From genes to human diseases incortical developmentGaelle Friocourt

In the last fifteen years, spectacular advances in molecular

genetics have permitted rapid progress in the exploration of

cerebral cortical development. In particular, major strides

have been made in the identification of genes involved in

cortical neurogenesis, fate determination, neuronal migration

and differentiation, and in the formation of connections.

In addition, transgenic mice, which make possible powerful

loss- and gain-of-function analysis, are increasingly used to

understand brain developmental disorders. However, some

human diseases, such as schizophrenia, mental retardation,

autism or dyslexia have proven difficult to model withmice and

although numerous susceptibility genes in humans have been

identified, there is still much to do to understand the

pathophysiology of these disorders. This was the subject of

themeeting that took place betweenFebruary 6th and 8th at the

Novartis Foundation in London. About 30 scientists from

around the world gathered to discuss and assess the latest

findings and their implications.

Defining the developmental mechanisms that govern area

patterningof theneocortex (‘‘arealization’’) is an issue thatwas

addressed by several speakers at this symposium. Several

studies have indicated that signallingmolecules, such asFgf8,

expressed by patterning centres, operate by generating the

graded expression of several transcription factors (Emx2,

COUP-TF1, Lhx2, Foxg1 and Pax6) in cortical progenitors

across the neocortex. To better understand how these

signalling molecules act, John L. Rubenstein (University

of California) has studied hypomorphic mouse mutants for

Fgf8. He showed that decreasing the expression of Fgf8 can

have profound effects on the relative size and the nature of

telencephalic subdivisions, and that Fgf8 mediates complex

interactions between rostral and dorsal patterning centres. He

also presented data about the Fgf17 knock-out mouse line.

Thesemiceareviable andmaintainFgf8expression.However,

they present hypoplasia of the dorsal prefrontal cortex and a

rostral expansion of the sensory cortex. Intriguingly, these

mice show selective behaviour defects related to social

interactions.

Dennis D. O’Leary (The Salk Institute, California)

presented the results of gain- and loss-of-function experi-

ments for the Emx2 gene, and showed that the expression of

Emx2 under the control of the nestin promoter increased the

size of the area V1, at the expense of M1 and S1. These data

suggest thatEmx2 specifies the area identity of neurons in the

cortical plate. Although the altered areas showed normal gene

expression and functional organisation, mice that had an

expanded V1 area displayed impaired behaviour in several

tests, suggesting that the area size is critically related to the

performance of such modality-specific behaviours. These

findings may have significance for human behaviour and may

be relevant to some cognitive disorders.

ArnoldR.Kriegstein (University ofCalifornia) reviewed

the latest findings on progenitor cells in cortical development.

Radial glial cells, in the ventricular zone (VZ) of the developing

cortex, have been shown to generate neurons and appear to

do so by asymmetric cell divisions. In addition, he described a

two-step pattern of neurogenesis in the rat developing cortex.

First, radial glial cells undergo asymmetric division, generat-

ing, with each division, one self-renewing radial glial cell and a

daughter progenitor cell, called an ‘‘intermediate progenitor

cell’’. The latter cells migrate to the overlying subventricular

zonewhere theygenerate neurons by symmetric divisions and

thus increase cell number within the same cortical layer. This

model could explain the generation of cell diversity and cell

number in the developing cortex.

Similarly, Henry Kennedy (INSERM, Lyon) presented

data about cell division as a mechanism of area patterning in

primates. His group studies the monkey cortical areas 17 and

18, which show striking cytoarchitectonic differences, as a

model system for studying how developmental events estab-

lish the boundaries of different areas. The cortex of the visual

area 17 has more cells per radial unit than the adjacent

area 18. Kennedy showed that, in this case, the variation in

the generation rate of cortical neurons is determined by

changes in cell cycle duration and the mode of division of

cortical precursors.

INSERM U613, Universite de Bretagne Occidentale, Faculte de

Medecine de Brest et des Sciences de la Sante, Centre Hospitalier

Universitaire (CHU) Brest, Hopital Morvan, 46 rue Felix Le Dantec,

29200 Brest, France. E-mail: gaelle.friocourt@univ-brest.fr

DOI 10.1002/bies.20603

Published online in Wiley InterScience (www.interscience.wiley.com).

706 BioEssays 29.7 BioEssays 29:706–709, � 2007 Wiley Periodicals, Inc.

Meetings

Christopher A. Walsh (Harvard Medical School)

discussed mechanisms controlling the shape and size of the

human cerebral cortex. Genes involved in microcephaly in

the mouse or human can be divided into several categories:

(1) genes acting on the localisation of cell fate determinants, in

particular, b-catenin or Pals1, (2) genes playing a role in

vesicular trafficking, such asARGEF2,COH1, and aSnap (thelatter identified via the hyhmutation) and (3) genes regulating

the position of mitotic spindle, such as ASPM, CDK5RAP2,

CENPJ and Nde1. In addition, molecular evolutionary data

indicate that some subtle sequence changes in both ASPM

and microcephalin genes had been subjected to positive

selection in the lineage leading to humans, suggesting that

the study of these genes can help not only for a better

understanding of some human diseases but also to give us

new insight into human evolutionary history.

Previous studies probing the site of origin of cortical

neurons demonstrated that, whereas cortical pyramidal

neurons are generated within the germinal zones of the dorsal

telencephalon, most, if not all, cortical interneurons derive

from the ventral telencephalon. Cortical interneurons repre-

sent a heterogeneous population of cell types, classified

according to their electrophysiology properties, their morphol-

ogy or their immunohistochemical content. To explorewhether

this diversity is controlled by intrinsic genetic programs,Gord

Fishell (New York University Medical Center) has used a

combination of gene loss-of-function analysis and fate

mapping. His findings suggest that both the spatial and

temporal origins of interneuron precursors predict the intrinsic

physiological properties of mature interneurons. For example,

at E13.5, the medial ganglionic eminence gives rise to fast-

spiking interneuronswhereas the caudal ganglionic eminence

generates predominantly regular-spiking interneurons. More-

over, he showed that the birth date of interneurons predicts

both their laminar fate and their subclass. Further studies are

still needed, however, to fully understand the respective roles

of intrinsic versus extrinsic factors that control the cortical

interneuron identities.

Fujio Murakami (Osaka University) presented several

very nice time-lapse movies showing the complexity of

movement of cortical interneurons once they reach the cortex.

Using either glutamate decarboxylase (GAD)67-GFPknock-in

mice or an electroporation-based gene transfer of DsRed into

the ganglionic eminence of mouse embryos, he showed that

the marked interneurons migrate in all directions within the

tangential plane, both in the marginal zone and the VZ. This

multidirectionalmigration of cells occurs throughout the cortex

over distances of up to 3 mm during a period of a few days. He

hypothesised that this migratory behaviour may contribute to

thedispersing and intermixingof cortical interneuron subtypes

and that it may therefore be important for achieving the

final and even distribution of interneuron subtypes throughout

the cortex.

PaskoRakic (Yale University) reviewed the role of newly

identified genes involved in radial neuronal migration. For

example, Numb and E-cadherin are important for maintaining

adherens junctions and promoting radial glial cell polarity.

Some recent studies from his group also demonstrate that

Notch knock-out mice have a similar phenotype to the reeler

mice and that stimulation of the Reelin signalling pathway

inhibits ubiquitinylation of active Notch. Similarly, the exocyst

component Exo70, which presumably tethers vesicles to

specific sites at the plasma membrane, is also required for

radial migration, as shown by the use of a dominant negative

allele of this gene. Recently, theMEKK4 pathway was found to

be involved in the initiationof radialmigration, by regulating key

modulators of the actin cytoskeleton. Another factor is calcium

fluctuations, which have been known to influence the rate of

migration for some time. New data fromRakic’s group confirm

this role by demonstrating that RNAi mediated-knockdown of

connexin-26 delays migration, possibly by impairing the

movement of the nucleus. Finally, SPARC-like 1 has been

found to be necessary to terminate neuronal migration, by

reducing neuron adhesion to the radial glia at the top of the

cortical plate. The identification and study of such factors,

involved in neuronalmigration,may help illuminate the basis of

several human cortical disorders. Indeed, even subtle mis-

positioning of cortical neurons is increasingly suspected to be

responsible for certain formsofmental retardation, autismand

epilepsy.

Seong-Seng Tan (University of Melbourne) created

genetic mosaics, introducing Reelin-positive cells in reeler

mice, to study the role of Reelin in cortical development.

Whereas chimeraswith a strongcontribution ofReelin-positive

cells had normal neuronal migration and layering, weak

chimeras displayed a secondary cortex in mirror inversion to

an adjacent normal cortex. Chimeras forDab1 showed similar

results. The presence of Dab1-positive cells failed to rescue

the inversion of cortical layers, suggesting that Dab1 functions

cell-autonomously with respect to radial migration and cortical

layering of pyramidal neurons. In contrast, p35 chimeras also

presented a secondary cortex, but displayed a partial rescue

of some layers, suggesting that p35 signalling can have both

cell-autonomous and non-autonomous consequences. In

addition, Tan presented some data suggesting that Reelin

affects both neuronal positioning and neuronal cohesion,

probably via its repelling action on migration.

Beside the intrinsic control of brain patterning by specific

genes, another major extrinsic source of patterning is provided

by the thalamocortical axons that convey specific information

from the sensory periphery to the neocortex and

impose functional specialisations on primary sensory areas.

Nobuhiko Yamamoto (Osaka University) is interested in

identifying the molecular mechanisms that underlie the

specific targeting of thalamocortical axons to layer IV in the

cerebral cortex. His group has focused on cell surface

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proteins expressed in the developing cortex, using a novel

protein-printing technique, which can mimic in vivo laminar

distributions of the candidate proteins. Using subtractive

cDNA libraries and clues from the literature, they isolated a

few proteins such as ephrin A5 or semaphorin 7A. In the

second part of his talk, he also showed that neuronal activity

can influence lamina-specific thalamocortical branching.

ZoltanMolnar (University of Oxford) reviewed the progress

made in the understanding of the role of the preplate and the

layers V and VI of projection neurons in the early intracortical

and extracortical circuitry. Several studies have shown that

defects in preplate splitting lead to aberrant thalamic axon

trajectories as shown in reeler or p35 mutant mice.

Andre M. Goffinet (University of Louvain Medical

School) presented data about the planar cell polarity (PCP)

genes, and in particular Celsr3, the murine orthologue of

Drosophila flamingo/starry night, which encodes a large

seven-pass transmembrane cadherin. Celsr3 mutant mice

die at birth of central hypoventilation and have major

anomalies of major tracts, in particular an absence of the

anterior commissure and of all components of the internal

capsule. A similar phenotype is generated by inactivation of

Frizzled3 (Fzd3), making Celsr3 and Fzd3 the first identified

members of a genetic pathway that controls the establishment

of the axonal blueprint. In order to study this pathway in

more detail, his group has generated two conditional Celsr3

knock-outs, inactivating Celsr3 in the forebrain and in the

cerebral cortex, by using mice that express Cre under the

Foxg1 andEmx2 promoters, respectively. They observed that,

although there was an absence of thalamocortical connec-

tions in Foxg1-Cre mice, the cortex was fine and the

hippocampus was almost normal. This knock-out thus

provides a model of cortical maturation in the absence of

extrinsic connections.

Linda J. Richards (University of Queensland) reviewed

latest data about corpus callosum (CC) formation and the

genes involved in this process, in particular Emx1 and the

nuclear factor I (Nfi) genes. Three of the four members of

the NFI gene family (Nfia, Nfib and Nfix) are involved in brain

patterning, glial development, cortical cell migration and axon

guidance. Mutations of these genes in the mouse all lead to

defects of theCC.Althoughdiscrepancies exist in the literature

about the description of CC defects in Emx1 mutant mice,

several studies now strongly implicate it as involved in CC

formation. These findings may help the identification of new

genes involved in CC agenesis in human.

Peter B. Crino (Hospital of the University of Pennsylva-

nia) described very interesting approaches to characterise

the molecular pathogenesis of focal cortical dysplasia (FCD).

FCD with balloon cells (FCDIIb), hemimegalencephaly

(HMEG) and ganglioglioma (GG) are sporadic focal mal-

formations of cortical development that are highly associated

with epilepsy. Histologically, these three malformations are

characterised by disordered cortical lamination and the

presence of markedly enlarged cell types that are similar to

giant cells in the tuberous sclerosis complex (TSC). Recent

work has shown that there is enhanced activation of two

important cell signalling pathways, the mTOR cascade and

the Wnt/b-catenin pathway in TSC, FCD, HMEG and GG,

suggesting a common pathogenesis for these disorders. To

investigate these malformations, Crino’s group has used two

different strategies: (1) targeted cDNA arrays from single

microdissected-cells, permitting identification of genes that

are differently regulated in patients versus normal brains, or

more specifically in giant cells, and (2) the use of SNP arrays

and gene sequencing to identify mutations in candidate genes

that could lead to the activation of one or the other above-

mentioned pathways. These strategies have allowed them to

identify relevant germline and somatic mutations and to study

the pathophysiology of these malformations associated with

aberrant cell size.

Paul J. Harrison (University of Oxford) presented new

data on susceptibility genes for schizophrenia. The cause of

schizophrenia is unknown, but it has a significant genetic

component. Based on epidemiological and neurobiological

evidence, this disease was first described as a neurodevelop-

mental disorder more than 20 years ago. Since then, a few

susceptibility genes have been identified, in particular neur-

egulin 1 (NRG1), DISC1 or dysbindin. NRG1 is a member of a

family of structurally related glycoproteinswithmultiple roles in

the central nervous system, and has several isoforms.

Variation in isoforms expression has been hypothesised as a

molecular mechanism for the genetic association of NRG1

with schizophrenia. In addition, mutant mice heterozygous for

either Nrg1 or its receptor, ErbB4, showed a behavioural

phenotype that overlapped with mouse models of schizo-

phrenia. Although the neurobiology of NRG1 is not fully

understood, several studies suggest that it involves glutamate

and dopamine neurotransmitter systems.

Jeffrey D. Macklis (Harvard Medical School) focused

on corticospinal motor neurons (CSMN), which are located

primarily in cortical layer V. Degeneration of these neurons is a

key component of motor neuron degenerative diseases,

including amyotrophic lateral sclerosis (ALS). A better under-

standing of the molecular control over CSMN development,

including neuron-type-specific differentiation, survival and

connectivity should help to develop strategies to prevent

neuronal death and enhanced neurogeneration, as well as

possibly identifying new disease genes. His group purified

CSMN and two closely inter-related neuronal subtypes

(callosal projection neurons and corticotectal projection

neurons) from murine neocortex at four distinct stages of

development. Using microarrays, they identified genes

that are specifically expressed in CSMN neurons, such

as Ctip2 or Fezl. Loss-of-function in null mutants or over-

expression experiments confirmed the role of these genes

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708 BioEssays 29.7

in the specification of CSMN neurons, controlling early

decisions regarding lineage-specific differentiation from

neural progenitors.

The meeting ended with a few words from everyone, on

what he/she thought were the most-important questions for

the future. Many emphasized the fact that our understanding

of brain development and human cortical malformations is

now limited by the lack of data that we have concerning the

targets and the pathways regulated by several transcription

factors important for brain patterning. It is expected that many

groups will focus in the coming years on the identification of

these pathways.

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