br STAR Methods br Introduction Secretase is a membrane
Introduction γ-Secretase is a membrane-embedded proteolytic complex belonging to a diverse family of intramembrane-cleaving proteases and is composed of four integral membrane proteins: presenilin (PS), nicastrin (NCT), anterior pharynx-defective 1 (Aph-1), and presenilin enhancer 2 (Pen-2), with presenilin being a catalytic subunit (Krishnaswamy et al., 2009) (Fig. 1A). Presenilins are nine-pass transmembrane proteins containing two conserved aspartate residues in adjacent transmembrane domains and are considered to have catalytic activity (Wolfe et al., 1999). Nicastrin is a single-pass transmembrane protein with a large ectodomain that is heavily glycosylated and plays a vital role in presenilin-mediated cleavage. Aph-1 is a seven-pass transmembrane protein that plays a role in forming a stable complex with nicastrin, sharing essential roles with Pen-2, a two-pass transmembrane protein, in γ-secretase activity and presenilin accumulation (Francis et al., 2002, Luo et al., 2003). The γ-secretase complex is assembled within early compartments of the endoplasmic reticulum (ER) and transported to late compartments and the cell surface, where they encounter substrates and are processed within respective transmembrane segments (Kim et al., 2004). γ-Secretase is involved in the intramembrane cleavage of various transmembrane substrates. PS-dependent γ-secretase cleavage is involved in the essential maturation of the amyloid precursor protein (APP) and the Notch receptor, leading to the production of amyloid-β (Aβ) peptides and the Notch intracellular domain (NICD), respectively (De Strooper et al., 1999, Takami and Funamoto, 2012) (Fig. 1B and 1C). Many other substrates of γ-secretase have recently been identified and studied, and a thorough review of those findings has been published (Haapasalo & Kovacs, 2011). Inflammation is a part of the body's complex biological defense system that is activated in response to harmful stimuli. In addition to playing a protective role in removing injury-related stimuli and initiating the healing process, inflammation can also contribute to pathological processes, such as in cancer, diabetes, heart disease, and Sulfamethazine synthesis diseases, resulting in further damage to tissue or organs. There is increasing evidence suggesting a role of γ-secretase in various inflammatory-related diseases, particularly through the regulation of the Notch ligand-receptor interaction. Notch signaling is a highly conserved signaling pathway that regulates a broad spectrum of cell fate decisions and differentiation during development and in adulthood (Fiúza & Arias, 2007). The canonical means of initiating signaling via the Notch pathway is through the binding of a ligand to a receptor. Five ligands (Jagged 1 and 2 and Delta-like 1, 3, and 4) and four Notch receptors (Notch 1–4) have been identified in mammals (Baron, 2003). Upon ligand binding, the Notch receptor undergoes a conformational change that allows for metalloprotease (tumor necrosis-factor-α-converting enzyme: TACE) shedding at the ectodomain. Immediately after cleavage of the TACE, the Notch receptor is cleaved at the transmembrane domain by γ-secretase and generates the NICD (Mumm & Kopan, 2000). The liberated NICD can then be translocated to the nucleus, where it interacts with the transcription factor RBP-Jκ (C-promoter-binding factor/suppressor-of-hairless/Lag1) and the co-activator protein Mastermind, which converts RBP-Jκ from a transcriptional repressor to an activator, leading to the expression of Notch target genes (Fortini, 2002). Increasing evidence suggests that Notch signaling plays an important role in the regulation of inflammatory responses in many disorders such as cardiovascular disease, rheumatoid arthritis, multiple sclerosis, asthma and stroke (Arumugam et al., 2006, Jiao et al., 2012, Kang et al., 2009, Minter et al., 2005, Quillard and Charreau, 2013). γ-Secretase PS gene expression is increased in the brain during ischemia (Pennypacker et al., 1999, Tanimukai et al., 1998), and γ-secretase activity is increased in ischemic stroke (Arumugam et al., 2006). In addition, γ-secretase-mediated Notch signaling has been shown to contribute to post-ischemic inflammation by modulating the microglial innate response in stroke (Wei et al., 2011). In Alzheimer's disease (AD), γ-secretase is involved in the generation of Aβ, which is the key pathology of the disease. In addition, there is emerging evidence showing the role of γ-secretase-associated neuroinflammation in AD (Sutinen et al., 2012). Furthermore, pro-inflammatory cytokines appear to increase the expression or activity of γ-secretase, also contributing to disease progression. These findings suggest a pivotal interaction between inflammation and γ-secretase to occur as a cause and/or result of inflammatory diseases (Fig. 2).