Carlos Belmonte, MD, PhD

Carlos Belmonte, MD, PhD

Personal Personal Profi Profilele Carlos Belmonte, MD, PhD euroscientist Carlos Belmonte has discovered most of what we know about corneal sensation and...

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Personal Personal Profi Profilele Carlos Belmonte, MD, PhD euroscientist Carlos Belmonte has discovered most of what we know about corneal sensation and the molecular and cellular processes that underlie it. In the 1990s, Dr. Belmonte developed a novel noncontact gas esthesiometer and began using it to pioneer scientific understanding of corneal innervation and sensitivity. Working with the University Miguel Hernández and the Consejo Superior de Investigaciones Científicas, in 1990 Dr. Belmonte founded— and until 2008 directed—the Neurosciences Institute of Alicante, which has become Spain’s primary center for neuroscience research. He is the current president of the International Brain Research Organization and travels regularly to the United States, where he has been Visiting Professor at Harvard University and adjunct professor of ophthalmology at the University of Utah Medical School. In 2009 King Juan Carlos I awarded Dr. Belmonte Spain’s National Research Prize in Biomedical Research for “his brilliant scientific work and his significant contribution towards the promotion of biomedical research in Spain and in the international arena.” In recent years, Dr. Belmonte has explored changes in corneal sensitivity in dry eye patients. In doing so, he discovered that ocular surface wetness is regulated, in part, by the cornea’s cold thermoreceptors and that dry eye disease may commonly result from damage or age-related loss of these sensory fibers.


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THE OCULAR SURFACE What were your early circumstances and what inspired you to pursue a medical career? CARLOS BELMONTE I was born into a family of ophthalmologists going back three generations. My grandfather, my father, and two of my uncles were ophthalmologists, as are my brother and cousins. I believe we were all inspired by my grandfather, who was charismatic in his love for this beautiful specialty. Like my family members, I intended to become a clinician. But after I began medical school at age 16, I experienced progressive hearing loss due to otosclerosis, an inherited disorder involving the bones of the middle ear. A few years later a surgical cure for this disorder was developed, but at the time none was available, and it appeared that my hearing loss would limit my ability to work with patients. As a result I began to pursue research, which seemed very natural, as I was already in love with histology and physiology. TOS Who were some of your most important mentors? BELMONTE As a researcher, I have been very lucky in my mentors. The first was Antonio Gallego, at the Universidad Complutense, in Madrid, where I pursued my PhD studies in physiology. More than a professor, he was almost a father to me, not only taking me to scientific congresses and guiding my research, but even providing material assistance to my wife and me. In those days [the 1960s], it was very difficult to be a young postdoc in

Spain, as the dictatorship had little interest in science or university students. I found another great mentor in 1971 when I arrived at the University of Utah, in Salt Lake City, as an i nt e r n at i on a l fellow of the US National Institutes of Health. Carlos Eyzaguirre was the chairman of the department of physiology and a world pioneer in chemoreception. Truly, he was one of the great gentlemen of science—a first-class scientist who was also an extremely helpful and warm person. He had a way of guiding my research that combined giving me my freedom and yet delicately supervising my progress. He was also a wonderful friend to me and my family, which at the time included two newborn children. TOS When and why did you begin your work on corneal innervation? BELMONTE Like so many important decisions in life, the reasons can be explained a posteriori, but in reality they were quite circumstantial. During my time at the University of Utah, my research concerned arterial chemoreceptors and the idea that acetylcholine was the associated neurotransmitter. I read a paper stating that the body tissue highest in acetylcholine was the cornea. It came as an exciting surprise to me and many others that acetylcholine might have something to do with sensory transmission in peripheral tissues.



PERSONAL PROFILE / Carlos Belmonte, MD, PhD

Also in Utah, I worked with Haldan K. Hartline, who had just received the Nobel Prize in Physiology for his analysis of the neurophysiological mechanisms of vision. Working with Dr. Hartline nudged me back a little in the direction of the eye. I guess you could say that the eye was somehow pursuing me despite my efforts to go somewhere else. And so, when I returned to Spain to pursue my own research, I decided to study the sensory fibers of the cornea and determine whether acetylcholine did, in fact, function as a neurotransmitter there. In the process I discovered that no one knew anything about corneal sensory physiology, and so my research began with defining the functions of the cornea’s sensory fibers—something wholly unknown at the time.1

search, the classical view was that the cornea was unique in lacking functional differences in its sensory fibers. This belief was based on the uniform physical morphology of the cornea’s sensory fiber endings, all of which were believed to be pain receptors. When we began using the gas esthesiometer to study the sensory fibers of the cornea, it became quite obvious that some of them respond only to mechanical stimuli, while others respond to a variety of stimuli, such as chemical irritants, CO2 , and heat.2-4 In other words, we discovered that the cornea had both pure mechanosensory fibers and polymodal nociceptor fibers—just like all the other tissues in the body. Next we discovered cold-sensitive fibers in the cornea, something that we had not thought to exist.5

TOS How did you become interested in developing your noncontact gas esthesiometer to measure corneal sensitivities?

TOS How is your research advancing understanding of age-related dry eye disease?

BELMONTE The challenge in studying corneal sensory nerves was to find a way to stimulate nociceptors, or pain fibers, without damaging the epithelium. In the scientific literature, I read about researchers using CO2 to stimulate smell and pain receptors inside nostrils. So we adapted this idea to the eye.2 Working first with laboratory animals, we found that CO2 was an excellent selective stimulus for polymodal nociceptors. Next we began using the same stimulus with humans—medical students. In this way, we showed that the sensation evoked by CO2 was painful, or at least unpleasant. From there we went on to use the device to stimulate other types of polymodal nociceptors, such as those that respond to temperature or mechanical force.3 By the mid-1980s, we had a patentable device, though we never bothered to protect the rights and allowed it to become an open-access instrument. TOS What important discoveries have you made with this device? BELMONTE When I began my re182

BELMONTE In November, our new paper in Nature Medicine introduced the concept that there may be an association between the cornea’s cold fibers and sensations of dryness.5 We found that cold fibers play a role in the regulation of ocular surface wetness by contributing to the regulation of tear secretion. For years, our suspicion was that the cornea’s cold thermoreceptor endings serve as wetness detectors that sense the gradual temperature decrease of the ocular surface that results from tear film evaporation. In this study, we knocked out the cold-transducing ion channel TRPM8 in mice and found that this both abrogated the corneal response to cold and reduced basal tearing, but did so without affecting irritation-evoked reflex tearing that depends on corneal pain polymodal fibers. In this way we demonstrated that the activity of the cornea’s cold sensory fibers influence basal tear secretion. An association between cold fiber activity and corneal dryness was independently suggested by Hirata and associates in a very nice paper published in parallel with our own.6 I suspect that this augmented cold sensory fiber activity does not produce

a conscious sensation of coolness but rather one of dryness, although we have not yet proven this. The question now is whether the reduced tear secretion that occurs with age is due to a lack of the input from the cornea’s cold sensory fibers—because we know that the number of these fibers declines with age. In addition, one can add a second speculation—that the reduced number of cold fibers that remain in old age will be more active because the compromised tear film will lead to more evaporation and more cooling. This increased activity in the remaining fibers, I think, may contribute to the unpleasant sensations associated with dry eye disease. These are the two things we are now exploring in mice: the clear association between age and decreasing numbers of cold fibers in the cornea and the association between this loss and reduced tear flow. Someday this research may lead to new drugs for dry eye that block or activate the ion channel that determines the activity in cold fibers. Perhaps by selectively manipulating this channel, one can activate basal tear secretion to help alleviate age-related dry eye disease. TOS How is your research advancing scientific understanding of the corneal nerve damage that can result from surgery or contact lens wear? BELMONTE One of our contributions has been the finding that damage to corneal sensory fibers—be it from trauma, surgery, or the long-term wear of contact lenses—can change the functional activity of nerves of the corneal surface. We are now obtaining convincing evidence that injured nerve fibers in the cornea fire quite abnormally and that this abnormal activity may produce unpleasant dryness sensations, or pain. We still need a better understanding of how the hypoxia or subclinical inflammation associated with contact lens wear also produces changes in the activity of corneal sensory fibers. The goal should be to develop lenses that are as innocuous as possible in terms of avoiding both inflammation around


PERSONAL PROFILE / Carlos Belmonte, MD, PhD

corneal nerve endings and direct mechanical or hypoxic damage to these nerve endings. In addition, I hope that our findings will inspire the development of surgical methods that minimize the cutting of nerve fibers in the cornea. In that regard, I must say that I am not a proponent of refractive surgery. Patients should weigh the evidence very carefully. I believe that there are many good reasons not to cut healthy tissue and damage nerves, and this feeling grows stronger as we gain more evidence that these nerves never fully recover.7 I am not saying refractive surgery or contact lenses are bad, per se, as there are millions of persons very happy with them. But I would like to see clinicians do a better job of screening patients to assure that they have normal corneal sensitivities prior to refractive surgery or contact lens wear. We know that there are people with fragile corneal innervation, and their low corneal sensitivity or disturbed sensitivity makes them prone to problems. TOS What gratifying applications of your work have you seen to this point? BELMONTE I see that my colleagues are increasingly interested in refining measurements of ocular sensitivity—something that was not even considered when I began this work. A few weeks ago, Todd Margolis of the University of California, San Francisco and I were invited by the National Eye Institute to organize a workshop

on ocular pain, which the institute now sees as deserving of much more attention than has been paid to it in the past. I also had the privilege of writing a 20-page chapter on ocular pain in the latest edition of Wall and Melzack’s Textbook of Pain—the “bible” in this field. By contrast, the edition that came out ten years ago had only a single page on ocular pain, and it was no more than a clinical description of pain associated with glaucoma and corneal ulcers. So people are devoting more attention to ocular pain and to the problems associated with ocular discomfort. I am gratified by this because, while the discomfort associated with ocular dryness may not reach the pain level of, say, a broken leg or gastric ulcer, it deserves more attention. Dry eye, in particular, is a continual irritant for a large segment of our society, particularly older people. So if there is more attention to this problem thanks to our work, I feel very gratified. TOS How would you like to see other researchers build upon your work? BELMONTE I would like to see nociception of the eye clarified at the same level that it has been for other parts of the body. Also I would like to stimulate young researchers to devote attention to the higher levels of pain processing in the central nervous system, as well as the molecular aspects of pain transduction at the level of the nerve endings. I think that the eye offers a

wonderful model for such studies because of the cornea’s simplicity. There is no other tissue as clean and well organized as the cornea, and it is here that we may best be able to study what happens inside individual sensory nerve endings. Such research will fuel progress in our understanding of both ocular pain and the general principles of pain transduction. ●

REFERENCES 1. Giraldez F, Geijo E, Belmonte C. Response characteristics of corneal sensory fibers to mechanical and thermal stimulation. Brain Res. 1979; 177:571-6 2. Chen X, Gallar J, Pozo MA, Baeza M, Belmonte C. CO2 stimulation of the cornea: a comparison between human sensation and nerve activity in polymodal nociceptive afferents of the cat. Eur J Neurosci. 1995 Jun;7(6):1154-63 3. Belmonte C, Acosta MC, Schmelz M, Gallar J. Measurement of corneal sensitivity to mechanical and chemical stimulation with a CO2 esthesiometer. Invest Ophthalmol Vis Sci. 1999 Feb; 40(2):513-9 4. Belmonte C, Gallar J, Pozo MA, Rebollo I. Excitation by irritant chemical substances of sensory afferent units in the cat’s cornea. J Physiol. 1991 437:709-25 5. Parra A, Madrid R, Echevarria D, et al. Ocular surface wetness is regulated by TRPM8-dependent cold thermoreceptors of the cornea. Nat Med. 2010 Dec;16(12):1396-9 6. Hirata H, Meng ID. Cold-sensitive corneal afferents respond to a variety of ocular stimuli central to tear production: implications for dry eye disease. Invest Ophthalmol Vis Sci. 2010 Aug;51(8):3969-76 7. Belmonte C, Acosta MC, Gallar J. Neural basis of sensation in intact and injured corneas. Exp Eye Res. 2004 Mar;78(3):513-25