OFF ganglion cells cannot drive the optokinetic reflex in zebrafish.

Whereas the zebrafish retina has long been an important model system for developmental and genetic studies, little is known about the responses of the inner retinal neurons. Here we report single-unit ganglion cell recordings from 5- to 6-day-old zebrafish larvae. In wild-type larvae we identify at least five subtypes of ganglion cell responses to full-field illumination, with ON-OFF and ON-type cells predominating. In the nrc mutant retina, in which the photoreceptor terminals develop abnormally, we observe normal OFF responses but abnormal ON-OFF responses and no ON responses. Previously characterized as blind, these mutants lack an optokinetic reflex (OKR), but in another behavioral assay nrc mutant fish have near-normal responses to the offset of light and slow and sluggish responses to the onset of light. Pharmacological block of the ON pathway mimics most of the nrc visual defects. We conclude that the abnormal photoreceptor terminals in nrc mutants predominantly perturb the ON pathway and that the ON pathway is necessary to drive the OKR in larval zebrafish.

Excessive Myosin activity in mbs mutants causes photoreceptor movement out of the Drosophila eye disc epithelium.

Neuronal cells must extend a motile growth cone while maintaining the cell body in its original position. In migrating cells, myosin contraction provides the driving force that pulls the rear of the cell toward the leading edge. We have characterized the function of myosin light chain phosphatase, which down-regulates myosin activity, in Drosophila photoreceptor neurons. Mutations in the gene encoding the myosin binding subunit of this enzyme cause photoreceptors to drop out of the eye disc epithelium and move toward and through the optic stalk. We show that this phenotype is due to excessive phosphorylation of the myosin regulatory light chain Spaghetti squash rather than another potential substrate, Moesin, and that it requires the nonmuscle myosin II heavy chain Zipper. Myosin binding subunit mutant cells continue to express apical epithelial markers and do not undergo ectopic apical constriction. In addition, mutant cells in the wing disc remain within the epithelium and differentiate abnormal wing hairs. We suggest that excessive myosin activity in photoreceptor neurons may pull the cell bodies toward the growth cones in a process resembling normal cell migration.

Immunoassay of rod visual pigment (opsin) in the eyes of rds mutant mice lacking receptor outer segments.

In 020/A mice, homozygous for the retinal degeneration slow (rds) gene, the photoreceptor cells fail to develop outer segments, and in the absorption spectra of retinal extracts the rhodopsin peak is lacking. Application of an enzyme-linked immunoassay using antisera against bovine opsin shows, however, that opsin is present in the homozygous mutant retina (0.010 nmol/eye) at 3% of the level of the normal retina (0.38 nmol/eye) of Balb/c mice. In the retina of heterozygous mice the opsin level (0.19 nmol/eye) is about half of the normal. Detection of opsin in the rds mutant retina demonstrates the functional basis for the reported electroretinographic response and light-mediated reduction in cyclic nucleotide levels in this mutant.

Differential effect of the rd mutation on rods and cones in the mouse retina.

The retinas of rd/rd C57BL/6J-rd le mice have been examined by light and electron microscopy to determine whether rod cell degeneration precedes cone cell degeneration. In all regions of the eye, a rapid rod degeneration precedes a much slower cone degeneration. Only about 2% of the rods remain in the posterior region at postnatal day 17, and none by the day 36. By contrast, at least 75% of the cone nuclei remain at day 17. Although most of these slowly disappear, about 1.5% of the original population of cone nuclei in the posterior retina is still present at 18 months of age. A central to peripheral temporal gradient of degeneration exists, such that some rod nuclei persist in the far periphery up to day 47, but none is found at day 65. About 5% of the cone nuclei are still present in the far periphery at 18 months of age.

An inherited retinopathy in collies. A light and electron microscopic study.

A recessively inherited retinopathy in collies aged 8 to 189 days was studied by light and electron microscopy. The disease is produced when the outer segments of rods and cones fail to develop normally. Retinal pigment epithelial changes found in several litters appeared to form a separate disease entity. We compared the collie retinopathy with other canine models and the collie ectasia syndrome.

Apoptosis in the murine rd1 retinal degeneration is predominantly p53-independent.

PURPOSE: To determine if p53 mediates apoptosis in photoreceptors of retinal degeneration, rd1, mice. METHODS: The rd1/rd1 mice were interbred with p53 null mice to generate p53-/- rd1/rd1 and p53+/+ rd1/rd1 mice. Rates of loss and incidence of apoptosis in rod photoreceptors were analyzed at appropriate ages (postnatal days 12, 14 and 16). RESULTS: The extent and kinetics of photoreceptor cell loss in rd1 mice were nearly indistinguishable in the p53+/+ and p53 null mice. CONCLUSIONS: Photoreceptor cell apoptosis in the rd1 mouse model occurs by a predominantly p53-independent molecular pathway.

Molecular diagnostic tests for ascertainment of genotype at the rod cone dysplasia 1 (rcd1) locus in Irish setters.

Rod-cone dysplasia type 1 (rcd1) is one of several canine photoreceptor degenerations, collectively termed progressive retinal atrophy (PRA), that afflict different breeds of dogs. The rcd1 phenotype is an early onset autosomal recessive disease caused by a nonsense amber mutation, at codon 807, in the canine gene for the beta-subunit of rod cyclic GMP phosphodiesterase (canine PDEB). The mutation involves a G to A transition at nucleotide position 2420, which presumably would cause premature termination of the canine PDEB protein by 49 amino acid residues. In both a small pedigree study of Irish setters from the United Kingdom and in larger canine pedigree studies in the United States, this gene defect has been found to be the only mutation causing rcd1. Here we report development of a diagnostic test which unequivocally distinguishes the three genotypes at the rcd1 locus: rcd1/rcd1 (homozygous mutant, affected); rcd1/+ (heterozygous, carrier); and +/+ (homozygous normal, wildtype).

Blue cone monochromatism.

Blue cone monochromatism (BCM) is a subtype of achromatopsia in which the blue cone mechanism predominates. Each of the four patients in this study had BCM proven by their having peak spectral sensitivities in the blue region of the visible spectrum (near 440 nm). Clinically, the diagnosis was suspected because of x-linked inheritance, the presence of acuities better than 20/200 in two patients and myopia ranging from -1.75 to -15.00 diopters in three patients. Congenital nystagmus was the presenting sign in three of the four patients. Examination of the fundi was uniformly normal. The distinctive spectral properties of BCM were demonstrated by the American Optical H-R-R and the Panel D-15 tests. All affected patients correctly identified three of the four blue-yellow plates and a variable number of the red-green plates in the American Optical H-R-R test. The study patients consistently made errors oriented along the protan and deutan axes but they made none along the tritan axis. The authors conclude that the results of these two color discrimination tests are useful in diagnosing BCM.

[C-wave changes in macular diseases]. / Onde C et pathologie maculaire.

C-wave and ERG records have been taken from patients with juvenile macular degeneration, cone dysfunction syndrome, non exudative macular degeneration and from patients who have taken hydroxychloroquine for more than a year. The study of these recordings showed that in juvenile macular degeneration the involved area might be larger than ophthalmoscopically visible. In cone dysfunction syndrome the prolonged latence implicit time can serve as a further diagnostic sign of the disease. In non exudative macular degeneration there is more extensive R.P.E. involvement than usually believed and finally in hydroxychloroquine retinopathy, the C-wave is a better criterion to monitor the disease than ERG.

Are rods and cones of dyslexics anomalous?

Grosser and Spafford (in this journal, 1989) have advanced an hypothesis and presented measurements which they believe support the idea of an excess of cones in the peripheral retinae of dyslexics. This note points out that their hypothesis is based on the erroneous assumption that normals have no peripheral cones. Further, their data can be explained by at least two alternative, though uninteresting, methodological hypotheses, that uncontrolled eye movements or experimenter suggestion (or both) could have produced their results. Finally, the requisite methods for assessing color vision, and the cones, were not met in the study.

The intraflagellar transport protein IFT57 is required for cilia maintenance and regulates IFT-particle-kinesin-II dissociation in vertebrate photoreceptors.

Defects in protein transport within vertebrate photoreceptors can result in photoreceptor degeneration. In developing and mature photoreceptors, proteins targeted to the outer segment are transported through the connecting cilium via the process of intraflagellar transport (IFT). In studies of vertebrate IFT, mutations in any component of the IFT particle typically abolish ciliogenesis, suggesting that IFT proteins are equally required for IFT. To determine whether photoreceptor outer segment formation depends equally on individual IFT proteins, we compared the retinal phenotypes of IFT57 and IFT88 mutant zebrafish. IFT88 mutants failed to form outer segments, whereas IFT57 mutants formed short outer segments with reduced amounts of opsin. Our phenotypic analysis revealed that IFT57 is not essential for IFT, but is required for efficient IFT. In co-immunoprecipitation experiments from whole-animal extracts, we determined that kinesin II remained associated with the IFT particle in the absence of IFT57, but IFT20 did not. Additionally, kinesin II did not exhibit ATP-dependent dissociation from the IFT particle in IFT57 mutants. We conclude that IFT20 requires IFT57 to associate with the IFT particle and that IFT57 and/or IFT20 mediate kinesin II dissociation.

Lens opacity and photoreceptor degeneration in the zebrafish lens opaque mutant.

The zebrafish lens opaque (lop) mutant was identified in a chemical mutagenesis screen. The lop mutant, which develops normally through 4 days postfertilization (dpf), exhibits several signs of lens and retinal degeneration at 7 dpf. Histology revealed disrupted lens fibers and increased numbers of nucleated cells within the mutant lens and anterior chamber. The mutant lens also exhibited aberrant epithelial cell morphologies and lacked a definitive transition zone, which suggests that secondary fiber differentiation was interrupted. In addition, the mutant exhibits severely reduced photoreceptors and a reduction in the number of horizontal cells at 7 dpf. Other retinal cell classes appeared unaffected in the mutant. Transmission electron microscopy and opsin immunohistochemistry showed that the different photoreceptor types were generated at the retinal margin, but the rods and cones failed to mature and disappeared. The mutant lens and retina also displayed increased cell proliferation based on proliferating cell nuclear antigen immunolabeling, suggesting that the lens opacity was due to unregulated cell proliferation and undifferentiated cell accumulation within the mutant lens. The lop mutant phenotype supports recent studies showing the lens has a role in regulating teleost retinal development.