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

Abstract

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 signaling in neurons. Our findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.

Excitatory GABAergic signalling is associated with acquired benzodiazepine resistance in status epilepticus

“…Previous reports suggest that on-going seizure activity results in reduced surface expression, and function, of the canonical Clextruder KCC2 (Rivera et al, 2004). KCC2 ensures that Clis maintained at levels lower than would be predicted by passive processes and also ameliorates activity-dependent Clloading (Düsterwald et al, 2018). Loss of KCC2, therefore, constitutes a double blow to the system, since it will cause a rise in baseline intraneuronal [Cl -], and reduce the rate of clearance of Cl -, meaning that activity-dependent Clloading is exacerbated.…”

Coincident glutamatergic depolarizations enhance GABAAreceptor-dependent Cl-influx in mature and suppress Cl-efflux in immature neurons

“…Glutamatergic co-stimulation had a direct effect on the GABAergic In this study we investigated the effect of co-activation of glutamatergic AMPA receptors on GABA receptor-dependent [Cl -]i changes over a wide range of initial [Cl -]i, which gave us the opportunity to evaluate the role of ionic plasticity for mature and developing nervous systems [3,40]. In the mature brain GABAergic activity causes an increase in [Cl -]i [11,16,25,41,42], thereby decreasing the inhibitory action of the GABAergic system [13,26,27,33]. The present study demonstrates that glutamatergic co-stimulation enhances the GABAergic Cl --influx and thus enhances the activity-dependent [Cl -]i increase, as has recently been shown in-vitro [32] and insilico [33].…”

“…Ionic plasticity plays an important role for physiological functions [20][21][22] as well as pathophysiological processes [23,24]. Theoretical assumptions and computational studies indicate that the amount and duration of [Cl -]i ionic plasticity directly depends on the relation between Cl − influx and the capacity of Cl − extrusion systems [3,[24][25][26][27][28]. In addition, the size and geometrical structure of the postsynaptic compartments critically affect the magnitude, duration and dimensions of [Cl -]i changes upon GABAergic activation [27,29].…”

Chloride dynamics alter the input-output properties of neurons

“…The idea that [Cl -]i can differ spatially within a particular neuron, depending on the subcellular compartment concerned, is important for understanding the effect of spatially targeted synaptic inhibition (10), as well as the consequences of the latest optogenetic silencing strategies which utilise light-activated Clchannels (32). Many interacting variables determine resting [Cl -]i in neurons (20,22), but subcellular differences [Cl -]i are typically attributed to differences in the activity of cation-chloride cotransporters such as KCC2 or NKCC1. In support of previous work (20,21), we show that the relative amounts of inhibitory and excitatory drive can also produce spatial variations in [Cl -]i despite uniform Clextrusion capacity.…”

“…Whilst the vast majority of theoretical models of inhibitory signalling and neuronal computation assume static values for EGABA, previous studies have addressed the biophysical underpinnings of Clhomeostasis and ionic plasticity in neurons (20,22), the effects of neuronal morphologies on Claccumulation (19,21,23), and how dynamic Claffects neural coding (24). However, how differences in the spatial targeting of synaptic inhibition interact with ionic plasticity to dynamically modulate the input-output function of neurons is not well understood.…”