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First published online August 12, 2008
doi: 10.1242/10.1242/dev.020974
Review |
Howard Hughes Medical Institute, Waksman Institute and Department of Molecular Biology and Biochemistry, Rutgers The State University of New Jersey, Piscataway, NJ 08854, USA.
* Author for correspondence (e-mail irvine{at}waksman.rutgers.edu)
SUMMARY
A cassette of cytoplasmic Drosophila tumor suppressors, including the kinases Hippo and Warts, has recently been linked to the transmembrane tumor suppressor Fat. These proteins act within interconnected signaling pathways, the principal functions of which are to control the growth and polarity of developing tissues. Recent studies have enhanced our understanding of the basis for signal transduction by Fat and Warts pathways, including the identification of a DNA-binding protein at the end of the pathway, have established the conservation of Fat and Warts signaling from flies to mammals, and have given us new insights into their regulation and biological functions.
Introduction
Both the patterning and the proportions of different organs and tissues are
strictly regulated during metazoan development. Much has been learned about
the signaling pathways that regulate developmental patterning, but until
recently the mechanisms responsible for developmental growth control have
remained poorly understood. With the discovery and characterization of the
Warts (or Hippo) and Fat pathways, this has begun to change, as these pathways
form an interconnected signaling network that plays a major role in
controlling growth (Fig. 1). A
distinctive feature of Fat-Hippo-Warts signaling is that it can influence
organ growth without affecting organ patterning, and indeed in
Drosophila acts downstream of the Decapentaplegic morphogen gradient
to influence wing growth (Rogulja et al.,
2008
).
fat encodes a large (>5000 amino acid) transmembrane protein
with 34 cadherin domains in its extracellular region
(Mahoney et al., 1991). Null
alleles of fat are lethal, and mutants have overgrown imaginal discs.
However, mutants with weak viable alleles exhibit a broadening of the abdomen
(hence the name) and wing, and a reduction in the distance between the two
wing cross-veins (Mohr, 1923;
Waddington, 1940). A reduced
distance between cross-veins is also characteristic of three other classical
Drosophila mutants, four-jointed (fj),
dachsous (ds) and dachs (d)
(Bridges and Morgan, 1919;
Waddington, 1940). Ds is a
large (>3000 amino acid) transmembrane protein with 27 cadherin domains
(Clark et al., 1995), Fj is a
Golgi protein kinase (Ishikawa et al.,
2008; Strutt et al.,
2004; Villano and Katz,
1995) and Dachs is an unconventional myosin
(Mao et al., 2006). A wealth
of observations have now established that these four genes function together
within a Fat signaling pathway that influences growth, gene expression and
planar cell polarity (PCP) (Fig.
1).
Many components of the Warts pathway, including Warts (Wts), Hippo (Hpo),
Salvador (Sav) and Mob-as-tumor suppressor (Mats), were first identified
through genetic screens for Drosophila tumor suppressors
(Harvey et al., 2003;
Jia et al., 2003;
Justice et al., 1995;
Kango-Singh et al., 2002;
Lai et al., 2005;
Pantalacci et al., 2003;
Tapon et al., 2002;
Udan et al., 2003;
Wu et al., 2003;
Xu et al., 1995). When any
one of these genes is mutant in a patch of cells in the body or head of the
fly, an overgrowth phenotype can occur, and this is accompanied by a
characteristic distortion and folding of the normally smooth cuticular
surface. These mutant phenotypes identified an essential, normal function for
these genes in limiting growth during the development of imaginal tissues in
Drosophila, and this appears to be the principle function of Warts
signaling. Nonetheless, it is now clear that theses genes can also regulate
other cellular behaviors, which are just now beginning to be identified. It
has become more popular over the past couple of years to refer to these genes
as functioning within the Hippo signaling pathway, but we prefer (and will
employ here) the term Warts signaling, reserving the term Hippo signaling for
pathways that act exclusively through the regulation of Hpo. We make this
distinction because some signaling through Wts is Hpo independent. This
terminology also has the advantage of using the antecedent gene name, as
wts was first discovered almost a decade before hpo.
Two years ago, our understanding of Fat and Warts signaling was greatly advanced by the realization that these pathways are interconnected, as Fat influences growth and gene expression through its effects on Warts. As will be described below, these and other recent studies have given us a framework of intertwined pathways, which extend from transmembrane receptors to DNA-binding transcription factors, and which influence growth, patterning and polarity. Although our understanding of these pathways continues to evolve, recent studies have clarified long-standing issues, including the identification of a DNA-binding protein at the end of the pathway, mechanisms by which the pathways are regulated and signals transduced, and the conservation of Fat and Warts signaling from flies to mammals. Here, we review our current understanding of Fat and Warts signaling, focusing on these most recent discoveries.
The Hippo kinase cassette in Drosophila
Genetic and biochemical studies have positioned Wts, Hpo, Sav and Mats at
the center of Warts signaling, and have identified a series of positively
reinforcing interactions among them. We will refer to these four proteins as
the Hippo kinase cassette (Fig.
2). Hpo and Wts are both Ser/Thr kinases, and their activity is
regulated by phosphorylation and by their association with Sav and Mats
(Fig. 2A). Studies of mammalian
homologues of Hpo (Mst1 and Mst2), have indicated that Hpo/Mst can be
activated by intermolecular autophosphorylation
(Glantschnig et al., 2002;
Lee and Yonehara, 2002).
Activated Hpo then phosphorylates Wts, Sav and Mats
(Wei et al., 2007;
Wu et al., 2003). The
phosphorylation of Wts by Hpo is facilitated by Sav, which binds to both Hpo
and Wts, thus acting as a scaffolding protein
(Wu et al., 2003). The
activation of Wts requires Mats, which acts as a co-factor
(Lai et al., 2005), and the
phosphorylation of Mats by Hpo promotes Mats-Wts binding
(Wei et al., 2007). The
activation of Wts is also associated with autophosphorylation
(Wei et al., 2007). Once
activated, Wts then phosphorylates and thereby inhibits the transcriptional
co-activator Yorkie (Yki), which is the crucial substrate of Wts in
transcriptional and growth regulation
(Huang et al., 2005).
|
|
;
Lai et al., 2005;
Wu et al., 2003). Comparisons
of mutant phenotypes in clones can be complicated by uncertainties over
whether particular alleles are null, and differences in perdurance of
wild-type gene products, but the current biochemical understanding of the
Hippo kinase cassette could explain these genetic differences. As Sav is
required only for the phosphorylation of Wts by Hpo
(Wu et al., 2003), and not
for the phosphorylation of Mats (Wei et
al., 2007), it makes sense that the sav mutant phenotype
is weaker than the wts mutant phenotype. Moreover, as Hpo
phosphorylation of Mats seems to work by promoting Mats-Wts binding, to the
extent that some association between Mats and Wts occurs even when they are
unphosphorylated (Wei et al.,
2007), it could explain why hpo mutant phenotypes appear
weaker than wts or mats. The Hippo kinase cassette in mammals
It has been known for some time that homologues of the Hippo kinase
cassette genes exist in mammals (Table
1). Indeed, in several cases it has been demonstrated that these
mammalian genes can rescue the phenotypes of Drosophila mutants
(Lai et al., 2005;
Tao et al., 1999;
Wu et al., 2003). However,
only more recently has cellular and biochemical evidence appeared to establish
that these mammalian proteins are linked in an analogous signaling cassette
(Fig. 2B), and that, as in
Drosophila, this signaling cassette plays a significant role in
mammalian growth control. Several regulatory steps that were first
characterized with Drosophila proteins have now been identified in
their mammalian homologues (Fig.
2B, compare with Fig.
2A), including: phosphorylation of Lats (Wts), Mob (Mats) and WW45
(Sav) proteins by Mst (Hpo) kinases
(Callus et al., 2006;
Chan et al., 2005;
Hirabayashi et al., 2008;
Praskova et al., 2008); the
association of WW45 with Mst and Lats
(Callus et al., 2006;
Lee et al., 2008); a
requirement for WW45 for the phosphorylation of Lats
(Lee et al., 2008); the
association of Mob and Lats and the consequent promotion of Lats
autophosphorylation (Praskova et al.,
2008); and the phosphorylation of the Yki homologue Yes-associated
protein (Yap) by Lats (Dong et al.,
2007; Hao et al.,
2008; Zhang et al.,
2008a; Zhao et al.,
2007). Studies of Mst in mammalian cells have identified
autophosphorylation as being a crucial regulatory step for Hpo/Mst
(Glantschnig et al., 2002;
Lee and Yonehara, 2002), and
have identified an association of Mst with Ras association domain family
proteins (Praskova et al.,
2004), which was subsequently also observed in Drosophila
(Polesello et al., 2006). In
mammalian cells, Mst proteins can also be activated by a caspase-mediated
cleavage (Graves et al., 2001;
Graves et al., 1998), which
has not yet been observed in Drosophila. Studies using Ww45
mutant mouse keratinocytes have also identified an unexpected influence of
WW45 on Mst1 autophosphorylation (Lee et
al., 2008), although evidence for a modest influence of Sav on Hpo
autophosphorylation has also been reported in cultured Drosophila
cells (Wei et al., 2007).
Another feature that has been described in cultured mammalian cells, but which
has not yet been documented in Drosophila, is nuclear-cytoplasmic
shuttling of Mst (Lee et al.,
2008; Lee and Yonehara,
2002). In parallel with Drosophila studies, genetic and
cell culture studies in mammalian cells have also linked the Hpo kinase
cassette to the phosphorylation of Yap and to the regulation of growth
(Hao et al., 2008;
Lee et al., 2008;
Zhang et al., 2008a;
Zhao et al., 2007).
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Since the discovery of Yki and its role in Hpo-mediated growth regulation,
the focus of the field has been on transcriptional regulation through Yki/Yap
as mediators of the effects of the Hpo kinase cassette genes. However, in
mammalian cells, Lats can also regulate the activity of transcriptional
co-activator with PDZ-binding motif (Taz)
(Fig. 2), which shares sequence
similarity to Yap and modulates mesenchymal differentiation
(Lei et al., 2008). In
addition, both Hpo/Mst and Wts/Lats may affect cell proliferation and survival
through non-transcriptional processes. Hpo has been reported to phosphorylate
Drosophila inhibitor of apoptosis protein 1 (Diap1), and this
activity might influence Diap1 stability
(Harvey et al., 2003;
Pantalacci et al., 2003).
Studies of Warts/Lats also imply that it has substrates crucial for cell
division (Fig. 1). In mammalian
cells, Lats proteins are phosphorylated in a cell cycle-dependent manner, and
negatively regulate Cdc2/Cyclin A (Tao et
al., 1999; Toji et al.,
2004). Lats1 has also been reported to act as a dynamic component
of the mitotic apparatus and to promote mitotic exit
(Bothos et al., 2005;
Morisaki et al., 2002). Lats
proteins localize to centrosomes during interphase, and to the mitotic spindle
during metaphase (Nishiyama et al.,
1999; Toji et al.,
2004). In addition, Lats proteins interact with and modulate the
functions of LIM (Lin11, Isl1, Mec3) domain proteins that participate in
spindle pole organization, actin filament assembly and cytokinesis
(Abe et al., 2006;
Hirota et al., 2000;
Yang et al., 2004). More
recent studies have identified that cell cycle-dependent changes occur in Mst
activity and in the Mst-dependent phosphorylation of Mob
(Praskova et al., 2008).
Additionally, Mats mutations have recently been reported to result in
aberrant chromosome segregation in the early Drosophila embryo
(Shimizu et al., 2008),
supporting both the conservation and the in vivo relevance of the association
of Lats and Mob with mitotic chromosomes in cultured cells
(Bothos et al., 2005;
Nishiyama et al., 1999;
Toji et al., 2004).
Altogether, these observations imply that the Hpo kinase cassette acts at
multiple steps to influence cell proliferation.
DNA-binding proteins for Warts signaling
Yki is a non-DNA-binding transcriptional co-activator, and since the
discovery of its role in Warts signaling
(Huang et al., 2005), a key
issue has been the identity of its relevant DNA-binding partner(s). Recently,
this has been at least partially answered by studies that have identified
Scalloped (Sd) as being a partner protein for Yki, and mammalian homologues of
Sd, the TEA domain/Transcription enhancer factor (Tead/Tef) proteins, as being
partners for Yap (Goulev et al.,
2008; Wu et al.,
2008; Zhang et al.,
2008b; Zhao et al.,
2008). Sd was first suggested as a candidate Yki-interacting
protein through a genome-wide yeast two-hybrid screen
(Giot et al., 2003). In
addition, mammalian Tead/Tef proteins are among the several DNA-binding
proteins that have been identified as partners for Yap in mammalian cells
(Espanel and Sudol, 2001;
Ferrigno et al., 2002;
Komuro et al., 2003;
Strano et al., 2001;
Vassilev et al., 2001;
Yagi et al., 1999;
Zaidi et al., 2004). However,
the functional significance of the Yki-Sd interaction was unclear, and, based
on prior genetic studies, Sd was not an obvious candidate to be the
DNA-binding partner of Yki, as sd is specifically required for wing
and neuronal development (Campbell et al.,
1992; Liu et al.,
2000), whereas yki appears to be required for normal
growth and survival in all imaginal cells
(Huang et al., 2005). Indeed,
in studies of sd mutant clones, sd was essential for cell
proliferation only in the wing (Liu et
al., 2000; Wu et al.,
2008; Zhang et al.,
2008b), where it functions as a DNA-binding partner for
vestigial (vg) (Halder
et al., 1998; Paumard-Rigal et
al., 1998; Simmonds et al.,
1998). However, genetic studies have clearly demonstrated that
sd is required for the overgrowth phenotype that is associated with
either the overexpression of Yki, or with the mutation of tumor suppressors in
the Warts signaling pathway (Goulev et
al., 2008; Wu et al.,
2008; Zhang et al.,
2008b). In cultured mammalian cells, experiments using either RNA
interference-mediated knockdown, or the expression of dominant-negative
proteins, indicated that Tead proteins are similarly required for Yap-mediated
gene expression and transformation (Zhao
et al., 2008). The linkage of Sd to Warts signaling in
Drosophila was further supported by the identification of an enhancer
within the downstream transcriptional target gene Diap1
(thread). This enhancer mediates Sd:Yki-dependent transcription in
vivo and in cultured cells, and is bound by Sd in vivo and in vitro
(Wu et al., 2008;
Zhang et al., 2008b).
Although these recent studies have provided convincing evidence that Sd is
a Yki partner, they left unanswered the question of why sd and
yki mutant phenotypes differ. Indeed it is striking that sd
is required for the effects of the overexpression of Yki on Diap1
expression, but outside of the Drosophila wing, sd is not
required for the endogenous expression of Diap1
(Wu et al., 2008;
Zhang et al., 2008b). One
possibility is that other DNA-binding transcription factors that partner with
Yki might contribute to Warts signaling
(Fig. 3B). Another possibility,
however, is that Sd might function as a transcriptional activator in the
presence of Yki, but as a transcriptional repressor in the absence of Yki
(Fig. 3A). Such repression of
normal Warts pathway targets might explain the observation that overexpression
of sd actually inhibits growth and promotes apoptosis
(Liu et al., 2000). Switching
from repressor to activator isoforms is typical of the DNA-binding
transcription factors at the end of many signaling pathways
(Barolo and Posakony, 2002). In
this case, the absence of sd would differ from the absence of
yki because target genes would be derepressed without Sd, but
repressed without Yki (Fig.
3).
|
;
Paumard-Rigal et al., 1998;
Simmonds et al., 1998), with
which it functions to promote wing development. Both loss-of-function and
gain-of-function experiments argue that Vg and Yki have different functions
and thus must have at least some distinct targets. Studies of Vg have
determined that, in addition to providing a transcriptional activation domain
to Sd, it also influences Sd DNA-binding specificity
(Halder and Carroll, 2001). If
this is also the case for Yki, it would support a simple explanation for how
they execute different functions, despite complexing with the same DNA-binding
protein. Although both yki and vg influence growth and some
of the same target genes in the wing, expression of yki in
vg mutant clones, or of vg in yki mutant clones
confirmed that yki and vg can function independently
(Wu et al., 2008). The issue
of how different co-activator proteins regulate different sets of downstream
genes using the same DNA-binding transcription factor is even more complex in
mammals, as there are four Tef/Tead proteins, and multiple Vg-related
proteins, one of which (Tondu) has also been shown to interact with Tef/Tead
proteins (Vaudin et al.,
1999), as does the Yap-related protein Taz
(Mahoney et al., 2005). Phosphorylation of Yki/Yap regulates its subcellular localization
Several recent studies have also increased our understanding of the
molecular and cellular basis for the regulation of Yki/Yap by Warts/Lats. One
crucial phosphorylation site is Ser168 of Yki (Ser127 of Yap)
(Dong et al., 2007;
Oh and Irvine, 2008;
Zhang et al., 2008b;
Zhao et al., 2007).
Phosphorylation of this Ser creates a binding site for 14-3-3 proteins
(Basu et al., 2003;
Dong et al., 2007;
Oh and Irvine, 2008;
Zhao et al., 2007), a class
of proteins that act as cytoplasmic anchors for several phosphorylated
transcription factors (Mackintosh,
2004). Indeed, experiments have shown that the phosphorylation of
Yki by Wts/Lats influences its subcellular localization: when Warts/Lats is
active, Yki/Yap is phosphorylated and is retained in the cytoplasm, but when
Warts/Lats are mutant or inactive, active Yki/Yap can enter the nucleus
(Dong et al., 2007;
Hao et al., 2008;
Oh and Irvine, 2008;
Zhang et al., 2008b;
Zhao et al., 2007). This
provides a simple mechanism for the regulation of Yki/Yap by Warts signaling.
However, complicating the story is the fact that both in vivo and cell culture
experiments indicate that Yki and Yap have multiple Wts/Lats sites
(Hao et al., 2008;
Oh and Irvine, 2008;
Zhao et al., 2007). Moreover,
even though the Yki-S168A/Yap-S127A mutation hyperactivates Yki/Yap, the
mutant protein still exhibits some sensitivity to Wts/Lats
(Oh and Irvine, 2008;
Zhao et al., 2007). The other
sites have not yet been as well characterized, but appear to fall within a
HXRXXS consensus motif (Hao et al.,
2008; Zhao et al.,
2007). As the site at 127/168 is the only 14-3-3 consensus binding
site within Yki/Yap, and mutation of 127/168 alone appears to eliminate 14-3-3
binding (Basu et al., 2003;
Dong et al., 2007;
Oh and Irvine, 2008;
Zhao et al., 2007), the
mechanism by which these other sites influence Yki remains to be
determined.
Downstream targets of Warts signaling
Warts signaling regulates gene expression, and studies in
Drosophila over the years have led to the identification of several
downstream genes that could contribute to the growth phenotypes associated
with pathway mutants. One important target is bantam
(Nolo et al., 2006;
Thompson and Cohen, 2006), a
gene that encodes a microRNA that is not obviously conserved in vertebrates.
The genes encoding other key growth regulators that are downstream of Warts
signaling in Drosophila include cyclin A, cyclin B, cyclin E,
E2F1 and Diap1 (Goulev et
al., 2008; Shimizu et al.,
2008; Silva et al.,
2006; Tapon et al.,
2002; Wu et al.,
2003). Microarray studies in cultured mammalian cells have
recently added substantially to the list of potential targets
(Dong et al., 2007;
Hao et al., 2008;
Zhang et al., 2008a;
Zhao et al., 2007), although
many of these may be indirect. There are many differences between the lists of
downstream genes identified in these different microarray studies, and more
needs to be done to define crucial downstream targets for growth control in
both flies and mammals.
Another important class of target genes in Drosophila imaginal
discs are upstream components of signaling pathways that influence Warts.
fj, a regulator of Fat signaling, and expanded
(ex), a regulator of Hippo signaling
(Table 1,
Fig. 1), are also both
downstream targets of Yki (Cho et al.,
2006; Hamaratoglu et al.,
2006; Yang et al.,
2002). The mammalian homologue of fj, four-jointed box 1
(Fjx1), is a Fat target gene in the mammalian kidney
(Saburi et al., 2008). Thus,
as in most signaling pathways, feedback regulation occurs in Fat and Warts
pathways.
A third class of targets are those involved in local cell fate and
patterning decisions. Activation of Yki in the proximal wing of
Drosophila induces expression of the Wingless (Wg) signaling molecule
(Cho et al., 2006;
Cho and Irvine, 2004), which
contributes to the overgrowth phenotypes associated with Fat signaling in this
region (Cho and Irvine, 2004),
but Wg is not induced by Yki in other regions of the wing disc. The Notch
ligand Serrate (Ser) is induced by Yki within the leg disc
(Cho et al., 2006;
Mao et al., 2006), but Ser is
not a Yki target in the Drosophila wing or eye. Components of Warts
signaling have also been implicated in a variety of other processes in
Drosophila, including regulating neural fate during early eye
development (Feng and Irvine,
2007; Maitra et al.,
2006; Pellock et al.,
2007), photoreceptor cell type during later eye development
(Mikeladze-Dvali et al.,
2005), posterior follicle cell fate in the ovary
(MacDougall et al., 2001;
Meignin et al., 2007;
Polesello and Tapon, 2007;
Yu et al., 2008) and
dendritic maintenance (Emoto et al.,
2006). Considering the variety of tissue-specific functions for
components of Warts signaling in Drosophila, one reason for some of
the differences in gene expression detected by microarray experiments on
cultured mammalian cells may be that they employed different cell types
(Dong et al., 2007;
Hao et al., 2008;
Zhang et al., 2008a;
Zhao et al., 2007).
Microarray targets identified in mammalian cells include not only genes
implicated in the regulation of cell proliferation and cell death but also
genes implicated in processes like epithelial-mesenchyme transition,
cytoskeletal organization, cell adhesion and cell migration, which also
supports the conclusion that Warts signaling has functions beyond growth
control, presumably involving the regulation of a variety of cell-type
specific targets.
Regulation by Merlin and Expanded
In Drosophila, two related genes, ex and Merlin
(Mer), have been identified as being upstream regulators of Hippo
signaling (Hamaratoglu et al.,
2006) (Fig. 1).
ex and Mer both encode members of the Band 4.1 super family,
a group of cytoplasmic proteins characterized by the inclusion of a FERM
(Four-point one, Ezrin, Radixin, Moesin) domain, which mediates membrane
association; many family members are also associated with cytoskeletal
regulation (Mangeat et al.,
1999). ex was first identified as a Drosophila
tumor suppressor (Boedigheimer and Laughon,
1993). Mer was identified as the Drosophila
homologue of a human tumor suppressor responsible for a congenital syndrome
(neurofibromatosis type 2, NF2) that is associated with a high frequency of
tumors in nervous tissue (LaJeunesse et
al., 1998; McClatchey and
Giovannini, 2005). Mutation of Drosophila Mer on its own
has only minor effects on growth, but characterization of Mer;
ex double mutants suggests that they are partially redundant
(McCartney et al., 2000). Each
gene also has unique functions (McCartney
et al., 2000; Pellock et al.,
2007; Silva et al.,
2006; Willecke et al.,
2006), but it is not yet clear whether these reflect unique
functions of each protein or simply differences in expression.
Mer and ex have been linked to Hpo signaling in
Drosophila by several observations. Mutation of these genes not only
influences growth and cell survival, resulting in phenotypes similar to the
effects of mutation of Hippo kinase cassette genes, they also influence the
same downstream target genes (Cho et al.,
2006; Hamaratoglu et al.,
2006). Genetic epistasis experiments have suggested that
ex and Mer act upstream of hpo
(Hamaratoglu et al., 2006),
and, consistent with this, they can influence Hpo and Wts phosphorylation in
cultured cells (Hamaratoglu et al.,
2006; Silva et al.,
2006), and can influence Yki phosphorylation and Yki subcellular
localization in vivo (Oh and Irvine,
2008).
The precise mechanisms by which these proteins influence Hpo signaling has
not yet been determined. One study identified an accumulation of several
different transmembrane receptors, including Fat, on the cell surface in
Mer; ex double mutant clones, and raised the possibility that
Mer and ex might exert a general influence on receptor
endocytosis (Maitra et al.,
2006). However, ex null mutant animals can be largely
rescued by overexpression of Wts (Feng and
Irvine, 2007), which suggests that the effects ex has on
the levels of cell surface receptors are a consequence, rather than a cause,
of its influence on the Hippo kinase cassette.
In Drosophila, Ex is the more crucial regulator of Hippo signaling
in most contexts, but it is not yet clear whether this is also the case in
mammals. Two mammalian genes with some sequence similarity to Ex have been
identified (Hamaratoglu et al.,
2006), but there are some differences in their domain structure
when compared with Drosophila Ex, and mutants have not been
described. Nonetheless, one Ex-related protein, Ex1/Frmd6, influenced Yap
activity in a cultured cell assay (Zhao
et al., 2007).
Regulation by contact inhibition
Mammalian Merlin has been extensively studied for its tumor suppressor
function (reviewed by McClatchey and
Giovannini, 2005). These studies have identified several proteins
that can interact with Merlin, and have tied Merlin to the activity of
cytoskeletal regulators, but the mechanisms by which the loss of Merlin leads
to tumor formation had remained unclear. However, one important clue comes
from experiments that implicate Merlin in the contact-dependent inhibition of
cell proliferation. Normal cells will proliferate in culture at low density,
but stop proliferating when they become confluent. Loss of contact information
is a hallmark of oncogenic transformation, and is not specific to Merlin.
However, Merlin has been tightly linked to contact inhibition by the
observation that it is subject to cell density-dependent phosphorylation in
culture (Morrison et al.,
2001), as many FERM-domain proteins are regulated by
phosphorylation (Mangeat et al.,
1999). This study also implicated CD44 in this regulation of
Merlin; CD44 is a transmembrane protein, the extracellular domain of which can
interact with the extracellular matrix, while its intracellular domain can
interact with Merlin.
More recent studies have now clearly implicated the Warts pathway in
contact inhibition (Zhao et al.,
2007). The phosphorylation status and subcellular localization of
Yap in cultured cells depends on cell density, and correlates with the
proliferative status of these cells. Thus, at low cell density, Yap is
predominantly unphosphorylated and nuclear, but when cells become confluent
and stop proliferating, Yap is predominantly phosphorylated and cytoplasmic
(Fig. 4). Moreover, the
expression of the YapS127A mutant overcomes contact inhibition.
This regulation of Yap was tied to Hippo signaling by the observations that
Lats2 kinase activity is also influenced by cell density, and that Yap remains
nuclear even at high cell density in a Merlin mutant cell line.
Warts signaling and cancer
Several studies have linked the Hippo kinase cassette to cancer in mammals.
Lats1 mutant mice are sensitive to carcinogen treatment, and develop
soft tissue sarcomas and ovarian tumors
(St John et al., 1999). A gene
targeted mutation of Lats2 causes embryonic lethality, but mutant
embryos show overgrowth in mesodermal lineages, and Lats2 embryonic
fibroblasts are refractory to contact inhibition
(McPherson et al., 2004). More
recently, studies in Ww45 mutant mice have uncovered a requirement
for Ww45 during cell cycle exit in epithelial tissues
(Lee et al., 2008).
Consequently, these tissues display hyperproliferation and are defective in
terminal differentiation. Ww45 mutations have also been identified in
some human renal cancer cell lines (Tapon
et al., 2002), and mutations in Mats were identified in a
human skin melanoma and a mouse mammary gland carcinoma
(Lai et al., 2005). Promoter
hypermethylation and decreased expression of MST1 and MST2
in soft tissue sarcomas, and of LATS1 and LATS2 in
aggressive breast cancers, have also been reported
(Seidel et al., 2007;
Takahashi et al., 2005).
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). Moreover, the amplification of the chromosomal region
that harbors Yap has been observed in several animal tumor models, including
mouse liver and mammary tumors (Zender et
al., 2006). Elevated Yap protein and nuclear localization was also
observed in human liver and prostrate cancers, and expression of
YapS127A in mice can cause overgrowth of the liver and other organs
(Camargo et al., 2007;
Dong et al., 2007). Linkage of Fat to Warts signaling
fat was identified as a Drosophila tumor suppressor 20
years ago (Bryant et al.,
1988), but the basis for its tumor suppressor activity was
unknown. Within the past few years, however, it has become clear that the
influence of fat on growth reflects its role as a receptor for an
intercellular signaling pathway that influences gene expression
(Fig. 4). The first gene
identified as a downstream effector of Fat signaling was dachs. The
mutation of dachs in Drosophila reduces growth, especially
in the wing and leg, and reduces the expression of Fat target genes
(Cho and Irvine, 2004;
Mao et al., 2006). These
phenotypes are opposite to those of fat mutants. Moreover,
dachs mutations completely suppresses the effects of fat
mutations on growth and gene expression. This epistasis of dachs to
fat suggested that dachs might act downstream of Fat, which
was confirmed by the observation that Fat regulates the subcellular
localization of Dachs protein (Mao et al.,
2006).
More recently, Fat and Warts signaling have been linked by the observation
that they regulate a common set of downstream target genes
(Bennett and Harvey, 2006;
Cho et al., 2006;
Silva et al., 2006;
Tyler and Baker, 2007;
Willecke et al., 2006). Thus,
fat regulates the expression of genes that were first identified as
Warts pathway targets, such as Diap1, Cyclin E and ex,
whereas components of Warts signaling regulate the expression of genes that
were first identified as being Fat pathway targets, such as wg, Ser
and fj. The inference that Fat signaling mediates its effects on gene
expression through the regulation of Yki is also supported by the observations
that heterozygosity for yki partially suppresses fat
phenotypes (Bennett and Harvey,
2006; Silva et al.,
2006; Willecke et al.,
2006), and that loss of fat influences Yki
phosphorylation and its subcellular localization in vivo
(Oh and Irvine, 2008).
Moreover, fat tumor suppressor phenotypes can be partially rescued by
the overexpression of Wts (Feng and
Irvine, 2007). One additional Drosophila tumor
suppressor, discs overgrown (dco), which encodes a Casein
kinase I
homologue (Zilian et al.,
1999), has also been linked to Fat-Warts signaling by its
regulation of common downstream target genes and by genetic epistasis
experiments that position the action of dco as being upstream of
dachs (Cho et al.,
2006).
Two distinct mechanisms by which Fat could intersect with Warts signaling
have been described. One involves an influence that Fat has on the levels of
Warts protein (Cho et al.,
2006). The mutation of fat or dco is associated
with a post-transcriptional reduction in Warts protein levels. dachs
is required for this influence on Warts, and Dachs can associate with Warts in
cultured cells, which suggests that Dachs might be involved in a turnover of
Warts protein. This effect on Warts levels is specific to Fat signaling, as
opposed to Hippo signaling, because it was not observed with mutations in
ex, sav or mats. A second proposed mechanism involves an
influence of Fat on the levels of Ex protein at the subapical membrane, which
are reduced in fat mutants
(Bennett and Harvey, 2006;
Silva et al., 2006;
Willecke et al., 2006); this
effect of fat also depends on dachs
(Feng and Irvine, 2007). This
reduction in Ex levels was hypothesized to influence Hippo signaling, which
was supported by the observation that the overexpression of the Fat
intracellular domain in cultured S2 cells could influence the expression of a
Yki-dependent reporter. Two observations indicate that this effect of
fat on Ex levels does not suffice to explain Fat signaling. First,
ex fat double mutants have more severe phenotypes
(Feng and Irvine, 2007;
Willecke et al., 2006), and
stronger effects on Yki phosphorylation and localization
(Oh and Irvine, 2008), than
either single mutant, consistent with the inference that they act in parallel
to influence Warts. Second, the reduction in Ex levels can be reversed by the
overexpression of Ex, yet Fat still affects tissue growth and gene expression
in these cells (Feng and Irvine,
2007). These observations indicate that Fat can signal
independently of Ex, but they do not exclude the possibility that Fat could
also signal through Ex, and hence influence Warts through two parallel
pathways, one affecting Warts levels and the other affecting Warts activation
(Figs 1,
4). Distinguishing the
respective contributions of these two mechanisms in different tissues in vivo
will require the development of reagents that can reliably detect the levels,
localization and phosphorylation status of components of the Hippo kinase
cassette in situ.
|
In addition to its affects on Warts signaling, Fat also affects planar cell
polarity (PCP). PCP is the polarization of cells within the plane of a tissue,
perpendicular to the apical-basal polarity of epithelial cells. Most studies
of PCP have focused on Frizzled-dependent PCP signaling, which involves a set
of core PCP proteins, including Frizzled, Dishevelled, Starry night and
Prickle (Klein and Mlodzik,
2005). Several years ago, however, PCP phenotypes were reported
for Drosophila fj, ds and fat mutants
(Adler et al., 1998;
Casal et al., 2002;
Rawls et al., 2002;
Strutt and Strutt, 2002;
Yang et al., 2002;
Zeidler et al., 1999;
Zeidler et al., 2000); a weak
PCP phenotype can also be seen in dachs mutants
(Held et al., 1986;
Mao et al., 2006). The
relationship between Fat PCP signaling and Frizzled PCP signaling remains
unclear. Some studies in the Drosophila eye and wing suggested that
Fat PCP signaling acts upstream of Frizzled PCP signaling
(Adler et al., 1998;
Ma et al., 2003;
Matakatsu and Blair, 2004;
Yang et al., 2002). More
recently, a detailed examination of the relationship between Fat and Frizzled
PCP signaling in the abdomen has indicated that these pathways can act in
parallel to influence PCP (Casal et al.,
2006). There is also at least one PCP phenotype that depends only
on Fat PCP signaling: the elongated shape of the wild-type Drosophila
wing depends in part on cell divisions that are oriented along the
proximodistal axis (Baena-Lopez et al.,
2005). The normal polarization of these cell divisions is lost in
ds mutants, and this correlates with the rounder shape of the wing
(Baena-Lopez et al., 2005),
whereas genes involved in Frizzled PCP signaling do not affect wing shape.
Although events downstream of Fat in PCP signaling remain poorly
understood, two genes have been implicated in this process
(Fig. 1). Atrophin has been
linked to Fat PCP signaling by the observations that Atrophin
(grunge) mutant clones have PCP phenotypes similar to fat
mutant clones, and that Atrophin can bind to the Fat cytoplasmic domain
(Fanto et al., 2003). Atrophin
is a transcriptional co-repressor, and influences the expression of
fj (Fanto et al.,
2003), but has not been reported to influence growth or the
expression of other Fat-Warts target genes
(Cho and Irvine, 2004;
Fanto et al., 2003). Dachs has
been linked to Fat PCP signaling by the observations that dachs
mutants partially suppress fat PCP phenotypes, and that the
subcellular localization of Dachs itself is polarized
(Mao et al., 2006).
|
PCP can be represented as a vector of polarity within a tissue. A
particularly striking aspect of Fat PCP signaling, then, is that fj
and ds are expressed in gradients in developing tissues, and these
vectors parallel their influence on PCP
(Fig. 5A-E). The instructive
nature of these gradients has been established by both loss- and
gain-of-function genetic mosaic experiments
(Adler et al., 1998;
Casal et al., 2006;
Casal et al., 2002;
Matakatsu and Blair, 2004;
Simon, 2004;
Strutt and Strutt, 2002;
Yang et al., 2002;
Zeidler et al., 1999). These
experiments also indicate that fj and ds have opposite
effects on PCP, which is consistent with the observation that they are
normally expressed in opposing gradients. Intriguingly, the PCP information in
these opposing gradients is partially redundant
(Simon, 2004;
Zeidler et al., 1999). Thus,
in the Drosophila eye, as long as ds expression is normally
graded, the loss of fj or the uniform expression of fj has
only minor effects on PCP, and strong PCP phenotypes are only observed when
there is a sharp difference in fj expression levels created by a
genetic mosaic (Fig. 5D,E).
Conversely, ds mutants have strong effects on PCP, but its expression
does not need to be graded as long as fj expression is graded. The
contributions of these gradients to PCP in different tissues can vary,
however, as neither the ds nor fj gradient is required for
normal PCP in much of the Drosophila wing
(Matakatsu and Blair, 2004;
Simon, 2004). Genetic
epistasis experiments suggest that fj and ds act upstream of
fat in regulating PCP, consistent with the conclusion that they act
as fat regulators (Yang et al.,
2002).
The hypothesis that fj and ds act as regulators of Fat is
also supported by their influence on gene expression. Wg is expressed in a
ring of cells in the proximal Drosophila wing. In the absence of
fat, Wg expression within the proximal wing is elevated and
broadened, and this effect of fat on Wg is cell autonomous
(Cho and Irvine, 2004).
Manipulations of fj and ds expression also influence Wg
expression, but their effects are non-autonomous
(Cho and Irvine, 2004).
Similarly, various studies have reported non-autonomous effects of fj
and ds on fj, Ser and Diap1 expression
(Buckles et al., 2001;
Cho et al., 2006;
Rogulja et al., 2008;
Zeidler et al., 1999),
whereas the expression of these genes is upregulated cell autonomously within
fat mutant clones (Cho et al.,
2006; Mao et al.,
2006; Yang et al.,
2002). The effects of fj and ds on
Diap1 expression and cell proliferation are suppressed in
dachs mutants (Rogulja et al.,
2008). Together, these observations indicate that Fj and Ds act on
the signaling side, and Fat on the receiving side, of a pathway that
influences gene expression. The hypothesis that Fat acts as a receptor is also
consistent with the observation that the expression of a truncated Fat protein
that is missing almost its entire extracellular domain can partially rescue
fat mutant phenotypes (Matakatsu
and Blair, 2006).
Direct support for Ds binding to Fat and has come from cell aggregation and
protein localization experiments. Cultured Drosophila S2 cells do not
normally aggregate, but can be induced to aggregate when they express
interacting proteins. Fat- and Ds-expressing cells specifically bind to each
other in this assay (Matakatsu and Blair,
2004). Studies of Fat and Ds protein localization in vivo also
suggest that they engage in heterophilic binding
(Cho and Irvine, 2004;
Ma et al., 2003;
Mao et al., 2006;
Strutt and Strutt, 2002). When
expression of Ds is manipulated in a mosaic fashion, such that a
Fat-expressing cell is confronted with neighbors that differ in the amount of
Ds expressed, Fat protein concentrates at the interface with neighbors that
express higher levels of Ds, and is lost from interfaces with neighbors that
express lower levels of Ds. Ds localization can be similarly affected by the
manipulation of Fat expression, and the localization of both proteins can be
affected by Fj. These observations imply that Fat and Ds bind to each other,
acting as a ligand-receptor pair, and further suggest that Fj influences this
binding. Although a fraction of Fj is secreted from cells
(Buckles et al., 2001), it
also localizes to the Golgi, and experiments with chimeric proteins have
indicated that the Golgi localization is relevant
(Strutt et al., 2004). A
biochemical explanation for the influence of Fj on Fat signaling has recently
been provided by the discovery that it is a protein kinase that can
phosphorylate some of the cadherin domains of Fat and Ds
(Ishikawa et al., 2008).
The mechanism by which Ds regulates Fat has not yet been determined.
However, as Fat antagonizes the localization of Dachs to the membrane, the
polarized localization of Dachs implies that Fat activity is normally
polarized within cells. This polarization parallels the fj and
ds expression gradients (Mao et
al., 2006; Rogulja et al.,
2008), which suggests that the polarization reflects an ability to
compare the relative levels of Ds presented on one side of a cell versus the
other, and, consistent with this, genetic experiments have confirmed that
Dachs localization can be altered by manipulating fj or ds
expression (Mao et al., 2006).
As Fat signaling can polarize Dachs localization, it could influence PCP
through a similar mechanism, but how might this be related to effects on Warts
signaling? A recent model proposes that polarization of Dachs could also
influence Warts signaling if the influences of Dachs on Warts levels and
activity, and if the influence of Warts on Yki phosphorylation, occur locally
at the membrane (Rogulja et al.,
2008) (Fig. 6).
This model provides an explanation for how Ds can act as a ligand that
activates Fat, yet inhibit Fat-Warts signaling when cells with different
levels of Ds expression are juxtaposed
(Rogulja et al., 2008).
Additionally, because in this model the influence of Fj and Ds on PCP depends
on the vector of their expression gradients, but their influence on Fat-Warts
signaling depends on the slope, it provides an explanation for why Fj and Ds
have opposite effects on Fat PCP signaling, but similar effects on Fat-Warts
signaling.
A variety of observations implicate Ds as a Fat ligand, but ds
mutants have weaker effects on growth than do fat mutants. Thus, some
degree of Fat activity might be ligand independent. Alternatively, there might
be other Fat ligands, although there are no obvious ds homologues
encoded by the Drosophila genome. There is another
fat-related gene in Drosophila, fat2, but its cytoplasmic
domain appears structurally distinct, and it has been implicated in the
morphogenesis and maintenance of tubular organs rather than in PCP or growth
control (Castillejo-Lopez et al.,
2004). In mammals, there are two ds-related genes,
Dchs1 and Dchs2 (Rock et
al., 2005), and four fat-related genes,
Fat1-Fat4 (Katoh and Katoh,
2006). Sequence analysis suggests that Fat4 is the
closest homologue of Drosophila fat, and this is supported by the
recent discovery that Fat4 mutant mice have PCP phenotypes and have
elevated mammalian fj (Fjx1) expression in the kidney
(Saburi et al., 2008).
Conclusion
The last few years have given us the basic outline of a novel set of
interconnected signaling pathways, the Fat-Warts signaling network (Figs
1,
4). This network has multiple
inputs and outputs. On the input side, Fat is the only transmembrane receptor
protein identified in Drosophila thus far. However, as double mutants
of fat with both Mer and ex have additive
phenotypes (Bennett and Harvey,
2006; Feng and Irvine,
2007; Silva et al.,
2006; Willecke et al.,
2006), we expect that there will be other transmembrane receptors
that regulate this pathway. CD44 may be one such protein in mammals
(Morrison et al., 2001), but
as CD44 is not obviously conserved in Drosophila, there must be
others, and their identification will be crucial for understanding the
regulation of Warts signaling.
Outputs of this network can be broadly classified as Warts dependent (Warts
signaling) or Warts independent (Fat PCP signaling). Although the influence on
PCP is largely Warts independent (Fanto et
al., 2003; Feng and Irvine,
2007; Mao et al.,
2006), feedback regulation, such as the regulation of fj
expression, is a complicating factor. Warts signaling incorporates both
effects on Warts levels (The Fat-Warts pathway), and effects on Warts
phosphorylation and activity (The Hippo pathway). The principal substrate of
Warts signaling in terms of effects on growth and gene expression is Yki/Yap.
The identification of a DNA-binding partner for Yki/Yap is an important
advance, but the divergence between sd and yki mutant
phenotypes indicates that there must be other proteins that participate in the
transcriptional regulation mediated by this pathway. Additionally, the cell
cycle-dependent localization and mitotic phenotypes of Warts/Lats and Mats/Mob
suggest that other Warts substrates that are not transcription factors will be
important for cell division.
In between these inputs and outputs, there is a series of identified biochemical interactions, and many unanswered questions, such as how does Dachs influence Warts levels and how does Ex influences Hpo activity? Indeed, current pathway models are best considered as frameworks, the details of which will continue to be added to over the coming years. A better understanding of the cell biology of Fat-Warts signaling, including the localization and dynamics of proteins and protein complexes, would be especially valuable. Nonetheless, tremendous progress has been made in just the past few years, and enough has been learnt to establish Fat-Warts signaling as one of the core conserved signaling pathways that acts throughout the metazoans to direct their growth and patterning.
ACKNOWLEDGMENTS
We thank C. Rauskolb for comments on the manuscript and the confocal images in Fig. 5. Research in K.D.I.'s laboratory is supported by the Howard Hughes Medical Institute and by the NIH.
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