Ephrin-B1 Controls the Columnar Distribution of Cortical Pyramidal Neurons by Restricting Their Tangential Migration

Ephrin signalling in the developing nervous system

Ephrin ligands and their Eph receptors hold our attention since their link to axon guidance almost twenty years ago. Since then, they have been shown to be critical for short distance cell-cell interactions in the nervous system. The interest in their function has not abated, leading to ever-more sophisticated studies generating as many surprising answers about their function as new questions. We discuss recent insights into their functions in the developing nervous system, including neuronal progenitor sorting, stochastic cell migration, guidance of neuronal growth cones, topographic map formation, as well as synaptic plasticity.

Lineage origins of GABAergic versus glutamatergic neurons in the neocortex

Neocortical circuits are assembled from subtypes of glutamatergic excitatory and GABAergic inhibitory neurons with divergent anatomical and molecular signatures and unique physiological properties. Excitatory neurons derive from progenitors in the pallium, whereas inhibitory neurons originate from progenitors in the subpallium. Both classes of neurons subsequently migrate along well-defined routes to their final target area, where they integrate into common neuronal circuits. Recent findings show that neuronal diversity within the lineages of excitatory and inhibitory neurons is in part already established at the level of progenitor cells before migration. This poses challenges for our understanding of how radial units of interconnected excitatory and inhibitory neurons are assembled from progenitors that are spatially segregated and diverse in nature.

Distinct temporal hierarchies in membrane and cytoskeleton dynamics precede the morphological polarization of developing neurons

Final morphological polarization of neurons, with the development of a distinct axon and of several dendrites, is preceded by phases of non-polarized architecture. The earliest of these phases is that of the round neuron arising from the last mitosis. A second non polarized stage corresponds to the bipolar neuron, with two morphologically identical neurites. Both phases have their distinctive relevance in the establishment of neuronal polarity. During the round cell stage a decision is made as to where from the cell periphery a first neurite will form, thus creating the first sign of asymmetry. At the bipolar stage a decision is made as to which of the two neurites becomes the axon in neurons polarizing in vitro and the leading edge in neurons in situ. In this study we analysed cytoskeletal and membrane dynamics in cells at these two “pre-polarity” stages. By mean of time lapse imaging in dissociated hippocampal neurons and ex vivo cortical slices we show that both stages are characterized by polarized intracellular arrangements, however with distinct temporal hierarchies: polarized actin dynamics marks the site of first polarization in round cells, whereas polarized membrane dynamics precedes asymmetric growth in the bipolar stage.