Development and mechanism of γ-secretase modulators for Alzheimer's disease

Abstract

γ-Secretase is an aspartyl intramembranal protease composed of presenilin, Nicastrin, Aph1, and Pen2 with 19 transmembrane domains. γ-Secretase cleaves the amyloid precursor proteins (APP) to release Aβ peptides that likely play a causative role in the pathogenesis of Alzheimer's disease (AD). In addition, γ-secretase cleaves Notch and other type I membrane proteins. γ-Secretase inhibitors (GSIs) have been developed and used for clinical studies. However, clinical trials have shown adverse effects of GSIs that are potentially linked with nondiscriminatory inhibition of Notch signaling, overall APP processing, and other substrate cleavages. Therefore, these findings call for the development of disease-modifying agents that target γ-secretase activity to lower levels of Aβ42 production without blocking the overall processing of γ-secretase substrates. γ-Secretase modulators (GSMs) originally derived from nonsteroidal anti-inflammatory drugs (NSAIDs) display such characteristics and are the focus of this review. However, first-generation GSMs have limited potential because of the low potency and undesired neuropharmacokinetic properties. This generation of GSMs has been suggested to interact with the APP substrate, γ-secretase, or both. To improve the potency and brain availability, second-generation GSMs, including NSAID-derived carboxylic acid and non-NSAID-derived heterocyclic chemotypes, as well as natural product-derived GSMs have been developed. Animal studies of this generation of GSMs have shown encouraging preclinical profiles. Moreover, using potent GSM photoaffinity probes, multiple studies unambiguously have showed that both carboxylic acid and heterocyclic GSMs specifically target presenilin, the catalytic subunit of γ-secretase. In addition, two types of GSMs have distinct binding sites within the γ-secretase complex and exhibit different Aβ profiles. GSMs induce a conformational change of γ-secretase to achieve modulation. Various models are proposed and discussed. Despite the progress of GSM research, many outstanding issues remain to be investigated to achieve the ultimate goal of developing GSMs as effective AD therapies.

Figure 1

(A) Illustration of APP processing by α-, β-, and γ-secretases and the corresponding products. (B) Sequence of the membrane and nearby regions of the β-CTF substrate and relevant cleavages. Thick horizontal arrows represent the hypothesized processive cleavage by γ-secretase. Vertical red arrows show locations of γ, ζ, and ε cleavages.

Figure 2

Illustration of the notch signaling cascade (A) depicting activation by a sending cell, which induces S2 cleavage by an ADAM protease, followed by S3 cleavage by γ-secretase within the membrane domain. Subsequently, notch intracellular domain (NICD) is released from the membrane and translocates to the nucleus where it can turn on target genes. (B) Sequence of the membrane domain and S3 site cleavage of the Notch-1 receptor.

Figure 3

The four essential components of γ-secretase. Presenilin, the catalytic center, is depicted in zymogen form before endoproteolysis of Exon 9 and according to the predicted structure by Li et al ⁽⁷³⁾. Stars represent the relative location of the two aspartic acid residues required for catalysis.

Figure 4

Structures of NSAID-based GSMs.

Figure 5

Structures of 2nd generation NSAID-derived GSMs with acetic acid chemotype.

Figure 6

Structures of non-NSAID-derived heterocyclic GSMs containing the aryl imidazole chemotype.

Figure 7

Additional GSMs with distinct chemotypes.

Figure 8

Structures of GSM derived photoaffinity probes containing (A) biotin or (B) clickable alkyne handle.

Figure 9

Model for different binding sites of GSMs and GSIs. The active site of γ-secretase is represented by a pair of scissors. GSMs alter the “handle” of the scissors, thereby manipulating the way the enzyme cuts and/or the location of the cleavage sites. In contrast, an allosteric GSI will shut the blades, whereas a transition state analog (TSA) will block the blades of the scissors, preventing substrate binding and cleavage of the substrate.

Figure 10

Proposed models for the mechanism of GSMs. A) GSM binding leads to a conformational change in the active site, such as the S1 subpocket. B) Sequential cleavage model: GSM binding has little effect on processivity of γ-secretase at 48 and 45 sites; however, a tighter association of γ-secretase with Aβ42 results in reduced release of Aβ42 and an increase in the generation of Aβ38. C) Independent cleavage model: all cleavage sites are parallel; GSM binding inhibits Aβ42 cleavage site, but enhances Aβ38 cleavage and has little effect on other cleavages including AICD production.