Comprehensive Modeling of Spinal Muscular Atrophy in Drosophila melanogaster

Abstract

Spinal muscular atrophy (SMA) is a neurodegenerative disorder that affects motor neurons, primarily in young children. SMA is caused by mutations in the Survival Motor Neuron 1 (SMN1) gene. SMN functions in the assembly of spliceosomal RNPs and is well conserved in many model systems including mouse, zebrafish, fruit fly, nematode, and fission yeast. Work in Drosophila has focused on the loss of SMN function during larval stages, primarily using null alleles or strong hypomorphs. A systematic analysis of SMA-related phenotypes in the context of moderate alleles that more closely mimic the genetics of SMA has not been performed in the fly, leading to debate over the validity and translational value of this model. We, therefore, examined 14 Drosophila lines expressing SMA patient-derived missense mutations in Smn , with a focus on neuromuscular phenotypes in the adult stage. Animals were evaluated on the basis of organismal viability and longevity, locomotor function, neuromuscular junction structure, and muscle health. In all cases, we observed phenotypes similar to those of SMA patients, including progressive loss of adult motor function. The severity of these defects is variable and forms a broad spectrum across the 14 lines examined, recapitulating the full range of phenotypic severity observed in human SMA. This includes late-onset models of SMA, which have been difficult to produce in other model systems. The results provide direct evidence that SMA-related locomotor decline can be reproduced in the fly and support the use of patient-derived SMN missense mutations as a comprehensive system for modeling SMA.

Intimate functional interactions between TGS1 and the Smn complex revealed by an analysis of the Drosophila eye development

“…Previous studies have shown that loss-of-function mutations in the Drosophila Smn gene result in a variety of phenotypes, including alterations in the sensory-motor neuronal network, abnormal neuromuscular junctions, and defective locomotion [44][45][46][47][48][49]. In addition, we have recently shown that Smn depletion in neurons results in unexpanded wings and unretracted ptilinum [43].…”

“…The unexpanded wing phenotype has been also observed in flies where Smn was specifically silenced in neurons using RNAi [43]. Smn mutant larvae are defective in the sensory-motor neuronal network and exhibit reduced muscle growth and defective locomotion [44,48,49,69]. These phenotypes have been attributed to reduced biogenesis of snRNAs leading to defective splicing of a subset U12 intron-containing RNAs, altering the expression of genes required for motor circuit function such as Stasimon [20].…”

Defective minor spliceosomes induce SMA-associated phenotypes through sensitive intron-containing neural genes in Drosophila

“…In Drosophila SMA models, Smn mutant and RNAi strains have reduced muscle size, motor rhythm, and motor neuron neurotransmission, and result in hindered locomotion 24 , 37 – 41 . Restored expression in the motor circuit of Stasimon , a U12 intron-containing gene, has been reported to correct defects in neuromuscular junction (NMJ) transmission and muscle growth in Drosophila Smn mutants 24 , whereas other studies show that the retention of the Stasimon U12 intron is instead a consequence of the developmental arrest 35 .…”

Assembly of higher-order SMN oligomers is essential for metazoan viability and requires an exposed structural motif present in the YG zipper dimer

“…Given that this C-terminal glycine (hsG279) is the least well-conserved position among the three G-motif residues (see Fig. S1A), the spG143V result suggests that the observed metazoan SMA phenotype (human: Type I, [42]; fruitfly: Class 2, [43]) is caused by a distinct mechanism-of-action. Indeed, the C-terminus of the nematode SMN orthologue diverges in this region and a chimeric fusion of the yeast and worm YG boxes fully complements the smn1 null mutation in vivo and displays an oligomerization profile that is very similar to that of the wild-type S.pombe protein in vitro (Fig.…”