Investigating the timing of pathology in motor neurons of the Smn2B/- mouse model of Spinal Muscular Atrophy

Centre for Integrative Physiology, Euan MacDonald Centre for Motor Neurone Disease Research, University of Edinburgh

Spinal muscular atrophy (SMA) is a motor neuron disease, caused by mutations and deletions within the SMN1 gene. This results in a reduction of SMN protein leading to a loss of lower motor neurons with subsequent muscle weakness and atrophy. Mouse models of SMA have revealed that motor neurons degenerate in a die-back fashion, with obvious distal to proximal withdrawal of the presynaptic terminals at the neuromuscular junction (NMJ). Because of this, it has often been assumed that synaptic loss precedes motor neuron death. However, recent data from our lab has suggested that there is an up-regulation of transcripts involved in cell death, specifically the P53 signalling pathway prior to symptom onset in 2 independent mouse models of SMA. It is therefore unclear whether synaptic defects cause motor neuron withdrawal or whether defects that occur elsewhere in the motor neuron cause the initiation of cell death pathways and subsequent presynaptic retraction. The Smn2B/- mouse model of SMA has an extended pre-symptomatic period compared to other mouse models of the disease which allows investigation into changes that may be occurring within motor neurons prior to obvious degeneration. We have therefore profiled neuromuscular junction pathology (swelling and endplate occupancy) and motor neuron cell body pathology (motor neuron cell body area and number, number of activated-caspase 3 positive cells) from p1 to P15. We also assessed the levels of transcripts associated with cell death. This detailed profiling of NMJ and motor neuron pathology has revealed that the earliest NMJ defects coincide with the up-regulation of the P53 signaling pathway at the cell body, and precede detectable motor neurons loss by up to 5 days. Future work will involve narrowing down the window in which pathology at the NMJ and at the cell body begins. Additionally, we are currently developing an inducible P53-knockdown Smn2B/- mouse model to determine whether synaptic withdrawal is a consequence of P53 activation.

References

1. Murray, L. M., Comley, L. H., Thomson, D., Parkinson, N., Talbot, K. & Gillingwater, T. H. Selective vulnerability of motor neurons and dissociation of pre- and post-synaptic pathology at the neuromuscular junction in mouse models of spinal muscular atrophy. Hum. Mol. Genet. 17, 949–62 (2008)
2. Murray, L. M., Beauvais, A., Gibeault, S., Courtney, N. L. & Kothary, R. Transcriptional profiling of differentially vulnerable motor neurons at pre-symptomatic stage in the Smn (2b/-) mouse model of spinal muscular atrophy. Acta Neuropathol. Commun. 3, 55 (2015)
3. Bowerman, M., Murray, L. M., Beauvais, A., Pinheiro, B. & Kothary, R. A critical smn threshold in mice dictates onset of an intermediate spinal muscular atrophy phenotype associated with a distinct neuromuscular junction pathology. Neuromuscul. Disord. 22, 263–76 (2012)

Funded by: This work was funded by Cure SMA, Fight SMA and the Gwendolyn Strong Foundation. NC is the recipient of a Euan MacDonald Centre PhD Scholarship.

* entered into the PhD student poster competition