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A synaptic trek to autism
Thomas Bourgeron1,2
Autism spectrum disorders (ASD) are diagnosed on the basisof three behavioral features namely deficits in social
communication, absence or delay in language, andstereotypy. The susceptibility genes to ASD remain largely
unknown, but two major pathways are emerging. Mutations in
TSC1/TSC2, NF1, or PTEN activate the mTOR/PI3K pathway
and lead to syndromic ASD with tuberous sclerosis,
neurofibromatosis, or macrocephaly. Mutations in NLGN3/4,
SHANK3, or NRXN1 alter synaptic function and lead to mental
retardation, typical autism, or Asperger syndrome. The mTOR/
PI3K pathway is associated with abnormal cellular/synaptic
growth rate, whereas the NRXN–NLGN–SHANK pathway is
associated with synaptogenesis and imbalance between
excitatory and inhibitory currents. Taken together, these datastrongly suggest that abnormal synaptic homeostasis
represent a risk factor to ASD.
Addresses
1
Human Genetics and Cognitive Functions, Institut Pasteur, 25 rue duDocteur Roux, 75015 Paris, France
2
University Denis Diderot, Paris 7, Paris, France
Corresponding author: Bourgeron, Thomas (thomasb@pasteur.fr)

by the recurrent observation of macrocephaly in 10–30%
of the patients with ASD [2–4]. The head circumference
may be normal at birth, but during the first four years oflife, an overgrowth of the brain is observed [5,6]. The
nature of the macrocephaly — too many neurons, glial
cells, synapses, or larger cells — remains difficult to establish. However, studies on neurofibromatosis, tuberous
sclerosis, and Cowden/Lhermitte–Duclos syndromes
have provided interesting information on the link between abnormal growth rate and ASD [7]. These genetic
syndromes associate both susceptibility to ASD andmacrocephaly and are caused by mutations in the tumor
suppressor genes NF1, TSC1/TSC2, and PTEN [7]. In
tuberous sclerosis, mutations of TSC1/TSC2 induce cortical developmental malformations called tubers. Thesetubers were originally thought to be the cause of ASD
when their locations in the brain were overlapping areas
important for social communication and language. However, studies in mice showing that loss of Tsc1/Tsc2 or Pten
results in neuronal hypertrophy have led to the hypothesis that susceptibility to ASD was not because of the
tubers, but to an abnormal shape and size of the neurons[8,9].

Current Opinion in Neurobiology 2009, 19:231–234
This review comes from a themed issue on
Development
Edited by Takao Hensch and Andrea Brand
Available online 21st June 20090959-4388/$ – see front matter# 2009 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.conb.2009.06.003

Introduction
Autism affects about 0.7% of children and is characterized
by deficits in social communication, absence or delay inlanguage, and stereotyped and repetitive behaviors.
Beyond this unifying definition, lies a spectrum of disorders/conditions, ranging from severe impairments tomild personality traits. Autism spectrum disorders(ASD) are diagnosed before three years of age, a periodcharacterized by intense synaptogenesis in the humanbrain [1]. This review reports recent genetic and neurobiological findings that highlight two routes leading to
ASD: abnormal cellular/synaptic growth and imbalance
between inhibitory and excitatory synaptic currents.

Abnormal cellular/synaptic growth in ASD
The hypothesis that abnormal cellular/synaptic growth
may increase the risk of having ASD, was first suggested
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Interestingly, NF1, TSC1/TSC2, and PTEN act in acommon pathway as negative effectors of the rapamycin-sensitive mTOR–raptor complex (mTORC1), a
major regulator of cellular growth in mitotic cells [10].
Mutations are predicted to enhance the mTORC1 complex, a signal activated by a sequential kinase cascade
downstream of phosphoinositide-3 kinase (PI3K) pathway. This pathway may also be modulated by serotoninsince macrocephaly and abnormal behaviors are exacerbated in mice with both Pten and serotonin transporter
mutations [11]. A stimulating hypothesis proposed by
Kelleher and Bear, suggests that the increase of themTOR pathway could lead to abnormal synaptic function
owing to an excess of protein synthesis at the synapse[10].

Abnormal balance between inhibitory andexcitatory currents in ASD
The possibility that alteration of synaptic functions could
lead to ASD was first indicated by the phenotypic overlap
between autism, fragile X syndrome, and Rett syndrome[12,13]. In addition, the key role of the excitatory/inhibitory currents in ASD was further supported by the observation that 10–30% of patients with ASD have epilepsy[14]. The synaptic hypothesis was confirmed by theidentification of mutations affecting the postsynaptic celladhesion molecules Neuroligins (NLGN) in individuals
with ASD [15,16]. At the functional level, the mutations
Current Opinion in Neurobiology 2009, 19:231–234

232 Development

Figure 1

Schematic representation of the different phases of synaptogenesis in the human brain. During the first three years of life, an excess of cell/synaptic
growth rate and inhibitory currents could increase the risk of ASD. Mutations within the mTOR/PI3K pathway lead to an excess of synaptic/cell growth.
Mutations within the NRXN–NLGN–SHANK pathway lead to abnormal synaptogenesis and excess of inhibitory currents. The arrows entering the redzone illustrate the excess of synaptic/cell growth and inhibitory currents during early brain development.

were found to alter the property of the NLGN to trigger
synapse formation in cultured neuronal cells [17]. NLGN
mutations probably concern a limited number of cases(<1% of the individuals), but following these initial
results, mutations in other synaptic proteins such as
SHANK3, NRXN1, CNTNAP2, CNTN3/4, and PCDH9/
10 were identified in patients with ASD [18–25]. Interestingly, NRXN1 codes for the presynaptic binding partner of NLGNs, CNTNAP2 (Caspr2) possess strong
homology to NRXN and SHANK3 is a scaffolding proteinof the postsynaptic density that binds to NLGN andregulates the size and shape of dendritic spines [26].
Only limited data are available for understanding the roleof these proteins in the human brain, but studies using
neuronal cell culture and animal models have provided
crucial information. Firstly, NLGNs and NRXNs
enhance synapse formation in vitro [27], but are notrequired for the generation of synapses in vivo [28].
Therefore, NLGNs may not establish synapses, but may
contribute to the activity-dependent formation of neuralcircuits [29]. Secondly, NLGNs and NRXNs are emerging as central organizing molecules for excitatory glutamatergic and inhibitory GABAergic synapses in the
mammalian brain [30,31]. The mutant mice carrying a
R451C Nlgn3 mutation identified in two brothers with
ASD displays an increased number of GABAergic
synapses and inhibitory currents [32]. An imbalance of
inhibition and excitation was also observed in MeCP2
Current Opinion in Neurobiology 2009, 19:231–234

knockout [33] and in several mice proposed as model ofautism such as the Caps2 knockout [34] or mice subject toprenatal valproate treatment [35]. Interestingly, the link
between GABA function and spine pruning has been
identified during a critical period of brain development
when individual experience is essential for the normal
development of the neuronal network [36]. Therefore,
impaired inhibitory–excitatory balance can be manifest asa shifted critical period for brain development [37] or analteration of sensory processing, such as reduced gamma
oscillations in FMRP knockout mice [38] as seen also in
ASD [39]. Taken together, these results strongly suggestthat synapse homeostasis and specificity play an important role in the susceptibility to ASD.

Atypical neuronal networks in ASD
In the human cerebral cortex, the first synapses areevident at the 40th day after conception. Thereafter,
the rate of synapse formation and pruning exhibit distinct
phases, the most dramatic change takes place during theperinatal period (Figure 1). During the first three years oflife, synaptic contacts are formed, but only some will bestabilized. This selection process represents a key step inthe cognitive development of the child. The NLGN–
NRXN–SHANK pathway is probably required during
this stabilization phase of the synapse in response toneuronal activity. Strikingly, the role of the NLGN–
NRXN–SHANK pathway in the development of social
interaction seems to be conserved in other species.
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A synaptic trek to autism Bourgeron 233

Indeed, knockout mice for Nlgn4 display reduced social
interactions and ultrasonic vocalizations (USV) at theadult stage [40]. Mice carrying the R451C mutationin Nlgn3 display normal [41] to reduced social interaction[32] at the adult stage and a reduction of isolation calls inpups [41]. However, knockout Nlgn4 and mutant knockin
Nlgn3 display normal to enhanced learning when compared with wild-type mice [32,40]. The same is true forthe mice carrying a null mutation of Shank1, which
exhibits increased anxiety-related behavior, but showenhanced spatial learning [42].
One of the main challenges for basic scientists andclinicians is to understand how far abnormal cell/synaptic
growth and synaptic function could be reversed. Remarkably, in mice with Tsc1/Tsc2 or Pten mutations, the use of
rapamycin, a specific inhibitor of mTORC1, can preventand reverse neuronal hypertrophy, resulting in the amelioration of the behavior [43,44]. Similarly, abnormal
synaptic functions could be reversed in adult mice model
for fragile X or Rett syndrome [45,46,47]. The possibility
to reverse the social and USV alterations of the Nlgn3/4
mutant mice has not been tested yet, but the recent
results obtained on mice model for fragile X or Rett
syndrome provide new hopes for the treatment of ASD.

New routes to ASD?
Two main pathways were identified in the susceptibility to
ASD, but most probably many other tracks can lead to thiscomplex syndrome. Furthermore, even when a pathway isidentified, the diversity of genotype–phenotype relationships observed in patients with ASD indicates that othermodulators such as serotonin and/or melatonin may play
crucial roles in the onset and severity of ASD [48,49]. The
recent results have shed light on the origin of ASD and weare confident that new pathways will be identified soon tobetter understand the many facets of ASD.

Acknowledgements
This work was supported by the Pasteur Institute, University Denis Diderot
Paris 7, INSERM, CNRS, Assistance Publique-Hôpitaux de Paris, FP6
ENI-NET, FP6 EUSynapse, Fondation Orange, Fondation de France, andFondation pour la Recherche Médicale, Fondation FondaMentale.

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