Somal Translocation Research Articles

Altered Speeds and Trajectories of Neurons Migrating in the Ventricular and Subventricular Zones of the Reeler Neocortex

The Reelin signaling pathway is essential for proper cortical development, but it is unclear to whether Reelin function is primarily important for cortical layering or neuron migration. It has been proposed that Reelin is perhaps required only for somal translocation but not glial-dependent locomotion. This implies that the location of neurons responding to Reelin is restricted to the outer regions of the cortical plate (CP). To determine whether Reelin is required for migration outside of the CP, we used time-lapse imaging to track the behavior of cells undergoing locomotion in the germinal zones. We focused on the migratory activity in the ventricular/subventricular zones where the first transition of bipolar to multipolar migration occurs and where functional Reelin receptors are known to be expressed. Despite Reelin loss, neurons had no difficulty in undergoing radial migration and indeed displayed greater migratory speed. Additionally, compared with the wild-type, reeler neurons displayed altered trajectories with greater deviation from a radial path. These results suggest that Reelin loss has early consequences for migration in the germinal zones that are portrayed as defective radial trajectories and migratory speeds. Together, these abnormalities can give rise to the increased cell dispersion observed in the reeler cortex.

Leading Tip Drives Soma Translocation via Forward F-Actin Flow during Neuronal Migration

Neuronal migration involves coordinated extension of the leading process and translocation of the soma, but the relative contribution of different subcellular regions, including the leading process and cell rear, in driving soma translocation remains unclear. By local manipulation of cytoskeletal components in restricted regions of cultured neurons, we examined the molecular machinery underlying the generation of traction force for soma translocation during neuronal migration. In actively migrating cerebellar granule cells in culture, a growth cone (GC)-like structure at the leading tip exhibits high dynamics, and severing the tip or disrupting its dynamics suppressed soma translocation within minutes. Soma translocation was also suppressed by local disruption of F-actin along the leading process but not at the soma, whereas disrupting microtubules along the leading process or at the soma accelerated soma translocation. Fluorescent speckle microscopy using GFP-alpha-actinin showed that a forward F-actin flow along the leading process correlated with and was required for soma translocation, and such F-actin flow depended on myosin II activity. In migrating neurons, myosin II activity was high at the leading tip but low at the soma, and increasing or decreasing this front-to-rear difference accelerated or impeded soma advance. Thus, the tip of the leading process actively pulls the soma forward during neuronal migration through a myosin II-dependent forward F-actin flow along the leading process.

Serotonergic Neurons Migrate Radially through the Neuroepithelium by Dynamin-Mediated Somal Translocation

Embryonic CNS neurons can migrate from the ventricular zone to their final destination by radial glial-guided locomotion. Another less appreciated mechanism is somal translocation, where the young neuron maintains its primitive ventricular and pial processes, through which the cell body moves. A major problem in studying translocation has been the identification of neuronal-specific markers that appear in primitive, radially shaped cells. We used enhanced yellow fluorescent protein under control of the Pet-1 enhancer/promoter region (ePet-EYFP), a specific marker of early differentiated serotonergic neurons, to study their migration via immunohistology and time-lapse imaging of living slice cultures. As early as E10.0, ePet-EYFP-expressing neurons were axonless, radially oriented, and spanned the entire neuroepithelium. The soma translocated within the pial process toward the pial surface and could also translocate through its neurites, which sprouted from the pial process. The dynamin inhibitor dynasore significantly reduced translocation velocity, while the nonmuscle myosin II inhibitor blebbistatin and the kinesin inhibitor AMP-PNP had no significant effect. Here we show for the first time that serotonergic neurons migrate by somal translocation mediated, in part, by dynamin.

Myosin II Motors and F-Actin Dynamics Drive the Coordinated Movement of the Centrosome and Soma during CNS Glial-Guided Neuronal Migration

Lamination of cortical regions of the vertebrate brain depends on glial-guided neuronal migration. The conserved polarity protein Par6alpha localizes to the centrosome and coordinates forward movement of the centrosome and soma in migrating neurons. The cytoskeletal components that produce this unique form of cell polarity and their relationship to polarity signaling cascades are unknown. We show that F-actin and Myosin II motors are enriched in the neuronal leading process and that Myosin II activity is necessary for leading process actin dynamics. Inhibition of Myosin II decreased the speed of centrosome and somal movement, whereas Myosin II activation increased coordinated movement. Ectopic expression or silencing of Par6alpha inhibited Myosin II motors by decreasing Myosin light-chain phosphorylation. These findings suggest leading-process Myosin II may function to "pull" the centrosome and soma forward during glial-guided migration by a mechanism involving the conserved polarity protein Par6alpha.

Differences of Migratory Behavior between Direct Progeny of Apical Progenitors and Basal Progenitors in the Developing Cerebral Cortex

Cerebral cortical neurons are known to be produced from both apical progenitors in the ventricular zone (VZ) and basal (intermediate) progenitors in the subventricular zone (SVZ). On the other hand, we have shown that many SVZ cells assume multipolar morphology and show a characteristic movement termed "multipolar migration." The relationship between multipolar cells and basal progenitors in the SVZ has yet to be investigated. Herein, we followed postmitotic cells generated in the VZ and found that they stayed for more than 10 h in the VZ after becoming postmitotic and then accumulated in the lower part of the SVZ (multipolar cell accumulation zone: MAZ) as multipolar cells (slowly exiting population: SEP), whereas basal progenitors rapidly migrated into the SVZ or intermediate zone (IZ) (rapidly exiting population: REP) with somal translocation morphology. Although REP reached the SVZ/IZ earlier than the SEP, REP stayed within in the SVZ/IZ, whereas SEP moved steadily and entered the CP prior to the REP. We also observed SEP to eventually differentiate into pyramidal neurons in layers II/III. This study provides in vivo evidence of differences in migration modes between postmitotic cells generated from apical progenitors and basal progenitors.

Boundary cap cells constrain spinal motor neuron somal migration at motor exit points by a semaphorin-plexin mechanism

BackgroundIn developing neurons, somal migration and initiation of axon outgrowth often occur simultaneously and are regulated in part by similar classes of molecules. When neurons reach their final destinations, however, somal translocation and axon extension are uncoupled. Insights into the mechanisms underlying this process of disengagement came from our study of the behaviour of embryonic spinal motor neurons following ablation of boundary cap cells. These are neural crest derivatives that transiently reside at motor exit points, central nervous system (CNS):peripheral nervous system (PNS) interfaces where motor axons leave the CNS. In the absence of boundary cap cells, motor neuron cell bodies migrate along their axons into the periphery, suggesting that repellent signals from boundary cap cells regulate the selective gating of somal migration and axon outgrowth at the motor exit point. Here we used RNA interference in the chick embryo together with analysis of null mutant mice to identify possible boundary cap cell ligands, their receptors on motor neurons and cytoplasmic signalling molecules that control this process.ResultsWe demonstrate that targeted knock down in motor neurons of Neuropilin-2 (Npn-2), a high affinity receptor for class 3 semaphorins, causes their somata to migrate to ectopic positions in ventral nerve roots. This finding was corroborated in Npn-2 null mice, in which we identified motor neuron cell bodies in ectopic positions in the PNS. Our RNA interference studies further revealed a role for Plexin-A2, but not Plexin-A1 or Plexin-A4. We show that chick and mouse boundary cap cells express Sema3B and 3G, secreted semaphorins, and Sema6A, a transmembrane semaphorin. However, no increased numbers of ectopic motor neurons are found in Sema3B null mouse embryos. In contrast, Sema6A null mice display an ectopic motor neuron phenotype. Finally, knockdown of MICAL3, a downstream semaphorin/Plexin-A signalling molecule, in chick motor neurons led to their ectopic positioning in the PNS.ConclusionWe conclude that semaphorin-mediated repellent interactions between boundary cap cells and immature spinal motor neurons regulates somal positioning by countering the drag exerted on motor neuron cell bodies by their axons as they emerge from the CNS at motor exit points. Our data support a model in which BC cell semaphorins signal through Npn-2 and/or Plexin-A2 receptors on motor neurons via a cytoplasmic effector, MICAL3, to trigger cytoskeletal reorganisation. This leads to the disengagement of somal migration from axon extension and the confinement of motor neuron cell bodies to the spinal cord.

Cell Migration: Catapulting Neurons from the Ventricular Zone?

In the developing vertebrate brain, newly born neurons migrate away from the proliferative zones. A new paper suggests that the initial phase of this migration may be propelled by the mechanical properties of the new neurons' processes rather than more conventional mechanisms of cell migration.

Twisting of Neocortical Progenitor Cells Underlies a Spring-like Mechanism for Daughter-Cell Migration

The mammalian neocortical wall thickens extensively during embryogenesis via proliferation of progenitor cells [1-4] and migration of daughter cells toward the pial surface [5-8]. Time-lapse imaging and functional experiments were carried out so that the possible involvement of mechanical forces in these processes could be examined. When bipolar cells connecting the ventricular and pial surfaces of the mouse cerebral wall lose their ventricular attachment, they undergo somal translocation toward the outer zones, which contain differentiated neurons. The pial process of these transitioning unipolar cells exhibits a coiled or hairpin-loop morphology, suggesting that twisting and stretching of the pial process establishes a spring-like mechanism that propels the daughter cell toward the pial surface upon ventricular detachment. This model is supported by morphological changes observed in microsurgically transected pial processes. Pharmacological experiments further reveal the involvement of intermediate filaments in twisting of pial processes. These results uncover a novel mechanism for cellular migration and provide valuable tools for the detailed study of the role of mechanical forces in 3D brain development.

HOW to Wrap Axons with Crooked Neck

How can RNA processing direct specific aspects of nervous system development? In this issue of Neuron, Edenfeld et al. identified a novel function for two regulators of mRNA splicing in Drosophila: peripheral glial cells require Crooked neck (Crn) and Held out wings (HOW) to mediate migration and ensheathment of peripheral axons.

Midline radial glia translocation and corpus callosum formation require FGF signaling

Midline astroglia in the cerebral cortex develop earlier than other astrocytes through mechanisms that are still unknown. We show that radial glia in dorsomedial cortex retract their apical endfeet at midneurogenesis and translocate to the overlaying pia, forming the indusium griseum. These cells require the fibroblast growth factor receptor 1 (Fgfr1) gene for their precocious somal translocation to the dorsal midline, as demonstrated by inactivating the Fgfr1 gene in radial glial cells and by RNAi knockdown of Fgfr1 in vivo. Dysfunctional astroglial migration underlies the callosal dysgenesis in conditional Fgfr1 knockout mice, suggesting that precise targeting of astroglia to the cortex has unexpected roles in axon guidance. FGF signaling is sufficient to induce somal translocation of radial glial cells throughout the cortex; furthermore, the targeting of astroglia to dorsolateral cortex requires FGFr2 signaling after neurogenesis. Hence, FGFs have an important role in the transition from radial glia to astrocytes by stimulating somal translocation of radial glial cells.

Impaired Neuronal Positioning and Dendritogenesis in the Neocortex after Cell-Autonomous Dab1 Suppression

Reelin and Disabled 1 (Dab1) are essential for positioning migrating neurons in the developing neocortex. Cell-autonomous RNA interference-mediated suppression of Dab1 in migrating neurons destined for layer 2/3 shifted the median position of these cells to deeper positions within the cortex. At the time of migration arrest [embryonic day 20 (E20) to E21], Dab1-suppressed cells were underrepresented in the upper approximately 40 microm of the cortex compared with controls, suggesting that Dab1 is essential for somal translocation through the cell-dense cortical plate. Closer examination of the morphology of Dab1-suppressed neurons at E20 revealed simplified leading processes that are less likely to contact the marginal zone (MZ), in which high levels of Reelin are expressed. Examination of Dab1-suppressed cells 3 d later (postnatal day 2) revealed simplified dendrites that are also less likely to contact the MZ. These data reveal a cell-autonomous role of Dab1 in dendritogenesis in the neocortex and suggest that remodeling of the leading process of a migrating neuron into a nascent dendrite by Reelin/Dab1 signaling plays an important role in cell positioning.

In vivo Evidence for Radial Migration of Neurons by Long-Distance Somal Translocation in the Developing Ferret Visual Cortex

During the development of the cerebral cortex, neurons generated in the cortical ventricular zone migrate radially toward the marginal zone. Radially migrating neurons are thought to display 1 of 2 morphologies: cells with a long, pia-contacting, apical process utilized for somal translocation early in development, when the cortex is still relatively thin; or cells with a short leading process, abundant at late stages of corticogenesis when neurons need to travel for longer distances. In large convoluted brains, like those of many primates and carnivores, radially migrating neurons must travel distances up to several millimeters before reaching their final destination, often following curvilinear trajectories. Here we analyze modes and morphologies of radially migrating neurons in convoluted brains by studying the visual cortex of developing ferrets. We provide in vivo and in vitro evidence for the existence of late-born cortical neurons that migrate radially by long-distance somal translocation within a long apical process extended to the cortical plate, in contrast to the early somal translocation observed in rodents. Long-distance translocating neurons in the ferret show a discontinuous rhythm of migration, alternating periods of advance with periods of stall. Furthermore, by combining different labeling methods we find the simultaneous presence in the developing ferret cortex of long-distance translocating neurons and neurons migrating within a short leading process.

The transmembrane semaphorin Sema6A controls cerebellar granule cell migration

The transmembrane semaphorin protein Sema6A is broadly expressed in the developing nervous system. Sema6A repels several classes of developing axons in vitro and contributes to thalamocortical axon guidance in vivo. Here we show that during cerebellum development, Sema6A is selectively expressed by postmitotic granule cells during their tangential migration in the deep external granule cell layer, but not during their radial migration. In Sema6A-deficient mice, many granule cells remain ectopic in the molecular layer where they differentiate and are contacted by mossy fibers. The analysis of ectopic granule cell morphology in Sema6a-/- mice, and of granule cell migration and neurite outgrowth in cerebellar explants, suggests that Sema6A controls the initiation of granule cell radial migration, probably through a modulation of nuclear and/or soma translocation. Finally, the analysis of mouse chimeras suggests that this function of Sema6A is primarily non-cell-autonomous.

LIS1 RNA interference blocks neural stem cell division, morphogenesis, and motility at multiple stages

Mutations in the human LIS1 gene cause the smooth brain disease classical lissencephaly. To understand the underlying mechanisms, we conducted in situ live cell imaging analysis of LIS1 function throughout the entire radial migration pathway. In utero electroporation of LIS1 small interference RNA and short hairpin dominant negative LIS1 and dynactin cDNAs caused a dramatic accumulation of multipolar progenitor cells within the subventricular zone of embryonic rat brains. This effect resulted from a complete failure in progression from the multipolar to the migratory bipolar state, as revealed by time-lapse analysis of brain slices. Surprisingly, interkinetic nuclear oscillations in the radial glial progenitors were also abolished, as were cell divisions at the ventricular surface. Those few bipolar cells that reached the intermediate zone also exhibited a complete block in somal translocation, although, remarkably, process extension persisted. Finally, axonal growth also ceased. These results identify multiple distinct and novel roles for LIS1 in nucleokinesis and process dynamics and suggest that nuclear position controls neural progenitor cell division.

Distinct migratory behavior of early- and late-born neurons derived from the cortical ventricular zone.

Time-lapse studies indicate that ventricular zone (VZ)-derived cells show two migratory modes in the cerebral cortex at different stages of mammalian embryogenesis: somal translocation and locomotion. We carried out a systematic analysis to examine whether the migratory behavior of cortical neurons derived from the cortical VZ is stage-dependent. We labeled VZ cells of mouse embryos with green fluorescent protein (gfp) -encoding plasmids by in utero electroporation and evaluated the labeled cells after appropriate survival periods. After electroporation at either embryonic day (E) 12.5 or E15.5, GFP+ VZ cells were initially spindle-shaped and radially oriented. After leaving the VZ, they transformed into round or horizontally oriented fusiform neurons with many short processes. They then seemed to gradually change into radially oriented bipolar cells as they moved upward. Whereas the earliest emigrants from the VZ labeled at E12.5 (early-born neurons) reached the top of the cortical plate (CP) after these changes, VZ cells labeled at E15.5 (late-born neurons) further migrated along the length of radial fibers to reach the top of the CP. A dominant negative form of the gene for cyclin-dependent kinase 5 (Cdk5DN) was then introduced into VZ cells. Transfection of E12.5 VZ with cdk5dn did not disrupt the migration of the early-born neurons. However, this caused a failure in migration of the late-born neurons, although they transformed into bipolar shapes in the intermediate zone. Thus, there appear to be at least two distinct migratory phases of cortical neurons: one common to the early- and late-born neurons, and the other specific to late-born neurons and Cdk5-dependent.

Multipolar Migration: The Third Mode of Radial Neuronal Migration in the Developing Cerebral Cortex

Two distinct modes of radial neuronal migration, locomotion and somal translocation, have been reported in the developing cerebral cortex. Although these two modes of migration have been well documented, the cortical intermediate zone contains abundant multipolar cells, and they do not resemble the cells migrating by locomotion or somal translocation. Here, we report that these multipolar cells express neuronal markers and extend multiple thin processes in various directions independently of the radial glial fibers. Time-lapse analysis of living slices revealed that the multipolar cells do not have any fixed cell polarity, and that they very dynamically extend and retract multiple processes as their cell bodies slowly move. They do not usually move straight toward the pial surface during their radial migration, but instead frequently change migration direction and rate; sometimes they even remain in almost the same position, especially when they are in the subventricular zone. Occasionally, the multipolar cells jump tangentially during their radial migration. Because the migration modality of these cells clearly differs from locomotion or somal translocation, we refer to their novel type of migration as "multipolar migration." In view of the high proportion of cells exhibiting multipolar migration, this third mode of radial migration must be an important type of migration in the developing cortex.

Neuronal migration in the developing cerebral cortex: observations based on real-time imaging.

We have used time-lapse imaging of acute cortical slices to study the migration of neurons from their sites of origin to their positions in the developing neocortex. We found that two distinct modes of cell movement, somal translocation and glia-guided locomotion, are responsible for the radial migration of neurons generated in the cortical ventricular zone. The former is the prevalent form of radial movement of the early-born cortical neurons, while the latter is adopted by those generated later in corticogenesis. Interneurons, found to originate in the ganglionic eminence, follow tangential migratory paths to reach the developing cortex. Upon reaching the cortex, these cells seek the ventricular zone using a mode of movement that we have termed 'ventricle-directed migration', before they migrate to their positions in the cortical plate. In addition to these forms of movement, we report here a unique morphological and migratory behavior for a population of cortical neurons. These cells are multipolar in form, and are highly motile in the formation and retraction of their processes. Based on these morphological features, we refer to this type of cells as 'branching cells' and attribute the phenotype to a subset of cortical interneurons.

Radial glia and somal translocation of radial neurons in the developing cerebral cortex.

A series of recent studies have demonstrated that radial glia are neural precursors in the developing cerebral cortex. These studies have further implied that these cells are the sole precursor constituents of the dorsal forebrain ventricular zone that generate the projection neurons of the cortex. In view of these new findings, this review discusses radial neurons, a progeny of cortical neurons that are generated by radial glia and adopt somal translocation as the mode of migration.

Modes of neuronal migration in the developing cerebral cortex.

The conventional scheme of cortical formation shows that postmitotic neurons migrate away from the germinal ventricular zone to their positions in the developing cortex, guided by the processes of radial glial cells. However, recent studies indicate that different neuronal types adopt distinct modes of migration in the developing cortex. Here, we review evidence for two modes of radial movement: somal translocation, which is adopted by the early-generated neurons; and glia-guided locomotion, which is used predominantly by pyramidal cells. Cortical interneurons, which originate in the ventral telencephalon, use a third mode of migration. They migrate tangentially into the cortex, then seek the ventricular zone before moving radially to take up their positions in the cortical anlage.

Two modes of radial migration in early development of the cerebral cortex.

Layer formation in the developing cerebral cortex requires the movement of neurons from their site of origin to their final laminar position. We demonstrate, using time-lapse imaging of acute cortical slices, that two distinct forms of cell movement, locomotion and somal translocation, are responsible for the radial migration of cortical neurons. These modes are distinguished by their dynamic properties and morphological features. Locomotion and translocation are not cell-type specific; although at early ages some cells may move by translocation only, locomoting cells also translocate once their leading process reaches the marginal zone. The existence of two modes of radial migration may account for the differential effects of certain genetic mutations on cortical development.