The molecular circadian clock and circadian rhythmicity Flashcards
Characteristics of circadian clocks
- a hierarchical network of central + peripheral clocks
- cell autonomous (pacemakers!)
- All clocks: same components, same properties but
○ peripheral clocks only affect the respective tissues + organs - Central clock (SCN):
○ entrains peripheral clocks
○ takes 4-12 h to synchronise the peripheral clocks
§ If one peripheral clock decided to be disrupted e.g. eating a meal in the middle of the night for several nights = that’s going to throw out the synchrony b/w the peripheral clocks + the body master clock = when eat in middle of night the enzymes for digestion are not there (not meant to be at night) = will really throw things off so there is no synchrony b/w the peripheral + master clocks
Yamazaki et al, 2000
Non-photic zeitgebers (feeding, activity, and stress):
- serve as time cues to peripheral tissue clocks
- can act on peripheral tissues:
○ influencing the phase of peripheral tissue clocks w/ the potential for putting them out of alignment w/ the phase set by the SCN (see later)
The molecular clock
- clock components regulate tissue function via expression of hundreds or thousands of genes in different organs
- networks of biochemical (transcriptional-translational) feedback loops that generate 24-h rhythms (i.e., a daily program of gene expression)
- During the day you will want to express a lot of genes = once these genes are expressed they produce target proteins e.g. target specific things e.g. when exercising to get fuel + energy need to activate the enzymes that can burn/mobilise carbs + fat to get ATP
Circadian rhythms = period
Circadian rhythms
* daily oscillations in gene expressions
PERIOD (per)
* Seymour Benzer (California Institute of Technology) + Ronald Konopka identified the gene PERIOD in fruit flies
* Nobel laureates Jeffrey C Hall, Michael Rosbash (Brandeis University) and Michael W Young (Rockefeller University) discovered the molecular mechanism that control the circadian rhythm
* This molecular mechanisms is the transcriptional-translational feedback loop
Transcriptional-translational feedback loops form the basis of the 24-hour rhythm
- –> Requires the process of transcription:
○ DNA makes RNA - –> Requires the process of translation:
○ RNA makes proteins
The promoter - a region of DNA where RNA polymerase begins to transcribe a gene
The 24-h transcriptional-translational feedback loops
Regulate the 24-h rhythms by linking tightly to:
* The daily light-dark cycle
* Day-night activities
○ Daytime: gene expressions – make target proteins for cellular tasks
○ Night-time: suppression of clock gene expression
§ Allows body to prepare for sleep + recovery (along w/ melatonin secretion + release
Transcriptional-translational feedback loop – the basis of the 24-hour rhythm
* Important transcriptional activators:
- Important transcriptional activators:
○ “BMAL1:CLOCK activator complex” of clock genes: Cry1, Cry2, Per1, Per2, Per3, CCGs - the Per gene encodes Period (PER)
- the Cry gene encodes Cryptochrome (CRY)
- PER-CRY dimers act to repress CLOCK:BMAL1 w/ a periodicity of ~24h
○ Become very stable when they attach = good as form an inhibitory process that suppress gene expressions at night
How circadian rhythms are generated..
- E-box = promoter = start where transcription can take place = for this to happen need the activator which is the complex BMAL1 + CLOCK
- Per + cry proteins in muscle not the same as in the skin or liver
At night turn off entire process
* As PER + CRY are being produced they also diffuse out into the cytoplasm, by nightfall a lot of these proteins are accumulating as well = but don’t want them at night in the cytoplasm = so we can turn off this pathway
* Per + cry can be destroyed by an enzyme known as proteasome = as they accumulate it will come along + chomp them up
* In addition per + cry but particularly the per protein is quite delicate - can easily be destroyed = so nature has it that they form a dimer (pair together) when they are paired they are more stable + is a good thing cause as a dimer they can come back into the nucleus + serve to inhibit the transcriptional-translational process = serve as a negative feedback loop on this system
The molecular clock has 3 key features:
- Entrainment
- Endogenous pacemaker
- Temp compensation
- Entrainment:
- The clock is entrained to the light-dark cycle
- Under light-dark cycle entrainment: –> Zeitgeber period T = 24h
- Zeitgebers can advance or delay the circadian clock
Actogram - activity-rest data in entrained organisms
* Diurnal – wakeful + active period occurs during the day
* Nocturnal – wakeful + active period occurs during the night
- eyes specialized for night vision
- Endogenous pacemaker
Endogenous:
* clock is internal to the organism
* clock keeps track of time persistently even w/o external cues
* daily rhythms have a natural period close to a solar day w/ a natural rhythm τ ~24.2h
What is a free-running rhythm?
- Rhythms under free-running conditions
○ –> absence of zeitgebers (endogenous circadian period, tau τ)
○ i.e., rhythms not synced to any env stimuli (light/dark, temp) or behaviour (rest/activity)
○ e.g., Mimosa plant placed in constant darkness - Free-running rhythm: an exclusive feature of the endogenous circadian clock
The cave experiment 1938
Findings:
* Study identified the endogenous circadian rhythm
* the 24-hr core body temp continued to fluctuate in an ~24-h cycle in the absence of external cues
* sleepiness was in sync w/ the temp cycle –> phase r/s
* Bedtime shifted 4h later each day during the Mammoth Cave study ∵ the circadian cycle was longer than 24-h (clock not reset)
Biological day and night
- Diurnal organism under entrained conditions of light-dark cycle
○ Beginning of biological day anchored to the onset of light - Subjective day and night
○ Diurnal organism under free-running conditions:
§ Beginning of subjective day anchored to the onset of activity
- The clock is temperature compensated
- The clock is not affected by external changes in temp –> a robust clock keeps time + its period length)
- seen in Drosophila, light-sensitive cyanobacteria, homoesthermic mammals
- Pittendrigh: studied Drosophila rhythm of eclosion over a temp range of 16–26°C.
The clock is temperature compensated
* Pittendrigh:
○ Drosophila rhythm of eclosion retained a 24 -h rhythmicity in total darkness over a temp 16 –26 ° C
Terminology in chronobiology
- Actogram - A graphical representation of an organism’s phases of activity + rest over the course of a day
- Temp compensation - The ability of circadian clocks to maintain a relatively constant free-running period despite fluctuations in environmental temp
- Endogenous - Growing or working from within an organism; intrinsic.
- Entrainment - The coupling of an observable rhythm in an organism to a Zeitgeber resulting in shared period, where (in contrast to masking) this change is caused by an alteration of the endogenous clock that schedules the observable rhythm. ‘entrain’ means ‘synchronise’
Circadian misalignment
- An inappropriate timed rhythm in relation to another rhythm e.g., the shifting of the body clock time w/ respect to the sleep-wake cycle = two rhythms are out of sync w/ each other
- Two types of misalignment:
○ phase advance = an event occurring much earlier = go to bed earlier
○ phase delay = go to bed at a later time
Misalignment of sleep-wake rhythms (with your light-dark cycle will cause sleep disturbances)
Negative impact:
* sleep disturbance
○ Difficulty in falling asleep
○ difficulty waking in the morning
○ –> poor sleep quality
○ –> ↑ health risks
* daytime sleepiness
○ –> impact on perf, errors, accidents
Assessing timing .. (circadian misalignment measurements)
- Assessing body clock time: dim-light melatonin onset (DLMO = onset release of melatonin is directly linked to the presence of a light source + therefore it can be measured quite readily even though it is quite a rigorous protocol to measure) = can also look at core body temp, cortisol rhythms but more difficult to assess
○ For example w/ body temp = because we are so active w/ different metabolic rates, that can alter your core temp somewhat + as a result you will have more variables to control for if want to measure core body temp
Assessing the timing of the sleep-wake cycle: mid-point of sleep bout
How is circadian misalignment measured?
- By assessing the phase angle of entrainment:
○ the time interval b/w the timing of the body clock + the timing of an external time cue, e.g.,
○ the time b/w DLMO (dim-light melatonin onset) + bedtime, sleep onset, sleep offset, mid-sleep or timing of light exposure
○ Need two lots of rhythms = the timing of the DLMO + the timing of the sleep-wake rhythm
Phase angle of entrainment: chronotypes
- For morning type the DLMO occurs just before 7:00pm
- For evening types it’s nearly at midnight
○ If you are a late chronotype the release of melatonin will be a lot later - For mid-sleep time for morning type = around 2:00am but for the late time its almost in the morning at around 5:15am
- Linking DLMO to the mid-sleep time
○ For the morning type = notice that the DLMO occurs early + mid-sleep time is there (as seen in image) = so the phase angle b/w DLMO and mid-sleep time is a lot longer compared to the phase angle b/w DLMO + mid-sleep for late chronotypes = a lot shorter
evening type:
- shorter phase angle
- longer tau
Morning/Evening type
- Inappropriate timing of sleep + wake behaviour to the light-dark cycle –> circadian misalignment
Compared to the Morning type, the Evening type:
* demonstrates a phase delay (go to bed later, wake up later); a shorter phase angle difference
* is predisposed to shorter sleep duration on weekdays, catch up on weekends
* 2.5x more likely to report poor/fair general health (2.5x ↑ in Type II diabetes, 1.3x ↑ hypertension prevalence)
* poorer lifestyle choices e.g. alcohol consumption, exercise
Circadian rhythm disorders
Examples of a normal shift in one’s internal clock
* Delayed sleep phase disorder (DSPD) in adolescents
* Advanced sleep phase disorder (ASPD) in older adults
DSPD
DSPD occurs during puberty due to
* a biological delay in the circadian rhythm = naturally occurring, not much we can do
* slow build-up of sleep pressure
○ difficulty in falling asleep (at a socially acceptable time of night)
○ difficulty waking in the morning
○ –>daytime sleepiness, poor school perf –> behavioural problems
* Thorpy et al, 1988
SPD Treatment: morning bright light exposure strengthens the sleep-wake cycle + advances circadian rhythms = to shift bed-time to an earlier time
