Combinatorial regulation of photoreceptor differentiation factor, neural retina leucine zipper gene NRL, revealed by in vivo promoter analysis.

Development and homeostasis require stringent spatiotemporal control of gene expression patterns that are established, to a large extent, by combinatorial action of transcription regulatory proteins. The bZIP transcription factor NRL (neural retina leucine zipper) is critical for rod versus cone photoreceptor cell fate choice during retinal development and acts as a molecular switch to produce rods from postmitotic precursors. Loss of Nrl in mouse leads to a cone-only retina, whereas ectopic expression of Nrl in photoreceptor precursors generates rods. To decipher the transcriptional regulatory mechanisms upstream of Nrl, we identified putative cis-control elements in the Nrl promoter/enhancer region by examining cross-species sequence conservation. Using in vivo transfection of promoter-reporter constructs into the mouse retina, we show that a 0.9-kb sequence upstream of the Nrl transcription initiation site is sufficient to drive reporter gene expression in photoreceptors. We further define a 0.3-kb sequence including a proximal promoter (cluster A1) and an enhancer (cluster B) that can direct rod-specific expression in vivo. Electrophoretic mobility shift assays using mouse retinal nuclear extracts, in combination with specific antibodies, demonstrate the binding of retinoid-related orphan nuclear receptor β (RORβ), cone rod homeobox, orthodenticle homolog 2, and cyclic AMP response element-binding protein to predicted consensus elements within clusters A and B. Our studies demonstrate Nrl as a direct transcriptional target of RORβ and suggest that combinatorial action of multiple regulatory factors modulates the expression of Nrl in developing and mature retina.