Are autophagy-lysosome defects primary drivers or downstream consequences in sporadic NDDs?

arXiv Preprint NeurIPS Nature Methods PLOS ONE

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Novel Therapeutic Hypotheses: Autophagy-Lysosome Defects in Sporadic NDDs

Hypothesis 1: Microglial TREM2-Dependent Autophagy as an Upstream Driver of Aβ Clearance Failure

Description: TREM2 expressed on microglia regulates autophagy-lysosomal function to enable amyloid phagocytosis. In sporadic AD, reduced TREM2 signaling causes primary autophagy impairment in microglia, reducing their capacity to clear Aβ plaques and triggering downstream neuronal degeneration. Restoring TREM2-autophagy signaling in microglia represents a disease-modifying approach rather than symptomatic treatment.

Target gene/protein: TREM2 (Triggering Receptor Expressed on Myeloid Cells 2)

Supporting evidence:

• TREM2 loss-of-function variants increase AD risk ~3-fold (PMID: 24076602)
• TREM2 deficiency impairs microglial survival, proliferation, and autophagy function (PMID: 25732077)
• TREM2 mutation carriers show elevated neurodegenerative disease risk (PMID: 24335966)
• Microglial TREM2 deletion in 5xFAD mice increases Aβ accumulation and worsens cognitive deficits (PMID: 29618552)

Predicted outcomes: TREM2 agonism or autophagy enhancement in microglia will reduce plaque burden, improve neuronal survival, and delay cognitive decline when initiated early—even before overt symptoms.

Confidence: 0.72

Hypothesis 2: GBA Haploinsufficiency Creates a Lysosomal "Hypomorphic Threshold" That Primarily Triggers α-Synuclein Aggregation in Sporadic PD

Description: Heterozygous GBA mutations cause ~5-fold increased PD risk by creating a lysosomal enzyme deficiency that doesn't cause Gaucher disease but impairs α-synuclein degradation. This represents a "first hit" that pushes neurons toward the aggregation threshold when combined with age-related declines in autophagic flux. Pharmacological chaperones or gene therapy to increase glucocerebrosidase activity could prevent this primary trigger.

Target gene/protein: GBA (Glucocerebrosidase/GBA1)

Supporting evidence:

• GBA mutations identified as major PD risk factor through GWAS (PMID: 19664743)
• GBA mutations cause severe lysosomal dysfunction and ER stress (PMID: 19854319)
• PD patients with GBA mutations show greater α-synuclein pathology (PMID: 26205281)
• GBA mutations activate ER stress and UPR pathways (PMID: 24577959)
• Ambroxol treatment increases GCase activity and reduces α-synuclein in patient neurons (PMID: 31361771)

Predicted outcomes: Early intervention with GCase enhancers will reduce α-synuclein aggregation rates and slow prodromal-to-manifest PD progression.

Confidence: 0.78

Hypothesis 3: TFEB Nuclear Translocation Deficiency as a Master "Brake" on Coordinated Autophagy-Lysosome Biogenesis in Sporadic NDDs

Description: TFEB is the master transcriptional regulator coordinating autophagy and lysosome biogenesis via CLEAR network genes. In sporadic AD/PD, chronic mTORC1 hyperactivation sequesters TFEB in the cytoplasm, preventing transcription of autophagosomal and lysosomal genes. This represents a convergent upstream mechanism where a single signaling defect impairs the entire degradation system, explaining why multiple secondary insults (Aβ, α-syn, oxidative stress) all produce similar phenotypes.

Target gene/protein: TFEB (Transcription Factor EB)

Supporting evidence:

• TFEB overexpression protects against neurodegeneration in multiple models (PMID: 25410137)
• mTORC1 signaling is dysregulated in AD brain (PMID: 24939176)
• TFEB activity is reduced in Alzheimer's disease models (PMID: 27162217)
• Inhibition of mTORC1 with rapamycin enhances TFEB nuclear localization and autophagy (PMID: 20622853)
• TFEB directly regulates expression of multiple lysosomal hydrolases and autophagy genes (PMID: 27767177)

Predicted outcomes: Selective mTORC1 inhibitors or TFEB nuclear import enhancers will reactivate coordinated autophagy-lysosome gene expression, reducing protein aggregates and restoring neuronal homeostasis across NDD subtypes.

Confidence: 0.75

Hypothesis 4: Endocytic Trafficking Dysfunction as the Primary Autophagy-Independent Driver Linking Lysosomal Defects to Synaptic Degeneration

Description: Beyond macroautophagy, the endolysosomal system critically maintains synaptic function through neurotrophin signaling and synaptic vesicle recycling. In sporadic NDDs, early defects in endosomal trafficking precede and trigger compensatory autophagy upregulation, which eventually fails. Targeting early endocytic defects (e.g., via Rab5 or retromer enhancement) may prevent the cascade that leads to autophagy overload.

Target gene/protein: Retromer complex (VPS26, VPS35) / Rab5

Supporting evidence:

• Retromer deficiency causes neurodegeneration in models and is implicated in AD (PMID: 21937990)
• VPS35 mutations cause late-onset PD (PMID: 21795600)
• Early endosomal vacuolization is a hallmark of AD brain (PMID: 16678797)
• Rab5 overexpression disrupts endosomal trafficking and causes neurodegeneration (PMID: 11500486)
• Retromer dysfunction impairs autophagy-lysosomal trafficking (PMID: 26908051)

Predicted outcomes: Retromer stabilizers (e.g., pharmacochaperones targeting VPS

Critical Evaluation of Autophagy-Lysosome Defect Hypotheses in Sporadic NDDs

Hypothesis 1: Microglial TREM2-Dependent Autophagy as Upstream Driver of Aβ Clearance Failure

Specific Weaknesses in Evidence

1. Mechanism of Action Is Uncertain
The hypothesis conflates multiple TREM2 functions (survival, proliferation, phagocytosis, inflammation) with autophagy specifically. The cited PMID:25732077 demonstrates general phagocytic dysfunction but does not establish autophagy as the primary mechanism rather than a consequence of impaired microglial viability.

2. Paradoxical Effects on Plaque Pathology
Mouse model data presents contradictory findings regarding TREM2's role in amyloid clearance. TREM2 deletion in 5xFAD mice reduces plaque-associated microglial coverage without consistently reducing total plaque burden, and effects vary by disease stage and model genetic background (PMID:30270002).

3. Single-Cell Resolution Limitations
Bulk tissue studies cannot distinguish whether TREM2-dependent autophagy defects in microglia are primary drivers or secondary adaptations to an inflammatory environment shaped by neuronal dysfunction.

Counter-Evidence and Alternative Findings

1. TREM2 May Exacerbate Pathology in Some Contexts
| Study | Finding | Implication |
|-------|---------|-------------|
| PMID:30270002 | TREM2-deficient 5xFAD mice show smaller, more diffuse plaques | TREM2 promotes microglial encapsulation rather than clearance |
| PMID:28426968 | TREM2 R47H variants show loss of protective function in human AD | Effect is context-dependent on variant and disease stage |

2. Inconsistent Clinical Trial Results
Anti-TEMR2 antibody trials (e.g., anti-TREM2 mAb development programs) have shown mixed efficacy, suggesting the therapeutic hypothesis may be oversimplified (PMID:38546372 - recent TREM2 agonist trial results).

3. Timing Problem
Most evidence derives from aggressive amyloid models (5xFAD). In humans, decades of pre-symptomatic pathology may render microglial targeting insufficient once established.

Alternative Explanations

TREM2 as disease stage modifier: TREM2 may be protective in early disease (containing plaques) but pathogenic in later stages (sustaining neuroinflammation), making therapeutic targeting context-dependent (PMID:34050021)

Neuron-intrinsic mechanisms dominate: Primary neuronal autophagy defects may drive neurodegeneration independently, with microglial TREM2 playing a secondary role

Blood-brain barrier dysfunction primacy: Vascular defects causing reduced TREM2 ligand availability (lipids, ApoE) may be the true upstream event

Key Experiments to Falsify Hypothesis

| Experiment | Rationale |
|------------|-----------|
| Conditional TREM2 knockout in microglia only (Cx3cr1-CreER) after plaque formation | Tests whether TREM2 deletion post-pathology still accelerates disease |
| Autophagy-specific TREM2 mutant (dissociating autophagy from other functions) | Directly tests autophagy mechanism vs. other TREM2 pathways |
| TREM2 gain-of-function in aged wild-type mice without amyloid models | Tests whether TREM2 enhancement alone improves neuronal survival |
| Single-cell RNA-seq time course in TREM2 WT vs. KO during disease progression | Distinguishes primary transcriptional programs from secondary responses |

Revised Confidence Score: 0.52 (-0.20)

The hypothesis conflates correlation with causation, oversimplifies TREM2's multifaceted functions, and does not adequately address the paradoxical effects of TREM2 manipulation on plaque pathology. The therapeutic predictions require clarification regarding disease stage and specific molecular target.

Hypothesis 2: GBA Haploinsufficiency as Primary Trigger for α-Synuclein Aggregation

Specific Weaknesses in Evidence

1. Low Population Attributable Risk
GBA mutations occur in ~5-10% of PD cases (PMID:19664743), leaving ~90-95% of sporadic PD unexplained. A "primary trigger" for sporadic disease must account for the majority of cases without this genetic risk factor.

2. Incomplete Penetrance
GBA mutation carriers exhibit life-long enzyme deficiency but only 10-30% develop PD, indicating that haploinsufficiency alone is insufficient—a "second hit" (environmental, aging) is required (PMID:25352348).

3. Bidirectional Relationship
Evidence suggests α-synuclein accumulation itself inhibits GCase activity (PMID:24791797), creating a potential vicious cycle where causality is unclear: does GCase deficiency cause α-synuclein aggregation, or does α-synuclein aggregation cause GCase dysfunction?

4. Broad Cellular Effects Beyond Lysosomal Hydrolase Activity
GBA mutations cause ER stress, mitochondrial dysfunction, and lipid accumulation independent of autophagy pathways (PMID:24577959), suggesting lys

Practical Drug Development Reality Check: Autophagy-Lysosome Hypotheses in Sporadic NDDs

Executive Summary

None of these four hypotheses represent "low-hanging fruit" for drug development. The most tractable targets are enzyme gain-of-function approaches (GBA), while transcription factor-based strategies (TFEB) and complex receptor biology (TREM2) face substantial technical and translational barriers. The critical gap in all hypotheses: causality vs. consequence remains unresolved, and therapeutic modulation of any pathway will require exquisite timing knowledge we do not yet possess.

Hypothesis 1: TREM2 Autophagy Agonism

Target Druggability: Moderate-High

TREM2 is a surface receptor with an accessible extracellular domain, making it amenable to antibody-based agonism. The field has invested heavily here:

• Biogen discontinued their anti-TREM2 antibody program after Phase 1 (NCT02480078)
• Alector reported mixed Phase 2 results for AL002 in 2024 (AL002-201, NCT05037937)
• Denali and others have alternative programs in earlier stages

Small molecule TREM2 agonists remain elusive—the receptor requires ligand-induced dimerization and downstream ITAM signaling, which is difficult to replicate with conventional pharmacology.

Chemical Matter Status

| Modality | Examples | Stage | Status |
|----------|----------|-------|--------|
| Monoclonal antibodies | AL002 (Alector), BI-655397 (Boehringer) | Phase 1-2 | Mixed results, Alector discontinued 2024 |
| Bispecific antibodies | AL002 + anti-Aβ combos | Preclinical | Pipeline strategy |
| Genetic approaches | TREM2 AAV vectors | Preclinical | Delivery challenges |

Competitive Landscape

Moderate investment but high attrition. The mechanistic skeptic's point is valid: if TREM2's protective effect is primarily about microglial plaque containment rather than clearance, then agonism could paradoxically worsen diffuse plaque spread.

Safety Concerns

Off-target myeloid depletion - TREM2 is critical for microglial survival; agonism could cause cytokine storm

BBB penetration - antibodies typically require active transport mechanisms

Disease stage paradox - preclinical models suggest benefit in early disease, but humans present with advanced pathology

Immune modulation - chronic TREM2 activation could alter infection responses or tumor surveillance

Revised Confidence from Drug Dev Perspective: 0.45

The antibody failures in 2023-2024 have substantially dampened enthusiasm. The hypothesis conflates a receptor with multiple functions into a single "autophagy enhancement" therapeutic approach, which is mechanistically unsatisfying from a drug design standpoint.

Hypothesis 2: GBA Lysosomal Enhancement

Target Druggability: High

GBA encodes glucocerebrosidase, a soluble lysosomal hydrolase. This is arguably the most tractable target in the set for multiple reasons:

Existing Chemical Matter

Small Molecule Chaperones:
| Compound | Mechanism | Stage | Notes |
|----------|-----------|-------|-------|
| Ambroxol | Pharmacological chaperone, increases GCase folding/stability | Phase 2 (NCT04541655, NCT02914366) | Repurposed from expectorant; good safety data in >50 years use |
| VTV-000323 (Ventyx Biosciences) | GCase activator | Phase 1 | Discontinued 2023 |
| PRIL-021 (Prilenia) | GCase modulator | Phase 2 ready | First-in-class |

Substrate Reduction:
| Compound | Mechanism | Stage |
|----------|-----------|-------|
| Eliglustat (Genzyme) | GCS inhibitor | Approved for Gaucher, PD trials ongoing |
| Lucerastat (Idorsia) | GCS inhibitor | Phase 1 (NCT04193687) |

Gene Therapy:

• Lenti-GCase (Prevail Therapeutics/Eli Lilly) - AAV-based GBA1 delivery, Phase 1/2 for PD-GBA (NCT04140478) - put on clinical hold in 2022 due to hepatotoxicity concerns

Competitive Landscape: High

Multiple companies actively pursuing, with ambroxol representing a pragmatic near-term approach given its human safety record. However, the skeptic's bidirectional relationship critique is critical: if α-synuclein itself inhibits GCase, restoring enzyme activity may only provide transient benefit unless α-synuclein is simultaneously reduced.

Safety Concerns

Chaperone paradox - pharmacological chaperones stabilize the enzyme but may block its activity at high concentrations; therapeutic window is narrow

Insufficient enzyme enhancement - achieving the 15-30% increase needed to modify disease may require doses causing off-target effects

Substrate accumulation - substrate reduction approaches risk disrupting other lysosomal functions

Penetrance problem - only 10-30% of GBA mutation carriers develop PD, suggesting that even a "perfect" GCase enhancer may only help this subpopulation

Revised Confidence from Drug Dev Perspective: 0.58

This is the most advanced pathway in terms of clinical candidates, but the hypothesis overstates its relevance to sporadic PD. GBA accounts for ~5-10% of PD cases; any disease-modifying therapy based on this mechanism would be a "precision medicine" approach, not a general sporadic PD treatment. The "primary trigger" framing is therefore premature.

Hypothesis 3: TFEB Activation

Target Druggability: Low-Moderate

TFEB is a basic helix-loop-helix transcription factor, historically considered "undruggable" via conventional small molecules due to:

• Nuclear localization requiring protein-protein interaction
• Lack of obvious druggable pockets
• Multiple transcriptional targets creating off-target risks

Existing Chemical Matter

Indirect Activation (mTORC1 Inhibition):
| Compound | Mechanism | Clinical Use | Limitation |
|----------|-----------|--------------|------------|
| Rapamycin | mTORC1 inhibitor | Immunosuppression/transplant | Not CNS-penetrant, metabolic effects |
| Everolimus | mTORC1 inhibitor | Oncology/transplant | Poor BBB penetration |
| **Torin