Chi3l1 deletion in APP/PS1 mouse model of Alzheimer's disease

PRIMARY OUTCOME

amyloid plaque burden and microglial phagocytic activity

EXPECTED OUTCOMES

1. **Amyloid plaque burden (Thioflavin-S)** in APP/PS1;Chi3l1^-/- will be 35–45% lower than APP/PS1;Chi3l1^+/+ at 6 months (expected: 12.4±3.2 vs 20.1±4.1 plaques/mm², p<0.01, Cohen's d=1.4) 2. **Aβ42 concentration (hippocampal ELISA)** in APP/PS1;Chi3l1^-/- will be 28–38% reduced at 6 months (expected: 847±156 vs 1182±203 pg/mg protein, p<0.005, d=1.6) 3. **Microglial process velocity** (2-photon imaging) will be 55–70% faster in APP/PS1;Chi3l1^-/- vs APP/PS1;Chi3l1^+/+ at 6 months (expected: 2.3±0.4 vs 1.2±0.3 µm/min, p<0.001, d=2.1) 4. **Phagocytosis assay (flow cytometry)** will show 2.8–3.5-fold increase in FITC+Aβ+ BV-2 cells with Chi3l1 deletion (expected: 34.2±6.1% vs 11.3±3.8%, p<0.0001, d=3.2) 5. **Colocalization of Aβ with CD68+ lysosomal compartments** will increase 2.1–2.6-fold (Pearson coefficient: 0.61±0.08 vs 0.28±0.07, p<0.001) 6. **CSF1R-PET signal** (global microglial density) will be 18–25% higher in APP/PS1;Chi3l1^-/- vs APP/PS1;Chi3l1^+/+ at 6 months (SUV: 1.42±0.18 vs 1.15±0.14, p<0.01, d=1.3) 7. **Nest building scores** will show 40% improvement in APP/PS1;Chi3l1^-/- vs APP/PS1;Chi3l1^+/+ at 6 months (score: 3.4±0.7 vs 2.4±0.8, p<0.05, d=1.1) ---

SUCCESS CRITERIA

- **Primary**: Thioflavine-S plaque burden reduction ≥30% in APP/PS1;Chi3l1^-/- vs APP/PS1;Chi3l1^+/+ at 6 months (two-tailed t-test, p<0.01, effect size d≥1.0) - **Secondary**: Ex vivo phagocytosis assay fold-change ≥2.0 in BV-2 cells (one-way ANOVA with Tukey post-hoc, F(3,44)=18.7, p<0.0001) - **Microglial morphology**: Process velocity increase ≥50% (Mann-Whitney U test, p<0.01) and branchpoint count increase ≥40% - **Biochemical validation**: Hippocampal Aβ42 reduction ≥25% (ELISA, p<0.01 after Bonferroni correction for 3 comparisons) - **Pathology staging**: Effect size at 3 months <effect size at 6 months <effect size at 9 months, confirming progressive phenotype interaction - **Confocal colocalization**: Pearson coefficient for Aβ/CD68 colocalization ≥0.50 in Chi3l1 KO (vs ≤0.30 in WT), indicating enhanced lysosomal processing - **Behavioral correlation**: If amyloid reduction is ≥30%, expect Y-maze alternation improvement of ≥15% relative to APP/PS1;Chi3l1^+/+ (mixed-model ANOVA with repeated measures, interaction p<0.05)

PROTOCOL

### Study Design - **Model**: APP/PS1 (APPswe/PSEN1dE9) x Chi3l1 knockout (C57BL/6J background) - **Animal Numbers**: n=12/genotype/group (power=0.80, α=0.05, expected effect size d=1.2) - **Sex**: Both sexes, balanced per group - **Housing**: Specific pathogen-free, 12h light/dark cycle --- ### PHASE 1: Colony Establishment & Genotyping (Week 0–4) **Timepoints**: 3–4 weeks of age (wean) **Methods**: - Cross APP/PS1 hemizygous mice with Chi3l1^+/- breeders on C57BL/6J - Genomic DNA extraction from ear punch: 50mM NaOH, 95°C 30min, neutralization - **Genotyping PCR**: - APP/PS1: primers 5'-GACTTGATGTTCTGTGTTCTGC-3' / 5'-TGACCACTCGACCAGGTT-3' (300bp mutant band) - Chi3l1 knockout: primers 5'-CTTCCTCGTGCTTTACGGTAT-3' / 5'-CCATGATGAAAGCTGGAAAG-3' (400bp WT, 600bp KO) - **Reagents**: DreamTaq PCR Master Mix (Thermo), 2% agarose gel, ethidium bromide **Controls**: - Positive control: known APP/PS1 and Chi3l1 KO DNA - Negative control: water template - Internal loading control: GAPDH primer pair --- ### PHASE 2: Baseline Behavioral Assessment (Week 8–10) **Timepoints**: 2 months of age, pre-intervention **Methods**: 1. **Open Field Test**: 40cm × 40cm arena, 5min session, automated tracking (Ethovision XT). Measure center time, total distance, velocity. 2. **Y-Maze Spontaneous Alternation**: 3-arm maze, 5min session, alternation % as hippocampal-independent working memory indicator. 3. **Nest Building Test**: 1g compressed cotton nest, 48h scoring (1–5 scale) as proxy for hippocampus-dependent behavior. **Equipment**: Noldus Ethovision XT 16, ANY-maze 7.0 --- ### PHASE 3: Experimental Intervention (Week 12–48) **Timepoints**: 3, 6, 9 months of age (early, mid, late pathology) **Methods**: - **APP/PS1;Chi3l1^+/+** (positive group): Standard chow - **APP/PS1;Chi3l1^-/-** (experimental group): Standard chow - **WT;Chi3l1^+/+** (negative control): Standard chow - **WT;Chi3l1^-/-** (knockout control): Standard chow **Reagents**: - Diet: Envigo Teklad 2918 irradiated chow - No pharmacological intervention (genetic deletion only) **Endpoint allocation**: 4 mice/sex/genotype at each timepoint (3, 6, 9 months) --- ### PHASE 4: In Vivo Microglial Imaging (Week 11, 23, 35) **Timepoints**: 1 week before sacrifice (2, 5, 8 months of age) **Methods**: 1. **TPH-1 Microglial Imaging** (5 animals/timepoint): - 5µg anti-Iba1-FITC (Wako #019-19741) + 50µg anti-P2RY12 (Abcam ab155624) IV - Window chamber implantation over somatosensory cortex - Intravital 2-photon microscopy (Zeiss LSM 7 MP, 910nm Ti:Sapphire) - **Parameters**: 1024×1024 pixels, 0.5Hz line scan, 60×1.0NA water immersion objective - Measures: microglial process velocity (µm/min), branchpoints/animal, dystrophy score (0–4) 2. **CSF1R-PET** (remaining animals, alternate week): - 250µCi ^64Cu-DOTA-anti-CSF1R (generated in-house, specific activity 1.2 GBq/µmol) - PET/CT scan at 2h post-injection (Siemens Inveon) - Standardized uptake value (SUV) normalized to muscle --- ### PHASE 5: Tissue Collection & Processing (Week 12, 24, 36) **Timepoints**: 3, 6, 9 months of age **Methods**: 1. **Perfusion**: 4% PFA in PBS, 50mL cardiac perfusion 2. **Brain dissection**: left hemisphere for biochemistry, right hemisphere for histology 3. **Cryopreservation**: 30% sucrose/PBS overnight, OCT-embedded, 40µm sagittal sections **Tissue allocation**: - Biochemical: cortex, hippocampus (flash-frozen in liquid N2, stored -80°C) - Histology: 12 sections per animal (Bregma -0.5 to -3.5mm) --- ### PHASE 6: Biochemical & Histological Analysis (Week 13–38) #### 6A: Amyloid Burden (ELISA) **Reagents**: - Amyloid Beta 40/42 High Sensitivity ELISA Kit (Thermo #010-020) - Sample: hippocampal homogenate in 8M urea/0.4% DEA - **Protocol**: 100µg protein per well, duplicate, overnight coating with anti-Aβ antibody 6E10 **Readout**: EnSpire 2300 plate reader, 450nm #### 6B: Thioflavin-S Amyloid Plaque Staining **Reagents**: - Thioflavin-S (Sigma T1892): 0.001% in PBS, 8min, destain 70%/50% EtOH 2min each **Quantification**: 5 regions × 3 sections per animal - Cortex (layers 4–6) - Hippocampus (CA1, CA3, dentate gyrus) - Subiculum **Equipment**: Zeiss Axio Imager Z2, 10 fields/region, automated particle analysis (ImageJ) #### 6C: Microglial Phagocytic Assay **Ex vivo assay**: - 100µg synaptically-enriched fraction from 6-month-old APP/PS1 brains - Incubate with 1×10^6 BV-2 cells (authenticated, mycoplasma-free) + 50µg FITC-labeled Aβ1-42 (rPeptide A-1003-1) - 24h incubation at 37°C, 5% CO2 **Reagents**: - Aβ1-42 fibrils: 2µM in PBS, 37°C 72h pre-aggregation - Phagocytosis inhibitor: Cytochalasin D 10µM (Sigma C8273) as control **Readout**: - Flow cytometry (BD FACSCanto II): FITC+ BV-2 cells (phagocytosis %) - Confocal microscopy: Aβ internalization colocalization with LAMP-1 (lysosome marker) **Markers by immunohistochemistry**: - Iba1 (1:500, Wako #019-19741) - CD68 (1:200, Abcam ab31636) - YKL-40 (Chi3l1) (1:200, R&D Systems AF2596) - 6E10 Aβ (1:1000, BioLegend #803001) **Secondary antibodies**: Alexa Fluor 488/594 (1:500, Invitrogen) ---

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