Motor Neuron Disease Flashcards
(27 cards)
What is amyotrophic lateral sclerosis?
- Characterised by the degeneration of upper and lower motor neurons
- Progressive muscle weakness and paralysis
- Mid-life onset and typically fatal between 1-5 years of diagnosis
Which mutations have been associated with FALS?
- SOD1 (20%)
- C9orf72(40%)
- TARDBD
- FUS
What causes, other than genetic, have been linked to ALS?
- Environmental and occupational risk factors
- Possibility of autoimmune disease
- Viruses and toxins have also been proposed
Which ALS disease mechanisms have been identified?
- Oxidative stress
- Mitochondrial dysfunction
- Axonal transport
- Protein transport
- Non-cell autonomous processes (glia&muscles )
- Glutamate mediated excitotoxicity
- RNA processing
How has oxidative stress been linked to ALS?
- Idea that an imbalance between the generation and removal of reactive oxidative species is disrupted
- SOD1 is a gain of function mutation, therefore a lack of oxidative species could be the problem
- Evidence for a decrease in EAAT2 (glutamate transporter) potentially leading to glutamate excitotoxicity
How has mitochondrial dysfunction been linked to ALS?
- Mitochondrial vacuoles are found in pre-sympotomatic ALS
- mSOD1 accumulates in these vacuoles and may impair several mitochondrial proteins and pathways (interrupting Ca2+ buffering)
How has neurofilament and axonal transport dysfunction been linked to ALS?
- Motorneurons are long and therefore require tight control of protein transport
- Dying-back pathology present, with disease progressing from synapses to axons to soma
- Accumulation of neurofilaments in MN axosn are present in the early stages of ALS with pre-sympotimatic deficit in axonal transport
- Mutations in dynactin, protein involved in axonal transport, found in ALS
How have protein aggregations been linked to ALS?
- Proteins found to be abnormally accumulate in many ALS-causative mutations including mSOD1, NF-l, TDP-43 and FUS
- Reflects failure of proteins to fold properly after synthesis
- Aggregates often contains ubiquitin which targets the protein for disposal via the proteosomes
How can the glia enact non-cell autonomous death in ALS?
- Loss of EEAT2 on glia results in a lack of glutamate clearance and excitotoxicity
- Evidence of toxic effects of astrocytes on motor neurons
- Removal of mSOD1 from astrocytes in animal models alters disease progression
What is the proposed mechanism of glutamate-mediated excitotoxicity in ALS?
- Reduced expression of EEAT2 and therefore reduced glutamate clearance
- Increased activation of glutamate receptors (Ca2+ permeable AMPA) thought to lead to increased Ca2+ influx which stimulates cell death pathways
How has RNA processing been implicated in TDP-43 ALS pathology?
- TDP-43 regulates the splicing of multiple transcripts
- Binds and regulates mRNA allowing for spatio-temporal specific translation of mRNA targets
How has RNA-processing been implicated in FUS ALS-pathology?
- Involved in microRNA processing
- Involved in nucleus-cytoplasm shuttling
How have electrophysiological changes been associated with ALS?
- Motor neuron excitability is reported very early in pathogenesis
- Appears to relate to increased Na+ currents
What is the normal function of superoxide dismutase? How has it been linked to ALS?
- SOD1 catalyses the removal of harmful Cu2+ ions
- Mutations in SOD1 were the first mutations associated with ALS, now over 180 identified
- mutations disrupt protein stability and folding often leading to aggregate formation
- Thought to be a gain of function mutation as the complete absence of SOD1 in mice does not induce the disease
What have ends-out homologous recombination studies of ALS shown?
Disorganised NMJs with fewer synaptic boutons and decreased transmission in adult flies. While larvae showed clear locomotor deficits, these were not matched with severe electrophysiological or morphological changes. Instead, they propose dysfunction of peripheral feedback downregulating motor function as the cause of this early ALS pathology
What is the proposed normal function of C9orf72? How has it been associated with ALS?
- Chromosome 9 open reading frame 72 thought to play a role in mRNA transport and support
- First intron contains hexanucleotide repeat domain G4C2, an increased number of which is associated with 40% of FALS cases
- Two modes of pathogenesis proposed: accumulation of RNA foci which sequester RNA binding proteins and thus disrupt their function, and translation of G4C2 repeats to form dipeptide repeat proteins through non-ATG translation
How has the pathology of C9orf72 in ALS pathogenesis been controversial?
Evidence from Drosophila:
- Transgenic expression of C9orf72 with repeats showed that RNA foci were formed and that these sequestered RNA binding proteins (pur-alpha) which when overexpressed mitigated toxicity
- However, by interspersing stop codons within hexanucleotide repeats can get plenty of RNA foci which bind RBPs without toxicity, therefore dipeptide repeat proteins may enact pathogenesis
How has C9orf72 been linked to cytoplasmic transport?
- RanGAP (key regulator of nucleo-cytoplasmic transport) was identified as a key target of C9orf72
- Evidence from both Drosophila and ALS-patient derived iPSCs showed that ranGAP function and nucleocytoplasmic transport were impaired in the presence of C9orf72 repeats
What is TDP-43 and how is it significant in ALS?
- DNA/RNA binding protein
- Forms the major component of highly phosphorylated and ubiquitinated protein aggregates observed in most cases of ALS (both sporadic and familial) as well as fronto-temporal dementia and is also cleaved to form C-terminal fractions
- Inclusions can be found within neurons and glia as well as in the brain
How may TDP-43 contribute to ALS pathology through both loss and gain of function?
Gain of function as cytoplasmic inclusions and loss of function within the nucleus (as it is partly cleared from the nucleus of neurons containing cytoplasmic aggregates)
What is FUS? How has it been linked to ALS?
- RNA/DNA binding protein
- Mutations in this gene have been identified in roughly 4% of cases of familial ALS with a dominant mutation pattern
- Similar patterns of cytoplasmic inclusions as TDP-43 however TDP-43 inclusions are absent from patients with these kinds of inclusions
What commonalities do TDP-43 and FUS share?
- Both bind directly to RNA, as well as single or double stranded DNA
- Both implicated in RNA maturation and splicing
- Both also strongly implicated in microRNA processing (
What has been a potential target for TDP-43 ALS therapy and why?
- TDP-43 mutants which cannot bind to RNA confer little toxicity
- Overexpression of TARDBP (TDPH in drosophila) in glia showed reduced expression of mRNA of glutamate transporter dEAAT1/2
- Therefore limiting interactions with RNA is a possible target, some success with inhibiting interaction of TDP-43 and C9orf72 interactions using small molecules however these results are still preliminary
What is VAPB and how has it been associated with ALS?
- VAMP-Associated Protein B is a member of the VAP protein family
- Mutations (proline to serine codon 56) first identified in several cases of FALS in Brazil however European cases and a Japanese case have been identified
