Control of tangential/non-radial migration of neurons in the developing cerebral cortex

Maternal immunoglobulins are distributed in the offspring's brain to support the maintenance of cortical interneurons in the postnatal period

It is known that maternal immunoglobulins (Igs) are transferred to the offspring across the placenta. However, receiving maternal Igs, especially before the blood-brain barrier (BBB) is formed in the offspring's brain, carries the risk of transferring some brain-reactive Igs. It is thus hypothesized that there may be some unknown benefit to the offspring's brain that overweighs this risk. In this study, we show that the Ig detected in the embryonic/perinatal mouse brain is IgG not produced by the pups themselves, but is basically transferred from the mother across the placenta using the neonatal Fc receptor (FcRn) during embryonic stages. The amount of IgG in the brain gradually decreases after birth, and almost disappears within 3 weeks postnatally. IgG is detected on axon bundles, microglia, and some meningeal cells, including border-associated macrophages (BAMs), endothelial cells, and fibroblasts. Using Fcer1g knock-out (KO) mice, we show that BAMs and microglia receive maternal IgG in an Fc receptor γ chain (FcRγ)-dependent manner, but IgG on other meningeal cells and axon bundles is received independently of the FcRγ. These results suggest that maternal IgG may be used in multiple ways by different mechanisms. In maternal IgG-deficient mice, the number of interneurons in the cerebral cortex is not altered around birth but is reduced postnatally, suggesting that receipt of maternal IgG is necessary for the maintenance of cortical interneurons in the postnatal period. These data suggest that maternal IgG has an important function in the developing brain, where neither obvious inflammation nor infection is observed.

Interactions between neural cells and blood vessels in central nervous system development

The sophisticated function of the central nervous system (CNS) is largely supported by proper interactions between neural cells and blood vessels. Accumulating evidence has demonstrated that neurons and glial cells support the formation of blood vessels, which in turn, act as migratory scaffolds for these cell types. Neural progenitors are also involved in the regulation of blood vessel formation. This mutual interaction between neural cells and blood vessels is elegantly controlled by several chemokines, growth factors, extracellular matrix, and adhesion molecules such as integrins. Recent research has revealed that newly migrating cell types along blood vessels repel other preexisting migrating cell types, causing them to detach from the blood vessels. In this review, we discuss vascular formation and cell migration, particularly during development. Moreover, we discuss how the crosstalk between blood vessels and neurons and glial cells could be related to neurodevelopmental disorders.

Patterning ganglionic eminences in developing human brain organoids using a morphogen-gradient-inducing device

In early neurodevelopment, the central nervous system is established through the coordination of various neural organizers directing tissue patterning and cell differentiation. Better recapitulation of morphogen gradient production and signaling will be crucial for establishing improved developmental models of the brain in vitro. Here, we developed a method by assembling polydimethylsiloxane devices capable of generating a sustained chemical gradient to produce patterned brain organoids, which we termed morphogen-gradient-induced brain organoids (MIBOs). At 3.5 weeks, MIBOs replicated dorsal-ventral patterning observed in the ganglionic eminences (GE). Analysis of mature MIBOs through single-cell RNA sequencing revealed distinct dorsal GE-derived CALB2+ interneurons, medium spiny neurons, and medial GE-derived cell types. Finally, we demonstrate long-term culturing capabilities with MIBOs maintaining stable neural activity in cultures grown up to 5.5 months. MIBOs demonstrate a versatile approach for generating spatially patterned brain organoids for embryonic development and disease modeling.

Activation of Sonic Hedgehog Signaling Promotes Differentiation of Cortical Layer 4 Neurons via Regulation of Their Cell Positioning

Neuronal subtypes in the mammalian cerebral cortex are determined by both intrinsic and extrinsic mechanisms during development. However, the extrinsic cues that are involved in this process remain largely unknown. Here, we investigated the role of sonic hedgehog (Shh) in glutamatergic cortical subtype specification. We found that E14.5-born, but not E15.5-born, neurons with elevated Shh expression frequently differentiated into layer 4 subtypes as judged by the cell positioning and molecular identity. We further found that this effect was achieved indirectly through the regulation of cell positioning rather than the direct activation of layer 4 differentiation programs. Together, we provided evidence that Shh, an extrinsic factor, plays an important role in the specification of cortical superficial layer subtypes.

N-cadherin (Cdh2) Maintains Migration and Postmitotic Survival of Cortical Interneuron Precursors in a Cell-Type-Specific Manner

The multiplex role of cadherin-based adhesion complexes during development of pallial excitatory neurons has been thoroughly characterized. In contrast, much less is known about their function during interneuron development. Here, we report that conditional removal of N-cadherin (Cdh2) from postmitotic neuroblasts of the subpallium results in a decreased number of Gad65-GFP-positive interneurons in the adult cortex. We also found that interneuron precursor migration into the pallium was already delayed at E14. Using immunohistochemistry and TUNEL assay in the embryonic subpallium, we excluded decreased mitosis and elevated cell death as possible sources of this defect. Moreover, by analyzing the interneuron composition of the adult somatosensory cortex, we uncovered an unexpected interneuron-type-specific defect caused by Cdh2-loss. This was not due to a fate-switch between interneuron populations or altered target selection during migration. Instead, potentially due to the migration delay, part of the precursors failed to enter the cortical plate and consequently got eliminated at early postnatal stages. In summary, our results indicate that Cdh2-mediated interactions are necessary for migration and survival during the postmitotic phase of interneuron development. Furthermore, we also propose that unlike in pallial glutamatergic cells, Cdh2 is not universal, rather a cell type-specific factor during this process.

Disruption of SynGAP–dopamine D1 receptor complexes alters actin and microtubule dynamics and impairs GABAergic interneuron migration

Disruption of γ-aminobutyric acid (GABA)–ergic interneuron migration is implicated in various neurodevelopmental disorders, including autism spectrum disorder and schizophrenia. The dopamine D1 receptor (D1R) promotes GABAergic interneuron migration, which is disrupted in various neurological disorders, some of which are also associated with mutations in the gene encoding synaptic Ras–guanosine triphosphatase–activating protein (SynGAP). Here, we explored the mechanisms underlying these associations and their possible connection. In prenatal mouse brain tissue, we found a previously unknown interaction between the D1R and SynGAP. This D1R-SynGAP interaction facilitated D1R localization to the plasma membrane and promoted D1R-mediated downstream signaling pathways, including phosphorylation of protein kinase A and p38 mitogen-activated protein kinase. These effects were blocked by a peptide (TAT-D1Rpep) that disrupted the D1R-SynGAP interaction. Furthermore, disrupting this complex in mice during embryonic development resulted in pronounced and selective deficits in the tangential migration of GABAergic interneurons, possibly due to altered actin and microtubule dynamics. Our results provide insights into the molecular mechanisms regulating interneuron development and suggest that disruption of the D1R-SynGAP interaction may underlie SYNGAP1 mutation–related neurodevelopmental disorders.

Cend1, a Story with Many Tales: From Regulation of Cell Cycle Progression/Exit of Neural Stem Cells to Brain Structure and Function

Neural stem/precursor cells (NPCs) generate the large variety of neuronal phenotypes comprising the adult brain. The high diversity and complexity of this organ have its origin in embryonic life, during which NPCs undergo symmetric and asymmetric divisions and then exit the cell cycle and differentiate to acquire neuronal identities. During these processes, coordinated regulation of cell cycle progression/exit and differentiation is essential for generation of the appropriate number of neurons and formation of the correct structural and functional neuronal circuits in the adult brain. Cend1 is a neuronal lineage-specific modulator involved in synchronization of cell cycle exit and differentiation of neuronal precursors. It is expressed all along the neuronal lineage, from neural stem/progenitor cells to mature neurons, and is associated with the dynamics of neuron-generating divisions. Functional studies showed that Cend1 has a critical role during neurogenesis in promoting cell cycle exit and neuronal differentiation. Mechanistically, Cend1 acts via the p53-dependent/Cyclin D1/pRb signaling pathway as well as via a p53-independent route involving a tripartite interaction with RanBPM and Dyrk1B. Upon Cend1 function, Notch1 signaling is suppressed and proneural genes such as Mash1 and Neurogenins 1/2 are induced. Due to its neurogenic activity, Cend1 is a promising candidate therapeutic gene for brain repair, while the Cend1 minimal promoter is a valuable tool for neuron-specific gene delivery in the CNS. Mice with Cend1 genetic ablation display increased NPC proliferation, decreased migration, and higher levels of apoptosis during development. As a result, they show in the adult brain deficits in a range of motor and nonmotor behaviors arising from irregularities in cerebellar cortex lamination and impaired Purkinje cell differentiation as well as a paucity in GABAergic interneurons of the cerebral cortex, hippocampus, and amygdala. Taken together, these studies highlight the necessity for Cend1 expression in the formation of a structurally and functionally normal brain.

Increased Anxiety-Related Behavior, Impaired Cognitive Function and Cellular Alterations in the Brain of Cend1-deficient Mice

Cend1 is a neuronal-lineage specific modulator involved in coordination of cell cycle exit and differentiation of neuronal precursors. We have previously shown that Cend1^-/- mice show altered cerebellar layering caused by increased proliferation of granule cell precursors, delayed radial granule cell migration and compromised Purkinje cell differentiation, leading to ataxic gait and deficits in motor coordination. To further characterize the effects of Cend1 genetic ablation we determined herein a range of behaviors, including anxiety and exploratory behavior in the elevated plus maze (EPM), associative learning in fear conditioning, and spatial learning and memory in the Morris water maze (MWM). We observed significant deficits in all tests, suggesting structural and/or functional alterations in brain regions such as the cortex, amygdala and the hippocampus. In agreement with these findings, immunohistochemistry revealed reduced numbers of γ amino butyric acid (GABA) GABAergic interneurons, but not of glutamatergic projection neurons, in the adult cerebral cortex. Reduced GABAergic interneurons were also observed in the amygdala, most notably in the basolateral nucleus. The paucity in GABAergic interneurons in adult Cend1^-/- mice correlated with increased proliferation and apoptosis as well as reduced migration of neuronal progenitors from the embryonic medial ganglionic eminence (MGE), the origin of these cells. Further we noted reduced GABAergic neurons and aberrant neurogenesis in the adult dentate gyrus (DG) of the hippocampus, which has been previously shown to confer spatial learning and memory deficits. Our data highlight the necessity of Cend1 expression in the formation of a structurally and functionally normal brain phenotype.

Subtype Specification of Cerebral Cortical Neurons in Their Immature Stages

The diversification of neuronal subtypes during corticogenesis is fundamental to the establishment of the complex cortical structure. Although subtype specification has been assumed to occur in neural progenitor cells, increasing evidence has begun to reveal the plasticity of subtype determination in immature neurons. Here, we summarize recent findings regarding the regulation of subtype specification during later periods of neuronal differentiation, such as the post-mitotic and post-migratory stages. We also discuss thalamocortical axons as an extra-cortical cue that provides information on the subtype determination of immature cortical neurons.

Psychiatric behaviors associated with cytoskeletal defects in radial neuronal migration

Normal development of the cerebral cortex is an important process for higher brain functions, such as language, and cognitive and social functions. Psychiatric disorders, such as schizophrenia and autism, are thought to develop owing to various dysfunctions occurring during the development of the cerebral cortex. Radial neuronal migration in the embryonic cerebral cortex is a complex process, which is achieved by strict control of cytoskeletal dynamics, and impairments in this process are suggested to cause various psychiatric disorders. Our recent findings indicate that radial neuronal migration as well as psychiatric behaviors is rescued by controlling microtubule stability during the embryonic stage. In this review, we outline the relationship between psychiatric disorders, such as schizophrenia and autism, and radial neuronal migration in the cerebral cortex by focusing on the cytoskeleton and centrosomes. New treatment strategies for psychiatric disorders will be discussed.

Neuronal Heterotopias Affect the Activities of Distant Brain Areas and Lead to Behavioral Deficits

Neuronal heterotopia refers to brain malformations resulting from deficits of neuronal migration. Individuals with heterotopias show a high incidence of neurological deficits, such as epilepsy. More recently, it has come to be recognized that focal heterotopias may also show a range of psychiatric problems, including cognitive and behavioral impairments. However, because focal heterotopias are not always located in the brain areas responsible for the symptoms, the causal relationship between the symptoms and heterotopias remains elusive. In this study, we showed that mice with focal heterotopias in the somatosensory cortex generated by in utero electroporation exhibited spatial working memory deficit and low competitive dominance behavior, which have been shown to be closely associated with the activity of the medial prefrontal cortex (mPFC) in rodents. Analysis of the mPFC activity revealed that the immediate-early gene expression was decreased and the local field potentials of the mPFC were altered in the mice with heterotopias compared with the control mice. Moreover, activation of these ectopic and overlying sister neurons using the DREADD (designer receptor exclusively activated by designer drug) system improved the working memory deficits. These findings suggest that cortical regions containing focal heterotopias can affect distant brain regions and give rise to behavioral abnormalities. Significance statement: Recent studies reported that patients with heterotopias have a variety of clinical symptoms, such as cognitive disturbance, psychiatric symptoms, and autistic behavior. However, the causal relationship between the symptoms and heterotopias remains elusive. Here we showed that mice with focal heterotopias in the somatosensory cortex generated by in utero electroporation exhibited behavioral deficits that have been shown to be associated with the mPFC activity in rodents. The existence of heterotopias indeed altered the neural activities of the mPFC, and direct manipulation of the neural activity of the ectopic neurons and their sister neurons in the overlying cortex improved the behavioral deficit. Thus, our results indicate that focal heterotopias could affect the activities of distant brain areas and cause behavioral abnormalities.

Involvement of GSK3 in the formation of the leading process and migration of neurons from the embryonic rat medial ganglionic eminence in vitro

Migrating neurons have leading processes that direct cell movement in response to guidance cues. We investigated the involvement of glycogen synthase kinase 3 (GSK3) in the formation of leading processes and migration of neurons in vitro. We used embryonic rat medial ganglionic eminence (MGE) neurons, which are precursors of inhibitory neurons that migrate into the cerebral cortex. When MGE neurons were placed on an astrocyte layer, they migrated freely with the highest speed among neurons from other parts of the embryonic forebrain. When they were cultured alone, they showed bipolar morphology and extended leading processes within 20 h. Their leading processes had large growth cones, but did not elongate during 3 days in culture, indicating that leading processes are distinct from short axons. Next, we examined the effect of GSK3 inhibitors on leading processes and the migratory behavior of MGE neurons. MGE neurons treated with GSK3 inhibitors showed multipolar morphology and altered process shapes. Moreover, migration of MGE neurons on the astrocyte layer was significantly decreased in the presence of GSK3 inhibitors. These data suggest that GSK3 is involved in the formation of leading processes and in the migration of MGE neurons.

Pyramidal cell development: postnatal spinogenesis, dendritic growth, axon growth, and electrophysiology

Here we review recent findings related to postnatal spinogenesis, dendritic and axon growth, pruning and electrophysiology of neocortical pyramidal cells in the developing primate brain. Pyramidal cells in sensory, association and executive cortex grow dendrites, spines and axons at different rates, and vary in the degree of pruning. Of particular note is the fact that pyramidal cells in primary visual area (V1) prune more spines than they grow during postnatal development, whereas those in inferotemporal (TEO and TE) and granular prefrontal cortex (gPFC; Brodmann's area 12) grow more than they prune. Moreover, pyramidal cells in TEO, TE and the gPFC continue to grow larger dendritic territories from birth into adulthood, replete with spines, whereas those in V1 become smaller during this time. The developmental profile of intrinsic axons also varies between cortical areas: those in V1, for example, undergo an early proliferation followed by pruning and local consolidation into adulthood, whereas those in area TE tend to establish their territory and consolidate it into adulthood with little pruning. We correlate the anatomical findings with the electrophysiological properties of cells in the different cortical areas, including membrane time constant, depolarizing sag, duration of individual action potentials, and spike-frequency adaptation. All of the electrophysiological variables ramped up before 7 months of age in V1, but continued to ramp up over a protracted period of time in area TE. These data suggest that the anatomical and electrophysiological profiles of pyramidal cells vary among cortical areas at birth, and continue to diverge into adulthood. Moreover, the data reveal that the “use it or lose it” notion of synaptic reinforcement may speak to only part of the story, “use it but you still might lose it” may be just as prevalent in the cerebral cortex.

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.

Selective loss of parvalbumin-positive GABAergic interneurons in the cerebral cortex of maternally stressed Gad1-heterozygous mouse offspring

Exposure to maternal stress (MS) and mutations in GAD1, which encodes the γ-aminobutyric acid (GABA) synthesizing enzyme glutamate decarboxylase (GAD) 67, are both risk factors for psychiatric disorders. However, the relationship between these risk factors remains unclear. Interestingly, the critical period of MS for psychiatric disorders in offspring corresponds to the period of GABAergic neuron neurogenesis and migration in the fetal brain, that is, in the late stage of gestation. Indeed, decrement of parvalbumin (PV)-positive GABAergic interneurons in the medial prefrontal cortex (mPFC) and hippocampus (HIP) has often been observed in schizophrenia patients. In the present study, we used GAD67-green fluorescent protein (GFP) knock-in mice (that is, mice in which the Gad1 gene is heterozygously deleted; GAD67(+/GFP)) that underwent prenatal stress from embryonic day 15.0 to 17.5 and monitored PV-positive GABAergic neurons to address the interaction between Gad1 disruption and stress. Administration of 5-bromo-2-deoxyuridine revealed that neurogenesis of GFP-positive GABAergic neurons, but not cortical plate cells, was significantly diminished in fetal brains during MS. Differential expression of glucocorticoid receptors by different progenitor cell types may underlie this differential outcome. Postnatally, the density of PV-positive, but not PV-negative, GABAergic neurons was significantly decreased in the mPFC, HIP and somatosensory cortex but not in the motor cortex of GAD67(+/GFP) mice. By contrast, these findings were not observed in wild-type (GAD67(+/+)) offspring. These results suggest that prenatal stress, in addition to heterozygous deletion of Gad1, could specifically disturb the proliferation of neurons destined to be PV-positive GABAergic interneurons.

RhoA and Cdc42 are required in pre-migratory progenitors of the medial ganglionic eminence ventricular zone for proper cortical interneuron migration

Cortical interneurons arise from the ganglionic eminences in the ventral telencephalon and migrate tangentially to the cortex. Although RhoA and Cdc42, members of the Rho family of small GTPases, have been implicated in regulating neuronal migration, their respective roles in the tangential migration of cortical interneurons remain unknown. Here we show that loss of RhoA and Cdc42 in the ventricular zone (VZ) of the medial ganglionic eminence (MGE) using Olig2-Cre mice causes moderate or severe defects in the migration of cortical interneurons, respectively. Furthermore, RhoA- or Cdc42-deleted MGE cells exhibit impaired migration in vitro. To determine whether RhoA and Cdc42 directly regulate the motility of cortical interneurons during migration, we deleted RhoA and Cdc42 in the subventricular zone (SVZ), where more fate-restricted progenitors are located within the ganglionic eminences, using Dlx5/6-Cre-ires-EGFP (Dlx5/6-CIE) mice. Deletion of either gene within the SVZ does not cause any obvious defects in cortical interneuron migration, indicating that cell motility is not dependent upon RhoA or Cdc42. These findings provide genetic evidence that RhoA and Cdc42 are required in progenitors of the MGE in the VZ, but not the SVZ, for proper cortical interneuron migration.

The control of precerebellar neuron migration by RNA-binding protein Csde1

Neuronal migration during brain development sets the position of neurons for the subsequent wiring of neural circuits. To understand the molecular mechanism regulating the migrating process, we considered the migration of mouse precerebellar neurons. Precerebellar neurons originate in the rhombic lip of the hindbrain and show stereotypic, long-distance tangential migration along the circumference of the hindbrain to form precerebellar nuclei at discrete locations. To identify the molecular components underlying this navigation, we screened for genes expressed in the migrating precerebellar neurons. As a result, we identified the following three genes through the screening; Calm1, Septin 11, and Csde1. We report here functional analysis of one of these genes, Csde1, an RNA-binding protein implicated in the post-transcriptional regulation of a subset of cellular mRNA, by examining its participation in precerebellar neuronal migration. We found that shRNA-mediated inhibition of Csde1 expression resulted in a failure of precerebellar neurons to complete their migration into their prospective target regions, with many neurons remaining in migratory paths. Furthermore, those that did reach their destination failed to invade the depth of the hindbrain via radial migration. These results have uncovered a crucial role of Csde1 in the proper control of both radial and tangential migration of precerebellar neurons.

Abnormal interneuron development in disrupted-in-schizophrenia-1 L100P mutant mice

Background

Interneuron deficits are one of the most consistent findings in post-mortem studies of schizophrenia patients and are likely important in the cognitive deficits associated with schizophrenia. Disrupted-in-Schizophrenia 1 (DISC1), a strong susceptibility gene for schizophrenia and other mental illnesses, is involved in neurodevelopment, including that of interneurons. However, the mechanism by which DISC1 regulates interneuron development remains unknown. In this study, we analyzed interneuron histology in the Disc1-L100P single point mutation mouse, that was previously shown to have behavioral abnormalities and cortical developmental defects related to schizophrenia.

Results

We sought to determine whether a Disc1-L100P point mutation in the mouse would alter interneuron density and location. First, we examined interneuron position in the developing mouse cortex during embryonic days 14–16 as an indicator of interneuron tangential migration, and found striking migration deficits in Disc1-L100P mutants. Further analysis of adult brains revealed that the Disc1-L100P mutants have selective alterations of calbindin- and parvalbumin-expressing interneurons in the cortex and hippocampus, decreased GAD67/PV co-localization and mis-positioned interneurons across the neocortex when compared to wild-type littermates.

Conclusion

Our results are consistent with the anomalies seen in post-mortem schizophrenia studies and other Disc1 mutant mouse models. Future research is required to determine the specific mechanisms underlying these cellular deficits. Overall, these findings provide further evidence that DISC1 participates in interneuron development and add to our understanding of how DISC1 variants can affect susceptibility to psychiatric illness.

DISC1: a key lead in studying cortical development and associated brain disorders

For the past decade, DISC1 has been studied as a promising lead to understand the biology underlying major mental illnesses, such as schizophrenia. Consequently, many review articles on DISC1 have been published. In this article, rather than repeating comprehensive overviews of research articles, we will introduce the utility of DISC1 in the study of cortical development in association with a wide range of developmental brain disorders. Cortical development involves cell autonomous and cell nonautonomous mechanisms as well as host responses to environmental factors, all of which involve DISC1 function. Thus, we will discuss the significance of DISC1 in forming an overall understanding of multiple mechanisms that orchestrate corticogenesis and can serve as therapeutic targets in diseases caused by abnormal cortical development.

Regulation of cortical neuron migration by the Reelin signaling pathway

Reeler is a mutant mouse with defects in layered structures of the central nervous system, such as the cerebral cortex, hippocampus, and cerebellum, and has been extensively examined for more than half a century. The full-length cDNA for the responsible gene for reeler, reelin, was serendipitously identified, revealing that Reelin encodes a large secreted protein. So far, two Reelin receptors, apolipoprotein E receptor 2 and very low-density lipoprotein receptor, and the cytoplasmic adaptor protein Disabled homolog 1 (Dab1) have been shown to be essential for Reelin signaling. Although a number of downstream cascades of Dab1 have also been reported using various experimental systems, the physiological functions of Reelin in vivo remain controversial. Here, we review recent advances in the understanding of the Reelin-Dab1 signaling pathway in the developing cerebral cortex.

Inhibition of cerebellar granule cell turning by alcohol

Ectopic neurons are often found in the brains of fetal alcohol spectrum disorders (FASD) and fetal alcohol syndrome (FAS) patients, suggesting that alcohol exposure impairs neuronal cell migration. Although it has been reported that alcohol decreases the speed of neuronal cell migration, little is known about whether alcohol also affects the turning of neurons. Here we show that ethanol exposure inhibits the turning of cerebellar granule cells in vivo and in vitro. First, in vivo studies using P10 mice demonstrated that a single intraperitoneal injection of ethanol not only reduces the number of turning granule cells but also alters the mode of turning at the EGL-ML border of the cerebellum. Second, in vitro analysis using microexplant cultures of P0-P3 mouse cerebella revealed that ethanol directly reduces the frequency of spontaneous granule cell turning in a dose-dependent manner. Third, the action of ethanol on the frequency of granule cell turning was significantly ameliorated by stimulating Ca(2+) and cGMP signaling or by inhibiting cAMP signaling. Taken together, these results indicate that ethanol affects the frequency and mode of cerebellar granule cell turning through alteration of the Ca(2+) and cyclic nucleotide signaling pathways, suggesting that the abnormal allocation of neurons found in the brains of FASD and FSA patients results, at least in part, from impaired turning of immature neurons by alcohol.

Morphological changes and synaptogenesis of corticothalamic neurons in the somatosensory cortex of rat during perinatal development

When rat fetuses grew from embryonic day (E) 18 to the day of birth (P0), the corticothalamic (CT) neurons, as identified by back labeling with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI), in the somatosensory cortex underwent gradual changes in the shape of their cell bodies, in their distribution in the cortical plate and in the complexity of dendritic branching. Fluorescence immunocytochemical studies indicated that in the marginal zone (MZ) the apical dendrites of the CT neurons formed contacts with horizontally oriented axons and contained putative glutamatergic, as clusters exhibiting both synaptophysin and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluR1 subunit immunoreactivities, and γ-aminobutyric acid (GABA)-ergic synapses, as clusters exhibiting both synaptophysin and gephyrin immunoreactivities. Quantitative analyses further revealed that during this perinatal period, the proportion of CT neurons containing glutamatergic synapses increased significantly, whereas the proportion of CT neurons containing GABAergic synapses remained virtually unchanged. Our results indicate that glutamatergic and GABAergic synapses between the CT neurons and the axons in the MZ are already formed in rat cortices as early as E18 and further suggest that the activities of the neural networks in the somatosensory cortex could be conveyed to their targets in the thalamus in rat brains at least 3 days before birth.

GABAergic interneuron lineages selectively sort into specific cortical layers during early postnatal development

It is of considerable interest to determine how diverse subtypes of γ-aminobutyric acidergic (GABAergic) interneurons integrate into the functional network of the cerebral cortex. Using inducible in vivo genetic fate mapping approaches, we found that interneuron precursors arising from the medial ganglionic eminence (MGE) and caudal ganglionic eminence (CGE) at E12.5, respectively, populate deep and superficial cortical layers in a complementary manner in the mature cortex. These age-matched populations initiate tangential migration into the cortex simultaneously, migrate above and below the cortical plate in a similar ratio, and complete their entrance into the cortical plate by P1. Surprisingly, while these 2 interneuron populations show a comparable layer distribution at P1, they subsequently segregate into distinct cortical layers. In addition, the initiation of the radial sorting within each lineage coincided well with the upregulation of the potassium/chloride cotransporter KCC2. Moreover, layer sorting of a later born (E16.5) CGE-derived population occurred with a similar time course to the earlier born E12.5 cohorts, further suggesting that this segregation step is controlled in a subtype specific manner. We conclude that radial sorting within the early postnatal cortex is a key mechanism by which the layer-specific integration of GABAergic interneurons into the emerging cortical network is achieved.

Inhibition of N-methyl-D-aspartate receptor activity resulted in aberrant neuronal migration caused by delayed morphological development in the mouse neocortex

Embryonic and neonatal neocortical neurons already express functional N-methyl-D-aspartate (NMDA) receptors before they form synapses. To elucidate the role of NMDA receptors in neuronal migration in the developing neocortex, we visualized radially migrating neurons by transferring the enhanced green fluorescent protein (EGFP) gene into the ventricular zone (VZ) of the mouse neocortex using in utero electroporation at E15.5. Two days later, we prepared neocortical slices and examined the EGFP-positive cells using time-lapse imaging in the presence of the NMDA receptor antagonist Cerestat. The EGFP-positive cells generated in the VZ in the control slices exhibited a multipolar morphology, but within several hours they became bipolar (with a leading process and an axon-like process) and migrated toward the pial surface. By contrast, many of the multipolar cells in the Cerestat-treated slices failed to extend either process and become bipolar, and frequently changed direction, although they ultimately reached their destination even after Cerestat-treatment. To identify the molecules responding for mediating NMDA signaling during neuronal migration and the changes in morphology observed above, we here focused on Src family kinases (SFKs), which mediate a variety of neuronal functions including migration and neurite extension. We discovered that the activity of Src and Fyn was reduced by Cerestat. These findings suggest that NMDA receptors are involved in neuronal migration and morphological changes into a bipolar shape, and in the activation of Src and Fyn in the developing neocortex.

GABAergic signaling increases through the postnatal development to provide the potent inhibitory capability for the maturing demands of the prefrontal cortex

The developmental profile of the firing patterns and construction of synapse connection were studied in LTS interneurons of prefrontal cortex (PFC) in rats with age (from P7 to P30). We used whole cell patch-clamp recordings to characterize electrophysiological properties of LTS interneurons in PFC at different age stages, including the action potentials (APs), short-term plasticity (STP), evoked excitatory postsynaptic currents (eEPSCs), spontaneous excitatory postsynaptic currents (sEPSC), and spontaneous inhibitory postsynaptic current (sIPSC). The developmental profile of LTS interneurons in our research showed two phases changes. The early phase from P7-P11 to P16-P19 during which the development of individual LTS interneuron dominated and just some simple synaptic connections formed, the synaptic inputs from pyramidal cells play a promoting role for the maturation of LTS interneurons to some extent. This was based on the changes of APs, eEPSCs, and STP such as the curtailment of time course of APs, the increasing facilitation of STP before P16-P19 group. The late phase from P20-P23 to P > 27 during which the function of inhibitory cortex network enhanced and the characters of this inhibitory cortex network continually changed although in the oldest age group (P > 27) in our research. The frequency and amplitude of sIPSC showed continually changes, and at the same age group, the frequency ratios and amplitude ratios of sIPSC was higher than that of sEPSC. Our study showed a foundation to clarify mechanisms underlying the evolution in time of intrinsic neuronal membrane properties and their important roles in balancing the cortex network, providing an academic foundation for the pathological researching on some psychiatric and neurological disorders.

CXCR4 is required for proper regional and laminar distribution of cortical somatostatin-, calretinin-, and neuropeptide Y-expressing GABAergic interneurons

Cortical GABAergic interneurons are divided into various subtypes, with each subtype contributing to rich variety and fine details of inhibition. Despite the functional importance of each interneuron subtype, the molecular mechanisms that contribute to sorting them to their appropriate positions within the cortex remain unclear. Here, we show that the chemokine receptor CXCR4 regulates the regional and layer-specific distribution of interneuron subtypes. We removed Cxcr4 specifically in a subset of interneurons at a specific mouse embryonic developmental stage and analyzed the number of interneurons and their laminar distribution in 9 representative cortical regions comprehensively in adults. We found that the number of Cxcr4-deleted calretinin- and that of neuropeptide Y-expressing interneurons were reduced in most caudomedial and lateral cortical regions, respectively, and also in superficial layers. In addition, Cxcr4-deleted somatostatin-expressing interneurons showed a reduction in the number of superficial layers in certain cortical regions but of deep layers in others. These findings suggest that CXCR4 is required for proper regional and laminar distribution in a wider interneuron subpopulation than previously thought and may regulate the establishment of functional cortical circuitry in certain cortical regions and layers.

Chemokine-like factor 1, a novel cytokine, induces nerve cell migration through the non-extracellular Ca2+-dependent tyrosine kinases pathway

Chemokine-like factor 1 (CKLF1) is a newly cloned chemotactic cytokine. The roles of CKLF1 in the immune system and the respiratory system have been reported, but its function in the nervous system is still remaining unclear. We aimed to investigate the role of CKLF1 in the nerve cell migration and its regulatory mechanisms. By chemotaxis assays and wound-healing assays, CKLF1 stimulated the migration of SH-SY5Y cells dose-dependently. By immunofluorescence staining, CKLF1 induced actin polymerization. By western blotting, proline-rich tyrosine kinase 2 (PYK2) was phosphorylated at Tyr-402 in response to CKLF1 and this phosphorylation was apparently suppressed by phospholipase C-gamma inhibitor U73122, but not extracellular Ca(2+) chelator EGTA. Furthermore, after transfection of dominant-negative mutant PYK2 plasmid, the chemotaxis upon CKLF1 was significantly attenuated in SH-SY5Y cells. Concluding, CKLF1 stimulates the migration of SH-SY5Y cells dose-dependently by activating non-extracellular Ca(2+)-dependent tyrosine kinases pathway and inducing actin polymerization.

Relationship between GABAergic interneurons migration and early neocortical network activity

Available evidence converges to suggest that during the early development of the cerebral cortex, the emergence of the spontaneous network activity chronologically overlap with the end of the cell migration period in the developing cortex. We approached the functional regulation of neuronal migration in a culture model of neocortical networks, using time lapses to detect migratory movements, calcium-imaging to assess the activity of migratory neurons, and immunocytochemical methods to identify the migratory cells retrospectively. In cell cultures, early physiological development and cell migration are reproduced at a local network level, thus allowing the study of the interrelationships between cell migration and network development independent of the topographical complexity. Neurons migrate at least until 12 days in vitro and GABAergic neurons migrate faster compared with non-GABAergic neurons. A decline of migratory activity was coincident with the development of spontaneous synchronous network activity. Migrating interneurons did not participate in synchronous network activity, but interneurons that ended cell migration during observation time frequently engaged in synchronous activity within less than an hour. Application of GABA(A) and ionotropic glutamate receptor antagonists significantly increased the number of migrating GABAergic neurons without changing the dynamics of the migratory movements. Thus, neurotransmitters released by early network activity might favor the termination of neuronal migration. These results reinforce the idea that network activity plays an important role in the development of late-born GABAergic cells.

The role of Robo3 in the development of cortical interneurons

A number of studies in recent years have shown that members of the Roundabout (Robo) receptor family, Robo1 and Robo2, play significant roles in the formation of axonal tracks in the developing forebrain and in the migration and morphological differentiation of cortical interneurons. Here, we investigated the expression and function of Robo3 in the developing cortex. We found that this receptor is strongly expressed in the preplate layer and cortical hem of the early cortex where it colocalizes with markers of Cajal–Retzius cells and interneurons. Analysis of Robo3 mutant mice at early (embryonic day [E] 13.5) and late (E18.5) stages of corticogenesis revealed no significant change in the number of interneurons, but a change in their morphology at E13.5. However, preliminary analysis on a small number of mice that lacked all 3 Robo receptors indicated a marked reduction in the number of cortical interneurons, but only a limited effect on their morphology. These observations and the results of other recent studies suggest a complex interplay between the 3 Robo receptors in regulating the number, migration and morphological differentiation of cortical interneurons.

COUP-TFII is preferentially expressed in the caudal ganglionic eminence and is involved in the caudal migratory stream

While the cortical interneurons derived from the medial ganglionic eminence (MGE) migrate rather diffusely into the cortex, interneurons that migrate out from the caudal ganglionic eminence (CGE) mainly move caudally into the caudal cerebral cortex and the hippocampus in the form of the caudal migratory stream (CMS) (Yozu et al., 2005). Although transplantation experiments at embryonic day 13.5 had revealed that the migrating cells in these two populations are already intrinsically different in regard to their ability to respond to the CGE environment (Yozu et al., 2005), it is not known how the CGE cells are specified and how their migratory behavior is determined. In this study we showed that, although CGE and lateral ganglionic eminence (LGE) express almost the same marker molecules, LGE cells do not migrate caudally when transplanted into the CGE, suggesting that LGE cells are intrinsically different from CGE cells. We therefore compared the transcriptomes of the CGE, MGE, and LGE, and the results showed that COUP-TFII was expressed preferentially in the CGE as well as in the migrating interneurons in the CMS. Transplantation experiments revealed that COUP-TFII is sufficient to change the direction of MGE cell migration to caudal when transplanted into the CGE environment, and knockdown of COUP-TFII inhibited the caudal migration of the CGE cells. These results suggest that COUP-TFII is both required and sufficient for the CGE-cell-specific migratory behavior in the caudal direction. Thus, a locally expressed transcription factor determines the migratory direction of the cortical interneurons in a region-specific manner.

Autonomous turning of cerebellar granule cells in vitro by intrinsic programs

External guidance cues play a role in controlling neuronal cell turning in the developing brain, but little is known about whether intrinsic programs are also involved in controlling the turning. In this study, we examined whether granule cells undergo autonomous changes in the direction of migration in the microexplant cultures of the early postnatal mouse cerebellum. We found that granule cells exhibit spontaneous and periodical turning without cell-cell contact and in the absence of external guidance cues. The frequency of turning was increased by stimulating the Ca(2+) influx and the internal Ca(2+) release, or inhibiting the cAMP signaling pathway, while the frequency was reduced by inhibiting the Ca(2+) influx. Granule cell turning in vitro was classified into four distinct modes, which were characterized by the morphological changes in the leading process and the trailing process, such as bifurcating, turning, withdrawing, and changing the polarity. The occurrence of the 1st and 2nd modes of turning was differentially affected by altering the Ca(2+) and cAMP signaling pathways. Collectively, the results demonstrate that intrinsic programs regulate the autonomous turning of cerebellar granule cells in vitro. Furthermore, the results suggest that extrinsic signals play a role as essential modulators of intrinsic programs.

Cortical interneurons require p35/Cdk5 for their migration and laminar organization

Projection neurons and interneurons populate the cerebral cortex in a layer-specific manner. Here, we studied the role of Cyclin-dependent kinase 5 (Cdk5) and its activator p35 in cortical interneuron migration and disposition in the cortex. We found that mice lacking p35 (p35(-/-)) show accumulation of interneurons in the upper part of the cortex. We also observed an inverted distribution of both early- and late-born interneurons, with the former showing a preference for the upper and the latter for the lower aspects of the cortex. We investigated the causes of the altered laminar organization of interneurons in p35(-/-) mice and found a cell-autonomous delay in their tangential migration that may prevent them from reaching correct positions. Incomplete splitting of the preplate in p35(-/-) mice, which causes accumulation of cells in the superficial layer and defects in the "inward" and "outward" components of their radial movement, may also account for the altered final arrangement of interneurons. We, therefore, propose that p35/Cdk5 plays a key role in guiding cortical interneurons to their final positions in the cortex.

The p21-activated kinase is required for neuronal migration in the cerebral cortex

The normal formation and function of the mammalian cerebral cortex depend on the positioning of its neurones, which occurs in a highly organized, layer-specific manner. The correct morphology and movement of neurones rely on synchronized regulation of their actin filaments and microtubules. The p21-activated kinase (Pak1), a key cytoskeletal regulator, controls neuronal polarization, elaboration of axons and dendrites, and the formation of dendritic spines. However, its in vivo role in the developing nervous system is unclear. We have utilized in utero electroporation into mouse embryo cortices to reveal that both loss and gain of Pak1 function affect radial migration of projection neurones. Overexpression of hyperactivated Pak1 predominantly caused neurones to arrest in the intermediate zone (IZ) with apparently misoriented and disorganized leading projections. Loss of Pak1 disrupted the morphology of migrating neurones, which accumulated in the IZ and deep cortical layers. Unexpectedly, a significant number of neurones with reduced Pak1 expression aberrantly entered into the normally cell-sparse marginal zone, suggesting their inability to cease migrating that may be due to their impaired dissociation from radial glia. Our findings reveal the in vivo importance of temporal and spatial regulation of the Pak1 kinase during key stages of cortical development.

The Pak1 kinase: an important regulator of neuronal morphology and function in the developing forebrain

The mammalian central nervous system (CNS) represents a highly complex unit, the correct function of which relies on the appropriate differentiation and survival of its neurones. It is becoming apparent that the Rho family of small GTPases and their downstream targets have a major function in regulating CNS development. Among the effectors, the role of the Pak family of kinases, especially Pak1, is becoming increasingly evident. Although highest levels of Pak1 expression and activation are detected in the developing nervous system, much remains undiscovered concerning its function in neurones. This review summarises what is currently known regarding the biological and molecular role of Pak1 in the mammalian forebrain. It emphasises the importance of Pak1 in regulating neuronal polarity, morphology, migration and synaptic function. Consequently, there are also strong indications that Pak1 is required for normal cognitive function. Furthermore, loss of Pak1 has been associated with the progression of neurodegenerative disorders, particularly Alzheimer's disease, while up-regulation and de-regulation may be responsible for oncogenic transformation of support cells within the CNS, especially astrocyte progenitors. Together, these new and exciting findings encourage the future exploration into the function of Pak1 in the nervous system, thus, paving the way for novel strategies towards improved diagnosis and therapeutic treatment of diseases that affect the CNS.

Cell-autonomous roles of ARX in cell proliferation and neuronal migration during corticogenesis

The aristaless-related homeobox (ARX) gene has been implicated in a wide spectrum of disorders ranging from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild forms of X-linked mental retardation without apparent brain abnormalities. To better understand its role in corticogenesis, we used in utero electroporation to knock down or overexpress ARX. We show here that targeted inhibition of ARX causes cortical progenitor cells to exit the cell cycle prematurely and impairs their migration toward the cortical plate. In contrast, ARX overexpression increases the length of the cell cycle. In addition, we report that RNA interference-mediated inactivation of ARX prevents cells from acquiring multipolar morphology in the subventricular and intermediate zones, resulting in decreased neuronal motility. In contrast, ARX overexpression appears to promote the development of tangentially oriented processes of cells in the subventricular and intermediate zones and affects radial migration of pyramidal neurons. We also demonstrate that the level of ARX expression is important for tangential migration of GABA-containing interneurons, because both inactivation and overexpression of the gene impair their migration from the ganglionic eminence. However, our data suggest that ARX is not directly involved in GABAergic cell fate specification. Overall, these results identify multiple and distinct cell-autonomous roles for ARX in corticogenesis.

Ethanol consumption during early pregnancy alters the disposition of tangentially migrating GABAergic interneurons in the fetal cortex

Consumption of alcohol (ethanol) during pregnancy can lead to developmental defects in the offspring, the most devastating being the constellation of symptoms collectively referred to as fetal alcohol syndrome (FAS). In the brain, a hallmark of FAS is abnormal cerebral cortical morphology consistent with insult during corticogenesis. Here, we report that exposure to a relatively low level of ethanol in utero (average maternal and fetal blood alcohol level of 25 mg/dl) promotes premature tangential migration into the cortical anlage of primordial GABAergic interneurons, including those originating in the medial ganglionic eminence (MGE). This ethanol-induced effect was evident in vivo at embryonic day 14.5 (E14.5) in GAD67 knock-in and BAC-Lhx6 embryos, as well as in vitro in isotypic telencephalic slice cocultures obtained from E14.5 embryos exposed to ethanol in utero. Analysis of heterotypic cocultures indicated that both cell-intrinsic and -extrinsic factors contribute to the aberrant migratory profile of MGE-derived cells. In this light, we provide evidence for an interaction between ethanol exposure in utero and the embryonic GABAergic system. Exposure to ethanol in utero elevated the ambient level of GABA and increased the sensitivity to GABA of MGE-derived cells. Our results uncovered for the first time an effect of ethanol consumption during pregnancy on the embryonic development of GABAergic cortical interneurons. We propose that ethanol exerts its effect on the tangential migration of GABAergic interneurons extrinsically by modulating extracellular levels of GABA and intrinsically by altering GABA(A) receptor function.