Through Proteostasis Network Modulation: Targeting Neurodegenerative Protein Aggregation

The Proteostasis Network: A Cellular Safeguard Against Misfolded Proteins

The proteostasis network (PN) is an intricate biological system responsible for maintaining protein homeostasis—ensuring proper folding, assembly, and degradation of proteins within cells. Comprised of molecular chaperones, ubiquitin-proteasome systems (UPS), autophagy-lysosomal pathways (ALP), and stress response mechanisms, the PN acts as a sentinel against protein misfolding and aggregation.

In neurodegenerative disorders such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), the PN becomes overwhelmed, leading to the accumulation of toxic protein aggregates. Amyloid-β plaques and tau tangles in AD, as well as α-synuclein Lewy bodies in PD, exemplify pathological protein misfolding events. Targeting the PN offers a promising therapeutic avenue to restore cellular balance and mitigate neurodegeneration.

Molecular Chaperones: The First Line of Defense

Molecular chaperones, including heat shock proteins (HSPs), play a pivotal role in guiding nascent polypeptides into their correct conformations. They prevent aggregation by binding to exposed hydrophobic regions of misfolded proteins, facilitating either proper refolding or targeted degradation.

• HSP70 and HSP90: These ATP-dependent chaperones assist in stabilizing partially folded proteins and preventing their aggregation. In AD models, overexpression of HSP70 reduces tau pathology by promoting its refolding or clearance.
• Small HSPs (e.g., HSP27): These ATP-independent chaperones act as "holdases," preventing irreversible aggregation. In PD, HSP27 has been shown to inhibit α-synuclein fibril formation.

Despite their protective roles, chaperone capacity diminishes with age—a key risk factor for neurodegenerative diseases. Enhancing chaperone expression or activity through pharmacological agents (e.g., HSP90 inhibitors that upregulate HSP70) is an active area of research.

Ubiquitin-Proteasome System: Precision Degradation of Misfolded Proteins

The ubiquitin-proteasome system (UPS) is responsible for the selective degradation of short-lived and misfolded proteins. Proteins tagged with ubiquitin chains are recognized and degraded by the 26S proteasome, a barrel-shaped protease complex.

In neurodegenerative diseases:

• UPS dysfunction: Impaired proteasomal activity has been observed in both AD and PD, contributing to the accumulation of toxic aggregates.
• Ubiquitin ligases (E3 enzymes): Parkin, an E3 ligase mutated in familial PD, plays a role in clearing damaged mitochondria and misfolded proteins. Its dysfunction exacerbates α-synuclein pathology.

Therapeutic Strategies Targeting UPS

• Proteasome activators: Compounds like PA28γ enhance proteasomal degradation of tau and α-synuclein in preclinical models.
• Ubiquitin-like modifiers: NEDD8ylation, a post-translational modification, regulates proteasome activity. Inhibiting NEDD8-activating enzyme (NAE) has shown promise in reducing tau accumulation.

Autophagy-Lysosomal Pathway: Bulk Clearance of Aggregates

When the UPS is overwhelmed, macroautophagy (hereafter autophagy) steps in as a bulk degradation mechanism. Autophagy engulfs protein aggregates and damaged organelles into double-membrane vesicles (autophagosomes), which fuse with lysosomes for enzymatic breakdown.

Key observations in neurodegeneration:

• Impaired autophagy: Defective autophagy is a hallmark of both AD and PD. Accumulation of autophagic vacuoles in neurons suggests a blockage in autophagosome-lysosome fusion.
• TFEB activation: Transcription factor EB (TFEB) is a master regulator of lysosomal biogenesis. Overexpression of TFEB enhances clearance of amyloid-β and α-synuclein in cellular and animal models.

Pharmacological Modulation of Autophagy

• mTOR inhibitors: Rapamycin and its analogs (rapalogs) induce autophagy by inhibiting mTORC1, a negative regulator of autophagy initiation.
• AMPK activators: Metformin, an AMPK activator, promotes autophagy and reduces amyloid-β burden in AD models.
• Lysosomal enzyme enhancement: Gene therapy approaches to increase glucocerebrosidase (GCase) activity—a lysosomal enzyme deficient in GBA-linked PD—improve α-synuclein clearance.

The Unfolded Protein Response: ER Stress and Beyond

The endoplasmic reticulum (ER) is a major site of protein folding. When misfolded proteins accumulate, the unfolded protein response (UPR) is activated to restore ER homeostasis. Persistent ER stress, however, triggers apoptosis—a contributor to neuronal loss in AD and PD.

UPR branches and neurodegeneration:

• PERK pathway: Phosphorylation of eIF2α attenuates global translation but upregulates ATF4, which promotes antioxidant responses. Chronic PERK activation exacerbates tau toxicity in AD.
• IRE1α-XBP1 pathway: XBP1 splicing enhances ER-associated degradation (ERAD). XBP1 deficiency worsens α-synuclein pathology in PD models.
• ATF6 pathway: ATF6 regulates chaperone expression. Its activation has been shown to reduce amyloidogenic processing of APP in AD.

Balancing UPR for Therapeutic Benefit

Modulating UPR requires precise control to avoid exacerbating stress responses. Small molecules like ISRIB (integrated stress response inhibitor) reverse eIF2α phosphorylation and improve memory in AD mice without triggering ER stress.

Emerging Technologies and Future Directions

The convergence of biotechnology and neuroscience is yielding novel tools to probe and manipulate the PN:

• PROTACs (Proteolysis-Targeting Chimeras): These bifunctional molecules recruit E3 ligases to degrade specific pathogenic proteins. Tau-targeting PROTACs have shown efficacy in reducing tau levels in neurons.
• CRISPR-based screens: Genome-wide screens identify PN modulators. For example, CRISPR knockout of USP14, a deubiquitinating enzyme, enhances proteasomal degradation of α-synuclein.
• Nanoparticle chaperones: Synthetic nanoparticles designed to mimic chaperones can selectively bind and dissolve amyloid fibrils.

The Challenge of Selective Modulation

A major hurdle in PN-targeted therapies is achieving cell-type specificity. Neurons and glia exhibit distinct PN dynamics, necessitating precise delivery systems (e.g., AAV vectors with neuronal promoters). Additionally, chronic PN modulation must avoid disrupting essential protein functions.

Conclusion: Toward Precision Medicine in Neurodegeneration

The proteostasis network represents a dynamic and multifaceted target for combating protein aggregation in AD and PD. While challenges remain—such as drug delivery across the blood-brain barrier and minimizing off-target effects—advances in PN modulation hold transformative potential. By harnessing molecular chaperones, UPS enhancers, autophagy inducers, and UPR modulators, we move closer to precision therapies that restore proteostasis and halt neurodegeneration at its root.