Functional demonstration of surface carbonic anhydrase IV activity on rat astrocytes

Abstract

Buffering of the brain extracellular fluid is catalyzed by carbonic anhydrase (CA) activity. Whereas the extracellular isoform CA XIV has been localized exclusively to neurons in the brain, and to glial cells in the retina, there has been uncertainty regarding the form or forms of CA on the surface of brain astrocytes. We addressed this issue using physiological methods on cultured and acutely dissociated rat astrocytes. Prior work showed that the intracellular lactate-induced acidification (LIA) of astrocytes is diminished by benzolamide, a poorly permeant, nonspecific CA inhibitor. We demonstrate that pretreatment of astrocytes with phosphatidylinositol-specific phospholipase C (PI-PLC) results in a similar inhibition of the mean LIA (by 66 +/- 3%), suggesting that the glycosylphosphatidylinositol-anchored CA IV was responsible. Pretreatment of astrocytes with CA IV inhibitory antisera also markedly reduced the mean LIA in both cultured cortical (by 46 +/- 4%) and acutely dissociated hippocampal astrocytes (by 54 +/- 8%). Pre-immune sera had no effect. The inhibition produced by PIPLC or CA IV antisera was not significantly less than that by benzolamide, suggesting that the majority of detectable surface CA activity was attributable to CA IV. Thus, our data collectively document the presence of CAIV on the surface of brain astrocytes, and suggest that this is the predominant CA isoform on these cells.

Carbonic anhydrase IV and XIV knockout mice: Roles of the respective carbonic anhydrases in buffering the extracellular space in brain

“…Both studies confirmed an earlier study showing that no CA IV was detected in retina (27). An example of the other extreme was suggested by the observation that CA IV is the dominant, if not the only, CA expressed on the surface of both isolated and cultured cortical astrocytes (24).…”

“…The only CA known at that time that would be expected to be removed by this treatment was the glycosylphosphatidylinositol-anchored CA IV. In addition, another study recently demonstrated that nearly all of the extracellular CA on the surface of rat astrocytes was attributable to CA IV (24). On the other hand, immunocytochemical studies showed that CA XIV was present on some neurons and axons in brain tissue (14), leading Parkkila et al (14) to suggest that CA XIV was also important.…”

Carbonic anhydrase XIV deficiency produces a functional defect in the retinal light response

“…As mentioned above, in mouse hippocampal slice preparations, CA IV has a greater effect on regulation of extracellular pH than CA XIV (12). CA IV is localized to neuronal and axonal membranes in the hippocampus (31) and has been found to be the only extracellular CA found on the surface of isolated rat astrocytes from neonatal brain (32). In contrast, in Müller glial cells in retina from both human and mouse, CA IV is not detectable and CA XIV expression is very high (8)(9)(10).…”

Intracellular and Extracellular Carbonic Anhydrases Cooperate Non-enzymatically to Enhance Activity of Monocarboxylate Transporters

“…As reported before, uptake and/or release of lactate in astrocytes are mediated via MCT1 and MCT4 (50 -52). Furthermore, astrocytes play a pivotal role in acid/base regulation in the brain, and thus highly express intracellular CAII and extracellular CAIV (31,53,54). Therefore, MCT1, MCT4, CAII, and CAIV might cooperate in astrocytes to ensure both rapid shuttling of lactate to neurons and efficient uptake of lactate from the extracellular space under different physiological conditions.…”