LARK activates posttranscriptional expression ofan essential mammalian clock protein, PERIOD1
Shihoko Kojima*†, Ken Matsumoto*, Matsumi Hirose*, Miyuki Shimada*, Mamoru Nagano‡, Yasufumi Shigeyoshi‡,Shin-ichi Hoshino§, Kumiko Ui-Tei¶, Kaoru Saigo¶, Carla B. Green†, Yoshiyuki Sakaki储, and Hajime Tei*,***Laboratory of Chronogenomics, Mitsubishi Kagaku Institute of Life Sciences, 11 Minamiooya, Machida, Tokyo 194-8511, Japan; ‡Department of Anatomy
and Neurobiology, Kinki University School of Medicine, 377-2 Ohno-Higashi, Osaka-Sayama, Osaka 589-8511, Japan; §Graduate School of Pharmaceutical
Sciences, Nagoya City University, 3-1 Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan; ¶Department of Biophysics and Biochemistry, Graduate School of
Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan; †Department of Biology, University of Virginia, Charlottesville, VA 22904-4328; and储Genomic Science Center, RIKEN, The Institute of Physical and Chemical Research, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045, JapanThe mammalian molecular clock is composed of feedback loops tokeep circadian 24-h rhythms. Although much focus has been on
transcriptional regulation, it is clear that posttranscriptional controls
also play important roles in molecular circadian clocks. In this study,
we found that mouse LARK (mLARK), an RNA binding protein,
activates the posttranscriptional expression of the mouse Period1(mPer1) mRNA. A strong circadian cycling of the mLARK protein isobserved in the suprachiasmatic nuclei with a phase similar to that of
mPER1, although the level of the Lark transcripts are not rhythmic. We
demonstrate that LARK causes increased mPER1 protein levels, most
likely through translational regulation and that the LARK1 protein
binds directly to a cis element in the 3ⴕ UTR of the mPer1 mRNA.
Alterations of mLark expression in cycling cells caused significant
changes in circadian period, with mLark knockdown by siRNA resulting in a shorter circadian period, and the overexpression of mLARK1
resulting in a lengthened period. These data indicate that mLARKs arenovel posttranscriptional regulators of mammalian circadian clocks.
circadian rhythms 兩 posttranscriptional regulation 兩 RNA binding protein 兩3⬘untranslated region 兩 suprachiasmatic nucleus

C

ircadian rhythms are generated by endogenous clocks, and theprincipal circadian pacemaker is located in the suprachiasmatic
nucleus (SCN) of the hypothalamus in mammals (1, 2). Circadianclocks regulate the daily fluctuations of biochemical, physiological,
and behavioral rhythms, and these rhythms persist under constantconditions and are entrained by the environmental light–dark (LD)cycles (3, 4). In mammals, the molecular circadian clock is comprised of interlocking transcriptional-translational feedback loops
containing several regulatory steps, transcriptional, posttranslational, and protein–protein interactions (reviewed in ref. 5). These
regulatory steps must be coordinated and orchestrated properly forthe fine-tuning of the 24-h periodicity. Although less well understood, it is clear that posttranscriptional regulation also plays a very
important role, contributing to the phase and amplitude of rhythmicmRNA expression in many organisms (reviewed in ref. 6).
One of several clock genes in mammals, Period1 (Per1) was
originally identified as a structural homologue of the Drosophila
circadian clock gene per (7). The transcription of Per1 is activatedby the CLOCK–BMAL1 heterodimer (8, 9) and repressed by acomplex containing PER and cryptochrome (CRY) proteins (10),thus comprising one of the core feedback loops. The molecularfunction of mPER1 is not yet clarified, but mPer1 is an essential
gene for maintenance of circadian rhythms, because loss of mPer1
in knockout mice results in an altered period (11–13). mPER1 isalso thought to be involved in resetting of the circadian oscillator(14). mPer1 expression is rhythmic, but the phase of the proteinrhythm is delayed 6–8 h relative to that of the mRNA in the mouse
SCN (15), indicating that mPER1 expression is regulated posttranscriptionally. A similar 6- to 8-h time lag between the expression of
Drosophila per (dper) mRNA and dPER protein has also been
observed (16), suggesting that these time lags are important evolutionarily conserved aspects of the clock mechanism.

www.pnas.org兾cgi兾doi兾10.1073兾pnas.0607567104

Posttranscriptional regulation of mRNA stability and translational efficiency are often mediated by cis elements in mRNAs thatinteract with trans-acting factors such as RNA-binding proteins
and/or microRNAs. In most cases, these cis elements reside in the3⬘ UTR, and several 3⬘ UTR motifs have been identified that are
critical for mRNA splicing, transport, stability, localization, andtranslation (17). Mouse Per1 (mPer1) 3⬘ UTR likely has an important regulatory role, because the human Per1 and mPer1 3⬘ UTRs
have high homology (78.0%) (18). In addition, two studies haveshown that mPer1 is regulated posttranscriptionally via its 3⬘ UTR,
but little is known about the mechanisms (18, 19). In Drosophila,
dper mRNA half-life also appears to be regulated by the circadianclock, resulting in different message stability during accumulating
and decay phases (20). Also, the circadian cycling of dPER levels
depends on the 3⬘ UTR of the dper mRNA (21).
Here, we identified an RNA-binding protein, called LARK, thatinteracts with the mPer1 3⬘ UTR and regulates mPer1 expression ina posttranscriptional manner. Alteration of mouse LARK(mLARK) expression resulted in changes in the circadian period.
Thus, we propose that mLARK is a novel posttranscriptional
regulator of the circadian clock.
Results
Because our previous work had shown that mPer1 expression was
regulated posttranscriptionally via sequences within its 3⬘ UTR(18), we began to investigate the potential trans-acting factors that
could be responsible for this regulation. At the time we began thesestudies the only protein that contained putative RNA-binding
motifs that had been tied to circadian rhythms was Drosophila
LARK (dLARK) (22–25). Using a candidate approach, we decided
to investigate a possible role for mammalian Lark homologs inposttranscriptional regulation of mPer1.
Circadian Expression of mLarks in the SCN. In our analysis, mouse

Rbm4a (mRbm4a) and Rbm4b (mRbm4b) encode polypeptides highly homologous to the N-terminal region of dLARK
that contain three RNA binding domains: two RNA recogniAuthor contributions: H.T. designed research; S.K., K.M., M.H., M.S., M.N., Y. Shigeyoshi,
and H.T. performed research; M.N., S.-i.H., K.U.-T., and K.S. contributed new reagents/
analytic tools; S.K., K.M., M.N., Y. Shigeyoshi, C.B.G., Y. Sakaki, and H.T. analyzed data; andS.K., C.B.G., and H.T. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Abbreviations: Per, Period; mPer, mouse Per; SCN, suprachiasmatic nucleus; DD, constantdark; LD, light– dark; CT, circadian time; ZT, Zeitgeber time; IRES, internal ribosome entry
site; Cry, cryptochrome; dper, Drosophila per; mLARK, mouse LARK; dLARK, Drosophila
LARK; mRbm4, mouse Rbm4; IR, immunoreactive; RMSA, RNA mobility-shift assay; F-luc,firefly luciferase; R-luc, Renilla luciferase.
**To whom correspondence should be addressed. E-mail: tei@mitils.jp.
This article contains supporting information online at www.pnas.org/cgi/content/full/
0607567104/DC1.
© 2007 by The National Academy of Sciences of the USA

PNAS 兩 February 6, 2007 兩 vol. 104 兩 no. 6 兩 1859 –1864

CELL BIOLOGY

Edited by Joseph S. Takahashi, Northwestern University, Evanston, IL, and approved December 5, 2006 (received for review September 1, 2006)

Fig. 1. Expression profiles of mLarks in mouse SCN. (A) (Left) Representative in situ hybridization results for mLark1 and mLark2 mRNAs in mouse SCN harvested at
different times of day from mice housed in either LD or DD conditions are shown. (Right) The graph shows mean quantification of mLark1 (䊐, LD; ■, DD) and mLark2(䊐, LD; F, DD) mRNAs. (B) (Left) Representative in situ hybridization results for mLark1 and mLark2 mRNAs in SCN harvested from mice after 600 lux of light exposurefor 30 min are shown. (Right) The graph shows mean quantitation of mLark1 (䊐) and mLark2 (■) mRNAs. (C) Representative immunohistochemical analysis of mLARKs
in mouse SCN (Left) in DD and quantitation of mLARK-IR cells (Right) are shown. One-way ANOVA revealed a highly significant (P ⬍ 0.0001) time effect. (D) Western
blot analysis of mLARK (Left) in extracts of microdissected mouse SCNs harvested in DD and quantification of mLARK levels (Right) are shown. One-way ANOVA revealed
a highly significant effect of time (P ⬍ 0.001). All data are representative of at least three independent experiments.

tion motifs and a CCHC-type zinc finger motif (22–27)[supporting information (SI) Fig. 7A]. The amino acid sequence identity between the protein encoded by mRbm4a andmRbm4b is 86% over the entire protein sequence. Both Rbm4aand Rbm4b were strongly expressed in the heart, brain, spleen,lung, liver, kidney, and testis, but not in the skeletal muscle (SIFig. 7B) and are independent genes located in adjacent regions
on both human and mouse chromosomes 11q13 (26) and 19A,
respectively. Therefore, we concluded that Lark1 and Lark2
are more suitable nomenclatures than Rbm4a and Rbm4b, andthey are used in this study. It has been shown that human
LARK1 and LARK2 (RBM4A and RBM4B) modulate alternative splicing; however, it is not clear whether this is the casefor mLARKS (27).
To determine the time dependence of mLark1 and mLark2
mRNA expression in the SCN, their levels were examined by in situ
hybridization. cRNA probes were designed to each 3⬘ UTR todistinguish the expression of the two mRNAs, because the nucleotide sequences of the coding regions of mRbm4a and mRbm4b arequite similar. Strong signals for both mLark1 and mLark2 weredetected in the mouse SCN, but neither mLark1 nor mLark2
mRNA showed distinct circadian fluctuation in the mouse SCN
under either LD or constant dark (DD) conditions (Fig. 1A).
Zeitgeber time (ZT) 0 is light onset, and ZT12 is light offset in a LD
cycle, whereas circadian time (CT) 0 indicates the beginning of
subjective day, and CT12 is the beginning of subjective night. Unlike
Per1 and Per2 transcripts that are rapidly induced by a short
exposure to light (28), neither mLark1 nor mLark2 transcripts
showed a response (Fig. 1B).
Next, we examined the mLARK protein in the SCN by using anantibody that recognizes both mLARK1 and mLARK2 (SI Fig. 8).
mLARK-immunoreactive (IR) cells were detected in the SCN, andthe number of mLARK-IR cells were rhythmic in DD conditions,with a peak at CT12 and a trough at CT4 (Fig. 1C). We also
1860 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0607567104

confirmed the expression levels of mLARKs under DD conditions
by Western blot analysis using proteins extracted from microdissected SCNs (Fig. 1D). The anti-LARK antibody detected two
major bands at ⬇42 kDa in the SCN, and both of these bands
disappeared by the preabsorption of the antigen peptide (data notshown). We presume that the upper and lower bands corresponded
to mLARK2 and mLARK1, respectively, judging from the estimated molecular weights of these two proteins (SI Fig. 8). The
expression profiles of the mLARK proteins in the SCN examinedby Western blot analysis were consistent with our observations of
the mLARK-IR cells. The mLARK expression was also rhythmicin cortex (data not shown). Because protein (but not mRNA)rhythmicity was observed even under DD conditions, it is clear thatmLarks’ expression is under the control of the circadian clock, andthat the temporal oscillation of mLARK is regulated at the posttranscriptional level.
Posttranscriptional Regulation of mPER1 Expression by mLARKs. Be-

cause the temporal profile of mLARK in the SCN was similar tothat of the mPER1 protein, in which the highest expression level wasobserved at CT12–16 and the lowest at CT0–4 (Fig. 1 C and D), theeffect of mLARK on mPer1 expression was examined by using tworeporter plasmids, pPLS and pPL3 (Fig. 2A) (18). These plasmids
contain a luciferase reporter gene under the control of the mPer1
promoter, and the luciferase gene is followed by either an simianvirus 40 poly(A) signal (pPLS) or the mPer1 3⬘ UTR (pPL3). These
plasmids were transfected into NIH 3T3 cells along with plasmids
expressing mCLOCK and mBMAL1 (activators of the Per1 promoter). Addition of mLARK1 and mLARK2 expression vectors
resulted in ⬇2.8- and 5.0-fold inductions of the luciferase activity of
pPL3, respectively, whereas no induction was observed in pPLS
(Fig. 2B). In addition, the induction of pPL3 by mLARK1 wasdose-dependent (Fig. 2C). Interestingly, mLARK1 did not enhance
the transcription of pPL3 nor pPLS, because no significant differKojima et al.

CELL BIOLOGY

Fig. 2. Posttranscriptional regulation of mPer1 chimeric reporter genes by
mLARKs. (A) Structures of chimeric luciferase genes are shown. Both constructs
contain a 6.8-kb mPer1 promoter driving a luciferase reporter gene followed byeither the simian virus 40 poly(A) signal (pPLS) or the mPer1 3⬘ UTR (pPL3). (B)
Relative luciferase activities of pPLS and pPL3 either with or without the presenceof mLARK expression plasmids (165 ng) with the transcriptional activators CLOCK
and BMAL1 are shown. (C) mLARK1 increases reporter gene expression in adose-dependent manner. The experiment was done as in B except with varying
levels of mLARK1 plasmid as indicated. (D) Relative luciferase mRNA levels are notaltered by mLARK1. The experiment was done as in B, except total RNA was
isolated and the level of luciferase mRNA was measured by real-time RT-PCR.
Data consist of two or three independent experiments.

ence between the mRNA levels of the luciferase mRNA in thepresence and absence of mLARK1 was detected, whereas similar
amounts of the transcripts were induced by mCLOCK/mBMAL1 inboth pPLS and pPL3 (Fig. 2D). These data indicate that mLARK1
regulates pPL3 posttransciptionally, most likely through the mPer1
3⬘ UTR.
Subsequently, we examined whether mLARK1 also affects theendogenous expression of mPER1 in NIH 3T3 cells. WhenmLARK1 was overexpressed in NIH 3T3 cells, the endogenous
mPER1 protein level was markedly increased; however, the endogenous level of mPer1 mRNA remained unchanged (Fig. 3). These
results are consistent with our observations with the pPL3 reporter
gene system and indicate that mLARK1 regulates the endogenous
mPER1 expression posttranscriptionally.
mLARK1 Interacts Directly with mPer1 RNA. Because mLARK posttranscriptionally activates mPER1 expression and contains several
RNA-binding domains, we tested whether mLARK1 could interact
directly with mPer1 3⬘ UTR. RNA mobility-shift assays (RMSAs)
were performed by using a series of RNA fragments covering theentire mPer1 3⬘ UTR (Fig. 4A and SI Table 2). Specific binding was
detected only in RNA fragment 19, which contains nucleotides
559–589 of mPer1 3⬘ UTR (Fig. 4A). Specificity of this binding was
confirmed by the demonstration that the intensities of the shifted

Fig. 3. Posttranscriptional regulation of endogenous mPER1 by mLARK1.
Measurement of endogenous mPER1, TUBULIN, and transfected mLARK1 byWestern blot (Left) and measurement of endogenous mPer1 and ␤-actin by
Northern blot and transfected mLark1 and ␤-actin by RT-PCR (Right) from NIH
3T3 cells with or without transfected mLARK1 are shown.

Kojima et al.

Fig. 4. Direct interaction between mLark1 and mPer1 3⬘ UTR. (A) RMSA of
His-mLARK1 and RNAs derived from mPer1 3⬘ UTR is shown. The numbers
correspond to RNA oligonucleotides that span the entire mPer1 3⬘ UTR as
diagrammed at the top (the sequences are listed in SI Table 2). The position of
the previously identified ARE and LOX-DICE elements are marked (18). Arrowhead denotes the LARK-specific retarded band. (B) Dose–response relationship between His-mLARK1 and RNA 19 is shown. Shown is an RMSA as inA, but using only RNA 19 as probe and with increasing doses of His-mLARK1
as indicated. (C) Competitive analyses verify specificity of interaction between
mLARK1 and RNA. RMSAs were done as in B but with the addition of increasing amounts of antibody or cold RNA competitors as indicated. (D) RMSA using
normal and mutated versions of RNA 19 is shown. Underlined nucleotides are
mutated nucleotides. The arrowhead indicates the RNA bands retarded by
mLARK1, and the arrow indicates a nonspecific band. (E) LARK1 only activates
reporter gene expression when the LARK binding site is intact. Reporter
analyses were done as in Fig. 2, except an additional reporter construct was
included in which the mPer1 3⬘ UTR contains the mutations shown in mut4 (D)[pPL3(m4)]. (F) The predicted RNA secondary structure is shown. Bold typeindicates nucleotides corresponding to RNA fragment 19. (G) The directinteraction between LARK1 protein and Per1 mRNA is shown. Extracts from
NIH 3T3 cells with either overexpressed mLARK1 or control plasmid were
immunoprecipitated with either anti-LARK or control antibody. Immunoprecipitates were analyzed by Western blot using LARK antibody (Upper) or
RT-PCR for mPer1 (Lower). Asterisk indicates the IgG heavy chain from theimmunoprecipitating antibody.
PNAS 兩 February 6, 2007 兩 vol. 104 兩 no. 6 兩 1861

Fig. 5. mLARK1 affects mPER1 translation. (A) Schematic representation of
bicistronic vectors. The promoter is minimal promoter containing five times
ecdysone/glucocorticoid (E/GRE) responsive element (18). See Results for further details. (B) Luciferase assay of bicistronic vector. The y axis represents theratio of R-luc/F-luc activity. Data consist of three independent experiments.

bands were proportional to the amount of mLARK1 (Fig. 4B), andexcess amounts of anti-LARK antibodies or cold RNA 19 fragments inhibited this interaction, whereas a unrelated control antibody or other cold RNA (RNA 12) did not (Fig. 4C). Althoughother retarded bands were observed in lanes 1 and 20 in Fig. 4A,
we concluded that these were nonspecific interactions, because
these bands were observed even in the presence of competitors(data not shown).
The sequences involved in this interaction were characterized by
the introduction of several mutations into the RNA 19 fragment.
When we used these mutated RNAs in RMSA, RNA–protein
interactions were detected only in the mut2 RNA (Fig. 4D). The
secondary structure of this region was predicted to form a stem–
loop structure by using the GENETYX-MAC program (Fig. 4F),
and such a structure would be disrupted in all mutant RNAs except
mut2. Therefore, it is possible that mLARK1 recognizes thestem–loop structure rather than specific RNA sequences. To clarify
the significance of this protein–RNA interaction for the posttranscriptional activation of mPer1 expression, we constructed anotherreporter plasmid pPL3(m4), in which the same mutation as mut4
was introduced into pPL3. The increased luciferase activity of pPL3
by mLARK1 was completely abolished in pPL3(m4) (Fig. 4E),
indicating that the interaction between the mPer1 3⬘ UTR andmLARK1 is necessary for the posttranscriptional activation ofmPer1 expression.
The direct interaction between mLARK1 and mPer1 mRNAs in
NIH 3T3 cells was examined by a UV cross-linking immunoprecipitation assay (29). Lysates were prepared from cells with orwithout overexpressed mLARK1 and UV-irradiated before immunoprecipitation. When RNA–protein complexes were analyzed
separately by SDS/PAGE and RT-PCR, both mLARK1 protein
and mPer1 mRNAs were detected only in the anti-LARK immunoprecipitate prepared from cells transfected with mLARK1 (Fig.
4G), indicating that mLARK1 physically interacts with endogenous
mPer1 RNA.
To investigate whether mLARK1 activates mPER1 expression bymodulating translation, we made use of a bicistronic reporter gene
system (30). In this system, Renilla luciferase (R-luc) and firefly
luciferase (F-luc) are under the control of a single inducible
promoter, but are translated by using two distinct mechanisms (Fig.
5A). R-luc activity represents eIF4G/poly(A) tail-dependent translation, because it is near the Cap structure, whereas F-luc activity
represents internal ribosome entry site (IRES)-dependent translation. F-luc activity also serves as an internal control for transfection
1862 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0607567104

Fig. 6. Circadian rhythms of bioluminescence in NIH-PL cells are affected by
LARK levels. Shown are representative recordings of NIH-PL cells over 4 days inculture, measuring bioluminescence as an output of the mPer1::luc reporter
gene. (A and B) Oscillations are not observed in these cells when treated withDMSO (A) but are induced after synchronization with dexamethasone (Dex) (B).
(C–H) Representative recordings of NIH-PL cells as in B, but with the addition ofsiRNAs against EGFP (C), Lark-B (D), Bmal11 (F), Cry1 (G), and Cry2 (H) or withoverexpression of mLARK1 (E). (I) The effect of siRNAs on the expression ofmLARKs was confirmed by immunoblotting. Extracts of cells containing siRNAs
against EGFP, Lark-A, and Lark-B were analyzed on Western blots using antibodies to LARK and TUBULIN.

efficiency. Both R-luc and F-luc translation are affected by 3⬘ UTRs
located downstream of F-luc, therefore, the ratio of the R-luc/F-luc
represents the efficiency of Cap/poly(A)-dependent translation. We
constructed three bicistronic reporter genes, each of which contained either the simian virus 40 poly(A) signal, the mPer1 3⬘ UTR,
or the mPer1 3⬘ UTR lacking the LARK1 binding domain (Fig. 5A).
Significant induction was observed when mLARK1 was overexpressed with pE-CAP-IRES-mP1, but not with pE-CAP-IRES-SV
and pE-CAP-IRES-mP1(⌬) (Fig. 5B). These results suggest that
LARK1 activates the translation of mPER1 via eIF4G/poly(A)interaction depending on mPer1 3⬘ UTR.
Functions of mLARKs in the Mammalian Circadian Clock System. To
analyze the role of mLark in the cellular circadian clock system, weused a NIH 3T3 cell line stably transfected with a mPer1::luc
reporter gene (NIH-PL cells). It should be noted that the pPLS
reporter construct that we used did not possess the mPer1 3⬘ UTR(Fig. 2 A) to observe the circadian property of the cells. Treatmentof NIH-PL with dexamethasone induced the circadian fluctuationsof bioluminescence with a circadian period of ⬇23.9 h, which wasclose to behavioral rhythm in mice (Fig. 6 A and B). When we
introduced a siRNA of mLark (siLark-B), which targeted andsignificantly repressed both mLark1 and mLark2 (Fig. 6I), theperiod of bioluminescence was ⬇0.3 h shorter than the control indexamethasone-induced NIH-PL cells (Fig. 6D and Table 1),
whereas another mLark siRNA (siLark-A), which targeted only
mLark1, did not repress the mLARK protein expression nor
Kojima et al.

DNA or RNA used fortransfection

No. of cells
testedMean period⫾ SEM, hMock transfection
siEGFP
siLark-A
siLark-B
siBmal1
siCry1siCry2mLARK1 overexpression

7
8
12
12
8
3
4
25

23.93 ⫾ 0.26
23.89 ⫾ 0.19
23.85 ⫾ 0.10
23.58 ⫾ 0.32*†
Arrythmic
23.23 ⫾ 0.50*†
24.35 ⫾ 0.10*†
24.25 ⫾ 0.62*

The length of the period shown is the average for the indicated number of
separate wells combined from at least three independent experiments. *, P ⬍0.05 vs. mock transfection; †, P ⬍ 0.01 vs. siEGFP (Student’s t test).

shorten the period (Fig. 6I and Table 1) (31). On the other hand,overexpression of mLARK1 in the cells resulted in a 0.3-h longer
period in Per1 oscillation (Fig. 6E and Table 1). The administrationof siRNAs against Bmal1, Cry1, or Cry2 to the NIH-PL cells resulted
in arrhythmic, 0.6- to 0.7 h-shortened, or 0.4- to 0.5-h-lengthened
circadian period of the reporter expression, respectively (Fig. 6 F–H
and Table 1), as expected from the behavioral phenotypes ofmutant mice of these genes.
Discussion
A great deal of attention has been paid to transcriptional regulationof the mammalian circadian clock mechanism and rhythmic gene
expression, including extensive studies using powerful tools such as
DNA microarrays (32, 33). In contrast, very little is known about themechanisms of posttranscriptional regulation despite accumulating
evidence that these mechanisms are important for proper circadianclock function. For example, a recent study (34) revealed that only
50% of genes that encode rhythmic proteins exhibited circadianvariation in mRNA levels, indicating that a large number of mRNAs
are regulated posttranscriptionally.
The identification of mLarks as mPer1 3⬘ UTR-binding proteinsprovides mechanistic insight into posttranscriptional control ofmammalian clock processes. Interestingly, decreased or increased
levels of lark in Drosophila pupae leads to an early- or late-eclosion
phenotype, respectively, presumably through posttranscriptional
mechanisms (22–25). In addition, the overexpression of dlark in thecentral clock cells of fly causes behavioral arrhythmicity without
affecting the cycling of core clock proteins such as per and timeless(tim) (25). However, interactions of LARK with RNA targets had
not been reported to our knowledge. Circadian control of translation has been observed in Gonyaulax polyedra and Chlamydomonas
reinhardtii, where CCTR and CHLAMY1 (35, 36), respectively,
regulate circadian translation by interaction with the UGcontaining region in corresponding mRNAs. Also in Arabidopsis, anuclear RNA-binding protein (AtGRP7) has been found as acomponent of the circadian clock system, although its effect ontranslation has not been clarified (37).
We have shown here that in mammals mLARK protein levels
oscillate in the SCN, with a peak at CT12–16 and a minimum at
CT0–4 (Fig. 2 E and F), but mLark mRNA expression remained
constant throughout the day. This is also the case for dLark;
dLARK protein fluctuates with a peak at ZT6 in pupae, but dLark
mRNA is not rhythmic (24). This result suggests that Larks, as well
as Pers, are under posttranscriptional control in both Drosophila andmammalian systems. Our finding that the oscillation profiles ofmLARK proteins were in-phase with the mPER1 protein is consistent with our proposed role for mLARK in regulating mPER1
expression.
Kojima et al.

In humans, RBM4a (human LARK1), appears to have a role asa modulator of alternative splicing of mRNAs (27, 38, 39); however,
it is unlikely that mLARK regulates mPer1 expression at the levelof splicing, because mLARK1 directly interacts with the mPer1 3⬘
UTR, not with a splicing junction, and up-regulates mPER1 protein
expression. The most parsimonious explanation is that mLARKs
act on mPER1 translation, because mLARKs activate mPER1
expression in a Cap/poly(A)-dependent manner (Fig. 5). By increasing efficiency of mPer1 translation, mLARKs may also contribute to the generation of the mPER1 protein rhythm, which isphase-lagged relative to its mRNA rhythm, although the exact
mechanism of this regulation remains to be elucidated.
We previously reported that within mPer1 3⬘ UTR, there aresequences that repress mPER1 expression posttranscriptionally(18), yet the regulatory molecule(s) still remains to be determined.
In this study, we found that LARK activates the mPer1 expression
via binding to the mPer1 3⬘ UTR at a distinct cis element. In ourcurrent working model, these two molecules could act on mPer1 atdifferent times of day; during the daytime, when mRNA expressionof mPer1 is high and protein expression is low, the translational
repression could be caused by the unidentified molecule(s) boundto the cis element, whereas during late afternoon/early night whenmPer1 mRNA expression decreases but protein expression reaches
its peak translational activation by rising LARK levels would
become dominant. Thus, there is a possibility that the effect ofmLark on mPer1 expression is to accentuate the amplitude andprovide robustness of the molecular cycling of PER1.
The alteration of circadian period as a result of changing
mLARK expression levels in NIH 3T3 cells demonstrates thefunctional relevance of mLarks in the circadian system. In our study,mLark gene knockdown shortened the period length of NIH-PL
cells ⬇0.3 h, and mLARK1 overexpression lengthened the period⬇0.3 h. Because mLARK levels regulate mPER1 levels, it seems
likely that mLARK’s effect on the circadian period is through theregulation of mPer1 expression. This idea is supported by data frommutant animals; Per1⫺/⫺ mice, which lack functional PER1 expression, exhibit a shortened period of locomotor activity (11–13),
whereas Per1 transgenic rats, which overexpress Per1, have a 1h-lengthened period (40). These results indicate that either loss oroverexpression of Per1 does not abolish the circadian oscillation, butdoes influence the length of its period, and the period of the rhythm
may have positive correlation with the expression levels of Per1.
Thus, we hypothesize that the decreased levels of mLARK upon theapplication of siLark-B (Fig. 6I) could lead to the decreased level
of mPER1 and shorten the period length, whereas the increased
level of mLARK1 up-regulated the mPER1 protein level (Fig. 3)
and lengthened the rhythm in NIH-PL cells.
Although our analysis of mLARK function was done in NIH 3T3cells, there is abundant evidence that this system is a good correlate
for circadian control in general (41, 42). For example, we also tested
several other kinds of siRNAs directed against clock genes (Bmal,
Cry1, and Cry2) in our NIH-PL cells and observed changes inrhythmicity that correspond closely to reported phenotypes ofmutant mice (although the effects of the siRNAs were somewhat
milder) (43, 44). The effect of siRNA against Larks was subtle, but
statistically significant, and the changes in circadian period in thesecells strongly suggest that LARK is important in maintaining
normal 24-h rhythmicity in the mammalian clock system. Future
analysis of mutant mice lacking the Lark genes will help to further
clarify the role of LARKs in the regulation of clock gene expression.
We propose that mLark is a novel clock gene that regulates
circadian gene expression at a posttranscriptional level. Identification of this regulatory mechanism begins to elucidate the role ofposttranscriptional regulation within mammalian circadian clocks.
In addition to mPer1, it is likely that mLark regulates other genes(perhaps even other core clock genes), and the identification of suchgenes will be important for understanding the molecular functionof mLARK and the role it plays in the circadian system.
PNAS 兩 February 6, 2007 兩 vol. 104 兩 no. 6 兩 1863

CELL BIOLOGY

Table 1. Period length of bioluminescent oscillation
in NIH-PL cells

Materials and Methods
Isolation and Sequencing of mLarks. mLark1 and Lark2 were isolated

from the mouse brain Marathon-Ready cDNA library (Clontech,
Mountain View, CA) by a PCR technique. Primer sequences are in
SI Text. The amplified DNA fragment was cloned into pCI-neo(Promega, Madison, WI) and verified by sequencing.
Animals and Histochemistry. Male BALB/c mice (JACJO, Osaka,
Japan) were housed in 12-h light (300 lux)/12-h dark (LD) cycles
from 2 to 4 weeks, then subjected to complete darkness (DD) as acontinuation of the dark phase of the last LD cycle. For a light-pulse
experiment, mice were exposed to a fluorescent light stimulus (600
lux, 30 min) at CT16. In situ hybridization and immunohistochemistry was performed as described (28, 45).
Transient Expression Assay. Mouse NIH 3T3 cells (JCBR0615) wereobtained from the Health Science Research Resources Bank (Tokyo, Japan), and luciferase assay and real-time RT-PCR analysis
were performed as described (18). Construction of pPL3(m4) andbicistronic vectors is described in SI Text.
Protein and mRNA Expression Analysis. Polyclonal antisera wereraised in rabbits by using a synthetic peptide (MARYEREQYADRARYSAF) corresponding to amino acids residues 344–361 ofmLARK1. Antisera were purified by using a Mab Trap kit (Amersham, Piscataway, NJ). Other antibodies were commerciallyavailable (␣PER1; ABR; ␣-Tubulin; Sigma, St. Louis, MO). For
Northern blot analysis poly(A)⫹ RNAs were extracted with a
QuickPrep Micro mRNA Purification kit (Amersham). Primersthat were used to make probes are described in SI Text.

cally synthesized (JBios, Saitama, Japan), and RMSAs were performed as described (46) with slight modifications. In a competition
assay, an anti-LARK antibody, an anti-c-MYC antibody (Roche,
Indianapolis, IN) (0, 1, 2, 4, and 8 ␮g), or heat-treated nonlabeled
RNA fragments (0, 10, 20, 40, and 100 pmol) were preincubatedwith His-mLARK1 at 30°C for 30 min before the addition ofradiolabeled RNA.
In Vivo Binding Assay. Cells were lysed in lysis-binding buffer (10mM Tris䡠HCl, pH 8.0/1 mM EDTA/150 mM NaCl/10 mM MgCl2/
0.05% Nonidet P-40). The lysed samples were collected andUV-irradiated (Funakoshi, Tokyo, Japan) for 15 min. The RNA–
protein complexes were immunoprecipitated with the anti-LARK
antibody or rabbit IgG (Sigma)–protein G Sepharose 4 fast flow
(Amersham), and RNAs were extracted with TRIzol (Gibco/BRL)
and reverse-transcribed by using SuperScript II (Gibco/BRL). PCR
conditions are described in SI Text.
Functional Analysis of mLark Using siRNAs. Stable transfectants ofmPer1::luc reporter gene (pPLS) in NIH 3T3 cells were generated
by selecting cells with G418 (Promega), and these reporter cells(designed as NIH-PL cells) were cultured in a 35-mm Petri dish with
2 ml of DMEM supplemented with 10% FBS. The siRNA transfection prodcedure and the sequences are described in SI Text. Thenbioluminescence was measured with a photomultiplier tube detector assemblies (Hamamatsu, Ichinocho, Japan). Origin 7.0 software(OriginLab, Northampton, MA) was used to calculate of the period
length of bioluminescent oscillation.

RMSA. Histidine (His)-tagged mLARK was partially purified with
the PROEX HT Prokaryotic Expression System (Gibco/BRL,
Gaithersburg, MD). mPer1 3⬘ UTR RNA fragments were chemi-

We thank Drs. Katsushi Tokunaga, Naoyuki Tsuchiya, Hirohiko Hohjoh,
and Jun Ohashi for helpful and thoughtful discussions and Drs. Atsuko
Fujioka and Akihito Adachi for excellent technical assistance. This work
was partially supported by research grants from the Japanese Ministry of
Education, Culture, Sports, Science, and Technology and the Japanese
Ministry of Health and Welfare.

1. Moore RY, Eichler VB (1972) Brain Res 42:201–206.
2. van den Pol AN, Dudek FE (1993) Neuroscience 56:793–811.
3. Pittendrigh CS (1993) Annu Rev Physiol 55:16–54.
4. Takahashi JS (1995) Annu Rev Neurosci 18:531–553.
5. Lowrey PL, Takahashi JS (2004) Annu Rev Genomics Hum Genet 5:407–441.
6. Harms E, Kivimae S, Young MW, Saez L (2004) J Biol Rhythms 19:361–373.
7. Tei H, Okamura H, Shigeyoshi Y, Fukuhara C, Ozawa R, Hirose M, Sakaki Y(1997) Nature 389:512–516.
8. Gekakis N, Staknis D, Nguyen HB, Davis FC, Wilsbacher LD, King DP,
Takahashi JS, Weitz CJ (1998) Science 280:1564–1569.
9. Hida A, Koike N, Hirose M, Hattori M, Sakaki Y, Tei H (2000) Genomics
65:224–233.
10. Kume K, Zylka MJ, Sriram S, Shearman LP, Weaver DR, Jin X, Maywood ES,
Hastings MH, Reppert SM (1999) Cell 98:193–205.
11. Bae K, Jin X, Maywood ES, Hastings MH, Reppert SM, Weaver DR (2001)
Neuron 30:525–536.
12. Cermakian N, Monaco L, Pando MP, Dierich A, Sassone-Corsi P (2001)
EMBO J 20:3967–3974.
13. Zheng B, Albrecht U, Kaasik K, Sage M, Lu W, Vaishnav S, Li Q, Sun ZS,
Eichele G, Bradley A, et al. (2001) Cell 105:683–694.
14. Albrecht U, Zheng B, Larkin D, Sun ZS, Lee CC (2001) J Biol Rhythms
16:100–104.
15. Field MD, Maywood ES, O’Brien JA, Weaver DR, Reppert SM, Hastings MH(2000) Neuron 25:437–447.
16. Zeng H, Hardin PE, Rosbash M (1994) EMBO J 13:3590–3598.
17. Moore MJ (2005) Science 309:1514–1518.
18. Kojima S, Hirose M, Tokunaga K, Sakaki Y, Tei H (2003) Biochem Biophys Res
Commun 301:1–7.
19. Wilsbacher LD, Yamazaki S, Herzog ED, Song EJ, Radcliffe LA, Abe M, Block
G, Spitznagel E, Menaker M, Takahashi JS (2002) Proc Natl Acad Sci USA
99:489–494.
20. So WV, Rosbash M (1997) EMBO J 16:7146–7155.
21. Chen Y, Hunter-Ensor M, Schotland P Sehgal A (1998) J Biol Rhythms
13:364–379.
22. Newby LM, Jackson FR (1993) Genetics 135:1077–1090.
23. Newby LM, Jackson FR (1996) J Neurobiol 31:117–128.
24. McNeil GP, Zhang X, Genova G, Jackson FR (1998) Neuron 20:297–303.

25. Schroeder AJ, Genova GK, Roberts MA, Kleyner Y, Suh J, Jackson FR (2003)
J Neurogenet 17:139–169.
26. Jackson FR, Banfi S, Guffanti A, Rossi E (1997) Genomics 41:444–452.
27. Lai MC, Kuo HW, Chang WC, Tarn WY (2003) EMBO J 22:1359–1369.
28. Shigeyoshi Y, Taguchi K, Yamamoto S, Takekida S, Yan L, Tei H, Moriya T,
Shibata S, Loros JJ, Dunlap JC, Okamura H (1997) Cell 91:1043–1053.
29. Imai T, Tokunaga A, Yoshida T, Hashimoto M, Mikoshiba K, Weinmaster G,
Nakafuku M, Okano H (2001) Mol Cell Biol 21:3888–3900.
30. Uchida N, Hoshino S, Imataka H, Sonenberg N, Katada T (2002) J Biol Chem
277:50286–50292.
31. Ui-Tei K, Naito Y, Takahashi F, Haraguchi T, Ohki-Hamazaki H, Juni A, Ueda
R, Saigo K (2004) Nucleic Acids Res 32:936–948.
32. Ueda HR, Chen W, Adachi A, Wakamatsu H, Hayashi S, Takasugi T, Nagano
M, Nakahama K, Suzuki Y, Sugano S, et al. (2002) Nature 418:534–539.
33. Panda S, Antoch MP, Miller BH, Su AI, Schook AB, Straume M, Schultz PG,
Kay SA, Takahashi JS, Hogenesch JB (2002) Cell 109:307–320.
34. Reddy AB, Karp NA, Maywood ES, Sage EA, Deery, M., O’Neill JS, Wong
GK, Chesham J, Odell M, Lilley KS, et al. (2006) Curr Biol 16:1107–1115.
35. Mittag M, Lee DH, Hastings JW (1994) Proc Natl Acad Sci USA 91:5257–5261.
36. Mittag M (1996) Proc Natl Acad Sci USA 93:14401–14404.
37. Heintzen C, Nater M, Apel K, Staiger D (1997) Proc Natl Acad Sci USA
94:8515–8520.
38. Lin JC, Tarn WY (2005) Mol Cell Biol 25:10111–10121.
39. Kar A, Havlioglu N, Tarn WY, Wu JY (2006) J Biol Chem 281:24479–24488.
40. Numano R, Yamazaki S, Umeda N, Samura T, Sujino M, Takahashi R, Ueda
M, Mori A, Yamada K, Sakaki Y, et al. (2006) Proc Natl Acad Sci USA
103:3716–3721.
41. Nagoshi E, Saini C, Bauer C, Laroche T, Naef F, Schibler U (2004) Cell
119:693–705.
42. Tsuchiya Y, Akashi M, Nishida E (2003) Genes Cells 8:713–720.
43. Bunger MK, Wilsbacher LD, Moran SM, Clendenin C, Radcliffe LA, Hogenesch JB, Simon MC, Takahashi JS, Bradfield CA (2000) Cell 103:1009–1017.
44. van der Horst GT, Muijtjens M, Kobayashi K, Takano R, Kanno S, Takao M,
de Wit J, Verkerk A, Eker AP, van Leenen D, et al. (1999) Nature 398:627–630.
45. Nagano M, Adachi A, Nakahama K, Nakamura T, Tamada M, MeyerBernstein E, Sehgal A, Shigeyoshi Y (2003) J Neurosci 23:6141–6151.
46. Fajardo MA, Butner KA, Lee K, Braun RE (1994) Dev Biol 166:643–653.

1864 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0607567104

Kojima et al.