a.i. Amyloid Precursor Protein

No documento Peter St George-Hyslop (páginas 35-39)

The first gene to be associated with inherited susceptibility to AD was the amyloid precursor protein gene (APP). The APP gene was cloned and

DISCOVERIES ARISING FROM BASIC SCIENCE STUDIES OF SPECIFIC NEURODEGENERATIVE DEMENTIAS

mapped to chromosome 21 by several groups16-18, including the author’s group15,19. We and others showed that the APP gene produces an alterna-tively spliced transcript which, in its longest isoform, encodes a single trans-membrane spanning polypeptide of 770 amino acids (Figure 7)16.

Following trafficking to the cell surface, the APP precursor protein is either recycled into the secretory pathway and then back to the cell surface via endosomes, or it undergoes a series of endoproteolytic cleavages (Figure 8). The majority of APP is either: 1) recycled back and forth between the cell surface and the retromer; or 2) it undergoes cleavage mediated by the membrane-associated ADAM10 and ADAM17 disintegrin metalloprote-ases. These membrane-bound enzymes cleave APP695 in the middle of the Aβ peptide domain. This cleavage is termed α-secretase and precludes the formation of Aβ.

Lesser amounts of APP (~5 - 10%) are processed by a minor cleavage pathway that occurs principally after internalization from the cell surface in the late endosome compartment, and involves sequential cleavages by β- and γ-secretases, which then generate a 40-42 amino acid peptide termed the Aβ peptide (Figure 8). The first of these cleavage events occurs at the beginning of the Aβ domain, and is mediated by β-secretase (BACE 1), a Type 1 trans-membrane glycosylated aspartyl protease101. This cleavage generates a

solu-Figure 7: Cartoon of the topology of the APP protein domains. The Aβ domain is depicted by the red box spanning the C-terminus of the extracellular domain and half of the TM donmain, and is generated by cleavage of the APP holoprotein.

TRANSLATING DISCOVERIES IN BASIC MOLECULAR BIOLOGY, CELL BIOLOGY, AND MOLECULAR GENETICS INTO TRANSFORMATIVE APPROACHES TO THE DIAGNOSIS AND TREATMENT OF CURRENTLY INCURABLE NEURODEGENERATIVE DEMENTIAS

ble N-terminal fragment (APP), and a membrane-bound stub (APP-C100).

The second set of cleavages, termed γ-, ζ- and ε-secretase cleavages, oc-cur at several sites between residues +38 and +59 of the membrane-bound APP-C100 stubs. The γ-, ζ- and ε-cleavages are performed by the presenilin complex (see Section III.1.aiii below). The N-terminal product of the γ/ε-secretase cleavage of APP-C100 is Aβ, while the C-terminal product is a labile fragment termed Amyloid Intra-Cellular Domain (AICD). The AICD might be translocated to the nucleus where it acts as a signal transduction molecule102. The γ/ε-secretase cleavage actually generates a mixture of Aβ peptides containing 38, 40, 42, or 43 amino acids. Aβ peptides ending at resi-due 42 or 43 (long tailed Aβ) are thought to be more fibrillogenic and more neurotoxic than Aβ ending at residue 40, which is the predominant isoform produced during normal metabolism of APP. Little is currently known about the physiological role (if any) of Aβ. Aβ is removed by several pathways, in-cluding degradation by a variety of peptidases such as neprilysin103, plasmin, and insulin degrading enzymes (IDE)104 (Figure 8).

The function of APP is currently unknown. Knockout of the murine APP gene leads only to minor weight loss, decreased locomotor activity, abnor-mal forelimb motor activity, and non-specific degrees of reactive gliosis in the cortex. However, double knockouts of APP and one or more of its

homo-Figure 8: Proteolytic processing pathways for APP. Only the BACE + γ-secretase pathway generates Aβ.

AICD is translocated to the nucleus.

Cell Surface Late endosomal pathways

DISCOVERIES ARISING FROM BASIC SCIENCE STUDIES OF SPECIFIC NEURODEGENERATIVE DEMENTIAS

logues (termed amyloid precursor-like proteins – APLPs) cause embryonic lethality, suggesting that APP and the APLPs have redundant but essential activities105. In vitro studies in cultured cells suggest that the N-terminal se-creted fragment of APP (APPs) can function as an autocrine factor, stimulat-ing cell proliferation and cell adhesion. Other studies have implied a role for APP in: 1) signal transduction by association of APP with heterotrimeric GTP-binding proteins; 2) a receptor for kinesin-1 during the fast axoplasmic transport of vesicles containing BACE and presenilins, where the cleavage of APP serves to stop the kinesin-mediated trafficking of the transport ves-icle; and/or 3) a signal transduction molecule in a manner similar to Notch signalling. The C-Terminal fragment of Notch that is equivalent to AICD is the Notch Intra-Cellular Domain (NICD). Following binding of Delta to Notch at the cell surface (during dorsal axis development in embryogenesis, The presenilin complex cleaves the Notch protein within the membrane, and releases NICD into the cytoplasm. NICD (and presumably also AICD) is then translocated to the nucleus, where it transcriptionally activates multiple downstream genes involved in Notch signalling (Figure 8).

Several observations made by the author and by many other researchers beginning with George Glenner11, led to the hypothesis that mutations in the APP gene might cause AD. First, a fragment of APP (Aβ) was part of the pathology of AD11. Second, individuals with trisomy 21 develop AD-like changes in their brains106. Third, the cloning and mapping of the APP gene to chromosome 21 nearby markers that co-segregated with AD in families by the author15,19 and colleagues20 implied that mutations in APP might cause AD. Fourth, sequencing of exons 16 and 17 in the APP gene of patients with Hereditary Cerebral Haemorrhage with Amyloidosis (HCHWA) led to the discovery of the first pathogenic mutations in APP107. Direct sequencing of the APP gene by several groups108-111, including the author’s group112, sub-sequently uncovered at least 25 different AD-associated missense mutations in the APP gene (most in exons 16 and 17) in families with early-onset AD (http://molgen-www.uia.ac.be/ADMutations).

All known AD-associated mutations in APP either alter APP process-ing and Aβ production, or alter the propensity of the resultprocess-ing Aβ peptide to aggregate into β-sheet amyloid fibrils. Some of the missense mutations in the APP gene result in the relative (APP717) or absolute (APP670/671)

over-TRANSLATING DISCOVERIES IN BASIC MOLECULAR BIOLOGY, CELL BIOLOGY, AND MOLECULAR GENETICS INTO TRANSFORMATIVE APPROACHES TO THE DIAGNOSIS AND TREATMENT OF CURRENTLY INCURABLE NEURODEGENERATIVE DEMENTIAS

production of full length Aβ species ending at residue 42. Other mutations cause the over-production of N-terminally truncated species of Aβ ending at residue 42 (APP715); or the production of Aβ species that have increased propensity to assemble into neurotoxic fibrils (APP692, APP693).

Multiple molecular mechanisms have been proposed to explain the neu-rotoxic effects of Aβ (and especially of small soluble aggregates variously called oligomers, amyloid-derived-diffusible-ligands or ADDLs, and pro-tofibrils) (See Section II). These include both direct effects (e.g. inducing apoptosis by effects on cell membranes) and by indirect effects (e.g. potenti-ating effects of excitatory amino acids on NMDA receptors, oxidative stress, and increases in intracellular calcium and free radicals). However, Aβ may not be the only cytotoxic product of β- and γ-secretase cleavage because the cytoplasmic C-terminal stub (C31-APP) is also toxic when over-expressed.

This series of discoveries has had huge practical significance. They have led to the invention of both novel biomarkers (e.g. CSF Aβ assays; position emission tomography (PET) imaging agents such as PIB); and also candi-date therapies (e.g. Aβ vaccines; Aβ aggregation inhibitors) (see Section IV below).

No documento Peter St George-Hyslop (páginas 35-39)