Definition: The A? peptide, which is the primary protein component of diffuse and neuritic plaques, originates through proteolysis from the amyloid-precursor protein (APP).

APP cleavage

A?, the main component of the extracellular amyloid deposits, is generated by sequential proteolytic cleavage of the amyloid precursor protein (APP).

APP is processed by at least two distinct proteolytic pathways. The non-amyloidogenic pathway involves cleavage by -secretases, which have been identified as members of the a disintegrin and metalloprotease (ADAM) family.

The *-secretase cleaves within the A? domain of APP, thus precluding the generation of A?. The amyloidogenic pathway involves cleavage by ?-secretase BACE1, a membrane-bound aspartyl protease that generates the N terminus of the A? peptide.

Both *- and ?-secretase cleave APP within its ectodomain (extracellular domain) at a short distance from the transmembrane domain, generating soluble forms of APP, sAPP and sAPP?, respectively.

The remaining membrane-bound fragments *-CTF and ?-CTF are further processed by ?-secretase, a multimeric enzyme complex. The ?-secretase cleavage of *-CTF releases the non-amyloidogenic peptide P3, whereas ?-secretase processing of ?-CTF generates the amyloidogenic A? peptide.

Independently of amyloidogenic or non-amyloidogenic processing of APP, the intracellular domain of APP (AICD) is released to the cytosol.

The multimeric enzyme complex ?-secretase consists of at least four proteins; the presenilins (PSENs), Aph-1, Pen-2 and nicastrin. Knockout (KO) of both PSs or the mutation of the aspartic residues D257 and D385 in PS results in complete loss of ?-secretase activity and A? production, thus identifying PSENs as the active centre of the ?-secretase complex.

The A? peptides generated by ?-secretase activity can vary in length; the most common forms contain 38, 40 or 42 amino acids (A?38, A?40 or A?42).

Because of the two additional amino acids isoleucine and alanine, A?42 aggregates more quickly than A?40 and is the major component of neuritic plaques in AD. The relevance of A?42 in AD is further supported by familiar forms of AD (FAD). Most of the missense mutations in the genes encoding APP and PS increase the production of A?42.

Structure

Amyloid ? peptide (A?)is a 40?42 amino acid peptide (?40 and ?42). The 42 amino acid form (?42) is the most toxic and prone to aggregation, and is the primary component of diffuse or neuritic plaques.

Amyloid precursor protein (encoded by the gene APP) is a member of an evolutionary conserved family of type I membrane proteins that contain a large N-terminal extracellular domain, a single transmembrane region and a short C-terminal cytoplasmic tail.

Pathology

Once formed, beta-amyloid molecules aggregate into ?plaques? within the brain, causing death and dysfunction of cells, especially in brain areas important for learning and memory (Alzheimer disease).

A? has been the focal point of Alzheimer disease research and is generally considered as the upstream causative factor.

The strongest evidence for this position derives from molecular genetic studies of the three genes (APP, PSEN1 and PSEN2) that underlie familial Alzheimer disease (FAD) cases, because they all modulate some aspect of A? metabolism, increasing the propensity for A? to aggregate.

In addition, the ?4 isoform of the apolipoprotein E gene, which is the major risk factor for late-onset disease, affects the rate of A? aggregation.

The central role of A? is trumpeted by the amyloid-cascade hypothesis, which states that A? is the trigger for all cases of AD. It is crucial to note that it is not yet established which form of A? (i.e. monomeric A?, oligomeric A? or fibrillar A?; intracellular A? versus extracellular A?) is the pathogenic culprit.

The amyloid hypothesis has not been embraced by all AD researchers, and it might be that A? is just an epiphenomenon and that a subtler event lies upstream.

According to the ?amyloid cascade? hypothesis, A? is the trigger for all cases of Alzheimer disease. The beta-amyloid peptide (A beta) is widely considered to be the molecule that causes Alzheimer?s disease (AD).

Besides APP, the amyloid precursor-like proteins APLP1 and APLP2 are members of the APP gene family (APPs) in mammals. The evolutionary conservation of this gene family also extends to invertebrates; for example, there is the Drosophila melanogaster homologue known as APPL and the Caenorhabditis elegans gene product named APL-1.

APP processing and Beta-amyloid formation

Beta-amyloid or A? peptide is formed by the actions of two enzymes on a much larger protein called APP, which extends through the cell membrane of brain cells. The two enzymes (BACE and PSEN1) cut the precursor protein APP in specific places, resulting in beta-amyloid formation.

APP is processed by at least two distinct proteolytic pathways. The ectodomain of APP can be cleaved by ?-secretase or ?-secretase, resulting in the release of soluble forms of APP (sAPP? and sAPP?, respectively). The remaining membrane-bound fragments ?-CTF and ?-CTF are further cleaved by ?-secretase, releasing p3 or A? and AICD.

The non-amyloidogenic pathway involves cleavage by ?-secretase, which have been identified as members of the a disintegrin and metalloprotease family (ADAMs). The ?-secretase cleaves within the A? domain of APP, thus precluding the generation of A?.

The amyloidogenic pathway involves cleavage by ?-secretase BACE1, a membrane-bound aspartyl protease that generates the N terminus of the A? peptide.

Both ?-secretase and ?-secretase cleave APP within its ectodomain (extracellular domain) at a short distance from the transmembrane domain, generating soluble forms of APP, sAPP? and sAPP?, respectively.

The remaining membrane-bound fragments ?-CTF and ?-CTF are further processed by ?-secretase, a multimeric enzyme complex.

The ?-secretase cleavage of ?-CTF releases the non-amyloidogenic peptide p3, whereas ?-secretase processing of ?-CTF generates the amyloidogenic A? peptide.

Independently of amyloidogenic or non-amyloidogenic processing of APP, the intracellular domain of APP (AICD) is released to the cytosol.

The multimeric enzyme complex ?-secretase consists of at least four proteins; the presenilins (PSENs), Aph-1, Pen-2 and nicastrin.

Knockout (KO) of both PSs or the mutation of the aspartic residues D257 and D385 in PS results in complete loss of ?-secretase activity and A? production, thus identifying PS as the active centre of the ?-secretase complex.

The A? peptides generated by ?-secretase activity can vary in length; the most common forms contain 38, 40 or 42 amino acids (A?38, A?40 or A?42).

Because of the two additional amino acids isoleucine and alanine, A?42 aggregates more quickly than A?40 and is the major component of neuritic plaques in AD.

The relevance of A?42 in AD is further supported by familiar forms of AD (FAD). Most of the missense mutations in the genes encoding APP and PS increase the production of A?42.

Lipid homeostasis

A beta also has an essential physiological role in lipid homeostasis. Cholesterol increases A beta production, and conversely A beta production causes a decrease in cholesterol synthesis.

The latter appears to be mediated by the inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase or HMGR), a key enzyme in cholesterol synthesis, in an action similar to that of statins.

Moreover, A beta regulates sphingolipid metabolism by directly activating sphingomyelinases (sphingomyeline phosphodiesterases or SMPDs).

APP and the A?-releasing secretases are transmembrane proteins, and their membrane positioning appears to be relevant because ?-secretase cleaves APP in the middle of the membranedomain.

Therefore, alterations in membrane thickness could have drastic consequences for the A?42/A?40 ratio. Accordingly, a membrane that has a smaller diameter might shift A? production toward A?42.

Indeed, it has been reported that mainly A?42 is produced in endoplasmic reticulum (ER) membranes, which have a smaller diameter in comparison to plasma membranes.

In addition, the cleavage site can be shifted by an altered length of the transmembrane domain. C-terminal insertion of two residues as well as N-terminal deletion of two residues strongly alters the A?42/A?40 ratio, whereas A? is still generated when residues within the transmembrane domain are mutated.

This model suggests that the ratio between the potentially pathogenic A?42 and the potentially non-pathogenic A?40 might be modulated by membrane composition and the physical state of the membrane.

Some lipids, especially cholesterol, sphingolipids and gangliosides, strongly influence A? production.

Pathology

 APP and Alzheimer?s disease (AD)

AD is characterized by two classical hallmark pathologies in the brain:

• (i) intracellular neurofibrillary tangles, composed of an abnormally phosphorylated form of the protein tau (TAU).

• (ii) extracellular neuritic plaques, composed of a dense amyloid core of ?-amyloid peptide (A?) surrounded by microglia and dystrophic neurites. Soluble oligomeric A? can also be found throughout the brain, and these molecules represent the potential precursors of insoluble amyloid plaques.

A?, the main component of the extracellular amyloid deposits, is generated by sequential proteolytic cleavage of the amyloid precursor protein (APP).

Animal models

Combined KOs of APP gene family members (APPs) in mice result in perinatal lethality and neurological deficits.

Mice deficient in APP and APLP2 have structurally and functionally defective neuromuscular synapses, reduced numbers of synaptic vesicles at presynaptic compartments, an aberrant apposition of presynaptic marker proteins with postsynaptic acetylcholine receptors and excessive nerve-terminal sprouting.

APP- and APLP1-deficient mice are viable but sterile and show no histological abnormalities. However, they also exhibit early postnatal death in conjunction with a haplodeficiency of APLP2.

The APP/APLP1/APLP2 triple-KO mice die prematurely and exhibit cranial abnormalities, including cortical dysplasias resembling the phenotype found in human type II lissencephaly and reduced numbers of cortical Cajal-Retzius cells, which have been implicated in the normal architectural development of the cerebral cortex.

These phenotypes of triple-KO mice highlight the importance of APP and the APP family members in regulating cell survival, neuronal cell adhesion and cell migration of neuroblasts, and thus brain development. In addition to the role of APP in brain development, it has been suggested that APP displays different functions in adulthood.

References

 Grimm MO, Grimm HS, Hartmann T.Amyloid beta as a regulator of lipid homeostasis.Trends Mol Med. 2007 Aug;13(8):337-44. PMID: 17644432