Impaired Spinal Cord Glutamate Transport Capacity and Reduced Sensitivity to Riluzole in a Transgenic Superoxide Dismutase Mutant Rat Model of Amyotrophic Lateral Sclerosis

Abstract

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.

Glutamate Transporters and the Excitotoxic Path to Motor Neuron Degeneration in Amyotrophic Lateral Sclerosis

“…In mutant SOD1 mice, several studies have shown decreased EAAT2 protein and downregulation of glutamate-transport activity in affected CNS areas (9,15,20,23,41,151,153,155). Similar results are found in the SOD1-G93A and H46R transgenic rats (41, 68) (see also Fig.…”

“…Another possibility is that, although EAAT2 could be properly translated and expressed, it loses function in ALS. This hypothesis is supported by the evidence that a lower V max for EAAT2 is measured in mutant SOD1 animal models (41). In addition, reactive oxygen species (ROS) formed in motor neurons after glutamate-receptor activation seem to be able to diffuse out of the motor neurons and induce oxidation and disruption of EAAT2-mediated glutamate uptake in neighboring astrocytes (108,109).…”

“…Several studies in the mouse model of ALS showed that changes in the EAAT2 expression levels and glutamateuptake activity are found only in the ventral horn of the affected spinal cord at a late stage of disease (9,23,41). An .…”

Oxidative and excitotoxic insults exert differential effects on spinal motoneurons and astrocytic glutamate transporters: Implications for the role of astrogliosis in amyotrophic lateral sclerosis

“…However, this area is controversial in terms of when astrogliosis occurs and whether it precedes or accompanies dysfunction of motoneurons (Barbeito et al, 2004;Boillee et al, 2006;Maragakis and Rothstein, 2006). Although motoneurons were intact histologically and physiologically when there was marked astrogliosis with loss of EAAT2 in the SOD1 mutant rat model of ALS (Howland et al, 2002), other studies do not show a presymptomatic loss of EAAT2 (Bendotti et al, 2001;Dunlop et al, 2003). Overall, these data and the evidence for EAAT2 loss in human and animal models of ALS (Barbeito et al, 2004;Boillee et al, 2006) strongly implicate an astrocytic excitotoxic component in the disease.…”

Neuronal Dysfunction in iPSC-Derived Medium Spiny Neurons from Chorea-Acanthocytosis Patients Is Reversed by Src Kinase Inhibition and F-Actin Stabilization

“…Riluzole is the only Food and Drug Administration-approved drug for the treatment of amyotrophic lateral sclerosis and has been reported to prevent excitatory neurotransmitter release from presynaptic nerve terminals by a reduction of cytosolic Ca 2ϩ , as well as stimulated transmitter clearance from the synapse (Azbill et al, 2000;Dunlop et al, 2003;Wang et al, 2004). It was also discussed as treatment for cerebellar ataxias (Romano et al, 2015).…”