSA countries based on growth accounting theory. Our results also show that capital stock per labor has persistent and significant positive impacts on SSA’s growth. In addition, capital per worker is another important determinant in growth
Cursed yet Satisfied Agents
In real life auctions, a widely observed phenomenon is the winner's curse --
the winner's high bid implies that the winner often over-estimates the value of
the good for sale, resulting in an incurred negative utility. The seminal work
of Eyster and Rabin [Econometrica'05] introduced a behavioral model aimed to
explain this observed anomaly. We term agents who display this bias "cursed
agents". We adopt their model in the interdependent value setting, and aim to
devise mechanisms that prevent the cursed agents from obtaining negative
utility. We design mechanisms that are cursed ex-post IC, that is, incentivize
agents to bid their true signal even though they are cursed, while ensuring
that the outcome is individually rational -- the price the agents pay is no
more than the agents' true value.
Since the agents might over-estimate the good's value, such mechanisms might
require the seller to make positive transfers to the agents to prevent agents
from over-paying. For revenue maximization, we give the optimal deterministic
and anonymous mechanism. For welfare maximization, we require ex-post budget
balance (EPBB), as positive transfers might lead to negative revenue. We
propose a masking operation that takes any deterministic mechanism, and imposes
that the seller would not make positive transfers, enforcing EPBB. We show that
in typical settings, EPBB implies that the mechanism cannot make any positive
transfers, implying that applying the masking operation on the fully efficient
mechanism results in a socially optimal EPBB mechanism. This further implies
that if the valuation function is the maximum of agents' signals, the optimal
EPBB mechanism obtains zero welfare. In contrast, we show that for sum-concave
valuations, which include weighted-sum valuations and l_p-norms, the welfare
optimal EPBB mechanism obtains half of the optimal welfare as the number of
agents grows large
Boolean versus continuous dynamics on simple two-gene modules
We investigate the dynamical behavior of simple modules composed of two genes
with two or three regulating connections. Continuous dynamics for mRNA and
protein concentrations is compared to a Boolean model for gene activity. Using
a generalized method, we study within a single framework different continuous
models and different types of regulatory functions, and establish conditions
under which the system can display stable oscillations. These conditions
concern the time scales, the degree of cooperativity of the regulating
interactions, and the signs of the interactions. Not all models that show
oscillations under Boolean dynamics can have oscillations under continuous
dynamics, and vice versa.Comment: 8 pages, 10 figure
Sperner type theorems with excluded subposets
Let F be a family of subsets of an n-element set. Sperner's theorem says that if there is no inclusion among the members of F then the largest family under this condition is the one containing all ⌊ frac(n, 2) ⌋-element subsets. The present paper surveys certain generalizations of this theorem. The maximum size of F is to be found under the condition that a certain configuration is excluded. The configuration here is always described by inclusions. More formally, let P be a poset. The maximum size of a family F which does not contain P as a (not-necessarily induced) subposet is denoted by La (n, P). The paper is based on a lecture of the author at the Jubilee Conference on Discrete Mathematics [Banasthali University, January 11-13, 2009], but it was somewhat updated in December 2010. © 2011 Elsevier B.V. All rights reserved
Gut microbes contribute to variation in solid organ transplant outcomes in mice
© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Microbiome 6 (2018): 96, doi:10.1186/s40168-018-0474-8.Solid organ transplant recipients show heterogeneity in the occurrence and timing of acute rejection episodes. Understanding the factors responsible for such variability in patient outcomes may lead to improved diagnostic and therapeutic approaches. Rejection kinetics of transplanted organs mainly depends on the extent of genetic disparities between donor and recipient, but a role for environmental factors is emerging. We have recently shown that major alterations of the microbiota following broad-spectrum antibiotics, or use of germ-free animals, promoted longer skin graft survival in mice. Here, we tested whether spontaneous differences in microbial colonization between genetically similar individuals can contribute to variability in graft rejection kinetics.
We compared rejection kinetics of minor mismatched skin grafts in C57BL/6 mice from Jackson Laboratory (Jax) and Taconic Farms (Tac), genetically similar animals colonized by different commensal microbes. Female Tac mice rejected skin grafts from vendor-matched males more quickly than Jax mice. We observed prolonged graft survival in Tac mice when they were exposed to Jax mice microbiome through co-housing or fecal microbiota transplantation (FMT) by gastric gavage. In contrast, exposure to Tac mice did not change graft rejection kinetics in Jax mice, suggesting a dominant suppressive effect of Jax microbiota. High-throughput sequencing of 16S rRNA gene amplicons from Jax and Tac mice fecal samples confirmed a convergence of microbiota composition after cohousing or fecal transfer. Our analysis of amplicon data associated members of a single bacterial genus, Alistipes, with prolonged graft survival. Consistent with this finding, members of the genus Alistipes were absent in a separate Tac cohort, in which fecal transfer from Jax mice failed to prolong graft survival.
These results demonstrate that differences in resident microbiome in healthy individuals may translate into distinct kinetics of graft rejection, and contribute to interpersonal variability in graft outcomes. The association between Alistipes and prolonged skin graft survival in mice suggests that members of this genus might affect host physiology, including at sites distal to the gastrointestinal tract. Overall, these findings allude to a potential therapeutic role for specific gut microbes to promote graft survival through the administration of probiotics, or FMT.This work was supported by NIH/NIAID R01 AI115716 to MLA
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