Carbonic anhydrase in the nervous system

Schizophrenia interactome derived repurposable drugs and randomized control trials of two candidates

There is a substantial unmet need for effective and patient-acceptable drugs to treat severe mental illnesses like schizophrenia. Computational analysis of genomic, transcriptomic, and pharmacologic data generated in the past two decades enables repurposing of drugs or compounds with acceptable safety profiles, namely those that are FDA-approved or reached late stages in clinical trials. We developed a rational approach to achieve this computationally for schizophrenia by studying drugs that target the proteins in its protein interaction network ('interactome'). This involved contrasting the transcriptomic modulations observed in the disorder and the drug; our analyses resulted in 12 candidate drugs, 9 of which had additional supportive evidence: their target networks were enriched for pathways relevant to schizophrenia etiology or for genes that had an association with diseases pathogenically similar to schizophrenia. To translate these computational results to the clinic, these shortlisted drugs must be tested empirically through randomized controlled trials (RCT), where their prior safety approvals obviate the need for time-consuming phase I and II studies. We selected two among the shortlisted candidates based on likely adherence and side effect profiles. We are testing them through adjunctive RCTs for patients with schizophrenia or schizoaffective disorder who experienced incomplete resolution of psychotic features with conventional treatment. The integrated computational analysis for identifying and ranking drugs for clinical trials can be iterated as additional data are obtained. Our approach could be expanded to enable disease subtype-specific drug discovery in future and should also be exploited for other psychiatric disorders.

New insights into carbonic anhydrase inhibition, vasodilation, and treatment of hypertensive-related diseases

Carbonic anhydrase (CA) and its inhibitors are relevant to many physiological processes and diseases. The enzyme is differentially expressed throughout the body, in concentration and subcellular location, and as 13 catalytically active isoforms. Blood vessels contain small amounts of CA, but the enzyme's role in vascular physiology and blood pressure regulation is uncertain. However, considerable recent evidence points to vasodilation by CA inhibitors. CA inhibition in vascular smooth muscle, endothelium, heart, blood cells, and nervous system could all contribute. It is equally plausible that other targets besides CA for all known CA inhibitors may account for their vascular effects. I will review this knowledge and important remaining gaps relating to treatment of hypertensive-related diseases with potent sulfonamide inhibitors, such as acetazolamide; but also the possibility that CA inhibition by thiazides and loop diuretics, although generally weaker, may have antihypertensive effects beyond their inhibition of renal sodium transporters.

GPI-anchored carbonic anhydrase IV displays both intra- and extracellular activity in cRNA-injected oocytes and in mouse neurons

Soluble cytosolic carbonic anhydrases (CAs) are well known to participate in pH regulation of the cytoplasm of mammalian cells. Membrane-bound CA isoforms--such as isoforms IV, IX, XII, XIV, and XV--also catalyze the reversible conversion of carbon dioxide to protons and bicarbonate, but at the extracellular face of the cell membrane. When human CA isoform IV was heterologously expressed in Xenopus oocytes, we observed, by measuring H(+) at the outer face of the cell membrane and in the cytosol with ion-selective microelectrodes, not only extracellular catalytic CA activity but also robust intracellular activity. CA IV expression in oocytes was confirmed by immunocytochemistry, and CA IV activity measured by mass spectrometry. Extra- and intracellular catalytic activity of CA IV could be pharmacologically dissected using benzolamide, the CA inhibitor, which is relatively slowly membrane-permeable. In acute cerebellar slices of mutant mice lacking CA IV, cytosolic H(+) shifts of granule cells following CO(2) removal/addition were significantly slower than in wild-type mice. Our results suggest that membrane-associated CA IV contributes robust catalytic activity intracellularly, and that this activity participates in regulating H(+) dynamics in the cytosol, both in injected oocytes and in mouse neurons.

Temperature, oxygen, and salt-sensing neurons in C. elegans are carbon dioxide sensors that control avoidance behavior

Homeostatic control of body fluid CO₂ is essential in animals but is poorly understood. C. elegans relies on diffusion for gas exchange and avoids environments with elevated CO₂. We show that C. elegans temperature, O₂, and salt-sensing neurons are also CO₂ sensors mediating CO₂ avoidance. AFD thermosensors respond to increasing CO₂ by a fall and then rise in Ca²⁺ and show a Ca²⁺ spike when CO₂ decreases. BAG O₂ sensors and ASE salt sensors are both activated by CO₂ and remain tonically active while high CO₂ persists. CO₂-evoked Ca²⁺ responses in AFD and BAG neurons require cGMP-gated ion channels. Atypical soluble guanylate cyclases mediating O₂ responses also contribute to BAG CO₂ responses. AFD and BAG neurons together stimulate turning when CO₂ rises and inhibit turning when CO₂ falls. Our results show that C. elegans senses CO₂ using functionally diverse sensory neurons acting homeostatically to minimize exposure to elevated CO₂.

Acetazolamide protects against posthypoxic unstable breathing in the C57BL/6J mouse

Acetazolamide (Acz), a carbonic anhydrase inhibitor, is used to manage periodic breathing associated with altitude and with heart failure. We examined whether Acz would alter posthypoxic ventilatory behavior in the C57BL/6J (B6) mouse model of recurrent central apnea. Experiments were performed with unanesthetized, awake adult male B6 mice (n = 9), ventilatory behavior was measured using flow-through whole body plethysmography. Mice were given an intraperitoneal injection of either vehicle or Acz (40 mg/kg), and 1 h later they were exposed to 1 min of 8% O(2)-balance N(2) (poikilocapnic hypoxia) or 12% O(2)-3% CO(2)-balance N(2) (isocapnic hypoxia) followed by rapid reoxygenation (100% O(2)). Hypercapnic response (8% CO(2)-balance O(2)) was examined in six mice. With Acz, ventilation, including respiratory frequency, tidal volume, and minute ventilation, in room air was significantly higher and hyperoxic hypercapnic ventilatory responsiveness was generally lower compared with vehicle. Poikilocapnic and isocapnic hypoxic ventilatory responsiveness were similar among treatments. One minute after reoxygenation, animals given Acz exhibited posthypoxic frequency decline, a lower coefficient of variability for frequency, and no tendency toward periodic breathing, compared with vehicle treatment. We conclude that Acz improves unstable breathing in the B6 model, without altering hypoxic response or producing short-term potentiation, but with some blunting of hypercapnic responsiveness.

Aquaporin-4 in the central nervous system: cellular and subcellular distribution and coexpression with KIR4.1

Aquaporin-4 (AQP4) is the predominant water channel in the neuropil of the central nervous system. It is expressed primarily in astrocytes, but also occurs in ependymocytes and endothelial cells. A striking feature of AQP4 expression is its polarized distribution in brain astrocytes and retinal Muller cells. Thus, immunogold analyses have revealed an enrichment of AQP4 in endfeet membranes in contact with brain microvessels or subarachnoidal space and a low but significant concentration in non-endfeet membranes, including those astrocyte membranes that ensheath glutamate synapses. The subcellular compartmentation of AQP4 mimics that of the potassium channel Kir4.1, which is implicated in spatial buffering of K(+). We propose that AQP4 works in concert with Kir4.1 and the electrogenic bicarbonate transporter NBC and that water flux through AQP4 contributes to the activity dependent volume changes of the extracellular space. Such volume changes are important as they affect the extracellular solute concentrations and electrical fields, and hence neuronal excitability. We conclude that AQP4-mediated water flux represents an integral element of brain volume and ion homeostasis.

CO2/H(+) sensing: peripheral and central chemoreception

H(+) is maintained constant in the internal environment at a given body temperature independent of external environment according to Bernard's principle of "milieu interieur". But CO2 relates to ventilation and H(+) to kidney. Hence, the title of the chapter. In order to do this, sensors for H(+) in the internal environment are needed. The sensor-receptor is CO2/H(+) sensing. The sensor-receptor is coupled to integrate and to maintain the body's chemical environment at equilibrium. This chapter dwells on this theme of constancy of H(+) of the blood and of the other internal environments. [H(+)] is regulated jointly by respiratory and renal systems. The respiratory response to [H(+)] originates from the activities of two groups of chemoreceptors in two separate body fluid compartments: (A) carotid and aortic bodies which sense arterial P(O2) and H(+); and (B) the medullary H(+) receptors on the ventrolateral medulla of the central nervous system (CNS). The arterial chemoreceptors function to maintain arterial P(O2) and H(+) constant, and medullary H(+) receptors to maintain H(+) of the brain fluid constant. Any acute change of H(+) in these compartments is taken care of almost instantly by pulmonary ventilation, and slowly by the kidney. This general theme is considered in Section 1. The general principles involving cellular CO2 reactions mediated by carbonic anhydrase (CA), transport of CO2 and H(+) are described in Section 2. Since the rest of the chapter is dependent on these key mechanisms, they are given in detail, including the role of Jacobs-Stewart Cycle and its interaction with carbonic anhydrase. Also, this section deals briefly with the mechanisms of membrane depolarization of the chemoreceptor cells because this is one mechanism on which the responses depend. The metabolic impact of endogenous CO2 appears in the section with a historical twist, in the context of acclimatization to high altitude (Section 3). Because low P(O2) at high altitude stimulates the peripheral chemoreceptors (PC) increasing ventilation, the endogenous CO2 is blown off, making the internal milieu alkaline. With acclimatization however ventilation increases. This alkalinity is compensated in the course of time by the kidney and the acidity tends to be restored, but the acidification is not great enough to increase ventilation further. The question is what drives ventilation during acclimatization when the central pH is alkaline? The peripheral chemoreceptor came to the rescue. Its sensitivity to P(O2) is increased which continues to drive ventilation further during acclimatization at high altitude even when pH is alkaline. This link of CO2 through the O2 chemoreceptor is described in Section 4 which led to hypoxia-inducible factor (HIF-1). HIF-1 is stabilized during hypoxia, including the carotid body (CB) and brain cells, the seat of CO2 chemoreception. The cells are always hypoxic even at sea level. But how CO2 can affect the HIF-1 in the brain is considered in this section. CO2 sensing in the central chemoreceptors (CC) is given in Section 5. CO(2)/H(+) is sensed by the various structures in the central nervous system but its respiratory and cardiovascular responses are restricted only to some areas. How the membranes are depolarized by CO2 or how it works through Na(+)/Ca(2+) exchange are discussed in this section. It is obvious, however, that CO2 is not maintained constant, decreasing with altitude as alveolar P(O2) decreases and ventilation increases. Rather, it is the [H(+)] that the organism strives to maintain at the expense of CO2. But then again, [H(+)] where? Perhaps it is in the intracellular environment. Gap junctions in the carotid body and in the brain are ubiquitous. What functions they perform have been considered in Section 6. CO2 changes take place in lung alveoli where inspired air mixes with the CO2 from the returning venous blood. It is the interface between the inspired and expired air in the lungs where CO2 change is most dramatic. As a result, various investigators have looked for CO2 receptors in the lung, but none have been found in the mammals. Instead, CO2/H(+) receptors were found in birds and amphibians. However, they are inhibited by increasing CO2/H(+), instead of stimulated. But the afferent impulses transmitted to the brain produced stimulation in the efferents. This reversal of afferent-efferent inputs is a curious situation in nature, and this is considered in Section 7. The NO and CO effects on CO2 sensing are interesting and have been briefly mentioned in Section 8. A model for CO2/H(+) sensing by cells, neurons and bare nerve endings are also considered. These NO effects, models for CO2/H(+) and O2-sensitive cells in the CNS have been considered in the perspectives. Finally, in conclusion, the general theme of constancy of internal environment for CO2/H(+) is reiterated, and for that CO2/H(+) sensors-receptors systems are essential. Since CO2/H(+) sensing as such has not been reviewed before, the recent findings in addition to defining basic CO2/H(+) reactions in the cells have been briefly summarized.

Posthaemorrhagic ventricular dilatation

Posthaemorrhagic ventricular dilatation is the most serious direct complication of intraventricular haemorrhage after preterm birth. It results initially from multiple small blood clots throughout the cerebrospinal fluid channels impeding circulation and reabsorption. Management is difficult and new treatment approaches are needed.

Diuretic therapy for newborn infants with posthemorrhagic ventricular dilatation

BACKGROUND

Intraventricular hemorrhage remains a serious complication of premature birth and post hemorrhagic hydrocephalus still has no satisfactory treatment. Acetazolamide and furosemide, which both reduce the production of cerebrospinal fluid, have been suggested as non-invasive therapies to reduce hydrocephalus and the need for ventriculo-peritoneal (V-P) shunting.

OBJECTIVES

The aim of this review was to determine whether the use of acetazolamide and furosemide improves outcome, especially shunt dependence, in infants developing post-hemorrhagic ventricular dilatation.

SEARCH STRATEGY

The standard search strategy of the Cochrane Collaboration was used.

SELECTION CRITERIA

Randomised, or quasi-randomised trials, of acetazolamide and/or furosemide compared with standard therapy in infants with IVH or post-hemorrhagic ventricular dilatation

DATA COLLECTION AND ANALYSIS

Data were extracted independently by each author and were analysed by the standard methods of the Cochrane Collaboration using relative risk (RR) and risk difference (RD), a fixed effect model and sensitivity analyses where appropriate.

MAIN RESULTS

There were two eligible trials: one randomized 16 infants and the other, 177. Neither showed a decreased risk for V-P shunt or for V-P shunt or death associated with acetazolamide and furosemide therapy. The larger trial showed that acetazolamide and furosemide treatment resulted in a borderline increase in the risk for motor impairment at one year (RR 1.27, CI 1.02 to 1.58; RD 0.16, CI 0.02 to 0.31), but did not significantly affect the risk for the combined outcome of delay, disability or motor impairment among survivors, or the risk of the combined outcome of death, delay, disability or impairment at one year. The larger trial showed that diuretic treatment increased the risk for nephrocalcinosis (RR 5.31, CI 1.90 to 14.84; RD 0.19, CI 0.09 to 0.29); meta-analysis confirmed this result.

REVIEWER'S CONCLUSIONS

Acetazolamide and furosemide therapy is neither effective nor safe in treating post hemorrhagic ventricular dilatation. Acetazolamide and furosemide cannot be recommended as therapy for post hemorrhagic hydrocephalus.

Transplantation of glial cell progenitors from brains of normal rats and mice into brains of neonatal carbonic anhydrase II-deficient mutant mice

Carbonic anhydrase II (CAII) is expressed in oligodendrocytes, astrocytes and myelin in brains of rats and normal mice, but not in the brains of CAII-deficient mutant mice. We have transplanted mixed glial-cell suspensions from normal mouse brains, and oligodendrocyte-enriched precursor cells cultured from normal rat brains, respectively, into the brains of neonatal CAII-deficient mutant mice. Some CAII-positive astrocytes and oligodendrocytes developed in the brains of the host CAII-deficient mice at 8, 14 and 18 days posttransplant (DPT). In transplants of either mixed glial cells or oligodendrocyte precursors, CAII-positive oligodendrocytes were less plentiful than CAII-positive astrocytes and appeared to be less healthy. CAII-positive astrocytes developed by 8 DPT, and there were some oligodendrocytes in cerebral cortex at 14 DPT and in brainstem by 18 DPT. The data suggested that if glial-cell progenitors were to be injected into demyelinated lesions, any oligodendrocytes descended from the donor would be accompanied by astrocytes also descended from donor cells.

Differential uptake of dextran beads by astrocytes, macrophages and oligodendrocytes in mixed glial-cell cultures from brains of neonatal rats

The present study addresses a controversy over the abilities of astrocytes to perform phagocytosis. Primary glial-cell cultures were prepared from the brains of neonatal rats and were incubated with fluorescently-labeled dextran beads (molecular weights approximately 10 and approximately 40 kDa). Astrocytes and oligodendrocytes were double-labeled by immunofluorescence staining of cell-specific markers, and microglia by lectin histochemistry. Cells were permitted to take up beads for 1 h, fixed, and incubated with primary antibodies, followed by fluorescent secondary antibodies or fluorescently-labeled lectin. Macrophages and astrocytes internalized beads of both sizes. In astrocyte processes the beads appeared to line up along glial filaments. The results, which provide direct evidence for uptake of beads by astrocytes in vitro and against equally rapid, if any, uptake by oligodendrocytes, bear upon issues of acid/base balance and glial cell development and are relevant to neuropathological observations in human disease.

Glial-cell cultures from brains of carbonic anhydrase II-deficient mutant mice: delay in oligodendrocyte maturation

Carbonic anhydrase II (CAII) is a multifunctional enzyme found in oligodendrocytes and astrocytes in normal mouse brains. We have begun to compare the glial cells in primary cultures from neonatal genetically CAII-deficient (Car) mice to those from normal (con) mice in order to detect developmental defects, if any, in Car glial cells. In con cultures intensely CAII-positive cells costained with antibodies against the oligodendrocytic markers, O4 and myelin basic protein (MBP), respectively. Most (82%) of the CAII-positive cells were O4-positive, but only approximately 60% were MBP-positive. Some clumps of GFAP-positive cells were CAII-positive. At each respective number of days in vitro (DIV) total numbers of O4-positive cells were similar in Car and con cultures, and total numbers of galactocerebroside-positive cells also were similar in Car and con cultures. However, compared to cells in con cultures at 7 DIV, a lower percent of Car cells in the oligodendrocyte lineage expressed MBP, and morphological differentiation also was subnormal in that the Car cells showed fewer processes and membrane sheets. Car and con cultures expressed similar numbers of MBP-positive cells by 10 DIV. The results suggest a temporary delay in the maturation of Car oligodendrocytes.