Three enzymatic activities are involved in APP processing, and were named α-secretase, β-secretase, and γ-secretase at a time when their molecular identities were unknown. APP exists in several isoforms ranging from 695 to 770 amino acids in length, including the domain from which the Aβ peptide derives. In APP695 (the most abundant isoform in the brain) this domain ranges from amino acids 597 to 638. In an initial step, APP is cleaved at a juxtamembrane position at the luminal side of the membrane. This cleavage is mediated at different positions by either α-secretase or β-secretase in different compartments of the cell [
4]. The majority of APP molecules in non-neuronal cells are initially cleaved by α-secretase between positions 16 and 17 of the Aβ domain. This is the so-called non-amyloidogenic pathway since the cleavage occurs within the Aβ domain, thereby preventing the production of Aβ. This event generates a stub called α-C-terminal fragment as well as a large ectodomain called sAPPα. Several members of the ADAM family of proteases are able to mediate this cleavage, but in neurons this function is likely to be exerted by the constitutively active ADAM10 protease [
5].
In the amyloidogenic pathway, leading to the production of Aβ peptides, β-secretase mediates the initial rate-limiting step. The membrane-bound aspartyl-protease BACE1 has been identified as the responsible enzyme. APP is cleaved by this enzyme before position 1 of the Aβ domain [
6], resulting in the release of a large ectodomain and the formation of a stub called β-C-terminal fragment. In addition, BACE1 can also cleave APP within the Aβ domain between positions 10 and 11 (β′ site) [
7]. Subsequently, both N-terminally cleaved precursors are further processed by γ-secretase, a complex that consists of at least the proteins APH-1, PEN-2, nicastrin and presenilin 1 or presenilin 2 [
8]. The transmembrane proteins presenilin 1 and presenilin 2 possess two critical aspartyl residues that are part of the catalytic domain of this γ-secretase subunit. The cleavage occurs within the transmembrane domain of APP, generating C-terminally truncated peptides ending with amino acids 37 to 43, due to an imprecise cleavage of these enzymes. The resulting peptides are liberated into extracellular fluids such as cerebrospinal fluid (CSF), plasma or interstitial fluid. This phenomenon is not fully understood, but endoproteolysis is thought to occur stepwise, cleaving the C-terminal stubs several times within their transmembrane domain. These cleavages are approximately three amino acids apart [
9,
10]: one at amino acid 48 or 49, followed by another one at position 45 or 46, and ending with a final cleavage most often at position 38, 40 or 42. At least in the CSF of nondemented controls, about one-half of the Aβ ends at amino acid 40, 16% ends at amino acid 38, and 10% ends at amino acid 42 [
11]. Aβ species ending with the alanine at position 42 have a stronger tendency to aggregate as compared with Aβ1–40. These species are thought to be the driving factor for the formation of amyloid plaques and the neurotoxic effects [
12]. During this last step, other minor cleavage sites have been observed at positions 34, 37, 39, and 43 [
9]. There are even shorter Aβ isoforms (Aβ1–17/18/19/20) that depend on γ-secretase [
13,
14] but the precise mechanism of their generation is unknown.
Both processing pathways lead to a large variety of peptides that start either at position 1 or position 11 caused by cleavage by BACE1 or at position 17 mediated by α-secretase. The latter peptides are called p3 [
15] but are not found in senile plaques, and neither do they yet appear to have any pathological or physiological role. Most of these N-terminal starting points are found in combination with the heterogeneity caused by the cleavage of γ-secretase.
To make things even more complicated, the amyloidogenic pathway and the nonamyloidogenic pathway seem not to be mutually exclusive. There are shorter isoforms of Aβ (Aβ1–14/15/16) present in the CSF that do not depend on γ-secretase cleavage but are sensitive to inhibitors of α-secretase. Interestingly, Aβ1–14/15/16 increase after γ-secretase treatment in CSF [
16,
17]. This leads to the conclusion that C-terminal BACE1-cleaved stubs (C99) are not exclusive substrates for γ-secretase, and that C99 can reach compartments with α-secretase activity resulting in the liberation of Aβ1–14/15/16 along alternative pathways [
13,
14,
18].