Using the mouse visual system, a team of Salk Institute for Biological Studies investigators led by Dennis O’Leary, Ph.D., identified an unanticipated factor that helps keep retinal axons from going astray. They report in the Sept. 11 issue of Neuron that p75, a protein previously known to regulate whether neurons live or die, leads a double life as an axon guidance protein.
“Historically, we thought that factors that mediate cell survival and those controlling axon guidance were part of two separate processes,” says O’Leary, a professor in the Molecular Neurobiology Laboratory, “But in this study we show a direct interaction between these two systems.”
Collaborating with Kuo-Fen Lee, Ph.D., professor in the Clayton Foundation Laboratories for Peptide Biology, the O’Leary team observed a defect in mice genetically engineered to lack p75. Through their synaptic connections, retinal axons develop a two-dimensional map of the retina in their targets in the brain. In the mice lacking p75, retinal axons stopped short of their final target and formed a map that was shifted forward in the superior colliculus, a major visual center in the brain.
Such a defect in p75-null mice was puzzling: researchers have studied p75 for decades and found it associated with activities as varied as neuronal growth, survival, and degeneration. Axonal migration was not among them.
Todd McLaughlin, Ph.D., a senior research associate in the lab and co-first author, says that insight came in a eureka moment: “We realized that what we were observing in these mice was similar to what would happen if you deleted a gene called ephrin-A from the retina.”
Unlike p75, ephrin-A was a well-characterized sender and receiver of axon guidance signals, but it lacked appendages normally seen on proteins controlling axon migration. p75, however, displayed those elements, suggesting that the proteins could pair up — one receiving the migration signal and the other transmitting it.
The research team then turned to biochemical analyses and with the added expertise of Tsung-Chang Sung, Ph.D., a research associate in Dr. Lee’s lab, obtained evidence that supported this hypothesis. The group found that ephrin-A and p75 complexes in axonal membranes and showed that when activated they could generate the signals required to guide axons and develop their map in the brain.
But the clincher was the “stripe assay,” a classical screen for guidance molecules that repel growing axons. In it, an immature neuron is placed on a microscopic running track, just as it starts to develop an axon. When flanking lanes are carpeted with repellant factors, the sprouting axon bursts from the block but remains in its lane like a well-coached runner, avoiding neighboring tracks.
Constructing tracks made from the repulsive factor sensed by ephrin-A, the researchers confirmed that axons from normal retinal neurons stayed in their lanes when flanked by the repellant. But neurons from mice lacking p75 were unreceptive to repulsive cues: when placed on the track their axons meandered all over the field, crossing lanes and running down repellant-covered stripes.
Why retinal neurons missed the target in the p75-minus mice became clear: they lacked the cellular machinery to respond to critical repellant signals encountered in the brain and stopped migrating prematurely.
Among its myriad functions, p75’s new role is a critical one. “Repulsion is probably the dominant force in axon guidance and a stronger influence than attraction,” explains McLaughlin, noting that providing axons with a lot of options is not the way to build a brain. “Attraction is like finding the best seat in an empty movie theater, but repulsion is like picking the lone empty seat in a full theater.”
“We have shown that ephrin-A cannot transduce an intracellular signal by itself and instead requires the co-receptor p75,” summarizes Yoo-Shick Lim, Ph.D., a postdoctoral fellow in the O’Leary lab and co-first author. “This interaction could operate in numerous events in neural development.”
O’Leary believes that identifying mechanisms underlying developmental events is fundamental to understanding the basis of any biological disorder. “These studies establish that two distinct molecular systems, neurotrophins and axon guidance, both critical for neural development directly collaborate to develop neural connectivity.
Findings such as these lend critical insight into how one might repair damage to the nervous system due to genetic defects, tumors or wounds to the brain or spinal cord,” he says. “We hope one day to be able to repair these defects and get cells to form functional connections again.”
Alicia Santiago, Ph.D., formerly of the O’Leary lab, also contributed to the study. Salk professor Tony Hunter and Sourav Ghosh, a former postdoctoral fellow in the Hunter lab, helped with preliminary biochemical experiments. Funding was from a grant from the National Eye Institute and from the Joseph Alexander Foundation.
For information on commercialization of this technology, please contact Mike White, Ph.D., Office of Technology Management (email@example.com, 858-453-4100 x 1703).
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.
Mike White | Newswise Science News
The Secret of the Rock Drawings
24.05.2019 | Max-Planck-Institut für Chemie
Chemical juggling with three particles
24.05.2019 | Rheinische Friedrich-Wilhelms-Universität Bonn
A new assessment of NASA's record of global temperatures revealed that the agency's estimate of Earth's long-term temperature rise in recent decades is accurate to within less than a tenth of a degree Fahrenheit, providing confidence that past and future research is correctly capturing rising surface temperatures.
The most complete assessment ever of statistical uncertainty within the GISS Surface Temperature Analysis (GISTEMP) data product shows that the annual values...
Physicists at the University of Basel are able to show for the first time how a single electron looks in an artificial atom. A newly developed method enables them to show the probability of an electron being present in a space. This allows improved control of electron spins, which could serve as the smallest information unit in a future quantum computer. The experiments were published in Physical Review Letters and the related theory in Physical Review B.
The spin of an electron is a promising candidate for use as the smallest information unit (qubit) of a quantum computer. Controlling and switching this spin or...
Engineers at the University of Tokyo continually pioneer new ways to improve battery technology. Professor Atsuo Yamada and his team recently developed a...
With a quantum coprocessor in the cloud, physicists from Innsbruck, Austria, open the door to the simulation of previously unsolvable problems in chemistry, materials research or high-energy physics. The research groups led by Rainer Blatt and Peter Zoller report in the journal Nature how they simulated particle physics phenomena on 20 quantum bits and how the quantum simulator self-verified the result for the first time.
Many scientists are currently working on investigating how quantum advantage can be exploited on hardware already available today. Three years ago, physicists...
'Quantum technologies' utilise the unique phenomena of quantum superposition and entanglement to encode and process information, with potentially profound benefits to a wide range of information technologies from communications to sensing and computing.
However a major challenge in developing these technologies is that the quantum phenomena are very fragile, and only a handful of physical systems have been...
29.04.2019 | Event News
17.04.2019 | Event News
15.04.2019 | Event News
24.05.2019 | Physics and Astronomy
24.05.2019 | Medical Engineering
24.05.2019 | Life Sciences