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Onishchenko N, Karpova N, Sabri F, Castrén E, Ceccatelli S.
Maneeshi Prasad
11/13/2009
Depression
• 1 in 6 individual in US is affected from depression during
their lifetime
• Approximately 18.8 million American adults, or about 9.5
percent of the U.S. population age 18 and older in a given
year, have a depressive disorder
• Nearly twice as many women (12.0 percent) as men (6.6
percent) are affected by a depressive disorder each year
• Depression affects all people regardless of age,
geographic location, demographic or social position
Depression
• Depression often exists with other diseases,
including chronic pain, arthritis, diabetes and
HIV
• Depression is also known to weaken the
immune system, making the body more
susceptible to other medical illnesses
• Anxiety disorders, such as post–traumatic
stress disorder (PTSD), obsessive–compulsive
disorder, panic disorder, social phobia and
generalized anxiety disorder, often accompany
depression
Causes of Depression
• Biochemical. People with depression have
physical changes in their brains with
neurotransmitters and also could be a culprit
may also play a role in depression
• Genes. Depression is more common in people
who are biologically related to a family
member who also has the condition
• Environment. Environmental factors such as
the loss of a loved one, financial problems and
high stress also leads to depression
Symptoms of depression
Depression symptoms can vary greatly with age. Some common symptoms are:
• Loss of interest in normal daily activities
• Feeling sad or down
• Feeling hopeless
• Crying spells for no apparent reason
• Problems sleeping
• Trouble focusing or concentrating
• Difficulty making decisions
• Unintentional weight gain or loss
• Irritability
• Restlessness
• Being easily annoyed
• Feeling fatigued or weak
• Feeling worthless
• Loss of interest in sex
• Thoughts of suicide or suicidal behavior
• Unexplained physical problems, such as back pain or headaches
Neural circuitry of depression
• Limbic regions have been implicated in
depression and antidepressant action
• Neural activity in amygdala and subgenual
cingulate cortex is highly increased in
individuals with depressive symptoms and
goes back to normal on successful treatment
Neural circuitry of depression
Neurotrophins
• Depressed patients show volumetric decrease
in the hippocampus and forebrain regions
• This relates to the decrease in neurotrophic
factors
• BDNF mediated signaling gets reduced due to
stress
• Chronic antidepressant treatment increases
BDNF mediated signaling
• Similar results have been seen in post-mortem
hippocampus of patients with depression: low
BDNF levels in hippocampus and serum
BDNF signaling
• BDNF signaling has region and antidepressant
specific effect
• Antidepressant effects can be inhibited by
blocking neurogenesis in hippocampus
• Antidepressants increase the amount of growth
factors (BDNF, VEGF) that influence
neurogenesis in hippocampus
Neurotrophic hypothesis of
depression
• Reduction in brain BDNF levels leads to
depression and increase in brain BDNF
levels leads to antidepressant action
• Antidepressant increase the TrKB activation
and signaling
– Activation of PLCg signaling
• Phosphorylation of CREB
– Increase in BDNF transcription
BDNF and depression
Neurodevelopmental disorders
• Learning disabilities, sensory deficits, attention deficit,
hyperactivity disorder
• Autism and autism spectrum disorders, traumatic brain
injury, communication, speech and language disorders
• Genetic disorders such as fragile-X syndrome, Down
syndrome and Rett syndrome, epilepsy, fetal alcohol
syndrome, learning disorders, neurological and psychiatric
• Environmental conditions affect expression
of genes, resulting in long lasting changes
in structure and function of brain
– Stress
– Drug treatments
– Maternal care
– Toxins
e.g.: Methyl mercury
Environmental contaminant present in sea food
How People Are Exposed to
Mercury
• Eating fish or shellfish that is contaminated with
methylmercury, which is the main source of general
human exposures to mercury
• Breathing air contaminated with elemental mercury
vapors (e.g., in workplaces such as dental offices and
industries that use mercury or in locations where a
mercury spill or release has occurred)
• Having dental fillings that contain mercury; and
• Practicing cultural or religious rituals that use
mercury.
How Mercury Affects People’s
Health
• Short-term exposure to extremely high levels
of elemental mercury vapors can result in lung
damage, nausea, diarrhea, increases in blood
pressure or heart rate, skin rashes, eye
irritation, and injury to the nervous system
• Prolonged exposure to lower levels of
elemental mercury can permanently damage
the brain and kidneys
• The developing brain of a fetus can be injured
if the mother is exposed to methylmercury
Levels of Mercury in U.S.
Population
• Survey period of 2005–2006: 95th percentile
levels for total blood mercury in children aged
1-5 years and females aged 16-49 years were
1.43μg/L and 4.48 μg/L, respectively
i.e. only 5 percent of the population will have
values at this level or higher
Mercury on neurodegeneration
http://www.epa.gov/mercury/effects.htm
http://www.epa.gov/mercury/advisories.htm
Long-lasting depression-like
behavior and epigenetic changes of
BDNF gene expression induced by
perinatal exposure to methylmercury
Experimental Animals and methods
• C57BL/6/Bkl mice
– Pregnant dams exposed to MeHg @ 0.5mg/kg/day via
drinking water from gestational day 7 till day 7 after
delivery
– No change in litter size, body weight gain
– Results in brain mercury concentration similar to infants in
human population who eat MeHg contaminated fish
In-situ hybridization, ChIP, Primer extension, Densitometry
Forced swim test
• Animals starting at 9 weeks were tested
• Individuals were tested for immobility for
6min after a pretest of 15min 24hrs before the
test
• Immobility- passively floating in water for
2sec or longer with only small movements
enough to keep the head above water
Results: Depression-like behavior of
MeHg-exposed mice
Results of FST
• Immobility time was longer in perinatally
MeHg-exposed mice than controls of 9 weeks,
12 weeks and 14 months
• Confirming the long-lasting effect of MeHg
• Treatment with fluoxetine (Prozac) reduced
the immobility time in the MeHg treated
animals
Hippocampal BDNF mRNA level
TrKB mRNA expression
Results of BDNF mRNA expression
levels
• MeHg lead to significant decrease in BDNF
mRNA level in dentate gyrus but not in CA1
and CA3 regions of 12 week old mice and
persisted in 14-month old mice
• No change was seen in TrKB mRNA
expression in the hippocampal formation
• Fluoxetine treatment was able to restore the
BDNF mRNA expression in dentate gyrus
Epigenetic regulation in depression.
H3K27-methylation : no transcription
H3K9 and H3K14- acetylating: transcription
Chromatin Immunoprecipitation (ChIP)
BDNF Gene locus with 8 promoters
Epigenetic state of BDNF gene
H3K27 methylation and H3 acetylation
regulate BDNF gene expression in
hippocampus
• Increased H3K27 tri-methylation and decrease in histone
H3 acetylation at the BDNF promoter IV of 12 week old
mice that persisted through 14-month old MeHg exposed
mice leading to silencing of BDNF gene
• Fluoxetine treatment had no effect on the H3k27 trimethylation but it did increase the histone H3 acetylation
to overcome the repression of BDNF transcription
Methylation of CpG sites
• Using MS-SNuPE (Methylation Sensitive Single
Nucleotide Primer Extension)
DNA treated with
bisulphite to convert
unmethylated
cytosine converted
to uracil
Primer extension
with P32 labeled
TTP and CTP,
extension will end
just 5’ to the C or U
Quantification of
CTP/TTP ratio to
estimate the amount
of methylated C
Methylated CpG result in repression of transcription
Epigenetic state of BDNF gene
Epigenetic state of BDNF gene
• Significant increase in DNA methylation at
BDNF promoter IV in hippocampus was seen
in 14 month old mice exposed to MeHg
prenatally
Discussion
• Previous studies have shown that MeHg exposure
during developmental stages have behavioral effects
in rodent animal models similar to depression
• Current data confirms the role of MeHg in
incorporating long term epigenetic changes in BDNF
gene locus leading to depression-like symptoms in
mice model exposed to MeHg prenatally
• Antidepressant treatment was able to overcome this
MeHg induced repression of BDNF expression in the
hippocampus
Discussion
• Impairment in memory, attention, language
and visuospatial perception has been seen
in children exposed to MeHg prenatally
• Depressive syndromes have been seen in
adults with occupational exposure to
mercury
• These patients have reduced hippocampal
and serum BDNF levels, which can be
restored by antidepressant treatment
Prozac (selective 5-HT reuptake inhibitor)
Discussion
• During depression there is decrease in
BDNF mRNA in dentate gyrus
• BDNF expression has been associated with
increase in neurogenesis in the subgranular
zone of dentate gyrus
• Suggesting that MeHg may be involved in
decrease in neurogenesis
Discussion
• Promoter IV of BDNF gene is regulated by neuronal
activity (CaRF and CREB)
• MeHg was able to induce long lasting epigenetic changes
in this promoter in the hippocampus by methylation of
H3K27 and decrease in acetylation of H3
• Antidepressant was able to increase the acetylation of H3
at this promoter to relieve the MeHg induced repression
of BDNF gene expression
• Antidepressants may act through downregulation of
histone deacetylases resulting in increased acetylation of
histones and thus gene transcription