The flashcards below were created by user
on FreezingBlue Flashcards.
Up to ___% of neurons due during development
When do most neurons die?
Around the time that their axons are invading their targets
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
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
What is apoptosis?
- Programmed cell death
- Motor neuron death
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
What is NGF?
Nerve growth factor
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)
___ was the first member of the NEUROTROPHIN family
What does neurotrophin family mean?
Trophic factors that promote neuronal survival/prevent cell death
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
DRG + sympathetic neurons alone
DRG + sympathetic neurons in culture + NGF
DRG + sympathetic neurons in culture + NGF + Ab vs NGF
DRG + sympathetic neurons in culture + NGF + Ab vs NGF + RNA or protein synthesis inhibitors
- Neurons SURVIVE
- NOTE: cell death dependent on protein synthesis
What are the different kinds of trophic factors?
- TGF-Beta family
- Interleukin-6 related cytokines (LIF, CNTF)
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
Different kinds of neurons depend on different types of neurotrophins for their survival
Cutaneous sensory afferent neuron needs what neurotrophic factor?
NGF on skin
Muscle sensory afferent neuron needs what neurotrophic factor?
NT3 on muscle
Motor neuron needs what neurotrophic factor?
GDNF, FGF, HGF on muscle spindle
Sympathetic ganglion neuron needs what neurotrophic factor?
NT3, GDNF, NGF on viscera
Motor neurons in culture alone
Motor neurons in culture + NGF
Motor neurons in culture + BDNF + NT3
What are the different ways for cells to die?
Apoptotic vs necrotic cell death
What is the difference between apoptotic and necrotic cell death?
- Apoptotic: INTRINSIC factors, suicide
- Necrotic: EXTERNAL factors, injury or murder
What are examples of apoptotic cell death?
- Genetically programmed
- Cell shrinkage
- Chromatin condensation
- Cellular fragmentation
- PHAGOCYTOSIS (eaten from inside out)
- Programmed cell death
What are examples of necrotic cell death?
- External factors
- Inflammatory response
Neurons deprived of ____ die by apoptosis.
Have pathways of apoptosis changed throughout evolution?
No: Pathways and molecules of apoptosis have been conserved through evolution
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)
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 theyre PRO-apoptotic
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
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
Neurotrophins _____ cell survival by _____ apoptosis
PROMOTE . . . INHIBITING
What is axon growth cone?
- A sensory-motor structure that recognizes and responds to guidance cues
- The expanding tip of the axon: its an enlargement at the end of growing axons
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
What is the structure of the growth cone?
It is full of MICROTUBULES at its core and ACTIN at the leading edges
What are the major structures of the axon growth cone?
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
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
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
____ 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
____ is concentrated in filopodia and lamellipodia
____ are concentrated in the central core of the growth cone
Filopodia and lamellipodia contain ___
ACTIN like filaments
The growth cone CORE or central domain contains ____, ____, and ____.
Axons require ____ at the growth cone to EXTEND.
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
What are the different ways axons are guided from point A to point B?
- Contact mediated
- Long range
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)
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
There are several families of different proteins that govern how an axon is guided. What are two?
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
Laminins are heterotrimers of ____
- Alpha, beta, and gamma subunits
- 5alpha, 4beta, 3gamma genes
What are integrins? Where are they expressed? What do they interact with?
- Integrins: Another contact mediated protein that guides axons
- Growth cone: is where theyre expressed/found
- Laminins: is what they interact with
Integrins are heterodimers of ____
- Alpha and beta subunits
- 16alpha and 8beta genes
- Different heterodimers interact with different heterodimers of the laminins
What are cell adhesion molecules responsible for? What are some examples?
- Cell surface attraction: is what theyre responsible for
- Examples: Cadherins and Immunoglobulins
Where are cadherins and immunoglobulins expressed?
Expressed in both the GROWTH CONE and the cells that are mediating the direct contact attraction
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
What is retinotectal map?
Its a topographic map of the visual field from the retina to the tectum (chick, frog, fish) or to the superior colliculus (mammals)
For each region of the visual field, there exists a specific location of the ____ that responds to stimuli in that visual field.
The retinotectal map starts at ___
What is the retinotectal projection?
- There is a point-to-point map from the retina, its 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
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
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
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
What is RGC?
Retinal ganglion cell
Where do RGC axons arborize?
- In their TARGET ZONE (TZ)
- Some take direct path, some overshoot and come back
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
What does Roger Sperry say about rotation of the retina?
RGCs still make connections according to their original (intrinsic) positions
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
What is positional identities?
Positional identities of axons and targets are matched up to establish the point-to-point topographic map
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 :/
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
Describe Friedrich Bonhoeffers 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
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
What is RAGS?
Repulsive Axon Guidance Signal
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 didnt have a preference
What did Bonhoeffers 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)
What is GPI? PI-PLC? RAG?
- GPI: Glycosyl Phosphatidyl Inositol
- PI-PLC: Phospho Inositide Phospho Lipase C
- RAG: Repulsive Axon Guidance
How was Bonhoeffers 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
What are receptor tyrosine kinases and their ligand?
This is the family that RAG is a part of
The original RAG that Bonhoeffer discovered was the ____
Ephrin A5 protein
Ephrins and their Eph receptors work together to ___
To be expressed in a complimentary manner
The more sensitive you are, the ____ amt you need to be exposed to in order to be affected. What does this mean?
- 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)
In the long range signals, there can either be ____ or ____
Chemoatractants or chemorepulsants
Upon exposure to chemorepulsants, the growth cone will ___and ____
Collapse and attempt to grow DOWN the gradient of the chemorepulsant
What are semaphorins?
A family of chemorepulsants
What was one of the first semaphorins discovered? Func?
- Func: it induced the collapse of the growth cone, hence the name
- NOTE: they dont induce growth cone collapse on ALL neurons, so they act more as signals for growth
Recombinant RAGS (ephrin A5) induces growth cone collapse of ____
Temporal axon growth cones
Recombinant RAGS show ____ in stripe assay
Where is EphA3 expressed?
In the retinal ganglion cells