Faculty Bibliography

In 1959, Ivar Sperber contrasted bile formation with that of urine and proposed that water flow into the canalicular conduit is in response to an osmotic, not a hydrostatic, gradient. Early attempts to support the hypothesis using a bile acid, sodium taurocholate, and the hormone secretin to stimulate bile flow led to conflicting data and a moratorium on attempts to further develop the initial proposal. However, current data amplify the initial proposal and indicate both paracellular and transcellular water flow into hepatic ductules and the canalicular conduit in response to an osmotic gradient. Also, the need to further modify the initial proposal became apparent with the recognition that bile acid aggregates (micelles), which form in the canalicular conduit, generate lecithin-cholesterol vesicles that contain water unrelated to an osmotic gradient. As part of this development is the recent introduction of the fluorescent localization after photobleaching technique for direct determination of hepatic duct flow and clarification of the role of biomarkers such as mannitol and polyethylene glycol 900. With the new paradigm, these biomarkers may prove useful for quantifying paracellular and transcellular water flow, respectively. SIGNIFICANCE STATEMENT: It is essential to identify and characterize all the sites for water flow during hepatic bile formation to obtain more precision in evaluating the causes and possible therapeutic approaches to cholestatic syndromes. Updating the Sperber proposal provides a new paradigm that addresses the advances in knowledge that have occurred.

BACKGROUND AND AIMS/OBJECTIVE:Faecal microbiota transplant [FMT] is effective in treating recurrent Clostridioides difficile infection [CDI] and restores gut microbiota composition. This is unlikely to account for its entire mechanism of efficacy, as studies have shown that factors such as bile acids influence the risk of infection by affecting Clostridioides difficile germination. We therefore aimed to investigate longitudinal changes in the gut bile acid composition after FMT performed for recurrent CDI, in children with and without inflammatory bowel disease [IBD]. METHODS:Eight children received FMT; five had underlying IBD. Primary and secondary faecal bile acids were measured by liquid chromatography-mass spectrometry in recipients [pre-FMT and longitudinally post-FMT for up to 6 months] and donors. RESULTS:Pre-FMT, recipients had higher primary and lower secondary bile acid proportions compared with donors. Post-FMT, there was a gradual increase of secondary and decrease of primary bile acids. Whereas gut bacterial diversity had been shown to be restored in all children shortly after FMT, normalisation of bile acids to donor levels occurred only by 6 months. In children with IBD, although microbiota diversity returned to pre-FMT levels within 6 months, secondary bile acids remained at donor levels. CONCLUSIONS:The differences in bile acid profiles compared with gut bacterial diversity post-FMT suggests that interactions between the two may be more complex than previously appreciated and may contribute to FMT efficacy in different ways. This initial finding demonstrates the need to further investigate gut metabolites in larger cohorts, with longitudinal sampling to understand the mechanisms of FMT effectiveness.

Vartak et al's recent publicatioin reviews their previous provocative data that water flow into the bile canaliculus is not in response to an osmotic gradient, as initially proposed by Sperber. However their technique, photoactivation of a non-fluorescent precursor of fluorescein, has not been validated. In contrast, a similar technique, enzymatic activation of a non-fluorescent precursor of fluorescein supported the concept of osmotically generated canalicular water flow and confirmed that peristalsis also participates in the flow of fluid from the canaliculi into the duct system as originally described by Philips.

Advances in molecular biology identifying the many carrier-mediated organic anion transporters and advances in microscopy that have provided a more detailed anatomy of the canalicular conduit make updating the concept of osmotically determined canalicular flow possible. For the most part water flow is not transmembrane but via specific pore proteins in both the hepatocyte and the tight junction. These pores independently regulate the rate at which water flows in response to an osmotic gradient and therefore are determinants of canalicular bile acid concentration. Review of the literature indicates that the initial effect on hepatic bile flow of cholestatic agents such as Thorazine and estradiol 17β-glucuronide are on water flow and not bile salt export pump-mediated bile acid transport and thus provides new approaches to the pathogenesis of drug-induced liver injury. Attaining a micellar concentration of bile acids in the canaliculus is essential to the formation of cholesterol-lecithin vesicles, which mostly occur in the periportal region of the canalicular conduit. The other regions, midcentral and pericentral, may transport lesser amounts of bile acid but augment water flow. Broadening the concept of how hepatic bile flow is initiated, provides new insights into the pathogenesis of canalicular cholestasis.

The recent report (1) that the increased cholesterol and lecithin content of hepatic bile after administration of the complex lipopeptide myrcludex B is attributable to its binding to the Sodium Taurocholate Cotransporting Polypeptide (NTCP) , which causes a re-distribution of canalicular bile transport to the peri-central zone appears to conflict with a previous study (2). After partial hepatectomy, NTCP expression significantly decreases in the remaining liver along with increased in hepatic bile flow and no change in cholesterol or lecithin output, findings that were also attributed to a shift in bile acid transport to the pericentral zone of the canaliculus.

BACKGROUNDMirabegron is a β3-adrenergic receptor (β3-AR) agonist approved only for the treatment of overactive bladder. Encouraging preclinical results suggest that β3-AR agonists could also improve obesity-related metabolic disease by increasing brown adipose tissue (BAT) thermogenesis, white adipose tissue (WAT) lipolysis, and insulin sensitivity.METHODSWe treated 14 healthy women of diverse ethnicities (27.5 ± 1.1 years of age, BMI of 25.4 ± 1.2 kg/m2) with 100 mg mirabegron (Myrbetriq extended-release tablet, Astellas Pharma) for 4 weeks in an open-label study. The primary endpoint was the change in BAT metabolic activity as measured by [18F]-2-fluoro-d-2-deoxy-d-glucose (18F-FDG) PET/CT. Secondary endpoints included resting energy expenditure (REE), plasma metabolites, and glucose and insulin metabolism as assessed by a frequently sampled intravenous glucose tolerance test.RESULTSChronic mirabegron therapy increased BAT metabolic activity. Whole-body REE was higher, without changes in body weight or composition. Additionally, there were elevations in plasma levels of the beneficial lipoprotein biomarkers HDL and ApoA1, as well as total bile acids. Adiponectin, a WAT-derived hormone that has antidiabetic and antiinflammatory capabilities, increased with acute treatment and was 35% higher upon completion of the study. Finally, an intravenous glucose tolerance test revealed higher insulin sensitivity, glucose effectiveness, and insulin secretion.CONCLUSIONThese findings indicate that human BAT metabolic activity can be increased after chronic pharmacological stimulation with mirabegron and support the investigation of β3-AR agonists as a treatment for metabolic disease.TRIAL REGISTRATIONClinicaltrials.gov NCT03049462.FUNDINGThis work was supported by grants from the Intramural Research Program of the NIDDK, NIH (DK075112, DK075116, DK071013, and DK071014).