Le fonctionnement moléculaire de l horloge circadienne

Transcription

1 CM: Le fonctionnement moléculaire de l horloge circadienne 1 1- Les composantes moléculaires de l horloge 2- Rôle de la lumière sur l expression des gènes 3- Le fonctionnement moléculaire des oscillateurs périphériques 4- De l expression des gènes à la physiologie UE19 Neuroendocrinologie et neurobiologie des rythmes circadiens: Coupe transversale au niveau de l hypothalamus chez un rat 2 Noyaux Suprachiasmatiques Atlas Paxinos du SNC de rat 1

2 Homologs of most genes involved in fly clockwork have now be cloned in mammals 3 Reppert and Weaver (2001) Proteins of the mammalian (mouse) circadian clock. Schematic diagrams of the full-length proteins encoded by the eight genes recently identified in mammals and likely to be components of the circadian clock (or involved in its entrainment by light). 4 2

3 Model of circadian clockwork within an individual suprachiasmatic nucleus neuron. 5 The clockwork is composed of interacting positive- (green lines) and negative- (red lines) feedback loops. The protein products are color-coded shapes (mcrys, gold diamond; mper1, blue oval; mper2, red rectangle; mper3, green hexagon; hypothetical activatorcoactivator of Bmal1 transcription, small gray circle). CLOCK (oval with C) and BMAL1 (oval with B) heterodimers activate (C) rhythmic transcription of mcry and mper genes. The mcry and mper proteins form complexes important for nuclear translocation of the mper proteins. The phosphorylation (P) state of the mper proteins may also regulate their cellular location and stability. mper2 positively regulates Bmal1 transcription by translocating an activator into the nucleus and/or acting as a coactivator (?). The nuclearlocalized mcry proteins directly interact with CLOCK and BMAL1 to negatively regulate ( ) CLOCK:BMAL1- mediated transcription. The functions of nuclear mper1 and mper3 have not yet been clearly defined (?). Reppert and Weaver (2001) Circadian clock gene expression in an ultradian mammal (Common voles (Microtus arvalis)). Clock gene expression is rhythmic in the SCN of voles, as shown by in situ hybridization of coronal brain sections to crna antisense probes for various clock and clock controlled genes. 6 SCN Stratman et al (2006) 3

4 7 Positive and negative feedback loop-transcription-translation loop. Preitner et al (2002) A network of transcriptional translational feedback loops constitutes the mammalian circadian clock. Ko and Takahashi (2006) 8 4

5 Effect of the Clock mutation at the behavioral and cellular level. 9 Effect of the Clock mutation at the behavioral and cellular level. 10 5

6 Effect of the Bmal1 (Mop3) mutation at the behavioral level. 11 Wild-Type Wild-Type Tau = 23.64±0.1 h 6 hr light pulse (300 lux) given at CT16 Mop32/2 Mop32/2 Tau = NO RHYTHM Bunger et al (2000) Circadian Locomotor Activity of Rev-erb- Proficient (/) and -Deficient (/) Mice Rev-erbα Rev-erbα 12 (A D) The voluntary locomotor activity was recorded as wheel-running activity for wild type mice and Rev-erb mutant mice in constant darkness (DD, A and B) and constant light (LL, C and D). In each actogram, the first few days were recorded under LD conditions (lights on at 7:00; lights off at 19:00). Time spans during which the lights were switched off are marked by gray shadowing. Rev-erbα Rev-erbα Preitner et al (2002) 6

7 Molecular framework of the mammalian circadian clock. 13 The multiple steps of the pathway of light from the eyes to the clock genes that it effects are depicted (discontinuous arrows). Avp Vasopressin gene. Schematic of light input pathway to the SCN clockwork. Highlighted in red are the elements known to be important for light activation of behaviorally relevant pathways. 14 7

8 Circadian rhythms of clock gene expression and clock protein levels in the mouse suprachiasmatic nuclei. 15 Clock gene expression in central and peripheral oscillators of wildtype mice housed under conditions of constant darkness (DD). Expression was determined by qpcr and expressed as fold-change relative to nadir of expression. Four tissues were investigated, SCN (A) and retina (B). Error bars indicate standard error of mean. P-values are indicated as follows: *P < 0.05, **P < 0.01, and ***P < Circadian Time Pierson et al (2006) 8

9 Hierarchical model of the circadian system in mammals. The master circadian pacemaker in the SCN drives rhythms in physiology and behavior. 17 glucocorticoids melatonin The SCN synchronizes to daily environmental signals such as the light cycle. Peripheral tissues, including the lungs, liver and skeletal muscle, express damped circadian oscillations that are sustained in vivo when the SCN is intact. In addition to signals from the SCN, peripheral oscillators respond independently to other inputs such as periodic food availability. The signals that couple these multiple oscillators follow both synaptic (arrows) and hormonal (circles) pathways. For example, the retinohypothalamic tract (RHT) represents one of the neural pathways, probably all of which are polysynaptic, that connects the eyes to the SCN. Clock gene expression in central and peripheral oscillators of wildtype mice 18 housed under conditions of constant darkness (DD). Expression was determined by qpcr and expressed as fold-change relative to nadir of expression. Four tissues were investigated, SCN (A) and heart (C). Error bars indicate standard error of mean. P-values are indicated as follows: *P < 0.05, **P < 0.01, and ***P < Circadian Time Pierson et al (2006) 9

10 Clock gene expression in central and peripheral oscillators of wildtype mice 19 housed under conditions of constant darkness (DD). Expression was determined by qpcr and expressed as fold-change relative to nadir of expression. Four tissues were investigated, SCN (A) and liver (D). Error bars indicate standard error of mean. P-values are indicated as follows: *P < 0.05, **P < 0.01, and ***P < Circadian Time Pierson et al (2006) Synchronization of the mammalian timing system. 20 A: The central pacemaker in the SCN is synchronized every day by the photoperiod. Rhythmic neuronal and hormonal SCN outputs drive circadian rest activity cycles, which in turn limit feeding time to certain time windows during the day. Feeding and/or fasting are the most dominant Zeitgebers for subsidiary clocks in peripheral tissues. However, the SCN also uses more direct timing cues, such as humoral and neuronal signals, to entrain the phases of peripheral clocks. B: Peripheral clocks continue to oscillate in SCN-lesioned mice, but their phases are no longer coordinated in these behaviorally arrhythmic animals 10

11 Circadian gene defects and biological consequences 21 Ko and Takahashi (2006) 11