Hypothalamic pituitary gonadal axis

Question 1

What type of feedbacks can be found in the HPG axis ?

Answer 1

Negative and positive feedback

Question 2

Give a rough outline of the HPG axis and the hormone at play here

Answer 2

Hormone release from the Hypothalamus which positively influences hormone release from the pituitary which will then release a pituitary hormone.
Through portal circulation this will cause hormone release from the gonads (steroid hormones )which will thus cause negative feedback of the hypothalamus and pituitary.

Question 3

How does the feedback from the gonad hormones differ in male and female?

Answer 3

In males the steroid hormones released by the gonads will always be negative however in females this can differ due to oestrogen which will have a positive feedback.
This is during the LH surge during ovulation

Question 4

What are the two important regions in the hypothalamic control?

Answer 4

1. The arcuate nucleus
   (infundibular nucleus )
   2.Medial preoptic nucleus

The AVPV
(Anteroventral periventricular nucleus)

These two regions express GNRH neurons

The neurons synthesise GNRH which makes its way through the medial eminence into the hypophyseal portal system (network of blood vessels) which transports GNRH from the hypothalamus to the gonadotrophic cells in the anterior pituitary

Question 5

What is a different term for Adenohypophyisis ?

Answer 5

This is also called the Anterior pituitary(gonadotrophin cells)

Question 6

Explain the hormone Kisspeptin and its role

Answer 6

Kisspeptin (KISS1/Kiss1)
Master controller of puberty
Controls the synthesis and secretion of GnRH from GnRH neurones
Upstream of GnRH for that reason
Expressed within hypothalamic areas especially the arcuate nucleus and AVPV
This is species dependent
Its expression is regulated by gonadal steroids, so Kisspeptin neurones actually express oestrogen and progestogen receptors
GnRH neurones do not express these receptors

Question 7

What is the AVPV ?

Answer 7

The Anteroventral Periventricular nucleus

Question 8

What was Kisspeptin called previously and why ?

Answer 8

It was called Metastin due to its ability to suppress metastatic spread of human melanomas and breast carcinomas.

Question 9

Outline the structure of Kisspeptin and how it is formed?

Answer 9

Peptide hormone, synthesised as a pre-pro-peptide hormone
undergoes proteolytic cleavage to release several peptide variants of Kisspeptin

1. Kisspeptin-54 (metastin)
2. Kisspeptin-14
3. Kisspeptin-13
4. Kisspeptin-10

largest is Kisspeptin-54 (metastin)

Question 10

Outline the use of Kisspeptin in in-vivo animal studies and why that variant is favoured

Answer 10

Kisspeptin-54

Is able to cross the blood brain barrier

Question 11

Outline the use of Kisspeptin in in-vitro studies and why that variant is favoured

Answer 11

Cell culture studies
Kisspeptin-10
Very active ligand of the GPR54 receptor and must cheaper to create

Question 12

Describe the GPR54 receptor

Answer 12

The Kisspeptin receptor is known as GPR54 (KISS1R second name ) and it is expressed on the GnRH neurons

Question 13

What kind the binding of KISS1R system lead to ?

Answer 13

This can trigger puberty , therefore inactivating mutations can cause failure to undergo spontaneous puberty

Question 14

What can happen if there is an inactivating mutation of KISS1R

Answer 14

This can lead to spontaneous puberty

Question 15

What can happen if there is an activating KISS1R mutation ?

Answer 15

This can cause precocious puberty

Question 16

How does KISS1 regulate GnRH secretion ?

Answer 16

Hypothalamic explants from mice shown Kisspeptin administration stimulates GnRH synthesis and secretion

Kisspeptin neurons send projections to GnRH neurons and bind to KISS1R expressed on GnRH neurons

Question 17

How does KISS1 regulate GnRH secretion ?

Answer 17

Kisspeptin neurones send projections to GnRH neurones
Bind to KISS1R on the GnRH neurones (upstream)
This causes a depolarisation of GnRH neurones
Leads to release of GnRH

Question 18

What can a bolus of Kisspeptin cause ?

Answer 18

If you administer a bolus of Kisspeptin in humans this correlates with a peak of LH secretion

LH is a good marker to check as it’s hard to measure GnRH

GnRH is difficult to access as we would need to access the hypophyseal portal in the brain or take a cerebrospinal fluid

Question 19

Why do we measure levels of LH as opposed to GnRH?

Answer 19

LH is a good marker as it’s hard to measure GnRH

GnRH is difficult to access as we would need to access the hypophyseal portal in the brain or take a cerebrospinal fluid

Question 20

What are the differences in the sexual dimorphic responses to Kisspeptin regulated GnRH secretion?

Answer 20

cyclical differences in male and female

Question 21

Outline the Kisspeptin-mediated regulation of GnRH

durring ovulation

Answer 21

The gonads will release sex steroids downstream of the hypothalamus and pituitary.WHen mediating feedback, oestrogen progesterone and testosterone will mediate their negative feedback via the Kisspeptin receptors on the Arcuate neurones.This will cause the downregulation of GnRH and the expression of LH and FSH.

However when the oestrogen threshold is reached and there is a LH surge and switch form negative to positive this feedback is regulated by the Kisspeptin neurons on the AVPV.

This is is rodents.

Question 22

Outline how Kisspeptin regulates feedback

Answer 22

When the H is subject to negative feedback via oestrogen the AC nucleus is mainly responsive to this
Decrease in KISS1 expression within the AVPV
When the critical threshold of oestrogen is reached there is switch to positive feedback for ovulation and LH surge
Increase in KISS1 expression from AVPV and increase in responsiveness
Increase drive to instigate positive feedback (increase in GnRH and LH)

Question 23

Describe the synthesis and release of GnRh

Answer 23

Synthesised and secreted from specialised neurons of of hypothalamus GnRH neuron.

Secreted in a pulsatile fashion and is regulated by pulse generator.

The pulsatility is very important

Question 24

Describe the synthesis of GnRH and how it becomes a mature protein

Answer 24

It is initially released as a pre-pro peptide from neurons found in the hypothalamus which then undergoes proteolytic cleavage to become GnRH which is a decapeptide (10 peptide sequence)

Expressed alongside it is GAP (GnRH associated protein)

Question 25

What is the GnRH pulsatility and how often is this released? | Why is this important ?

Answer 25

GnRH is secreted in pulses from the hypothalamus and this occours every 30-120 minutes. This is crucial for GnRH function A GnRH pulse will stimulate a pulse of LH and FSH secretion from pituitary. Pulsatile GnRH secreyion is important to stimulate the synthesis and secretion of LH/FSH.

Question 26

What are the pulses of GnRh which correspond with LH and FSH ?

Answer 26

Slow frequency pulse favours FSHb transcription and FSH release whereas rapid pulse frequency will favour LHb transcription and LH release,.

Question 27

Why is maintaining the pulsatility of GnRH so important ?

Answer 27

This is important because continuous release will result in the cessation of response of FSH/LH of the HPG axis.

Question 28

What was some reserach carried out for GnRH and what has been found ?

Answer 28

Common alpha subunit transcription is not strictly GnRH dependent. The synthesis occurs independent of GnRH pulse frequency and in excess of that required for FSH and LH production and secretion. Continuous release is mimicked in the start of IVF* treatment using GnRH analogues to downregulate endogenous GnRH production to allow the cycle to be manipulated and prevent premature ovulation when oestrogen levels are high.

Question 29

What is the GnRH pulse generator ?

Answer 29

A population of neurons in the arcuate nucleus with possible modulation of pulse frequency by other brain regions. Act on the GnRH neurons and modulate them to act in a pulsatile manner. (Single pulse generator)

Question 30

What was the GnRh pulse generator in Mice ?

Answer 30

This was found to be a sub-population of Arcuate Nucleus Kisspeptin Neurons. The evidence was that these were exhibiting periods of synchronised electrical activity which had a near perfect correlation with pulsatile LH secretion. Electrical activity in brain neurons tends to cause neurotransmitter release. The second evidence was that inhibition of these sub populations in the Arcuate Nucleus in the mid caudal region suppressed pulsatile LH secretion.

Question 31

What is the mode of GnRH action?

Answer 31

GnRH will bind to its receptor (GnRHR) This will cause signal transduction of the FSH and LH beta gene expression. This is followed by the synthesis ,secretion and packaging of FSH and LH (Gondadotrophins).

Question 32

What are the gonadotrophin hormones?

Answer 32

``` These are glycoprotein hormones : (Contains sugar residue attached ) HETERODIMERS(two different subunits) LH FSH hCG ``` Heterodimeric Comprised of a common alpha subunit They all have different N-linked carbohydrate side chains 🡪 microheterogeneity These are also secreted in a pulsatile fashion but pulsatility is not essential for their function The beta subunit gives them their unique properties and they are crucial for pulsatility. Beta subunit is produced in response to GnRH pulsatility.

Question 33

What is the structure of Gonadotrophins ?

Answer 33

A heterodimeric configuration made up of a alpha and beta subunit. These are held together by disulphide bonds. They both share a common alpha subunit but have variable beta units. Contain Y structures and these are the sugar residues attached to the peptide sequences via glycosylation. The beta subunit becomes unique through : Its length and the sugar which attaches as well as where it attaches.

Question 34

Describe the structure of gonadotrophin receptors

Answer 34

Components are : 1. Contain very large extracellular region : This is the region where FSH and LH tend to bind to This is also where activating and inactivating mutations tend to happen. 2.Transmembrane 3.Intracellular

Question 35

What are the functions of the gonadotrophins (FSH)?

Answer 35

Follicle-stimulating hormone (via FSHR) TESTIS: Regulation of Sertoli cell metabolism (important for nurturing spermatogenesis) OVARY: Critical for follicular maturation (without FSH or FSHR maturation arrests at preantral follicle stage) Granulosa cell oestrogen synthesis

Question 36

What are the functions of the Gonadotrophins ?(LH)

Answer 36

TESTIS: Stimulates Leydig cell mediated androgen synthesis OVARY: Stimulates Theca cell mediated androgen synthesis Ovulation Progesterone production of corpus luteum (via the LH receptor) Important for dominant follicle selection (LHR present on dominant follicle)

Question 37

What are the steroid hormones ?

Answer 37

Progesterone Oestrogens (Oestradiol and oestrone) Androgens (Androstenedione, testosterone, dihydrotestosterone)

Question 38

Male gonadal steroid production

Answer 38

(Cross-section of semineferous tubulues ) LHR are expressed on Leydig cells This stimulates androgens (androstenedione and DHEA production ) Then… FSHR are expressed on Sertoli cells and directs Sertoli cell metabolism (crucial for spermatogenesis) Also in Sertoli cells you get conversion of testosterone to DHT

Question 39

Female gonadal steroid production

Answer 39

LHR is expressed on Theca cells This causes synthesis of testosterone and androstenedione from cholesterol These androgens are transported to granulosa cells and converted by Aromatase to form Oestrogens. (Oestradiol and Oestrone) LHR are also present on granulosa cells especially larger antral follicles Then… FSHR are expressed on Granulosa cells CORPUS LUTEUM LHR and FSHR are also expressed on corpus luteum The remodelled ovarian remnant has both receptors LH in particular directs progesterone production Progesterone and Oestrogen production in the luteal phase.

Question 40

What other peptide is important and works alongside steroid hormones in the HPG axis?

Answer 40

Peptide hormones Inhibin's which are heterodimers. 2 isoforms of inhibin which are A and B Inhibin =Gonadal so inhibits FSH secretion via direct negative feedback to anterior pituitary.

Question 41

What are the structures of Inhibin ?What are found in males and females?

Answer 41

Common alpha subunit 2 different Beta subunit: Beta A and Beta B Depending on what subunit is present it is either Inhibin A or Inhibin B Males: Inhibin B Females: Cyclical dependent