Developmental guidance of the retroflex tract at its bending point involves Robo1-Slit2-mediated floor plate repulsion

Abstract

The retroflex tract contains medial habenula efferents that target the hindbrain interpeduncular complex and surrounding areas. This tract displays a singular course. Initially, the habenular axons extend ventralwards in front of the pretectum until they reach the basal plate. Next, they avoid crossing the local floor plate, sharply changing course caudalwards (the retroflexion alluded by the tract name) and navigate strictly antero-posteriorly across basal pretectum, midbrain and isthmus. Once they reach rhombomere 1, the habenular axons criss-cross the floor plate several times within the interpeduncular nuclear complex as they innervate it. Here we explored in vitro the timing and details of growth phenomena as these axons first change course, and examined the corresponding molecular background. The first dorsoventral course apparently obeys Ntn1 attraction. The decision to bend caudalwards next to the repelling thalamic floor plate seems related to Slit2 repulsive cues. We checked the role of local floor plate signaling by studying Gli2 knockout mice. We found reduced expression of the Slit repulsive cues, suggesting involvement of the floor-derived Robo-Slit system in the normal guidance of this tract. The Gli2−/− phenotype caused a contralateral projection of most habenular axons, plus ulterior bizarre navigation rostralwards. Using Slit and Robo mutant mice, and open neural tube and co-culture assays, we determined that a Robo1-Slit2 interaction is specifically required for impeding that habenular axons cross the thalamic floor plate. This pathfinding mechanism is essential to establish the functionally important habenulo-interpeduncular connection.

Netrin 1-Mediated Role of the Substantia Nigra Pars Compacta and Ventral Tegmental Area in the Guidance of the Medial Habenular Axons

“…The fr pathfinding mechanisms have been largely neglected until the last years. The reinterpretation of the fr trajectory under the prosomeric model has opened the possibility of properly understanding the behavior of this tract ( Moreno-Bravo et al, 2016 ). Among the different key decision points adopted by the Hb axons during their trajectory, one of the most elusive has been the caudal bending in the basal p2.…”

“…In the case of the fr, we reinterpreted its trajectory in the context of the prosomeric model paradigm. This interpretation allowed us to describe a key decision point: the arrival to the p2 basal plate where the axons bend caudally by navigating longitudinally until its final targets ( Moreno-Bravo et al, 2016 ). Following this interpretation, we selected the SNc and VTA as candidate territories to control this decision point.…”

“…We described the fr trajectory in the prosomeric paradigm context (see Figures 1 , 3 of Moreno-Bravo et al, 2016 ). This precise anatomic interpretation of the axon path was a prerequisite to understanding which mechanisms are involved in the guidance of these axons.…”

“…Focusing on the mHb, the first step of the mHb axons is to grow dorsoventrally, presumably repulsed by semaphorins from the diencephalic alar plates ( Funato et al, 2000 ; Sahay et al, 2003 ; Kantor et al, 2004 ) and then attracted by a netrin 1–deleted in colorectal cancer ( Ntn1 – Dcc ) interaction from the ventral region ( Funato et al, 2000 ). Once they arrive at the thalamic tegmentum, they are repulsed by the floor plate through a Robo1 – Slit2 interaction ( Moreno-Bravo et al, 2016 ). The axons turn caudalward toward the IP, at r1, following a long pathway through the mesodiencephalic basal plates.…”

Shared and differential features of Robo3 expression pattern in amniotes

“…Robo3 expression in neurons was also detected using in toto in situ hybridization on E12 mouse nervous system, showing strong expression of Robo3 in the rhombic lip in the hindbrain, and weaker expression in Ge, Mes and Sc neurons (Figure 1q). Robo3+ axons were found in the fasciculus retroflexus (Fr) which connects medial habenula neurons in the dorsal epithalamus to the interpeduncular nucleus (Ipn) in r1 (Moreno-Bravo et al, 2016). At…”