Downregulation of N1 gene expression inhibits the initial heartbeating and heart development in axolotls.

Recessive mutant gene c in the axolotl results in a failure of affected embryos to develop contracting hearts. This abnormality can be corrected by treating the mutant heart with RNA isolated from normal anterior endoderm or from endoderm conditioned medium. A cDNA library was constructed from the total conditioned medium RNA using a random priming technique in a pcDNAII vector. We have previously identified a clone (designated as N1) from the constructed axolotl cDNA library, which has a unique nucleotide sequence. We have also discovered that the N1 gene product is related to heart development in the Mexican axolotl [Cell Mol. Biol. Res. 41 (1995) 117]. In the present studies, we further investigate the role of N1 on heartbeating and heart development in axolotls. N1 mRNA expression has been determined by using semi-quantitative RT-PCR with specifically designed primers. Normal embryonic hearts (at stages 30-31) have been transfected with anti-sense oligonucleotides against N1 to determine if downregulation of N1 gene expression has any effect on normal heart development. Our results show that cardiac N1 mRNA expression is partially blocked in the hearts transfected with anti-sense nucleotides and the downregulation of N1 gene expression results in a decrease of heartbeating in normal embryos, although the hearts remain alive as indicated by calcium spike movement throughout the hearts. Confocal microscopy data indicate some myofibril disorganization in the hearts transfected with the anti-sense N1 oligonucleotides. Interestingly, we also find that N1 gene expression is significantly decreased in the mutant axolotl hearts. Our results suggest that N1 is a novel gene in Mexican axolotls and it probably plays an important role in myofibrillogenesis and in the initiation of heartbeating during heart development.