Brain Development: Normal Processes and the Effects of Alcohol ...

(a) suggest that the response to a cue can be modified
by bot h outside-i n (recepto r t o signalin g pathway)
and inside-ou t (signaling molecule to altered stat e of
receptor responsiveness) mechanisms and (b) support
the ide a that growt h cones hav e multipl e mean s by
which to finely tune their responses .
Adhesion and Guidance
Growth cones explore their environment by regulating
their attachment to other neurons, glia, or components
of the extracellular matrix (ECM). Contact-dependent
interactions betwee n cel l adhesio n molecule s
(CAMs) o f th e immunoglobuli n superfamily , cad -
herin superfamily , integrins, and th e EC M generat e
traction sufficient fo r growth cone extension. Interactions
between CAMs and ECMs that are inhibitory to
growth creat e boundarie s (Sno w an d Letourneau ,
1992). It has long been suspecte d that standing gradients
o f ECM s ma y als o b e abl e t o orien t o r stee r
growth cones , bu t th e dat a directl y supportin g thi s
role a s a guidanc e cu e ar e quit e recen t becaus e o f
the difficult y o f generatin g continuous , substrate -
bound gradient s (Adam s et al., 2005) . Cell-cell an d
cell-substrate interaction s can promote axon bundling
(fasciculation) an d defasciculation , thereb y makin g
the process of guidance mor e efficient . O n th e othe r
hand, there is little evidence indicating that selective
fasciculation serve s as a critica l guidanc e cu e (Ben -
son et al, 2001).
Contact-dependent interaction s ca n sto p axo n
extension when i t has reached it s final target region,
preceding th e formatio n of terminal branches an d
synapses. Axons usually terminate within small neural
groups or a restricted number o f layers, greatly reducing
th e numbe r o f potentia l individua l neural
targets. Homophilic N-cadherin adhesion appears to
prevent thalamocortica l an d retinocollicula r axon s
from extendin g pas t thei r targe t layer s (Inou e an d
Sanes, 1997 ; Poskanzer et al., 2003) and can apically
restrict mossy-fibe r terminal field s i n hippocampu s
(Bekirov e t al. , 2002) . Similarly , attractive interactions
betwee n axonin-1/transien t axona l glycopro -
tein 1 an d neuron-gli a CA M (Ng-CAM) , an d
between Fll/contactin an d Ng-CAM-relate d CA M
(Nr-CAM) contribut e t o lamina-specifi c termina -
tions o f nociceptiv e an d proprioceptiv e fibers , re -
spectively, in chick spinal cord (Perrin et al., 2001).
Thus, CAM s appea r to encode laminar recognitio n
cues, bu t the y ma y als o provid e a sto p signal ,
NEURONAL DIFFERENTIATION: FROM AXONS TO SYNAPSE S 5 5
preventing axon s fro m extendin g further . Whethe r
recognition an d stoppin g ar e mediate d concomi -
tantly by a particular cue or combination of cues has
yet to be investigated.
Few experiments have addressed directly how adhesive
cue s becom e integrate d wit h othe r guidanc e
cues. Th e emergin g pictur e promise s a complicate d
answer. The CAM L I can suppor t rapid axon extension,
but in a cis complex with the receptor neuropilin-
1, i t als o appear s t o b e require d fo r repulsio n o f
cortical axons by the solubl e ligand Sema3 A (Castellani
et al., 2000). Additionally, soluble LI (whic h can
be generate d b y metalloproteas e cleavage ) can convert
a repulsive Sema3A response to attraction (Castellani
et al., 2000) in a manner reminiscent o f laminin's
ability to convert an attractive netrin response to on e
of repulsio n (se e Cue s Ar e Bifunctional , above )
(Hopkeretal., 1999) .
Interactions betwee n substrat e and solubl e cue s
may underli e ho w commissura l axon s ca n travers e
the neutra l midline an d exten d laterally toward longitudinal
tracts . Rob o activatio n by Sli t ca n elimi -
nate N-cadherin-base d adhesio n tha t normall y
promotes rapi d extensio n (Rhe e e t al. , 2002) . This
activation correlates with the formation of a complex
between Robo , N-cadherin, an d Abelson kinas e and
is accompanie d b y phosphorylatio n o f p-catenin , a
necessary cadherin binding partner. Participation in
this complex, i n turn, occludes interactio n of N-cadherin
wit h the acti n cytoskeleton . One ca n imagin e
that rapi d N-cadherin-mediate d outgrowt h concur -
rent with high Rig-1 expression would suffice t o permit
progressio n throug h a n otherwis e neutra l
midline t o th e poin t a t which Rob o concentrations
increase, wher e axon s would disengage fro m N-cad -
herin, turn , an d gro w longitudinally by a differen t
mechanism.
Thus, severa l attractiv e and repulsive , short - and
long-ranged cue s ac t hierarchically and occasionally
collaborate t o generat e a fina l stereotype d targetin g
outcome. Som e cue s ac t similarl y throughou t th e
rostral-caudal extent of the developing nervous system,
but th e action s o f others appear to differ acros s CN S
regions. Additionally, certain molecule s ar e co-opte d
during multiple epochs of development for slightly different
purposes. The temporal sensitivities of such cues
are just beginning to be appreciated with the advent of
conditional geneti c tool s an d improve d method s fo r
transient transfections. These techniques ar e useful for
determining th e virtuall y unknow n mechanisms tha t