Neurodesarrollo - Migración neuronal

Available online at www.sciencedirect.com

Neuronal migration mechanisms in development and disease
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Manuel Valiente and Oscar Marın

Neuronal migration is a fundamental process that determines molecular requirements and whose disruption causes
the final allocation of neurons in the nervous system, specific neurological syndromes [2,3].
establishing the basis for the subsequent wiring of neural
circuitry. From cell polarization to target identification, neuronal Recent work has shed light on the molecular mechanisms
migration integrates multiple cellular and molecular events that regulating each of the different aspects of the migratory
enable neuronal precursors to move across the brain to reach cycle of neurons, the topic of the present review. We will
their final destination. In this review we summarize novel also summarize recent findings illustrating how abnormal
findings on the key processes that govern the cell biology of neuronal migration may cause neurological disorders.
migrating neurons, describing recent advances in their The transcriptional mechanisms regulating neuronal
molecular regulation in different migratory pathways of the migration will not be addressed in this article, as it has
brain, spinal cord, and peripheral nervous system. We will also recently been reviewed elsewhere [4,5].
review how this basic knowledge is contributing to a better
understanding of the etiology and pathophysiology of multiple Leading process dynamics
neurological syndromes in which neuronal migration is Real time imaging experiments have begun to shed light
disrupted. into the dynamic behavior of the leading process in
migrating neurons. Somehow surprisingly, we have dis-
Address covered that the leading process of migrating neurons has
Instituto de Neurociencias, Consejo Superior de Investigaciones relatively distinct morphologies in different classes of
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Cientıficas & Universidad Miguel Hernandez, 03550 Sant Joan d’Alacant,
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neurons, probably reflecting their adaptation to specific
Spain
microenvironments. In contrast to the simplicity of the
Corresponding author: Marın, Oscar (o.marin@umh.es)
´ single leading process reported for radially migrating
pyramidal cells [6], tangentially migrating neurons dis-
play very elaborated leading process morphologies. This
Current Opinion in Neurobiology 2010, 20:68–78 is the case, for example, of precerebellar neurons, cortical
This review comes from a themed issue on
interneurons, or immature enteric neurons, all of which
Development have branched leading processes [7–10]. In these cells,
Edited by Francois Guillemot and Oliver Hobert leading process branches are stabilized or retracted in
binary decisions that determine the direction followed by
Available online 5th January 2010
the nucleus during nucleokinesis [11] (Figure 2), and this
0959-4388/$ – see front matter process is intimately linked to their directional guidance
# 2009 Elsevier Ltd. All rights reserved. [8,11,12]. Thus, guidance cues influence the frequency
DOI 10.1016/j.conb.2009.12.003
and orientation at which new leading process branches
emerge, which allow migrating neurons to rapidly change
direction without having to reorient pre-existing
branches. This guidance mechanism is different from
Introduction that described for growing axons, in which growth cone
Instructed by extracellular cues, the activation of gui- steering determine the direction of movement [13].
dance receptors and their downstream signaling pathways
enable newborn neurons to migrate through the devel- It has been suggested that leading process branching
oping nervous system until they reach their destination. represents a general, default strategy for neuronal
The migratory cycle of neurons involves leading process migration, which might be suppressed under certain
dynamics, through which directional migration is circumstances, for example, during glial-guided radial
achieved, and somal translocation, which involves the migration [14]. Consistent with this view, the leading
movement of perinuclear material and organelles as well process of pyramidal cells become progressively unipolar
as the nucleus. Different classes of neurons adjust and as neurons transit from the progenitor region toward the
modify this basic migratory cycle depending on the intermediate zone (Figure 1), where they start actively
specific requirements of their migratory pathway, which interacting with radial glial fibers [1,3]. Moreover,
may also change through time. For example, pyramidal preventing the normal interaction of pyramidal cells
cells transit through at least three different phases during with radial glial fibers increases the number of leading
their migration toward the cortical plate [1] (Figure 1). process branches in these cells [15] (Figure 1). One
During these stages, pyramidal cells adopt different of the proteins that may regulate this process is the
morphologies, each of which probably involves distinct small GTP-binding protein Rnd2 [16]. Rnd2 silencing

Current Opinion in Neurobiology 2010, 20:68–78 www.sciencedirect.com

Neuronal migration mechanisms in development and disease Valiente and Marın 69
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Figure 1

Regulators of cell morphology during glial-guided migration. Radial glia cells present in the ventricular zone (VZ) are the progenitors of cortical
projection neurons and extend a long basal process that ends in the pial basement membrane over the marginal zone (MZ). Newborn neurons polarize
and begin their migration close to the progenitor radial glia cell (1). At the subventricular zone (SVZ), however, migrating neurons halt their progression
toward the cortical plate (CP) and modify their morphology (2), becoming multipolar cells. Cells at this state are characterized by the generation of
multiple processes all around the cell and can make small tangential displacements. Eventually, cells polarize again, acquire a clear bipolar
morphology (3), and restart their migration through the intermediate zone (IZ) and into the CP. During this later phase, projection neurons migrate
closely attached to the basal process of radial glia cells. Interaction with the cellular scaffold of radial glia is mediated, among other molecules, by
connexins Cx26 and Cx43. The coupling of the nucleus (n) and the centrosome (c) is also schematically depicted in these drawings. The distance
between the nucleus and centrosome (white dashed line) changes during migration owing to the forward displacement of the centrosome and the
saltatory movement of the nucleus. Knockdown of several regulators of the morphology of pyramidal cells impairs their migration. For example, RNAi
against Cx43 or Cx26 disrupts the progression of projection neurons from the multipolar stage, probably owing to a failure in their ability to interact with
radial glial fibers. Of note, cells that manage to progress toward the CP develop a leading process with multiple branches. Rnd2 knockdown causes a
similar phenotype, although it is presently unclear how this small GTP-binding protein (expressed by cells in the SVZ/IZ transition) controls this
process. Other proteins that regulate the stability of microtubules (e.g., p600, MARK2/Par-1, and Lkb1) are essential for the progression of neurons
from the multipolar stage into glial-guided migration. Loss of p600 leads to abnormal development of the leading process, which becomes thin and
wavy. Reduction in the levels of MARK2/Par-1 or Lkb1 causes different defects in centrosome–nucleus coupling. Axons of migrating and positioned
neurons were not drawn for simplicity. For axonal phenotypes see corresponding references.

perturbs the transition of pyramidal cells from the multi- tural support to the leading process, but also allows the
polar stage to the unipolar stage, and even those cells that ﬂow of vesicles required for intracellular communication.
manage to polarize in the absence of Rnd2 have many This process seems to be directly regulated by p600 (also
more leading process branches than normal cells known as Ubr4), a novel microtubule associated protein
(Figure 1). (MAP) that interacts with the endoplasmic reticulum
(ER). Knockdown of p600 in pyramidal cells leads to a
Leading process stability depends on a system of longi- prominent reduction in acetylated tubulin and an almost
tudinal microtubules that links the leading edge of the complete loss of the ER in the leading process, which
cell with the soma. This system not only provides struc- acquires a wavy appearance (Figure 1) and disrupts

www.sciencedirect.com Current Opinion in Neurobiology 2010, 20:68–78

70 Development

Figure 2

The cell biology of neuronal migration. (A) Schematic drawing of representative structures in a migrating interneuron. A high magnification of the
nucleus and associated cytoskeleton and organelles is also depicted. Note that actin and Myosin II are enriched at the rear part of the nucleus and at
the proximal aspect of the leading process. (B) Schematic of the migratory cycle of cortical interneurons. The cycle starts with the extension of a
branch of the leading process (green arrow) and the forward movement of the perinuclear dilatation or swelling, containing the Golgi apparatus and the
centrosome (blue arrow). Extension and retraction of different branches of the leading process coexist until one of them is selected. At this time, the
nucleus moves forward until the branch point (yellow arrow), which represents the main static point for the cell in the entire cycle. Non-selected
processes retract while a new process starts to grow, and the cycle is repeated. (C) A high magnification schema of nucleus and cytoskeleton during
nucleokinesis. tyr-Tub, tyrosinated tubulin; ace-tub, acetylated tubulin; and t, time.

migration [17]. These experiments reinforce the notion cerebellar granule cells [18]. First, a cytoplasmic dilata-
that microtubule stability and membrane trafﬁcking are tion or swelling appears in the proximal part of the leading
required for leading process maintenance. process, toward which the centrosome and the Golgi
apparatus moves. Second, the nucleus advances forward
The signaling mechanisms that control leading process and invades the cytoplasmic dilatation (Figure 2).
branching are presently unknown, although recent work
in cerebellar granule cells suggests that this process is The movement of centrosome and nucleus is highly
modulated by intracellular Ca2+ and cAMP levels. Thus, dependent on the integrity of a rich network of micro-
experimentally increasing the concentration of intracellu- tubules with different post-transcriptional modiﬁcations
lar Ca2+ from extracellular sources or internal organelle that extends between both [18,19,20,21,22]. Most
deposits or stimulating the adenylyl cyclase promotes microtubules surrounding the nucleus are tyrosinated,
leading process branching, while reducing the intracellu- which makes them highly dynamic. By contrast, micro-
lar Ca2+ concentration or inhibiting PKA decreases its tubules at the anterior pole of the nucleus, near the
frequency [12]. These results suggest that guidance cues centrosome, are acetylated and therefore more stable.
regulating intracellular Ca2+ and cAMP levels are likely Although the function of these two types of microtubules
regulators of leading process branching during neuronal is not completely understood, they seem to play different
migration. roles in somal translocation [20,21].

Molecular regulation of nucleokinesis It has been hypothesized that the microtubule network
While the leading process determines the direction in surrounding the nucleus is connected with the centro-
which the neuron moves, effective neuronal migration is some, which is the main microtubule-organizing center
only achieved by the subsequent translocation of the cell (MTOC) in these cells. Consequently, it was suggested
soma, including the small organelles and the nucleus. that the microtubule-dependent pulling forces on the
Soma translocation occurs in two consecutive phases in nucleus converge at the centrosome [18,23,24]. Recent
most neurons studied so far, as initially described in experiments, however, have put this hypothesis into


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question [21]. Analysis of migrating granule cells in enriched in the proximal region of the leading process
organotypic slices, a preparation that resembles the of migrating cerebellar granule cells [32]. Rapid F-actin
microenvironment found in vivo, suggests that most peri- turnover powered by Myosin II in this region appears to
nuclear microtubules are not anchored to the centrosome contribute to the forward movement of the centrosome,
but rather extend into the leading process. In addition, which in turn facilitates the subsequent displacement of
the authors frequently observed that the nucleus in these the nucleus [32]. These results are nevertheless contro-
cells advance forward pass the centrosome, which would versial, because previous experiments in pyramidal cells
argue against a role of this structure in pulling the led to the suggestion that centrosome movement is inde-
nucleus. Further experiments would be needed to elu- pendent of Myosin II function [19].
cidate whether these observations represent a specializ-
ation of granule cells or indeed define a general principle Recent studies suggest that signals emanating from the
for migrating neurons. cell surface influence centrosome–nucleus coupling, and
therefore somal translocation. During cerebellar devel-
MAPs and related motor proteins are important regulators opment, Sema6A is expressed in the deep aspect of the
of the movement of organelles. RNAi experiments have external granule layer, where it promotes the inward
shown that Lissencephaly 1 (Lis1) and the related motor migration of granule cells through its PlexinA2 receptor
protein dynein are both independently required during [33]. Interestingly, disruption of Sema6A/PlexinA2 sig-
centrosome progression and nucleokinesis [19]. In naling impairs centrosome–nucleus coupling, which leads
addition, several proteins involved in cell polarity have to defective migration [33]. Although the precise mech-
been described to play important roles in coordinating the anisms linking PlexinA2 activation with the cytoskeleton
movement of the centrosome and the nucleus in each are currently unknown, these results nicely illustrate that
migratory cycle. Par6a, for instance, localizes to the guidance and somal displacement are probably coordi-
centrosome and is required for its forward movement nated downstream of surface receptors during neuronal
[18]. Similarly, loss of MARK2 (microtubule affinity- migration.
regulating kinase 2, also known as Par-1) perturbs centro-
some dynamics and blocks the transition of radially Adhesion during neuronal migration
migrating pyramidal cells through the multipolar stage Cell migration requires the dynamic regulation of
(Figure 1) [25]. MARK2 levels are inversely related to adhesion complexes between migrating cells and sur-
the stability of microtubules, and so loss of MARK2 rounding extracellular matrix (ECM) proteins. In many
function may impair migration owing to excessive micro- cell types, this process involves integrin-mediated
tubule stabilization. Interestingly, the lissencephaly- adhesion [34], but the function of this signaling system
associated gene Doublecortin (DCX) is a substrate of in neuronal migration has remained controversial. In the
MARK2, and loss of DCX function increases microtubule CNS, cell adhesion dynamics have been best character-
dynamics [26]. Another polarity protein, LKB1 (a Ser/ ized during glial-guided migration in developing cortical
Thr kinase related to Par-4), may also be involved in structures [35]. Early in vitro studies emphasized a role for
regulating the transition of migrating neurons through the neuronal a3b1 integrin receptors in mediating the inter-
multipolar stage (Figure 1) [27]. Knockdown of LKB1 in action of migrating pyramidal neurons with radial glial
migrating neurons impairs neuronal migration, probably cells [36]. Subsequent analyses of mouse mutants have in
owing to interfering with centrosome–nucleus coupling turn focused the attention to radial glial cells, as these
[27]. It should be noted, however, that genetic loss of cells and their endfeet anchorage to the meningeal base-
Lkb1 function does not lead to layering defects in the ment membrane are dramatically affected in the absence
cerebral cortex [28], and so the precise involvement of of b1 integrin signaling [37–39]. According to this later
this protein in neuron migration requires further exam- view, integrin function is only indirectly required for
ination. Nevertheless, disruption of Lkb1 leads to migrating neurons, which would otherwise migrate nor-
exuberant leading process branching, which is likely to mally in the absence of integrin-mediated adhesion. As
perturb neuronal migration [27,28,29]. migration on two-dimensional substrates naturally over-
emphasizes the role of adhesion, it is very likely that in
Independently of the precise function of MAPs and vivo studies may offer a much more precise idea of the
associated motor proteins in somal translocation, it is contribution of integrin signaling during neuronal
unlikely that they alone provide the forces required for migration. Nevertheless, mouse genetic analyses may
the movement of these structures. Several studies have have overlooked subtle defects in the motility of integrin
shown that the motor protein Myosin II is enriched mutant neurons [40], which may only cause a minor delay
behind the nucleus of migrating neurons and that in migration and not a major disruption of cortical layer-
pharmacological blocking its ATPase activity inhibits ing. Consistent with this notion, two recent studies
nucleokinesis, suggesting that rear contraction of acto- suggest that integrin signaling cell-autonomously influ-
myosin fibers propels the nucleus [30,31]. Recent work ence neuronal migration. Conditional deletion of
suggests that F-actin and Myosin II motors are also neuronal laminin g1, a component of the ECM deposited


72 Development

around migrating neurons and radial glial fibers that binds phosphorylation of another two tyrosine residues, the
integrin receptors, disrupts the migration of pyramidal cd sites (Y220 and Y232). Previous work has shown that
neurons [41]. In addition, specific removal of a3 integrins phosphorylation of the cd tyrosines is necessary to termi-
from tangentially migrating interneurons perturbs their nate glial-guided migration, because neurons expressing
normal distribution as they invade the embryonic cortex, non-phosphorylatable forms of these residues (Y220F and
suggesting that these receptors are required to recognize Y232F mutants, respectively) fail to downregulate the
specific guidance cues present in the migratory route of expression of a3 integrin in the cortical plate, and con-
interneurons [42]. sequently remain attached to the radial glia [51]. This
function might be mediated by other elements in the
Recent studies have provided a new perspective to the signaling cascade, such as the adaptor proteins Crk and
role of cell–cell adhesions during radial migration. The Crk-like (CrkL) and the Ras family guanine nucleotide
Gap junction proteins Connexin 26 and 43 are expressed exchange factor C3G, because their recruitment to the
at the contact points between radial glial fibers and Dab1 complex also requires intact cd sites [50]. Moreover,
migrating neurons during glial-guided migration loss of these proteins causes phenotypes that are similar to
[15,43] (Figure 1). Gap junctions are best known for those described in reeler [52,53] (Figure 3). In sum, Dab1
their channel capabilities, which allow the electrical and seems to operate both as an activator and scaffold protein
chemical coupling of cells or the release of small mol- through the phosphorylation of the ab and cd domains,
ecules to the extracellular space. During radial migration, respectively, and both functions are required for normal
however, Gap junctions provide dynamic adhesive con- cortical development in vivo.
tacts between neurons and radial glial fibers that enable
the stabilization of the leading process of migrating Recent work has revealed additional elements in the
neurons along radial glial processes [15]. Very little is signaling cascade downstream of Dab1. Biochemical
known about the mechanisms regulating Gap adhesion experiments suggest that activated Dab1 prevents the
dynamics during radial migration, but recent work degradation of the intracellular, active form of Notch in
suggests that the C-terminal tail of connexins is required migrating pyramidal neurons [54]. It is presently unclear
in this process [44]. This domain has been shown to how this process works, since Reelin signaling also pro-
regulate mobility in other cell types, and it is known to motes the degradation of Dab1 as part of a feedback loop
interact with several cytoskeleton proteins, such as zona that exists in the pathway [55,56,57]. In any case,
occludens-1 (ZO-1) and N-cadherin. Notch-ICD is cell-autonomously required for the normal
positioning of neurons both in the neocortex and in the
News from the Reelin front dentate gyrus [54,58], suggesting that one of the
Reelin is crucial for the lamination of cortical structures possible functions of Reelin signaling in cortical devel-
[45], but the molecular mechanisms underlying its action opment would be to modulate Notch signaling (Figure 4).
remain unclear [46]. Genetic studies have positioned This would be consistent with the hypothesis that Reelin
Reelin, ApoER2, Vldlr and Dab1 into a common signaling functions as a signal that incites migrating neurons to
pathway that leads to the phosphorylation of Dab1 in adapt their cytoskeleton during glial-guided migration
migrating neurons (Figure 3), an event that is required for [59]. In Drosophila growth cones, for instance, Notch
normal layering of the cortex [47,48]. ApoER2 and Vldlr controls actin cytoskeletal dynamics through its inter-
are the canonical Reelin receptors, but their relative action with Abl [60]. Consistently, pyramidal cells lacking
contribution to neuronal migration is controversial. Notch display abnormal leading processes [54].
Recent experiments suggest that the neuronal positioning
of late-born pyramidal neurons is largely attributable to Halting migrations
ApoER2. On the contrary, lack of Vldlr causes ectopic Upon arrival to their final destination, neurons cancel
accumulation of pyramidal neurons into layer I, which their migratory program and continue their differentiation
suggest that both phenotypes (layering and invasion) are into mature neurons. Understanding the mechanisms
molecularly distinguishable [49] (Figure 3). through which neurons perceive that they have reached
their target position has been a puzzling question for a
The mechanisms underlying Reelin function in neuronal long time, but recent studies have begun to elucidate this
migration are only beginning to be elucidated. Recent issue [61,62]. It has been suggested that early patterns of
experiments have shown that Dab1 tyrosine phosphoryl- activity generated in the target region may influence this
ation sites have distinct signaling properties in vivo [50] process. Migrating interneurons, for example, sense ambi-
(Figure 3). Two of the putative tyrosine phosphorylation ent GABA and glutamate in their way to the cortex using
sites, named ab sites (Y185 and Y198), are sufficient for GABAA and AMPA/NMDA receptors [63]. At early stages
the recruitment of Src-family kinases (SFKs), phosphoryl- of cortical development, both neurotransmitters seem to
ation of Dab1 at these sites, stimulation of Akt/PI3K depolarize migrating cells, inducing intracellular Ca2+
signaling, and targeting activated Dab1 for degradation. transients that promote cell movement. Upon arrival to
The ab sites seem also important for the subsequent the cortex, interneurons upregulate the expression of the


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Figure 3

The genetics of Reelin signaling. Schematic drawings of the distribution of cortical pyramidal cells in wild type mice and in different mouse mutants
related to the Reelin signaling pathway. In wild type mice, early born pyramidal cells split the preplate in two layers, the marginal zone (MZ, light green
circles) and the subplate (sp, dark green circles), to form the cortical plate. Here, subsequent cohorts of pyramidal cells are organized according to an
inside-out sequence to progressively form layers VI to II (blue, yellow and red circles). The absence of Reelin (reeler mice), both Reelin receptors
(AporER2 and Vldlr) or the intracellular adaptor protein Dab1 all cause the same phenotype: the preplate does not split and the layering pattern is
grossly inverted. Mouse mutants for individual Reelin receptors have milder phenotypes than double mutants and reveal specific differences in their
function. Loss of ApoER2 leads to a phenotype that is similar to that found in the double receptor mutant, although milder. On the contrary, the
absence of Vldlr primarily affects the late-born pyramidal cells, which end up migrating all the way to the basement membrane in the MZ. Although
layering has been investigated in these mice, the preplate state was not specifically addressed; the schema thus depicts the most likely situation. The
function of Dab1 involves tyrosine phosphorylation by the Src-family kinases Fyn and Src. Mice with point mutations in all tyrosine residues
phosphorylated in Dab1 upon Reelin activation (Dab15F/5F) or with simultaneous loss of both kinases (FynÀ/À; SrcÀ/À) reproduce the phenotype found
in reeler or Dab1 mutants (scramble, yotari or Dab1À/À mice). When point mutations only affect the so-called ab residues (Y185, Y198), the phenotype
is very similar to complete loss of Dab1. By contrast, point mutation of the cd residues (Y220, Y232) does not impair preplate splitting and causes a
relatively milder layering phenotype than that found in Dab1 mutants. Biochemical experiments suggest that Crk, Crk-like (CrkL) and C3G are
downstream effectors of the Reelin signaling pathway, whose activation is dependent on Dab1 phosphorylation at the cd residues. Mice that are
homozygous for C3G hypomorphic alleles (C3Ggt/gt) fail to split the preplate and have inverted layering, but in addition show disruption of the basal
lamina (red line) and many neurons fail to migrate altogether. Conditional deletion of both Crk and CrkL using a Nestin-Cre mouse line leads prominent
cortical defects, with preplate splitting failure and roughly inverted layering. It should be noted that the layering defects in both C3Ggt/gt and CrkÀ/
À
;CrkLÀ/À double mutants requires a more detailed examination. Apo, ApoER2; IZ, intermediate zone; SVZ, subventricular zone; Vl, Vldlr; and VZ,
ventricular zone.

potassium/chloride (K+/ClÀ) exchanger KCC2, which important to understand the dynamics of KCC2 expres-
modifies their response to GABA from depolarizing to sion in interneurons, as these cells tend to invade the
hyperpolarizing. This later event leads to a reduction in cortex in different waves according to their birth date [65].
the Ca2+ influx, and since this is ultimately needed for
movement, the net result is that neurons end up halting Migration and disease
their migration [64]. According to this model, KCC2 Neurological syndromes caused by disruption of neuronal
expression determines the end of the migratory period for migration are among the most devastating human dis-
cortical interneurons, although the underlying mechan- orders (Table 1). They typically arise during mid-
isms remain to be clarified. To this end, it would be embryonic stages and are mostly due to genetic causes


74 Development

Figure 4

Notch signaling meets Reelin. The schemas depict the analysis of the laminar allocation of neurons born at E14.5 in the embryonic mouse cortex,
which are normally allocated to supragranular layers. In reeler of Dab1 mutants (in this particular case Dab15F/5F mice, which carry point mutations in all
tyrosine residues phosphorylated in Dab1 upon Reelin activation), many neurons born at E14.5 end up located in deep layers of the cortex. A very
similar phenotype is observed when Notch is inactivated in migrating pyramidal cells (Cre;Notch1F/F;Notch2F/F). Remarkably, reintroduction of the
activated intracellular domain of Notch (Notch-ICD) in pyramidal cells rescue the layering phenotype in both reeler and Dab15F/5F mice, suggesting that
Notch-ICD is downstream of Dab1 in the Reelin signaling pathway.

(for an extensive review see, for example, Refs. [2,66]). Migration disorders are considered untreatable, but
Recent studies have shown that de novo mutations in a recent ﬁndings have opened new venues for therapeutic
and b tubulin genes cause lissencephaly, pachygyria, and intervention [69]. Double cortex syndrome or subcor-
polymicrogyria syndromes in humans [67,68]. Analysis tical band heterotopia (SBH) is caused by mutations in
of orthologous mutations in mice revealed that they also the X-linked gene DCX. It is characterized by periven-
disrupt cortical neuronal migration, probably owing to tricular accumulation of a large population of neurons,
impaired formation of tubulin heterodimers. These which fail to migrate during embryonic development.
results reinforce the view that disruption of microtu- Remarkably, postnatal re-induction of the Dcx gene in
bule-based processes underlies a large spectrum of the cortex of a rat model of SBH (generated by in utero
neuronal migration disorders. RNAi against the Dcx) restores the ability of arrested cells

Table 1

Genes implicated in migration leading to human neurological syndromes

Gene Migratory defect Molecular defect Syndrome Ref.
Tuba3a Abnormal lamination of hippocampus Tubulin polymerization defects Lissencephaly, pachygyria [67]
and cortex
Tubb2b Accumulation of late-born neurons in Tubulin polymerization defects Polymicrogyria [68]
deep cortical layers (SVZ/IZ)
Gpr56 Basal lamina disruption in Defective interaction Polymicrogyria (BFPP) [70,71]
neocortex and cerebellum glial endfeet-pial membrane
Drd1 and Drd2 Disrupted tangential migration of Unknown Fetal cocaine syndrome [73]
cortical interneurons
Slc6a4 Disrupted tangential migration of Unknown Depression related to [74]
cortical interneurons polymorphism 5-HTTLP
in SLC6A4
Nrg1/ErbB4 Disrupted tangential migration of Decreased PI3K recruitment Schizophrenia [75,76]
cortical interneurons and activation
Disc1 Accumulation of late-born neurons Abnormal dynein function. Disrupted Schizophrenia, depression, [77–79]
in deep cortical layers (SVZ/IZ). Disc1–NDEL1 interaction bipolar disorder
Abnormal migration in hippocampus
App Accumulation of late-born neurons Abnormal Dab1 activation Alzheimer’s disease a [80]
in deep cortical layers (SVZ/IZ)
a
APP dysfunction in Alzheimer’s disease does not seem to involve neurodevelopmental mechanisms, but recent work suggests that APP might also
be implicated in the regulation of neuronal migration during development.


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to migrate into their proper layers in the cortex. Moreover, cited in this review. Regrettably, space was too limited to cover more than a
few selected topics and to cite all significant original articles. Work in our
this intervention rescues the phenotypic deficits linked to laboratory is supported by grants from Spanish Government SAF2008-
this model of SBH, including the epilepsy that is also 00770, CONSOLIDER CSD2007-00023, and the EURYI scheme award
associated to the human disorder [69]. (see www.esf.org/euryi) to OM. MV was supported by a fellowship from the
Generalitat Valenciana CTBPRA/2005/021.

Mutations in GPR56, an orphan G-protein-coupled re- References and recommended reading
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review, have been highlighted as:
(BFPP), a severe disorder characterized by mental retar-
dation and seizures. Analysis of the cerebellum of GPR56 of special interest
mouse mutants has revealed major morphological of outstanding interest
abnormalities [70], reminiscent of those described in
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5. ´
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Conclusions ´ ´
11. Martini FJ, Valiente M, Lopez Bendito G, Szabo G, Moya F,
Our knowledge of the mechanisms underlying the cell Valdeolmillos M, Marın O: Biased selection of leading process
´
branches mediates chemotaxis during tangential neuronal
biology of migration is rapidly expanding, in particular in migration. Development 2009, 136:41-50.
relation to nucleokinesis. There are, however, important In this manuscript, the authors demonstrate that tangentially migrating
interneurons (and probably many other cell types throughout the nervous
questions that remain to be addressed. For example, we system) continuously branch their leading processes as part of their
know very little about the mechanisms coordinating the migratory cycle. They also show that interneurons use binary choices
of leading process branches instead of leading process steering to
guidance of the leading process with the movement of the achieve directional migration.
nucleus. We also ignore the mechanisms that allow the
12. Kumada T, Jiang Y, Kawanami A, Cameron DB, Komuro H:
coordinated allocation of different classes of neurons to Autonomous turning of cerebellar granule cells in vitro by
the same region of the brain, as it is the case for the intrinsic programs. Dev Biol 2009, 326:237-249.
This study shows that intrinsic programs may dictate the behavior of
cerebral cortex. Finally, several genes linked to important migrating neurons in the absence of external cues, and that the later
neurological and neuropsychiatric disorders, such as influence migration by modulating Ca2+ influx and cAMP levels.
NRG1 [75,76], DISC1 [77–79], or the b-amyloid precursor 13. Lin AC, Holt CE: Local translation and directional steering in
protein APP [80], are required for neuronal migration, but axons. EMBO J 2007, 26:3729-3736.
we still ignore the relevance of these findings in the 14. Marın O, Valdeolmillos M, Moya F: Neurons in motion: same
´
disease process. Detailed analysis of all these questions principles for different shapes? Trends Neurosci 2006,
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should expand our knowledge of this central process in
the development of the nervous system. 15. Elias LA, Wang DD, Kriegstein AR: Gap junction adhesion is
necessary for radial migration in the neocortex. Nature 2007,
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This paper reveals that the adhesive properties of Gap junctions mediate
Acknowledgements the dynamic interaction between pyramidal neurons and radial glial fibers
´
We are grateful to members of the Marın, Rico, and Borrell labs for helpful during glial-guided migration. RNAi experiments suggest that both Cx26
discussions and comments. We apologize to colleagues whose work is not and Cx43 seem to be required for pyramidal cell migration. The precise


76 Development

mechanisms through which connexins interact with the cytoskeleton ing cell polarity in these cells (see Ref. [28] for an elegant analysis of the
remain to be elucidated, although the authors suggest that signaling function of LKB1 in this context), LKB1 seem to participate in the
through the carboxy-terminal domains of connexins is not required in this molecular machinery required for centrosome–nucleus coupling during
process. This later result is in conflict with reports from a different migration.
laboratory (see annotation in Ref. [44]).
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Besides providing the first example of the transcriptional regulation of the 29. Shelly M, Cancedda L, Heilshorn S, Sumbre G, Poo MM: Lkb1/
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the movement of the centrosome and the nucleus, and that their function Experimental evidence suggests that actomyosin forces are required for
in both processes is independent. Their experiments also suggest that the forward movement of the centrosome. This result disputes previous
myosin contributes to nuclear movement, but is dispensable for centro- findings on the role of myosin in this process (see annotation in Ref.
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from the Hatten laboratory (see annotation in Ref. [32]).
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Connexin43 in neuronal migration. J Neurosci 2009, demonstrate that downregulation of Dab1 is important for proper cortical
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