The HPG Axis

Question 1

Briefly describe the HPG axis

Answer 1

• Hypothalamus synthesises and secretes GnRH to anterior pituitary
• Gonadotroph cells in AP synthesis and secrete FSH and LH to the gonads
• Cellular compartments of the gonads synthesise + secrete steroid hormones (progesterone, oestrogen, testosterone)
• These hormones feedback at different points on the anterior pituitary and hypothalamus
• In males: feedback is always negative
• In females: feedback is primarily negative except for LH surge required for ovulation

Question 2

Where are GnRH neurones expressed?

Answer 2

• GnRH neurones are expressed in the hypothalamus in two regions.
o Arcuate nucleus
o Medial preoptic nucleus
• These neurones synthesise + secrete GnRH
• GnRH makes its way through the medial eminence to the gonadotroph cells of the anterior pituitary via the hypophyseal portal system

Question 3

Explain Kisspeptin.

Answer 3

Kisspeptin is a peptide hormone.

Known for its role as the ‘gatekeeper of puberty’

Kisspeptin neurones are expressed in the hypothalamus in the Arcuate Nucleus (ARC) and the anteroventral periventricular nucleus (AVPV).

Controls the synthesis and secretion of GnRH in GnRH neurones.

Kisspeptin expression is regulated by gonadal steroids. Regions in the kisspeptin neurones express oestrogen and progesterone receptors.

Question 4

Describe Kisspeptins Structure

Answer 4

• Synthesised as preproprotein, undergoes proteolytic cleavage to become Kisspeptin 54
• Kisspeptin-54 undergoes further cleavage to become kisspeptin 10, 13, 14.
o Kisspeptin 54 – used in in vivo animal studies – can cross blood-brain barrier efficiently

o Kisspeptin 10 – used in invitro/cell culture studies – very active ligands of GPR54 receptor + cheaper to make

Question 5

Describe Kisspeptin Receptor.

Answer 5

• Expressed on GnRH neurons.
• The Kisspeptin/KISS1R system is a component trigger of puberty.
• Inactivating mutations of KISS1R and Kisspeptin = failure to undergo spontaneous puberty.
o KISS1R = mutations that render it inactive, Kisspeptin = mutations that cause deficiency in expression
• Activating KISS1R mutations can lead to precocious puberty

Question 6

How does KISS1 regulate GnRH secretion?

Answer 6

• Hypothalamic explants from mice shown kisspeptin administration stimulates GnRH synthesis and secretion.
• Kisspeptin neurons send projections to GnRH neurons, and binding to KISS1R expressed on GnRH neurons.
• A bolus of Kisspeptin correlates with a peak of LH secretion.
o LH is used to clinically measure GnRH activity – very GnRH driven
o LH is measured than GnRH due to practically + ethical consideration
o GnRH samples requires access to hypophyseal portal circulation or CSF = impractical

Question 7

Describe Kisspeptin-mediated regulation of GnRH

Answer 7

When Oestrogen, Progesterone, Testosterone mediate their feedback via Kisspeptin neurones located on the Arcuate nucleus. GnRH is downregulated and expression of LH/FSH is downregulated. This is negative feedback.

When oestrogen threshold reaches + there is a LH surge for positive feedback: it is mediated via the kisspeptin neurones on AVPV (anteroventral periventricular nucleus) causing GnRH to be upregulated causing the LH surge

Important to note this is how it works in rodents – in humans, there is more to be understood

Question 8

Describe GnRH

Answer 8

• Synthesised and secreted from specialised neurons of hypothalamus – GnRH neurons.
• Secreted in a pulsatile fashion – pulse generator orchestrated.
• Stimulates the synthesis and secretion of pituitary gonadotrophin hormones - LH and FSH.
• Pulse generator = series of specialised neurones that modulate the pulsatility of GnRH

Question 9

Describe GnRH Structure

Answer 9

• Initially synthesised as a preproprotein  undergoes proteolytic cleavage  GnRH (deca-peptide)
• GAP = GnRH Associated Protein
• GAP is also expressed alongside GnRH

Question 10

Explain GnRH pulsatility

Answer 10

• GnRH is secreted in pulses from hypothalamus every 30-120 min
• A GnRH pulse stimulates a pulse of LH and FSH secretion from the pituitary
• Pulsatile GnRH secretion is vital for stimulation of synthesis and secretion of LH/FSH
o Slow frequency pulse = favours FSHb transcription and FSH release
o Rapid pulse frequency = favours LHb transcription and LH release
• Continuous release results in cessation of response

Question 11

What does GnRH slow frequency pulse favour?

Answer 11

favours FSHb transcription and FSH release

Question 12

What does GnRH fast frequency pulse favour?

Answer 12

favours LHb transcription and LH release

Question 13

What is the GnRH pulse generator?

Answer 13

• Studies by Clarkson et al. (2017) provided evidence for the identity of the GnRH pulse generator in mice:
• A sub-population of Arcuate Nucleus Kisspeptin Neurons (ARNKISS) were all found to exhibit intermittent periods of synchronized electrical activity which had a near-perfect with pulsatile LH secretion.
• Inhibition of ARNKISS in the mid-caudal region of the arcuate nucleus suppressed pulsatile LH secretion
• Possible input from other regions of the brain

Question 14

Name the 3 gonadotrophins and describe their structure.

Answer 14

LH, FSH (and hCG)
• Glycoprotein hormones
o Glycoproteins = sugar residue attached to protein
• Heterodimeric
• LH, FSH, HCG all share a common alpha sub-unit – beta subunit gives distinct properties
o beta subunits are synthesised and expressed in response to GnRH pulsatility
o alpha subunit is produced constitutively
• Pulsatile

• alpha and beta subunits held together by disulfide bonds
• Y structures = sugar residues attached (glycosylation)

• beta subunits unique properties is due to:
o Length of sequence
o Type of glycosylation
o Location of glycosylation

Question 15

describe structure of gonadotrophin receptors

Answer 15

• Large extracellular region
o where FSH and LH bind
o region where activating or inactivating mutations occur

Question 16

what is the function of gonadotrophins

Answer 16

Support germ cell growth, development, and maturation to produce oocyte and sperm competent for fertilisation

Question 17

What does FSH do to the testis

Answer 17

regulation of Sertoli cell metabolism

Question 18

What does FSH do to the ovary

Answer 18

 follicular maturation

 granulosa cell oestrogen synthesis

Question 19

what does LH do to the testis

Answer 19

 stimulation of Leydig cell androgen synthesis

Question 20

what does LH do to the ovary

Answer 20

 theca cell androgen synthesis
 ovulation
 progesterone production of corpus luteum

Question 21

Describe Leydig Cells

Answer 21

located in the interstitial region in seminiferous tubules in males

o Express LH receptor
o LH moderates’ production of androgens

Question 22

Describe Sertoli cells

Answer 22

located further inside seminiferous tubules in males

o Nurture developing germ cells
o Express FSH receptor
o FSH modulates Sertoli cell metabolism = crucial for spermatogenesis
o FSH is responsible for the concentration of testosterone to DHT

Question 23

Describe Theca Cells

Answer 23

express LH receptor

o LH produces testosterone + androstenedione

Question 24

Describe Granulosa cells

Answer 24

express FSH receptor

o FSH modulates the conversion of androgens (theca cells) to oestradiol + oestrone (via aromatase)

Question 25

Describe steroid production in the corpus luteum

Answer 25

expresses BOTH receptors | o Produces progesterone + oestrogen in the luteal phase

Question 26

Describe inhibin

Answer 26

• Peptide hormone • Heterodimers • 2 isoforms of inhibin: A and B. • Inhibin produced by the gonads • inhibits FSH secretion via direct negative feedback to anterior pituitary. • Males: Inhibin B • Females: cyclical dependent – o In females, Inhibin expression is cyclical depending on the phase of the menstrual cycle o Switch from inhibin B in the follicular phase  Inhibin A in the luteal phase.