Cellular Thermal Shift Assay (CETSA) for Intracellular Target Engagement of Small-Molecule Inhibitors in Intact A549 Cells

Two arms: (A) Melt-curve CETSA — cells +/- saturating compound (e.g. 30 µM) heated across a 10-temperature gradient (37-67 C in ~3 C steps); (B) ITDRF — cells treated with an 8-point compound dose-response (0.01-30 µM) then challenged at a single temperature near the apparent Tagg. Each condition is run in technical duplicate per experiment across 3 independent biological replicates. A DMSO vehicle melt curve and a known engager (tool compound) are included. Soluble target is quantified by immunoblot densitometry.

BSL-1 cell culture in a Class II biosafety cabinet. Liquid nitrogen for freeze-thaw is a cryogenic and asphyxiation hazard — use a face shield, cryo-gloves, and a ventilated area. SDS-PAGE and transfer involve acrylamide-based gels (handle pre-cast gels to avoid monomer), methanol-containing transfer buffer (flammable, toxic), and ECL reagents — wear nitrile gloves, lab coat, and goggles. Dispose of cell waste as biohazard and chemical waste per EHS.

Vehicle (DMSO) melt curve defines the unstabilized baseline Tagg; the compound curve is compared against it for delta-Tagg. Positive control: a known cellular engager (tool compound) must produce a measurable rightward melt shift, validating the antibody and workflow. Negative control: a structurally similar non-binding analog (or a non-target protein melt curve, e.g. a soluble housekeeping protein that should NOT shift) confirms specificity. A 37 C (unheated) sample defines 100% soluble target. Loading-control immunoblot normalizes for input.

A specific, engaging compound shifts the target Tagg by >= 1.5-2 C versus vehicle (delta-Tagg). Melt curves are sigmoidal with the unheated 37 C sample near 100% soluble and high-temperature points approaching 0%. The non-target loading control shows no compound-dependent shift. ITDRF curves are sigmoidal; cellular engagement EC50 is typically within ~3-10 fold of the biochemical IC50. Reproducible delta-Tagg across 3 replicates with SD < 1 C is the success criterion.

• A549 cells (human lung carcinoma), mycoplasma-free
• Growth medium: DMEM + 10% FBS + 1% penicillin/streptomycin
• PBS, pH 7.4, calcium/magnesium-free
• Test compound, 10 mM in DMSO; tool engager positive control
• Protease inhibitor cocktail (EDTA-free)
• Lysis by freeze-thaw in PBS + protease inhibitors (no detergent)
• Primary antibody against the target protein (validated for WB)
• Loading/normalization antibody (e.g. anti-GAPDH or vinculin)
• HRP-conjugated secondary antibody + ECL substrate
• SDS-PAGE gels (4-20%), transfer membranes (PVDF)
• Thin-wall PCR tubes/strips and a thermal cycler with gradient

To demonstrate that a candidate inhibitor binds its intended protein target inside living cells, not merely in a purified biochemical assay, by exploiting ligand-induced thermal stabilization. This CETSA protocol produces protein melt curves (with and without compound) and an isothermal dose-response fingerprint (ITDRF) at a fixed challenge temperature to quantify cellular target-engagement potency and corroborate biochemical potency with an orthogonal, cell-based readout.

1. Grow A549 to ~80% confluence; harvest and resuspend in PBS + protease inhibitors at ~3-5 x 10^6 cells/mL.
2. Split into +compound and +vehicle pools; incubate 1-3 h at 37 C to allow intracellular equilibration.
3. For melt curves: aliquot 100 µL per temperature point into PCR strips; heat each at its assigned temperature for 3 min in a gradient thermal cycler, then cool to RT 3 min.
4. Lyse by 3 freeze-thaw cycles (liquid N2 / 37 C heat block).
5. Clear lysates by centrifugation at 20,000 x g for 20 min at 4 C; collect soluble supernatant only.
6. For ITDRF: pre-treat cell aliquots with the 8-point compound series, then heat all at the chosen challenge temperature; lyse and clear as above.
7. Resolve soluble fractions by SDS-PAGE; transfer to PVDF.
8. Immunoblot for target and loading control; develop with ECL and image.
9. Quantify band intensity by densitometry.

Independent variables: temperature (melt-curve arm) and compound concentration (ITDRF arm). Dependent variable: soluble (non-denatured) target band intensity from immunoblot densitometry. Controlled variables: cell number, compound pre-incubation time and temperature, heating time (3 min), centrifugation speed/time, lysis method, protein load per lane, antibody lots, and ECL exposure time.

Binding of the compound to the folded target protein lowers its free energy of unfolding, shifting its aggregation/denaturation temperature (Tagg) to higher values. We hypothesize that compound-treated cells show a rightward shift in the target's melt curve (delta-Tagg) and that, at a fixed temperature on the melt midpoint, soluble target signal increases with compound concentration, yielding an EC50-like cellular engagement value (ITDRF EC50).

Normalize each lane's target band to the maximal (37 C / lowest-temperature or vehicle-max) signal after loading-control correction. For melt curves, fit normalized soluble fraction vs temperature to a Boltzmann sigmoidal in GraphPad Prism to extract Tagg (melt midpoint); compute delta-Tagg = Tagg(compound) - Tagg(vehicle). For ITDRF, fit soluble fraction vs log[compound] to a 4PL to obtain the engagement EC50. Use ImageJ/Image Lab for densitometry with local background subtraction.

1. No melt curve (flat signal): heating insufficient or antibody not WB-validated — verify the gradient block and confirm antibody on a denaturation ladder. 2. No delta-Tagg for a known binder: compound not reaching the target (poor permeability) or too short pre-incubation — extend to 3 h and confirm cytosolic exposure. 3. High lane-to-lane variability: uneven protein load — measure total protein and load equal input; normalize to loading control. 4. Smeared/aggregated bands: detergent or incomplete clearing — use detergent-free lysis and centrifuge at full 20,000 x g. 5. Saturated blots: reduce ECL exposure and stay in the densitometer's linear range.

Compare Tagg between compound and vehicle by unpaired two-tailed t-test across n=3 biological replicates at alpha = 0.05; delta-Tagg is the effect size. For multiple compounds, use one-way ANOVA with Dunnett's correction vs vehicle. With n=3 and intra-assay Tagg SD ~0.5-1 C, the design detects delta-Tagg >= 1.5 C with ~80% power. ITDRF EC50 is reported as geometric mean +/- 95% CI. Replicates with failed positive control are excluded.