Biology subjects

Kay, A. R.

Publications and source records attributed to Kay, A. R..

6 recordsLinked to original sources

bioRxiv2026

A pump-leak model of epithelia reveals the roles of paracellular pathways in water transport & the importance of boundary conditions

We develop a pump-leak model of epithelial transport, where the flux of water through the epithelium is modeled as a compartment, or sink, whose volume grows linearly with time. A method is described for calculating the voltage in the sink compartment, as well as analytical solutions to the system of equations. We show how growing boundary conditions can be used to model a scenario where water and ion fluxes are possible in the steady state. It is possible to drive a continuous flux of water through an epithelium that is powered by the Na+ pump establishing small osmotic gradients between compartments. We show that paracellular pathways play a crucial role in the operation of epithelial transport by ensuring that the flux through the epithelium is electroneutral. For the Na+ pump situated on the basolateral membrane, water fluxes can be directed in the same direction as the Na+ ion by having a high Na+ conductance on the apical membrane and high K+ conductance on the basolateral membrane. Flipping these conductances changes the direction of water fluxes. We introduce a graphic formalism that makes clear the precise mechanism of water transfer, showing the osmotic and electrical constraints. We demonstrate that it is possible when the extracellular osmolarity is low (~10mOsm) for the intracellular osmolarity to exceed that of the former considerably and produce a hyperosmolar solution.

physiology↗

bioRxiv2024

Macromolecular condensation is unlikely to buffer intracellular osmolality

The flux of water across biological membrane is determined by gradients in transmembrane osmolality and the water permeability of membranes. Watson et al. (2023) have proposed that the reversible formation and disassembly of molecular condensates could act as the primary buffer of cytoplasmic osmolality in the face of changes in extracellular osmolality. In this communication, we show using well-established membrane biophysics, that the water permeability of plasma membranes is likely to overwhelm any cytoplasmic water buffers.

cell biology↗

bioRxiv2023

The Physical Basis of Osmosis

Osmosis is an important force in all living organisms, yet the molecular basis of osmosis is widely misunderstood as arising from differences in water concentration in solutions of differing osmolarities. In 1923 Debye proposed a physical model for a semipermeable membrane that was hardly noticed at the time and slipped out of view. We show that Debyes analysis of vant Hoffs law for osmotic equilibrium provides a consistent and plausible explanation for osmotic flow. A difference in osmolyte concentrations in solutions separated by a semipermeable membrane generates different pressures at the two water-membrane interfaces. Water is therefore driven through the membrane for exactly the same reason that pure water flows in response to an imposed hydrostatic pressure difference. In this paper we present the Debye model in both equilibrium and flow conditions. We point out its applicability regardless of the nature of the membrane with examples ranging from predominantly convective flow of water through synthetic membranes to purely diffusive flow of independent water molecules through a lipid bilayer and the flow of strongly interacting water molecules in single file across narrow protein channels.

biophysics↗

bioRxiv2022

Mathematical modeling of ion homeostasis & cell volume stabilization: impact of ion transporters, impermeant molecules, & Donnan effect

The pump-leak mechanism (PLM) first, described by Tosteson and Hoffman (1960), demonstrates how the activity of the Na+ - K+ ATPase (NKA) can counteract the osmotic influx of water stimulated by the presence of impermeant intracellular molecules. We derive analytical solutions for the steady state ion concentrations, voltage, and volume of a cell, by including impermeant extracellular molecules, variable impermeant charge, and Cation-Chloride Co-transporters (CCC). We demonstrate that impermeant extracellular molecules could stabilize a cell without NKA activity but argue that it is unlikely to play a significant role in vivo. Significantly we have shown that the precise form of the NKA is unimportant for determining the steady state in PLMs. We have derived an analytical expression for the steady state of the PLM with one of the Cation-Chloride Co-transporters, either KCC, NCC, or NKCC, active. Notably, we have demonstrated that NCC at high pump rates can destabilize cells, which could account for the rarity of this co-transporter. In addition, we show that the reversal of any of the CCCs is unlikely. Importantly, we link the thermodynamics of the NKA to the PLM to show that there is a natural limit to the energy utilized by the PLM that prevents futile cycles. We show that the average charge on the intracellular impermeant molecules influences ion distributions but has no impact on energy utilization. Our study shows that analytical mathematical solutions from physically well-grounded models provide insight into ion transport systems that could only be obtained from numerical simulations with great difficulty. Significance StatementThe regulation of cell volume is fundamental to the stability of all tissue. Animal cells regulate their volume by actively pumping sodium and potassium ions, preventing the waters osmotic influx from blowing up the cell. Based on the physical laws that determine ion and water fluxes, we derive equations that allow one to predict how pump rates and ion conductances combine to stabilize cell volume. The action of the sodium pump consumes about 30% of a cells energy budget, and we demonstrate the rate of ion pumping is constrained so that cells do not consume excessive energy. Our work also demonstrates the power of closed-form mathematical equations in characterizing such pump-leak systems.

physiology↗

bioRxiv2021

A sensitive and specific genetically encodable biosensor for potassium ions

Potassium ions (K+) play a critical role as an essential electrolyte in all biological systems. Here we report the crystal structure-guided optimization and directed evolution of an improved genetically encoded fluorescent K+ biosensor, GINKO2. GINKO2 is highly sensitive and specific for K+ and enables in vivo detection of K+ dynamics in multiple species.

bioengineering↗

bioRxiv2017

Biophysical models reveal the relative importance of transporter proteins and impermeant anions in chloride homeostasis

Fast synaptic inhibition in the nervous system depends on the transmembrane flux of Cl- ions based on the neuronal Cl- driving force. Established theories regarding the determinants of Cl- driving force have recently been questioned. Here we present biophysical models of Cl- homeostasis using the pump-leak model. Using numerical and novel analytic solutions, we demonstrate that the Na+/K+-ATPase, ion conductances, impermeant anions, electrodiffusion, water fluxes and cation-chloride cotransporters (CCCs) play roles in setting the Cl- driving force. Our models, together with experimental validation, show that while impermeant anions can contribute to setting [Cl-]i in neurons, they have a negligible effect on the driving force for Cl- locally and cell-wide. In contrast, we demonstrate that CCCs are well-suited for modulating Cl- driving force and hence inhibitory signalling in neurons. Our findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.

neuroscience↗