Laboratory of Molecular Biology, National Institute of Diabetes, Digestive, and Kidney Diseases, NIH, Bethesda, MD 20892, USA
* Author for correspondence (e-mail: mwkrause{at}helix.nih.gov)
Accepted 10 February 2005
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SUMMARY |
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Key words: C. elegans, MyoD, Muscle, Myogenesis
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Introduction |
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In C. elegans, the striated body wall muscle cells are comparable
to vertebrate skeletal muscle. This musculature provides the locomotive force
for the animal and consists of 95 mononucleated cells, arranged in four
quadrants along the length of the body
(Waterston, 1988). Eighty-one
of these cells are born and differentiate during embryogenesis
(Sulston et al., 1983
), while
14 more are added post-embryonically
(Sulston and Horvitz, 1977
).
Although previous studies have identified several factors required for
post-embryonic muscle development (Harfe
et al., 1998a
; Harfe et al.,
1998b
; Liu and Fire,
2000
; Corsi et al.,
2000
), there has been little progress in understanding embryonic
striated myogenesis. One general conclusion of these studies is that embryonic
and post-embryonic muscle development are controlled by different sets of
transcription factors, an unexpected finding given that muscle cells born
during these two different periods of development appear morphologically and
functionally equivalent.
In C. elegans, fertilization is followed by a rapid set of
embryonic cellular divisions that generate five somatic founder blastomeres
called AB, MS, E, C and D, and the germline blastomere P4. The 81 embryonic
body wall muscles are derived from four of the five somatic founders AB (1),
MS (28), C (32) and D (20). The D lineage gives rise exclusively to body wall
muscle whereas the other lineages give rise to multiple cell fates. Maternal
effect mutations have been identified that alter the fate of one body wall
muscle-producing founder without affecting other lineages. For example,
skn-1 mutants lack the 28 MS-derived body wall muscles but body wall
muscle from the C and D lineages are present
(Bowerman et al., 1992;
Bowerman, 1995
;
Bowerman et al., 1997
).
Conversely, pal-1 mutants lack the C and D lineage-derived body wall
muscle cells without affecting those derived from MS
(Hunter and Kenyon, 1996
;
Ahringer, 1997
). This founder
blastomere autonomy, with respect to body wall muscle formation, demonstrates
that there are several genetically distinct pathways for embryonic striated
muscle development. However, it is yet to be determined if these independent
genetic pathways converge on a common molecular nodal point to regulate body
wall muscle cell fate.
A single C. elegans gene, hlh-1, encodes a transcription
factor (HLH-1, a.k.a. CeMyoD) that is related to the vertebrate MRFs.
hlh-1 is zygotically expressed in embryonic body wall muscle precursors
and their differentiated descendants, beginning at the 90 cell stage of
development (Krause et al.,
1990
). This expression pattern suggested that hlh-1 might
represent a nodal point for body wall muscle development, analogous to the
role of Myf-5 and MyoD in vertebrate myogenesis. Homozygous
hlh-1 null mutant animals complete embryogenesis but are paralyzed
upon hatching, have severe morphological defects, and usually die during the
first larval stage, revealing an essential role for HLH-1 in muscle
development and function (Chen et al.,
1992
; Chen et al.,
1994
). However, hlh-1 null mutants have 81 embryonic body
wall muscle cells and express many terminal muscle products at wild-type
levels. Thus, HLH-1 is not essential for body wall muscle cell fate, and one
or more additional factors must be involved in myogenic determination. In
fact, the hlh-1 knockout allele phenotypes challenge the notion that
HLH-1 is itself myogenic, suggesting instead that it may be downstream of
myogenic factors in the body wall muscle transcriptional cascade. Similar
results in Drosophila for the MRF-related gene nautilus have
suggested that there may be a fundamental difference between vertebrates and
invertebrates with regard to the regulation of striated muscle development
(Michelson et al., 1990
;
Balagopalan et al., 2001
).
The present study further defines the function of HLH-1 and its relationship to intrinsic and extrinsic factors regulating early development. By ectopically activating HLH-1 in the early C. elegans embryo, we show that HLH-1 alone is sufficient to convert most cells of the early embryo into a body wall muscle-like fate. This is true for cells that would normally give rise to either ectoderm or endoderm, demonstrating that HLH-1 is a bona fide MRF with potent myogenic activity. The ectopic myogenic activity of HLH-1 is limited to undifferentiated blastomeres and spans several hours of early development, including times when non-muscle, lineage-restricted markers are normally expressed. We find that the Caudal-related factor PAL-1 can activate hlh-1, providing a link between maternal factors and HLH-1 activation, and we explore the role of Wnt/MAP kinase signaling in making cells competent for myogenesis. These studies demonstrate a level of developmental plasticity in C. elegans that had previously not been appreciated.
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Materials and methods |
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The heat-shock hlh-1 construct was made by transferring a fragment
containing the full-length hlh-1 cDNA
(Krause et al., 1990) into the
hsp 16.41 (Stringham et al.,
1992
) vector pPD49.83 (kindly provided by A. Fire) to yield the
plasmid pKM1211. Animals harboring pKM1211 were generated by standard
techniques using 100 µg of pKM1211 and 50 µg of the selectable dominant
rol-6 plasmid pRF4 (Mello and
Fire, 1995
). Integrated heat-shock hlh-1 lines were
generated by gamma irradiation (Egan et
al., 1995
) of extrachromosomal transformants and were back-crossed
twice with the wild-type strain (N2) prior to use.
The heat-shock experiments
One and two cell stage embryos were isolated from transgenic hermaphrodites
containing heat-shock expression constructs or from N2 controls. Embryos were
heat shocked immediately or incubated for various times at room temperature
(22°C) prior to heat shock. For all treatments, heat shock consisted
of a single 30 minute pulse at 34°C. The heat-shocked embryos were
examined over time for the expression of cell type-specific reporter genes or
incubated and fixed for antibody staining with markers for muscle, gut,
hypodermis and germline (see below). Using the absence of hypodermal marker
staining as a sensitive assay for the degree of myogenic conversion, we
determined that the optimal conditions for HLH-1 myogenic activity were to
isolate one- to two-cell embryos and incubate for 60 minutes prior to heat
shock. Heat shock induction of PAL-1 was carried out after 20 minutes
incubation of isolated embryos.
Antibody staining
Embryos were fixed with 5% paraformaldehyde in phosphate-buffered saline
(PBS) on ice for 15 minutes and transferred onto 0.1% gelatin-coated slides,
placed on an aluminum block on dry ice, freeze-cracked, methanol fixed at
20°C for 6 minutes, and rehydrated in PBS at room temperature. The
primary antibodies used were raised against myosin heavy chain A [MHC A
(Miller et al., 1986)], HLH-1
(Krause et al., 1990
), ELT-2
(Fukushige et al., 1998
) (a
gift from Jim McGhee), LIN-26 (Labouesse
et al., 1996
) (a gift from Michel Labouesse), a pharyngeal muscle
epitope [3NB12 (Priess and Thomson,
1987
)], UNC-89, UNC-98 (Benian
et al., 1996
; Mercer et al.,
2003
) (a gift from Guy Benian), PAT-3
(Gettner et al., 1995
) (a gift
from Don Moerman), and germline P-granules [OIC1D4
(Strome and Wood, 1983
)].
Secondary antibodies were fluorescein- or rhodamine-conjugated donkey
anti-rabbit or goat anti-mouse IgG (Jackson Immunological).
RNAi
Double-stranded RNA corresponding to the genes mex-1, mex-3, lit-1,
wrm-1, skn-1 and pal-1 were amplified from cDNA clones that were
generated by reverse transcriptase-polymerase chain reaction (RT-PCR) with the
primers listed below. These partial cDNA PCR products were inserted into the
vector L4440 (Timmons et al.,
2001) and served as a template for in vitro transcription to
produce double stranded RNA for injection. The pop-1 RNAi plasmid
RL499 was a gift from R. Lin.
Primers used for PCR amplification of cDNA clones for RNAi:
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Results |
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Under optimal conditions, heat shock-induced hlh-1 was robust and high levels of nuclear localized protein could be detected in most cells of the embryos within 30 minutes after the end of heat shock induction. Levels of HLH-1 remained high throughout embryogenesis, although nuclear localization was less pronounced after overnight (16-20 hours) incubation of the treated embryos. Embryos incubated overnight arrested with 400-500 cells and appeared healthy, as assayed by Nomarski optics, indicating that neither the heat shock treatment nor high HLH-1 levels had any obvious deleterious effect on cellular viability or proliferation.
In almost all treatment paradigms, over-expression of hlh-1
resulted in widespread myogenesis; most cells had adopted a muscle-like fate.
The muscle marker routinely used to assay myogenesis was myosin heavy chain A
[MHC A (Miller et al., 1986)
Fig. 1]. Although high levels
of MHC A were detected, filaments were disorganized and these muscle-like
cells fail to contract. To determine the extent to which these cells adopted a
true muscle-like fate, we tested several additional muscle markers, including
several structural proteins. The muscle-like cells resulting from heat
shock-induced HLH-1 activated a myo-3::gfp reporter gene
(Fire and Waterston, 1989
) and
were positive for filamentous actin, UNC-89
(Benian et al., 1996
), UNC-98
(Mercer et al., 2003
) and
PAT-3 (Gettner et al., 1995
;
Francis and Waterston, 1985
)
(data not shown). These muscle cells were not positive for the pharyngeal
muscle-specific marker 3NB12 (Priess and
Thomson, 1987
). The continued presence of HLH-1 and activation of
several body wall muscle markers is consistent with cells terminally
differentiating and adopting a fate most closely resembling body wall muscle
cells.
|
The excess number of muscle-like cells observed after ectopic activation of
HLH-1 could be due to excessive proliferation of myogenic blastomeres,
conversion of other cell types to muscle, or both. The total number of cells
in HLH-1-activated, terminally arrested embryos was similar to that normally
born during embryogenesis, suggesting that there was not a general and
widespread hyper-proliferation of blastomeres induced by HLH-1. We therefore
assayed the number of embryonic cells adopting one of several cell fates by
scoring them for LIN-26 (hypodermis)
(Labouesse et al., 1996),
ELT-2 (intestine) (Fukushige et al.,
1998
), 3NB12 (pharyngeal muscle)
(Priess and Thomson, 1987
) and
P-granules (germline) (Strome and Wood,
1983
) following heat shock induction of HLH-1. Although the
majority of activated HLH-1 embryos had the normal number of germline
precursors (two), there was a complete elimination of all somatic cell types
assayed in almost all embryos (Fig.
1). Similar heat shock of wild-type embryos did not affect normal
development and these embryos were positive for all cell-type markers tested.
These results suggested that early expression of hlh-1 was able to
convert most, if not all, somatic cells of the embryo into muscle-like
cells.
It was possible that HLH-1 was not actively converting cells to muscle but
instead blocking normal development and revealing a default muscle cell fate
program of early blastomeres. To address this, we eliminated two maternal
factors needed to specify several founder blastomere fates. In wild-type
embryos, the loss of skn-1 and pal-1 gene products prevents
the proper specification of the MS, C and D lineages and greatly reduces, or
eliminates, the cell types derived from each lineage
(Fig. 2)
(Bowerman et al., 1992;
Bowerman et al., 1993
;
Hunter and Kenyon, 1996
). Such
embryos arrest with 400-500 cells, most of which fail to express body wall
muscle markers. In HLH-1-activated embryos that have also been depleted of
both skn-1 and pal-1 gene products by RNAi, almost all cells
adopted the body wall muscle-like fate
(Fig. 2). These results further
demonstrated the myogenic potential of HLH-1 activity and that myogenesis is a
consequence of HLH-1 activity.
|
Myogenic conversion by HLH-1 acts in a broad window of early development
Stable hlh-1 gene expression is normally detectable in all body
wall muscle precursors shortly after they are born, beginning with the
daughters of the D founder about 2 hours after fertilization
(Krause et al., 1990). HLH-1
is present in body wall myoblasts as they proliferate and in all
differentiated body wall muscle cells in embryos, larvae and adults. To
determine if the ability of HLH-1 to convert cells to a muscle-like fate was
restricted to a specific time of embryogenesis, embryos harboring the heat
shock hlh-1 transgene were isolated and incubated for various periods
of time prior to induction. The percentage of embryos that were positive for
MHC A, and each of several different non-body wall muscle cell type markers,
was used to determine the efficiency of HLH-1-induced myogenesis. For embryos
incubated for more than 90 minutes prior to HLH-1 induction (
2 hours
post-fertilization), we used the intestine-specific marker elt-2::GFP
to count the number of E cell descendants present at the onset of the heat
shock under our experimental conditions
(Table 1). Embryos that had
anywhere between one and eight E cell descendants at the time ectopic HLH-1
activation was initiated showed widespread myogenic conversion and a nearly
complete loss of intestine, hypodermis and pharyngeal cell markers
(Table 1). At 210 minutes of
incubation (
4 hours post-fertilization), when embryos averaged 10.2 E
descendants, markers indicative of other cell fates became evident in most
embryos. This defined a window of competence for blastomeres to respond to
ectopic HLH-1 to the first 3 hours of development, up to the eight E cell
stage of embryogenesis. After this time, the ability of non-myogenic cells to
respond to HLH-1 declines rapidly over the subsequent hour of development and
is lost completely in terminally differentiated cells.
|
To determine if hlh-1 is a downstream target of PAL-1 in myogenic
lineages, we ectopically activated PAL-1 in early embryos harboring an
hlh-1::gfp reporter gene. In mex-3 RNAi-treated embryos,
ectopic PAL-1 activated the hlh-1 reporter within 3 hours in a subset
of blastomeres that differentiated as body wall muscle-like cells. The extent
of differentiation of muscle-like cells in mex-3 RNAi-treated embryos was
indistinguishable from that observed after heat shock induction of HLH-1
activity. The embryos also had ectopic hypodermal cells that were LIN-26
positive, as was expected from previous characterizations of mex-3
mutants (Draper et al., 1996;
Bowerman et al., 1997
). To
determine if the effects of mex-3 RNAi were attributable to PAL-1
mis-regulation, we also ectopically over-expressed PAL-1 using a heat shock
promoter-driven pal-1 cDNA clone, kindly provided by Julie Ahringer.
We found that heat shock-induced PAL-1 activity in early embryos also resulted
in widespread hlh-1 reporter gene activation and myogenesis
(Fig. 3), as well as hypodermal
development (data not shown). Although similar, the effects of mex-3
RNAi and heat shock-induced PAL-1 were not identical and could be
distinguished with the gut cell marker ELT-2. All mex-3 RNAi-treated
embryos (100%, n=16) were positive for ELT-2 demonstrating that the
gut lineage was largely unaffected in these embryos, consistent with earlier
studies (Draper et al., 1996
).
However, only 58% (n=268) of heat shock-induced PAL-1 embryos were
ELT-2 positive, suggesting that over-expression of PAL-1 interfered with
normal gut development.
|
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|
The ability of PAL-1 to induce muscle is independent of HLH-1 activity
In all experiments in which ectopic PAL-1 activity, in concert with low or
no POP-1 activity, resulted in a muscle-like fate, HLH-1 was activated and
localized to the nucleus of myogenic cells prior to the expression of terminal
muscle markers. Previous genetic studies have demonstrated that HLH-1 is not
necessary for cells to adopt a body wall muscle fate during embryogenesis
(Chen et al., 1992;
Chen et al., 1994
). To
determine if the PAL-1-induced muscle-like fate was dependent on HLH-1, we
repeated the ectopic PAL-1 experiments in an hlh-1 null mutant
background. Hermaphrodites heterozygous for the balanced hlh-1(cc450)
null allele (Chen et al.,
1992
) were treated with mex-3, pop-1 double RNAi and
progeny embryos collected. If HLH-1 was required for PAL-1 to induce muscle,
25% of the embryos (corresponding to the homozygous hlh-1(cc450)
animals) should not respond to PAL-1. However, all treated embryos resulted in
widespread body wall muscle-like myogenesis demonstrating that HLH-1 activity
was not required for PAL-1-induced myogenesis
(Fig. 7). We did notice that
22% (n=196) of the embryos showed less robust myogenesis, based on
MHC A filament formation (Fig.
7B). These experiments were repeated using the
temperature-sensitive hlh-1 allele cc561
(Harfe et al., 1998a
), or
hlh-1 RNAi for which the genotype of each embryo was unambiguous and
the same results were obtained (Fig.
7C); myogenesis occurred but was not as robust in the absence of
HLH-1. These results demonstrated that ectopic PAL-1-induced myogenesis was
slightly more robust with, but not dependent on, HLH-1.
|
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Discussion |
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The myogenic activity of HLH-1 is robust, possibly reflecting its ability
to auto-activate its expression in a positive feedback loop
(Krause et al., 1994). We find
that induction of HLH-1 as late as the eight E cell (>100 total cells)
stage of embryogenesis still results in almost all somatic cells adopting a
muscle-like fate. Within the E lineage, several tissue-specific markers (e.g.
end-1 and elt-2) are being expressed by the eight E cell
stage, indicating that these cells have already initiated the intestinal cell
fate (reviewed by Maduro and Rothman,
2002
). The over-expression of HLH-1 is able to extinguish the gut
program and redirect these early intestinal cells into body wall muscle-like
cells. This distinguishes our studies from previous work in which blastomere
fate-switching was induced much earlier in development (two E cell stage)
(Fukushige et al., 1998
;
Zhu et al., 1998
). We did not
observe cells expressing terminal markers of multiple fates, suggesting that
cell fate decisions are mutually exclusive. How competing transcriptional
factors result in all-or-none developmental fate decisions at the mechanistic
level is an interesting and unanswered question. Regardless of the mechanism,
the potency of HLH-1 reveals a remarkable level of developmental plasticity in
cells that have already initiated a cell fate program; within the first 3
hours of development somatic blastomeres are not irreversibly committed to a
single fate.
The decision to adopt a body wall muscle cell fate can be cell autonomous.
The blastomeres adopting the muscle-like fate are not related by lineage and
are not in a fixed location within the embryo. If exogenous signals are
required for HLH-1-mediated myogenesis, they must originate in the germline
precursors as that is the only non-muscle lineage that remains identifiable in
these HLH-1-activated embryos. Such signals would have to be far-reaching to
affect the most anterior blastomeres that do not physically contact the P cell
lineage after the four cell stage. Consequently, we think it is unlikely that
signals from outside the muscle lineage are required for HLH-1 to activate the
muscle program, although we can not exclude a `community effect'
(Gurdon, 1988) among muscle
cells.
One question that arises from our current work is the extent to which HLH-1
is able to drive myogenesis. That is, are these muscle-like cells exhibiting
most of the characteristics of terminally differentiated cells, or have they
merely initiated a small part of the muscle program? Studies in both
Xenopus (Hopwood and Gurdon,
1990; Hopwood et al.,
1991
) and the mouse (Miner et
al., 1992
; Faerman et al.,
1993
) have demonstrated that ectopic MRF activity in vivo is able
to activate some genes of the skeletal muscle program but fails to drive
terminal muscle differentiation. However, more recent work has shown terminal
differentiation of skeletal muscle after transfection of the chicken embryonic
neural tube with Myf5 or MyoD expression transgenes
(Delfini and Duprez, 2004
). We
have assayed five major markers of body wall muscle in C. elegans and
found that all are present in the muscle-like cells generated by ectopic
HLH-1. This includes gene products encoding structural components needed for
terminal differentiation. However, these muscle-like cells lack clearly
defined sarcomeres and contraction has not been observed. This may reflect the
fact that these embryos also lack somatic non-muscle cell types that might be
important for normal sarcomere assembly and function. This includes the
hypodermal cells, which play a role in sarcomere organization (reviewed by
Rogalski et al., 2001
;
Labouesse and Georges-Labouesse,
2003
) and neurons needed for innervation. Additional studies will
be required to determine if the failure to make functional sarcomeres reflects
the lack of an appropriate cellular environment, a failure in expression in
all requisite muscle cell genes, or a combination of these factors.
PAL-1 is sufficient for myogenesis in cells with little or no POP-1 activity
Our results demonstrate that PAL-1 is sufficient to activate hlh-1
and that this is part of the mechanism of body wall muscle development in the
C and D lineages. Interestingly, this function of PAL-1 is not completely
HLH-1 dependent, demonstrating that one or more factors must act redundantly
with HLH-1 in driving body wall myogenesis. It is not clear if PAL-1 directly
activates hlh-1. The hlh-1 promoter has been extensively
characterized and essential cis-acting elements for expression delineated
(Krause et al., 1994) (J. Liu,
personal communication). In addition, the DNA-binding site preferences for
Caudal and related factors have been defined in Drosophila and
mammalian tissue culture studies (Dearolf
et al., 1989
; Suh et al.,
1994
; Charite et al.,
1998
; Xu et al.,
1999
). However, we have yet to uncover a direct interaction
between PAL-1 and the hlh-1 promoter using either bioinformatic or
experimental approaches. This analysis is complicated by the AT-rich binding
site preferences of Caudal-related factors and our lack of understanding of
which, if any, co-factors act in concert with PAL-1 to regulate transcription
in C. elegans. This is an important question that needs to be
answered in future studies.
The effects of PAL-1 on cell fate specification are altered in the presence of POP-1 activity. Cells lacking POP-1 (e.g. the D lineage) respond to PAL-1 by activating the body wall muscle program. However, in the presence of POP-1 activity, PAL-1 instead promotes hypodermal fate. Our results show that POP-1 activity must be down-regulated in cells that become body wall muscle in both the C and MS lineages. The down-regulation of POP-1 via Wnt/MAP kinase signaling is, therefore, an important component of embryonic body wall muscle development in all lineages except D. The source(s) of Wnt/MAP kinase signaling to descendants of the C lineage is unknown. However, once hlh-1 is activated within body wall muscle precursors, Wnt/MAP kinase signaling is dispensable.
Mechanistically, the combination of PAL-1 and Wnt/MAP kinase signaling in
the C lineage acts in a manner that is analogous to E and MS founder fate
specification. The transcription factor SKN-1 is required for both the MS and
E founder fates that give rise to mesoectoderm and endoderm, respectively
(Bowerman et al., 1992;
Bowerman, 1995
). In the absence
of POP-1, SKN-1 initiates a gut cell fate, analogous to PAL-1 initiating a
muscle-like fate. In the presence of high POP-1 activity, SKN-1 results in the
MS fate, analogous to PAL-1 resulting in a hypodermal fate. In both cases,
Wnt/MAP kinase signaling is responsible for the down-regulation of POP-1
(Rocheleau et al., 1997
;
Thorpe et al., 1997
;
Lin et al., 1998
;
Meneghini et al., 1999
).
The anatomically simple body wall musculature arises from a complex genetic program
The body wall muscle cells are the only striated musculature in C.
elegans. All 81 embryonically born body wall muscle cells are arranged
along the length of the animal in one of four parallel quadrants
(Waterston, 1988). These cells
are morphologically nearly identical to each other, making C. elegans
body wall muscle one of the simplest striated muscle systems under study.
Despite this anatomical simplicity, these 81 cells arise by a surprisingly
complex number of different genetic programs. There are at least two maternal
transcription factors, SKN-1 and PAL-1, regulating embryonic myogenesis in a
manner that is distinct from each other and distinct from the regulation of
post-embryonic body wall muscle development
(Bowerman et al., 1992
;
Hunter and Kenyon, 1996
;
Edgar et al., 2001
). In
addition, ablation experiments reveal a complicated interplay between
different founder blastomere lineages in regulating myogenesis
(Schnabel, 1995
). Finally,
blastomere culture experiments reveal that cell-cell interactions within the C
lineage influence cell fate decisions
(Mickey, 2000
). Taken
together, these studies reveal a surprising level of complexity for the
genesis of an anatomically simple striated musculature.
Similarities between myogenesis in C. elegans and vertebrates
Previous studies of the transcriptional regulation of body wall myogenesis
in C. elegans have highlighted numerous differences between the
vertebrate and nematode systems. In vertebrates, MRFs heterodimerize with
members of the broadly distributed E protein family to activate transcription
of muscle-specific genes (reviewed by
Weintraub et al., 1991;
Weintraub, 1993
). In C.
elegans, the only E-related factor, E/DA, is not detected in striated
muscle cells and HLH-1 appears to function as a homodimer to activate
transcription (Krause et al.,
1997
). In addition, members of the Twist and MEF-2 transcription
factor families, which play important roles in vertebrate mesoderm
specification and muscle differentiation (reviewed by
Black and Olson, 1998
;
Castanon and Baylies, 2002
),
play little or no apparent role in embryonic body wall muscle development in
C. elegans (Corsi et al.,
2000
; Dichoso et al.,
2000
). The transcriptional cascade regulating myogenesis in C.
elegans embryogenesis is clearly distinct from that operating in the
vertebrates.
However, the current study also highlights the similarities between nematode and vertebrate striated myogenesis. HLH-1 is a bona fide MRF with potent myogenic activity in vivo, demonstrating conservation of function throughout evolution. It would appear that function is redundant to other, as yet unidentified, factors in C. elegans and Drosophila whereas vertebrates may rely predominantly on the redundancy provided by multiple MRFs.
The role of Wnt signaling in C. elegans body wall muscle
specification also suggests parallels to other systems. During vertebrate
embryogenesis, Wnts are needed for the dorsal neural tube and surface ectoderm
to induce myogenesis in the adjacent somites in the trunk region
(Munsterberg et al., 1995;
Tajbakhsh et al., 1998
). Wnt
signaling is also important for activating myogenesis in resident stem cells
during muscle regeneration following injury in mice
(Polesskaya et al., 2003
).
During both of these vertebrate developmental events, Wnt signaling precedes
the activation of the MRFs. C. elegans Wnt/MAPK signaling similarly
precedes expression of hlh-1, suggesting that at least some aspects
of the regulation of MRF genes may also be evolutionarily conserved.
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ACKNOWLEDGMENTS |
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