Circadian Rhythm + Sleep Flashcards
What is the circadian clock?
= an endogenous molecular pacemaker
= drives daily rhythms affecting physiology, biochemistry, behavior, other functions
What are the oscillations
= translated to physiological and behavioural rhythms
= expression of large parts of transcriptome and proteome = shows daily oscillations
Where were circadian clock mutants identified?
First = in Drosophila
Later = other organisms e.g. mice
= indicated genetic nature of mechanism involving network of interacting proteins
What is the central dogma?
most conserved mechanism of circadian clock
= negative feedback loop
Studying circadian clock in drosophila (locomotor activity)
Through infared detection
Free run
= a biological rhythm existing without any external input or cues
e.g. dark-dark conditions
Entrainment
= biological rhythm synchronised to an external oscillation
e.g. light/ dark cycle
Drosphila clock mutants? What gene is involved?
= proved that single gene mutations could affect circadian rhythms
gene 1= period
= per0 mutant alleles produced arrhythmicity (no circadian rhythm)
= perL = long period rhtyhms (28 hrs)
= perS = short period ryhtms (19 hrs)
= per homologues found in mice and humans
What have studies of genetically mosaic and trasgenic Drosophila indicated?
that circadian behavioural locomotor rhythms are controlled by 20-30 neurons of the central brain
What are the 4 properties of circadian rhythms?
- The oscillations persist, or free run, under constant conditions
= indicating presence of a self-susatining clock - The clock-driven events recur approximately, but not precisely every 24 hrs
- Rhythms are entrained by sun-driven changes in light and temperature
- Period of the clock is remarkably stable over a wide temperature range (temperature compensation)
What are the 3 main parts the circadian systems are organised into?
- The core clock
= keeps time - Input pathways
= synchronise the clock to the environment - Output pathways
= transmit information to temporally organise behaviour and physiology
How does the negative feedback loop form?
CLOCK / CYCLE protein complex
= binds to E-box elements in the promoters of the period (per) and timeless (tim) genes
Doubletime, Casein Kinase 2, Shaggy, phosphates: PP2A, PP1
= modify the PER and TIM proteins
PER and TIM
= dimerise and move to nucleus
= repress CLK / CYC activity (inhibit) scr
= also bind E3 ubiquitin ligase SLIMB = leading to their proteolysis
How does a clock cell act at different times of the day? (molecular mechanisms)
Light regulates adaption of the clock to its environment = entrainment
Sun does not rise + set at same time each day
= therefore ZEITGEBERS harmonse clock neurons to their environment (light, temp, social cues)
What is the most important Zeitgeber
= Light
= reaches clock neurons via photorecpeptive organs containing rhodopsins
= e.g eyes, ocelli and HB-eyelets
Cryptochrome = blue light photpigment, activated by light
Active Cry binds to Tim = triggers its degradation = ‘lights on’
What are some similarities between clock oscillators in mouse and fly?
E-BOX conserved
Negative feedback loop conserved
Homologous proteins - not always same names though
What is the role of PDF neurons?
Work as an output transmitter of the clock
Coordinated and synchronized the oscillations of the clock neurons when external signals are absent
Phases the oscillations of the clock neurons and activity peaks to the right time of the day
Works as a light-input and arousal factor of the clock
(under natural conditions = PDF may help system to adapt to seasonal variations in photoperiod)
How do different clock neuron subsets contribute to the Light:Dark (L:D) behaviour?
s-LNvs
= promote morning activity
LNds / fifth PDF-negative s-LNv
= promote evening activity
DN1s
= promote morning and evening activity
= evening output very weak in high light but strongly increases in low light
What behaviours and physiology are regulated by the circadian clock?
Sleep
Learning + Memory
Feeding + Metabolism
Chemosensation
Courtship + Mating
Immunity
What are the 2 categories of sleep regulation?
Circadian
= preferred bedtime when to sleep
Homeostatic
= changes in sleep quantity / quality after sleep
= performance impairment after sleep loss
What is the circadian regulation of sleep? (shown through mutants)
Circadian genes affect timing of sleep / possibly its homeostatic regulation
Transcriptional activators = Clk and cyc
= Null mutants = exhibit less overall sleep that WT
cyc mutants (BUT not other mutants)
= hypersensitive to lethal effects (Death) of sleep deprivation
Methods for measuring sleep in Drosphila?
Actigraphy
= infared beam
= video tracking
Neurophysiology
= Local field potentials recorded, locomotor activator simultaneously monitored
Metabolic Rate
= whole body calorimetry
Arousal Threshold
= provide flies with increasingly intense stimuli, simulateoudly monitor locomotor activity
The circadian regulation of sleep? What structural elements are involved?
Light activates I-vLN (large ventral lateral neurons)
= releases PDF onto s-vLNs = projects to other clock neurons and other brain regions
Both I-vLNs and s-vLNs express GABA receptors
= may enable sleep-promoting GABAergic neurons to suppress wakefulness
Activation of ILNv by selective channel expression
= disrupts night time sleep
Ablation
= increases sleep
GABAergic inputs
= inhibit arousal promoting function of these neurons
Data suggests non-PDF evening cells may oppose effects of ILNv
What is the neuronanatomy involved in sleep and circadian rhythms in flies?
Neuronal sleep centres
= kenyon cells comprise mushroom bodies (MB) , fan shaped body (FSB) , ellpsoid (EB)
= major sleep-regulating centres in the brain
= regulated by many different neuromodulatory neurons (DPMs, dopaminergic PPL1 neurons)
Circadian clock neuron network in left hemisphere
= have wake/ sleep - promoting subsets
Circadian clock neuron network in right hemisphere
= DN1ps = promote sleep through inhibition of wake-promoting LNs classes
Wake-promoting circadian output pathways
= DN1p neuorns receive inputs from s-LNvs
= relay time of day info to wake-promoting neurons
Pathways linking clock neuron network to homeostatic sleep centre
= DN1ps project to tubercular-bulbar neurons = which project to EB ring
= wake-promoting LNs project to dopaminergic PPM3 neurons = also innervate EB ring
Homeostatic reponses to different types of sleep deprivation?
No sleep rebound
= sleep-feeding conflict
= failed copulation (PI neurons)
= starvation (Dilp2 neurons)
Sleep rebound
= caffeine feeding (dopamine neurons)
= mechanical sleep deprivation (ellipsoid body neurons)
The Sleep:Feeding Conflict
Starvation impairs sleep initiation but not maintenance
Can test using drosophila = get either food or agar
= starved flies sleep less than fed group
= total sleep bouts decreased in starved flies BUT length does not differ
Extra Reading - other functions of circadian clock
Circadia clock involved in many other physiological processes:
Nature, 2021 = disruptions to circadian clock in mice led to increased risk of developing breast cancer
(gene = Bmal1 in mammary gland cells)
Nature, 2020 = circadian clock in liver essential for maintaining metabolic health = disruption led to metabolic dysfunction and obesity in mice
Science Advances, 2020 = also affects plants = regulating time of plant growth and development = disruption led to abnormal growth and reduced yields
Science Advances, 2020 = circadian clock plays role in regulating timing of ovulation in Drosophila
eLife, 2020 = circadian clock regulates timing of feeding behaviour in Drosophila
