Research published online this week by the Proceedings of the National Academy of Sciences could lead to therapeutic advances for recovery from injury and diseases affecting the central nervous system. Bart Borghuis, PhD, assistant professor in the University of Louisville Department of Anatomical Sciences and Neurobiology, worked with researchers in Idaho and Puerto Rico on the research, which stimulated the formation of new neural connections in adult retinal cells through genetic modification.
Typically, adult neurons cannot make new synaptic connections as easily as developing neurons. That limits the potential for recovery from injury to the brain and spinal cord. One type of neurons in the retinas of mice, OFF-type retinal bipolar cells, has the unusual ability to make new connections into adulthood. Under normal conditions however, these cells only develop new connections with a few cells and within a limited area known as a tile. The function of these cells is to receive information from photoreceptor cells and send it along the optic nerve to the brain.
“These neurons continue to develop and elaborate their connections within their established group of cone cells in the retina,” Borghuis said. “This suggests synaptic plasticity, or the ability for the neurons to create new connections with other neurons. This is significant because in brain disease, you want to transplant and regenerate neurons and integrate them through the formation of new synapses with other neurons.”
In the first stage of the work, a team of researchers at the University of Idaho led by Peter Fuerst, PhD, determined that removing the gene encoding a protein known as Down syndrome cell-adhesion molecule (DSCAM) allows these cells to extend neuronal connections beyond their normal tile barriers. They genetically modified the mice to omit DSCAM from those cells, after which the cells were seen to form apparent contacts with neurons outside their tiles.
However, the researchers were unable to determine whether those apparent neural connections were, in fact, functional and capable of transmitting visual information.
That’s where Borghuis comes in. Using a unique imaging and recording technique pioneered in his laboratory at UofL, two-photon fluorescence-guided electrophysiology in deep layers of the neural retina, Borghuis recorded the bipolar cells’ responses to visual stimulation. His measurements showed enlarged visual receptive fields in the genetically manipulated retinal neurons, demonstrating that the extended cells made new, functional synapses onto cones.
“Right off the bat we could see that the receptive fields were larger, so we could tell that their visual responses were consistent with neural outgrowth and new synapse formation,” Borghuis said.
These tests proved the neural outgrowth seen by the Idaho team led to stable, functional connections with new cells.
A new joint research grant from the National Institutes of Health, awarded equally to Borghuis and Fuerst, will fund the collaborative research for another two years. During that time, they will induce the DSCAM knockout later in the lifespan to determine the identity and strength of the new synapses. In addition, they also will perform studies of neurons at later synaptic stages within the retina to determine other potential consequences of increased neuron growth at the level of the visual input.
“We have known about the tiling or mosaic structure of these cells for decades, and there are models and ideas for why neurons should tile. Now that we have a genetic tool that allows us to disrupt tiling within a neuron population experimentally, we can finally test these models,” Borghuis said.
The ability to stimulate neural outgrowth with new synaptic connections may ultimately improve humans’ ability to recover from brain and spinal cord injury or disease by supplying new neural connections. Even more promising, it could lead to neural regeneration and transplantation-based therapies for restoring visual function in retinal diseases such as diabetic retinopathy and macular degeneration.