Seahorse XFe96 Mito Stress Test for Oxidative Phosphorylation Profiling in Adherent C2C12 Myoblasts

Cells are seeded in a Seahorse XF96 cell culture microplate at 1.5 x 10^4 cells/well in the 80 µL central wells (background-correction wells A1, A12, H1, H12 receive medium only, no cells). Each experimental condition uses a minimum of 5-6 technical replicate wells; conditions are interleaved across the plate (not blocked by column) to control for positional/edge effects, with column 1 and 12 reserved as edge-effect monitors. Run 3 independent biological replicates (separate passages/seedings on different days). Time points: 3 baseline OCR measurements followed by 3 measurements after each of oligomycin, FCCP, and rotenone/antimycin A injections (12 measurement cycles total, ~90 min). Final drug concentrations are titrated per cell line; defaults: oligomycin 1.5 µM, FCCP 1.0 µM, rotenone 0.5 µM + antimycin A 0.5 µM.

BSL-1; C2C12 is a non-pathogenic murine line. FCCP, oligomycin, rotenone, and antimycin A are acutely toxic mitochondrial poisons — handle stocks in a fume hood, wear nitrile gloves and lab coat, and avoid skin contact and aerosols. DMSO enhances dermal absorption of co-dissolved toxins. Collect drug-containing medium as chemical hazardous waste; do not pour down the drain. Decontaminate the cartridge and plate as biohazard solid waste.

Negative/background control: cell-free wells (A1, A12, H1, H12) containing only assay medium correct for non-cellular sensor drift. Vehicle control: wells treated with the same DMSO volume (<0.1% v/v) as the perturbation condition. Positive technical control: the rotenone/antimycin A injection itself is a built-in positive control that should collapse OCR to non-mitochondrial levels in every well, confirming the assay measured genuine mitochondrial respiration. An untreated baseline condition establishes the reference bioenergetic phenotype. Include a known uncoupler-response control well set to verify FCCP titration is optimal.

Healthy C2C12 baseline OCR is typically 80-200 pmol/min/well at 1.5 x 10^4 cells. Oligomycin should drop OCR by 40-70% (ATP-linked fraction). FCCP should raise OCR above baseline to maximal respiration (1.5-3x basal); spare respiratory capacity is typically 50-150% of basal. Rotenone/antimycin A should reduce OCR to 10-30% of basal (non-mitochondrial). A bioenergetic defect appears as reduced maximal respiration and spare capacity; CV across technical replicates should be <15%.

• C2C12 mouse myoblast line (ATCC CRL-1772), passage <15, mycoplasma-negative
• Seahorse XF96 cell culture microplate and XFe96 sensor cartridge (Agilent 102416-100)
• Seahorse XF DMEM medium, pH 7.4 (Agilent 103575-100) supplemented with 10 mM glucose, 1 mM pyruvate, 2 mM glutamine
• Seahorse XF Calibrant solution (Agilent 100840-000)
• Growth medium: DMEM high-glucose (Gibco 11965) + 10% FBS + 1% penicillin/streptomycin
• Oligomycin A (Sigma 75351), 5 mM stock in DMSO
• FCCP (Sigma C2920), 5 mM stock in DMSO
• Rotenone (Sigma R8875) and Antimycin A (Sigma A8674), 5 mM stocks in DMSO
• Hoechst 33342 (Thermo 62249) for post-run nuclear count normalization, or BCA protein assay kit (Pierce 23225)
• Non-CO2 37 °C incubator

To measure real-time oxygen consumption rate (OCR) in adherent C2C12 mouse myoblasts using the Agilent Seahorse XFe96 analyzer and the sequential injection of oligomycin, FCCP, and rotenone/antimycin A, thereby resolving the canonical Mito Stress Test bioenergetic parameters. The protocol generates per-well kinetic OCR traces that are decomposed into basal respiration, ATP-linked respiration, proton leak, maximal respiration, spare respiratory capacity, and non-mitochondrial oxygen consumption, normalized to cell number or total protein.

1. Day 0: Hydrate the XFe96 sensor cartridge by adding 200 µL Seahorse Calibrant to each well of the utility plate, seat the sensor, and incubate overnight at 37 °C in a non-CO2 incubator.
2. Day 0: Seed C2C12 at 1.5 x 10^4 cells/well in 80 µL growth medium; leave background wells with medium only. Incubate 1 h at RT in the hood to ensure even attachment, then 18-24 h at 37 °C/5% CO2.
3. Day 1: Confirm a uniform near-confluent monolayer by microscopy. Prepare assay medium (XF DMEM + 10 mM glucose, 1 mM pyruvate, 2 mM glutamine), warm to 37 °C, adjust pH to 7.4.
4. Wash cells twice with 180 µL assay medium, leaving 180 µL final volume. Equilibrate 45-60 min at 37 °C in a non-CO2 incubator.
5. Prepare 10x injection stocks in assay medium: port A oligomycin (15 µM), port B FCCP (10 µM), port C rotenone+antimycin A (5 µM each). Load 20 µL (A), 22 µL (B), 25 µL (C) into the cartridge ports.
6. Calibrate the cartridge in the XFe96, then run the Mito Stress Test template: 3x baseline, then injections A/B/C each followed by 3 measurement cycles (mix 3 min, wait 0 min, measure 3 min).
7. After the run, aspirate medium, add 50 µL Hoechst (10 µg/mL), image and count nuclei per well (or run BCA) for normalization.

Independent variable: the metabolic perturbation (treatment vs. vehicle) and the sequential ETC drug injections. Dependent variables: OCR (pmol O2/min) at each measurement and the derived parameters (basal respiration, ATP-linked respiration, proton leak, maximal respiration, spare respiratory capacity, non-mitochondrial OCR). Controlled variables: cell seeding density, passage number, assay medium composition and pH (7.4), temperature (37 °C non-CO2), measurement cycle timing, drug concentrations, and normalization method (cells/well or µg protein).

We hypothesize that a treatment impairing electron transport chain function (e.g., complex I inhibition or mitochondrial biogenesis suppression) will reduce maximal respiration and spare respiratory capacity by at least 25% relative to vehicle-treated controls, while non-mitochondrial oxygen consumption remains unchanged. If the perturbation instead uncouples respiration, proton leak will rise and ATP-linked respiration will fall without a drop in maximal respiration.

Export OCR data from Wave software (Agilent). Subtract non-mitochondrial OCR (post rotenone/antimycin A) from all values. Compute basal = (last baseline) - (non-mito); ATP-linked = (basal) - (post-oligomycin); proton leak = (post-oligomycin) - (non-mito); maximal = (post-FCCP) - (non-mito); spare capacity = maximal - basal. Normalize each parameter to nuclei count (Hoechst) or µg protein (BCA) per well. Exclude wells with <50% of median cell count or negative drug responses. Aggregate technical replicates to a per-condition mean per biological replicate.

1. High well-to-well variability: ensure even seeding (pre-incubate plate at RT before 37 °C) and confirm monolayer confluence; edge wells often read low — interleave conditions. 2. No FCCP response (flat maximal): FCCP concentration is sub- or supra-optimal — run an FCCP titration (0.25-2 µM); over-uncoupling collapses OCR. 3. Negative OCR or sensor drift: incomplete cartridge hydration or wrong calibrant — re-hydrate overnight. 4. Low basal OCR: too few cells, wrong assay medium substrates, or pH off — verify 10 mM glucose/1 mM pyruvate and pH 7.4. 5. Oligomycin doesn't reduce OCR: cells are highly glycolytic or oligomycin degraded — make fresh stock and verify ATP-linked fraction with a complex V-competent control.

Use the biological replicate (n=3 independent seedings) as the unit of analysis, not technical replicate wells, to avoid pseudoreplication. For two conditions, apply a paired two-tailed t-test on each bioenergetic parameter; for >2 conditions use one-way ANOVA with Tukey HSD post-hoc correction. Set alpha = 0.05. A priori power analysis (effect size d=1.5, power 0.8) supports n=3-4 biological replicates for detecting a 25% change. Report mean ± SEM and exact p-values.