Molecular Bases for Circadian Clocks

Molecular Bases for Circadian Clocks

Molecular Bases forCircadian ClocksJay C. Dunlap, Department ofBiochemistry, DartmouthMedical SchoolGenevieve KendallMolecular SeminarApril 18, 2007

Outline• Background• Common elementsin designingcircadian clocks• Synechococcusspp. Model• Mouse Model

Background• Chronobiology- study of periodiccycles in living organisms (i.e..Circadian clocks)• Circadian clocks are an internalclock that is… “the lens throughwhich we survey acute externalfactors; it takes the lead indetermining how we perceivetime”• Internal clock that guides theday from within

Background• Each cell has its own independent clocknot affected by cell to cell interactions• Work on the circadian clock has beenongoing since the 1970’s, but morerecent work is the most valuable– By mid 1980s they did not realize that abrain isn’t necessary to have a clock(cyanobacteria don’t even have a nucleusto help regulate!)

BackgroundLine lengthmeasuresrates ofchange insmall subunitrRNABlue=describedcircadianrhythmsRed=significantprogress

Background• Exist in some cyanobacteria, in mostEukaryota, and are in nearly all “CrownEukaryotes” of the lineage Plantae,Fungi and Animalia• Have not been found in Archaebacteria(not many investigations) or inEubacteria• Circadian clocks are adaptive

Background• Oscillator vs. System• Circadian oscillator is the “minimal setof molecular causes and effectssufficient to describe circadian cycles asthey operate”• Circadian system pertains to eventsoutside clock cells and how they willaffect the clock’s progress

Background• 3 Questions needed to describe the clock– How does the clock work?• Biochemical and genetic answers– Inputs• How does it synchronize with the world outside of thecell?–Outputs• How is “molecular time” generated and how does it bringabout changes in the cell and ultimately changes in thebehavior of the organism?

Common Elements of Design• Clock genes (different in each organism)– Positive elements- activate clock genes to drivetheir transcription which yields clock proteins– Negative elements- clock proteins which areproduced due to the actions of positive elements,“feedback” and inhibit the actions of positiveelements• All oscillators use loops that close within cellsand do not require cell to cell interaction

Synechococcus spp.• Cyanobacteria• Oxygenic phototrophs that canphotolyze either H2O or H2S• Circadian rhythm is an intracellularfeedback loop that involves transcriptionand translation– Circadian clock regulates cell division,amino acid uptake, N fixation,photosynthesis, carbohydrate synthesis,and respiration

Kai Genes* Expression ofgenes drivenby 2promoters; 1for KaiA, andthe other forboth KaiB andKaiCCR-circadian rhythmCT- circadian time, normalizes subjective biological time underconstant conditions; CT0 corresponds to subjective dawn andCT12 corresponds to subjective duskARR- arrhythmic

Temporal Regulation of KaiGenesYellow- positive element; Blue- negative elementKai A rhythmically expressed with a RNA peak late in the subjective day CT 9-12

Instead of directlybinding to theDNA, Kai proteinsmay also regulatethe activity of thepolymerase itselfor essentialcomponents of thepolymerasePossible Model

Circadian Requirements• Synechococcus fulfills the requirements…– Period length of about a day– Entrainable to environmental cues of lightand temperature– Period length remains about the same whenmeasured at different ambient temperaturesfor organism– Positive and negative demands of theory

• One of the best andmost accessiblemodels for studyinghumansMouse

Clock Genes

Temporal Regulation of Per1,2,3,tim, Clock and bmal1/mop3 genes

Possible MechanismArrowspositiveregulationLines endingin barsnegativeregulationDashed linespossibleregulatoryconnections

Clock Resetting by LightLight induces mPer1 with a peak of 60 min. after light exposure, andmPer2 peaks at about 90 min., mPer3 is not light inducedLight induction is gated- mPer1 and mPer2 only induced late in the dayor at night- autoregulation where clock output loops back and affectsinput

Review of Mouse Clocks• Negative feedback transcription basedoscillators• Heterodimeric PAS domain containingproteins as positive elements (Clock,bMAL1/MOP3)• 3 gene family of similar negativeelements (mPer1, mPer2, mPer3)

Literature Cited• Jay C. Dunlap. 1999. Molecular Basesfor Circadian Clocks. Cell, Vol. 96, 271-290.


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