A Drosophila melanogaster model of spinal muscular atrophy reveals a function for SMN in striated muscle

Abstract

Mutations in human survival motor neurons 1 (SMN1) cause spinal muscular atrophy (SMA) and are associated with defects in assembly of small nuclear ribonucleoproteins (snRNPs) in vitro. However, the etiological link between snRNPs and SMA is unclear. We have developed a Drosophila melanogaster system to model SMA in vivo. Larval-lethal Smn-null mutations show no detectable snRNP reduction, making it unlikely that these animals die from global snRNP deprivation. Hypomorphic mutations in Smn reduce dSMN protein levels in the adult thorax, causing flightlessness and acute muscular atrophy. Mutant flight muscle motoneurons display pronounced axon routing and arborization defects. Moreover, Smn mutant myofibers fail to form thin filaments and phenocopy null mutations in Act88F, which is the flight muscle–specific actin isoform. In wild-type muscles, dSMN colocalizes with sarcomeric actin and forms a complex with α-actinin, the thin filament crosslinker. The sarcomeric localization of Smn is conserved in mouse myofibrils. These observations suggest a muscle-specific function for SMN and underline the importance of this tissue in modulating SMA severity.

Gemin3Is an Essential Gene Required for Larval Motor Function and Pupation inDrosophila

“…The Flag-Gemin3 product was cloned into pUAST and sent to BestGene, (Chino Hills, CA) for embryo injection and transgene screening. The YFP-Smn transgene (a gift from J. Gall, Carnegie Institution of Washington, Baltimore, MD) has been previously characterized (Liu et al, 2006;Rajendra et al, 2007). Western blotting demonstrated a specific, direct interaction with GST-dSMN.…”

“…We utilized an Sm core assembly assay to investigate whether dGem3 plays a similar conserved role in flies. As described in Rajendra et al (2007), this assay uses cytoplasmic extracts prepared from S2 cell lysates depleted for individual components by RNAi. Radiolabeled U1 snRNA was incubated in these lysates, and its assembly with Sm proteins was assayed by coimmunoprecipitation with anti-Sm antibodies (mAb Y12).…”

“…Smn E33 homozygotes exhibit reduced dSMN protein levels in the thorax of the adult fly. This deficiency leads to severe neuromuscular defects, including flightlessness, all of which can be rescued by expression of a YFP-Smn transgene (Rajendra et al, 2007). Notably, SMN is a sarcomeric protein in both flies and mice, and because snRNPs are absent from myofibrils, SMN likely performs a tissue-specific function in muscle (Rajendra et al, 2007).…”

“…Genetic analysis in model organisms provides a unique opportunity to study factors contributing to disease pathogenesis. Drosophila SMN (dSMN) has been identified on the basis of sequence and functional conservation, and null mutations within the gene are larval lethal in the second and third instar stages (Chan et al, 2003;Rajendra et al, 2007). These larvae exhibit motor and neuromuscular defects.…”

“…We have also generated an adult model for Drosophila SMA. A hypomorphic mutation, called Smn E33 , was created by imprecise excision of a P-element residing in the upstream control region (Rajendra et al, 2007). Smn E33 homozygotes exhibit reduced dSMN protein levels in the thorax of the adult fly.…”