Researchers at Johns Hopkins Medicine say they have successfully demonstrated that disrupting an eye structure long suspected of blocking the growth and survival of transplanted nerve cells may help restore vision in people with optic nerve damage.
A report on the National Institutes of Health-funded experiments with animals, stem cells and donated eye tissue was published in Science Translational Medicine on April 29. It suggests that altering or removing a thin layer of tissue called the internal limiting membrane, which separates the light-sensing retinal tissue at the back of the eye from the gel-like vitreous fluid that fills the eye, could help transplanted retinal ganglion cells (RGCs) survive and grow in people with blinding optic nerve damage.
Such damage, also known as optic neuropathy, occurs when retinal ganglion cells die of disease, inflammation or injury and stop carrying electrical signals to the brain. Common causes of damage include glaucoma, optic nerve inflammation (optic neuritis) and ischemic optic neuropathy (sudden loss of blood flow to the optic nerve).
Healthy, functional human RGCs can be grown in a lab, but most die when transplanted, says principal investigator and corresponding author Thomas Vincent Johnson III, M.D., Ph.D., the Shelley and Allan Holt Rising Professor of Ophthalmology at the Johns Hopkins Wilmer Eye Institute.
"Even when the retinal ganglion cells survive, they remain on the retina's surface and do not migrate into the tissue or form the connections with other nerve cells necessary to detect light," Johnson says.
Vision restoration researchers have speculated that the internal limiting membrane, present in many vertebrates including humans, may be causing transplant failures, but an absence of solid evidence in living organisms has prevented other possible causes from being ruled out.
Starting with immunosuppressed rodents, the researchers injected lab-grown human retinal ganglion cells (hRGCs) into the vitreous humors of mice with an inborn gene mutation that caused an incomplete, patchy internal limiting membrane to form; injected the hRGCs into a second group of mice treated with an enzyme solution known to partially digest the membrane without damaging the eye; and injected a third, control group of mice treated with an inactive sterile solution. After two weeks, they observed transplantation survival in 95% of eyes (45/50) with the inborn structural defect, 80% of enzymatically disrupted eyes (32/40), and 75% of control group eyes (12/16).
The researchers then traced where the surviving human retinal ganglion cells settled and grew in the mice, noting that a much greater percentage reached the retinal ganglion cell layer (the primary structure responsible for vision) in mice born with a patchy internal limiting membrane and in those treated with the enzyme.
Capturing 3D images of the migrated cells, the researchers say they observed that 2% plus or minus 0.6% and 7.1% plus or minus 1.6% surviving cells in enzyme-treated and mutant eyes, respectively, matured to form dendrites, structures that allow nerve cells to communicate with each other and process light information. In contrast, migration and maturation only occurred in 0.01% plus or minus 0.01% of surviving control human retinal ganglion cells.
Conducting similar experiments in larger eyes and donated eye tissue replicated the group's findings, establishing evidence that the inner limiting membrane is indeed a structural obstacle to neuron replacement, the researchers say. They also established a surgical procedure for RGC transplantation that could be used in clinical trials, thus advancing potential methods for restoring vision in humans with optic neuropathy.
While the study's results are promising, Johnson cautions that further work is still needed before their experimental findings can be applied to people.
"We know our methods are effective, but we don't know if completely removing the internal limiting membrane helps or harms the retinal ganglion cells in the long run," says Johnson, "It will likely take several years before our findings could become available as an experimental therapy, but the methods we developed will guide the field moving forward."
Funding for this research was provided by the National Institutes of Health (K08EY031801, R21EY034332 and P30EY001765), the Department of Defense Focused Translational Team Science Award (#13752359), the Research to Prevent Blindness Career Development Award and Unrestricted Grant, the BrightFocus Foundation National Glaucoma Research Award, the Zenkel Family Foundation Research Award and the Glaucoma Foundation Rajen Savjani and Joe Rosen Grant Awards.
Other Johns Hopkins Medicine researchers who conducted the study include Erika A. Aguzzi, who is also affiliated with the University College London; Behnoosh Bonakdar; Jian Du; Dayn Romero Godinez; James T. Handa; Shreya Hariharakumar, who is also affiliated with the University of Texas Southwestern Medical Center; Elizabeth Kimball; Stella Mary; Arumugam Nagalingam; Harry A. Quigley; Sarah Quillen; Marzieh Mowlavi Vardanjani; Donald J. Zack; and Kevin Y. Zhang.
The authors affiliated with Johns Hopkins University did not declare any conflicts of interest under Johns Hopkins University policies.
DOI: 10.1126/scitranslmed.adr1062
