Biomedical subjects

Karsten Kruse

Publications and source records attributed to Karsten Kruse.

9 recordsLinked to original sources

PubMed2005

A self-organized vortex array of hydrodynamically entrained sperm cells.

Many patterns in biological systems depend on the exchange of chemical signals between cells. We report a spatiotemporal pattern mediated by hydrodynamic interactions. At planar surfaces, spermatozoa self-organized into dynamic vortices resembling quantized rotating waves. These vortices formed an array with local hexagonal order. Introducing an order parameter that quantifies cooperativity, we found that the array appeared only above a critical sperm density. Using a model, we estimated the hydrodynamic interaction force between spermatozoa to be approximately 0.03 piconewtons. Thus, large-scale coordination of cells can be regulated hydrodynamically, and chemical signals are not required.

Animals↗

PubMed2005

Theory of mitotic spindle oscillations.

During unequal cell division the mitotic spindle is positioned away from the center of the cell before cell cleavage. In many biological systems this repositioning is accompanied by oscillatory movements of the spindle. We present a theoretical description for mitotic spindle oscillations. We show that the cooperative attachment and detachment of cortical force generators to astral microtubules leads to spontaneous oscillations beyond a critical number of force generators. This mechanism can quantitatively describe the spindle oscillations observed during unequal division of the one cell stage Caenorhabditis elegans embryo.

Animals↗

PubMed2005

Filament depolymerization by motor molecules.

Motor proteins that specifically interact with the ends of cytoskeletal filaments can induce filament depolymerization. A phenomenological description of this process is presented. We show that under certain conditions motors dynamically accumulate at the filament ends. We compare simulations of two microscopic models to the phenomenological description. The depolymerization rate can exhibit maxima and dynamic instabilities as a function of the bulk motor density for processive depolymerization. We discuss our results in relation to experimental studies of Kin-13 family motor proteins.

Models, Biological↗

PubMed2005

Cellular organization by self-organization: mechanisms and models for Min protein dynamics.

We use the oscillating Min proteins of Escherichia coli as a prototype system to illustrate the current state and potential of modeling protein dynamics in space and time. We demonstrate how a theoretical approach has led to striking new insights into the mechanisms of self-organization in bacterial cells and indicate how these ideas may be applicable to more complex structure formation in eukaryotic cells.

Adenosine Triphosphatases↗

PubMed2005

Oscillations in cell biology.

Oscillations play an important role in many dynamic cellular processes. They can emerge as the collective dynamic behavior of an ensemble of interacting components in the cell. Examples include oscillations in cytoskeletal structures such as the axonemes of cilia. Spontaneous oscillations of mechano-sensitive hair bundles have been shown to give frequency selectivity and amplification to mechano-sensation. In some bacteria, oscillations of Min proteins are important for division site selection. Genetic oscillators form the basis of circadian clocks. All these oscillations share many general features. Models and theoretical approaches are essential for an understanding of the principles underlying these dynamic cellular processes.

Animals↗

PubMed2005

Min-oscillations in Escherichia coli induced by interactions of membrane-bound proteins.

During division it is of primary importance for a cell to correctly determine the site of cleavage. The bacterium Escherichia coli divides in the center, producing two daughter cells of equal size. Selection of the center as the correct division site is in part achieved by the Min-proteins. They oscillate between the two cell poles and thereby prevent division at these locations. Here, a phenomenological description of these oscillations is presented, where lateral interactions between proteins on the cell membrane play a key role. Solutions to the dynamic equations are compared to experimental findings. In particular, the temporal period of the oscillations is measured as a function of the cell length and found to be compatible with the theoretical prediction.

Adenosine Triphosphatases↗

PubMed2004

Dpp gradient formation by dynamin-dependent endocytosis: receptor trafficking and the diffusion model.

Developing cells acquire positional information by reading the graded distribution of morphogens. In Drosophila, the Dpp morphogen forms a long-range concentration gradient by spreading from a restricted source in the developing wing. It has been assumed that Dpp spreads by extracellular diffusion. Under this assumption, the main role of endocytosis in gradient formation is to downregulate receptors at the cell surface. These surface receptors bind to the ligand and thereby interfere with its long-range movement. Recent experiments indicate that Dpp spreading is mediated by Dynamin-dependent endocytosis in the target tissue, suggesting that extracellular diffusion alone cannot account for Dpp dispersal. Here, we perform a theoretical study of a model for morphogen spreading based on extracellular diffusion, which takes into account receptor binding and trafficking. We compare profiles of ligand and surface receptors obtained in this model with experimental data. To this end, we monitored directly the pool of surface receptors and extracellular Dpp with specific antibodies. We conclude that current models considering pure extracellular diffusion cannot explain the observed role of endocytosis during Dpp long-range movement.

Amino Acid Sequence↗

PubMed2003

Self-organization and mechanical properties of active filament bundles.

A general framework for the description of active bundles of polar filaments is presented. The activity of the bundle results from mobile cross-links that induce relative displacements between the aligned filaments. Our generic description is based on momentum conservation within the bundle. By specifying the internal forces, a simple minimal model for the bundle dynamics can be derived, capturing a rich variety of dynamic behaviors. In particular, contracted states as well as solitary and oscillatory waves appear through dynamic instabilities. We present the full bifurcation diagram of this model and study the effects of a dynamic motor distribution on the bundle dynamics. Furthermore, we discuss the mechanical properties of the bundle in the presence of externally applied forces. Our description is motivated by dynamic phenomena in the cytoskeleton and could apply to in vitro experiments as well as to stress fibers and to self-organization phenomena during cell locomotion.

Actin Cytoskeleton↗

PubMed2002

A dynamic model for determining the middle of Escherichia coli.

Proper placement of the division septum is an essential part of bacterial cell division. In Escherichia coli, this process depends crucially on the proteins MinC, MinD, and MinE. The detailed mechanism by which these proteins determine the correct position of the division plane is currently unknown, but observed pole-to-pole oscillations of the corresponding distributions are thought to be of functional importance. Here, a theoretical approach toward an explanation of this dynamical behavior is reported. Emphasizing generic properties of the protein dynamics, two features are found to be sufficient for generating oscillations: first, a tendency of membrane bound MinD to cluster; and second, attachment to and detachment from the cell wall, which depends on the amount of molecules already attached. The model is in qualitative agreement with the presently existing experimental results and further tests of the underlying model assumptions are suggested. Finally, based on the analysis of the model a simple mechanism is proposed on how these proteins might initiate septal growth. In addition, to ensure correct positioning of the septum, the MinCDE complex could therefore also play an important role in cell cycle control.

Adenosine Triphosphatases↗