The neuronal-specific K-C1 cotransporter (KCC2) exhibits high apparent affinity for external K+: Implications for external K+ buffering in the brain

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Abstract

A unique isoform of the K-C1 cotransporter (KCC2) has been identified in our lab that is expressed specifically in neurons throughout the brain. We have functionally expressed KCC2 in a stable HEK cell line (KCC2-9) and characterized the ouabahvinsensitive 86Rb influx in these cells. The KCC2-9 cells exhibited a significant constitutively active 86Rb influx that could be increased further by ImM N-ethylmaleimide (NEM) but not by cell swelling. Both furosemide (Ki≈25μM) and bumetanide (Ki=55μM) dramatically inhibited the NEM-stimulated 86Rb influx in the KCC2-9 cells. This diuretic-sensitive 86Rb influx, which we operationally defined as KCC2-mediated, exhibited a low apparent affinity for external Cl- (gluconate replacement; Km>50mM); however, it displayed a remarkably high apparent affinity for external K+ (Km=5.2±0.9mM; mean±SEM, n=5). This external K+ affinity is at least 5 times greater than that previously reported for the ubiquitously expressed KCC1 isoform. We have previously proposed that KCC2 is involved in postsynaptic inhibition by maintaining low [Cl-], (i.e. ECI<Em) for the proper function of ligand-gated Cl- channels. As such, most neurons have low [Cl-]i (<10mM), and therefore, K-Cl cotransport is very near thermodynamic equilibrium. Consequently, modest activity-dependent increases in [K+]o, which are common in the brain, will reverse the force driving net K-Cl cotransport from outward to inward. Based on thermodynamic considerations as well as the unique kinetic properties of the KCC2 isoform, we hypothesize that KCC2 can buffer increases in [K+]o, and this function can significantly modulate neuronal activity as KCC2-dependent K+ buffering will shift ECi to more positive values.

Original languageEnglish (US)
JournalFASEB Journal
Volume11
Issue number3
StatePublished - 1997

Fingerprint

Brain
Protein Isoforms
Ethylmaleimide
brain
Neurons
Thermodynamics
thermodynamics
Bumetanide
neurons
Furosemide
cells
Ligand-Gated Ion Channels
Diuretics
furosemide
diuretics
Swelling
Buffers
Cells
Ligands
Kinetics

ASJC Scopus subject areas

  • Agricultural and Biological Sciences (miscellaneous)
  • Biochemistry, Genetics and Molecular Biology(all)
  • Biochemistry
  • Cell Biology

Cite this

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title = "The neuronal-specific K-C1 cotransporter (KCC2) exhibits high apparent affinity for external K+: Implications for external K+ buffering in the brain",
abstract = "A unique isoform of the K-C1 cotransporter (KCC2) has been identified in our lab that is expressed specifically in neurons throughout the brain. We have functionally expressed KCC2 in a stable HEK cell line (KCC2-9) and characterized the ouabahvinsensitive 86Rb influx in these cells. The KCC2-9 cells exhibited a significant constitutively active 86Rb influx that could be increased further by ImM N-ethylmaleimide (NEM) but not by cell swelling. Both furosemide (Ki≈25μM) and bumetanide (Ki=55μM) dramatically inhibited the NEM-stimulated 86Rb influx in the KCC2-9 cells. This diuretic-sensitive 86Rb influx, which we operationally defined as KCC2-mediated, exhibited a low apparent affinity for external Cl- (gluconate replacement; Km>50mM); however, it displayed a remarkably high apparent affinity for external K+ (Km=5.2±0.9mM; mean±SEM, n=5). This external K+ affinity is at least 5 times greater than that previously reported for the ubiquitously expressed KCC1 isoform. We have previously proposed that KCC2 is involved in postsynaptic inhibition by maintaining low [Cl-], (i.e. ECI<Em) for the proper function of ligand-gated Cl- channels. As such, most neurons have low [Cl-]i (<10mM), and therefore, K-Cl cotransport is very near thermodynamic equilibrium. Consequently, modest activity-dependent increases in [K+]o, which are common in the brain, will reverse the force driving net K-Cl cotransport from outward to inward. Based on thermodynamic considerations as well as the unique kinetic properties of the KCC2 isoform, we hypothesize that KCC2 can buffer increases in [K+]o, and this function can significantly modulate neuronal activity as KCC2-dependent K+ buffering will shift ECi to more positive values.",
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T1 - The neuronal-specific K-C1 cotransporter (KCC2) exhibits high apparent affinity for external K+

T2 - Implications for external K+ buffering in the brain

AU - Payne, John A

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N2 - A unique isoform of the K-C1 cotransporter (KCC2) has been identified in our lab that is expressed specifically in neurons throughout the brain. We have functionally expressed KCC2 in a stable HEK cell line (KCC2-9) and characterized the ouabahvinsensitive 86Rb influx in these cells. The KCC2-9 cells exhibited a significant constitutively active 86Rb influx that could be increased further by ImM N-ethylmaleimide (NEM) but not by cell swelling. Both furosemide (Ki≈25μM) and bumetanide (Ki=55μM) dramatically inhibited the NEM-stimulated 86Rb influx in the KCC2-9 cells. This diuretic-sensitive 86Rb influx, which we operationally defined as KCC2-mediated, exhibited a low apparent affinity for external Cl- (gluconate replacement; Km>50mM); however, it displayed a remarkably high apparent affinity for external K+ (Km=5.2±0.9mM; mean±SEM, n=5). This external K+ affinity is at least 5 times greater than that previously reported for the ubiquitously expressed KCC1 isoform. We have previously proposed that KCC2 is involved in postsynaptic inhibition by maintaining low [Cl-], (i.e. ECI<Em) for the proper function of ligand-gated Cl- channels. As such, most neurons have low [Cl-]i (<10mM), and therefore, K-Cl cotransport is very near thermodynamic equilibrium. Consequently, modest activity-dependent increases in [K+]o, which are common in the brain, will reverse the force driving net K-Cl cotransport from outward to inward. Based on thermodynamic considerations as well as the unique kinetic properties of the KCC2 isoform, we hypothesize that KCC2 can buffer increases in [K+]o, and this function can significantly modulate neuronal activity as KCC2-dependent K+ buffering will shift ECi to more positive values.

AB - A unique isoform of the K-C1 cotransporter (KCC2) has been identified in our lab that is expressed specifically in neurons throughout the brain. We have functionally expressed KCC2 in a stable HEK cell line (KCC2-9) and characterized the ouabahvinsensitive 86Rb influx in these cells. The KCC2-9 cells exhibited a significant constitutively active 86Rb influx that could be increased further by ImM N-ethylmaleimide (NEM) but not by cell swelling. Both furosemide (Ki≈25μM) and bumetanide (Ki=55μM) dramatically inhibited the NEM-stimulated 86Rb influx in the KCC2-9 cells. This diuretic-sensitive 86Rb influx, which we operationally defined as KCC2-mediated, exhibited a low apparent affinity for external Cl- (gluconate replacement; Km>50mM); however, it displayed a remarkably high apparent affinity for external K+ (Km=5.2±0.9mM; mean±SEM, n=5). This external K+ affinity is at least 5 times greater than that previously reported for the ubiquitously expressed KCC1 isoform. We have previously proposed that KCC2 is involved in postsynaptic inhibition by maintaining low [Cl-], (i.e. ECI<Em) for the proper function of ligand-gated Cl- channels. As such, most neurons have low [Cl-]i (<10mM), and therefore, K-Cl cotransport is very near thermodynamic equilibrium. Consequently, modest activity-dependent increases in [K+]o, which are common in the brain, will reverse the force driving net K-Cl cotransport from outward to inward. Based on thermodynamic considerations as well as the unique kinetic properties of the KCC2 isoform, we hypothesize that KCC2 can buffer increases in [K+]o, and this function can significantly modulate neuronal activity as KCC2-dependent K+ buffering will shift ECi to more positive values.

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