Cx43 and GJA1-20k overexpression effects on mitochondrial transfer

Phase 1: Cell Culture & Baseline Characterization (Days 1-7)

Day 1-2: MSC and chondrocyte culture establishment

• Obtain human bone marrow-derived MSCs (Lonza, cat. #PT-2501) and immortalized human chondrocytes (TC28a2, Sigma-Aldrich, cat. #94020204)
• Culture MSCs in α-MEM (Gibco, cat. #12571063) supplemented with 10% FBS (Gibco, cat. #16000044), 1% penicillin-streptomycin (Gibco, cat. #15140122), and 2 mM L-glutamine (Gibco, cat. #25030081) at 37°C, 5% CO₂
• Culture chondrocytes in DMEM high glucose (Gibco, cat. #11965092) supplemented with 10% FBS and 1% penicillin-streptomycin at 37°C, 5% CO₂
• Confirm MSC identity by flow cytometry: CD73⁺, CD90⁺, CD105⁺, CD34⁻, CD45⁻ (BD Biosciences, cat.

Phase 1: Cell Culture & Baseline Characterization (Days 1-7)

Day 1-2: MSC and chondrocyte culture establishment

• Obtain human bone marrow-derived MSCs (Lonza, cat. #PT-2501) and immortalized human chondrocytes (TC28a2, Sigma-Aldrich, cat. #94020204)
• Culture MSCs in α-MEM (Gibco, cat. #12571063) supplemented with 10% FBS (Gibco, cat. #16000044), 1% penicillin-streptomycin (Gibco, cat. #15140122), and 2 mM L-glutamine (Gibco, cat. #25030081) at 37°C, 5% CO₂
• Culture chondrocytes in DMEM high glucose (Gibco, cat. #11965092) supplemented with 10% FBS and 1% penicillin-streptomycin at 37°C, 5% CO₂
• Confirm MSC identity by flow cytometry: CD73⁺, CD90⁺, CD105⁺, CD34⁻, CD45⁻ (BD Biosciences, cat. #562245, #555596, #55061)
• Confirm chondrocyte identity by collagen II immunostaining (Abcam, cat. #ab185430, 1:200) and aggrecan expression (Abcam, cat. #ab3778, 1:200)
• Assess baseline mitochondrial content using MitoTracker Green FM (Thermo Fisher, cat. #M7514, 100 nM, 30 min at 37°C) and MitoSOX Red (Thermo Fisher, cat. #M36008, 5 µM, 10 min at 37°C) for superoxide detection

• Seed MSCs at 5 × 10⁴ cells/cm² in 6-well transwell inserts (Corning, cat. #3450, 0.4 µm pore) for indirect co-culture
• Seed chondrocytes at 8 × 10⁴ cells/cm² in the lower chamber
• Allow cells to attach for 24 hours before experimental manipulation

• Pre-label MSC mitochondria with MitoTracker Deep Red FM (Thermo Fisher, cat. #M22413, 200 nM, 30 min at 37°C) according to manufacturer's protocol
• Wash cells 3× with PBS and incubate in fresh medium for 1 hour to remove excess dye
• Image baseline:chondrocytes using EVOS M5000 (Thermo Fisher) at 20× magnification, n=5 fields/well, n=3 wells/condition
• Quantify mitochondrial transfer by counting MitoTracker-positive particles in chondrocytes using ImageJ (FIJI) with a custom macro (see Endpoint Measurement section)

Phase 2: Viral Vector Generation & Transduction (Days 8-16)

Day 8-10: Plasmid preparation

• Obtain pLVX-Cx43-Puro and pLVX-GJA1-20k-Puro expression vectors (custom constructs, available from Addgene or generated via Gibson assembly)
• Sequence-verify constructs using Sanger sequencing (Eurofins) with primers spanning the GJA1 coding region and 3' UTR
• Transform into NEB 5-alpha competent E. coli (NEB, cat. #C2987) and purify using EndoFree Plasmid Maxi Kit (Qiagen, cat. #12362)

• Culture HEK293T cells (ATCC, cat. #CRL-3216) in DMEM high glucose + 10% FBS at 37°C, 5% CO₂
• Co-transfect HEK293T with: (i) pLVX-Cx43 or pLVX-GJA1-20k, (ii) psPAX2 packaging plasmid (Addgene, cat. #12260), and (iii) pMD2.G envelope plasmid (Addgene, cat. #12259) using Lipofectamine 3000 (Thermo Fisher, cat. #L3000001) at 1:1 DNA:Lipofectamine ratio
• Use 2 µg pLVX plasmid + 1.5 µg psPAX2 + 0.5 µg pMD2.G per 10 cm dish
• Harvest viral supernatant at 48 hours and 72 hours post-transfection
• Concentrate virus using Lenti-X Concentrator (Takara, cat. #631232) according to manufacturer protocol
• Titrate virus using Lenti-X qRT-PCR Titration Kit (Takara, cat. #631280)

• Seed MSCs at 3 × 10⁴ cells/cm² in 6-well plates
• Transduce with lentivirus at MOI of 5 in the presence of 8 µg/mL polybrene (Sigma-Aldrich, cat. #TR-1003-G)
• Select with 1 µg/mL puromycin (Gibco, cat. #A1113803) for 7 days starting 48 hours post-transfection
• Confirm overexpression by: (i) Western blot using anti-Cx43 antibody (Cell Signaling Technology, cat. #3512S, 1:1000) and anti-GJA1-20k antibody (custom anti-GJA1-20k, 1:500), (ii) immunofluorescence (same antibodies, 1:200), (iii) qRT-PCR (see below)

Phase 3: Overexpression Validation (Days 17-21)

qRT-PCR validation

• Extract total RNA using RNeasy Mini Kit (Qiagen, cat. #74104)
• Reverse transcribe 1 µg RNA using SuperScript IV VILO (Thermo Fisher, cat. #11754050)
• Perform qRT-PCR on QuantStudio 7 (Applied Biosystems):
• Cx43 (GJA1): Forward: 5'-CCCTCCAGCTCTTCTCTG-3', Reverse: 5'-GGCATTTACACTTTGGCAGC-3' (targeting all GJA1 splice variants)
• GJA1-20k specific: Forward: 5'-GGCTGGTCAAGAGCAGTG-3', Reverse: 5'-CCAGCCCATGAGAAGCAG-3' (targeting exon 5-specific splice junction)
• Housekeeping: GAPDH (Applied Biosystems, cat. # Hs99999905_m1)
• Calculate fold change using ΔΔCt method; expected ΔCt for Cx43-OE: cycle threshold shift of ≥3 cycles vs. empty vector control

• Lyse cells in RIPA buffer (Thermo Fisher, cat. #89900) with protease/phosphatase inhibitor (Thermo Fisher, cat. #78440)
• Load 30 µg protein/lane on 12% SDS-PAGE gel
• Transfer to PVDF membrane (Bio-Rad, cat. #1620177)
• Block in 5% BSA/TBST for 1 hour at room temperature
• Primary antibodies: Cx43 (CST #3512S, 1:1000), GJA1-20k (custom, 1:500), β-actin (Cell Signaling Technology, cat. #4970S, 1:2000) as loading control
• Secondary: HRP-conjugated anti-rabbit IgG (Cell Signaling Technology, cat. #7074S, 1:3000)
• Develop with SuperSignal West Pico PLUS (Thermo Fisher, cat. #34580) and image on ChemiDoc (Bio-Rad)
• Expected: Cx43-OE shows ~3-5× band intensity vs. empty vector; GJA1-20k-OE shows band at ~20 kDa absent in vector control

• Culture transduced MSCs on glass-bottom 96-well plates (Brooks Life Sciences, cat. #MGB096-1-2-LG-L)
• Load donor cells with calcein AM (Thermo Fisher, cat. #C3100MP, 1 µM, 30 min at 37°C) and CMRA (Thermo Fisher, cat. #C34554, 2 µM, 30 min at 37°C)
• Co-culture with unlabelled acceptor chondrocytes at 1:1 ratio
• Measure fluorescent dye transfer by live-cell imaging on Opera Phenix (PerkinElmer) at 10× every 5 minutes for 60 minutes
• Quantify gap junction functional coupling as the rate of calcein transfer from MSCs to chondrocytes

Phase 4: Mitochondrial Transfer Assay (Days 22-35)

Transwell co-culture model

• Seed MitoTracker Deep Red-labeled Cx43-OE or GJA1-20k-OE MSCs (5 × 10⁴ cells/cm²) in transwell inserts (0.4 µm pore)
• Seed chondrocytes (8 × 10⁴ cells/cm²) in lower chamber
• Use the following conditions (n=6 per condition):

• For siRNA conditions: transfect with Lipofectamine RNAiMAX (Thermo Fisher, cat. #13778100) at 25 nM siRNA per well of a 6-well plate, 48 hours before co-culture
• Incubate co-cultures for 72 hours at 37°C, 5% CO₂

• Seed chondrocytes at 4 × 10⁴ cells/cm² in glass-bottom 24-well plates
• After 24 hours, add MitoTracker-labeled MSCs at 2 × 10⁴ cells/cm² directly to chondrocyte monolayer (direct cell-cell contact)
• Co-culture for 24, 48, and 72 hours

Phase 5: Endpoint Measurements (Days 29-35)

5.1 Mitochondrial transfer quantification

• Fix cells with 4% paraformaldehyde (Electron Microscopy Sciences, cat. #15710) for 15 minutes at room temperature
• Permeabilize with 0.1% Triton X-100 (Sigma-Aldrich, cat. #T9284) for 10 minutes
• Block with 5% normal goat serum (Jackson ImmunoResearch, cat. #005-000-121) for 1 hour
• Stain with anti-Tom20 (mitochondrial import receptor, Santa Cruz, cat. #sc-17764, 1:200) to confirm mitochondrial identity
• Counterstain nuclei with DAPI (Thermo Fisher, cat. #62248, 1:1000)
• Acquire images on Zeiss LSM 980 Airyscan 2 confocal microscope at 63× oil immersion (NA 1.4)
• Use Z-stack imaging (0.5 µm step, 15 stacks per field) to resolve mitochondrial particles within chondrocyte cytoplasm
• Quantify mitochondrial transfer using Imaris 10.0 (Bitplane):
• Create surfaces for MitoTracker-positive particles in chondrocyte region
• Measure total MitoTracker signal intensity per chondrocyte nucleus
• Count number of discrete mitochondrial particles per chondrocyte
• n=50 chondrocytes per condition across 3 independent experiments

• Cell viability: Seed chondrocytes in 96-well plates (5 × 10³ cells/well) after co-culture recovery. Measure ATP content using CellTiter-Glo (Promega, cat. #G7570) after 24 hours culture
• Apoptosis assay: Stain with Annexin V-FITC / 7-AAD (BD Biosciences, cat. #559763) and analyze on BD LSRFortessa; gate: Annexin V⁺/7-AAD⁻ = early apoptosis, Annexin V⁺/7-AAD⁺ = late apoptosis
• Mitochondrial membrane potential (ΔΨm): Stain with TMRE (Thermo Fisher, cat. #T669, 100 nM, 30 min at 37°C) and measure on plate reader (EnVision, PerkinElmer) at ex/em 549/575 nm
• Intracellular ROS: Stain with CellROX Green (Thermo Fisher, cat. #C10444, 5 µM, 30 min at 37°C) and measure on flow cytometer

• IL-1β treatment model: Treat chondrocytes with 10 ng/mL IL-1β (R&D Systems, cat. #201-LB) for 24 hours to induce osteoarthritic phenotype before co-culture
• Inflammatory cytokine panel: Measure conditioned medium from co-cultures using Luminex Human Cytokine 44-plex (Thermo Fisher, cat. #EPX450-12123-901) focusing on: IL-6, IL-8, TNF-α, MMP-1, MMP-13, ADAMTS-4, ADAMTS-5
• ECM deposition: Measure sulfated glycosaminoglycan (sGAG) content in culture medium using DMMB assay (Biocolor, cat. #BMPDGAG1000); normalize to cell number
• Collagen II expression: Stain with anti-collagen II (Abcam, cat. #ab185430, 1:200) and analyze by flow cytometry; measure mean fluorescence intensity

• Assess Cx43, GJA1-20k, p-Akt (Cell Signaling Technology, cat. #4060S, 1:1000), total Akt (CST #4691S, 1:1000), PGC-1α (CST #72487S, 1:1000), TFAM (CST #7495S, 1:1000), and mitochondrial dynamics proteins: MFN1 (CST #14739S), MFN2 (CST #9482S), OPA1 (CST #80471S)

• Dissociate co-cultures with TrypLE Express (Gibco, cat. #12604021)
• Stain surface with anti-CD90-FITC (MSC marker, BD Biosciences, cat. #555595) and anti-CD73-PE (BD Biosciences, cat. #550257)
• Fix and permeabilize for mitochondrial staining with anti-Tom20-AF647 (Santa Cruz, cat. #sc-17764 AF647)
• Gate on CD90⁻/CD73⁻ (chondrocytes) and measure Tom20-AF647 MFI
• Analyze on BD LSRFortessa; collect 20,000 events per sample

Phase 6: Data Analysis & Statistical Framework (Days 36-42)

Data collection

• Export all raw data to GraphPad Prism 10.2 for statistical analysis
• Blinded analysis: all image quantification performed by two independent observers blinded to condition

• One-way ANOVA with Tukey's post-hoc test for multi-group comparisons
• Two-way ANOVA for time-course experiments (time × condition interaction)
• Effect size reported as Cohen's d or partial η²
• Normality assessed by Shapiro-Wilk test; non-parametric alternatives (Kruskal-Wallis) used when appropriate
• Significance threshold: α = 0.05

• Based on pilot data (n=3): expected effect size for mitochondrial transfer = 0.8 (Cohen's d)
• Power = 0.80, α = 0.05 → minimum n = 6 per group
• Three independent biological replicates (different donor MSC lines) required