Cholangiocytes

In the intricate landscape of liver physiology, cholangiocytes emerge as fundamental players in maintaining hepatic function. These dynamic cells are the architects behind one of the body’s most essential processes: bile production. Bile is a substance synthesized by the liver to aid in the digestion of lipids and the elimination of bilirubin (a substance produced by the breakdown of red blood cells). Nestled along the bile ducts, cholangiocytes act as the sophisticated gatekeepers, orchestrating the flow and composition of bile with precision. Their role extends to influencing liver homeostasis and responding to various physiological and pathological stimuli. Cholangiocytes serve as the initial defense mechanism of the biliary system against pathogenic microorganisms to ensure proper liver functioning.

Morphological heterogeneity

Originating from hepatoblasts in the peribiliary glands, cholangiocytes exhibit distinct morphological and functional heterogeneity, setting them apart from other epithelial cells. These specialized epithelial cells lining the intrahepatic bile ducts account for approximately 3% to 5% of the total liver cell mass. As the biliary lumen’s diameter increases, these cells enlarge and exhibit greater cytoplasmic complexity, evidenced by organelles like the Golgi apparatus, vesicles, and mitochondria. Small cholangiocytes in the Hering canals and distal branches possess a cuboidal shape, while large cholangiocytes lining the larger-diameter ducts display a cylindrical/columnar shape (1,2).

Cells that keep the bile flowing and the liver thriving

Under the powerful gaze of the electron microscope, large cholangiocytes unveil a fascinating array of structural features, most notably the presence of primary cilia dancing on their apical surface. These cilia aren’t just for show; they act as sophisticated mechanosensors, chemosensors, and osmosensors within the dynamic environment of the bile duct lumen (3). Cilia are also crucial in directing bile flow and are pivotal in cell proliferation, senescence, activation of progenitor cell compartments, regeneration, and development. Moreover, extracellular vesicles released by cells can accumulate in the lumen of bile ducts and bind to cilia influencing the proliferation of bile duct cells (4). Remarkably, cholangiocytes are responsible for up to 40% of salt-independent bile flow, expertly regulated by both basal and hormone-driven events (5).

Functional Diversity

At the functional level, cholangiocytes exhibit incredible diversity. Small cholangiocytes possess proliferative capabilities and display functional plasticity in response to disease states, while large cholangiocytes are primarily involved in regulating bile secretion mediated by hormones. Small and large cholangiocytes have different profiles of receptor, enzyme, and transporter expression. Large cholangiocytes are characterized by the presence of CFTR, the anion exchanger, the bile acid transporter, and the secretin receptor. They start proliferating in response to bile duct ligation. In contrast, small cholangiocytes do not possess a hormone-regulated ductal secretory response, do not respond to bile duct ligation, and demonstrate resistance to carbon tetrachloride. Upon exposure to carbon tetrachloride, large cholangiocytes undergo apoptosis, whereas small cholangiocytes proliferate in a compensatory manner. Additionally, cholangiocytes express cell-matrix adhesion molecules, such as integrins, reflecting their role as polarized epithelial cells on a basement membrane (6,7).

Such diversity suggests that cholangiocytes exhibit significant versatility and complexity in their functions. For example, cholangiocytes facilitate bile synthesis and transport via transmembrane proteins on their apical and basolateral membranes. These include channels (e.g., aquaporins), transporters (e.g., SGLT1), and exchangers (e.g., SLC4A2). Additionally, cholangiocytes interact with resident and non-resident bile duct cells through inflammatory mediators such as tumor necrosis factor-alpha and interleukin 6, while damaged cholangiocytes may trigger biliary inflammation and fibrosis. They also regulate cell-cycle processes to maintain biliary tissue homeostasis through molecules affecting apoptosis (e.g., AKT1), senescence (e.g., N-RAS), and proliferation (e.g., platelet-derived growth factor) (8–10).

Understanding Body’s Detox Squad: Journey Through the Biliary Highway

Cholangiocytes line the intricate network of bile ducts through which bile, excreted by hepatocytes in the bile canaliculus, is conveyed to the duodenum (11,12). This network of bile ducts is crucial for bile transport. Hepatocytes release the primary bile into the canalicular space and subsequently into the canals of Hering. These canals are lined with both hepatocytes and cholangiocytes. Aside from their role in the conduction of bile, cholangiocytes secrete water, bicarbonate, and cations into the bile, thereby actively contributing to bile composition. This way, cholangiocytes are responsible for approximately 30% of bile volume and regulate the reabsorption of various solutes from bile, including glucose, glutamate, urate, and, most notably, bile acids. The bile acids reabsorbed by the biliary epithelium are recirculated to hepatocytes through the peribiliary capillary plexus. This mechanism establishes a chole-hepatic shunt pathway, facilitating bile acid-dependent bile flow (13).

The bile is then conveyed through the bile ducts to the gall bladder for storage. The biliary tree comprises an intricate network of tubular structures, encompassing intrahepatic and extrahepatic compartments with distinct embryonic origins and specialized functions. It originates from the Hering canals in the hepatic lobules and progresses into interlobular, septal, and major ducts, ultimately forming the extrahepatic bile ducts, thereby facilitating the transport of bile from the liver to the gallbladder and then into the small intestine for digestion.

Cholangiopathies

Dysfunction in cholangiocytes can significantly disrupt the intricate systems of both intrahepatic and extrahepatic biliary trees. Collectively known as “cholangiopathies,” these chronic liver diseases predominantly target cholangiocytes, and they can arise from a mix of genetic, viral, and environmental influences, or from unknown triggers. The consequences are often serious: impaired bile flow, exaggerated immune responses, and abnormal cholangiocyte growth characterize these conditions. Over time, these chronic challenges can lead to biliary fibrosis, damage to liver tissue, and eventually, life-threatening end-stage liver disease, which may require transplantation for survival (9,14).

Cholangiopathies come in various forms, including Primary Biliary Cirrhosis, Primary Sclerosing Cholangitis, Cystic Fibrosis, Biliary Atresia, Polycystic Liver Disease, and Cholangiocarcinoma. Unfortunately, the tools we have to diagnose several of these conditions remain limited, which can result in subpar treatment or worse, inappropriate therapeutic approaches. Given their progressive nature and the complexities involved in managing these diseases, cholangiopathies pose significant challenges, contributing to high rates of morbidity and mortality.

Exploring Cholangiocyte Organoids

However, there is hope on the horizon. Cholangiocytes step up as unsung heroes in the liver regeneration process, playing a vital “pinch-hitter” role when hepatocytes can no longer rise to the occasion. By delving deeper into cholangiocyte biology and pinpointing the genetic and non-genetic factors linked to these diseases, we can enhance our understanding and potentially improve prognostic accuracy. In this regard, cholangiocyte organoids are revolutionizing our knowledge of cholangiopathies by offering an exciting in vitro model for exploring their pathogenesis and potential treatments. These remarkable structures hold immense promise for regenerative therapies. What’s truly groundbreaking is the use of patient-derived organoids, which pave the way for personalized medicine. Recently, researchers have successfully established branching cholangiocyte organoids (BRCOs) derived from human adult intrahepatic bile ducts. In the lab, these BRCOs exhibit remarkable growth, forming self-renewing and self-organizing branching structures that mimic the complexity of cholangiocytes found in the body. At a single-cell transcriptome level, BRCO cells strikingly resemble mature cholangiocytes, capturing the diverse characteristics seen in vivo (15). What’s more, advancements in pharmacology, along with innovative organoid and in vitro culture systems, are paving the way for exciting opportunities to fast-track the evaluation of new drug candidates. This could lead us toward exciting pharmacological strategies, seamlessly transitioning these discoveries into animal studies and clinical trials (16).

Yet, there are still some translational barriers, such as regulatory, ethical, and technical challenges, which must be addressed for the clinical application of organoid technology. However, understanding the cellular and molecular mechanisms of biliary disorders has the potential to transform diagnosis and treatment, leading to improved outcomes for patients with cholangiopathies. The journey toward better care is just beginning, and each step forward is significant.

Recognizing and appreciating the labs working in this space

• Dr. Nicholas F. LaRusso, Mayo Clinic, USA. https://www.mayo.edu/research/labs/cholangiopathies/overview
• Dr. Sergio Gradilone, University of Minnesota, USA. https://hi.umn.edu/research/faculty/sergio-gradilone-phd, LinkedIn: Sergio Gradilone X: @GradiloneSergio
• Dr. Gregory J. Gores, Mayo Clinic, USA. https://www.mayo.edu/research/faculty/gores-gregory-j-m-d/bio-00078325
• Dr. Sita Kugel, Fred Hutch Cancer Center, USA. https://research.fredhutch.org/kugel/en.html, LinkedIn: Sita Kugel
• Tom Hemming Karlsen, Oslo University Hospital, Norway. https://www.ous-research.no/home/karlsen/Group%20members/6913, X: @tomhemmingk
• Dr. Andrew Feranchak, University of Pittsburgh School Medicine, USA. https://www.pediatrics.pitt.edu/divisions/gastroenterology-hepatology-and-nutrition/labs-and-faculty-pages/feranchak-lab
• Dr. L.J.W. (Luc) van der Laan, Erasmus MC, Netherlands. https://www.erasmusmc.nl/en/research/groups/laboratory-of-experimental-transplantation-and-intestinal-surgery#
• Robert C. Huebert, Mayo Clinic, USA. https://www.mayo.edu/research/faculty/huebert-robert-c-m-d/bio-20124638
• Dr. Stuart J. Forbes, University of Edinburgh, UK. https://regenerative-medicine.ed.ac.uk/research/stuart-forbes
• Dr Fotios Sampaziotis, University of Cambridge, UK. https://www.stemcells.cam.ac.uk/people/pi/sampaziotis

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References

1. Maroni L, Haibo B, Ray D, Zhou T, Wan Y, Meng F, et al. Functional and Structural Features of Cholangiocytes in Health and Disease. Cellular and Molecular Gastroenterology and Hepatology. 2015 Jun 3;1(4):368.
2. Glaser S, Gaudio E, Rao A, Pierce L, Onori P, Franchitto A, et al. Morphological and Functional Heterogeneity of the Mouse Intrahepatic Biliary Epithelium. Laboratory investigation; a journal of technical methods and pathology. 2009 Feb 9;89(4):456.
3. Masyuk AI, Masyuk TV, LaRusso NF. Cholangiocyte primary cilia in liver health and disease. Dev Dyn. 2008 Aug;237(8):2007–12.
4. Tabibian JH, Masyuk AI, Masyuk TV, O’Hara SP, LaRusso NF. Physiology of cholangiocytes. Compr Physiol. 2013 Jan;3(1):541–65.
5. Franchitto A, Onori P, Renzi A, Carpino G, Mancinelli R, Alvaro D, et al. Recent advances on the mechanisms regulating cholangiocyte proliferation and the significance of the neuroendocrine regulation of cholangiocyte pathophysiology. Annals of Translational Medicine. 2013 Oct;1(3):27.
6. Yoo KS, Lim WT, Choi HS. Biology of Cholangiocytes: From Bench to Bedside. Gut and Liver. 2016 Sep 15;10(5):687–98.
7. Marzioni M, Glaser SS, Francis H, Phinizy JL, LeSage G, Alpini G. Functional heterogeneity of cholangiocytes. Semin Liver Dis. 2002 Aug;22(3):227–40.
8. Bogert PT, LaRusso NF. Cholangiocyte biology. Curr Opin Gastroenterol. 2007 May;23(3):299–305.
9. Lazaridis KN, LaRusso NF. The Cholangiopathies. Mayo Clin Proc. 2015 Jun;90(6):791–800.
10. Lazaridis KN, Strazzabosco M, Larusso NF. The cholangiopathies: disorders of biliary epithelia. Gastroenterology. 2004 Nov;127(5):1565–77.
11. Cardinale V, Wang Y, Carpino G, Mendel G, Alpini G, Gaudio E, et al. The biliary tree — a reservoir of multipotent stem cells. Nat Rev Gastroenterol Hepatol. 2012 Feb 28;9(4):231–40.
12. Nathanson MH, Boyer JL. Mechanisms and regulation of bile secretion. Hepatology. 1991 Sep;14(3):551–66.
13. Qin L, Crawford JM. 1 — Anatomy and Cellular Functions of the Liver. In: Sanyal AJ, Boyer TD, Lindor KD, Terrault NA, editors. Zakim and Boyer’s Hepatology (Seventh Edition). Philadelphia: Elsevier; 2018 [cited 2024 Nov 16]. p. 2–19.e4.
14. Tam PK, Yiu RS, Lendahl U, Andersson ER. Cholangiopathies — Towards a molecular understanding. EBioMedicine. 2018 Sep 17;35:381.
15. Roos FJM, Tienderen GS van, Wu H, Bordeu I, Vinke D, Albarinos LM, et al. Human branching cholangiocyte organoids recapitulate functional bile duct formation. Cell Stem Cell. 2022 May 5;29(5):776–794.e13.
16. Rejas C, Junger H. Cholangiocyte Organoids in Liver Transplantation; a Comprehensive Review. Transpl Int. 2024;37:12708.

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