Regulating Axon Growth within
the Postnatal Central Nervous System
Fenghua, Hu, and Stephen M. Strittmatter
As neuronal development enters its final stages, axonal growth is restricted. Recent work
indicates that several myelin-derived proteins, Nogo, MAG and OMgp, play a critical role
in restricting axonal growth in the mature central nervous system (CNS). These proteins
function by binding to an axonal NgR protein that limits axonal growth by activating Rho in
neurons. Hypoxic conditions during the later stages of neuronal development have a
prominent effect on oligodendrocytes and hence on the expression of these axon growth
inhibitors. Reduced expression of these proteins caused by the developmental hypoxia, or
direct blockade of the myelin inhibitor pathways in the adult CNS leads to axonal sprouting
and the formation of new neuronal connections. The regulation of axonal growth, sprouting
and connections in the postnatal brain by myelin proteins is an area of important investigation and potential therapeutic intervention.
Semin Perinatol 28:371-378 © 2004 Elsevier Inc. All rights reserved.
D
uring the early prenatal development of the nervous
system, neurons project axons over long distances to
reach their final targets. Successful targeting of axons depends both on the supply of neurotrophic factors and the
directed steering of axons by the extracellular distribution of
guidance factors. At the leading edge of a growing axon is the
growth cone, a structure capable of sensing and rapidly responding to its environment. Attractive and repellent axon
guidance cues are sensed by growth cones and affect growth
cone dynamics and motility. Several families of guidance
molecules and their receptors have been identified and studied during the past few years, including RGMs/Neogenin,
netrins/DCC, slits/Robos, semaphorins/neuropilins/plexins,
ephrins/Eph tyrosine kinase receptors, and cell adhesion receptors.1-4
Axons progressively stop growing as neuronal networks
are formed during the final stages of development. If there is
injury to the nervous system in the later stages of development or in the adult, the axonal growth cones formed after
the resealing of the injured axons interact with secreted factors, extracellular matrix and neighboring cells, all of which
affect growth cone dynamics and the extent of the axon regeneration process. Neurons in the mammalian peripheral
nervous system (PNS) can regrow functional axons after injury but neurons from adult mammalian central nervous sysDepartment of Neurology, Yale University School of Medicine, New Haven, CT.
Address reprint requests to: Dr. Stephen M. Strittmatter, Department of
Neurology, Yale University School of Medicine, P.O. Box 208018, New
Haven, CT 06520. E-mail: stephen.strittmatter@yale.edu
0146-0005/04/$-see front matter © 2004 Elsevier Inc. All rights reserved.
doi:10.1053/j.semperi.2004.10.001
tem (CNS) cannot. Experiments have demonstrated that the
inability of regeneration in the CNS is not solely an intrinsic
deficit of the CNS neurons but depends at least in part on an
unfavorable environment for axon regeneration in the
CNS.5-7 When provided with a permissive milieu, damaged
CNS neurons can regenerate axons. Several inhibitory factors
prevent axon outgrowth at the site of a CNS lesion, including
CNS myelin produced by oligodendrocytes and a glial scar
formed primarily by astrocytes. Recently, many inhibitory
molecules have been identified and characterized in the CNS
myelin and this review will update some of recent findings.
Apart from their role as inhibitors of CNS regeneration after
injury, it is hypothesized that CNS myelin proteins might
also help preserve an appropriate CNS neuronal network, by
preventing over-exuberant axonal sprouting with aberrant
misconnections. An example of the correlation between a
downregulation of myelin protein and increased axonal
sprouting occurs in a rodent model for periventricular leukomalacia (PVL), which is a common brain lesion seen in
premature human infants.8
CNS Myelin Derived Inhibitors
The importance of CNS myelin derived proteins for inhibiting CNS axon growth has been demonstrated by several studies. In mice immunized with CNS myelin, significant regeneration of injured corticospinal tract (CST) fibers is observed
while virtually no axon regeneration is seen in control mice.9
Other studies including sciatic nerve transplant studies, tissue culture experiments and olfactory ensheathing cell trans371
F. Hu and S.M. Strittmatter
372
Figure 1 Inhibition of neurite outgrowth by CNS myelin proteins. The myelin protein components, Nogo, MAG and
OMGP, inhibit axon regeneration by binding to a common receptor, NgR. NgR lacks a transmembrane domain and is
thought to form a functional receptor complex with p75NTR and Lingo-1 to transduce intracellular signals. RhoA and
PKC are activated in response to myelin ligands to induce growth cone collapse and inhibit outgrowth. Upregulation
of cAMP attenuates the inhibitory effect of myelin. Nogo-A may exist in an alternative topology with amino Nogo
extracellular. Amino Nogo might inhibit axon outgrowth and fibroblast spreading through a distinct receptor.
plant studies have also confirmed the inhibitory effect of CNS
myelin. Multiple inhibitors have been identified in the CNS
myelin, including Nogo, myelin-associated glycoprotein
(MAG), oligodendrocyte-myelin glycoprotein (OMGP) and
chondroitin sulfate proteoglycan (CSPG) (Fig. 1). Although
each of these molecules have been shown to inhibit neurite
outgrowth in vitro, the relative inhibitory activity of these
molecules in the CNS myelin is not fully defined.
Nogo
A monoclonal antibody (IN-1) raised against a fraction of
myelin enriched for inhibitory activity has been shown to
enhance regeneration of corticospinal tract (CST), corticorubral and corticopontine fibers after spinal cord injury.
Furthermore, this axon growth is correlated with improved
functional recovery following injury.10,11 The antigen for the
IN-1 antibody is a protein of 250 kDa (NI-250). With partial
protein sequence data derived from a proteolytic digest of
bovine NI-250, three groups identified the cDNA for this
major inhibitory protein, termed Nogo.12-14 Alternative promoter usage and differential splicing of the Nogo gene give
rise to three major transcript (Nogo-A, -B and –C). Three
isoforms of Nogo have overlapping expression patterns15:
Nogo-A is expressed by CNS myelin forming oligodendrocytes and some neuronal subpopulations, heart and testis,
but not by peripheral myelin forming Schwann cells. Nogo-B
has a widespread expression pattern while Nogo-C is present
in CNS neurons and is particularly enriched in skeletal muscles.
The three isoforms of Nogo share a common carboxylterminal of 188 amino acids, which is homologous to the
reticulon protein family and contains two transmembrane
domains. The loop between the two transmembrane domains
of Nogo (Nogo66) has been shown to be extracellular and has
an axon growth inhibitory effect on its own.12 Nogo-A contains a unique sequence (amino-Nogo) that has been shown
to inhibit both axon growth and fibroblast spreading in
vitro.16,17 The topology of Nogo (and reticulons) remains
controversial. There are two proposed topologies of Nogo,
one with amino-Nogo being intracellular and the other with
amino-Nogo facing extracellular. The majority of Nogo in the
cell has been found to associate with endoplasmic reticulum
(ER), just like other members of the reticulon family.15 Nogo
lacks a conventional signal peptide at its amino-termini and
translocation into the ER is assumed to be directed by the two
hydrophobic domains. The two putative transmembrane domains in reticulons are very large and could span the membrane either once or twice. Antibodies against Nogo66 and
amino-Nogo can stain surface of differentiated oligodendrocyte in culture, suggesting that, at least in these cells, both
Nogo66 and amino-Nogo can be extracellular.17 In transfected Cos7 cells, only low levels of surface staining for
Nogo66, and none for amino-Nogo, can be detected.12 It is
possible that Nogo can assume different topologies in differ-
Regulating axon growth
ent cell types or its topology might be regulated in response
to signals, such as contact with axons or in response to axon
damage. Further studies on Nogo topology and factors that
might regulate its topology and intracellular distribution will
be very interesting.
The importance of Nogo in inhibiting axon regeneration in
the CNS has been addressed through genetic studies. Transgenic mice expressing Nogo-A or Nogo-C in peripheral
Schwann cells show delayed axonal regeneration and decreased recovery rate after sciatic nerve injury,18,19 demonstrating that Nogo is sufficient to inhibit axon regeneration in
vivo. Three groups independently generated Nogo knockout
mice and determined axon regeneration and functional recovery after spinal cord injury in these mice.20-22 All Nogo
mutant mice exhibit normal brain histology and behavior.
Myelin appears to be normal and there are no obvious defects
in axons and oligodendrocytes, suggesting that Nogo does
not play an essential role in CNS development or the maintenance of CNS integrity in the absence of injury. In vitro
assays demonstrate that myelin lacking Nogo-A is significantly less inhibitory for axon growth.
After spinal injury, one gene-trap derived strain of NogoA/B deficient mice exhibits regeneration of CST fibers and
improved functional recovery of locomotion.20 A lesser degree of CST fiber growth is detected in mice selectively lacking Nogo-A; this might be explained by increased Nogo-B
expression in oligodendrocytes.21 On the other hand, no CST
spouting or regeneration is apparent in additional strains of
Nogo-A/B or Nogo-A/B/C deficient mice.22 The basis for the
variable phenotypes in the mutant mice requires further investigation to determine the exact role that Nogo-A plays in
CNS regeneration.
Despite variable phenotypes in Nogo gene deletion mice,
several other experiments demonstrate a role for Nogo in
limiting CNS axon regeneration in vivo. Treatment with IN-1
antibodies or implantation of mAb IN-1-secreting hybridoma
cells,23,24 or direct administration of a partially humanized,
recombinant Fab fragment (rIN-1 Fab) derived from the original mAb IN-125 improve axon regeneration and functional
recovery after spinal cord injury, as judged by the BBB locomotor score. A 40-residue peptide (NEP1-40) from the
Nogo66 region behaves as a Nogo66 antagonist and also
promotes CST axon growth and functional recovery in vivo
following spinal cord injury.26
Nogo has been implicated in physiological functions other
inhibition of axon growth. Recently, Nogo has also been reported to interact with the Caspr-F3 complex at paranodes
and this interaction might have a role in modulating axoglial
architecture and possibly potassium channel localization
during development.27 Nogo-B might have a function as proapoptotic protein.28 A recent report demonstrates that
Nogo-B has a nonneuronal role in regulating vascular homeostasis and remodeling.29
MAG
Myelin associated glycoprotein (MAG) contains five Ig-like
domains and is a member of the immunoglobin superfamily.
373
MAG is present in both PNS and CNS myelin and is a potent
inhibitor of neurite outgrowth from a variety of neurons in
vitro.30-32 It appears that MAG has a dual function in regulating axon growth: it stimulates neurite outgrowth from immature neurons but inhibits outgrowth from mature neurons.31-34 MAG is thought to contribute significantly to the
inhibitory activity of myelin in vitro.30 However, in one
study, myelin from the MAG-deficient mouse behaves similarly to myelin prepared from wild type mice in axon outgrowth assays and the extent of axonal regeneration after
spinal cord injury is similar in MAG-deficient and wild-type
mice.35 It is not clear if there are other inhibitory molecules
up-regulated in the absence of MAG or if MAG is not essential
for the inhibition of CNS axon regeneration. In mutant backgrounds with delayed Wallerian degeneration, MAG does
appear to have some role in limiting axon regeneration rate.36
The physiologic function of MAG might be more involved in
axoglial interactions necessary for myelin formation and integrity.37
OMGP
Oligodendrocyte-myelin glycoprotein (OMGP) is a glycosylphosphatidylinositol (GPI)-anchored CNS myelin protein
and belongs to the leucine rich repeat protein family. OMGP
is expressed on the surface of mature oligodendrocyte and it
is a potent inhibitor of neurite outgrowth from cultured neurons.38,39 OMGP is also highly expressed by some neurons,
but its neuronal functions are ill-defined.40 OMGP gene deletion studies should help reveal the relative significance of
OMGP in inhibiting CNS regeneration.
CSPG
The scar that forms at the site of CNS lesion is comprised
largely of reactive astrocytes and contributes to the failure of
axon regeneration in the CNS. Chondroitin sulfate proteoglycans (CSPGs) are thought to be major inhibitory components of the glial scar. Inhibitory CSPGs have also been isolated from CNS myelin.41 Specific CSPGs, including NG2,
versican, neurocan and phosphocan, are expressed at high
levels at the sites of CNS injury and have been shown to
inhibit neurite outgrowth.42,43 Removal of the chondroitin
sulfate glycosaminoglycan chains from the core proteins by
chondroitinase ABC treatment neutralizes the inhibitory effect of many CSPGs and promotes axon regeneration in
vivo.44,45 However, chondroitinase ABC treatment does not
appear to reduce the inhibitory activity of NG2 proteoglycan,
suggesting that the inhibitory activity of NG2 also resides the
protein core.46-48 The mechanisms by which CSPGs inhibit
neurite outgrowth are still unknown.
Nogo Receptor (NgR)
Nogo receptor was identified as a protein that interacts
with Nogo66 in an expression cloning screen.16 NgR expression is confined to neurons in the adult nervous system. It binds Nogo66 with high affinity. Release of NgR
from membrane with phosphatidylinositol-specific phos-
374
pholipase C (PI-PLC) treatment abolishes Nogo66
responsiveness and transfection of NgR confers a Nogo66
response in otherwise nonresponsive neurons, demonstrating that NgR is the receptor for Nogo66. NgR contains
a leucine rich repeat (LRR) structure and the carboxylterminal (CT) domain, which does not have sequence homology to known protein sequences. The LRR domain of
NgR is comprised of eight typical LRR segments flanked by
cysteine-rich LRR amino-terminal (LRRNT) and the LRR
carboxyl-terminal (LRRCT) domains. The crystal structure
of the NgR LRR domain has been determined.49,50 The NgR
LRR domain is most closely related to that of platelet glycoprotein-1b-␣ (GP1b-␣). LRR segments in NgR are arranged in a parallel way as ␤-sheet segments, creating a
banana-shaped structure that has a concave and a convex
surface capable of protein-protein interaction. NgR lacks a
transmembrane domain and it is tethered to the plasma
membrane through a glycosylphosphatidylinositol (GPI)
moiety, suggesting that NgR might associate with a transmembrane coreceptor to transduce intracellular signals.
Interestingly, two other inhibitory components of myelin,
MAG and OMGP also interact with NgR and require NgR for
their inhibitory activity in vitro.39,51,52 All three ligands,
Nogo66, MAG and OMGP bind to the leucine rich repeat
domains of NGR. It is interesting that NgR can interact with
three ligands that share no homology to each other: Nogo 66
does not have a defined domain structure; MAG is a protein
with IgG domain and OMGP contains leucine rich repeat
domains. How these various ligands interact with NgR is still
unclear. Detailed mutagenesis studies of NgR and cocrystallization of NgR with its ligands will help define the ligand
binding regions in NgR and give us more insight into the
molecular basis of NgR interaction with three structurally
distinct ligands.
Blockade of NgR signaling in mice using a soluble version of NgR (NgR-Ecto) which blocks NgR function and
the inhibitory effects of NgR ligands in vitro has been
shown to improve axon growth and/or recovery after optic
nerve injury and after stroke, suggesting that signaling
through NgR plays an important role in limiting CNS axon
growth.53,54
Recently, two homologs of NgR (now termed NgR1) have
been identified, termed NgR2 (also known as NgRH1) and
NgR3 (also known as NgRH2).49,55 NgR2 and NgR3 do not
appear to interact with known NgR ligands, at least in the
affinity range for NgR. The expression pattern of NgR2 and
NgR3 are quite similar to NgR1 and the functions of these
two homologs are waiting to be explored.
The inhibitory effect of amino-Nogo on fibroblast
spreading and neurite outgrowth does not seem to be dependent on NgR, suggesting that there might exist a distinct receptor for amino-Nogo.17 Work by Schwab’s group
suggest that there exist two distinct inhibitory regions in
amino-Nogo: an amino-terminal region common to
Nogo-A and B which can inhibit fibroblast spreading and a
Nogo-A specific stretch that inhibits neurite outgrowth
and cell spreading (NiG-⌬20).17 Amino-Nogo binds to the
cell surface of responsive cells and to rat brain cortical
F. Hu and S.M. Strittmatter
membranes, suggesting the existence of specific binding
receptors.17 Amino-Nogo does not bind to NgR2 or NgR3
(Hu and Strittmatter, unpublished data). It will be very
interesting to identify the receptor for amino-Nogo and to
explore the mechanisms that amino-Nogo inhibits fibroblast spreading and neurite outgrowth.
Coreceptors:
p75, Lingo, Others?
NgR lacks a transmembrane domain, suggesting that it
might need a coreceptor or coreceptors for signaling. Recent studies have identified two possible coreceptors for
NgR: p75NTR and Lingo-1.56-58 p75NTR interacts with a variety of ligands and is well known to function as a neurotrophin receptor in concert with the Trk receptor tyrosine
kinase family.59,60 The first hint that p75NTR might function in the NgR pathway came from the observation that
MAG-dependent inhibition of neurite outgrowth and
RhoA activation is impaired in neurons from p75NTR-deficient mice.61 Two groups explored the possibility that
p75NTR might be the coreceptor for p75 and they showed
that these two proteins bind to each other and p75NTR is
required for the inhibitory activity of Nogo, MAG and
OMGP.56,57 In a yeast two hybrid screen, the cytoplasmic
domain of p75NTR was found to interact with Rho, a small
GTPase implicated in mediating growth inhibition by myelin. Later, the p75-Rho interaction was described as being
indirect and mediated via the Rho-guanine dissociation
inhibitor (GDI).62 p75NTR is postulated to activate Rho by
acting as a displacement factor that releases Rho from
Rho-GDI.62
However, there are several indications that p75-NTR does
not constitute the sole mediator of NgR signaling.63,64 Immunohistochemical data show that p75NTR is expressed in only a
very small subset of neurons.63 Furthermore, the depletion of
the functional p75NTR or local administration of a dominant
negative p75NTR -Fc molecule does not promote axon regeneration after spinal cord injury, suggesting that p75NTR may
not be a critical molecule mediating the function of myelinassociated inhibitory factors in vivo.63
Recently, a nervous system specific leucine-rich repeat
protein named Lingo-1 (LRR and Ig domain-containing,
Nogo receptor-interacting protein) was found to be a third
component of the NgR signaling complex.58 Lingo-1 interacts
with NgR1 and p75 physically. Coexpression of NgR, p75NTR
and Lingo-1 in nonneuronal cells conferred cellular activation of RhoA in response to OMGP treatment, while coexpression of NgR and p75 did not. Interfering Lingo-1 function using a dominant-negative construct or exogenously
added soluble Lingo-1-Fc fusion protein attenuated the inhibitory effect of myelin and myelin ligands to neurons.
These data suggest that Lingo-1 is a functional component of
the NgR-p75 signaling complex.
It remains an open question whether there are additional
or alternative components in NgR receptor complexes. There
might be other transmembrane proteins interacting with
Regulating axon growth
375
NgR-p75-Lingo-1 complex and required for RhoA activation.
Alternatively, different neurons might utilize different coreceptors to transduce signals from myelin ligands. p75NTR is
only expressed by a subset of neurons and it is possible that
there are other coreceptors functioning together with NgR in
p75-negative neurons.
Intracellular Signaling:
Rho, cAMP, PKA, PKC
The Rho family of small guanine triphosphatases (GTPase),
including Rho, Rac and Cdc42, regulates actin cytoskeleton
and growth cone dynamics. Myelin and individual NgR ligands have been shown to activate RhoA. Blockade of RhoA
by C3 transferase mediated ADP-ribosylation can attenuate
myelin and NgR ligand-mediated axon growth inhibition.65-67 C3 treatment can promote axon regeneration and
functional recovery in vivo.67,68 Some studies have demonstrated that Rho-associated kinase (ROCK) might play a
prominent role in downstream signaling of Rho in response
to NgR activation. The ROCK inhibitor Y-27632 reverses
myelin inhibition in vitro and promotes axon regeneration
and locomotor recovery in vivo.66,67
The level of cytosolic nucleotides can modulate the neuronal response to a number of factors involved in neurite attraction and repulsion. Recently the relative ratio of cAMP to
cGMP has been shown to play an important role in determining growth cone turning behavior.69 MAG/myelin dependent
inhibition can be overcome by priming neurons with neurotrophins and this effect is mediated by the cAMP-PKA pathway.70 cAMP level is dramatically higher in young neurons
than in the same types of older neurons and only older neuron are inhibited by MAG. Elevating cAMP in older neurons
blocks MAG inhibition of neurite outgrowth.33 Elevation of
cAMP by direct injection into dorsal root ganglion (DRG)
overcomes inhibition by MAG and myelin and results in extensive regeneration of dorsal column axons lesioned one
week later.71 Rolipram elevates cAMP in the brain by inhibiting phosphodiesterase 4, and enhances SCI recovery in
combination with Schwann cell transplants.72 Arginase I and
polyamines are likely to act downstream of cAMP in overcoming inhibition of axonal growth by MAG and myelin in
vitro.73 Thus, modulation of cAMP levels and downstream
signaling might be important for axon regeneration.
Recently, protein kinase C (PKC) was also found to play
an important role in mediating inhibitory effects of myelin
components and CSPGs.74 Both the myelin inhibitors and
CSPGs induce PKC activation. Blocking PKC activity pharmacologically and genetically attenuates the ability of CNS
myelin and CSPGs to activate Rho and inhibit neurite
outgrowth. Intrathecal infusion of a PKC inhibitor into the
site of dorsal hemisection promotes regeneration of dorsal
column axons across and beyond the lesion site in adult
rats. Since PKC is involved in many cellular events,
whether the activation of PKC is a direct effect of myelin
ligands or occurs through interference with other pathways remains to be determined.
Figure 2 Changes in axonal sprouting and myelin axon growth inhibitors in PVL. In response to hypoxia induced damage, Nogo-A
and MAG are selectively downregulated in oligodendrocytes. The
decreased expression of Nogo-A and MAG in developing CNS myelin might contribute to the axon sprouting observed in animals
subjected to chronic sublethal hypoxia (CSH) and in humans with
PVL.
Suppression of
Oligodendrocyte
Nogo-A Expression in
Response to Neonatal Hypoxia
Recently, in a rodent model for periventricular leukomalacia (PVL), Nogo-A expression was found to be selectively
lost from oligodendrocytes8 (Fig. 2). As discussed elsewhere in this volume, PVL is characterized a reduction of
central myelinated tracts and is a common brain white
matter lesion in infants born with very low birth weight
(VLBW).75 Both mental retardation and attention deficit
hyperactivity disorder occur with increased frequency in
VLBW infants. Hypoxia occurs after birth in most VLBW
infants and is believed to be the primary cause of PVL.75,76
Hypoxia both causes direct neuronal damage and modifies
the environment for axonal re-growth. Oligodendrocytes
precursors are the CNS cells most sensitive to hypoxic
damage in premature infants.77
Rodent chronic sublethal hypoxia (CSH) from P3 to 33
(postnatal day 3-33) provides a model for PVL. Rats exposed
to these conditions experience changes comparable to premature humans including failure of brain growth, progressive cerebral ventriculomegaly, decreased subcortical white
F. Hu and S.M. Strittmatter
376
matter, decreased corpus callosum size and decreased cortical volume.78 Mice exposed to CSH from P3-P33 followed by
normoxia from P33-P75 continue to exhibit a locomotor hyperactivity and anxiety that resembles behavioral changes
observed in some human children born with VLBW, confirming the validity of CSH as a model for PVL.8
In microarray studies, many oligodendrocyte-specific proteins are reduced in abundance in the hypoxic neonatal
brain.8,79 Both MAG and myelin basic protein (MBP) protein
levels from P12 hypoxic brain are decreased and return to
normal after exposure to normoxic conditions from P33 to
P75.8 There is more dramatic oligodendrocyte-selective loss
of Nogo-A during the P3-P12 period in response to hypoxia
compared with MAG.8 Oligodendrocyte selective Nogo-A
loss is also transient during hypoxia and resolves by adulthood (P75). In contrast, axonal NgR expression levels are
indistinguishable between normoxic and hypoxic animals.8
Although myelin protein expression returns to normal by
maturity (P75), persistent abnormalities in axonal trajectories are detectable.8 Anterograde axonal tracing from motor
cortex demonstrates ectopic corticofugal fibers in the CST,
corpus callosum and caudate nucleus of adult animals reared
in CSH. Clinically, misrouted axonal trajectories have been
documented in VLBW infants.80 The loss of myelin inhibitors
in combination with the mild hypoxic insult to the neuron
itself is likely to cause the observed fiber misrouting since
absence or blockade of myelin-derived inhibitors alone do
not result in CST fibers sprouting in uninjured animals.20,26 It
is still not clear whether ectopic corticofugal fibers ameliorate
or exacerbate behavioral deficits in CSH animals. In the presence of NgR blockade, spinal-injured animals exhibit enhanced functional recovery despite many ectopic fibers. By
analogy, increased axonal sprouting in the absence of myelin
inhibitors induced by hypoxia might ameliorate deficits in
CSH mice and PVL humans. Alternatively, supratentorial
sprouting and axonal misconnections may contribute to the
hyperactivity and anxiety observed in the CSH mice.
Conclusion and
Future Directions
Identification of Nogo as an inhibitory component of myelin
and the cloning of its receptor (NgR) not only shed light on the
inhibitory mechanisms of the CNS myelin but also provides
therapeutic approaches to modulate the inhibitory effect of CNS
myelin on axon growth. Animals treated with a Nogo antibody
or a peptide antagonist of Nogo 66 (NEP1-40) showed enhanced axon regeneration and improved functional recovery
after spinal cord injury.23-26,81 Nogo, MAG and OMGP bind to a
common receptor NgR to inhibit neurite outgrowth and blockade of NgR with NgR-Ecto attenuates the inhibitory effects of all
three ligands16,39,51,52,82 (Fig. 1). Animals treated with NgR-Ecto
showed improved axon growth and/or recovery after optic
nerve injury and after stroke, raising the hope that NgR-Ecto or
other possible NgR antagonists might be used as a drug to treat
spinal cord injury.53,54
The organization of the NgR receptor complex and the
downstream signaling mechanisms need to be studied in
more details. p75NTR and Lingo-1 appear to be functional
components of the NgR receptor complex56-58 (Fig. 1). However, it is still not clear whether there are other necessary
components. RhoA and PKC seem to signal downstream of
NgR,64,74 but how Rho and PKC become activated on NgR
ligand binding needs to determined (Fig. 1).
It is very interesting that in rodent models of PVL, oligodendrocyte Nogo-A is selectively downregulated.8 The
mechanisms of altered Nogo-A expression are still not clear,
and may occur at the mRNA level, the protein level or both.
Nogo-A and MAG downregulation correlates with increased
axon sprouting and hyperactivity in animals subjected to
hypoxia neonatally. Enhanced axon sprouting is part of the
pathology of PVL, and may have simultaneous adaptive and
maladaptive consequences.
Acknowledgments
This work was supported by grants to S.M.S. from the NIH.
References
1. Huber AB, Kolodkin AL, Ginty DD, et al: Signaling at the growth cone:
Ligand-receptor complexes and the control of axon growth and guidance. Annu Rev Neurosci 26:509-563, 2003
2. Dickson BJ: Molecular mechanisms of axon guidance. Science 298:
1959-1964, 2002
3. Monnier PP, Sierra A, Macchi P, et al: RGM is a repulsive guidance
molecule for retinal axons. Nature 419:392-395, 2002
4. Rajagopalan S, Deitinghoff L, Davis D, et al: Neogenin mediates the
action of repulsive guidance molecule. Nat Cell Biol 6:756-762, 2004
5. Richardson PM, McGuinness UM, Aguayo AJ: Axons from CNS neurons regenerate into PNS grafts. Nature 284:264-265, 1980
6. David S, Aguayo AJ: Axonal elongation into peripheral nervous system
“bridges” after central nervous system injury in adult rats. Science 214:
931-933, 1981
7. Benfey M, Aguayo AJ: Extensive elongation of axons from rat brain into
peripheral nerve grafts. Nature 296:150-152, 1982
8. Weiss J, Takizawa B, McGee A, et al: Neonatal hypoxia suppresses
oligodendrocyte Nogo-A and increases axonal sprouting in a rodent
model for human prematurity. Exp Neurol 189:141-149, 2004
9. Huang DW, McKerracher L, Braun PE, et al: A therapeutic vaccine
approach to stimulate axon regeneration in the adult mammalian spinal
cord. Neuron 24:639-647, 1999
10. von Meyenburg J, Brosamle C, Metz GA, et al: Regeneration and sprouting of chronically injured corticospinal tract fibers in adult rats promoted by NT-3 and the mAb IN-1, which neutralizes myelin-associated
neurite growth inhibitors. Exp Neurol 154:583-594, 1998
11. Z’Graggen WJ, Metz GA, Kartje GL, et al: Functional recovery and
enhanced corticofugal plasticity after unilateral pyramidal tract lesion
and blockade of myelin-associated neurite growth inhibitors in adult
rats. J Neurosci 18:4744-4757, 1998
12. GrandPre T, Nakamura F, Vartanian T, et al: Identification of the Nogo
inhibitor of axon regeneration as a Reticulon protein. Nature 403:439444, 2000
13. Chen MS, Huber AB, van der Haar ME, et al: Nogo-A is a myelinassociated neurite outgrowth inhibitor and an antigen for monoclonal
antibody IN-1. Nature 403:434-439, 2000
14. Prinjha R, Moore SE, Vinson M, et al: Inhibitor of neurite outgrowth in
humans. Nature 403:383-384, 2000
15. Oertle T, Schwab ME: Nogo and its paRTNers. Trends Cell Biol 13:187194, 2003
16. Fournier AE, GrandPre T, Strittmatter SM: Identification of a receptor
mediating Nogo-66 inhibition of axonal regeneration. Nature 409:341346, 2001
17. Oertle T, van der Haar ME, Bandtlow CE, et al: Nogo-A inhibits neurite
Regulating axon growth
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
outgrowth and cell spreading with three discrete regions. J Neurosci
23:5393-5406, 2003
Kim JE, Bonilla IE, Qiu D, et al: Nogo-C is sufficient to delay nerve
regeneration. Mol Cell Neurosci 23:451-459, 2003
Pot C, Simonen M, Weinmann O, et al: Nogo-A expressed in Schwann
cells impairs axonal regeneration after peripheral nerve injury. J Cell
Biol 159:29-35, 2002
Kim JE, Li S, GrandPre T, et al: Axon regeneration in young adult mice
lacking Nogo-A/B. Neuron 38:187-199, 2003
Simonen M, Pedersen V, Weinmann O, et al: Systemic deletion of the
myelin-associated outgrowth inhibitor Nogo-A improves regenerative
and plastic responses after spinal cord injury. Neuron 38:201-211,
2003
Zheng B, Ho C, Li S, et al: Lack of enhanced spinal regeneration in
Nogo-deficient mice. Neuron 38:213-224, 2003
Bregman BS, Kunkel-Bagden E, Schnell L, et al: Recovery from spinal
cord injury mediated by antibodies to neurite growth inhibitors. Nature
378:498-501, 1995
Merkler D, Metz GA, Raineteau O, et al: Locomotor recovery in spinal
cord-injured rats treated with an antibody neutralizing the myelinassociated neurite growth inhibitor Nogo-A. J Neurosci 21:3665-3673,
2001
Brosamle C, Huber AB, Fiedler M, et al: Regeneration of lesioned corticospinal tract fibers in the adult rat induced by a recombinant, humanized IN-1 antibody fragment. J Neurosci 20:8061-8068, 2000
GrandPre T, Li S, Strittmatter SM: Nogo-66 receptor antagonist peptide
promotes axonal regeneration. Nature 417:547-551, 2002
Nie DY, Zhou ZH, Ang BT, et al: Nogo-A at CNS paranodes is a ligand
of Caspr: Possible regulation of K(⫹) channel localization. EMBO J
22:5666-5678, 2003
Watari A, Yutsudo M: Multi-functional gene ASY/Nogo/RTN-X/RTN4:
Apoptosis, tumor suppression, and inhibition of neuronal regeneration. Apoptosis 8:5-9, 2003
Acevedo L, Yu J, Erdjument-Bromage H, et al: A new role for Nogo as a
regulator of vascular remodeling. Nat Med 10:382-388, 2004
McKerracher L, David S, Jackson DL, et al: Identification of myelinassociated glycoprotein as a major myelin-derived inhibitor of neurite
growth. Neuron 13:805-811, 1994
Mukhopadhyay G, Doherty P, Walsh FS, et al: A novel role for myelinassociated glycoprotein as an inhibitor of axonal regeneration. Neuron
13:757-767, 1994
DeBellard ME, Tang S, Mukhopadhyay G, et al: Myelin-associated glycoprotein inhibits axonal regeneration from a variety of neurons via
interaction with a sialoglycoprotein. Mol Cell Neurosci 7:89-101, 1996
Cai D, Qiu J, Cao Z, et al: Neuronal cyclic AMP controls the developmental loss in ability of axons to regenerate. J Neurosci 21:4731-4739,
2001
Johnson PW, Abramow-Newerly W, Seilheimer B, et al: Recombinant
myelin-associated glycoprotein confers neural adhesion and neurite
outgrowth function. Neuron 3:377-385, 1989
Bartsch U, Bandtlow CE, Schnell L, et al: Lack of evidence that myelinassociated glycoprotein is a major inhibitor of axonal regeneration in
the CNS. Neuron 15:1375-1381, 1995
Schafer M, Fruttiger M, Montag D, et al: Disruption of the gene for the
myelin-associated glycoprotein improves axonal regrowth along myelin in C57BL/Wlds mice. Neuron 16:1107-1113, 1996
Li C, Trapp B, Ludwin S, et al: Myelin associated glycoprotein modulates glia-axon contact in vivo. J Neurosci Res 51:210-217, 1998
Kottis V, Thibault P, Mikol D, et al: Oligodendrocyte-myelin glycoprotein (OMGP) is an inhibitor of neurite outgrowth. J Neurochem 82:
1566-1569, 2002
Wang KC, Koprivica V, Kim JA, et al: Oligodendrocyte-myelin glycoprotein is a Nogo receptor ligand that inhibits neurite outgrowth. Nature 417:941-944, 2002
Habib AA, Marton LS, Allwardt B, et al: Expression of the oligodendrocyte-myelin glycoprotein by neurons in the mouse central nervous
system. J Neurochem 70:1704-1711, 1998
Niederost BP, Zimmermann DR, Schwab ME, et al: Bovine CNS myelin
377
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
contains neurite growth-inhibitory activity associated with chondroitin
sulfate proteoglycans. J Neurosci 19:8979-8989, 1999
Morgenstern DA, Asher RA, Fawcett JW: Chondroitin sulphate proteoglycans in the CNS injury response. Prog Brain Res 137:313-332, 2002
Properzi F, Asher RA, Fawcett JW: Chondroitin sulphate proteoglycans
in the central nervous system: Changes and synthesis after injury. Biochem Soc Trans 31:335-336, 2003
Bradbury EJ, Moon LD, Popat RJ, et al: Chondroitinase ABC promotes
functional recovery after spinal cord injury. Nature 416:636-640, 2002
Moon LD, Asher RA, Rhodes KE, et al: Regeneration of CNS axons back
to their target following treatment of adult rat brain with chondroitinase ABC. Nat Neurosci 4:465-466, 2001
Dou CL, Levine JM: Inhibition of neurite growth by the NG2 chondroitin sulfate proteoglycan. J Neurosci 14:7616-7628, 1994
Chen ZJ, Ughrin Y, Levine JM: Inhibition of axon growth by oligodendrocyte precursor cells. Mol Cell Neurosci 20:125-139, 2002
Ughrin YM, Chen ZJ, Levine JM: Multiple regions of the NG2 proteoglycan inhibit neurite growth and induce growth cone collapse. J Neurosci 23:175-186, 2003
Barton WA, Liu BP, Tzvetkova D, et al: Structure and axon outgrowth
inhibitor binding of the Nogo-66 receptor and related proteins. EMBO
J 22:3291-3302, 2003
He XL, Bazan JF, McDermott G, et al: Structure of the Nogo receptor
ectodomain: A recognition module implicated in myelin inhibition.
Neuron 38:177-185, 2003
Liu BP, Fournier A, GrandPre T, et al: Myelin-associated glycoprotein
as a functional ligand for the Nogo-66 receptor. Science 297:11901193, 2002
Domeniconi M, Cao Z, Spencer T, et al: Myelin-associated glycoprotein
interacts with the Nogo66 receptor to inhibit neurite outgrowth. Neuron 35:283-290, 2002
Fischer D, He Z, Benowitz LI: Counteracting the Nogo receptor enhances optic nerve regeneration if retinal ganglion cells are in an active
growth state. J Neurosci 24:1646-1651, 2004
Lee JK, Kim JE, Sivula M, et al: Nogo receptor antagonism promotes
stroke recovery by enhancing axonal plasticity. J Neurosci 24:62096217, 2004
Pignot V, Hein AE, Barske C, et al: Characterization of two novel proteins, NgRH1 and NgRH2, structurally and biochemically homologous
to the Nogo-66 receptor. J Neurochem 85:717-728, 2003
Wong ST, Henley JR, Kanning KC, et al: A p75(NTR) and Nogo receptor complex mediates repulsive signaling by myelin-associated glycoprotein. Nat Neurosci 5:1302-1308, 2002
Wang KC, Kim JA, Sivasankaran R, et al: P75 interacts with the Nogo
receptor as a co-receptor for Nogo, MAG and OMgp. Nature 420:7478, 2002
Mi S, Lee X, Shao Z, et al: LINGO-1 is a component of the Nogo-66
receptor/p75 signaling complex. Nat Neurosci 7:221-228, 2004
Dechant G, Barde YA: The neurotrophin receptor p75(NTR): Novel
functions and implications for diseases of the nervous system. Nat
Neurosci 5:1131-1136, 2002
Roux PP, Barker PA: Neurotrophin signaling through the p75 neurotrophin receptor. Prog Neurobiol 67:203-233, 2002
Yamashita T, Higuchi H, Tohyama M: The p75 receptor transduces the
signal from myelin-associated glycoprotein to Rho. J Cell Biol 157:565570, 2002
Yamashita T, Tohyama M: The p75 receptor acts as a displacement
factor that releases Rho from Rho-GDI. Nat Neurosci 6:461-467, 2003
Song XY, Zhong JH, Wang X, et al: Suppression of p75NTR does not
promote regeneration of injured spinal cord in mice. J Neurosci 24:
542-546, 2004
McGee AW, Strittmatter SM: The Nogo-66 receptor: Focusing myelin
inhibition of axon regeneration. Trends Neurosci 26:193-198, 2003
Niederost B, Oertle T, Fritsche J, et al: Nogo-A and myelin-associated
glycoprotein mediate neurite growth inhibition by antagonistic regulation of RhoA and Rac1. J Neurosci 22:10368-10376, 2002
Fournier AE, Takizawa BT, Strittmatter SM: Rho kinase inhibition enhances axonal regeneration in the injured CNS. J Neurosci 23:14161423, 2003
378
67. Dergham P, Ellezam B, Essagian C, et al: Rho signaling pathway targeted to promote spinal cord repair. J Neurosci 22:6570-6577, 2002
68. Lehmann M, Fournier A, Selles-Navarro I, et al: Inactivation of Rho
signaling pathway promotes CNS axon regeneration. J Neurosci 19:
7537-7547, 1999
69. Nishiyama M, Hoshino A, Tsai L, et al: Cyclic AMP/GMP-dependent
modulation of Ca2⫹ channels sets the polarity of nerve growth-cone
turning. Nature 423:990-995, 2003
70. Cai D, Shen Y, De Bellard M, et al: Prior exposure to neurotrophins
blocks inhibition of axonal regeneration by MAG and myelin via a
cAMP-dependent mechanism. Neuron 22:89-101, 1999
71. Qiu J, Cai D, Dai H, et al: Spinal axon regeneration induced by elevation
of cyclic AMP. Neuron 34:895-903, 2002
72. Pearse DD, Pereira FC, Marcillo AE, et al: cAMP and Schwann cells
promote axonal growth and functional recovery after spinal cord injury. Nat Med 10:610-616, 2004
73. Cai D, Deng K, Mellado W, et al: Arginase I and polyamines act downstream from cyclic AMP in overcoming inhibition of axonal growth
MAG and myelin in vitro. Neuron 35:711-719, 2002
74. Sivasankaran R, Pei J, Wang KC, et al: PKC mediates inhibitory effects
of myelin and chondroitin sulfate proteoglycans on axonal regeneration. Nat Neurosci 7:261-268, 2004
F. Hu and S.M. Strittmatter
75. Rezaie P, Dean A: Periventricular leukomalacia, inflammation and
white matter lesions within the developing nervous system. Neuropathology 22:106-132, 2002
76. Volpe JJ: Perinatal brain injury: From pathogenesis to neuroprotection.
Ment Retard Dev Disabil Res Rev 7:56-64, 2001
77. Back SA, Luo NL, Borenstein NS, et al: Late oligodendrocyte progenitors coincide with the developmental window of vulnerability for human perinatal white matter injury. J Neurosci 21:1302-1312, 2001
78. Ment LR, Schwartz M, Makuch RW, et al: Association of chronic sublethal hypoxia with ventriculomegaly in the developing rat brain. Brain
Res Dev Brain Res 111:197-203, 1998
79. Curristin SM, Cao A, Stewart WB, et al: Disrupted synaptic development in the hypoxic newborn brain. Proc Natl Acad Sci U S A 99:
15729-15734, 2002
80. Huppi PS, Murphy B, Maier SE, et al: Microstructural brain development after perinatal cerebral white matter injury assessed by diffusion
tensor magnetic resonance imaging. Pediatrics 107:455-460, 2001
81. Li S, Strittmatter SM: Delayed systemic Nogo-66 receptor antagonist
promotes recovery from spinal cord injury. J Neurosci 23:4219-4227,
2003
82. Fournier AE, Gould GC, Liu BP, et al: Truncated soluble Nogo receptor
binds Nogo-66 and blocks inhibition of axon growth by myelin. J Neurosci 22:8876-8883, 2002