Salud Mental ISSN: 0185-3325 [email protected] Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz México

Flores Cruz, María Guadalupe; Escobar, Alfonso Normal neuronal migration Salud Mental, vol. 34, núm. 1, enero-febrero, 2011, pp. 61-66 Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz Distrito Federal, México

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Normal neuronal migration

María Guadalupe Flores Cruz,1 Alfonso Escobar1

Artículo original

SUMMARY with cytoplasmic dilatation, and then the centrosome and Golgi apparatus approach it, finally nucleus advances to the cytoplasmic Ontogenesis of both central and peripheral nervous systems depends dilatation. Movement of centrosome and nucleus depends on integrity on basic, molecular and cellular mechanisms of the normal neuronal of a microtubule network. Most of the microtubules surrounding the migration. Any deviation leads to neural malformations. All neural nucleus are tyrosinated, making them dynamic; microtubules at the cells and structures derive from the neural ectoderm, which under the anterior pole of the nucleus, near the centrosome, are acetylated. influence of the and the molecules Noggin and Chordin, is Once neurons reach their final destination, they need to cancel transformed consecutively into , , the migratory program and differentiate. The mechanisms are and primary vesicles. Of the latter, the most rostral, the prosencephalon, unknown; possibly early patterns of activity in the target region could two vesicles are bilaterally generated, the telencephalon and in the influence. Ca2+ influx is a proposed mechanism for halting migration. middle, the unpaired diencephalons. The telencepahlic vesicles generate the cerebral hemispheres and the ; the latter Key words: Neuronal migration, , molecular constitutes the main source of progenitor neuroepithelial cells (NEC) mechanisms. in the . The NEC massively migrates to constitute the cerebral cortex and other hemispheric structures in the telencephalon and diencephalon. The NEC expresses a broad RESUMEN repertory of markers: BLBP, GLAST, vimentin, tenascin, S100β and, in primates GFAP; in a sequential order the NEC form the first cortical La ontogenia de los Sistemas Nervioso Central y Nervioso Periférico layer formed by the marginal zone and the subplate. The marginal depende de procesos como la proliferación, diferenciación y migra- zone harbors the Cajal-Retzius positive neurons and reelin ción neuronal, entre otros. Cualquier desviación resulta en malfor- negative neurons. maciones. Las estructuras y células nerviosas derivan del ectodermo, Reelin, besides signaling stop to migrating neurons, also la notocorda induce la formación de la placa neural mediante la participates in ordering the cortical layers; it is known that in mutant secreción de las moléculas Noggin y Chordin; posteriormente la placa mice lacking reelin cortical layers are disrupted. Genetic studies neural se convierte en surco y tubo neurales. indicate that ApoER2, Vldr (both reelin receptors) and Dab1, reelin Una vez que el tubo neural está formado, las células signaling adaptor protein, enter into a common pathway leading neuroepiteliales (CNE), futuras neuronas y glía, en la zona subventricular Dab1 to phosphorylation in migrating neurons. migran masivamente para constituir la corteza cerebral y otras Cortical pyramidal neurons generate at germinal zone; estructuras. Las CNE, al ser células gliales, expresan múltples generate both in Vz and SVZ in medial ganglionic marcadores: BLBP, GLAST, vimentin, tenascin, S100β y en primates GFAP. eminence and caudal GE. Las CNE forman la primera capa cortical, también llamada Two types of neuronal migration coexist, radial and tangential. preplato. Las siguientes divisiones celulares darán origen a la zona In radial migration, the neurons move perpendicular to marginal marginal y al subplato. Las subsecuentes neuronas que arriban al zone and radial serves as a scaffold to migrating cells; in the subplato desplazan a las anteriores de modo que en las capas tangential way, neurons migrate in parallel to brain surface guided superficiales se encuentran las últimas neuronas que llegaron. La by semaphorins, neuropilins, cell adhesion molecules, neuregulins, capa marginal o capa I contiene células de Cajal-Retzius chemokines and the slit and robo families of attractant and repellent inmunorreactivas a reelin y neuronas reelin-negativas situadas más molecules. profundamente. The migratory cycle of neurons involves leading process La proteína reelin, además de servir como señal de alto a las dynamics and somal translocation, which involves the movement of neuronas migratorias, también interviene en el orden de la perinuclear material, organelles and nucleus. laminación cortical, la cual es desordenada en los ratones que Leading process stability depends on the microtubular array carecen de reelin. No se conoce en su totalidad el mecanismo that links the leading edge of the cell with the soma. The centrosome molecular mediante el cual reelin regula los procesos antes is a microtubule center to control microtubule polymerization. In mencionados. Hasta el momento se conoce que ApoER2, Vldlr radially migrating neurons, the centrosome establishes a link between (ambos receptors de reelin) y Dab1, proteína adaptadora en la centrioles and nuclear membrane. The effective neuronal migration señalización por reelin, participan en una vía común que lleva a la is only completed by translocation of the cell soma, which occurs fosforilación de Dab1 en las neuronas en migración.

1 Departamento de Biología Celular y Fisiología, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México. Corresponding: María Guadalupe Flores Cruz, Departamento de Biología Celular y Fisiología, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, México, D.F. Tel. & Fax. +52 55 5622-3850. E-mail: [email protected] Recibido: 13 de octubre de 2010. Aceptado: 16 de diciembre de 2010.

Vol. 34, No. 1, enero-febrero 2011 61 Flores Cruz y Escobar

Las neuronas piramidales corticales se generan en el telencéfalo delante de los centriolos deforma el conjunto perinuclear de dorsal, mientras que las interneuronas se generan en la zona y subzona microtúbulos. Se debe a la elasticidad de ese conjunto microtubular ventriculares del telencéfalo ventral, en las bien definidas subdivisiones y sus proteínas motoras asociadas al desplazamiento del núcleo. de la eminencia gangliónica (EG): lateral, medial y caudal. La nucleocinesis o movimiento del núcleo determina la dirección La migración neuronal puede ser radial o tangencial; la del movimiento nuclear, la migración neuronal efectiva sólo se migración radial emplea a la glía radial mientras que en la tangencial completa por la translocación subsecuente del soma, lo cual ocurre las neuronas migran paralelamente a la superficie cortical. En los por la dilatación del citoplasma y el movimiento del centrosoma y dos tipos hay formación de neuritas, translocación somática y del aparato de Golgi hacía el mecanismo; finalmente el núcleo núcleocinesis. Varios factores participan en la migración tangencial: avanza e invade la dilatación del citoplasma. semaforinas, neuropilinas, moléculas de adhesion celular, El movimiento del centrosoma y del núcleo depende de la neuregulinas, quimiocinas y moléculas atrayentes y repelentes de integridad de la red microtubular y de las modificaciones post- las familias slit y robo. transcripción. La mayoría de los micotúbulos perinucleares están El ciclo migratorio de las neuronas incluye la translocación del tirosinados, lo cual los hace extremadamente dinámicos; en cambio, soma con movilización de material perinuclear, organelos y del los microtúbulos del polo anterior del núcleo, vecinos del centrosoma, núcleo. Así mismo, dicho ciclo aparece con morfología bien definida están acetilados y por ende más estables. Se ha dicho que los en una variedad de neuronas lo que refleja adaptación a ambientes microtúbulos perinucleares se hallan conectados con el centrosoma específicos. De tal modo que las claves guías influyen en la frecuencia que en sí es el centro que los organiza. Además, se han descrito y orientación de la emergencia dendrítica, que a su vez permite a otras proteínas asociadas con la polaridad celular que desempeñan las neuronas migrantes cambiar de dirección sin reorientar las un papel esencial en la coordinación del movimiento del centrosoma dendritas preexistentes. La estabilidad del mecanismo depende de y del núcleo en cada ciclo migratorio. la organización microtubular que asocia el borde celular con el soma; Finalmente, una vez que las neuronas alcanzan su posición ya que el sistema de microtúbulos apoya dicho mecanismo y también definitiva, requieren cancelar el programa migratorio y continuar su permite el flujo de vesículas. diferenciación hasta alcanzar las características morfológicas y En las células animales el centrosoma es el centro que organiza funcionales que les corresponden. el citoesqueleto, la polimerización, el arreglo de los microtúbulos perinucleares y establece el contacto de los centriolos con la Palabras clave: Migración neuronal, corteza cerebral, mecanismos membrana nuclear. En la migración radial el movimiento hacia moleculares.

INTRODUCTION forebrain. Shortly afterwards the prosencephalon laterally generates two optic vesicles and two telencephalic vesicles The concept of neuronal migration refers to the essential plus the unpaired structure in the middle, the diencephalon. mechanisms, molecular and cellular aspects that, during The two telencephalic vesicles constitute the primitive , constitute the structures of both cerebral hemispheres; inside the telencephalon two fluid- the Central and the Peripheral Nervous Systems. Early filled spaces constitute the lateral ventricles. The wall of neuroscience studies back in the 19th century, contributed the ventricles constitutes the main source of neuroepithelial to firmly establish knowledge in the precise sequential progenitor cells, the future neurons that will migrate to form stages of the embryonic development in vertebrates, and the cerebral cortex and other structures in the developing also recognized that any failure in the stage sequence would brain.1 lead to malformations of the . All the Nervous System, structures and cells, without exception, are derived from the ectoderm −the THE EMBRYONIC EMERGENCE −, the outer layer of the primitive embryo, OF THE CEREBRAL CORTEX which under the influence of the underlying notochord, generated by the mesoderm, gives the signal, the neural Delimitation of the zones induction coupled with the molecules Noggin and Chordin, to transform that zone of the ectoderm into the neural plate, At early stages of embryonic development, most neuro- actually the skin of the embryo that has thickened. By the epithelial progenitor cells in the undergo third week of gestation in the human embryonic develop- symmetric division, this mechanism allows the expansion ment, due to significant stem cell proliferation, the neural of the population; meanwhile, at the onset of , plate grooves in to form the neural groove followed in neuroepithelial cells take the features of glial cells. They sequence to form the neural tube. The neural tube closes express a broad repertory of markers: BLBP, GLAST, its open ends –the neuropores- by the fourth week. The vimentin, tenascin, S100β and, in primates, GFAP.2 rostral end of the neural tube expands to form three Radial glial cells are one of the key components of the swellings, called the primary vesicles. The most rostral developing cerebral cortex; radial glia undergo asymmetric vesicle is called the prosencephalon, also called the division, generating with each division another glial cell and

62 Vol. 34, No. 1, enero-febrero 2011 Normal neuronal migration one daughter cell that becomes a neuron. The aforementioned DIVERSE ORIGIN OF CORTICAL NEURONS divisions take place in the ventricular zone.3 The first postmitotic cells, that underwent asymmetric Cortical pyramidal neurons are generated at the germinal division, migrate in a radial way out of the neuroephitelium zones of the dorsal telencephalon.9 to form the first cortical layer or preplate. The subsequent Interneurons are generated in the VZ and SVZ of the developing cortical plate is formed within the preplate and ventral telencephalon, or subpallium, in the distinct divides this layer and its neuronal population into a subdivisions of the ganglionic eminence −GE−.10 The GE is superficial zone, named the marginal zone, and a deep, subdivided into three principal progenitor domains: the lateral lower zone called the subplate.4 As additional waves ganglionic eminence −LGE−, the medial ganglionic eminence migrating neurons arrive in the cortical plate, they bypass −MGE−, and the caudal ganglionic eminence −CGE.5 previously generated neurons to form the cortical layers; hence the deeper layers are the first to form, while the superficial layers are the last, excepting the marginal zone TYPES OF MOVEMENT: or layer I.5 RADIAL AND TANGENTIAL MIGRATION At this time-point, the marginal zone contains at least two types of neurons, named subpial or pioneer cells: large Across the embryonic stages, two types of neuronal reelin-positive Cajal-Retzius neurons that extend their migration predominate, radial and tangential movement. in the marginal zone, and other large, reelin-negative The first one consists in movement in perpendicular neurons located deeper in the marginal zone.6 direction to the marginal zone, and a scaffold of radial glial The extracellular matrix protein reelin serves as a stop cells guides migrating neurons. The second movement, the signal for migrating neurons, it also controls the formation tangential type, describes neurons that migrate in parallel of cortical layers. Lack of reelin, as in the reeler mutants, to the brain surface, or marginal zone; tangential migration causes disorders in cortical lamination.4,7 Besides the does not depend on radial glia, unlike radial migration.11 importance of the molecule for the lamination of the cortical Both types of movement involve processes such as the structures, the molecular mechanisms underlying its action extension of a leading neurite, somal translocation, nucleo- remain unclear. kinesis.12 Several genetic studies have positioned Reelin, ApoER2, Vldlr (both of them reelin receptors) and Dab1 (an adaptor Radial migration protein in reelin signaling) into a common signaling pathway that leads to the phosphorylation of Dab1 in Radial migration of neurons along radial glial fibers is also migrating neurons.8 termed gliophilic migration. The neurons generated in the dorsal cortex proliferative zones pass through a series of distinct migrational stages characterized by changes in cell shape, direction of movement and speed of migration. The stages of radial migration are four: first, neurons generated at the ventricular zone move radially to the subventricular zone; in the second stage, neurons pause in the intermediate zone-SVZ for as long as 24h and become multipolar. A fraction of neurons pass through a third stage in which they extend a process toward the ventricle (and sometimes they also translocate the cell body toward the ventricle). Once the neurons reach the ventricle, they enter in stage four of migration. Neurons reverse multipolar to bipolar morphology and extend a pia-directed leading process and begin radial migration to the cortical plate.5 Correct migration in the cortical plate requires coordination of adhesive and cytoskeletal dynamics between neurons and radial glia and changes in the disposability of Figure 1. Schematic representation of the subdivisions of the extracellular matrix molecules interacting with them. developing cortex. In A, developing cortex showing the principal subdivisions; in MZ is possible to appreciate the terminal process of the radial glia attached to the subpial surface and the most abundant Tangential migration pioneer cells, the CRc. In B, amplified view of a migrating neuron attached to radial glia. Abbreviations: MZ= marginal zone, CP= cortical plate, IZ= Tangential migration has been shown to be guided by a intermediate zone, SVZ= subventricular zone, CRc= Cajal-Retzius cell. variety of factors. These factors include semaphorins,

Vol. 34, No. 1, enero-febrero 2011 63 Flores Cruz y Escobar neuropilins, cell adhesion molecules, neuregulins, a delayed migration and cannot invade the cortical plate. chemokines and the slit and robo families of attractant and Those results suggest that the Dlx-dependent switch from repellent molecules.5,9 tangential to radial migration might be mediated by Arx.14 The polysialylated form of the neural adhesion The migratory failure in Dlx mutants has been molecule (PSA-N-CAM), a member of the immunoglobulin attributed to premature differentiation, associated with the superfamily that mediates homo-and heterophilic cell-cell extension of longer and highly branched processes; this interactions, is important in the migratory process. feature is shared with Arx mutants. Homeodomain Mutation of N-CAM in mice results in a small olfactory transcription factors appear to control bulb and the accumulation of olfactory interneuron migration by influencing the expression of cytoskeletal and precursors in the SVZ.13 cell dynamics related genes, like PAK3 (p21-activated Homeodomain transcription factors are essential in serine/threonine kinases), MAP2 (microtubule associated specification and differentiation of proliferating GE protein2), Tau and GAP43.9 progenitors, and in later phases, orchestrate neuronal Many downstream genes appear to be selectively migration away from the GE. controlled by either Arx or Dlx proteins, suggesting that Dlx genes promote GABAergic phenotype and these transcription factors may also display distinct interneuron migration. In Dlx null brains, GE tangentially activities in subpallial neurons. The LIM (Lin-11, Isl-1, Mec- migrating neurons are blocked in the SVZ. A primary target 3) homeodomain transcription factor Lhx6 downregulation of DLX genes is another transcription factor, Arx (X-linked results in MGE-derived neurons accumulation at the aristaless-related homeobox gene); Arx is located on X cortico-striatal boundary; in Dlx and Arx ko, Lhx6 is still chromosome and is involved in human neurological expressed by interneurons, suggesting that this disorders including epilepsy, lisencephaly and mental transcription factor is not sufficient to trigger a normal retardation. A Dlx protein binds and activates an Arx migration program. Nkx2.1 is another transcription factor enhancer element. Male Arx ko mouse embryos have a essential to the migration of cortical interneurons. Nkx2.1 smaller , and due to is expressed in MGE progenitor domains and is maintained in neurons migration to the but it is downregulated in interneurons targeting the neocortex.9 Another influence for both neurogenesis and tangential migration is dopaminergic signaling. Activation of the D1-like receptors promotes GABA interneuron migration from the basal forebrain to the cerebral cortex, whereas the activation of D2-like receptors decreases it. The activation of D1-like receptors mobilizes cytoplasmic dynein heavy chain and tubulin to cellular process, whereas activation of D2-like receptors produces a condensation of these proteins around the nucleus.15

MIGRATION MECHANISMS

The migratory cycle of neurons involves leading process dynamics and somal translocation, which involves the movement of perinuclear material, organelles and nucleus. The leading process of migrating neurons has relatively distinct morphologies in different classes of neurons, probably reflecting their adaptation to specific micro- environments. Thus guidance cues influence the frequency and orientation at which new leading process branches emerge, which allows migrating neurons to rapidly change Figure 2. Diverse origin of interneuron population. Interneurons direction without having to reorient pre-existing branches.8 are generated in the distinct subdivisions of the ganglionic eminence, the lateral ganglionic eminence –LGE- the birthplace of the Leading process stability depends on the microtubular interneurons that will populate the neocortex; the medial ganglionic array that links the leading edge of the cell with the soma; eminence –MGE-, giving rise to interneurons that migrate to the the microtubule system supports the leading process and striatum, and the caudal ganglionic eminence –CGE. The GE also generates neurons that populate other structures such as the septum, also allows the flow of vesicles required for intracellular olfactory bulb and . communication. P600 is a microtubule associated protein

64 Vol. 34, No. 1, enero-febrero 2011 Normal neuronal migration that interacts with the endoplasmic reticulum; knockdown differentiation. The mechanisms through which neurons of p600 in cortical pyramidal cells leads to an important detect that they have reached their target position remain reduction in acetylated tubulin and an almost complete loss elusive. It has been suggested that early patterns of activity of the endoplasmic reticulum in the leading process, generated in the target region could be influencing this conferring a wavy appearance and disrupting migration.8 process. Ca2+ influx is one proposed mechanism for halting In animal cells, the centrosome is a microtubule- migration.8 organizing center that controls microtubule polymerization, organizing the cytoskeleton. In radially migrating neurons, COROLLARY the centrosome controls the formation of a microtubule array that surrounds the nucleus as a cage and establishes a link The central nervous development could be compared with between centrioles and nuclear membrane. During radial a symphony, a complex molecular symphony, in which migration, the forward movement of centrioles leads to the precise participation in time and space of each marker deformation of the perinuclear microtubule cage. The is crucial for the harmonic evolution of the movements. elasticity of the cage together with the microtubule associated For example, core cell cycle proteins provide the tempo and 13 motor proteins cooperates in pulling the nucleus. establish the duration of each movement; and the tonal features are given by the extracellular markers. Nucleokinesis The neuronal migration is one of the most beautiful and dynamic process in the Nervous System, and because of its While the leading process determines the direction in which complexity, it is vulnerable to a broad range of disruptive the neuron moves, the effective neuronal migration is only events such as mutations, infections and chemical insults. completed by the subsequent translocation of the cell soma. Learning about the molecular interactions that regulate Soma translocation occurs in the following way: first embryonic neuronal migration could help us to understand cytoplasmic dilatation appears in the proximal part of the more than the genesis of severe cortical malformations; it leading process, then the centrosome and the Golgi is a promissory way to aboard stem cell investigation in apparatus moves toward it; finally the nucleus advances neurodegeneration and neuronal plasticity. forward and invades the cytoplasmic dilatation.8 The movement of both the centrosome and the nucleus ACKNOWLEDGEMENT is dependent on the integrity of a microtubules network with different post-transcriptional modifications. Most of Supported by research grant PAPIIT IN207510-3 from DGAPA, to the microtubules surrounding the nucleus are tyrosinated, Mariana Flores for the illustrations of the manuscript. which make them extremely dynamic; by contrast, microtubules at the anterior pole of the nucleus, near the REFERENCES centrosome, are acetylated and therefore more stable. It has been hypothesized that the microtubule array surrounding 1. Bear MF, Connors BW, Paradiso MA. Neuroscience. Exploring the bra- the nucleus is connected with the centrosome, which is the in. Segunda edición. Baltimore: Lippincott Williams & Wilkins; 2001. main microtubule-organizing center. In addition, several 2. Casanova MF, Trippe J. Regulatory mechanisms of cortical laminar de- velopment. Brain Res Rev 2006;51:72-84. proteins involved in cell polarity have been described to 3. Kriegstein A, Noctor S, Martínez-Cerdeño V. Patterns of neuronal stem play central roles in coordinating the movement of the and division may underlie evolutionary cortical expan- 8 centrosome and the nucleus in each migratory cycle. sion. Nat Rev Neurosci 2006;7:883-890. Another type of factors that need to be revised here are 4. Supèr H, Soriano E, Uylings HBM. The functions of the preplate in de- the core-cell cycle regulators. The cell cycle coordination velopment and evolution of the neocortex and hippocampus. Brain Res Rev 1998;27:40-64. requires both positive and negative regulators. Cip/Kip and 5. Kriegstein AR, Noctor SC. Patterns of neuronal migration in the embr- INK4 CKIs are negative regulators. In addition to their roles yonic cortex. TINS 2004;27:392-399. as CKIs, Cip/Kip proteins regulate cell motility and 6. Tissir F, Lambert de Rouvroit C, Goffinet AM. The role of reelin in the migration by facilitating actin cytoskeleton rearrangement. development and evolution of the cerebral cortex. Braz J Med Biol Res Cip/Kip proteins promote cell motility and migration by 2002;35:1473-1484. 7. Luhmann HJ, Hanganu I, Kilb W. Cellular physiology of the neonatal inhibiting the Rho signaling pathway. Another core-cell cycle rat cerebral cortex. Brain Res Bull 2003;60:345-353. regulators that regulate neuronal migration are Rb and E2F 8. Valiente M, Marín O. Neuronal migration mechanisms in development transcription factors; Rb’s major function in the cell cycle is and disease. Curr Opin Neurobiol 2010;20:68-78. to sequester and inhibit E2F transcription factors in order to 9. Hatanaka Y, Murakami F. In vitro analysis of the origin, migratory be- control the timing of the DNA replication. Rb loss of function havior, and maturation of cortical pyramidal cells. J Comp Neurol 2002;454:1-14. elicits radial and tangential migration defects.16 10. Chédotal A, Rijli FM. Transcriptional regulation of tangential neuro- Finally, neurons reach their final destination, so they nal migration in the developing forebrain. Curr Opin Neurobiol need to cancel their migratory program and continue their 2009;19:139-145.

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11. Keays DA. Neuronal migration: unraveling the molecular pathway with or ARX in cell proliferation and neuronal migration during corticogene- humans, mice, and fungus. Mamm Genome 2007;18:425-430. sis. J Neurosci 2008;28:5794-5805. 12. Nadarajah B, Brunstrom JE, Grutzendler J, Wong RO et al. Two modes 15. Metín C, Vallee RB, Rakic P, Bhide PG. Modes and mishaps of neuronal of radial migration in early development of the cerebral cortex. Nat migration in the mammalian brain. J Neurosci 2008;28:11746-11752. Neurosci 2001;4:143-150. 16. Frank CL, Tsai LH. Alternative functions of core cell cycle regulators in 13. Marín O, Rubenstein JLR. Cell migration in the forebrain. Annu Rev neuronal migration, neuronal maturation, and synaptic plasticity. Neu- Neurosci 2003;26:441-483. ron 2009;62:312-326. 14. Friocourt G, Kanatani S, Tabata H, Yozu M et al. Cell autonomous roles

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