A map of retinal disease generated by Moran’s Marclab for Connectomics offers a deeper understanding of a host of neurodegenerative diseases.
As Rebecca Pfeiffer, PhD, points to a 2-D image produced by a transmission electron microscope, her excitement is palpable.
“That’s one of the most gorgeous I’ve ever seen,” said Pfeiffer, a research associate in the John A. Moran Eye Center's Marclab for Connectomics directed by Bryan W. Jones, PhD. It’s a green blob and a blue blob with a tiny gap of space between them to the untrained onlooker. But Pfeiffer can explain it as a revelation three years in the making.
The blobs are neurons in the retina of the eye. The green one allows us to detect darkness—the blue one, light. And the gap of space between is actually a new connection formed between them. The two types of neurons shouldn’t be able to communicate with each other, but in retinal disease they do. Data from 946 retinal tissue samples is clear: The rewiring is a previously unknown way a disease-stricken eye keeps trying to do its job.
“There are several rules in the way neurons can connect, and these two didn’t have the right proteins to do that,” Pfeiffer said. “So seeing that these cells are making a whole new type of connection is really, really surprising.”
The discovery was one of many made as the Marclab in 2020 published the world’s first “pathoconnectome,”or map showing how the retina rewires itself in disease. The pathoconnectome was the next historic chapter for a lab that, in 2011, became the first to publish a connectome detailing the circuitry of a healthy retina.
Developing the Pathoconnectome Model
The lab developed the pathoconnectome from a model of early-stage retinitis pigmentosa (RP), an inherited retinal disease that can lead to blindness. RP impacts 1 in 4,000 people, and symptoms begin as a person starts to have difficulty seeing at night. The change progresses to a loss of peripheral and daytime vision as more retina cells start to die.
Yet the promise of the pathoconnectome extends far beyond one rare eye disease. It offers the potential to be used as a model to study a host of neurodegenerative diseases that attack the body’s central nervous system, such as Alzheimer’s, Parkinson’s, epilepsy, and amyotrophic lateral sclerosis (ALS).
“The components of neurodegeneration we see in the eye seem to mimic those we see in the brain,” explained Pfeiffer. “A pathoconnectome allows us to learn how neurodegenerative diseases alter neural networks in general. The ultimate goal is to identify how we might develop new therapies based on preventing or interfering with the network rewiring that happens during disease.”
The Marclab is now working to produce two more pathoconnectomes that will show how rewiring occurs in later stages of RP.
“For our first pathoconnectome, we wanted to focus on the earliest stage of disease we could because there are real implications for being able to rescue eyesight if therapies are developed at that point,” said Jones. “We had data going back years suggesting changes start really early, so we deliberately picked an early timepoint with the hypothesis that rewiring changes were already happening.”
The hypothesis was proven: The team found far more extensive rewiring than expected—so much so that scientists had to identify, classify, and understand connections never seen before in the eye.
A Dating Game
Pfeiffer found that neurons are reaching out continually to seek new inputs and partners in the face of disease.
“One of the main things we were looking at is as these cells degenerate, what happens to their downstream partners in the eye,” said Pfeiffer. “What we find is they make new partners. But what are the rules for that? How do they make new partners, and what sorts of partners do they tend to try to find?
For Pfeiffer’s curious mind, connectomics is the perfect research area, although she continues to work in metabolomics, or the study of small molecules.
“The world is a really, really big place, and I’m working on these tiny, tiny pieces of it that I just can’t put together yet,” she said. “I don’t think that anyone who works in connectomics for more than three months can walk away from it again because there is just more to know. It’s a really fun puzzle, and I want to put the next piece in.”
Since the first pathoconnectome has been completed, the second and third should be faster to construct. The lab could finish the second as soon as the end of 2021.
Like its predecessor, the pathoconnectome data set has been open-sourced for use by other scientists around the world.
“There is so much data, no one lab could mine it all,” said Jones. “We’re not so much interested in fame as we are in creating a body of knowledge that can be used for epiphany moments in several other fields, like electrophysiology or genetics. Connectomes are a discovery tool.”
Now, researchers around the world can use the data to begin to ask their own questions. In the myriad answers will be new hope.
In addition to Pfeiffer and Jones, 11 other Marclab researchers are authors on the new pathoconnectome publication, titled “A pathoconnectome of early neurodegeneration: Network changes in retinal degeneration.”
They are James R. Anderson, PhD; Jeebika Dahal; Jessica C. Garcia; Jia-Hui Yang; Crystal L. Sigulinsky, PhD; Kevin Rapp; Daniel P. Emrich; Carl B. Watt, PhD; Hope AB Johnstun; Alexis R. Houser; and lab founder and professor emeritus Robert E. Marc, PhD.
The pathoconnectome research was supported by NIH grants RO1 EY015128(BWJ), RO1 EY028927(BWJ), P30 EY014800(Core), T32 EY024234(RLP)], and an Unrestricted Research Grant from Research to Prevent Blindness, New York, NY, to the Department of Ophthalmology & Visual Sciences, University of Utah.
