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Alzheimer's Disease: Misguided Slicing of Amyloid Precursor Protein by Secretase
 | | | | | | Alzheimer's disease (AD) is characterized by a progressive deposition of the 4 kDa b-amyloid peptide (Ab) in senile plaques and accumulation of Tau protein as neurofibrillary tangles. In normal healthy individuals, Ab peptides are present only in small quantities as soluble monomers that circulate in cerebrospinal fluid and blood. However, in AD patients, their levels increase significantly and they begin to accumulate as insoluble, fibrillar plaques. Ab's are 40 to 43 amino acid peptides that originate from the proteolytic cleavage of the amyloid precursor protein (APP). APP is reported to occur in three common isoforms, APP695, APP751, and APP770. APP695 is expressed exclusively in neurons, whereas APP751 and APP770 are present in both neural and non-neural cells. The primary structure of APP contains a small signal sequence, a large extramembranous N-terminal region, a single transmembrane domain, and a cytoplasmic C-terminal tail. Processing of APP in vivo occurs by two major pathways. Cleavage of APP at the N-terminus of the Ab region by b-secretase and at the C-terminus by g-secretase represents the amyloidogenic pathway for processing of APP. The b-secretase cleaves APP between residues Met671 and Asp672 and yields sAPPs and the C99 fragment. The b-secretase has also been identified as an aspartyl protease (BACE or Asp-2) of unusual nature. It has a C-terminal transmembrane domain and two active site motifs located in the luminal domain. Newly synthesized BACE contains a propeptide domain, which is cleaved at residue E46 to produce the mature enzyme. The active site of BACE and the b-secretase cleavage site of APP are in precise topological orientation for endoproteinases. Succeeding the b-secretase cleavage, a second cleavage occurs at the C-terminus of Ab peptide that releases Ab from C99. This cleavage occurs in the vicinity of residue 712 of the C-terminus. The g-secretase can cleave the C-terminal region at either Val711 or Ile713 to produce the shorter Ab peptide (Ab1-40) or the longer Ab peptide (Ab1-42). The predominant form of Ab found in the cerebrospinal fluid is the shorter Ab1-40 peptide. Despite its lower rate of synthesis, Ab1-42 is the peptide that is initially deposited within the extracellular plaques of AD patients. In addition, Ab1-42 is shown to aggregate at a much lower concentration than the Ab1-40 form.
APP can also be processed by a-secretase (TACE), which cleaves within the Ab domain between Lys687 and Leu688 and produces a large soluble a-APP domain and the C-terminal fragment containing P3 and C83. The latter can then be cleaved by g-secretase at residue 711 or 713 to release P3 fragment. This pathway does not yield Ab peptide. Hence, shunting APP towards the a-secretase pathway may have a beneficial effect in lowering Ab peptide levels. It is reported that a-secretase shares many of its properties with the secretase that cleaves angiotensin-converting enzyme and is believed to be a zinc metalloproteinase of the ADAMs family. Muscarinic agonists (M1 and M3) and some PKC activators are reported to enhance a-secretase activity and are under consideration for their therapeutic value as AD treatment tools.
Presenilin (PS) and nicastrin have also been reported to play an important role in bAPP processing. PS1 and PS2 are polytopic transmembrane proteins that share extensive amino acid sequence identity. They are functional components of separate high molecular weight complexes in the endoplasmic reticulum (ER) and Golgi apparatus and are essential for the proteolytic cleavage of several proteins, including bAPP and Notch. g-secretase cleavage of bAPP and production of Ab1-42 is reported to increase as a result of mutations in PS1 and PS2. Some researchers have suggested that PS may have some catalytic activity of their own and may even be g-secretases, while others argue that this activity requires interactions between the presenilins and other proteins. PS1 and PS2 also complex with nicastrin, a transmembrane glycoprotein that is important in PS-mediated bAPP and Notch processing. Nicastrin binds both full-length bAPP and g-secretase substrates (C99- and C83-bAPP fragments), and modulates the activity of g-secretase. Mutations in the conserved domain of nicastrin (312-369 in the hydrophilic N-terminus) increase bAPP cleavage by g-secretase; however, binding of nicastrin to C99-/ C83- is not significantly altered.
In mammalian cells, endogenous as well as overexpressed nicastrin undergoes post-translational N-glycosylation during trafficking from the endoplasmic reticulum to the Golgi apparatus. PS1 is shown to interact preferentially with mature glycosylated nicastrin. Studies using an RNA interference approach has shown that down-regulation of the nicastrin level leads to accumulation of the C-terminal fragments of the bAPP, destabilizes PS, and leads to reduction in Ab production. It is suggested that nicastrin and PS regulate each other and determine g-secretase activity via formation of a complex. Interestingly, inhibition of nicastrin expression is shown to reduce g-secretase activity and PS1 complex formation. This indicates that perhaps nicastrin is a limiting factor for the assembly of PS complex. In addition to nicastrin, recent genetic studies have identified a number of additional genes encoding Aph-1a, Aph-1b, and Pen-2 proteins, as part of the g-secretase complex with PS1. Over-expression of these proteins can also increase the levels of Ab, which indicates their limiting nature for g-secretase activity. Most cases of familial AD (FAD) are reported to result from mutations in one of the three genes, APP, PS1 and PS2. The mutation in the APP gene, located on chromosome 21, accounts for about 2% of all cases of FAD and approximately 5 - 20% of early-onset FAD. Over 40 different mutations have been reported in PS1, which account for about 30 to 50% of all presenile FAD. The PS2 gene mutations are rather rare and account for less than 2% of all early-onset FAD. Mutations in both PS1 and PS2 are associated with an increased production of the Ab1-42 peptide.
The characterization of the APP secretases during the past few years has provided significant advancement in therapeutic strategies that may lead to limiting the build up of Ab peptide in the brain and eliminating or delaying the pathological effects of AD. Recent characterization of secretases has uncovered several common features, particularly their sensitivity to certain metalloprotease inhibitors and the upregulation of their activity by phorbol esters. Presenilins and g-secretases are considered to be the most promising molecular targets for developing therapeutic agents that may minimize the debilitating effects of AD. Major focuses in AD research are identifying more genetic and environmental factors responsible for Ab build-up in nerve cells. | | | | | | | |
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