Amyloid precursor protein (APP) contributes to pathology in the SOD1G93A mouse model of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), the progressive loss of motor neurons is accompanied by extensive muscle denervation, resulting in paralysis and ultimately death. Upregulation of amyloid beta (A4) precursor protein (APP) in muscle fibres coincides with symptom onset in both sporadic ALS patients and the SOD1(G93A) mouse model of familial ALS. We have further characterized this response in SOD1(G93A) mice and also revealed elevated levels of β-amyloid (Aβ) peptides in the SOD1(G93A) spinal cord, which were predominantly localized within motor neurons and their surrounding glial cells. We therefore examined the effect of genetic ablation of APP on disease progression in SOD1(G93A) mice, which significantly improved multiple disease parameters, including innervation, motor function, muscle contractile characteristics, motor unit and motor neuron survival. These results therefore strongly suggest that APP actively contributes to SOD1(G93A)-mediated pathology. Together with observations from ALS cases, this study indicates that APP may contribute to human ALS pathology.     

Increased expression of the glial glutamate transporter EAAT2 modulates excitotoxicity and delays the onset but not the outcome of ALS in mice

The glial glutamate transporter EAAT2 is primarily responsible for clearance of glutamate from the synaptic cleft and loss of EAAT2 has been previously reported in amyotrophic lateral sclerosis (ALS) and Alzheimer's disease. The loss of functional EAAT2 could lead to the accumulation of extracellular glutamate, resulting in cell death known as excitotoxicity. However, it is still unknown whether it is a primary cause in the cascade leading to neuron degeneration or a secondary event to cell death. The goals of this study were to generate transgenic mice overexpressing EAAT2 and then to cross these mice with the ALS-associated mutant SOD1(G93A) mice to investigate whether supplementation of the loss of EAAT2 would delay or rescue the disease progression. We show that the amount of EAAT2 protein and the associated Na+-dependent glutamate uptake was increased about 2-fold in our EAAT2 transgenic mice. The transgenic EAAT2 protein was properly localized to the cell surface on the plasma membrane. Increased EAAT2 expression protects neurons from L-glutamate induced cytotoxicity and cell death in vitro. Furthermore, our EAAT2/G93A double transgenic mice showed a statistically significant (14 days) delay in grip strength decline but not in the onset of paralysis, body weight decline or life span when compared with G93A littermates. Moreover, a delay in the loss of motor neurons and their axonal morphologies as well as other events including caspase-3 activation and SOD1 aggregation were also observed. These results suggest that the loss of EAAT2 may contribute to, but does not cause, motor neuron degeneration in ALS.     

alpha-Amino-3-hydroxy-5-methyl-isoxazole-4-propionate receptors in spinal cord motor neurons are altered in transgenic mice overexpressing human Cu,Zn superoxide dismutase (Gly93-->Ala) mutation

There are many evidences implicating glutamatergic toxicity as a contributory factor in the selective neuronal injury occurring in amyotrophic lateral sclerosis (ALS). This neurodegenerative disorder is characterized by the progressive loss of motor neurons, whose pathogenesis is thought to involve Ca(2+) influx mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate receptors (AMPARs). In the present study we report alterations in the AMPARs function in a transgenic mouse-model of the human SOD1(G93A) familial ALS. Compared with those expressed in motor neurons carrying the human wild type gene, AMPAR-gated channels expressed in motor neurons carrying the human mutant gene exhibited modified permeability, altered agonist cooperativity between the sites involved in the process of channel opening and were responsible for slower spontaneous synaptic events. These observations demonstrate that the SOD1(G93A) mutation induces changes in AMPAR functions which may underlie the increased vulnerability of motor neurons to glutamatergic excitotoxicity in ALS.     

Lack of TDP-43 abnormalities in mutant SOD1 transgenic mice shows disparity with ALS

Mislocalization of the TAR-DNA binding protein (TDP-43) from the nucleus to the cytoplasm of diseased motor neurons and association with intraneuronal ubiquitinated inclusions has recently been reported in amyotrophic lateral sclerosis (ALS). Here, we have investigated TDP-43 immunoreactivity in three lines of mutant SOD1 transgenic mice, G93A, G37R and G85R and compared with labeling in one sporadic ALS case and two familial ALS cases carrying mutations in SOD1, A4T and I113T. Our findings show that there is no mislocalization of TDP-43 to the cytoplasm in motor neurons of mutant SOD1 transgenic mice, nor association of TDP-43 with ubiquitinated inclusions. In contrast, mislocalization of TDP-43 to the cytoplasm and association with ubiquitinated inclusions was found in the ALS cases, including those carrying mutations in SOD1. Interestingly, there was no association of TDP-43 with ubiquitinated hyaline conglomerate inclusions, pathology closely associated with ALS cases carrying mutations in SOD1. Our findings indicate that the process of motor neuron degeneration in mutant SOD1 transgenic mice is unlikely to involve the abnormalities of TDP-43 described in the human disease.     

Deficiency in the ALS2 gene does not affect the motor neuron degeneration in SOD1(G93A) transgenic mice

Dysfunction of the ALS2 gene has been linked to one form of juvenile onset autosomal recessive amyotrophic lateral sclerosis (ALS). Previous in vitro studies suggest that over-expression of ALS2 protects cells from mutant Cu/Zn superoxide dismutase (SOD1)-induced cytotoxicity. To test whether ALS2 plays a protective role against mutant SOD1-mediated motor neuron degeneration in vivo, we examined the progression of motor neuron disease in SOD1(G93A) mice on an ALS2 null background. Our data suggest that deficiency in the ALS2 gene does not affect the pathogenesis of SOD1(G93A) mice.     

Overexpression of neurofilament subunit NF-L and NF-H extends survival of a mouse model for amyotrophic lateral sclerosis

Mutations in superoxide dismutase 1 (SOD1) cause amyotrophic lateral sclerosis (ALS) in a subset of patients. Neurofilaments (NFs), the most abundant protein in motoneurons, may play a role in motoneuron degeneration. To investigate this role, we crossed transgenic mice expressing SOD1 mutant G93A (G93A mice) with mice overexpressing mouse neurofilament subunit H (H mice) or L (L mice). G93A mice overexpressing either NF-L or NF-H developed ALS later and survived longer than the G93A mice on a wild type background. These results illustrate a beneficial role of neurofilaments in ALS and call into question of several hypotheses regarding the role of neurofilaments in the development of ALS.     

Mechanisms underlying altered striatal synaptic plasticity in old A53T-α synuclein overexpressing mice

The interactions between certain α-synuclein (SNCA) conformations and dopamine (DA) metabolism cause selective DA neuron degeneration in Parkinson's disease (PD). Preclinical research on PD took advantage of increasing studies involving different animal models which express different forms of mutated SNCA. Transgenic animals expressing mutant α-synucleins such as mice transgenic for A53T-SNCA (TG) are considered valuable models to assess specific aspects of the pathogenesis of synucleinopathies and PD. In this study we performed electrophysiological recordings in corticostriatal slice preparations from young TG overexpressing mice, in which extracellular striatal DA levels appeared to be normal, and in old TG mice, characterized by abnormalities in striatal DA signaling and impaired long-term depression (LTD). We report no difference in TG mice from the two groups of age of either the basal membrane properties and synaptic striatal excitability in respect to age-matched wild-type mice. Furthermore, in old TG mice, showing plastic abnormalities and motor symptoms, we investigated the mechanisms at the basis of the altered LTD. In old TG mice LTD could not be restored by treatments with acute application of DA or by subchronic treatment with L-3,4-dihydroxyphenylalanine (L-DOPA). Conversely, the application of the phosphodiesterase inhibitor zaprinast fully restored LTD to normal conditions via the stimulation of a cyclic guanosine monophosphate (GMP)-protein kinase G-dependent intracellular signaling pathway. These results suggest that, in addition to the dopaminergic alterations reported in this genetic model of PD, other signal transduction pathways linked to striatal synaptic plasticity are altered in an age-dependent manner.     

Distribution and cellular localization of high mobility group box protein 1 (HMGB1) in the spinal cord of a transgenic mouse model of ALS

Although the aetiology of amyotrophic lateral sclerosis (ALS) is still elusive, increased attention has been put forward on events related to neuroinflammation and an active participation of glial cells in the ALS pathogenesis has been suggested. However, the specific role of many proinflammatory mediators that usually accompany the inflammatory changes is still largely unknown. High mobility group box protein 1 (HMGB1) is an ubiquitous nuclear protein that exerts numerous extranuclear and extracellular functions, including a proinflammatory activity, able to induce cytokines expression and activate inflammatory cells. To investigate whether this protein may play a role in the inflammatory events in ALS, we examined both expression and localization of HMGB1 in the lumbar spinal cord of SOD1G93A transgenic mice, a well established mouse model of familial ALS, at different stages of the disease. Intense HMGB1 reactivity was detected in ventral horn motor neurons of both non-transgenic and SOD1G93A mice and there was no difference in its expression between presymptomatic SOD1G93A mice and controls. With the progression of the disease, degenerating neurons showed a reduction of HMGB1 immunoreactivity which could reflect an extracellular release of this protein. By contrast, in the reactive glial cells HMGB1 was remarkably expressed in the nucleus, but not in the cytosol, likely contributing to the proliferation and/or hypertrophy of these cells. These results suggest that HMGB1 may have a different involvement in the motor neurons and glial cells in response to the neurotoxic environment in the spinal cord of SOD1G93A mice, and it may contribute to the progression of inflammatory and neurodegenerative processes.     

Neuron-specific overexpression of the co-chaperone Bcl-2-associated athanogene-1 in superoxide dismutase 1(G93A)-transgenic mice

Bcl-2-associated athanogene-1 (BAG1) binds heat-shock protein 70 (Hsp70)/Hsc70, increases intracellular chaperone activity in neurons and proved to be protective in several models for neurodegeneration. Mutations in the superoxide dismutase 1 (SOD1) gene account for approximately 20% of familial amyotrophic lateral sclerosis (ALS) cases. A common property shared by all mutant SOD1 (mtSOD1) species is abnormal protein folding and the propensity to form aggregates. Toxicity and aggregate formation of mutant SOD1 can be overcome by enhanced chaperone function in vitro. Moreover, expression of mtSOD1 decreases BAG1 levels in a motoneuronal cell line. Thus, several lines of evidence suggested a protective role of BAG1 in mtSOD1-mediated motoneuron degeneration. To explore the therapeutic potential of BAG1 in a model for ALS, we generated SOD1G93A/BAG1 double transgenic mice expressing BAG1 in a neuron-specific pattern. Surprisingly, substantially increased BAG1 protein levels in spinal cord neurons did not significantly alter the phenotype of SOD1G93A-transgenic mice. Hence, expression of BAG1 is not sufficient to protect against mtSOD1-induced motor dysfunction in vivo. Our work shows that, in contrast to the in vitro situation, modulation of multiple cellular functions in addition to enhanced expression of a single chaperone is required to protect against SOD1 toxicity, highlighting the necessity of combined treatment strategies for ALS.     

Neurodegeneration and NLRP3 inflammasome expression in the anterior thalamus of SOD1(G93A) ALS mice

Nowadays, amyotrophic lateral sclerosis (ALS) is considered as a multisystem disorder, characterized by a primary degeneration of motor neurons as well as neuropathological changes in non‐motor regions. Neurodegeneration in subcortical areas, such as the thalamus, are believed to contribute to cognitive and behavioral abnormalities in ALS patients. In the present study, we investigated neurodegenerative changes including neuronal loss and glia pathology in the anterodorsal thalamic nucleus (AD) of SOD1(G93A) mice, a widely used animal model for ALS. We detected massive dendrite swelling and neuronal loss in SOD1(G93A) animals, which was accompanied by a mild gliosis. Furthermore, misfolded SOD1 protein and autophagy markers were accumulating in the AD. Since innate immunity and activation inflammasomes seem to play a crucial role in ALS, we examined protein expression of Nod‐like receptor protein 3 (NLRP3), apoptosis‐associated speck‐like protein containing a caspase‐1 recruitment domain (ASC) and the cytokine interleukin 1 beta (IL1β) in AD glial cells and neurons. NLRP3 and ASC were significantly up‐regulated in the AD of SOD1(G93A) mice. Finally, co‐localization studies revealed expression of NLRP3, ASC and IL1β in neurons. Our study yielded two main findings: (i) neurodegenerative changes already occur at an early symptomatic stage in the AD and (ii) increased inflammasome expression may contribute to neuronal cell death. In conclusion, neurodegeneration in the anterior thalamus may critically account for cognitive changes in ALS pathology.     

AIF translocates to the nucleus in the spinal motor neurons in a mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by selective loss of motor neurons in the brain stem and the spinal cords. One of the causes for the familial ALS has been attributed to the mutations in copper-zinc superoxide dismutase (SOD1). Although the toxic function of the mutant enzyme has not been fully understood, the final cell death pathway has been suggested as caspase-dependent. In the present study, we present evidence that the activation of apoptosis inducing factor (AIF) may play a role to induce motor neuron death during ALS pathogenesis. In the spinal cord of SOD1 G93A transgenic mice, expression of AIF was detected in the motor neurons and astrocytes. The level of AIF expression increased as the disease progressed. In the symptomatic SOD1 G93A transgenic mice, AIF released from the mitochondria and translocated into the nucleus in the motor neurons as evidenced by confocal microscopy and biochemical analysis. These results suggest that AIF may play a role to induce motor neuron death in a mouse model of ALS.     

Voltage-dependent sodium channels in spinal cord motor neurons display rapid recovery from fast inactivation in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by a substantial loss of motor neurons in the spinal cord, brain stem, and motor cortex. Previous evidence showed that in a mouse model of a familial form of ALS expressing high levels of the human mutated protein Cu,Zn superoxide dismutase (Gly(93)-->Ala, G93A), the firing properties of single motor neurons are altered to induce neuronal hyperexcitability. To determine whether the functionality of the macroscopic voltage-dependent Na(+) currents is modified in G93A motor neurons, in the present work their physiological properties were examined. The voltage-dependent sodium channels were studied in dissociated motor neurons in culture from nontransgenic mice (Control), from transgenic mice expressing high levels of the human wild-type protein [superoxide dismutase 1 (SOD1)], and from G93A mice, using the whole cell configuration of the patch-clamp recording technique. The voltage dependency of activation and of steady-state inactivation, the kinetics of fast inactivation and slow inactivation of the voltage-dependent Na(+) channels were not modified in the mutated mice. Conversely, the recovery from fast inactivation was significantly faster in G93A motor neurons than that in Control and SOD1. The recovery from fast inactivation was still significantly faster in G93A motor neurons exposed for different times (3-48 h) and concentrations (5-500 microM) to edaravone, a free-radical scavenger. Clarification of the importance of these changes in membrane ion channel functionality may have diagnostic and therapeutic implications in the pathogenesis of ALS.     

Protective effects of cardiotrophin-1 adenoviral gene transfer on neuromuscular degeneration in transgenic ALS mice

Amyotrophic lateral sclerosis (ALS) is mainly a sporadic neurodegenerative disorder characterized by loss of cortical and spinal motoneurons. Some familial ALS cases (FALS) have been linked to dominant mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1). Transgenic mice overexpressing a mutated form of human SOD1 with a Gly93Ala substitution develop progressive muscle wasting and paralysis as a result of spinal motoneuron loss and die at 5 to 6 months. We investigated the effects of neurotrophic factor gene delivery in this FALS model. Intramuscular injection of an adenoviral vector encoding cardiotrophin-1 (CT-1) in SOD1G93A newborn mice resulted in systemic delivery of CT-1, supplying motoneurons with a continuous source of trophic factor. CT-1 delayed the onset of motor impairment as assessed in the rotarod test. Axonal degeneration was slowed and skeletal muscle atrophy was largely reduced by CT-1 treatment. By monitoring the amplitude of the evoked motor response, we showed that the time-course of motor impairment was significantly decreased by CT-1 treatment. Thus, adenovirus-mediated gene transfer of neurotrophic factors might delay neurogenic muscular atrophy and progressive neuromuscular deficiency in ALS patients.     

Mislocalization of TDP-43 in the G93A mutant SOD1 transgenic mouse model of ALS

Previous evidence demonstrates that TAR DNA binding protein (TDP-43) mislocalization is a key pathological feature of amyotrophic lateral sclerosis (ALS). TDP-43 normally shows nuclear localization, but in CNS tissue from patients who died with ALS this protein mislocalizes to the cytoplasm. Disease specific TDP-43 species have also been reported to include hyperphosphorylated TDP-43, as well as a C-terminal fragment. Whether these abnormal TDP-43 features are present in patients with SOD1-related familial ALS (fALS), or in mutant SOD1 over-expressing transgenic mouse models of ALS remains controversial. Here we investigate TDP-43 pathology in transgenic mice expressing the G93A mutant form of SOD1. In contrast to previous reports we observe redistribution of TDP-43 to the cytoplasm of motor neurons in mutant SOD1 transgenic mice, but this is seen only in mice having advanced disease. Furthermore, we also observe rounded TDP-43 immunoreactive inclusions associated with intense ubiquitin immunoreactivity in lumbar spinal cord at end stage disease in mSOD mice. These data indicate that TDP-43 mislocalization and ubiquitination are present in end stage mSOD mice. However, we do not observe C-terminal TDP-43 fragments nor TDP-43 hyperphosphorylated species in these end stage mSOD mice. Our findings indicate that G93A mutant SOD1 transgenic mice recapitulate some key pathological, but not all biochemical hallmarks, of TDP-43 pathology previously observed in human ALS. These studies suggest motor neuron degeneration in the mutant SOD1 transgenic mice is associated with TDP-43 histopathology.     

Non-cell autonomous effect of glia on motor neurons in an embryonic stem cell-based ALS model

Here we report an in vitro model system for studying the molecular and cellular mechanisms that underlie the neurodegenerative disease amyotrophic lateral sclerosis (ALS). Embryonic stem cells (ESCs) derived from mice carrying normal or mutant transgenic alleles of the human SOD1 gene were used to generate motor neurons by in vitro differentiation. These motor neurons could be maintained in long-term coculture either with additional cells that arose during differentiation or with primary glial cells. Motor neurons carrying either the nonpathological human SOD1 transgene or the mutant SOD1(G93A) allele showed neurodegenerative properties when cocultured with SOD1(G93A) glial cells. Thus, our studies demonstrate that glial cells carrying a human SOD1(G93A) mutation have a direct, non-cell autonomous effect on motor neuron survival. More generally, our results show that ESC-based models of disease provide a powerful tool for studying the mechanisms of neural degeneration. These phenotypes displayed in culture could provide cell-based assays for the identification of new ALS drugs.     

Hepatocyte growth factor (HGF) attenuates gliosis and motoneuronal degeneration in the brainstem motor nuclei of a transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive loss of brainstem and spinal motoneurons. Although prevention of motoneuronal degeneration has been postulated as the primary target for a cure, accumulating evidence suggests that microglial accumulation contributes to disease progression. This study was designed to assess the ability of HGF to modulate microglial accumulation and motoneuronal degeneration in brainstem motor nuclei, using double transgenic mice overexpressing mutated SOD1(G93A) and HGF (G93A/HGF). Histological and immunohistochemical analyses of the tissues of G93A/HGF mice revealed a marked decrease in the number of microglia and reactive astrocytes and an attenuation of the loss of motoneurons in facial and hypoglossal nuclei compared with G93A mice. HGF overexpression attenuated monocyte chemoattractant protein-1 (MCP-1) induction, predominantly in astrocytes; suppressed activation of caspase-1, -3 and -9; and, increased X chromosome-linked inhibition of apoptosis protein (XIAP) in the motoneurons of G93A mice. The implication is that HGF reduces microglial accumulation by suppressing MCP-1 induction and prevents motoneuronal death through inhibition of pro-apoptotic protein activation. These findings suggest that, in addition to direct neurotrophic activity on motoneurons, HGF-suppression of gliosis may retard disease progression, making HGF a potential therapeutic agent for the treatment of ALS patients.     

Increased persistent Na+ current and its effect on excitability in motoneurones cultured from mutant SOD1 mice

Mutations in the enzyme superoxide dismutase 1 (SOD1) initiate a progressive motoneurone degeneration in amyotrophic lateral sclerosis (ALS). Transgenic mice overexpressing this mutation develop a similar progressive motoneurone degeneration. In spinal motoneurones cultured from presymptomatic mice expressing the glycine to alanine mutation at base pair 93 (G93A) SOD1 mutation, a marked increase in the persistent component of the Na⁺ current was observed, without changes in passive properties. This increase only enhanced neuronal excitability in high input conductance cells, as low input conductance cells exhibited a compensatory outward shift in the current remaining after Na⁺ blockade. High input conductance motoneurones tend to be large, so these results may explain the tendency of large motoneurones to degenerate first in ALS. Riluzole, at the therapeutic concentration used to treat ALS, decreased neuronal excitability and persistent Na⁺ current in G93A motoneurones to levels observed in the control motoneurones. Aberrations in the intrinsic electrical properties may be among the first symptoms to emerge in SOD1-linked ALS.     

Hepatocyte growth factor (HGF) attenuates gliosis and motoneuronal degeneration in the brainstem motor nuclei of a transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive loss of brainstem and spinal motoneurons. Although prevention of motoneuronal degeneration has been postulated as the primary target for a cure, accumulating evidence suggests that microglial accumulation contributes to disease progression. This study was designed to assess the ability of HGF to modulate microglial accumulation and motoneuronal degeneration in brainstem motor nuclei, using double transgenic mice overexpressing mutated SOD1^G93A and HGF (G93A/HGF). Histological and immunohistochemical analyses of the tissues of G93A/HGF mice revealed a marked decrease in the number of microglia and reactive astrocytes and an attenuation of the loss of motoneurons in facial and hypoglossal nuclei compared with G93A mice. HGF overexpression attenuated monocyte chemoattractant protein-1 (MCP-1) induction, predominantly in astrocytes; suppressed activation of caspase-1, -3 and -9; and, increased X chromosome-linked inhibition of apoptosis protein (XIAP) in the motoneurons of G93A mice. The implication is that HGF reduces microglial accumulation by suppressing MCP-1 induction and prevents motoneuronal death through inhibition of pro-apoptotic protein activation. These findings suggest that, in addition to direct neurotrophic activity on motoneurons, HGF-suppression of gliosis may retard disease progression, making HGF a potential therapeutic agent for the treatment of ALS patients.     

Intranasal delivery of insulin and a nitric oxide synthase inhibitor in an experimental model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neuromuscular disorder in which motor neurons may be targeted by oxidative and nitrergic stress without sufficient compensation by intrinsic support mechanisms. In this work, we addressed two key tenets of this hypothesis for the pathogenesis of ALS. Using superoxide dismutase (SOD) 1G93A mice, we studied the impact of reduction of nitrergic stress within the CNS with the use of a broad spectrum nitric oxide synthase (NOS) inhibitor, NG-nitro-l-arginine methyl ester. A separate cohort of SOD1G93A mice received direct insulin neurotrophic support, ligating receptors expressed upon motor neurons, to attempt protection against neuronal and functional motor dropout. For direct access, we used a novel form of intranasal delivery that provides peak concentration levels in the CNS within 1 h of delivery without systemic side effects at doses which previously rescued retrograde loss of motor axons after axotomy. To identify even minor impacts of these interventions on the outcome, we utilized an intensive program of serial behavioral and electrophysiological testing weekly, combined with endpoint quantitative morphometry and molecular analysis. This intensive evaluation enhanced our knowledge of the time course in SOD1G93A mice and impact of the SOD1G93A mutation upon motor neurons and their function. Neither intervention had even minimal impact upon slowing progression of disease in SOD1G93A mice. Our data argue against significant roles for nitrergic stress in promoting motor neuron loss and the importance of alternative neurotrophic support mechanisms that might support motor neurons and prevent disease progression in SOD1G93A mice.