Problems in retinal nourishing cells may drive juvenile Batten vision loss
Study sheds light on the cellular, molecular mechanisms that lead to retina's degeneration
Structural and functional changes in the retinal pigment epithelium (RPE) — a cell layer in the eye that nourishes the light-sensing cells — may be behind the progressive vision loss seen in juvenile Batten disease, or CLN3 disease, according to a recent study.
Using lab-grown patient-derived and healthy RPE cells, researchers found that a healthy version of CLN3, the gene that’s mutated in juvenile Batten, is critical for the RPE’s proper functioning, which is needed for vision.
“It is important to understand how vision loss is triggered in this disease, what is primary and what is secondary, and this will allow us to develop new therapeutic strategies,” said Ruchira Singh, PhD, the study’s senior author, in a university press release.
Singh, who is also an associate professor in the department of ophthalmology and at the Center for Visual Science of the University of Rochester, New York, said the findings could help researchers design gene therapies that target specific eye cells to combat vision loss in CLN3 disease.
The study, “A human model of Batten disease shows role of CLN3 in phagocytosis at the photoreceptor–RPE interface,” was published in Communications Biology.
Juvenile Batten, also called juvenile neuronal ceroid lipofuscinosis (JNCL), is caused by mutations in the CLN3 gene, which result in the production of a faulty CLN3 protein (also called battenin). While the protein’s functions remain largely unclear, its abnormal form drives the toxic buildup of a fatty substance called lipofuscin inside cells.
A hallmark of the disease is rapid and progressive vision loss leading to blindness. Vision problems are among the disease’s earliest symptoms, emerging as early as age 4.
These visual declines are driven by degeneration of the retina, the back part of the eyeball that contains a layer of photoreceptors, along with other layers. Specialized light-sensing cells, photoreceptors convert light into signals that are sent to the brain, enabling vision.
This early and aggressive retinal degeneration, which seems to affect multiple retinal layers, “makes it likely that cellular processes that are compromised in JNCL are critical for health and function of the retina,” Singh said.
However, “the primary cellular and molecular mechanisms leading to retinal degeneration in CLN3 disease are not known,” the researchers wrote.
A barrier to understanding these mechanisms is the fact that CLN3 disease mouse models don’t exhibit the same retinal degeneration or vision loss that patients do.
Deriving retinal cells from patients
Singh and colleagues at the University of Rochester and other U.S. institutions developed a way to study this issue in lab-grown patient-derived retinal cells by using induced pluripotent stem cells (iPSCs), which are generated from fully matured cells (often blood and skin cells) reprogrammed back to a stem cell-like state, where they can develop into almost every type of human cell given certain biochemical cues. When derived directly from patients, these cells can be used as cellular models that mimic a disease’s genetic and clinical diversity.
Skin cells were obtained from two juvenile Batten patients and three people unaffected by the disease to generate iPSCs that were transformed into RPE cells.
The RPE is a tight-knit group of cells forming the outermost layer of the retina, or the farthest from the light, that’s attached to the photoreceptor layers. It plays a number of critical roles, including nourishing and promoting photoreceptor survival.
Phagocytosis is one of the key processes by which RPE cells maintain photoreceptors’ health and function. It involves engulfing and digesting cells or other particles that are damaged or unneeded.
Since the outer segments of photoreceptors are vulnerable to oxidative damage, a type of cellular damage, these must be continuously shed and renewed. These shed particles are phagocytosed by RPEs.
RPE changes’ effect on retinal degeneration
The researchers found changes in RPE function and structure that could underlie retinal degeneration in juvenile Batten patients.
Patient-derived RPE cells showed significantly reduced phagocytosis than cells from healthy controls. CLN3 mutations were found to disrupt the binding of RPEs to the old photoreceptor fragments, hindering their ability to clear them.
A proportion of the CLN3 protein was found to be localized in the microvilli — projections needed to be able to phagocytose — of RPE cells derived from healthy controls. The density of these microvilli was significantly reduced in patient cells.
These findings suggest that “reduced … microvilli density directly contributes to decreased [photoreceptor fragment] binding and uptake by” CLN3 disease patient-derived RPE cells, the researchers wrote.
Delivering a healthy version of the CLN3 gene to patient-derived cells reversed these RPE deficits, confirming the CLN3 protein’s role in RPE function and in maintaining photoreceptor health.
The reduced uptake of photoreceptor fragments by RPE cells from CLN3 disease patients “can also explain the buildup of excess [lipofuscin] in the photoreceptor layer and subsequent photoreceptor cell death due to accumulation of [photoreceptor] debris in the CLN3 disease retina,” said the researchers, who noted that retinal data from a deceased CLN3 disease patient were generally consistent with this hypothesis.
The findings “suggest a role of primary RPE dysfunction in CLN3-associated retinal degeneration,” the researchers wrote, adding, “if RPE dysfunction and [photoreceptor fragment] phagocytosis defect are central to photoreceptor cell loss and retinal degeneration in CLN3 disease,” gene therapies that target RPE cells may help prevent retina degeneration in juvenile Batten.
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