| Mitf and Otx transcription factors have been identified as essential to the development of the retinal pigmented epithelium (RPE), but the relationship between these factors and their specific role in the RPE developmental pathway have not been clearly defined. The role of the two Mitf transcription factors (Mitfa and Mitfb) and two Otx transcription factors (Otx1a and Otx2) in zebrafish RPE development was explored in these experiments. The loss of Mitf activity in mitfa, mitfb, or double mitf null mutant fish lines had no effect on RPE pigmentation or development. The loss of Otx2 activity through morpholino knockdown produced a RPE deficient phenotype in a small percentage of embryos, while the additional knockdown of Otx1a caused widespread and severe RPE developmental abnormalities. Analysis of ocular sections revealed that the retinal layers remain unaffected in mitf mutants, as well as in most RPE-deficient otx morphants. Mitf and Otx combined loss of function experiments suggest that Mitfa and Mitfb may still play a role in zebrafish RPE development. Expression analysis through in situ hybridization has demonstrated that Otx transcription factors are necessary for the proper expression of mitfa and mitfb while Mitf transcriptions factors are not required for the expression of otx genes. The transcriptional regulation of Mitf by Otx transcription factors may explain why only Otx transcription factors are necessary for zebrafish RPE development despite the somewhat overlapping functions of Mitf and Otx transcription factors. |
