Myelin is the secret weapon of the nervous system, the insulation that allows neurons to transmit electrical signals at the speed of thought. Wrapping itself around the long projecting axons of neurons, myelin acts like the insulation on a power cable, allowing electricity to travel more efficiently down nerve cells that can stretch for several feet. The importance of this function is revealed by diseases such as multiple sclerosis, where the loss of myelin causes severe movement difficulties and seizures.
The myelinating machines of the central nervous system have been known for decades: cells called oligodendrocytes, which produce myelin sheets that wrap around axons. But what genes and signals drive oligodendrocytes to do their work properly – and which of those factors go wrong in diseases like MS – remains a mystery. But the laboratory of Brian Popko, professor of neurology at the University of Chicago Medical Center, recently found a key piece of the myelination puzzle thanks to an odd, shivering mouse.
Published last week in the journal Genes & Development, Popko and his team used a method called forward genetics to launch an unbiased hunt for genes that had not previously been identified as important for myelination. Unlike reverse genetics, where researchers choose a gene of interest and make a knockout mouse lacking that gene, forward genetics relies upon the spontaneous creation of an interesting mutant. In this case, Popko and colleagues were looking for a mouse with the muscle tremors and seizures you would expect from deficient myelination.
“There were a number of knockout mutants available to us that disrupted the myelination process. They all resulted in mice that had a tremor, with names like shiverer, jumpy, trembler – you can get an idea of what the phenotype is,” Popko said. “Our screen was about as unsophisticated as you can imagine; we just selected for mice that have a tremor.”
Soon, Jackson Laboratory, a company that breeds and distributes mice for lab studies, came up with just such a mouse, who in its brief life (only about 3-1/2 weeks) has a chronic tremor in its hind legs, like it is constantly shivering in the cold.
Sure enough, when the brain and spinal cord of this mouse was dissected, it had little to no white matter, the informal term for myelinated neurons. Even more promising, the oligodendrocytes of this mutant were present, and appeared mature, but were frozen in the state just before actually initiating myelination – an interruption that resembles the malfunctioning oligodendrocytes in MS.
From there, the team worked backwards to the gene mutation that was preventing proper myelination in this mouse mutant, which was named hypomyelinating CNS – hmcns, to friends. Popko said that the sequencing of the mouse genome, which was largely completed in 2002, simplified this process, leading the researchers to a small stretch of 29 genes and then to the specific culprit, a gene called Zfp191. Further experiments with knockout mice lacking Zfp191 confirmed that mice lacking the gene suffered from arrested myelination, and that the disruption was caused by the gene’s improper expression in oligodendrocytes.
But what does Zfp191 do? Its involvement in myelination was a surprise – “We would never have predicted that this gene was involved in myelination,” Popko said. Turns out that Zfp191 codes for a transcription factor – a protein that turns around and subsequently activates more genes. Using a genechip microarray, Popko’s team was able to characterize proteins where expression levels are very different between normal mice and hmcns mice, proteins likely produced after Zfp191 activation of their corresponding genes.
That list contains 22 proteins, some of which were already known players in the myelination process, some of which were new arrivals on the scene. Popko suggested that one or more of those proteins may be a key piece of what goes wrong in demyelinating disorders such as MS where oligodendrocytes freeze in their late stages of maturation.
“Where I think that this is helpful is that we don’t know as much about that final stage of myelination, such that we don’t know the susceptibility of it and the factors that are critical,” Popko said. “What this mouse now does for us is narrow down our search for critical molecules that are important to that final stage of myelination.”