Extracellular matrix (ECM)
The organic matter (and water) found between the cells in multicellular organisms is called extracellular matrix. It glues the cells together and provides the framework for maintaining shape. ECM is composed of collagens, glycoproteins, proteoglycans, glycosaminoglycans, and elastin. Collagens are the most abundant proteins in the body, and serve also as a storage of amino acids.
ECM proteins are very large, partly because of extensive glycosylation and other posttranslational modifications. ECM molecules self-assemble to form ordered multidomain structures that are very stable despite the mostly noncovalent interactions. In addition, covalent cross-links are formed by specific extracellular enzymes, for example transglutaminase type 2 (TG2). Slow non-enzymatic cross-linking happens through glycation and transglutamination. Connective tissue cells control the structure and composition of the ECM. Cells of the tissue are bound to the surrounding ECM by specific membrane receptors, most importantly integrins. Interactions between ECM and the cells mediate adhesion, cell migration, growth, differentiation, apoptosis, and survival. ECM contains fibroblasts, which are the primary producers of the ECM proteins, and is also a reservoir of stem cells. Fibronectin is the most important glycoprotein constituent of the ECM which interacts with many of the integrins on the cell membrane. Other cell membrane binding glycoproteins include fibrinogen, laminins, tenascins, thrombospondins, entactins, vitronectin, and nephronectin.
Basement membranes (BMs) are specialized ECM that are found as thin sheets at the basal surfaces of endothelial and epithelial cells, surrounding also muscle and fat cells, the central neural system, and peripheral nerves. Basement membrane supports the integrity and organization of the endothelium and epithelium, and serves also as a barrier for macromolecules. Blood-brain barrier is composed of two basement membranes. The BMs of the extracellular matrix are formed by collagen (Type IV) in association with laminin, entactin, and heparan sulfate proteoglycans. Heparan sulfate polysaccharides are negatively charged, attracting cations such as 68Ga3+.
Fibroblasts are found in the ECM of different parenchymal tissues, where they synthesize the ECM proteins and enzymes responsible for the build-up and degradation of ECM. Fibroblasts extracted from different anatomical locations retain different patterns of gene expression ("positional memory"), defined by homeobox (HOX) family transcription factors. Immature fibroblasts ("mesenchymal fibroblasts") can differentiate into several cell lineages, and fibroblast precursors can circulate in the blood ("mesenchymal stem cells"). Extracellular signals can induce resident mature fibroblasts or their progenitor cells to proliferate and migrate, differentiate into myofibroblasts, remodel the ECM, and to secrete growth factors, cytokines, and chemokines. During inflammation, tissue macrophages produce pro-inflammatory cytokines, which activate the ECM remodelling functions of fibroblasts and production of chemokines and prostaglandins. In chronic inflammation the persistent activation of fibroblasts leads to continued recruitment of leukocytes into the tissue, and may lead to overproduction of ECM and fibrosis. Fibroblasts constitutively express class I MHC molecules and can express also class II MHC molecules, permitting fibroblasts to present antigens to T cells. Myofibroblasts are especially active in ECM protein production during healing processes and scar formation. Fibroblast activation and ECM remodelling is important in growth and spreading of tumours.
Fibroblast growth factor (FGF) family is comprised of secreted proteins that can interact with four tyrosine kinase FGF receptors (FGFR1-4), and intracellular proteins (iFGFs). Secreted FGFs are expressed in nearly all tissues and organs, participating in the regulation of cell proliferation, differentiation, migration, and metabolism (Beenken & Mohammadi, 2009; Ornitz & Itoh, 2015). FGFRs are single-chained glycoproteins, with tyrosine kinase region in the intracellular part, and three immunoglobulin-like domains in the extracellular part. Alternative gene splicing can produce multiple isoforms of FGFRs. Mutations, amplifications, and fusions of FGFR genes have been observed in tumours. In breast cancer the genetic alterations in FGFR1 and FGFR2 are found especially in treatment resistant tumours, offering a target for new therapies.
Fibroblast activation protein (FAP) is an atypical type II transmembrane serine protease, which is expressed on activated fibroblasts, for instance during wound healing. Fibroblasts express FAP also during chronic inflammation, atherosclerosis, and fibrosis (Hamson et al., 2014). Cancer-associated fibroblasts (CAFs) overexpress FAP, promoting tumour cell migration and angiogenesis. Isotope-labelled FAP inhibitors (FAPIs) have been developed for diagnostic and theranostic use (Dendl et al., 2021). Small-molecule FAPI radioligands may provide good tumour lesion detection from a short and early (∼10 min) PET scan, increasing the scan volume in clinical work (Fernandus et al., 2021).
Lysyl oxidase (LOX) and lysyl oxidase-like 1-4 (LOXL1, LOXL2, LOXL3, LOXL4) are enzymes that oxidize peptidyl lysine in collagen and elastin to form inter- and intrachain cross-links, stabilizing the ECM proteins. LOX is upregulated in fibrotic diseases, and during formation of atherosclerotic plaques.
Extracellular proteases, including serine proteases, meprins, and matrix metalloproteinases (MMPs), have a role in degradation of ECM and release of signalling molecules (growth factors) from the ECM. MMPs have several natural inhibitors, and proteases also degrade other proteases. MMP-9 is considered as an inflammatory proteinase, and especially linked to neuroinflammation. Increased expression of MMPs have been detected in MS. Many tumours have increased MMP-9 activity (Ujula et al., 2010). After ischemic myocardial injury, expression of MMP-2 and MMP-9 are increased, MMP-9 during the first days, and MMP-2 later during remodelling phase (Kiugel et al., 2018a). MMP activity is increased in inflamed atherosclerotic plaques (Kiugel et al., 2018b).
ACE inhibitors enalapril and imidapril inhibit matrix metalloproteinases, and can be used to prevent myocardial tissue remodelling with acute myocardial infarction (Yokota et al., 2014).
Aminopeptidase N (AP-N, ANPEP, CD13) is located in plasma membranes, and is overexpressed on angiogenic blood vessels and tumour cells. Asparagine-glycine-arginine (NGR) peptide analogues bind to CD13, and can be labelled with positron emitting radioisotopes (Zhang et al., 2014; Gyuricza et al., 2021).
Dynamic remodelling of the ECM is essential for normal organ homeostasis, growth, and healing processes. Excessive ECM degradation is linked to diseases such as osteoarthritis and cardiomyopathy. Excessive proliferation of fibroblasts and differentiation into myofibroblasts leads to overproduction of ECM proteins. In fibrotic diseases, such as cardiovascular disease, liver cirrhosis, lung fibrosis, and renal fibrosis the amount and proportion of collagen I and III and other ECM macromolecules is increased, leading to tissue stiffness. Excess ECM substitutes normal tissue by non-functional fibrotic tissue. Fibrosis tends to be a self-reinforcing process, that also increases the risk of cancer.
Chronic inflammation and infiltrated inflammatory cells are participating or even driving the fibrosis, although their normal function should be in phagocytosis of ECM and cell debris after acute tissue injury. Macrophages produce TGF-β which is one of the activators of ECM protein synthesis. RAGE-knockout mice develop pulmonary fibrosis, and later lung carcinomas (Kumar et al., 2017). Transcriptional repressor protein YY1 (Yin Yang 1) is involved in fibrotic diseases.
Fibro-adipogenic progenitors (FAPs) are precursors of fibrogenic cells, which produce ECM, supporting stem cells during tissue regeneration. FAPs can also differentiate into adipocytes. Also many other cell types can transdifferentiate into fibroblast-like cells, or otherwise start to (over)produce ECM proteins. In the early phases of fibrosis, somatostatin receptors can be upregulated.
The enlargement of extracellular matrix can be detected using tracers that are rapidly distributed in extracellular space, but cannot diffuse and are not transported into intracellular space. For example the uptake of [64Cu]DOTA is higher in fibrotic part of the myocardium than in the healthy tissue (Kim et al., 2016).
Collagelin is a cyclic peptide with micromolar affinity to collagen, and could be labelled with positron-emitting radionuclides (Velikyan et al., 2014).
- Transglutaminase type 2
- Infection and inflammation
- Wound healing
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Updated at: 2022-01-03
Created at: 2016-05-10
Written by: Vesa Oikonen