Ascl1 promotes tangential migration and confines migratory routes by induction of Ephb2 in the telencephalon

Abstract

During development, cortical interneurons generated from the ventral telencephalon migrate tangentially into the dorsal telencephalon. Although Achaete-scute family bHLH transcription factor 1 (Ascl1) plays important roles in the developing telencephalon, whether Ascl1 regulates tangential migration remains unclear. Here, we found that Ascl1 promoted tangential migration along the ventricular zone/subventricular zone (VZ/SVZ) and intermediate zone (IZ) of the dorsal telencephalon. Distal-less homeobox 2 (Dlx2) acted downstream of Ascl1 in promoting tangential migration along the VZ/SVZ but not IZ. We further identified Eph receptor B2 (Ephb2) as a direct target of Ascl1. Knockdown of EphB2 disrupted the separation of the VZ/SVZ and IZ migratory routes. Ephrin-A5, a ligand of EphB2, was sufficient to repel both Ascl1-expressing cells in vitro and tangentially migrating cortical interneurons in vivo. Together, our results demonstrate that Ascl1 induces expression of Dlx2 and Ephb2 to maintain distinct tangential migratory routes in the dorsal telencephalon.The cerebral cortex is important for executing high-order brain functions, such as sensory perception, motor control and cognition. Proper functions of the cerebral cortex require coordination between excitatory (glutamatergic) projection neurons and inhibitory (GABAergic) interneurons. Many neurological and psychological disorders, such as epilepsy, autism, schizophrenia, and Alzheimer's disease, may be resulted from an imbalance of excitatory and inhibitory neuronal activities or the dysfunction of interneurons 1,2 . Interestingly, glutamatergic neurons and GABAergic interneurons are produced from different regions of the developing brain. Glutamatergic neurons are generated from neural progenitors in the dorsal telencephalon, which develops into the cerebral cortex. GABAergic cortical interneurons are remotely derived from neural progenitors in the ventral telencephalon, which develops into the striatum and globus pallidus 3 . Around 85% of cortical interneurons are generated from the medial ganglionic eminence (MGE) and the rest are generated from the caudal ganglionic eminence (CGE) 4,5 . Once generated in the ventricular zone (VZ), glutamatergic neurons and GABAergic interneurons migrate toward final destinations through distinct paths. Glutamatergic neurons undergo radial migration to reach the cortical plate (CP) 6 . Interneurons take a long tangentially migratory path from the ventral telencephalon toward the striatum or the cerebral cortex 3 . When cortical interneurons reach the dorsal telencephalon, they continue migrating tangentially through three migratory routes: the marginal zone (MZ), intermediate zone (IZ) or ventricular/subventricular zone (VZ/SVZ) of the dorsal telencephalon 7,8 . Upon arrival at destined region, interneurons switch to migrate radially until they reach specific layers of the cerebral cortex, where they integrate into local neuronal circuits. Since cortical interneurons go through such a complicated migratory path, it is impo...

Interneuron odyssey: molecular mechanisms of tangential migration

“…Further, a recent study demonstrated that EphB2/EphrinA5 signaling maintains the segregation of the SVZ and SP migratory routes. EphrinA5, highly expressed in the deep ventricular zone (VZ), upper SVZ, deep intermediate zone (IZ) and the CP of the developing telencephalon, confines EphB2-expressing interneurons to the SVZ and SP streams through a repulsive effect ( Liu et al, 2017 ). Similarly, fibronectin leucine-rich transmembrane proteins (FLRT2 and 3), expressed by pyramidal cells and previously known for their roles in axon guidance, excitatory neuron migration and synaptogenesis ( Yamagishi et al, 2011 ; O'Sullivan et al, 2012 ; Leyva-Díaz et al, 2014 ; Del Toro et al, 2017 , 2020 ), have recently been shown to exert repulsive effects on Unc5B/D-expressing cINs in vitro ( Fleitas et al, 2021 ).…”

“…In utero electroporation of both Ascl1 and Dlx2 in the dorsal telencephalon of mice promotes tangential migration along the SVZ/VZ and IZ. Further, knockdown of Dlx2 and overexpression of Ascl1 leads to a reduction in the number of INs migrating through the SVZ/VZ, but increases the number of INs migrating through the IZ ( Liu et al, 2017 ). Moreover, chromatin immunoprecipitation assays confirmed that EphB2 receptor, which binds to the repulsive molecule EphrinA5, is a direct downstream target of Ascl1 .…”

“…Moreover, chromatin immunoprecipitation assays confirmed that EphB2 receptor, which binds to the repulsive molecule EphrinA5, is a direct downstream target of Ascl1 . Disruption of EphB2/EphrinA5 signaling alters tangential migration as INs fail to confine to the DMS ( Liu et al, 2017 ). These results indicate that Ascl1 promotes tangential migration in two distinct ways: through the induction of Dlx2 expression and in a Dlx2 -independent fashion through the induction of Ephb2 expression.…”