Glycosphingolipid dysregulation and lysosomal dysfunction in motor neuron disease
Keywords: edaravone
SOD1(G93A)
motor neuron disease
TDP-43(M337V)
glucosylceramide
SOD1(G86R)
glycosphingolipids
NP-HPLC

Historically, the early-onset rare neurodegenerative lysosomal storage disorders (LSDs) have been studied as discrete metabolic diseases in their own right. However, some links with common late-onset neurodegenerative diseases, including motor neurone disease (MND), have recently emerged. Reports show lipid dysregulation, including increased levels of various glycosphingolipids (GSLs) and upregulation of enzymes responsible for GSL hydrolysis, in the spinal cord of MND patients and the SOD1G93A mouse model. Inhibition of glucosylceramide (GlcCer) synthesis, the precursor of the vast majority of GSLs, accelerated disease progression and decreased lifespan, whereas infusion with GM3 ganglioside significantly delayed disease progression and increased survival, in the same mouse model. Previous work in our laboratory showed that GlcCer and downstream metabolism is significantly altered in spinal cord and muscle of SOD1G86R mice, and inhibition of GlcCer catabolism preserved motor function and delayed disease onset. Together, these studies highlight the importance of GlcCer and other GSLs for maintenance and regeneration of motor units. However, the relationship between the basic biochemical mechanisms linking lipid dysregulation, lysosomal dysfunction and altered lysosomal enzymatic activity in MND remains poorly understood.
In this thesis, the basic biological processes that cause lipid changes and lysosomal dysfunction in MND were investigated. After optimising a protocol for quantification of GlcCer by normal-phase high-performance liquid chromatography (NP-HPLC), general changes in patterns of GSL expression in samples from SOD1G86R, SOD1G93A and TDP-43M337V mouse models of MND in different stages of disease development were characterised. In agreement with previous reports, in spinal cord and soleus muscle of SOD1G86R mice, total GSL levels in gastrocnemius and tibialis anterior of SOD1G93A mice were elevated at later stages of disease. However, the GSL content in these tissues from SOD1G93A mice and in spinal cord and tibialis anterior of SOD1G86R mice remained unchanged at the time of symptoms onset. In TDP-43M337V mice, which show progressive motor function deficits much later than the SOD1 models, GSL levels in the spinal cord remained unchanged in asymptomatic and initial stages of disease, but some changes were found in the brain of these mice. Importantly, changes in the activity of the enzymes that regulate GSL catabolism were found both in the spinal cord and brain of TDP-43M337V mice. These data suggest different changes in GSL catabolism occur in different stages of disease depending on the mutation or the aetiology of MND.
Finally, a pilot study exploring the effect of edaravone, a free radical scavenger used for the treatment of MND, in the TDP-43M337V mouse model of MND and the Hexb-/- mouse model of the LSD Sandhoff disease showed no effect in the disease progression or the development of the main symptoms associated to MND or Sandhoff disease. Additional pharmacological studies are necessary to better investigate the potential common mechanisms between MND and LSDs.