Lecture 18: Seasonal clocks II

Question 1

what is the target tissue for melatonin?

Answer 1

pars tuberalis, part of the main pituitary, Interface between the main pituitary gland and hypothalamus.

MT1 receptors in pars tuberalis.
expressed in both short/long photoperiod

Question 2

describe melatonin receptors

Answer 2

Major mammalian subtypes: MT1 and MT2

G-protein-coupled receptors

MT1 receptors implicated in seasonal biology

Melatonin receptors expressed in many neuroendocrine tissues

Question 3

what is the cellular makeup of the pars tuberalis?

Answer 3

contrasts to main putuitary which makes many different hormones.

only contains Folliculostellate cells and thyrotrophs which make TSH (thyroid stim hormone).

don’t possess receptors for thyrotropin releasing hormone (TRH) however.
So unlike thyrotrophs in main pituitary don’t respond to conventional hypothalamic signaling.

Question 4

what is the thyroid system?

Answer 4

Hypothalamus produces TRH

TRH acts on the anterior pituitary gland

Production of TSH from thyrotrophs

TSH stimulates thyroid gland to produce thyroid hormones

T4 (thyroxine) is converted to T3 -the active hormone

leads to increased metabolism, growth and development, increased catecholamine effect.

Question 5

effects of a thyoidectomy?

Answer 5

Thyroidectomy blocks seasonal reproductive responses in birds and mammals. ie ducks, starlings, quail, sheep, red deer.

T4 injection restores

Question 6

thyroid hormones in siberian hamsters?

Answer 6

T3 implants prevents short photoperiod activation of the gonads. testis remain large.

if given when testis are small/winter, T3 implant causes testis size to increase. reactivates gonadal axis.

T3 implants reactivate the gonadal axis in short photoperiod adapted animals.

Question 7

What are tanycytes?

Answer 7

Specialised ependymal cells

Found within 3rd ventricle of the hypothalamus extend to the pars tuberalis.

Can convert thyroid hormone into active and inactive forms using Deiodinase enzymes.

main pituitary do have TRH receptors, pars tuberalis no TRH.
tanycyctes form bridge between pars tuberalis and hypothalamus.

Question 8

describe thyroid hormones

Answer 8

Thyroid hormones

Triiodothyronine (T3) - active
Thyroxine (T4) – inactive (relatively)

In blood ratio of T4 > T3 approximately 20:1

In brain almost all T3 is derived from local conversion of T4.

T4 to T3 conversion requires deiodinases (Deiodinase 2).

slide has pathways

Question 9

what controls hypothalamic

de-iodinase activity?

Answer 9

seasonal clockwok.

in long photoperiods:
more Dio2 so T4–>T3 so active thyroid hormones.

in short:
less Dio2, so Dio3 causes more T4–>rT3 (reverse T3).

Expression of the converting enzyme DIO2 in the hypothalamus is photoperiodic.

sheep kept in LD 16:8 high DIO2, SD 8:16 less DIO2 in hypothalamus.

Question 10

how is DIO2 expression controlled seasonally?

Answer 10

TSH drives ependymal DIO2 expression, via TSH-receptor mediated cAMP activation.

TSH is composed of 2 sub-units: αGSU and TSHβ

αGSU does not change with photoperiod

The β sub-unit is strongly activated by long days in the PT.

Question 11

what stimulates the conversion of T4-T3?

Answer 11

βTSH photoperiodically regulated signal that

stimulates the conversion of T4 to T3

Question 12

TSH-DIO switch?

Answer 12

photoperiod info to scn –> melatonin.

long/short day melatonin signals perceived in pars tuberalis, produces BTSH in a circannual fashion.

BTSH used by tanocytes to produce DIO2, converts T4-T3.

T3 causes metabolic and reproductive changes.

conserved in vertebrates.
birds fish too.

Question 13

deep brain photoreceptors in birds?

Answer 13

extra retinal photoreceptors in the pineal organ and deep brain.

blind duck shows gonadal development, but this response disappears if a black cap is place on the head (Benoit 1935).

Question 14

other signals driving seasonal biology?

Answer 14

Signals other than thyroid hormone important for driving seasonal biology

Thyroid hormone is essential for some, but not all seasonal changes, e.g prolactin

Additional signals from pars tuberalis (not TSH) affect prolactin

Prolactin responsible for coat colour change, moulting (birds)

Question 15

Discovery of Eya3?

Answer 15

Discovery of Eya3 as a long-day activated gene in mammals.

Sheep pars tuberalis tissue

Collected after 7 or 28 days in long photoperiod versus short photoperiod

Compared transcript levels using microarray

Identified Eya3 as up-regulated in long-photoperiod in sheep pars tuberalis

coincides with increase in TSHB

Question 16

What is Eya3?

Answer 16

Discovered in drosophila.
Eya/eyes absent.
mutant affects development of eye badly.

4 EYA genes in mammals

EYA1, 2 and 4 are essential for development

EYA3 is a transcriptional co-activator
(increases gene expression by binding to transcription factor)

≈ 90% TSHβ cells co-express EYA3

All TSHβ cells co-express αGSU

Question 17

How does EYA3 drive TSHB expression?

Answer 17

Transcription co-factor, synergises with TEF (a transcription factor) to maximise TSHβ transcription

BTSH promotor has D-Box element which TEF binds to to enhance gene transcription.

In short day, TEF can bind to D-box but not effective, absence of EYA3 low tshβ expression.

Long photoperiod, high EYA3 production, acts as transcriptional co factor, binds with TEF to D-Box, TEF is more effective, increased transcription of BTSH -> more BTSH protein.

Question 18

What regulates Eya3 expression?

Answer 18

Eya3 expression is under circadian control, peak 12h after melatonin onset.

Nocturnal melatonin levels suppress Eya3 expression.

Results in strong morning peak of Eya3 under long day.