hnf1b Genes in Zebrafish Hindbrain Development

Abstract

The Hnf1b transcription factor acts during formation of rhombomeres (r) 5 and 6 in the hindbrain. To determine if hnf1b is absolutely required in r5/r6, we examined the hnf1b^hi2169 and hnf1b^hi1843 retroviral insertion alleles. Hnf1b^hi2169 shows highly variable residual expression of several genes in r5/r6, but this is not due to full-length hnf1b transcripts persisting in hnf1b^hi2169 embryos, nor to hnf1bl, a novel hnf1 family member expressed in r5 that we identified. Instead, we find evidence for a virus-hnf1b fusion transcript in hnf1b^hi2169 embryos and demonstrate that morpholino-mediated knockdown of this transcript leads to near-undetectable r5 gene expression. The hnf1b^hi1843 allele has a more severe phenotype with near-undetectable expression of r5/r6 genes. We next examined if hoxb1b, which functions upstream of hnf1b in r5/r6 formation, can induce expression of r5/r6 genes in hnf1b mutants. We find that microinjected hoxb1b mRNA induces ectopic gene expression anterior to the hindbrain in hnf1b^hi2169 and hnf1b^hi1843 embryos, but cannot restore gene expression in r5/r6 of the mutants. We conclude that hnf1b^hi2169 is hypomorphic to hnf1b^hi1843 and that, while hnf1b is required for r5/r6 gene expression in the hindbrain, r5/r6 gene expression can be experimentally induced independently of hnf1b anterior to the hindbrain.