By Dean Haycock
Special To BioWorld Today
There is more to the eye than previously seen.
Obviously, it grabs light to form an image of the world. But even the eyes of sightless individuals grab light for another purpose that has little to do with recognizing objects in a visual field. Somewhere in the retina is an unidentified photoreceptor that sets and resets the biological clock, the internal regulatory system that influences activity, sleep, hormonal function and other biological processes. You may never think about these processes until you fly to another continent and find yourself stumbling around like a zombie. Jet lag is a symptom of a biological clock out of kilter.
For Russell Foster, reader in zoology at the Imperial College of Science, Technology and Medicine, London, and his colleagues, the dual function of the eye is an important observation to emerge from their recent work. Their papers, ¿Regulation of Mammalian Circadian Behavior by Non-rod, Non-cone, Ocular Photoreceptors¿ and ¿Regulation of the Mammalian Pineal by Non-rod, Non-cone, Ocular Photoreceptors¿ will both appear this Friday in Science.
¿We can shift the biological clock to local time, primarily by exposure to the light environment,¿ Foster told BioWorld Today. ¿We know that is an ocular response. It is mediated by the eye, because if you cover the eye, or if an individual has no eyes, this capacity to lock onto local time is gone. So, the question has been, What are the cells within the eye that mediate this response?¿¿
Earlier in this decade, researchers showed that mice with hereditary retinal disorders that eliminated their classical visual responses still nevertheless had intact biological clocks. ¿Now, we know in those animals that there were a few rod and cone cells [pigment containing cells in the retina] left,¿ Foster said. ¿What we wanted to do was to show that the rods and the cones, those cells that we know are light-sensitive in the eye, are not required for the regulation of biological time.¿
To do it, he and his co-workers used two assays, each described in one of their two papers. One involved shifting circadian behavior ¿ in this case, the propensity of rodents to be active at night. The other involved a biochemical event closely linked to the biological clock ¿ suppression of melatonin in the pineal gland by light.
The crux of the research, of course, was the type of mice they ran through these assays. Their subjects were equipped with genes that destroyed the rod and cone cells in their retinas, thus eliminating the doubt left by the possibility that mice used in earlier studies had some residual rods and cones. The rodless, coneless mice respond to light by resetting their biological clocks just as normal mice do. They also respond to light by suppressing pineal melatonin just as normal mice do.
¿We have shown that the classical image-forming visual system, the rod and cone system, is not required [for resetting the clock],¿ Foster said. ¿But this is definitely an ocular response, because if you remove the eyes of these mice, this capacity is gone. So, there has to be something left in the eye regulating biological time.¿
Two candidates for that ¿something¿ were cryptochrome proteins named Cry1 and 2. These proteins are found in cells beneath the rod and cone layer in the retina, in a different type of cell called ganglion cells.
Graduate student Gijsbertus van der Horst, professor of molecular genetics Jan Hoeijmakers, at Erasmus University, in the Netherlands, and their colleagues recently collaborated with Akira Yasui of the Tohoku University at, Sendai, Japan, to find out what would happen if they knocked out Cry1 and 2 in mice. Their results are described in a report in today¿s issue of Nature, ¿Mammalian Cry1 and Cry2 are essential for maintenance of circadian rhythms.¿
Their results do not appear to indicate that the proteins are the missing photoreceptors responsible for receiving the light signals that set the clock. Instead, they suggest that Cry proteins work at another level in the circadian system. That is, they appear to be crucial components of the clock machinery.
¿There are two pathways for light input,¿ Hoeijmakers said. ¿One is a pathway that uses visual pigment with which we can see and that influences behavior of the mice. The other uses the Cry proteins, located in the ganglion cells, which are sensitive to blue light via the Cry proteins. They provide a direct input to the biological clock that also influences behavior. If we leave our Cry1 and Cry2 single mutant mice, and also the double mutant mice, in a normal day light/dark regimen, they display running activity only in the dark. That is because mice are nocturnal.¿
The mice lacking both functional genes appear completely healthy and normal. Only when the light clue is eliminated do the effects of removing Cry proteins become apparent.
¿In the Cry1 knockout mice, the clock clicks faster so the clock is shorter by 1 = hour per day,¿ he added. ¿In Cry2 knockout mice, the day becomes longer. And in constant darkness, double mutants, which have no Cry proteins at all, no longer have a functioning biological clock. So, they display an activity pattern that is not concentrated in the period when it would normally be dark. Now, it is spread all over the entire 24-hour period of a day. Their ability to concentrate their activity during waking hours [nighttime for mice] is totally lost.¿
Noting the results of the Dutch-Japanese team show that removing cryptochrome 1 and 2 causes the animals to become arrhythmic, Foster told BioWorld Today, ¿That suggests to us that these genes are coding for perhaps clock components or components that may link the clock with behavior. It might be part of the output pathway.¿
In principle, and in the long run, the findings may give a clue about how to interfere with this pathway, Hoeijmakers suggested.
Foster agreed. ¿By identifying the pigments and knowing what connections the sensory cells [that contain them] make with the circadian system, one could think of using very highly directed pharmacological approaches,¿ he said.