NKCC proteins are
membrane transport proteins that transport
sodium (Na),
potassium (K), and
chloride (Cl) ions across the
cell membrane. Because they move each solute in the same direction, they are considered
symporters. They maintain electroneutrality by moving two positively charged solutes (sodium and potassium) alongside two parts of a negatively charged solute (chloride). Thus the
stoichiometry of the transported solutes is 1Na:1K:2Cl. Although
squid giant axons are the only notable exception with a stoichiometry of 2Na:1K:3Cl, electroneutrality across the protein transporter is still maintained. The rate of transport of these ions are regulated by
phosphorylation sites, which present on both NKCC isoforms.
NKCC1 The NKCC1 isoform consists of about 1,200
amino acids, with about 500 amino acids residues giving rise to twelve hydrophobic
transmembrane regions. However, evidence of a shorter NKCC1
mRNA transcript (6.7 kb to 7-7.5 kb) in skeletal muscle cells gives support that further NKCC1 variants exists in a tissue-specific manner. The
carboxy-terminal of the NKCC1 cotransporter contains multiple
phosphorylation sites and is highly conserved across species, while in contrast, the
amino-terminal contains at least one phosphorylation site and is poorly conserved across species In cells of these organs, NKCC1 is commonly found in the
basolateral membrane, Thus, NKCC1 cotransporters that have been alternatively spliced to exclude exon 21 will be translocated to the
apical membrane rather than the basolateral membrane. Its basolateral location gives NKCC1 the ability to transport sodium, potassium, and chloride from the blood into the cell. Other transporters assist in the movement of these solutes out of the cell through its apical surface. The end result is that solutes from the blood, particularly chloride, are secreted into the lumen of these exocrine glands, increasing the luminal
concentration of solutes and causing water to be secreted by
osmosis. In addition to exocrine glands, NKCC1 is necessary for establishing the potassium-rich
endolymph that bathes part of the
cochlea, an organ necessary for hearing. Inhibition of NKCC1, as with
furosemide or other
loop diuretics, can result in
deafness. Specifically in the cochlea, NKCC1 is present in the
stria vascularis,
spiral ligament, and
spiral ganglia. Additionally, NKCC1 is present in the
dark cells of the
vestibule and contributes to generation of the
endolymph of the
vestibular system. NKCC1 is also expressed in many regions of the
brain during early development, but not in adulthood. This change in NKCC1 presence seems to be responsible for altering responses to the neurotransmitters
GABA and
glycine from excitatory to inhibitory, which was suggested to be important for early neuronal development. As long as NKCC1 transporters are predominantly active, internal chloride concentrations in neurons is raised in comparison with mature chloride concentrations, which is important for GABA and glycine responses, as respective ligand-gated anion channels are permeable to chloride. With higher internal chloride concentrations, outward driving force for this ions increases, and thus channel opening leads to chloride leaving the cell, thereby depolarizing it. Put another way, increasing internal chloride concentration increases the
reversal potential for chloride, given by the
Nernst equation. Later in development expression of NKCC1 is reduced, while expression of a
KCC2 K-Cl cotransporter increased, thus bringing internal chloride concentration in neurons down to adult values. Activity-dependent regulation of NKCC1 during early neuronal development has been suggested to contribute, together with the upregulation of KCC2, to the developmental shift of GABAergic signalling from depolarizing to hyperpolarizing responses. NKCC1 has been identified in
Sertoli cells,
spermatocytes, and
spermatids in the
male reproductive system. NKCC1 function appears to be critical for
spermatogenesis, as
knockdown of NKCC1 in mice results in
spermatocytes failing to mature into
spermatozoa, resulting in
infertility. NKCC2 is specifically found in cells of the
thick ascending limb of the loop of Henle and the
macula densa in
nephrons, the basic functional units of the
kidney. Within these cells, NKCC2 resides in the
apical membrane abutting the nephron's
lumen, which is the hollow space containing
urine. It thus serves both in sodium absorption and in
tubuloglomerular feedback. The thick ascending limb of the loop of Henle begins at the deeper portion of the renal outer medulla. Here, the urine has a relatively high concentration of sodium. As urine moves towards the more superficial portion of the thick ascending limb, NKCC2 is the major transport protein by which sodium is reabsorbed from the urine. This outward movement of sodium and the lack of water permeability in the thick ascending limb, creates a more diluted urine. According to the stoichiometry outlined above, each sodium ion reabsorbed brings one potassium ion and two chloride ions. Sodium goes on to be reabsorbed into the
blood, where it contributes to the maintenance of
blood pressure.
Furosemide and other
loop diuretics inhibit the activity of NKCC2, thereby impairing sodium reabsorption in the thick ascending limb of the loop of Henle. The action of these
loop diuretics also reduces potassium reabsorption through the NKCC2 cotransporter and consequently increases tubular flow rate which enhances potassium secretion and emphasises the hypokalaemic effect. Impaired sodium reabsorption increases diuresis by three mechanisms: • Increases the amount of active
osmolytes in urine by decreasing absorption of sodium • Erases the papillar gradient • Inhibits
tubuloglomerular feedback Loop diuretics therefore ultimately result in decreased blood pressure. The hormone
vasopressin also stimulates the activity of NKCC2. Vasopressin stimulates sodium chloride reabsorption in the thick ascending limb of the nephron by activating signaling pathways. Vasopressin increases the traffic of NKCC2 to the membrane and phosphorylates some
serine and
threonine sites on the cytoplasmic N-terminal of the NKCC2 located in the membrane, increasing its activity. Increased NKCC2 activity aids in water reabsorption in the collecting duct through
aquaporin 2 channels by creating a hypo-osmotic filtrate. ==Genetics==