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Results for “Biological Clocks”

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At least 127 records · Page 7Linked to original sources
PubMed1992

Perinatal development of human circadian rhythms: role of the foetal biological clock.

The development of circadian rhythms and the neuronal mechanisms underlying their generation (particularly the suprachiasmatic nucleus of the hypothalamus) were reviewed. Based on perinatal animal studies and data from human foetuses and/or preterm infants it was concluded that human circadian rhythms are present as early as at 30 weeks of gestation. The significance of the mother and/or the environment regarding the entrainment of the "endogenous" foetal biological clock was emphasized.

Circadian Rhythm↗
PubMed1992

Endogenous synthesis of peptidoglycan in eukaryotic cells; a novel concept involving its essential role in cell division, tumor formation and the biological clock.

Degradation products of peptidoglycan, the universal bacterial cell wall constituent, were previously found in animal tissues and urine. Reassessment and quantitative analysis of available data lead to an original concept, i.e. that eukaryotic cells synthesize peptidoglycan. We present a model in which this endogenously synthesized peptidoglycan is essential for the processes of eukaryotic cell division and sleep induction in animals. Genes for peptidoglycan metabolism, like those for lysine biosynthesis in plants, are probably inherited from endosymbiotic bacteria, the ancestors of mitochondria and chloroplasts. Corollaries of this concept, i.e. roles for peptidoglycan metabolism in tumor formation and in the biological clock, are supported by abundant evidence. We propose that many interactions between bacteria and eukaryotes are conditioned by their common genetic heritage.

Animals↗
PubMed2006

The proteasome-dependent degradation of CKB4 is regulated by the Arabidopsis biological clock.

Most organisms have evolved an internal timing mechanism, the circadian clock, that is able to generate and maintain 24 h rhythmic oscillation in molecular, biochemical and metabolic activities. In Arabidopsis, the clock-dependent synchronization of physiology with the environment is essential for successful growth and development. The mechanisms of the Arabidopsis clockwork have been described as transcriptional feedback loops at the core of the oscillator. However, an increasing body of evidence points towards a key role of post-translational regulation of clock components as an essential mechanism of circadian function. Here, we identify CKB4, a CK2 regulatory subunit, as a component of the Arabidopsis circadian system. We demonstrate that the nuclear-localized CKB4 protein exists in vivo as different isoforms, resulting from phosphorylation on serine residues. Our findings show that the phosphorylated isoforms are the preferred substrate for ubiquitination and degradation by the proteasome pathway. We provide evidence of the involvement of the biological clock in the circadian regulation of CKB4 protein abundance, which itself is important for an accurate control of circadian period by the clock. Overexpression of CKB4 results in elevated CK2 overall activity and period-shortening of clock-controlled genes peaking at different phase angles. Restriction of CKB4 protein phosphorylation and/or degradation to specific phases within the circadian cycle might provide the cell with a fine-tuning mechanism to selectively regulate the CK2 phosphorylation activity on specific substrates.

Amino Acid Sequence↗
PubMed2006

Fluoxetine modulates the circadian biological clock via phase advances of suprachiasmatic nucleus neuronal firing.

BACKGROUND: The documented ability of serotonin (5-HT) to directly modulate circadian rhythms prompted interest in a similar role for therapeutic agents that readily enhance 5-HT neurotransmission, namely the selective serotonin reuptake inhibitors (SSRIs). METHODS: Extracellular recordings of unit firing of suprachiasmatic nucleus (SCN) neurons maintained in slice culture enabled determinations of circadian rhythmicity. Shifts in the peak of activity were determined during the next circadian cycle following drug exposure. RESULTS: Fluoxetine (10 microm, 60 minutes incubation) produced robust phase advances only in the presence of L-tryptophan (.5 microm), added to maintain serotonergic tone. CONCLUSIONS: Actions of SSRIs at the level of the circadian biological clock add to the list of pharmacological effects for this drug class and encourage speculation as to their importance clinically.

Animals↗
PubMed1983

Regulation of Cell Shape in Euglena gracilis: I. Involvement of the Biological Clock, Respiration, Photosynthesis, and Cytoskeleton.

The alga Euglena gracilis Z. changes its shape two times per day when grown under the synchronizing effect of a daily light-dark cycle. At the beginning of the light period when photosynthetic capacity is low, the population of cells is largely spherical in shape. The mean cell length of the population increases to a maximum in the middle of the light period when photosynthetic capacity is greatest, and then decreases for the remainder of the 24-hour period. The population becomes spherical by the end of the 24-hour period when the cycle reinitiates. These changes are also observed under constant dim light conditions (up to 72 hours) and are therefore controlled by the biological clock and represent a circadian rhythm in cell shape. In constant dim light, the cell division rhythm is either arrested or slowed considerably, while the cell shape rhythm continues.The involvement of respiratory and photosynthetic pathways in the cell shape changes was investigated with energy pathway inhibitors. Antimycin A and NaN(3) both inhibited the round to long and long to round shape changes, indicating that the respiratory pathways are involved. DCMU and atrazine inhibited the round to long shape change but did not affect the long to round transition, indicating that light-induced electron flow is necessary only for the round to long shape change.The influence of the cell shape changes on the photosynthetic reactions was investigated by altering cell shape with the cytoskeletal inhibitors cytochalasin and colchicine. Both inhibitors blocked the round to long and long to round shape changes. Cytochalasin B was found to have minimal cytotoxic effects on the photosynthetic reactions, but colchicine significantly inhibited light-induced electron flow and the in vivo expression of the photosynthetic rhythm.

Journal Article↗
PubMed1983

Dependence of memory of meal time upon circadian biological clock in rats.

Water intake was measured in rats under a free feeding schedule and under a meal feeding schedule (food was presented for 3 hours each day from 14:00 to 17:00 hr), both under LD condition. Both schedules allowed animals free access to water. Under the meal feeding schedule, drinking occurred mainly during the food presentation time, but it was also observed during the dark period, when food was not present. After 3 weeks of meal feeding, the rats were transferred to the free feeding schedule. They showed a habit of drinking at the meal time of the previous schedule (meal-feeding-induced drinking) up to 7 days of free feeding. When the rats were made blind in the transition from the meal feeding to the 2nd free feeding, the meal-feeding induced drinking occurred with a progressively increasing delay relative to the clock time, in the same way as seen with the nocturnal drinking. Rats with bilateral lesions of the suprachiasmatic nucleus did not show nocturnal drinking during the meal feeding schedule, and no signs of the meal-feeding-induced drinking were observed after the 2nd free feeding schedule was started. It was concluded that the persistence of the memory of the time of meal feeding in the following free feeding period could be traced with water-drinking as an index and such memory seems to be formed depending on the biological clock which is ascribed to activity of the suprachiasmatic nucleus under LD condition.

Animals↗
PubMed2001

Earth--moon evolution: implications for the mechanism of the biological clock?

The geophysical characteristics of the planet Earth dictate the physiological traits of living organisms. Changes in the geophysical conditions over the course of geological time are responsible for major evolutionary changes in life emergence and evolvement. Calendar day length is one of earth's geophysical characteristics which is under a constant, if extremely small, progressive change. This enforces an adjustment of circadian rhythmicity throughout geological time. The calendar day has extended approximately 9 hours in the last 3.5 billion years. Two mechanisms for circadian-rhythm adjustment are suggested: a directional selection mechanism -- an endogenous -- oriented explanation regarding a genetic drift in the population's endogenous oscillation toward a lengthened daily cycle; and an exogenous calibration mechanism - a hypothesis on the existence of a geophysical responsive element which senses a geophysical stimuli and calibrates the inner cellular oscillation in accordance with the length of the calendar day. A distinguishing experiment between the two explanations is suggested and discussed. Circadian rhythm mechanism and the evolution of circadian rhythmicity are tightly connected. Circadian rhythms' evolutionary theories are discussed in light of their contribution to our understanding of the selective pressures being applied throughout geological time and of how, once the clock has been established, it maintains an ongoing adjustment to a continuous change in the length of day.I argue that the exogenous calibration mechanism combines with the endosymbiont coordination theory, together, present an explanation to the path by which the calendar day adjustment was acquired and maintained. This hypothesis suggests a role for gravity cyclic force and for cytoskeleton's components in calendar day adjustment mechanism and circadian rhythm entrainment.

Animals↗
PubMed1988

Butyrate infusions in the ovine fetus delay the biologic clock for globin gene switching.

The switch from fetal to adult hemoglobin expression is regulated in many mammalian species by a developmental clock-like mechanism and determined by the gestational age of the fetus. Prolonging fetal globin gene expression is of considerable interest for therapeutic potential in diseases caused by abnormal beta-globin genes. Butyric acid, which is found in increased plasma concentrations in infants of diabetic mothers who have delayed globin gene switching, was infused into catheterized fetal lambs in utero during the time of the normal globin gene switch period. The globin gene switch was significantly delayed in three of four butyrate-treated fetuses compared with controls and was entirely prevented in one fetus in whom the infusion was begun before the globin switch was under way. These data provide a model for investigating and arresting the biologic clock of hemoglobin switching.

Animals↗
PubMed1994

The biological clock that measures the mitotic life-span of mouse embryo fibroblasts continues to function in the presence of simian virus 40 large tumor antigen.

Normal mammalian fibroblasts cultured in vitro undergo a limited number of divisions before entering a senescent phase in which they can be maintained for long periods but cannot be induced to divide. In rodent fibroblasts senescence can be prevented by expression of simian virus 40 large tumor antigen (T antigen). Cells expressing T antigen can proliferate indefinitely; however, such cells are absolutely dependent upon continued expression of T antigen for maintenance of growth; inactivation of T antigen results in a rapid and irreversible entry into a postmitotic state. To determine when, after the initial expression of T antigen, fibroblasts become dependent upon it for continued growth, we serially cultivated embryonic fibroblasts prepared from H-2Kb-tsA58 transgenic mice. We show that these fibroblasts become dependent upon T antigen for maintenance of proliferation only when their normal mitotic life-span has elapsed and that the biological clock that limits the mitotic potential continues to function normally, even in cells expressing this immortalizing gene. Our results suggest that random accumulation of cellular damage is unlikely to be the factor that limits fibroblast division but support the hypothesis that senescence is regulated via a genetic program.

Animals↗
PubMed1987

The biological clock of Chlamydomonas reinhardii in space.

The overt circadian rhythm in a wildtype (wt+) and a short period (s-) strain of Chlamydomonas reinhardii has been studied in space using the photoaccumulation behavior as the recorded parameter. The period of the wt+ was 29.6 h, of the s- 21.4 h and did not deviate significantly from ground controls performed exactly at the same time. The phase was delayed in space by 4.2 h in the wt+, but was not altered in the s-. In both strains the amplitudes were significantly higher in space than in the ground controls. During the recording period of 6.5 days the cell density increased in both strains. The survival rate, i.e. the ability to form colonies on agar petri dishes, was higher in space than on ground. The period was in both strains by 1.1 h longer in Florida (Kennedy Space Center) in both the flight and the control samples than in Europe. The significance of these results is discussed with respect to the endogenous nature of the biological clock and the role of the microgravity environment.

Cell Division↗
PubMed1975

Ontogenesis of a biological clock for serotonin:acetyl coenzyme A N-acetyltransferase in pineal gland of rat.

Serotonin:acetyl coenzyme A N-acetyltransferase (EC 2.3.1.5) Activity in pineal gland was assayed in rats which were born and raised under diurnal lighting conditions, in continuous darkness, or in constant light. N-Acetyltransferase activity in the pups raised under diurnal lighting showed a rhythmic pattern, with high enzyme activity during dark period. The pups raised in continuous darkness also showed a rhythmic pattern, the phase of which was delayed by 8 hr in 7-week-old pups; the rhythmic phase of N-acetyltransferase began in the evening in 12-day-old pups and was regularly delayed by 1 hr every week. The pups raised in constant illumination also showed a rhythmic pattern; the rhythmic phase was delayed by 3 hr every week. When the mother rats were coupled in darkness and maintained in darkness during pregnancy and after the pups were born, their pups again showed a rhythmic pattern. These observations indicate that the biological clock for N-acetyltransferase is generated independently of environmental lighting. When mothers were coupled under reversed lighting conditions and transferred into darkness or light, the rhythmic pattern in their pups was inverted 180 degrees from that of pups born of diurnal mothers. When the pups were raised in darkness, the rhythmic phase of N-acetyltransferase in the pups was similar to that of their mothers. It is suggested that in the absence of light-darkness cycle, the mother rat sets the rhythm of the pups to synchronize with her own rhythm. When pups were reared by a foster mother with a different rhythmic pattern from that of their original mother, the rhythmic phase in the pups was closer to that of the original mother, suggesting that the original mother plays the predominant role setting the rhythm of the pups.

Acetyltransferases↗
PubMed2003

Cellular location and circadian rhythm of expression of the biological clock gene Period 1 in the mouse retina.

The cellular location and rhythmic expression of Period 1 (Per1) circadian clock gene were examined in the retina of a Per1::GFP transgenic mouse. Mouse Per1 (mPer1) RNA was localized to inner nuclear and ganglion cell layers but was absent in the outer nuclear (photoreceptor) layer. Green fluorescent protein (GFP), which was shown to colocalize with PER1 protein, was found in a few subtypes of amacrine neuron, including those containing tyrosine hydroxylase, calbindin, and calretinin, but not in cholinergic amacrine cells. A small subset of ganglion cells also contained GFP immunoreactivity (GFP-IR), but the melanopsin-containing subtype, which projects to the suprachiasmatic nuclei (SCN), lacked GFP-IR. Although the intensity of GFP-IR varied among the populations of amacrine cells at each time point that was examined, both diurnal and circadian rhythms were found for the fraction of neurons showing strong GFP-IR, with peak expression between Zeitgeber/circadian (ZT/CT) times 10 and 14. In SCNs that were examined in the same mice used for the retinal measures, the peak in GFP-IR also occurred at approximately ZT/CT 10. Our results are the first to demonstrate a circadian rhythm of a biological clock component in identified neurons of a mammalian retina.

Amacrine Cells↗