Expression of membrane-associated carbonic anhydrase XIV on neurons and axons in mouse and human brain

The potential of carbonic anhydrase enzymes as a novel target for anti-cancer treatment

Carbonic anhydrase (CA) is a zinc-dependent metal enzyme that maintains the pH and carbon dioxide (CO2) homeostasis in cells by catalyzing the reversible hydration and dehydration of CO2 and bicarbonate (HCO3-). In mammals, there are 16 isozymes of CA existed, namely CAI to CAXIV, but only 15 isozymes are found in humans except CAXV. Human CAs have highly conserved catalytic domains, all of which are distributed in different tissues and play important physiological roles. Changes in their functions may disrupt the typical distribution of CAs throughout human body and therefore CAs can be used as diagnostic biomarkers for many diseases. Furthermore, the expression of CAs is correlated to the progression of numerous tumors, therapeutic sensitivity and patient prognosis. In this review, we discuss thoroughly the structure of CAs, their functional activities in human physiology, dysregulations and diseases related to CAs, and different types of CA inhibitors that can reverse their dysregulation.

pH regulating mechanisms of astrocytes: A critical component in physiology and disease of the brain

Strict homeostatic control of pH in both intra- and extracellular compartments of the brain is fundamentally important, primarily due to the profound impact of free protons ([H+]) on neuronal activity and overall brain function. Astrocytes, crucial players in the homeostasis of various ions in the brain, actively regulate their intracellular [H+] (pHi) through multiple membrane transporters and carbonic anhydrases. The activation of astroglial pHi regulating mechanisms also leads to corresponding alterations in the acid-base status of the extracellular fluid. Notably, astrocyte pH regulators are modulated by various neuronal signals, suggesting their pivotal role in regulating brain acid-base balance in both health and disease. This review presents the mechanisms involved in pH regulation in astrocytes and discusses their potential impact on extracellular pH under physiological conditions and in brain disorders. Targeting astrocytic pH regulatory mechanisms represents a promising therapeutic approach for modulating brain acid-base balance in diseases, offering a potential critical contribution to neuroprotection.

The circulating proteome and brain health: Mendelian randomisation and cross-sectional analyses

Decline in cognitive function is the most feared aspect of ageing. Poorer midlife cognitive function is associated with increased dementia and stroke risk. The mechanisms underlying variation in cognitive function are uncertain. Here, we assessed associations between 1160 proteins' plasma levels and two measures of cognitive function, the digit symbol substitution test (DSST) and the Montreal Cognitive Assessment in 1198 PURE-MIND participants. We identified five DSST performance-associated proteins (NCAN, BCAN, CA14, MOG, CDCP1), with NCAN and CDCP1 showing replicated association in an independent cohort, GS (N = 1053). MRI-assessed structural brain phenotypes partially mediated (8-19%) associations between NCAN, BCAN, and MOG, and DSST performance. Mendelian randomisation analyses suggested higher CA14 levels might cause larger hippocampal volume and increased stroke risk, whilst higher CDCP1 levels might increase intracranial aneurysm risk. Our findings highlight candidates for further study and the potential for drug repurposing to reduce the risk of stroke and cognitive decline.

Cerebrospinal fluid pH regulation

The cerebrospinal fluid (CSF) fills the brain ventricles and the subarachnoid space surrounding the brain and spinal cord. The fluid compartment of the brain ventricles communicates with the interstitial fluid of the brain across the ependyma. In comparison to blood, the CSF contains very little protein to buffer acid-base challenges. Nevertheless, the CSF responds efficiently to changes in systemic pH by mechanisms that are dependent on the CO2/HCO3- buffer system. This is evident from early studies showing that the CSF secretion is sensitive to inhibitors of acid/base transporters and carbonic anhydrase. The CSF is primarily generated by the choroid plexus, which is a well-vascularized structure arising from the pial lining of the brain ventricles. The epithelial cells of the choroid plexus host a range of acid/base transporters, many of which participate in CSF secretion and most likely contribute to the transport of acid/base equivalents into the ventricles. This review describes the current understanding of the molecular mechanisms in choroid plexus acid/base regulation and the possible role in CSF pH regulation.

Antihistamines, phenothiazine-based antipsychotics, and tricyclic antidepressants potently activate pharmacologically relevant human carbonic anhydrase isoforms II and VII

Carbonic anhydrases (CAs) are important regulators of pH homeostasis and participate in many physiological and pathological processes. CA activators (CAAs) are becoming increasingly important in the biomedical field since enhancing CA activity may have beneficial effects at neurological level. Here, we investigate selected antihistamines, phenothiazine-based antipsychotics, and tricyclic antidepressants (TCAs) as potential activators of human CAs I, II, IV, and VII. Our findings indicate that these compounds are more effective at activating hCA II and VII compared to hCA I and IV. Overall, hCA VII was the most efficiently activated isoform, particularly by phenothiazines and TCAs. This is especially relevant since hCA VII is the most abundant isoform in the central nervous system (CNS) and is implicated in neuronal signalling and bicarbonate balance regulation. This study offers additional insights into the pharmacological profiles of clinically employed drugs and sets the ground for the development of novel optimised CAAs.

Carbonic anhydrase IX: A tumor acidification switch in heterogeneity and chemokine regulation

The primary physiological process of respiration produces carbon dioxide (CO₂) that reacts with water molecules which subsequently liberates bicarbonate (HCO-₃) and protons. Carbonic anhydrases (CAs) are the primary catalyst involved in this conversion. More than 16 isoforms of human CAs show organ or subcellular specific activity. Dysregulation of each CA is associated with multiple pathologies. Out of these members, the overexpression of membrane-bound carbonic anhydrase IX (CAIX) is associated explicitly with hypoxic tumors or various solid cancers. CAIX helps tumors deal with higher CO₂ by sequestering it with bicarbonate ions and helping cancer cells to grow in a comparatively hypoxic or acidic environment, thus acting as a pH adaptation switch. CAIX-mediated adaptations in cancer cells include angiogenesis, metabolic alterations, tumor heterogeneity, drug resistance, and regulation of cancer-specific chemokines. This review comprehensively collects and describe the cancer-specific expression mechanism and role of CAIX in cancer growth, progression, heterogeneity, and its structural insight to develop future combinatorial targeted cancer therapies.

Acetazolamide modulates intracranial pressure directly by its action on the cerebrospinal fluid secretion apparatus

Background

Elevated intracranial pressure (ICP) is observed in many neurological pathologies, e.g. hydrocephalus and stroke. This condition is routinely relieved with neurosurgical approaches, since effective and targeted pharmacological tools are still lacking. The carbonic anhydrase inhibitor, acetazolamide (AZE), may be employed to treat elevated ICP. However, its effectiveness is questioned, its location of action unresolved, and its tolerability low. Here, we determined the efficacy and mode of action of AZE in the rat .

Methods

We employed in vivo approaches including ICP and cerebrospinal fluid secretion measurements in anaesthetized rats and telemetric monitoring of ICP and blood pressure in awake rats in combination with ex vivo choroidal radioisotope flux assays and transcriptomic analysis.

Results

AZE effectively reduced the ICP, irrespective of the mode of drug administration and level of anaesthesia. The effect appeared to occur via a direct action on the choroid plexus and an associated decrease in cerebrospinal fluid secretion, and not indirectly via the systemic action of AZE on renal and vascular processes. Upon a single administration, the reduced ICP endured for approximately 10 h post-AZE delivery with no long-term changes of brain water content or choroidal transporter expression. However, a persistent reduction of ICP was secured with repeated AZE administrations throughout the day.

Conclusions

AZE lowers ICP directly via its ability to reduce the choroid plexus CSF secretion, irrespective of mode of drug administration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00348-6.

Carbonic Anhydrases in Metazoan Model Organisms: Molecules, Mechanisms, and Physiology

During the past three decades, mice, zebrafish, fruit flies, and Caenorhabditis elegans have been the primary model organisms used for the study of various biological phenomena. These models have also been adopted and developed to investigate the physiological roles of carbonic anhydrases (CAs) and carbonic anhydrase-related proteins (CARPs). These proteins belong to eight CA families and are identified by Greek letters: α, β, γ, δ, ζ, η, θ, and ι. Studies using model organisms have focused on two CA families, α-CAs and β-CAs, which are expressed in both prokaryotic and eukaryotic organisms with species-specific distribution patterns and unique functions. This review covers the biological roles of CAs and CARPs in light of investigations performed in model organisms. Functional studies demonstrate that CAs are not only linked to the regulation of pH homeostasis, the classical role of CAs but also contribute to a plethora of previously undescribed functions.

Comparable Number of Genes Having Experienced Positive Selection among Great Ape Species

Simple Summary

It is of great interest to quantify adaptive evolution in human lineage by studying genes under positive selection, since these genes could reveal insights into our own adaptive evolutionary history compared to our closely related species and often these genes are functionally important. We used the great apes as the subjects to detect gene-level adaptive evolution signals in all the great ape lineages and investigated the evolutionary patterns and functional relevance of these adaptive evolution signals. Even the differences in population size among these closely related great apes have resulted in differences in their ability to remove deleterious alleles and to adapt to changing environments, we found that they experienced comparable numbers of positive selection. Notably, we identified several genes that offer insights into great ape and human evolution. For example, SOD1, a gene associated with aging in humans, experienced positive selection in the common ancestor of the great ape and this positive selection may contribute to the aging evolution in great apes. Overall, an updated list of positively selected genes reported by this study not only informs us of adaptive evolution during great ape evolution, but is also helpful to the further study of non-human primate models for disease and other fields.

Abstract

Alleles that cause advantageous phenotypes with positive selection contribute to adaptive evolution. Investigations of positive selection in protein-coding genes rely on the accuracy of orthology, models, the quality of assemblies, and alignment. Here, based on the latest genome assemblies and gene annotations, we present a comparative analysis on positive selection in four great ape species and identify 211 high-confidence positively selected genes (PSGs). Even the differences in population size among these closely related great apes have resulted in differences in their ability to remove deleterious alleles and to adapt to changing environments, we found that they experienced comparable numbers of positive selection. We also uncovered that more than half of multigene families exhibited signals of positive selection, suggesting that imbalanced positive selection resulted in the functional divergence of duplicates. Moreover, at the expression level, although positive selection led to a more non-uniform pattern across tissues, the correlation between positive selection and expression patterns is diverse. Overall, this updated list of PSGs is of great significance for the further study of the phenotypic evolution in great apes.

Carbonic Anhydrase Inhibitors and Epilepsy: State of the Art and Future Perspectives

Carbonic anhydrases (CAs) are a group of ubiquitously expressed metalloenzymes that catalyze the reversible hydration/dehydration of CO₂/HCO₃. Thus, they are involved in those physiological and pathological processes in which cellular pH buffering plays a relevant role. The inhibition of CAs has pharmacologic applications for several diseases. In addition to the well-known employment of CA inhibitors (CAIs) as diuretics and antiglaucoma drugs, it has recently been demonstrated that CAIs could be considered as valid therapeutic agents against obesity, cancer, kidney dysfunction, migraine, Alzheimer's disease and epilepsy. Epilepsy is a chronic brain disorder that dramatically affects people of all ages. It is characterized by spontaneous recurrent seizures that are related to a rapid change in ionic composition, including an increase in intracellular potassium concentration and pH shifts. It has been reported that CAs II, VII and XIV are implicated in epilepsy. In this context, selective CAIs towards the mentioned isoforms (CAs II, VII and XIV) have been proposed and actually exploited as anticonvulsants agents in the treatment of epilepsy. Here, we describe the research achievements published on CAIs, focusing on those clinically used as anticonvulsants. In particular, we examine the new CAIs currently under development that might represent novel therapeutic options for the treatment of epilepsy.

Anticonvulsant Effects of Carbonic Anhydrase Inhibitors: The Enigmatic Link Between Carbonic Anhydrases and Electrical Activity of the Brain

Acetazolamide (ACZ), a sulfonamide carbonic anhydrase (CA) inhibitor, was first introduced into medical use as a diuretic in the1950s. Shortly after its introduction, its antiglaucoma and anticonvulsant properties came to light. Subsequently, studies of ACZ have explored a plethora of neurophysiological functions of CAs in the CNS. In addition, topiramate (TPM) and zonisamide (ZNS), which were developed as antiepileptic drugs (AEDs) in the1990s, were found to have the ability to inhibit CAs. How CA inhibition prevents seizures is elusive. CA expression and activity are extensively detected in neurons, the choroid plexus, oligodendrocytes and astrocytes. TPM and ZNS appear to produce multimodal actions in the CNS as well as CA inhibition unlike ACZ. Nonetheless, CA inhibitors share some common denominators. They do not only affect the fine equilibrium among CO₂, H⁺ and HCO₃^- in the extraneuronal and intraneuronal milieu, but also modulate the activity of ligand gated ion channels at the neuronal level such as GABA-A signaling through inhibiting CA-replenished HCO₃^- efflux. In addition, there are studies reporting their ability to alter Ca²⁺ kinetics through modulation of ligand gated Ca²⁺ channels, voltage gated Ca²⁺ channels (VGCC) or Ca²⁺-induced Ca²⁺ release channels (CICRC). The present study will review the involvement of CAs in the formation of epileptogenesis, and likely mechanisms by which CA inhibitors suppress the electrical activity of the brain. The common properties of CA inhibitors provide some clues for a possible link among metabolism, CAs, Ca²⁺ and GABA signaling.

The Expression of Carbonic Anhydrases II, IX and XII in Brain Tumors

Carbonic anhydrases (CAs) are zinc-containing metalloenzymes that participate in the regulation of pH homeostasis in addition to many other important physiological functions. Importantly, CAs have been associated with neoplastic processes and cancer. Brain tumors represent a heterogeneous group of diseases with a frequently dismal prognosis, and new treatment options are urgently needed. In this review article, we summarize the previously published literature about CAs in brain tumors, especially on CA II and hypoxia-inducible CA IX and CA XII. We review here their role in tumorigenesis and potential value in predicting prognosis of brain tumors, including astrocytomas, oligodendrogliomas, ependymomas, medulloblastomas, meningiomas, and craniopharyngiomas. We also introduce both already completed and ongoing studies focusing on CA inhibition as a potential anti-cancer strategy.

A New Kid on the Block? Carbonic Anhydrases as Possible New Targets in Alzheimer's Disease

The increase in the incidence of neurodegenerative diseases, in particular Alzheimer's Disease (AD), is a consequence of the world's population aging but unfortunately, existing treatments are only effective at delaying some of the symptoms and for a limited time. Despite huge efforts by both academic researchers and pharmaceutical companies, no disease-modifying drugs have been brought to the market in the last decades. Recently, several studies shed light on Carbonic Anhydrases (CAs, EC 4.2.1.1) as possible new targets for AD treatment. In the present review we summarized preclinical and clinical findings regarding the role of CAs and their inhibitors/activators on cognition, aging and neurodegeneration and we discuss future challenges and opportunities in the field.

Carbonic anhydrases as disease markers

INTRODUCTION

The physiologic importance of fast CO₂/HCO₃^- interconversion in various tissues requires the presence of carbonic anhydrase (CA, EC 4.2.1.1). Fourteen CA isozymes are present in humans, all of them being used as biomarkers.

AREAS COVERED

A great number of patents and articles were focused on the use of CA isozymes as biomarkers for various diseases and syndromes in the recent years, in an ascending trend over the last decade. The review highlights the most important studies related with each isozyme and covers the most recent patent literature.

EXPERT OPINION

The CAs biomarker research area expanded significantly in recent years, shifting from the predominant use of CA IX and CA XII in cancer diagnostic, staging, and prognosis towards a wider use of CA isozymes as disease biomarkers. CA isozymes are currently used either alone, in tandem with other CA isozymes and/or in combination with other proteins for the detection, staging, and prognosis of a huge repertoire of human dysfunctions and diseases, ranging from mild transformation of the normal tissues to extreme shifts in tissue organization and function. The techniques used for their detection/quantitation and the state-of-the-art in each clinical application are presented through relevant clinical examples and corresponding statistical data.

Long non-coding RNA and mRNA analysis of Ang II-induced neuronal dysfunction

The sustained activation of Angiotensin II (Ang II) induces the remodelling of neurovascular units, inflammation and oxidative stress reactions in the brain. Long non-coding RNAs (lncRNAs) play a crucial regulatory role in the pathogenesis of hypertensive neuronal damage. The present study aimed to substantially extend the list of potential candidate genes involved in Ang II-related neuronal damage. This study assessed apoptosis and energy metabolism with Annexin V/PI staining and a Seahorse assay after Ang II exposure in SH-SY5Y cells. The expression of mRNA and lncRNA was investigated by transcriptome sequencing. The integrated analysis of mRNA and lncRNAs and the molecular mechanism of Ang II on neuronal injury was analysed by bioinformatics. Ang II increased the apoptosis rate and reduced the energy metabolism of SH-SY5Y cells. The data showed that 702 mRNAs and 821 lncRNAs were differentially expressed in response to Ang II exposure (244 mRNAs and 432 lncRNAs were upregulated, 458 mRNAs and 389 lncRNAs were downregulated) (fold change ≥ 1.5, P < 0.05). GO and KEGG analyses showed that both DE mRNA and DE lncRNA were enriched in the metabolism, differentiation, apoptosis and repair of nerve cells. This is the first report of the lncRNA-mRNA integrated profile of SH-SY5Y cells induced by Ang II. The novel targets revealed that the metabolism of the vitamin B group, the synthesis of unsaturated fatty acids and glycosphingolipids are involved in the Ang II-related cognitive impairment. Sphingolipid metabolism, the Hedgehog signalling pathway and vasopressin-regulated water reabsorption play important roles in nerve damage.

Carbonic anhydrase enzymes: Likely targets for inhalational anesthetics

Inhalational anesthetics such as isoflurane, desflurane and halothane are the mainstay medications for surgical procedures; upon inhalation, they produce anesthesia described as reversible unconsciousness with the features of amnesia, sleep, immobility and analgesia. To date, how they produce anesthesia is unknown. This study proposes that carbonic anhydrase enzymes are likely targets mediating the actions of inhalational anesthetics. Carbonic anhydrase enzymes, commonly expressed in living organisms, utilize carbon dioxide (CO₂) as a substrate and can generate H⁺ and HCO₃^- from CO₂ with a great efficiency. There are remarkable lines of evidence for their likely roles in mediating anesthetic actions. Firstly, carbonic anhydrase enzymes are extensively expressed in the brain and spinal cord, and their importance in the brain activity, especially for the GABA and NMDA receptor signaling pathways, has been demonstrated in numerous studies. According to these studies, they provide HCO₃^- for GABA-A receptor activities and also buffer HCO₃^- excess resulting from NMDA receptor activation. Activation of GABA-A and inhibition of NMDA receptors are associated with the induction of anesthesia by the intravenous general anesthetics propofol and ketamine, respectively. Secondly, the carbonic anhydrase inhibitors topiramate and zonisamide are effectively used in the treatment of epilepsy for decades; their chronic use results in the requirement of increased levels of amobarbital in order to produce anesthesia in the epileptic patients during WADA test. In addition, given that CO₂ is a substrate for these enzymes, their tertiary structure is likely has a hydrophobic pocket suitable for the anesthetic molecules to bind. Inhalational anesthetic molecules, which are lipophilic and inert in nature, have an ability to cross the membranes and inhibit carbonic anhydrases, which might not be accessible by topiramate and zonisamide. Unlike carbonic anhydrase inhibitors, they could bind to the hydrophobic pocket for CO₂ molecules and produce a profound effect called anesthesia. Finally, there is a great deal of similarities between the physiological actions of inhalational anesthetics and carbonic anhydrase inhibitors; moreover well-known side effects of inhalational anesthetics could be associated with the inhibition of carbonic anhydrases. Therefore, this article presents a hypothesis that the anesthetic actions of inhalational anesthetics could be due to their inhibitory effects on the carbonic anhydrases. Investigating this hypothesis might lead to the development of new safer anesthetics, and more importantly it might reveal an endogenous anesthetic pathway, in which the carbonic anhydrase system is a component along with the GABA-A and NMDA receptor systems.

UFLC-Derived CSF Extracellular Vesicle Origin and Proteome

Cerebrospinal fluid (CSF) extracellular vesicles (EVs) show promise as a source of neurological disease biomarkers, although their precise origin is poorly understood. Current extraction techniques produce disappointing yield and purity. This study describes the application of ultrafiltration LC (UFLC) to CSF-EVs, compared with ultracentrifugation (UC), and explores CSF-EV origin. EVs are extracted from human CSF by UC and UFLC and characterized using nanoparticle tracking analysis, electron microscopy, and immunoblotting. EV and CSF proteomes are analyzed by LC-MS/MS. UFLC-isolated particles have size, morphology, and marker expression characteristic of EVs. UFLC provides greater EV yield (UFLC 7.90 × 10⁸  ± SD 1.31 × 10⁸ EVs mL^-1 CSF, UC 1.06 × 10⁸  ± 0.57 × 10⁸ p < 0.001). UFLC enhances purity, proteomic depth (UFLC 622 ± 49, UC 298 ± 50, p = 0.001), and consistency of quantification (CV 17% vs 23%). EVs contain more intracellular proteins (Odds ratio [OR] 2.63 p < 0.001) and fewer plasma proteins than CSF (OR 0.60, p < 0.001). CSF and EV-enriched proteomes show overrepresentation of brain-specific proteins (EV OR 3.18, p < 0.001; CSF OR 3.37, p < 0.001). Overrepresentation of cerebral white matter (OR 1.99, p = 0.015) and choroid plexus proteins (OR 1.87, p<0.001) is observed in EVs. UFLC improves yield and purity of CSF-EVs. The EV-enriched proteome better reflects the intracellular and white matter proteome than whole CSF.

Light-enhanced expression of Carbonic Anhydrase 4-like supports shell formation in the fluted giant clam Tridacna squamosa

Giant clams represent symbiotic associations between a host clam and its extracellular zooxanthellae. They are able to grow in nutrient-deficient tropical marine environments and conduct light-enhanced shell formation (calcification) with the aid of photosynthates donated by the symbiotic zooxanthellae. In light, there is a high demand for inorganic carbon (C_i) to support photosynthesis in the symbionts and light-enhanced calcification in the host. In this study, we cloned and characterized a host Carbonic Anhydrase 4 homolog (CA4-like) from the whitish inner mantle of the giant clam Tridacna squamosa. The full cDNA coding sequence of CA4-like consisted of 1002 bp, encoding for 334 amino acids of 38.5 kDa. The host CA4-like was phenogramically distinct from algal CAs. The transcript level of CA4-like in the inner mantle was ~3-fold higher than those in the colorful outer mantle and the ctenidium. In the inner mantle, CA4-like was immunolocalized in the apical membrane of the seawater-facing epithelial cells, but absent from the shell-facing epithelium. Hence, CA4-like was positioned to catalyze the conversion of HCO₃^- to CO₂ in the ambient seawater which would facilitate CO₂ uptake. The absorbed CO₂ could be converted back to HCO₃^- by the cytoplasmic CA2-like. As the protein abundance of CA4-like increased in the inner mantle after 6 or 12 h of light exposure, there could be an augmentation of the total CA4-like activity to increase C_i uptake in light. It is plausible that the absorbed C_i was allocated preferentially for shell formation due to the close proximity of the seawater-facing epithelium to the shell-facing epithelium in the inner mantle that contains only few zooxanthellae.

Differential expression and function of CAIX and CAXII in breast cancer: A comparison between tumorgraft models and cells

Carbonic anhydrase IX (CAIX) and XII (CAXII) are transmembrane proteins that are associated with cancer progression. We have previously described the catalytic properties of CAIX in MDA-MB-231 breast cancer cells, a line of cells that were derived from a patient with triple negative breast cancer. We chose this line because CAIX expression in breast cancer is a marker of hypoxia and a prognosticator for reduced survival. However, CAXII expression is associated with better survival statistics than those patients with low CAXII expression. Yet CAIX and CAXII have similar catalytic activities. Here we compare the potential roles of CAIX and CAXII in the context of TNBC and estrogen receptor (ER)-positive breast cancer. In tumor graft models, we show that CAIX and CAXII exhibit distinct expression patterns and non-overlapping. We find the same pattern across a panel of TNBC and luminal breast cancer cell lines. This affords an opportunity to compare directly CAIX and CAXII function. Our data suggest that CAIX expression is associated with growth potentiation in the tumor graft model and in a TNBC line using knockdown strategies and blocking activity with an impermeant sulfonamide inhibitor, N-3500. CAXII was not associated with growth potentiation. The catalytic activities of both CAIX and CAXII were sensitive to inhibition by N-3500 and activated at low pH. However, pH titration of activity in membrane ghosts revealed significant differences in the catalytic efficiency and pKa values. These features provide evidence that CAIX is a more efficient enzyme than CAXII at low pH and that CAIX shifts the equilibrium between CO₂ and bicarbonate in favor of CO₂ production by consuming protons. This suggests that in the acidic microenvironment of tumors, CAIX plays a role in stabilizing pH at a value that favors cancer cell survival.

An Update on the Metabolic Roles of Carbonic Anhydrases in the Model Alga Chlamydomonas reinhardtii

Carbonic anhydrases (CAs) are metalloenzymes that are omnipresent in nature. CAs catalyze the basic reaction of the reversible hydration of CO₂ to HCO₃⁻ and H⁺ in all living organisms. Photosynthetic organisms contain six evolutionarily different classes of CAs, which are namely: α-CAs, β-CAs, γ-CAs, δ-CAs, ζ-CAs, and θ-CAs. Many of the photosynthetic organisms contain multiple isoforms of each CA family. The model alga Chlamydomonas reinhardtii contains 15 CAs belonging to three different CA gene families. Of these 15 CAs, three belong to the α-CA gene family; nine belong to the β-CA gene family; and three belong to the γ-CA gene family. The multiple copies of the CAs in each gene family may be due to gene duplications within the particular CA gene family. The CAs of Chlamydomonas reinhardtii are localized in different subcellular compartments of this unicellular alga. The presence of a large number of CAs and their diverse subcellular localization within a single cell suggests the importance of these enzymes in the metabolic and biochemical roles they perform in this unicellular alga. In the present review, we update the information on the molecular biology of all 15 CAs and their metabolic and biochemical roles in Chlamydomonas reinhardtii. We also present a hypothetical model showing the known functions of CAs and predicting the functions of CAs for which precise metabolic roles are yet to be discovered.

Carbonic Anhydrases: Role in pH Control and Cancer

The pH of the tumor microenvironment drives the metastatic phenotype and chemotherapeutic resistance of tumors. Understanding the mechanisms underlying this pH-dependent phenomenon will lead to improved drug delivery and allow the identification of new therapeutic targets. This includes an understanding of the role pH plays in primary tumor cells, and the regulatory factors that permit cancer cells to thrive. Over the last decade, carbonic anhydrases (CAs) have been shown to be important mediators of tumor cell pH by modulating the bicarbonate and proton concentrations for cell survival and proliferation. This has prompted an effort to inhibit specific CA isoforms, as an anti-cancer therapeutic strategy. Of the 12 active CA isoforms, two, CA IX and XII, have been considered anti-cancer targets. However, other CA isoforms also show similar activity and tissue distribution in cancers and have not been considered as therapeutic targets for cancer treatment. In this review, we consider all the CA isoforms and their possible role in tumors and their potential as targets for cancer therapy.

Carbonic anhydrase XII functions in health and disease

Human CAXII was initially identified as a cancer marker in different cancers and tumors. Expression of CAXII is regulated by hypoxia and estrogen receptors. CAXII expression has been also detected in several tissues, whereas in cancer and tumor tissues its expression is several fold higher. In brain tumors, an alternatively spliced form of CAXII is expressed. Higher expression of CAXII in breast cancer is indicative of lower grade disease. CAXII plays a key role in several physiological functions. Mutation in the CAXII gene causes cystic fibrosis-like syndrome and salt wasting disease. CAXII is also seen in nuclear pulposus cells of the vertebrae. Aging dependent stiffness or degeneration of backbone correlates with CAXII expression level. This finding suggests a possible implication of CAXII as a biomarker for chronic back pain and a pharmacological target for possible treatment of chronic back pain.

Probing Molecular Interactions between Human Carbonic Anhydrases (hCAs) and a Novel Class of Benzenesulfonamides

On the basis of X-ray crystallographic studies of the complex of hCA II with 4-(3,4-dihydro-1H-isoquinoline-2-carbonyl)benzenesulfonamide (3) (PDB code 4Z1J ), a novel series of 4-(1-aryl-3,4-dihydro-1H-isoquinolin-2-carbonyl)benzenesulfonamides (23-33) was designed. Specifically, our idea was to improve the selectivity toward druggable isoforms through the introduction of additional hydrophobic/hydrophilic functionalities. Among the synthesized and tested compounds, the (R,S)-4-(6,7-dihydroxy-1-phenyl-3,4-tetrahydroisoquinoline-1H-2-carbonyl)benzenesulfonamide (30) exhibited a remarkable inhibition for the brain-expressed hCA VII (Ki = 0.20 nM) and selectivity over wider distributed hCA I and hCA II isoforms. By enantioselective HPLC, we solved the racemic mixture and ascertained that the two enantiomers (30a and 30b) are equiactive inhibitors for hCA VII. Crystallographic and docking studies revealed the main interactions of these inhibitors into the carbonic anhydrase (CA) catalytic site, thus highlighting the relevant role of nonpolar contacts for this class of hCA inhibitors.

Regulation of high glucose-induced apoptosis of brain pericytes by mitochondrial CA VA: A specific target for prevention of diabetic cerebrovascular pathology

Events responsible for cerebrovascular disease in diabetes are not fully understood. Pericyte loss is an early event that leads to endothelial cell death, microaneurysms, and cognitive impairment. A biochemical mechanism underlying pericyte loss is rapid respiration (oxidative metabolism of glucose). This escalation in respiration results from free influx of glucose into insulin-insensitive tissues in the face of high glucose levels in the blood. Rapid respiration generates superoxide, the precursor to all reactive oxygen species (ROS), and results in pericyte death. Respiration is regulated by carbonic anhydrases (CAs) VA and VB, the two isozymes expressed in mitochondria, and their pharmacologic inhibition with topiramate reduces respiration, ROS, and pericyte death. Topiramate inhibits both isozymes; therefore, in the earlier studies, their individual roles were not discerned. In a recent genetic study, we showed that mitochondrial CA VA plays a significant role in regulation of reactive oxygen species and pericyte death. The role of CA VB was not addressed. In this report, genetic knockdown and overexpression studies confirm that mitochondrial CA VA regulates respiration in pericytes, whereas mitochondrial CA VB does not contribute significantly. Identification of mitochondrial CA VA as a sole regulator of respiration provides a specific target to develop new drugs with fewer side effects that may be better tolerated and can protect the brain from diabetic injury. Since similar events occur in the capillary beds of other insulin-insensitive tissues such as the eye and kidney, these drugs may also slow the onset and progression of diabetic disease in these tissues.

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.

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.

Membrane associated carbonic anhydrase IV (CA IV): a personal and historical perspective

Carbonic anhydrase IV is one of 12 active human isozymes and one of four expressed on the extracellular surfaces of certain endothelial and epithelial cells. It is unique in being attached to the plasma membrane by a glycosyl-phosphatiydyl-inositol (GPI) anchor rather than by a membrane-spanning domain. It is also uniquely resistant to high concentrations of sodium dodecyl sulfate (SDS), which allows purification from tissues by inhibitor affinity chromatography without contamination by other isozymes. This unique resistance to SDS and recovery following denaturation is explained by the two disulfide bonds. The 35-kDa human CA IV is a "high activity" isozyme in CO2 hydration activity, like CA II, and has higher activity than other isozymes in catalyzing the dehydration of HCO3 (-). Human CA IV is also unique in that it contains no oligosaccharide chains, where all other mammalian CA IVs are glycoproteins with one to several oligosaccharide side chains.Although CA IV has been shown to be active in mediating CO2 and HCO3 (-) transport in many important tissues like kidney and lung, and in isolated cells from brain and muscle, the gene for CA IV appears not to be essential. The CA IV knockout mouse produced by targeted mutagenesis, though slightly smaller and produced in lower than expected numbers, is viable and has no obvious mutant phenotype. Conversely, several dominant negative mutations in humans are associated with one form of reitinitis pigmentosa (RP-17), which we attribute to unfolded protein accumulation in the choreocapillaris, leading to apoptosis of cells in the overlying retina.

Synthesis of 5-arylidine amino-1,3,4-thiadiazol-2-[(N-substituted benzyol)]sulphonamides endowed with potent antioxidants and anticancer activity induces growth inhibition in HEK293, BT474 and NCI-H226 cells

Abstract

A series of imines 5-amino-1,3,4-thiadiazol-2-[(N-substituted benzyol)]sulphonamide derivatives were synthesized from various aromatic aldehydes and substituted with benzoyl acetazolamides under different reaction conditions and were evaluated for their antioxidant and free radical scavenging, antimitotic activity by Allium cepa meristem root model and cytotoxicity activity against HEK 293 (human epidermal kidney cell line), BT474 (breast cancer cell line) and NCI-H226 (lung cancer cell line) by MTT assay. Some of the synthesized compounds showed moderately potent cytotoxicity compared to indisulam.

Graphical abstract

A series of imines 5-amino-1,3,4-thiadiazol-2-[(N-substituted benzyol)]sulphonamide derivatives (9a–j); 5-amino-1,3,4-thiadiazol-2-[N-(substituted benzoyl)]sulphonamide (4a–g); 5-(4-acetamido phenyl sulphonamido)-1,3,4-thiadiazol-2-[N-(substituted benzoyl)]sulphonamide (6a–g); and 5-(4-amino phenyl sulphonamido)-1,3,4-thiadiazol-2-[N-(substituted benzoyl)]sulphonamide (7a–g) were synthesized from acetazolamide and were investigated for the in vitro anticancer by MTT assay, free radical scavenging and antimitotic activity by Allium cepa root meristem model. Experimental observations indicate that synthesized compounds were moderately potent anticancer agents.

Minireview: pH and synaptic transmission

As a general rule a rise in pH increases neuronal activity, whereas it is dampened by a fall of pH. Neuronal activity per se also challenges pH homeostasis by the increase of metabolic acid equivalents. Moreover, the negative membrane potential of neurons promotes the intracellular accumulation of protons. Synaptic key players such as glutamate receptors or voltage-gated calcium channels show strong pH dependence and effects of pH gradients on synaptic processes are well known. However, the processes and mechanisms that allow controlling the pH in synaptic structures and how these mechanisms contribute to normal synaptic function are only beginning to be resolved.

Targeted mutagenesis of mitochondrial carbonic anhydrases VA and VB implicates both enzymes in ammonia detoxification and glucose metabolism

Prior studies with carbonic anhydrase (CA) inhibitors implicated mitochondrial CA in ureagenesis and gluconeogenesis. Subsequent studies identified two mitochondrial CAs. To distinguish the contribution of each enzyme, we studied the effects of targeted disruption of the murine CA genes, called Car5A and Car5B. The Car5A mutation had several deleterious consequences. Car5A null mice were smaller than wild-type littermates and bred poorly. However, on sodium-potassium citrate-supplemented water, they produced offspring in expected numbers. Their blood ammonia concentrations were markedly elevated, but their fasting blood sugars were normal. By contrast, Car5B null mice showed normal growth and normal blood ammonia levels. They too had normal fasting blood sugars. Car5A/B double-knockout (DKO) mice showed additional abnormalities. Impaired growth was more severe than for Car5A null mice. Hyperammonemia was even greater as well. Although fertile, DKO animals were produced in less-than-predicted numbers even when supplemented with sodium-potassium citrate in their drinking water. Survival after weaning was also reduced, especially for males. In addition, fasting blood glucose levels for DKO mice were significantly lower than for controls (153 ± 33 vs. 230 ± 24 mg/dL). The enhanced hyperammonemia and lower fasting blood sugar, which are both seen in the DKO mice, indicate that both Car5A and Car5B contribute to both ammonia detoxification (ureagenesis) and regulation of fasting blood sugar (gluconeogenesis). Car5A, which is expressed mainly in liver, clearly has the predominant role in ammonia detoxification. The contribution of Car5B to ureagenesis and gluconeogenesis was evident only on a Car5A null background.

NMDA receptor-dependent afterdepolarizations are curtailed by carbonic anhydrase 14: regulation of a short-term postsynaptic potentiation

In the hippocampus, extracellular carbonic anhydrase (Car) speeds the buffering of an activity-generated rise in extracellular pH that impacts H(+)-sensitive NMDA receptors (NMDARs). We studied the role of Car14 in this brain structure, in which it is expressed solely on neurons. Current-clamp responses were recorded from CA1 pyramidal neurons in wild-type (WT) versus Car14 knock-out (KO) mice 2 s before (control) and after (test) a 10 pulse, 100 Hz afferent train. In both WT and KO, the half-width (HW) of the test response, and its number of spikes, were augmented relative to the control. An increase in presynaptic release was not involved, because AMPAR-mediated EPSCs were depressed after a train. The increases in HW and spike number were both greater in the Car14 KO. In 0 Mg(2+) saline with picrotoxin (using a 20 Hz train), the HW measures were still greater in the KO. The Car inhibitor benzolamide (BZ) enhanced the test response HW in the WT but had no effect on the already-prolonged HW in the KO. With intracellular MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]-cyclohepten-5,10-imine maleate], the curtailed WT and KO responses were indistinguishable, and BZ caused no change. In contrast, the extracellular alkaline changes evoked by the train were not different between WT and KO, and BZ amplified these alkalinizations similarly. These data suggest that Car14 regulates pH transients in the perisynaptic microenvironment and govern their impact on NMDARs but plays little role in buffering pH shifts in the broader, macroscopic, extracellular space.

The transcriptomic response of mixed neuron-glial cell cultures to 1,25-dihydroxyvitamin d3 includes genes limiting the progression of neurodegenerative diseases

Seasonal or chronic vitamin D deficiency and/or insufficiency is highly prevalent in the human population. Receptors for 1,25-dihydroxyvitamin D3, the hormonal metabolite of vitamin D, are found throughout the brain. To provide further information on the role of this hormone on brain function, we analyzed the transcriptomic profiles of mixed neuron-glial cell cultures in response to 1,25-dihydroxyvitamin D3. 1,25-dihydroxyvitamin D3 treatment increases the mRNA levels of 27 genes by at least 1.9 fold. Among them, 17 genes were related to neurodegenerative and psychiatric diseases, or brain morphogenesis. Notably, 10 of these genes encode proteins potentially limiting the progression of Alzheimer's disease. These data provide support for a role of 1,25-dihydroxyvitamin D3 in brain disease prevention. The possible consequences of circannual or chronic vitamin D insufficiencies on a tissue with a low regenerative potential such as the brain should be considered.

Characterization of the carbonic anhydrases 15b expressed in PGCs during early zebrafish development

The main function of carbonic anhydrases is to regulate acid-base balance. In the present study, the zebrafish CA15b sequence was identified from the National Center for Biotechnology Information database (accession No. NM_213182). The 1716 base pair full-length cDNA of CA15b was obtained by 3' and 5'-rapid amplification of cDNA ends analysis. It was expressed (reverse transcription polymerase chain reaction and Western blot analysis) in the ovary, heart, brain, and muscle, but not in testis or liver. Based on in situ hybridization, CA15b mRNA was transcribed in the ooplasm of stage I to stage II oocytes, in the cortex of stage III oocytes, and along the periphery of stage IV oocytes. Furthermore, this protein was localized (immunohistochemistry) in the plasma membrane of oocytes. Based on whole-mount in situ hybridization, CA15b mRNA was present in every blastomere of embryos from one-cell to blastula stages. Strong signals of the transcripts were present along cleavage furrows of two- and eight-cell stage embryos, which subsequently condensed into four clusters of cells during the blastula stage. During subsequent stages, the four groups of CA15b-expressing cells appeared to move toward the dorsal side of the embryos, clustered into two groups on either side of the midline, and remained visible as they migrated toward the region of the gonad in embryos at 24 hours postfertilization. Expression patterns of CA15b were similar to those of vasa, a marker of primordial germ cells. Thus, we hypothesized that CA15b might be necessary for development of primordial germ cells and female germ cells in zebrafish.

Structure, function and applications of carbonic anhydrase isozymes

The carbonic anhydrases enzymes (CAs, EC 4.2.1.1) are zinc containing metalloproteins, which efficiently catalyse the reversible conversion of carbon dioxide to bicarbonate and release proton. These enzymes are essentially important for biological system and play several important physiological and patho-physiological functions. There are 16 different alpha-carbonic anhydrase isoforms studied, differing widely in their cellular localization and biophysical properties. The catalytic domains of all CAs possess a conserved tertiary structure fold, with predominately β-strands. We performed an extensive analysis of all 16 mammalian CAs for its structure and function in order to establish a structure-function relationship. CAs have been a potential therapeutic target for many diseases. Sulfonamides are considered as a strong and specific inhibitor of CA, and are being used as diuretics, anti-glaucoma, anti-epileptic, anti-ulcer agents. Currently CA inhibitors are widely used as a drug for the treatment of neurological disorders, anti-glaucoma drugs, anti-cancer, or anti-obesity agents. Here we tried to emphasize how CAs can be used for drug discovery, design and screening. Furthermore, we discussed the role of CA in carbon capture, carbon sensor and metabolon. We hope this review provide many useful information on structure, function, mechanism, and applications of CAs in various discipline.

Lactate flux in astrocytes is enhanced by a non-catalytic action of carbonic anhydrase II

Rapid exchange of metabolites between different cell types is crucial for energy homeostasis of the brain. Besides glucose, lactate is a major metabolite in the brain and is primarily produced in astrocytes. In the present study, we report that carbonic anhydrase 2 (CAII) enhances both influx and efflux of lactate in mouse cerebellar astrocytes. The augmentation of lactate transport is independent of the enzyme's catalytic activity, but requires direct binding of CAII to the C-terminal of the monocarboxylate transporter MCT1, one of the major lactate/proton cotransporters in astrocytes and most tissues. By employing its intramolecular proton shuttle, CAII, bound to MCT1, can act as a ‘proton collecting antenna' for the transporter, suppressing the formation of proton microdomains at the transporter-pore and thereby enhancing lactate flux. By this mechanism CAII could enhance transfer of lactate between astrocytes and neurons and thus provide the neurons with an increased supply of energy substrate.

Carbonic anhydrase XIV in the normal and hypertrophic myocardium

Two AE3 transcripts, full-length (AE3fl) and cardiac (AE3c) are expressed in the heart. AE3 catalyzes electroneutral Cl(-)/HCO(3)(-) exchange across cardiomyocyte sarcolemma. AE proteins associate with carbonic anhydrases (CA), including CAII and CAIV, forming a HCO(3)(-) transport metabolon (BTM), increasing HCO(3)(-) fluxes and regulating cardiomyocytes pH. CAXIV, which is also expressed in the heart's sarcolemma, is a transmembrane enzyme with an extracellular catalytic domain. Herein, AE3/CAXIV physical association was examined by coimmunoprecipitation using rodent heart lysates. CAXIV immunoprecipitated with anti-AE3 antibody and both AE3fl and AE3c were reciprocally immunoprecipitated using anti-CAXIV antibody, indicating AE3fl-AE3c/CAXIV interaction in the myocardium. Coimmunoprecipitation experiments on heart lysates from a mouse with targeted disruption of the ae3 gene, failed to pull down AE3 with the CAXIV antibody. Confocal images demonstrated colocalization of CAXIV and AE3 in mouse ventricular myocytes. Functional association of AE3fl and CAXIV was examined in isolated hypertrophic rat cardiomyocytes, using fluorescence measurements of BCECF to monitor cytosolic pH. Hypertrophic cardiomyocytes of spontaneously hypertensive rats (SHR) presented elevated myocardial AE-mediated Cl(-)/HCO(3)(-) exchange activity (J(HCO3-) mM.min(-1)) compared to normal (Wistar) rats (7.5±1.3, n=4 versus 2.9±0.1, n=6, respectively). AE3fl, AE3c, CAII, CAIV, and CAIX protein expressions were similar in SHR and Wistar rat hearts. However, immunoblots revealed a twofold increase of CAXIV protein expression in the SHR myocardium compared to normal hearts (n=11). Furthermore, the CA-inhibitor, benzolamide, neutralized the stimulatory effect of extracellular CA on AE3 transport activity (3.7±1.5, n=3), normalizing AE3-dependent HCO(3)(-) fluxes in SHR. CAXIV/AE3 interaction constitutes an extracellular component of a BTM which potentiates AE3-mediated HCO(3)(-) transport in the heart. Increased CAXIV expression and consequent AE3/CAXIV complex formation would render AE3 hyperactive in the SHR heart.

T tubules and surface membranes provide equally effective pathways of carbonic anhydrase-facilitated lactic acid transport in skeletal muscle

We have studied lactic acid transport in the fast mouse extensor digitorum longus muscles (EDL) by intracellular and cell surface pH microelectrodes. The role of membrane-bound carbonic anhydrases (CA) of EDL in lactic acid transport was investigated by measuring lactate flux in muscles from wildtype, CAIV-, CAIX- and CAXIV-single ko, CAIV-CAXIV double ko and CAIV-CAIX-CAXIV-triple ko mice. This was complemented by immunocytochemical studies of the subcellular localization of CAIV, CAIX and CAXIV in mouse EDL. We find that CAXIV and CAIX single ko EDL exhibit markedly but not maximally reduced lactate fluxes, whereas triple ko and double ko EDL show maximal or near-maximal inhibition of CA-dependent lactate flux. Interpretation of the flux measurements in the light of the immunocytochemical results leads to the following conclusions. CAXIV, which is homogeneously distributed across the surface membrane of EDL fibers, facilitates lactic acid transport across this membrane. CAIX, which is associated only with T tubular membranes, facilitates lactic acid transport across the T tubule membrane. The removal of lactic acid from the lumen of T tubuli towards the interstitial space involves a CO2-HCO3- diffusional shuttle that is maintained cooperatively by CAIX within the T tubule and, besides CAXIV, by the CAIV, which is strategically located at the opening of the T tubules. The data suggest that about half the CA-dependent muscular lactate flux occurs across the surface membrane, while the other half occurs across the membranes of the T tubuli.

Comparative transcriptome analysis of the CO2 sensing pathway via differential expression of carbonic anhydrase in Cryptococcus neoformans

Carbon dioxide (CO(2)) sensing and metabolism via carbonic anhydrases (CAs) play pivotal roles in survival and proliferation of pathogenic fungi infecting human hosts from natural environments due to the drastic difference in CO(2) levels. In Cryptococcus neoformans, which causes fatal fungal meningoencephalitis, the Can2 CA plays essential roles during both cellular growth in air and sexual differentiation of the pathogen. However the signaling networks downstream of Can2 are largely unknown. To address this question, the present study employed comparative transcriptome DNA microarray analysis of a C. neoformans strain in which CAN2 expression is artificially controlled by the CTR4 (copper transporter) promoter. The P(CTR4)CAN2 strain showed growth defects in a CO(2)-dependent manner when CAN2 was repressed but resumed normal growth when CAN2 was overexpressed. The Can2-dependent genes identified by the transcriptome analysis include FAS1 (fatty acid synthase 1) and GPB1 (G-protein beta subunit), supporting the roles of Can2 in fatty acid biosynthesis and sexual differentiation. Cas3, a capsular structure designer protein, was also discovered to be Can2-dependent and yet was not involved in CO(2)-mediated capsule induction. Most notably, a majority of Can2-dependent genes were environmental stress-regulated (ESR) genes. Supporting this, the CAN2 overexpression strain was hypersensitive to oxidative and genotoxic stress as well as antifungal drugs, such as polyene and azole drugs, potentially due to defective membrane integrity. Finally, an oxidative stress-responsive Atf1 transcription factor was also found to be Can2-dependent. Atf1 not only plays an important role in diverse stress responses, including thermotolerance and antifungal drug resistance, but also represses melanin and capsule production in C. neoformans. In conclusion, this study provides insights into the comprehensive signaling networks orchestrated by CA/CO(2)-sensing pathways in pathogenic fungi.

The most recently discovered carbonic anhydrase, CA XV, is expressed in the thick ascending limb of Henle and in the collecting ducts of mouse kidney

Background

Carbonic anhydrases (CAs) are key enzymes for physiological pH regulation, including the process of urine acidification. Previous studies have identified seven cytosolic or membrane-bound CA isozymes in the kidney. Recently, we showed by in situ hybridization that the mRNA for the most novel CA isozyme, CA XV, is present in the renal cortex. CA XV is a unique isozyme among mammalian CAs, because it has become a pseudogene in primates even though expressed in several other species.

Methodology/Principal Findings

In the present study, we raised a polyclonal antibody against recombinant mouse CA XV that was produced in a baculovirus/insect cell expression system, and the antibody was used for immunohistochemical analysis in different mouse tissues. Positive immunoreactions were found only in the kidney, where the enzyme showed a very limited distribution pattern. Parallel immunostaining experiments with several other anti-CA sera indicated that CA XV is mainly expressed in the thick ascending limb of Henle and collecting ducts, and the reactions were most prominent in the cortex and outer medulla.

Conclusion/Significance

Although other studies have proposed a role for CA XV in cell proliferation, its tightly limited distribution may point to a specialized function in the regulation of acid-base homeostasis.