Mareike Zink

The impact of jamming on boundaries of collectively moving weak-interacting cells

Collective cell migration is an important feature of wound healing, as well as embryonic and tumor development. The origin of collective cell migration is mainly intercellular interactions through effects such as a line tension preventing cells from detaching from the boundary. In contrast, in this study, we show for the first time that the formation of a constant cell front of a monolayer can also be maintained by the dynamics of the underlying migrating single cells. Ballistic motion enables the maintenance of the integrity of the sheet, while a slowed down dynamics and glass-like behavior cause jamming of cells at the front when two monolayers?even of the same cell type?meet. By employing a velocity autocorrelation function to investigate the cell dynamics in detail, we found a compressed exponential decay as described by the Kohlrausch?William?Watts function of the form , with 1.5????(t)???1.8. This clearly shows that although migrating cells are an active, non-equilibrium system, the cell monolayer behaves in a glass-like way, which requires jamming as a part of intercellular interactions. Since it is the dynamics which determine the integrity of the cell sheet and its front for weakly interacting cells, it becomes evident why changes of the migratory behavior during epithelial to mesenchymal transition can result in the escape of single cells and metastasis.

Biocompatibility of single crystalline Fe70Pd30 ferromagnetic shape memory films

Controllable by an external magnetic field, ferromagnetic shape memory materials reveal a high potential for actuators in biomedical applications. Simulated body fluid (SBF) and cell tests were performed to assess the biocompatibility of Fe70Pd30 ferromagnetic shape memory thin films as grown on MgO substrates. Calcium-phosphate aggregates were detected on the film surface after soaking in SBF. Biocompatibility tests with NIH 3T3 fibroblasts revealed adhesion and proliferation on the film surface but morphological modifications with a reduced cell size became evident as well as changes in cell viability for continuous and noncontinuous FePd films. The results are compared to FePd on SiO2.

Fe–Pd based ferromagnetic shape memory actuators for medical applications: Biocompatibility, effect of surface roughness and protein coatings

Ferromagnetic shape memory (FMSM) alloys constitute an exciting new class of smart materials that can yield magnetically switchable strains of several percent at constant temperatures and frequencies from quasi-static up to some kilohertz. In addition to their FMSM properties, these alloys can still be operated as conventional shape memory materials and also exhibit related superelasticity, which are both important features for use in medical devices. In this study, extensive in vitro assessments demonstrate for the first time that vapor-deposited single crystalline Fe(70)Pd(30) thin films and roughness graded polycrystalline splats of the same stoichiometry exhibit excellent biocompatibility and even bioactivity in contact with different cell types-a prerequisite for medical applications. The present study shows that fibroblast and epithelial cell lines, as well as primary osteoblast cells, proliferate well on Fe-Pd. The number of focal contacts, important for strong tissue bonding, can be improved with different binding agents from the extracellular matrix. However, even without coating, there is clear evidence that cells on Fe-Pd substrates behave similarly to control experiments. Additionally, cytotoxic effects of polycrystalline surfaces with various roughness profiles can be excluded, giving another tunable parameter for applying Fe-Pd magnetically switchable membranes in, e.g., stents and valves.

Testing the differential adhesion hypothesis across the epithelial−mesenchymal transition

We analyze the mechanical properties of three epithelial/mesenchymal cell lines (MCF-10A, MDA-MB-231, MDA-MB-436) that exhibit a shift in E-, N- and P-cadherin levels characteristic of an epithelial−mesenchymal transition associated with processes such as metastasis, to quantify the role of cell cohesion in cell sorting and compartmentalization. We develop a unique set of methods to measure cell–cell adhesiveness, cell stiffness and cell shapes, and compare the results to predictions from cell sorting in mixtures of cell populations. We find that the final sorted state is extremely robust among all three cell lines independent of epithelial or mesenchymal state, suggesting that cell sorting may play an important role in organization and boundary formation in tumours. We find that surface densities of adhesive molecules do not correlate with measured cell–cell adhesion, but do correlate with cell shapes, cell stiffness and the rate at which cells sort, in accordance with an extended version of the differential adhesion hypothesis (DAH). Surprisingly, the DAH does not correctly predict the final sorted state. This suggests that these tissues are not behaving as immiscible fluids, and that dynamical effects such as directional motility, friction and jamming may play an important role in tissue compartmentalization across the epithelial−mesenchymal transition.

Effect of microgrooved surface topography on osteoblast maturation and protein adsorption

Microgrooved surfaces have been used extensively to influence cell contact guidance. Guiding cell growth, extracellular matrix deposition, and mineralization is important for bone implant longevity. In this study, we investigated the osteoblast response to microgrooved metallic surfaces in serum-supplemented medium. Groove spacing was comparable with the spread osteoblast size. Focal adhesions were observed to confine to the intervening ridge/groove boundaries. Osteoblasts bridged over the grooves and were unable to conform to the concave shape of the underlying grooves. Microgrooved surfaces induced higher osteoblast proliferation and metabolic activity after 14 days in osteogenic medium compared with as-received surfaces, resulting in higher mineralization and alignment of cell-secreted collagen after 28 days. To establish whether preferential cell attachment at the ridge/groove boundaries was influenced by the adhesion proteins contained in the serum-supplemented media, fluorescently labeled fibronectin was adsorbed onto the microgrooved substrates at low concentrations, mimicking the concentrations found in blood serum. Fibronectin was found to selectively adsorb onto the ridge/groove boundaries, the osteoblast focal adhesion sites, suggesting that protein adsorption may have influenced the cell attachment pattern.

Directed persistent motion maintains sheet integrity during multi-cellular spreading and migration

Multi-cellular migration plays an important role in physiological processes such as embryogenesis, cancer metastasis and tissue repair. Collective cell migration involves specific single cell motility behaviour that maintains the integrity of the monolayer and the fluid-like behavior of the sheet at long time scales. We have studied the dynamics of MCF-10A, MDA-MB-231 epithelial cell monolayers and that of a NIH-3t3 fibroblast monolayer. For the case where MCF-10A cell–cell interactions are so weak that single cells can detach from the monolayer, a collective cell front can be maintained by interplay between the directed persistent motion of the monolayer and the random motion of escaping single cells. The dynamics of the MCF-10A monolayer is contrasted with the MDA-MB-231 monolayer where single cells did not detach, with non-interacting NIH-3t3 fibroblast cells that are always random walkers, and with a MCF-10A monolayer treated with a calcium chelating agent that reduces intercellular interactions.

Tailoring the material properties of gelatin hydrogels by high energy electron irradiation

Natural hydrogels such as gelatin are highly desirable biomaterials for application in drug delivery, biosensors, bioactuators and extracellular matrix components due to strong biocompatibility and biodegradability. Typically, chemical crosslinkers are used to optimize material properties, often introducing toxic byproducts into the material. In this present work, electron irradiation is employed as a reagent-free crosslinking technique to precisely tailor the viscoelasticity, swelling behavior, thermal stability and structure of gelatin. With increasing electron dose, changes in swelling behavior and rheology indicate increasing amounts of random coils and dangling ends as opposed to helical content, a result confirmed through Fourier transform infrared spectroscopy. Gel fraction, rheology and swelling measurements at 37 °C were used to verify thermal stability in biological conditions. Scanning electron microscopy images of dried gelatin samples support these conclusions by revealing a loss of free volume and apparent order in the fracture patterns. The degree of crosslinking and mesh size are quantified by rubber elasticity theory and the Flory-Rehner equation. Overall, precise control of material properties is demonstrated through the interplay of concentration and irradiation dose, while providing an extensive parameter-property database suitable for optimized synthesis.

ERBB2 overexpression triggers transient high mechanoactivity of breast tumor cells

Biomechanical properties of tumor cells play an important role for the metastatic capacity of cancer. Cellular changes of viscoelastic features are prerequisite for cancer progression since they are essential for proliferation and metastasis. However, only little is known about the way how expression of oncogenes influences these biomechanical properties. To address this aspect we used a breast cancer cell line with inducible expression of an oncogenic version of ERBB2. ERBB2 is known to be correlated with bad prognosis in breast cancer. Cell elasticity was determined by the Optical Stretcher, where suspended cells are deformed by two slightly divergent laser beams. We found that induction of ERBB2 caused remarkable biomechanical alterations of the MCF‐7 cells after 24 h: the cells actively contracted in response to mechanical stimuli, a phenomenon known as mechanoactivation. After this period, as the cells became senescent, the mechanoactivity returned to control levels. Time‐resolved gene array analysis revealed that mechanoactivation was accompanied by temporal upregulation of 46 cytoskeletal genes. A possible role of these genes in tumor progression was investigated by expression analyses of 766 breast cancer patients. This showed an association of 12 out of these 46 genes with increased risk of metastasis. Our results demonstrate that overexpression of ERBB2 causes mechanoactivation of tumor cells, which may enhance tumor cell motility fostering distant metastasis.

Thermal instability of cell nuclei

DNA is known to be a mechanically and thermally stable structure. In its double stranded form it is densely packed within the cell nucleus and is thermo-resistant up to . In contrast, we found a sudden loss of cell nuclei integrity at relatively moderate temperatures ranging from 45 to . In our study, suspended cells held in an optical double beam trap were heated under controlled conditions while monitoring the nuclear shape. At specific critical temperatures, an irreversible sudden shape transition of the nuclei was observed. These temperature induced transitions differ in abundance and intensity for various normal and cancerous epithelial breast cells, which clearly characterizes different cell types. Our results show that temperatures slightly higher than physiological conditions are able to induce instabilities of nuclear structures, eventually leading to cell death. This is a surprising finding since recent thermorheological cell studies have shown that cells have a lower viscosity and are thus more deformable upon temperature increase. Since the nucleus is tightly coupled to the outer cell shape via the cytoskeleton, the force propagation of nuclear reshaping to the cell membrane was investigated in combination with the application of cytoskeletal drugs.

Calcium imaging in the optical stretcher

The Microfluidic Optical Stretcher (MOS) has previously been shown to be a versatile tool to measure mechanical properties of single suspended cells. In this study we combine optical stretching and fluorescent calcium imaging. A cell line transfected with a heat sensitive cation channel was used as a model system to show the versatility of the setup. The cells were loaded with the Ca(2+) dye Fluo-4 and imaged with confocal laser scanning microscopy while being stretched. During optical stretching heat is transferred to the cell causing a pronounced Ca(2+) influx through the cation channel. The technique opens new perspectives for investigating the role of Ca(2+) in regulating cell mechanical behavior.