While for patients, there is nothing but a void when it comes to effective treatments for amyotrophic lateral sclerosis (ALS), scientists have no shortage of ideas under development. Researchers presented several potential approaches at the Society for Neuroscience annual meeting, held 12-16 November 2011 in Washington, DC, and at RNA-Binding Proteins in Neurological Disease, a satellite conference held 10-11 November 2011 in Arlington, Virginia. Researchers at both meetings presented drug screens and other attempts to stymie ALS pathology by sidestepping or destroying toxic aggregates of the disease-linked proteins TDP-43 and FUS; blocking the destruction of protective molecules or the production of pathogenic ones; or silencing damaging genes. Fen-Biao Gao of the University of Massachusetts in Worcester and Paul Taylor of St. Jude Children’s Research Hospital in Memphis, Tennessee, chaired the satellite meeting.
Gregory Petsko of Brandeis University in Waltham, Massachusetts, works with yeast models of TDP-43 and FUS. At the satellite meeting, he noted that FUS is one of the more toxic proteins we have ever worked with. In yeast, both wild-type and mutant FUS localize to the cytoplasm and form toxic aggregates. Looking for medicines that would ameliorate this toxicity, Petsko set up a screen to identify compounds with which yeast survive FUS induction. Even though the screen itself is simple, it took a year of tweaking to come up with a clean assay that has low signal-to-background noise but can still find small effects, Petsko said. His group has now screened some 150,000 compounds, of which 20 ameliorate FUS toxicity. The team is still working to discover what the drugs do, and whether their mechanism of action would be useful in human neurons suffering from ALS.
Petsko is also studying a suppressor of FUS toxicity called UPF1, a protein that is able to keep yeast alive in the presence of cytoplasmic aggregates of FUS or TDP-43 (see ARF related news story on Ju et al., 2011). UPF1 is an RNA-binding protein involved in degrading miscreant RNAs. Like TDP-43 and FUS, it has several functions, and Petsko’s team has not yet sorted out the protective mechanism. In collaboration with Steven Finkbeiner at the University of California in San Francisco, Petsko is studying the gene’s effects in motor neurons. As in yeast, FUS kills the neurons, and UPF1 rescues them. We are pretty excited about this, Petsko said. This suggests that the yeast model is good enough and that the drug screen is worth doing. At the same time, he wondered if perhaps UPF1, provided as a gene or a peptide, or upregulated or stabilized by medication, might be the treatment he is looking for.
TDP-43 Aggregate Busters
If aggregated TDP-43 and FUS are toxic to neurons, then some scientists argue that dissolving the aggregates should save the cells—and with that, perhaps people with ALS. Marisa Feiler in the Boston University laboratory of Ben Wolozin is taking that approach in her drug screen. Feiler presented a poster at the Neuroscience meeting and Wolozin discussed the work in a talk. Feiler transfected PC12 (rat adrenal cancer) cells with a green fluorescent protein-tagged TDP-43 construct and treated the cells with the stressor arsenite to cause the protein to aggregate in cytoplasmic stress granules (see related ARF Webinar on stress granules in disease). After screening more than 75,000 drugs, Feiler found 22 candidates that wipe away those TDP-43 clusters. On her poster, she focused on a drug known as compound #8, which cut the number of stress granule by 40 percent.
Feiler and Wolozin collaborated with Brian Kraemer of the University of Washington in Seattle to try out compound #8 in nematodes expressing both wild-type and mutant (alanine-315-threonine) TDP-43. These worms have movement phenotypes, but under the treatment they wriggled more quickly across their plates. This was not a slam dunkbut it did increase their movement about twofold, Wolozin told ARF. Worms with mutant TDP-43 normally lose five of their 19 motor neurons and three neuromuscular junctions; with treatment, they lost an average of 2.5 motor neurons and one junction. The researchers did not report if the treatment ablated aggregates. While the data suggest that dissolving TDP-43 aggregates is effective in this simple animal model, the investigators do not yet know how the drug acts.
At the South San Francisco biotechnology company iPierian, Ashkan Javaherian and colleagues are also working to do away with TDP-43 aggregates. The team, Javaherian reported at the Arlington symposium, has collected skin cells from 20 people with sporadic ALS and reprogrammed them into induced pluripotent stem cells, and then into motor neurons. TDP-43 pathology seems to be a dominant type of pathology across sporadic ALS types, said Javaherian, adding that TDP-43 aggregation is one among several potential disease mechanisms the company is targeting. In motor neurons derived from skin cells of healthy people, TDP-43 was nuclear and not aggregated, while three of the 20 sporadic ALS cases yielded motor neurons in which TDP-43 formed distinct nuclear aggregates. The company is looking for drugs that will block or destroy those TDP-43 globs. In a small screen of some 2,000 compounds, the team came up with 39 hits that decrease the percentage of cells with aggregates.
Virginia Lee of the University of Pennsylvania in Philadelphia questioned the pathology seen at iPierian, since most researchers have reported TDP-43 aggregation in the cytoplasm, not nucleus, of sick cells. I worry that they are looking at an artifact, she told ARF. In response, Javaherian noted that one of the three skin donors whose cells produced nuclear inclusions later died, and the researchers found the same kinds of nuclear aggregates in his spinal cord motor neurons. Thus, he thinks some scientists might have missed this nuclear pathology, or it might only be present in a subset of people with ALS.
Blocking Undesirable Pairings
Also at the Arlington satellite, Leonard Petrucelli of the Mayo Clinic in Jacksonville, Florida, discussed a potential treatment for frontotemporal lobar dementia (FTLD). This disease shares features with ALS, such as genetic mutations and pathology in TDP-43 and FUS. Petrucelli is interested in progranulin, which is mutated in some people with FTLD, leading to haploinsufficiency of the protein. Uptake of progranulin by the membrane receptor sortilin enhances progranulin clearance (see ARF related news story on Hu et al., 2010; reviewed in Ward and Miller, 2011), so Petrucelli hypothesized that blocking this process should boost progranulin levels and alleviate disease. His team screened for drugs that would interfere with the progranulin-sortilin interaction, and discovered compounds that boost extracellular progranulin levels in HeLa (cervical cancer) cells in culture.
Beka Solomon of Tel Aviv University in Israel also sought to block an undesirable interaction—in her case, between amyloid precursor protein (APP) and the β-secretase that cleaves it to form amyloid-β and soluble APP β (sAPPβ). At the Neuroscience meeting, Solomon described an antibody that shields the β-cleavage site on APP. She is testing this antibody in mouse models for Alzheimer’s disease (Arbel-Ornath et al., 2010), but some evidence indicates that sAPPβ may be involved in ALS as well (see ARF related news story; Steinacker et al., 2009; Koistinen et al., 2006). To look into this, Solomon treated 70-day-old ALS model mice overexpressing mutant human superoxide dismutase 1 (SOD1) with her APP antibody and observed a reduction in spinal cord sAPPβ levels. She reported seeing some improvement in the animals’ ability to balance on a rotating rod, as well as a two-week extension in lifespan. In male transgenics, the treatment tripled the number of motor neurons present at an age of 104 days, 14 days after treatment started, compared to untreated animals which lost neurons at a more rapid rate. At the same time point, the antibody treatment had reduced astrogliosis in both genders.
Gene therapy is another approach that Petsko and others are considering for ALS. This would require a reliable method to deliver nucleic acids to the motor neurons struggling in the spinal cord. Brian Kaspar of Nationwide Children’s Research Institute in Columbus, Ohio, is working on a vehicle, but as he said at the Arlington meeting, crossing the blood-brain barrier is ever the challenge. He has succeeded in getting adeno-associated virus (AAV) vectors to traverse the barrier (see ARF related news story on Foust et al., 2009), and at the meeting he discussed using the virus to deliver short hairpin RNA (shRNA) against SOD1. This approach is being trialed for familial ALS due to SOD1 mutations. Kaspar’s group recently reported that it also works on astrocytes derived from neural precursor cells from people with sporadic ALS, suggesting the treatment might have a broader reach (see ARF related news story on Haidet-Phillips et al., 2011). When Kaspar and colleagues used AAV9 to deliver the shRNA to three-week-old SOD1-G93A mice, it effectively knocked down the enzyme in astrocytes. Preliminary tests of grip strength indicated the treatment might delay disease onset, Kaspar said. While all of the treatment possibilities presented at the meeting are in preliminary stages of development, they offer varied approaches that may one day pan out as treatment options for ALS and related disorders.
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