Functional properties of aquaporin-1 ion channels in choroid plexus

Abstract

Aquaporins (also known as water channels) are members of the Major Intrinsic Protein family. Ion channel function has been shown for several members of the aquaporin family and the related neurogenic gene product Big Brain. Aquaporin-1 (AQP1) is a transmembrane channel that mediates osmotically-driven water flux. Prior work demonstrated that AQP1 channels expressed in Xenopus oocytes mediate a cGMP-dependent cationic current. Based on amino acid sequence alignments with cyclic nucleotide-gated channels and cGMP-selective phosphodiesterases, I found that the efficacy of ion channel activation is decreased by mutations of AQP1 at conserved residues in the C-terminal domain (aspartate D237 and lysine K243). These data provide direct evidence for the involvement of the AQP1 carboxyl terminal domain in cGMP-mediated ion channel activation. Because the proportion of active AQP1 ion channels seen in heterologous expression systems is low, it was of fundamental importance to investigate the functional properties of this channel in a physiological context. Using rat choroid plexus, a brain tissue that secretes cerebral spinal fluid (CSF) and endogenously expresses abundant AQP1, I demonstrated the existence of native AQP1 ion channels that show properties similar to those described previously in the oocyte expression system. They mediate a cGMP-dependent cationic conductance, are blocked by cadmium, and show a single-channel conductance of 166 pS. Given the skull's rigidity, pathological increases in CSF secretion (tumors, hydrocephalus, stroke) can result in brain damage. In the choroid plexus several proteins work in concert to regulate CSF secretion. The findings presented in this dissertation are first to demonstrate that AQP1 mediates a cationic current in response to intracellular signals that regulate CSF secretion such as ANP signaling. Fluxes of water and Na⁺ across confluent choroid plexus cell monolayers showed a decreased flow rate following treatment with ANP, and Cd²⁺ reversed the inhibitory effect. These results suggest that activation and block of the AQP1-mediated ionic current may alter net fluid transport across the choroid plexus barrier, and therefore be physiologically relevant in the regulation of net fluid transport in choroid plexus. This places AQP1 as one of the important targets for clinical intervention in brain volume disorders.