Mitochondrial dysfunction and Alzheimer's disease Flashcards
What does an advanced Alzheimer’s disease brain look like ?
The brain is shrunken- there is substantial loss of both grey and white matter and the ventricles are enlarged
What are pathological indicators of AD?
amyloid beta plaques and neurofibrillary tangles
Who proposed the amyloid hypothesis ?
John Hardy
- proposed that amyloid beta peptides was the key initial event in AD- idea they form and are toxic
What is a potentially suggestion for why amyloid beta plaques form ?
They may actually form because it could possibly be the safest way of storing amyloid beta peptide
What happens to APP when it is processed by beta secretase?
sAPPbeta is released which doesn’t appear to have a function but the remaining protein is degraded by gamma secretase and this causes the release of amyloid beta peptide
amyloid beta comes in different forms (1-40) is the most prevalent while (1-42) is the most toxic
What happens to the amyloid beta peptide?
It can undergo spontaneous aggregation leading to the formation of oligomers which are very damaging
then the oligomers can form amyloid beta plaques which is probably the safest way of storing them
What is oligomerization ?
globular structures= small aggregates of amyloid beta peptide
What is amyloid beta like on its own and what happens when it is within saline solution ?
amyloid beta is pretty non-toxic but once it is put within saline solution at 37 degrees oligomers form= protofibrils which are prefibrilar fragments - thought to be nanotubes= soft tubular structures of amyloid beta peptide
What happened when 1 day old amyloid beta peptide aggregates were applied to model cells ?
Carried out cell viability assay and it showed that amyloid 1-42 causes reductions in cell viability as its concentration is increased
- the reverse (amyloid beta 42-1) didn’t induce toxic effect
amyloid beta was even more toxic in the presence of prion proteins
therefore amyloid beta is non-toxic on its own but when it forms aggregates it is damaging
What are some issues with the amyloid beta hypothesis ?
- there are individuals which are clearly heavily demented and have AD yet when you look at their brains, there is little or no evidence of plaques or neurofibrillary tangles
- there are also individuals which are cognitively normal and dont demonstrate any signs of AD yet they have lots of plaques within their brains
What is the mitochondrial cascade hypothesis ?
suggested for sporadic AD
- this hypothesis suggests AD really is a disease of waging
- as you age the mitochondria become more damaged, there will be more mitochondrial DNA damage, more ROS production as they are not as efficient at mopping up the ROS they produce causing further damage
What is the reset, remove and replace system ?
RESET= ROS will be produced and this causes insoluble amyloid beta to form, reducing ETC activity and leading to the production of more amyloid beta plaques REMOVE= removal, apoptosis, cell loss and synaptic degeneration- in an attempt to try and remove the damaged material REPLACE= end up with tau phosphorylation and this forms tangles called neurofibrillary tangles within cells
What determines the efficiency of our ETS?
We inherit maternal ETS genes and it is this which determines ETS efficiency and the degree of ROS production
- it will define the rate at which mitochondrial DNA alterations occur
- once this reaches a pathological level a critical threshold can be reached and can lead to AD histopathology
What is a cybrid?
it is a eukaryotic cell line produced by the fusion of a whole cell with a cytoplast
What are rho0 cells?
They don’t have functional mitochondria because they have been incubated with ethidium bromide for a long time and this inhibits mitochondrial DNA replication
- therefore you can make these cells uptake specific mitochondrial DNA
- they are kept alive by maintaining them in highly supplemented culture medium
What experiments can be carried out with cybrids?
You can cause rho0 cells to express either mitochondria from subjects with a disease and other rho0 cells to express mitochondria from a control patient and then compare the cells because their nuclear DNA will be the same
can compare the phenotypes of the cells to determine what effects the different mitochondrial DNA has
How do you cause the rho0 cells to take up the mitochondria?
polyethylene glycol is used to merge both the membranes
What are the differences seen in AD brains and AD cybrids compared to controls ?
- low cytochrome oxidase Vmax activity
- increased oxidative stress markers
- increased amyloid beta
- activated stress signalling pathways
- reduced PGC1 alpha mRNA
- reduced HIF1 alpha protein
- activated apoptotic signalling
- NFkbeta activation
- increased COX2 protein
- reduced mTOR protein
- increased mitochondrial fission
- decreased SIRT1
- decreased oxygen consumption
- decreased glucose utilisation- as both OXPHOS and TCA cycle are damaged
What is induced by respiratory inefficiency ?
reductions in NAD/NADH ratio, oxidative stress and energy stress
- reductions in NAD/NADH ratio leads to reduced glycolysis
What bioenergetic effects occur in AD?
- reductions in NAD/NADH ratio
- AMPK and p38 are activated while SIRT1 declines
- increased ROS and AMPkinase activity
The AMPK is activated by energy stress and this can reduce mTOR activity and initiate autophagy
What is the relevance of the changes in bioenergetic factors in AD?
many of the factors altered in AD are involved in metabolic activities such as those seen in diabetes, insulin resistance and obesity therefore this underlies the theory that AD could potentially be classified as a type 3 diabetes
What have Brown uni suggested?
suggested that insulin is produced in the brain and there are insulin receptors in the brain therefore they suggested that the kind of insulin resistance seen in diabetes may also be seen in the brain so they described AD as type 3 diabetes but this is still controversial
What is the current mitochondrial cascade hypothesis ?
1) baseline mitochondrial function is genetically determined
2) age associated mitochondrial decline- rate is genetically and environmentally determined
3) metabolically compensated brain aging which is asymptomatic - can cause amyloid beta production acceleration
4) metabolically uncompensated brain aging which is symptomatic - can cause amyloid beta production deceleration
What are the predictions associated with the mitochondrial cascade hypothesis ?
MCI doesn’t always progress to AD
asymptomatic at first
amyloidogenesis causes increased amyloid beta production which then reduced and returns back to normal levels- during this time there are no cognitive defects
BUT theory suggests that as this process carries on you reach a point in which the mitochondrial damage causes you to tip over the edge causing MCI and then onto severe dementia
What effect did mitoQ have on amyloid beta ?
it prevented the increased amyloid beta induced ROS production and it also prevented amyloid beta induced mitochondrial membrane depolarization in C57BL/6 mouse cortical neurones in cell culture
What is mitoQ?
mitochondrial directed ubiquinone
What did confocal microscopy show about the effects of amyloid beta (22-35)?
the ROS were labelled by CM-H2DCFDA
24 hour treatments of amyloid beta (22-35) treated cells demonstrated much higher levels of ROS staining
What effect did mitoQ and NOS inhibitor INNA have on the level of ROS produced by amyloid beta ?
they suppressed the ROS produced by amyloid beta
How is dTTP different to mitoQ?
dTTP mimics the mitochondrial targeting moiety of mitoQ but doesn’t have any ubiquinone so it didn’t induce any significant effects on amyloid beta treatments
What is FCCP and what does it do?
FCCP= uncoupling agent which blocks ROS at a concentration that eliminates the mitochondrial membrane potential in neurones
What effect did mitoQ, I-NNA and dTTP have on mitochondrial membrane potential?
mitoQ and I-NNA blocked depolarisation of the mitochondrial membrane potential while dTTP didn’t have an effect
What effect did mitoQ have on triple transgenic AD mice?
prevented the onset of cognitive deficits in young females triple transgenic AD mice - determined by the morris water maze - measured latencies to find the hidden platform in opaque water after training
Which mice showed better results in the morris water maze test ?
non-transgenic mice
non-transgenic mice treated with mitoQ
triple transgenic mice treated with mitoQ
these mice faired better than the untreated mice or the triple transgenic mice treated with dTTP
How long were the treated mice exposed to dTTP or mitoQ?
for 2-7 months after birth
What effect did mitoQ have on spatial memory?
it prevented the loss of short and long term spatial memory retention in triple transgenic AD mice
What do morris water maze probe trials assess ?
assess memory retention after 1.5 to 24 hours after training
- the platform is removed and the mouse are allowed to swim for 60seconds, during which the swimming tracks are mapped and the amount of times they cross the location of the platform is assessed
What effect did dTTP have on memory and learning ?
it didn’t significantly affect learning or memory in the triple transgenic AD mice
What did cued acquisition trial demonstrate in the different groups of mice ?
showed analogous results in all mice when the platform was visible
What effect did mitoQ have on synaptic loss and proliferation ?
prevented synaptic loss and proliferation of reactive astrocytes in the brains of young triple transgenic AD mice
- prevented the loss of synaptic protein, synaptophysin and increase the astrocyte marker GFAP in the brains of these mice
What effect did mitoQ have on amyloid beta (1-42)?
it reduced the burden of amyloid beta 1-42 on young triple transgenic AD mice- less cytosolic staining for amyloid beta in the treated mice
What did cybrids indicate about MCI patients?
showed that MCI patients have got mitochondrial defects and they think the mitochondrial defects occur very early on when patients are asymptomatic
- so for a treatment to be effective it would have to be given extremely early on
using the cybrids of MCI what was shown about mitochondrial transport ?
looked at mitochondrial transport and it showed that it was going the wrong way - the anterograde and retrograde transport was severely damaged which is not good because mitochondria are localised to the synaptic terminal however they are not produced there, so they require transport from the soma to the synaptic terminal
When they looked at the structure of the mitochondria in MCI cybrids what was seen ?
noticed there was virtually no sign of an inner mitochondrial membrane or cristae- severely altered morphology
- also severe loss of oxidative activity
What metabolomic differences were seen in APP, PS1 and APP/PS1 mice?
these are familial mutations
- reductions in amino acid metabolism
- changes in energy production
What does APP do to the mitochondria?
it can be transported to the mitochondria where it can interact with TOM and TIM and disturbing the mitochondrial protein import
What effect can amyloid beta have on mitochondria?
it can be imported into mitochondria via TIM and TOM and associate with the inner mitochondrial membrane, disrupting mitochondrial respiration and leading to excessive production of ROS
What enzymes present in the mitochondrial matrix can amyloid beta interact with ?
amyloid beta alcohol dehydrogenase ABAD
- it inhibits the enzyme activity and leads to an up regulation of AD biomarkers endophilin 1 and periredoxin 2
What can amyloid beta interact with at the inner mitochondrial membrane ?
it can interact with cyclophilin D which is important for the formation of mPTP and calcium release from mitochondria
What can amyloid beta disrupt when it is present in the cytosol?
it can disrupt cell signalling, protein degradation and cause ROS production leading to an increase in cytosolic calcium
How can cyclin depedent kinase 5 be activated and what effect can it have ?
it can be activated by calcium and this can inhibit Prdx-2’s antioxidant function
How can mitochondria potentially create amyloid beta ?
they can contain beta secrete and gamma secretase and therefore they may be able to produce amyloid beta within them and this may disrupt the ETS
What effect is thought to occur between amyloid beta and ABAD?
thought that ABAD gets switched off when amyloid beta reaches a certain level causing proteins to be upregulated
What happens when amyloid beta directly interacts with cytofilin D?
this is important for opening the mitochondrial permeability pore leading to a permanent opening which causes the calcium within the mitochondria to flood out into the cytosol
- this subsequently signals to a kinase which inhibits antioxidants - creating a positive feedback loop causing more and more damage leading to cytochrome c release which will then activate apoptosis
What can tau interact with ?
it interacts with complex 1 and 5 and inhibits them which directly affects the ETS
What are the numerous dysfunctions that occur in AD?
1) amyloid beta affects the kreb cycle by causing a decline in NADH
2) tau is a hyperphosphorylation of a tublin which severely affects transport down the axon to the synaptic terminal
3) amyloid beta is inhibitory upon mitofusin 1 and 2, preventing fusion and it also reduces OPA1
4) also affects fission as it stimulating it so therefore it causes mitochondrial fragmentation which is not good
5) damage to calcium signalling
6) reductions in COX activation and increased ROS production which is damaging to the cell and mitochondrial DNA and proteins
7) voltage dependent ion channels are blocked by amyloid beta and tau
What occurs before amyloid beta accumulation ?
see mitochondrial damage
What causes optic neuropathies?
thought that both genetic and environmental factors are involved
they are inherited by maternal mitochondrial DNA mutations
the end point is mitochondrial dysfunction
What does mitochondrial dysfunction cause in optic neuropathies?
leads to damage of the retinal ganglion cells which require lots of energy - any damage is likely to cause degeneration in retina and optic nerve deregeneration causing visual failure
- many are mitochondrial diseases, not dysfunctions
What is apparent in optic atrophy ?
typical fundal appearance in dominant optic atrophy showing bilateral optic disc pallor- more marked in the temporal quadrant
- loss of colouration
dominant optic atrophy affects both eyes and the retinal cells in the pale area are almost non-functional
What can the inheritance of optic atrophy be ?
dominant or recessive - it can be x-linked or purely mitochondrial
- nuclear genes can be involved because they can be involved in protein formation in the mitochondria
What are the mutations like in nuclear genetic optic atrophy ?
mutations involving structural subunits of the mitochondria respiratory chain
- optic atrophy and ataxia in complex 2 deficiency= mutations in SDHA
What is mohr-tranebjaerg syndrome?
also known as deafness dystonia optic neuropathy (ddon)
MUTATIONS IN TIMM8A
What are the mutations in chronic progressive external ophthalomoplegia?
POLG1, POLG2, PEO1, SCL25A4, RRM2B and OPA1
Whata re the mutations in alpers syndrome ?
POLG1 and MPV17
