γ-Secretase Inhibitors – from Molecular Probes to New Therapeutics?

Publisher Summary

The hallmark lesions in Alzheimer's disease (AD) brains described by Alois Alzheimer are extracellular proteinaceous deposits, found either as amyloid plaques in the brain parenchyma or as vascular amyloid surrounding the brain blood vessels, and intracellular neurofibrillary tangles consisting of an abnormally phosphorylated microtubule-associated protein tau. Over the past several years, extraordinary progress has been made in understanding mechanistic details of the amyloid hypothesis of AD. A part of this understanding has arisen as a result of the availability of molecular probes targeted toward γ-secretase. Although the final proof of the identity of γ-secretase by reconstitution of the γ-secretase complex in vitro is a difficult task, substantial evidence generated by affinity labeling of presenilins by transition state analog inhibitors points toward a critical role of polypeptides in the enzyme complex.

Introduction

In 1906, the neuropathologist Alois Alzheimer drew the attention of the scientific community to neuropathological changes that he had observed in the brains of elderly patients and suggested a possible connection to the neuronal dysfunction underlying the disease [1], which was later to bear his name. Subsequent progress in the understanding of the molecular basis of Alzheimer's disease (AD) proceeded at a relatively slow rate. During the last decade, however, new genetic and biochemical evidence has provided a rationale for a potential disease-modifying therapeutic approach. Since both age and genetic predisposition are the major known risk factors for AD, the increased life span of the population of the Western world in recent decades has resulted in an urgent need for an effective therapy. One of the main aims will be to prevent or slow down the rapid decline in cognitive function and memory, which is a consequence of this irreversible and progressive neurodegenerative disease affecting the central nervous system (CNS).

The hallmark lesions in AD brains described by Alois Alzheimer are extracellular proteinaceous deposits found either as amyloid plaques in the brain parenchyma or as vascular amyloid surrounding the brain blood vessels, and intracellular neurofibrillary tangles consisting of an abnormally phosphorylated microtubule-associated protein tau [2]. Until 1984, the identity of the main component of the extracellular deposits was unknown, but amino acid sequencing of the major peptide entity purified from vascular amyloid [3] or senile plaques [4] revealed that they are composed primarily of the 40–42 amino acid amyloid-β (Aβ) peptide. The Aβ peptide is a product of post-translational processing, which is derived by sequential proteolytic cleavage of a type I transmembrane protein, the β-amyloid precursor protein (βAPP) [5], by enzymes referred to as β- and γ-secretase [6] (Figure 3.1). β-Secretase cleaves within the βAPP ectodomain close to the extracellular membrane surface to generate a membrane-bound intermediate, the β-C-terminal fragment (β-CTF, C99). A predominant alternative processing pathway, which precludes Aβ peptide formation, involves cleavage of βAPP within the Aβ sequence by a protease termed α-secretase, producing an alternative membrane-bound stub, the α-C-terminal fragment (α-CTF, C83). This activity appears to be mediated by members of the disintegrin and metalloprotease family TACE [7] and ADAM-10 [8] and leads to the secretion of the soluble βAPP ectodomain as secretory βAPP [9]. Both processing events (β- and α-secretase cleavage) generate truncated membrane-bound substrates for a third protease, γ-secretase. These substrates are cleaved within the lipid bilayer by γ-secretase to release either Aβ peptides as a product of sequential β/γ-cleavage or a peptide termed p3 as a result of α/γ-cleavage [6].

A causative role for the Aβ peptide, and especially the more hydrophobic C-terminally elongated variant Aβ(1–42), in AD in which accumulation in extracellular protein deposits occurs, has been postulated (the so-called amyloid cascade hypothesis) [10]. This hypothesis is substantially supported by the identification of autosomal dominant mutations causing familial Alzheimer's disease (FAD) which is an inherited form of AD leading to an early onset of the disease. Such FAD mutations were found either in the βAPP gene itself, clustering around β- and γ-secretase cleavage sites of the corresponding Aβ peptide sequence (Figure 3.2) or in two alternative genes encoding presenilin 1 and 2 (PS1 and PS2) (Figure 3.3), two proteins which were shown to have high homology with each other. Mechanistically, all these mutations increase the production of Aβ(1–42) peptide, which is more hydrophobic, more prone to aggregation and is deposited earlier in the time course of the disease than the shorter (and more predominant) species Aβ(1–40) (for review see 10, 11). Although the prevalence of FAD is much lower than of late onset sporadic AD, the pathology observed in both types is similar, and it seems likely that the disease mechanisms involved may be the same. It is noteworthy that the occurrence of familial and sporadic forms of several neurodegenerative diseases has been observed, including Parkinsonism and amyotrophic lateral sclerosis (ALS) [12]. If one considers a causative role for Aβ peptide in the manifestation of AD, inhibition of either of the two critical enzymes involved in Aβ peptide generation provides target opportunities to develop drugs which could slow down or even halt the progression of the disease.