Chloride Homeostasis in Neurons With Special Emphasis on the Olivocerebellar System: Differential Roles for Transporters and Channels

Abstract

The intraneuronal ionic composition is an important determinant of brain functioning. There is growing evidence that aberrant homeostasis of the intracellular concentration of Cl^- ([Cl^-]_i) evokes, in addition to that of Na⁺ and Ca²⁺, robust impairments of neuronal excitability and neurotransmission and thereby neurological conditions. More specifically, understanding the mechanisms underlying regulation of [Cl^-]_i is crucial for deciphering the variability in GABAergic and glycinergic signaling of neurons, in both health and disease. The homeostatic level of [Cl^-]_i is determined by various regulatory mechanisms, including those mediated by plasma membrane Cl^- channels and transporters. This review focuses on the latest advances in identification, regulation and characterization of Cl^- channels and transporters that modulate neuronal excitability and cell volume. By putting special emphasis on neurons of the olivocerebellar system, we establish that Cl^- channels and transporters play an indispensable role in determining their [Cl^-]_i and thereby their function in sensorimotor coordination.

Keywords: GABAergic inhibition; cerebellar motor learning; chloride homeostasis; chloride transporters and channels; olivocerebellar system.

Figure 1

GABAergic signaling in olivo-cerebellar circuit. (A) Schematic representation of a sagittal section of the mouse olivo-cerebellar system (left). Inferior olivary neurons (shown in blue) project to the cerebellar cortex (gray) and excite Purkinje cells (brown), as well as the deeply located cerebellar nuclei (CN) neurons (pink). A particular subset of CN neurons projects back to the inferior olive (IO), forming the olivo-cortico-nuclear loop. The right panel demonstrates the anatomical circuit of the cerebellar cortical neurons and their connectivity with CN and the IO. ML, molecular layer; PCL, Purkinje cell layer; GCL, granule cell layer; PC, Purkinje cell; GrC, granule cell; SC, stellate cell; BC, basket cell; GoC, Golgi cell; PF, parallel fiber; CF, climbing fiber; MF, mossy fiber; CN, cerebellar nuclei; IO, inferior olive. (B) The level of intracellular chloride concentration ([Cl⁻]_i) dictates the polarity of the current through GABA_A receptors (GABA_ARs). If [Cl⁻]_i is low, the reversal potential of Cl⁻ (E_Cl) becomes negative compared to the resting membrane potential (RMP). In this condition GABA_ARs mediate an inward Cl⁻ current that results in hyperpolarization of the cell membrane (a). In contrast, high [Cl⁻]_i results in a positive shift of E_Cl and leads to an outward Cl⁻ current through GABA_AR and depolarization of the cell membrane that potentially induces action potential firing (c). In conditions where E_Cl shifts to values similar to RMP, there will be no net Cl⁻ current through GABA_ARs (b).

Figure 2

Diagram of chloride channels and transporters that are expressed in the olivocerebellar neurons. The intracellular Cl⁻ concentration in all neurons, including the neurons of cerebellum and inferior olive, is maintained by activation of various transmembrane anion channels and transporters. These anion transporters/channels also interact with H⁺ exchangers/channels, such as NHE1 and H⁺-ATPase, and thereby are involved in intracellular pH regulation. Cell membrane depolarization, through voltage-gated Na⁺ channels (VGSCs), activates several Cl⁻ channels/transporters, such as SLC26A11 (KBAT), to repolarize the cell by mediating inward Cl⁻ currents. In addition, depolarizations activate voltage-gated Ca²⁺ channels (VGCCs) and the resulting rise in intracellular Ca²⁺ levels, which may be aided by internal Ca²⁺ stores, may lead to activation of Ca²⁺ sensitive Cl⁻ channels, such as anoctamin-2 (Ano2). Transmembrane movements of Cl⁻ together with cations such as K⁺ and Na⁺ also control cell volume through transport of water molecules. Channels and transporters which belong to the same family of proteins are shown with a similar color.