Proximal tubule Na+/H+ exchanger activity in adult NHE8(-/-), NHE3(-/-), and NHE3(-/-)/NHE8(-/-) mice

1UT Southwestern Medical Center at Dallas.
AJP Renal Physiology (Impact Factor: 3.25). 10/2012; 303(11). DOI: 10.1152/ajprenal.00415.2012
Source: PubMed


NHE3 is the predominant Na(+)/H(+) exchanger on the brush border membrane (BBM) of the proximal tubule in adults. However, NHE3 null mice still have significant renal BBM Na(+)/H(+) activity. NHE8 has been localized to the BBM of proximal tubules and is more highly expressed in neonates than adult animals. The relative role of NHE8 in adult renal H(+) transport is unclear. This study examined if there was compensation by NHE8 in NHE3(-/-) mice and by NHE3 in NHE8(-/-) mice. NHE3(-/-) mice had significant metabolic acidosis and renal BBM NHE8 protein abundance was greater in NHE3(-/-) mice than control mice indicating that there may be compensation by NHE8 in NHE3(-/-) mice. NHE8(-/-) mice had serum bicarbonate levels and pH that were not different from controls. NHE3 protein expression on brush border membranes was greater in NHE8(-/-) mice than in wild type mice indicating that there may be compensation by NHE3 in NHE8(-/-) mice. Both BBM NHE3 and NHE8 protein abundance increase in response to acidosis. Blood pressure and Na(+)/H(+) exchanger activity were comparable in NHE8(-/-) mice to that of controls, but both were significantly lower in NHE3(-/-) mice compared to control mice. Compared to NHE3(-/-) mice, NHE3(-/-)/NHE8(-/-) mice had lower blood pressures. While serum bicarbonate was comparable in NHE3(-/-) mice and NHE3(-/-)/NHE8(-/-) mice, proximal tubule Na(+)/H(+) exchange activity was less in NHE3(-/-)/NHE8(-/-) mice compared to NHE3(-/-) mice. In conclusion, NHE3 is the predominant Na(+)/H(+) exchanger in adult mice. NHE8 may play a compensatory role in renal acidification and blood pressure regulation in NHE3(-/-) mice.

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Available from: Orson W Moe, Oct 05, 2015
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    • "Although the basolateral membrane of PTs contains Na+-dependent and Na+-independent Cl−/HCO3− exchangers [7], these transporters cannot effectively compensate for the loss of NBCe1 function. By contrast, the loss of NHE3 function may be at least partially compensated by the other luminal transporters such as NHE8 [8]. "
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    ABSTRACT: Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) plays a pivotal role in the maintenance of systemic acid/base balance. Indeed, mutations in the Na(+)-HCO3 (-) cotransporter NBCe1, which mediates a majority of bicarbonate exit from PTs, cause severe proximal renal tubular acidosis associated with ocular and other extrarenal abnormalities. Sodium transport in PTs also plays an important role in the regulation of blood pressure. For example, PT transport stimulation by insulin may be involved in the pathogenesis of hypertension associated with insulin resistance. Type 1 angiotensin (Ang) II receptors in PT are critical for blood pressure homeostasis. Paradoxically, the effects of Ang II on PT transport are known to be biphasic. Unlike in other species, however, Ang II is recently shown to dose-dependently stimulate human PT transport via nitric oxide/cGMP/ERK pathway, which may represent a novel therapeutic target in human hypertension. In this paper, we will review the physiological and pathophysiological roles of PT transport.
    BioMed Research International 05/2014; 2014(3):504808. DOI:10.1155/2014/504808 · 2.71 Impact Factor
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    • "Functional analysis using isolated renal proximal tubules from NHE3/8 double-KO mice confirmed this view (Baum et al., 2012). Surprisingly, however, the acidosis of NHE3/8 double-KO mice was also mild and comparable to that in NHE3 KO mice (Baum et al., 2012). "
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    ABSTRACT: 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.
    Frontiers in Physiology 10/2013; 4:270. DOI:10.3389/fphys.2013.00270 · 3.53 Impact Factor
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    • "The protein obtained from brush border vesicle or total lysates were mixed with 5× loading buffer (2.5 mM Tris HCl [pH 6.8], 2.5% β-mercaptoethanol, 25% glycerol, and 2.5% SDS). The proteins were heated to 85°C for 5 min (NHE3) and 37°C for 5 min (NHE8) and then loaded on an 8% polyacrylamide gel and separated using SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) as has been described (Baum et al. 2012; Joseph et al. 2012). Proteins were then transferred to a polyvinylidene difluoride membrane (Immobilon; Millipore, Billerica, MA) at 400 mA for 1 h at 4°C (Baum et al. 1998; Shah et al. 2000; Bobulescu et al. 2005). "
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    ABSTRACT: The proximal tubule reabsorbs most of the filtered bicarbonate which is mediated in large part by Na(+)/H(+) exchange (NHE). We have previously demonstrated that there is an isoform switch during postnatal maturation from NHE8 to NHE3 that is concordant with the postnatal increase in serum glucocorticoid levels. To examine if glucocorticoids may be responsible for this isoform switch, we administered dexamethasone daily to mice at 7-10 days of age, a time prior to the normal isoform switch. We show that compared to vehicle treated controls, dexamethasone caused a premature increase in renal NHE3 and decrease in NHE8 mRNA, total protein and brush border membrane protein abundance. To examine if there was a direct epithelial action of dexamethasone on NHE8, we studied NRK cells in vitro which express NHE8 on their apical membrane. Dexamethasone decreased NHE8 mRNA, total protein, and apical protein abundance. Dexamethasone also decreased Na(+)/H(+) exchanger activity. These studies provide evidence that glucocorticoids may play a role in the developmental isoform switch from NHE8 to NHE3 and cause a decrease in NHE8 expression and activity.
    07/2013; 1(2). DOI:10.1002/phy2.31
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