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Lecture 8

lecture 8

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Martin Ralph

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Lecture 8 Kai Oscillator Ordered KaiC autokinase and autophosphatase activities drive the circadian oscillator Operates based on Kai C protein o Generally in its kinase form and self phosphorylates itself until 5 of the 6 hexamers are phosphorylates then it dephosphorlyates itself to a autophosphates (oscillating back and forth) o Kai A promotes phosphylations o KaiB inhibits Kai A Kai C is also a transcription factor that drives the Kai cluster in the genome The period can be anywhere from 16 to 24h, thus not a very accurate clock However, the concentration of Kai proteins are heavily regulated in nature and therefore likely have a much stricter clock than in lab Do not rely on a continuous supply of new proteins to produce oscillations thus they are considered independent of transcription and translation o In the other systems it is a transcription rhythm (oscillations of genes) that determines the circadian clock A mutation of Kai changes the period of the cyanobacteria rhythm; even without the rest of the cell those proteins have a approximately 24h o Mutant in vitro has an oscillation of 22h proving that oscillations are intrinsic to the proteins and independent of gene transcription Light creates ATP which leads to quicker phosphorylations of Kai C that drives a quicker cycle o More ADP slows down phosphorylation of Kai C Dark produces phase shifts by inhibition phosphorylation step Certain mechanisms are conserved all over phylogeny • Cryptochrome: is to transduce blue light into an energetic form that is essential to the cell o Photolyases: repair light caused DNA damage o Other affect circadian clocks Cryptochrome Functions • Mammals: direct role in rhythm production • Plants: blue light photoreception o inhibition of stem elongation o photoperiodism o light-induced changes in gene expression Peripheral Circadian Oscillators and the Circadian Hierarchy Input in the eye à RHT à main oscillator in the SCN à releases hormones à sync peripheral clocks à output Peripheral organs: directly pay attention to the demands placed on them and indirectly to light signals • The SCN is not the only circadian oscillator • Other brain region, organs, tissues, and single cells produce circadian oscillators autonomously • The TTFL oscillator may operate in all cells • Cells exhibit PTOs, and there may be multiple PTOs within a cell Unique Oscillators of Circadian Rhythms Food Entrainment • Rat tends to wheel run in anticipation to the presence of food • Reward can cause the same anticipatory behavior than persists even longer Amphetamine Induced Oscillations • Amphetamine is able to produce an approximate 24h rhythm without the presence of SCN • Methamphetamine produces long period locomotor rhythms in both intact and SCN lesioned, arrhythmic mice and rats. • Methamphetamine rhythms show relative coordination with the SCN oscillator • Methamphetamine rhythm is entrained by restricted feeding schedules • Therefore an amphetamine senstitive oscillator is responsible for rhythmic locomotor activity, and this oscillator is entrained by the SCN. • It is likely that the methamphetamine oscillator has a natural period that is close to 24 hours, and is lengthened in the presence of the drug, so that it eventually breaks away from the SCN pacemaker. Time Memory Learning and Memory 1
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