Hyperpolarizing GABAergic transmission requires the KCC2 C-terminal ISO domain

Neuronal K+-Cl- cotransporter KCC2 as a promising drug target for epilepsy treatment

Epilepsy is a prevalent neurological disorder characterized by unprovoked seizures. γ-Aminobutyric acid (GABA) serves as the primary fast inhibitory neurotransmitter in the brain, and GABA binding to the GABAA receptor (GABAAR) regulates Cl- and bicarbonate (HCO3-) influx or efflux through the channel pore, leading to GABAergic inhibition or excitation, respectively. The neuron-specific K+-Cl- cotransporter 2 (KCC2) is essential for maintaining a low intracellular Cl- concentration, ensuring GABAAR-mediated inhibition. Impaired KCC2 function results in GABAergic excitation associated with epileptic activity. Loss-of-function mutations and altered expression of KCC2 lead to elevated [Cl-]i and compromised synaptic inhibition, contributing to epilepsy pathogenesis in human patients. KCC2 antagonism studies demonstrate the necessity of limiting neuronal hyperexcitability within the brain, as reduced KCC2 functioning leads to seizure activity. Strategies focusing on direct (enhancing KCC2 activation) and indirect KCC2 modulation (altering KCC2 phosphorylation and transcription) have proven effective in attenuating seizure severity and exhibiting anti-convulsant properties. These findings highlight KCC2 as a promising therapeutic target for treating epilepsy. Recent advances in understanding KCC2 regulatory mechanisms, particularly via signaling pathways such as WNK, PKC, BDNF, and its receptor TrkB, have led to the discovery of novel small molecules that modulate KCC2. Inhibiting WNK kinase or utilizing newly discovered KCC2 agonists has demonstrated KCC2 activation and seizure attenuation in animal models. This review discusses the role of KCC2 in epilepsy and evaluates its potential as a drug target for epilepsy treatment by exploring various strategies to regulate KCC2 activity.

Both chloride-binding sites are required for KCC2-mediated transport

The K+-Cl- cotransporter 2 (KCC2) plays an important role in inhibitory neurotransmission, and its impairment is associated with neurological and psychiatric disorders, including epilepsy, schizophrenia, and autism. Although KCCs transport K+ and Cl- in a 1:1 stoichiometry, two Cl- coordination sites were indicated via cryo-EM. In a comprehensive analysis, we analyzed the consequences of point mutations of residues coordinating Cl- in Cl1 and Cl2. Individual mutations of residues in Cl1 and Cl2 reduce or abolish KCC2WT function, indicating a crucial role of both Cl- coordination sites for KCC2 function. Structural changes in the extracellular loop 2 by inserting a 3xHA tag switches the K+ coordination site to another position. To investigate, whether the extension of the extracellular loop 2 with the 3xHA tag also affects the coordination of the two Cl- coordination sites, we carried out the analogous experiments for both Cl- coordinating sites in the KCC2HA construct. These analyses showed that most of the individual mutation of residues in Cl1 and Cl2 in the KCC2HA construct reduces or abolishes KCC2 function, indicating that the coordination of Cl- remains at the same position. However, the coupling of K+ and Cl- in Cl1 is still apparent in the KCC2HA construct, indicating a mutual dependence of both ions. In addition, the coordination residue Tyr569 in Cl2 shifted in KCC2HA. Thus, conformational changes in the extracellular domain affect K+ and Cl--binding sites. However, the effect on the Cl--binding sites is subtler.

SLC12A cryo-EM: analysis of relevant ion binding sites, structural domains, and amino acids

The solute carrier family 12A (SLC12A) superfamily of membrane transporters modulates the movement of cations coupled with chloride across the membrane. In doing so, these cotransporters are involved in numerous aspects of human physiology: cell volume regulation, ion homeostasis, blood pressure regulation, and neurological action potential via intracellular chloride concentration modulation. Their physiological characterization has been largely studied; however, understanding the mechanics of their function and the relevance of structural domains or specific amino acids has been a pending task. In recent years, single-particle cryogenic electron microscopy (cryo-EM) has been successfully applied to members of the SLC12A family including all K+:Cl- cotransporters (KCCs), Na+:K+:2Cl- cotransporter NKCC1, and recently Na+:Cl- cotransporter (NCC); revealing structural elements that play key roles in their function. The present review analyzes the data provided by these cryo-EM reports focusing on structural domains and specific amino acids involved in ion binding, domain interactions, and other important SCL12A structural elements. A comparison of cryo-EM data from NKCC1 and KCCs is presented in the light of the two recent NCC cryo-EM studies, to propose insight into structural elements that might also be found in NCC and are necessary for its proper function. In the final sections, the importance of key coordination residues for substrate specificity and their implication on various pathophysiological conditions and genetic disorders is reviewed, as this could provide the basis to correlate structural elements with the development of novel and selective treatments, as well as mechanistic insight into the function and regulation of cation-coupled chloride cotransporters (CCCs).

Chloride transporters controlling neuronal excitability

Synaptic inhibition plays a crucial role in regulating neuronal excitability, which is the foundation of nervous system function. This inhibition is largely mediated by the neurotransmitters GABA and glycine that activate Cl^--permeable ion channels, which means that the strength of inhibition depends on the Cl^- gradient across the membrane. In neurons, the Cl^- gradient is primarily mediated by two secondarily active cation-chloride cotransporters (CCCs), NKCC1 and KCC2. CCC-mediated regulation of the neuronal Cl^- gradient is critical for healthy brain function, as dysregulation of CCCs has emerged as a key mechanism underlying neurological disorders including epilepsy, neuropathic pain, and autism spectrum disorder. This review begins with an overview of neuronal chloride transporters before explaining the dependent relationship between these CCCs, Cl^- regulation, and inhibitory synaptic transmission. We then discuss the evidence for how CCCs can be regulated, including by activity and their protein interactions, which underlie inhibitory synaptic plasticity. For readers who may be interested in conducting experiments on CCCs and neuronal excitability, we have included a section on techniques for estimating and recording intracellular Cl^-, including their advantages and limitations. Although the focus of this review is on neurons, we also examine how Cl^- is regulated in glial cells, which in turn regulate neuronal excitability through the tight relationship between this nonneuronal cell type and synapses. Finally, we discuss the relatively extensive and growing literature on how CCC-mediated neuronal excitability contributes to neurological disorders.

WNK3 Maintains the GABAergic Inhibitory Tone, Synaptic Excitation and Neuronal Excitability via Regulation of KCC2 Cotransporter in Mature Neurons

The activation of chloride (Cl⁻)permeable gamma (γ)-aminobutyric acid type A(GABA_A) receptors induces synaptic inhibition in mature and excitation in immature neurons. This developmental “switch” in GABA function controlled by its polarity depends on the postnatal decrease in intraneuronal Cl⁻ concentration mediated by KCC2, a member of cation-chloride cotransporters (CCCs). The serine-threonine kinase WNK3 (With No Lysine [K]), is a potent regulator of all CCCs and is expressed in neurons. Here, we characterized the functions of WNK3 and its role in GABAergic signaling in cultured embryonic day 18 (E18) hippocampal neurons. We observed a decrease in WNK3 expression as neurons mature. Knocking down of WNK3 significantly hyperpolarized E_GABA in mature neurons (DIV13–15) but had no effect on immature neurons (DIV6–8). This hyperpolarized E_GABA in WNK3-deficient neurons was not due to the total expression of NKCC1 and KCC2, that remained unchanged. However, there was a reduction in phosphorylated KCC2 at the membrane, suggesting an increase in KCC2 chloride export activity. Furthermore, hyperpolarized E_GABA observed in WNK3-deficient neurons can be reversed by the KCC2 inhibitor, VU024055, thus indicating that WNK3 acts through KCC2 to influence E_GABA. Notably, WNK3 knockdown resulted in morphological changes in mature but not immature neurons. Electrophysiological characterization of WNK3-deficient mature neurons revealed reduced capacitances but increased intrinsic excitability and synaptic excitation. Hence, our study demonstrates that WNK3 maintains the “adult” GABAergic inhibitory tone in neurons and plays a role in the morphological development of neurons and excitability.

Structural changes in the extracellular loop 2 of the murine KCC2 potassium chloride cotransporter modulate ion transport

K+-Cl- cotransporters (KCCs) play important roles in physiological processes such as inhibitory neurotransmission and cell-volume regulation. KCCs exhibit significant variations in K+ affinities, yet recent atomic structures demonstrated that K+- and Cl--binding sites are highly conserved, raising the question of whether additional structural elements may contribute to ion coordination. The termini and the large extracellular domain (ECD) of KCCs exhibit only low sequence identity and were already discussed as modulators of transport activity. Here, we used the extracellular loop 2 (EL2) that links transmembrane helices (TMs) 3 and 4, as a mechanism to modulate ECD folding. We compared consequences of point mutations in the K+-binding site on the function of WT KCC2 and in a KCC2 variant, in which EL2 was structurally altered by insertion of a IFYPYDVPDYAGYPYDVPDYAGSYPYDVPDYAAHAAA (3xHA) tag (36 amino acids). In WT KCC2, individual mutations of five residues in the K+-binding site resulted in a 2- to 3-fold decreased transport rate. However, the same mutations in the KCC2 variant with EL2 structurally altered by insertion of a 3xHA tag had no effect on transport activity. Homology models of mouse KCC2 with the 3xHA tag inserted into EL2 using ab initio prediction were generated. The models suggest subtle conformational changes occur in the ECD upon EL2 modification. These data suggest that a conformational change in the ECD, for example, by interaction with EL2, might be an elegant way to modulate the K+ affinity of the different isoforms in the KCC subfamily.

Depolarizing GABA Transmission Restrains Activity-Dependent Glutamatergic Synapse Formation in the Developing Hippocampal Circuit

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mature brain but has the paradoxical property of depolarizing neurons during early development. Depolarization provided by GABA_A transmission during this early phase regulates neural stem cell proliferation, neural migration, neurite outgrowth, synapse formation, and circuit refinement, making GABA a key factor in neural circuit development. Importantly, depending on the context, depolarizing GABA_A transmission can either drive neural activity or inhibit it through shunting inhibition. The varying roles of depolarizing GABA_A transmission during development, and its ability to both drive and inhibit neural activity, makes it a difficult developmental cue to study. This is particularly true in the later stages of development when the majority of synapses form and GABA_A transmission switches from depolarizing to hyperpolarizing. Here, we addressed the importance of depolarizing but inhibitory (or shunting) GABA_A transmission in glutamatergic synapse formation in hippocampal CA1 pyramidal neurons. We first showed that the developmental depolarizing-to-hyperpolarizing switch in GABA_A transmission is recapitulated in organotypic hippocampal slice cultures. Based on the expression profile of K⁺−Cl⁻ co-transporter 2 (KCC2) and changes in the GABA reversal potential, we pinpointed the timing of the switch from depolarizing to hyperpolarizing GABA_A transmission in CA1 neurons. We found that blocking depolarizing but shunting GABA_A transmission increased excitatory synapse number and strength, indicating that depolarizing GABA_A transmission can restrain glutamatergic synapse formation. The increase in glutamatergic synapses was activity-dependent but independent of BDNF signaling. Importantly, the elevated number of synapses was stable for more than a week after GABA_A inhibitors were washed out. Together these findings point to the ability of immature GABAergic transmission to restrain glutamatergic synapse formation and suggest an unexpected role for depolarizing GABA_A transmission in shaping excitatory connectivity during neural circuit development.

Chloride homeodynamics underlying modal shifts in cellular and network oscillations

γ-Aminobutyric acid (GABA) generally induces hyperpolarization and inhibition in the adult brain, but causes depolarization (and can be excitatory) in the immature brain. Depolarizing GABA actions are necessary for neurogenesis, differentiation, migration, and synaptogenesis. Additionally, the conversion of GABA responses from inhibition to excitation can be induced in adults by pathological conditions. Because GABA_A receptors are Cl^- channels, alternating GABA actions between hyperpolarization (Cl^- influx) and depolarization (Cl^- efflux) are induced by changes in the Cl^- gradient, which is regulated by C^- transporters. Thus, the dynamics of neural functions are modulated by active Cl^- homeostasis (Cl^- homeodynamics), alternating inhibition and excitation, and could underlie the modal shifts in cellular and network oscillations. Such a modal shift in GABA actions is required for normal development. Thus disturbances in this developmental GABA modal shift and/or the induction of excitatory GABA action in adult could underlie the pathogenesis of diverse neurological diseases (so-called network diseases).

Role of NKCC1 and KCC2 in Epilepsy: From Expression to Function

As a main inhibitory neurotransmitter in the central nervous system, γ-aminobutyric acid (GABA) activates chloride-permeable GABAa receptors (GABAa Rs) and induces chloride ion (Cl⁻) flow, which relies on the intracellular chloride concentration ([Cl⁻]_i) of the postsynaptic neuron. The Na-K-2Cl cotransporter isoform 1 (NKCC1) and the K-Cl cotransporter isoform 2 (KCC2) are two main cation-chloride cotransporters (CCCs) that have been implicated in human epilepsy. NKCC1 and KCC2 reset [Cl⁻]_i by accumulating and extruding Cl⁻, respectively. Previous studies have shown that the profile of NKCC1 and KCC2 in neonatal neurons may reappear in mature neurons under some pathophysiological conditions, such as epilepsy. Although increasing studies focusing on the expression of NKCC1 and KCC2 have suggested that impaired chloride plasticity may be closely related to epilepsy, additional neuroelectrophysiological research aimed at studying the functions of NKCC1 and KCC2 are needed to understand the exact mechanism by which they induce epileptogenesis. In this review, we aim to briefly summarize the current researches surrounding the expression and function of NKCC1 and KCC2 in epileptogenesis and its implications on the treatment of epilepsy. We will also explore the potential for NKCC1 and KCC2 to be therapeutic targets for the development of novel antiepileptic drugs.

Calpain fosters the hyperexcitability of motoneurons after spinal cord injury and leads to spasticity

Up-regulation of the persistent sodium current (INaP) and down-regulation of the potassium/chloride extruder KCC2 lead to spasticity after spinal cord injury (SCI). We here identified calpain as the driver of the up- and down-regulation of INaP and KCC2, respectively, in neonatal rat lumbar motoneurons. Few days after SCI, neonatal rats developed behavioral signs of spasticity with the emergence of both hyperreflexia and abnormal involuntary muscle contractions on hindlimbs. At the same time, in vitro isolated lumbar spinal cords became hyperreflexive and displayed numerous spontaneous motor outputs. Calpain-I expression paralleled with a proteolysis of voltage-gated sodium (Nav) channels and KCC2. Acute inhibition of calpains reduced this proteolysis, restored the motoneuronal expression of Nav and KCC2, normalized INaP and KCC2 function, and curtailed spasticity. In sum, by up- and down-regulating INaP and KCC2, the calpain-mediated proteolysis of Nav and KCC2 drives the hyperexcitability of motoneurons which leads to spasticity after SCI.

Crossing the Chloride Channel: The Current and Potential Therapeutic Value of the Neuronal K+-Cl- Cotransporter KCC2

Chloride (Cl-) homeostasis is an essential process involved in neuronal signalling and cell survival. Inadequate regulation of intracellular Cl- interferes with synaptic signalling and is implicated in several neurological diseases. The main inhibitory neurotransmitter of the central nervous system is γ-aminobutyric acid (GABA). GABA hyperpolarises the membrane potential by activating Cl- permeable GABAA receptor channels (GABAAR). This process is reliant on Cl- extruder K+-Cl- cotransporter 2 (KCC2), which generates the neuron's inward, hyperpolarising Cl- gradient. KCC2 is encoded by the fifth member of the solute carrier 12 family (SLC12A5) and has remained a poorly understood component in the development and severity of many neurological diseases for many years. Recent advancements in next-generation sequencing and specific gene targeting, however, have indicated that loss of KCC2 activity is involved in a number of diseases including epilepsy and schizophrenia. It has also been implicated in neuropathic pain following spinal cord injury. Any variant of SLC12A5 that negatively regulates the transporter's expression may, therefore, be implicated in neurological disease. A recent whole exome study has discovered several causative mutations in patients with epilepsy. Here, we discuss the implications of KCC2 in neurological disease and consider the evolving evidence for KCC2's potential as a therapeutic target.

KCC2 membrane diffusion tunes neuronal chloride homeostasis

Neuronal Cl^- homeostasis is regulated by the activity of two cation chloride co-transporters (CCCs), the K⁺-Cl^- cotransporter KCC2 and the Na⁺-K⁺-Cl^- cotransporter NKCC1, which are primarily extruding and importing chloride in neurons, respectively. Several neurological and psychiatric disorders including epilepsy, neuropathic pain, schizophrenia and autism are associated with altered neuronal chloride (Cl^-) homeostasis. A current view is that the accumulation of intracellular Cl^- in neurons as a result of KCC2 down-regulation and/or NKCC1 up-regulation may weaken inhibitory GABA signaling and thereby promote the development of pathological activities. CCC activity is determined mainly by their level of expression in the plasma membrane. Furthermore, CCCs undergo "diffusion-trapping" in the membrane, a mechanism that is rapidly adjusted by activity-dependent post-translational modifications i.e. phosphorylation/dephosphorylation of key serine and threonine residues. This represents probably the most rapid cellular mechanism for adapting CCC function to changes in neuronal activity. Therefore, interfering with these mechanisms may help restoring Cl^- homeostasis and inhibition under pathological conditions. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Ionotropic and metabotropic kainate receptor signalling regulates Cl- homeostasis and GABAergic inhibition

KEY POINTS

Potassium-chloride co-transporter 2 (KCC2) plays a critical role in regulating chloride homeostasis, which is essential for hyperpolarizing inhibition in the mature nervous system. KCC2 interacts with many proteins involved in excitatory neurotransmission, including the GluK2 subunit of the kainate receptor (KAR). We show that activation of KARs hyperpolarizes the reversal potential for GABA (EGABA ) via both ionotropic and metabotropic signalling mechanisms. KCC2 is required for the metabotropic KAR-mediated regulation of EGABA , although ionotropic KAR signalling can hyperpolarize EGABA independent of KCC2 transporter function. The KAR-mediated hyperpolarization of EGABA is absent in the GluK1/2-/- mouse and is independent of zinc release from mossy fibre terminals. The ability of KARs to regulate KCC2 function may have implications in diseases with disrupted excitation: inhibition balance, such as epilepsy, neuropathic pain, autism spectrum disorders and Down's syndrome.

ABSTRACT

Potassium-chloride co-transporter 2 (KCC2) plays a critical role in the regulation of chloride (Cl- ) homeostasis within mature neurons. KCC2 is a secondarily active transporter that extrudes Cl- from the neuron, which maintains a low intracellular Cl- concentration [Cl- ]. This results in a hyperpolarized reversal potential of GABA (EGABA ), which is required for fast synaptic inhibition in the mature central nervous system. KCC2 also plays a structural role in dendritic spines and at excitatory synapses, and interacts with 'excitatory' proteins, including the GluK2 subunit of kainate receptors (KARs). KARs are glutamate receptors that display both ionotropic and metabotropic signalling. We show that activating KARs in the hippocampus hyperpolarizes EGABA , thus strengthening inhibition. This hyperpolarization occurs via both ionotropic and metabotropic KAR signalling in the CA3 region, whereas it is absent in the GluK1/2-/- mouse, and is independent of zinc release from mossy fibre terminals. The metabotropic signalling mechanism is dependent on KCC2, although the ionotropic signalling mechanism produces a hyperpolarization of EGABA even in the absence of KCC2 transporter function. These results demonstrate a novel functional interaction between a glutamate receptor and KCC2, a transporter critical for maintaining inhibition, suggesting that the KAR:KCC2 complex may play an important role in excitatory:inhibitory balance in the hippocampus. Additionally, the ability of KARs to regulate chloride homeostasis independently of KCC2 suggests that KAR signalling can regulate inhibition via multiple mechanisms. Activation of kainate-type glutamate receptors could serve as an important mechanism for increasing the strength of inhibition during periods of strong glutamatergic activity.

Reciprocal Regulation of KCC2 Trafficking and Synaptic Activity

The main inhibitory neurotransmitter receptors in the adult central nervous system (CNS) are type A γ-aminobutyric acid receptors (GABA_ARs) and glycine receptors (GlyRs). Synaptic responses mediated by GlyR and GABA_AR display a hyperpolarizing shift during development. This shift relies mainly on the developmental up-regulation of the K⁺-Cl^- co-transporter KCC2 responsible for the extrusion of Cl^-. In mature neurons, altered KCC2 function-mainly through increased endocytosis-leads to the re-emergence of depolarizing GABAergic and glycinergic signaling, which promotes hyperexcitability and pathological activities. Identifying signaling pathways and molecular partners that control KCC2 surface stability thus represents a key step in the development of novel therapeutic strategies. Here, we present our current knowledge on the cellular and molecular mechanisms governing the plasma membrane turnover rate of the transporter under resting conditions and in response to synaptic activity. We also discuss the notion that KCC2 lateral diffusion is one of the first parameters modulating the transporter membrane stability, allowing for rapid adaptation of Cl^- transport to changes in neuronal activity.

Locally Reducing KCC2 Activity in the Hippocampus is Sufficient to Induce Temporal Lobe Epilepsy

Mesial temporal lobe epilepsy (mTLE) is the most common form of epilepsy, believed to arise in part from compromised GABAergic inhibition. The neuronal specific K+/Cl- co-transporter 2 (KCC2) is a critical determinant of the efficacy of GABAergic inhibition and deficits in its activity are observed in mTLE patients and animal models of epilepsy. To test if reductions of KCC2 activity directly contribute to the pathophysiology of mTLE, we locally ablated KCC2 expression in a subset of principal neurons within the adult hippocampus. Deletion of KCC2 resulted in compromised GABAergic inhibition and the development of spontaneous, recurrent generalized seizures. Moreover, local ablation of KCC2 activity resulted in hippocampal sclerosis, a key pathological change seen in mTLE. Collectively, our results demonstrate that local deficits in KCC2 activity within the hippocampus are sufficient to precipitate mTLE.

KCC2-GABAA pathway correlates with the analgesic effect of electro-acupuncture in CCI rats

Potassium-chloride cotransporter 2 (KCC2) has been indicated to serve a crucial role during chronic neuropathic pain (NP). Following the emergence of NP, γ‑aminobutyric acid (GABA) A receptor‑mediated signaling may be further impaired by the changes of KCC2 chloride anion gradient. In the present study, the authors investigate the effect of electro-acupuncture (EA) on the behavior and the expression of KCC2 and GABAA receptor γ2 subunit in the spinal cord of chronic constriction injury (CCI) model rats. A total of 60 adult male Sprague‑Dawley rats were divided into four groups: Normal group, sham‑CCI group, CCI group and CCI+EA group. The effect of EA was assessed via the values of mechanical withdrawal threshold and thermal withdrawal latency, which were significantly improved upon stimulation of the ST‑36 and GB‑34 acupoints. In addition, a marked reduction in both the mRNA and protein levels of KCC2 and GABAA receptor γ2 subunit was observed in the spinal cord following loose ligation of the sciatic nerve. The reductions in KCC2 and GABAA receptor γ2 subunit expression were reversed by EA treatment. These results support the notion that KCC2 and GABAA receptor γ2 subunit contribute to NP following peripheral nerve injury and extend the understanding of the analgesic effects of EA on NP.

Inability to suppress the stress-induced activation of the HPA axis during the peripartum period engenders deficits in postpartum behaviors in mice

The stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis is normally suppressed during pregnancy. Dysregulation of the HPA axis has been proposed to play a role in postpartum depression. However, direct investigation into the relationship between the HPA axis and postpartum depression has been hindered by the lack of useful animal models. Building on our discovery of a role for the K+/Cl-co-transporter, KCC2, in the GABAergic regulation of CRH neurons in the paraventricular nucleus of the hypothalamus (PVN), critical for mounting the body's physiological response to stress, we assessed the role of KCC2 in the regulation of the HPA axis during pregnancy and the postpartum period. Here we demonstrate that the normal suppression of the stress-induced activation of the HPA axis during the peripartum period involves maintenance of KCC2 in the PVN. Mice lacking KCC2 specifically in corticosterone-releasing hormone (CRH) neurons, which govern the activity of the HPA axis (KCC2/Crh mice), exhibit dysregulation of the HPA axis and abnormal postpartum behaviors. Loss of KCC2 specifically in CRH neurons in the PVN is sufficient to reproduce the depression-like phenotype and deficits in maternal behaviors during the postpartum period. Similarly, chemogenetic activation of CRH neurons in the PVN is sufficient to induce abnormal postpartum behaviors and chemogenetic silencing of CRH neurons in the PVN can ameliorate abnormal postpartum behaviors observed in KCC2/Crh mice. This study demonstrates that dysregulation of the HPA axis is sufficient to induce abnormal postpartum behaviors and deficits in maternal behaviors in mice, providing empirical support for a role of HPA axis dysfunction in the pathophysiology of postpartum depression.

Emerging Mechanisms Underlying Dynamics of GABAergic Synapses

Inhibitory circuits are diverse, yet with a poorly understood cell biology. Functional characterization of distinct inhibitory neuron subtypes has not been sufficient to explain how GABAergic neurotransmission sculpts principal cell activity in a relevant fashion. Our Mini-Symposium brings together several emerging mechanisms that modulate GABAergic neurotransmission dynamically from either the presynaptic or the postsynaptic site. The first two talks discuss novel developmental and neuronal subtype-specific contributions to the excitatory/inhibitory balance and circuit maturation. The next three talks examine how interactions between cellular pathways, lateral diffusion of proteins between synapses, and chloride transporter function at excitatory and inhibitory synapses and facilitate inhibitory synapse adaptations. Finally, we address functional differences within GABAergic interneurons to highlight the importance of diverse, flexible, and versatile inputs that shape network function. Together, the selection of topics demonstrates how developmental and activity-dependent mechanisms coordinate inhibition in relation to the excitatory inputs and vice versa.

K+-Cl- co-transporter 2 (KCC2) - a membrane trafficking perspective

K⁺-Cl^- co-transporter 2 (KCC2/SLC12A5) is a neuronal specific cation chloride co-transporter which is active under isotonic conditions, and thus a key regulator of intracellular Cl^- levels. It also has an ion transporter-independent structural role in modulating the maturation and regulation of excitatory glutamatergic synapses. KCC2 levels are developmentally regulated, and a postnatal upregulation of KCC2 generates a low intracellular chloride concentration that allows the neurotransmitters γ-aminobutyric acid (GABA) and glycine to exert inhibitory neurotransmission through its Cl^- permeating channel. Functional expression of KCC2 at the neuronal cell surface is necessary for its activity, and impairment in KCC2 cell surface transport and/or internalization may underlie a range of neuropathological conditions. Although recent advances have shed light on a range of cellular mechanisms regulating KCC2 activity, little is known about its membrane trafficking itinerary and regulatory proteins. In this review, known membrane trafficking signals, pathways and mechanisms pertaining to KCC2's functional surface expression are discussed.

Implications of the N-terminal heterogeneity for the neuronal K-Cl cotransporter KCC2 function

The neuron-specific K-Cl cotransporter KCC2 maintains the low intracellular chloride concentration required for the fast hyperpolarizing responses of the inhibitory neurotransmitters γ-aminobutyric acid (GABA) and glycine. The two KCC2 isoforms, KCC2a and KCC2b differ by their N-termini as a result of alternative promoter usage. Whereas the role of KCC2b in mediating the chloride transport is unequivocal, the physiological role of KCC2a in neurons has remained obscure. We show that KCC2a isoform can decrease the intracellular chloride concentration in cultured neurons and attenuate calcium responses evoked by application of the GABAA receptor agonist muscimol. While the biotinylation assay detected both KCC2 isoforms at the cell surface of cultured neurons, KCC2a was not detected at the plasma membrane in immunostainings, suggesting that the N-terminal KCC2a epitope is masked. Confirming this hypothesis, KCC2a surface expression was detected by the C-terminal KCC2 pan antibody but not by the N-terminal KCC2a antibody in KCC2b-deficient neurons. One possible cause for the epitope masking is the binding site of Ste20-related proline-alanine-rich kinase (SPAK) in the KCC2a N-terminus. SPAK, a known regulator of K-Cl cotransporters, was co-immunoprecipitated in a complex with KCC2a but not KCC2b isoform. Moreover, SPAK overexpression decreased the transport activity of KCC2a but not that of KCC2b, as revealed by rubidium flux assay in HEK293 cells. Thus, our data indicate that both KCC2 isoforms perform as chloride cotransporters in neuronal cells, while their N-terminal heterogeneity could play an important role in fine-tuning of the K-Cl transport activity.

Native KCC2 interactome reveals PACSIN1 as a critical regulator of synaptic inhibition

KCC2 is a neuron-specific K+-Cl- cotransporter essential for establishing the Cl- gradient required for hyperpolarizing inhibition in the central nervous system (CNS). KCC2 is highly localized to excitatory synapses where it regulates spine morphogenesis and AMPA receptor confinement. Aberrant KCC2 function contributes to human neurological disorders including epilepsy and neuropathic pain. Using functional proteomics, we identified the KCC2-interactome in the mouse brain to determine KCC2-protein interactions that regulate KCC2 function. Our analysis revealed that KCC2 interacts with diverse proteins, and its most predominant interactors play important roles in postsynaptic receptor recycling. The most abundant KCC2 interactor is a neuronal endocytic regulatory protein termed PACSIN1 (SYNDAPIN1). We verified the PACSIN1-KCC2 interaction biochemically and demonstrated that shRNA knockdown of PACSIN1 in hippocampal neurons increases KCC2 expression and hyperpolarizes the reversal potential for Cl-. Overall, our global native-KCC2 interactome and subsequent characterization revealed PACSIN1 as a novel and potent negative regulator of KCC2.

A Novel View on the Role of Intracellular Tails in Surface Delivery of the Potassium-Chloride Cotransporter KCC2

A plethora of neurological disorders are associated with alterations in the expression and localization of potassium-chloride cotransporter type 2 (KCC2), making KCC2 a critical player in neuronal function and an attractive target for therapeutic treatment. The activity of KCC2 is determined primarily by the rates of its surface insertion and internalization. Currently the domains of KCC2 dictating its trafficking and endocytosis are unknown. Here, using live-cell immunolabeling and biotinylation of KCC2 proteins expressed in murine neuroblastoma N2a cells, human embryonic kidney 293 cells, or primary cultures of rat hippocampal neurons, we identified a novel role for the intracellular N and C termini in differentially regulating KCC2 surface expression. We report that the N terminus is required for KCC2 insertion into the plasma membrane, whereas the C terminus is critical for the membrane stability of KCC2. Our results provide novel insights into the structure-function role of specific KCC2 domains and open perspectives in exploring structural organization of this protein.

A kainate receptor subunit promotes the recycling of the neuron-specific K+-Cl- co-transporter KCC2 in hippocampal neurons

Synaptic inhibition depends on a transmembrane gradient of chloride, which is set by the neuron-specific K+-Cl- co-transporter KCC2. Reduced KCC2 levels in the neuronal membrane contribute to the generation of epilepsy, neuropathic pain, and autism spectrum disorders; thus, it is important to characterize the mechanisms regulating KCC2 expression. In the present study, we determined the role of KCC2-protein interactions in regulating total and surface membrane KCC2 expression. Using quantitative immunofluorescence in cultured mouse hippocampal neurons, we discovered that the kainate receptor subunit GluK2 and the auxiliary subunit Neto2 significantly increase the total KCC2 abundance in neurons but that GluK2 exclusively increases the abundance of KCC2 in the surface membrane. Using a live cell imaging assay, we further determined that KCC2 recycling primarily occurs within 1-2 h and that GluK2 produces an ∼40% increase in the amount of KCC2 recycled to the membrane during this time period. This GluK2-mediated increase in surface recycling translated to a significant increase in KCC2 expression in the surface membrane. Moreover, we found that KCC2 recycling is enhanced by protein kinase C-mediated phosphorylation of the GluK2 C-terminal residues Ser-846 and Ser-868. Lastly, using gramicidin-perforated patch clamp recordings, we found that the GluK2-mediated increase in KCC2 recycling to the surface membrane translates to a hyperpolarization of the reversal potential for GABA (EGABA). In conclusion, our results have revealed a mechanism by which kainate receptors regulate KCC2 expression in the hippocampus.

Gain-of-function missense variant in SLC12A2, encoding the bumetanide-sensitive NKCC1 cotransporter, identified in human schizophrenia

Perturbations of γ-aminobutyric acid (GABA) neurotransmission in the human prefrontal cortex have been implicated in the pathogenesis of schizophrenia (SCZ), but the mechanisms are unclear. NKCC1 (SLC12A2) is a Cl(-)-importing cation-Cl(-) cotransporter that contributes to the maintenance of depolarizing GABA activity in immature neurons, and variation in SLC12A2 has been shown to increase the risk for schizophrenia via alterations of NKCC1 mRNA expression. However, no disease-causing mutations or functional variants in NKCC1 have been identified in human patients with SCZ. Here, by sequencing three large French-Canadian (FC) patient cohorts of SCZ, autism spectrum disorders (ASD), and intellectual disability (ID), we identified a novel heterozygous NKCC1 missense variant (p.Y199C) in SCZ. This variant is located in an evolutionarily conserved residue in the critical N-terminal regulatory domain and exhibits high predicted pathogenicity. No NKCC1 variants were detected in ASD or ID, and no KCC3 variants were identified in any of the three neurodevelopmental disorder cohorts. Functional experiments show Y199C is a gain-of-function variant, increasing Cl(-)-dependent and bumetanide-sensitive NKCC1 activity even in conditions in which the transporter is normally functionally silent (hypotonicity). These data are the first to describe a functional missense variant in SLC12A2 in human SCZ, and suggest that genetically encoded dysregulation of NKCC1 may be a risk factor for, or contribute to the pathogenesis of, human SCZ.

The K(+)-Cl(-) Cotransporter KCC2 and Chloride Homeostasis: Potential Therapeutic Target in Acute Central Nervous System Injury

The K(+)-Cl(-) cotransporter-2 (KCC2) is a well-known member of the electroneutral cation-chloride cotransporters with a restricted expression pattern to neurons. This transmembrane protein mediates the efflux of Cl(-) out of neurons and exerts a critical role in inhibitory γ-aminobutyric acidergic (GABAergic) and glycinergic neurotransmission. Moreover, KCC2 participates in the regulation of various physiological processes of neurons, including cell migration, dendritic outgrowth, spine morphology, and dendritic synaptogenesis. It is important to note that down-regulation of KCC2 is associated with the pathogenesis of multiple neurological diseases, which is of particular relevance to acute central nervous system (CNS) injury. In this review, we aim to survey the pathogenic significance of KCC2 down-regulation under the condition of acute CNS injuries. We propose that further elucidation of the molecular mechanisms regarding KCC2 down-regulation after acute CNS injuries is necessary because of potential promising avenues for prevention and treatment of acute CNS injury.

Regulatory domain or CpG site variation in SLC12A5, encoding the chloride transporter KCC2, in human autism and schizophrenia

Many encoded gene products responsible for neurodevelopmental disorders (NDs) like autism spectrum disorders (ASD), schizophrenia (SCZ), intellectual disability (ID), and idiopathic generalized epilepsy (IGE) converge on networks controlling synaptic function. An increase in KCC2 (SLC12A5) Cl⁻ transporter activity drives the developmental GABA excitatory-inhibitory sequence, but the role of KCC2 in human NDs is essentially unknown. Here, we report two rare, non-synonymous (NS), functionally-impairing variants in the KCC2 C-terminal regulatory domain (CTRD) in human ASD (R952H and R1049C) and SCZ (R952H) previously linked with IGE and familial febrile seizures, and another novel NS KCC2 variant in ASD (R1048W) with highly-predicted pathogenicity. Exome data from 2517 simplex families in the ASD Simon Simplex Collection (SSC) revealed significantly more KCC2 CTRD variants in ASD cases than controls, and interestingly, these were more often synonymous and predicted to disrupt or introduce a CpG site. Furthermore, full gene analysis showed ASD cases are more likely to contain rare KCC2 variants affecting CpG sites than controls. These data suggest genetically-encoded dysregulation of KCC2-dependent GABA signaling may contribute to multiple human NDs.

Neto2-null mice have impaired GABAergic inhibition and are susceptible to seizures

Neto2 is a transmembrane protein that interacts with the neuron-specific K⁺-Cl⁻ cotransporter (KCC2) in the central nervous system (CNS). Efficient KCC2 transport is essential for setting the neuronal Cl⁻ gradient, which is required for fast GABAergic inhibition. Neto2 is required to maintain the normal abundance of KCC2 in neurons, and increases KCC2 function by binding to the active oligomeric form of this cotransporter. In the present study, we characterized GABAergic inhibition and KCC2-mediated neuronal chloride homeostasis in pyramidal neurons from adult hippocampal slices. Using gramicidin perforated patch clamp recordings we found that the reversal potential for GABA (E_GABA) was significantly depolarized. We also observed that surface levels of KCC2 and phosphorylation of KCC2 serine 940 (Ser940) were reduced in Neto2^−/− neurons compared to wild-type controls. To examine GABAergic inhibition we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) and found that Neto2^−/− neurons had significant reductions in both their amplitude and frequency. Based on the critical role of Neto2 in regulating GABAergic inhibition we rationalized that Neto2-null mice would be prone to seizure activity. We found that Neto2-null mice demonstrated a decrease in the latency to pentylenetetrazole (PTZ)-induced seizures and an increase in seizure severity.

Molecular and evolutionary insights into the structural organization of cation chloride cotransporters

Cation chloride cotransporters (CCC) play an essential role for neuronal chloride homeostasis. K(+)-Cl(-) cotransporter (KCC2), is the principal Cl(-)-extruder, whereas Na(+)-K(+)-Cl(-) cotransporter (NKCC1), is the major Cl(-)-uptake mechanism in many neurons. As a consequence, the action of the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine strongly depend on the activity of these two transporters. Knowledge of the mechanisms involved in ion transport and regulation is thus of great importance to better understand normal and disturbed brain function. Although no overall 3-dimensional crystal structures are yet available, recent molecular and phylogenetic studies and modeling have provided new and exciting insights into structure-function relationships of CCC. Here, we will summarize our current knowledge of the gross structural organization of the proteins, their functional domains, ion binding and translocation sites, and the established role of individual amino acids (aa). A major focus will be laid on the delineation of shared and distinct organizational principles between KCC2 and NKCC1. Exploiting the richness of recently generated genome data across the tree of life, we will also explore the molecular evolution of these features.

A novel regulatory locus of phosphorylation in the C terminus of the potassium chloride cotransporter KCC2 that interferes with N-ethylmaleimide or staurosporine-mediated activation

The neuron-specific cation chloride cotransporter KCC2 plays a crucial role in hyperpolarizing synaptic inhibition. Transporter dysfunction is associated with various neurological disorders, raising interest in regulatory mechanisms. Phosphorylation has been identified as a key regulatory process. Here, we retrieved experimentally observed phosphorylation sites of KCC2 from public databases and report on the systematic analysis of six phosphorylated serines, Ser(25), Ser(26), Ser(937), Ser(1022), Ser(1025), and Ser(1026). Alanine or aspartate substitutions of these residues were analyzed in HEK-293 cells. All mutants were expressed in a pattern similar to wild-type KCC2 (KCC2(WT)). Tl(+) flux measurements demonstrated unchanged transport activity for Ser(25), Ser(26), Ser(1022), Ser(1025), and Ser(1026) mutants. In contrast, KCC2(S937D), mimicking phosphorylation, resulted in a significant up-regulation of transport activity. Aspartate substitution of Thr(934), a neighboring putative phosphorylation site, resulted in a comparable increase in KCC2 transport activity. Both KCC2(T934D) and KCC2(S937D) mutants were inhibited by the kinase inhibitor staurosporine and by N-ethylmaleimide, whereas KCC2(WT), KCC2(T934A), and KCC2(S937A) were activated. The inverse staurosporine effect on aspartate versus alanine substitutions reveals a cross-talk between different phosphorylation sites of KCC2. Immunoblot and cell surface labeling experiments detected no alterations in total abundance or surface expression of KCC2(T934D) and KCC2(S937D) compared with KCC2(WT). These data reveal kinetic regulation of transport activity by these residues. In summary, our data identify a novel key regulatory phosphorylation site of KCC2 and a functional interaction between different conformation-changing post-translational modifications. The action of pharmacological agents aimed to modulate KCC2 activity for therapeutic benefit might therefore be highly context-specific.

Genetically encoded impairment of neuronal KCC2 cotransporter function in human idiopathic generalized epilepsy

The KCC2 cotransporter establishes the low neuronal Cl(-) levels required for GABAA and glycine (Gly) receptor-mediated inhibition, and KCC2 deficiency in model organisms results in network hyperexcitability. However, no mutations in KCC2 have been documented in human disease. Here, we report two non-synonymous functional variants in human KCC2, R952H and R1049C, exhibiting clear statistical association with idiopathic generalized epilepsy (IGE). These variants reside in conserved residues in the KCC2 cytoplasmic C-terminus, exhibit significantly impaired Cl(-)-extrusion capacities resulting in less hyperpolarized Gly equilibrium potentials (EG ly), and impair KCC2 stimulatory phosphorylation at serine 940, a key regulatory site. These data describe a novel KCC2 variant significantly associated with a human disease and suggest genetically encoded impairment of KCC2 functional regulation may be a risk factor for the development of human IGE.

Kainate receptors coexist in a functional complex with KCC2 and regulate chloride homeostasis in hippocampal neurons

KCC2 is the neuron-specific K+-Cl(-) cotransporter required for maintaining low intracellular Cl(-), which is essential for fast inhibitory synaptic transmission in the mature CNS. Despite the requirement of KCC2 for inhibitory synaptic transmission, understanding of the cellular mechanisms that regulate KCC2 expression and function is rudimentary. We examined KCC2 in its native protein complex in vivo to identify key KCC2-interacting partners that regulate KCC2 function. Using blue native-polyacrylamide gel electrophoresis (BN-PAGE), we determined that native KCC2 exists in a macromolecular complex with kainate-type glutamate receptors (KARs). We found that KAR subunits are required for KCC2 oligomerization and surface expression. In accordance with this finding, acute and chronic genetic deletion of KARs decreased KCC2 function and weakened synaptic inhibition in hippocampal neurons. Our results reveal KARs as regulators of KCC2, significantly advancing our growing understanding of the tight interplay between excitation and inhibition.

Current view on the functional regulation of the neuronal K(+)-Cl(-) cotransporter KCC2

In the mammalian central nervous system (CNS), the inhibitory strength of chloride (Cl(-))-permeable GABAA and glycine receptors (GABAAR and GlyR) depends on the intracellular Cl(-) concentration ([Cl(-)]i). Lowering [Cl(-)]i enhances inhibition, whereas raising [Cl(-)]i facilitates neuronal activity. A neuron's basal level of [Cl(-)]i, as well as its Cl(-) extrusion capacity, is critically dependent on the activity of the electroneutral K(+)-Cl(-) cotransporter KCC2, a member of the SLC12 cation-Cl(-) cotransporter (CCC) family. KCC2 deficiency compromises neuronal migration, formation and the maturation of GABAergic and glutamatergic synaptic connections, and results in network hyperexcitability and seizure activity. Several neurological disorders including multiple epilepsy subtypes, neuropathic pain, and schizophrenia, as well as various insults such as trauma and ischemia, are associated with significant decreases in the Cl(-) extrusion capacity of KCC2 that result in increases of [Cl(-)]i and the subsequent hyperexcitability of neuronal networks. Accordingly, identifying the key upstream molecular mediators governing the functional regulation of KCC2, and modifying these signaling pathways with small molecules, might constitute a novel neurotherapeutic strategy for multiple diseases. Here, we discuss recent advances in the understanding of the mechanisms regulating KCC2 activity, and of the role these mechanisms play in neuronal Cl(-) homeostasis and GABAergic neurotransmission. As KCC2 mediates electroneutral transport, the experimental recording of its activity constitutes an important research challenge; we therefore also, provide an overview of the different methodological approaches utilized to monitor function of KCC2 in both physiological and pathological conditions.

Modulation of neuronal activity by phosphorylation of the K-Cl cotransporter KCC2

The K-Cl cotransporter KCC2 establishes the low intraneuronal Cl- levels required for the hyperpolarizing inhibitory postsynaptic potentials mediated by ionotropic γ-aminobutyric acid receptors (GABAARs) and glycine receptors (GlyRs). Decreased KCC2-mediated Cl- extrusion and impaired hyperpolarizing GABAAR- and/or GlyR-mediated currents have been implicated in epilepsy, neuropathic pain, and spasticity. Recent evidence suggests that the intrinsic ion transport rate, cell surface stability, and plasmalemmal trafficking of KCC2 are rapidly and reversibly modulated by the (de)phosphorylation of critical serine, threonine, and tyrosine residues in the C terminus of this protein. Alterations in KCC2 phosphorylation have been associated with impaired KCC2 function in several neurological diseases. Targeting KCC2 phosphorylation directly or indirectly via upstream regulatory kinases might be a novel strategy to modulate GABA- and/or glycinergic signaling for therapeutic benefit.

Activity-dependent regulation of the K/Cl transporter KCC2 membrane diffusion, clustering, and function in hippocampal neurons

The neuronal K/Cl transporter KCC2 exports chloride ions and thereby influences the efficacy and polarity of GABA signaling in the brain. KCC2 is also critical for dendritic spine morphogenesis and the maintenance of glutamatergic transmission in cortical neurons. Because KCC2 plays a pivotal role in the function of central synapses, it is of particular importance to understand the cellular and molecular mechanisms underlying its regulation. Here, we studied the impact of membrane diffusion and clustering on KCC2 function. KCC2 forms clusters in the vicinity of both excitatory and inhibitory synapses. Using quantum-dot-based single-particle tracking on rat primary hippocampal neurons, we show that KCC2 is slowed down and confined at excitatory and inhibitory synapses compared with extrasynaptic regions. However, KCC2 escapes inhibitory synapses faster than excitatory synapses, reflecting stronger molecular constraints at the latter. Interfering with KCC2-actin interactions or inhibiting F-actin polymerization releases diffusion constraints on KCC2 at excitatory but not inhibitory synapses. Thus, F-actin constrains KCC2 diffusion at excitatory synapses, whereas KCC2 is confined at inhibitory synapses by a distinct mechanism. Finally, increased neuronal activity rapidly increases the diffusion coefficient and decreases the dwell time of KCC2 at excitatory synapses. This effect involves NMDAR activation, Ca(2+) influx, KCC2 S940 dephosphorylation and calpain protease cleavage of KCC2 and is accompanied by reduced KCC2 clustering and ion transport function. Thus, activity-dependent regulation of KCC2 lateral diffusion and clustering allows for a rapid regulation of chloride homeostasis in neurons.

Refuting the challenges of the developmental shift of polarity of GABA actions: GABA more exciting than ever!

During brain development, there is a progressive reduction of intracellular chloride associated with a shift in GABA polarity: GABA depolarizes and occasionally excites immature neurons, subsequently hyperpolarizing them at later stages of development. This sequence, which has been observed in a wide range of animal species, brain structures and preparations, is thought to play an important role in activity-dependent formation and modulation of functional circuits. This sequence has also been considerably reinforced recently with new data pointing to an evolutionary preserved rule. In a recent “Hypothesis and Theory Article,” the excitatory action of GABA in early brain development is suggested to be “an experimental artefact” (Bregestovski and Bernard, 2012). The authors suggest that the excitatory action of GABA is due to an inadequate/insufficient energy supply in glucose-perfused slices and/or to the damage produced by the slicing procedure. However, these observations have been repeatedly contradicted by many groups and are inconsistent with a large body of evidence including the fact that the developmental shift is neither restricted to slices nor to rodents. We summarize the overwhelming evidence in support of both excitatory GABA during development, and the implications this has in developmental neurobiology.

Activity-dependent cleavage of the K-Cl cotransporter KCC2 mediated by calcium-activated protease calpain

The K-Cl cotransporter KCC2 plays a crucial role in neuronal chloride regulation. In mature central neurons, KCC2 is responsible for the low intracellular Cl(-) concentration ([Cl(-)](i)) that forms the basis for hyperpolarizing GABA(A) receptor-mediated responses. Fast changes in KCC2 function and expression have been observed under various physiological and pathophysiological conditions. Here, we show that the application of protein synthesis inhibitors cycloheximide and emetine to acute rat hippocampal slices have no effect on total KCC2 protein level and K-Cl cotransporter function. Furthermore, blocking constitutive lysosomal degradation with leupeptin did not induce significant changes in KCC2 protein levels. These findings indicate a low basal turnover rate of the total KCC2 protein pool. In the presence of the glutamate receptor agonist NMDA, the total KCC2 protein level decreased to about 30% within 4 h, and this effect was blocked by calpeptin and MDL-28170, inhibitors of the calcium-activated protease calpain. Interictal-like activity induced by incubation of hippocampal slices in an Mg(2+)-free solution led to a fast reduction in KCC2-mediated Cl(-) transport efficacy in CA1 pyramidal neurons, which was paralleled by a decrease in both total and plasmalemmal KCC2 protein. These effects were blocked by the calpain inhibitor MDL-28170. Taken together, these findings show that calpain activation leads to cleavage of KCC2, thereby modulating GABAergic signaling.