The phospholipid flippase ATP8B1 is involved in the pathogenesis of Ulcerative Colitis via establishment of intestinal barrier function

OBJECTIVE

Patients with mutations in ATP8B1 develop Progressive Familial Intrahepatic Cholestasis type 1 (PFIC1), a severe liver disease that requires life-saving liver transplantation. PFIC1 patients also present with gastrointestinal problems, including intestinal inflammation and diarrhea, which are aggravated after liver transplantation. Here we investigate the intestinal function of ATP8B1 in relation to inflammatory bowel diseases.

DESIGN

ATP8B1 expression was investigated in intestinal samples of patients with Crohn's Disease (CD) or Ulcerative Colitis (UC) as well as in murine models of intestinal inflammation. Colitis was induced in ATP8B1-deficient mice with Dextran Sodium Sulphate (DSS) and intestinal permeability was investigated. Epithelial barrier function was assessed in ATP8B1 knock-down Caco2-BBE cells. Co-immunoprecipitation experiments were performed in Caco2-BBE cells overexpressing ATP8B1-eGFP. Expression and localization of ATP8B1 and tight junction proteins were investigated in cells and in biopsies of UC and PFIC1 patients.

RESULTS

ATP8B1 expression was decreased in UC and DSS-treated mice, and associated with a decreased Tight Junctional pathway transcriptional program. ATP8B1-deficient mice were extremely sensisitve to DSS-induced colitis, evidenced by increased intestinal barrier leakage. ATP8B1 knockdown cells showed delayed barrier establishment that associated with affected Claudin-4 (CLDN4) levels and localization.. CLDN4 immunohistochemistry showed a tight-junctional staining in control tissue, whereas in UC and intestinal PFIC1 samples, CLDN4 was not properly localized.

CONCLUSION

ATP8B1 is important in the establishment of the intestinal barrier Downregulation of ATP8B1 levels in UC, and subsequent altered localization of tight junctional proteins, including CLDN4, might therefore be an important mechanism in UC pathophysiology.

The phospholipid flippase ATP8B1 is required for lysosomal fusion in macrophages

ATP8B1 is a phospholipid flippase and member of the type 4 subfamily of P-type ATPases (P4-ATPase) subfamily. P4-ATPases catalyze the translocation of phospholipids across biological membranes, ensuring proper membrane asymmetry, which is crucial for membrane protein targeting and activity, vesicle biogenesis, and barrier function. Here we have investigated the role of ATP8B1 in the endolysosomal pathway in macrophages. Depletion of ATP8B1 led to delayed degradation of content in the phagocytic pathway and in overacidification of the endolysosomal system. Furthermore, ATP8B1 knockdown cells exhibited large multivesicular bodies filled with intraluminal vesicles. Similar phenotypes were observed in CRISPR-generated ATP8B1 knockout cells. Importantly, induction of autophagy led to accumulation of autophagosomes in ATP8B1 knockdown cells. Collectively, our results support a novel role for ATP8B1 in lysosomal fusion in macrophages, a process crucial in the terminal phase of endolysosomal degradation.

ATP8B1 Deficiency Results in Elevated Mitochondrial Phosphatidylethanolamine Levels and Increased Mitochondrial Oxidative Phosphorylation in Human Hepatoma Cells

ATP8B1 is a phospholipid flippase that is deficient in patients with progressive familial intrahepatic cholestasis type 1 (PFIC1). PFIC1 patients suffer from severe liver disease but also present with dyslipidemia, including low plasma cholesterol, of yet unknown etiology. Here we show that ATP8B1 knockdown in HepG2 cells leads to a strong increase in the mitochondrial oxidative phosphorylation (OXPHOS) without a change in glycolysis. The enhanced OXPHOS coincides with elevated low-density lipoprotein receptor protein and increased mitochondrial fragmentation and phosphatidylethanolamine levels. Furthermore, expression of phosphatidylethanolamine N-methyltransferase, an enzyme that catalyzes the conversion of mitochondrial-derived phosphatidylethanolamine to phosphatidylcholine, was reduced in ATP8B1 knockdown cells. We conclude that ATP8B1 deficiency results in elevated mitochondrial PE levels that stimulate mitochondrial OXPHOS. The increased OXPHOS leads to elevated LDLR levels, which provides a possible explanation for the reduced plasma cholesterol levels in PFIC1 disease.

Amino acid metabolism, transport and signalling in the liver revisited

The liver controls the systemic exposure of amino acids entering via the gastro-intestinal tract. For most amino acids except branched chain amino acids, hepatic uptake is very efficient. This implies that the liver orchestrates amino acid metabolism and also controls systemic amino acid exposure. Although many amino acid transporters have been identified, cloned and investigated with respect to substrate specificity, transport mechanism, and zonal distribution, which of these players are involved in hepatocellular amino acid transport remains unclear. Here, we aim to provide a review of current insight into the molecular machinery of hepatic amino acid transport. Furthermore, we place this information in a comprehensive overview of amino acid transport, signalling and metabolism.

CDC50A is required for aminophospholipid transport and cell fusion in mouse C2C12 myoblasts

Myoblast fusion is essential for the formation of multinucleated muscle fibers and is promoted by transient changes in the plasma membrane lipid distribution. However, little is known about the lipid transporters regulating these dynamic changes. Here, we show that proliferating myoblasts exhibit an aminophospholipid flippase activity that is downregulated during differentiation. Deletion of the P4-ATPase flippase subunit CDC50A (also known as TMEM30A) results in loss of the aminophospholipid flippase activity and compromises actin remodeling, RAC1 GTPase membrane targeting and cell fusion. In contrast, deletion of the P4-ATPase ATP11A affects aminophospholipid uptake without having a strong impact on cell fusion. Our results demonstrate that myoblast fusion depends on CDC50A and may involve multiple CDC50A-dependent P4-ATPases that help to regulate actin remodeling.

Role of the IgG4-related cholangitis autoantigen annexin A11 in cholangiocyte protection

BACKGROUND & AIMS

Annexin A11 was identified as autoantigen in IgG4-related cholangitis (IRC), a B-cell driven disease. Annexin A11 modulates calcium-dependent exocytosis, a crucial mechanism for insertion of proteins into their target membranes. Human cholangiocytes form an apical 'biliary bicarbonate umbrella' regarded as defense against harmful hydrophobic bile acid influx. The bicarbonate secretory machinery comprises the chloride/bicarbonate exchanger AE2 and the chloride channel ANO1. We aimed to investigate the expression and function of annexin A11 in human cholangiocytes and a potential role of IgG1/IgG4-mediated autoreactivity against annexin A11 in the pathogenesis of IRC.

METHODS

Expression of annexin A11 in human liver was studied by immunohistochemistry and immunofluorescence. In human control and ANXA11 knockdown H69 cholangiocytes, intracellular pH, AE2 and ANO1 surface expression, and bile acid influx were examined using ratio microspectrofluorometry, cell surface biotinylation, and 22,23-3H-glycochenodeoxycholic acid permeation, respectively. The localization of annexin A11-mEmerald and ANO1-mCherry was investigated by live-cell microscopy in H69 cholangiocytes after incubation with IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies or disease control serum.

RESULTS

Annexin A11 was strongly expressed in human cholangiocytes, but not hepatocytes. Knockdown of ANXA11 led to reduced plasma membrane expression of ANO1, but not AE2, alkalization of intracellular pH and uncontrolled bile acid influx. High intracellular calcium conditions led to annexin A11 membrane shift and colocalization with ANO1. Incubation with IRC patient serum inhibited annexin A11 membrane shift and reduced ANO1 surface expression.

CONCLUSION

Cholangiocellular annexin A11 mediates apical membrane abundance of the chloride channel ANO1, thereby supporting biliary bicarbonate secretion. Insertion is inhibited by IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies. Anti-annexin A11 autoantibodies may contribute to the pathogenesis of IRC by weakening the 'biliary bicarbonate umbrella'.

LAY SUMMARY

We previously identified annexin A11 as a specific autoantigen in immunoglobulin G4-related cholangitis (IRC), a B-cell driven disease affecting the bile ducts. Human cholangiocytes are protected against harmful hydrophobic bile acid influx by a defense mechanism referred to as the 'biliary bicarbonate umbrella'. We found that annexin A11 is required for the formation of a robust bicarbonate umbrella. Binding of patient-derived annexin A11 autoantibodies inhibits annexin A11 function, possibly contributing to bile duct damage by weakening the biliary bicarbonate umbrella in patients with IRC.

Long-Term Effects of Biliverdin Reductase a Deficiency in Ugt1-/- Mice: Impact on Redox Status and Metabolism

Accumulation of neurotoxic bilirubin due to a transient neonatal or persistent inherited deficiency of bilirubin glucuronidation activity can cause irreversible brain damage and death. Strategies to inhibit bilirubin production and prevent neurotoxicity in neonatal and adult settings seem promising. We evaluated the impact of Bvra deficiency in neonatal and aged mice, in a background of unconjugated hyperbilirubinemia, by abolishing bilirubin production. We also investigated the disposal of biliverdin during fetal development. In Ugt1^−/− mice, Bvra deficiency appeared sufficient to prevent lethality and to normalize bilirubin level in adults. Although biliverdin accumulated in Bvra-deficient fetuses, both Bvra^−/− and Bvra^−/−Ugt1^−/− pups were healthy and reached adulthood having normal liver, brain, and spleen histology, albeit with increased iron levels in the latter. During aging, both Bvra^−/− and Bvra^−/−Ugt1^−/− mice presented normal levels of relevant hematological and metabolic parameters. Interestingly, the oxidative status in erythrocytes from 9-months-old Bvra^−/− and Bvra^−/−Ugt1^−/− mice was significantly reduced. In addition, triglycerides levels in these 9-months-old Bvra^−/− mice were significantly higher than WT controls, while Bvra^−/−Ugt1^−/− tested normal. The normal parameters observed in Bvra^−/−Ugt1^−/− mice fed chow diet indicate that Bvra inhibition to treat unconjugated hyperbilirubinemia seems safe and effective.

Collagen release by human hepatic stellate cells requires vitamin C and is efficiently blocked by hydroxylase inhibition

Liver fibrosis is characterized by the accumulation of extracellular matrix proteins, mainly composed of collagen. Hepatic stellate cells (HSCs) mediate liver fibrosis by secreting collagen. Vitamin C (ascorbic acid) is a cofactor of prolyl-hydroxylases that modify newly synthesized collagen on the route for secretion. Unlike most animals, humans cannot synthesize ascorbic acid and its role in liver fibrosis remains unclear. Here, we determined the effect of ascorbic acid and prolyl-hydroxylase inhibition on collagen production and secretion by human HSCs. Primary human HSCs (p-hHSCs) and the human HSCscell line LX-2 were treated with ascorbic acid, transforming growth factor-beta (TGFβ) and/or the pan-hydroxylase inhibitor dimethyloxalylglycine (DMOG). Expression of collagen-I was analyzed by RT-qPCR (COL1A1), Western blotting, and immunofluorescence microscopy. Collagen secretion was determined in the medium by Western blotting for collagen-I and by HPLC for hydroxyproline concentrations. Expression of solute carrier family 23 members 1 and 2 (SLC23A1/SLC23A2), encoding sodium-dependent vitamin C transporters 1 and 2 (SVCT1/SVCT2) was quantified in healthy and cirrhotic human tissue. In the absence of ascorbic acid, collagen-I accumulated intracellularly in p-hHSCs and LX-2 cells, which was potentiated by TGFβ. Ascorbic acid co-treatment strongly promoted collagen-I excretion and enhanced extracellular hydroxyproline concentrations, without affecting collagen-I (COL1A1) mRNA levels. DMOG inhibited collagen-I release even in the presence of ascorbic acid and suppressed COL1A1 and alpha-smooth muscle actin (αSMA/ACTA2) mRNA levels, also under hypoxic conditions. Hepatocytes express both ascorbic acid transporters, while p-hHSCs and LX-2 express the only SVCT2, which is selectively enhanced in cirrhotic livers. Human HSCs rely on ascorbic acid for the efficient secretion of collagen-I, which can be effectively blocked by hydroxylase antagonists, revealing new therapeutic targets to treat liver fibrosis.

Glycochenodeoxycholate Promotes Liver Fibrosis in Mice with Hepatocellular Cholestasis

Hydrophobic bile salts are considered to promote liver fibrosis in cholestasis. However, evidence for this widely accepted hypothesis remains scarce. In established animal models of cholestasis, e.g., by Mdr2 knockout, cholestasis and fibrosis are both secondary to biliary damage. Therefore, to test the specific contribution of accumulating bile salts to liver fibrosis in cholestatic disease, we applied the unique model of inducible hepatocellular cholestasis in cholate-fed Atp8b1^G308V/G308V mice. Glycochenodeoxycholate (GCDCA) was supplemented to humanize the murine bile salt pool, as confirmed by HPLC. Biomarkers of cholestasis and liver fibrosis were quantified. Hepatic stellate cells (HSC) isolated from wild-type mice were stimulated with bile salts. Proliferation, cell accumulation, and collagen deposition of HSC were determined. In cholestatic Atp8b1^G308V/G308V mice, increased hepatic expression of αSMA and collagen1a mRNA and excess hepatic collagen deposition indicated development of liver fibrosis only upon GCDCA supplementation. In vitro, numbers of myofibroblasts and deposition of collagen were increased after incubation with hydrophobic but not hydrophilic bile salts, and associated with EGFR and MEK1/2 activation. We concluded that chronic hepatocellular cholestasis alone, independently of biliary damage, induces liver fibrosis in mice in presence of the human bile salt GCDCA. Bile salts may have direct pro-fibrotic effects on HSC, putatively involving EGFR and MEK1/2 signaling.

Blocking Sodium-Taurocholate Cotransporting Polypeptide Stimulates Biliary Cholesterol and Phospholipid Secretion in Mice

Active secretion of bile salts into the canalicular lumen drives bile formation and promotes biliary cholesterol and phospholipid output. Disrupting hepatic bile salt uptake, by inhibition of sodium-taurocholate cotransporting polypetide (NTCP; Slc10a1) with Myrcludex B, is expected to limit bile salt flux through the liver and thereby to decrease biliary lipid excretion. Here, we show that Myrcludex B-mediated NTCP inhibition actually causes an increase in biliary cholesterol and phospholipid excretion whereas biliary bile salt output and bile salt composition remains unchanged. Increased lysosomal discharge into bile was excluded as a potential contributor to increased biliary lipid secretion. Induction of cholesterol secretion was not a consequence of increased ATP-binding cassette subfamily G member 5/8 activity given that NTCP inhibition still promoted cholesterol excretion in Abcg8^-/- mice. Stimulatory effects of NTCP inhibition were maintained in Sr-b1^-/- mice, eliminating the possibility that the increase in biliary lipids was derived from enhanced uptake of high-density lipoprotein-derived lipids. NTCP inhibition shifts bile salt uptake, which is generally more periportally restricted, toward pericentral hepatocytes, as was visualized using a fluorescently labeled conjugated bile salt. As a consequence, exposure of the canalicular membrane to bile salts was increased, allowing for more cholesterol and phospholipid molecules to be excreted per bile salt. Conclusion: NTCP inhibition increases biliary lipid secretion, which is independent of alterations in bile salt output, biliary bile salt hydrophobicity, or increased activity of dedicated cholesterol and phospholipid transporters. Instead, NTCP inhibition shifts hepatic bile salt uptake from mainly periportal hepatocytes toward pericentral hepatocytes, thereby increasing exposure of the canalicular membrane to bile salts linking to increased biliary cholesterol secretion. This process provides an additional level of control to biliary cholesterol and phospholipid secretion.

Liver-directed gene therapy results in long-term correction of progressive familial intrahepatic cholestasis type 3 in mice

BACKGROUND & AIMS

Progressive familial intrahepatic cholestasis type 3 (PFIC3), for which there are limited therapeutic options, often leads to end-stage liver disease before adulthood due to impaired ABCB4-dependent phospholipid transport to bile. Using adeno-associated virus serotype 8 (AAV8)-mediated gene therapy, we aimed to restore the phospholipid content in bile to levels that prevent liver damage, thereby enabling stable hepatic ABCB4 expression and long-term correction of the phenotype in a murine model of PFIC3.

METHODS

Ten-week-old Abcb4-/- mice received a single dose of AAV8-hABCB4 (n = 10) or AAV8-GFP (n = 7) under control of a liver specific promoter via tail vein injection. Animals were sacrificed either 10 or 26 weeks after vector administration to assess transgene persistence, after being challenged with a 0.1% cholate diet for 2 weeks. Periodic evaluation of plasma cholestatic markers was performed and bile duct cannulation enabled analysis of biliary phospholipids. Liver fibrosis and the Ki67 proliferation index were assessed by immunohistochemistry.

RESULTS

Stable transgene expression was achieved in all animals that received AAV8-hABCB4 up to 26 weeks after administration. AAV8-hABCB4 expression restored biliary phospholipid excretion, increasing the phospholipid and cholesterol content in bile to levels that ameliorate liver damage. This resulted in normalization of the plasma cholestatic markers, alkaline phosphatase and bilirubin. In addition, AAV8-hABCB4 prevented progressive liver fibrosis and reduced hepatocyte proliferation for the duration of the study.

CONCLUSION

Liver-directed gene therapy provides stable hepatic ABCB4 expression and long-term correction of the phenotype in a murine model of PFIC3. Translational studies that verify the clinical feasibility of this approach are warranted.

LAY SUMMARY

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a severe genetic liver disease that results from impaired transport of lipids to bile, which makes the bile toxic to liver cells. Because therapeutic options are currently limited, this study aims to evaluate gene therapy to correct the underlying genetic defect in a mouse model of this disease. By introducing a functional copy of the missing gene in liver cells of mice, we were able to restore lipid transport to bile and strongly reduce damage to the liver. The proliferation of liver cells was also reduced, which contributes to long-term correction of the phenotype. Further studies are required to evaluate whether this approach can be applied to patients with PFIC3.

The P4-ATPase ATP9A is a novel determinant of exosome release

Extracellular vesicles (EVs) released by cells have a role in intercellular communication to regulate a wide range of biological processes. Two types of EVs can be recognized. Exosomes, which are released from multi-vesicular bodies upon fusion with the plasma membrane, and ectosomes, which directly bud from the plasma membrane. How cells regulate the quantity of EV release is largely unknown. One of the initiating events in vesicle biogenesis is the regulated transport of phospholipids from the exoplasmic to the cytosolic leaflet of biological membranes. This process is catalyzed by P4-ATPases. The role of these phospholipid transporters in intracellular vesicle transport has been established in lower eukaryotes and is slowly emerging in mammalian cells. In Caenorhabditis elegans (C. elegans), deficiency of the P4-ATPase member TAT-5 resulted in enhanced EV shedding, indicating a role in the regulation of EV release. In this study, we investigated whether the mammalian ortholog of TAT-5, ATP9A, has a similar function in mammalian cells. We show that knockdown of ATP9A expression in human hepatoma cells resulted in a significant increase in EV release that was independent of caspase-3 activation. Pharmacological blocking of exosome release in ATP9A knockdown cells did significantly reduce the total number of EVs. Our data support a role for ATP9A in the regulation of exosome release from human cells.

Vps33b is crucial for structural and functional hepatocyte polarity

BACKGROUND & AIMS

In the normal liver, hepatocytes form a uniquely polarised cell layer that enables movement of solutes from sinusoidal blood to canalicular bile. Whilst several cholestatic liver diseases with defects of hepatocyte polarity have been identified, the molecular mechanisms of pathogenesis are not well defined. One example is arthrogryposis, renal dysfunction and cholestasis syndrome, which in most patients is caused by VPS33B mutations. VPS33B is a protein involved in membrane trafficking that interacts with RAB11A at recycling endosomes. To understand the pathways that regulate hepatocyte polarity better, we investigated VPS33B deficiency using a novel mouse model with a liver-specific Vps33b deletion.

METHODS

To assess functional polarity, plasma and bile samples were collected from Vps33b liver knockout (Vps33bfl/fl-AlfpCre) and control (Vps33bfl/fl) mice; bile components or injected substrates were quantitated by mass spectrometry or fluorometry. For structural analysis, livers underwent light and transmission electron microscopy. Apical membrane and tight junction protein localisation was assessed by immunostaining. Adeno-associated virus vectors were used for in vivo gene rescue experiments.

RESULTS

Like patients, Vps33bfl/fl-AlfpCre mice showed mislocalisation of ATP-binding cassette proteins that are specifically trafficked to the apical membrane via Rab11a-positive recycling endosomes. This was associated with retention of bile components in blood. Loss of functional tight junction integrity and depletion of apical microvilli were seen in knockout animals. Gene transfer partially rescued these defects.

CONCLUSIONS

Vps33b has a key role in establishing structural and functional aspects of hepatocyte polarity and may be a target for gene replacement therapy.

LAY SUMMARY

Hepatocytes are liver cells with tops and bottoms; that is, they are polarised. At their bottoms they absorb substances from blood. They then, at their tops, secrete these substances and their metabolites into bile. When polarity is lost, this directional flow of substances from blood to bile is disrupted and liver disease follows. In this study, using a new mouse model with a liver-specific mutation of Vps33b, the mouse version of a gene that is mutated in most patients with arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, we investigated how the Vps33b gene product contributes to establishing hepatocyte polarity. We identified in these mice abnormalities similar to those in children with ARC syndrome. Gene transfer could partly reverse the mouse abnormalities. Our work contributes to the understanding of VPS33B disease and hepatocyte polarity in general, and may point towards gene transfer mediated treatment of ARC liver disease.

Extended Abstract: Deficiency of Sodium Taurocholate Cotransporting Polypeptide (SLC10A1): A New Inborn Error of Metabolism with an Attenuated Phenotype

We present the first patient with a defect in the Na+-taurocholate cotransporting polypeptide SLC10A1 (NTCP), which plays a key role in the enterohepatic circulation of bile salts. The clinical presentation of the child was mild and the child showed no signs of liver dysfunction or pruritus despite extremely elevated plasma bile salt levels (>100-fold upper-limit of normal). A homozygous point mutation was found in the SLC10A1 gene (resulting in amino acid change R252H) and functional studies confirmed the pathogenicity of the mutation. This confirms the role of NTCP as the major transporter of conjugated bile salts into the liver as part of the enterohepatic circulation and shows that other transporters partly can take over its function, resulting in a relatively mild phenotype. This work was published previously in [Vaz et al.: Hepatology 2015;61:260-267] and supplemented with some follow-up information of the patient.

Phospholipid flippases attenuate LPS-induced TLR4 signaling by mediating endocytic retrieval of Toll-like receptor 4

P4-ATPases are lipid flippases that catalyze the transport of phospholipids to create membrane phospholipid asymmetry and to initiate the biogenesis of transport vesicles. Here we show, for the first time, that lipid flippases are essential to dampen the inflammatory response and to mediate the endotoxin-induced endocytic retrieval of Toll-like receptor 4 (TLR4) in human macrophages. Depletion of CDC50A, the β-subunit that is crucial for the activity of multiple P4-ATPases, resulted in endotoxin-induced hypersecretion of proinflammatory cytokines, enhanced MAP kinase signaling and constitutive NF-κB activation. In addition, CDC50A-depleted THP-1 macrophages displayed reduced tolerance to endotoxin. Moreover, endotoxin-induced internalization of TLR4 was strongly reduced and coincided with impaired endosomal MyD88-independent signaling. The phenotype of CDC50A-depleted cells was also induced by separate knockdown of two P4-ATPases, namely ATP8B1 and ATP11A. We conclude that lipid flippases are novel elements of the innate immune response that are essential to attenuate the inflammatory response, possibly by mediating endotoxin-induced internalization of TLR4.

Soluble Adenylyl Cyclase Regulates Bile Salt-Induced Apoptosis in Human Cholangiocytes

Anion exchanger 2 (AE2), the principal bicarbonate secretor in the human biliary tree, is down-regulated in primary biliary cholangitis. AE2 creates a "bicarbonate umbrella" that protects cholangiocytes from the proapoptotic effects of bile salts by maintaining them deprotonated. We observed that knockdown of AE2 sensitized immortalized H69 human cholangiocytes to not only bile salt-induced apoptosis (BSIA) but also etoposide-induced apoptosis. Because the toxicity of etoposide is pH-independent, there could be a more general mechanism for sensitization of AE2-depleted cholangiocytes to apoptotic stimuli. We found that AE2 deficiency led to intracellular bicarbonate accumulation and increased expression and activity of soluble adenylyl cyclase (sAC), an evolutionarily conserved bicarbonate sensor. Thus, we hypothesized that sAC regulates BSIA. H69 cholangiocytes and primary mouse cholangiocytes were used as models. The sAC-specific inhibitor KH7 not only reversed sensitization to BSIA in AE2-depleted H69 cholangiocytes but even completely prevented BSIA. sAC knockdown by tetracycline-inducible short hairpin RNA also prevented BSIA. In addition, sAC inhibition reversed BSIA membrane blebbing, nuclear condensation, and DNA fragmentation. Furthermore, sAC inhibition also prevented BSIA in primary mouse cholangiocytes. Mechanistically, sAC inhibition prevented Bax phosphorylation at Thr167 and mitochondrial translocation of Bax and cytochrome c release but not c-Jun N-terminal kinase activation during BSIA. Finally, BSIA in H69 cholangiocytes was inhibited by intracellular Ca(2+) chelation, aggravated by thapsigargin, and unaffected by removal of extracellular calcium.

CONCLUSIONS

BSIA is regulated by sAC, depends on intracellular Ca(2+) stores, and is mediated by the intrinsic apoptotic pathway; down-regulation of AE2 in primary biliary cholangitis sensitizes cholangiocytes to apoptotic insults by activating sAC, which may play a crucial role in disease pathogenesis. (Hepatology 2016;64:522-534).

ATP11C targets basolateral bile salt transporter proteins in mouse central hepatocytes

ATP11C is a homolog of ATP8B1, both of which catalyze the transport of phospholipids in biological membranes. Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type1 in humans, which is characterized by a canalicular cholestasis. Mice deficient in ATP11C are characterized by a conjugated hyperbilirubinemia and an unconjugated hypercholanemia. Here, we have studied the hypothesis that ATP11C deficiency interferes with basolateral uptake of unconjugated bile salts, a process mediated by organic anion-transporting polypeptide (OATP) 1B2. ATP11C localized to the basolateral membrane of central hepatocytes in the liver lobule of control mice. In ATP11C-deficient mice, plasma total bilirubin levels were 6-fold increased, compared to control, of which ∼65% was conjugated and ∼35% unconjugated. Plasma total bile salts were 10-fold increased and were mostly present as unconjugated species. Functional studies in ATP11C-deficient mice indicated that hepatic uptake of unconjugated bile salts was strongly impaired whereas uptake of conjugated bile salts was unaffected. Western blotting and immunofluorescence analysis demonstrated near absence of basolateral bile salt uptake transporters OATP1B2, OATP1A1, OATP1A4, and Na(+) -taurocholate-cotransporting polypeptide only in central hepatocytes of ATP11C-deficient liver. In vivo application of the proteasome inhibitor, bortezomib, partially restored expression of these proteins, but not their localization. Furthermore, we observed post-translational down-regulation of ATP11C protein in livers from cholestatic mice, which coincided with reduced OATP1B2 levels.

CONCLUSIONS

ATP11C is essential for basolateral membrane localization of multiple bile salt transport proteins in central hepatocytes and may act as a gatekeeper to prevent hepatic bile salt overload. Conjugated hyperbilirubinemia and unconjugated hypercholanemia and loss of OATP expression in ATP11C-deficient liver strongly resemble the characteristics of Rotor syndrome, suggesting that mutations in ATP11C can predispose to Rotor syndrome. (Hepatology 2016;64:161-174).

Rescue of defective ATP8B1 trafficking by CFTR correctors as a therapeutic strategy for familial intrahepatic cholestasis

BACKGROUND & AIMS

ATP8B1 deficiency is an autosomal recessive liver disease characterized by intrahepatic cholestasis. ATP8B1 mutation p.I661T, the most frequent mutation in European patients, results in protein misfolding and impaired targeting to the plasma membrane. Similarly, mutations in cystic fibrosis transmembrane conductance regulator (CFTR), associated with cystic fibrosis, impair protein folding and trafficking. The aim of this study was to investigate whether compounds that rescue CFTR F508del trafficking are capable of improving p.I661T-ATP8B1 plasma membrane expression.

METHODS

The effect of CFTR corrector compounds on plasma membrane expression of p.I661T-ATP8B1 was evaluated by cell surface biotinylation and immunofluorescence. ATPase activity was evaluated of a purified analogue protein carrying a mutation at the matching position (p.L622T-ATP8A2).

RESULTS

The clinically used compounds, 4-phenylbutyric acid (4-PBA), suberoylanilide hydroxamic acid (SAHA) and N-butyldeoxynojirimycin (NB-DNJ) improved p.I661T-ATP8B1 plasma membrane targeting. Compounds C4, C5, C13 and C17 also significantly increased plasma membrane expression of p.I661T-ATP8B1. SAHA and compound C17 upregulated ATP8B1 transcription. p.I661T-ATP8B1 was partly targeted to the canalicular membrane in polarized cells, which became more evident upon treatment with SAHA and/or C4. p.L622T-ATP8A2 showed phospholipid-induced ATPase activity, suggesting that mutations at a matching position in ATP8B1 do not block functionality. Combination therapy of SAHA and compound C4 resulted in an additional improvement of ATP8B1 cell surface abundance.

CONCLUSIONS

This study shows that several CFTR correctors can improve trafficking of p.I661T-ATP8B1 to the plasma membrane in vitro. Hence, these compounds may be suitable to be part of a future therapy for ATP8B1 deficiency and other genetic disorders associated with protein misfolding.

LAY SUMMARY

Compounds that improve the cellular machinery dealing with protein homeostasis (proteostasis) and allow for proper folding of proteins with (mild) missense mutations are called proteostasis regulators (Balch, Science 2008). Such compounds are potentially of high therapeutic value for many (liver) diseases. In this manuscript, we investigated whether compounds identified in screens as CFTR folding correctors are actually proteostasis regulators and thus have a broader application in other protein folding diseases. Using these compounds, we could indeed show improved trafficking to the (apical) plasma membrane of a mutated ATP8B1 protein, carrying the p.I661T missense mutation. This is the most frequently identified mutation in this rare cholestatic disorder. Importantly, ATP8B1 shows no similarity to CFTR. These data are important in providing support for the concept that rare, genetic liver diseases can potentially be treated using a generalized strategy.

ATP8B1-mediated spatial organization of Cdc42 signaling maintains singularity during enterocyte polarization

The disease-associated phospholipid flippase ATP8B1 decreases Cdc42 mobility at the apical membrane to ensure the formation of a single apical domain and to maintain healthy lumen architecture.

During yeast cell polarization localization of the small GTPase, cell division control protein 42 homologue (Cdc42) is clustered to ensure the formation of a single bud. Here we show that the disease-associated flippase ATPase class I type 8b member 1 (ATP8B1) enables Cdc42 clustering during enterocyte polarization. Loss of this regulation results in increased apical membrane size with scattered apical recycling endosomes and permits the formation of more than one apical domain, resembling the singularity defect observed in yeast. Mechanistically, we show that to become apically clustered, Cdc42 requires the interaction between its polybasic region and negatively charged membrane lipids provided by ATP8B1. Disturbing this interaction, either by ATP8B1 depletion or by introduction of a Cdc42 mutant defective in lipid binding, increases Cdc42 mobility and results in apical membrane enlargement. Re-establishing Cdc42 clustering, by tethering it to the apical membrane or lowering its diffusion, restores normal apical membrane size in ATP8B1-depleted cells. We therefore conclude that singularity regulation by Cdc42 is conserved between yeast and human and that this regulation is required to maintain healthy tissue architecture.

Characterization of P4 ATPase Phospholipid Translocases (Flippases) in Human and Rat Pancreatic Beta Cells: THEIR GENE SILENCING INHIBITS INSULIN SECRETION

The negative charge of phosphatidylserine in lipid bilayers of secretory vesicles and plasma membranes couples the domains of positively charged amino acids of secretory vesicle SNARE proteins with similar domains of plasma membrane SNARE proteins enhancing fusion of the two membranes to promote exocytosis of the vesicle contents of secretory cells. Our recent study of insulin secretory granules (ISG) (MacDonald, M. J., Ade, L., Ntambi, J. M., Ansari, I. H., and Stoker, S. W. (2015) Characterization of phospholipids in insulin secretory granules in pancreatic beta cells and their changes with glucose stimulation. J. Biol. Chem. 290, 11075-11092) suggested that phosphatidylserine and other phospholipids, such as phosphatidylethanolamine, in ISG could play important roles in docking and fusion of ISG to the plasma membrane in the pancreatic beta cell during insulin exocytosis. P4 ATPase flippases translocate primarily phosphatidylserine and, to a lesser extent, phosphatidylethanolamine across the lipid bilayers of intracellular vesicles and plasma membranes to the cytosolic leaflets of these membranes. CDC50A is a protein that forms a heterodimer with P4 ATPases to enhance their translocase catalytic activity. We found that the predominant P4 ATPases in pure pancreatic beta cells and human and rat pancreatic islets were ATP8B1, ATP8B2, and ATP9A. ATP8B1 and CDC50A were highly concentrated in ISG. ATP9A was concentrated in plasma membrane. Gene silencing of individual P4 ATPases and CDC50A inhibited glucose-stimulated insulin release in pure beta cells and in human pancreatic islets. This is the first characterization of P4 ATPases in beta cells. The results support roles for P4 ATPases in translocating phosphatidylserine to the cytosolic leaflets of ISG and the plasma membrane to facilitate the docking and fusion of ISG to the plasma membrane during insulin exocytosis.

ATP8B1 and ATP11C: Two Lipid Flippases Important for Hepatocyte Function

P4 ATPases are lipid flippases and transport phospholipids from the exoplasmic to the cytosolic leaflet of biological membranes. Lipid flipping is important for the biogenesis of transport vesicles. Recently it was shown that loss of the P4 ATPases ATP8B1 and ATP11C are associated with severe Cholestatic liver disease. Mutation of ATP8B1 cause progressive familial Intrahepatic Cholestasis type 1 (PFIC1)and benign recurrent intrahepatic cholestasis type 1 (BRIC 1). From our observations we hypothesized that ATP8B1 deficiency causes a phospholipids randomization at the canalicular membrane, which results in extraction of cholesterol due to increase sensitivity of the canalicular membrane. Deficiency of ATP11C causes conjugated hyperbilirubinemia. In our preliminary result we observed accumulation of unconjugated bile salts in Atp11c deficient mice probably because of regulation in the expression or function of OATP1B2. Similar to ATP8B1, ATP11C have regulation on membrane transporters.

Sodium taurocholate cotransporting polypeptide (SLC10A1) deficiency: conjugated hypercholanemia without a clear clinical phenotype

The enterohepatic circulation of bile salts is an important physiological route to recycle bile salts and ensure intestinal absorption of dietary lipids. The Na(+)-taurocholate cotransporting polypeptide SLC10A1 (NTCP) plays a key role in this process as the major transporter of conjugated bile salts from the plasma compartment into the hepatocyte. Here we present the first patient with NTCP deficiency, who was clinically characterized by mild hypotonia, growth retardation, and delayed motor milestones. Total bile salts in plasma were extremely elevated (up to 1,500 μM, ref. <16.3) but there were no clinical signs of cholestatic jaundice, pruritis, or liver dysfunction. Bile salt synthesis and intestinal bile salt signaling were not affected, as evidenced by normal plasma 7α-hydroxy-4-cholesten-3-one (C4) and FGF19 levels. Importantly, the presence of secondary bile salts in the circulation suggested residual enterohepatic cycling of bile salts. Sequencing of the SLC10A1 gene revealed a single homozygous nonsynonymous point mutation in the coding sequence of the gene, resulting in an arginine to histidine substitution at position 252. Functional studies showed that this mutation resulted in a markedly reduced uptake activity of taurocholic acid. Immunofluorescence studies and surface biotinylation experiments demonstrated that the mutant protein is virtually absent from the plasma membrane.

CONCLUSION

We describe the identification of NTCP deficiency as a new inborn error of metabolism with a relatively mild clinical phenotype. The identification of NTCP deficiency confirms that this transporter is the main import system for conjugated bile salts into the liver but also indicates that auxiliary transporters are able to sustain the enterohepatic cycle in its absence.

Hepatic cytochrome P450 deficiency in mouse models for intrahepatic cholestasis predispose to bile salt-induced cholestasis

Progressive familial intrahepatic cholestasis (PFIC) types 1 and 3 are severe cholestatic liver diseases caused by deficiency of ATB8B1 and ABCB4, respectively. Mouse models for PFIC display mild phenotypes compared with human patients, and this can be explained by the difference in bile salt pool composition. Mice, unlike humans, have the ability to detoxify hydrophobic bile salts by cytochrome P450-mediated (re)hydroxylation and thus have a less toxic bile salt pool. We have crossed mouse models for PFIC1 and PFIC3 with Hrn mice that have a reduced capacity to (re)hydroxylate bile salts. Double transgenes were obtained by backcrossing Atp8b1(G308V/G308V) and Abcb4(-/-) mice with Hrn mice that have a liver-specific disruption of the cytochrome P450 reductase gene and therefore have markedly reduced P450 activity. In these mice, a more hydrophobic bile salt pool was instilled by cholic acid supplementation of the diet, and bile formation and liver pathology was studied. As opposed to single transgenes, Atp8b1(G308V/G308V)/Hrn and Abcb4(-/-)/Hrn mice rapidly developed strong cholestasis that was evidenced by increased plasma bilirubin and bile salt levels. The bile salt pool was more toxic in both models; Atp8b1(G308V/G308V)/Hrn mice had a more hydrophobic plasma pool compared with the single transgene, whereas Abcb4(-/-)/Hrn mice had a more hydrophobic biliary pool compared with the single transgene. In line with these findings, liver damage was not aggravated in Atp8b1(G308V/G308V)/Hrn but was more severe in Abcb4(-/-)/Hrn mice. These data indicate that bile salt pool composition is a critical determinant in the initiation and progression of cholestasis and liver pathology in PFIC1 and PFIC3. Most importantly, our data suggest that the hydrophobicity of the plasma bile salt pool is an important determinant of the severity of cholestasis, whereas the hydrophobicity of the biliary bile salt pool is an important determinant of the severity of liver pathology.

The lipid flippase heterodimer ATP8B1-CDC50A is essential for surface expression of the apical sodium-dependent bile acid transporter (SLC10A2/ASBT) in intestinal Caco-2 cells

Deficiency of the phospholipid flippase ATPase, aminophospholipid transporter, class I, type 8B, member 1 (ATP8B1) causes progressive familial intrahepatic cholestasis type 1 (PFIC1) and benign recurrent intrahepatic cholestasis type 1 (BRIC1). Apart from cholestasis, many patients also suffer from diarrhea of yet unknown etiology. Here we have studied the hypothesis that intestinal ATP8B1 deficiency results in bile salt malabsorption as a possible cause of PFIC1/BRIC1 diarrhea. Bile salt transport was studied in ATP8B1-depleted intestinal Caco-2 cells. Apical membrane localization was studied by a biotinylation approach. Fecal bile salt and electrolyte contents were analyzed in stool samples of PFIC1 patients, of whom some had undergone biliary diversion or liver transplantation. Bile salt uptake by the apical sodium-dependent bile salt transporter solute carrier family 10 (sodium/bile acid cotransporter), member 2 (SLC10A2) was strongly impaired in ATP8B1-depleted Caco-2 cells. The reduced SLC10A2 activity coincided with strongly reduced apical membrane localization, which was caused by impaired apical membrane insertion of SLC10A2. Moreover, we show that endogenous ATP8B1 exists in a functional heterodimer with transmembrane protein 30A (CDC50A) in Caco-2 cells. Analyses of stool samples of post-transplant PFIC1 patients demonstrated that bile salt content was not changed, whereas sodium and chloride concentrations were elevated and potassium levels were decreased. The ATP8B1-CDC50A heterodimer is essential for the apical localization of SLC10A2 in Caco-2 cells. Diarrhea in PFIC1/BRIC1 patients has a secretory origin to which SLC10A2 deficiency may contribute. This results in elevated luminal bile salt concentrations and consequent enhanced electrolyte secretion and/or reduced electrolyte resorption.

FXR-dependent reduction of hepatic steatosis in a bile salt deficient mouse model

It has been established that bile salts play a role in the regulation of hepatic lipid metabolism. Accordingly, overt signs of steatosis have been observed in mice with reduced bile salt synthesis. The aim of this study was to identify the mechanism of hepatic steatosis in mice with bile salt deficiency due to a liver specific disruption of cytochrome P450 reductase. In this study mice lacking hepatic cytochrome P450 reductase (Hrn) or wild type (WT) mice were fed a diet supplemented with or without either 0.1% cholic acid (CA) or 0.025% obeticholic acid, a specific FXR-agonist. Feeding a CA-supplemented diet resulted in a significant decrease of plasma ALT in Hrn mice. Histologically, hepatic steatosis ameliorated after CA feeding and this was confirmed by reduced hepatic triglyceride content (115.5±7.3mg/g liver and 47.9±4.6mg/g liver in control- and CA-fed Hrn mice, respectively). The target genes of FXR-signaling were restored to normal levels in Hrn mice when fed cholic acid. VLDL secretion in both control and CA-fed Hrn mice was reduced by 25% compared to that in WT mice. In order to gain insight in the mechanism behind these bile salt effects, the FXR agonist also was administered for 3weeks. This resulted in a similar decrease in liver triglycerides, indicating that the effect seen in bile salt fed Hrn animals is FXR dependent. In conclusion, steatosis in Hrn mice is ameliorated when mice are fed bile salts. This effect is FXR dependent. Triglyceride accumulation in Hrn liver may partly involve impaired VLDL secretion.

P4 ATPases: flippases in health and disease

P4 ATPases catalyze the translocation of phospholipids from the exoplasmic to the cytosolic leaflet of biological membranes, a process termed “lipid flipping”. Accumulating evidence obtained in lower eukaryotes points to an important role for P4 ATPases in vesicular protein trafficking. The human genome encodes fourteen P4 ATPases (fifteen in mouse) of which the cellular and physiological functions are slowly emerging. Thus far, deficiencies of at least two P4 ATPases, ATP8B1 and ATP8A2, are the cause of severe human disease. However, various mouse models and in vitro studies are contributing to our understanding of the cellular and physiological functions of P4-ATPases. This review summarizes current knowledge on the basic function of these phospholipid translocating proteins, their proposed action in intracellular vesicle transport and their physiological role.

Cellular localization and biochemical analysis of mammalian CDC50A, a glycosylated β-subunit for P4 ATPases

CDC50 proteins are β-subunits for P4 ATPases, which upon heterodimerization form a functional phospholipid translocation complex. Emerging evidence in mouse models and men links mutations in P4 ATPase genes with human disease. This study analyzed the tissue distribution and cellular localization of CDC50A, the most abundant and ubiquitously expressed CDC50 homologue in the mouse. The authors have raised antibodies that detect mouse and human CDC50A and studied CDC50A localization and glycosylation status in mouse liver cells. CDC50A is a terminal-glycosylated glycoprotein and is expressed in hepatocytes and liver sinusoidal endothelial cells, where it resides in detergent-resistant membranes. In pancreas and stomach, CDC50A localized to secretory vesicles, whereas in the kidney, CDC50A localized to the apical region of proximal convoluted tubules of the cortex. In WIF-B9 cells, CDC50A partially costains with the trans-Golgi network. Data suggest that CDC50A is present as a fully glycosylated protein in vivo, which presumes interaction with distinct P4 ATPases.

A biliary HCO3- umbrella constitutes a protective mechanism against bile acid-induced injury in human cholangiocytes

Human cholangiocytes are continuously exposed to millimolar levels of hydrophobic bile salt monomers. We recently hypothesized that an apical biliary HCO3- umbrella might prevent the protonation of biliary glycine-conjugated bile salts and uncontrolled cell entry of the corresponding bile acids, and that defects in this biliary HCO3- umbrella might predispose to chronic cholangiopathies. Here, we tested in vitro whether human cholangiocyte integrity in the presence of millimolar bile salt monomers is dependent on (1) pH, (2) adequate expression of the key HCO3- exporter, anion exchanger 2 (AE2), and (3) an intact cholangiocyte glycocalyx. To address these questions, human immortalized cholangiocytes and cholangiocarcinoma cells were exposed to chenodeoxycholate and its glycine/taurine conjugates at different pH levels. Bile acid uptake was determined radiochemically. Cell viability and apoptosis were measured enzymatically. AE2 was knocked down by lentiviral short hairpin RNA. A cholangiocyte glycocalyx was identified by electron microscopy, was enzymatically desialylated, and sialylation was quantified by flow cytometry. We found that bile acid uptake and toxicity in human immortalized cholangiocytes and cholangiocarcinoma cell lines in vitro were pH and AE2 dependent, with the highest rates at low pH and when AE2 expression was defective. An apical glycocalyx was identified on cholangiocytes in vitro by electron microscopic techniques. Desialylation of this protective layer increased cholangiocellular vulnerability in a pH-dependent manner.

CONCLUSION

A biliary HCO3- umbrella protects human cholangiocytes against damage by bile acid monomers. An intact glycocalyx and adequate AE2 expression are crucial in this process. Defects of the biliary HCO3- umbrella may lead to the development of chronic cholangiopathies.

Complementary functions of the flippase ATP8B1 and the floppase ABCB4 in maintaining canalicular membrane integrity

BACKGROUND & AIMS

Progressive familial intrahepatic cholestasis can be caused by mutations in ABCB4 or ATP8B1; each encodes a protein that translocates phospholipids, but in opposite directions. ABCB4 flops phosphatidylcholine from the inner to the outer leaflet, where it is extracted by bile salts. ATP8B1, in complex with the accessory protein CDC50A, flips phosphatidylserine in the reverse direction. Abcb4(-/-) mice lack biliary secretion of phosphatidylcholine, whereas Atp8b1-deficient mice have increased excretion of phosphatidylserine into bile. Each system is thought to have a role protecting the canalicular membrane from bile salts.

METHODS

To investigate the relationship between the mechanisms of ABCB4 and ATP8B1, we expressed the transporters separately and together in cultured cells and studied viability and phospholipid transport. We also created mice with disruptions in ABCB4 and ATP8B1 (double knockouts) and studied bile formation and hepatic damage in mice fed bile salts.

RESULTS

Overexpression of ABCB4 was toxic to HEK293T cells; the toxicity was counteracted by coexpression of the ATP8B1-CDC50A complex. In Atp8b1-deficient mice, bile salts induced extraction of phosphatidylserine and ectoenzymes from the canalicular membrane; this process was not observed in the double-knockout mice.

CONCLUSIONS

ATP8B1 is required for hepatocyte function, particularly in the presence of ABCB4. This is most likely because the phosphatidylserine flippase complex of ATP8B1-CDC50A counteracts the destabilization of the membrane that occurs when ABCB4 flops phosphatidylcholine. Lipid asymmetry is therefore important for the integrity of the canalicular membrane; ABCB4 and ATP8B1 cooperate to protect hepatocytes from bile salts.

Progressive familial intrahepatic cholestasis type 1

Progressive familial intrahepatic cholestasis type 1 is a rare genetic liver disease that presents in the first year of life. Bile salts are elevated and these patients are often jaundiced. Despite the cholestasis, serum gamma-glutamyltransferase activity is normal or reduced. Pruritus is a major symptom in these patients. Partial external biliary diversion is helpful in several patients as it reduces the pruritus and postpones or even avoids liver transplantation. The disease is caused by mutations in the gene ATP8B1 that preclude the normal expression of ATP8B1. ATP8B1 is a protein that acts as a lipid flippase, transporting phosphatidylserine from the exoplasmic to the cytoplasmic leaflet of the canalicular membrane of hepatocytes. The authors have shown that the canalicular membrane of ATP8B1-deficient hepatocytes is less stable as evidenced by enhanced extraction of membrane constituents by bile salts. Recent evidence suggests membrane instability in ATP8B1-deficient hair cells of the ear, providing an explanation for hearing loss in ATP8B1 deficiency. Although the exact etiology of cholestasis is incompletely understood, it is hypothesized that ATP8B1 deficiency results in enhanced cholesterol extraction from the canalicular membrane, which impairs the function of the bile salt export pump (BSEP), resulting in cholestasis. Mutations in ATP8B1 also cause benign recurrent intrahepatic cholestasis, a milder variant of the disease characterized by episodes of cholestasis. The onset and resolution of the cholestatic episodes in these patients is still not well understood.

Strain background modifies phenotypes in the ATP8B1-deficient mouse

Background

Mutations in ATP8B1 (FIC1) underlie cases of cholestatic disease, ranging from chronic and progressive (progressive familial intrahepatic cholestasis) to intermittent (benign recurrent intrahepatic cholestasis). The ATP8B1-deficient mouse serves as an animal model of human ATP8B1 deficiency.

Methodology/Principal Findings

We investigated the effect of genetic background on phenotypes of ATP8B1-deficient and wild-type mice, using C57Bl/6 (B6), 129, and (B6-129) F1 strain backgrounds. B6 background resulted in greater abnormalities in ATP8B1-deficient mice than did 129 and/or F1 background. ATP8B1-deficient pups of B6 background gained less weight. In adult ATP8B1-deficient mice at baseline, those of B6 background had lower serum cholesterol levels, higher serum alkaline phosphatase levels, and larger livers. After challenge with cholate-supplemented diet, these mice exhibited higher serum alkaline phosphatase and bilirubin levels, greater weight loss and larger livers. ATP8B1-deficient phenotypes in mice of F1 and 129 backgrounds are usually similar, suggesting that susceptibility to manifestations of ATP8B1 deficiency may be recessive. We also detected differences in hepatobiliary phenotypes between wild-type mice of differing strains.

Conclusions/Significance

Our results indicate that the ATP8B1-deficient mouse in a B6 background may be a better model of human ATP8B1 deficiency and highlight the importance of informed background strain selection for mouse models of liver disease.

Differential effects of progressive familial intrahepatic cholestasis type 1 and benign recurrent intrahepatic cholestasis type 1 mutations on canalicular localization of ATP8B1

Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type 1 (PFIC1) and benign recurrent intrahepatic cholestasis type 1 (BRIC1), forming a spectrum of cholestatic disease. Whereas PFIC1 is a progressive, endstage liver disease, BRIC1 patients suffer from episodic periods of cholestasis that resolve spontaneously. At present it is not clear how the type and location of the mutations relate to the clinical manifestations of PFIC1 and BRIC1. ATP8B1 localizes to the canalicular membrane of hepatocytes where it mediates the inward translocation of phosphatidylserine. ATP8B1 interacts with CDC50A, which is required for endoplasmic reticulum exit and plasma membrane localization. In this study we analyzed a panel of missense mutations causing PFIC1 (G308V, D554N, G1040R) or BRIC1 (D70N, I661T). In addition, we included two mutations that have been associated with intrahepatic cholestasis of pregnancy (ICP) (D70N, R867C). We examined the effect of these mutations on protein stability and interaction with CDC50A in Chinese hamster ovary cells, and studied the subcellular localization in WIF-B9 cells. Protein stability was reduced for three out of six mutations studied. Two out of three PFIC1 mutant proteins did not interact with CDC50A, whereas BRIC1/ICP mutants displayed reduced interaction. Importantly, none of the PFIC1 mutants were detectable in the canalicular membrane of WIF-B9 cells, whereas all BRIC1/ICP mutants displayed the same cellular staining pattern as wild-type ATP8B1. Our data indicate that PFIC1 mutations lead to the complete absence of canalicular expression, whereas in BRIC1/ICP residual protein is expressed in the canalicular membrane.

CONCLUSION

These data provide an explanation for the difference in severity between the phenotypes of PFIC1 and BRIC1.

P4 ATPases - lipid flippases and their role in disease

P4 ATPases (type 4 P-type ATPases) are multispan transmembrane proteins that have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Studies in Saccharomyces cerevisiae have indicated that P4 ATPases are important in vesicle biogenesis and are required for vesicular trafficking along several intracellular vesicular transport routes. Although little is known about mammalian P4 ATPases, some members of this subfamily appear to be associated with human disease or mouse pathophysiology. ATP8B1, a phosphatidylserine translocase, is the most extensively studied mammalian P4 ATPase. This protein is important for maintaining the detergent resistant properties of the apical membrane of the hepatocyte. Mutations in ATP8B1 give rise to severe liver disease. Furthermore, a role for Atp8b3 in mouse sperm cell capacitation has been suggested, whereas deficiency of Atp10a and Atp10d leads to insulin resistance and obesity in mice. Here we review the present status on the pathophysiological consequences of P4 ATPase deficiency.

Activity of the bile salt export pump (ABCB11) is critically dependent on canalicular membrane cholesterol content

Mutations in ATP8B1 cause severe inherited liver disease. The disease is characterized by impaired biliary bile salt excretion (cholestasis), but the mechanism whereby impaired ATP8B1 function results in cholestasis is poorly understood. ATP8B1 is a type 4 P-type ATPase and is a flippase for phosphatidylserine. Atp8b1-deficient mice display a dramatic increase in the biliary extraction of cholesterol from the canalicular (apical) membrane of the hepatocyte. Here we studied the hypothesis that disproportionate cholesterol extraction from the canalicular membrane impairs the activity of the bile salt transporter, ABCB11, and as a consequence causes cholestasis. Using single pass liver perfusions, we show that not only ABCB11-mediated transport but also Abcc2-mediated transport were reduced at least 4-fold in Atp8b1 deficiency. We show that canalicular membranes of cholestatic Atp8b1-deficient mice have a dramatically reduced cholesterol to phospholipid ratio, i.e. 0.75 +/- 0.24 versus 2.03 +/- 0.71 for wild type. In vitro depletion of cholesterol from mouse liver plasma membranes using methyl-beta-cyclodextrin demonstrated a near linear relation between cholesterol content of the membranes and ATP-dependent taurocholate transport. Abcc2-mediated transport activity was not affected up to 30% of membrane cholesterol depletion but declined to negligible levels at 70% of membrane cholesterol depletion. These effects were reversible as cholesterol repletion of the liver membranes completely restored Abcb11- and Abcc2-mediated transport. Our data demonstrate that membrane cholesterol content is a critical determinant of ABCB11/ABCC2 transport activity, provide an explanation for the etiology of ATP8B1 disease, and suggest a novel mechanism protecting the canalicular membrane against luminal bile salt overload.

Abcg5/8 independent biliary cholesterol excretion in Atp8b1-deficient mice

BACKGROUNDS & AIMS

ATP8B1 is a phosphatidylserine flippase in the canalicular membrane; patients with mutations in ATP8B1 develop severe chronic (PFIC1) or periodic (BRIC1) cholestatic liver disease. We have observed that Atp8b1 deficiency leads to enhanced biliary cholesterol excretion. It has been established that biliary cholesterol excretion depends on transport by the heterodimer Abcg5/Abcg8. We hypothesized that the increased cholesterol output was due to enhanced extraction from the altered canalicular membrane rather than to higher Abcg5/Abcg8 activity. We therefore studied the relation between Abcg5/Abcg8 expression and biliary cholesterol excretion in mice lacking Atp8b1, Abcg8, or both (GF mice).

METHODS

Bile formation was studied in LXR agonist-fed wild-type mice as well as mice lacking Atp8b1 or Abcg8, or in GF mice upon infusion of taurocholate. Bile samples were analyzed for cholesterol, bile salt, phospholipids, and ectoenzyme content.

RESULTS

LXR agonist increased Abcg5/8 expression, and this was accompanied by increased biliary cholesterol output in both wild-type and Atp8b1(G308V/G308V) mice. However, Atp8b1(G308V/G308V) mice maintained higher cholesterol output. Although in Abcg8(-/-) mice biliary cholesterol output was severely reduced, GF mice displayed high biliary cholesterol output, which was comparable with wild-type mice. Bile of both Atp8b1(G308V/G308V) and GF mice displayed elevated levels of phosphatidylserine and sphingomyelin, indicating membrane stress.

CONCLUSIONS

Our data demonstrate that the increased biliary cholesterol excretion in Atp8b1-deficient mice is independent of Abcg5/8 activity. This implicates that Atp8b1 deficiency leads to a decrease in the detergent resistance and subsequent nonspecific extraction of cholesterol from the canalicular membrane by bile salts.

ATP8B1 requires an accessory protein for endoplasmic reticulum exit and plasma membrane lipid flippase activity

Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type 1 and benign recurrent intrahepatic cholestasis type 1. Previously, we have shown in mice that Atp8b1 deficiency leads to enhanced biliary excretion of phosphatidylserine, and we hypothesized that ATP8B1 is a flippase for phosphatidylserine. However, direct evidence for this function is still lacking. In Saccharomyces cerevisiae, members of the Cdc50p/Lem3p family are essential for proper function of the ATP8B1 homologs. We have studied the role of two human members of this family, CDC50A and CDC50B, in the routing and activity of ATP8B1. When only ATP8B1 was expressed in Chinese hamster ovary cells, the protein localized to the endoplasmic reticulum. Coexpression with CDC50 proteins resulted in relocalization of ATP8B1 from the endoplasmic reticulum to the plasma membrane. Only when ATP8B1 was coexpressed with CDC50 proteins was a 250%-500% increase in the translocation of fluorescently labeled phosphatidylserine observed. Importantly, natural phosphatidylserine exposure in the outer leaflet of the plasma membrane was reduced by 17%-25% in cells coexpressing ATP8B1 and CDC50 proteins in comparison with cells expressing ATP8B1 alone. The coexpression of ATP8B1 and CDC50A in WIF-B9 cells resulted in colocalization of both proteins in the canalicular membrane.

CONCLUSION

Our data indicate that CDC50 proteins are pivotal factors in the trafficking of ATP8B1 to the plasma membrane and thus may be essential determinants of ATP8B1-related disease. In the plasma membrane, ATP8B1 functions as a flippase for phosphatidylserine. Finally, CDC50A may be the potential beta-subunit or chaperone for ATP8B1 in hepatocytes.

Alteration of viral lipid composition by expression of the phospholipid floppase ABCB4 reduces HIV vector infectivity

Background

The presence of cholesterol in the Human Immunodeficiency Virus (HIV) lipid envelop is important for viral function as cholesterol depleted viral particles show reduced infectivity. However, it is less well established whether other viral membrane lipids are also important for HIV infection.

The ABCB4 protein is a phosphatidyl choline (PC) floppase that mediates transport of PC from the inner to the outer membrane leaflet. This property enabled us to modulate the lipid composition of HIV vectors and study the effects on membrane composition and infection efficiency.

Results

Virus generated in the presence of ABCB4 was enriched in PC and cholesterol but contained less sphingomyelin (SM). Viral titers were reduced 5.9 fold. These effects were not observed with an inactive ABCB4 mutant. The presence of the ABC transport inhibitor verapamil abolished the effect of ABCB4 expression on viral titers.

The ABCB4 mediated reduction in infectivity was caused by changes in the viral particles and not by components co purified with the virus because virus made in the presence of ABCB4 did not inhibit virus made without ABCB4 in a competition assay.

Incorporation of the envelope protein was not affected by the expression of ABCB4. The inhibitory effect of ABCB4 was independent of the viral envelope as the effect was observed with two different envelope proteins.

Conclusion

Our data indicate that increasing the PC content of HIV particles reduces infectivity.

ATP8B1 requires an accessory protein for endoplasmic reticulum exit and plasma membrane lipid flippase activity

Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type 1 and benign recurrent intrahepatic cholestasis type 1. Previously, we have shown in mice that Atp8b1 deficiency leads to enhanced biliary excretion of phosphatidylserine, and we hypothesized that ATP8B1 is a flippase for phosphatidylserine. However, direct evidence for this function is still lacking. In Saccharomyces cerevisiae, members of the Cdc50p/Lem3p family are essential for proper function of the ATP8B1 homologs. We have studied the role of two human members of this family, CDC50A and CDC50B, in the routing and activity of ATP8B1. When only ATP8B1 was expressed in Chinese hamster ovary cells, the protein localized to the endoplasmic reticulum. Coexpression with CDC50 proteins resulted in relocalization of ATP8B1 from the endoplasmic reticulum to the plasma membrane. Only when ATP8B1 was coexpressed with CDC50 proteins was a 250%-500% increase in the translocation of fluorescently labeled phosphatidylserine observed. Importantly, natural phosphatidylserine exposure in the outer leaflet of the plasma membrane was reduced by 17%-25% in cells coexpressing ATP8B1 and CDC50 proteins in comparison with cells expressing ATP8B1 alone. The coexpression of ATP8B1 and CDC50A in WIF-B9 cells resulted in colocalization of both proteins in the canalicular membrane.

CONCLUSION

Our data indicate that CDC50 proteins are pivotal factors in the trafficking of ATP8B1 to the plasma membrane and thus may be essential determinants of ATP8B1-related disease. In the plasma membrane, ATP8B1 functions as a flippase for phosphatidylserine. Finally, CDC50A may be the potential beta-subunit or chaperone for ATP8B1 in hepatocytes.

Intestinal bile salt absorption in Atp8b1 deficient mice

BACKGROUND/AIMS

Mutations in the ATP8B1 gene can cause Progressive Familial Intrahepatic Cholestasis type 1. We have previously reported that Atp8b1(G308V/G308V) mice, a model for PFIC1, have slightly, but significantly, higher baseline serum bile salt (BS) concentrations compared to wt mice. Upon BS feeding, serum BS concentrations strongly increased in Atp8b1-deficient mice. Despite these findings, we observed only mildly impaired canalicular BS transport. In the present report we tested the hypothesis that Atp8b1(G308V/G308V) mice hyperabsorb BS in the intestine during BS feeding.

METHODS

Intestinal BS absorption was measured in intestinal perfusion and in intestinal explants. In addition, we measured BS concentrations in portal blood. Ileal expression of the Fxr-targets Asbt, Ilbp and Shp was assessed.

RESULTS

In wt and Atp8b1(G308V/G308V) mice, intestinal taurocholate absorption is primarily mediated by the ileal bile salt transporter Asbt. Neither of the experimental systems revealed enhanced absorption of BS in Atp8b1(G308V/G308V) mice compared to wt mice. In line with these observations, we found no difference in the ileal protein expression of Asbt. Induction of Shp expression during BS feeding also demonstrated that Fxr signalling is intact in Atp8b1(G308V/G308V) mice.

CONCLUSIONS

The accumulation of BS in plasma of Atp8b1(G308V/G308V) mice during BS feeding is not caused by increased intestinal BS absorption.

Atp8b1 deficiency in mice reduces resistance of the canalicular membrane to hydrophobic bile salts and impairs bile salt transport

Progressive familial intrahepatic cholestasis type 1 (PFIC1, Byler disease, OMIM 211600) is a severe inherited liver disease caused by mutations in ATP8B1. ATP8B1 is a member of the type 4 subfamily of P-type ATPases, which are phospholipid flippases. PFIC1 patients generally develop end-stage liver disease before the second decade of life. The disease is characterized by impaired biliary bile salt excretion, but the mechanism whereby impaired ATP8B1 function results in cholestasis is unclear. In a mouse model for PFIC1, we observed decreased resistance of the hepatocanalicular membrane to hydrophobic bile salts as evidenced by enhanced biliary recovery of phosphatidylserine, cholesterol, and ectoenzymes. In liver specimens from PFIC1 patients, but not in those from control subjects, ectoenzyme expression at the canalicular membrane was markedly deficient. In isolated mouse livers Atp8b1 deficiency impaired the transport of hydrophobic bile salts into bile. In conclusion, our study shows that Atp8b1 deficiency causes loss of canalicular phospholipid membrane asymmetry that in turn renders the canalicular membrane less resistant toward hydrophobic bile salts. The loss of phospholipid asymmetry may subsequently impair bile salt transport and cause cholestasis.

Diseases of intramembranous lipid transport

The maintenance of transbilayer distribution of phospholipids is crucial for proper cell function. Intramembrane transport of lipids is mediated by three activities termed floppases, flippases, and scramblases. Members of the ATP-binding cassette transporter family and P-type ATPase superfamily have been implicated in the translocation of lipids. The importance of these activities is exemplified by several severe human inherited disorders that are caused by defects in intramembranous transport of lipids. In order to elucidate the molecular mechanisms that underlie these disorders, the combination of in vivo, biochemical, and structural analyses on intramembrane transporters is crucial.

Function and pathophysiological importance of ABCB4 (MDR3 P-glycoprotein)

Like several other ATP-binding cassette (ABC) transporters, ABCB4 is a lipid translocator. It translocates phosphatidylcholine (PC) from the inner to the outer leaflet of the canalicular membrane of the hepatocyte. Its function is quite crucial as evidenced by a severe liver disease, progressive familial intrahepatic cholestasis type 3, which develops in persons with ABCB4 deficiency. Translocation of PC makes the phospholipid available for extraction into the canalicular lumen by bile salts. The primary function of biliary phospholipid excretion is to protect the membranes of cells facing the biliary tree against these bile salts: the uptake of PC in bile salt micelles reduces the detergent activity of these micelles. In this review, we will discuss the functional aspects of ABCB4 and the regulation of its expression. Furthermore, we will describe the clinical and biochemical consequences of complete and partial deficiency of ABCB4 function.

Multidrug resistance associated protein 2 mediates transport of prostaglandin E2

BACKGROUND/AIM

Inactivation of prostaglandin E(2) (PGE(2)) in the liver is a rapid process and occurs mainly through beta-oxidation in the peroxisome of the hepatocyte. Biliary excretion of PGE(2) is also a means of elimination from the liver. We investigated the role of multidrug resistance-associated protein 2 (MRP2) in the transport of PGE(2).

METHODS

Biliary PGE(2) elimination was measured in liver perfusions in Wistar and MRP2-deficient TR(-) rats. Furthermore, transport experiments were performed in membrane vesicles from human MRP2-infected Spodoptera frugiperda 21 (Sf21) insect cells.

RESULTS

The liver perfusions showed a 3.5 times higher percentage of undegraded [(3)H]PGE(2) in bile of Wistar rats in comparison with MRP2 deficient (TR(-)) rats (3.6% vs. 1.1%, respectively; P<0.05). MRP2-mediated transport of the model substrate [(3)H]DNP-SG was inhibited by PGE(2). Half maximal inhibition was achieved at a concentration of approximately 15 microM PGE(2). In addition, [(3)H]PGE(2) uptake in these vesicles was detected, and determined to be ATP dependent.

CONCLUSION

MRP2 mediates the transport of PGE(2) and its breakdown products. The biliary excretion of PGE(2) via MRP2 may contribute to rapid elimination of the prostaglandin but might also serve to relay prostaglandin signalling to the biliary tree.

Hepatocanalicular transport defects: pathophysiologic mechanisms of rare diseases

The apical membrane of the hepatocyte fulfils a unique function in the formation of primary bile. For all important biliary constituents a primary active transporter is present that extrudes or translocates its substrate toward the canalicular lumen. Most of these transporters are ATP-binding cassette (ABC) transporters. Two types of transporters can be recognized: those having endogenous metabolites as substrates (which could be referred to as "physiologic" transporters) and those involved in the elimination of drugs, toxins, and waste products. It should be emphasized that this distinction cannot be strictly made as some endogenous metabolites can be regarded as toxins as well. The importance of the canalicular transporters has been recognized by the pathologic consequence of their genetic defects. For each of the physiologic transporter genes an inherited disease has now been identified and most of these diseases have a quite serious clinical phenotype. Strikingly, complete defects in drug transporter function have not been recognized (yet) or only cause a mild phenotype. In this review we only briefly discuss the inherited defects in transporter function, and we focus on the pathophysiologic concepts that these diseases have generated.