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Clk
BMAL1
Clk
BMAL1
Cry
Per
Clk
BMAL1
DEC1
DEC1
Cry
Cry
Rev-Erb
a
Rev-Erb a
Per
Per
RORA
RORA
Clock controlled genes
Clock output
Figure 9.7 Mammalian circadian clock genes. Clk and Bmal1 dimerize and drive the
expression of DEC1, Rev-erba, RORA, Cry, and Per, along with clock controlled genes
that drive rhythmic processes of the body. Per and Cry then dimerize and feed back
to inhibit Clk and Bmal1. DEC1 and Rev-erba inhibit transcription of Bmal1, while RORA
drives transcription of Bmal1. The rhythmicity of these autoregulatory feedback loops
drives the circadian rhythms in all cells. The master circadian clock is located in the sup-
rachiasmatic nucleus of the hypothalamus and is entrained to the environmental pho-
toperiod through a direct input from the retina. Bmal1, brain and muscle aryl
hydrocarbon receptor nuclear translocator (ARNT)-like; Clk, Clock; Cry, Cryptochrome;
DEC1, differentially expressed in chondrocytes 1; Per, Period; Rev-erba, nuclear receptor
subfamily 1; group D; member 1; RORA, retinoic acid receptor-related orphan receptor
alpha. Model is based on Kawamoto et al. (2004) and Ko and Takahashi (2006) .
Circadian clock genes could also be playing a role in seasonal timing.
Clock genes were constantly expressed throughout hibernation in European
hamsters ( Cricetus cricetus L.) but their 24-h rhythm was eliminated ( Revel
et al., 2007 ). Specifically, Per1 was elevated during hibernation to daytime
levels of normothermic hamsters, while Per2 was expressed at approximately
nighttime levels. Bmal1 expression was intermediate between day and night
levels. In the thirteen-lined ground squirrel hypothalamus, two clock genes
were differentially expressed: BMAL1 ( ARNTL ) and DEC1 ( BHLHE40 ).
BMAL1 exhibits low expression in April compared to every other collection
point, while DEC1 is elevated during torpor ( Schwartz et al., 2013 ).
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