Amyloid precursor protein

Amyloid precursor protein (APP) is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. Its primary function is not known, though it has been implicated as a regulator of synapse formation and neural plasticity. APP is best known and most commonly studied as the precursor molecule whose proteolysis generates amyloid beta, a 39- to 42-amino acid peptide whose amyloid fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients.

Genetics
In humans, the gene for APP is located on chromosome 21 and contains at least 18 exons in 240 kilobases. Several alternative splicing isoforms of APP have been observed in humans, ranging in length from 365 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. Homologous proteins have been identified in other organisms such as Drosophila (fruit flies), C. elegans (roundworms), and all mammals. 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.

Mutations in critical regions of APP, including the region that generates amyloid beta, are known to cause familial susceptibility to Alzheimer's disease. For example, several mutations outside the Aβ region associated with familial Alzheimer's have been found to dramatically increase production of Aβ.

Structure
A number of distinct, largely independently-folding structural domains have been identified in the APP sequence. The extracellular region, much larger than the intracellular region, is divided into the E1 and E2 domains; E1 contains several subdomains including a growth factor-like domain (GFLD), a metal-binding motif, and a serine protease inhibitor domain that is absent from the isoform differentially expressed in the brain. The E2 domain contains a coiled coil dimerization motif and may bind proteoglycans in the extracellular matrix. The complete crystal structure of APP has not yet been solved; however, individual domains have been successfully crystallized, including the copper-binding domain in multiple configurations and ion-binding states and the E2 dimerization domain.

Post-translational processing
APP undergoes extensive post-translational modification including glycosylation, phosphorylation, and tyrosine sulfation, as well as many types of proteolytic processing to generate peptide fragments. 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. Cleavage by gamma secretase within the membrane-spanning domain 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.

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. Gamma secretase activity has also been associated with lipid rafts. 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.

Biological function
Although the native biological role of APP is of obvious interest to Alzheimer's research, thorough understanding has remained elusive. The most-substantiated role for APP is in synaptic formation and repair; 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. In particular, similarities in post-translational processing have invited comparisons to the signaling role of the surface receptor protein Notch. APP knockout mice are viable and have relatively minor phenotypic effects including impaired long-term potentiation and memory loss without general neuron loss. On the other hand, transgenic mice with upregulated APP expression have also been reported to show impaired long-term potentiation.