Regulation of Axon Guidance by Slit and Netrin Signaling in the Drosophila Ventral Nerve Cord

Abstract

Netrin and Slit signaling systems play opposing roles during the positioning of longitudinal tracts along the midline in the ventral nerve cord of Drosophila embryo. It has been hypothesized that a gradient of Slit from the midline interacts with three different Robo receptors to specify the axon tract positioning. However, no such gradient has been detected. Moreover, overexpression of Slit at the midline has no effect on the positioning of these lateral tracts. In this article, we show that Slit is present outside of the midline along the longitudinal and commissural tracts. Sli from the midline, in a Robo-independent manner, is initially taken up by the commissural axon tracts when they cross the midline and is transported along the commissural tracts into the longitudinal connectives. These results are not consistent with a Sli gradient model. We also find that sli mRNA is maternally deposited and embryos that are genetically null for sli can have weaker guidance defects. Moreover, in robo or robo3 mutants, embryos with normal axon tracts are found and such robo embryos reach pupal stages and die, while robo3 mutant embryos develop into normal individuals and produce eggs. Interestingly, embryos from robo3 homozygous individuals fail to develop but have axon tracts ranging from normal to various defects: robo3 phenotype, robo phenotype, and slit-like phenotype, suggesting a more complex functional role for these genes than what has been proposed. Finally, our previous results indicated that netrin phenotype is epistatic to sli or robo phenotypes. However, it seems likely that this previously reported epistatic relationship might be due to the partial penetrance of the sli, robo, robo3 (or robo2) phenotypes. Our results argue that double mutant epistasis is most definitive only if the penetrance of the phenotypes of the mutants involved is complete.

Ancestry-independent fate specification and plasticity in the developmental timing of a typicalDrosophilaneuronal lineage

“…Among the mid alleles, mid 1 was genetically the strongest. We also observed variation in the penetrance of the defect in some of the alleles depending on whether the embryos were collected from balanced (CyO balancer) parents or parents in a wild-type background (Table 1), indicating a parental balancer-induced influence on the phenotype (see Bhat et al, 2007).…”

The glycosylation pathway is required for the secretion of Slit and for the maintenance of the Slit receptor Robo on axons

“…In the Drosophila embryo, Slit is produced only by the ventral midline glial cells (1–4) and interacts with Robo1, Robo2, and Robo3, which are present on growth cones and axons of the longitudinal tracts. Robo1 is also present in commissures but in very small amounts.…”

“…In cultured cells, Slit appears to be secreted and deposited into the culture matrix (9). In embryos, Slit is detected only in the midline glia and in longitudinal axon tracts (4) but has not been reported to form an extracellular gradient. Thus, no direct proof exists that Slit is secreted in vivo.…”

“…Signaling molecules that function as a gradient are often very sensitive to gene dosage effects (12), yet slit does not exhibit haploinsufficiency nor does duplication of slit cause axon guidance defects (1, 4). Moreover, overexpression of slit at the midline does not alter the positioning of axon tracts (1, 4), an essential feature of a gradient model. We previously showed that Slit from the midline glia is transported to the longitudinal connectives through commissural tracts (4).…”

Netrin-guided accessory cell morphogenesis dictates the dendrite orientation and migration of a Drosophila sensory neuron

“…Netrins elicit diverse, and in some cases opposing, cellular responses, depending on the developmental context, raising the question of how this single class of molecules can exert such different effects. The netrin receptor Frazzled (Fra), the Drosophila homologue of DCC/UNC-40, can act either in a cell-autonomous fashion in the netrin-responding cell or noncell-autonomously by localising netrin distribution (Bhat et al, 2007;Garbe and Bashaw, 2007;Hiramoto and Hiromi, 2006;Hiramoto et al, 2000).…”