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.

Neurotrophic factors are agents with a promising ability to retard progression of neurodegenerative diseases and are effective in slowing photoreceptor degeneration in animal models of retinitis pigmentosa. Here we report a human clinical trial of a neurotrophic factor for retinal neurodegeneration. In this Phase I safety trial, human ciliary neurotrophic factor (CNTF) was delivered by cells transfected with the human CNTF gene and sequestered within capsules that were surgically implanted into the vitreous of the eye. The outer membrane of the encapsulated cell implant is semipermeable to allow CNTF to reach the retina. Ten participants received CNTF implants in one eye. When the implants were removed after 6 months, they contained viable cells with minimal cell loss and gave CNTF output at levels previously shown to be therapeutic for retinal degeneration in rcd1 dogs. Although the trial was not powered to form a judgment as to clinical efficacy, of seven eyes for which visual acuity could be tracked by conventional reading charts, three eyes reached and maintained improved acuities of 10 -15 letters, equivalent to two-to three-line improvement on standard Snellen acuity charts. A surgically related choroidal detachment in one eye resulted in a transient acuity decrease that resolved with conservative management. This Phase I trial indicated that CNTF is safe for the human retina even with severely compromised photoreceptors. The approach to delivering therapeutic proteins to degenerating retinas using encapsulated cell implants may have application beyond disease caused by genetic mutations.clinical trial ͉ neurodegeneration ͉ retinitis pigmentosa ͉ photoreceptor ͉ macular degeneration R etinitis pigmentosa (RP) describes a set of neurodegenerative retinal diseases that cause the death of photoreceptor cells and lead to progressive vision loss and blindness. More than 39 genetic loci and genes have been implicated in monogenic forms of RP (1), underscoring the complexity of its pathogenic mechanisms. With the exception of vitamin A nutritional supplementation (2), no treatments have been shown to be effective across the range of these disorders. Regardless of the initial causative genetic defect, the end result is photoreceptor cell death. The multiplicity of mechanisms stimulated a search for therapeutic agents that are effective in slowing photoreceptor death regardless of the causative genetic mutation.Intervention studies have indicated the possibility of using neurotrophic factors as therapeutic agents for RP (3). Specifically, ciliary neurotrophic factor (CNTF) is effective at retarding retinal degeneration in at least 13 animal RP models, including rd-PDE6b mice (4, 5), rds-peripherin mice (4, 6-10), transgenic rats expressing P23H or S334ter mutant rhodopsin (7, 10), Rdy cats (11), rcd1-PDE6b dogs (7), rhodopsin-knockout mice (9), and rd͞rd mice, nr͞nr mice, and Q334ter rhodopsin transgenic mice (4).CNTF also appears effective for retarding cellular and functional losses in neurodegenerative diseases ...