Smn deficiency causes neuritogenesis and neurogenesis defects in the retinal neurons of a mouse model of spinal muscular atrophy.

The eye is an excellent model for the study of neuronal development and pathogenesis of central nervous system disorders because of its relative ease of accessibility and the well-characterized cellular makeup. We have used this model to study spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disease caused by deletions or mutations in the survival of motor neuron 1 gene (SMN1). We have investigated the expression pattern of mouse Smn mRNA and protein in the neural retina and the optic nerve of wild type mice. Smn protein is present in retinal ganglion cells and amacrine cells within the neural retina as well as in glial cells in the optic nerve. Histopathological analysis in phenotype stage SMA mice revealed that Smn deficiency is associated with a reduction in ganglion cell axon and glial cell number in the optic nerve, as well as compromised cellular processes and altered organization of neurofilaments in the neural retina. Whole mount preparation and retinal neuron primary culture provided further evidence of abnormal synaptogenesis and neurofilament accumulation in the neurites of Smn-deficient retinal neurons. A subset of amacrine cells is absent, in a cell-autonomous fashion, in the retina of SMA mice. Finally, the retinas of SMA mice have altered electroretinograms. Altogether, our study has demonstrated defects in axodendritic outgrowth and cellular composition in Smn-depleted retinal neurons, indicating a role for Smn in neuritogenesis and neurogenesis, and providing us with an insight into pathogenesis of SMA.