 |
 Previous Article | Next Article 
The Journal of Neuroscience, March 15, 1998, 18(6):1953-1962
Myelin-Associated Glycoprotein Is a Myelin Signal that Modulates
the Caliber of Myelinated Axons
Xinghua
Yin1,
Thomas O.
Crawford2,
John W.
Griffin2, 3,
Pang-hsien
Tu4,
Virginia M.-Y.
Lee4,
Chumei
Li5,
John
Roder5, and
Bruce D.
Trapp1
1 Department of Neurosciences, Lerner Research
Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44195, Departments of 2 Neurology and 3 Neurosciences,
Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, 4 Department of Pathology and Laboratory Medicine, Division
of Anatomic Pathology, University of Pennsylvania School of Medicine,
Philadelphia, Pennsylvania 19104, and 5 Samuel Lunenfeld
Research Institute, Mt. Sinai Hospital, Toronto, Ontario M5G 1X5 Canada
 |
ABSTRACT |
Myelination increases neuronal conduction velocity through its
insulating properties and an unidentified extrinsic effect that
increases axonal caliber. Although it is well established that
demyelination can cause axonal atrophy, the myelin molecule that
regulates axonal caliber is not known. Loss of the structural proteins
of compact peripheral nervous system (PNS) myelin, P0 protein, and myelin basic protein does not lead to axonal atrophy. This
study demonstrates that mice with a null mutation of the myelin-associated glycoprotein (MAG) gene have a chronic atrophy of
myelinated PNS axons that results in paranodal myelin tomaculi and
axonal degeneration. Absence of MAG was correlated with reduced axonal
calibers, decreased neurofilament spacing, and reduced neurofilament
phosphorylation. Because axonal atrophy and degeneration in
MAG-deficient mice occur in the absence of inflammation,
hypomyelination, significant demyelination-remyelination, or gain of
function mutations, these data support a functional role for MAG in
modulating the maturation and viability of myelinated axons.
Key words:
myelin-associated glycoprotein; axonal atrophy; neurofilament phosphorylation; axonal caliber; neurofilament spacing; paranodal tomaculi
| |
INTRODUCTION |
Peripheral nerve axons are
ensheathed by Schwann cells. Axons with larger calibers are surrounded
by myelin, which is a multilayered, tightly compacted extension of the
Schwann cell plasma membrane. Myelination is essential for rapid
propagation of action potentials and for normal neurological function
(Waxman, 1980 ). Unmyelinated axons, usually of smaller caliber, are
ensheathed by single uncompacted Schwann cell membranes. Myelination of
peripheral nervous system (PNS) axons is initiated by an axonally
derived signal (Aguayo et al., 1976; Weinberg and Spencer, 1976), and
in most settings maintenance of PNS myelin internodes depends on
physical contact with a viable axon (Griffin et al., 1993). Although
axonal regulation of Schwann cell phenotype is well established, it is
also clear that myelination influences the axon locally. Myelination is
associated with increases in axonal caliber (Windebank et al., 1985).
This effect is important functionally because axonal caliber is a
determinant of neuronal conduction velocity (Arbuthnott et al., 1980).
Axonal caliber is influenced by neurofilament number (Hoffman and
Griffin, 1992; Ohara et al., 1993; Eyer and Peterson, 1994) and
neurofilament spacing (Friede and Samorajski, 1971; Hoffman and
Griffin, 1992).
The extrinsic effect of myelination on axonal caliber was demonstrated
in developing rat optic nerve (Sanchez et al., 1996), and by comparing
axonal caliber in myelinated regions with the nonmyelinated initial
segment of dorsal root ganglion axons (Hsieh et al., 1994). The trophic
effect of myelination on axonal caliber is also reversible. In
demyelinated internodes of the Trembler mouse, axonal calibers were
smaller, and neurofilaments (NFs) were spaced closer and were less
phosphorylated than those in control mice (deWaegh et al., 1992).
Reduced axonal calibers and neurofilament spacing after demyelination
is also reversed by remyelination (Hsieh et al., 1993).
Neurofilaments consist of three subunits with apparent molecular
weights of 68, 150, and 200 kDa that are referred to as NFL, NFM, and
NFH, respectively (Hoffman and Lasek, 1975; Hirokawa et al., 1984).
All subunits share a highly conserved central rod domain but differ in
their C-terminal region. NFL forms the core of the 10 nm
neurofilaments, and the C-terminal regions of NFH and NFM include
sidearms that extend from the core filament. The C-terminal region of
NFH and NFM contains 40-50 and 10-15 lysine-serine-proline (KSP)
repeats, respectively, which are potential phosphorylation sites. It
has been proposed that phosphorylation of NFH and NFM increases the
total negative charge and the lateral extension of neurofilament
sidearms (Hirokawa et al., 1984; Nakagawa et al., 1995). Sidearm
extension, in turn, increases neurofilament spacing and axonal
caliber.
The studies discussed above support a positive correlation
between myelination, neurofilament number, neurofilament spacing, neurofilament phosphorylation, and axonal caliber. Although the mechanism by which myelination regulates axonal cytoskeleton is unclear, one tenable possibility is that a molecule(s) in the adaxonal
membrane of the myelin internode regulates neurofilament number or
neurofilament phosphorylation or both. The following observations
support this hypothesis. Ensheathment of axons by oligodendrocyte
processes that do not form myelin because of mutations in myelin
protein genes still increase axonal caliber and neurofilament spacing
(Sanchez et al., 1996). Increased axonal calibers, therefore, can occur
in the absence of myelination as long as the axon is ensheathed by a
myelin-forming cell. However, axonal ensheathment by Schwann cells does
not guarantee increased axonal caliber of unmyelinated fibers,
suggesting that a molecule enriched in the adaxonal membrane of
myelinating Schwann cells and absent from the adaxonal membrane of
nonmyelinating Schwann cells regulates axonal caliber.
Although little is known about the molecules that regulate the
myelinotropic effect on axonal cytoskeleton, the myelin-associated glycoprotein (MAG) is an attractive candidate. MAG is a member of the
I-type lectin subgroup of the Ig gene superfamily (Arquint et al.,
1987; Lai et al., 1987; Salzer et al., 1987) and is expressed exclusively by myelinating cells where it is enriched in the adaxonal membrane of the myelin internode (Sternberger et al., 1979; Trapp and
Quarles, 1982; Trapp et al., 1989). MAG was initially considered a
potential receptor for an axonal ligand important to the initiation and
progression of myelination. This was based on the biochemical properties of MAG and enrichment in periaxonal membranes of myelinated fibers and by studies of transfected Schwann cells in vitro
that correlated MAG overexpression with accelerated myelination (Owens et al., 1990) and MAG underexpression with hypomyelination (Owens and
Bunge, 1991). Normal myelination in MAG-deficient mice (Li et al.,
1994; Montag et al., 1994), however, has ruled out an essential role
for MAG in myelin formation in vivo. In vitro, neurite outgrowth can be inhibited by MAG (McKerracher et al., 1994; Mukhopadhyay et al., 1994), and MAG-coated beads can cause growth
cone collapse (Li et al., 1996). These data raised the possibility that
MAG directly or indirectly modulates axonal cytoskeleton. This study
investigated whether alterations in axonal cytoskeleton occur in
MAG-deficient myelinated fibers. The results, for the first time,
implicate MAG in regulating the levels of neurofilament spacing and
axonal caliber in mature myelinated axons. The data are consistent with
the hypothesis that MAG exerts a major extrinsic influence on the
organization of the axonal cytoskeleton and the viability of axons.
| |
MATERIALS AND METHODS |
Morphological analysis and morphometrics. Mice
deficient in MAG at the mRNA and protein levels (Li et al., 1994) and
age-matched controls were perfused with 4% paraformaldehyde, 2.5%
glutaraldehyde, and 0.08 M of Sorrenson's buffer,
post-fixed in OSO4, and embedded in
Epon. For light microscopy, 1-µm-thick sections were stained with
toluidine blue. For electron microscopy, approximately 120-nm-thick sections were stained with uranyl acetate and lead citrate, and were
examined in a Philips CM100 electron microscope. Axonal caliber of
35-d-old fibers was determined by analysis of electron micrographs (final magnification 3000×) with the aid of a computer-based
morphometric system, as described previously (Xu et al., 1996). For 3-, 6-, 9-, and 16-month-old nerves, similar analysis was performed on 1-µm-thick Epon sections.
Axonal caliber. Mean axon calibers of
myelinated fibers were obtained from 1-µm-thick Epon sections of
sciatic nerve as described previously (Wong et al., 1995). Continuous,
nonoverlapping images in regions of good cross-sectional orientation
containing ~1000 axons were digitized with a standard video frame
grabber (Targa M8). The optimal gray scale and lighting were determined
for each image individually, and all axons with a cross-sectional area >1.5 µm2 centered within the frame were
enumerated with a computerized morphometric program (Bioquant MEGx, R & M Biometrics, Memphis, TN). Axonal caliber is determined by the
diameter of a circle of area equivalent to each axon. Four nerves from
control and MAG-deficient mice were analyzed at 3, 6, 9, and 16 months
of age. To combine different age groups for statistical comparison, the
average axonal diameter of each nerve was transformed by subtracting the appropriate age control group mean diameter value. Statistical analysis was performed by the two-tailed Student's t
test.
Neurofilament spacing. Nearest neighbor
distance was used to measure neurofilament spacing. Electron
micrographs were illuminated on a view box and imaged with a video
camera. Neurofilament bundles that were oriented in true transverse
section were accepted and digitized with Bioquant software (R & M
Biometrics). Nearest neighbor distance was determined as described
previously (Xu et al., 1996). For statistical analysis, the mean
neurofilament nearest neighbor distance for 11 or more axons for each
age was compared by the two-tailed Student's t
test.
Neurofilament content and phosphorylation. Three
pairs of sciatic nerves from 35-d-old, 3-month-old, and 9-month-old
MAG-deficient and age-matched wild-type mice were used for quantitative
Western blot analysis of NF proteins with a panel of NF-specific mouse monoclonal antibodies (mAbs) and rabbit polyclonal antisera. These studies were performed as described previously (Tu et al., 1995), with
minor modifications. Briefly, sciatic nerves were homogenized, sonicated in 200 µl of BUST buffer (50 mM Tris-HC1, pH
7.4, 8 M urea, 2%  -mercaptoethanol, and 0.5% SDS), and
centrifuged at 40 × 103 rpm, at 25°C for 30 min. Protein concentrations in the supernatants were determined using
Coomassie protein assay according to the manufacturer's instructions.
Nerves from three separate animals were analyzed at each time point.
Each sample was loaded in triplicate, and each lane contained 10 µg
of total protein. Blots were then cut into three parts for analyses.
The top third was incubated overnight with RM024 (a mAb specific for
highly phosphorylated NFH, NFHP+++), Rmd09 (a mAb specific for the
poorly or nonphosphorylated NFH, NFHP ), or rabbit polyclonal
antiserum raised against the last 20 amino acids of NFH for total NFH.
The middle third was probed with RM0189 (a mAb for rod domain of NFM)
for total NFM level or with RM055 (a mAb specific for highly
phosphorylated NFM, NFMP+++). Finally, the lower third was probed with
rabbit polyclonal antiserum raised against the C-terminal 20 amino
acids of NFL for total NFL. The blots were incubated for 1 hr with 10 µCi125I-conjugated goat anti-mouse IgG for the
mouse mAbs (RM024, Rmd09, RM055, and RM0189) or
125I-conjugated protein A for the rabbit anti-NFH and
anti-NFL polyclonal antisera. The dried blots were exposed to
phosphorimager plates for various time periods, and individual bands
were visualized and quantified with ImageQuant software (Molecular
Dynamics, Sunnyvale, CA). Statistical analysis was performed by the
Student's t test.
Electron microscopic immunocytochemistry. Segments of
sciatic nerve from control and 9-month-old MAG-deficient mice were
fixed in 2.5% glutaraldehyde and 4% paraformaldehyde, infiltrated
with 2.3 M sucrose and 30% polyvinylpyrrolidone, placed on
specimen stubs, and frozen in liquid nitrogen. Ultrathin cryostections (~120-nm-thick) were cut on glass knives in an ultracryomicrotome (Ultracut S, Reichert Scientific Instruments) maintained at 100°C. The sections were placed on carbon- and formvar-coated grids and immunostained by previously described immunogold procedures (Trapp et
al., 1989). After application of primary antibodies and gold-labeled secondary antibodies, the sections were placed on 2.5% glutaraldehyde for 15 min, stained with uranyl acetate, and mounted in 1.5%
methycellulose containing 3.0% uranyl acetate. Immunostained sections
were examined in a Philips CM100 electron microscope. The antibodies
were the same as those used in Western blots and include RM024
(anti-NFHP+++; 1:5000 dilution) and Rmd09 (anti-NFHP; 1:250
dilution). The number of gold particles per area of axoplasm was
quantified on electron micrographs printed at a magnification of
28,500×. Axoplasm (21 µm2) from 15 control and 15 MAG-deficient fibers was analyzed. Sampling was restricted to axons
surrounded by normal-appearing myelin containing 53-67 lamellae.
Statistical analysis was performed by the Student's t
test.
Teased fiber preparations. Glutaraldehyde-fixed
sciatic nerves from 3- and 9-month-old control and MAG-deficient mice
were osmicated and placed in glycerol. Individual fibers were removed with forceps, placed on slides, and analyzed with a Zeiss axiophot microscope. Thick myelin segments were quantified and scored as to
paranodal or internodal location. Statistical analysis was performed by
the Student's t test.
| |
RESULTS |
To study pathological changes in MAG-deficient myelinated fibers,
the light microscopic appearance of sciatic nerves from control and
MAG-deficient mice was compared. Epon sections (1-µm-thick) of
sciatic nerves from postnatal day (P) 7, P14, and P35, and 3-, 6-, 9- and 16-month-old mice were studied. The appearance of myelinated fibers
in sections from 7- and 14-d-old MAG-deficient sciatic nerves did not
differ significantly from age-matched control nerves (data not shown).
At P35 (Fig.
1A,B),
sections of MAG-deficient nerves contained two to three profiles with
classic features of Wallerian degeneration: multiple myelin ovoids, no
axon, and scattered macrophages. Sections from 3-month-old
MAG-deficient mice contained four to five fibers undergoing
Wallerian-like degeneration, redundant or asymmetrically positioned
myelin, and an apparent reduction in the caliber of myelinated axons
(Fig. 1C,D). Sections from 9- and 16-month-old
MAG-deficient nerves contained a greater proportion of redundant myelin
profiles, four to nine degenerating myelinated fibers, and an apparent
decrease in the calibers of myelinated axons (Fig.
1E-H). Degenerating myelinated
fibers were detected in control nerves at 16 months of age and averaged
less than one per section. Compared with control sciatic nerves,
Student's t test analysis detected a statistically
significant increase in the incidence of Wallerian degeneration in P35
and older MAG-deficient nerves (Table 1).
These observations extend previous reports of axonal pathology and
Wallerian degeneration in adult MAG-deficient nerves (Fruttiger et al.,
1995; Carenini et al., 1997), and they raise the possibility that
myelinated fibers in MAG-deficient mice have reduced axonal
calibers.
View larger version (202K):
[in this window]
[in a new window]
|
Figure 1.
MAG-deficient sciatic nerves contain degenerating
myelin internodes and redundant myelin.
A-D, Epon sections of sciatic nerves from P35 (A = MAG+/+, B = MAG/) and 3-month-old mice (C = MAG+/+, D = MAG/). Occasional MAG-deficient myelinated
fibers are degenerating (B, D,
arrows), and some axons are surrounded by redundant or excessive myelin (B, D,
arrowheads). E-H, At 9 months (E = MAG+/+, F = MAG/) and 16 months (G = MAG+/+,
H = MAG/) of age, degenerating myelin
internodes (arrows) and redundant myelin
(arrowheads) persist in MAG-deficient sciatic nerves.
Compared with control nerves, myelinated fibers in MAG-deficient nerves
appear to have reduced axonal calibers. Scale bar, 10 µm.
|
|
To test whether a generalized reduction in axonal caliber occurred in
MAG-deficient sciatic nerves, a mean average axonal caliber of ~1000
myelinated fibers from four control and four MAG-deficient mice was
obtained at P35 and 3, 6, 9, and 16 months of age. Axonal calibers in
35-d-old MAG-deficient and control nerves were similar (data not
shown). By 3 months and thereafter, myelinated axons in MAG-deficient
nerves were significantly smaller (all ages combined, p < 0.005) than control axons (Fig.
2).
View larger version (K):
[in this window]
[in a new window]
|
Figure 2.
Average axonal calibers of myelinated fibers in
control and MAG-deficient sciatic nerves. MAG-deficient myelinated
axons have reduced calibers. Axons surrounded by normal-appearing
myelin in MAG-deficient nerves are significantly smaller than those in control nerves (p < 0.005, for all ages
combined). Statistical analysis was performed by the two-tailed
Student's t test. Values are mean ± SD.
|
|
Interneurofilament spacing is reduced in MAG-deficient mice
Because decreased axonal caliber can correlate with decreased
neurofilament spacing (deWaegh et al., 1992; Hsieh et al., 1994; Sanchez et al., 1996), the nearest neighbor neurofilament distance was
compared in myelinated axons from MAG-deficient and control mice.
Analysis was restricted to those axons surrounded by normal-appearing myelin. In cross sections of myelinated axons, neurofilaments appear as
10-nm-thick dots (Fig. 3A,
arrowheads) from which thread-like sidearms extend (Fig.
3A, arrows). The spacings between neurofilaments appear greater in control axons (Fig. 3A) than in
MAG-deficient axons (Fig. 3B). Myelinated axons were sampled
randomly in electron micrographs of 14- and 35-d-old and 3- and
9-month-old MAG-deficient and control sciatic nerves. Between 1420 and
3930 neurofilaments were identified, and the distance to their nearest
neighbor was measured. Neurofilament spacing was similar in
MAG-deficient and wild-type myelinated axons at P14 (data not shown).
Statistically significant differences in the nearest neighbor
neurofilament distance in MAG-deficient and control nerves were
detected at P35, 3 months, and 9 months of age (Fig.
3C-E).
View larger version (80K):
[in this window]
[in a new window]
|
Figure 3.
MAG-deficient nerves have reduced neurofilament
spacing. A, B, In electron micrographs,
neurofilaments (arrowheads) appear as 10-nm-thick
structures from which sidearms (arrows) extend. Compared
with myelinated axons in 9-month-old control nerves
(A), neurofilaments are spaced closer together in
9-month-old MAG-deficient myelinated axons (B).
Scale bar, 0.1 µm. C-E, Compared with
myelinated axons in control nerves, the neurofilament nearest neighbor
distance in myelinated axons in MAG-deficient nerves was significantly reduced at P35 (C), 3 months
(D), and 9 months (E).
Statistical analysis by Student's t test analysis;
n = number of neurofilaments analyzed.
F, Neurofilament spacing in unmyelinated axons was
similar in control and MAG-deficient nerves from 9-month-old mice;
n = number of neurofilaments analyzed.
|
|
MAG is not present in the adaxonal Schwann cell membrane of
unmyelinated fibers (Trapp et al., 1989). If alterations in
neurofilament spacing are regulated by MAG, then the nearest neighbor
neurofilament distance in unmyelinated axons should be identical in
MAG-deficient and control mice. Therefore, ~950 neurofilaments in
unmyelinated axons from 9-month-old MAG-deficient and control mice were
identified, and the distance to their nearest neighbor was measured.
The spacing of neurofilaments in unmyelinated fibers was identical in
MAG-deficient and control mice (Fig. 3F). These
studies correlated reduced neurofilament spacing with the loss of MAG
in the adaxonal Schwann cell membrane.
MAG-deficient nerves have decreased
neurofilament phosphorylation
Alterations in axonal diameter and neurofilament spacing have been
correlated with the phosphorylation state of NFH and NFM (deWaegh et
al., 1992; Tu et al., 1995) and with alterations in NF subunit ratios
(Tu et al., 1995; Xu et al., 1996). Based on these observations and the
data in Figures 1-3, the neurofilament subunit composition and the
extent of neurofilament protein phosphorylation were compared in
sciatic nerve homogenates from MAG-deficient and wild-type mice.
Significant decreases in the levels of total NFL and NFH were detected
in MAG-deficient nerve homogenates from P35, 3- and 9-month-old mice,
whereas levels of total NFM were comparable to those in the age-matched
control nerves (Fig. 4). In the absence
of MAG, therefore, NF subunit ratios are altered. Relative levels of
NFH and NFM phosphorylation (P+++) were also significantly decreased in
the MAG-deficient nerves. The reduction in NFHP+++ paralleled that of
total NFH, but there was no decrease in nonphosphorylated (P) NFH
epitopes in 3- and 9-month-old MAG-deficient nerves. These observations
indicate reduced NFH phosphorylation levels in the MAG-deficient mice
relative to NFH nonphosphorylated epitopes. Although total NFM remained
unchanged in MAG-deficient nerves, a significant reduction in NFM
phosphorylation was detected in 3- and 9-month-old nerves (Fig. 4). NFM
is also a structural component of cross-bridges, and these
cross-bridges can control the spacing of neurofilaments (Nakagawa et
al., 1995).
View larger version (84K):
[in this window]
[in a new window]
|
Figure 4.
Neurofilament proteins and their
phosphorylation states are reduced in sciatic nerves of MAG-deficient
mice. A shows comparison between quantitative Western
blots of neurofilament epitopes in control (+/+) and MAG-deficient
(/) sciatic nerve homogenates from P35 and 3- and 9-month old mice.
NFHP, Poorly or nonphosphorylated NFH; NFHP+++ and NFMP+++, highly
phosphorylated NFH and NFM. Three MAG-deficient and three control
nerves were analyzed in triplicate at each time point, and
representative bands were quantitated and expressed as
MAG-deficient/wild-type ratios ± SD (B).
Total NFL, total NFH, and NFHP+++ were significantly reduced at all ages. NFHP was reduced in 35-d-old MAG-deficient homogenates but not
at later time points. Total NFM remained unchanged at all ages, whereas
NFMP+++ was significantly reduced in 3- and 9-month-old MAG-deficient
homogenates. C-E, Ultrathin cryosections of 9-month-old control (C) and MAG-deficient
(D) sciatic nerves were immunostained with
NFHP+++ antibodies by immunogold procedures. Compared with control
nerves (C), MAG-deficient nerves
(D) contained a 50% decrease in NFHP+++ labeling
(E). In contrast, NFHP labeling was
significantly increased in MAG-deficient nerves
(E). Scale bars, 0.1 µm.
|
|
The 20-30% decrease in the amount of highly phosphorylated NFM and
NFH in MAG-deficient mice is a minimum estimate because the homogenates
of sciatic nerve contained both myelinated and unmyelinated axons, and
our electron microscopy data showed that the NFs in the unmyelinated
axons remained unchanged (Fig. 3F). To overcome these
possible limitations and to analyze NFH phosphorylation states in
myelinated axons, ultrathin cryosections of adult sciatic nerves were
immunostained with NFHP+++ and NFHP antibodies using immunogold
procedures (Fig. 4C, D). Gold particles per unit
area of axoplasm were quantified in axons surrounded by 53-67
normal-appearing myelin lamellae. When compared with control fibers, a
50% reduction in NFHP+++ epitope labeling and a fivefold increase in
NFHP epitope labeling was detected in MAG-deficient fibers (Fig.
4E). Because neurofilament density is higher in
MAG-deficient fibers, analysis of these NF epitopes per unit area of
axoplasm indicates significant reductions in NFH phosphorylation in
myelinated axons of MAG-deficient nerves.
Collapse of paranodal axons and formation of redundant myelin
A significant number of myelinated fibers in transverse sections
from adult MAG-deficient sciatic nerves had excessive myelin thickness
(Fig. 1). The extent and location of thick myelin was studied in teased
fibers from control and MAG-deficient sciatic nerves. Thick myelin
segments were quantified in 85 and 150 internodes from 3- and
9-month-old mice, respectively (Fig. 5).
Paranodal regions (57 and 93%) from 3- and 9-month-old
MAG-deficient mice had thick-appearing myelin segments (Fig.
5B). Compared with control fibers, segments of internodal
thick-appearing myelin were also increased in 3- and 9-month-old
MAG-deficient mice (Fig. 5B), but they were less frequent
than those in paranodal regions. In electron micrographs of
MAG-deficient paranodes, the excessive myelin was positioned
asymmetrically around axons with remarkably small calibers (Fig.
5C). Electron microscopic examination of longitudinal nerve
sections established that small axonal calibers characteristic of nodes
of Ranvier extended into paranodal regions of MAG-deficient fibers.
Figure 5D shows the dramatic difference between axonal
caliber in nodal and paranodal regions in 9-month-old control nerve. In
contrast, axonal calibers in 9-month-old MAG-deficient fibers did not
increase significantly in paranodal regions that contained redundant
myelin (Fig. 5E). Thick-appearing para-nodal myelin,
therefore, is generated in part by axonal shrinkage and collapse of
myelin.
View larger version (98K):
[in this window]
[in a new window]
|
Figure 5.
Tomaculi result from axonal shrinkage and myelin
collapse. A, Teased fibers from 9-month-old control
(top) and MAG-deficient (center and
bottom) sciatic nerves. Thick paranodal myelin was rare
in control fibers but abundant in MAG-deficient fibers
(center, arrowheads). Internodal segments
also had thick myelin in MAG-deficient fibers (bottom,
arrowhead). Scale bar, 20 µm. B,
Thick-appearing myelin segments were quantified, grouped by location
(paranodal and internodal), and found at significantly greater numbers
in MAG-deficient fibers when compared with control fibers.
C-E, Electron micrographs of paranodal
regions of MAG-deficient (C, E) and
control (D) myelin internodes. In transverse
section, thick redundant myelin (C,
arrowheads) partially surrounds an axon
(C, arrows) with remarkably small
caliber. In longitudinal orientation, the difference between nodal
(*) and internodal (arrows) axonal
caliber is much greater in control (D) than in
MAG-deficient (E) fibers. Redundant paranodal
myelin surrounds paranodal axons with reduced caliber in MAG-deficient
fibers (E, arrowheads). Scale bars:
C, 2.0 µm; D, E, 1.0 µm.
|
|
| |
DISCUSSION |
This report establishes for the first time that MAG
modulates caliber, neurofilament spacing, and neurofilament
phosphorylation of myelinated axons. In addition, absence of MAG
results in axonal atrophy and Wallerian degeneration of myelinated
fibers. Axonal atrophy was most prominent in paranodal regions where it
resulted in formation of tomaculi. Decreased axonal calibers,
neurofilament spacing, and neurofilament phosphorylation have been
described previously in peripheral nerves from Trembler mice (deWaegh
et al., 1992). Trembler mice have a point mutation in the PMP22 gene that causes a dysmyelinating peripheral neuropathy (Suter et al., 1992)
mediated by a "gain of function" of the mutated PMP22. The mechanism by which mutated PMP22 modulates axonal cytoskeleton and
reduced axonal caliber is unclear, and the chronic demyelination and
remyelination of Trembler axons complicates the interpretation of
underlying mechanisms. Nevertheless, the striking similarities between
our data and those described in Trembler mice raised the possibility
that reductions in axonal caliber, neurofilament spacing, and
neurofilament phosphorylation can be caused by two separate molecules
(MAG and PMP22) that participate in a common pathway. In contrast to
Trembler mice, axonal changes in MAG-deficient mice result from a
"loss of function," and interpretation of this effect is not
compounded by chronic hypomyelination, demyelination, and
remyelination. In addition, near-normal calibers can be obtained when
myelin-forming cells ensheath but fail to myelinate axons (Sanchez et
al., 1996), supporting the notion that axonal calibers are regulated by
a molecule in the adaxonal membrane of myelin-forming cells.
Loss of MAG results in reduced neurofilament spacing
How does MAG modulate axonal caliber? Reduced axonal calibers in
MAG-deficient mice could, in principle, reflect decreased neurofilament
numbers or spacings or both. Our results indicate that the primary
determinant of reduced axon calibers in MAG-deficient mice is reduced
neurofilament spacing. Furthermore, our data support the hypothesis
that MAG affects phosphorylation of NFH and NFM. This is best supported
by our quantitative immunolabeling studies in which highly
phosphorylated NFH epitopes were reduced by 50%. We sampled fibers
that theoretically should have had the largest axonal calibers based on
their number of compact myelin lamella. A fivefold increase in
nonphosphorylated NFH epitopes was also detected in sections from
MAG-deficient nerves. Decreased NFH phosphorylation was also confirmed
by Western blot analysis. Compared with control nerve
homogenates, MAG-deficient nerves contained reduced NFHP+++
levels relative to nonphosphorylated NFH and a 25% reduction in
NFMP+++ epitopes. These data are consistent with previous reports that
correlated reduced axonal caliber with reduced NF spacing and NF
phosphorylation (deWaegh et al., 1992; Tu et al., 1995). If NF spacing
is regulated by bulk NF phosphorylation, then one would predict greater
reductions in NFHP+++ and NFMP+++ than observed in our Western blot
analysis. It is possible, therefore, that phosphorylation of select KSP
or other sites regulates lateral extension of NF sidearms.
Collectively, our data are most consistent with the hypothesis that the
absence of MAG causes reduced phosphorylation of NFH and NFM,
retraction of neurofilament sidearms, reduced neurofilament spacing,
and smaller axonal calibers.
Loss of MAG caused reduced neurofilament phosphorylation
How does MAG regulate neurofilament spacing? The biochemical
properties and periaxonal of location of MAG are consistent with the
possibility that MAG regulates axonal cytoskeleton through interaction
with an axolemmal receptor. In support of this function, MAG-containing
liposomes adhere to neurons (Poltorak et al., 1987; Johnson et al.,
1989), and MAG inhibits neurite extension (McKerracher et al., 1994;
Mukhopadhyay et al., 1994) and can cause axonal growth cone collapse
(Li et al., 1996). Neurofilament phosphorylation depends on a delicate
balance between the activities of axonal kinases and phosphatases (Pant
and Veeranna, 1995) that include glycogen synthase kinase 3 (Yang et
al., 1995), cyclin-dependent kinase 5 (cdk5) (Shetty et al., 1993), and
protein phosphatase 2a (Veeranna et al., 1995). Western blot analysis
detected similar levels of cdk5 in homogenates of control and
MAG-deficient sciatic nerve (X. Yin, unpublished observations). Kinase
activity of cdk5 depends on the formation of a multimeric complex that
has several activators (Lew et al., 1992). It is possible that MAG
directly or indirectly initiates a signaling cascade that modulates
kinase complexes or activators. It is also possible that the overall organization of axonal cytoskeleton may be compromised in MAG-deficient mice, and this may reduce neurofilament phosphorylation and
neurofilament spacing by a more indirect mechanism. Although
microtubule distributions were not studied extensively in this report,
microtubule-enriched and microtubule-free regions were conspicuously
apparent in electron micrographs of axons from MAG-deficient mice (Yin,
unpublished observations). Because alteration in microtubules and
axonal transport accompany reduced axonal calibers and neurofilament
spacing in Trembler peripheral nerve (Kirkpatrick and Brady, 1994),
similar studies may provide additional insights into the mechanisms of axonal atrophy in MAG-deficient mice.
Because neurofilaments are obligate heteropolymers, changes in
total NF subunits or subunit ratios could also affect axonal caliber by
changing neurofilament number and phosphorylation. Although total NFL
and NFH subunits were reduced in MAG-deficient nerves, NF numbers were
similar to those in control nerves. These findings suggest a smaller
nonfilamentous NF subunit pool in MAG-deficient mice and support
previous in vitro (Ching and Liem, 1993; Nakagawa et al.,
1995) and in vivo (Veeranna et al., 1995; Xu et al., 1996) studies that demonstrate NF assembly over a wide range of NF subunit stoichiometries. The contribution of NF subunits to radial axonal growth has been studied by overexpressing each subunit or a combination of subunits in transgenic mice. One- to twofold increases in individual neurofilament subunit ratios inhibit radial growth of axons (Cote et
al., 1993; Xu et al., 1996). The 25% increase in NFM/NFL ratios in
9-month-old MAG-deficient nerves could contribute, in principle, to
reduce neurofilament spacing and axonal calibers.
Loss of MAG causes paranodal tomaculi
Regional variations in axonal calibers were prominent within
individual myelin fibers of MAG-deficient mice. Most striking was a
dramatic reduction of caliber and neurofilament spacing in paranodal
axons. In MAG-deficient mice, paranodal axonal calibers expanded
appropriately during early postnatal development. Between P35 and P90,
paranodal axonal calibers in normal mice continue to expand, whereas
those in MAG-deficient mice shrink. Reduction in paranodal axonal
calibers resulted in paranodal tomaculi in 57% of fibers in 90-d-old
MAG-deficient mice and in 95% of fibers in 9-month-old MAG-deficient
mice. Tomaculi described in another line of MAG knockout mice (Carenini
et al., 1997) and in heterozygous PMP22 knockout mice (Adlkofer et al.,
1997) were attributed to paranodal hypermyelination. The symmetrical
distribution of redundant myelin around paranodal axons in heterozygous
PMP22 mice (Adlkofer et al., 1997) supports hypermyelination as a cause
of tomaculi formation. The dramatic reduction in paranodal axonal
calibers and the asymmetrical distribution of the redundant myelin
around the axon in MAG-deficient mice is more consistent with collapse of myelin that formed around larger axonal cylinders (Fig.
6).
View larger version (27K):
[in this window]
[in a new window]
|
Figure 6.
Schematic representation of tomaculi formation in
MAG-deficient paranodal regions. A-C,
Relationship between axonal caliber and myelin sheath in transverse
section of normal myelinated fiber (A). In
MAG-deficient nerves, paranodal axonal calibers shrink (B) and result in myelin sheath collapse and
tomaculi formation (C). D, In
MAG+/+ paranodal regions, increased neurofilament spacings correlate
with large axonal caliber. In MAG/ paranodal regions surrounded by
tomaculi, neurofilament spacings and axonal calibers are similar to
those in nodal regions.
|
|
Regional variations in axonal caliber of MAG-deficient mice raise
several additional possibilities regarding mechanisms of Schwann
cell-axon interactions. First, the role of MAG in modulating axonal
cytoskeleton may differ based on location along the internode. Second,
other Schwann cell molecules that are concentrated within or
transported to paranodal regions (and possibly to axons) may affect
neurofilament spacing and axonal caliber. Third, MAG helps segregate
nodal axoplasmic environments that favor NF sidearm retraction (i.e.,
NF dephosphorylation) from paranodal environments that favor NF sidearm
extension (i.e., NF phosphorylation). In chronic absence of MAG, these
nodal components or microenvironments may diffuse into paranodal
axoplasm and cause sidearm retraction.
Summary
This report has established that MAG-deficient mice develop a
chronic atrophy of myelinated axons that can lead to axonal degeneration. Although the numbers of degenerating myelinated fibers in
MAG-deficient nerves were small (two to nine per section), they were
present in all sections examined from P35 and older mice, and were
detected only in control nerves from 16-month-old mice. Axonal atrophy
and degeneration as a consequence of demyelination have been reported
in multiple sclerosis brains (Trapp et al., 1998) and in chronic
relapsing experimental allergic encephalomyelitis (Raine and Cross,
1989). MAG-deficient mice are an excellent model for investigating
mechanisms of axonal atrophy and degeneration, and they provide a
unique setting to test potential therapeutics that may delay or stop
axonal atrophy associated with chronic diseases of myelin.
| |
FOOTNOTES |
Received Nov. 10, 1997; revised Dec. 22, 1997; accepted Dec. 29, 1997.
This work was supported by National Institutes of Health Grant NS29818.
We thank Karen Toil for typing this manuscript, and Robert Wesley for
technical assistance.
Correspondence should be addressed to Dr. Bruce Trapp, Department of
Neurosciences/NC30, Lerner Research Institute, The Cleveland Clinic
Foundation, 9500 Euclid Avenue, Cleveland, OH
44195.
| |
REFERENCES |
-
Adlkofer K,
Frei R,
Neuberg DH-H,
Zielasek J,
Toyka KV,
Suter U
(1997)
Heterozygous peripheral myelin protein 22-deficient mice are affected by a progressive demyelinating tomaculous neuropathy.
J Neurosci
17:4662-4671[Abstract/Free Full Text].
-
Aguayo AJ,
Epps J,
Charron L,
Bray GM
(1976)
Multipotentiality of Schwann cells in cross anastomosed and grafted myelinated and unmyelinated nerves: quantitative microscopy and radioautography.
Brain Res
104:1-20[Web of Science][Medline].
-
Arbuthnott ER,
Boyd IA,
Kalu KU
(1980)
Ultrastructural dimensions of myelinated peripheral nerve fibres in the cat and their relation to conduction velocity.
J Physiol (Lond)
308:125-157[Abstract/Free Full Text].
-
Arquint M,
Roder J,
Chia L-S,
Down J,
Wilkinson O,
Bayley H,
Braun P,
Dunn R
(1987)
Molecular cloning and primary structure of myelin-associated glycoproteins.
Proc Natl Acad Sci USA
84:600-604[Abstract/Free Full Text].
-
Carenini S,
Montag D,
Cremer H,
Schachner M,
Martini R
(1997)
Absence of the myelin-associated glycoprotein (MAG) and the neural cell adhesion molecule (N-CAM) interferes with the maintenance, but not with the formation of peripheral myelin.
Cell Tissue Res
287:3-9[Web of Science][Medline].
-
Ching GY,
Liem RK
(1993)
Assembly of type IV neuronal intermediate filaments in non-neuronal cells in the absence of pre-existing cytoplasmic intermediate filaments.
J Cell Biol
122:1323-1335[Abstract/Free Full Text].
-
Cote F,
Collard JF,
Julien JP
(1993)
Progressive neuronopathy in transgenic mice expressing the human neurofilament heavy gene: a mouse model of amyotrophic lateral sclerosis.
Cell
73:35-46[Web of Science][Medline].
-
deWaegh SM,
Lee VM-Y,
Brady ST
(1992)
Local modulation of neurofilament phosphorylation, axonal caliber, and slow axonal transport by myelinating Schwann cells.
Cell
68:451-463[Web of Science][Medline].
-
Eyer J,
Peterson A
(1994)
Neurofilament-deficient axons and perikaryal aggregates in viable transgenic mice expressing a neurofilament--galactosidase fusion protein.
Neuron
12:389-405[Web of Science][Medline].
-
Friede RL,
Samorajski T
(1971)
Axonal caliber related to neurofilaments and microtubules in sciatic nerve fibers of rats and mice.
Anat Rec
167:379-387.
-
Fruttiger M,
Montag D,
Schachner M,
Martini R
(1995)
Crucial role for the myelin-associated glycoprotein in the maintenance of axon-myelin integrity.
Eur J Neurosci
7:511-515[Web of Science][Medline].
-
Griffin JW,
Kidd G,
Trapp BD
(1993)
Interactions between axons and Schwann cells.
In: Peripheral neuropathy (Dyck PJ,
Thomas PK,
eds), pp 317-330. Philadelphia: Saunders.
-
Hirokawa N,
Glicksman MA,
Willard MB
(1984)
Organization of mammalian neurofilament polypeptides within the neuronal cytoskeleton.
J Cell Biol
98:1523-1536[Abstract/Free Full Text].
-
Hoffman PN,
Griffin JW
(1992)
Regulation of axonal caliber.
In: Peripheral neuropathy (Dyck PJ,
Thomas PK,
Griffin JW,
Low PA,
Poduslo J,
eds), pp 389-402. New York: Raven.
-
Hoffman PN,
Lasek RJ
(1975)
The slow component of axonal transport: identification of major structural polypeptides of the axon and their generality among mammalian neurons.
J Cell Biol
66:351-366[Abstract/Free Full Text].
-
Hsieh S-T,
Crawford TO,
Bouldin TW,
Griffin JW
(1993)
Influence of demyelination and remyelination on axonal organization.
Brain Pathol
3:307.
-
Hsieh S-T,
Kidd GJ,
Crawford TO,
Xu Z,
Trapp BD,
Cleveland DW,
Griffin JW
(1994)
Regional modulation of neurofilament organization by myelination in normal axons.
J Neurosci
14:6392-6401[Abstract].
-
Johnson PW,
Abramow-Newerly W,
Seilheimer B,
Sadoul R,
Tropak MB,
Arquint M,
Dunn RJ,
Schachner M,
Roder JC
(1989)
Recombinant myelin-associated glycoprotein confers neural adhesion and neurite outgrowth function.
Neuron
3:377-385[Web of Science][Medline].
-
Kirkpatrick LL,
Brady ST
(1994)
Modulation of the axonal microtubule cytoskeleton by myelinating Schwann cells.
J Neurosci
12:7440-7450.
-
Lai C,
Brow MA,
Nave K-A,
Noronha AB,
Quarles RH,
Bloom FE,
Milner RJ,
Sutcliffe JG
(1987)
Two forms of 1B236/myelin-associated glycoprotein (MAG), a cell adhesion molecule for postnatal neural development, are produced by alternative splicing.
Proc Natl Acad Sci USA
84:4337-4341[Abstract/Free Full Text].
-
Lew J,
Winkfein RJ,
Paudel HK,
Wang JH
(1992)
Brain proline-directed protein kinase is a neurofilament kinase which displays high sequence homology to p34cdc2.
J Biol Chem
267:25922-25926[Abstract/Free Full Text].
-
Li C,
Tropak MB,
Gerial R,
Clapoff S,
Abramow-Newerly W,
Trapp B,
Peterson A,
Roder J
(1994)
Myelination in the absence of myelin-associated glycoprotein.
Nature
369:747-750[Medline].
-
Li M,
Shibata A,
Li C,
Braun PE,
McKerracher L,
Roder J,
Kater SB,
David S
(1996)
Myelin-associated glycoprotein inhibits neurite/axon growth and causes growth cone collapse.
J Neurosci Res
46:404-414[Web of Science][Medline].
-
McKerracher L,
David S,
Jackson DL,
Kottis V,
Dunn RJ,
Braun PE
(1994)
Identification of myelin-associated glycoprotein as a major myelin-derived inhibitor of neurite growth.
Neuron
13:805-811[Web of Science][Medline].
-
Montag D,
Giese KP,
Bartsch U,
Martini R,
Land Y,
Blüthmann H,
Karthigasan J,
Kirschner DA,
Wintergerst ES,
Nave K-A,
Zielasek J,
Toyka KV,
Lipp H,
Schachner M
(1994)
Mice deficient for the myelin-associated glycoprotein show subtle abnormalities in myelin.
Neuron
13:229-246[Web of Science][Medline].
-
Mukhopadhyay G,
Doherty P,
Walsh FS,
Crocker PR,
Filbin MT
(1994)
A novel role for myelin-associated glycoprotein as an inhibitor of axonal regeneration.
Neuron
13:757-767[Web of Science][Medline].
-
Nakagawa T,
Chen J,
Zhang Z,
Kanai Y,
Hirokawa N
(1995)
Two distinct functions of the carboxyl-terminal tail domain of NF-M upon neurofilament assembly: cross-bridge formation and longitudinal elongation of filaments.
J Cell Biol
129:411-429[Abstract/Free Full Text].
-
Ohara O,
Gahara Y,
Miyake T,
Teraoka H,
Kitamura T
(1993)
Neurofilament deficiency in quail caused by nonsense mutation in neurofilament-L gene.
J Cell Biol
121:387-395[Abstract/Free Full Text].
-
Owens GC,
Bunge RP
(1991)
Schwann cells infected with a recombinant retrovirus expressing myelin-associated glycoprotein antisense RNA do not form myelin.
Neuron
7:565-575[Web of Science][Medline].
-
Owens GC,
Boyd CJ,
Bunge RP,
Salzer JL
(1990)
Expression of recombinant myelin-associated glycoprotein in primary Schwann cells promotes the initial investment of axons by myelinating Schwann cells.
J Cell Biol
111:1171-1182[Abstract/Free Full Text].
-
Pant HC,
Veeranna
(1995)
Neurofilament phosphorylation.
Biochem Cell Biol
73:575-592[Web of Science][Medline].
-
Poltorak M,
Sadoul R,
Keilhauer G,
Landa C,
Fahrig T,
Schachner M
(1987)
Myelin-associated glycoprotein, a member of the L2/HNK-1 family of neural cell adhesion molecules, is involved in neuron-oligodendrocyte and oligodendrocyte-oligodendrocyte interaction.
J Cell Biol
105:1893-1899[Abstract/Free Full Text].
-
Raine CS,
Cross AH
(1989)
Axonal dystrophy as a consequence of long-term demyelination.
Lab Invest
60:714-725[Web of Science][Medline].
-
Salzer JL,
Holmes WP,
Colman DR
(1987)
The amino acid sequences of the myelin-associated glycoproteins: homology to the immunoglobulin gene superfamily.
J Cell Biol
104:957-965[Abstract/Free Full Text].
-
Sanchez I,
Hassinger L,
Paskevich PA,
Shine HD,
Nixon RA
(1996)
Oligodendroglia regulate the regional expansion of axon caliber and local accumulation of neurofilaments during development independently of myelin formation.
J Neurosci
16:5095-5105[Abstract/Free Full Text].
-
Shetty KT,
Link WT,
Pant HC
(1993)
cdc2-like kinase from rat spinal cord specifically phosphorylates KSPXK motifs in neurofilament proteins: isolation and characterization.
Proc Natl Acad Sci USA
90:6844-6848[Abstract/Free Full Text].
-
Sternberger NH,
Quarles RH,
Itoyama Y,
Webster HdeF
(1979)
Myelin-associated glycoprotein demonstrated immunocytochemically in myelin and myelin-forming cells of developing rats.
Proc Natl Acad Sci USA
76:1510-1514[Abstract/Free Full Text].
-
Suter U,
Welcher AA,
Ozcelik T,
Snipes GJ,
Kosaras B,
Francke U,
Billings-Gagliardi S,
Sidman RL,
Shooter EM
(1992)
Trembler mouse carries a point mutation in a myelin gene.
Nature
356:241-244[Medline].
-
Trapp BD,
Quarles RH
(1982)
Presence of the myelin-associated glycoprotein correlates with alterations in the periodicity of peripheral myelin.
J Cell Biol
92:877-882[Abstract/Free Full Text].
-
Trapp BD,
Andrews SB,
Cootauco C,
Quarles RH
(1989)
The myelin-associated glycoprotein is enriched in multivesicular bodies and periaxonal membranes of actively myelinating oligodendrocytes.
J Cell Biol
109:2417-2426[Abstract/Free Full Text].
-
Trapp BD,
Peterson J,
Ransohoff RM,
Rudick R,
Mörk S,
Bö L
(1998)
Axonal transection in multiple sclerosis lesions.
N Engl J Med
338:278-285[Abstract/Free Full Text].
-
Tu P-H,
Elder G,
Lazzarini RA,
Nelson D,
Trojanowski JQ,
Lee VM-Y
(1995)
Overexpression of the human NFM subunit in transgenic mice modifies the level of endogenous NFL and the phosphorylation state of NFH subunits.
J Cell Biol
129:1629-1640[Abstract/Free Full Text].
-
Veeranna,
Shetty KT,
Link WT,
Jaffe H,
Wang J,
Pant HC
(1995)
Neuronal cyclin-dependent kinase-5 phosphorylation sites in neurofilament protein (NF-H) are dephosphorylated by protein phosphatase 2A.
J Neurochem
64:2681-2690[Web of Science][Medline].
-
Waxman SG
(1980)
Determinants of conduction velocity in myelinated nerve fiber.
Muscle Nerve
3:141-150[Web of Science][Medline].
-
Weinberg HJ,
Spencer PS
(1976)
Studies on the control of myelinogenesis. II. Evidence for neuronal regulation of myelin production.
Brain Res
113:363-378[Web of Science][Medline].
-
Windebank AJ,
Word P,
Bunge RP,
Dyck PJ
(1985)
Myelination determines the caliber of dorsal root ganglion neurons in culture.
J Neurosci
5:1563-1567[Abstract].
-
Wong PC,
Marszalek J,
Crawford TO,
Xu Z,
Hsieh S-T,
Griffin JW,
Cleveland DW
(1995)
Increasing neurofilament subunit NF-M expression reduces axonal NF-H, inhibits radial growth, and results in neurofilamentous accumulation in motor neurons.
J Cell Biol
130:1413-1422[Abstract/Free Full Text].
-
Xu Z,
Marszalek JR,
Lee MK,
Wong PC,
Folmer J,
Crawford TO,
Hsieh S-T,
Griffin JW,
Cleveland DW
(1996)
Subunit composition of neurofilaments specifies axonal diameter.
J Cell Biol
133:1061-1069[Abstract/Free Full Text].
-
Yang SD,
Huang JJ,
Huang TJ
(1995)
Protein kinase FA/glycogen synthase kinase 3 alpha predominantly phosphorylates the in vivo sites of Ser502, Ser506, Ser603, and Ser666 in neurofilament.
J Neurochem
64:1848-1854[Web of Science][Medline].
Copyright © 1998 Society for Neuroscience 0270-6474/98/1861953-10$05.00/0
This article has been cited by other articles:
|
|
|
|
 
M. Furusho, J. L. Dupree, M. Bryant, and R. Bansal
Disruption of Fibroblast Growth Factor Receptor Signaling in Nonmyelinating Schwann Cells Causes Sensory Axonal Neuropathy and Impairment of Thermal Pain Sensitivity
J. Neurosci.,
February 11, 2009;
29(6):
1608 - 1614.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. L. Garcia, M. V. Rao, J. Fujimoto, V. B. Garcia, S. B. Shah, J. Crum, T. Gotow, Y. Uchiyama, M. Ellisman, N. A. Calcutt, et al.
Phosphorylation of Highly Conserved Neurofilament Medium KSP Repeats Is Not Required for Myelin-Dependent Radial Axonal Growth
J. Neurosci.,
February 4, 2009;
29(5):
1277 - 1284.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
T. Nguyen, N. R. Mehta, K. Conant, K.-J. Kim, M. Jones, P. A. Calabresi, G. Melli, A. Hoke, R. L. Schnaar, G.-L. Ming, et al.
Axonal Protective Effects of the Myelin-Associated Glycoprotein
J. Neurosci.,
January 21, 2009;
29(3):
630 - 637.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
X. Yin, G. J. Kidd, K.-A. Nave, and B. D. Trapp
P0 Protein Is Required for and Can Induce Formation of Schmidt-Lantermann Incisures in Myelin Internodes
J. Neurosci.,
July 9, 2008;
28(28):
7068 - 7073.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Lasiene, L. Shupe, S. Perlmutter, and P. Horner
No Evidence for Chronic Demyelination in Spared Axons after Spinal Cord Injury in a Mouse
J. Neurosci.,
April 9, 2008;
28(15):
3887 - 3896.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
N. R. Mehta, P. H. H. Lopez, A. A. Vyas, and R. L. Schnaar
Gangliosides and Nogo Receptors Independently Mediate Myelin-associated Glycoprotein Inhibition of Neurite Outgrowth in Different Nerve Cells
J. Biol. Chem.,
September 21, 2007;
282(38):
27875 - 27886.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Schumacher, R. Guennoun, A. Ghoumari, C. Massaad, F. Robert, M. El-Etr, Y. Akwa, K. Rajkowski, and E.-E. Baulieu
Novel Perspectives for Progesterone in Hormone Replacement Therapy, with Special Reference to the Nervous System
Endocr. Rev.,
June 1, 2007;
28(4):
387 - 439.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Dutta and B. D. Trapp
Pathogenesis of axonal and neuronal damage in multiple sclerosis
Neurology,
May 29, 2007;
68(22_suppl_3):
S22 - S31.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
X. Yin, R. C. Baek, D. A. Kirschner, A. Peterson, Y. Fujii, K.-A. Nave, W. B. Macklin, and B. D. Trapp
Evolution of a neuroprotective function of central nervous system myelin
J. Cell Biol.,
January 30, 2006;
172(3):
469 - 478.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Imitola, T. Chitnis, and S. J. Khoury
Insights Into the Molecular Pathogenesis of Progression in Multiple Sclerosis: Potential Implications for Future Therapies
Arch Neurol,
January 1, 2006;
63(1):
25 - 33.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
E. M. Frohman, M. Filippi, O. Stuve, S. G. Waxman, J. Corboy, J. T. Phillips, C. Lucchinetti, J. Wilken, N. Karandikar, B. Hemmer, et al.
Characterizing the Mechanisms of Progression in Multiple Sclerosis: Evidence and New Hypotheses for Future Directions
Arch Neurol,
September 1, 2005;
62(9):
1345 - 1356.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J Versijpt, J C Debruyne, K J Van Laere, F De Vos, J Keppens, K Strijckmans, E Achten, G Slegers, R A Dierckx, J Korf, et al.
Microglial imaging with positron emission tomography and atrophy measurements with magnetic resonance imaging in multiple sclerosis: a correlative study
Multiple Sclerosis,
April 1, 2005;
11(2):
127 - 134.
[Abstract]
[PDF]
|
|
|
|
|
|
|
A. Bolino, A. Bolis, S. C. Previtali, G. Dina, S. Bussini, G. Dati, S. Amadio, U. Del Carro, D. D. Mruk, M. L. Feltri, et al.
Disruption of Mtmr2 produces CMT4B1-like neuropathy with myelin outfolding and impaired spermatogenesis
J. Cell Biol.,
November 22, 2004;
167(4):
711 - 721.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
G. Corfas, M. O. Velardez, C.-P. Ko, N. Ratner, and E. Peles
Mechanisms and Roles of Axon-Schwann Cell Interactions
J. Neurosci.,
October 20, 2004;
24(42):
9250 - 9260.
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Sun, N. L. Shaper, S. Itonori, M. Heffer-Lauc, K. A. Sheikh, and R. L. Schnaar
Myelin-associated glycoprotein (Siglec-4) expression is progressively and selectively decreased in the brains of mice lacking complex gangliosides
Glycobiology,
September 1, 2004;
14(9):
851 - 857.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
G. C. DeLuca, G. C. Ebers, and M. M. Esiri
Axonal loss in multiple sclerosis: a pathological survey of the corticospinal and sensory tracts
Brain,
May 1, 2004;
127(5):
1009 - 1018.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
X. Yin, G. J. Kidd, E. P. Pioro, J. McDonough, R. Dutta, M. L. Feltri, L. Wrabetz, A. Messing, R. M. Wyatt, R. J. Balice-Gordon, et al.
Dysmyelinated Lower Motor Neurons Retract and Regenerate Dysfunctional Synaptic Terminals
J. Neurosci.,
April 14, 2004;
24(15):
3890 - 3898.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. M. Taylor, C. B. Marta, R. J. Claycomb, D. K. Han, M. N. Rasband, T. Coetzee, and S. E. Pfeiffer
Proteomic mapping provides powerful insights into functional myelin biology
PNAS,
March 30, 2004;
101(13):
4643 - 4648.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. L. Garcia, C. S. Lobsiger, S. B. Shah, T. J. Deerinck, J. Crum, D. Young, C. M. Ward, T. O. Crawford, T. Gotow, Y. Uchiyama, et al.
NF-M is an essential target for the myelin-directed "outside-in" signaling cascade that mediates radial axonal growth
J. Cell Biol.,
December 8, 2003;
163(5):
1011 - 1020.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. V. Rao, J. Campbell, A. Yuan, A. Kumar, T. Gotow, Y. Uchiyama, and R. A. Nixon
The neurofilament middle molecular mass subunit carboxyl-terminal tail domains is essential for the radial growth and cytoskeletal architecture of axons but not for regulating neurofilament transport rate
J. Cell Biol.,
December 8, 2003;
163(5):
1021 - 1031.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
D T Chard, P A Brex, O Ciccarelli, C M Griffin, G J M Parker, C Dalton, D R Altmann, A J Thompson, and D H Miller
The longitudinal relation between brain lesion load and atrophy in multiple sclerosis: a 14 year follow up study
J. Neurol. Neurosurg. Psychiatry,
November 1, 2003;
74(11):
1551 - 1554.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
Y.-l. Zheng, B.-S. Li, Veeranna, and H. C. Pant
Phosphorylation of the Head Domain of Neurofilament Protein (NF-M): A FACTOR REGULATING TOPOGRAPHIC PHOSPHORYLATION OF NF-M TAIL DOMAIN KSP SITES IN NEURONS
J. Biol. Chem.,
June 20, 2003;
278(26):
24026 - 24032.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
I. Kobsar, M. Berghoff, M. Samsam, C. Wessig, M. Maurer, K. V. Toyka, and R. Martini
Preserved myelin integrity and reduced axonopathy in connexin32-deficient mice lacking the recombination activating gene-1
Brain,
April 1, 2003;
126(4):
804 - 813.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Samsam, W. Mi, C. Wessig, J. Zielasek, K. V. Toyka, M. P. Coleman, and R. Martini
The Wlds Mutation Delays Robust Loss of Motor and Sensory Axons in a Genetic Model for Myelin-Related Axonopathy
J. Neurosci.,
April 1, 2003;
23(7):
2833 - 2839.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. D. Fields and B. Stevens-Graham
NEUROSCIENCE: New Insights into Neuron-Glia Communication
Science,
October 18, 2002;
298(5593):
556 - 562.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. V. Rao, M. L. Garcia, Y. Miyazaki, T. Gotow, A. Yuan, S. Mattina, C. M. Ward, N. A. Calcutt, Y. Uchiyama, R. A. Nixon, et al.
Gene replacement in mice reveals that the heavily phosphorylated tail of neurofilament heavy subunit does not affect axonal caliber or the transit of cargoes in slow axonal transport
J. Cell Biol.,
August 19, 2002;
158(4):
681 - 693.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. P. T. Lunn, T. O. Crawford, R. A. C. Hughes, J. W. Griffin, and K. A. Sheikh
Anti-myelin-associated glycoprotein antibodies alter neurofilament spacing
Brain,
April 1, 2002;
125(4):
904 - 911.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Pujol, C. Hindelang, N. Callizot, U. Bartsch, M. Schachner, and J. L. Mandel
Late onset neurological phenotype of the X-ALD gene inactivation in mice: a mouse model for adrenomyeloneuropathy
Hum. Mol. Genet.,
March 1, 2002;
11(5):
499 - 505.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
D. T. Chard, C. M. Griffin, G. J. M. Parker, R. Kapoor, A. J. Thompson, and D. H. Miller
Brain atrophy in clinically early relapsing-remitting multiple sclerosis
Brain,
February 1, 2002;
125(2):
327 - 337.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Franzen, S. L. Tanner, S. M. Dashiell, C. A. Rottkamp, J. A. Hammer, and R. H. Quarles
Microtubule-associated protein 1B: a neuronal binding partner for myelin-associated glycoprotein
J. Cell Biol.,
December 10, 2001;
155(6):
893 - 898.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Forghani, L. Garofalo, D. R. Foran, H. F. Farhadi, P. Lepage, T. J. Hudson, I. Tretjakoff, P. Valera, and A. Peterson
A Distal Upstream Enhancer from the Myelin Basic Protein Gene Regulates Expression in Myelin-Forming Schwann Cells
J. Neurosci.,
June 1, 2001;
21(11):
3780 - 3787.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. J. Tifft and R. L. Proia
Stemming the tide: glycosphingolipid synthesis inhibitors as therapy for storage diseases
Glycobiology,
December 1, 2000;
10(12):
1249 - 1258.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. C. Previtali, A. Quattrini, M. Fasolini, M. C. Panzeri, A. Villa, M. T. Filbin, W. Li, S.-Y. Chiu, A. Messing, L. Wrabetz, et al.
Epitope-Tagged P0Glycoprotein Causes Charcot-Marie-Tooth-Like Neuropathy in Transgenic Mice
J. Cell Biol.,
November 27, 2000;
151(5):
1035 - 1046.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
T. Uschkureit, O. Sporkel, J. Stracke, H. Bussow, and W. Stoffel
Early Onset of Axonal Degeneration in Double (plp-/-mag-/-) and Hypomyelinosis in Triple (plp-/-mbp-/-mag-/-) Mutant Mice
J. Neurosci.,
July 15, 2000;
20(14):
5225 - 5233.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. Kornek, M. K. Storch, R. Weissert, E. Wallstroem, A. Stefferl, T. Olsson, C. Linington, M. Schmidbauer, and H. Lassmann
Multiple Sclerosis and Chronic Autoimmune Encephalomyelitis : A Comparative Quantitative Study of Axonal Injury in Active, Inactive, and Remyelinated Lesions
Am. J. Pathol.,
July 1, 2000;
157(1):
267 - 276.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. R. Chan, P. M. Rodriguez-Waitkus, B. K. Ng, P. Liang, and M. Glaser
Progesterone Synthesized by Schwann Cells during Myelin Formation Regulates Neuronal Gene Expression
Mol. Biol. Cell,
July 1, 2000;
11(7):
2283 - 2295.
[Abstract]
[Full Text]
|
|
|
|
|
|
|
L. Wrabetz, M. L. Feltri, A. Quattrini, D. Imperiale, S. Previtali, M. D'Antonio, R. Martini, X. Yin, B. D. Trapp, L. Zhou, et al.
P0 Glycoprotein Overexpression Causes Congenital Hypomyelination of Peripheral Nerves
J. Cell Biol.,
March 6, 2000;
148(5):
1021 - 1034.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
X. Yin, G.J. Kidd, L. Wrabetz, M.L. Feltri, A. Messing, and B.D. Trapp
Schwann Cell Myelination Requires Timely and Precise Targeting of P0 Protein
J. Cell Biol.,
March 6, 2000;
148(5):
1009 - 1020.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. E. Collins, T. J. Fralich, S. Itonori, Y. Ichikawa, and R. L. Schnaar
Conversion of cellular sialic acid expression from N-acetyl- to N-glycolylneuraminic acid using a synthetic precursor, N-glycolylmannosamine pentaacetate: inhibition of myelin-associated glycoprotein binding to neural cells
Glycobiology,
January 1, 2000;
10(1):
11 - 20.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. E. Collins, H. Ito, N. Sawada, H. Ishida, M. Kiso, and R. L. Schnaar
Enhanced Binding of the Neural Siglecs, Myelin-associated Glycoprotein and Schwann Cell Myelin Protein, to Chol-1 (alpha -Series) Gangliosides and Novel Sulfated Chol-1 Analogs
J. Biol. Chem.,
December 31, 1999;
274(53):
37637 - 37643.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C.A. Haney, Z. Sahenk, C. Li, V.P. Lemmon, J. Roder, and B.D. Trapp
Heterophilic Binding of L1 on Unmyelinated Sensory Axons Mediates Schwann Cell Adhesion and Is Required for Axonal Survival
J. Cell Biol.,
September 6, 1999;
146(5):
1173 - 1184.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. Sancho, J. P. Magyar, A. Aguzzi, and U. Suter1
Distal axonopathy in peripheral nerves of PMP22-mutant mice
Brain,
August 1, 1999;
122(8):
1563 - 1577.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Frei, S. Motzing, I. Kinkelin, M. Schachner, M. Koltzenburg, and R. Martini
Loss of Distal Axons and Sensory Merkel Cells and Features Indicative of Muscle Denervation in Hindlimbs of P0-Deficient Mice
J. Neurosci.,
July 15, 1999;
19(14):
6058 - 6067.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
K. A. Sheikh, J. Sun, Y. Liu, H. Kawai, T. O. Crawford, R. L. Proia, J. W. Griffin, and R. L. Schnaar
Mice lacking complex gangliosides develop Wallerian degeneration and myelination defects
PNAS,
June 22, 1999;
96(13):
7532 - 7537.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. D. Trapp, R. M. Ransohoff, E. Fisher, and R. A. Rudick
Neurodegeneration in Multiple Sclerosis: Relationship to Neurological Disability
Neuroscientist,
January 1, 1999;
5(1):
48 - 57.
[Abstract]
[PDF]
|
|
|
|
|
|
|
M. V. Rao, M. K. Houseweart, T. L. Williamson, T. O. Crawford, J. Folmer, and D. W. Cleveland
Neurofilament-dependent Radial Growth of Motor Axons and Axonal Organization of Neurofilaments Does Not Require the Neurofilament Heavy Subunit (NF-H) or Its Phosphorylation
J. Cell Biol.,
October 5, 1998;
143(1):
171 - 181.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
G. A. Elder, V. L. Friedrich Jr., C. Kang, P. Bosco, A. Gourov, P.-H. Tu, B. Zhang, V. M.-Y. Lee, and R. A. Lazzarini
Requirement of Heavy Neurofilament Subunit in the Development of Axons with Large Calibers
J. Cell Biol.,
October 5, 1998;
143(1):
195 - 205.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
Top
Abstract
Introduction
