Cytoskeleton

The mechanical properties of the nucleus are critical for maintaining nuclear integrity and function. We previously showed that chromatin dominates short-extension mechanics whereas lamins provide long-extension strain stiffening. To distinguish the roles of lamin isoforms, micromanipulation nucleus force measurements were performed on isolated nuclei from lamin A/C (Lmna-/-) and lamin B1 (Lmnb1-/-) knockout mouse embryonic fibroblast cells. Lamin A/C knockout does not alter short-extension nuclear stiffness but is essential for strain stiffening at longer extensions. Oppositely, lamin B1 loss reduced short-extension stiffness due to facultative heterochromatin loss while long-extension strain stiffening was slightly increased. Loss of lamin A/C and B1 resulted in similar lamin-chromatin linkers effects as LBR did not change and LAP2{beta} decreased in both. A simulation model of a polymeric lamin shell with stiff lamin A/C and softer lamin B1 subunits can qualitatively recapitulate experimental measurements of lamin knockout cells. Lamin A/C knockout resulted in abnormal nuclear shape but not nuclear blebbing or rupture whereas lamin B1 knockout, similar to other perturbations that cause heterochromatin loss, resulted in increased nuclear blebbing and rupture. This work illuminates the distinct mechanical roles of lamin A/C and B1 in determining nuclear structure and integrity.

Injuries to individual cells happen frequently as a result of physiological and environmental stresses during their normal daily functions that can lead to a ruptured cell cortex (plasma membrane and underlying cortical cytoskeleton). The capacity of cells to rapidly repair general daily injuries, as well as ones resulting from trauma, infection, or diseases/cancer, is essential for their survival. While we know the general cell biological outline of the highly-conserved physiological events taking place during cell wound repair, our knowledge of the molecular mechanisms governing the repair process is still fairly limited, due in large part to the lack of information regarding the molecules, machineries, and pathways involved. Here, we performed a genetic screen of 1322 fluorescent-tagged proteins to identify cell wound repair components that are recruited upon laser wounding or whose expression is lost and/or altered upon laser wounding. We identified 129 proteins that are recruited to wounds during the cell repair process through high resolution spatio-temporal expression analyses of these gene fusions in conjunction with a fluorescent actin reporter. Strikingly, we find that many members of the Rab family GTPases are recruited to wounds where, in addition to their well-known roles in intracellular membrane trafficking, they are affecting actin cytoskeletal organization and dynamics during the repair process. These studies are allowing us to define the earliest acting proteins, as well as those required at specific steps in the repair process based on their recruitment patterns and the precise timing of their recruitment to wounds. Thus, our imaging-based screen is providing us with a global view of the repair processes, as well as a large number of genes/gene families that provide new entry points for examining specific steps in the cell wound repair process. Author SummaryCells in our bodies get injured every day from normal activity, environmental stress, infection, or disease. To survive, they must quickly repair these injuries and restore normal function. While some molecules have been identified as key players of cell wound repair, many of the molecules involved and their roles remain unknown. In this study, we identified new molecules that are involved in different steps of cell wound repair. Using laser-induced injury in the Drosophila model, we examined 1322 proteins and observed their spatial and temporal dynamics in a cell after injury. From the 1322 proteins examined, we identified 129 proteins recruited to distinct regions around the damage site during cell wound repair, suggesting roles in specific steps of the repair process. Interestingly, a subset of these proteins are Rab family GTPase members, highlighting new roles for these proteins in regulating actin dynamics. By identifying new candidate repair molecules, we provide a foundation for understanding how cells maintain their integrity and how repair processes may be influenced by factors such as wound size, infection, aging, and disease.

Focal adhesions maintain force-bearing attachment between cells and the extracellular matrix but can also undergo dynamic remodeling. Their assembly and actomyosin tension are coupled through mechanochemical feedback. The processes underlying this feedback are not instantaneous and therefore involve a time delay. However, how this delayed feedback gives rise to stable adhesion maintenance or dynamic remodeling remains unclear. Here, paired time-lapse measurements of vinculin fluorescence and traction stress revealed distinct local adhesion-force dynamics, including low-fluctuation and recurrent fluctuation patterns. To examine how these patterns could arise, we formulated a minimal mechanochemical model coupling focal adhesion assembly and actomyosin force through delayed reciprocal feedback. The model exhibited stable and oscillatory modes depending on feedback strength, the balance of opposing feedback effects, and the effective feedback delay. Bistability and hysteretic switching also occurred in a subset of parameter space, and the oscillation period followed a power-law relation with the delay. These results suggest that stable adhesion maintenance and dynamic remodeling can emerge from a common mechanochemical feedback architecture.

Wound repair is an important mechanism to preserve tissue integrity in organisms after injury. However, why different tissues exhibit different mechanisms to repair wounds is a long-standing question that remains unanswered. In this work, we theoretically explore the role of the purse string, an actomyosin contractile cable used by tissues to close small wounds. Does the tissue 3D geometry influence the efficiency of the purse string in driving wound closure? Using a 3D biophysical model, we study in silico tissues with the same cell volumes but different aspect ratios, ranging from squamous to thick and tall tissues. The model predicts that taller cells are easily deformed by the purse string. In contrast, very squamous cells require a very strong purse string that might demand additional cellular mechanisms to close the gap. These findings establish a theoretical framework to predict the optimal biophysical mechanisms of wound healing in different tissues. Graphical abstractCells of different aspect ratios can be observed in a range of organisms with different function and mechanics. The wound healing efficiency of the purse string increases with the cell aspect ratio in our theoretical exploration. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=135 SRC="FIGDIR/small/743165v1_ufig1.gif" ALT="Figure 1"> View larger version (23K): org.highwire.dtl.DTLVardef@d44ab0org.highwire.dtl.DTLVardef@1737cbaorg.highwire.dtl.DTLVardef@101b5d4org.highwire.dtl.DTLVardef@1487f26_HPS_FORMAT_FIGEXP M_FIG C_FIG

Activated leukocyte cell adhesion molecule A (Alcama) plays a role in axonal guidance, cell differentiation, and retinal lamination in a developing retina and was identified as a marker for activated Muller glial cells in adult zebrafish. However, its spatiotemporal localization and its involvement in retina regeneration remains unclear. Here we induced focal photoreceptor damage in zebrafish using laser photocoagulation and examined the expression and localization of Alcama at different time points post lesion. Immunohistochemistry in wild type fish and Tg(kdrl-EGFP) fish showed Alcama localized to the blood retina barrier with increased expression in Muller glial end feet and radial processes in a regenerating retina. To confirm its role in retina regeneration, alcama expression was transiently knocked down using morpholinos in adult fish. Scanning laser ophthalmoscopy, Zpr1 immunostaining and EdU staining showed delayed retina regeneration in alcama knockdown fish, indicating a possible role for Alcama in zebrafish retina regeneration.

Adult zebrafish efficiently regenerate skeletal muscle following different types of injury; however, the molecular programs involved in repair after extensive cryoinjury remain to be comprehensively characterized. Here, we explored the transcriptomic response of adult zebrafish skeletal muscle at 7 days post cryoinjury (dpci), a stage marked by ongoing tissue clearance, progenitor expansion, myogenic differentiation, and nascent myofiber formation, and compared it with phase-matched stab wound injury. Cryoinjury induced a broad transcriptional response, with 5,330 differentially expressed genes. Integrated enrichment and protein-protein interaction analyses revealed that, at 7 dpci, zebrafish skeletal muscle functions as an integrated regenerative system in which immune remodeling, progenitor expansion, myogenic differentiation, extracellular matrix reconstruction, mechanotransduction, biosynthetic adaptation, proteostasis, and intracellular trafficking operate simultaneously. In parallel, mature sarcomeric and oxidative metabolic programs were suppressed, consistent with ongoing tissue reconstruction and structural immaturity. Comparison with stab-wounded skeletal muscle revealed substantial transcriptional conservation, as 612 of 717 stab-wound-responsive genes (85%) were also differentially expressed after cryoinjury. Shared upregulated genes formed coherent functional modules related to proliferation, extracellular matrix organization and signaling, immune regulation, muscle differentiation, and protein processing. Thus, distinct injury modalities converge on a common regenerative program, while cryoinjury elicits a quantitatively broader transcriptional response. These findings support a conserved regenerative architecture of adult zebrafish skeletal muscle repair, in which interconnected biological modules act coordinately, with the breadth of transcriptional engagement reflecting regenerative demand.

Tumor spheroid mechanics arise from both the mechanical properties of individual cells and the adhesive interactions that organize them into tissues. The relative contribution of these two factors to the bulk mechanical behavior, however, remains difficult to disentangle experimentally. Here, we develop a computational model of micropipette aspiration to compare the mechanical response of isolated cells and multicellular spheroids within a common computational framework. By independently varying single-cell stiffness and cell-cell adhesion, we quantify their effects on aspiration dynamics, effective elastic modulus, and viscoelastic relaxation. Our results show that increasing single-cell stiffness substantially alters the mechanics of isolated cells but has limited influence on the effective elastic modulus of multicellular spheroids. In contrast, changes in cell-cell adhesion produce pronounced effects on spheroid effective elastic modulus. Nevertheless, both parameters increase the retardation time governing the transition from the initial elastic response to long-time viscous deformation. These findings suggest that multicellular elasticity is governed primarily by intercellular mechanical coupling, whereas the dynamical response to applied stress depends jointly on cell-scale mechanics and cell-cell adhesion.

Low-density lipoprotein (LDL) is a major atherogenic lipoprotein, yet its potential to directly modify the fibrin scaffold of blood clots is incompletely understood. Here, we investigated how LDL alters plasma fibrin network architecture and internal fibrinolysis across defined fibrinogen/thrombin conditions. Pooled normal human plasma was supplemented with LDL and clotted with controlled concentrations of fibrinogen and thrombin. Fibrin architecture was visualized by confocal microscopy and quantified by pore-size analysis; clot formation and lysis were monitored turbidimetrically in the presence of tissue plasminogen activator (tPA). Increasing LDL produced a pronounced reduction in fibrin-network pore size across the tested fibrinogen/thrombin conditions. The LDL dependence of pore diameter was well described by a power-law relationship, D_pore=(6.54 +/- 0.11)[LDL]^(-0.12 +/- 0.02) , (R^2 = 0.90), with a significant negative LDL exponent (p = 4 x 10^5). Increasing LDL also prolonged clot lysis time and altered turbidity kinetics. These findings extend epidemiologic and clinical associations between ApoB-containing lipoproteins and hypofibrinolytic clot phenotypes by demonstrating, in a controlled plasma system, that LDL itself can modify fibrin network architecture and fibrinolytic susceptibility. The results support a structure-function role for LDL within the fibrin biomaterial and motivate direct tests of LDL incorporation, protofibril packing, fibrinolytic-protein binding, and single-fiber mechanics.

Endothelial-to-mesenchymal transition (EndMT) has become a central mechanism in developmental biology, fibrosis, vascular disease, and cancer. We previously reported on an integrated signaling model in which TGF-{beta}2 induces EndMT through coordinated activation of Smad-dependent and Smad-independent signaling pathways converging on Snail, while GSK-3{beta} regulates Snail activity. We performed a systematic figure-by-figure reproducibility analysis of the original publication. Independent studies published between 2011 and 2026 were identified and curated according to predefined inclusion criteria. Each original experimental conclusion was evaluated for independent confirmation. In parallel, selected biochemical experiments were independently reproduced using newly acquired reagents and contemporary Western blot methodologies. Independent publications consistently reproduced each major mechanistic conclusion of the original study, including activation of Smad, ERK, PI3K/AKT, and p38 MAPK signaling, regulation of Snail expression, EndMT-associated marker switching, and GSK-3{beta}-dependent control of Snail activity. Independent laboratory experiments reproduced the principal biochemical findings using contemporary reagents and experimental workflows. The combined literature analysis and independent laboratory replication demonstrate that the mechanistic framework in our previous study has remained reproducible across multiple laboratories, endothelial cell types, disease models, and fifteen years of investigation. This work illustrates a complementary framework for assessing reproducibility that integrates direct experimental replication with cumulative independent validation.

Photoendosymbiosis between the ciliate Paramecium tritobursaria and the green alga Chlorella variabilis provides a model for understanding stable photoendosymbiosis. A defining feature of this association is the perialgal vacuole (PV) membrane, a host-derived membrane that encloses each alga and prevents its digestion. However, the timing of PV membrane maturation remains poorly understood because of the lack of molecular markers to distinguish between immature and mature PV membranes. Previous studies have shown that the establishment of symbiosis proceeds through multiple regulated steps following algal uptake; however, the molecular maturation of the PV membrane has not been directly examined. Here, we report a monoclonal antibody that specifically recognizes the PV membrane in symbiotic P. tritobursaria. Time-course immunofluorescence analysis showed that the PV membrane antigen was absent in the early stages after algal uptake, appeared at 48 h, and was detected in all PV membranes by 72 h. The antigen persisted before and after synchronous PV swelling, an experimentally inducible state associated with the loss of normal PV membrane function, but was absent from the membranes surrounding the digested algae. Our findings provide the first molecular evidence that PV membrane maturation is a temporally regulated checkpoint during the establishment of photoendosymbiosis.

Bipolar spindle assembly is essential for accurate chromosome segregation. KIFC1, a conserved Ran- regulated minus-end-directed kinesin-14 motor, accumulates in the nucleus during interphase and promotes chromatin-mediated spindle assembly during mitosis and meiosis. In human oocytes, reduced KIFC1 levels destabilize meiotic spindles, a defect that can be rescued by increasing KIFC1 expression. However, how KIFC1 expression levels affect mitotic spindle stability during cleavage divisions in vertebrates remains unclear. Here, we show that whereas an approximately 50% reduction in KIFC1 causes no detectable defects in spindle assembly, approximately 10-fold overexpression of KIFC1 induces monopolar spindle formation, leading to chromosome mis-segregation and embryonic lethality in medaka early embryos. KIFC1 overexpression results in ectopic centrosomal localization during interphase, impairing the separation of duplicated centrosomes before mitotic entry. Analyses of KIFC1 mutants demonstrated that these centrosome separation defects require KIFC1s microtubule-binding and motor activities and are further enhanced by deletion of KIFC1s nuclear localization sequences. Together, our findings demonstrate that tight regulation of KIFC1 expression and its nuclear sequestration is essential for the proper separation and positioning of duplicated centrosomes before mitotic entry, thereby ensuring efficient bipolar spindle assembly during the rapid cleavage divisions of vertebrate embryos. HighlightsO_LIKIFC1 accumulates in the nucleus and at the embryonic spindle midplane via the Ran pathway. C_LIO_LIPartial KIFC1 depletion does not impair spindle assembly in medaka early embryos. C_LIO_LIKIFC1 overexpression induces monopolar spindles by preventing centrosome separation. C_LIO_LICentrosome separation defects require KIFC1 microtubule-binding and motor activity. C_LI

Recent experiments show that protein condensates sitting on opposite surfaces of a flat lipid membrane move together and prefer to overlap, even though they cannot touch each other. This points to an indirect, membrane-mediated interaction. Two mechanisms could be responsible: a curvature-induced interaction, which is energetic in origin, and a fluctuation-induced interaction, which is entropic. Here we study both with coarse-grained molecular dynamics simulations, using Cookes implicit-solvent lipid model together with a generic bead-spring polymer model for the condensate. We compute the potential of mean force between two condensates across the membrane. For condensates of the same size, full overlap is unfavorable, and the pair instead settles into a partially overlapping state that bends the membrane into an S-like shape. When the two condensates differ strongly in size, full overlap becomes favorable. We explain this with a simple geometric picture. The condensate wets the membrane as a thin film and imposes curvature only along its rim, while membrane tension flattens the membrane under its interior. The resulting ring of curvature can trap a smaller condensate on the opposite side. We also compare the bending undulations and the effective bending modulus of a bare membrane, a membrane with one condensate, and a membrane with condensates on both sides. A wetting condensate suppresses the undulation modes and stiffens the membrane, but whether this makes overlap entropically favorable remains inconclusive. Our results indicate that the coupling is driven mainly by curvature, and that it depends on the wetting mechanism and on the membrane tension.

Three-dimensional (3D) spheroid models have become essential in cancer biology, drug screening, and tissue engineering. However, their small size, fragile structure, and tendency to disintegrate during routine histoprocessing present persistent technical challenges. Conventional paraffin embedding often results in tissue fragmentation, loss of spatial orientation, and poor section quality, whereas cryosectioning often compromises cellular morphology. Here, we present a robust, cost-effective protocol for preserving and sectioning fragile 3D spheroids, resulting in high-quality histological sections with intact architecture and excellent cellular detail. The method involves optimized handling and embedding procedures that stabilize spheroids during standard formalin fixation, paraffin infiltration, and microtomy, eliminating mechanical distortion and preserving spherical integrity for consistent sectioning. We demonstrate successful application across different cell line spheroids, with subsequent compatibility with hematoxylin and eosin (H&E) staining protocols. Compared to conventional methods, our approach significantly reduces sample loss, improves inter-section reproducibility, and preserves fine structural features such as necrotic cores, proliferative zones, and extracellular matrix components. This protocol provides a reliable, accessible solution for routine histological analysis of fragile 3D spheroids, facilitating more accurate morphological and molecular assessment in translational research settings. Key featuresO_LIMaintains spheroid integrity: Prevents mechanical distortion, fragmentation, and loss of spatial orientation during processing. C_LIO_LISignificantly reduces sample loss: Decreases failure rate compared to traditional methods, conserving valuable samples. C_LIO_LIBroad spheroid compatibility: Works effectively with primary tumor-derived, stem cell-derived, and co-culture spheroid models. C_LIO_LIEnables high-quality sectioning and staining: Delivers consistent, reproducible sections that are fully compatible with H&E, IHC, and IF. C_LI Graphical overview O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=140 SRC="FIGDIR/small/743094v1_ufig1.gif" ALT="Figure 1"> View larger version (44K): org.highwire.dtl.DTLVardef@1670c4org.highwire.dtl.DTLVardef@145810aorg.highwire.dtl.DTLVardef@1accb1org.highwire.dtl.DTLVardef@17481c0_HPS_FORMAT_FIGEXP M_FIG C_FIG

Intertidal mudflats are low hydrodynamic energy environments hosting microphytobenthic communities that experience strong spatiotemporal variability in light regimes, including changes in spectral quality and light intensity that can lead to cellular photooxidative stress. To cope with these fluctuations, autotrophs exhibit diverse and highly plastic adaptations that are often species-dependent and shaped by their ecological niches. This study investigates photophysiological responses and metabolic remodeling in a diatom assemblage originating from a natural winter microphytobenthic biofilm under contrasting red and blue light intensities. To this end, photosynthetic parameters were monitored alongside changes in lipophilic metabolites, including untargeted lipids and lipophilic pigments. While few metabolites showed temporal remodeling, rapid and contrasting changes were observed within 30 minutes in response to both spectral quality and light intensity. Red light treatments induced broader remodeling of lipophilic metabolites than blue light, whereas blue light appeared to have a greater impact on photosynthetic parameters. Moreover, red light induced xanthophyll-cycle responses comparable to those observed under blue light at equivalent incident intensity. We discuss these metabolic responses in relation to diatom photoadaptive strategies, placing these findings within the intertidal environmental framework. This work further underlines the importance of understanding rapid metabolic plasticity in coping with light fluctuations, providing new insights into the photoregulatory strategies of natural microphytobenthic communities.

The methyltransferase EZH2 (Enhancer of Zest Homolog 2) and the demethylase KDM6B (Lysine Demethylase 6B) have been associated with epithelial to mesenchymal transition (EMT) and poor prognosis in various cancers. These enzymes methylate and demethylate H3K27me3 and regulate distinct sets of genes controlling EMT induction, despite having opposite catalytic activities. This could be due to their recruitment or the modulation of their activity by partner proteins on specific loci. This work sought to identify proteins associated with chromatin and interacting with EZH2 or with KDM6B during EMT. To do so, co-immunoprecipitation and mass spectroscopy was used under TGF{beta} (Tumor growth factor {beta}) and TNF (Tumor necrosis factor ) treatment to induce EMT in A549 lung cancer cells. Surprisingly, numerous proteins related to focal adhesions were identified to interact with EZH2 or KDM6B. These proteins are part of a nuclear protein interaction network previously described as nucleo-adhesome. Among these proteins, TGFB1I1 (transforming growth factor induced peptide 1) and CSRP2 (cysteine and glycine rich protein 2) were further confirmed to interact with KDM6B in the nucleus and even more so during EMT. The target genes of these complexes were then sought by knocking down KDM6B, TGFB1I1 or CSRP2. Three genes (coding Integrin alpha 5, Laminin y2 and Matrix Metalloproteinase 9) were confirmed to be regulated by KDM6B, TGFB1I1 and CSRP2. These findings may have clinical relevance, as immunohistochemistry analyses performed on a cohort of lung cancer patients revealed increased nuclear localization of TGFB1I1 and CSRP2 in cells undergoing EMT.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterised by progressive motor neuron degeneration. Mutations in the SOD1 gene represent the second most common genetic cause of ALS (ALS), and distinct SOD1 missense variants present with markedly different clinical profiles. A4V leads to an aggressive form of the disease (median survival [~]1y), H46R confers a mild, slowly progressive course and I113T exhibits an intermediate phenotype. The molecular basis by which these mutations produce divergent clinical outcomes remains poorly understood. We performed extensive classical molecular dynamics simulations of wild-type SOD1 and the three ALS-associated variants in the apo monomeric state to attempt to investigate the mechanisms behind such phenotypic differences. Structural stability, global compactness, and conformational flexibility, as well as analysis of collective motions between residues and estimation of free energy, were assessed. The H46R, A4V, and I113T variants exhibited distinct dynamic behaviours, highlighting differences in structural stability, local flexibility, and intramolecular interactions. These findings suggest that specific structural regions may contribute differently to protein dysfunction and could represent key elements for understanding the relationship between molecular dynamic properties and the differing clinical severity associated with these variants. Most strikingly, H46R exhibited exceptional structural stability across every analytical level, the lowest global deviation, most attenuated local flexibility, strongest internal dynamic coordination, and the deepest, most confined free energy basins of any system examined. This convergent multi-layered evidence of structural restraint provides a compelling mechanistic basis for the mild and slowly progressive clinical course of H46R ALS, suggesting that enhanced conformational rigidity, rather than bulk destabilisation, is the defining biophysical feature of this variant, and that its pathogenic mechanism operates through a route fundamentally decoupled from the aggregation-driven toxicity that characterises the more aggressive SOD1-ALS mutations.

Abstract Calcific deposits in the arterial media have been associated with a number of metabolic and genetic disorders including diabetes, chronic kidney disease and generalized arterial calcification of infancy. While medial calcification and physiologic hard tissue mineralization in the skeleton are both regulated by several common determinants, emerging data suggest that there might be fundamental differences in the mechanisms underlying these two processes. Objective: We previously demonstrated that elastin haploinsufficiency delays medial calcification in MGP-deficient mice. Here, using mice in which a human ELN transgene rescues mouse elastin deficiency, we investigated whether the origin and abundance of arterial elastin differentially affect the initiation and progression of medial calcification. Approach and Results: We pursued a transgenic approach to alter the arterial elastin scaffold in MGP-deficient mice. Our analyses of a humanized MGP-deficient model with 40% reduction of medial elastin content showed a complete absence of the early-stage vascular calcification. Additionally, we showed that mouse and human elastin orthologues affect vascular calcification in a comparable manner. Conclusion: Arterial elastin abundance, rather than orthologue origin, modulates the initiation and progression of medial calcification in MGP-deficient mice. A further reduction in arterial elastin beyond that achieved by elastin haploinsufficiency profoundly delays mineral deposition and maturation, whereas restoration of elastin abundance through transgenic human ELN expression restores arterial calcification.

In the active loop extrusion model, the cohesin protein complex creates chromatin loops in eukaryotic cells. Extrusion maintains topologically associated domains (TADs), which are contiguous segments of chromatin that preferentially colocalize in space and are typically bounded by CTCF proteins that pause cohesin translocation. Here, we model active loop extrusion with hybrid molecular dynamics - Monte Carlo simulations in entangled flexible linear polymer melts. Intra-chain contact probabilities of polymers with active loop extrusion are enhanced compared to their equilibrium, passive counterparts. Extrusion causes the size of chain segments to be much smaller than in passive melts. While the overlap parameter in passive melts without extrusion monotonically increases with segment length, it is nonmonotonic in active melts and on the order of unity within the parameters of this study. Active loop extrusion suppresses contacts between TADs in favor of intra-TAD contacts. Reduction of overlaps between chain segments dilutes entanglements in active melts. Depending on parameters, active extrusion without TADs may induce more compact conformations than with TADs, due in part to fractal loopy globule-like dynamics. This work suggests that active loop extrusion reduces overlaps between TADs, contributing to effective gene regulation by cis-regulatory elements.

Binding of ligands to partially or fully unfolded proteins can play a key role in the mechanism of cellular and pharmacological chaperones, facilitating proper folding. However, it is challenging to quantify the binding affinity of ligands for unfolded states in a protein that is normally folded, as the methods standardly used to destabilize the native fold also affect ligand binding. We used single-molecule force spectroscopy to unfold single protein molecules without altering solution condi-tions and observe interactions of a ligand with unfolded states. Focusing on pentosan polysulfate (PPS), an anti-prion pharmacological chaperone previously shown to interact with both the native and partially or fully unfolded states of the prion protein, we measured the concentration-dependent effects of PPS binding on the conformational dynamics of bank vole prion protein (BvPrP) molecules held in optical tweezers. We found that PPS stabilized certain partially unfolded intermediate states of BvPrP as well as the fully unfolded state. Strikingly, the tendency to bind unfolded states instead of the folded state increased as the PPS concentration was reduced, implying a higher affinity to unfolded states. From the relative amount of binding to unfolded versus folded states, we estimated that PPS bound roughly 100-fold more tightly to unfolded states than to the native state of PrP. These results reinforce the likely importance of unfolded states in prion misfolding and propagation. More generally, they show how binding affinity to transient, unstable states can be estimated.

Zebrafish bone regeneration is a highly efficient process, enabling the complete restoration of an amputated fin within few weeks. The hallmark of this epimorphic regeneration is the formation of a blastema atop of a bony fin ray. Osteoblasts near the injury site dedifferentiate and migrate off the bone to contribute to the developing blastema. We show that an injury or a blastema alone is not sufficient to trigger off-bone migration of osteoblasts. Surprisingly, we found that blastema cells themselves possess intrinsic migratory properties. Moreover, when multiple injury sites are present, a preferential distal migration could be observed. We conclude that multiple injuries are hierarchical organized, and that injuries with the highest regenerative potential take priority.