Year: 2006
Type: review
Abstract: The earth rotates on its axis every 24 h, with the result that any position on the earth's surface alternately faces toward or away from the sun—day and night. That the metabolism, physiology, and behavior of most organisms changes profoundly between day and night is obvious to even the most casual observer. These biological oscillations are apparent as diurnal rhythms. It is less obvious that most organisms have the innate ability to measure time. Indeed, most organisms do not simply respond to sunrise but, rather, anticipate the dawn and adjust their biology accordingly. When deprived of exogenous time cues, many of these diurnal rhythms persist, indicating their generation by an endogenous biological circadian clock. Until recently, the molecular mechanisms by which organisms functioned in this fourth dimension, time, remained mysterious. However, over the last 30 or so years, the powerful approaches of molecular genetics have revealed the molecular underpinnings of a cellular circadian clockwork as complicated and as beautiful as the wonderful chronometers developed in the 18th century. Then, the need to accurately measure time to precisely determine longitude sparked an international competition to claim a prize, the princely sum of 20,000 pounds sterling, offered by the British Crown (Sobel, 1995). Critical Terms Used to Describe Circadian Rhythms. Period is defined as the time to complete one cycle. It is commonly measured from peak to peak but could equally be measured from trough to trough or from any specified phase marker. Phase is the time of day for any given event. For example, if the peak in a rhythm occurred at dawn, the phase of the peak would be defined as 0 h. If a rhythm peaked 6 h after dawn, its phase would be 6 h, and so on. Phase is often defined in zeitgeber time (ZT). Zeitgeber is German for time giver, and any stimulus that imparts time information to the clock is a zeitgeber. The onset of light is a powerful zeitgeber, and dawn is defined as ZT0. The amplitude of the rhythm is defined as one-half the peak-to-trough distance. Only in exceptional circumstances, such as in the laboratory, is an organism deprived of environmental time cues, such as light/dark cycles or temperature cycles, that derive from the alternation of day and night. These environmental time cues, termed zeitgebers (German for time givers), entrain the endogenous timing system to a period of 24 h, precisely corresponding to the exogenous period of the earth's rotation. The ability of a stimulus to reset the clock is a function of the time of day (phase; see Figure 1) at which the stimulus is administered. A pulse of light given before dawn will advance the phase of the clock, yet the same pulse of light given after dusk will delay the phase. If given at noon, the same pulse of light will have no effect at all. From this it is apparent that the clock regulates its own sensitivity to environmental stimuli. This varying sensitivity can be quantified and displayed as a phase response curve, in which one plots the shift in phase in response to a stimulus applied at different times across the circadian cycle (Dunlap et al., 2004). The first writings, at least in the western canon, to recognize diurnal rhythms come from the fourth century BC. Androsthenes described the observation of daily leaf movements of the tamarind tree, Tamarindus indicus, that were observed on the island of Tylos (now Bahrein) in the Persian Gulf during the marches of Alexander the Great (Bretzl, 1903). There was no suggestion that the endogenous origin of these rhythms was suspected at the time, and it took more than two millennia for this to be experimentally tested. The scientific literature on circadian rhythms began in 1729 when the French astronomer de Mairan reported that the daily leaf movements of the sensitive heliotrope plant (probably Mimosa pudica) persisted in constant darkness, demonstrating their endogenous origin (de Mairan, 1729). Presciently, de Mairan suggested that these rhythms were related to the sleep rhythms of bedridden humans. It took 30 years before de Mairan's observations were independently repeated (Hill, 1757; Duhamel duMonceau, 1759; Zinn, 1759). These studies excluded temperature variation as a possible zeitgeber driving the leaf movement rhythms. Leaf Movements of a Representative Species. (A) Sleep movements of Phaseolus coccineus. The position of the primary leaves of a seedling at night is at the left and during the day is at the right. (B) Circadian rhythm of leaf movements of P. coccineus entrained to light/dark cycles and monitored in continuous light. As can be inferred from the leaf positions in (A), the peaks of the curve represent the nighttime leaf position. The vertical lines indicate 24-h intervals. The period for this trace is ∼27 h. (A) was originally published as Figure 14 and (B) as Figure 4 in Chapter 2 in Bünning (1973). Both are reproduced with kind permission of Springer Science and Business Media. Not surprisingly, that these circadian rhythms in leaf movements were truly endogenous was disputed. Pfeffer (1873), for example, suspected that light leaking into the darkrooms (and wine cellars and caves) employed in these studies foiled the attempts to provide constant conditions and invalidated the claims that these rhythms had endogenous origins. However, the critics ultimately were persuaded by the accumulating mass of evidence. Pfeffer himself extensively studied leaf movements and provided many examples of the free-running periods of leaf movement rhythms differing from 24 h (Pfeffer, 1915). That the rhythms were circadian and not exactly 24 h was an extremely important point because it was the best evidence, until experiments on the fungus Neurospora crassa were conducted in space (Sulzman et al., 1984), that these rhythms were truly endogenous and not driven by some subtle and undetected geophysical cue associated with the rotation of the earth on its axis. The third key criterion of circadian rhythms is temperature compensation, and it took much longer for this attribute to become appreciated. The rationale for examining temperature dependence of the period length emerged from the expectation that the clock mechanism was based on alternating chemical processes. Thus, it was anticipated that, like chemical processes, the clock should exhibit marked temperature dependence. The rate of a typical chemical reaction doubles with a 10° increase in temperature (Q10 = 2). However, the period of leaf movement in P. coccineus exhibited a Q10 of only 1.2 (Bünning, 1931). By the 1960s, this observation had been extended to many other plants as well as to animals (Sweeney and Hastings, 1960). That the clocks were not temperature independent but, instead, exhibited less than expected temperature dependence strongly supported the concept of a temperature compensation mechanism that was imperfect. Consistent with this view was the observation of Q10 values of <1.0: an imperfect compensation mechanism could lengthen the period either insufficiently or too greatly at higher temperatures or, conversely, shorten the period too little or too much at lower temperatures. As early as 1880, Charles and Francis Darwin suggested the heritability of circadian rhythms (Darwin and Darwin, 1880), as opposed to the imprinting of a 24-h period by exposure to diurnal cycles during development. This was initially explored in the 1930s by two strategies. In one, plants or animals were raised in constant conditions for multiple generations. One of the most grueling among such studies demonstrated the retention of stable rhythms among fruit flies reared in constant conditions for 700 generations (reviewed in Johnson, 2005). In a second strategy, seedlings or animals were exposed to cycles that differed from 24 h in an effort to imprint novel periods; such studies could sometimes impose the novel period length during the novel cycles, but upon release into continuous conditions, the endogenous circadian period was restored (Bünning, 1973). The inheritance of period length among progeny from crosses of parents with distinct period lengths was first reported in Phaseolus; hybrids had period length intermediates between those of the parents (Bünning, 1932, 1935). Forward genetic analysis to identify components of circadian clocks began in the 1970s. Although now it seems axiomatic that circadian clocks are composed of the products of genes, just how this might be so was the source of considerable controversy. It was argued that forward genetic efforts would be fruitless because clocks were sufficiently complex to reasonably be expected to exhibit polygenic inheritance (Bünning, 1935) and would not yield easily to standard genetic approaches. However, mutations conferring altered period length were identified and characterized in the fruitfly Drosophila melanogaster (Konopka and Benzer, 1971), the green alga Chlamydomonas reinhardtii (Bruce, 1972), and the filamentous fungus N. crassa (Feldman and Hoyle, 1973). It took more than a decade to clone the first clock gene, the Drosophila period (per) gene (Bargiello and Young, 1984; Zehring et al., 1984), and another 5 years to clone the second, the Neurospora frequency gene (McClung et al., 1989). However, the decade of the 1990s saw rapid progress toward the identification of clock components and the elucidation of oscillator mechanisms central to the circadian clock in a number of organisms, most notably Drosophila, Neurospora, and mice (Dunlap, 1999). In plants, it was realized that the leaf movement rhythm was only one among many rhythms that included germination, growth, enzyme activity, stomatal movement and gas exchange, photosynthetic activity, flower opening, and fragrance emission (Cumming and Wagner, 1968). However, genetic studies of plant clocks languished after Bünning's first experiments. Two critical discoveries changed this. First, Kloppstech (1985) described a circadian rhythm in pea in the abundance of three nuclear-encoded transcripts encoding the light-harvesting chlorophyll a/b binding protein (LHCB; also called CAB), the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, and an early light-induced protein. This observation was replicated and extended in wheat, where it was shown that the transcription rate for the Cab-1 gene was under circadian control (Nagy et al., 1988). Neither pea nor wheat was particularly suitable for positional gene cloning, but Arabidopsis thaliana was emerging as a powerful system in which to combine forward genetic analysis with molecular gene cloning techniques (Somerville and Koornneef, 2002). It was soon established that the transcription rate and transcript accumulation of Arabidopsis LHCB (Millar and Kay, 1991) and a number of other genes (McClung and Kay, 1994) were also under circadian control. These initial Arabidopsis experiments were quite labor intensive, as tissues for RNA extraction, RNA gel blotting, and nuclear run-on analyses had to be harvested at frequent intervals over fairly lengthy time courses. Such experiments inevitably became exercises in sleep deprivation for the experimenters and provided considerable disincentive to the recruitment of graduate students into the field. Moreover, forward genetic analysis required a sensitive, reliable, and nondestructive assay that could score the circadian activity of individual seedlings without killing them. The luciferases offered a of the of with the release of a at this light emission can be quantified with or with sensitive et al., 2005). et demonstrated that a of the Arabidopsis would transcription and accumulation of as light emission from individual Arabidopsis seedlings the There was an of in as it that the protein was quite and protein to in However, protein activity after only a cycles, with the result that light de that is by transcript The is sufficiently that its accumulation the transcription when driven by the LHCB is this initial in in circadian studies to other organisms, Drosophila and et al., 2005). 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Source: The Plant Cell
Author: C. Robertson McClung
Institution: Dartmouth College
Language: English
Subfield: Endocrine and Autonomic Systems
Field: Neuroscience
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W2147948709