Accentuate the Positive
Some Center scientists, eying a cure, seek the cause of ALS. Others study how it damages cells. But a third group’s work may lessen immediate misery: They’re learning the basics of damage control and repair.

Jonathan Glass and Ahmet Höke wrestle with fine points of regenerative biology.

A variation on the small, garden-variety laboratory mouse sold for years in Europe has a gift that ALS researchers wish they could pass on to patients: The animals’ motor neurons resist damage and still transmit impulses up to a month after they’re cut in two. They’re the biological equivalent of Superman’s chest in withstanding assault.

“Injure a normal neuron and it begins to die within 48 hours,” says neuroscientist Jonathan Glass, who’s trying to determine what accounts for the animals’ good fortune; “but the WldS mice defy those rules of biology.”

Recently, Glass has turned to ALS as a disease that might benefit from a better understanding of the unusual mice. His basic studies on the nervous system—and that of Ahmet Höke and others who make up one arm of Center research—are helping to uncover tactics to rescue or help regrow ALS-targeted motor neurons.

In the case of WldS mice, Glass says, the rodents have a rare mutation that juxtaposes parts of two normally separate genes. The protein products of this “supergene” are responsible, somehow, for the neurons’ unusual survival.

To test the gene’s capabilities, Glass and his team transferred it into normal mice and exposed them to vincristine, a known neuro-poison. WldS wonderfully protected their neurons, which resumed growing after the team flushed the toxin away.

Recently, Glass bred WldS mice with mouse models of ALS to see if the gene protects their offspring, which would otherwise die. “It does,” he says, “though the good effects mostly appear in female mice.” Now Glass is exploring why that’s the case, monitoring the hybrid mice for signs of illness and analyzing tissues at every stage from birth. Because he suspects that more copies of WldS would boost the protective effect, his team’s also looking into ways to get more of the genes into cells.

Glass has other work where the biology’s even clearer. Most encouraging are his experiments that center on calpains—common enzymes that’re a “smoking gun” in a variety of nervous system damage. Calpains are active in the nerve-cell breakdown that follows stroke or spinal cord injury. They also participate in the normal tearing down of internal “scaffolds” that temporarily appear when neural cells develop. “If you’re going to break down tissue in the human body,” says Glass, “calpains are probably going to be there.”

Because calpains are destructive, blocking them might become part of a new therapeutic approach. In studies with animal models of peripheral nerve disease, Glass indeed found that inhibiting the enzymes with a drug called AK295 was “extraordinarily useful” in staving off injury. Soon he’ll administer the drug to ALS rat models, easing it into spinal fluid where it could reach vulnerable motor neurons.

One reason Glass feels justified in giving calpain-blocking a try stems from the fact that the enzyme is calcium-activated. “And ALS is a disease where calcium ions flood the interior of neurons,” he explains, “making that environment an ideal place for the enzyme to operate. If blocking calpain doesn’t help in the rat models, I’d be really surprised.”

It’s not just unbridled optimism to say that there’ll come a time, scientists believe, when they have enough of a handle on ALS to slow its pace dramatically. Whether it’ll be at an early or a more advanced stage, no one can say. But in that held-breath interval before an actual cure, how fine it would be to be able to heal already-damaged motor neurons! To that end, studies by Center scientist Ahmet Höke are laying necessary groundwork.

Höke’s work focuses on peripheral nerves—those that extend beyond the brain and spinal cord—because they hold a key property: Peripheral nerves can heal if the repair process isn’t long delayed. Exactly how repair occurs isn’t clear, but Höke hopes his studies will apply to recovery of ALS-ravaged neurons in the brain and spinal cord.

Newly cut peripheral nerve cells, seen end on, maintain their axon centers for a while.

Cut peripheral nerve cells in two, Höke explains, and their axons eventually disintegrate. Fortunately, the intact cell bodies can sprout new axons. But for those new axons to survive and reach needed targets, say, to muscle, they must have a clear path to follow, free of the debris the dying axons left behind. Also, to survive and grow, new nerve cells apparently need a mix of specific agents called neurotrophins.

Normally, cells known as Schwann cells come to the rescue, says Höke. Living in direct contact with neurons, Schwann cells produce neurotrophins while they summon the immune system to clear debris. Unfortunately, Schwann cells’ repair capability comes with a statute of limitations—an interval that lasts a far shorter time than do chronic diseases such as ALS. Höke has shown this in a model of nerve injury he recently developed in rats. There’s a clear difference between newly injured and chronically injured nerve cells in their ability to regenerate properly.

So Höke is now clarifying Schwann cell biology with an aim to mimic the cells’ nurturing role in patients or, even better, to keep the Schwann cells from fading away in the first place. He’s found, for example, that Schwann cells in the vicinity of newly injured motor neurons produce GDNF, the most potent neurotrophin. Adding GDNF artificially might encourage nerve repair.

But because dropping GDNF on nerve cells is technically difficult to do, Höke has begun working with a line of mouse stem cells engineered to produce large quantities of GDNF—some 50 times normal. By gently injecting these cells into injured nerves in his model, he’s sparked regeneration in nerves considered “over the hill.” “We’ve watched nerve function come back in the rats week by week,” he says.

Höke’s studies branch out to human stem cell lines derived from embryos, from fetal tissue and adults—all to see which have the best potential to heal.

Next > The Aggregate Dilemma: Too Obvious to Ignore
For years, scientists have noted obvious clumps of protein in motor neurons of patients with both sporadic and inherited forms of ALS—those who have a mutated gene for the SOD1 enzyme.