Expression of novel isoforms of the CIC-1 chloride channel in astrocytic glial cells in vitro

Chloride channels play an important role in glial astrocyte function. However, in astrocytes, no chloride channels besides the gamma-aminobutyric acid (GABA)A receptor, glycine receptor, and ClC-2 chloride channels have been molecularly identified. In this study, we examined the expression of the ClC-1 chloride channel in rat astrocytic glioma C6 cells and rat primary astrocytes. Five isoforms of ClC-1, but not skeletal muscle ClC-1 (SM ClC-1), were found to be expressed in C6 cells. Comparison with rat SM ClC-1 showed that common features shared by these isoforms are a short 3' end with a deletion of the nucleotides from 3115 to 3197 and a substitution of T by C at nucleotides 480 and 1733. Three of the five isoforms, M1, M2, and M3, were produced by partial deletion of ClC-1 exon 7, partial insertion of ClC-1 exon 7a, and a TAG insertion at nucleotide 858, respectively. One of the two remaining isoforms, M4, was produced by partial deletion of ClC-1 exon 8 at nucleotide 937; the other, M5, was the same as SM ClC-1 except for the short 3' end and substitutions at the two positions. Only the M5 isoform could be expressed as a functional channel in Xenopus oocytes. This glial isoform exhibited less dependence on voltage and extracellular Cl- than rat SM ClC-1. However, the anion selectivity sequence and the anthracene-9-carboxylic acid (9-AC) sensitivity of this channel were the same as for SM ClC-1. Since whole-cell recordings failed to detect ClC-1-like Cl- currents in C6 cells, it appears that the ClC-1 isoform is functioning in intracellular organelles. In rat primary astrocytes, we found that the M2 isoform as well as two additional distinct isoforms were expressed. The present study showed that astrocytic glial cells express multiple isoforms of the ClC-1 chloride channel, which has been thought to be expressed almost exclusively in the skeletal muscle.