Mutations in the gene Fused in Sarcoma cause amyotrophic lateral sclerosis, but does the problem arise from a lack of normal FUS or some new, toxic action by the mutant protein? A paper in the April 25 Acta Neuropathologica Communications makes a strong case for the latter. Mice missing the FUS gene lived to nearly two years with no signs of motor neuron disease. Curiously, some developed vacuoles in the hippocampus and were unusually active, report researchers led by Nobuyuki Nukina of Doshisha University in Kyoto, Japan. The animals appear to have few similarities to ALS, but they might make a good model for attention deficit/hyperactivity disorder, Nukina speculated.
Brain bubbles: Compared to wild-type mice (left), FUS knockouts (middle) looked mostly normal, though they developed some vacuoles in the hippocampus (magnified on right). [Image courtesy of Kino et al., Acta Neuropathologica Communications.]
FUS regulates the transcription and splicing of numerous mRNAs (see Oct 2012 news; Ishigaki et al., 2012). Substitution and truncation mutations cause ALS, and the protein also appears in inclusions in a subset of frontotemporal lobar degeneration cases (see Feb 2009 news; Jul 2009 news). Mice with mutant FUS or excess wild-type FUS develop a progressive movement disorder, suggesting that the protein can turn toxic (see Feb 2013 news; Michell et al., 2013; Shelkovnikova et al., 2013). However, a key datum has been absent from the literature, noted Nukina and Yoshihiro Kino, first author of the new study: No one had analyzed neurons in mice without FUS.
Perhaps that was because the knockout mice are tricky to make. Back in 2000, study co-author Geoff Hicks of the University of Manitoba in Winnipeg, Canada, tried it, but his homozygous FUS-null mice died within a day of birth—too soon to look for signs of an aging disease (Hicks et al., 2000). Around the same time, David Ron and colleagues at the New York University Medical Center succeeded in making FUS knockouts. They were sterile and those researchers did not examine neural phenotypes (Kuroda et al., 2000). Ron's group had used an outbred group of mice, with a mixture of genotypes, rather than the highly homogenous inbred C57 black-6 strain that Hicks used. “Each inbred mouse genetic background is going to have its own vulnerabilities,” pointed out Lawrence Hayward of the University of Massachusetts Medical School in Worcester, who was not involved in any of these studies. “This background may lack genes required to overcome the effects of a lack of FUS.”
Kino and Nukina started their experiments with Hicks’ black-6 animals that were missing just one copy of the FUS gene. Co-author Toru Takumi had a colony of these animals at the RIKEN Brain Science Institute in Wako, Japan. To shuffle the background genotype, Kino crossed them with another strain—white ICR mice (named for Philadelphia’s Institute for Cancer Research, where the stock was developed). He then intercrossed the heterozygous offspring to obtain total FUS knockouts with a hodgepodge of black-6/ICR genomes. Unlike the original Hicks homozygotes, these mixed-background knockout pups survived more than a day, but they were puny, and most died before they were weaned—typically within a month. Kino realized that they could not compete with their FUS-positive littermates for milk, and when he removed some of those robust siblings from the cages, the FUS knockouts survived to adulthood.
Not only did they live, they moved just fine. Atop a rotating rod—a common challenge for ALS mouse models—the knockouts were as quick-footed and balanced as their littermates sporting two copies of FUS. At 90 weeks, Kino collected their spinal cords, and saw the knockouts had the same number of motor neurons as controls. Most of the mice have been sacrificed now, Nukina said, and the authors saw no new symptoms in those surviving beyond two years. “The [lack of motor neuron deficits] suggest that FUS gain of function is the main pathophysiology in ALS,” Nukina concluded.
How did the neurons—and other body tissues—survive without FUS to manage their gene expression? Nukina and colleagues speculate that related RNA regulators may have compensated. The knockouts made extra mRNA for TAF15 and EWS, which are in the same protein family as FUS (Tan and Manley, 2009). “The outbred mouse line may have a better capacity to upregulate these two proteins and thereby to compensate for the loss of FUS,” wrote Jernej Ule of University College London, who was not involved in the study. Hayward noted that it remains to be seen how much overlap exists between the mRNA targets of FUS and other RNA binding proteins.
As evident by their small size and difficulty scrambling for milk after birth, the mice were not entirely normal. Unexpectedly, as the knockouts aged Kino observed a surfeit of boldness and activity throughout their lifespan. Compared with control mice, they roved their cages more frequently, and spent more time exploring open and lighted areas. What made them so frisky? Their brains looked fairly normal, but many of the knockouts had vacuoles in the hippocampus (see image above). While it was not possible to ascertain the origin of these vacuoles, some were lined with MAP2, a marker for dendrites. Perhaps these structures were related to some kind of synaptic abnormality, Nukina suggested. Similar structures occur in some prion disorders and one severe ALS mouse model, Hayward added, though not in people with ALS. Many different problems can cause vacuolization.
“This paper makes a pretty persuasive argument for a gain-of-function mechanism for FUS. This is about as definitive as you can get,” said Randal Tibbetts of the University of Wisconsin School of Medicine and Public Health in Madison. Even so, Tibbetts could still think of ways that FUS loss of function might cause problems in people. Mice only live for two years, and it may take decades of a human lifespan to see the effects of the missing protein, he theorized. Alternatively, a second hit such as environmental stress might trigger neurodegeneration in animals with no FUS, he speculated. Ule also noted that even if other proteins take over the role of FUS in mice, the human versions might not have the same flexibility. The debate between loss-of-function and gain-of-function hypotheses for FUS is not settled.—Amber Dance
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