Isolation of the bile canalicular actin-myosin II motor

Alteration of liver enzymes is a feature of the MYH9-related disease syndrome

Background

MYH9-related disease (MYH9-RD) is a rare autosomal dominant genetic syndrome characterized by congenital thrombocytopenia associated with the risk of developing progressive nephropathy, sensorineural deafness, and presenile cataract. During the collection of a large case-series of patients with MYH9-RD we noticed several cases with unexplained elevation of liver enzymes. Our aim was to evaluate if the alteration of liver tests is a feature of the MYH9-RD and to define its clinical significance.

Methods and Findings

Data concerning liver tests, prospectively recorded in the Italian Registry for MYH9-RD, were collected and compared with those of three control populations: patients with autoimmune thrombocytopenia, patients with inherited thrombocytopenias other than MYH9-RD, and the participants to a large epidemiologic survey in an Italian geographic isolate. Thirty-eight of 75 evaluable MYH9-RD patients (50.7%) showed an elevation of ALT and/or AST, and 17 of 63 (27.0%) an increase of GGT. The increases ranged from 1.9±0.7 to 2.7±1.6 fold the upper normal limit. The prevalence of liver test alterations was significantly higher in MYH9-RD patients than in each of the control populations, with odds ratios ranging from 8.2 (95% CIs 2.2–44.8) to 24.7 (14.8–40.8). Clinical follow-up and more detailed liver studies of a subset of patients, including ultrasound liver scan, liver elastography and liver biopsy in one case, did not show any significant structural damage or evolution towards liver insufficiency.

Conclusions

Elevation of liver enzymes is a frequent and previously unrecognized feature of the MYH9-RD syndrome; however, this defect does not appear to have poor prognostic value.

Compensatory role of P-glycoproteins in knockout mice lacking the bile salt export pump

Bile salt export pump (BSEP; ATP-binding cassette, subfamily B, member 11) mutations in humans result in progressive familial intrahepatic cholestasis type 2, a fatal liver disease with greatly reduced bile flow. However in mice, Bsep knockout leads only to mild cholestasis with substantial bile flow and up-regulated P-glycoprotein genes (multidrug resistance protein 1a [Mdr1a] and Mdr1b). To determine whether P-glycoprotein is responsible for the relatively mild phenotype observed in Bsep knockout mice, we have crossed mouse strains knocked out for Bsep and the two P-glycoprotein genes and generated a triple knockout mouse. We found that a knockout of the three genes leads to a significantly more severe phenotype with impaired bile formation, jaundice, flaccid gallbladder, and increased mortality. The triple knockout mouse is the most severe genetic model of intrahepatic cholestasis yet developed.

CONCLUSION

P-glycoprotein functions as a critical compensatory mechanism, which reduces the severity of cholestasis in Bsep knockout mice.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.

New insights into the dynamics of sinusoidal endothelial fenestrae in liver sinusoidal endothelial cells

Ultrastructural studies have shown that liver sinusoidal endothelial cells (LSECs) contain a cytoskeletal framework of filamentous actin, and that the presence of actin in the form of a calmodulin-actomyosin complex is responsible for regulation of the diameter of sinusoidal endothelial fenestrae (SEF). Rho has emerged as an important regulator of the actin cytoskeleton and consequently of cell morphology. We investigated actin filaments in relation to SEF in LSEC using heavy meromyosin decorated reaction and elucidated the roles of Rho and actin cytoskeleton in morphological and functional alterations of SEF. Second, according to intracytoplasmic Ca2+ concentration, plasma membrane Ca2+Mg2+-ATPase activities were clearly demonstrated on the outer surface of the labyrinth-like SEF in the isolated LSECs. Furthermore, by investigating intracytoplasmic Ca2+ concentration, we have demonstrated plasma membrane Ca2+Mg2+-ATPase activities on the outer surface of the labyrinth-like SEF in the isolated LSECs. Currently, the majority of fenestral studies are focused on finding ways to increase the liver sieve's porosity, which is reduced through pathological mechanisms.

Taurolithocholate impairs bile canalicular motility and canalicular bile secretion in isolated rat hepatocyte couplets

AIM

To investigate the effects of taurolithocholate (TLC) on the canalicular motility in isolated rat hepatocyte couplets (IRHC).

METHODS

TLC was added to IRHC at concentrations of 10 and 50 mumol/L, respectively. In each group, five time-lapse movies containing 3 representative bile canaliculi were taken under phase-contrast microscopy for 12 h. The number of bile canalicular contractions and the intervals between consecutive canalicular contractions were calculated. Furthermore, the effects of TLC on IRHC were examined by transmission electron microscopy.

RESULTS

The bile canalicular contractions were spontaneous and forceful in the controls. Active vesicular movement was observed in the pericanalicular region. Immediately after the addition of TLC, the bile canaliculi were deformed, and canalicular bile was incorporated into the vacuoles. The canaliculi were gradually dilated, and canalicular contractions were markedly inhibited by TLC. The vesicular movements became extremely slow in the pericanalicular region. The number of canalicular contractions significantly decreased in the TLC-treated groups, as compared with that in the controls. The time intervals were prolonged, as the TLC dosage increased, indicating that bile secretion into the canaliculi was impaired with TLC. Transmission electron microscopy revealed the lamellar transformation of the canalicular membranes in IRHC treated with TLC.

CONCLUSION

TLC impairs both the bile canalicular contractions and the canalicular bile secretion, possibly by acting directly on the canalicular membranes in TLC-induced cholestasis.

Myosin II regulatory light chain is required for trafficking of bile salt export protein to the apical membrane in Madin-Darby canine kidney cells

BSEP, MDR1, and MDR2 ATP binding cassette transporters are targeted to the apical (canalicular) membrane of hepatocytes, where they mediate ATP-dependent secretion of bile acids, drugs, and phospholipids, respectively. Sorting to the apical membrane is essential for transporter function; however, little is known regarding cellular proteins that bind ATP binding cassette proteins and regulate their trafficking. A yeast two-hybrid screen of a rat liver cDNA library identified the myosin II regulatory light chain, MLC2, as a binding partner for BSEP, MDR1, and MDR2. The interactions were confirmed by glutathione S-transferase pulldown and co-immunoprecipitation assays. BSEP and MLC2 were overrepresented in a rat liver subcellular fraction enriched in canalicular membrane vesicles, and MLC2 colocalized with BSEP in the apical domain of hepatocytes and polarized WifB, HepG2, and Madin-Darby canine kidney cells. Expression of a dominant negative, non-phosphorylatable MLC2 mutant reduced steady state BSEP levels in the apical domain of polarized Madin-Darby canine kidney cells. Pulse-chase studies revealed that Blebbistatin, a specific myosin II inhibitor, severely impaired delivery of newly synthesized BSEP to the apical surface. These findings indicate that myosin II is required for BSEP trafficking to the apical membrane.

Severe cholestasis induced by cholic acid feeding in knockout mice of sister of P-glycoprotein

Intrahepatic cholestasis is often associated with impairment of biliary bile acid secretion, a process mediated by the sister of P-glycoprotein (Spgp or Abcb11) also known as the bile salt export pump (Bsep). In humans, mutations in the Spgp gene are associated with a fatal childhood disease, type 2 progressive familial intrahepatic cholestasis (PFIC2). However in mice, the "knockout" of Spgp only results in mild cholestasis. In this study, we fed spgp(-/-) knockout mice with a cholic acid (CA)-supplemented diet to determine whether a more pronounced PFIC2-like phenotype could be induced. Such mice developed severe cholestasis characterized by jaundice, weight loss, elevated plasma bile acid, elevated transaminase, cholangiopathy (proliferation of bile ductules and cholangitis), liver necrosis, high mortality, and wide-ranging changes in the mRNA expression of major liver genes (16/36 examined). A surprising observation was that the bile acid output and bile flow in CA-fed mutant mice was significantly higher than anticipated. This suggests that the spgp(-/-) mice are able to utilize an alternative bile salt transport system. However, unlike Spgp, this system is insufficient to protect the knockout mice from cholestasis despite its high capacity. In conclusion, the spgp(-/-) mice provide a unique model to investigate molecular pathways associated with cholestasis and related diseases.

Abnormalities in villin gene expression and canalicular microvillus structure in progressive cholestatic liver disease of childhood

BACKGROUND

The molecular basis of clinical cholestasis is a subject of intense investigation. Villin is an actin binding, bundling, and severing protein needed for maintenance of structural integrity of canalicular microvilli, in which membrane transporters required for bile secretion are located. We aimed to investigate the role of canalicular cytoskeletal proteins in three genetically unrelated children with a biliary atresia-like clinical disorder, each of whom developed liver failure requiring liver transplantation.

METHODS

Explanted livers from the three patients were examined by standard pathological methods followed by transmission and cryoimmunoelectron microscopy. With archival tissue samples, a panel of cytoskeletal proteins was investigated by immunohistochemistry and western blotting, with purified canalicular membrane preparations. Villin mRNA analyses were undertaken on liver homogenates, with primers from coding regions of the human villin gene. Classic biliary atresia, other types of cholestasis, and normal livers served as controls.

FINDINGS

In patients, pronounced ultrastructural deformities of canaliculi and especially of their microvilli were noted, which correlated with absence of villin protein by immunostaining of liver tissue sections and by western blot analysis. Additionally, villin mRNA was strikingly reduced or absent. These results differed greatly from those in controls.

INTERPRETATION

These results suggest that the disorder described mimics biliary atresia, but structural and molecular pathological findings differ. We propose that a functional abnormality in villin gene expression is key to the mechanism of cholestasis in patients with progressive cholestasis and hepatic failure.

Cholestatic liver disease: pathophysiology and therapeutic options

Cholestasis results from defective canalicular secretion of bile or obstruction to bile flow distal to the canaliculus. In early primary biliary cirrhosis, bile secretion continues, because of the secretory pressure of bile or because some ductules are not obstructed. With complete cholestasis, a bile acid deficiency occurs in the small intestinal lumen leading to lipid maldigestion and fat-soluble vitamin malabsorption. Bacterial proliferation, bacterial translocation to lymph nodes and endotoxemia may also occur leading to an acute phase reaction. Retention of bile acids in the hepatocyte leads to apoptosis. Accumulation of bile acids in the systemic circulation leads to pruritus, and may contribute to endothelial injury in the lungs and kidney. Early attempts to mimic hepatic excretory function by hemoperfusion over adsorbent columns were unsuccessful for a variety of reasons. Extracorporeal dialysis against albumin offers promise of a realistic albeit partial simulation of hepatic excretory function.

Intracellular calcium in the isolated rat liver: correlation to glucose release, K(+) balance and bile flow

This study correlates whole organ measurements of intracellular calcium concentration ([Ca(2+)](i)) with hormone-induced (epinephrine, vasopressin) changes of liver functions (glucose release, K(+) balance and bile flow). [Ca(2+)](i) was measured in the isolated perfused rat liver using the sensor Fura-2 and applying liver surface fluorescence spectroscopy. The technique was improved by (i) minimizing biliary elimination of the sensor by employing a rat strain deficient in canalicular organic anion transport (TR(-) mutation) and (ii) by correcting for changes of interfering intrinsic organ fluorescence that was shown to depend on the oxidation-reduction state (NAD(P)H content) of the organ. Epinephrine (50 nM) elicits an instantaneous peak rise of [Ca(2+)](i) to approx. 400 nM, followed by a sustained elevation that depends on the presence of extracellular Ca(2+). The rise of [Ca(2+)](i) coincides with initiation of glucose release, transient K(+) uptake, and transient stimulation of bile flow. Vasopressin (2 nM) exerts qualitatively similar effects. The transient rise of bile flow is attributed to Ca(2+)-mediated contraction of the pericanalicular actin-myosin web of hepatocytes.

Targeted inactivation of sister of P-glycoprotein gene (spgp) in mice results in nonprogressive but persistent intrahepatic cholestasis

Mutations in the sister of P-glycoprotein (Spgp) or bile salt export pump (BSEP) are associated with Progressive Familial Intrahepatic Cholestasis (PFIC2). Spgp is predominantly expressed in the canalicular membranes of liver. Consistent with in vitro evidence demonstrating the involvement of Spgp in bile salt transport, PFIC2 patients secrete less than 1% of biliary bile salts compared with normal infants. The disease rapidly progresses to hepatic failure requiring liver transplantation before adolescence. In this study, we show that the knockout of spgp gene in mice results in intrahepatic cholestasis, but with significantly less severity than PFIC2 in humans. Some unexpected characteristics are observed. Notably, although the secretion of cholic acid in mutant mice is greatly reduced (6% of wild-type), total bile salt output in mutant mice is about 30% of wild-type. Also, secretion of an unexpectedly large amount of tetra-hydroxylated bile acids (not detected in wild-type) is observed. These results suggest that hydroxylation and an alternative canalicular transport mechanism for bile acids compensate for the absence of Spgp function and protect the mutant mice from severe cholestatic damage. In addition, the spgp(-/-) mice display a significant increase in the secretion of cholesterol and phospholipids into the bile. This latter observation in spgp(-/-) mice suggests that intrahepatic, rather than intracanalicular, bile salts are the major driving force for the biliary lipid secretion. The spgp(-/-) mice thus provide a unique model for gaining new insights into therapeutic intervention for intrahepatic cholestasis and understanding mechanisms associated with lipid homeostasis.

Targeted inactivation of sister of P-glycoprotein gene (spgp) in mice results in nonprogressive but persistent intrahepatic cholestasis

Mutations in the sister of P-glycoprotein (Spgp) or bile salt export pump (BSEP) are associated with Progressive Familial Intrahepatic Cholestasis (PFIC2). Spgp is predominantly expressed in the canalicular membranes of liver. Consistent with in vitro evidence demonstrating the involvement of Spgp in bile salt transport, PFIC2 patients secrete less than 1% of biliary bile salts compared with normal infants. The disease rapidly progresses to hepatic failure requiring liver transplantation before adolescence. In this study, we show that the knockout of spgp gene in mice results in intrahepatic cholestasis, but with significantly less severity than PFIC2 in humans. Some unexpected characteristics are observed. Notably, although the secretion of cholic acid in mutant mice is greatly reduced (6% of wild-type), total bile salt output in mutant mice is about 30% of wild-type. Also, secretion of an unexpectedly large amount of tetra-hydroxylated bile acids (not detected in wild-type) is observed. These results suggest that hydroxylation and an alternative canalicular transport mechanism for bile acids compensate for the absence of Spgp function and protect the mutant mice from severe cholestatic damage. In addition, the spgp(-/-) mice display a significant increase in the secretion of cholesterol and phospholipids into the bile. This latter observation in spgp(-/-) mice suggests that intrahepatic, rather than intracanalicular, bile salts are the major driving force for the biliary lipid secretion. The spgp(-/-) mice thus provide a unique model for gaining new insights into therapeutic intervention for intrahepatic cholestasis and understanding mechanisms associated with lipid homeostasis.

Molecular and functional analysis of the human prothrombinase gene (HFGL2) and its role in viral hepatitis

In the present studies, we report the cloning and structural characterization of the HFGL2 gene and its functional role in human fulminant hepatitis. The HFGL2 gene is approximately 7 kb in length with 2 exons. The putative promoter contains cis element consensus sequences that strongly suggest the inducibility of its expression. From the nucleotide sequence of the human gene, a 439-amino acid long protein is predicted. The overall identity between the murine fgl2 and hfgl2 coded proteins is over 70%. About 225 amino acids at the carboxyl end of these molecules are almost 90% identical, and correspond to a well-conserved fibrinogen-related domain. Both HFGL2 and FGL2 encode a type II transmembrane protein with a predicted catalytic domain toward the amino terminus of the protein. Transient transfection of Chinese hamster ovary (CHO) cells with a full-length cDNA of HFGL2 coding region resulted in high levels of prothrombinase activity. Livers from 8 patients transplanted for fulminant viral hepatitis were examined for extent of necrosis, inflammation, fibrin deposition, and HFGL2 induction. In situ hybridization showed positive staining of macrophages in areas of active hepatocellular necrosis. Fibrin stained positively in these areas and was confirmed by electron microscopy. These studies define a unique prothrombinase gene (HFGL2) and implicate its importance in the pathogenesis of fulminant viral hepatitis.