Neurite arborization and mosaic spacing in the mouse retina require DSCAM

Abstract

To establish functional circuitry, retinal neurons occupy spatial domains by arborizing their processes, which requires the selfavoidance of neurites from an individual cell, and by spacing their cell bodies, which requires positioning the soma and establishing a zone within which other cells of the same type are excluded 1 . The mosaic patterns of distinct cell types form independently and overlap. The cues that direct these processes in the vertebrate retina are not known 2,3 . Here we show that some types of retinal amacrine cells from mice with a spontaneous mutation in Down syndrome cell adhesion molecule (Dscam), a gene encoding an immunoglobulin-superfamily member adhesion molecule 4,5 , have defects in the arborization of processes and in the spacing of cell bodies. In the mutant retina, cells that would normally express Dscam have hyperfasciculated processes, preventing them from creating an orderly arbor. Also, their cell bodies are randomly distributed or pulled into clumps rather than being regularly spaced mosaics. Our results indicate that mouse DSCAM mediates isoneuronal self-avoidance for arborization and heteroneuronal self-avoidance within specific cell types to prevent fasciculation and to preserve mosaic spacing. These functions are analogous to those of Drosophila DSCAM (ref. 6) and DSCAM2 (ref. 7). DSCAM may function similarly in other regions of the mammalian nervous system, and this role may extend to other members of the mammalian Dscam gene family.We have identified a spontaneous mutation in mice that creates a loss-of-function allele of Dscam, the Drosophila homologues of which function in both isoneuronal self-avoidance for dendrite arborization and heteroneuronal self-avoidance for axon tiling [7][8][9][10][11][12] . The recessive mutation arose in the BALB/cByJ genetic background and caused an overt neurological phenotype. Mutant and wild-type mice are indistinguishable at birth, but are severely uncoordinated by postnatal day 3 (P3); as adults, the mutant mice have spontaneous seizures and kyphosis, but are fertile and long-lived (.24 months; see http://www.jax.org/research/media/wild_type.html for video). Through positional cloning (see Methods), a mutation was identified in Dscam. Sequencing of genomic and complementary DNA from affected mice revealed a 38-bp deletion in exon 17, causing a frame shift resulting in ten unique amino acids followed by a premature stop codon (Supplementary Fig. 1). This mutation truncates the protein in the second fibronectin repeat (Fig. 1a). Dscam messenger RNA levels in the brain were reduced by 70% in affected mice, consistent with nonsense-mediated decay (Fig. 1b).

Dscam-mediated repulsion controls tiling and self-avoidance

“…Burgess and colleagues identified defects in the organization of retinal amacrine cell neurites in a DSCAM mutant mouse, consistent with a role in both processes [17]. Amacrine cells make synaptic connections with retinal ganglion cells in specific layers within the inner plexiform layer.…”

The Conserved Ig Superfamily Member Turtle Mediates Axonal Tiling in Drosophila

“…Mutual repulsion has been suggested to be required for columnar restriction of L1 neurons in the Drosophila visual system, which is mediated by Dscam2 (Millard et al, 2007). Up to now, Dscam2 and its mouse homolog Dscam are only known cell surface transmembrane proteins to be implicated in mediating mutual repulsion for axonal/dendrite tiling (Millard et al, 2007;Fuerst et al, 2008). In addition, Dscam and the receptor tyrosine phosphatase Hm-LAR2 have been shown to be involved in mediating selfavoidance, a phenomenon referring to the avoidance of sister neurites projected from a single neuron, in sensory neurons in Drosophila (Hughes et al, 2007;Matthews et al, 2007;Soba et al, 2007) and the comb cell in Leech (Gershon et al, 1998), respectively.…”

“…The establishment of tiling pattern involves both repulsion between same-type neurites (Wässle et al, 1981;Perry and Linden, 1982;Kramer and Kuwada, 1983;Grueber et al, 2003) and the intrinsic control of neurite growth (Lin et al, 2004;Ting et al, 2007). Recent studies identify Dscam as a repulsive tiling receptor in a small subset of neurons in Drosophila and mice (Millard et al, 2007;Fuerst et al, 2008). The identity of other cell surface proteins for tiling different neuronal cell types in the complex nervous system, however, remains unknown.…”

Protogenin Defines a Transition Stage during Embryonic Neurogenesis and Prevents Precocious Neuronal Differentiation

“…The human Down's syndrome cell adhesion molecule gene (DSCAM ) was originally identified as a gene associated with mental retardation (Yamakawa et al, 1998). Recent studies indicate that DSCAM is involved in neurite arborization, cell body spacing, and lamina-specific synaptic targeting in the vertebrate retina via homophilic interactions (Fuerst et al, 2008;Yamagata and Sanes, 2008). Intriguingly, DSCAM also acts as a receptor for netrindependent commissural axon outgrowth, turning responses and pathfinding in collaboration with DCC (Ly et al, 2008;Liu et al, 2009).…”