Choroid plexus epithelial cells use NKCC1 cotransporters as sensors and regulators of cerebrospinal fluid potassium levels
Forming the blood-cerebrospinal fluid (CSF) barrier, the choroid plexus resides within the brain ventricles; choroid plexus epithelial cells (CPECs) secrete CSF and regulate its electrolyte composition. Ions freely diffuse between CSF and interstitial brain fluid (IBF); thus, CSF and IBF ion compositions are interdependent. Potassium (K+) is of fundamental importance in CSF and IBF; small changes in extracellular K+ concentration ([K+]o) profoundly affect neuronal excitability and water balance. Brain [K+]o is tightly regulated, we hypothesize the Na+/K+/2Cl-cotransporter 1 (NKCC1) is pivotal in CSF K+ homeostasis. CPECs express NKCC1 protein on their luminal membrane (CSF-facing), deviating from typical basolateral membrane (blood-facing) location in secretory epithelia. NKCC1-mediated ion and water fluxes are tightly coupled, thus the magnitude and direction of these fluxes can be inferred by measuring cell water volume (CWV) changes. CPECs from NKCC1 KO mice (NKCC1-/-) are half the sizes of CPECs from WT, indicating: physiological NKCC1-mediated solute and water fluxes are inwards, NKCC1 maintains normal CWV, and NKCC1 is constitutively active. Measuring relative CWV changes in isolated CPECs, using the fluorescent dye calcein, reveals NKCC1-mediated fluxes are exquisitely sensitive to [K+]o in the physiologically relevant range (1-5mM). Reducing [K+]o to 1mM from physiological CSF concentration (?3mM) produces a ?16% reduction in CWV, while increasing [K+]o to 5mM produces a ?20% increase in CWV. NKCC1-/- mouse CPECs loose this sensitivity to [K+]o. Thus, NKCC1 works near its thermodynamic equilibrium in CPECs; small changes in [K+]o alter direction and magnitude of NKCC1-mediated fluxes. We propose NKCC1 senses and regulates CSF [K+].