SMN, profilin IIa and plastin 3: a link between the deregulation of actin dynamics and SMA pathogenesis.

Spinal muscular atrophy (SMA) is the most common human genetic disease resulting in infant mortality. SMA is caused by mutations or deletions in the ubiquitously expressed survival motor neuron 1 (SMN1) gene. Why SMA specifically affects motor neurons remains poorly understood. We have shown that Smn deficient PC12 cells have increased levels of the neuronal profilin IIa protein, leading to an inappropriate activation of the RhoA/ROCK pathway. This suggests that mis-regulation of neuronal actin dynamics is central to SMA pathogenesis. Here, we demonstrate an increase in profilin IIa and a decrease in plastin 3 protein levels in a SMA mouse model. Furthermore, knock-out of profilin II upregulates plastin 3 expression in a Smn-dependent manner. However, the depletion of profilin II and the restoration of plastin 3 are not sufficient to rescue the SMA phenotype. Our study suggests that additional regulators of actin dynamics must also contribute to SMA pathogenesis.