Molecular, functional, and genomic characterization of human KCC2, the neuronal K–Cl cotransporter

Abstract

The expression level of the neuronal-specific K–Cl cotransporter KCC2 (SLC12A5) is a major determinant of whether neurons will respond to GABA with a depolarizing, excitatory response or a hyperpolarizing, inhibitory response. In view of the potential role in human neuronal excitability we have characterized the hKCC2 cDNA and gene. The 5.9 kb hKCC2 transcript is specific to brain, and is induced during in vitro differentiation of NT2 teratocarcinoma cells into neuronal NT2-N cells. The 24-exon SLC12A5 gene is on human chromosome 20q13, and contains a polymorphic dinucleotide repeat within intron 1 near a potential binding site for neuron-restrictive silencing factor. Expression of hKCC2 cRNA in Xenopus laevis oocytes results in significant Cl⁻-dependent ⁸⁶Rb⁺ uptake under isotonic conditions; cell swelling under hypotonic conditions causes a 20-fold activation, which is blocked by the protein phosphatase inhibitor calyculin-A. In contrast, oocytes expressing mouse KCC4 do not mediate isotonic K–Cl cotransport but express much higher absolute transport activity than KCC2 oocytes under hypotonic conditions. Initial and steady state kinetics of hKCC2-injected oocytes were performed in both isotonic and hypotonic conditions, revealing K_ms for K⁺ and Cl⁻ of 9.3±1.8 mM and 6.8±0.9 mM, respectively; both affinities are significantly higher than KCC1 and KCC4. The K_m for Cl⁻ is close to the intracellular Cl⁻ activity of mature neurons, as befits a neuronal efflux mechanism.

Introduction

GABA and glycine are the major inhibitory neurotransmitters in the mature mammalian brain, however there is a brief developmental window in the first week of postnatal life during which they evoke an excitatory response. The activation of glycine and GABA_A receptors in immature neurons thus causes membrane depolarization due to the efflux of Cl⁻ through ligand-gated Cl⁻ channels, resulting in neuronal excitation [46]. The subsequent influx of Ca²⁺, via both voltage-dependent and NMDA-gated Ca²⁺ channels [12], [37], has significant trophic [41] and neurodevelopmental consequences [2], [13], [29]. In contrast, intracellular Cl⁻ in mature neurons is much lower, and activation of GABA_A receptors has a hyperpolarizing and inhibitory effect.

The developmental switch in the effect of GABA and glycine is generated in part by the differential expression of electroneutral cation–chloride transporters [64]. There is thus robust expression of the Na–K–2Cl cotransporter NKCC1 in immature neurons (postnatal day 1–14), with subsequent downregulation in mature neurons [58]. In contrast, expression of the K–Cl cotransporter KCC2 is minimal in immature neurons and increases dramatically after postnatal day 7 [4], [40], [60]. In consequence, immature neurons have a surfeit of inward-directed chloride transport (NKCC1); in mature neurons, the efflux of K–Cl through KCC2 and/or other K–Cl cotransporters is dominant, resulting in lower intracellular chloride and neuronal inhibition in response to GABA and glycine. Selected adult neurons may also express NKCC1, resulting in a depolarizing response to GABA [10]. The functional importance of both NKCC1 and KCC2 in GABA signaling has been shown by both targeted deletion [10], [20], [67] and anti-sense inhibition [60]. GABA itself was recently shown to promote the switch in the neuronal GABA response from excitatory to inhibitory, via direct induction of KCC2 [13].

The gene family of cation–chloride cotransporters (HUGO SLC12, solute carrier family 12) encompasses two Na–K–2Cl cotransporters [48], one Na–Cl cotransporter [48], four K–Cl cotransporters (KCCs) [15], [18], [34], [49], [54], and one homologous protein (CIP) with undefined transport specificity [3]. The corresponding amino acid sequences predict a similar membrane topology, with cytoplasmic amino- and carboxy-terminal domains separated by a central core of 12 hydrophobic transmembrane (TM) domains. The published sequences also predict a glycosylated extracellular loop, which is situated between TM5 and TM6 in CIP and the KCCs, and between TM7 and TM8 in the Na⁺-dependent cotransporters. Shared characteristics of the functional family members include electroneutrality of ion transport, functional dependence on the presence of each transported ion, variable sensitivity to diuretics and other anion transport inhibitors, and regulation by cell volume [34], [48].

The basic functional characteristics of K–Cl cotransport have largely been established in red cells, the cell type in which this transport pathway was first discovered [11], [34], [35]. The emerging characteristics of the four KCCs, as expressed in HEK293 cells [15], [18], [53], [59] and/or Xenopus oocytes [45], [49], [65], [66], are similar in most respects to those of red cell K–Cl cotransport. K–Cl cotransport is activated by cell swelling and the cysteine-alkylating agent N-ethylmaleimide; the effect of both stimuli on red cell K–Cl cotransport is abrogated by inhibition of protein phosphatases, in particular protein phosphatase-1 [6], [34]. Pharmacological inhibitors of variable efficacy include furosemide, bumetanide, DIDS, and the alkaloid DIOA [9], [34], [45], [53]. Kinetic studies indicate that KCC1 transports ⁸⁶rubidium (⁸⁶Rb⁺, a congener of K⁺) with low affinity, whereas KCC2 and KCC4 have higher cation affinities [44], [45], [53]. Phylogenetic comparison also splits the four KCCs into two groups, KCC1 paired with KCC3 and KCC2 with KCC4. Although heterologous expression of the four KCCs reveals a common requirement for serine–threonine phosphatases in their activation by cell swelling, they differ in their respective responses to cell volume. KCC2 appears to be unique in expressing constitutive isotonic activity in both Xenopus oocytes [65] and HEK293 cells [53], whereas the other three KCCs have no activity under isotonic conditions. The four KCCs differ further in their relative activation by cell swelling, however the molecular determinants of this variation are as yet undefined.

KCC2 is thus distinguished from the other KCCs by a restricted expression pattern [40], [73], a defined physiological role [7], [13], [24], [60], constitutive isotonic activity [53], [65], and high cation affinity [53]. A thoughtful analysis of kinetic and electrochemical data leads to the suggestion that rat KCC2 might function in both K–Cl efflux and influx [53]. As extracellular K⁺ accumulates during neuronal activity, the driving force for net K–Cl cotransport will thus switch from efflux to influx. In this regard, the high K⁺ affinity of KCC2 indicates that it is particularly well suited to function as a buffer of external K⁺ and internal Cl⁻. The reversibility of K–Cl cotransport mediated by neuronal KCCs (mostly but not exclusively KCC2) has been verified experimentally [7], [24], [27]. This bi-directional nature of KCC2 also provides an explanation for activity-dependent disinhibition [68], whereby repetitive activation of GABA receptors results in increased extracellular K⁺, an increase in neuronal Cl⁻ due to K–Cl influx through KCC2, and a reduction in the inhibitory GABA effect.

The role of KCC2 in both GABA-ergic inhibition and activity-dependent disinhibition suggest a role for this transporter in seizure generation. Indeed, targeted deletion of the mouse KCC2 gene results in repetitive seizures and early neonatal lethality [10]. To begin to explore the role of the human KCC2 gene in epilepsy and other disorders [69] we have characterized the human KCC2 gene and cDNA. The functional study of hKCC2 in Xenopus oocytes also provides an opportunity to compare this KCC with the other KCC isoforms, as studied in the same expression system [44], [45].