Transcriptional profile of primary astrocytes expressing ALS-linked mutant SOD1.

Amyotrophic lateral sclerosis (ALS) is caused by the progressive degeneration of motor neurons. Mutations in the Cu/Zn superoxide dismutase (SOD1) are found in approximately 20% of patients with familial ALS. Mutant SOD1 causes motor neuron death through an acquired toxic property. Although the molecular mechanism underlying this toxic gain-of-function remains unknown, evidence support the role of mutant SOD1 expression in nonneuronal cells in shaping motor neuron degeneration. We have previously found that in contrast to nontransgenic cells, SOD1(G93A)-expressing astrocytes induced apoptosis of cocultured motor neurons. This prompted us to investigate whether the effect on motor neuron survival was related to a change in the gene expression profile. Through high-density oligonucleotide microarrays, we found changes in the expression of genes involved in transcription, signaling, cell proliferation, extracellular matrix synthesis, response to stress, and steroid and lipid metabolism. The most up-regulated gene was decorin (Dcn), a small multifunctional extracellular proteoglycan. Down-regulated genes included the insulin-like growth factor-1 receptor (Igf-1r) and the RNA binding protein ROD1. Rod1 was also found down-regulated in purified motor neurons expressing SOD1(G93A). Changes in the expression of Dcn, Igf-1r, and Rod1 were found in the spinal cord of asymptomatic animals, suggesting these changes occur before overt neuronal degeneration and potentially influence astrocyte-motor neuron interaction in the course of the disease. The astrocyte-specific gene expression profile might contribute to the identification of possible candidates for cell type-specific therapies in ALS.

Mitochondrial superoxide production and nuclear factor erythroid 2-related factor 2 activation in p75 neurotrophin receptor-induced motor neuron apoptosis.

Nerve growth factor (NGF) can induce apoptosis by signaling through the p75 neurotrophin receptor (p75(NTR)) in several nerve cell populations. Cultured embryonic motor neurons expressing p75(NTR) are not vulnerable to NGF unless they are exposed to an exogenous flux of nitric oxide (*NO). In the present study, we show that p75(NTR)-mediated apoptosis in motor neurons involved neutral sphingomyelinase activation, increased mitochondrial superoxide production, and cytochrome c release to the cytosol. The mitochondria-targeted antioxidants mitoQ and mitoCP prevented neuronal loss, further evidencing the role of mitochondria in NGF-induced apoptosis. In motor neurons overexpressing the amyotrophic lateral sclerosis (ALS)-linked superoxide dismutase 1(G93A) (SOD1(G93A)) mutation, NGF induced apoptosis even in the absence of an external source of *NO. The increased susceptibility of SOD1(G93A) motor neurons to NGF was associated to decreased nuclear factor erythroid 2-related factor 2 (Nrf2) expression and downregulation of the enzymes involved in glutathione biosynthesis. In agreement, depletion of glutathione in nontransgenic motor neurons reproduced the effect of SOD1(G93A) expression, increasing their sensitivity to NGF. In contrast, rising antioxidant defenses by Nrf2 activation prevented NGF-induced apoptosis. Together, our data indicate that p75(NTR)-mediated motor neuron apoptosis involves ceramide-dependent increased mitochondrial superoxide production. This apoptotic pathway is facilitated by the expression of ALS-linked SOD1 mutations and critically modulated by Nrf2 activity.