Splice-Blocking Morpholino Knockdown of tbx5a in Zebrafish Embryos with RT-PCR Validation and Fin/Heart Phenotyping

One-cell-stage AB wild-type embryos are randomized across injection groups by pooling clutches from >=3 independent crosses. Five groups: uninjected, vehicle (Danieau + phenol red), standard control MO (8 ng), tbx5a e2i2 MO dose series (2, 4, 8 ng), and a tbx5a MO + 1.5 ng p53 MO co-injection arm to control for off-target apoptosis. n >= 50 embryos/group/replicate, 3 biological replicates (independent clutches on separate days). Scoring at 48 and 72 hpf is performed blinded (tubes coded by a second operator). A subset (n=30 pooled) per group is harvested at 30 hpf for RT-PCR splice validation.

BSL-1; zebrafish work under approved IACUC protocol. MS-222 (tricaine) is a hazardous anesthetic — weigh in a fume hood, wear nitrile gloves, and buffer to neutral pH. PTU is toxic (thyroid inhibitor) — avoid skin contact, dispose as chemical waste. TRIzol contains phenol/guanidine — use in a fume hood, wear gloves and eye protection; never mix guanidine waste with bleach. Morpholinos are low-hazard but handled with gloves. Euthanize larvae beyond 5 dpf per IACUC by rapid chilling or tricaine overdose. Dispose biological waste per institutional guidelines.

Negative controls: uninjected and vehicle define baseline defect rate. Specificity: standard 25-N control MO at 8 ng controls for injection trauma and generic toxicity. Off-target: tp53 MO co-injection distinguishes p53-driven apoptosis from specific phenotype. Molecular positive control: actb1 RT-PCR confirms cDNA integrity. Biological positive reference: comparison to the heartstrings (tbx5a) mutant phenotype.

A dose-dependent increase in pectoral fin loss and cardiac edema, reaching >=70% affected at 8 ng. RT-PCR should show near-complete loss of the correct ~350 bp product and appearance of a larger intron-retained band in tbx5a MO embryos at >=4 ng. Control MO and vehicle should show <10% defects and a normal RT-PCR pattern. The +p53 co-injection arm should retain the fin/heart phenotype (>60% affected) while reducing nonspecific necrosis, supporting specificity. Mortality should remain <15% across all groups.

• AB wild-type zebrafish (Danio rerio), adult breeders, 6-18 months
• tbx5a e2i2 splice MO (Gene Tools, ~25-mer, lyophilized, resuspended to 1 mM stock in nuclease-free water)
• Standard control MO (Gene Tools, 5'-CCTCTTACCTCAGTTACAATTTATA-3')
• p53 (tp53) MO (Gene Tools) for off-target control
• Danieau buffer (1x: 58 mM NaCl, 0.7 mM KCl, 0.4 mM MgSO4, 0.6 mM Ca(NO3)2, 5 mM HEPES, pH 7.6)
• 0.05% phenol red injection tracer
• E3 medium (5 mM NaCl, 0.17 mM KCl, 0.33 mM CaCl2, 0.33 mM MgSO4) + 0.0001% methylene blue
• 0.003% PTU (1-phenyl-2-thiourea) for pigment suppression (imaging subset only)
• Tricaine (MS-222) 0.016% for anesthesia
• TRIzol reagent; SuperScript IV reverse transcriptase; gene-specific exon1-F/exon3-R primers; Taq polymerase
• 1.2% agarose/TAE gel, 100 bp ladder

To establish a reproducible, validated transient knockdown of zebrafish tbx5a using a splice-site-blocking morpholino (MO) targeting the exon2/intron2 boundary (e2i2), and to quantify the resulting pectoral fin agenesis and cardiac looping/edema phenotypes. The protocol couples gold-standard MO validation (RT-PCR demonstrating intron retention/exon skipping) with quantitative morphological scoring so that off-target/p53-related toxicity can be distinguished from specific phenotype, addressing the well-known reproducibility concerns of MO experiments.

1. Set up 3 pairwise crosses the evening before; collect embryos within 15 min of light onset to maximize 1-cell-stage windows.
2. Prepare MO working dilutions in 1x Danieau + 0.05% phenol red; heat MO stock to 65 C for 5 min and vortex before diluting to disrupt aggregates.
3. Pull and bevel borosilicate needles (1.0 mm OD); calibrate droplet to 1 nL in mineral oil over a stage micrometer.
4. Inject 1 nL into the yolk just below the blastomere at the 1-cell stage; deliver doses of 2, 4, 8 ng (adjust concentration, hold volume at 1 nL).
5. Incubate embryos in E3 at 28.5 C; remove dead/unfertilized at 6 and 24 hpf and record mortality.
6. At 30 hpf, pool 30 embryos/group, homogenize in TRIzol, extract RNA, DNase-treat, synthesize cDNA with SuperScript IV.
7. PCR across the targeted junction (exon1-F to exon3-R), 30 cycles; resolve on 1.2% agarose. Confirm aberrant (larger/intron-retained) band in MO group vs single correct band in controls; gel-purify and Sanger-sequence to confirm intron retention.
8. At 48 and 72 hpf, anesthetize in tricaine and score blinded under stereomicroscope: pectoral fin presence/absence, cardiac edema (none/mild/severe), heart looping (D-loop normal vs unlooped), circulation.
9. Image representative larvae of each class on a compound scope after PTU treatment.

Independent: MO identity (tbx5a vs control vs +p53) and MO dose (0, 2, 4, 8 ng). Dependent: fraction of larvae with fin agenesis, cardiac edema severity score, heart-looping defect rate, and qualitative/quantitative RT-PCR splice disruption. Controlled: embryo stage at injection (1-cell), injection volume (1 nL), incubation temperature (28.5 C), clutch source (randomized across replicates), scorer blinding, and developmental timepoints (48/72 hpf).

Injection of 4-8 ng of an e2i2 splice-blocking MO against tbx5a will disrupt normal splicing (producing a detectable larger/aberrant RT-PCR product), leading to dose-dependent pectoral fin loss and cardiac edema in >70% of injected embryos, phenocopying the heartstrings (tbx5a) mutant, while a 5-mismatch control MO at the same dose produces <10% nonspecific defects.

Phenotype counts per category are tabulated per replicate and expressed as percentage affected. RT-PCR band intensities are quantified by densitometry in ImageJ/Fiji; the ratio of aberrant to correct product is computed and normalized to actb1. Dose-response is fit to a sigmoidal (Hill) curve in GraphPad Prism to estimate the effective MO dose for 50% penetrance. Images are scored against a predefined edema rubric (0-3). All raw scoring sheets retain coded tube IDs until analysis is complete to preserve blinding.

1. No phenotype at any dose: MO may be degraded or mis-targeted — re-heat/vortex stock, verify splice disruption by RT-PCR before concluding; re-order if no aberrant band. 2) High nonspecific mortality/necrosis: lower dose, ensure 1-cell injection, add p53 MO co-injection, and check for needle clogging delivering boluses. 3) RT-PCR shows only the correct band: injection missed the cell or dose too low — confirm phenol red distribution at injection and increase to 8 ng. 4) Variable penetrance across clutches: standardize embryo staging and inject all groups from the same pooled clutch within one session. 5) Smeared/multiple PCR bands: optimize annealing temperature and re-design primers to span 2-3 exons.

Per-category phenotype frequencies are compared across groups using Fisher's exact test or chi-square with Bonferroni correction across the dose series (alpha = 0.05, corrected). Ordinal edema scores are compared by Kruskal-Wallis with Dunn's post hoc. Replicate effects are modeled with a mixed-effects logistic regression (clutch as random effect). With 3 replicates of n>=50, the design has >80% power to detect a 30-percentage-point difference in penetrance at alpha = 0.05.