Organ-Specific Endothelial Cell Differentiation and Impact of Microenvironmental Cues on Endothelial Heterogeneity

Abstract

Endothelial cells throughout the body are heterogeneous, and this is tightly linked to the specific functions of organs and tissues. Heterogeneity is already determined from development onwards and ranges from arterial/venous specification to microvascular fate determination in organ-specific differentiation. Acknowledging the different phenotypes of endothelial cells and the implications of this diversity is key for the development of more specialized tissue engineering and vascular repair approaches. However, although novel technologies in transcriptomics and proteomics are facilitating the unraveling of vascular bed-specific endothelial cell signatures, still much research is based on the use of insufficiently specialized endothelial cells. Endothelial cells are not only heterogeneous, but their specialized phenotypes are also dynamic and adapt to changes in their microenvironment. During the last decades, strong collaborations between molecular biology, mechanobiology, and computational disciplines have led to a better understanding of how endothelial cells are modulated by their mechanical and biochemical contexts. Yet, because of the use of insufficiently specialized endothelial cells, there is still a huge lack of knowledge in how tissue-specific biomechanical factors determine organ-specific phenotypes. With this review, we want to put the focus on how organ-specific endothelial cell signatures are determined from development onwards and conditioned by their microenvironments during adulthood. We discuss the latest research performed on endothelial cells, pointing out the important implications of mimicking tissue-specific biomechanical cues in culture.

Keywords: endothelial cell; extracellular matrix; heterogeneity; mechanobiology; microenvironment; organ-specific signature; phenotypic drift; vascular development.

Figure 2

Endothelial cell heterogeneity throughout embryonic development and main factors involved. Mesodermal cells differentiate into vascular and hematopoietic progenitors under the influence of BMP4, secreted by the endoderm. These endothelial progenitor cells differentiate under the influence of VEGF-signaling, driven by ETV2 expression. In turn, they populate arteries, veins, and capillaries. During arteriovenous differentiation, endothelial cells take on an arterial or venous identity, characterized by expression of Nrp1, EphrinB2, and Notch signaling components, or Nrp2, EphB4, and Coup-TFII expression, respectively. Endothelial cells from capillaries are influenced by the microenvironment of their respective organ. Furthermore, a subset of organ-specific endothelial cells also has non-mesodermal origins, based on co-expression of endothelial markers and tissue-specific endoderm markers. Adapted from [32].

Figure 1

Representation of the three main structural phenotypes in organ-specific microvasculature. Discontinuous endothelium is mainly found in the sinusoidal microvasculature of the liver, spleen and in bone marrow, and is characterized by large fenestrations and pores within and in between endothelial cells, respectively. It has a poorly developed basement membrane and contains clathrin-coated pits and vesicles that dramatically increase permeability. Non-fenestrated endothelium is characterized by low permeability and a high abundance of tight junctions and caveolae. It is mostly found in the microvasculature of the brain, heart, and lung and in larger vessels. Fenestrated endothelium has an intermediate permeability and is characterized by fenestrations covered with a diaphragm. These fenestrations and sparse tight junctions ensure proper filtration and transendothelial transport, as found in the microvasculature of kidney, gastric and intestinal mucosa, and endocrine glands.

Figure 3

Morphological, molecular, and functional characterization of liver endothelial cells. Central panel. Schematic representation of a liver sinusoid with the different cell types and the zonated organization of the liver sinusoidal endothelium. Note the increasing gradient of cKit expression from the portal nodules to the central venule. HA: hepatic arteriole; BD: bile duct; PV: portal venule; CV: central venule; LY: lymphatic capillaries. (A) Morphological characterization of the liver lobular vasculature (top), liver sinusoids (bottom, right) and freshly isolated liver sinusoidal endothelial cells (LSECs; bottom, left) by scanning electron microscopy revealing fenestrae organized in sieve plates. (B) Molecular characterization of liver endothelial cells at different omics levels (left) and at different cellular resolutions (right). The dotted line indicates that single-cell resolution is currently routinely possible for the epigenome and the transcriptome, but not (yet) for the (phospho)proteome or glycome. The glycome can be analyzed indirectly by specific lectin binding patterns. P: phosphate. (C) Functional characterization of LSECs showing three commonly used assays, i.e., ligand binding and lysosomal degradation (top), Factor VIII secretion (middle), and virus binding (bottom).