The enzymatic degradation of amyloid-beta (Aβ) peptides represents a critical clearance pathway in the brain. Multiple proteolytic systems have evolved to degrade Aβ, with neprilysin (NEP) and insulin-degrading enzyme (IDE) being the two most important Aβ-degrading peptidases [1]. Dysfunction of these clearance pathways contributes to Aβ accumulation in Alzheimer's disease (AD), making them attractive therapeutic targets [2].
| Enzyme | Location | Substrate Specificity | Efficiency |
|---|---|---|---|
| Neprilysin | Neurons, Endothelium | Aβ40 > Aβ42 | Highest |
| IDE | Neurons, Glia | Aβ42 > Aβ40 | High |
| MMP-2/9 | Astrocytes, Microglia | Aβ40, Aβ42 | Moderate |
| Cathepsin B | Lysosomes | Aβ40 | Moderate |
| Cathepsin D | Lysosomes | Aβ42 | High |
Neprilysin (CD10, EC 3.4.24.11) is a zinc-dependent metalloprotease [3]:
NEP is the most efficient Aβ-degrading enzyme [4]:
| Factor | Effect | Mechanism |
|---|---|---|
| Aβ itself | ↑ NEP | Feedback upregulation |
| Aging | ↓ NEP | Epigenetic silencing [6] |
| APOE4 | ↓ NEP | Reduced expression [7] |
| Exercise | ↑ NEP | Transcriptional activation |
| Estrogen | ↑ NEP | ER-mediated signaling |
Approaches to enhance NEP activity:
Challenges:
IDE (EC 3.4.24.56) is a zinc-binding metalloprotease [9]:
IDE has several unique features [10]:
IDE degrades multiple substrates:
| Factor | Effect on IDE |
|---|---|
| Diabetes | ↓ IDE (insulin competition) [11] |
| Aging | ↓ IDE expression [12] |
| APOE4 | ↓ IDE activity [7:1] |
| Exercise | ↑ IDE expression |
| Statins | ↑ IDE (PPAR-α mediated) |
Strategies:
Considerations:
MMPs contribute to Aβ degradation [14]:
| Feature | MMP-2 | MMP-9 |
|---|---|---|
| Expression | Astrocytes, Neurons | Inflammatory cells |
| Aβ specificity | Moderate | Moderate |
| Activity in AD | ↓ | ↑ (but inactive) |
The lysosomal system provides a major Aβ clearance route [16]:
Aβ clearance involves multiple overlapping systems:
| Approach | Target | Status |
|---|---|---|
| NEP gene therapy (AAV) | NEP | Preclinical |
| NEP activators | NEP | Discovery |
| IDE gene therapy | IDE | Preclinical |
| IDE modulators | IDE | Discovery |
| MMP activators | MMP-2/9 | Preclinical |
Miners JS, et al. Abeta-degrading enzymes in Alzheimer's disease. J Affect Disord. 2011. ↩︎
Nalivaeva NN, et al. Amyloid-degrading enzymes as therapeutic targets in Alzheimer's disease. Trends Neurosci. 2012. ↩︎
Saido TC, et al. Neprilysin degrades both amyloid beta peptides 1-40 and 1-42 most efficiently. Biochem Biophys Res Commun. 1998. ↩︎
El-Charnabi MK, et al. Neprilysin activity in brain and cerebrospinal fluid in Alzheimer's disease. J Neurochem. 2024. ↩︎ ↩︎
Cordy JM, et al. Neprilysin activity in brain microvessels: role in amyloid clearance. Neurobiol Aging. 2003. ↩︎
Jacobsen JS, et al. Neprilysin activity and amyloid clearance in aging and Alzheimer's disease. Alzheimers Res Ther. 2019. ↩︎
Tanaka Y, et al. APOE4 impairs neprilysin and insulin-degrading enzyme expression. Brain. 2022. ↩︎ ↩︎ ↩︎
Baranello RJ, et al. AAV-mediated gene delivery of neprilysin for Alzheimer's disease. Mol Ther. 2015. ↩︎
Ferguson SS, et al. Insulin-degrading enzyme: a therapeutic target for Alzheimer's disease. Trends Pharmacol Sci. 2019. ↩︎
Kurochkin IV, et al. Insulin-degrading enzyme in the centre of the Alzheimer's disease amyloid metabolism. Int J Biochem Cell Biol. 2004. ↩︎
Zhang Y, et al. Insulin-degrading enzyme: a promising therapeutic target for AD. Curr Alzheimer Res. 2009. ↩︎
Miners JS, et al. Aβ-degrading enzymes in Alzheimer's disease. J Affect Disord. 2018. ↩︎ ↩︎
Shen Y, et al. Targeting IDE for Alzheimer's disease therapy. Mol Cells. 2020. ↩︎
Malik S, et al. Matrix metalloproteinases and Aβ degradation. Sci Rep. 2021. ↩︎
Bozycki K, et al. MMP-9 and MMP-2 activity in Alzheimer's disease brain. Front Aging Neurosci. 2021. ↩︎
Kim H, et al. Cathepsin B and D in amyloid-beta degradation in Alzheimer's disease. Brain Sci. 2023. ↩︎
Cuajungco MP, et al. Astrocytic lysosomal dysfunction in Alzheimer's disease. Biochim Biophys Acta Mol Basis Dis. 2020. ↩︎
Li C, et al. Autophagy impairment in Alzheimer's disease and therapeutic potential. Pharmacol Ther. 2022. ↩︎