Develop. Growth Differ. (2001) 43, 693–698

Cloning and characterization of the Xenopus laevis
p8 geneToshime Igarashi,1 Hiroki Kuroda,1 Shuji Takahashi2 andMakoto Asashima1,2*
1

Department of Life Sciences (Biology) and 2CREST Project, Graduate School of Arts and Sciences,
University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, 153-8902, Japan.

The p8 gene encodes a transcription factor with a basic helix-loop-helix motif and has been cloned in rat, mouseand human. It is upregulated in acute pancreatitis. In the present study, the Xenopus laevis homolog of p8(Xp8) was isolated by PCR. The full-length Xp8 cDNA consists of 677 bp and encodes 82 amino acids. The
basic helix-loop-helix region is well conserved between X. laevis and mammals. Overexpression of an
Xp8-EGFP fusion protein indicated that Xp8 was localized to nucleus, as in mammals. Analysis by RT–PCR
showed that expression of Xp8 was increased after gastrulation and maintained into later developmental stages.
Expression was detected in the prospective neural region at the neurula stage by whole-mount in situ hybridization. At the larval stage, Xp8 was expressed in central nervous regions, such as ventrolateral brain, trigeminal
nerve, vestibulocochlear nerve and dorsal neural tube. Expression was also detected in the basal cement gland,
olfactory placode, ear vesicle, notochord and anus. This specific pattern of expression suggests that Xp8 may
play a role in the development of the central nervous system in X. laevis.
Key words: basic helix-loop-helix, development, nerve, p8, Xenopus.

Introduction
p8 was first cloned from a rat pancreatic cDNA libraryas a gene that was upregulated in pancreatic acinar
cells during the acute phase of pancreatitis (Malloet al. 1997). Rat p8 encodes an 80-amino acidpolypeptide with a basic helix-loop-helix (bHLH)
motif and shows homology to several homeotic genes(Mallo et al. 1997). p8 homologs have subsequently
been cloned and sequenced in human and mouse(Vasseur et al. 1999a,b). Human p8 (hp8) has DNAbinding activity, which is increased when the protein
is phosphorylated (Encinar et al. 2001). hp8 also
has sequence similarity to the transcription factor
HMG-I/Y (Bustin & Reeves 1996; Encinar et al. 2001).
Analysis of the mouse p8 promoter has identified
putative transfactor binding sites (Vasseur et al. 1999b).
In adult, p8 is expressed in various normal organs, suchas the pancreas, lung, liver and gut (Mallo et al. 1997;
Vasseur et al. 1999a), and is upregulated after injury
and during regeneration, and in pancreatic cancer (Suet al. 2001). p8 is also structurally similar to candidate*Author to whom all correspondence should be addressed.
Email: asashi@bio.c.u-tokyo.ac.jp
Received 27 April 2001; revised 28 May 2001; accepted
26 July 2001.

of metastasis-1, a novel factor in human breast cancer(Ree et al. 1999).
In a search for novel genes expressed in theX. laevis embryo using degenerate oligonucleotide
primers ‘GRRMFP’ and ‘VTAYQN’, complementary to a
DNA-binding sequence (Hayata et al. 1999), we isolated a clone with a bHLH motif, which was sequenced
and found to be the X. laevis homolog of p8. We
obtained a full-length clone and characterized its
expression pattern in early X. laevis development.

Materials and Methods
Embryos
Xenopus laevis eggs were obtained from females injected with 400 units of human chorionic gonadotropin(Gestron; Denka Seiyaku Co., Kawasaki, Japan)
and were fertilized in vitro with minced testis, then cultured in 1  Steinberg’s solution (58 mM NaCl, 0.67 mM
KCl, 0.34 mM Ca(NO3)2, 0.83 mM MgSO4, 100 mg/L
kanamycin sulfate and 5 mM Tris-HCl (pH 7.4)). The
X. laevis embryos were staged according to Nieuwkoop
and Faber (1967). The jelly coats were removed with
3% cysteine hydrochloride in 1  Steinberg’s solution(pH 7.8) and the vitelline membranes were removed
manually with fine forceps. All operations were carriedout under sterile conditions.

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Cloning of full-length X. laevis p8 cDNA
A phagemid library containing approximately 2  106
p.f.u. was prepared from X. laevis larvae cultured for
3 days. To isolate a full-length X. laevis p8 cDNA, weused a modified polymerase chain reaction (PCR)
method. Phage were eluted from NZY medium to
SM buffer (100 mM NaCl, 10 mM MgSO4-H2O, 0.01%
gelatin, 50 mM Tris-HCl, pH 7.5) and then positive
pools were identified by PCR with primers 5-AACGTCGTACATTGAGGCC-3 (forward) and 5-CTTCTTCTTGCACTCGCTGC-3 (reverse) designed against theoriginal Xp8 partial clone. In the first round, phage wereplated on 15 NZY plates at 5  104 p.f.u./plate, cultured
and screened by PCR. Second-round screening
involved the transfer of soft agar from the positive plate
to 16 tubes of SM buffer (500 µL) with sterile wooden
toothpicks, to give less than 5  103 p.f.u./tube. Tubes
were screened by PCR, then a 1 µL aliquot was takenfrom a positive tube and diluted 1  10–3, 1  10–4 and1  10–5 and 100 µL of each dilution was plated onto
NZY plates. A plate with approximately 5  103 p.f.u.
was screened in a third round, and three further roundsof screening were needed to obtain a single positiveplaque.

Overexpression of enhanced green fluorescentprotein
The Xp8 coding region linked to enhanced green
fluorescent protein (EGFP) or only EGFP was inserted
into the pCS2 plasmid vector (Fig. 3a). Xp8-EGFP and
EGFP transcripts were generated in vitro and 0.1–1 ng
of mRNA was injected into the animal pole of each
blastomere at the 2-cell stage and cultured. The putative ectoderm region, called the animal cap, was cut
away at stage 9 and immediately observed under afluorescent microscope.

Fig. 1. Sequence of Xenopus
laevis p8 cDNA and the deduced
peptide sequence. The polyadenylation site AATAAA is underlined. A full-length Xp8 clone
was obtained by a modified
PCR method. The Xp8 cDNA is677 bp in length and encodes
82 amino acids, including abasic helix-loop-helix motif. It has
246 bp in the ORF, 114 bp in the3 UTR and 317 bp in the 5 UTR.
Fig. 2. Comparisons of p8 peptide sequences among Xenopus
laevis, human, mouse and rat.
(a) Alignments of Xp8 and thethree mammalian p8 peptides.
Conserved amino acids arehighlighted. A putative basic
helix-loop-helix (bHLH) region is
illustrated under the sequence.
The region defined by arrows
indicates a nuclear targeting
sequence (Mallo et al. 1997). (b)Full-length amino acid sequence
identity between the four species.
(c) Sequence identity within the p8
bHLH regions of the four species.
Comparison of the full-length and bHLH regions indicates that only the bHLH region is well conserved. Arrowheads show conserved
serine, threonine and tyrosine residues, which may be targets for phosphorylation.

Cloning of the Xenopus p8 gene

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Fig. 3. Intracellular localizationof the Xp8. (a) Xp8-EGFP construct. An Xp8-EGFP or EGFP-onlysequence was ligated into thepCS2 plasmid vector. Enhanced
green fluorescent protein (EGFP)
or Xp8-EGFP transcripts were
injected into the animal pole at the
2-cell stage. The enclosed regionsin (b) and (e) are enlarged in (c)and (f), respectively. Animal cap
cells injected with 2 ng EGFP
had fluorescence throughout thecytoplasm (b,c), whereas Xp8EGFP was specifically localized to
the nucleus (e,f). Arrows show
the nucleus. Injected embryos
were cultured to stage 38 andobserved. Larvae injected with
0.2 ng EGFP mRNA or 2 ng
Xp8-EGFP mRNA are shown in(d) and (g), respectively. (d) The
fluorescence remained strongin whole embryo. (g) Only low
level fluorescence was observedon the abdominal surface.

Whole-mount in situ hybridization
Whole-mount in situ hybridization was performed
according to standard methods (Harland 1991) on
X. laevis albino embryos using a full-length Xp8 cDNA
as a template to synthesize RNA probe. The RNA probe(200 ng) was added to 1 mL hybridization buffer.
Hybridized embryos were stained with BM purple atroom temperature for 3 h and at 4°C overnight.
Embryos were refixed with Bouin’s fluid, dehydrated
in ethanol series, replaced in xylene, embedded inparaffin and sectioned at 12 µm.
Fig. 4. Reverse transcription–polymerase chain reaction analysis of Xp8 expression. Expression of Xp8 mRNA was increased
from stages 12 to 15 and was maintained until stage 40. Maternal
Xp8 mRNA was detected at very low levels.

Reverse transcription–polymerase chain reaction
Reverse transcription–polymerase chain reaction (RT–
PCR) was used to detect Xp8 mRNA. The following
primers were used: Xp8 5-AGACGGACCAAGAGAACTGG-3 (forward) and 5-CAACTGATTCTCACATGCAGC-3 (reverse); ODC 5-GTCAATGATGGAGTGTATGGATC-3 (forward) and 5-TCCATTCCCTCTCCTGAGCAC-3 (reverse). Both genes were amplified using
an annealing temperature of 56°C for 30 cycles.

Results
Cloning of the X. laevis p8 gene
A full-length Xp8 clone comprising 677 nucleotides
was isolated from a 3-day-old X. laevis embryo cDNA
library using a modified PCR method. Xp8 is predicted
to encode an 82 amino acid polypeptide with abHLH motif (Fig. 1). A putative polyadenylation signal(AATAAA) is present at 20 bp upstream of the poly(A)extension. There were 114 nucleotides and 317
nucleotides in the 5 and 3 untranslated regions,
respectively. Comparison of the X. laevis p8 aminoacid sequence with that of rat, mouse, and human(Fig. 2) showed a 38–39% similarity to the mammaliansequences (Fig. 2b). The bHLH motif in Xp8 (Fig. 2a,

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T. Igarashi et al.

Mallo et al. 1997) had 75% similarity with the mammalian p8 sequences (Fig. 2c). In addition, the candidate phosphorylated amino acids, serine, threonine
and tyrosine, were well conserved in all species(Fig. 2a).

Intracellular localization of Xp8
Animal cap cells were injected with Xp8-EGFP mRNAand observed under fluorescent microscope. Xp8EGFP fluorescence was stronger in the nucleus thanin the cytoplasm (Fig. 3f), suggesting that Xp8 is localized in the nucleus. Some injected embryos were

cultured to stage 38. Although strong EGFP fluorescence was maintained to this stage (Fig. 3d), Xp8-EGFP
fluorescence was weak (Fig. 3g).

Pattern of Xp8 expression in early development
Analysis by RT–PCR was used to detect Xp8 mRNA
expression in the developing embryo (Fig. 4). The
expression of Xp8 increased from stage 12 to stage
15 and then remained at a steady level until stage
40.
Whole-mount in situ hybridization was performed todetermine the spatial pattern of Xp8 expression in

Fig. 5. Whole-mount in situ
hybridization. Expression patternof Xp8 at stages (a, b) 14, (c, d)
18, (e, f) 22 and (g, h) 30. (a,c,e)Dorsal (D) view, (d,f–h) lateralview, (b) ventroposterior view.
The anterior (A) is situated on
the right side in all parts except(b). Arrowheads show the central
nervous system. V, ventral; P,
posterior; AN, anus; NC, notochord; CG, cement gland; VN,
vestibulocochlear nerve; TN,trigeminal nerve; OP, olfactory
placode.

Cloning of the Xenopus p8 genedeveloping embryos (Figs 5,6). Xp8 expression was
detected in the middle body of the prospective neuralregion at stage 14 (Fig. 5a,b), where it was maintained
to stage 18 (Fig. 5c,d). Expression in the prospective
neural region extended to the anteroposterior area andbranched in the brain area at stage 22 (Fig. 5e,f). The
neural tube showed patchy signals, which were thought
to be along the somites. At stage 30, signals were
present in the central nervous regions, including theventrolateral brain (Fig. 6B,C), vestibulocochlear nerve(Fig. 5h), trigeminal nerve (Fig. 5h) and dorsal neuraltube (Fig. 5g). Expression was also detected in thebasal cement gland, olfactory placode, notochord andanus (Fig. 5g). Xp8 expression was detected in the
notochord at stage 30, but thereafter moved anteriorly
and then disappeared (data not shown). Embryos
were sectioned (Fig. 6) and expression was detected
throughout the brain and eye, but was particularly
strong in the dorsal and ventrolateral brain and dorsal
and ventral retina (Fig. 6B). The dorsal ear vesicle
adjacent to the hindbrain also showed a strong signal(Fig. 6C). In the bulbar region, Xp8 was expressed in
the dorsal and ventrolateral neural tube (Fig. 6D).

Discussion
This is the first report of a non-mammalian p8 gene.
Xp8 is a small gene with a full-length sequence of only
677 bp, encoding 82 amino acids. The Xp8 product,consistent with other p8 proteins, has no signal peptide or transmembrane regions, but contains a bHLH
motif. The sequence of this bHLH region showedsignificant similarity between X. laevis and mammals

Fig. 6. After whole-mount in situ
hybridization, embryos wereembedded in paraffin and sectioned at 12 µm. The larva shown(stage 30) was sectioned at lines
A–D and sections are shown in(A–D). CG, cement gland; Re,retina; Ev, ear vesicle; Nt, neural
tube.

697

(Fig. 2c) and, given that Xp8 is localized to the nucleus,
it is likely to be a transcription factor. The DNAbinding activity of human p8 is increased with phosphorylation (Encinar et al. 2001) and, while we could
not identify any putative phosphorylation sites on Xp8,
there were a couple of well-conserved amino acids
outside the bHLH region (Fig. 2a) that may be targets
for phosphorylation.
The Xp8 clone reported here was isolated by a modified PCR screening method. This method has several
advantages over standard filter hybridization methods.
Screening time is significantly reduced: it took only
48 h to isolate one positive plaque by this method. The
method is also technically simple, with no requirement
for radiolabeled or fluorescent probes or hybridization
filters. In addition, the rate of signal to noise is very high,decreasing the rate of false positives.
Analysis by RT–PCR showed that Xp8 expression
was increased after gastrulation and persisted tostage 40 (Fig. 4). Reports of p8 expression in rat (Malloet al. 1997) and human adult organs (Vasseur et al.
1999a) have suggested that Xp8 may be expressed in
endoderm during development and in adult X. laevis.
However, whole-mount in situ hybridization (Figs 5,6)
showed that Xp8 is expressed in the prospective neuralregion in X. laevis embryos, and then in limited regionsof nerves. These results suggest that Xp8 may have different functions in development and in the adult, as has
been seen for the Pax genes, which also have a HLH
motif (Dohrmann et al. 2000).
Injection of Xp8-EGFP mRNA into 2-cell stageembryos led to very minimal fluorescence in thelarva. Microinjection of Xp8 mRNA into X. laevis

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embryos led to no identifiable mutant phenotype. The
pattern of Xp8 expression in the embryo suggested
a possible role in neural development, but otherexperimental strategies, such as loss-of-function or
dominant-negative approaches may be needed tofurther clarify the role of this gene in X. laevis
development.

Acknowledgments
This work was supported by grants from the Ministryof Education, Sciences, Sports and Culture, Japan, andby CREST (Core Research for Evolution Science andTechnology) of the Japan Science and Technology
Corporation.

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