Transcript 03/07 PPT
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 Expt: Look at the effect of removing or adding a limb on 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 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 How Nerve Growth Factor was isolated 1. 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 Does NGF really promote cell survival? DRG + Sympathetic Neurons They DIE + 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 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 There are many different trophic factors •Neurotrophins •NGF TrkA Receptor •BDNF TrkB Receptor •NT3 TrkC Receptor They all also bind the p75 receptor •NT4/5 TrkB Receptor •TGF-Beta Family •Interleukin – 6 related cytokins •FGFs •SHH 7 Neurotrophins act through receptor tyrosine kinase signaling cascades 8 Signal transduction for Neurotrophins 9 Other actions of neurotrophins • Neuronal survival • Nerve growth • Nerve sprouting • Differentiation • Modulation of synaptic transmission • Electrical properties 10 The Neurotrophic Hypothesis: neurons compete for limiting amounts of a neurotrophin some neurons survive, other neurons die A Neurons approach target Target source of neurotrophic factor Degenerating neuron Limited supply of neurotrophic factor B 11 How do neurons die? Two kinds of cell death 1) Necrosis Death by accident, ie trauma to tissue 2) Programmed Cell Death (Apoptosis) Death by design Controlled cellular self-destruction 12 The process of Programmed Cell Death (PCD) Normal Cell Cell shrinks away from neighbours Plasma membrane blebbing Cytoplasmic and nuclear condensation Chromatin condenses Nuclear and cellular fragmentation Apoptotic Bodies Phagocytosis 13 Morphological Differences between Programmed Cell Death and Necrosis PCD Nuclei vs Chromatin condensation, fragmentation Necrosis Irregular chromatin clumping Cytoplasmic Organelles Membranes intact Disrupted DNA Cleaved into fragments No damage 14 Morphological Appearance of PCD normal A A A A PCD Nucleus is fragmented, blebby 15 Morphological appearance of PCD DNA is fragmented into 180 bp pieces normal PCD 16 What molecules cause Programmed Cell Death? C. elegans cell death Program Complete lineage description of all 1090 cells - 131 cells die during development Mutagen Increased survival Decreased survival Mutated Gene ced3 ced4 ced9(g.o.f) Isolate gene Phenotype blocked all 131 embryonic cell deaths blocked all 131 embryonic cell deaths absence of cell death, embryonic lethality 17 The apoptotic pathway is conserved in worms and humans 18 Most of the molecules involved in PCD are novel CED3 caspase is a cysteine protease….cleaves proteins CED4 adaptor activates CED3 CED9 inhibits CED4 EGL-1 inhibits CED9 19 Do trophic factors inhibit Programmed Cell Death? A Neurons approach target Target source of neurotrophic factor Degenerating neuron Limited supply of neurotrophic factor B 20 Neurotrophins increase Bcl-2 expression to inhibit PCD 21 Overexpression (OE) of Bcl-2 gives bigger brains bcl-2 OE normal Normal 12% volume increase in Bcl-2 OE brains Bcl-2 OE 22 Why is PCD used during development? 1. sculpting structures 2. deleting unneeded structures 3. controlling cell numbers 4. eliminating non-functional or harmful cells 23 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 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 24 Why does a neuron live or die? 1. Neurotrophins promote cell survival by inhibiting Programmed Cell Death 2. Programmed Cell Death allows death to occur without harming nearby cells 25