Almost a century and a half after it was first described by Jean-Martin Charcot, and 74 years after New York Yankees star Lou Gehrig’s case of it made it widely known, amyotrophic lateral sclerosis (ALS) remains a mystery. Researchers don’t know how it starts, how it progresses, or why it mostly harms motor neurons and not other types of neuron. There is no treatment that can delay the progression of the disease significantly, let alone halt or reverse it. In very rare cases, like that of the physicist Stephen Hawking, the disease strikes relatively early in life, and then stops on its own, for reasons unknown. But ALS typically hits in middle age and kills within a few years, by spreading to the spinal cord motor neurons that control breathing.
Even so, there are reasons to hope that now is a good time for a breakthrough. One is that researchers are shifting their focus away from motor neurons, having found clues that suggest the disease might originate at least partly in other cells.
Researchers at Johns Hopkins University, for example, recently reported that brain and spine cells called oligodendrocytes degenerate before motor neurons do, at least in mice with a form of ALS. Such findings might explain why progress has lagged, and they clearly suggest new strategies for research and potential treatment.
Meanwhile scientists have been turning up hints that ALS involves a cell-harming spread of protein aggregates similar to that found in Alzheimer’s, Parkinson’s, and other neurodegenerative diseases. Whether these two seemingly disparate lines of investigation can converge remains to be seen. But if they do, that convergence could result not only in a better understanding of ALS and how to treat it, but also in new ways of thinking about neurodegenerative diseases in general.
Helper cells die first
Evidence that the ALS disease process occurs—and may even begin—outside motor neurons has been building for more than a decade. In 2001, for example, McGill University researchers reported the surprising finding that a mutant version of the antioxidant gene SOD1, which causes ALS in some human cases and is used to trigger disease in ALS-model mice, failed to cause the degeneration of mouse motor neurons when its activity was confined to just those cells. Two years later, Don Cleveland’s laboratory at the University of California–San Diego reported inducing the ALS-like condition in mice by activating mutant SOD1 in non-neuronal cells only. Similarly, in 2008, Cleveland and his colleagues found that knocking down mutant SOD1 activity just in motor-neuron helper cells called astrocytes delayed disease progression by about 50 days in mice.
New findings suggest that oligodendrocytes, which produce and maintain the myelin sheaths surrounding nerve fibers, might be even more important than astrocytes in the progress of ALS. In July 2012 Jeffrey Rothstein’s lab at Johns Hopkins reported that in addition to their myelin-maintenance role, oligodendrocytes provide a key source of nourishment to motor neurons: They convert blood-borne glucose into lactate—a precursor of the basic cellular energy molecule ATP—and transport the lactate into nearby motor-neuron nerve fibers. Rothstein’s team found that the primary carrier molecule used for this lactate transport, MCT1, is abundant in oligodendrocytes—and when its function is disrupted, neurons that normally receive this basic nourishment start to die. The researchers noted too that oligodendrocytes’ MCT1 levels are reduced in ALS patients and mouse models. Perhaps a cutoff of lactate supply could be a major cause of motor neuron death in ALS.
In the latest study, Rothstein’s lab teamed up with the lab of Hopkins neurobiologist Dwight Bergles and took a closer look at oligodendrocytes in ALS. Within the spinal cords of young mutant-SOD1 mice, months before motor neurons would show signs of disease, they observed that oligodendrocytes were already diseased and dying—and among other dysfunctions, expressed lower levels of MCT1 transporters. Immature precursor cells that should have been able to become new and functional oligodendrocytes failed to mature. “It’s likely due to changes in the environment in which the cells were trying to develop,” says Bergles—and he notes that these changes included signs of inflammation.
The degeneration of oligodendrocytes seemed to affect nearby motor neurons, which soon began to lose their myelin sheaths. The Hopkins researchers found similar signs of oligodendrocyte degeneration and motor neuron demyelination in autopsied brain tissue from human patients—most of whom had died of ordinary, “sporadic” ALS, of unknown cause. “That suggests that this is a widespread phenomenon [within ALS], not unique to the rare people who carry SOD1 mutations,” Bergles says.
Working with UCSD’s Cleveland, whose lab had developed a cell-specific SOD1-knockdown technique, Bergles and Rothstein analyzed what happened when they reduced the mutant gene’s activity just in oligodendrocytes. Whereas Cleveland’s previously reported knockdown of the gene in astrocytes had delayed the disease course by about 50 days on average, a knockdown in oligodendrocytes turned out to slow the disease course by about 85 days on average: about one-eighth of a lab mouse’s normal lifespan. In addition, it delayed the initial appearance of disease signs by about 60 days—an effect not seen when mutant SOD1 was reduced in astrocytes. “It tells us that oligodendrocytes really affect the course of the disease, and appear to do so quite early in the disease, at least in this mouse model,” says Rothstein.
Oligodendrocytes and the myelin sheaths they make are known to be harmed in another neurological disease, multiple sclerosis (MS). Rothstein hopes that recently developed candidate MS drugs, meant to boost oligodendrocyte function and restore myelin, will have a beneficial effect on ALS too. He also hopes that MRI and related imaging techniques, long ago applied to MS, can now be targeted to affected areas in the brains and spinal cords of people with ALS—“to begin to delineate this pathology, and to use it for diagnosis and to monitor the response to drugs,” he says.
What about the aggregates?
Another line of investigation in ALS research has aimed at understanding the abnormal protein aggregates that are known to accumulate in affected motor neurons. In mutant-SOD1 ALS cases, for example, these clumps are made chiefly of mutant SOD1 proteins, which appear to be toxic somehow—mice with ALS-causing mutant SOD1 fare much worse than mice that have no SOD1 at all.
“There is a common defect that arises from many SOD1 mutations, which is an increased propensity for the SOD1 protein to expose its hydrophobic or ‘sticky’ surfaces,” says Anne Bertolotti, a researcher at Cambridge University. That makes SOD1—normally a very stable protein—more likely to clump up. Bertolotti and her colleagues have shown in lab-dish studies that mutant SOD1 can still take a long time to aggregate spontaneously within a given neuron. But it can aggregate much more rapidly when “seeded” with aggregates from other cells—and in prion-like fashion these self-propagating aggregates can spread through a cell population.
That appears to fit the clinical picture of ALS that is caused by the most common SOD1 mutation in North America: decades of normal life, followed by a sudden outbreak of degeneration in one small set of motor neurons, and a rapid spread from there. “The disease progression is really fast and furious,” Bertolotti says. “Half of patients die within a year and a half of symptom onset.”
It also fits well with the emerging understanding of other neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and Huntington’s diseases, all of which appear to be driven by the spread of prion-like protein aggregates. Even in people with ordinary “sporadic” ALS, unconnected to SOD1 or any other known gene mutation, motor neurons accumulate aggregates. In these cases the aggregates are made of two other proteins, TDP-43 and FUS, which are thought to play a similar role to SOD1 in causing disease. (Rare mutations to the TDP-43 or FUS genes also can cause familial forms of ALS.)
A big question now is how to reconcile the toxic-spreading-aggregate hypothesis with the evidence that oligodendrocytes and astrocytes are affected first. Do SOD1 and TDP-43 aggregates somehow harm oligodendrocytes, astrocytes, and other glial cells even before they harm motor neurons? “Certainly in the pathological analysis of tissue from human ALS patients, there’s evidence of protein inclusions in oligodendrocytes,” notes Bergles. Bertolotti emphasizes that more work needs to be done in this area, but says: “I could see the glial cells as important players in capturing aggregates that transfer from cell to cell.”
Although there have been hints from research on other brain diseases that glial cell failure drives neurodegeneration, that hypothesis hasn’t caught on widely. In any case, Bergles says that he and Rothstein are now doing experiments that might help resolve the question in ALS: repeating their deletion of the mutant-gene from oligodendrocytes, this time in TDP-43 mouse models. “It’s going to be very interesting to see how this affects the oligodendrocytes,” he says.