Exp Neurobiol 2014; 23(3): 191-199
Published online September 30, 2014
© The Korean Society for Brain and Neural Sciences
Euna Lee and Eun Young Kim*
Department of Biomedical Sciences, Department of Brain Science, Ajou University School of Medicine, Suwon 443-380, Korea
Correspondence to: *To whom correspondence should be addressed.
TEL: 82-31-219-4546, FAX: 82-31-219-4530
By means of a circadian clock system, all the living organisms on earth including human beings can anticipate the environmental rhythmic changes such as light/dark and warm/cold periods in a daily as well as in a yearly manner. Anticipating such environmental changes provide organisms with survival benefits via manifesting behavior and physiology at an advantageous time of the day and year. Cell-autonomous circadian oscillators, governed by transcriptional feedback loop composed of positive and negative elements, are organized into a hierarchical system throughout the organisms and generate an oscillatory expression of a clock gene by itself as well as clock controlled genes (ccgs) with a 24 hr periodicity. In the feedback loop, hetero-dimeric transcription factor complex induces the expression of negative regulatory proteins, which in turn represses the activity of transcription factors to inhibit their own transcription. Thus, for robust oscillatory rhythms of the expression of clock genes as well as ccgs, the precise control of subcellular localization and/or timely translocation of core clock protein are crucial. Here, we discuss how sub-cellular localization and nuclear translocation are controlled in a time-specific manner focusing on the negative regulatory clock proteins.
Keywords: circadian rhythms, nuclear translocation, phosphorylation, posttranslational modification, O-GlcNAcylation
The molecular clock present in nearly every cell is composed of transcriptional/translational feedback loop, namely TTFL . Although specific components of TTFL are different, the governing rules of TTFL are well conserved from fungi to vertebrate, including humans . Current understanding of the underlying biochemical mechanisms in animal circadian clockworks is largely based on earlier studies using the
In the mammalian system, a similar circuitry operates. CLK and CYC homolog BMAL1, posit in the center of interlocked TTFL produce
At a systemic level, cell-autonomous oscillator is orchestrated in a hierarchical network of master and peripheral oscillators. In the
Although transcriptional control via interlocked feedback loop posits as a framework for the molecular clock, diverse regulations employed after the clock genes are transcribed also play crucial roles to finely adjust the clock speed to a 24 hr period. Post-translational modification, most notably phosphorylation of the clock proteins, has been extensively studied so far [22, 23, 24, 25, 26]. The first example was the
One important issue in circadian rhythm is to generate oscillation in such a long 24 hr period. Based on a simple oscillator model , synthetic feedback loop only generates rhythmic oscillation with a 2 hr period; thus, imposing a time delay between transcriptional activation and repression is inevitable to generate such a long rhythm period [44, 45, 46, 47, 48, 49, 50, 51]. The observation that nuclear accumulation of PER is lagged in both
Traffic between the nucleus and the cytoplasm is carried out through specialized apertures, nuclear pore complexes (NPCs) [53, 54]. Various carrier proteins are involved in the translocation of cargo proteins through NPCs. Cargo proteins are targeted for nuclear import by a short nuclear localization signal (NLS) sequence motif. A well-known NLS is composed of one (monopartite) or two (bipartite) basic amino acid clusters. The classic nuclear import pathway uses importin β1 (Impβ1) as a carrier, which recognizes NLS as cargo and binds through the adaptor molecule importin α (Impα). It is the trimeric cargo containing protein complex Impα/Impβ1/NLS that can enter the nucleus [55, 56, 57].
Similar to the
In mammals, the regulation of nuclear entry via phosphorylation of mPER is inconsistent depending on types of cells and kinds of proteins studied. Casein kinase 1 delta (CKIδ) and CKIε are two paralogs of mammalian CKI, both target mPERs as substrates regulating their stability and subcellular localization [79, 80]. Although some degree of functional redundancy of CKIε and CKIδ was observed, depending on the tissues, one might act more dominant than the other, as shown in the study where the depletion of CKIδ resulted in a long circadian period in the absence of behavioral effect with the depletion of CKIε . In HEK293T cells, ectopically expressed mPER1 enters the nucleus while mPER2 resides in the cytoplasm . This observation provided the idea that there must be a mechanism to prevent the premature nuclear entry of mPER1. This turned out to be a CKIε mediated phosphorylation of mPER1 via masking of NLS motif on mPER1 . Interestingly, the co-expression of mCRY1 in the presence of CKIε and mPER1 brings all three components in the vicinity, and this trimeric complex can enter the nucleus . Thus, mCRY1 is able to negate the CKIε mediated cytoplasmic retention of mPER1. This phenomenon is very closely related to the situation where DBT dependent phosphorylation retards the nuclear entry of dPER in
Recent findings have indicated that the extent of
mPER2 is also modified with
To be able to sustain a 24 hr rhythm period, timely control of nuclear translocation is crucial in both