KCC2, best known as the neuron-specific chloride-extruder that sets the strength and polarity of GABAergic currents during neuronal maturation, is a multifunctional molecule that can regulate cytoskeletal dynamics via its C-terminal domain (CTD). We describe the molecular and cellular mechanisms involved in the multiple functions of KCC2 and its splice variants, ranging from developmental apoptosis and the control of early network events to the formation and plasticity of cortical dendritic spines. The versatility of KCC2 actions at the cellular and subcellular levels is also evident in mature neurons during plasticity, disease, and aging. Thus, KCC2 has emerged as one of the most important molecules that shape the overall neuronal phenotype. KCC2 in the Fundamental Machinery Underlying Neuronal Development, Signaling, and StructureFast synaptic transmission relies on ion fluxes through ligand-gated ion channels. Although the default state of a cell is to have a high intracellular chloride concentration ([Cl − ] i ), mature neurons in the CNS have evolved a unique ability to maintain a low [Cl − ] i , which is needed for the generation of hyperpolarizing Cl − currents across GABA A and glycine receptors (GABA A Rs and GlyRs) [1]. This specialization comes at a high energy cost [2], and is brought about by upregulation of the neuron-specific K-Cl cotransporter KCC2 during neuronal maturation [1,3,4]. KCC2 belongs to the evolutionarily ancient family of SLC12 cation/chloride cotransporters (CCCs) (Box 1; for KCCs and NKCCs, see Glossary) that have their roots in a single gene in Archaea, from which numerous duplication events in both archaeans and eukaryotes have led to the divergence and neofunctionalization of the paralogous CCC subfamilies [5]. Possibly because of a primordial role in cellular volume regulation, CCCs have evolved to communicate with the actin cytoskeleton. Indeed, KCC2 has emerged as an important player in controlling actin dynamics during neuronal development and plasticity [6][7][8][9].The functional upregulation of KCC2-mediated K-Cl cotransport in hippocampal and neocortical neurons underlies the hyperpolarizing shift in GABAergic currents which takes place postnatally in rats and mice [3], but it has become clear that KCC2 is expressed at low but functionally significant levels pre-and perinatally. KCC2 acts in an ion transport-independent manner as an antiapoptotic factor in projection neurons in the prenatal mouse neocortex [10]; in the perinatal mouse and rat hippocampus transport-functional KCC2 decreases the depolarizing driving force of GABA A R-mediated currents (DF GABA ), thereby influencing spontaneous network events at their developmental onset [11].In this review we highlight the developmental paths from KCC2 protein expression to its multiple functions, and discuss the wide variety of post-translational mechanisms which control these phenomena. The versatility of KCC2 regulation at the cellular and subcellular levels is also evident in mature neurons during plasticity, disease,...
Background General anesthetics potentiating γ-aminobutyric acid (GABA)–mediated signaling are known to induce a persistent decrement in excitatory synapse number in the cerebral cortex when applied during early postnatal development, while an opposite action is produced at later stages. Here, the authors test the hypothesis that the effect of general anesthetics on synaptogenesis depends upon the efficacy of GABA receptor type A (GABAA)–mediated inhibition controlled by the developmental up-regulation of the potassium-chloride (K-Cl) cotransporter 2 (KCC2). Methods In utero electroporation of KCC2 was used to prematurely increase the efficacy of (GABAA)–mediated inhibition in layer 2/3 pyramidal neurons in the immature rat somatosensory cortex. Parallel experiments with expression of the inward-rectifier potassium channel Kir2.1 were done to reduce intrinsic neuronal excitability. The effects of these genetic manipulations (n = 3 to 4 animals per experimental group) were evaluated using iontophoretic injection of Lucifer Yellow (n = 8 to 12 cells per animal). The total number of spines analyzed per group ranged between 907 and 3,371. Results The authors found a robust effect of the developmental up-regulation of KCC2–mediated Cl− transport on the age-dependent action of propofol on dendritic spines. Premature expression of KCC2, unlike expression of a transport-inactive KCC2 variant, prevented a propofol-induced decrease in spine density. In line with a reduction in neuronal excitability, the above result was qualitatively replicated by overexpression of Kir2.1. Conclusions The KCC2–dependent developmental increase in the efficacy of GABAA–mediated inhibition is a major determinant of the age-dependent actions of propofol on dendritic spinogenesis.
KCC2, encoded in humans by the SLC12A5 gene, is a multifunctional neuron‐specific protein initially identified as the chloride (Cl−) extruder critical for hyperpolarizing GABAA receptor currents. Independently of its canonical function as a K‐Cl cotransporter, KCC2 regulates the actin cytoskeleton via molecular interactions mediated through its large intracellular C‐terminal domain (CTD). Contrary to the common assumption that embryonic neocortical projection neurons express KCC2 at non‐significant levels, here we show that loss of KCC2 enhances apoptosis of late‐born upper‐layer cortical projection neurons in the embryonic brain. In utero electroporation of plasmids encoding truncated, transport‐dead KCC2 constructs retaining the CTD was as efficient as of that encoding full‐length KCC2 in preventing elimination of migrating projection neurons upon conditional deletion of KCC2. This was in contrast to the effect of a full‐length KCC2 construct bearing a CTD missense mutation (KCC2R952H), which disrupts cytoskeletal interactions and has been found in patients with neurological and psychiatric disorders, notably seizures and epilepsy. Together, our findings indicate ion transport‐independent, CTD‐mediated regulation of developmental apoptosis by KCC2 in migrating cortical projection neurons.
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