CFTR mediated chloride secretion in the avian renal proximal tubule

ClC-5 Downregulation Induces Osteosarcoma Cell Apoptosis by Promoting Bax and tBid Complex Formation

Osteosarcoma is the most common malignant bone tumor. Chloride (Cl^-) channels-mediated Cl^- movement plays an important role in regulating the functions of various cancer cells, but its role in osteosarcoma remains unclear. In this study, we found that ClC-5 was increased in osteosarcoma tissues compared with normal bone tissues. Patients with high ClC-5 expression showed poor overall survival relative to those patients with low ClC-5 expression. Higher ClC-5 expression and lower intracellular Cl^- concentration ([Cl^-]_i) were observed in osteosarcoma cells compared with normal osteoblasts. Lowering [Cl^-]_i increased the viability of osteosarcoma cells, which was markedly blocked by ClC-5 downregulation. Knockdown of ClC-5 significantly induced osteosarcoma cell apoptosis and increased the release of cytochrome c from mitochondria to cytosol, concomitantly with cleavage of caspase-9, caspase-3, and PARP. The effect of ClC-5 downregulation on osteosarcoma cell apoptosis and viability was abolished by caspase-3 and caspase-9 inhibitors, but not caspase-8 inhibitor. Furthermore, ClC-5 inhibition promoted Bax translocation from cytosol to mitochondria. Immunoprecipitation showed that ClC-5 interacted with Bax and ClC-5 downregulation enhanced Bax and tBid complex formation. Collectively, we demonstrate that ClC-5 downregulation induces osteosarcoma cell apoptosis via mitochondria-dependent apoptotic pathway activation by promoting Bax and tBid association and subsequent Bax translocation.

ClC-5 alleviates renal fibrosis in unilateral ureteral obstruction mice

Renal fibrosis is the major feature of end-stage renal disease with high mortality. Chloride (Cl^-) moving along Cl^- channels has been suggested to play to an important role in renal function. This study aims to investigate the role of ClC-5 in renal fibrosis in unilateral ureteral occlusion (UUO) mice. C57BL/6 mice received UUO surgery followed by delivery of adeno-associated virus encoding ClC-5 cDNA (AAVClC-5). Western blotting, real-time PCR and histological analysis were used to investigate the effects of ClC-5 on renal fibrosis and underlying mechanisms. The expression of ClC-5 was significantly decreased in renal cortex of UUO mice and transforming growth factor-β1 (TGF-β1)-stimulated HK2 cells. Overexpression of ClC-5 in vivo markedly ameliorated UUO-induced renal injury and fibrosis. The increased expressions of plasminogen activator inhibitor type 1, connective tissue growth factor, collagen III and collagen IV were also inhibited by ClC-5 upregulation. Moreover, UUO-induced immune cell infiltration and inflammatory cytokines release were attenuated in mice infected with AAVClC-5. In addition, the in vivo and in vitro results showed that ClC-5 overexpression prevented epithelial-to-mesenchymal transition (EMT), concomitantly with a restoration of E-cadherin expression and a decrease of vimentin, α-SMA and S100A4 expressions. Furthermore, ClC-5 overexpression inhibited UUO- or TGF-β1-induced increase in nuclear factor kappa B (NF-κB) acetylation and matrix metalloproteinases-9 (MMP-9) expression. However, downregulation of ClC-5 in HK2 cells further potentiated TGF-β1-induced EMT and increase in NF-κB acetylation and MMP-9 expression. ClC-5 upregulation ameliorates renal fibrosis via inhibiting NF-κB/MMP-9 pathway signaling activation, suggesting that ClC-5 may be a novel therapeutic target for treating renal fibrosis and chronic kidney disease.

Pathophysiology of avian intestinal ion transport

The gut has great importance for the commercial success of poultry production. Numerous ion transporters, exchangers, and channels are present on both the apical and the basolateral membrane of intestinal epithelial cells, and their differential expression along the crypt-villus axis within the various intestinal segments ensures efficient intestinal absorption and effective barrier function. Recent studies have shown that intensive production systems, microbial exposure, and nutritional management significantly affect intestinal physiology and intestinal ion transport. Dysregulation of normal intestinal ion transport is manifested as diarrhoea, malabsorption, and intestinal inflammation resulting into poor production efficiency. This review discusses the basic mechanisms involved in avian intestinal ion transport and the impact of development during growth, nutritional and environmental alterations, and intestinal microbial infections on it. The effect of intestinal microbial infections on avian intestinal ion transport depends on factors such as host immunity, pathogen virulence, and the mucosal organisation of the particular intestinal segment.

A role for the cystic fibrosis transmembrane conductance regulator in the nitric oxide-dependent release of Cl- from acidic organelles in amacrine cells

γ-Amino butyric acid (GABA) and glycine typically mediate synaptic inhibition because their ligand-gated ion channels support the influx of Cl^- However, the electrochemical gradient for Cl^- across the postsynaptic plasma membrane determines the voltage response of the postsynaptic cell. Typically, low cytosolic Cl^- levels support inhibition, whereas higher levels of cytosolic Cl^- can suppress inhibition or promote depolarization. We previously reported that nitric oxide (NO) releases Cl^- from acidic organelles and transiently elevates cytosolic Cl^-, making the response to GABA and glycine excitatory. In this study, we test the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) is involved in the NO-dependent efflux of organellar Cl^- We first establish the mRNA and protein expression of CFTR in our model system, cultured chick retinal amacrine cells. Using whole cell voltage-clamp recordings of currents through GABA-gated Cl^- channels, we examine the effects of pharmacological inhibition of CFTR on the NO-dependent release of internal Cl^- To interfere with the expression of CFTR, we used clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 genome editing. We find that both pharmacological inhibition and CRISPR/Cas9-mediated knockdown of CFTR block the ability of NO to release Cl^- from internal stores. These results demonstrate that CFTR is required for the NO-dependent efflux of Cl^- from acidic organelles.NEW & NOTEWORTHY Although CFTR function has been studied extensively in the context of epithelia, relatively little is known about its function in neurons. We show that CFTR is involved in an NO-dependent release of Cl^- from acidic organelles. This internal function of CFTR is particularly relevant to neuronal physiology because postsynaptic cytosolic Cl^- levels determine the outcome of GABA- and glycinergic synaptic signaling. Thus the CFTR may play a role in regulating synaptic transmission.

Duplicated CFTR isoforms in eels diverged in regulatory structures and osmoregulatory functions

Two cystic fibrosis transmembrane conductance regulator (CFTR) isoforms, CFTRa and CFTRb, were cloned in Japanese eel and their structures and functions were studied in different osmoregulatory tissues in freshwater (FW) and seawater (SW) eels. Molecular phylogenetic results suggested that the CFTR duplication in eels occurred independently of the duplication event in salmonid. CFTRa was expressed in the intestine and kidney and downregulated in both tissues in SW eels, while CFTRb was specifically expressed in the gill and greatly upregulated in SW eels. Structurally, the CFTR isoforms are similar in most functional domains except the regulatory R domain, where the R domain of CFTRa is similar to that of human CFTR but the R domain of CFTRb is unique in having high intrinsic negative charges and fewer phosphorylation sites, suggesting divergence of isoforms in terms of gating properties and hormonal regulation. Immunohistochemical results showed that CFTR was localized on the apical regions of SW ionocytes, suggesting a Cl(-) secretory role as in other teleosts. In intestine and kidney, however, immunoreactive CFTR was mostly found in the cytosolic vesicles in FW eels, indicating that Cl(-) channel activity could be low at basal conditions, but could be rapidly increased by membrane insertion of the stored channels. Guanylin (GN), a known hormone that increases CFTR activity in mammalian intestine, failed to redistribute CFTR and to affect its expression in eel intestine. The results suggested that GN-independent CFTR regulation is present in eel intestine and kidney.

cAMP-stimulated Cl- secretion is increased by glucocorticoids and inhibited by bumetanide in semicircular canal duct epithelium

Background

The vestibular system controls the ion composition of its luminal fluid through several epithelial cell transport mechanisms under hormonal regulation. The semicircular canal duct (SCCD) epithelium has been shown to secrete Cl^- under β₂-adrenergic stimulation. In the current study, we sought to determine the ion transporters involved in Cl^- secretion and whether secretion is regulated by PKA and glucocorticoids.

Results

Short circuit current (I_sc) from rat SCCD epithelia demonstrated stimulation by forskolin (EC₅₀: 0.8 μM), 8-Br-cAMP (EC₅₀: 180 μM), 8-pCPT-cAMP (100 μM), IBMX (250 μM), and RO-20-1724 (100 μM). The PKA activator N6-BNZ-cAMP (0.1, 0.3 & 1 mM) also stimulated I_sc. Partial inhibition of stimulated I_sc individually by bumetanide (10 & 50 μM), and [(dihydroindenyl)oxy]alkanoic acid (DIOA, 100 μM) were additive and complete. Stimulated I_sc was also partially inhibited by CFTR_inh-172 (5 & 30 μM), flufenamic acid (5 μM) and diphenylamine-2,2^′-dicarboxylic acid (DPC; 1 mM). Native canals of CFTR^+/− mice showed a stimulation of I_sc from isoproterenol and forskolin+IBMX but not in the presence of both bumetanide and DIOA, while canals from CFTR^−/− mice had no responses. Nonetheless, CFTR^−/− mice showed no difference from CFTR^+/− mice in their ability to balance (rota-rod). Stimulated I_sc was greater after chronic incubation (24 hr) with the glucocorticoids dexamethasone (0.1 & 0.3 μM), prednisolone (0.3, 1 & 3 μM), hydrocortisone (0.01, 0.1 & 1 μM), and corticosterone (0.1 & 1 μM) and mineralocorticoid aldosterone (1 μM). Steroid action was blocked by mifepristone but not by spironolactone, indicating all the steroids activated the glucocorticoid, but not mineralocorticoid, receptor. Expression of transcripts for CFTR; for KCC1, KCC3a, KCC3b and KCC4, but not KCC2; for NKCC1 but not NKCC2 and for WNK1 but only very low WNK4 was determined.

Conclusions

These results are consistent with a model of Cl^- secretion whereby Cl^- is taken up across the basolateral membrane by a Na⁺-K⁺-2Cl^- cotransporter (NKCC) and potentially another transporter, is secreted across the apical membrane via a Cl^- channel, likely CFTR, and demonstrate the regulation of Cl^- secretion by protein kinase A and glucocorticoids.