Cation-Chloride Cotransporters, Na/K Pump, and Channels in Cell Water/Ionic Balance Regulation Under Hyperosmolar Conditions: In Silico and Experimental Studies of Opposite RVI and AVD Responses of U937 Cells to Hyperosmolar Media.

Valentina E Yurinskaya,Alexey A Vereninov

doi:10.3389/fcell.2021.830563

Valentina E Yurinskaya, Alexey A Vereninov

Open Access

https://doi.org/10.3389/fcell.2021.830563

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Abstract

Studying the transport of monovalent ions across the cell membrane in living cells is complicated by the strong interdependence of fluxes through parallel pathways and requires therefore computational analysis of the entire electrochemical system of the cell. Current paper shows how to calculate changes in the cell water balance and ion fluxes caused by changes in the membrane channels and transporters during a normal regulatory increase in cell volume in response to osmotic cell shrinkage (RVI) followed by a decrease in cell volume associated with apoptosis (AVD). Our recently developed software is used as a computational analysis tool and the established human lymphoid cells U937 are taken as an example of proliferating animal cells. It is found that, in contrast to countless statements in the literature that cell volume restoration requires the activation of certain ion channels and transporters, the cellular responses such as RVI and AVD can occur in an electrochemical system like U937 cells without any changes in the state of membrane channels or transporters. These responses depend on the types of chloride cotransporters in the membrane and differ in a hyperosmolar medium with additional sucrose and in a medium with additional NaCl. This finding is essential for the identification of the true changes in membrane channels and transporters responsible for RVI and AVD in living cells. It is determined which changes in membrane parameters predicted by computational analysis are consistent with experimental data obtained on living human lymphoid cells U937, Jurkat, and K562 and which are not. An essential part of the results is the developed software that allows researchers without programming experience to calculate the fluxes of monovalent ions via the main transmembrane pathways and electrochemical gradients that move ions across the membrane. The software is available for download. It is useful for studying the functional expression of the channels and transporters in living cells and understanding how the cell electrochemical system works.

Highlights

Many processes at the physiological, proteomic, and transcriptomic levels are triggered in cells already in the first hour after the increase in the osmolarity of the external environment, which is often called “osmotic stress” (Burg et al, 2007; Lambert et al, 2008; Hoffmann et al, 2009; Koivusalo et al, 2009; Wang et al, 2014)
There is no quantitative description of transient processes in the cell electrochemical system caused by replacing the isoosmolar medium with a hyperosmolar medium, which would consider, in addition to the sodium pump and electrically conductive channels, all the main types of cation-chloride cotransporters
We tried to fill this gap using a mathematical analysis of the complex interdependence of ion fluxes via the main pathways across the cell membrane and an experimental study of living U937 cells included determination of cell water content by buoyant density, cell ion content using flame photometry, and optical methods using flow cytometry

Summary

Introduction

Many processes at the physiological, proteomic, and transcriptomic levels are triggered in cells already in the first hour after the increase in the osmolarity of the external environment, which is often called “osmotic stress” (Burg et al, 2007; Lambert et al, 2008; Hoffmann et al, 2009; Koivusalo et al, 2009; Wang et al, 2014). It is found by this way that in U937 cells studied as example: 1) an effect like RVI can take place in hyperosmolar media with addition of NaCl without changes in the membrane channels and transporters if certain cation-chloride cotransporters present in the cell membrane, 2) time-dependent decrease in cell volume, such as AVD, can occur in the hyperosmolar medium of sucrose without changes in membrane ion channels and transporters; 3) the response of living cells to a hypoosmolar challenge is more complex than the response of their electrochemical model due to regulation of transporters by intracellular signaling mechanisms and due to changes in the content of intracellular impermeable osmolytes. These specific effects are identified by eliminating the “physical” effects found by mathematical analysis of the entire electrochemical system of the cell

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