SLC12A5 encodes KCC2 (K-Cl cotransporter 2), a neuron-specific, electroneutral potassium-chloride cotransporter of the SLC12 cation-chloride cotransporter family. As a multi-pass plasma-membrane protein it couples the outward movement of K+ and Cl- down the K+ gradient, acting as the major extruder of intracellular chloride in mature central neurons. By keeping intraneuronal [Cl-] low, KCC2 sets the equilibrium potential for chloride that makes GABA-A and glycine receptor activation hyperpolarizing (inhibitory); its developmental upregulation underlies the excitatory-to-inhibitory switch of GABAergic signaling. KCC2 is concentrated on the somatodendritic plasma membrane, including dendritic spines and the vicinity of inhibitory and excitatory synapses, and is essentially absent from axons. Beyond ion transport it has a documented transport-independent structural role in the morphogenesis and maturation of dendritic spines, mediated by its large C-terminal cytoplasmic domain. Transport activity is suppressed by WNK-SPAK/OSR1 phosphorylation and is activated by cell swelling. Loss-of-function and hypomorphic variants cause developmental and epileptic encephalopathy (DEE34 / epilepsy of infancy with migrating focal seizures) and confer susceptibility to idiopathic generalized epilepsy (EIG14).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: KCC2 is a multi-pass plasma-membrane cotransporter; the plasma membrane is its primary functional location. Reason: Correct and core. UniProt curates the subcellular location as Cell membrane (multi-pass membrane protein), consistent with transport across the plasma membrane. This is the site where the symporter activity acts. Supporting Evidence: file:human/SLC12A5/SLC12A5-uniprot.txt Cell membrane {ECO:0000250|UniProtKB:Q91V14}; |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-426155 | ACCEPT | Summary: Plasma-membrane localization, asserted by Reactome for the K-Cl cotransport reaction. Reason: Correct and core; concordant with the IEA plasma-membrane row and with the UniProt curated cell-membrane location. |
| GO:0006811 monoatomic ion transport | IDA PMID:12106695 Molecular, functional, and genomic characterization of human... | TOO GENERAL | Summary: General ion-transport process, entailed by the specific K+ and Cl- transmembrane transport KCC2 performs. Reason: True but uninformative. The specific accepted processes GO:0071805 potassium ion transmembrane transport and GO:1902476 chloride transmembrane transport both entail monoatomic ion transport. Judges the term, not the IDA evidence. |
| GO:0006811 monoatomic ion transport | IEA GO_REF:0000002 | TOO GENERAL | Summary: General ion-transport process from InterPro mapping. Reason: Entailed by the specific transmembrane-transport terms accepted in this review (GO:0071805, GO:1902476). |
| GO:0006813 potassium ion transport | IEA GO_REF:0000107 | TOO GENERAL | Summary: Potassium transport, the broader parent of the specific K+ transmembrane transport performed by KCC2. Reason: Entailed by GO:0071805 potassium ion transmembrane transport (accepted). |
| GO:0006821 chloride transport | IBA GO_REF:0000033 | TOO GENERAL | Summary: Chloride transport inherited phylogenetically across the cation-chloride cotransporter family. Reason: The phylogenetic chloride-transport assertion is correct for KCC2, but the term is entailed by the more specific GO:1902476 chloride transmembrane transport (accepted here). Judges the term, not the IBA node placement, which is sound. |
| GO:0006821 chloride transport | IEA GO_REF:0000107 | TOO GENERAL | Summary: Chloride transport from ortholog transfer. Reason: Entailed by GO:1902476 chloride transmembrane transport (accepted). |
| GO:0006971 hypotonic response | IDA PMID:12106695 Molecular, functional, and genomic characterization of human... | KEEP AS NON CORE | Summary: KCC2 transport is strongly activated by cell swelling under hypotonic conditions (20-fold in oocytes). Reason: A genuine, directly observed property of KCC2 (swelling-activated K-Cl cotransport), shared across the KCC subfamily, but peripheral to the core neuronal chloride-extrusion function, which in neurons is substantially constitutive under isotonic conditions. Supporting Evidence: PMID:12106695 cell swelling under hypotonic conditions causes a 20-fold activation |
| GO:0008519 ammonium channel activity | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: NH4+ can substitute for K+ in K-Cl cotransport, but KCC2 is not an ammonium channel. Reason: Mechanistically incorrect molecular-function characterization. GO:0008519 is defined as energy-independent facilitated diffusion of ammonium through a transmembrane pore/channel; KCC2 is a secondary-active electroneutral symporter, not a channel. Any ammonium movement occurs by NH4+ substituting for K+ in the coupled cotransport cycle, which is better captured by GO:0072488 ammonium transmembrane transport. |
| GO:0015108 chloride transmembrane transporter activity | IDA PMID:26333769 Mutations in SLC12A5 in epilepsy of infancy with migrating f... | TOO GENERAL | Summary: Chloride transporter activity measured directly; subsumed by the specific K:Cl symporter activity. Reason: Correct activity, but entailed by the specific accepted MF GO:0015379 potassium:chloride symporter activity, which already asserts chloride transport. Judges the term, not the IDA evidence (impaired chloride extrusion by DEE34 mutants). |
| GO:0015377 chloride:monoatomic cation symporter activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: Generic cation-chloride symporter activity from InterPro mapping. Reason: Direct parent of the specific accepted MF GO:0015379 potassium:chloride symporter activity, which entails it. |
| GO:0015379 potassium:chloride symporter activity | IBA GO_REF:0000033 | ACCEPT | Summary: Core K+:Cl- symporter activity, inherited across the KCC clade and experimentally grounded on KCC2 itself. Reason: This is the defining molecular function of KCC2. The IBD node for K:Cl symporter activity is well supported, and Q9H2X9 appears in its own WITH/FROM, marking experimental grounding on the target (expected, not circular). Supporting Evidence: file:human/SLC12A5/SLC12A5-uniprot.txt Reaction=K(+)(in) + chloride(in) = K(+)(out) + chloride(out); |
| GO:0015379 potassium:chloride symporter activity | IDA PMID:12106695 Molecular, functional, and genomic characterization of human... | ACCEPT | Summary: Direct demonstration of Cl--dependent K+ (Rb+) cotransport by human KCC2 in oocytes. Reason: Core molecular function with direct experimental evidence, including kinetics (KM K+ 9.3 mM, Cl- 6.8 mM) matching a neuronal efflux mechanism. Supporting Evidence: PMID:12106695 Expression of hKCC2 cRNA in Xenopus laevis oocytes results in significant Cl(-)-dependent (86)Rb(+) uptake under isotonic conditions |
| GO:0015379 potassium:chloride symporter activity | IEA GO_REF:0000120 | ACCEPT | Summary: K:Cl symporter activity from combined IEA methods. Reason: Concordant with the IBA, IDA and TAS rows for the same core molecular function. |
| GO:0015379 potassium:chloride symporter activity | TAS Reactome:R-HSA-426155 | ACCEPT | Summary: K:Cl cotransport activity asserted by Reactome. Reason: Concordant curated assertion of the core molecular function. |
| GO:0016020 membrane | IDA PMID:12106695 Molecular, functional, and genomic characterization of human... | MODIFY | Summary: Generic membrane localization; KCC2 specifically resides in the plasma membrane. Reason: Correct but less specific than the evidence supports. KCC2 is a plasma-membrane transporter; refine to GO:0005886 plasma membrane, which the gene already carries. Proposed replacements: plasma membrane |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: Generic membrane localization from InterPro mapping. Reason: Refine to the specific supported location GO:0005886 plasma membrane. Proposed replacements: plasma membrane |
| GO:0016324 apical plasma membrane | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Apical plasma membrane is an epithelial location propagated from epithelial SLC12 members; KCC2 is a neuronal transporter. Reason: The IBD node for apical plasma membrane reflects epithelial cation-chloride cotransporters (e.g. NKCC2/SLC12A1, NCC/SLC12A3). KCC2 is expressed in neurons, which are not apical/basolaterally polarized epithelia; it localizes to the somatodendritic plasma membrane, captured by GO:0005886 and GO:0032590. The apical qualifier is an over-propagation for this gene. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: COMPARTMENT OR COMPLEX MISMATCH WRONG ORTHOLOG OR PARALOG Sources checked: PANTHER:PTN002274228 SUPPORTS SOURCE BUT NOT TARGET IBD node seeded largely by epithelial cation-chloride cotransporters (e.g. NKCC2/SLC12A1, NCC/SLC12A3) that reside in the apical plasma membrane; the apical qualifier does not transfer to the neuronal KCC2. |
| GO:0022857 transmembrane transporter activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: Root transporter activity. Reason: Entailed by the specific accepted MF GO:0015379 potassium:chloride symporter activity. |
| GO:0030425 dendrite | IEA GO_REF:0000044 | ACCEPT | Summary: KCC2 is enriched on dendrites, a principal functional site. Reason: Consistent with the UniProt curated cell-projection (dendrite) location and with its role in dendritic chloride extrusion and spine maturation. |
| GO:0030644 intracellular chloride ion homeostasis | IDA PMID:26333769 Mutations in SLC12A5 in epilepsy of infancy with migrating f... | ACCEPT | Summary: KCC2 maintains low intraneuronal chloride; disease mutants impair chloride extrusion. Reason: Core biological process. Direct evidence that SLC12A5/KCC2 mediates neuronal chloride extrusion, loss of which causes epileptic encephalopathy. Supporting Evidence: PMID:26333769 with impaired KCC2-mediated chloride extrusion |
| GO:0032502 developmental process | IEA GO_REF:0000117 | TOO GENERAL | Summary: Extremely general developmental-process term from ARBA. Reason: Uninformative; entailed by the specific developmental role GO:0060996 dendritic spine development, which is retained in this review. |
| GO:0032590 dendrite membrane | IEA GO_REF:0000107 | ACCEPT | Summary: KCC2 resides in the dendritic plasma membrane. Reason: A specific and accurate location; KCC2 is a plasma-membrane protein enriched on the dendritic surface, where it extrudes chloride. Consistent with UniProt dendrite localization. |
| GO:0043005 neuron projection | IBA GO_REF:0000033 | ACCEPT | Summary: KCC2 is active in neuron projections (dendrites). Reason: Correct functional location, supported phylogenetically and by direct data; the target's own experimental annotation grounds the IBD node (self in WITH/FROM expected). |
| GO:0043005 neuron projection | IDA PMID:16227993 NKCC1 transporter facilitates seizures in the developing bra... | ACCEPT | Summary: Direct localization of KCC2 to neuron projections. Reason: Consistent with the IBA and IEA neuron-projection rows and with KCC2's somatodendritic distribution. |
| GO:0043005 neuron projection | IEA GO_REF:0000117 | ACCEPT | Summary: Neuron projection localization from ARBA. Reason: Concordant with the IDA and IBA rows for the same location. |
| GO:0043025 neuronal cell body | IBA GO_REF:0000033 | ACCEPT | Summary: KCC2 is present and active at the neuronal soma plasma membrane. Reason: KCC2 is somatodendritic; the cell-body surface is a genuine functional location. Self appearing in WITH/FROM reflects experimental grounding on the target. |
| GO:0043025 neuronal cell body | IDA PMID:16227993 NKCC1 transporter facilitates seizures in the developing bra... | ACCEPT | Summary: Direct localization to neuronal cell bodies. Reason: Concordant with the IBA/ISS/IEA rows for this location. |
| GO:0043025 neuronal cell body | IEA GO_REF:0000120 | ACCEPT | Summary: Neuronal cell body localization from combined IEA methods. Reason: Concordant with the experimental and phylogenetic rows. |
| GO:0043025 neuronal cell body | ISS GO_REF:0000024 | ACCEPT | Summary: Neuronal cell body localization by similarity to mouse KCC2. Reason: Concordant with the IDA/IBA/IEA rows for the same location. |
| GO:0043204 perikaryon | IEA GO_REF:0000107 | ACCEPT | Summary: Perikaryon (soma cytoplasmic region) localization transferred from the rat ortholog. Reason: Consistent with the well-supported somatic/cell-body localization of KCC2; the perikaryal surface is part of the somatodendritic compartment where KCC2 functions. |
| GO:0051452 intracellular pH reduction | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: KCC2 mediates electroneutral K-Cl cotransport, which does not directly reduce intracellular pH. Reason: Electroneutral, pH-neutral K+/Cl- cotransport does not itself acidify the cytosol. This process term most likely derives from NH4+ (ammonium) substitution for K+ in the cotransport cycle; attributing an intracellular-pH-reduction role to KCC2 overstates the biology for this gene. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | TOO GENERAL | Summary: Root transmembrane-transport process. Reason: Entailed by the specific accepted processes GO:0071805 and GO:1902476. |
| GO:0060996 dendritic spine development | IDA PMID:24668262 A variant of KCC2 from patients with febrile seizures impair... | KEEP AS NON CORE | Summary: KCC2 has a transport-independent structural role in the formation and maturation of dendritic spines. Reason: A genuine, directly supported function distinct from ion transport. The human KCC2-R952H variant has a compromised ability to induce dendritic spines, and rodent KCC2 acts as a structural protein for spine maturation independently of its transport activity. Retained as a real but non-core (second) function relative to the defining chloride-extrusion activity. Supporting Evidence: PMID:24668262 rodent KCC2 has (independently of its role as an ion transporter) been shown to play an important role as a structural protein required for the morphological and functional maturation of cortical dendritic spines |
| GO:0060996 dendritic spine development | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Dendritic spine development transferred from the rat ortholog. Reason: Concordant with the IDA row; a real transport-independent structural function, non-core relative to chloride extrusion. |
| GO:0071805 potassium ion transmembrane transport | IBA GO_REF:0000033 | ACCEPT | Summary: K+ transmembrane transport, the K+ arm of K-Cl cotransport. Reason: Core process directly entailed by the K:Cl symporter activity and inherited across the KCC clade; specific and informative. |
| GO:0071805 potassium ion transmembrane transport | IEA GO_REF:0000107 | ACCEPT | Summary: K+ transmembrane transport from ortholog transfer. Reason: Concordant with the IBA row and with the core cotransport activity. |
| GO:0071944 cell periphery | IDA PMID:16227993 NKCC1 transporter facilitates seizures in the developing bra... | MODIFY | Summary: Cell periphery is a general location; KCC2 specifically resides in the plasma membrane. Reason: Correct but less specific than warranted. Refine to GO:0005886 plasma membrane, the supported descendant the gene already carries. Proposed replacements: plasma membrane |
| GO:0071944 cell periphery | IEA GO_REF:0000107 | MODIFY | Summary: Cell periphery from ortholog transfer. Reason: Refine to the specific supported location GO:0005886 plasma membrane. Proposed replacements: plasma membrane |
| GO:0071944 cell periphery | ISS GO_REF:0000024 | MODIFY | Summary: Cell periphery by similarity to mouse KCC2. Reason: Refine to the specific supported location GO:0005886 plasma membrane. Proposed replacements: plasma membrane |
| GO:0072488 ammonium transmembrane transport | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: KCCs can carry NH4+ in place of K+ during cotransport. Reason: A documented secondary property of K-Cl cotransporters (NH4+ substitutes for K+ in the coupled cycle), non-core relative to the K+/Cl- physiology. Note this is coupled cotransport, not the channel-type mechanism implied by the GO:0008519 molecular-function row, which is marked over-annotated. |
| GO:0098970 postsynaptic neurotransmitter receptor diffusion trapping | IEA GO_REF:0000107 | UNDECIDED | Summary: KCC2's C-terminal domain participates in confining postsynaptic receptors via cytoskeletal scaffolding. Reason: Plausibly consistent with the documented transport-independent scaffolding role of KCC2 at synapses, but this is an electronic (IEA) transfer from the rat ortholog with no cached primary publication to verify receptor diffusion-trapping specifically, so it is left UNDECIDED rather than asserted as a non-core function. |
| GO:0098978 glutamatergic synapse | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: KCC2 is enriched at excitatory (glutamatergic) synapses and dendritic spines. Reason: A real synaptic localization consistent with KCC2's enrichment in spines, but peripheral to the defining somatodendritic chloride-extrusion function. |
| GO:0099634 postsynaptic specialization membrane | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: KCC2 localizes near postsynaptic specialization membranes. Reason: Consistent with KCC2's enrichment at postsynaptic sites (spines); a non-core location relative to the general somatodendritic plasma membrane. |
| GO:0150052 regulation of postsynapse assembly | IEA GO_REF:0000107 | UNDECIDED | Summary: KCC2's structural role extends to shaping postsynaptic/spine assembly. Reason: Plausibly consistent with the documented transport-independent structural role of KCC2 in dendritic spine and synapse morphogenesis, but this is an electronic (IEA) transfer from the rat ortholog with no cached primary publication to verify regulation of postsynapse assembly specifically, so it is left UNDECIDED rather than asserted non-core. |
| GO:1902476 chloride transmembrane transport | TAS Reactome:R-HSA-426117 | ACCEPT | Summary: Chloride transmembrane transport, the Cl- arm of K-Cl cotransport. Reason: Core process; the specific chloride transmembrane transport performed by KCC2 as it extrudes chloride from neurons. Informative and well supported. |
Loading supporting content…
Download this section (compressed HTML)Q: Does human KCC2's transport-independent structural role in dendritic spine morphogenesis warrant a distinct core molecular-function term, and what domain/mechanism (e.g. C-terminal cytoskeletal scaffolding) mediates it?
Q: What is the physiological relevance, if any, of KCC2-mediated NH4+ (ammonium) cotransport in neurons, as distinct from the K+/Cl- transport that defines the protein?
Experiment: Use transport-dead but structurally intact KCC2 mutants (separating ion transport from the C-terminal structural role) in neuronal cultures and knock-in models to dissect the contributions of chloride extrusion versus dendritic spine scaffolding to inhibitory synapse maturation and epileptogenesis.
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)