Biology subjects

Thomas, E. C.

Publications and source records attributed to Thomas, E. C..

12 recordsLinked to original sources

bioRxiv2026

Taccalonolide AJ uncouples tubulin GTP hydrolysis from microtubule depolymerization

Microtubules are dynamic polymers of tubulin subunits that are vital for a variety of eukaryotic cell functions, including intracellular trafficking and cell division. Microtubule-targeting drugs are among the oldest and most widely used chemotherapeutics. Despite their extensive use, the development and optimization of these drugs is hindered by our limited understanding of the molecular impacts on microtubules. Recent high-resolution structures have begun to explain how paclitaxel (Taxol) stabilizes microtubules: it reverses the hydrolysis-driven lateral strain at the microtubule seam and induces an expanded, GTP-like conformational state that promotes polymerization. In this work, we demonstrate that the microtubule-targeting natural product taccalonolide AJ (taccaAJ) stabilizes microtubules in a manner completely distinct from paclitaxel. Using cryo-EM, we show that taccaAJ stabilizes a compacted, post-GTP hydrolysis state, which is usually prone to depolymerization. In vitro assays show that taccaAJ-stabilized microtubules are more rigid than paclitaxel-stabilized microtubules, and reconstituted dynamics with taccaAJ exhibit increased rescue close to the plus-end. Finally, we use fluorescent biosensors for the compacted tubulin conformation to confirm that taccaAJ-stabilized microtubules in cells are compacted yet stable against depolymerization. We propose that taccaAJ preferentially binds a GDP lattice, promotes lateral interactions between protofilaments, and thus creates a rigid, stable microtubule in a compacted, post-GTP hydrolysis state.

biochemistry↗

bioRxiv2026

Taxol exploits molecular switches within tubulin to stabilize microtubules.

Microtubules are dynamic filaments of tubulin heterodimers that comprise an essential part of the eukaryotic cytoskeleton1. The nucleotide state of tubulin controls microtubule dynamics: stable GTP-microtubules favor polymerization, whereas unstable GDP-microtubules drive depolymerization2. Anticancer compounds such as Taxol (paclitaxel) target microtubule dynamicity by preventing microtubule depolymerization3,4. Despite decades of work, the molecular basis of microtubule dynamics remains poorly defined. Using cryo-EM, we determined [~]2.2 [A] structures of human microtubules in GTP-like (GMPCPP) and GDP states. Comparison of these two states revealed switch-like structural changes as tubulins transition from the pre-hydrolysis (GMPCPP) to the post-hydrolysis (GDP) state. Additional structure determination of Taxol-bound microtubules at [~]2.2 [A] showed that Taxol binding converts the microtubule lattice into a pre-hydrolysis state by reversing the structural switches flipped during GTP hydrolysis. Focusing our analysis on the microtubule seam shows that the pre-hydrolysis conformation of GMPCPP or Taxol-GDP exhibits favorable lateral interactions at the seam, with lattice deformations clearly visible at the GDP seam. Together, our data show the existence of structural switches in tubulin that are coupled to the nucleotide state and are exploited by Taxol to stabilize microtubules into a pre-hydrolysis-like state. (191 words)

molecular biology↗

bioRxiv2026

Tubulin C-terminal tails are pH sensors that regulate microtubule function

Changes in intracellular pH are critical for maintaining homeostasis, mediating signaling pathways, and enabling cellular responses to stress, injury, and disease. There is increasing evidence that clusters of acidic residues, primarily glutamates, are both highly prevalent and conserved in disordered regions of proteins and can play an important role in cellular pH response. Tubulin C-terminal tails (CTTs) are glutamate rich regions which protrude from the microtubule surface. These tails are a primary site of for both post-translational modifications and binding of microtubule-associated proteins. Motivated by these observations, we measured the pH response of tubulin CTTs using NMR spectroscopy, circular dichroism, and computational simulations. We find that glutamate residues in CTTs taken from organisms across eukaryotes exhibit a robust upshift in their pKa values, that the sequential context of glutamate residues creates hot spots for protonation, and that hydrogen bonding between side chains stabilizes interactions that alter the conformation of the CTT. To determine whether the CTT pH response plays a potentially important role in microtubule interactions, we measured the pH dependence of the binding of the yeast kinesin-5, Cin8, to microtubules. We find that Cin8 binding is modulated by pH in a CTT-dependent manner. Our results demonstrate that acidic clusters are important mediators of cellular pH response and establish that pH can regulate interactions at the microtubule surface. Significance StatementVariation in cellular pH is important for cell function in changing environmental conditions or developmental states. Here we probe protonation of the glutamate-rich C-terminal tails of tubulin, revealing the existence of and mechanism driving the anomalously high pH response and subsequent regulation of microtubule binding. Our results demonstrate that acidic clusters are important mediators of cellular pH response and establish pH-based regulation of interactions at the microtubule surface.

biophysics↗

bioRxiv2025

Accessibility of the unstructured α-tubulin C-terminal tail is controlled by microtubule lattice conformation

Microtubules are cytoskeletal filaments that self-assemble from the protein tubulin, a heterodimer of -tubulin and {beta}-tubulin, and are important for cell mechanics, migration, and division. Much work has focused on how the nucleotide state of {beta}-tubulin regulates the structure and dynamics of microtubules. In contrast, less is known about the structure and function of the C-terminal tails (CTTs) of - and {beta}-tubulin which are thought to freely protrude from the surface of the microtubule. To study the CTT of -tubulin, we developed three different biosensors that bind the tyrosinated -tubulin CTT (Y-CTT). Surprisingly, live imaging of the probes indicates that the Y-CTT is minimally accessible along the microtubule lattice under normal cellular conditions. Lattice binding of the Y-CTT probes can be increased by three different ways of changing the tubulin conformational state: the drug Taxol, expression of microtubule-associated proteins (MAPs) that recognize or promote an expanded tubulin conformation, or expression of tubulin that cannot hydrolyze GTP. Molecular dynamics simulations indicate that the Y-CTT undergoes numerous transient interactions with the bodies of -tubulin and {beta}-tubulin in the lattice, and that the frequency of these interactions is regulated by the tubulin nucleotide state. These findings suggest that accessibility of the Y-CTT is locally governed by nucleotide- and MAP-dependent conformational changes to tubulin subunits within the microtubule lattice.

cell biology↗

bioRxiv2024

Selective regulation of kinesin-5 function by β-tubulin carboxy-terminal tails

The tubulin code hypothesis predicts that carboxy-terminal tails of tubulins create programs for selective regulation of microtubule-binding proteins, including kinesin motors. However, the molecular mechanisms that determine selective regulation and their relevance in cells are poorly understood. In this work, we report selective regulation of budding yeast kinesin-5 motors, Cin8 and Kip1, by the {beta}-tubulin tail. Cin8, but not Kip1, requires the {beta}-tubulin tail for its recruitment to the mitotic spindle. This creates a balance of both kinesin-5 motors in the spindle, and efficient mitotic progression. We identify a negatively charged patch of acidic amino acids in the {beta}-tubulin tail that mediates the interaction with Cin8 in vivo. Using in vitro reconstitution with genetically modified yeast tubulin, we demonstrate that the charged patch in the {beta}-tubulin tail increases Cin8 plus-end directed velocity and processivity. Finally, we determine that the positively charged amino-terminal extension from the Cin8 motor domain coordinates interactions with the {beta}-tubulin tail. This work provides the first demonstration of a molecular mechanism underlying selective regulation of closely related kinesin motors by tubulin tails, and how this regulation promotes proper function of the mitotic spindle.

cell biology↗

bioRxiv2023

Tissue stiffness mapping by light sheet elastography

Tissue stiffness plays a crucial role in regulating morphogenesis. The ability to measure and monitor the dynamic progression of tissue stiffness is important for generating and testing mechanistic hypotheses. Methods to measure tissue properties in vivo have been emerging but present challenges with spatial and temporal resolution especially in 3D, by their reliance on highly specialised equipment, and/or due to their invasive nature. Here, introduce light sheet elastography, a noninvasive method that couples low frequency shear waves with light sheet fluorescence microscopy by adapting commercially available instruments. With this method, we achieved in toto stiffness mapping of organ-stage mouse and zebrafish embryos at cellular resolution. Versatility of the method enabled time-lapse stiffness mapping during tissue remodelling and of the beating embryonic heart. This method expands the spectrum of tools available to biologists and presents new opportunities for uncovering the mechanical basis of morphogenesis.

bioengineering↗

bioRxiv2023

Mesenchymal Wnts are required for morphogenetic movements of calvarial osteoblasts during apical expansion

Apical expansion of calvarial osteoblast progenitors from the cranial mesenchyme (CM) above the eye is integral for calvarial growth and enclosure of the brain. The cellular behaviors and signals underlying the morphogenetic process of calvarial expansion are unknown. During apical expansion, we found that mouse calvarial primordia have consistent cellular proliferation, density, and survival with complex tissue scale deformations, raising the possibility that morphogenetic movements underlie expansion. Time lapse light sheet imaging of mouse embryos revealed that calvarial progenitors intercalate in 3D to converge supraorbital arch mesenchyme mediolaterally and extend it apically. In contrast, progenitors located further apically exhibited protrusive and crawling activity. CM cells express non-canonical Wnt/Planar Cell Polarity (PCP) core components and calvarial osteoblasts are bidirectionally polarized. We found non-canonical ligand, Wnt5a-/- mutants have less dynamic cell rearrangements, protrusive activity, and a flattened head shape. Loss of cranial mesenchyme-restricted Wntless (CM-Wls), a gene required for secretion of all Wnt ligands, led to diminished apical expansion of OSX+ calvarial osteoblasts in the frontal bone primordia in a non-cell autonomous manner without perturbing proliferation or survival. Calvarial osteoblast polarization, progressive cell elongation and enrichment for actin cytoskeleton protein along the baso-apical axis were dependent on CM-Wnts. Thus, CM-Wnts regulate cellular behaviors during calvarial morphogenesis and provide tissue level cues for efficient apical expansion of calvarial osteoblasts. These findings also offer potential insights into the etiologies of calvarial dysplasias.

developmental biology↗

bioRxiv2023

Fluctuations and entropy enable neural crest cell ingression

The second law of thermodynamics explains the dissipative nature of embryonic development as an exchange of energy-dependent order for proportionately greater output of heat and waste. Recent work on granular matter provides a path by which to define the roles of passive, stochastic mechanisms in nonequilibrium systems. Here, we apply such a framework to examine the role of thermodynamic parameters to cell ingression, the movement of cells from one tissue layer to another that has been attributed, in part, to directional cues. Using the murine neural crest as a model system, we provide evidence that a stochastic mechanism, rather than a proposed stiffness gradient, underlies cell ingression. Cortical fluctuations representing effective temperature and cell packing configurations generate an entropic trap that promotes cell ingression. The results imply dissipative mechanisms that transiently disorder tissue underlie some morphogenetic events.

developmental biology↗

bioRxiv2023

Durotaxis bridges phase transition as a function of tissue stiffness in vivo

Physical processes ultimately drive morphogenetic cell movements. Two proposals are that 1) cells migrate toward stiffer tissue (durotaxis) and that 2) the extent of cell rearrangements reflects liquid-solid tissue phase. It is unclear whether and how these concepts are related. Here, we identify fibronectin-dependent tissue stiffness as a control variable that underlies and unifies these phenomena in vivo. In murine limb bud mesoderm, cells are either caged, move directionally by durotaxis or intercalate as a function of their location along a stiffness gradient. A unifying stiffness-phase transition model that is based on a Landau equation accurately predicts cell diffusivity upon loss or gain of fibronectin. Fibronectin is regulated by a WNT5A-YAP positive feedback pathway that controls cell movements, tissue shape and skeletal pattern. The results identify a key determinant of phase transition and show how durotaxis emerges in a mixed phase environment in vivo.

developmental biology↗

bioRxiv2022

Shape driven confluent rigidity transition in curved biological tissues

Collective cell motions underlie structure formation during embryonic development. Tissues exhibit emergent multicellular characteristics such as jamming, rigidity transitions, and glassy dynamics, but there remain questions about how those tissue scale dynamics derive from local cell level properties. Specifically, there has been little consideration of the interplay between local tissue geometry and cellular properties influencing larger scale tissue behaviours. Here we consider a simple two dimensional computational vertex model for confluent tissue monolayers, which exhibits a rigidity phase transition controlled by the shape index (ratio of perimeter to square root area) of cells, on surfaces of constant curvature. We show that the critical point for the rigidity transition is a function of curvature such that positively curved systems are likely to be in a less rigid, more fluid, phase. Likewise, negatively curved systems (saddles) are likely to be in a more rigid, less fluid, phase. A phase diagram we generate for the curvature and shape index constitutes a testable prediction from the model. The curvature dependence is interesting because it suggests a natural explanation for more dynamic tissue remodelling and facile growth in regions of higher surface curvature, without invoking the need for biochemical or other physical differences. This has potential ramifications for our understanding of morphogenesis of budding and branching structures.

biophysics↗

bioRxiv2021

In Vivo Simultaneous Nonlinear Absorption Raman and Fluorescence (SNARF) Imaging of Mouse Brain Cortical Structures

AO_SCPLOWBSTRACTC_SCPLOWTwo-photon excited fluorescence (TPEF) microscopy is a widely used optical imaging technique that has revolutionized neurophotonics through a diverse palette of dyes, specialized transgenic models, easy implementation, and straightforward data analysis. However, in vivo TPEF imaging is often limited in the number of contrasts available to distinguish different cells, structures, or functions. We propose using two label-free multiphoton microscopy techniques - stimulated Raman scattering (SRS) microscopy and transient absorption microscopy (TAM) - as complementary and orthogonal imaging modalities to TPEF for in vivo brain imaging. In this study, we construct a simultaneous nonlinear absorption, Raman, and fluorescence (SNARF) microscope and image several cortical structures up to 250-300 m below the pial surface, the highest reported in vivo imaging depth for SRS or TAM. We further demonstrate the capabilities of our SNARF microscope through the quantification of age-dependent myelination, hemodynamics, vessel structure, cell density, and cell identity in vivo. Using machine learning, we report the use of label-free SRS and TAM features to predict capillary-lining cell identities with 90% accuracy. The SNARF microscope and methodology outlined herein provide a powerful platform to study several research topics, including neurovascular coupling, blood-brain barrier, neuronal and axonal degeneration in aging, and neurodegenerative diseases.

neuroscience↗

bioRxiv2020

Ase1 domains dynamically slow anaphase spindle elongation and recruit Bim1 to the midzone

How cells regulate microtubule crosslinking activity to control the rate and duration of spindle elongation during anaphase is poorly understood. In this study, we test the hypothesis that PRC1/Ase1 proteins use distinct microtubule-binding domains to control spindle elongation rate. Using budding-yeast Ase1, we identify unique contributions for the spectrin and carboxy-terminal domains during different phases of spindle elongation. We show that the spectrin domain uses conserved, basic residues to promote the recruitment of Ase1 to the midzone before anaphase onset and slow spindle elongation during early anaphase. In contrast, a partial Ase1 carboxy-terminal truncation fails to form a stable midzone in late anaphase, produces faster elongation rates after early anaphase, and exhibits frequent spindle collapses. We find that the carboxy-terminal domain interacts with the plus-end tracking protein EB1/Bim1 and recruits Bim1 to the midzone to maintain midzone length. Overall, our results suggest that the Ase1 domains provide cells with a modular system to tune midzone activity and control elongation rates.

cell biology↗