SCI Awakens Developmental Signals

Posted by Sam Maddox in Research News on June 02, 2016

When the nervous system is formed, before birth and during development, numerous biochemical signals guide and shape the process, some by attracting nerve growth, others by repelling it. Turns out many of these signal molecules remain in the body, hidden. They show up again after an injury to the brain or spinal cord, which has led scientists to figure out that they can manipulate these signals to enhance recovery of both sensory and motor function.

A research study, Ryk controls remapping of motor cortex during functional recovery after spinal cord injury, was published a few weeks ago in Nature Neuroscience by the Yimin Zou lab at UC San Diego. Ryk is the receptor for a repulsive guidance signal. Zou was able to turn it off, thus promoting recovery from spinal cord injury.

For a number of years, Zou, a neurobiologist, has been deciphering a complex system of signaling proteins called Wnts; these proteins control many important aspects of nervous system wiring, and as it turns out, post-trauma rewiring.

Wnts proteins, when expressed during development, are detected by guidance receptors, such as Ryk, which repels axons by sensing Wnt molecules. In an adult, Wnt is hidden, but for some reason it reappears in the lesion area after spinal cord injury. That’s good that the wiring program is preserved into adulthood, but Ryk shows up too.

In a spinal cord injury animal model, Zou showed how to turn off Ryk. Doing so promotes growth of nerve circuits in the corticospinal tract – the most important nerves for fine motor control. The modified nervous system is able to remap and rewire itself for recovery of fine motor skills.

In a previous study, Zou and his group neutralized Ryk using polyclonal antibodies after injury and observed increased axon growth. They applied a monoclonal Ryk antibody in this new study, and too is effective; animals with SCI improved forepaw grasping function.

Zou says these studies are encouraging, particularly for therapeutic designs, but one could not rule out the possibility that the antibodies may function by modifying the local environment and not directly by regulating axon responses. So this time Zou also used a knock-out mouse strain, deleting the gene that encodes Ryk only in the neurons that saw the improved function.

Although these results still do not rule out a possible function of the reappeared Wnt signaling system in the local environment, they are at least certain that axons will respond to the reappeared Wnt system.

Using optical stimulation, Zou observed that motor cortical wiring maps “underwent massive changes after injury and that hind limb cortical areas were recruited to control the forelimb over time.” It’s known in the clinic that motor maps change after injury. One can now monitor those changes in an animal model.

Training, it seems, is a key variable. The Zou lab found that functional recovery and remapping requires task-specific training. Without training, return of grasping function and remapping do not take place even in Ryk conditional knockout mice.

I spoke with Zou about the background for this work, which he thinks will “put the Wnt/Ryk system on the map in central nervous system injury.”

We published a paper in 2008 that showed that in the adult spinal cord, the Wnt system is expressed at low levels, or not at all. But after spinal cord injury, it is reactivated. It’s a puzzle why this axon guidance system would be reactivated. Once we blocked the receptor that mediates Wnt repulsion, Ryk, using our polyclonal antibodies, this made the axons grow more.

We continued to ask whether Wnt-Ryk signaling affects other types of axons, and then, in 2012, we published another paper showing that if we did a nerve crush preconditioning injury before the spinal cord injury, seven days later, Ryk is expressed in sensory nerves. We found that by inhibiting Ryk, sensory nerves could regenerate much better.

In another more recent paper in 2014, we showed that blocking Wnt repulsion using a Wnt inhibitor enhances recovery of proprioception, a sensory function that allows the animal to sense the position and movement of the body.

What is really exciting in this new paper is that we completed a study using a conditional knock-out mouse model we generated. Before, when blocking Wnt signaling with antibodies or inhibitors, we were not sure whether this was because of affecting the glial environment locally. With the new genetic evidence, it is now clear that the Wnt-Ryk pathway is important in the injured axons.

Zou says his experiments also show how the nervous system rewires itself to compensate for injury.

The system is able to reshape its motor maps. This is important to understand how neural circuits reorganize, and how molecular manipulation can enhance this reorganization.

The paper suggests that the antibody to Ryk is therapeutic:

.... Ryk monoclonal antibody can be a therapeutic tool, as blocking Ryk function after lesion leads to improved functional recovery. We show here that maximal recovery of the forelimb can be achieved by combining targeted plasticity for the forelimb function (continued reaching and grasping training) and molecular manipulation. Therefore, we anticipate that combining targeted plasticity of other functions with molecular manipulation may allow recovery of other motor or sensory functions.

A large proportion of patients have incomplete spinal cord injuries, providing a substrate for recovery. Our work illustrates that promoting circuit plasticity is a promising approach to restore maximal function following incomplete spinal cord injury.

In order to address the circuit mechanisms with which the primary motor cortex regains control over the remodeled spinal cord, we used an optogenetic approach to monitor cortical output. Cortical motor maps have been studied using intracortical electrical stimulation in rodents and primates, as well as transcranial magnetic stimulation in human. Recent advances in optogenetic tools allow for the stimulation of specific neural populations in a minimally invasive manner.

We observed massive remapping of cortical motor output immediately after spinal cord injury in the mouse. ... In order to understand how neural circuits reorganize to regain function after injury, we performed functional, anatomical and behavioral analyses. We demonstrate here that the motor cortex remaps such that the cortical areas are no longer used for the hindlimb are recruited to control the forelimb to achieve functional recovery after a C5 dorsal column lesion and this reorganization requires continued training.

Zou, from a UCSD press release:

Our new study now provides the first genetic evidence that those signaling proteins, important in wiring the nervous system in development, have a profound influence on how central nervous system axons respond to spinal cord injury. This suggests that many other guidance cues, in addition to these signaling proteins, may also play roles in adult spinal cord repair. This opens up new opportunities to apply what we've learned in nervous system development to treat paralysis in adulthood.

We found that manipulating Ryk will not only improve the results of functional recovery, but also speed up recovery. If this can be achieved in humans, it will significantly improve the recovery and quality of life for individuals with partial spinal cord injuries.