A Systems Biology Approach Identifies Hidden Regulatory Connections Between the Circadian and Cell-Cycle Checkpoints.

Circadian rhythms form a self-sustaining, endogenous, time-keeping system that allows organisms to anticipate daily environmental changes. The core of the clock network consists of interlocking transcriptional-translational feedback loops that ensures that metabolic, behavioral, and physiological processes run on a 24 h timescale. The hierarchical nature of the clock manifests itself in multiple points of control on the daily cell division cycle, which relies on synthesis, degradation, and post-translational modification for progression.

Distinct control of PERIOD2 degradation and circadian rhythms by the oncoprotein and ubiquitin ligase MDM2.

The circadian clock relies on posttranslational modifications to set the timing for degradation of core regulatory components, which drives clock progression. Ubiquitin-modifying enzymes that target clock components for degradation mainly recognize phosphorylated substrates. Degradation of the circadian clock component PERIOD 2 (PER2) is mediated by its phospho-specific recognition by β-transducin repeat-containing proteins (β-TrCPs), which are F-box-containing proteins that function as substrate recognition subunits of the SCF ubiquitin ligase complex.

Model-driven experimental approach reveals the complex regulatory distribution of p53 by the circadian factor Period 2.

The circadian clock and cell cycle networks are interlocked on the molecular level, with the core clock loop exerting a multilevel regulatory role over cell cycle components. This is particularly relevant to the circadian factor Period 2 (Per2), which modulates the stability of the tumor suppressor p53 in unstressed cells and transcriptional activity in response to genotoxic stress. Per2 binding prevents Mdm2-mediated ubiquitination of p53 and, therefore, its degradation, and oscillations in the peaks of Per2 and p53 were expected to correspond.

Chronotherapy: Intuitive, Sound, Founded…But Not Broadly Applied.

Circadian rhythms are a collection of endogenously driven biochemical, physiological, and behavioral processes that oscillate in a 24-h cycle and can be entrained by external cues. Circadian clock molecules are responsible for the expression of regulatory components that modulate, among others, the cell's metabolism and energy consumption. In clinical practice, the regulation of clock mechanisms is relevant to biotransformation of therapeutics.

Association of the circadian factor Period 2 to p53 influences p53's function in DNA-damage signaling.

Circadian period proteins influence cell division and death by associating with checkpoint components, although their mode of regulation has not been firmly established. hPer2 forms a trimeric complex with hp53 and its negative regulator Mdm2. In unstressed cells, this association leads to increased hp53 stability by blocking Mdm2-dependent ubiquitination and transcription of hp53 target genes.

The circadian factor Period 2 modulates p53 stability and transcriptional activity in unstressed cells.

Human Period 2 (hPer2) is a transcriptional regulator at the core of the circadian clock mechanism that is responsible for generating the negative feedback loop that sustains the clock. Its relevance to human disease is underlined by alterations in its function that affect numerous biochemical and physiological processes. When absent, it results in the development of various cancers and an increase in the cell's susceptibility to genotoxic stress.

Regulatory pathways coordinating cell cycle progression in early Xenopus development.

The African clawed frog, Xenopus laevis, is used extensively as a model organism for studying both cell development and cell cycle regulation. For over 20 years now, this model organism has contributed to answering fundamental questions concerning the mechanisms that underlie cell cycle transitions--the cellular components that synthesize, modify, repair, and degrade nucleic acids and proteins, the signaling pathways that allow cells to communicate, and the regulatory pathways that lead to selective expression of subsets of genes. In addition, the remarkable simplicity of the Xenopus early cell cycle allows for tractable manipulation and dissection of the basic components driving each transition.

Phosphorylation of Claspin is triggered by the nucleocytoplasmic ratio at the Xenopus laevis midblastula transition.

At the Xenopus midblastula transition (MBT), cell cycles lengthen, and checkpoints that respond to damaged or unreplicated DNA are established. The MBT is triggered by a critical nucleocytoplasmic (N/C) ratio; however, the molecular basis for its initiation remains unknown. In egg extracts, activation of Chk1 checkpoint kinase requires the adaptor protein Claspin, which recruits Chk1 for phosphorylation by ATR.

Near-infrared laser delivery of nanoparticles to developing embryos: a study of efficacy and viability.

Targeted delivery of materials to individual cells remains a challenge in nanoscience and nanomedicine. Near infrared (NIR) laser injection may be a promising alternative to manual injection (where the micropipet diameter limits targeting to small cells) or other laser techniques (such as picosecond green and UV lasers, which can be damaging to cells). However, the efficiency with which NIR pulses can deliver nanoparticles and any adverse effects on living cells needs thorough testing.

Cyclin E2 is required for embryogenesis in Xenopus laevis.

In mammalian cells, E-type cyclins (E1 and E2) are generally believed to be required for entry into S phase. However, in mice, cyclin E is largely dispensable for normal embryogenesis. Moreover, Drosophila cyclin E plays a critical role in cell fate determination in neural lineages independently of proliferation.

NESH (Abi-3) is present in the Abi/WAVE complex but does not promote c-Abl-mediated phosphorylation.

Abl interactor (Abi) was identified as an Abl tyrosine kinase-binding protein and subsequently shown to be a component of the macromolecular Abi/WAVE complex, which is a key regulator of Rac-dependent actin polymerization. Previous studies showed that Abi-1 promotes c-Abl-mediated phosphorylation of Mammalian Enabled (Mena) and WAVE2. In addition to Abi-1, mammals possess Abi-2 and NESH (Abi-3).

Emi1-mediated M-phase arrest in Xenopus eggs is distinct from cytostatic factor arrest.

Oocytes of most vertebrates arrest at metaphase of the second meiosis (meta-II) to await fertilization, thus preventing parthenogenetic activation. This arrest is caused by a cytoplasmic activity called cytostatic factor (CSF), which was first identified in the frog Rana pipiens oocyte >30 years ago. CSF arrest is executed by maintaining the activity of cyclin B-Cdc2 at elevated levels largely through prevention of cyclin B destruction.