High levels of Rnd2 expression in the preplate of the cortex at E12.5.

From Nature 6 August 2008; doi: 10.1038/nature07198.

A key step in nervous system development is the migration of neurons from their birthplace to a final permanent location. Proneural transcription factors are known to control the differentiation of neural stem cells into neurons, as well as promoting neuronal radial migration in the embryonic cerebral cortex. However, the mechanisms underlying this migration-promoting activity have remained unclear. Reporting in Nature, François Guillemot and colleagues have identified the small Rho GTPase Rnd2 as a direct target of the transcription factor Neurogenin 2 (Neurog2), which is involved in neuronal migration.

The authors used microarray expression analysis to identify Rnd2 as a gene that is downregulated in Neurog2-mutant mice embryos, and upregulated when Neurog2 is overexpressed. Expression studies revealed that Rnd2 is restricted to migrating cortical neurons and their immediate precursors, and confirmed that this expression was dependent on Neurog2.

Rnd2 silencing resulted in the disruption of all stages of migration through the cortex and the alteration of neuronal morphology. Conversely, other features such as progenitor proliferation and cortical neuron specification, as well as the organization of radial glia along which neurons migrate, were unaffected. These findings indicate that Rnd2 functions cell-autonomously to regulate both the shape and migration of cortical neurons.

Expression of Rnd2 via the NeuroD1 promoter, which is transiently and moderately active in newborn cortical neurons, rescued the early radial migration defects seen in Neurog2 mutant neurons, but did not rescue the final migration phase for correct neuronal positioning in the cortical plate. Thus, Rnd2 appears to be the main downstream effector of Neurog2 that promotes radial migration, whereas other factors might regulate later stages of these processes.

But is Rnd2 a direct target of Neurog2 or is it possible that its expression is induced via a transcriptional cascade? Using a luciferase reporter assay, the authors showed that Neurog2 was able to efficiently activate transcription from an Rnd2 3' enhancer containing conserved E-boxes specific to Neurog2. Furthermore, Neurog2 bound to the Rnd2 3' enhancer in cortical cells in vivo. Together these results show that Neurog2 directly induces Rnd2 expression in the embryonic cortex.

This work reports for the first time the spatio-temporal regulation of a small GTPase at the transcriptional level. Unlike other Rho family members, Rnd proteins are not regulated by guanine nucleotide exchange factors (GEFs) or GTPase-activated proteins (GAPs), and their expression is thought to have an important regulatory role. Interestingly, whereas Neurog2 is downregulated when neuronal progenitors stop dividing, Rnd2 continues to be expressed. This suggests that other factors might be responsible for the maintenance of Rnd2 expression.

Author: Kim Baumann