Lack of Charge Interaction in the Ion Binding Site Determines Anion Selectivity in the Sodium Bicarbonate Cotransporter NBCe1

The Na/HCO3 cotransporter NBCe1 is a member of SLC4A transporters that move HCO3− across cell membranes and regulate intracellular pH or transepithelial HCO3 transport. NBCe1 is highly selective to HCO3− and does not transport other anions; the molecular mechanism of anion selectivity is presently unclear. We previously reported that replacing Asp555 with a Glu (D555E) in NBCe1 induces increased selectivity to other anions, including Cl−. This finding is unexpected because all SLC4A transporters contain either Asp or Glu at the corresponding position and maintain a high selectivity to HCO3−. In this study, we tested whether the Cl− transport in D555E is mediated by an interaction between residues in the ion binding site. Human NBCe1 and mutant transporters were expressed in Xenopus oocytes, and their ability to transport Cl− was assessed by two-electrode voltage clamp. The results show that the Cl− transport is induced by a charge interaction between Glu555 and Lys558. The bond length between the two residues is within the distance for a salt bridge, and the ionic strength experiments confirm an interaction. This finding indicates that the HCO3− selectivity in NBCe1 is established by avoiding a specific charge interaction in the ion binding site, rather than maintaining such an interaction.

Multiphysics and Multiscale Modeling of Coupled Transport of Chloride Ions in Concrete

Chloride ions (Cl−)-induced corrosion is one of the main degradation mechanisms in reinforced concrete (RC) structures. In most situations, the degradation initiates with the transport of Cl− from the surface of the concrete towards the reinforcing steel. The accumulation of Cl− at the steel-concrete interface could initiate reinforcement corrosion once a threshold Cl− concentration is achieved. An accurate numerical model of the Cl− transport in concrete is required to predict the corrosion initiation in RC structures. However, existing numerical models lack a representation of the heterogenous concrete microstructure resulting from the varying environmental conditions and the indirect effect of time dependent temperature and relative humidity (RH) on the water adsorption and Cl− binding isotherms. In this study, a numerical model is developed to study the coupled transport of Cl− with heat, RH and oxygen (O2) into the concrete. The modeling of the concrete microstructure is performed using the Virtual Cement and Concrete Testing Laboratory (VCCTL) code developed by the U.S. National Institute of Standards and Technology (NIST). The concept of equivalent maturation time is utilized to eliminate the limitation of simulating concrete microstructure using VCCTL in specific environmental conditions such as adiabatic. Thus, a time-dependent concrete microstructure, which depends on the hydration reactions coupled with the temperature and RH of the environment, is achieved to study the Cl− transport. Additionally, Cl− binding isotherms, which are a function of the pH of the concrete pore solution, Cl− concentration, and weight fraction of mono-sulfate aluminate (AFm) and calcium-silicate-hydrate (C-S-H), obtained from an experimental study by the same authors are utilized to account for the Cl− binding of cement hydration products. The temperature dependent RH diffusion was considered to account for the transport of Cl− with moisture transport. The temperature and RH diffusion in the concrete domain, composite theory, and Cl− binding and water adsorption isotherms are used in combination, to estimate the ensuing Cl− diffusion field within the concrete. The coupled transport process of heat, RH, Cl−, and O2 is implemented in the Multiphysics Object-Oriented Simulation Environment (MOOSE) developed by the U.S. Idaho National Laboratory (INL). The model was verified and validated using data from multiple experimental studies with different concrete mixture proportions, curing durations, and environmental conditions. Additionally, a sensitivity analysis was performed to identify that the water-to-cement (w/c) ratio, the exposure duration, the boundary conditions: temperature, RH, surface Cl− concentration, Cl− diffusion coefficient in the capillary water, and the critical RH are the important parameters that govern the Cl− transport in RC structures. In a case study, the capabilities of the developed numerical model are demonstrated by studying the complex 2D diffusion of Cl− in a RC beam located in two different climatic regions: warm and humid weather in Galveston, Texas, and cold and dry weather in North Minnesota, Minnesota, subjected to time varying temperature, RH, and surface Cl− concentrations.

Interactions between Membrane Resistance, GABA-A Receptor Properties, Bicarbonate Dynamics and Cl−-Transport Shape Activity-Dependent Changes of Intracellular Cl− Concentration

The effects of ionotropic γ-aminobutyric acid receptor (GABA-A, GABAA) activation depends critically on the Cl−-gradient across neuronal membranes. Previous studies demonstrated that the intracellular Cl−-concentration ([Cl−]i) is not stable but shows a considerable amount of activity-dependent plasticity. To characterize how membrane properties and different molecules that are directly or indirectly involved in GABAergic synaptic transmission affect GABA-induced [Cl−]i changes, we performed compartmental modeling in the NEURON environment. These simulations demonstrate that GABA-induced [Cl−]i changes decrease at higher membrane resistance, revealing a sigmoidal dependency between both parameters. Increase in GABAergic conductivity enhances [Cl−]i with a logarithmic dependency, while increasing the decay time of GABAA receptors leads to a nearly linear enhancement of the [Cl−]i changes. Implementing physiological levels of HCO3−-conductivity to GABAA receptors enhances the [Cl−]i changes over a wide range of [Cl−]i, but this effect depends on the stability of the HCO3− gradient and the intracellular pH. Finally, these simulations show that pure diffusional Cl−-elimination from dendrites is slow and that a high activity of Cl−-transport is required to improve the spatiotemporal restriction of GABA-induced [Cl−]i changes. In summary, these simulations revealed a complex interplay between several key factors that influence GABA-induced [Cl]i changes. The results suggest that some of these factors, including high resting [Cl−]i, high input resistance, slow decay time of GABAA receptors and dynamic HCO3− gradient, are specifically adapted in early postnatal neurons to facilitate limited activity-dependent [Cl−]i decreases.