Chronic activation in presymptomatic amyotrophic lateral sclerosis (ALS) mice of a feedback loop involving Fas, Daxx, and FasL

Abstract

The reasons for the cellular specificity and slow progression of motoneuron diseases such as ALS are still poorly understood. We previously described a motoneuron-specific cell death pathway downstream of the Fas death receptor, in which synthesis of nitric oxide (NO) is an obligate step. Motoneurons from ALS model mice expressing mutant SOD1 showed increased susceptibility to exogenous NO as compared with controls. Here, we report a signaling mechanism whereby NO leads to death of mutant, but not control, motoneurons. Unexpectedly, exogenous NO triggers expression of Fas ligand (FasL) in cultured motoneurons. In mutant SOD1 G93A and SOD1 G85R , but not in control motoneurons, this up-regulation results in activation of Fas, leading through Daxx to phosphorylation of p38 and further NO synthesis. This Fas͞NO feedback amplification loop is required for motoneuron death in vitro. In vivo, mutant SOD1 G93A and SOD1 G85R mice show increased numbers of positive motoneurons and Daxx nuclear bodies weeks before disease onset. Moreover, FasL up-regulation is reduced in the presence of transgenic dominant-negative Daxx. We propose that chronic low-level activation of the Fas͞NO feedback loop may underlie the motoneuron loss that characterizes familial ALS and may help to explain its slowly progressive nature.cell death ͉ motoneuron disease ͉ NO ͉ p38 kinase ͉ neurodegeneration A myotrophic lateral sclerosis (ALS) is the most frequent adultonset motoneuron disease in humans. ALS is characterized by the selective degeneration of motoneurons in spinal cord, brainstem, and cerebral cortex leading to muscle atrophy and paralysis and ultimately to death. About 1 to 2% of all human ALS forms are caused by dominantly inherited mutations in the Cu͞Zn superoxide dismutase (SOD1) gene. Mice transgenic for the ALS-linked SOD1 mutations G37R (1), H46R͞H48Q (2), G85R (3), and G93A (4) develop an adult-onset motoneuron disorder that remarkably resembles human ALS.Despite much intensive study, many questions remain concerning the mechanism(s) by which mutant SOD1 triggers specific motoneuron death. One unresolved issue is the cellular site of action of the gain-of-function mutations. Clement et al. (5) generated chimeric mice carrying a mixture of WT and mutant SOD1-expressing cells in the spinal cord. In these mice, WT motoneurons eventually showed stigmata of degeneration, whereas some mutant SOD1 motoneurons were protected from degeneration when surrounded by WT nonneuronal cells. These results suggest that mutant SOD1 in both motoneurons and surrounding cells may play a role in the disease process.Our earlier studies using cultures of purified embryonic motoneurons reached similar conclusions. We found that motoneurons from SOD1 G93A , SOD1 G37R , and SOD1 G85R mice survived normally in the presence of optimal trophic support or when challenged by excitotoxic agonists. In marked contrast, compared with controls, they displayed a 10-to 100-fold increase in sensitivity to extracellular agonists of the Fas receptor or to exogenous nitr...

IFNγ triggers a LIGHT-dependent selective death of motoneurons contributing to the non-cell-autonomous effects of mutant SOD1

“…14 We first determined the phosphorylation levels of nuclear p38 by quantitative confocal miscroscopy. 15,22 Treatment of motoneurons with sLIGHT, as well as sFasL, resulted in a significant increase in levels of p38 phosphorylation (Figure 2b and Supplementary Figure 2a). SB203580, a pyridinyl imidazole inhibitor of p38, prevented phosphorylation of p38, consistent with its inhibitory effect on the autophosphorylation-dependent activation of p38.…”

“…(b) Quantification of phospho-p38 kinase fluorescence in Hb9HGFP motoneurons treated (or not) with sLIGHT in the presence or the absence of SB203580 (10 mM) for 1 h. Treatment with sFasL served as a control. 15 Confocal fluorescence imaging and ImageJ image analysis were used to determine the nuclear mean fluorescence intensity of p38 in Hb9HGFP neurons (a.u, arbitrary unit, *Po0.05, **Po0.01). (c) Motoneurons were maintained in culture for 24 h and treated or not with sLIGHT (100 ng/ml) and SB203580 (10 mM).…”

“…9 We previously demonstrated that Fas triggers a motoneuron-restricted death pathway, which is exacerbated in a cell-autonomous manner by mutant SOD1. 14,15 Interestingly, a functional involvement of the Fas death pathway in motoneuron degeneration in mutant SOD1 mice has been shown. [15][16][17] Regarding the pathogenic processes, the mutant astrocyte-mediated toxicity to motoneurons would occur independently of the Fas death pathway, 8 suggesting that other sources, such as microglia or serum, activate Fas.…”

“…14,15 Interestingly, a functional involvement of the Fas death pathway in motoneuron degeneration in mutant SOD1 mice has been shown. [15][16][17] Regarding the pathogenic processes, the mutant astrocyte-mediated toxicity to motoneurons would occur independently of the Fas death pathway, 8 suggesting that other sources, such as microglia or serum, activate Fas. 14,18 Our understanding of the selective degenerative process integrating external death triggers remains, however, incomplete.…”

Les cellules gliales

“…Ces résul-tats ne signifient pas, cependant, que les astrocytes sont nécessaires pour entraîner la mort des motoneurones. En effet, les pionniers dans le domaine des cultures de motoneurones ont, quelques années auparavant, montré que les motoneurones exprimant la SOD1 mutée étaient plus susceptibles de mourir que des motoneurones contrôles et qu'ils pouvaient auto-entraî-ner leur mort [7,8]. …”