Wednesday, April 4, 2012

Circadian rhythms and neuroendocrine regulation II

1CLOCK¶

• suprachiasmatic nuclues (SCN) is the central clock in mammals.
  • located next to 3rd ventricle, right above optic chiasm, where the two optic nerves cross each other.
  • lesioning SCN disrupt circadian rhythm as shown on the actograms.
    • SCN graft restores circadian rhythm... to that of the donor!!!!
  • circadian rhythm of electrical activity is shown in isolated SCN neurons
    • which suggests that mechanisms of regulation is at the cellular level.
• however, SCN is not the only clock in the organism. also in other brain regions and periphery organs.
  • different clock genes could also have different functions/regulations/circadian rhythms in different locations.
    • as demonstrated by Per1-luciferase transgene's expression rhythms in different tissue cultures.
• clock genes:
  • conserved among species (ex: rat's and drosophila's clock genes have similar names)
  • conserved feature:
    • oscillating mRNAs, proteins, or activities
      • protein level lags behind mRNA level (both are in rhythms)
    • autoregulatory feedback loops
      • ex: Bmal1 and Clock form dimer and act as TF(transcription factor)
      • the TF activate transcription for more clock genes: Per1/2/3, Cyr1/2
      • Cyr1/2 and P1/2/3, in conjunction w/ CKI (a kinase), dimerize and suppress Bmal+Clock complex and thereby their own expression
      • as Cyr1/2 and P1/2/3 are degraded over time, suppression on Bmal+Clock becomes less, so transcription occurs again on the next day.
      • hence the rhythm
• Bmal-knockout shows very low expression of Per1 in SCN
  • locomotor activity is still in control, but when environmental clue is removed (put in DD cycle), circadian rhythm is completely disrupted.
  • so rhythm in Bmal1-knockout is due to masking.

2Mutation¶

2.1CKI¶

• CKI does many things:
  • 1.phosphorylate Per1 and thereby regulate Per1's location in the cell.
  • 2.phosphorylate PER1 and targets it for degradation
  • 3.phosphoryalte Per1/Cry complex and the complex then inhibit Clock/Bmal1.
  • 4.phosphoryalte BMAL1 to stimulate BMAL1/Clock-induced transcription
• tau mutation in hamster: mutation of CKI
  • less stalb ePER protein and thus decreased duration of inhibition of CLOCK/BMAL1.
  • heterogenous mutation: period decrease from 24hr to 22hr
  • homozygous mutation : period is 20 hr. (even worse)

2.2Per2, CKI¶

• familiar advanced sleep phase disorder (FASPD)
  • otherwise healthy individuals.
  • caused by mutation in PER2 and CK1-gamma
  • shorter period than normal (sleep from 6pm to 2am)
• problems may lie in the 1st and the 2nd functions of CKI.

3Components of a circadian clock¶

• input (light, clues, etc)
• pacemaker (SCN genes)
• output (physiological rhythms)
  • there must be a link between pacemaker and clock mechanism --> ans: clock-controlled genes (ccg)

3.1(ccg)Clock-controlled Genes¶

• ccgs are controlled by block machinery and thus the RNAs are expressed rhythmically.
• they can encode neuropeptides such as vasopressin, transcription factors (DBP), receptors, ion channels, metabolic enzymes, etc.
• ex: vasopressin as a ccg
  • In front of vasopressin gene, there's a Ebox in promoter. the Ebox binds Clock/Bmal1.
• regulation of ccgs can be different in different tissues
  • in SCN, mutant mice's amp of vasopressin is flattens and has little rhythms comparing to wild type
  • in SON, mutant mice's amp and rhythm of VP is pretty much the same as the wild type.
  • so there are many clock-controlled genes and most ccgs are different in SCN and periphery (like liver).
    • less than 10% of ccgs shows the same rhythm in more than 1 tissue, so genes have different functions in different tissues (whether it's rhythmic or non-rhythmic).
    • this is also proven w/ micrarray analysis, showing that different clock genes are expressed in different tissues.

3.2SCN-periphery communication¶

• hierarchy :
  • central clock
    • peripheral clocks, which drives rhythms.
    • directly drives rhythms.
  • environments gives direct input to central clock and indirectly to peripheral clock
• putative signals
  • glucocorticoids, retinoic acid, neuronal pathway (autonomic nervous systems)
  • temperature cycles (by peripheral clock, not central)
  • feeding rhythms (also directly by SCN, by peripheral clock.
• SCN sends projections to many nuclei in the brain: PVN, PVT, DMH, CRH, GnRH,etc.

4Melatonin and Cortisol¶

4.1melatonin¶

• melatonin is secreted by pineal gland, which must have connection w/ SCN cuz lesion at SCN abolishes melatonin production.
• pathway of projections: SCN --> PVN --> IMC --> SCG --(via noradrenaline)--> (alpha-1 beta-1 adrenergic receptors) PIN (pineal gland) --> high malatonin at night and low during a day.
• melatonin production:
  • serotonin --(AANAT)--> N-acelylserotonin) --(HIOMT)--> Melatonin
    • AANAT: has a dramatic rhythm : very high at night and very low during the day.
    • HIOMT: more or less level throughout the day.
    • (therefore AANAT is the rate-limiting step)
• noradrenaline regulation of AANAT via alpha-1/beta-1-adrenergic receptors lead to more transcription of AANAT and less degradation.
  • regulation at transcription and degradation differ among species.

4.2cortisol ##¶

• regulated by 3 factors:
  • neuroendocrine: SCN --> PVN --> CRH (aka CRF) --> pituitary (produces ACTH) --> adrenal gland (produces cortisol)
  • parasympathetic" SCN --> PVN --> DMV --> adrenal gland
  • sympathetic: SCN --> PVN --> IML --> adrenal gland.
• but there are also circaidan clock in the adrenal gland itself. so SCN is not the only source of rhythm for cortisol.
  • ex: Bmal1 and Per1 genes expression oscillates in the cortex and medulla of the adrenal gland.
  • many other clock-controlled genes are involved in the control of adrenal cortisol biosynthesis and secretion.

4.2.1Adrenal clock experiment¶

• rodents with mutated Per2/Cry1 do not show difference between level of cortisol during early day and early dark.
• whereas wild type rodents, cortisol is much higher in early dark than in early day.
  • (rodents are active at night)
• therefore, while same amount of ACTH act on adrenal gland, different amount of cortisol were secreted in wild type and mutant.
  • so clock in adrenal cortext has gating effect on the ability of ACTH to evoke cortisol release
  • clock determines how strongly tissue would response to ACTh and secrete cortisol.

5Circadian and photoperiodism¶

• photoperiod: longer in the summer. duration of light phase.
• photoperiodic regulation of prolactin and pelage/horn growth
  • hamsters secrete high prolactin in LD and low in SD (short day)
    • they're white in winte rand agouti in summer
  • but if SD number of days persist, there's a SD refractory: prolactin level goes up again and animals become agouti.
    • due to intrinsic regulation
  • same goes with sheep and their prolactin level
    • summer/LD: high prolactin/horn growth
    • winter/SD: low prolatin/no horn growth
    • prolonged LD: refractoriness (intrinsic regulation)
• photoperiod on fertility of male hamster:
  • wild type: testis atrophy if night >12hr. more fertile when days are long
  • mutant : testis atrophy if night >10hr.
    • mutation in the circadian clock alters photoperiodic time measurement
• photoperiod on oscillation of Per genes in SCN
  • Per1 is high in light phase
  • in SD: higher and narrower peak
  • in LD: similar behavior, but lower and broader peak.
    • seems to mimics length of day.
• photoperiod and pineal melatonin rhythms
  • just like Per genes, but opposite
    • melatonin peaks at night and low during day.
  • pituitary cells response to melatonin from pineal gland, determine whether it's SD or LD, and influence hormonal release
    • sheep are reproductive in SD while hamsters aren't
    • hamsters are reproductive in LD while sheep aren't.