Transgenic overexpression of Cu ؉2 ͞Zn ؉2 superoxide dismutase 1 (SOD1) harboring an amyotrophic lateral sclerosis (ALS)-linked familial genetic mutation (SOD1 G93A ) in a Sprague-Dawley rat results in ALS-like motor neuron disease. Motor neuron disease in these rats depended on high levels of mutant SOD1 expression, increasing from 8-fold over endogenous SOD1 in the spinal cord of young presymptomatic rats to 16-fold in end-stage animals. Disease onset in these rats was early, Ϸ115 days, and disease progression was very rapid thereafter with affected rats reaching end stage on average within 11 days. Pathological abnormalities included vacuoles initially in the lumbar spinal cord and subsequently in more cervical areas, along with inclusion bodies that stained for SOD1, Hsp70, neurofilaments, and ubiquitin. Vacuolization and gliosis were evident before clinical onset of disease and before motor neuron death in the spinal cord and brainstem. Focal loss of the EAAT2 glutamate transporter in the ventral horn of the spinal cord coincided with gliosis, but appeared before motor neuron͞axon degeneration. At end-stage disease, gliosis increased and EAAT2 loss in the ventral horn exceeded 90%, suggesting a role for this protein in the events leading to cell death in ALS. These transgenic rats provide a valuable resource to pursue experimentation and therapeutic development, currently difficult or impossible to perform with existing ALS transgenic mice.A myotrophic lateral sclerosis (ALS) is a late-onset neuromuscular disorder characterized by progressive motor dysfunction that leads to paralysis and eventually death. The pathology of the disease results from the death of large motor neurons in the spinal cord and brainstem (1, 2). ALS occurs in both sporadic and familial forms (3). Familial ALS accounts for Ϸ5-10% of all reported cases. Approximately 15-20% of familial ALS cases has been linked to inheritance in an autosomal dominant fashion of a mutant form of Cu ϩ2 ͞Zn ϩ2 superoxide dismutase 1 (SOD1) (4, 5). SOD1 normally functions in the regulation of oxidative stress by conversion of free radical superoxide anions to hydrogen peroxide and molecular oxygen. Over 90 distinct familial SOD1 mutations have been found to date. SOD1 mutations that have been tested in transgenic mice result in ALS-like motor neuron disease (6-8), but SOD1-null mice do not develop motor neuron disease (9). Furthermore, crossing SOD1-null mice with transgenic ALS mice does not alter disease onset or progression (10). Taken together, these results indicate that familial ALS does not result from loss of SOD1 function but rather an unidentified gain of function. There is no consensus as to the mechanism, and theories include alterations in SOD1 folding, oxidative stress from aberrant catalysis (11), or cytoplasmic aggregates (12). New studies also suggest that the disease is not cell autonomous-that nonneuronal cells are necessary for motor neuron degeneration (13, 14, ¶).Transgenic mouse models expressing mutant forms of SOD1 (15-21) develop...

We characterized synaptosomal glutamate transport activity in a recently developed transgenic rat model of amyotrophic lateral sclerosis (ALS) overexpressing the G93A Cu(2+)/Zn(2+) superoxide dismutase (SOD1) mutation. Using spinal cord synaptosomes, a significant reduction (43%) in the maximal velocity for high-affinity, Na(+)-dependent glutamate uptake was observed at disease end stage in G93A rats compared with age-matched controls. Similarly, a 27% reduction in maximum velocity (V(max)) was measured at disease onset, but no difference in spinal cord V(max) values were observed with presymptomatic animals compared with controls. In comparison, we observed no differences in the V(max) for glutamate clearance at disease end stage with synaptosomes from cortex, hippocampus, striatum, cerebellum, and brainstem, indicating a specific deficit in the spinal cord. The pharmacological sensitivity of spinal cord uptake to dihydrokainate suggests that the GLT-1 (glutamate transporter-1) subtype primarily mediates the transport activity. Expression analysis revealed a loss of GLT-1 as well as qualitative changes in GLAST (glutamate/aspartate transporter) but no measurable changes in EAAC1 (excitatory amino acid carrier 1) in spinal cord of end-stage G93A rats, indicating that deficits in glutamate transporters in this rat model may be glial specific. Riluzole, a neuroprotective agent used clinically to slow the progression of ALS, produced an enhancement of spinal cord synaptosomal glutamate uptake in control animals and early-stage disease G93A rats, but this effect was lost in end-stage animals. Altered expression of astroglial glutamate transporters accompanied by reduced capacity for spinal cord clearance of extracellular glutamate in the G93A SOD1 transgenic rat may account for a dampened effect of riluzole to enhance glutamate uptake at end-stage disease.

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