Tangential migration in neocortical development

During cortical development, different cell populations arise in the basal telencephalon and subsequently migrate tangentially to the neocortex. However, it is not clear whether these cortical cells are generated in the lateral ganglionic eminence (LGE), the medial ganglionic eminence (MGE), or both. In this study, we have generated a three-dimensional reconstruction to study the morphological formation of the two ganglionic eminences and the interganglionic sulcus. As a result, we have demonstrated the importance of the development of these structures for this tangential migration to the neocortex. We have also used the tracers DiI and BDA in multiple experimental paradigms (whole embryo culture, in utero injections, and brain slice cultures) to analyze the routes of cell migration and to demonstrate the roles of both eminences in the development of the cerebral cortex. These results are further strengthened, confirming the importance of the MGE in this migration and demonstrating the early generation of tangential migratory cells in the LGE early in development. Finally, we show that the calcium-binding protein Calretinin is expressed in some of these tangentially migrating cells. Moreover, we describe the spatiotemporal sequence of GABA, Calbindin, and Calretinin expression, showing that these three markers are expressed in the cortical neuroepithelium over several embryonic days, suggesting that the cells migrating tangentially form a heterogeneous population.     

Control of cortical interneuron migration by neurotrophins and PI3-kinase signaling

During telencephalic development, cells from the medial ganglionic eminence (MGE) are thought to migrate to the neocortex to give rise to a majority of cortical GABAergic interneurons. By combining time-lapse video-microscopy, immunofluorescence and pharmacological perturbations in a new in vitro migration assay, we find that MGE-derived cells migrate through the entire extent of the cortex and into the CA fields of the hippocampus, but avoid the dentate gyrus. Migrating neurons initially travel within the marginal zone and intermediate zone, and can enter the cortical plate from either location. Tangential migration is strongly stimulated by BDNF and NT4 and attenuated by the Trk-family inhibitor, K252a, suggesting that migration is regulated by TrkB signaling. Furthermore, TrkB-null mice show a significant decrease in the number of calbindin-positive neurons migrating tangentially in the embryonic cortex. BDNF and NT4 cause rapid activation of PI3-kinase in MGE cells, and inhibition of PI3-kinase (but not of MAP kinase or PLCgamma) dramatically attenuates tangential migration. These observations suggest that TrkB signaling, via PI3-kinase activation, plays an important role in controlling interneuron migration in the developing cerebral cortex.     

The COUP-TF nuclear receptors regulate cell migration in the mammalian basal forebrain

Cells migrate via diverse pathways and in different modes to reach their final destinations during development. Tangential migration has been shown to contribute significantly to the generation of neuronal diversity in the mammalian telencephalon. GABAergic interneurons are the best-characterized neurons that migrate tangentially, from the ventral telencephalon, dorsally into the cortex. However, the molecular mechanisms and nature of these migratory pathways are only just beginning to be unravelled. In this study we have first identified a novel dorsal-to-ventral migratory route, in which cells migrate from the interganglionic sulcus, located in the basal telencephalon between the lateral and medial ganglionic eminences, towards the pre-optic area and anterior hypothalamus in the diencephalon. Next, with the help of transplantations and gain-of-function studies in organotypic cultures, we have shown that COUP-TFI and COUP-TFII are expressed in distinct and non-overlapping migratory routes. Ectopic expression of COUP-TFs induces an increased rate of cell migration and cell dispersal, suggesting roles in cellular adhesion and migration processes. Moreover, cells follow a distinct migratory path, dorsal versus ventral, which is dependent on the expression of COUP-TFI or COUP-TFII, suggesting an intrinsic role of COUP-TFs in guiding migrating neurons towards their target regions. Therefore, we propose that COUP-TFs are directly involved in tangential cell migration in the developing brain, through the regulation of short- and long-range guidance cues.     

Multidirectional and multizonal tangential migration of GABAergic interneurons in the developing cerebral cortex

Most GABAergic interneurons originate from the basal forebrain and migrate tangentially into the cortex. The migratory pathways and mode of interneuron migration within the developing cerebral cortex, however, previously was largely unknown. Time-lapse imaging and in vivo labelling with glutamate decarboxylase (GAD)67-green fluorescence protein (GFP) knock-in embryonic mice with expression of GFP in gamma-aminobutyric acid (GABA)ergic neurons indicated that multidirectional tangential (MDT) migration of interneurons takes place in both the marginal zone (MZ) and the ventricular zone (VZ) of the cortex. Quantitative analysis of migrating interneurons showed that rostrocaudally migrating neurons outnumber those migrating mediolaterally in both of these zones. In vivo labelling with a lipophilic dye showed that the MDT migration in the MZ occurs throughout the cortex over distances of up to 3 mm during a period of a few days. These results indicate that MZ cortical interneurons undergo a second phase of tangential migration in all directions and over long distances, after reaching the cortex by dorsomedial tangential migration. The MDT migration in the MZ may disperse and intermix interneurons within the cortex, resulting in a balanced distribution of interneuron subtypes.     

Distinct origin of GABA-ergic neurons in forebrain of man, nonhuman primates and lower mammals

In this mini-review we present recent data about origin of GABA-ergic (gama-aminobutyric acid) neurons in the mammalian forebrain, including the diencephalon and telencephalon. The interest in GABA-ergic neurons, which in cerebral cortex mostly correspond to local circuit neurons (interneurons), has increased in the past decade. Many studies have shown that in lower mammals all hippocampal and almost all neo-cortical GABA-ergic neurons are born in the specific region named ganglionic eminence, and not locally in proliferative layers all around telencephalic vesicle. The ganglionic eminence, that represents a region with thick proliferative-subventricular layer in the ventral (basal) part of telencephalon, was classically thought to give neurons to basal ganglia and septal nuclei, whereas proliferative layers of dorsal telencephalon give neurons to cerebral cortex including hippocampus. It was thought that neurons migrate from proliferative layer to their target region following a radial orientation. However, data in lower mammals showed that this is the case only for glutamatergic principal cells, i.e. projection neurons. GABA-ergic neurons use long distance tangentional migration, parallel to pial surface to reach, from ganglionic eminence, their targeting layer in the cerebral cortex. Especially intriguing, but frequently neglecting, several studies suggest that mammalian evolution might use different developmental rules to provide GABA-ergic neurons to an expending brain. In this review we focus on specific events underlying GABA-ergic neuron development in human and non-human primates. Disturbances of the GABAergic network are found in many neurological and psychiatric disorders, some of them might result from altered production or migration of these neurons during development. Therefore, it is crucial to understand human-specific mechanisms that regulate the development of GABA-ergic neurons.     

GABA regulates the multidirectional tangential migration of GABAergic interneurons in living neonatal mice

Cortical GABAergic interneurons originate from ganglionic eminences and tangentially migrate into the cortical plate at early developmental stages. To elucidate the characteristics of this migration of GABAergic interneurons in living animals, we established an experimental design specialized for in vivo time-lapse imaging of the neocortex of neonate mice with two-photon laser-scanning microscopy. In vesicular GABA/glycine transporter (VGAT)-Venus transgenic mice from birth (P0) through P3, we observed multidirectional tangential migration of genetically-defined GABAergic interneurons in the neocortical marginal zone. The properties of this migration, such as the motility rate (distance/hr), the direction moved, and the proportion of migrating neurons to stationary neurons, did not change through P0 to P3, although the density of GABAergic neurons at the marginal zone decreased with age. Thus, the characteristics of the tangential motility of individual GABAergic neurons remained constant in development. Pharmacological block of GABA_A receptors and of the Na⁺-K⁺-Cl⁻ cotransporters, and chelating intracellular Ca²⁺, all significantly reduced the motility rate in vivo. The motility rate and GABA content within the cortex of neonatal VGAT-Venus transgenic mice were significantly greater than those of GAD67-GFP knock-in mice, suggesting that extracellular GABA concentration could facilitate the multidirectional tangential migration. Indeed, diazepam applied to GAD67-GFP mice increased the motility rate substantially. In an in vitro neocortical slice preparation, we confirmed that GABA induced a NKCC sensitive depolarization of GABAergic interneurons in VGAT-Venus mice at P0-P3. Thus, activation of GABA_AR by ambient GABA depolarizes GABAergic interneurons, leading to an acceleration of their multidirectional motility in vivo.     

The adhesion molecule TAG-1 mediates the migration of cortical interneurons from the ganglionic eminence along the corticofugal fiber system.

Cortical nonpyramidal cells, the GABA-containing interneurons, originate mostly in the medial ganglionic eminence of the ventral telencephalon and follow tangential migratory routes to reach the dorsal telencephalon. Although several genes that play a role in this migration have been identified, the underlying cellular and molecular cues are not fully understood. We provide evidence that the neural cell adhesion molecule TAG-1 mediates the migration of cortical interneurons. We show that the migration of these neurons occurs along the TAG-1-expressing axons of the developing corticofugal system. The spatial and temporal pattern of expression of TAG-1 on corticofugal fibers coincides with the order of appearance of GABAergic cells in the developing cortex. Blocking the function of TAG-1, but not of L1, another adhesion molecule and binding partner of TAG-1, results in a marked reduction of GABAergic neurons in the cortex. These observations reveal a mechanism by which the adhesion molecule TAG-1, known to be involved in axonal pathfinding, also takes part in neuronal migration.     

Radial glial dependent and independent dynamics of interneuronal migration in the developing cerebral cortex

Interneurons originating from the ganglionic eminence migrate tangentially into the developing cerebral wall as they navigate to their distinct positions in the cerebral cortex. Compromised connectivity and differentiation of interneurons are thought to be an underlying cause in the emergence of neurodevelopmental disorders such as schizophrenia. Previously, it was suggested that tangential migration of interneurons occurs in a radial glia independent manner. Here, using simultaneous imaging of genetically defined populations of interneurons and radial glia, we demonstrate that dynamic interactions with radial glia can potentially influence the trajectory of interneuronal migration and thus the positioning of interneurons in cerebral cortex. Furthermore, there is extensive local interneuronal migration in tangential direction opposite to that of pallial orientation (i.e., in a medial to lateral direction from cortex to ganglionic eminence) all across the cerebral wall. This counter migration of interneurons may be essential to locally position interneurons once they invade the developing cerebral wall from the ganglionic eminence. Together, these observations suggest that interactions with radial glial scaffold and localized migration within the expanding cerebral wall may play essential roles in the guidance and placement of interneurons in the developing cerebral cortex.     

Cell migration from the ganglionic eminences is required for the development of hippocampal GABAergic interneurons

GABAergic interneurons have major roles in hippocampal function and dysfunction. Here we provide evidence that, in mice, virtually all of these cells originate from progenitors in the basal telencephalon. Immature interneurons tangentially migrate from the basal telencephalon through the neocortex to take up their final positions in the hippocampus. Disrupting differentiation in the embryonic basal telencephalon (lateral and medial ganglionic eminences) through loss of Dlx1/2 homeobox function blocks the migration of virtually all GABAergic interneurons to the hippocampus. On the other hand, disrupting specification of the medial ganglionic eminence through loss of Nkx2.1 homeobox function depletes the hippocampus of a distinct subset of hippocampal interneurons. Loss of hippocampal interneurons does not appear to have major effects on the early development of hippocampal projection neurons nor on the pathfinding of afferrent tracts.     

Ephrins guide migrating cortical interneurons in the basal telencephalon

Cortical interneurons are born in the proliferative zones of the ganglionic eminences in the subpallium and migrate to the developing cortex along well-defined tangential routes. The mechanisms regulating interneuron migration are not completely understood. Here we examine the role of class-A members of the Eph/ephrin system in directing the migration of interneurons. In situ hybridizations demonstrated that ephrin-A3 is expressed in the developing striatum, an area that is strictly avoided by migrating cortical interneurons in vivo, which express the EphA4 receptor. We then examined interneuron migration in grafting experiments, where explants of the medial ganglionic eminence (MGE) from enhanced green fluorescent protein-expressing transgenic mice were homotopically grafted into host slices from wild-type littermate embryos. After blocking ephrin-A ligands, many interneurons invaded the striatal anlage. Moreover, stripe assay experiments revealed that ephrin-A3 acts as a repellent cue for neurons from the medial ganglionic eminence. Downregulation of the EphA4 receptor via siRNA transfection reduced the repulsive effect of ephrin-A3, indicating that EphA4 mediates at least in part the repulsive effect of ephrin-A3 on these cells. Together, these results suggest that ephrin-A3 acts as a repulsive cue that restricts cortical interneurons from entering inappropriate regions and thus contributes to define the migratory route of cortical interneurons.Key words: interneuron migration, cortical development, neuronal guidance cues, ephrin, Eph receptors, organotypic slice cultures     

Netrin 1 regulates ventral tangential migration of guidepost neurons in the lateral olfactory tract

In the developing nervous system, functional neural networks are constructed with intricate coordination of neuronal migrations and axonal projections. We have previously reported a ventral tangential migration of a special type of cortical neurons, lot cells, in the mouse embryo. These neurons originate from the ventricular zone of the entire neocortex, tangentially migrate in the surface layer of the neocortex into the ventral direction, align in the future pathway of the lateral olfactory tract (LOT) and eventually guide the projection of LOT axons. In this study, we developed an organotypic culture system to investigate the regulation of this cell migration in the developing telencephalon. Our data show that the neocortex contains the signals that direct lot cells ventrally, that the ganglionic eminence excludes lot cells by repelling the migration and that lot cells are attracted to netrin 1, an axon guidance factor. Furthermore, we demonstrate that mutations in the genes encoding netrin 1 and its functional receptor Dcc lead to inappropriate distribution of lot cells and subsequent partial disruption of LOT projection. These results suggest that netrin 1 regulates the migration of lot cells and LOT projections, possibly by ensuring the correct distribution of these guidepost neurons.     

Distinct cortical migrations from the medial and lateral ganglionic eminences

Recent evidence suggests that projection neurons and interneurons of the cerebral cortex are generally derived from distinct proliferative zones. Cortical projection neurons originate from the cortical ventricular zone (VZ), and then migrate radially into the cortical mantle, whereas most cortical interneurons originate from the basal telencephalon and migrate tangentially into the developing cortex. Previous studies using methods that label both proliferative and postmitotic cells have found that cortical interneurons migrate from two major subdivisions of the developing basal telencephalon: the medial and lateral ganglionic eminences (MGE and LGE). Since these studies labeled cells by methods that do not distinguish between the proliferating cells and those that may have originated elsewhere, we have studied the contribution of the MGE and LGE to cortical interneurons using fate mapping and genetic methods. Transplantation of BrdU-labeled MGE or LGE neuroepithelium into the basal telencephalon of unlabeled telencephalic slices enabled us to follow the fate of neurons derived from each of these primordia. We have determined that early in neurogenesis GABA-expressing cells from the MGE tangentially migrate into the cerebral cortex, primarily via the intermediate zone, whereas cells from the LGE do not. Later in neurogenesis, LGE-derived cells also migrate into the cortex, although this migration occurs primarily through the subventricular zone. Some of these LGE-derived cells invade the cortical plate and express GABA, while others remain within the cortical proliferative zone and appear to become mitotically active late in gestation. In addition, by comparing the phenotypes of mouse mutants with differential effects on MGE and LGE migration, we provide evidence that the MGE and LGE may give rise to different subtypes of cortical interneurons.     

Formation and preservation of cortical layers in slice cultures.

During cortical development, neurons generated at the same time in the ventricular zone migrate out into the cortical plate and form a cortical layer (Berry and Eayrs, 1963, Nature 197:984-985; Berry and Rogers, 1965, J. Anat. 99:691-709). We have been studying both the formation and maintenance of cortical layers in slice cultures from rat cortex. The bromodeoxyuridine (BrdU) method was used to label cortical neurons on their birthday in vivo. When slice cultures were prepared from animals at different embryonic and postnatal ages, all cortical layers that have already been established in vivo remained preserved for several weeks in vitro. In slice cultures prepared during migration in the cortex, cells continued to migrate towards the pial side of the cortical slice, however, migration ceased after about 1 week in culture. Thus, cortical cells reached their final laminar position only in slice cultures from postnatal animals, whereas in embryonic slice, migrating cells became scattered throughout the cortex. Previous studies demonstrated that radial glia fibers are the major substrate for migrating neurons (Rakic, 1972, J. Comp. Neurol. 145:61-84; Hatten and Mason, 1990, Experientia 46:907-916). Using antibodies directed against the intermediate filament Vimentin, radial glial cells were detected in all slice cultures where cell migration did occur. Comparable to the glia development in vivo, radial glial fibers disappeared and astrocytes containing the glia fibrillary-associated protein (GFAP) differentiated in slice cultures from postnatal cortex, after the neurons have completed their migration. In contrast, radial glial cells were detected over the whole culture period, and very few astrocytes differentiated in embryonic slices, where cortical neurons failed to finish their migration. The results of this study indicate that the local environment is sufficient to sustain the layered organization of the cortex and support the migration of cortical neurons. In addition, our results reveal a close relationship between cell migration and the developmental status of glial cells.     

Tangential migration of glutamatergic neurons and cortical patterning during development: Lessons from Cajal‐Retzius cells

Tangential migration is a mode of cell movement, which in the developing cerebral cortex, is defined by displacement parallel to the ventricular surface and orthogonal to the radial glial fibers. This mode of long‐range migration is a strategy by which distinct neuronal classes generated from spatially and molecularly distinct origins can integrate to form appropriate neural circuits within the cortical plate. While it was previously believed that only GABAergic cortical interneurons migrate tangentially from their origins in the subpallial ganglionic eminences to integrate in the cortical plate, it is now known that transient populations of glutamatergic neurons also adopt this mode of migration. These include Cajal‐Retzius cells (CRs), subplate neurons (SPs), and cortical plate transient neurons (CPTs), which have crucial roles in orchestrating the radial and tangential development of the embryonic cerebral cortex in a noncell‐autonomous manner. While CRs have been extensively studied, it is only in the last decade that the molecular mechanisms governing their tangential migration have begun to be elucidated. To date, the mechanisms of SPs and CPTs tangential migration remain unknown. We therefore review the known signaling pathways, which regulate parameters of CRs migration including their motility, contact‐redistribution and adhesion to the pial surface, and discuss this in the context of how CR migration may regulate their signaling activity in a spatial and temporal manner. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 847–881, 2016     

Ephrins guide migrating cortical interneurons in the basal telencephalon

Cortical interneurons are born in the proliferative zones of the ganglionic eminences in the subpallium and migrate to the developing cortex along well-defined tangential routes. The mechanisms regulating interneuron migration are not completely understood. Here we examine the role of class-A members of the Eph/ephrin system in directing the migration of interneurons. In situ hybridizations demonstrated that ephrin A3 is expressed in the developing striatum, an area that is strictly avoided by migrating cortical interneurons in vivo, which express the EphA4 receptor. We then examined interneuron migration in grafting experiments, where explants of the medial ganglionic eminence (MGE) from enhanced green fluorescent protein-expressing transgenic mice were homotopically grafted into host slices from wild-type littermate embryos. After blocking ephrin-A ligands, many interneurons invaded the striatal anlage. Moreover, stripe assay experiments revealed that ephrin-A3 acts as a repellent cue for neurons from the medial ganglionic eminence. Downregulation of the EphA4 receptor via siRNA transfection reduced the repulsive effect of ephrin-A3, indicating that EphA4 mediates at least in part the repulsive effect of ephrin A3 on these cells. Together, these results suggest that ephrin-A3 acts as a repulsive cue that restricts cortical interneurons from entering inappropriate regions and thus contributes to define the migratory route of cortical interneurons.     

Short- and long-range attraction of cortical GABAergic interneurons by neuregulin-1

Most cortical interneurons arise from the subcortical telencephalon, but the molecules that control their migration remain largely unidentified. Here, we show that different isoforms of Neuregulin-1 are expressed in the developing cortex and in the route that migrating interneurons follow toward the cortex, whereas a population of the migrating interneurons express ErbB4, a receptor for Neuregulin-1. The different isoforms of Neuregulin-1 act as short- and long-range attractants for migrating interneurons, and perturbing ErbB4 function in vitro decreases the number of interneurons that tangentially migrate to the cortex. In vivo, loss of Neuregulin-1/ErbB4 signaling causes an alteration in the tangential migration of cortical interneurons and a reduction in the number of GABAergic interneurons in the postnatal cortex. These observations provide evidence that Neuregulin-1 and its ErbB4 receptor directly control neuronal migration in the nervous system.     

Formation and preservation of cortical layers in slice cultures

During cortical development, neurons generated at the same time in the ventricular zone migrate out into the cortical plate and form a cortical layer (Berry and Eayrs, 1963, Nature 197:984–985; Berry and Rogers, 1965, J. Anat. 99:691–709). We have been studying both the formation and maintenance of cortical layers in slice cultures from rat cortex. The bromodexyuridine (BrdU) method was used to label cortical neurons on their birthday in vivo. When slice cultures were prepared from animals at different embryonic and postnatal ages, all cortical layers that have already been established in vivo remained preserved for several weeks in vitro. In slice cultures prepared during migration in the cortex, cells contiuned to migrate towards the pial side of the cortical slice, however, migration ceased after about 1 week in culture. Thus, cortical cells reached their final laminar position only in slice cultures from postnatal animals, whereas in embryonic slices, migrating cells became scattered throughout the cortex. Previous studies demonstrated that radial glia fibers are the major substrate for migrating neurons (Rakic, 1972, J. Comp. Neurol. 145:61–84; Hatten and Mason, 1990, Experientia 46:907–916). Using antibodies directed against the intermediate filament Vimentin, radial glial cells were detected in all slice cutures where cell migration did occur. Comparable to the glia development in vivo, radial glial fibers disappeared and astrocytes containing the glia fibrillary-associated protein (GFAP) differentiated in slice cultures from postnatal cortex, after the neurons have completed their migration. In contrast, radial glial cells were detected over the whole culture period, and very few astrocytes differentiated in embryonic slices, where cortical neurons failed to finish their migration. The results of this study indicate that the local environment is sufficient to sustain the layered organization of the cortex and support the migration of cortical neurons. In addition, our results reveal a close relationship between cell migration and the developmental status of glial cells. © 1992 John Wiley & Sons, Inc.