Early loss of axons limits the therapeutic window in a mouse model of spinal muscular atrophy

Abstract

Current treatments for spinal muscular atrophy have significantly improved patient prognosis, but still fall short of a full cure, particularly when administered after symptom onset. To improve future therapies, it is essential to identify the early pathological events that currently limit treatment efficacy. Here we have investigated the vulnerability and maturation of intramuscular motor axons in the SmnΔ7 mouse model of spinal muscular atrophy using a panel of cranial muscles with differing susceptibility to disease. We demonstrate that although there is intermuscular heterogeneity in neuromuscular junction pathology, they are united by a common loss of motor axons. We show that the timing of motor axon loss differs between muscles, but that motor axon loss precedes structural denervation and can occur by postnatal day 1 in highly vulnerable muscles. Electron microscopy of intramuscular axons revealed that axonal maturation and myelination are grossly normal prior to subsequent postnatal degeneration. We note that axons lack the traditional morphological correlates of axon degeneration but can be visualised using antibodies which detect degenerating neurofilaments. Treatment with SMN-upregulating therapy SMN-C8 at best preserves intramuscular axons when administered before the onset of degeneration but is unable to reverse damage or promote regeneration once axon loss has occurred. Collectively this data demonstrates that SMN-upregulating therapy can prevent motor axon loss, but the capacity is limited by the number of intramuscular axons which remain at the time of treatment onset. This work highlights the importance of developing complementary therapies which can promote motor axon regeneration to act in synergy with SMN-upregulating compounds.