Liver sinusoidal endothelial cell

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Liver sinusoidal endothelial cell
Details
System Circulatory system
Location blood vessels of liver
Anatomical terms of microanatomy

Liver sinusoidal endothelial cells (LSECs) form the lining of the smallest blood vessels in the liver, also called the hepatic sinusoids. LSECs are highly specialized endothelial cells with characteristic morphology and function. They constitute an important part of the reticuloendothelial system (RES).

Contents

Structure

Although the LSECs make up only about 3% of the total liver cell volume, their surface in a normal adult human liver is about 210 m2, or nearly the size of a tennis court. [1]

The LSEC structure differs from other endothelia. The cells contain many open pores, or fenestrae, with diameters from 100 to 150 nm. The fenestrae occupy 20% of the LSEC surface and are arranged in groups referred to as "sieve plates". [2] Filtering fluid between the sinusoidal lumen and the space of Disse, the fenestrae are crucial for lipoprotein traffic between the hepatocytes and the sinusoidal lumen. [3] The LSECs lack an organized basal lamina. [4] The LSECs contain 45% and 17% of the liver's total mass of pinocytic vesicles and lysosomes, and contain twice as many clathrin-coated pits per membrane unit, compared with two other major liver cells, Kupffer cells and hepatocytes, [5] reflecting the high capacity clathrin-mediated endocytic activity of LSECs.

Physiological functions

LSECs play a central role in the clearance of blood borne waste. The cells express endocytosis receptors that mediate extremely rapid internalization of waste molecules. In rat it has been shown that LSECs express scavenger receptors (SR) class A, B, E and H. [6] The latter exists as stabilin-1 (SR-H1) and stabilin-2 (SR-H2) in LSECs. In the liver stabilin-2, the most important SR on LSECs, is uniquely expressed in these cells. Moreover, LSECs also express high levels of the macrophage mannose receptor (MMR) [7] and the Fc-gamma receptor IIb2 (FcγRIIb2), [8] both highly active in clathrin-mediated endocytosis just like the two stabilins. Other important receptors on LSECs are L-SIGN (liver/lymph node-specific ICAM-3 grabbing nonintegrin), [9] LSECtin (liver and lymph node sinusoidal endothelial cell C-type lectin), [10] Lyve-1 (lymphatic vessel endothelial hyaluronan receptor‐1), [11] and LRP‐1 (low‐density lipoprotein receptor‐related protein‐1). [12]

The capacity of LSECs as scavengers of blood borne waste assigns an important role of these cells in innate immunity. The abundant expression of receptors such as the endocytic FcγRIIb2 and pattern recognition receptors (PRRs) i.e. toll like receptors (TLRs), MMR and SRs, as well as the high expression of inflammasome molecules NLRP-1, NLRP-3, and AIM2 [13] point to innate immune functions of LSECs. In addition, LSECs display features of adaptive immunity, contributing to hepatic immune tolerance. [14]

Pathobiology

Liver fibrosis

LSECs have been reported to play a role in the development of liver fibrosis. Liver fibrosis is associated with decreased LSEC fenestration, and appearance of an organized basal lamina in the space of Disse, a process called capillarization, which precedes the onset of liver fibrosis. [15] Normally differentiated LSECs prevent hepatic stellate cell activation and promote reversion to quiescence, whereas capillarized LSECs do not. [16]

Atherosclerosis

Chylomicrons produced by the intestinal epithelial cells from dietary lipids have diameter up to 1000 nm which prevents them from passing through the LSEC fenestrae. [17] The size of circulating chylomicrons is gradually reduced to chylomicron remnants by lipoprotein lipase on endothelial cells of systemic capillaries. When the chylomicron remnants become small enough (30–80 nm), they pass through the LSEC fenestrations, leading to their metabolism in hepatocytes. Reduced porosity, as in liver cirrhosis, diabetes mellitus or old age may lead to prolonged postprandial lipoproteinemia and increased circulatory cholesterol levels, with increased risk for development of atherosclerosis. [18]

Autoimmunity

It has been suggested that reduced Fc receptor function in humans, causing increased circulating levels of soluble immune complexes is important in the etiology of autoimmune diseases such as systemic lupus erythematosus (SLE) and Sjögren's syndrome. [19] Moreover, the observation that small soluble IgG-antigen immune complexes are cleared in the mouse mainly via the LSEC FcγRIIb2 (8), along with the observation that deletion of same receptor causes spontaneous auto-immunity and SLE-like disease in mice, [20] point to a pivotal role of LSEC FcγRIIb2 in the disease mechanism of SLE. Furthermore, the finding that scavenging of blood borne DNAs is chiefly by SR-mediated uptake in LSECs, [21] along with the fact that SLE is associated with generation of anti-DNA antibody, lend additional support to the hypothesis that LSECs participate in the onset of SLE.

Liver toxicology

LSECs may sometimes be the initial target of injury in a condition referred to as sinusoidal obstruction syndrome (SOS, formerly hepatic veno-occlusive disease, VOD), which is described as a change of the sinusoid that may lead to hepatocyte hypoxia, with liver dysfunction and disruption of the portal circulation. Major causes of SOS are dietary ingestion of pyrrolizidine alkaloids, treatment with several chemotherapeutic drugs, and acetaminophen. [22] Moreover, since LSECs are geared to (generally unwanted) active blood clearance of large molecule compounds and nano formulations (7) these cells may be easily intoxicated by off-target mechanisms, causing subsequent hepatotoxicity. [23]

Origin and renewal

Normal LSEC turnover is maintained by the liver resident population of LSEC progenitor cells; in addition, recruitment of bone marrow derived cells contributes to replenish the LSEC population when needed. [24]

History

By the end of the 19th and beginning of the 20th centuries researchers observed avid accumulation of intravenously administered colloidal vital dyes in cells lining the sinusoids of some tissues, with the highest uptake in the littoral cells of the hepatic sinusoids. These very active blood clearance cells were collectively named "The reticuloendothelial system", or RES. [25] For several decades it was believed that the cells comprising the RES were the macrophages of the mononuclear phagocyte system (MPS). Hence the hepatic clearance of circulating waste was attributed to the liver macrophages, or Kupffer cells. [26] However, by a recent re-investigation of the original vital stain experiments carried out 100–140 years ago [27] it was concluded that the vital stain accumulated mainly in LSECs. It is increasingly accepted that LSECs and Kupffer cells play complementary roles in the hepatic blood clearance process, referred to as the dual cell principle of waste clearance (6): LSECs clear macromolecules and nanoparticles roughly <200 nm by clathrin-mediated endocytosis whereas Kupffer cells clear larger particles >200 nm by phagocytosis.

In a comparative context

All vertebrates carry a population of endothelial cells that are remarkably active in blood clearance of macromolecules and nano-substances. The great majority of these cells are located in the liver sinusoids of land-based vertebrates (mammals, birds, reptiles and amphibia). However, in bony fishes, these specialized endothelial cells are located either in the heart endocardium or in endothelial cells of the kidney sinusoidal lining, depending on the fish species. In cartilaginous fishes and the jawless fishes, these endothelial cells constitute the lining of the gill capillaries. The name scavenger endothelial cells (SECs) has been coined to denote the endothelial cells in vertebrates that are geared to blood clearance. [28] Thus, LSECs in mammals and other land-based vertebrates are a member of the vertebrate SEC family.

See also

Related Research Articles

<span class="mw-page-title-main">Lipoprotein</span> Biochemical assembly whose purpose is to transport hydrophobic lipid molecules

A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, surrounded by a phospholipid outer shell, with the hydrophilic portions oriented outward toward the surrounding water and lipophilic portions oriented inward toward the lipid center. A special kind of protein, called apolipoprotein, is embedded in the outer shell, both stabilising the complex and giving it a functional identity that determines its role.

<span class="mw-page-title-main">Phagocytosis</span> Process by which a cell uses its plasma membrane to engulf a large particle

Phagocytosis is the process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is called a phagocyte.

<span class="mw-page-title-main">CD32</span> Surface receptor glycoprotein

CD32, also known as FcγRII or FCGR2, is a surface receptor glycoprotein belonging to the Ig gene superfamily. CD32 can be found on the surface of a variety of immune cells. CD32 has a low-affinity for the Fc region of IgG antibodies in monomeric form, but high affinity for IgG immune complexes. CD32 has two major functions: cellular response regulation, and the uptake of immune complexes. Cellular responses regulated by CD32 include phagocytosis, cytokine stimulation, and endocytic transport. Dysregulated CD32 is associated with different forms of autoimmunity, including systemic lupus erythematosus. In humans, there are three major CD32 subtypes: CD32A, CD32B, and CD32C. While CD32A and CD32C are involved in activating cellular responses, CD32B is inhibitory.

<span class="mw-page-title-main">Hepatocyte</span> Liver cell type

A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in:

<span class="mw-page-title-main">Kupffer cell</span> Macrophages located in the liver

Kupffer cells, also known as stellate macrophages and Kupffer–Browicz cells, are specialized cells localized in the liver within the lumen of the liver sinusoids and are adhesive to their endothelial cells which make up the blood vessel walls. Kupffer cells comprise the largest population of tissue-resident macrophages in the body. Gut bacteria, bacterial endotoxins, and microbial debris transported to the liver from the gastrointestinal tract via the portal vein will first come in contact with Kupffer cells, the first immune cells in the liver. It is because of this that any change to Kupffer cell functions can be connected to various liver diseases such as alcoholic liver disease, viral hepatitis, intrahepatic cholestasis, steatohepatitis, activation or rejection of the liver during liver transplantation and liver fibrosis. They form part of the mononuclear phagocyte system.

Scavenger receptors are a large and diverse superfamily of cell surface receptors. Its properties were first recorded in 1970 by Drs. Brown and Goldstein, with the defining property being the ability to bind and remove modified low density lipoproteins (LDL). Today scavenger receptors are known to be involved in a wide range of processes, such as: homeostasis, apoptosis, inflammatory diseases and pathogen clearance. Scavenger receptors are mainly found on myeloid cells and other cells that bind to numerous ligands, primarily endogenous and modified host-molecules together with pathogen-associated molecular patterns(PAMPs), and remove them. The Kupffer cells in the liver are particularly rich in scavenger receptors, includes SR-A I, SR-A II, and MARCO.

<span class="mw-page-title-main">Perisinusoidal space</span>

The perisinusoidal space is a location in the liver between a hepatocyte and a sinusoid. It contains the blood plasma. Microvilli of hepatocytes extend into this space, allowing proteins and other plasma components from the sinusoids to be absorbed by the hepatocytes. Fenestration and discontinuity of the endothelium facilitates this transport. This space may be obliterated in liver disease, leading to decreased uptake by hepatocytes of nutrients and wastes such as bilirubin.

<span class="mw-page-title-main">Liver sinusoid</span> Hepatic sinusoidal blood vessel

A liver sinusoid is a type of capillary known as a sinusoidal capillary, discontinuous capillary or sinusoid, that is similar to a fenestrated capillary, having discontinuous endothelium that serves as a location for mixing of the oxygen-rich blood from the hepatic artery and the nutrient-rich blood from the portal vein.

The mannose receptor is a C-type lectin primarily present on the surface of macrophages, immature dendritic cells and liver sinusoidal endothelial cells, but is also expressed on the surface of skin cells such as human dermal fibroblasts and keratinocytes. It is the first member of a family of endocytic receptors that includes Endo180 (CD280), M-type PLA2R, and DEC-205 (CD205).

CD16, also known as FcγRIII, is a cluster of differentiation molecule found on the surface of natural killer cells, neutrophils, monocytes, macrophages, and certain T cells. CD16 has been identified as Fc receptors FcγRIIIa (CD16a) and FcγRIIIb (CD16b), which participate in signal transduction. The most well-researched membrane receptor implicated in triggering lysis by NK cells, CD16 is a molecule of the immunoglobulin superfamily (IgSF) involved in antibody-dependent cellular cytotoxicity (ADCC). It can be used to isolate populations of specific immune cells through fluorescent-activated cell sorting (FACS) or magnetic-activated cell sorting, using antibodies directed towards CD16.

<span class="mw-page-title-main">LRP1</span> Mammalian protein found in Homo sapiens

Low density lipoprotein receptor-related protein 1 (LRP1), also known as alpha-2-macroglobulin receptor (A2MR), apolipoprotein E receptor (APOER) or cluster of differentiation 91 (CD91), is a protein forming a receptor found in the plasma membrane of cells involved in receptor-mediated endocytosis. In humans, the LRP1 protein is encoded by the LRP1 gene. LRP1 is also a key signalling protein and, thus, involved in various biological processes, such as lipoprotein metabolism and cell motility, and diseases, such as neurodegenerative diseases, atherosclerosis, and cancer.

<span class="mw-page-title-main">STAB1</span> Protein-coding gene in the species Homo sapiens

Stabilin-1 is a protein that in humans is encoded by the STAB1 gene.

<span class="mw-page-title-main">STAB2</span> Protein-coding gene in the species Homo sapiens

Stabilin-2 is a protein that in humans is encoded by the STAB2 gene.

<span class="mw-page-title-main">Liver</span> Vertebrate organ involved in metabolism

The liver is a major metabolic organ only found in vertebrate animals, which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it is located in the right upper quadrant of the abdomen, below the diaphragm and mostly shielded by the lower right rib cage. Its other metabolic roles include carbohydrate metabolism, the production of hormones, conversion and storage of nutrients such as glucose and glycogen, and the decomposition of red blood cells.

In anatomy the term "reticuloendothelial system", often associated nowadays with the mononuclear phagocyte system (MPS), was originally launched by the beginning of the 20th century to denote a system of specialised cells that effectively clear colloidal vital stains from the blood circulation. The term is still used today, but its meaning has changed over the years, and is used inconsistently in present-day literature. Although RES is commonly associated exclusively with macrophages, recent research has revealed that the cells that accumulate intravenously administered vital stain belong to a highly specialised group of cells called scavenger endothelial cells (SECs), that are not macrophages.

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The term scavenger endothelial cell (SEC) was initially coined to describe a specialized sub-group of endothelial cells in vertebrates that express a remarkably high blood clearance activity. The term SEC has now been adopted by several scientists.

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