Timeless (gene)

Timeless (''tim'') is a gene in multiple species but is most notable for its role in ''Drosophila'' for encoding TIM, an essential protein that regulates circadian rhythm. ''Timeless'' mRNA and protein oscillate rhythmically with time as part of a transcription-translation negative feedback loop involving the '' period'' (''per'') gene and its protein.

Discovery

In 1994, ''timeless'' was discovered through forward genetic screening performed by Jeffery L. Price while working in the lab of Michael W. Young. This gene was found when they noticed an arrhythmic ''tim''⁰¹ mutant via a P element screen. The tim⁰¹ mutation caused arrhythmic behavior, defined by the lack of ability to establish proper circadian rhythms. In 1995, the timeless gene was cloned by Amita Sehgal and partners in the lab of Michael W. Young. Unlike the Drosophila ''timeless'' gene, homologs have been discovered in other species that are non-essential for circadian rhythm. The discovery of ''timeless'' followed the discovery of the ''period'' mutants in 1971 through forward genetic screening, the cloning of ''per'' in 1984, and an experiment determining that ''per'' is circadian in 1990. This occurred during a period of rapid expansion in the field of chronobiology in the 1990s.

Structure

The length of the coding region of the Drosophila ''timeless'' gene is 4029 base pairs, from which a 1398 amino acid protein is transcribed. The gene starts at a consensus cap site upstream of a methionine codon. It contains 11 exons and 10 introns. In various ''Drosophila'' species, the ''timeless'' protein TIM contains more highly conserved functional domains and amino acid sequence than its counterpart, PER (protein encoded by ''per''). CLD was the least conserved of these regions between ''D. virilis'' and ''D. melanogaster''. These conserved parts include: the PER interaction domain, the nuclear localization signal (NLS), cytoplasmic localization domain (CLD), N-terminal end (non-functional), and C-terminal end. TIM is also known to have a basic region, which interacts with the PAS domain of the PER protein, and a central acidic region. There is also a region of unknown function near the N-terminus of the TIM protein that contains a 32 amino acid sequence that, when deleted, causes arrhythmic behavior in the fly. In various species of Drosophila, such as ''D. virilis'' and ''D. melanogaster'', the initiating methionine for translation of the ''timeless'' gene into TIM is in different places, with the ''D. virilis'' start site downstream of the start site in ''D. melanogaster''.

''Timeless'' homologs

Drosophila homolog

The ''timeless'' gene is an essential component of the molecular circadian clock in ''Drosophila''. It acts as part of an autoregulatory feedback loop in conjunction with the period (per) gene product as noted in collaborative studies performed by the labs of Michael W. Young and Amita Sehgal. Further studies by the labs of Young, Sehgal, Charles Weitz, and Michael Rosbash indicated that ''timeless'' protein (TIM) and ''period'' protein (PER) form a heterodimer that exhibits circadian rhythms in wild type ''Drosophila''. Researchers in Rosbash's lab also showed that ''tim'' mRNA levels and TIM protein levels have circadian rhythms that are similar to those of the ''period'' (''per'') mRNA and its product. Experiments done jointly by the Weitz, Young, and Sehgal labs using yeast 2-hybrid proved that TIM directly binds with PER. During the early evening, PER and TIM dimerize and accumulate. Late at night, the dimer travels into the nucleus to inhibit ''per'' and ''tim'' transcription. In 1996, the teams of Sehgal, Edery, and Young found that exposure to light leads to the degradation of TIM and subsequently PER.

The PER/TIM heterodimer negatively regulates transcription of ''period'' (''per'') and ''timeless'' (''tim'') genes. Within this negative feedback loop, first the PER/TIM heterodimers form in the cytoplasm, accumulate, and then translocate to the nucleus. The complex then blocks the positive transcription factors clock (CLK) and cycle (CYC), thereby repressing the transcription of ''per''.

As part of the circadian clock, ''timeless'' is essential for entrainment to light-dark (LD) cycles. The typical period length of free-running ''Drosophila'' is 23.9 hours, requiring adaptations to the 24-hour environmental cycle. Adaptation first begins with exposure to light. This process leads to the rapid degradation of the TIM protein, allowing organisms to entrain at dawn to environmental cycles. In light-dark cycles, TIM protein level decreases rapidly in late night/early morning, followed by the similar but more gradual changes in PER protein level. TIM degradation is independent of ''per'' and its protein, and releases PER from the PER/TIM complex. In some cell types, the photoreceptor protein cryptochrome (CRY) physically associates with TIM and helps regulate light-dependent degradation. CRY is activated by blue light, which binds to TIM and tags it for degradation. This ends the PER/TIM repression of the CLK/CYC-mediated transcription of ''per'' and ''tim'' genes, allowing ''per'' and ''tim'' mRNA to be produced to restart the cycle.

This mechanism allows entrainment of flies to environmental light cues. When ''Drosophila'' receive light inputs in the early subjective night, light-induced TIM degradation causes a delay in TIM accumulation, which creates a phase delay. When light inputs are received in the late subjective night, a light pulse causes TIM degradation to occur earlier than under normal conditions, leading to a phase advance.

In ''Drosophila'', the negative regulator PER, from the PER/TIM complex, is eventually degraded by a casein kinase-mediated phosphorylation cycle, allowing fluctuations in gene expression according to environmental cues. These proteins mediate the oscillating expression of the transcription factor VRILLE (VRI), which is required for behavioral rhythmicity, ''per'' and ''tim'' expression, and accumulation of PDF (pigment-dispersing factor).

''Gryllus bimaculatus'' (two-spotted cricket) homolog

''Timeless'' does not appear to be essential for oscillation of the circadian clock for all insects. In wild type ''Gryllus bimaculatus,'' ''tim'' mRNA shows rhythmic expression in both LD and DD (dark-dark cycles) similar to that of ''per'', peaking during the subjective night. When injected with ''tim'' double-stranded RNA (''dstim''), ''tim'' mRNA levels were significantly reduced and its circadian expression rhythm was eliminated. After the ''dstim'' treatment, however, adult crickets showed a clear locomotor rhythm in constant darkness, with a free-running period significantly shorter than that of control crickets injected with Discosoma sp. Red2 (DsRed2) dsRNA. These results suggest that in the cricket, ''tim'' plays some role in fine-tuning of the free-running period but may not be essential for oscillation of the circadian clock.

Mammalian homolog

In 1998, researchers identified a mouse homolog and a human homolog of the ''Drosophila'' ''timeless'' gene. The exact role of TIM in mammals is still unclear,. Recent work on the mammalian ''timeless'' (''mTim'') in mice has suggested that the gene may not play the same essential role in mammals as in ''Drosophila'' as an necessary function of the circadian clock. While ''Tim'' is expressed in the Suprachiasmatic Nucleus (SCN) which is thought to be the primary oscillator in humans, its transcription does not oscillate rhythmically in constant conditions, and the TIM protein remains in the nucleus.

However, ''mTim'' is shown to be necessary for embryonic development in mice, indicating a different gene function than in ''Drosophila''. This suggests a divergence between mammalian clocks and the ''Drosophila'' clock. Moreover, mammalian ''tim'' is more orthologous to the ''Tim-2'' (''Timeout'') paralog of the ''Drosophila'' ''Timeless'' gene than the actual gene itself. Like ''tim-2'', the mamallian orthologs has a C-terminal PARP1-binding (PAB) domain. The complex they from promotes homologous recombination DNA repair.

The ''timeless'' protein is thought to directly connect the cell cycle with the
circadian rhythm in mammals. In this model. referred to as a “direct coupling,” the two cycles share a key protein whose expression exhibits a circadian pattern. The essential role of ''Tim'' in ''Drosophila'' in creating circadian rhythm is accomplished by ''Cry'' in mammals. In mammals, ''Cry'' and ''Per'' transcription is activated by the CLOCK/BMAL1 complex, and repressed by the PER/CRY complex.

Humans

The human ''timeless'' protein (hTIM) has been shown to be required for the production of electrical oscillations output by the suprachiasmatic nucleus (SCN), the major clock governing all tissue-specific circadian rhythms of the body. This protein also interacts with the products of major clock genes CLOCK, BMAL, PER1, PER2 and PER3.

Sancar and colleagues investigated whether hTIM played a similar role to orthologs in ''C. elegans'' and types of yeast, which are known to play important roles in the cell cycle. Their experiments suggested that hTIM plays an integral role in the G2/M and intra-S cell cycle checkpoints. With respect to the G2/M checkpoint, hTIM binds to the ATRIP subunit on ATR – a protein kinase sensitive to DNA damage. This binding between hTIM and ATR then leads to the phosphorylation of Chk1, resulting in cell cycle arrest or apoptosis. This process serves as an important control to stop the proliferation of cells with DNA damage prior to mitotic division. The role of hTIM in the intra-S checkpoint is less clear at the molecular level; however, down-regulation of hTIM leads to an increase in the rate of generation of replication forks – even in the presence of DNA damage and other regulatory responses.

Current research

The ''Timeless'' gene has also been found to influence the development of disease in humans. Downregulation of the ''timeless'' gene in human carcinoma cells leads to shortened telomeres, indicating its role in telomere length maintenance. Telomere-associated DNA damage also increases in ''timeless'' depleted cells, along with the delay of telomere replication. Swi1 is a ''timeless''-related protein that is required for DNA replication in the telomere region. This association between ''timeless'' and telomeres is indicative of the gene's possible association with cancer.

A single nucleotide polymorphism substitution that results in the transformation of glutamine to arginine in the amino acid sequence in the human ''timeless gene'' shows no association with either morningness or eveningness tendencies in humans. This is consistent with other research, suggesting that ''htim'' is not important in the circadian rhythm of humans.

Timeless is now frequently found to be overexpressed in many different tumor types. In a study that used Timeless-targeting siRNA oligos, followed by a whole-genome expression microarray as well as network analysis. Further testing of Timeless down-regulation on cell proliferation rates of a cervical and breast cancer cell line. It was found that elevated expression of Timeless was significantly associated with more advanced tumor stage and poorer breast cancer prognosis. Similarity in gene expression signatures has allowed for TIMELESS to be identified as Kinase Suppressor of Ras-1 (KSR1)-like and a potential target required for cancer cell survival. TIMELESS overexpression represents a vulnerability in Ras-driven tumors that offers potential insight into novel and selective targets found in Ras-driven cancers, which can be leveraged to develop selective and more effective therapeutics.

See also

* Clock gene
* Period gene
* Suprachiasmatic nucleus
* Oscillating gene
* PDF (gene)

References

Further reading

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External links

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HHMI - The Drosophila Molecular Clock Model
{{Webarchive, url=https://web.archive.org/web/20130217032321/http://www.hhmi.org/biointeractive/clocks/drosophila_clock.html , date=2013-02-17

Science Magazine Neurobiology
Drosophila melanogaster genes