Injuries that involve spinal cord damage usually take a lot of time to heal properly and usually still leave traces in the life of the person. It is because healing of the spinal cord damage is extremely difficult, because neurons have to be reconnected in a precise fashion.
In fact, there are still many mysteries surrounding how this happens. However, now scientists at the RIKEN Brain Science Institute in Japan have found that as important as proteins are in the healing of the spinal cord, lipids are also necessary for guiding axons.
Recently scientists have published a study, explaining how a phospholipid released by glial cells controls the positioning of sensory neurons within the spinal cord. Glial cells the cells in the nervous system that support neurons. This phospholipid is extremely important in the healing process, because it helps guiding axons, which are long extensions of neurons. They acts as roads that allow neural information to travel from one are of the body to another.
Pathways of these axons are typically specific for each of our senses, because information about different sense has to reach different brain regions. There is a patterned distribution of molecules that either attract or repel axons as they grow. This guides their growth very precisely and there are many proteins that guide axons and help with network formation, but now scientists have better knowledge about these processes and role of lipids.
Hiroyuki Kamiguchi, leader of the research team, said: “we discovered that glial cells have the ability to release membrane structural lipids in specific patterns that can then control axon migration and neuron organization. In this case, we found that a lipid called LysoPtdGlc has a major role in separating axons of pain- and position-sensing neurons from each other.”
Spinal cord in the pathway of information is like a highway – before reaching the brain, sensory information from our skin and muscles goes the spinal cord. Axons, through which this information is delivered, enter the spinal cord together but soon separate – nociception (responsible for sensation of pain) travel along the side of the spinal cord, while proprioception (allows us to know where our muscles are) travels in a neighbouring region closer to the midline.
In order to study these pathways, scientists used animal models – this time they used chicks. Scientists labelled spinal cord sections from chicks with markers for LysoPtdGlc and the two different types of sensory neurons and found that this phospholipid was located only near the midline region where position-sensing axons are located. This allowed scientists to think that LysoPtdGlc helps guiding pain-sensing neurons, which are repulsed by the LysoPtdGlc from this midline area and have to choose a different route – a more lateral region of the spinal cord.
In order to check if this is correct, researchers analysed how cultured pain-sensing neurons reacted to the LysoPtdGlc. They found that gradients of LysoPtdGlc repelled axons from the pain-sensing neurons. This was further confirmed when scientists used experimental antibody to block access to the lipid, which prevented pain-sensing neurons from being repelled.
While continuing research, scientists injected the antibody into the spinal cord of chick embryos. This proved once again that LysoPtdGlc is responsible for directing axon growth – antibody did its job and the axons of pain-sensing neurons were no longer repelled. They grew threw the region of the spinal cord reserved for position-sensitive neurons.
Team of researchers then tested over 100 protein receptors in order to find the one that responds well to LysoPtdGlc. Scientists found that this protein is also expressed in the spinal cord. Scientists labelled axons in GPR55 knockout mice and found that pain-sensing axons have mistakenly entered the upper-medial portion of the spinal cord. It is similar to results of the experiment when they had blocked LysoPtdGlc function by injecting the antibody into chick spinal cords.
This knowledge is extremely important. Scientists say that now they can investigate possibilities to create new therapies, if they will confirm this lipid-based signalling system as a therapeutic target for spinal cord injury. This means that in the future there may be more effective ways to heal spinal cord damage that currently is an extremely hard task even for modern medicine.