Dscam and Sidekick proteins direct lamina-specific synaptic connections in vertebrate retina

Abstract

Synaptic circuits in the retina transform visual input gathered by photoreceptors into messages that retinal ganglion cells (RGCs) send to the brain. Processes of retinal interneurons (amacrine and bipolar cells) form synapses on dendrites of RGCs in the inner plexiform layer (IPL). The IPL is divided into at least 10 parallel sublaminae; subsets of interneurons and RGCs arborize and form synapses in just one or a few of them [1][2][3] . These lamina-specific circuits determine the visual features to which RGC subtypes respond [3][4][5] . Here we show that four closely related immunoglobulin superfamily (IgSF) adhesion molecules-Dscam (Down's syndrome cell adhesion molecule), DscamL (refs 6-9), Sidekick-1 and Sidekick-2 (ref. 10)-are expressed in chick by non-overlapping subsets of interneurons and RGCs that form synapses in distinct IPL sublaminae. Moreover, each protein is concentrated within the appropriate sublaminae and each mediates homophilic adhesion. Loss-and gain-of-function studies in vivo indicate that these IgSF members participate in determining the IPL sublaminae in which synaptic partners arborize and connect. Thus, vertebrate Dscams, like Drosophila Dscams 11-19 , play roles in neural connectivity. Together, our results on Dscams and Sidekicks suggest the existence of an IgSF code for laminar specificity in retina and, by implication, in other parts of the central nervous system.We previously showed that Sidekick-1 and Sidekick-2, large transmembrane adhesion molecules of the IgSF, are expressed by complementary subsets of amacrine, bipolar and ganglion cells in chick retina 10 . Double-label in situ hybridization revealed that only 10-15% of RGCs were positive for each Sidekick (Supplementary Table 1). We speculated that Sidekick-negative retinal neurons might express genes encoding related molecules. Homology searches of genomic databases revealed that their closest relatives are the Dscams 6-9 , which, like Sidekicks, contain multiple immunoglobulin and fibronectin type III domains, plus a carboxy terminus predicted to bind PDZ domains (Fig. 1a and Supplementary Fig. 1). Moreover, vertebrate Dscams are related to Drosophila Dscams, which have been implicated in neural specificity [11][12][13][14][15][16][17][18][19] , although they lack the complex pattern of alternative splicing found in Drosophila Dscam-1. We therefore cloned chick Dscam and DscamL, the orthologues of mammalian Dscam and Dscam-like-1, respectively, and assessed their expression in retina. Dscam and DscamL, like Sidekicks, were expressed by non-overlapping subsets of retinal neurons between embryonic day (E) 12 and hatching (E21) (Fig. 1b and Supplementary Fig. 2). None of the Dscam-positive neurons expressed either Sidekick (Fig. 1c and Supplementary Fig. 3). Thus, Dscams and Sidekicks mark four distinct sets of presynaptic (amacrine and bipolar) and postsynaptic cells (RGCs) that arborize in the IPL.To localize Dscam and Sidekick proteins, we generated antibodies specific for each ( Supplementary Fig. 4) and used them to i...

Dscam-mediated repulsion controls tiling and self-avoidance

“…Interestingly, cells that express the same DSCAM target to the same layer, suggesting that the DSCAMs may be instructive targeting cues. Consistent with this, knocking down DSCAM in retinal ganglion cells causes abnormal targeting and misexpressing DSCAM in cells that do not normally express this protein causes them to target to DSCAM-positive layers [22]. Thus, DSCAM proteins may have different functions in different developmental contexts.…”

Neurite arborization and mosaic spacing in the mouse retina require DSCAM

“…Alternatively, additional co-signals may distinguish distinct cell populations, consistent with the continued lack of interaction between bNOS and dopaminergic cells in the Dscam 2/2 retina. Additional complexity for self-recognition could also be introduced by other Dscam gene family members, such as Dscam-like 1 (Dscaml1), which is expressed in a similar but discrete population of retinal neurons 27,28 (P.G.F. and R.W.B., unpublished) or the highly homologous Sidekick proteins 29 .…”

“…Mouse DSCAM functions in a highly analogous way, promoting self-avoidance for both arborization (isoneuronal) and the prevention of fasciculation (heteroneuronal). Although the vertebrate genes are not extensively alternatively spliced, they do undergo homophilic and paralogue-specific binding 5,27,28 . The coexistence of Dscam-expressing amacrine cell populations suggests that stratification in different IPL layers may allow reuse of DSCAM as a recognition signal.…”

DSCAM functions as a netrin receptor in commissural axon pathfinding

“…Mouse DSCAM is expressed widely in the developing nervous system (16,24). Recent studies indicate that DSCAM plays an important role in neurite arborization, cell body spacing, and lamina-specific synaptic targeting in vertebrate retina (25,26).…”