Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
mined. Moreover, the blinking mechanism of water soluble QDs has not
yet been systematically studied. In this work we present the study of single
CdSe/ZnS QDs by means of Total Internal Reflection Fluorescence
Microscopy (TIRFM). A comparison in blinking behavior of QDs with
organic soluble Tri-n-octylphosphine oxide (TOPO) and water soluble
mercaptoundecanoic acid (MUA) shells is performed.
AKB 50.13 Fr 10:30 B
Artificial actin cortices on microfabricated pillar arrays —
•Wouter Roos 1 , Alexander Roth 2 , Roman Glass 1 , Erich
Sackmann 2 , and Joachim P. Spatz 1 — 1 Universitaet Heidelberg,
Institut fuer physikalische Chemie, Biophysikalische Chemie, 69120
Heidelberg — 2 Technische Universitaet Muenchen, Physik-Department
E22, 85747 Garching
Arrays of microfabricated pillars are constructed to serve as a template
for mimicking the actin cortex. Different methods to fabricate pillar arrays
will be discussed, these involve top-down and bottom-up approaches
using photolithographic techniques, plasma etching processes and epitaxial
ZnO growth. A two-dimensional network of actin filaments, that
is pending from the pillar tops, is fabricated. Due to the 3-dimensional
template surface interaction of the filaments hanging in between the pillars
with substrate surfaces is prevented. This opens new possibilities
to study the mechanics of 2-dimensional actin networks as a function of
actin-crosslinkers, and the active diffusion of molecular motors operating
on pending networks. The behaviour of this artificial actin cortex will be
compared to models of networks and to cortices in living cells.
AKB 50.14 Fr 10:30 B
Dynamics of cell adhesion contacts and forces on rigid nanoadhesive
templates — •Christine Selhuber 1 , Marco Arnold 1 ,
Roman Glass 1 , Jacques Blümmel 1 , Horst Kessler 2 , and
Joachim Spatz 1 — 1 Universität Heidelberg, Biophysikalische Chemie,
INF 253, 69120 Heidelberg — 2 TU München, Institut für Organische
Chemie und Biochemie, Lichtenbergstrasse 4 , 85747 Garching
Cooperative processes in cell adhesion are one of the most fundamental
issues in cell sciences which control many cell functions. Nanometer
sized RGD based adhesive dots of an extension smaller 8 nm are applied
as bindin g sites for the activation of a single integrin per dot. The dots
assemble with high precision on interfaces where the pattern geometry
and the separation of single dots can be controlled in a flexible way. Thus,
the pattern resembles a rigid adhesive template on which cell or vesicle
adhesion can be probed with single receptor resolution. If adhesive dots
are separated by more than 73 nm cell adhesion fails due to extended
singl e integrin-integrin separation, as well as cell spreading, formation
of foca l contact clusters and actin stress fibre formation is constricted.
The adhesion forces generated on such substrates are measured as a function
of nanopattern geometry and adhesion time. The dynamic change of
the cell adhesive area is determined using reflection interference contrast
microscopy.
AKB 50.15 Fr 10:30 B
Probing a Biomimetic Model of the Actin Cortex with Dynamic
Holographic Optical Tweezers — •Christian Schmitz, Jennifer
Curtis, and Joachim Spatz — Biophysikalische Chemie, Institut für
Physikalische Chemie, Universität Heidelberg
The actin cortex is an adaptive chemo-mechanical polymer network
located beneath the cell membrane. A thin, quasi two-dimensional (2D)
network, the actin cortex plays a leading role in controlling cellular viscoelasticity,
shape, and motility. Regulated by internal and external stimuli,
the actin cortex varies its properties with controlled reversible polymerisation
of actin. We construct a freely-floating 2D biomimetic actin
network to address key questions such as what are the viscoelastic properties
of a 2D actin network, and how are these mechanical properties
modified by active, biochemical components like molecular motors. The
actin network is arranged and probed using holographic optical tweezers,
which produce and independently steer one to hundreds of optical traps.
Using a bed of optical traps, microspheres are arranged into a geometric
array onto which actin is deposited or grown. The tweezers coordinatively
exert distorting forces on the network and when calibrated, they measure
the response of the network at each microsphere.
AKB 50.16 Fr 10:30 B
Optical Control of Neuronal Growth — •Björn Stuhrmann,
Josef Käs, Allen Ehrlicher, Michael Gögler, Daniel Koch,
and Timo Betz — Lehrstuhl für die Physik weicher Materie, Universität
Leipzig, Fakultät für Physik und Geowissenschaften, Linnéstr. 5,
D-04103 Leipzig, Germany
The control of neuronal growth is an essential tool in neuroscience,
cell biology, biophysics, biomedicine, and is particularly important for
the formation of neural circuits in vitro, as well as nerve regeneration in
vivo. We have shown experimentally that we can use weak optical forces
to influence the motility of a growth cone by biasing the polymerizationdriven
intracellular machinery. In actively extending growth cones, a laser
spot placed at specific areas of the nerve’s leading edge affects the following
three potentially important elements of controlled neuronal network
formation: the growth speed, the direction taken by a growth cone, and
the splitting of a growth cone [Ehrlicher et al. ”Guiding neuronal growth
with light” PNAS (2002)]. We have also succeeded to establish transient
cell-cell contacts between growth cones and cell bodies of other nerve
cells. Our results open a new venue to control neuronal growth with a
simple, noncontact technique with potential applications in the formation
of neural networks and in understanding the cytoskeleton driven
morphological changes in growth cones.
AKB 50.17 Fr 10:30 B
Elektrische Charakterisierung von Metall-Glas-
Elektrodenstrukturen zur Stimulation von Neuronen —
•S. Günther 1 , A. Krtschil 1 , A. Reiher 1 , H. Witte 1 , A. Krost 1 ,
A. de Lima 2 , T. Opitz 2 und T. Voigt 2 — 1 Institut für Experimentelle
Physik, Otto-von-Guericke-Universität Magdeburg, PF 4120, D-39016
Magdeburg — 2 Institut für Physiologie, Otto-von-Guericke-Universität
Magdeburg, Leipziger Str. 44, D-39120 Magdeburg
Durch in vitro-Kultivierung von Neuronen aus dem Kortex embryonaler
Ratten ist es möglich, 2-dimensionale neuronale Netzwerke zu erzeugen,
in die planare Elektrodenstrukturen zur elektrischen Kommunikation
mit den Neuronen integriert werden können. Die Stimulation
und das Auslesen von Aktionspotentialen wird dabei maßgeblich von den
elektrischen Eigenschaften des Systems Elektrodenstruktur, Nährlösung
und neuronales Netzwerk bestimmt. Die Untersuchungen wurden an
Streifen- und Fingerstrukturen von Ti/Au-Schichten durchgeführt. Auf
der Grundlage von elektrischen Untersuchungen der Einzelkomponenten,
sowie der vollständigen Anordnung wurde ein Ersatzschaltbild zur Beschreibung
der DC/AC-Eigenschaften erstellt. Als ein kritischer Parameter
des Interfaces erwies sich das elektrische Verhalten der Nährlösung,
verbunden mit elektrolytischen Effekten. Aufgrund dieser Analysen wurde
ein geeignetes Parameterfeld gefunden und optimiert, so dass neuronale
Netzwerke stimuliert werden konnten.
AKB 50.18 Fr 10:30 B
Mechanism of Model Membrane Fusion Determined from
Monte Carlo Simulation — •Marcus Mueller 1 , Kirill
Katsov 2 , and Michael Schick 2 — 1 Institut fuer Physik, WA331,
Johannes Gutenberg Universitaet, D55099 Mainz, Germany — 2 Dept.
of Physics, University of Washington, Seattle, WA 98195-1560, USA
Fusion of membranes is essential to living systems, but its mechanism
is not well understood. We have carried out extensive Monte Carlo simulations
of the fusion of tense apposed bilayers formed by amphiphilic
molecules within the framework of a coarse grained lattice model. Our
model exhibits a phase diagram which is similar to that of biological
lipids. The fusion pathway, however, differs from the “hemifusion hypothesis”.
First stalks form between the apposed bilayers, but rather than expanding
radially to form an axial-symmetric hemifusion diaphragm of
the cis leaves of both bilayers, they promote hole nucleation of the bilayers
in their vicinity. Two subsequent paths are observed: (i) A second
hole is formed in the opposite bilayer, and the stalk aligns both hole as
it encircles them and forms the fusion pore. (ii) The stalk encircles the
hole completely before a second hole is formed in the opposite bilayer.
A diphragm is formed out of both leaves of the intact bilayer. The rupture
of this diaphragm completes fusion. Both pathways give rise to an
increase in mixing between the cis and trans leaves of the bilayer and
allow for leakage.