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

Two Signaling Modes Are Better than One: Flux-Independent Signaling by Ionotropic Glutamate Receptors Is Coming of Age

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamatergic transmission can be mediated by ionotropic glutamate receptors (iGluRs), which mediate rapid synaptic depolarization that can be associated with Ca2+ entry and activity-dependent change in the strength of synaptic transmission, as well as by metabotropic glutamate receptors (mGluRs), which mediate slower postsynaptic responses through the recruitment of second messenger systems. A wealth of evidence reported over the last three decades has shown that this dogmatic subdivision between iGluRs and mGluRs may not reflect the actual physiological signaling mode of the iGluRs, i.e., α-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA) receptors (AMPAR), kainate receptors (KARs), and N-methyl-D-aspartate (NMDA) receptors (NMDARs). Herein, we review the evidence available supporting the notion that the canonical iGluRs can recruit flux-independent signaling pathways not only in neurons, but also in brain astrocytes and cerebrovascular endothelial cells. Understanding the signaling versatility of iGluRs can exert a profound impact on our understanding of glutamatergic synapses. Furthermore, it may shed light on novel neuroprotective strategies against brain disorders.

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.

How Staying Negative Is Good for the (Adult) Brain: Maintaining Chloride Homeostasis and the GABA-Shift in Neurological Disorders

Excitatory-inhibitory (E-I) imbalance has been shown to contribute to the pathogenesis of a wide range of neurodevelopmental disorders including autism spectrum disorders, epilepsy, and schizophrenia. GABA neurotransmission, the principal inhibitory signal in the mature brain, is critically coupled to proper regulation of chloride homeostasis. During brain maturation, changes in the transport of chloride ions across neuronal cell membranes act to gradually change the majority of GABA signaling from excitatory to inhibitory for neuronal activation, and dysregulation of this GABA-shift likely contributes to multiple neurodevelopmental abnormalities that are associated with circuit dysfunction. Whilst traditionally viewed as a phenomenon which occurs during brain development, recent evidence suggests that this GABA-shift may also be involved in neuropsychiatric disorders due to the “dematuration” of affected neurons. In this review, we will discuss the cell signaling and regulatory mechanisms underlying the GABA-shift phenomenon in the context of the latest findings in the field, in particular the role of chloride cotransporters NKCC1 and KCC2, and furthermore how these regulatory processes are altered in neurodevelopmental and neuropsychiatric disorders. We will also explore the interactions between GABAergic interneurons and other cell types in the developing brain that may influence the GABA-shift. Finally, with a greater understanding of how the GABA-shift is altered in pathological conditions, we will briefly outline recent progress on targeting NKCC1 and KCC2 as a therapeutic strategy against neurodevelopmental and neuropsychiatric disorders associated with improper chloride homeostasis and GABA-shift abnormalities.

Gephyrin Interacts with the K-Cl Cotransporter KCC2 to Regulate Its Surface Expression and Function in Cortical Neurons

The K+-Cl- cotransporter KCC2, encoded by the Slc12a5 gene, is a neuron-specific chloride extruder that tunes the strength and polarity of GABAA receptor-mediated transmission. In addition to its canonical ion transport function, KCC2 also regulates spinogenesis and excitatory synaptic function through interaction with a variety of molecular partners. KCC2 is enriched in the vicinity of both glutamatergic and GABAergic synapses, the activity of which in turn regulates its membrane stability and function. KCC2 interaction with the submembrane actin cytoskeleton via 4.1N is known to control its anchoring near glutamatergic synapses on dendritic spines. However, the molecular determinants of KCC2 clustering near GABAergic synapses remain unknown. Here, we used proteomics to identify novel KCC2 interacting proteins in the adult rat neocortex. We identified both known and novel candidate KCC2 partners, including some involved in neuronal development and synaptic transmission. These include gephyrin, the main scaffolding molecule at GABAergic synapses. Gephyrin interaction with endogenous KCC2 was confirmed by immunoprecipitation from rat neocortical extracts. We showed that gephyrin stabilizes plasmalemmal KCC2 and promotes its clustering in hippocampal neurons, mostly but not exclusively near GABAergic synapses, thereby controlling KCC2-mediated chloride extrusion. This study identifies gephyrin as a novel KCC2 anchoring molecule that regulates its membrane expression and function in cortical neurons.SIGNIFICANCE STATEMENT Fast synaptic inhibition in the brain is mediated by chloride-permeable GABAA receptors (GABAARs) and therefore relies on transmembrane chloride gradients. In neurons, these gradients are primarily maintained by the K/Cl cotransporter KCC2. Therefore, understanding the mechanisms controlling KCC2 expression and function is crucial to understand its physiological regulation and rescue its function in the pathology. KCC2 function depends on its membrane expression and clustering, but the underlying mechanisms remain unknown. We describe the interaction between KCC2 and gephyrin, the main scaffolding protein at inhibitory synapses. We show that gephyrin controls plasmalemmal KCC2 clustering and that loss of gephyrin compromises KCC2 function. Our data suggest functional units comprising GABAARs, gephyrin, and KCC2 act to regulate synaptic GABA signaling.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Kainate receptors regulate the functional properties of young adult-born dentate granule cells

Both inhibitory and excitatory neurotransmitter receptors can influence maturation and survival of adult-born neurons in the dentate gyrus; nevertheless, how these two neurotransmitter systems affect integration of new neurons into the existing circuitry is still not fully characterized. Here, we demonstrate that glutamate receptors of the kainate receptor (KAR) subfamily are expressed in adult-born dentate granule cells (abDGCs) and that, through their interaction with GABAergic signaling mechanisms, they alter the functional properties of adult-born cells during a critical period of their development. Both the intrinsic properties and synaptic connectivity of young abDGCs were affected. Timed KAR loss in a cohort of young adult-born neurons in mice disrupted their performance in a spatial discrimination task but not in a hippocampal-dependent fear conditioning task. Together, these results demonstrate the importance of KARs in the proper functional development of young abDGCs.

Kainate receptors in the developing neuronal networks

Kainate receptors (KARs) are highly expressed in the immature brain and have unique developmentally regulated functions that may be important in linking neuronal activity to morphogenesis during activity-dependent fine-tuning of the synaptic connectivity. Altered expression of KARs in the developing neural network leads to changes in glutamatergic connectivity and network excitability, which may lead to long-lasting changes in behaviorally relevant circuitries in the brain. Here, we summarize the current knowledge on physiological and morphogenic functions described for different types of KARs at immature neural circuitries, focusing on their roles in modulating synaptic transmission and plasticity as well as circuit maturation in the rodent hippocampus and amygdala. Finally, we discuss the emerging evidence suggesting that malfunction of KARs in the immature brain may contribute to the pathophysiology underlying developmentally originating neurological disorders.

NETO2 promotes esophageal cancer progression by inducing proliferation and metastasis via PI3K/AKT and ERK pathway

Esophageal squamous cell carcinoma (ESCC) causes aggressive and lethal malignancies with extremely poor prognoses, and accounts for about 90% of cases of esophageal cancer. Neuropilin and tolloid-like 2 (NETO2) protein coding genes have been associated with various human cancers. Nevertheless, little information is reported about the phenotypic expression and its clinical significance in ESCC progression. Here, our study found that NETO2 expression in ESCC patients was associated with tumor clinical stage and lymph node metastasis status. Gain-of-function and loss-of-function analyses showed that NETO2 stimulated ESCC cell proliferation while suppressing apoptosis in vitro and enhanced tumor growth in vivo. Moreover, knockdown of NETO2 significantly inhibited migration and invasion in combination with regulation of epithelial-mesenchymal transition (EMT) related markers. Mechanistically, overexpression of NETO2 increased the phosphorylation of ERK, PI3k/AKT, and Nuclear factor erythroid-2-related factor 2(Nrf2), whereas silencing NETO2 decreased the phosphorylation of these targets. Our data suggest that Nrf2 was a critical downstream event responsible for triggering the PI3K/AKT and ERK signaling pathways and plays a crucial role in NETO2-mediated tumorigenesis. Taken together, NETO2 acts as an oncogene and might serve as a novel therapeutic target or prognostic biomarker in ESCC patients.

Kainate Receptor Auxiliary Subunit NETO2-Related Cued Fear Conditioning Impairments Associate with Defects in Amygdala Development and Excitability

NETO2 is an auxiliary subunit for kainate-type glutamate receptors that mediate normal cued fear expression and extinction. Since the amygdala is critical for these functions, we asked whether Neto2 ^-/- mice have compromised amygdala function. We measured the abundance of molecular markers of neuronal maturation and plasticity, parvalbumin-positive (PV⁺), perineuronal net-positive (PNN⁺), and double positive (PV⁺PNN⁺) cells in the Neto2 ^-/- amygdala. We found that Neto2 ^-/- adult, but not postnatal day (P)23, mice had 7.5% reduction in the fraction of PV⁺PNN⁺ cells within the total PNN⁺ population, and 23.1% reduction in PV staining intensity compared with Neto2 ^+/+ mice, suggesting that PV interneurons in the adult Neto2 ^-/- amygdala remain in an immature state. An immature PV inhibitory network would be predicted to lead to stronger amygdalar excitation. In the amygdala of adult Neto2 ^-/- mice, we identified increased glutamatergic and reduced GABAergic transmission using whole-cell patch-clamp recordings. This was accompanied by increased spine density of thin dendrites in the basal amygdala (BA) compared with Neto2 ^+/+ mice, indicating stronger glutamatergic synapses. Moreover, after fear acquisition Neto2 ^-/- mice had a higher number of c-Fos-positive cells than Neto2 ^+/+ mice in the lateral amygdala (LA), BA, and central amygdala (CE). Altogether, our findings indicate that Neto2 is involved in the maturation of the amygdala PV interneuron network. Our data suggest that this defect, together with other processes influencing amygdala principal neurons, contribute to increased amygdalar excitability, higher fear expression, and delayed extinction in cued fear conditioning, phenotypes that are common in fear-related disorders, including the posttraumatic stress disorder (PTSD).

The Kainate Receptor Subunit GluK2 Interacts With KCC2 to Promote Maturation of Dendritic Spines

Kainate receptors (KAR) play a crucial role in the plasticity and functional maturation of glutamatergic synapses. However, how they regulate structural plasticity of dendritic spines is not known. The GluK2 subunit was recently shown to coexist in a functional complex with the neuronal K-Cl cotransporter KCC2. Apart from having a crucial role in the maturation of GABAergic transmission, KCC2 has a morphogenic role in the maturation of dendritic spines. Here, we show that in vivo local inactivation of GluK2 expression in CA3 hippocampal neurons induces altered morphology of dendritic spines and reduction in mEPSC frequency. GluK2 deficiency also resulted in a strong change in the subcellular distribution of KCC2 as well as a smaller somatodendritic gradient in the reversal potential of GABA_A. Strikingly, the aberrant morphology of dendritic spines in GluK2-deficient CA3 pyramidal neurons was restored by overexpression of KCC2. GluK2 silencing in hippocampal neurons significantly reduced the expression of 4.1N and functional form of the actin filament severing protein cofilin. Consistently, assessment of actin dynamics using fluorescence recovery after photobleaching (FRAP) of β-actin showed a significant increase in the stability of F-actin filaments in dendritic spines. In conclusion, our results demonstrate that GluK2-KCC2 interaction plays an important role in the structural maturation of dendritic spines. This also provides novel insights into the connection between KAR dysfunction, structural plasticity, and developmental disorders.

Kainate receptor regulation of synaptic inhibition in the hippocampus

Kainate receptors (KARs) are glutamate-type receptors that mediate both canonical ionotropic currents and non-canonical metabotropic signalling. While KARs are expressed widely throughout the brain, synaptic KAR currents have only been recorded at a limited set of synapses, and the KAR currents that have been recorded are relatively small and slow, which has led to the question, what is the functional significance of KARs? While the KAR current itself is relatively modest, its impact on inhibition in the hippocampus can be profound. In the CA1 region of the hippocampus, presynaptic KAR activation bidirectionally regulates γ-aminobutyric acid (GABA) release in a manner that depends on the glutamate concentration; lower levels of glutamate facilitate GABA release via an ionotropic pathway, while higher levels of glutamate depress GABA release via a metabotropic pathway. Postsynaptic interneuron KAR activation increases spike frequency through an ionotropic current, which in turn can strengthen inhibition. In the CA3 region, postsynaptic KAR activation in pyramidal neurons also strengthens inhibition, but in this case through a metabotropic pathway which regulates the neuronal chloride gradient and hyperpolarizes the reversal potential for GABA (E_GABA ). Taken together, the evidence for KAR-mediated regulation of the strength of inhibition via pre- and postsynaptic mechanisms provides compelling evidence that KARs are ideally positioned to regulate excitation-inhibition balance - through sensing the excitatory tone and concomitantly tuning the strength of inhibition.

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.

Reducing NETO2 expression prevents human nasopharyngeal carcinoma (NPC) progression by suppressing metastasis and inducing apoptosis

Nasopharyngeal carcinoma (NPC), the most common cancer in head and neck regions, is a serious health problem worldwide. Neuropilin and tolloid-like 2 (NETO2), a member of the subfamily of CUB domain and LDLa-containing proteins, has been suggested to be involved in tumor progression. Nevertheless, little is known about the function and molecular mechanism of NETO2 in NPC progression. In the study, NETO2 was found to be significantly up-regulated in clinical tissues and NPC cell lines. NETO2 expression was positively correlated with tumor size. NETO2 knockdown inhibited cell proliferation, migration and invasion in NPC cell lines. Significantly, NETO2 knockdown promoted the radiotherapy in vitro, as evidenced by the further reduced cell proliferation and metastasis in NPC cells using 3-[4, 5-dimethylthiazol-2-yl]-2, 5 diphenyl tetrazolium bromide (MTT), colony formation and transwell analysis. In addition, NETO2 inhibition markedly induced apoptosis in NPC cells through activating Caspase-3 signaling. Also, the knockdown of NETO2 obviously promoted the efficacy of radiotherapy in apoptosis induction, along with higher expression of cleaved Caspase-3. NETO2 knockdown-triggered apoptosis in NPC cells were considerably diminished by Caspase-3 inactivation, demonstrating the essential role of Caspase-3 in NETO2-regulated NPC development. Moreover, in vivo experiments suggested that NETO2 knockdown promoted radiation-induced tumor growth suppression in the absence of significant side effects. Collectively, reducing NETO2 expression might elevate the efficiency of radiotherapy in NPC patients.

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.

NETO2 promotes invasion and metastasis of gastric cancer cells via activation of PI3K/Akt/NF-κB/Snail axis and predicts outcome of the patients

Aberrant expression of neuropilin and tolloid-like 2 (NETO2) has been observed during the progression of some human carcinomas. However, the expression pattern and clinical relevance of NETO2 in gastric cancer (GC) remain to be elucidated. In this study, we found that NETO2 expression was higher in GC tissues compared with paired non-cancerous tissues. Moreover, the expression of NETO2 was positively correlated with clinical stage, invasion depth, lymph node metastasis, and tumor size, but inversely correlated with overall and disease-free survival rates. Cox regression analysis identified NETO2 as an independent prognostic indicator for GC patients. Overexpression of NETO2 facilitated migration and invasion of GC cells in vitro and metastasis in vivo in association with induction of epithelial-mesenchymal transition. Conversely, knockdown of NETO2 had the opposite effects. Mechanistically, silencing NETO2 reduced the phosphorylation of PI3K, AKT, and NF-κB p65 as well as the expression of Snail, whereas NETO2 overexpression achieved the opposite results. Furthermore, we identified TNFRSF12A as a mediator for NETO2 to activate PI3K/AKT/NF-κB/Snail axis. Collectively, our results demonstrate that NETO2 promotes invasion and metastasis of GC cells and represents a novel prognostic indicator as well as a potential therapeutic target in GC.

Activation of 5-HT2A Receptors Restores KCC2 Function and Reduces Neuropathic Pain after Spinal Cord Injury

Downregulation of the potassium chloride cotransporter type 2 (KCC2) after a spinal cord injury (SCI) disinhibits motoneurons and dorsal horn interneurons causing spasticity and neuropathic pain, respectively. We showed recently (Bos et al., 2013) that specific activation of 5-HT2A receptors by TCB-2 [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide] upregulates KCC2 function, restores motoneuronal inhibition and reduces SCI-induced spasticity. Here, we tested the potential analgesic effect of TCB-2 on central (thoracic hemisection) and peripheral [spared nerve injury (SNI)] neuropathic pain. We found mechanical and thermal hyperalgesia reduced by an acute administration of TCB-2 in rats with SCI. This analgesic effect was associated with an increase in dorsal horn membrane KCC2 expression and was prevented by pharmacological blockade of KCC2 with an intrathecal injection of DIOA [(dihydroindenyl)oxy]alkanoic acid]. In contrast, the SNI-induced neuropathic pain was not attenuated by TCB-2 although there was a slight increase of membrane KCC2 expression in the dorsal horn ipsilateral to the lesion. Up-regulation of KCC2 function by targeting 5-HT2A receptors, therefore, has therapeutic potential in the treatment of neuropathic pain induced by SCI but not by SNI.

Chloride transporters and GABA polarity in developmental, neurological and psychiatric conditions

Neuronal chloride regulation is a determinant factor for the dynamic tuning of GABAergic inhibition during and beyond brain development. This regulation is mainly dependent on the two co-transporters K⁺/Cl^- co-transporter KCC2 and Na⁺/K⁺/Cl^- co-transporter NKCC1, whose activity can decrease or increase neuronal chloride concentrations respectively. Altered expression and/or activity of either of these co-transporters has been associated with a wide variety of brain disorders including developmental disorders, epilepsy, schizophrenia and stroke. Here, we review current knowledge on chloride transporter expression and activity regulation and highlight the intriguing potential for existing and future interventions to support chloride homeostasis across a wide range of mental disorders and neurological conditions.

Exciting Times: New Advances Towards Understanding the Regulation and Roles of Kainate Receptors

Kainate receptors (KARs) are glutamate-gated ion channels that play fundamental roles in regulating neuronal excitability and network function in the brain. After being cloned in the 1990s, important progress has been made in understanding the mechanisms controlling the molecular and cellular properties of KARs, and the nature and extent of their regulation of wider neuronal activity. However, there have been significant recent advances towards understanding KAR trafficking through the secretory pathway, their precise synaptic positioning, and their roles in synaptic plasticity and disease. Here we provide an overview highlighting these new findings about the mechanisms controlling KARs and how KARs, in turn, regulate other proteins and pathways to influence synaptic function.

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.

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.