Low expression of the ClC-2 chloride channel during postnatal development: a mechanism for the paradoxical depolarizing action of GABA and glycine in the hippocampus

Abstract

In early postnatal development, during the period of synapse formation, -aminobutyric acid (GABA) and glycine, the main inhibitory transmitters in the adult brain, paradoxically excite and depolarize neuronal membranes by an outward £ux of chloride. The mechanisms of chloride homeostasis are not fully understood. It is known that in adult neurons intracellular chloride accumulation is prevented by a particular type of chloride channel, the ClC-2. This channel strongly recti¢es in the inward direction at potentials negative to E Cl thus ensuring chloride e¥ux. We have tested the hypothesis that in the developing hippocampus, a di¡erential expression or regulation of ClC-2 channels may contribute to the depolarizing action of GABA and glycine. We have cloned a truncated form of ClC-2 (ClC-2nh) from the neonatal hippocampus which lacks the 157 bp corresponding to exon 2. In situ hybridization experiments show that ClC-2nh is the predominant form of ClC-2 mRNA in the neonatal brain. ClC-2nh mRNA is unable to encode a full-length protein due to a frameshift, consequently it does not induce any currents upon injection into Xenopus oocytes. Low expression of the full-length ClC-2 channel, could alter chloride homeostasis, lead to accumulation of [Cl 7 ] i and thereby contribute to the depolarizing action of GABA and glycine during early development.

Mutations in CLCN2 encoding a voltage-gated chloride channel are associated with idiopathic generalized epilepsies

“…All affected family members were found to be heterozygous with respect to the mutation G2144A (G715E). [13][14][15] . We sequenced all 24 exons and adjacent splice sites of CLCN2 in two affected individuals from 46 unrelated families with IGE.…”

Mutations affecting GABAergic signaling in seizures and epilepsy

“…This permits the hyperpolarizing actions of GABA A receptors. The developmental increase in CLC-2 has been proposed as one of the factors that contribute to the gradual decline in E Cl in neurons and the appearance of hyperpolarizing GABA A currents [95, 96]. The localization of its gene (CLCN2) within the 3q24 chromosomal region, previously linked to IGEs, prompted the screening of patients with IGEs for mutations in CLCN2.…”

ClC-2 Voltage-Gated Channels Constitute Part of the Background Conductance and Assist Chloride Extrusion

“…Importantly, however, an inhibitory outcome does not require the chloride reversal potential to be more negative than the resting membrane potential, because a major component of inhibition is mediated by shunting (Bartos et al, 2007). It has been proposed that ClC-2 plays a role in chloride homeostasis (Staley, 1994;Staley et al, 1996;Mladinić et al, 1999;Ben-Ari, 2002); however, the extrusion of chloride was thus far only indirectly shown. Here we provide direct evidence that ClC-2 mediates an efflux pathway for chloride in CA1 neurons, which normally have a low [Cl Ϫ ] i .…”