Week in Review: Number 37

Corneal Epithelial Stem Cell Topical Therapy in DED
Dry eye disease is estimated to affect up to half of the U.S. population. Depending on the underlying etiology, a wide range of therapies are available. Pharmaceutical scientists, along with clinical colleagues, are exploring a topical delivery of corneal epithelial stem cells, located and obtained solely at the limbus, as a potentially safer and more effective treatment option. They report excellent results in the first stage of clinical trials. Collaborating with a Good Manufacturing Practice (GMP) facility, they isolated and expanded the human corneal stem cells, obtained from donor tissues from an eye bank, to derive a topically applied medicine in this case. The study recruited 17 participants (34 eyes) suffering from advanced dry eye disease, each of whom have documented attempts to treat their dry eyes using between 6 and 14 conventional dry eye therapies. These participants also qualified for the trial based on a score greater than or equal to 14 on the Standardized Patient Evaluation of Eye Dryness (SPEED™) questionnaire and a score greater than or equal to 40 on the Ocular Surface Disease Index (OSDI©). The lead researcher of the study remarks, “Nothing was helping them; they were in a very severe condition. Some of them could only go outside in the dark because their inflammation was so bad that sunlight would burn their eyes.”

During the trial, each patient self-administered the topical corneal epithelial stem cell-derived product four times daily in both eyes for 12 weeks. Compared to baseline, there was a 23% improvement in SPEED™ scores and a 17% improvement in OSDI© scores (but not statistically significant). An additional dry eye assessment questionnaire (University of North Carolina Dry Eye Management Scale) showed a 14% score improvement. Objective measurements included best-corrected visual acuity (BCVA), corneal topography, and tear film osmolarity, none of which changed significantly in the 12 weeks. None of the study participants reported any adverse effects and, as the authors are keen to note, all asked to resume use of the drops after the study. The project lead speculates that the success of the therapy is likely because it both reduces inflammation and regenerates damaged tissue, as stem cells would. They conclude that topical corneal epithelial stem cell-derived supernatant that can be self-administered by the patient shows promise at improving patient symptoms and quality of life in severe DED that is unresponsive to conventional therapies. They next plan a double-blinded study and have filed patents for their product.

RGC Dendrite Mosaics are Optimized for Efficiency
Neurobiologists studying the cellular organization of the retina found that it closely mimics a concept known as efficient coding theory, an optimization model of sensory coding in the nervous system. In two papers on retinal structure, they show that natural selection and evolution shaped the patterns of sensitivity in the retina to closely follow what efficient coding theory would predict. Specifically, these sensitivity patterns are seen in the layer of dendrites of the retinal ganglion cells (RGCs), i.e., the inner plexiform layer. Although the ganglion cells downstream only output in binary fashion, either depolarization or not, it is the three-dimensional dendrite mosaic that is sensitive to different stimuli. One of the researchers explains, “The mosaics don’t just randomly overlap, but they don’t overlap in a highly ordered way.” And those mosaics adapt to current conditions. This results in a retina that is not merely one mosaic but many stacked mosaics that each encodes something different about the visual field, parsing as many as 40 different features that together add up to form an image. Furthermore, the depth of the mosaic serves as a kind of address for the type of information that that layer encodes. For example, the deeper layers receive "off" signals, while the more shallow layers get "on" signals, meaning that even when the same (cross-section) area of the retina is stimulated, different layers of the dendrite mosaic can convey different kinds of signals.

One reason the array is so efficient is that the cells conserve energy by not responding to some stimuli. In environments that are "noisy," the receptors tune out most of the static and only respond to something that’s very bright. One of the researchers states, “The more noise there is in the world, the pickier the cell can be about what it will respond to...And when they get pickier, it turns out that there's less redundancy in them.” Moreover, the more noise, the greater the offset between on and off RGC detector pairs. In technical terms, "[I]nformation is maximized when these mosaic pairs are anti-aligned," when the distances between them are greater than average. In other words, the retina is optimized to handle high noise conditions in order to detect things that stand out, and it does so by minimizing the amount of redundant information it encodes. The researchers hope that studying how the retina is optimized for efficiency will help to design smartphone sensors; however, they acknowledge that we are a long ways away from replicating the natural arrangement in the retina.

NeuroD1-Mediated Gene Therapy Restores Visual Function in Mice after Stroke
Strokes happen when blood flow to neural tissue stops, leading to neuronal loss and gliosis. A large portion of the cerebrum is devoted to processing vision, thus when an artery is blocked in these regions, vision loss can result. Researchers are exploring the use of gene therapy to directly reprogram endogenous astrocytes into neurons in situ as a means to restore vision in the aftermath of irreversible death of neurons. As compared to the plasticity found when the brain remaps its pathways, a slow and inefficient process, the new gene therapy offers a more efficient solution. The technique shows promise thus far in a model of ischemic stroke affecting the visual centers in the brains of mice, in particular using adeno-associated viruses to deliver transcription factor NeuroD1 in vivo to glial cells in the affected area of the brain. These cells were observed to reprogram into neurons and integrate into the microcircuits of the visual cortex. Furthermore, following visual experience, the reprogrammed neurons demonstrated maturation of orientation selectivity and functional connectivity. One of the researchers comments, “We don’t have to implant new cells, so there’s no immunogenic rejection. This process is easier to do than stem cell therapy, and there’s less damage to the brain. We are helping the brain heal itself. We can see the connections between the old neurons and the newly reprogrammed neurons get reestablished. We can watch the mice get their vision back.” They conclude, "Our results show that NeuroD1-reprogrammed neurons can successfully develop and integrate into the visual cortical circuit leading to vision recovery after ischemic injury." The researchers hope that the techniques they develop to restore function in the visual cortex will then help in perfecting techniques to restore motor function after stroke.

CRISPR Gene Therapy in LCA Patients Shows Positive Results
Scientists report encouraging results for the first few cases of CRISPR gene-editing for Leber congenital amaurosis (LCA), presenting their findings at the International Symposium on Retinal Degeneration. The gene-editing was notable in this case for being the first trial to inject CRISPR in vivo in human subjects, as compared to explanting cells, editing them in vitro, and then infusing them back into tissues of the body. Thus far, seven patients have volunteered to have the experimental therapy. NPR interviewed two of those patients, Carlene Knight and Michael Kalberer. Knight reports, "I was bumping into the cubicles and really scaring people that were sitting at them." With vision improved enough to make out doorways, navigate hallways, spot objects and even see colors, she says she no longer scares people and has fewer bruises from bumping into things. Knight also says that colors are more vivid, which she has appreciated since she was a kid. "I've always loved colors. Since I was a kid it's one of those things I could enjoy with just a small amount of vision. But now I realize how much brighter they were as a kid because I can see them a lot more brilliantly now," she says. Similarly, Kalberer reports being thrilled at his improved vision, which he noticed starting one month after the treatment. For example, he is now able to recognize shapes and light much better, and has regained more peripheral vision. It was an especially joyous moment for him to watch the DJ's strobe lights change color at his cousin's wedding and seeing the sunset again for the first time. One of the clinical researchers at HMS says, "We're thrilled about this. This is the first time we're having evidence that gene editing is functioning inside somebody and it's improving — in this case — their visual function." They next plan on trials at higher doses, and in age brackets that have the best chance of benefiting. Although the treatment is far from a cure, vision never returned to normal, and visual improvement is not seen in all seven patients at this point, for some patients in the trial, the changes experienced are enough to have a meaningful impact on their daily lives.

Handheld Screening Device to Detect Amblyopia
Colloquially called "lazy eye," amblyopia results when there is an underdeveloped connection between the eye and the brain, leading the brain to favor the vision in the better-seeing eye as those synapses strengthen and the synapses with the worse-seeing eye diminish. This can occur as a result of a variety of underlying causes, with anisometropia (or unequal refractive error) being the most common cause. Other factors leading to amblyopia include strabismus or tropia (an eye misalignment), visual deprivation, and high astigmatism. Where strabismus is the underlying etiology, detection of an eye misalignment in early childhood can prevent difficulties in academic performance. However, many pediatricians and primary care providers are not exposed to vision testing beyond basic visual acuity. Funded by the National Eye Institute, researchers are exploring a prototype handheld screening device to assess the eyes' ability to fixate together. The Pediatric Vision Scanner (PVS) simultaneously scans both retinas, specifically the fovea, with a polarized laser to detect even small-angle deviations. The device then provides a binocularity score, which is used to determine whether the child needs referral for more specialized assessment. The study recruited 300 children ages 2 to 6 with no known eye disorders. Two non-ophthalmic research associates were trained to use the device to screen each child, and the results were compared to eye examination performed by a pediatric ophthalmologist who was masked to the device's results. The device showed a 100% sensitivity, detecting all 6 cases of amblyopia and/or strabismus that agreed with the professional eye examination. However, the device also flagged an additional 45 children as possibly having amblyopia and/or strabismus who were later determined by the eye examination to be normal. The study did not compare this prototype with photo-screening devices which detect risk factors of amblyopia via differences in light reflexes between the two eyes. Despite a relatively low threshold of sensitivity (resulting in a high rate of false positives) in this prototype, the test requires only 2.5 seconds to perform, which could be a beneficial addition to busy pediatric practices.

In Other News
(1) Why can’t we identify music notes as well as colors?
(2) CRISPR gene editing in Leber congenital amaurosis
(3) Children's visual perception continues to develop up to age 10