BIO254:Pacemaker

Overview

Pacemakers are internal biological oscillators in organisms that control the period and phase of some biological rhythms such as the circadian rhythm and the heartbeat. These rhythms involve many separate oscillating systems (or cells in the case of the heartbeat) that are coordinated by the pacemaker. Thus pacemakers are ‘master clocks’. This controlling feature distinguishes pacemakers from the other types of biological oscillators.

The study of biological rhythms, which pacemakers control, is one of the oldest fields of science. They are mentioned in the writings of Aristotle (384-322 BC) (Ward, 1971). The periods of biological rhythms range from sub-microsecond in the electric organ of eels (Moortgat, 1998), to 24 hours in circadian rhythm, to 1 month in the menstrual cycle, to 12 months in hibernation, to longer than 1 year in the flowering cycle of bamboo (Fig. 1).

Figure 1. Image from (Engelmann, 2004).

In the nervous system, rhythms underlie sensory coding, attention, memory and sleep (Fontanini, 2006). Rhythms are found in all organisms including bacteria (Engelmann, 2004), fungi, plants, insects, and mammals. Rhythms that are linked to external phenomena are called circa and those that are not are called non-circa (Hosn, 1997). Non-circa rhythms are not synchronized to external cycles but may still be influenced by external cues. Non-circa rhythms include the heartbeat.

It is likely that most or all of the biological rhythms in Fig. 1 are controlled by oscillators that have the defining characteristics of pacemakers listed above; however, it has not been common to refer to them all as pacemakers. The two most studied pacemakers are the circadian pacemaker and the cardiac pacemaker.

Circadian pacemaker

Circadian pacemakers coordinate the timing of biological events in an organism with the day/night cycle caused by the rotation of the Earth about its axis. Circadian rhythms are found in fungi, mammals, insects, fish, bacteria, and plants (Panda, 2002). Synchronizing an organism with the day/night cycle conveys selective advantages. For example, priming photosynthesis in plants before the sun comes up allows the maximum amount of light to be converted to useable energy (Merrow, 2005).

Defining Characteristics

Circadian pacemakers are self-sustaining, have a characteristic period, are capable of entrainment, and are temperature compensated.

Self-sustaining

Pacemakers will continue to oscillate in the absence of external cues. This is in contrast to external regulators of activity such as thermoregulation of lizard activity level by the warming of the sun.

Characteristic period

Circadian pacemakers have a period of approximately 24 hours, . If kept in constant darkness, some mice will have a period of activity and rest that is slightly less than 24 hours (Pittendrigh, 1993).

Entrainment

Circadian pacemakers would not be much use if they were not synchronized with the day/night cycle. Therefore, despite being self-sustaining, circadian pacemakers can be entrained (synchronized) by input from external cues such as light or social activity. The mechanisms of light entrainment are the same for yeast and Drosophila but different for mammals. For more information on light entrainment see Term 8.1 in the BIOSCI 154/254 Wikipedia.

Temperature compensation

Despite being based on biochemical reactions that are individually temperature sensitive, the circadian pacemaker is relatively insensitive to temperature variations.

History

The earliest record of an observation of a circadian pacemaker was made by de Mairan, a French astronomer, in 1729 (Hosn, 1997). He observed that clover leaves moved even in total darkness, despite the fact that the purpose of their movement seemed to be to track the sun. Linaeus (1707-1778) introduced the concept of a ‘flower clock’. In 1914 Szymanski showed that even under constant conditions goldfish had a circadian rhythm. A genetic basis for the circadian rhythms was indicated in experiments in bean plants by Bunning in 1932. Experiments by Pittendrigh around 1954 showed that the circadian rhythm in Drosophila was temperature compensated. Experiments by Ronald J. Konopka in Seymour Benzer’s lab identified a gene in which mutations caused an alteration of the circadian pacemaker’s period, and was thus named period (per). Different mutations of per either shortened or lengthened the period, or abolished the rhythm altogether (Chandrashekaran, 1999). Since then many other genes involved in circadian pacemaker have been identified in various species.

Locations

The locations of the circadian pacemakers in Drosophila and mice are shown in Fig. 3.

Figure 3. Image from (Panda, 2002).

The circadian pacemaker in Drosophila is composed of six cell types, with only between 4 and 40 cells of each type (Fig. 2a). One of these types controls the morning activity level, three are synchronized with the evening activity level, and the other two have unknown function (Stoleru, 2005). In mice, the suprachiasmatic nuclus (SCN) of the hypothalamus contains the circadian pacemaker (Fig. 2b, c).

Mechanisms

A unifying theme in circadian pacemaker mechanisms is a transcription/translation feedback loop found in fungi, insects, and mammals (Fig. 3).

Figure 3. Image from (Dunlap, 1998).

The binding of PAS proteins to DNA causes transcription of clock proteins, which inhibit the PAS proteins and are degraded. This results in an oscillation, whose period is determined by how long it takes to degrade the clock proteins. The rate of clock protein degradation depends on their rate of phosphorylation. The details differ among species, and are as follows.

1) Drosophila

Since the pacemaker gene, per, was identified in Drosophila, it is one of the best understood mechanisms. The period of the clock is tuned by the multiple phosphorylation of per. Entrainment (phase) is modulated by the light-induced degradation of Timeless protein. The proposed mechanism is outlined in Fig. 4.

Figure 4. Image from (Bae, 2006).

2) Mice

The SCN controls circadian oscillators in the retina, liver, heart, lung (Peirson, 2006), and kidney (Merrow, 2005). Circadian pacemakers control circulating hormone levels, cognitive and locomotor activity levels. Circadian pacemakers make use of a transcription/translation feedback loop. Several experiments in which single cells are isolated have shown that single cells can generate circadian rhythms of gene expression and circadian pacemakers are thus composed of a collection of single-cell circadian rhythm generators. The mechanism within each cell is diagrammed in Fig. 5.

Figure 5. Image from (Ko, 2006).

The period of the clock is again tuned by the multiple phosphorylation of Per. The output is transcription rate of clock controlled genes (Ccgs).

3) Fungi

The network in yeast is shown in Fig. 6.

Figure 6. Image from (Dunlap, 2006).

4) Fish

Many of the clock genes in mammals have homologs in zebrafish and the interactions are largely similar (Cahill, 2002).

Cardiac pacemakers

There are two cardiac pacemakers, a primary and secondary; the secondary acts as a backup should the primary cease to function. The cardiac pacemaker uses a different mechanism than the circadian pacemaker, which accounts for their different periods. The cardiac pacemaker is similar to the circadian pacemaker in that it controls many other oscillators, the cells of the heart, each of which has the ability to generate spontaneous rhythmic depolarization. The cardiac pacemaker cells in the sinoatrial node control the heart rate because they have a faster rhythm than the other cells in the heart, thus forcing them to depolarize before they normally would. A secondary cardiac pacemaker is in the atrioventricular node and acts as a backup in case the primary pacemaker fails. The mechanism that causes spontaneous depolarization is based on the unique properties of the potassium, sodium, and calcium channels in the pacemaker cells of the heart. The cells spontaneously depolarize due to both a feedback mechanism in which the rate of potassium eflux decreases as the cell depolarizes, depolarizing it further, and a ‘funny current’, which results from sodium leaking in.

Artificial pacemakers

Artificial pacemakers are currently made useing microelectronics however genetically engineered pacemakers may begin to replace microelectronic pacemakers (Boink, 2006).

Cardiac

Artificial pacemakers made with microelectronics are used in patients with deficient pacemakers, for example when the sinus node of the heart does not function correctly.

Neural

Micoelectronic pacemakers implanted in the brain of epilepsy patients at the precise (with 1 mm accuracy) locations that are the origins of seizures in a given patient create signals that help prevent seizures (http://www.clevelandclinic.org/health/health-info/docs/1900/1937.asp?index=8782).

References

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Boink GJ, Seppen J, de Bakker JM, Tan HL (2006) .Gene therapy to create biological pacemakers. Med Biol Eng Comput. Oct 18; [Epub ahead of print]

Buhusi CV, Meck WH (2005) What makes us tick? Functional and neural mechanisms of interval timing. Nat Rev Neurosci. 6(10):755-65.

Cahill GM (2002) Clock mechanisms in zebrafish. Cell Tissue Res. 2002 Jul;309(1):27-34. Epub 2002 May 25.

Chandrashekaran (1999) The Egg With Two Yellows. Indian Academy of Sciences. Journal of Genetics. 78(3):181-185.

Dunlap J (1998) Circadian rhythms. An end in the beginning. Science 280(5369):1548-9.

Dunlap JC, Loros JJ (2006) How fungi keep time: circadian system in Neurospora and other fungi. Curr Opin Microbiol. 2006 Oct 23; [Epub ahead of print]

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Fontanini A, Bower JM.Slow-waves in the olfactory system: an olfactory perspective on cortical rhythms.Trends Neurosci. 2006 Aug;29(8):429-37. Epub 2006 Jul 13.

Hosn A (1997) Analysis of Growth Rhythms and Activity Patterns of Brook Trout. Salvelinur fontinalis.Thesis, McGill University.

Ko CH, Takahashi JS (2006) Molecular components of the mammalian circadian clock. Hum Mol Genet. 15 Spec No 2:R271-7.

Merrow M, Spoelstra K, Roenneberg T (2005) The circadian cycle: daily rhythms from behaviour to genes. EMBO Rep. 6(10):930-5.

Moortgat KT, Clifford H. Keller§, Theodore H. Bullock, and Terrence J. Sejnowski (1998) Submicrosecond pacemaker precision is behaviorally modulated: The gymnotiform electromotor pathway. PNAS Vol. 95, Issue 8, 4684-4689.

Panda S, Hogenesch JB, Kay SA (2002) Circadian rhythms from flies to human. Nature 417(6886):329-35.

Peirson SN, Butler JN, Duffield GE, Takher S, Sharma P, Foster RG (2006) Comparison of clock gene expression in SCN, retina, heart, and liver of mice. Biochem Biophys Res Commun. Dec 29;351(4):800-807. Epub 2006 Oct 30.

Pittendrigh CS (1967) Circadian Systems, I. The Driving Oscillation and its Assay in Drosophila melanogaster PNAS 58:1762-1767

Ward RR (1971) The living clock. Alfred K ed. New York. 385 pages.