Identification of membrane‐bound carbonic anhydrase in white matter coated vesicles: The fate of carbonic anhydrase and other white matter coated vesicle proteins in triethyl tin‐induced leukoencephalopathy

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Abstract
We have extended our studies on the content of white matter derived coated vesicles (WMCVs) to show that they are enriched in membrane‐bound carbonic anhydrase. Within the myelin complex membrane‐bound carbonic anhydrase is concentrated in the periaxolemmal domain; however, this protein is enriched almost sevenfold in the bilayer of coated vesicles even relative to this myelin membrane region. These data suggest that some vesicles are derived from a site at which this enzyme is highly localized. The enrichment observed for membrane‐bound carbonic anhydrase is unique since other periaxolemmal proteins such as CNPase and plasmolipin are only present in equal amounts in periaxolemmal‐myelin fractions and WMCVs. Based on their known localization, the presence of CNPase coupled with the absence of MAG in WMCVs suggest that these vesicles are derived from the paranodal region. The identification in WMCVs of periaxolemmal‐myelin proteins associated with ion and fluid movement, such as carbonic anhydrase, Na+, K+ ATPase, and the putative K+ channel protein plasmolipin, prompted us to examine the status of these vesicles in triethyl tin (TET)‐induced myelin edema. Coated vesicles and other membrane fractions were isolated from whole brains of control and TET‐treated rats. Whole brains were used so we could compare the effects of TET on WMCV proteins with the effect on proteins enriched in gray matter coated vesicles. The results indicated that TET had no detectable effect on compact or periaxolemmal‐myelin, however, Western blot analysis showed that WMCV proteins, such as carbonic anhydrase, CNPase, and plasmolipin, were virtually absent or greatly diminished from the whole brain coated vesicle fraction. In contrast, gray matter coated vesicle proteins such as synaptophysin, SV2, and the general vesicle protein, cation‐independent mannose‐6‐phosphate receptor were unchanged. These results indicate that coated vesicles may be a useful probe for membrane changes associated with leukopathologies.