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Expression of mutant huntingtin in glial cells contributes to neuronal excitotoxicity.

Shin JY, Fang ZH, Yu ZX, Wang CE, Li SH, Li XJ - J. Cell Biol. (2005)

Bottom Line: Here, we report that mutant huntingtin accumulates in glial nuclei in HD brains and decreases the expression of glutamate transporters.Mutant htt in cultured astrocytes decreased their protection of neurons against glutamate excitotoxicity.These findings suggest that decreased glutamate uptake caused by glial mutant htt may critically contribute to neuronal excitotoxicity in HD.

View Article: PubMed Central - PubMed

Affiliation: Department of Human Genetics, Emory University School of Medicine, Atlanta, GA 30322, USA.

ABSTRACT
Huntington disease (HD) is characterized by the preferential loss of striatal medium-sized spiny neurons (MSNs) in the brain. Because MSNs receive abundant glutamatergic input, their vulnerability to excitotoxicity may be largely influenced by the capacity of glial cells to remove extracellular glutamate. However, little is known about the role of glia in HD neuropathology. Here, we report that mutant huntingtin accumulates in glial nuclei in HD brains and decreases the expression of glutamate transporters. As a result, mutant huntingtin (htt) reduces glutamate uptake in cultured astrocytes and HD mouse brains. In a neuron-glia coculture system, wild-type glial cells protected neurons against mutant htt-mediated neurotoxicity, whereas glial cells expressing mutant htt increased neuronal vulnerability. Mutant htt in cultured astrocytes decreased their protection of neurons against glutamate excitotoxicity. These findings suggest that decreased glutamate uptake caused by glial mutant htt may critically contribute to neuronal excitotoxicity in HD.

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Electron micrographs of HD mouse brains. (A–D) EM48 immunogold labeling of the striatum (A) and cortex (B–D) of R6/2 mice at 12 wk of age. Immunogold-labeled aggregates are present in glial cells (arrows), which show a more condensed nuclear membrane and a smaller sized cytoplasm than do neuronal cells (arrowheads). Bars, 1 μm.
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fig1: Electron micrographs of HD mouse brains. (A–D) EM48 immunogold labeling of the striatum (A) and cortex (B–D) of R6/2 mice at 12 wk of age. Immunogold-labeled aggregates are present in glial cells (arrows), which show a more condensed nuclear membrane and a smaller sized cytoplasm than do neuronal cells (arrowheads). Bars, 1 μm.

Mentions: We used an antibody (EM48) to htt and performed immunogold labeling to examine brains from R6/2 mice that express HD exon1 protein with a 115–150-glutamine repeat. EM48 sensitively detects aggregated htt in HD brain (Li et al., 2000), enabling us to identify htt nuclear aggregates in glial cells in the striatum of R6/2 mice (Fig. 1 A). Glia can be classified as microglia, astrocytes, or oligodendrocytes. They are distinguished from neurons by a condensed nuclear envelope, a small and irregular shape, and a limited cytoplasmic area with sparse content. Microglial cells often show highly condensed nuclear membranes. Identification of astrocytes is primarily based on the presence of fibrils within their processes, and oligodendrocytes are often recognized by their association with groups of myelinated nerve fibers. The ultrathin sections used for electron microscopy might not have allowed us to definitively identify astrocytes containing htt aggregates, as a single plane of an ultrathin section could have been too thin to show both htt aggregates and the distinguishing morphological features of the cell. However, some glial cells, which displayed a highly condensed nuclear membrane and a small cytoplasmic space, contained intranuclear aggregates (Fig. 1, A–C). The nuclear htt aggregates in these glial cells are clearly smaller than neuronal nuclear inclusions (Fig. 1 D). Small glial nuclear inclusions were observed in the striatum (Fig. 1 A) and cortex (Fig. 1, B and C) of R6/2 mice 11–12 wk old. We also observed some hypertrophic and dark glial cells with increased electron density in R6/2 mice. These cells did not show visible cytoplasmic organelles and, in many cases, engulfed neuronal bodies or processes (Fig. S1, A and B, available at http://www.jcb.org/cgi/content/full/jcb.200508072/DC1). The hypertrophic glial cells are smaller and darker than the previously identified dark neurons (Turmaine et al., 2000; Yu et al., 2003), which often have long neuronal processes that have begun to degenerate (Fig. S1, C and D). Like dark neurons, the degenerating glial cells were not frequently observed and did not show htt aggregates, suggesting that glial dysfunction rather than degeneration is more important for HD pathology.


Expression of mutant huntingtin in glial cells contributes to neuronal excitotoxicity.

Shin JY, Fang ZH, Yu ZX, Wang CE, Li SH, Li XJ - J. Cell Biol. (2005)

Electron micrographs of HD mouse brains. (A–D) EM48 immunogold labeling of the striatum (A) and cortex (B–D) of R6/2 mice at 12 wk of age. Immunogold-labeled aggregates are present in glial cells (arrows), which show a more condensed nuclear membrane and a smaller sized cytoplasm than do neuronal cells (arrowheads). Bars, 1 μm.
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Related In: Results  -  Collection

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fig1: Electron micrographs of HD mouse brains. (A–D) EM48 immunogold labeling of the striatum (A) and cortex (B–D) of R6/2 mice at 12 wk of age. Immunogold-labeled aggregates are present in glial cells (arrows), which show a more condensed nuclear membrane and a smaller sized cytoplasm than do neuronal cells (arrowheads). Bars, 1 μm.
Mentions: We used an antibody (EM48) to htt and performed immunogold labeling to examine brains from R6/2 mice that express HD exon1 protein with a 115–150-glutamine repeat. EM48 sensitively detects aggregated htt in HD brain (Li et al., 2000), enabling us to identify htt nuclear aggregates in glial cells in the striatum of R6/2 mice (Fig. 1 A). Glia can be classified as microglia, astrocytes, or oligodendrocytes. They are distinguished from neurons by a condensed nuclear envelope, a small and irregular shape, and a limited cytoplasmic area with sparse content. Microglial cells often show highly condensed nuclear membranes. Identification of astrocytes is primarily based on the presence of fibrils within their processes, and oligodendrocytes are often recognized by their association with groups of myelinated nerve fibers. The ultrathin sections used for electron microscopy might not have allowed us to definitively identify astrocytes containing htt aggregates, as a single plane of an ultrathin section could have been too thin to show both htt aggregates and the distinguishing morphological features of the cell. However, some glial cells, which displayed a highly condensed nuclear membrane and a small cytoplasmic space, contained intranuclear aggregates (Fig. 1, A–C). The nuclear htt aggregates in these glial cells are clearly smaller than neuronal nuclear inclusions (Fig. 1 D). Small glial nuclear inclusions were observed in the striatum (Fig. 1 A) and cortex (Fig. 1, B and C) of R6/2 mice 11–12 wk old. We also observed some hypertrophic and dark glial cells with increased electron density in R6/2 mice. These cells did not show visible cytoplasmic organelles and, in many cases, engulfed neuronal bodies or processes (Fig. S1, A and B, available at http://www.jcb.org/cgi/content/full/jcb.200508072/DC1). The hypertrophic glial cells are smaller and darker than the previously identified dark neurons (Turmaine et al., 2000; Yu et al., 2003), which often have long neuronal processes that have begun to degenerate (Fig. S1, C and D). Like dark neurons, the degenerating glial cells were not frequently observed and did not show htt aggregates, suggesting that glial dysfunction rather than degeneration is more important for HD pathology.

Bottom Line: Here, we report that mutant huntingtin accumulates in glial nuclei in HD brains and decreases the expression of glutamate transporters.Mutant htt in cultured astrocytes decreased their protection of neurons against glutamate excitotoxicity.These findings suggest that decreased glutamate uptake caused by glial mutant htt may critically contribute to neuronal excitotoxicity in HD.

View Article: PubMed Central - PubMed

Affiliation: Department of Human Genetics, Emory University School of Medicine, Atlanta, GA 30322, USA.

ABSTRACT
Huntington disease (HD) is characterized by the preferential loss of striatal medium-sized spiny neurons (MSNs) in the brain. Because MSNs receive abundant glutamatergic input, their vulnerability to excitotoxicity may be largely influenced by the capacity of glial cells to remove extracellular glutamate. However, little is known about the role of glia in HD neuropathology. Here, we report that mutant huntingtin accumulates in glial nuclei in HD brains and decreases the expression of glutamate transporters. As a result, mutant huntingtin (htt) reduces glutamate uptake in cultured astrocytes and HD mouse brains. In a neuron-glia coculture system, wild-type glial cells protected neurons against mutant htt-mediated neurotoxicity, whereas glial cells expressing mutant htt increased neuronal vulnerability. Mutant htt in cultured astrocytes decreased their protection of neurons against glutamate excitotoxicity. These findings suggest that decreased glutamate uptake caused by glial mutant htt may critically contribute to neuronal excitotoxicity in HD.

Show MeSH
Related in: MedlinePlus