Keeping muscles strong and active in a mouse model of amyotrophic lateral sclerosis allows the animals to perform better in the face of dwindling motor neuron input, but ultimately cannot slow their demise, according to new research reported in the May 2 Cell Metabolism. Researchers in the laboratory of Don Cleveland at the University of California, San Diego, overexpressed the mitochondria-boosting gene PGC-1α in the skeletal muscles of mice that systemically produced mutant superoxide dismutase 1 (SOD1). The double transgenic mice could run farther and faster than single-mutant mSOD1 mice, but their disease progressed at the same rate. The work confirms that while mSOD1 in muscles causes atrophy, the muscle woes do not contribute to the neurodegeneration that is the primary ALS pathology, said co-first authors Sandrine Da Cruz and Philippe Parone in an interview with ARF.
The study addresses a question ALS researchers have debated—whether muscle pathology could cause the degeneration of neuromuscular junctions (NMJs), axons, and ultimately motor neurons themselves (Wong and Martin, 2010), wrote Ashu Johri and Flint Beal of the Weill Medical College of Cornell University, New York, in a commentary accompanying the Cell Metabolism paper. However, the fact that improving muscle function did not prevent denervation of motor neurons shows “that the disease is not a consequence of a dying back of axons following damage to NMJs in muscle,” Johri and Beal concluded.
PGC-1α (peroxisome proliferator-activated receptor gamma (PPARγ) coactivator-1α) is “the master regulator of mitochondrial biogenesis and oxidative metabolism,” Parone said. Researchers have already shown that PGC-1α not only increases motor ability, but also extends survival of ALS model mice when constitutively expressed (Zhao et al., 2011; Liang et al., 2011). Parone and Da Cruz, in order to understand the cell-specific contributions of mitochondrial pathology to disease, are developing mSOD1 mouse models that selectively express PGC-1α in motor neurons, astrocytes, or skeletal muscle. The muscle model is the first to be published.
Muscular PGC-1α noticeably benefited the double transgenic mice, even after symptoms developed. Their muscles possessed more mitochondria than did plain mSOD1 mice, and the organelles were bigger. The mice ran farther on a treadmill and faster on a running wheel, and spent more time exploring an open area, than the single mutants. When the researchers electrically stimulated the hind limb muscles, bypassing the motor neurons, the PGC-1α-expressing tissues took twice as long to fatigue as the muscle with only mSOD1. “Even in a mouse that is 100 percent paralyzed [in the hind limbs], the muscle is still very much active,” Parone said.
The scientists reasoned that if dysfunctional muscle sends some toxic signal to motor neurons, then the PGC-1α treatment should dampen that signal and improve the neurological side of the disease. This was not the case. In terms of disease onset, weight loss (of fat), and time of death, “there is absolutely no difference between the single transgenics and the double transgenics,” Parone said. “They still progress through disease at exactly the same speed as the mutant SOD1 animals.” There was also no difference in denervation of NMJs or motor neuron degeneration.
Parone and Da Cruz concluded that muscle cells could not be the most crucial source of ALS pathology. In support of this standpoint, Da Cruz noted that mice with mSOD1 only in muscle do not exhibit motor neuron degeneration (Dobrowolny et al., 2008). SOD1 toxicity in the central nervous system, and the subsequent retraction of neurons from NMJs, is what causes disease, Parone said. Other researchers agree. “The results clearly demonstrate that there is a dissociation between the health of muscle fibers and the denervation process,” wrote Giovanni Manfredi in an e-mail to Alzforum (see full comment below). Manfredi, at Weill Medical College of Cornell University, New York, was not involved in the study.
However, the paper supports the concept that muscle cells suffer direct toxicity from mSOD1, wrote Lee Martin, of the Johns Hopkins Medical School in Baltimore, Maryland, in an e-mail to Alzforum (see full comment below). Martin’s work has supported the contribution of muscles to ALS-like pathology (Wong and Martin, 2010). While that toxicity could be due to mitochondrial dysfunction, as explored by the Cleveland team, it could also be the result of other mechanisms such as misregulated RNA processing, Martin added. Antonio Musarò of the University of Rome, Italy, whose studies have also shown that muscle suffers in the presence of mSOD1 (Dobrowolny et al., 2008), argued in an e-mail to Alzforum that this makes muscles a primary target of the mutant protein, but agreed with the Cleveland team that mSOD1 muscle toxicity is not the main cause of neurodegeneration (see full comment below).
The results, the authors wrote, suggest that amping up muscle PGC-1α expression could improve quality of life for people with ALS, allowing them greater movement for longer. “Anything that could make the muscles stronger…that would translate into a very meaningful treatment,” said Robert Miller of the Forbes Norris MDA/ALS Research Center at the California Pacific Medical Center in San Francisco. Miller was not involved in the Cleveland paper. At this point, there are no drugs specific for PGC-1α that would be appropriate, Parone and Da Cruz said.
There are other options. For example, blocking myostatin activity improved strength and muscle size, but not survival, in ALS mice (Morrison et al., 2009; Holzbaur et al., 2006). Scientists are already testing another muscle-maximizing therapy, CK-2017357, in people (see ARF related news story on Russell et al., 2012). Although model mice with very severe motor neuron disease derive no survival benefit from these treatments, it remains possible that in people, the treatment would extend life by keeping the diaphragm working for longer, speculated Jeremy Shefner of the State University of New York Upstate Medical University in Syracuse. Shefner is lead investigator on a clinical trial in ALS—Amber Dance
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