Biomedical subjects

Albrecht Ott

Publications and source records attributed to Albrecht Ott.

5 recordsLinked to original sources

PubMed2006

Optical study of DNA surface hybridization reveals DNA surface density as a key parameter for microarray hybridization kinetics.

We investigate the kinetics of DNA hybridization reactions on glass substrates, where one 22 mer strand (bound-DNA) is immobilized via phenylene-diisothiocyanate linker molecule on the substrate, the dye-labeled (Cy3) complementary strand (free-DNA) is in solution in a reaction chamber. We use total internal reflection fluorescence for surface detection of hybridization. As a new feature we perform a simultaneous real-time measurement of the change of free-DNA concentration in bulk parallel to the total internal reflection fluorescence measurement. We observe that the free-DNA concentration decreases considerably during hybridization. We show how the standard Langmuir kinetics needs to be extended to take into account the change in bulk concentration and explain our experimental results. Connecting both measurements we can estimate the surface density of accessible, immobilized bound-DNA. We discuss the implications with respect to DNA microarray detection.

Computer Simulation↗

PubMed2006

A master relation defines the nonlinear viscoelasticity of single fibroblasts.

Cell mechanical functions such as locomotion, contraction, and division are controlled by the cytoskeleton, a dynamic biopolymer network whose mechanical properties remain poorly understood. We perform single-cell uniaxial stretching experiments on 3T3 fibroblasts. By superimposing small amplitude oscillations on a mechanically prestressed cell, we find a transition from linear viscoelastic behavior to power law stress stiffening. Data from different cells over several stress decades can be uniquely scaled to obtain a master relation between the viscoelastic moduli and the average force. Remarkably, this relation holds independently of deformation history, adhesion biochemistry, and intensity of active contraction. In particular, it is irrelevant whether force is actively generated by the cell or externally imposed by stretching. We propose that the master relation reflects the mechanical behavior of the force-bearing actin cytoskeleton, in agreement with stress stiffening known from semiflexible filament networks.

3T3 Cells↗

PubMed2006

Osmotically driven shape transformations in axons.

We report a cylindrical-peristaltic shape transformation in axons exposed to a controlled osmotic perturbation. The peristaltic shape relaxes and the axon recovers its original geometry within minutes. We show that the shape instability depends critically on the swelling rate and that volume and membrane area regulation are responsible for the shape relaxation. We propose that volume regulation occurs via leakage of ions driven by elastic pressure, and analyze the peristaltic shape dynamics taking into account the internal structure of the axon. The results obtained provide a framework for understanding peristaltic shape dynamics in nerve fibers occurring in vivo.

Animals↗

PubMed2005

Mechanical response analysis and power generation by single-cell stretching.

To harvest useful information about cell response due to mechanical perturbations under physiological conditions, a cantilever-based technique was designed, which allowed precise application of arbitrary forces or deformation histories on a single cell in vitro. Essential requirements for these investigations are a mechanism for applying an automated cell force and an induced-deformation detection system based on fiber-optical force sensing and closed loop control. The required mechanical stability of the setup can persist for several hours since mechanical drifts due to thermal gradients can be eliminated sufficiently (these gradients are caused by local heating of the cell observation chamber to 37 degrees C). During mechanical characterization, the cell is visualized with an optical microscope, which enables the simultaneous observation of cell shape and intracellular morphological changes. Either the cell elongation is observed as a reaction against a constant load or the cell force is measured as a response to constant deformation. Passive viscoelastic deformation and active cell response can be discriminated. The active power generated during contraction is in the range of Pmax= 10(-16) Watts, which corresponds to 2500 ATP molecules s(-1) at 10 k(B)T/molecule. The ratio of contractive to dissipative power is estimated to be in the range of 10(-2). The highest forces supported by the cell suggest that about 10(4) molecular motors must be involved in contraction. This indicates an energy-conversion efficiency of approximately 0.5. Our findings propose that, in addition to the recruitment of cell-contractile elements upon mechanical stimulation, the cell cytoskeleton becomes increasingly crosslinked in response to a mechanical pull. Quantitative stress-strain data, such as those presented here, may be employed to test physical models that describe cellular responses to mechanical stimuli.

3T3 Cells↗

PubMed2003

Kinesin motion in the absence of external forces characterized by interference total internal reflection microscopy.

We study the motion of the kinesin molecular motor along microtubules using interference total internal reflection microscopy. This technique achieves nanometer scale resolution together with a fast time response. We describe the first in vitro observation of kinesin stepping at high ATP concentration in the absence of an external load, where the 8-nm step can be clearly distinguished. The short-time resolution allows us to measure the time constant related to the relative motion of the bead-motor connection; we deduce the associated bead-motor elastic modulus.

Adenosine Triphosphate↗