Getting to the Heart of It
With ALS, many of the simplest questions remain unanswered. ‘That just won’t do,’ say Center scientists.

Don Cleveland: His team plans a gutsy approach to the ALS mystery.

Sluggish isn’t a word that describes researcher Don Cleveland: His walk’s more of a trot, he speaks fast and he’s as quick-witted as they come. And when he presented his work at the Center’s March investigators’ meeting, there’s no doubt it welled from a deep pool of impatience with the status quo. All the years of ALS research, all the money that’s been spent worldwide, he’d likely say, and nobody even knows where in the nervous system the miserable problem STARTS!

So Cleveland, who heads a respected scientific institute at the University of California, and Center co-researcher Larry Goldstein, also at UCSD, have made finding the disease’s target cells a personal quest. In the past year, they and several other Packard scientists have quietly overturned dogma about ALS’s attack on the nervous system.

“Because the disease kills motor neurons, people have thought those cells are damaged directly by the disease process. But that’s not necessarily true,” says Cleveland. “We’re learning neighboring cells such as astrocytes and the immune cells, microglia, also have a role in the decline, perhaps a major one.”

Work on targeting relies on SOD1 mice, an ALS mimic that Cleveland helped “invent.” They’re the standard animal model for the illness. Though SOD1 mice become paralyzed because they carry a mutant human gene for inherited ALS, researchers believe the model also sheds light on the disease’s more common, sporadic form. After a certain point, both inherited and sporadic ALS likely follow the same cell paths.

Chimeras Prove Helpful
To pinpoint where things start in SOD1 mice, the two scientists tried an unusual approach that uses chimeric animals, “calico” mice engineered to be a mix of normal, or wild type cells, and cells containing the mutant human SOD1 gene. The researchers tagged the cells with molecular flags to make it clear which were which.

Healthy neurons: victims of bad company?

As expected, each of the 42 mice they produced was a blend of the cell types. But luckily, two of the animals weren’t such an even blend. They showed an unusual mix of cells that proved tremendously helpful.

In those animals, all their motor neurons carried the SOD1 mutation. That meant their slip into death was pretty much a given. But, surprisingly, not all of the neurons took that downward turn, and why they didn’t was revealing.

While motor neurons on both halves of the spinal cord carried the mutant gene, adjacent, non-neural cells varied in each half. Through a quirk of distribution, motor neurons in the left half of the spinal cord nestled among “good” wild type cells. On the right side, motor neurons were close to “bad” cells with the SOD1 mutation.

Did that make a difference? “No doubt,” says Cleveland. Twice as many motor neurons to the left survived as to the right. “The environment is apparently important,” he explains. “For motor neurons, being surrounded by healthy wild type cells seems to offer protection.”

Moreover, in mice where the opposite happened—normal motor neurons surrounded by mutant cells—the healthy neurons now showed signs of decline. “Mutant cells, we now believe, can transfer toxic characteristics to normal ones.”

More proof
While Goldstein and Cleveland were sorting out their chimeras, Center scientist Jean-Pierre Julien, at McGill University in Montreal, had slightly different chimeras that further bolstered the others’ results. In some of Julien’s chimeric mice, as many as a third of the motor neurons in the animals’ spinal cords carried the ALS mutation. But you’d never know it. The neurons, surrounded by healthy cells, showed no signs of disease. Nor did the mice that carried them.

Can good cells keep bad neurons healthy?

“What we’re seeing,” Cleveland says, “is a real-life metaphor. Living in a bad environment can damage good cells. And more important, restoring a better environment to “bad” neurons by surrounding them with healthy neighboring cells can significantly lessen their toxic effects. In some cases, the presence of normal cells completely stops motor neuron death. We’re seeing doomed animals saved by what we’d consider a small percentage of normal neighboring cells.

“All this has great implications for stem cell therapy: We now believe delivery of normal, non-neuronal cells to spinal cords could be completely protective, even without replacement of a single motor neuron.”

Still Not Solved
Knowing a motor neuron’s environment is important, however, isn’t the same as teasing out where things first go awry. “We haven’t answered our question,” says Cleveland. This winter, he and Goldstein were awarded a two-year Center grant for a different approach—one some fellow scientists describe as “gutsy.”

“We thought we’d start with an animal that we know will get the disease because all its cells have a mutant gene. Then, one by one, we’ll turn that gene off in specific groups of cells and see what’s critical.” A novel technique lets the researchers put the equivalent of an off switch for the gene in specific groups of cells. They’ll try turning the gene off in motor neurons, in neighboring astrocytes, in inflammatory cells called microglia and even in muscle cells.

Also, an equivalent of a master switch will let them shut off most all the toxic SOD1 genes at one swoop. “That way we can see if the gene needs to be continually active for ongoing sickness or if shutting it off early on would save the animals.” The technique is fairly new and untested, Cleveland explains, “But we won’t know unless we try.”

Next > One Step Closer to the Bedside
The Basics Bolster Stem Cell Therapy.