Characterization and biosynthesis of soluble and membrane-bound carbonic anhydrase in brain

Expression of Carbonic Anhydrase I in Motor Neurons and Alterations in ALS

Carbonic anhydrase I (CA1) is the cytosolic isoform of mammalian α-CA family members which are responsible for maintaining pH homeostasis in the physiology and pathology of organisms. A subset of CA isoforms are known to be expressed and function in the central nervous system (CNS). CA1 has not been extensively characterized in the CNS. In this study, we demonstrate that CA1 is expressed in the motor neurons in human spinal cord. Unexpectedly, a subpopulation of CA1 appears to be associated with endoplasmic reticulum (ER) membranes. In addition, the membrane-associated CA1s are preferentially upregulated in amyotrophic lateral sclerosis (ALS) and exhibit altered distribution in motor neurons. Furthermore, long-term expression of CA1 in mammalian cells activates apoptosis. Our results suggest a previously unknown role for CA1 function in the CNS and its potential involvement in motor neuron degeneration in ALS.

Resistance of hippocampal synaptic transmission to hypoxia in carbonic anhydrase II-deficient mice

Mutant Car2n/Car2n mice deficient in carbonic anhydrase II (CA II; a major brain CA isozyme) suffer from systemic acidosis and are more resistant to experimental seizures than their normal littermates (+/+ or +/Car2n). The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor has been shown to contribute to long-term potentiation (LTP) of synaptic transmission, hypoxic/ischemic neuronal injury and to be blocked by extracellular protons (acidosis). We compared the effects of hypoxia on synaptic transmission and LTP in field CA1 of hippocampal slices from CA II-deficient mice to their normal littermates. Slices were subjected to successive 5, 10 and 15 min-periods of hypoxia with 30 min-recovery periods in between. Hippocampal slices from mutant, CA II-deficient mice, were more resistant to all periods of hypoxia tested than slices from normal littermates. In a separate set of mutant and normal slices, there were no differences in LTP of population spike amplitude. The relative resistance of CA II-deficient mice to hypoxia-induced damage may be a consequence of severe interstitial acidosis. The sustained influence of increased extracellular proton concentrations may change the characteristics of NMDA receptors resulting in an increased resistance of synaptic transmission in CA II-deficient mice to hypoxia compared to controls.

A competitive dual-label time-resolved immunofluorometric assay for simultaneous detection of carbonic anhydrase I and II in cerebrospinal fluid

Carbonic anhydrase (CA) is functionally an important enzyme in the central nervous system (CNS) where it is involved in the control of acid-base balance and regulation of the production of cerebrospinal fluid (CSF). Isoenzyme II (CAII) is the most widely distributed CA in the CNS being specifically present in CNS glial tissue and therefore it is expected to be leaked to CSF in degenerative CNS diseases. A competitive dual-labeled time-resolved immunofluorometric assay was developed for simultaneous quantification of human CAI (HCA I) and II (HCA II) in CSF. HCA I was measured to determine the blood contamination in the samples. This solid-phase immunoassay is based on competition between europium (Eu3+)- or samarium (Sm3+)-labeled antigen and the sample antigens for polyclonal rabbit antibodies which are attached to microtiter-plate wells precoated with sheep anti-rabbit IgG. The subsequent immunoassay, including the separation of free and bound HCA I and II, requires only one incubation step, after which an enhancement solution dissociates Sm3+ and Eu3+ ions from the labeled HCA I and II, respectively, into a solution where they form highly fluorescent chelates. Spectra of the fluorescent chelates in the microtitration strip wells were run on time-resolved fluorometers equipped with filters for Eu3+ (613 nm) and Sm3+ (643 nm), the fluorescence from each sample being inversely proportional to the concentration of antigens. The detection limit of the HCA II assay was 0.3 micrograms/l and that of the HCA I assay was 5.2 micrograms/l. The intra- and inter-assay imprecisions (C.V.s) were 8.0% and 8.8% for HCA I and 6.3% and 4.8% for HCA II, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for two isoforms of carbonic anhydrase II in the leech (Hirudo medicinalis) central nervous system

1. We dissected, homogenized and prepared ganglia and connectives from the central nervous system of medicinal leeches for SDS gel electrophoresis. The isolated proteins were transferred to nitrocellulose and incubated with affinity column-purified antibodies. 2. The immunoblots showed a strong positive reaction of a bovine carbonic anhydrase standard at a molecular weight of 29 kDa, and a distinct double-bond at the same molecular weight in the analyzed material. 3. We demonstrated with rat antibodies that carbonic anhydrase II is detectable in the leech central nervous system as the main isoenzyme. 4. The biochemical knowledge of carbonic anhydrase reported here agrees well with the immunocytochemical locations, thus affirming the validity of specific staining.

Glial cell abnormalities in the CNS of the carbonic anhydrase II deficient mutant mouse

The Car-2n/Car-2n mutant mouse is entirely lacking in carbonic anhydrase II (CA), which is normally found in myelin and glial cells in the CNS. CNS tissue from CA-deficient mutant mice was examined to see whether abnormalities could be detected which might help in understanding the function(s) of CA in the normal brain. Analyses of myelin yields and myelin proteins showed no differences between mutants and normal littermates. To visualize the oligodendrocytes and astrocytes in tissue sections, immunocytochemical staining was performed using antibodies against the Pi form of glutathione-S-transferase and glial-fibrillary acidic protein, respectively. In both gray and white matter from the mutants' brains the oligodendrocytes appeared to be shrinking and, possibly, degenerating. Hypertrophy of astrocytes occurred in the white matter, and the astrocytes in gray matter appeared swollen. It is suggested that imbalances in the HCO3-/CO2 ratios in various glial-cell compartments may produce abnormal distributions of water and ions in the brains of the CAII-deficient mice.

pH-dependent association of carbonic anhydrase (CA) with gastric light microsomal membranes isolated from bovine abomasum. Partial characterization of membrane-associated activity

1. The effect of pH on the association of carbonic anhydrase (CA) with bovine gastric light microsomal membranes (LMMs) was investigated (a) by washing LMMs containing CA activity with solutions of different pHs; (b) by studying the adsorption at various pHs of soluble bovine erythrocyte CA to washed gastric LMMs. In both cases, the association of CA with gastric LMMs was dependent on pH, being lower at neutral or alkaline pH. 2. The amount of soluble CA associated with gastric LMMs at pHs 8.0 and 9.0 was reduced when 140 mM K+/10 mM Na+ was added to the incubation medium. 3. Two sources of CA activity in bovine gastric LMMs were assumed: a loosely- and a firmly-membrane-associated activity. Both CA activities were dose-dependently inhibited by acetazolamide (I50: 3.6 x 10(-9) and 8.4 x 10(-9) M, respectively) and by chloride, acetate, iodide, bromide and nitrate at 100 mM. Firmly-membrane-associated activity appeared to be less sensitive to inhibition by acetazolamide, chloride and iodide. 4. Both activities exhibited different behavior and stability following treatment with alkaline Triton X-100. 5. The possible importance of a membrane-associated CA activity in gastric LMMs related to gastric acid secretion is discussed.

Developmental changes in seizure susceptibility in carbonic anhydrase II-deficient mice and normal littermates

Mice deficient in carbonic anhydrase II (CA II) were tested along with their normal littermates for susceptibility to seizures induced by flurothyl and loud sound at ages 10-180 days. In the flurothyl seizure model, CA II-deficient mice displayed increased resistance to clonic seizures from 32 to 90 days of age, whereas tonic-clonic seizures were suppressed at all ages. The mortality of CA II-deficient mice was significantly decreased at ages 19-40 days. The incidence of sound-induced seizures was very low and no difference between CA II-deficient and normal mice was found. The anticonvulsant effect of CA II deficiency appears to be dependent on seizure model and seizure type and to have age-specific characteristics.

Reduced susceptibility to seizures in carbonic anhydrase II deficient mutant mice

The seizure susceptibility of carbonic anhydrase II (CA) deficient mice and their normal littermates was determined and compared. In flurothyl-induced seizures, CA deficient mice displayed longer latencies to the onset of both clonic and tonic-clonic seizures. In pentylenetetrazole-induced seizures mutant mice exhibited a lower incidence of clonic seizures than did their normal littermates. Acetazolamide (a CA blocker) was used for the pretreatment of normal mice to compare them to CA deficient littermates. The pretreated mice displayed a lower incidence of flurothyl-induced tonic-clonic seizures and of both types of pentylenetetrazole seizures. The attempts to elicit audiogenic seizure did not reveal any difference between normal and mutant littermates. However, when the mice were primed by a loud sound during the critical period and retested for audiogenic seizures again at age 1.5 months, the CA deficient mice displayed a significantly lower incidence of seizures. The similarity between the anticonvulsant action of CA deficiency and the anticonvulsant action of acetazolamide suggests an important role of CA in seizures. The exact mechanism of anticonvulsant action by CA inhibition, however, remains to be elucidated.

Subcellular distribution of carbonic anhydrase in the enterocyte of the rabbit ileum and the Caco-2 cell. Evidence for the presence of two isozymes bound to brush-border membranes

The activity of CA has been determined in the membranes of enterocytes from rabbit ileum and of Caco-2 cells. No CA activity was detected in the BLM, but the activity in the BBM (43 and 7 WAU/mg protein for rabbit and Caco-2, respectively) was doubled by the addition of Triton. These two types of activity could be distinguished in rabbit ileum by their different IC50 in the presence of acetazolamide (10(-5) and 5 x 10(-7) M) and their different sensitivities to heat. They were not modified by inhibitors of cytoplasmic isozymes and seem to correspond to two forms of CA, one situated in the extracellular leaflet of the BBM and the other one in the intracellular leaflet.

Carbonic anhydrase IV on brain capillary endothelial cells: a marker associated with the blood-brain barrier

Carbonic anhydrase (CA) activity plays an important role in controlling cerebrospinal fluid production and also influences neuroexcitation and susceptibility to seizures. Until recently, CA II was the only CA demonstrated in brain. Its distribution is limited to the epithelial cells of the choroid plexus and to the myelin-forming cells, the oligodendrocytes. In this report, we present immunoblots, using an antibody raised to CA IV from rat lung, that show that CA IV is also present in rat and mouse brain. Results of immunohistochemistry and immunoelectron microscopy on sections from rat and mouse brain are presented that show the distribution of CA IV to be quite distinct from that of CA II. CA IV is expressed on and is limited to the luminal surface of endothelial cells of cerebral capillaries. These results establish CA IV as a cytochemical marker associated with the blood-brain barrier and suggest an important role for CA IV in CO2 and HCO3- homeostasis in brain.

Isolation and characterization of periaxolemmal and axolemmal enriched membrane fractions from the rat central nervous system

In this report, we describe the fractionation of crude axolemmal fractions from rat lower brainstem into subfractions enriched in markers for either periaxolemmal myelin or axolemma. These subfractions were isolated on density gradients as bands layering on 0.8M and 1.0M sucrose. Both subfractions consisted of unilamellar vesicles. Relative to myelin purified from the same starting material, the 0.8M subfraction was enriched in MAG, CNPase, carbonic anhydrase and Na+, K+ ATPase but was extremely low in PLP and MBP. In addition, this fraction exhibited a protein profile distinct from myelin. The 1.0M fraction was also highly enriched in Na+, K+ ATPase and had an overall composition similar to the 0.8M subfraction. However, it differed from the 0.8M subfraction by being low in MAG, CNPase, and carbonic anhydrase, but enriched in voltage-dependent Na+ channel, axon-specific fodrin, and MAP-1B. Based on these characteristics we concluded that the 0.8M and 1.0M subfractions were highly enriched in periaxolemmal myelin and axolemmal membrane, respectively. Plasmolipin10 was unique with equally high levels in myelin and in the 0.8M and 1.0M subfractions. Both subfractions were enriched, relative to myelin, in the alpha subunit of the GTP binding protein, Go, and the alpha subunit common to all G proteins, GA/1. Electrophysiology with membrane subfractions fused to lipid bilayers showed that both membranes contained sets of K+ and Cl- channels, which based on channel sizes and open times, are largely distinct from one another.

Effects of detergents on Ca(2+)-activated neural proteinase activity (calpain) in neural and non-neural tissue: a comparative study

Calcium activated neutral proteinase (mcalpain) activity was determined in brain and other tissue of rat. More than 60% of the brain mcalpain activity was present in the particulate fraction while only 30% was in cytosol. In contrast, particulate fractions of liver, kidney, muscle, and heart contained about 8-12% of tissue mcalpain activity while 88% was present in cytosol. Removal of the endogenous inhibitor calpastatin increased the tissue mcalpain activity severalfold. Triton X-100 and deoxycholate (DOC) stimulated the neural calpain activity by ten-fold while activity in non-neural tissue was unaffected. Incubation with other detergents, e.g. Triton N-57 and thioglucopyranoside, stimulated brain calpain activity five-fold while Brij-35 did not have any effect. Sodiumdodecylsulphate (SDS), on the other hand, inhibited the enzyme activity. Brain contained the lowest calpain activity compared to non-neural tissue. The calpain activity in muscle, kidney and heart was three-fold greater than liver. Immunoblot identification of the enzyme revealed that calpain was predominantly in the particulate fraction and less in cytosol of brain while it was present mainly in cytosol and less in the pellet fractions of non-neural tissue.

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.

Comparison of immunocytochemical staining of astrocytes, oligodendrocytes, and myelinated fibers in the brains of carbonic anhydrase II-deficient mice and normal littermates

Immunostaining for carbonic anhydrase (CA) was performed in paraffin sections from the brains of CA II-deficient mutant mice and their normal littermates. Double immunofluorescence staining showed CA in myelinated tracts and oligodendrocytes in the cerebellum of the normal but not the CA II-deficient mice, and also in astrocytes in the cerebral cortex of the normal mice but not the mutants. The data show that the CA in normal oligodendrocytes, astrocytes, and myelin is the II isoenzyme, because these structures in the mutants would be positively stained if the staining normally were due to a contaminant in the antiserum or an antibody against a different isoenzyme. The findings in normal gray matter also suggest that many neuronal cell bodies are surrounded by a network of fine, CA-positive astrocytic processes.

Sequence of a novel carbonic anhydrase-related polypeptide and its exclusive presence in Purkinje cells

I isolated a mouse cDNA clone encoding a novel polypeptide which has strong homology with carbonic anhydrase. Unlike the other carbonic anhydrases, it has an additional N-terminal domain with a glutamic acid stretch and an arginine substitutes one of the three histidine residues which bind zinc ion. In the central nervous system, carbonic anhydrase is known to be expressed only in glia cells, but this gene is expressed in neuron, but only in Purkinje cells.

Identification and pharmacological properties of binding sites for the atypical thiazide diuretic, indapamide

[3H]Indapamide bound to a single class of binding sites in pig renal cortex membranes with a dissociation constant Kd = 35 +/- 13 nM and a binding site density Bmax = 40 +/- 9 pmol/mg of protein. [3H]Indapamide binding was inhibited by the carbonic anhydrase inhibitor, acetazolamide, and by thiazide diuretics with the following rank order of potency: chlorothiazide greater than hydrochlorothiazide approximately metolazone greater than hydroflumethiazide. The effect of the latter drugs to inhibit [3H]indapamide binding was not related to their activity as thiazide diuretics, but was significantly correlated with their inhibitory effect on carbonic anhydrase II. These results suggest that the major renal binding site of [3H]indapamide is a membrane form of carbonic anhydrase. Inhibition of carbonic anhydrase may play a role in the antihypertensive effect of indapamide.

Reactive gliosis in the brains of Lewis rats with experimental allergic encephalomyelitis

This paper assesses reactive gliosis in the optic tracts and other regions of brain in Lewis rats with experimental autoimmune encephalomyelitis (EAE). Enhanced immunostaining for glial fibrillary acidic protein (GFAP) in brains from rats with EAE occurred primarily in the white-matter tracts and was not restricted to sites of inflammation. Immunocytochemical staining for other putative astrocytic antigens demonstrated glutathione-S-transferase (Yb isoenzyme) to be localized extensively in GFAP-positive cells and vimentin to be present both in inflammatory cells and in some GFAP-positive astroglial cells. Positive staining for carbonic anhydrase and glutamine synthetase was observed in oligodendrocytes. In the optic tracts glutamine synthetase, but not carbonic anhydrase, was also observed in some astrocytes.

Enhancement of carbonic anhydrase activity by erythrocyte membranes

The human erythrocyte membrane is an efficient enhancer of both high (CA II) and low (CA I) activity isozymes of red blood cell carbonic anhydrase. The presence of membrane increased CO2 hydration catalyzed by bovine CA II 1.6-fold, human CA II 3.5-fold, and human CA I 1.6-fold. With the high activity CA isozymes, maximal stimulation was observed in the presence of 1-3 micrograms membrane protein/ml. The Vmax for bovine CA II (4 nM) rose from 0.302 to 0.839 mM/s, while that for human CA II (6 nM) increased from 0.113 to 0.414 mM/s in the absence and presence of membrane, respectively. The apparent Km for CO2 increased from 13.2 to 51.2 mM for bovine CA II, and from 6.5 to 38.5 mM for human CA II. Mixtures of membrane plus enzyme, upon centrifugation through linear sucrose density gradients, displayed enhanced Ca activity only in membrane-containing gradient fractions, verifying the stimulatory ability of membranes on enzyme activity and indicating tight and stable complex formation. Membrane enhancement of CA activity appears to be a general phenomenon in that mouse hepatocyte membranes also stimulated CA activity, although less efficiently than erythrocyte membranes. Of the many soluble putative effectors assayed, only imidazole enhanced CA II activity to an extent comparable with erythrocyte membranes; imidazole did not, however, stimulate the activity of human CA I. The data are consistent with a model of CA II activation by membrane association that may effect a distortion of the enzyme conformation in such a way as to facilitate intra- and/or intermolecular proton transfer between membrane-bound and enzyme-bound proton shuttling residues (perhaps the imidazole moiety of histidine) and the Zn-bound hydroxide at the catalytic site of the enzyme.

The regional and subcellular distribution of calcium activated neutral proteinase (CANP) in the bovine central nervous system

Calcium-activated neutral proteinase (CANP) activity was determined in subcellular fractions and in different regions of bovine brain. The CANP specific activity in spinal cord and corpus callosum, areas rich in myelin, were almost six-fold greater than cerebral cortex and cerebellum. Treatment of whole homogenate and myelin with 0.1% Triton X-100 increased the CANP activity by tenfold. Subcellular fractions were prepared from bovine brain gray and white matter. Most of the CANP activity (70%) was in the primary particulate fractions P1 (nuclear), P2 (mitochondrial) and P3 (microsomal). On subfractionation of each particulate fraction, the majority of the activity (greater than 50%) was recovered in the myelin-enriched fractions (P1A, P2A, P3A) which separate at the interphase of 0.32 M- and 0.85 M-sucrose. The distribution of activity was P2A greater than P1A greater than P3A. Further purification of myelin (of P2A) increased the specific activity over homogenate by more than three-fold. The same myelin fractions contained the highest proportion (60%) and specific activity (five-fold increase) of CNPase. The enzyme activity in different regions of brain and in subcellular fractions was increased by 20-39% after the inhibitor was removed. Electron microscopic study confirmed that the myelin fractions were highly purified. The cytosolic fraction contained 20-30% of the total homogenate CANP activity. Other fractions contained low enzyme activity. CANP was identified in the purified myelin fraction by electroimmublot-technique. It is concluded that the bulk of CANP in CNS is tightly bound to the membrane, may be masked or hidden and is intimately associated with the myelin sheath.

Renal membrane-bound carbonic anhydrase. Purification and properties

Microsomes from perfused human donor kidneys were separated by differential centrifugation in sucrose, and thoroughly washed before solubilization by the nonionic detergent nonyl-beta-D-glucoside. The solubilized material was first applied onto an affinity chromatographic column of acetazolamide-oxirane-SepharoseR-CL-4B to remove contaminating cytoplasmic carbonic anhydrase isozymes CA I and CA II. It was then added onto an affinity column of p-aminomethylbenzene sulfonamide coupled to CM Bio-gel AR to purify the membrane-bound carbonic anhydrase activity. This resulted in a 50% pure enzyme. It was then concentrated and fractionated on an anion-exchange column, and desalted and purified to homogeneity (SDS-PAGE and isoelectric focusing) by gel filtration. The enzyme was now purified 411-fold from extractable membrane protein. Its molecular weight was 34.4 kDa from gel filtration and SDS-PAGE, and 36.7 kDa from amino acid analysis. The amino acid composition differed from that of the cytoplasmic isozymes CA I, II, and III. Antisera, produced in rabbits against the purified SDS-treated enzyme, reacted with native nondenatured membrane enzyme protein but only weakly with CA II. Kinetically the enzyme was similar to CA II with respect to hydrase and esterase activities and to inhibition by various sulfonamides. Considered together, the data suggest that the human kidney contains a membrane-bound carbonic anhydrase protein that differs from the cytoplasmic isozymes CA I, II, and III and the secretory form (CA VI) in the saliva.

Expression of carbonic anhydrase II gene in early brain cells as revealed by in situ hybridization and immunohistochemistry

A mouse carbonic anhydrase (CA II) complementary(c) DNA probe was used for in situ hybridization on mouse brain cultured cells in order to follow CA II gene expression during brain development. An improved method was established using biotinated probes that resulted in a high sensitivity and an absence of background; this method could be combined with immunohistochemistry. Hypothalamic cells of embryonic day (ED) 12-14 mice were cultured for various periods. Chronologic appearance of CA II messenger(m)RNA and protein was studied. The CA II gene transcripts are detectable as early as ED 12-13, although the protein they encode is not detectable until ED 17-18. Gene expression is restricted to 0.1% of the total population. Northern blot analysis confirmed the presence of CA II transcripts in embryonic hypothalamus. At postnatal stage, the majority of glial cells express both the CA II mRNA and the protein. Our results favour the early appearance of a glial lineage in a precise area of the developing CNS. The precocity of CA II gene transcription makes in situ hybridization an invaluable approach in defining the onset of nerve cell lineages during embryonic development.

Gliosis in the spinal cords of rats with experimental allergic encephalomyelitis: immunostaining of carbonic anhydrase and vimentin in reactive astrocytes

Spinal cord sections from rats sensitized to develop experimental allergic encephalomyelitis (EAE) were immunostained with antibodies against glial fibrillary acidic protein (GFAP), carbonic anhydrase, and vimentin, to see whether the latter two antigens could be detected in GFAP-positive reactive astrocytes. Sixteen days after sensitization (16 dpi) there was intense carbonic anhydrase immunostaining in GFAP-positive cells in the spinal cords of EAE rats, particularly in the white matter. At 13 and 20 dpi carbonic anhydrase immunostaining in astrocytes was less intense, and in the spinal cord white matter of control animals carbonic anhydrase was not detected in the few GFAP-positive cells. In the spinal cords of EAE rats vimentin immunostaining was observed in inflammatory cells and astrocytes. In the latter, GFAP and carbonic anhydrase were colocalized with vimentin. The data suggest that carbonic anhydrase expression in astrocytes is an acute response to injury and that vimentin can be detected in astrocytes, as well as inflammatory cells, as early as 16 dpi.

Light-evoked changes in extracellular pH in frog retina

Light-induced changes in extracellular H+ concentration (delta pH0) were studied with intraretinal H(+)-sensitive double-barreled microelectrodes in frog eyecup and isolated retina preparations. The most prominent delta pH0 were found in the inner plexiform layer, as pH increases (alkalinizations) at light onset and offset. With a small-spot stimulus (0.3 mm dia.), 30 sec in duration, the delta pH0 were relatively small (0.03 pH units), and long lasting (peak at 25-30 sec). They were enhanced by flicker (0.3 Hz). Depth profiles paralleled those of the field potentials (PNR/M-wave), the ON delta pH0 peaking 40 microns more proximal than the OFF response. The delta pH0 exhibited surround antagonism, which was blocked by tetrodotoxin (TTX), indicating an independence from action potentials. The mechanism for these pH increases in proximal retina is not yet understood. In the subretinal space diffuse retinal illumination produced a small pH increase, consistent with a presumed decrease in photoreceptor lactate production. Inhibition of carbonic anhydrase (CA) with acetazolamide or methazolamide increased both the proximal and distal retinal delta pH0, suggesting that CA is involved in buffering retinal pH.

The astrocyte as a locus of carbonic anhydrase in the brains of normal and dysmyelinating mutant mice

There is some controversy in the literature whether carbonic anhydrase occurs in astrocytes, as well as in oligodendrocytes and myelin, in the mammalian brain. In the present study this issue was addressed by double immunostaining for carbonic anhydrase and two astrocytic "markers" in the brains of normal mice and two dysmyelinating mutants, jimpy and shiverer. In the brains of young mice, carbonic anhydrase and glutamine synthetase were colocalized in astrocytes in the cortical gray matter. In gray matter of the adult mouse brain, it was possible to immunostain both carbonic anhydrase and glial fibrillary acidic protein (GFAP) in the same cells. However, in contrast to the findings in gray matter, in and near subcortical white matter carbonic anhydrase could be detected only in oligodendrocytes and myelinated fibers. In the brains of jimpy mice, virtually all the carbonic-anhydrase-positive cells were also GFAP positive, even in regions normally occupied by white matter. In the brains of young and adult shiverer mice, carbonic anhydrase was localized in astrocytes in the gray matter, but in and near the tracts normally occupied by white matter carbonic anhydrase could be detected only in oligodendrocytes and their abundant processes. The findings confirmed the oligodendrocyte-myelin unit to be the primary locus of carbonic anhydrase in the normal mouse brain and showed the astrocytes in gray matter normally to be a secondary locus of carbonic anhydrase. The immunostaining in the jimpy mouse brain suggested further that reactive astrocytes, in particular, might be rich in carbonic anhydrase.

Calcium-activated neutral proteinase in rat brain myelin and subcellular fractions

The activity of calcium-activated neutral proteinase (mM CANP) was determined in subcellular fractions of rat brain. The CANP activity in whole homogenate and its membrane fractions including myelin was increased ten-fold following treatment with Triton X-100. The majority of the activity (60%) was in the primary particulate fractions P1 (nuclear), P2 (mitochondrial), and P3 (microsomal). Following subfractionation of each particulate fraction, most of the activity (50%) was found in the myelin-enriched fractions (P1A, P2A, and P3A) and separated at the interface of 0.32-0.85 M sucrose. Only 20-30% of the total homogenate activity was in cytosol. The enrichment in the myelin fractions resembled that for 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) activity. Immunoblotting revealed that the CANP was mainly in myelin and cytosol. In addition to the presence of 72-76 Kd and 80 Kd bands, there were faint high-molecular-weight CANP bands ranging from 110-150 Kd and lower-molecular-weight forms in the region of 30-50 Kd in both purified myelin and cytosol. These studies suggested that CANP is present in myelin and cytosol and that it exists in the brain in membrane-bound and soluble forms.

Distribution of carbonic anhydrase activity in the rat central nervous system, as revealed by a new semipermeable technique

Results obtained with a new method provided evidence for the extraneuronal localization of CA and supports the idea that the enzyme is very widespread in non-neuronal cell types of the CNS. Most of these cells were considered to be oligodendroglia, but probably the astrocytes also contributed to the reactivity of the neuropil. The perineuronal CA activity observed in the spinal cord, brainstem, cerebellum, and hippocampus could be astrocytic in origin. Our observations concerning the widespread CA staining of the CNS vessels pointed to the possible functional significance of CA in the vessel wall. This activity could be due not only to pericytes but also to astrocytic processes. We have not found stained myelin sheats although biochemically the myelin contained the enzyme. Might be that our histochemical reaction was not sensitive to the membrane bound form of the CA. The fact that the reaction of the nucleoli did not disappear after acetazolamide treatment could be explained on the basis of binding of the cobalt-phosphate complex to the proteins of the nucleolus.

Distribution of calcium activated neutral proteinase (mM CANP) in myelin and cytosolic fractions in bovine brain white matter

The activity of calcium-activated neutral proteinase (mM CANP) was determined in homogenate, myelin and supernatant of bovine brain corpus callosum. The enzyme activity in homogenate and myelin was increased eleven and thirteen-fold respectively by Triton X-100. Myelin prepared by the method of Norton and Poduslo as well as by a modified method, was shown to contain most (more than 50%) of homogenate mM CANP activity. The specific activity was highest in myelin, and increased almost three times more than the homogenate. Supernatant only contained 17% of enzyme activity. It is concluded from these studies that mM CANP is tightly bound to the membrane and predominantly associated with the myelin sheath.

Carbonic anhydrase activity develops postnatally in the rat optic nerve

We examined the appearance of carbonic anhydrase (CA) activity in rat optic nerves (RONs) 5-77 postnatal days of age and correlated the appearance of enzyme activity with structural and physiological alterations. CA activity was nearly absent before 10 days of age and appeared in this CNS white matter tract with a developmental time-course similar to that of oligodendrogliogenesis and myelinogenesis. When oligodendrocytes and myelin were depleted in the RON by treatment with a mitotic inhibitor, CA activity was markedly reduced. These observations support the hypothesis that CA is contained primarily in oligodendrocytes and myelin. Neural activity in the RON caused changes in extracellular pH (pHo) and the character of these pHo responses was very age dependent; older nerves exhibited much larger acid shifts than neonatal nerves. The development of CA activity may be a factor contributing to this physiological alteration.

Characterization and biosynthesis of the plasma membrane proteolipid protein in neural tissue

In this study we have characterized, in brain, the expression of a plasma membrane proteolipid protein (PM-PLP) complex that can form cation-selective channels in lipid bilayers. We isolated PLP fractions from synaptic plasma membrane and glial microsomes and found a high degree of similarity in both size and amino acid composition to the complex we had previously isolated from kidney. Antibodies specific to the kidney PM-PLP were prepared, and, on the basis of immunoblot and immunoprecipitation studies, the PM-PLP complex isolated from neural membranes was shown to be immunologically related to the kidney PM-PLP. These proteolipid proteins exhibited a molecular weight of approximately 14K and contained a high percentage of hydrophobic amino acids with an apparent absence of cysteine. The biogenesis of PM-PLP in brain was studied by in vitro translation of free and bound polysomes and total RNA in a rabbit reticulocyte lysate followed by immunoprecipitation of the translation products. From these studies it is concluded that the PM-PLP complex is synthesized on the rough endoplasmic reticulum. On the basis of the identical electrophoretic mobility of material isolated from plasma membranes and material immunoprecipitated after translation of bound polysomes and isolated RNA, it appears that the PM-PLP does not undergo detectable posttranslational processing between its site of synthesis and its incorporation into the plasma membrane.

In vitro studies of the biosynthesis of brain tubulin on membranes

Membrane elements in brain tissue contain relatively large amounts of alpha- and beta-tubulin (FIGURES 2 and 3). We have investigated the subcellular sites of tubulin biosynthesis in order to determine the origin of this membrane-associated tubulin. Free and membrane-bound polysomes from rat forebrain were separated by differential centrifugation, and the products of translation from these polysome populations were analyzed by 2DGE (FIGURES 4 and 6). Alpha- and beta-tubulin subunits were synthesized by the free polysome population (FIGURES 4 and 5A and B). The membrane-bound polysome fraction synthesized a protein with similar (but not identical) characteristics to alpha-tubulin (denoted as "MB" in FIGURE 6), including isoelectric point, molecular weight, peptide map, and copurification with microtubules after aggregation-disaggregation. Tubulin subunits synthesized in vitro by free polysomes could associate posttranslationally with a microsome fraction (FIGURE 7A). The association of the tubulin translation products with membranes was not disrupted by high salt; the associated tubulin, however, was susceptible to proteolytic digestion, with the exception of one of the beta-tubulin subunits (FIGURE 7B). There was an identical protease-resistant beta-tubulin subunit among the native proteins of the smooth microsome fractions. Our data is consistent with the conclusion that at least one beta subunit of membrane-associated tubulin is synthesized by free polysomes and becomes posttranslationally added to membrane structures. It is unlikely that a cotranslational mechanism is responsible, in which there is a signal-mediated insertion of a growing polypeptide chain to membrane. Our results, however, are consistent with a "membrane trigger" mechanism proposed by Wickner in which the membrane lipid bilayer triggers the folding of a polypeptide into a configuration that allows integral membrane insertion. The association of tubulin with membranes may also be secondary to the interaction of hydrophobic elements. The amino acid sequence of beta tubulin is known to contain several hydrophobic domains. Tubulin can be incorporated into phospholipid vesicles and various subcellular membrane elements. In our studies, in vitro synthesized tubulin from free polysome was found to be purified by hydrophobic affinity chromatography with ethane-sepharose (FIGURE 8). Thus, the hydrophobic characteristics of newly synthesized tubulin could be partially responsible for the posttranslational association of tubulin subunit with membranes. Native tubulin in a soluble fraction of CNS tissue was not purified by hydrophobic affinity chromatography.(ABSTRACT TRUNCATED AT 400 WORDS)

Histochemical study of the distribution of carbonic anhydrase in the cat brain

The distribution of carbonic anhydrase (CA) in cerebrum, cerebellum and medulla oblongata of the cat brain has been examined by a histochemical method. Neuron cell bodies and dendrites are stained in some locations. Many axons are distinctly stained and different intensities of staining can be seen even in adjacent axons. One of the most intensely stained structures is the capillary endothelium and stained capillaries are found in all parts examined. Glial cells are intensely stained in agreement with biochemical and earlier histochemical works. Myelin sheaths are never stained, possibly due to enzyme loss during embedding. The localization of the enzyme shows regional differences. In this respect, the medulla oblongata has been examined in more detail. A small area close to the ventral surface, medial to the roots of the hypoglossal nerve, is characterized by a high CA staining of the neuropil. The cell membrane of some large neurones and the capillary endothelium in this area were also stained. With regard to position and CA content, this area corresponds well to the characteristics of the medullary chemosensitive area as defined by previous experimental studies.

Oligodendroglial structures and distribution shown by carbonic anhydrase immunostaining in the spinal cords of developing normal and shiverer mice

The spinal cords of young and adult normal and dysmyelinating mutant (shiverer) mice were immunostained with anticarbonic anhydrase to investigate the distribution of oligodendroglial populations into the gray- and white-matter regions in the developing normal and mutant animals; the morphology of oligodendrocytes and their processes at the light microscopic level in gray matter and white matter; and the apparent gliosis in the gray matter, as well as the white matter, of the mutants. Immunocytochemistry and enzyme assays revealed consistent increases in carbonic anhydrase antigenicity and specific activity in controls and mutants between the ages of approximately 15 days and approximately 60 days. As shown previously in adult animals, oligodendroglia in larger than normal proportions were situated at the periphery of the "white-matter" columns, as compared to gray matter, in the shiverers, with, however, significant numbers of oligodendroglia were heterogeneous with respect to shapes, configuration of processes, and intensity of carbonic anhydrase immunostaining. In the shiverer "white matter" the oligodendrocytes were smaller than normal, and their shapes and arrangement were relatively irregular. In the normal gray matter short oligodendroglial processes appeared to be associated with neuronal perikarya, and those processes were more pronounced at approximately 90 days than at approximately 20 days of age. Background staining in normal gray matter suggested that oligodendroglial processes were, in addition, tightly wound around many axons. In shiverer gray matter the oligodendrocytes were smaller, and their processes appeared to be wrapped more loosely around smaller numbers of conspicuous axons and to be associated less frequently with neuronal perikarya. This finding suggests that the deficiency in the myelin basic protein in the mutant may affect interactions between oligodendrocytes and neurons in the gray matter as well as in the white matter. The astrocytic "marker," glial fibrillary acidic protein, was detected in gray and white matter of shiverers as young as 16 days, and the differences from carbonic anhydrase localization supported the conclusion that the processes enwrapping axons in the shiverer mouse CNS are derived from oligodendrocytes, not astrocytes.

Properties and function of brain carbonic anhydrase

This chapter has described the characterization and biogenesis of soluble and membrane-bound CA in the central nervous system. The two forms of the enzyme appear to be quite similar in their molecular characteristics, however the data strongly indicate that they are synthesized on separate polysomal populations; the membrane-bound form resulting from synthesis on the RER. Our preliminary data suggest that the partitioning of mRNA for CA on the different polysomes results from the interaction of partial nascent chains with a specific receptor on the RER. We feel a function of membrane-associated synthesis is for the targeting of CA to sites in the cell where there are enzymes that can rapidly utilize the protons and bicarbonate produced by CA catalytic activity for ion exchange reactions. We have also presented arguments that CA may function as a bicarbonate source in the control of metabolism specifically in the acceleration of fatty acid synthesis in the oligodendrocyte.

The value of inherited deficiencies of human carbonic anhydrase isozymes in understanding their cellular roles

Very little light has been shed on the role of the low-activity CA I isozyme in humans by studies on CA I-deficient individuals. On the other hand, CA II-deficient individuals exhibit abnormalities of bone, kidney and brain, implicating a functional role for the high-activity CA II isozyme in cells from these tissues and organs. It also appears that the CA II-deficient red cell is capable of normal respiratory function under unstressed conditions. In addition, there is some preliminary evidence that those organs such as the eye which primarily contain the CA II isozyme, may be able to function effectively in the absence of CA II.

Characterization of translation products of the polyadenylated RNA of free and membrane-bound polyribosomes of rat forebrain

Poly(A)+ RNA (polyadenylated RNA) isolated from membrane-bound and free polyribosomes was translated in reticulocyte lysates, and the products were analysed by two-dimensional gel electrophoresis. Several translation products were specific to membrane-bound polyribosomal mRNA, including polypeptides of 47kDa, 35kDa and 21 kDa, whereas others (e.g. of 37 kDa, 17 kDa and 14 kDa) were specific to free polyribosomal mRNA. Although many products were common to both mRNA species, cross-contamination could be ruled out on the basis of the presence of these and other specific products. The common products included a 68 kDa microtubule-associated protein, tubulin, actin, the brain form of creatine kinase, neuron-specific enolase and protein 14-3-3 and calmodulin, all of which were identified on the basis of two-dimensional gel and peptide analyses. The 35 kDa protein product of membrane-specific mRNA was co-translationally processed in vitro by microsomal membranes, resulting in its cleavage to 33 kDa (and partial glycosylation). The 33 kDa processed protein (but not the 35 kDa precursor) was integrated into both dog pancreas and rat brain microsomal membranes. The occurrence of the enzymes and calmodulin as products of membrane-bound polyribosomal mRNA is discussed in the light of their presence on rat brain synaptic plasma membranes [Lim, Hall, Leung, Mahadevan & Whatley (1983) J. Neurochem. 41, 1177-1182] and their existence in a specific component of axonal flow. It is suggested that some of these translation products of the rough endoplasmic reticulum may represent proteins destined for the plasma membrane. However, the identity and location of the 35 kDa membrane-specific product (or its processed form) still remain unestablished.

Cerebral circulation and metabolism

Recent developments in the field of cerebral circulation and metabolism are reviewed, with emphasis on circulatory and metabolic events that have a bearing on brain damage incurred in ischemia. The first part of the treatise reviews aspects of cerebral metabolism that provide a link to the coupling of metabolism and blood flow, notably those that lead to a perturbation of cellular energy state, ionic homeostasis, and phospholipid metabolism. In the second part, attention is focused on the derangement of energy metabolism and its effects on ion fluxes, acid-base homeostasis, and lipid metabolism. It is emphasized that gross brain damage, involving edema formation and infarction, is enhanced by tissue acidosis, and that neuronal damage, often showing a pronounced selectivity in localization, appears related to a disturbed Ca2+ homeostasis, and to Ca2+-triggered events such as lipolysis and proteolysis.