RAGE PET imaging
Receptor for Advanced glycation end-products (RAGE) is a pattern recognition receptor, and member of the immunoglobulin (Ig) superfamily, which includes Igs, certain cell surface receptors, and adhesion molecules. RAGE is encoded by AGER in humans (Ager in mice). Both DNA and RAGE protein are highly conserved in mammalian species. Several RAGE isoforms exist, including soluble isoforms, but the most common isoform is the membrane bound form transcript variant 1 (Tv1-RAGE). In humans, fl-RAGE, sRAGE, and esRAGE variant proteins have been detected.
Cell surface receptor RAGE in humans is a 55-kDa protein, consisting of a large extracellular region of 321 amino acids, a transmembrane-spanning domain of 19 amino acids, and a cytosolic tail of 41 amino acids. The extracellular region has three Ig-like domains: one N-terminal V-type (variable) and two C-type (constant, C1 and C2) domains. C2 domain connects the V and C1 domains into a structural and functional unit (VC1), which directly binds most of the RAGE ligands. RAGE recognizes Advanced glycation end-products (AGEs), but also numerous other ligands, including certain S100 proteins, nucleic acids, HMGB1, amyloid-β-peptide and β-sheet fibrils, phosphatidylserine, β2-integrin, and complement C3a. It can also bind LPS, and prion proteins. V domain has N-glycosylation sites which affect the substrate binding. Cytosolic tail is essential for starting intracellular signalling cascades via a number of adaptor molecules. Depending on the ligand, RAGEs tend to associate via their V or C1 domains, and heterodimerize with toll-like receptors. RAGE can also interact with GPCRs, such as formyl peptide receptors (FPRs) and leukotriene B4 receptor 1 (BLT1) (Riuzzi et al., 2018).
RAGE is an important mediator of several physiological and pathological processes, including cell differentiation and tissue remodelling, inflammation, apoptosis and necrosis, metabolic syndrome, neurodegenerative diseases, and cancer. RAGE is abundantly expressed during embryonic development, but later high levels are found only in the skin and lungs. Low levels of RAGE are found in other tissues and cell types, including vascular endothelial cells, smooth muscle cells, CNS, and most white blood cells. Increased local RAGE ligand accumulation leads to increased expression of RAGE in pathological conditions such as atherosclerosis and cancer (Riuzzi et al., 2018). RAGE expression can be seen early in the glycolytic and autophagic switch during carcinogenesis.
RAGE is found not only on the cell membranes, but also in cytoplasm, nucleus, and mitochondria Nuclear RAGE is an important regulator of double-strand break DNA repair (Kumar et al., 2017).
AGEs (advanced glycation end-products) are a heterogeneous group of non-enzymatically modified proteins, lipids, and nucleic acids. Modifications include Nε-carboxymethyl-lysine (CML), pentosidine, Nε-carboxyethyl-lysine (CEL), argpyrimidine, and many others. These modifications happen by time, but are enhanced by hyperglycaemia (increased glycation) and ROS (Riuzzi et al., 2018). Due to their relatively slow turnover, extracellular matrix proteins are especially vulnerable to these processes (Rojas et al., 2018).
Multigenic S100 protein family includes several Ca2+-binding and Ca2+-modulated proteins that are implicated in Ca2+ homeostasis and Ca2+-dependent regulation of intracellular activities, but can also function like cytokines when released into extracellular space or blood (Laube et al., 2019).
Since RAGE its ligands are involved in the development of many diseases, they provide an interesting target for serum markers and for imaging, for example in Alzheimer disease (Cary et al., 2016), coronary artery disease (Fishman et al., 2018), lung diseases (Blondonnet et al., 2017), and skeletal disorders (Plotkin et al., 2019).
Cary et al (2016) have developed a small-molecule radioligand, [18F]RAGER, which is not applicable to PET studies, but provides a proof-of-concept that brain-penetrating RAGE radioligands can be developed. [18F]RAGER is based on RAGE inhibitor FPS-ZM1, and also other PEt radioligands based on it are being developed (Drake & Scott, 2018). CML-modified albumin attached to 64Cu-labelled nanoparticle was developed by Konopka et al (2018), and it has shown promising results in mouse model of peripheral arterial damage. However, neither the nanoparticle or [18F]RAGER have shown uptake in the lungs, despite that pulmonary RAGE expression is high (Drake & Scott, 2018).
Labelled monoclonal antibodies, targeted against AGEs and RAGE, have been developed. Antibody fragments have more favourable in vivo kinetics, and a 64Cu-labelled ∼25 kDa single-chain Fv has shown promising results in mice (Kim et al., 2018).
Recombinant S100s have been labelled 18F and used in small animal models (Laube et al., 2019).
Bierhaus A, Humpert PM, Morcos M, Wendt T, Chavakis T, Arnold B, Stern DM, Nawroth PP. Understanding RAGE, the receptor for advanced glycation end products. J Mol Med. 2005; 83(11): 876-886. doi: 10.1007/s00109-005-0688-7.
Bongarzone S, Savickas V, Luzi F, Gee AD. Targeting the receptor for advanced glycation endproducts (RAGE): A medicinal chemistry perspective. J Med Chem. 2017; 60(17): 7213-7232. doi: 10.1021/acs.jmedchem.7b00058.
Hudson BI, Carter AM, Harja E, Kalea AZ, Arriero M, Yang H, Grant PJ, Schmidt AM. Identification, classification, and expression of RAGE gene splice variants. FASEB J. 2008; 22(5): 1572-1580. doi: 10.1096/fj.07-9909com.
Updated at: 2019-03-11
Created at: 2019-03-10
Written by: Vesa Oikonen