Multiple Phosphorylated Isoforms of NRL Are Expressed in Rod Photoreceptors

Abstract

NRL, a bZIP transcription factor of the Maf subfamily, interacts with the homeodomain protein CRX and synergistically regulates rhodopsin expression. Here we report that six isoforms of NRL (29 -35 kDa) are generated by phosphorylation and expressed specifically in the mammalian retina. The anti-NRL antibody also crossreacts with a cytosolic 45-kDa protein, which is detected in neuronal tissues but is not encoded by the NRL gene. In both human retinal cell cultures and sections of fetal and adult human retina, NRL is present in the nuclei of developing and mature rods but not cones. We propose that NRL regulates rod photoreceptor-specific gene expression and is involved in rod differentiation.Retinal photoreceptors are highly specialized neurons that capture photons and convert them to chemical signals. Humans and Old World primates have four distinct photoreceptor types, each with a specific visual pigment (1) and a characteristic retinal distribution (2). Rhodopsin is the photopigment in rod photoreceptors, which dominate primate retina. Rhodopsin provides high sensitivity, but the rod synaptic circuitry yields low spatial resolution. The red, green, and blue visual pigments define the three cone types, whose neural circuits mediate color vision and high spatial resolution but require bright light.Extensive anatomical, lineage, and birth dating studies have demonstrated that the genesis of specific photoreceptor types from retinal progenitors is guided by intrinsic genetic programs, inductive cell-cell interactions, and extrinsic factors (3-6). Postmitotic neurons committed to a photoreceptor cell fate exhibit varying delays before expressing their cell typespecific photopigment, suggesting that the specification of a differentiated rod or cone phenotype requires additional cues (7-10). It is envisaged that these inductive cues turn on a "molecular switch," which leads to expression of a specific visual pigment and other components of the transduction machinery.Cell type-specific gene expression is achieved by combinatorial and synergistic actions of specific activator proteins that recruit the basal transcription machinery to the promoter region (11, 12). Several transcription factor genes are expressed in retina (13-17), and a number of cis-regulatory elements have been identified in retinal gene promoters (18 -20). However, only two transcription factors, NRL and CRX, have so far been implicated directly in modulating photoreceptor-specific gene expression. NRL was isolated from a subtracted human retinal cDNA library and encodes a basic motif-leucine zipper (bZIP) protein (21). It displays strong homology to Maf proteins, which are involved in differentiation and gene regulation (22). NRL was the first transcription factor shown to bind to a cis-regulatory sequence (called NRE or NRL response element) in the rhodopsin promoter and transactivate its activity in cultured cells (23,24). CRX is a photoreceptor-and pineal-specific homeodomain protein that appears to modulate several retinal gene promoters (25-2...

Transformation of cone precursors to functional rod photoreceptors by bZIP transcription factor NRL

“…NRL's role as a transcriptional repressor is not surprising, because key regulatory proteins can control transcription through context-dependent combinatorial mechanisms (52). Notably, distinct phosphorylated isoforms of NRL are expressed during retinal development (27), and phosphorylation differences are suggested to modulate transcriptional activity of NRL by altering nuclear translocation, DNA binding, or protein interactions (63). Additional studies are required to evaluate whether different NRL isoforms participate in transcriptional activation versus repression during early photoreceptor development.…”

“…Interestingly, ETS transcription factors are selectively expressed during motor and sensory neuron development but only after their axons reach the periphery, suggesting that these proteins confer postmitotic subtype identity during the establishment of selective connections (24,25). These findings indicate that neural identity can be specified even after the terminal cell cycle exit; however, direct in vivo evidence of postmitotic plasticity has not been reported.The Maf-family transcription factor Nrl is expressed specifically in postmitotic rod photoreceptors of the retina and in the pineal gland (26,27). NRL interacts with the homeodomain protein CRX (28), orphan nuclear receptor NR2E3 (29), and other retinal proteins (30-32) to regulate the expression of rod-specific genes (26, 33).…”

“…The Maf-family transcription factor Nrl is expressed specifically in postmitotic rod photoreceptors of the retina and in the pineal gland (26,27). NRL interacts with the homeodomain protein CRX (28), orphan nuclear receptor NR2E3 (29), and other retinal proteins (30-32) to regulate the expression of rod-specific genes (26, 33).…”

The Minimal Transactivation Domain of the Basic Motif-Leucine Zipper Transcription Factor NRL Interacts with TATA-binding Protein

“…Multiple phosphorylated isoforms of NRL are expressed in rod and not cone photoreceptors (14,15). Rhodopsin is the photopigment in rods, and its expression is stringently controlled at the level of transcription (16 -19).…”

Conserved Transcriptional Activators of the Xenopus Rhodopsin Gene

“…Nrl, a bZIP containing transcription factor, is a member of the large Maf family (30,31). Nrl is expressed specifically in developing and mature rods (32), and has been shown to activate expression of rhodopsin both in non-retinal and retinal cell cultures (24 -26). Furthermore, Nrl has been shown to bind to Crx (33), and with Crx, can synergistically activate expression of the rhodopsin promoter (24).…”