Biology subjects

Tyska, M. J.

Publications and source records attributed to Tyska, M. J..

At least 19 recordsLinked to original sources

bioRxiv2026

IRTKS promotes Tir membrane insertion for intimate bacterial attachment and subsequent pedestal formation

Enterohemorrhagic Escherichia coli (EHEC) is a foodborne pathogen that causes bloody diarrhea and hemolytic uremic syndrome by disrupting the intestinal brush border. During infection, EHEC injects the transmembrane virulence factor Tir into enterocytes; upon insertion into the apical membrane, this factor mediates bacterial attachment and drives formation of actin-rich pedestals needed for colonization. How Tir is inserted into the host plasma membrane remains unclear. Here, we investigated the role of brush border resident IRTKS, a Tir- and membrane-binding protein, in this process. Using multiple IRTKS gain- and loss-of-function models, we analyzed pedestal organization and component localization. Whereas canonical models position IRTKS downstream of Tir as a scaffolding link to F-actin, we found that perturbing IRTKS disrupted the distribution and abundance of Tir. Moreover, ectopic IRTKS expression enhanced Tir membrane insertion in the absence of other virulence factors. We conclude that IRTKS functions early in pedestal formation to promote Tir accumulation in the plasma membrane and in turn, facilitate bacterial attachment. SUMMARYEHEC attaches to intestinal epithelial cells using injected virulence factor Tir, which forms actin pedestals and promotes bacterial colonization. We found that host protein IRTKS promotes Tir accumulation in the plasma membrane, to facilitate intimate bacterial attachment and pedestal formation.

cell biology↗

bioRxiv2026

EPS8 dampens the growth dynamics and prolongs the lifetime of actin-based protrusions

Actin-based membrane protrusions such as filopodia, microvilli, and stereocilia support a range of cell functions, from nutrient absorption to mechanosensation. In each case, membrane deformation is supported by a core bundle of actin filaments, organized in a unipolar barbed-end out manner. Although their structures and proteomes are well characterized, mechanisms governing the growth and stability of these protrusions remain less clear. Factors that localize to the distal tips of these structures are of particular interest, as they are well positioned to control actin assembly at filament barbed ends. One such factor, EPS8, localizes to distal tip puncta in multiple protrusion types. While early biochemical studies suggested a role in filament capping, loss of EPS8 in multiple models shortened microvilli and stereocilia, suggesting roles in elongation. More recent studies in differentiating epithelial cells suggested that EPS8 promotes protrusion growth and stability. To clarify EPS8s function in the distal tip compartment, we leveraged acute loss-of-function experiments and titrated gain-of-function approaches in combination with live imaging. Acute sequestration of EPS8 led to rapid depletion of filopodia. Conversely, increasing cellular EPS8 levels elevated EPS8 per distal tip punctum, increased F-actin content within individual filopodia, reduced filopodia elongation rates, increased protrusion lifetimes, and protected filopodia against cytochalasin D-induced collapse. These findings suggest that EPS8 binds filament barbed ends as a leaky capper, slowing monomer addition while stabilizing bundles and preventing collapse. These activities are likely critical for building and maintaining the large arrays of protrusions that are assembled by diverse epithelial cell types.

cell biology↗

bioRxiv2025

Brush border intermicrovillar adhesion limits bacterial attachment to the small intestine brush border

Enterocytes present a continuous array of uniform, tightly packed, apical microvilli known as the "brush border". Adjacent microvilli tips are linked by an intermicrovillar adhesion complex (IMAC) composed of cadherins that are required for microvillar packing, length uniformity, and junctional integrity, with disruption linked to intestinal autoimmune disease and infections. We found that IMAC-deficient mice have dysbiosis with increased adherent (actin-dependent) and mucosal (actin-independent) bacteria in the terminal ileum. The bacteria are primarily commensals without differences in diversity between groups. Primary segments of Segmented filamentous bacteria (SFB) recruit the microvilli-associated proteins EPS8 and IRTKS, as well as the Arp2/3 complex, to remodel actin. SFB not only anchor adjacent to epithelial cell junctions but also incorporate ZO-1 into their attachment structures. Together, these data reveal maturation of the SFB-host interface that mirrors the complex SFB lifecycle. Our work indicates a previously unrecognized role for the brush border as a key component of host defense against luminal microbes.

cell biology↗

bioRxiv2025

Spef1 is a microvillar component that limits apical actomyosin contractility and preserves intestinal barrier function

Epithelial sheet integrity is established by adherent contacts that form between cells, at the interface between their apical and basolateral domains. Although cell contacts are reinforced by actomyosin contractility, which generates tension that propagates across the apical surface. How epithelial cells tune tension to reinforce junctions without compromising their physical barrier properties remains unclear. Herein, we report that Sperm Flagellar 1 (Spef1) is a microvillar component enriched in the apical domain and terminal web of enterocytes that prevents actomyosin hypercontractility. Loss-of-function in Caco-2 BBE cells showed that Spef1 depletion induced invaginations of the apical domain at tricellular contacts, with a redistribution of the tricellular and tight junction components. These changes, which were paralleled by increased activity of NM2A across the apical surface, compromised intestinal barrier function. These findings highlight Spef1 as a microvillar resident that tunes actomyosin contractility across the apical surface, to a level appropriate for junctional reinforcement and maintenance of epithelial function.

cell biology↗

bioRxiv2024

A non-muscle α-actinin is an intrinsic component of the cardiac Z-disc and regulates sarcomere turnover, contractility, and heart remodeling

Cardiac sarcomeres generate the fundamental forces behind each heartbeat and are thought to contain only muscle-specific cytoskeletal proteins. We show that a widely expressed actin cross-linking protein, -actinin 4 (ACTN4), is a sarcomere component of the human and zebrafish heart in vivo and in human iPSC-derived cardiac myocytes (CMs) in vitro. A confluence of biochemical experiments, immunofluorescence, and AI modeling suggest ACTN4 forms a heterodimeric complex with muscle-specific ACTN2 at the cardiac Z-disc, the cardiac sarcomere border. ACTN4 depletion from human iPSC-CMs stabilizes canonical sarcomere proteins and drives contractility-dependent cellular hypertrophy while ACTN4 overexpression destabilizes sarcomeres. ACTN4 depletion from zebrafish embryos specifically increases ventricular contractility which drives atrial enlargement, suggesting biomechanically driven atrial remodeling. ACTN4-associated phenotypes in both model systems lack hallmarks of cardiac disease models and an ACTN4 variant in humans is associated with reduced risk for disease. Our findings suggest a "non-muscle" actinin regulates heart contractility and influences clinical outcomes related to heart failure.

cell biology↗

bioRxiv2024

Apical clathrin-coated endocytic pits control the growth and size of epithelial microvilli

Epithelial cells from diverse contexts assemble apical specializations to serve tissue-specific functions. In virtually all cases, these features consist of arrays of microvilli: micron-scale, actin bundle-supported protrusions that mediate biochemical and physical interactions with the external environment. Despite their importance for epithelial physiology, how microvilli grow during cellular differentiation remains poorly understood. Using genetic and small molecule perturbations, we found that an epithelial cells potential for growing microvilli of normal size is limited by an adjacent actin-dependent process: apical clathrin-mediated endocytosis. Unexpectedly, timelapse imaging of individual microvillar growth events revealed tight spatial and temporal coupling to sites of clathrin-mediated endocytosis. Ultrastructural characterization of undifferentiated epithelial monolayers also showed that most nascent microvilli are in contact with an apical endocytic pit. Finally, inhibition of the Arp2/3 branched nucleation complex, which drives actin polymerization on coated pits, significantly reduced the accumulation of new microvilli on the surface of differentiating epithelial cells. Based on these discoveries, we conclude that clathrin-mediated endocytosis and its associated Arp2/3-based actin nucleation activity control the timing and location of microvillar growth, as well as the dimensions of the resulting protrusions.

cell biology↗

bioRxiv2024

Molecular counting of myosin force generators in growing filopodia

Animal cells build actin-based surface protrusions to enable biological activities ranging from cell motility to mechanosensation to solute uptake. Long-standing models of protrusion growth suggest that actin filament polymerization provides the primary mechanical force for "pushing" the plasma membrane outward at the distal tip. Expanding on these actin-centric models, our recent studies used a chemically inducible system to establish that plasma membrane-bound myosin motors, which are abundant in protrusions and accumulate at the distal tips, can also power robust filopodial growth. How protrusion resident myosins coordinate with actin polymerization to drive elongation remains unclear, in part because the number of force generators and thus, the scale of their mechanical contributions remain undefined. To address this gap, we leveraged the SunTag system to count membrane-bound myosin motors in actively growing filopodia. Using this approach, we found that the number of myosins is log-normally distributed with a mean of 12.0 {+/-} 2.5 motors [GeoMean {+/-} GeoSD] per filopodium. Together with unitary force values and duty ratio estimates derived from biophysical studies for the motor used in these experiments, we calculate that a distal tip population of myosins could generate a time averaged force of [~]tens of pN to elongate filopodia. This range is comparable to the expected force production of actin polymerization in this system, a point that necessitates revision of popular physical models for protrusion growth. SIGNIFICANCE STATEMENTThis study describes the results of in-cell molecular counting experiments to define the number of myosin motors that are mechanically active in growing filopodia. This data should be used to constrain future physical models of the formation of actin-based protrusions.

cell biology↗

bioRxiv2024

Loss of intermicrovillar adhesion impairs basolateral junctional complexes in transporting epithelia

Transporting epithelial cells in the gut and kidney rely on protocadherin-based apical adhesion complexes to organize microvilli that extend into the luminal space. In these systems, CDHR2 and CDHR5 localize to the distal ends of microvilli, where they form an intermicrovillar adhesion complex (IMAC) that links the tips of these structures, promotes the formation of a well-ordered array of protrusions, and in turn maximizes apical membrane surface area. Recently, we discovered that IMACs can also form between microvilli that extend from neighboring cells, across cell-cell junctions. As an additional point of physical contact between cells, transjunctional IMACs are well positioned to impact the integrity of canonical tight and adherens junctions that form more basolaterally. Here, we sought to test this idea using cell culture and mouse models that lacked CDHR2 expression and were unable to form IMACs. CDHR2 knockout perturbed cell and junction morphology, led to loss of key components from tight and adherens junctions, and impaired barrier function and wound healing. These results indicate that, in addition to organizing apical microvilli, IMACs provide a layer of cell-cell contact that functions in parallel with canonical tight and adherens junctions to support the physiological functions of transporting epithelia.

cell biology↗

bioRxiv2024

Intestinal tuft cells assemble a cytoskeletal superstructure composed of co-aligned actin bundles and microtubules

Background & AimsAll tissues consist of a distinct set of cell types, which collectively support organ function and homeostasis. Tuft cells are a rare epithelial cell type found in diverse epithelia, where they play important roles in sensing antigens and stimulating downstream immune responses. Exhibiting a unique polarized morphology, tuft cells are defined by an array of giant actin filament bundles that support [~]2 m of apical membrane protrusion and extend over 7 m towards the cells perinuclear region. Despite their established roles in maintaining intestinal epithelial homeostasis, tuft cells remain understudied due to their rarity (e.g. [~] 1% in the small intestinal epithelium). Details regarding the ultrastructural organization of the tuft cell cytoskeleton, the molecular components involved in building the array of giant actin bundles, and how these cytoskeletal structures support tuft cell biology remain unclear. MethodsTo begin to answer these questions, we used advanced light and electron microscopy to perform quantitative morphometry of the small intestinal tuft cell cytoskeleton. ResultsWe found that tuft cell core bundles consist of actin filaments that are crosslinked in a parallel "barbed-end out" configuration. These polarized structures are also supported by a unique group of tuft cell enriched actin-binding proteins that are differentially localized along the giant core bundles. Furthermore, we found that tuft cell actin bundles are co-aligned with a highly ordered network of microtubules. ConclusionsTuft cells assemble a cytoskeletal superstructure that is well positioned to serve as a track for subcellular transport along the apical-basolateral axis and in turn, support the dynamic sensing functions that are critical for intestinal epithelial homeostasis. SYNOPSISThis research leveraged advanced light and electron microscopy to perform quantitative morphometry of the intestinal tuft cell cytoskeleton. Three-dimensional reconstructions of segmented image data revealed a co-aligned actin-microtubule superstructure that may play a fundamental role in tuft cell function.

cell biology↗

bioRxiv2023

Mitotic spindle positioning protein (MISP) is an actin bundler that senses ADP-actin and binds near the pointed ends of filaments

Actin bundling proteins crosslink filaments into polarized structures that shape and support membrane protrusions including filopodia, microvilli, and stereocilia. In the case of epithelial microvilli, mitotic spindle positioning protein (MISP) is an actin bundler that localizes specifically to the basal rootlets, where the pointed ends of core bundle filaments converge. Previous studies established that MISP is prevented from binding more distal segments of the core bundle by competition with other actin binding proteins. Yet whether MISP holds a preference for binding directly to rootlet actin remains an open question. Using in vitro TIRF microscopy assays, we found that MISP exhibits a clear binding preference for filaments enriched in ADP-actin monomers. Consistent with this, assays with actively growing actin filaments revealed that MISP binds at or near their pointed ends. Moreover, although substrate attached MISP assembles filament bundles in parallel and antiparallel configurations, in solution MISP assembles parallel bundles consisting of multiple filaments exhibiting uniform polarity. These discoveries highlight nucleotide state sensing as a mechanism for sorting actin bundlers along filaments and driving their accumulation near filament ends. Such localized binding might drive parallel bundle formation and/or locally modulate bundle mechanical properties in microvilli and related protrusions.

cell biology↗

bioRxiv2023

Adhesion-based capture stabilizes nascent microvilli at epithelial cell junctions

Differentiated transporting epithelial cells present an extensive apical array of microvilli - a "brush border" - where neighboring microvilli are linked together by intermicrovillar adhesion complexes (IMACs) composed of protocadherins CDHR2 and CDHR5. Although loss-of-function studies provide strong evidence that IMAC function is needed to build a mature brush border, how the IMAC contributes to the stabilization and accumulation of nascent microvilli remains unclear. We found that, early in differentiation, the apical surface exhibits a marginal accumulation of microvilli, characterized by higher packing density relative to medial regions of the surface. While medial microvilli are highly dynamic and sample multiple orientations over time, marginal protrusions exhibit constrained motion and maintain a vertical orientation. Unexpectedly, we found that marginal microvilli span the junctional space and contact protrusions on neighboring cells, mediated by complexes of CDHR2/CDHR5. FRAP analysis indicated that these transjunctional IMACs are highly stable relative to adhesion complexes between medial microvilli, which explains the restricted motion of protrusions in the marginal zone. Finally, long-term live imaging revealed that the accumulation of microvilli at cell margins consistently leads to accumulation in medial regions of the cell. Collectively, our findings suggest that nascent microvilli are stabilized by a capture mechanism that is localized to cell margins and enabled by the transjunctional formation of IMACs. These results inform our understanding of how apical specializations are assembled in diverse epithelial systems.

cell biology↗

bioRxiv2022

Protrusion growth driven by myosin-generated force

Actin-based protrusions are found on the surface of all eukaryotic cells, where they support diverse biological activities essential for life. Models of protrusion growth hypothesize that actin filament assembly provides the mechanical force for bending the plasma membrane outward. However, membrane-associated myosin motors are also abundant in protrusions, though their potential for contributing growth-promoting force remains unexplored. Using a novel inducible system that docks myosin motor domains to membrane binding modules with temporal control, we found that the application of myosin-generated force to the plasma membrane is sufficient for driving robust elongation of protrusions. Protrusion growth scaled with motor accumulation, required active, barbed end-directed force, and was independent of cargo delivery or the recruitment of canonical barbed end elongation factors. Application of growth-promoting force was also supported by structurally distinct myosin motor domains and membrane binding modules. We conclude that myosin-generated force can drive protrusion growth and this mechanism is likely active in diverse biological contexts.

cell biology↗

bioRxiv2021

Mitotic Spindle Positioning (MISP) is an actin bundler that selectively stabilizes the rootlets of epithelial microvilli

Microvilli are conserved actin-based surface protrusions that have been repurposed throughout evolution to fulfill diverse cell functions. In the case of transporting epithelia, microvilli are supported by a core of actin filaments bundled in parallel by villin, fimbrin, and espin. Remarkably, microvilli biogenesis persists in mice lacking all three of these factors, suggesting the existence of unknown bundlers. We identified Mitotic Spindle Positioning (MISP) as an actin binding factor that localizes specifically to the rootlet end of the microvillus. MISP promotes rootlet elongation in cells, and purified MISP exhibits potent filament bundling activity in vitro. MISP-bundled filaments also recruit fimbrin, which further elongates and stabilizes bundles. MISP confinement to the rootlet is enforced by ezrin, which prevents decoration of the membrane-wrapped distal end of the core bundle. These discoveries reveal how epithelial cells optimize apical membrane surface area and offer insight on the remarkable robustness of microvilli biogenesis.

cell biology↗

bioRxiv2020

Direct visualization of epithelial microvilli biogenesis

Microvilli are actin bundle supported surface protrusions that play essential roles in diverse epithelial cell functions. To develop our understanding of microvilli biogenesis, we used live imaging to directly visualize protrusion growth at early stages of epithelial differentiation. Time-lapse data revealed that an "initiation complex" enriched in EPS8 and IRTKS appears at future sites of microvillus growth minutes before core actin bundle assembly. Elongation of a new core bundle occurs in parallel with the arrival of EZRIN and plasma membrane encapsulation. In addition to de novo growth, we also observed that new microvilli emerge from pre-existing protrusions. Additionally, we found that new microvilli can also collapse, characterized first by loss of membrane wrapping and Ezrin enrichment, followed by a sharp decrease in distal tip EPS8 and IRTKS. These studies are the first to offer a temporally resolved microvillus growth mechanism and highlight critical factors that drive this process.

cell biology↗

bioRxiv2020

A heterologous in-cell assay for investigating intermicrovillar adhesion complex interactions and function

Solute transporting epithelial cells build arrays of microvilli on their apical surface to increase membrane scaffolding capacity and enhance function potential. In epithelial tissues such as the kidney and gut, microvilli are length-matched and assembled into tightly packed brush borders, which are organized by [~]50 nm thread-like links that form between the distal tips of adjacent protrusions. Composed of protocadherins CDHR2 and CDHR5, adhesion links are stabilized at the tips by a cytoplasmic tripartite module containing the scaffolds USH1C and ANKS4B, and the actin-based motor, MYO7B. As several questions about the formation and function of this intermicrovillar adhesion complex remain open, we devised a system that allows one to study individual binary interactions between specific complex components and MYO7B. Our approach employs a chimeric myosin consisting of the motor domain of MYO10 fused to the cargo-binding tail domain of MYO7B. When expressed in HeLa cells, which do not normally produce adhesion complex proteins, this motor exhibited robust trafficking to the tips of filopodia and was also able to transport individual components to these sites. Unexpectedly, the MYO10/MYO7B chimera was able to deliver CDHR2 and CDHR5 to distal tips in the absence USH1C or ANKS4B. Cells engineered to localize high levels of CDHR2 at filopodial tips acquired inter-filopodial adhesion and exhibited a striking dynamic length matching activity that aligned distal tips over time. These observations reveal a robust adhesion-dependent mechanism for matching the lengths of adjacent surface protrusions, and may offer insight on how epithelial cells minimize microvillar length variability.

cell biology↗

bioRxiv2020

SPECIES-DEPENDENT HETEROPHILIC AND HOMOPHILIC CADHERIN INTERACTIONS IN INTESTINAL INTERMICROVILLAR LINKS

Enterocytes are specialized epithelial cells lining the luminal surface of the small intestine that build densely packed arrays of microvilli known as brush borders. These microvilli drive nutrient absorption and are arranged in a hexagonal pattern maintained by intermicrovillar links formed by two non-classical members of the cadherin superfamily of calcium-dependent cell adhesion proteins: protocadherin-24 (PCDH24, also known as CDHR2) and the mucin-like protocadherin (CDHR5). The extracellular domains of these proteins are involved in heterophilic and homophilic interactions important for intermicrovillar function, yet the structural determinants of these interactions remain unresolved. Here we present X-ray crystal structures of the PCDH24 and CDHR5 extracellular tips and analyze their species-specific features relevant for adhesive interactions. In parallel, we use binding assays to identify the PCDH24 and CDHR5 domains involved in both heterophilic and homophilic adhesion for human and mouse proteins. Our results suggest that homophilic and heterophilic interactions involving PCDH24 and CDHR5 are species dependent with unique and distinct minimal adhesive units.

biochemistry↗

bioRxiv2020

Non-muscle myosin-2 contractility-dependent actin turnover limits the length of epithelial microvilli

Epithelial brush borders are large arrays of microvilli that enable efficient solute uptake from luminal spaces. In the context of the intestinal tract, brush border microvilli drive functions that are critical for physiological homeostasis, including nutrient uptake and host defense. However, cytoskeletal mechanisms that regulate the assembly and morphology of these protrusions are poorly understood. The parallel actin bundles that support microvilli have their pointed-end rootlets anchored in a highly crosslinked filamentous meshwork referred to as the "terminal web". Although classic EM studies revealed complex ultrastructure, the composition, organization, and function of the terminal web remains unclear. Here, we identify non-muscle myosin-2C (NM2C) as a major component of the brush border terminal web. NM2C is found in a dense, isotropic layer of puncta across the sub-apical domain, which transects the rootlets of microvillar actin bundles. Puncta in this network are separated by [~]210 nm, dimensions that are comparable to the expected size of filaments formed by NM2C. In primary intestinal organoid cultures, the terminal web NM2C network is highly dynamic and exhibits continuous remodeling. Using pharmacological and genetic perturbations to disrupt NM2C activity in cultured intestinal epithelial cells, we found that this motor controls the length of growing microvilli by regulating actin turnover in a manner that requires a fully active motor domain. Our findings answer a decades old question on the function of terminal web myosin and hold broad implications for understanding apical morphogenesis in diverse epithelial systems.

cell biology↗

bioRxiv2020

SETD2 is an actin lysine methyltransferase

SET-domain-containing-2 (SETD2) was identified as the methyltransferase responsible for the histone 3 lysine 36 trimethyl (H3K36me3) mark of the histone code. Most recently, SETD2 has been shown to be a dual-function remodeler that regulates genome stability via methylation of dynamic microtubules during mitosis and cytokinesis. Here we show that actin is a bona fide target for methylation by SETD2 in vitro and in cells. Antibodies against the SETD2 trimethyl lysine epitope recognize methylated actin, with this methyl mark localizing to areas of active actin cytoskeleton reorganization in migrating cells. Disruption of this methylation activity causes defects in actin polymerization and impairs collective cell migration. Together, these data identify SETD2 as a multifunctional cytoskeletal remodeler regulating methylation and polymerization of actin filaments, and provide new avenues for understanding how defects in SETD2 drive disease via aberrant cytoskeletal methylation.

cell biology↗