Reviewer #3 (Public Review):
The submitted paper by Hoving et al addresses the role of N-cadherin in Schwann cell collective cell migration and its previously unknown relationship with the slit/robo signaling pathway. The main conclusion is that N-cadherin has two distinct functions. One that is dependent on its classical role as a cell-cell junction protein promoting cell clustering and one that promotes cell repulsion and polarity independently of the formation of cell adhesion complexes. The second function is mediated by the Slit/Robo pathway. It is proposed that N-cadherin and Glypican-4 act together to present Slit2/3 at the surface of Schwann cells in order to trigger Robo signaling on neighboring cells.
The data about N-cadherin loss of function and the associated rescue experiments with the various truncated forms of N-cadherin are well substantiated by proper controls for efficiency and specificity. They show that the extracellular domain of Ncadherin is the one required for the repulsive effect. The experiments performed to distinguish the roles in adhesion and repulsion seem clear and conclusive. In addition, the fact the slit signal needs to be provided in a polarized manner for directional migration to occur is also clearly demonstrated in vitro and on slice assays. Overall the model that Ncadherin plays two different roles, a repulsive one via presentation of slit at the cell surface and a cell adhesion one via formation of adherens junctions, is well supported by the data and will be of interest beyond the subfield of the authors.
However, other parts of the manuscript seem weaker. If N-cadherin presentation of the Slit signal is so critical why are repulsion rates still very high in cells without N-cadherin? Same is observed with Glypican4 knockdowns. In both loss of function 50% of cell collisions lead to repulsion (compared to 70% amongst control cells). While significant such drop remains modest. The authors propose a cooperative role of Glypican-4 and N-cadherin at the cell surface as co-binding factors for Slit2/3 but they have not checked whether double knockdown of N-cad and Glypican4 might have a stronger effect. Could Glypican and N-cadherin present Slit at the cell surface independently in a somewhat redundant manner? Can Glypican and Slit interact physically in absence of N-cadherin? They also have not further analyzed the putative colocalization fo Ncad and Glypican at the cell surface.
The data supporting a role for N-cadherin in Slit's trafficking to the cell surface seem also circumstantial. While western blot data seem to indicate no change in Slit protein level after N-cad knockdown, immunostaining for Slit in such condition show a dramatic loss of Slit signal. These two independent data sets are difficult to reconcile and are not designed to address whether Slit reaches the cell surface in control or N-cadherin knockdown conditions.
If Slit signaling is so critical for repulsion why in double sit2/3 knockdown 40% of collisions still lead to repulsion. Also, no analysis of cell collision are provided upon Robo1/2 knockdown for comparison with Slit knockdowns. Altogether, these relatively mild effects of n-cad, slit or glypican knockdown on repulsion seem to indicate that other signals might contribute to contact-inhibition and polarization/repulsion of cells upon physical contact but this is unfortunately not discussed. All statements related to cell polarity stem from the overall cell morphology without being substantiated by actual polarity analysis (using markers such as detection of Rac-GTP or using a proxy such as the golgi-nucleus axis). The authors present the cell cluster generated after Sox2 expression and Sox2 + exposure to recombinant Slit2 as lacking polarity, however in one case cells do not present any flat membrane at their free edge whereas in the other case they do. This suggests a minimal cell polarity with a protrusive-like organization away from the contact. Finally, Robo1/2 siRNA knockdown are used but contrary to the other loss of functions it seems that controls for knockdown efficiency/specificity were not provided.
Therefore, while the study is overall well documented and based on solid data, some weaknesses exist.
The overall topic is clearly of broad interest as N-cadherin is protein essential in various biological settings from development to disease but the range of its biological functions remains to be fully explored. This study clearly adds to the current knowledge and how N-cadherin might act in vivo and in particular how it could mediate crosstalks between various signaling pathways.

