PARADOX OF VISION

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by Academician Mikhail OSTROVSKY, Head of laboratory, N. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences

 

Removal of cataract and implantation of artificial intraocular lens is now routine eye surgery that makes up about 80 percent of the total number of operations on the eye.

Eye doctors are making wide use of technologies predicated on medical and basic biological studies.

Back in the mid - 1980s Russian scientists developed a new generation of photoprotective artificial lenses for the eye, yellow intraocular lenses, called "Spektr".

These helped reduce by 90 percent the number of post-operative complications associated with the macula edema of the retina-when compared with techniques involving the use of conventional achromatic, colorless lenses.

In fact, foreign-made lenses with similar spectral transmittance entered the world market nearly two decades later - only in 2002 - 2003.

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"SOLARITY OF THE EYE"

"You cannot understand the eye unless you know the sun. This is why the eye is solar." Such are the closing words of the wonderful book "The Eye and the Sun", written by Acad. Sergei Vavilov (President of the USSR Academy of Sciences in 1945 to 1951), the father of the Soviet school of physical optics. First published in 1927, this work has had at least a dozen subsequent editions, and it is still the best book about the intersection of two disciplines, the physics of light and the physiology of vision. With the progress of science the metaphor, "solarity of the eye", has gained many dimensions. How does this wondrous phenomenon work?

Solar radiation or, rather, its spectrum gives rise to light-sensitive pigments, which initiate the visual process. This is what we call "visible light", or light in the visible range. A photon is the smallest portion of light. In the course of biological evolution, the visual cell has reached the limit of light sensitivity. The rod cells of the retina, which are responsible for twilight (achromatic, black-and-white) vision, are capable of detecting only one absorbed photon. The following chain of events obtains as a result: one photons → one molecule of the visual pigment that absorbs it → one excited rod-cell. Meanwhile the brain responds to as many as 10 - 15 photons in perceiving them as a very weak visible flash, that is, it picks up signals from ten to fifteen rods. Owing to this ability the human eye can make out barely visible stars in the night sky-of the sixth and occasionally, of the seventh and even eighth magnitudes. The sunlight reaching the earth constitutes only a 1012 to 1014th fraction of total solar radiation. So, our eyesight can perform within a vast span of illuminance - all the way from dark night to bright day. The daylight world appears to us in a thrilling palette because our eye can perceive all the colors of the rainbow. Responsible for color perception are cells other than rods, namely the retinal cones that contain blue-, green- and red-sensitive pigments, respectively.

Thus, the human eye is best adapted to the perception of natural sunlight, which supplies the brain with more than 90 percent of information about the world we live in. Yes, the eye is solar indeed!

However, light also spells danger - that's the paradox of vision. The molecular machinery of the eye is not only photosensitive, but also fragile, subject to lesions. As is shown by studies conducted in Russia and other countries over recent decades, rod-cells ought to "burn up" in sunlight. Just a mere glance at the light of the sun should make us blind. But this does not happen. Why? Because in the course of biological evolution the eye has developed a reliable multilevel system to protect it against such light-induced impairments.

The decoding of the intricate molecular mechanism of vision may help towards progress in the prevention and treatment of grave eye diseases. Learning the

Shorthand image of rod visual cell: rod, photoreceptor disk of the rod outer segment, photoreceptor membrane of the disk and a rhodopsin molecule with the chromophore group, 11 -cis-retinal (center), covalently bound to the protein moiety of the molecule (opsin).

 

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nature of the "solarity of the eye", we come to the rather simple idea of a yellow, cut-off artificial lens instead of the achromatic, colorless one.

VISUAL PURPLE (RHODOPSIN)

Purple is a sacred, majestic color-that of the rising Sun and of the mantles worn by Roman Catholic cardinals. Purple dyes were used in Ancient Egypt and on the island of Crete to dye clothes with. In Assyria and in the Byzantine (Eastern Roman) Empire it symbolized power and noble birth. Seafarers and merchants would sacrifice their lives in search of the regal paint extracted from purple sea mollusks in faraway exotic lands. However, the ancients had no inkling of the main thing: that purple is the color of a unique substance that bestows the gift of vision on us. This is visual purple. In modern terminology its name is rhodopsin. Its discovery harks back to 1851 when the German physiologist Heinrich Muller isolated a frog eye retina and examined it. It proved to be pink-purple in color. This curious fact attracted little, if any, attention from the scientific community. A quarter century after the event, Franz Boll of Austria repeated the experiment. "The roseate retina isolated from the eye is bleached in light and turns whitish. Such bleaching is directly associated with the process of vision," Dr. Boll wrote in 1876. His article, when published in the press, ushered in systematic studies of visual pigments. Today light-sensitive visual rhodopsin is one of the best-studied membrane proteins.

Therefore, we are dealing with a pigment present in the retinal rods of man and other vertebrates. Its molecule contains a chromophore group, two oligosaccharide chains and the water-insoluble membrane protein, called opsin. The rhodopsin molecular mass is relatively small, about 40 kDa, and its polypeptide chain has 348 amino acid residues. The rhodopsin chromophore group alike is represented by vitamin A aldehyde, or retinal- namely by just one isomer from among the many - the 11-cis retinal*.

Rhodopsin is the first animal membrane protein whose amino acid sequence and topography in the membrane were determined in the 1980s almost simultaneously by Soviet (Yuri Ovchinnikov and his team) and American (Pul Hargrave and his team) groups. At the beginning of the twenty-first century its three-dimensional structure was also determined by joint Japanese-American group, -first with a resolution of 2.8, and then - 2.6 A.

Rhodopsin photoconversion includes a photochemical reaction of the cis-trans isomerization of the chromophore group, 11 -cis retinal, and subsequent temperature-dependent reactions in the dark. This is one of the fastest photochemical reactions known in photochemistry, and it proceeds as less as 200 femtoseconds (1 fs = 1015 s). Thereby the first photoproduct, photorhodopsin, is formed in which the 11 -cis retinal is present in all-trans isomeric form. The follow-up conversions within the rhodopsin molecule take place in darkness. Meanwhile the photorhodopsin - in a matter of several dozen picoseconds (1 ps = 1012s) - transforms into another product, bathorhodopsin, which is followed by other products of rhodopsin photoconversion (photolysis). Photoconversion in case of vertebrate rhodopsin ends in a break of covalent chemical bond between the all-trans retinal and the protein, called opsin. All-trans retinal is finally released from opsin to find itself in the phospholipids environment of the photoreceptor membrane. At this point all-trans retinal should be eliminated from the membrane as soon as possible, because all-trans retinal per se is phototoxic. The elimination mechanism is there. However, if for

 

Left: seven transmembrane "strands" of rhodopsin with 11-cis-retinal in between. Right: chemical formulae of 11-cis-and all-trans-retinals.

* With reference to cis-trans-isomerism, or the spatial (three-dimensional) arrangement of the atoms of chemical compounds positioned either on one side (cis-isomer) or on different sides (trans-isomer) of the double bond (C=C, C=N) plane. Isomer is any of two or more chemical compounds having the same molecular formula and thus the same composition, but differing in properties because their atoms are arranged differently. - Ed.

стр. 20

some reason retinal is accumulated in the photoreceptor membrane nonetheless, it may become responsible for the light-effected impairment of the retina.

PHOTOTRANSDUCTION MECHANISM

Phototransduction is the process of transformation and amplification of a light signal within the visual cell, whereby the signal is intensified nearly millionfold. A bioelectric signal is obtained as a result. How?

A photon is by the chromophore group of the rhodopsin molecule R, that is by 11-cis retinal, which turns into all-trans isomeric form, the all-trans retinal. The retinal cis-trans transfer causes a conformational realignment of the protein moiety of the molecule that is opsin, to make rhodopsin capable of interacting with the next protein in the sophisticated chain of multistep reactions of phototransduction. This protein is called transducin (TD). In its turn, the activated TD activates the next protein in the cascade, the enzyme phosphodi-esterase (PDE). In a vigorous, high-speed reaction this enzyme destroys cyclic guanosine monophosphate (cGMP), which is an intracellular messenger or signal transmitter. The declining concentration of free cGMP within the cytoplasm of the photoreceptor cell results in a change of bioelectrical potential on the cell membrane, i.e. hyperpolarization. In fact, this phenomenon of hyperpolarization evolves as a bioelectric signal transmitted to nerve cells of the retina and further on to the brain.

Thus, the light signal is intensified in three consecutive stages of the phototransduction cascade. First, the light-activated rhodopsin molecule activates a set of TD molecules. Next, the TD-activated PDE destroys a large number of cGMP molecules. And finally, the decrease in cGMP concentration blocks the specific ionic channels in the cell membrane. Their obstruction gives rise to a hyperpolarized electrical potential of the visual cell, that is a bioelectric signal, in response to rhodopsin-effected light absorption.

"PHOTOFRAGILITY" OF THE RETINA

Light in the ultraviolet and blue parts of solar spectrum can be harmful. Such radiation can trigger the destructive photochemical reactions of free-radical oxidation (photo-initiated oxidation). Three factors are both necessary and sufficient for initiating such oxidation. First, the presence of colored substances (photo-sensitizers). Second, oxidation substrates, i.e. molecules subject to oxidation. And last, the implication of oxygen. These three factors are all there, both in the retina and in the retinal pigment epithelium (a mono-layer of cells sited in the eyeball behind the retina and before the vascular coat of the eye). Acting as photo-sensitizers can be both the all-trans retinal released from the rhodopsin molecule at the final stage of its photolysis and the subsequent products of its conversion. Proteins and lipids of visual or retinal pigment epithelium cells are the target of photo-initiated oxidation. As to oxygen, the eye retina is amply supplied with it - just as well as the brain.

Getting into the eye, light is first absorbed by the retina and thereupon by the retinal pigment epithelium. This epithelium contains a large number of melanosomes, which are black or dark-brown granules. Melanosomes that belong to eye screening pigments protect both visual cells and retinal pigment epithelium cells from excessive light entering the eye. As we demonstrated way back at the end of the 1970s and in the early 1980s, melanosomes are not only a passive screening pigment, but also possess a manifest antioxidant ability, an important thing for prevention the light-induced oxidation.

Because rod and cone outer segments are permanently renewed all along, and their fragments are phagocyted by retinal pigment epithelium cells, a significant amount of such fragments, so called phagosomes, comes to be accumulated with age. Their incomplete "digestion" culminates in formation and deposition

Schematic representation of phototransduction process. R - rhodopsin molecule, T- transducin molecule, or G - protein that is protein that is binding guanosine 5'-triphosphate (GTP), PDE - phosphodiesterase molecule. GTP-guanosine triphosphate, GC-guanylacetcyclase, GMP-guanosine monophosphate, cGMP-cyclic guanosine monophosphate.

An ionic channel is shown in the cell membrane.

 

стр. 21

 

Scheme of retinal pigment epithelium cell and rod outer segments (ROS): left - in young age, right - in middle or old age.

within the retinal pigment epithelium cells. Amassed in the course of one's life, such granules stay in, unfortunately. In the beginning of the 1990s we found that the "age pigment" is not just inert material or passive "trash" within the pigment epithelium cells. Lipofuscin granules are capable of generating free radicals-the active high-toxicity forms of oxygen. Since these granules readily absorb ultraviolet and blue light, this part of solar spectrum may be dangerous for the cell. The hazard becomes real if the natural, physiological photoprotective system of the retina and retinal pigment epithelium is inadequate.

NATURAL DEFENSES

The combination of light, photosensitizers, oxygen, and readily oxidized substrates (lipids and proteins) is thus a source of real danger for the retina and the retinal pigment epithelium. Such is the paradox of vision when light acts both as an information carrier and as a potentially harmful factor. The paradox has been resolved with much success by evolution through multilevel defenses. This protective system provides for a variety of counteracting factors, such as the sufficiently rapid renewal of the photoreceptor membranes of visual cells; the potent complex of endogenous antioxidants; the effective mechanism for the transportation and elimination of free all-trans retinal; and finally, the system of optical color cut-off filters, where the crystalline lens of the eye is assigned a key role.

The most radical mode of protection consists, of course, in the permanent renewal of the visual cells photoreceptor membranes. Such renewal helps avoid accumulation of molecular defects. That is why the molecular "machinery" of photoreception, one that implicates the primary processes of vision, is effective throughout an individual's life span.

The next line of defense involves antioxidants. These are vitamins E (α-tocopherol) and С (ascorbic acid), taurine, and a set of antioxidant enzymes. Their cumulative effect is intensified by melanosomes, the screen-

 

Optical defense of the retina and retinal pigment epithelium: crystalline lens as a cut - off filter. The phenomenon of age-related yellowing of the lens. Spectra (left to right): 1 - for newborns; 2 - at age 8 to 29; 3 - from 31 to 49 years; 4 - from 52 to 65; 5 - above 70 years of age.

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ing pigment. The shortage of this pigment makes pink-eyed albinos shy of light and much sensitive to light-initiated damages of the eye.

And now comes the most vital defense line of the retina and retinal pigment epithelium, an optical one. It comprises a sequential set of cut-off filters: the cornea, iris and lens as well as the screening pigments - melanosomes and yellow macular pigment. They keep off the UV and in part, blue light shortwave radiation harmful to the retina and retinal pigment epithelium. In addition the yellow filter works to diminish the chromatic aberration, i.e. to improve the acuity of vision.

The crystalline lens plays the main light-filtering role in the system of optical protection. The lens turns yellow with age. It looks like a natural physiological process instrumental in prohibiting more of the blue light from the retina. That is to say, this is tantamount to an additional yellow cut-off filter. Working on an artificial intraocular lens of the eye (which, as we saw it, imitates the light-filtering properties of natural lens), we carried out a detailed study of this physiological phenomenon. It is remarkable that many animals living in a bright light environment have an intensely yellow lens from the very birth.

Surgical removal of a cataractous lens and implantation of a colorless intraocular lens (let alone one with no UV absorber) means a dramatic change in the spectral composition of the light reaching the retina and retinal pigment epithelium. Such intraocular lenses cause too much of the blue light to reach the retina and pigment epithelium, which is even more dangerous in the absence of a UV absorber. That is why we developed a photoprotective artificial lens with natural spectral characteristics, which is a yellow intraocular lens, now with a record of clinical use behind it. We got an author's certificate from the USSR as well as patents from the Russian Federation and the United States. Working like a natural crystalline lens in a person about 50 years of age, our intraocular lens "Spektr" keeps out the ultraviolet and much of the violet-blue component of the optical spectrum. We as well as many other physiologists of vision and ophthalmologists conclude that the implantation of a yellow intraocular lens (not a colorless one, even if equipped with a UV filter!) should become a routine clinical practice.

According to information supplied by Svyatoslav Fyodorov's Institute "Microsurgery of the Eye", over a million intraocular lenses of "Spektr" format have been produced and implanted since 1986. The available clinical evidence on the remote consequences of the implantation of yellow intraocular lenses confirms their light-protective effect for the retina. While not interfering with normal color perception, they keep chromatic aberration down. In his latest article the French ophthalmologist C. Malbrel argues that a yellow intraocular lens bringing the blue light component down to 450 nm offers effective protection for cases of macula degeneration, one of the most widespread and grave retina diseases. It damages the central (macular) region of the retina, i.e. central vision.

Absorption spectra of photosensitizers as well as light transmission spectra of natural human lens and by yellow intraocular lens "Spektr".

 

стр. 23

 

A - a human lens (age, ca. 50 years), B - an intraocular lens "Spektr"

AGGRAVATING FACTOR

Age-dependent diseases of the retina are a thorny problem for present-day ophthalmology. In Russia and in many other countries such pathologies that result in complete or partial blindness and incapacitation escalate into a grave social problem. A good deal of time and effort has been spent in recent years in collecting information on similar maladies. Such data show that light aggravates the clinical picture of the early and late forms of age-related macula degeneration of the retina (age-related maculopathies).

There is a clear connection between the surgical removals of a cataractous lens that is usual dark-yellow or brown colored and the implantation of an artificial colorless, even UV-absorbing, intraocular lens, on the one hand. As demonstrated recently by work carried out in the United States by a team of Ronald Klein, an eminent ophthalmologist and epidemiologist, five years after the excision of a cataract and implantation of a colorless (not stained) intraocular lens, the probability of the late stages of age-related macula degeneration increases 5.7 times over (statistically accurate findings).

Therefore, cataract surgery cases showing even the earliest symptoms of the pathology are the best candidates for the implantation of a yellow artificial lens. When out of doors, especially in bright sunlight, patients should wear special protective spectral glasses. Such kind glasses as a cut-off filters may delay the progression macula degenerations of the retina. The causes of this and related diseases are not yet clear, unfortunately. What we know is that many of them are of hereditary nature. No effective ways of treatment have been developed up to present time. But observing light hygiene precautions, a patient can slow down the progress of such anomalies. This is very important. Now we are only beginning to understand the mechanisms of aggravating action of UV and violet-blue light. In our research we are drawing upon fundamental studies into the molecular machinery of vision, including the nature of its photobiological paradox and ways of its solving. In the course of evolution of eye it has been developed multilevel defense system against a hazard of light-initiated lesions. The yellow cut-off artificial intraocular lenses developed by us just imitates one of the natural defensive lines.

A research team that has developed, tested and introduced for clinical practice photoprotective artificial intraocular lenses with natural spectral characteristics has been awarded a Prize of the Russian Federation Government (Science and Engineering) for 2005. These are Academician Mikhail Ostrovsky (who supervised the project); Pavel Zak, Cand. Sc. (Biol.), a leading research associate at the N. Emanuel Institute of Biochemical Physics; Leonid Linnik, Dr. Sc. (Medicine), a chief scientific consultant of the Stanislav Fyodorov Institute "Microsurgery of the Eye"; and Khristo Takhchidi, Dr. Sc. (Medicine), Director General of the same Institute.


Опубликовано 27 сентября 2018 года




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