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Human Molecular Genetics Advance Access originally published online on August 4, 2004
Human Molecular Genetics 2004 13(19):2183-2196; doi:10.1093/hmg/ddh246
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Human Molecular Genetics, Vol. 13, No. 19 © Oxford University Press 2004; all rights reserved

Calcium-permeable AMPA receptors promote misfolding of mutant SOD1 protein and development of amyotrophic lateral sclerosis in a transgenic mouse model

Minako Tateno1, Hisako Sadakata1, Mika Tanaka2, Shigeyoshi Itohara2, Ryong-Moon Shin3, Masami Miura3, Masao Masuda3, Toshihiko Aosaki3, Makoto Urushitani1, Hidemi Misawa4 and Ryosuke Takahashi1,*

1Laboratory for Motor System Neurodegeneration and 2Laboratory for Behavioral Genetics, Brain Science Institute, RIKEN, Wako, Saitama 351-0198, Japan, 3Neural Circuits Dynamics Research Group, Tokyo Metropolitan Institute of Gerontology, Itabashi, Tokyo 173-0015, Japan and 4Department of Neurology, Tokyo Metropolitan Institute for Neuroscience, Fuchu, Tokyo 183-8526, Japan

Received April 21, 2004; Accepted July 20, 2004


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 SUPPLEMENTARY MATERIAL
 REFERENCES
 
Mutant Cu/Zn-superoxide dismutase (SOD1) protein aggregation has been suggested as responsible for amyotrophic lateral sclerosis (ALS), although the operative mediating factors are as yet unestablished. To evaluate the contribution of motoneuronal Ca2+-permeable (GluR2 subunit-lacking) {alpha}-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors to SOD1-related motoneuronal death, we generated chat-GluR2 transgenic mice with significantly reduced Ca2+-permeability of these receptors in spinal motoneurons. Crossbreeding of the hSOD1G93A transgenic mouse model of ALS with chat-GluR2 mice led to marked delay of disease onset (19.5%), mortality (14.3%) and the pathological hallmarks such as release of cytochrome c from mitochondria, induction of cox2 and astrogliosis. Subcellular fractionation analysis revealed that unusual SOD1 species first accumulated in two fractions dense with neurofilaments/glial fibrillary acidic protein/nuclei and mitochondria long time before disease onset, and then concentrated into the former fraction by disease onset. All these processes for unusual SOD1 accumulation were considerably delayed by GluR2 overexpression. Ca2+-influx through atypical motoneuronal AMPA receptors thus promotes a misfolding of mutant SOD1 protein and eventual death of these neurons.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 SUPPLEMENTARY MATERIAL
 REFERENCES
 
Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset neurodegenerative disease characterized by a selective loss of motoneurons in the spinal cord and brainstem (1). Mutation of Cu/Zn-superoxide dismutase (SOD1) is the most frequent cause of familial ALS (2). Introduction of such mutated SOD1 genes into mice causes ALS-like symptoms characterized by the selective death of spinal motoneurons, despite a ubiquitous expression of mutant proteins (3). Several lines of evidence have demonstrated that mutant SOD1 toxicity is not essentially due to decreased dismutase activity, but rather to a ‘gain of toxic function’ (4). This so-called ‘oligomerization hypothesis’ has recently attracted attention from ALS researchers. The hypothesis maintains that mutant SOD1 proteins are misfolded, and consequently oligomerized and aggregated, gaining toxic properties at some stage in their formation (5). The hypothesis is based on the numerous observations that SOD1-containing inclusions/high-molecular-weight-shifted protein complexes are specifically found in spinal motoneurons and their surrounding astrocytes from autopsied patients and transgenic mice carrying mutant SOD1 genes (6–8), in spinal cord extracts from mutant SOD1 transgenic mice (9–12) and in cultured motoneurons into which mutant SOD1 has been microinjected (13).

However, in addition to this line of evidence, glutamate-induced excitotoxicity has also been implicated in the pathophysiology of ALS patients and mutant SOD1 transgenic mice (14–17). Pharmacological experiments have strongly suggested that the excitotoxicity of spinal motoneurons largely depends on Ca2+-permeable {alpha}-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors specifically expressed in a subset of neurons, including spinal motoneurons (18,19). AMPA receptors, major mediators for fast excitatory neurotransmission in the mammalian central nervous system, are composed of a heteromeric complex of four subunits GluR1–GluR4, and the absence of GluR2 renders the receptor Ca2+-permeable (20). As this unique property of GluR2 is generated posttranscriptionally by RNA editing, an editing failure can also produce Ca2+-permeable AMPA receptors (21). Reduced editing efficiency of GluR2 mRNA has been specifically reported in spinal motoneurons from human sporadic ALS patients (22), further suggesting that Ca2+-permeable AMPA receptor-mediated excitotoxicity is closely linked to the vulnerability of spinal motoneurons in ALS. However, whether and in what manner this atypical type of AMPA receptor affects mutant SOD1-induced motoneuronal degeneration remains to be elucidated.

The purpose of the present study was to explore a mechanistic link between glutamate toxicity and the conversion of mutant SOD1 into aberrant forms by modification of the electrophysiological properties of motoneuronal AMPA receptors in an ALS mouse model. The chat-GluR2 transgenic mouse line was generated to overexpress GluR2 in a cholinergic neuron-specific manner, resulting in a large reduction in Ca2+-permeability of motoneuronal AMPA receptors. We detected various types of abnormally folded SOD1 proteins in fractions derived from different cellular compartments from hSOD1G93A mice spinal cords. Double transgenic mice carrying both chat-GluR2 and hSOD1G93A displayed a marked delay of disease onset, followed by delayed formation of all the abnormal SOD1 species. These results indicate that Ca2+-permeable AMPA receptors in motoneurons contribute to the conformational changes of mutant SOD1 and the subsequent neurodegeneration associated with these changes.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
DISCUSSION
 MATERIALS AND METHODS
 SUPPLEMENTARY MATERIAL
 REFERENCES
 
Generation and characterization of chat-GluR2 transgenic mice: cholinergic neuron-specific GluR2 overexpression results in substantial reduction of Ca2+-permeable AMPA receptors in spinal motoneurons
A mouse line with reduced numbers of Ca2+-permeable AMPA receptors in spinal motoneurons was generated. Spinal motoneurons are typical cholinergic neurons, constituting a minor population among total spinal neurons. Thus, a cholinergic neuron-specific promoter, i.e. the choline acetyltransferase (ChAT) gene promoter (23) (Fig. 1A) was used to preferentially increase GluR2 expression in spinal motoneurons. Three independent chat-GluR2 transgenic lines, Tg3, Tg7 and Tg10, were established. To examine the copy number of the chat-GluR2 transgene, Taqman quantitative DNA PCR and genomic Southern blotting were performed. Results from Taqman PCR indicated that the Tg3, Tg7 and Tg10 lines contained ~2, 10 and 16 copies of the chat-GluR2 transgene, respectively, a finding which was also supported by genomic Southern blotting (Fig. 1B). Expression patterns of the transgenes in the spinal cord were examined using in situ hybridization (Fig. 1C), revealing a preferential transcription of transgenes in cholinergic neurons in chat-GluR2 mice. To quantify the GluR2 mRNA level in spinal motoneurons, motoneurons were carefully purified from frozen slices of spinal cord using laser microdissection, because other neuronal populations such as dorsal horn neurons express high level of GluR2. Quantitative PCR analysis revealed that spinal motoneurons in Tg7 expressed levels of GluR2 mRNA nearly 5-fold higher than those of non-transgenic control mice (Table 1). Tg10 included numerous copies of the transgene, but displayed lower levels of GluR2 expression than Tg7, probably owing to DNA methylation of transgenes (data not shown). No significant changes in mRNA levels of ChAT, endogenous SOD1, GluR3 or GluR4 were observed in chat-GluR2 transgenic mice compared with non-transgenic mice. Western blotting of the extracts prepared from the spinal cord ventral region also revealed a significant increase of the GluR2 protein level in Tg7 compared with that in non-transgenic littermates (Fig. 1D). GluR2 expression was thus significantly increased in spinal motoneurons in chat-GluR2 mice without affecting the expression of other AMPA receptor subunits. Next, the Ca2+-permeability of AMPA receptors in spinal motoneurons was examined. Whole-cell patch-clamp recordings were performed on motoneurons in spinal cord slices. The first two graphs in Figure 1E represent typical I-V relationships, showing distinct inward rectification in wild-type (wt), whereas a linear relationship is seen in Tg7. Normalized I–V relationships reveal a clear difference between wt and Tg7 (P<0.001). The rectification index, an index of Ca2+-permeability calculated as the ratio of chord conductance at +40 and –70 mV, was estimated as 0.262±0.024 for wt and 0.436±0.038 for Tg7 (mean±SEM, P<0.001). Thus, the majority of AMPA receptors in spinal motoneurons were Ca2+-impermeable in Tg7 mice, but were Ca2+-permeable in non-transgenic controls.



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  		Figure 1. Generation and characterization of chat-GluR2 transgenic mice. (A) The DNA fragment introduced into chat-GluR2 transgenic mice contains the rat GluR2 and EGFP coding sequences driven by mouse ChAT promoter. GluR2 and EGFP are transcribed into the same mRNA, but are independently translated owing to the IRES. The striped and filled boxes represent exons of the endogenous ChAT gene and SV40 polyadenylation signal, respectively. (B) Transgene copy number was examined by genomic Southern blotting using the probe indicated in (A) (24) and by Taqman quantitative DNA PCR. The arrowhead indicates the XhoI–SalI-digested 9.0 kb fragment of the transgene. (C) Transgene expression is predominant in cholinergic neurons. Either digoxigenin-labeled EGFP or ChAT riboprobe was used to hybridize to spinal cord sections. Both EGFP and ChAT were preferentially expressed in large cells (diameter ≥25 µm) located in the ventral horn, representing spinal motoneurons. (D) The GluR2 protein level is increased in the ventral half of the spinal cord in chat-GluR2 transgenic mice. The ventral half was carefully dissected from T10–L5 segments of spinal cords. Extracts of ventral spinal cords (40 µg, n=4) and cerebrum (10 µg, n=2) were immunoblotted. {dagger} P<0.05, {ddagger} P<0.01. (E) The majority of AMPA receptors in spinal motoneurons were Ca2+-impermeable in chat-GluR2 transgenic mice. The excitatory postsynaptic potential (EPSC) of AMPA components was measured from 23 motoneurons of chat-GluR2 transgenic mice (Tg7, n=11) and 22 motoneurons of non-transgenic C57BL/6J mice (wt, n=9), using the whole-cell patch-clamp method. Insets represent synaptic currents at holding potentials of –70 and +40 mV.

 

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  		Table 1. GluR2 mRNA level is to increased in motoneurons of chat-GluR2 transgenic mice
 
Crossbreeding hSOD1G93A transgenic mice with chat-GluR2 transgenic mice markedly delays disease onset and mortality
The chat-GluR2 transgenic mouse was mated with a hSOD1G93A transgenic ALS mouse to generate double transgenic (GS) mice. Most of the spinal motoneuronal AMPA receptors were actually Ca2+-impermeable in GS mice, but Ca2+-permeable in littermates carrying only the hSOD1G93A transgene (S mice, Fig. 2A). The G1L line of hSOD1G93A mice develops overt symptoms defined as disease onset at around 7 months, a classification based on a sudden decrease in motor performance in behavioral tests such as the rotarod test (25,26). Death occurs at around 8.5–9 months. To evaluate the effects of reduced Ca2+-permeability in AMPA receptors on the clinical course of ALS, motor performance was assessed by the rotarod test. Figure 2B depicts the rotarod score of each mouse measured every week, clearly showing that mice carrying the SOD1G93A gene are rapidly declining in performance score after a certain period. The day just before the decline in score was defined as the day of disease onset, and the mean time of disease onset was compared between S and GS littermates. Disease onset in GS mice was delayed, by 42.9 days (19.5%) in Tg7 and 18.7 days (8.5%) in Tg10, as compared with S mice. Lifespan was also prolonged in GS mice, by 37.5 days (14.3%) in Tg7 and 15.2 days (5.7%) in Tg10 (Fig. 2C, Table 2). No significant difference in rotarod score or lifespan was observed between the chat-GluR2 and wt mice (data not shown). Furthermore, no prolongation of lifespan was observed in the GS mice generated from Tg3 (data not shown), which animals displayed no additional GluR2 expression in spinal motoneurons (Table 1). The number of motoneurons in the spinal cord was counted, revealing that degeneration of motoneurons was also delayed in GS mice from the Tg7 line (Fig. 2D). All these results indicate that reducing Ca2+-permeability in AMPA receptors delays disease onset and motoneuron death caused by mutant SOD1, presumably in a dose-dependent manner.



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  		Figure 2. GluR2 overexpression markedly delays disease onset and prolongs survival in hSOD1G93A transgenic mice. (A) The majority of AMPA receptors in spinal motoneurons were Ca2+-impermeable in GS (chat-GluR2/+;hSOD1G93A/+) littermates, but Ca2+-permeable in S (hSOD1G93A/+) littermates. EPSCs of AMPA components were measured from 19 (S, n=5) and 21 (GS, n=4) spinal motoneurons in the same way as in Figure 1E. Rectification index was estimated as 0.321±0.036 for S and 0.535±0.018 for GS (mean±SEM, P<0.01). (B) GluR2 overexpression delayed the decline of motor performance assessed by the rotarod test. Each point represents the mean of four measurements per day every week on each mouse. Significance of difference in comparison of GS versus S littermates was analyzed by repeated measured ANOVA followed by Fisher's PLSD post hoc test (Tg7: P<0.0001 and Tg10: P=0.0005). In both lines, P<0.001 after 32 weeks of age. (C) Cumulative probability of disease onset and survival was compared between S and GS littermates. Data were analyzed by Kaplan-Meier life test and log-rank test, and the result is summarized in Table 2. (D) Degeneration of spinal motoneurons was significantly delayed by GluR2 overexpression. The 30 µm thick frozen sections were prepared from T10–L5 segments of spinal cords and stained with 0.01% toluidine blue. Large neurons with diameter >25 µm in the ventral horn, which are most severely depleted in hSOD1G93A mice, were counted serially in all sections. Data represent the means±SEM from 4–6 mice in each stage. {dagger} P<0.05, {ddagger} P<0.01 and # P<0.001.

 

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  		Table 2. Prolonging effects of GluR2 overexpression on the disease onset and survival
 
Cytochrome c-release from mitochondria, cox2-induction and gliosis are delayed by GluR2 overexpression
We next investigated whether pathological changes related to disease onset are verifiably affected by overexpression of GluR2. Of the numerous events accompanying disease onset in hSOD1G93A mice, we focused on cytochrome c-release from mitochondria and induction of cyclooxygenase-2 (cox-2), as a treatment of hSOD1G93A mice with agents inhibiting these events delays disease onset (26,27). Cytochrome c, which is normally localized to the intermembrane space of mitochondria, activates caspases and subsequent apoptosis after release into the cytosol (28). Cox-2 catalyzes the synthesis of prostaglandin E2, which stimulates glutamate release from astrocytes and plays a key role in the inflammatory process (29). Cytosolic extracts (26) and RNA were prepared from the spinal cord lumbar region, as this is the most severely affected region in ALS. Cytochrome c became clearly detectable in the cytosolic fraction around 7 months in S, but was only faintly detectable even at 8 months in GS littermates, indicating that the release of cytochrome c is considerably delayed in GS mice (Fig. 3A). Induction of cox-2 transcription was also significantly delayed in GS in comparison to S littermates (Fig. 3B). After disease onset, hSOD1G93A mice exhibit severe gliosis in the spinal cord owing to exacerbated inflammation (30). We also found that astrogliosis was remarkably delayed in GS mice (Fig. 3C). Reducing Ca2+-permeability of AMPA receptors is thus likely to affect the upstream events of cytochrome c-release and cox-2 induction among the processes triggered by mutant SOD1 proteins.



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  		Figure 3. GluR2 overexpression delays cytochrome c-release from mitochondria, cox-2 induction and subsequent astrogliosis. (A) Cytochrome c release from mitochondria into the cytosol was markedly delayed by GluR2 overexpression. Cytosolic (15 µg) and crude mitochondrial (2 µg) fractions from the lumbar spinal cords were immunoblotted. Hsp60 was used as a marker protein for mitochondria. The anti-SOD1 antibody used in this paper recognizes both human (upper band, 22 kDa) and mouse (lower band, 16 kDa) SOD1. {dagger} P<0.05, in comparison of S and GS littermates. (B) Induction of cox-2 transcription was significantly delayed by GluR2 overexpression. RT–PCR analysis was performed using total RNA extracted from lumbar spinal cords. To confirm exponential amplification in each PCR condition, results using two dilution series of template, which differed in concentration by an order of magnitude, are shown. Significance of difference was assessed from the results using larger amounts of templates. {dagger} P<0.05 and {ddagger} P<0.01 (S versus GS). (C) Astrogliosis was prominent in 8-month-old S, but was barely detected in GS littermates at the same age. The lumbar regions of mouse spinal cords were immunostained with anti-GFAP antibody.

 
Mutant SOD1 protein is converted into various unusual forms in different cellular compartments, but the conversion is markedly delayed by GluR2 overexpression
The misfolding and subsequent conformational changes in mutant SOD1 proteins are hypothesized to be responsible for the death of motoneurons in SOD1-related ALS (5,9,10). Most SOD1 proteins are located in the cytosol, but very small populations are found in organelles such as mitochondria (31) and nuclei (32). Therefore, we roughly divided a homogenate from the lumbar spinal cord into a crude mitochondrial fraction and a post-mitochondrial fraction by simple centrifugation (31), and analyzed in which fraction the misfolded and hence high-molecular-weight-shifted SOD1 proteins were detectable. Most organelles and cytoskeletons were found to be contained in the crude mitochondrial fraction, whereas cytosolic proteins were in the post-mitochondrial fraction (data not shown).

As the post-mitochondrial fraction contained an extremely large amount of SOD1 proteins, it was a formidable task to detect high-molecular-weight-shifted SOD1 species in this fraction by conventional western blotting (data not shown). To enhance the sensitivity of detection, the post-mitochondrial fraction was size-fractionated using size-exclusive chromatography with high-performance liquid chromatography (HPLC), and the HPLC fractions were immunoblotted. The results as shown in Figure 4A indicated that, in addition to the very large amount of SOD1 monomers, high-molecular-weight-shifted SOD1-immunopositive species corresponding to dimer (*2), trimer (*3) and tetramer (*4) sizes of mutant SOD1 were detectable in 2-, 6- and 8-month-old S mice, respectively (Fig. 4A). These oligomer-sized species were not observably detected in the lumbar spinal cord from wt or the cerebrum from S littermates even at 8 months, suggesting that the conversion of the SOD1 protein into oligomer-sized forms preferentially occurs in the spinal cord. The molecular shifts of those species were not due to ubiquitination, as they were not detected by anti-ubiquitin antibody (Supplementary Material, Fig. S1). Formation of oligomer-sized SOD1 aberrant forms was significantly delayed in GS compared with S littermates. On the other hand, western blotting of the crude mitochondrial fractions revealed that a significant population of SOD1 proteins in this fraction was converted into species distinct from those found in the post-mitochondrial fractions (Fig. 4B). In addition to dimer-sized SOD1, two major species between the monomer and dimer sizes (~25 and 35 kDa) were detected in symptomatic mice. Formation of all the unusual species in crude mitochondrial fractions was also delayed by more than 1 month in GS compared with S littermates.



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  		Figure 4. Distinctive patterns of unusual SOD1 species are found in the fractions derived from the cytosol and organelle/cytoskeleton, the formation of which is markedly delayed by GluR2 overexpression. (A) A very small population of SOD1 proteins is converted into oligomer-sized species in the cytosol long before disease onset, which was effectively delayed by GluR2 overexpression. HPLC fractions derived from ~100 µg of post-mitochondrial (cytosolic protein-enriched) fractions were immunoblotted. Formation of unusual SOD1 species corresponding to a dimer (*2), a trimer (*3) and a tetramer (*4) in size was significantly delayed in GS compared with S littermates. {dagger} P<0.05, {ddagger} P<0.01 and # P<0.001. Non-specific bands appear in the void fraction. (B) Unusual SOD1 species differing from those in post-mitochondrial fractions were detected in crude mitochondrial (organelle/cytoskeleton-enriched) fractions at disease onset, which was effectively delayed by GluR2 overexpression. The crude mitochondrial fractions (5 µg) were immunoblotted. Asterisks indicate usual SOD1 species corresponding to a dimer, ~25 and 35 kDa sizes. {dagger} P<0.05 (S versus GS littermates). (C) Filter-trapped SOD1, which might represent misfolded or aggregated forms, increased markedly before disease onset in S, but very slowly in GS littermates. Aliquots 12.5 µg of the post-mitochondrial fractions were solubilized with TBST (0.025% Tween-20) and filtered using cellulose acetate membrane (0.2 µm diameter) followed by immunostaining (10). {dagger} P<0.05 (S versus GS littermates).

 
Certain populations of SOD1 proteins, probably growing aggregates, can be efficiently trapped onto membranes composed of cellulose acetate, and these filter-trapped SOD1 species extensively increased in tandem with disease progression in mutant SOD1 transgenic mice (10). Figure 4C indicates that the filter-trapped SOD1 aggregates were considerably increased during disease onset in S mice, but increased more slowly in GS compared with S littermates.

Unusual SOD1 species first accumulate in the fractions dense with neurofilaments, GFAP, nuclei and mitochondria, which accumulation is markedly delayed by GluR2 overxpression
The finding of unusual SOD1 species depicted in Figure 4 led us to do further subcellular fractionation analysis in order to define in which cellular component such unusual species are localized. We thus divided the lumbar spinal cord into four different organelle-enriched fractions (P1–P4) and a supernatant fraction (S) consisting of cytosolic proteins (Fig. 5A). Immunoblots of these fractions revealed that unusual SOD1 species (*) first appeared in the P1 and P2 fractions long time before disease onset, and then, intensively accumulated into the P1 fraction by the disease onset (Fig. 5B). Nuclei and certain kinds of cytoskeletons such as neurofilaments and glial fibrillary acidic protein (GFAP) were effectively concentrated into the P1 fraction, whereas mitochondria are concentrated into the P2 fraction. In S mice, the dimer-sized species were first detected in the P1 and P2 fractions at 4 months. At 7 months, the stage of disease onset, the P1 fraction contained a considerable amount of unusual SOD1 species of approximately the size of a dimer, 25 and 35 kDa, which were very similar to those detected in the crude mitochondrial fractions as depicted in Figure 4B. These species were only weakly detected in P2, P3 and P4 fractions at the stage of disease onset, but then accumulated with disease progression. All these unusual SOD1 species were hardly detectable in the cerebrum, cerebellum, testis and muscle from S mice even at end stage (data not shown for cerebellum and muscle), and were hardly detectable in the spinal cord from 9-month-old wt littermates. In GS littermates, dimer-sized species were faintly detected in the P1 and P2 fractions at 6 months of age. Other species accumulated to an enormous extent in the P1 fraction at 8 months of age, the stage of disease onset in GS mice, indicating that the formation of these unusual SOD1 species was delayed concomitantly with the delay of disease onset in GS compared with S littermates. These observations strongly suggest that the misfolding and subsequent conformational changes of mutant SOD1 proteins are delayed when the Ca2+-permeability of AMPA receptors is significantly reduced.



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  		Figure 5. Unusual SOD1 species first appear in the fraction enriched in neurofilaments, GFAP, nuclei and mitochondria, which is markedly delayed by GluR2 overexpression. (A) The procedure for subcellular fractionation. The representative organelles and cytoskeletons enriched in each fraction are indicated. (B) Unusual SOD1 species (asterisks) were first detected in P1 and P2 before disease onset, and then accumulated in other organelle fractions with disease progression. Aliquots of 5 µg of each fraction was immunoblotted. The distribution of each organelle and cytoskeletons was evaluated using antibodies for each marker protein. Accumulation of unusual SOD1 species in spinal cords was significantly delayed in GS compared with S littermates. {dagger} P<0.05, {ddagger} P<0.01 and # P<0.001 (S versus GS littermates).

 
The increase of oxidatively modified proteins is attenuated by GluR2 overexpression
Although the mechanism underlying the marked effects of reduced AMPA receptor Ca2+-permeability on the conformational changes of mutant SOD1 is currently unclear, the attenuation of cellular oxidative stress may be involved. Oxidation of human SOD1 proteins in vitro causes cleavage and/or conjugation (33), resulting in the formation of various types of unusual SOD1 species (34,35). Moreover, elevated cellular oxidative stress and resulting oxidative modification of proteins and lipids such as carbonylation are reported in spinal cords from hSOD1G93A mice (36–38). Thus, we compared the level of carbonylated proteins in spinal cord extracts between S and GS littermates, taking it as a marker of cellular oxidative stress. The results in Figure 6 reveal that carbonylated proteins in spinal cords increased only gradually before disease onset, then, increased substantially at disease onset in both S and GS mice. Such drastic increase was not observed in the extracts from cerebrum even in 9-month-old S mice. Statistical analysis revealed that the increase of carbonylation was significantly delayed, by at least 2 months, in GS compared with S littermates. This delay of protein oxidative modification might help explain why conformational changes of SOD1 proteins are delayed in GS mice.



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  		Figure 6. Oxidative modification of proteins is slowly increased by GluR2 overexpression. Tissue extracts were treated with DNPH, which specifically reacted with carbonyl groups in the protein side chains and converted into DNP–hydrazone. The levels of carbonylated proteins in spinal cords, which were evaluated from the absorption of DNP–hydrazone (375 nm), were significantly higher in the 6- and 7-month-old S littermates and 8-month-old GS littermates compared with G and wt littermates (P<0.05). In comparison between the S and GS littermates, the carbonylated protein levels were significantly higher in S than GS littermates at the age of 6 and 7 months ({dagger} P<0.05). n=3–4 mice per each group.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
MATERIALS AND METHODS
 SUPPLEMENTARY MATERIAL
 REFERENCES
 
Motoneuronal Ca2+-permeable AMPA receptors contribute to selective cell death in SOD1-related ALS
The present study demonstrates that motoneuronal Ca2+-permeable AMPA receptors contribute to the development of SOD1-related ALS. Reducing permeability by motoneuron-preferential GluR2 overexpression significantly prolongs the lifespan of ALS mice by delaying disease onset (Fig. 2B and C). The mutant SOD1 protein level in the ventral spinal cord was not significantly different between S and GS littermates (Supplementary Material, Fig. S2), and GluR2 mRNA level in spinal motoneurons did not significantly change during the course of disease in hSOD1G93A mice (data not shown). Thus, the beneficial effects of GluR2 overexpression do not result from either a reduction of mutant SOD1 expression or a simple compensation of decreased GluR2 expression, but from reduced Ca2+-influx through motoneuronal AMPA receptors.

A recent study on chimeric mice between wild-type and mutant SOD1 transgenic mice revealed that the death of motoneurons expressing mutant SOD1 can be delayed when the surrounding non-neuronal cells do not express mutant SOD1 (39). This finding indicates that motoneuronal death triggered by mutant SOD1 is not cell-autonomous, but also depends on the interactions with surrounding glial cells expressing mutant SOD1. However, there must be a reason why only motoneurons die among the neurons surrounded by non-neuronal cells expressing mutant SOD1. The present study provides evidence that the expression of Ca2+-permeable AMPA receptors confers a critical feature on motoneurons such that they undergo death in response to mutant SOD1 effects within themselves and surrounding cells.

Ca2+-influx though motoneuronal AMPA receptors promotes conversion of SOD1 protein into aberrant forms
In addition to delaying the disease onset, GluR2 overexpression succeeded in delaying the conversion of SOD1 protein into unusual forms (Figs 4 and 5B). To date, more than 20 papers have reported the delay of disease onset in mutant SOD1 transgenic mice by various pharmacological or transgenic techniques (1). However, the effects of such trials on SOD1 conformational changes have yet to be shown. Here, we provide the first evidence that disease onset is delayed when conversion of SOD1 into unusual forms is delayed, consistent with the hypothesis that SOD1 aggregation participates in ALS pathogenesis.

This study also provides an outline of the temporal profile for the formation of unusual SOD1 species during the clinical course of mouse ALS. Previous studies with western blots detected two distinct patterns of unusual SOD1 species in spinal cord extracts from mutant SOD1 transgenic mice. One is a set of oligomer-sized species (9), and another contains species between a monomer and dimer in size (10–12). Here, we show that trimer- and tetramer-sized species are detectable in the post-mitochondrial fractions containing cytosolic proteins (Fig. 4A), whereas species between a monomer and dimer in size are detected in the crude mitochondrial fractions containing major cellular components such as organelles and cytoskeleton. On the other hand, dimer-sized species were detectable in both fractions (Figs 4B and 5B). The amount of trimer and tetramer-sized species in the cytosol seemed comparatively very small to the unusual species in the organelle/cytoskeleton fractions, as these species are no longer detectable in Figure 5B, an experiment in which the same subcellular fraction amounts were loaded to the respective lanes. The first detectable unusual SOD1 species are dimer-sized species that appear several months before disease onset. Subsequently, a very small population of SOD1 proteins grows into trimer- and tetramer-sized species in the cytosol. On the other hand, a large amount of SOD1 protein is converted into ~25 and 35 kDa-sized species. These unusual species first appear in the P1 and P2 fractions and then extensively accumulate into the P1 fraction by disease onset. The presence in the P2 mitochondrial fraction might be related to the dysfunction of mitochondria that has been reported in hSOD1G93A mice around disease onset (1,31). In the P1 fraction, nuclei and certain kind of filamentous cytoskeletons such as neurofilaments and GFAP, but not actin filaments or microtubiles, are effectively concentrated. The extensive accumulation of unusual SOD1 species in the P1 fraction implies that these species are fundamentally associated with neurofilaments or GFAP, because a further fractionation study revealed that these species do not independently accumulate in the nuclei (data not shown). Abnormalities in the neurofilaments observed in ALS patients and mutant SOD1 mice, such as an accumulation of neurofilament inclusions (1) and a defect in axonal transport (4), might be closely related to this phenomenon. Alternatively, those SOD1 species might be involved in aggresome-like structures, as observed in HEK293 cells transfected with G85R and G93A SOD1 mutants (9). After disease onset, unusual species spread to other organelle fractions enriched in lysosomes, peroxysomes and microsomes derived from the endoplasmic reticulum (ER), Golgi apparatus and plasma membrane. These alterations in SOD1 proteins are thought to predominantly occur in motoneurons and surrounding astrocytes, as SOD1-containing proteinacious inclusions, which are likely to have developed from high-molecular-weight-shifted SOD1 species, are specifically detected in motoneurons and neighboring astrocytes in end-stage hSOD1G93A mice (6–8). The reduction of Ca2+-influx through motoneuronal AMPA receptors successfully delayed the formation of the entire range of unusual SOD1 species in the spinal cord extracts (Figs 4 and 5A) as well as the development of astrogliosis (Fig. 3C). These results suggest that Ca2+-influx through motoneuronal AMPA receptors can affect the physiology of neighboring astrocytes as well as their own, and contributes to the misfolding and subsequent conversion of SOD1 protein.

Ca2+-influx through AMPA receptors enhances ROS production, which may induce the misfolding of SOD1 proteins
Ca2+-influx through motoneuronal AMPA receptors seems to enhance oxidative stress primarily in motoneurons and secondarily in neighboring astrocytes. Activation of Ca2+-permeable AMPA receptors causes rapid increases in the level of cytosolic calcium, which are rapidly attenuated by trapping with Ca2+-chelating proteins and by incorporation of calcium into mitochondria and ER (40). As spinal motoneurons are less capable of buffering increased calcium levels, probably due to a lack of major Ca2+-chelating proteins such as parvalbumin and calbindin D28K (41), a large proportion of free calcium is reported to be incorporated into mitochondria, resulting in enhanced ROS production (42). Evidence suggests that ROS generated in motoneurons can exit from the motoneurons and cause oxidative disruption of glutamate transporters and increased ROS level in neighboring astrocytes (43). The loss of astrocytic glutamate transporters, which has been preferentially observed in the affected area in ALS patients and mouse models (15,43,44), accelerates AMPA receptor-mediated Ca2+-influx and ROS generation within motoneurons, resulting in a vicious cycle to enhance the oxidative stress in motoneurons and neighboring astrocytes (45).

Enhanced ROS levels might, in part, account for the formation of unusual SOD1 species, as oxidative modification by ROS has been shown to convert SOD1 proteins into a variety of unusual forms, at least in vitro (34,35). We found a good correlation between the levels of carbonylated proteins and unusual SOD1 species in the spinal cord extracts from S and GS mice. Both levels were rapidly increased at disease onset, but these increases were similarly attenuated when the Ca2+-permeability of AMPA receptors was reduced (Figs 4, 5B and 6). This correlation may imply that the increased cellular oxidative stress resulting from activation of motoneuronal Ca2+-permeable AMPA receptors induces the misfolding and subsequent conversion of SOD1 protein within motoneurons and adjacent glial cells.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
SUPPLEMENTARY MATERIAL
 REFERENCES
 
The sequence information for the primers and probes is described in Supplementary Material, Table S1. All the data shown are representative of three mice per group when the number of mice (n) used for experiments is not mentioned. Signals in immunoblots and RT–PCR were quantified with NIH image software (1.61J). Statistical significance was assessed by the two-tailed Student's t-test when the statistical method is not mentioned. All protein electrophoreses in this paper utilized SDS–PAGE under reducing conditions. Antibodies were used for immunoblots and immunohistochemistry, which comprised of: anti-SOD1 (Stressgen, SOD-100), anti-GluR2 (BD PharMingen, 556341), anti-ChAT (Chemicon, AB144P), anti-cytochrome c (BD PharMingen, 556433), anti-Hsp60 (Sigma, H4149), anti-actin (Chemicon, MAB1501), anti-GFAP (Chemicon, MsX GFAP), anti-prohibitin (NeoMarkers, MS-261-P0), anti-CoxIV (Molecular Probes, A-6431), anti-nucleoporin p62 (BD Transduction Laboratories, N43620), anti-ribophorin I (Santa Cruz, sc-12164), anti-Lamp1 (BD PharMingen, 553792), anti-adaptin gamma (BD Transduction Laboratories, A36120), anti-neurofilament-M (Chemicon, AB1987), and anti-beta-tubulin (Sigma, T 4026) antibodies.

Generation of GluR2 transgenic mice
The transgene construct contained the 6.4 kb promoter region of the mouse ChAT gene (AF019045), the 2.6 kb rat GluR2 coding sequence (CDS, M85035), internal ribosome entry sequence (IRES), EGFP CDS and the SV40 polyadenylation signal, in that order. As the ChAT promoter region contains an open reading frame of a vesicular acetylcholine transporter (VAChT) intronless gene (23), we introduced a stop codon at the 55th amino acid position to avoid producing a functional VAChT protein from this construct. The region from IRES to the polyadenylation signal was derived from the pIRES2–EGFP vector (Clonetech), with a base substitution so as to delete a NotI site in the EGFP 3'-untranslated region. The NotI-digested 10.9 kb DNA fragment was injected into C57BL/6J mouse eggs, and three transgenic lines, Tg3, Tg7 and Tg10, were established.

Taqman quantitative DNA PCR was performed using probe–primer sets specific to EGFP and two internal control genes, SOD1 and CPTI, on an ABI7700 thermal cycler (PE Biosystems) under the conditions recommended by the manufacturer. Data were normalized using results from the EGFP knock-in mouse in the Cx43 gene (M. Tanaka and S. Itohara, unpublished data), which displays a single copy of EGFP in the genome. Then absolute EGFP copy numbers in Tg3 (n=3) Tg7 (n=3) and Tg10 (n=5) of chat-GluR2 mice were calculated as 2.10±0.04, 9.31±0.14 and 16.44±0.67 copies for the SOD1 control, and 2.13±0.07, 10.20±0.09 and 16.43±1.28 copies for the CPTI control, respectively (mean±SD). The transgene copy number was thus represented as the mean of these two values.

Electrophysiological recordings
Electrophysiological experiments were performed as previously described (46) using 200–250 µm slices of spinal cord lumbar regions from mice at the postnatal ages of 4–7 days. Whole-cell patch-clamp recordings were performed with motoneurons identified using biocytin-containing Cs-based intracellular solution. To isolate the AMPA current, the extracellular solution contained 20 µM bicuculline, 25 µM D-2-amino-phosphonovaleric acid (D-APV) and 10 µM strychnine, which blocks the GABA, NMDA and glycine receptors, respectively. A glass electrode containing artificial cerebrospinal fluid positioned in the spinal cord was used for synaptic stimulation.

Quantitative analysis of gene expression levels in motoneurons
Spinal cords from 6–8-month-old mice were dissected without fixation, immediately embedded in OCT compound (Tissue-Tek), and frozen in liquid nitrogen. Frozen sections 30 µm thick were processed and stained using 0.01% toluidine blue, and motoneurons in spinal cords were clipped out of sections using laser microdissection (AS LMD, Leica) according to the manufacturer's protocol. About 1000 clipped slices of motoneurons were collected per mouse and subjected to RNA purification using an RNAeasy kit (Qiagen), followed by cDNA synthesis primed with oligo-dT using Superscript II (Gibco-Brl). The gene expression level was examined by Taqman real-time quantitative PCR using probe–primer sets specific to target genes. PCR was performed on an ABI7700 thermalcycler (PE Biosystems) under the manufacturer's recommended conditions, using cDNA derived from 60 (for GluR2-4 and ChAT) or 10 (for SOD1 and GAPDH) clipped slices of motoneurons as templates. Data were normalized with the expression level of GAPDH, and presented as a relative expression level compared with the level in the C57BL/6J non-transgenic control mouse.

Animals
Non-transgenic littermates without any transgene are indicated as wt to distinguish them from non-transgenic control mice, C57BL/6J mice. All data except for Table 1 and Figure 1E are comparisons among littermates. The G1L line of transgenic mice harboring the G93A-mutated human SOD1 gene (B6SJL-TgN(SOD1-G93A)1Gurdl) was purchased from Jackson Laboratories and backcrossed with C57BL/6J mice. We used the littermates generated by crossing male hSOD1G93A mice (fourth backcross-generation) with female chat-GluR2 transgenic mice for all analyses except for a study of disease onset and mortality in Tg10 line (Table 2 and Fig. 2B and C), that involved three S and three GS littermates generated from the third backcrossed hSOD1G93A mouse. To determine the day of disease onset, mice were subjected to the rotarod test (47). The retention time on a rotating wheel, the rotarod score, was measured four times per day with a 1-week interval in a blind fashion. Each trial lasted for a maximum of 5 min, during which time the wheel rotates with a linear acceleration from 4 to 40 rpm. The day of disease onset was defined as the day just before the mean retention time of four trials was sequentially shortened to <80% of the previous time. The end time was defined as the day of death or the day when the mouse was unable to right itself within 30 s (25).

Preparation of crude mitochondrial and post-mitochondrial fractions
Crude mitochondrial and post-mitochondrial fractions were prepared according to an established method (31) with certain modifications. The bovine strum albumin concentration in buffer H was reduced to 0.1%. The spinal cord L1–L5 segments were homogenized in buffer H (1 mg tissue/10 µl buffer H) on ice and centrifuged at 600g for 5 min at 4°C to remove unbroken cells. The supernatant was centrifuged at 13 500g for 10 min at 4°C, dividing into pellet (crude mitochondrial fraction) and supernatant (post-mitochondrial fraction).

Size-exclusive chromatography
Approximately 200 µg of cytosolic extracts were filtered through Millex-HV PVDF filters (Millipore, 0.45 µm diameter), concentrated with a Vivaspin column (Vivascience, cut-off size, MW 10 000), then resolved on a Superdex200 PC3.2/30 column (linear fractionation range, MW 10 000–600 000; bed volume, 2.4 ml; Pharmacia Biotech) at a flow rate of 40 µl/min in 50 mM sodium phosphate with 150 mM NaCl, pH 7.0. Fractionation started when 800 µl was eluted, and a 30 µl/tube of elutant was collected for a total of 48 tubes. Void volume was determined by the elution profile of dextran blue (2000 kDa). The column was calibrated using gel filtration calibration kits for high- and low-molecular weights (Amersham Bioscience).

Subcellular fractionation
Figure 5A represents a schematic of this procedure. Whole spinal cords were gently homogenized in modified buffer H [0.22 M D-mannitol, 0.07 M sucrose, 20 mM HEPES, pH 7.4, 1 mM EGTA and complete protease inhibitor cocktail (Roche), at 1 mg tissue/10 µl buffer] with a glass–Teflon homogenizer (10 up-and-down strokes) on ice, and centrifuged at 600g for 10 min. The supernatants were sequentially centrifuged at 5000g for 15 min, 8000g for 15 min and 100 000g for 1 h, to obtain three pellets (P2, P3 and P4) and the resulting supernatant (S). The pellets generated by the first brief centrifugation were very gently suspended with 2.2 M sucrose containing complete protease inhibitor cocktail (1 mg starting tissue/15 µl), and centrifuged at 40 000g for 1 h. The resulting pellets (P1) were rinsed with modified buffer H followed by centrifugation at 12 000g for 10 min. The pellets of P1, P2, P3 and P4 were finally resuspended with 1/4, 2, 1/2 and 1/2 volume (µl) of modified buffer H per starting tissue weight (mg), respectively. All centrifugations were performed at 4°C.

Measurement of protein carbonylation
Freshly dissected tissues were sonicated in buffer [50 mM Tris–HCl, pH 7.6, 20 mM Na4P2O7, 20 mM sodium fluoride, 1 mM EGTA, 5 mM EDTA, 5 mM DTT and complete protease inhibitor cocktail (Roche), at 1 mg tissue/20 µl buffer] and centrifuged at 500g for 5 min. The supernatants were further centrifuged at 100 000g for 1 h, and the resulting supernatants were used. The levels of carbonylated proteins in these supernatants were evaluated by measuring absorbance derived from dinitrophenyl (DNP)–hydrazone as previously described (36) with slight modifications. We started with 40 µg of protein and used 6% SOD for a solubilization of the trichloroacetic acid precipitates.


   SUPPLEMENTARY MATERIAL
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 SUPPLEMENTARY MATERIAL
REFERENCES
 
Supplementary Material is available at HMG Online.


   ACKNOWLEDGEMENTS
 
We wish to thank Drs Shin Kwak and Hiroshi Funakoshi for critical reading of the manuscript, Pacific Edit and Ms Bonnie Lee La Madeleine for review prior to submission for publication, Drs Takashi Sakurai, Haruhisa Inoue, Yasuyuki Suzuki and Runa Araya for helpful suggestions, and Ms Sachiko Iita, Yoshie Yoshida (LMD sampling) and Tomoko Yoda (rotarod test) for technical supports. We would also like to thank Drs Stephen F. Heinemann and Yuzuru Imai for kindly donating rat GluR2-4 cDNAs and anti-ubiquitin antibody, respectively. This work was funded by research grants from RIKEN BSI, a Grant-in-Aid for Scientific Research on Priority Area (Advanced Brain Science Project) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and grants from the Ministry of Health, Labor and Welfare, Japan.


   FOOTNOTES
 
* To whom correspondence should be addressed at: Laboratory for Motor System Neurodegeneration, Brain Science Institute, Riken, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. Tel: +81 484676072; Fax: +81 484624796; Email: ryosuke{at}brain.riken.jp


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 INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 SUPPLEMENTARY MATERIAL
 REFERENCES
 

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