Ciliary neurotrophic factor-induced sprouting preserves motor function in a mouse model of mild spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is caused by homozygous loss or mutation of the SMN1 gene on human chromosome 5. Depending on the levels of SMN protein produced from a second SMN gene (SMN2), different forms of the disease are distinguished. In patients with milder forms of the disease, type III or type IV SMA that normally reach adulthood, enlargement of motor units is regularly observed. However, the underlying mechanisms are not understood. Smn(+/-) mice, a mouse model of type III/IV SMA, reveal progressive loss of motor neurons and denervation of motor endplates starting at 4 weeks of age. Loss of spinal motor neurons between 1 month and 12 months reaches 40%, whereas muscle strength is not reduced. In these animals, amplitude of single motor unit action potentials in the gastrocnemic muscle is increased more than 2-fold. Confocal analysis reveals pronounced sprouting of innervating motor axons. As ciliary neurotrophic factor (CNTF) is highly expressed in Schwann cells, we investigated its role for a compensatory sprouting response and maintenance of muscle strength in this mouse model. Genetic ablation of CNTF results in reduced sprouting and decline of muscle strength in Smn(+/-) mice. These findings indicate that CNTF is necessary for a sprouting response and thus enhances the size of motor units in skeletal muscles of Smn(+/-) mice. This compensatory mechanism could guide the way to new therapies for this motor neuron disease.