Effect of simultaneously replacing putative TM6 and TM12 of human NBCe1-A with those from NBCn1 on surface abundance in Xenopus oocytes

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.

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.

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.

Structure, function, and regulation of the SLC4 NBCe1 transporter and its role in causing proximal renal tubular acidosis

PURPOSE OF REVIEW

There has been significant progress in our understanding of the structural and functional properties and regulation of the electrogenic sodium bicarbonate cotansporter NBCe1, a membrane transporter that plays a key role in renal acid-base physiology. The NBCe1 variant NBCe1-A mediates basolateral electrogenic sodium-base transport in the proximal tubule and is critically required for transepithelial bicarbonate absorption. Mutations in NBCe1 cause autosomal recessive proximal renal tubular acidosis (pRTA). The review summarizes recent advances in this area.

RECENT FINDINGS

A topological model of NBCe1 has been established that provides a foundation for future structure-functional studies of the transporter. Critical residues and regions have been identified in NBCe1 that play key roles in its structure, function (substrate transport, electrogenicity) and regulation. The mechanisms of how NBCe1 mutations cause pRTA have also recently been elucidated.

SUMMARY

Given the important role of proximal tubule transepithelial bicarbonate absorption in systemic acid-base balance, a clear understanding of the structure-functional properties of NBCe1 is a prerequisite for elucidating the mechanisms of defective transepithelial bicarbonate transport in pRTA.

NBCe1 as a model carrier for understanding the structure-function properties of Na⁺ -coupled SLC4 transporters in health and disease

SLC4 transporters are membrane proteins that in general mediate the coupled transport of bicarbonate (carbonate) and share amino acid sequence homology. These proteins differ as to whether they also transport Na(+) and/or Cl(-), in addition to their charge transport stoichiometry, membrane targeting, substrate affinities, developmental expression, regulatory motifs, and protein-protein interactions. These differences account in part for the fact that functionally, SLC4 transporters have various physiological roles in mammals including transepithelial bicarbonate transport, intracellular pH regulation, transport of Na(+) and/or Cl(-), and possibly water. Bicarbonate transport is not unique to the SLC4 family since the structurally unrelated SLC26 family has at least three proteins that mediate anion exchange. The present review focuses on the first of the sodium-dependent SLC4 transporters that was identified whose structure has been most extensively studied: the electrogenic Na(+)-base cotransporter NBCe1. Mutations in NBCe1 cause proximal renal tubular acidosis (pRTA) with neurologic and ophthalmologic extrarenal manifestations. Recent studies have characterized the important structure-function properties of the transporter and how they are perturbed as a result of mutations that cause pRTA. It has become increasingly apparent that the structure of NBCe1 differs in several key features from the SLC4 Cl(-)-HCO3 (-) exchanger AE1 whose structural properties have been well-studied. In this review, the structure-function properties and regulation of NBCe1 will be highlighted, and its role in health and disease will be reviewed in detail.

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.

Effects of optional structural elements, including two alternative amino termini and a new splicing cassette IV, on the function of the sodium-bicarbonate cotransporter NBCn1 (SLC4A7)

The SLC4A7 gene encodes the electroneutral sodium/HCO3 cotransporter NBCn1, which plays important physiological and pathophysiological roles in many cell types. Previous work identified six NBCn1 variants differing in the sequence of the extreme N terminus--MEAD in rat only, MERF in human only--as well as in the optional inclusion of cassettes I, II, and III. Earlier work also left open the question of whether optional structural elements (OSEs) affect surface abundance or intrinsic (per-molecule) transport activity. Here, we demonstrate for the first time that SLC4A7 from one species can express both MEAD- and MERF-NBCn1. We also identify a novel cassette IV of 20 aa, and extend by 10 the number of full-length NBCn1 variants. The alternative N termini and four cassettes could theoretically produce 32 major variants. Moreover, we identify a group of cDNAs predicted to encode just the cytosolic N-terminal domain (Nt) of NBCn1. A combination of electrophysiology and biotinylation shows that the OSEs can affect surface abundance and intrinsic HCO3(-) transport activity of NBCn1, as expressed in Xenopus oocytes. Specifically, MEAD tends to increase whereas novel cassette IV reduces surface abundance. Cassettes II, III and novel cassette IV all appear to increase the intrinsic activity of NBCn1.

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.