Amyloid-beta precursor protein (APP) is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. It functions as a cell surface receptor [5] and has been implicated as a regulator of synapse formation, [6] neural plasticity, [7] antimicrobial activity, [8] and iron export. [9] It is coded for by the gene APP and regulated by substrate presentation. [10] APP is best known as the precursor molecule whose proteolysis generates amyloid beta (Aβ), a polypeptide containing 37 to 49 amino acid residues, whose amyloid fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients.
Amyloid-beta precursor protein is an ancient and highly conserved protein. [11] In humans, the gene APP 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]
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]
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.
APP undergoes extensive post-translational modification including glycosylation, phosphorylation, sialylation, and tyrosine sulfation, as well as many types of proteolytic processing to generate peptide fragments. [29] It is commonly cleaved by proteases in the secretase family; alpha secretase and beta secretase both remove nearly the entire extracellular domain to release membrane-anchored carboxy-terminal fragments that may be associated with apoptosis. [16] Cleavage by gamma secretase within the membrane-spanning domain after beta-secretase cleavage generates the amyloid-beta fragment; gamma secretase is a large multi-subunit complex whose components have not yet been fully characterized, but include presenilin, whose gene has been identified as a major genetic risk factor for Alzheimer's. [30]
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 E genotype are major risk factors for Alzheimer's disease. [33]
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.
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.
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]
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.
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]
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.
Amyloid precursor protein has been shown to interact with:
APP interacts with reelin, a protein implicated in a number of brain disorders, including Alzheimer's disease. [71]
Beta-secretase 2 is an enzyme that cleaves Glu-Val-Asn-Leu!Asp-Ala-Glu-Phe in the Swedish variant of Alzheimer's amyloid precursor protein. BACE2 is a close homolog of BACE1.
Amyloid beta denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid-beta precursor protein (APP), which is cleaved by beta secretase and gamma secretase to yield Aβ in a cholesterol-dependent process and substrate presentation. Both neurons and oligodendrocytes produce and release Aβ in the brain, contributing to formation of amyloid plaques. Aβ molecules can aggregate to form flexible soluble oligomers which may exist in several forms. It is now believed that certain misfolded oligomers can induce other Aβ molecules to also take the misfolded oligomeric form, leading to a chain reaction akin to a prion infection. The oligomers are toxic to nerve cells. The other protein implicated in Alzheimer's disease, tau protein, also forms such prion-like misfolded oligomers, and there is some evidence that misfolded Aβ can induce tau to misfold.
Amyloid plaques are extracellular deposits of amyloid beta (Aβ) protein that present mainly in the grey matter of the brain. Degenerative neuronal elements and an abundance of microglia and astrocytes can be associated with amyloid plaques. Some plaques occur in the brain as a result of aging, but large numbers of plaques and neurofibrillary tangles are characteristic features of Alzheimer's disease. The plaques are highly variable in shape and size; in tissue sections immunostained for Aβ, they comprise a log-normal size distribution curve, with an average plaque area of 400-450 square micrometers (μm2). The smallest plaques, which often consist of diffuse deposits of Aβ, are particularly numerous. Plaques form when Aβ misfolds and aggregates into oligomers and longer polymers, the latter of which are characteristic of amyloid.
A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, multiple system atrophy, tauopathies, and prion diseases. Neurodegeneration can be found in the brain at many different levels of neuronal circuitry, ranging from molecular to systemic.Because there is no known way to reverse the progressive degeneration of neurons, these diseases are considered to be incurable; however research has shown that the two major contributing factors to neurodegeneration are oxidative stress and inflammation. Biomedical research has revealed many similarities between these diseases at the subcellular level, including atypical protein assemblies and induced cell death. These similarities suggest that therapeutic advances against one neurodegenerative disease might ameliorate other diseases as well.
Beta-secretase 1, also known as beta-site amyloid precursor protein cleaving enzyme 1, beta-site APP cleaving enzyme 1 (BACE1), membrane-associated aspartic protease 2, memapsin-2, aspartyl protease 2, and ASP2, is an enzyme that in humans is encoded by the BACE1 gene. Expression of BACE1 is observed mainly in neurons and oligodendrocytes.
The biochemistry of Alzheimer's disease, the most common cause of dementia, is not yet very well understood. Alzheimer's disease (AD) has been identified as a proteopathy: a protein misfolding disease due to the accumulation of abnormally folded amyloid beta (Aβ) protein in the brain. Amyloid beta is a short peptide that is an abnormal proteolytic byproduct of the transmembrane protein amyloid-beta precursor protein (APP), whose function is unclear but thought to be involved in neuronal development. The presenilins are components of proteolytic complex involved in APP processing and degradation.
Gamma secretase is a multi-subunit protease complex, an integral membrane protein, that cleaves single-pass transmembrane proteins at residues within the transmembrane domain. Proteases of this type are known as intramembrane proteases. The most well-known substrate of gamma secretase is amyloid precursor protein, a large integral membrane protein that, when cleaved by both gamma and beta secretase, produces a short 37-43 amino acid peptide called amyloid beta whose abnormally folded fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients. Gamma secretase is also critical in the related processing of several other type I integral membrane proteins, such as Notch, ErbB4, E-cadherin, N-cadherin, ephrin-B2, or CD44.
Presenilins are a family of related multi-pass transmembrane proteins which constitute the catalytic subunits of the gamma-secretase intramembrane protease protein complex. They were first identified in screens for mutations causing early onset forms of familial Alzheimer's disease by Peter St George-Hyslop. Vertebrates have two presenilin genes, called PSEN1 that codes for presenilin 1 (PS-1) and PSEN2 that codes for presenilin 2 (PS-2). Both genes show conservation between species, with little difference between rat and human presenilins. The nematode worm C. elegans has two genes that resemble the presenilins and appear to be functionally similar, sel-12 and hop-1.
Alpha secretases are a family of proteolytic enzymes that cleave amyloid precursor protein (APP) in its transmembrane region. Specifically, alpha secretases cleave within the fragment that gives rise to the Alzheimer's disease-associated peptide amyloid beta when APP is instead processed by beta secretase and gamma secretase. The alpha-secretase pathway is the predominant APP processing pathway. Thus, alpha-secretase cleavage precludes amyloid beta formation and is considered to be part of the non-amyloidogenic pathway in APP processing. Alpha secretases are members of the ADAM family, which are expressed on the surfaces of cells and anchored in the cell membrane. Several such proteins, notably ADAM10, have been identified as possessing alpha-secretase activity. Upon cleavage by alpha secretases, APP releases its extracellular domain - a fragment known as APPsα - into the extracellular environment in a process known as ectodomain shedding.
Nicastrin, also known as NCSTN, is a protein that in humans is encoded by the NCSTN gene.
Low-density lipoprotein receptor-related protein 8 (LRP8), also known as apolipoprotein E receptor 2 (ApoER2), is a protein that in humans is encoded by the LRP8 gene. ApoER2 is a cell surface receptor that is part of the low-density lipoprotein receptor family. These receptors function in signal transduction and endocytosis of specific ligands. Through interactions with one of its ligands, reelin, ApoER2 plays an important role in embryonic neuronal migration and postnatal long-term potentiation. Another LDL family receptor, VLDLR, also interacts with reelin, and together these two receptors influence brain development and function. Decreased expression of ApoER2 is associated with certain neurological diseases.
Insulin-degrading enzyme, also known as IDE, is an enzyme.
Presenilin-1(PS-1) is a presenilin protein that in humans is encoded by the PSEN1 gene. Presenilin-1 is one of the four core proteins in the gamma secretase complex, which is considered to play an important role in generation of amyloid beta (Aβ) from amyloid-beta precursor protein (APP). Accumulation of amyloid beta is associated with the onset of Alzheimer's disease.
Presenilin-2 is a protein that is encoded by the PSEN2 gene.
Amyloid beta A4 precursor protein-binding family B member 1 is a protein that in humans is encoded by the APBB1 gene.
Amyloid-like protein 1, also known as APLP1, is a protein that in humans is encoded by the APLP1 gene. APLP1 along with APLP2 are important modulators of glucose and insulin homeostasis.
Amyloid beta A4 precursor protein-binding family A member 2 is a protein that in humans is encoded by the APBA2 gene.
Early-onset Alzheimer's disease (EOAD), also called younger-onset Alzheimer's disease (YOAD), is Alzheimer's disease diagnosed before the age of 65. It is an uncommon form of Alzheimer's, accounting for only 5–10% of all Alzheimer's cases. About 60% have a positive family history of Alzheimer's and 13% of them are inherited in an autosomal dominant manner. Most cases of early-onset Alzheimer's share the same traits as the "late-onset" form and are not caused by known genetic mutations. Little is understood about how it starts.
The biomarkers of Alzheimer's disease are neurochemical indicators used to assess the risk or presence of the disease. The biomarkers can be used to diagnose Alzheimer's disease (AD) in a very early stage, but they also provide objective and reliable measures of disease progress. It is imperative to diagnose AD disease as soon as possible, because neuropathologic changes of AD precede the symptoms by years. It is well known that amyloid beta (Aβ) is a good indicator of AD disease, which has facilitated doctors to accurately pre-diagnose cases of AD. When Aβ peptide is released by proteolytic cleavage of amyloid-beta precursor protein, some Aβ peptides that are solubilized are detected in CSF and blood plasma which makes AB peptides a promising candidate for biological markers. It has been shown that the amyloid beta biomarker shows 80% or above sensitivity and specificity, in distinguishing AD from dementia. It is believed that amyloid beta as a biomarker will provide a future for diagnosis of AD and eventually treatment of AD.
p3 peptide also known as amyloid β- peptide (Aβ)17–40/42 is the peptide resulting from the α- and γ-secretase cleavage from the amyloid precursor protein (APP). It is known to be the major constituent of diffuse plaques observed in Alzheimer's disease (AD) brains and pre-amyloid plaques in people affected by Down syndrome. However, p3 peptide's role in these diseases is not truly known yet.