Retrograde axonal transport and motor neuron disease

Role of Autophagy and Mitophagy in Neurodegenerative Disorders

Autophagy is a self-destructive cellular process that removes essential metabolites and waste from inside the cell to maintain cellular health. Mitophagy is the process by which autophagy causes disruption inside mitochondria and the total removal of damaged or stressed mitochondria, hence enhancing cellular health. The mitochondria are the powerhouses of the cell, performing essential functions such as ATP (adenosine triphosphate) generation, metabolism, Ca2+ buffering, and signal transduction. Many different mechanisms, including endosomal and autophagosomal transport, bring these substrates to lysosomes for processing. Autophagy and endocytic processes each have distinct compartments, and they interact dynamically with one another to complete digestion. Since mitophagy is essential for maintaining cellular health and using genetics, cell biology, and proteomics techniques, it is necessary to understand its beginning, particularly in ubiquitin and receptor-dependent signalling in injured mitochondria. Despite their similar symptoms and emerging genetic foundations, Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS) have all been linked to abnormalities in autophagy and endolysosomal pathways associated with neuronal dysfunction. Mitophagy is responsible for normal mitochondrial turnover and, under certain physiological or pathological situations, may drive the elimination of faulty mitochondria. Due to their high energy requirements and post-mitotic origin, neurons are especially susceptible to autophagic and mitochondrial malfunction. This article focused on the importance of autophagy and mitophagy in neurodegenerative illnesses and how they might be used to create novel therapeutic approaches for treating a wide range of neurological disorders.

Therapeutic Effects of Combination of Nebivolol and Donepezil: Targeting Multifactorial Mechanisms in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive loss of motor neurons in the spinal cord. Although the disease's pathophysiological mechanism remains poorly understood, multifactorial mechanisms affecting motor neuron loss converge to worsen the disease. Although two FDA-approved drugs, riluzole and edaravone, targeting excitotoxicity and oxidative stress, respectively, are available, their efficacies are limited to extending survival by only a few months. Here, we developed combinatorial drugs targeting multifactorial mechanisms underlying key components in ALS disease progression. Using data analysis based on the genetic information of patients with ALS-derived cells and pharmacogenomic data of the drugs, a combination of nebivolol and donepezil (nebivolol-donepezil) was identified for ALS therapy. Here, nebivolol-donepezil markedly reduced the levels of cytokines in the microglial cell line, inhibited nuclear factor-κB (NF-κB) nucleus translocation in the HeLa cell and substantially protected against excitotoxicity-induced neuronal loss by regulating the PI3K-Akt pathway. Nebivolol-donepezil significantly promoted the differentiation of neural progenitor cells (NPC) into motor neurons. Furthermore, we verified the low dose efficacy of nebivolol-donepezil on multiple indices corresponding to the quality of life of patients with ALS in vivo using SOD1G93A mice. Nebivolol-donepezil delayed motor function deterioration and halted motor neuronal loss in the spinal cord. Drug administration effectively suppressed muscle atrophy by mitigating the proportion of smaller myofibers and substantially reducing phospho-neurofilament heavy chain (pNF-H) levels in the serum, a promising ALS biomarker. High-dose nebivolol-donepezil significantly prolonged survival and delayed disease onset compared with vehicle-treated mice. These results indicate that the combination of nebivolol-donepezil efficiently prevents ALS disease progression, benefiting the patients' quality of life and life expectancy.

Kinesin-1 autoinhibition facilitates the initiation of dynein cargo transport

The functional significance of Kinesin-1 autoinhibition has been unclear. Kinesin-1 transports multiple cargoes including cytoplasmic dynein to microtubule plus ends. From a genetic screen for Aspergillus mutants defective in dynein-mediated early endosome transport, we identified a kinesin-1 mutation kinAK895* at the C-terminal IAK motif involved in autoinhibition. The kinA∆IAK and kinAK895E mutants exhibited a similar defect in dynein-mediated early endosome transport, verifying the importance of kinesin-1 autoinhibition in dynein-mediated transport. Kinesin-1 autoinhibition is not critical for dynein accumulation at microtubule plus ends or for the secretory vesicle cargoes of kinesin-1 to reach the hyphal tip. However, it facilitates dynein to initiate early endosome transport. This is unrelated to a direct competition between dynein and kinesin-1 on early endosomes because kinesin-3 rather than kinesin-1 drives the plus-end-directed early endosome movement. This effect of kinesin-1 autoinhibition on dynein-mediated early endosome transport is related to cargo adapter-mediated dynein activation but at a step beyond the switching of dynein from its autoinhibited conformation.

CNS Delivery of Nucleic Acid Therapeutics: Beyond the Blood-Brain Barrier and Towards Specific Cellular Targeting

Nucleic acid-based therapeutic molecules including small interfering RNA (siRNA), microRNA(miRNA), antisense oligonucleotides (ASOs), messenger RNA (mRNA), and DNA-based gene therapy have tremendous potential for treating diseases in the central nervous system (CNS). However, achieving clinically meaningful delivery to the brain and particularly to target cells and sub-cellular compartments is typically very challenging. Mediating cell-specific delivery in the CNS would be a crucial advance that mitigates off-target effects and toxicities. In this review, we describe these challenges and provide contemporary evidence of advances in cellular and sub-cellular delivery using a variety of delivery mechanisms and alternative routes of administration, including the nose-to-brain approach. Strategies to achieve subcellular localization, endosomal escape, cytosolic bioavailability, and nuclear transfer are also discussed. Ultimately, there are still many challenges to translating these experimental strategies into effective and clinically viable approaches for treating patients.

Kinesin-1, -2, and -3 motors use family-specific mechanochemical strategies to effectively compete with dynein during bidirectional transport

Bidirectional cargo transport in neurons requires competing activity of motors from the kinesin-1, -2, and -3 superfamilies against cytoplasmic dynein-1. Previous studies demonstrated that when kinesin-1 attached to dynein-dynactin-BicD2 (DDB) complex, the tethered motors move slowly with a slight plus-end bias, suggesting kinesin-1 overpowers DDB but DDB generates a substantial hindering load. Compared to kinesin-1, motors from the kinesin-2 and -3 families display a higher sensitivity to load in single-molecule assays and are thus predicted to be overpowered by dynein complexes in cargo transport. To test this prediction, we used a DNA scaffold to pair DDB with members of the kinesin-1, -2, and -3 families to recreate bidirectional transport in vitro, and tracked the motor pairs using two-channel TIRF microscopy. Unexpectedly, we find that when both kinesin and dynein are engaged and stepping on the microtubule, kinesin-1, -2, and -3 motors are able to effectively withstand hindering loads generated by DDB. Stochastic stepping simulations reveal that kinesin-2 and -3 motors compensate for their faster detachment rates under load with faster reattachment kinetics. The similar performance between the three kinesin transport families highlights how motor kinetics play critical roles in balancing forces between kinesin and dynein, and emphasizes the importance of motor regulation by cargo adaptors, regulatory proteins, and the microtubule track for tuning the speed and directionality of cargo transport in cells.

Calcium-Mediated Calpain Activation and Microtubule Dissociation in Cell Model of Hereditary Sensory Neuropathy Type-1 Expressing V144D SPTLC1 Mutation

Hereditary sensory neuropathy type 1A (HSN1A) is an autosomal, dominantly inherited peripheral neuropathy caused by mutations in serine palmitoyl transferase long chain 1 (SPTLC1), involved in the de novo synthesis of sphingolipids. We have previously reported calcium imbalance, as well as mitochondrial and ER stress in both HSN1 patient lymphoblasts and a transiently transfected cell model. In this study, we investigated the role of the Ca2+-activated protease calpain in destabilizing the cell cytoskeleton, by examining calpain activity in SH-SY5Y cells overexpressing the V144D mutant and changes in microtubule-associated proteins (MAP). Intramitochondrial Ca2+ was found to be significantly depleted and cytoplasmic Ca2+ increased in the V144D mutant. Subsequently, calpain and proteasome activity were increased and calpain substrates, microtubule associated proteins MAP2, and tau were significantly reduced in the microtubule fraction of the mutant. Significant changes were also found in motor proteins dynein and KIF2A detected in the microtubule fraction of cells overexpressing the V144D mutation. There was also a reduction in anterograde and retrograde mitochondrial transport velocities in the V144D mutant. These findings strongly implicate cytoskeletal aberration caused by Ca2+ dysregulation and subsequent loss of microtubule transport functions as the cause of axonal dying back that is characteristic of HSN1.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Clearance of peripheral nerve misfolded mutant protein by infiltrated macrophages correlates with motor neuron disease progression

Macrophages expressing C-C chemokine receptor type 2 (CCR2) infiltrate the central and peripheral neural tissues of amyotrophic lateral sclerosis (ALS) patients. To identify the functional role of CCR2⁺ macrophages in the pathomechanisms of ALS, we used an ALS animal model, mutant Cu/Zn superoxide dismutase 1^G93A (mSOD1)-transgenic (Tg) mice. To clarify the CCR2 function in the model, we generated SOD1^G93A/CCR2^{Red fluorescence protein (RFP)/Wild type (WT)}/CX3CR1^{Green fluorescence protein (GFP)/WT}-Tg mice, which heterozygously express CCR2-RFP and CX3CR1-GFP, and SOD1^G93A/CCR2^RFP/RFP-Tg mice, which lack CCR2 protein expression and present with a CCR2-deficient phenotype. In mSOD1-Tg mice, mSOD1 accumulated in the sciatic nerve earlier than in the spinal cord. Furthermore, spinal cords of SOD1^G93A/CCR2^RFP/WT/CX3CR1^GFP/WT mice showed peripheral macrophage infiltration that emerged at the end-stage, whereas in peripheral nerves, macrophage infiltration started from the pre-symptomatic stage. Before disease onset, CCR2⁺ macrophages harboring mSOD1 infiltrated sciatic nerves earlier than the lumbar cord. CCR2-deficient mSOD1-Tg mice showed an earlier onset and axonal derangement in the sciatic nerve than CCR2-positive mSOD1-Tg mice. CCR2-deficient mSOD1-Tg mice showed an increase in deposited mSOD1 in the sciatic nerve compared with CCR2-positive mice. These findings suggest that CCR2⁺ and CX3CR1⁺ macrophages exert neuroprotective functions in mSOD1 ALS via mSOD1 clearance from the peripheral nerves.

Cell-Clearing Systems Bridging Repeat Expansion Proteotoxicity and Neuromuscular Junction Alterations in ALS and SBMA

The coordinated activities of autophagy and the ubiquitin proteasome system (UPS) are key to preventing the aggregation and toxicity of misfold-prone proteins which manifest in a number of neurodegenerative disorders. These include proteins which are encoded by genes containing nucleotide repeat expansions. In the present review we focus on the overlapping role of autophagy and the UPS in repeat expansion proteotoxicity associated with chromosome 9 open reading frame 72 (C9ORF72) and androgen receptor (AR) genes, which are implicated in two motor neuron disorders, amyotrophic lateral sclerosis (ALS) and spinal-bulbar muscular atrophy (SBMA), respectively. At baseline, both C9ORF72 and AR regulate autophagy, while their aberrantly-expanded isoforms may lead to a failure in both autophagy and the UPS, further promoting protein aggregation and toxicity within motor neurons and skeletal muscles. Besides proteotoxicity, autophagy and UPS alterations are also implicated in neuromuscular junction (NMJ) alterations, which occur early in both ALS and SBMA. In fact, autophagy and the UPS intermingle with endocytic/secretory pathways to regulate axonal homeostasis and neurotransmission by interacting with key proteins which operate at the NMJ, such as agrin, acetylcholine receptors (AChRs), and adrenergic beta2 receptors (B2-ARs). Thus, alterations of autophagy and the UPS configure as a common hallmark in both ALS and SBMA disease progression. The findings here discussed may contribute to disclosing overlapping molecular mechanisms which are associated with a failure in cell-clearing systems in ALS and SBMA.

Proteostasis is essential during cochlear development for neuron survival and hair cell polarity

Protein homeostasis is essential to cell function, and a compromised ability to reduce the load of misfolded and aggregated proteins is linked to numerous age-related diseases, including hearing loss. Here, we show that altered proteostasis consequent to Elongator complex deficiency also impacts the proper development of the cochlea and results in deafness. In the absence of the catalytic subunit Elp3, differentiating spiral ganglion neurons display large aggresome-like structures and undergo apoptosis before birth. The cochlear mechanosensory cells are able to survive proteostasis disruption but suffer defects in polarity and stereociliary bundle morphogenesis. We demonstrate that protein aggregates accumulate at the apical surface of hair cells, where they cause a local slowdown of microtubular trafficking, altering the distribution of intrinsic polarity proteins and affecting kinocilium position and length. Alleviation of protein misfolding using the chemical chaperone 4-phenylbutyric acid during embryonic development ameliorates hair cell polarity in Elp3-deficient animals. Our study highlights the importance of developmental proteostasis in the cochlea and unveils an unexpected link between proteome integrity and polarized organization of cellular components.

Consideration of gravity as a possible etiological factor in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with an unknown mechanism of onset that predominantly impairs the upper and lower motor neurons. Components of the sensory and autonomic nervous system were once thought to be spared in the disease, but more recently they have been identified to be impaired at various levels. However, some of the motor neurons such as oculomotor, abducens, or pudendal nerves are spared even in the later stages of ALS. The mechanism of such complex and heterogeneous neuronal loss in typical ALS is still unknown. In this study, the characteristics of the nervous system involved in the pathogenesis of ALS were comprehensively reviewed. As a result, the direction of the axon in the anatomical position, rather than the functional type or length of the axon, was suggested to contribute the most to the onset of ALS. This finding suggested that downward directed axons, represented by motor neurons, require extra energy to move waste or unnecessary substances from the synapse side to the neural cell body with retrograde fast axonal transport. Based on this theory, the extra energy that is required in vertically directed axons due to the effect of gravity was mathematically estimated. As a result, several percent more adenosine triphosphate molecules were suggested to be consumed in vertical axonal transport by gravity, compared to those consumed in transverse axonal transport. Because most of the motor neurons project downward in the anatomical position, unretrieved waste will gradually sediment in axon terminals by gravity, which could eventually result in motor neuron-dominant neuronal loss. Although the theory that gravity is one of the mechanisms responsible for ALS is still hypothetical, it is theoretically reasonable and compatible with the clinical manifestations of the disease. Further basic research studies with cultured neurons or animal models are necessary to confirm the association between gravity and the onset of ALS.

Autophagy as a common pathway in amyotrophic lateral sclerosis

Age-dependent neurodegenerative diseases are associated with a decline in protein quality control systems including autophagy. Amyotrophic lateral sclerosis (ALS) is a motor neuron degenerative disease of complex etiology with increasing connections to other neurodegenerative conditions such as frontotemporal dementia. Among the diverse genetic causes for ALS, a striking feature is the common connection to autophagy and its associated pathways. There is a recurring theme of protein misfolding as in other neurodegenerative diseases, but importantly there is a distinct common thread among ALS genes that connects them to the cascade of autophagy. However, the roles of autophagy in ALS remain enigmatic and it is still unclear whether activation or inhibition of autophagy would be a reliable avenue to ameliorate the disease. The main evidence that links autophagy to different genetic forms of ALS is discussed.

scAAV9-VEGF-165 inhibits neuroinflammatory responses and invasion of macrophages into the peripheral nervous system of ALS transgenic mice

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder that leads to paralysis and death within 3-5 years. Although the vast majority of studies have focused on vulnerable neurons, growing evidence has shown that non-neuronal cells contribute to the pathogenesis and disease progression. Here, we showed that intrathecal injection of scAAV9-VEGF at 60 days of age significantly reduced the number of microglia and inhibited the neuroinflammatory response in the CNS. Meanwhile, we found that administration of VEGF inhibited the invasion of macrophages into the PNS, including ventral nerve roots, sciatic nerves and muscles. Overall, our study indicated the anti-inflammation effect of VEGF in the CNS and PNS of ALS mice when delivered by intrathecal injection.

The where, what, and when of membrane protein degradation in neurons

Membrane protein turnover and degradation are required for the function and health of all cells. Neurons may live for the entire lifetime of an organism and are highly polarized cells with spatially segregated axonal and dendritic compartments. Both longevity and morphological complexity represent challenges for regulated membrane protein degradation. To investigate how neurons cope with these challenges, an increasing number of recent studies investigated local, cargo-specific protein sorting, and degradation at axon terminals and in dendritic processes. In this review, we explore the current answers to the ensuing questions of where, what, and when membrane proteins are degraded in neurons. © 2017 The Authors Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 283-297, 2018.

Mechanical coupling of microtubule-dependent motor teams during peroxisome transport in Drosophila S2 cells

BACKGROUND

Intracellular transport requires molecular motors that step along cytoskeletal filaments actively dragging cargoes through the crowded cytoplasm. Here, we explore the interplay of the opposed polarity motors kinesin-1 and cytoplasmic dynein during peroxisome transport along microtubules in Drosophila S2 cells.

METHODS

We used single particle tracking with nanometer accuracy and millisecond time resolution to extract quantitative information on the bidirectional motion of organelles. The transport performance was studied in cells expressing a slow chimeric plus-end directed motor or the kinesin heavy chain. We also analyzed the influence of peroxisomes membrane fluidity in methyl-β-ciclodextrin treated cells. The experimental data was also confronted with numerical simulations of two well-established tug of war scenarios.

RESULTS AND CONCLUSIONS

The velocity distributions of retrograde and anterograde peroxisomes showed a multimodal pattern suggesting that multiple motor teams drive transport in either direction. The chimeric motors interfered with the performance of anterograde transport and also reduced the speed of the slowest retrograde team. In addition, increasing the fluidity of peroxisomes membrane decreased the speed of the slowest anterograde and retrograde teams.

GENERAL SIGNIFICANCE

Our results support the existence of a crosstalk between opposed-polarity motor teams. Moreover, the slowest teams seem to mechanically communicate with each other through the membrane to trigger transport.

Deleterious variants in TRAK1 disrupt mitochondrial movement and cause fatal encephalopathy

Cellular distribution and dynamics of mitochondria are regulated by several motor proteins and a microtubule network. In neurons, mitochondrial trafficking is crucial because of high energy needs and calcium ion buffering along axons to synapses during neurotransmission. The trafficking kinesin proteins (TRAKs) are well characterized for their role in lysosomal and mitochondrial trafficking in cells, especially neurons. Using whole exome sequencing, we identified homozygous truncating variants in TRAK1 (NM_001042646:c.287-2A > C), in six lethal encephalopathic patients from three unrelated families. The pathogenic variant results in aberrant splicing and significantly reduced gene expression at the RNA and protein levels. In comparison with normal cells, TRAK1-deficient fibroblasts showed irregular mitochondrial distribution, altered mitochondrial motility, reduced mitochondrial membrane potential, and diminished mitochondrial respiration. This study confirms the role of TRAK1 in mitochondrial dynamics and constitutes the first report of this gene in association with a severe neurodevelopmental disorder.

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Amyotrophic lateral sclerosis (ALS) is an aggressive multifactorial disease converging on a common pathology: the degeneration of motor neurons (MNs), their axons and neuromuscular synapses. This vulnerability and dysfunction of MNs highlights the dependency of these large cells on their intracellular machinery. Neuronal microtubules (MTs) are intracellular structures that facilitate a myriad of vital neuronal functions, including activity dependent axonal transport. In ALS, it is becoming increasingly apparent that MTs are likely to be a critical component of this disease. Not only are disruptions in this intracellular machinery present in the vast majority of seemingly sporadic cases, recent research has revealed that mutation to a microtubule protein, the tubulin isoform TUBA4A, is sufficient to cause a familial, albeit rare, form of disease. In both sporadic and familial disease, studies have provided evidence that microtubule mediated deficits in axonal transport are the tipping point for MN survivability. Axonal transport deficits would lead to abnormal mitochondrial recycling, decreased vesicle and mRNA transport and limited signaling of key survival factors from the neurons peripheral synapses, causing the characteristic peripheral "die back". This disruption to microtubule dependant transport in ALS has been shown to result from alterations in the phenomenon of microtubule dynamic instability: the rapid growth and shrinkage of microtubule polymers. This is accomplished primarily due to aberrant alterations to microtubule associated proteins (MAPs) that regulate microtubule stability. Indeed, the current literature would argue that microtubule stability, particularly alterations in their dynamics, may be the initial driving force behind many familial and sporadic insults in ALS. Pharmacological stabilization of the microtubule network offers an attractive therapeutic strategy in ALS; indeed it has shown promise in many neurological disorders, ALS included. However, the pathophysiological involvement of MTs and their functions is still poorly understood in ALS. Future investigations will hopefully uncover further therapeutic targets that may aid in combating this awful disease.

Amyloid precursor protein-mediated endocytic pathway disruption induces axonal dysfunction and neurodegeneration

The endosome/lysosome pathway is disrupted early in the course of both Alzheimer's disease (AD) and Down syndrome (DS); however, it is not clear how dysfunction in this pathway influences the development of these diseases. Herein, we explored the cellular and molecular mechanisms by which endosomal dysfunction contributes to the pathogenesis of AD and DS. We determined that full-length amyloid precursor protein (APP) and its β-C-terminal fragment (β-CTF) act though increased activation of Rab5 to cause enlargement of early endosomes and to disrupt retrograde axonal trafficking of nerve growth factor (NGF) signals. The functional impacts of APP and its various products were investigated in PC12 cells, cultured rat basal forebrain cholinergic neurons (BFCNs), and BFCNs from a mouse model of DS. We found that the full-length wild-type APP (APPWT) and β-CTF both induced endosomal enlargement and disrupted NGF signaling and axonal trafficking. β-CTF alone induced atrophy of BFCNs that was rescued by the dominant-negative Rab5 mutant, Rab5S34N. Moreover, expression of a dominant-negative Rab5 construct markedly reduced APP-induced axonal blockage in Drosophila. Therefore, increased APP and/or β-CTF impact the endocytic pathway to disrupt NGF trafficking and signaling, resulting in trophic deficits in BFCNs. Our data strongly support the emerging concept that dysregulation of Rab5 activity contributes importantly to early pathogenesis of AD and DS.

The cytoskeleton as a novel therapeutic target for old neurodegenerative disorders

Cytoskeleton defects, including alterations in microtubule stability, in axonal transport as well as in actin dynamics, have been characterized in several unrelated neurodegenerative conditions. These observations suggest that defects of cytoskeleton organization may be a common feature contributing to neurodegeneration. In line with this hypothesis, drugs targeting the cytoskeleton are currently being tested in animal models and in human clinical trials, showing promising effects. Drugs that modulate microtubule stability, inhibitors of posttranslational modifications of cytoskeletal components, specifically compounds affecting the levels of tubulin acetylation, and compounds targeting signaling molecules which regulate cytoskeleton dynamics, constitute the mostly addressed therapeutic interventions aiming at preventing cytoskeleton damage in neurodegenerative disorders. In this review, we will discuss in a critical perspective the current knowledge on cytoskeleton damage pathways as well as therapeutic strategies designed to revert cytoskeleton-related defects mainly focusing on the following neurodegenerative disorders: Alzheimer's Disease, Parkinson's Disease, Huntington's Disease, Amyotrophic Lateral Sclerosis and Charcot-Marie-Tooth Disease.

Adipose-derived Stem Cell Conditioned Media Extends Survival time of a mouse model of Amyotrophic Lateral Sclerosis

Adipose stromal cells (ASC) secrete various trophic factors that assist in the protection of neurons in a variety of neuronal death models. In this study, we tested the effects of human ASC conditional medium (ASC-CM) in human amyotrophic lateral sclerosis (ALS) transgenic mouse model expressing mutant superoxide dismutase (SOD1^G93A). Treating symptomatic SOD1^G93A mice with ASC-CM significantly increased post-onset survival time and lifespan. Moreover, SOD1^G93A mice given ASC-CM treatment showed high motor neuron counts, less activation of microglia and astrocytes at an early symptomatic stage in the spinal cords under immunohistochemical analysis. SOD1^G93A mice treated with ASC-CM for 7 days showed reduced levels of phosphorylated p38 (pp38) in the spinal cord, a mitogen-activated protein kinase that is involved in both inflammation and neuronal death. Additionally, the levels of α-II spectrin in spinal cords were also inhibited in SOD1^G93A mice treated with ASC-CM for 3 days. Interestingly, nerve growth factor (NGF), a neurotrophic factor found in ASC-CM, played a significant role in the protection of neurodegeneration inSOD1^G93A mouse. These results indicate that ASC-CM has the potential to develop into a novel and effective therapeutic treatment for ALS.

Bidirectional cargo transport: moving beyond tug of war

Vesicles, organelles and other intracellular cargo are transported by kinesin and dynein motors, which move in opposite directions along microtubules. This bidirectional cargo movement is frequently described as a 'tug of war' between oppositely directed molecular motors attached to the same cargo. However, although many experimental and modelling studies support the tug-of-war paradigm, numerous knockout and inhibition studies in various systems have found that inhibiting one motor leads to diminished motility in both directions, which is a 'paradox of co-dependence' that challenges the paradigm. In an effort to resolve this paradox, three classes of bidirectional transport models--microtubule tethering, mechanical activation and steric disinhibition--are proposed, and a general mathematical modelling framework for bidirectional cargo transport is put forward to guide future experiments.

Cytoplasmic dynein: a key player in neurodegenerative and neurodevelopmental diseases

Cytoplasmic dynein is the most important molecular motor driving the movement of a wide range of cargoes towards the minus ends of microtubules. As a molecular motor protein, dynein performs a variety of basic cellular functions including organelle transport and centrosome assembly. In the nervous system, dynein has been demonstrated to be responsible for axonal retrograde transport. Many studies have revealed direct or indirect evidence of dynein in neurodegenerative diseases such as amyotrophic lateral sclerosis, Charcot-Marie-Tooth disease, Alzheimer's disease, Parkinson's disease and Huntington's disease. Among them, a number of mutant proteins involved in various neurodegenerative diseases interact with dynein. Axonal transport disruption is presented as a common feature occurring in neurodegenerative diseases. Dynein heavy chain mutant mice also show features of neurodegenerative diseases. Moreover, defects of dynein-dependent processes such as autophagy or clearance of aggregation-prone proteins are found in most of these diseases. Lines of evidence have also shown that dynein is associated with neurodevelopmental diseases. In this review, we focus on dynein involvement in different neurological diseases and discuss potential underlying mechanisms.

Abnormal mitochondrial transport and morphology are common pathological denominators in SOD1 and TDP43 ALS mouse models

Neuronal mitochondrial morphology abnormalities occur in models of familial amyotrophic lateral sclerosis (ALS) associated with SOD1 and TDP43 mutations. These abnormalities have been linked to mitochondrial axonal transport defects, but the temporal and spatial relationship between mitochondrial morphology and transport alterations in these two distinct genetic forms of ALS has not been investigated in vivo. To address this question, we crossed SOD1 (wild-type SOD1(WT) and mutant SOD1(G93A)) or TDP43 (mutant TDP43(A315T)) transgenic mice with mice expressing the fluorescent protein Dendra targeted to mitochondria in neurons (mitoDendra). At different time points during the disease course, we studied mitochondrial transport in the intact sciatic nerve of living mice and analyzed axonal mitochondrial morphology at multiple sites, spanning from the spinal cord to the motor terminals. Defects of retrograde mitochondrial transport were detected at 45 days of age, before the onset of symptoms, in SOD1(G93A) and TDP43(A315T) mice, but not in SOD1(WT). At later disease stages, also anterograde mitochondrial transport was affected in both mutant mouse lines. In SOD1(G93A) mice, mitochondrial morphological abnormalities were apparent at 15 days of age, thus preceding transport abnormalities. Conversely, in TDP43(A315T) mice, morphological abnormalities appeared after the onset of transport defects. Taken together, these findings demonstrate that neuronal mitochondrial transport and morphology abnormalities occur in vivo and that they are common denominators of different genetic forms of the ALS. At the same time, differences in the temporal and spatial manifestation of mitochondrial abnormalities between the two mouse models of familial ALS imply that different molecular mechanisms may be involved.

Inhibition of fast axonal transport by pathogenic SOD1 involves activation of p38 MAP kinase

Dying-back degeneration of motor neuron axons represents an established feature of familial amyotrophic lateral sclerosis (FALS) associated with superoxide dismutase 1 (SOD1) mutations, but axon-autonomous effects of pathogenic SOD1 remained undefined. Characteristics of motor neurons affected in FALS include abnormal kinase activation, aberrant neurofilament phosphorylation, and fast axonal transport (FAT) deficits, but functional relationships among these pathogenic events were unclear. Experiments in isolated squid axoplasm reveal that FALS-related SOD1 mutant polypeptides inhibit FAT through a mechanism involving a p38 mitogen activated protein kinase pathway. Mutant SOD1 activated neuronal p38 in mouse spinal cord, neuroblastoma cells and squid axoplasm. Active p38 MAP kinase phosphorylated kinesin-1, and this phosphorylation event inhibited kinesin-1. Finally, vesicle motility assays revealed previously unrecognized, isoform-specific effects of p38 on FAT. Axon-autonomous activation of the p38 pathway represents a novel gain of toxic function for FALS-linked SOD1 proteins consistent with the dying-back pattern of neurodegeneration characteristic of ALS.

α -Synuclein Modification in an ALS Animal Model

Amyotrophic lateral sclerosis (ALS) is a progressively paralytic neurodegenerative disease that can be caused by mutations in Cu/Zn-superoxide dismutase 1 (SOD1). Transgenic mice that overexpress mutant SOD1 develop paralysis and accumulate aggregates of mutant protein in the brainstem and spinal cord. Bee venom (BV), which is also known as apitoxin, is extracted from honeybees and is commonly used in oriental medicine for the treatment of chronic rheumatoid arthritis and osteoarthritis. The purpose of the present study was to determine whether BV affects misfolded protein aggregates such as alpha-synuclein, which is a known pathological marker in Parkinson disease, and ubiquitin-proteasomal activity in hSOD1^G93A mutant mice. BV was bilaterally administered into a 98-day-old hSOD1^G93A animal model. We found that BV-treated hSOD1^G93A transgenic mice showed reduced detergent-insoluble polymerization and phosphorylation of α-synuclein. Furthermore, phosphorylated or nitrated α-synuclein was significantly reduced in the spinal cords and brainstems of BV-treated hSOD1^G93A mice and reduced proteasomal activity was revealed in the brainstems of BV-treated symptomatic hSOD1^G93A. From these findings, we suggest that BV treatment attenuates the dysfunction of the ubiquitin-proteasomal system in a symptomatic hSOD1^G93A ALS model and may help to slow motor neuron loss caused by misfolded protein aggregates in ALS models.

A compartmental model of neuropeptide circulation and capture between the axon soma and nerve terminals

This paper is motivated by a recent experimental research by Levitan and colleagues (Wong MY, Zhou C, Shakiryanova D, Lloyd TE, Deitcher DL, Levitan ES. Neuropeptide delivery to synapses by long-range vesicle circulation and sporadic capture. Cell 2012; 148(5): 1029-1038), which discovered and explained the circulation of dense core vesicles (DCVs) between the neuron soma and nerve terminals. We developed a compartmental mechanistic model to simulate this circulation. The model includes five compartments, the axonal compartment and four en passant boutons, which are small axonal varicosities that are present in many axon terminals. By postulating expressions for DCV fluxes between the compartments and for the rates of DCV capture in the boutons and utilizing conservation of DCVs, ODEs modeling concentrations in each of the compartments were developed. The equations were then solved numerically. The obtained results provide insight into how DCV circulation develops, what the circulation is at steady state, and how it may be affected by defects in retrograde transport.

A novel phenotype for the dynein heavy chain mutation Loa: altered dendritic morphology, organelle density, and reduced numbers of trigeminal motoneurons

Dynein, the retrograde motor protein, is essential for the transport of cargo along axons and proximal dendrites in neurons. The dynein heavy chain mutation Loa has been reported to cause degeneration of spinal motor neurons, as well as defects of spinal sensory proprioceptive neurons, but cranial nerve nuclei have received little attention. Here, we examined the number and morphology of neurons in cranial nerve nuclei of young, adult, and aged heterozygous Loa mice, with a focus on the trigeminal, facial, and trochlear motor nuclei, as well as the proprioceptive mesencephalic trigeminal nucleus. By using stereological counting techniques, we report a slowly progressive and significant reduction, to 75% of wild-type controls, in the number of large trigeminal motoneurons, whereas normal numbers were found for sensory mesencephalic trigeminal, facial, and trochlear motoneurons. The morphology of many surviving large trigeminal motoneurons was substantially altered, in particular the size and length of perpendicularly extending primary dendrites, but not those of facial or trochlear motoneurons. At the ultrastructural level, proximal dendrites of large trigeminal motoneurons, but not other neurons, were significantly depleted in organelle content such as polyribosomes and showed abnormal (vesiculated) mitochondria. These data indicate primary defects in trigeminal α-motoneurons more than γ-motoneurons. Our findings expand the Loa heterozygote phenotype in two important ways: we reveal dendritic in addition to axonal defects or abnormalities, and we identify the Loa mutation as a mouse model for mixed motor-sensory loss when the entire neuraxis is considered, rather than a model primarily for sensory loss.

Dysregulation of the autophagy-endolysosomal system in amyotrophic lateral sclerosis and related motor neuron diseases

Amyotrophic lateral sclerosis (ALS) is a heterogeneous group of incurable motor neuron diseases (MNDs) characterized by a selective loss of upper and lower motor neurons in the brain and spinal cord. Most cases of ALS are sporadic, while approximately 5–10% cases are familial. More than 16 causative genes for ALS/MNDs have been identified and their underlying pathogenesis, including oxidative stress, endoplasmic reticulum stress, excitotoxicity, mitochondrial dysfunction, neural inflammation, protein misfolding and accumulation, dysfunctional intracellular trafficking, abnormal RNA processing, and noncell-autonomous damage, has begun to emerge. It is currently believed that a complex interplay of multiple toxicity pathways is implicated in disease onset and progression. Among such mechanisms, ones that are associated with disturbances of protein homeostasis, the ubiquitin-proteasome system and autophagy, have recently been highlighted. Although it remains to be determined whether disease-associated protein aggregates have a toxic or protective role in the pathogenesis, the formation of them results from the imbalance between generation and degradation of misfolded proteins within neuronal cells. In this paper, we focus on the autophagy-lysosomal and endocytic degradation systems and implication of their dysfunction to the pathogenesis of ALS/MNDs. The autophagy-endolysosomal pathway could be a major target for the development of therapeutic agents for ALS/MNDs.

Neuropeptide delivery to synapses by long-range vesicle circulation and sporadic capture

Neurotransmission requires anterograde axonal transport of dense core vesicles (DCVs) containing neuropeptides and active zone components from the soma to nerve terminals. However, it is puzzling how one-way traffic could uniformly supply sequential release sites called en passant boutons. Here, Drosophila neuropeptide-containing DCVs are tracked in vivo for minutes with a new method called simultaneous photobleaching and imaging (SPAIM). Surprisingly, anterograde DCVs typically bypass proximal boutons to accumulate initially in the most distal bouton. Then, excess distal DCVs undergo dynactin-dependent retrograde transport back through proximal boutons into the axon. Just before re-entering the soma, DCVs again reverse for another round of anterograde axonal transport. While circulating over long distances, both anterograde and retrograde DCVs are captured sporadically in en passant boutons. Therefore, vesicle circulation, which includes long-range retrograde transport and inefficient bidirectional capture, overcomes the limitations of one-way anterograde transport to uniformly supply release sites with DCVs.

Neurotrophin signaling via long-distance axonal transport

Neurotrophins are a family of target-derived growth factors that support survival, development, and maintenance of innervating neurons. Owing to the unique architecture of neurons, neurotrophins that act locally on the axonal terminals must convey their signals across the entire axon for subsequent regulation of gene transcription in the cell nucleus. This long-distance retrograde signaling, a motor-driven process that can take hours or days, has been a subject of intense interest. In the last decade, live-cell imaging with high sensitivity has significantly increased our capability to track the transport of neurotrophins, their receptors, and subsequent signals in real time. This review summarizes recent research progress in understanding neurotrophin-receptor interactions at the axonal terminal and their transport dynamics along the axon. We emphasize high-resolution studies at the single-molecule level and also discuss recent technical advances in the field.

Cargo distributions differentiate pathological axonal transport impairments

Axonal transport is an essential process in neurons, analogous to shipping goods, by which energetic and cellular building supplies are carried downstream (anterogradely) and wastes are carried upstream (retrogradely) by molecular motors, which act as cargo porters. Impairments in axonal transport have been linked to devastating and often lethal neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis, Huntington's, and Alzheimer's. Axonal transport impairment types include a decrease in available motors for cargo transport (motor depletion), the presence of defective or non-functional motors (motor dilution), and the presence of increased or larger cargos (protein aggregation). An impediment to potential treatment identification has been the inability to determine what type(s) of axonal transport impairment candidates that could be present in a given disease. In this study, we utilize a computational model and common axonal transport experimental metrics to reveal the axonal transport impairment general characteristics or "signatures" that result from three general defect types of motor depletion, motor dilution, and protein aggregation. Our results not only provide a means to discern these general impairments types, they also reveal key dynamic and emergent features of axonal transport, which potentially underlie multiple impairment types. The identified characteristics, as well as the analytical method, can be used to help elucidate the axonal transport impairments observed in experimental and clinical data. For example, using the model-predicted defect signatures, we identify the defect candidates, which are most likely to be responsible for the axonal transport impairments in the G93A SOD1 mouse model of ALS.

In vivo pathogenic role of mutant SOD1 localized in the mitochondrial intermembrane space

Mutations in Cu,Zn superoxide dismutase (SOD1) are associated with familial amyotrophic lateral sclerosis (ALS). Mutant SOD1 causes a complex array of pathological events, through toxic gain of function mechanisms, leading to selective motor neuron degeneration. Mitochondrial dysfunction is among the well established toxic effects of mutant SOD1, but its mechanisms are just starting to be elucidated. A portion of mutant SOD1 is localized in mitochondria, where it accumulates mostly on the outer membrane and inside the intermembrane space (IMS). Evidence in cultured cells suggests that mutant SOD1 in the IMS causes mitochondrial dysfunction and compromises cell viability. Therefore, to test its pathogenic role in vivo we generated transgenic mice expressing G93A mutant or wild-type (WT) human SOD1 targeted selectively to the mitochondrial IMS (mito-SOD1). We show that mito-SOD1 is correctly localized in the IMS, where it oligomerizes and acquires enzymatic activity. Mito-G93ASOD1 mice, but not mito-WTSOD1 mice, develop a progressive disease characterized by body weight loss, muscle weakness, brain atrophy, and motor impairment, which is more severe in females. These symptoms are associated with reduced spinal motor neuron counts and impaired mitochondrial bioenergetics, characterized by decreased cytochrome oxidase activity and defective calcium handling. However, there is no evidence of muscle denervation, a cardinal pathological feature of ALS. Together, our findings indicate that mutant SOD1 in the mitochondrial IMS causes mitochondrial dysfunction and neurodegeneration, but per se it is not sufficient to cause a full-fledged ALS phenotype, which requires the participation of mutant SOD1 localized in other cellular compartments.

A high-throughput screening method for small-molecule inhibitors of the aberrant mutant SOD1 and dynein complex interaction

Aberrant protein-protein interactions are attractive drug targets in a variety of neurodegenerative diseases due to the common pathology of accumulation of protein aggregates. In amyotrophic lateral sclerosis, mutations in SOD1 cause the formation of aggregates and inclusions that may sequester other proteins and disrupt cellular processes. It has been demonstrated that mutant SOD1, but not wild-type SOD1, interacts with the axonal transport motor dynein and that this interaction contributes to motor neuron cell death, suggesting that disrupting this interaction may be a potential therapeutic target. However, it can be challenging to configure a high-throughput screening (HTS)-compatible assay to detect inhibitors of a protein-protein interaction. Here we describe the development and challenges of an HTS for small-molecule inhibitors of the mutant SOD1-dynein interaction. We demonstrate that the interaction can be formed by coexpressing the A4V mutant SOD1 and dynein intermediate complex in cells and that this interaction can be disrupted by compounds added to the cell lysates. Finally, we show that some of the compounds identified from a pilot screen to inhibit the protein-protein interaction with this method specifically disrupt the interaction between the dynein complex and mtSOD1 but not the dynein complex itself when applied to live cells.

Familial amyotrophic lateral sclerosis, a historical perspective

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease of the upper and lower motor neuron of unknown etiology. Although a familial cause for this disease has been suspected early one, it is only in the past two decades that advances in modern genetics led to the identification of more than 10 genes linked to familial ALS and helped us understand some of the complex genetic and environmental interactions that may contribute to sporadic ALS. In this article, we chronologically summarize the genetic breakthroughs in familial and sporadic ALS and depict how it shaped our understanding of disease pathogenesis and our quest for rational therapies.

ALS genetic modifiers that increase survival of SOD1 mice and are suitable for therapeutic development

Amyotrophic lateral sclerosis (ALS) is a frequently fatal motor neuron disease without any cure. To find molecular therapeutic targets, several studies crossed transgenic ALS murine models with animals transgenic for some ALS target genes. We aimed to revise the new discoveries and new works in this field. We selected the 10 most promising genes, according to their capability when down-regulated or up-regulated in ALS animal models, for increasing life span and mitigating disease progression: XBP-1, NogoA and NogoB, dynein, heavy and medium neurofilament, NOX1 and NOX2, MLC-mIGF-1, NSE-VEGF, and MMP-9. Interestingly, some crucial modifier genes have been described as being involved in common pathways, the most significant of which are inflammation and cytoskeletal activities. The endoplasmic reticulum also seems to play an important role in ALS pathogenesis, as it is involved in different selected gene pathways. In addition, these genes have evident links to each other, introducing the hypothesis of a single unknown, common pathway involving all of these identified genes and others to be discovered.

Dysregulation of axonal transport and motorneuron diseases

MNDs (motorneuron diseases) are neurodegenerative disorders in which motorneurons located in the motor cortex, in the brainstem and in the spinal cord are affected. These diseases in their inherited or sporadic forms are mainly characterized by motor dysfunctions, occasionally associated with cognitive and behavioural alterations. Although these diseases show high variability in onset, progression and clinical symptoms, they share common pathological features, and motorneuronal loss invariably leads to muscle weakness and atrophy. One of the most relevant aspect of these disorders is the occurrence of defects in axonal transport, which have been postulated to be either a direct cause, or a consequence, of motorneuron degeneration. In fact, due to their peculiar morphology and high energetic metabolism, motorneurons deeply rely on efficient axonal transport processes. Dysfunction of axonal transport is known to adversely affect motorneuronal metabolism, inducing progressive degeneration and cell death. In this regard, the understanding of the fine mechanisms at the basis of the axonal transport process and of their possible alterations may help shed light on MND pathological processes. In the present review, we will summarize what is currently known about the alterations of axonal transport found to be either causative or a consequence of MNDs.

Wild-type and mutant SOD1 share an aberrant conformation and a common pathogenic pathway in ALS

Many mutations confer upon copper/zinc superoxide dismutase-1 (SOD1) one or more toxic function(s) that impair motor neuron viability and cause familial amyotrophic lateral sclerosis (FALS). Using a conformation-specific antibody that detects misfolded SOD1 (C4F6), we demonstrate that oxidized WT-SOD1 and mutant-SOD1 share a conformational epitope that is not present in normal WT-SOD1. In a subset of human sporadic ALS (SALS) cases, motor neurons in the lumbosacral spinal cord displayed striking C4F6 immunoreactivity, denoting the presence of aberrant WT-SOD1 species. Recombinant, oxidized WT-SOD1 and WT-SOD1 immunopurified from SALS tissues inhibited kinesin-based fast axonal transport in a manner similar to FALS-linked mutant SOD1. Studies here suggest that WT-SOD1 can be pathogenic in SALS and identifies an SOD1-dependent pathogenic mechanism common to FALS and SALS.

Effects of ALS-related SOD1 mutants on dynein- and KIF5-mediated retrograde and anterograde axonal transport

Transport of material and signals between extensive neuronal processes and the cell body is essential to neuronal physiology and survival. Slowing of axonal transport has been shown to occur before the onset of symptoms in amyotrophic lateral sclerosis (ALS). We have previously shown that several familial ALS-linked copper-zinc superoxide dismutase (SOD1) mutants (A4V, G85R, and G93A) interacted and colocalized with the retrograde dynein-dynactin motor complex in cultured cells and affected tissues of ALS mice. We also found that the interaction between mutant SOD1 and the dynein motor played a critical role in the formation of large inclusions containing mutant SOD1. In this study, we showed that, in contrast to the dynein situation, mutant SOD1 did not interact with anterograde transport motors of the kinesin-1 family (KIF5A, B and C). Using dynein and kinesin accumulation at the sciatic nerve ligation sites as a surrogate measurement of axonal transport, we also showed that dynein mediated retrograde transport was slower in G93A than in WT mice at an early presymptomatic stage. While no decrease in KIF5A-mediated anterograde transport was detected, the slowing of anterograde transport of dynein heavy chain as a cargo was observed in the presymptomatic G93A mice. The results from this study along with other recently published work support that mutant SOD1 might only interact with and interfere with some kinesin members, which, in turn, could result in the impairment of a selective subset of cargos. Although it remains to be further investigated how mutant SOD1 affects different axonal transport motor proteins and various cargos, it is evident that mutant SOD1 can induce defects in axonal transport, which, subsequently, contribute to the propagation of toxic effects and ultimately motor neuron death in ALS.

Mitochondrial base excision repair in mouse synaptosomes during normal aging and in a model of Alzheimer's disease

Brain aging is associated with synaptic decline and synaptic function is highly dependent on mitochondria. Increased levels of oxidative DNA base damage and accumulation of mitochondrial DNA (mtDNA) mutations or deletions lead to mitochondrial dysfunction, playing an important role in the aging process and the pathogenesis of several neurodegenerative diseases. Here we have investigated the repair of oxidative base damage, in synaptosomes of mouse brain during normal aging and in an AD model. During normal aging, a reduction in the base excision repair (BER) capacity was observed in the synaptosomal fraction, which was associated with a decrease in the level of BER proteins. However, we did not observe changes between the synaptosomal BER activities of presymptomatic and symptomatic AD mice harboring mutated amyolid precursor protein (APP), Tau, and presinilin-1 (PS1) (3xTgAD). Our findings suggest that the age-related reduction in BER capacity in the synaptosomal fraction might contribute to mitochondrial and synaptic dysfunction during aging. The development of AD-like pathology in the 3xTgAD mouse model was, however, not associated with deficiencies of the BER mechanisms in the synaptosomal fraction when the whole brain was analyzed.

Neurotrophic growth factors for the treatment of amyotrophic lateral sclerosis: where do we stand?

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that results in progressive loss of motoneurons, motor weakness and death within 3–5 years after disease onset. Therapeutic options remain limited despite substantial number of approaches that have been tested clinically. Many neurotrophic growth factors are known to promote the survival of neurons and foster regeneration in the central nervous system. Various neurotrophic factors have been investigated pre-clinically and clinically for the treatment of ALS. Although pre-clinical data appeared promising, no neurotrophic factors succeeded yet in a clinical phase III trial. In this review we discuss the rationale behind those factors, possible reasons for clinical failures, and argue for a renewal of hope in this powerful class of drugs for the treatment of ALS.

Integrative gene-tissue microarray-based approach for identification of human disease biomarkers: application to amyotrophic lateral sclerosis

Advances in genomics and proteomics permit rapid identification of disease-relevant genes and proteins. Challenges include biological differences between animal models and human diseases, high discordance between DNA and protein expression data and a lack of experimental models to study human complex diseases. To overcome some of these limitations, we developed an integrative approach using animal models, postmortem human material and a combination of high-throughput microarray methods to identify novel molecular markers of amyotrophic lateral sclerosis (ALS). We used laser capture microdissection coupled with microarrays to identify early transcriptome changes occurring in spinal cord motor neurons or surrounding glial cells. Two models of familial motor neuron disease, SOD1(G93A) and TAU(P301L), transgenic mice were used at the presymptomatic stage. Identified gene expression changes were predominantly model-specific. However, several genes were regulated in both models. The relevance of identified genes as clinical biomarkers was tested in the peripheral blood transcriptome of presymptomatic SOD1(G93A) animals using custom-designed ALS microarray. To confirm the relevance of identified genes in human sporadic ALS (SALS), selected corresponding protein products were examined by high-throughput immunoassays using tissue microarrays constructed from human postmortem spinal cord tissues. Genes that were identified by these experiments and located within a linkage region associated with familial ALS/frontotemporal dementia were sequenced in several families. This large-scale gene and protein expression study pointing to distinct molecular mechanisms of TAU- and SOD1-induced motor neuron degeneration identified several new SALS-relevant proteins (CNGA3, CRB1, OTUB2, MMP14, SLK, DDX58, RSPO2) and putative blood biomarkers, including Nefh, Prph and Mgll.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.

Pathology of protein synthesis and degradation systems in ALS

Protein synthesis and degradation systems in neurons are among the major subjects of study in neurobiology. These systems are believed to be the main pathways involved in ALS; however, the essential pathomechanisms that underlie this disease remain obscure. In addition to the ubiquitin-proteasomal and autophagic systems, several cytoplasmic organelles are also involved in ALS. Here, we present our data and discuss the main morphological abnormalities detected in the anterior horn cells of ALS patients.

Activation of innate and humoral immunity in the peripheral nervous system of ALS transgenic mice

During injury to the nervous system, innate immune cells mediate phagocytosis of debris, cytokine production, and axon regeneration. In the neuro-degenerative disease amyotrophic lateral sclerosis (ALS), innate immune cells in the CNS are activated. However, the role of innate immunity in the peripheral nervous system (PNS) has not been well defined. In this study, we characterized robust activation of CD169/CD68/Iba1+ macrophages throughout the PNS in mutant SOD1(G93A) and SOD1(G37R) transgenic mouse models of ALS. Macrophage activation occurred pre-symptomatically, and expanded from focal arrays within nerve bundles to a tissue-wide distribution following symptom onset. We found a striking dichotomy for immune cells within the spinal cord and PNS. Flow cytometry and GFP bone marrow chimeras showed that spinal cord microglia were mainly tissue resident derived, dendritic-like cells, whereas in peripheral nerves, the majority of activated macrophages infiltrated from the circulation. Humoral antibodies and complement localized to PNS tissue in tandem with macrophage recruitment, and deficiency in complement C4 led to decreased macrophage activation. Therefore, cross-talk between nervous and immune systems occurs throughout the PNS during ALS disease progression. These data reveal a progressive innate and humoral immune response in peripheral nerves that is separate and distinct from spinal cord immune activation in ALS transgenic mice.

Mitochondrial dysfunction in amyotrophic lateral sclerosis

The etiology of motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains to be better understood. Based on the studies from ALS patients and transgenic animal models, it is believed that ALS is likely to be a multifactorial and multisystem disease. Many mechanisms have been postulated to be involved in the pathology of ALS, such as oxidative stress, glutamate excitotoxicity, mitochondrial damage, defective axonal transport, glia cell pathology and aberrant RNA metabolism. Mitochondria, which play crucial roles in excitotoxicity, apoptosis and cell survival, have shown to be an early target in ALS pathogenesis and contribute to the disease progression. Morphological and functional defects in mitochondria were found in both human patients and ALS mice overexpressing mutant SOD1. Mutant SOD1 was found to be preferentially associated with mitochondria and subsequently impair mitochondrial function. Recent studies suggest that axonal transport of mitochondria along microtubules and mitochondrial dynamics may also be disrupted in ALS. These results also illustrate the critical importance of maintaining proper mitochondrial function in axons and neuromuscular junctions, supporting the emerging "dying-back" axonopathy model of ALS. In this review, we will discuss how mitochondrial dysfunction has been linked to the ALS variants of SOD1 and the mechanisms by which mitochondrial damage contributes to the disease etiology.

A common property of amyotrophic lateral sclerosis-associated variants: destabilization of the copper/zinc superoxide dismutase electrostatic loop

At least 119 mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis by an unidentified toxic gain of function. We compared the dynamic properties of 13 as-isolated, partially metallated, SOD1 variant enzymes using hydrogen-deuterium exchange. We identified a shared property of these familial amyotrophic lateral sclerosis-related SOD1 variants, namely structural and dynamic change affecting the electrostatic loop (loop VII) of SOD1. Furthermore, SOD1 variants that have severely compromised metal binding affinities demonstrated additional structural and dynamic changes to the zinc-binding loop (loop IV) of SOD1. Although the biological consequences of increased loop VII mobility are not fully understood, this common property is consistent with the hypotheses that SOD1 mutations exert toxicity via aggregation or aberrant association with other cellular constituents.

Mitochondrial function, morphology, and axonal transport in amyotrophic lateral sclerosis

Perturbation of organellar axonal transport is increasingly recognized as an important contributor in a number of neurodegenerative diseases. Although the specificity of this impairment remains to be elucidated, growing evidence suggests that in certain disease conditions, mitochondria are affected primarily by transport defects. Many hypotheses have been formulated to explain the pathogenic mechanisms involved in amyotrophic lateral sclerosis (ALS). The mutations described so far in genetic forms of ALS (familial ALS, fALS) affect proteins involved in a wide variety of cellular mechanisms, including free radical scavenging, energy metabolism, axonal transport, RNA processing, DNA repair, vesicular transport, and angiogenesis. Here we review the current knowledge on mitochondrial transport and its role in ALS.