Cis mutagenesis in vivo reveals extensive noncanonical functions of Dscam1 isoforms in neuronal wiring

Abstract

Drosophila Dscam1 encodes tens of thousands of cell recognition molecules via alternative splicing, which are required for neural function. A canonical self-avoidance model seems to provide a central mechanistic basis for Dscam1 functions in neuronal wiring. Here, we reveal extensive noncanonical functions of Dscam1 isoforms in neuronal wiring. We generated a series of allelic cis mutations in Dscam1, encoding a normal number of isoforms, but with an altered isoform composition. Despite normal dendritic self-avoidance and self/non-self discrimination in dendritic arborization (da) neurons, which is consistent with the canonical self-avoidance model, these mutants exhibited strikingly distinct spectra of phenotypic defects in the three types of neurons: up to ∼60% defects in mushroom bodies, a significant increase in branching and growth in da neurons, and mild axonal branching defects in mechanosensory neurons. Remarkably, the altered isoform composition resulted in increased dendrite growth yet inhibited axon growth. Moreover, reducing Dscam1 dosage exacerbated axonal defects in mushroom bodies and mechanosensory neurons, but reverted dendritic branching and growth defects in da neurons. This splicing-tuned regulation strategy suggests that axon and dendrite growth in diverse neurons cell-autonomously require Dscam1 isoform composition. These findings provide important insights into the functions of Dscam1 isoforms in neuronal wiring.

A systematic CRISPR screen reveals redundant and specific roles for Dscam1 isoform diversity in neuronal wiring

“…This idea is supported by genetic evidence that altering the variant composition by deleting docking sites in exon 4 or 6 clusters leads to <5% subtle to mild MB defects [ 27 , 36 ]. However, deleting upstream docking sites in the exon 9 cluster causes up to 60% of MB defects [ 28 ]. Diverse neurons exhibit considerable correlation curve differences in individual exon clusters ( Fig 7G–7I ).…”

“…Recent knockdown of Dscam1 revealed its diverse roles in central neuron arbor differentiation [ 26 ]. Recently, we showed that alterations of Dscam1 exon 9 variants result in obvious mushroom body (MB) axonal defects, partially supporting the specific function of Dscam1 isoforms [ 27 , 28 ]. Genetic analysis of a series of mutant flies with a single exon 6 or 9 variant revealed that 396 and 576 Dscam1 isoforms are insufficient for normal patterning of MB axonal branches [ 22 ].…”