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Robust discrimination between self and non-self neurites requires thousands of Dscam1 isoforms

Abstract

Down Syndrome cell adhesion molecule (Dscam) genes encode neuronal cell recognition proteins of the immunoglobulin superfamily1,2. In Drosophila, Dscam1 generates 19,008 different ectodomains by alternative splicing of three exon clusters, each encoding half or a complete variable immunoglobulin domain3. Identical isoforms bind to each other, but rarely to isoforms differing at any one of the variable immunoglobulin domains4,5. Binding between isoforms on opposing membranes promotes repulsion6. Isoform diversity provides the molecular basis for neurite self-avoidance6,7,8,9,10,11. Self-avoidance refers to the tendency of branches from the same neuron (self-branches) to selectively avoid one another12. To ensure that repulsion is restricted to self-branches, different neurons express different sets of isoforms in a biased stochastic fashion7,13. Genetic studies demonstrated that Dscam1 diversity has a profound role in wiring the fly brain11. Here we show how many isoforms are required to provide an identification system that prevents non-self branches from inappropriately recognizing each other. Using homologous recombination, we generated mutant animals encoding 12, 24, 576 and 1,152 potential isoforms. Mutant animals with deletions encoding 4,752 and 14,256 isoforms14 were also analysed. Branching phenotypes were assessed in three classes of neurons. Branching patterns improved as the potential number of isoforms increased, and this was independent of the identity of the isoforms. Although branching defects in animals with 1,152 potential isoforms remained substantial, animals with 4,752 isoforms were indistinguishable from wild-type controls. Mathematical modelling studies were consistent with the experimental results that thousands of isoforms are necessary to ensure acquisition of unique Dscam1 identities in many neurons. We conclude that thousands of isoforms are essential to provide neurons with a robust discrimination mechanism to distinguish between self and non-self during self-avoidance.

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Figure 1: Thousands of isoforms are required for dendrites of dendritic arborization neurons to distinguish between self and non-self.
Figure 2: Thousands of isoforms are required for mushroom body lobe development.
Figure 3: Branching of posterior scutellar neurons requires thousands of isoforms independent of their identity.
Figure 4: Mathematical model showing that thousands of isoforms are required to give rise to many unique Dscam1 identities.

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Acknowledgements

We thank T. Lee and B. Dickson for fly stocks, B. Dickson and E. Demir for advice on homologous recombination procedures, S. Miura for staining of embryos, and N. Nocera and other members of the Zipursky laboratory for help in screening homologous recombinants. We thank D. Eisenberg for helpful discussion on mathematical modelling. We thank R. Axel, A. Nern, W. Wojtowicz and other members of the Zipursky and Grueber laboratories for discussion and comments on the manuscript. This work was supported by a grant from the NIH (S.L.Z.), and NRSA from NIH/NINDS (B.J.M.).W.B.G. is a Searle Scholar, McKnight Scholar and Klingenstein Fellow. S.L.Z. is an investigator of the Howard Hughes Medical Institute.

Author Contributions D.H. generated knock-in mutants. B.J.M. and W.B.G. analysed dendritic arborization neuron patterning. D.H. and Y.C. performed analyses of mushroom body and posterior scutellar neurons. L.S. and D.H. conducted mathematical modelling. C.S. performed statistical analysis on the branching of posterior scutellar neuron and linear regression analysis on dendritic arborization neuron data. D.H. and S.L.Z. designed the study and wrote the paper.

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Correspondence to S. Lawrence Zipursky.

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Hattori, D., Chen, Y., Matthews, B. et al. Robust discrimination between self and non-self neurites requires thousands of Dscam1 isoforms. Nature 461, 644–648 (2009). https://doi.org/10.1038/nature08431

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