A novel basic helix–loop–helix (bHLH) transcriptional repressor, NeuroAB, expressed in bipolar and amacrine cells in the chick retina

Basic helix–loop–helix (bHLH) transcription factors are implicated in cell fate determination and differentiation in neurogenesis. We identified a novel chick bHLH transcription factor, NeuroAB. A phylogenetic tree prepared from bHLH sequences suggested that NeuroAB belongs to the BETA3 group in the Atonal-related protein family (ARPs). In situ hybridization and immunostaining indicated that NeuroAB is expressed predominantly in postmitotic bipolar cells and GABAergic amacrine cells in the retina. Reporter and DNA pull down assays indicated that NeuroAB functions as a transcriptional repressor by binding to the E-box sequence, and its activity is modulated by phosphorylation at a specific serine residue that fits the consensus phosphorylation site for glycogen synthase kinase 3β (GSK3β). Since members of the BETA3 group possess this consensus site, it is suggested that their activities are commonly regulated by GSK3β or other kinases bearing the same substrate specificity. We found that the expression of GSK3β is spatially and temporally regulated in the developing retina; its strong expression was observed in ganglion cells from E8 and a subset of amacrine cells from E12. These findings suggest that NeuroAB is involved in the maturation and maintenance of bipolar cells and GABAergic amacrine cells and regulation by GSK3β plays an important role in retinogenesis.