In amyotrophic lateral sclerosis and related diseases, damaged motor neurons commit cellular suicide, calling on the apoptosis program to finish their lives. But what has been unclear is whether apoptosis is a core process early in the disease, or merely the swan song of cells that have lost the ability to do anything other than make a graceful exit. In the October 1 Journal of Clinical Investigation, scientists from the University of California in San Francisco report that blocking apoptosis in ALS model mice keeps motor neurons alive, suggesting apoptosis is a central part of the pathological process.

First author Nichole Reyes and senior author Scott Oakes led the study. They reasoned that if apoptosis happens early in ALS, and helps to push motor neurons to their final hour, then blocking apoptosis should slow the disease. Conversely, if apoptosis simply finishes off cells that are already beyond repair, then blocking the process should not affect symptoms.

To get to the answer they sought, Reyes and colleagues chose to knock out apoptosis at a key gateway: mitochondrial permeability mediated by BCL2-associated X protein (Bax) and BCL2-homologous antagonist/killer (Bak). Bax and Bak share a similar function, sitting in the mitochondrial membrane and waiting for cell death signals. Upon receiving those signals, they form pores, allowing cytochrome c to leak out and initiating cellular suicide.

Other studies have also targeted the apoptotic pathway. Blocking caspases or other cell death genes, or overexpressing BCL2 (which puts a damper on apoptosis) increases lifespan in ALS model mice (see ARF related news story on Li et al., 2000; Friedlander et al., 1997; Kostic et al., 1997). And the drug minocycline, which prevents cytochrome c release, also prolongs survival (Zhu et al., 2002).

The scientists generated mice that lack the Bak gene, but carry a floxed version of Bax. They expressed Cre recombinase under the nervous system-specific rat nestin promoter. Around day 12 of embryonic development, when nestin turns on, the recombinase cuts Bax out of brain and spinal cord tissue. “We could really, truly, for the first time, completely block the apoptotic pathway in vivo,” Oakes said. They crossed these mice with animals overexpressing human superoxide dismutase 1 (SOD1)-G93A, a mutant associated with familial ALS. As controls, they used SOD1-G93A mice carrying only the nestin-Cre construct, with no changes to their Bak or Bax levels. These control animals evince apoptosis in spinal cord motor neurons and paralysis starting at 100 days of age. They die within a month of symptom onset.

In the transgenic Bax/Bak double knockouts, disease onset was delayed by more than three weeks, and the animals survived for nearly a month longer than controls. When the control mice were dying, the double knockouts “were still walking around, pretty much fine,” Oakes said. The double knockout transgenic mice not only had more motor neurons than age-matched controls, they also preserved neuromuscular junctions for longer. The motor neurons they had were also functional, the scientists concluded, because the double knockouts maintained their ability to balance on a rotarod. Oakes and colleagues concluded that apoptosis must contribute directly to the disease process.

“It is terrific that they have been able to look into this very specific trigger of mitochondrial apoptosis,” said Robert Friedlander of the University of Pittsburgh, Pennsylvania, who noted the work confirms previous results on the importance of mitochondria and mitochondrial cell death pathways in ALS. For example, SOD1-G93A mice deficient in only Bax also exhibit extended survival (Gould et al., 2006).

The study also poses further questions, said Serge Przedborski of Columbia University in New York. Now, he wonders what pathway leads from ALS, to Bak and Bax, and beyond to motor neuron death. “They have a very elegant model that will allow us, perhaps, to dissect a little further, and in more detail, what is the molecular cascade,” he said.

Przedborski added, “It re-stimulates the idea that targeting things like Bak and Bax for therapy may be a valuable strategy.” However, it could also be a dangerous strategy. “A lot of cells, in the brain and outside the brain, rely on the machinery of apoptosis to be eliminated,” he said. “You run the risk to have major side effects, including tumorigenesis.”

For that reason, Oakes is interested in targeting apoptosis at a different point than the Bax/Bak gateway. This experiment, he said, was a proof of principle that blocking apoptosis can extend motor neuron survival. Now, he and his colleagues are searching for upstream signals—such as those from the unfolded protein response—that activate Bak and Bax in ALS. Those upstream signals should be amenable to drugs, he said, and might allow doctors to prevent apoptosis specifically in struggling neurons.—Amber Dance.

Reference:
Reyes NA, Fisher JK, Austgen K, VandenBerg S, Huang EJ, Oakes SA. Blocking the mitochondrial apoptotic pathway preserves motor neuron viability and function in a mouse model of amyotrophic lateral sclerosis. J Clin Invest. 2010 Oct 1;120(10):3673-9. Abstract

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  1. Reyes et al. report that the mitochondrial apoptotic pathway crucially contributes to degeneration and death of motor neurons in ALS of SOD1-G93A mice. This pathway is also induced by the neurotrophin receptor p75 (see, e.g., Troy et al., 2002), and earlier reports have linked p75 to ALS (e.g., Lowry et al., 2001). In particular, NGF from reactive astrocytes can cause motor neuronal apoptosis via p75 (Pehar et al., 2004).

    However, neither p75 knockout (deletion of p75 exon 3) in SOD1-G93A mice (Küst et al., 2003) nor application of a cyclic decapeptide against the N-terminal region of p75 (Turner et al., 2004) significantly influence onset and progression of ALS; on the other hand, application of antisense peptide nucleotides against p75 delays and attenuates ALS (Turner et al., 2003). These seemingly contradictory results can be reconciled by considering the (normally weak) expression of the truncated form of p75, s-p75, that can also be found in p75 exon 3 knockout mice and that (like p75) reaches significant levels in sciatic nerves of SOD1-G93A mice by endstage ALS (Turner et al., 2009). The s-p75 lacks the N-terminal cysteine-rich domains of p75, and hence the neurotrophin- and Aβ-binding sites within this region, but still encompasses a second Aβ-binding site in the extracellular juxtamembrane region of p75; evidence of this binding site is presented in my Aβ-crosslinker-hypothesis).

    The hypothesis suggests that oligomeric Aβ can crosslink p75, via this extracellular juxtamembrane binding site, with other surface proteins such as APP, α-synuclein, and prion protein, and that this crosslinking represents an important physiological function of Aβ. Under pathophysiological conditions, however, β-sheet aggregates of certain amyloidogenic proteins can cause p75-mediated cell degeneration and apoptosis when they form complexes with Aβ species and use Aβ to interact with the juxtamembrane binding site of p75. Amyloid aggregates that can activate p75 and s-p75 through this binding site include, for example, β-sheet Aβ and NAC, a natural fragment of α-synuclein, and may also include secreted SOD1 aggregates since SOD1 and especially SOD1-G93A can directly interact with Aβ (Yoon et al., 2009); TDP-43, too, has been reported to interact with Aβ (Higashi et al., 2010), but it is unknown if TDP-43 is secreted as well. In addition, Aβ aggregates may contribute to peripheral motor neuron degeneration and to motor neuronal death by irregular activation of p75 and s-p75 as APP and Aβ are increased in certain muscle groups of ALS patients and of SOD1-G93A mice (Koistinen et al., 2006). Although not the basic cause of ALS, p75 and s-p75 signaling may crucially aggravate the disease in the described ways.

    Reyes et al. conclude that targeting processes that induce the mitochondrial apoptotic pathway might be a useful therapeutic strategy for ALS and related motor neuron diseases. The neurotrophin receptor p75 and its truncated form s-p75 certainly are worthwhile objects of such research.

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    . Elevated levels of amyloid precursor protein in muscle of patients with amyotrophic lateral sclerosis and a mouse model of the disease. Muscle Nerve. 2006 Oct;34(4):444-50. PubMed.

    . Reduced p75NTR expression delays disease onset only in female mice of a transgenic model of familial amyotrophic lateral sclerosis. Amyotroph Lateral Scler Other Motor Neuron Disord. 2003 Jun;4(2):100-5. PubMed.

    . A potential role for the p75 low-affinity neurotrophin receptor in spinal motor neuron degeneration in murine and human amyotrophic lateral sclerosis. Amyotroph Lateral Scler Other Motor Neuron Disord. 2001 Sep;2(3):127-34. PubMed.

    . Astrocytic production of nerve growth factor in motor neuron apoptosis: implications for amyotrophic lateral sclerosis. J Neurochem. 2004 Apr;89(2):464-73. PubMed.

    . Mechanisms of p75-mediated death of hippocampal neurons. Role of caspases. J Biol Chem. 2002 Sep 13;277(37):34295-302. PubMed.

    . Antisense peptide nucleic acid-mediated knockdown of the p75 neurotrophin receptor delays motor neuron disease in mutant SOD1 transgenic mice. J Neurochem. 2003 Nov;87(3):752-63. PubMed.

    . Effect of p75 neurotrophin receptor antagonist on disease progression in transgenic amyotrophic lateral sclerosis mice. J Neurosci Res. 2004 Oct 15;78(2):193-9. PubMed.

    . Dismutase-competent SOD1 mutant accumulation in myelinating Schwann cells is not detrimental to normal or transgenic ALS model mice. Hum Mol Genet. 2010 Mar 1;19(5):815-24. PubMed.

    . Intracellular amyloid beta interacts with SOD1 and impairs the enzymatic activity of SOD1: implications for the pathogenesis of amyotrophic lateral sclerosis. Exp Mol Med. 2009 Sep 30;41(9):611-7. PubMed.

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  6. . Blocking the mitochondrial apoptotic pathway preserves motor neuron viability and function in a mouse model of amyotrophic lateral sclerosis. J Clin Invest. 2010 Oct 1;120(10):3673-9. PubMed.

Further Reading

Papers

  1. . Blocking the mitochondrial apoptotic pathway preserves motor neuron viability and function in a mouse model of amyotrophic lateral sclerosis. J Clin Invest. 2010 Oct 1;120(10):3673-9. PubMed.
  2. . Inhibitors of cytochrome c release with therapeutic potential for Huntington's disease. J Neurosci. 2008 Sep 17;28(38):9473-85. PubMed.
  3. . Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases. Nature. 2006 Oct 19;443(7113):787-95. PubMed.
  4. . A network dysfunction perspective on neurodegenerative diseases. Nature. 2006 Oct 19;443(7113):768-73. PubMed.
  5. . Apoptosis in amyotrophic lateral sclerosis--what is the evidence?. Lancet Neurol. 2005 Aug;4(8):500-9. PubMed.
  6. . Abolishing Bax-dependent apoptosis shows beneficial effects on spinal muscular atrophy model mice. Mol Ther. 2006 Jun;13(6):1149-55. PubMed.
  7. . Caspase-1 and -3 are sequentially activated in motor neuron death in Cu,Zn superoxide dismutase-mediated familial amyotrophic lateral sclerosis. Proc Natl Acad Sci U S A. 2000 Dec 5;97(25):13901-6. PubMed.

Primary Papers

  1. . Blocking the mitochondrial apoptotic pathway preserves motor neuron viability and function in a mouse model of amyotrophic lateral sclerosis. J Clin Invest. 2010 Oct 1;120(10):3673-9. PubMed.