Available online at www.sciencedirect.com

ScienceDirect Gastrointestinal HCO3S transport and epithelial protection in the gut: new techniques, transport pathways and regulatory pathways Ursula E Seidler The concept of a protective alkaline gastric and duodenal mucus layer is a century old, yet it is amazing how much new information on HCO3 transport pathways has emerged recently, made possible by the extensive utilization of genedeleted and transgenic mice and novel techniques to study HCO3 transport. This review highlights recent findings regarding the importance of HCO3 for mucosal protection of duodenum and other gastrointestinal epithelia against luminal acid and other damaging factors. Recently, methods have been developed to visualize HCO3 transport in vivo by assessing the surface pH in the mucus layer, as well as the epithelial pH. New information about HCO3 transport pathways, and emerging concepts about the intricate regulatory network that governs duodenal HCO3 secretion are described, and new perspectives for drug therapy discussed. Addresses Department of Gastroenterology, Hannover Medical School, Hannover, Germany Corresponding author: Seidler, Ursula E. ([email protected])

Current Opinion in Pharmacology 2013, 13:900–908 This review comes from a themed issue on Gastrointestinal Edited by David T Thwaites For a complete overview see the Issue and the Editorial Available online 25th October 2013 1471-4892/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coph.2013.10.001

A century of research in bicarbonate transport and mucosal protection The ability of the gastrointestinal tract to self-regulate luminal acidity was described a century ago by the Russian physiologist Vasilii Boldyreff (see in [1] for review). Active epithelial HCO3 secretion by the gastric and duodenal mucosa was recognized in the 1970s [2–4]. In chambered gastric or duodenal mucosa in vitro, a pH gradient forms between the surface epithelium and the acidic lumen [5–7]. For decades, the importance of an alkaline pH in the mucus layer for mucosal protection has been controversial [8,9], possibly because this question could not be directly studied in vivo. More recently, it has been demonstrated that in all GI epithelia, HCO3 secretion plays an important role in protection against injury of the epithelial cells by luminal contents: gastric Current Opinion in Pharmacology 2013, 13:900–908

acid, drugs, reactive oxygen species, bile acids and bacterial products in the esophagus, stomach and intestine [9,10,11]; trypsin, bile acids and alcohol in the pancreatic ducts [12]; and toxic bile acids in the biliary tract [13]. The lack of HCO3 ions in the pancreatic secretions of children with cystic fibrosis was recognized in the 1960s and the significance for impaired mucus release discussed [14,15]. It is now evident that CFTR expression is essential for HCO3 secretion in most gastrointestinal epithelia, such as the esophagus [16], the small intestine [17], the biliary tract [18,19], and the pancreatic ducts [20], as well as the reproductive tract [21,22] and the airways [23,24]. The low pH in the acinar-ductal unit after release of the zymogen granules needs to be quickly neutralized to prevent acinar damage [13]. Similarly, the bile ducts need a ‘biliary HCO3 umbrella’ to keep toxic bile acids ionized and thereby membrane-impermeable [13,25], and the esophagus needs HCO3 secretion to protect the epithelial surface from acid reflux [26], and this is possibly mediated also by CFTR-dependent mechanisms [16]. HCO3 is essential for the release and proper expansion of mucin molecules [27,28,29,30]. CF patients and CFTR-deficient mice have impaired lipid absorption, which in mice has been experimentally linked with the duodenal HCO3 deficit [31]. Thus the HCO3 secretory defect of cystic fibrosis patients is closely linked to many of the pathophysiological GI manifestations of CF. The luminal pH can be very acidic in the colon of ulcerative colitis patients [32], which may be caused at least in part by disturbed ion transport in inflamed colon [33,34]. A dramatic downregulation of the expression and membrane localization of the Cl /HCO3 exchanger Slc26a3 (DRA) was observed in inflamed colon [35,36]. This may explain the frequent failure of release of colon-targeted pH-triggered drug delivery formulations and influence the microbial composition and possibly the mucus layer and the absorptive functions of the inflamed colon.

New techniques HCO3 secretion cannot be assessed by tracer methods, and cell culture models for the measurement of gastrointestinal HCO3 secretion and its regulation are not available. Short circuit (Isc) measurements in isolated www.sciencedirect.com

HCO3 transport and epithelial protection in the gut Seidler 901

epithelia in the presence and absence of CO2/HCO3 are fraught with pitfalls because of the HCO3 dependency of basolateral Cl and Na+ import mechanisms. Only direct titration of base equivalents secreted from isolated mucosae or perfused whole intestinal segments into unbuffered luminal perfusate allows direct assessment of HCO3 secretory rate [40]. Damage to the epithelium occurs easily and allows passive diffusion of HCO3 from the serosal side of the chambered mucosa or the interstitium in an anesthetized animal, into the luminal fluid [37–39]. New techniques for the study of HCO3 transport in vivo and the recognition of novel HCO3 transporters and regulatory pathways have recently become available and greatly moved the field forward. Major advances have been made in the assessment of surface epithelial function using video-imaging and two-photon confocal techniques to

assess the intracellular and juxtamucosal pH in anesthetized rodents and isolated tissues using dye methods. The measurement of intracellular pH and juxta-mucosal pH using fluorescent pH-sensitive dyes in vivo and in isolated intestinal mucosa in vitro [40,41] or a combination of pH-microelectrodes, optical techniques and Laser Doppler technology to simultaneously assess mucus thickness, juxtamucosal pH and changes in mucosal blood flow [42] have provided new insights into HCO3 transport pathways, its regulation, and its relation to mucosal protection. Acid or alkaline secretions were measured with blood flow parameters and mucosal injury score in exteriorized stomach and duodena of anesthetized rodents [43,44]. Laser light was used to induce microscopic mucosal injury and simultaneously study the surface pH conditions that occur during repair of these lesions [45]. The use of fluorescent beads as well as pH-sensitive dyes allow simultaneous assessment of surface pH and the build-up of a

Figure 1

(a) Custom made Confocal Chamber

Water bath Heating Control device

Exteriorized intestinal segment

Isoflurane

IN

OUT

Luminal Chamber

Anesthesia Unit

Heating pad

(b)

Surface pH gradient

Beads

Scanning from top to bottom

Mucus Thickness = Distance between beads and epithelial surface

Mucus Layer

Current Opinion in Pharmacology

Schematic diagram of a method for in vivo measurement of intestinal surface pH within the mucus layer. (a) A two-photon microscope set-up for the study of intracellular pH, mucus layer thickness, or surface pH within the mucus layer, in any exteriorized segment of the GI tract. The mouse is lying on its side on a temperature-controlled support, and the exteriorized gut segment with its surface cells loaded with the pH-sensitive dye SNARF-1 is being superfused and optically assessed under a two-photon microscope. (b) The method of measuring mucus layer build-up and surface pH, using a fluorescent dye that will, after calibration, allow an in vivo assessment of the surface pH gradient [29]. www.sciencedirect.com

Current Opinion in Pharmacology 2013, 13:900–908

902 Gastrointestinal

New data demonstrate that the Ca2+ activated TMEM16a anion channel (anoctamin 1) also conducts HCO3 when intracellular Ca2+ levels rise strongly [50]. However, the contribution of this channel to anion secretion in the intestinal tract is still under debate [51–53].

mucus layer in exteriorized vascularly-perfused intestinal mucosa of anesthetized mice (Figure 1) [29]. These novel techniques allow us to dynamically assess the acid/ base parameters important for protection of gastrointestinal mucosa. New aspects of HCO3 transporter physiology are also likely to be gained by the use of intestinal segment-specific organoids [46], which may allow molecular manipulation of ion transporter regulation that has, so far, only been described for pancreatic ducts in organ culture.

In most epithelia, including the GI tract, high HCO3 secretory rates under ‘basal’ conditions appear to require the presence of Cl /HCO3 exchangers of the Slc26A anion transporter family in the apical membranes [9,35]. Each GI epithelium expresses a number of different Slc26A isoforms. Expressed heterologously, these isoforms may exchange Cl and HCO3 either electroneutrally (1:1) or electrogenically (1:2;2:1) [54,55]. This suggests that the transport direction and overall contribution of these isoforms to the HCO3 secretory rate in a given segment of the GI tract may be highly variable, depending on the luminal pH, the membrane potential, and co-expression with other ion transporters. In murine intestine, Slc26a3 (DRA) is involved in Cl absorption and luminal alkalinization in a largely CFTR-independent fashion and is responsible for the high HCO3 output rates in the cecum and the mid-distal colon [56]. In the pancreatic ducts, Slc26a6 (PAT-1) appears to be strictly coupled to CFTR activity [57], and to operate in a 2 HCO3 /1 Cl fashion [58]. The CFTRrich ‘high expressor’ cells in the rat and human jejunum, which are single cells interspersed in the villous

New HCO3S transport pathways In the intestine, the CFTR anion channel remains the most important conductive pathway for HCO3 exit under agonist-stimulated conditions. Recent data have shown how the CFTR channel may switch from a predominantly Cl -conductive to an HCO3 -conductive channel through the activation of WNK (with-no-lysine) kinase and the phosphorylation of downstream kinases SPAK and OSR1 in pancreatic ducts cells [47], although investigations at the single channel level are still missing. In murine duodenum, phosphorylated SPAK and OSR1 are very abundant [48], which may be one reason why, even without exogenous stimulation, the percentage of HCO3 secretory rate that is dependent on luminal Cl (and therefore presumably mediated by Cl /HCO3 exchange) is minor. Another reason may be the low duodenal pH, since acid was found to induce SPAK phosphorylation [49].

Figure 2

Slc26a6 (PAT1)

Slc9a2/a3 (NHE2/3)

2HCO3- HCO3-

Cl-/HCO3-

Na+

Apical membrane Cl-

Cl-

H+

CFTR

Slc26a3 (DRA) Slc4a4 (pNBC1, NBCe1)

Slc4a7 (NBCn1)

KCNQ1/KCNE3 and other K+ channels Na+

Na+ 2HCO3- Na+ HCO3- HCO3-

Basolateral membrane

Cl-

H+

Slc9a1 (NHE1)

Slc4a2 (AE2) Current Opinion in Pharmacology

Schematic diagram of a duodenal villous enterocyte highlighting the acid/base transporters (with documented expression and localization in duodenal villous enterocytes and a documented function) in duodenal HCO3 transport. Duodenal villous enterocytes strongly express CFTR, in contrast to other intestinal segments, where CFTR expression is crypt-based [81]. Duodenal villous enterocytes express two Slc26a isoforms and two Slc9 isoforms in the brush border membrane, as well as two major Na+HCO3 cotransporters and the AE2 anion exchanger [[47,48,49] see [45,46] for review]. Additional anion transporters have been found by PCR in duodenal scrapings but not localized. Current Opinion in Pharmacology 2013, 13:900–908

www.sciencedirect.com

HCO3 transport and epithelial protection in the gut Seidler 903

gut-expressed subtype of the electrogenic Na+HCO3 cotransporter Slc4a4 (also called pancreatic subtype pNBC, or NHEe1B), which differs from the kidney subtype kNBC1, or NBCe1A, in its N-terminal end, its stoichiometry and therefore its presumed transport direction [63], is strongly expressed in villus enterocytes in the small intestine and surface colonocytes in the large intestine [64]. Slc4a4 KO mice have very low blood pH and die within 1–3 weeks after birth [65]. When isolated duodenal, jejunal, cecal and proximal colonic mucosa of these mice were studied in Ussing chambers, basal and forskolin-stimulated HCO3 secretion was compromised only after inhibition of carbonic anhydrases, demonstrating that multiple uptake pathways for HCO3 exist in the intestine [66]. Figure 2 shows a schematic

epithelium, as well as the Brunner’s gland cells in the proximal duodenum, co-express V-type H+ ATPase, not a Cl /HCO3 exchanger in the luminal membrane [59,60]. Despite this, CFTR activation by cAMP-dependent agonists nevertheless results in an increase in the duodenal and jejunal HCO3 secretory rate, which may in part be due to the inhibition of apical proton extrusion by NHE3 [49]. New molecular information also exists regarding the mechanisms of basolateral HCO3 uptake into GI epithelia. In the duodenum, the electroneutral Na+HCO3 cotransporter Slc4a7 (NBCn1) is highly expressed in the basolateral membrane of the villus epithelium [61] and is involved in agonist- and particularly strongly in acid-induced HCO3 secretory response [62,50]. The Figure 3

H+ H2O2

HCO3-

LPS

ATP TLRs CFTR

P2Y1 PLCβ

Pgp (Mdr1a)

ATP

MDP

ADO

IAP

H2O2

Pam3

Duox2

H+

Duox2

PepT1 cPLA2

Gq NOD2

AA

Ca2+ COX

Ca2+

? IKK

cAMP

NFkB

Gs

Genomic effects

AC

EP4

PGE2 Current Opinion in Pharmacology

Schematic diagram of the duodenal enterocyte to explain the hypothetical local surface pH control elicited by luminal acid or bacteria. Reading across the figure from left to right: Luminal acid exposure causes cellular ATP release via P-glycoprotein (Mdr1a), and simultaneously inhibits intestinal alkaline phosphatase (IAP) activity, which results in increased ATP concentration in the lumen. Luminal ATP stimulates apical P2Y receptors, resulting in an increase in intracellular Ca2+, which activates apical dual oxidase-2 (Duox2), an NADPH oxidase, generating luminal H2O2. H2O2 activates cPLA2, followed by COX activation and PGE2 synthesis. PGE2 stimulates EP4 receptors, resulting in increased cellular cAMP levels. This stimulates CFTRmediated HCO3 secretion. This may be beneficial in preventing luminal bacterial attachment to the mucosal surface, in addition to mucosal defenses. Once juxtamucosal pH has increased to the optimal pH range for IAP activity, ATP is cleaved and the ATP-P2Y signal is turned off. When bacterial translocation to the apical surface occurs, luminal bacterial components are also sensed by apical toll-like receptors (TLRs) and NOD2; these increase ATP release and Duox2-mediated H2O2 generation, an anti-bacterial molecule. Therefore, acid and bacterial sensing pathways may provide duodenal mucosal protection as well as host defense [82–84]. www.sciencedirect.com

Current Opinion in Pharmacology 2013, 13:900–908

904 Gastrointestinal

diagram of a duodenal villous enterocyte highlighting the acid/base transporters with documented apical or basolateral localization and role in duodenal HCO3 transport. Enterocytes in other segments of the intestine share some of the transporters but also express distinctly different pathways.

gained attention through its ability to detoxify LPS and strengthen the gut barrier against bacterial invasion [72,73], curbs this stress response through ATP breakdown ([74], see also Figure 3). More recent work also showed an involvement of toll-like, taste, adenosine, bile acid, and estrogen receptors in regulating duodenal HCO3 secretion [75,76,77]. A comprehensive model of luminal acid- as well as bacteria (i.e. Helicobacter pylori) induced signalling that ultimately results in a local stimulation of HCO3 secretion is suggested in Figure 3.

New regulatory pathways Both the NHERF family of PDZ adaptor proteins [67], the WNK (with no lysine) kinases and their downstream targets [47,48,68] and the regulatory protein IRBIT [(IP(3)) receptor-binding protein released with IP(3)] [68,69,70], have recently been demonstrated to play important regulatory roles in coordinating the interplay of different HCO3 transporters with the signalling proteins that regulate their transport activity. Recent major progress has also been made regarding the duodenal mechanisms that sense the luminal milieu and respond with a protective HCO3 secretory response. Luminal CO2 and duodenocyte carbonic anhydrase II was recognized as important for ‘acid sensing’ [71]. ATP release via cellular stress (including high luminal acidity) induces purinergic signalling, eliciting a HCO3 secretory response via Ca2+. As surface pH rises, the intestinal alkaline phosphatase (IAP), a brush border-associated enzyme that has recently

Despite this interesting local regulatory network at the level of the epithelium, it has been known since the late 1990s that a large part of the acid-induced HCO3 secretory response is mediated via axonal reflexes. Gasotransmitter release (NO, CO and H2S), prostaglandin generation and capsaicin-sensitive sensory neurons are involved in acid sensing and/or HCO3 stimulation [see [78] for review, and Figure 4]. On the basis of duodenal acid exposure in anesthetized mice with cell-specific knockout of the cGMP-dependent kinase I (cGKI), Singh et al. recently suggested that approximately 70% of the acid-induced HCO3 secretory response was dependent on cGKI-dependent signal transduction in the nervous system [79].

Figure 4

Acid HCO3Epithelial Cell

EP3 Receptor GC

Gq

EP4 Receptor

AC Gs

Gs

HCO3-

CSAN Ca2+ GTPcGMP

H2S (CSE)

PGE2

NO

ATP

(cNOS)

cAMP

CO

(HO-1, HO-2) COX-1 Current Opinion in Pharmacology

Roles of gas mediators (NO, H2S and CO) in the regulatory mechanism of acid-induced duodenal HCO3 secretion. Acidification of the mucosa releases nitric oxide (NO) locally, probably through the activation of capsaicin-sensitive sensory neurons [85,86]. This in turn increases PGE2 production via the GC/cGMP pathway (the cells where this takes place are not known, but likely reside in the lamina propria). PGE2 stimulates HCO3 secretion via the activation of EP3/EP4 receptors, mediated intracellularly by the AC/cAMP and Ca2+ pathways. Cyclooxygenase 1 (COX-1) and neuronal nitric oxide synthase (nNOS) are major enzymes responsible for the production of PGE2 and NO, respectively, following mucosal acidification. Other gas mediators such as hydrogen sulfide (H2S) and carbon monoxide (CO) are also actively involved in the regulation of this process. H2S is mainly derived from cystathionine g-lyase (CSE), modulates the secretion mediated by PGE2 and NO and sensory neurons. CO is probably derived from both heme oxygenase 1 and 2 (HO-1 and HO-2) and modulates this process mainly mediated by endogenous PGE2. The cellular location of many of these processes in the duodenal epithelium and subepithelial tissues is not exactly known. Figure and text adapted from Takeuchi K., et al. Curr Med Chem 2012 [78] with permission of the author. Current Opinion in Pharmacology 2013, 13:900–908

www.sciencedirect.com

HCO3 transport and epithelial protection in the gut Seidler 905

New perspectives for drug development Pharmacological manipulation to increase epithelial HCO3 secretion is an unmet therapeutic need, particularly in CF patients. Direct application of HCO3 to the epithelia may be a challenge. A recent study found that even a low amount of the F508del CFTR mutant, when expressed in the plasma membrane, enhanced HCO3 secretory rate in the small and large intestine of mice out of proportion to the increase in Isc [48]. This suggests that for CF patients, ‘corrector therapy’, using recently developed or discovered substances that allow CFTR mutations to escape premature degradation and enhance surface expression [80], may enhance both fluid and HCO3 secretion. The realization that the major exit pathway for HCO3 in most epithelia are the Slc26A anion transporters, which have their own inhibitory profile (i.e. certain NSAIDs), and need luminal Cl for HCO3 exit, may influence current management of luminal therapies during acute obstructive (CF) or bleeding episodes. For patients with inflammatory, metabolic or druginduced impairment of epithelial HCO3 secretion, it is currently unclear whether the activation of receptors that have recently been implicated in stimulating epithelial HCO3 secretion may be feasible and may provide therapeutic benefit. The development of small molecules that target CFTR, regulatory proteins, or stimulatory receptors, suggest that such strategies may become available in the future.

Acknowledgements Anurag Kumar Singh kindly provided Figure 1, Yasutada Akiba kindly provided Figure 3, Koji Takeuchi kindly provided Figure 4, and the work was in part funded by the DFG grant Se 460/13-4 and SFB621/C9. Marion Lange helped in compiling the literature list.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Seidler U: Acta Physiologica symposium: acid-base transporters and epithelial electrolyte transport. Acta Physiol (Oxf) 2011, 201:1-2.

2.

Flemstro¨m G: Active alkalinization by amphibian gastric fundic mucosa in vitro. Am J Physiol 1977, 233:E1-E12.

3.

Graper WP, Crass RA, Halpern NB: Secretion of base by the in vitro amphibian antrum. Surg Forum 1976, 27:435-437.

4.

Flemstro¨m G: Stimulation of HCO3S transport in isolated proximal bullfrog duodenum by prostaglandins. Am J Physiol 1980, 239:G198-G203.

5.

Flemstro¨m G, Kivilaakso E: Demonstration of a pH gradient at the luminal surface of rat duodenum in vivo and its dependence on mucosal alkaline secretion. Gastroenterology 1983, 84:787-794.

6.

7.

Takeuchi K, Magee D, Critchlow J, Matthews J, Silen W: Studies of the pH gradient and thickness of frog gastric mucus gel. Gastroenterology 1983, 84:331-340. Williams SE, Turnberg LA: Demonstration of a pH gradient across mucus adherent to rabbit gastric mucosa: evidence for a ‘mucus-bicarbonate’ barrier. Gut 1981, 22:94-96.

www.sciencedirect.com

8.

Wallace JL: Gastric resistance to acid: is the ‘‘mucusbicarbonate barrier’’ functionally redundant? Am J Physiol 1989, 256(1 Pt 1):G31-G38.

9.

Seidler U, Sjo¨blom M: Gastroduodenal Bicarbonate Secretion. Chapter 48. In Physiology of the Gastrointestinal Tract, 5th Edition. Edited by Johnson Leonard Road, Ghishan Fayez KyzK, Kaunitz Jonathan Dnta, Merchant Juanita Laia, Said Hamid MmdM, Wood Jackie Dci . Elseviers; 2012.

10. Kaji I, Akiba Y, Kaunitz JD: Involvement of gut chemosensing in  the regulation of mucosal barrier function and defense mechanisms. J Anim Sci 2013, 91:1957-1962. Comprehensive review of the recently detected chemosensing receptors in the duodenum. 11. Ambort D, Johansson ME, Gustafsson JK, Edmund A,  Hansson GC: Perspectives on mucus properties and formation — lessons from the biochemical world. Cold Spring Harb Perspect Med 2012, 2. Excellent overview of recent data concerning the generation and significance of a firm colonic mucus layer 12. Hegyi P, Male´th J, Venglovecz V, Rakonczay Z Jr: Pancreatic  ductal bicarbonate secretion: challenge of the acinar acid load. Front Physiol 2011, 2:36. Concept of the danger to the acinar ductal unit by loss of bicarbonate neutralisation of acidic acinar secretions. 13. Hohenester S, Wenninger LM, Paulusma CC, van Viliet SJ,  Jefferson DM, Elferink RP, Beuers U: A biliary HCO3S umbrella constitutes a protective mechanism against bile acid-induced injury in human cholangiocytes. Hepatology 2012, 55:173-183. Experimental evidence for the ‘biliary HCO3 umbrella’ against toxic bile acids. 14. Hadorn B, Johansen PG, Anderson CM: Pancreozymin secretin test of exocrine pancreatic function in cystic fibrosis and the significance of the result for the pathogenesis of the disease. Can Med Assoc J 1968, 98:377-385. 15. Johansen PG, Anderson CM, Hadorn B: Cystic fibrosis of the pancreas. A generalised disturbance of water and electrolyte movement in exocrine tissues. Lancet 1968, 1:455-460. 16. Abdulnour-Nakhoul S, Nakhoul HN, Kalliny MI, Gyftopoulos A,  Rabon E, Doetjes R, Brown K, Nakhoul NL: Ion transport mechanisms linked to bicarbonate secretion in the esophageal submucosal glands. Am J Physiol Regul Integr Comp Physiol 2011, 301:R83-R96. First detailed immunohistochemical study of ion transport proteins expressed in esophageal submucosal glands. 17. Seidler U, Blumenstein I, Kretz A, Viellard-Baron D, Rossmann H,  Colledge WH, Evans M, Ratcliff R, Gregor M: A functional CFTR protein is required for mouse intestinal cAMP-, cGMP- and Ca2+-dependent HCO3S secretion. J Physiol 1997, 505:411-442. Loss of agonist-stimulated but not basal HCO3 secretion in duodenal isolated mucosa of CFTR null mice. 18. Cohn JA, Strong TV, Picciotto MR, Nairn AC, Collins FS, Fitz JG:  Localization of the cystic fibrosis transmembrane conductance regulator in human bile duct epithelial cells. Gastroenterology 1993, 105:1857-1864. Description of CFTR expression in biliary epithelium. 19. Zsembery A, Jessner W, Sitter G, Spirli C, Strazzabosco M, Graf J: Correction of CFTR malfunction and stimulation of Ca2+activated Cl channels restore HCO3S secretion in cystic fibrosis bile ductular cells. Hepatology 2002, 35:95-104. 20. Lee MG, Ohana E, Park HW, Yang D, Muallem S: Molecular  mechanism of pancreatic and salivary gland fluid and HCO3S secretion. Physiol Rev 2012, 92:39-74. Excellent review on the topic of pancreatic ductal HCO3 secretion and its regulation. 21. Liu Y, Wang DK, Chen LM: The physiology of bicarbonate transporters in mammalian reproduction. Biol Reprod 2012, 86:99. 22. Chan HC, Ruan YC, He Q, Chen MH, Chen H, Xu WM, Chen WY,  Xie C, Zhang XH, Zhou Z: The cystic fibrosis transmembrane conductance regulator in reproductive health and disease. J Physiol 2009, 587(Pt 10):2187-2195. Excellent review on the role of CFTR in reproductive organs. Current Opinion in Pharmacology 2013, 13:900–908

906 Gastrointestinal

23. Quinton PM: Cystic fibrosis: impaired bicarbonate secretion and mucoviscidosis. Lancet 2008, 372:415-417. 24. Bridges RJ: Mechanisms of bicarbonate secretion: lesson from the airways. Cold Spring Harb Perspect Med 2012, 2. 25. Beuers U, Maroni L, Elferink RO: The biliary HCO3S umbrella: experimental evidence revisited. Curr Opin Gastroenterol 2012, 28:253-257. 26. Long JD, Orlando RC: Esophageal submucosal glands: structure and function. Am J Gastroenterol 1999, 94:2818-2824. 27. Garcia MA, Yang N, Qinton PM: Normal mouse intestinal mucus  release requires cystic fibrosis transmembrane regulatordependent bicarbonate secretion. J Clin Invest 2009, 119:2613-2622. Elegant study on the importance of bicarbonate secretion for mucus release. 28. Yang J, Ye M, Tian C, Yang M, Wang Y, Shu Y: Dopaminergic modulation of axonal potassium channels and action potential waveform in pyramidal neurons o f prefrontal cortex. J Physiol 2013, 591(Pt 13):3233-3251. 29. Singh AK, Xia W, Riederer B, Juric M, Li J, Zheng W, Cinar A, Xiao F, Bachmann O, Song P et al.: Essential role of the  electroneutral Na+-HCO3S cotransporter NBCn1 in murine duodenal acid-base balance and colonic mucus layer build-up in vivo. J Physiol 2013, 591(Pt 8):2189-2204. Surprising findings for the role of the electroneutral NBC (Slc4a7) in the intestine. 30. Gustafsson JK, Ermund A, Ambort D, Johansson ME, Nilsson HE, Thorell K, Hebert H, Sjo¨vall H, Hansson GC: Bicarbonate and functional CFTR channel are required for proper mucin secretion and link cystic fibrosis with its mucus phenotype. J Exp Med 2012, 209:1263-1272. 31. Bijvelds MJ, Bronsveld I, Havinga R, Sinaasappel M, de Jonge HR,  Verkade HJ: Fat absorption in cystic fibrosis mice is impeded by defective lipolysis and post-lipolytic events. Am J Physiol Gastrointest Liver Physiol 2005, 288:G646-G653. Meticulous study on the potential reasons for defective intestinal fat absorption in CF mice. 32. Nugent SG, Kumar D, Rampton DS, Evans DF: Intestinal luminal pH in inflammatory bowel disease: possible determinants and implications for therapy with aminosalicylates and other drugs. Gut 2001, 48:571-577. 33. Binder HJ, Rajendran V, Sadasivan V, Geibel JP: Bicarbonate secretion: a neglected aspect of colonic ion transport. J Clin Gastroenterol 2005, 39:S53-S58. 34. Bachmann O, Seidler U: News from the end of the gut — how the highly segmental pattern of colonic HCO3S transport relates to absorptive function and mucosal integrity. Biol Pharm Bull 2011, 34:794-802. 35. Xiao F, Juric M, Li J, Riederer B, Yeruva S, Singh AK, Zheng L, Glage S, Kollias G, Dudeja P et al.: Loss of downregulated in  adenoma (DRA) impairs mucosal HCO3S secretion in murine ileocolonic inflammation. Inflamm Bowel Dis 2012, 18:101-111. 36. Farkas K, Yeruva S, Rakonczay Z Jr, Ludolph L, Molna´r T, Nagy F, Szepes Z, Schnu´r A, Wittmann T, Hubricht J et al.: New  therapeutic targets in ulcerative colitis: the importance of ion transporters in the human colon. Inflamm Bowel Dis 2011, 17:884-898. Well-performed study of defective Na+/H+ and Cl /HCO3 exchange in human colonic crypts isolated from biopsies from a large cohort of UC patients.

40. Hug MJ, Clarke LL, Gray MA: How to measure CFTR-dependent  bicarbonate transport: from single channels to the intact epithelium. Methods Mol Biol 2011, 741:489-509. Excellent technical review. 41. Chu S, Tanaka S, Kaunitz JD, Montrose MH: Dynamic regulation  of gastric surface pH by luminal pH. J Clin Invest 1999, 103:605-612. Innovative use of two photon microscopy for surface pH measurements in the murine stomach in vivo. 42. Phillipson M, Atuma C, Henriksna¨s J, Holm L: The importance of  mucus layers and bicarbonate transport in preservation of gastric juxtamucosal pH. Am J Physiol Gastrointest Liver Physiol 2002, 282:G211-G219. Seminal paper on gastric surface pH regulation measured with pHmicroelectrodes. 43. Kato S, Takeuchi K, Okabe S: Mechanism by which histamine increases gastric mucosal blood flow in the rat. Role of luminal H+. Dig Dis Sci 1993, 38:1224-1232. 44. Mimaki H, Kagawa S, Aoi M, Kato S, Satoshi T, Kohama K, Takeuchi K: Effect of lafutidine, a histamine H2-receptor antagonist, on gastric mucosal blood flow and duodenal HCO3S secretion in rats: relation to capsaicin-sensitive afferent neurons. Dig Dis Sci 2002, 47:2696-2703. 45. Xue L, Aihara E, Wang TC, Montrose MH: Trefoil factor 2 requires Na+/H+ exchanger 2 activity to enhance mouse gastric epithelial repair. J Biol Chem 2011, 286:38375-38382. 46. Liu J, Walker NM, Cook MT, Ootani A, Clarke LL: Functional CFTR in crypt epithelium of organotypic enteroid cultures from murine small intestine. Am J Physiol Cell Physiol 2012, 302:C1492-C1503. 47. Park HW, Nam JH, Kim JY, Namkung W, Yoon JS, Lee JS, Kim KS,  Venglovecz V, Gray MA, Kim KH et al.: Dynamic regulation of CFTR bicarbonate permeability by [ClS]i and its role in pancreatic bicarbonate secretion. Gastroenterology 2010, 139:620-631. Excellent and very detailed study suggesting the involvement of the WNK pathway in CFTR-mediated HCO3 secretion. 48. Xiao F, Li J, Singh AK, Riederer B, Wang J, Sultan A, Park H,  Lee MG, Lamprecht G, Scholte BJ et al.: Rescue of epithelial HCO3S secretion in murine intestine by apical membrane expression of the cystic fibrosis transmembrane conductance regulator mutant F508del. J Physiol 2012, 590:5317-5334. Low amount of membrane delF508 mutants have a surprisingly higher intestinal HCO3 secretory rate compared to CFTR null murine intestine in vitro and in vivo. 49. Singh AK, Liu Y, Riederer B, Engelhardt R, Thakur BK,  Soleimani M, Seidler U: Molecular transport machinery involved in orchestrating luminal acid-induced duodenal bicarbonate secretion in vivo. J Physiol 2013. 2013 Sep 9. [Epub ahead of print]. This article delineates the multitude of ion transport proteins and signalling events involved in mediating the duodenal HCO3 secretory response to epithelial contact with luminal acid. 50. Jung J, Noam JH, Park HW, Oh U, Yoon JH, Lee MG: Dynamic  modulation of ANO1/TMEM16A HCO3S permeability by Ca2+/ calmodulin. Proc Natl Acad Sci USA 2013, 110:360-365. First description of HCO3 permeability via TMEM16A. 51. Ousingsawat J, Mirza M, Tian Y, Roussa E, Schreiber R, Cook DI, Kunzelmann K: Rotavirus toxin NSP4 induces diarrhea by activation of TMEM16A and inhibition of Na+ absorption. Pflu¨gers Arch 2011, 461:579-589.

37. Vattay P, Feil W, Klimesch S, Wenl E, Starlinger M, Schiessel R: Acid stimulated alkaline secretion in the rabbit duodenum is passive and correlates with mucosal damage. Gut 1988, 29:284-290.

52. Namkung W, Phuan PW, Verkman AS: TMEM16A inhibitors reveal TMEM16A as a minor component of calcium-activated chloride channel conductance in airway and intestinal epithelial cells. J Biol Chem 2011, 286:2365-2374.

38. Shorrock CJ, Prescott RJ, Rees WD: The effects of indomethacin on gastroduodenal morphology and mucosal pH gradient in the healthy human stomach. Gastroenterology 1990, 99:334-339.

53. Quanhua H, Susan T, Halm JZ, Halm DR: Activation of the basolateral membrane ClS conductance essential for electrogenic K+ secretion suppresses electrogenic ClS secretion. Exp Physiol 2011, 96:305-316.

39. Nylander O, Kvietys P, Granger DN: Effects of hydrochloric acid on duodenal and jejunal mucosal permeability in the rat. Am J Physiol 1989, 257(4 Pt 1):G653-G660.

54. Ohana E, Shcheynikov N, Yang D, So I, Muallem S: Determinants of  coupled transport and uncoupled current by the electrogenic SLC26 transporters. J Gen Physiol 2011, 137:239-251.

Current Opinion in Pharmacology 2013, 13:900–908

www.sciencedirect.com

HCO3 transport and epithelial protection in the gut Seidler 907

Excellent study on Slc26A family member stochiometry. 55. Walker NM, Simpson JE, Hoover EE, Brazill JM, Schweinfest CW,  Soleimani M, Clarke LL: Functional activity of Pat-1 (Slc26a6) ClS/HCO3S exchange in the lower villus epithelium of murine duodenum. Acta Physiol (Oxf) 2011, 201:21-31. Technically superb and ingenious study on PAT-1 mediated transport mode in intact duodenal mucosa.

This, and the following two papers, give insight into a very exciting new regulatory pathway for anion transport. 69. Hong JH, Yang D, Shcheynikov N, Ohana E, Shin DM, Muallem S:  Convergence of IRBIT, phosphatidylinositos (4,5) bisphosphate, and WNK/SPAK kinases in regulation of the Na+-HCO3S cotransporters family. Proc Natl Acad Sci USA 2013, 110:4105-4110.

56. Xiao F, Singh AK, Li J, Johansson MF, Xia W, Riederer B, Engelhardt R, Schweinfest C, Soleimani M, Montrose MH et al.: Colonic SLC26A3 Establishes Alkaline Mucus Layer and Its Loss Results in Barrier Impairment and Mucosal Inflammation in Mice. Gastroenterology 2012, 142 1:74.

70. Park S, Shcheynikov N, Hong JH, Zheng C, Suh SH, Kawaai K,  Ando H, Mizutani A, Abe T, Kiyonari H et al.: Irbit mediates synergy between Ca2+ and cAMP signalling pathways during epithelial transport in mice. Gastroenterology 2013, 145:232-241.

57. Wang Y, Soyombo AA, Shcheynikov N, Zeng W, Dorwart M,  Marino CR, Thomas PJ, Muallem S: Slc26a6 regulates CFTR activity in vivo to determine pancreatic duct HCO3S secretion: relevance to cystic fibrosis. EMBO J 2006, 25:5049-5057. Seminal paper on CFTR and Slc26a6 interaction in pancreatic ductal cells.

71. Sjo¨blom M, Singh AK, Zheng W, Wang J, Tuo BG, Krabbenho¨ft A,  Riederer B, Gros G: Duodenal acidity ‘‘sensing’’ but not epithelial HCO3S supply is critically dependent on carbonic anhydrase II expression. Proc Natl Acad Sci USA 2009, 106:13094. Surprising role of carbonic anhydrase II in duodenal acid-induced HCO3 secretory response.

58. Song Y, Yamamoto A, Steward MC, Ko SB, Stewart AK,  Soleimani M, Liu BC, Kondo T, Jin CX, Ishiguro H: Deletion of Slc26a6 alters the stoichiometry of apical ClS/HCO3S exchange in mouse pancreatic duct. Am J Physiol Cell Physiol 2012, 303:C815-C824. Evidence for 2:1 HCO3 :Cl coupling for endogenous murine pancreatic PAT-1.

72. Beumer C, Wolferink M, Raaben W, Fiechter D, Brands R, Seinen W: Calf intestinal alkaline phosphatise, a novel therapeutic drug for lipopolysacharide (LPS)-mediated diseases, attenuates LPS toxicity in mice and piglets. J Pharmacol Exp Ther 2002, 307:737-744.

59. Collaco AM, Jakab RL, Hoekstra N, Mitchell K, Brooks A, Ameen NA: Regulated traffic of anion transporters in mammalian Brunner’s glands: a role for water and fluid transport. Am J Physiol Gastrointest Liver Physiol 2013, 305:G258-G275. 60. Collaco A, Jakab RL, Gorelick FS, Ameen NA: The vacuolar ATPase associates with CFTR in apical endocytic and recycling vesicles and undergoes cAMP regulated trafficking in intestinal epithelial cells. Gastroenterology 2010, 138 1:589. 61. Praetorius J, Hager H, Nielsen S, Aalkjaer C, Friis UG, Ainsworth MA, Johansen T: Molecular and functional evidence for electrogenic and electroneutral Na+-HCO3S cotransporters in murine duodenum. Am J Physiol Gastrointest Liver Physiol 2001, 280:G332-G343. 62. Chen M, Praetorius J, Zheng W, Xiao F, Riederer B, Singh AK, Stieger N, Wang J, Shull GE, Aalkjaer C et al.: The electroneutral Na+-HCO3S cotransporter NBCn1 is a major pHi regulator in murine duodenum. J Physiol 2012, 590:3317-3333. 63. Parker MD, Boron WF: The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters. Physiol Rev 2013, 93:803-959. 64. Jakab RL, Collaco AM, Ameen NA: Physiological relevance of  cell-specific distribution patterns of CFTR, NKCC1, NBCe1, and NHE3 along the crypt-villus axis in the intestine. Am J Physiol Gastrointest Liver Physiol 2011, 300:G82-G98. Fantastic immunohistochemical localization of a variety of ion transporters along the rat GI tract. 65. Gawenis LR, Bradford EM, Prasad V, Lorenz JN, Simpson JE, Clarke LL, Woo AL, Grisham C, Sanford LP, Doetschman T et al.: Colonic anion secretory defects and metabolic acidosis in mice lacking the NBC1 Na+/HCO3S cotransporter. J Biol Chem 2007, 282:9042-9052. 66. Yu Q, Liu X, Riederer B, Tian DA, Tuo BG, Shull GE, Seidler U: Role of the electrogenic Na+HCO3S cotransporter NBCe1 in pHi regulation and anion secretion in murine intestine. Gastroenterology 2013, 144:S131.

73. Goldberg RF, Austen WG Jr, Zhang X, Munene G, Mostafa G, Biswas S, McCormack M, Eberlin KR, Nguyen JT, Tatlidede HS et al.: Intestinal alkaline phosphatase is a gut mucosal defense factor maintained by enteral nutrition. Proc Natl Acad Sci USA 2008, 105:3551-3556. 74. Mizumori M, Ham M, Guth PH, Engel E, Kaunitz JD, Akiba Y: Intestinal alkaline phosphatase regulates protective surface microclimate pH in rat duodenum. J Physiol 2009, 587:3651-3663. 75. Ham M, Mizumori M, Watanabe C, Wang JH, Inoue T, Nakano T, Guth PH, Engel E, Kaunitz JD, Akiba Y: Endogenous luminal surface adenosine signaling regulates duodenal bicarbonate secretion in rats. J Pharmacol Exp Ther 2010, 335:607-613. 76. Wang JH, Inoue T, Higashiyama M, Guth PH, Engel E, Kaunitz JD, Akiba Y: Umami receptor activation increases duodenal bicarbonate secretion via glucagon-like peptide-2 release in rats. J Pharmacol Exp Ther 2011, 339:464-473. 77. Tuo B, Wen G, Wie J, Liu X, Wang X, Zhang Y, Wu H, Dong X,  Chow JY, Vallon V et al.: Estrogen regulation of duodenal bicarbonate secretion and sex-specific protection of human duodenum. Gastroenterology 2011, 141:854-863. 78. Takeuchi K, Aihara E, Kimura M, Dogishi K, Hara T, Hayashi S: Gas  mediators involved in modulating duodenal HCO3S secretion. Curr Med Chem 2012, 19:43-54. Excellent overview over recent knowledge on gas transmitter-mediated modulation of HCO3 secretion. 79. Singh AK, Spießberger B, Zheng W, Xiao F, Lukowski R,  Wegener JW, Weinmeister P, Saur D, Klein S, Schemann M et al.: Neuronal cGMP kinase I is essential for stimulation of duodenal bicarbonate secretion by luminal acid. FASEB J 2012:1745-1754. This paper demonstrates, by using a battery of conditional knockout and cell-specific knockin mice for the cGMP kinase I, that the acid-induced duodenal HCO3 secretory response is largely mediated via a reflex located, in part, in the central nervous system. 80. Merk D, Schubert-Zsilavecz M: Repairing mutated proteins — development of small molecules targeting defects in the cystic fibrosis transmembrane conductance regulator. Expert Opin Drug Discov 2013, 8:691-708.

67. Singh AK, Riederer B, Krabbenho¨ft A, Rausch B, Bonhagen J, Lehmann U, de Jonge HR, Donowitz M, Yun C, Weinman EJ et al.: Differential roles of NHERF1, NHERF2, and PDZK1 in regulating CFTR-mediated intestinal anion secretion in mice. J Clin Invest 2009, 119:540-550.

81. Akiba Y, Inoue T, Engel E, Guth PH, Kaunitz JD: Luminal ATP release is enhanced in multidrug resistant protein Mdr1a knockout mouse duodenum. Gastroenterology 2011, 140:S32.

68. Yang D, Li Q, So I, Huang CL, Ando H, Mizutani A, Seki G,  Mikoshiba K, Thomas PJ, Muallem S: IRBIT governs epithelial secretion in mice by antagonizing the WNK/SPAK kinase pathway. J Clin Invest 2011, 121:956-965.

82. Higashiyama M, Akiba Y, Rudenkyy S, Guth PH, Engel E, Kaunitz JD: Dual oxidase: a novel antimicrobial duodenal defense mechanism. Gastroenterology 2012, 142:S489.

www.sciencedirect.com

Current Opinion in Pharmacology 2013, 13:900–908

908 Gastrointestinal

83. Akiba Y, Higashiyama M, Rudenkyy S, Guth PH, Engel E, Kaunitz JD: Novel nongenomic bacterial component sensing in duodenum via pattern recognition receptors. Gastroenterology 2012, 142:S491. 84. Holzer P, Livingston EH, Guth PH: Sensory neurons signal via an increase in rat gastric mucosal blood flow in the

Current Opinion in Pharmacology 2013, 13:900–908

face of pending acid injury. Gastroenterology 1991, 101:416-423. 85. Kagawa S, AOI M, Kubo Y, Kotani T, Takeuchi K: Stimulation by capsaicin of duodenal HCO3S secretion via afferent neurons and vanilloid receptors in rats: comparison with acid-induced HCO3S response. Dig Dis Sci 2003, 48:1850-1856.

www.sciencedirect.com

Gastrointestinal HCO3- transport and epithelial protection in the gut: new techniques, transport pathways and regulatory pathways.

The concept of a protective alkaline gastric and duodenal mucus layer is a century old, yet it is amazing how much new information on HCO3(-) transpor...
1MB Sizes 0 Downloads 0 Views