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Life and Death for Neurons We lose 20-80% of our neurons during development

Life and Death for Neurons We lose 20-80% of our neurons during development Most neurons die right around the time that axons are invading the target. 1. Why do neurons live or die?. Viktor Hamburger, 1920s.

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Life and Death for Neurons We lose 20-80% of our neurons during development

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  1. Life and Death for Neurons We lose 20-80% of our neurons during development Most neurons die right around the time that axons are invading the target 1

  2. Why do neurons live or die? Viktor Hamburger, 1920s Expt: Look at the effect of removing or adding a limb on motor neuron survival in the spinal cord of the Xenopus frog Result: Adding a limb increases neuronal survival, while removing one reduces it. Model: The target is making something in limited quantities that promotes cell survival. 2

  3. Nobel 1986: Rita Levi-Montalcini and Stanley Cohen Identification of nerve growth factor Experiment : Transplant mouse sarcoma tumors near limb bud. Result: More neurons in the dorsal root ganglia survive. What makes neurons survive? They isolated the factor and called it nerve growth factor. 3

  4. How Nerve Growth Factor was isolated • Develop an assay: cultured Dorsal Root Ganglia in a dish • Added tumor extract, • neuron outgrowth is dramatically enhanced 2. Identify a rich source of NGF: purified NGF from snake venom 4

  5. Does NGF really promote cell survival? They DIE DRG + Sympathetic Neurons + NGF They SURVIVE venom + NGF Ab They DIE • NGF is the trophic factor that promotes cell survival • NGF is made by the target neuron 5

  6. The Neurotrophic Hypothesis • The target cells release a factor that promotes cell survival • This factor is found in limiting quantities • Cells compete to get enough factor to survive 6

  7. There are many different trophic factors • Neurotrophins • NGF TrkA Receptor • BDNF  TrkB Receptor • NT3 TrkC Receptor • NT4/5 TrkB Receptor • TGF-Beta Family • Interleukin – 6 related cytokins • FGFs • SHH They all also bind the p75 receptor 7

  8. Neurotrophins act through receptor tyrosine kinase signaling cascades 8

  9. Signal transduction for Neurotrophins 9

  10. Other actions of neurotrophins • Neuronal survival • Nerve growth • Nerve sprouting • Differentiation • Modulation of synaptic transmission • Electrical properties 10

  11. The Neurotrophic Hypothesis: neurons compete for limiting amounts of a neurotrophin some neurons survive, other neurons die 11

  12. How do neurons die? • Two kinds of cell death • Necrosis • Death by accident, ie trauma to tissue • Apoptosis • Death by design • Controlled cellular self-destruction 12

  13. The process of Apoptosis Normal Cell Cell shrinks away from neighbours Plasma membrane blebbing Cytoplasmic and nuclear condensation Chromatin condenses Nuclear and cellular fragmentation Apoptotic Bodies Phagocytosis 13

  14. Morphological Differences between Apoptosis and Necrosis Apoptosis Necrosis vs Nuclei Dense Condensation of Chromatin Irregular Chromatin Clumping Cytoplasmic Organelles Morphologically Intact Disrupted Cell Membrane Apoptotic Bodies, Blebbing Blebbing and Loss of integrity Cell Volume Cells shrink Cells swell DNA Cleaved into fragments No damage 14

  15. A A A A Morphological Appearance of Apoptosis Nucleus is fragmented, blebby 15

  16. Morphological appearance of apoptosis DNA is fragmented into 180 bp pieces 16

  17. What molecules cause apoptosis? C. elegans cell death Program Complete lineage description of all 1090 cells - 113 cells die during embryogenesis - 18 cells die later in life Mutagen Increased survival Isolate gene Decreased survival Mutated GenePhenotype ced3 blocked all 131 embryonic cell deaths ced4 blocked all 131 embryonic cell deaths ced9(g.o.f) absence of cell death, embryonic lethality 17

  18. The apoptotic pathway is conserved in worms and humans 18

  19. Most of the molecules involved in apoptosis are novel CED3 caspase is a cysteine protease….cleaves proteins CED4 adaptor activates CED3 CED9 inhibits CED4 EGL-1 inhibits CED9 19

  20. Do trophic factors inhibit apoptosis? 20

  21. Neurotrophins increase Bcl-2 expression to inhibit apoptosis 21

  22. P75 on its own can activate the apoptotic pathway 22

  23. Why is apoptosis used during development? • sculpting structures • 2. deleting unneeded structures • 3. controlling cell numbers • 4. eliminating non-functional or • harmful cells 23

  24. Inhibition of Apoptosis Cancer Follicular lymphomas carinomas with p53 mutations hormone dependent tumours: breast cancer, prostate cancer, ovarian cancer Autoimmune Disorders Systemic lupus erythematosus Immune-mediated glomerulonephritus Viral Infections Herpesvirus, poxvirus, adenovirus Diseases Associated with Deregulated Apoptosis Increased Apoptosis AIDS Neurodegernative disorders Alzeheimer’s disease, Parkinson’s disease, Amyotrophic lateral sclerosis Retinitis pigmentosa Myelodysplastic syndromes Aplastic anaemia Ischaemic Injury Myocardial infarction, Stroke, Reperfusion injury Toxin-Induced liver disease Alcohol 24

  25. Why does a neuron live or die? • Neurotrophins promote cell survival • by inhibiting apoptosis • Apoptosis allows cell death to occur • without harming nearby cells 25

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