Abnormal Protein Phosphorylation in Human Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by loss of neurons in cortex, brain stem and spinal cord. Currently, ALS is believed to be triggered by a number of distinct factors including glutamate excitotoxicity and mutations in the sod1 gene (mSOD). To explore the role of protein kinase C (PKC) in N-methyl-D-aspartate (NMDA)- mediated cytotoxicity, cell death assays were performed in NR 1 AINR2A-transfected human embryonic kidney (HEK) cells. NMDA-mediated cell death was potentiated by the activation of ca2+- and lipid-dependent isoforms of PKC, specifically PKCP 1. A discrete segment of the C-terminus of NR2A subunit contributed to this potentiation by PKC. These data demonstrate that the elevation of PKC activity increases cell death induced by NMDA receptor activation. To examine the involvement of abnormal protein phosphorylation in ALS, the expression of over 130 proteins were evaluated in postmortem spinal cord tissues from patients having sporadic ALS and controls. There was increased expression of protein kinases and phosphoproteins in ALS tissue such as PKCaIP, protein kinase B a (PKBa) and phospho-PKCalP. This suggests that both pro- and anti-apoptotic signaling pathways are up-regulated in ALS. It is possible that the final outcome for each individual cell is determined by which pathway dominates over the other. Transgenic mice over-expressing mSOD have been used extensively as a model of familial ALS. Comparative studies have revealed a striking similarity in pathology between mSOD mouse and human ALS. The morphological analogy between these two was investigated and we found that in G93A mSOD mice, the spinal nucleus of bulbocavernosus (SNB) is spared from degeneration, paralleling the survival of its functional and anatomical homologue, Onuf s nucleus. This provides evidence that mSOD mice may suitably models ALS. However, a distinct profile of changes in protein expression were observed in the CNS tissues of G93A mSOD mice compared with their control littermates, which was dissimilar to that between ALS patients and controls. This observation indicates that the activation of protein kinases is different with neuron loss in mSOD mouse compared with that seen in patients with sporadic ALS. These findings suggest an important role for abnormal protein phosphorylation in ALS.