Studying glial cells is technically difficult as they are essential for neuronal cell survival: disturbing them in any way puts the organism's life in jeopardy. Shaham and Perens show that worms are the perfect model system to study the function of these cells in the nervous system, because the glial cells can be manipulated and the neurons still form and function, though not entirely as normal.
"Glial cells have been traditionally hard to study in vertebrates because it is difficult to ask how they influence neurons beyond how they affect a neuron's survival," says Shaham, head of the Strang Laboratory of Developmental Genetics. "This is the first paper to take a serious crack at glial cells in C. elegans. It shows that the worm really is a great system in which to study glial cells, because we are able to get the kind of answers that could help us understand how they are functioning in the human brain."
The story began 30 years ago with the daf-6 mutant worm, the sixth type of mutant worm found that was defective in dauer formation. Dauer is when worms enter a kind of suspended animation state because of overcrowding or starvation. Research by other scientists hinted that the mutation in this daf-6 worm was involved in glial cell development, but no one designed experiments to directly ask, and the mutant was forgotten.
"We had this mutant worm that nobody had looked at in more than 15 years," says Perens, an M.D.-Ph.D. student in Shaham's lab. "We started with the knowledge that it was somehow affecting glial cells. From there, we tried to determine what was actually wrong with the glial cells in this mutant worm, and we found that they don't form properly."
Worms have a pair of neuron bundles in their heads, each with eight neurons reserved for senses such as taste and smell. Each bundle of neurons works by extending through a small tube in the head of the worm, like a nostril, that is open to the outside environment. The glial cells wrap around the neurons to create the tube and protect the neuronal endings. In daf-6 mutants, the glial cells don't make the tube properly and the neurons have no connection with the outside world. It's as if their nostrils are plugged, and the worms have no sense of smell or taste.
"At the surface of any cell, the amount of membrane added to the surface, and the amount taken away, are equal, so the cell stays the same in size," says Shaham. "To generate a tube, you still need to add membrane to the surface. However, in order to help the tube grow, you also need to make sure that no membrane is taken away, so there is a net increase of membrane at the cell surface."
Perens and Shaham think that the DAF-6 protein is involved in making sure that membrane isn't taken away from the surface. Another protein, CHE-14, seems to be responsible for adding the membrane. If both the daf-6 and che-14 genes are mutated, not only is the glial tube not formed, but all other organs in the worm that are made up of tubes, such as the intestine and kidneys, are disrupted.
Perens also observed that the neurons are important to give instructions to the glial cells when they are forming into the tube. When the neurons are disrupted, the DAF-6 protein doesn't end up in the right place, making Perens and Shaham think that there is some cross-talk going on between the glial cell and the neurons.
"The neuronal endings are telling the glial cell how big to make the tube and what its proper dimensions should be," says Perens. "So the neurons need the glia, and the glia need the neurons in order for the whole structure to be formed properly."
In big picture terms, this process is also very similar to the process of myelination, which is critical for neuron function in human beings, says Shaham. Myelination, when glial cells form an insulating layer of specialized cell membrane that wraps around neurons in the brain, ensures smooth nerve impulses through its insulation. Loss of myelination can cause many problems, and is the cause of multiple sclerosis.
"You have a neuronal ending that becomes surrounded by a glial cell," says Shaham. "There is a little bit known about the late stages in myelination, but these discoveries could give us an insight abut how the whole process is initiated."
The protein made by the daf-6 gene is related to a gene called patched, encoding a receptor protein, which is the most common gene mutated in medulloblastomas, a major form of brain cancer found in children. The DAF-6 branch of this family of proteins seems to be conserved throughout many different organisms, but roles for the conserved proteins haven't been studied. Perens plans to see if the DAF-6 protein is made in the glial cells of other organisms, such as fruit flies and mice, to determine if it is acting in a similar way.
This research was supported by funds from the Patterson Trust to Shaham and a National Institutes of Health MSTP grant to Perens.
Journal
Developmental Cell