During embryonic development, billions of neurons nimbly reposition themselves within the brain and spinal cord, and connect branches to form the neural circuits that ultimately control our movements, perception, and memory. Scientists have long sought to understand the driving forces in this meticulously choreographed dance, in which each cell seems to know precisely where to be to at any given time, and with what other cells it must link. In particular, intensive research has been focused on axon guidance, a process in which a neuron sends a wiry extension to seek out a partner cell and connect to it. Now, a study from researchers at Rockefeller University has identified a previously unknown molecule secreted by cells in the spinal cord that helps guide axons during a critical stage of central nervous system (CNS) development. The study is published in the journal Science.
Previous studies show that a type of cell called a commissural neuron projects axons from one side of the nervous system to the other, across an embryonic structure known as the midline. Commissural neurons build circuits that connect the right and left sides of the CNS. A number of inborn diseases have been linked to early developmental failures in which commissural axons lose their way and make inappropriate connections. In the past two decades, work led by the researchers and others has identified a number of cell-secreted molecules, including two groups of proteins called Netrins and Slits, that direct the routes of these axons. These factors can directionally attract or repel axons by binding to receptors on their tips. However, scientists still have an incomplete understanding of how neurons receive and integrate inputs from several guidance cues at once, or how their responsiveness to certain factors changes over time.
Earlier studies from the team in mice identified the new axon guidance factor, NELL2, and elucidated how it makes commissural axons creep toward the midline, and how the axons then depart from this area to reach their final destination. They showed that commissural neurons are able to get to the midline because they are attracted there by certain factors, including Netrins. However, once they’ve crossed the midline, they need to move on, and to do so the neurons somehow switch off their sensitivity to the attractants. At the same time, the neurons become sensitive to repelling factors in the midline, such as Slits. This prevents them from backtracking and helps them move forward. From previous work, the researchers knew that a receptor protein called Robo3 is a key controller of the switch in which commissural neurons are initially attracted to the midline and then begin to avoid it. They also knew that it did so by boosting the action of attractants (Netrins) and blocking the action of repellents (Slits). The current study shows that Robo3 has a previously unseen third action that dovetails with the first two.
The current study biochemically isolated proteins that bind to Robo3 from a vast pool of cell-secreted factors, and showed that a protein called NELL2 binds to Robo3. In a series of mouse experiments, data findings show that NELL2 is present in the developing spinal cord and directs the paths of commissural axons through repulsion by activating Robo3 receptors.
Results show that NELL2 is an axon-repulsion factor that helps the axon get to the midline. Data findings show that the Robo3 receptor receives and coordinates input from at least three diverse signaling molecules, NELL2, Netrins, and Slits, to fine tune an axon’s journey across and away from the midline.
The team surmise that their findings shed new light on the biology of axon guidance in a general sense. For the future, the researchers state they could also help in understanding what goes wrong in a rare disease called horizontal gaze palsy with progressive scoliosis, which has been linked to mutations in the gene that codes for Robo3. They go on to explain that people affected by the condition often suffer from abnormal spine curvature, and are unable to move their eyes horizontally from side to side, complications that arise early in development when commissural axons fail to cross the midline.
Source: The Rockefeller University