CIC-2: a developmentally dependent chloride channel expressed in the fetal lung and downregulated after birth

Growth and differentiation of the fetal lung are dependent on chloride and fluid secretion, yet the specific molecular identities of fetal chloride channels have not been fully determined. In this study, we demonstrate mRNA expression of the volume-activated chloride channel, CIC-2, in fetal rat lung using reverse-transcriptase polymerase chain reaction (RT-PCR) and ribonuclease (RNase) protection assay. By RNase protection assay, CIC-2 mRNA expression is most abundant in fetal lung and diminishes after birth until it is almost undetectable in adult rat lung. To confirm this result at the protein level, a C-terminal fragment of CIC-2 cDNA derived from 19-day fetal rat lung was cloned into an expression plasmid. The truncated 33-kD CIC-2 protein was expressed in Escherichia coli and purified by column chromatography. Polyclonal antibodies to this antigen were raised in chickens, and the antisera detected a 94-kD protein in fetal rat lung homogenates by Western blotting. Protein expression of CIC-2 was most abundant in mid and late gestation and decreased significantly shortly after birth, as would be predicted by the RNase protection data. CIC-2 protein was localized along the apical surface of fetal airway epithelium by immunocytochemistry. The abundant fetal expression of CIC-2 RNA and protein supports the hypothesis that CIC-2 is important to fetal lung development, and its apical location suggests that it may be involved in fluid secretion during normal lung morphogenesis.