The protein neural retina leucine zipper (Nrl) is a basic motif-leucine zipper transcription factor that is preferentially expressed in rod photoreceptors. It acts synergistically with Crx to regulate rhodopsin transcription. Missense mutations in human NRL have been associated with autosomal dominant retinitis pigmentosa. Here we report that deletion of Nrl in mice results in the complete loss of rod function and super-normal cone function, mediated by S cones. The photoreceptors in the Nrl-/- retina have cone-like nuclear morphology and short, sparse outer segments with abnormal disks. Analysis of retinal gene expression confirms the apparent functional transformation of rods into S cones in the Nrl-/- retina. On the basis of these findings, we postulate that Nrl acts as a 'molecular switch' during rod-cell development by directly modulating rod-specific genes while simultaneously inhibiting the S-cone pathway through the activation of Nr2e3.

The Maf-family transcription factor Nrl is a key regulator of photoreceptor differentiation in mammals. Ablation of the Nrl gene in mice leads to functional cones at the expense of rods. We show that a 2.5-kb Nrl promoter segment directs the expression of enhanced GFP specifically to rod photoreceptors and the pineal gland of transgenic mice. GFP is detected shortly after terminal cell division, corresponding to the timing of rod genesis revealed by birthdating studies. In Nrl ؊/؊ retinas, the GFP؉ photoreceptors express S-opsin, consistent with the transformation of rod precursors into cones. We report the gene profiles of freshly isolated flow-sorted GFP؉ photoreceptors from wild-type and Nrl ؊/؊ retinas at five distinct developmental stages. Our results provide a framework for establishing gene regulatory networks that lead to mature functional photoreceptors from postmitotic precursors. Differentially expressed rod and cone genes are excellent candidates for retinopathies.gene profiling ͉ gene regulation ͉ neuronal differentiation ͉ retina ͉ transcription factor E volution of higher-order sensory and behavioral functions in mammals is accompanied by increasingly complex regulation of gene expression (1). As much as 10% of the human genome is presumably dedicated to the control of transcription. Exquisitely timed expression of cell-type-specific genes, together with spatial and quantitative precision, depends on the interaction between transcriptional control machinery and extracellular signals (2, 3). Neuronal heterogeneity and functional diversity result from combinatorial and cooperative actions of regulatory proteins that form complicated yet precise transcriptional networks to generate unique gene expression profiles. A key transcription factor, combined with its cognate regulatory cis-sequence codes, specifies a particular node in the gene regulatory networks that guide differentiation and development (4).The retina offers an ideal paradigm for investigating regulatory networks underlying neuronal differentiation. The genesis of six types of neurons and Müller glia in the vertebrate retina proceeds in a predictable sequence during development (5). Subsets of multipotent retinal neuroepithelial progenitors exit the cell cycle at specific time points and acquire a particular cell fate under the influence of intrinsic genetic program and extrinsic factors (5-7). Pioneering studies using thymidine labeling and retroviral vectors established the order and birthdates of neurons in developing retina (5,(8)(9)(10)). The current model of retinal differentiation proposes that a heterogeneous pool of progenitors passes through states of competence, where it can generate a distinct subset of neurons (5). One can predict that, at the molecular level, this competence is acquired by combinatorial action of specific transcriptional regulatory proteins. Genetic ablation studies of transcription factors involved in early murine eye specification are consistent with combinatorial regulation (11-13).Rod and cone photorecep...

Many morphological, molecular, and electrophysiological features of the Nrl(-/-) photoreceptors are cone-like, and strongly distinguish these cells from rods. This retina provides a model for the investigation of cone function and cone-specific genetic disease.

Genetic linkage, genome mismatch scanning, and analysis of patients with alterations of chromosome 6 have indicated that a major locus for development of the anterior segment of the eye, IRID1, is located at 6p25. Abnormalities of this locus lead to glaucoma. FKHL7 (also called "FREAC3"), a member of the forkhead/winged-helix transcription-factor family, has also been mapped to 6p25. DNA sequencing of FKHL7 in five IRID1 families and 16 sporadic patients with anterior-segment defects revealed three mutations: a 10-bp deletion predicted to cause a frameshift and premature protein truncation prior to the FKHL7 forkhead DNA-binding domain, as well as two missense mutations of conserved amino acids within the FKHL7 forkhead domain. Mf1, the murine homologue of FKHL7, is expressed in the developing brain, skeletal system, and eye, consistent with FKHL7 having a role in ocular development. However, mutational screening and genetic-linkage analyses excluded FKHL7 from underlying the anterior-segment disorders in two IRID1 families with linkage to 6p25. Our findings demonstrate that, although mutations of FKHL7 result in anterior-segment defects and glaucoma in some patients, it is probable that at least one more locus involved in the regulation of eye development is also located at 6p25.

X-linked retinitis pigmentosa (XLRP) is a clinically and genetically heterogeneous degenerative disease of the retina. At least five loci have been mapped for XLRP; of these, RP2 and RP3 account for 10%-20% and 70%-90% of genetically identifiable disease, respectively. However, mutations in the respective genes, RP2 and RPGR, were detected in only 10% and 20% of families with XLRP. Mutations in an alternatively spliced RPGR exon, ORF15, have recently been shown to account for 60% of XLRP in a European cohort of 47 families. We have performed, in a North American cohort of 234 families with RP, a comprehensive screen of the RP2 and RPGR (including ORF15) genes and their 5' upstream regions. Of these families, 91 (39%) show definitive X-linked inheritance, an additional 88 (38%) reveal a pattern consistent with X-linked disease, and the remaining 55 (23%) are simplex male patients with RP who had an early onset and/or severe disease. In agreement with the previous studies, we show that mutations in the RP2 gene and in the original 19 RPGR exons are detected in <10% and approximately 20% of XLRP probands, respectively. Our studies have revealed RPGR-ORF15 mutations in an additional 30% of 91 well-documented families with X-linked recessive inheritance and in 22% of the total 234 probands analyzed. We suggest that mutations in an as-yet-uncharacterized RPGR exon(s), intronic changes, or another gene in the region might be responsible for the disease in the remainder of this North American cohort. We also discuss the implications of our studies for genetic diagnosis, genotype-phenotype correlations, and gene-based therapy.

The rod photoreceptor-specific neural retina leucine zipper protein Nrl is essential for rod differentiation and plays a critical role in regulating gene expression. In the mouse retina, rods account for 97% of the photoreceptors; however, in the absence of Nrl (Nrl-/-), no rods are present and a concomitant increase in cones is observed. A functional all-cone mouse retina represents a unique opportunity to investigate, at the molecular level, differences between the two photoreceptor subtypes. Using mouse GeneChips (Affymetrix), we have generated expression profiles of the wild-type and Nrl-/- retina at three time-points representing distinct stages of photoreceptor differentiation. Comparative data analysis revealed 161 differentially expressed genes; of which, 78 exhibited significantly lower and 83 higher expression in the Nrl-/- retina. Hierarchical clustering was utilized to predict the function of these genes in a temporal context. The differentially expressed genes primarily encode proteins associated with signal transduction, transcriptional regulation, intracellular transport and other processes, which likely correspond to differences between rods and cones and/or retinal remodeling in the absence of rods. A significant number of these genes may serve as candidates for diseases involving rod or cone dysfunction. Chromatin immunoprecipitation assay showed that in addition to the rod phototransduction genes, Nrl might modulate the promoters of many functionally diverse genes in vivo. Our studies provide molecular insights into differences between rod and cone function, yield interesting candidates for retinal diseases and assist in identifying transcriptional regulatory targets of Nrl.

Sonic hedgehog (Shh) is an indispensable, extrinsic cue that regulates progenitor and stem cell behavior in the developing and adult mammalian central nervous system. Here, we investigate the link between the Shh signaling pathway and Hes1, a classical Notch target. We show that Shh-driven stabilization of Hes1 is independent of Notch signaling and requires the Shh effector Gli2. We identify Gli2 as a primary mediator of this response by showing that Gli2 is required for Hh (Hedgehog)-dependent up-regulation of Hes1. We also show using chromatin immunoprecipitation that Gli2 binds to the Hes1 promoter, which suggests that Hes1 is a Hh-dependent direct target of Gli2 signaling. Finally, we show that Shh stimulation of progenitor proliferation and cell diversification requires Gli2 and Hes1 activity. This paper is the first demonstration of the mechanistic and functional link between Shh, Gli, and Hes1 in the regulation of progenitor cell behavior.

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...