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

Abstract

Slit proteins act as repulsive axon guidance cues by activating receptors of the Roundabout (Robo) family. During early neurogenesis in Drosophila melanogaster, Slit prevents the growth cones of longitudinal tract neurons from inappropriately crossing the midline, thus restricting these cells to trajectories parallel to the midline. Slit is expressed in midline glial cells, and Robo is present in longitudinal axon tracts and growth cones. We showed that the enzyme mummy (Mmy) controlled Slit-Robo signaling through mechanisms that affected both the ligand and the receptor. Mmy was required for the glycosylation of Slit, which was essential for Slit secretion. Mmy was also required for maintaining the abundance and spatial distribution of Robo through an indirect mechanism that was independent of Slit secretion. Moreover, secretion of Slit was required to maintain the fasciculation and position of longitudinal axon tracts, thus maintaining the hardwiring of the nervous system. Thus, Mmy is required for Slit secretion and for maintaining Robo abundance and distribution in the developing nervous system in Drosophila.

Post-guidance signaling by extracellular matrix-associated Slit/Slit-N maintains fasciculation and position of axon tracts in the nerve cord

“…The slit gene is transcribed in the glial cells at the midline [ 1 , 2 ; see Fig 2A, 2B and 2E ). However, the protein is present in the midline glia as well as in axon tracts of the commissures and connectives ( Fig 2A and 2B )[ 7 , 13 , see also ref. 1 – 3 , 14 ].…”

“…These results indicate that Slit has a direct role in maintaining the position of tracts along the midline, in addition to the initial growth cone guidance [see also ref. 13 ].…”

“…Signaling by Slit-Robo appears to have a significant role in these events since loss of Slit later during neurogenesis causes loss of positioning and fasciculation of tracts [see also ref. 13 ]. These results also illuminate how Slit signaling could mediate these functions, which also sheds light on the significance of the presence of Slit in axon tracts.…”

“…A previous study had explored the role of Slit on the fasciculation of tracts and their spacing in the mouse diaphragm [ 12 ], but not much is known if the position of tracts and their fasciculation are maintained within the CNS. By examining a mutation in the enzyme involved in the glycosylation of Slit, we have recently shown that Slit-Robo signaling contributes to the maintenance of axon tracts [ 13 ]. This issue of maintenance of tracts has major implications since it is essential for maintaining the integrity of the hardwiring of the nervous system, thus, neural function, or dysfunction later in development or in life.…”

“…While Slit is expressed in the midline glia [ 1 – 3 ], Slit is also found in tracts [ 7 , 13 , see also 1 – 3 , 14 ]. We ruled out Slit in tracts as cross-reactivity to the antibody or a background staining and showed that tracts-Slit is transported from the midline to the longitudinal connectives along the commissural tracts [ 7 , 13 ]. Since both Slit and Robo are present in tracts of the longitudinal connectives, it is most likely that they would interact with each other in tracts to mediate certain function.…”