The Cholangiocyte Glycocalyx Stabilizes the 'Biliary HCO3 Umbrella': An Integrated Line of Defense against Toxic Bile Acids

BACKGROUND Destruction of cholangiocytes is the hallmark of chronic cholangiopathies such as primary biliary cirrhosis. Under physiologic conditions, cholangiocytes display a striking resistance to the high, millimolar concentrations of toxic bile salts present in bile. We recently showed that a 'biliary HCO3(-) umbrella', i.e. apical cholangiocellular HCO3(-) secretion, prevents cholangiotoxicity of bile acids, and speculated on a role for extracellular membrane-bound glycans in the stabilization of this protective layer. This paper summarizes published and thus far unpublished evidence supporting the role of the glycocalyx in stabilizing the 'biliary HCO3(-) umbrella' and thus preventing cholangiotoxicity of bile acids. KEY MESSAGES The apical glycocalyx of a human cholangiocyte cell line and mouse liver sections were visualized by electron microscopy. FACS analysis was used to characterize the surface glycan profile of cultured human cholangiocytes. Using enzymatic digestion with neuraminidase the cholangiocyte glycocalyx was desialylated to test its protective function. Using lectin assays, we demonstrated that the main N-glycans in human and mouse cholangiocytes were sialylated biantennary structures, accompanied by high expression of the H-antigen (\textgreeka1-2 fucose). Apical neuraminidase treatment induced desialylation without affecting cell viability, but lowered cholangiocellular resistance to bile acid-induced toxicity: both glycochenodeoxycholate and chenodeoxycholate (pKa \geq4), but not taurochenodeoxycholate (pKa \textless2), displayed cholangiotoxic effects after desialylation. A 24-hour reconstitution period allowed cholangiocytes to recover to a pretreatment bile salt susceptibility pattern. CONCLUSION Experimental evidence indicates that an apical cholangiocyte glycocalyx with glycosylated mucins and other glycan-bearing membrane glycoproteins stabilizes the 'biliary HCO3(-) umbrella', thus aiding in the protection of human cholangiocytes against bile acid toxicity

Biliary Bicarbonate Secretion Constitutes a Protective Mechanism against Bile Acid-Induced Injury in Man

Background: Cholangiocytes expose a striking resistance against bile acids: while other cell types, such as hepatocytes, are susceptible to bile acid-induced toxicity and apoptosis already at micromolar concentrations, cholangiocytes are continuously exposed to millimolar concentrations as present in bile. We present a hypothesis suggesting that biliary secretion of HCO(3)(-) in man serves to protect cholangiocytes against bile acid-induced damage by fostering the deprotonation of apolar bile acids to more polar bile salts. Here, we tested if bile acid-induced toxicity is pH-dependent and if anion exchanger 2 (AE2) protects against bile acid-induced damage. Methods: A human cholangiocyte cell line was exposed to chenodeoxycholate (CDC), or its glycine conjugate, from 0.5 mM to 2.0 mM at pH 7.4, 7.1, 6.7 or 6.4, or after knockdown of AE2. Cell viability and apoptosis were determined by WST and caspase-3/-7 assays, respectively. Results: Glycochenodeoxycholate (GCDC) uptake in cholangiocytes is pH-dependent. Furthermore, CDC and GCDC (pK(a) 4-5) induce cholangiocyte toxicity in a pH-dependent manner: 0.5 mM CDC and 1 mM GCDC at pH 7.4 had no effect on cell viability, but at pH 6.4 decreased viability by >80% and increased caspase activity almost 10- and 30-fold, respectively. Acidification alone had no effect. AE2 knockdown led to 3- and 2-fold enhanced apoptosis induced by 0.75 mM CDC or 2 mM GCDC at pH 7.4. Discussion: These data support our hypothesis of a biliary HCO(3)(-) umbrella serving to protect human cholangiocytes against bile acid-induced injury. AE2 is a key contributor to this protective mechanism. The development and progression of cholangiopathies, such as primary biliary cirrhosis, may be a consequence of genetic and acquired functional defects of genes involved in maintaining the biliary HCO(3)(-) umbrella. Copyright (C) 2011 S. Karger AG, Base

Bile salts and cholestasis

Bile salts have a crucial role in hepatobiliary and intestinal homeostasis and digestion. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Bile salts are efficiently reabsorbed from the intestinal lumen to undergo enterohepatic circulation. In addition to their function as a surfactant involved in the absorption of dietary lipids and fat-soluble vitamins bile salts are potent signaling molecules in both the liver and intestine. Under physiological conditions the bile salt pool is tightly regulated, but the adaptive capacity may fall short under cholestatic conditions. Elevated serum and tissue levels of potentially toxic hydrophobic bile salts during cholestasis may cause mitochondrial damage, apoptosis or necrosis in susceptible cell types. Therapeutic nontoxic bile salts may restore impaired hepatobiliary secretion in cholestatic disorders. The hydrophilic bile salt ursodeoxycholate is today regarded as the effective standard treatment of primary biliary cirrhosis and intrahepatic cholestasis of pregnancy, and is implicated for use in various other cholestatic conditions. Novel therapeutic bile salts that are currently under evaluation may also prove valuable in the treatment of these diseases. (C) 2010 Published by Elsevier Ltd on behalf of Editrice Gastroenterologica Italiana S.r.

Focal Nodular Hyperplasia and Hepatic Adenoma: Epidemiology and Pathology

Focal nodular hyperplasia (FNH) and hepatic adenoma (HA) represent the most frequent non-vascular benign liver tumors. They are often asymptomatic. The widespread use of high-resolution imaging modalities leads to an increase of incidental detection of FNH and HA. Physicians are thus increasingly confronted with these formerly rarely recognized conditions, stressing the need for concise but adequate information on the optimal clinical strategies for these patients. FNH is the most common non-vascular benign tumor of the liver. It probably arises as a polyclonal, hyperplastic response to a locally disturbed blood flow. It is typically found in asymptomatic women. Histologically, FNH can be described as a focal form of cirrhosis. Complications of FNH are extremely rare and surgical resection is generally not advised. HA is a rare monoclonal, but benign liver tumor primarily found in young females using estrogen-containing contraceptives. Although its exact etiology is unknown, a direct link between sex steroid exposure and the uncontrolled hepatocellular growth is suspected. Complications of HA are spontaneous bleeding and malignant transformation. Withdrawal of estrogen treatment and excision of large tumors (>5 cm) are established therapeutic strategies. In conclusion, although FNH and HA are reasonably well- described clinical and histopathological entities, their epidemiology and pathophysiology need to be further unraveled. Copyright (C) 2010 S. Karger AG, Base

Exposure to occupational antigens might predispose to IgG4-related disease

Evidence is mounting that the immune system of patients with IgG4-related disease (IgG4-RD) shows indications of chronic antigenic stimulation. Hypothesizing a possible role for occupational antigenic exposure, we observed in two independent cohorts of patients with IgG4-RD that the majority had had a career in blue collar occupations with prolonged exposures to potentially hazardous occupational antigens. We postulate that the chronic antigenic load associated with 'dirty' jobs could be one of the factors contributing to the development of IgG4-related disease. (Hepatology 2014;

Testicular inflammation as a new manifestation of IgG4-associated disease

IgG4-related disease has properties of a systemic disorder but simultaneously is associated with a growing list of organ-specific manifestations including autoimmune pancreatitis, IgG4-associated cholangitis, IgG4-related kidney disease, and IgG4-associated prostatitis. In this study, we present, to the best of our knowledge, the first case of a patient with multiorgan IgG4-related disease who lost his testes because of IgG4-related testicular inflammation. We postulate that IgG4-related disease in the urogenital tract is not restricted to IgG4-related kidney disease and prostatitis, but that this rare disorder may also affect the testi

Clinical review of retinotopy

Two observations made 29 years apart are the cornerstones of this review on the contributions of Dr Gordon T. Plant to understanding pathology affecting the optic nerve. The first observation laid the anatomical basis in 1990 for the interpretation of optical coherence tomography (OCT) findings in 2009. Retinal OCT offers clinicians detailed in vivo structural imaging of individual retinal layers. This has led to novel observations which were impossible to make using ophthalmoscopy. The technique also helps to re-introduce the anatomically grounded concept of retinotopy to clinical practise. This review employs illustrations of the anatomical basis for retinotopy through detailed translational histological studies and multimodal brain-eye imaging studies. The paths of the prelaminar and postlaminar axons forming the optic nerve and their postsynaptic path from the dorsal lateral geniculate nucleus to the primary visual cortex in humans are described. With the mapped neuroanatomy in mind we use OCT-MRI pairings to discuss the patterns of neurodegeneration in eye and brain that are a consequence of the hard wired retinotopy: anterograde and retrograde axonal degeneration which can, within the visual system, propagate trans-synaptically. The technical advances of OCT and MRI for the first time enable us to trace axonal degeneration through the entire visual system at spectacular resolution. In conclusion, the neuroanatomical insights provided by the combination of OCT and MRI allows us to separate incidental findings from sinister pathology and provides new opportunities to tailor and monitor novel neuroprotective strategies

Costimulatory ligand CD70 is delivered to the immunological synapse by shared intracellular trafficking with MHC class II molecules

TNF family member CD70 is the ligand of CD27, a costimulatory receptor that shapes effector and memory T cell pools. Tight control of CD70 expression is required to prevent lethal immunodeficiency. By selective transcription, CD70 is largely confined to activated lymphocytes and dendritic cells (DC). We show here that, in addition, specific intracellular routing controls its plasma membrane deposition. In professional antigen-presenting cells, such as DC, CD70 is sorted to late endocytic vesicles, defined as MHC class II compartments (MIIC). In cells lacking the machinery for antigen presentation by MHC class II, CD70 travels by default to the plasma membrane. Introduction of class II transactivator sufficed to reroute CD70 to MIIC. Vesicular trafficking of CD70 and MHC class II is coordinately regulated by the microtubule-associated dynein motor complex. We show that when maturing DC make contact with T cells in a cognate fashion, newly synthesized CD70 is specifically delivered via MIIC to the immunological synapse. Therefore, we propose that routing of CD70 to MIIC serves to coordinate delivery of the T cell costimulatory signal in time and space with antigen recognition