Anion exchanger 1b, but not sodium-bicarbonate cotransporter 1b, plays a role in transport functions of zebrafish H⁺-ATPase-rich cells

Abstract

ϩ -ATPase rich (HR) cells in zebrafish skin and gills are also responsible for Na ϩ uptake and acid secretion functions. However, the basolateral transport pathways in HR cells are still unclear. In the present study, we tested the hypothesis if there are specific slc4 members involved in basolateral ion transport pathways in HR cells. Fourteen isoforms were identified in the zebrafish(z) slc4 family, and the full-length cDNAs of two novel isoforms, zslc4a1b (anion exchanger, zAE1b) and zslc4a4b (Na ϩ /HCO 3 Ϫ cotransporter, zNBCe1b), were sequenced. mRNA signals of zslc4a1b and zslc4a4b were mainly detected in certain groups of ionocytes in zebrafish skin/gills. Further double immunocytochemistry or in situ hybridization demonstrated that zAE1b, but not zNBCe1b, was localized to basolateral membranes of HR cells. Acclimation to low-Na ϩ or acidic environments stimulated the mRNA expression of zslc4a1b in zebrafish gills, and loss-of-function of zslc4a1b with specific morpholinos caused significant decreases in both the whole body Na ϩ content and the skin H ϩ activity in the morphants. On the basis of these results, it was concluded that zAE1b, but not zNBCe1b, is involved in the basolateral transport pathways in Na ϩ uptake/acid secretion mechanisms in zebrafish HR cells.acid-base regulation; fish; ion regulation; mitochondrion-rich cell; Na uptake BICARBONATE (HCO 3 Ϫ ) is a single-carbon molecule that plays crucial roles in diverse animal physiological processes, including whole body and cellular pH regulation, cellular volume mediation, and NaCl absorption (1,4,34). The solute carrier family 4 (SLC4) was identified as the transporters responsible for the transmembrane movement of HCO 3 Ϫ , and the family contains 10 members in mammals (34). In mammalian nephrons, about 70 -80% of filtered sodium and bicarbonate is reabsorbed in proximal tubules (43), and the electrogenic Na ϩ -HCO 3 Ϫ cotransporter (NBCe1/SLC4A4) is present in the basolateral membrane of proximal tubular cells for the epithelial transport of bicarbonate from the lumen to the blood (34). Human mutations in NBCe1 are associated with proximal renal tubular acidosis (pRTA), which results in increased renal HCO 3Ϫ levels due to a failure of bicarbonate reabsorption (4). On the other hand, in ␣-intercalated cells of the collecting duct, 5% filtered bicarbonate is reabsorbed, and the major basolateral bicarbonate transporter is the kidney form of the anion exchanger (kAE1/SLC4A1) (1). Different sets of AE1 mutations, such as AE1R589H (21) and AE1901X (6, 40), result in distal RTA, which is due to the failure of acid secretion by ␣-intercalated cells of the cortical collecting duct in the kidney. In proximal tubular cells and collecting duct ␣-intercalated cells, cytosolic carbonic anhydrase generates protons and bicarbonate, protons are excreted by the apical Na ϩ /H ϩ exchanger (NHE) and/or H ϩ -ATPase (HA), and the generated HCO 3 Ϫ is reabsorbed into the blood by the basolateral NBCe1 (proximal tubular cells) or kAE1 (␣-intercalated cells) (1,4...

Branchial NH4+-dependent acid–base transport mechanisms and energy metabolism of squid (Sepioteuthis lessoniana) affected by seawater acidification

“…Due to the lack of sequence information the existence and role of anion exchangers (e.g. AE1) which were demonstrated to contribute to acid-base homeostasis in teleosts [46] remains unexplored for cephalopods.…”

Oestrogen-related receptor α is required for transepithelial H + secretion in zebrafish

“…Knockdown of ERRa decreased the expression of metabolic genes (electronic supplementary material, figure S2) as well as the transporter genes related to H þ secretion (figure 4). Zebrafish HR cells express a set of transporters, including H þ -ATPase [26], NHE3b [49] and AE1 [30], that are critical for H þ secretion of HR cells; knockdown or inhibition of these transporters has been demonstrated to impair H þ secretion in zebrafish embryos [27,29,30]. In this study, it is shown that the decrease in H þ secretion caused by ERRa knockdown (figure 3a) was accompanied by a significant reduction in gene expression of these three transporters (figure 4), suggesting the actions of ERRa on H þ secretion can be attributed to the impairment of transporter expression.…”

“…To examine whether ERRa regulates H þ secretion through affecting the expression of related transporters, we used Q-PCR to measure the expression of genes encoding transporters related to H þ secretion. While the majority of H þ secretion in HR cells is mediated by H þ -ATPase [29], the sodium hydrogen exchanger 3b (NHE3b) and anion exchanger 1b (AE1b) are also involved, as evidenced by observations that inhibition or knockdown of these transporters impaired H þ secretion [27,30]. As compared with the control group, the expression levels of apt6v1a (a subunit of H þ -ATPase) (figure 4a), slc9a3.2 (nhe3b) (figure 4b) and slc4a1b (ae1b) (figure 4c) were all significantly downregulated (e) Reduced HR cell number at yolk-sac skin in ERRa morphants…”

Recent advances in understanding trans-epithelial acid-base regulation and excretion mechanisms in cephalopods

“…12,13 In vertebrate systems the basolateral import of HCO 3 ¡ is achieved by various transporters of the SLC4 family, including anion exchangers, and Na C /HCO 3 ¡ co-transporters. 96,97 Two potential candidates for the transport of HCO 3 ¡ ions including an electrogenic Na C /HCO 3 ¡ co-transporter (NBC e ) and a Na C -dependent Cl ¡ /HCO 3 ¡ exchanger (NDCBE) were cloned from the squid giant axon. 98,99 While NDCBE had comparatively low transcript abundance in branchial epithelia but higher RNA levels in neurons, the electrogenic NBC was highly expressed in gill epithelia and expression of this gene was stimulated upon hypercapnic exposure.…”