Molecular regulation of mammalian hepatic architecture

Abstract

The essential liver exocrine and endocrine functions require a precise spatial arrangement of the hepatic lobule consisting of the central vein, portal vein, hepatic artery, intrahepatic bile duct system, and hepatocyte zonation. This allows blood to be carried through the liver parenchyma sampled by all hepatocytes and bile produced by the hepatocytes to be carried out of the liver through the intrahepatic bile duct system composed of cholangiocytes. The molecular orchestration of multiple signaling pathways and epigenetic factors is required to set up lineage restriction of the bipotential hepatoblast progenitor into the hepatocyte and cholangiocyte cell lineages, and to further refine cell fate heterogeneity within each cell lineage reflected in the functional heterogeneity of hepatocytes and cholangiocytes. In addition to the complex molecular regulation, there is a complicated morphogenetic choreography observed in building the refined hepatic epithelial architecture. Given the multifaceted molecular and cellular regulation, it is not surprising that impairment of any of these processes can result in acute and chronic hepatobiliary diseases. To enlighten the development of potential molecular and cellular targets for therapeutic options, an understanding of how the intricate hepatic molecular and cellular interactions are regulated is imperative. Here, we review the signaling pathways and epigenetic factors regulating hepatic cell lineages, fates, and epithelial architecture.

Keywords: Bile duct development; Biliary system; Cholangiocyte; Hepatoblast; Hepatocyte zonation; Liver development.

Fig. 1

Initial chromatin targeting of pioneer factor and subsequent events. (A) Closed chromatin configuration in either a low signal or repressed state. (B) Pioneer factors (gray shape) locally open gene regulatory regions in chromatin, referred to as a “poised” state. This enables cooperative binding with other transcription factors (purple shape) and establishes competence to respond to signals. (C) An active chromatin state is established (active histone marks, orange triangles). Additional transcription factors (orange and blue shapes) are recruited promoting cooperative and stable binding.

Fig. 2

Specification of the foregut endoderm. (A) During embryogenesis, the thyroid, lung, liver, and ventral pancreas arise by the budding of diverticula from the ventral foregut endoderm that contains multipotent progenitor cells. (B) Hepatic and pancreatic organogenesis requires complex and temporally precise FGF and BMP signaling. At the 3–4 somite stage of mouse development, FGF target genes are activated in lateral endoderm, whereas BMP targets are activated in the ventral midline endodermal lip (VMEL). Therefore, hepatic domains initially reside in a region of high BMP activity at the ventral midline endodermal lip and high FGF activity at the lateral endoderm.(C) Once specified, the Hex-positive hepatoblasts then delaminate from the epithelium and migrate into the septum transversum (STM).

Fig. 3

Hepatic architecture. (A) Schematic of the approximate hexagonal or pentagonal hepatic lobule. (B) One radius of the approximate hexagonal or pentagonal hepatic lobule. Hepatocyte cords run along the radius of the lobule between portal veins (PV) and central veins (CV), and are arranged into three zones: zone 1 near the PV, zone 2, and zone 3 near the CV. Bile is secreted from hepatocytes into the canalicular channels and transported to the intrahepatic bile ducts (IHBD). The sinusoidal capillaries carry oxygenated blood from the PV and hepatic artery (HA) past the hepatocytes to the CV.(C) Localization of multidrug resistance (Mdr1) protein encoded by ATP binding cassette subfamily B member 1 (ABCB1) on the apical hepatocyte canalicular membrane and apical surface of cholangiocytes comprising IHBDs in mouse liver.

Fig. 4

Hierarchical branching architecture of the intrahepatic bile duct (IHBD) system. Retrograde ink injections into the left lobe of the mouse liver IHBD system. Liver lobe cleared with benzyl benzoate and benzyl alcohol (BABB) solution to visualize hierarchical structure.

Fig. 5

Mechanistic models of hepatocyte differentiation. (A) Schematic demonstrating progressive assembly of transcription complexes. Fetal/neonatal hepatoblasts have less complicated transcription factor complexes associated at enhancers of the same genetic locus compared to adult hepatocytes. (B) Schematic of differentiation-dependent enhancers. Example demonstrates that the enhancer at genetic locus X is occupied in fetal/neonatal hepatoblasts but not occupied in adult hepatocytes, and the enhancer at genetic locus Y is not occupied in fetal/neonatal hepatoblasts but is occupied in adult hepatocytes. (C) Schematic of a differentiation-independent enhancer. Example demonstrates that the enhancer at genetic locus Z is similarly occupied in both fetal/neonatal hepatoblasts and adult hepatocytes. (B and C) Representation of H3K4me1 average enrichment profiles of binding (red peaks) at differentiation-dependent and -independent enhancers in adult hepatocytes. Distinct binding patterns (bimodal and monomodal distribution) are present, depending on whether FoxA2 and Hnf4a are bound to enhancers.

Fig. 6

Zonal hepatocyte functions. Dark orange color indicates more activity for hepatocyte functions.

Fig. 7

Three-dimensional association of the portal vein (PV) system (white) and the intrahepatic bile duct (IHBD) system (black). Retrograde ink injections into the left lobe of the mouse liver PV and IHBD system. Liver lobe cleared with benzyl benzoate and benzyl alcohol (BABB) solution to visualize hierarchical structure.

Fig. 8

Schematic of temporal cholangiocyte specification and morphogenesis process.(A) Hepatoblasts that are in close association with the portal vein (PV) myofibroblasts begin to express Sox9 and enter into the cholangiocyte transcriptional program in addition to repressing the hepatocyte program. These tightly associated Sox9+ cells form a temporary structure termed the ductal plate as they appear to form a cover surrounding the PV. (B) Primitive ductal structures (PDS) or luminal structures surrounded by asymmetrical gene expressing cells begin to form. These are luminal spaces are surrounded by Sox9+ Hnf4a−Tgfbr2− cells on the PV side and Sox9− Hnf4a+ Tgfbr2+ cells on the parenchymal side. The origin of these cells is uncertain. They may arise from the parenchymal hepatoblast-like cells or cells contributing from the initially formed ductal plate (black arrows). (C) Symmetrical ductal structures or mature ducts are formed of Sox9+ Hnf4a− Tgfbr2− cells encircling the luminal structure. The remaining ductal plate cells that are not incorporated into an intrahepatic bile duct (IHBD) regress back to Sox9+ Hnf4a+ hepatocytes-like cells.

Fig. 9

Three-dimensional model of intrahepatic bile duct formation (IHBD) formation.(A) At the beginning of IHBD development, cholangiocytes are specified in the region adjacent to the portal vein (PV) system and are quickly incorporated into a dense homogeneous network that is communicating with the extrahepatic bile duct. (B) Upon hepatocyte bile production, secretion, and canalicular membrane lengthening, the homogenous network begins to reorganize into a hierarchical network between mouse E17 and E18. (C) As the liver parenchyma expands, new IHBDs are generated peripherally and the distance between the fine network structures surrounding the PV increases.

Fig. 10

Mechanistic model of cholangiocyte differentiation. Basic schematic of the regulatory nodes and pathways involved in setting up the cholangiocyte transcriptional program. Green outlined cell represents cholangiocyte and blue outlined cell represents portal vein (PV) myofibroblasts. Evidence of functional requirement indicated by solid lines and unknown level of interaction is indicated by dotted lines. Unidentified binding partners/coactivators or targets during cholangiocyte differentiation indicated by “?”. Double-headed arrow indicates mutual cross regulation between Lkb1 and Notch signaling.