Lecture 33 mcb60

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  1. Lecture 33
  2. Up to ___% of neurons due during development
    80%
  3. When do most neurons die?
    Around the time that their axons are invading their targets
  4. What did viktor hamburger look at and when?
    • Around 1920s
    • Looked at the effect of removing or adding a limb (TARGET) on motor neuron survival in the spinal cord
  5. Removing a limb ___ the number of surviving motor neurons and adding a limb ____ the number of surviving motor neurons. What is normal survival? What does this suggest?
    • Normal survival: 50%
    • Removing: REDUCES (to 10% in experiment)
    • Adding: INCREASES (to 75% in experiment)
    • Suggests: that the target produces some factor in limiting quantities that promotes motor neuron survival
  6. What is apoptosis?
    • Programmed cell death
    • Motor neuron death
  7. What is the neurotrophic hypothesis?
    • That target cells release a factor (present in limiting quantities) that promotes cell survival
    • Neurons reaching the target compete for these factors in order to survive
  8. What is NGF?
    Nerve growth factor
  9. What did Levi-Montalcini and Cohen find?
    • Found that mouse sarcoma tumor release a factor that promotes survival of DRG and sympathetic neurons in culture
    • NGF: they isolated this factor and called it nerve growth factor
    • They found that neurons die in the absence of trophic factor (in this case NGF)
  10. ___ was the first member of the NEUROTROPHIN family
    NGF
  11. What does neurotrophin family mean?
    Trophic factors that promote neuronal survival/prevent cell death
  12. Levi-Montalcini and Cohen found that if you add factor culture or remove/block its function, the following happens
    • Add factor to culture: neurons live
    • Remove or block its function: neurons die
  13. DRG + sympathetic neurons alone
    Neurons DIE
  14. DRG + sympathetic neurons in culture + NGF
    Neuron SURVIVE
  15. DRG + sympathetic neurons in culture + NGF + Ab vs NGF
    Neurons DIE
  16. DRG + sympathetic neurons in culture + NGF + Ab vs NGF + RNA or protein synthesis inhibitors
    • Neurons SURVIVE
    • NOTE: cell death dependent on protein synthesis
  17. What are the different kinds of trophic factors?
    • • Neurotrophins
    • • TGF-Beta family
    • • Interleukin-6 related cytokines (LIF, CNTF)
    • • FGFs
  18. What are some neurotrophins?
    • NGF: TrkA receptor
    • BDNF: TrkB receptor
    • NT3: TrkC receptor
    • NT4/5: TrkB receptor
    • NOTE: these all also bind p75 receptors, and they can bind the other receptors but with lower affinities
  19. Different kinds of neurons depend on different types of neurotrophins for their survival
    YES
  20. Cutaneous sensory afferent neuron needs what neurotrophic factor?
    NGF on skin
  21. Muscle sensory afferent neuron needs what neurotrophic factor?
    NT3 on muscle
  22. Motor neuron needs what neurotrophic factor?
    GDNF, FGF, HGF on muscle spindle
  23. Sympathetic ganglion neuron needs what neurotrophic factor?
    NT3, GDNF, NGF on viscera
  24. Motor neurons in culture alone
    Neurons DIE
  25. Motor neurons in culture + NGF
    Neurons DIE
  26. Motor neurons in culture + BDNF + NT3
    Neurons SURVIVE
  27. What are the different ways for cells to die?
    Apoptotic vs necrotic cell death
  28. What is the difference between apoptotic and necrotic cell death?
    • Apoptotic: INTRINSIC factors, suicide
    • Necrotic: EXTERNAL factors, injury or murder
  29. What are examples of apoptotic cell death?
    • Intrinsic
    • • Genetically programmed
    • • Cell shrinkage
    • • Chromatin condensation
    • • Cellular fragmentation
    • • PHAGOCYTOSIS (eaten from inside out)
    • • Programmed cell death
  30. What are examples of necrotic cell death?
    • External factors
    • • Trauma/injury
    • • Messy
    • • Lytic
    • • Inflammatory response
  31. Neurons deprived of ____ die by apoptosis.
    Neurotrophic factors
  32. Have pathways of apoptosis changed throughout evolution?
    No: Pathways and molecules of apoptosis have been conserved through evolution
  33. What are the molecules involved in apoptosis which immediately lead to cell death?
    • Ced-3: in roundworm C elegans
    • Caspase: in humans
    • NOTE: these are PROTEASES (effector)
  34. What are the molecules involved in apoptosis that affect Ced-3 and Caspase (which lead to cell death). Are these considered pro or anti apoptotic?
    • Ced-4: in roundworm c elegans
    • Apaf-1: in humans
    • NOTE: these ACTIVATE Ced-3 and Caspase (which lead to cell death) so they’re PRO-apoptotic
  35. What are the molecules involved in apoptosis that affect Ced-4 and Apaf-1? Are these considered pro or anti apoptotic?
    • Ced-9: in roundworm c elegans
    • Bcl-2: in humans
    • NOTE: these INHIBIT the ACTIVATORS so they are ANTI –apoptotic
  36. What are the molecules involved that affect Ced-9 and Blc-2? Are these considered pro or anti apoptotic?
    • Egl-1: in roundworms c elegans
    • BAD: in humans
    • NOTE: These INHIBIT the INHIBITORS so they are considered PRO-apoptotic
  37. Neurotrophins _____ cell survival by _____ apoptosis
    PROMOTE . . . INHIBITING
  38. What is axon growth cone?
    • A sensory-motor structure that recognizes and responds to guidance cues
    • The expanding tip of the axon: it’s an enlargement at the end of growing axons
  39. What does the expanding tip of the axon growth cone do? How does it do it?
    • It is the expanding tip that is a sensory-motor structure that can recognize and respond to guidance cues
    • by promoting growth towards or away from those cues or signals
  40. What is the structure of the growth cone?
    It is full of MICROTUBULES at its core and ACTIN at the leading edges
  41. What are the major structures of the axon growth cone?
    • Lamellipodia
    • Filopodia
  42. What is lamellipodia made of and what is the function?
    • Lamellipodium: broader, foot-like or sheet-like projections made of branched actin that represent the ‘’leading edge’’ of the cell
    • Function: help treadmill the axon forward
  43. What is filopodia made of and what is the function?
    • Filopodia: finger-like structures made mostly of actin that extend out of the growth cone and beyond the lamellipodia
    • Function: responsible for sensory ability of the growth cone
  44. What pushes filopodia forward? What does this lead to?
    • Vesicel fusion and Actin polymerization pushes the filopodia forward
    • This will drag the lamellipodia and the microtubules from the center core to advance, creating new regions of the axon
  45. ____ will reduce F-actin assembly and inhibit F-actin retrograde flow. This will promote ___. ____ will do the opposite.
    • ATTRACTIVE cues: will reduce F-actin assembly.
    • This will promote filopodia growth.
    • REPULSIVE cues: will reduce filopodia growth
  46. ____ is concentrated in filopodia and lamellipodia
    ACTIN
  47. ____ are concentrated in the central core of the growth cone
    MICROTUBULES
  48. Filopodia and lamellipodia contain ___
    ACTIN like filaments
  49. The growth cone CORE or central domain contains ____, ____, and ____.
    • Microtubules
    • Mitochondria
    • Vesicles
  50. Axons require ____ at the growth cone to EXTEND.
    ACTIN
  51. What is Cytochalasin B? what is the function?
    • A drug that binds to actin filaments and prevents their polymerization
    • Function: add it locally to the growth cone, quickly inhibit movement
  52. What are the different ways axons are guided from point A to point B?
    • Contact mediated
    • Long range
  53. What is contact mediated guidance mechanism?
    Using DIRECT contact, the axon can adhere to parts of extracellular matrix, cell surfaces, other axons (fasciculation), or be repulsed by contact (contact INHIBITION)
  54. What is long range guidance mechanism? What does it lead to?
    • Axons can use long range signals, like gradients of signaling proteins.
    • This will lead to chemo-ATTRACTION or chemo-REPULSION
  55. There are several families of different proteins that govern how an axon is guided. What are two?
    • 1) Laminins
    • 2) Integrins
  56. What are laminins? Where are they found? What does it account for?
    • Laminins: Major components of basal laminae
    • Extracellular matrix: is where theyre found
    • Accounts for: much of the axon outgrowth promoting ability of the extracellular matrix
  57. Laminins are heterotrimers of ____
    • Alpha, beta, and gamma subunits
    • 5alpha, 4beta, 3gamma genes
  58. What are integrins? Where are they expressed? What do they interact with?
    • Integrins: Another contact mediated protein that guides axons
    • Growth cone: is where they’re expressed/found
    • Laminins: is what they interact with
  59. Integrins are heterodimers of ____
    • Alpha and beta subunits
    • 16alpha and 8beta genes
    • Different heterodimers interact with different heterodimers of the laminins
  60. What are cell adhesion molecules responsible for? What are some examples?
    • Cell surface attraction: is what theyre responsible for
    • Examples: Cadherins and Immunoglobulins
  61. Where are cadherins and immunoglobulins expressed?
    Expressed in both the GROWTH CONE and the cells that are mediating the direct contact attraction
  62. What does homophilic interactions mean and give example.
    Means the Cadherin or Immunoglobulin expressed by the GROWHT CONDE is attached to the same Cadherin or Immunoglobulin in the CELL SURFACE of the attracting cell
  63. What is retinotectal map?
    It’s a topographic map of the visual field from the retina to the tectum (chick, frog, fish) or to the superior colliculus (mammals)
  64. For each region of the visual field, there exists a specific location of the ____ that responds to stimuli in that visual field.
    TECTUM
  65. The retinotectal map starts at ___
    The RETINA
  66. What is the retinotectal projection?
    • There is a point-to-point map from the retina, it’s just INVERTED over the A/P and D/V axes.
    • Both the retinal and Tectum are rightside up, dorsal on top and ventral on bottom, but the image crosses in the middle so that the image is inverted or upside down
  67. For the inversion to happen, what has to be there?
    There has to be signaling coming from the specific regions of the tectum or SC to tell the neurons (projecting from the retina) where to go
  68. A number of discrete steps and decision points along the way
    • 1) to leave the retina at the optic nerve head (ONH)
    • 2) To cross (or not) at the optic chiasm (and where)
    • 3) A/P and D/V positions
  69. Pre-target sorting, Branch formation, Final refinement: what happens during each?
    • Pre-target formation: the axons (dendrites?) just shoot out from retina to tectum
    • Branch formation: branches form and they find where they need to go
    • Final refinement: branches have found their location and everything is nice and neat
  70. What is RGC?
    Retinal ganglion cell
  71. Where do RGC axons arborize?
    • In their TARGET ZONE (TZ)
    • Some take direct path, some overshoot and come back
  72. Who developed the chemoaffinity hypothesis and what does it state?
    • Roger Sperry: da man of 1963
    • Hypothesis: states that there must be a unique molecular address in the tectum or SC
    • Also that each RGC that projects to the tectum of SC has a unique set of receptors for the tags expressed by the tectum/SC
  73. What does Roger Sperry say about rotation of the retina?
    RGCs still make connections according to their original (intrinsic) positions
  74. What are the three tenets of Sperrys Chemoaffinity hypothesis of 1963?
    • 1) Molecular address/tag: Each position in the optic tectum has a unique MOLECULAR ADDRESS or MOLECULAR TAG
    • 2) Identity: Each RGC has a unique set of receptors for these tags (i.e. an identity) resulting in a position-dependent response dependent on differential affinities or differential intracellular responses
    • 3) A/P, D/V: unlikely to be unique molecules for each position (would be insufficient info in the genome to wire up entire brain this way), rather, info is probably encoded in orthogonal gradients
  75. What is positional identities?
    Positional identities of axons and targets are matched up to establish the point-to-point topographic map
  76. Sperry and frog eyes
    • If you were to rotate frog eyes 180 degrees, the projections of retinal will be maintained
    • Cells that were originally ventral projecting dorsal will still project dorsal even when switched to a dorsal position
    • Conclusion: where the cells is MADE determines where it will project to, not the position from where it grows
    • I hope this makes sense to you :/
  77. Who is bonhoeffer and what did he do?
    • Scientist who developed the Stripe Assay to demonstrate sensitivity of temporal retinal axons to a repellent activity in posterior tectal membranes
    • To test the affinity of the different projecting retinal ganglion cells in the retina
  78. Describe Friedrich Bonhoeffer’s STRIPE assay.
    • He wanted to design a unique way to culture developing RGCs
    • • He would make ALTERNATING STRIPES of cell homogenates from either the posterior or anterior portion of the tectum
    • • These stripes would contain the FACTORS expressed in the posterior or anterior parts of the tectum
    • • He would then GROW different parts of the retinal tissue, either TEMPORAL or NASAL
  79. Temporal vs Nasal
    • Temporal: is considered posterior and it will project anteriorly in the tectum
    • Nasal: is considered anterior and it will project posteriorly in the tectum
    • TP?
    • NA!
  80. What is RAGS?
    Repulsive Axon Guidance Signal
  81. What did Bonhoeffer discover?
    • He found that if we exposed axons from posterior/temporal region of the retina, it would preferentially grow in the region of the plate that had anterior portion of the tectum
    • The anterior/nasal region didn’t have a preference
  82. What did Bonhoeffer’s experiment show?
    That there was a repulsive cue expressed by cells in a specific region in the tectum (e.g. posterior) that only affected cells expressed by that region of the retina (e.g. posterior/temporal)
  83. What is GPI? PI-PLC? RAG?
    • GPI: Glycosyl Phosphatidyl Inositol
    • PI-PLC: Phospho Inositide Phospho Lipase C
    • RAG: Repulsive Axon Guidance
  84. How was Bonhoeffer’s activity abolished? What did they find about RAG?
    • By administering PI-PLC which meant that the signal, or protein, they were looking for had a GPI anchor
    • Eph Receptors and Ephrins: The protein they purified, RAG, ended up being part of a larger family of proteins called Eph receptors, and their targets, Ephrins
  85. What are receptor tyrosine kinases and their ligand?
    This is the family that RAG is a part of
  86. The original RAG that Bonhoeffer discovered was the ____
    Ephrin A5 protein
  87. Ephrins and their Eph receptors work together to ___
    To be expressed in a complimentary manner
  88. The more sensitive you are, the ____ amt you need to be exposed to in order to be affected. What does this mean?
    • LESS
    • So if the gradient of the expression of ephrinB goes from HIGH dorsal to LOW ventral, the cells in the dorsal portion will be very sensitive to EphB receptors bc they express so much of the ligand
    • So if those cells then project to the tectum, they will grow only into low gradient (bc they are so sensitive and will be inhibited from growing up the gradient, bc even small amts will inhibit growth)
  89. In the long range signals, there can either be ____ or ____
    Chemoatractants or chemorepulsants
  90. Upon exposure to chemorepulsants, the growth cone will ___and ____
    Collapse and attempt to grow DOWN the gradient of the chemorepulsant
  91. What are semaphorins?
    A family of chemorepulsants
  92. What was one of the first semaphorins discovered? Func?
    • Collapsin-1
    • Func: it induced the collapse of the growth cone, hence the name
    • NOTE: they don’t induce growth cone collapse on ALL neurons, so they act more as signals for growth
  93. Recombinant RAGS (ephrin A5) induces growth cone collapse of ____
    Temporal axon growth cones
  94. Recombinant RAGS show ____ in stripe assay
    Repellent activity
  95. Where is EphA3 expressed?
    In the retinal ganglion cells
Author:
Mursizzle
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Card Set:
Lecture 33 mcb60
Updated:
2015-12-14 15:03:38
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lecture 33
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