Epigenetics

Molecular Biology Recap

1. Synthesis of mRNA in the nucleus

2. Movement of mRNA into cytoplasm via nuclear pore

3. Synthesis of protein

• DNA is copied into messenger RNA via transcription

• Translation is the process in which mRNA is decoded to produce an amino acid chain

• The amino acid chain gets folded into a protein

  • Neurons, astrocytes, microglia

  • Receptors

  • Cytoskeletal elements

  • Enzymes

  • Etc., the building blocks of everything

• The human body requires approx a billion proteins each day

• Closed Chromatin - Gene switched off

  • Methylation of DNA and histones cause nucleosomes to pack tightly together

  • Transcription factors cannot bind the DNA, and genes are not expressed

• Open Chromatin - Gene switched on

  • Histone acetylation results in loose packing of nucleosomes

  • Transcription factors can bind the DNA and genes are expressed

Histone Modifications

• Modifications can alter the charge of proteins, and therefore how closely they are associated

  • Histone acetyltransferases (HATs) catalyse acetylation (gene open)

  • Histone deacetylases (HDACs) catalyse deacetylation (gene closed)

What is epigenetics?

• Epigenetics is the switching on or off of genes

  • This switch can stay that way for a lifetime

• Epigenetics can also flick a gene on or off in a dynamic way within a lifetime

• A explanation how our genes interact with our environment?

  • Exposure to environmental change may alter how the brain works and the subsequent behaviour expressed

    • Environmental chemicals, Drugs, Ageing, Diet

  • Epigenetic factors are involved in over 30 human neurodevelopmental and mental disorders

    • Autism, Schizophrenia, Depression, Addiction

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Drugs of abuse → Epigenetic changes

Epigenetic changes → drug use

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Epigenetics in Learning and Memory

• A brief summary of LTP and how this relates to epigenetics

1. an AP arrives

2. Glutamate is released

3. Glutamate binds to AMPA receptors allowing Na+ to enter the cell

4. If enough Na+ depolarises the membrane, Mg2+ is expelled from the NMDA receptor

5. The NMDA receptor can now open, allowing Ca2+ to enter

6. The influx of Ca2+ activates a cascade of intracellular events, including activating protein kinase pathways

7. One function is to chaperone AMPA receptors held in stores to the membrane

8. A second is to activate retrograde messenger systems which can enhance presynaptic neurotransmitter release

9. This process is early LTP is rapid, short-lived (hours) and protein independent

10. With high frequency stimulation, enough Ca2+ enters the cell to trigger a second cascade of events

11. The signalling pathways ultimately terminates with binding of the transcription factor CREB (cAMP response element binding protein) to CRE DNA sequences

12. This leads to transcription of various genes, including those important for axonal growth

13. This strengthens the connection between neurons that encode important memories

• But the induction of late LTP is induced by the state of chromatin in the promoter region of the genes necessary for synaptic plasticity

• Stefanko et al 2009

  • Object recognition task

    • Rat, put in box, give it 2 objects so they become familiar

    • Delay

    • Change one of the objects in the box (rats should prefer novelty)

    • Rats should investigate the new object

    • If memory is impaired or delay is too long, they will explore both

  • 2 habituation durations (3 or 10 mins)

  • Sodium butyrate after training

  • HDAC inhibition generates a type of long-term memory that persists beyond a point at which normal object recognition memory fails

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Learning event → Synaptic Plasticity (LTP) → Long-Term Memory

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If drugs of abuse are used:

A role for epigenetics in drug addiction

• Drug addiction can be viewed as maladaptive neural plasticity that occurs in vulnerable individuals in response to repeated exposure to a drug of abuse (nestler 2013)

• That vulnerability is determined roughly half of genetic factors and half by non-genetic factors (environmental exosures, stochastic events during development)

Three interacting possibilities

1. Repeated exposure to a drug of abuse in vulnerable individuals may lead to addiction via stable epigenetic changes

   1. Drugs → epigenetic change → vulnerability to addiction

   2. Kumar et al 2007 → cocaine increases histone acetylation → this process promotes open chromatin

2. Exposure to various environmental factors may lead to epigenetic changes that determine vulnerability later in life

   1. Environmental → epigenetic change → vulnerability to addiction

3. Drugs may induce epigenetic changes in sperm or ova that are then passed on to offspring

   1. Addiction in parents → epigenetic change → vulnerability to addiction in offspring

• These changes are not genome wide but can occur at genes that are really important for learning and memory, reward or stress responses

• Aberrant learning hypothesis again:

  • “Based in associative learning theory and assumes that drugs of abuse lead to enduring changes in the circuits underlying normal learning and memory processes”

  • As a consequence, otherwise innocuous stimuli can take on powerfully reinforcing properties.

  • (HDAC inhibition generates a type of long-term memory that persists beyond a point at which normal object recognition memory fails)

Gateway Hypothesis

• Nicotine appears to be an HDAC inhibitor

• Levine et al 2011

  • Pretreatment with nicotine → facilitates behavioural and neuronal responses to cocaine

    • Cocaine → nicotine had no such effect

  • This was associated with

    • Increases in FosB gene expression (TF that promotes plasticity)

    • Increased H3K9 acetylation at the FosB promoter

    • Overall increased acetylated H3 protein

    • Overall decreased HDAC activity

  • Numerous changes in histone acetylation, methylation and DNA methylation have been show in the NAc following:

    • Cocaine

    • Methamphetamine

    • Amphetamine

    • MDMA

    • Ethanol

    • Opioids

    • Nicotine

    • Cannabis

    • Inhalants

• Drugs of abuse appear to alter the epigenetic landscape (Waddington) of the brain against which learning occurs

Epigenetic changes may determine vulnerability to drugs

• Short-term increase in histone acetylation can promote behavioural response to the drugs

• Depends on the type of drug, the type of modification and the timing of administration

• Not clear whether environmental factors can promote long-term changes

Do early life stressors epigenetically prime addiction?

• Prenatal environments (exposure to maternal stress, viral infections, drugs, and toxins) can disrupt normal brain development, increasing risk of mental disease later in life

• Prenatal stress can lead to epigenetic changes (maternal care studies)

• Pre natal stress:

  • Pregnant dams exposed to restraint stress (Kippin 2008)

  • Enhanced responsiveness to cocaine

  • Prenatal stress leads to widespread epigenetic changes (Boersma et al 2014)

Drugs-induced epigenetic changes passed onto offspring

• Vassoler et al 2013

  • Male rats trained to self-administer cocaine or saline for 60 days

  • Males mated with drug naive females

  • Examine cocaine self-administration in offspring

• No difference in rates of acquisition of cocaine self-administration in the female offspring

• Delayed acquisition in male offspring of cocaine experienced males

• Not evident with a food reward

• Not related to maternal behaviour

• The difference was related to increased BDNF in the mPFC

  • BDNF is related to neuronal health and growth, regulates plasticity

  • Related to an increased association of AcH3 with BDNF exon IV

  • Acetylation of histones in areas where BDNF is produced

  • BDNF suppresses cocaine self-administration and reinstatement (region and procedure dependent)

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• Epigenetics may provide a mechanism through which drugs interact with the brain to produce long-lasting changes

• It may highlight a possible new avenue for the development of pharmacotherapies

• It may also alter the way we think about learning and memory more generally </aside>