New insights in the understanding of motor neuron disease by longitudinal brain and muscle MRI analysis and characterization of spinal cord-derived stem cells in G93-SOD1 mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal, neurodegenerative disorder caused by the degeneration of motor neurons in the CNS, which results in complete paralysis of skeletal muscles. To establish the timeframe of motor neuron degeneration in relation to muscle atrophy in motor neuron disease, we have used MRI to monitor changes throughout disease in brain and skeletal muscle of G93A-SOD1 mice, a purported model of ALS. Longitudinal MRI examination of the same animals indicated that muscle volume in the G93A-SOD1 mice was significantly reduced from as early as week 8 of life, four weeks prior to clinical onset. Progressive muscle atrophy from week 8 onwards was confirmed by histological analysis. In contrast, brain MRI indicated that neurodegeneration occurs later in G93A-SOD1 mice, with hyperintensity MRI signals detected only at weeks 10-18. Neurodegenerative changes were observed only in the motor nuclei areas of the brainstem; MRI changes indicative of neurodegeneration were not detected in the motorcortex where first motor neurons originate, even at the late disease stage. This longitudinal MRI study establishes unequivocally that, in the experimental murine model of ALS, muscle degeneration occurs before any evidence of neurodegeneration and clinical signs, supporting the postulate that motor neuron disease can initiate from muscle damage and result from retrograde dying-back of the motor neurons. In G93A-SOD1 ALS mice the response to neurodegeneration comprises proliferation and migration of ependymal stem progenitor cells (epSPCs), normally present and quiescent in spinal cord. We isolated epSPCs from G93A-SOD1 mice at 8 (asymptomatic) and 18 (symptomatic) weeks of age, and characterized the ability of epSPC cultures to proliferate and differentiate into the three neural cell lineages. G93A epSPCs produced neurospheres of self-renewing cells, and differentiated into more neurons and fewer astrocytes than control epSPCs, whereas oligodendrocytes did not show difference between the examined groups. The G93A-SOD1 neurons were small and the astrocytes were consistently activated. MicroRNA analysis revealed that miR-9 and miR-124a, involved in neural cell fate, were upregulated in differentiating G93A-SOD1 epSPCs, particularly at 18 weeks. miR-19a and miR-19b, implicated in cell-cycle regulation, were differentially expressed during epSPC differentiation in G93A-SOD1 compared with controls. Our findings demonstrated that G93A-SOD1 epSPCs have neurogenic potential constituting a source of multipotent cells useful for understanding the ALS pathogenesis and for identifying new therapeutic targets.