Nuclear erythroid 2-related factor 2-antioxidative response element signaling pathway in motor cortex and spinal cord in amyotrophic lateral sclerosis

Absence of Nrf2 or its selective overexpression in neurons and muscle does not affect survival in ALS-linked mutant hSOD1 mouse models

The nuclear factor erythroid 2-related factor 2 (Nrf2) governs the expression of antioxidant and phase II detoxifying enzymes. Nrf2 activation can prevent or reduce cellular damage associated with several types of injury in many different tissues and organs. Dominant mutations in Cu/Zn-superoxide dismutase (SOD1) cause familial forms of amyotrophic lateral sclerosis (ALS), a fatal disorder characterized by the progressive loss of motor neurons and subsequent muscular atrophy. We have previously shown that Nrf2 activation in astrocytes delays neurodegeneration in ALS mouse models. To further investigate the role of Nrf2 in ALS we determined the effect of absence of Nrf2 or its restricted overexpression in neurons or type II skeletal muscle fibers on symptoms onset and survival in mutant hSOD1 expressing mice. We did not observe any detrimental effect associated with the lack of Nrf2 in two different mutant hSOD1 animal models of ALS. However, restricted Nrf2 overexpression in neurons or type II skeletal muscle fibers delayed disease onset but failed to extend survival in hSOD1^G93A mice. These results highlight the concept that not only the pharmacological target but also the cell type targeted may be relevant when considering a Nrf2-mediated therapeutic approach for ALS.

Emerging roles of Nrf2 and phase II antioxidant enzymes in neuroprotection

Phase II metabolic enzymes are a battery of critical proteins that detoxify xenobiotics by increasing their hydrophilicity and enhancing their disposal. These enzymes have long been studied for their preventative and protective effects against mutagens and carcinogens and for their regulation via the Keap1 (Kelch-like ECH associated protein 1)/Nrf2 (Nuclear factor erythroid 2 related factor 2)/ARE (antioxidant response elements) pathway. Recently, a series of studies have reported the altered expression of phase II genes in postmortem tissue of patients with various neurological diseases. These observations hint at a role for phase II enzymes in the evolution of such conditions. Furthermore, promising findings reveal that overexpression of phase II genes, either by genetic or chemical approaches, confers neuroprotection in vitro and in vivo. Therefore, there is a need to summarize the current literature on phase II genes in the central nervous system (CNS). This should help guide future studies on phase II genes as therapeutic targets in neurological diseases. In this review, we first briefly introduce the concept of phase I, II and III enzymes, with a special focus on phase II enzymes. We then discuss their expression regulation, their inducers and executors. Following this background, we expand our discussion to the neuroprotective effects of phase II enzymes and the potential application of Nrf2 inducers to the treatment of neurological diseases.

Nrf2/ARE Signaling Pathway: Key Mediator in Oxidative Stress and Potential Therapeutic Target in ALS

Nrf2 (nuclear erythroid 2-related factor 2) is a basic region leucine-zipper transcription factor which binds to the antioxidant response element (ARE) and thereby regulates the expression of a large battery of genes involved in the cellular antioxidant and anti-inflammatory defence as well as mitochondrial protection. As oxidative stress, inflammation and mitochondrial dysfunctions have been identified as important pathomechanisms in amyotrophic lateral sclerosis (ALS), this signaling cascade has gained interest both with respect to ALS pathogenesis and therapy. Nrf2 and Keap1 expressions are reduced in motor neurons in postmortem ALS tissue. Nrf2-activating compounds have shown therapeutic efficacy in the ALS mouse model and other neurodegenerative disease models. Alterations in Nrf2 and Keap1 expression and dysregulation of the Nrf2/ARE signalling program could contribute to the chronic motor neuron degeneration in ALS and other neurodegenerative diseases. Therefore, Nrf2 emerges as a key neuroprotective molecule in neurodegenerative diseases. Our recent studies strongly support that the Nrf2/ARE signalling pathway is an important mediator of neuroprotection and therefore represents a promising target for development of novel therapies against ALS, Parkinson's disease (PD), Huntington's disease (HD), and Alzheimer's disease (AD).

Molecular pathways of motor neuron injury in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a genetically diverse disease. At least 15 ALS-associated gene loci have so far been identified, and the causative gene is known in approximately 30% of familial ALS cases. Less is known about the factors underlying the sporadic form of the disease. The molecular mechanisms of motor neuron degeneration are best understood in the subtype of disease caused by mutations in superoxide dismutase 1, with a current consensus that motor neuron injury is caused by a complex interplay between multiple pathogenic processes. A key recent finding is that mutated TAR DNA-binding protein 43 is a major constituent of the ubiquitinated protein inclusions in ALS, providing a possible link between the genetic mutation and the cellular pathology. New insights have also indicated the importance of dysregulated glial cell-motor neuron crosstalk, and have highlighted the vulnerability of the distal axonal compartment early in the disease course. In addition, recent studies have suggested that disordered RNA processing is likely to represent a major contributing factor to motor neuron disease. Ongoing research on the cellular pathways highlighted in this Review is predicted to open the door to new therapeutic interventions to slow disease progression in ALS.

Impaired antioxydative Keap1/Nrf2 system and the downstream stress protein responses in the motor neuron of ALS model mice

The Kelch-like ECH-associated protein 1 (Keap1)/Nuclear erythroid 2-related factor 2 (Nrf2) system is the major cellular defense mechanism under oxidative stress, but the role in motor neuron degeneration under amyotrophic lateral sclerosis (ALS) pathology has not yet been fully elucidated. Here we examined temporal and spatial changes of Keap1, Nrf2, and their downstream stress response proteins heme oxgenase-1 (HO-1), glutathione, thioredoxin (TRX), and heat shock protein 70 (HSP70) throughout the course of motor neuron (MN) degeneration in the spinal cord of ALS model mice. Keap1 protein levels progressively decreased in the MN and anterior lumbar cord of ALS mice to 63% at early symptomatic 14 weeks and 58% at end symptomatic 18 weeks, while Nrf2 dramatically increased in the anterior lumbar cord with accumulation in the MN nucleus to 229% at 14 weeks and 471% at 18 weeks when glial like cells became also positive. In contrast, downstream stress response proteins such as HO-1, glutathione, TRX, and HSP70 showed only a small increase in MN with a significant increase to 149% to 280% in the number of glial-like cells after symptomatic 14 weeks. Our present observation suggests that MN selectively lost inductions of these important downstream protective proteins without regard to the Keap1/Nrf2 system activation, which could be a pivotal mechanism of neurodegenerative processes of ALS.

Dysfunction of constitutive and inducible ubiquitin-proteasome system in amyotrophic lateral sclerosis: implication for protein aggregation and immune response

The ubiquitin-proteasome system (UPS) is the major intracellular proteolytic mechanism controlling the degradation of misfolded/abnormal proteins. A common hallmark in amyotrophic lateral sclerosis (ALS) and in other neurodegenerative disorders is the accumulation of misfolded/abnormal proteins into the damaged neurons, leading to the formation of cellular inclusions that are mostly ubiquitin-positive. Although proteolysis is a complex mechanism requiring the participation of different pathways, the abundant accumulation of ubiquitinated proteins strongly suggests an important contribution of UPS to these neuropathological features. The use of cellular and animal models of ALS, particularly those expressing mutant SOD1, the gene mutation most represented in familiar ALS, has provided significant evidence for a role of UPS in protein inclusions formation and motor neuron death. This review will specifically discuss this piece of evidence and provide suggestions of potential strategies for therapeutic intervention. We will also discuss the finding that, unlike the constitutive proteasome subunits, the inducible subunits are overexpressed early during disease progression in SOD1 mice models of ALS. These subunits form the immunoproteasome and generate peptides for the major histocompatibility complex class I molecules, suggesting a role of this system in the immune responses associated with the pathological features of ALS. Since recent discoveries indicate that innate and adaptive immunity may influence the disease process, in this review we will also provide evidence of a possible connection between immune-inflammatory reactions and UPS function, in the attempt to better understand the etiopathology of ALS and to identify appropriate targets for novel treatment strategies of this devastating disease.

Neuroprotective effect of Nrf2/ARE activators, CDDO ethylamide and CDDO trifluoroethylamide, in a mouse model of amyotrophic lateral sclerosis

Oxidative damage, neuroinflammation, and mitochondrial dysfunction contribute to the pathogenesis of amyotrophic lateral sclerosis (ALS), and these pathologic processes are tightly regulated by the Nrf2/ARE (NF-E2-related factor 2/antioxidant response element) signaling program. Therefore, modulation of the Nrf2/ARE pathway is an attractive therapeutic target for neurodegenerative diseases such as ALS. We examined two triterpenoids, CDDO (2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid) ethylamide and CDDO trifluoroethylamide (CDDO-TFEA), that potently activate Nrf2/ARE in a cell culture model of ALS and in the G93A SOD1 mouse model of ALS. Treatment of NSC-34 cells stably expressing mutant G93A SOD1 with CDDO-TFEA upregulated Nrf2 expression and resulted in translocation of Nrf2 into the nucleus. Western blot analysis showed an increase in the expression of Nrf2/ARE-regulated proteins. When treatment started at a "presymptomatic age" of 30days, both of these compounds significantly attenuated weight loss, enhanced motor performance, and extended the survival of G93A SOD1 mice. Treatment started at a "symptomatic age," as assessed by impaired motor performance, was neuroprotective and slowed disease progression. These findings provide further evidence that compounds that activate the Nrf2/ARE signaling pathway may be useful in the treatment of ALS.

The Nrf2-inducible antioxidant defense in astrocytes can be both up- and down-regulated by activated microglia:Involvement of p38 MAPK

The effects of microglia-conditioned medium (MCM) on the inducible Nrf2 system in astrocyte-rich cultures were investigated by determination of glutathione (GSH) levels, γglutamylcysteine ligase (γGCL) activity, the protein levels of Nrf2, Keap1, the modulatory subunit of γGCL (γGCL-M) and activated MAP kinases (ERK1/2, JNK and p38). Microglia were either cultured for 24 h in serum-free culture medium to achieve microglia-conditioned medium from non-activated cells (MCM(0) ), used as control condition, or activated with different concentrations (0.1-1,000 ng mL(-1) ) of lipopolysaccharide (LPS) to produce MCM(0.1-1,000) . Acute exposure (24 h) to MCM(100) increased GSH, γGCL activity, the protein levels of γGCL-M, Nrf2, and activated JNK and ERK1/2 in astrocyte-rich cultures. In contrast, treatment with MCM(10) for 24 h decreased components of the Nrf2 system in parallel with activation of p38 MAPK. Stimulation of the Nrf2 system by tBHQ was partly intact after 24 h but blocked after 72 h treatment with MCM(10) and MCM(100) . This down-regulation after 72 h correlated with activation of p38 MAPK and lack of ERK1/2 and JNK activation. The negative effects were partly reversed by an inhibitor of p38 which restored tBHQ mediated protection against oxidative stress. In conclusion, the study showed a negative effect of MCM(10) on the inducible anti-oxidant defense in astrocyte-rich cultures at both 24 and 72 h that correlated with activation of p38 and was partly reversed by a p38 inhibitor. A transient protective effect of MCM(100) on astrocyte-rich cultures against H(2)O(2) toxicity was observed at 24 h which coincided with activation of JNK and ERK1/2.

Amyotrophic lateral sclerosis pathogenesis: a journey through the secretory pathway

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset motoneuron degenerative disease characterized by the selective loss of motoneurons in the spinal ventral horn, most brainstem nuclei, and the cerebral cortex. Although approximately 90% of ALS cases are sporadic (sALS), analyses of familial ALS (fALS)-causative genes have generated relevant insight into molecular events involved in the pathology. Here we overview an emerging concept indicating the occurrence of secretory pathway stress in the disease process. These alterations include a failure in the protein folding machinery at the endoplasmic reticulum (ER), engagement of the unfolded protein response (UPR), modifications of the Golgi apparatus network, impaired vesicular trafficking, inhibition of protein quality control mechanisms, oxidative damage to ER proteins, and sustained activation of degradative pathways such as autophagy. A common feature predicted for most of these alterations is abnormal protein homeostasis associated with the accumulation of misfolded proteins at the ER, possibly leading to chronic ER stress and neuronal dysfunction. Signs of ER stress are observed even during presymptomatic stages in fALS mouse models, and pharmacological strategies to alleviate protein misfolding slow disease progression. Because the secretory pathway stress occurs in both sALS and several forms of fALS, it may offer a unique common target for possible therapeutic strategies to treat this devastating disease.

Motor neuron-immune interactions: the vicious circle of ALS

Because microglial cells, the resident macrophages of the CNS, react to any lesion of the nervous system, they have for long been regarded as potential players in the pathogenesis of several neurodegenerative disorders including amyotrophic lateral sclerosis, the most common motor neuron disease in the adult. In recent years, this microglial reaction to motor neuron injury, in particular, and the innate immune response, in general, has been implicated in the progression of the disease, in mouse models of ALS. The mechanisms by which microglial cells influence motor neuron death in ALS are still largely unknown. Microglial activation increases over the course of the disease and is associated with an alteration in the production of toxic factors and also neurotrophic factors. Adding to the microglial/macrophage response to motor neuron degeneration, the adaptive immune system can likewise influence the disease process. Exploring these motor neuron-immune interactions could lead to a better understanding in the physiopathology of ALS to find new pathways to slow down motor neuron degeneration.

Stress-activated cap'n'collar transcription factors in aging and human disease

Cap'n'collar (Cnc) transcription factors are conserved in metazoans and have important developmental and homeostatic functions. The vertebrate Nrf1, Nrf2, and Nrf3; the Caenorhabditis elegans SKN-1; and the Drosophila CncC comprise a subgroup of Cnc factors that mediate adaptive responses to cellular stress. The most studied stress-activated Cnc factor is Nrf2, which orchestrates the transcriptional response of cells to oxidative stressors and electrophilic xenobiotics. In rodent models, signaling by Nrf2 defends against oxidative stress and aging-associated disorders, such as neurodegeneration, respiratory diseases, and cancer. In humans, polymorphisms that decrease Nrf2 abundance have been associated with various pathologies of the skin, respiratory system, and digestive tract. In addition to preventing disease in rodents and humans, Cnc factors have life-span-extending and anti-aging functions in invertebrates. However, despite the pro-longevity and antioxidant roles of stress-activated Cnc factors, their activity paradoxically declines in aging model organisms and in humans suffering from progressive respiratory disease or neurodegeneration. We review the roles and regulation of stress-activated Cnc factors across species, present all reported instances in which their activity is paradoxically decreased in aging and disease, and discuss the possibility that the pharmacological restoration of Nrf2 signaling may be useful in the prevention and treatment of age-related diseases.

Oxidative stress in ALS: key role in motor neuron injury and therapeutic target

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by death of motor neurons leading to muscle wasting, paralysis, and death, usually within 2-3 years of symptom onset. The causes of ALS are not completely understood, and the neurodegenerative processes involved in disease progression are diverse and complex. There is substantial evidence implicating oxidative stress as a central mechanism by which motor neuron death occurs, including elevated markers of oxidative damage in ALS patient spinal cord and cerebrospinal fluid and mutations in the antioxidant enzyme superoxide dismutase 1 (SOD1) causing approximately 20% of familial ALS cases. However, the precise mechanism(s) by which mutant SOD1 leads to motor neuron degeneration has not been defined with certainty, and the ultimate trigger for increased oxidative stress in non-SOD1 cases remains unclear. Although some antioxidants have shown potential beneficial effects in animal models, human clinical trials of antioxidant therapies have so far been disappointing. Here, the evidence implicating oxidative stress in ALS pathogenesis is reviewed, along with how oxidative damage triggers or exacerbates other neurodegenerative processes, and we review the trials of a variety of antioxidants as potential therapies for ALS.

COP9 limits dendritic branching via Cullin3-dependent degradation of the actin-crosslinking BTB-domain protein Kelch

Components of the COP9 signalosome (CSN), a key member of the conserved 26S proteasome degradation pathway, have been detected to be altered in patients of several debilitating syndromes. These findings suggest that CSN acts in neural circuits, but the exact function of CSN in brain remains unidentified. Previously, using Drosophila peripheral nervous system (PNS) as a model system, we determined that CSN is a critical regulator of dendritic morphogenesis. We found that defects in CSN led to the strikingly contrast phenotype of either reducing or stimulating dendritic branching. In particular, we have reported that CSN stimulates dendritic branching via Cullin1-mediated proteolysis. Here we describe that CSN inhibits dendritic arborization in PNS neurons acting via control of Cullin3 function: loss of Cullin3 causes excessive dendritic branching. We also identified a downstream target for Cullin3-dependent degradation in neurons – the actin-crosslinking BTB-domain protein Kelch. Inappropriate accumulation of Kelch, either due to the impaired Cullin3-dependent turnover, or ectopic expression of Kelch, leads to uncontrolled dendritic branching. These findings indicate that the CSN pathway modulates neuronal network in a multilayer manner, providing the foundation for new insight into the CSN role in human mental retardation disorders and neurodegenerative disease.

The Nrf2-ARE cytoprotective pathway in astrocytes

The expression of phase-II detoxification and antioxidant enzymes is governed by a cis-acting regulatory element named the antioxidant response element (ARE). ARE-containing genes are regulated by the nuclear factor erythroid-2-related factor 2 (Nrf2), a member of the Cap'n'Collar basic-leucine-zipper family of transcription factors. ARE-regulated genes are preferentially activated in astrocytes, which consequently have more efficient detoxification and antioxidant defences than neurons. Astrocytes closely interact with neurons to provide structural, metabolic and trophic support, as well as actively participating in the modulation of neuronal excitability and neurotransmission. Therefore, functional alterations in astrocytes can shape the interaction with surrounding cells, such as neurons and microglia. Activation of Nrf2 in astrocytes protects neurons from a wide array of insults in different in vitro and in vivo paradigms, confirming the role of astrocytes in determining the vulnerability of neurons to noxious stimuli. Here, we review the current data supporting Nrf2 activation in astrocytes as a viable therapeutic approach, not only in acute neuronal damage, but also in chronic neurodegeneration related to oxidative stress.