Superoxide dismutase is an abundant component in cell bodies, dendrites, and axons of motor neurons and in a subset of other neurons

Mechanism of selective motor neuronal death after exposure of spinal cord to glutamate: involvement of glutamate-induced nitric oxide in motor neuron toxicity and nonmotor neuron protection

In this study, we analyzed the mechanism of selective motor neuronal death, a characteristic of amyotrophic lateral sclerosis, using embryonic rat spinal cord culture. When dissociated cultures were exposed to low-level glutamate (Glu) coadministered with the Glu transporter inhibitor L-trans-pyrrolidine-2,4-decarboxylate (PDC) for 24 hours, motor neurons were selectively injured through N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptors. Nitric oxide synthase (NOS) inhibitors attenuated this toxicity, and long-acting nitric oxide (NO) donors damaged motor neurons selectively. Nonmotor neurons survived after exposure to low-dose Glu/PDC, but Glu-induced toxicity was potentiated by coadministration of an NO-dependent guanylyl cyclase inhibitor. In addition, 8-bromo-cyclic GMP, a soluble cyclic GMP analogue, rescued nonmotor neurons, but not motor neurons, exposed to high-dose Glu/PDC. Twenty-four hours' incubation with PDC elevated the number of neuronal NOS-immunoreactive neurons by about twofold compared with controls, and a double-staining study, using the motor neuron marker SMI32, revealed that most of them were nonmotor neurons. These findings suggest that selective motor neuronal death caused by chronic low-level exposure to Glu is mediated by the formation of NO in nonmotor neurons, which inversely protects nonmotor neurons through the guanylyl cyclase-cyclic GMP cascade. Induction of neuronal NOS in nonmotor neurons might enhance both the toxicity of motor neurons and the protection of nonmotor neurons, which could explain the pathology of amyotrophic lateral sclerosis.

Aggregation and motor neuron toxicity of an ALS-linked SOD1 mutant independent from wild-type SOD1

Analysis of transgenic mice expressing familial amyotrophic lateral sclerosis (ALS)-linked mutations in the enzyme superoxide dismutase (SOD1) have shown that motor neuron death arises from a mutant-mediated toxic property or properties. In testing the disease mechanism, both elimination and elevation of wild-type SOD1 were found to have no effect on mutant-mediated disease, which demonstrates that the use of SOD mimetics is unlikely to be an effective therapy and raises the question of whether toxicity arises from superoxide-mediated oxidative stress. Aggregates containing SOD1 were common to disease caused by different mutants, implying that coaggregation of an unidentified essential component or components or aberrant catalysis by misfolded mutants underlies a portion of mutant-mediated toxicity.

Absence of neurofilaments reduces the selective vulnerability of motor neurons and slows disease caused by a familial amyotrophic lateral sclerosis-linked superoxide dismutase 1 mutant

Mutations in superoxide dismutase 1 (SOD1), the only proven cause of amyotrophic lateral sclerosis (ALS), provoke disease through an unidentified toxic property. Neurofilament aggregates are pathologic hallmarks of both sporadic and SOD1-mediated familial ALS. By deleting NF-L, the major neurofilament subunit required for filament assembly, onset and progression of disease caused by familial ALS-linked SOD1 mutant G85R are significantly slowed, while selectivity of mutant-mediated toxicity for motor neurons is reduced. In NF-L-deleted animals, levels of the two remaining neurofilament subunits, NF-M and NF-H, are markedly reduced in axons but are elevated in motor neuron cell bodies. Thus, while neither perikaryal nor axonal neurofilaments are essential for SOD1-mediated disease, the absence of assembled neurofilaments both diminishes selective vulnerability and slows SOD1(G85R) mutant-mediated toxicity to motor neurons.

Differential expression of superoxide dismutase isoforms in neuronal and glial compartments in the course of excitotoxically mediated neurodegeneration: relation to oxidative and nitrergic stress

To examine the cellular distribution of radical scavenging enzymes in glia, in comparison to that in neurons and their behaviour during excitotoxically induced neurodegenerative processes, protein levels and the cellular localization of cytosolic and mitochondrial superoxide dismutase (Cu/Zn- and Mn-SOD) were investigated in the rat brain undergoing quinolinic acid (Quin)-induced neurodegeneration. Evidence for the specificity of the applied antibodies to detect immunocytochemically these SOD isoforms was obtained from electron microscopy and Western blotting. In control striatum Mn-SOD was clearly confined to neurons, whereas Cu/Zn-SOD was found, rather delicately, only in astrocytes. Microglia failed to stain with antibodies to both SOD isoforms. Quin application resulted in an initial formation of oxygen and nitrogen radicals as determined by the decline in the ratio of ascorbic to dehydroascorbic acid and by increased levels of nitrated proteins, an indicator for elevated peroxynitrite formation. Morphologically, massive neuronal damage was seen in parallel. Astroglia remained intact but showed initially decreased glutamine synthetase activities. The levels of Mn-SOD protein increased 2-fold 24 h after Quin injection (Western blotting) and declined only slowly over the time period considered (10 days). Cu/Zn-SOD levels increased only 1.3-fold. Immunocytochemical studies revealed that the increase in Mn-SOD is confined to neurons, whereas that of Cu/Zn-SOD was observed only in astroglial cells. Quiescent microglial cells were, as a rule, free of immunocytochemically detectable SOD, whereas in activated microglia a few Mn-SOD immunolabeled mitochondria occurred. Our results suggest a differential protective response in the Quin lesioned striatum in that Mn-SOD is upregulated in neurons and Cu/Zn-SOD in astroglia. Both SOD-isoforms are assumed to be induced to prevent oxidative and nitric oxide/peroxynitrite-mediated damage. In the border zone of the lesion core this strategy may contribute to resist the noxious stimulus.

Axonal transport of mutant superoxide dismutase 1 and focal axonal abnormalities in the proximal axons of transgenic mice

Superoxide dismutase 1 (SOD1), a ubiquitously expressed enzyme, detoxifies superoxide radicals and participates in copper homeostasis. Mutations in this enzyme have been linked to a subset of autosomal dominant cases of familial amyotrophic lateral sclerosis (FALS), a disorder characterized by selective degeneration of motor neurons. Transgenic mice expressing FALS mutant human (Hu) SOD1 at high levels develop a motor neuron disease, indicating that mutant Hu SOD1 gains properties that are particularly toxic to motor neurons. In this report, we demonstrate that transgenic mice expressing Hu SOD1 with the G37R FALS mutation, but not mice expressing wild-type enzyme, develop focal increases in immunoreactivity in the proximal axons of spinal motor neurons. This SOD1 immunoreactivity and immunoreactivity to hypophosphorylated neurofilament H epitopes are found adjacent to small vacuoles in axons. Using metabolic radiolabeling methods, we show that mutant G37R HuSOD1 as well as endogenous mouse SOD1 are transported anterograde in slow component b in motor and sensory axons of the sciatic nerve. Together, these findings suggest that anterogradely transported mutant SOD1 may act locally to damage motor axons.

Advanced glycation endproducts in neurofilament conglomeration of motoneurons in familial and sporadic amyotrophic lateral sclerosis

BACKGROUND

Massive neurofilament conglomeration in motor neurons has been described to occur in the early stages of both familial and sporadic amyotrophic lateral sclerosis (ALS). Previously, neurofilament conglomerates were immunolabeled for both superoxide dismutase (SOD1) and nitrotyrosine, suggesting the potential for oxidative nitration damage to neurofilament protein by peroxynitrite. Long-lived neurofilaments may also undergo modification by advanced glycation endproducts (AGEs) with concomitant generation of free radicals, including superoxide. This radical species may then react with nitric oxide to form the potent oxidant, peroxynitrite, which in turn can nitrate neurofilament protein. Such a glycated and nitrated neurofilament protein may become resistant to proteolytic systems, forming high-molecular-weight protein complexes and cytotoxic, neuronal inclusions.

MATERIALS AND METHODS

Paraffin sections containing both neurofilament conglomerates and neuronal inclusions were obtained from patients with sporadic (n = 5) and familial (n = 2) ALS and were probed with specific antibodies directed against the AGEs cypentodine/piperidine-enolone, arginine-lysine imidazole, pentosidine, and pyrraline.

RESULTS

Neurofilament conglomerates, but not neuronal inclusions, were intensely immunolabeled with each of the anti-AGE antibodies tested. The immunoreactivity was selective for neurofilament conglomerates and suggested that AGEs may form inter- or intramolecular cross-links in neurofilament proteins.

CONCLUSIONS

These data support the hypothesis that AGE formation affects neurofilament proteins in vivo and is associated with the concomitant induction of SODI and protein nitration in neurofilament conglomerates. AGE formation in neurofilament protein may not only cause covalent cross-linking but also generate superoxide and block nitric oxide-mediated responses, thereby perpetuating neuronal toxicity in patients with ALS.

Overexpression of human copper, zinc-superoxide dismutase (SOD1) prevents postischemic injury

Superoxide and superoxide-derived oxidants have been hypothesized to be important mediators of postischemic injury. Whereas copper, zinc-superoxide dismutase, SOD1, efficiently dismutates superoxide, there has been controversy regarding whether increasing intracellular SOD1 expression would protect against or potentiate cellular injury. To determine whether increased SOD1 protects the heart from ischemia and reperfusion, studies were performed in a newly developed transgenic mouse model in which direct measurement of superoxide, contractile function, bioenergetics, and cell death could be performed. Transgenic mice with overexpression of human SOD1 were studied along with matched nontransgenic controls. Immunoblotting and immunohistology demonstrated that total SOD1 expression was increased 10-fold in hearts from transgenic mice compared with nontransgenic controls, with increased expression in both myocytes and endothelial cells. In nontransgenic hearts following 30 min of global ischemia a reperfusion-associated burst of superoxide generation was demonstrated by electron paramagnetic resonance spin trapping. However, in the transgenic hearts with overexpression of SOD1 the burst of superoxide generation was almost totally quenched, and this was accompanied by a 2-fold increase in the recovery of contractile function, a 2.2-fold decrease in infarct size, and a greatly improved recovery of high energy phosphates compared with that in nontransgenic controls. These results demonstrate that superoxide is an important mediator of postischemic injury and that increasing intracellular SOD1 dramatically protects the heart from this injury. Thus, increasing intracellular SOD1 expression may be a highly effective approach to decrease the cellular injury that occurs following reperfusion of ischemic tissues.

Heme oxygenase in the experimental ALS mouse

Heme oxygenase-1 (HO-1) is a stress protein inducible in some cells by oxidative stress. The status of heme oxygenase was investigated in a transgenic mouse model of amyotrophic lateral sclerosis (ALS) since oxidative mechanisms are postulated in neuronal injury. Three ALS mice [(SOD1-G93A)1Gur] and three controls [(SOD-1)2Gur] were obtained from The Jackson Laboratory. Behavioral differences suggestive of neurodegeneration in ALS mice developed at 4-5 months of age. All mice were killed at 7-8 months of age. Tissue vacuolation, cell loss, and the presence of GFAP+ cells were noted in the spinal cords of ALS mice. Spinal cord motor neurons in both control and ALS mice stained positive for heme oxygenase-2 (HO-2). While not precluding the presence of low levels of HO-1 neither immunohistochemical staining nor Western blot analysis provided evidence for significant HO-1 induction in degenerating spinal cord.

In cortical cultures of trisomy 16 mouse brain the upregulated metallothionein-I/II fails to respond to H2O2 exposure or glutamate receptor stimulation

To assess whether a defective oxidative defense may contribute to Down's syndrome, we studied the regulation of the metallothionein(MT)-I/II isoforms in primary cultures of cerebral cortex from fetal trisomy 16 mice and their euploid littermates. Western blot analysis showed that MT-I/II was upregulated and the protein carbonyl content was higher in trisomy 16 compared with euploid cultures. Addition of N-acetyl-l-cysteine to the culture medium reduced the increment of MT-I/II in trisomy 16 cortical cells. In euploid, but not trisomic cortical cultures, kainic acid, trans-(+/-)-ACPD, or H2O2 exposure elicited a dose-dependent increase of the MT-I/II immunoblots. In trisomic cells, the MT-I/II immunoblot densities were not increased beyond their elevated basal levels. In contrast, 25 microM Pb induced MT-I/II, to a similar extent, in cortical cultures from euploid and trisomy 16 mice. This suggests that the antioxidant-but not the metal-response element of the MT-I/II promoter was altered by increased oxidative stress. Our data suggest that, in the trisomy 16 mouse, the effects of increased production of reactive oxygen species, due to the increased SOD-1, GluR5, or amyloid precursor protein gene dosage, is exacerbated by an insufficient or missing antioxidant response.

Subunit asymmetry in the three-dimensional structure of a human CuZnSOD mutant found in familial amyotrophic lateral sclerosis

The X-ray crystal structure of a human copper/zinc superoxide dismutase mutant (G37R CuZnSOD) found in some patients with the inherited form of Lou Gehrig's disease (FALS) has been determined to 1.9 angstroms resolution. The two SOD subunits have distinct environments in the crystal and are different in structure at their copper binding sites. One subunit (subunit[intact]) shows a four-coordinate ligand geometry of the copper ion, whereas the other subunit (subunit[broken]) shows a three-coordinate geometry of the copper ion. Also, subunit(intact) displays higher atomic displacement parameters for backbone atoms ((B) = 30 +/- 10 angstroms2) than subunit(broken) ((B) = 24 +/- 11 angstroms2). This structure is the first CuZnSOD to show large differences between the two subunits. Factors that may contribute to these differences are discussed and a possible link of a looser structure to FALS is suggested.

A role for superoxide in protein kinase C activation and induction of long-term potentiation

The induction of several forms of long-term potentiation (LTP) of synaptic transmission in the CA1 region of the mammalian hippocampus is dependent on N-methyl-D-aspartate receptor activation and the subsequent activation of protein kinase C (PKC), but the mechanisms that underlie the regulation of PKC in this context are largely unknown. It is known that reactive oxygen species, including superoxide, are produced by N-methyl-D-aspartate receptor activation in neurons, and recent studies have suggested that some reactive oxygen species can modulate PKC in vitro. Thus, we have investigated the role of superoxide in both the induction of LTP and the activation of PKC during LTP. We found that incubation of hippocampal slices with superoxide scavengers inhibited the induction of LTP. The effects of superoxide on LTP induction may involve PKC, as we observed that superoxide was required for appropriate modulation of PKC activation during the induction of LTP. In this respect, superoxide appears to work in conjunction with nitric oxide, which was required for a portion of the LTP-associated changes in PKC activity as well. Our observations indicate that superoxide and nitric oxide together regulate PKC in a physiologic context and that this type of regulation occurs during the induction of LTP in the hippocampus.

Regional and ultrastructural immunolocalization of copper-zinc superoxide dismutase in rat central nervous system

We examined the distribution of copper-zinc superoxide dismutase (CuZnSOD) in adult rat central nervous system by light and electron microscopic immunocytochemistry, using an affinity-purified polyclonal antibody. The enzyme appeared to be exclusively localized in neurons. No immunoreactivity was seen in non-neuronal cells. The staining intensity was variable, depending on the brain region and, within the same region, on the neuron type. Highly immunoreactive elements included cortical neurons evenly distributed in the different layers, hippocampal interneurons, neurons of the reticular thalamic nucleus, and Golgi, stellate, and basket cells of the cerebellar cortex. Other neurons, i.e., pyramidal cells of the neocortex and hippocampus, Purkinje and granule cells of the cerebellar cortex, and the majority of thalamic neurons, showed much weaker staining. In the spinal cord, intense CuZnSOD immunoreactivity was present in many neurons, including motor neurons. Pre-embedding immunoelectron microscopy of the neocortex, hippocampus, reticular thalamic nucleus, and cerebellar cortex showed cytosolic and nucleoplasmic labeling. Moreover, single membrane-limited immunoreactive organelles identified as peroxisomes were often found, even in neurons that appeared weakly stained at the light microscopic level. In double immunogold labeling experiments, particulate CuZnSOD immunoreactivity co-localized with catalase, a marker enzyme for peroxisomes, thus demonstrating that in neural tissue CuZnSOD is also present in peroxisomes.

Ethanol metabolism in the brain

Acetaldehyde is suspected of being involved in the central mechanism of central nervous system depression and addiction to ethanol, but in contrast to ethanol, it can not penetrate easily from blood into the brain because of metabolic barriers. Therefore, the possibility of ethanol metabolism and acetaldehyde formation inside the brain has been one of the crucial questions in biomedical research of alcoholism. This article reviews the recent progress in this area and summarizes the evidence on the first stage of ethanol oxidation in the brain and the specific enzyme systems involved. The brain alcohol dehydrogenase and microsomal ethanol oxidizing systems, including cytochrome P450 II E1 and catalase are considered. Their physicochemical properties, the isoform composition, substrate specificity, the regional and subcellular distribution in CNS structures, their contribution to brain ethanol metabolism, induction under ethanol administration and the role in the neurochemical mechanisms of psychopharmacological and neurotoxic effects of ethanol are discussed. In addition, the nonoxidative pathway of ethanol metabolism with the formation of fatty acid ethyl esters and phosphatidylethanol in the brain is described.

The enzymatic activity of Cu/Zn superoxide dismutase does not fluctuate in mouse spermatogenic cells despite mRNA changes

In the mammalian testis, multiple mRNAs encoding the copper zinc superoxide dismutase (SOD-1) are expressed in postmeiotic male germ cells. Here we relate SOD-1 mRNA levels to SOD-1 protein and enzyme activity levels in mouse spermatogenic cells. Although the sizes and relative amounts of the multiple SOD-1 mRNAs vary as male germ cells enter meiosis and proceed into the postmeiotic stages of spermatogenesis, the amount of SOD-1 protein and enzyme activity does not fluctuate significantly, suggesting a precise control of SOD-1 activity in male germ cells.

Midbrain dopaminergic neuronal degeneration in a transgenic mouse model of familial amyotrophic lateral sclerosis

Familial amyotrophic lateral sclerosis has been linked in 15% of families to mutations in the gene encoding for copper-zinc superoxide dismutase (Cu/Zn-SOD), a key enzyme in the cellular defense mechanisms against free radical attack. We used a transgenic mouse model of familial amyotrophic lateral sclerosis (transgenic G1H mice) based on expression of mutant human Cu/Zn-SOD to examine the influence of the transgene expression on midbrain dopaminergic neurons, cells that contain conspicuous amounts of this enzyme. At the time that 50% of motor neurons of the spinal cord were lost, we observed concurrent reductions in dopamine levels in the caudate-putamen and the nucleus accumbens of transgenic G1H mice. In addition, numbers of tyrosine hydroxylase-immunostained neurons were significantly reduced in both the substantia nigra (26%) and the ventral tegmental area (16%) compared to those in their nontransgenic littermates. Similar abnormalities were not observed in the brains of transgenic mice overexpressing wild-type Cu/Zn-SOD. These findings indicate that overexpression of the mutated Cu/Zn-SOD protein caused a significant loss of midbrain dopaminergic neurons in addition to the loss of spinal motor neurons. The potential of the mutated enzyme to induce cell death extending beyond the motor neurons is consistent with the description of substantia nigra degeneration in some patients with familial amyotrophic lateral sclerosis. Furthermore, if mutated Cu/Zn-SOD is conclusively shown to kill cells by oxidative stress, such an observation would be in keeping with the known sensitivity of dopaminergic neurons to free radical attack.

Murine thioredoxin peroxidase delays neuronal apoptosis and is expressed in areas of the brain most susceptible to hypoxic and ischemic injury

Thioredoxin peroxidase (TPx) is an antioxidant protein that limits the activity of reactive oxygen species (ROS). We cloned the cDNA encoding the mouse homolog of TPx from an E14.5 brain cDNA library and analyzed its distribution and function in murine tissues. Comparison of the amino acid sequence of mouse TPx with those of other species revealed that TPx was highly conserved across all species. Mouse TPx had broad tissue distribution, but its expression was especially marked in cells that metabolize oxygen molecules at high levels such as erythroid cells, renal tubular cells, cardiac and skeletal muscle cells, and certain types of neurons. Levels of increased expression of TPx in the brain were coincident with regions known to be especially sensitive to hypoxic and ischemic injury in humans. Models of erythroid differentiation and neuronal survival were employed to study the function of TPx. Murine erythroleukemia cells (MEL cells) increased TPx transcription when in a chemically differentiated state. Furthermore, expression of mouse TPx in PC12 pheochromocytoma cells prolonged their survival in the absence of nerve growth factor (NGF) and serum, indicating that TPx could promote neuronal cell survival. We propose that TPx contributes to antioxidant defense in erythrocytes and neuronal cells by limiting the destructive capacity of oxygen radicals. These findings identify a novel gene that appears to be relevant to hypoxic brain injury and may be of importance in development of new approaches to abrogate the effects of ischemic- and hypoxic-related injury in the central nervous system (CNS).

ALS-linked SOD1 mutant G85R mediates damage to astrocytes and promotes rapidly progressive disease with SOD1-containing inclusions

High levels of familial Amyotrophic Lateral Sclerosis (ALS)-linked SOD1 mutants G93A and G37R were previously shown to mediate disease in mice through an acquired toxic property. We report here that even low levels of another mutant, G85R, cause motor neuron disease characterized by an extremely rapid clinical progression, without changes in SOD1 activity. Initial indicators of disease are astrocytic inclusions that stain intensely with SOD1 antibodies and ubiquitin and SOD1-containing aggregates in motor neurons, features common with some cases of SOD1 mutant-mediated ALS. Astrocytic inclusions escalate markedly as disease progresses, concomitant with a decrease in the glial glutamate transporter (GLT-1). Thus, the G85R SOD1 mutant mediates direct damage to astrocytes, which may promote the nearly synchronous degeneration of motor neurons.

Mutations in SOD1 associated with amyotrophic lateral sclerosis cause novel protein interactions

A subset of familial and sporadic amyotrophic lateral sclerosis (ALS-a fatal disorder characterised by progressive motor neuron degeneration) cases are due to mutations in the gene encoding Cu,Zn superoxide dismutase (SOD1). Two mutations which have been successfully used to generate transgenic mice that develop an ALS-like syndrome are glycine 85 to arginine (G85R) and glycine 93 to alanine (G93A) with the mutant SOD1 allele overexpressed in a normal mouse genetic background. No ALS-like phenotype is observed in mice overexpressing wild-type SOD1 or mice without any SOD1 activity. These dominant mutations, which do not necessarily decrease SOD1 activity, may confer a gain of function that is selectively lethal to motor neurons. The yeast interaction trap system allowed us to determine whether these mutations in SOD1 caused novel protein interactions not observed with wild-type SOD1 and which might participate in the generation of the ALS phenotype. Two proteins, lysyl-tRNA synthetase and translocon-associated protein delta, interact with mutant forms of SOD1 but not with wild-type SOD1. The specificity of the interactions was confirmed by the coimmunoprecipitation of mutant SOD1 and the expressed proteins. These proteins are expressed in ventral cord, lending support to the relevance of this interaction to motor neuron disease.

Familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is sporadic in ninety percent of cases and familial (FALS) in ten percent. Both forms of FALS whether transmitted as an autosomal dominant (DFALS) or as an autosomal recessive (RFALS) trait is genetically heterogeneous. The locus for one form of RFALS maps to chromosome 2q33. Fifteen percent of DFALS families have mutations in the gene for Cu, Zn superoxide dismutase (SOD1) gene which is coded on chromosome 21. These mutations result in decreased SOD1 activity and shortened half-life of the protein in most instances. Transgenic mice overexpressing mutated SOD1 protein develop an ALS-like disease which suggests that the degeneration of motor neurons in DFALS is caused by the gain of a novel toxic function by mutated SOD1 rather than by the decrease of SOD1 activity. Possible mechanisms of the novel neurotoxic function of mutated SOD1 are discussed.

Identification of an apo-superoxide dismutase (Cu,Zn) pool in human lymphoblasts

Copper incorporation (64Cu(II)) into Cu,Zn-superoxide dismutase (SOD) was studied in human lymphoblasts. Rapid incorporation of copper with a proportionate increase in SOD activity was detected. No copper incorporation or SOD activation was detected when 64Cu(II) was added to cell cytosols rather than to intact cells. Thus, incorporation of 64Cu was not due to isotopic exchange. Cycloheximide had no significant effect on copper incorporation and activation of SOD when the data were corrected for total cell copper. Thus, the data were consistent with copper incorporation into a preexisting apoSOD pool rather than newly synthesized SOD, and no new SOD synthesis was detected over a 15-h incubation period. The size of the apoSOD pool was estimated to be approximately 35% of the total SOD in lymphoblasts. When cells were preincubated for 15 h with excess copper (15 microM Cu(II)), the size of the apo pool markedly decreased but was not eliminated, suggesting that the apoSOD was not due to copper deficiency. These experiments also indicated that newly arrived copper was preferentially incorporated into the apoSOD pool, while the function(s) of an apoSOD pool remains unknown. Copper binding to apoSOD may provide a rapid protective response against copper toxicity.

Copper incorporation into superoxide dismutase in Menkes lymphoblasts

The incorporation of copper into Cu,Zn-superoxide dismutase (SOD) was examined in Menkes lymphoblasts that express a genetic defect of copper metabolism. SOD activity was approximately 40% higher in Menkes than normal lymphoblasts. Since Menkes lymphoblasts contain elevated copper levels, the higher SOD activity is most likely due to near copper saturation of an apoSOD pool that is in normal lymphoblasts. Cycloheximide markedly inhibited 64Cu(II) incorporation into SOD in Menkes lymphoblasts under conditions in which no significant, de novo synthesis of SOD protein was detected with normal lymphoblasts. The maximal amount of 64Cu incorporation into newly synthesized SOD in Menkes lymphoblasts was approximately equal to the maximal amount of 64Cu that could be incorporated into the apoSOD pool in normal lymphoblasts. The increased synthesis of SOD in Menkes lymphoblasts may play a protective role against copper toxicity in Menkes lymphoblasts. The protonophore, CCCP markedly inhibited 64Cu incorporation into SOD in both normal and Menkes lymphoblasts, which is consistent with 64Cu incorporation into SOD within a membrane-bounded compartment in both cell types. When 64Cu-incorporation into SOD was blocked with CCCP, copper accumulated in a Superose column fraction that contains S-adenosylhomocysteine hydrolase (SAHH), which has a high affinity for copper. SAHH may play a role in delivering copper to SOD.

Immunocytochemical localization of seleno-glutathione peroxidase in the adult mouse brain

Cytoplasmic seleno-glutathione peroxidase, by reducing hydrogen peroxide and fatty acid hydroperoxides, may be a major protective enzyme against oxidative damage in the brain. Oxidative damage is strongly suspected to contribute to normal aging and neurodegenerative process of Alzheimer's and Parkinson's diseases. We report here an immunocytochemical analysis of the localization of glutathione peroxidase in the adult mouse brain, carried out with an affinity-purified polyclonal antibody. Most of the brain areas analysed showed weak to strong glutathione peroxidase immunoreactivity, expressed in both neurons and glial cells. The strongest immunoreactivity was found in the reticular thalamic and red nuclei. Highly immunoreactive neurons were observed in the cerebral cortex (layer II), the CA1, dentate gyrus and pontine nucleus. Other regions, such as the caudate-putamen, septum nuclei, diagonal band of Broca, hippocampus, thalamus and hypothalamus, showed moderate staining. This study provides original information about the wide distribution of glutathione peroxidase in the mouse brain. Double-staining experiments indicated that specific subsets of cholinergic neurons in septal and diagonal band nuclei were negative for this antigen. Similarly, many dopaminergic neurons of the substantia nigra pars compacta expressed low levels of glutathione peroxidase antigen, in contrast to the ventral tegmental area, wherein most catecholaminergic cells were strongly positive. A lack of glutathione peroxidase in subsets of dopaminergic or cholinergic neurons may thus confer a relative sensitivity of these cells to oxidative injury of various origins, including catecholamine oxidation, neurotoxins and excitotoxicity.

Transgenic models of neurodegenerative diseases

Identification of genetic mutations linked to familial neurodegenerative diseases have made it possible to generate useful transgenic animal models. Studies using these transgenic animals indicate that many familial neurodegenerative diseases, such as motor neuron disease, Alzheimer's disease, prion diseases and trinucleotide repeat diseases, result from a gain of deleterious properties. The disease-specific pathology in transgenic mice demonstrates the utility of these models in elucidating pathogenic mechanisms of the disease and in developing therapeutic strategies.

Mechanisms of selective motor neuron death in ALS: insights from transgenic mouse models of motor neuron disease

Concerning the mechanism(s) of disease underlying amyotrophic lateral sclerosis (ALS), transgenic mouse models have provided (i) a detailed look at the pathogenic progression of disease, (ii) a tool for testing hypotheses concerning the mechanism of neuronal death, and (iii) a host appropriate for testing therapeutic strategies. Thus far, these efforts have proved that mutation in a neurofilament subunit can cause progressive disease displaying both selective motor neuron death and aberrant neurofilament accumulation similar to that reported in human disease. Additional mice expressing point mutations in the cytoplasmic enzyme superoxide dismutase (SOD1), the only known cause of ALS, have proved that disease arises from a toxic property of the mutant enzyme rather than loss of enzymatic activity.

Therapeutic horizons for amyotrophic lateral sclerosis

Recent theories on the pathogenesis of motor neuron disease and research on motor neuron injury have resulted in new putative therapies, which include treatment with various neurotrophic factors, antioxidants and anti-excitotoxicity agents. Clinical and preclinical studies have now provided the first agents that reproducibly alter the course of amyotrophic lateral sclerosis.

Quantitative immunocytochemical analysis of the spinal cord in G86R superoxide dismutase transgenic mice: neurochemical correlates of selective vulnerability

Transgenic mice with a G86R mutation in the mouse superoxide dismutase (SOD-1) gene, which corresponds to a mutation that has been observed in familial amyotrophic lateral sclerosis (ALS), display progressive loss of motor function and provide a valuable model of ALS. The pathology in the spinal cords of these mice was evaluated to determine whether there are chemically identified populations of neurons that are either highly vulnerable or resistant to degeneration. Qualitatively, there were phosphorylated neurofilament protein (NFP)-immunoreactive inclusions and a pronounced loss of motoneurons in the ventral horn of the spinal cord without the presence of vacuoles that has been reported in other SOD-1 transgenic mice. Neuron counts from SOD-1 and control spinal cords revealed that the percentage loss of NFP-, choline acetyltransferase (ChAT)-, and calretinin (CR)-immunoreactive neurons was greater than the percentage loss of total neurons, suggesting that these neuronal groups are particularly vulnerable in SOD-1 transgenic mice. In contrast, calbindin-containing neurons did not degenerate significantly and represent a protected population of neurons. Quantitative double-labeling experiments suggested that the vulnerability of ChAT- and CR-immunoreactive neurons was due primarily to the presence of NFP within a subset of these neurons, which degenerated preferentially to ChAT- and CR-immunoreactive neurons that did not colocalize with NFP. Our findings suggest that NFP, which has been demonstrated previously to be involved mechanistically in motoneuron degeneration, may also be important in the mechanism of degeneration that is initiated by the SOD-1 mutation.

Inductions of Cu/Zn superoxide dismutase- and nitric oxide synthase-like immunoreactivities in rabbit spinal cord after transient ischemia

The distributions and inductions of Cu/Zn superoxide dismutase (SOD), neuronal and endothelial nitric oxide (NO) synthase (nNOS and eNOS), and nitrotyrosine (NT) were immunohistochemically examined in rabbit spinal cords after 5 and 15 min of transient ischemia. The neurons in the anterior horns (AH) were selectively lost 7 days after 15-min ischemia as compared with those of sham-operated controls. In the normal spinal cords, a number of neurons in the AHs were positive for the nNOS, and only slightly positive for the Cu/Zn SOD and the eNOS. Immunoreactivities for the proteins were induced at 8-24 h both after 5- and 15-min ischemia. In contrast, NT-like immunoreactivity was negative both in the normal and postischemic spinal cords. These results suggest that Cu/Zn SOD- and nNOS-, and eNOS-like immunoreactivities are induced, but that, even though an interaction of Cu/Zn SOD with NO could be present, NT was not detected in the motor neurons in the rabbit spinal cords after transient ischemia. Other factors could be required for NT formation found in degenerative motor neuron death in humans.

The roles of free radicals in amyotrophic lateral sclerosis

The mutations of the Cu,Zn superoxide dismutase (Cu,Zn-SOD) gene observed in amyotrophic lateral sclerosis (ALS) patients suggest that free radicals play a role in this fatal disease. Free radicals trigger oxidative damage to proteins, membrane lipids, and DNA, thereby destroying neurons. Mutations of the SOD gene may reduce its superoxide dismutase activity, thereby elevating free radical levels. In addition, the mutant SOD protein may function as a peroxidase to oxidize cellular components, and it may also react with peroxynitrite-a product of the reaction between superoxide and nitric oxide-to ultimately form nitrate proteins. The selective degeneration of motor neurons in ALS may be caused by the high level of Cu,Zn-SOD present in and the large number of glutamatergic synapses projecting to these neurons. Free radical-triggered and age-accumulated oxidation may modify the program controlling motor neuron death, thereby initiating apoptosis of motor neurons in young adults.

A gain-of-function of an amyotrophic lateral sclerosis-associated Cu,Zn-superoxide dismutase mutant: An enhancement of free radical formation due to a decrease in Km for hydrogen peroxide

Cu,Zn-superoxide dismutase (SOD) is known to be a locus of mutation in familial amyotrophic lateral sclerosis (FALS). Transgenic mice that express a mutant Cu,Zn-SOD, Gly-93--> Ala (G93A), have been shown to develop amyotrophic lateral sclerosis (ALS) symptoms. We cloned the FALS mutant, G93A, and wild-type cDNA of human Cu,Zn-SOD, overexpressed them in Sf9 insect cells, purified the proteins, and studied their enzymic activities for catalyzing the dismutation of superoxide anions and the generation of free radicals with H2O2 as substrate. Our results showed that both enzymes contain one copper ion per subunit and have identical dismutation activity. However, the free radical-generating function of the G93A mutant, as measured by the spin trapping method, is enhanced relative to that of the wild-type enzyme, particularly at lower H2O2 concentrations. This is due to a small, but reproducible, decrease in the value of Km for H2O2 for the G93A mutant, while the kcat is identical for both enzymes. Thus, the ALS symptoms observed in G93A transgenic mice are not caused by the reduction of Cu,Zn-SOD activity with the mutant enzyme; rather, it is induced by a gain-of-function, an enhancement of the free radical-generating function. This is consistent with the x-ray crystallographic studies showing the active channel of the FALS mutant is slightly larger than that of the wild-type enzyme; thus, it is more accessible to H2O2. This gain-of-function, in part, may provide an explanation for the association between ALS and Cu,Zn-SOD mutants.

Transgenic and knockout mice in the study of neurodegenerative diseases

Accurate animal models are essential for detailed analysis of the mechanisms underlying human neurodegenerative diseases. In addition, they can offer useful paradigms for the development and evaluation of new therapeutic strategies. We review the most popular techniques for modification of the mammalian genome in vivo, and provide a critical evaluation of the available transgenic mouse models for several neurological conditions of humans, including prion diseases, human retroviral diseases, Alzheimer's disease, and motor neuron diseases.

Benefit of vitamin E, riluzole, and gabapentin in a transgenic model of familial amyotrophic lateral sclerosis

Familial amyotrophic lateral sclerosis (FALS) has been linked in some families to dominant mutations of the SOD1 gene encoding Cu,Zn superoxide dismutase (Cu,ZnSOD). We have used a transgenic model of FALS based on expression of mutant human Cu,ZnSOD to explore the etiology and therapy of the genetic disease. Expression of mutant, but not wild-type, human Cu,ZnSOD in mice places the brain and spinal cord under oxidative stress. This causes depletion of vitamin E, rather than the typical age-dependent increase in vitamin E content as occurs in nontransgenic mice and in mice expressing wild-type human Cu,ZnSOD. Dietary supplementation with vitamin E delays onset of clinical disease and slows progression in the transgenic model but does not prolong survival. In contrast, two putative inhibitors of the glutamatergic system, riluzole and gabapentin, prolong survival. However, riluzole did not delay disease onset. Thus, there was clear separation of effects on onset, progression, and survival by the three therapeutics tested. This suggests the hypothesis that oxidative damage produced by the expression of mutant Cu,ZnSOD causes slow or weak excitotoxicity that can be inhibited in part by alerting glutamate release or biosynthesis presynaptically.

Altered reactivity of superoxide dismutase in familial amyotrophic lateral sclerosis

A subset of individuals with familial amyotrophic lateral sclerosis (FALS) possesses dominantly inherited mutations in the gene that encodes copper-zinc superoxide dismutase (CuZnSOD). A4V and G93A, two of the mutant enzymes associated with FALS, were shown to catalyze the oxidation of a model substrate (spin trap 5,5'-dimethyl-1-pyrroline N-oxide) by hydrogen peroxide at a higher rate than that seen with the wild-type enzyme. Catalysis of this reaction by A4V and G93A was more sensitive to inhibition by the copper chelators diethyldithiocarbamate and penicillamine than was catalysis by wild-type CuZnSOD. The same two chelators reversed the apoptosis-inducing effect of mutant enzymes expressed in a neural cell line. These results suggest that oxidative reactions catalyzed by mutant CuZnSOD enzymes initiate the neuropathologic changes in FALS.

Characterisation of the chicken Cu,Zn superoxide dismutase gene

A PCR product was generated from embryonic chicken spinal cord cDNA using primers designed to conserved regions of the human and bovine amino and carboxyl-terminal coding sequences of the Cu,Zn superoxide dismutase (SOD1, EC 1.15.1.1) gene. DNA sequencing confirmed this product to be the chicken homologue of the SOD1 gene. This sequence was compared to SOD1 from bovine, human and Xenopus laevis. Important structural features of SOD1 are shown to be conserved in the chicken gene.

Copper/zinc superoxide dismutase expression in the human central nervous system. Correlation with selective neuronal vulnerability

Oxidative stress has been implicated in the pathogenesis of several neurological disorders. We examined the regional distribution of copper/zinc superoxide dismutase (SOD-1), one of the key antioxidant enzymes, in the human central nervous system using in situ hybridization. Our results show that the enzyme is present at high levels of constitutive expression in alpha-motor neurons, oculomotor neurons, nucleus basalis, substantia nigra, neocortex, and the hippocampal sector resistant to hypoxia (H2). Relatively lower levels were found in Sommer's sector (H1) and Purkinje cells. We conclude that a lower constitutive level of SOD-1 expression may play a role in the selective vulnerability of certain neuronal populations to hypoxia but does not correlate with the patterns of neurodegeneration observed in amyotrophic lateral sclerosis. Parkinson's disease, and Alzheimer's disease.

A role for neurofilaments in the pathogenesis of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late-onset degenerative disease of motor neurons, characterized by abnormal accumulation of neurofilaments (NFs) in perikarya and proximal axons. Two lines of evidence suggest that neurofilament accumulation can play a crucial role in ALS pathogenesis. First, transgenic mouse models overexpressing NF proteins were found to develop motor neuron degeneration and, second, variant alleles of the NF heavy-subunit (NF-H) gene have been found in some human ALS patients. Our axonal transport studies with transgenic mice overexpressing the human NF-H gene, a model of ALS, revealed defects of intracellular transport not only for neurofilament proteins but also for other cytoskeletal proteins and organelles such as mitochondria. Therefore, we propose that neurofilament accumulation in mice causes neurodegeneration by disrupting axonal transport, a mechanism that may account for the pathogenesis of ALS.