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
- Wong M, Martin LJ. Skeletal muscle-restricted expression of human SOD1 causes motor neuron degeneration in transgenic mice. Hum Mol Genet. 2010 Jun 1;19(11):2284-302. PubMed.
- Zhao W, Varghese M, Yemul S, Pan Y, Cheng A, Marano P, Hassan S, Vempati P, Chen F, Qian X, Pasinetti GM. Peroxisome proliferator activator receptor gamma coactivator-1alpha (PGC-1α) improves motor performance and survival in a mouse model of amyotrophic lateral sclerosis. Mol Neurodegener. 2011;6(1):51. PubMed.
- Liang H, Ward WF, Jang YC, Bhattacharya A, Bokov AF, Li Y, Jernigan A, Richardson A, Van Remmen H. PGC-1α protects neurons and alters disease progression in an amyotrophic lateral sclerosis mouse model. Muscle Nerve. 2011 Dec;44(6):947-56. PubMed.
- Dobrowolny G, Aucello M, Rizzuto E, Beccafico S, Mammucari C, Boncompagni S, Bonconpagni S, Belia S, Wannenes F, Nicoletti C, Del Prete Z, Rosenthal N, Molinaro M, Protasi F, Fanò G, Sandri M, Musarò A. Skeletal muscle is a primary target of SOD1G93A-mediated toxicity. Cell Metab. 2008 Nov;8(5):425-36. PubMed.
- Morrison BM, Lachey JL, Warsing LC, Ting BL, Pullen AE, Underwood KW, Kumar R, Sako D, Grinberg A, Wong V, Colantuoni E, Seehra JS, Wagner KR. A soluble activin type IIB receptor improves function in a mouse model of amyotrophic lateral sclerosis. Exp Neurol. 2009 Jun;217(2):258-68. PubMed.
- Holzbaur EL, Howland DS, Weber N, Wallace K, She Y, Kwak S, Tchistiakova LA, Murphy E, Hinson J, Karim R, Tan XY, Kelley P, McGill KC, Williams G, Hobbs C, Doherty P, Zaleska MM, Pangalos MN, Walsh FS. Myostatin inhibition slows muscle atrophy in rodent models of amyotrophic lateral sclerosis. Neurobiol Dis. 2006 Sep;23(3):697-707. Epub 2006 Jul 11 PubMed.
- Russell AJ, Hartman JJ, Hinken AC, Muci AR, Kawas R, Driscoll L, Godinez G, Lee KH, Marquez D, Browne WF, Chen MM, Clarke D, Collibee SE, Garard M, Hansen R, Jia Z, Lu PP, Rodriguez H, Saikali KG, Schaletzky J, Vijayakumar V, Albertus DL, Claflin DR, Morgans DJ, Morgan BP, Malik FI. Activation of fast skeletal muscle troponin as a potential therapeutic approach for treating neuromuscular diseases. Nat Med. 2012 Mar;18(3):452-5. PubMed.
- Hervias I, Beal MF, Manfredi G. Mitochondrial dysfunction and amyotrophic lateral sclerosis. Muscle Nerve. 2006 May;33(5):598-608. PubMed.
- Lin J, Handschin C, Spiegelman BM. Metabolic control through the PGC-1 family of transcription coactivators. Cell Metab. 2005 Jun;1(6):361-70. PubMed.
- Miller TM, Kim SH, Yamanaka K, Hester M, Umapathi P, Arnson H, Rizo L, Mendell JR, Gage FH, Cleveland DW, Kaspar BK. Gene transfer demonstrates that muscle is not a primary target for non-cell-autonomous toxicity in familial amyotrophic lateral sclerosis. Proc Natl Acad Sci U S A. 2006 Dec 19;103(51):19546-51. PubMed.
- Dupuis L, Gonzalez de Aguilar JL, Oudart H, de Tapia M, Barbeito L, Loeffler JP. Mitochondria in amyotrophic lateral sclerosis: a trigger and a target. Neurodegener Dis. 2004;1(6):245-54. PubMed.
- Dobrowolny G, Giacinti C, Pelosi L, Nicoletti C, Winn N, Barberi L, Molinaro M, Rosenthal N, Musarò A. Muscle expression of a local Igf-1 isoform protects motor neurons in an ALS mouse model. J Cell Biol. 2005 Jan 17;168(2):193-9. PubMed.
- Zhou J, Yi J, Fu R, Liu E, Siddique T, Ríos E, Deng HX. Hyperactive intracellular calcium signaling associated with localized mitochondrial defects in skeletal muscle of an animal model of amyotrophic lateral sclerosis. J Biol Chem. 2010 Jan 1;285(1):705-12. PubMed.
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- Johri A, Beal MF. Muscling in on PGC-1α for improved quality of life in ALS. Cell Metab. 2012 May 2;15(5):567-9. PubMed.
- Da Cruz S, Parone PA, Lopes VS, Lillo C, McAlonis-Downes M, Lee SK, Vetto AP, Petrosyan S, Marsala M, Murphy AN, Williams DS, Spiegelman BM, Cleveland DW. Elevated PGC-1α activity sustains mitochondrial biogenesis and muscle function without extending survival in a mouse model of inherited ALS. Cell Metab. 2012 May 2;15(5):778-86. PubMed.