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A Hypothesis About the Relationship of Myelin Associated Glycoprotein's Function in Myelinated Axons to Its Capacity to Inhibit Neurite GrowthThis paper is not endorsed by the uploader.TRANSCRIPT
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ORIGINAL PAPER
A Hypothesis About the Relationship of Myelin-AssociatedGlycoproteins Function in Myelinated Axons to its Capacityto Inhibit Neurite Outgrowth
Richard H. Quarles
Accepted: 12 March 2008 / Published online: 12 April 2008
Springer Science+Business Media, LLC 2008
Abstract The myelin-associated glycoprotein (MAG) is
selectively localized in periaxonal Schwann cell and oli-
godendroglial membranes of myelin sheaths suggesting
that it functions in gliaaxon interactions in the PNS and
CNS, and this is supported by much experimental evi-
dence. In addition, MAG is now well known as one of
several white matter inhibitors of neurite outgrowth in
vitro and axonal regeneration in vivo, and this latter area of
research has provided a substantial amount of information
about neuronal receptors or receptor complexes for MAG.
This article makes the hypothesis that the capacity of MAG
to inhibit outgrowth of immature developing or regener-
ating neurites is an aberration of its normal physiological
function to promote the maturation, maintenance, and
survival of myelinated axons. The overview summarizes
the literature on the function of MAG in PNS and CNS
myelin sheaths and its role as an inhibitor of neurite out-
growth to put this hypothesis into perspective. Additional
research is needed to determine if receptors and signaling
systems similar to those responsible for MAG inhibition of
neurite outgrowth also promote the maturation, mainte-
nance, and survival of myelinated axons as hypothesized
here, or if substantially different MAG-mediated signaling
mechanisms are operative at the gliaaxon junction.
Keywords Axonglia interactions Gangliosides Myelin-associated glycoprotein Myelinated axons Neurite outgrowth inhibition Nogo receptor Sialic acid
Abbreviations
MAG Myelin-associated glycoprotein
MS Multiple sclerosis
NgR Nogo receptor
p75NtR p75 Neurotrophin receptor
OMgp Oligodendrocyte-Myelin glycoprotein
2,3- or 2,6-SA a2,3- or a2,6-linked sialic acidsiglec Sialic acid-binding immunoglobulin-like
lectin
Introduction
The selective localization of the myelin-associated glyco-
protein (MAG) in periaxonal Schwann cell and
oligodendroglial membranes of myelin sheaths suggests
that it functions in gliaaxon interactions in the PNS and
CNS. Furthermore, there is a substantial body of published
experimental data supporting this concept (reviewed in
[1]). In addition, MAG is now well known as one of several
white matter inhibitors of neurite outgrowth in vitro and
axonal regeneration in vivo (reviewed in [2, 3]), and this
latter area of research has provided a substantial amount of
information about neuronal receptors or receptor com-
plexes for MAG. However, It is not yet clear how the
information obtained from MAG inhibition of neurite
outgrowth relates to its function in the periaxonal glial
membranes of myelinated axons. The principal objective of
this article is to propose the hypothesis that the capacity of
MAG to inhibit outgrowth of immature developing or
Special issue article in honor of Dr. George DeVries.
R. H. Quarles (&)National Institute of Neurological Disorders and Stroke,
NIH, 5625 Fishers Lane, Rm. 4S-30, MSC 9407,
Bethesda, MD 20892, USA
e-mail: [email protected]
123
Neurochem Res (2009) 34:7986
DOI 10.1007/s11064-008-9668-y
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regenerating neurites is an aberration of its normal physi-
ological function to promote the maturation, maintenance,
and survival of myelinated axons.
The Role of MAG in GliaAxon Interactions
Within Myelinated Axons
The periaxonal localization of MAG, its five immuno-
globulin-like domains, and the abnormalities in MAG-
deficient mice all implicate MAG in gliaaxon interactions
related to the formation and maintenance of myelin
sheaths. Furthermore, it appears that MAG may participate
in signaling in both directions between axons and glia. It
had long been thought that MAG was likely to be a glial
receptor for an axonal signal that promotes myelination,
and some findings do support such a role, especially in the
CNS [1]. MAG expression begins very early in the process
of myelination [48], so it could function in the initial
interactions of oligodendroglial processes with axons and
in the molecular mechanisms by which oligodendroglial
and Schwann cell membranes wrap around axons to form
myelin. However, it also appears that MAG is a ligand that
binds to a receptor on the axonal surface of myelinated
axons and thereby affects axonal properties. MAG
expression continues at a relatively high level in mature
animals consistent with important functions in the main-
tenance of myelin and myelinated axons. It is the signaling
from glial cells to the axons that will be emphasized here,
because that is the signaling that is most likely to be related
to MAGs capacity to inhibit neurite outgrowth.
Research on two lines of MAG knockout mice [9, 10]
has played a key role in forming our concepts of MAG
function. Representative electron micrographs illustrating
some of the key points described here are included in two
review articles that emphasize MAG-null mice [5, 11].
Behavioral studies on MAG-null mice have produced
variable results, but as a whole they demonstrate functional
and locomotor neurological deficits [9, 10, 12, 13]. These
mice exhibit subtle structural abnormalities in the periax-
onal region of myelin sheaths, consistent with MAGs
periaxonal localization. Although they form compact
myelin relatively normally, myelination in the CNS is
significantly delayed. Other CNS anomalies include aber-
rant or redundant myelin loops, supernumerary myelin
sheaths, and mild disorganization of the gliaaxon junc-
tions in the paranodal regions. However, the most
pronounced abnormality as the MAG-null mice age was
first reported in the PNS and consists of degeneration of
myelinated axons in sciatic nerves [14, 15]. The amount of
compact myelin in the nerves appears normal. Rather, the
pathology is associated with decreased axonal caliber,
increased neurofilament density, reduced expression, and
phosphorylation of neurofilaments and eventual axonal
degeneration. Early electrophysiological studies on rela-
tively young MAG-null mice (less than 5 months) did not
reveal abnormalities [10, 12]. However, evaluation of sci-
atic nerves in 1-year old MAG-null mice demonstrated a
mild, but statistically significant, reduction of conduction
velocity and small decreases in compound muscle action
potential amplitudes that did not reach statistical signifi-
cance [15]. Overall, these findings, taken in the context of
the ultrastructural and biochemical studies on these
mutants, are consistent with an axonopathy of the PNS that
progresses with age in the absence of MAG, rather than a
demyelinating neuropathy.
The capacity of myelination to increase the caliber of
axons in internodes via a mechanism involving expression
and phosphorylation of neurofilaments has been known for
many years (reviewed in [16]). The findings in MAG-null
mice suggest that MAG is a ligand at the inside of myelin
sheaths that binds to an axonal receptor to mediate this
effect. However, the structural disruption of the periaxonal
region in myelinated axons of MAG-null mice raises the
possibility that a general loosening of the gliaaxon junc-
tion in vivo could interfere with other signaling systems in
which MAG does not participate directly. That uncertainty
was circumvented by in vitro models in which neurons
were co-cultured with MAG-transfected COS cells or
treated with a soluble MAG-Fc chimera [17]. The presence
of MAG resulted in increased expression and phosphory-
lation of neurofilament subunits and microtubule
associated proteins together with up-regulation of cyclin
dependent kinase-5 and extracellular signal regulated
kinases 1 & 2, both of which phosphorylate axonal cyto-
skeletal components and are decreased in MAG-null mice.
These in vitro results substantially reinforce the concept
that MAG itself is a component of a signaling system that
affects the axonal cytoskeleton
Early studies on the CNS of aging MAG-null mice
indicated that myelinated axons appeared normal [12, 18],
but more recent reports demonstrate axonal pathology in
the CNS as the mice age. In one report [19], the axonal
injury consisted of focal swellings and spheroids, whereas
in a different report [13] quantitatively and qualitatively
similar decreases in axonal diameter and neurofilament
spacing as well as axonal degeneration were reported for
both the CNS and PNS. The mice in the latter study had
been backcrossed from the mixed background on which
they were generated to the C57BL/6 background, and it
was suggested that the importance of MAG for the main-
tenance of CNS axons may depend on the overall genetic
background. Other possibilities are that the two indepen-
dent lines of MAG knockout mice [9, 10] are different in
some respects or that there are regional differences in the
extent of axonal degeneration in the CNS. Further
80 Neurochem Res (2009) 34:7986
123
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investigations are needed before a full understanding of the
relative importance of MAG for the maintenance and
structural integrity of CNS and PNS myelinated axons is
achieved. Nevertheless, it is clear that MAG participates in
a signaling system within the periaxonal region of myelin
sheaths that is necessary for the maintenance and survival
of some axons.
Information About Biochemical Mechanisms of MAG
Function Revealed by its Inhibition of Neurite
Outgrowth
In recent years, a substantial amount of information has
been obtained about a neuronal receptor complex for MAG
from investigation of several white matter inhibitors of
neurite outgrowth in vitro and neuronal regeneration
in vivo (Fig. 1). The inhibitors include MAG, oligoden-
drocyte-myelin glycoprotein (OMgp) and Nogo, all three
of which appear to act via the Nogo receptor (NgR) despite
the fact that they have little structural similarity. MAG can
inhibit or promote neurite outgrowth depending on the
developmental status of the neuron and other factors, but
most emphasis has been placed on its inhibitory properties
because of the potential relevance to regeneration follow-
ing neural injury. Recent reviews of this active area of
research are available [2, 3], and it will not be considered
in detail here. Nevertheless, the information obtained about
a neuronal receptor for MAG from this research may be
relevant to MAGs physiological function in the formation
and maintenance of myelinated axons. However, it is not
obvious how the capacity of MAG to inhibit outgrowth of
immature or regenerating neurites would relate to its nor-
mal physiological function. MAG is not present early in
development during the most active phase of neurite out-
growth. Later in development it is sequestered at the inside
of myelin sheaths, where it would not be accessible to
interact with growing neurites. Therefore, the hypothesis of
Raisman [20] that the physiological role of myelin-related
inhibitors is actually to facilitate rapid longitudinal axonal
growth along pre-existing myelinated tracts via a repulsive
guidance-contact mechanism may apply to Nogo and
OMgp, but probably not to MAG. A possible functional
role for MAG and the other inhibitors is that they prevent
the sprouting of neuronal processes that could interfere
with the spiraling of glial membranes around axons to form
myelin early in development. Huang et al. [21] reported
that OMgp in the nodal axoglial apparatus is particularly
important in preventing collateral axonal sprouting at
nodes of Ranvier, although some MAG was detected in
these structures and also could play a role. Nevertheless,
expression of MAG continues at high levels in adults, and
the observations on MAG-null mice demonstrate that it has
essential functions in the maintenance of mature myelin-
ated axons. Actually, the capacity of MAG to inhibit
outgrowth of immature neurites is consistent with a MAG-
mediated signaling mechanism that could promote the
maturation of axons during the formation of adult mye-
linated axons. Thus, a physiologically important signal
promoting the maturation and stability of myelinated axons
could be interpreted inappropriately by a plastic developing
neurite in vitro or a regenerating neurite in vivo, thereby
inhibiting its growth. Whether or not the inhibition of
neurite outgrowth by MAG, which is well established
in vitro, is also a significant factor in preventing regener-
ation in vivo following neural injury is not as clear. MAG
may be released from its sequestered periaxonal localiza-
tion during degeneration following tissue injury so it could
be accessible to regenerating neurons. Indeed in the CNS, a
proteolytic derivative of MAG [22], consisting of its sol-
uble extracellular domain [23], is released in some
neurological disorders [24] and has been shown to inhibit
neurite outgrowth in vitro [25].
In addition to the NgR, some gangliosides have also
been shown to be MAG receptors involved in the inhibition
of neurite outgrowth [26, 27] (Fig. 1). MAG is in the
siglec (sialic acid-binding immunolgobulin-like lectin)
subgroup of the Ig superfamily [11, 28, 29], whose mem-
bers exhibit high homology in the first two amino terminal
Ig-like domains and bind to sialic acid-containing oligo-
saccharides [30]. In the siglec nomenclature, MAG is
siglec-4a, and siglec-4b is Schwann cell myelin protein in
birds that could be the avian ortholog of MAG. The linkage
of sialic acid to the underlying sugar is a very important
determinant of siglec binding, and MAG shows high
specificity for a2,3-linked sialic acid (2,3-SA) in compar-ison to a2,6-linked sialic acid (2,6-SA). It binds well tooligosaccharides with 2,3-SA on a core structure of Galb1-3GalNAc found in some gangliosides, such as the major
GD1a and GT1b brain gangliosides as well as some minor
ones [28, 31], and in O-linked oligosaccharides on glyco-
proteins. However, MAG also binds to N-linked
oligosaccharides with 2,3-SA on glycoproteins [32, 33].
The fact that MAG binds to oligosaccharides on both
glycoproteins and gangliosides indicates that there are
likely to be many binding-partners on neuronal mem-
branes. Furthermore, a potentially important general
consideration with regard to MAGs sialic-acid binding
properties in signal transduction is the substantial differ-
ence in the predominant sialic acid linkages on
glycoproteins of the CNS and PNS. Whereas most of the
sialic acid on glycoproteins in the CNS is 2,3-linked, most
is 2,6-linked in the PNS [34]. Another difference is that the
PNS also contains lower levels of gangliosides, especially
those in the ganglio-series including GD1a and GT1b [35].
Binding and signal transduction to adjacent cells by MAG
Neurochem Res (2009) 34:7986 81
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and other siglecs may be regulated by overall levels of both
cis and trans sialic acid [30, 36]. Thus if binding to any
glycoconjugate containing 2,3-SA on the adjacent cell is
sufficient for a MAG function, that would occur more
readily in the CNS. However, if interaction must be with a
specific binding partner, this could occur more readily in
the PNS because the high level of 2,3-SA in the CNS could
mask the correct binding partner. The latter circumstance
might explain the higher levels of MAG in the CNS than
the PNS, which would be needed to overcome the greater
degree of masking.
There have been two sets of findings about the receptor
that transmits the MAG-mediated signal that inhibits neu-
rite outgrowth that initially seemed to differ with regard to
the relevance of the sialic acid-binding capacity of MAG
(Fig. 1) (reviewed in [2, 3]). Early reports on the inhibition
indicated that it was sialic acid-dependent [37], although
this might only reflect a MAG docking mechanism and a
Inhibition of neurite outgrowth ? Maturation, maintenance and survival of myelinated axons
"raft"
Sialic acid-dependent
Glialmembrane
Neuronalmembrane
MAGMAG
Combined model
glycoprotein ganglioside ganglioside / p75NtR NgR1 (or NgR2) / p75NtR (or TROY) / LINGO-1Receptors:
Glialmembrane
Neuronalmembrane
Nogo receptor complex
OMgpOMgp
NogoNogo MAGMAG
Fig. 1 Neuronal receptor complexes for MAG and the role of sialicacidBinding of MAG (yellow) to a component (green) on the
axolemma is likely to be sialic acid-dependent because MAG is a
lectin in the siglec family. Some published results support this (see
text), and three possibilities are schematically represented at the upper
left. The simplest would be MAG directly binding to a sialyloligo-
saccharide on a glycoprotein or ganglioside (both green), and one
report described a somewhat more complex situation in which a
ganglioside was an intermediary for the interaction of MAG with the
p75NtR. However, the initial reports that MAG was one of several
ligands for the NgR (also green) (upper right) indicated that MAG
binding was sialic acid-independent and that p75NtR interacted
directly with NgR. This receptor complex is located in lipid rafts
(pink). Other ligands for this receptor are Nogo itself (brown) and
OMgp (dark blue). In addition, TROY can substitute for p75NtR in
the receptor complex, and LINGO-1 (red) is another component of the
complex that interacts with both NgR and p75NtR, but does not bind
any of the ligands. A more recent report modified the situation with
regard to sialic acid dependence with the demonstration that both
NgR1 and its NgR2 isoform expressed in neurons bind MAG in a
sialic acid-dependent manner. Furthermore, NgR2 binds MAG more
strongly than NgR1, and does not bind Nogo or OMgp. See text for
original references about the various components in the NgR
complexes. A possible combined model incorporating many compo-
nents of both schemes is illustrated at the bottom of the figure. Here
components of the receptor complex are localized together in a lipid
raft by a variety of different interactions with each other and with
MAG, some of which are sialic acid-dependent and others sialic acid-
independent. The sialic acid-dependent interactions could include
recruitment of MAG and p75NtR to the complex by interaction with
gangliosides and the strong binding of MAG to NgR2. It remains to
be established if a receptor complex such as this, which may function
in the inhibition of neurite outgrowth (arrow to the left), is also
involved in gliaaxon interactions within the periaxonal space of
myelinated axons that promotes their maturation, maintenance and
survival (arrow to the right). Furthermore, it should be noted that of
the three ligands shown for the NgR, only MAG is selectively
localized in the periaxonal glial membrane, whereas Nogo and OMgp
are much more widely distributed. Also this figure depicts all three as
being localized in the adjacent glial membrane, whereas they are
likely to be in degrading membrane fragments or released proteins
when they act to inhibit neurite outgrowth following neural injury.
Modified from Fig. 3 in Ref. [1] with permission
82 Neurochem Res (2009) 34:7986
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separate polypeptide-binding site is needed for outgrowth
inhibition [38]. Indeed, this polypeptide-binding site was
recently identified to be in Ig-domain 5 of MAG that is
closest to the glial membrane and far removed from the
sialic acid-binding site in terminal Ig-domain 1 [39].
However, the first reports of MAG interaction with the
NgR expressed in non-neural cells indicated that the
binding was sialic acid-independent [40, 41]. However, a
more recent report demonstrated sialic acid-dependent
binding of MAG both to NgR1 and its NgR2 homologue
expressed in neurons [42]. NgR2 bound MAG with higher
affinity than NgR1 and does not bind Nogo or OMgp,
raising the interesting possibility that NgR2 could be the
MAG receptor most relevant to its function in myelinated
axons. As mentioned above, other reports indicated that
gangliosides containing terminal 2,3-SA are functional
binding partners for MAG inhibition of neurite outgrowth
[26, 27]. Also, the p75 neurotrophin receptor (p75NtR) was
implicated as a signal transducing partner in the receptor
complex for MAG that was needed because neither the
phosphatidylinositol-linked NgR nor gangliosides span
the membrane to interact with signaling molecules on the
cytoplasmic side. Some reports demonstrated a direct
interaction between the p75NtR and the NgR [43, 44], but
one implicated an interaction of GT1b ganglioside with the
p75NtR, suggesting a role for sialic acid-binding in
bringing MAG and p75NtR together in the receptor com-
plex [45] (Fig. 1). Recent reports have identified LINGO-1
as another component of the complex and demonstrated
that Troy/TAJ can serve as an alternative co-receptor with
NgR in place of p75NtR (Fig. 1) (see [2, 3] for review).
The MAG receptor complex is present in lipid rafts [42, 46,
47], in which both gangliosides and phosphtidylinositol-
linked proteins are characteristic components. The NgR
and gangliosides may be part of independent receptor
systems for MAG, which could function in the same or
different types of neurons. Indeed, there are recent reports
showing that there are differences between types of neu-
rons with regard to the importance of sialic acid,
gangliosides, and NgRs for MAG inhibition of outgrowth
[48, 49]. However, it also seems possible that the variable
results with regard to sialic acid dependence could be
explained by the complex properties of raft-localized
receptor complexes with a variety of interactions between
the different components, as well as variable techniques
that were used by different investigators. A model com-
bining many aspects of the various reports about the sialic
acid-dependence or independence of signaling is shown at
the bottom of Fig. 1. The report that sialidase treatment
enhances spinal axon outgrowth in a model of traumatic
injury to rat nerve roots suggests that sialic acid-dependent
MAG signaling could be involved in outgrowth inhibition
in vivo [50].
The findings summarized above indicate that the
molecular mechanisms mediating the effects of MAG and
other white matter inhibitors on neuronal properties are
complex. However, taken as a whole, the results clearly
demonstrate that there is one or more receptor complexes
on the surface of developing or regenerating neurites that
transmit signals induced by MAG that cause inhibition of
neurite outgrowth. Also, much has been learned about the
intracellular signaling pathways that are affected by MAG
and the other white matter inhibitors and how they are
regulated [2, 3]. The signaling within neurons involves
activation of Rho and Rho kinase, activation of protein
kinase C, influx of calcium and eventual changes in the
actin and microtubular cytoskeleton that cause growth cone
collapse. More recent findings have implicated intramo-
lecular proteolysis of the p75 NtR by a- and b-secretases,downstream activation of the epidermal growth factor
receptor and inhibition of microtubule assembly via the
collapsing response mediator protein-2 [51]. With regard to
the normal physiological role of MAG, an important out-
standing question is whether or not similar neuronal
receptor complexes and intracellular signaling pathways
mediate the effects of MAG within myelinated axons that
are needed for their maturation, maintenance, and survival
(Fig. 1).
Localization of MAG Receptors
In order, for the potential MAG receptor complexes dis-
cussed above in the context of neurite outgrowth inhibition
to function in the physiological signaling affecting axonal
properties of myelinated axons in vivo, they must be
localized in the axolemma of myelinated axons. Ganglio-
sides are wide spread in neuronal membranes. Ganglioside
analysis of rat axolemma isolated from myelinated axons
of rat brain demonstrated the presence of all the major
brain gangliosides, including GD1a and GT1b that are
binding partners for MAG [52]. GT1b was also suggested
to be in the axolemma of myelinated axons in peripheral
nerve based on histochemical staining with tetanus toxin
fragment C [53].
Similarly to gangliosides, NgR1 and NgR2 are also
widely distributed in neurons of the postnatal brain,
although their distributions are somewhat different [42].
Immunohistochemistry showed that they are present
throughout the axons in internodes of myelinated fibers, but
not selectively concentrated in the axolemma [42, 54].
Recently, we used biochemical subfractionation procedures
to investigate whether components of the NgR receptor
complex are present in axolemma of rat brain myelinated
axons (R. Quarles, A. Sedlock, and R. Giger, unpublished
results). A modification of the procedure of DeVries et al.
Neurochem Res (2009) 34:7986 83
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[55] for isolating axolemma enriched fractions (AEF) was
employed, and the fractions were analyzed by western
blotting. Both NgR1 and NgR2 as well as LINGO-1 were
in the AEF, suggesting that a NgR complex could partic-
ipate in MAG signaling within the periaxonal compartment
of myelinated axons. However, we were unable to detect
the p75NtR or TROY in the axolemma fraction, suggesting
than another transmembrane signal transducing component
could be involved. Alternative signal transducing agents
for NgRs have also been suggested by studies of MAG
inhibition of neurite outgrowth in different types of neu-
rons [48]. In addition, we attempted to demonstrate an
interaction between the endogenous MAG and NgR mol-
ecules by co-immunoprecipiation experiments using both
anti-MAG and anti-NgR antibodies. For these experiments,
we modified the gradients for subfractionation so that the
membrane fractions used included both AEF and a heavy
myelin fraction enriched in MAG-containing periaxonal
oligodendroglial membranes. However, these co-immuno-
precipitation experiments did not provide convincing
evidence for a significant interaction of MAG and NgR
within these membranes from myelinated axons. In sum-
mary, these experiments demonstrated the presence of key
components of the NgR complex within the axolemma of
myelinated axons consistent with their involvement in
MAG-mediated signaling at this location, but attempts to
demonstrate an interaction of the NgR with MAG in these
fractions by co-immunoprecipitation experiments were
unsuccessful in our hands. Clearly, further research is
needed to determine if and in what form a NgR complex is
involved in transmitting MAG-mediated signals to mye-
linated axons.
Conclusions and Perspectives
It is clear from studies on MAG-null mice that this gly-
coprotein is essential for the maintenance of myelinated
axons in the PNS. However, its importance for a similar
function in the CNS remains to be clarified in view of
differing reports. Factors requiring further investigation
that may be related to these differences include possible
CNS regional differences in the MAG-requirement for
axonal stability and the effect of genetic background of the
mice. It is also well established that MAG-mediated sig-
naling affects the caliber of PNS myelinated axons by
increasing the expression of phosphorylated neurofila-
ments, but essentially nothing is known about why PNS
axons actually degenerate in the absence of this signaling.
Much has been learned in the context of inhibition of
outgrowth of immature neurites about the neuronal NgR
complex for MAG and other white matter ligands as well
as MAG binding to ganglioside receptors. In addition much
is known about the intracellular signaling pathways within
neurons that are affected by MAG in this context. The
specificity of the NgR2 isoform for MAG, and not Nogo or
OMgp, provides an intriguing suggestion that it could be an
important specific receptor for MAG with regard to the
effect of MAG-mediated signaling to myelinated axons.
The underlying hypothesis of this paper is that the capacity
of MAG to inhibit neurite outgrowth is a manifestation of a
physiological MAG-mediated signaling mechanism for
promoting maturation and stability of axons to optimize
their properties for rapid saltatory conduction in myelin-
ated fibers.
Further research to test this hypothesis will be partic-
ularly challenging because the signaling occurs within the
sequestered periaxonal space of myelinated axons, which
is not readily modeled by in vitro systems. The presence
of MAG-binding gangliosides and some components of
the NgR receptor complex in the axolemma of myelinated
axons is consistent with the hypothesis. Furthermore,
mice lacking GM2/GD2 synthase and not expressing
complex gangliosides have been shown to exhibit simi-
larities to MAG-null mice including axonal degeneration
in the PNS and CNS [56], suggesting that gangliosides
could be part of functional receptors for MAG in the
axolemma. Careful examination of axonal structure and
stability in transgenic mice lacking various components of
the NgR complex could also be informative. An inter-
esting possibility is that the amount of 2,3-SA on trans
and cis glycoconjugates at the axonglia junction regu-
lates MAG-mediated signaling as appears to be the case
for other siglecs [30, 36].
The MAG-mediated signaling mechanisms affecting the
stability of myelinated axons are of potential clinical sig-
nificance, because MAG is lost earlier than most other
myelin components in the progression of many multiple
sclerosis (MS) lesions (reviewed in detail in [1]). Its loss
could contribute to axonal degeneration that is important
for the irreversible neurological deficits in MS and other
disorders of myelin [57]. An important objective of future
MAG research will be to determine if signaling systems
similar to those responsible for MAG inhibition of neurite
outgrowth also promote the maturation, maintenance, and
survival of myelinated axons as hypothesized here, or if
substantially different MAG-mediated signaling mecha-
nisms are operative at the gliaaxon junction.
Acknowledgments This article is dedicated to George DeVries, along time colleague and good friend. The subject is very appropriate
because George followed the MAG story closely as it evolved over
the years. Preparation of this review and covered research from our
laboratory were supported by the Intramural Research Program of
NINDS, NIH.
84 Neurochem Res (2009) 34:7986
123
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References
1. Quarles RH (2007) Myelin-associated glycoprotein (MAG): past,
present and beyond. J Neurochem 100:14311448
2. Filbin MT (2006) Recapitulate development to promote axonal
regeneration: good or bad approach? Philos Trans R Soc Lond B
Biol Sci 361:15651574
3. Yiu G, He Z (2006) Glial inhibition of CNS axon regeneration.
Nat Rev Neurosci 7:617627
4. Quarles RH (1989) Glycoproteins of myelin and myelin-forming
cells. In: Margolis RU, Margolis RK (eds) Neurobiology of
glycoconjugates. Plenum Publishing Corporation, New York, pp
243275
5. Schachner M, Bartsch U (2000) Multiple functions of the myelin-
associated glycoprotein MAG (siglec-4a) in formation and
maintenance of myelin. Glia 29:154165
6. Keita M, Magy L, Heape A, Richard L, Piaser M, Vallat JM
(2002) Immunocytological studies of L-MAG expression regu-
lation during myelination of embryonic brain cell cocultures. Dev
Neurosci 24:495503
7. Nakahara J, Takemura M, Gomi H, Tsunematsu K, Itohara S,
Asou H, Ogawa M, Aiso S, Tan-Takeuchi K (2003) Role of radial
fibers in controlling the onset of myelination. J Neurosci Res
72:279289
8. Paivalainen S, Heape AM (2007) Myelin-associated glycoprotein
and galactosylcerebroside expression in Schwann cells during
myelination. Mol Cell Neurosci 35:436446
9. Li C, Tropak MB, Gerlai R, Clapoff S, Abramow NW, Trapp B,
Peterson A, Roder J (1994) Myelination in the absence of myelin-
associated glycoprotein. Nature 369:747750
10. Montag D, Giese KP, Bartsch U, Martini R, Lang Y, Bluthmann
H, Karthigasan J, Kirschner DA, Wintergerst ES, Nave KA et al
(1994) Mice deficient for the myelin-associated glycoprotein
show subtle abnormalities in myelin. Neuron 13:229246
11. Georgiou J, Tropak MP, Roder JC (2004) Myelin-associated
glycoprotein gene. In: Lazzarini RA (ed) Myelin biology and
disorders. ElsevierAcademic Press, San Diego, pp 421467
12. Uschkureit T, Sporkel O, Stracke J, Bussow H, Stoffel W (2000)
Early onset of axonal degeneration in double (plp-/-mag-/-) and
hypomyelinosis in triple (plp-/-mbp-/-mag-/-) mutant mice.
J Neurosci 20:52255233
13. Pan B, Fromholt SE, Hess EJ, Crawford TO, Griffin JW, Sheikh
KA, Schnaar RL (2005) Myelin-associated glycoprotein and
complementary axonal ligands, gangliosides, mediate axon sta-
bility in the CNS and PNS: neuropathology and behavioral
deficits in single- and double-null mice. Exp Neurol 195:208
217
14. Yin X, Crawford TO, Griffin JW, Tu P, Lee VM, Li C, Roder J,
Trapp BD (1998) Myelin-associated glycoprotein is a myelin
signal that modulates the caliber of myelinated axons. J Neurosci
18:19531962
15. Weiss MD, Luciano CA, Quarles RH (2001) Nerve conduction
abnormalities in aging mice deficient for myelin-associated gly-
coprotein. Muscle Nerve 24:13801387
16. Pigano G, Kirkpatrick LL, Brady ST (2006) The cytoskeleton of
neurons and glia. In: Siegel GJ, Albers RW, Brady ST, Price DL
(eds) Basuc neurochemistry: molecular, cellular and medical
aspects. Academic Press Elsevier, Boston, pp 123137
17. Dashiell SM, Tanner SL, Pant HC, Quarles RH (2002) Myelin-
associated glycoprotein modulates expression and phosphoryla-
tion of neuronal cytoskeletal elements and their associated
kinases. J Neurochem 81:12631272
18. Lassmann H, Bartsch U, Montag D, Schachner M (1997) Dying-
back oligodendrogliopathy: a late sequel of myelin-associated
glycoprotein deficiency. Glia 19:104110
19. Loers G, Aboul-Enein F, Bartsch U, Lassmann H, Schachner M
(2004) Comparison of myelin, axon, lipid, and immunopathology
in the central nervous system of differentially myelin-compro-
mised mutant mice: a morphological and biochemical study. Mol
Cell Neurosci 27:175189
20. Raisman G (2004) Myelin inhibitors: does NO mean GO? Nat
Rev Neurosci 5:157161
21. Huang JK, Phillips GR, Roth AD, Pedraza L, Shan W, Belkaid
W, Mi S, Fex-Svenningsen A, Florens L, Yates JR III, Colman
DR (2005) Glial membranes at the node of Ranvier prevent
neurite outgrowth. Science 310:18131817
22. Sato S, Quarles RH, Brady RO (1982) Susceptibility of the
myelin-associated glycoprotein and basic protein to a neutral
protease in highly purified myelin from human and rat brain.
J Neurochem 39:97105
23. Stebbins JW, Jaffe H, Fales HM, Moller JR (1997) Determination
of a native proteolytic site in myelin-associated glycoprotein.
Biochemistry 36:22212226
24. Moller JR, Yanagisawa K, Brady RO, Tourtellotte WW, Quarles RH
(1987) Myelin-associated glycoprotein in multiple sclerosis lesions:
a quantitative and qualitative analysis. Ann Neurol 22:469474
25. Tang S, Qiu J, Nikulina E, Filbin MT (2001) Soluble myelin-
associated glycoprotein released from damaged white matter
inhibits axonal regeneration. Mol Cell Neurosci 18:259269
26. Vinson M, Strijbos PJ, Rowles A, Facci L, Moore SE, Simmons
DL, Walsh FS (2001) Myelin-associated glycoprotein interacts
with ganglioside GT1b. A mechanism for neurite outgrowth
inhibition. J Biol Chem 276:2028020285
27. Vyas AA, Patel HV, Fromholt SE, Heffer-Lauc M, Vyas KA,
Dang J, Schachner M, Schnaar RL (2002) Gangliosides are
functional nerve cell ligands for myelin-associated glycoprotein
(MAG), an inhibitor of nerve regeneration. Proc Natl Acad Sci
USA 99:84128417
28. Kelm S, Pelz A, Schauer R, Filbin MT, Tang S, de Bellard ME,
Schnaar RL, Mahoney JA, Hartnell A, Bradfield P et al (1994)
Sialoadhesin, myelin-associated glycoprotein and CD22 define a
new family of sialic acid-dependent adhesion molecules of the
immunoglobulin superfamily. Curr Biol 4:965972
29. Quarles RH (2002) Myelin sheaths: glycoproteins involved in
their formation, maintenance and degeneration. Cell Mol Life Sci
59:18511871
30. Varki A, Angata T (2006) Siglecsthe major subfamily of I-type
lectins. Glycobiology 16:1R27R
31. Vyas AA, Schnaar RL (2001) Brain gangliosides: functional
ligands for myelin stability and the control of nerve regeneration.
Biochimie 83:677682
32. Strenge K, Schauer R, Bovin N, Hasegawa A, Ishida H, Kiso M,
Kelm S (1998) Glycan specificity of myelin-associated glyco-
protein and sialoadhesin deduced from interactions with synthetic
oligosaccharides. Eur J Biochem 258:677685
33. Strenge K, Schauer R, Kelm S (1999) Binding partners for the
myelin-associated glycoprotein of N2A neuroblastoma cells.
FEBS Lett 444:5964
34. Bartoszewicz ZP, Lauter CJ, Quarles RH (1996) The myelin-
associated glycoprotein of the peripheral nervous system in
trembler mutants contains increased alpha 23 sialic acid and
galactose. J Neurosci Res 43:587593
35. Ogawa-Goto K, Abe T (1998) Gangliosides and glycosphingoli-
pids of peripheral nervous system myelinsa minireview.
Neurochem Res 23:305310
36. Tropak MB, Roder JC (1997) Regulation of myelin-associated
glycoprotein binding by sialylated cis-ligands. J Neurochem
68:17531763
37. DeBellard ME, Tang S, Mukhopadhyay G, Shen YJ, Filbin MT
(1996) Myelin-associated glycoprotein inhibits axonal
Neurochem Res (2009) 34:7986 85
123
-
regeneration from a variety of neurons via interaction with a
sialoglycoprotein. Mol Cell Neurosci 7:89101
38. Tang S, Shen YJ, DeBellard ME, Mukhopadhyay G, Salzer JL,
Crocker PR, Filbin MT (1997) Myelin-associated glycoprotein
interacts with neurons via a sialic acid binding site at ARG118
and a distinct neurite inhibition site. J Cell Biol 138:13551366
39. Cao Z, Qiu J, Domeniconi M, Hou J, Bryson JB, Mellado W,
Filbin MT (2007) The inhibition site on myelin-associated gly-
coprotein is within Ig-domain 5 and is distinct from the sialic acid
binding site. J Neurosci 27:91469154
40. Domeniconi M, Cao Z, Spencer T, Sivasankaran R, Wang K,
Nikulina E, Kimura N, Cai H, Deng K, Gao Y, He Z, Filbin M
(2002) Myelin-associated glycoprotein interacts with the Nogo66
receptor to inhibit neurite outgrowth. Neuron 35:283290
41. Liu BP, Fournier A, GrandPre T, Strittmatter SM (2002) Myelin-
associated glycoprotein as a functional ligand for the Nogo-66
receptor. Science 297:11901193
42. Venkatesh K, Chivatakarn O, Lee H, Joshi PS, Kantor DB,
Newman BA, Mage R, Rader C, Giger RJ (2005) The Nogo-66
receptor homolog NgR2 is a sialic acid-dependent receptor
selective for myelin-associated glycoprotein. J Neurosci 25:808
822
43. Wang KC, Kim JA, Sivasankaran R, Segal R, He Z (2002) P75
interacts with the Nogo receptor as a co-receptor for Nogo, MAG
and OMgp. Nature 420:7478
44. Wong ST, Henley JR, Kanning KC, Huang KH, Bothwell M, Poo
MM (2002) A p75(NTR) and Nogo receptor complex mediates
repulsive signaling by myelin-associated glycoprotein. Nat Neu-
rosci 5:13021308
45. Yamashita T, Higuchi H, Tohyama M (2002) The p75 receptor
transduces the signal from myelin-associated glycoprotein to
Rho. J Cell Biol 157:565570
46. Fujitani M, Kawai H, Proia RL, Kashiwagi A, Yasuda H,
Yamashita T (2005) Binding of soluble myelin-associated gly-
coprotein to specific gangliosides induces the association of
p75NTR to lipid rafts and signal transduction. J Neurochem
94:1521
47. Vinson M, Rausch O, Maycox PR, Prinjha RK, Chapman D,
Morrow R, Harper AJ, Dingwall C, Walsh FS, Burbidge SA,
Riddell DR (2003) Lipid rafts mediate the interaction between
myelin-associated glycoprotein (MAG) on myelin and MAG-
receptors on neurons. Mol Cell Neurosci 22:344352
48. Venkatesh K, Chivatakarn O, Sheu SS, Giger RJ (2007) Molec-
ular dissection of the myelin-associated glycoprotein receptor
complex reveals cell type-specific mechanisms for neurite out-
growth inhibition. J Cell Biol 177:393399
49. Mehta NR, Lopez PH, Vyas AA, Schnaar RL (2007) Ganglio-
sides and Nogo receptors independently mediate myelin-
associated glycoprotein inhibition of neurite outgrowth in
different nerve cells. J Biol Chem 282:2787527886
50. Yang LJ, Lorenzini I, Vajn K, Mountney A, Schramm LP,
Schnaar RL (2006) Sialidase enhances spinal axon outgrowth in
vivo. Proc Natl Acad Sci USA 103:1105711062
51. Mimura F, Yamagishi S, Arimura N, Fujitani M, Kubo T, Kai-
buchi K, Yamashita T (2006) MAG inhibits microtubule
assembly by a Rho-kinase dependent mechanism. J Biol Chem
281:1597015979
52. De Vries GH, Zmachinski CJ (1980) The lipid composition of rat
CNS axolemma-enriched fractions. J Neurochem 34:424430
53. Sheikh KA, Deerinck TJ, Ellisman MH, Griffin JW (1999) The
distribution of ganglioside-like moieties in peripheral nerves.
Brain 122:449460
54. Wang X, Chun SJ, Treloar H, Vartanian T, Greer CA, Strittmatter
SM (2002) Localization of Nogo-A and Nogo-66 receptor pro-
teins at sites of axon-myelin and synaptic contact. J Neurosci
22:55055515
55. DeVries G (1981) Isolation of axolemma-enriched fractions from
mammalian CNS. Res Method Neurochem 5:325
56. Sheikh KA, Sun J, Liu Y, Kawai H, Crawford TO, Proia RL,
Griffin JW, Schnaar RL (1999) Mice lacking complex ganglio-
sides develop Wallerian degeneration and myelination defects.
Proc Natl Acad Sci USA 96:75327537
57. Bjartmar C, Wujek JR, Trapp BD (2003) Axonal loss in the
pathology of MS: consequences for understanding the progres-
sive phase of the disease. J Neurol Sci 206:165171
86 Neurochem Res (2009) 34:7986
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A Hypothesis About the Relationship of Myelin-Associated Glycoproteins Function in Myelinated Axons to its Capacity to Inhibit Neurite OutgrowthAbstractIntroductionThe Role of MAG in Glia-Axon Interactions Within Myelinated AxonsInformation About Biochemical Mechanisms of MAG Function Revealed by its Inhibition of Neurite OutgrowthLocalization of MAG ReceptorsConclusions and PerspectivesAcknowledgmentsReferences
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