Role of the neuronal K-Cl co-transporter KCC2 in inhibitory and excitatory neurotransmission

Could tuning of the inhibitory tone involve graded changes in neuronal chloride transport?

Hyperpolarizing synaptic inhibition through GABAA and glycine receptors depends on the presence of the neuronal cation-chloride-cotransporter protein, KCC2. Several transcriptional and post-transcriptional mechanisms have been shown to regulate KCC2 and thereby influence the polarity and efficacy of inhibitory synaptic transmission. It is unclear however whether regulation of KCC2 enables the transporter to attain different levels of activity thus allowing a neuron to modulate the strength of inhibitory synaptic transmission to its changing requirements. We therefore investigated whether phosphorylation can allow KCC2 to achieve distinct levels of [Cl(-)]i in neurons. We generated a variety of KCC2 alanine dephosphorylation mimics and used NH4(+)-induced pHi shifts in cultured hippocampal neurons to quantify the rate of KCC2 transport activity exhibited by these mutants. To explore the relationship between KCC2 transport and GABAA receptor-mediated current amplitudes we performed gramicidine perforated-patch recordings. The correlation between EGABA and NH4(+)-induced pHi shifts enabled an estimate of the range of chloride extrusion possible by kinase/phosphatase regulation of KCC2. Our results demonstrate that KCC2 transport can vary considerably in magnitude depending on the combination of alanine mutations present on the protein. Transport can be enhanced to sufficiently high levels that hyperpolarizing GABAA responses may be obtained even in neurons with an extremely negative resting membrane potential and at high extracellular K(+) concentrations. Our findings highlight the significant potential for regulating the inhibitory tone by KCC2-mediated chloride extrusion and suggest that cellular signaling pathways may act combinatorially to alter KCC2 phosphorylation/dephosphorylation and thereby tune the strength of synaptic inhibition.

KCC2 regulates actin dynamics in dendritic spines via interaction with β-PIX

Chloride extrusion in mature neurons is largely mediated by the neuron-specific potassium-chloride cotransporter KCC2. In addition, independently of its chloride transport function, KCC2 regulates the development and morphology of dendritic spines through structural interactions with the actin cytoskeleton. The mechanism of this effect remains largely unknown. In this paper, we show a novel pathway for KCC2-mediated regulation of the actin cytoskeleton in neurons. We found that KCC2, through interaction with the b isoform of Rac/Cdc42 guanine nucleotide exchange factor β-PIX, regulates the activity of Rac1 GTPase and the phosphorylation of one of the major actin-regulating proteins, cofilin-1. KCC2-deficient neurons had abnormally high levels of phosphorylated cofilin-1. Consistently, dendritic spines of these neurons exhibited a large pool of stable actin, resulting in reduced spine motility and diminished density of functional synapses. In conclusion, we describe a novel signaling pathway that couples KCC2 to the cytoskeleton and regulates the formation of glutamatergic synapses.

K-Cl cotransporter KCC2--a moonlighting protein in excitatory and inhibitory synapse development and function

The K-Cl cotransporter KCC2 has two entirely independent biological actions as either an ion transporter or a structural protein orchestrating the organization of the cytoskeleton in neuronal structures. The K-Cl cotransport by KCC2 is central for hyperpolarizing inhibitory signaling, which is based on chloride currents mediated by γ-aminobutyric acid (GABA)- or glycine-gated receptor channels. In contrast, the structural role of KCC2 seems to be crucially involved in the maturation and regulation of excitatory glutamatergic synapses. This dual role at GABAergic/glycinergic and glutamatergic synapses makes KCC2 a key molecule in the regulation of inhibitory and excitatory signaling. Therefore, KCC2 is most likely involved in the synchronization of the two types of activity during network formation in the immature system and a similar synchronizing role might also be important under physiological and pathological conditions in mature neuronal networks. In this review, we explore new findings on the regulation of KCC2 by protease-mediated cleavage and on the structural role of KCC2 in spine morphogenesis and glutamate receptor clustering. We then discuss the implications of the putative interaction between the independent functions of the transporter and overlapping regulatory mechanisms in a neurophysiological context. In addition, we look at the multifunctional properties of KCC2 in the light of evolution and propose that KCC2 belongs to the group of moonlighting (multifunctional) proteins.

The effect of propofol postconditioning on the expression of K(+)-Cl(-)-co-transporter 2 in GABAergic inhibitory interneurons of acute ischemia/reperfusion injury rats

It has been shown in our previous study that propofol postconditioning enhanced the activity of phosphatidylinositol-3-kinase (PI3K) and prevented the internalization of GluR2 subunit of α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, thus provided neuroprotection in cerebral ischemia/reperfusion (I/R) injury. Regarding inhibitory system in CNS, K(+)-Cl(-)-co-transporter 2 (KCC2), a Cl(-) extruder, plays a critical role in gamma-aminobutyric acid (GABA) inhibitory effect in mature central neurons. However, the effect of propofol postconditioning on the expression of KCC2 in GABAergic interneurons is unclear. Therefore, in this article we describe the role of KCC2 in GABAergic interneurons in the ipsilateral hippocampal CA1 region of adult rats and the effects of propofol postconditioning on this region. Herein we demonstrate that propofol postconditioning (20mg/kg/h, 2h) improved rats' neurobehavioral abilities, increased the number of survival neurons, and up-regulated neuronal KCC2 expression in glutamic acid decarboxylase 67 (GAD67) expressing GABAergic interneurons in hippocampal CA1 region at 24h after I/R. In contrast, when rats were injected with the KCC2 antagonist, [(dihydroindenyl)oxy] alkanoic acid (DIOA), the neuroprotective effects induced by propofol postconditioning were reversed. Our study indicated that propofol postconditioning increased the expression of KCC2 in inhibitory GABAergic interneurons, thus providing acute neuroprotection to rats who had undergone cerebral I/R injury.

High salt intake increases blood pressure via BDNF-mediated downregulation of KCC2 and impaired baroreflex inhibition of vasopressin neurons

The mechanisms by which dietary salt promotes hypertension are unknown. Previous work established that plasma [Na(+)] and osmolality rise in proportion with salt intake and thus promote release of vasopressin (VP) from the neurohypophysis. Although high levels of circulating VP can increase blood pressure, this effect is normally prevented by a potent GABAergic inhibition of VP neurons by aortic baroreceptors. Here we show that chronic high salt intake impairs baroreceptor inhibition of rat VP neurons through a brain-derived neurotrophic factor (BDNF)-dependent activation of TrkB receptors and downregulation of KCC2 expression, which prevents inhibitory GABAergic signaling. We show that high salt intake increases the spontaneous firing rate of VP neurons in vivo and that circulating VP contributes significantly to the elevation of arterial pressure under these conditions. These results provide the first demonstration that dietary salt can affect blood pressure through neurotrophin-induced plasticity in a central homeostatic circuit.

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.

BDNF is required for seizure-induced but not developmental up-regulation of KCC2 in the neonatal hippocampus

A robust increase in the functional expression of the neuronal K-Cl cotransporter KCC2 during CNS development is necessary for the emergence of hyperpolarizing ionotropic GABAergic transmission. BDNF-TrkB signaling has been implicated in the developmental up-regulation of KCC2 and, in mature animals, in fast activity-dependent down-regulation of KCC2 function following seizures and trauma. In contrast to the decrease in KCC2 expression observed in the adult hippocampus following trauma, seizures in the neonate trigger a TrkB-dependent up-regulation of neuronal Cl(-) extrusion capacity associated with enhanced surface expression of KCC2. Here, we show that this effect is transient, and impaired in the hippocampus of Bdnf(-/-) mice. Notably, however, a complete absence of BDNF does not compromise the increase in KCC2 protein or K-Cl transport functionality during neuronal development. Furthermore, we present data indicating that the functional up-regulation of KCC2 by neonatal seizures is temporally limited by calpain activity.

Down-regulation of KCC2 expression and phosphorylation in motoneurons, and increases the number of in primary afferent projections to motoneurons in mice with post-stroke spasticity

Spasticity obstructs motor function recovery post-stroke, and has been reported to occur in spinal cord injury and electrophysiological studies. The purpose of the present study was to assess spinal cord circuit spasticity in post-stroke mice. At 3, 7, 21, and 42 d after photothrombotic ischemic cortical injury in C57BL/6J mice, we observed decreased rate-dependent depression (RDD) of the Hoffmann reflex (H reflex) in the affected forelimb of mice compared with the limbs of sham mice and the non-affected forelimb. This finding suggests a hyper-excitable stretch reflex in the affected forelimb. We then performed immunohistochemical and western blot analyses to examine the expression of the potassium-chloride cotransporter 2 (KCC2) and phosphorylation of the KCC2 serine residue, 940 (S940), since this is the main chloride extruder that affects neuronal excitability. We also performed immunohistochemical analyses on the number of vesicular glutamate transporter 1 (vGluT1)-positive boutons to count the number of Ia afferent fibers that connect to motoneurons. Western bolts revealed that, compared with sham mice, experimental mice had significantly reduced KCC2 expression at 7 d post-stroke, and dephosphorylated S940 at 3 and 7 d post-stroke in motoneuron plasma membranes. We also observed a lower density of KCC2-positive areas in the plasma membrane of motoneurons at 3 and 7 d post-stroke. However, western blot and immunohistochemical analyses revealed that there were no differences between groups 21 and 42 d post-stroke, respectively. In addition, at 7 and 42 d post-stroke, experimental mice exhibited a significant increase in vGluT1 boutons compared with sham mice. Our findings suggest that both the down-regulation of KCC2 and increases in Ia afferent fibers are involved in post-stroke spasticity.

Cation-chloride cotransporters in neuronal development, plasticity and disease

Electrical activity in neurons requires a seamless functional coupling between plasmalemmal ion channels and ion transporters. Although ion channels have been studied intensively for several decades, research on ion transporters is in its infancy. In recent years, it has become evident that one family of ion transporters, cation-chloride cotransporters (CCCs), and in particular K(+)-Cl(-) cotransporter 2 (KCC2), have seminal roles in shaping GABAergic signalling and neuronal connectivity. Studying the functions of these transporters may lead to major paradigm shifts in our understanding of the mechanisms underlying brain development and plasticity in health and disease.

Effects of VU0240551, a novel KCC2 antagonist, and DIDS on chloride homeostasis of neocortical neurons from rats and humans

The normal function of GABAA receptor-mediated inhibition is governed by several factors, including release of GABA, subunit composition and density of the receptors and in particular by the appropriate ionic gradient. In the human epileptogenic neocortex an impaired chloride (Cl(-)) gradient has been proposed, due to decreases of potassium-coupled chloride transport (KCC2) and voltage-gated Cl(-) channels (ClC). Regarding sodium- and potassium-coupled Cl(-) transport (NKCC1) both up- and downregulations have been proposed. We investigated changes of Cl(-) homeostasis of human and rat neocortical neurons (layer 2/3) with intracellular recordings and iontophoretic Cl(-) loading employing selective compounds. After cessation of iontophoresis, the IPSPA amplitudes of rat neurons recovered with a time constant (τrec) of 6.5s (n=21). In human neurons, τrec averaged 17.8s (n=36; 23 resections). Application of the novel KCC2 blocker VU0240551 (1 μM) caused in rat neurons a reversible prolongation of τrec from 5.7 to 8.1s (n=11), corresponding to a VU0240551-sensitive Cl(-) transport rate (1/Δτrec) of 0.0504s(-1). In human neurons, τrec increased on application of 1μM VU0240551, on average from 15.1 to 20.3s (n=17). The human neurons comprised two subgroups with different τrec when segregated according to a border given by the mean+2s.d. of rat neurons. In one group, τrec averaged 8.7s (n=6) and reversibly increased to 14.6s in the presence of 1μM VU0240551, corresponding to a Cl(-) transport rate of 0.0504s(-1). The other group had an average τrec of 18.5s which increased in the presence of 1μM VU0240551 to 23.3s (n=11), indicating a much smaller rate (0.0151s(-1)). Addition of DIDS, a presumed blocker of anion exchanger (AE), increased the τrec of rat neurons from 7.5 to 8.8s (n=6) corresponding to a DIDS-sensitive rate of 0.0185s(-1). In human neurons, DIDS increased τrec from 23.3 to 50.7s (n=7), corresponding to a DIDS-sensitive rate of 0.0200s(-1). These data suggest a greatly reduced KCC2-mediated transport rate in most of the human neurons. The two subgroups observed in human tissue indicate a considerable variability of Cl(-) transport within a given tissue from almost normal to greatly impeded, predominated by a decline of KCC2 whereas AE is unaltered.

Analgesic effect of intrathecal bumetanide is accompanied by changes in spinal sodium-potassium-chloride co-transporter 1 and potassium-chloride co-transporter 2 expression in a rat model of incisional pain

Accumulating evidence has demonstrated that the sodium-potassium-chloride co-transporter 1 and potassium-chloride co-transporter 2 have a role in the modulation of pain transmission at the spinal level through chloride regulation in the pain pathway and by effecting neuronal excitability and pain sensitization. The present study aimed to investigate the analgesic effect of the specific sodium-potassium-chloride co-transporter 1 inhibitor bumetanide, and the change in spinal sodium-potassium-chloride co-transporter 1 and potassium-chloride co-transporter 2 expression in a rat model of incisional pain. Results showed that intrathecal bumetanide could decrease cumulative pain scores, and could increase thermal and mechanical pain thresholds in a rat model of incisional pain. Sodium-potassium-chloride co-transporter 1 expression increased in neurons from dorsal root ganglion and the deep laminae of the ipsilateral dorsal horn following incision. By contrast, potassium-chloride co-transporter 2 expression decreased in neurons of the deep laminae from the ipsilateral dorsal horn. These findings suggest that spinal sodium-potassium-chloride co-transporter 1 expression was up-regulated and spinal potassium-chloride co-transporter 2 expression was down-regulated following incision. Intrathecal bumetanide has analgesic effects on incisional pain through inhibition of sodium-potassium-chloride co-transporter 1.

Serotonergic modulation of post-synaptic inhibition and locomotor alternating pattern in the spinal cord

The central pattern generators (CPGs) for locomotion, located in the lumbar spinal cord, are functional at birth in the rat. Their maturation occurs during the last few days preceding birth, a period during which the first projections from the brainstem start to reach the lumbar enlargement of the spinal cord. Locomotor burst activity in the mature intact spinal cord alternates between flexor and extensor motoneurons through reciprocal inhibition and between left and right sides through commisural inhibitory interneurons. By contrast, all motor bursts are in phase in the fetus. The alternating pattern disappears after neonatal spinal cord transection which suppresses supraspinal influences upon the locomotor networks. This article will review the role of serotonin (5-HT), in particular 5-HT₂ receptors, in shaping the alternating pattern. For instance, pharmacological activation of these receptors restores the left-right alternation after injury. Experiments aimed at either reducing the endogenous level of serotonin in the spinal cord or blocking the activation of 5-HT₂ receptors. We then describe recent evidence that the action of 5-HT₂ receptors is mediated, at least in part, through a modulation of chloride homeostasis. The postsynaptic action of GABA and glycine depends on the intracellular concentration of chloride ions which is regulated by a protein in the plasma membrane, the K⁺-Cl⁻ cotransporter (KCC2) extruding both K⁺ and Cl⁻ ions. Absence or reduction of KCC2 expression leads to a depolarizing action of GABA and glycine and a marked reduction in the strength of postsynaptic inhibition. This latter situation is observed early during development and in several pathological conditions, such as after spinal cord injury, thereby causing spasticity and chronic pain. It was recently shown that specific activation of 5-HT_2A receptors is able to up-regulate KCC2, restore endogenous inhibition and reduce spasticity.

Prevention of NKCC1 phosphorylation avoids downregulation of KCC2 in central sensory pathways and reduces neuropathic pain after peripheral nerve injury

Neuropathic pain after peripheral nerve injury is characterized by loss of inhibition in both peripheral and central pain pathways. In the adult nervous system, the Na(+)-K(+)-2Cl(-) (NKCC1) and neuron-specific K(+)-Cl(-) (KCC2) cotransporters are involved in setting the strength and polarity of GABAergic/glycinergic transmission. After nerve injury, the balance between these cotransporters changes, leading to a decrease in the inhibitory tone. However, the role that NKCC1 and KCC2 play in pain-processing brain areas is unknown. Our goal was to study the effects of peripheral nerve injury on NKCC1 and KCC2 expression in dorsal root ganglia (DRG), spinal cord, ventral posterolateral (VPL) nucleus of the thalamus, and primary somatosensory (S1) cortex. After sciatic nerve section and suture in adult rats, assessment of mechanical and thermal pain thresholds showed evidence of hyperalgesia during the following 2 months. We also found an increase in NKCC1 expression in the DRG and a downregulation of KCC2 in spinal cord after injury, accompanied by later decrease of KCC2 levels in higher projection areas (VPL and S1) from 2 weeks postinjury, correlating with neuropathic pain signs. Administration of bumetanide (30 mg/kg) during 2 weeks following sciatic nerve lesion prevented the previously observed changes in the spinothalamic tract projecting areas and the appearance of hyperalgesia. In conclusion, the present results indicate that changes in NKCC1 and KCC2 in DRG, spinal cord, and central pain areas may contribute to development of neuropathic pain.

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.

Nitric oxide signaling modulates synaptic inhibition in the superior paraolivary nucleus (SPN) via cGMP-dependent suppression of KCC2

Glycinergic inhibition plays a central role in the auditory brainstem circuitries involved in sound localization and in the encoding of temporal action potential firing patterns. Modulation of this inhibition has the potential to fine-tune information processing in these networks. Here we show that nitric oxide (NO) signaling in the auditory brainstem (where activity-dependent generation of NO is documented) modulates the strength of inhibition by changing the chloride equilibrium potential. Recent evidence demonstrates that large inhibitory postsynaptic currents (IPSCs) in neurons of the superior paraolivary nucleus (SPN) are enhanced by a very low intracellular chloride concentration, generated by the neuronal potassium chloride co-transporter (KCC2) expressed in the postsynaptic neurons. Our data show that modulation by NO caused a 15 mV depolarizing shift of the IPSC reversal potential, reducing the strength of inhibition in SPN neurons, without changing the threshold for action potential firing. Regulating inhibitory strength, through cGMP-dependent changes in the efficacy of KCC2 in the target neuron provides a postsynaptic mechanism for rapidly controlling the inhibitory drive, without altering the timing or pattern of the afferent spike train. Therefore, this NO-mediated suppression of KCC2 can modulate inhibition in one target nucleus (SPN), without influencing inhibitory strength of other target nuclei (MSO, LSO) even though they are each receiving collaterals from the same afferent nucleus (a projection from the medial nucleus of the trapezoid body, MNTB).

A variant of KCC2 from patients with febrile seizures impairs neuronal Cl- extrusion and dendritic spine formation

Genetic variation in SLC12A5 which encodes KCC2, the neuron-specific cation-chloride cotransporter that is essential for hyperpolarizing GABAergic signaling and formation of cortical dendritic spines, has not been reported in human disease. Screening of SLC12A5 revealed a co-segregating variant (KCC2-R952H) in an Australian family with febrile seizures. We show that KCC2-R952H reduces neuronal Cl(-) extrusion and has a compromised ability to induce dendritic spines in vivo and in vitro. Biochemical analyses indicate a reduced surface expression of KCC2-R952H which likely contributes to the functional deficits. Our data suggest that KCC2-R952H is a bona fide susceptibility variant for febrile seizures.

Pharmacotherapeutic targeting of cation-chloride cotransporters in neonatal seizures

Seizures are a common manifestation of acute neurologic insults in neonates and are often resistant to the standard antiepileptic drugs that are efficacious in children and adults. The paucity of evidence-based treatment guidelines, coupled with a rudimentary understanding of disease pathogenesis, has made the current treatment of neonatal seizures empiric and often ineffective, highlighting the need for novel therapies. Key developmental differences in γ-aminobutyric acid (GABA)ergic neurotransmission between the immature and mature brain, and trauma-induced alterations in the function of the cation-chloride cotransporters (CCCs) NKCC1 and KCC2, probably contribute to the poor efficacy of standard antiepileptic drugs used in the treatment of neonatal seizures. Although CCCs are attractive drug targets, bumetanide and other existing CCC inhibitors are suboptimal because of pharmacokinetic constraints and lack of target specificity. Newer approaches including isoform-specific NKCC1 inhibitors with increased central nervous system penetration, and direct and indirect strategies to enhance KCC2-mediated neuronal chloride extrusion, might allow therapeutic modulation of the GABAergic system for neonatal seizure treatment. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.

Isolating specific cell and tissue compartments from 3D images for quantitative regional distribution analysis using novel computer algorithms

BACKGROUND

Isolating specific cellular and tissue compartments from 3D image stacks for quantitative distribution analysis is crucial for understanding cellular and tissue physiology under normal and pathological conditions. Current approaches are limited because they are designed to map the distributions of synapses onto the dendrites of stained neurons and/or require specific proprietary software packages for their implementation.

NEW METHOD

To overcome these obstacles, we developed algorithms to Grow and Shrink Volumes of Interest (GSVI) to isolate specific cellular and tissue compartments from 3D image stacks for quantitative analysis and incorporated these algorithms into a user-friendly computer program that is open source and downloadable at no cost.

RESULTS

The GSVI algorithm was used to isolate perivascular regions in the cortex of live animals and cell membrane regions of stained spinal motoneurons in histological sections. We tracked the real-time, intravital biodistribution of injected fluorophores with sub-cellular resolution from the vascular lumen to the perivascular and parenchymal space following a vascular microlesion, and mapped the precise distributions of membrane-associated KCC2 and gephyrin immunolabeling in dendritic and somatic regions of spinal motoneurons.

COMPARISON WITH EXISTING METHODS

Compared to existing approaches, the GSVI approach is specifically designed for isolating perivascular regions and membrane-associated regions for quantitative analysis, is user-friendly, and free.

CONCLUSIONS

The GSVI algorithm is useful to quantify regional differences of stained biomarkers (e.g., cell membrane-associated channels) in relation to cell functions, and the effects of therapeutic strategies on the redistributions of biomolecules, drugs, and cells in diseased or injured tissues.

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.

Novel functions of GABA signaling in adult neurogenesis

Neurotransmitter gamma-aminobutiric acid (GABA) through ionotropic GABA_A and metabotropic GABA_B receptors plays key roles in modulating the development, plasticity and function of neuronal networks. GABA is inhibitory in mature neurons but excitatory in immature neurons, neuroblasts and neural stem/progenitor cells (NSCs/NPCs). The switch from excitatory to inhibitory occurs following the development of glutamatergic synaptic input and results from the dynamic changes in the expression of Na⁺/K⁺/2Cl^- co-transporter NKCC1 driving Cl^- influx and neuron-specific K⁺/Cl^- co-transporter KCC2 driving Cl^- efflux. The developmental transition of KCC2 expression is regulated by Disrupted-in-Schizophrenia 1 (DISC1) and brain-derived neurotrophic factor (BDNF) signaling. The excitatory GABA signaling during early neurogenesis is important to the activity/experience-induced regulation of NSC quiescence, NPC proliferation, neuroblast migration and newborn neuronal maturation/functional integration. The inhibitory GABA signaling allows for the sparse and static functional networking essential for learning/memory development and maintenance.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

An unexpected role of neuroligin-2 in regulating KCC2 and GABA functional switch

BACKGROUND

GABAA receptors are ligand-gated Cl- channels, and the intracellular Cl- concentration governs whether GABA function is excitatory or inhibitory. During early brain development, GABA undergoes functional switch from excitation to inhibition: GABA depolarizes immature neurons but hyperpolarizes mature neurons due to a developmental decrease of intracellular Cl- concentration. This GABA functional switch is mainly mediated by the up-regulation of KCC2, a potassium-chloride cotransporter that pumps Cl- outside neurons. However, the upstream factor that regulates KCC2 expression is unclear.

RESULTS

We report here that KCC2 is unexpectedly regulated by neuroligin-2 (NL2), a cell adhesion molecule specifically localized at GABAergic synapses. The expression of NL2 precedes that of KCC2 in early postnatal development. Upon knockdown of NL2, the expression level of KCC2 is significantly decreased, and GABA functional switch is significantly delayed during early development. Overexpression of shRNA-proof NL2 rescues both KCC2 reduction and delayed GABA functional switch induced by NL2 shRNAs. Moreover, NL2 appears to be required to maintain GABA inhibitory function even in mature neurons, because knockdown NL2 reverses GABA action to excitatory. Gramicidin-perforated patch clamp recordings confirm that NL2 directly regulates the GABA equilibrium potential. We further demonstrate that knockdown of NL2 decreases dendritic spines through down-regulating KCC2.

CONCLUSIONS

Our data suggest that in addition to its conventional role as a cell adhesion molecule to regulate GABAergic synaptogenesis, NL2 also regulates KCC2 to modulate GABA functional switch and even glutamatergic synapses. Therefore, NL2 may serve as a master regulator in balancing excitation and inhibition in the brain.

GABAergic interneurons shape the functional maturation of the cortex

From early embryonic development to adulthood, GABA release participates in the construction of the mammalian cerebral cortex. The maturation of GABAergic neurotransmission is a protracted process which takes place in discrete steps and results from the dynamic interaction between developmentally directed gene expression and brain activity. During the course of development, GABAergic interneurons contribute to key aspects of the functional maturation of the cortex in different ways, from exerting a trophic role to pacing immature neural networks. In this review, we provide an overview of the maturation of GABAergic neurotransmission and discuss the role of GABAergic interneurons in cortical wiring, plasticity, and network activity during pre- and postnatal development. We also discuss psychiatric diseases that may be considered at least in part developmental disorders of the GABAergic system.

Asphyxia induced by umbilical cord occlusion alters glutamatergic and GABAergic synaptic transmission in neurons of the superior colliculus in fetal rats

Using optical recordings, we studied the effects of asphyxia on intracellular Cl(-) and Ca(2+) concentrations ([Cl(-)]i; [Ca(2+)]i) in the superior colliculus of fetal rats, which were connected via the umbilical cord to the dam. Acute asphyxia was induced by umbilical cord occlusion. The number of fetal superior colliculus neurons showing GABA-mediated increases in [Cl(-)]i (leading to hyperpolarization) following local synaptic electrical stimulation had decreased by 3 h post-asphyxiation, while the number showing GABA-mediated decreases in [Cl(-)]i (leading to depolarization) increased. [Ca(2+)]i rise, which occurred after acute asphyxiation, was antagonized by both non-NMDA and NMDA receptor antagonists. The increase in [Ca(2+)]i following focal superior colliculus stimulation was markedly attenuated at 3 h post-asphyxiation. These findings suggest that asphyxia induced by umbilical occlusion induces changes in glutamatergic and GABAergic synaptic transmission in the fetal brain.

Possible alterations in GABAA receptor signaling that underlie benzodiazepine-resistant seizures

Benzodiazepines have been used for decades as first-line treatment for status epilepticus (SE). For reasons that are not fully understood, the efficacy of benzodiazepines decreases with increasing duration of seizure activity. This often forces clinicians to resort to more drastic second- and third-line treatments that are not always successful. The antiseizure properties of benzodiazepines are mediated by γ-aminobutyric acid type A (GABA(A) ) receptors. Decades of research have focused on the failure of GABAergic inhibition after seizure onset as the likely cause of the development benzodiazepine resistance during SE. However, the details of the deficits in GABA(A) signaling are still largely unknown. Therefore, it is necessary to improve our understanding of the mechanisms of benzodiazepine resistance so that more effective strategies can be formulated. In this review we discuss evidence supporting the role of altered GABA(A) receptor function as the major underlying cause of benzodiazepine-resistant SE in both humans and animal models. We specifically address the prevailing hypothesis, which is based on changes in the number and subtypes of GABA(A) receptors, as well as the potential influence of perturbed chloride homeostasis in the mature brain.

Changes in expression of the chloride homeostasis-regulating genes, KCC2 and NKCC1, in the blood of cirrhotic patients with hepatic encephalopathy

Hepatic encephalopathy (HE), a neuropsychiatric abnormality that commonly accompanies cirrhosis of the liver, is often difficult to treat and manage. Changes in chloride homeostasis are involved in the generation of a number of brain disorders. In this study, we considered whether chloride homeostasis is associated with HE. The mRNA levels of the Cl⁻ extrusion system (KCC2) and the Cl⁻ intrusion system (NKCC1) were detected by real-time RT-PCR in the plasma of 29 cirrhotic patients with HE of grade I-II, 36 cirrhotic patients with HE of grade III–IV, 20 cirrhotic patients without HE and 15 healthy controls. The mRNA levels of KCC2 in cirrhotic patients with mild and severe HE were significantly lower compared to those in cirrhotic patients without HE or in the healthy controls. However, NKCC1 mRNA levels did not differ between the different groups. In addition, for cirrhotic patients with HE, there were significant negative correlations between KCC2 levels and the levels of blood ammonia and hepatic function scores (Child-Pugh and model for end-stage liver disease scores); there was also a significant positive correlation between KCC2 levels and neurological status (Glasgow scores). In conclusion, our study indicates that an imbalance of KCC2 and NKCC1 may be a novel biomarker for detecting HE and for monitoring disease development.