Light microscopic histochemistry of the postnatal development and localization of carbonic anhydrase activity in glial and neuronal cell types of the rat central nervous system

Carbonic anhydrase seven bundles filamentous actin and regulates dendritic spine morphology and density

Intracellular pH is a potent modulator of neuronal functions. By catalyzing (de)hydration of CO2 , intracellular carbonic anhydrase (CAi ) isoforms CA2 and CA7 contribute to neuronal pH buffering and dynamics. The presence of two highly active isoforms in neurons suggests that they may serve isozyme-specific functions unrelated to CO2 -(de)hydration. Here, we show that CA7, unlike CA2, binds to filamentous actin, and its overexpression induces formation of thick actin bundles and membrane protrusions in fibroblasts. In CA7-overexpressing neurons, CA7 is enriched in dendritic spines, which leads to aberrant spine morphology. We identified amino acids unique to CA7 that are required for direct actin interactions, promoting actin filament bundling and spine targeting. Disruption of CA7 expression in neocortical neurons leads to higher spine density due to increased proportion of small spines. Thus, our work demonstrates highly distinct subcellular expression patterns of CA7 and CA2, and a novel, structural role of CA7.

Carbonic anhydrases and brain pH in the control of neuronal excitability

H(+) ions are remarkably efficient modulators of neuronal excitability. This renders brain functions highly sensitive to small changes in pH which are generated "extrinsically" via mechanisms that regulate the acid-base status of the whole organism; and "intrinsically", by activity-induced transmembrane fluxes and de novo generation of acid-base equivalents. The effects of pH changes on neuronal excitability are mediated by diverse, largely synergistically-acting mechanisms operating at the level of voltage- and ligand-gated ion channels and gap junctions. In general, alkaline shifts induce an increase in excitability which is often intense enough to trigger epileptiform activity, while acidosis has the opposite effect. Brain pH changes show a wide variability in their spatiotemporal properties, ranging from long-lasting global shifts to fast and highly localized transients that take place in subcellular microdomains. Thirteen catalytically-active mammalian carbonic anhydrase isoforms have been identified, whereof 11 are expressed in the brain. Distinct CA isoforms which have their catalytic sites within brain cells and the interstitial fluid exert a remarkably strong influence on the dynamics of pH shifts and, consequently, on neuronal functions. In this review, we will discuss the various roles of H(+) as an intra- and extracellular signaling factor in the brain, focusing on the effects mediated by CAs. Special attention is paid on the developmental expression patterns and actions of the neuronal isoform, CA VII. Studies on the various functions of CAs will shed light on fundamental mechanisms underlying neuronal development, signaling and plasticity; on pathophysiological mechanisms associated with epilepsy and related diseases; and on the modes of action of CA inhibitors used as CNS-targeting drugs.

Up-regulation of cerebral carbonic anhydrase by anoxic stress in piglets

The resuscitation of asphyxiated babies is associated with changes in cerebral protein synthesis that can influence the neurological outcome. Insufficient gas exchange results in rapid shifts in extracellular and intracellular pH. Carbonic anhydrase (CA) plays an important role in buffering acute changes in pH in the brain. We investigated whether asphyxia/re-ventilation influences the expression of cerebral CA isoforms (CA-II, CA-III and CA-IV) in anaesthetized newborn pigs. The cerebral cortex, hippocampus, cerebellum and retina were sampled, and prepared for either CA immunohistochemistry or CA immunoblotting from piglets subjected to asphyxia (10 min) followed by 2-4 h of re-ventilation, and also from normoxic controls. The CA immunoreactivity (IR) of all the isoforms studied was weak in the controls, apart from staining of a few oligodendrocytes in the subcortical white matter, some astrocytes in the superficial layer of the cerebral cortex, the cerebellar Purkinje cells and the retinal Müller cells that possessed moderate CA-II IR. However, asphyxia induced a marked increase in the CA IR of all isoforms in all the cerebral regions investigated and the retina after 4 h of survival. The pyramidal cells of the frontal cortex and hippocampus displayed the most conspicuous increase in CA IR. Immunoblotting confirmed increased levels of all the CA isoenzymes. We conclude that raised CA levels after asphyxia may contribute to the compensatory mechanisms that protect against the pathological changes in the neonatal CNS.

Carbonic anhydrase II and carbonic anhydrase-related protein in the cerebellar cortex of normal and lurcher mice

The developmental profiles of carbonic anhydrase II (CA-II) and a carbonic anhydrase related protein (CARP) were studied in rat and mouse cerebella. Enzyme histochemistry, immunohistochemistry, in situ hybridisation and Western blotting were used to study the synthesis and expression of these enzymes in cerebellar sections from age matched control, CA-II deficient and lurcher mice, the latter being characterised by Purkinje cell degeneration. Both CA-II and CARP were first found to be expressed in the Purkinje cells in the 9 day old mouse, and the immunoreactivity of both peptides increased with time. Immunohistochemistry showed more intense staining of CARP than of CA-II in Purkinje cells throughout the developmental profile of the mouse, and this was mirrored by the mRNA levels determined by in situ hybridisation. Immunohistochemistry of CA-II and CARP also demonstrated the progressive dendritic growth of the mouse and rat Purkinje cells. CA-II and CARP immunoreactivity ceased by the end of cerebellar maturation. The onset of Purkinje cell degeneration was detected at day 10 in the lurcher mouse, with concomitant marked decrease in CA-II level: however CARP expression was found to be unchanged. By postnatal day 16 neither CA-II mRNA, protein, nor activity was detectable in contrast to CARP which remained at a decreased level unit the Purkinje cells population had completely degenerated. Our findings suggest a role of CA-II in the degenerative processes of the lurcher Purkinje cells, with CARP playing an important role in the development and maturation of the cerebellar cortex.

Carbonic anhydrase II in microglia in forebrains of neonatal rats

In the brains of adult rodents carbonic anhydrase II (CA) immunoreactivity has been observed in the choroid plexus and in oligodendrocytes, astrocytes, and myelin. Localization and functions of CA in the neonatal brain, however, have been controversial. One issue is whether the CAII-immunopositive round and ameboid cells in the corpus callosum and cingulum in the rat CNS during the first postnatal week are oligodendrocytes or microglia. Colocalization of CAII with the microglial antigen, ED1, and the microglia-specific isolectin, BSI-B4, suggested that most (approx. 60%) of the CAII-positive round and ameboid cells in rat brain during the first postnatal week were, indeed, macrophages and microglia. During that initial week, some CAII-positive protoplasmic astrocytes (approx. 40%) were observed as well. At the end of the first postnatal week smooth-surfaced CAII-positive cells began to appear in the corpus callosum. Those cells also bound MAbO4, a marker for the oligodendrocyte cell line. We conclude that during the first postnatal week most of the CAII-positive cells are macrophages and microglia, and that some are protoplasmic astrocytes. During the second postnatal week CAII-positive cells in the oligodendrocyte lineage become apparent, and by the end of that week there are few CAII-positive microglia. Confocal microscopy suggests that in brains of three-day-old rats the ameboid microglia are associated with nerve fibers, where they may perform phagocytosis of axons, directional guidance of axons, or disinhibition of axonal growth.

A reappraisal of ganglioside GD3 expression in the CNS

GD3 ganglioside is a major glycolipid component of the developing central nervous system but diminishes considerably as the CNS matures. Despite consistent biochemical data, the cellular localization of GD3 expression has been controversial. In this commentary we will review the cellular expression of GD3 during CNS development and in neuropathological circumstances as determined by studies with the two most commonly used anti GD3 monoclonal antibodies, R24 and LB1. GD3 is not restricted to any one cell lineage, being expressed in development to varying degrees by immature neuroectodermal cells, oligodendrocyte progenitors, ameboid microglia, and subpopulations of developing neurons and astrocytes. In the adult CNS, GD3 is expressed in low amounts by some neuronal subpopulations, on reactive and resting microglia, and by reactive astrocytes. In the appropriate contexts of development or neuropathology, anti-GD3 antibodies are useful for cell type identification and for cell isolation, but caution should be exercised because of the lack of cellular specificity.

Strongly GD3+ cells in the developing and adult rat cerebellum belong to the microglial lineage rather than to the oligodendrocyte lineage

A recent study has shown that ramified microglia in the adult rat optic nerve express the ganglioside GD3 [Wolswijk Glia 10:244-249, 1994], thereby raising the possibility that some GD3+ in the developing rat central nervous system (CNS) belong to the microglial lineage rather than to the oligodendrocyte lineage, as previously thought. To examine this possibility, sections of postnatal and adult cerebellum were double-labelled with markers for rat microglia [the B4 isolectin derived from Griffonia simplicifolia (GSI-B4), the ED1 monoclonal antibody (mAb), and the OX-42 mAb] and anti-GD3 mAbs (the mAbs R24 and LB1). These immunolabellings showed that ramified microglia as well as amoeboid microglia are strongly GD3+ in vivo. Moreover, most, if not all, cells that express high levels of GD3 in sections of developing cerebellum appear to belong to the microglial lineage. These observations contradict previous suggestions that the strongly GD3+ cells in the putative white matter regions of the developing brain are oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells; the cells that give rise to oligodendrocytes in the CNS. The present study did, however, confirm that some O-2A progenitor cells in sections of postnatal cerebellum are weakly GD3+ in vivo. Amoeboid microglia are present in areas of the developing cerebellum where newly generated oligodendrocytes are found, suggesting that these cells play a role in the phagocytosis of the large numbers of oligodendrocytes that die as part of CNS development.

Differential expression of carbonic anhydrase isozymes in microglial cell types

Microglia cells have been shown to express carbonic anhydrase. Using carbonic anhydrase histochemistry and immunohistochemistry, different types of central nervous system microglial cells were detected, which expressed two main carbonic anhydrase (CA) isozymes during the early postnatal stage of development and after peripheral nerve injury in the spinal cord of adult rats. Amoeboid and reactive microglial cells were heavily immunostained for CA-II and CA-III and showed colocalization with complement receptor type 3 and Griffonia Simplicifolia B4 isolectin. Resting microglial cells in the brain and spinal cord showed faint CA-III staining and were negative for CA-II. These results show that not only CA-II, but also CA-III isozyme is represented in the central nervous system and carbonic anhydrase activity may correlate with metabolic and immunological changes of microglial cells. These data also further strengthen the idea of the mesodermal origin of central nervous system macrophages.

Localization of acetazolamide-resistant carbonic anhydrase III in human and rat choroid plexus by immunocytochemistry and in situ hybridisation

Carbonic anhydrase is an essential metabolic enzyme of the central nervous system and has an important role in the production and regulation of cerebrospinal fluid. Although it has been known for over 30 years that inhibition of the enzyme with acetazolamide dramatically but not completely reduces the production of cerebrospinal fluid, the precise mechanism of the inhibitory action has been only recently revealed. In this study we present evidence that apart from carbonic anhydrase II, the catalytically highly active isozyme, carbonic anhydrase III, an acetazolamide-resistant and kinetically different isozyme could be demonstrated in the epithelial cells of the developing and mature rodent and human choroid plexuses. Both isozymes express intense immunostaining revealed with specific antisera, and by using in situ hybridisation histochemistry, carbonic anhydrase III mRNA was also observed. Since the kinetic properties and proportion of brain carbonic anhydrase III in the human choroid plexus are not revealed the function of this isozyme in choroid plexus is still to be determined.

Carbonic anhydrase in distinct precursors of astrocytes and oligodendrocytes in the forebrains of neonatal and young rats

Carbonic anhydrase is present in oligodendrocytes and astrocytes in the mature rat brain. Whereas carbonic anhydrase-positive oligodendrocyte precursors had been identified during the first postnatal week, no information was available about the earliest occurrence of carbonic anhydrase in the astrocytic cell line, nor had carbonic anhydrase been detected in astrocytes in neonatal rat brains. Beginning on the first postnatal day, rat brains were double immunostained with anti-carbonic anhydrase II and respective 'markers' for immature and mature astrocytes and oligodendrocytes. During the first postnatal week there were intensely carbonic anhydrase-positive cells which were ovoid or had broad processes. On the basis of their shapes and antigen contents these were considered to be precursors of oligodendrocytes. Beginning on the first postnatal day carbonic anhydrase II was also observed in some vimentin-positive radial glia and in other vimentin-positive cells that differed in their appearance from the immature oligodendrocytes. The vimentin-positive, carbonic anhydrase-positive cells were less smooth-surfaced, and had much finer processes, than the oligodendrocyte precursors. By the third postnatal day there appeared carbonic anhydrase-positive, glial fibrillary acidic protein (GFAP)-positive cells that resembled the vimentin-positive cells. It is concluded that the latter are immature astrocytes and that carbonic anhydrase is in distinct precursors of oligodendrocytes and astrocytes as early as the first postnatal day.

Expression and quantitative changes of carbonic anhydrase in developing neurones of rat central nervous system

Postnatal changes in carbonic anhydrase activity were investigated in the islands of Calleja, which have been previously reported to contain the enzyme. Results obtained with a new modified method of Hansson provided further evidence for the distinction between the medial and lateral islands of Calleja. The enzyme was localized mainly in the nucleus and cytoplasm of granule cells without showing binding to any cytoplasmic organelle. No large neurons of the islands displayed carbonic anhydrase reactivity. The time course and rate of increase of carbonic anhydrase expression were different in the giant island of Calleja and lateral islands and this finding may strengthen the hypothesis regarding the medio-lateral diversity of Calleja's islands. On the other hand, at the end of the maturation process the granule cell complexes showed no significant difference in the proportion of carbonic anhydrase positive neurones. The almost equal rate of appearance of carbonic anhydrase reactive granule cells raises the possibility of a basic common role of both medial and lateral islets.