Amyloid-beta precursor protein is an ancient and highly conserved protein.[11] In humans, the geneAPP is located on chromosome 21 and contains 18 exons spanning 290 kilobases.[12][13] Several alternative splicing isoforms of APP have been observed in humans, ranging in length from 639 to 770 amino acids, with certain isoforms preferentially expressed in neurons; changes in the neuronal ratio of these isoforms have been associated with Alzheimer's disease.[14]Homologous proteins have been identified in other organisms such as Drosophila (fruit flies), C. elegans (roundworms),[15] and all mammals.[16] The amyloid beta region of the protein, located in the membrane-spanning domain, is not well conserved across species and has no obvious connection with APP's native-state biological functions.[16]
Mutations in critical regions of amyloid precursor protein, including the region that generates amyloid beta (Aβ), cause familial susceptibility to Alzheimer's disease.[17][18][19][20] For example, several mutations outside the Aβ region associated with familial Alzheimer's have been found to dramatically increase production of Aβ.[21]
A mutation (A673T) in the APP gene protects against Alzheimer's disease. This substitution is adjacent to the beta secretase cleavage site and results in a 40% reduction in the formation of amyloid beta in vitro.[22]
Structure
A number of different structural domains that fold mostly on their own have been found in the APP sequence. The extracellular region, much larger than the intracellular region, is divided into the E1 and E2 domains, linked by an acidic domain (AcD); E1 contains two subdomains including a growth factor-like domain (GFLD) and a copper-binding domain (CuBD) interacting tightly together.[24] A serine protease inhibitor domain, absent from the isoform differentially expressed in the brain, is found between acidic region and E2 domain.[25] The complete crystal structure of APP has not yet been solved; however, individual domains have been successfully crystallized, the growth factor-like domain,[26] the copper-binding domain,[27] the complete E1 domain[24] and the E2 domain.[23]
Isoform diversity
Amyloid-beta precursor protein is highly versatile with several isoforms generated through alternative splicing of its mRNA. The primary isoforms include APP695, APP751, and APP770, differing in their inclusion of certain exons, mainly exon 7 and 8. APP695 is predominantly expressed in neuronal cells and is crucial for normal neuronal function. APP751 and APP770 are more widely expressed in non-neuronal tissues but exhibit distinct expression patterns during neuron differentiation.[28] The differential expression of these isoforms plays a significant role in cellular processes such as neurodevelopment, synaptic plasticity, and the pathogenesis of Alzheimer's disease. Understanding the isoform diversity of APP is essential for deciphering its various physiological and pathological roles.
The amyloidogenic processing of APP has been linked to its presence in lipid rafts. When APP molecules occupy a lipid raft region of membrane, they are more accessible to and differentially cleaved by beta secretase, whereas APP molecules outside a raft are differentially cleaved by the non-amyloidogenic alpha secretase.[31] Gamma secretase activity has also been associated with lipid rafts.[32] The role of cholesterol in lipid raft maintenance has been cited as a likely explanation for observations that high cholesterol and apolipoprotein Egenotype are major risk factors for Alzheimer's disease.[33]
Biological function
Although the native biological role of APP is of obvious interest to Alzheimer's research, thorough understanding has remained elusive. Experimental models of Alzheimer's disease are commonly used by researchers to gain better understandings about the biological function of APP in disease pathology and progression.
Synaptic formation and repair
The most-substantiated role for APP is in synaptic formation and repair;[6] its expression is upregulated during neuronal differentiation and after neural injury. Roles in cell signaling, long-term potentiation, and cell adhesion have been proposed and supported by as-yet limited research.[16] In particular, similarities in post-translational processing have invited comparisons to the signaling role of the surface receptor protein Notch.[34]
APP knockout mice are viable and have relatively minor phenotypic effects including impaired long-term potentiation and memory loss without general neuron loss.[35] On the other hand, transgenic mice with upregulated APP expression have also been reported to show impaired long-term potentiation.[36]
The logical inference is that because Aβ accumulates excessively in Alzheimer's disease its precursor, APP, would be elevated as well. However, neuronal cell bodies contain less APP as a function of their proximity to amyloid plaques.[37] The data indicate that this deficit in APP results from a decline in production rather than an increase in catalysis. Loss of a neuron's APP may affect physiological deficits that contribute to dementia.
Somatic recombination
In neurons of the human brain, somatic recombination occurs frequently in the gene that encodes APP.[38]Neurons from individuals with sporadic Alzheimer's disease show greater APP gene diversity due to somatic recombination than neurons from healthy individuals.[38]
Anterograde neuronal transport
Molecules synthesized in the cell bodies of neurons must be conveyed outward to the distal synapses. This is accomplished via fast anterograde transport. It has been found that APP can mediate interaction between cargo and kinesin and thus facilitate this transport. Specifically, a short peptide 15-amino-acid sequence from the cytoplasmic carboxy-terminus is necessary for interaction with the motor protein.[39]
Additionally, it has been shown that the interaction between APP and kinesin is specific to the peptide sequence of APP.[40] In a recent experiment involving transport of peptide-conjugated colored beads, controls were conjugated to a single amino acid, glycine, such that they display the same terminal carboxylic acid group as APP without the intervening 15-amino-acid sequence mentioned above. The control beads were not motile, which demonstrated that the terminal COOH moiety of peptides is not sufficient to mediate transport.
Iron export
A different perspective on Alzheimer's is revealed by a mouse study that has found that APP possesses ferroxidase activity similar to ceruloplasmin, facilitating iron export through interaction with ferroportin; it seems that this activity is blocked by zinc trapped by accumulated Aβ in Alzheimer's.[9] It has been shown that a single nucleotide polymorphism in the 5'UTR of APP mRNA can disrupt its translation.[41]
The hypothesis that APP has ferroxidase activity in its E2 domain and facilitates export of Fe(II) is possibly incorrect since the proposed ferroxidase site of APP located in the E2 domain does not have ferroxidase activity.[42][43]
As APP does not possess ferroxidase activity within its E2 domain, the mechanism of APP-modulated iron efflux from ferroportin has come under scrutiny. One model suggests that APP acts to stabilize the iron efflux protein ferroportin in the plasma membrane of cells thereby increasing the total number of ferroportin molecules at the membrane. These iron-transporters can then be activated by known mammalian ferroxidases (i.e. ceruloplasmin or hephaestin).[44]
Hormonal regulation
The amyloid-β precursor protein (AβPP), and all associated secretases, are expressed early in development and play a key role in the endocrinology of reproduction – with the differential processing of AβPP by secretases regulating human embryonic stem cell (hESC) proliferation as well as their differentiation into neural precursor cells (NPC). The pregnancy hormone human chorionic gonadotropin (hCG) increases AβPP expression[45] and hESC proliferation while progesterone directs AβPP processing towards the non-amyloidogenic pathway, which promotes hESC differentiation into NPC.[46][47][48]
AβPP and its cleavage products do not promote the proliferation and differentiation of post-mitotic neurons; rather, the overexpression of either wild-type or mutant AβPP in post-mitotic neurons induces apoptotic death following their re-entry into the cell cycle.[49] It is postulated that the loss of sex steroids (including progesterone) but the elevation in luteinizing hormone, the adult equivalent of hCG, post-menopause and during andropause drives amyloid-β production[50] and re-entry of post-mitotic neurons into the cell cycle.
Interactions
Amyloid precursor protein has been shown to interact with:
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^Chen F, Hasegawa H, Schmitt-Ulms G, Kawarai T, Bohm C, Katayama T, Gu Y, Sanjo N, Glista M, Rogaeva E, Wakutani Y, Pardossi-Piquard R, Ruan X, Tandon A, Checler F, Marambaud P, Hansen K, Westaway D, St George-Hyslop P, Fraser P (Apr 2006). "TMP21 is a presenilin complex component that modulates gamma-secretase but not epsilon-secretase activity". Nature. 440 (7088): 1208–12. doi:10.1038/nature04667. PMID16641999. S2CID4349251.
^Selkoe D, Kopan R (2003). "Notch and Presenilin: regulated intramembrane proteolysis links development and degeneration". Annual Review of Neuroscience. 26 (1): 565–97. doi:10.1146/annurev.neuro.26.041002.131334. PMID12730322.
^Phinney AL, Calhoun ME, Wolfer DP, Lipp HP, Zheng H, Jucker M (1999). "No hippocampal neuron or synaptic bouton loss in learning-impaired aged beta-amyloid precursor protein-null mice". Neuroscience. 90 (4): 1207–16. doi:10.1016/S0306-4522(98)00645-9. PMID10338291. S2CID6001957.
^Matsuyama S, Teraoka R, Mori H, Tomiyama T (2007). "Inverse correlation between amyloid precursor protein and synaptic plasticity in transgenic mice". NeuroReport. 18 (10): 1083–7. doi:10.1097/WNR.0b013e3281e72b18. PMID17558301. S2CID34157306.
^Porayette P, Gallego MJ, Kaltcheva MM, Meethal SV, Atwood CS (Dec 2007). "Amyloid-beta precursor protein expression and modulation in human embryonic stem cells: a novel role for human chorionic gonadotropin". Biochemical and Biophysical Research Communications. 364 (3): 522–7. doi:10.1016/j.bbrc.2007.10.021. PMID17959150.
^Tanahashi H, Tabira T (Feb 1999). "X11L2, a new member of the X11 protein family, interacts with Alzheimer's beta-amyloid precursor protein". Biochemical and Biophysical Research Communications. 255 (3): 663–7. doi:10.1006/bbrc.1999.0265. PMID10049767.
^Tanahashi H, Tabira T (Feb 1999). "Molecular cloning of human Fe65L2 and its interaction with the Alzheimer's beta-amyloid precursor protein". Neuroscience Letters. 261 (3): 143–6. doi:10.1016/S0304-3940(98)00995-1. PMID10081969. S2CID54307954.
^Chauhan VP, Ray I, Chauhan A, Wisniewski HM (May 1999). "Binding of gelsolin, a secretory protein, to amyloid beta-protein". Biochemical and Biophysical Research Communications. 258 (2): 241–6. doi:10.1006/bbrc.1999.0623. PMID10329371.
^Yan SD, Fu J, Soto C, Chen X, Zhu H, Al-Mohanna F, Collison K, Zhu A, Stern E, Saido T, Tohyama M, Ogawa S, Roher A, Stern D (Oct 1997). "An intracellular protein that binds amyloid-beta peptide and mediates neurotoxicity in Alzheimer's disease". Nature. 389 (6652): 689–95. Bibcode:1997Natur.389..689D. doi:10.1038/39522. PMID9338779. S2CID4343238.
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1aap: X-RAY CRYSTAL STRUCTURE OF THE PROTEASE INHIBITOR DOMAIN OF ALZHEIMER'S AMYLOID BETA-PROTEIN PRECURSOR
1amb: SOLUTION STRUCTURE OF RESIDUES 1-28 OF THE AMYLOID BETA-PEPTIDE
1amc: SOLUTION STRUCTURE OF RESIDUES 1-28 OF THE AMYLOID BETA-PEPTIDE
1aml: THE ALZHEIMER`S DISEASE AMYLOID A4 PEPTIDE (RESIDUES 1-40)
1ba4: THE SOLUTION STRUCTURE OF AMYLOID BETA-PEPTIDE (1-40) IN A WATER-MICELLE ENVIRONMENT. IS THE MEMBRANE-SPANNING DOMAIN WHERE WE THINK IT IS? NMR, 10 STRUCTURES
1ba6: SOLUTION STRUCTURE OF THE METHIONINE-OXIDIZED AMYLOID BETA-PEPTIDE (1-40). DOES OXIDATION AFFECT CONFORMATIONAL SWITCHING? NMR, 10 STRUCTURES
1brc: RELOCATING A NEGATIVE CHARGE IN THE BINDING POCKET OF TRYPSIN
1ca0: BOVINE CHYMOTRYPSIN COMPLEXED TO APPI
1iyt: Solution structure of the Alzheimer's disease amyloid beta-peptide (1-42)
1mwp: N-TERMINAL DOMAIN OF THE AMYLOID PRECURSOR PROTEIN
1owt: Structure of the Alzheimer's disease amyloid precursor protein copper binding domain
1rw6: human APP core domain
1taw: BOVINE TRYPSIN COMPLEXED TO APPI
1tkn: Solution structure of CAPPD*, an independently folded extracellular domain of human Amyloid-beta Precursor Protein
1z0q: Aqueous Solution Structure of the Alzheimer's Disease Abeta Peptide (1-42)
1zjd: Crystal Structure of the Catalytic Domain of Coagulation Factor XI in Complex with Kunitz Protease Inhibitor Domain of Protease Nexin II
2beg: 3D Structure of Alzheimer's Abeta(1-42) fibrils
2fjz: Structure of the Alzheimer's Amyloid Precursor Protein (APP) copper binding domain (residues 133 to 189) in 'small unit cell' form, metal-free
2fk1: Structure of the Alzheimer's Amyloid Precursor Protein (APP) Copper Binding Domain in 'small unit cell' form, Cu(II)-bound
2fk2: Structure of the Alzheimer's Amyloid Precursor Protein (APP) Copper Binding Domain in 'small unit cell' form, Cu(I)-bound
2fk3: Structure of the Alzheimer's Amyloid Precursor Protein (APP) Copper Binding Domain in 'large unit cell' form
2fkl: Structure of the Alzheimer's Amyloid Precursor Protein (APP) Copper Binding Domain (Residues 126- 189 of APP)
2fma: Structure of the Alzheimer's Amyloid Precursor Protein (APP) Copper Binding Domain in 'small unit cell' form, atomic resolution
2g47: Crystal structure of human insulin-degrading enzyme in complex with amyloid-beta (1-40)