Sortilin (SORT1) is a protein that in humans is encoded by the SORT1 gene on chromosome 1. [5] This protein is a type I membrane glycoprotein in the vacuolar protein sorting 10 protein (Vps10p) family of sorting receptors. While it is ubiquitously expressed in many tissues, [6] sortilin is most abundant in the central nervous system. [7] At the cellular level, sortilin functions in protein transport between the Golgi apparatus, endosome, lysosome, and plasma membrane, leading to its involvement in multiple biological processes such as glucose and lipid metabolism as well as neural development and cell death. [8] [9] [10] [11] [12] Moreover, the function and role of sortilin is now emerging in several major human diseases such as hypertension, atherosclerosis, coronary artery disease, Alzheimer’s disease, and cancer. [13] [14] [15] [16] The SORT1 gene also contains one of 27 loci associated with increased risk of coronary artery disease. [17]
The SORT1 gene resides on chromosome 1 at the band 1p13.3 and includes 23 exons. [5] This gene encodes 2 isoforms through alternative splicing. [18]
Sortilin is a member of the Vps10p sorting receptor family. [7] Crystallization studies of the protein reveal that, when complexed with the ligand neurotensin, the Vps10 ectodomain of sortilin forms a ten-bladed beta-propeller structure with an inner tunnel that contains multiple ligand binding sites. [19] To prevent premature ligand binding during its synthesis, the precursor protein of sortilin contains a 44-amino acid pro-peptide that serves as a chaperone for the Vps10p domain. [20] In addition, two hydrophobic loops have been detected in this domain and act to anchor the protein in the cell membrane. [21] Sortilin has also been shown to undergo a conformational change and form a protein dimer in acidic conditions similar to ones found in the endosome, indicating a double mechanism for low pH-induced ligand release and possibly signaling towards recycling of the receptor. [22]
In humans, sortilin is expressed over a wide range of cell types and tissues such as the brain, spinal cord, adrenal gland, thyroid, B-lymphocytes, adipocytes, skeletal muscle, and heart. [23] As a sorting receptor on the cell surface and on the endoplasmic reticulum-Golgi apparatus within the cell, sortilin is involved in the transport of a wide variety of intracellular proteins between the trans-Golgi network, endosome, lysosome, and secretory granules, as well as the plasma membrane. [8] This molecular function enables sortilin to participate in various biological processes, including the transport of GLUT4 to the plasma membrane of fat and skeletal muscle cells in response to insulin. [9] It also mediates the interaction between proNGF and the p75NTR:sortilin complex by acting as a co-receptor to signal cell death. [12] [20] The fine regulation of the brain-derived neurotrophic factor (BDNF) by sortilin is required for both neuronal and tumor cell survival. [24] Moreover, sortilin has been implicated in LDL-cholesterol metabolism, VLDL secretion, and PCSK9 secretion, and thus plays a role in the development of atherosclerotic lesions. [10] [11] It modulates lipid metabolism in adipocytes, hepatocytes, and macrophages. [25] Other processes involving sortilin include endocytosis, [8] negative regulation of lipoprotein lipase activity, [26] myotube differentiation, [27] ossification, [28] and regulation of gene expression. [27]
Given its function in facilitating lysosomal degradation or recycling of ligands in lipid metabolism [11] [14] [29] [30] [31] and the neural system, [32] sortilin likely plays an important role in the underlying mechanisms and pathophysiology of atherogenesis and coronary artery disease, as well as in neurological disorders. For example, sortilin has been identified as an important receptor for brain apolipoprotein E (APOE) metabolism, which is implicated in the underlying mechanisms of Alzheimer’s disease. [32] [33] [34] [35] A significant role for sortilin has recently also been reported in the field of oncology, as it has been detected in several cancer cell lines. Notably, human cancerous epithelial cells exhibited increased levels of sortilin as compared to normal epithelial tissues. Furthermore, it appears that sortilin participates in the progression of breast cancer and contributes to tumor cell adhesion and invasion. [15] [16]
In 2007, chromosome 1p13.3 was identified as a promising locus through a genome-wide approach in patients with coronary artery disease. [36] Subsequently, accumulating evidence suggests that the SORT1 gene at the 1p13 locus is an important risk factor for coronary artery disease, which is attributed to lipid metabolism disorders. [36] [37] [38] Several single nucleotide polymorphisms of the SORT1 gene have a genetic association between serum blood lipid levels and the pathogenesis of cardiometabolic syndrome, including obesity, hypertension, and coronary artery disease. [25] As the role of sortilin in lipid metabolism and the development of atherosclerosis has been established, a recent study further reported that increased release of soluble sortilin from platelets, measured as circulating sortilin, may be associated with in vivo platelet activation. [39] This observation also indicates that sortilin has a potential application as a clinical biomarker for diagnosis and prognosis. [10] [39] Additionally, a multi-locus genetic risk score study, based on a combination of 27 loci including the SORT1 gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmo Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22). [17]
Sortilin has been shown to interact with GGA1 [40] and GGA2. [8] [40]
Sortilin participates in interactions within the trans-Golgi network vesicle budding and BDNF signaling pathways.
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.
The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor that recognizes apolipoprotein B100 (ApoB100), which is embedded in the outer phospholipid layer of very low-density lipoprotein (VLDL), their remnants—i.e. intermediate-density lipoprotein (IDL), and LDL particles. The receptor also recognizes apolipoprotein E (ApoE) which is found in chylomicron remnants and IDL. In humans, the LDL receptor protein is encoded by the LDLR gene on chromosome 19. It belongs to the low density lipoprotein receptor gene family. It is most significantly expressed in bronchial epithelial cells and adrenal gland and cortex tissue.
Retromer is a complex of proteins that has been shown to be important in recycling transmembrane receptors from endosomes to the trans-Golgi network (TGN) and directly back to the plasma membrane. Mutations in retromer and its associated proteins have been linked to Alzheimer's and Parkinson's diseases.
The p75 neurotrophin receptor (p75NTR) was first identified in 1973 as the low-affinity nerve growth factor receptor (LNGFR) before discovery that p75NTR bound other neurotrophins equally well as nerve growth factor. p75NTR is a neurotrophic factor receptor. Neurotrophic factor receptors bind Neurotrophins including Nerve growth factor, Neurotrophin-3, Brain-derived neurotrophic factor, and Neurotrophin-4. All neurotrophins bind to p75NTR. This also includes the immature pro-neurotrophin forms. Neurotrophic factor receptors, including p75NTR, are responsible for ensuring a proper density to target ratio of developing neurons, refining broader maps in development into precise connections. p75NTR is involved in pathways that promote neuronal survival and neuronal death.
The liver X receptor (LXR) is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, FXR and RXR. Liver X receptors (LXRs) are important regulators of cholesterol, fatty acid, and glucose homeostasis. LXRs were earlier classified as orphan nuclear receptors, however, upon discovery of endogenous oxysterols as ligands they were subsequently deorphanized.
Sortilin-related receptor, L(DLR class) A repeats containing is a protein that in humans is encoded by the SORL1 gene.
Neurotensin receptors are transmembrane receptors that bind the neurotransmitter neurotensin. Two of the receptors encoded by the NTSR1 and NTSR2 genes contain seven transmembrane helices and are G protein coupled. Numerous crystal structures have been reported for the neurotensin receptor 1 (NTS1). The third receptor has a single transmembrane domain and is encoded by the SORT1 gene.
Low density lipoprotein receptor-related protein-associated protein 1 also known as LRPAP1 or RAP is a chaperone protein which in humans is encoded by the LRPAP1 gene.
CX3C motif chemokine receptor 1 (CX3CR1), also known as the fractalkine receptor or G-protein coupled receptor 13 (GPR13), is a transmembrane protein of the G protein-coupled receptor 1 (GPCR1) family and the only known member of the CX3C chemokine receptor subfamily.
Peroxisome proliferator-activated receptor alpha (PPAR-α), also known as NR1C1, is a nuclear receptor protein functioning as a transcription factor that in humans is encoded by the PPARA gene. Together with peroxisome proliferator-activated receptor delta and peroxisome proliferator-activated receptor gamma, PPAR-alpha is part of the subfamily of peroxisome proliferator-activated receptors. It was the first member of the PPAR family to be cloned in 1990 by Stephen Green and has been identified as the nuclear receptor for a diverse class of rodent hepatocarcinogens that causes proliferation of peroxisomes.
Oxidized low-density lipoprotein receptor 1 also known as lectin-type oxidized LDL receptor 1 (LOX-1) is a protein that in humans is encoded by the OLR1 gene.
Low density lipoprotein receptor-related protein 2 also known as LRP-2 or megalin is a protein which in humans is encoded by the LRP2 gene.
ADP-ribosylation factor-binding protein GGA1 is a protein that in humans is encoded by the GGA1 gene.
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.
Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an enzyme encoded by the PCSK9 gene in humans on chromosome 1. It is the 9th member of the proprotein convertase family of proteins that activate other proteins. Similar genes (orthologs) are found across many species. As with many proteins, PCSK9 is inactive when first synthesized, because a section of peptide chains blocks their activity; proprotein convertases remove that section to activate the enzyme. The PCSK9 gene also contains one of 27 loci associated with increased risk of coronary artery disease.
Neurotensin receptor type 2 is a protein that in humans is encoded by the NTSR2 gene.
Neurotensin receptor type 1 is a protein that in humans is encoded by the NTSR1 gene. For a crystal structure of NTS1, see pdb code 4GRV. In addition, high-resolution crystal structures have been determined in complex with the peptide full agonist NTS8-13, the non-peptide full agonist SRI-9829, the partial agonist RTI-3a, and the antagonists / inverse agonists SR48692 and SR142948A, as well as in the ligand-free apo state., see PDB codes 6YVR (NTSR1-H4X:NTS8–13), 6Z4V (NTSR1-H4bmX:NTS8–13), 6Z8N (NTSR1-H4X:SRI-9829), 6ZA8 (NTSR1-H4X:RTI-3a), 6Z4S (NTSR1-H4bmX:SR48692), 6ZIN (NTSR1-H4X:SR48692), 6Z4Q, and 6Z66.
ADP-ribosylation factor-binding protein GGA2 is a protein that in humans is encoded by the GGA2 gene.
Vacuolar protein sorting-associated protein 26A is a protein that in humans is encoded by the VPS26A gene.
The SorCS2 gene is found on chromosome 4 (4p16.1), and is composed of 28 exons. The N-terminal exons which encode the Vps10p domain are spaced by large introns. The functional receptor protein is largely present in the brain. It is 1109 amino acids long, largely neutral, and has a single transmembrane pass....