Biology subjects

Franze, K.

Publications and source records attributed to Franze, K..

30 records · Page 2Linked to original sources

bioRxiv2021

The Orsay Virus as a model for population-wide viral infection dynamics

To this day, epidemics pose a considerable threat to mankind. Experimental models that simulate the spread of infectious diseases are thus crucial to the inception of effective control policies. Current models have had great success incorporating virulence and host immune response but do rarely take host genetics, behavior and host environment into account. Here, we present a full-scale imaging setup that utilizes the infection of the nematode C. elegans with a positive-stranded RNA virus (Orsay Virus) to probe key epidemiological parameters and simulate the spread of infection in a whole population. We demonstrate that our system is able to quantify infection levels and host behavior at a high sampling rate and show that different host genetic backgrounds can influence viral spread, while also highlighting the influence of infection on various host behaviors. Future work will allow the isolation of key behavioral and environmental factors that affect viral spread, potentially enabling novel policies to combat the spread of viral infections. Significance StatementIn the ongoing COVID-19 pandemic, we struggle to find effective control policies that "stop the spread". While current animal models of virus spread in populations are highly sophisticated, they rarely explore effects of host behavior and its environment. We developed an experimental animal model system that allows us to visualize virus transmission in whole populations of C. elegans while also measuring behaviors. We were able to demonstrate how C. elegans genetics influences the progression of viral infection in a population and how animals adjust their behavior when infected. In the future, we envision that animal model systems like ours are used to test the effects of viral control policies on viral spread before they are applied in real world scenarios.

immunology↗

bioRxiv2020

An optic ray theory for durotactic axon guidance

During the development of the nervous system, neurons extend bundles of axons that grow and meet other neurons to form the neuronal network. Robust guidance mechanisms are needed for these bundles to migrate and reach their functional target. Directional information depends on external cues such as chemical or mechanical gradients. Unlike chemotaxis that has been extensively studied, the role and mechanism of durotaxis, the directed response to variations in substrate rigidity, remain unclear. We model bundle migration and guidance by rigidity gradients by using the theory of morphoelastic rods. We show that at a rigidity interface, the motion of axon bundles follows a simple behavior analogous to optic ray theory and obeys Snells law for refraction and reflection. We use this powerful analogy to demonstrate that axons can be guided by the equivalent of optical lenses and fibers created by regions of different stiffnesses.

neuroscience↗

bioRxiv2020

Cell surface fluctuations regulate early embryonic lineage sorting

In development, lineage segregation of multiple lineages must be coordinated in time and space. An important example is the mammalian inner cell mass (ICM), in which the primitive endoderm (PrE, founder of the yolk sac) physically segregates from the epiblast (EPI, founder of the foetus). The physical mechanisms that determine this spatial segregation between EPI and PrE are still poorly understood. Here, we identify an asymmetry in cell-cell affinity, a mechanical property thought to play a significant role in tissue sorting in other systems, between EPI and PrE precursors (pEPI and pPrE). However, a computational model of cell sorting indicated that these differences alone appeared insufficient to explain the spatial segregation. We also observed significantly greater surface fluctuations in pPrE compared to pEPI. Including the enhanced surface fluctuation in pPrE in our simulation led to robust cell sorting. We identify phospho-ERM regulated membrane tension as an important mediator of the increased surface fluctuations in pPrE. Using aggregates of engineered cell lines with different surface fluctuation levels cells with higher surface fluctuations were consistently excluded to the outside of the aggregate. These cells behaved similarly when incorporated in the embryo. Surface fluctuations-driven segregation is reminiscent of activity-induced phase separation, a sorting phenomenon in colloidal physics. Together, our experiments and model identify dynamic cell surface fluctuations, in addition to static mechanical properties, as a key factor for orchestrating the correct spatial positioning of the founder embryonic lineages.

developmental biology↗

bioRxiv2020

Robust T-cell signaling triggered on soft polydimethylsiloxane-supported lipid-bilayers

The T-cell receptor (TCR) is thought to be triggered either by mechano-transduction or local tyrosine phosphatase exclusion at cell-cell contacts. However, the effects of the mechanical properties of activating surfaces have only been tested for late-stage T-cell activation, and phosphatase segregation has mostly been studied on glass-supported lipid bilayers that favor imaging but are orders-of-magnitude stiffer than typical cells. We developed a method for attaching lipid bilayers to polydimethylsiloxane polymer supports, producing soft bilayers with physiological levels of mechanical resistance (Youngs modulus of 4 kPa). Comparisons of T-cell behavior on soft and glass-supported bilayers revealed that early calcium signaling is unaffected by substrate rigidity, implying that early steps in TCR triggering are not mechanosensitive. Robust phosphatase exclusion was observed on the soft bilayers, however, suggesting it likely occurs at cell-cell contacts. This work sets the stage for an imaging-based exploration of receptor signaling under conditions closely mimicking physiological cell-cell contact.

biophysics↗

bioRxiv2020

Spatial heterogeneity of cell-matrix adhesive forces predicts human glioblastoma migration

BackgroundGlioblastoma (GBM) is a highly aggressive incurable brain tumor. The main cause of mortality in GBM patients is the invasive rim of cells migrating away from the main tumor mass and invading healthy parts of the brain. Although motion is driven by forces, our current understanding of the physical factors involved in glioma infiltration remains limited. This study aims to investigate the adhesion properties within and between patients tumors on a cellular level and test whether these properties correlate with cell migration. MethodsNine tissue samples were taken from spatially separated sections during 5-aminolevulinic acid (5-ALA) fluorescence guided surgery. Navigated biopsy samples were collected from strongly fluorescent tumor cores, a weak fluorescent tumor rim, and non-fluorescent tumor margins. A microfluidics device was built to induce controlled shear forces to detach cells from monolayer cultures. Cells were cultured on low modulus polydimethylsiloxane representative of the stiffness of brain tissue. Cell migration and morphology were then obtained using time lapse microscopy. ResultsGBM cell populations from different tumor fractions of the same patient exhibited different migratory and adhesive behaviors. These differences were associated with sampling location and amount of 5-ALA fluorescence. Cells derived from weak- and non-fluorescent tumor tissue were smaller, adhered less well, and migrated quicker than cells derived from strongly fluorescent tumor mass. ConclusionGBM tumors are biomechanically heterogeneous. Selecting multiple populations and broad location sampling are therefore important to consider for drug testing. Key pointsGBM tumors are biomechanically heterogeneous GBM cell migration is inversely correlated with cell-matrix adhesion strength 5-ALA fluorescence intensity during surgery correlates with the motility properties of GBM cells Importance of the studyThis is the first study to compare single cell migration and cell-matrix adhesion strength of GBM, using cell lines derived from different tumors and from different regions within the same tumor. Not accounting for internal sampling location within each tumor obscures differences in cell morphology, motility and adhesion properties between patients. Peripherical and marginal tumor cells have different adhesion profiles and are highly migratory compared to those found in the core of the tumor. Aggressive regions of the tumor (highly motile) are linked to the spatial distribution of adhesion strength and are strongly associated with 5-ALA fluorescence intensity. Preclinical tests aimed at developing a treatment for GBM using anti-invasive drugs or adhesion inhibitors, would benefit from using cell lines derived from the tumor periphery (with low 5-ALA intensity) rather than cell lines derived from the tumor core.

biophysics↗

bioRxiv2019

Cortical cell stiffness is independent of substrate mechanics

Cortical stiffness is an important cellular property that changes during migration, adhesion, and growth. Previous atomic force microscopy (AFM) indentation measurements of cells cultured on deformable substrates suggested that cells adapt their stiffness to that of their surroundings. Here we show that the force applied by AFM onto cells results in a significant deformation of the underlying substrate if it is softer than the cells. This soft substrate effect leads to an underestimation of a cells elastic modulus when analyzing data using a standard Hertz model, as confirmed by finite element modelling (FEM) and AFM measurements of calibrated polyacrylamide beads, microglial cells, and fibroblasts. To account for this substrate deformation, we developed the composite cell-substrate model (CoCS model). Correcting for the substrate indentation revealed that cortical cell stiffness is largely independent of substrate mechanics, which has significant implications for our interpretation of many physiological and pathological processes.

biophysics↗

bioRxiv2019

Mechanical matching of implant to host minimises foreign body reaction

Medical implants offer a unique and powerful therapeutic approach in many areas of medicine. However, their lifetime is often limited as they may cause a foreign body reaction (FBR) leading to their encapsulation by scar tissue1-4. Despite the importance of this process, how cells recognise implanted materials is still poorly understood5, 6. Here, we show how the mechanical mismatch between implants and host tissue leads to FBR. Fibroblasts and macrophages, which are both crucially involved in mediating FBR, became activated when cultured on materials just above the stiffness of healthy tissue. Coating stiff implants with a thin layer of hydrogel or silicone with a tissue-like elastic modulus ([~]20 kPa in subcutaneous and [~]2 kPa in peripheral nerve implants) or softer significantly reduced inflammation and fibrosis three months after implantation. Materials stiffer than the host tissue led to nuclear localisation of the mechanosensitive transcriptional regulator YAP in neighbouring cells in vivo, confirming mechanotransduction. The alleviation of FBR by soft coatings not exceeding the stiffness of the host tissue provides a strategy to achieve long-term implant stability without extensive modification of current implant manufacturing techniques, facilitating clinical translation.

bioengineering↗

bioRxiv2019

StemBond hydrogels optimise the mechanical microenvironment for embryonic stem cells

Studies of mechanical signalling are typically performed by comparing cells cultured on soft and stiff hydrogel-based substrates. However, it is challenging to independently and robustly control both substrate stiffness and tethering of extracellular matrix (ECM) to substrates, making ECM tethering a potentially confounding variable in mechanical signalling investigations. Moreover, poor ECM tethering can lead to weak cell attachment. To address this, we developed StemBond hydrogels, a hydrogel formulation in which ECM tethering is stable and can be varied independently of stiffness. We show that soft StemBond hydrogels provide an optimal format for culturing embryonic stem (ES) cells. We find that soft StemBond substrates improve the homogeneity of ES cell populations, boost their self-renewal, and increase the efficiency of cellular reprogramming. Our findings underline how soft microenvironments impact mechanosensitive signalling pathways regulating self-renewal and differentiation, indicating that optimising the complete mechanical microenvironment will offer greater control over stem cell fate specification.

cell biology↗

bioRxiv2018

KymoButler: A deep learning software for automated kymograph tracing and analysis

Kymographs are graphical representations of spatial position over time, which are often used in biology to visualise the motion of fluorescent particles, molecules, vesicles, or organelles moving along a predictable path. Although in kymographs tracks of individual particles are qualitatively easily distinguished, their automated quantitative analysis is much more challenging. Kymographs often exhibit low signal-to-noise-ratios (SNRs), and available tools that automate their analysis usually require manual supervision. Here we developed KymoButler, a Deep Learning-based software to automatically track dynamic processes in kymographs. We demonstrate that KymoButler performs as well as expert manual data analysis on kymographs with complex particle trajectories from a variety of different biological systems. The software was packaged in a web-based \"one-click\" application for use by the wider scientific community. Our approach significantly speeds up data analysis, avoids unconscious bias, and represents another step towards the widespread adaptation of Machine Learning techniques in biological data analysis.

cell biology↗

bioRxiv2018

Predicting local tissue mechanics using immunohistochemistry

Local tissue stiffness provides an important signal to which cells respond in vivo. However, assessing tissue mechanics is currently challenging and requires sophisticated technology. We here developed a model quantitatively predicting nervous tissue stiffness heterogeneities at cellular resolution based on cell density, myelin and GFAP fluorescence intensities. These histological parameters were identified by a correlation analysis of atomic force microscopy-based elasticity maps of spinal cord sections and immunohistochemical stainings. Our model provides a simple tool to estimate local stiffness distributions in nervous tissue, and it can easily be expanded to other tissue types, thus paving the way for studies of the role of mechanical signals in development and pathology.

biophysics↗

bioRxiv2018

Simultaneous in vivo time-lapse stiffness mapping and fluorescence imaging of developing tissue

Tissue mechanics is important for development; however, the spatio-temporal dynamics of in vivo tissue stiffness is still poorly understood. We here developed tiv-AFM, combining time-lapse in vivo atomic force microscopy with upright fluorescence imaging of embryonic tissue, to show that in the developing Xenopus brain, a stiffness gradient evolves over time because of differential cell proliferation. Subsequently, axons turn to follow this gradient, underpinning the importance of time-resolved mechanics measurements.

developmental biology↗

bioRxiv2017

Long-Term Imaging of Cellular Forces with High Precision by Elastic Resonator Interference Stress Microscopy

Cellular forces are crucial for many biological processes but current methods to image them have limitations with respect to online analysis, resolution and throughput. Here, we present a robust approach to measure mechanical cell-substrate interactions in diverse biological systems by interferometrically detecting deformations of an elastic micro-cavity. Elastic Resonator Interference Stress Microscopy (ERISM) yields stress maps with exceptional precision and large dynamic range (2 nm displacement resolution over a >1 m range, translating into 1 pN force sensitivity). This enables investigation of minute vertical stresses (<1 Pa) involved in podosome protrusion, protein specific cell-substrate interaction and amoeboid migration through spatial confinement in real time. ERISM requires no zero-force reference and avoids phototoxic effects, which facilitates force monitoring over multiple days and at high frame rates and eliminates the need to detach cells after measurements. This allows observation of slow processes like differentiation and further investigation of cells, e.g. by immunostaining.

biophysics↗