NAALADase (GCP II) inhibitors protect in models of amyotrophic lateral sclerosis (ALS)

Chronic glutamate toxicity is implicated in the pathogenesis of ALS. The neuropeptide N-acetyl-aspartyl glutamate (NAAG) appears to function both as a storage form for glutamate and as a neuromodulator at glutamatergic synapses. Catabolism of NAAG by N-acetylated-α-linked acidic dipeptidase (NAALADase; also termed glutamate carboxypeptidase II), yields N-acetyl aspartate (NAA) and glutamate. Since prior studies demonstrate an up-regulation of NAALADase in motor cortex and increased levels of NAA and glutamate in the CSF of ALS patients, we hypothesized that inhibition of NAALADase could protect against neuronal degeneration in ALS. Neuroprotective effects of two NAALADase inhibitors were assessed. 2-(Phosphonomethyl)pentanedioic acid (2-PMPA) decreased motor neuron loss and prevented loss of choline acetyltransferase (ChAT) activity in an in vitro model of ALS wherein chronic glutamate toxicity was induced by blocking glutamate transport. Gross morphology was preserved in 2-PMPA-treated cultures. In a SOD-1 transgenic mouse model of ALS, oral administration of a structurally different NAALADase inhibitor (GPI 5693) increased survival by 29 days and delayed onset of clinical symptoms by 17 days. Preliminary analysis of spinal cord pathology revealed severe neuronal depletion and astrocytosis with white matter changes in control mice. In mice treated with GPI 5693, normal neuronal populations with modest vacuolar changes were observed. These data suggest that NAALADase inhibition may provide an exciting therapeutic approach to the devastating disease, ALS.