Slit2, a Branching–Arborization Factor for Sensory Axons in the Mammalian CNS

Abstract

Axons that carry information from the sensory periphery first elongate unbranched and form precisely ordered tracts within the CNS. Later, they begin collateralizing into their proper targets and form terminal arbors. Target-derived factors that govern sensory axon elongation and branching-arborization are not well understood. Here we report that Slit2 is a major player in branching-arborization of central trigeminal axons in the brainstem. Embryonic trigeminal axons initially develop unbranched as they form the trigeminal tract within the lateral brainstem; later, they emit collateral branches into the brainstem trigeminal nuclei and form terminal arbors therein. In whole-mount explant cultures of this pathway, embryonic day 15 (E15) rat central trigeminal axons retain their unbranched growth within the tract, whereas E17 trigeminal axons show branching and arborization in the brainstem trigeminal nuclei, much like that seen in vivo. Similar observations were made in E13 and E15 mouse embryos. We cocultured Slit2-expressing tissues or cells with the whole-mount explant cultures of the central trigeminal pathway derived from embryonic rats or mice. When central trigeminal axons are exposed to ectopic Slit2 during their elongation phase, they show robust and premature branching and arborization. Blocking available Slit2 reverses this effect on axon growth. Spatiotemporal expression of Slit2 and Robo receptor mRNAs within the brainstem trigeminal nuclei and the trigeminal ganglion during elongation and branching-arborization further corroborates our experimental results.

Dual Branch-Promoting and Branch-Repelling Actions of Slit/Robo Signaling on Peripheral and Central Branches of Developing Sensory Axons

“…We also studied the central afferent branching of sensory neurons, because Slits have been proposed to control central collateral formation and have been shown to stimulate trigeminal ganglia to sprout collaterals into the brainstem in an organotypic culture (Ozdinler and Erzurumlu, 2002). However, we did not observe any clear defects in the formation of central collateral branches inside the spinal cord.…”

“…Using dorsal root ganglion (DRG) cell cultures, we found that an N-terminal fragment of Slit2 (Slit2N) stimulated branching and elongation of rat DRG axons from embryonic day 14 (E14) when they normally arborize in their targets in vivo. Evidence to support this branching effect was also obtained using a trigeminal ganglion explant culture (Ozdinler and Erzurumlu, 2002) and by misexpression in zebrafish (Yeo et al, 2004). In the CNS, Slit1 was shown to regulate cortical dendritic branching in vitro (Whitford et al, 2002).…”

Molecular determinants of the face map development in the trigeminal brainstem

“…We recently reported that a member of the Slit family of proteins, Slit2, can induce arborization of central trigeminal axons at a time when they are in the elongation phase (Fig. 5) (Ozdinler and Erzurumlu, 2002). There are at least three members of the Slit family (Slit1, Slit2, Slit3) and three members of the Robo family, Robo1, Robo2, Robo3Rig1 (Rajagopalan et al, 2000;Simpson et al, 2000), which bind Slits.…”

Functional Disruption of the Netrin-1 Guidance Gue Leads to Disruption in Mammary Gland Development and Increased Tumor Incidence

“…It is currently unknown how expression of the DUTTI mutant results in this phenotype, although a likely explanation is that normal signals transduced by ROBO1 are perturbed, either through inappropriate interactions with SLIT, or another ROBO receptor, or an as yet unidentified co-factor. In the nervous system, SLIT signals through ROBO to promote axon branching (Ozdinler and Erzurumlu, 2002;, therefore one possibility is that these irregularly shaped bronchioles are a result of inappropriate branching. This possibility, however, has not been directly tested, and additional data that could aid the interpretation of this phenotype, such as lung defects in other Slit or Robo null mice, have not yet been reported.…”