1,891 research outputs found
Bistability of cell-matrix adhesions resulting from non-linear receptor-ligand dynamics
Bistability is a major mechanism for cellular decision making and usually
results from positive feedback in biochemical control systems. Here we show
theoretically that bistability between unbound and bound states of adhesion
clusters results from positive feedback mediated by structural rather than
biochemical processes, namely by receptor-ligand dissociation and association
dynamics which depend non-linearly on mechanical force and receptor-ligand
separation. For small cell-matrix adhesions, we find rapid switching between
unbound and bound states, which in the initial stages of adhesion allows the
cell to explore its environment through many transient adhesions.Comment: Revtex, 3 pages, 3 postscript figures included, to appear in
Biophysical Journal as Biophysical Lette
Focal adhesions as mechanosensors: the two-spring model
Adhesion-dependent cells actively sense the mechanical properties of their
environment through mechanotransductory processes at focal adhesions, which are
integrin-based contacts connecting the extracellular matrix to the
cytoskeleton. Here we present first steps towards a quantitative understanding
of focal adhesions as mechanosensors. It has been shown experimentally that
high levels of force are related to growth of and signaling at focal adhesions.
In particular, activation of the small GTPase Rho through focal adhesions leads
to the formation of stress fibers. Here we discuss one way in which force might
regulate the internal state of focal adhesions, namely by modulating the
internal rupture dynamics of focal adhesions. A simple two-spring model shows
that the stiffer the environment, the more efficient cellular force is built up
at focal adhesions by molecular motors interacting with the actin filaments.Comment: Latex, 17 pages, 5 postscript figures include
The role of spatial averaging in the precision of gene expression patterns
During embryonic development, differentiating cells respond via gene
expression to positional cues from morphogen gradients. While gene expression
is often highly erratic, embryonic development is precise. We show by theory
and simulations that diffusion of the expressed protein can enhance the
precision of its expression domain. While diffusion lessens the sharpness of
the expression boundary, it also reduces super-Poissonian noise by washing out
bursts of gene expression. Balancing these effects yields an optimal diffusion
constant maximizing the precision of the expression domain.Comment: 5 pages, 4 EPS figure
Focal adhesions as mechanosensors: the two-spring model
Adhesion-dependent cells actively sense the mechanical properties of their
environment through mechanotransductory processes at focal adhesions, which are
integrin-based contacts connecting the extracellular matrix to the
cytoskeleton. Here we present first steps towards a quantitative understanding
of focal adhesions as mechanosensors. It has been shown experimentally that
high levels of force are related to growth of and signaling at focal adhesions.
In particular, activation of the small GTPase Rho through focal adhesions leads
to the formation of stress fibers. Here we discuss one way in which force might
regulate the internal state of focal adhesions, namely by modulating the
internal rupture dynamics of focal adhesions. A simple two-spring model shows
that the stiffer the environment, the more efficient cellular force is built up
at focal adhesions by molecular motors interacting with the actin filaments.Comment: Latex, 17 pages, 5 postscript figures include
Ăberlagerung von sozialer und ethnischer Segregation in Hamburg
Der Beitrag analysiert die kleinrÀumige Verteilung der Hamburger Bevölkerung im Hinblick auf ihren
sozialen Status ihre ethnische Zugehörigkeit auf der Basis von Daten aus der Bevölkerungs- und
Sozialhilfestatistik.
Die Stadtteile mit einem hohen Armenanteil unter der deutschen Bevölkerung befinden sich
hauptsÀchlich in Hafen-InnenstadtnÀhe sowie in den durch sozialen Mietwohnungsbau geprÀgten
Gegenden am Stadtrand. Es handelt sich dabei oftmals um die klassischen Arbeiterwohngebiete, die
durch ein ungĂŒnstiges Wohnumfeld sowie einen unterdurchschnittlichen Bildungsstand und
vergleichsweise geringe Einkommen der Bewohner gekennzeichnet
sind. Die Armutshochburgen der
auslĂ€ndischen Bevölkerung sind dagegen ungleichmĂ€Ăiger im Stadtgebiet verteilt. Besonders in
einigen siedlungsstrukturell unauffÀlligen Vierteln
am Stadtrand, in denen zahlreiche Asylbewerber
untergebracht sind, erreichen die Armutsquoten der
Nichtdeutschen ĂŒberdurchschnittliche Werte. In
den traditionellen AuslÀnderwohnquartieren in HafennÀhe sind dagegen nichtdeutsche Personen kaum
hÀufiger von Armut betroffen
als in der Gesamtstadt.
Die stadtteilbezogenen Unterschiede in Bezug auf die Armutsquote der Deutschen waren im
Zeitverlauf weitgehend stabil, wogegen sich bei den AuslÀndern infolge ihrer höheren MobilitÀt
gröĂere VerĂ€nderungen
ergeben hatten.
Beim Vergleich der regionalen
Verteilung der einzelnen nach Sozialstatus und ethnischer
Zugehörigkeit abgegrenzten Bevölkerungsgruppen zeigt
sich fĂŒr arme Deutsche und arme AuslĂ€nder
die gröĂte rĂ€umliche NĂ€he. Auch bei armen Deutschen und wohlhabenden AuslĂ€ndern besteht noch
eine vergleichsweise groĂe Ăbereinstimmung der
Wohnorte. Dagegen weisen
wohlhabende Deutsche
eine deutlich gröĂere rĂ€umliche Distanz sowohl zu
armen als auch zu wohlhabenden AuslÀndern auf.
Dies gilt auffĂ€lligerweise auch fĂŒr
das VerhÀltnis zwischen wohlhabenden und armen AuslÀndern.
Im Zeitverlauf sind wohlhabende Deutsche und wohlhabende AuslÀnder etwas stÀrker
zusammengerĂŒckt, was als zunehmende gesellschaftliche Integration von AuslĂ€ndern gedeutet werden
kann. Auf der anderen Seite lÀsst sich aber innerhalb der nichtdeutschen Einwohnerschaft eine
deutliche rÀumliche Entmischung zwischen wohlhabenden und armen Bevölkerungsteilen erkennen.
Obwohl AuslÀnder in nahezu allen Stadtteilen deutlich hÀufiger von Einkommensarmut betroffen sind
als deutsche Einwohner, war die wechselseitige Beeinflussung zwischen der sozialen und der
ethnischen Segregation nur gering. Die regionale
Ungleichverteilung von Deutschen und AuslÀndern
kann nicht als Folge der sozialen Segregation gedeutet werden.This article analyses the distribution of Hamburg's population in a small space with a view towards its
social status and ethnic affiliation based upon data
from the population and social welfare statistics.
The boroughs with a large proportion of poor persons among the German population are mainly in the
area of the port and city centre and on the outskirts of the city with rented social housing. These are
often the classical workers' quarters and their main
features are an unfavourable housing environment,
below-average education and comparably low incomes among the inhabitants. In contrast, the
strongholds of poverty of the foreign population are
distributed more unevenly in the urban area. The
poverty rates among the non-German population reach
above-average values especially in some
quarters on the outskirts of the city with an unobtrusive settlement structure where many asylum
seekers are housed. In contrast, non-German persons are hardly affected more frequently by poverty in
the traditional foreigners' quarters in the area of the
port than in the city as a whole. The differences
with reference to the poverty rate
of Germans based upon boroughs remained stable to a great extent in
the course of time. But in contrast, there were greater differences among foreigners because of their
greater mobility.
When comparing the regional distribution of the individual population groups delineated according to
social status and ethnic affiliation, we can see that
poor Germans and poor foreigners are very close to
one another. But the residential locations of poor
Germans and prosperous foreigners coincide to a
great extent. In contrast, prosperous Germans have
a significantly greater distance both to poor and
prosperous foreigners. This also noticeably applies to the relationship between prosperous and poor
foreigners.
Prosperous Germans and foreigners have come closer together in the course of time, which we may
interpret as increasing social integration of foreigners. On the other hand, we can see a clear spatial
demixing among prosperous and poor population segments within the group of non-German
inhabitants.
Although foreigners are clearly more frequently affected by income poverty in almost all boroughs
than the German inhabitants, the alternating influence between social and ethnic segregation was only
slight. The unequal distribution of Germans and foreigners in the region cannot be interpreted as a
consequence of social segregation
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