Circadian Control

Question 1

Circadian pacemakers

Answer 1

cells have circadian clock, and these are coordinated by the SCN

Question 2

Circadian mismatch

Answer 2

• if behaviour doesn’t match circadian clock
• cannot anticipate environmental changes
• so more likely to develop e.g. cancer, diabetes, as a shift worker
• same diet eaten at different times of day can cause an obesity phenotype = put down more adipose if eating at a time when your body isn’t expecting it

Question 3

Theory of endogenous rhythms - objections

Answer 3

1. no biological explanation of temperature compensation
2. no explanation for free-running at 24 hours
3. entrainment - can’t exclude the role of endogenous rhythms
4. just because something is rhythmic, doesn’t mean it is a clock!

• scientist who said these believed rhythms are always caused by the environment
• e.g. gravity
• he is wrong, but some of these objections are valid!
• just because something is rhythmically expressed, doesn’t mean protein levels oscillate as degradation can oscillate in phase
• period is rhythmically expressed but not a clock

Question 4

CLOCK KO

Answer 4

• CLOCK not required for circadian oscillator function
• mice not arrhythmic as may expect
• rhythm actually a little faster
• may be due to redundancy of something that takes on the role of CLOCK when it is KO
• Npas2 also binds BMAL1 so could take this role
• but isn’t expressed in the SCN

Question 5

CRY1/2 double KO

Answer 5

• still exhibit some rhythmicity
• more rhythms in gene expression than in wildtype
• more fluctuations in proteasome than in wildtype
• but at 16 hours, rather than 24

Question 6

BMAL1 KO

Answer 6

• synchronicity lost
• but still some rhythmicity

Question 7

Why 24 hours?

Answer 7

• PTM determine clock period
• phosphorylation promotes ubiquitination and degradation of clock proteins
• therefore controls periodicity
• kinases and phosphatases very important - often targeted by drugs targeting circadian rhythms

Question 8

Cyanobacteria

Answer 8

• have TTFL
• but have a TTFL-less circadian rhythm in vitro
• have a true molecular clock
• in vivo, have a TTFL which is an output of the clock, rather than the clock itself
• mammals do not have these equivalent proteins and so we continue to use the TTFL model as our closest approximation

Question 11

Comparative chronobiology

Answer 11

• animal clocks thought to show common ancestry in some period-like protein
• not related to plants and fungi
• components differ but models favour a conserved mechanistic logic e.g. TTFL
• negative feedback loop creates periodicity but not sure what makes this period of 24 hours

Question 11

Acetabularia

Answer 11

• eukaryotic algae
• single cell
• one nucleus in root
• circadian rhythm of chloroplast movement from leaf down root depends on time of day
• circadian control of photosynthesis so faster in daylight
• cutting off section with nucleus does not stop rhythmicity (will die in a few days)

Question 12

Red blood cells

Answer 12

• ribosomes not active in rbcs so gene expression cannot occur after
• but rbcs have a 24 hour circadian clock which is entrainable to zeitgebers
• how as this cannot be a TTFL?

Question 13

Why have a cellular clock?

Answer 13

• in SCN and peripheral cells, period gene expression is an input, output and core clock mechanism

liver-specific BMAL1 KO = increased glucose clearance
- circadian hypoglycaemia at normal fasting times
- hepatic clock anticipates and accommodates metabolic load