Chmiel, T. A. | bioRxiv Science

bioRxiv2022

Confluence and tight junction dependence of volume regulation in epithelial tissue

Epithelial cell volume regulation is a key component to tissue stability and dynamics. In particular, how cells respond to osmotic stresses is of significant physiological interest in kidney epithelial tissue. For individual mammalian cells, it is well established that Na-K-2Cl cotransporter (NKCC) channels mediate cell volume homeostasis in response to hyperosmotic stress. However, whether mature epithelium respond similarly is not well known. Here we show that while small colonies of MDCK epithelial cells behave similarly to single cells and exhibit volume homeostasis that is dependent on the NKCC channel function, mature epithelial tissue does not. Instead, the cell volume decreases by 33% when confluent monolayers or acini formed from MDCK are subjected to hyperosmotic stress. We show that the tight junction protein, ZO-1, and Rho-associated kinase (ROCK) are essential for osmotic regulation of cell volume in mature epithelium. Since these both are known to be essential for tight junction assembly, this strongly suggest a role for tight junctions in changing volume response in mature epithelium. Thus, tight junctions act either directly or indirectly in osmotic pressure response of epithelial tissue to suppress volume homeostasis common to isolated epithelial cells.

biophysics↗

bioRxiv2021

Asymmetric Contraction of Adherens Junctions arises through RhoA and E-cadherin feedback

Tissue morphogenesis often arises from the culmination of discrete changes in cell-cell junction behaviors, namely ratcheted junction contractions that lead to collective cellular rearrangements. Mechanochemical signaling in the form of RhoA underlies these ratcheted contractions, which occur asymmetrically as one highly motile vertex contracts toward a relatively less motile tricellular vertex. The underlying mechanisms driving asymmetric vertex movement remains unknown. Here, we use optogenetically controlled RhoA in model epithelia together with biophysical modeling to uncover the mechanism lending to asymmetric vertex motion. We find that both local and global RhoA activation leads to increases in junctional tension, thereby facilitating vertex motion. RhoA activation occurs in discrete regions along the junction and is skewed towards the less-motile vertex. At these less-motile vertices, E-cadherin acts as an opposing factor to limit vertex motion through increased frictional drag. Surprisingly, we uncover a feedback loop between RhoA and E-cadherin, as regional optogenetic activation of specified junctional zones pools E-cadherin to the location of RhoA activation. Incorporating this circuit into a mathematical model, we find that a positive feedback between RhoA-mediated tension and E-cadherin-induced frictional drag on tricellular vertices recapitulates experimental data. As such, the location of RhoA determines which vertex is under high tension, pooling E-cadherin and increasing the frictional load at the tricellular vertex to limit its motion. This feedback drives a tension-dependent intercellular "clutch" at tricellular vertices which stabilizes vertex motion upon tensional load.

cell biology↗