Expression of carbonic anhydrases IX and XII during mouse embryonic development

Mechanisms of cerebrospinal fluid secretion by the choroid plexus epithelium: Application to various intracranial pathologies

The choroid plexus (CP) is a small yet highly active epithelial tissue located in the ventricles of the brain. It secretes most of the CSF that envelops the brain and spinal cord. The epithelial cells of the CP have a high fluid secretion rate and differ from many other secretory epithelia in the organization of several key ion transporters. One striking difference is the luminal location of, for example, the vital Na+-K+-ATPase. In recent years, there has been a renewed focus on the role of ion transporters in CP secretion. Several studies have indicated that increased membrane transport activity is implicated in disorders such as hydrocephalus, idiopathic intracranial hypertension, and posthemorrhagic sequelae. The importance of the CP membrane transporters in regulating the composition of the CSF has also been a focus in research in recent years, particularly as a regulator of breathing and hemodynamic parameters such as blood pressure. This review focuses on the role of the fundamental ion transporters involved in CSF secretion and its ion composition. It gives a brief overview of the established factors and controversies concerning ion transporters, and finally discusses future perspectives related to the role of these transporters in the CP epithelium.

Unveiling tomorrow: Carbonic anhydrase activators and inhibitors pioneering new frontiers in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder and a principal basis of dementia in the elderly population globally. Recently, human carbonic anhydrases (hCAs, EC 4.2.1.1) were demonstrated as possible new targets for treating AD. hCAs are vital for maintaining pH balance and performing other physiological processes as they catalyze the reversible hydration of carbon dioxide to bicarbonate and a proton. Current research indicates that hCA plays a role in brain functions critical for transmitting neural signals. Activation of carbonic anhydrase (CA) has emerged as a promising avenue in addressing memory loss and cognitive issues. Conversely, the exploration of CA inhibition represents a novel frontier in this field. By enhancing glial fitness and cerebrovascular health and blocking amyloid-β (Aβ)-induced mitochondrial dysfunction pathways, cytochrome C (CytC) release, caspase 9 activation, and H2O2 generation in neurons, CA inhibitors improve cognition and lessen the pathology caused by Aβ. Recent research has pushed hCAs into the spotlight as critical players in AD pathogenesis and precise therapeutic targets. The captivating dilemma of choosing between hCA inhibitors and activators looms large, as inhibitors reduce Aβ aggregation and improve cerebral blood flow, while activators enhance cerebrovascular functions and restore pH balance. The current review sheds light on the clinical evidence for hCAs and the roles of inhibitors and activators in AD. Additionally, this review offers a fascinating outlook on the data that may aid medicinal chemists in designing and developing new leads that are more effective and selective for upcoming in vitro and in vivo studies, allowing for the discovery and introduction of novel drug candidates for the treatment of AD to the market and into the clinical pipeline.

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.

SLC-0111, an inhibitor of carbonic anhydrase IX, attenuates hepatoblastoma cell viability and migration

Background

In response to hypoxia, tumor cells undergo transcriptional reprogramming including upregulation of carbonic anhydrase (CA) IX, a metalloenzyme that maintains acid-base balance. CAIX overexpression has been shown to correlate with poor prognosis in various cancers, but the role of this CA isoform in hepatoblastoma (HB) has not been examined.

Methods

We surveyed the expression of CAIX in HB specimens and assessed the impact of SLC-0111, a CAIX inhibitor, on cultured HB cells in normoxic and hypoxic conditions.

Results

CAIX immunoreactivity was detected in 15 out of 21 archival pathology HB specimens. The CAIX-positive cells clustered in the middle of viable tumor tissue or next to necrotic areas. Tissue expression of CAIX mRNA was associated with metastasis and poor clinical outcome of HB. Hypoxia induced a striking upregulation of CAIX mRNA and protein in three HB cell models: the immortalized human HB cell line HUH6 and patient xenograft-derived lines HB-295 and HB-303. Administration of SLC-0111 abrogated the hypoxia-induced upregulation of CAIX and decreased HB cell viability, both in monolayer and spheroid cultures. In addition, SLC-0111 reduced HB cell motility in a wound healing assay. Transcriptomic changes triggered by SLC-0111 administration differed under normoxic vs. hypoxic conditions, although SLC-0111 elicited upregulation of several tumor suppressor genes under both conditions.

Conclusion

Hypoxia induces CAIX expression in HB cells, and the CAIX inhibitor SLC-0111 has in vitro activity against these malignant cells.

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.

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.

Carbonic Anhydrase IX-Mouse versus Human

In contrast to human carbonic anhydrase IX (hCA IX) that has been extensively studied with respect to its molecular and functional properties as well as regulation and expression, the mouse ortholog has been investigated primarily in relation to tissue distribution and characterization of CA IX-deficient mice. Thus, no data describing transcriptional regulation and functional properties of the mouse CA IX (mCA IX) have been published so far, despite its evident potential as a biomarker/target in pre-clinical animal models of tumor hypoxia. Here, we investigated for the first time, the transcriptional regulation of the Car9 gene with a detailed description of its promoter. Moreover, we performed a functional analysis of the mCA IX protein focused on pH regulation, cell-cell adhesion, and migration. Finally, we revealed an absence of a soluble extracellular form of mCA IX and provided the first experimental evidence of mCA IX presence in exosomes. In conclusion, though the protein characteristics of hCA IX and mCA IX are highly similar, and the transcription of both genes is predominantly governed by hypoxia, some attributes of transcriptional regulation are specific for either human or mouse and as such, could result in different tissue expression and data interpretation.

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.

Targeting CAIX with [64Cu]XYIMSR-06 Small Molecular Radiotracer Enables Noninvasive PET Imaging of Malignant Glioma in U87 MG Tumor Cell Xenograft Mice

Carbonic anhydrase IX (CAIX) plays an important role in glioma cell proliferation, invasion, metastasis, and resistance to radiotherapy and chemotherapy. An effective and noninvasive PET molecular imaging agent targeting CAIX would help its diagnosis and treatment but is not currently available. Recently, a low-molecular-weight (LMW) CAIX targeting agent, [⁶⁴Cu]XYIMSR-06, was reported to have significantly improved properties for targeting clear cell renal cell carcinoma (ccRCC). We are encouraged to investigate the feasibility of adapting this agent for the diagnosis and treatment of CAIX-overexpressing malignant glioma. In vitro cell uptake and binding affinity assays were used to verify the binding capacity of [⁶⁴Cu]XYIMSR-06 to U87 MG tumor cells in which CAIX overexpression was confirmed. The U87 MG tumor-bearing mouse (in situ and subcutaneous) model was built, and mice were injected with the radiotracer and/or coinjected with acetazolamide (0.2 g/kg) as a blocking agent for noninvasive micro-PET imaging. Micro-PET imaging was performed at 2, 4, and 8 h postinjection. ROI (region of interest)-based semiquantification was performed in an orthotopic glioma tumor model. Biodistribution throughout each organ was performed at 2, 4, 4 h block, 8, and 24 h postinjection. Hematoxylin and eosin (HE) staining and immunofluorescence or immunohistochemistry (IF/IHC) staining were implemented postimaging to assess the expression of CAIX in tumor organs. In vitro, [⁶⁴Cu]XYIMSR-06 exhibits greater uptake in glioma cells (high CAIX expression) than in HCT116 cells (low CAIX expression). The binding affinity of [⁶⁴Cu]XYIMSR-06 to U87 MG cell lines reaches up to 4.22 nM. Both orthotopic and subcutaneous tumors were clearly visualized at 2-8 h postinjection. Biodistribution studies demonstrated a maximum tumor uptake of 3.13% ID/g at 4 h postinjection, and the tumor to brain ratio (T/brain) was 6.51 at 8 h postinjection. The ROI-based T/brain values were 7.03 and 5.46 at 2 and 8 h postinjection, respectively. Histopathological analysis confirmed the overexpression of CAIX in gliomas, and the area of CAIX-positive IF staining is extremely consistent with the morphology on micro-PET imaging. In this study, [⁶⁴Cu]XYIMSR-06 demonstrated specific accumulation in CAIX-expressing U87 MG glioma tumors, indicating that the radiotracer has the potential for noninvasively monitoring and guiding personalized treatment of malignant glioma and other tumors overexpressing CAIX.

Cancer Drug Development of Carbonic Anhydrase Inhibitors beyond the Active Site

Carbonic anhydrases (CAs) catalyze the reversible hydration of carbon dioxide to produce bicarbonate and a proton. Multiple CA isoforms are implicated in a range of diseases, including cancer. In solid tumors, continuously dividing cells create hypoxic conditions that eventually lead to an acidic microenvironment. Hypoxic tumor cells have different mechanisms in place to regulate and adjust the surrounding microenvironment for survival. These mechanisms include expression of CA isoform IX (CA IX) and XII (CA XII). These enzymes help maintain a physiological intracellular pH while simultaneously contributing to an acidic extracellular pH, leading to tumor cell survival. Expression of CA IX and CA XII has also been shown to promote tumor cell invasion and metastasis. This review discusses the characteristics of CA IX and CA XII, their mechanism of action, and validates their prospective use as anticancer targets. We discuss the current status of small inhibitors that target these isoforms, both classical and non-classical, and their future design in order to obtain isoform-specificity for CA IX and CA XII. Biologics, such as monoclonal antibodies, monoclonal-radionuclide conjugated chimeric antibodies, and antibody-small molecule conjugates are also discussed.

Genetic ablation of carbonic anhydrase IX disrupts gastric barrier function via claudin-18 downregulation and acid backflux

Aim

This study aimed to explore the molecular mechanisms for the parietal cell loss and fundic hyperplasia observed in gastric mucosa of mice lacking the carbonic anhydrase 9 (CAIX).

Methods

We assessed the ability of CAIX‐knockout and WT gastric surface epithelial cells to withstand a luminal acid load by measuring the pH_i of exteriorized gastric mucosa in vivo using two‐photon confocal laser scanning microscopy. Cytokines and claudin‐18A2 expression was analysed by RT‐PCR.

Results

CAIX‐knockout gastric surface epithelial cells showed significantly faster pH_i decline after luminal acid load compared to WT. Increased gastric mucosal IL‐1β and iNOS, but decreased claudin‐18A2 expression (which confer acid resistance) was observed shortly after weaning, prior to the loss of parietal and chief cells. At birth, neither inflammatory cytokines nor claudin‐18 expression were altered between CAIX and WT gastric mucosa. The gradual loss of acid secretory capacity was paralleled by an increase in serum gastrin, IL‐11 and foveolar hyperplasia. Mild chronic proton pump inhibition from the time of weaning did not prevent the claudin‐18 decrease nor the increase in inflammatory markers at 1 month of age, except for IL‐1β. However, the treatment reduced the parietal cell loss in CAIX‐KO mice in the subsequent months.

Conclusions

We propose that CAIX converts protons that either backflux or are extruded from the cells rapidly to CO₂ and H₂O, contributing to tight junction protection and gastric epithelial pH_i regulation. Lack of CAIX results in persistent acid backflux via claudin‐18 downregulation, causing loss of parietal cells, hypergastrinaemia and foveolar hyperplasia.

Bicarbonate Recycling by HIF-1-Dependent Carbonic Anhydrase Isoforms 9 and 12 Is Critical in Maintaining Intracellular pH and Viability of Nucleus Pulposus Cells

Intervertebral disc degeneration is a ubiquitous condition closely linked to chronic low-back pain. The health of the avascular nucleus pulposus (NP) plays a crucial role in the development of this pathology. We tested the hypothesis that a network comprising HIF-1α, carbonic anhydrase (CA) 9 and 12 isoforms, and sodium-coupled bicarbonate cotransporters (NBCs) buffer intracellular pH through coordinated bicarbonate recycling. Contrary to the current understanding of NP cell metabolism, analysis of metabolic-flux data from Seahorse XF analyzer showed that CO2 hydration contributes a significant source of extracellular proton production in NP cells, with a smaller input from glycolysis. Because enzymatic hydration of CO2 is catalyzed by plasma membrane-associated CAs we measured their expression and function in NP tissue. NP cells robustly expressed isoforms CA9/12, which were hypoxia-inducible. In addition to increased mRNA stability under hypoxia, we observed binding of HIF-1α to select hypoxia-responsive elements on CA9/12 promoters using genomic chromatin immunoprecipitation. Importantly, in vitro loss of function studies and analysis of discs from NP-specific HIF-1α null mice confirmed the dependency of CA9/12 expression on HIF-1α. As expected, inhibition of CA activity decreased extracellular acidification rate independent of changes in HIF activity or lactate/H+ efflux. Surprisingly, CA inhibition resulted in a concomitant decrease in intracellular pH that was mirrored by inhibition of sodium-bicarbonate importers. These results suggested that extracellular bicarbonate generated by CA9/12 is recycled to buffer cytosolic pH fluctuations. Importantly, long-term intracellular acidification from CA inhibition lead to compromised cell viability, suggesting that plasma-membrane proton extrusion pathways alone are not sufficient to maintain homeostatic pH in NP cells. Taken together, our studies show for the first time that bicarbonate buffering through the HIF-1α-CA axis is critical for NP cell survival in the hypoxic niche of the intervertebral disc. © 2017 American Society for Bone and Mineral Research.

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.

Transport across the choroid plexus epithelium

The choroid plexus epithelium is a secretory epithelium par excellence. However, this is perhaps not the most prominent reason for the massive interest in this modest-sized tissue residing inside the brain ventricles. Most likely, the dominant reason for extensive studies of the choroid plexus is the identification of this epithelium as the source of the majority of intraventricular cerebrospinal fluid. This finding has direct relevance for studies of diseases and conditions with deranged central fluid volume or ionic balance. While the concept is supported by the vast majority of the literature, the implication of the choroid plexus in secretion of the cerebrospinal fluid was recently challenged once again. Three newer and promising areas of current choroid plexus-related investigations are as follows: 1) the choroid plexus epithelium as the source of mediators necessary for central nervous system development, 2) the choroid plexus as a route for microorganisms and immune cells into the central nervous system, and 3) the choroid plexus as a potential route for drug delivery into the central nervous system, bypassing the blood-brain barrier. Thus, the purpose of this review is to highlight current active areas of research in the choroid plexus physiology and a few matters of continuous controversy.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Expression of the CHOP-inducible carbonic anhydrase CAVI-b is required for BDNF-mediated protection from hypoxia

Carbonic anhydrases (CAs) comprise a family of zinc-containing enzymes that catalyze the reversible hydration of carbon dioxide. CAs contribute to a myriad of physiological processes, including pH regulation, anion transport and water balance. To date, 16 known members of the mammalian alpha-CA family have been identified. Given that the catalytic family members share identical reaction chemistry, their physiologic roles are influenced greatly by their tissue and sub-cellular locations. CAVI is the lone secreted CA and exists in both saliva and the gastrointestinal mucosa. An alternative, stress-inducible isoform of CAVI (CAVI-b) has been shown to be expressed from a cryptic promoter that is activated by the CCAAT/Enhancer-Binding Protein Homologous Protein (CHOP). The CAVI-b isoform is not secreted and is currently of unknown physiological function. Here we use neuronal models, including a model derived using Car6 and CHOP gene ablations, to delineate a role for CAVI-b in ischemic protection. Our results demonstrate that CAVI-b expression, which is increased through CHOP-signaling in response to unfolded protein stress, is also increased by oxygen-glucose deprivation (OGD). While enforced expression of CAVI-b is not sufficient to protect against ischemia, CHOP regulation of CAVI-b is necessary for adaptive changes mediated by BDNF that reduce subsequent ischemic damage. These results suggest that CAVI-b comprises a necessary component of a larger adaptive signaling pathway downstream of CHOP.

Cerebrospinal Fluid Secretion by the Choroid Plexus

The choroid plexus epithelium is a cuboidal cell monolayer, which produces the majority of the cerebrospinal fluid. The concerted action of a variety of integral membrane proteins mediates the transepithelial movement of solutes and water across the epithelium. Secretion by the choroid plexus is characterized by an extremely high rate and by the unusual cellular polarization of well-known epithelial transport proteins. This review focuses on the specific ion and water transport by the choroid plexus cells, and then attempts to integrate the action of specific transport proteins to formulate a model of cerebrospinal fluid secretion. Significant emphasis is placed on the concept of isotonic fluid transport across epithelia, as there is still surprisingly little consensus on the basic biophysics of this phenomenon. The role of the choroid plexus in the regulation of fluid and electrolyte balance in the central nervous system is discussed, and choroid plexus dysfunctions are described in a very diverse set of clinical conditions such as aging, Alzheimer's disease, brain edema, neoplasms, and hydrocephalus. Although the choroid plexus may only have an indirect influence on the pathogenesis of these conditions, the ability to modify epithelial function may be an important component of future therapies.

Recent developments in targeting carbonic anhydrase IX for cancer therapeutics

Carbonic anhydrase IX (CAIX) is a hypoxia-inducible enzyme that is overexpressed by cancer cells from many tumor types, and is a component of the pH regulatory system invoked by these cells to combat the deleterious effects of a high rate of glycolytic metabolism. CAIX functions to help produce and maintain an intracellular pH (pHi) favorable for tumor cell growth and survival, while at the same time participating in the generation of an increasingly acidic extracellular space, facilitating tumor cell invasiveness. Pharmacologic interference of CAIX catalytic activity using monoclonal antibodies or CAIX-specific small molecule inhibitors, consequently disrupting pH regulation by cancer cells, has been shown recently to impair primary tumor growth and metastasis. Many of these agents are in preclinical or clinical development and constitute a novel, targeted strategy for cancer therapy.

The spatial organization of proton and lactate transport in a rat brain tumor

Tumors create a heterogeneous acidic microenvironment which assists their growth and which must be taken into account in the design of drugs and their delivery. In addition, the acidic extracellular pH (pHe) is itself exploited in several experimental techniques for drug delivery. The way the acidity is created is not clear. We report here the spatial organization of key proton-handling proteins in C6 gliomas in rat brain. The mean profiles across the tumor rim of the Na⁺/H⁺ exchanger NHE1, and the lactate-H⁺ cotransporter MCT1, both showed peaks. NHE1, which is important for extension and migration of cells in vitro, showed a peak 1.55 times higher than in extratumoural tissue at 0.33 mm from the edge. MCT1 had a broader peak, further into the tumor (maximum 1.76 fold at 1.0 mm from the edge). In contrast, MCT4 and the carbonic anhydrase CAIX, which are associated with hypoxia, were not significantly upregulated in the rim. The spatial distribution of MCT4 was highly correlated with that of CAIX, suggesting that their expression is regulated by the same factors. Since protons extruded by NHE1 diffuse away through extracellular clefts, NHE1 requires a continuous source of intracellular protons. From the stoichiometries of metabolic pathways that produce or consume H⁺, and the greater availability of glucose compared to oxygen in most parts of a tumor, we support the classic view that most of the net proton efflux from C6 gliomas originates in glycolytic formation of lactate and H⁺ inside the tumor, but add that some lactate is taken up into cells in the rim on MCT1, and some lactate diffuses away, leaving its associated protons available to re-enter cells for extrusion on NHE1. Therapeutic inhibition of NHE1, MCT1 or CAIX is predicted to affect different parts of a tumor.

Embryonic frog epidermis: a model for the study of cell-cell interactions in the development of mucociliary disease

Specialised epithelia such as mucociliary, secretory and transporting epithelia line all major organs, including the lung, gut and kidney. Malfunction of these epithelia is associated with many human diseases. The frog embryonic epidermis possesses mucus-secreting and multiciliated cells, and has served as an excellent model system for the biogenesis of cilia. However, ionic regulation is important for the function of all specialised epithelia and it is not clear how this is achieved in the embryonic frog epidermis. Here, we show that a third cell type develops alongside ciliated and mucus-secreting cells in the tadpole skin. These cells express high levels of ion channels and transporters; therefore, we suggest that they are analogous to ionocytes found in transporting epithelia such as the mammalian kidney. We show that frog ionocytes express the transcription factor foxi1e, which is required for the development of these cells. Depletion of ionocytes by foxi1e knockdown has detrimental effects on the development of multiciliated cells, which show fewer and aberrantly beating cilia. These results reveal a newly identified role for ionocytes and suggest that the frog embryonic skin is a model system that is particularly suited to studying the interactions of different cell types in mucociliary, as well as in secretory and transporting, epithelia.

Hyperchlorhidrosis caused by homozygous mutation in CA12, encoding carbonic anhydrase XII

Excessive chloride secretion in sweat (hyperchlorhidrosis), leading to a positive sweat test, is most commonly indicative of cystic fibrosis yet is found also in conjunction with various metabolic, endocrine, and dermatological disorders. There is conflicting evidence regarding the existence of autosomal-recessive hyperchlorhidrosis. We now describe a consanguineous Israeli Bedouin kindred with autosomal-recessive hyperchlohidrosis whose sole symptoms are visible salt precipitates after sweating, a preponderance to hyponatremic dehydration, and poor feeding and slow weight gain at infancy. Through genome-wide linkage analysis, we demonstrate that the phenotype is due to a homozygous mutation in CA12, encoding carbonic anhydrase XII. The mutant (c.427G>A [p.Glu143Lys]) protein showed 71% activity of the wild-type enzyme for catalyzing the CO₂ hydration to bicarbonate and H(+), and it bound the clinically used sulfonamide inhibitor acetazolamide with high affinity (K(I) of 10 nM). Unlike the wild-type enzyme, which is not inhibited by chloride, bromide, or iodide (K(I)s of 73-215 mM), the mutant is inhibited in the submicromolar range by these anions (K(I)s of 0.37-0.73 mM).

Carbonic anhydrases in the mouse harderian gland

The harderian gland is located within the orbit of the eye of most terrestrial vertebrates. It is especially noticeable in rodents, in which it synthesises lipids, porphyrins, and indoles. Various functions have been ascribed to the harderian gland, such as lubrication of the eyes, a site of immune response, and a source of growth factors. Carbonic anhydrases (CAs) are zinc-containing metalloenzymes that catalyse the reaction CO₂ + H₂O <--> H+ + HCO₃. They are involved in the adjustment of pH in the secretions of different glands. Thirteen enzymatically active isozymes have been described in the mammalian α-CA family. Here, we first investigated the mRNA expression of all 13 active CAs in the mouse harderian gland by quantitative real-time PCR. Nine CA mRNAs were detectable in the gland. Car5b and Car13 showed the highest signals. Car4, Car6, and Car12 showed moderate expression levels, whereas Car2, Car3, Car7, and Car15 mRNAs were barely within the detection limits. Immunohistochemical staining was performed to study the expression of Car2, Car4, Car5b, Car12, and Car13 at the protein level. The epithelial cells were intensively stained for CAVB, whereas only weak signal was detected for CAXIII. Positive signals for CAIV and CAXII were observed in the capillary endothelial cells and the basolateral plasma membrane of the epithelial cells, respectively. This study provides an expression profile of all CAs in the mouse harderian gland. These results should improve our understanding of the distribution of CA isozymes and their potential roles in the function of harderian gland. The high expression of mitochondrial CAVB at both mRNA and protein levels suggests a role in lipid synthesis, a key physiological process of the harderian gland.

The tumour-associated carbonic anhydrases CA II, CA IX and CA XII in a group of medulloblastomas and supratentorial primitive neuroectodermal tumours: an association of CA IX with poor prognosis

Background

Medulloblastomas (MBs) and supratentorial primitive neuroectodermal tumours (PNETs) are the most common highly aggressive paediatric brain tumours. In spite of extensive research on these tumours, there are only few known biomarkers or therapeutic target proteins, and the prognosis of patients with these tumours remains poor. Our aim was to investigate whether carbonic anhydrases (CAs), enzymes commonly overexpressed in various tumours including glioblastomas and oligodendrogliomas, are present in MBs and PNETs, and whether their expression can be correlated with patient prognosis.

Methods

We determined the expression of the tumour-associated carbonic anhydrases CA II, CA IX and CA XII in a series of MB/PNET specimens (n = 39) using immunohistochemistry.

Results

Endothelial CA II, cytoplasmic CA II, CA IX and CA XII were expressed in 49%, 73%, 23% and 11% of the tumours, respectively. CA II was detected in the neovessel endothelium and the tumour cell cytoplasm. CA IX was mainly expressed in the tumour cells located in perinecrotic areas. CA XII showed the most homogenous distribution within the tumours. Importantly, CA IX expression predicted poor prognosis in both univariate (p = 0.041) and multivariate analyses (p = 0.016).

Conclusions

We suggest that CA IX should be considered a potential prognostic and therapeutic target in MBs and PNETs.

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.

Tumour hypoxia induces a metabolic shift causing acidosis: a common feature in cancer

Maintenance of cellular pH homeostasis is fundamental to life. A number of key intracellular pH (pHi) regulating systems including the Na⁺/H⁺ exchangers, the proton pump, the monocarboxylate transporters, the HCO₃⁻ transporters and exchangers and the membrane-associated and cytosolic carbonic anhydrases cooperate in maintaining a pHi that is permissive for cell survival. A common feature of tumours is acidosis caused by hypoxia (low oxygen tension). In addition to oncogene activation and transformation, hypoxia is responsible for inducing acidosis through a shift in cellular metabolism that generates a high acid load in the tumour microenvironment. However, hypoxia and oncogene activation also allow cells to adapt to the potentially toxic effects of an excess in acidosis. Hypoxia does so by inducing the activity of a transcription factor the hypoxia-inducible factor (HIF), and particularly HIF-1, that in turn enhances the expression of a number of pHi-regulating systems that cope with acidosis. In this review, we will focus on the characterization and function of some of the hypoxia-inducible pH-regulating systems and their induction by hypoxic stress. It is essential to understand the fundamentals of pH regulation to meet the challenge consisting in targeting tumour metabolism and acidosis as an anti-tumour approach. We will summarize strategies that take advantage of intracellular and extracellular pH regulation to target the primary tumour and metastatic growth, and to turn around resistance to chemotherapy and radiotherapy.

Expression of transmembrane carbonic anhydrases, CAIX and CAXII, in human development

Background

Transmembrane CAIX and CAXII are members of the alpha carbonic anhydrase (CA) family. They play a crucial role in differentiation, proliferation, and pH regulation. Expression of CAIX and CAXII proteins in tumor tissues is primarily induced by hypoxia and this is particularly true for CAIX, which is regulated by the transcription factor, hypoxia inducible factor-1 (HIF-1). Their distributions in normal adult human tissues are restricted to highly specialized cells that are not always hypoxic. The human fetus exists in a relatively hypoxic environment. We examined expression of CAIX, CAXII and HIF-1α in the developing human fetus and postnatal tissues to determine whether expression of CAIX and CAXII is exclusively regulated by HIF-1.

Results

The co-localization of CAIX and HIF-1α was limited to certain cell types in embryonic and early fetal tissues. Those cells comprised the primitive mesenchyma or involved chondrogenesis and skin development. Transient CAIX expression was limited to immature tissues of mesodermal origin and the skin and ependymal cells. The only tissues that persistently expressed CAIX protein were coelomic epithelium (mesothelium) and its remnants, the epithelium of the stomach and biliary tree, glands and crypt cells of duodenum and small intestine, and the cells located at those sites previously identified as harboring adult stem cells in, for example, the skin and large intestine. In many instances co-localization of CAIX and HIF-1α was not evident. CAXII expression is restricted to cells involved in secretion and water absorption such as parietal cells of the stomach, acinar cells of the salivary glands and pancreas, epithelium of the large intestine, and renal tubules. Co-localization of CAXII with CAIX or HIF-1α was not observed.

Conclusion

The study has showed that: 1) HIF-1α and CAIX expression co- localized in many, but not all, of the embryonic and early fetal tissues; 2) There is no evidence of co-localization of CAIX and CAXII; 3) CAIX and CAXII expression is closely related to cell origin and secretory activity involving proton transport, respectively. The intriguing finding of rare CAIX-expressing cells in those sites corresponding to stem cell niches requires further investigation.

An analysis of expression patterns of genes encoding proteins with catalytic activities

BACKGROUND

In situ hybridization (ISH) is a powerful method for visualizing gene expression patterns at the organismal level with cellular resolution. When automated, it is capable of determining the expression of a large number of genes.

RESULTS

The expression patterns of 662 genes that encode enzymes were determined by ISH in the mid-gestation mouse embryo, a stage that models the complexity of the adult organism. Forty-five percent of transcripts encoding metabolic enzymes (n = 297) showed a regional expression pattern. A similar percentage was found for the 190 kinases that were also analyzed. Many mRNAs encoding glycolytic and TCA cycle enzymes exhibited a characteristic expression pattern. The annotated expression patterns were deposited on the Genepaint database and are retrievable by user-defined queries including gene name and sites of expression.

CONCLUSION

The 662 expression patterns discussed here comprised gene products with activities associated with catalysis. Preliminary analysis of these data revealed that a significant number of genes encoding housekeeping functions such as biosynthesis and catabolism were expressed regionally, so they could be used as tissue-specific gene markers. We found no difference in tissue specificity between mRNAs encoding housekeeping functions and those encoding components of signal transduction pathways, as exemplified by the kinases.

Water and solute secretion by the choroid plexus

The cerebrospinal fluid (CSF) provides mechanical and chemical protection of the brain and spinal cord. This review focusses on the contribution of the choroid plexus epithelium to the water and salt homeostasis of the CSF, i.e. the secretory processes involved in CSF formation. The choroid plexus epithelium is situated in the ventricular system and is believed to be the major site of CSF production. Numerous studies have identified transport processes involved in this secretion, and recently, the underlying molecular background for some of the mechanisms have emerged. The nascent CSF consists mainly of NaCl and NaHCO(3), and the production rate is strictly coupled to the rate of Na(+) secretion. In contrast to other secreting epithelia, Na(+) is actively pumped across the luminal surface by the Na(+),K(+)-ATPase with possible contributions by other Na(+) transporters, e.g. the luminal Na(+),K(+),2Cl(-) cotransporter. The Cl(-) and HCO(3) (-) ions are likely transported by a luminal cAMP activated inward rectified anion conductance, although the responsible proteins have not been identified. Whereas Cl(-) most likely enters the cells through anion exchange, the functional as well as the molecular basis for the basolateral Na(+) entry are not yet well-defined. Water molecules follow across the epithelium mainly through the water channel, AQP1, driven by the created ionic gradient. In this article, the implications of the recent findings for the current model of CSF secretion are discussed. Finally, the clinical implications and the prospects of future advances in understanding CSF production are briefly outlined.