The mechanisms involved in selective motor neuron degeneration in amyotrophic lateral sclerosis remain unknown more than 135 years after the disease was first described. Although most cases have no ...known cause, mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1) have been implicated in a fraction of familial cases of the disease. Transgenic mouse models with mutations in the SOD1 gene and other ALS genes develop pathology reminiscent of the disorder, including progressive death of motor neurons, and have provided insight into the pathogenesis of the disease but have consistently failed to predict therapeutic efficacy in humans. However, emerging research has demonstrated that mutations and pathology associated with the TDP‐43 gene and protein may be more common than SOD1 mutations in familial and sporadic ALS. Putative mechanisms of toxicity targeting motor neurons include oxidative damage, accumulation of intracellular aggregates, mitochondrial dysfunction, defects in axonal transport, growth factor deficiency, aberrant RNA metabolism, glial cell pathology, and glutamate excitotoxicity. Convergence of these pathways is likely to mediate disease onset and progression. Ann Neurol 2009;65 (suppl):S3–S9
Astrocytes make up 20–40% of glial cells within the central nervous system (CNS) and provide several crucial functions, ranging from metabolic and structural support to regulation of synaptogenesis ...and synaptic transmission. Although these cells are morphologically and functionally complex, astrocytes have been historically regarded as homogenous cell populations and studied as one population of cells. Fortunately, recent evidence in RNA profiling and imaging data has begun to refute this view. These studies suggest heterogeneity of astrocytes across brain regions, differing in many aspects such as morphology, function, physiological properties, developmental origins, and response to disease. Increased understanding of astrocyte heterogeneity is critical for investigations into the function of astrocytes in the brain and neuro–glia interactions. Furthermore, insights into astrocyte heterogeneity can help better understand their role in neurological disorders and potentially produce novel approaches to treating these diseases.
A nucleotide repeat expansion (NRE) within the chromosome 9 open reading frame 72 (C9orf72) gene was the first of this type of mutation to be linked to multiple neurological conditions, including ...amyotrophic lateral sclerosis and frontotemporal dementia. The pathogenic mechanisms through which the C9orf72 NRE contributes to these disorders include loss of C9orf72 function and gain-of-function mechanisms of C9orf72 driven by toxic RNA and protein species encoded by the NRE. These mechanisms have been linked to several cellular defects - including nucleocytoplasmic trafficking deficits and nuclear stress - that have been observed in both patients and animal models.
Highlights • The pathophysiological mechanisms underlying amyotrophic lateral sclerosis (ALS) are multifactorial. • Genetic factors and dysfunction of vital molecular pathways underlie ALS ...pathogenesis. • Novel therapeutic approaches have been developed in ALS.
The mammalian CNS contains a ubiquitous population of glial progenitors known as NG2
+ cells that have the ability to develop into oligodendrocytes and undergo dramatic changes in response to injury ...and demyelination. Although it has been reported that NG2
+ cells are multipotent, their fate in health and disease remains controversial. Here, we generated
PDGFαR-CreER transgenic mice and followed their fate in vivo in the developing and adult CNS. These studies revealed that NG2
+ cells in the postnatal CNS generate myelinating oligodendrocytes, but not astrocytes or neurons. In regions of neurodegeneration in the spinal cord of ALS mice, NG2
+ cells exhibited enhanced proliferation and accelerated differentiation into oligodendrocytes but remained committed to the oligodendrocyte lineage. These results indicate that NG2
+ cells in the normal CNS are oligodendrocyte precursors with restricted lineage potential and that cell loss and gliosis are not sufficient to alter the lineage potential of these progenitors.
► NG2
+ cells generate myelinating oligodendrocytes in the developing and adult CNS ► Gray matter NG2
+ cells are not postmitotic and retain the ability to form OLs ► NG2
+ cells are not progenitors of astrocytes or neurons in the postnatal CNS ► Proliferating NG2
+ cells in ALS mice do not exhibit lineage plasticity
Amyotrophic Lateral Sclerosis (ALS) is a motor neuron disease affecting upper and lower motor neurons in the central nervous system. Patients with ALS develop extensive muscle wasting and atrophy ...leading to paralysis and death 3 to 5 years after disease onset. The condition may be familial (fALS 10%) or sporadic ALS (sALS, 90%). The large majority of fALS cases are due to genetic mutations in the Superoxide dismutase 1 gene (SOD1, 15% of fALS) and repeat nucleotide expansions in the gene encoding C9ORF72 (∼ 40% to 50% of fALS and ∼ 10% of sALS). Studies suggest that ALS is mediated through aberrant protein homeostasis (i.e., ER stress and autophagy) and/or changes in RNA processing (as in all non-SOD1-mediated ALS). In all of these cases, animal models suggest that the disorder is mediated non-cell autonomously, i.e., not only motor neurons are involved, but glial cells including microglia, astrocytes, and oligodendrocytes, and other neuronal subpopulations are also implicated in the pathogenesis. Provided in this unit is a review of ALS rodent models, including discussion of their relative advantages and disadvantages. Emphasis is placed on correlating the model phenotype with the human condition and the utility of the model for defining the disease process. Information is also presented on RNA processing studies in ALS research, with particular emphasis on the newest ALS rodent models.
Oligodendrocytes associate with axons to establish myelin and provide metabolic support to neurons. In the spinal cord of amyotrophic lateral sclerosis (ALS) mice, oligodendrocytes downregulate ...transporters that transfer glycolytic substrates to neurons and oligodendrocyte progenitors (NG2(+) cells) exhibit enhanced proliferation and differentiation, although the cause of these changes in oligodendroglia is unknown. We found extensive degeneration of gray matter oligodendrocytes in the spinal cord of SOD1 (G93A) ALS mice prior to disease onset. Although new oligodendrocytes were formed, they failed to mature, resulting in progressive demyelination. Oligodendrocyte dysfunction was also prevalent in human ALS, as gray matter demyelination and reactive changes in NG2(+) cells were observed in motor cortex and spinal cord of ALS patients. Selective removal of mutant SOD1 from oligodendroglia substantially delayed disease onset and prolonged survival in ALS mice, suggesting that ALS-linked genes enhance the vulnerability of motor neurons and accelerate disease by directly impairing the function of oligodendrocytes.
Amyotrophic lateral sclerosis (ALS) is a progressive, adult onset neurodegenerative disease that is always fatal. The history of ALS drug discovery is fraught with many stops and starts. It took 22 ...years after the FDA approval of the anti-excitotoxic drug Riluzole before another drug was found to be effective in altering ALS progression: the anti-oxidant Edaravone.
Amyotrophic lateral sclerosis (ALS) is a progressive, adult onset neurodegenerative disease that is always fatal. The history of ALS drug discovery is fraught with many stops and starts. It took 22 years after the FDA approval of the anti-excitotoxic drug Riluzole before another drug was found to be effective in altering ALS progression: the anti-oxidant Edaravone.
The term neurodegenerative disease refers to the principal pathology associated with disorders such as amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease and Parkinson's ...disease, and it is presumed that neurodegeneration results in the clinical findings seen in patients with these diseases. Decades of pathological and physiological studies have focused on neuronal abnormalities in these disorders, but it is becoming increasingly evident that astrocytes are also important players in these and other neurological disorders. Our understanding of the normative biology of astrocytes has been aided by the development of animal models in which astrocyte-specific proteins and pathways have been manipulated, and mouse models of neurodegenerative diseases have also revealed astrocyte-specific pathologies that contribute to neurodegeneration. These models have led to the development of targeted therapies for pathways in which astrocytes participate, and this research should ultimately influence the clinical treatment of neurodegenerative disorders.
The hexanucleotide repeat expansion (HRE) GGGGCC (G4C2) in C9orf72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Recent studies support an HRE RNA ...gain-of-function mechanism of neurotoxicity, and we previously identified protein interactors for the G4C2 RNA including RanGAP1. A candidate-based genetic screen in Drosophila expressing 30 G4C2 repeats identified RanGAP (Drosophila orthologue of human RanGAP1), a key regulator of nucleocytoplasmic transport, as a potent suppressor of neurodegeneration. Enhancing nuclear import or suppressing nuclear export of proteins also suppresses neurodegeneration. RanGAP physically interacts with HRE RNA and is mislocalized in HRE-expressing flies, neurons from C9orf72 ALS patient-derived induced pluripotent stem cells (iPSC-derived neurons), and in C9orf72 ALS patient brain tissue. Nuclear import is impaired as a result of HRE expression in the fly model and in C9orf72 iPSC-derived neurons, and these deficits are rescued by small molecules and antisense oligonucleotides targeting the HRE G-quadruplexes. Nucleocytoplasmic transport defects may be a fundamental pathway for ALS and FTD that is amenable to pharmacotherapeutic intervention.
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