Angiodiversity and organotypic functions of sinusoidal endothelial cells

Abstract

'Angiodiversity' refers to the structural and functional heterogeneity of endothelial cells (EC) along the segments of the vascular tree and especially within the microvascular beds of different organs. Organotypically differentiated EC ranging from continuous, barrier-forming endothelium to discontinuous, fenestrated endothelium perform organ-specific functions such as the maintenance of the tightly sealed blood-brain barrier or the clearance of macromolecular waste products from the peripheral blood by liver EC-expressed scavenger receptors. The microvascular bed of the liver, composed of discontinuous, fenestrated liver sinusoidal endothelial cells (LSEC), is a prime example of organ-specific angiodiversity. Anatomy and development of LSEC have been extensively studied by electron microscopy as well as linage-tracing experiments. Recent advances in cell isolation and bulk transcriptomics or single-cell RNA sequencing techniques allowed the identification of distinct LSEC molecular programs and have led to the identification of LSEC subpopulations. LSEC execute homeostatic functions such as fine tuning the vascular tone, clearing noxious substances from the circulation, and modulating immunoregulatory mechanisms. In recent years, the identification and functional analysis of LSEC-derived angiocrine signals, which control liver homeostasis and disease pathogenesis in an instructive manner, marks a major change of paradigm in the understanding of liver function in health and disease. This review summarizes recent advances in the understanding of liver vascular angiodiversity and the functional consequences resulting thereof.

Keywords: Angiocrine signaling; Angiodiversity; Endothelial cell heterogeneity; Liver vasculature; Sinusoids.

Fig. 1

Angiodiversity of the hepatic endothelium. Nutrient-rich blood from the gastrointestinal tract enters the sinusoid via the portal vein and mixes with oxygen-rich blood from the hepatic artery. The blood then moves through the fenestrated sinusoidal vasculature and exits the liver via the central vein creating an oxygen and blood flow gradient along the axis of the liver lobule. These physical gradients establish a “zonation” of hepatocytes and LSEC. Hepatocytes adjacent to the portal vein show high gluconeogenesis, urea synthesis, and beta oxidation activity. In turn, pericentral hepatocytes are characterized with increased glycolysis, bile synthesis, xenobiotics metabolism, and triglyceride synthesis. Fenestrations of LSEC are larger in the periportal area, whereas they are more abundant in pericentral regions. Pericentral LSEC modulate the spatial division of hepatocytes by secreting angiocrine Wnt ligands and Wnt-signaling enhancer Rspo3. The hepatic microenvironment is mainly determined by the interplay of the four major cell populations, namely hepatocytes, LSEC, HSC, and KC. While KC are located in the sinusoids, HSC reside in the space of Disse, which is a perisinusoidal space between LSEC and hepatocytes

Fig. 2

Development of liver sinusoidal endothelial cells. Multiple source may give rise to liver EC, but a unifying concept of LSEC specification is still missing. a Genetic in vivo studies have shown that hepatoblasts in the septum transversum mesenchyme coordinate LSEC development through the VEGF signaling pathway. b Recent fate mapping studies have indicated a pivotal role of cardiac endothelial progenitor cells in establishing the liver vasculature. Sinus venous-derived NFATC1+ and NPR3- endothelial progenitors may contribute to liver endothelium. c LSEC may also be derived from hemangioblasts and/or erythro-myeloid progenitors

Fig. 3

Self-renewal of liver sinusoidal endothelial cells. LSEC are highly plastic and can self-renew upon different challenges. Resident LSEC progenitors have a unique molecular signature expressing CD157 and ABC transporters. CD157-positive LSEC are self-renewable and can replenish the liver microvasculature following challenge. In addition to resident LSEC progenitors, BM-derived progenitor cells may be recruited to the liver and contribute to the regenerating liver vasculature following severe, resident EC damaging challenge such as irradiation-induced vascular injury.

Fig. 4

Clearance and immunoregulatory functions of liver sinusoidal endothelial cell. LSEC have distinct organotypic functions capable of acting as professional scavengers and immunomodulators. a LSEC-lined sinusoids serve as conduits for the passive transfer of nutrients and soluble factors including circulating chylomicrons through open fenestrations. b LSEC execute active scavenger functions clearing pathogens, macromolecules, and waste products through expression of scavenger receptors such as mannose receptor, CD16, CD32b, Stabilin-1, Stabilin-2, VEGFR3, and C-type lectins. (C) LSEC play key roles in innate immunity by expressing Toll-like receptors (TLRs). They are also involved in regulating adaptive immunity. LSEC express major histocompatibility complex class I (MHC I) receptors to present antigens to CD8+ T cells and major histocompatibility complex class II (MHC II) receptors to present antigens to CD4+ T cells

Fig. 5

Angiocrine factors control liver function. The hepatic endothelium is not just a passive conduit for blood. Instead, it acts as instructive gatekeeper regulating the hepatic microenvironment through EC-derived angiocrine factors. Pericentral LSEC are a prime example of angiocrine signaling. They modulate hepatocytic function, by secreting angiocrine Wnt2, Wnt9b, and Rspo3 to establish the spatial division of labor of hepatocytes (“metabolic liver zonation”). Concurrently, angiocrine Wnt-signaling fuels Axin2⁺, Lgr5⁺, and glutamine synthase (GS)⁺ pericentral hepatocytes to maintain the pericentral niche during tissue homeostasis. Moreover, angiocrine Wnt-signaling plays a pivotal role in regulating xenobiotic functions by modulating the hepatic cytochrome activity such as CYP2E1. In the portal zone, hepatocytes are actively involved in gluconeogenesis and urea cycle with increased expression of arginase (Arg1). Portal vein EC and periportal LSEC have high Notch activity with upregulated Dll4 expression. Angiocrine DLL4 may orchestrate monocyte recruitment to establish a niche for KC. Likewise, the hepatic immune zonation is further maintained by LSEC driven CXCL9 and through MYD88 pathway. Periportal LSEC also serve as a niche for resident LSEC progenitors expressing CD157. Midlobular LSEC control iron homeostasis by secretion of the angiocrine factors BMP2 and BMP6

Fig. 6

Angiocrine control of liver regeneration. Angiocrine signaling is indispensable for liver regeneration. a During the early inductive phase of liver regeneration, downregulation of EC ANGPT2 leads to downregulation of TGF-β production, thereby facilitating hepatocyte proliferation. During the later angiogenic phase, EC ANGPT2 production rebounds to control LSEC proliferation by regulating VEGFR2 in an autocrine Tie2-dependent manner in response to hepatocyte-derived VEGF. EC ANGPT2 thereby serves as a spatiotemporally regulated rheostat dynamically controlling regenerating hepatocytes and angiogenic EC. b The balance between pro-regenerative (CXCR7-Id1) and pro-fibrotic (CXCR4) pathways modulates liver regeneration. The pro-regenerative CXCR7-Id1 pathway is upregulated during liver regeneration, to induce Wnt2 and HGF. In addition to CXCR7, the VEGFR2-Id1-mediated pathway triggers the angiocrine factors Wnt2 and HGF to boost hepatocyte proliferation. c Activated platelets from the injured area release stromal-derived factor-1 (SDF1) to activate pro-regenerative CXCR7-mediated signaling. d Angiocrine HGF can be induced by blood perfusion-regulated mechano-sensing mechanisms. Increased blood flow resulting from hepatic injury upregulates VEGFR3 and β1-integrins for HGF production

Fig. 7

Characteristics of liver sinusoidal endothelial cells during disease progression. Chronic liver damage can be experimentally induced by toxic substances such as carbon tetrachloride (CCl₄) and thioacetamide (TAA) administration, surgical interventions such as bile duct ligation, as well as dietary models of non-alcoholic steatohepatitis (NASH) including methionine- and choline-deficient diets (MCD) and the choline-deficient L-amino-defined diet (CDAA). LSEC maintain liver homeostasis through nitric oxide (NO) synthesis and secreting angiocrine factors such as Wnt2, Wnt9b, HGF, BMP2, p300 mediated CCL2, and heparin-binding EGF-like growth factor (HB-EGF). The transcription factor GATA4 controls the sinusoidal phenotype including the absence of a basement membrane. During liver homeostasis, autophagic activity of LSEC is increased to protect against liver injury, and there is only little deposition of ECM molecules in the space of Disse. GATA4 is downregulated and continuous EC genes including the transcription factor Myc and the angiocrine factor Pdgfb are upregulated in NASH-induced perisinusoidal fibrosis. The balance between CXCR7 and CXCR4 shifts and further favors the pro-fibrotic pathways upon toxic liver injury. During fibrosis, angiocrine factors including TGF-β, PDGFB, SDF1, and Hh are dynamically upregulated. Activated LSEC may further trigger HSC to produce excessive ECM. NO bioavailability is lost and the autophagic activity is reduced