Inward-rectifying anion channels are expressed in the epithelial cells of choroid plexus isolated from ClC-2 'knock-out' mice

Choroid plexus epithelial cells express inward-rectifying anion channels which have a high HCO3- permeability. These channels are thought to have an important role in the secretion of cerebrospinal fluid. The possible relationship between these channels and the ClC-2 Cl- channel was investigated in the present study. RT-PCR, using specific ClC-2 primers, amplified a 238 bp fragment of mRNA from rat choroid plexus, which was 99% identical to the 5' sequence of rat ClC-2. A 2005 bp clone was isolated from a rat choroid plexus cDNA library using a probe for ClC-2. The clone showed greater than 99% identity with the sequence of rat CIC-2. Inward-rectifying anion channels were observed in whole-cell. recordings of choroid plexus epithelial cells isolated from CIC-2 knock-out mice. The mean inward conductance was 19.6 +/- 3.6 nS (n = 8) in controls (3 heterozygote animals), and 22.5 +/- 3.1 nS (n = 10) in three knock-out animals. The relative permeability of the conductances to I- and Cl- (P-I: P-Cl) was determined. I- was more permeant than Cl- in both heterozygotes (P-I:P-Cl = 4.0 +/- 0.9, n = 3) and knock-out animals (P-I: P-Cl = 4.1 +/- 1.4, n = 3). These results indicate that rat choroid plexus expresses the ClC-2 variant that was originally reported in other tissues. CIC-2 does not contribute significantly to inward-rectifying anion conductance in mouse choroid plexus, which must therefore express a novel inward-rectifying anion channel.