Mechanisms of preferential vulnerability of motor neurons in a familial form of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive loss of upper and lower motor neurons. A subset of familial ALS cases (20%; FALS-1) are linked to mutations in the Cu/Zn-superoxide dismutase (SOD-1). In order to study the mechanisms of mutant SOD-1 (mSOD-1) toxicity and preferential vulnerability of motor neurons, human wild type and seven different mSOD-1 were expressed in motor neurons or dorsal root ganglion neurons of dissociated cultures of murine spinal cord-dorsal root ganglia, or in cultured hippocampal neurons. Expression was obtained by intranuclear microinjection of plasmid expression vectors. Many features of the motor neuron disease observed in humans with ALS and in mSOD-1 transgenic mice were reproduced in this new culture model, including preferential vulnerability of motor neurons and abnormal distribution of hyperphosphorylated neurofilament proteins. A novel observation was the formation of cytoplasmic SOD-1 aggregates in motor neurons expressing mSOD-1 followed by evidence of apoptotic cell death. Both formation of aggregates and motoneuronal death were reduced by coexpression of the stress-inducible heat shock protein, HSP70, indicating that insufficiency of molecular chaperones may be involved in FALS-1. Using specific antagonists of glutamate receptor subtypes, we have demonstrated that normally nontoxic glutamatergic input, particularly via calcium-permeable AMPA/kainate receptors, is a major factor in the vulnerability of motor neurons to the toxicity of mSOD-1. Partial protection was obtained by blockade of L-type voltage-gated calcium channels, implicating Ca2+ entry through these channels in the toxicity of mSOD-1. Dramatic neuroprotection was obtained by coexpressing the calcium-binding protein calbindin-D28k indicating that the toxicity of mSOD-1 is calcium-dependent. However, increasing intracellular glutathione levels or treatment with N-tert-butyl-alpha-phenylnitrone (PBN) had no significant effect on moton