Biology subjects

Sixt, M.

Publications and source records attributed to Sixt, M..

At least 19 recordsLinked to original sources

bioRxiv2025

Dynamic protrusions mediate unique crawling motility in Asgard Archaea (Promethearchaeota)

Crawling motility is a hallmark of eukaryotic cells and requires a dynamic actin cytoskeleton, regulated adhesion, and spatially organized signalling pathways1-3. Asgard archaea (phylum Promethearchaeota) which are considered the closest known prokaryotic relatives of eukaryotes potentially encode these functions within their large set of eukaryotic signature proteins4-9. The few cultivated members show a complex cell morphology, consisting of a central cell body from which several protrusions extend, filled with an actin-based cytoskeleton10,11. Here, live cell microscopy of two organisms of the Loki- and Hodarchaea lineages10,12 showed that they dynamically and drastically change their cell shape on a minute time scale and grow and retract their extensive protrusions with a speed of 1.5 to 5.3 {micro}m/min, respectively. After adhering to a glass surface, cells employ their protrusions to undergo active crawling motion. In the presence of selected actin inhibitors however, the observed dynamics were arrested, suggesting a central role of actin in these processes. The observed cellular plasticity and motility are unique features among prokaryotes and might have been crucial for the emergence of the first eukaryotic cells that are thought to have formed through the association of a member of the Promethearchaeota and an alphaproteobacterium, the ancestor of mitochondria.

microbiology↗

bioRxiv2025

Chemotaxis of branched cells in complex environments

Cell migration in vivo is often guided by chemical signals. Such chemotaxis, such as performed by immune cells migrating to a wound site, is complicated by the complex geometry inside living tissues. In this study, we extend our theoretical model of branched-cell migration on a network by introducing chemokine sources to explore the cellular response. The model predicts a speed-accuracy tradeoff, whereby slow cells are significantly more accurate and able to follow efficiently a weak chemoattractant signal. We then compare the models predictions with experimental observations of neutrophils migrating to the site of laser-inflicted wound in a zebrafish larva fin, and migrating in-vitro inside a regular lattice of pillars. We find that the model captures the details of the sub-cellular response to the chemokine gradient, as well as the large-scale migration response. This comparison suggests that the neutrophils behave as fast cells, compromising their chemotaxis accuracy, which explains the functionality of these immune cells.

biophysics↗

bioRxiv2025

Substrate heterogeneity promotes cancer cell dissemination

While tumor malignancy has been extensively studied under the prism of genetic and epigenetic heterogeneity, tumor cell states also critically depend on reciprocal interactions with the microenvironment. This raises the hitherto untested possibility that heterogeneity of the untransformed tumor stroma can actively fuel malignant progression. As biological heterogeneity is inherently difficult to control, we adopted a reductionist approach and let tumor cells invade micro-engineered environments harboring obstacles with precision-controlled geometry. We find that not only the presence of obstacles, but more surprisingly their spatial disorder, causes a drastic shift from a collective to a single-cell mode of invasion - comparable in strength to cadherin loss. Combining live-imaging and perturbation experiments with minimal biophysical modeling, we demonstrate that cell detachments result both from local geometrical constraints and a global integration of spatial disorder over time. We show that different types of microenvironments map onto different universality classes of invasion dynamics - homogeneous substrates follow Kardar-Parisi-Zhang (KPZ) scaling, while disordered ones exhibit exponents consistent with KPZ with quenched disorder (KPZq). Our findings highlight generic physical principles for how the mode of cancer cell invasion depends on environmental heterogeneity, with potential implications to understand tumor evolution in vivo.

biophysics↗

bioRxiv2025

Edited Filamin A in myeloid cells reduces intestinal inflammation and protects from colitis

Patho-mechanistic origins and disease dynamics of ulcerative colitis are still poorly understood. The actin-crosslinker Filamin A (FLNA) impacts cellular responses through interaction with cytosolic proteins. FLNA exists in two forms that differ only in one amino acid: genome-encoded FLNAQ and FLNAR - generated by post-transcriptional A-to-I editing. FLNA is edited in fibroblasts, smooth muscle- and endothelial cells in the colon. We identified the FLNA editing status as a key determinant of colitis severity. FLNA editing was highest in healthy colons and reduced during acute murine and human colitis. Mice that exclusively express edited FLNAR and do not downregulate editing upon challenge were highly resistant to DSS-induced colitis, whereas fully unedited FLNAQ animals developed severe inflammation. While the genetic induction of FLNA editing influenced transcriptional states of structural cells and the microbiome composition, we found that FLNAR exerts protection specifically via its influence on myeloid cells, which are not edited under physiological conditions. Introducing fixed, fully edited FLNAR did not hamper normal cell migration but reduced macrophage inflammation and rendered neutrophils less prone to NETosis. In conclusion, loss of FLNA editing correlates with colitis severity, and targeted FLNA editing of myeloid cells might serve as a novel therapeutic approach in intestinal inflammation. SummaryIn this study, Gawish et al. show that RNA editing of the actin cross-linker FLNA is similarly regulated in mice and humans and that the targeted induction of edited FLNAR in myeloid cells governs resistance to DSS-induced colitis, revealing its potential in IBD therapy.

molecular biology↗

bioRxiv2024

Self-generated chemotaxis of mixed cell populations

Cell and tissue movement in development, cancer invasion, and immune response relies on chemical or mechanical guidance cues. In many systems, this behavior is locally guided by self-generated signaling gradients rather than long-range, pre-patterned cues. However, how heterogeneous mixtures of cells interact non-reciprocally and navigate through self-generated gradients remains largely unexplored. Here we introduce a theoretical framework for the self-organized chemotaxis of heterogeneous cell populations. We find that relative chemotactic sensitivity of cell populations controls their long-time coupling and co-migration dynamics, with boundary conditions such as external cell- and attractant reservoirs substantially influencing the migration patterns. We further predict an optimal parameter regime that leads to robust and colocalized migration. We test our theoretical predictions with in vitro experiments demonstrating the co-migration of different immune cell populations, and quantitatively reproduce observed migration patterns under wild-type and perturbed conditions. Interestingly, immune cell co-migration appears close to the predicted optimal regime. Finally, we incorporate mechanical interactions into our framework, revealing a phase diagram that illustrates non-trivial interplays between chemotactic and mechanical non-reciprocity in driving collective migration. Together, our findings suggest that self-generated chemotaxis is a robust strategy for emergent multicellular navigation.

biophysics↗

bioRxiv2024

Global coordination of protrusive forces in migrating immune cells

Efficient immune responses rely on the capacity of leukocytes to traverse diverse and complex tissues. To meet such changing environmental conditions, leukocytes usually adopt an amoeboid configuration, utilizing their forward-positioned nucleus as a probe to identify and follow the path of least resistance among pre-existing pores. We show that in dense environments, where even the largest pores preclude free passage, leukocytes switch polarity and position their nucleus behind centrosome and organelles. In this mesenchymal configuration, local compression of the cell body triggers assembly of a central F-actin pool, located between cell front and nucleus. Central actin pushes outward to transiently dilate a path for organelles and nucleus. Pools of central and front actin are tightly coupled and experimental depletion of the central pool enhances actin accumulation and protrusion formation at the cell front. Although this shifted balance speeds up cells in permissive environments, migration in restrictive environments is impaired, as the unleashed leading edge dissociates from the trapped cell body. Our findings establish an actin regulatory loop that balances path dilation with advancement of the leading edge to maintain cellular coherence.

cell biology↗

bioRxiv2023

Unveiling the ultrastructural landscape of native extracellular matrix via lift-out cryo-FIBSEM and cryo-ET

The extracellular matrix (ECM) is a highly hydrated, three-dimensional network composed of various macromolecules and signaling factors. It serves as a structural scaffold for cells and plays an essential role in the regulation of numerous cellular processes, including cell migration, adhesion, and proliferation. Despite its importance in metazoans, structural knowledge is rudimentary on how the components of the matrisome are secreted, remodeled, and interact with each other and with surrounding cells. Specifically, the exact molecular assembly of important ECM fibers, such as fibronectin fibrils, fibrillin microfibrils, or Collagen-VI filaments has remained enigmatic. This is largely due to methodological limitations in specimen preparation for conventional room-temperature electron microscopy (EM). To overcome these limitations, we have developed a cell culture-based 3D-ECM platform compatible with sample thinning by cryo-lift out focused ion beam (FIB) milling and cryo-electron tomography (cryo-ET). Our workflow involves the implementation of cell-derived matrices (CDMs) grown on EM grids, resulting in a highly adaptable and versatile tool to closely mimic ECM environments. This allows us to visualize native ECM and its components for the first time in their fully hydrated, cellular context. Our data reveals an intricate network of ECM fibers and their positioning with respect to matrix-secreting cells. In addition to D-spaced collagen fibers, we visualize previously unresolved fibrous structures, and an amorphous matrix co-assembling in proximity to ECM fibers and delineating the boundary between ECM and empty extra-cellular space. Intra- and extracellular granules presumably represent assembly intermediates of the ECM. Our results add to the structural atlas of the ECM and provide novel insights into ECM secretion, assembly and maintenance.

cell biology↗

bioRxiv2022

Synchronization in collectively moving inanimate and living active matter

Regardless of whether one considers swarming insects, flocking birds, or bacterial colonies, collective motion arises from the coordination of individuals and entails the adjustment of their respective velocities. In particular, in close confinement, such as those encountered by dense cell populations during development or regeneration, collective migration can only arise coordinately. Yet, how individuals unify their velocities is often not understood. Focusing on a finite number of cells in circular confinements, we identify waves of polymerizing actin that function as a pacemaker governing the speed of individual cells. We show that the onset of collective motion coincides with the synchronization of the wave nucleation frequencies across the population. Employing a simpler and more readily accessible mechanical model system of active spheres, we identify the essential requirements to reach the corresponding collective state, i.e. the synchronization of the individuals internal oscillators. The mechanical toy experiment illustrates that the global synchronous state is achieved by nearest neighbor coupling. We suggest by analogy that local coupling and the synchronization of actin waves are essential for emergent, self-organized motion of cell collectives.

biophysics↗

bioRxiv2022

Plasmacytoid dendritic cells regulate megakaryocyte and platelet homeostasis

Platelet homeostasis is essential for vascular integrity and immune defense. While the process of platelet formation by fragmenting megakaryocytes (thrombopoiesis) has been extensively studied, the cellular and molecular mechanisms required to constantly replenish the pool of megakaryocytes by their progenitor cells (megakaryopoiesis) remains unclear. Here we use intravital 2 photon microscopy to track individual megakaryopoiesis over days. We identify plasmacytoid dendritic cells (pDCs) as crucial bone marrow niche cells that regulate megakaryopoiesis. pDCs monitor the bone marrow for platelet-producing megakaryocytes and deliver IFN- to the megakaryocytic niche to trigger local on-demand proliferation of megakaryocyte progenitors. This fine-tuned coordination between thrombopoiesis and megakaryopoiesis is crucial for megakaryocyte and platelet homeostasis in steady state and stress. However, uncontrolled pDC function within the megakaryocytic niche is detrimental. Accordingly, we show that pDCs activated by SARS-CoV2 drive inappropriate megakaryopoiesis associated with thrombotic complications. Together, we uncover a hitherto unknown megakaryocytic bone marrow niche maintained by the constitutive delivery of pDC-derived IFN-.

immunology↗

bioRxiv2022

Sinking the way: a dual role for CCR7 in collective leukocyte migration

Immune responses crucially rely on the rapid and coordinated locomotion of leukocytes. While it is well established that single-cell migration is often guided by gradients of chemokines and other chemoattractants, it remains poorly understood how such gradients are generated, maintained and modulated. Combining experiment and theory on leukocyte chemotaxis guided by the G protein-coupled receptor (GPCR) CCR7, we demonstrate that in addition to its role as the sensory receptor that steers migration, CCR7 also acts as a generator and modulator of chemotactic gradients. Upon exposure to the CCR7 ligand CCL19, dendritic cells (DCs) effectively internalize the receptor and ligand as part of the canonical GPCR-desensitization response. We show that CCR7 internalization also acts as an effective sink for the chemoattractant, thereby dynamically shaping the spatio-temporal distribution of the chemokine. This mechanism drives complex collective migration patterns, enabling DCs to create or sharpen chemotactic gradients. We further show that these self-generated gradients can sustain the long-range guidance of DCs, adapt collective migration patterns to the size and geometry of the environment, as well as provide a guidance cue for other co-migrating cells. Such dual role of CCR7 as a GPCR that both senses and consumes its ligand can thus provide a novel mode of cellular self-organization.

cell biology↗

bioRxiv2021

Type 1 piliated uropathogenic Escherichia coli hijack the host immune response by binding to CD14

A key attribute of persistent or recurring bacterial infections is the ability of the pathogen to evade the hosts immune response. Many Enterobacteriaceae express type 1 pili, a pre-adapted virulence trait, to invade host epithelial cells and establish persistent infections. However, the molecular mechanisms and strategies by which bacteria actively circumvent the immune response of the host remain poorly understood. Here, we identified CD14, the major co-receptor for lipopolysaccharide detection, on dendritic cells as a previously undescribed binding partner of FimH, the protein located at the tip of the type 1 pilus of Escherichia coli. The FimH amino acids involved in CD14 binding are highly conserved across pathogenic and non-pathogenic strains. Binding of pathogenic bacteria to CD14 lead to reduced dendritic cell migration and blunted expression of co-stimulatory molecules, both rate-limiting factors of T cell activation. While defining an active molecular mechanism of immune evasion by pathogens, the interaction between FimH and CD14 represents a potential target to interfere with persistent and recurrent infections, such as urinary tract infections or Crohns disease.

immunology↗

bioRxiv2021

Multi-tier mechanics control stromal adaptations in swelling lymph nodes

Lymph nodes (LNs) comprise two main structural elements: Fibroblastic reticular cells (FRCs) that form dedicated niches for immune cell interaction and capsular fibroblasts that build a shell around the organ. While LNs are fairly stable in size during homeostatic conditions, immunological challenge causes more than 10-fold increase in size within only a few days. How a solid organ can accommodate such extreme volumetric changes is poorly understood. Here, we characterize the biomechanics of LN swelling on the cellular and organ scale. We identify lymphocyte trapping by influx and proliferation as drivers of an outward pressure force, causing FRCs and their associated conduits to stretch. After an initial phase of relaxation, FRCs sense the resulting strain via cell matrix adhesions, which coordinates local growth and remodeling of the stromal network. While the expanded FRC network adopts its typical configuration, a massive fibrotic reaction of the organ capsule sets in and counters further organ expansion. Thus, different fibroblast populations mechanically control LN swelling in a multi-tier fashion.

immunology↗

bioRxiv2021

Mechanosensitivity of amoeboid cells crawling in 3D

Efficient immune-responses require migrating leukocytes to be in the right place at the right time. When crawling through the body amoeboid leukocytes must traverse complex three-dimensional tissue-landscapes obstructed by extracellular matrix and other cells, raising the question how motile cells adapt to mechanical loads to overcome these obstacles. Here we reveal the spatio-temporal configuration of cortical actin-networks rendering amoeboid cells mechanosensitive in three-dimensions, independent of adhesive interactions with the microenvironment. In response to compression, Wiskott-Aldrich syndrom protein (WASp) assembles into dot-like structures acting as nucleation sites for actin spikes that in turn push against the external load. High precision targeting of WASp to objects as delicate as collagen fibers allows the cell to locally and instantaneously deform its viscoelastic surrounding in order to generate space for forward locomotion. Such pushing forces are essential for fast and directed leukocyte migration in fibrous and cell-packed tissues such as skin and lymph nodes. In BriefWASp-driven actin spikes counter compressive loads of crowded tissue-landscapes.

cell biology↗

bioRxiv2021

Curvature sensing steers actin-driven cell migration

Cell migration is a hallmark out-of-equilibrium process in biology. In addition to persistent self-propelled motion, many cells display remarkable adaptive behaviors when they navigate complex environments within the body. Combining theory and experiments, we identify a curvature-sensing mechanism underlying obstacle avoidance in immune-like cells. The genetic perturbation of this machinery leads to a reduced capacity to evade obstructions combined with faster and more persistent cell migration in obstacle-free environments. We propose that the active polymerization of the actin cytoskeleton at the advancing edge of migrating cells is locally inhibited by the curvature-sensitive BAR protein Snx33 in regions with inward plasma membrane curvature. This coupling between actin and membrane dynamics leads to a mechanochemical instability that generates complex protrusive patterns at the cellular front. Adaptive motility thus arises from two simultaneous curvature-dependent effects, i) the specific reduction of propulsion in regions where external objects deform the plasma membrane and ii) the intrinsic patterning capacity due to the membrane-actin coupling that promotes spontaneous changes in the cells protrusions. Our results show how cells utilize actin- and plasma membrane biophysics to sense their environment, allowing them to adaptively decide if they should move ahead or turn away. On the basis of our findings, we propose that the natural diversity of BAR proteins may allow cells to tune their curvature sensing machinery to match the shape characteristics in their environment.

cell biology↗

bioRxiv2020

Dendritic cell actin dynamics controls T cell priming efficiency at the immunological synapse

Dendritic cells (DCs) are crucial for the priming of naive T cells and the initiation of adaptive immunity. Priming is initiated at a heterologous cell-cell contact, the immunological synapse (IS). While it is established that actin dynamics regulates signalling at the T cell side of the contact, little is known about the cytoskeletal contribution on the DC side. We show that that the DC cytoskeleton is decisive for the formation of a multifocal synaptic structure, which correlates with T cell priming efficiency. We demonstrate that DC actin appears in transient foci at the IS and that these foci are dynamized by the WAVE complex. Absence of WAVE in DCs leads to stabilized contacts with T cells, caused by an increase in ICAM1-integrin mediated cell-cell adhesions. This results in a lower number of activated and proliferating T cells. Our results reveal an important role of DC actin in the regulation of synaptic contacts with crucial relevance for full T cell expansion.

cell biology↗

bioRxiv2020

Loss of Hem1 disrupts macrophage function and impacts on migration, phagocytosis and integrin-mediated adhesion

The hematopoietic-specific protein 1 (Hem1) comprises an essential subunit of the WAVE Regulatory Complex (WRC) in immune cells. WRC has a fundamental role in Arp2/3 complex activation and the protrusion of branched actin networks in motile cells. Hem1 deficiency leads to suppression of the entire WRC in immune cells. Defective WRC function in macrophages results in loss of lamellipodia and migration defects. Moreover, phagocytosis, commonly accompanied by lamellipodium protrusion during cup formation, is altered in Hem1 null cells concerning frequency and efficacy. When analyzing cell spreading, adhesion and podosome formation, we found that Hem1 null cells are capable, in principle, of podosome formation and consequently, do not show any quantitative differences in extracellular matrix degradation. Their adhesive behavior, however, was significantly altered. Specifically, adhesion as well as de-adhesion of Hem1 null cells was strongly compromised, likely contributing to the observed reduced efficiency of phagocytosis. In line with this, phosphorylation of the prominent adhesion component paxillin was diminished. Non-hematopoietic somatic cells disrupted in expression for both Hem1 and its ubiquitous orthologue Nck-associated protein 1 (Nap1) or the essential WRC components Sra-1/PIR121 did not only confirm defective paxillin phosphorylation, but also revealed that paxillin turnover in focal adhesions is accelerated in the absence of WRC. Finally, adhesion assays using platelets lacking functional WRC as model system unmasked radically decreased IIb{beta}3 integrin activation. Our results thus demonstrate that WRC-driven actin networks impact on integrin-dependent processes controlling formation and dismantling of different types of cell-substratum adhesion. One sentence summaryInterference of Hem1 function in mice and cells uncovers a hitherto unrecognized role in integrin-mediated cell adhesion that is crucial for macrophage function and connects to recently discovered immunodeficiencies in patients carrying Hem1 mutations.

cell biology↗

bioRxiv2020

Loss of Ena/VASP interferes with lamellipodium architecture, motility and integrin-dependent adhesion

Cell migration entails networks and bundles of actin filaments termed lamellipodia and microspikes or filopodia, respectively, as well as focal adhesions, all of which recruit Ena/VASP family members hitherto thought to antagonize efficient cell motility. However, we find these proteins to act as positive regulators of migration in different cell lines. CRISPR/Cas9-mediated loss of Ena/VASP proteins reduced lamellipodial actin assembly and perturbed lamellipodial architecture, as evidenced by changed network geometry as well as reduction of filament length and number that was accompanied by abnormal Arp2/3 complex and heterodimeric capping protein accumulation. Loss of Ena/VASP function also abolished the formation of microspikes normally embedded in lamellipodia, but not of filopodia capable of emanating without lamellipodia. Ena/VASP-deficiency also impaired integrin-mediated adhesion accompanied by reduced traction forces exerted through these structures. Our data thus uncover novel, cellular Ena/VASP functions of these actin polymerases that are fully consistent with their promotion of cell migration.

cell biology↗

bioRxiv2019

Adhesion-free cell migration by topography-based force transduction

Eukaryotic cells migrate by coupling the intracellular force of the actin cytoskeleton to the environment. While force-coupling is usually mediated by transmembrane adhesion receptors, especially these of the integrin family, amoeboid cells like leukocytes can migrate extremely fast despite very low adhesive forces1. We show that leukocytes cannot only migrate under low adhesion but indeed can transduce forces in the complete absence of transmembrane force coupling. When confined within three-dimensional environments, they use the topographic features of the substrate to propel themselves. Here, the retrograde flow of the actin cytoskeleton follows the texture of the substrate, creating shear forces sufficient to drive deformations towards the back of the cell. Notably, adhesion dependent and adhesion independent migration are not exclusive but rather variants of the same principle of coupling retrograde actin flow to the environment and thus can potentially operate simultaneously. As adhesion free migration is independent of the chemical composition of the environment it renders cells completely autonomous in their locomotive behavior.

cell biology↗