Identification of molecules preferentially expressed beneath the marginal zone in the developing cerebral cortex

Transcription factor 4 controls positioning of cortical projection neurons through regulation of cell adhesion

The establishment of neural circuits depends on precise neuronal positioning in the cortex, which occurs via a tightly coordinated process of neuronal differentiation, migration, and terminal localization. Deficits in this process have been implicated in several psychiatric disorders. Here, we show that the transcription factor Tcf4 controls neuronal positioning during brain development. Tcf4-deficient neurons become mispositioned in clusters when their migration to the cortical plate is complete. We reveal that Tcf4 regulates the expression of cell adhesion molecules to control neuronal positioning. Furthermore, through in vivo extracellular electrophysiology, we show that neuronal functions are disrupted after the loss of Tcf4. TCF4 mutations are strongly associated with schizophrenia and cause Pitt-Hopkins syndrome, which is characterized by severe intellectual disability. Thus, our results not only reveal the importance of neuronal positioning in brain development but also provide new insights into the potential mechanisms underlying neurological defects linked to TCF4 mutations.

Adhesion dynamics in the neocortex determine the start of migration and the post-migratory orientation of neurons

The neocortex is stereotypically organized into layers of excitatory neurons arranged in a precise parallel orientation. Here we show that dynamic adhesion both preceding and following radial migration is essential for this organization. Neuronal adhesion is regulated by the Mowat-Wilson syndrome-associated transcription factor Zeb2 (Sip1/Zfhx1b) through direct repression of independent adhesion pathways controlled by Neuropilin-1 (Nrp1) and Cadherin-6 (Cdh6). We reveal that to initiate radial migration, neurons must first suppress adhesion to the extracellular matrix. Zeb2 regulates the multipolar stage by transcriptional repression of Nrp1 and thereby downstream inhibition of integrin signaling. Upon completion of migration, neurons undergo an orientation process that is independent of migration. The parallel organization of neurons within the neocortex is controlled by Cdh6 through atypical regulation of integrin signaling via its RGD motif. Our data shed light on the mechanisms that regulate initiation of radial migration and the postmigratory orientation of neurons during neocortical development.

Grina/TMBIM3 modulates voltage-gated CaV2.2 Ca2+ channels in a G-protein-like manner

Grina/TMBIM3 is a poorly characterized transmembrane protein with a broad expression pattern in mammals and with a very ancient origin within eukaryotes. Although initially characterized as an NMDA-receptor associated subunit, there is increasing evidence that Grina/TMBIM3 is involved in the unfolded protein response and controls apoptosis via regulation of Ca²⁺ homeostasis. Here, we investigate a putative direct interaction of Grina/TMBIM3 with voltage gated Ca²⁺ channels, in particular with the Ca_V2.2 α1-subunit and describe its modulatory effects on the current through Ca_V2.2 N-type channels. Direct interaction was confirmed by co-immunoprecipitation studies and membrane localization was proven. Co-expression of Grina/TMBIM3 with Ca_V2.2 channels resulted in a significant decrease of the current amplitude and in a slowing of the kinetics of current activation. This effect was accompanied by a significant shift of the voltage dependencies of activation time constants towards more depolarized voltages. Application of a stimulus protocol including a strong depolarizing pulse relieved inhibition of current amplitude by Grina/TMBIM3. When Grina/TMBIM3 was present, inactivation by an action potential-like train of pulses was diminished. Both observations resemble mechanisms that are well-studied modulatory effects of G-protein βγ subunits on Ca_V2 channels. The impact of Grina/TMBIM3 and G-protein βγ subunits are rather comparable with respect to suppression of current amplitude and slowing of activation kinetics. Furthermore, both modulators had the same effect on current inactivation when evoked by an action potential-like train of pulses.

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.

Control of Neuronal Migration and Aggregation by Reelin Signaling in the Developing Cerebral Cortex

The mammalian cerebral neocortex has a well-organized laminar structure, achieved by the highly coordinated control of neuronal migration. During cortical development, excitatory neurons born near the lateral ventricle migrate radially to reach their final positions to form the cortical plate. During this process, dynamic changes are observed in the morphologies and migration modes, including multipolar migration, locomotion, and terminal translocation, of the newborn neurons. Disruption of these migration processes can result in neuronal disorders such as lissencephaly and periventricular heterotopia. The extracellular protein, Reelin, mainly secreted by the Cajal-Retzius neurons in the marginal zone during development, plays a crucial role in the neuronal migration and neocortical lamination. Reelin signaling, which exerts essential roles in the formation of the layered neocortex, is triggered by the binding of Reelin to its receptors, ApoER2 and VLDLR, followed by phosphorylation of the Dab1 adaptor protein. Accumulating evidence suggests that Reelin signaling controls multiple steps of neuronal migration, including the transition from multipolar to bipolar neurons, terminal translocation, and termination of migration beneath the marginal zone. In addition, it has been shown that ectopically expressed Reelin can cause neuronal aggregation via an N-cadherin-mediated manner. This review attempts to summarize our knowledge of the roles played by Reelin in neuronal migration and the underlying mechanisms.

Identity of neocortical layer 4 neurons is specified through correct positioning into the cortex

Many cell-intrinsic mechanisms have been shown to regulate neuronal subtype specification in the mammalian neocortex. However, how much cell environment is crucial for subtype determination still remained unclear. Here, we show that knockdown of Protocadherin20 (Pcdh20), which is expressed in post-migratory neurons of layer 4 (L4) lineage, caused the cells to localize in L2/3. The ectopically positioned "future L4 neurons" lost their L4 characteristics but acquired L2/3 characteristics. Knockdown of a cytoskeletal protein in the future L4 neurons, which caused random disruption of positioning, also showed that those accidentally located in L4 acquired the L4 characteristics. Moreover, restoration of positioning of the Pcdh20-knockdown neurons into L4 rescued the specification failure. We further suggest that the thalamocortical axons provide a positional cue to specify L4 identity. These results suggest that the L4 identity is not completely determined at the time of birth but ensured by the surrounding environment after appropriate positioning.

The COUP-TFII/Neuropilin-2 is a molecular switch steering diencephalon-derived GABAergic neurons in the developing mouse brain

The preoptic area (POa) of the rostral diencephalon supplies the neocortex and the amygdala with GABAergic neurons in the developing mouse brain. However, the molecular mechanisms that determine the pathway and destinations of POa-derived neurons have not yet been identified. Here we show that Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII)-induced expression of Neuropilin-2 (Nrp2) and its down-regulation control the destination of POa-derived GABAergic neurons. Initially, a majority of the POa-derived migrating neurons express COUP-TFII and form a caudal migratory stream toward the caudal subpallium. When a subpopulation of cells steers toward the neocortex, they exhibit decreased expression of COUP-TFII and Nrp2. The present findings show that suppression of COUP-TFII/Nrp2 changed the destination of the cells into the neocortex, whereas overexpression of COUP-TFII/Nrp2 caused cells to end up in the medial part of the amygdala. Taken together, these results reveal that COUP-TFII/Nrp2 is a molecular switch determining the pathway and destination of migrating GABAergic neurons born in the POa.

How does Reelin control neuronal migration and layer formation in the developing mammalian neocortex?

The mammalian neocortex has a laminar structure that develops in a birth-date-dependent "inside-out" pattern. Its layered structure is established by neuronal migration accompanied by sequential changes in migratory mode regulated by several signaling cascades. Although Reelin was discovered about two decades ago and is one of the best known molecules that is indispensable to the establishment of the "inside-out" neuron layers, the cellular and molecular functions of Reelin in layer formation are still largely unknown. In this review article, we summarize our recent understanding of Reelin's functions during neuronal migration. Reelin acts in at least two different steps of neuronal migration: the final step of neuronal migration (somal/terminal translocation) just beneath the marginal zone (MZ) and the regulation of cell polarity step when the neurons change their migratory mode from multipolar migration to locomotion. During the translocation mode, Reelin activates integrin α5β1 through an intracellular pathway that triggers the translocation and activates N-cadherin in concert with the nectin-afadin system. Reelin is also involved in the termination of neuronal migration by degrading Dab1 via the SOCS7-Cullin5-Rbx2 system, and Reelin has been found to induce the birth-date-dependent neuronal aggregation in vivo. Based on these findings, we hypothesize that the molecular function of Reelin during neuronal migration is to control cell-adhesiveness during development by regulating the expression/activation of cell adhesion molecules.

Reelin controls neuronal positioning by promoting cell-matrix adhesion via inside-out activation of integrin α5β1

Birthdate-dependent neuronal layering is fundamental to neocortical functions. The extracellular protein Reelin is essential for the establishment of the eventual neuronal alignments. Although this Reelin-dependent neuronal layering is mainly established by the final neuronal migration step called "terminal translocation" beneath the marginal zone (MZ), the molecular mechanism underlying the control by Reelin of terminal translocation and layer formation is largely unknown. Here, we show that after Reelin binds to its receptors, it activates integrin α5β1 through the intracellular Dab1-Crk/CrkL-C3G-Rap1 pathway. This intracellular pathway is required for terminal translocation and the activation of Reelin signaling promotes neuronal adhesion to fibronectin through integrin α5β1. Since fibronectin is localized in the MZ, the activated integrin α5β1 then controls terminal translocation, which mediates proper neuronal alignments in the mature cortex. These data indicate that Reelin-dependent activation of neuronal adhesion to the extracellular matrix is crucial for the eventual birth-date-dependent layering of the neocortex.

Regulation of motility of myogenic cells in filling limb muscle anlagen by Pitx2

Cells of the ventrolateral dermomyotome delaminate and migrate into the limb buds where they give rise to all muscles of the limbs. The migratory cells proliferate and form myoblasts, which withdraw from the cell cycle to become terminally differentiated myocytes. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed specifically in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Microarrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1(EGFP/+), which resulted from the loss of Pitx2. Very few genes showed changes in expression. Many small-fold, yet significant, changes were observed in genes encoding cytoskeletal and adhesion proteins which play a role in cell motility. Myogenic cells from genetically-tagged mice were cultured and subjected to live cell-tracking analysis using time-lapse imaging. Myogenic cells lacking Pitx2 were smaller, more symmetrical, and had more actin bundling. They also migrated about half of the total distance and velocity. Decreased motility may prevent myogenic cells from filling pre-patterned regions of the limb bud in a timely manner. Altered shape may prevent proper assembly of higher-order fibers within anlagen. Pitx2 therefore appears to regulate muscle anlagen development by appropriately balancing expression of cytoskeletal and adhesion molecules.

Segregation and pathfinding of callosal axons through EphA3 signaling

The corpus callosum, composed of callosal axons, is the largest structure among commissural connections in eutherian animals. Axon pathfinding of callosal neurons has been shown to be guided by intermediate targets, such as midline glial structures. However, it has not yet been understood completely how axon-axon interactions, another major mechanism for axon pathfinding, are involved in the pathfinding of callosal neurons. Here, we show that callosal axons from the medial and lateral regions of the mouse cerebral cortex pass through the dorsal and ventral parts, respectively, of the corpus callosum. Using an explant culture system, we observed that the axons from the medial and lateral cortices were segregated from each other in vitro, and that this segregation was attenuated by inhibition of EphA3 signaling. We also found that knockdown of EphA3, which is preferentially expressed in the lateral cortex, resulted in disorganized segregation of the callosal axons and disrupted axon pathfinding in vivo. These results together suggest the role of axonal segregation in the corpus callosum, mediated at least in part by EphA3, in correct pathfinding of callosal neurons.

The outermost region of the developing cortical plate is crucial for both the switch of the radial migration mode and the Dab1-dependent "inside-out" lamination in the neocortex

Mammalian neocortex has a laminated structure that develops in a birth-date-dependent "inside-out" pattern. This layered structure is established by neuronal migration with sequential changes of the migratory mode regulated by several signaling cascades, including the Reelin-Disabled homolog 1 (Dab1) pathway. Although the importance of "locomotion," the major migratory mode, has been well established, the physiological significance of the mode change from locomotion to "terminal translocation," the final migratory mode, is unknown. In this study, we found that the outermost region of the mouse cortical plate has several histologically distinct features and named this region the primitive cortical zone (PCZ). Time-lapse analyses revealed that "locomoting" neurons paused transiently just beneath the PCZ before migrating into it by "terminal translocation." Furthermore, whereas Dab1-knockdown (KD) neurons could reach beneath the PCZ, they failed to enter the PCZ, suggesting that the Dab1-dependent terminal translocation is necessary for entry of the neurons into the PCZ. Importantly, sequential in utero electroporation experiments directly revealed that failure of the Dab1-dependent terminal translocation resulted in disruption of the inside-out alignment within the PCZ and that this disrupted pattern was still preserved in the mature cortex. Conversely, Dab1-KD locomoting neurons could pass by both wild-type and Dab1-KD predecessors beneath the PCZ. Our data indicate that the PCZ is a unique environment, passage of neurons through which involves molecularly and behaviorally different migratory mechanisms, and that the migratory mode change from locomotion to terminal translocation just beneath the PCZ is critical for the Dab1-dependent inside-out lamination in the mature cortex.

Transcriptional programs in transient embryonic zones of the cerebral cortex defined by high-resolution mRNA sequencing

Characterizing the genetic programs that specify development and evolution of the cerebral cortex is a central challenge in neuroscience. Stem cells in the transient embryonic ventricular and subventricular zones generate neurons that migrate across the intermediate zone to the overlying cortical plate, where they differentiate and form the neocortex. It is clear that not one but a multitude of molecular pathways are necessary to progress through each cellular milestone, yet the underlying transcriptional programs remain unknown. Here, we apply differential transcriptome analysis on microscopically isolated cell populations, to define five transcriptional programs that represent each transient embryonic zone and the progression between these zones. The five transcriptional programs contain largely uncharacterized genes in addition to transcripts necessary for stem cell maintenance, neurogenesis, migration, and differentiation. Additionally, we found intergenic transcriptionally active regions that possibly encode unique zone-specific transcripts. Finally, we present a high-resolution transcriptome map of transient zones in the embryonic mouse forebrain.

The ancient cell death suppressor BAX inhibitor-1

Bax inhibitor-1 (BI-1) was initially identified for its ability to inhibit BAX-induced apoptosis in yeast cells and is the founding member of a family of highly hydrophobic proteins localized in diverse cellular membranes. It is evolutionarily conserved and orthologues from plants can substitute for mammalian BI-1 in regard to its anti-apoptotic function suggesting a high degree of functional conservation. BI-1 interacts with BCL-2 and BCL-XL and, similar to these two anti-apoptotic proteins, the effect of BI-1 on cell death involves changes in the amount of Ca(2+) releasable from intracellular stores. However, BI-1 is also a negative regulator of the endoplasmic reticulum stress sensor IRE1 α, it interacts with G-actin and increases actin polymerization, enhances cancer metastasis by altering glucose metabolism and activating the sodium-hydrogen exchanger, and reduces the production of reactive oxygen species through direct interaction with NADPH-P450 reductase. In this contribution, we summarize what is known about the expression, intracellular localization and structure of BI-1 and specifically illuminate its effects on the intracellular Ca(2+) homeostasis and how this might relate to its other functions. We also present a thorough phylogenetic analysis of BI-1 proteins from major phyla together with paralogues from all BI-1 family members.

Ectopic Reelin induces neuronal aggregation with a normal birthdate-dependent "inside-out" alignment in the developing neocortex

Neurons in the developing mammalian neocortex form the cortical plate (CP) in an "inside-out" manner; that is, earlier-born neurons form the deeper layers, whereas later-born neurons migrate past the existing layers and form the more superficial layers. Reelin, a glycoprotein secreted by Cajal-Retzius neurons in the marginal zone (MZ), is crucial for this "inside-out" layering, because the layers are inverted in the Reelin-deficient mouse, reeler (Reln(rl)). Even though more than a decade has passed since the discovery of reelin, the biological effect of Reelin on individual migrating neurons remains unclear. In addition, although the MZ is missing in the reeler cortex, it is unknown whether Reelin directly regulates the development of the cell-body-sparse MZ. To address these issues, we expressed Reelin ectopically in the developing mouse cortex, and the results showed that Reelin caused the leading processes of migrating neurons to assemble in the Reelin-rich region, which in turn induced their cell bodies to form cellular aggregates around Reelin. Interestingly, the ectopic Reelin-rich region became cell-body-sparse and dendrite-rich, resembling the MZ, and the late-born neurons migrated past their predecessors toward the central Reelin-rich region within the aggregates, resulting in a birthdate-dependent "inside-out" alignment even ectopically. Reelin receptors and intracellular adaptor protein Dab1 were found to be necessary for formation of the aggregates. The above findings indicate that Reelin signaling is capable of inducing the formation of the dendrite-rich, cell-body-sparse MZ and a birthdate-dependent "inside-out" alignment of neurons independently of other factors/structures near the MZ.

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.