SOST is a ligand for LRP5/LRP6 and a Wnt signaling inhibitor, M Semënov, K Tamai, X He

Tags: Van Hul, SOST, Wnt signaling, molecular weight, BMP, function, Sclerosteosis, Van Buchem, bone growth, Xenopus embryos, J. C., Yu, IgG antibodies, SOST RNA, Van Eerdewegh, Van Wesenbeeck, WISE, Harvard Medical School, Xi He Division, gene expression, Mikhail Semenov, antagonist
Content: JBC Papers in Press. Published on May 20, 2005 as Manuscript M504308200
SOST IS A LIGAND FOR LRP5/LRP6 AND A WNT SIGNALING INHIBITOR* Mikhail Semлnov, Keiko Tamai and Xi He Division of Neuroscience, Children's Hospital, Harvard Medical School, Boston, MA 02115, USA Running Title: SOST is a ligand for LRP5/LRP6 and a Wnt signaling inhibitor Address correspondence to: Mikhail Semenov or Xi He, Division of Neuroscience, Children's Hospital, 300 Longwood Avenue, Boston, MA 02115, Tel. 617-919-2260 or 617-355-6885; Fax. 617-730-0243; E-Mail: [email protected], [email protected]
Sclerosteosis is an autosomal recessive disease that is characterized by overgrowth of bone tissue and is linked to mutations in the gene encoding the secreted protein SOST. Sclerosteosis shares remarkable similarities with "high bone mass" (HBM) diseases caused by "gain-of-function" mutations in LRP5 gene, which encodes a coreceptor for Wnt signaling proteins. We show here that SOST antagonizes Wnt signaling in Xenopus embryos and mammalian cells by binding to the extracellular domain of Wnt coreceptors LRP5 and LRP6 and disrupting Wnt-induced Fz-LRP complex formation. Our findings suggest that SOST is an antagonist for Wnt signaling, and that the loss of SOST function likely leads to hyperactivation of Wnt signaling that underlies bone overgrowth seen in Sclerosteosis patients. Sclerosteosis (1) and Van Buchem (2) disease are rare forms of autosomal recessive severe craniotubular hyperostoses. Both diseases are characterized by generalized overgrowth of bone tissue mostly manifested in cranial bones and the diaphysis of the tubular bones (3). Bone overgrowth appeared as early as at the age of 5 years and becoming more prominent with time. Sclerosteosis is linked to a loss of function of the SOST gene product (4,5), whereas Van Buchem disease is linked to a 52 kb-deletion downstream of the SOST gene which causes downregulation of SOST gene expression (6,7). SOST gene encodes a secreted protein. During embryogenesis SOST expression is first detected in the mesenchyme at the sites of osteogenesis (8,9), and SOST expression is confined specifically to osteoblasts and osteocytes postnatally (9,10). Increased rate of bone formation and elevated levels of serum alkaline phosphatase and osteocalcin in SOST mutation
carriers suggest that excessive bone accumulation is most likely due to an increase in osteoblast activity upon the loss or decrease of SOST expression (5,11). Some studies suggest that the ability of SOST to decrease osteogenic activity of osteoblasts may be explained by its anti-BMP activity (8-10). However, SOST clearly has additional activities. SOST induces apoptosis of human osteoblastic cells, an activity that other BMP antagonists such as noggin, chordin, gremlin do not possess (12). Furthermore, despite the ability of SOST to antagonize osteoblast differentiation induced by BMP, SOST does not inhibit BMP-induced SMAD protein phosphorylation or luciferase reporters driven by BMP responsive elements (10). These data suggest that SOST may affect other signaling pathways that complements or mediates BMP effect on osteoblasts. Sclerosteosis and Van Buchem disease share a remarkable similarity with "high bone mass" (HBM) phenotype as these diseases are all caused by an increase of osteogenic activity of osteoblasts and osteocytes and classified as "craniotubular hyperostoses" (13). HBM is associated with `activating' mutations in LRP5 gene (14-16), which encodes a Wnt coreceptor (17-19). Thus a lack of SOST function exhibits phenotypes related to excessive LRP5 function, implying the possibility that SOST may antagonize LRP5 function. Interestingly, SOST is related to WISE, a secreted protein that binds to Wnt coreceptor LRP6 and modulates (activates or inhibits) Wnt signaling in a cell context-dependent manner (20). Here we show that SOST binds to both LRP5 and LRP6, and inhibits the canonical Wnt pathway. Our results suggest that Sclerosteosis and Van Buchem disease may be a result of hyperactive Wnt signaling.
1 Copyright 2005 by The American Society for Biochemistry and Molecular Biology, Inc.
materials and methods SOST cDNA was cloned by PCR using IMAGE cDNA clone 2380708 as the template. SOST/pcDNA3.1+ contains the full length SOST. SOST-Myc/pcDNA3 and SOST-IgG/pcDNA3.1+ were generated by fusion at 5' end of full length SOST cDNA without the stop codon with the 6myc epitope tag from CS2+MT or with the IgG tag from IgG/pRK5 (21). Kremen2/pCS (22), DKK1-flag/CS2+ (23), LRP6N-myc/pcDNA3, Xwnt8/CS2+ (19), LRP5N-myc/pcDNA3, LDLRN-myc/pcDNA3 (24), Fz8CRD-IgG (21), tBR/pSP64T+ (25), LRP5/pcDNA3 (26), LRP6/pcDNA3.1 (27), LRP6N/CS2+ (28), Wnt1/LNCX (29), and PSecAP-MH (30) constructions have been described previously. Precipitation and immunoblotting were done similarly to Hsieh et al. (21). Fz-LRP6 Complex formation (19), cytosolic -catenin assay, and LRP6N-Frizzled8 complex disruption by SOST or DKK1 were performed as previously described (24) Xenopus injection and RT-PCR were performed as previously described (24). Rat2 and HEK293T cells were maintained in high-glucose DMEM medium with 10% newborn calf serum. Wnt1 CM was collected from retrovirally infected Rat2 cells. Other CMs were collected from transiently transfected HEK293T cell. TOPflash reporter assays were performed using the dualluciferase reporter assay system (Promega). HEK293T cells were seeded into 24-well plates one day before transfection. Cells were transfected with 100 ng superTOPflash reporter plasmid (31) and 10 ng pRL-TK plasmid (Promega) with other plasmids as indicated in experiment descriptions using Lipofectamine reagent (Invitrogen). DNA amounts were balanced with pcDNA3.1+ plasmid. Cell extracts were prepared 36 hours after transfection and assayed sequentially for firefly and renilla luciferase activity. Firefly luciferase readings were normalized against renilla luciferase. RESULTS Xenopus axis duplication assay provides a sensitive and reliable way to test Wnt stimulatory and Wnt inhibitory activities (32). Injection of 5 pg of Xwnt8 or 50 pg of LRP6N mRNA (an
constitutively active mutant LRP6) into the ventral marginal zone of four-cell Xenopus embryo caused ectopical axis formation in more than 80% of injected embryos (Fig.1A). SOST mRNA ventral injection alone did not induce ectopic axis formation, even when injected at a high dose (2000 pg/embryo). However, SOST mRNA efficiently blocked axis duplication by Xwnt8 mRNA (Fig.1A). Thus SOST antagonizes Xwnt8 activity. LRP6N activated Wnt signaling in a Wnt-independent manner and was not inhibited by SOST (Fig.1A). Two versions of SOST, an untagged SOST and a SOST with a carboxy- terminal Myc tag, showed the same activity in antagonizing Wnt signaling in this assay (Fig.1A). To ensure that SOST directly antagonizes Wnt signaling, we examined the ability of SOST to inhibit nodal-related 3 (Xnr3) induction by Wnt8 in animal pole explants, since Xnr3 is a direct downstream target of Wnt signaling (32). SOST blocked Xnr3 activation by Xwnt8 but not by LRP6N (Fig.1B). SOST mRNA injection alone did not induce Xnr3 expression (Fig.1B). We also examined whether SOST possesses anti-BMP activity in Xenopus embryos (33,34). Injection of 250 pg of truncated BMP receptor (tBR) mRNA ventrally efficiently blocked BMP signaling in developing Xenopus embryos and caused trunk duplication in more than 80% of injected embryos. By contrast, SOST mRNA injection even at 2000 pg could not induce trunk axis duplication (Fig.1C). Further, dorsal injection of SOST mRNA (2000 pg per embryo) at 4-cell stage resulted in exaggerated anterior development such as enlargement of the cement gland (not shown), a phenotype similar to that observed by injection of other Wnt antagonists (33). Thus SOST shows potent anti-Wnt activity and does not exhibit anti-BMP activity in Xenopus embryos. To test the ability of SOST to inhibit Wnt signaling in mammalian cells, we employed the SuperTOP-Flash reporter (31,35), which is driven by TCF/LEF-binding elements and responsive to Wnt signaling. The SuperTOP-flash reporter was readily activated in HEK293T cells transfected with Wnt1 expressing plasmid. Co-expression of a SOST expression plasmid inhibited Wnt signaling in dose-dependent manner (Fig.2A). To further study the SOST protein biochemically, we generated conditioned medium (CM) from
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HEK293T cells transfected with the SOST-myc plasmid. We found that the majority of the SOST protein remains cell-associated (Fig.2B). The major SOST isoform associated with cells and in the CM had an apparent molecular weight of 40 kD, as compared to the calculated molecular weight of 33 kD. In addition, SOST isoforms with apparent molecular weight from 40 to 50 kD and from 70 to 110 kD were also detected in the CM (Fig.2B). These higher molecular weight isoforms may be attributed to post-translational modifications, since SOST has two predicted sites for Nglycosylation at N53 and N175 (http://www.cbs.dtu.dk/services/NetNGlyc/) and one site for O-linked glycosylation at T55 (http://www.cbs.dtu.dk/services/NetOGlyc/) (36). We tested SOST CM for inhibition of Wnt signaling. Indeed, SOST CM inhibited cytosolic -catenin accumulation induced by Wnt-1 CM in Rat2 cells (Fig.2C). Thus secreted SOST directly antagonizes Wnt1 activity extracellularly. Similarities between sclerosteosis (associated with the loss of SOST function) and HBM disease linked to hyperactive LRP5 signaling suggest that SOST may antagonize Wnt signaling via direct binding to LRP5. To test the binding between SOST and LRP5 as well as LRP6, we produced CM containing a SOST-IgG fusion protein, which tags SOST with the constant region of immunoglobulin heavy chain, and the extracellular portion of LRP5 and LRP6 tagged with the myc epitope (LRP5N-myc and LRP6N-myc). We found that SOST showed specific interactions with LRP5 and LRP6, but not with a similarly tagged LDLR (LDLRN-myc)(Fig.3A). Further, when LDLRN-myc was mixed in large excess together with LRP5N-myc and LRP6N-myc, SOST specifically bound to LRP5 and LRP6 (Fig. 3A). As an additional control for the binding specificity, we used Fz8CRD-IgG, which has a similar molecular weight to that of SOST-IgG. Despite much higher abundance in the CM, Fz8CRD-IgG was unable to precipitate any detectable amount of LRP5 or LRP6 protein (Fig. 3A). To further demonstrate SOST-LRP5 and SOST-LRP6 interaction functionally, we examined whether SOST could inhibit LRP5 or LRP6 signaling activities. While LRP5 and LRP6 overexpression alone showed little activation of
Wnt signaling in the superTOP-flash assay, LRP5 or LRP6 synergized robustly with Wnt1 (Fig. 3B). SOST efficiently inhibited signaling activated by Wnt1 plus LRP5 or Wnt1 plus LRP6 (Fig. 3B), showing that SOST functionally antagonizes signaling by LRP5 and LRP6. Formation of complexes between LRP5 or LRP6 and Frizzled (Fz) proteins in the presence of Wnt proteins has been proposed to be an initial step in Wnt signaling activation (19). As we showed previously (19), Fz8CRD-IgG showed no interaction with the extracellular domain of LRP6 (Fig. 3A and 4A), but addition of Wnt1 CM induced formation of complexes between Fz8 and LRP6 (Fig.4A). Addition of CM containing either untagged or Myc-tagged SOST efficiently blocked such complex formation. CM containing alkaline phosphatase (AP), used as a negative control, had no effect on complex formation (Fig.4A). Thus SOST appears to be able to disrupt Wnt-induced Fz-LRP6 complex formation. We have shown that another Wnt antagonist, DKK1, also interacts with LRP5 and LRP6 and prevents Wnt-induced complex formation between LRP5/6 and Fz proteins (24). Anti-Wnt activity of DKK1 can be greatly enhanced by Kremen proteins, which also bind DKK1 (22). We thus compared the relationship between Kremen and SOST or DKK1 in Wnt signaling inhibition. We activated Wnt signaling by co-transfecting Wnt1 plus LRP5 in HEK293T cells (Fig.4B), and used a suboptimal amount of SOST or DKK1 that exhibited slight inhibition of Wnt effect (Fig. 4B). Expression of Kremen2 down-regulated Wnt signaling activity by ~40% in cells expressing LRP5 with Wnt1 (this was likely due to endogenous Dkk1 gene induction by activated Wnt signaling (37)). While DKK1 plus Kremen2 resulted in strong Wnt signaling inhibition, SOST and Kremen2 did not exhibit synergy in antagonizing Wnt signaling (Fig. 4B). Thus unlike DKK1, SOST does not cooperate with Kremen2 in Wnt signaling inhibition. DISCUSSION In this study we demonstrated that SOST, the product of the gene mutated in Sclerosteosis and Van Buchem disease, is an antagonistic ligand for Wnt coreceptor LRP5 and LRP6, and an inhibitor of the canonical Wnt/-catenin signaling
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in both mammalian cells and Xenopus embryos. These findings not only expand the repertoire of secreted antagonistic ligands that bind to Wnt coreceptor LRP5 and LRP6, but also have implications to our understanding of Sclerosteosis, Van Buchem disease and bone density diseases associated with LRP5 mutations, such as HBM syndrome and osteoporosis. It has become evident that the Wnt/-catenin signaling mediated by LRP5 and LRP6 plays a central role in mammalian bone density regulation (13). Loss-of-function mutations of LRP5 are associated with the recessive familial osteoporosispseudoglioma (OPPG) syndrome (38), whereas "gain-of-function" mutations of LRP5 are associated with HBM diseases (14,15). Intriguingly, all known LRP5 mutations from HBM families are single amino acid substitutions clustered in the first so-called YWTD--propeller of the LRP5 extracellular domain (16). The molecular mechanism by which these mutations cause the increase of LRP5/-catenin signaling during bone growth remains unclear. For one particular LRP5 mutation, G171V (glycine 171 mutated to valine), decreased inhibition of LRP5 by DKK-1 was suggested to account for increased LRP5 signaling (14,39). Because SOST is highly expressed in osteoblasts and osteocytes and a loss or down-regulation of SOST function in Sclerosteosis and Van Buchem disease exhibits increased bone growth, SOST appears to be an endogenous negative regulator of bone growth, likely through the inhibition of LRP5 and LRP6 function. It will be interesting to examine whether LRP5 mutations associated with HBM disease results in compromised SOST inhibition of LRP5 function. SOST shares 36% identity with WISE, which was shown to bind LRP6 and to activate or inhibit Wnt signaling in a context-dependent manner (20). These two closely related proteins share the `cysteine knot' domain that occupies the central part of proteins. One noticeable difference between SOST and WISE is that while SOST behaves exclusively as an antagonist for Wnt/LRP5/6 signaling in mammalian cells and Xenopus embryos, WISE alone can function as a
weak agonist that activates -catenin signaling to a limited extent (20). The mechanism and function of this Wnt agonist activity of WISE remains unclear. It is also of interest to compare SOST with the prototypic LRP5/6 antagonist DKK-1 (33), which does not show any amino acid sequence similarity with SOST. Like DKK-1 (24), SOST has the ability to disrupt Wnt1-induced Fz8LRP6 complex formation in an in vitro assay, suggesting a potential mechanism for the observed SOST action. However, unlike DKK-1, SOST inhibition of Wnt signaling is insensitive to the presence of Kremen2, a transmembrane protein that binds to DKK-1 (22). Thus while both are LRP5/6 ligands, the anti-Wnt activities of DKK-1 and SOST can be differently modulated by other co-factors. Some studies suggest that SOST and WISE can bind BMP proteins and act as BMP antagonists (8-10,40,41). However, in Xenopus embryo experiments neither SOST nor WISE exhibits any detectable antagonist activities towards BMP while both strongly affect Wnt signaling. Indeed, whereas prototypic BMP antagonists such as Noggin and Chordin induce neural tissue and trunk formation (via inhibition of BMP signaling), SOST and WISE failed to do so even when they were expressed at high levels (Fig. 1C, (20)). Thus SOST and WISE may at best have weak BMP antagonist activities. Consistent with this notion, the binding affinities of SOST and WISE for BMPs are significantly weaker than those of Noggin and Chordin (8,41). Further studies will be required to clarify whether SOST and WISE only function as Wnt/LRP5/6 antagonists or they function as antagonists for both Wnt and BMP signaling. Nonetheless, given the specific bone growth phenotypes of loss of SOST function and LRP5 mutations, modulating SOST and LRP5 interaction may be a potential therapeutic strategy for treatment of bone density diseases such as osteoporosis. While this manuscript is in preparation, Li et al. also reported that SOST can bind to LRP5/6 and function as a Wnt signaling antagonist (42).
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FOOTNOTES *We thank members of the Xi He lab for suggestions and help. This work was supported by a grant (GM057603) from NIH to X.H., who is a W. M. Keck Foundation DistinguishEd Young Scholar in medical research. 1The abbreviations used are: HMB, high bone mass; tBR, truncated BMP receptor; CM, conditioned media; IP, immunoprecipitation. FIGURE LEGENDS Fig. 1. SOST inhibits ectopic axis duplication and Xnr3 expression induced by Xwnt8 in Xenopus embryos. A, SOST antagonizes axis induction by Xwnt-8, but not by LRP6N. Ventral injection of SOST RNA did not induce ectopic axis formation (lanes 1 and 2). Xwnt8 RNA injection induced ectopic axis formation (lane 3). SOST RNA injection antagonized, in a dose-dependent manner, axis duplication by Xwnt-8 (lanes 4-6). LRP6N RNA injection induced ectopic axis formation (lane 8), and SOST RNA co-injection was unable to inhibit axis duplication by LRP6N (lanes 9-11). Similar results were obtained when globin RNA was included to equalize the dose of injected RNAs. B, SOST antagonizes Xnr3 induction by Xwnt-8, but not by LRP6N in Xenopus animal pole explants. SOST RNA injection did not induce Xnr3 expression (lanes 2-4). Xwnt8 RNA injection induced Xnr3 expression (lane5); SOST RNA co-injection inhibited Xnr3 expression induced by Xwnt8 (lanes 6-8). LRP6N RNA injection activated Xnr3 expression (lane 9), and SOST co-injection did not inhibit Xnr3 expression (lanes 10-12). C, SOST does not exhibit anti-BMP signaling activity in Xenopus embryos. Ventral injection of SOST RNA was unable to induce ectopic trunk formation (lanes 1-3). Dominant negative BMP receptor (tBR) RNA injection induced ectopic trunk (lane 4). RNA amounts are shown in pg per embryo. Fig. 2. SOST inhibits Wnt1 signaling in mammalian cells. A, SOST inhibits superTOP-flash reporter expression activated by Wnt1. HEK293T cells were untransfected (lane 1) or transfected either with Wnt1 expressing plasmid (lane 2) or co-transfected with Wnt1 plus different amounts of the SOST expressing plasmid (lanes 3-5). DNA amounts are shown in ng. B, SOST-myc protein is expressed and secreted into CM in transiently transfected HEK293T cells. SOST protein associated with cells (lane 1) and secreted into CM (lane 3) were detected with anti-myc antibodies. Control cell lysate (lane 2) and CM (lane 4) were from untransfected cells. C, SOST CM inhibits cytosolic -catenin stabilization induced by Wnt1 CM in Rat2 cells. Cells were either untreated (lane 1) or treated with Wnt1 CM (line 3) or Wnt1 CM plus SOST CM (lane 2). Cytosolic fractions were obtained and assayed for -catenin. Fig. 3. SOST binds specifically to LRP5 and LRP6, and inhibits signaling by LRP5 and LRP6. A, LRP5N-myc and LRP6N-myc, but not LDLRN-myc, were coprecipitated with SOST-IgG. None was coprecipitated with Fz8CRD-IgG. SOST-IgG CM (lanes 1-4) or Fz8CRD-IgG CM (lanes 5-8) were mixed with LRP6N-myc CM (lanes 1 and 5), LRP5N-myc (lanes 2 and 6), LDLRN-myc (lanes 3 and 7), or LRP6N-myc plus LRP5N-myc plus LDLRN-myc (lanes 4 and 8), and were precipitated with Protein G agarose beads. Precipitates were immunoblotted with anti-myc (a) or anti-human IgG antibodies (d). CM mixtures before precipitation were immunoblotted with anti-myc (b) or anti-human IgG antibodies (c). Note that SOST-IgG level in the input was much lower than that of Fz8CRD-IgG (c). B, SOST inhibits superTOP-flash reporter expression activated by Wnt1 plus LRP5 or Wnt1 plus LRP6. HEK293T cells were untransfected (lane 1) or transfected with Wnt1 plus indicated plasmids (lanes 2-6). DNA amounts are shown in ng. Fig. 4. SOST disrupts Fz-LRP6 complex formed in the presence of Wnt1, and unlike DKK1, does not cooperate with Kremen2 in Wnt1 inhibition. A, SOST blocks Fz-LRP6 complex formation induced by 7
Wnt1. Fz8CRD-IgG and LRP6N-myc CMs were mixed with control CM (lane 1), Wnt1 CM (lane 2), Wnt1 plus SOST CM (lane 3), Wnt1 plus SOST-myc CM (lane 4), or Wnt1 plus AP-myc CM (lane 5). CM mixtures were precipitated with Protein G agarose beads and precipitates were immunoblotted with anti-myc (a) or anti-human IgG antibodies (d). CM mixtures before precipitation were immunoblotted with anti-myc (b) or anti-human IgG antibodies (c). B, SOST does not cooperate with Kremen2 to inhibit TOP-flash reporter expression activated by Wnt1 and LRP6. HEK293T cells were untransfected (lane 1) or transfected with Wnt1 plus LRP6 expressing plasmids or other indicated plasmids (lanes 2-7). DNA amounts are shown in ng. 8
Ectopic Axis Induction (%) Ectopic Trunk Induction (%)
Figure 1
A
100
80
n=44
n=19 n=18 n=21 n=20
60
40
20
n=35
n=18 n=20
0
1
2
3
n=38 n=31 n=17
4
5
6
7
8
9 10 11
SOST
2000
SOST-myc
2000
Xwnt8
5
LRP6N
2000
100 500 2000
100 500 2000
5555
50 50 50 50
B Xnr3
EF-1
1 SOST Xwnt8 LRP6N
2
3
4
5
100 500 2000 5
6
7
8
100 500 2000 555
9 10 11 12 100 500 2000 50 50 50 50
C
100
80
60
n=29
40
20
n=25 n=23 n=22 0
1
2
3
4
SOST-myc 100 500 2000
tBR
250
Figure 2
A 16
B
Fold Activation
12
8
4
0 1 Wnt1 SOST
2
3
4
5
0.1 0.1 0.1 0.1 10 30 100
C Cytosolic extracts
-catenin
1 Wnt1 SOST
23 ++ +
130 kD 100 kD 72 kD 55 kD 40 kD 33 kD
70-110 kD 40-50 kD
24 kD
12 + SOST-myc Cell Lysate
34 + Cell Media
Figure 3
A
SOST-IgG
a IP
b Input
Fz8CRD-IgG LRP6N-myc LRP5N-myc LDLRN-myc LRP6N-myc LRP5N-myc LDLRN-myc
c Input
SOST-IgG Fz8CRD-IgG
d IP
SOST-IgG Fz8CRD-IgG
1
2
3
4
5
6
7
8
+ LRP6N-myc
++
+
LRP5N-myc
+
+
+
+
LDLRN-myc
++
++
SOST-IgG + + + +
Ez8CRD-IgG
++++
B 90 60
Fold Activation
30
0 1 Wnt1 LRP5 LRP6 SOST
2
3
4
5
6
0.1 0.1 0.1 0.1 0.1
11
11
100
100
Figure 4 A a IP
B 300
LRP6N-myc
200
LRP6N-myc
100
Fold Activation
b Input
AP-myc
SOST-myc
c Input
Fz8CRD-IgG
d IP
Fz8CRD-IgG
1
2
3
4
5
SOST
+
SOST-myc
+
AP-myc
+
Wnt1
++++
+ Fz8CRD-IgG + + + +
+ LRP6N-myc + + + +
0 1 Wnt1 LRP6 SOST DKK1 Kremen2
2
3
4
5
6
7
111111
111111
10
10
10
10
10 10 10

M Semënov, K Tamai, X He

File: sost-is-a-ligand-for-lrp5lrp6-and-a-wnt-signaling-inhibitor.pdf
Title: Microsoft Word - SOST IS A LIGAND FOR LRP5.rtf
Author: M Semënov, K Tamai, X He
Author: semenov
Published: Sun May 15 21:48:53 2005
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