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 ecto-domains 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-avoidance6–11. 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.

Topographic Mapping--The Olfactory System

“…Cite this article as Cold Spring Harb Perspect Biol 2010;2:a001776 whether the diversity of DSCAM isoforms instructs glomerular specificity of either or both cell types, although an instructional role for DSCAM has been shown at other fly synapses (Hattori et al 2009). …”

Cartography of neurexin alternative splicing mapped by single-molecule long-read mRNA sequencing

“…The genome size is insufficient to encode such diversity, but a mixture of combinatorial expression patterns that pair different synaptic cell-adhesion molecules with each other and of distinct alternative splicing patterns that amplify the number of cell-adhesion molecules into a large number of isoforms may generate the number of transsynaptic interactions needed to account for the enormous diversity of synaptic connections. In Drosophila melanogaster, alternative splicing of the mRNAs encoding the Down syndrome cell-adhesion molecule can generate nearly 20,000 protein isoforms whose structures specify axon bundling but not synapse formation (6). In mammals, alternative splicing of neurexin and some protocadherin mRNAs can also produce thousands of isoforms, which may at least in the case of neurexins be involved in synapse formation (7)(8)(9)(10).…”

The Combinatorics of Neurite Self-Avoidance

“…When their neurites contact, they undergo homophilic binding, which generates repulsion, whereas when other neurons, which have a different set of isoforms, are encountered, homophilic binding and repulsion do not occur (see Figure 1). The number of Dscam1 isoforms expressed by each neuron is believed to be in the range 10 to 50, and it has been proposed that this set of isoforms is chosen stochastically from the set of all possible isoforms (Hattori et al, 2009). Although little is known biologically about how this might occur, understanding the consequences of this assumption reduces to a problem in combinatorics.…”