The primary cilium regulates neuronal migration via cyclic AMP production activating protein kinase A at the centrosome.

In the forebrain, ventrally derived oligodendrocyte precursor cells (vOPCs) travel tangentially toward the cortex together with cortical interneurons. Here, we tested in the mouse whether these populations interact during embryogenesis while migrating. By coupling histological analysis of genetic models with live imaging, we show that although they are both attracted by the chemokine Cxcl12, vOPCs and cortical interneurons occupy mutually exclusive forebrain territories enriched in this chemokine. Moreover, first-wave vOPC depletion selectively disrupts the migration and distribution of cortical interneurons. At the cellular level, we found that by promoting unidirectional contact repulsion, first-wave vOPCs steered the migration of cortical interneurons away from the blood vessels to which they were both attracted, thereby allowing interneurons to reach their proper cortical territories.

The primary cilium (PC) is a microtubule-based tiny sensory organelle emanating from the centrosome and protruding from the surface of most eukaryotic cells, including neurons. The extremely severe phenotypes of ciliopathies have suggested their paramount importance for multiple developmental events, including brain formation. Neuronal migration is an essential step of neural development, with all neurons traveling from their site of birth to their site of integration. Neurons perform a unique type of cellular migration called cyclic saltatory migration, where their soma periodically jumps along with the stereotyped movement of their centrosome. We will review here how the role of the PC on cell motility was first described in non-neuronal cells as a guide pointing to the direction of migration. We will see then how these findings are extended to neuronal migration. In neurons, the PC appears to regulate the rhythm of cyclic saltatory neuronal migration in multiple systems. Finally, we will review recent findings starting to elucidate how extracellular cues sensed by the PC could be intracellularly transduced to regulate the machinery of neuronal migration. The PC of migrating neurons was unexpectedly discovered to display a rhythmic extracellular emergence during each cycle of migration, with this transient exposure to the external environment associated with periodic transduction of cyclic adenosine monophosphate (cAMP) signaling at the centrosome. The PC in migrating neurons thus uniquely appears as a beat maker, regulating the tempo of cyclic saltatory migration.

The saltatory migration of neurons is essential for brain formation. Whether mechanical stimuli regulate this process is unknown. Here, we show that the primary cilium acts as a mechanical sensor through GPR161. Using an ex vivo neuronal migration model and microfluidic assays, we demonstrate that fluid shear stress induces migration via the mechanoreceptor GPR161 at the primary cilium, with its mechanosensitive helix 8 being essential. We demonstrate that GPR161 activates a recently discovered cAMP/PKA signaling pathway leading to the phosphorylation of NDE1, a dynein complex regulator, and microtubule organization to regulate migration. These findings unveil a critical role of mechanosensation in neuronal migration, regulating the rhythmicity of migration, in concert with the externalization/internalization dynamics of the primary cilium.

Morphogen gradients are used repeatedly during development to pattern embryonic tissues. Absolute concentrations, duration or even temporal derivative of morphogen concentration have all been proposed to carry positional information. However, establishing the causal relationship between the gradient spatio-temporal profile and resulting cellular diversity and tissue patterning is difficult to address in live embryos because of lack of tools to control those variables. Here, we developed microfluidics devices able to apply spatio-temporally well-defined morphogen landscapes on the basal side of human embryonic stem cells colonies, thus mimicking how BMP4 is delivered to the pluripotent epiblast during mouse gastrulation. Combining live imaging and theoretical modelling, we show that in this configuration the absolute concentration of BMP4 provides positional information and the cell identities emerging during differentiation can vary according to the shape of the gradient. Our toolbox provides a powerful means to dissect the logic of patterning and to engineer tissues precisely.

Dynamique de la signalisation AMPc pendant la migration neuronale: L'AMPc joue un rôle crucial dans le développement du système nerveux central. Cependant, le rôle de l'AMPc lors de la migration neuronale est encore largement inexploré. Pour analyser la dynamique de l'AMPc dans les neurones en migration, nous avons effectué de l'imagerie sur tissu vivant d'un biosenseur spécifique de l’AMPc dans les neurones en migration le long du courant de migration rostral (CMR). Ces cellules présentent un mode de migration saltatoire cyclique avec une alternance de nucléokinese (NK) et de pauses, ainsi qu'un mouvement stéréotypé du centrosome (CTR). Nous avons découvert un hotspot dynamique riche en AMPc entourant le CTR pendant les NKs. Le CTR est situé à la base du cil primaire (CP), ce qui suggère que le CP pourrait être le lieu de production de l’AMPc. L'ablation génétique du CP dans les cellules en migration ralentit la migration et entraîne des défauts de mouvement du CTR, ainsi que la disparition du hotspot riche en AMPc. L'adénylate cyclase 3 (AC3) est localisée dans le CP et son inhibition par ARN interférence conduit à des défauts de migration similaires à l'ablation du CP ainsi que la disparition du hotspot, ce qui suggère l'importance de la production d'AMPc ciliaire pour la migration neuronale. La protéine kinase A dépendante de l’AMPc (PKA) est localisée au niveau du CTR des neurones en migration. Pour tester si le hotspot riche en AMPc pouvait agir sur la PKA centrosomale, nous avons exprimé une PKA dominante négative non localisée au centrosome, qui a conduit aux mêmes défauts de migration que l'ablation du CP et l'inhibition de l’AC3. Nos données montrent ainsi un nouveau mécanisme régulant la migration saltatoire neuronale par la production ciliaire d'AMPc diffusant au centrosome et activant localement la PKA. Le hotspot d'AMPc est également présent lors de la migration tangentielle adulte du CMR, lors de la migration radiaire postnatale dans le bulbe olfactif et lors de la migration radiaire embryonnaire ce qui suggère son importance générale pour la migration neuronale. Le syndrome de l’X fragile (SXF) est un trouble neurodéveloppemental et la forme héréditaire la plus courante de déficience intellectuelle. Nous avons découvert que les neurones en migration des souris modèle du SXF affichent plusieurs hotspot d’AMPc ainsi qu'une migration anormale. Les neurones mutants migrent plus lentement et font plus de pauses. Des recherches plus approfondies sur le mécanisme moléculaire sous-jacent à ces phénotypes sont nécessaires pour déterminer comment et si les défauts de migration neuronale sont liés à la dérégulation de l'AMPc. Cela pourrait mettre en évidence un nouveau phénotype neurodéveloppemental impliqué dans le syndrome de l’X fragile.