Radial glial cells are the primary neural progenitor cells in the developing neocortex 1. Consecutive asymmetric divisions of individual radial glial progenitor cells produce a number of sister excitatory neurons that migrate along the elongated radial glial fibre, resulting in the formation of ontogenetic columns 2–4. Moreover, sister excitatory neurons in ontogenetic columns preferentially develop specific chemical synapses with each other rather than with nearby non-siblings 5. While these findings provide crucial insights into the emergence of functional columns in the neocortex, little is known about the basis for this lineage-dependent assembly of excitatory neuron microcircuits with single-cell resolution. Here we show that transient electrical coupling between radially aligned sister excitatory neurons regulates the subsequent formation of specific chemical synapses in the neocortex. Multiple-electrode whole-cell recordings revealed that sister excitatory neurons preferentially form strong electrical coupling with each other rather than with adjacent non-sister excitatory neurons during early postnatal stages. This coupling allows selective electrical communication between sister excitatory neurons, promoting their action potential generation and synchronous firing. Interestingly, while this electrical communication largely disappears prior to the appearance of chemical synapses, its blockade impairs the subsequent formation of specific chemical synapses between sister excitatory neurons in ontogenetic columns. These results suggest a strong link between a lineage-dependent transient electrical coupling and the assembly of precise excitatory neuron microcircuits in the neocortex.

The neocortex contains excitatory neurons and inhibitory interneurons. Clones of neocortical excitatory neurons originating from the same progenitor cell are spatially organized and contribute to the formation of functional microcircuits. In contrast, relatively little is known about the production and organization of neocortical inhibitory interneurons. We found that neocortical inhibitory interneurons were produced as spatially organized clonal units in the developing ventral telencephalon. Furthermore, clonally related interneurons did not randomly disperse but formed spatially isolated clusters in the neocortex. Individual clonal clusters consisting of interneurons expressing the same or distinct neurochemical markers exhibited clear vertical or horizontal organization. These results suggest that the lineage relationship plays a pivotal role in the organization of inhibitory interneurons in the neocortex.

Despite their therapeutic potential, progress in generating fully differentiated forebrain neurons from embryonic stem cells (ESCs) has lagged behind that from more caudal regions of the neuraxis. GABAergic interneuron precursors have the remarkable ability to migrate extensively and survive after transplantation into postnatal cortex, making them an attractive candidate for use in cell-based therapy for seizures or other neuropsychiatric disorders. We have modified a mouse ESC line with an Lhx6-GFP reporter construct that allows for the isolation of newly generated cortical interneuron precursors. When transplanted into postnatal cortex, these cells can migrate into the cortical parenchyma, survive for months, and display morphological, neurochemical, and electrophysiological properties characteristic of mature interneurons. This work demonstrates that forebrain neuronal subtypes with complex traits can be generated from embryonic stem cells, and provides a novel approach to the study of cortical interneuron development and to the establishment of cell-based therapies for neurological disease.

SUMMARY The neocortex contains glutamatergic excitatory neurons and GABAergic inhibitory interneurons. Extensive studies have revealed substantial insights into excitatory neuron production. However, our knowledge of the generation of GABAergic interneurons remains limited. Here we show that periventricular blood vessels selectively influence neocortical interneuron progenitor behavior and neurogenesis. Distinct from those in the dorsal telencephalon, radial glial progenitors (RGPs) in the ventral telencephalon responsible for producing neocortical interneurons progressively grow radial glial fibers anchored to periventricular vessels. This progenitor-vessel association is robust and actively maintained as RGPs undergo interkinetic nuclear migration and divide at the ventricular zone surface. Disruption of this association by selective removal of INTEGRIN β1 in RGPs leads to a decrease in progenitor division, a loss of PARVALBUMIN and SOMATOSTATIN-expressing interneurons, and defective synaptic inhibition in the neocortex. These results highlight a prominent interaction between RGPs and periventricular vessels important for proper production and function of neocortical interneurons.