N-cadherin directs the collective Schwann cell migration required for nerve regeneration through Slit2/3-mediated contact inhibition of locomotion
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eLife assessment
This study presents an interesting set of findings that connects N-cadherin and glypican-4 to Slit signaling during the regulation of contact inhibition of locomotion of Schwann cells in culture. Solid evidence is provided showing that N-cadherin not only regulates cell recognition but also proper trafficking of Slit to the cell surface. An ex-vivo model demonstrates the importance of Slit signaling during cell migration but the molecular details of how N-cadherin traffics Slit to the surface and role of glypican are unclear. The data would have been strengthened with a similar interrogation of N-cadherin in this system. The work will be of interest to cell biologists studying the mechanisms behind peripheral nervous system regeneration.
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Abstract
Collective cell migration is fundamental for the development of organisms and in the adult for tissue regeneration and in pathological conditions such as cancer. Migration as a coherent group requires the maintenance of cell–cell interactions, while contact inhibition of locomotion (CIL), a local repulsive force, can propel the group forward. Here we show that the cell–cell interaction molecule, N-cadherin, regulates both adhesion and repulsion processes during Schwann cell (SC) collective migration, which is required for peripheral nerve regeneration. However, distinct from its role in cell–cell adhesion, the repulsion process is independent of N-cadherin trans -homodimerisation and the associated adherens junction complex. Rather, the extracellular domain of N-cadherin is required to present the repulsive Slit2/Slit3 signal at the cell surface. Inhibiting Slit2/Slit3 signalling inhibits CIL and subsequently collective SC migration, resulting in adherent, nonmigratory cell clusters. Moreover, analysis of ex vivo explants from mice following sciatic nerve injury showed that inhibition of Slit2 decreased SC collective migration and increased clustering of SCs within the nerve bridge. These findings provide insight into how opposing signals can mediate collective cell migration and how CIL pathways are promising targets for inhibiting pathological cell migration.
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eLife assessment
This study presents an interesting set of findings that connects N-cadherin and glypican-4 to Slit signaling during the regulation of contact inhibition of locomotion of Schwann cells in culture. Solid evidence is provided showing that N-cadherin not only regulates cell recognition but also proper trafficking of Slit to the cell surface. An ex-vivo model demonstrates the importance of Slit signaling during cell migration but the molecular details of how N-cadherin traffics Slit to the surface and role of glypican are unclear. The data would have been strengthened with a similar interrogation of N-cadherin in this system. The work will be of interest to cell biologists studying the mechanisms behind peripheral nervous system regeneration.
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Reviewer #1 (Public Review):
Hoving and colleagues investigated the mechanisms of contact inhibition of locomotion (CIL) in Schwann cells using cell migration assays, in combination with siRNA as well as an ex-vivo model for collective cell migration of the peripheral nervous system. They found that N-cadherin is needed for proper cell repulsion during CIL. Schwann Cells depleted of N-cadherin failed CIL when encountering other Schwann cells depleted of N-cadherin, however they maintained CIL when encountering Schwann cells expressing N-cadherin. Depletion of alpha-catenin and to some degree p120 did not have the same effect as N-cadherin depletion. Further, they determined that the extracellular domain is needed for CIL as well as an interaction with Glypican-4. Glypicans often act as co-receptors for other signaling molecules, and so …
Reviewer #1 (Public Review):
Hoving and colleagues investigated the mechanisms of contact inhibition of locomotion (CIL) in Schwann cells using cell migration assays, in combination with siRNA as well as an ex-vivo model for collective cell migration of the peripheral nervous system. They found that N-cadherin is needed for proper cell repulsion during CIL. Schwann Cells depleted of N-cadherin failed CIL when encountering other Schwann cells depleted of N-cadherin, however they maintained CIL when encountering Schwann cells expressing N-cadherin. Depletion of alpha-catenin and to some degree p120 did not have the same effect as N-cadherin depletion. Further, they determined that the extracellular domain is needed for CIL as well as an interaction with Glypican-4. Glypicans often act as co-receptors for other signaling molecules, and so the authors further narrowed CIL's dependence to Slit signaling. N-cadherin was needed for proper Slit surface expression, again, dependent on the extracellular domain, and depletion of both Slit2 and 3 lead to a cell clumping and rounding phenotype. Finally, using an ex-vivo model of Schawnn cell migration they showed that rSlit lead to a similar cell rounding and clumping phenotype, ultimately leading to an inhibition of cell migration.
Strengths
This was a very methodical examination of what is needed for CIL in cultured Schwann cells. The data presented largely supports the findings and the linking of N-cad to glypican-4 to Slit signaling further illuminates this process helping to define the molecular players. The mechanistic insight goes further in that they demonstrate the Slit does not get to the cell surface without the expression of the extracellular domain of N-cad.
Weaknesses
The conclusions that can be drawn from this study remain a little narrow since only Schwann cells were used. This is not so much a weakness in that authors were indeed investigating the periphery nervous system regeneration but it does limit their findings. The experiments carried out in the ex-vivo system only touch on one aspect of their cell culture work, the mechanism of Slit. No other aspects of their cell culture system was tested ex-vivo which
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Reviewer #2 (Public Review):
Schwann cells actively repair and regenerate peripheral nerves following tissue injury. Central to this process is the collective cell migration of 'cords' of Schwann cells, which guide the regenerating axons across an injury site. Previously published research from the Lloyd lab shows that at the injury site, Schwann cell cords are maintained via N-cadherin-based cell-cell adhesions; however, when cultured under low density conditions, Schwann cells display cell repulsion and contact inhibition of locomotion (CIL) phenotypes, suggesting Schwann cell behaviour is plastic. In this study, Hoving, Lloyd and colleagues build upon their previous work to show that Slit2/3/Robo signalling triggers cell repulsion between Schwann cells in an N-cadherin-dependent manner. This in turn induces contact inhibition of …
Reviewer #2 (Public Review):
Schwann cells actively repair and regenerate peripheral nerves following tissue injury. Central to this process is the collective cell migration of 'cords' of Schwann cells, which guide the regenerating axons across an injury site. Previously published research from the Lloyd lab shows that at the injury site, Schwann cell cords are maintained via N-cadherin-based cell-cell adhesions; however, when cultured under low density conditions, Schwann cells display cell repulsion and contact inhibition of locomotion (CIL) phenotypes, suggesting Schwann cell behaviour is plastic. In this study, Hoving, Lloyd and colleagues build upon their previous work to show that Slit2/3/Robo signalling triggers cell repulsion between Schwann cells in an N-cadherin-dependent manner. This in turn induces contact inhibition of locomotion to propel Schwann cells to migrate collectively and with direction. The authors show that N-cadherin has a dual function in Schwann cell migration: to keep migrating Schwann cells together as a group, and concomitantly present Slit2/3 repulsive cues to cells to trigger cell repulsion locally. Their data also show that extracellular N-cadherin is required for cell repulsion, independent of cell-cell adhesion functions. The authors use a combination of in vitro Schwann cell cultures and live cell imaging, with an ex vivo precision cut tissue slice system to show that Slit2/3-dependent CIL underpins proper Schwann cell migration in an injury model.
This is a very well executed and important study, which provides new insights into mechanisms of CIL and places CIL in the context of tissue repair and regeneration in adult tissues. The experiments are well designed, and the main findings and conclusions are based on robust and convincing data.
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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 …
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.
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