Survival Motor Neuron Affects Plastin 3 Protein Levels Leading to Motor Defects

Abstract

The actin binding protein plastin 3 (PLS3) has been identified as a modifier of the human motoneuron disease spinal muscular atrophy (SMA). SMA is caused by decreased levels of the survival motor neuron protein (SMN) and in its most severe form causes death in infants and young children. To understand the mechanism of PLS3 in SMA, we have analyzed pls3 RNA and protein in zebrafish smn mutants. We show that Pls3 protein levels are severely decreased in smn−/− mutants without a reduction in pls3 mRNA levels. Moreover, we show that both pls3 mRNA and protein stability are unaffected when Smn is reduced. This indicates that SMN affects PLS3 protein production. We had previously shown that in smn mutants, the presynaptic protein SV2 is decreased at neuromuscular junctions (NMJs). Transgenically driving human PLS3 in motoneurons rescues the decrease in SV2 expression. To determine whether PLS3 could also rescue function, we performed behavioral analysis on smn mutants and found that they had a significant decrease in spontaneous swimming and turning. Driving PLS3 transgenically in motoneurons rescued both of these defects. These data show that PLS3 protein levels are dependent on SMN and that PLS3 is able to rescue the neuromuscular defects and corresponding movement phenotypes caused by low levels of Smn suggesting that decreased PLS3 contributes to SMA motor phenotypes.

Therapeutic strategies for spinal muscular atrophy: SMN and beyond

“…Indeed, overexpression of plastin 3 in a zebrafish model of SMA significantly rescues the axonal growth and branching defects caused by smn1 gene depletion ( Oprea et al, 2008 ). Further analysis of smn1 mutant zebrafish reveals that reduced SMN levels lead to decreased plastin 3 protein expression, NMJ defects and aberrant motor function, and these effects can be corrected by plastin 3 overexpression ( Hao et al, 2012 ). More recently, studies in mice have shown that increased expression of plastin 3 delays axonal degeneration and improves NMJ function ( Ackermann et al, 2013 ) as well as ameliorates survival and neuromuscular phenotype ( Kaifer et al, 2017 ), possibly through the modulation of endocytic pathways ( Hosseinibarkooie et al, 2016 ).…”

SMN affects membrane remodelling and anchoring of the protein synthesis machinery

“…PLS3 is an F-actin-bundling protein, primarily involved in the regulation of the actin cytoskeleton (Delanote et al, 2005). Interestingly, zebrafish SMN mutants display reduced PLS3 protein production, without changes at the transcriptional level (Hao et al, 2012). Importantly, proteins encoded by SMA modifier genes operate within distinct cellular networks, specifically, actin dynamics, endocytosis and mRNA translational control (Dimitriadi et al, 2010).…”

Small Molecule Suppressors of Drosophila Kinesin Deficiency Rescue Motor Axon Development in a Zebrafish Model of Spinal Muscular Atrophy

“…9). The different efficacies of SBL-154 in the morpholino and genetic models of SMA may indicate that the compound’s therapeutic effect is restricted to motor neurons, since previously reported suppressor strategies that target motor neurons also failed to increase survival [93], [94]. Even increased SMN levels – if restricted to motor neurons – have limited effects on survival of zebrafish and mouse models of SMA [95]–[98], which is consistent with the emerging view of SMA as multisystem disorder [99], [100].…”