Ion channel function of aquaporin-1 natively expressed in choroid plexus

Aquaporins are known as water channels; however, an additional ion channel function has been observed for several including aquaporin-1 (AQP1). Using primary cultures of rat choroid plexus, a brain tissue that secretes CSF and abundantly expresses AQP1, we confirmed the ion channel function of AQP1 and assessed its functional relevance. The cGMP-gated cationic conductance associated with AQP1 is activated by an endogenous receptor guanylate cyclase for atrial natriuretic peptide (ANP). Fluid transport assays with confluent polarized choroid plexus cultures showed that AQP1 current activation by 4.5 mu M ANP decreases the normal basal-to-apical fluid transport in the choroid plexus; conversely, AQP1 block with 500 mu M Cd2+ restores fluid transport. The cGMP-gated conductance in the choroid plexus is lost with targeted knockdown of AQP1 by small interfering RNA (siRNA), as confirmed by immunocytochemistry and whole-cell patch electrophysiology of transiently transfected cells identified by enhanced green fluorescent protein. The properties of the current (permeability to Na+, K+, TEA(+), and Cs+; voltage insensitivity; and dependence on cGMP) matched properties characterized previously in AQP1-expressing oocytes. Background K+ and Cl+ currents in the choroid plexus were dissected from AQP1 currents using Cs-methanesulfonate recording salines; the background currents recorded in physiological salines were not affected by AQP1-siRNA treatment. These results confirm that AQP1 can function as both a water channel and a gated ion channel. The conclusion that the AQP1-associated cation current contributes to modulating CSF production resolves a lingering concern as to whether an aquaporin ionic conductance can have a physiologically relevant function.