Effects of carbonic anhydrase II (CAII) deficiency on CNS structure and function in the myelin‐deficient CAII‐deficient double mutant mouse

In the choroid plexus carbonic anhydrase II (CAII) supports the transport of bicarbonate ions, sodium ions, and water from blood to the CSF, and in the myelin sheath CAII supports compaction of myelin by stimulating cotransport of ions and water out from between the myelin membranes. In view of the latter, it is surprising that mutant mice deficient in CAII (Car-2n) have compact myelin. Since myelin basic protein also takes part in myelin compaction, we bred double CAII-deficient, myelin-deficient (MId) mutant mice, in which the adults would have some compact myelin sheaths and a partial deficiency in myelin basic protein, with a view to examining oligodendrocytes and myelin sheaths in the double mutant. Like the parent MId strain, the double mutants displayed tremors and seizures; however, the onset of seizures was delayed significantly in the double mutants, and the lifespan increased by several months. Like the brains of Car-2n mutants, those of double mutants (MldCar-2n) were deficient in mRNA and protein for CAII and showed upregulation of a different isozyme, CAIV. In the double mutants, oligodendrocytes were reduced in number, and the myelin sheaths and oligodendrocytes were swollen. The partial protection against seizures, which CAII deficiency conferred, suggests that acidosis in the central nervous system (CNS) of the Car-2n and MldCar-2n mice, due to absence of CAII from the choroid plexus, may downregulate the activity of NMDA receptors. The swelling of myelin and oligodendrocytes only in the MldCar-2n suggests that CAIV activity is not sufficient for maintaining compact myelin when the myelin is partly deficient in myelin basic protein and severely deficient in CAII. The findings provide the first direct evidence for a role of carbonic anhydrase II in the compaction of myelin membranes. © 1995 Wiley-Liss, Inc.