K⁺-Cl⁻co-transporter 2 (KCC2) – a membrane trafficking perspective

Abstract

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.

Developmental Formation of the GABAergic and Glycinergic Networks in the Mouse Spinal Cord

“…The activity of KCC2 is regulated not only by expression level but also through various other mechanisms, such as phosphorylation/de-phosphorylation [130][131][132] and membrane trafficking [133][134][135]. For example, phosphorylation of threonine residues 906 and 1007 decreases the activity of KCC2.…”

“…The phosphorylation of tyrosine residue 1087 is involved in internalization of KCC2, which results in the down-regulation of KCC2 activity [135]. In addition to tyrosine residue phosphorylation, other complex mechanisms may play roles in trafficking and endocytosis of KCC2 and regulate the activity of KCC2 in developing neurons [133][134][135].…”

Amyloid Precursor Protein (APP) and GABAergic Neurotransmission

“…The authors showed with co-transfection experiments that full-length APP, but not its proteolytic fragments, stabilized KCC2 levels. Functional expression of KCC2 at the neuronal cell surface is necessary for its Cl − efflux activity, and the trafficking of KCC2 to the cell surface and its subsequent endocytic internalization is regulated by different cellular mechanisms, with defects in these known to underlie a range of neuropathological conditions [58]. One such regulatory mechanism is the tyrosine phosphorylation of KCC2 mediated by tyrosine kinases, such as Src [81,82,83], which promotes KCC2 internalization from the plasma membrane and its subsequent lysosomal degradation.…”

“…GABA A R functions as ligand-gated chloride (Cl − ) channels and whether GABA binding would be depolarizing or hyperpolarizing is largely determined by intracellular Cl − concentrations and the GABA reversal potential (E GABA ). Resting Cl − concentration in central nervous system (CNS) neurons is determined by the activity of two major cation-chloride cotransporters, namely the Cl − influx-mediating Na + -K + -2Cl − cotransporter 1 (NKCC1) and the efflux-mediating K + -Cl − cotransporter 2 (KCC2) [58]. In the adult brain, GABA is mainly hyperpolarizing and inhibitory, but it is primarily depolarizing and excitatory in developing neurons, as demonstrated using rat embryonic and neonatal cortical slices [59].…”

The Expanding Therapeutic Potential of Neuronal KCC2

“…KCC2 transcript levels could be downregulated by neuronal activity [11,12] and brain-derived growth factor (BDNF)-TrkB signaling [13], and its post-translational functional activity is mainly determined by the level of expression at the plasma membrane. The 12-transmembrane KCC2 protein oligomerises, and its membrane trafficking [14], as well as "diffusion-trapping" [15] at the plasma membrane is modulated by neuronal activity-dependent phosphorylation/dephosphorylation of key serine and threonine residues at its cytoplasmic loops. Phosphorylation of S940 of KCC2 by protein kinase C (PKC) [16], for example, is known to stabilize KCC2 at the cell surface and reduces its internalization, whereas excitatory input through the NMDA receptor dephosphorylate S940 by protein phosphatase 1 (PP1) [12] downregulates KCC2's Cl − efflux activity.…”