As expected, heat shocks performed before or during axon elongation along the tract (between
30 and 48 hpf) fully rescued the phenotype ( Figures 3F and 3G), indicating that restoring Ext2 activity at early times is sufficient to produce HS that rescues sorting at later stages. Interestingly, heat shocks performed after axons had grown along the tract, from 48 to 69 hpf, also rescued missorting defects in dak. Rescue was no longer observed when heat shocks were performed selleck chemicals llc after 72 hpf. This later period coincides with the onset of visually evoked responses from tectal neurons ( Niell and Smith, 2005), suggesting that missorted axons might be stabilized by connections with synaptic partners. Thus, restoring HS synthesis at late stages in dak is sufficient to restore pretarget topographic sorting, indicating that the correction mechanism editing missorted axons along the tract requires the presence of HS. HS is carried by diverse core proteins present at GS-7340 the cell surface or in the extracellular matrix. It could therefore act non-cell-autonomously in the environment and/or cell-autonomously at the surface of missorted DN axons. To determine where HS is required to correct missorted DN axons, we performed topographic transplantations of DN
RGCs between WT and dak embryos carrying the isl2b:EGFP or isl2b:TagRFP transgenes, which are expressed in RGCs ( Pittman et al., 2008) ( Figure 4A). DN donor RGCs were transplanted between 30 and 34 hpf into the DN quadrant of the host retina, and their axonal projections were analyzed at 4 dpf. As in WT > WT transplants, dak RGCs transplanted into a WT host projected axons that were correctly sorted into the ventral branch of the tract ( Figures 4C, 4C′, 4E, and 4E′). In contrast, some axons of WT RGCs transplanted into a dak host were clearly missorted and elongated along the dorsal branch of
the tract, as observed in dak > dak transplants ( Figures 4B, 4B′, 4D, and 4D′). Thus, HS is required non-cell-autonomously for correcting missorted DN axons and establishing pretarget topographic sorting. Pretarget axon sorting is an important process contributing to the formation of topographic maps, yet the cellular else and molecular mechanisms ordering axonal projections remain largely unknown. In this Report, we took advantage of the unique accessibility of the zebrafish embryo to determine how retinal axons are sorted along the dorsoventral axis in the optic tract before reaching the tectum. We showed that topographic sorting of retinal axons is not precisely established during initial growth cone pathfinding along the tract but is rather achieved through the selective degeneration of missorted axons. We further demonstrated that this specific developmental degeneration is regulated non-cell-autonomously by HS.