Chronic psychosocial stress enhances long-term depression in a subthreshold amyloid-beta rat model of Alzheimer's disease

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Chronic psychosocial stress enhances long-term
depression in a subthreshold amyloid-beta rat
model of Alzheimer's disease

• Tran, Trinh, Srivareerat,et al.

Khalil Khlifi Pharm D. Candidate 2012 University
of Georgia, College of Pharmacy
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Alzheimer's Disease

• Progressive, irreversible brain disorder that
  gradually destroys memory and cognitive abilities
• Clinically, patient's are seen to have marked
  memory loss and impairment of social and
  occupational functions
• Pathologically, it is characterized by the
  accumulation of amyloid-beta peptides and tau
  protein
•  Accumulation is caused by abnormally folded
  amyloid-beta and tau proteins
• The plaques are made up of small peptides 39-43
  amino acids in length and deposit outside neurons
• Tau is used to stabilize the microtubules and a
  chemical change causes it to be
  hyperphosphorylated and thereby creates
  neurofibrillary tangles and disintegrates the
  neuronal transport system

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Stress and AD

• Genetic factors have a very definitive role on
  the progression of the disease but what does the
  effect of non-genetic factors have on progression
  of these patients?
• Exposure to severe and/or prolonged stress
  challenges homeostasis mechanisms and may cause
  over-activation and dysregulation of
  stress-activated systems
• These may lead to subsequent negative changes in
  the brain
• The hippocampus is particularly susceptible to
  stress functions and is one of the first regions
  to be affected by AD
• The hippocampus is involved in learning and
  memory processes

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Methods and Materials

• Adult male Wistar rats weighing 200-225g at the
  beginning of the experiment
• Housed in groups of 6 with free access to food
  and water in a temperature and light controlled
  environment
• 4 Treatment groups were designated in this study
• Control
• Stress
• Sub-amyloid-beta (SAB)
• Stress and sub-amyloid-beta (SSAB)
• The SAB and SSAB groups were infused with
  subpathogenic doses of AB1-42
• The Control and Stress groups were infused with
  an inactive reverse peptide AB42-1

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Methods and Materials

• Both SAB and SSAB were subjected to chronic
  psychosocial stress using a form of "intruder"
  stress
• Rats were kept in the same cages for 1 week then
  2 rats were swapped in each cage daily for 6
  weeks total
• Stress was measured by marked increases in
  corticosterone levels
• Hippocampal Dissection
• Animals were killed under urethane anaesthesia
  and the right hippocampi was removed and the
  dorsal and vental sides of area CA1 were
  separated and placed in microcentrifuge tubes and
  stored at -80C until processed

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Method and Materials

• Immunoblotting
• Hippocampal tissue was placed in an isotonic
  buffered cocktail containing protease and
  phosphatase inhibitors
• Following antibodies were used in the study
• Amyloid precursor protein (APP)
• Beta-site amyloid precursor protein cleaving
  enzyme (BACE)
• p-CaMKII
• CaMKII
• Calcineurin
• Brain-derived neutrotrophic fator
• GAPDH antibody

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Statistics

• Carried out with a two-way analysis of variance
  (ANOVA) followed by Tukey's post hoc test
• Minimum significance of plt0.05

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ANOVA

• Provides statistical test of whether or not the
  means of several groups are all equal and
  therefore generalizes t-test to more than two
  groups
• Doing multiple t-tests between 2 groups would
  result in an increased chance of committing a
  type I error (rejects a true null hypothesis)

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Results

• Effects on chronic stress on long term depression
  (LTD) in subAB rats
• Magnitude of LTD was similar in both control and
  subAB rats however it was significantly enhanced
  in the stress compared with the control and subAB
  rats as shown as a decline in fEPSP (field
  exictatory post-synaptic potential) slope
• Control 78.38 /- 1.97
• Stress 62.93 /- 4.35
• SubAB 75.98 /- 3.41
• The slope of fEPSP was significantly (lt0.001)
  lower in SSAB than those in control, stress, and
  SAB groups

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Results

• Levels of calcineurin during expression of LTD
• The levels of calcineurin were markedly increased
  by 30 /- 6 in stimulated stress group and 26
  /- 3 in SSAB compared to unstimulated control
  group 
• Levels of p-CaMkII and total CaMKII during
  expression of LTD
• Levels of p-CaMKII were significantly increased
  by 116/- 31 in SAB and 125/-47 in SSAB rats
  as compared to unstimulated control group
• Significant increases in levels of CaMKII in all
  the stimulated groups as compared to the
  unstimulated groups
• Basal levels of APP
• There was no significant change in the APP levels
  across all groups

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Discussion

• Human studies have never revealed an association
  between the number of stressful or negative life
  events and the effects on cognitive decline
• However, studies involving those with AD have
  shown that increased cortisol levels with one
  APOE-4 allele showed poorer coginitive decline
• The researchers agree with the above statement
  based on the data in this study that points
  towards accelerated cognitive impairment in
  chronic stress situations
• Chronic stress compounded with amyloid-beta
  causes an enhanced LTD shown in the SSAB group
• Total CaMKII was increased in all groups whereas
  p-CaMKII was only increased in the stress groups
  and the normal brain both work to reverse LTD but
  in the SAB and SSAB group it potentiated LTD

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Discussion

• As the great majority of the cases of AD are of
  the late onset sporadic type, it is presumed that
  it may be triggered or accelerated by non-genetic
  risk factors, such as chronic stress
• Exposure to stress lead to an increase in LTD but
  was enhanced with the addition of amyloid-beta
• The exact mechanism of acceleration is still to
  be explained but there are currently 2 ideas
• Excessive secretion or prolonged exposure to
  glucocorticoids increases neuronal vulnerability
  to age-related disease
• Alter the processing and production of various
  AD-related proteins resulting in negative
  alterations in cognition and synaptic plasticity

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Author's Conclusions   
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Resources

1. "Analysis of Variance." Wikipedia, the Free
   Encyclopedia. Web. 08 Feb. 2012.
   lthttp//en.wikipedia.org/wiki/ANOVAgt. 
2. Alzheimer, Alois. "Alzheimer's Disease." Wikipedia
   , the Free Encyclopedia. Web. 08 Feb. 2012.
   lthttp//en.wikipedia.org/wiki/Alzheimergt.