Reduced Survival of Motor Neuron (SMN) Protein in Motor Neuronal Progenitors Functions Cell Autonomously to Cause Spinal Muscular Atrophy in Model Mice Expressing the Human Centromeric (SMN2) Gene

Abstract

Spinal muscular atrophy (SMA) is a common (ϳ1:6400) autosomal recessive neuromuscular disorder caused by a paucity of the survival of motor neuron (SMN) protein. Although widely recognized to cause selective spinal motor neuron loss when deficient, the precise cellular site of action of the SMN protein in SMA remains unclear. In this study we sought to determine the consequences of selectively depleting SMN in the motor neurons of model mice. Depleting but not abolishing the protein in motor neuronal progenitors causes an SMA-like phenotype. Neuromuscular weakness in the model mice is accompanied by peripheral as well as central synaptic defects, electrophysiological abnormalities of the neuromuscular junctions, muscle atrophy, and motor neuron degeneration. However, the disease phenotype is more modest than that observed in mice expressing ubiquitously low levels of the SMN protein, and both symptoms as well as early electrophysiological abnormalities that are readily apparent in neonates were attenuated in an age-dependent manner. We conclude that selective knock-down of SMN in motor neurons is sufficient but may not be necessary to cause a disease phenotype and that targeting these cells will be a requirement of any effective therapeutic strategy. This realization is tempered by the relatively mild SMA phenotype in our model mice, one explanation for which is the presence of normal SMN levels in non-neuronal tissue that serves to modulate disease severity.

Dysregulation of synaptogenesis genes antecedes motor neuron pathology in spinal muscular atrophy

“…Importantly, similar NMJ defects have been reported in type I (the most severe type) SMA human fetuses (10). SMA MNs have also been shown to be hyperexcitable and loss of proprioceptive synapses on the somata, and proximal dendrites of SMA MNs (MN deafferentation) occurs no later than P4 (11)(12)(13). These findings indicate that both peripheral (NMJs) and central (sensory-motor) synapses are affected in SMA mice at early stage of disease.…”

“…These findings indicate that both peripheral (NMJs) and central (sensory-motor) synapses are affected in SMA mice at early stage of disease. Although SMA is clinically manifested primarily in MNs, recent studies have shown that other cell types and nonneuronal tissues are also involved (13,14,15). SMN, as part of the SMN-Gemins complex (16), functions in the assembly of Sm protein cores on snRNAs (U1, U2, U4, U5, U11, U12, and U4atac), a key step in snRNP biogenesis (17-20).…”

Motor neuron cell-nonautonomous rescue of spinal muscular atrophy phenotypes in mild and severe transgenic mouse models

“…On the other hand, in several studies, including our own, selective depletion of SMN in motor neurons induces NMJ defects, and restoration of SMN rescues them, indicating a cell-autonomous mechanism (Park et al 2010;Gogliotti et al 2012;Lee et al 2012;Martinez et al 2012;Sahashi et al 2012Sahashi et al , 2013. This apparent discrepancy may be due to the extremely low SMN levels in motor neurons in these studies.…”

“…This unexpected finding indicates that the basal SMN protein levels in the CNS are already sufficient for normal mouse development, provided that a sufficient increase in SMN levels in peripheral tissues can be achieved. Consistent with this interpretation, using a conditional knockout strategy, Park et al (2010) recently found that 70% of mice with selective depletion of SMN in motor neurons survived over a year. Porensky et al (2012) reported that ICV injection of PMO ASO10-29 at a dose of 27, 54, or 81 mg in the SMND7 model (which has a slightly milder phenotype than the severe model that we used here) led to a sixfold to sevenfold survival increase; the investigators proposed that early and sustained increase of SMN in the CNS is essential for SMA therapy.…”

“…Sensory motor synaptic defects in the spinal cord have also been observed (Ling et al 2010;Park et al 2010;Mentis et al 2011). Three recent studies demonstrated that both peripheral and central synaptic defects are autonomously caused by insufficient levels of SMN in motor neurons (Gogliotti et al 2012;Lee et al 2012;Martinez et al 2012).…”