Circadian and light-driven regulation of rod dark adaptation

Continuous visual perception and the dark adaptation of vertebrate photoreceptors after bright light exposure require recycling of their visual chromophore through a series of reactions in the retinal pigmented epithelium (RPE visual cycle). Light-driven chromophore consumption by photoreceptors is greater in daytime vs. nighttime, suggesting that correspondingly higher activity of the visual cycle may be required. However, as rod photoreceptors are saturated in bright light, the continuous turnover of their chromophore by the visual cycle throughout the day would not contribute to vision. Whether the recycling of chromophore that drives rod dark adaptation is regulated by the circadian clock and light exposure is unknown. Here, we demonstrate that mouse rod dark adaptation is slower during the day or after light pre-exposure. This surprising daytime suppression of the RPE visual cycle was accompanied by light-driven reduction in expression of Rpe65, a key enzyme of the RPE visual cycle. Notably, only rods in melatonin-proficient mice were affected by this daily visual cycle modulation. Our results demonstrate that the circadian clock and light exposure regulate the recycling of chromophore in the RPE visual cycle. This daily melatonin-driven modulation of rod dark adaptation could potentially protect the retina from light-induced damage during the day

Chromophore supply modulates cone function and survival in retinitis pigmentosa mouse models.

Retinitis pigmentosa (RP) is an ocular disease characterized by the loss of night vision, followed by the loss of daylight vision. Daylight vision is initiated in the retina by cone photoreceptors, which are gradually lost in RP, often as bystanders in a disease process that initiates in their neighboring rod photoreceptors. Using physiological assays, we investigated the timing of cone electroretinogram (ERG) decline in RP mouse models. A correlation between the time of loss of the cone ERG and the loss of rods was found. To investigate a potential role of the visual chromophore supply in this loss, mouse mutants with alterations in the regeneration of the retinal chromophore, 11-cis retinal, were exam- ined. Reducing chromophore supply via mutations in Rlbp1 or Rpe65 resulted in greater cone function and survival in a RP mouse model. Conversely, overexpression of Rpe65 and Lrat, genes that can drive the regeneration of the chromophore, led to greater cone degeneration. These data suggest that abnormally high chromophore supply to cones upon the loss of rods is toxic to cones, and that a potential therapy in at least some forms of RP is to slow the turnover and/or reduce the level of visual chromophore in the retina

CRALBP supports the mammalian retinal visual cycle and cone vision

Mutations in the cellular retinaldehyde-binding protein (CRALBP, encoded by RLBP1) can lead to severe cone photoreceptor-mediated vision loss in patients. It is not known how CRALBP supports cone function or how altered CRALBP leads to cone dysfunction. Here, we determined that deletion of Rlbp1 in mice impairs the retinal visual cycle. Mice lacking CRALBP exhibited M-opsin mislocalization, M-cone loss, and impaired cone-driven visual behavior and light responses. Additionally, M-cone dark adaptation was largely suppressed in CRALBP-deficient animals. While rearing CRALBP-deficient mice in the dark prevented the deterioration of cone function, it did not rescue cone dark adaptation. Adeno-associated virus-mediated restoration of CRALBP expression specifically in Müller cells, but not retinal pigment epithelial (RPE) cells, rescued the retinal visual cycle and M-cone sensitivity in knockout mice. Our results identify Müller cell CRALBP as a key component of the retinal visual cycle and demonstrate that this pathway is important for maintaining normal cone-driven vision and accelerating cone dark adaptation

The role of retinol dehydrogenase 10 in the cone visual cycle

Pigment regeneration is critical for the function of cone photoreceptors in bright and rapidly-changing light conditions. This process is facilitated by the recently-characterized retina visual cycle, in which Müller cells recycle spent all-trans-retinol visual chromophore back to 11-cis-retinol. This 11-cis-retinol is oxidized selectively in cones to the 11-cis-retinal used for pigment regeneration. However, the enzyme responsible for the oxidation of 11-cis-retinol remains unknown. Here, we sought to determine whether retinol dehydrogenase 10 (RDH10), upregulated in rod/cone hybrid retinas and expressed abundantly in Müller cells, is the enzyme that drives this reaction. We created mice lacking RDH10 either in cone photoreceptors, Müller cells, or the entire retina. In vivo electroretinography and transretinal recordings revealed normal cone photoresponses in all RDH10-deficient mouse lines. Notably, their cone-driven dark adaptation both in vivo and in isolated retina was unaffected, indicating that RDH10 is not required for the function of the retina visual cycle. We also generated transgenic mice expressing RDH10 ectopically in rod cells. However, rod dark adaptation was unaffected by the expression of RDH10 and transgenic rods were unable to use cis-retinol for pigment regeneration. We conclude that RDH10 is not the dominant retina 11-cis-RDH, leaving its primary function in the retina unknown

The Mechanism and Regulation of Mammalian Photoreceptor Dark Adaptation

The visual perception of vertebrates begins in rod and cone photoreceptors. Both photoreceptors require visual pigments to detect light. At the first step of light detection, a chromophore molecule (i.e. 11-cis retinal), which is conjugated to the visual pigment in photoreceptor outer segment, absorbs a photon. Photoisomerization of the chromophore activates the visual pigment, triggers the phototransduction cascade, and produces electrical signals. After photoisomerization, the chromophore is ultimately converted to all-trans retinol, which must be recycled to regenerate the visual pigment. This visual pigment regeneration process is called the visual cycle. It is the rate-limiting step of the photoreceptor dark adaptation after extensive light activation. The chromophore is recycled through retinal pigment epithelium (RPE) cells. In addition, cones can access a second visual cycle through the retinal Müller cells. This second visual cycle is cone-specific and fast-operating. However, it is unknown how important this retina visual cycle is to mammalian cone function and dark adaptation. To address this question, we studied whether this pathway could be impaired by deleting one of its components, the cellular retinaldehyde binding protein (CRALBP), and how this impairment would affect cone function and survival. We found that the deletion of CRALBP in mice led to impaired retina visual cycle and cone overall dark adaption, causing chronic chromophore deprivation, which desensitized M-cones, mislocalized M-opsin, and decreased M-cone numbers. We discovered that only rescuing the retina, but not RPE visual cycle, could partially restore the cone function. Considering the changes in ambient luminance, chromophore consumption is vastly different at day compared to at night. It is not clear whether the efficiency of the RPE visual cycle is modulated to reflect this chromophore consumption difference. To explore this question, we conducted rod dark adaptation experiments at subjective day, subjective night and objective day using electroretinography (ERG) on both melatonin-proficient and melatonin-deficient mouse strains. We observed that in melatonin-proficient mice the RPE visual cycle during the day is slightly down-regulated by the circadian clock and dramatically down-regulated by light exposure. We did not observe any such differences in melatonin-deficient strains, suggesting that this daytime down-regulation is melatonin-dependent. Cones, but not rods can oxidize the 11-cis retinol produced by the retina visual cycle. However, the 11-cis retinol dehydrogenase (RDH) driving this reaction in cones has not been unidentified. To address this question, we examined how knocking out RDH10, an 11-cis RDH candidate, selectively in cones or in the retina affects the retina visual cycle. We did not observe any alteration in cone function and the retina visual cycle, suggesting that RDH10 is not necessary for the retina visual cycle. In addition, the transgenic RDH10 rods did not accelerate rod dark adaptation in vivo, suggesting that RDH10 is not sufficient for rods to access the retina visual cycle. The identity of the cone 11-cis RDH(s) is still unclear. In summary, we first reported that the retina visual cycle supports cone function and dark adaptation. CRALBP plays a crucial role in retina visual cycle, whereas RDH10 appears not to be involved in this pathway. The RPE visual cycle is down-regulated to decrease the chromophore turnover for saturated rods during the day. These findings strongly support the existence of a functional retina visual cycle and provide hints for future study on the evolution of this pathway

Research on Optical Spectrum Processing for Photonic-Assisted Broadband RF Cross- Eye Jamming System

Cross-eye jamming technology has been used in electronic warfare, which attempts to protect a military platform from monopulse tracking radars. The cross-eye jamming technology has the ability of inducing angular errors to the monopulse tracking radars by transmitting two jamming signals with equal amplitudes and opposite phases. At present, high operation frequency and broadband cross-eye jamming system has been rarely demonstrated. Therefore, the present cross-eye jamming systems are hard to jam frequency-agile radar or multi-band radar, whose carrier frequency covers a large spectral range. In this paper, a photonic-assisted broadband radio frequency (RF) cross-eye jamming system is proposed and experimentally demonstrated. To achieve effective jamming effect, the intercepted radar signal is modulated to optical carriers and the phase shift is realized by optical spectrum processing. The relationship between system parameter tolerances and jamming effects are also simulated. Furthermore, the RF transfer function of X-band, Ku-band and K-band has been obtained in the experiments. The experimental results show that the amplitude and phase mismatch are below 2.15 degrees and 0.4 dB, respectively. Calculated cross-eye gain is 39 dB