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Date of Award
Doctor of Philosophy (PhD)
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.
Chair and Committee
Joseph Corbo, Daniel Kerschensteiner, Peter Lukasiewicz, Paul Taghert
Xue, Yunlu, "The Mechanism and Regulation of Mammalian Photoreceptor Dark Adaptation" (2015). Arts & Sciences Electronic Theses and Dissertations. 692.
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