Pörsti, A. | bioRxiv Science

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↗

bioRxiv2026

Deep Invaginations of Nuclear Envelope Coordinate Spatial Organization of Chromatin in Epithelium

Cell nuclei are often used to assess cell health, but how their shapes vary in normal tissues and how they respond to mechanical forces is not well understood. Here, we describe deep invaginations of the nuclear envelope (DINEs) as common features of epithelial cell nuclei. After their formation, DINEs exist independently of the cytoskeleton, depend on A-type lamins, and emerge in response to cell crowding, contact inhibition, and tissue maturation. High-resolution imaging shows that, in contrast to the peripheral nuclear lamina, DINEs contain densely packed chromatin with regions of active gene transcription. They also remodel dynamically during confined migration, allowing nuclei to adapt to physical constraints. Mechanistically, DINE formation is linked to suppression of MAPK signaling, while activation of growth-promoting pathways reduces their occurrence. These findings reveal DINEs as intrinsic, mechanosensitive structures that coordinate nuclear shape, chromatin organization, and gene activity, providing new insight into how epithelial cells integrate mechanical and biochemical cues to maintain tissue homeostasis. TeaserDeep nuclear envelope invaginations organize chromatin and gene activity in response to epithelial crowding.

cell biology↗