GnRH

Question 1

What is the role of GnRH?

Answer 1

Master controller of reproduction 
Roles include: 
- Neuroendocrine - HPG
- Paracrine (placenta/gonads)
- Autocrine (prostate/breast cancer)
- Neurotransmitter (Regions of the brain)

Question 2

Describe the structure of GnRH

Answer 2

Gene located on chromosome 8
23 isoforms in vertebrates - GnRH I / GnRH II most common
Pre-prohormone

Question 3

Describe the cleavage steps of GnRH

Answer 3

Cleavage steps = from 69 aa to Mature GnRH (10aa) and GAP

Question 4

What is the role of the GAP peptide?

Answer 4

GAP peptide co-secreted with GnRH, unknown function

Question 5

Where is GnRH located in embryonic period?

Answer 5

Originate outside CNS, in medial olfactory placode

Question 6

Where do GnRH containing cells migrate for GnRH release?

Answer 6

Nasal region 🡪 brain 🡪 medio-basal hypothalamus

GnRH-producing cells migrate through nasal system into forebrain
Numerous genes involved

Question 7

What is a placode?

Answer 7

Area of thickening of embryonic epithelial later where organ / structure later develops

Question 8

What are the possible disorders of incorrect GnRH neurone migration?

Answer 8

Kallmann Syndrome

Hypogonadotropic Hypogonadism

Question 9

How does Hypogonadotropic Hypogonadism occur due to incorrect GnRH migration?

Answer 9

Hypogonadotropic Hypogonadism due to regulatory gene mutations

Question 10

Give examples of known gene mutations causing Hypogonadotropic Hypogonadism

Answer 10

KAL1
FGFR1, FGF8
PROK2, PROKR2, 
NELF
CHD7
GNRH1, GNRHR
GPR54
TAC3, TAC3R
NKB/NK3R
WDR11

Question 11

What causes kallman syndrome?

Answer 11

Mutation in KAL-1 gene
Premature termination of migration
⇒ anosmia & hypogonadotrophic hypogonadism

Question 12

Which cells are responsible for GnRH production?

Answer 12

Parvocellular system (medial preoptic nucleus & arcuate nucleus) produce GnRH

Question 13

How is GnRH prepared for release?

Answer 13

GnRH processed + packaged into storage granules

Transported down axons to external zone of median eminence

Question 14

Where is GnRH released?

Answer 14

GnRH released in synchronized pulses from GnRH nerve endings into hypophyseal portal system

Question 15

How often is GnRH released?

Answer 15

Rhythmic pulses- every 30-120 minutes – “circhoral pulses”

Question 16

What generates GnRH pulses?

Answer 16

collection of hypothalamic neurons producing endogenous secretory rhythms

Question 17

What is the half life of GnRH?

Answer 17

GnRH t1/2 2-4 minutes

Question 18

What does GnRH release cause?

Answer 18

GnRH stimulates synthesis and secretion of gonadotrophins.

Question 19

What is the significance of GnRH pulse release?

Answer 19

Differential frequency + amplitude alters pattern of FSH and LH secretion, therefore impact gonadal response
Determines dimerisation of subunits and glycosylation

Question 20

Describe the structure of the GnRHr

Answer 20

G-protein-coupled receptor (GPCR)

Question 21

How many types of GnRHr are there?

Answer 21

Two variants Type I and II GnRHR
Type 1 - full length
Type 2 - missense truncation (humans)

Question 22

Where is the GnRHr expressed?

Answer 22

Expressed on gonadotroph cells of anterior pituitary

Question 23

What is the role of the GnRHr?

Answer 23

GnRH-R = regulatory molecule

Transduces GnRH action (to stimulate synthesis and release of gonadotrophins).

Question 24

Which tissues is GnRH receptors found in?

Answer 24

GnRH receptors are found in tissues like human anterior pituitary, breast, placenta and gonads and certain tumour cell lines

Question 25

What are gonadotrophins?

Answer 25

Glycoproteins with α & β chains

Question 26

Describe the structure of gonadotrophins

Answer 26

α-chains identical in FSH & LH | β-chains unique & confer biological actions

Question 27

Which gonadotrophin is favoured slow GnRH expression?

Answer 27

Slow frequency / low amp GnRH pulse ⇒ ⇧FSHb gene expression

Question 28

Which gonadotrophin does a rapid GnRH frequency favour?

Answer 28

Fast frequency GnRH pulse ⇒ ⇧LHb transcription

Question 29

Which signalling pathway is activated by GnRH?

Answer 29

MAP Kinase pathway via Gq and Gs activation

Question 30

What is a major sign that ovulation has occurred?

Answer 30

presence / formation of the corpus luteum is a good indicator of ovulation

Question 31

Describe ovulation in DKO(ERK1/2-/-) mice

Answer 31

Anovulatory Follicles weren't making it past antral follicle stage...no ovulation LHβ expression is decreased

Question 32

What is the essential pathway for LHb expression in mice?

Answer 32

ERK-MAPK activation is essential for LHβ transcription and translation

Question 33

What were the effects of ERK-MAPK absence in mice?

Answer 33

Transcription / Translation: Absence of ERK caused drastic reduction in LH expression in females No effect on FSH production.

Question 34

Outline the proposed mechanism of LHb expression via ERK_MAPK

Answer 34

1. GnRH binds to GnRHR 2. Triggers downstream signalling via Gs and Gq pathways 3. PKA and PKC upregulated 4. Undergo diverging ERK1/2 pathways 5. Upregulation of EGR (early growth response) factor 6. Results in upregulation of LHβ

Question 35

What is the consequence of ERK-MAPK blockade?

Answer 35

When ERK pathway is blocked there is a subsequent downregulation in LHβ expression

Question 36

What is CREB?

Answer 36

cAMP response element binding protein - binds on FSHbr

Question 37

What is essential for CREB activity?

Answer 37

CREB activity requires cyclic AMP

Question 38

What is ICER?

Answer 38

A protein that inhibits cAMP dependent pathways

Question 39

How does ICER affect FSHb expression?

Answer 39

ICER inhibits CREB binding to response element on FSHβ gene | => downregulates FSH expression

Question 40

How does the menstrual cycle affect GnRH pulsatility?

Answer 40

GnRH pulses change frequency during menstrual cycle

Question 41

How does GnRH pulsatility vary during menstrual cycle?

Answer 41

Early Follicular phase – pulses slow (every 90-120mins) »FSH Mid-late F phase – pulse freq increases (every hr.) »LH After ovulation – pulses slow (every 3-5h) »FSH production End of luteal phase – increase GnRH pulse secretion » FSH

Question 42

How is GnRH negative feedback stopped?

Answer 42

1. Follicular phase: FSH secretion = Increase in estrogen levels 2. -ve feedback resulting in FSH decline = DF selection (due to follicle expressing LHr) 3. Switch from FSH pulses to LH pulses 4. E2 levels continue to rise: 48hr sustained high level E2 expression reaching threshold of 300pg/mL 5. Switch to +ve feedback – rapid pulses 6. LH surge - ovulation

Question 43

Describe the gonadal steroid feedback on LH in rhesus monkey males

Answer 43

Testosterone from Leydig cells reduces LH secretion (Rhesus monkey experiments)

Question 44

How is FSH affected by steroid feedback in males?

Answer 44

Inhibin ↓FSH secretion from pituitary | Activin ↑FSH secretion from pituitary

Question 45

Describe the HPG control of sperm production

Answer 45

1. Hypothalamus secretes GnRH 2. Stimulates synthesis + secretion of FSH & LH 3. LH binds to LHr stimulating testosterone production from Leydig cells 4. FSH binds to FSHr on Sertoli cells + stimulates production of Sertoli cell proteins e.g. inhibin B, activin, ABP, and transferrin 5. Inhibin B + activin feedback to pituitary, regulating LH/FSH production and secretion

Question 46

How does sperm mature?

Answer 46

Maturation of sperm to spermatid occurs in epididymis through contributions from prostate and seminal vesicle.

Question 47

Describe the -ve feedback of gonadal steroids in females?

Answer 47

Progesterone & low plasma oestrogen ⇒ -ve feedback Net effect = reduced LH & FSH

Question 48

How is +ve feedback of gonadal steroids produced in females?

Answer 48

High sustained (48h) plasma [E2] = enhanced LH & FSH ⇒ +ve feedback

Question 49

How do gonadal steroids affect GnRH release?

Answer 49

P4 ↓GnRH pulse freq | E2 ↓GnRH pulse amplitude

Question 50

Where are the proposed sites of steroid feedback?

Answer 50

Either anterior pituitary or hypothalamus E2 & P4 receptors on anterior pituitary and hypothalamus Inhibin receptors found only in pituitary

Question 51

How is steroid feedback proposed to occur in the anterior pituitary?

Answer 51

Direct regulation of GnRHR - Hormones directly regulating FSH / LH to decrease/increase their sensitivity to GnRH pulses Plenty of receptors of E2, P4 and inhibins here via GnRH receptor regulation

Question 52

How is steroid feedback proposed in the hypothalamus?

Answer 52

Directly affecting GnRH neurones Indirectly: changing activity of neural systems that influence GnRH release e.g. ovarian hormones