|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online June 22, 2006
doi: 10.1242/10.1242/dev.02452
Meeting Review |
1 Neurobiology Program, Children's Hospital and Harvard Medical School, Boston,
MA 02115-5724, USA.
2 Pathology Department, Stanford University School of Medicine, Stanford, CA
94305-5324, USA.
e-mail: xi.he{at}childrens.harvard.edu; jaxelrod{at}stanford.edu
SUMMARY
The Keystone Symposium on `Wnt and ß-catenin signaling in development and disease' was held recently in Snowbird, UT, USA. Organized by Mariann Bienz and Hans Clevers, this meeting covered a wide range of topics, including Wnt protein biogenesis, Wnt receptors and signaling pathways, ß-catenin/Tcf complexes and gene expression, Wnt signaling in development, cancer, stem cell biology and regeneration, and therapeutics that target the Wnt/ß-catenin pathway.
Introduction
The Wnt family of secreted signaling molecules regulates many aspects of
animal development and tissue homeostasis, and abnormal Wnt signaling has been
associated with human diseases, such as cancer and osteoporosis
(Logan and Nusse, 2004
;
Clevers, 2006). The study of
Wnt signaling has transcended a vast landscape of biomedical research, from
the basic mechanism of embryogenesis to stem cell biology and carcinogenesis,
and therapeutics and drug discovery.
Wnt proteins: modification, secretion and gradient formation
Wnt proteins exhibit both short- and long-range signaling capabilities and
are thought to function as morphogens
(Zecca et al., 1996). The
molecular basis of these properties is not well understood, but is probably
related to Wnt protein biogenesis. Wnt proteins are poorly secreted and
notoriously difficult to purify. Roel Nusse (Stanford University, Stanford,
CA, USA) described the successful purification of several Wnt proteins,
including Drosophila Wingless (Wg), and murine Wnt3a and Wnt5a. In
addition to facilitating the analysis of early signaling responses, the
purification of Wnt3a revealed that Wnt proteins are palmitoylated on a
conserved cysteine residue (Cys77 in Wnt3a)
(Willert et al., 2003). This
lipid modification, which is crucial for Wnt function, was discussed by Shinji
Takada (Okazaki Institute for Integrative Bioscience, Okazaki, Japan) and
Laura Burrus (San Francisco State University, San Francisco, CA, USA). Wg
palmitoylation and secretion depend on Porcupine, a multi-pass transmembrane
(TM) protein and putative O-acyltransferase that resides in the endoplasmic
reticulum (Kadowaki et al.,
1996; van den Heuvel et al.,
1993; Zhai et al.,
2004). Takada and Burrus each showed that Wnt3a palmitoylation and
secretion require the mammalian Porcupine ortholog. They also suggested that
in addition to Cys77, another Wnt3a palmitoylation site (or sites) is/are
likely to exist and to be important for Wnt3a activity, secretion and gradient
formation in the developing Xenopus blastula and chick neural
tube.
Konrad Basler (University of Zurich, Zurich, Switzerland) and Hendrik
Korswagen (Netherlands Institute of Developmental Biology, Utrecht, The
Netherlands) described two new components involved in Wnt secretion. Basler
identified the wntless (wls) gene in a suppressor screen for a Wg
gain-of-function phenotype (Banziger et
al., 2006) (see also
Bartscherer et al., 2006). The
wls gene encodes a seven-pass TM protein that is conserved from
C. elegans to humans. The C. elegans wls ortholog,
mom-3, is required for mom-2/Wnt function
(Thorpe et al., 1997). Wg/Wnt
proteins are not secreted in the Drosophila wls mutant or from
mammalian cells deprived of Wls, indicating that Wls is essential for Wnt
protein maturation/secretion (Fig.
1A). Korswagen identified vps-35 in an RNAi screen
performed in C. elegans to identify components of the
egl-20/Wnt pathway (Coudreuse et
al., 2006) (also see Prasad
and Clark, 2006). VPS-35 is a subunit of the `retromer' complex,
which is required for intracellular protein trafficking in yeast. Similar to
Wls (Banziger et al., 2006),
expression of vps-35 in EGL-20/Wnt-producing cells, but not in
responding cells, rescues mutant phenotypes. However, VPS-35 does not appear
to affect Wnt secretion per se, but rather disrupts long-range, but not
short-range, EGL-20/Wnt signaling. VPS-35 and the retromer complex may guide
Wnt to a specific secretory pathway for long-range gradient formation
(Coudreuse et al., 2006)
(Fig. 1A). Like Porcupine, Wls
and VPS-35 seem to be dedicated specifically to Wnt biogenesis.
Wnt receptors and downstream components
Nusse showed that purified Wnt5a can activate canonical ß-catenin
signaling in the presence of Frizzled4 (Fz4), one of the Fz Wnt receptors, and
a co-receptor, LDL receptor-related protein 5 (Lrp5). But Wnt5a can also
antagonize Wnt3a signaling via a putative tyrosine kinase receptor Ror2,
downstream of ß-catenin stabilization
(Mikels and Nusse, 2006)
(Fig. 1B,C). Nusse argued that
the receptor complement determines Wnt signaling output and that Wnt proteins
should not be classified as being intrinsically canonical or non-canonical. Xi
He (Children's Hospital/Harvard Medical School, Boston, MA, USA), Christof
Niehrs (German Cancer Research Center, Heidelberg, Germany), Akira Kikuchi
(Hiroshima University, Hiroshima, Japan), Paul Polakis (Genentech, South San
Francisco, CA, USA) and Anna Bafico (Mount Sinai School of Medicine, New York,
NY, USA) discussed the co-receptor Lrp6. Lrp6 is essential for ß-catenin
pathway activation (He et al.,
2004) and is activated via Wnt-induced phosphorylation at its
PPPSPxS motifs (Tamai et al.,
2004), which are docking sites for Axin
(Mao et al., 2001) (see
Fig. 1C). He demonstrated that
Gsk3 and Ck1 are sequential kinases for Lrp6 phosphorylation and activation,
and that Wnt signaling induces Gsk3 phosphorylation of PPPSP
(Zeng et al., 2005). This
model challenges the view that Gsk3 is solely an inhibitor of
Wnt/ß-catenin signaling (via ß-catenin degradation) and implies that
Gsk3 intricately regulates Wnt signaling. In agreement, Niehrs identified
Ck1
as a kinase for Lrp6, but showed that Ck1 phosphorylation,
but not PPPSP phosphorylation, is Wnt-inducible
(Davidson et al., 2005).
Polakis demonstrated MAP kinase phosphorylation of the PPPSP motif in vitro.
Further experimentation will clarify the Lrp6 phosphorylation issue. Kikuchi
discussed Lrp6 interaction with Caveolin, a structural protein of caveolae
(vesicular invaginations of the plasma membrane). Wnt3a induces Lrp6
internalization and Lrp6-Caveolin association, and siRNA depletion of Caveolin
(but not of Clathrin) or a dominant-negative Dynamin mutant inhibits Lrp6
internalization and Wnt signaling. Kikuchi suggested that Caveolin-mediated
Lrp6 endocytosis is important for Wnt/ß-catenin signaling. Bafico showed
that Lrp6 mediates an autocrine Wnt signal in some breast cancer cell lines
(Bafico et al., 2004), some of
which exhibit Lrp6 overexpression. She also observed in an ovarian cancer line
the expression of a Lrp6 variant that is Wnt-responsive but is resistant to
inhibition by Dkk1 (a Lrp6 antagonist) (He
et al., 2004).
|
). Mariann Bienz (MRC Laboratory of Molecular
Biology, Cambridge, UK) demonstrated that the cytoplasmic Dvl2 `dots' are not
vesicles but rather dynamic Dvl protein multimers
(Schwarz-Romond et al., 2005)
(also see Smalley et al.,
2005), which forms via the DIX domain of the protein. She showed
that recombinant Dvl2 DIX domain polymerizes gradually and reversibly in
vitro. As Dvl2 can recruit Axin into its assemblies, Bienz proposed that DIX
domain-mediated Dvl polymerization promotes the efficient recruitment of Axin,
resulting in ß-catenin signaling. ß-Catenin and Tcf/Lef complex
The `simple' model that Wnt signaling stabilizes ß-catenin to lead to
ß-catenin-Tcf/Lef complex formation, which activates downstream gene
expression, is widely cited. In the absence of Wnt, Tcf/Lef associates with
co-repressors, including Groucho/Tle to suppress gene expression
(Stadali et al., 2006)
(Fig. 2A). Bill Weis (Stanford
University, Stanford, CA, USA) demonstrated, by using recombinant proteins,
that ß-catenin-Lef1 and Groucho-Lef1 complexes are mutually exclusive,
owing to ß-catenin binding to a previously unrecognized low-affinity site
within Lef1 that overlaps with the Groucho-binding domain
(Daniels and Weis, 2005).
Wenqing Xu (University of Washington, Seattle, WA, USA) discussed the crystal
structure of full-length ß-catenin, revealing additional structural
components that may provide new interfaces for known and novel ß-catenin
partners. A crystal structure that contained fragments of Tcf, ß-catenin
and Bcl9 (a co-activator) (Fig.
2B) also revealed that the ß-catenin-Bcl9 interface is
distinct from most other ß-catenin-interacting proteins, highlighting a
potential target for specific therapeutic intervention.
Basler and Kathy Jones (Salk Institute, La Jolla, CA, USA) described
several novel ß-catenin co-activators. Basler identified
Hyrax/Parafibromin, which is required for ß-catenin-dependent
transcription in Drosophila and human cells and is a component of the
PAF1 (polymerase associated factor 1) complex
(Mosimann et al., 2006).
Hyrax/Parafibromin and Bcl9/Legless-pygopus
(Stadali et al., 2006) bind
ß-catenin C- and N-terminal transactivation domains (CTD and NTD)
(Fig. 2B), respectively, and
act in parallel or sequentially in ß-catenin-mediated transcription.
Jones identified the TRRAP/TIP60 histone acetylation complex, the ISW1
nucleosome remodeling complex and MLL1/MLL2 histone methylation complexes as
binding partners of ß-catenin CTD
(Fig. 2B). ß-catenin-Lef1
recruits these complexes, and Bcl9-pygopus to the Myc (previously
known as c-Myc) gene, and promotes histone H4 acetylation and histone
H3K4 (lysine 4) trimethylation (Sierra et
al., 2006). The APC tumor suppressor, the adenomatous
polyposis coli gene product, antagonizes Myc expression, but
(surprisingly) by acting directly on the Myc gene together with
co-repressors, including CtBP (Hamada and Bienz, 2005)
(Fig. 2C). Apc and CtBP are
subsequently replaced by other co-repressors, such as Groucho and Hdac1
(histone deacetylase 1) (Sierra et al.,
2006). Jones proposed that Apc has a direct role in promoting the
exchange of co-activators with co-repressors on ß-catenin-Tcf/Lef target
genes. Ken Cadigan (University of Michigan, Ann Arbor, MI, USA) examined the
role of CtBP in the Wg pathway in fly cells, reporting that CtBP directly
represses the naked cuticle gene in parallel to Tcf-Groucho
(Fang et al., 2006)
(Fig. 2A). This mechanism is
distinct from previous reports (Hamada and Bienz, 2005;
Sierra et al., 2006), in that
CtBP repression occurs in the absence of ß-catenin/Armadillo. Cadigan
also provided data that CtBP plays a positive role in Wg signaling, being
recruited by ß-catenin/Armadillo to some Wg targets
(Fang et al., 2006).
|
). Multiple
isoforms can be derived from alternative promoters and/or by alternative
splicing of each Tcf/Lef gene. Waterman reported that Tcf1E and Tcf4E contain
a novel DNA-binding domain in the `E-tail', which binds double-stranded DNA
(but with little sequence specificity) and seems to allow Tcf1E and Tcf4E to
regulate a larger set of genes. In colon tissues, Tcf1E predominantly acts as
a repressor because it lacks the ß-catenin-binding domain, and
antagonizes Tcf4E transactivation. In colon cancer cells, however, Tcf1E
becomes cytoplasmic, leaving Tcf4E unopposed in the nucleus and presumably
enabling higher Tcf4E-ß-catenin transactivation. Development
Wnt signaling induces vertebrate limb bud mesenchyme to proliferate
(Capdevila and Izpisua Belmonte,
2001). Wnt3a-induced mesenchymal proliferation, reported Nusse,
requires the induction and function of N-Myc (Mycn - Mouse Genome
Informatics), as proliferation is prevented in N-Myc knockout mice.
Wnt3a simultaneously blocks differentiation (chondrogenesis), which,
interestingly, is N-Myc-independent and is probably mediated via Sox9
inhibition. Christine Perret (Institut Cochin, Paris, France) reported a key
role for Wnt signaling in liver patterning and function from studies of mice
with acute Apc deletion or forced Dkk1 expression. Differential gene
expression along the perivenous to periportal axis depends on the activation
and suppression of ß-catenin signaling, respectively, which correlates
with the complementary expression of active ß-catenin in the perivenous
region and of Apc in the periportal region
(Benhamouche et al., 2006). By
conditionally deleting or activating ß-catenin, Walter Birchmeier (Max
Delbruck Center for Molecular Medicine, Berlin, Germany) explored the function
of ß-catenin and BMP signaling in patterning the dorsal neural tube,
particularly in the allocation of dl2-dl6 neuronal fates. He also demonstrated
two `opposing' roles of ß-catenin signaling in lens induction and in the
restriction of the lens field.
Pygopus and Legless/Bcl9 are essential partners/co-activators of
Wg/ß-catenin signaling in flies
(Belenkaya et al., 2002;
Kramps et al., 2002;
Parker et al., 2002;
Thompson et al., 2002). Xing
Dai (University of California, Irvine, CA, USA) knocked out the gene encoding
pygopus 2 in mice and found phenotypes consistent with decreased, but not
abolished, Wnt signaling in mammary glands, hair follicles, and the lung.
Given the restricted expression pattern of the other homolog, pygopus 1
(Li et al., 2004), the results
suggest that pygopus is not essential for Wnt signaling but facilitates
signaling amplification. Birchmeier and Michel Aguet (Swiss Institute for
Experimental Cancer Research, Epalinges, Switzerland) each made pygopus 1 and
2 double knockout mice. Each found failure of lens development associated with
loss of Pax6 expression. Aguet also reported the incomplete migration
of cardiac neural crest and failed aortic arch formation, associated with the
decreased expression of specific Wnt target genes, in addition to a general
attenuation (but not elimination) of a Tcf reporter expression (BAT-Gal) in
these double mutants. Strikingly, in all three laboratories, the mutants fail
to recapitulate the full spectrum of Wnt/ß-catenin signaling defects.
Aguet also deleted both Bcl9 and Bcl9-2, and pygopus 1 and
2, in the intestine, which requires Wnt/ß-catenin signaling for
development and homeostasis (Gregorieff
and Clevers, 2005). Surprisingly, he observed no overt
gastrointestinal abnormality. However, intestinal regeneration after injury
did occur in the pygopus 1 and 2 double mutant, but not in the
Bcl9/Bcl9-2 double mutant intestines. There was a spirited debate
about whether pygopus1 and 2, and Bcl9 and 9-2 are only required for a subset
of Wnt signaling in mice or whether the described deletions generate null
alleles. Perhaps multiple co-activator complexes
(Sierra et al., 2006) can lead
to ß-catenin transactivation in different contexts.
Jeff Axelrod (Stanford University, Stanford, CA, USA) was one of several
speakers that discussed non-canonical Wnt or Fz signaling. He described a
local feedback loop model for planar cell polarity (PCP) signaling that
depends on the mutual recruitment of Fz and Van Gogh/Strabismus to adjacent
membranes of neighboring cells, and that serves to align their polarities in a
domino fashion in fly wing epithelium. By combining mathematical modeling with
experimentation (Amonlirdviman et al.,
2005), he demonstrated how some alleles of Fz affect the polarity
of neighboring wild-type cells. He also showed that PCP propagation is
sensitive to cell geometry. Ping Chen (Emory University, Atlanta, GA, USA)
described the role of PCP signaling in stereocilia orientation and cellular
intercalation in the organ of Corti. She showed that, analogous to the
Drosophila wing, Vangl2 (a Van Gogh homolog) and Dvl2 exhibit
polarized medial and lateral localization, respectively, in a PCP
signaling-dependent manner (Wang et al.,
2005; Wang et al.,
2006). Wnt5a and Frzb, a Wnt antagonist, are candidates for
establishing PCP. Wnt5a-/- mice show cochlear duct
malformation, which is consistent with Wnt5a functioning in cochlear
convergent extension, but no apparent stereocilia orientation defect.
Sergei Sokol (Mount Sinai School of Medicine, New York, NY, USA) discussed
the interaction of Dsh with Lgl (lethal giant larvae), a protein important for
epithelial apical basal polarity. In Xenopus and Drosophila
embryos, loss of Dsh function causes defective epithelial polarity and Lgl to
mislocalize from its normal basolateral cortex. Fz8 (but not Fz7)
overexpression results in Lgl mislocalization, suggesting a mechanism by which
Fz signaling regulates apical basal polarity
(Dollar et al., 2005). Lgl
depletion in Xenopus embryos causes defective gastrulation movements
similar to those seen in embryos with abnormal Fz/Dsh PCP signaling,
indicating that Fz/Dsh regulation of PCP and apical basal polarity may be
coupled. Katherine Harris (University of California, Berkeley, CA, USA)
reported that multiple Wnt pathways regulate fly salivary gland migration
mediated by the repellents Wnt5 and Wnt4. Mutations in the Wnt5 receptor gene
derailed (Yoshikawa et al.,
2003; He, 2005), and in fz, fz2, dsh and tcf all
lead to abnormal salivary gland migration. Harris proposed that Wnt5-Derailed
and Wnt4-Fz/Fz2 activate ß-catenin-independent and -dependent pathways,
respectively, to coordinate the repulsive response during migration.
Cancer
Marc van de Wetering from Hans Clevers' group (Netherlands Institute of
Developmental Biology, Utrecht, The Netherlands) described continuing effects
to dissect the Tcf/ß-catenin regulatory network in colon cancers. Their
analyses indicate that Wnt/ß-catenin signaling controls three aspects of
intestinal homeostasis: stem cell renewal, proliferation of transient
amplifying progenitors, and Paneth cell maturation. Mark Taketo (Kyoto
University, Kyoto, Japan) showed that activated Tcf/ß-catenin signaling,
via APC loss or activated ß-catenin, induces chromosomal instability in
mouse intestinal polyps and embryonic stem cells. Alan Clarke (Cardiff
University, Cardiff, UK) reported the events that follow acute Apc
loss in the mouse intestine, which include increased proliferation, failure of
differentiation and migration, and aberrant apoptosis
(Sansom et al., 2004).
Expression profiling revealed that many `known' Tcf-ß-catenin targets are
not induced until adenoma development, and are therefore indirect targets,
such as cyclin D1 (Sansom et al.,
2005). Clarke discussed two novel targets, Sparc (an extracellular
protein) and Mbd2 (a methylated CpG binding protein); the absence of each
suppresses tumorigenesis induced by Apc loss (e.g.
Sansom et al., 2003) (whereas
the absence of Cyclin D1, Tcf1, or p53 does not). Using the same paradigm,
Owen Sansom (Beatson Institute of Cancer Research, Glasgow, UK) demonstrated
that tumors induced by Apc loss are completely inhibited when Myc is
removed; normal crypt architecture is restored despite high levels of nuclear
ß-catenin. Interestingly, over half of known ß-catenin target genes
were not induced in the absence of Myc. Thus Myc, like Sparc and Mbd2, is
essential for tumorigenesis caused by Apc loss; they may therefore represent
attractive therapeutic targets. Mark Peifer (University of North Carolina,
Chapel Hill, NC, USA), in collaboration with Brooke McCartney (Carnegie Mellon
University, Pittsburgh, PA, USA), investigated an allelic series of
Drosophila APC1 and APC2 mutations, including a protein
null. In APC1 and APC2 double-null embryos, several cellular
processes in which APC functions have been implicated are generally normal,
including mitotic spindle formation, epithelial cell division and
cadherin-based cell junction formation
(McCartney et al., 2006).
These results argue that many phenotypes caused by existing alleles and, by
analogy, by APC truncations in cancer, probably reflect dominant-negative
(neomorphic) functions. Their results also indicate that the APC truncations
are strong, but not null, with regard to Wg/ß-catenin signaling
(McCartney et al., 2006).
David Jones (University of Utah, Salt Lake City, UT, USA) explored the
relationship between APC and retinoic acid (RA) in colon tissue and in a
zebrafish model (Nadauld et al.,
2004; Nadauld et al.,
2005). He showed that APC induces RA biosynthesis through a
ß-catenin-independent mechanism and blocks its catabolism through a
ß-catenin-dependent mechanism. RA is in turn required for colonocyte
differentiation and suppresses Cox2 (cyclooxygenase 2) expression. Raymond
DuBois (Vanderbilt-Ingram Cancer Center, Nashville, TN, USA) also discussed
Cox2, as it is upregulated in invasive colon cancers and lies downstream of
many tumor promoters, including Wnt/ß-catenin signaling. Recent evidence
suggests that Cox2-dependent PGE2 (prostaglandin E2) production feeds back to
augment ß-catenin stabilization through the EP2 G-protein coupled
receptor (GPCR) (Castellone et al.,
2005). DuBois presented yet another Cox2 pathway, in which PGE2
binds the EP4 GPCR and activates a ß-arrestin/Src/Egf receptor pathway,
leading to the activation of the Akt kinase and to tumor metastasis
(Buchanan et al., 2006).
|
Elaine Fuchs (Rockefeller University, New York, NY, USA) and Tannishtha
Reya (Duke University, Durham, NC, USA) focused on the role of
Wnt/ß-catenin signaling in stem cell regulation. Fuchs discussed adult
hair follicle stem cells (SCs), which reside in the niche (bulge)
(Alonso and Fuchs, 2003) and
require ß-catenin for their maintenance
(Lowry et al., 2005). Tcf3
maintains SCs in a growth- and differentiation-inhibited state. During normal
hair cycle or injury, SCs are activated by ß-catenin signaling to
proliferate and exit the niche. Lef1-ß-catenin signaling is then required
to direct these activated SCs down the hair cell lineage
(Lowry et al., 2005). Thus,
the transition from Tcf3 suppression to Lef1-ß-catenin activation governs
stem cell maintenance versus activation and differentiation. Reya examined the
role of ß-catenin in hematopoietic stem cells (HSCs)
(Reya and Clevers, 2005). She
reported that conditional loss of ß-catenin in HSCs allows a normal
hematopoietic compartment to develop but leads to a significant reduction in
the ability of HSCs to renew and maintain hematopoiesis in vivo following
transplantation. She also found that ß-catenin loss in a Bcr-Abl-driven
leukemia mouse model slowed disease progression and significantly reduced the
incidence of chronic myelogenous leukemia, a neoplasm that arises in stem
cells. Therefore, ß-catenin is required for long term self-renewal of
normal and malignant stem cells in the hematopoietic system.
Several talks addressed Wnt in regeneration. Randall Moon (University of Washington, Seattle, WA, USA) showed that Wnt genes and Tcf/ß-catenin signaling are induced in all regenerating tissues examined, from zebrafish, frog to mouse. Wnt/ß-catenin signaling is required, via promoting proliferation, for fish fin regeneration (Fig. 3), as forced expression of Dkk1 or blocking Tcf prevents regeneration. Nusse reported similar observations in the study of lung epithelial regeneration. Moon showed that Wnt5a is also induced during, but antagonizes, fin regeneration, and probably by counterbalancing ß-catenin signaling. Whether Wnt/ß-catenin signaling regulates rare stem cells or promotes dedifferentiation at injury sites remains unanswered. Juan Carlos Izpisua-Belmonte (Salk Institute, La Jolla, CA, USA) considered the difference between animals that retain the ability to regenerate a limb after birth, such as urodele amphibians and teleost fish, versus other animals, and hypothesized that the key difference may be the absence of the apical ectodermal ridge (AER), a stratified epithelium and a source of many signals, in regeneration-competent limbs. Indeed, he observed regeneration in the chick limb after removing the AER and expressing ß-catenin in the underlying mesenchyme.
Cnidarians are remarkable models for regeneration studies
(Holstein et al., 2003).
Cnidarian genomes have recently been shown to encode an unexpected diversity
of Wnt genes, possessing at least 11 of the 12 Wnt subfamilies carried by
bilaterians (Kusserow et al.,
2005). In Nematostella vectensis, a basal cnidarian,
Thomas Holstein (Heidelberg University) showed that eight Wnt genes have
serial, overlapping expression in rings around the blastopore, and that Dkk
proteins (Wnt antagonists) are expressed in an inverse gradient
(Guder et al., 2006)
(Fig. 3). Pharmacological
intervention indicates a role for Wnt signaling in head regeneration. In
Hydra, there is also evidence of ß-catenin-dependent signaling during
head induction, and of ß-catenin-independent signaling during tentacle
and bud morphogenesis.
Therapeutic targets
In the therapeutic targets talks, Kyung-Ah Kim (Nuvelo, San Carlos, CA,
USA) discussed secreted R-spondin proteins, which can induce intestinal
epithelial proliferation via ß-catenin signaling
(Kim et al., 2005;
Kazanskaya et al., 2004), but
their mechanism of action and relationship with Wnt proteins remain unclear.
Xiaoyan Zhang (Curis, Cambridge, MA, USA) discussed two small molecule
antagonists identified in a cell-based Wnt/Tcf-reporter assay. Their targets
remain to be identified, although one may act at the receptor level. Ramesh
Shivdasani (Dana Farber Cancer Institute and Harvard Medical School, Boston,
MA, USA) described a compound, identified from a screen performed with
Novartis, that appears to specifically disrupt the Tcf4-ß-catenin
interaction (Lepourcelet et al.,
2004). Jie Zheng (St Jude Children's Hospital, Memphis, TN, USA)
described an in silico screen to identify an inhibitor of the Dsh PDZ
domain-Fz interaction, which partially blocks Wnt signaling in vivo
(Shan et al., 2005). He also
identified a compound that binds to a domain of Lrp5 and interferes with
Dkk1-Lrp5 interaction, thereby activating the pathway. Polakis described his
effort to use phage display to identify small peptides that bind Dvl and
inhibit Tcf-reporter expression and showed that Wnt inhibitors such as Dkk1
and the Fz8CRD (cysteine-rich domain) can reduce tumor growth in mice.
Conclusion
Many challenges await the Wnt field. We have much to learn about Wnt proteins and their biosynthetic pathways, and Wnt-receptor interactions and signaling propagation, as well as the plethora of cytoplasmic and nuclear changes triggered by Wnt proteins. We also need a better understanding of the involvement of Wnt/ß-catenin and other Wnt pathways in embryogenesis, cancer/stem cell biology and regeneration. A WNTer wonderland is on the horizon.
ACKNOWLEDGMENTS
We thank M. Bienz and H. Clevers for the superb meeting program; all speakers for excellent presentations and for allowing us to discuss their unpublished results; and all attendees for sharing data, posters and discussion. We apologize to many colleagues whose outstanding work could not be discussed fully owing to space constraints. X.H. is a W. M. Keck Foundation Distinguished Young Scholar and a Leukemia and Lymphoma Society Scholar, and is supported by grants from NIH. J.A. is supported by grants from NIH and American Cancer Society.
REFERENCES
Alonso, L. and Fuchs, E. (2003). Stem cells in
the skin: waste not, Wnt not. Genes Dev.
17,1189
-1200.
Amonlirdviman, K., Khare, N., Tree, D. R., Chen, W.-S., Axelrod,
J. D. and Tomlin, C. J. (2005). Mathematical modeling of
planar cell polarity to understand domineering nonautonomy.
Science 307,423
-426.
Bafico, A., Liu, G., Goldin, L., Harris, V. and Aaronson, S. A. (2004). An autocrine mechanism for constitutive Wnt pathway activation in human cancer cells. Cancer Cell 6, 497-506.[CrossRef][Medline]
Banziger, C., Soldini, D., Schutt, C., Zipperlen, P., Hausmann, G. and Basler, K. (2006). Wntless, a conserved membrane protein dedicated to the secretion of Wnt proteins from signaling cells. Cell 125,509 -522.[CrossRef][Medline]
Bartscherer, K., Pelte, N., Ingelfinger, D. and Boutros, M. (2006). Secretion of Wnt ligands requires Evi, a conserved transmembrane protein. Cell 125,523 -533.[CrossRef][Medline]
Belenkaya, T. Y., Han, C., Standley, H. J., Lin, X., Houston, D.
W., Heasman, J. and Lin, X. (2002). pygopus Encodes a nuclear
protein essential for wingless/Wnt signaling.
Development 129,4089
-4101.
Benhamouche, S., Decaens, T., Godard, C., Chambrey, R., Rickman, D., Moinard, C., Vasseur-Cognet, M., Kuo, C. J., Kahn, A., Perret, C. et al. (2006). Apc Tumor suppressor gene is the `zonation-keeper' of the mouse liver. Dev. Cell 10,759 -770.[CrossRef][Medline]
Buchanan, F. G., Gorden, D. L., Matta, P., Shi, Q., Matrisian,
L. M. and DuBois, R. N. (2006). Role of beta-arrestin 1 in
the metastatic progression of colorectal cancer. Proc. Natl. Acad.
Sci. USA 103,1492
-1497.
Capdevila, J. and Izpisua Belmonte, J. C. (2001). Patterning mechanisms controlling vertebrate limb development. Annu. Rev. Cell Dev. Biol. 17, 87-132.[CrossRef][Medline]
Castellone, M. D., Teramoto, H., Williams, B. O., Druey, K. M.
and Gutkind, J. S. (2005). Prostaglandin E2 promotes colon
cancer cell growth through a Gs-axin-beta-catenin signaling axis.
Science 310,1504
-1510.
Clevers, H. (2006). Wnt/ß-catenin signaling in development and disease. Cell (in press).
Coudreuse, D. Y. M., Roel, G., Betist, M. C., Destree, O. and
Korswagen, H. C. (2006). Wnt gradient formation requires
retromer function in Wnt producing cells. Science
312,921
-924.
Daniels, D. L. and Weis, W. I. (2005). ßeta-catenin directly displaces Groucho/TLE repressors from Tcf/Lef in Wnt-mediated transcription activation. Nat. Struct. Mol. Biol. 12,364 -371.[CrossRef][Medline]
Davidson, G., Wu, W., Shen, J., Bilic, J., Fenger, U., Stannek, P., Glinka, A. and Niehrs, C. (2005). Casein kinase 1 gamma couples Wnt receptor activation to cytoplasmic signal transduction. Nature 438,867 -872.[CrossRef][Medline]
Dollar, G. L., Weber, U., Mlodzik, M. and Sokol, S. Y. (2005). Regulation of Lethal giant larvae by Dishevelled. Nature 437,1376 -1380.[CrossRef][Medline]
Fang, M., Li, Z., Blauwkamp, T., Bhambhani, C., Campbell, N. and Cadigan, K. M. (2006). C-terminal-binding protein directly activates and represses Wnt transcriptional targets in Drosophila. EMBO J. doi:10.1038/sj.emboj.7601153[CrossRef][Medline]
Gregorieff, A. and Clevers, H. (2005). Wnt
signaling in the intestinal epithelium: from endoderm to cancer.
Genes Dev.19,877
-890.
Guder, C., Pinho, S., Nacak, T. G., Schmidt, H. A., Hobmayer,
B., Niehrs, C. and Holstein, T. W. (2006). An ancient
Wnt-Dickkopf antagonism in Hydra. Development
133,901
-911.
Hamada, F. and Bienz, M. (2004). The APC tumor suppressor binds to C-terminal binding protein to divert nuclear beta-catenin from TCF. Dev. Cell 7,677 -685.[CrossRef][Medline]
He, X. (2004). Wnt signaling went derailed again: a new track via the LIN-18 receptor? Cell 118,668 -670.[Medline]
He, X., Semenov, M., Tamai, K. and Zeng, X.
(2004). LDL receptor-related proteins 5 and 6 in Wnt/beta-catenin
signaling: arrows point the way. Development
131,1663
-1677.
Hobmayer, B., Rentzsch, F., Kuhn, K., Happel, C. M., Laue, C. C., Snyder, P., Rothbächer, U. and Holstein, T. W. (2000). Wnt signaling and axis formation in the diploblastic metazoan Hydra. Nature 407,186 -189.[CrossRef][Medline]
Holstein, T. W., Hobmayer, E. and Technau, U. (2003). Cnidarians: an evolutionarily conserved model system for regeneration? Dev. Dyn. 226,257 -267.[CrossRef][Medline]
Kadowaki, T., Wilder, E., Klingensmith, J., Zachary, K. and
Perrimon, N. (1996). The segment polarity gene porcupine
encodes a putative multitransmembrane protein involved in Wingless processing.
Genes Dev. 10,3116
-3128.
Kazanskaya, O., Glinka, A., del Barco Barrantes, I., Stannek, P., Niehrs, C. and Wu, W. (2004). R-Spondin2 is a secreted activator of Wnt/beta-catenin signaling and is required for Xenopus myogenesis. Dev. Cell 7,525 -534.[CrossRef][Medline]
Kim, K. A., Kakitani, M., Zhao, J., Oshima, T., Tang, T.,
Binnerts, M., Liu, Y., Boyle, B., Park, E., Emtage, P. et al.
(2005). Mitogenic influence of human R-spondin1 on the intestinal
epithelium. Science 309,1256
-1259.
Kramps, T., Peter, O., Brunner, E., Nellen, D., Froesch, B., Chatterjee, S., Murone, M., Zullig, S. and Basler, K. (2002). Wnt/wingless signaling requires BCL9/legless-mediated recruitment of pygopus to the nuclear beta-catenin-TCF complex. Cell 109, 47-60.[CrossRef][Medline]
Kusserow, A., Pang, K., Sturm, C., Hrouda, M., Lentfer, J., Schmidt, H. A., Technau, U., von Haeseler, A., Hobmayer, B., Martindale, M. Q. et al. (2005). Unexpected complexity of the Wnt gene family in a sea anemone. Nature 433,156 -160.[CrossRef][Medline]
Lepourcelet, M., Chen, Y. N., France, D. S., Wang, H., Crews, P., Petersen, F., Bruseo, C., Wood, A. W. and Shivdasani, R. A. (2004). Small-molecule antagonists of the oncogenic Tcf/beta-catenin protein complex. Cancer Cell 5, 91-102.[CrossRef][Medline]
Li, B., Mackay, D. R., Ma, J. and Dai, X. (2004). Cloning and developmental expression of mouse pygopus 2, a putative Wnt signaling component. Genomics 84,398 -405.[CrossRef][Medline]
Liu, F., van den Broek, O., Destrée, O. and Hoppler,
S. (2005). Distinct roles for Xenopus Tcf/Lef genes in
mediating specific responses to Wnt/b-catenin signalling in mesoderm
development. Development
132,5375
-5385.
Logan, C. Y. and Nusse, R. (2004). The Wnt signaling pathway in development and disease. Annu. Rev. Cell Dev. Biol. 20,781 -810.[CrossRef][Medline]
Lowry, W. E., Blanpain, C., Nowak, J. A., Guasch, G., Lewis, L.
and Fuchs, E. (2005). Defining the impact of beta-catenin/Tcf
transactivation on epithelial stem cells. Genes Dev.
19,1596
-1611.
Mao, J., Wang, J., Liu, B., Pan, W., Farr, G. H., 3rd, Flynn, C., Yuan, H., Takada, S., Kimelman, D., Li, L. et al. (2001). Low-density lipoprotein receptor-related protein-5 binds to Axin and regulates the canonical Wnt signaling pathway. Mol. Cell 7, 801-809.[CrossRef][Medline]
McCartney, B. M., Price, M. H., Webb, R., Hayden, M. A., Holot,
L., Zhou, M., Bejsovec, A. and Peifer, M. (2006). Testing
hypotheses for the functions of APC family proteins using null and truncation
alleles in Drosophila. Development
133,2407
-2418.
Mikels, A. J. and Nusse, R. (2006). Purified Wnt5a protein activates or inhibits beta-catenin-TCF signaling depending on receptor context. PLoS Biol. 4, e115.[CrossRef][Medline]
Mosimann, C., Hausmann, G. and Basler, K. (2006). Parafibromin/Hyrax activates Wnt/Wg target gene transcription by direct association with ß-catenin/Armadillo. Cell 125,327 -341.[CrossRef][Medline]
Nadauld, L. D., Sandoval, I. T., Chidester, S., Yost, H. J. and
Jones, D. A. (2004). Adenomatous polyposis coli control of
retinoic acid biosynthesis is critical for zebrafish intestinal development
and differentiation. J. Biol. Chem.
279,51581
-51589.
Nadauld, L. D., Shelton, D. N., Chidester, S., Yost, H. J. and
Jones, D. A. (2005). The zebrafish retinol dehydrogenase,
rdh1l, is essential for intestinal development and is regulated by the tumor
suppressor adenomatous polyposis coli. J. Biol. Chem.
280,30490
-30495.
Parker, D. S., Jemison, J. and Cadigan, K. M.
(2002). Pygopus, a nuclear PHD-finger protein required for
Wingless signaling in Drosophila. Development
129,2565
-2576.
Prasad, B. C. and Clark, S. G. (2006). Wnt
signaling establishes anteroposterior neuronal polarity and requires retromer
in C. elegans. Development
133,1757
-1766.
Reya, T. and Clevers, H. (2005). Wnt signalling in stem cells and cancer. Nature 434,843 -850.[CrossRef][Medline]
Sansom, O. J., Berger, J., Bishop, S. M., Hendrich, B., Bird, A. and Clarke, A. R. (2003). Deficiency of Mbd2 suppresses intestinal tumorigenesis. Nat. Genet. 34,145 -147.[CrossRef][Medline]
Sansom, O. J., Reed, K. R., Hayes, A. J., Ireland, H.,
Brinkmann, H., Newton, I. P., Batlle, E., Simon-Assmann, P., Clevers, H.,
Nathke, I. S. et al. (2004). Loss of Apc in vivo immediately
perturbs Wnt signaling, differentiation, and migration. Genes
Dev. 18,1385
-1390.
Sansom, O. J., Reed, K. R., van de Wetering, M., Muncan, V.,
Winton, D. J., Clevers, H. and Clarke, A. R. (2005). Cyclin
D1 is not an immediate target of beta-catenin following Apc loss in the
intestine. J. Biol. Chem.
280,28463
-28467.
Schwarz-Romond, T., Merrifield, C., Nichols, B. J. and Bienz,
M. (2005). The Wnt signalling effector Dishevelled forms
dynamic protein assemblies rather than stable associations with cytoplasmic
vesicles. J. Cell Sci.
118,5269
-5277.
Shan, J., Shi, D. L., Wang, J. and Zheng, J. (2005). Identification of a specific inhibitor of the dishevelled PDZ domain. Biochemistry 44,15495 -15503.[CrossRef][Medline]
Sierra, J., Yoshida, T., Joazeiro, C. A. and Jones, K. A.
(2006). The APC tumor suppressor counteracts beta-catenin
activation and H3K4 methylation at Wnt target genes. Genes
Dev. 20,586
-600.
Smalley, M. J., Signoret, N., Robertson, D., Tilley, A., Hann,
A., Ewan, K., Ding, Y., Paterson, H. and Dale, T. C. (2005).
Dishevelled (Dvl-2) activates canonical Wnt signalling in the absence of
cytoplasmic puncta. J. Cell Sci.
118,5279
-5289.
Stadali, R., Hoffmans, R. and Basler, K. (2006). Transcription under the control of nuclear Arm/ß-catenin. Curr. Biol. 16,R378 -R385.[CrossRef][Medline]
Tamai, K., Zeng, X., Liu, C., Zhang, X., Harada, Y., Chang, Z. and He, X. (2004). A mechanism for Wnt coreceptor activation. Mol. Cell 13,149 -156.[CrossRef][Medline]
Thompson, B., Townsley, F., Rosin-Arbesfeld, R., Musisi, H. and Bienz, M. (2002). A new nuclear component of the Wnt signalling pathway. Nat. Cell Biol. 4, 367-373.[CrossRef][Medline]
Thorpe, C. J., Schlesinger, A., Carter, J. C. and Bowerman, B. (1997). Wnt signaling polarizes an early C. elegans blastomere to distinguish endoderm from mesoderm. Cell 90,695 -705.[CrossRef][Medline]
van den Heuvel, M., Harryman-Samos, C., Klingensmith, J., Perrimon, N. and Nusse, R. (1993). Mutations in the segment polarity genes wingless and porcupine impair secretion of the wingless protein. EMBO J. 12,5293 -5302.[Medline]
Wallingford, J. and Habas, R. (2005). The
developmental biology of Dishevelled: an enigmatic protein governing cell fate
and cell polarity. Development
132,4421
-4436.
Wang, J., Mark, S., Zhang, X., Qian, D., Yoo, S. J., Radde-Gallwitz, K., Zhang, Y., Lin, X., Collazo, A., Wynshaw-Boris, A. et al. (2005). Regulation of polarized extension and planar cell polarity in the cochlea by the vertebrate PCP pathway. Nat. Genet. 37,980 -985.[CrossRef][Medline]
Wang, J., Hamblet, N. S., Mark, S., Dickinson, M. E., Brinkman,
B. C., Segil, N., Fraser, S. E., Chen, P., Wallingford, J. B. and
Wynshaw-Boris, A. (2006). Dishevelled genes mediate a
conserved mammalian PCP pathway to regulate convergent extension during
neurulation. Development
133,1767
-1778.
Willert, K., Brown, J. D., Danenberg, E., Duncan, A. W., Weissman, I. L., Reya, T., Yates, J. R., 3rd and Nusse, R. (2003). Wnt proteins are lipid-modified and can act as stem cell growth factors. Nature 423,448 -452.[CrossRef][Medline]
Yoshikawa, S., McKinnon, R. D., Kokel, M. and Thomas, J. B. (2003). Wnt-mediated axon guidance via the Drosophila Derailed receptor. Nature 422,583 -588.[CrossRef][Medline]
Zecca, M., Basler, K. and Struhl, G. (1996). Direct and long-range action of a wingless morphogen gradient. Cell 87,833 -844.[CrossRef][Medline]
Zeng, X., Tamai, K., Doble, B., Li, S., Huang, H., Habas, R., Okamura, H., Woodgett, J. and He, X. (2005). A dual-kinase mechanism for Wnt coreceptor phosphorylation and activation. Nature 438,873 -877.[CrossRef][Medline]
Zhai, L., Chaturvedi, D. and Cumberledge, S.
(2004). Drosophila wnt-1 undergoes a hydrophobic modification and
is targeted to lipid rafts, a process that requires porcupine. J.
Biol. Chem. 279,33220
-33227.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  T. Jin and L. Liu Minireview: The Wnt Signaling Pathway Effector TCF7L2 and Type 2 Diabetes Mellitus Mol. Endocrinol., November 1, 2008; 22(11): 2383 - 2392. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
