Chloride potassium symporter 5

Solute carrier family 12 (potassium/chloride transporter), member 5
Identifiers
Symbols	SLC12A5; KCC2; KIAA1176
External IDs	OMIM: 606726 MGI: 1862037 HomoloGene: 10665 GeneCards: SLC12A5 Gene
Gene Ontology Molecular function • transporter activity • symporter activity • potassium:chloride symporter activity • potassium:chloride symporter activity • ammonia transmembrane transporter activity Cellular component • plasma membrane • integral to membrane • dendrite membrane • perikaryon Biological process • transport • ion transport • potassium ion transport • sodium ion transport • chloride transport • cellular ion homeostasis • synaptic transmission • learning • ammonium transport • multicellular organism growth • thermosensory behavior • response to drug • intracellular pH reduction • transmembrane transport Sources: Amigo / QuickGO
RNA expression pattern
More reference expression data
Orthologs
Species	Human	Mouse
Entrez	57468	57138
Ensembl	ENSG00000124140	ENSMUSG00000017740
UniProt	Q9H2X9	Q3UHQ2
RefSeq (mRNA)	NM_001134771.1	NM_020333.2
RefSeq (protein)	NP_001128243.1	NP_065066.2
Location (UCSC)	Chr 20: 44.65 – 44.69 Mb	Chr 2: 164.79 – 164.83 Mb
PubMed search	[1]	[2]

Potassium-chloride transporter member 5 (aka: KCC2 and SLC12A5) is a neuron-specific chloride potassium symporter responsible for establishing the chloride ion gradient in neurons through the maintenance of low intracellular chloride concentrations^[1]. It is a critical mediator of synaptic inhibition, cellular protection against excitotoxicity^[2][3] and may also act as a modulator of neuroplasticity.^[4][5][6] Potassium-chloride transporter member 5 is also known by the names: KCC2 (potassium chloride cotransporter 2) for its ionic substrates, and SLC12A5 for its genetic origin from the SLC12A5 gene in humans.^[1]

Animals with reduced expression of this transporter exhibit severe motor deficits, epileptiform activity, and spasticity.^[4] KCC2 knockout animals, in which KCC2 is completely absent, die postnatally due to respiratory failure.^[4]

Location

KCC2 is a neuron-specific membrane protein expressed throughout the central nervous system, including the hippocampus, hypothalamus, brainstem, and motoneurons of the ventral spinal cord.^[5]

At the subcellular level, KCC2 has been found in membranes of the somata and dendrites of neurons,^[7][4] with no evidence of expression on axons.^[4] KCC2 has also been shown to colocalize with GABA_A receptors, which serve as ligand-gated ion channels to allow chloride ion movement across the cell membrane. Under normal conditions, the opening of GABA_A receptors permits the hyperpolarizing influx of chloride ions to inhibit postsynaptic neurons from firing.^[3]

Counterintuitively, KCC2 has also been shown to colocalize at excitatory synapses.^[2] One suggested explanation for such colocalization is a potential protective role of KCC2 against excitotoxicity.^[2][3] Ion influx due to the excitatory synaptic stimulation of ion channels in the neuronal membrane causes osmotic swelling of cells as water is drawn in alongside the ions. KCC2 may help to eliminate excess ions from the cell in order to re-establish osmotic homeostasis.

Structure

KCC2 is a member of the cation-chloride cotransporter (CCC) superfamily of proteins.^[8]

As with all CCC proteins, KCC2 is an integral membrane protein with 12 transmembrane domains and both N- and C-terminal cytoplasmic domains. The terminal cytoplasmic domains can be phosphorylated by kinases within the neuron for rapid regulation.

Two Isoforms: KCC2a, KCC2b

There are two isoforms of KCC2: KCC2a and KCC2b.^[4][9] The two isoforms arise from alternative promoters on the SLC12A5 gene and differential splicing of the first mRNA exon.^[4][9] The isoforms differ in their N-termini, with the KCC2a form constituting the larger of the two splice variants.^[10]

KCC2a levels remain relatively constant during pre- and postnatal development.^[10]

KCC2b, on the other hand, is scarcely present during prenatal development and is strongly upregulated during postnatal development. The upregulation of KCC2b expression is thought to be responsible for the “developmental shift” observed in mammals from depolarizing postsynaptic effects of inhibitory synapses in early neural networks to hyperpolarizing effects in mature neural networks.^[4]

KCC2b knockout mice can survive up to postnatal day 17 (P17) due to the presence of functional KCC2a alone, but they exhibit low body weight, motor deficits and generalized seizures.^[4] Complete KCC2 knockouts (both KCC2a and KCC2b absent) die after birth due to respiratory failure.^[4]

Oligomerization

Both KCC2 isoforms can form homomultimers, or heteromultimers with other K-Cl symporters on the cell membrane to maintain chloride homeostasis in neurons.^[1] Dimers, trimers, and tetramers involving KCC2 have been identified in brainstem neurons.^[11] Oligomerization may play an important role in transporter function and activation, as it has been observed that the oligomer to monomer ratio increases in correlation to the development of the chloride ion gradient in neurons.^[11]

Developmental changes in expression

KCC2 levels are low during mammalian embryonic development, when neural networks are still being established and neurons are highly plastic (changeable). During this stage, intracellular chloride ion concentrations are high due to low KCC2 expression and high levels of a transporter known as NKCC1 (Na+/K+ chloride cotransporter 1), which moves chloride ions into cells.^[12] Thus, during embryonic development, the chloride gradient is such that stimulation of GABA_A receptors and glycine receptors at inhibitory synapses causes chloride ions to flow out of cells, making the internal neuronal environment less negative (ie. more depolarized) than it would be at rest. At this stage, GABA_A receptors and glycine receptors act as excitatory rather than inhibitory effectors on postsynaptic neurons, resulting in depolarization and hyperexcitability of neural networks.^[4][5][6]

During postnatal development, KCC2 levels are strongly upregulated while NKCC1 levels are down regulated.^[12] This change in expression correlates to a developmental shift of the chloride ion concentration within neurons from high to low intracellular concentration. Effectively, as the chloride ion concentration is reduced, the chloride gradient across the cellular membrane is reversed such that GABA_A receptor and glycine receptor stimulation causes chloride ion influx, making the internal neuronal environment more negative (ie. more hyperpolarized) than it would be at rest. This is the developmental shift of inhibitory synapses from the excitatory postsynaptic responses of the early neural development phase to the inhibitory postsynaptic responses observed throughout maturity.

Function

Current literature suggests that KCC2 serves three primary roles within neurons:

1. Establishing the chloride ion gradient necessary for postsynaptic inhibition
2. As a protective mechanism against stimulation-induced excitotoxicity
3. As a regulator of neuroplasticity and morphogenicity

Postsynaptic inhibition

KCC2 is a potassium (K+)/chloride (Cl-) symporter that maintains chloride homeostasis in neurons. The electrochemical chloride gradient established by KCC2 activity is crucial for classical postsynaptic inhibition through GABA_A receptors and glycine receptors in the central nervous system. KCC2 utilizes the potassium gradient generated by the Na+/K+ pump to drive chloride extrusion from neurons.^[4] In fact, any disruption of the neuronal K⁺ gradient would indirectly affect KCC2 activity.

Loss of KCC2 following neuronal damage (ie. ischemia, spinal cord damage, physical trauma to the central nervous system) results in the loss of inhibitory regulation and the subsequent development of neuronal hyperexcitability, motor spasticity, and seizure-like activity^[5] as GABA_A receptors and glycine receptors revert from hyperpolarizing to depolarizing postsynaptic effects.

Cellular protection

High levels of stimulation and subsequent ionic influx through activated ion channels can result in cellular swelling as osmotically-obliged water is drawn into neurons along with ionic solutes. This phenomenon is known as excitotoxicity.^[2] KCC2 has been shown to be activated by cell-swelling, and may therefore play a role in eliminating excess ions following periods of high stimulation in order to maintain steady-state neuronal volume and prevent cells from bursting.^[2]

This role may also account for the fact that KCC2 has been known to colocalize near excitatory synapses, even though its primary role is to establish the chloride gradient for postsynaptic inhibition.^[2][3]

Neuroplasticity

KCC2 may play a role in dendritic development within the central nervous system. Studies on hippocampal tissue in KCC2 knockout animals demonstrate that neurons lacking KCC2 have stunted dendritic growth and malformed dendritic spines.^[4] Excitatory synapses characteristically occur on dendritic spines, and therefore KCC2 may have an effect on the formation of excitatory synapses during development due to its influence on the morphology of developing neurons.

It has been proposed that the downregulation of KCC2 observed following neuronal trauma, and the consequent depolarizing shift of GABA_A-mediated synapses, may be an aspect of neuronal de-differentiation. De-differentiation of damaged portions of the nervous system would allow for neuronal networks to return to higher levels of plasticity in order to rewire of surviving neurons to compensate for damage in the network.^[4][5][6]

Regulation

Transcriptional regulation: TrkB receptor signalling

KCC2 is transcriptionally downregulated following central nervous system injury by the TrkB receptor signalling transduction cascade (activated by BDNF and NT-4/5).^[13][14][15]

Post-translational regulation: phosphorylation

It is conventionally thought that phosphorylation inactivates or downregulates KCC2, however there is recent evidence to suggest that phosphorylation at different sites on the KCC2 protein determines different regulational outcomes:

• Wnk1/ Wnk3 and tyrosine kinase (ie. TrkB) phosphorylation downregulates KCC2 activity.^{[13][14][15][16]}
• PKC phosphorylation of the C-terminus Ser940 residue of the KCC2 protein upregulates KCC2 activity by increasing surface stability.^[4]

KCC2 has an extremely high rate of turnover at the plasmalemma (minutes),^[4] suggesting that phosphorylation serves as the primary mechanism for rapid regulation.

Activity-dependent downregulation

KCC2 is downregulated by excitatory glutamate activity on NMDA receptor activity and Ca²⁺ influx.^[16][6]

Glutamate release occurs not only at excitatory synapses, but is also known to occur after neuronal damage or ischemic insult.^[6] Thus, activity-dependent downregulation may be the underlying mechanism by which KCC2 downregulation occurs following central nervous system injury.

See also

• Solute carrier family

References

1. ^ ^{a b c} "Entrez Gene: SLC12A5 solute carrier family 12, (potassium-chloride transporter) member 5". http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=57468. 
2. ^ ^{a b c d e f} Watanabe M, Wake H, Moorhouse AJ, Nabekura J (October 2009). "Clustering of neuronal K+-Cl- cotransporters in lipid rafts by tyrosine phosphorylation". J. Biol. Chem. 284 (41): 27980–8. doi:10.1074/jbc.M109.043620. PMC 2788850. PMID 19679663. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2788850. 
3. ^ ^{a b c d} Gulyás AI, Sík A, Payne JA, Kaila K, Freund TF (June 2001). "The KCl cotransporter, KCC2, is highly expressed in the vicinity of excitatory synapses in the rat hippocampus". Eur. J. Neurosci. 13 (12): 2205–17. doi:10.1046/j.0953-816x.2001.01600.x. PMID 11454023. 
4. ^ ^{a b c d e f g h i j k l m n o p} Blaesse P, Airaksinen MS, Rivera C, Kaila K (March 2009). "Cation-chloride cotransporters and neuronal function". Neuron 61 (6): 820–38. doi:10.1016/j.neuron.2009.03.003. PMID 19323993. 
5. ^ ^{a b c d e} Vinay L, Jean-Xavier C (January 2008). "Plasticity of spinal cord locomotor networks and contribution of cation-chloride cotransporters". Brain Res Rev 57 (1): 103–10. doi:10.1016/j.brainresrev.2007.09.003. PMID 17949820. 
6. ^ ^{a b c d e} Ginsberg MD (September 2008). "Neuroprotection for ischemic stroke: past, present and future". Neuropharmacology 55 (3): 363–89. doi:10.1016/j.neuropharm.2007.12.007. PMC 2631228. PMID 18308347. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2631228. 
7. ^ Báldi R, Varga C, Tamás G (October 2010). "Differential distribution of KCC2 along the axo-somato-dendritic axis of hippocampal principal cells". Eur. J. Neurosci. 32 (8): 1319–25. doi:10.1111/j.1460-9568.2010.07361.x. PMID 20880357. 
8. ^ Lee HH, Walker JA, Williams JR, Goodier RJ, Payne JA, Moss SJ (October 2007). "Direct protein kinase C-dependent phosphorylation regulates the cell surface stability and activity of the potassium chloride cotransporter KCC2". J. Biol. Chem. 282 (41): 29777–84. doi:10.1074/jbc.M705053200. PMID 17693402. 
9. ^ ^{a b} Stil A, Jean-Xavier C, Liabeuf S, Brocard C, Delpire E, Vinay L, Viemari JC (April 2011). "Contribution of the potassium-chloride co-transporter KCC2 to the modulation of lumbar spinal networks in mice". Eur. J. Neurosci. 33 (7): 1212–22. doi:10.1111/j.1460-9568.2010.07592.x. PMID 21255132. 
10. ^ ^{a b} Uvarov P, Ludwig A, Markkanen M, Soni S, Hübner CA, Rivera C, Airaksinen MS (May 2009). "Coexpression and heteromerization of two neuronal K-Cl cotransporter isoforms in neonatal brain". J. Biol. Chem. 284 (20): 13696–704. doi:10.1074/jbc.M807366200. PMC 2679471. PMID 19307176. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2679471. 
11. ^ ^{a b} Blaesse P, Guillemin I, Schindler J, Schweizer M, Delpire E, Khiroug L, Friauf E, Nothwang HG (October 2006). "Oligomerization of KCC2 correlates with development of inhibitory neurotransmission". J. Neurosci. 26 (41): 10407–19. doi:10.1523/JNEUROSCI.3257-06.2006. PMID 17035525. 
12. ^ ^{a b} Stil A, Liabeuf S, Jean-Xavier C, Brocard C, Viemari JC, Vinay L (December 2009). "Developmental up-regulation of the potassium-chloride cotransporter type 2 in the rat lumbar spinal cord". Neuroscience 164 (2): 809–21. doi:10.1016/j.neuroscience.2009.08.035. PMID 19699273. 
13. ^ ^{a b} Rivera C, Li H, Thomas-Crusells J, Lahtinen H, Viitanen T, Nanobashvili A, Kokaia Z, Airaksinen MS, Voipio J, Kaila K, Saarma M (December 2002). "BDNF-induced TrkB activation down-regulates the K+-Cl- cotransporter KCC2 and impairs neuronal Cl- extrusion". J. Cell Biol. 159 (5): 747–52. doi:10.1083/jcb.200209011. PMC 2173387. PMID 12473684. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2173387. 
14. ^ ^{a b} Rivera C, Voipio J, Thomas-Crusells J, Li H, Emri Z, Sipilä S, Payne JA, Minichiello L, Saarma M, Kaila K (May 2004). "Mechanism of activity-dependent downregulation of the neuron-specific K-Cl cotransporter KCC2". J. Neurosci. 24 (19): 4683–91. doi:10.1523/JNEUROSCI.5265-03.2004. PMID 15140939. 
15. ^ ^{a b} Kovalchuk Y, Holthoff K, Konnerth A (October 2004). "Neurotrophin action on a rapid timescale". Curr. Opin. Neurobiol. 14 (5): 558–63. doi:10.1016/j.conb.2004.08.014. PMID 15464888. 
16. ^ ^{a b} Lee HH, Deeb TZ, Walker JA, Davies PA, Moss SJ (May 2011). "NMDA receptor activity downregulates KCC2 resulting in depolarizing GABA(A) receptor-mediated currents". Nat Neurosci 14 (6): 736–43. doi:10.1038/nn.2806. PMC 3102766. PMID 21532577. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3102766. 

Further reading

• Hebert SC, Mount DB, Gamba G (2004). "Molecular physiology of cation-coupled Cl- cotransport: the SLC12 family". Pflugers Arch. 447 (5): 580–93. doi:10.1007/s00424-003-1066-3. PMID 12739168. 
• Rivera C, Voipio J, Kaila K (2005). "Two developmental switches in GABAergic signalling: the K+-Cl- cotransporter KCC2 and carbonic anhydrase CAVII". J. Physiol. (Lond.) 562 (Pt 1): 27–36. doi:10.1113/jphysiol.2004.077495. PMC 1665491. PMID 15528236. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1665491. 
• Andersson B, Wentland MA, Ricafrente JY et al. (1996). "A "double adaptor" method for improved shotgun library construction". Anal. Biochem. 236 (1): 107–13. doi:10.1006/abio.1996.0138. PMID 8619474. 
• Yu W, Andersson B, Worley KC et al. (1997). "Large-scale concatenation cDNA sequencing". Genome Res. 7 (4): 353–8. doi:10.1101/gr.7.4.353. PMC 139146. PMID 9110174. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=139146. 
• Hirosawa M, Nagase T, Ishikawa K et al. (2000). "Characterization of cDNA clones selected by the GeneMark analysis from size-fractionated cDNA libraries from human brain". DNA Res. 6 (5): 329–36. doi:10.1093/dnares/6.5.329. PMID 10574461. 
• Hübner CA, Stein V, Hermans-Borgmeyer I et al. (2001). "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition". Neuron 30 (2): 515–24. doi:10.1016/S0896-6273(01)00297-5. PMID 11395011. 
• Sallinen R, Tornberg J, Putkiranta M et al. (2001). "Chromosomal localization of SLC12A5/Slc12a5, the human and mouse genes for the neuron-specific K(+)-Cl(-) cotransporter (KCC2) defines a new region of conserved homology". Cytogenet. Cell Genet. 94 (1–2): 67–70. doi:10.1159/000048785. PMID 11701957. 
• Deloukas P, Matthews LH, Ashurst J et al. (2002). "The DNA sequence and comparative analysis of human chromosome 20". Nature 414 (6866): 865–71. doi:10.1038/414865a. PMID 11780052. 
• Song L, Mercado A, Vázquez N et al. (2002). "Molecular, functional, and genomic characterization of human KCC2, the neuronal K-Cl cotransporter". Brain Res. Mol. Brain Res. 103 (1–2): 91–105. doi:10.1016/S0169-328X(02)00190-0. PMID 12106695. 
• Strausberg RL, Feingold EA, Grouse LH et al. (2003). "Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences". Proc. Natl. Acad. Sci. U.S.A. 99 (26): 16899–903. doi:10.1073/pnas.242603899. PMC 139241. PMID 12477932. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=139241. 
• Bräuer M, Frei E, Claes L et al. (2003). "Influence of K-Cl cotransporter activity on activation of volume-sensitive Cl- channels in human osteoblasts". Am. J. Physiol., Cell Physiol. 285 (1): C22–30. doi:10.1152/ajpcell.00289.2002. PMID 12637262. 
• Ota T, Suzuki Y, Nishikawa T et al. (2004). "Complete sequencing and characterization of 21,243 full-length human cDNAs". Nat. Genet. 36 (1): 40–5. doi:10.1038/ng1285. PMID 14702039. 
• Lee H, Chen CX, Liu YJ et al. (2005). "KCC2 expression in immature rat cortical neurons is sufficient to switch the polarity of GABA responses". Eur. J. Neurosci. 21 (9): 2593–9. doi:10.1111/j.1460-9568.2005.04084.x. PMC 2945502. PMID 15932617. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2945502. 
• Mercado A, Broumand V, Zandi-Nejad K et al. (2006). "A C-terminal domain in KCC2 confers constitutive K+-Cl- cotransport". J. Biol. Chem. 281 (2): 1016–26. doi:10.1074/jbc.M509972200. PMID 16291749. 
• Vanhatalo S, Palva JM, Andersson S et al. (2006). "Slow endogenous activity transients and developmental expression of K+-Cl- cotransporter 2 in the immature human cortex". Eur. J. Neurosci. 22 (11): 2799–804. doi:10.1111/j.1460-9568.2005.04459.x. PMID 16324114. 
• Lee HH, Walker JA, Williams JR et al. (2007). "Direct protein kinase C-dependent phosphorylation regulates the cell surface stability and activity of the potassium chloride cotransporter KCC2". J. Biol. Chem. 282 (41): 29777–84. doi:10.1074/jbc.M705053200. PMID 17693402. 
• Uvarov P, Ludwig A, Markkanen M et al. (2007). "A novel N-terminal isoform of the neuron-specific K-Cl cotransporter KCC2". J. Biol. Chem. 282 (42): 30570–6. doi:10.1074/jbc.M705095200. PMID 17715129. 

External links

• How Brain Injury Leads To Seizures, Memory Problems - medical news, 20 Oct 2006.

This article incorporates text from the United States National Library of Medicine, which is in the public domain.