Biology subjects

Tervonen, A.

Publications and source records attributed to Tervonen, A..

4 recordsLinked to original sources

bioRxiv2026

Junction-level imaging and modeling of transient leaks in epithelia predicts a dynamic tight junction strand network

Epithelial tight junctions restrict paracellular permeability, but how small solutes leak through the barrier is not fully understood, because permeability is usually measured globally. Here, we combined fast zinc-based barrier imaging (ZnUMBA) with computational modelling of the dynamic tight junction strand network to study transient leaks at single bicellular junctions in MDCK II cells. Leaks were short-lived, with a median duration of 17 s in wild-type cells, and occurred in all cell lines studied, including ZO-1 knockout, ZO-2 knockout, and ZO-1-overexpressing cells. In ZO-2 knockout and ZO-1-overexpressing cells, leaks lasted longer without changes in their frequency or transepithelial electrical resistance (TER). ZO-1 knockout increased the junctional ZnUMBA signal and reduced TER and the number of detected leaks. Simulations showed that stochastic breaking and sealing of tight junction strands is sufficient to explain the observed leak frequencies and durations, and that a single pair of strand break formation and sealing rates reproduces both junction-level leaks and global permeability. Whereas leak frequency mainly reflects the ratio of these rates, leak duration provides independent information on strand sealing that global measurements cannot resolve. Modulating myosin II activity had little effect on leaks, whereas actin depolymerization compromised the barrier. Together, these results link junction-level leaks of ions and small solutes to tight junction strand dynamics.

cell biology↗

bioRxiv2025

Viscoelasticity driven deformation dynamics of the substrate affect mechanically induced Ca2+ signals

The viscoelastic properties of the substrate are known to affect many aspects of cellular behavior from migration to differentiation. Cells are also known to sense deformations in the extracellular matrix (ECM). However, these mechanotransduction events have mostly been studied separately, and the emphasis has been on purely elastic materials although most tissues in the body are viscoelastic. We wanted to understand how the viscosity and elasticity of the substrate affect cells mechanosensation, i.e., how cells integrate passive (substrate viscoelasticity) and active cues (dynamic substrate deformation) into calcium transients. We used a light-controllable Disperse Red 1-glass (DR1-glass) coating together with a thin polyacrylamide (PAA) hydrogel with controllable viscoelasticity. This allowed us to generate local deformations in the cell-substrate interface on substrates with different viscoelastic properties. Inspecting the resulting calcium transients in a Madin Darby canine kidney type II (MDCK II) monolayer with the genetically encoded calcium indicator jRCaMP1b revealed that cells responded to deformation differently depending on the viscoelasticity of the substrate. On stiff elastic gels cells exhibited all in all the largest calcium responses as well as increased sensitivity to the magnitude of deformation, with larger deformations leading to stronger calcium signals, whereas on soft elastic and soft viscoelastic gels the magnitudes of deformation had less effect on the degree of calcium signals. Indeed, immunostainings showed that cells formed the strongest focal adhesions (FA) on stiff gels, albeit differences were surprisingly small. Instead, computational modeling revealed that forces generated at FAs were strongly dependent on the viscoelasticity of the substrate, with increased elasticity and decreased viscosity leading to larger forces. Moreover, viscoelasticity affected the dynamics of the force generation. On stiff elastic gels force increased in fast steps, whereas on soft gels the buildup was gradual and was further slowed down by increased viscosity. Surprisingly, experiments with PIEZO1 KO cells showed that the calcium responses to the substrate deformation did not require PIEZO1 channels. Instead, depleting the ER with thapsigargin (TG), depolymerizing the actin cytoskeleton with cytochalasin D (cyto D) and latrunculin A (Lat A), and inhibiting actomyosin II with y-27632, showed that the calcium was mainly released from the ER in an actin, but not actomyosin, dependent mechanism. The data therefore suggests that the forces subjected to FAs could be directly transmitted to the ER via an actin mediated tension that results in the opening of ER residing calcium release channels. Our results illustrate that the viscoelasticity of the cell niche controls cell behavior in two ways. Firstly, it affects how cells build adhesions to the ECM and thus the accumulation of mechanosensitive proteins. Secondly, it affects the dynamics of the deformations and forces that are sensed by FAs. In line with our previous findings, we show that the mechanically induced calcium responses are dependent on dynamics, with fast mechanical cues resulting in larger calcium responses. Moreover, our research suggests that cells may possess distinct response mechanisms that are selectively activated depending on the mechanical properties of their niche, and the type and dynamics of the mechanical stimulation.

cell biology↗

bioRxiv2025

Nucleus softens during herpesvirus infection

Nuclear mechanics is remodeled not only by extracellular forces but also by internal modifications, such as those induced by viral infections. During herpes simplex virus type 1 infection, the nuclear structures undergo drastic reorganization, but little is known about how nuclear mechanobiology changes as a result. We show that the nucleus softens dramatically during the infection. To understand the phenomenon, we used advanced microscopy and computational modeling. We discovered that the enlarged viral replication compartment had a low biomolecular density, partially explaining the observed nuclear softening. The mobility of the nuclear lamina decreased, which suggests increased rigidity and an inability to induce softening. However, computational modeling supported by experimental data showed that reduced outward forces, such as cytoskeletal pull and intranuclear osmotic pressure acting both on and within the nucleus, can explain the decreased nuclear stiffness. Our findings reveal that during infection, the nucleus is subject to changes in multiple mechanical forces, leading to decreased nuclear stiffness. Author SummaryDNA viruses take over the host cell nucleus, inducing dramatic structural modifications. There is currently very little knowledge of how the progression of viral infection modifies the mechanical properties of the nucleus, which are essential for various cellular processes, including gene expression and cell migration. Here, we show that the nucleus softens when herpesvirus infection progresses. We discovered that the viral replication compartment established in the central parts of the nucleus had a low biomolecular density, which may contribute to the nuclear softening. The shape and motion of the nuclear lamina suggested that it became more rigid, indicating that another mechanism was involved in the decreased elasticity. Our mechanical simulations and experiments showed that a reduction in outward forces, such as actin cytoskeleton pull or osmotic pressure, is the most likely factor in the nuclear softening. Our study provides new insights into the effects of DNA viruses on the mechanics of host cell nuclei, significantly expanding the knowledge of viral infection mechanobiology.

cell biology↗

bioRxiv2021

The effect of substrate stiffness on tensile force transduction in the epithelial monolayers

In recent years, the importance of mechanical signaling and the cellular mechanical microenvironment in affecting cellular behavior has been widely accepted. Cells in epithelial monolayers are mechanically connected to each other and the underlying extracellular matrix (ECM), forming a highly connected mechanical system subjected to various mechanical cues from their environment, such as the ECM stiffness. Changes in the ECM stiffness have been linked to many pathologies, including tumor formation. However, our understanding of how ECM stiffness and its heterogeneities affect the transduction of mechanical forces in epithelial monolayers is lacking. To investigate this, we used a combination of experimental and computational methods. The experiments were conducted using epithelial cells cultured on an elastic substrate and applying a mechanical stimulus by moving a single cell by micromanipulation. To replicate our experiments computationally and quantify the forces transduced in the epithelium, we developed a new model that described the mechanics of both the cells and the substrate. Our model further enabled the simulations with local stiffness heterogeneities. We found the substrate stiffness to distinctly affect the force transduction as well as the cellular movement and deformation following an external force. Also, we found that local changes in the stiffness can alter the cells response to external forces over long distances. Our results suggest that this long-range signaling of the substrate stiffness depends on the cells ability to resist deformation. Furthermore, we found that the cells elasticity in the apico-basal direction provides a level of detachment between the apical cell-cell junctions and the basal focal adhesions. Our simulation results show potential for increased ECM stiffness, e.g. due to a tumor, to modulate mechanical signaling between cells also outside the stiff region. Furthermore, the developed model provides a good platform for future studies on the interactions between epithelial monolayers and elastic substrates. Author summaryCells can communicate using mechanical forces, which is especially important in epithelial tissues where the cells are highly connected. Also, the stiffness of the material under the cells, called the extracellular matrix, is known to affect cell behavior, and an increase in this stiffness is related to many diseases, including cancers. However, it remains unclear how the stiffness affects intercellular mechanical signaling. We studied this effect using epithelial cells cultured on synthetic deformable substrates and developed a computational model to quantify the results better. In our experiments and simulations, we moved one cell to observe how the substrate stiffness impacts the deformation of the neighboring cells and thus the force transduction between the cells. Our model also enabled us to study the effect of local stiffness changes on the force transduction. Our results showed that substrate stiffness has an apparent impact on the force transduction within the epithelial tissues. Furthermore, we found that the cells can communicate information on the local stiffness changes over long distances. Therefore, our results indicate that the cellular mechanical signaling could be affected by changes in the substrate stiffness which may have a role in the progression of diseases such as cancer.

cell biology↗