Synaptic vulnerability in spinal muscular atrophy
Mounting evidence suggests that synaptic connections are early pathological targets in many neurodegenerative diseases, including motor neuron disease. A better understanding of synaptic pathology is therefore likely to be critical in order to develop effective therapeutic strategies. Spinal muscular atrophy (SMA) is a common autosomal recessive childhood form of motor neuron disease. Previous studies have highlighted nerve- and muscle-specific events in SMA, including atrophy of muscle fibres and postsynaptic motor endplates, loss of lower motor neuron cell bodies and denervation of neuromuscular junctions caused by loss of pre-synaptic inputs. Here I have undertaken a detailed morphological investigation of neuromuscular synaptic pathology in the Smn-/- ;SMN2 and Smn-/-;SMN2;Δ7 mouse models of SMA. Results imply that synaptic degeneration is an early and significant event in SMA, with progressive denervation and neurofilament accumulation being present at early symptomatic time points. I have identified selectively vulnerable motor units, which appear to conform to a distinct developmental subtype compared to more stable motor units. I have also identified significant postsynaptic atrophy which does no correlate with pre-synaptic denervation, suggesting that there is a requirement for Smn in both muscle and nerve and pathological events can occur in both tissues independently. Rigorous investigation of lower motor neuron development, connectivity and gene expression at pre-symptomatic time points revealed developmental abnormalities do not underlie neuromuscular vulnerability in SMA. Equivalent gene expression analysis at end-stage time points has implicated growth factor signalling and extracellular matrix integrity in SMA pathology. Using an alternative model of early onset neurodegeneration, I provide evidence that the processes regulating morphologically distinct types of synaptic degeneration are also mechanistically distinct. In summary, in this work I highlight the importance and incidence of synaptic pathology in mouse models of spinal muscular atrophy and provide mechanistic insight into the processes regulating neurodegeneration.