Life and death of neurons Flashcards
What are the two forms of transport in the neuron?
Kinesin (anterograde)
Dyenin (retrograde)
What does kinesin typically transport?
Synaptic components, cytoskeletal components and ion channels
What does dyenin typically transport?
cargo for degradation, neurotrophic signals
What are the 3 main stages of extracellular signalling?
1) Reception
2) Transduction
3) Response (changes in gene expression, enzyme modification, metabolic change ect.)
What are the 3 extracellular cues which promote neuron survival?
Synaptic transmission - correct connections cause electrical signals promoting survival
Extracellular matrix - integrins and lanins
Extracellular growth factoes - neurotrophic factors
What is the neurotrophic factor hypothesis?
Developing neurons compete for limited neurotrophic factor released by TARGET FIELDS
- Proliferation of neuroblasts and differentiation into neurons
- Extension of axons and dendrites guided by neurotrophins
- Developmental neuronal loss
What are the members of the neurotrophic growth factor family?
- BDNF (brain derived neurotrophic factor)
- NT-3
- NT -4/5
Name 4 different families of neuron growth factor
- GDNF
- Neurotrophic cytokines
- Peptide hormones
- Steroid hormones
What receptors do the NGF family bind to?
Tyrosine kinase receptors
Trk - bind with high affinity with specificity for types
P75 - binds with low affinity to all of the family
What types of trk receptor do each of the NGF family bind to?
NGF - TrkA
BDNF and NT4/5 - TrkB
NT-3 - TrkC
What do activation of the PI3 kinase, Ras MAPK and PLC pathways by trk receptors lead to?
PI3 kinase - survival
Ras MAPK/PLC - neurite growth and neuronal differentiation
Describe how receptors travel around the neuron
- When made transported by kinesin to axon terminals
- When binding occurs transported back to cell body for signalling by dyenin
How what effect do neurotrophins NGF and Nt-3 have upon binding to trk A?
NGF - complex budded off by endocytosis, travels towards cell body and signals survival and differentiation
NT-3 - remains at cell surface and signals locally to promote axonal growth
How does p75 sinal for cell death and survival?
- When trk is present it is linked by ARMS molecule which results in cell survival signalling
- If not present leads to cell apoptosis
Give an example if neurons dying due to no longer being needed
Sexual dimorphism - male and female mice born with equal number of estrogen receptors however males loose majority during development
Give an example of neurons dying after having served their pupose
Motor neurons controlling movement upon development of tadpole to frog, change from swimming to 4 legged locomotion
How can neuron death lead to pattern formation and morphogenesis?
During development e.g certain cells must die in order to form the neural tube
What is size matching?
Neuronal adaptation for body size
Name 2 other situationsmwhich may promote neuronal cell death
Error correction - elimination of erroneous connections (e.g optic chiasm)
Damage - killing of infected neurons to protect the rest of the population
Name 5 mechanisms of cell death
- Apoptosis
- Necrosis
- Autophagy
- Park cell death
- Parapoptosis
What are the features of apoptosis?
- Sporadic
- No inflammatory response
- Slow at first followed by cascade
- If there are no engulfing cells (e.g cell culture) scondary necrosis may occur
What can cause apoptosis?
- Neurotrophic factor deprivation
- Excitotoxicity
- Metamorphosis
- Neurodegeneration
- UV exposure
- Reactive oxygen species
Describe the process of apopstosis
- Cells shrink in size
- Chromatin condenses to form pyknotic nucleus where DNA has a laddered appearance
- Membrane and organelles remain relatively intact (no leaks!)
- Phagocytosed
How can neuronal apoptosis be triggered?
- ‘death receptors’ TNF alpha can initiate cell death via JNK pathway
- Intrinsic factors (UV, withdrawal of trophic factors) can activate BAX proteins to form holes in the mitochondria (otherwise inhibited by BCL2 which activates BH3)
How does the destruction of the mitochondria by intrinsic factors lead to apoptosis?
- Cytochrome C leaks from matric to cytoplasm, binding to APAf1 monomer and causing a conformational change in the rail allowing 8 of the molecules to assemble and form the apoptosome
- Caspase 9 is recruited which cleaves and activates caspase 3
- Activated casp3 mediates downstream signalling cascades that leads to breakdown of cellular structures
What are the features of necrosis?
- Accidental/pathological injury which causes rapid cell death
- Contagious area of cells
- Immune response and inflammation
How can necrosis be triggered?
- Environmental stress
- Extracellular termed necroptosis
Describe the process of necrosis
- Swelling of the cell
- Chromatin condenses into small irregular clumps
- Cell membranes and organelles desintegrate, cell contents spilling out
What types of receptor have been linked to necroptosis?
- TNF (tumor necrosis factor) and Fas (same family as p75)
How is the necrosome formed?
- RIP-1 molecule is poly-ubiquitinated
- CYLD enzyme is activated by receptors and removes poly-ubiquitin tail
- RIP-1 then binds to caspase 8, if in active state downregulates the necrosome by inactivating RIP-1, if inactive RIP-1 molecule is maintained, phosphorylated and able to complex with RIPK3 then making an alloy like filamental coplex which then triggers necroptosis
Define neurodegeneration
The progressive loss of neuronal structure/number, usually affecting a defined subset of neurons
What are the risk factors of neurodegeneration?
- Age
- Genetic (familial and sporadic/epigenetic changes)
- Lifestyle (cardiovascular risk)
- Mild traumatic brain injury
- Metabolic aberrations (link with diabetes)
What are the cellular features of Alzheimers?
- Cell loss
- Amyloid plaques accumulating both intraand extra-cellular
- Accumulation of hyper-phosphorylated tau
- Neuroinflammation
- Cell death
- Mitochondria dysfunction and oxidative stress
Where is cell loss most prominent in Alzheimer’s disease?
The cerebral cortex and hippocampus
Where is cell loss most prominent in Parkinson’s disease?
The substantia nigra (dopaminergic neurons)
What are the cellular features of Parkinson’s disease?
- Accumulation of Lewy bodies filled with alpha-synuclein (protein ‘bags’)
- Neuroinflammation
- Cell death
- Mitochondrial dysfunction and oxidative stress
Why do proteins aggregate in neurodegeneration?
- Misfolded proteins tend to stick together forming oligomers, protofibrils and then larger aggregates
- Argument that this could be a defensive mechanism, however can also block function
What is the function of the healthy tau protein?
- Associated with microtubules and aids in function: promotes neural outgrowth and axonal transport
- Reversible phosphorylated by PKA and MARK at Ser199/202 and Thr205
How does tau become hyperphosphorylated?
- Phosphorylation at Ser262 and Thr231, tau then aggregates to form tangles
- Further phosphorylated at Ser422 mediated by GSK-3B
How is amyloid cleaved in a healthy cell?
- APP cleaved by alpha secretase to form sAPP (possible function of being neuroprotective, role in neurite growth and survival) and gamma-secreatse to form p3 (unknown function)
- AICD left embedded in membrane
How is amyloid cleaved in an unhealthy cell?
- APP cleaved by beta-secretase and again by gamma-secretase forming beta-amyloid which then aggregates
Which systems usually prevent protein from aggregating?
- Targetting by poly-ubiquitin
- Escort proteins target it for lysosome
What is oxidative stress?
When reactive oxygen species (ROS) overwhelm the antioxidant defense leading to damage of nucleoides, proteins and lipids
How do ROS produced by the respiratory chain produce an aggressive cycle?
- May damage proteins in the respiratory chain
- May damage mitochondrial DNA causing more faulty proteins to be made
- May damage lipid membrane causing cytochrome c to leak out which may promote apoptosis/necrosis
Describe the process of microglial activation
- Upon recieving activation signals, withdraw processes and become motile locally looking for things to engulf
- Can also travel further in locomotor phase
- Produce inflammatory signals which can be neurotoxic causing more neurons to enter a similar phase
Which mediators of neuroinflammation have been shown to have an effect on Parkinson’s disease?
TNF-alpha (tumour necrosis factor) binds to TNFR1 leading to cell death pathway, ablation of receptor in rodents has been shown to prevent neuronal loss
Why is neurodegeneration hard to treat?
- Multiple models
- Non-linear chain reaction
What is the cholinergic hypothesis?
- Cholinergic neurons rely on NGF which activates cell survival system by TrkA and p75
- AD patients have damage in basal forebrain (cholinergic) which correlates with extent of loss of function
- Cholinergic replacement strategy could be used to treat AD
How do healthy cholinergic neurons usually function?
- Ach acts on neurons to produce proNGF which matures to mature NGF which is detected by neuron TrkA to stimulate cell survival
- Loop promoting survival between nucleus basilis and cerebral cortex
How do cholinergic neurons function in AD?
- Decrease in Ach means that pro NGF is produced but does not mature
- Neurons die, leading to less Ach being produced
What barriers are there to treating AD patients with NGF?
Large polar molecule that cannot cross the blood-brain barrier
How can NGF be administered to AD patients?
- Intraventricular administration
- Grafting of NGF secreting brain tissue
- Viral vectors
What are the results of using NGF in treatment?
Improved cognitive function however NGF had leeched into cerebreal spinal fluid causing extreme back pin as it activates noicireceptors
What have been current NGF treatment strategies?
- Implantation of NGF secreting fibroblasts
- Aim for physiological dosing
- Seen improvement in neurite outgrowth
What does GDNF stand for?
- Glial cell-line derived neurotrophic factor
- Important for maintenance of midbrain dopaminergic neurons (Parkinson’s)
How has GDNF been used as a potential treatment for Parkinsons?
- GDNF transfected macrophages in animal models has preserved dopaminergic neurons from toxicity
- Initial trials in humans were positive but double-blind trial showed no benefits with patients developing anti-GDNF antibodies
What other delivery methods have been proposed for GDNF?
- Gene therapy (no effect)
- Perfusion pump devices (no effect)
- Encapsulated genetically modified cells (not tried yet)
What is leptin?
- Naturally occuring anti-obesity hormone
- Activates neuronal pathways such as PI3-kinase which promotes neuronal growth
What is a lack of leptin associated with?
- Degenerative changes in the synapse
- Upregulation of AD linked proteins
- Neuronal vunerability
- Increased AD risk
What are the advantages and disadvantages of using leptin as a drug?
Adv.
- Targets AD at multiple levels
- Well tolerated in humans, already used to target obesity
disav. - Big molecule which is hard to deliver (cannot be tablet) and expensive (however chopped down version is cheaper!)
How is methionine metabolism a possible contributor to AD?
- Methionine becomes a methyl donor and forms homocysteine which is tocix and a risk factor for AD
- Initiates oxidative stress, produces more beta-amyloid and phosphorylated tau
How could increasing levels of vitamin D help prevent AD?
- Increases efficiency of recycling of homocysteine
- Does not slow onset of disease but can slow cognitive decline in those in pre-Alzheimers stages
What are the preventative strategies for AD?
- Identifying at-risk populations
- Lifestyle modification
What are the properties of the NGF family of neurotrophins?
- Synthesized as large proteins, processed to 120 alpha helix proteins
- Dimers and soluble with two subunits
- Excreted by many cells
- Bind to trk receptors
