cue integration Flashcards
What is meant by integration in this context?
‘where am i and does it matter’’
What is the level of intracellular cyclic nucleotides important for?
indicating the polarity/direction of growth i.e. attract/repel
e.g. 1980- showed cyclic AMP can turn a growth cone in frogs in vitro
Higher cAMP is attractive for a growth cone
How can these cyclic nucleotides be manipulated?
- manipulation can affect the response to a wide range of guidance cues
- netrin is attractive, if you inhibit cAMP (block PKA) you turn netrin in to a negative response
What ratio of two cyclic nucleotides is the most important for determining how responses are received?
Levels of cyclic AMP and cGMP are the most important- the RATIO OF THESE TWO MOLECULES IS THE MOST IMPORTANT FOR DETERMINING
What can affect levels of cAMP and cGMP?
- Anything that influences cAMP and cGAMP can also change the way the growth cone responds
- the set of the receptors
- Modulators like nitric oxide, laminin and glutamate
How is integrin and laminin important in regulating cAMP levels?
- Integrin binding to laminin reduces intracellular cAMP levels. Laminin is an extracellular matrix protein, and its receptors are part of the integrin family.
- higher cAMP levels are attractive for growth cones, while lower levels can result in repulsion or altered guidance
give an example of how integrin and laminin can change the environmental cue effect
- retinal ganglion cells on their way to the tectum are attracted to netrin expressed by cells in optic nerve
- after this, contact with laminin in the optic nerve reverses the response to netrin and directs RG axons away
- also affects PKA downstream to affect response
What has been found to be involved in the failure of the adult CNS to regenerate?
Myelin and NOGO lower cAMP levels by promoting rhoA
In more detail, why cant the CNS regenerate compared to the PNS?
PNS = injury to peripheral nerve (e.g.) –> macrophages remove debris –> expression of growth related genes –> prolif of shwann cells –> axon regneration
CNS = injury –> myelin debris clearance –> inhibitory factors –> astrocytes and glia come in to create a glial scar –> physical barrier to regeneration
In what organisms can the CNS regenerate?
- embryonic CNS in mammals
- fish and other verts can regenerate most of their limbs
What experiment showed that the environment determines the CNS axons ability to regenerate?
Crushed rodent axon, presented peripheral nerve as an alternative to where they can grow (i.e. a promoting environment) do the retinal ganglion cells regrow?
Found it worked - shows environment is important
Why in an experiment was it shown that cutting the peripheral nerve followed by CNS injury induces regeneration?
- Cutting the peripheral nerve leads to elevated cAMP in the cell body to the dorsal root ganglion
- If you inject cAMP around the DRG to elevate the levels, it has the same effect as cutting the peripheral nerve allowing regrowth
- Its not as strong though- it isn’t enough for full regrowth, cutting the PNS nerve before allows a lot more changes
How do inhibitory molecules like NOGO work?
i.e MAG/oMgp/NOGO
- Activate RhoA leading to growth cone collapse
Therapeutic opporunity:
- Elevated cAMP levels activate PKA which in turn phosphorylates RhoA leading to its inactivation.
Detail the experiment which showed that Rho-kinase inhibition promotes Corticospinal tract regrowth
- corticospinal tract projects from the cortex
- You can do a lesion of this tract testing if the corticospinal neurons can regenerate
- found that C3 transferase (RhoA inhibitor) effects regrowth in cultures but not in vivo
- ROCK inhibitor administered at the same as the lesion promotes regrowth
What was found with iburoprofen?
- Via inhibiting rhoA at spinal injury site
- Found mildly enhanced recovery of motor function
- After a combination of different behavioural tests- the BBB score was positive
Why does iburoprofen work but not naproxen?
Iburoprofen activates a specific TF which upregulates a phosphatase that inhibits a RhoA
What did Park et al, 2008 reason?
- Pathways involved in regulating cell growth may also regulate the ability of axons to grow
- Tested KO mice for major growth control genes such as retinoblastoma, P53, Smad4 , PTEN etc. to regrow optic nerve after injury
What did Park et al, 2008 find?
- Only PTEN significantly enhanced neuronal survival, allowing RGC axon regrowth in adult mice
- KOs were done by virus induced cre-recombinase
How is PTEN relevant in the mTOR pathway?
- PTEN negatively regulates the mTOR pathway
- mTOR pathway is inhibited as we become more developed by PTEN
- As mTOR controls p-S6, it means levels of p-S6 fall which usually promote protein synthesis
- Removing PTEN removes the ‘block’ on this pathway, allowing axonal growth + regeneration
How does PTEN deletion enhance spinal axon regrowth?
- Used same technique as before and found axon regrowth with synapses
- NO functional recovery though!
Why was there no functional recovery under PTEN inhibition?
Anderson et al 2018:
There are three requirements for functional recovery:
1. Neuron intrinsic growth capacity
2. growth-supportive substrate
3. chemoattraction
**studies including growth supporting substrates such as EGF and including GDNF as a chemoattractant found functional recovery but no behavioural recovery
Name 3 new research ideas regarding axon regeneration and strategies being tested to encourage it?
- RhoA pathway inhibitors were implemented but failed in humans
- Blocking glial scar formation with anti-inflammatories
- Neurotrophins administered to try and turn on the regrowth program
