Protein retention in the endoplasmic reticulum, blockade of programmed cell death and autophagy selectively occur in spinal cord motoneurons after glutamate receptor-mediated injury

We previously showed that, in contrast to the acute administration of NMDA, chronic treatment of chick embryos from embryonic day (E) 5 to E9 with this excitotoxin rescues motoneurons (MNs) from programmed cell death. Following this protocol, MNs are also protected against later acute excitotoxic cell death. Previously, we found that MNs treated from E5 to E9 develop long-lasting changes involving vesicular trafficking and other organelle pathology similar to the abnormalities observed in certain chronic neurological diseases including amyotrophic lateral sclerosis (ALS). Here we extend these previous results by showing that protein aggregation within the endoplasmic reticulum (ER) takes place selectively in MNs as an early event of chronic excitotoxicity. Although protein aggregates do not induce appreciable MN death, they foreshadow the activation of a conspicuous autophagic response leading to long-lasting degenerative changes that causes dysfunction but not immediate cell death. Chronic early treatment with NMDA results in a transient (between E6 and E10) lack of vulnerability to undergo cell death induced by different types of stimuli. It is suggested that blockade of protein translation in stressed ER may inhibit apoptosis in NMDA-treated MNs. However, in embryos older than E12, degenerating MNs are sensitized to die after limb ablation (axotomy) and accumulate hyperphosphorylated neurofilaments. Moreover, chronic NMDA treatment does not induce the upregulation of molecular chaperones in spinal cord. These results represent a new model of glutamate receptor-mediated neurotoxicity that selectively occurs in spinal cord MNs and also demonstrate an experimental system that may be valuable for understanding the mechanisms involved in chronic MN degeneration and in certain cytological hallmarks of ALS-diseased MNs such as inclusion bodies.