Migrating neurons navigate their environment by responding to various extracellular guidance cues. Coordination of the behaviour of the growth cone and the translocation of the cell body is essential, yet the means by which this is achieved are unknown. Poo and colleagues now reveal a mechanism by which growth cones can relay such guidance signals to the cell body.
The guidance molecule SLIT2 causes neurons to halt or to reverse their migration. Here, the researchers showed that presenting a SLIT2 gradient in front of the growth cone of isolated rat cerebellar granule cells caused the growth cone to collapse or retract. It also reversed the direction of translocation of the cell body, yet the presentation of SLIT2 at the cell body itself had no effect. This suggested that only the growth cone can respond to SLIT2, which was supported by the finding that the SLIT2 receptor ROBO2 is enriched at the growth cone membrane in these cells.
These findings implied that a long-range signal from the growth cone must trigger the reversal of cell soma translocation. The authors revealed that intracellular Ca2+ levels increased within seconds of SLIT2 exposure. Moreover, stimulating Ca2+ release from intracellular stores with low-level ryanodine treatment triggered the reversal of soma translocation, whereas blocking Ca2+ release from these stores with high levels of ryanodine abolished the SLIT2-triggered reversal of translocation. This indicated that a propagated Ca2+wave triggers SLIT2-induced reversal of translocation.
To elucidate the downstream effectors involved in this process, the authors turned to the RhoGTPases, which are known to mediate cytoskeletal rearrangements. They transfected granule cells with dominant-negative forms of three RhoGTPases — RhoA, Rac1 and Cdc42 — revealing that RhoA is required for SLIT2-triggered reversal of soma translocation. A specific RhoA inhibitor eliminated the response to SLIT2 or low-level ryanodine treatment, confirming that RhoA acts downstream of Ca2+ elevation.
The authors further showed that both SLIT2 and low-level ryanodine reduced RhoA activity in cultured cells. They revealed that active RhoA accumulates towards the leading front of the cell body and found that this becomes redistributed after SLIT2 exposure, leading them to propose this as the underlying mechanism for the reversal of the direction of migration.
These findings present a new mechanism by which long range migration may be coordinated. It is unknown whether similar mechanisms apply in other circumstances, such as the control of forward migration and the response to other repulsive or attractive cues; however, these results provide an example of how long-range intracellular signals can coordinate the motility of two relatively distant neuronal domains.