Identification of a new form of AQP4 mRNA that is developmentally expressed in mouse brain.

The water channel aquaporin 4 (AQP4) is abundantly expressed in the brain, and also in lung and kidney. Previous studies have suggested that there are at least two AQP4 mRNA. The two mRNA encode for two AQP4 proteins that differ with regard to the length of the N-terminal: AQP4.M1 and AQP4.M23. Here we report, by use of reverse transcriptase PCR and comparison of genomic and cDNA structures, the presence of a third form of mouse AQP4 mRNA. The upstream sequence of this form of mRNA originates from an additional exon, interspaced between exon 0 and exon 1, and an alternatively spliced form of exon 1. Analysis of nucleotide sequence suggests that this new form of AQP4 mRNA also encodes for the AQP4.M23 protein. The two forms of AQP4 mRNA that presumably both encode for M23 have a tissue- and age-specific expression. The new AQP4 mRNA was predominantly expressed in brain. The expression was approximately twofold higher in the adult brain than in the infant brain. In contrast, the expression levels of the new mRNA were low in both infant and adult lung and kidney. The previously described mRNA encoding for AQP4.M23 was predominantly expressed in lung and kidney. In lung, the expression of this form was higher in infancy than in adulthood. In conclusion, we have identified a new form of AQP4 mRNA that is predominantly expressed in the brain and that is developmentally regulated.