Early Retinal Rewiring in RP Visualized

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A massive new study of retinal degeneration has concluded that, long before retinitis pigmentosa (RP) has progressed to complete blindness, de­terioration of rod photoreceptors leads to rewiring events within the retina—a process that previously was thought to commence much later in the disease.¹

“RP in many ways is a black box mystery,” said lead author Rebecca L. Pfeiffer, PhD, at the University of Utah’s Moran Eye Center in Salt Lake City. “We know these changes in retinal circuitry occur as photoreceptors die, leading to blindness. But our research points to much smaller-scale processes before that, which could be important to developing therapeutics in the future,” she said.

The scientists’ painstaking exam­ination of thousands of transmission electron microscope images revealed that rod photoreceptor postsynaptic target cells begin making new neural/synaptic connections to cone photo­receptors prior to complete loss of rod photoreceptors. These results, and evi­dence of other kinds of neural rewiring, were found by an ambitious project to construct a “pathoconnectome” for RP retinas.¹

GAP JUNCTION. This image shows a gap junction in a transmission electron micrograph between an Aii glycinergic amacrine cell (green) and a rod bipolar cell (blue). The gap junction between them (inset) is shown magnified to 25,000×.

3-D visualization. The researchers detected the structural changes in ret­inal neural networks by stacking serial images from transmission electron microscopy into a 3-D visualization of a retinal segment. The visual­ization showed not only the cells present but also the syn­aptic connections between them (a “connectome”), Dr. Pfeiffer said. (Molecular probes helped the research­ers identify phenotypes of the visualized cells.)

This study builds upon earlier research: In 2011, the researchers assembled the first-ever retinal connec­tome, built with images from a normal rabbit’s retina.² In the current study, they followed up by building a pathocon­nectome that shows neural network alterations in a rabbit model of retinal degeneration demonstrating a form of early, cone-sparing RP.

At this stage, some rods had already died, but many others remained mor­phologically intact. “The remaining rod photoreceptors, along with the inner retinal cell population, allowed for connectomics-based evaluation of early stage RP rewiring, corresponding to a clinical point where human patients would still have vision,” the researchers wrote. However, the patients “would likely be suffering from scotopic deficits as well as adaptation deficits.”¹

Mixed signaling. Examination of the rod synaptic network showed that rod bipolar cells, which normally synapse with rod photoreceptors, react to the partial loss of the rods by sprouting neurites to connect with cone photo­receptors while maintaining contact with the remaining rods, Dr. Pfeiffer said. Simultaneously, rod bipolar cells form gap junctions with other cell types, including Aii amacrine cells (a common synaptic partner in the inner retina), although the rod bipolar cells never make gap junctions in the healthy retina, she said.

“While they’re maintaining some input from the rod photoreceptors that are still in place, they’re also getting input from the cone photoreceptors as well. There is mixed signaling going on,” Dr. Pfeiffer said. “You have a bleeding of signals between the day and night vision pathways that doesn’t exist in a healthy retina.”

How the pathoconnectome was built. The studies are being performed in an NIH-funded connectomics research laboratory headed by Bryan William Jones, PhD, at the University of Utah’s Brain Institute and Moran Eye Center. “We are taking apart and reconstructing the retina at nanometer scale—and exploring how those con­nections are broken in retinal disease,” he said.

The lab constructed the pathocon­nectome by studying 946 tissue sections, just 70 nm thick, obtained with two transmission electron microscopes.

What’s next? The researchers have already gathered images and data for two other retinal pathoconnectomes, reflecting later stages of RP, but these must be further studied before publica­tion, Dr. Jones said.

The analyses likely will be helpful for understanding not only retinal neuro­degeneration but also other degenera­tive conditions, such as Parkinson and Alzheimer diseases, he said.

—Linda Roach

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1 Pfeiffer RL et al. Exp Eye Res. 2020;199:108196.

2 Anderson JR et al. Mol Vis. 2011;17:355-379.

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Relevant financial disclosures—Dr. Pfeiffer: None; Dr. Jones: University of Utah Research Foundation: P.

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