Mutations in superoxide dismutase-1 (SOD1) cause a form of the fatal paralytic disorder amyotrophic lateral sclerosis (ALS), presumably by a combination of cell-autonomous and non-cell-autonomous processes. Here, we show that expression of mutated human SOD1 in primary mouse spinal motor neurons does not provoke motor neuron degeneration. Conversely, rodent astrocytes expressing mutated SOD1 kill spinal primary and embryonic mouse stem cell-derived motor neurons. This is triggered by soluble toxic factor(s) through a Bax-dependent mechanism. However, mutant astrocytes do not cause the death of spinal GABAergic or dorsal root ganglion neurons or of embryonic stem cell-derived interneurons. In contrast to astrocytes, fibroblasts, microglia, cortical neurons and myocytes expressing mutated SOD1 do not cause overt neurotoxicity. These findings indicate that astrocytes may play a role in the specific degeneration of spinal motor neurons in ALS. Identification of the astrocyte-derived soluble factor(s) may have far-reaching implications for ALS from both a pathogenic and therapeutic standpoint.

SUMMARY Most cases of neurodegenerative disease are sporadic, hindering the use of genetic mouse models to analyze disease mechanisms. Focusing on the motor neuron (MN) disease amyotrophic lateral sclerosis (ALS) we therefore devised a fully humanized co-culture model composed of human adult primary sporadic ALS (sALS) astrocytes and human embryonic stem cell-derived MNs. The model reproduces the cardinal features of human ALS: sALS astrocytes, but not those from control patients, trigger selective death of MNs. The mechanisms underlying this non-cell-autonomous toxicity were investigated in both astrocytes and MNs. Although causal in familial ALS (fALS), SOD1 does not contribute to the toxicity of sALS astrocytes. Death of MNs triggered by either sALS or fALS astrocytes occurs through necroptosis, a form of programmed necrosis involving receptor-interacting protein 1 and the mixed lineage kinase domain-like protein. The necroptotic pathway therefore constitutes a novel potential therapeutic target for this incurable disease.

ALS is a fatal paralytic disorder characterized by a progressive loss of spinal cord motor neurons. Herein, we show that NADPH oxidase, the main reactive oxygen species-producing enzyme during inflammation, is activated in spinal cords of ALS patients and in spinal cords in a genetic animal model of this disease. We demonstrate that inactivation of NADPH oxidase in ALS mice delays neurodegeneration and extends survival. We also show that NADPH oxidase-derived oxidant products damage proteins such as insulin-like growth factor 1 (IGF1) receptors, which are located on motor neurons. Our in vivo and in vitro data indicate that such an oxidative modification hinders the IGF1͞Akt survival pathway in motor neurons. These findings suggest a non-cell-autonomous mechanism through which inflammation could hasten motor neuron death and contribute to the selective motor neuronal degeneration in ALS.LS is the most common adult-onset paralytic disease and is characterized by a loss of motor neurons in the cerebral cortex, brainstem, and spinal cord (1). It is invariably fatal, usually within 3-5 years after onset (1). Insights into its neurodegenerative mechanisms followed the discovery that dominant mutations in the gene for superoxide dismutase 1 (SOD1) cause familial ALS (2, 3). Overexpression of SOD1 mutants in rodents emulate clinical and pathological hallmarks of ALS through a toxic gain of function (4). Development of chimeric mice provided animals with a mixture of neuronal and nonneuronal cells expressing wild-type or mutant SOD1 (5); investigation of these animals suggested that nonneuronal cells influence the fate of spinal cord motor neurons (5). Corroborating this hypothesis is the demonstration that reduction of mutant SOD1, selectively in microglia, extended survival in transgenic SOD1 G37R mice (6). In light of the latter results, microglia are now thought to contribute to the non-cell-autonomous killing of motor neurons.Among the microglia-derived mediators that could promote neurodegeneration are reactive oxygen species (ROS) produced by the enzyme NADPH oxidase complex (7). Although this ROSgenerating multimeric oxidase is indispensable for protecting the host against invading microorganisms in infectious disorders (8), its inappropriate activation may be harmful in noninfectious neurodegenerative disorders. Such bystander cytotoxicity is thought to lead to the death of developing oligodendrocytes in periventricular leukomalacia (9), one of the most important causes of cerebral palsy. In light of these facts, we undertook the study of NADPH oxidase in both human ALS and one of its genetic models. Our results for both human and mouse postmortem tissues indicate that spinal cord microgliosis in ALS is accompanied with an upregulation of NADPH oxidase. Furthermore, by using mutant deficient mice in functional NADPH oxidase as well as in neuronlike cell culture systems, we provide compelling evidence that supports the concept that this microglial ROS-generating enzymatic complex promotes spinal cord motor neuro...

Indices of neuroinflammation are found in a variety of diseases of the CNS including amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). Over the years, neuroinflammation, in degenerative disorders of the CNS, has evolved from being regarded as an innocent bystander accomplishing its housekeeping function secondary to neurodegeneration to being considered as a bona fide contributor to the disease process and, in some situations, as a putative initiator of the disease. Herein, we will review not only neuroinflammation in both ALS and SMA from the angle of neuropathology, but also from the angle of its potential role in the pathogenesis and treatment of these two dreadful paralytic disorders.

Summary Neurodegenerative phenotypes reflect complex, time-dependent molecular processes, whose elucidation may reveal neuronal class-specific therapeutic targets. The current focus in neurodegeneration has been on individual genes and pathways. In contrast, we assembled a genome-wide regulatory model (henceforth, interactome), whose unbiased interrogation revealed 23 candidate causal master regulators of neurodegeneration in an in vitro model of amyotrophic lateral sclerosis (ALS), characterized by a loss of spinal motor neurons (MNs). Of these, eight were confirmed as specific MN death drivers in our model of familial ALS, including NF-κB, which has long been considered a pro-survival factor. Through an extensive array of molecular, pharmacological and biochemical approaches we have confirmed that neuronal NF-κB drives the degeneration of MNs in both familial and sporadic models of ALS, thus providing proof-of-principle that regulatory network analysis is a valuable tool for studying cell-specific mechanisms of neurodegeneration.