Zebrafish hmx1 promotes retinogenesis

Ocular development is controlled by a complex network of transcription factors, cell cycle regulators, and diffusible signaling molecules. Together, these molecules regulate cell proliferation, apoptosis and specify retinal fate. In the zebrafish (Danio rerio), hmx1 is a homeobox transcription factor implicated in eye and brain development. Hmx1 transcripts were detected in the nasal retina and lens as well as otic vesicles and pharyngeal arches by 24-32 hpf. Before this stage, transcripts were more uniformly expressed in the optic vesicle. Knockdown of hmx1 led to microphthalmia. Delayed withdrawal of retinal progenitors from the cell cycle resulting in retarded retinal differentiation was observed in morphant. The retina and brain also showed an increased cell death at 24 hpf. The polarized expression of hmx1 to the nasal part in the zebrafish retina strongly suggested an involvement in the nasal-temporal patterning. However, the key patterning genes tested so far were not regulated by hmx1. Altogether, these results suggest an important role for hmx1 in retinogenesis.