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SRF’ing Around the Clock

Xuan Zhao,1,3 Han Cho,1,3 and Ronald M. Evans1,2,*1Gene Expression Laboratory2Howard Hughes Medical InstituteSalk Institute for Biological Studies, La Jolla, CA 92037, USA3These authors contributed equally to this work

*Correspondence: evans@salk.edu

http://dx.doi.org/10.1016/j.cell.2013.01.028

Central circadian clock oscillators, located in the suprachiasmatic nucleus (SCN) of the hypothal-amus, align peripheral clock systems with geosynchronous time. Gerber et al. (2013) identify actinpolymerization and serum response factor (SRF) activation as key steps linking the central masterclock to peripheral oscillators.

Circadian clocks are the intrinsic time-

keeping system that coordinates our

internal physiology and behavior with

daily light/dark cycles. Increasing lines of

evidence suggest that disruptions of

normal circadian clocks are closely linked

to many diseases, including cancers, as

well as metabolic, sleep, and mental

disorders (Takahashi et al., 2008). How

central clock pacemakers communicate

and entrain peripheral clock oscillators

remains unknown. In this issue of Cell,

Schibler and colleagues (Gerber et al.,

2013) identify a signaling pathway that

responds to hormone-like cues to effi-

ciently align peripheral and central clock

cycling.

Historically, the circadian rhythm was

believed to be a neuronal phenomenon,

controlled by the SCN located in the

hypothalamus of the brain. Steve Kay

and Ueli Schibler’s groups (Balsalobre

et al., 1998; Plautz et al., 1997) trans-

formed the field by showing that cell-

autonomous and self-sustained circadian

clock control systems also exist in many,

if not all, peripheral tissues. Mechanisti-

cally, both central and peripheral clock

control systems utilize similar factors

such as Bmal1/Clock heterodimer and

nuclear hormone receptor Rev-erbs

(Bugge et al., 2012; Cho et al., 2012) in

transcription/translation feedback loops

to generate circadian oscillation. Bmal1/

Clock controlled loop and nuclear-

receptor-directed pathways crosstalk

with each other in order to coordinately

regulate circadian clock and physiology.

For example, Lamia et al. (2011) show

that the repression of widely expressed

nuclear receptors by the transcriptional

cofactor cryptochrome links the circadian

clock to metabolism. Peripheral clocks

remain synchronized by the central

master clock despite their sensitivity to

environmental cues such as feeding and

temperature, thus ensuring system-wide

integration (Mohawk et al., 2012). There-

fore, studying molecular mechanisms

governing system-regulated gene ex-

pression in peripheral tissues is impor-

tant to understanding the coordination

between central and peripheral clocks.

The authors start by searching for tran-

scription signaling pathways that are acti-

vated by circadian signals in serum. The

screening approach utilizes a library of

expression reporters containing a diverse

collection of DNA response elements

paired with sera harvested at different

circadian time points. This scanning

strategy yields one reporter construct

that exhibits opposing oscillatory

responses to human and rodent sera

collected around the clock. This synthetic

reporter harbors a perfect serum

response factor (SRF) binding site. SRF,

a highly conserved and ubiquitously ex-

pressed transcription factor, has previ-

ously been implicated in many biological

processes such as cell proliferation,

growth, and the dynamics of the actin

cytoskeleton (Posern and Treisman,

2006). As expected, the knockdown of

SRF in U2OS cells eliminates the

response of this reporter to circadianly

collected sera, suggesting that SRF may

be the long-sought immediate early tran-

scription factor that mediates the align-

ment of the peripheral clock with the

Cell 152

central clock. The chemical nature of

the blood-borne factor that induces the

responsiveness of this reporter remains

elusive, although preliminary evidence

indicates that peptides instead of lipids

or other small molecules may regulate

the oscillation of the peripheral clocks.

What might be the cellular signal that

awakens the dormant SRF pathway? It

has been known that SRF activation

depends on the myocardin-related tran-

scription factor (MRTF), which is seques-

tered in the cytoplasm by actin monomers

(G-actin). The activation of Rho GTPases

in turn, however, promotes actin polymer

(F-actin) assembly, therefore releasing

MRTF into the nucleus to activate the

SRF transcription program (Posern and

Treisman, 2006) (Figure 1). Consistent

with this model, the authors demonstrate

that the manipulation of actin polymeriza-

tion strongly influences the serum-

induced activation of the reporter

harboring the SRF binding site, and the

knockdown of MRTF also dramatically

attenuates the serum responsiveness of

the reporter. In liver cells, both MRTF

nuclear accumulation and the actin

dynamics are under diurnal control, sug-

gesting that this Rho GTPases-actin-

MRTF signaling pathway regulates the

peripheral clock in vivo. It should be

emphasized that, although the control of

actin polymerization has been extensively

studied, its link with the circadian clock

has not been well established. In addition

to modulating gene expression, this new

observation suggests that circadian

clocks may also be a driving force for

dynamic regulation of other biological

, January 31, 2013 ª2013 Elsevier Inc. 381

Figure 1. Serum Response Factor Links Central Circadian Oscilla-

tions into Peripheral TissuesDaily oscillation in the light/dark cycle is perceived by the retina, which signalsto the suprachiasmatic nucleus (SCN) in the hypothalamus. Themaster centralcircadian clock in the SCN generates neural and hormonal cues for peripheraltissues to properly align their own molecular clocks to the light/dark cycle.Gerber et al. (2013) identify the activation of serum response factor (SRF) asa key pathway in this process. Actin polymerization (F-actin) triggered byyet-to-be-identified hormonal cue(s) and receptor(s), presumably via RhoGTPases pathways, decreases the cytoplasmic pool of actin monomer(G-actin), which induces the nuclear import of myocardin-related transcriptionfactor (MRTF) and subsequent SRF activation. Target genes of SRF includePer2, whose expression pulse provides a resetting cue to the local clock, andseveral cytoskeletal genes to ensure effective communications betweencentral clock signaling and peripheral physiology through cytoskeleton-mediated cellular functions.

processes involving cytoskel-

eton such as cell migration,

intracellular transport, and

cell division (Figure 1). Finally,

both knockdown and whole

genome-wide localization

analyses show that SRF and

MRTF mediate the serum

induction of rhythmically ex-

pressed genes, such as

Per2, whose peripheral ex-

pression is tightly controlled

by systemic cues (Kornmann

et al., 2007). Taken together,

the model suggests that

the blood-borne factor modu-

lates actin dynamics, MRTF

nuclear accumulation, and

SRF activation, which in turn

control circadian gene ex-

pression in peripheral tissues

(Figure 1).

This work provides insights

into mechanisms integrating

the central clock and periph-

eral clocks while simulta-

neously placing Rho GTPase-

actin polymerization and SRF

into the bigger picture of

circadian clock regulation.

Like any important advance,

it also raises additional ques-

tions. First, some critical

pieces of the picture are

still missing. The chemical

nature of the blood-borne

factor and the molecular

identity of the receptor and

the cellular events upstream

of actin polymerization re-

main to be clarified. Second,

the definitive roles of the

actin dynamics-SRF activa-

tion pathway in mediating

the connection between the

central and peripheral clocks

in vivo remain to be estab-

382 Cell 152, January 31, 2013 ª2013 Elsevier Inc.

lished. Finally and most

importantly, could the pertur-

bation of this pathway be an

underlying mechanism of cir-

cadian-clock-related disease

conditions such as metabolic

disorders, jet lag, or daytime

sleepiness? This work helps

identify new cogs in the ever

expanding circadian clock

mechanism and makes us

realize that many others are

still missing.

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