Amyotrophic Lateral Sclerosis — Are Microglia Killing Motor Neurons?
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《新英格兰医药杂志》
Amyotrophic lateral sclerosis (ALS) is a disease characterized by the selective degeneration of motor neurons. The only known genetic cause of the disease, which accounts for about 2% of cases, is a mutation in the gene encoding superoxide dismutase 1 (SOD1) protein. Therefore, the effect of mutant SOD1 has been intensely studied, with emphasis on the question of whether it causes cell-autonomous degeneration of motor neurons or, rather, compromises the function of other types of cells in the spinal cord (such as astrocytes and microglia), which, in turn, damages neurons. A recent study by Boillee et al.1 sheds some light on the answer.
SOD1 scavenges superoxide radicals in the cytoplasm. The effect of the mutation varies greatly, from little or no loss to complete loss of scavenging activity. Even though the mutant protein is expressed in every tissue and cell type in the body, it seems to damage motor neurons alone. In transgenic mouse models of the disease, for example, expression of the mutant protein in all tissues may be elevated by a factor of 10, but degeneration is confined to the spinal motor neurons.
To examine the role of individual types of cells in SOD1-mediated ALS, Boillee et al. manipulated the expression of mutant SOD1 in motor neurons or microglia. (Microglia are specialized macrophages that protect the central nervous system from injury.) The investigators generated mice that expressed a mutant version of SOD1 in a manner that allowed them to "turn down" the level of the mutant protein in specific types of cells. First, they generated mice in which the mutant gene was flanked by short sequences of DNA that are recognized by a bacterial enzyme (called Cre recombinase) that catalyzes the excision of the short sequences together with all intervening DNA (in this case, the mutated SOD1 gene) from the genome. The researchers established several lines of such mice with multiple copies of the gene inserted in tandem into germ-line DNA. They then determined the age of the onset of disease and the timing of progression in the mice, establishing a baseline for comparison. Previous work had shown that the more mutated SOD1 genes a mouse carried, the greater the level of mutant SOD1 protein and the shorter its life. Boillee et al. replicated this finding.
The mice were then mated to mice that express Cre recombinase in a particular type of cell (that is, the motor neuron or the microglia), which resulted in progeny with repressed levels of mutant SOD1 in that cell type. (The mutant SOD1 is expressed in all cells, but the expression of Cre recombinase in, say, the microglia excises the mutated gene and hence diminishes synthesis of mutant SOD1.)
The investigators observed that in mice carrying even a modestly diminished load of mutant SOD1 in their motor neurons, the onset and progression of the disease were substantially delayed. This finding is consistent with a delayed onset of disease in SOD1-mutant mice treated with small interfering RNAs that specifically target the expression of mutant SOD1. The investigators noted that the selective diminution of the mutant protein in microglia greatly delayed the progression of late-stage disease — a period characterized by axonal degeneration and a loss of motor neurons. This finding implies that an activity of mutant SOD1 in microglia initiates events that hasten the demise of motor neurons.
Activation of microglia is accompanied by increased expression of surface-membrane proteins that form gap junctions between cells. Moreover, infiltrating macrophages, which replenish microglial populations, may become fusogenic when activated. Close apposition of microglia with motor neurons has been observed in neurodegenerative conditions, but whether actual connections are made, allowing for the passage of molecules through a cytoplasmic bridge, is uncertain. If Cre recombinase could be transferred to motor neurons, any resultant decrease in the level of expression of mutant SOD1 could delay the progression of ALS. The fact that only the late stage of disease was affected in these mice fits with this scenario, since activation of microglia tends to occur during the later stages of disease. Another possibility — and one that is much more interesting with regard to the disease mechanism — is that activated microglia supply toxins to the motor neurons or otherwise induce neuronal degeneration (Figure 1). Or perhaps toxins (or some other property of the mutant SOD1) damage microglia, which lose their protective function. The question is whether microglia expressing mutant SOD1 directly injure motor neurons or whether a protective function of microglia is diminished. Activated microglia, which are present in the murine models of ALS and are typically found in patients with the disease, have been shown to have both neurotoxic and neuroprotective properties in other disease settings.
Figure 1. Potential Mechanisms of Injury to Motor Neurons by Microglia.
A recent study by Boillee et al.1 suggests that the expression of mutant SOD1 in microglia accelerates the death of motor neurons in mouse models of SOD1-linked ALS. The cause may be toxin production or the transfer of toxins to nearby motor neurons or astrocytes by mutant microglia, or the mutant protein may damage the microglia and prevent them from producing yet-to-be-identified protective factors. Other possibilities exist, including the production of diffusible toxins (such as ions or cytokines) by damaged microglia.
Although the study by Boillee et al. suggests a role of microglia in the modification of disease progression, it is not certain that mutant SOD1 in microglia is critical to disease pathogenesis. Recent studies show that a fatal ALS-like disease develops in mice expressing particularly unstable mutants of SOD1; the unstable protein is rapidly degraded and does not accumulate to appreciable levels in cells other than degenerating motor neurons.2,3,4
Questions remain about the mechanism by which mutant SOD1 in microglia contributes to the degeneration of motor neurons, but the study by Boillee et al. provides compelling evidence that even a modest reduction in the level of expression of mutant SOD1 in motor neurons, and perhaps in other non-neuronal cells, could substantially alter the course of disease. The therapeutic implications for an approach that lowers the level of expression of mutant protein might extend to other autosomal dominant neurodegenerative diseases, such as Huntington's disease.
No potential conflict of interest relevant to this article was reported.
Source Information
From the Department of Neuroscience, McKnight Brain Institute, SantaFe Health Alzheimer's Disease Research Center, University of Florida, Gainesville.
References
Boillee S, Yamanaka K, Lobsiger CS, et al. Onset and progression in inherited ALS determined by motor neurons and microglia. Science 2006;312:1389-1392.
Wang J, Xu G, Li H, et al. Somatodendritic accumulation of misfolded SOD1-L126Z in motor neurons mediates degeneration: B-crystallin modulates aggregation. Hum Mol Genet 2005;14:2335-2347.
Jonsson PA, Ernhill K, Andersen PM, et al. Minute quantities of misfolded mutant superoxide dismutase-1 cause amyotrophic lateral sclerosis. Brain 2004;127:73-88.
Watanabe Y, Yasui K, Nakano T, et al. Mouse motor neuron disease caused by truncated SOD1 with or without C-terminal modification. Brain Res Mol Brain Res 2005;135:12-20.(David R. Borchelt, Ph.D.)
SOD1 scavenges superoxide radicals in the cytoplasm. The effect of the mutation varies greatly, from little or no loss to complete loss of scavenging activity. Even though the mutant protein is expressed in every tissue and cell type in the body, it seems to damage motor neurons alone. In transgenic mouse models of the disease, for example, expression of the mutant protein in all tissues may be elevated by a factor of 10, but degeneration is confined to the spinal motor neurons.
To examine the role of individual types of cells in SOD1-mediated ALS, Boillee et al. manipulated the expression of mutant SOD1 in motor neurons or microglia. (Microglia are specialized macrophages that protect the central nervous system from injury.) The investigators generated mice that expressed a mutant version of SOD1 in a manner that allowed them to "turn down" the level of the mutant protein in specific types of cells. First, they generated mice in which the mutant gene was flanked by short sequences of DNA that are recognized by a bacterial enzyme (called Cre recombinase) that catalyzes the excision of the short sequences together with all intervening DNA (in this case, the mutated SOD1 gene) from the genome. The researchers established several lines of such mice with multiple copies of the gene inserted in tandem into germ-line DNA. They then determined the age of the onset of disease and the timing of progression in the mice, establishing a baseline for comparison. Previous work had shown that the more mutated SOD1 genes a mouse carried, the greater the level of mutant SOD1 protein and the shorter its life. Boillee et al. replicated this finding.
The mice were then mated to mice that express Cre recombinase in a particular type of cell (that is, the motor neuron or the microglia), which resulted in progeny with repressed levels of mutant SOD1 in that cell type. (The mutant SOD1 is expressed in all cells, but the expression of Cre recombinase in, say, the microglia excises the mutated gene and hence diminishes synthesis of mutant SOD1.)
The investigators observed that in mice carrying even a modestly diminished load of mutant SOD1 in their motor neurons, the onset and progression of the disease were substantially delayed. This finding is consistent with a delayed onset of disease in SOD1-mutant mice treated with small interfering RNAs that specifically target the expression of mutant SOD1. The investigators noted that the selective diminution of the mutant protein in microglia greatly delayed the progression of late-stage disease — a period characterized by axonal degeneration and a loss of motor neurons. This finding implies that an activity of mutant SOD1 in microglia initiates events that hasten the demise of motor neurons.
Activation of microglia is accompanied by increased expression of surface-membrane proteins that form gap junctions between cells. Moreover, infiltrating macrophages, which replenish microglial populations, may become fusogenic when activated. Close apposition of microglia with motor neurons has been observed in neurodegenerative conditions, but whether actual connections are made, allowing for the passage of molecules through a cytoplasmic bridge, is uncertain. If Cre recombinase could be transferred to motor neurons, any resultant decrease in the level of expression of mutant SOD1 could delay the progression of ALS. The fact that only the late stage of disease was affected in these mice fits with this scenario, since activation of microglia tends to occur during the later stages of disease. Another possibility — and one that is much more interesting with regard to the disease mechanism — is that activated microglia supply toxins to the motor neurons or otherwise induce neuronal degeneration (Figure 1). Or perhaps toxins (or some other property of the mutant SOD1) damage microglia, which lose their protective function. The question is whether microglia expressing mutant SOD1 directly injure motor neurons or whether a protective function of microglia is diminished. Activated microglia, which are present in the murine models of ALS and are typically found in patients with the disease, have been shown to have both neurotoxic and neuroprotective properties in other disease settings.
Figure 1. Potential Mechanisms of Injury to Motor Neurons by Microglia.
A recent study by Boillee et al.1 suggests that the expression of mutant SOD1 in microglia accelerates the death of motor neurons in mouse models of SOD1-linked ALS. The cause may be toxin production or the transfer of toxins to nearby motor neurons or astrocytes by mutant microglia, or the mutant protein may damage the microglia and prevent them from producing yet-to-be-identified protective factors. Other possibilities exist, including the production of diffusible toxins (such as ions or cytokines) by damaged microglia.
Although the study by Boillee et al. suggests a role of microglia in the modification of disease progression, it is not certain that mutant SOD1 in microglia is critical to disease pathogenesis. Recent studies show that a fatal ALS-like disease develops in mice expressing particularly unstable mutants of SOD1; the unstable protein is rapidly degraded and does not accumulate to appreciable levels in cells other than degenerating motor neurons.2,3,4
Questions remain about the mechanism by which mutant SOD1 in microglia contributes to the degeneration of motor neurons, but the study by Boillee et al. provides compelling evidence that even a modest reduction in the level of expression of mutant SOD1 in motor neurons, and perhaps in other non-neuronal cells, could substantially alter the course of disease. The therapeutic implications for an approach that lowers the level of expression of mutant protein might extend to other autosomal dominant neurodegenerative diseases, such as Huntington's disease.
No potential conflict of interest relevant to this article was reported.
Source Information
From the Department of Neuroscience, McKnight Brain Institute, SantaFe Health Alzheimer's Disease Research Center, University of Florida, Gainesville.
References
Boillee S, Yamanaka K, Lobsiger CS, et al. Onset and progression in inherited ALS determined by motor neurons and microglia. Science 2006;312:1389-1392.
Wang J, Xu G, Li H, et al. Somatodendritic accumulation of misfolded SOD1-L126Z in motor neurons mediates degeneration: B-crystallin modulates aggregation. Hum Mol Genet 2005;14:2335-2347.
Jonsson PA, Ernhill K, Andersen PM, et al. Minute quantities of misfolded mutant superoxide dismutase-1 cause amyotrophic lateral sclerosis. Brain 2004;127:73-88.
Watanabe Y, Yasui K, Nakano T, et al. Mouse motor neuron disease caused by truncated SOD1 with or without C-terminal modification. Brain Res Mol Brain Res 2005;135:12-20.(David R. Borchelt, Ph.D.)