Mitochondrial ubiquitin ligase alleviates Alzheimer's disease pathology via blocking the toxic amyloid-β oligomer generation

The study investigates how mitochondrial dysfunction, specifically via downregulation or deletion of the mitochondrial E3 ubiquitin ligase MITOL/MARCH5, influences amyloid-β (Aβ) aggregation pathways and Alzheimer’s disease (AD) pathology. While Aβ plaques are hallmarks of AD, accumulating evidence indicates soluble Aβ oligomers are highly pathogenic and correlate better with clinical severity. Mitochondrial dynamics (fusion/fission) are critical for maintaining mitochondrial quality, and AD-related conditions disrupt these processes. Prior database analyses (Alzbase) suggested MITOL is downregulated in AD, and MITOL regulates mitochondrial morphology and ER–mitochondria contacts. The central question is whether perturbing mitochondrial dynamics through MITOL loss shifts Aβ aggregation toward toxic oligomer production by enhancing fibril-mediated seeding, thereby exacerbating cognitive decline, despite not increasing spontaneous plaque formation.

Prior work shows soluble Aβ oligomers are potent synaptotoxins and correlate with cognitive impairment; familial Aβ variants (e.g., E22A) preferentially form oligomers without plaques and cause early-onset AD. Mitochondrial dysfunction is a common hallmark in AD, with fusion/fission dynamics essential for mitochondrial quality control. AD-related pathologies can impair these dynamics. MITOL/MARCH5 is a mitochondrial outer membrane E3 ligase that ubiquitinates fusion/fission machinery and modulates ER–mitochondria tethering, thereby tuning mitochondrial morphology and function. An integrative AD gene dysregulation database (Alzbase) indicates MITOL downregulation in AD. The literature on how mitochondrial pathophysiology affects Aβ aggregation in vivo is mixed: different mitochondrial perturbations have been reported to both increase and decrease plaque burden. Emerging biophysical literature emphasizes secondary nucleation/assembly on fibril surfaces as a dominant pathway for Aβ aggregate generation compared to spontaneous self-assembly, implying that fibril properties can catalyze further oligomer/fibril formation.

• Models: APPswe/PSEN1dE9 (APP/PS1) transgenic mice as an Aβ pathology model; neuron-specific MITOL knockout achieved by crossing MITOL floxed (MITOL F/F) mice with Emx1-Cre to delete MITOL in cerebral cortex and hippocampus. Groups included Non-Tg, MITOL F/F, APP/PS1, and MITOL−/− APP/PS1.
• MITOL expression analyses: Immunoblotting and qRT-PCR in mouse cortex; cellular model using SH-SY5Y cells stably expressing APPswe with siRNA knockdown of PS1 (siPS1), with γ-secretase inhibitor DAPT to assess Aβ-dependence of MITOL regulation.
• Mitochondrial structure and function: Transmission electron microscopy (TEM) to quantify mitochondrial size distribution and cristae morphology (classified as normal Class I or abnormal Classes II–III). Cytochrome c oxidase (COX) histochemical staining in brain sections to assess respiratory activity. In vitro mitochondrial ATP production assay on isolated mitochondria. Oxygen consumption rate (Seahorse) in SH-SY5Y APPswe+siPS1 cells with or without MITOL knockdown.
• Behavioral assays: Novel object recognition (recognition memory), Y-maze (spatial working memory), Barnes maze (learning and memory), open-field locomotor activity controls.
• Neurodegeneration and neuroinflammation: Nissl (cresyl violet) staining to count neurons and soma size; hippocampal synaptic markers (synaptophysin, PSD-95) by immunoblot; microglia activation by IBA1 immunostaining and CD68 mRNA; hippocampal cytokine mRNAs (TNFα, IL-1β, IL-6) by qRT-PCR.
• Aβ metabolism and plaques: Human app mRNA by qRT-PCR; APP, α-CTF, β-CTF by immunoblot; Aβ-degrading enzyme mRNAs (IDE, NEP, MMP2, MMP9). Plaque assessment with dual staining: anti-Aβ 6E10 (overall plaque area) and Thioflavin S (ThS; fibrillar core). ImageJ quantification of plaque number, overall area (Aβ+), and core area (ThS+); classification of plaques (toxic ThS+ vs non-toxic ThS−).
• Aβ fibril purification and seeding assays: Amyloid-enriched fractions purified from brain; Thioflavin T (ThT) kinetics to assess secondary fibril formation from seed-free Aβ40 monomers with equal amounts of purified fibril seeds from APP/PS1 vs MITOL−/− APP/PS1 brains; calculation of lag phase and elongation rates. ELISAs for Aβ species (Aβ38/40/42/43) in fibril fractions.
• Oligomer-focused assays: TBS-soluble brain fractions analyzed by ELISA for Aβ40, Aβ42, and oligomer-specific ELISA (24B3). Dot blot with oligomer-specific antibodies (11A1). Bis-ANS fluorescence assay to monitor secondary oligomerization seeded by purified fibrils. Conformation-specific ELISA with A11 (off-pathway, antiparallel β-sheet) and OC (on-pathway, parallel β-sheet) antibodies to categorize oligomer types. Time-course analyses including pre-plaque age (3 months).
• Cell toxicity assays: SH-SY5Y exposure to mixtures of seed-free Aβ40 monomers and purified brain-derived fibrils; LDH release (cell death) and Caspase-Glo 3/7 activity after 48 h.
• Pharmacology: Rifampicin (RFP) administration per prior protocols to inhibit Aβ oligomerization; assessment of effects on soluble oligomers (ELISA), synaptophysin, microglia activation (IBA1), and cognition (Barnes maze).

• MITOL downregulation by Aβ: In 15-month APP/PS1 mouse cortex, MITOL protein and mRNA were reduced compared to non-transgenic controls; similar reductions occurred in APPswe+siPS1 cells, reversible by γ-secretase inhibition (DAPT), indicating Aβ-dependent transcriptional suppression of MITOL.
• Mitochondrial pathology with MITOL loss: In APP/PS1 brains, MITOL deletion markedly increased small mitochondria and inner membrane abnormalities (dilated cristae, empty matrices). COX staining intensity decreased and mitochondrial ATP production was reduced. In APPswe+siPS1 cells, MITOL knockdown lowered oxygen consumption. Residual MITOL (~40% of non-Tg) in APP/PS1 appeared protective for morphology.
• Worsened cognition and neurodegeneration: MITOL−/− alone did not impair behavior, while APP/PS1 showed mild deficits; MITOL−/− APP/PS1 mice had the most severe impairments in novel object recognition, Y-maze alternation, and Barnes maze latency. Nissl-positive neuron number and soma size declined; synaptophysin and PSD-95 decreased in hippocampus; microglial activation increased (IBA1, CD68), with elevated TNFα, IL-1β, IL-6 transcripts.
• Aβ metabolism unaffected: MITOL deletion did not alter human app mRNA, APP full-length or CTFs, nor hippocampal expression of Aβ-degrading enzymes (IDE, NEP, MMP2, MMP9), indicating effects occur independently of Aβ production/clearance changes.
• Plaque burden unchanged but cores expanded: Total plaque number and overall Aβ-positive plaque area (6E10) were unchanged by MITOL deletion. However, ThS-positive fibrillar core areas increased and the fibrillized-region rate within plaques rose from 0.34 (APP/PS1) to 0.60 (MITOL−/− APP/PS1), with Spearman correlations between total and cored areas increasing from R=0.66 to R=0.82. Non-fibrillar ThS− plaques decreased in MITOL−/− APP/PS1.
• Elevated seeding activity of fibrils: Purified, normalized Aβ fibrils from MITOL−/− APP/PS1 brains shortened the ThT lag time more than APP/PS1-derived fibrils, demonstrating stronger seeding of fibril formation. Aβ43 was specifically enriched in fibrils from MITOL−/− APP/PS1 brains.
• Excess soluble Aβ and oligomers: TBS-soluble Aβ42 and Aβ40 increased in MITOL−/− APP/PS1 cortex; oligomer-specific ELISA and dot blots showed higher oligomer levels. At 3 months (pre-plaque), soluble Aβ levels were similar between groups, suggesting oligomer increases depend on plaque-associated secondary processes.
• Secondary oligomerization enhanced: Bis-ANS assays showed that fibrils from MITOL−/− APP/PS1 brains not only shortened lag time but also accelerated secondary oligomer formation from seed-free Aβ40 monomers more than APP/PS1 fibrils. A11-positive (off-pathway) oligomers were preferentially increased, while OC-positive (on-pathway) oligomers were unchanged by MITOL status.
• Toxicity requires monomers plus fibril seeds and is higher with MITOL−/− seeds: Fibrils alone had little toxicity, but in combination with free Aβ monomers they increased LDH release and caspase-3/7 activity, with stronger effects for MITOL−/− APP/PS1-derived fibrils.
• Oligomer-targeting therapy rescues phenotypes: Rifampicin reduced soluble oligomers, restored synaptophysin and reduced microglial activation, and improved Barnes maze performance in MITOL−/− APP/PS1 mice, without affecting locomotion, supporting oligomer-dependent pathology.

The findings demonstrate that neuronal MITOL deletion perturbs mitochondrial dynamics and function and, crucially, enhances the seeding activity of Aβ fibrils. This does not increase spontaneous plaque formation but expands the fibrillar core within existing plaques and drives a strong secondary assembly pathway that preferentially generates toxic, dispersible, off-pathway (A11+) Aβ oligomers. This mechanism explains worsened neuroinflammation, synaptic loss, and cognitive decline in MITOL−/− APP/PS1 mice and aligns with clinical observations that soluble oligomers correlate with disease severity more than plaques. The data support a model in which AD pathogenesis can be driven either by increased self-assembly (yielding both oligomers and plaques) or by enhanced secondary assembly on fibril surfaces (yielding a predominance of toxic oligomers with limited additional plaque growth). The observed enrichment of Aβ43 in fibrils from MITOL-deficient brains suggests that altered fibril composition and morphology may underlie increased catalytic seeding efficiency and bias towards generating off-pathway oligomers, though causality remains to be established. Overall, mitochondrial morphology and MITOL expression emerge as modulators that direct Aβ aggregation pathways toward oligomer versus plaque production, with significant implications for disease classification and therapy targeting oligomeric Aβ.

This study identifies MITOL/MARCH5 as a protective regulator in AD-related Aβ pathology. Neuronal MITOL loss disrupts mitochondrial dynamics and enhances the seeding activity of Aβ fibrils, expanding fibrillar plaque cores and, critically, promoting excessive secondary generation of toxic, off-pathway Aβ oligomers that drive neurodegeneration and cognitive deficits. MITOL expression levels and mitochondrial pathophysiology may help classify AD patients by dominant Aβ aggregation mechanisms and guide oligomer-focused interventions. Future work should elucidate molecular links between mitochondrial dysfunction and Aβ seeding, define how fibril composition (e.g., Aβ43 enrichment) alters secondary assembly and oligomer structure, investigate neuronal primary seeding sources, and assess translational strategies to restore MITOL function or block fibril-catalyzed oligomerization in vivo.

• Mechanistic gaps: The precise molecular connection between mitochondrial pathophysiology from MITOL deletion and increased Aβ fibril seeding activity is not defined. Whether Aβ43 enrichment directly causes enhanced seeding and off-pathway oligomer generation remains unresolved.
• Model specificity: Findings are in APP/PS1 mice with neuron-specific MITOL deletion and in human neuroblastoma cells; generalizability to sporadic human AD may be limited.
• Extracellular vs intracellular origins: The proposed role of intraneuronal seeds and their modulation by mitochondrial dysfunction is hypothesized but not directly demonstrated.
• Scope of Aβ metabolism: Although major APP processing and key degrading enzymes were assayed, other metabolic pathways or clearance mechanisms may contribute and were not exhaustively tested.
• Plaque assessment methods: Differences between antibody and ThS staining emphasize that conclusions depend on detection modality; ultrastructural and compositional plaque analyses could be expanded.