Biology subjects

Rieger, H.

Publications and source records attributed to Rieger, H..

12 recordsLinked to original sources

bioRxiv2025

Ubiquitin-dependent signal amplification in lipid saturation sensing

Cellular membranes are dynamic platforms whose composition and biophysical properties are surveyed by sensor proteins to maintain homeostasis. Failure to preserve membrane homeostasis, however, results in cellular stress and organelle dysfunction. Using the prototypical lipid saturation sensor Mga2, we explored how weak physical cues that modulate rotational movements in the transmembrane region are converted into decisive biochemical outputs that ultimately control the production of unsaturated fatty acids. Quantitative in vitro ubiquitylation assays and kinetic modeling reveal vastly distinct rates of Mga2 ubiquitylation controlled by the membrane environment. Mga2 ubiquitylation dominates in tightly packed, saturated membranes, while loosely packed environments favor an inhibitory autoubiquitylation of the cognate E3 ligase Rsp5. This mechanism provides a means of signal amplification, which can function even in the absence of deubiquitylating enzymes. Our findings provide a mechanistic framework for how membrane property sensors convert weak, fluctuating physical cues into robust biochemical outcomes, and put a spotlight on the regulatory potential of E3 ligase autoubiquitylation in cellular surveillance.

biochemistry↗

bioRxiv2025

Modelling Actin-MT Crosstalk in Migrating Cells

Actin-Microtubule crosstalk regulates the polarity and morphology of migrating cells and encompasses mechanical interactions, mediated by crosslinkers, molecular motors, and cytoskeletal regulators. Recent experiments indicate that local microtubule depolymerization promotes local actomyosin retraction, whereas local microtubule polymerization promotes local actin-polymerization. Based on these observations, we develop a computational whole-cell model involving dynamic microtubules interacting mechanically and chemically with an active cell boundary. Specifically, the tips of microtubules send signals for local expansion or contraction to the active cell boundary, depending on whether they are in the growth or shrink phase. A rich, self-organized, dynamic behavior emerges, characterized by the repositioning of the microtubule-organizing center relative to the nucleus and the direction of migration. This also includes a variety of migration patterns, cell morphologies, and complex responses to obstacles in microfluidic and obstacle park environments. We demonstrate that microtubule length and cell boundary stiffness have a significant impact on these features, highlighting the need for new experimental investigations. Thus, the model provides a unified framework that explains a wide range of experimental observations and setups where actin-microtubule crosstalk plays a crucial role.

biophysics↗

bioRxiv2024

Multiple clustered centrosomes in antigen-presenting cells foster T cell activation without MTOC polarization

Cellular polarization plays a pivotal role in regulating immunological processes and is often associated with centrosome reorientation. During immune synapse (IS) formation centrosome repositioning in lymphocytes assists in T cell activation. While a single centrosome, consisting of two centrioles, is present in T cells, antigen-presenting cells (APCs) such as dendritic cells (DCs) amplify centrioles during maturation leading to increased centrosome numbers upon immune activation. How centrosome amplification in DCs affects IS formation and T cell activation is unclear. In this study, we combine experimental data with mathematical and computational modelling to provide evidence that centrosome amplification in DCs enhances antigen-specific T cell activation. Extra centrioles in DCs form active centrosomes, which cluster during DC-T cell interactions and unlike in T cells, localize close to the cell center. Perturbing either centriole numbers or centrosome configuration in DCs results in impaired T cell activation. Collectively, our results highlight a crucial role for centrosome amplification and optimal centrosome positioning in APCs for controlling T cell responses.

cell biology↗

bioRxiv2022

Amoeboid Cell Migration through Regular Arrays of Micropillars under Confinement

Migrating cells often encounter a wide variety of topographic features--including the presence of obstacles--when navigating through crowded biological environments. Unravelling the impact of topography and crowding on the dynamics of cells is key to better understand many essential physiological processes such as the immune response. We study how migration and search efficiency of HL-60 cells differentiated into neutrophils in quasi two-dimensional environments are influenced by the lateral and vertical confinement and spatial arrangement of obstacles. A microfluidic device is designed to track the cells in confining geometries between two parallel plates with distance h, in which identical micropillars are arranged in regular pillar forests. We find that at each cell-pillar contact event, the cell spends a finite time near the pillar surface, which is independent of the height h and the interpillar spacing e. At low pillar density regime, the directional persistence of cells reduces with decreasing h or e, influencing their diffusivity and first-passage properties. The dynamics is strikingly different at high pillar density regime, where the cells are in simultaneous contact with more than one pillar; the cell velocity and persistence are distinctly higher compared to dilute pillar configurations with the same h. Our simulations reveal that the interplay between cell persistence and cell-pillar interactions can dramatically affect cell diffusivity and, thus, its first-passage properties.

biophysics↗

bioRxiv2021

Stochastic model of T Cell repolarization during target elimination (II)

Cytotoxic T lymphocytes (T cells) and natural killer cells form a tight contact, the immunological synapse (IS), with target cells, where they release their lytic granules containing perforin/granzyme and cytokine containing vesicles. During this process the cell repolarizes and moves the microtubule organizing center (MTOC) towards the IS. In the first part of our work we developed a computational model for the molecular-motor-driven motion of the MT cytoskeleton during T cell polarization and analyzed effects of cortical sliding and capture-shrinkage mechanisms. Here we use this model to analyze the dynamics of the MTOC repositioning in situations in which a) the IS is in an arbitrary position with respect to the initial position of the MTOC and b) the T cell has two IS at two arbitrary positions. In the case of one IS, we found that the initial position determines which mechanism is dominant and that the time of repositioning does not rise monotonously with the MTOC-IS distance. In the case of two IS, we observe several scenarios that have also been reported experimentally: the MTOC alternates stochastically (but with a well defined average transition time) between the two IS; it wiggles in between the two IS without transiting to one of the two; or it is at some point pulled to one of the two IS and stays there. Our model allows to predict which scenario emerges in dependency of the mechanisms in action and the number of dyneins present. We report that the presence of capture-shrinkage mechanism at least in one IS is necessary to assure the transitions in every cell configuration. Moreover, the frequency of transitions does not decrease with the distance between the two IS and is the highest when both mechanisms are present in both IS.

biophysics↗

bioRxiv2020

Migration of Cytotoxic T Lymphocytes in 3D Collagen Matrices

CD8+ cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are the main cytotoxic killer cells of the human body to eliminate pathogen-infected or tumorigenic cells (= target cells). To find their targets they have to navigate and migrate through a complex biological microenvironments, a key component of which is the extracellular matrix (ECM). The mechanisms underlying killer cells navigation are not well understood. To mimic an ECM we use a matrix formed by different collagen concentrations, and analyze migration trajectories of primary human CTLs. Different migration patterns are observed and can be grouped into three motility types: slow, fast and mixed. The dynamics are well described by a two-state persistent random walk model which allows cells to switch between slow motion with low persistence, and fast motion with high persistence. We hypothesize that the slow motility mode describes CTLs creating channels through the collagen matrix by deforming and tearing apart collagen fibers, and that the fast motility mode describes CTLs moving within these channels. Experimental evidence supporting this scenario is presented by visualizing migrating T cells following each other on exactly the same track and showing cells moving quickly in channel-like cavities within the surrounding collagen matrix. Consequently, the efficiency of the stochastic search process of CTLs in the ECM should strongly be influenced by a dynamically changing channel network produced by the killer cells themselves.

biophysics↗

bioRxiv2019

Stochastic model of T Cell repolarization during target elimination (I)

Cytotoxic T lymphocytes (T) and natural killer (NK) cells are the main cytotoxic killer cells of the human body to eliminate pathogen-infected or tumorigenic cells (i.e. target cells). Once a NK or T cell has identified a target cell, they form a tight contact zone, the immunological synapse (IS). One then observes a re-polarization of the cell involving the rotation of the microtubule (MT) half-spindle and a movement of the microtubule organizing center (MTOC) to a position that is just underneath the plasma membrane at the center of the IS. Concomitantly a massive relocation of organelles attached to MTs is observed, including the Golgi apparatus, lytic granules and mitochondria. Since the mechanism of this relocation is still elusive we devise a theoretical model for the molecular motor driven motion of the MT half-spindle confined between plasma membrane and nucleus during T cell polarization. We analyze different scenarios currently discussed in the literature, the cortical sliding and the capture-shrinkage mechanisms, and compare quantitative predictions about the spatio-temporal evolution of MTOC position and spindle morphology with experimental observations. The model predicts the experimentally observed biphasic nature of the repositioning process due to an interplay between spindle geometry and motor forces and confirms the dominance of the capture-shrinkage over the cortical sliding mechanism when MTOC and IS are initially diametrically opposed. We also find that the two mechanisms act synergetically, thereby reducing the resources necessary for repositioning. Moreover, it turns out that the localization of dyneins in the pSMAC facilitates their interaction with the MTs. Our model also opens a way to infer details of the dynein distribution from the experimentally observed features of the MT half-spindle dynamics. In a subsequent publication we will address the issue of general initial configurations and situations in which the T cell established two immunological synapses.

biophysics↗

bioRxiv2019

Fine-grained simulations of the microenvironment of vascularized tumours

One of many important features of the tumour microenvironment is that it is a place of active Darwinian selection where different tumour clones become adapted to the variety of ecological niches that make up the microenvironment. These evolutionary processes turn the microenvironment into a powerful source of tumour heterogeneity and contribute to the development of drug resistance in cancer. Here, we describe a computational tool to study the ecology of the microenvironment and report results about the ecology of the tumour microenvironment and its evolutionary dynamics.

biophysics↗

bioRxiv2018

Adaption of artificial microvascular networks

We explored the application of the adaption algorithm introduced by Secomb et. al. to artificial blood vessel networks and find that the resulting networks to not obey Murrays law. The reported analysis and visual examples document the contrast between two different types of networks. We conjecture that an interrupt of an established signaling cascade could be a hallmark of solid vascular tumors.

cancer biology↗

bioRxiv2018

Understanding the regulation of STIM1-ORAI1 interaction using a reaction-diffusion model

Release of Ca2+ from endoplasmatic retriculum (ER) Ca2+ stores causes stromal interaction molecules (STIM) in the ER membrane and ORAI proteins in the plasma membrane (PM) to interact and form the Ca2+ release activated Ca2+ (CRAC) channels, which represent a major Ca2+ entry route in non-excitable cells and thus control various cell functions. Extracellular reactive oxygen species (ROS) can modulate the CRAC current Icrac via oxidation of ORAI1. We formulate a reaction-diffusion model to quantify the STIM1-ORAI1 interaction during CRAC channel formation and analyze different ORAI1 channel stoichiometries and different ratios of STIM1 and ORAI1 in comparison with experimental data. We incorporate the inhibition of ORAI1 channels by ROS into our model and calculate its contribution to the CRAC channel amplitude. We find that the possibility of reactions between CRAC channel subunits and established CRAC channels tunes the total amount of Ca2+ influx and determines which CRAC channel configuration is mostly preferred. High numbers of ROS-inhibited ORAI1 dimers (in comparison to non-inhibited ORAI1 dimers) are needed to induce a strong decrease of Ca2+ influx compared to the wildetype (WT) case.

biophysics↗

bioRxiv2017

Tumorcode - A framework to simulate vascularized tumors

During the past years our group published several articles using computer simulations to address the complex interaction of tumors and the vasculature as underlying transport network. Advances in imaging and lab techniques pushed in vitro research of tumor spheroids forward and animal models as well as clinical studies provided more insights to single processes taking part in tumor growth, however, an overall picture is still missing. Computer simulations are a none-invasive option to cumulate current knowledge and form a quasi in vivo system. In our software, several known models were assembled into a multi-scale approach which allows to study length scales relevant for clinical applications.\n\nWe release our code to the public domain, together with a detailed description of the implementation and several examples, with the hope of usage and futher development by the community. Justification for the included algorithms and the biological models was obtained in previous publications, here we summarize technical aspects following the workflow of a typical simulation procedure.

biophysics↗

bioRxiv2017

Spatial cytoskeleton organization supports targeted intracellular transport

The efficiency of intracellular cargo transport from specific source to target locations is strongly dependent upon molecular motor-assisted motion along the cytoskeleton. Radial transport along microtubules and lateral transport along the filaments of the actin cortex underneath the cell membrane are characteristic for cells with a centrosome. The interplay between the specific cytoskeleton organization and the motor performance realizes a spatially inhomogeneous intermittent search strategy. In order to analyze the efficiency of such intracellular search strategies we formulate a random velocity model with intermittent arrest states. We evaluate efficiency in terms of mean first passage times for three different, frequently encountered intracellular transport tasks: i) the narrow escape problem, which emerges during cargo transport to a synapse or other specific region of the cell membrane, ii) the reaction problem, which considers the binding time of two particles within the cell, and iii) the reaction-escape problem, which arises when cargo must be released at a synapse only after pairing with another particle. Our results indicate that cells are able to realize efficient search strategies for various intracellular transport tasks economically through a spatial cytoskeleton organization that involves only a narrow actin cortex rather than a cell body filled with randomly oriented actin filaments.

biophysics↗