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Originally published In Press as doi:10.1074/jbc.M305530200 on July 2, 2003
J. Biol. Chem., Vol. 278, Issue 37, 35805-35811, September 12, 2003
Heterogeneity of the Chondroitin Sulfate Portion of Phosphacan/6B4 Proteoglycan Regulates Its Binding Affinity for Pleiotrophin/Heparin Binding Growth-associated Molecule*
Nobuaki Maeda ,
Jue He ,
Yuki Yajima ¶,
Tadahisa Mikami ¶,
Kazuyuki Sugahara ¶ and
Tomio Yabe
From the
Department of Developmental Neuroscience,
Tokyo Metropolitan Institute for Neuroscience, 2-6 Musashidai, Fuchu, Tokyo
183-8526, and the ¶Department of Biochemistry,
Kobe Pharmaceutical University, Higashinada-ku, Kobe 658-8558, Japan
Received for publication, May 27, 2003
, and in revised form, July 1, 2003.
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ABSTRACT
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PTP is a receptor-type protein-tyrosine phosphatase that is
synthesized as a chondroitin sulfate proteoglycan and uses pleiotrophin as a
ligand. The chondroitin sulfate portion of this receptor is essential for high
affinity binding to pleiotrophin. Here, we purified phosphacan, which
corresponds to the extracellular domain of PTP , from postnatal day 7
(P7) and P12 rat cerebral cortex (PG-P7 and PG-P12, respectively) and from P20
rat whole brain (PG-P20). The chondroitin sulfate of these preparations
displayed immunologically and compositionally different structures. In
particular, only PG-P20 reacted with the monoclonal antibody MO-225, which
recognizes chondroitin sulfate containing the
GlcA(2S) 13GalNAc(6S) disaccharide unit (D
unit). Analysis of the chondroitinase digestion products revealed that
GlcA 13GalNAc(4S) disaccharide unit (A unit) was the
major component in these preparations and that PG-P20 contained 1.3% D unit,
which was not detected in PG-P7 and PG-P12. Interaction analysis using a
surface plasmon resonance biosensor indicated that PG-P20 had 5-fold
stronger affinity for pleiotrophin (dissociation constant
(KD) = 0.14 nM) than PG-P7 and PG-P12, although
all these preparations showed similar low affinity binding to pleiotrophin
after chondroitinase ABC digestion (KD = 1.4 1.6
nM). We also found that shark cartilage chondroitin sulfate D
containing 20% D unit bound to pleiotrophin with moderate affinity
(KD = 2.7 nM), whereas whale cartilage
chondroitin sulfate A showed no binding to this growth factor. These results
suggest that variation of chondroitin sulfate plays important roles in the
regulation of signal transduction in the brain.
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INTRODUCTION
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Recent studies reveal that proteoglycans play pivotal roles in various
developmental processes such as cell migration, recognition, and morphogenesis
(14).
Accumulating evidence suggests that glycosaminoglycans with specific sequences
selectively bind with a wide range of proteins including growth factors,
morphogens, and proteases, regulating their biological activities
(14).
Although this paradigm is principally derived from the findings on heparan
sulfate proteoglycans, increasing evidence suggests that chondroitin sulfate
proteoglycans also play such roles
(3).
PTP 1 is a
receptor-type protein-tyrosine phosphatase abundantly expressed in the brain
as a chondroitin sulfate proteoglycan
(5,
6). The extracellular domain of
this receptor is secreted in the brain as a major soluble chondroitin sulfate
proteoglycan called phosphacan/6B4 proteoglycan/DSD-1-PG
(68).
PTP binds to pleiotrophin/heparin-binding growth-associated molecule
with high affinity (KD = 0.25 3 nM)
(911).
The binding of pleiotrophin to PTP depends on the chondroitin sulfate
portion of this receptor, and removal of chondroitin sulfate resulted in a
drastic decrease in the binding affinity and signal transduction
(911).
This binding was inhibited strongly by squid cartilage chondroitin sulfate E
(CS-E) and shark cartilage CS-D, moderately by shark cartilage CS-C, and very
weakly by whale cartilage CS-A
(9,
10).
An in vitro cell migration assay indicated that pleiotrophin
induces migration of cortical neurons
(10). This activity of
pleiotrophin was also inhibited by CS-C, -D, and -E but not by CS-A.
Furthermore, using an organotypic slice culture system of the rat cerebellum,
we revealed that disruption of PTP -pleiotrophin signaling leads to the
aberrant morphogenesis of Purkinje cells
(12). The number of Purkinje
cells with abnormal dendrites such as multiple and disoriented dendrites
markedly increased when cerebellar slices were treated with polyclonal
antibodies against PTP , chondroitinase ABC, CS-C, CS-D, or CS-E, whereas
CS-A was also ineffective in this system
(12). These findings suggested
that a specific structural motif containing oversulfated portion in
chondroitin sulfate is involved in the high affinity binding between PTP
and pleiotrophin and their signal transduction.
Recently, Deepa et al.
(13) report that squid
cartilage CS-E interacted with various heparin binding growth factors. CS-E
bound strongly to pleiotrophin, midkine, fibroblast growth factor-16 (FGF-16),
FGF-18, and heparin binding epidermal growth factor-like growth factor with
comparable affinities to those of heparin
(13). Zou et al.
(14) also found that
chondroitin sulfate artificially enriched with E unit
(GlcA 13GalNAc(4S,6S)) bound strongly with
midkine, which shows a 45% amino acid sequence identity to pleiotrophin,
forming a heparin binding growth factor family together with this protein.
However, these chondroitin sulfate preparations contained unusually high
amounts of oversulfated disaccharide units (more than 60% was E unit), which
are only minor components in the brain
(15). In fact, Ueoka et
al. (15) report that
chondroitin sulfate proteoglycan from embryonic day 18 rat brain contained
only 1.7% D (GlcA(2S) 13GalNAc(6S)) and 1.2% E
units. Thus, to evaluate the contribution of the chondroitin sulfate structure
to the signal transduction of phosphacan/PTP , it is necessary to use
proteoglycan samples purified from the brain.
It has been suggested that phosphacan is composed of several subpopulations
of molecules bearing chondroitin sulfate chains with different structures
(16,
17). In this study we found
that the immunological and compositional structure of chondroitin sulfate of
phosphacan changed dramatically during development of the brain. Using a
BIAcore system we observed that the binding affinity of phosphacan for
pleiotrophin highly depends on the structure of chondroitin sulfate,
especially on the presence of D unit. Our observations suggest that structural
variation of chondroitin sulfate plays important roles in the regulation of
signal transduction.
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EXPERIMENTAL PROCEDURES
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MaterialsHuman recombinant pleiotrophin was purchased from
R&D systems (Minneapolis, MN). CS-A from whale cartilage (average
molecular mass 34 kDa), CS-B from pig skin (average molecular mass 32 kDa),
CS-C (average molecular mass 64 kDa) and CS-D (average molecular mass 30 kDa)
from shark cartilage, CS-E from squid cartilage (average molecular mass 70
kDa), chondroitinase ABC, chondroitinase AC-II, unsaturated
chondro-disaccharide kit, monoclonal anti-chondroitin sulfate A (2H6), and
monoclonal anti-chondroitin sulfate D (MO-225) were purchased from Seikagaku
Corp. (Tokyo, Japan). Heparin from porcine intestinal mucosa (average
molecular mass 14 kDa) was obtained from Calbiochem. YMC pack PA-03 column was
purchased from YMC Co (Kyoto, Japan). Monoclonal anti-chondroitin sulfate
(CS-56), QuantiPro BCA assay kit, and actinase E were purchased from Sigma.
Vectastain ABC kit was from Vector Labs (Burlingame, CA). Immobilon membrane
and Microcon YM-30 were purchased from Millipore (Bedford, MA).
Streptavidin-conjugated alkaline phosphatase and Sepharose CL-6B were
purchased from Amersham Biosciences. A research grade sensor chip CM5 and an
amine coupling kit containing N-hydroxysuccinimide,
N-ethyl-N'-(3-dimethylaminopropyl)-carbodiimide
hydrochloride, and 1 M ethanolamine hydrochloride were purchased
from Biacore AB (Uppsala, Sweden). The biosensing system used for surface
plasmon resonance measurements consisted of BIAcore 1000 and BIAevaluation
software 3.2 (Biacore AB). Phosphacan/6B4 proteoglycan was purified from rat
brain as described previously
(18). Polyclonal antibodies
against phosphacan/6B4 proteoglycan (anti-6B4PG) were prepared as described
previously (5).
ImmunohistochemistryAfter ether anesthesia, BALB/c mice
were perfused with a solution containing 4% paraformaldehyde and 0.1
M sodium phosphate buffer, pH 7.4, via the left ventricle. The
solution was washed out from the right atrium, and the brains were dissected
out and embedded in paraffin after dehydration through a graded alcohol
series. Paraffin-embedded samples were cut into sections 6-µm thick, which
were then deparaffinized and equilibrated in phosphate-buffered saline (PBS).
The sections were incubated sequentially in the following solutions: 1) 2.5%
H2O2, PBS for 30 min; 2) 1% bovine serum albumin (BSA),
4% goat serum, PBS for 30 min; 3) primary antibody solution diluted in 1% BSA,
PBS (1:100) for 60 min; 4) biotinylated anti-mouse IgM solution for 60 min; 5)
ABC solution for 30 min; and 6) 0.1% diaminobenzidine, 0.02% hydrogen
peroxide, PBS. A Vectastain ABC kit was used according to the supplier's
protocol.
ImmunoblottingPurified phosphacan/6B4 proteoglycan (1
µg) was precipitated with 3 volumes of ethanol containing 1.3% potassium
acetate and then dissolved in 20 µl of the solution containing 30
mM sodium acetate, 1 mM phenylmethylsulfonyl fluoride,
0.1 mM pepstatin A, 10 mM EDTA, 10 mM
N-ethylmaleimide, and 0.1 M Tris-HCl, pH 7.5. The samples
were incubated for 60 min at 37 °C in the presence or absence of 2 mIU of
protease-free chondroitinase ABC and then mixed with the same volume of 4%
SDS, 20% glycerol, 10% 2-mercaptoethanol, and 0.125 M Tris-HCl, pH
6.8. The samples were boiled for 5 min and processed for 5% SDS-PAGE, and then
the proteins were transferred to Immobilon membranes as described previously
(18). The membranes were
soaked in 3% BSA, Tris-buffered saline (TBS) for 60 min and then incubated in
0.1% BSA, TBS containing a mouse monoclonal anti-chondroitin sulfate A (2H6;
1:100), a mouse monoclonal anti-chondroitin sulfate D (MO-225; 1:100), a mouse
monoclonal anti-chondroitin sulfate (CS-56; 1:100), or rabbit anti-6B4PG
(1:500). The membranes were washed 3 times with 0.05% Tween 20, TBS and then
incubated for 30 min with biotinylated anti-mouse IgM (1:200; for 2H6, MO-225,
and CS-56) or biotinylated anti-rabbit IgG (1:200; for anti-6B4PG). After
washing 3 times with 0.05% Tween 20, TBS, the membranes were incubated for 30
min with streptavidin-conjugated alkaline phosphatase (1:1000). The membranes
were washed 3 times with 0.05% Tween 20, TBS and then treated with 0.3 mg/ml
nitro blue tetrazolium, 0.8 mg/ml 5-bromo-4-chloro-3-indolyl phosphate, 0.1
M NaCl, 50 mM MgCl2, and 0.1 M
Tris-HCl, pH 9.5.
Characterization of Chondroitin SulfatePurified phosphacan
(100 nmol as hexuronate) was treated with 0.2 M NaOH at room
temperature for 24 h. After neutralization with glacial acetic acid, the
sample was digested with 5 µg/ml actinase E at 50 °C for 24 h in the
presence of 0.5 M Tris-HCl, pH 7.8. After heating at 100 °C for
3 min, the solution was applied to a Sepharose CL-6B column (7 mm x 30
cm), and the chondroitin sulfate was eluted with 0.4 M ammonium
acetate. The size of chondroitin sulfate was estimated from the
Kav values as described by Wasteson
(19). Analysis of the
disaccharide composition of chondroitin sulfate was performed by the method of
Yoshida et al. (20)
after chondroitinase ABC digestion of the phosphacan preparations. The
disaccharide composition of the preparations were also confirmed by the method
reported by Kinoshita and Sugahara
(21). Briefly, phosphacan
preparations (30 nmol as hexuronate) were incubated with 10 mIU of
chondroitinase AC-II in a total volume of 30 µl of 50 mM sodium
acetate buffer, pH 6.0, at 37 °C for 30 min. The reaction was terminated
by heating at 100 °C for 1 min. The samples were dried and derivatized
with 2-aminobenzamide, and the excess 2-aminobenzamide reagent was removed by
paper chromatography. An aliquot of the 2-aminobenzamide-derivative (1 nmol as
GlcUA) was subjected to HPLC on an amine-bound silica PA-03 column (4.6
x 250 mm) using a linear gradient of NaH2PO4 from
16 to 530 mM over 60 min at a flow rate of 1 ml/min. Eluates were
monitored by fluorescence intensity with excitation and emission wavelength of
330 and 420 nm, respectively.
Immobilization of Pleiotrophin on the Sensor
SurfacePleiotrophin was immobilized on the surface of a CM5 sensor
chip by amine coupling, where the primary amino groups on the protein were
coupled to the carboxymethylated dextran on a sensor surface. The
carboxymethylated dextran surface on the sensor chip was activated by
injection of 35 µl of a mixture of
N-ethyl-N'-(3-dimethylaminopropyl)-carbodiimide
hydrochloride/N-hydroxysuccinimide (0.2/0.05 M). Then 10
µg/ml pleiotrophin in 10 mM maleate buffer, pH 6.0, was injected
onto the activated sensor surface. The remaining unreacted sites were blocked
by injecting 35 µl of 1 M ethanolamine, pH 8.5. The amounts of
pleiotrophin immobilized onto the sensor surface were controlled within the
range 30004500 resonance units by changing the injection time. All
steps were carried out in a continuous flow of a solution containing 10
mM HEPES, pH 7.4, 0.15 M NaCl, 3 mM EDTA,
0.005% Tween 20 (HBS running buffer) at 5 µl/min.
Surface Plasmon Resonance AnalysisAll experiments were
carried out at flow rate of 20 µl/min at 25 °C. Buffer exchange of
phosphacan solution was performed using Microcon YM-30 by repeated
concentration and reconstitution with HBS running buffer. The phosphacan
preparations (10 µl of a 200 µg/ml solution) were treated with 25 mIU of
protease-free chondroitinase ABC for 1 h at 37 °C. Each sample was diluted
in HBS running buffer and injected onto the sensor surface. The sensor
surfaces were regenerated with 10 µl of 2 M NaCl after a
dissociation phase. To correct for the bulk effects and the nonspecific
bindings of samples, the equivalent sample solutions were injected onto an
untreated sensor surface, and the obtained responses were subtracted from the
pleiotrophin-immobilized sensor surface data. The kinetic parameters were
evaluated with BIAevaluation software 3.2 using a 1:1 (Langmuir) binding
model.
Other MethodsProtein concentration was determined using a
QuantiPro BCA assay kit using BSA as a standard. Hexuronate concentration was
determined as described by Bitter and Muir
(22).
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RESULTS
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Immunohistochemical Localization of Chondroitin Sulfate Epitopes in the
Developing Mouse BrainTo analyze systematically the changes in
chondroitin sulfate structure during development of the brain, we
immunohistochemically stained the mouse brain sections using three kinds of
monoclonal antibodies against native chondroitin sulfate: CS-56, 2H6, and
MO-225. Among these monoclonal antibodies, MO-225 specifically recognizes
tetrasaccharides containing D unit
(23). CS-56 strongly reacts
with whale cartilage CS-A and shark cartilage CS-C
(24), and 2H6 selectively
recognizes whale cartilage CS-A
(25); however, the structures
of their epitopes are unknown. Fig.
1 shows the immunohistochemically stained sagittal sections from
the brains of postnatal day 7 (P7), P12, and P20 mice. 2H6 and CS-56 epitopes
were highly expressed in the P7 cerebral cortex, but their expression in this
region decreased thereafter (Fig.
1, Cx). On the other hand, the expression of MO-225
epitope was very low in the cerebral cortex during these postnatal ages. In
contrast, the cerebellum was strongly stained by MO-225 from P7 to P20
(Fig. 1, Ce). The
expression of CS-56 epitopes was also observed in the P7 and P12 cerebellum,
but the expression disappeared in the P20 cerebellar cortex. 2H6 epitope was
barely detected in the postnatal cerebellar cortex. These results indicated
that the chondroitin sulfate structure changes regionally and developmentally
in the mouse brain. Similar results were obtained using sections from the rat
brain (data not shown), and a detailed description of the immunohistochemical
analysis will be reported elsewhere.

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FIG. 1. Immunohistochemical localization of chondroitin sulfate epitopes in the
developing mouse brain. Sagittal sections from P7 (A, B, and
C), P12 (D, E, and F), and P20 (G, H, and
I) mouse brains were stained immunohistochemically with CS-56 (A,
D, and G), 2H6 (B, E, and H), and MO-225
(C, F, and I) monoclonal antibodies. No staining was
observed when chondroitinase ABC-treated tissue sections were processed for
immunohistochemistry (data not shown). Staining of cerebral cortex
(Cx) and cerebellum (Ce) dynamically changed during
development.
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Purification of Phosphacan Bearing Chondroitin Sulfate with Different
StructuresFrom the immunohistochemical analysis described above,
we anticipated that phosphacan bearing chondroitin sulfate chains with
different structures could be purified from the rat brain by selecting
appropriate regions and ages, because this molecule is the major chondroitin
sulfate proteoglycan in the brain. Our expectations were as follows. 1)
Phosphacan samples with low content of D unit would be purified from P7 and
P12 cerebral cortex. 2) Phosphacan bearing chondroitin sulfate chains with a
substantial amount of D unit would be purified from P20 whole brain. 3) These
three phosphacan samples would show different overall chondroitin sulfate
structures.
Fig. 2 shows the immunoblot
analysis of phosphacan samples from P7 cerebral cortex (PG-P7), P12 cerebral
cortex (PG-P12), and P20 whole brain (PG-P20). Analysis of the purified
phosphacan with the polyclonal antibodies against core protein (anti-6B4 PG)
indicated that the sizes of native proteoglycan and core glycoprotein were
indistinguishable among these preparations
(Fig. 2A).
Reactivities to CS-56 and 2H6 monoclonal antibodies were highly detected in
PG-P7, moderately in PG-P12 and barely in PG-P20
(Fig. 2, B and
C). On the other hand, the MO-225 epitope was highly
expressed in PG-P20 but was not detected in PG-P7 and PG-P12
(Fig. 2D).

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FIG. 2. Immunoblot analysis of phosphacan preparations. Phosphacan samples
purified from P7 cerebral cortex (a and d), P12 cerebral
cortex (b and e), and P20 whole brain (c and
f) were applied to 5% SDS-PAGE before (a, b, and c)
or after (d, e, and f) chondroitinase ABC digestion. The
samples were analyzed by immunoblotting using anti-6B4PG (A), CS-56
(B), 2H6 (C), and MO-225 (D). The arrows
indicate the positions of the core glycoprotein of phosphacan.
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Analysis of the disaccharide composition of each preparation indicated that
PG-P20 contained 1.3% D unit, which was not detected in PG-P7 and PG-P12
(Table I). The contents of A
unit (GlcA 13GalNAc(4S)) and C unit
(GlcA 13GalNAc(6S)) were variable among these
preparations. In particular, the contents of C unit drastically decreased from
P7 to P20 (Table I). Despite
the large difference in the disaccharide composition, the chain length of
chondroitin sulfate and GlcA contents of the each preparation were not
significantly different from each other
(Table I).
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TABLE I Characterization of chondroitin sulfate on phosphacan/6B4
proteoglycan
The disaccharide composition was determined by the HPLC analysis of the
chondroitinase ABC digestion products of the each sample. Chondroitinase AC-II
digestion products gave essentially the same results. ND, not detected.
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Analysis of Interaction between Phosphacan and Pleiotrophin with the
BIAcore SystemReal-time analysis of the interaction between
phosphacan and pleiotrophin was performed using the BIAcore system. Various
concentrations of phosphacan samples were passed over the CM5 sensor chip
containing immobilized pleiotrophin, and the change in response was evaluated
as a function of time (Fig. 3).
The binding of phosphacan to pleiotrophin was monitored by the increase in the
response with time up to 12 min, which corresponds to the association phase.
This phase was followed by a dissociation phase generated by the stream
flowing over the sensor chip containing only running buffer. The affinity of
phosphacan for pleiotrophin was quantified by determining the dissociation
constant KD (KD =
kd/ka), where kd
and ka represent dissociation and association rate
constants calculated from the dissociation and association phases,
respectively.

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FIG. 3. Sensorgrams of binding of phosphacan preparations to immobilized
pleiotrophin. Various concentrations (as protein) of PG-P7 (A and
B), PG-P12 (C and D), and PG-P20 (E and
F) were injected onto a pleiotrophin-immobilized CM5 sensor chip
before (A, C, and E) or after (B, D, and
F) chondroitinase ABC digestion. The samples were injected at 80 s,
and the dissociation phase began at 800 s. RU, resonance units.
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The sensorgrams of the bindings of PG-P7 and PG-P12 to pleiotrophin were
very similar to each other (Fig. 3,
A and C), and showed KD
values of 0.66 and 0.57 nM, respectively
(Table II). PG-P20 exhibited a
higher affinity for pleiotrophin (KD = 0.14 nM)
than PG-P7 and PG-P12 (Fig.
3E). The association rate constant for the
PG-P20-pleiotrophin interaction (ka = 2.0 x
106 M1
s1) was 34-fold faster than those for
PG-P7- and PG-P12-pleiotrophin interactions (ka = 5.3 and
6.1 x 105 M1
s1, respectively)
(Table II). On the other hand,
their dissociation rate constants were not significantly different from each
other (kd = 2.7 3.5 x
104 s1).
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TABLE II Kinetic parameters for the interaction between phosphacan and
pleiotrophin
The ka, kd, KD, and
Rmax values were calculated from the sensorgrams using six
or more concentrations of analytes (phosphacan or chondroitinase ABC-treated
phosphacan) in two or three independent experiments. The values are expressed
as the mean ± S.D. PG-P7-Ch, PG-P12-Ch, and PG-P20-Ch mean the
chondroitinase ABC-treated samples of PG-P7, PG-P12, and PG-P20, respectively.
RU, resonance units.
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When the proteoglycan samples were treated with chondroitinase ABC to
remove chondroitin sulfate, their affinities for pleiotrophin remarkably
decreased (Fig. 3, B, D, and
F), and all the preparations showed similar, relatively
low affinity binding to pleiotrophin (KD = 1.4 1.6
nM) (Table II). At
higher concentrations of phosphacan, the binding of intact proteoglycans
reached near equilibrium in the association phase, whereas chondroitinase
ABC-treated proteoglycans exhibited a nearly linear increase in responses
during the association phase even at the concentration of 1.3 µg/ml
(Fig. 3). This was caused
mainly by the decrease in the association rate constants of
phosphacan-pleiotrophin interaction after chondroitinase ABC digestion
(Table II). On the other hand,
the values of the dissociation rate constant were barely influenced by the
chondroitinase ABC digestion (Table
II). All the phosphacan preparations displayed similar values of
association and dissociation rate constants after chondroitinase ABC digestion
(ka = 1.6 2.0 x 105
M1 s1 and
kd = 2.2 3.2 x
104 s1). These
results indicated that the presence of chondroitin sulfate critically
influences the association rate, and the value of this rate constant is
regulated by the structural variation of chondroitin sulfate chains.
It is noteworthy that the calculated values of Rmax,
which correspond to the maximum responses reached after equilibrium, increased
2-fold after chondroitinase ABC digestion of the phosphacan preparations.
This might be caused by the decrease in steric hindrance after chondroitinase
ABC digestion, where intact proteoglycans cannot efficiently use the ligands
immobilized in the carboxymethylated dextran matrix because of their large
molecular size.
Analysis of the Interaction between Glycosaminoglycans and Pleiotrophin
with the BIAcore SystemThe experiments described above indicated
that the structural differences in chondroitin sulfate chains regulate the
binding affinity of phosphacan for pleiotrophin. Thus, we analyzed the
interaction of various chondroitin sulfate preparations with pleiotrophin
using the BIAcore system. The measurements were performed for whale cartilage
CS-A, pig skin CS-B, shark cartilage CS-C, shark cartilage CS-D, and squid
cartilage CS-E, and these preparations exhibited quite characteristic
sensorgrams (Fig. 4). Among
these chondroitin sulfate samples, CS-E showed the strongest affinity for
pleiotrophin with a KD of 0.76 nM
(Fig. 4E and
Table III). Deepa et
al. (13) previously
reported that CS-E bound pleiotrophin with a lower affinity
(KD = 11.4 nM). The cause of this difference is
unknown, but they observed the association between soluble pleiotrophin and
immobilized CS-E. This might result in the different affinity of
pleiotrophin-CS-E interaction. In this study, we found for the first time that
CS-D also had strong affinity for pleiotrophin with a KD
of 2.7 nM (Fig.
4D).

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FIG. 4. Sensorgrams of binding of glycosaminoglycans to immobilized
pleiotrophin. Various concentrations of whale cartilage chondroitin
sulfate A (A), pig skin chondroitin sulfate B (B), shark
cartilage chondroitin sulfate C (C), shark cartilage chondroitin
sulfate D (D), squid cartilage chondroitin sulfate E (E),
and porcine intestinal mucosa heparin (F) were injected onto a
pleiotrophin-immobilized CM5 sensor chip. The arrow in B
indicates the first phase of response observed for chondroitin sulfate B. The
samples were injected at 75 s, and the dissociation phases began at 425 s.
RU, resonance units.
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TABLE III Kinetic parameters for the interaction between glycosaminoglycans and
pleiotrophin
The ka, kd, and KD
values were calculated from the sensorgrams using six or more concentrations
of analytes in two or three independent experiments. The values are expressed
as the mean ± S.D. IC50 values are the concentrations of
glycosaminoglycans at which 50% inhibition of the binding between
125I-labeled phosphacan and pleiotrophin was attained. These values
were calculated from the experimental results reported previously
(9). ND, not detected.
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CS-B had 45-fold less affinity (KD = 34
nM) in comparison with CS-E. However, it should be noted that the
sensorgram of this type of chondroitin sulfate showed a two-phase reaction.
There was a small, rapid rise in the responses
(Fig. 4B,
arrow) followed by a slow, large increase. Although the former phase
was observed even at 1.3 µg/ml of CS-B, the latter became evident at higher
concentrations (Fig.
4B), suggesting that this preparation is composed of a
small population of high affinity components and a large population of low
affinity components with a KD of 34 nM, the
value of which was calculated by neglecting the first phase of binding.
Vacherot et al. (26)
also report that CS-B binds with pleiotrophin/heparin affin regulatory peptide
with a KD of 51 nM, although they did not
detect the first phase. CS-C also showed specific binding to pleiotrophin;
however, we could not determine the kd and
ka values, because the reaction was too slow to make that
determination (Fig.
4C). In contrast, CS-A showed no binding even at 50
µg/ml (Fig.
4A).
The sensorgram of the binding of heparin with pleiotrophin was very similar
to that of CS-E (Fig.
4F), where heparin showed strong affinity for
pleiotrophin with a KD of 1.7 nM
(Table III). Previously, we
reported that various types of chondroitin sulfate and heparin inhibited the
binding of phosphacan to pleiotrophin
(9). The IC50 values
for these inhibitory effects roughly coincided with the KD
values obtained in this study (Table
III), supporting the reliability of our BIAcore analysis. These
results indicated that the affinity of chondroitin sulfate for pleiotrophin is
highly dependent on the structural variation of this type of
glycosaminoglycan.
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DISCUSSION
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In this study, we demonstrated dynamic changes in the chondroitin sulfate
structure in the developing brain. This regional and developmental change in
the chondroitin sulfate structure was reflected in the chondroitin sulfate
portion of phosphacan. Real-time analysis of the interaction between
pleiotrophin and various phosphacan preparations indicated that the
differences in the chondroitin sulfate structure on phosphacan markedly
influence their binding affinity for pleiotrophin. This suggests that the
ligand binding affinity and the strength of signal transduction of
phosphacan/PTP are regulated by the structural heterogeneity of its
chondroitin sulfate portion.
The three phosphacan preparations purified in this study showed
immunologically and compositionally different chondroitin sulfate structures.
PG-P7 strongly reacted with CS-56 and 2H6 but was not recognized by MO-225.
PG-P12 showed medium reactivity to CS-56 and 2H6 without reactivity to MO-225.
PG-P20 strongly reacted with MO-225 with low reactivities to CS-56 and 2H6.
MO-225 recognizes chondroitin sulfate containing D unit
(23), which is consistent with
our data that D unit was only detected in PG-P20 by an HPLC analysis of the
chondroitinase digestion products. On the other hand, it was reported that 2H6
strongly reacted with whale cartilage CS-A
(25), and this antibody is
sold as anti-chondroitin sulfate A. However, there was no correlation between
the contents of A unit in phosphacan preparations and their reactivities to
2H6. In fact, PG-P7 with the lowest content of A unit (64%) showed the
strongest reactivity to 2H6, and PG-P20 bearing chondroitin sulfate containing
86% A unit displayed very low reactivity to this antibody, suggesting that the
epitope of 2H6 is not a simple stretch of A unit but some complex sequences in
chondroitin sulfate. These observations suggest that chondroitin sulfate
chains of phosphacan have a differential combination of structural motifs
depending on the regions and developmental stages of the brain.
Analysis of the interaction between pleiotrophin and phosphacan
preparations with the BIAcore system revealed that differences in the
chondroitin sulfate structure on phosphacan lead to a large difference in the
binding affinity for pleiotrophin. PG-P20 displayed high affinity binding to
pleiotrophin (KD = 0.14 nM), and PG-P7 and
PG-P12 showed 45-fold lower affinity for pleiotrophin
(KD = 0.6 nM) than PG-P20. The difference
in affinity disappeared after chondroitinase ABC digestion, and all the
preparations showed KD values of 1.4 1.6
nM. This indicated that chondroitin sulfate chains of PG-P7 and
PG-P12 increased the binding affinity of phosphacan 2.5-fold, whereas
those of PG-P20 increased the affinity more than 11-fold.
From experiments using various midkine mutants, we previously suggested
that there is a hierarchy with three steps in the binding between phosphacan
and midkine: 1) low affinity binding between midkine and core glycoprotein
(KD = 8 nM); 2) medium affinity binding
between midkine and phosphacan bearing a general structure of chondroitin
sulfate (KD = 3 nM); and 3) high affinity
binding between midkine and phosphacan bearing a specific structural motif of
chondroitin sulfate (KD = 0.5 nM), which
involves a specific contribution of Arg78 of midkine
(27). An in vitro
cell migration assay indicated that the third high affinity binding mediated
by Arg78 is necessary for the full signal transduction of PTP
(27). It seems highly possible
that the binding of pleiotrophin to phosphacan follows a similar hierarchy,
because the characteristics of the binding between these molecules are very
similar to those of phosphacan and midkine
(9,
27), which shares the basic
structural motif with pleiotrophin
(28). According to this
scheme, it is likely that the chondroitin sulfate chains of PG-P7 and PG-P12
correspond to the general structure, and those of PG-P20 correspond to the
polysaccharide with a specific structural motif. At present, we do not know
what components contribute to the low affinity binding of chondroitinase
ABC-treated phosphacan to pleiotrophin. Kurosawa et al.
(29) indicate that midkine
bound specifically to sulfatide with a KD of 5.1
nM, suggesting that midkine could bind with sulfated
monosaccharides. Unsaturated disaccharides of chondroitin sulfate stubs
remained on the core glycoprotein after chondroitinase ABC digestion, or
sulfated oligosaccharides such as HNK-1 carbohydrate
(16,
30,
31) might be involved in the
low affinity binding.
Recently, Deepa et al.
(13) report that squid
cartilage CS-E interacted with various heparin binding growth factors
including pleiotrophin and midkine. However, squid CS-E contains a substantial
amount of unusual 3-O-sulfated glucuronic acid. Accordingly, Zou
et al. (14) prepared
artificial CS-E without 3-O-sulfation of glucuronic acid using
N-acetylgalactosamine 4-sulfate 6-O-sulfotransferase and
demonstrated that this preparation also bound strongly with midkine. These
findings suggested that oversulfated portions in chondroitin sulfate
contribute to the strong binding with heparin binding growth factors. In this
study, we revealed that, in addition to CS-E, shark cartilage CS-D strongly
bound with pleiotrophin. Shark cartilage CS-C also showed binding to a
pleiotrophin-immobilized sensor chip. CS-C contains substantial amounts of D
unit (9.6% for CS-C and 21.2% for CS-D)
(32), which could contribute
to the binding to pleiotrophin. On the other hand, no binding of whale
cartilage chondroitin sulfate A to pleiotrophin was observed by our BIAcore
analysis. Our results revealed unexpectedly diverse characteristics of each
type of chondroitin sulfate and are consistent with the view that oversulfated
chondroitin sulfate has strong affinity for pleiotrophin.
Although chondroitin sulfate chains of PG-P7 and PG-P12 displayed different
A unit/C unit ratios (2.0 and 5.9, respectively), their affinities for
pleiotrophin were not significantly different from each other
(KD = 0.66 and 0.57 nM, respectively). On the
other hand, chondroitin sulfate chains of PG-P20 had an A unit/C unit ratio of
14.3 and contained 1.3% D unit. From the observations that oversulfated
chondroitin sulfate had a strong affinity for pleiotrophin and whale cartilage
CS-A did not bind to pleiotrophin, we suggest that D unit in the chondroitin
sulfate chains of PG-P20 contributes to the high affinity binding of this
proteoglycan to pleiotrophin. Clement et al.
(17) report that chondroitin
sulfate chains of DSD-1-PG contained 5% D unit
(17). DSD-1-PG corresponds to
the subpopulation of phosphacan bearing D unit-rich chondroitin sulfate
(17), suggesting that the D
unit content of phosphacan is strictly regulated between 0 and 5%. Overall, it
seems likely that even a small content of oversulfated disaccharide units is
sufficient for the high affinity binding between phosphacan and pleiotrophin.
In addition to pleiotrophin and midkine, phosphacan binds with various
molecules including fibroblast growth factor-2, tenascin, L1, N-CAM,
contactin, and TAG-1 (33,
34). The next important
question is whether structural variation of chondroitin sulfate affects the
interaction between phosphacan and these molecules.
Recent studies in neuroscience reveal that neurons extend or retract their
neurites by recognizing the structure of chondroitin sulfate in their
environment (4,
12,
17,
35). In this study, we
demonstrated that the structure of chondroitin sulfate on phosphacan changes
dynamically during development of the brain concomitant with the changes in
the affinity for pleiotrophin. This suggests that the signal transduction
system of PTP -pleiotrophin is one of the molecular sensors that
distinguishes differences in the chondroitin sulfate structure. Further
studies are required to characterize the nature of this sensor to elucidate
the molecular mechanism involved in neurite extension.
 |
FOOTNOTES
|
|---|
* This study was supported in part by the grants from the Ministry of
Education Science, Sports, and Culture of Japan (to N. M. and K. S.), the
Takeda Science Foundation (to N. M.), and the Science Research Promotion Fund
of Japan Private School Promotion Foundation (to K. S.). The costs of
publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section 1734
solely to indicate this fact. 
To whom correspondence should be addressed. Tel.: 81-42-325-3881; Fax:
81-42-321-8678; E-mail:
maedan{at}tmin.ac.jp.
1 The abbreviations used are: PTP, protein-tyrosine phosphatase; CS,
chondroitin sulfate; PBS, phosphate-buffered saline; BSA, bovine serum
albumin; TBS, Tris-buffered saline; P, postnatal day; HPLC, high performance
liquid chromatography; ABC, avidin-biotin-peroxidase complex. 
 |
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