Editors' ChoiceCell Migration

One Step at a Time

Science Signaling  12 Apr 2011:
Vol. 4, Issue 168, pp. ec100
DOI: 10.1126/scisignal.4168ec100

Cortical neurons undergo a complex series of morphological changes as they migrate from their “birth site,” the ventricular zone and subventricular zone, to their final destination, the cortical plate. In mammals, the neurons are bipolar when they begin to migrate from their birth site; they become transiently multipolar in the intermediate zone; and then, before they enter the cortical plate, the cells reassume a bipolar morphology. Bipolar neurons exhibit nucleokinesis, a process by which the nucleus translocates into the leading process, and then the trailing process retracts, resulting in net forward motion. Rnd2 and Rnd3 are two members of an atypical family of Rho proteins that lack guanosine triphosphatase (GTPase) activity and are thus constitutively bound to GTP. These proteins function in cellular migration and inhibit RhoA. Expression of Rnd2 is stimulated by the proneural factor Neurog2 (also known as Neurogenin2), and deficiency in Rnd2 causes cortical neuron migration defects. Pacary et al. showed that expression of Rnd3 in cortical progenitors is stimulated by another proneural transcription factor, Ascl1, and that loss of Ascl1 or Rnd3 activity impaired cortical neuron migration. With in utero electroporation into the cerebral cortex, the authors showed that coexpression of Rnd2 and the Rnd3-silencing construct failed to rescue the migration defects of Rnd3-silenced neurons, and coexpression of Rnd3 with the Rnd2-silencing construct failed to rescue the migration defects of Rnd2-silenced neurons. In vivo examination of the morphologies of the silenced neurons that reached the cortical plate revealed that the Rnd3-silenced neurons exhibited enlarged leading processes and multiple thin processes that extended from the cell body, suggesting that nucleokinesis may be impaired in these cells. The distance from the nucleus to the centrosome was increased in the Rnd3-silenced neurons compared with wild-type or Rnd2-silenced neurons. The Rnd2-silenced neurons that reached the cortical plate had normal leading processes; however, most of the Rnd2-silenced neurons that did not complete migration failed to leave the intermediate zone, where they exhibited a multipolar morphology. Measurement of RhoA activity in the silenced cortical neurons in vivo showed that RhoA activity was enhanced by knockdown of either Rnd2 or Rnd3, and knockdown of RhoA rescued the migration defects produced by knockdown of either Rnd2 or Rnd3. Rescue experiments with mutant forms of Rnd3 that disrupted its ability either to inhibit the Rho effector ROCK1 (Rho-associated kinase 1) or to stimulate the Rho GTPase–activating protein p190RhoGAP suggested that the interaction with p190RhoGAP was required for the role of Rnd3 in migration. In contrast, Rnd2 mutants that could not stimulate p190RhoGAP were capable of rescuing the migration defects of the Rnd2-silenced neurons. Although knockdown of either Rnd2 or Rnd3 disrupted the actin cytoskeleton, rescue experiments with a constitutively active mutant form of cofilin, an active depolymerizing protein, restored the migration of Rnd3-silenced neurons only. These rescue experiments indicate that although both Rnd3 and Rnd2 inhibit RhoA to control migration, they regulate this process through different mechanisms. Immunofluorescence analysis of isolated cortical neurons showed that Rnd3 was present in both the cell processes and the soma, whereas Rnd2 was present only in the soma. Furthermore, Rnd3 was present at the plasma membrane and in endosomal compartments, whereas Rnd2 was detected only in endosomal compartments. Mutations that disrupted the plasma membrane localization of Rnd3 impaired its ability to rescue the migration defects of Rnd3-silenced neurons, and forcing Rnd2 to the plasma membrane allowed this protein to rescue the migration defects of the Rnd3-silenced neurons. Thus, Rnd2 and Rnd3 appear to function in distinct cellular compartments to inhibit RhoA activity locally and thus control different steps in cortical neuron migration.

E. Pacary, J. Heng, R. Azzarelli, P. Riou, D. Castro, M. Lebel-Potter, C. Parras, D. M. Bell, A. J. Ridley, M. Parsons, F. Guillemot, Proneural transcription factors regulate different steps of cortical neuron migration through Rnd-mediated inhibition of RhoA signaling. Neuron 69, 1069–1084 (2011). [PubMed]