Mutational and functional analysis of SLC4A4 in a patient with proximal renal tubular acidosis

Left Ventricular Systolic Dysfunction in NBCe1-B/C-Knockout Mice

Congenital proximal renal tubular acidosis (pRTA) is a rare systemic disease caused by mutations in the SLC4A4 gene that encodes the electrogenic sodium bicarbonate cotransporter, NBCe1. The major NBCe1 protein variants are designated NBCe1-A, NBCe1-B, and NBCe1-C. NBCe1-A expression is kidney-specific, NBCe1-B is broadly expressed and is the only NBCe1 variant expressed in the heart, and NBCe1-C is a splice variant of NBCe1-B that is expressed in the brain. No cardiac manifestations have been reported from patients with pRTA, but studies in adult rats with virally induced reduction in cardiac NBCe1-B expression indicate that NBCe1-B loss leads to cardiac hypertrophy and prolonged QT intervals in rodents. NBCe1-null mice die shortly after weaning, so the consequence of congenital, global NBCe1 loss on the heart is unknown. To circumvent this issue, we characterized the cardiac function of NBCe1-B/C-null (KOb/c) mice that survive up to 2 months of age and which, due to the uninterrupted expression of NBCe1-A, do not exhibit the confounding acidemia of the globally null mice. In contrast to the viral knockdown model, cardiac hypertrophy was not present in KOb/c mice as assessed by heart-weight-to-body-weight ratios and cardiomyocyte cross-sectional area. However, echocardiographic analysis revealed reduced left ventricular ejection fraction, and intraventricular pressure-volume measurements demonstrated reduced load-independent contractility. We also observed increased QT length variation in KOb/c mice. Finally, using the calcium indicator Fura-2 AM, we observed a significant reduction in the amplitude of Ca2+ transients in paced KOb/c cardiomyocytes. These data indicate that congenital, global absence of NBCe1-B/C leads to impaired cardiac contractility and increased QT length variation in juvenile mice. It remains to be determined whether the cardiac phenotype in KOb/c mice is influenced by the absence of NBCe1-B/C from neuronal and endocrine tissues.

The role of Na+-coupled bicarbonate transporters (NCBT) in health and disease

Cellular and organism survival depends upon the regulation of pH, which is regulated by highly specialized cell membrane transporters, the solute carriers (SLC) (For a comprehensive list of the solute carrier family members, see: https://www.bioparadigms.org/slc/ ). The SLC4 family of bicarbonate (HCO3-) transporters consists of ten members, sorted by their coupling to either sodium (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE), chloride (AE1, AE2, AE3), or borate (BTR1). The ionic coupling of SLC4A9 (AE4) remains controversial. These SLC4 bicarbonate transporters may be controlled by cellular ionic gradients, cellular membrane voltage, and signaling molecules to maintain critical cellular and systemic pH (acid-base) balance. There are profound consequences when blood pH deviates even a small amount outside the normal range (7.35-7.45). Chiefly, Na+-coupled bicarbonate transporters (NCBT) control intracellular pH in nearly every living cell, maintaining the biological pH required for life. Additionally, NCBTs have important roles to regulate cell volume and maintain salt balance as well as absorption and secretion of acid-base equivalents. Due to their varied tissue expression, NCBTs have roles in pathophysiology, which become apparent in physiologic responses when their expression is reduced or genetically deleted. Variations in physiological pH are seen in a wide variety of conditions, from canonically acid-base related conditions to pathologies not necessarily associated with acid-base dysfunction such as cancer, glaucoma, or various neurological diseases. The membranous location of the SLC4 transporters as well as recent advances in discovering their structural biology makes them accessible and attractive as a druggable target in a disease context. The role of sodium-coupled bicarbonate transporters in such a large array of conditions illustrates the potential of treating a wide range of disease states by modifying function of these transporters, whether that be through inhibition or enhancement.

A novel I551F variant of the Na+/HCO3- cotransporter NBCe1-A shows reduced cell surface expression, resulting in diminished transport activity

Homozygous mutations in SLC4A4, which encodes the electrogenic Na⁺/[Formula: see text] cotransporter (NBCe1), cause proximal renal tubular acidosis associated with extrarenal symptoms. Although 17` mutated sites in SLC4A4 have thus far been identified among patients with proximal renal tubular acidosis, the physiological significance of other nonsynonymous single-nucleotide variants (SNVs) remains largely undetermined. Here, we investigated the functional properties of SNVs in NBCe1. From the National Center for Biotechnology Information dbSNP database, we identified 13 SNVs that have not previously been characterized in the highly conserved, transmembrane domains of NBCe1-A. Immunocytochemical analysis revealed that the I551F variant was present predominantly in the cytoplasm in human embryonic kidney (HEK)-293 cells, whereas all other SNVs did not show as dramatic a change in subcellular distribution. Western blot analysis in HEK-293 cells demonstrated that the I551F variant showed impaired glycosylation and a 69% reduction in cell surface levels. To determine the role of I551 in more detail, we examined the significance of various artificial mutants in both nonpolarized HEK-293 cells and polarized Madin-Darby canine kidney cells, which indicated that only I551F substitution resulted in cytoplasmic retention. Moreover, functional analysis using Xenopus oocytes demonstrated that the I551F variant had a significantly reduced activity corresponding to 39% of that of the wild-type, whereas any other SNVs and artificial I551 mutants did not show significant changes in activity. Finally, immunofluorescence experiments in HEK-293 cells indicated that the I551F variant retained wild-type NBCe1-A in the cytoplasm. These data demonstrate that the I551F variant of NBCe1-A shows impaired transport activity predominantly through cytoplasmic retention and suggest that the variant can have a dominant negative effect by forming complexes with wild-type NBCe1-A.NEW & NOTEWORTHY Electrogenic Na⁺/[Formula: see text] cotransporter 1-A (NBCe1-A) in the proximal tubule regulates the acid/base balance and fluid volume homeostasis. From the National Center for Biotechnology Information dbSNP database, we identified the I551F variant of NBCe1-A, which showed reduced glycosylation, cell surface expression, and transport activity. We also found that the I551F variant can exert a dominant negative effect on wild-type NBCe1-A, suggesting its physiological significance.

Epilepsy, status epilepticus, and hemiplegic migraine coexisting with a novel SLC4A4 mutation

BACKGROUND

Recessive mutations in the SLC4A4 gene cause a syndrome characterised by proximal renal tubular acidosis (pRTA), mental retardation, dental and ocular abnormalities, and hemiplegic migraine. Rare cases involving the development of epilepsy or its severe complication-status epilepticus-have been described.

METHODS

The clinical and genetic status of four affected members in a Spanish family was studied. The SLC4A4 gene mutation was detected with a next-generation sequencing (NGS) panel in the proband, and Sanger confirmed the putative mutations in affected relatives. In silico analysis was performed to elucidate the putative effect of mutation on the splicing process.

RESULTS

A novel mutation, c.2562+2T>G, was identified in the homozygous state in all diseased members of the family. This mutation affected a canonical splice site and is predicted to abolish the wild-type donor site, which predicts a premature truncated NBCe1 protein with cotransport activity. The resulting protein lacks the 190 amino acids of the carboxyl-terminus, and the effect is likely to be a loss of function. All patients suffered from severe pRTA and ocular abnormalities, and the adults also suffered from neurological complications, such as hemiplegic migraine and/or epilepsy. Two developed life-threatening status epilepticus, although they fully recovered and remained free of seizures with valproate.

CONCLUSION

These results expand the clinical and mutational spectra of SLC4A4-related disease and have implications for understanding the potential role of NBCe1 in the pathophysiologic processes of hemiplegic migraine and epilepsy/status epilepticus associated with the mutation.

Carbonic anhydrases enhance activity of endogenous Na-H exchangers and not the electrogenic Na/HCO3 cotransporter NBCe1-A, expressed in Xenopus oocytes

KEY POINTS

According to the HCO 3 - metabolon hypothesis, direct association of cytosolic carbonic anhydrases (CAs) with the electrogenic Na/HCO3 cotransporter NBCe1-A speeds transport by regenerating/consuming HCO 3 - . The present work addresses published discrepancies as to whether cytosolic CAs stimulate NBCe1-A, heterologously expressed in Xenopus oocytes. We confirm the essential elements of the previous experimental observations, taken as support for the HCO 3 - metabolon hypothesis. However, using our own experimental protocols or those of others, we find that NBCe1-A function is unaffected by cytosolic CAs. Previous conclusions that cytosolic CAs do stimulate NBCe1-A can be explained by an unanticipated stimulatory effect of the CAs on an endogenous Na-H exchanger. Theoretical analyses show that, although CAs could stimulate non- HCO 3 - transporters (e.g. Na-H exchangers) by accelerating CO2 / HCO 3 - -mediated buffering of acid-base equivalents, they could not appreciably affect transport rates of NBCe1 or other transporters carrying HCO 3 - , CO 3 = , or NaCO 3 - ion pairs.

ABSTRACT

The HCO 3 - metabolon hypothesis predicts that cytosolic carbonic anhydrase (CA) binds to NBCe1-A, promotes HCO 3 - replenishment/consumption, and enhances transport. Using a short step-duration current-voltage (I-V) protocol with Xenopus oocytes expressing eGFP-tagged NBCe1-A, our group reported that neither injecting human CA II (hCA II) nor fusing hCA II to the NBCe1-A carboxy terminus affects background-subtracted NBCe1 slope conductance (GNBC ), which is a direct measure of NBCe1-A activity. Others - using bovine CA (bCA), untagged NBCe1-A, and protocols keeping holding potential (Vh ) far from NBCe1-A's reversal potential (Erev ) for prolonged periods - found that bCA increases total membrane current (ΔIm ), which apparently supports the metabolon hypothesis. We systematically investigated differences in the two protocols. In oocytes expressing untagged NBCe1-A, injected with bCA and clamped to -40 mV, CO2 / HCO 3 - exposures markedly decrease Erev , producing large transient outward currents persisting for >10 min and rapid increases in [Na+ ]i . Although the CA inhibitor ethoxzolamide (EZA) reduces both ΔIm and d[Na+ ]i /dt, it does not reduce GNBC . In oocytes not expressing NBCe1-A, CO2 / HCO 3 - triggers rapid increases in [Na+ ]i that both hCA II and bCA enhance in concentration-dependent manners. These d[Na+ ]i /dt increases are inhibited by EZA and blocked by EIPA, a Na-H exchanger (NHE) inhibitor. In oocytes expressing untagged NBCe1-A and injected with bCA, EIPA abolishes the EZA-dependent decreases in ΔIm and d[Na+ ]i /dt. Thus, CAs/EZA produce their ΔIm and d[Na+ ]i /dt effects not through NBCe1-A, but endogenous NHEs. Theoretical considerations argue against a CA stimulation of HCO 3 - transport, supporting the conclusion that an NBCe1-A- HCO 3 - metabolon does not exist in oocytes.

Transcriptomic response to three osmotic stresses in gills of hybrid tilapia (Oreochromis mossambicus female × O. urolepis hornorum male)

Background

Osmotic stress is a widespread phenomenon in aquatic animal. The ability to cope with salinity stress and alkaline stress is quite important for the survival of aquatic species under natural conditions. Tilapia is an important commercial euryhaline fish species. What’s more tilapia is a good experimental material for osmotic stress regulation research, but the molecular regulation mechanism underlying different osmotic pressure of tilapia is still unexplored.

Results

To elucidate the osmoregulation strategy behind its hyper salinity, alkalinity and salinity-alkalinity stress of tilapia, the transcriptomes of gills in hybrid tilapia (Oreochromis mossambicus ♀ × O. urolepis hornorum ♂) under salinity stress (S: 25‰), alkalinity stress(A: 4‰) and salinity-alkalinity stress (SA: S: 15‰, A: 4‰) were sequenced using deep-sequencing platform Illumina/HiSeq-2000 and differential expression genes (DEGs) were identified. A total of 1958, 1472 and 1315 upregulated and 1824, 1940 and 1735 downregulated genes (P-value < 0.05) were identified in the salt stress, alkali stress and saline-alkali stress groups, respectively, compared with those in the control group. Furthermore, Kyoto Encyclopedia of Genes and Genomes pathway analyses were conducted in the significant different expression genes. In all significant DEGs, some of the typical genes involved in osmoregulation, including carbonic anhydrase (CA), calcium/calmodulin-dependent protein kinase (CaM kinase) II (CAMK2), aquaporin-1(AQP1), sodium bicarbonate cotransporter (SLC4A4/NBC1), chloride channel 2(CLCN2), sodium/potassium/chloride transporter (SLC12A2 / NKCC1) and other osmoregulation genes were also identified. RNA-seq results were validated with quantitative real-time PCR (qPCR), the 17 random selected genes showed a consistent direction in both RNA-Seq and qPCR analysis, demonstrated that the results of RNA-seq were reliable.

Conclusions

The present results would be helpful to elucidate the osmoregulation mechanism of aquatic animals adapting to saline-alkali challenge. This study provides a global overview of gene expression patterns and pathways that related to osmoregulation in hybrid tilapia, and could contribute to a better understanding of the molecular regulation mechanism in different osmotic stresses.

Regulation of renal NaDC1 expression and citrate excretion by NBCe1-A

Citrate is critical for acid-base homeostasis and to prevent calcium nephrolithiasis. Both metabolic acidosis and hypokalemia decrease citrate excretion and increase expression of Na+-dicarboxylate cotransporter 1 (NaDC1; SLC13A2), the primary protein involved in citrate reabsorption. However, the mechanisms transducing extracellular signals and mediating these responses are incompletely understood. The purpose of the present study was to determine the role of the Na+-coupled electrogenic bicarbonate cotransporter (NBCe1) A variant (NBCe1-A) in citrate metabolism under basal conditions and in response to acid loading and hypokalemia. NBCe1-A deletion increased citrate excretion and decreased NaDC1 expression in the proximal convoluted tubules (PCT) and proximal straight tubules (PST) in the medullary ray (PST-MR) but not in the PST in the outer medulla (PST-OM). Acid loading wild-type (WT) mice decreased citrate excretion. NaDC1 expression increased only in the PCT and PST-MR and not in the PST-MR. In NBCe1-A knockout (KO) mice, the acid loading change in citrate excretion was unaffected, changes in PCT NaDC1 expression were blocked, and there was an adaptive increase in PST-MR. Hypokalemia in WT mice decreased citrate excretion; NaDC1 expression increased only in the PCT and PST-MR. NBCe1-A KO blocked both the citrate and NaDC1 changes. We conclude that 1) adaptive changes in NaDC1 expression in response to metabolic acidosis and hypokalemia occur specifically in the PCT and PST-MR, i.e., in cortical proximal tubule segments; 2) NBCe1-A is necessary for normal basal, metabolic acidosis and hypokalemia-stimulated citrate metabolism and does so by regulating NaDC1 expression in cortical proximal tubule segments; and 3) adaptive increases in PST-OM NaDC1 expression occur in NBCe1-A KO mice in response to acid loading that do not occur in WT mice.

Extrarenal Signs of Proximal Renal Tubular Acidosis Persist in Nonacidemic Nbce1b/c-Null Mice

BACKGROUND

The SLC4A4 gene encodes electrogenic sodium bicarbonate cotransporter 1 (NBCe1). Inheritance of recessive mutations in SLC4A4 causes proximal renal tubular acidosis (pRTA), a disease characterized by metabolic acidosis, growth retardation, ocular abnormalities, and often dental abnormalities. Mouse models of pRTA exhibit acidemia, corneal edema, weak dental enamel, impacted colons, nutritional defects, and a general failure to thrive, rarely surviving beyond weaning. Alkali therapy remains the preferred treatment for pRTA, but it is unclear which nonrenal signs are secondary to acidemia and which are a direct consequence of NBCe1 loss from nonrenal sites (such as the eye and enamel organ) and therefore require separate therapy. SLC4A4 encodes three major NBCe1 variants: NBCe1-A, NBCe1-B, and NBCe1-C. NBCe1-A is expressed in proximal tubule epithelia; its dysfunction causes the plasma bicarbonate insufficiency that underlies acidemia. NBCe1-B and NBCe1-C exhibit a broad extra-proximal-tubular distribution.

METHODS

To explore the consequences of Nbce1b/c loss in the absence of acidemia, we engineered a novel strain of Nbce1b/c-null mice and assessed them for signs of pRTA.

RESULTS

Nbce1b/c-null mice have normal blood pH, but exhibit increased mortality, growth retardation, corneal edema, and tooth enamel defects.

CONCLUSIONS

The correction of pRTA-related acidemia should not be considered a panacea for all signs of pRTA. The phenotype of Nbce1b/c-null mice highlights the physiologic importance of NBCe1 variants expressed beyond the proximal tubular epithelia and potential limitations of pH correction by alkali therapy in pRTA. It also suggests a novel genetic locus for corneal dystrophy and enamel hypomineralization without acidemia.

How pH is regulated during amelogenesis in dental fluorosis

Amelogenesis is a complicated process that concerns the interaction between growing hydroxyapatite crystals and extracellular proteins, which requires the tight regulation of pH. In dental fluorosis, the balance of pH regulation is broken, leading to abnormal mineralization. The current review focuses on the electrolyte transport processes associated with pH homeostasis, particularly regarding the changes in ion transporters that occur during amelogenesis, following exposure to excessive fluoride. Furthermore, the possible mechanism of fluorosis is discussed on the basis of acid hypothesis. There are two main methods by which F^- accelerates crystal formation in ameloblasts. Firstly, it induces the release of protons, lowering the pH of the cell microenvironment. The decreased pH stimulates the upregulation of ion transporters, which attenuates further declines in the pH. Secondly, F^- triggers an unknown signaling pathway, causing changes in the transcription of ion transporters and upregulating the expression of bicarbonate transporters. This results in the release of a large amount of bicarbonate from ameloblasts, which may neutralize the pH to form a microenvironment that favors crystal nucleation. The decreased pH stimulates the diffusion of F^- into the cytoplasm of amelobalsts along the concentration gradient formed by the release of protons. The retention of F^- causes a series of pathological changes, including oxidative and endoplasmic reticulum stress. If the buffering capacity of ameloblasts facing F^- toxicity holds, normal mineralization occurs; however, if F^- levels are high enough to overwhelm the buffering capacity of ameloblasts, abnormal mineralization occurs, leading to dental fluorosis.

SLC4A4 compound heterozygous mutations in exon-intron boundary regions presenting with severe proximal renal tubular acidosis and extrarenal symptoms coexisting with Turner's syndrome: a case report

Background

Congenital NBCe1A deficiency with the SLC4A4 mutation causes severe proximal renal tubular acidosis, which often comprises extrarenal symptoms, such as intellectual disability and developmental delay, glaucoma, cataract and band keratopathy. To date, almost all mutations have been found to be homozygous mutations located in exons.

Case presentation

We performed direct nucleotide sequencing analysis of exons and exon–intron boundary regions of the SLC4A4 in a patient presenting with severe renal proximal tubule acidosis, glaucoma and intellectual disability and her parents without these signs. The examination revealed compound heterozygous mutations in exon–intron boundary regions, c.1076 + 3A > C and c.1772 − 2A > T, neither of which have been reported previously. While the former mutation was found in the mother, the latter was found in the father. The transcript of the SLC4A4 gene was almost undetectable, and the patient was also diagnosed with Turner’s syndrome.

Conclusions

We identified two novel SLC4A4 mutations, c.1076 + 3A > C and c.1772 − 2A > T. When presented in a compound heterozygous state, these mutations caused a phenotype of severe renal proximal tubular acidosis along with glaucoma and mental retardation. This is the first report of congenital proximal renal tubular acidosis carrying compound heterozygous SLC4A4 mutations in exon–intron boundary regions. We suggest that an mRNA surveillance mechanism, nonsense-mediated RNA decay, following aberrant splicing was the reason that the SLC4A4 transcript was almost undetectable in the proband.

NBCe1-A Regulates Proximal Tubule Ammonia Metabolism under Basal Conditions and in Response to Metabolic Acidosis

Renal ammonia metabolism is the primary mechanism through which the kidneys maintain acid-base homeostasis, but the molecular mechanisms regulating renal ammonia generation are unclear. In these studies, we evaluated the role of the proximal tubule basolateral plasma membrane electrogenic sodium bicarbonate cotransporter 1 variant A (NBCe1-A) in this process. Deletion of the NBCe1-A gene caused severe spontaneous metabolic acidosis in mice. Despite this metabolic acidosis, which normally causes a dramatic increase in ammonia excretion, absolute urinary ammonia concentration was unaltered. Additionally, NBCe1-A deletion almost completely blocked the ability to increase ammonia excretion after exogenous acid loading. Under basal conditions and during acid loading, urine pH was more acidic in mice with NBCe1-A deletion than in wild-type controls, indicating that the abnormal ammonia excretion was not caused by a primary failure of urine acidification. Instead, NBCe1-A deletion altered the expression levels of multiple enzymes involved in proximal tubule ammonia generation, including phosphate-dependent glutaminase, phosphoenolpyruvate carboxykinase, and glutamine synthetase, under basal conditions and after exogenous acid loading. Deletion of NBCe1-A did not impair expression of key proteins involved in collecting duct ammonia secretion. These studies demonstrate that the integral membrane protein NBCe1-A has a critical role in basal and acidosis-stimulated ammonia metabolism through the regulation of proximal tubule ammonia-metabolizing enzymes.

The role of disease-linked residue glutamine-913 in support of the structure and function of the human electrogenic sodium/bicarbonate cotransporter NBCe1-A

Mutations in the sodium bicarbonate cotransporter NBCe1 (SLC4A4) cause proximal renal tubular acidosis (pRTA). We recently described a novel pRTA mutation p.Gln913Arg (Q913R), inherited in compound heterozygous form with p.Arg510His (R510H). Q913R causes intracellular retention of NBCe1 and a 'gain of function' Cl- leak. To learn more about the importance of glutamine at position 913, we substituted a variety of alternative amino-acid residues (Cys, Glu, Lys, Leu, Ser) at position 913. Studying cRNA-injected Xenopus oocytes by voltage clamp, we find that most de novo mutants exhibit close-to-normal NBCe1 activity; only Q913K expresses a Cl- leak. Studying transiently-transfected, polarised kidney cells by fluorescence microscopy we find that most de novo mutants (except Q913E) are intracellularly retained. A 3D homology model predicts that Gln913 is located in the gating domain of NBCe1 and neighbours the 3D space occupied by another pRTA-associated residue (Arg881), highlighting an important and conformationally-sensitive region of NBCe1. We conclude that the intracellular retention of Q913R is caused by the loss of Gln at position 913, but that the manifestation of the Cl- leak is related to the introduction of Arg at position 913. Our findings will inform future studies to elucidate the nature and the consequences of the leak.

Mouse models of SLC4-linked disorders of HCO3--transporter dysfunction

The SLC4 family Cl^-/[Formula: see text] cotransporters (NBCe1, NBCe2, NBCn1, and NBCn2) contribute to a variety of vital physiological processes including pH regulation and epithelial fluid secretion. Accordingly, their dysfunction can have devastating effects. Disorders such as epilepsy, hemolytic anemia, glaucoma, hearing loss, osteopetrosis, and renal tubular acidosis are all genetically linked to SLC4-family gene loci. This review summarizes how studies of Slc4-modified mice have enhanced our understanding of the etiology of SLC4-linked pathologies and the interpretation of genetic linkage studies. The review also surveys the novel disease signs exhibited by Slc4-modified mice which could either be considered to presage their description in humans, or to highlight interspecific differences. Finally, novel Slc4-modified mouse models are proposed, the study of which may further our understanding of the basis and treatment of SLC4-linked disorders of [Formula: see text]-transporter dysfunction.

Bicarbonate Transport During Enamel Maturation

Amelogenesis (tooth enamel formation) is a biomineralization process consisting primarily of two stages (secretory stage and maturation stage) with unique features. During the secretory stage, the inner epithelium of the enamel organ (i.e., the ameloblast cells) synthesizes and secretes enamel matrix proteins (EMPs) into the enamel space. The protein-rich enamel matrix forms a highly organized architecture in a pH-neutral microenvironment. As amelogenesis transitions to maturation stage, EMPs are degraded and internalized by ameloblasts through endosomal-lysosomal pathways. Enamel crystallite formation is initiated early in the secretory stage, however, during maturation stage the more rapid deposition of calcium and phosphate into the enamel space results in a rapid expansion of crystallite length and mineral volume. During maturation-stage amelogenesis, the pH value of enamel varies considerably from slightly above neutral to acidic. Extracellular acid-base balance during enamel maturation is tightly controlled by ameloblast-mediated regulatory networks, which include significant synthesis and movement of bicarbonate ions from both the enamel papillary layer cells and ameloblasts. In this review we summarize the carbonic anhydrases and the carbonate transporters/exchangers involved in pH regulation in maturation-stage amelogenesis. Proteins that have been shown to be instrumental in this process include CA2, CA6, CFTR, AE2, NBCe1, SLC26A1/SAT1, SLC26A3/DRA, SLC26A4/PDS, SLC26A6/PAT1, and SLC26A7/SUT2. In addition, we discuss the association of miRNA regulation with bicarbonate transport in tooth enamel formation.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

The role of renal proximal tubule transport in the regulation of blood pressure

The electrogenic sodium/bicarbonate cotransporter 1 (NBCe1) on the basolateral side of the renal proximal tubule plays a pivotal role in systemic acid-base homeostasis. Mutations in the gene encoding NBCe1 cause severe proximal renal tubular acidosis accompanied by other extrarenal symptoms. The proximal tubule reabsorbs most of the sodium filtered in the glomerulus, contributing to the regulation of plasma volume and blood pressure. NBCe1 and other sodium transporters in the proximal tubule are regulated by hormones, such as angiotensin II and insulin. Angiotensin II is probably the most important stimulator of sodium reabsorption. Proximal tubule AT_1A receptor is crucial for the systemic pressor effect of angiotensin II. In rodents and rabbits, the effect on proximal tubule NBCe1 is biphasic; at low concentration, angiotensin II stimulates NBCe1 via PKC/cAMP/ERK, whereas at high concentration, it inhibits NBCe1 via NO/cGMP/cGKII. In contrast, in human proximal tubule, angiotensin II has a dose-dependent monophasic stimulatory effect via NO/cGMP/ERK. Insulin stimulates the proximal tubule sodium transport, which is IRS2-dependent. We found that in insulin resistance and overt diabetic nephropathy, stimulatory effect of insulin on proximal tubule transport was preserved. Our results suggest that the preserved stimulation of the proximal tubule enhances sodium reabsorption, contributing to the pathogenesis of hypertension with metabolic syndrome. We describe recent findings regarding the role of proximal tubule transport in the regulation of blood pressure, focusing on the effects of angiotensin II and insulin.

Ammonia Transporters and Their Role in Acid-Base Balance

Acid-base homeostasis is critical to maintenance of normal health. Renal ammonia excretion is the quantitatively predominant component of renal net acid excretion, both under basal conditions and in response to acid-base disturbances. Although titratable acid excretion also contributes to renal net acid excretion, the quantitative contribution of titratable acid excretion is less than that of ammonia under basal conditions and is only a minor component of the adaptive response to acid-base disturbances. In contrast to other urinary solutes, ammonia is produced in the kidney and then is selectively transported either into the urine or the renal vein. The proportion of ammonia that the kidney produces that is excreted in the urine varies dramatically in response to physiological stimuli, and only urinary ammonia excretion contributes to acid-base homeostasis. As a result, selective and regulated renal ammonia transport by renal epithelial cells is central to acid-base homeostasis. Both molecular forms of ammonia, NH₃ and NH₄⁺, are transported by specific proteins, and regulation of these transport processes determines the eventual fate of the ammonia produced. In this review, we discuss these issues, and then discuss in detail the specific proteins involved in renal epithelial cell ammonia transport.

A novel mutant Na+ /HCO3- cotransporter NBCe1 in a case of compound-heterozygous inheritance of proximal renal tubular acidosis

KEY POINTS

The inheritance of two defective alleles of SLC4A4, the gene that encodes the widely-expressed electrogenic sodium bicarbonate cotransporter NBCe1, results in the bicarbonate-wasting disease proximal renal tubular acidosis (pRTA). In the present study, we report the first case of compound-heterozygous inheritance of pRTA (p.Arg510His/p.Gln913Arg) in an individual with low blood pH, blindness and neurological signs that resemble transient ischaemic attacks. We employ fluorescence microscopy on non-polarized (human embryonic kidney) and polarized (Madin-Darby canine kidney) renal cell lines and electrophysiology on Xenopus oocytes to characterize the mutant transporters (R510H and Q913R). Both mutant transporters exhibit enhanced intracellular retention in renal cells, an observation that probably explains the HCO₃^- transport deficit in the individual. Both mutants retain a close-to-normal per molecule Na⁺ /HCO₃^- cotransport activity in Xenopus oocytes, suggesting that they are suitable candidates for folding-correction therapy. However, Q913R expression is uniquely associated with a depolarizing, HCO₃^- independent, Cl^- -conductance in oocytes that could have pathological consequences if expressed in the cells of patients.

ABSTRACT

Proximal renal tubular acidosis (pRTA) is a rare, recessively-inherited disease characterized by abnormally acidic blood, blindness, as well as below average height and weight. pRTA is typically associated with homozygous mutation of the solute carrier 4 family gene SLC4A4. SLC4A4 encodes the electrogenic sodium bicarbonate cotransporter NBCe1, a membrane protein that acts to maintain intracellular and plasma pH. We present the first description of a case of compound-heterozygous inheritance of pRTA. The individual has inherited two mutations in NBCe1: p.Arg510His (R510H) and p.Gln913Arg (Q913R), one from each parent. In addition to the usual features of pRTA, the patient exhibits unusual signs, such as muscle spasms and fever. We have recreated these mutant transporters for expression in model systems. We find that both of the mutant proteins exhibit substantial intracellular retention when expressed in mammalian renal cell lines. When expressed in Xenopus oocytes, we find that the R510H and Q913R-mutant NBCe1 molecules exhibit apparently normal Na⁺ /HCO₃^- cotransport activity but that Q913R is associated with an unusual HCO₃^- independent anion-leak. We conclude that a reduced accumulation of NBCe1 protein in the basolateral membrane of proximal-tubule epithelia is the most probable cause of pRTA in this case. We further note that the Q913R-associated anion-leak could itself be pathogenic if expressed in the plasma membrane of mammalian cells, compromising the benefit of strategies aiming to enhance mutant NBCe1 accumulation in the plasma membrane.

Acidosis and Urinary Calcium Excretion: Insights from Genetic Disorders

Metabolic acidosis is associated with increased urinary calcium excretion and related sequelae, including nephrocalcinosis and nephrolithiasis. The increased urinary calcium excretion induced by metabolic acidosis predominantly results from increased mobilization of calcium out of bone and inhibition of calcium transport processes within the renal tubule. The mechanisms whereby acid alters the integrity and stability of bone have been examined extensively in the published literature. Here, after briefly reviewing this literature, we consider the effects of acid on calcium transport in the renal tubule and then discuss why not all gene defects that cause renal tubular acidosis are associated with hypercalciuria and nephrocalcinosis.

NBCe1 expression is required for normal renal ammonia metabolism

The mechanisms regulating proximal tubule ammonia metabolism are incompletely understood. The present study addressed the role of the proximal tubule basolateral electrogenic Na(+)-coupled bicarbonate cotransporter (NBCe1; Slc4a4) in renal ammonia metabolism. We used mice with heterozygous and homozygous NBCe1 gene deletion and compared these mice with their wild-type littermates. Because homozygous NBCe1 gene deletion causes 100% mortality before day 25, we studied mice at day 8 (±1 day). Both heterozygous and homozygous gene deletion caused a gene dose-related decrease in serum bicarbonate. The ability to lower urinary pH was intact, and even accentuated, with NBCe1 deletion. However, in contrast to the well-known effect of metabolic acidosis to increase urinary ammonia excretion, NBCe1 deletion caused a gene dose-related decrease in ammonia excretion. There was no identifiable change in proximal tubule structure by light microscopy. Examination of proteins involved in renal ammonia metabolism showed decreased expression of phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase, key enzymes in proximal tubule ammonia generation, and increased expression of glutamine synthetase, which recycles intrarenal ammonia and regenerates glutamine. Expression of key proteins involved in ammonia transport outside of the proximal tubule (rhesus B glycoprotein and rhesus C glycoprotein) was not significantly changed by NBCe1 deletion. We conclude from these findings that NBCe1 expression is necessary for normal proximal tubule ammonia metabolism.

Cation-coupled bicarbonate transporters

Cation-coupled HCO3(-) transport was initially identified in the mid-1970s when pioneering studies showed that acid extrusion from cells is stimulated by CO2/HCO3(-) and associated with Na(+) and Cl(-) movement. The first Na(+)-coupled bicarbonate transporter (NCBT) was expression-cloned in the late 1990s. There are currently five mammalian NCBTs in the SLC4-family: the electrogenic Na,HCO3-cotransporters NBCe1 and NBCe2 (SLC4A4 and SLC4A5 gene products); the electroneutral Na,HCO3-cotransporter NBCn1 (SLC4A7 gene product); the Na(+)-driven Cl,HCO3-exchanger NDCBE (SLC4A8 gene product); and NBCn2/NCBE (SLC4A10 gene product), which has been characterized as an electroneutral Na,HCO3-cotransporter or a Na(+)-driven Cl,HCO3-exchanger. Despite the similarity in amino acid sequence and predicted structure among the NCBTs of the SLC4-family, they exhibit distinct differences in ion dependency, transport function, pharmacological properties, and interactions with other proteins. In epithelia, NCBTs are involved in transcellular movement of acid-base equivalents and intracellular pH control. In nonepithelial tissues, NCBTs contribute to intracellular pH regulation; and hence, they are crucial for diverse tissue functions including neuronal discharge, sensory neuron development, performance of the heart, and vascular tone regulation. The function and expression levels of the NCBTs are generally sensitive to intracellular and systemic pH. Animal models have revealed pathophysiological roles of the transporters in disease states including metabolic acidosis, hypertension, visual defects, and epileptic seizures. Studies are being conducted to understand the physiological consequences of genetic polymorphisms in the SLC4-members, which are associated with cancer, hypertension, and drug addiction. Here, we describe the current knowledge regarding the function, structure, and regulation of the mammalian cation-coupled HCO3(-) transporters of the SLC4-family.

MicroRNA 224 Regulates Ion Transporter Expression in Ameloblasts To Coordinate Enamel Mineralization

Enamel mineralization is accompanied by the release of protons into the extracellular matrix, which is buffered to regulate the pH value in the local microenvironment. The present study aimed to investigate the role of microRNA 224 (miR-224) as a regulator of SLC4A4 and CFTR, encoding the key buffering ion transporters, in modulating enamel mineralization. miR-224 was significantly downregulated as ameloblasts differentiated, in parallel with upregulation of SLC4A4 and CFTR. Overexpression of miR-224 downregulated SLC4A4 and CFTR expression in cultured human epithelial cells. A microRNA luciferase assay confirmed the specific binding of miR-224 to the 3' untranslated regions (UTRs) of SLC4A4 and CFTR mRNAs, thereby inhibiting protein translation. miR-224 agomir injection in mouse neonatal incisors resulted in normal enamel length and thickness, but with disturbed organization of the prism structure and deficient crystal growth. Moreover, the enamel Ca/P ratio and microhardness were markedly reduced after miR-224 agomir administration. These results demonstrate that miR-224 plays a pivotal role in fine tuning enamel mineralization by modulating SLC4A4 and CFTR to maintain pH homeostasis and support enamel mineralization.

Therapeutic effect of prenatal alkalization and PTC124 in Na(+)/HCO3(-) cotransporter 1 p.W516* knock-in mice

We created Na(+)/HCO3(-) cotransporter 1 (NBCe1) p.W516* knock-in mice as a model of isolated proximal renal tubular acidosis showing early lethality associated with severe metabolic acidosis to investigate the therapeutic effects of prenatal alkalization or posttranscriptional control 124 (PTC124). NBCe1(W516*/W516*) mice were treated with non-alkalization (control, n=12), prenatal alkalization postcoitus (prenatal group, n=7) and postnatal alkalization from postnatal day 6 (postnatal group, n=12). Mutation-specific therapy, PTC124 (60 mg kg(-1)) or gentamicin (30 mg kg(-1)), was administered intraperitoneally from postnatal day 6. Blood and urine biochemistry, acid-base analysis, survival rate and renal histology were examined. NBCe1 protein, mRNA abundance and activity ex vivo were assessed after PTC124 and gentamicin treatment. Prenatal group mice had similar initial body weight to wild-type mice and achieved significant weight gain thereafter compared with controls. They had higher serum bicarbonate level (15.5 ± 1.4 vs 5.5 ± 0.1 mmol l(-1), P<0.05) on postnatal day 14 and better renal function, histology and survival rates (60.8 ± 23.5 vs 41.1 ± 15.8 days, P<0.05) than the postnatal group. Compared with the control and gentamicin therapies, PTC124 therapy significantly increased NBCe1 protein abundance despite unchanged mRNA transcription. Only PTC124 therapy significantly increased survival rate and partially rescued NBCe1 activity ex vivo. In NBCe1(W516*/W516*) mice, prenatal alkali therapy achieved higher survival rates and ameliorated organ dysfunction. PTC124 therapy for this nonsense mutation was partially effective in increasing NBCe1 expression and activity.

Bicarbonate transport in health and disease

Bicarbonate (HCO3(-)) has a central place in human physiology as the waste product of mitochondrial energy production and for its role in pH buffering throughout the body. Because bicarbonate is impermeable to membranes, bicarbonate transport proteins are necessary to enable control of bicarbonate levels across membranes. In humans, 14 bicarbonate transport proteins, members of the SLC4 and SLC26 families, function by differing transport mechanisms. In addition, some anion channels and ZIP metal transporters contribute to bicarbonate movement across membranes. Defective bicarbonate transport leads to diseases, including systemic acidosis, brain dysfunction, kidney stones, and hypertension. Altered expression levels of bicarbonate transporters in patients with breast, colon, and lung cancer suggest an important role of these transporters in cancer.

Prevention of the disrupted enamel phenotype in Slc4a4-null mice using explant organ culture maintained in a living host kidney capsule

Slc4a4-null mice are a model of proximal renal tubular acidosis (pRTA). Slc4a4 encodes the electrogenic sodium base transporter NBCe1 that is involved in transcellular base transport and pH regulation during amelogenesis. Patients with mutations in the SLC4A4 gene and Slc4a4-null mice present with dysplastic enamel, amongst other pathologies. Loss of NBCe1 function leads to local abnormalities in enamel matrix pH regulation. Loss of NBCe1 function also results in systemic acidemic blood pH. Whether local changes in enamel pH and/or a decrease in systemic pH are the cause of the abnormal enamel phenotype is currently unknown. In the present study we addressed this question by explanting fetal wild-type and Slc4a4-null mandibles into healthy host kidney capsules to study enamel formation in the absence of systemic acidemia. Mandibular E11.5 explants from NBCe1^−/− mice, maintained in host kidney capsules for 70 days, resulted in teeth with enamel and dentin with morphological and mineralization properties similar to cultured NBCe1^+/+ mandibles grown under identical conditions. Ameloblasts express a number of proteins involved in dynamic changes in H⁺/base transport during amelogenesis. Despite the capacity of ameloblasts to dynamically modulate the local pH of the enamel matrix, at least in the NBCe1^−/− mice, the systemic pH also appears to contribute to the enamel phenotype. Extrapolating these data to humans, our findings suggest that in patients with NBCe1 mutations, correction of the systemic metabolic acidosis at a sufficiently early time point may lead to amelioration of enamel abnormalities.

Expression patterns and subcellular localization of carbonic anhydrases are developmentally regulated during tooth formation

Carbonic anhydrases (CAs) play fundamental roles in several physiological events, and emerging evidence points at their involvement in an array of disorders, including cancer. The expression of CAs in the different cells of teeth is unknown, let alone their expression patterns during odontogenesis. As a first step towards understanding the role of CAs during odontogenesis, we used immunohistochemistry, histochemistry and in situ hybridization to reveal hitherto unknown dynamic distribution patterns of eight CAs in mice. The most salient findings include expression of CAII/Car2 not only in maturation-stage ameloblasts (MA) but also in the papillary layer, dental papilla mesenchyme, odontoblasts and the epithelial rests of Malassez. We uncovered that the latter form lace-like networks around incisors; hitherto these have been known to occur only in molars. All CAs studied were produced by MA, however CAIV, CAIX and CARPXI proteins were distinctly enriched in the ruffled membrane of the ruffled MA but exhibited a homogeneous distribution in smooth-ended MA. While CAIV, CAVI/Car6, CAIX, CARPXI and CAXIV were produced by all odontoblasts, CAIII distribution displayed a striking asymmetry, in that it was virtually confined to odontoblasts in the root of molars and root analog of incisors. Remarkably, from initiation until near completion of odontogenesis and in several other tissues, CAXIII localized mainly in intracellular punctae/vesicles that we show to overlap with LAMP-1- and LAMP-2-positive vesicles, suggesting that CAXIII localizes within lysosomes. We showed that expression of CAs in developing teeth is not confined to cells involved in biomineralization, pointing at their participation in other biological events. Finally, we uncovered novel sites of CA expression, including the developing brain and eye, the olfactory epithelium, melanoblasts, tongue, notochord, nucleus pulposus and sebaceous glands. Our study provides important information for future single or multiple gene targeting strategies aiming at deciphering the function of CAs during odontogenesis.

Proximal renal tubular acidosis mediated by mutations in NBCe1-A: unraveling the transporter's structure-functional properties

NBCe1 belongs to the SLC4 family of base transporting membrane proteins that plays a significant role in renal, extrarenal, and systemic acid-base homeostasis. Recent progress has been made in characterizing the structure-function properties of NBCe1 (encoded by the SLC4A4 gene), and those factors that regulate its function. In the kidney, the NBCe1-A variant that is expressed on the basolateral membrane of proximal tubule is the key transporter responsible for overall transepithelial bicarbonate absorption in this nephron segment. NBCe1 mutations impair transepithelial bicarbonate absorption causing the syndrome of proximal renal tubular acidosis (pRTA). Studies of naturally occurring NBCe1 mutant proteins in heterologous expression systems have been very helpful in elucidation the structure-functional properties of the transporter. NBCe1 mutations are now known to cause pRTA by various mechanisms including the alteration of the transporter function (substrate ion interaction, electrogenicity), abnormal processing to the plasma membrane, and a perturbation in its structural properties. The elucidation of how NBCe1 mutations cause pRTA in addition to the recent studies which have provided further insight into the topology of the transporter have played an important role in uncovering its critically important structural-function properties.

Molecular mechanisms of renal and extrarenal manifestations caused by inactivation of the electrogenic Na(+)-HCO3 (-) cotransporter NBCe1

The electrogenic Na(+)-HCO3 (-) cotransporter NBCe1 plays an essential role in bicarbonate absorption from renal proximal tubules, but also mediates the other biological processes in extrarenal tissues such as bicarbonate secretion from pancreatic ducts, maintenance of tissue homeostasis in eye, enamel maturation in teeth, or local pH regulation in synapses. Homozygous mutation in NBCe1 cause proximal renal tubular acidosis (pRTA) associated with extrarenal manifestations such as short stature, ocular abnormalities, enamel abnormalities, and migraine. Functional analyses of NBCe1 mutants using different expression systems suggest that at least a 50% reduction of the transport activity may be required to induce severe pRTA. In addition to functional impairments, some NBCe1 mutants show trafficking defects. Some of the pRTA-related NBCe1 mutants showing the cytoplasmic retention have been shown to exert a dominant negative effect through hetero-oligomer complexes with wild-type NBCe1 that may explain the occurrence of extrarenal manifestations in the heterozygous carries of NBCe1 mutations. Both NBCe1 knockout (KO) and W516X knockin (KI) mice showed very severe pRTA and reproduced most of the clinical manifestations observed in human pRTA patients. Functional analysis on isolated renal proximal tubules from W516X KI mice directly confirmed the indispensable role of NBCe1 in bicarbonate absorption from this nephron segment. In this review, we will focus on the molecular mechanisms underling the renal and extrarenal manifestations caused by NBCe1 inactivation.

The SLC4 family of bicarbonate (HCO₃⁻) transporters

The SLC4 family consists of 10 genes (SLC4A1-5; SLC4A7-11). All encode integral membrane proteins with very similar hydropathy plots-consistent with 10-14 transmembrane segments. Nine SLC4 members encode proteins that transport HCO3(-) (or a related species, such as CO3(2-)) across the plasma membrane. Functionally, eight of these proteins fall into two major groups: three Cl-HCO3 exchangers (AE1-3) and five Na(+)-coupled HCO3(-) transporters (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE). Two of the Na(+)-coupled transporters (NBCe1, NBCe2) are electrogenic; the other three Na(+)-coupled HCO3(-) transporters and all three AEs are electroneutral. In addition, two other SLC4 members (AE4, SLC4A9 and BTR1, SLC4A11) do not yet have a firmly established function. Most, though not all, SLC4 members are functionally inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS). SLC4 proteins play important roles many modes of acid-base homeostasis: the carriage of CO2 by erythrocytes, the transport of H(+) or HCO3(-) by several epithelia, as well as the regulation of cell volume and intracellular pH.

Proximal renal tubular acidosis: a not so rare disorder of multiple etiologies

Proximal renal tubular acidosis (RTA) (Type II RTA) is characterized by a defect in the ability to reabsorb HCO₃ in the proximal tubule. This is usually manifested as bicarbonate wastage in the urine reflecting that the defect in proximal tubular transport is severe enough that the capacity for bicarbonate reabsorption in the thick ascending limb of Henle's loop and more distal nephron segments is overwhelmed. More subtle defects in proximal bicarbonate transport likely go clinically unrecognized owing to compensatory reabsorption of bicarbonate distally. Inherited proximal RTA is more commonly autosomal recessive and has been associated with mutations in the basolateral sodium-bicarbonate cotransporter (NBCe1). Mutations in this transporter lead to reduced activity and/or trafficking, thus disrupting the normal bicarbonate reabsorption process of the proximal tubules. As an isolated defect for bicarbonate transport, proximal RTA is rare and is more often associated with the Fanconi syndrome characterized by urinary wastage of solutes like phosphate, uric acid, glucose, amino acids, low-molecular-weight proteins as well as bicarbonate. A vast array of rare tubular disorders may cause proximal RTA but most commonly it is induced by drugs. With the exception of carbonic anhydrase inhibitors which cause isolated proximal RTA, drug-induced proximal RTA is associated with Fanconi syndrome. Drugs that have been recently recognized to cause severe proximal RTA with Fanconi syndrome include ifosfamide, valproic acid and various antiretrovirals such as Tenofovir particularly when given to human immunodeficiency virus patients receiving concomitantly protease inhibitors such as ritonavir or reverse transcriptase inhibitors such as didanosine.

Identification of dominant negative effect of L522P mutation in the electrogenic Na⁺-HCO₃⁻ cotransporter NBCe1

Homozygous mutations in the electrogenic Na(+)-HCO3 (-) cotransporter NBCe1 cause proximal renal tubular acidosis (pRTA) associated with extrarenal manifestations such as ocular abnormalities and migraine. Previously, the NBCe1 cytosolic mutant S982NfsX4 was shown to have a dominant negative effect by forming hetero-oligomer complexes with wild type (WT), which might be responsible for the occurrence of glaucoma and migraine in the heterozygous family members. In this study, we investigated whether the NBCe1 L522P mutant has a similar dominant negative effect. Functional analyses in Xenopus oocytes and HEK293 cells revealed that the L522P mutant had no transport activity due to defective membrane expression. Furthermore, when coexpressed with WT, L522P significantly reduced the transport activity of WT. In HEK293 cells, the cytosolic mutant L522P reduced the membrane expression of NBCe1 by forming hetero-oligomer complexes with WT. Among the artificial Leu(522) mutants, L522I showed proper membrane expression and normal transport activity. However, the other mutants L522R, L522K, L522D, and L522E showed a predominant cytosolic retention. Moreover, L522R had a dominant negative effect, when coexpressed with WT. These results indicate that Leu(522) plays an important role in the structure and trafficking of NBCe1. They also suggest that the NBCe1 mutants retaining in cytoplasm may have the dominant negative effect in common, which may induce some clinical manifestations.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Functional Roles of Electrogenic Sodium Bicarbonate Cotransporter NBCe1 in Ocular Tissues

Electrogenic Na⁺-HCO₃^- cotransporter NBCe1 is expressed in several tissues such as kidney, eye, and brain, where it may mediate distinct biological processes. In particular, NBCe1 in renal proximal tubules is essential for the regulation of systemic acid/base balance. On the other hand, NBCe1 in eye may be indispensable for the maintenance of tissue homeostasis. Consistent with this view, homozygous mutations in NBCe1 cause severe proximal renal tubular acidosis associated with ocular abnormalities such as band keratopathy, glaucoma, and cataract. The widespread expression of NBCe1 in eye suggests that the inactivation of NBCe1 per se may be responsible for the occurrence of these ocular abnormalities. In this review, we discuss about physiological and pathological roles of NBCe1 in eye.

HCO(3)(-)-independent conductance with a mutant Na(+)/HCO(3)(-) cotransporter (SLC4A4) in a case of proximal renal tubular acidosis with hypokalaemic paralysis

The renal electrogenic Na(+)/HCO(3)(−) cotransporter (NBCe1-A) contributes to the basolateral step of transepithelial HCO(3)(−) reabsorption in proximal tubule epithelia, contributing to the buffering of blood pH. Elsewhere in the body (e.g. muscle cells) NBCe1 variants contribute to, amongst other processes, maintenance of intracellular pH. Others have described a homozygous mutation in NBCe1 (NBCe1-A p.Ala799Val) in an individual with severe proximal renal tubular acidosis (pRTA; usually associated with defective HCO(3)(−) reabsorption in proximal tubule cells) and hypokalaemic periodic paralysis (hypoPP; usually associated with leaky cation channels in muscle cells). Using biotinylation and two-electrode voltage-clamp on Xenopus oocytes expressing NBCe1, we demonstrate that the mutant NBCe1-A (A(A799V)) exhibits a per-molecule transport defect that probably contributes towards the observed pRTA. Furthermore, we find that A(A799V) expression is associated with an unusual HCO(3)(−)-independent conductance that, if associated with mutant NBCe1 in muscle cells, could contribute towards the appearance of hypokalaemic paralysis in the affected individual. We also study three novel lab mutants of NBCe1-A: p.Ala799Ile, p.Ala799Gly and p.Ala799Ser. All three exhibit a per-molecule transport defect, but only A(A799I) exhibits an A(A799V)-like ion conductance. A(A799G) and A(A799S) exhibit unusual outward rectification in their HCO(3)(−)-dependent conductance and A(A799G) exhibits reduced sensitivity to both DIDS and tenidap. A799G is the first mutation shown to affect the apparent tenidap affinity of NBCe1. Finally we show that A(A799V) and A(A799I), which accumulate poorly in the plasma membrane of oocytes, exhibit signs of abnormal intracellular accumulation in a non-polarized renal cell-line.

Requirements for ion and solute transport, and pH regulation during enamel maturation

Transcellular bicarbonate transport is suspected to be an important pathway used by ameloblasts to regulate extracellular pH and support crystal growth during enamel maturation. Proteins that play a role in amelogenesis include members of the ABC transporters (SLC gene family and CFTR). A number of carbonic anhydrases (CAs) have also been identified. The defined functions of these genes are likely interlinked during enamel mineralization. The purpose of this study is to quantify relative mRNA levels of individual SLC, Cftr, and CAs in enamel cells obtained from secretory and maturation stages on rat incisors. We also present novel data on the enamel phenotypes for two animal models, a mutant porcine (CFTR-ΔF508) and the NBCe1-null mouse. Our data show that two SLCs (AE2 and NBCe1), Cftr, and Car2, Car3, Car6, and Car12 are all significantly up-regulated at the onset of the maturation stage of amelogenesis when compared to the secretory stage. The remaining SLCs and CA gene transcripts showed negligible expression or no significant change in expression from secretory to maturation stages. The enamel of CFTR-ΔF508 adult pigs was hypomineralized and showed abnormal crystal growth. NBCe1-null mice enamel was structurally defective and had a marked decrease in mineral content relative to wild-type. These data demonstrate the importance of many non-matrix proteins to amelogenesis and that the expression levels of multiple genes regulating extracellular pH are modulated during enamel maturation in response to an increased need for pH buffering during hydroxyapatite crystal growth.

The SLC16A family of monocarboxylate transporters (MCTs)--physiology and function in cellular metabolism, pH homeostasis, and fluid transport

The SLC16A family of monocarboxylate transporters (MCTs) is composed of 14 members. MCT1 through MCT4 (MCTs 1-4) are H(+)-coupled monocarboxylate transporters, MCT8 and MCT10 transport thyroid hormone and aromatic amino acids, while the substrate specificity and function of other MCTs have yet to be determined. The focus of this review is on MCTs 1-4 because their role in lactate transport is intrinsically linked to cellular metabolism in various biological systems, including skeletal muscle, brain, retina, and testis. Although MCTs 1-4 all transport lactate, they differ in their transport kinetics and vary in tissue and subcellular distribution, where they facilitate "lactate-shuttling" between glycolytic and oxidative cells within tissues and across blood-tissue barriers. However, the role of MCTs 1-4 is not confined to cellular metabolism. By interacting with bicarbonate transport proteins and carbonic anhydrases, MCTs participate in the regulation of pH homeostasis and fluid transport in renal proximal tubule and corneal endothelium, respectively. Here, we provide a comprehensive review of MCTs 1-4, linking their cellular distribution to their functions in various parts of the human body, so that we can better understand the physiological roles of MCTs at the systemic level.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the electrogenic Na+-HCO3- cotransporter NBCe1A

The electrogenic Na(+)-HCO(3)(-) cotransporter NBCe1 encoded by SLC4A4 plays essential roles in the regulation of intracellular/extracellular pH. Homozygous mutations in NBCe1 cause proximal renal tubular acidosis associated with ocular abnormalities. In the present study, we tried to perform functional characterization of the four nonsynonymous single nucleotide polymorphisms (SNPs), E122G, S356Y, K558R, and N640I in NBCe1A. Functional analysis in Xenopus oocytes revealed that while the K558R variant had a significantly reduced transport activity corresponding to 47% of the wild-type activity, the remaining variants E122G, S356Y, and N640I did not change the NBCe1A activity. Apparent Na(+) affinity of K558R was not different from that of wild-type NBCe1A. Immunohistological analyses in HEK293 cells and MDCK cells indicated that none of these SNPs changed the trafficking behaviors of NBCe1A. Functional analysis in HEK293 cells also revealed that only the K558R variant had a reduced transport activity, corresponding to 41-47% of the wild-type activity. From these results, we conclude that among four SNPs, only the K558R variant, which is predicted to lie in transmembrane segment 5, significantly reduces the NBCe1A activity without changing the trafficking behavior or the apparent extracellular Na(+) affinity.

Severe metabolic acidosis causes early lethality in NBC1 W516X knock-in mice as a model of human isolated proximal renal tubular acidosis

We have identified a novel homozygous nonsense mutation (W516X) in the kidney-type electrogenic sodium bicarbonate cotransporter 1 (NBC1) in a patient with isolated proximal renal tubular acidosis (pRTA). To specifically address the pathogenesis of this mutation, we created NBC1 W516X knock-in mice to match the patient's abnormalities. The expression of NBC1 mRNA and protein in the kidneys of NBC1(W516X/W516X) mice were virtually absent, indicating that nonsense-mediated mRNA decay (NMD) is involved in the defective transcription and translation of this mutation. These mice not only recapitulated the phenotypes of this patient with growth retardation, pRTA, and ocular abnormalities, but also showed anemia, volume depletion, prerenal azotemia, and several organ abnormalities, culminating in dehydration and renal failure with early lethality before weaning. In isolated renal proximal tubules, both NBC1 activity and the rate of bicarbonate absorption were markedly reduced. Unexpectedly, there was no compensatory increase in mRNA of distal acid/base transporters. Sodium bicarbonate but not saline administration to these mutant mice markedly prolonged their survival, decreased their protein catabolism and attenuated organ abnormalities. The prolonged survival time uncovered the development of corneal opacities due to corneal edema. Thus, NBC1(W516X/W516X) mice with pRTA represent an animal model for metabolic acidosis and may be useful for testing therapeutic inhibition of NMD in vivo.

Role of an extracellular loop in determining the stoichiometry of Na+-HCO₃⁻ cotransporters

The Na+–HCO₃⁻ cotransporters (NBCs) of the solute carrier 4 family (SLC4) are critical for regulating pH in cells as well as in fluids such as blood and cerebrospinal fluid. Moreover, mutations and gene disruptions in NBC are linked to a wide range of pathologies. NBCe1 (SLC4A4) is electrogenic because it has an apparent Na+:HCO₃⁻ stoichiometry of 1:2 or 1:3, whereas NBCn1 (SLC4A7) is electroneutral because it has an apparent stoichiometry of 1:1. Because stoichiometry influences the effect of transport on membrane potential and vice versa, a central question is what structural features underlie electrogenicity versus electroneutrality. A previous study on rat NBCe1/n1 chimeras demonstrated that the structural elements determining the electrogenicity of NBCe1-A are located within the transmembrane domain, excluding the large third extracellular loop. In the present study we generated a series of chimeras of human NBCe1-A and human NBCn1-A. We found that replacing merely the predicted fourth extracellular loop (EL4) – containing 32 amino acid residues that include 7 prolines – of human NBCe1-A with EL4 of NBCn1-A creates an electroneutral NBC. The opposite switch converts an electroneutral construct to one with electrogenic properties. The introduction of an N-glycosylation site into EL4 confirms that at least a part of it is exposed to the extracellular fluid. We hypothesize that putative EL4 either contributes to the substrate-binding vestibule or indirectly influences substrate binding by interacting with one or more transmembrane segments, thereby controlling the nature of transport.

Defining a new candidate gene for amelogenesis imperfecta: from molecular genetics to biochemistry

Amelogenesis imperfecta is a group of genetic conditions that affect the structure and clinical appearance of tooth enamel. The types (hypoplastic, hypocalcified, and hypomature) are correlated with defects in different stages of the process of enamel synthesis. Autosomal dominant, recessive, and X-linked types have been previously described. These disorders are considered clinically and genetically heterogeneous in etiology, involving a variety of genes, such as AMELX, ENAM, DLX3, FAM83H, MMP-20, KLK4, and WDR72. The mutations identified within these causal genes explain less than half of all cases of amelogenesis imperfecta. Most of the candidate and causal genes currently identified encode proteins involved in enamel synthesis. We think it is necessary to refocus the search for candidate genes using biochemical processes. This review provides theoretical evidence that the human SLC4A4 gene (sodium bicarbonate cotransporter) may be a new candidate gene.

Defective membrane expression of the Na(+)-HCO(3)(-) cotransporter NBCe1 is associated with familial migraine

Homozygous mutations in SLC4A4, encoding the electrogenic Na(+)-HCO(3)(-) cotransporter NBCe1, have been known to cause proximal renal tubular acidosis (pRTA) and ocular abnormalities. In this study, we report two sisters with pRTA, ocular abnormalities, and hemiplegic migraine. Genetic analysis ruled out pathological mutations in the known genes for familial hemiplegic migraine, but identified a homozygous 65-bp deletion (Delta65bp) in the C terminus of NBCe1, corresponding to the codon change S982NfsX4. Several heterozygous members of this family also presented glaucoma and migraine with or without aura. Despite the normal electrogenic activity in Xenopus oocytes, the Delta65bp mutant showed almost no transport activity due to a predominant cytosolic retention in mammalian cells. Furthermore, coexpression experiments uncovered a dominant negative effect of the mutant through hetero-oligomer formation with wild-type NBCe1. Among other pRTA pedigrees with different NBCe1 mutations, we identified four additional homozygous patients with migraine. The immunohistological and functional analyses of these mutants demonstrate that the near total loss of NBCe1 activity in astrocytes can cause migraine potentially through dysregulation of synaptic pH.

The sodium bicarbonate cotransporter (NBCe1) is essential for normal development of mouse dentition

Proximal renal tubular acidosis (pRTA) is a syndrome caused by abnormal proximal tubule reabsorption of bicarbonate resulting in metabolic acidosis. Patients with mutations to the SLC4A4 gene (coding for the sodium bicarbonate cotransporter NBCe1), have pRTA, growth delay, ocular defects, and enamel abnormalities. In an earlier report, we provided the first evidence that enamel cells, the ameloblasts, express NBCe1 in a polarized fashion, thereby contributing to trans-cellular bicarbonate transport. To determine whether NBCe1 plays a critical role in enamel development, we studied the expression of NBCe1 at various stages of enamel formation in wild-type mice and characterized the biophysical properties of enamel in NBCe1(-/-) animals. The enamel of NBCe1(-/-) animals was extremely hypomineralized and weak with an abnormal prismatic architecture. The expression profile of amelogenin, a known enamel-specific gene, was not altered in NBCe1(-/-) animals. Our results show for the first time that NBCe1 expression is required for the development of normal enamel. This study provides a mechanistic model to account for enamel abnormalities in certain patients with pRTA.

Regulation of pH During Amelogenesis

During amelogenesis, extracellular matrix proteins interact with growing hydroxyapatite crystals to create one of the most architecturally complex biological tissues. The process of enamel formation is a unique biomineralizing system characterized first by an increase in crystallite length during the secretory phase of amelogenesis, followed by a vast increase in crystallite width and thickness in the later maturation phase when organic complexes are enzymatically removed. Crystal growth is modulated by changes in the pH of the enamel microenvironment that is critical for proper enamel biomineralization. Whereas the genetic bases for most abnormal enamel phenotypes (amelogenesis imperfecta) are generally associated with mutations to enamel matrix specific genes, mutations to genes involved in pH regulation may result in severely affected enamel structure, highlighting the importance of pH regulation for normal enamel development. This review summarizes the intra- and extracellular mechanisms employed by the enamel-forming cells, ameloblasts, to maintain pH homeostasis and, also, discusses the enamel phenotypes associated with disruptions to genes involved in pH regulation.

Modular structure of sodium-coupled bicarbonate transporters

Mammalian genomes contain 10 SLC4 genes that, between them, encode three Cl-HCO(3) exchangers, five Na(+)-coupled HCO(3) transporters (NCBTs), one reported borate transporter, and what is reported to be a fourth Cl-HCO(3) exchanger. The NCBTs are expressed throughout the body and play important roles in maintaining intracellular and whole-body pH, as well as contributing to transepithelial transport processes. The importance of NCBTs is underscored by the genetic association of dysfunctional NCBT genes with blindness, deafness, epilepsy, hypertension and metal retardation. Key to understanding the action and regulation of NCBTs is an appreciation of the diversity of NCBT gene products. The transmembrane domains of human NCBT paralogs are 50-84% identical to each other at the amino acid level, and are capable of a diverse range of actions, including electrogenic Na/HCO(3) cotransport (i.e. NBCe1 and NBCe2) and electroneutral Na/HCO(3) cotransport (i.e. NBCn1 and NBCn2), as well as Na(+)-dependent Cl-HCO(3) exchange (i.e. NDCBE). Furthermore, by the use of alternative promoters and alternative-splicing events, individual SLC4 genes have the potential to generate multiple splice variants (as many as 16 in the case of NBCn1), each of which could have unique temporal and spatial patterns of distribution, unitary transporter activity (i.e. flux mediated by one molecule), array of protein-binding partners, and complement of regulatory stimuli. In the first section of this review, we summarize our present knowledge of the function and distribution of mammalian NCBTs and their multiple variants. In the second section of this review we consider the molecular consequences of NCBT variation.

Bicarbonate transport in cell physiology and disease

The family of mammalian bicarbonate transport proteins are involved in a wide-range of physiological processes. The importance of bicarbonate transport follows from the biochemistry of HCO(3)(-) itself. Bicarbonate is the waste product of mitochondrial respiration. HCO(3)(-) undergoes pH-dependent conversion into CO(2) and in doing so converts from a membrane impermeant anion into a gas that can diffuse across membranes. The CO(2)-HCO(3)(-) equilibrium forms the most important pH buffering system of our bodies. Bicarbonate transport proteins facilitate the movement of membrane-impermeant HCO(3)(-) across membranes to accelerate disposal of waste CO(2), control cellular and whole-body pH, and to regulate fluid movement and acid/base secretion. Defects of bicarbonate transport proteins manifest in diseases of most organ systems. Fourteen gene products facilitate mammalian bicarbonate transport, whose physiology and pathophysiology is discussed in the present review.

Electrogenic NBCe1 (SLC4A4), but not electroneutral NBCn1 (SLC4A7), cotransporter undergoes cholinergic-stimulated endocytosis in salivary ParC5 cells

Cholinergic agonists are major stimuli for fluid secretion in parotid acinar cells. Saliva bicarbonate is essential for maintaining oral health. Electrogenic and electroneutral Na(+)-HCO(3)(-) cotransporters (NBCe1 and NBCn1) are abundant in parotid glands. We previously reported that angiotensin regulates NBCe1 by endocytosis in Xenopus oocytes. Here, we studied cholinergic regulation of NBCe1 and NBCn1 membrane trafficking by confocal fluorescent microscopy and surface biotinylation in parotid epithelial cells. NBCe1 and NBCn1 colocalized with E-cadherin monoclonal antibody at the basolateral membrane (BLM) in polarized ParC5 cells. Inhibition of constitutive recycling with the carboxylic ionophore monensin or the calmodulin antagonist W-13 caused NBCe1 to accumulate in early endosomes with a parallel loss from the BLM, suggesting that NBCe1 is constitutively endocytosed. Carbachol and PMA likewise caused redistribution of NBCe1 from BLM to early endosomes. The PKC inhibitor, GF-109203X, blocked this redistribution, indicating a role for PKC. In contrast, BLM NBCn1 was not downregulated in parotid acinar cells treated with constitutive recycling inhibitors, cholinergic stimulators, or PMA. We likewise demonstrate striking differences in regulation of membrane trafficking of NBCe1 vs. NBCn1 in resting and stimulated cells. We speculate that endocytosis of NBCe1, which coincides with the transition to a steady-state phase of stimulated fluid secretion, could be a part of acinar cell adjustment to a continuous secretory response. Stable association of NBCn1 at the membrane may facilitate constitutive uptake of HCO(3)(-) across the BLM, thus supporting HCO(3)(-) luminal secretion and/or maintaining acid-base homeostasis in stimulated cells.

G418-mediated ribosomal read-through of a nonsense mutation causing autosomal recessive proximal renal tubular acidosis

Autosomal recessive proximal renal tubular acidosis is caused by mutations in the SLC4A4 gene encoding the electrogenic sodium bicarbonate cotransporter NBCe1-A. The mutations that have been characterized thus far result in premature truncation, mistargeting, or decreased function of the cotransporter. Despite bicarbonate treatment to correct the metabolic acidosis, extrarenal manifestations persist, including glaucoma, cataracts, corneal opacification, and mental retardation. Currently, there are no known therapeutic approaches that can specifically target mutant NBCe1-A proteins. In the present study, we tested the hypothesis that the NBCe1-A-Q29X mutation can be rescued in vitro by treatment with aminoglycoside antibiotics, which are known for their ability to suppress premature stop codons. As a model system, we cloned the NBCe1-A-Q29X mutant into a vector lacking an aminoglycoside resistance gene and transfected the mutant cotransporter in HEK293-H cells. Cells transfected with the NBCe1-A-Q29X mutant failed to express the cotransporter because of the premature stop codon. Treatment of the cells with G418 significantly increased the expression of the full-length cotransporter, as assessed by immunoblot analysis. Furthermore, immunocytochemical studies demonstrated that G418 treatment induced cotransporter expression on the plasma membrane whereas in the absence of G418, NBCe1-A-Q29X was not expressed. In HEK293-H cells transfected with the NBCe1-A-Q29X mutant not treated with G418, NBCe1-A-mediated flux was not detectable. In contrast, in cells transfected with the NBCe1-A-Q29X mutant, G418 treatment induced Na(+)- and HCO(3)(-)-dependent transport that did not differ from wild-type NBCe1-A function. G418 treatment in mock-transfected cells was without effect. In conclusion, G418 induces ribosomal read-through of the NBCe1-A-Q29X mutation in HEK293-H cells. These findings represent the first evidence that in the presence of the NBCe1-A-Q29X mutation that causes proximal renal tubular acidosis, full-length functional NBCe1-A protein can be produced. Our results provide the first demonstration of a mutation in NBCe1-A that has been treated in a targeted and specific manner.

Role of NBCe1 and AE2 in secretory ameloblasts

The H(+)/base transport processes that control the pH of the microenvironment adjacent to ameloblasts are not currently well-understood. Mice null for the AE2 anion exchanger have abnormal enamel. In addition, persons with mutations in the electrogenic sodium bicarbonate co-transporter NBCe1 and mice lacking NBCe1 have enamel abnormalities. These observations suggest that AE2 and NBCe1 play important roles in amelogenesis. In the present study, we aimed to understand the roles of AE2 and NBCe1 in ameloblasts. Analysis of the data showed that NBCe1 is expressed at the basolateral membrane of secretory ameloblasts, whereas AE2 is expressed at the apical membrane. Transcripts for AE2a and NBCe1-B were detected in RNA isolated from cultured ameloblast-like LS8 cells. Our data are the first evidence that AE2 and NBCe1 are expressed in ameloblasts in vivo in a polarized fashion, thereby providing a mechanism for ameloblast transcellular bicarbonate secretion in the process of enamel formation and maturation.