Figure 1: Dscam1 isoform diversity is generated through alternative splicing.
The Dscam1 gene is alternatively spliced to give rise to a vast number of different isoforms. Dscam1 proteins are single-pass transmembrane proteins of the immunoglobulin (Ig) superfamily. Each isoform shares the same domain structure but differs in amino acid sequences within three variable Ig domains (Ig2, Ig3, and Ig7; indicated in colors red, blue, and green, respectively).
From Schmucker, D., Clemens, J.C., Shu, H., Worby, C.A., Xiao, J., Muda, M., Dixon, J.E., and Zipursky, S.L. 2000. Cell 101:671–684. © 2000 Elsevier Inc.
Figure 2: Dscam1 protomers undergo a conformational change upon homophilic binding.
Dscam1 isoforms form trans dimers through matching of Ig2, Ig3, and Ig7. X-ray crystal structures of Ig1–8 indicate that Dscam1 forms a twofold symmetric double S-shape structure. Binding requires that the two monomers share the same Ig2 (red), Ig3 (blue), and Ig7 (green) domain. Interfaces form through an antiparallel matching of interfaces within segments of paired Ig domains. The Zipursky lab speculates that matching induces a conformational change and this in turn leads to signaling from the cytoplasmic domain promoting repulsion.
From Sawaya, M.R., Wojtowicz, W.M., Andre, I., Qian, B., Wu, W., Baker, D., Eisenberg, D., and Zipursky, S.L. 2008. Cell 134:1007–1018. © 2008 Elsevier Inc. See also Wojtowicz, W.M., Flanagan, J.J., Millard, S.S., Zipursky, S.L., and Clemens, J.C. 2004. Cell 118:619–633; and Wojtowicz, W.M., Wu, W., Andre, I., Qian, B., Baker, D., and Zipursky, S.L. 2007. Cell 130:1134–1145.
Figure 3: Dscam1 mediates dendrite self-avoidance.
Dscam1-mediated self-avoidance is best described in the dendritic arborization (da) neurons in the fly larval body wall. Upper panels: wild-type, type I da neurons arborize within the body wall, and their dendrites seldom cross or fasciculate (left). By contrast, in null mutants, these dendrites cross and fasciculate with one another (right).
Lower panels: type I (magenta) and type III (green) da neurons overlap with the dendrites crossing one another (left). Expression of the same isoform in each cell induces repulsion between them (right). These and other studies argue that Dscam1 isoform diversity is essential for self-avoidance. Diversity and largely random expression of isoforms in different neurons ensure that neurites of different neurons are sufficiently different for neurites to discriminate between neurites from the same and different cells.
From the lab of Wesley Grueber (Columbia University). See also Matthews, B.J., Kim, M.E., Flanagan, J.J., Hattori, D., Clemens, J.C., Zipursky, S.L., and Grueber, W.B. 2007. Cell 129:593–604.
Figure 4: Tetrad synapses in the fly visual system.
Synapses in the fly central nervous system are typically of the multiple-contact type. These include a single presynaptic release site and multiple postsynaptic elements. The best characterized of these are the tetrad synapses in the lamina neuropil. The lamina neuropil comprises some 750 microcircuits, called cartridges, and each cartridge contains processes from some 19 different neuronal cell types, and these processes are interlinked by more than 1,000 synaptic connections. Tetrad synapses are the output synapses for R1–R6 photoreceptor neurons. In the cartoon view in the center of the figure, the R cell axon terminals are shown as gray cylinders. The four postsynaptic elements include L1 and L2. These are found paired at all tetrads and thus represent an invariant pair. The other two postsynaptic elements are contributed by other neurons, and in this schematic they include one each from an amacrine (Am) and L3 neuron. In the lower panel, the electron micrograph shows a presynaptic element with a characteristic T-bar and two postsynaptic elements from L1 and L2. A series of studies in the lab using newly developed genetic methods and light microscopy, RNAseq, and traditional genetic approaches have been taken to identify the cell recognition molecules regulating this process.
Adapted from Millard, S.S., Lu, Z., Zipursky, S.L., and Meinertzhagen, I.A. 2010. Neuron 67:761–768.
Figure 5: Dscam1 and Dscam2 act redundantly to control tetrad composition.
Using serial EM reconstruction, the Zipursky lab sought to assess whether Dscam1 and Dscam2 play a role in tetrad assembly. Surprisingly, these two genes act in parallel and in a largely redundant fashion to prevent inappropriate pairing of L1 and L2. The L1 postsynaptic elements are from the same L1 cells; this is also the case for pairs of L2 postsynaptic elements. Horse-radish peroxidase (HRP) was expressed as a transgene selectively in L2 to aid in identifying the L2 axon. It produces an electron-dense precipitation along the membrane. R, photoreceptor cell axon.
From the Zipursky lab. See also Millard, S.S., Lu, Z., Zipursky, S.L., and Meinertzhagen, I.A. 2010. Neuron 67:761–768.
Figure 6: N-cadherin, Semaphorin-1a, and Dscam2 regulate L1 targeting.
Two left panels: in wild type, L1 axons terminate in the M5 layer of the medulla. In the absence of both N-cadherin and Semaphorin-1a, L1 axons extend beyond M5 into deeper regions of the visual system. N-cadherin mediates binding between L1 and other axons within M5, and Semaphorin-1a acts as a repulsive receptor for Plexin A, acting as a ligand, expressed in neurons in the layer just below M6.
Two right panels: in wild type, L1 arbors in the M1 and M5 layers are restricted to a single column. In the absence of Dscam2, arbors extend into adjacent columns. This result and other genetic experiments support a model wherein Dscam2 promotes repulsion between L1 arbors in adjacent columns.
From the Zipursky lab. Two left panels, see also Pecot, M.Y., Tadros, W., Nern, A., Bader, M., Chen, Y., and Zipursky, S.L. 2013. Neuron 77:299–310. Two right panels, see also Millard, S.S., Flanagan, J.J., Pappu, K.S., Wu, W., and Zipursky, S.L. 2007. Nature 447:720–724.
