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<italic>hand2</italic> mRNA rescues the cardiomyocyte deficiency at 12ss and ventricular cardiomyocyte deficiency at 28hpf.(A) Wholemount preparations of control and MZjnk1a/MZjnk1b embryos labelled with anti-gfp antibody to detect myl7;gfp-positive cells and DAPI to identify nuclei at 12ss. The bar indicates the extent of the cardiomyocyte distribution, which is deficient in the posterior part of the field in MZjnk1a/MZjnk1b embryos but is rescued by hand2 mRNA. (B) Quantification indicates restoration of cardiomyocyte numbers in MZjnk1a/MZjnk1b embryos injected with hand2 mRNA. (C) At 28 hpf the deficit in FHF ventricular cardiomyocytes (red) in MZjnk1a/MZjnk1b embryos is rescued with hand2 mRNA. (D) Counting of cardiomyocytes reveals almost complete rescue of ventricular cardiomyocytes with no change in atrial numbers. (E) Crystal structure of JNK1 indicating position of C-terminal extension (yellow and yellow arrow) and alternatively spliced exon 6a/b (blue) threonine residue (T228) present in JNK1 Ex6b (red and red arrow). based on crystal structure at https://www.uniprot.org/uniprot/P45983. (F) Mechanism of FHF ventricular hypoplasia. (i) Although all 8 possible transcripts are produced by jnk1a and jnk1b, only jnk1a Lg and jnk1b Sh forms are expressed at high levels within the heart. (ii) Of these, jnk1a Ex7Lg modulates the posterior expression domain of hand2 (directly or indirectly). (iii) Overlaying the expression fields of transcription factors directly taken from WISH in Fig 7I reveals a reduction in the posterior cardiomyocyte field which gives rise to ventricular cardiomyocytes. (iv) This reduction in cardiomyocytes produces a smaller FHF ventricular segment, with normal SHF ventricular addition which does not compensate for the FHF hypoplasia. * = p<0.05, *** = p<0.001, **** = p<0.0001, ns = not significant.
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