Absence of long-wavelength photic potentiation of murine intrinsically photosensitive retinal ganglion cell firing in vitro

Melanopsin is an opsin-family photopigment required for photosensitivity of the intrinsically photosensitive retinal ganglion cells (ipRGCs), which subserve photic entrainment of circadian rhythms in mammals. The melanopsin photocycle is presently unknown but is independent of the enzymatic photocycle employed by rhodopsin and cone opsins. Recent experiments have demonstrated that red-light exposure potentiates circadian phase-shifting responses to blue-light stimuli, consistent with the hypothesis that melanopsin functions as a bistable photopigment. To further test this hypothesis, we analyzed ipRGC firing activity in response to 480-nm blue light with or without intervening long-wavelength 620-nm red-light stimulation, using in vitro multielectrode array recording of postnatal day 8 to 10 murine retina. Cell-firing responses to 480-nm light were highly reproducible. No significant potentiating or bleaching effect of intervening subthreshold 620-nm light on ipRGC firing to 480-nm light could be discerned. Further physiologic and biochemical analysis of the ipRGC photoreception is required to reconcile the presence of long-wavelength potentiation at the level of the SCN with its absence in light-induced ipRGC firing.

Nonvisual ocular photoreception in the mammal

Rodents blind from outer retinal (rod and cone) degeneration still retain several light-dependent phenomena, including entrainment of the circadian clock and pupillary light responsiveness. This paradox is explained by the presence of intrinsically photosensitive retinal ganglion cells in the inner retina. These cells have unique properties, including a novel action spectrum, resistance to bleaching and adaptation under continuous light, and resistance to vitamin A depletion. Two candidate classes of photopigment have been proposed: melanopsin and cryptochromes. Physiologic analysis of circadian entrainment and pupillary light responsiveness in mice lacking these proteins leads to three conclusions: (1) outer and inner retinal photoreceptors provide partially redundant information to the inner retina, (2) melanopsin is required for inner retinal phototransduction in the absence of rod and cone signaling, and (3) cryptochromes contribute to the amplitude of inner retinal phototransduction but are not strictly required.

Prospective comparison of microbial culture and polymerase chain reaction in the diagnosis of corneal ulcer

PURPOSE

To compare polymerase chain reaction (PCR) to microbial culture for the detection and identification of bacterial and fungal pathogens in microbial keratitis.

DESIGN

Prospective cohort study.

METHODS

A total of 108 consecutive corneal ulcers were cultured and analyzed by PCR using pan-bacterial and pan-fungal primers. PCR products were cloned, sequenced, and compared to culture results using standard bioinformatics tools.

RESULTS

Of the 108 samples, 56 were culture-positive, 25 for bacteria and 31 for fungi; 52 were culture-negative. After eliminating false-positive PCR products, 94 of 108 were positive by PCR, 37 for bacteria and 57 for fungi. Nineteen of 25 bacterial culture-positive samples were positive by PCR, and 29 of 31 samples culture-positive for fungi were positive by PCR. The majority of sequenced PCR products matched the positive culture results. Of the 52 culture-negative samples, 46 (88%) yielded pathogen deoxyribonucleic acid (DNA) PCR products, 18 bacterial and 28 fungal. These represented a variety of species, including at least three novel previously uncultured microbes.

CONCLUSIONS

PCR detects microbial DNA in the majority of bacterial and fungal corneal ulcers, and identifies potentially pathogenic organisms in a high proportion of culture-negative cases. Yield and concordance with culture are higher for fungal than bacterial ulcers. Practical use of the technique is limited by artefactual amplification of nonpathogenic organisms. PCR may be used as an adjunct to culture to identify potential pathogens in microbial keratitis.

Non-visual photoreception: sensing light without sight

Recent work in blind human subjects has confirmed the presence of a non-visual ocular photoreceptive mechanism similar to that described in blind mice. This system appears to subserve circadian photic entrainment, the pupillary light response, and a number of other aspects of neurophysiology and behavior.

The imprinted gene Magel2 regulates normal circadian output

Mammalian circadian rhythms of activity are generated within the suprachiasmatic nucleus (SCN). Transcripts from the imprinted, paternally expressed Magel2 gene, which maps to the chromosomal region associated with Prader-Willi Syndrome (PWS), are highly enriched in the SCN. The Magel2 message is circadianly expressed and peaks during the subjective day. Mice deficient in Magel2 expression entrain to light cycles and express normal running-wheel rhythms, but with markedly reduced amplitude of activity and increased daytime activity. These changes are associated with reductions in food intake and male fertility. Orexin levels and orexin-positive neurons in the lateral hypothalamus are substantially reduced, suggesting that some of the consequences of Magel2 loss are mediated through changes in orexin signaling. The robust rhythmicity of Magel2 expression in the SCN and the altered behavioral rhythmicity of null mice reveal Magel2 to be a clock-controlled circadian output gene whose disruption results in some of the phenotypes characteristic of PWS.

Melanopsin-dependent persistence and photopotentiation of murine pupillary light responses

PURPOSE

To determine the relative contributions of inner and outer retinal photoreception to the pupillary light response.

METHODS

Wild-type, retinal degenerate (rd/rd), and melanopsin mutant (opn4(-/-)) mice were tested for pupillary light responsiveness by video pupillometry before, during, and after exposure to supersaturating light intensities. Similar lighting protocols were used to probe responses of intrinsically photosensitive retinal ganglion cells (ipRGCs) recorded with multielectrode arrays ex vivo.

RESULTS

Both outer retinal photoreceptors (rods and cones) and inner retinal photoreceptors (intrinsically photosensitive retinal ganglion cells [ipRGCs]) are sufficient to drive the pupillary light response in mice. After supersaturating light exposure, rather than bleaching or adapting, rd/rd mice showed paradoxical potentiation of responses to subsaturating light exposure. opn4(-/-) mice, in contrast, could not sustain pupillary constriction under continuous bright illumination, and showed desensitization after bright-light exposure. Both the intensity of light necessary to induce potentiation and the spectral sensitivity for sustained and potentiated responses differed from that necessary to trigger pupillary constriction, suggesting that photopotentiation is dependent on a pigment-state distinct from that triggering the pupillary light response itself. Multielectrode array recordings of ipRGCs from rd/rd retinas demonstrated persistent cell firing under continuous light exposure but did not show potentiation.

CONCLUSIONS

Unique photoreceptive properties of intrinsically photosensitive RGCs confer resistance to bleaching and/or adaptation under continuous bright illumination to the pupillary light response and suggest the presence of a photopigment with multiple absorption states.

Action Spectrum of Drosophila Cryptochrome

Cryptochromes are a highly conserved class of UV-A/blue light photoreceptors. In Drosophila, cryptochrome is required for the normal entrainment of circadian rhythms to light dark cycles. The photocycle and molecular mechanism of animal cryptochrome photoreception are presently unknown. Drosophila cryptochrome undergoes light-dependent degradation when heterologously expressed in Schneider-2 cells. We have generated Drosophila luciferase-cryptochrome fusion proteins to more precisely monitor light-dependent cryptochrome degradation. We found that the luciferase-cryptochrome fusion protein undergoes light-dependent degradation with luciferase activity declining approximately 50% within 5 min of light exposure and approximately 85% within 1 h of light exposure. Degradation is inhibited by MG-132, consistent with a proteasomal degradation mechanism. Irradiance-response curves yield an action spectrum similar to absorption spectra for prokaryotic and eukaryotic cryptochromes with highest sensitivity in the UV-A. A luciferase-cryptochrome fusion protein lacking the terminal 15 amino acids is stably expressed in the dark but demonstrates increased sensitivity to light-induced degradation. The conferral of light-dependent degradation on a heterologous protein by fusion to cryptochrome may be a useful tool for probing protein function in cell expression systems.

Rubeosis and anterior segment ischemia associated with systemic cryoglobulinemia

PURPOSE

To report two cases of iris neovascularization associated with systemic cryoglobulinemia.

DESIGN

Retrospective case report.

METHODS

Patient chart review and review of literature.

RESULTS

Two patients with iris neovascularization in the absence of retinal ischemia were subsequently found to have systemic cryoglobulinemia. Successful treatment of one patient's underlying lymphoma led to stabilization and resolution of neovascularization.

CONCLUSIONS

Systemic cryoglobulinemia may be associated with anterior segment ischemia and neovascularization, and should be considered in the differential diagnosis of iris neovascularization in the absence of apparent retinal ischemia.

Metaplastic squamous epithelial downgrowth after clear corneal cataract surgery

PURPOSE

To report a case of metaplastic squamous epithelial downgrowth after cataract surgery.

DESIGN

Interventional case report.

METHODS

Clinical, laboratory, and histologic findings are presented. Our study is in compliance with institutional review board guidelines.

RESULTS

A 76-year-old man developed anterior chamber inflammation five months after uncomplicated clear corneal cataract surgery. Despite antimicrobial and anti-inflammatory therapies, the inflammation persisted. An extensive examination failed to demonstrate an infectious etiology or lymphoma. Subsequently, the patient developed an incipient limbal lesion and iris mass. Immunostaining of a biopsy specimen from the iris mass indicated an epithelial-derived tumor. The prephthisical and painful eye was enucleated; histopathology of the globe revealed a contiguous lesion extending from the limbal mass to the iris tumor through the surgical incision site, a finding consistent with metaplastic squamous epithelial downgrowth. Systemic evaluation was negative.

CONCLUSIONS

After intraocular surgery, metaplastic epithelial downgrowth may occur as a consequence of occult ocular surface squamous neoplasia and masquerade as chronic inflammation; clinicians should be aware of this rare complication.

Inner retinal photoreception independent of the visual retinoid cycle

Mice lacking the visual cycle enzymes RPE65 or lecithin-retinol acyl transferase (Lrat) have pupillary light responses (PLR) that are less sensitive than those of mice with outer retinal degeneration (rd/rd or rdta). Inner retinal photoresponses are mediated by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting that the melanopsin-dependent photocycle utilizes RPE65 and Lrat. To test this hypothesis, we generated rpe65(-/-); rdta and lrat(-/-); rd/rd mutant mice. Unexpectedly, both rpe65(-/-); rdta and lrat(-/-); rd/rd mice demonstrate paradoxically increased PLR photosensitivity compared with mice mutant in visual cycle enzymes alone. Acute pharmacologic inhibition of the visual cycle of melanopsin-deficient mice with all-trans-retinylamine results in a near-total loss of PLR sensitivity, whereas treatment of rd/rd mice has no effect, demonstrating that the inner retina does not require the visual cycle. Treatment of rpe65(-/-); rdta with 9-cis-retinal partially restores PLR sensitivity. Photic sensitivity in P8 rpe65(-/-) and lrat(-/-) ipRGCs is intact as measured by ex vivo multielectrode array recording. These results demonstrate that the melanopsin-dependent ipRGC photocycle is independent of the visual retinoid cycle.

Abnormal anterior chamber associated immune deviation (ACAID) in 129-strain mice

PURPOSE

To characterize anterior chamber immune deviation (ACAID) in 129-strain and mixed 129-strain mice.

METHODS

ACAID was assayed using standard protocols with herpes simplex-1 (HSV-1) and trinitrophenol-hapten-spleen cells (TNP-spleen) in C57B1/6, 129P2, 129X1, and intercrossed strains. Systemic tolerance induction was assayed using an ultraviolet light skin tolerance protocol to 2,-4,6-trinitro-l-chlorobenzene (TNCB).

RESULTS

129X1 and C57Bl/6xl29Xl Fl mice did not show ACAID to HSV-1. C57Bl/6xl29P2 mice did not show ACAID to TNP-spleen. C57Bl/6xl29P2 mice did show normal peripheral immune deviation to TNCB. (C57Bl/6xl29Xl) x C57B1/6 N2 backcrossed mice showed a bimodal ACAID response to HSV-1 suggesting a single dominant allele in the 129X1 background responsible for suppressing ACAID.

CONCLUSION

ACAID to multiple antigens is significantly reduced in 129-strain mice and their outcrossed progeny. Since 129-strain embryonic stem cells are widely used to generate knockout and transgenic mice, care must be taken to extensively backcross resultant strains in order to assess the effect of particular genes on ACAID.

Physiologic diversity and development of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate numerous nonvisual phenomena, including entrainment of the circadian clock to light-dark cycles, pupillary light responsiveness, and light-regulated hormone release. We have applied multielectrode array recording to characterize murine ipRGCs. We find that all ipRGC photosensitivity is melanopsin dependent. At least three populations of ipRGCs are present in the postnatal day 8 (P8) murine retina: slow onset, sensitive, fast off (type I); slow onset, insensitive, slow off (type II); and rapid onset, sensitive, very slow off (type III). Recordings from adult rd/rd retinas reveal cells comparable to postnatal types II and III. Recordings from early postnatal retinas demonstrate intrinsic light responses from P0. Early light responses are transient and insensitive but by P6 show increased photosensitivity and persistence. These results demonstrate that ipRGCs are the first light-sensitive cells in the retina and suggest previously unappreciated diversity in this cell population.

Presumed ocular histoplasmosis syndrome

PURPOSE OF REVIEW

Presumed ocular histoplasmosis syndrome remains a significant cause of visual morbidity. We review recent literature on the pathogenesis and particularly on the treatment of complications of presumed ocular histoplasmosis syndrome.

RECENT FINDINGS

The pathogenesis of presumed ocular histoplasmosis syndrome remains mysterious; although some recent molecular evidence suggests a direct link between Histoplasma capsulatum and presumed ocular histoplasmosis syndrome, other reports document nearly identical disease occurring in the absence of H. capsulatum seropositivity. Treatment options have advanced rapidly in the past few years. Small case series and clinical trials suggest excellent efficacy for photodynamic therapy in the treatment of subretinal neovascularization associated with presumed ocular histoplasmosis syndrome. Preliminary studies also suggest efficacy of intravitreal corticosteroids. A large randomized controlled clinical trial failed to show efficacy for subretinal surgery in the management of presumed ocular histoplasmosis syndrome, however.

SUMMARY

Despite continued debate on the etiology of presumed ocular histoplasmosis syndrome, there have been significant advances in treatment of the blinding complications of presumed ocular histoplasmosis syndrome.

Pharmacological and rAAV gene therapy rescue of visual functions in a blind mouse model of Leber congenital amaurosis

BACKGROUND

Leber congenital amaurosis (LCA), a heterogeneous early-onset retinal dystrophy, accounts for approximately 15% of inherited congenital blindness. One cause of LCA is loss of the enzyme lecithin:retinol acyl transferase (LRAT), which is required for regeneration of the visual photopigment in the retina.

METHODS AND FINDINGS

An animal model of LCA, the Lrat-/- mouse, recapitulates clinical features of the human disease. Here, we report that two interventions--intraocular gene therapy and oral pharmacologic treatment with novel retinoid compounds--each restore retinal function to Lrat-/- mice. Gene therapy using intraocular injection of recombinant adeno-associated virus carrying the Lrat gene successfully restored electroretinographic responses to approximately 50% of wild-type levels (p < 0.05 versus wild-type and knockout controls), and pupillary light responses (PLRs) of Lrat-/- mice increased approximately 2.5 log units (p < 0.05). Pharmacological intervention with orally administered pro-drugs 9-cis-retinyl acetate and 9-cis-retinyl succinate (which chemically bypass the LRAT-catalyzed step in chromophore regeneration) also caused long-lasting restoration of retinal function in LRAT-deficient mice and increased ERG response from approximately 5% of wild-type levels in Lrat-/- mice to approximately 50% of wild-type levels in treated Lrat-/- mice (p < 0.05 versus wild-type and knockout controls). The interventions produced markedly increased levels of visual pigment from undetectable levels to 600 pmoles per eye in retinoid treated mice, and approximately 1,000-fold improvements in PLR and electroretinogram sensitivity. The techniques were complementary when combined.

CONCLUSION

Intraocular gene therapy and pharmacologic bypass provide highly effective and complementary means for restoring retinal function in this animal model of human hereditary blindness. These complementary methods offer hope of developing treatment to restore vision in humans with certain forms of hereditary congenital blindness.

Resetting the clock: Dexras1 defines a path

The signaling pathways by which light and activity shift the circadian clock are not well understood. In this issue of Neuron, Cheng et al. analyze mice lacking Dexras1 (a Ras family GTPase protein) and demonstrate an important role for G(i/o) signaling mediating both photic and nonphotic phase shifts of the circadian clock.

Nonvisual Photoreception in the Chick Iris

The embryonic chicken iris constricts to light ex vivo, but with characteristics atypical of visual phototransduction. The chick iris was most sensitive to short-wavelength light, demonstrating an action spectrum consistent with cryptochrome rather than with opsin pigments. Pupillary responses did not attenuate after saturating light exposure, but showed paradoxical potentiation. Iris photosensitivity was not affected by retinoid depletion or inhibitors of visual phototransduction. Knockdown of cryptochrome expression, but not of melanopsin expression, decreased iris photosensitivity. These data characterize a non-opsin photoreception mechanism in a vertebrate eye and suggest a conserved photoreceptive role for cryptochromes in vertebrates.

Sleep disturbances in young subjects with visual dysfunction

PURPOSE

To determine whether the type of ophthalmic disease is predictive of sleep and wakefulness disturbances in young subjects with visual dysfunction.

DESIGN

Prospective cohort study.

PARTICIPANTS AND CONTROLS

Twenty-five subjects (ages 12-20) were recruited from the Missouri School for the Blind. Twelve controls with normal sight were recruited from a residential school.

METHODS

Daily activity was monitored for 14 days using wrist actigraphy. Sleep and wakefulness measures were derived from actigraphy records by automated analysis. Visually impaired subjects were prospectively stratified by presence or absence of optic nerve disease.

MAIN OUTCOME MEASURES

Daytime napping and regularity of awakening time (wake-up time instability).

RESULTS

Subjects with optic nerve disease napped in the daytime significantly more than other visually impaired children or normal sighted controls: 28.1+/-4.0 minutes per day (mean +/- standard error) versus 11.9+/-2.4 minutes per day in equally visually impaired subjects with intact optic nerve function versus 6.2+/-2.2 minutes per day in subjects with normal sight (P<0.0001). These subjects also showed significantly more variable awakening times than the other groups. Logistic regression revealed that subjects with optic nerve disease are 9.1 times more likely to demonstrate daily napping of more than 20 minutes per day than equally blind subjects without optic nerve disease (95% confidence interval [CI] = 1.4-58.7, P = 0.02). Blind subjects with optic nerve disease are 21.3 times more likely than children with normal sight to nap more than 20 minutes on average per day (95% CI = 1.2-378, P = 0.04).

CONCLUSIONS

Optic nerve disease is predictive of increased daytime napping in young visually impaired subjects, suggesting that the nature and presence of ophthalmic disease affect the probability of concomitant sleep timing disorders.

Recent Insights into Mammalian Circadian Rhythms

This short review highlights recent progress in understanding the mammalian circadian clock. Advances in the understanding of the neuroanatomy of circadian rhythms, the molecular biology of the core clock mechanism, mechanisms of light entrainment of the circadian clock, clock synchronization among multiple tissues, and recent work on the relationship of the mouse circadian clock and cancer are discussed. This review is intended as an overview of recent research activity for the interested sleep disorders clinician or researcher.

Lecithin-retinol acyltransferase is essential for accumulation of all-trans-retinyl esters in the eye and in the liver

Lecithin-retinol acyltransferase (LRAT), an enzyme present mainly in the retinal pigmented epithelial cells and liver, converts all-trans-retinol into all-trans-retinyl esters. In the retinal pigmented epithelium, LRAT plays a key role in the retinoid cycle, a two-cell recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. We disrupted mouse Lrat gene expression by targeted recombination and generated a homozygous Lrat knock-out (Lrat-/-) mouse. Despite the expression of LRAT in multiple tissues, the Lrat-/- mouse develops normally. The histological analysis and electron microscopy of the retina for 6-8-week-old Lrat-/- mice revealed that the rod outer segments are approximately 35% shorter than those of Lrat+/+ mice, whereas other neuronal layers appear normal. Lrat-/- mice have trace levels of all-trans-retinyl esters in the liver, lung, eye, and blood, whereas the circulating all-trans-retinol is reduced only slightly. Scotopic and photopic electroretinograms as well as pupillary constriction analyses revealed that rod and cone visual functions are severely attenuated at an early age. We conclude that Lrat-/- mice may serve as an animal model with early onset severe retinal dystrophy and severe retinyl ester deprivation.