Fig. 11.

Proposed mechanism of apical Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1) function in choroid plexus epithelial cells (CPECs) under basal conditions. We propose that by virtue of being continuously active (CA) and working in the inward direction under basal conditions, apical NKCC1 maintains intracellular Cl⁻ concentration ([Cl⁻]_i) and cell water volume (CWV) at a certain level (set point), playing an indirect but critical role in cerebrospinal fluid (CSF) secretion. Intracellular [Cl⁻] and CWV result from a delicate balance between active Cl⁻ uptake via apical NKCC1 along with associated water [osmotically driven by NKCC1 via aquaporin 1 (AQP1) and/or cotransported by NKCC1] and various Cl⁻ and water efflux pathways, working in a concerted and coordinated manner. Putative Cl⁻ efflux pathways in CPECs are the basolateral K⁺-Cl⁻ cotransporters (KCC3a and/or KCC1), the apical KCC4, and 2 types of apical Cl⁻ conductances, i.e., the volume-activated (VA) and the inward-rectifying (IR) Cl⁻ channels. This homeostatic molecular machinery contributes to both CSF secretion and K⁺ absorption from CSF to blood. Pharmacological or genetic inactivation of NKCC1 disrupts the homeostatic equilibrium of CWV and [Cl⁻]_i, thereby resulting in unbalanced Cl⁻ and water efflux leading to cell shrinkage and a subsequent decrease in CSF secretion. A: diagram depicting some of the cotransporters and channels proposed to be involved in the maintenance of CPEC volume and [Cl⁻]_i needed for both CSF secretion and K⁺ absorption under basal conditions. They include the apical KCC4, NKCC1, the Na⁺-K⁺-ATPase, AQP1 water channels, and VA and IR Cl⁻ channels. Mechanisms by which water permeates the basolateral membrane are unknown. B: diagram depicting the factors determining the set point for the basal activity of both NKCC1 and KCCs, which maintain the CWV and [Cl⁻]_i necessary for both simultaneous CSF secretion and K⁺ absorption. NKCC1 and KCCs cotransport activity (in arbitrary units 0–100) is plotted as a function of relative (normalized) cell water volume [V_o is the basal steady-state cell water volume (to normalize V_o = 1), and V_t is the steady-state cell water volume reached after a change in intracellular solute and water content (V_t/V_o)]. In the ordinate, 100 represents maximal cotransport activity. In the abscissa, basal V_t/V_o = 1.0. NKCC1 and KCCs cotransport activity is reciprocally regulated by numerous interdependent factors that include V_t/V_o, [Cl⁻]_i, and the relative fraction of phosphorylated cotransporter molecules (P*/P), where P* represents the number of phosphorylated cotransporters and P the number of dephosphorylated cotransporters. Phosphorylation of NKCC1 and KCCs is likely to be effected by the WNK-SPS1-related proline/alanine-rich kinase (SPAK)/oxidative stress-responsive kinase 1 (OSR1) kinase complex as in other epithelial cells (1, 86). Sigmoid curves are only for schematic purposes.