METABOLIC BASIS OF VISUAL PIGMENT CONVERSION IN METAMORPHOSING RANA CATESBEIANA

Vitamin A1/A2 chromophore exchange: Its role in spectral tuning and visual plasticity

Vertebrate rod and cone photoreceptors detect light via a specialized organelle called the outer segment. This structure is packed with light-sensitive molecules known as visual pigments that consist of a G-protein-coupled, seven-transmembrane protein known as opsin, and a chromophore prosthetic group, either 11-cis retinal ('A₁') or 11-cis 3,4-didehydroretinal ('A₂'). The enzyme cyp27c1 converts A₁ into A₂ in the retinal pigment epithelium. Replacing A₁ with A₂ in a visual pigment red-shifts its spectral sensitivity and broadens its bandwidth of absorption at the expense of decreased photosensitivity and increased thermal noise. The use of vitamin A₂-based visual pigments is strongly associated with the occupation of aquatic habitats in which the ambient light is red-shifted. By modulating the A₁/A₂ ratio in the retina, an organism can dynamically tune the spectral sensitivity of the visual system to better match the predominant wavelengths of light in its environment. As many as a quarter of all vertebrate species utilize A₂, at least during a part of their life cycle or under certain environmental conditions. A₂ utilization therefore represents an important and widespread mechanism of sensory plasticity. This review provides an up-to-date account of the A₁/A₂ chromophore exchange system.

Phototransduction: Making the Chromophore to See Through the Murk

A candidate gene approach has finally identified the 3,4-dehydrogenase that converts vitamin A1 into vitamin A2 to supply the chromophore for rhodopsin that freshwater vertebrates need for long-wavelength vision.

Transient expression of thyroid hormone nuclear receptor TRβ2 sets S opsin patterning during cone photoreceptor genesis

Cone photoreceptors in the murine retina are patterned by dorsal repression and ventral activation of S opsin. TR beta 2, the nuclear thyroid hormone receptor beta isoform 2, regulates dorsal repression. To determine the molecular mechanism by which TR beta 2 acts, we compared the spatiotemporal expression of TR beta 2 and S opsin from embryonic day (E) 13 through adulthood in C57BL/6 retinae. TR beta 2 and S opsin are expressed in cone photoreceptors only. Both are transcribed by E13, and their levels increase with cone genesis. TR beta 2 is expressed uniformly, but transiently, across the retina. mRNA levels are maximal by E17 at completion of cone genesis and again minimal before P5. S opsin is also transcribed by E13, but only in ventral cones. Repression in dorsal cones is established by E17, consistent with the occurrence of patterning during cone cell genesis. The uniform expression of TR beta 2 suggests that repression of S opsin requires other dorsal-specific factors in addition to TR beta 2. The mechanism by which TR beta 2 functions was probed in transgenic animals with TR beta 2 ablated, TR beta 2 that is DNA binding defective, and TR beta 2 that is ligand binding defective. These studies show that TR beta 2 is necessary for dorsal repression, but not ventral activation of S opsin. TR beta 2 must bind DNA and the ligand T3 (thyroid hormone) to repress S opsin. Once repression is established, T3 no longer regulates dorsal S opsin repression in adult animals. The transient, embryonic action of TR beta 2 is consistent with a role (direct and/or indirect) in chromatin remodeling that leads to permanent gene silencing in terminally differentiated, dorsal cone photoreceptors.

Seasonal cycle in vitamin A1/A2-based visual pigment composition during the life history of coho salmon (Oncorhynchus kisutch)

Microspectrophotometry of rod photoreceptors was used to follow variations in visual pigment vitamin A1/A2 ratio at various life history stages in coho salmon. Coho parr shifted their A1/A2 ratio seasonally with A2 increasing during winter and decreasing in summer. The cyclical pattern was statistically examined by a least-squares cosine model, fit to the 12-month data sets collected from different populations. A1/A2 ratio varied with temperature and day length. In 1+ (>12 month old) parr the A2 to A1 shift in spring coincided with smoltification, a metamorphic transition preceding seaward migration in salmonids. The coincidence of the shift from A2 to A1 with both the spring increase in temperature and day length, and with the timing of seaward migration presented a challenge for interpretation. Our data show a shift in A1/A2 ratio correlated with season, in both 0+ (<12 months old) coho parr that remained in fresh water for another year and in oceanic juvenile coho. These findings support the hypothesis that the A1/A2 pigment pair system in coho is an adaptation to seasonal variations in environmental variables rather than to a change associated with migration or metamorphosis.

Substitution of porphyropsin for rhodopsin in mouse retina

Weanling, male albino mice were placed on a vitamin A-free diet for three months to deplete their vitamin A stores. The vitamin A-deficient mice were injected intraperitoneally with all-trans 3-dehydroretinol. 3-dehydroretinol was rapidly incorporated into the liver as a fatty acid ester. The chromophore of visual pigment increased gradually and reached a normal level 13 days after the injection. 3-Dehydroretinal accounted for 95% of the total chromophore in the retina. The high proportion of 3-dehydroretinal was observed also in the long-term experiment which was continued for six weeks with the injection of 3-dehydroretinol once a week. When the animal was injected with a mixture of 3-dehydroretinol and retinol, the ratio of dehydroretinal/retinal in the retina was far lower than the ratio of dehydroretinol/retinol in the liver. These results indicate that 3-dehydroretinol is not converted to retinol in mouse and is used less efficiently than retinol for the chromophore of visual pigment. The synthesis of visual pigment was observed even when the animal was kept in complete darkness after the injection of all-trans 3-dehydroretinol. This fact indicates that light is not required for the production of 11-cis chromophore of visual pigment.

Conversion of retinol to 3,4-didehydroretinol in the tadpole

The conversion of retinol to 3,4-didehydroretinol in bullfrog tadpoles was studied by injecting [3H] all-trans retinol into the peritoneal cavity. The specific activities of retinoids in the eye and the rest of the body at various time intervals after the injection were then determined by HPLC (high-performance liquid chromatography). Radioactivity was observed in ocular 3,4-didehydroretinyl esters after 2 days and their specific activity increased throughout the 2 weeks of experiment. This demonstrates that tadpoles can convert retinol to its 3,4-didehydro derivative. In vitro experiments performed on isolated eye cups also suggested that the ocular tissues could convert retinol to 3,4-didehydroretinol. In the eye, the specific activity of porphyropsin or all-trans 3,4-didehydroretinal (extracted by the denaturing solvent acetone) exceeded that of the all-trans 3,4-didehydroretinyl esters in storage. This suggests that the main ocular store of 3,4-didehydroretinyl esters does not constitute a precursor pool for porphyropsin synthesis.

Visual pigments and the labile scotopic visual system of fish

Among mammals, birds, most reptiles and chondrichthians, only rhodopsins are present. Among agnathans, osteichthians, amphibians and certain freshwater turtles there are species having only porphyropsins or only rhodopsins or, more interestingly, both pigments, either sequentially or together. This latter grouping represents the paired-pigment species. Associated with the presence of paired-pigments is the possibility that the proportions of rhodopsin and porphyropsin may change. Depending on the characteristics of each paired-pigment species, naturally occurring changes in visual pigment ratios are related to migrations in anadromous and catadromous teleosts and anadromous cyclostomes and to seasonal variation in several teleosts. In addition, the visual pigment composition of certain species of teleosts has been altered by the specific effects of light, temperature, diet and hormones. Of two possible mechanisms for altering spectral sensitivity, varying the proportion of rhodopsin and porphyropsin is far more common than utilizing a single chromophore and changing the opsin. In addition to the long established evidence that extractable rod pigment ratios may change during the life cycle or in response to specific exogenous factors, there is the more recent recognition from microspectrophotometry that cone pigment ratios may also change in concert. The effect of lighting conditions and temperature on the visual pigment composition of certain paired-pigment species is presented.

Prolactin-thyroxine antagonism and the metamorphosis of visual pigments in Rana catesbeiana tadpoles

The relationship between prolactin-thyroxin antagonism and the metamorphosis of visual pigments in larval amphibians has been investigated using bullfrog (Rana catesbeiana) tadpoles. Althought prolactin-thyroxine antagonism is demonstrable by morphological criteria, ovineprolactin does not appear to anatagonize thyroxine-induced rhodopsin synthesis. The hypothesisis offered that prolactin-thyroxine antagonism is the result of differential gene activities which are opposite in their physiological effects.

Rhodopsin and porphyropsin fields in the adult bullfrog retina

Though it had been supposed earlier that the bullfrog undergoes a virtually complete metamorphosis of visual systems from vitamin A(2) and porphyropsin in the tadpole to vitamin A(1) and rhodopsin in the adult, the present observations show that the retina of the adult frog may contain as much as 30-40% porphyropsin, all of it segregated in the dorsal zone. The most dorsal quarter of the adult retina may contain 81-89% porphyropsin mixed with a minor amount of rhodopsin; the ventral half contains only rhodopsin. Further, the dorsal zone contains a two to three times higher concentration of visual pigments than the ventral retina. The pigment epithelium underlying the retina contains a corresponding distribution of vitamins A(1) and A(2), predominantly vitamin A(2) in the dorsal pigment epithelium, exclusively vitamin A(1) in the ventral zone. The retina accepts whatever vitamin A the pigment epithelium provides it with, and turns it into the corresponding visual pigment. Thus, a piece of light-adapted dorsal retina laid back on ventral pigment epithelium regenerates rhodopsin, whereas a piece of light-adapted ventral retina laid back on dorsal pigment epithelium regenerates predominantly porphyropsin. Vitamin A(2) must be made from vitamin A(1), by dehydrogenation at the 3,4-bond in the ring. This conversion must occur in the pigment epithelium, presumably through the action of a vitamin A-3,4-dehydrogenase. The essential change at metamorphosis is to make much less of this dehydrogenase, and to sequester it in the dorsal pigment epithelium. Some adult bullfrogs, perhaps characteristically taken in the summer, contain very little porphyropsin-only perhaps 5%-still sequestered in the dorsal retina. The gradient of light over the retinal surface has little if any effect on this distribution. The greater density of visual pigments in the dorsal retina, and perhaps also-although this is less clear-the presence of porphyropsin in this zone, has some ecological importance in increasing the retinal sensitivity to the dimmer and, on occasion, redder light received from below.