there has been remarkable progress in our understanding of the complex intracellular mechanisms that generate and regulate circadian rhythms across multiple species . in mammals ,
the molecular clock arises from a transcriptional translational - feedback loop , consisting of the transcription factors clock and bmal1 that drive the expression of their regulators , period and cryptochrome ( reppert and weaver , 2002 ) .
these elements form part of an autoregulatory - feedback loop , the period of which is approximately 24  hr . most cells of the body appear to possess such a molecular oscillator ( albrecht , 2012 ; balsalobre et  al . , 1998 ) , and
these autonomous clocks are synchronized to each other and the external environment primarily by a circadian pacemaker within the suprachiasmatic nuclei ( scn ) ( ralph et  al . ,
the scn is in - turn aligned ( entrained ) to the solar day by exposure to light at dawn and dusk , detected by photoreceptors located exclusively within the retina of the eye .
photosensitive retinal ganglion cells ( prgcs ) , utilizing the photopigment melanopsin ( opn4 ) , provide the primary light input to the scn , exemplified by the fact that mice or human subjects lacking all their rods and cones still possess a full cohort of circadian responses to light ( freedman et  al . , 1999 ;
although rod and cone photoreceptors are not required for entrainment , they can partially compensate for the loss of melanopsin in opn4 mice ( hattar et  al . ,
in addition , a growing body of evidence demonstrates that rods , and the different classes of cone photoreceptor , can complement the role of the prgcs in circadian entrainment by expanding the irradiance range of the response and in the detection of light transitions ( lall et  al . , 2010 ;
there remains , however , only a very basic understanding of the molecular mechanisms whereby cellular oscillators within the scn are entrained to environmental light .
this can be summarized as follows : light activation of prgcs causes the release of the neurotransmitters glutamate and pacap ( hannibal et  al . , 2000 ) at the terminals of the retinohypothalamic tract ( rht ) that projects directly to the scn .
neurotransmitter release results in a ca influx , which activates  phosphorelay  signaling pathways .
a number of different kinases have been implicated in these pathways , including pka , pkg , camk , and mapk , although their relative roles remain unclear ( meijer and schwartz , 2003 ) .
all these pathways are thought to converge on the camp response element binding protein ( creb ) , which is activated by phosphorylation at ser133 ( ginty et  al . , 1993 ) and ser142 ( gau et  al . , 2002 ) .
phosphorylated creb then binds to camp response elements ( cre ) in the promoters of light - regulated genes .
the promoters of many previously identified light - responsive genes in the scn including fos , nr4a1 , egr1 , egr3 ( lin et  al . , 1997 ;
morris et  al . , 1998 ; porterfield et  al . , 2007 ) , the map kinase phosphatase dusp1 ( doi et  al . , 2007 ) , and mirnas mir-132 and mir-217 ( cheng et  al
significantly , the clock genes per1 and per2 also contain conserved cres , which mediate their light induction ( albrecht et  al . , 1997 ; schwartz et  al . , 2011 ) .
elevated per1 and per2 adjust the molecular - feedback loop of the circadian oscillator to the light / dark ( ld ) cycle .
this linear description of light - induced per gene induction can not explain a key feature of light - regulated circadian behavior .
namely , that circadian entrainment is slow , taking multiple days to adjust to an advanced or delayed ld cycle . in most mammals , including jet - lagged humans , behavioral shifts are limited to approximately 1  hr ( one time zone ) per day ( aschoff , 1981 ) .
furthermore , at a molecular level , the effect of light on per gene induction is limited .
for example , under extended illumination , per1 mrna light - induction peaks after 1  hr and then returns to baseline ( vitaterna et  al . , 2006 ) , indicating creb transcription is arrested one hour after it is induced . mirroring this molecular response ,
high - irradiance light pulses greater than 1 hr cease to have a phase - shifting effect upon the murine clock .
this saturation of the response suggests the presence of an unidentified negative - feedback mechanism that effectively limits the magnitude of phase - shifting responses to light within the scn .
our aims in this study were to understand the molecular mechanisms that limit the effects of entrainment stimuli on the clock . here ,
we show both in  vitro ( via serum shock ) and in  vivo ( via light ) that an entraining stimulus causes creb - regulated transcription coactivator 1 ( crtc1 ) to coactivate creb , inducing the expression of both per1 and sik1 .
critically , we show that knockdown of sik1 within the scn results in extended light - induced phase shifts and an enhanced rate of re - entrainment to an advanced ld cycle . as sik1 acts to suppress the effects of light on the clock , this pathway provides a pharmacological target for the treatment of disturbed circadian rhythms , including jet lag , shift - work , and illnesses that disturb circadian timing .
in order to identify signaling pathways involved in photic input , we examined the light - regulated transcriptome of the wild - type ( opn4 ) scn across a period of nocturnal light exposure .
we collected scn from mice that received a nocturnal light pulse delivered 4  hr after activity onset ( circadian time / ct16 ) over a 2  hr time course and identified 536 genes as light - regulated in the wild - type scn using affymetrix whole - genome mouse exon arrays ( figure  1a and figure  s1 and table s1 available online ) .
surprisingly , the majority ( 81% , 436 genes ) of transcripts were downregulated in response to light , with a smaller number of genes showing upregulation ( 19% , 100 genes ) .
these included the acutely upregulated immediate early genes ( iegs ) previously shown to respond to light in the scn , including fos ( lupi et  al . , 2006 ) ,
nr4a1 , and egr1 ( porterfield et  al . , 2007 ) , clock genes ( per1 ) , and clock - controlled genes ( rorb ) .
in addition , we confirmed rrad ( porterfield et  al . , 2007 ) , a gtpase modulating calcium - binding proteins such as camkii , which has an identified role in photic resetting ( fukushima et  al . , 1997 ) and
dusp1 , a map kinase phosphatase , characterized as light - induced and clock - controlled in the scn ( doi et  al . , 2007 ) .
among the genes we identified were dusp4 and sik1 , which were found to cluster with per1 ( figure  1a ) .
sik1 encodes salt inducible kinase 1 , a ser / thr protein kinase of the amp - dependent protein kinases ( ampk ) family of which ampk rhythmically phosphorylates cry1 ( lamia et  al . , 2009 ) and dusp4 encodes a dual ser / thr and tyr phosphatase selectively controlling erk ( cagnol and rivard , 2013 ) .
the downregulated transcripts include tac1 , which encodes substance p precursor , known to have a role in phase shifting ( kim et  al . , 2001 ) ; sim1 , a transcription factor of the bhlh - pas family that also includes clock ; plekho1 , which shows homology with casein kinase and a number of receptors including the serotonin receptor htr7 ; lactrophilin ( lphn3 ) ; and the glycine receptors glra1 and  glra2 . to gain insight into the pathways represented by these transcripts
, we performed a gene - enrichment analysis based on gene ontology ( go ) terms ( figure  1b ) .
one category of interest that is significantly responsive to light includes a number of transcripts relating to synaptic remodeling and plasticity , which were upregulated at a late stage , and included raspgrp1 , grit , and ntng1 .
a number of previous studies have highlighted the central role of creb in phase shifting ( gau et  al .
1993 ) and the promoters of many previously identified light - responsive genes in the scn including fos , per1 , nr4a1 , and dusp1 contain conserved cres ( porterfield et  al . , 2007 ; travnickova - bendova et  al . ,
we found that of the genes upregulated more than 1.5-fold , there was significant enrichment of those containing cre sites near the tata box ( figure  1c ) .
this correlation implies that creb drives the high - magnitude gene induction that underlies phase - shifting responses in the scn .
we provide a detailed characterization of creb target genes that are induced by light in the scn ( table s2 ) .
analysis of individual clusters of the opn4 scn light - induced transcriptome showed that in addition to creb , binding sites for sp1 and krox ( in transcripts upregulated at 120  min ) and tal1 , hnfalpha3 , and wt1 ( transcripts showing early repression followed by upregulation ) were enriched . to identify any genes from our screen that could play a direct role in the core circadian oscillator
, we reviewed data obtained from bioluminescent recordings from a genome - wide rnai screen on per2-luc transfected u2os cells ( zhang et  al . , 2009 ) .
silencing of several of the genes caused significant changes in period length , which would indicate a role in regulating phase delays of the clock ( pendergast et  al .
csnk2a1 ( period  = 31.2  hr versus 25.2 for control ) , which is downregulated by light , is a serine threonine kinase .
it is closely related to csnke , which is mutated in the tau mutant hamster , which also shows period lengthening ( lowrey et  al . , 2000 ) .
silencing of sik1 , which is upregulated by light in the scn , in a manner similar to per1 ( figure  1a ) also shows period lengthening ( 28.0  hr ) .
melanopsin - expressing retinal ganglion cells provide the major retinal projection to the scn ( gler et  al . , 2008 ) .
the endogenous light sensitivity of these cells conferred by melanopsin ( opn4 ) is important for circadian entrainment .
mice lacking melanopsin ( opn4 ) show attenuated phase - shifting responses to light , which correlate with attenuated induction of fos and per1 in the scn ( lupi et  al . , 2006 ;
we compared scn samples from opn4 mice with the data from wild - type animals ( opn4 ) ; 92% of the genes that were upregulated in response to light in the wild - type showed an attenuated light - regulated pattern of expression in the opn4 scn ( figures s2a and s2b ) .
we verified the attenuated induction of fos , per1 , and egr1 ( figure  s2c ) in the opn4 scn using qpcr .
analysis of transcription factor binding site ( tfbs ) showed that those transcripts with the greatest attenuation in opn4mice , including fos , nr4a1 and per1 , were enriched for wt1 , a repressor of egr1 ( tables s3 and s4 ) .
our results indicate that in opn4 mice , transcriptional responses in the scn are broadly attenuated , mirroring the attenuated fos induction and circadian responses to light previously observed .
these results suggest that in the absence of melanopsin , there is an overall reduction in signal amplitude reaching the scn via the rht . as creb
transcription plays a critical role in photic entrainment ( ginty et  al . , 1993 ) , and because sik1 acts both as a repressor of creb transcription ( katoh et  al . ,
2006 ) and as a target of creb regulation itself , sik1 emerged as a strong candidate for an inducible repressor acting as a brake on photic input to the scn .
sik1 functions by phosphorylating and thereby deactivating the creb coactivator crtc ( creb - regulated transcription coactivator ) , which exists in three isoforms , crtc13 .
these isoforms exhibit a tissue - specific expression pattern and the dual requirement for creb phosphorylation and the presence of crtc provides the complexity and specificity required in the response to camp signaling .
indeed , the sik / crtc pathway can regulate creb transcription in other cellular systems ( berdeaux et  al . , 2007 ;
ca and camp signaling triggers calcineurin - mediated dephosphorylation of crtc ( altarejos and montminy 2011 ; screaton et  al . , 2004 ) , and its nuclear translocation ( chng et  al . , 2012 ) .
the resulting induction of sik1 represses creb transcription through the phosphorylation of crtc . in such a manner
, sik1 could provide negative feedback on photic input to the scn by inhibiting the light - induced , creb - mediated expression of per1 . to test this hypothesis ,
we investigated the function of the crtc - sik1 pathway in regulating the circadian clock . to this end , we studied the nuclear translocation of crtc1 ; the resultant creb - mediated sik1 and per1 induction ; and the effect of sik1 on the transcription of per1 and on re - entrainment in both cellular clock models and the intact animal . in order to test the effects of sik1 inhibition on the molecular oscillator
, we used the immortalized mouse embryonic fibroblast cell line nih 3t3 as an in  vitro model of the circadian clock .
these cells can be phase shifted using serum stimulation ( balsalobre et  al . , 1998 ) .
we found that crtc1 translocated to the nucleus following a serum shock ( figure  2a ) and sik1 , which contains a conserved cre / atf ( figure  2b top ) was induced ( figure  2b bottom ) . increased levels of crtc phosphorylation measured in cell lysates collected before and after serum shock ,
were observed as a consequence of sik1 induction ( figure  2c ) . in order to specifically inhibit sik1 activity , we used a gene - specific rnai - based approach ( figures 2d and s3a  s3d and table 1 ) .
lysates of sik1 knockdown cells showed no increase in crtc phosphorylation after serum shock ( figure  s3e ) .
cells transfected with sirna against sik1 ( sisik1 ) showed severely attenuated induction of sik1 resulting from serum treatment ( figure  2d and table 1 ) with no significant change over time . to test the proposed role of sik1 and crtc in phase shifting of the clock , per1
induction was assayed following silencing of sik1 and crtc isoforms expressed in nih 3t3 cells ( crtc1 and crtc3 , figure  s3b ) .
significant changes of per1 levels over the duration of the experiment were apparent after treatment with nontargeting control ( sint ) and sisik1 ( figure  2e and table 1 ) . however , knockdown of sik1 resulted in an increase in the duration and levels of per1 induction as measured by a significant increase in the area under the curve for sik1 ( figure  2 g , tables 1 and 2 ) . in order to test that sik1 acts via crtc to attenuate creb - mediated transcription of per1 , we assessed the effect of cosilencing crtc1 and crtc3 ( sicrtc ) on serum - shock - induced per1 expression .
knockdown of crtc attenuated the induction of per1 ( figures 2f and 2 g , tables 1 and 2 ) with no significant difference over time after serum treatment .
this result supports the proposed model whereby the effects of sik1 are mediated through crtc .
crucially , this result also demonstrates that crtc is an important creb coactivator required in the phase - shifting response .
circadian expression of per1 is regulated by multiple promoters , including the e box ; therefore , we confirmed this requirement for crtc in the phase - shifting response by examining the induction of two other light / serum - responsive genes ( egr1 and nr4a1 ) that contain cres in their promoters ( as identified previously [ zhang et  al . , 2005 ] ) and show a robust response to phase - shifting stimuli .
these transcripts showed increased induction in cells treated with sisik1 and attenuation with sicrtc ( figures 2h , 2i , s3f , and s3 g and tables 1 and 2 ) .
these data support our hypothesis that sik1 acts to modulate the magnitude of phase shifts via the negative regulation of creb - mediated transcription of per1 .
findings from the in  vitro study using rnai in fibroblasts were further supported by data from the direct pharmacological inhibition of sik1 by indirubin-3-monoxime ( i3 m ) ( hashimoto et  al . , 2008 )
( figure  3a ) , which produced both an increase in the level and the duration of serum - induced per1 and per2 expression in nih 3t3 cells following a serum shock ( figures 3b and 3c ) . in order to quantify
the phase shift induced , we used per2::luc mouse embryonic fibroblasts ( mefs ) in a double - serum treatment experiment ( balsalobre et  al . , 1998 ) .
the first serum shock was used to synchronize the cells and the second to allow precise quantification of the phase shift .
we observed a 10.90  1.02  hr phase delay of the per2::luc rhythm in i3m - treated cells , compared with 7.59  1.00  hr delay in the control , a difference of 3.3  hr ( figures 3d and 3e ) . in order to assess the effect of i3 m on the scn
, per1:luc mice were exposed to light at zt14 for 10  min , and the scn was removed and maintained in culture .
the rhythms of scn treated with i3 m peaked approximately 1  hr later than dmso - treated controls ( figure  3f ) .
it should be noted that i3 m is a nonspecific inhibitor of sik1 , and although directly inhibiting sik1 ( figure  3a ) , its targets also include the kinase gsk3b , which has a role in the circadian clock ( hashimoto et  al . , 2008 ) .
nevertheless , these results using i3 m and those using sik1-specific rnai are consistent ( figure  2 ) .
collectively , these data show that inhibition of  sik1 increases the magnitude of phase delays in the molecular clock . of the different crtc isoforms
, crtc1 is the most abundantly expressed in the scn ( figure  4a ) .
therefore , we investigated whether phase - shifting stimuli trigger the nuclear translocation of crtc1 in the scn .
application of a serum shock to dissociated scn cells in culture resulted in the translocation of crtc1 from the nucleus to cytoplasm within 10  min ( figure  4b ) and an upregulation of sik1 in the scn at 120  min after the light pulse ( figure  4c ) .
we verified the expression patterns of sik1 in the scn seen on the exon arrays using qpcr ( figure  4d ) .
sik1 was light - induced with a peak mrna expression at 60  min following both a ct16 phase - delaying and ct22 phase - advancing light pulse in wild - type mice , confirming a role for sik1 in both phase advances as well as phase delays .
further , sik1 induction is higher at ct22 when compared with ct16 . in agreement with a role of sik1 in negative feedback , egr1 and nr4a1 expression
were found to be correspondingly lower at  ct22 . to address effect of sik1 on circadian behavior
, we used in  vivo rnai to provide an acute and specific inhibition of sik1 within the scn .
cy3-labeled nontargeting sirna - lipid complex was administered via stereotaxic injection into the third ventricle adjacent to the scn .
subsequent examination of the scn showed the accumulation of sirna in the cell bodies ( figure  s4a ) , confirming transfection .
we designed and tested sirnas against sik1 that achieved significant in  vivo silencing in the scn at both mrna and protein levels ( figure  s4b ) , causing little or no interferon responses ( figure  s4c ) .
we then assessed behavioral responses in sik1-silenced mice to three light regimes : ( 1 ) a phase - delaying nocturnal light pulse ; ( 2 ) re - entrainment to a 6  hr phase advanced ld cycle ( jet - lag protocol ) and ( 3 ) period length in constant darkness ( dd ) . for the phase - shifting paradigm ,
mice injected with either a control sirna or sik1 sirna were exposed 4  days later to a 30  min , type ii phase - shifting light pulse ( 400 lux ) at ct14.5 ( figure  5a ) . compared with the controls , the mice with sik1 silenced showed significantly enhanced phase shifts ( 98  min compared to 50  min , p  = 0.036 , figure  5b ) . for the jet - lag protocol ,
2  days after sirna administration , the ld cycle was advanced 6  hr , and 10  days later , the cycle was advanced 6  hr again .
sik1 silenced mice showed significantly faster re - entrainment when compared to the controls in both cycles ( figure  5c ) . to illustrate the magnitude of this effect , on day 2
the difference in phase was over 3  hr in the sint - injected animals but was less than 1  hr in the sik1-silenced mice ( figure  5d ) .
for period length , we measured period length ( tau ) in both cell lines ( per2::luc u2os ) as well as in  vivo after sik1 knockdown .
period length was markedly increased in cell lines ( 24.8  hr with greater than 90% knockdown of sik1 versus 22.1  hr with sint - treated cells ) , and less so in  vivo ( 23.27  hr with approximately 50% knockdown of sik1 versus 23.04 sint , figure  s5 ) .
these data suggest that in addition to acute effects on gene expression , creb may play a role in constitutive clock gene regulation . indeed
, recent reports show that creb silencing results in overall lower expression of clock genes and output hormones such as avp and vip ( lee et  al . , 2010 ) .
in this case , basal levels of sik1 would be important in regulating the activity of crtc , which in turn acts as a creb coactivator . as a result
, basal levels of sik1 may affect period length via this creb - dependent mechanism .
collectively , our in  vitro studies combined with these behavioral data , provide critical support for a role of sik1 acting as an inducible repressor inhibiting the effect of phase - shifting stimuli on the circadian clock by attenuating per1 induction .
the circadian system is entrained to environmental zeitgebers ( time givers ) , which allow the appropriate alignment of internal and external time . however , the adjustment of the clock to such zeitgebers is rarely instantaneous , often occurring over repeated 24  hr cycles .
what limits the effects of entraining stimuli on the circadian system represents a key , yet largely unanswered question .
we have identified > 500 genes that are light regulated within the scn . from these , we defined a key role for the crtc - sik1 pathway in regulating creb - mediated clock gene expression .
our data show both in  vitro and in  vivo that crtc1 translocates to the nucleus after phase - shifting stimuli and acts as a coactivator of creb - driven transcription of sik1  and per1 . following in  vitro crtc silencing , induction of per1 and other creb - mediated genes
elevated per1 aligns the transcriptional / translational - feedback loop to the zeitgeber stimulus ( albrecht , 2012 ; albrecht et  al . , 1997 ) .
we also show that sik1 phosphorylates and deactivates crtc with an appropriate time delay , which effectively blocks further creb - driven per1 transcription .
critically , we show that in  vivo rnai knockdown of sik1 within the scn greatly enhances light - induced behavioral phase shifts and re - entrainment to ld cycles .
collectively , these data provide empirical evidence for a molecular mechanism that limits the impact of an entraining stimulus on the circadian system .
it is worth emphasizing that the sik1-crtc regulation of creb - mediated gene expression we describe within the context of the circadian system has also been demonstrated in several nonclock pathways .
for example , in skeletal myocytes , sik1 functions as a ca - responsive creb - regulated hdac kinase ( berdeaux et  al . , 2007 ) and a ca - induced regulator of na transport in kidney cells ( sjstrm et  al . , 2007 ) .
in hippocampal neurons , crtc1 functions as a sensor of stimulation , by traveling from the synapse to the nucleus in response to activation , triggering creb mediated transcription and thereby regulating hippocampal plasticity ( chng et  al . , 2012 ) .
an appreciation of the role of crtc1-sik1 regulation in these noncircadian pathways very much supports our interpretation of how this signaling pathway functions within the molecular clock .
the same regulatory processes that induce per1 also induce sik1 . following the induction of these genes ,
the phosphorylation of core clock proteins is a major posttranslational event whereby delays are introduced into the transcriptional - translational - feedback loop to generate a 24  hr oscillation .
several proteins have been described in this process , including fbxl3 , an f box protein that targets phosphorylated cry for degradation ( godinho et  al . , 2007 ) ; csnk1e , in the tau mutant hamster , which phosphorylates per1 , cry1 , and bmal1 ( lowrey et  al . , 2000 ) and nemo / nlk that phosphorylates per to initiate a time - delay phosphorylation circuit ( chiu et  al . , 2011 ) .
in contrast to these kinases , sik1 does not target the core clock proteins to control their degradation ; rather , it attenuates the induction of clock genes in response to entraining stimuli , functioning as a negative - feedback mechanism to deactivate the molecular pathways that are activated following a phase - shifting stimulus . but why limit the impact of an entraining stimulus on the clock ?
almost all physiological systems benefit from buffering mechanisms that prevent sudden and large changes , and in this regard the circadian system is no exception
. this would be particularly important for the clock because it has to maintain internal synchrony between the multiple cell - autonomous circadian oscillators throughout the body .
if sudden and large shifts are imposed upon one part of the circadian network , such as the scn , internal desynchrony among the cellular clocks of the body will result . under such circumstances normal physiology will be severely compromised ( albrecht , 2012 ) . by limiting the phase - shifting effects of light and other clock - resetting stimuli , such as food ( stokkan et  al . , 2001 ) or activity (
mrosovsky and salmon , 1987 ) the circadian system would be shielded from such disruption .
such a buffering mechanism in the natural world would protect the clock from abnormal zeitgeber stimuli .
in addition , limiting the impact of light on the scn would be useful in nonequatorial latitudes during the equinoxes when the time of dawn and dusk , along with activity and food availability , moves very rapidly . with a rapidly changing twilight transition , the circadian system has to maintain a balance between entrainment and the prevention of internal de - synchrony .
sleep and circadian rhythms disruption ( scrd ) is endemic within the 24/7 economies of the developed and developing industrialized societies ( foster and wulff , 2005 ) .
in addition , scrd is a common feature of both neuropsychiatric and neurodegenerative disease ( wulff et  al . , 2010 ) .
in shift - work , jet lag , and brain abnormalities in general , scrd is associated with chronic diseases such as cancer , metabolic disorders , cardiovascular disease , immune dysfunction , and impaired cognition ( hastings et  al . , 2003 )
. currently , there are very few options available to stabilize or adjust the circadian axis and sleep timing in individuals with scrd . however , the discovery of the molecular mechanisms that generate and regulate the circadian system provides targets for the development of therapeutic agents that regulate internal time .
indeed , there have been recent efforts in this direction , including the use of small molecule activators of cryptochrome ( hirota et  al .
as sik1 acts to suppress the effects on light on the clock , this pathway presents a tractable target .
opn4 mice ( hattar et  al . , 2002 ; lucas et  al . , 2003 ) were maintained on a c57bl/6_129sv background as heterozygous breeders . for scn sample collection and rna extraction ,
sense probes were prepared for hybridization from 300  ng of total rna for opn4 shams scn ( n  = 6 ) , lp30 , lp60 , and lp120 ( n  = 4 ) and opn4 shams ( n  = 4 ) , lp30 , lp60 , and lp120 ( n  = 3 ) using the genechip wt sense target labeling kit ( affymetrix , santa clara , ca , usa ) and hybridized overnight to genechip mouse exon 1.0 st arrays ( affymetrix santa clara , ca , usa ) .
cel file data were extracted , log transformed and quantile normalized with expression console ( v1.1 ) software from affymetrix using the rma - sketch algorithm at the exon - core probe set level including the detection above background ( dabg ) p value for each probe set .
exon array quality control was assessed using expression console ( v1.1 ) and any outliers removed .
probe sets were filtered by dabg p value for each array sequentially , discarding data for probe sets for which p  0.05 . only probe sets called as 
were further analyzed ( 110,026 probe sets ) by one - way anova and filtered by p value ( p 0.05 ) ( corresponding to 4,384 probe sets that respond to light ) .
probe set enrichment was evaluated by comparing the number of significant probe sets for an individual gene to ( 1 ) number of probe sets on the array for that gene , ( 2 ) number of probe sets significantly changing , and ( 3 ) the total number of probe sets on the array based upon a binomial distribution .
finally , probe sets were filtered by probe set enrichment p value of p  < 0.05 ( 1,417 probe sets corresponding to 536 genes with the false - positive rate determined to be 0.02 ) . the same probe sets were subsequently analyzed in the opn4 data to assess attenuation .
refer to extended experimental procedures for hierarchical clustering , functional pathway analysis and tfbs prediction . for qpcr , primer sequences , western blotting , immunostaining , microscopy , crtc phosphorylation assay , and i3m - based studies ,
sirnas were synthesized as sigenome ( dharmacon ) for in  vitro experiments , sistable ( dharmacon ) or custom synthesized by roche for in  vivo work .
the sirnas supplied by roche were moderately stabilized for in  vivo use and tagged with a cholesterol conjugate to aid transfection .
nih 3t3 cells ( mouse embryonic fibroblast cell line ) were cultured in dmem supplemented with 10% fbs at 37c , 5% co2 .
where required , cells were transfected with 10  nm sirna using lipofectamine rnaimax ( invitrogen , life technologies ) according to manufacturer s instructions .
the cells were maintained for at least 24  hr in 1% fbs in dmem at confluence . at 0  min ,
the cells were synchronized by application of 50% horse serum ( balsalobre et  al . , 1998 ; duffield et  al . , 2002 ) returned to 1% fbs in dmem after 30  min ( serum shock  treatment ) .
rna was  extracted using the rneasy mini kit ( qiagen ) and qpcr was performed as discussed in supplementary material to measure knockdown / gene expression .
sirna complexed with invivofectamine ( invitrogen ) prepared according to manufacturer s instructions were delivered using microinjection into the 3v with sterotaxic equipment with coordinates ( paxinos and franklin , 2008 ) as described previously ( butcher et  al .
mice were allowed to recover and returned to the light - tight chambers for further experiments . for visualization of sirna ,
5 cy3-labeled nontargeting sirna was injected into the 3v , and 24  hr later , the animals were sacrificed .
tissue processing and imaging were conducted as described in supplemental information . to measure silencing of sik1
, mice were injected with either sint or sisik1 and 96  hr later were sacrificed , scn collected on which rna extraction and qpcr were conducted for the required mrnas . to check for activation of an interferon response , a common off - target effect of sirna , oas1 mrna levels ( bauer et  al . , 2009 ) from sirna ( sint and sisik1)-injected mice
were compared with noninjected controls ( figure  s4 ) . for phase - shifting studies ,
c57bl/6 mice were maintained on running wheels in light tight chambers on a 12:12  hr ld cycle ( 400 lux from white led lamps ) and injected with sirna as above .
four days after the injection , the mice were placed in dd for 24  hr and received a 30  min light pulse at approximately ct14.5 .
this time point was chosen to allow all experimental subjects to receive a light pulse within the maximal phase delay zone of ct1416 ( pendergast et  al . , 2010 ) .
the mice were then allowed to free run for 14  days in dd , running wheel activity data were collected and analyzed on clocklab ( actimetrics , wilmette , il , usa ) . for jet - lag studies ,
c57bl/6 mice were maintained on running wheels in light tight chambers on a 12:12  hr ld cycle ( 400 lux from white led lamps ) and injected with sirna as above .
two days after injection , the ld cycle was shifted 6  hr in advance .
onset of activity on each day was used to measure phase relative to the ld cycle .
extended experimental proceduresscn tissue collectionanimals were housed under a reversed 12:12  hr ld cycle for 2  weeks with food and water ad libitum . a light pulse ( lp ) of 400 - 500 lux was administered at ct16 for 30  min , using a spectranova halogen white light ( schott ag , mainz , germany ) .
animals were sacrificed under dim red light by cervical dislocation at 30 , 60 , and 120  min after the onset of the light pulse .
sham - treated mice not given a light pulse were dissected at each time point .
brains were removed and placed into a brain matrix ( kent scientific , torrington ct , usa ) .
a skin graft blade ( swann - morton , sheffield , uk ) was positioned at bregma 0.10  mm ( figure  s1 ) .
a second blade was placed 1  mm ( bregma 1.10 ) caudal from the first , and a 1  mm thick brain slice was dissected .
scn punches were taken using a sample corer ( 1  mm internal diameter , fine science tools gmbh , heidelberg , germany ) from the brain slice ( n  = 4 ) , flash frozen on dry ice and stored at 80c prior to rna extraction .
all procedures were performed in accordance with the uk home office animals ( scientific procedures ) act 1986 and the university of oxford s policy on the use of animals in scientific research.rna extraction and sample preparationtotal rna was extracted using the microrneasy column method ( qiagen , hilden , germany ) .
quality and quantity of rna were measured using an agilent bioanalyzer and a nanodrop1000 ( thermo fisher scientific , waltham , ma usa ) , respectively . to ensure that punches of the defined areas were accurately dissected , qpcr of the scn - specific mrna six6 and per1
revealing selective enrichment ( figure  s1).functional pathway analysisthe transcripts found to be significantly altered following the light pulse in the opn4 animals were and those then downregulated in the opn4 were examined using the david ontology tools ( huang da et  al .
536 transcript cluster ids from the opn4 corresponded to 527 david identifiers and these were used to generate functional annotation charts using david default ontologies for go , pathways and protein domains ( as of 28 nov 2011 ) .
these annotation charts were visualized using the enrichment map ( merico et  al . , 2010 )
, 2003).transcription factor binding site predictionwe used a comparative genomics approach based on multiple sequence alignments of genomic sequences from several mammalian species : human , dog , horse , cow , pig , rabbit , mouse and rat .
tfbss are represented by position - specific scoring matrices ( pssms ) ( badis et  al . ,
2009 ) as obtained from the transfac database ( version 11.4 ) ( matys et  al . , 2006 ) and the jaspar ( vlieghe et  al . , 2006 ) collection .
we used the profilestats package ( rahmann et  al . , 2003 ) to regularize the count matrices .
the resulting scoring matrices were then used to scan genomic sequences ( excluding known protein - coding stretches ) from all species studied individually , and on both forward and reverse strands .
we considered those matches that were conserved in at least two species.tfbss are characterized by two parameters : an  expectation ratio  ( er ) , estimating the relative probability of finding a pssm match of the given quality in a random sequence of 500  bp length and genome background base composition , and the number of allowed mismatching species in the alignment .
thus , predictions with a low er and few or none mismatching species have the strongest support.quantitative pcrrna samples were prepared as described for microarray hybridization .
cdna was synthesized with a qscript cdna synthesis kit ( quanta biosciences , gaithersburg , md ) , and quantitative pcr ( qpcr ) was conducted with sybr green i and an sds7700 thermal cycler ( applied biosystems , foster city , ca ) . relative quantification of transcript levels was done as described previously ( peirson et  al . , 2003 ) .
the geometric mean of three housekeeping genes was used for normalization ( gapdh , actb , and arbp ) .
primer sequences are provided in table s5.dissociation of scn cells and culturep0 neonatal mice(from c57 bl/6 and c3h backgrounds ) were killed by decapitation and the brains dissected in to ice - cold pbs .
micro - dissection was carried out under a stereomicroscope to isolate the optic chiasm and the surrounding hypothalamus up to 1  mm caudal and ventral to the chiasm .
this region should contain the scn , but was not considered to yield purely scn - specific cells .
the tissue was then dissociated using a papain dissociation system ( worthington biochemical corp , lakewood , nj , usa ) , using incubation times from 5070  min .
single - cell suspensions were generated using a series of sterile plastic pasteur pipettes with decreasing bores ( appleton woods , uk ) .
final cell densities of 10 cells / ml were prepared in dmem as before ( with the addition of b27 supplement , invitrogen , oregon , usa ) and cells were plated on sterile , poly - l - lysine - treated glass slides , prepared using silicone chamber gaskets ( coverwell , invitrogen , oregon , usa ) and allowed to recover for at least 48hr.crtc1 immunostainingimages are nih
3t3 cells / dissociated scn cell culture grown on glass slides using coverwell perfusion chamber gaskets .
individual chambers on the same slide were changed to media containing 50% horse serum at 10 and 0  min prior to washing with pbs and fixation with pbs  + 4% pfa .
slides were processed using a monoclonal rabbit antibody to crtc1 ( c71d11 , cell signaling technology , danvers , ma , usa ) ( jeong et  al . , 2011 ) at a concentration of 1:100 .
images were collected using a zeiss lsm710 confocal microscope ( carl zeiss ltd . , cambridge , uk).crtc phosphorylationnih 3t3 cells ( mouse embryonic fibroblast cell line ) were cultured in dmem supplemented with 10% fbs at 37c , 5% co2 . 
cells  were cultured to confluence in 6 well plates , incubated in 1% fbs for at least 24  hr followed by application of 50% horse serum for 30  min ( serum shock ) to synchronize cellular rhythms and induce expression of sik1 and per1 as shown  in figure  3 .
cells then were then washed with pbs lysed at the required time points in 200  l kinase lysis buffer ( jansson  et  al . , 2008 )
( 25  mm tris ( ph  7.5 ) , 150  mm nacl , 50  mm naf , 5% glycerol , 5  mm -glycerol phosphate , 1  mm edta , 1  mm navo4 , 1  mm dtt , complete mini protease inhibitor cocktail ( roche diagnostics , penzberg , germany ) and phosphatase inhibitor cocktail ( sigma  aldrich , dorset , england ) . 10  g crtc peptide ( rlpsalnrts sdsalhtsvm , containing the sik1 phosphorylation site of all three crtc isoforms ) was blotted onto circles of whatmann nitrocellulose 0.25um membrane ( ge healthcare , waukesha , wi , usa ) punched with a paper punch and allowed to dry .
35  l lysate or 0.1  g sik1 ( ab89693 , abcam , cambridge , uk ) or 0.1  g sik1  + drug ( 20  m indirubin-3-monoxime or 100  nm staurosporine ) in 50  l kinase buffer ( jansson et  al . ,
2008 ) and 2  ci ( p)atp were added to the membrane in a 50  l reaction and allowed to incubate at rt for 20  min .
0.2% tween-20 , rinsed with water and detected using a typhoon phosphor imager system ( ge  healthcare , waukesha , wi , usa).determining the effect of indirubin-3-monoxime on serum - induced per1 and per2 expressionmouse embryonic fibroblasts ( mefs ) derived from per2::luciferase c57bl/6 mice ( per2:luc ) ( gamsby et  al .
, 2009 ; yoo et  al . , 2004 ) ; were cultured in dmem  + 10% fbs  + antibiotic ( hyclone , logan , ut ) .
confluent cells were maintained in dmem  + 1% fbs  + antibiotics for 2448  hr .
confluent cells were treated with i3 m ( 20  m ) ( sigma - aldrich , st .
louis , mo ) with dmso ( 0.06% ) or dmso alone ( vehicle control ) 30  min before time 0  min .
i3 m has been shown to inhibit sik1 activity on torc by preventing the phosphorylation and consequent activation of sik1 ( hashimoto et  al . , 2008 ) and also directly inhibits sik1 activity as shown by the crtc phosphorylation assay as described above .
the autonomous cellular rhythms were synchronized with a serum treatment ( balsalobre et  al . , 1998 ; duffield et  al . , 2002 ) .
at 0  min , cells were treated with 50% horse serum ( hs ) plus i3 m  + dmso or dmso in dmem , and rna was isolated from cells at 30  min intervals over 4  hr .
qrt - pcr using cdna prepared from each time point was used to measure relative per1 and per2 mrna levels and conducted as previously described using the standard curve method and normalized to gapdh ( duffield et  al .
per2 taqman reagents ( forward 5-cccatcccacacttgcctc-3 , reverse 5-cactgtgccagccggg-3 ; probe 6fam - ccaggctgagttccctagtcggacct - tamra).measuring i3 m modulation of serum - induced phase shifts of per2::luc cellsper2::luc mefs were cultured in 35  mm petri dishes in dmem  + 10% fbs  + antibiotic ( hyclone ) .
once confluent , the cells were maintained in dmem  + 1% fbs  + antibiotics for 24  48  hr .
ten hours later , the cells were treated with a second 30  min 50% hs pulse with either i3 m ( 20  m ) and dmso ( 0.06% ) or dmso alone ( vehicle control ) , which results in a delay shift of the rhythm of a defined magnitude .
experimental cells were pretreated 30  min prior with either i3 m or dmso alone .
the cells were then placed in leibovitzs-15 media ( gibco , grand island , ny ) with 1% fbs and 0.1m luciferin ( gold bio technology , st .
louis , mo ) , dishes sealed and placed into a lumicycle instrument ( actimetrics , wilmette , il ) . for the first 3  hr , media also contained i3 m  + dmso or dmso .
rhythmic expression of per2::luc activity was measured by luminometry ( gamsby et  al . , 2009 ; yamazaki and takahashi , 2005 ) .
peaks were determined using the actimetrics lumicycle analysis software and determining the time after the start of the first serum pulse at which each peak reached its maximum .
phase differences between treatment groups were determined using one - way anova with post hoc tukey s hsd tests ( p  < 0.05 ; n  = 6 - 13 cultures per treatment group).measuring i3 m modulation of light - induced phase shifts of scn per1:luciferase expressionadult per1:luciferase mice ( wilsbacher et  al . ,
2002 ) ( per1:luc , cd1 background ; 34  months ; n  = 8) maintained on a 12:12  hr ld cycle were exposed to a 10  min light pulse ( 400 lux ) at zt14 - 14.5 .
mice were immediately sacrificed and scn tissue isolated and cultured as previously described ( yamazaki and takahashi , 2005 ) .
scn were maintained in media containing 0.1  m luciferin and either 20  m i3 m  + dmso or dmso alone .
after 3  hr , the treatment media was replaced with fresh recording media , including 2x 15  min washes .
explant cultures were monitored for several days in the lumicycle instrument , per1:luc expression monitored by luminescence , and peak phase within the first circadian cycle determined .
phase differences between treatment groups were determined using student s t test ( p  < 0.05 ) .
all procedures were approved by the university of notre dame animal care and use committee and performed in accordance with nih guidelines for the care and use of laboratory animals.rnaisirna sequences are as follows ; sik1 ( in  vivo ) : 5 gcucaacccuccuugcaua 3 ; nontargeting sirna : 5 cuuacgcugaguacuucga 3 ; sik1 ( 2 , in  vitro ) : 5 ugaacaagaucaaaggguu 3 ; sik1 ( 3 , in  vitro ) : 5 aaacgcagguugcaauaaa 3 ; crtc1 : 5 uggacagaguauaucguga 3 ; crtc3 : 5 guuucgagcugaccggcua 3 ; sik1 ( human ) : 5 acgauuagauucaagcaau 3 ; cry1 ( human ) : 5 ggaguuaauuauccuaaac 3.western blottingas shown in figure  4c , 2  g scn lysate in ripa buffer were run on 4%20% sds - page gels ( bio - rad , hercules , ca , usa ) ,  transferred using a semi - dry protocol on the turbo transfer system ( bio - rad ) onto pvdf membranes , blocked with 3% bsa , incubated with rabbit polyclonal antibodies to sik1 ( sik1 ( y-20 ) sc-83754 , santa cruz biotechnology , santa cruz ,  ca , usa , 1:1000 ) and beta - actin ( ab8227 , abcam , 1:5000 ) , subsequently with a donkey anti - rabbit igg hrp conjugated antibody ( ab98503 , abcam , 1:5000 ) and developed with a standard ecl protocol ( luminata classico , millipore , billerica , ma ,  usa).as shown in figure  4e : 5  g of scn ( from nt or sik1 sirna - injected animals ) in ripa buffer were run on 4%20% sds - page gels ( nupage , life technologies ) , transferred using standard protocols ( bio - rad ) onto immobilon fl pvdf membranes ( millipore ) , blocked with odyssey blocking buffer ( li - cor biosciences , lincoln , ne , usa ) incubated with rabbit polyclonal antibodies to sik1 ( ab64428 , abcam , 1:500 ) and beta - actin ( ab8227 , abcam , 1:1000 ) , subsequently with donkey anti - rabbit igg 488 dylite conjugated secondary antibody ( pierce biotechnology , rockford , il , usa , 1:200 ) and scanned with a typhoon scanner ( ge healthcare).assays using per2::luc u2os cellsu2os cells stably transfected with a per2::luc reporter were cultured in dmem supplemented with 10% fbs . for sirna - based experiments ,
cells were seeded at 5,000 per well into white 384-well plates and transfected with 20  nm sirna the next day .
two days later , the cells were synchronized with 100  nm dexamethasone and the medium replaced with phenol - red free dmem supplemented with b-27 and 100  m luciferin potassium salt ( buhr et  al . , 2010 ) and sealed .
per2::luc rhythms were recorded from a bmg labtech fluostar omega plate reader maintained at 36c and readings taken from each well every hour .
animals were housed under a reversed 12:12  hr ld cycle for 2  weeks with food and water ad libitum . a light pulse ( lp ) of 400 - 500 lux was administered at ct16 for 30  min , using a spectranova halogen white light ( schott ag , mainz , germany ) .
animals were sacrificed under dim red light by cervical dislocation at 30 , 60 , and 120  min after the onset of the light pulse .
sham - treated mice not given a light pulse were dissected at each time point .
brains were removed and placed into a brain matrix ( kent scientific , torrington ct , usa ) . a skin graft blade ( swann - morton , sheffield , uk ) was positioned at bregma 0.10  mm ( figure  s1 ) .
a second blade was placed 1  mm ( bregma 1.10 ) caudal from the first , and a 1  mm thick brain slice was dissected .
scn punches were taken using a sample corer ( 1  mm internal diameter , fine science tools gmbh , heidelberg , germany ) from the brain slice ( n  = 4 ) , flash frozen on dry ice and stored at 80c prior to rna extraction .
all procedures were performed in accordance with the uk home office animals ( scientific procedures ) act 1986 and the university of oxford s policy on the use of animals in scientific research .
total rna was extracted using the microrneasy column method ( qiagen , hilden , germany ) .
quality and quantity of rna were measured using an agilent bioanalyzer and a nanodrop1000 ( thermo fisher scientific , waltham , ma usa ) , respectively . to ensure that punches of the defined areas were accurately dissected , qpcr of the scn - specific mrna six6 and per1
were conducted ( conte et  al . , 2005 ) , revealing selective enrichment ( figure  s1 ) .
the transcripts found to be significantly altered following the light pulse in the opn4 animals were and those then downregulated in the opn4 were examined using the david ontology tools ( huang da et  al .
536 transcript cluster ids from the opn4 corresponded to 527 david identifiers and these were used to generate functional annotation charts using david default ontologies for go , pathways and protein domains ( as of 28 nov 2011 ) .
these annotation charts were visualized using the enrichment map ( merico et  al . , 2010 )
we used a comparative genomics approach based on multiple sequence alignments of genomic sequences from several mammalian species : human , dog , horse , cow , pig , rabbit , mouse and rat .
tfbss are represented by position - specific scoring matrices ( pssms ) ( badis et  al . ,
2009 ) as obtained from the transfac database ( version 11.4 ) ( matys et  al . , 2006 ) and the jaspar ( vlieghe et  al . , 2006 ) collection .
we used the profilestats package ( rahmann et  al . , 2003 ) to regularize the count matrices .
the resulting scoring matrices were then used to scan genomic sequences ( excluding known protein - coding stretches ) from all species studied individually , and on both forward and reverse strands .
tfbss are characterized by two parameters : an  expectation ratio  ( er ) , estimating the relative probability of finding a pssm match of the given quality in a random sequence of 500  bp length and genome background base composition , and the number of allowed mismatching species in the alignment .
thus , predictions with a low er and few or none mismatching species have the strongest support .
cdna was synthesized with a qscript cdna synthesis kit ( quanta biosciences , gaithersburg , md ) , and quantitative pcr ( qpcr ) was conducted with sybr green i and an sds7700 thermal cycler ( applied biosystems , foster city , ca ) . relative quantification of transcript levels was done as described previously ( peirson et  al . , 2003 ) .
the geometric mean of three housekeeping genes was used for normalization ( gapdh , actb , and arbp ) .
bl/6 and c3h backgrounds ) were killed by decapitation and the brains dissected in to ice - cold pbs .
micro - dissection was carried out under a stereomicroscope to isolate the optic chiasm and the surrounding hypothalamus up to 1  mm caudal and ventral to the chiasm .
this region should contain the scn , but was not considered to yield purely scn - specific cells .
the tissue was then dissociated using a papain dissociation system ( worthington biochemical corp , lakewood , nj , usa ) , using incubation times from 5070  min .
single - cell suspensions were generated using a series of sterile plastic pasteur pipettes with decreasing bores ( appleton woods , uk ) .
final cell densities of 10 cells / ml were prepared in dmem as before ( with the addition of b27 supplement , invitrogen , oregon , usa ) and cells were plated on sterile , poly - l - lysine - treated glass slides , prepared using silicone chamber gaskets ( coverwell , invitrogen , oregon , usa ) and allowed to recover for at least 48hr .
3t3 cells / dissociated scn cell culture grown on glass slides using coverwell perfusion chamber gaskets .
individual chambers on the same slide were changed to media containing 50% horse serum at 10 and 0  min prior to washing with pbs and fixation with pbs  + 4% pfa .
slides were processed using a monoclonal rabbit antibody to crtc1 ( c71d11 , cell signaling technology , danvers , ma , usa ) ( jeong et  al . , 2011 ) at a concentration of 1:100 .
images were collected using a zeiss lsm710 confocal microscope ( carl zeiss ltd . , cambridge , uk ) .
nih 3t3 cells ( mouse embryonic fibroblast cell line ) were cultured in dmem supplemented with 10% fbs at 37c , 5% co2 . 
cells  were cultured to confluence in 6 well plates , incubated in 1% fbs for at least 24  hr followed by application of 50% horse serum for 30  min ( serum shock ) to synchronize cellular rhythms and induce expression of sik1 and per1 as shown  in figure  3 .
cells then were then washed with pbs lysed at the required time points in 200  l kinase lysis buffer ( jansson  et  al . , 2008 ) ( 25  mm tris ( ph  7.5 ) , 150  mm nacl , 50  mm naf , 5% glycerol , 5  mm -glycerol phosphate , 1  mm edta , 1  mm navo4 , 1  mm dtt , complete mini protease inhibitor cocktail ( roche diagnostics , penzberg , germany ) and phosphatase inhibitor cocktail ( sigma  aldrich , dorset , england ) .
10  g crtc peptide ( rlpsalnrts sdsalhtsvm , containing the sik1 phosphorylation site of all three crtc isoforms ) was blotted onto circles of whatmann nitrocellulose 0.25um membrane ( ge healthcare , waukesha , wi , usa ) punched with a paper punch and allowed to dry .
35  l lysate or 0.1  g sik1 ( ab89693 , abcam , cambridge , uk ) or 0.1  g sik1  + drug ( 20  m indirubin-3-monoxime or 100  nm staurosporine ) in 50  l kinase buffer ( jansson et  al . ,
2008 ) and 2  ci ( p)atp were added to the membrane in a 50  l reaction and allowed to incubate at rt for 20  min .
0.2% tween-20 , rinsed with water and detected using a typhoon phosphor imager system ( ge  healthcare , waukesha , wi , usa ) .
mouse embryonic fibroblasts ( mefs ) derived from per2::luciferase c57bl/6 mice ( per2:luc ) ( gamsby et  al .
, 2009 ; yoo et  al . , 2004 ) ; were cultured in dmem  + 10% fbs  + antibiotic ( hyclone , logan , ut ) .
confluent cells were maintained in dmem  + 1% fbs  + antibiotics for 2448  hr .
confluent cells were treated with i3 m ( 20  m ) ( sigma - aldrich , st .
louis , mo ) with dmso ( 0.06% ) or dmso alone ( vehicle control ) 30  min before time 0  min .
i3 m has been shown to inhibit sik1 activity on torc by preventing the phosphorylation and consequent activation of sik1 ( hashimoto et  al . ,
2008 ) and also directly inhibits sik1 activity as shown by the crtc phosphorylation assay as described above .
the autonomous cellular rhythms were synchronized with a serum treatment ( balsalobre et  al . , 1998 ; duffield et  al . , 2002 ) .
at 0  min , cells were treated with 50% horse serum ( hs ) plus i3 m  + dmso or dmso in dmem , and rna was isolated from cells at 30  min intervals over 4  hr .
qrt - pcr using cdna prepared from each time point was used to measure relative per1 and per2 mrna levels and conducted as previously described using the standard curve method and normalized to gapdh ( duffield et  al .
per2 taqman reagents ( forward 5-cccatcccacacttgcctc-3 , reverse 5-cactgtgccagccggg-3 ; probe 6fam - ccaggctgagttccctagtcggacct - tamra ) .
per2::luc mefs were cultured in 35  mm petri dishes in dmem  + 10% fbs  + antibiotic ( hyclone ) .
once confluent , the cells were maintained in dmem  + 1% fbs  + antibiotics for 24  48  hr .
ten hours later , the cells were treated with a second 30  min 50% hs pulse with either i3 m ( 20  m ) and dmso ( 0.06% ) or dmso alone ( vehicle control ) , which results in a delay shift of the rhythm of a defined magnitude .
experimental cells were pretreated 30  min prior with either i3 m or dmso alone .
the cells were then placed in leibovitzs-15 media ( gibco , grand island , ny ) with 1% fbs and 0.1m luciferin ( gold bio technology , st .
louis , mo ) , dishes sealed and placed into a lumicycle instrument ( actimetrics , wilmette , il ) . for the first 3  hr , media also contained i3 m  + dmso or dmso .
rhythmic expression of per2::luc activity was measured by luminometry ( gamsby et  al . , 2009 ; yamazaki and takahashi , 2005 ) .
peaks were determined using the actimetrics lumicycle analysis software and determining the time after the start of the first serum pulse at which each peak reached its maximum .
phase differences between treatment groups were determined using one - way anova with post hoc tukey s hsd tests ( p  < 0.05 ; n  = 6 - 13 cultures per treatment group ) .
( per1:luc , cd1 background ; 34  months ; n  = 8) maintained on a 12:12  hr ld cycle were exposed to a 10  min light pulse ( 400 lux ) at zt14 - 14.5 .
mice were immediately sacrificed and scn tissue isolated and cultured as previously described ( yamazaki and takahashi , 2005 ) .
scn were maintained in media containing 0.1  m luciferin and either 20  m i3 m  + dmso or dmso alone .
after 3  hr , the treatment media was replaced with fresh recording media , including 2x 15  min washes .
explant cultures were monitored for several days in the lumicycle instrument , per1:luc expression monitored by luminescence , and peak phase within the first circadian cycle determined .
phase differences between treatment groups were determined using student s t test ( p  < 0.05 ) .
all procedures were approved by the university of notre dame animal care and use committee and performed in accordance with nih guidelines for the care and use of laboratory animals .
sirna sequences are as follows ; sik1 ( in  vivo ) : 5 gcucaacccuccuugcaua 3 ; nontargeting sirna : 5 cuuacgcugaguacuucga 3 ; sik1 ( 2 , in  vitro ) : 5 ugaacaagaucaaaggguu 3 ; sik1 ( 3 , in  vitro ) : 5 aaacgcagguugcaauaaa 3 ; crtc1 : 5 uggacagaguauaucguga 3 ; crtc3 : 5 guuucgagcugaccggcua 3 ; sik1 ( human ) : 5 acgauuagauucaagcaau 3 ; cry1 ( human ) : 5 ggaguuaauuauccuaaac 3. as shown in figure  4c , 2  g scn lysate in ripa buffer were run on 4%20% sds - page gels ( bio - rad , hercules , ca , usa ) ,  transferred using a semi - dry protocol on the turbo transfer system ( bio - rad ) onto pvdf membranes , blocked with 3% bsa , incubated with rabbit polyclonal antibodies to sik1 ( sik1 ( y-20 ) sc-83754 , santa cruz biotechnology , santa cruz ,  ca , usa , 1:1000 ) and beta - actin ( ab8227 , abcam , 1:5000 ) , subsequently with a donkey anti - rabbit igg hrp conjugated antibody ( ab98503 , abcam , 1:5000 ) and developed with a standard ecl protocol ( luminata classico , millipore , billerica , ma ,  usa ) . as shown in figure  4e : 5  g of scn ( from nt or sik1 sirna - injected animals ) in ripa buffer were run on 4%20% sds - page gels ( nupage , life technologies ) , transferred using standard protocols ( bio - rad ) onto immobilon fl pvdf membranes ( millipore ) , blocked with odyssey blocking buffer ( li - cor biosciences , lincoln , ne , usa ) incubated with rabbit polyclonal antibodies to sik1 ( ab64428 , abcam , 1:500 ) and beta - actin ( ab8227 , abcam , 1:1000 ) , subsequently with donkey anti - rabbit igg 488 dylite conjugated secondary antibody ( pierce biotechnology , rockford , il , usa , 1:200 ) and scanned with a typhoon scanner ( ge healthcare ) .
u2os cells stably transfected with a per2::luc reporter were cultured in dmem supplemented with 10% fbs . for sirna - based experiments ,
cells were seeded at 5,000 per well into white 384-well plates and transfected with 20  nm sirna the next day .
two days later , the cells were synchronized with 100  nm dexamethasone and the medium replaced with phenol - red free dmem supplemented with b-27 and 100  m luciferin potassium salt ( buhr et  al . , 2010 ) and sealed .
per2::luc rhythms were recorded from a bmg labtech fluostar omega plate reader maintained at 36c and readings taken from each well every hour .
 summaryretinal photoreceptors entrain the circadian system to the solar day . 
 this photic resetting involves camp response element binding protein ( creb)-mediated upregulation of per genes within individual cells of the suprachiasmatic nuclei ( scn ) . 
 our detailed understanding of this pathway is poor , and it remains unclear why entrainment to a new time zone takes several days . by analyzing the light - regulated transcriptome of the scn , we have identified a key role for salt inducible kinase 1 ( sik1 ) and creb - regulated transcription coactivator 1 ( crtc1 ) in clock re - setting . 
 an entrainment stimulus causes crtc1 to coactivate creb , inducing the expression of per1 and sik1 . 
 sik1 then inhibits further shifts of the clock by phosphorylation and deactivation of crtc1 . 
 knockdown of sik1 within the scn results in increased behavioral phase shifts and rapid re - entrainment following experimental jet lag . 
 thus sik1 provides negative feedback , acting to suppress the effects of light on the clock . 
 this pathway provides a potential target for the regulation of circadian rhythms . 
Introduction
Results
Discussion
Experimental Procedures