Self-avoidance alone does not explain the function of Dscam1 in mushroom body axonal wiring

Alternative splicing of Drosophila Dscam1 into 38,016 isoforms provides neurons with a unique molecular code for self-recognition and self-avoidance. A canonical model suggests that the homophilic binding of identical Dscam1 isoforms on the sister branches of mushroom body (MB) axons supports segregation with high fidelity, even when only a single isoform is expressed. Here, we generated a series of mutant flies with a single exon 4, 6, or 9 variant, encoding 1,584, 396, or 576 potential isoforms, respectively. Surprisingly, most of the mutants in the latter two groups exhibited obvious defects in the growth, branching, and segregation of MB axonal sister branches. This demonstrates that the repertoires of 396 and 576 Dscam1 isoforms were not sufficient for the normal patterning of axonal branches. Moreover, reducing Dscam1 levels largely reversed the defects caused by reduced isoform diversity, suggesting a functional link between Dscam1 expression levels and isoform diversity. Taken together, these results indicate that canonical self-avoidance alone does not explain the function of Dscam1 in MB axonal wiring.

Intron-targeted mutagenesis reveals roles for Dscam1 RNA pairing architecture-driven splicing bias in neuronal wiring

Drosophila melanogaster Down syndrome cell adhesion molecule (Dscam1) can generate 38,016 different isoforms through largely stochastic, yet highly biased, alternative splicing. These isoforms are required for nervous functions. However, the functional significance of splicing bias remains unknown. Here, we provide evidence that Dscam1 splicing bias is required for mushroom body (MB) axonal wiring. We generate mutant flies with normal overall protein levels and an identical number but global changes in exon 4 and 9 isoform bias (DscamΔ4D^-/- and DscamΔ9D^-/-), respectively. In contrast to DscamΔ4D^-/-, DscamΔ9D^-/- exhibits remarkable MB defects, suggesting a variable domain-specific requirement for isoform bias. Importantly, changes in isoform bias cause axonal defects but do not influence the self-avoidance of axonal branches. We conclude that, in contrast to the isoform number that provides the molecular basis for neurite self-avoidance, isoform bias may play a role in MB axonal wiring by influencing non-repulsive signaling.

Dscam1 in Pancrustacean Immunity: Current Status and a Look to the Future

The Down syndrome cell adhesion molecule 1 (Dscam1) gene is an extraordinary example of diversity: by combining alternatively spliced exons, thousands of isoforms can be produced from just one gene. So far, such diversity in this gene has only been found in insects and crustaceans, and its essential part in neural wiring has been well-characterized for Drosophila melanogaster. Ten years ago evidence from D. melanogaster showed that the Dscam1 gene is involved in insect immune defense and work on Anopheles gambiae indicated that it is a hypervariable immune receptor. These exciting findings showed that via processes of somatic diversification insects have the possibility to produce unexpected immune molecule diversity, and it was hypothesized that Dscam1 could provide the mechanistic underpinnings of specific immune responses. Since these first publications the quest to understand the function of this gene has uncovered fascinating insights from insects and crustaceans. However, we are still far from a complete understanding of how Dscam1 functions in relation to parasites and pathogens and its full relevance for the immune system. In this Hypothesis and Theory article, we first briefly introduce Dscam1 and what we know so far about how it might function in immunity. By focusing on seven questions, we then share our sometimes contrasting thoughts on what the evidence tells us so far, what essential experiments remain to be done, and the future prospects, with the aim to provide a multiangled view on what this fascinating gene has to do with immune defense.

Complete deletion of all alpha-dystrobrevin isoforms does not reveal new neuromuscular junction phenotype

The dystrophin glycoprotein complex (DGC) is critical for muscle stability, and mutations in DGC proteins lead to muscular dystrophy. The DGC also contributes to the maturation and maintenance of the neuromuscular junction (NMJ). The gene encoding the DGC protein alpha-dystrobrevin undergoes alternative splicing to produce at least five known isoforms. Isoform-specific antibody staining and reverse transcription PCR in mutant mice with a deletion of exon 3 of the alpha-dystrobrevin gene suggested the existence of a remaining synaptic isoform, which might be compensating for alpha-dystrobrevin function. To test this possibility and to more completely understand the synaptic function of alpha-dystrobrevin, we used a two-step homologous recombination strategy combined with in vivo Cre-mediated excision to generate mice with a large deletion of the alpha-dystrobrevin gene to disrupt all isoforms. However, these mice did not exhibit a more severe NMJ phenotype than that observed in the exon 3-deleted mice. Nonetheless, these mice not only eliminate possible compensation by remaining isoforms of alpha-dystrobrevin, but also offer a conditional allele that could be used to identify tissue-specific and developmental functions of alpha-dystrobrevin. This work also demonstrates a successful strategy to achieve deletion of a large genomic sequence, which can be a valuable tool for functional studies of genes encoding multiple isoforms that span a large genomic region.