AQP4 transfected into mouse cholangiocytes promotes water transport in biliary epithelia

Expression profiling and immunolocalization of Na+-D-glucose-cotransporter 1 in mice employing knockout mice as specificity control indicate novel locations and differences between mice and rats

The expression and localization of sodium-D-glucose cotransporter SGLT1 (SLC5A1), which is involved in small intestinal glucose absorption and renal glucose reabsorption, is of high biomedical relevance because SGLT1 inhibitors are currently tested for antidiabetic therapy. In human and rat organs, detailed expression profiling of SGLT1/Sglt1 mRNA and immunolocalization of the transporter protein has been performed. Using polyspecific antibodies and preabsorption with antigenic peptide as specificity control, in several organs, different immunolocalizations of SGLT1/Sglt1 between human and rat were obtained. Because the preabsorption control does not exclude cross-reactivity with similar epitopes, some localizations remained ambiguous. In the present study, we performed an immunocytochemical localization of Sglt1 in various organs of mice. Specificities of the immunoreactions were evaluated using antibody preabsorption with the Sglt1 peptide and the respective organs of Sglt1 knockout mice. Because staining in some locations was abolished after antibody preabsorption but remained in the knockout mice, missing staining in knockout mice was used as specificity criterion. The immunolocalization in mouse was identical or similar to rat in many organs, including small intestine, liver, and kidney. However, the male-dominant renal Sglt1 protein expression in mice differed from the female-dominant expression in rats, and localization in lung, heart, and brain observed in rats was not detected in mice. In mice, several novel locations of Sglt1, e.g., in eyes, tongue epithelial cells, pancreatic ducts, prostate, and periurethral glands were detected. Using end-point and quantitative RT-PCR in various organs, different Sglt1 expression in mice and rats was confirmed.

Biology of Cholangiocytes: From Bench to Bedside

Cholangiocytes, the lining epithelial cells in bile ducts, are an important subset of liver cells. They are activated by endogenous and exogenous stimuli and are involved in the modification of bile volume and composition. They are also involved in damaging and repairing the liver. Cholangiocytes have many functions including bile production. They are also involved in transport processes that regulate the volume and composition of bile. Cholangiocytes undergo proliferation and cell death under a variety of conditions. Cholangiocytes have functional and morphological heterogenecity. The immunobiology of cholangiocytes is important, particularly for understanding biliary disease. Secretion of different proinflammatory mediators, cytokines, and chemokines suggests the major role that cholangiocytes play in inflammatory reactions. Furthermore, paracrine secretion of growth factors and peptides mediates extensive cross-talk with other liver cells, including hepatocytes, stellate cells, stem cells, subepithelial myofibroblasts, endothelial cells, and inflammatory cells. Cholangiopathy refers to a category of chronic liver diseases whose primary disease target is the cholangiocyte. Cholangiopathy usually results in end-stage liver disease requiring liver transplant. We summarize the biology of cholangiocytes and redefine the concept of cholangiopathy. We also discuss the recent progress that has been made in understanding the pathogenesis of cholangiopathy and how such progress has influenced therapy.

A comprehensive analysis of aquaporin and secretory related gene expression in neonate and adult cholangiocytes

Canalicular bile is secreted by hepatocytes and then passes through the intrahepatic bile ducts, comprised of cholangiocytes, to reach the extrahepatic biliary system. In addition to providing a conduit for bile to drain from the liver, cholangiocytes play an active role in modifying bile composition. Bile formation is the result of a series of highly coordinated intricate membrane-transport interactions. Proper systematic regulation of solute and water transport is critical for both digestion and the health of the liver, yet our knowledge of cholangiocyte water and ion transporters and their relative expression patterns remains incomplete. In this report, we provide a comprehensive expression profile of the aquaporin (AQP) family and three receptors/channels known to regulate ion transport in the murine cholangiocyte. In murine intrahepatic cholangiocytes, we found mRNA expression for all twelve of the members of the AQP family of proteins and found temporal changes in the expression profile occurring with age. Using AQP4, an established marker within cholangiocyte physiology, we found that AQP2, AQP5 and AQP6 expression levels to be significantly different between the neonatal and adult time points. Furthermore, there were distinct temporal expression patterns, with that of AQP12 unique in that its expression level decreased with age, whilst the majority of AQPs followed an increasing expression level trend with age. Of the three receptors/channels regulating ion transport in the murine cholangiocyte, only the cystic fibrosis transmembrane conductance regulator was found to follow a consistent trend of decreasing expression coincident with age. We have further validated AQP3 and AQP8 protein localization in both hepatocytes and cholangiocytes. This study emphasizes the need to further appreciate and consider the differences in cholangiocyte biology when treating neonatal and adult hepatobiliary diseases.

Physiology of cholangiocytes

Cholangiocytes are epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. The main physiologic function of cholangiocytes is modification of hepatocyte-derived bile, an intricate process regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules through intracellular signaling pathways and cascades. The mechanisms and regulation of bile modification are reviewed herein.

Water channel proteins in bile formation and flow in health and disease: when immiscible becomes miscible

An essential function of the liver is the formation and secretion of bile, a complex aqueous solution of organic and inorganic compounds essential as route for the elimination of body cholesterol as unesterified cholesterol or as bile acids. In bile, a considerable amount of otherwise insoluble cholesterol is solubilized by carriers including two other classes of lipids, namely phospholipid and bile acids. Formation of bile and generation of bile flow are driven by the active secretion of bile acids, lipids and electrolytes into the canalicular and bile duct lumens followed by the parallel movement of water. Thus, water has to cross rapidly into and out of the cell interior driven by osmotic forces. Bile as a fluid, results from complicated interplay of hepatocyte and cholangiocyte uptake and secretion, concentration, by involving a number of transporters of lipids, anions, cations, and water. The discovery of the aquaporin water channels, has clarified the mechanisms by which water, the major component of bile (more than 95%), moves across the hepatobiliary epithelia. This review is focusing on novel acquisitions in liver membrane lipidic and water transport and functional participation of aquaporin water channels in multiple aspects of hepatobiliary fluid balance. Involvement of aquaporins in a series of clinically relevant hepatobiliary disorders are also discussed.

Preparative scale production of functional mouse aquaporin 4 using different cell-free expression modes

The continuous progress in the structural and functional characterization of aquaporins increasingly attracts attention to study their roles in certain mammalian diseases. Although several structures of aquaporins have already been solved by crystallization, the challenge of producing sufficient amounts of functional proteins still remains. CF (cell free) expression has emerged in recent times as a promising alternative option in order to synthesize large quantities of membrane proteins, and the focus of this report was to evaluate the potential of this technique for the production of eukaryotic aquaporins. We have selected the mouse aquaporin 4 as a representative of mammalian aquaporins. The protein was synthesized in an E. coli extract based cell-free system with two different expression modes, and the efficiencies of two modes were compared. In both, the P-CF (cell-free membrane protein expression as precipitate) mode generating initial aquaporin precipitates as well as in the D-CF (cell-free membrane protein expression in presence of detergent) mode, generating directly detergent solubilized samples, we were able to obtain mg amounts of protein per ml of cell-free reaction. Purified aquaporin samples solubilized in different detergents were reconstituted into liposomes, and analyzed for the water channel activity. The calculated P(f) value of proteoliposome samples isolated from the D-CF mode was 133 µm/s at 10°C, which was 5 times higher as that of the control. A reversible inhibitory effect of mercury chloride was observed, which is consistent with previous observations of in vitro reconstituted aquaporin 4. In this study, a fast and convenient protocol was established for functional expression of aquaporins, which could serve as basis for further applications such as water filtration.

Revised immunolocalization of the Na+-d-glucose cotransporter SGLT1 in rat organs with an improved antibody

Previously, we characterized localization of Na+-glucose cotransporter SGLT1 ( Slc5a1) in the rat kidney using a polyclonal antibody against the synthetic COOH-terminal peptide of the rat protein (Sabolić I, Škarica M, Gorboulev V, Ljubojević M, Balen D, Herak-Kramberger CM, Koepsell H. Am J Physiol Renal Physiol 290: 913–926, 2006). However, the antibody gave some false-positive reactions in immunochemical studies. Using a shortened peptide for immunization, we have presently generated an improved, more specific anti-rat SGLT1 antibody (rSGLT1-ab), which in immunochemical studies with isolated membranes and tissue cryosections from male (M) and female (F) rats exhibited 1) in kidneys and small intestine, labeling of a major protein band of ∼75 kDa; 2) in kidneys of adult animals, localization of rSGLT1 to the proximal tubule (PT) brush-border membrane (S1 < S2 < S3) and intracellular organelles (S1 > S2 > S3), with zonal (cortex < outer stripe) and sex differences (M < F) in the protein expression, which correlated well with the tissue expression of its mRNA in RT-PCR studies; 3) in kidneys of castrated adult M rats, upregulation of the protein expression; 4) in kidneys of prepubertal rats, weak and sex-independent labeling of the 75-kDa protein band and immunostaining intensity; 5) in small intestine, sex-independent regional differences in protein abundance (jejunum > duodenum = ileum); and 6) thus far unrecognized localization of the transporter in cortical thick ascending limbs of Henle and macula densa in kidney, bile ducts in liver, enteroendocrine cells and myenteric plexus in the small intestine, and initial ducts in the submandibular gland. Our improved rSGLT1-ab may be used to identify novel sites of SGLT1 localization and thus unravel additional physiological functions of this transporter in rat organs.

Aquaporins in the hepatobiliary tract. Which, where and what they do in health and disease

The biological importance of the aquaporin family of water channels was recently acknowledged by the 2003 Nobel Prize for Chemistry awarded to the discovering scientist Peter Agre. Among the pleiotropic roles exerted by aquaporins in nature in both health and disease, the review addresses the latest acquisitions about the expression and regulation, as well as physiology and pathophysiology of aquaporins in the hepatobiliary tract. Of note, at least seven out of the thirteen mammalian aquaporins are expressed in the liver, bile ducts and gallbladder. Aquaporins are essential for bile water secretion and reabsorption, as well as for plasma glycerol uptake by the hepatocyte and its conversion to glucose during starvation. Novel data are emerging regarding the physio-pathological involvement of aquaporins in multiple diseases such as cholestases, liver cirrhosis, obesity and insulin resistance, fatty liver, gallstone formation and even microparasite invasion of intrahepatic bile ducts. This body of knowledge represents the mainstay of present and future research in a rapidly expanding field.

Gene expression profile analysis of the spontaneous reversal of rat hepatic fibrosis by cDNA microarray

Our aim was to gain insight into the gene expression profile during hepatic fibrosis autoreversal. Spontaneous recovery from hepatic fibrosis was created in SD rats by CCl(4) exposure for 8 weeks and then withdrawal for 6 weeks. Then differentially expressed genes during regression of fibrosis were analyzed using cDNA microarray. Results obtained were further subjected to hierarchical clustering and validated by semiquantitative RT-PCR. Expression of Mapk1 and Rps6ka1, which are critical members of the mitogen-activated protein kinase (MAPK) signaling pathway, was also investigated by Northern blot and immunohistochemistry. Microarray hybridization identified 254 genes differentially expressed throughout resolution of fibrosis. Being verified by RT-PCR, up- or down-regulated genes were classified into various groups according to clustering and function: (1) metabolic enzymes, (2) facilitated diffusion proteins/transporters/symporters, (3) gastrointestinal hormones/receptors, (4) lipoproteins/fatty acid binding proteins, (5) transcription factors/nuclear factors, and (6) the MAPK signaling pathway. The mRNA level of Mapk1 increased greatly as hepatic fibrosis reversed. Meanwhile Mapk1 and Rps6ka1 were proven to be expressed in hepatocytes and absent from mesenchymal cells. Six groups of genes exhibit a close relation to the recovery of CCl(4)-induced hepatic fibrosis. The MAPK signaling-dependent pathway, representing one of the gene groups, may contribute to the reversal of hepatic fibrosis.

Dominant-negative suppression of big brain ion channel activity by mutation of a conserved glutamate in the first transmembrane domain

The neurogenic protein Drosophila big brain (BIB), which is involved in the process of neuroblast determination, and the water channel aquaporin-1 (AQP1) are among a subset of the major intrinsic protein (MIP) channels that have been found to show gated monovalent cation channel activity. A glutamate residue in the first transmembrane (M1) domain is conserved throughout the MIP family. Mutation of this residue to asparagine in BIB (E71N) knocks out ion channel activity, and when coexpressed with BIB wild-type as shown here generates a dominant-negative effect on ion channel function, measured in the Xenopus oocyte expression system using two-electrode voltage clamp. cRNAs for wild-type and mutant BIB or AQP1 channels were injected individually or as mixtures. The magnitude of the BIB ionic conductance response was greatly reduced by coexpression of the mutant E71N subunit, suggesting a dominant-negative mechanism of action. The analogous mutation in AQP1 (E17N) did not impair ion channel activation by cGMP, but did knock out water channel function, although not via a dominant-negative effect. This contrast in sensitivity between BIB and AQP1 to mutation of the M1 glutamate suggests the possibility of interesting structural differences in the molecular basis of the ion permeation between these two classes of channels. The dominant-negative construct of BIB could be a tool for testing a role for BIB ion channels during nervous system development in Drosophila. The neurogenic protein Drosophila big brain (BIB), which is involved in the process of neuroblast determination, and the water channel aquaporin-1 (AQP1) are among a subset of the major intrinsic protein (MIP) channels that have been found to show gated monovalent cation channel activity. A glutamate residue in the first transmembrane (M1) domain is conserved throughout the MIP family. Mutation of this residue to asparagine in BIB (E71N) knocks out ion channel activity, and when coexpressed with BIB wild-type as shown here generates a dominant-negative effect on ion channel function, measured in the Xenopus oocyte expression system using two-electrode voltage clamp. cRNAs for wild-type and mutant BIB or AQP1 channels were injected individually or as mixtures. The magnitude of the BIB ionic conductance response was greatly reduced by coexpression of the mutant E71N subunit, suggesting a dominant-negative mechanism of action. The analogous mutation in AQP1 (E17N) did not impair ion channel activation by cGMP, but did knock out water channel function, although not via a dominant-negative effect. This contrast in sensitivity between BIB and AQP1 to mutation of the M1 glutamate suggests the possibility of interesting structural differences in the molecular basis of the ion permeation between these two classes of channels. The dominant-negative construct of BIB could be a tool for testing a role for BIB ion channels during nervous system development in Drosophila.

Cholangiocyte anion exchange and biliary bicarbonate excretion

Primary canalicular bile undergoes a process of fluidization and alkalinization along the biliary tract that is influenced by several factors including hormones, innervation/neuropeptides, and biliary constituents. The excretion of bicarbonate at both the canaliculi and the bile ducts is an important contributor to the generation of the so-called bile-salt independent flow. Bicarbonate is secreted from hepatocytes and cholangiocytes through parallel mechanisms which involve chloride efflux through activation of Cl^- channels, and further bicarbonate secretion via AE2/SLC4A2-mediated Cl^-/HCO₃^- exchange. Glucagon and secretin are two relevant hormones which seem to act very similarly in their target cells (hepatocytes for the former and cholangiocytes for the latter). These hormones interact with their specific G protein-coupled receptors, causing increases in intracellular levels of cAMP and activation of cAMP-dependent Cl^- and HCO₃^- secretory mechanisms. Both hepatocytes and cholangiocytes appear to have cAMP-responsive intracellular vesicles in which AE2/SLC4A2 colocalizes with cell specific Cl^- channels (CFTR in cholangiocytes and not yet determined in hepatocytes) and aquaporins (AQP8 in hepatocytes and AQP1 in cholangiocytes). cAMP-induced coordinated trafficking of these vesicles to either canalicular or cholangiocyte lumenal membranes and further exocytosis results in increased osmotic forces and passive movement of water with net bicarbonate-rich hydrocholeresis.

Aquaporins in the hepatobiliary system

The review focuses on the potential physiological and pathophysiological roles of aquaporins (AQPs), a family of water channel proteins, in the hepatobiliary system. Among 13 aquaporins (AQP0-AQP12) cloned in mammals, seven AQPs have been identified in the liver and biliary tree. Accumulating evidence suggests that AQPs are likely involved in canalicular and ductal bile secretion, gluconeogenesis and microbial infection and may have other novel roles that affect liver function.

Diagnostic significance of aquaporin-1 in liver tumors

The diagnostic utility of aquaporin (AQP)-1 in liver tumors was tested and compared with other well-established markers. In 30 cholangiocarcinomas (CCs), 20 hepatocellular carcinomas (HCCs), and 10 metastatic colorectal carcinomas (MCCs) of the liver, expression of AQP-1, CD10, cytokeratin (CK) 7, CK20, and polyclonal carcinoembryonic antigen (pCEA) was tested. In addition, staining patterns of CD10 and pCEA were analyzed. To compare the selectivity of AQP-1 and CK7 as possible markers for differentiated cholangiocytes, liver biopsies of cholestatic disease were also analyzed. Aquaporin-1 expression was found in 93% of all CCs compared with 0% of HCC (P < .000001) and with 30% of MCC (P < .01). CD10 was positive in 16.7% of CC compared with 40% of HCC (P < .04) and to 20% of MCC (not significant). Cytokeratin 7 was positive in 90% of CC compared with 10% of HCC (P < .00001) and with 20% of MCC (P < .0001). Cytokeratin 20 was positive in 90% of MCC compared with 16.7% of CC (P < .0001) and with 20% of HCC (P < .00001). Canalicular staining patterns of CD10 and pCEA were observed in HCC (100% and 89.5%, respectively) but typically not in CC (0% and 6.7%, respectively) and never in MCC. In cholestatic disease, AQP-1 was expressed in differentiated epithelial cells of the bile ducts, whereas CK7-positive hepatocytes of Rappaport zone 1 did not show any AQP-1 reactivity. Therefore, AQP-1 seems to be a highly selective marker for differentiated cholangiocytes and can be very helpful in the differential diagnosis of liver tumors.

Cholangiocyte biology

PURPOSE OF REVIEW

Cholangiocytes are increasingly recognized as biologically important because of the diverse array of cellular processes in which they participate. Collectively, these processes define normal function and, when disturbed, account for abnormalities that cause disease. Advances in animal models and sophisticated technology in imaging and gene silencing have allowed progress in defining the roles that cholangiocytes play in signaling, transport of water, ions and solutes, and alterations that result in cholestasis. The pace of advances in technology justifies a yearly summary to identify the most important developments in cholangiocyte biology.

RECENT FINDINGS

The main areas of recent progress include insights into the molecular mechanisms of bile secretion and the development of new experimental models and technologies.

SUMMARY

Understanding the critical components and key biologic processes in cholangiocytes responsible for regulation of ductal bile secretion is an initial and required step in generating hypotheses relevant to disease. With regard to the pathologic relevance of this work, cholestatic liver diseases represent a broad group of hepatobiliary disorders with which hepatologists must deal. In addition to genetic defects, the study of the normal and altered trafficking of cholangiocyte transport systems involved in bile secretion may provide a molecular correlate for the functional changes that occur in disease. Critical to this understanding is the ongoing development of experimental models and techniques to interpret data to answer key hypothesis-driven questions. Second, the collegial sharing and exchange of novel concepts, ideas, reagents, and probes promotes positive advances in the field.