Much of the potential for iPS cells lies in the possibility of new models for human disease (see ARF related news story). Researchers studying ALS rely heavily on mice carrying mutant superoxide dismutase 1 (SOD1), which exhibit many symptoms of the disease. However, only a few percent of ALS cases are caused by SOD1 mutations. The majority of people with the disease have a sporadic form with no known genetic cause, and even among familial cases, SOD1 explains only one-fifth.

Enter iPS cells: patient-specific cell lines that can be nudged in the direction of a number of cell fates. With a little Sonic hedgehog, and a little retinoic acid, pluripotent cells can become motor neurons. Kevin Eggan of Harvard University reported on progress in turning fibroblasts from people with ALS—both familial and sporadic—into iPS cells. “We really see this as one more arrow in the quiver” for ALS research, Eggan said. Last year, Eggan’s group reported on an iPS line from an 82-year-old woman with ALS (see ARF related news story on Dimos et al., 2008). Those cells expressed the SOD1-L144F mutant; Eggan has added to his repertoire SOD1-G85R and SOD1-D90A cells. In addition, the group has generated five iPS lines from people with sporadic ALS. They are working on another SOD1 mutant line, SOD1-A4V, and a TDP-43 mutant line.

However, much work remains, Eggan noted, in the “painstaking and important, but rather mind-numbing” process of validation. The researchers must show that the cells are truly pluripotent, by checking biochemical markers and assaying their ability to form teratomas. They must confirm that the retroviral genes they used to induce pluripotency are truly silenced. And, they must verify that the cells can indeed differentiate into motor neurons. Of the 12 lines tried so far, all formed motor neurons, Eggan said, but he plans to test them further using electrophysiology. Overall, he said, the process is going well: “Version 1.0 of the technology is ending up being more robust than I had hoped for.”

Once valid iPS lines are available, they could be a testing ground for drug screening, and Haruhisa Inoue, of Kyoto University, reported on his progress with that approach. Inoue focused his search on iPS-derived, mSOD1-overexpressing astrocytes. These support cells are important players in ALS (Yamanaka et al., 2008). Inoue and colleagues engineered cells to express luciferase under the human SOD1 promoter, and screened 12,000 drugs for an effect on expression. They also examined SOD1 protein expression in cells treated with promising candidates.

The work led to a potential therapeutic Inoue is calling “Drug X,” as its identity is not yet public. The compound, which he said is currently used in hospitals for a non-neurological condition, decreased levels of both mutant and wild-type SOD1. The researchers are now testing the drug in mSOD1 mice, as well as researching its mechanism of action in iPS cells, Inoue wrote in an e-mail to ARF.

With a new preponderance of stem and iPS cell-derived lines poised to enter the ALS research laboratory, a key question is how well these lines model the “real thing,” motor neurons in a person or animal. Certainly, Eggan said, “These cells have not experienced exactly the same experiences” as natural motor neurons. However, he said they are more like the body’s motor neurons than any other model system.

Victor Rafuse of Dalhousie University in Halifax, Canada, also argued that differentiated stem cells are close facsimiles of motor neurons. In a paper last year, Rafuse and colleagues showed that embryonic stem cell-derived motor neurons, when injected into adult mice, form functional synapses (Yohn et al., 2008). “These cells live very happily in the peripheral nerve, and become myelinated,” Rafuse said. He also reported that when differentiated motor neurons are injected into chick embryos, they migrate to the correct location to form neuromuscular junctions.

François Berthod of Laval University in Québec City, who chaired the stem cell session, found the presentations encouraging. “The iPS cells will certainly transform our way to study these diseases in vitro in a couple of years,” he said. “That really will help to understand the disease.”

Rafuse suggested that stem cell-derived saviors might someday home to the right place in humans needing motor neuron replacements. With FDA approval now in place, other clinicians are poised to begin a safety study of spinal cord stem cells in people with ALS. The trial, led by Eva Feldman of the University of Michigan in Ann Arbor and Jonathan Glass of Emory University in Atlanta, Georgia, will test a patented neural stem cell line from Neuralstem, Inc. of Rockville, Maryland. These cells have already shown promise in mSOD1 rats, where they differentiated into motor neurons and formed synapses with host cells, and delayed disease onset and progression (Xu et al., 2006).

A similar strategy using purified motor neurons derived from stem cells was recently shown to improve pathology in mouse models of spinal muscular atrophy with respiratory distress (see ARF related news story). For the first stage of the human ALS trial, 12 participants will receive five to 10 spinal injections of neural stem cells. Safety is the primary outcome, but the researchers will also be looking for a slowing of disease. Final results are expected in two years.—Amber Dance.