Developmental expression of the rat rod photoreceptor cGMP-gated cation channel

Stanniocalcin-1 rescued photoreceptor degeneration in two rat models of inherited retinal degeneration

Oxidative stress and photoreceptor apoptosis are prominent features of many forms of retinal degeneration (RD) for which there are currently no effective therapies. We previously observed that mesenchymal stem/stromal cells reduce apoptosis by being activated to secrete stanniocalcin-1 (STC-1), a multifunctional protein that reduces oxidative stress by upregulating mitochondrial uncoupling protein-2 (UCP-2). Therefore, we tested the hypothesis that intravitreal injection of STC-1 can rescue photoreceptors. We first tested STC-1 in the rhodopsin transgenic rat characterized by rapid photoreceptor loss. Intravitreal STC-1 decreased the loss of photoreceptor nuclei and transcripts and resulted in measurable retinal function when none is otherwise present in this rapid degeneration. We then tested STC-1 in the Royal College of Surgeons (RCS) rat characterized by a slower photoreceptor degeneration. Intravitreal STC-1 reduced the number of pyknotic nuclei in photoreceptors, delayed the loss of photoreceptor transcripts, and improved function of rod photoreceptors. Additionally, STC-1 upregulated UCP-2 and decreased levels of two protein adducts generated by reactive oxygen species (ROS). Microarrays from the two models demonstrated that STC-1 upregulated expression of a similar profile of genes for retinal development and function. The results suggested that intravitreal STC-1 is a promising therapy for various forms of RD including retinitis pigmentosa and atrophic age-related macular degeneration (AMD).

Retinal degeneration in Aipl1-deficient mice: a new genetic model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most severe inherited retinopathy, with the earliest age of onset. Because this currently incurable disease is present from birth and is a relatively rare disorder, the development of animal models that closely resemble the phenotype in patients is especially important. Our previous genetic analyses of LCA patients identified mutations in the aryl-hydrocarbon interacting protein-like 1 (AIPL1) gene. Here we present development of an animal model of AIPL1-associated LCA, the Aipl1-deficient mouse. Aipl1 is expressed at low levels throughout human and mouse retinal development and is rapidly upregulated as photoreceptors differentiate. The mouse displays rapid retinal degeneration and massive Müler cell gliosis, resembling the phenotype of the rd mouse, which is caused by a mutation in the gene for the beta-subunit of the rod-specific phosphodiesterase. We confirm that this phenotype is consistent with the human disease using electroretinograms, and document the disease pathogenesis by analyzing the development of all retinal cell types and synaptogenesis during retinal histogenesis. Ectopic expression of AIPL1 led to deregulated retinal progenitor cell proliferation and alterations in cell fate specification; however, no gross abnormalities of proliferation during retinal development were detected. Data from analysis of proliferation and cell fate specification during retinal development of Aipl1-deficient mice suggests that there may be redundancy or compensation for Aipl1 loss by other related proteins. Because this mouse model closely mimics the human retinopathy caused by homozygous mutations in this gene, it provides a preclinical model for testing therapies to rescue the vision of children whose blindness is caused by AIPL1 mutations.

Defects in the MITF(mi/mi) apical surface are associated with a failure of outer segment elongation

The loss of MITF function in the MITF(mi/mi)mouse affects not only RPE differentiation, but also the development of rod photoreceptor outer segments. Our data indicate that opsin immunoreactivity is detected in the cell membrane and along the ONL/RPE border of developing MITF(mi/mi)rod photoreceptors and that rod outer segment morphogenesis is initiated. Although molecules associated with the outer segment continued to be expressed, outer segments did not elongate and develop stacked organized discs perpendicular to the RPE. The MITF(mi/mi)RPE also failed to form apical microvilli and lacked the apical network of the phosphoprotein ezrin seen in wild type tissue. The MITF(mi/mi)RPE basal surface was loosely organized and retained ezrin labelling which indicated some degree of differentiation. The correlation seen in our data suggest that there may be a link between the failure of the RPE apical domain to form and lack of rod outer segment elongation.

Developmental expression of retinal cone cGMP-gated channels: evidence for rapid turnover and trophic regulation

The cyclic GMP-gated cationic channels of vertebrate photoreceptors are essential for visual phototransduction. We have examined the developmental regulation of cGMP-gated channels in morphologically identified cones in the chick retina. Expression of cone-type cGMP-gated channel mRNA can be detected at embryonic day 6 (E6), but expression of functional channels, as accessed by patch-clamp recordings, cannot be detected until E8. Plasma membrane channels in embryonic cones have a high turnover rate because inhibition of protein synthesis or disruption of the Golgi apparatus causes an almost complete loss of functional cGMP-gated channels within 12 hr. Different subpopulations of cones begin to express functional channels at different developmental stages, but all cones express channels by E10. Expression of cGMP-gated channels in at least one cone subpopulation appears to require one or more soluble differentiation factors, which are presumably present in the normal microenvironment of the developing retina. Application of chick embryo extract (CEE), a rich source of trophic factors, causes marked stimulation of cGMP-gated channel expression in chick cones at E8, but not at E6. Inhibition of MAP kinase (Erk) signaling using PD98059, or inhibition of PI3 kinase signaling by LY294002, blocked the stimulatory effects of CEE on E8 cones. Several recombinant trophic factors were also tested, but none could mimic the stimulatory effects of CEE on channel expression. In summary, the developmental expression of cGMP-gated cationic channels in embryonic cones appears to be regulated by epigenetic factors. The ability of cones to respond to these epigenetic factors is also developmentally regulated.