Carbonic anhydrase XII mRNA encodes a hydratase that is differentially expressed along the rabbit nephron

Molecular Cloning and Characterization of Carbonic Anhydrase XII from Pufferfish (Takifugu rubripes)

In this study, an 1888-bp carbonic anhydrase XII (CA XII) sequence was cloned from the brain of the pufferfish, Takifugu rubripes. The cloned sequence contained a coding region of 1470-bp, which was predicted to translate into a protein of 490 amino acid residues. The predicted protein showed between 68–56% identity with the large yellow croaker (Larimichthys crocea), tilapia (Oreochromis niloticus), and Asian arowana (Scleropages formosus) CA XII proteins. It also exhibited 36% and 53% identity with human CA II and CA XII, respectively. The cloned sequence contained a 22 amino acid NH₂-terminal signal sequence and three Asn-Xaa-Ser/Thr sequons, among which one was potentially glycosylated. Four cysteine residues were also identified (Cys-21, Cys-201, Cys-355, and Cys-358), two of which (Cys-21 and Cys-201) could potentially form a disulfide bond. A 22-amino acid COOH-terminal cytoplasmic tail containing a potential site for phosphorylation by protein kinase A was also found. The cloned sequence might be a transmembrane protein, as predicted from in silico and phylogenetic analyses. The active site analysis of the predicted protein showed that its active site residues were highly conserved with tilapia CA XII protein. Homology modeling of the pufferfish CA XII was done using the crystal structure of the extracellular domain of human carbonic anhydrase XII at 1.55 Å resolution as a template. Semi-quantitative reverse transcription (RT)-PCR, quantitative PCR (q-PCR), and in situ hybridization confirmed that pufferfish CA XII is highly expressed in the brain.

Lithium induces aerobic glycolysis and glutaminolysis in collecting duct principal cells

Lithium, given to bipolar disorder patients, causes nephrogenic diabetes insipidus (Li-NDI), a urinary-concentrating defect. Li-NDI occurs due to downregulation of principal cell AQP2 expression, which coincides with principal cell proliferation. The metabolic effect of lithium on principal cells, however, is unknown and investigated here. In earlier studies, we showed that the carbonic anhydrase (CA) inhibitor acetazolamide attenuated Li-induced downregulation in mouse-collecting duct (mpkCCD) cells. Of the eight CAs present in mpkCCD cells, siRNA and drug treatments showed that downregulation of CA9 and to some extent CA12 attenuated Li-induced AQP2 downregulation. Moreover, lithium induced cell proliferation and increased the secretion of lactate. Lithium also increased urinary lactate levels in wild-type mice that developed Li-NDI but not in lithium-treated mice lacking ENaC, the principal cell entry site for lithium. Inhibition of aerobic glycolysis with 2-deoxyglucose (2DG) attenuated lithium-induced AQP2 downregulation in mpkCCD cells but did not attenuate Li-NDI in mice. Interestingly, NMR analysis demonstrated that lithium also increased the urinary succinate, fumarate, citrate, and NH₄⁺ levels, which were, in contrast to lactate, not decreased by 2DG. Together, our data reveal that lithium induces aerobic glycolysis and glutaminolysis in principal cells and that inhibition of aerobic glycolysis, but not the glutaminolysis, does not attenuate Li-NDI.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Membrane-bound carbonic anhydrases in osteoclasts

Osteoclasts are multinucleated bone-resorbing cells that use multiple pH regulation mechanisms to create an acidic pH in the resorption lacuna. Carbonic anhydrase II and vacuolar H(+)-ATPases produce and transport protons, while chloride channels provide a Cl(-) flux into the resorption site. These activities are required for inorganic matrix dissolution that precedes enzymatic removal of organic bone matrix. In other cell types it has become evident that carbonic anhydrase isoenzymes interact with AE proteins to form transport metabolons that regulate intracellular pH. Membrane-bound carbonic anhydrase isoenzymes may also compensate for the lack of cytoplasmic carbonic anhydrase II. Therefore, our goal was to explore the expression of membrane-bound carbonic anhydrase (CA) isoenzymes CA IV, CA IX, CA XII and CA XIV in bone-resorbing osteoclasts. Immunohistochemistry and confocal microscopy showed expression of CA IV, CA XII and CA XIV in cultured rat and human osteoclasts. To confirm these results, RT-PCR was used. Immunohistochemistry revealed distinct staining patterns for CA IV, CA XII and CA XIV in rat trabecular bone specimens. A plasma membrane staining was observed in bone lining cells with the CA XII antibody while osteoclast plasma membranes were stained with CA IV and CA XIV antibodies. Confocal microscopy of cultured human osteoclasts showed a punctated intracellular CA IV staining and a perinuclear CA XIV staining while no CA IX or CA XII staining was observed. To evaluate the physiological role of membrane-bound CAs in osteoclasts, we used PCS, a novel membrane-impermeable CA inhibitor. Increased osteoclast number and bone resorption activity was observed in rat osteoclast cultures exposed to a low concentration of PCS while higher concentrations affected cell survival. PCS treatment also disturbed intracellular acidification in osteoclasts, as determined by live cell microscopy. In conclusion, our data shows that membrane-bound carbonic anhydrase isoenzymes CA IV and CA XIV are expressed both at mRNA and protein levels in osteoclasts in vivo and in vitro. In addition, the inhibitor experiments provide novel evidence to support the hypothesis that intracellular pH regulation in osteoclasts may indeed involve transport metabolons.

Expression of membrane-associated carbonic anhydrase isoforms IV, IX, XII, and XIV in the rabbit: induction of CA IV and IX during maturation

Several carbonic anhydrase (CA) isoforms are associated with plasma membranes. It is probable that these enzymes interact with anion transporters to facilitate the movement of HCO3- into or out of the cell. A better knowledge of CA isoform expression in a given tissue would facilitate a systematic examination of any associations with such transporters. We examined the expression of CAs IV, IX, XII, and XIV mRNAs in rabbit tissues, including kidney, heart, lung, skeletal muscle, liver, pancreas, gall bladder, stomach, small intestine, colon, and spleen, using quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). CA IV mRNA was mainly in kidney, heart, lung, colon, and gall bladder. CA IX mRNA was restricted to stomach, gall bladder, duodenum, and early jejunum. CA XII mRNA was found in kidney and colon. CA XIV mRNA was localized to heart, lung, skeletal muscle, and liver. The data indicate that there are different patterns of CA expression in various tissues: CA IX was expressed in the proximal gastrointestinal tract, whereas CA XII and CA IV were more distal. CA IV and CA XII are important kidney isoforms. CA XIV was abundant in metabolically active tissues such as liver, heart, lung, and skeletal muscle. Some significant species differences were noted in the expression of some of these isoforms; for example, CA XIV is not expressed in rabbit kidney, despite being abundant in mouse kidney. Maturational studies showed that the expression of CA IX mRNA and protein increased markedly with weaning ( approximately 3-4 postnatal wk) and was well correlated with the maturational expression of the alpha-subunit of the gastric H+,K+-ATPase, suggesting that function of CA IX and the gastric H+ pump might be linked in the digestion of adult foodstuffs. The unique pattern of membrane-bound CA isoforms suggests different functional associations with transporters, depending on the physiological demands on the tissue.