Epigenetic: Brain and Behaviour Flashcards
1
Q
History
In 1942 the Term Epigenetics was coined by
A
Conrad Waddington
2
Q
History
What did Conrad Waddington do in 1942? (3)
A
- Used the term epigenetics to describe processes from gene to phenotype
- Envisioned that presence/abscence of particular genes determines which phenotypic path is followed
- Pioneering experiments in Drosophila: temperature shock influenced wing phenotype
3
Q
Explain Conrad’s experiment
A
He heat shock Drosophila embryos (puparium) at a critical time in development. This caused many of them to develop an unusual crossveinless wing phenotype (normally, their wings have crossveins). He kept breeding them and some offspring began showing the crossveinless phenotype even without heat shock. There is something in environment that affect expression of gene.
4
Q
Epigenetic processes are essential during:
A
embryonic development and cell differentiaton
5
Q
Epigentic changes are also documented in
A
Mature organisms
6
Q
Talk about epigentics and the fertilized embryo:
A
- Fertilization begins with two terminally differentiated (have distinct phenotype) germ cells and end with a totipotent zygote that can become any type of cells.
- Epigentic modifications in gametes must be reset to basal state with a wave of gene demethylation inorder to bring zygote to toitipotent state. (demethylation occurs right after fertilization)
- There is a second wave of gene methylation where genes are turned on/off. (Right around blastocyst stage)
- Between the two points (unmethylated everything and then remethylated genes), vulnerable state for embryo. If you experience things unanticipated things in environment that might alter DNA methlation pattern you can end up with a disease or neurological disorder.
7
Q
The —— is prone to epigenetic modification
A
developmental
8
Q
Examples of epigenetic remodelling during embryonic development:
Zygote:
A
Maternal and paternal genomes undergo widespread demethylation (preimplantation) and that produces wave of zygotic gene activation which induces pluripotency. Methylation re-established in lineage-specific regions (depending on cell type) during post-implantation likely influencing cell fate desicions.
9
Q
—– mutations in —— can resut in embryonic lethality. Example is —–
A
- Homozygous
- epigentic regulator genes
- Dnmt1 (maintenance methyltransferase)
10
Q
Mutations in MECP2 gene:
A
Responsible for 90% (or more) of cases of Rett syndrome (RTT) because MECP2 gene is a transcriptional repressor that recruit co-repressor, for example HDACs)
Loss-of-function mutations in MECP2 disrupt its ability to repress gene expression.
11
Q
Embryos are likely particularly vulnerable to environmental influences which may alter —–
A
the epigenome.
12
Q
Epigenetic modifications are —-
+ talk about the signifigance
A
- reversible
- You can use drugs to target epigenetic machinery to restore normal gene activity. For example, too much downregulation you can use drugs to inhibit HDACs
13
Q
Modification by environmental factors likely contributes to development of:
A
both normal and pathological phenotypes
14
Q
mC binding proteins (3)
What + structure + chromatin
A
- Recognize and bind to methylated cytosines (5-mC) in DNA — especially in CpG dinucleotides.
- Have a methyl binding domain that recognize methylated DNA.
- Promotes silent chromatin
15
Q
What are 3 proteins associated with silent chomatin?
A
- DNA DNMT
- mC binding proteins
- HDACs
16
Q
What are 3 proteins associated with active chomatin?
A
- Transcription factors
- coactivators
- HATs
17
Q
The — contains some of the highest levels of DNA methylation and methyl-CpG binding domain (MBD) proteins of any tissue
A
brain
18
Q
Radial glial cells (2)
What + exhibit
A
- Neural stem cell that produce every cell type in brain including glial cells
- Exhibit a spatiotemporal pattern of gene expression that is regulated by epigenetic mechanisms, including DNA methylation and histone modifications
19
Q
DNA methylation vs demethylation patterns
A
- DNA methylation patterns are stable while demethylation is relatively rare but does occur in the mature nervous system and the early zygote (two most plastic tissue where we see demethylation)
20
Q
Explain how epigenetic mechanisms in adult neurons redefined the field of neuroepigenetics (3):
A
- Epigenetic mechanisms in adult neurons redefined the field of neuroepigenetics by demonstrating that the epigenome is not static after development, but remains dynamically regulated throughout life, even in post-mitotic neurons.
- Epigenetic mechanisms originally were defined as heritable (via cell devision or procreation).
- Emerging roles of such mechanisms in mature cells with regards to acquired behaviours, neurological disorders, plasticity, addition, etc involved epigenetic modifications in addition to genetic roles.
21
Q
Rett syndrome and MeCP2 (5)
What + affects + symptoms + why it occurs + autism
A
- Rare neurodevelopmental disorder where individuals are born with it
- Almost exclusively affects girls. In boys it is embryonically lethal and they are born as stillborn
- Characterized by profound intellectual disability, poor expressive language (producing language then understanding), and clumsy and repetitive stereotyped hand movements, microephaly (especially reduced volume of cerebellum and cortex which are involved in autism)
- Usually de novo (sparatic), dominant, loss of function mutations of the X-linked methyl-CpG binding protein 2 (MECP2) gene.
- Not characterized as autism because happens in every cell (breathing, muscle) due to mutation in MECP2 which is expressed in every cell. So it affects other tissues outside the brain as well.
22
Q
Rett syndrome and MeCP2
Who do MeCP2 protein associate with and what do they participate in (2)?
A
- Traditionally thought to associate with represor complexes (HDACs) but can also associate with activator complexes
- Participates in chromatin condensation via association with posttranslational modifiers (e.g. HDAC)
23
Q
MeCP2 protein bind to —— via their ——.
A
- methylated DNA
- MBD
24
Q
Where are MeCP2 proteins sually expressed (2)?
+ glial
A
- Widely expressed, highest expression in brain, lung and spleen. Expecially cortex and cerebellum.
- Predominatly in neurons (7x higher of MeCP2 in neuron then glial) but also detected in glial cells.
25
MeCP2 expression correlates with:
- Postnatal maturation and neuronal differentiation but percise function is unknown.
26
In the brains of individuals with MeCP2 mutation there is (2):
1. Dysfunction in Neurotransmitter signalling
2. Dysfunction in Growth factor signalling: Brain derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1) as downstream transcriptional target
27
Dendrite and synapse in children with Rett syndrome/ MeCP2 mutation
- There is a decrease in number and length of dendrite, synapse and overall communication in the brain which is influenced by GF, NT signalling
28
What are psychiatric disorders?
- Behavioural disorders characterized by abnormalities in emotional and or cognitive functioning without obvious brain lesions. For example in stroke/alzhiemer you can pinpoint a certain area of the brain but in neurophyiatric disorder, not always obvious.
29
Neurological disorder vs pyschiatrict disorder
Treated by Neurologists or psychiatrists + psychologists
30
Early life adversity is associated with an increased risk of psychiatric disorder, explain (2):
| linked to + why?
- In humans, early life stress linked to increased rate of diabetes, heart disease, susceptibility to drug abuse, depression, schizophrenia, anxiety, etc.
- Epigenetic mechanisms may explain how early life experiences can affect brain and behaviour into adulthood
31
Relationship between ACE and genectic risk factor
ACE + genectic risk factor = more risk in developing pyschiatric disorder
32
Transgenerational impacts of stress example (5):
| Experiment + offspring behaviour +altered response + cortisol + dna meth
- Mice pups subjected to unpredictable maternal seperation and maternal stress from P1-P14. Take pup away from mom ~3 hours daily and usually stress mom by doing force swim. When pup is given back, she is also stresed and affects mothering behaviour (less attentive)
- Induces depressive-like behviours as adults in F1, F2, and F3 generations.
- Altered responses to novel and aversive environments as adults. When you stress later on, might not cope, more anxious, fearful. Put in box and see how many squares crossed. Anxious animals adventure less to middle even though mice are curious.
- Spike in cortisol higher/longer to taper off.
- Altered DNA methylation patterns in promoters of several genes in the brain (metabolic, GF pathways).
33
Affective mood disorders
34
----- brain structures such as ------ evaluate incoming sensory information
- Limbic
- Hippocampus, amygdla, PFC
35
Stressors trigger the stress response (2):
1. Activation of sympathetic NS (PNS: flight or fight NE, EP, adrenalin)
2. Activation of HPA axis: Cortisol production= main stress hormone in humans
| 1 occurs first*
36
Coping
stress response is efficiently terminated. If stress reponse is going on too long, you are not coping well with that stress.
37
Inappropriate stress response (2):
| increaed + what
- increased risk of affective disorders in genectically predisposed individuals.
- Smaller innocoulous stimuli leading to inappropriate stress resposne where it shouldn't be initiated in the 1st place or should have been terminated.
38
Chronic production of cortisol (2):
| what it does + feedback system
- damages our stress system
- There is negative feedback where producing cortisol turns off production of cortisol
39
# The limbic system
Distrupted cognition is a ---------
feature of depression
| Memory and executive function
40
Hippocampus is important in:
spatial navigation and episodic memory
41
PFC is important in:
executive function
42
Hypothalamus is important in:
homeostatic breathing and hormone production
43
Neurochemical aspects of major depression (2):
| + antidepressants
- Reduction in monoamines (NT/ small molecules from AA precursors) including NE, DA, 5-HT
- Mechanisms of antidepressants largely unknown
44
Examples of antidepressant actions (2):
- Inhibit the reuptake of monoamines into presynaptic neurons. Excess NT in synaptic cleft is pulled back into presynaptic cell, enzyme monoamine oxidase break down and terminate response. Examples like SSRI increase monoamine in cleft.
- Inhibit the breakdown of monoamines in presynaptic storage vessicles. Ex: monoamine oxidase inhibitor inhibit the breakdown of NT.
45
# HPA axis
Explain how HPA is involved in stress response (5):
1. Monoamine sends message to amgdyla that something stressful is happening and signals hypothalamus.
2. Release CRH when stressed (corticotrophin releasing hormone)
3. CRH stimulates the anterior pituitary gland to release ACTH (adrenocorticotropic hormone).
4. ACTH travels through the bloodstream to the adrenal cortex, which then releases cortisol (a glucocorticoid hormone).
5. Cortisol circulates through the body and mobilizes glucose for energy (via gluconeogenesis), Suppresses non-essential functions (like immunity, digestion), feeds back to the hypothalamus, hippocampus, and amygdala to downregulate the stress response and it's own production
46
Excessive chronic cortisol may:
Damage the feedback loop that turns off the stress response
47
Hyperactivity of the HPA axis occurs in ------
- MDD (major depressive disorder) + anxiety
- You are constantly living in fearful state
48
In rats it is not cortisol but -----
corticosterone
49
Cortisol actions (2):
| organs + binds to
1. Reaches every organ via circulation
2. Binds to mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs)
50
In hyperactive HPA axis, the receptors that seem to be affected is:
glucocorticoid receptors (GRs)
51
GRs and MRs are highly expressed in:
- hippocampus, PFC, amygdala
*to be affected in neuropsychiatric disorders*
52
GRs and MRs difference (2):
- MRs are involved in appraisal/stress onset
- GRs are involved in stress recovery (inhibition of HPA axis). Less present means damage to negative feedback
53
Dysregulation of MR and/or GR gene expression/function may contribute to development of ----- because -----
- MDD/anxiety
- excess cortisol + reduced expression of GRs/MRs mean more stress, more potential damage to HPA axis. Less GR not going to get that negative feedback and produce even more cortisol
54
GRs regulate the transcription of many genes (3):
| Gene + what it is + cortisol process
- NR3C1 (nuclear receptor subfamily 3, group C, member 1) the gene that encodes the glucocorticoid receptor (GR).
- GR is a ligand-activated transcription factor that mediates the effects of glucocorticoids (e.g., cortisol). Binds to specific glucocorticoid responsive elements (GREs)
- Cortisol, being lipophilic, diffuses across the cell membrane. In the cytoplasm, GR is bound to a complex of chaperone proteins (e.g., heat shock proteins like HSP90) that keep it in an inactive conformation. Upon cortisol binding, the receptor undergoes a conformational change, the chaperone complex dissociates, the GR–cortisol complex translocates into the nucleus. In the nucleus, GR binds to specific glucocorticoid response elements (GREs) on DNA
55
Genes and environment interact to produce vulnerability to affective disorders, explain:
Genectic predisposition + priming life events (individual's history, parental care, ACEs)
Can cause chronic stress
If axis is not regulated properly, rise in cortisol doesnt taper off. Cortisol activates GR which are TF and results in gene expression changes. GR is rich in brain and can affect brain in high extent.
In 3 weeks later: MR/GR downregulation, hippocampus dendritic tree atrophy, cell turnover in DG slowed down, reduced expression of 5-HT, LTP reduced + congnitive impairment (learning and memory impairment because affects hippocampus, amgdyla and PFC rich in GR).
56
HPA axis allows us to adapt to external stimuli such as:
- maternal care, diet, immune challenge, stress, enrichment etc
57
Explain the relationship between environment, epigenetic state and brain function and behaviour:
Environment (prenatal, perinatal and beyond) influences epigenetic states through acetylation and methylation.
Epigectic state influences:
Brain function and behaviour (pathological and resilient)
which influences the environment you surround yourself.
58
What are the three neuroepigenetics of early life experiences?
1. Maternal care
2. Prenatal stress
3. Environmental enrichment/excercise
59
# Epigenetic programming and the long-term effects of maternal care:
Maternal behaviour may affect epigenome of offspring, explain:
| two groups + adult offspring
- Rats: high licking/grooming and arched back nursing mothers vs low LG-ABN mothers (lay on side, no interaction, more passive)
- Adult offspring of high LG mothers show:
Increase hippocampal GR expression (enhances negative feedback on the HPA axis, helping to shut down cortisol production more efficiently.”)
Low CRF expression: Low basal state of stress
Low stress responses (less fearful, more modest (low) HPA responses)
60
How does maternal behaviour alter methylation status of a GR promoter (2)?
| What + how do you know its due to that?
- Adult offspring born to low LG-ABN mothers have: increased methylation of hippocampal GR promoter at the nerve growth factor-inducible protein A (NGFI-A) binding sequence (usually need a TF to induce expression but in this case methylated). As a result, levels of glucocorticoid receptor is not as higher for offspring born to those Low LG_ABN mothers.
- Affects are abolished by cross-fostering. Rats of LG-ABN but raised by HG-ABN affects goes away. This is due to maternal care and epigenectic effects, not genectic.
61
Explain ChIP process (5):
1. Stabilization of protein and DNA complex using formaldehyde. Because the interaction is usually transient, you want to form covalent bond so they get stuck: Snapshot in time
2. Cell membrane dissolved with detergents. DNA is sheared into small pieces using mechanical or enzymatic.
3. AB used to isolate specific protein. We want to understand what DNA element it is interacting with.
4. Reverse cross-linking with heat or enzymatic digestion
5. qPCR with southern blot
62
Maternal behaviour alters histone acetylation and transcription factor binding, explain:
- Adult offsprings born to high LG-ABN mothers have high histone H3-K9 acetylation at GR promoter and high binding of NGFI-A protein to hippocampal GR promoter (glucocortiod receptor upregulated = better negative feedback = better regulation of stress later in life)
63
GR more upregulated in HG offspring means ----- phenotype
resilient
64
# Impact of maternal care is reversible:
Explain trichostatin A (TSA) and it's effect (2)
| rescues + increases
- The HDAC inhibitor, TSA rescues H3K9 acetylation in adult offspring of low LG-ABN mothers
- Increased H3K9 acetylation and NGFI-A biniding of the GR promoter in hippocampus of TSA-treated offspring
65
TSA treatment ---- hippocampal GR expression in the hippocampus of offsprings born to ---- LG_ABN mothers which -----.
- increases
- low
- enhances multiple gene
66
Offsprings born to ----- LG-ABN mothers normally display ---------. ------ eliminated this effect
- low
- heightened stress responses
- TSA
67
Offsprings born to high LG_ABN mothers (2):
| In terms of GR expression + response to stress
- Increase GR expression (shuts off stress response) in hippocampus and more modest responses to stress
68
------ NGFI-A binding to ----acetylated, ----methylated GR promoters liklely increases gene expression which ------- of the HPA axis to negative feedback.
- Increase
- hyper
- hypo
- enhances sensitivity
69
--- acetylated + ---- methylated are better copers in life
- More
- less
70
Prenatal (the period before birth) stress also effects the epigenome and emotionality of offsprings:
| Experiment on mom + male offspring effect + reversed by
- Give mice mom stress everyday for 7 days during early, mid or late preganancy. Stressors that dont impact eating habit such as wet bedding, light ot cage change frequency.
- Males who have early gestational stress have maladaptive behavioural stress responsivity, increased GR gene methylation and decreased GR expression and increased HPA axis responsivity (higher levels of cort that are not being turned off)
- Reversed by citalopram (SSRI- antidepressant)
71
There is a ---- GR expression in prenatally stressed male mice and ---- of GR promoter also reported. ----- HPA anxis responsivity in prenatally stressed male mice. Elevated corticsterone levels that ------ in 15 min restraint stress.
- reduced
- hypermethylation
- Increased
- remained elevated for longer
72
Prenatal stress and methylation status and HPA axis of human fetus (2):
| methylation + HPA axis
- Increased NR3C1 promoter methylation associated with prenatal stress but maternal tactile (skin to skin) stimulation appears to reduce negative impact both on methylation and behaviour
- Elevated maternal cortisol may cause overactivity/dysregulation of fetal HPA axis potentially increasing risk for psychopathology such as anxiety and depression
73
McGowan et al, (2009) and sucide victim experiment (3):
| WHat it was + groups + result
- Examined NR3C1 promoter methylation in hippocampal samples from suicide victims vs control subjects (unrelated causes of death)
- Within suicide victims theres two groups, one with childhood abuse or neglect and one with not.
- There is a decreased GR expression, increase promoter methylation and decrease NGFI-A TF binding in suicide victims with a history of child abuse
## Footnote
NGFI-A binding to exon 1F promoter of NR3C1 increases GR expression in the hippocampus
