The Asparaginyl Endopeptidase Legumain: An Emerging Therapeutic Target and Potential Biomarker for Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is incurable dementia closely associated with aging. Most cases of AD are sporadic, and very few are inherited; the pathogenesis of sporadic AD is complex and remains to be elucidated. The asparaginyl endopeptidase (AEP) or legumain is the only recognized cysteine protease that specifically hydrolyzes peptide bonds after asparagine residues in mammals. The expression level of AEPs in healthy brains is far lower than that of peripheral organs. Recently, growing evidence has indicated that aging may upregulate and overactivate brain AEPs. The overactivation of AEPs drives the onset of AD through cleaving tau and amyloid precursor proteins (APP), and SET, an inhibitor of protein phosphatase 2A (PP2A). The AEP-mediated cleavage of these peptides enhances amyloidosis, promotes tau hyperphosphorylation, and ultimately induces neurodegeneration and cognitive impairment. Upregulated AEPs and related deleterious reactions constitute upstream events of amyloid/tau toxicity in the brain, and represent early pathological changes in AD. Thus, upregulated AEPs are an emerging drug target for disease modification and a potential biomarker for predicting preclinical AD. However, the presence of the blood–brain barrier greatly hinders establishing body-fluid-based methods to measure brain AEPs. Research on AEP-activity-based imaging probes and our recent work suggest that the live brain imaging of AEPs could be used to evaluate its predictive efficacy as an AD biomarker. To advance translational research in this area, AEP imaging probes applicable to human brain and AEP inhibitors with good druggability are urgently needed.

The Role of Aldose Reductase in Beta-Amyloid-Induced Microglia Activation

“…The interaction of APP and β-amyloid with cell adhesion molecules subsequently induces intracellular signaling contributing to cytotoxicity [ 2 ]. In addition, the overactivation of asparaginyl endopeptidases (AEPs) cleaves tau and APP, which enhances amyloidosis and drives the onset of AD [ 3 ]. When insoluble β-amyloid accumulates in the brain, microglia infiltrate the plaque site, secreting pro-inflammatory cytokines such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), and nitric oxide [ 4 ].…”

Discovery of Orally Available and Brain Penetrant AEP Inhibitors

“…The AEP-cleaved fragment Tau N368 was recently shown to augment BACE1 expression and Aβ production via binding to the BACE1 transcription factor STAT1. 12 We hypothesized that AEP inhibition may offer therapeutic benefits to AD patients, 13 protein, AEP has been reported to have a broad substrate scope, 14 including other proteins relevant in neurodegenerative diseases, such as α-synuclein 15 and TDP-43. 16 AEP is ubiquitously produced in human tissues, with relatively high levels in kidney and lymphoid organs compared to brain.…”

“…The AEP-cleaved fragment Tau N368 was recently shown to augment BACE1 expression and Aβ production via binding to the BACE1 transcription factor STAT1 . We hypothesized that AEP inhibition may offer therapeutic benefits to AD patients, either by direct reduction of TauN368 fragments or indirect suppression of the BACE1 pathway. Beyond the Tau protein, AEP has been reported to have a broad substrate scope, including other proteins relevant in neurodegenerative diseases, such as α-synuclein and TDP-43 …”

“…13 C NMR (151 MHz, DMSO-d6): δ 171.7, 169.4, 169.2, 146.5, 139.9, 127.2, 121.1, 120.3, 119.1, 59.4, 47.3, 37.2, 37.1, 29.6, 29.0, 24.4, 16.1, 15.2. m/z: ES + [M + H] + = 439.2. LC-HRMS (m/z): a M…”

“…Figure 5. Scatter plot of enzymatic potency against13 C shift of carbon atom C el , in ppm relative to TMS in d6-DMSO. Heteroaryl fragments with ortho nitrogen atom (blue) tend to give higher potency than fragments without this feature (red).…”