A balance of noncanonical Semaphorin signaling from the cerebrospinal fluid regulates apical cell dynamics during corticogenesis.

During corticogenesis, dynamic regulation of apical adhesion is fundamental to generate correct numbers and cell identities. While radial glial cells (RGCs) maintain basal and apical anchors, basal progenitors and neurons detach and settle at distal positions from the apical border. Whether diffusible signals delivered from the cerebrospinal fluid (CSF) contribute to the regulation of apical adhesion dynamics remains fully unknown. Secreted class 3 Semaphorins (Semas) trigger cell responses via Plexin-Neuropilin (Nrp) membrane receptor complexes. Here, we report that unconventional Sema3-Nrp preformed complexes are delivered by the CSF from sources including the choroid plexus to Plexin-expressing RGCs via their apical endfeet. Through analysis of mutant mouse models and various ex vivo assays mimicking ventricular delivery to RGCs, we found that two different complexes, Sema3B/Nrp2 and Sema3F/Nrp1, exert dual effects on apical endfeet dynamics, nuclei positioning, and RGC progeny. This reveals unexpected balance of CSF-delivered guidance molecules during cortical development.

The role of intraspinal sensory neurons in the control of quadrupedal locomotion.

From swimming to walking and flying, animals have evolved specific locomotor strategies to thrive in different habitats. All types of locomotion depend on the integration of motor commands and sensory information to generate precisely coordinated movements. Cerebrospinal-fluid-contacting neurons (CSF-cN) constitute a vertebrate sensory system that monitors CSF composition and flow. In fish, CSF-cN modulate swimming activity in response to changes in pH and bending of the spinal cord; however, their role in mammals remains unknown. We used mouse genetics to study their function in quadrupedal locomotion. We found that CSF-cN are directly integrated into spinal motor circuits. The perturbation of CSF-cN function does not affect general motor activity nor the generation of locomotor rhythm and pattern but results in specific defects in skilled movements. These results identify a role for mouse CSF-cN in adaptive motor control and indicate that this sensory system evolved a novel function to accommodate the biomechanical requirements of limb-based locomotion.

The Transcription Factor LHX1 Regulates the Survival and Directed Migration of POA-derived Cortical Interneurons.

The delicate balance of excitation and inhibition is crucial for proper function of the cerebral cortex, relying on the accurate number and subtype composition of inhibitory gamma-aminobutyric (GABA)-expressing interneurons. Various intrinsic and extrinsic factors precisely orchestrate their multifaceted development including the long-range migration from the basal telencephalon to cortical targets as well as interneuron survival throughout the developmental period. Particularly expressed guidance receptors were described to channel the migration of cortical interneurons deriving from the medial ganglionic eminence (MGE) and the preoptic area (POA) along distinct routes. Hence, unveiling the regulatory genetic networks controlling subtype-specific gene expression profiles is key to understand interneuron-specific developmental programs and to reveal causes for associated disorders. In contrast to MGE-derived interneurons, little is known about the transcriptional networks in interneurons born in the POA. Here, we provide first evidence for the LIM-homeobox transcription factor LHX1 as a crucial key player in the post-mitotic development of POA-derived cortical interneurons. By transcriptional regulation of related genes, LHX1 modulates their survival as well as the subtype-specific expression of guidance receptors of the Eph/ephrin family, thereby affecting directional migration and layer distribution in the adult cortex.

Thalamic afferents influence cortical progenitors via ephrin A5-EphA4 interactions.

The phenotype of excitatory cerebral cortex neurons is specified at the progenitor level, orchestrated by various intrinsic and extrinsic factors. Here, we provide evidence for a subcortical contribution to cortical progenitor regulation by thalamic axons via ephrin A5-EphA4 interactions. Ephrin A5 is expressed by thalamic axons and represents a high-affinity ligand for EphA4 receptors detected in cortical precursors. Recombinant ephrin A5-Fc protein, as well as ephrin A ligand-expressing, thalamic axons affect the output of cortical progenitor division in vitro. Ephrin A5-deficient mice show an altered division mode of radial glial cells (RGCs) accompanied by increased numbers of intermediate progenitor cells (IPCs) and an elevated neuronal production for the deep cortical layers at E13.5. In turn, at E16.5 the pool of IPCs is diminished, accompanied by reduced rates of generated neurons destined for the upper cortical layers. This correlates with extended infragranular layers at the expense of superficial cortical layers in adult ephrin A5-deficient and EphA4-deficient mice. We suggest that ephrin A5 ligands imported by invading thalamic axons interact with EphA4-expressing RGCs, thereby contributing to the fine-tuning of IPC generation and thus the proper neuronal output for cortical layers.

A dual role of EphB1/ephrin-B3 reverse signaling on migrating striatal and cortical neurons originating in the preoptic area: should I stay or go away?

During embryonic development the preoptic area (POA) gives rise to two populations of neurons which are generated at the same time, cortical interneurons and striatal cells. POA-derived cortical interneurons take a superficial path and avoid the developing striatum (Str) when they migrate to their target region. We found that EphB1, which is expressed in the striatal anlage, prevents cortical interneurons from entering the Str via ephrin-B3 reverse signaling. In contrast, for striatal neurons which also express ephrin-B3, EphB1 acts as a stop signal. This dual role of EphB1 is due to differences in ephrin-B3 reverse signaling cascades. For striatal neurons, binding of EphB1 to ephrin-B3 reduces endogenously high levels of pSrc and pFAK, which then causes the cells to stop migration. In contrast, in cortical interneurons EphB1-ephrin-B3 reverse signaling leads to phosphorylation of Src and focal adhesion kinase (FAK) which then mediates repulsion. Consistent with these in vitro findings, in an ephrin-B3 knockout mouse line, we discovered misrouted cortical interneurons in the Str and an over-migration of striatal neurons in their target region. Thus, EphB1/ephrin-B3 reverse signaling has a different impact on two sets of neurons which are generated at the same time and place: it can act as a repulsive cue for migrating neurons or it can terminate neuronal migration, a novel role of the Eph/ephrin system.

Bidirectional ephrinB3/EphA4 signaling mediates the segregation of medial ganglionic eminence- and preoptic area-derived interneurons in the deep and superficial migratory stream.

The integration of interneuron subtypes into specific microcircuits is essential for proper cortical function. Understanding to what extent interneuron diversity is regulated and maintained during development might help to reveal the principles that govern their role as synchronizing elements as well as causes for dysfunction. Particular interneuron subtypes are generated in a temporally regulated manner in the medial ganglionic eminence (MGE), the caudal ganglionic eminence, and the preoptic area (POA) of the basal telencephalon. Long-range tangential migration from their site of origin to cortical targets is orchestrated by a variety of attractive, repulsive, membrane-bound, and secreted signaling molecules, to establish the critical balance of inhibition and excitation. It remains unknown whether interneurons deriving from distinct domains are predetermined to migrate in particular routes and whether this process underlies cell type-specific regulation. We found that POA- and MGE-derived cortical interneurons migrate within spatially segregated corridors. EphrinB3, expressed in POA-derived interneurons traversing the superficial route, acts as a repellent signal for deeply migrating interneurons born in the MGE, which is mediated by EphA4 forward signaling. In contrast, EphA4 induces repulsive ephrinB3 reverse signaling in interneurons generated in the POA, restricting this population to the superficial path. Perturbation of this bidirectional ephrinB3/EphA4 signaling in vitro and in vivo leads to a partial intermingling of cells in these segregated migratory pathways. Thus, we conclude that cell contact-mediated bidirectional ephrinB3/EphA4 signaling mediates the sorting of MGE- and POA-derived interneurons in the deep and superficial migratory stream.