Cell death (DONE)

Question 1

Cell types in the CNS

Answer 1

90% glia (can’t transmit AP)
Oligodendrocytes- produce myelin, facilitate transmission
Astrocytes- enable homeostasis, physical barrier/connector
Microglia- immune cells of the brain, phagocytose dead cells and debris
10% neurons
All cell types may be affects- damaging in itself, damage neurons

Question 2

Specific issues in the CNS

Answer 2

In the mature CNS, most neurons are post mitotic (not dividing so can’t be replaced)
Cell death in CNS neurons is more severe than in other tissue
May occur during embryonic development (normal), acute e.g. traumatic event, chronic e.g. disease state
May share common mechanisms of cell death

Question 3

What is important about a neuron?

Answer 3

Long axon
Metabolic requirements- very energy demanding, cell bodies full of mitochondria, constant use of energy to maintain excitable membrane
Trophic requirements- need energy to move proteins around, growth factors to sustain survival

Question 4

Types of injury to neurons

Answer 4

Neuronopathy- death of a neuron, loss of all cytoplasmic processes and the myelin sheath, irreversible
Axonopathy- the toxic effect begins in the axon (associated myelin sheath will also degenerate)
Myelinopathy- the myelinating cell or the myelin sheath is the primary target

Question 5

Axonal transport

Answer 5

Transport intracellular material over considerable distances, energy dependent, disturbance of metabolism/ATP may interfere leading to degeneration of the axon, disturbance of cytoskeleton leads to degeneration of the axon
Degeneration of the axon will occur if it is physically cut

Question 6

Axonal transport structures

Answer 6

Tubulin exists in alpha and beta heterodimers, polymerise to form micro tubules
MAPs bind to tubulin subunits to regulate their stability/ cross linking
May be stabilizing or destabilizing or guide microtubules
Any disruption in tubulin or MAPs may interfere with axon transport, leading to degeneration

Question 7

Axonapathies

Answer 7

PNS axons can regenerate, CNS cannot
Peripherally, if toxin removed, Schwann cells distal to the transection divide and act as a guide for axonal sprouts and synthesise nerve growth factor

Question 8

Axonal transection

Answer 8

Axonal transection (chemical or physical)- accumulation of mitochondira, vesicles, filaments and lysosomes (esp. distal) within axonal swellings on either side of the lesion
Distal portion degenerates, macrophages enter the myelin sheath to take up the axonal debris (more rapid peripherally)
Proximal portion may atrophy as the cell body produces fewer cytoskeletal elements

Question 9

Myelinopathies

Answer 9

Direct to the myelin or to the myelinating cell
Separation of the myelin lamellae leading to intramyelinic oedema
In the early stages this may be reversible
With severe/prolonged exposure to toxin, segmental demyelination follows

Question 10

Myelinopathies- macrophages

Answer 10

Derived from microglia, remove most of the degenerate myelin

Astrocytes, oligodendrocytes and Schwann cells may also take up some components

Question 11

Myelinopathies- remyelination limited in CNS

Answer 11

In the periphery, if the axon is intact it may be complete

The distances between nodes of Ranvier in the remyelinated sections are shorter

Question 12

Processes involved in cell death

Answer 12

Morphological/physiological characteristics
Biochemical events
Implications of the type of cell death

Question 13

Apoptosis

Answer 13

Process of a cell suicide under any condition when carried out by a cascade of executioner proteases; a type of programmed cell death- physiological suicide programme critical for the development and maintenance of healthy tissues

Question 14

Necrosis

Answer 14

Accidental death of a group of cells secondary to traumatic injury

Question 15

Mechanism of necrosis

Answer 15

Passive disintegration of cell
Dramatic and rapid- every compartment
Environmental insult- excessive injury, normal responses overwhelmed
Neurons have marked dysregulation of ion homeostasis
Breakdown/loss of plasma membrane integrity, swelling, vacuolation of cytoplasm, mitochondrial and ER dilation, gene transcription stops
Take in water and lyse
Contents of cell leaks, lysosomal enzymes escape, inflammatory process to neighbouring cells

Question 16

Apoptosis mechanism

Answer 16

Active process
Toxic intracellular material packaged into vesicles- phagocytosed by near cells and metabolised
Damaged cells detach and round up
Cell contents packaged into membrane bound apoptotic bodies, engulfed by neighbouring cells
Reduce any inflammatory processes

Question 17

When do apoptosis and necrosis take place?

Answer 17

Apoptosis: physiological process, development, degenerative diseases
Necrosis: pathological, ischaemia, injury, infection, cancer

Question 18

Apoptosis death signals

Answer 18

Controlled by a range of cell signals
Intrinsic: response to stress, diverse array of non-receptor mediated stimuli, produce intracellular signals that act directly on targets within the cell, withdrawal of growth factors, hormones and cytokines, DNA damage
Extrinsic: involve transmembrane receptor, mediated interactions

Question 19

Major players are the caspase enzymes

Answer 19

Family of proteins > 11 cysteine proteases
Inactive until cleaved
Initiator caspases (regulatory)- 2, 8, 9, 10
Effector caspases (disassembly e.g. activated DNAse, endonucleases)- 3, 6, 7
Cleaved in sequence (initiator caspases activate effector caspases)

Question 20

How are caspases activated? Role of BCL proteins

Answer 20

Bcl-2 (B cell lymphoma 2) family- control integrity and response of mitochondria
Anti and pro apoptotic members:
Anti-apoptotic- Bcl2- stabilises mitochondrial function
Pro-apoptotic- Bad, Bid, Bax and Bak- pore formation in mitochondrial membrane
Other proteins may interact with Bcl2- mitochondrial dysfunction

Question 21

Activation of caspases by BCL

Answer 21

When cell death triggers occur:
Pro-apoptotic BCL members increase in relation to anti-apoptotic members and oligomerise
Opening of the mitochondrial pore, permeabilising the outer membrane
Permits release of pro-death proteins- cytochrome C from mitochondrial inter membrane space

Question 22

Apaf-1 in the activation of caspases

Answer 22

Apaf-1 apoptotic protease activating factor exists normally in cytosol as inactive monomer
Interacts with cytochrome C release from mitochondria and caspase 9- forms the apatosome
Transmits apoptotic signals- activate caspase 3 (effector caspase)

Question 23

Apoptosis growth factor

Answer 23

Growth factors activate PI3K or Akt
These phosphorylate proapoptotic BAD, inhibiting its ability to build up or oligomerise and activate its pro-apoptotic activity
No mitochondrial pore formation

Question 24

What triggers apoptosis or necrosis in CNS?

Answer 24

Triggers of cell death: withdrawal of growth factors, DNA damage, increased calcium, excitotoxicity, oxidative stress, mitochondrial dysfunction
Each may contribute to apoptosis or necrosis depending on severity and duration of the trigger as well as the type and age of tissue involved
Necrosis- sustained duration triggers- severe intensity
Apoptosis- transient duration triggers- less severe intensity

Question 25

Calcium functions

Answer 25

Calcium may originate extracellularly or from intracellular stores
Timing and duration of signalling events important

Question 26

The control of calcium

Answer 26

Plasma membrane channels: voltage operated ca channels, receptor operated ca channels, store operated ca channels, non-selective channels
Intracellular Ca channels- InsP3 channels, ryanodine receptors

Question 27

How does calcium kill cells?

Answer 27

Activate proteases
Mitochondrial dysfunction
Oxidative stress
Excitotoxicity

Question 28

Calcium activated proteases

Answer 28

Calpain- cleaves cytoskeletal proteins including microtubule subunits and MAPs
Phospholipases e.g. PLA2- hydrolyses membrane phospholipids- may disrupt neuronal membrane

Question 29

Mitochondrial dysfunction

Answer 29

Excess calcium in the cytosol- opening of the mitochondrial permeability transition pore
Mitochondria swell and release proteins that can lead to apoptosis
Can also cause mitochondria to release more calcium
Production of ATP may be stopped

Question 30

Oxidative stress

Answer 30

The steady state level of damage caused to cells or tissue by the generation of reactive oxygen species (ROS)
ROS are formed as a product of biological reactions and metabolism
Such as free radicals and peroxides
Normally controlled by the cell, in excess can cause damage to DNA leading to release of cytochrome C, high levels lead to necrosis

Question 31

Membrane associated oxidative stress

Answer 31

Lipid peroxidation by hydroxyl radicals and peroxynitrite
Other receptor activation
Disruption of ion movement across the membrane
Promotes depolarisation and Ca influx through NMDA receptors
Ca concentrations continue to rise as NMDA channels over activate beyond tolerable levels

Question 32

Combatting cell death- therapeutic implications

Answer 32

Caspase inhibitors prevent the apoptotic cascade
Growth factors prevent apoptosis, stimulate survival, outgrowth
Anti-oxidants- reduce oxidative stress
Anti-glutamatergics- reduce excitotoxicity

Question 33

What type of damage/disease can lead to the generation of these triggers in the CNS?

Answer 33

Traumatic brain injury
Change in/damage to the BBB
Toxins- common mechanisms: reduced axonal transport, interference in metabolism- oxidative stress
Ischaemic event
Neurodegenerative diseases

Question 34

Cell death in neurodegenerative diseases

Answer 34

Huntington’s disease: striatum and related cortical structures
Parkinson’s disease: substantia nigra and related cortical structures
Alzheimer’s disease: hippocampus and related cortical and limbic structures
Amyotrophic lateral sclerosis: spinal cord and precentral gyrus

Question 35

Ageing

Answer 35

Major risk factor for neurodegenerative diseases
Causes: increased oxidative stress, protein accumulation, DNA damage, mitochondrial instability
Additional factors contribute to disease onset: lack of trophic support, excitotoxicity, oxidative stress

Question 36

Common features of neurodegenerative diseases

Answer 36

All progressive disease states
All have specific patterns of cell death in the brain with predominantly affected regions
All have silent period before disease onset whilst degeneration is occurring
Protein deposits
Genetic vs environment: most have genetic components, genetic risk factors and environmental causes
No predictive testing for any, except genetic testing

Question 37

Apoptosis in neurodegeneration

Answer 37

Most ND associated with specific proteins
Accumulation in cells
Gene mutations

Question 38

Features of neurodegenerative diseases

Answer 38

All have protein deposits- end stage
Before deposited soluble monomers build up into oligomers
Currently believe that these oligomers are the toxic form not the insoluble protein deposits

Question 39

Can the brain repair itself?

Answer 39

Areas of cell proliferation in the adult brain
New neurons can be formed
But it is not enough