Steady-State and Dose-Response Characterization of a Genetically-Encoded Zn2+ FRET Biosensor (eCALWY-4) in HEK293T Cells

Three biological replicates (independent transfections on separate days), each with 4 technical replicate wells. Per well, ratiometric imaging is acquired every 5 s for 30 min. Free Zn2+ is clamped at 9 buffered concentrations (0, 0.1, 0.3, 1, 3, 10, 30, 100 nM, plus saturating) using N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) for Rmax and pyrithione/ZnSO4 for Rmin. Wells are randomized across the plate to avoid edge/positional bias; the analyst is blinded to clamp concentration during ROI selection by coded filenames. Minimum 40 cells/condition (power calc below).

BSL-2 for HEK293T culture; work in a Class II biosafety cabinet. TPEN and pyrithione are toxic/irritant — handle with nitrile gloves, lab coat, goggles; avoid skin contact. ZnSO4 is an irritant. Collect transfection and chelator waste as chemical hazardous waste; autoclave biological waste. Lentiviral steps are not used here. Dispose of sharps in approved containers.

Positive control: cells co-treated with TPEN (forces Rmax) and pyrithione/Zn (forces Rmin) define the response envelope. Negative control: untransfected HEK293T wells confirm autofluorescence/bleed-through is negligible. Vehicle control: DMSO carrier (TPEN/pyrithione solvent) at matched 0.1% v/v. Spectral control: donor-only (Cerulean) and acceptor-only (Citrine) constructs imaged to compute bleed-through correction factors. Photobleaching control: a continuously illuminated well with no perturbation tracks ratio drift.

A clean response shows Rmax/Rmin dynamic range of ~1.6-2.0 (donor/acceptor ratio rising ~60-100% from Zn-saturated to Zn-free). The dose-response fits a single-site isotherm with apparent Kd ~0.5-2 nM. Reversibility cycles return to within 5% of baseline. Healthy cells give ratio CV <10% across technical replicates; bleaching drift <3% over 30 min.

• HEK293T cells (ATCC CRL-3216), passage 8-25, in DMEM (high glucose, GlutaMAX) + 10% FBS
• eCALWY-4 plasmid (Cerulean-Atox1-linker-WD4-Citrine), 1 ug/well, in pcDNA3.1
• Lipofectamine 3000 transfection reagent
• Imaging buffer: 20 mM HEPES pH 7.4, 140 mM NaCl, 5 mM KCl, 1 mM MgCl2, 1 mM CaCl2, 10 mM glucose
• TPEN (Zn2+ chelator), 50 uM final
• Zinc pyrithione, 5 uM, + ZnSO4 100 uM (saturating clamp)
• Glass-bottom 35 mm dishes (No. 1.5) or 96-well glass-bottom plates
• Poly-D-lysine coating, 50 ug/mL

To characterize the in-cell FRET response of the genetically-encoded eCALWY-4 zinc sensor (Cerulean donor / Citrine acceptor) expressed in HEK293T cells, determining its dynamic range (Rmax/Rmin), apparent dissociation constant (Kd), response kinetics, and reversibility. The output is a calibration curve relating the donor/acceptor emission ratio to buffered free Zn2+ concentration, enabling absolute quantification of cytosolic free zinc in downstream experiments.

1. Seed HEK293T at 6x10^4 cells/well on poly-D-lysine-coated glass-bottom plates; culture 24 h to ~70% confluence.
2. Transfect 1 ug eCALWY-4 with Lipofectamine 3000 per manufacturer ratio; incubate 24-36 h.
3. On imaging day, wash 2x with imaging buffer; equilibrate 15 min at 37 C.
4. Mount on a confocal/widefield system with 37 C, 5% CO2 stage incubator; excite Cerulean at 440 nm.
5. Collect donor (470-490 nm) and acceptor (520-540 nm) emission channels every 5 s.
6. Acquire 3 min baseline, then perfuse 50 uM TPEN to obtain Rmax (Zn-free, maximal FRET).
7. Perfuse zinc pyrithione/ZnSO4 to obtain Rmin (Zn-saturated).
8. For dose-response wells, equilibrate each buffered Zn2+ clamp 10 min before recording steady-state ratio.
9. Record reversibility by alternating TPEN and Zn clamps x3 cycles.

Independent: buffered free Zn2+ concentration (clamp). Dependent: Cerulean/Citrine emission ratio (FRET readout) and derived occupancy. Controlled: excitation power, exposure, gain, temperature (37 C), pH 7.4, expression time post-transfection, cell passage, ROI cytosolic placement (nuclei excluded).

The intramolecular FRET efficiency of eCALWY-4 decreases monotonically as cytosolic free Zn2+ rises, because Zn2+ binding to the Atox1/WD4 metal-binding domains separates the Cerulean and Citrine fluorophores. We predict the in-cell emission ratio will follow a single-site binding isotherm with an apparent Kd in the high-picomolar range (~1 nM), distinguishable from buffer at alpha=0.05.

Background-subtract each channel using untransfected ROI median. Apply donor/acceptor bleed-through correction. Compute R = Idonor/Iacceptor per cell per frame. Normalize occupancy = (Rmax - R)/(Rmax - Rmin). Fit occupancy vs [Zn2+] to a Hill/single-site model in GraphPad Prism or Python (scipy curve_fit) to extract Kd and Hill coefficient. Segment cells in Fiji/ImageJ with cytosolic ROIs; export per-cell traces.

1. Low dynamic range (<1.3): expression too high causing aggregation — reduce plasmid to 0.5 ug and shorten expression to 24 h. 2. No FRET change with TPEN: sensor mislocalized or Zn already depleted — verify both fluorophores express via spectral controls. 3. Rapid bleaching: lower excitation power and exposure, add oxygen scavenger or reduce frame rate. 4. High well-to-well variability: ensure even poly-D-lysine coating and consistent confluence. 5. Acceptor photobleaching artifact inflating ratio: include a bleach-only control and correct drift.

Apparent Kd compared across biological replicates by one-way ANOVA on log-Kd with Tukey HSD; alpha=0.05. Dose-response fit reported with 95% CI on Kd. Power analysis: to detect a 20% ratio shift (effect size d=0.8, SD ~12% ratio) at 80% power and alpha=0.05 requires n>=26 cells/condition; we collect >=40. Multiple Zn2+ concentrations corrected via Bonferroni for pairwise contrasts.