Molecular mechanisms of labour Flashcards
What are the potential mechanisms of the onset of labour
Binary switch
Placental clock- CRH/ACTH/HPA axis
Fetal derived signal
Infection
Describe the placental clock mechanism of labour
Increased placental corticotrophin releasing hormone towards term stimulates fetal pituitary to release ACTH
ACTH increases fetal adrenal DHEA
DHEA is major oestrogenic precursor
Oestrogens increase myometrial gap junctions which facilitate regular coordinated uterine contractions
Why is the placental clock mechanism not the sole mechanism for labour
Anencephalic pregnancies are post term but deliver spontaneously
Lack part of the pathway for placental clock theory
Describe the fetal derived signal mechanism for labour
ACTH may be related to fetal surfactant proteins
Increased surfactant secretion at term activates amniotic fluid derived macrophages
AF macrophages migrate to uterine wall and activate inflammatory gene expression
What chemicals are required for onset of labour
Prostaglandins
Cytokines
Chemoattractant cytokines
What happens in preparation for labour
Myocyte depolarisation
Elevation of intracellular Ca2+
Increased expression of gap junction proteins
Functional progesterone withdrawal
What is functional progesterone withdrawal
there is no decline in plasma progesterone levels prior to parturition but progesterone receptor agonists induce termination
Something blocks progesterone at term
Possibly: different isoforms of receptor becoming dominant, estrogen activity becoming dominant, local metabolism to inactive form
What are the 3 layers of smooth muscle in the uterus
Inner circular layer- underlies endometrium, junctional zone, responsive to oestrogen
Interlocking middle layer
Outer longitudinal layer
Describe the spatial expression patterns of protein in the myometrium
Different levels of protein expression between each region
Possibly reflects function
Proteins include gap junction proteins, NFkB, RNA splicing proteins
Describe myometrial quiescence
Balance between contraction and relaxation
Balance between proquiescent and contractile molecules
Not just absence of contraction
What are the proquiescent molecules
Progesterone, Gas, CRH, cAMP, PKA, PKC, cGMP, NO
What are the contractile molecules
Oestrogen, CRH, Gaq, oxytocin, PGE2, PGF2a, Ca2+, IP3
Describe how signalling cascades influence myometrial contraction
L and T type Ca2+ channels Agonists cause calcium spikes Calcium modulated by calmodulin Chain reaction resulting in activation of actomyosin ATPase Causes myocyte contraction
What are the 3 Ga subunits
Gas
Gai
Gaq
Describe Gas signalling
Positive signalling
cAMP second messenger
B2 agonists eg. PGE2 act through EP2 receptor
Cause uterine relaxation and quiescence
Describe Gai signalling
Negative signalling Prevents cAMP production Involves alpha andrenergic agonists eg. ergometrine Opiod receptors PGE2 acting via EP1 and EP3 receptors Causes uterine contraction Can be targeted by misoprostol
Describe Gaq signalling
Positive signalling IP3 + DAG second messengers Oxytocin receptor PGF2 acting via FP2a receptor Causes uterine contraction Targeted by carboprost
What are the 3 signalling pathways involved in myometrial quiescence
Gas receptors increasing cAMP
Calcium regulated potassium channels causing myocyte hyperpolarisation
PKA phosphorylation of intracellular proteins activating acto-myosin ATPase
What are B2 agonists
Eg. Ritodrine, salbutamol
Respond to adrenaline and noradrenaline
Increase cAMP, promote smooth muscle relaxation + quiescence
Promotes myocyte hyperpolarisation and open K+ channels
Tocolytic activity
What is Gas
GTP-binding protein
Elevates cAMP and PKA activity
Present during pregnancy, downregulated during labour
Loss of Gas may cause contractions
Describe how inflammation disturbs the Gas promoter
Normal: No TNF, limited amount of CBP, acetylated histones, open chromatin, expression of Gas
Inflammation: TNF, elevated HDAC1, reduced CBP, reduced histone acetylation, closed chromatin, gene repression
What regulates inflammatory factors
NFkB
How does TSA inhibit myometrial contraction
Inhibits HDACs
HDACs remove acetylation so close chromatin
TSA prevents Gas being turned off
High levels of TNF can overcome repression
Describe canonical activation of NFkB
Involves TNFa, IL18, LPS
IkBa on NFkB targeted for degradation by 26s proteasome
NFkB binds promoter to increase IkBa expression
Describe non-canonical activation of NFkB
Involves BAFF, CD40, LPS
P100 CTD degradation by 26s proteasome
Promotes unknown gene expression
Describe hypoxic activation of NFkB
Promotes HIF expression
What proinflammatory factors are associated with labour
COX2, IL1B, IL6, IL8, TNF
Regulated by NFkB
Describe gene analysis experiment during inflammation
TNF stimulation- 13300 enriched regions, 46% intronic
Unstimulated- 11110 regions, 54% intronic
What are the 3 stages of parturition
Cervical dilation/remodelling Fetal expulsion (myometrial contraction) Placental delivery
What is involved in the transition from quiescence to contractile
Inflammation
Differential gene expression
Elevated Ca2+
Describe the role of Ca2+ in contractility
Calcium regulated contraction pathways
Extracellular sources target voltage gated Ca2+ channels
Intracellular stores act on channels on sarcoplasmic reticulum
What treatment for premature contractions targets Ca2+ channels
Nifedipine
Ca2+ channel blocker
Normally treatment for high blood pressure. Relaxes smooth muscle, increase luminal volume
Describe the role of oxytocin in contractions
Increased receptor numbers at term in fundal myometrium
Cervical stimulation/ myometrial stretch increases oxytocin secretion from paraventricular nuclei of hypothalamus
Positive feedback mechanism = ferguson reflex
What oxytocin analogue is used to induce labour
Syntocinon
Describe an oxytocin receptor agonist
Atosiban
Can inhibit premature myometrial contractions
Specific to oxytocin receptors
Doesn’t revert to full quiescence
What signals regulate myometrial quiescence
G-protein signalling particularly Gas B2 adrenergic receptor activation facilitating K+ extrusion Myocyte hyperpolarisation Nitric oxide hCG PGER1 + PGER3
