Multiple sclerosis (MS) is an autoimmune disease characterized by damage to the myelin sheath surrounding nerve fibers. The cause remains unknown, and it is a chronic illness affecting the central nervous system that has no cure.
MS has long been considered a disease of white matter, a reference to the white-colored bundles of myelin-coated axons that project from the main body of a brain cell. But researchers have discovered that the condition also affects myelinated axons scattered in gray matter that contains main bodies of brain cells, and specifically the hippocampus region, which is important for learning and memory.
Up to half of MS patients suffer cognitive deficits in addition to physical symptoms. Researchers suspect that cognitive problems are caused by abnormal electrical activities of the demyelinated axons extending from hippocampal cells, but until now have not been able to test myelin's role in this part of the brain.
Ohio State University researchers have created a system in which two types of cells interact in a dish as they do in nature: neurons from the hippocampus and other brain cells, called oligodendrocytes, whose role is to wrap myelin around the axons.
Now that the researchers can study how myelination is switched on and off for hippocampal neurons, they also can see how myelin does more than provide insulation -- it also has a role in controlling nerve impulses traveling between distant parts of the nervous system. Identifying this mechanism when myelin is present will help improve understanding of what happens when axons in this critical area of the brain lose myelin as a result of MS, researchers say.
So far, the scientists have used the system to show that myelin regulates the placement and activity of a key protein, called a Kv1.2 voltage-gated potassium channel, that is needed to maintain ideal conditions for the effective transmission of electrical signals along these hippocampal axons.
"This channel is important because it is what leads to electrical activity and how neurons communicate with each other downstream," said Chen Gu, assistant professor of neuroscience at Ohio State and lead author of the study. "If that process is disrupted by demyelination, disease symptoms may occur."
The study appears in the current (July 22, 2011) issue of the Journal of Biological Chemistry.
To create the cell culture system, the researchers began with hippocampus neurons from a rodent brain -- a cell type that Gu has worked with for years. In culture, these cells can grow and develop dendrites -- other branch-like projections off of neurons -- and axons as well as generate electrical activity and synaptic connections, the same events that occur in the brain.
The researchers then added oligodendrocytes, along with some of their precursor cells, to the same dish as the neurons. And eventually, after maturing, these oligodendrocytes began to wrap myelin around the axons of the hippocampal neurons.
This system takes about five weeks to create, but the trickiest part, Gu said, was developing the proper solution for this culture so that both kinds of cells would behave as nature intended.
"In the end, the composition of the culture medium is basically half from a solution that supports the neurons and half from a medium in which the oligodendrocytes function well. We know that all the cells were happy because we got myelin," said Gu, also an investigator in Ohio State's Center for Molecular Neurobiology.
With the system established, they then turned to experimentation to test the effects of the myelin's presence on these specific brain cells.
Nerve cells send their signals encoded in electrical impulses over long distances. Concerted actions of various ion channels are required for properly generating these nerve impulses. Potassium channels are involved at the late phase in an impulse, and its role is to return a nerve cell to a resting state after the impulse has passed through it and gear up for the next one. The Kv1.2 ion channel helps ensure that this process works smoothly.
By experimentally manipulating signal conditions with the new co-culture system, Gu and his colleague were able to establish part of the sequence of events required for myelinated hippocampal neurons to effectively get their signals to their targets. Starting with a protein known to be produced by myelin and axons, called TAG-1, a cell adhesion molecule, they traced a series of chemical reactions indicating that myelin on the hippocampal axons was controlling the placement and activity of the Kv1.2 ion channel.
"The analysis allowed us to see the signaling pathways involving myelin's regulation of the Kv1.2 channel's placement along the axon as well as fine-tuning of the channel's activity," Gu said.
When MS demyelinates these axons, the affected nerve cells don't get the message to rest, and subsequently can't prepare adequately to receive and transmit the next signal that comes along.
"This means a nerve impulse will have a hard time traveling through the demyelinated region," Gu said. "This shows that the ion channel is probably involved in the downstream disease progression of MS."
Gu envisions many additional uses for the new co-culture system, including additional studies of how myelin affects the behavior of other channels, proteins and molecules that function within axons, as well as to screen the effects of experimental drugs on these myelinated cells.
This work was supported by a Career Transition Fellowship Award from the National Multiple Sclerosis Society and a grant from the National Institute for Neurological Disorders and Stroke.
Gu conducted this study with Yuanzheng Gu, a research associate in the Department of Neuroscience at Ohio State.
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