Biology subjects

Wittmann, T.

Publications and source records attributed to Wittmann, T..

11 recordsLinked to original sources

bioRxiv2026

JCAD couples tight junction condensates to actin and RhoA to maintain the endothelial barrier

How endothelial cell-cell junctions integrate cytoskeletal, adhesive, and local signaling networks to maintain vascular barrier integrity remains incompletely defined. Here, we identify junctional cadherin 5-associated protein (JCAD) as a modular scaffold that organizes endothelial tight junction architecture by coupling junctional condensates to actin and RhoA signaling. Genetic deletion of Jcad in mice does not affect baseline vascular permeability but causes inflammation-dependent barrier hyperpermeability. JCAD depletion in primary human endothelial cells disrupts tight junction continuity and increases paracellular permeability. Mechanistically, JCAD localizes to ZO-1-positive tight junctions independently of VE-cadherin, directly binds filamentous actin, and forms dynamic actin-associated condensates at cell-cell contacts. Structure-function analysis reveals separable domains mediating tight junction targeting and actin binding, establishing a bipartite architecture that distinctly coordinates junctional signaling and cytoskeletal coupling. Together, these findings identify JCAD as a cell-cell adhesion scaffold that integrates the phase-separated tight junction plaque with actin and RhoA-dependent mechanics, enabling endothelial barrier adaptation to inflammatory stress.

cell biology↗

bioRxiv2026

StrataChip: a microphysiological system capturing dynamic keratinocyte fate and mechanical transitions during human epidermal morphogenesis

Epidermal development and homeostasis require precise coordination between keratinocyte differentiation and mechanics. Still, the mechanisms integrating these processes remain poorly understood in part due to limitations of existing experimental systems. Here, we introduce StrataChip, a tractable microphysiological system that enables dynamic, multimodal interrogation of human epidermal morphogenesis. The platform integrates a media perfused dermal tissue with human epidermal keratinocytes within a microfluidic device and supports rapid epidermal stratification following establishment of an air-liquid interface. High-resolution confocal imaging and single-cell RNA-sequencing demonstrate that the StrataChip recapitulates key architectural and molecular features of human epidermis, including distinct basal, spinous, and granular layers defined by canonical differentiation markers and adhesion molecule organization. Single-cell profiling reveals transcriptionally distinct basal and spinous subpopulations, including transitional states associated with suprabasal commitment. Live 3D imaging in situ captures keratinocyte morphodynamics including basal cell delamination and asymmetric division, linking dynamic cellular behaviors to defined differentiation fates and stratification. Altogether, StrataChip provides a robust platform for a dynamic and mechanistic interrogation of how gene regulation and cell mechanics are coupled during epidermal morphogenesis.

bioengineering↗

bioRxiv2024

Quantitative Comparison of Monomeric StayGold Variants Using Protein Nanocages in Living Cells

To standardize comparison of fluorescent proteins and independently determine which monomeric StayGold variant is best for live microscopy, we analyzed fluorescent protein tagged I3-01 peptides that self-assemble into stable sixty subunit dodecahedrons inside live cells. We find mStayGold is 3-fold brighter and 3-fold more photostable compared with EGFP and superior to other monomeric variants in mammalian cytoplasm. In addition, analysis of intracellular nanocage diffusion confirms the monomeric nature of mStayGold.

cell biology↗

bioRxiv2024

Doublecortin reinforces microtubules to promote growth cone advance in soft environments

Doublecortin (DCX) is a microtubule-associated protein critical for brain development. Although most highly expressed in the developing central nervous system, the molecular function of DCX in neuron morphogenesis remains unknown and controversial. We demonstrate that DCX function is intimately linked to its microtubule-binding activity. By using human induced pluripotent stem cell (hiPSC)- derived cortical i3Neurons genome engineered to express mEmerald-tagged DCX from the endogenous locus, we find that DCX-MT interactions become highly polarized very early during neuron morphogenesis. DCX becomes enriched only on straight microtubules in advancing growth cones with approximately 120 DCX molecules bound per micrometer of growth cone microtubule. At a similar saturation, microtubule-bound DCX molecules begin to impede lysosome transport, and thus can potentially control growth cone organelle entry. In addition, by comparing control, DCX-mEmerald and knockout DCX -/Y i3Neurons, we find that DCX stabilizes microtubules in the growth cone peripheral domain by reducing the microtubule catastrophe frequency and the depolymerization rate. DCX -/Y i3Neuron morphogenesis was inhibited in soft microenvironments that mimic the viscoelasticity of brain tissue and DCX -/Y neurites failed to grow toward brain-derived neurotrophic factor (BDNF) gradients. Together with high resolution traction force microscopy data, we propose a model in which DCX-decorated, rigid growth cone microtubules provide intracellular mechanical resistance to actomyosin generated contractile forces in soft physiological environments in which weak and transient adhesion-mediated forces in the growth cone periphery may be insufficient for productive growth cone advance. These data provide a new mechanistic understanding of how DCX mutations cause lissencephaly-spectrum brain malformations by impacting growth cone dynamics during neuron morphogenesis in physiological environments.

cell biology↗

bioRxiv2024

Protein-protein interactions with G3BPs drive stress granule condensation and gene expression changes under cellular stress

Stress granules (SGs) are macromolecular assemblies that form under cellular stress. Formation of these condensates is driven by the condensation of RNA and RNA-binding proteins such as G3BPs. G3BPs condense into SGs following stress-induced translational arrest. Three G3BP paralogs (G3BP1, G3BP2A, and G3BP2B) have been identified in vertebrates. However, the contribution of different G3BP paralogs to stress granule formation and stress-induced gene expression changes is incompletely understood. Here, we identified key residues for G3BP condensation such as V11. This conserved amino acid is required for formation of the G3BP-Caprin-1 complex, hence promoting SG assembly. Total RNA sequencing and ribosome profiling revealed that disruption of G3BP condensation corresponds to changes in mRNA levels and ribosome engagement during the integrated stress response (ISR). Moreover, we found that G3BP2B preferentially condenses and promotes changes in mRNA expression under endoplasmic reticulum (ER) stress. Together, this work suggests that stress granule assembly promotes changes in gene expression under cellular stress, which is differentially regulated by G3BP paralogs.

cell biology↗

bioRxiv2022

Growth cone advance requires EB1 as revealed by genomic replacement with a light-sensitive variant

A challenge in analyzing dynamic intracellular cell biological processes is the dearth of methodologies that are sufficiently fast and specific to perturb intracellular protein activities. We previously developed a light-sensitive variant of the microtubule plus end tracking protein EB1 by inserting a blue light-controlled protein dimerization module between functional domains. Here we describe an advanced method to replace endogenous EB1 with this light-sensitive variant in a single genome editing step enabling this approach in human induced pluripotent stem cells and derived neurons. We demonstrate that acute and local optogenetic EB1 inactivation in developing cortical neurons induces microtubule depolymerization in the growth cone periphery and subsequent neurite retraction. In addition, advancing growth cones are repelled from areas of blue light exposure. These phenotypes were independent of the neuronal EB1 homolog EB3, revealing a direct dynamic role of EB1-mediated microtubule plus end interactions in neuron morphogenesis and neurite guidance.

cell biology↗

bioRxiv2021

Optogenetic EB1 inactivation shortens metaphase spindles by disrupting cortical force-producing interactions with astral microtubules

Chromosome segregation is accomplished by the mitotic spindle, a bipolar micromachine built primarily from microtubules. Different microtubule populations contribute to spindle function: Kinetochore microtubules attach and transmit forces to chromosomes, antiparallel interpolar microtubules support spindle structure, and astral microtubules connect spindle poles to the cell cortex [1,2]. In mammalian cells, End Binding (EB) proteins associate with all growing microtubule plus ends throughout the cell cycle and serve as adaptors for a diverse group of +TIPs that control microtubule dynamics and interactions with other intracellular structures [3]. Because binding of many +TIPs to EB1 and thus microtubule-end association is switched off by mitotic phosphorylation [4-6] the mitotic function of EBs remains poorly understood. To analyze how EB1 and associated +TIPs on different spindle microtubule populations contribute to mitotic spindle dynamics, we use a light sensitive EB1 variant, {pi}-EB1, that allows local, acute and reversible inactivation of +TIP association with growing microtubule ends in live cells [7]. We find that acute {pi}-EB1 photoinactivation results in rapid and reversible metaphase spindle shortening and transient relaxation of tension across the central spindle. However, in contrast to interphase, {pi}-EB1 photoinactivation does not inhibit microtubule growth in metaphase, but instead increases astral microtubule length and number. Yet, in the absence of EB1 activity astral microtubules fail to engage the cortical dynein/dynactin machinery and spindle poles move away from regions of {pi}-EB1 photoinactivation. In conclusion, our optogenetic approach reveals mitotic EB1 functions that remain hidden in genetic experiments likely due to compensatory molecular systems regulating vertebrate spindle dynamics.

cell biology↗

bioRxiv2020

The E3 ligase TRIM1 ubiquitinates LRRK2 and controls its localization, degradation, and toxicity

Missense mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinsons Disease (PD); however, pathways regulating LRRK2 subcellular localization, function, and turnover are not fully defined. We performed quantitative mass spectrometry-based interactome studies to identify 48 novel LRRK2 interactors, including the microtubule-associated E3 ubiquitin ligase TRIM1 (Tripartite Motif Family 1). TRIM1 recruits LRRK2 to the microtubule cytoskeleton for ubiquitination and proteasomal degradation by binding LRRK2911-920, a nine amino acid segment within a flexible interdomain region (LRRK2853-981), which we designate the "Regulatory Loop" (RL). Phosphorylation of LRRK2 Ser910/Ser935 within LRRK2 RL serves as a molecular switch controlling LRRK2s association with cytoplasmic 14-3-3 versus microtubule-bound TRIM1. Association with TRIM1 modulates LRRK2s interaction with Rab29 and prevents upregulation of LRRK2 kinase activity by Rab29 in an E3-ligase-dependent manner. Finally, TRIM1 rescues neurite outgrowth deficits caused by PD-driving mutant LRRK2 G2019S. Our data suggest that TRIM1 is a critical regulator of LRRK2, controlling its degradation, localization, binding partners, kinase activity, and cytotoxicity.

molecular biology↗

bioRxiv2020

pHLARE: A Genetically Encoded Ratiometric Lysosome pH Biosensor

Many lysosome functions are determined by a lumenal pH of ~5.0, including the activity of resident acid-activated hydrolases. Lysosome pH (pHlys) is increased in neurodegenerative disorders and predicted to be decreased in cancers, making it a potential target for therapeutics to limit the progression of these diseases. Accurately measuring pHlys, however, is limited by currently used dyes that accumulate in multiple intracellular compartments and cannot be propagated in clonal cells for longitudinal studies or in vivo determinations. To resolve this limitation, we developed a genetically encoded ratiometric pHlys biosensor, pHLARE (pH Lysosomal Activity REporter), which localizes predominantly in lysosomes, has a dynamic range of pH 4.0 to 6.5, and can be stably expressed in cells. Using pHLARE we show decreased pHlys with inhibiting activity of the mammalian target of rapamycin complex 1 (mTORC1), in breast and pancreatic cancer cells compared with tissue-matched untransformed cells, and with the activated oncogene H-RasV12. pHLARE is a new tool to accurately measure pHlys, for improved understanding of lysosome dynamics that could be a promising therapeutic target. Summary StatementMost lysosome functions require a low lumenal pH, which is dysregulated in many human diseases. We report a new genetically biosensor to accurately measure lysosome pH dynamics, which we use to show decreased lysosome pH in cancer cell lines.

cell biology↗

bioRxiv2018

Conserved tau microtubule-binding repeat histidines confer pH-dependent tau-microtubule association

Tau, a member of the MAP2/tau family of microtubule-associated proteins, functions to stabilize and organize axonal microtubules in healthy neurons. In contrast, tau dissociates from microtubules and forms neurotoxic extracellular aggregates in neurodegenerative tauopathies. MAP2/tau family proteins are characterized by three to five conserved, intrinsically disordered repeat regions that mediate electrostatic interactions with the microtubule surface. We use molecular dynamics, microtubule-binding experiments and live cell microscopy to show that highly conserved histidine residues near the C terminus of each MT-binding repeat are pH sensors that can modulate tau-MT interaction strength within the physiological intracellular pH range. At lower pH, these histidines are positively charged and form cation-{pi} interactions with phenylalanine residues in a hydrophobic cleft between adjacent tubulin dimers. At higher pH, tau deprotonation decreases microtubule-binding both in vitro and in cells. However, electrostatic and hydrophobic characteristics of histidine are required for tau-MT-binding as substitution with constitutively positively charged, non-aromatic lysine or uncharged alanine greatly reduces or abolishes tau-MT binding. Consistent with these findings, tau-MT binding is reduced in a cancer cell model with increased intracellular pH but is rapidly rescued by decreasing pH to normal levels. Thus, these data add a new dimension to the intracellular regulation of tau activity and could be relevant in normal and pathological conditions.

cell biology↗

bioRxiv2017

Local Control of Intracellular Microtubule Dynamics by End Binding Protein 1 (EB1) Photo-Dissociation

Dynamic remodelling of the microtubule cytoskeleton and local interactions with intracellular targets are central to many polarized cell biological processes, an idea first formalized as search-and-capture hypothesis three decades ago1. However, because of the rapid timescale of microtubule polymerization dynamics, it is difficult to directly ask how, when and where dynamic microtubules participate in specific biological processes. Here, we employ a blue light-sensitive interaction with the oat phototropin LOV2 domain2 to generate a photo-inactivated variant of the microtubule end-binding protein EB1, a small adaptor that is central to the interaction of functionally and structurally diverse proteins with growing microtubule ends3,4, that can replace endogenous EB1 function. Acute and reversible blue light-mediated n-EB1 photo-dissociation allows spatially and temporally precise control of intracellular microtubule polymerization dynamics. In addition to demonstrating that neither the GTP cap nor the MT polymerase CKAP5 are sufficient to sustain persistent MT polymerization at physiological growth rates, our data illustrate accurate subcellular control of a freely diffusible, cytoplasmic protein at the second and micrometer scale. This novel design may serve as a template for precise control of many other intracellular protein activities.

cell biology↗