Although severe deficiencies of canalicular transporter enzymes due to biallelic mutations are well known as causes of progressive cholestatic liver disease in children, it is increasingly recognized that milder disease may occur if a single, heterozygous gene mutation is present. This mild disease, generally presenting initially in adulthood, may have a variety of clinical and histological appearances. Bland canalicular cholestasis is the prototypic change but it is now clear that some gene mutations, particularly in ABCB4 (encoding MDR3), can cause other patterns that include early cholesterol calculus formation, bile duct injury and disappearance, ductular reactions mimicking large duct obstruction and, in rare cases, progressive fibrosis. Since the features can be subtle and not diagnostic in isolation, it is generally the combination of a biliary pattern of injury combined with a suggestive clinical and family history that allows the diagnosis to be suspected. Increased awareness and improved access to genetic testing is likely to result in more frequent diagnosis of these disorders.

ABCB4/MDR3 is located in the canalicular membrane of hepatocytes and translocates PC-lipids from the cytoplasmic to the extracellular leaflet. ABCB4 is an ATP-dependent transporter that reduces the harsh detergent effect of the bile salts by counteracting self-digestion. To do so, ABCB4 provides PC lipids for extraction into bile. PC lipids account for 40% of the entire pool of lipids in the canalicular membrane with an unknown distribution over both leaflets. Extracted PC lipids end up in so-called mixed micelles. Mixed micelles are composed of phospholipids, bile salts, and cholesterol. Ninety to ninety-five percent of the phospholipids are members of the PC family, but only a subset of mainly 16.0-18:1 PC and 16:0-18:2 PC variants are present. To elucidate whether ABCB4 is the key discriminator in this enrichment of specific PC lipids, we used in vitro studies to identify crucial determinants in substrate selection. We demonstrate that PC-lipid moieties alone are insufficient for stimulating ABCB4 ATPase activity, and that at least two acyl chains and the backbone itself are required for a productive interaction. The nature of the fatty acids, like length or saturation has a quantitative impact on the ATPase activity. Our data demonstrate a two-step enrichment and protective function of ABCB4 to mitigate the harsh detergent effect of the bile salts, because ABCB4 can translocate more than just the PC-lipid variants found in bile.

Keywords: ATP binding cassette subfamily B member 4; adenosine 5′-triphosphatase activity; adenosine 5′-triphosphate binding cassette transporter; bile; fatty acids.

ABCB4/MDR3 is located in the canalicular membrane of hepatocytes and translocates PC-lipids from the cytoplasmic to the extracellular leaflet. ABCB4 is an ATP-dependent transporter that reduces the harsh detergent effect of the bile salts by counteracting self-digestion. To do so, ABCB4 provides PC lipids for extraction into bile. PC lipids account for 40% of the entire pool of lipids in the canalicular membrane with an unknown distribution over both leaflets. Extracted PC lipids end up in so-called mixed micelles. Mixed micelles are composed of phospholipids, bile salts, and cholesterol. Ninety to ninety-five percent of the phospholipids are members of the PC family, but only a subset of mainly 16.0-18:1 PC and 16:0-18:2 PC variants are present. To elucidate whether ABCB4 is the key discriminator in this enrichment of specific PC lipids, we used in vitro studies to identify crucial determinants in substrate selection. We demonstrate that PC-lipid moieties alone are insufficient for stimulating ABCB4 ATPase activity, and that at least two acyl chains and the backbone itself are required for a productive interaction. The nature of the fatty acids, like length or saturation has a quantitative impact on the ATPase activity. Our data demonstrate a two-step enrichment and protective function of ABCB4 to mitigate the harsh detergent effect of the bile salts, because ABCB4 can translocate more than just the PC-lipid variants found in bile.

Keywords: ATP binding cassette subfamily B member 4; adenosine 5′-triphosphatase activity; adenosine 5′-triphosphate binding cassette transporter; bile; fatty acids.

Directed intercellular movement of diverse small molecules, including metabolites, signal molecules and xenobiotics, is a key feature of multicellularity. Networks of small molecule transporters (SMTs), including several ATP Binding Cassette (ABC) transporters, are central to this process. While small molecule transporters are well described in differentiated organs, little is known about their patterns of expression in early embryogenesis. Here we report the pattern of ABC-type SMT expression and activity during the early development of sea urchins. Of the six major ABCs in this embryo (ABCB1, -B4, -C1, -C4, -C5 and -G2), three expression patterns were observed: 1) ABCB1 and ABCC1 are first expressed ubiquitously, and then become enriched in endoderm and ectoderm-derived structures. 2) ABCC4 and ABCC5 are restricted to a ring of mesoderm in the blastula and ABCC4 is later expressed in the coelomic pouches, the embryonic niche of the primordial germ cells. 3) ABCB4 and ABCG2 are expressed exclusively in endoderm-fated cells. Assays with fluorescent substrates and inhibitors of transporters revealed a ring of ABCC4 efflux activity emanating from ABCC4⁺ mesodermal cells. Similarly, ABCB1 and ABCB4 efflux activity was observed in the developing gut, prior to the onset of feeding. This study reveals the early establishment of unique territories of small molecule transport during embryogenesis. A pattern of ABCC4/C5 expression is consistent with signaling functions during gut invagination and germ line development, while a later pattern of ABCB1/B4 and ABCG2 is consistent with roles in the embryonic gut. This work provides a conceptual framework with which to examine the function and evolution of SMT networks and define the specific developmental pathways that drive the expression of these genes.

Keywords: ABC transporters; Gut development; MRP; Mesoderm signaling; Sea urchin; Small molecule.

Objectives: Cholestasis is caused by a wide variety of etiologies, often genetic in origin. Broad overlap in clinical presentations, particularly in newborns, renders prioritizing diagnostic investigations challenging. In this setting, a timely, comprehensive assessment using a multigene panel by a clinical diagnostic laboratory would likely prove useful. We summarize initial findings from a testing program designed to discover genetic causes of cholestasis.

Methods: A neonatal/adult sequencing panel containing 66 genes (originally 57; 9 added March 2017) relevant to cholestasis was used. A broad range of eligible patients were enrolled with current/past history of cholestasis without an identified cause, or unexplained chronic liver disease. DNA sequencing utilized a custom-designed capture library, and variants were classified and reported as benign, likely benign, variant of unknown significance (VOUS), likely pathogenic (LP), or pathogenic (P), according to the clinical interpretation workflow at EGL Genetics.

Results: A total of 2433 samples were submitted between February 2016 and December 2017; 2171 results were reported. Median turnaround time was 21 days. Results from the 2171 subjects (57% <1 year old) included 583 P variants, 79 LP variants, and 3117 VOUS; 166 P/LP variants and 415 VOUS were novel. The panel's overall diagnostic yield was 12% (n=265/2171) representing 32 genes. The top 5 genetic diagnoses for the group, in order: JAG1 + NOTCH2 (Alagille Syndrome), ABCB11, SERPINA1, ABCB4, and POLG.

Conclusions: These findings support the utility of comprehensive rapid multigene testing in diagnosing cholestasis and highlight the evolving understanding of genetic variants contributing to the pathogenesis of cholestasis.

An infographic is available for this article at:http://links.lww.com/MPG/C250.

ABCB4 is described as an ATP-binding cassette (ABC) transporter that primarily transports lipids of the phosphatidylcholine (PC) family but is also capable of translocating a subset of typical multidrug-resistance-associated drugs. The high degree of amino acid identity of 76% for ABCB4 and ABCB1, which is a prototype multidrug-resistance-mediating protein, results in ABCB4's second subset of substrates, which overlap with ABCB1's substrates. This often leads to incomplete annotations of ABCB4, in which it was described as exclusively PC-lipid specific. When the hydrophilic amino acids from ABCB4 are changed to the analogous but hydrophobic ones from ABCB1, the stimulation of ATPase activity by 1,2-dioleoyl-sn-glycero-3-phosphocholine, as a prime example of PC lipids, is strongly diminished, whereas the modulation capability of ABCB1 substrates remains unchanged. This indicates two distinct and autonomous substrate binding sites in ABCB4.

Keywords: ABC transporter; ABCB1; ABCB4; ATPases; MD simulation; MDR3; PC lipids; credit card swipe; drugs; two-site access model.

Objective: Intrahepatic Cholestasis of Pregnancy (ICP) is a rare pregnancy specific disorder. Genetic variants of ABCB4 gene increase ICP risk. This study was conducted to determine frequency of ICP cases presented at a tertiary care hospital in Rawalpindi, Pakistan and to screen for genetic variants of exon 6 and 14 of ABCB4 gene in ICP cases.

Methods: This analytical study included ICP patients presenting at Department of Gynaecology and Obstetrics, Holy Family Hospital Rawalpindi, from February 2017 to May 2017. Sanger's sequencing was performed using genomic DNA extracted from blood samples of patients and controls.

Results: Twenty pregnant women out of 1150 (1.74%) had ICP and were enrolled during study period. Overall (19/20) 95% patients had pruritus and among them (8/20) 40%, (4/20) 20% and (2/20) 10% had a history of miscarriages, stillbirths and familial ICP respectively. Genetic analysis revealed an already reported variant i.e., c.504C>T in exon 6 in thirteen patients and a novel variant i.e., c.1686A>G in exon 14 in five patients. Both variants were not present in controls. In silico analysis suggested that both variants might affect pre-mRNA splicing of ABCB4 transcript.

Conclusions: ICP had a frequency of 1.74% among pregnant women. Identification of a novel heterozygous variant in five patients and an already reported variant in thirteen patients reaffirms genetic heterogeneity and role of ABCB4 in ICP etiology.

Keywords: intrahepatic cholestasis of pregnancy, ABCB4 gene, Single Nucleotide Polymorphism (SNP), DNA sequencing..

Background & aims: The endocannabinoid system is involved in the modulation of inflammatory, fibrotic, metabolic, and carcinogenesis-associated signaling pathways via the cannabinoid receptors 1 and 2 (CB1 and CB2). We hypothesized that the pharmacological antagonization of CB1 receptor improves cholestasis in Abcb4^-/- mice.

Methods: After weaning, male Abcb4^-/- mice were treated orally with rimonabant (an specific antagonist of CB1) or ACEA (an agonist of CB1) for up to 16 weeks of age. Liver tissue and serum were isolated and examined by means of serum analysis, qRT-PCR, western blot, immunohistochemistry, and enzyme function. Untreated Abcb4^-/- and BALB/c wild type mice served as controls.

Results: Cholestasis induced symptoms such as liver damage, bile duct proliferation, and enhanced circulating bile acids were improved by CB1 antagonization. Rimonabant treatment also improved PEPCK expression, reduced inflammation and the acute phase response. The carcinogenesis-associated JNK/c-JUN and STAT3 signaling pathways activated in Abcb4^-/- mice were reduced to wild type level by CB1 antagonization.

Conclusions: We demonstrated a protective effect of oral CB1 antagonization in chronic cholestasis using the established Abcb4^-/- model. Our results suggest that a pharmacologic antagonization of the CB1 receptor could have a therapeutic benefit in cholestasis associated metabolic changes, liver damage, inflammation, and carcinogenesis.

Keywords: acute phase; bile acid; fibrosis; liver; rimonabant.

Introduction: Heterozygous defects in genes implicated in Progressive Familial Intrahepatic Cholestasis have been described in milder forms of cholestatic diseases. Our aim is to describe clinical, laboratory and imaging characteristics as well as treatment and outcome of a cohort of pediatric patients with heterozygous mutations in ATP8B1, ABCB11 or ABCB4.

Patients and methods: We present a retrospective descriptive study including pediatric patients with at least one heterozygosis defect in ATP8B1, ABCB11 or ABCB4 diagnosed after a cholestatic episode. Clinical, diagnostic and outcome data were collected including gene analysis (panel of PFIC NextGeneDx®).

Results: 7 patients showed a heterozygous mutation: 3 patients in ABCB4, 1 in ABCB11, 2 in ABCB4 and ABCB11 and 1 in ATP8B1. The median onset age was 5.5 years with a median time of follow-up of 6 years. The initial presentation was pruritus followed by asymptomatic hypertransaminasemia and persistent cholestasis. Two patients had family history of gallbladder stones and mild hepatitis. All showed elevated transaminases and bile acids, high gamma glutamyl-transferase (GGT) in 3 and conjugated bilirubin in 2 patients. Liver biopsy showed inflammatory infiltrate or mild fibrosis with normal immunohistochemistry. All patients were treated with ursodeoxycholic acid, two patients requiring the addition of resincholestyramine. During follow-up, 3 patients suffered limited relapses of pruritus. No disease progression was observed.

Conclusion: Heterozygous mutations in genes coding proteins of the hepatocellular transport system can cause cholestatic diseases with great phenotypic variability. The presence of repeated episodes of hypertransaminasemia or cholestasis after a trigger should force us to rule out the presence of these heterozygous mutations in genes involved in CIFP.

Keywords: ABCB11; ABCB4; ATP8B1; BSEP; Colestasis intrahepática progresiva familiar; FIC1; MDR3; Progressive Familial Intrahepatic Cholestasis.

The aim of the present study was to investigate the effects and the underlying mechanisms of Yinchenhao Decoction (YCHD), a traditional Chinese medicine formulation, on C57BL/6 mice with lithogenic diet (LD)-induced cholelithiasis. The condition of cholelithiasis was evaluated using a six-level criteria. Levels of alanine aminotransferase (ALT), alkaline phosphatase (ALP), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) in the serum and liver tissue were measured using enzyme colorimetry. Concentrations of TC, phospholipids (PL) and total bile acids (TBA) in the bile were measured to calculate the cholesterol saturation index. Liver histopathology was microscopically observed and mRNA expression levels of ABCG5, ABCG8, SRBI, ABCB4, ABCB11 and NPC1L1 involved in cholesterol metabolism were measured using reverse transcription-quantitative PCR. The results showed that feeding mice the LD induced cholelithiasis, along with abnormal serum biochemical indices and imbalances in biliary cholesterol homeostasis. Increased ALT and ALP levels in the serum and ALT, ALP, TC and LDL-C levels in the serum and liver indicated the existence of hepatocyte injury, which were consistent with the pathological changes. YCHD treatment ameliorated the serum and hepatic biochemical abnormalities and adjusted the biliary imbalance. In addition, elevated expression of ATP-binding cassette subfamily G member 5/8, scavenger receptor class B type I and Niemann-Pick C1 Like 1 in the liver and small intestine were observed at the onset of cholelithiasis but were reversed by YCHD. Taken together, results from the present study suggest that YCHD ameliorated LD-induced cholelithiasis mice, which may be caused by improvements in biliary cholesterol supersaturation and regulation of cholesterol metabolism.

Keywords: Yinchenhao decoction; biliary cholesterol supersaturation; cholelithiasis; cholesterol metabolism.

Introduction and objectives: Progressive familial intrahepatic cholestasis type 3 (PFIC-3) is a rare autosomal recessive cholestatic liver disorder caused by mutations in the ABCB4 gene. The aim of this study was to present the phenotypic and genotypic spectrum of 4 Polish PFIC-3 patients diagnosed in a one-referral centre.

Materials and methods: The study included 4 patients with cholestasis and pathogenic variants in the ABCB4 gene identified by next-generation sequencing (NGS) of a targeted-gene panel or whole exome sequencing (WES). Clinical, laboratory, histological, and molecular data were collected.

Results: Four patients (three males) were identified. The age at first noted clinical signs and symptoms was 6, 2.5, 14, and 2 years respectively; the mean age was 6 years. Those signs and symptoms include pruritus (2 out of 4 patients) and hepatomegaly with splenomegaly (4 out of 4 patients). The age at the time of referral to our centre was 9, 3, 15, and 2.5 years respectively, while the mean age was 7 years. Chronic cholestatic liver disease of unknown aetiology was established in all of them. The NGS analysis was performed in all patients at the last follow-up visit. Three novel variants including c.902T>A, p.Met301Lys, c.3279+1G>A, p.?, and c.3524T>A, p.Leu1175His were identified. The time from the first consultation to the final diagnosis was 14, 9, 3, and 1 year respectively; the mean was 6.8 years. A detailed follow-up was presented.

Conclusions: The clinical phenotype of PFIC-3 could be variable. The clinical and biochemical diagnosis of PFIC-3 is difficult, thus the NGS study is very useful in making a proper diagnosis.

Keywords: children; cholestasis; liver transplantation; next-generation sequencing; progressive familial intrahepatic cholestasis.

Postępująca rodzinna cholestaza wewnątrzwątrobowa typu 3 (PFIC-3) należy do grupy rodzinnych cholestaz wewnątrzwątrobowych, dziedziczonych w sposób autosomalny recesywny. Patogeneza choroby wiąże się z obecnością patogennych wariantów molekularnych w genie ABCB4. Dotychczas, w literaturze opisano ok. 200 pacjentów z różnymi schorzeniami wątroby i dróg żółciowych, stanowiących ekspresję kliniczną PFIC-3.Celem pracy jest charakterystyka patogenezy, obrazu klinicznego, diagnostyki oraz leczenia PFIC-3 na podstawie aktualnego przeglądu piśmiennictwa.

Progressive familial intrahepatic cholestasis is caused by mutations in the ABCB4 gene and belongs to the family of familial intrahepatic cholestais disorders inherited in an autosomal recessive pattern. To date, about 200 patients with various hepatobiliary disorders associated with ABCB4 gene mutations have been described in the literature.The aim of this manuscript was to describe the pathogenesis, clinical presentation, diagnostic process and treatment of progressive familial intrahepatic cholestais type 3, based on the literature review.

Keywords: ABCB4 gene; MDR3 protein; białko MDR3; cholestasis; cholestaza; gen ABCB4; kwas ursodeoksycholowy; liver cirrhosis; liver transplantation; marskość wątroby; postępująca rodzinna cholestaza wewnątrzwątrobowa; progressive familial intrahepatic cholestasis; przeszczepienie wątroby; ursodeoxycholic acid.

Background and aims: Fibroblast growth factor (FGF) 21 has recently been shown to play a potential role in bile acid metabolism. We aimed to investigate the FGF21 response in an ethanol-induced acute-on-chronic liver injury (ACLI) model in Abcb4^-/- mice with deficiency of the hepatobiliary phospholipid transporter.

Methods: Total RNA was extracted from wild-type (WT, C57BL/6J) and Abcb4^-/^- (KO) mice, which were either fed a control diet (WT-Cont and KO-Cont groups; n = 28/group) or ethanol diet, followed by an acute ethanol binge (WT-EtOH and KO-EtOH groups; n = 28/group). A total of 58 human subjects were recruited into the study, including patients with alcohol-associated liver disease (AALD; n = 31) and healthy controls (n = 27). The hepatic and ileal expressions of genes involved in bile acid metabolism, plasma FGF levels, and bile acid and its precursors 7α- and 27-hydroxycholesterol (7α- and 27-OHC) concentrations were determined. Primary mouse hepatocytes were isolated for cell culture experiments.

Results: Alcohol feeding significantly induced plasma FGF21 and decreased hepatic Cyp7a1 levels. Hepatic expression levels of Fibroblast growth factor receptor 1 (Fgfr1), Fgfr4, Farnesoid X-activated receptor (Fxr), and Small heterodimer partner (Shp) and plasma FGF15/FGF19 levels did not differ with alcohol challenge. Exogenous FGF21 treatment suppressed Cyp7a1 in a dose-dependent manner in vitro. AALD patients showed markedly higher FGF21 and lower 7α-OHC plasma levels while FGF19 did not differ.

Conclusions: The simultaneous upregulation of FGF21 and downregulation of Cyp7a1 expressions upon chronic plus binge alcohol feeding together with the invariant plasma FGF15 and hepatic Shp and Fxr levels suggest the presence of a direct regulatory mechanism of FGF21 on bile acid homeostasis through inhibition of CYP7A1 by an FGF15-independent pathway in this ACLI model. Lay Summary: Alcohol challenge results in the upregulation of FGF21 and repression of Cyp7a1 expressions while circulating FGF15 and hepatic Shp and Fxr levels remain constant both in healthy and pre-injured livers, suggesting the presence of an alternative FGF15-independent regulatory mechanism of FGF21 on bile acid homeostasis through the inhibition of Cyp7a1.

Keywords: ATP binding cassette subfamily B member 4 (Abcb4) knock-out mouse; acute-on-chronic liver failure (ACLF); alcohol-associated liver disease (AALD); bile acid; cholesterol 7α-hydroxylase (Cyp7A1); fibroblast growth factor 21 (FGF21).

Background: The induction, progression and resolution of liver fibrosis are influenced by multiple chemokines. The inhibition of CCR1 signalling by a specific non-peptide inhibitor (BX471) reduces kidney fibrosis after unilateral ureteral obstruction via suppression of leukocyte recruitment in mice. However, it remains unclear whether selective CCR1 inhibition also affects hepatic fibrogenesis. Therefore we aimed to study the effect of this intervention on liver fibrosis in prevention (CCl₄ administration) and rescue (ABCB4-deficient mice) mouse models.

Methods: In the prevention model, hepatic fibrosis was induced by repeated injections of CCl₄. Additionally, the verum group was treated with subcutaneous injections of BX471, while controls received vehicle only. ABCB4 deficient mice (on the BALB/c-background) with sclerosing cholangitis and biliary fibrosis received BX471 or vehicle, respectively (rescue model). Liver histopathology was assessed after Sirius red staining of collagen, and hepatic collagen contents were measured. In addition, we performed gene expression analyses of fibrosis-related genes.

Results: BX471 injections were tolerated moderately well by all mice, and all mice developed hepatic fibrosis. Significant differences were neither observed in serum aminotransferase activities after 6 weeks of treatment between the two groups in the prevention nor in the rescue model. Interestingly, hepatic collagen contents were significantly higher in mice treated with BX471 in the prevention model as compared to controls but histological stages of liver sections did not differ. Of note, we observed only moderate effects on liver fibrosis in the ABCB4 knock-out model.

Conclusions: Our data indicate that BX471 treatment did neither affect serum and tissue markers of liver injury and fibrosis in the CCl₄ model and only moderately in the Abcb4 ^-/- model of biliary fibrosis. The animal models indicate that treatment with BX471 alone is unlikely to exert major beneficial effects in chronic liver disease.

Keywords: C-C chemokine receptor type 1; Fibrogenesis; Hepatic fibrosis.

ABCB4 is expressed in hepatocytes and translocates phosphatidylcholine into bile canaliculi. The mechanism of specific lipid recruitment from the canalicular membrane, which is essential to mitigate the cytotoxicity of bile salts, is poorly understood. We present cryogenic electron microscopy structures of human ABCB4 in three distinct functional conformations. An apo-inward structure reveals how phospholipid can be recruited from the inner leaflet of the membrane without flipping its orientation. An occluded structure reveals a single phospholipid molecule in a central cavity. Its choline moiety is stabilized by cation-π interactions with an essential tryptophan residue, rationalizing the specificity of ABCB4 for phosphatidylcholine. In an inhibitor-bound structure, a posaconazole molecule blocks phospholipids from reaching the central cavity. Using a proteoliposome-based translocation assay with fluorescently labeled phosphatidylcholine analogs, we recapitulated the substrate specificity of ABCB4 in vitro and confirmed the role of the key tryptophan residue. Our results provide a structural basis for understanding an essential translocation step in the generation of bile and its sensitivity to azole drugs.

Keywords: ABC transporter; cryo-EM; hepatocyte; membrane transport; phosphatidylcholine.

Background and aims: Parenteral nutrition (PN) associated cholestasis (PNAC) complicates the care of patients with intestinal failure. In PNAC, phytosterol containing PN synergizes with intestinal injury and IL-1β derived from activated hepatic macrophages to suppress hepatocyte FXR signaling and promote PNAC. We hypothesized that pharmacological activation of FXR would prevent PNAC in a mouse model.

Methods: To induce PNAC, male C57BL/6 mice were subjected to intestinal injury (2% dextran sodium sulfate for 4 days) followed by central venous catheterization and 14 day-infusion of PN with or without the FXR agonist GW4064. Following sacrifice, hepatocellular injury, inflammation, biliary and sterol transporter expression were determined.

Results: GW4064 (30mg/kg/day) added to PN on days 4-14 prevented hepatic injury and cholestasis, reversed the suppressed mRNA expression of Nr1h4/FXR, Abcb11/BSEP, Abcc2, Abcb4, and Abcg5/8, and normalized serum bile acids. Chromatin-immunoprecipitation of liver showed that GW4064 increased FXR binding to the Abcb11 promoter. Furthermore, GW4064 prevented DSS-PN-induced hepatic macrophage accumulation, hepatic expression of genes associated with macrophage recruitment and activation (ll1b, Ccr2, Itgam, Ly6C), and hepatic macrophage cytokine transcription in response to LPS in vitro. In primary mouse hepatocytes, GW4064 activated transcription of FXR canonical targets, irrespective of IL-1β exposure. Intestinal inflammation and ileal mRNAs (Nr1h4, Fgf15 and Ostα) were not different among groups, supporting a liver specific effect of GW4064 in this model CONCLUSION: GW4064 prevents PNAC in mice through restoration of hepatic FXR signaling resulting in increased expression of canalicular bile, sterol and phospholipid transporters, and suppression of macrophage recruitment and activation. These data support augmenting FXR activity as a therapeutic strategy to alleviate or prevent PNAC.

Keywords: GW4064; IL-1β; Intestinal failure associated liver disease; Liver X receptor; bile acids.

Background: ABCB4-gene mutations are responsible for several cholestatic diseases with a heterogeneous clinical spectrum.

Aims: To analyse phenotype/genotype relationships in ABCB4-mutations.

Methods: Retrospective characterization of adult patients with ABCB4-variations diagnosed between 2015 and 2020. Genotype-phenotype correlations were analysed and compared with previously reported data.

Results: Twenty patients from 12 families were included. Thirteen patients presented recurrent elevated liver tests, eight fulfilled Low-Phospholipid-Associated-Cholelithiasis syndrome criteria, five had Intrahepatic Cholestasis of Pregnancy and three patients developed Drug-Induced-Liver-Injury. ABCB4 screening identified eight different mutations. Five patients were homozygotes to the variant c.504T > C. Ten patients had one mutation in heterozygote-state and five patients had two mutations in compound-heterozygosity. Portal fibrosis occurred in two patients. One of these patients presented progressive fibrosis and progression of cholestasis despite ursodeoxycholic-acid treatment, this patient also harbours a ABCB11 polymorphism.

Conclusion: Although, phenotype-genotype relationships have not been clearly defined, an early diagnosis of ABCB4-variants may have an important role in management decisions and patient outcomes. To our knowledge, we describe a not previously reported deletion (c.1181delT) in ABCB4. The c.504T > C polymorphism, although a silent mutation at the protein level, seems to be associated to different cholestatic diseases. The role of other genes variants, namely ABCB11, as co-factor for progression, needs to be clarified.

Keywords: ABCB4; Idiopathic chronic cholestasis; Intrahepatic cholestasis of pregnancy; Low phospholipid-associated cholelithiasis syndrome; MDR3 protein.

Many inherited conditions cause cholestasis in the neonate or infant. Next-generation sequencing methods can facilitate a prompt diagnosis in some of these cases; application of these methods in patients with liver diseases of unknown cause has also uncovered novel gene-disease associations and improved our understanding of physiological bile secretion and flow. By helping to define the molecular basis of certain cholestatic disorders, these methods have also identified new targets for therapy as well patient subgroups more likely to benefit from specific therapies. At the same time, sequencing methods have presented new diagnostic challenges, such as the interpretation of single heterozygous genetic variants. This article discusses those challenges in the context of neonatal and infantile cholestasis, focusing on difficulties in predicting variant pathogenicity, the possibility of other causal variants not identified by the genetic screen used, and phenotypic variability among patients with variants in the same genes. A prospective, observational study performed between 2010-2013, which sequenced six important genes (ATP8B1, ABCB11, ABCB4, NPC1, NPC2 and SLC25A13) in an international cohort of 222 patients with infantile liver disease, is given as an example of potential benefits and challenges that clinicians could face having received a complex genetic result. Further studies including large cohorts of patients with paediatric liver disease are needed to clarify the spectrum of phenotypes associated with, as well as appropriate clinical response to, single heterozygous variants in cholestasis-associated genes.

Keywords: heterozygous pathogenic variants; infantile cholestasis; neonatal cholestasis; next-generation sequencing.

Polar auxin transport through plant tissue strictly requires polarly localized PIN proteins and uniformly distributed ABCB proteins. A functional synergy between the two types of membrane protein where their localizations overlap may create the degree of asymmetric auxin efflux required to produce polar auxin transport. We investigated this possibility by expressing ABCB4 and PIN2 in human embryonic kidney cells and measuring whole-cell ionic currents with the patch-clamp technique and CsCl-based electrolytes. ABCB4 activity was 1.81-fold more selective for Cl^- over Cs⁺ and for PIN2 the value was 2.95. We imposed auxin gradients and determined that ABCB4 and PIN2 were 12-fold more permeable to the auxin anion (IAA^-) than Cl^-. This measure of the intrinsic selectivity of the transport pathway was 21-fold when ABCB4 and PIN2 were co-expressed. If this increase occurs in plants, it could explain why asymmetric PIN localization is not sufficient to create polar auxin flow. Some form of co-action or synergy between ABCB4 and PIN2 that increases IAA^- selectivity at the cell face where both occur may be important. We also found that auxin stimulated ABCB4 activity, which may contribute to a self-reinforcement of auxin transport known as canalization.

ABCB4 (ATP-binding cassette subfamily B member 4) is an ABC transporter expressed at the canalicular membrane of hepatocytes where it ensures phosphatidylcholine secretion into bile. Genetic variations of ABCB4 are associated with several rare cholestatic diseases. The available treatments are not efficient for a significant proportion of patients with ABCB4-related diseases and liver transplantation is often required. The development of novel therapies requires a deep understanding of the molecular mechanisms regulating ABCB4 expression, intracellular traffic, and function. Using an immunoprecipitation approach combined with mass spectrometry analyses, we have identified the small GTPase RAB10 as a novel molecular partner of ABCB4. Our results indicate that the overexpression of wild type RAB10 or its dominant-active mutant significantly increases the amount of ABCB4 at the plasma membrane expression and its phosphatidylcholine floppase function. Contrariwise, RAB10 silencing induces the intracellular retention of ABCB4 and then indirectly diminishes its secretory function. Taken together, our findings suggest that RAB10 regulates the plasma membrane targeting of ABCB4 and consequently its capacity to mediate phosphatidylcholine secretion.

Keywords: MDR3; RAB GTPase; bile secretion; intracellular traffic; phosphatidylcholine.

Progressive familial intrahepatic cholestasis (PFIC) is a rare genetically heterogeneous group of autosomal recessive liver disorders that manifests as intrahepatic cholestasis during the neonatal period. ATP8B1, ABCB11, and ABCB4 genes are responsible for PFIC type 1, PFIC type 2, and PFIC type 3, respectively. To determine the underlying molecular etiology of PFIC, 80 patients from 77 families were investigated. The molecular genetic diagnosis was applied by using next-generation sequencing (NGS) and revealed 29 different variants from 32 patients. In this study, we evaluated these variants according to mechanisms, clinical sub-groups, and genotype-phenotype correlation.

Keywords: ABCB11; ABCB4; ATP8B1; Next-generation sequencing; progressive familial intrahepatic cholestasis.

Capillary endothelial cells of the human blood-brain barrier (BBB) express high levels of P-glycoprotein (P-gp, encoded by ABCB1) and ABCG2 (encoded by ABCG2). However, little information is available regarding ATP-binding cassette transporters expressed at the zebrafish BBB, which has emerged as a potential model system. We report the characterization and tissue localization of two genes that are similar to ABCB1, zebrafish abcb4 and abcb5. When stably expressed in HEK293 cells, both Abcb4 and Abcb5 conferred resistance to P-gp substrates; however, Abcb5 poorly transported doxorubicin and mitoxantrone compared to zebrafish Abcb4. Additionally, Abcb5 did not transport the fluorescent P-gp probes BODIPY-ethylenediamine or LDS 751, while they were transported by Abcb4. High-throughput screening of 90 human P-gp substrates confirmed that Abcb4 has an overlapping substrate specificity profile with P-gp. In the brain vasculature, RNAscope probes for abcb4 colocalized with staining by the P-gp antibody C219, while abcb5 was not detected. The abcb4 probe also colocalized with claudin-5 in brain endothelial cells. Abcb4 and Abcb5 had different tissue localizations in multiple zebrafish tissues, potentially indicating different functions. The data suggest that zebrafish Abcb4 functionally phenocopies P-gp and that the zebrafish may serve as a model to study the role of P-gp at the BBB.