development

DevelopmentEditorial overviewAnirvan Ghosh and Christine E Holt

Current Opinion in Neurobiology 2006, 16:1–4

Available online 19th January 2006

0959-4388/$ – see front matter

# 2006 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.conb.2006.01.012

www.sciencedirect.com

IntroductionThe past decade has seen rapid progress in understanding the molecular

mechanisms that govern many aspects of the development of the nervous

system. Although many key molecular players seem to have been identified,

the past couple of years have yielded some notable surprises, some of which

have caused re-investigations of old problems that were thought to be

relatively well understood. In this collection, we have brought together 16

reviews that explore up-to-date issues of neuronal development. An emerging

theme is that the same molecules are used multiple times in development to

achieve different outcomes. Molecules that were once classified neatly into

categories such as ‘morphogens’ or ‘transcription factors’ are now known to

function more broadly depending on developmental age and context, herald-

ing a new era of understanding how all of the known players work together to

activate different andmeaningful signaling pathways that give rise to specific

cell behaviours at the right time and place. For example, now we must

consider Wnts as molecules that exert broad patterning effects, that later

guide axons, and that later still are involved in making synaptic connections.

This issue emphasizes this emerging viewpoint: that the CNS uses a limited

set of players in a sequential and combinatorial way. The challenge for future

studies will be trying to understand how similar signals can lead to such

different outcomes andhow cells integrate all the information for their various

distinct developmental tasks.

Patterning in the nervous systemOur series of reviews on early patterning begins with the review by Brand

and co-workers, in which they discuss recent advances in our under-

standing of neural plate patterning. The role of signals such as noggin

and chordin in neural induction have been known for some time, but how

the neural plate gets regionally patterned is only now beginning to be

understood. The authors discuss evidence that the patterning signals

function at the same time as the neural induction signals in the early

embryo. These early patterning signals include FGF3 and FGF 8, Sonic

Hedgehog (Shh) andWnts, all of which set up the major subdivisions of the

vertebrate nervous system. One of the themes that emerges from the set of

reviews on neural patterning is the central role that these three signaling

molecules play in regulating patterning in various parts of the nervous

system.

The theme of regulation of patterning by FGFs and Shh continues in the

review by Esteve and Bovolenta on eye morphogenesis. The eyes originate

froma single primordium located in the anterior neural tube,which is between

presumptive telencephalon and diencephalon. A series of inductive signals

under the control of Shh and TGF-ß signaling leads the eye field to split into

two fields that give rise to the two eyes. At the same time, TGF-ß/BMP and

Anirvan Ghosh

Stephen Kuffler Professor, Division of Biology,

0366, University of California San Diego, La

Jolla, CA 92093, USA

Email: [email protected]

Anirvan Ghosh’s laboratory studies the

mechanisms that regulate the

specification of connections in the

developing cerebral cortex. There are

two major areas of research in the lab.

First, the lab is investigating the

mechanisms that regulate synapse

formation and synaptic specificity in

the developing cortex. Second, the lab

is investigating the mechanisms by

which synaptic activity influences the

assembly and plasticity of neural

circuits.

Christine E Holt

Professor of Developmental Neuroscience,

Department of Physiology, Development and

Neuroscience, University of Cambridge,

Cambridge CB2 3DY, UK.

Email: [email protected]

Christine Holt’s laboratory investigates

the molecular mechanisms that guide

the growth of axons in the developing

visual system of vertebrates. A

combination of in vitro and in vivo

approaches are used to: first, study the

signaling mechanisms that underlie

directional guidance of growth cones;

second, identify the guidance cues

that function to direct axon growth at

specific points in the pathway (e.g. the

optic chiasm and the optic nerve

head); and third, discover the nature of

the rules that govern topographic

mapping.


2 Development

FGF signaling are involved in specifying neural retina and

the retinal pigment epithelium.

Huge strides have been made in recent years in under-

standing the molecular mechanisms underlying dorsoven-

tral patterning in the spinal cord. Althoughmuch attention

has focused on how Shh secreted by the floorplate leads to

the specification ofmotorneurons in theventral spinal cord,

the issue of interneuron specification in the dorsal cord is

less well understood. Zhuang and Sockanathan discuss

evidence that illuminates this process. They show that

BMP, secreted by the roofplate, is involved in the speci-

fication of two main classes of class A dorsal interneurons.

Higher levels of BMP signaling are associated with more

dorsal identity and lower signaling levels withmore ventral

identity.However, some dorsal interneurons, those of class

B, are still present in BMP knockout mice. This suggests

that the specification process is more complicated than the

mechanism operating in ventral spinal cord where a simple

gradient of Shh appears to specify all the ventral neuron

types. The homeodomain protein Lbx1 is a ‘determinant’

of class B fate, whereas the bHLH protein Olig3 is

expressed in class A neurons and determines their fate.

Similar to specification ventrally, there is evidence for

cross-repressive interactions between these proteins that

probably refine the dorsal progenitor domains.

One of the most impressive examples of patterning in the

nervous system involves specification of different areas in

the cerebral cortex that mediate different functions. The

past few years have seen major advances in our under-

standing of themolecular control of cortical patterning, and

these are discussed in the review byRash andGrove.They

discuss the evidence supporting the idea of an early pro-

tomap in the telencephalon that contains information to

specify areal organization. This map is created by organiz-

ing centres that secrete factors to establish the coordinates

for the map. As in the case of neural plate patterning, an

FGF8 gradient specifies position in the anterior–posterior

axis and BMP andWnt gradients appear to specify dorsal–

ventral position.These gradients are read out as patterns of

transcription factor expression and specify area and laminar

identity. The review also discusses molecular mechanisms

of laminar fate specification, in which many of the relevant

genes are being identified. For example, Ngn1 and 2

appear to be crucial for specification of deep layer fates

andTlx and Pax6 appear to be required for specification of

upper layers. The rapid progress in identifying molecules

that regulate areal and laminar specification in the cortex

provides amolecular framework for understandinghow the

most complex part of the vertebrate nervous system is

patterned.

Axonogenesis and axon guidancemechanismsDifferent aspects of how axons navigate to their targets

are explored in four reviews. The reviews by Stoeckli and


Van Vactor and co-workers focus on molecular mechan-

isms recently identified for directing axon growth in vivo,and the review by Wen and Zheng discusses the intra-

cellular pathways that mediate attractive versus repulsive

turning in vitro. During the past decade much has been

learnt about the cues that direct axon pathfinding along

the dorsoventral axis of the CNS but little is known about

how longitudinal axons choose to grow in an anterior

versus posterior direction. Key studies reviewed by

Stoeckli reveal the surprising finding that Wnt and Shh

proteins play an active role in longitudinal guidance. Wnt

and Shh now join an increasing number of ‘morphogens’

that play a dual role in CNS development — first they

function to pattern the embryonic neuroepithelium and

later they appear to direct the growth of axons along the

anterior–posterior axis. As might be expected from their

well-characterized early patterning action, Wnts function

in gradients and are expressed in an increasing anterior-

to-posterior gradient. Shh together with SFRPs (secreted

frizzled-related protein), inhibitors of Wnt activity, are

expressed in an opposing gradient and, because Shh can

upregulate SFRP, Shh might function to sharpen the

effect of Wnt. Many guidance cues, such as Wnts and

Shh, require interactions with glycosaminoglycans such as

heparan sulfates (HS) to exert their action.

Van Vactor, Wall and Johnson discuss recent genetic

studies that reveal the importance of heparan sulfate

proteoglycans (HSPGs) in axon guidance and connectiv-

ity. There is now good evidence that HSPGs function via

ligand–receptor systems, such as Slit–Robo, and that

HSPG-mediated behaviour is highly dependent on the

molecular context. The possibility that HS modifications

provide a complex ‘HS code’ sufficient to endow different

cells with distinct profiles of ligand sensitivities which, in

turn, leads to discrete cell-type specificity is appealing but

awaits further experimental testing. Some fascinating

evolutionary aspects of HSPGs are also reviewed. The

HS family are ancient molecules that pre-date the appear-

ance of axon guidance molecules. For example, ortholo-

gues exist in Cnidaria in which the first nervous system is

thought to appear. This means that HSPGs existed long

before many of the ligand–receptor systems that they now

regulate. Because synapses appeared well before long-

range axon guidance in evolutionary terms, the authors

suggest the intriguing idea that HSPGs first acted alone to

mediate the formation of functional connections in early

neuronal networks.

Axon navigation requires a growth cone to integrate infor-

mation from multiple signals in space and time and to

generate an appropriate directional response through coor-

dinating the dynamics of its membrane and cytoskeleton.

Wen and Zheng discuss recent findings that shed light on

the complex signaling network that controls this process.

Howmight different receptors, for example, activate sepa-

rate intracellular signaling pathways? The view that


Editorial overview Ghosh and Holt 3

emerges is that membrane microdomains, or lipid rafts,

might serve as platforms that mediate distinct signaling

pathways. Ephrins and Eph receptors, for example, co-

exist on the same growth cone surface but are thought to

reside in separate microdomains enabling the selective

coupling of different ligand–receptor systems to distinct

signaling pathways. Intracellular calcium is well estab-

lished as a regulator of directional guidance but recent

evidence indicates that the switch between repulsion and

attraction hinges on the focal levels of calcium that are

controlled by aCaMKII/CaN-PP1 switch. Exactly how the

influx of calcium is influenced in growth cones during

navigation is not well understood. The authors discuss

new findings that indicate that members of the transient

receptor potential (TRP) channel family function as key

mediators for the calcium influx that regulates directional

turning in response to guidance cues such as netrin, brain

derived neurotrophic factor (BDNF) and myelin–asso-

ciated glycoprotein (MAG). How TRP channels are spe-

cifically activated by each guidancemolecule remains to be

determined.

After sensory axons navigate to their targets they com-

monly arrange themselves in topographic order, thereby

preserving the spatial arrangement of sensory informa-

tion. Flanagan reviews recent developments in mapping

of the visual system and considers broader questions of

the mechanisms of mapping in the CNS. The author

discusses why gradients are used for mapping and puts

forward the attractive idea that maps are discrete devel-

opmental modules in which the same sets of molecular

gradients, particularly ephrins and EphRs, together with

competitive fibre–fibre interactions, are used broadly in

different systems to achieve smooth mapping. The puz-

zle of why multiple overlapping gradients of ephrins and

Eph receptors exist is discussed and it is suggested,

interestingly, that this complexity functions to influence

the precision and scale of representation of specific fea-

tures of the map.

Wiring specificity and synaptogenesisAlthough axon guidance molecules enable neurons to

project to appropriate target areas, they do not have

sufficient information to specify synaptic connections.

The problems of wiring specificity and synapse formation

are discussed in a set of reviews, beginning with the

review by Komiyama and Luo on wiring the olfactory

system. In the olfactory system the olfactory sensory

neurons extend axons to glomeruli in the olfactory bulb,

and second order neurons convey this information to the

brain. In vertebrates the olfactory receptors themselves

contribute to the selection of appropriate glomerular

targets. This mechanism appears not to be used in Dro-sophila, in which the problem is solved at least in part by

the action of guidance signals such as ephrins, semaphor-

ins, slits, and Down syndrome cell adhesion molecule

(Dscams). The development of second order neurons in


Drosophila is particularly fascinating. These neurons

extend a dendrite to a specific glomerulus and an axon

to a specific target area in the brain. The targeting of

both the dendrites and the axons appears to be regulated

by transcription factors such as Acj6 and Drifter. How

transcription factors might specify connectivity is also

discussed.

The problem of synapse formation is addressed in reviews

by Sanes, Biederer, Patrick, Sheng and their co-workers.

The review by Sanes and co-workers revisits the problem

of synapse formation at the neuromuscular junction. This

paradigmatic synapse has been intensively studied for the

past 40 years. With the identification of synaptogenic

signals, such as Agrin, the basic framework of neuromus-

cular junction formation appeared to be understood.

Somewhat surprisingly, many aspects of that model have

been challenged in the past few years, largely on the basis

of phenotypes of mice that lack putative synaptogenic

genes. The review synthesizes the recent data and pro-

poses a new model for neuromuscular junction formation

that gives the target muscle a more prominent role in the

process.

The problem of CNS synapse formation is dealt with in

the review by Akins and Biederer. This problem is now

beginning to be tackled by several labs and a number of

synaptogenic signals for CNS synapses have been iden-

tified. The authors discuss the role of cell adhesion

molecules and other cell surface proteins in synapse

formation, although for most of these molecules genetic

evidence demonstrating necessity is not yet available.

The contribution of synaptogenic molecules to target

selection promises to be an active area of investigation

in the next few years.

Patrick discusses recent evidence that the ubiquitin pro-

teasome system (UPS) might play a crucial role in axon

guidance in addition to synapse formation. The author

discusses genetic evidence in invertebrates that supports

a role of this system in presynaptic development and

molecular evidence in vertebrates that suggests a role for

the UPS in axon guidance and synapse formation. This

system probably plays a key role in regulating protein

turnover at the growth cone and at the synapse.

The problem of dendritic spine morphogenesis is dis-

cussed in the review by Tada and Sheng. The control of

spine morphogenesis has been one of the most active

areas of neuronal cell biology in the past few years

because spines function as sites of excitatory transmission

and their dynamic regulation probably contributes to

synaptic plasticity. The authors discuss the evidence that

spine size expansion and reduction might correspond to

long-term potentiation (LTP) and long-term depression

(LTD) and emphasize the central role of Rho and Rac

GTPases in regulating spine size.


4 Development

Plasticity, regeneration and local translationSome aspects of synaptic plasticity require local transla-

tion and a diverse array of mRNAs and RNA-binding

proteins (RBPs) are known to reside in dendrites and

synapses. Ule and Darnell consider the issue of how

synaptic plasticity is regulated and discuss exciting new

advances that point to a combinatorial model of regulation

of alternative splicing by RBPs. In addition, recent stu-

dies show that different RBPs bind to subsets of mRNAs

that encode sets of proteins that are functionally related.

For example, many of the target mRNAs of the RBP

Nova encode proteins that function at the synapse, sug-

gesting that they form a synaptic functional module.

Interestingly, gene knockout studies have linked three

different RBPs (Nova, CPEB and FMRP) to specific

aspects of synaptic plasticity. The hypothesis that

emerges is that RBPs might regulate functionally coher-

ent subsets of RNAs to mediate different aspects of

synaptic plasticity.

In the past five years mounting evidence has indicated

that local translation of mRNA occurs in the axons of

many different types of neurons. This has led to the idea

that the differential regenerative capacity of mature axons

might be related to their ability to activate local transla-

tion following injury. Willis and Twiss explore this fas-

cinating new area and discuss recent findings that suggest

that the ability to synthesize proteins locally can, indeed,

facilitate axon regeneration. Axonal protein synthesis can

both initiate and maintain axon regeneration and suggests

a way for the development of therapeutic strategies to

facilitate neural repair in future studies. It will be exciting

in the future to test whether or not neurons with poor

regenerative capacity can be made to regenerate success-

fully by augmenting axonal translation.

Glial influence on neuronal developmentDespite the fact that glial cells far out-number neurons in

the vertebrate CNS, neurons are generally considered the

luminaries with glia as the passive bystanders. The review

by Freeman highlights recent evidence that firmly dispels


this view and shows instead that glia are active partici-

pants in the development of neuronal form and number.

Microglia in the cerebellum, for example, can promote

cell death of excess Purkinje neurons through apoptosis.

There is also accumulating evidence that glia play an

important role in sculpting axon arbors by engulfing

degenerating branches. The degree to which glia instruct

or inform these degenerative processes is not yet clear and

is an area for future experimentation. For example, it will

be important to establish whether neuronal branches

destined for removal are marked for destruction by glia

or whether the neuron first identifies branches for removal

and then recruits glia to do the job.

ConclusionsThe reviews in this issue reflect the spectacular progress

and dynamic changes that have characterized recent

research in neural development. Although the field

remains focused on understanding how a nervous system

is assembled, the approaches used are constantly evolving,

and with that new frontiers are revealed. Problems such as

patterning of the nervous system,which have been studied

for a long time, are beginning to be understood in terms of

unifying themes and mechanisms, such as regulation of

transcriptional programs by gradients of morphogens.

Research on axon guidance, which had been driven by

the identification of guidance molecules for some time, is

expanding to explore the role of newmechanisms, such as

the contribution of membrane microdomains and HSPG

codes. Research on synapse formation, after focusing on

neuromuscular junction as the primary model, is making

major inroads intoCNSsynaptogenesis.This is leading to a

serious effort towards understanding how functional cir-

cuits are assembled. Finally, newer areas of investigation

such as control of RNA trafficking and stability, axonal

protein synthesis, and synaptic protein turnover, are affect-

ing virtually all areas of developmental neurobiology,

including plasticity and regeneration. One has the distinct

impression that exploring the ‘unsolved mysteries’ in

neural development will keep us hooked for some time

with the promise of revelations still to come.


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