Endothelial signals modulate hepatocyte apicobasal polarization in zebrafish

Abstract

Emerging evidence indicates that paracrine signals from endothelial cells play a role in tissue differentiation and organ formation [1-3]. Here, we identify a novel role for endothelial cells in modulating hepatocyte polarization during liver organogenesis. We find that in zebrafish, the apical domain of the hepatocytes predicts the location of the intrahepatic biliary network. The refinement of hepatocyte polarization coincides with the invasion of endothelial cells into the liver, and these endothelial cells migrate along the maturing basal surface of the hepatocytes. Using genetic, pharmacological, and transplantation experiments, we provide evidence that endothelial cells influence the polarization of the adjacent hepatocytes. This influence of endothelial cells on hepatocytes is mediated at least in part by the cell-surface protein Heart of glass and contributes to the establishment of coordinately aligned hepatocyte apical membranes and evenly spaced intrahepatic conduits.

Figure 1

The Vascular and Biliary Networks in the Developing Zebrafish Liver (A and A′) Projected confocal image of Tg(flk1:EGFP)^s843 expression in the liver at 80 hpf. The intrahepatic vascular network visualized by Tg(flk1:EGFP)^s843 expression is schematically presented in (A′). (B and B′) Projected confocal image of Alcam expression at 80 hpf. The intrahepatic biliary network visualized by Alcam staining is schematically presented in (B′). (C and C′) Merged image of Tg(flk1:EGFP)^s843 (green) and Alcam (red) expression shows that the intrahepatic vascular and biliary networks always maintain a certain distance between each other. The intrahepatic vascular (green) and biliary (red) networks are schematically represented in (C′). (D and E) Projected confocal image (D) and z-plane confocal image (E) of Tg(flk1:EGFP)^s843 larvae visualized for GFP (green), Prox1 (blue), and Alcam (red) expression. Monolayers of hepatocytes separate the vascular and biliary networks, as illustrated in the inset.

Figure 2

Coordinated Development of the Intrahepatic Vascular and Biliary Networks Projected (A–D) and z-plane (E–H) confocal images of the liver at 50 (A and F), 55 (B and G), 60 (C and H), and 80 (D and I) hpf. The images present ventral views, anterior to the top. (A–D) Tg(flk1:EGFP)^s843 larvae visualized for GFP (green) and Prox1 (blue) expression. (A) By 50 hpf, endothelial cells have established contact with the liver on its dorsolateral side (as indicated by the arrow). (B) By 55 hpf, endothelial cells have surrounded the liver. (C) By 60 hpf, endothelial cells have started to invade the liver (as indicated by white arrowheads). (D) By 80 hpf, the endothelial cells have formed a highly branched network in the liver. (E–H) Tg(flk1:EGFP)^s843 larvae visualized for GFP (green), Prox1 (blue), and Alcam (red). The outlined areas are magnified and shown in bottom corners. (E′–H′) Alcam immunostainings are shown separately. By 60 hpf, Alcam expression has become restricted to the cell membrane opposite to the one contacted by endothelial cells (as indicated by white arrowheads), whereas it is still localized along the entire cell membrane of cells (as indicated by the black arrowhead) located far from the invading endothelial cells. L, liver; P, pancreas.

Figure 3

Alcam Localizes to the Apical Membranes of Hepatocytes Z-plane confocal images of the liver at 55 (A), 70 (E and F), and 80 (B–D) hpf. The images present ventral views, anterior to the top. (A and B) Wild-type Tg(gutGFP)^s854 larvae visualized for GFP (pseudocolored blue), Alcam (red), and aPKCs (pseudocolored green) expression. The outlined areas in (A) and (B) are magnified and shown in bottom right corners. At 55 hpf, aPKCs show restricted expression in hepatocytes prior to Alcam restriction. Alcam and aPKCs colocalize more extensively at 80 hpf. (C) Tg(gutGFP)^s854 heart and soul^m567(has)/prkci mutant larvae visualized for GFP (pseudocolored blue) and Alcam expression at 80 hpf. The Alcam staining of the outlined area is magnified and shown in the bottom right corner. At 80 hpf, whereas Alcam localization is restricted to the apical membrane of hepatocytes in wild-type larvae, in has ^m567 mutants, it is still localized along the entire cell membrane of most cells. (D) Tg(flk1:EGFP)^s843 larvae visualized for GFP (pseudocolored blue), Alcam (red), and Ntcp (green) expression. Alcam and Ntcp immunostainings are shown separately in (D′). Alcam and Tg(flk1:EGFP)^s843 expressions are shown separately in (D″). (E and F) Wild-type (E) and cloche^la1164 (clo) mutant (F) larvae visualized for Tg(flk1:EGFP)^s843 (green), Alcam (red), and Prox1 (blue) expression at 70 hpf. At this stage, whereas Alcam has localized to the apical membranes of most cells in wild-type livers, it is still localized along the entire cell membrane of many cells in clo ^la1164 mutant livers.

Figure 4

Endothelial Cells Can Influence Alcam Subcellular Localization (A–D) Tg(flk1:EGFP)^s843 larvae treated with the VEGF-receptor inhibitor SU5416 from 55 to 80 hpf were visualized for GFP (green), Alcam (red), and Prox1 (blue) expression at 80 hpf. (A) Projected confocal image of Tg(flk1:EGFP)^s843 expression in the liver of an SU5416-treated larva in which the intrahepatic vascular network (schematically presented in [A′]) is disrupted and endothelial cells have formed a dead-end vascular branch (indicated by arrow). (B) Projected confocal image of the same liver visualized for Alcam expression at 80 hpf. The pattern of the Alcam-positive network (schematically presented in [B′]) reflects the disrupted intrahepatic vascular network. (C) Merged image of Alcam (red) and Tg(flk1:EGFP)^s843 (green) expression shows that the intrahepatic networks still do not intersect. (D) Z-plane confocal image of the same larva. Alcam (indicated by white arrowheads) accumulates on membranes away from the dead-end vascular branch (indicated by arrow). The outlined area is magnified and shown in (D′). Alcam immunostaining in (D′) is shown separately in (D″). (E–I) Tg(flk1:EGFP)^s843 larva containing vegf₁₂₁ mRNA-overexpressing cells visualized for GFP (green), Alcam (pseudocolored red), and donor cell tracer (rhodamine dextran; pseudocolored white) at 80 hpf. (E) Projected confocal image of Tg(flk1:EGFP)^s843 expression in the liver. The transplanted vegf₁₂₁ mRNA-overexpressing cells (indicated by black arrowheads) disrupt the intrahepatic vascular network (schematically presented in [E′]). (F) Projected confocal image of the same liver visualized for Alcam expression at 80 hpf. The pattern of the Alcam-positive network (schematically presented in [F′]) reflects the disrupted intrahepatic vascular network. (G) Merged image of Alcam (red) and Tg(flk1:EGFP)^s843 (green) expression shows that the intrahepatic vascular networks still do not intersect. (H) Z-plane confocal image of the same larva. The outlined area is magnified and shown in (I).

Figure 5

The heart of glass Gene Regulates Hepatocyte Polarization (A–F) Tg(flk1:EGFP)^s843 wild-type (A and D), valentine^s259 (vtn) mutant (B and E), and heart of glass^m552 (heg) mutant (C and F) larvae, visualized for GFP (green), Prox1 (blue), and Alcam (red) expression. Z-plane (A–C) and projected (D–F) confocal images of the liver at 80 hpf. (A and D) In wild-type larvae, the vascular network marked by Tg(flk1:EGFP)^s843 expression and the biliary network marked by Alcam expression maintain a certain distance from each other and usually do not intersect. (B and E) In vtn ^s259 mutants, the intrahepatic vascular and biliary networks frequently intersect (as indicated by arrowheads). (C) In heg ^m552 mutants, the phenotype is similar to that of vtn ^s259 mutants, but milder; the intrahepatic vascular and biliary networks occasionally intersect (as indicated by arrowheads). (G) Number of intersections between the intrahepatic vascular and biliary networks in wild-type, Vegf-receptor-inhibitor-treated, vegf₁₂₁ mRNA-overexpressing cell hosts, vtn ^s259 mutants, and heg ^m552 mutants at 80 hpf. The error bars represent standard deviations. (H and I) vtn (H) and heg (I) expression in 80 hpf wild-type larvae. The images show lateral views, anterior to the left. Both genes appear to be expressed in endothelial cells. Asterisks indicate the common cardinal vein. Black arrowheads point to the intrahepatic vascular network. (J) Projected confocal image of the liver at 80 hpf. Larva transplanted with heg mRNA-overexpressing cells in the endoderm visualized for Alcam (pseudocolored red) and rhodamine dextran (donor cell marker; pseudocolored green). The arrowhead points to the transplanted heg mRNA-overexpressing cells inside the liver. Alcam appears to accumulate away from the heg mRNA-overexpressing cells. Alcam-expression pattern is schematically presented in (J′).