2.A.30.1.4
Bumetanide-sensitive NaCl/KCl symporter (basolateral), NKCC1 or SLC12A2 (may also transport NH₄⁺ and water); (Worrell et al., 2008; Hamann et al., 2010). Loop diuretic and ion binding residues have been identified (Somasekharan et al., 2012).  NKCC1 is the major Cl^--loader responsible for the depolarizing action of GABA/glycine receptors at postnatal days 3-5 in cochlear nucleus neurons (Witte et al. 2014). Its activity is stimulated by ammonia (Hertz et al. 2015).  NKCC maintains Cl^- gradients to sustain pacemaker activity (TC# 1.A.1.5.10) in interstitial cells of Cajal (Zhu et al. 2016). It uses the existing Na⁺ and/or K⁺ gradients to move Cl^- into or out of cells. NKCC1 plays fundamental roles in regulating trans-epithelial ion movement, cell volume, chloride homeostasis and neuronal excitability. Yang et al. 2020 reported a cryo-EM structure of human NKCC1 captured in a partially loaded, inward-open state. NKCC1 assembles into a dimer, with the first ten TMSs harboring the transport core and TM11-TM12 helices lining the dimer interface. TMSs 1 and 6 break alpha-helical geometry halfway across the lipid bilayer where ion binding sites are organized around these discontinuous regions. NKCC1 may harbor multiple extracellular entryways and intracellular exit sites, raising the possibility that K⁺, Na⁺, and Cl^- ions may traverse along their own routes for translocation (Yang et al. 2020). It is present in the basolateral membrane of crypt epithelial cells and mediates uptake of these three ions into these cells (A. Quach, personal communication). NKCC1 mediates trans-epithelial Cl^- secretion and regulates excitability of some neurons, while NKCC2 is critical to renal salt reabsorption. Bumetanide is a mainstay for treating edema and hypertension. Cryo-EM structures reveal an outward-facing conformation of NKCC1, showing bumetanide wedged into a pocket in the extracellular ion translocation pathway. Zhao et al. 2022 defined the translocation pathway and the conformational changes necessary for ion translocation. An NKCC1 dimer was observed with separated transmembrane domains and extensive transmembrane and C-terminal domain interactions. An N-terminal phosphoregulatory domain that interacts with the C-terminal domain, suggests a mechanism whereby (de)phosphorylation regulates NKCC1 by tuning the strength of this domain association (Zhao et al. 2022).

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