May 23, 2012 — Interventions resulting in optic nerve regeneration restored some components of vision, according to the results of a mouse model study published online May 21 issue of the Proceedings of the National Academy of Sciences. If these findings are confirmed and extended to other models, they may ultimately offer promise to patients with glaucoma or optic nerve damage.
"It had been widely thought that the central nervous system does not regenerate anatomically or functionally," G. Astrid Limb, PhD, FRCPath, group leader of ocular biology and therapeutics at the University College London Institute of Ophthalmology in the United Kingdom, told Medscape Medical News in an email interview. Dr. Limb was not involved in the current study. "However, in the past few years there have been several demonstrations that anatomical regeneration of the optic nerve can be partially induced after injury, following stimulation with various neurotrophic and neuroprotective factors."
In the present study, Silmara de Lima, from the Laboratory for Neuroscience Research in Neurosurgery and F.M. Kirby Neurobiology Center, Children’s Hospital, Boston, Massachusetts, and the Program of Basic and Clinical Neuroscience, Institute of Biomedical Sciences, Centre of Health Sciences, Universidade Federal do Rio de Janeiro, Brazil, and colleagues evaluated, using mice models, the ability of the mature optic nerve to regenerate when injured and to reenter the brain, navigate to appropriate target areas, and restore vision.
"We are able to regenerate nerve cells in the eye which normally cannot regenerate, as is true for all central nervous system cells," senior author Larry Benowitz, PhD, from the F.M. Kirby Neurobiology Center and Laboratories for Neuroscience Research in Neurosurgery at Boston Children's Hospital, told Medscape Medical News in a telephone interview. "We can stimulate these cells to regenerate axons all the way back into the brain. It previously was unknown if these cells could 'find their way' back into the brain, and we see now that they can, with early signs of functional improvement."
This study used an optic nerve crush model in mice treated with 3 interventions shown to act synergistically to stimulate growth of optic nerve fibers. These were a conditional deletion of the phosphatase and tensin homolog (PTEN) gene, combined intraocular injections of zymosan (a microglia activator stimulating a growth-promoting compound called oncomodulin), and using a cyclic adenosine monophosphate (cAMP) analog (4-chlorophenylthio) adenosine, CPT) to increase cAMP levels.
"This is a proof-of-concept study showing that rewiring of neural circuits is possible," said Dr. Benowitz, who is also professor of surgery and ophthalmology at Harvard Medical School in Boston. "Normally, many signals are required for this to occur during development, and it has not previously been clear if this competence persists in the adult brain, but now it appears that it does."
Adequate stimulation of retinal ganglion cells with these interventions enabled them to partially regenerate myelinated axons over the full length of the visual pathway and to navigate into the lateral geniculate nucleus, superior colliculus, and other visual centers. By forming synapses with other neurons in these regions, the regenerated axons allowed partial restoration of visual circuits.
"For the first time, Dr. Benowitz and colleagues showed that partial anatomical regeneration of the optic nerve is accompanied by partial recovery of function," Dr. Limb said. "In comparison with controls, treated animals regained some depth perception, showed an improved pupillary light reflex and photoentrainment of the circadian rhythm, and improvement of the optomotor response 7 to 10 weeks after injury."
The investigators tested depth perception by having mice walk on clear Plexiglas platforms above a checkerboard pattern that appeared to drop off abruptly. Although untreated blind mice were equally likely to walk over either end, mice that had optic nerve regeneration in response to the interventions spent less time over the "deep" end.
Untreated mice with optic nerve injury lost synchrony with the room's day/night light cycle, whereas treated mice had restored patterns of circadian entrainment.
In addition, unlike untreated mice with optic nerve injury, those that received the intervention had a positive optomotor response: When placed on a platform surrounded by rotating vertical stripes, treated mice moved their heads reflexively to follow the pattern.
Limitations and Implications
"It is well recognized that there are differences between experimental models of optic nerve damage, and the model used by the investigators might not be the most representative of human optic nerve disease," Dr. Limb said. "In addition, the molecules investigated that promoted axon regeneration in mice may need to be tested in other models before they can be developed for safety studies in humans."
Despite these limitations, the findings offer hope for patients with blindness resulting from optic nerve damage from trauma or from glaucoma, which is thought to affect more than 4 million people in the United States. Treatment of macular degeneration or some other forms of vision loss may sometimes restore visual acuity to a limited extent, but there are currently no available treatments to improve vision in patients with optic nerve damage.
"The strength of the study is the demonstration that functional regeneration of the optic nerve can be achieved upon partial axon regeneration," Dr. Limb said. "This has important implications for the development of human therapies, where partial recovery of light perception can lead to a significant improvement in the quality of life."
Dr. Benowitz cautions that the experimental interventions regenerated only a small percentage of the total number of visual nerve fibers that would normally enter the brain, and that functional improvements in vision were limited. Most likely, the treated mice were still unable to discriminate objects.
Possibilities Far From Clinical Practice: Could Gene Therapy Be on the Distant Horizon?
"The process we tested is not yet applicable for clinical use, and we need to see how this could be translated into the clinic," Dr. Benowitz explained. "To modify gene expression in the neurons could involve gene therapy methods to alter the genetic pathway to mimic what we did in the animal model. Gene therapy has been used in other ophthalmological disorders with some signs of functional improvement."
Leber's hereditary neuropathy, a rare genetic disease condition causing vision loss, has been shown to be amenable to gene therapy. An advantage of using gene therapy in the eye is that virus vectors used to introduce genes into nerve cells can easily reach retinal ganglion cells and mostly remain in the eye.
"Patients shouldn't get too excited yet, because these methods are not yet ready for clinical use, and the treatment in animals only works if given shortly after the injury occurs," Dr. Benowitz concluded. "Nerve cells begin to atrophy after injury, giving us only a limited window of opportunity because we don't yet know how to repair atrophied cells. "
The National Eye Institute, the Department of Defense, the Dr. Miriam and Sheldon G. Adelson Medical Research Foundation, and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior funded this research. The study authors and Dr. Limb have disclosed no relevant financial relationships.
Proc Natl Acad Sci. Published online May 21, 2012. Abstract
Medscape Medical News © 2012 WebMD, LLC
Send comments and news tips to firstname.lastname@example.org.
Cite this: Optic Nerve Regeneration in Mice Linked to Improved Vision - Medscape - May 23, 2012.