NBCe1 Na+-HCO3-cotransporter ablation causes reduced apoptosis following cardiac ischemia-reperfusion injury in vivo

AIM To investigate the hypothesis that cardiomyocytespecific loss of the electrogenic NBCe1 Na^+-HCO3^- cotransporter is cardioprotective during in vivo ischemiareperfusion(IR)injury.METHODS An NBCe1 (Slc4a4 gene) conditional knockout mouse(KO)model was prepared by gene targeting.Cardiovascular performance of wildtype (WT) and cardiac-specific NBCe1 KO mice was analyzed by intraventricular pressure measurements,and changes in cardiac gene expression were determined by RNA Seq analysis.Response to in vivo IR injury was analyzed after 30 min occlusion of the left anterior descending artery followed by 3 h of reperfusion. RESULTS Loss of NBCe1 in cardiac myocytes did not impair cardiac contractility or relaxation under basal conditions or in response toβ-adrenergic stimulation,and caused only limited changes in gene expression patterns,such as those for electrical excitability.However,following ischemia and reperfusion,KO heart sections exhibited significantly fewer apoptotic nuclei than WT sections.CONCLUSION These studies indicate that cardiac-specific loss of NBCe1 does not impair cardiovascular performance,causes only minimal changes in gene expression patterns,and protects against IR injury in vivo.

Physiological and pathophysiological roles of the electrogenic Na⁺-HCO₃^–cotransporter NBCe1

The electrogenic Na+-HCO3– cotransporter NBCe1 encoded by SLC4A4 gene plays essential roles in the regulation of intracellular/extracellular pH. Three NBCe1 variants are thought to mediate distinct physiological roles with different modes of transport stoichiometry. Homozygous inactivating mutations in NBCe1 cause the isolated proximal renal tubular acidosis (pRTA) invariably associated with ocular abnormalities. Functional analyses indicate that more than 50% reduction in NBCe1 activity may be required to induce severe acidemia. Some of the pRTA- related NBCe1 mutations, which show defective me-mbrane expression in mammalian cells, are also associated with migraine. Dysregulation of local pH in brain due to the loss of NBCe1 activity in astrocytes may underlie this association. Two types of NBCe1 deficient animals, NBCe1 knockout and W516X knockin mice, have been reported. Both of them show severe acidemia and early lethality unless they are treated with alkali. In isolated renal proximal tubules from W516X knockin mice, both NBCe1 activity and the rate of bicarbonate absorption are severely reduced, confirming the essential role of NBCe1 in bicarbonate absorption from this nephron segment. In this review, we summarize the recent data about physiological and pathophysiological roles of NBCe1 in health and diseases.

The acid-base regulation by renal proximal tubule

The kidney plays quite an important role in the regulation of acid-base homeostasis. The dysfunction of renal acid-base regulation causes diseases such as developmental disorder, bone malformation, calcification of eye and brain, etc. In the kidney, this regulation is performed, to a considerable part, in the proximal tubule of the nephron. In the luminal side the key player is sodium-proton exchanger type 3 (NHE3), whereas sodium-bicarbonate cotransporter (NBCe1) plays the critical role in the basolateral side. In the cytoplasm there is carbonic anhydrase type 2 (CAII) that intermediates the conversion of CO2/ . Interestingly, in human, mutations have been found in NBCe1 and CAII but not in NHE3 so far. Mutations of NBCe1 lead to severe proximal renal tubular acidosis (pRTA) and other systemic manifestations. In animal model studies, however, the relative contribution of NHE3 to proximal tubule functions remains controversial. Recently, V-ATPase with renal specific subunits is suggested to have some roles in the regulation of proximal tubule functions. In this review, we will discuss the regulation of acid-base transport in the proximal tubule and the updates.

Role of renal proximal tubule transport in thiazolidinedioneinduced volume expansion

Thiazolidinediones(TZDs), pharmacological activators of peroxisome-proliferator-activated receptors γ(PPARγ), significantly improve insulin resistance and lower plasma glucose concentrations. However, the use of TZDs is associated with plasma volume expansion, the mechanism of which has been a matter of controversy. Originally, PPARγ-mediated enhanced transcription of the epithelial Na channel(ENaC) γ subunit was thought to play a central role in TZD-induced volume expansion. However, later studies suggested that the activation of ENaC alone could not explain TZD-induced volume expansion. We have recently shown that TZDs rapidly stimulate sodium-coupled bicarbonate absorption from renal proximal tubule(PT) in vitro and in vivo. TZD-induced transport stimulation was dependent on PPARγ/Src/EGFR/ERK, and observed in rat, rabbit and human. However, this stimulation was not observed in mouse PTs where Src/EGFR is constitutively activated. Analysis in mouse embryonic fibroblast cells confirmed the existence of PPARγ/Src-dependent non-genomic signaling, which requires the ligand binding ability but not the transcriptional activity of PPARγ. The TZD-induced enhancement of association between PPARγ and Src supports an obligatory role for Src in this signaling. These results support the view that TZD-induced volume expansion is multifactorial. In addition to the PPARγ-dependent enhanced expression of the sodium transport system(s) in distal nephrons, the PPARγ-dependent non-genomic stimulation of renal proximal transport may be also involved in TZD-induced volume expansion.