Axonal degeneration in the peripheral nervous system: Implications for the pathogenesis of amyotrophic lateral sclerosis

Sarm1 knockout modifies biomarkers of neurodegeneration and spinal cord circuitry but not disease progression in the mSOD1G93A mouse model of ALS

The mechanisms underlying the loss of motor neuron axon integrity in amyotrophic lateral sclerosis (ALS) are unclear. SARM1 has been identified as a genetic risk variant in sporadic ALS, and the SARM1 protein is a key mediator of axon degeneration. To investigate the role of SARM1 in ALS-associated axon degeneration, we knocked out Sarm1 (Sarm1^KO) in mSOD1^G93ATg (mSOD1) mice. Animals were monitored for ALS disease onset and severity, with motor function assessed at pre-symptomatic and late-stage disease and lumbar spinal cord and sciatic nerve harvested for immunohistochemistry at endpoint (20 weeks). Serum was collected monthly to assess protein concentrations of biomarkers linked to axon degeneration (neurofilament light (NFL) and tau), and astrogliosis (glial fibrillary acidic protein (GFAP)), using single molecule array (Simoa®) technology. Overall, loss of Sarm1 in mSOD1 mice did not slow or delay symptom onset, failed to improve functional declines, and failed to protect motor neurons. Serum NFL levels in mSOD1 mice increased between 8 -12 and 16-20 weeks of age, with the later increase significantly reduced by loss of SARM1. Similarly, loss of SARM1 significantly reduced an increase in serum GFAP between 16 and 20 weeks of age in mSOD1 mice, indicating protection of both global axon degeneration and astrogliosis. In the spinal cord, Sarm1 deletion protected against loss of excitatory VGluT2-positive puncta and attenuated astrogliosis in mSOD1 mice. In the sciatic nerve, absence of SARM1 in mSOD1 mice restored the average area of phosphorylated neurofilament reactivity towards WT levels. Together these data suggest that Sarm1^KO in mSOD1 mice is not sufficient to ameliorate functional decline or motor neuron loss but does alter serum biomarker levels and provide protection to axons and glutamatergic synapses. This indicates that treatments targeting SARM1 could warrant further investigation in ALS, potentially as part of a combination therapy.

Genetic and pharmacological PARP inhibition reduces axonal degeneration in C. elegans models of ALS

Axonal degeneration is observed in early stages of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). This degeneration generally precedes apoptosis and therefore may be a promising therapeutic target. An increasing number of genes have been identified to actively regulate axonal degeneration and regeneration, however, only a few potential therapeutic targets have been identified in the context of neurodegenerative diseases. Here we investigate DLK-1, a major axonal regeneration pathway and its contribution to axonal degeneration phenotypes in several C. elegans ALS models. From this pathway, we identified the PAR polymerases (PARP) PARP-1 and PARP-2 as the most consistent modifiers of axonal degeneration in our models of ALS. Genetic and pharmacological inhibition of PARP-1 and PARP-2 reduces axonal degeneration and improves related motor phenotypes.

Neuromuscular Development and Disease: Learning From in vitro and in vivo Models

The neuromuscular junction (NMJ) is a specialized cholinergic synaptic interface between a motor neuron and a skeletal muscle fiber that translates presynaptic electrical impulses into motor function. NMJ formation and maintenance require tightly regulated signaling and cellular communication among motor neurons, myogenic cells, and Schwann cells. Neuromuscular diseases (NMDs) can result in loss of NMJ function and motor input leading to paralysis or even death. Although small animal models have been instrumental in advancing our understanding of the NMJ structure and function, the complexities of studying this multi-tissue system in vivo and poor clinical outcomes of candidate therapies developed in small animal models has driven the need for in vitro models of functional human NMJ to complement animal studies. In this review, we discuss prevailing models of NMDs and highlight the current progress and ongoing challenges in developing human iPSC-derived (hiPSC) 3D cell culture models of functional NMJs. We first review in vivo development of motor neurons, skeletal muscle, Schwann cells, and the NMJ alongside current methods for directing the differentiation of relevant cell types from hiPSCs. We further compare the efficacy of modeling NMDs in animals and human cell culture systems in the context of five NMDs: amyotrophic lateral sclerosis, myasthenia gravis, Duchenne muscular dystrophy, myotonic dystrophy, and Pompe disease. Finally, we discuss further work necessary for hiPSC-derived NMJ models to function as effective personalized NMD platforms.

Two distinct skeletal muscle microRNA signatures revealing the complex mechanism of sporadic ALS

Skeletal muscle pathology is thought to have an important role in the onset and/or progression of amyotrophic lateral sclerosis (ALS), which is a neurodegenerative disorder characterized by progressive muscle weakness. Since miRNAs are recognized as important regulatory factors of essential biological processes, we aimed to identify differentially expressed miRNAs in the skeletal muscle of sporadic ALS patients through the combination of molecular-omic technologies and bioinformatic tools. We analyzed the miRnome profiles of skeletal muscle biopsies acquired from ten sALS patients and five controls with Affymetrix GeneChip miRNA 4.0 Array. To find out differentially expressed miRNAs in patients, data were analyzed by The Institute for Genomic Research-Multi Experiment Viewer (MeV) and miRNAs whose expression difference were statistically significant were identified as candidates. The potential target genes of these miRNAs were predicted by miRWalk 2.0 and were functionally enriched by gene ontology (GO) analysis. The expression level of priority candidates was validated by quantitative real-time PCR (qRT-PCR) analysis. We identified ten differentially expressed miRNAs in patients with a fold change threshold ≥ 2.0, FDR = 0. We identified ten differentially expressed miRNAs in patients with a fold change threshold ≥ 2.0, FDR = 0. Nine out of the ten miRNAs were found to be related to top three enriched ALS-related terms. Based on the qRT-PCR validation of candidate miRNAs, patients were separated into two groups: those with upregulated miR-4429 and miR-1825 expression and those with downregulated miR-638 expression. The different muscle-specific miRNA profiles in sALS patients may indicate the involvement of etiologic heterogeneity, which may allow the development of novel therapeutic strategies.

Skeletal Muscle Metabolism: Origin or Prognostic Factor for Amyotrophic Lateral Sclerosis (ALS) Development?

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive and selective loss of motor neurons, amyotrophy and skeletal muscle paralysis usually leading to death due to respiratory failure. While generally considered an intrinsic motor neuron disease, data obtained in recent years, including our own, suggest that motor neuron protection is not sufficient to counter the disease. The dismantling of the neuromuscular junction is closely linked to chronic energy deficit found throughout the body. Metabolic (hypermetabolism and dyslipidemia) and mitochondrial alterations described in patients and murine models of ALS are associated with the development and progression of disease pathology and they appear long before motor neurons die. It is clear that these metabolic changes participate in the pathology of the disease. In this review, we summarize these changes seen throughout the course of the disease, and the subsequent impact of glucose–fatty acid oxidation imbalance on disease progression. We also highlight studies that show that correcting this loss of metabolic flexibility should now be considered a major goal for the treatment of ALS.

Stathmin-2: adding another piece to the puzzle of TDP-43 proteinopathies and neurodegeneration

Cytoplasmic aggregated proteins are a common neuropathological feature of neurodegenerative diseases. Cytoplasmic mislocalization and aggregation of TAR-DNA binding protein 43 (TDP-43) is found in the majority of patients with amyotrophic lateral sclerosis (ALS) and in approximately 50% of patients dying of frontotemporal lobar degeneration (FTLD). In this issue of the JCI, Prudencio, Humphrey, Pickles, and colleagues investigated the relationship of TDP-43 pathology with the loss of stathmin-2 (STMN2), an essential protein for axonal growth and maintenance. Comparing genetic, cellular, and neuropathological data from patients with TDP-43 proteinopathies (ALS, ALS-frontotemporal dementia [ALS-FTD], and FTLD-TDP-43 [FTLD-TDP]) with data from patients with non-TDP-related neurodegenerations, they demonstrate a direct relationship between TDP-43 pathology and STMN2 reduction. Loss of the normal transcription suppressor function of TDP-43 allowed transcription of an early termination cryptic axon, resulting in truncated, nonfunctional mRNA. The authors suggest that measurement of truncated STMN2 mRNA could be a biomarker for discerning TDP proteinopathies from other pathologies.

Neuromuscular junction-on-a-chip: ALS disease modeling and read-out development in microfluidic devices

Amyotrophic lateral sclerosis (ALS) is a fatal and progressive neurodegenerative disease affecting upper and lower motor neurons with no cure available. Clinical and animal studies reveal that the neuromuscular junction (NMJ), a synaptic connection between motor neurons and skeletal muscle fibers, is highly vulnerable in ALS and suggest that NMJ defects may occur at the early stages of the disease. However, mechanistic insight into how NMJ dysfunction relates to the onset and progression of ALS is incomplete, which hampers therapy development. This is, in part, caused by a lack of robust in vitro models. The ability to combine microfluidic and induced pluripotent stem cell (iPSC) technologies has opened up new avenues for studying molecular and cellular ALS phenotypes in vitro. Microfluidic devices offer several advantages over traditional culture approaches when modeling the NMJ, such as the spatial separation of different cell types and increased control over the cellular microenvironment. Moreover, they are compatible with 3D cell culture, which enhances NMJ functionality and maturity. Here, we review how microfluidic technology is currently being employed to develop more reliable in vitro NMJ models. To validate and phenotype such models, various morphological and functional read-outs have been developed. We describe and discuss the relevance of these read-outs and specifically illustrate how these read-outs have enhanced our understanding of NMJ pathology in ALS. Finally, we share our view on potential future directions and challenges.

Ammonia induces calpain-dependent cleavage of CRMP-2 during neurite degeneration in primary cultured neurons

Hyperammonemia in the CNS induces irreversible damages to neurons due to ultimate cell loss. Neurite degeneration, a primary event that leads to neuronal cell death, remains less elucidated especially in hyperammonemia circumstances. Here, we found that the administration of ammonia induced neurite degeneration in cultured cerebellar granule neurons. The resulting altered neuronal morphology, rupture of neurites, and disassembly of the cytoskeleton led to cell death. Calcein and Fluo-4 staining revealed that ammonia induced intracellular calcium dysregulation. Subsequently activated calpain cleaved CRMP-2, a microtubule assembly protein. Pharmacologically inhibition of calpain, but not caspases or GSK-3, suppressed the cleavage of CRMP-2 and reversed neurite degeneration under ammonia treatment. Exposure to ammonia decreased whereas inhibition of calpain restored the amplitude and frequency of miniature excitatory postsynaptic currents. These data suggest a mechanism by which elevated ammonia level may induce neuronal dysfunction via abnormal calcium influx and calpain-dependent CRMP-2 cleavage, leading to abnormal synaptic transmission, cytoskeletal collapse, and neurite degeneration.

The Peripheral Nervous System in Amyotrophic Lateral Sclerosis: Opportunities for Translational Research

Although amyotrophic lateral sclerosis (ALS) has been considered as a disorder of the motor neuron (MN) cell body, recent evidences show the non-cell-autonomous pathogenic nature of the disease. Axonal degeneration, loss of peripheral axons and destruction of nerve terminals are early events in the disease pathogenic cascade, anticipating MN degeneration, and the onset of clinical symptoms. Therefore, although ALS and peripheral axonal neuropathies should be differentiated in clinical practice, they also share damage to common molecular pathways, including axonal transport, RNA metabolism and proteostasis. Thus, an extensive evaluation of the molecular events occurring in the peripheral nervous system (PNS) could be fundamental to understand the pathogenic mechanisms of ALS, favoring the discovery of potential disease biomarkers, and new therapeutic targets.

Apoptosis versus axon pruning: Molecular intersection of two distinct pathways for axon degeneration

Neurons are capable of degenerating their axons for the physiological clearance and refinement of unnecessary connections via the programmed degenerative pathways of apoptosis and axon pruning. While both pathways mediate axon degeneration they are however distinct. Whereas in apoptosis the entire neuron, both axons and cell body, degenerates, in the context of axon pruning only the targeted axon segments are selectively degenerated. Interestingly, the molecular pathways mediating axon degeneration in these two contexts have significant mechanistic overlap but also retain distinct differences. In this review, we describe the peripheral neuronal cell culture models used to study the molecular pathways of apoptosis and pruning. We outline what is known about the molecular mechanisms of apoptosis and axon pruning and focus on highlighting the similarities and differences of these two pathways.

Ca2+-activated K+ channels modulate microglia affecting motor neuron survival in hSOD1G93A mice

Recent studies described a critical role for microglia in amyotrophic lateral sclerosis (ALS), where these CNS-resident immune cells participate in the establishment of an inflammatory microenvironment that contributes to motor neuron degeneration. Understanding the mechanisms leading to microglia activation in ALS could help to identify specific molecular pathways which could be targeted to reduce or delay motor neuron degeneration and muscle paralysis in patients. The intermediate-conductance calcium-activated potassium channel KCa3.1 has been reported to modulate the "pro-inflammatory" phenotype of microglia in different pathological conditions. We here investigated the effects of blocking KCa3.1 activity in the hSOD1G93AALS mouse model, which recapitulates many features of the human disease. We report that treatment of hSOD1G93A mice with a selective KCa3.1 inhibitor, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), attenuates the "pro-inflammatory" phenotype of microglia in the spinal cord, reduces motor neuron death, delays onset of muscle weakness, and increases survival. Specifically, inhibition of KCa3.1 channels slowed muscle denervation, decreased the expression of the fetal acetylcholine receptor γ subunit and reduced neuromuscular junction damage. Taken together, these results demonstrate a key role for KCa3.1 in driving a pro-inflammatory microglia phenotype in ALS.

Application of diffusion tensor imaging in quantitatively monitoring chronic constriction injury of rabbit sciatic nerves: correlation with histological and functional changes

OBJECTIVE

To investigate the potential of diffusion tensor imaging (DTI) in quantitatively monitoring chronic constriction injuri (CCI) of sciatic nerves and to analyse the association of DTI parameters with nerve histology and limb function.

METHODS

CCI was created on sciatic nerves in the right hind legs of 20 rabbits with the left as control. DTI parameters-fractional anisotropy (FA), apparent diffusion coefficient (ADC), axial diffusivity (AD) and radial diffusivity (RD)-and limb function were longitudinally evaluated. Pathology analysis was performed on day 3 (d3), week 1 (w1), 2, 4, 6, 8 and 10.

RESULTS

FA of the constricted nerves decreased on d3 (0.316 ± 0.044) and increased from w1 to w10 (0.331 ± 0.018, 0.354 ± 0.044, 0.375 ± 0.015, 0.394 ± 0.020, 0.42 ± 0.03 and 0.464 ± 0.039). ADC increased on d3 until w2 (1.502 ± 0.126, 1.462 ± 0.058 and 1.473 ± 0.124 × 10^-3 mm² s^-1) and decreased to normal from w4 to w10 (1.356 ± 0.129, 1.375 ± 0.107, 1.290 ± 0.064 and 1.298 ± 0.026 × 10^-3 mm² s^-1). AD decreased and stayed low from d3 to w10 (2.042 ± 0.160, 2.005 ± 0.095, 2.057 ± 0.124, 1.952 ± 0.213, 1.988 ± 0.180, 1.947 ± 0.106 and 2.097 ± 0.114). RD increased on d3 (1.233 ± 0.152) and declined from w1 to w10 (1.19 ± 0.06, 1.181 ± 0.14, 1.071 ± 0.102, 1.068 ± 0.084, 0.961 ± 0.063 and 0.923 ± 0.058). FA, ADC and RD correlated significantly with limb functional scores (all Ps < 0.0001) and their changes were associated with histological changes.

CONCLUSION

FA, ADC and RD are promising to monitor CCI. AD may be a stable indicator for injury. Histological changes, oedema, axon loss and demyelination, and fibrosis, accompanied the changes of these parameters. Advances in knowledge: DTI parameters can detect and monitor acute and chronic changes after nerve compression.

Loss of SIRT2 leads to axonal degeneration and locomotor disability associated with redox and energy imbalance

Sirtuin 2 (SIRT2) is a member of a family of NAD⁺‐dependent histone deacetylases (HDAC) that play diverse roles in cellular metabolism and especially for aging process. SIRT2 is located in the nucleus, cytoplasm, and mitochondria, is highly expressed in the central nervous system (CNS), and has been reported to regulate a variety of processes including oxidative stress, genome integrity, and myelination. However, little is known about the role of SIRT2 in the nervous system specifically during aging. Here, we show that middle‐aged, 13‐month‐old mice lacking SIRT2 exhibit locomotor dysfunction due to axonal degeneration, which was not present in young SIRT2 mice. In addition, these Sirt2^−/− mice exhibit mitochondrial depletion resulting in energy failure, and redox dyshomeostasis. Our results provide a novel link between SIRT2 and physiological aging impacting the axonal compartment of the central nervous system, while supporting a major role for SIRT2 in orchestrating its metabolic regulation. This underscores the value of SIRT2 as a therapeutic target in the most prevalent neurodegenerative diseases that undergo with axonal degeneration associated with redox and energetic dyshomeostasis.

Molecular analysis of axonal-intrinsic and glial-associated co-regulation of axon degeneration

Wallerian degeneration is an active program tightly associated with axonal degeneration, required for axonal regeneration and functional recovery after nerve damage. Here we provide a functional molecular foundation for our undertstanding of the complex non-cell autonomous role of glial cells in the regulation of axonal degeneration. To shed light on the complexity of the molecular machinery governing axonal degeneration we employ a multi-model, unbiased, in vivo approach combining morphological assesment and quantitative proteomics with in silico-based higher order functional clustering to genetically uncouple the intrinsic and extrinsic processes governing Wallerian degeneration. Highlighting a pivotal role for glial cells in the early stages fragmenting the axon by a cytokinesis-like process and a cell autonomous stage of axonal disintegration associated to mitochondrial dysfunction.

Reversible Axonal Dystrophy by Calcium Modulation in Frataxin-Deficient Sensory Neurons of YG8R Mice

Friedreich’s ataxia (FRDA) is a peripheral neuropathy involving a loss of proprioceptive sensory neurons. Studies of biopsies from patients suggest that axonal dysfunction precedes the death of proprioceptive neurons in a dying-back process. We observed that the deficiency of frataxin in sensory neurons of dorsal root ganglia (DRG) of the YG8R mouse model causes the formation of axonal spheroids which retain dysfunctional mitochondria, shows alterations in the cytoskeleton and it produces impairment of axonal transport and autophagic flux. The homogenous distribution of axonal spheroids along the neurites supports the existence of continues focal damages. This lead us to propose for FRDA a model of distal axonopathy based on axonal focal damages. In addition, we observed the involvement of oxidative stress and dyshomeostasis of calcium in axonal spheroid formation generating axonal injury as a primary cause of pathophysiology. Axonal spheroids may be a consequence of calcium imbalance, thus we propose the quenching or removal extracellular Ca²⁺ to prevent spheroids formation. In our neuronal model, treatments with BAPTA and o-phenanthroline reverted the axonal dystrophy and the mitochondrial dysmorphic parameters. These results support the hypothesis that axonal pathology is reversible in FRDA by pharmacological manipulation of intracellular Ca²⁺ with Ca²⁺ chelators or metalloprotease inhibitors, preventing Ca²⁺-mediated axonal injury. Thus, the modulation of Ca²⁺ levels may be a relevant therapeutic target to develop early axonal protection and prevent dying-back neurodegeneration.

The Importance of Rare Subtypes in Diagnosis and Treatment of Peripheral Neuropathy: A Review

IMPORTANCE

Peripheral neuropathy is a prevalent condition that usually warrants a thorough history and examination but has limited diagnostic evaluation. However, rare localizations of peripheral neuropathy often require more extensive diagnostic testing and different treatments.

OBJECTIVE

To describe rare localizations of peripheral neuropathy, including the appropriate diagnostic evaluation and available treatments.

EVIDENCE REVIEW

References were identified from PubMed searches conducted on May 29, 2015, with an emphasis on systematic reviews and randomized clinical trials. Articles were also identified through the use of the authors' own files. Search terms included common rare neuropathy localizations and their causes, as well as epidemiology, pathophysiology, diagnosis, and treatment.

FINDINGS

Diffuse, nonlength-dependent neuropathies, multiple mononeuropathies, polyradiculopathies, plexopathies, and radiculoplexus neuropathies are rare peripheral neuropathy localizations that often require extensive diagnostic testing. Atypical neuropathy features, such as acute/subacute onset, asymmetry, and/or motor predominant signs, are frequently present. The most common diffuse, nonlength-dependent neuropathies are Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, and amyotrophic lateral sclerosis. Effective disease-modifying therapies exist for many diffuse, nonlength-dependent neuropathies including Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, and some paraprotein-associated demyelinating neuropathies. Vasculitic neuropathy (multiple mononeuropathy) also has efficacious treatment options, but definitive evidence of a treatment effect for IgM anti-MAG neuropathy and diabetic amyotrophy (radiculoplexus neuropathy) is lacking.

CONCLUSIONS AND RELEVANCE

Recognition of rare localizations of peripheral neuropathy is essential given the implications for diagnostic testing and treatment. Electrodiagnostic studies are an important early step in the diagnostic evaluation and provide information on the localization and pathophysiology of nerve injury.

Dysregulated axonal RNA translation in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult‐onset motor neuron disease that has been associated with a diverse array of genetic changes. Prominent among these are mutations in RNA‐binding proteins (RBPs) or repeat expansions that give rise to toxic RNA species. RBPs are additionally central components of pathologic aggregates that constitute a disease hallmark, suggesting that dysregulation of RNA metabolism underlies disease progression. In the context of neuronal physiology, transport of RNAs and localized RNA translation in axons are fundamental to neuronal survival and function. Several lines of evidence suggest that axonal RNA translation is a central process perturbed by various pathogenic events associated with ALS. Dysregulated translation of specific RNA groups could underlie feedback effects that connect and reinforce disease manifestations. Among such candidates are RNAs encoding proteins involved in the regulation of microtubule dynamics. Further understanding of axonally dysregulated RNA targets and of the feedback mechanisms they induce could provide useful therapeutic insights. WIREs RNA 2016, 7:589–603. doi: 10.1002/wrna.1352

For further resources related to this article, please visit the WIREs website.

The Role of Skeletal Muscle in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal adult-onset disease primarily characterized by upper and lower motor neuron degeneration, muscle wasting and paralysis. It is increasingly accepted that the pathological process leading to ALS is the result of multiple disease mechanisms that operate within motor neurons and other cell types both inside and outside the central nervous system. The implication of skeletal muscle has been the subject of a number of studies conducted on patients and related animal models. In this review, we describe the features of ALS muscle pathology and discuss on the contribution of muscle to the pathological process. We also give an overview of the therapeutic strategies proposed to alleviate muscle pathology or to deliver curative agents to motor neurons. ALS muscle mainly suffers from oxidative stress, mitochondrial dysfunction and bioenergetic disturbances. However, the way by which the disease affects different types of myofibers depends on their contractile and metabolic features. Although the implication of muscle in nourishing the degenerative process is still debated, there is compelling evidence suggesting that it may play a critical role. Detailed understanding of the muscle pathology in ALS could, therefore, lead to the identification of new therapeutic targets.

MicroRNAs as potential circulating biomarkers for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a condition primarily characterized by the selective loss of upper and lower motor neurons. Motor neuron loss gives rise to muscle tissue malfunctions, including weakness, spasticity, atrophy, and ultimately paralysis, with death typically due to respiratory failure within 2 to 5 years of symptoms' onset. The mean delay in time from presentation to diagnosis remains at over 1 year. Biomarkers are urgently needed to facilitate ALS diagnosis and prognosis as well as to act as indicators of therapeutic response in clinical trials. MicroRNAs (miRNAs) are small molecules that can influence posttranscriptional gene expression of a variety of transcript targets. Interestingly, miRNAs can be released into the circulation by pathologically affected tissues. This review presents therapeutic and diagnostic challenges associated with ALS, highlights the potential role of miRNAs in ALS, and discusses the diagnostic potential of these molecules in identifying ALS-specific miRNAs or in distinguishing between the various genotypic and phenotypic forms of ALS.

A role for the p75 neurotrophin receptor in axonal degeneration and apoptosis induced by oxidative stress

The p75 neurotrophin receptor (p75(NTR)) mediates the death of specific populations of neurons during the development of the nervous system or after cellular injury. The receptor has also been implicated as a contributor to neurodegeneration caused by numerous pathological conditions. Because many of these conditions are associated with increases in reactive oxygen species, we investigated whether p75(NTR) has a role in neurodegeneration in response to oxidative stress. Here we demonstrate that p75(NTR) signaling is activated by 4-hydroxynonenal (HNE), a lipid peroxidation product generated naturally during oxidative stress. Exposure of sympathetic neurons to HNE resulted in neurite degeneration and apoptosis. However, these effects were reduced markedly in neurons from p75(NTR-/-) mice. The neurodegenerative effects of HNE were not associated with production of neurotrophins and were unaffected by pretreatment with a receptor-blocking antibody, suggesting that oxidative stress activates p75(NTR) via a ligand-independent mechanism. Previous studies have established that proteolysis of p75(NTR) by the metalloprotease TNFα-converting enzyme and γ-secretase is necessary for p75(NTR)-mediated apoptotic signaling. Exposure of sympathetic neurons to HNE resulted in metalloprotease- and γ-secretase-dependent cleavage of p75(NTR). Pharmacological blockade of p75(NTR) proteolysis protected sympathetic neurons from HNE-induced neurite degeneration and apoptosis, suggesting that cleavage of p75(NTR) is necessary for oxidant-induced neurodegeneration. In vivo, p75(NTR-/-) mice exhibited resistance to axonal degeneration associated with oxidative injury following administration of the neurotoxin 6-hydroxydopamine. Together, these data suggest a novel mechanism linking oxidative stress to ligand-independent cleavage of p75(NTR), resulting in axonal fragmentation and neuronal death.

Role of mitochondria in mutant SOD1 linked amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with an adult onset characterized by loss of both upper and lower motor neurons. In ~10% of cases, patients developed ALS with an apparent genetic linkage (familial ALS or fALS). Approximately 20% of fALS displays mutations in the SOD1 gene encoding superoxide dismutase 1. There are many proposed cellular and molecular mechanisms among which, mitochondrial dysfunctions occur early, prior to symptoms occurrence. In this review, we modeled the effect of mutant SOD1 protein via the formation of a toxic complex with Bcl2 on mitochondrial bioenergetics. Furthermore, we discuss that the shutdown of ATP permeation through mitochondrial outer membrane could lead to both respiration inhibition and temporary mitochondrial hyperpolarization. Moreover, we reviewed mitochondrial calcium signaling, oxidative stress, fission and fusion, autophagy and apoptosis in mutant SOD1-linked ALS. Functional defects in mitochondria appear early before symptoms are manifested in ALS. Therefore, mitochondrial dysfunction is a promising therapeutic target in ALS.

Chronic proximal axonopathy in rats is associated with long-standing neurofilament depletion in neuromuscular junctions and behavioral deficits

In rodents exposed to 3,3'-iminodipropionitrile (IDPN), neurofilaments (NFs) accumulate in swollen proximal axon segments; this also occurs in motor neurons of patients with amyotrophic lateral sclerosis. We hypothesized that early loss of NFs in neuromuscular junctions (NMJs) in IDPN proximal neuropathy would result in neuromuscular dysfunction and lead to neuromuscular detachment. Adult male rats were given 0 or 15 mmol/L IDPN in drinking water for up to 1 year. The IDPN-exposed rats dragged their tails and had impaired endurance in a grip test. Neuromuscular junctions and distal axons were examined in the levator auris longus muscle after 3, 6, 9, and 12 months. Neuromuscular junctions showed a progressive reduction in NF immunolabeling, which became undetectable in up to 70% of the NMJs after 12 months. Neurofilament labeling was also reduced in preterminal axons and in a more proximal axon level within the muscle. Triple-label analysis with antisyntaxin demonstrated that the terminals remained in place and usually contained a few minute NF bundles. Electron microscopy revealed the disappearance of terminal NFs, reduced content in synaptic vesicles, and accumulation of multilamellar bodies, but scant degeneration. Thus, IDPN proximal neurofilamentous axonopathy is associated with NF depletion in motor terminals; motor weakness and structural changes in the NMJs suggest impaired synaptic function despite long-term preservation of the NMJs.

Blood biomarkers for amyotrophic lateral sclerosis: myth or reality?

Amyotrophic lateral sclerosis (ALS) is a fatal condition primarily characterized by the selective loss of upper and lower motor neurons. At present, the diagnosis and monitoring of ALS is based on clinical examination, electrophysiological findings, medical history, and exclusion of confounding disorders. There is therefore an undeniable need for molecular biomarkers that could give reliable information on the onset and progression of ALS in clinical practice and therapeutic trials. From a practical point of view, blood offers a series of advantages, including easy handling and multiple testing at a low cost, that make it an ideal source of biomarkers. In this review, we revisited the findings of many studies that investigated the presence of systemic changes at the molecular and cellular level in patients with ALS. The results of these studies reflect the diversity in the pathological mechanisms contributing to disease (e.g., excitotoxicity, oxidative stress, neuroinflammation, metabolic dysfunction, and neurodegeneration, among others) and provide relatively successful evidence of the usefulness of a wide-ranging panel of molecules as potential biomarkers. More studies, hopefully internationally coordinated, would be needed, however, to translate the application of these biomarkers into benefit for patients.

The role of oxidative stress in degeneration of the neuromuscular junction in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive loss of motoneurons and degradation of the neuromuscular junctions (NMJ). Consistent with the dying-back hypothesis of motoneuron degeneration the decline in synaptic function initiates from the presynaptic terminals in ALS. Oxidative stress is a major contributory factor to ALS pathology and affects the presynaptic transmitter releasing machinery. Indeed, in ALS mouse models nerve terminals are sensitive to reactive oxygen species (ROS) suggesting that oxidative stress, along with compromised mitochondria and increased intracellular Ca(2+) amplifies the presynaptic decline in NMJ. This initial dysfunction is followed by a neurodegeneration induced by inflammatory agents and loss of trophic support. To develop effective therapeutic approaches against ALS, it is important to identify the mechanisms underlying the initial pathological events. Given the role of oxidative stress in ALS, targeted antioxidant treatments could be a promising therapeutic approach. However, the complex nature of ALS and failure of monotherapies suggest that an antioxidant therapy should be accompanied by anti-inflammatory interventions to enhance the restoration of the redox balance.

Characterization of intercostal muscle pathology in canine degenerative myelopathy: a disease model for amyotrophic lateral sclerosis

Dogs homozygous for missense mutations in the SOD1 gene develop a late-onset neuromuscular disorder called degenerative myelopathy (DM) that has many similarities to amyotrophic lateral sclerosis (ALS). Both disorders are characterized by widespread progressive declines in motor functions, accompanied by atrophic changes in the descending spinal cord tracts. Some forms of ALS are also associated with SOD1 mutations. In end-stage ALS, death usually occurs as a result of respiratory failure from severe functional impairment of respiratory muscles. The mechanisms that lead to this loss of function are not known. Dogs with DM are euthanized at all stages of disease progression, providing an opportunity to characterize the onset and progression of any pathological changes in the respiratory muscles that may precede respiratory failure. To characterize such potential disease-related pathology, we evaluated intercostal muscles from Boxer and Pembroke Welsh Corgi dogs that were euthanized at various stages of DM disease progression. DM was found to result in intercostal muscle atrophy, fibrosis, increased variability in muscle fiber size and shape, and alteration in muscle fiber type composition. This pathology was not accompanied by retraction of the motor neuron terminals from the muscle acetylcholine receptor complexes, suggesting that the muscle atrophy did not result from physical denervation. These findings provide a better understanding of the mechanisms that likely lead to respiratory failure in at least some forms of ALS and will be useful in the development and evaluation of potential therapeutic interventions using the DM model.