Chloride potassium symporter 5

There are two isoforms of KCC2: KCC2a and KCC2b.[8][14] The two isoforms arise from alternative promoters on the SLC12A5 gene and differential splicing of the first mRNA exon.[8][14] The isoforms differ in their N-termini, with the KCC2a form constituting the larger of the two splice variants.[15]

KCC2a levels remain relatively constant during pre- and postnatal development.[15]

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.[8]

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.[8] Complete KCC2 knockouts (both KCC2a and KCC2b absent) die after birth due to respiratory failure.[8]

Both KCC2 isoforms can form homomultimers, or heteromultimers with other K-Cl symporters on the cell membrane to maintain chloride homeostasis in neurons.[5] Dimers, trimers, and tetramers involving KCC2 have been identified in brainstem neurons.[16] 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.[16]

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.[8] In fact, any disruption of the neuronal K⁺ gradient would indirectly affect KCC2 activity.

Loss of KCC2 following neuronal damage (i.e. 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[10] as GABA_A receptors and glycine receptors revert from hyperpolarizing to depolarizing postsynaptic effects.

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.[6] 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.[6]

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.[6][7]

In addition to controlling the efficacy of GABAergic synapses through chloride homeostasis, KCC2 play a critical role in the morphogenesis and function of glutamatergic synapses within the central nervous system. Studies on hippocampal tissue in KCC2 knockout animals showed that neurons lacking KCC2 have stunted dendritic growth and malformed dendritic spines.[8] Recent studies demonstrate that KCC2 plays a critical role in the structure and function of dendritic spines[9] which host most excitatory synapses in cortical neurons. Through an interaction with actin cytoskeleton, KCC2 forms a molecular barrier to the diffusion of transmembrane proteins within dendritic spines, thereby regulating the local confinement of AMPA receptors and synaptic potency.[9]

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 surviving neurons to compensate for damage in the network.[8][10][11] In addition, reduced glutamatergic transmission upon KCC2 downregulation may serve as a homeostatic process to compensate for the reduced GABA transmission due to altered chloride extrusion.[9]

Mutations in SLC12A5 are associated with colon cancer.[18]

KCC2 is transcriptionally downregulated following central nervous system injury by the TrkB receptor signalling transduction cascade (activated by BDNF and NT-4/5).[19][20][21]

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 (i.e. TrkB) phosphorylation downregulates KCC2 activity.[19][20][21][22]
• PKC phosphorylation of the C-terminus Ser940 residue of the KCC2 protein upregulates KCC2 activity by increasing surface stability.[8] Conversely, Ser940 dephosphorylation leads to enhanced membrane diffusion and endocytosis of KCC2.[23]

KCC2 has an extremely high rate of turnover at the plasmalemma (minutes),[8] suggesting that phosphorylation serves as the primary mechanism for rapid regulation.

KCC2 is downregulated by excitatory glutamate activity on NMDA receptor activity and Ca²⁺ influx.[11][22] This process involves rapid dephosphorylation on Ser940 and calpain protease cleavage of KCC2, leading to enhanced membrane diffusion and endocytosis of the transporter,[23] as demonstrated in experiments using single particle tracking.

Glutamate release occurs not only at excitatory synapses, but is also known to occur after neuronal damage or ischemic insult.[11] Thus, activity-dependent downregulation may be the underlying mechanism by which KCC2 downregulation occurs following central nervous system injury.