image: USC researchers use 3D microscopy and genetically engineered viruses to visualize and distinguish specific retinal ganglion cell types (shown here in green) from other cells in the retina (purple).
Credit: Image by Jim Stanis, courtesy of Dr. Arthur W. Toga and the USC Stevens Neuroimaging and Informatics Institute. Data provided by Dr. Michael S. Bienkowski, USC Stevens Neuroimaging and Informatics Institute.
A team of researchers at USC and the University of Utah has received a $2.7 million grant from the National Institutes of Health to map out how an incurable eye disease affects the wiring that powers vision in the eye, in hopes of discovering ways to slow or prevent blindness. Retinitis pigmentosa (RP) is a progressive disease with four known stages that affects the retina, the area at the back of the eye where light is turned into electrical signals that the brain processes to produce sight. The research team seeks to develop comprehensive maps for each stage, detailing nerve connections in the retina and how they break down.
RP, an inherited condition affecting about 2 million people globally, causes cells in the retina to die off. Usually arising during childhood, RP leads to gradual loss of peripheral vision, narrowing the field of sight to nothing over time.
“We’re optimistic that this is the right time for this project,” said principal investigator Gianluca Lazzi, PhD, a USC Provost Professor of Ophthalmology at the Keck School of Medicine of USC and of Electrical and Computer Engineering, Clinical Entrepreneurship, and Biomedical Engineering at the USC Viterbi School of Engineering. “We know how to handle data in a way that was unimaginable even five years ago. We believe this will keep growing while we’re doing the work, enabling us to handle even more.”
The map of the retina’s nerve network, referred to as a “connectome,” functions similarly to a wiring diagram, showing how nerve cells, or neurons, transmit signals. By linking changes in structure to changes in function, the team hopes to gain valuable insights for battling RP.
“There’s an old saying, ‘Don’t tell me what it is; tell me what it does,’” said Lazzi, who is also the Fred H. Cole Professor of Engineering at USC Viterbi. “We want to bring the entire network to life with functional models — showing what the disease does — that can be used by everybody working in this field. Collectively, we’ll have a better shot at developing effective therapies.”
The death of retinal cells in RP kicks off a secondary problem that is of particular interest to Lazzi and his colleagues. Casualties leave healthy neurons disconnected. They, in turn, link up with the wrong cells and block correct connections, escalating damage.
“One neuron dies, and the other realizes no one is receiving the signal, but talking to somebody else turns out to be a bad idea,” said Lazzi, director of the USC Institute for Technology and Medical Systems. “If we can unlock what drives this change, we can imagine a future where we can drive the neuron to make a better connection, one that slows progression of the disease.”
The team, which unites USC Viterbi, the Keck School of Medicine of USC, the USC Dornsife College of Letters, Arts and Sciences, and the University of Utah School of Medicine, uses two complementary imaging techniques for the connectome: two-photon excitation microscopy and transmission electron microscopy.
Two-photon excitation microscopy, which can look deep into tissue without causing damage, allows the researcher to visualize the entire retinal system while tagging individual populations of neurons with different fluorescent colors. This work is led by Michael Bienkowski, PhD, a USC assistant professor of physiology and neuroscience, the director of the USC Center for Integrative Connectomics and a member of the USC Laboratory of Neuro Imaging at the USC Mark and Mary Stevens Neuroimaging and Informatics Institute. He has developed a way to selectively deliver fluorescent dye to neurons using a neutered form of the rabies virus.
“Mike’s technique allows us to image a precise layer of the retina,” Lazzi said. “We can capture the entire image of, say, all the ganglion cells, so there’s no confusion between different types of neurons. This is a huge advantage.”
The other imaging technique, transmission electron microscopy, records details smaller than the wavelength of light, enabling the scientists to zoom in to examine the individual synapses where neurons connect. Retinal neuroscientist Bryan Jones, PhD, of the University of Utah oversees the electron imaging and the specialized laboratory models needed for the research.
Lazzi focuses on computation and modeling with the assistance of Jean-Marie Bouteiller, a USC research associate professor of biomedical engineering. That venture that will incorporate artificial intelligence to recognize attributes of healthy neurons. Electrophysiologist Steven Walston, PhD, assistant professor of research ophthalmology at the Keck School of Medicine, will play a crucial role in validating the team’s investigations by profiling electrical signaling in the retina.
“Steve’s contributions enable us to correlate the results of the computational models with experimental measurements,” said Lazzi. “Right now we don’t have a very clear answer to how a given cell in stage four of degeneration responds to stimuli. We’re looking for those answers.”
The grant builds on previous studies funded by the NIH and the National Science Foundation, including electron microscopy–based connection models of the retina in health and in the earliest stage of RP. Lazzi describes the efforts of the combined team as convergent research, a mission-driven collaboration engaging experts from different spheres in all phases of the project.
“This is true cross-fertilization,” he said. “We’re working together on an entire mesh of activities. In that environment, you learn and adapt. You operate in areas that might push the borders of the box you’re used to. But that’s entirely the point — there shouldn’t be a box, right?”