| Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by a loss of α-motoneurons in the spinal cord. SMA is caused by low levels of the ubiquitously expressed survival motor neuron (SMN) protein. Mouse models of SMA have been an extremely valuable tool for understanding the genetics of SMA. One of the drawbacks with the mouse system, however, is that it is not ideal for studying neural anatomy due to its neuromuscular complexity. Therefore, another vertebrate model system with more simplified and stereotypic neuromuscular organization, the zebrafish, will be used to model and further study SMA.;To mimic SMA in zebrafish, antisense morpholino oligonucleotides have been utilized to reduce Smn levels in the developing embryo. When Smn levels are reduced throughout the entire embryo, motor axon pathfinding defects are observed. These defects are specific to motor axons; other sensory and interneuron axons were unaffected by Smn knockdown. Reduction of Smn in individual motoneurons revealed that smn is acting cell-autonomously. These results show that Smn functions in motor axon development and suggest that these early developmental defects may lead to subsequent motoneuron loss.;Because SMN is a ubiquitously expressed protein, a paramount question in SMA research revolves around why reduced levels of SMN lead to a motoneuron-specific disease. It has been hypothesized that SMN may have a dual function: a well-characterized role in mediating snRNP assembly and a novel motor axonal-specific function. To begin to identify which function is important for axonal outgrowth and potentially SMA, non-Smn components in these separate pathways have been knocked-down by morpholino in zebrafish. Knockdown of Gemin2, a Smn interacting snRNP assembly component, does not yield motor axon defects. If Smn does have a role in motor axons, it is in the proper place to do so, as shown by hSMN-RFP localization to motor axons. While not conclusive evidence, these data suggest a non-snRNP assembly function for Smn in motor axon outgrowth and pathfinding.;Because morpholinos are a transient knockdown of protein and have an inherent variability in their phenotypes, a genetic model of SMA utilizing a zebrafish smn mutation can address questions that morpholinos alone cannot. Because targeted mutagenesis is unavailable in zebrafish, rapid high-throughput screens for mutations in a particular gene of interest are necessary to further study gene function. (Abstract shortened by UMI.). |
