GnRH Flashcards
What is the role of GnRH?
Master controller of reproduction Roles include: - Neuroendocrine - HPG - Paracrine (placenta/gonads) - Autocrine (prostate/breast cancer) - Neurotransmitter (Regions of the brain)
Describe the structure of GnRH
Gene located on chromosome 8
23 isoforms in vertebrates - GnRH I / GnRH II most common
Pre-prohormone
Describe the cleavage steps of GnRH
Cleavage steps = from 69 aa to Mature GnRH (10aa) and GAP
What is the role of the GAP peptide?
GAP peptide co-secreted with GnRH, unknown function
Where is GnRH located in embryonic period?
Originate outside CNS, in medial olfactory placode
Where do GnRH containing cells migrate for GnRH release?
Nasal region 🡪 brain 🡪 medio-basal hypothalamus
GnRH-producing cells migrate through nasal system into forebrain
Numerous genes involved
What is a placode?
Area of thickening of embryonic epithelial later where organ / structure later develops
What are the possible disorders of incorrect GnRH neurone migration?
Kallmann Syndrome
Hypogonadotropic Hypogonadism
How does Hypogonadotropic Hypogonadism occur due to incorrect GnRH migration?
Hypogonadotropic Hypogonadism due to regulatory gene mutations
Give examples of known gene mutations causing Hypogonadotropic Hypogonadism
KAL1 FGFR1, FGF8 PROK2, PROKR2, NELF CHD7 GNRH1, GNRHR GPR54 TAC3, TAC3R NKB/NK3R WDR11
What causes kallman syndrome?
Mutation in KAL-1 gene
Premature termination of migration
⇒ anosmia & hypogonadotrophic hypogonadism
Which cells are responsible for GnRH production?
Parvocellular system (medial preoptic nucleus & arcuate nucleus) produce GnRH
How is GnRH prepared for release?
GnRH processed + packaged into storage granules
Transported down axons to external zone of median eminence
Where is GnRH released?
GnRH released in synchronized pulses from GnRH nerve endings into hypophyseal portal system
How often is GnRH released?
Rhythmic pulses- every 30-120 minutes – “circhoral pulses”
What generates GnRH pulses?
collection of hypothalamic neurons producing endogenous secretory rhythms
What is the half life of GnRH?
GnRH t1/2 2-4 minutes
What does GnRH release cause?
GnRH stimulates synthesis and secretion of gonadotrophins.
What is the significance of GnRH pulse release?
Differential frequency + amplitude alters pattern of FSH and LH secretion, therefore impact gonadal response
Determines dimerisation of subunits and glycosylation
Describe the structure of the GnRHr
G-protein-coupled receptor (GPCR)
How many types of GnRHr are there?
Two variants Type I and II GnRHR
Type 1 - full length
Type 2 - missense truncation (humans)
Where is the GnRHr expressed?
Expressed on gonadotroph cells of anterior pituitary
What is the role of the GnRHr?
GnRH-R = regulatory molecule
Transduces GnRH action (to stimulate synthesis and release of gonadotrophins).
Which tissues is GnRH receptors found in?
GnRH receptors are found in tissues like human anterior pituitary, breast, placenta and gonads and certain tumour cell lines
What are gonadotrophins?
Glycoproteins with α & β chains
Describe the structure of gonadotrophins
α-chains identical in FSH & LH
β-chains unique & confer biological actions
Which gonadotrophin is favoured slow GnRH expression?
Slow frequency / low amp GnRH pulse ⇒ ⇧FSHb gene expression
Which gonadotrophin does a rapid GnRH frequency favour?
Fast frequency GnRH pulse ⇒ ⇧LHb transcription
Which signalling pathway is activated by GnRH?
MAP Kinase pathway via Gq and Gs activation
What is a major sign that ovulation has occurred?
presence / formation of the corpus luteum is a good indicator of ovulation
Describe ovulation in DKO(ERK1/2-/-) mice
Anovulatory
Follicles weren’t making it past antral follicle stage…no ovulation
LHβ expression is decreased
What is the essential pathway for LHb expression in mice?
ERK-MAPK activation is essential for LHβ transcription and translation
What were the effects of ERK-MAPK absence in mice?
Transcription / Translation: Absence of ERK caused drastic reduction in LH expression in females
No effect on FSH production.
Outline the proposed mechanism of LHb expression via ERK_MAPK
- GnRH binds to GnRHR
- Triggers downstream signalling via Gs and Gq pathways
- PKA and PKC upregulated
- Undergo diverging ERK1/2 pathways
- Upregulation of EGR (early growth response) factor
- Results in upregulation of LHβ
What is the consequence of ERK-MAPK blockade?
When ERK pathway is blocked there is a subsequent downregulation in LHβ expression
What is CREB?
cAMP response element binding protein - binds on FSHbr
What is essential for CREB activity?
CREB activity requires cyclic AMP
What is ICER?
A protein that inhibits cAMP dependent pathways
How does ICER affect FSHb expression?
ICER inhibits CREB binding to response element on FSHβ gene
=> downregulates FSH expression
How does the menstrual cycle affect GnRH pulsatility?
GnRH pulses change frequency during menstrual cycle
How does GnRH pulsatility vary during menstrual cycle?
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
How is GnRH negative feedback stopped?
- Follicular phase: FSH secretion = Increase in estrogen levels
- -ve feedback resulting in FSH decline = DF selection (due to follicle expressing LHr)
- Switch from FSH pulses to LH pulses
- E2 levels continue to rise: 48hr sustained high level E2 expression reaching threshold of 300pg/mL
- Switch to +ve feedback – rapid pulses
- LH surge - ovulation
Describe the gonadal steroid feedback on LH in rhesus monkey males
Testosterone from Leydig cells reduces LH secretion (Rhesus monkey experiments)
How is FSH affected by steroid feedback in males?
Inhibin ↓FSH secretion from pituitary
Activin ↑FSH secretion from pituitary
Describe the HPG control of sperm production
- Hypothalamus secretes GnRH
- Stimulates synthesis + secretion of FSH & LH
- LH binds to LHr stimulating testosterone production from Leydig cells
- FSH binds to FSHr on Sertoli cells + stimulates production of Sertoli cell proteins e.g. inhibin B, activin, ABP, and transferrin
- Inhibin B + activin feedback to pituitary, regulating LH/FSH production and secretion
How does sperm mature?
Maturation of sperm to spermatid occurs in epididymis through contributions from prostate and seminal vesicle.
Describe the -ve feedback of gonadal steroids in females?
Progesterone & low plasma oestrogen ⇒ -ve feedback
Net effect = reduced LH & FSH
How is +ve feedback of gonadal steroids produced in females?
High sustained (48h) plasma [E2] = enhanced LH & FSH ⇒ +ve feedback
How do gonadal steroids affect GnRH release?
P4 ↓GnRH pulse freq
E2 ↓GnRH pulse amplitude
Where are the proposed sites of steroid feedback?
Either anterior pituitary or hypothalamus
E2 & P4 receptors on anterior pituitary and hypothalamus
Inhibin receptors found only in pituitary
How is steroid feedback proposed to occur in the anterior pituitary?
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
How is steroid feedback proposed in the hypothalamus?
Directly affecting GnRH neurones
Indirectly: changing activity of neural systems that influence GnRH release
e.g. ovarian hormones
