A flippase-independent function of ATP8B1, the protein affected in familial intrahepatic cholestasis type 1, is required for apical protein expression and microvillus formation in polarized epithelial cells

The Phospholipid Flippase ATP8B1 is Involved in the Pathogenesis of Ulcerative Colitis via Establishment of Intestinal Barrier Function

AIMS

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 diarrhoea, which are aggravated after liver transplantation. Here we investigate the intestinal function of ATP8B1 in relation to inflammatory bowel diseases.

METHODS

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 knockdown 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 was associated with a decreased tight junctional pathway transcriptional programme. ATP8B1-deficient mice were extremely sensitive to DSS-induced colitis, as evidenced by increased intestinal barrier leakage. ATP8B1 knockdown cells showed delayed barrier establishment that 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.

Asymmetric Distribution of Plasmalogens and Their Roles-A Mini Review

Plasmalogens are a unique family of cellular glycerophospholipids that contain a vinyl-ether bond. The synthesis of plasmalogens is initiated in peroxisomes and completed in the endoplasmic reticulum. Plasmalogens are transported to the post-Golgi compartment, including endosomes and plasma membranes, in a manner dependent on ATP, but not vesicular transport. Plasmalogens are preferentially localized in the inner leaflet of the plasma membrane in a manner dependent on P4-type ATPase ATP8B2, that associates with the CDC50 subunit. Plasmalogen biosynthesis is spatiotemporally regulated by a feedback mechanism that senses the amount of plasmalogens in the inner leaflet of the plasma membrane and controls the stability of fatty acyl-CoA reductase 1 (FAR1), the rate-limiting enzyme for plasmalogen biosynthesis. The physiological consequences of such asymmetric localization and homeostasis of plasmalogens are discussed in this review.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

ATP8B1: A prognostic prostate cancer biomarker identified via genetic analysis

BACKGROUND

Controlling the asymmetric distribution of phospholipids across biological membranes plays a pivotal role in the life cycle of cells; one of the most important contributors that maintain this lipid asymmetry are phospholipid-transporting adenosine triphosphatases (ATPases). Although sufficient information regarding their association with cancer exists, there is limited evidence linking the genetic variants of phospholipid-transporting ATPase family genes to prostate cancer in humans.

METHODS

In this study, we investigated the association of 222 haplotype-tagging single-nucleotide polymorphisms (SNPs) in eight phospholipid-transporting ATPase genes with cancer-specific survival (CSS) and overall survival (OS) of 630 patients treated with androgen-deprivation therapy (ADT) for prostate cancer.

RESULTS

After multivariate Cox regression analysis and multiple testing correction, we found that ATP8B1 rs7239484 was remarkably associated with CSS and OS after ADT. A pooled analysis of multiple independent gene-expression datasets demonstrated that ATP8B1 was under-expressed in tumor tissues and that a higher ATP8B1 expression was associated with a better patient prognosis. Moreover, we established highly invasive sublines using two human prostate cancer cell lines to mimic cancer progression traits in vitro. The expression of ATP8B1 was consistently downregulated in both highly invasive sublines.

CONCLUSION

Our study indicates that rs7239484 is a prognostic factor for patients treated with ADT and that ATP8B1 can potentially attenuate prostate cancer progression.

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.

Ileal Bile Acid Transporter Inhibition Reduces Post-Transplant Diarrhea and Growth Failure in FIC1 Disease—A Case Report

Familial intrahepatic cholestasis 1 (FIC1) disease is a genetic disorder characterized by hepatic and gastrointestinal disease due to ATP8B1 deficiency, often requiring liver transplantation (LT). Extrahepatic symptoms, such as diarrhea, malabsorption, and failure to thrive, do not improve and instead may be aggravated after LT. We describe a patient with FIC1 disease who underwent LT at 2 years, 8 months of age. After LT, the child developed severe refractory diarrhea and failed to thrive. The response to bile acid resins was unsatisfactory, and the parents declined our recommendation for partial external biliary diversion (PEBD). Quality of life was extremely impaired, especially due to severe diarrhea, making school attendance impossible. Attempting to reduce the total bile acids, we initiated off-label use of the ileal bile acid transporter (IBAT) inhibitor Elobixibat (Goofice^™), later converted to Odevixibat (Bylvay^™). After six months of treatment, the patient showed less stool output, increased weight and height, and improved physical energy levels. The child could now pursue higher undergraduate education. In our patient with FIC1 disease, the use of IBAT inhibitors was effective in treating chronic diarrhea and failure to thrive. This approach is novel; further investigations are needed to clarify the exact mode of action in this condition.

Matrix Metalloproteinase 7 Expression and Apical Epithelial Defects in Atp8b1 Mutant Mouse Model of Pulmonary Fibrosis

Abnormalities in airway epithelia and lung parenchyma are found in Atp8b1 mutant mice, which develop pulmonary fibrosis after hyperoxic insult. Microarray and ingenuity pathway analysis (IPA) show numerous transcripts involved in ciliogenesis are downregulated in 14-month (14 M) -old Atp8b1 mouse lung compared with wild-type C57BL/6. Lung epithelium of Atp8b1 mice demonstrate apical abnormalities of ciliated and club cells in the bronchial epithelium on transmission electron microscopy (TEM). Matrix metalloproteinase 7 (MMP7) regulates of ciliogenesis and is a biomarker for idiopathic pulmonary fibrosis (IPF) in humans. Mmp7 transcript and protein expression are significantly upregulated in 14 M Atp8b1 mutant mouse lung. MMP7 expression is also increased in bronchoalveolar lavage fluid (BAL). Immunohistochemistry is localized MMP7 to bronchial epithelial cells in the Atp8b1 mutant. In conclusion, MMP7 is upregulated in the aged Atp8b1 mouse model, which displays abnormal ciliated cell and club cell morphology. This mouse model can facilitate the exploration of the role of MMP7 in epithelial integrity and ciliogenesis in IPF. The Atp8b1 mutant mouse is proposed as a model for IPF.

Microvilli-derived extracellular vesicles carry Hedgehog morphogenic signals for Drosophila wing imaginal disc development

Morphogens are secreted molecules that regulate and coordinate major developmental processes, such as cell differentiation and tissue morphogenesis. Depending on the mechanisms of secretion and the nature of their carriers, morphogens act at short and long range. We investigated the paradigmatic long-range activity of Hedgehog (Hh), a well-known morphogen, and its contribution to the growth and patterning of the Drosophila wing imaginal disc. Extracellular vesicles (EVs) contribute to Hh long-range activity; however, the nature, the site, and the mechanisms underlying the biogenesis of these vesicular carriers remain unknown. Here, through the analysis of mutants and a series of Drosophila RNAi-depleted wing imaginal discs using fluorescence and live-imaging electron microscopy, including tomography and 3D reconstruction, we demonstrate that microvilli of the wing imaginal disc epithelium are the site of generation of small EVs that transport Hh across the tissue. Further, we show that the Prominin-like (PromL) protein is critical for microvilli integrity. Together with actin cytoskeleton and membrane phospholipids, PromL maintains microvilli architecture that is essential to promote its secretory function. Importantly, the distribution of Hh to microvilli and its release via these EVs contribute to the proper morphogenesis of the wing imaginal disc. Our results demonstrate that microvilli-derived EVs are carriers for Hh long-range signaling in vivo. By establishing that members of the Prominin protein family are key determinants of microvilli formation and integrity, our findings support the view that microvilli-derived EVs conveying Hh may provide a means for exchanging signaling cues of high significance in tissue development and cancer.

Lipid flippases as key players in plant adaptation to their environment

Lipid flippases (P4 ATPases) are active transporters that catalyse the translocation of lipids between the two sides of the biological membranes in the secretory pathway. This activity modulates biological membrane properties, contributes to vesicle formation, and is the trigger for lipid signalling events, which makes P4 ATPases essential for eukaryotic cell survival. Plant P4 ATPases (also known as aminophospholipid ATPases (ALAs)) are crucial for plant fertility and proper development, and are involved in key adaptive responses to biotic and abiotic stress, including chilling tolerance, heat adaptation, nutrient deficiency responses and pathogen defence. While ALAs present many analogies to mammalian and yeast P4 ATPases, they also show characteristic features as the result of their independent evolution. In this Review, the main properties, roles, regulation and mechanisms of action of ALA proteins are discussed.

Impact of Genotype, Serum Bile Acids, and Surgical Biliary Diversion on Native Liver Survival in FIC1 Deficiency

BACKGROUND AND AIMS

Mutations in ATPase phospholipid transporting 8B1 (ATP8B1) can lead to familial intrahepatic cholestasis type 1 (FIC1) deficiency, or progressive familial intrahepatic cholestasis type 1. The rarity of FIC1 deficiency has largely prevented a detailed analysis of its natural history, effects of predicted protein truncating mutations (PPTMs), and possible associations of serum bile acid (sBA) concentrations and surgical biliary diversion (SBD) with long-term outcome. We aimed to provide insights by using the largest genetically defined cohort of patients with FIC1 deficiency to date.

APPROACH AND RESULTS

This multicenter, combined retrospective and prospective study included 130 patients with compound heterozygous or homozygous predicted pathogenic ATP8B1 variants. Patients were categorized according to the number of PPTMs (i.e., splice site, frameshift due to deletion or insertion, nonsense, duplication), FIC1-A (n = 67; no PPTMs), FIC1-B (n = 29; one PPTM), or FIC1-C (n = 34; two PPTMs). Survival analysis showed an overall native liver survival (NLS) of 44% at age 18 years. NLS was comparable among FIC1-A, FIC1-B, and FIC1-C (% NLS at age 10 years: 67%, 41%, and 59%, respectively; P = 0.12), despite FIC1-C undergoing SBD less often (% SBD at age 10 years: 65%, 57%, and 45%, respectively; P = 0.03). sBAs at presentation were negatively associated with NLS (NLS at age 10 years, sBAs < 194 µmol/L: 49% vs. sBAs ≥ 194 µmol/L: 15%; P = 0.03). SBD decreased sBAs (230 [125-282] to 74 [11-177] μmol/L; P = 0.005). SBD (HR 0.55, 95% CI 0.28-1.03, P = 0.06) and post-SBD sBA concentrations < 65 μmol/L (P = 0.05) tended to be associated with improved NLS.

CONCLUSIONS

Less than half of patients with FIC1 deficiency reach adulthood with native liver. The number of PPTMs did not associate with the natural history or prognosis of FIC1 deficiency. sBA concentrations at initial presentation and after SBD provide limited prognostic information on long-term NLS.

Triggers of benign recurrent intrahepatic cholestasis and its pathophysiology: a review of literature

Benign recurrent intrahepatic cholestasis (BRIC) is a rare genetic disorder that is characterized by episodes of cholestasis followed by complete resolution. The episodic nature of BRIC raises concerns about its possible trigger factors. Indeed, case reports of this orphan disease have associated BRIC to some triggers. In the absence of any reviews, we reviewed BRIC trigger factors and its pathophysiology. The study consisted of a systematic search for case reports using PubMed. Articles describing a clear case of BRIC associated with a trigger were included resulting in 22 articles that describe 35 patients. Infection was responsible for 54.3% of triggered episodes, followed by hormonal, drugs, and miscellaneous causes reporting as 30%, 10%, and 5.7% respectively. Females predominated with 62.9%. The longest episode ranged between 3 months to 2 years with a mean of 32.37 weeks. The mean age of the first episode was 14.28 ranging between 3 months to 48 years. Winter and autumn were the major seasons during which episodes happened. Hence, BRIC is potentially triggered by infection, which is most commonly a viral infection, hormonal disturbances as seen in oral contraceptive pills and pregnancy state, and less commonly by certain drugs and other causes. The appearance of cholestasis during the first two trimesters of pregnancy compared to intrahepatic cholestasis of pregnancy could help to differentiate between the two conditions. The possible mechanism of BRIC induction implicates a role of BSEP and ATP8B1. While estrogen, drugs, and cytokines are known to affect BSEP, less is known about their action on ATP8B1.

The transport mechanism of P4 ATPase lipid flippases

P4 ATPase lipid flippases are ATP-driven transporters that translocate specific lipids from the exoplasmic to the cytosolic leaflet of biological membranes, thus establishing a lipid gradient between the two leaflets that is essential for many cellular processes. While substrate specificity, subcellular and tissue-specific expression, and physiological functions have been assigned to a number of these transporters in several organisms, the mechanism of lipid transport has been a topic of intense debate in the field. The recent publication of a series of structural models based on X-ray crystallography and cryo-EM studies has provided the first glimpse into how P4 ATPases have adapted the transport mechanism used by the cation-pumping family members to accommodate a substrate that is at least an order of magnitude larger than cations.

Molecular overview of progressive familial intrahepatic cholestasis

Cholestasis is a clinical condition resulting from the imapairment of bile flow. This condition could be caused by defects of the hepatocytes, which are responsible for the complex process of bile formation and secretion, and/or caused by defects in the secretory machinery of cholangiocytes. Several mutations and pathways that lead to cholestasis have been described. Progressive familial intrahepatic cholestasis (PFIC) is a group of rare diseases caused by autosomal recessive mutations in the genes that encode proteins expressed mainly in the apical membrane of the hepatocytes. PFIC 1, also known as Byler’s disease, is caused by mutations of the ATP8B1 gene, which encodes the familial intrahepatic cholestasis 1 protein. PFIC 2 is characterized by the downregulation or absence of functional bile salt export pump (BSEP) expression via variations in the ABCB11 gene. Mutations of the ABCB4 gene result in lower expression of the multidrug resistance class 3 glycoprotein, leading to the third type of PFIC. Newer variations of this disease have been described. Loss of function of the tight junction protein 2 protein results in PFIC 4, while mutations of the NR1H4 gene, which encodes farnesoid X receptor, an important transcription factor for bile formation, cause PFIC 5. A recently described type of PFIC is associated with a mutation in the MYO5B gene, important for the trafficking of BSEP and hepatocyte membrane polarization. In this review, we provide a brief overview of the molecular mechanisms and clinical features associated with each type of PFIC based on peer reviewed journals published between 1993 and 2020.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

Identification of ATP8B1 as a Tumor Suppressor Gene for Colorectal Cancer and Its Involvement in Phospholipid Homeostasis

Homeostasis of membrane phospholipids plays an important role in cell oncogenesis and cancer progression. The flippase ATPase class I type 8b member 1 (ATP8B1), one of the P4-ATPases, translocates specific phospholipids from the exoplasmic to the cytoplasmic leaflet of membranes. ATP8B1 is critical for maintaining the epithelium membrane stability and polarity. However, the prognostic values of ATP8B1 in colorectal cancer (CRC) patients remain unclear. We analyzed transcriptomics, genomics, and clinical data of CRC samples from The Cancer Genome Atlas (TCGA). ATP8B1 was the only potential biomarker of phospholipid transporters in CRC. Its prognostic value was also validated with the data from the Gene Expression Omnibus (GEO). Compared to the normal group, the expression of ATP8B1 was downregulated in the tumor group and the CRC cell lines, which declined with disease progression. The lower expression level of ATP8B1 was also significantly associated with worse survival outcomes in both the discovery samples (359 patients) and the validation samples (566 patients). In multivariate analyses, low ATP8B1 levels predicted unfavorable OS (adjusted HR 1.512, 95% CI: 1.069-2.137; P = 0.019) and were associated with poor progress-free interval (PFI) (adjusted HR: 1.62, 95% CI: 1.207-2.174; P = 0.001). The pathway analysis results showed that the underexpression of ATP8B1 was negatively associated with phospholipid transport, phospholipid metabolic process, and cell-cell adherent junction and positively associated with the epithelial-mesenchymal transition in CRC. Our analysis suggests that ATP8B1 is a potential cancer suppressor in CRC patients and may offer new strategies for CRC therapy.

New paradigms of USP53 disease: normal GGT cholestasis, BRIC, cholangiopathy, and responsiveness to rifampicin

Biallelic variants in the USP53 gene have recently been reported to segregate with normal gamma glutamyltransferase (GGT) cholestasis. Using whole-exome sequencing (WES), we detected two USP53 homozygous variants (c.951delT; p. Phe317fs and c.1744C>T; p. Arg582*) in five additional cases, including an unpublished cousin of a previously described family with intractable itching and normal GGT cholestasis. Three patients, a child and two adults, presented with recurrent episodes of normal GGT cholestasis, consistent with a diagnosis of benign recurrent intrahepatic cholestasis (BRIC). Cholangiopathic changes, possibly autoimmune in origin, were recognized in some patients. Additional phenotypic details in one patient included an enlarged left kidney, and speech/developmental delay. Notably, two patients exhibited a complete response to rifampicin, and one responded to ursodeoxycholic acid (UDCA). Two adult patients were suspected to have autoimmune liver disease and treated with steroids. This report describes new cases of USP53 disease presenting with normal GGT cholestasis or BRIC in three children and two adults. We also describe the novel finding of a dramatic response to rifampicin. The association of cholangiopathy with normal GGT cholestasis provides a diagnostic challenge and remains poorly understood.

Potential of Intestine-Selective FXR Modulation for Treatment of Metabolic Disease

Farnesoid X receptor controls bile acid metabolism, both in the liver and intestine. This potent nuclear receptor not only maintains homeostasis of its own ligands, i.e., bile acids, but also regulates glucose and lipid metabolism as well as the immune system. These findings have led to substantial interest for FXR as a therapeutic target and to the recent approval of an FXR agonist for treating primary biliary cholangitis as well as ongoing clinical trials for other liver diseases. Given that FXR biology is complex, including moderate expression in tissues outside of the enterohepatic circulation, temporal expression of isoforms, posttranscriptional modifications, and the existence of several other bile acid-responsive receptors such as TGR5, clinical application of FXR modulators warrants thorough understanding of its actions. Recent findings have demonstrated remarkable physiological effects of targeting FXR specifically in the intestine (iFXR), thereby avoiding systemic release of modulators. These include local effects such as improvement of intestinal barrier function and intestinal cholesterol turnover, as well as systemic effects such as improvements in glucose homeostasis, insulin sensitivity, and nonalcoholic fatty liver disease (NAFLD). Intriguingly, metabolic improvements have been observed with both an iFXR agonist that leads to production of enteric Fgf15 and increased energy expenditure in adipose tissues and antagonists by reducing systemic ceramide levels and hepatic glucose production. Here we review the recent findings on the role of intestinal FXR and its targeting in metabolic disease.

Identification of novel loci for pediatric cholestatic liver disease defined by KIF12, PPM1F, USP53, LSR, and WDR83OS pathogenic variants

PURPOSE

Genetic testing in pediatric cholestasis can be very informative but genetic causes have not been fully characterized.

METHODS

Exome sequencing and positional mapping in seven families with cholestatic liver disease and negative clinical testing for known disease genes.

RESULTS

KIF12, which encodes a microtubule motor protein with a tentative role in cell polarity, was found to harbor three homozygous likely deleterious variants in three families with sclerosing cholangitis. KIF12 expression is dependent on HNF-1β, deficiency which is known to cause bile duct dysmorphogenesis associated with loss of KIF12 expression. In another extended family, we mapped an apparently novel syndrome of sclerosing cholangitis, short stature, hypothyroidism, and abnormal tongue pigmentation in two cousins to a homozygous variant in PPM1F (POPX2), a regulator of kinesin-mediated ciliary transport. In the fifth family, a syndrome of normal gamma glutamyltransferase (GGT) cholestasis and hearing loss was found to segregate with a homozygous truncating variant in USP53, which encodes an interactor with TJP2. In the sixth family, we mapped a novel syndrome of transient neonatal cholestasis, intellectual disability, and short stature to a homozygous variant in LSR, an important regulator of liver development. In the last family of three affected siblings, a novel syndrome of intractable itching, hypercholanemia, short stature, and intellectual disability was mapped to a single locus that contains a homozygous truncating variant in WDR83OS (C19orf56), known to interact with ATP13A2 and BSEP.

CONCLUSION

Our results expand the genetic heterogeneity of pediatric cholestatic liver disease and highlight the vulnerability of bile homeostasis to a wide range of molecular perturbations.

Disruption of Tmem30a results in cerebellar ataxia and degeneration of Purkinje cells

Phospholipids are asymmetrically distributed across mammalian plasma membrane with phosphatidylserine (PS) and phosphatidylethanolamine concentrated in the cytoplasmic leaflet of the membrane bilayer. This asymmetric distribution is dependent on a group of P4-ATPases named PS flippases. The proper transport and function of PS flippases require a β-subunit transmembrane protein 30 A (TMEM30A). Disruption of PS flippases led to several human diseases. However, the roles of TMEM30A in the central nervous system remain elusive. To investigate the role of Tmem30a in the cerebellum, we developed a Tmem30a Purkinje cell (PC)-specific knockout (KO) mouse model. The Tmem30a KO mice displayed early-onset ataxia and progressive PC death. Deficiency in Tmem30a led to an increased expression of Glial fibrillary acidic protein and astrogliosis in regions with PC loss. Elevated C/EBP homologous protein and BiP expression levels indicated the presence of endoplasmic reticulum stress in the PCs prior to visible cell loss. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analysis suggested that apoptotic cell death occurred in the cerebellum. Our data demonstrate that loss of Tmem30a in PCs results in protein folding and transport defects, a substantial decrease in dendritic spine density, increased astrogliosis and PC death. Taken together, our data demonstrate an essential role of Tmem30a in the cerebellum PCs.

Total internal biliary diversion during liver transplantation for type 1 progressive familial intrahepatic cholestasis: a novel approach

LT for PFIC type 1 is often complicated by postoperative diarrhea and recurrent graft steatosis. A 26-month-old female child with cholestatic jaundice, pruritus, diarrhea, and growth retardation revealed total bilirubin 9.1 mg/dL, gamma-glutamyl transpeptidase 64 IU/L, and TBA 295.8 μmol/L. Genetic analysis confirmed ATP8B1 defects. A LT (segment 2, 3 graft) from the heterozygous father was performed. Biliary diversion was performed by a 35-cm jejunum conduit between the graft hepatic duct and the mid-transverse colon. Stools became pigmented immediately. Follow-up at 138 days revealed resolution of jaundice and pruritus and soft-to-hard stools (6-8 daily). Radioisotope hepato-biliary scintigraphy (days 26, 68, and 139) confirmed unobstructed bile drainage into the colon (t_1/2 34, 27, and 19 minutes, respectively). Contrast meal follow-through at day 62 confirmed the absence of any colo-jejuno-hepatic reflux. At 140 days, contrast follow-through via the biliary stent revealed patent jejuno-colonic anastomosis and satisfactory transit. Graft biopsy at LT, 138 days, and 9 months follow-up revealed comparable grades of macrovesicular steatosis (<20%). TIBD during LT may be a clinically effective stoma-free biliary diversion and may prevent recurrent graft steatosis following LT for PFIC type 1.

Directed evolution of a sphingomyelin flippase reveals mechanism of substrate backbone discrimination by a P4-ATPase

Phospholipid flippases in the type IV P-type ATPase (P4-ATPases) family establish membrane asymmetry and play critical roles in vesicular transport, cell polarity, signal transduction, and neurologic development. All characterized P4-ATPases flip glycerophospholipids across the bilayer to the cytosolic leaflet of the membrane, but how these enzymes distinguish glycerophospholipids from sphingolipids is not known. We used a directed evolution approach to examine the molecular mechanisms through which P4-ATPases discriminate substrate backbone. A mutagenesis screen in the yeast Saccharomyces cerevisiae has identified several gain-of-function mutations in the P4-ATPase Dnf1 that facilitate the transport of a novel lipid substrate, sphingomyelin. We found that a highly conserved asparagine (N220) in the first transmembrane segment is a key enforcer of glycerophospholipid selection, and specific substitutions at this site allow transport of sphingomyelin.

The phospholipid flippase ATP8B1 mediates apical localization of the cystic fibrosis transmembrane regulator

Progressive familial intrahepatic cholestasis type 1 (PFIC1) is caused by mutations in the gene encoding the phospholipid flippase ATP8B1. Apart from severe cholestatic liver disease, many PFIC1 patients develop extrahepatic symptoms characteristic of cystic fibrosis (CF), such as pulmonary infection, sweat gland dysfunction and failure to thrive. CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel essential for epithelial fluid transport. Previously it was shown that CFTR transcript levels were strongly reduced in livers of PFIC1 patients. Here we have investigated the hypothesis that ATP8B1 is important for proper CFTR expression and function. We analyzed CFTR expression in ATP8B1-depleted intestinal and pulmonary epithelial cell lines and assessed CFTR function by measuring short-circuit currents across transwell-grown ATP8B1-depleted intestinal T84 cells and by a genetically-encoded fluorescent chloride sensor. In addition, we studied CFTR surface expression upon induction of CFTR transcription. We show that CFTR protein levels are strongly reduced in the apical membrane of human ATP8B1-depleted intestinal and pulmonary epithelial cell lines, a phenotype that coincided with reduced CFTR activity. Apical membrane insertion upon induction of ectopically-expressed CFTR was strongly impaired in ATP8B1-depleted cells. We conclude that ATP8B1 is essential for correct apical localization of CFTR in human intestinal and pulmonary epithelial cells, and that impaired CFTR localization underlies some of the extrahepatic phenotypes observed in ATP8B1 deficiency.

On the molecular mechanism of flippase- and scramblase-mediated phospholipid transport

Phospholipid flippases are key regulators of transbilayer lipid asymmetry in eukaryotic cell membranes, critical to many trafficking and signaling pathways. P4-ATPases, in particular, are responsible for the uphill transport of phospholipids from the exoplasmic to the cytosolic leaflet of the plasma membrane, as well as membranes of the late secretory/endocytic pathways, thereby establishing transbilayer asymmetry. Recent studies combining cell biology and biochemical approaches have improved our understanding of the path taken by lipids through P4-ATPases. Additionally, identification of several protein families catalyzing phospholipid 'scrambling', i.e. disruption of phospholipid asymmetry through energy-independent bi-directional phospholipid transport, as well as the recent report of the structure of such a scramblase, opens the way to a deeper characterization of their mechanism of action. Here, we discuss the molecular nature of the mechanism by which lipids may 'flip' across membranes, with an emphasis on active lipid transport catalyzed by P4-ATPases. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Hypothyroidism Associated with ATP8B1 Deficiency

OBJECTIVE

To examine whether hypothyroidism is an extrahepatic feature of ATPase, aminophospholipid transporter, class I, type 8B, member 1 (ATP8B1) deficiency.

STUDY DESIGN

Children with normal γ-glutamyltransferase cholestasis (n = 47; 13 patients with ATP8B1 deficiency, 19 with ATP-binding cassette, subfamily B (MDR/TAP), member 11 (ABCB11) deficiency, and 15 without either ATP8B1 or ABCB11 mutations) were enrolled. Clinical information and thyroid function test results were retrospectively retrieved from clinical records and compared. Hypothyroidism was diagnosed by clinical-biochemistry criteria (thyroid function test results).

RESULTS

Three out of 13 patients with ATP8B1 deficiency were diagnosed as hypothyroid and 2 as subclinically hypothyroid. The frequency of hypothyroidism and subclinical hypothyroidism was significantly higher than in patients with ABCB11 deficiency (5/13 vs 0/19, P = .006) and in patients without ATP8B1 or ABCB11 mutations (5/13 vs 0/15, P = .013). Thyroid function test results normalized after hormone replacement in all 5 patients, with no relief of cholestasis.

CONCLUSIONS

As hypothyroidism can be another extrahepatic feature of ATP8B1 deficiency, thyroid function should be monitored in these patients.

Structural and functional hepatocyte polarity and liver disease

Hepatocytes form a crucially important cell layer that separates sinusoidal blood from the canalicular bile. They have a uniquely organized polarity with a basal membrane facing liver sinusoidal endothelial cells, while one or more apical poles can contribute to several bile canaliculi jointly with the directly opposing hepatocytes. Establishment and maintenance of hepatocyte polarity is essential for many functions of hepatocytes and requires carefully orchestrated cooperation between cell adhesion molecules, cell junctions, cytoskeleton, extracellular matrix and intracellular trafficking machinery. The process of hepatocyte polarization requires energy and, if abnormal, may result in severe liver disease. A number of inherited disorders affecting tight junction and intracellular trafficking proteins have been described and demonstrate clinical and pathophysiological features overlapping those of the genetic cholestatic liver diseases caused by defects in canalicular ABC transporters. Thus both structural and functional components contribute to the final hepatocyte polarity phenotype. Many acquired liver diseases target factors that determine hepatocyte polarity, such as junctional proteins. Hepatocyte depolarization frequently occurs but is rarely recognized because hematoxylin-eosin staining does not identify the bile canaliculus. However, the molecular mechanisms underlying these defects are not well understood. Here we aim to provide an update on the key factors determining hepatocyte polarity and how it is affected in inherited and acquired diseases.

[P4-ATP-ase Atp8b1/FIC1: structural properties and (patho)physiological functions]

P4-ATP-ases comprise an interesting family among P-type ATP-ases, since they are thought to play a major role in the transfer of phospholipids such as phosphatydylserine from the outer leaflet to the inner leaflet. Isoforms of P4-ATP-ases are partially interchangeable but peculiarities of tissue-specific expression of their genes, intracellular localization of proteins, as well as regulatory pathways lead to the fact that, on the organismal level, serious pathologies may develop in the presence of structural abnormalities in certain isoforms. Among P4-ATP-ases a special place is occupied by ATP8B1, for which several mutations are known that lead to serious hereditary diseases: two forms of congenital cholestasis (PFIC1 or Byler disease and benign recurrent intrahepatic cholestasis) with extraliver symptoms such as sensorineural hearing loss. The physiological function of the Atp8b1/FIC1 protein is known in general outline: it is responsible for transport of certain phospholipids (phosphatydylserine, cardiolipin) for the outer monolayer of the plasma membrane to the inner one. It is well known that perturbation of membrane asymmetry, caused by the lack of Atp8B1 activity, leads to death of hairy cells of the inner ear, dysfunction of bile acid transport in liver-cells that causes cirrhosis. It is also probable that insufficient activity of Atp8b1/FIC1 increases susceptibility to bacterial pneumonia.Regulatory pathways of Atp8b1/FIC1 activity in vivo remain to be insufficiently studied and this opens novel perspectives for research in this field that may allow better understanding of molecular processes behind the development of certain pathologies and to reveal novel therapeutical targets.

Inner workings and biological impact of phospholipid flippases

The plasma membrane, trans-Golgi network and endosomal system of eukaryotic cells are populated with flippases that hydrolyze ATP to help establish asymmetric phospholipid distributions across the bilayer. Upholding phospholipid asymmetry is vital to a host of cellular processes, including membrane homeostasis, vesicle biogenesis, cell signaling, morphogenesis and migration. Consequently, defining the identity of flippases and their biological impact has been the subject of intense investigations. Recent work has revealed a remarkable degree of kinship between flippases and cation pumps. In this Commentary, we review emerging insights into how flippases work, how their activity is controlled according to cellular demands, and how disrupting flippase activity causes system failure of membrane function, culminating in membrane trafficking defects, aberrant signaling and disease.

Byler disease: early natural history

OBJECTIVES

Byler disease, originally described in Amish kindred, results from mutations in ATPase Class I Type 8b Member 1 (ATP8b1). Specific clinical reports of Amish Byler disease were last published 40 years ago. These investigations were directed at the present detailed clinical understanding of the early course of hepatic manifestations of Byler disease.

METHODS

This study analyzed routine clinical practice and outcomes of children with Byler disease (defined by homozygous c.923G>T mutation in ATP8b1), who initially presented to Children's Hospital of Pittsburgh of UPMC between January 2007 and October 2014. Data were analyzed to the earlier of 24 months of age or partial external biliary diversion.

RESULTS

Six children presented between 1 and 135 days of life: 2 presented with newborn direct hyperbilirubinemia, 2 had complications of coagulopathy, 1 had failure to thrive and rickets, and 1 sibling was identified by newborn genetic testing. Intensive fat-soluble vitamin supplementation was required to prevent insufficiencies in vitamins D, E, and K. Hyperbilirubinemia was variable both over time and between children. Serum bile acid levels were elevated, whereas γ-glutamyltranspeptidase levels were low normal. Scratching behavior (pruritus) was intractable in 4 of 6 children with onset between 6 and 12 months of age. Features of portal hypertension were not observed. Partial external biliary diversion was used during the second year of life in 4 children.

CONCLUSIONS

Detailed analysis of Byler disease revealed varied disease presentation and course. Nutritional issues and pruritus dominated the clinical picture in the first 2 years of life.

P4-ATPases: lipid flippases in cell membranes

Cellular membranes, notably eukaryotic plasma membranes, are equipped with special proteins that actively translocate lipids from one leaflet to the other and thereby help generate membrane lipid asymmetry. Among these ATP-driven transporters, the P4 subfamily of P-type ATPases (P4-ATPases) comprises lipid flippases that catalyze the translocation of phospholipids from the exoplasmic to the cytosolic leaflet of cell membranes. While initially characterized as aminophospholipid translocases, recent studies of individual P4-ATPase family members from fungi, plants, and animals show that P4-ATPases differ in their substrate specificities and mediate transport of a broader range of lipid substrates, including lysophospholipids and synthetic alkylphospholipids. At the same time, the cellular processes known to be directly or indirectly affected by this class of transporters have expanded to include the regulation of membrane traffic, cytoskeletal dynamics, cell division, lipid metabolism, and lipid signaling. In this review, we will summarize the basic features of P4-ATPases and the physiological implications of their lipid transport activity in the cell.

Phospholipid flippase activities and substrate specificities of human type IV P-type ATPases localized to the plasma membrane

Type IV P-type ATPases (P4-ATPases) are believed to translocate aminophospholipids from the exoplasmic to the cytoplasmic leaflets of cellular membranes. The yeast P4-ATPases, Drs2p and Dnf1p/Dnf2p, flip nitrobenzoxadiazole-labeled phosphatidylserine at the Golgi complex and nitrobenzoxadiazole-labeled phosphatidylcholine (PC) at the plasma membrane, respectively. However, the flippase activities and substrate specificities of mammalian P4-ATPases remain incompletely characterized. In this study, we established an assay for phospholipid flippase activities of plasma membrane-localized P4-ATPases using human cell lines stably expressing ATP8B1, ATP8B2, ATP11A, and ATP11C. We found that ATP11A and ATP11C have flippase activities toward phosphatidylserine and phosphatidylethanolamine but not PC or sphingomyelin. By contrast, ATPase-deficient mutants of ATP11A and ATP11C did not exhibit any flippase activity, indicating that these enzymes catalyze flipping in an ATPase-dependent manner. Furthermore, ATP8B1 and ATP8B2 exhibited preferential flippase activities toward PC. Some ATP8B1 mutants found in patients of progressive familial intrahepatic cholestasis type 1 (PFIC1), a severe liver disease caused by impaired bile flow, failed to translocate PC despite their delivery to the plasma membrane. Moreover, incorporation of PC mediated by ATP8B1 can be reversed by simultaneous expression of ABCB4, a PC floppase mutated in PFIC3 patients. Our findings elucidate the flippase activities and substrate specificities of plasma membrane-localized human P4-ATPases and suggest that phenotypes of some PFIC1 patients result from impairment of the PC flippase activity of ATP8B1.

Effect of drug transporter pharmacogenetics on cholestasis

INTRODUCTION

The liver is the central place for the metabolism of drugs and other xenobiotics. In the liver cell, oxidation and conjugation of compounds take place, and at the same time, bile formation helps in extrusion of these compounds via the biliary route. A large number of transporters are responsible for drug uptake into the liver cell and excretion into bile or efflux to the sinusoidal blood.

AREAS COVERED

Genetic variants of these transporters and their transactivators contribute to changes in drug handling and are also responsible for cholestatic syndromes of different severity. This review summarizes the current knowledge regarding the influence of these genetic changes. The review covers progressive hereditary cholestatic syndromes as well as recurrent or transient cholestatic syndromes such as drug-induced liver injury, intrahepatic cholestasis of pregnancy, and benign recurrent intrahepatic cholestasis.

EXPERT OPINION

Polymorphisms in transporter genes are frequent. For clinically relevant cholestatic syndromes, it often requires a combination of genetic variants or acquired triggers such as pregnancy or drug treatment. In combination with other pathogenetic aspects, genetic variants in drug transporters may contribute to our understanding of not only cholestatic diseases such as primary sclerosing cholangitis or primary biliary cirrhosis, but also the natural course of chronic liver disease in general.

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.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) is a group of rare disorders which are caused by defect in bile secretion and present with intrahepatic cholestasis, usually in infancy and childhood. These are autosomal recessive in inheritance. The estimated incidence is about 1 per 50,000 to 1 per 100,000 births, although exact prevalence is not known. These diseases affect both the genders equally and have been reported from all geographical areas. Based on clinical presentation, laboratory findings, liver histology and genetic defect, these are broadly divided into three types-PFIC type 1, PFIC type 2 and PFIC type 3. The defect is in ATP8B1 gene encoding the FIC1 protein, ABCB 11 gene encoding BSEP protein and ABCB4 gene encoding MDR3 protein in PFIC1, 2 and 3 respectively. The basic defect is impaired bile salt secretion in PFIC1/2 whereas in PFIC3, it is reduced biliary phospholipid secretion. The main clinical presentation is in the form of cholestatic jaundice and pruritus. Serum gamma glutamyl transpeptidase (GGT) is normal in patients with PFIC1/2 while it is raised in patients with PFIC3. Treatment includes nutritional support (adequate calories, supplementation of fat soluble vitamins and medium chain triglycerides) and use of medications to relieve pruritus as initial therapy followed by biliary diversion procedures in selected patients. Ultimately liver transplantation is needed in most patients as they develop progressive liver fibrosis, cirrhosis and end stage liver disease. Due to the high risk of developing liver tumors in PFIC2 patients, monitoring is recommended from infancy. Mutation targeted pharmacotherapy, gene therapy and hepatocyte transplantation are being explored as future therapeutic options.

Role of phosphatidylserine in phospholipid flippase-mediated vesicle transport in Saccharomyces cerevisiae

Phospholipid flippases translocate phospholipids from the exoplasmic to the cytoplasmic leaflet of cell membranes to generate and maintain phospholipid asymmetry. The genome of budding yeast encodes four heteromeric flippases (Drs2p, Dnf1p, Dnf2p, and Dnf3p), which associate with the Cdc50 family noncatalytic subunit, and one monomeric flippase Neo1p. Flippases have been implicated in the formation of transport vesicles, but the underlying mechanisms are largely unknown. We show here that overexpression of the phosphatidylserine synthase gene CHO1 suppresses defects in the endocytic recycling pathway in flippase mutants. This suppression seems to be mediated by increased cellular phosphatidylserine. Two models can be envisioned for the suppression mechanism: (i) phosphatidylserine in the cytoplasmic leaflet recruits proteins for vesicle formation with its negative charge, and (ii) phosphatidylserine flipping to the cytoplasmic leaflet induces membrane curvature that supports vesicle formation. In a mutant depleted for flippases, a phosphatidylserine probe GFP-Lact-C2 was still localized to endosomal membranes, suggesting that the mere presence of phosphatidylserine in the cytoplasmic leaflet is not enough for vesicle formation. The CHO1 overexpression did not suppress the growth defect in a mutant depleted or mutated for all flippases, suggesting that the suppression was dependent on flippase-mediated phospholipid flipping. Endocytic recycling was not blocked in a mutant lacking phosphatidylserine or depleted in phosphatidylethanolamine, suggesting that a specific phospholipid is not required for vesicle formation. These results suggest that flippase-dependent vesicle formation is mediated by phospholipid flipping, not by flipped phospholipids.

Cholestasis in a patient with gallstones and a normal gamma-glutamyl transferase

Cholestasis with normal gamma glutamyl transferase characterizes functional deficiencies in the gene ABCB11, which encodes the bile salt export pump (BSEP), a liver-specific adenosine triphosphate (ATP)-binding cassette transporter. Here we report the case of a patient presenting with features of benign recurrent intrahepatic cholestasis associated with a heterozygous mutation in the ABCB11 gene. Immunohistochemistry showed a gradual decrease of BSEP from zone 1 to zone 3 of the liver lobule, suggesting that the mutation identified here may predispose patients to cholestasis through a delocalization process of BSEP at the lobular level. (HEPATOLOGY 2013;57:2539-2541).

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.

Phospholipase D2 mediates signaling by ATPase class I type 8B membrane 1

Functional defects in ATPase class I type 8B membrane 1 (ATP8B1 or familial intrahepatic cholestasis 1, FIC1) lead to cholestasis by mechanism(s) that are not fully understood. One proposed pathophysiology involves aberrant signaling to the bile acid sensor, the farnesoid X receptor (FXR), via protein kinase C ζ (PKCζ). The following cell line-based studies investigated whether phospholipase D2 may transduce a signal from FIC1 to FXR. PLD2 gain of function led to activation of the bile salt export pump (BSEP) promoter, a well-characterized FXR response. BSEP activation by PLD2 could be blocked by abrogating either PKCζ or FXR signaling. PLD2 loss of function led to a reduction in BSEP promoter activity. In addition, a variety of proteins that are activated by FXR, including BSEP, were reduced in HepG2 cells treated with PLD2 siRNA. Similar effects were observed in freshly isolated human hepatocytes. Activation of BSEP by FIC1 gain of function was blocked when PLD2 but not PLD1 was silenced. Overexpression of wild-type but not Byler mutant FIC1 led to an increase in membrane associated PLD activity. An intermediate level of activation of PLD activity was induced when a benign recurrent intrahepatic cholestasis FIC1 mutant construct was expressed. These studies show that FIC1 signals to FXR via a signaling pathway including PLD2 and PKCζ.

ATP8B1 gene expression is driven by a housekeeping-like promoter independent of bile acids and farnesoid X receptor

Background

Mutations in ATP8B1 gene were identified as a cause of low γ-glutamyltranspeptidase cholestasis with variable phenotype, ranging from Progressive Familial Intrahepatic Cholestasis to Benign Recurrent Intrahepatic Cholestasis. However, only the coding region of ATP8B1 has been described. The aim of this research was to explore the regulatory regions, promoter and 5′untranslated region, of the ATP8B1 gene.

Methodology/Principal Findings

5′Rapid Amplification of cDNA Ends using human liver and intestinal tissue was performed to identify the presence of 5′ untranslated exons. Expression levels of ATP8B1 transcripts were determined by quantitative reverse-transcription PCR and compared with the non-variable part of ATP8B1. Three putative promoters were examined in vitro using a reporter gene assay and the main promoter was stimulated with chenodeoxycholic acid. Four novel untranslated exons located up to 71 kb upstream of the previously published exon 1 and twelve different splicing variants were found both in the liver and the intestine. Multiple transcription start sites were identified within exon −3 and the proximal promoter upstream of this transcription start site cluster was proven to be an essential regulatory element responsible for 70% of total ATP8B1 transcriptional activity. In vitro analysis demonstrated that the main promoter drives constitutive ATP8B1 gene expression independent of bile acids.

Conclusions/Significance

The structure of the ATP8B1 gene is complex and the previously published transcription start site is not significant. The basal expression of ATP8B1 is driven by a housekeeping-like promoter located 71 kb upstream of the first protein coding exon.

Endoscopic nasobiliary drainage improves jaundice attack symptoms in benign recurrent intrahepatic cholestasis: A case report

A 66-year-old male with unbearable pruritus and jaundice was admitted for detailed examination. Blood tests on admission showed increased bilirubin with a dominant direct fraction. Ultrasonography and computed tomography performed subsequent to admission showed no narrowing or distension of the bile ducts. As the jaundice symptoms were not improved by the oral administration of ursodeoxycholic acid (300 mg/day) that had been started immediately after admission, endoscopic retrograde cholangiopancreatography (ERCP) was performed on hospital day 14. This also showed no abnormalities of the bile ducts. After considerating its potential effects for improving jaundice, endoscopic nasobiliary drainage (ENBD) was performed on the same day and was followed by immediate improvements in pruritus and jaundice. Detailed examinations were performed to identify the cause of the jaundice, which was suspected to be viral hepatitis, autoimmune hepatitis or drug-induced liver injury, however, there were no findings suggestive of any of these conditions. Following a further increase in bilirubin levels, confirmed by additional blood tests, a liver biopsy was performed. Histological findings were consistent with the histological features of benign recurrent intrahepatic cholestasis (BRIC). Although ursodeoxycholic acid is used as a first-line treatment in most cases of BRIC, ENBD should also be considered for patients not responding to this treatment.

Mammalian P4-ATPases and ABC transporters and their role in phospholipid transport

Transport of phospholipids across cell membranes plays a key role in a wide variety of biological processes. These include membrane biosynthesis, generation and maintenance of membrane asymmetry, cell and organelle shape determination, phagocytosis, vesicle trafficking, blood coagulation, lipid homeostasis, regulation of membrane protein function, apoptosis, etc. P(4)-ATPases and ATP binding cassette (ABC) transporters are the two principal classes of membrane proteins that actively transport phospholipids across cellular membranes. P(4)-ATPases utilize the energy from ATP hydrolysis to flip aminophospholipids from the exocytoplasmic (extracellular/lumen) to the cytoplasmic leaflet of cell membranes generating membrane lipid asymmetry and lipid imbalance which can induce membrane curvature. Many ABC transporters play crucial roles in lipid homeostasis by actively transporting phospholipids from the cytoplasmic to the exocytoplasmic leaflet of cell membranes or exporting phospholipids to protein acceptors or micelles. Recent studies indicate that some ABC proteins can also transport phospholipids in the opposite direction. The importance of P(4)-ATPases and ABC transporters is evident from the findings that mutations in many of these transporters are responsible for severe human genetic diseases linked to defective phospholipid transport. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Biochemical characterization of P4-ATPase mutations identified in patients with progressive familial intrahepatic cholestasis

Mutations in the P4-ATPase ATP8B1 cause the inherited liver disease progressive familial intrahepatic cholestasis. Several of these mutations are located in conserved regions of the transmembrane domain associated with substrate binding and transport. Assays for P4-ATPase-mediated transport in living yeast cells were developed and used to characterize the specificity and kinetic parameters of this transport. Progressive familial intrahepatic cholestasis mutations were introduced into the yeast plasma membrane P4-ATPase Dnf2p, and the effect of these mutations on its catalysis of phospholipid transport were determined. The results of these measurements have implications for the basis of the disease and for the mechanism of phospholipid transit through the enzyme during the reaction cycle.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) refers to a heterogeneous group of autosomal-recessive disorders of childhood that disrupt bile formation and present with cholestasis of hepatocellular origin. The exact prevalence remains unknown, but the estimated incidence varies between 1/50,000 and 1/100,000 births. Three types of PFIC have been identified and associated with mutations in hepatocellular transport-system genes involved in bile formation. PFIC1 and PFIC2 usually appear in the first months of life, whereas onset of PFIC3 may arise later in infancy, in childhood or even during young adulthood. The main clinical manifestations include cholestasis, pruritus and jaundice. PFIC patients usually develop fibrosis and end-stage liver disease before adulthood. Serum gamma-glutamyltransferase (GGT) activity is normal in PFIC1 and PFIC2 patients, but is elevated in PFIC3 patients. Both PFIC1 and PFIC2 are caused by impaired bile salt secretion due to defects in ATP8B1 encoding the FIC1 protein and in ABCB11 encoding bile salt export pump (BSEP) protein, respectively. Defects in ABCB4, encoding multidrug resistance 3 protein (MDR3), impair biliary phospholipid secretion, resulting in PFIC3. Diagnosis is based on clinical manifestations, liver ultrasonography, cholangiography and liver histology, as well as on specific tests to exclude other causes of childhood cholestasis. MDR3 and BSEP liver immunostaining, and analysis of biliary lipid composition should help to select PFIC candidates for whom genotyping could be proposed to confirm the diagnosis. Antenatal diagnosis may be proposed for affected families in which a mutation has been identified. Ursodeoxycholic acid (UDCA) therapy should be initiated in all patients to prevent liver damage. In some PFIC1 and PFIC2 patients, biliary diversion may also relieve pruritus and slow disease progression. However, most PFIC patients are ultimately candidates for liver transplantation. Monitoring of liver tumors, especially in PFIC2 patients, should be offered from the first year of life. Hepatocyte transplantation, gene therapy and specific targeted pharmacotherapy may represent alternative treatments in the future.

A putative plant aminophospholipid flippase, the Arabidopsis P4 ATPase ALA1, localizes to the plasma membrane following association with a β-subunit

Plasma membranes in eukaryotic cells display asymmetric lipid distributions with aminophospholipids concentrated in the inner leaflet and sphingolipids in the outer leaflet. This unequal distribution of lipids between leaflets is, amongst several proposed functions, hypothesized to be a prerequisite for endocytosis. P4 ATPases, belonging to the P-type ATPase superfamily of pumps, are involved in establishing lipid asymmetry across plasma membranes, but P4 ATPases have not been identified in plant plasma membranes. Here we report that the plant P4 ATPase ALA1, which previously has been connected with cold tolerance of Arabidopsis thaliana, is targeted to the plasma membrane and does so following association in the endoplasmic reticulum with an ALIS protein β-subunit.

Phospholipid flippases: building asymmetric membranes and transport vesicles

Phospholipid flippases in the type IV P-type ATPase family (P4-ATPases) are essential components of the Golgi, plasma membrane and endosomal system that play critical roles in membrane biogenesis. These pumps flip phospholipid across the bilayer to create an asymmetric membrane structure with substrate phospholipids, such as phosphatidylserine and phosphatidylethanolamine, enriched within the cytosolic leaflet. The P4-ATPases also help form transport vesicles that bud from Golgi and endosomal membranes, thereby impacting the sorting and localization of many different proteins in the secretory and endocytic pathways. At the organismal level, P4-ATPase deficiencies are linked to liver disease, obesity, diabetes, hearing loss, neurological deficits, immune deficiency and reduced fertility. Here, we review the biochemical, cellular and physiological functions of P4-ATPases, with an emphasis on their roles in vesicle-mediated protein transport. This article is part of a Special Issue entitled Lipids and Vesicular Transport.