Chloride secretion in a morphologically differentiated human colonic cell line that expresses the epithelial Na+ channel

Functional characterization of ion channels expressed in kidney organoids derived from human induced pluripotent stem cells

Kidney organoids derived from human or rodent pluripotent stem cells have glomerular structures and differentiated/polarized nephron segments. Although there is an increasing understanding of the patterns of expression of transcripts and proteins within kidney organoids, there is a paucity of data regarding functional protein expression, in particular on transporters that mediate the vectorial transport of solutes. Using cells derived from kidney organoids, we examined the functional expression of key ion channels that are expressed in distal nephron segments: the large-conductance Ca2+-activated K+ (BKCa) channel, the renal outer medullary K+ (ROMK, Kir1.1) channel, and the epithelial Na+ channel (ENaC). RNA-sequencing analyses showed that genes encoding the pore-forming subunits of these transporters, and for BKCa channels, key accessory subunits, are expressed in kidney organoids. Expression and localization of selected ion channels was confirmed by immunofluorescence microscopy and immunoblot analysis. Electrophysiological analysis showed that BKCa and ROMK channels are expressed in different cell populations. These two cell populations also expressed other unidentified Ba2+-sensitive K+ channels. BKCa expression was confirmed at a single channel level, based on its high conductance and voltage dependence of activation. We also found a population of cells expressing amiloride-sensitive ENaC currents. In summary, our results show that human kidney organoids functionally produce key distal nephron K+ and Na+ channels.NEW & NOTEWORTHY Our results show that human kidney organoids express key K+ and Na+ channels that are expressed on the apical membranes of cells in the aldosterone-sensitive distal nephron, including the large-conductance Ca2+-activated K+ channel, renal outer medullary K+ channel, and epithelial Na+ channel.

LRRC8A-containing chloride channel is crucial for cell volume recovery and survival under hypertonic conditions

Rapid regulatory volume increase (RVI) is important for cell survival under hypertonic conditions. RVI is driven by Cl⁻ uptake via the Na–K–Cl cotransporter (NKCC), which is activated by WNK kinases following a reduction in intracellular [Cl⁻]. However, how intracellular [Cl⁻] is regulated to modulate the WNK–NKCC axis and engage a protective RVI remains unknown. Our work reveals that LRRC8A-containing chloride channel is a key protective factor against hypertonic shocks. Considering that LRRC8A (SWELL1) is typically activated by low ionic strength under hypotonic stress, our results posed another interesting question: what activates this chloride channel under hypertonic stress? We demonstrated that, upon hyperosmotic activation, the p38-MSK1 pathway gates LRRC8A-containing chloride channel to facilitate activation of WNK–NKCC and an effective RVI.

Regulation of cell volume is essential for tissue homeostasis and cell viability. In response to hypertonic stress, cells need rapid electrolyte influx to compensate water loss and to prevent cell death in a process known as regulatory volume increase (RVI). However, the molecular component able to trigger such a process was unknown to date. Using a genome-wide CRISPR/Cas9 screen, we identified LRRC8A, which encodes a chloride channel subunit, as the gene most associated with cell survival under hypertonic conditions. Hypertonicity activates the p38 stress-activated protein kinase pathway and its downstream MSK1 kinase, which phosphorylates and activates LRRC8A. LRRC8A-mediated Cl⁻ efflux facilitates activation of the with-no-lysine (WNK) kinase pathway, which in turn, promotes electrolyte influx via Na⁺/K⁺/2Cl⁻ cotransporter (NKCC) and RVI under hypertonic stress. LRRC8A-S217A mutation impairs channel activation by MSK1, resulting in reduced RVI and cell survival. In summary, LRRC8A is key to bidirectional osmotic stress responses and cell survival under hypertonic conditions.

A functional CFTR assay using primary cystic fibrosis intestinal organoids

We recently established conditions allowing for long-term expansion of epithelial organoids from intestine, recapitulating essential features of the in vivo tissue architecture. Here we apply this technology to study primary intestinal organoids of people suffering from cystic fibrosis, a disease caused by mutations in CFTR, encoding cystic fibrosis transmembrane conductance regulator. Forskolin induces rapid swelling of organoids derived from healthy controls or wild-type mice, but this effect is strongly reduced in organoids of subjects with cystic fibrosis or in mice carrying the Cftr F508del mutation and is absent in Cftr-deficient organoids. This pattern is phenocopied by CFTR-specific inhibitors. Forskolin-induced swelling of in vitro-expanded human control and cystic fibrosis organoids corresponds quantitatively with forskolin-induced anion currents in freshly excised ex vivo rectal biopsies. Function of the CFTR F508del mutant protein is restored by incubation at low temperature, as well as by CFTR-restoring compounds. This relatively simple and robust assay will facilitate diagnosis, functional studies, drug development and personalized medicine approaches in cystic fibrosis.

Gastrin inhibits a novel, pathological colon cancer signaling pathway involving EGR1, AE2, and P-ERK

Human anion exchanger 2 (AE2) is a plasma membrane protein that regulates intracellular pH and cell volume. AE2 contributes to transepithelial transport of chloride and bicarbonate in normal colon and other epithelial tissues. We now report that AE2 overexpression in colon cancer cells is correlated with expression of the nuclear proliferation marker, Ki67. Survival analysis of 24 patients with colon cancer in early stage or 33 patients with tubular adenocarcinoma demonstrated that expression of AE2 is correlated with poor prognosis. Cellular and molecular experiments indicated that AE2 expression promoted proliferation of colon cancer cells. In addition, we found that transcription factor EGR1 underlies AE2 upregulation and the AE2 sequester p16INK4a (P16) in the cytoplasm of colon cancer cells. Cytoplasmic P16 enhanced ERK phosphorylation and promoted proliferation of colon cancer cells. Gastrin inhibited proliferation of colon cancer cells by suppressing expression of EGR1 and AE2 and by blocking ERK phosphorylation. Taken together, our data describe a novel EGR1/AE2/P16/P-ERK signaling pathway in colon carcinogenesis, with implications for pathologic prognosis and for novel therapeutic approaches.

Rab27a negatively regulates CFTR chloride channel function in colonic epithelia: Involvement of the effector proteins in the regulatory mechanism

Cystic fibrosis, an autosomal recessive disorder, is caused by the disruption of biosynthesis or function of CFTR. CFTR regulatory mechanisms include channel transport to plasma membrane and protein-protein interactions. Rab proteins are small GTPases involved in vesicle transport, docking, and fusion. The colorectal epithelial HT-29 cells natively express CFTR and respond to cAMP with an increase in CFTR-mediated currents. DPC-inhibited currents could be completely eliminated with CFTR-specific SiRNA. Over-expression of Rab27a inhibited, while isoform specific SiRNA and Rab27a antibody stimulated CFTR-mediated currents in HT-29 cells. CFTR activity is inhibited both by Rab27a (Q78L) (constitutive active GTP-bound form of Rab27a) and Rab27a (T23N) (constitutive negative form that mimics the GDP-bound form). Rab27a mediated effects could be reversed by Rab27a-binding proteins, the synaptotagmin-like protein (SLP-5) and Munc13-4 accessory protein (a putative priming factor for exocytosis). The SLP reversal of Rab27a effect was restricted to C2A/C2B domains while the SHD motif imparted little more inhibition. The CFTR-mediated currents remain unaffected by Rab3 though SLP-5 appears to weakly bind it. The immunoprecipitation experiments suggest protein-protein interactions between Rab27a and CFTR. Rab27a appears to impair CFTR appearance at the cell surface by trapping CFTR in the intracellular compartments. Munc13-4 and SLP-5, on the other hand, limit Rab27a availability to CFTR, thus minimizing its effect on channel function. These observations decisively prove that Rab27a is involved in CFTR channel regulation through protein-protein interactions involving Munc13-4 and SLP-5 effector proteins, and thus could be a potential target for cystic fibrosis therapy.

Restoration of ENaC expression by glucocorticoid receptor transfection in human HT-29/B6 colon cells

The epithelial sodium channel (ENaC) controls colonic sodium absorption. So far, investigation of ENaC was limited by an unexplained lack of steroid-dependent ENaC expression in cultured intestinal cells, which we aimed to resolve. HT-29/B6 cells constitutively expressed the alpha-ENaC subunit, while beta- and gamma-ENaC subunits could not be detected due to deficient basal as well as corticosteroid-induced transcription. This was due to a lack of expression of both activating and inhibiting isoforms of glucocorticoid receptor (GR-alpha, -beta) and mineralocorticoid receptor. Stable transfection of GR-alpha restored intestine-specific glucocorticoid upregulation of beta- and gamma-ENaC in HT-29/B6 cells, which was followed by intact targeting of ENaC channels to the apical cell membrane and dose-dependent induction of electrogenic sodium absorption. In conclusion, ENaC deficiency is due to a lack of steroid receptors and not the consequence of a crypt-like phenotype of cultured intestinal cells. By stable GR transfection we obtained a model, in which ENaC regulation can be studied.

Luminally active, nonabsorbable CFTR inhibitors as potential therapy to reduce intestinal fluid loss in cholera

Enterotoxin-mediated secretory diarrheas such as cholera involve chloride secretion by enterocytes into the intestinal lumen by the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. We previously identified glycine hydrazide CFTR blockers that by electrophysiological studies appeared to block the CFTR anion pore at its lumen-facing surface. Here, we synthesize highly water-soluble, nonabsorbable malondihydrazides by coupling 2,4-disulfobenzaldehyde, 4-sulfophenylisothiocyante, and polyethylene glycol (PEG) moieties to 2-naphthalenylamino-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] propanedioic acid dihydrazide, and aminoacethydrazides by coupling PEG to [(N-2-naphthalenyl)-2-(2-hydroxyethyl)]-glycine-2-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] hydrazide. Compounds rapidly, fully and reversibly blocked CFTR-mediated chloride current with Ki of 2-8 microM when added to the apical surface of epithelial cell monolayers. Compounds did not pass across Caco-2 monolayers, and were absorbed by <2%/hr in mouse intestine. Luminally added compounds blocked by >90% cholera toxin-induced fluid secretion in mouse intestinal loops, without inhibiting intestinal fluid absorption. These orally administered, nonabsorbable, nontoxic CFTR inhibitors may reduce intestinal fluid losses in cholera.

Cyclic AMP-dependent Cl- secretion induced by thromboxane A2 in isolated human colon

Increased release of thromboxane A(2) (TXA(2)) has been shown to be involved in inflammatory bowel diseases. In the present study, we have investigated the effect of a stable TXA(2) analogue (STA(2)) on the electrical parameters in isolated human colonic mucosa. In the human mucosa set between Ussing chambers, STA(2) stimulated Cl- secretion in a concentration-dependent manner with an EC(50) of 0.06 microm. The STA(2)-induced Cl- secretion was significantly inhibited by ONO-3708 (10 microm), a specific TXA(2) receptor antagonist. The effect of STA(2) (0.3 microm) was independent of the colonic segment from which the tissue was obtained, from caecum to rectum. Chromanol 293B, an inhibitor of the cAMP-dependent KvLQT1 channel, attenuated the STA(2)-induced Cl- secretion in the human colonic mucosa (IC(50) value 1.18 microm). We found that KvLQT1 mRNA and protein were expressed in all the tested segments of the human colon. The STA(2)-induced Cl- secretion was significantly inhibited by 8-bromo-2'-monobutyryladenosine-3',5'-cyclic monophosphorothioate (50 microm), a membrane-permeant cAMP antagonist. STA(2) (0.3 microm) significantly increased the intracellular cAMP levels and the short-circuit current via TXA(2) receptor in a human colonic cell line. These results suggest that the TXA(2)-induced Cl- secretion in the colon is mediated via the cAMP pathway in addition to the Ca(2+)-calmodulin pathway which was previously reported.

Effects of in ovo feeding of carbohydrates and beta-hydroxy-beta-methylbutyrate on the development of chicken intestine

Early development of the digestive tract is crucial for achieving maximal growth and development of chickens. Because the late-term embryo naturally consumes the amniotic fluids, insertion of a nutrient solution into the embryonic amniotic fluid [in ovo (IO) feeding] may enhance development. This study examined the effect of IO feeding on d 17.5 of incubation of carbohydrates (CHO) and beta-hydroxy-beta-methylbutyrate (HMB) on small intestinal development of chickens during the pre and posthatch periods. Results shows that 48 h post-IO feeding procedure all IO feeding treatments exhibited increased villus width and surface area compared with the control group. At d 3 posthatch the surface area of an average villi was increased by 45% for the HMB IO group and by 33% for the CHO and CHO+HMB IO groups compared with controls (noninjected fertile eggs). The activity of jejunal sucrase-isomaltase (SI) was higher (P < 0.05) 48 h after IO feeding in all the IO fed embryos, whereas at day of hatch and at d 3 the CHO+HMB IO group had the highest maltase activity (P < 0.05), which was approximately 50% greater than control embryos. These observations indicated that small intestines of IO fed hatchlings were functionally at a similar stage of development as a conventionally fed 2-d-old chick. Body weight of all IO fed hatchlings was greater than controls, and these differences (P < 0.05) were sustained until the end of the experiment (10 d). At d 10 chicks that were IO fed with CHO had BW that were 2.2% higher, whereas HMB and CHO+HMB IO fed chicks showed 5 to 6.2% BW increase, respectively, compared with controls. The current study shows that the administration of exogenous nutrients into the amnion enhanced intestinal development by increasing the size of the villi and by increasing the intestinal capacity to digest disaccharides. This advantage probably leads to higher BW in IO fed chicks.