1 Department of Biology, Faculty of Sciences, Kyushu University Graduate School, Fukuoka 812-8581, Japan
Author for correspondence (e-mail:
yohshscb{at}mbox.nc.kyushu-u.ac.jp)
Accepted 16 December 2002
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SUMMARY |
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Key words: che-1, Chemotaxis, Transcription factor, ASE neuron, C. elegans
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INTRODUCTION |
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A C. elegans hermaphrodite has 32 neurons of 14 types, which are
thought to be chemosensory because they have ciliated endings exposed to the
environment through an opening of the cuticle at the end of amphid, phasmid
and inner labial sensory neurons (Ward et
al., 1975; Ware et al.,
1975
; White et al.,
1986
). The amphid and phasmid neurons consist of a pair of similar
neurons, each on the left and right sides of the animal. Functions of the
chemosensory neurons have been determined by killing individual neurons with
laser microbeam and analysis of the resultant behavioral responses
(Bargmann et al., 1993
;
Bargmann and Horvitz, 1991a
;
Bargmann and Horvitz, 1991b
;
Kaplan and Horvitz, 1993
;
Troemel et al., 1995
). Amphid
neurons were found to be responsible for chemosensory behaviors, while phasmid
neurons were reported recently to function as chemosensory cells that
negatively modulate reversals to repellents
(Hilliard et al., 2002
). One
pair of amphid neurons, the ASE neurons, were found to be uniquely important
for chemotaxis to Na+, Cl-, cAMP and biotin: killing the
ASE neurons greatly reduced chemotaxis to these chemicals
(Bargmann and Horvitz,
1991a
).
Several genes involved in the function of the ASE neurons for chemotaxis
have been molecularly cloned. Among a number of che
(chemotaxis-defective) and tax (chemotaxis abnormal) genes that have
been identified in the studies of mutants that fail to respond to NaCl
(Dusenbery et al., 1975;
Lewis and Hodgkin, 1977
),
tax-2 and tax-4 encode cyclic nucleotide-gated channels
(Coburn and Bargmann, 1996
;
Komatsu et al., 1996
).
daf-11 gene, a mutation of which leads to the constitutive dauer
larva phenotype, encodes a transmembrane guanylate cyclase
(Birnby et al., 2000
). Although
no mutants are obtained, gcy-5, gcy-6 and gcy-7 encode
putative transmembrane guanylate cyclases expressed only in the right side ASE
(ASER) or the left side ASE (ASEL), and they are thought to function as
chemoreceptors (Yu et al.,
1997
). lim-6 encodes a LIM homeobox transcription factor
expressed in ASEL (Hobert et al.,
1999
). In lim-6 mutants, gcy-5 is expressed in
ASEL in addition to ASER (Hobert et al.,
1999
), and the functional asymmetry of ASE for discriminating
Na+ and Cl- is lost
(Pierce-Shimomura et al.,
2001
).
che-1 mutants were originally isolated as mutants defective in
chemotaxis to NaCl. They also show chemotaxis defects to water-soluble
attractants such as cAMP and biotin, but not to volatile odorants
(Bargmann et al., 1993;
Dusenbery, 1976
;
Dusenbery et al., 1975
;
Lewis and Hodgkin, 1977
).
che-1 mutants have no significant structural defects in ASE and the
other chemosensory neurons of the amphid in electron micrographs
(Lewis and Hodgkin, 1977
). The
che-1 gene is likely to affect the ASE function mediating chemotaxis.
We show here that the che-1 gene encodes a C2H2-type zinc-finger
protein similar to the GLASS transcription factor required for photoreceptor
cell differentiation in Drosophila
(Moses et al., 1989
) and that
che-1 is required for the identity of ASE neurons.
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MATERIALS AND METHODS |
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PR672 che-1(p672)=tax-5(p672) I, PR674 che-1(p674) I, PR679 che-1(p679) I, PR680 che-1(p680) I, PR692 che-1(p692) I, PR696 che-1(p696) I, CB1034 che-1(e1034) I and BC700 sDf4/bli-4(e937) dpy-14(e188) I.
kyIs5[ceh-23::gfp, lin-15(+)] IV
(Zellen et al., 1999)
(Forrester et al., 1998
) and
NW1229 dpy-20; evIs111[F25B3.3::gfp, dpy-20(+)]. OH811
otIs3[gcy-7::gfp], OH812 otIs114[lim-6::gfp, rol-6(d)] and
OH813 ntIs1[gcy-5::gfp] (O. Hobert, personal communication).
kyIs37[odr-10::gfp] (Sengupta
et al., 1996).
Chemotaxis assay
Chemotaxis assays to NaCl were performed essentially as described
(Dusenbery et al., 1975) but
with some modification. A radial concentration gradient of NaCl was
established by spotting 2 µl of 5 M NaCl to the center of a 9 cm plate
containing 8 ml of agar medium [2% agar, 0.25% Tween 20, 10 mM HEPES (pH
7.2)], and leaving the plates at room temperature for 12-16 hours. For assays,
the animals were placed on the surface of agar 1 cm distant from the periphery
and allowed to move freely for 1 hour.
Genetic mapping of che-1
bli-4(e937) dpy-14(e188) hermaphrodites were mated with
che-1(696) males. F1 hermaphrodites were then placed on separate
plates and allowed to self-fertilize for F2 progeny. Among the F2 animals,
recombinant animals (Bli non-Dpy, Dpy non-Bli) were selected. Homozygotes,
bli-4 + or + dpy-14, derived from the recombinants were
mated with che-1 males, and the non-Bli non-Dpy progeny were
subjected to chemotaxis assays.
Cloning of che-1
General molecular biology manipulations were carried out using standard
methods (Sambrook et al.,
1989). Cosmid DNA was prepared from 200 ml of a liquid culture of
E. coli at
0.2 of OD600 using QIAfilter Plasmid Maxi Kit
(Qiagen). PCR products for rescue experiments were amplified from wild-type N2
genomic DNA with a high accuracy polymerase, LA Taq polymerase (Takara) and
purified using QIAquick PCR purification kit (Qiagen).
Germline transformation (Mello et al.,
1991) was carried out by co-injecting test DNA at a concentration
of 5-50 ng/µl and marker DNA at a concentration of 5-100 ng/µl into the
gonads. Rescue experiments for identification of the che-1 gene were
performed by injecting a cosmid or a PCR product at 5-10 ng/µl together
with pTOCV01 GFP marker DNA (Oka et al.,
1997
) at 90-100 ng/µl into che-1 (p696). Transgenic
animals recognized by GFP expression were subjected to chemotaxis assays.
To identify molecular lesions in the che-1 alleles, the genomic sequence of che-1 was amplified from p672, p674, p679, p680, p692, p696 and e1034 mutants, and the products were directly sequenced.
che-1 cDNA was amplified from total RNA of a mixed-stage population of wild-type N2 by RT-PCR with a set of primers spanning the entire putative che-1 ORF and the product was directly sequenced. The PCR product was cloned into pGEM-T vector so as to be pche-1cDNA. The sequence of the cDNA was verified by sequencing.
Expression constructs and generation of transgenic animals
A che-1 promoter construct, pche-1p::gfp, was prepared by
amplifying 5.4 kb of che-1 upstream sequence of the predicted
initiation codon from the wild-type genome. A C-terminal tag construct,
pche-1::gfpC, was prepared by amplifying the 5.4 kb upstream sequence and the
entire coding region of che-1 from the wild-type genome. A
SphI site and a PstI site engineered into the PCR primers
were used to insert the amplified products into the GFP vector pPD95.77. Two
internal GFP tag constructs, pche-1::gfpBamHI and
pche-1::gfpBglII were constructed by inserting gfp fragments
amplified from pPD95.77 in frame into the BamHI site and the
BglII site of pche-1HindIII, a che-1 gene subclone
of 6.2kb HindIII fragment from ZC130 cosmid in pHSG398 (a
Cmr plasmid vector, TAKARA), respectively. For amplification by
PCR, LA-PCR kit (TAKARA) was used. The coding region of che-1 and gfp
originating from the PCR amplification in these constructs were verified by
sequencing.
pF55E10.7::gfp and pR13H7.2::gfp transcriptional gfp fusions were
constructed by ligating 4.5 kb and 4.3 kb of their upstream sequences
amplified by PCR between the PstI and BamHI sites of pGFP-TT
vector (Y. Jin, personal communication) and the SalI and
BamHI sites of pPD95.75 vector (A. Fire, S. Xu, J. Ahnn and G.
Seydoux, personal communication), respectively. ptax-2p::gfp was constructed
by ligating 2.0 kb of the tax-2 upstream sequence amplified by PCR
between the PstI and BamHI sites of pPD95.77. gcy-5::GFP,
gcy-6:: GFP and gcy-7:: GFP were gifts from D. Garbers
(Yu et al., 1997).
pgpa-10p::che-1 and pgpa-14p::che-1 expression constructs were prepared by
using pPD49.26 vector (A. Fire, S. Xu, J. Ahnn and G. Seydoux, personal
communication) as a backbone. The 3 kb SphI-SmaI fragment of
pgpa10p-gfp or pgpa14p-gfp (Murakami et
al., 2001) containing an upstream sequence of gpa-10 or gpa-14 and
a blunt-ended SacII-SacI fragment of pche-1 cDNA containing
a che-1 full-length cDNA were inserted between the SphI and
SmaI sites and at the blunt-ended KpnI site of pPD49.26,
respectively, through several subcloning steps.
Transgenic animals were obtained by germline transformation. These
constructs were injected at 5-50 ng/µl without or with injection markers,
pMTG25-4 (a flr-1::gfp expressed in the intestine)
(Take-Uchi et al., 1998) at 50
ng/µl or plin44-gfp (a lin-44::gfp construct expressed in ASI)
(Murakami et al., 2001
) at
33-50 ng/µl into wild-type N2 or PR679 che-1(p679) animals.
Once the transformant lines had been established, the animals were tested
for rescue of the che-1 defect or were observed under the fluorescent
microscope for the GFP expression. Cells that expressed GFP were identified by
positions mainly in L1 and L2 animals under Normarski optics. The position of
the cells in C. elegans has been described by Sulston et al.
(Sulston et al., 1983) and
White et al. (White et al.,
1986
).
Dye-filling assay
DiI stock solution was made by dissolving 2 mg DiI in 1 ml dimethly
formamide, and stored at -20°C. Worms on a growth plate were washed off
with M9 buffer into a test tube, washed at least twice with M9 buffer, and
then suspended in 400 µl M9 buffer, to which 2 µl of DiI solution was
added. The tube was shielded from the light with aluminum foil and incubated
for 2-3 hours at room temperature. After incubation, the animals were washed
with M9 at least three times, put onto a growth plate and cultivated
overnight. The animals were put on an agar pad containing 50 mM sodium azide
and observed with a fluorescence microscope (AxioPhoto2, Zeiss).
Dauer formation assay
About 10 adult hermaphrodites were allowed to lay eggs for 12-24 hours on a
6 cm NGM agar plate with E. coli OP50. After the parental animals had
been removed, plates were incubated for 2 days at 20°C. Then, dauer and
non-dauer (L3 to adult) F1 progeny with or without the injection marker were
counted, respectively.
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RESULTS |
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|
che-1 encodes a zinc-finger protein like GLASS transcription
factor
The genomic sequence and the sequence of che-1 cDNA amplified by
RT-PCR representing the entire che-1 coding region were determined.
The genomic organization of che-1 is shown in
Fig. 2B. The cDNA has a single
long open reading frame encoding a predicted protein product of 272 amino
acids (Fig. 2A).
|
Comparison of the amino acid sequence of CHE-1 to other sequences in the
databases indicated that CHE-1 has four C2H2 type zinc-finger motifs in the
C-terminal that are most similar to those of GLASS, a transcription factor for
photoreceptor differentiation in Drosophila
(Moses et al., 1989). GLASS
has five zinc-finger domains and the last three C-terminal zinc finger domains
alone are necessary and sufficient for DNA binding
(O'Neill et al., 1995
). The
first to the fourth zinc-finger domains of CHE-1 correspond to the second,
third, fourth and fifth zinc-finger domains of GLASS, respectively. These
combinations show higher similarities (71%, 71%, 68% and 57%) than any other
combinations (Fig. 2C). These
high similarities and the same order of corresponding zinc-finger domains
along the primary structure indicate that che-1 may be a C.
elegans counterpart of glass. The other regions of CHE-1 apart
from zinc-finger domains did not show a significant homology to any proteins
or motifs.
The genomic DNA, including all exons and introns of six che-1 alleles, was sequenced (Fig. 2A,B). The p672, p674, p679 and p680 mutations were found to be C to T transitions resulting in the conversion of Arg213 to a stop codon in the fifth exon of che-1. These mutations should result in a truncated protein with only the first zinc-finger domain intact but the second impaired, and the third and fourth lacking. The p692 and p696 mutations were G to A transitions resulting in the conversion of Gly263 to Arg in the last zinc-finger domain. This glycine is conserved between CHE-1 and GLASS. The e1034 mutation was a A to G transversion, resulting in the conversion of His268 to Pro. This histidine is an invariant residue among the C2H2-type zinc fingers, and one of the four residues to bind a zinc ion. All these mutations led to alterations of CHE-1 in the last three zinc-finger domains, which are known to be essential for DNA binding in GLASS. These results show that the last three zinc-finger domains are essential for CHE-1 activity, or for the potential to bind to DNA.
che-1 is expressed in the ASE chemosensory neurons
To determine where che-1 functions for chemotaxis behavior, four
gfp fusion genes were constructed
(Fig. 3A). Among them, only two
fusions showed gfp expression, as observed in ASE chemosensory
neurons and a few other neurons (Fig.
3B,C). The ASE was the only neuron class in which both of the
fusion genes showed constant and strong gfp expression. The ASEs are
the major neurons that mediate chemotaxis to water-soluble attractants such as
Na+, Cl-, biotin and cAMP, which is affected by
che-1 mutations (Bargmann and
Horvitz, 1991a). These results suggest that che-1
functions in the ASE neuron for chemotaxis. Neither of the fusion genes
rescued the chemotaxis defects of the che-1 mutants, although they
have the entire coding region and the promoter included in the original
genomic clone (pche-1-HindIII) that rescues chemotaxis defect
(Fig. 3A,F). The GFP insertion
may disrupt an unidentified functional domain or the GFP tag very close to the
last zinc-finger domain may perturb the DNA binding activity of CHE-1. As
these che-1::gfp constructs were expressed in the che-1
mutant (data not shown) as well as in the wild type animals, che-1 is
not required for expression of itself.
|
che-1 mutations affect the identity of ASE
In the che-1 mutants, ASE neurons were found at the normal
position and their morphology looked normal under DIC or fluorescent
microscopes. In addition, it was reported that in animals with a
che-1 mutation, e1034 or e1035, the cilia of ASE
had no significant defect in ultrastructure except for a mild alteration in
the patterning of cilia of ASE and other amphidial cells in the way their tips
were bundled distally within the channel formed by the sheath cell opening to
the outside of the worms (Lewis and
Hodgkin, 1977). Based on these results, CHE-1 was expected to be a
transcription factor mainly required for expression of genes involved in
ASE-specific functions for chemotaxis to water-soluble attractants after
morphological differentiation. To examine this idea further, expression of
such genes was determined using gfp reporter constructs
(Fig. 4;
Table 1). Two putative seven
transmembrane receptor genes, F55E10.7 and R13H7.2, were identified by the
C. elegans Sequence Consortium. Among the amphid neurons,
pF55E10.7::gfp transcriptional fusion construct was expressed in ASE, AFD and
AWC, and pR13H7.2::gfp fusion was expressed in ASE, AFD and ASJ in wild-type
animals (Fig. 4A,C). But in the
che-1 mutant, the expression of both these genes was lost
specifically in ASE neurons (Fig.
4B,D). gcy-5, gcy-6 and gcy-7, which are
membrane-spanning guanylate cyclase genes expressed specifically on the left
or right side ASE in wild-type animals (Yu
et al., 1997
) (Fig.
4E,G,I), lost their expression in the che-1 mutant
(Fig. 4F,H,J). tax-2,
a cyclic nucleotide-gated channel gene required for chemotaxis and thermotaxis
(Coburn and Bargmann, 1996
),
also lost expression specifically in ASE neurons of the che-1 mutant
(Fig. 4L). The loss of
tax-2 expression in the ASE neurons is thought to be sufficient to
cause chemotaxis defects in the che-1 mutant. Expression of a
homeobox gene ceh-23 (Wang et
al., 1993
) and a LIM-homeobox gene lim-6
(Hobert et al., 1999
) was also
lost specifically in ASE neurons (Fig.
4N,P). However, a pan-neuronal marker gene F25B3.3 encoding a
putative Ca2+-regulated Ras nucleotide exchange factor
(Altun-Gultekin et al., 2001
),
was expressed in the ASE neurons of the che-1 mutant
(Fig. 4R). These results are
consistent with the idea outlined above.
|
|
We found three kinds of mutations among the seven che-1 alleles, as described (Fig. 2B). p692, e1034 and p679, which represent each of the three kinds of mutations were examined to see whether there are differences in effect on the expression of two ASE specific marker genes (gcy-5 and gcy-6). We did not find any difference (Table 2). We did not find any difference in the defect of chemotaxis to NaCl among these alleles, either (data not shown). Therefore, these mutations are expected to lack the function of CHE-1 to the same extent.
|
Wild-type animals take up a fluorescent dye such as DiI into AWB, ASH, ASJ,
ASK, ADL and ASI amphid neurons, PHA and PHB phasmid neurons, but not into ASE
(Hedgecock et al., 1985;
Starich et al., 1995
).
che-1 mutants take DiI into ASE as well
(Fig. 4S,T and
Table 3). DiI is taken by the
sensory cilia exposed to the outside through amphid or phasmid pores. But not
all such cilia take up the dye in the wild type. Therefore, a specific
mechanism should be involved in the dye filling, although little is known for
such a mechanism. This unusual dye uptake into ASE in the che-1
mutants suggests that a character of ASE cilia changed in the presence of a
che-1 mutation.
|
Ectopic expression of che-1 induces misexpression of
ASE-specific marker genes, dye-filling defect and dauer formation
To determine whether che-1 is sufficient for ASE fate
specification, che-1 cDNA was ectopically expressed using
gpa-10 or gpa-14 promoters
(Jansen et al., 1999). The
gpa-10 promoter drives expression in ADF, ASI and ASJ amphid neurons,
in ALN, CAN, LUA neurons and spermatheca. The gpa-14 promoter drives
expression in ASI, ASJ, ASH, ASK amphid neurons, PHA, PHB, ADE, ALA, AVA, CAN,
DVA, PVQ, RIA neurons, and in vulval muscles. gcy-5, 6 and
7::gfp reporters and a gpa-10p::che-1 or gpa-14::che-1
cDNA construct were injected into wild-type animals
(Fig. 5B,D). As negative
controls, promoter-only constructs, in which che-1 cDNA was removed
from gpa-10p::che-1 cDNA or gpa-14::che-1 cDNA construct,
were injected with the gcy::gfp reporters
(Fig. 5A,C). In the transgenic
animals with gpa-10p::che-1 cDNA
(Fig. 5B), GFP fluorescence was
observed not only in ASE but also constantly in ADF and occasionally in ASJ
and ASI. In the transgenic animals with gpa-14p::che-1 cDNA, GFP
fluorescence was observed constantly in PHA and PHB, and often or occasionally
in ASI, ASJ, ASK, AVA, ADE and RIA (Fig.
5D). However, in transgenic animals with the promoter-only
constructs, no ectopic gfp expression was observed out of 16 lines
for the gpa-10 promoter-only construct and 15 lines for the
gpa-14 promoter-only construct
(Fig. 5A,C). Because, in these
experiments, the che-1 cDNA expression construct and the GFP reporter
constructs were in the same extrachromosomal array, it may be possible that
the ectopic gfp expression was due to an artificial interaction of
these constructs within the same extrachromosomal array. To examine this
possibility, we used two integrated strains, OH811 otIs3[gcy-7::gfp]
and OH813 ntIs1[gcy-5::gfp]. gpa-10p::che-1 cDNA construct was
introduced into OH811 animals as an extrachromosomal array. In this transgenic
line, ectopic gfp expression was observed in cells likely to be
amphid neurons (six cells in 21% animals, five in 25%, four in 19%, three in
19%, two in 17% and one in 4% out of the 48 animals observed), although cell
identification was impossible under Nomarski optics because of abnormal cell
positions in OH811 strain. Next, the extrachromosomal array was transferred to
the wild-type N2 strain, then to OH813. In the wild-type background, no
ectopic gfp expression was observed, while in OH813 background,
ectopic gfp expression was observed in the cells in which
che-1 cDNA expression was expected to be driven by the
gpa-10 promoter (Table
4). These results indicate that ectopic expression of
che-1 is sufficient to confer ASE-specific gene expression on several
classes of neurons.
|
|
Furthermore, to examine whether ectopic expression of che-1 cDNA provides another ASE property, namely inability to take up DiI, transgenic animals shown in Table 5 were observed for their Dil filling and gfp expression. As shown in Table 5, significant proportions of cells in which che-1 cDNA is expected to be expressed showed dye-filling defect, whereas almost all of these cells took up DiI when the promoter only (empty) constructs were introduced. Thus, che-1 cDNA expression changes the property of these cells not to take up DiI as is observed in ASE.
|
gpa-10 and gpa-14 promoters drive expression in ASI and
ASG (gpa-10 promoter), and ASI (gpa-14 promoter) that are
known to be involved in inhibition of dauer formation under favorable
conditions (Bargmann and Horvitz,
1991b). Therefore, we examined dauer formation of transgenic lines
in which che-1 cDNA was ectopically expressed by each of these
promoters (Table 6). In fact, a
significant proportion of the animals formed dauers. This result suggests that
the dauer-inhibiting function of ASI and ASG is perturbed by che-1
ectopic expression in these cells.
|
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DISCUSSION |
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How does che-1 affects the ASE character? In che-1
mutants, expression of neuron subtype-specific gfp-marker genes that
we examined were affected specifically in ASE but not in others. In addition,
the number and position of ASE neurons, and ASJ, AUA, AWB and ADF, which are
closely related to ASE in the cell lineage
(Sulston et al., 1983), and
the other amphid neurons were normal. These findings indicate that
che-1 mutations do not cause alterations of the cell lineage. As ASE
neurons of che-1 mutants took up DiI, it is possible that
che-1 mutations alter ASE to another cell type which takes DiI, such
as AWB, ASH, ASI, ASJ, ASK and ADL. However, ASH, ASI, ASJ and ADL can be
excluded from such candidate cell types, because R13H7.2::gfp,
tax-2::gfp and ceh-23::gfp expression were lost in ASE but not
affected in ASH, ASI, ASJ and ADL in the che-1 mutant (ASI is not
visible in Fig. 4L, and ADL and
ASI are not visible in Fig. 4N,
because they are out of the focus). In addition, ASK and AWB can be excluded,
because srg-8::gfp (used as an ASK marker gene)
(Troemel et al., 1995
),
lim-4::gfp (used as an AWB marker gene)
(Sagasti et al., 1999
) and
odr-3::gfp (used as an AWB and AWC marker gene)
(Roayaie et al., 1998
) were
not expressed in ASE of a che-1 mutant
(Table 7). Furthermore, there
is another reason for exclusion of AWB and ADL from such candidates. In the
che-1 mutants: ASE has a single, long and slender cilium that is
identical to that in wild type and very different from those of AWB and ADL.
An AWA marker odr-10::gfp was not expressed in ASE of a
che-1 mutant (Table
7). ADF does not take up DiI and its cilium morphology is
different from that of ASE. Therefore, given the above discussion, it is
difficult to consider that CHE-1 represses ASE fate and that che-1
mutations cause a homeotic change of ASE to another cell type of amphid
neurons. These results, together with the findings that che-1 was
expressed in ASE and that ectopic expression of CHE-1 was sufficient to confer
ASE-specific gene expression onto several classes of neurons, indicate that
che-1 is a positive regulator capable of inducing a gene expression
sequence in ASE neurons after their development to amphid neurons.
|
In development of a particular cell, the process may be divided into two
parts, the first is responsible for determination of the basic cell fate and
the second for terminal differentiation, such as expression of cell-specific
genes and functions. che-1 is likely to act at the point that links
the first and second parts of the development of the ASE neurons, because a
che-1 mutation leads to loss of the second part. mec-3 and
ttx-3 genes are likely to be in the same category, although
mec-3 and ttx-3 mutations cause some morphological change in
touch receptor neurons and AIY neurons, respectively. They encode LIM
homeodomain proteins, which are required for expression of the final
differentiated features in touch receptor neurons and AIY interneurons,
respectively (Way and Chalfie,
1988; Mitani et al.,
1993
; Hobert et al.,
1997
; Altun-Gultekin et al.,
2001
). In this category, a mutation causes loss of identity of a
specific neuron class without alteration of cell lineages. odr-7,
ttx-1 and lim-4 may also be in this category, although they have
an additional character. odr-7, ttx-1 and lim-4, which
encode a nuclear receptor superfamily member, an OTD/OTX homeodomain protein
and a LIM homeodomain protein, are required for expression of cell specific
features in AWA olfactory neurons, AFD thermosensory neurons and AWB olfactory
neurons, respectively (Sengupta et al.,
1994
; Sengupta et al.,
1996
; Sagasti et al.,
1999
; Satterlee et al.,
2001
). However, mutations in these genes brought about some
expression of AWC olfactory neuron fate in AWA, AFD and AWB
(Sagasti et al., 1999
;
Satterlee et al., 2001
). This
phenotype could be crucial for putting these genes into another category,
which involves another hierarchy of cell specification process. Alternatively,
if a default cell fate of AWA, AFD and AWB is AWC, odr-7, ttx-1 and
lim-4 genes should be in the same category as che-1, mec-3
and ttx-3.
che-1 and glass genes were shown to encode highly
homologous zinc-finger proteins and to be required for differentiation of ASE
chemosensory neurons in C. elegans and the photoreceptor cell in
Drosophila, respectively. This conservation in both structure and
function suggests that che-1 and glass are homologous in
evolution, and that ASE chemosensory neurons of C. elegans and the
photoreceptor cells of Drosophila may be counterparts in the
evolution of these two species. AFD thermosensory neurons in C.
elegans are thought to have an evolutionary relationship to photoreceptor
cells of a vertebrate (Satterlee et al.,
2001). Therefore, ASE and AFD in C. elegans and
photoreceptors cells in vertebrates and invertebrates might have a close
relation in evolution.
To reveal developmental processes of ASE, che-1 is a good starting point from which to trace the process by asking how che-1 gene expression is regulated and what genes are targets of CHE-1.
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ACKNOWLEDGMENTS |
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Footnotes |
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REFERENCES |
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Altun-Gultekin, Z., Andachi, Y., Tsalik, E. L., Pilgrim, D.,
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