Regeneration in the CNS and PNS Flashcards
LO
- Analyse the strategies to promote axonal regeneration in the PNS and CNS
- Compare and contrast the factors responsible for regeneration failure in the CNS
- Evaluate the potential as well as the limitations of regenerative strategies in the CNS
- Consider the efficacy and use of combinatorial therapies to restore CNS function
- Read Bradke et al. paper
What are the current treatments for spinal cord injury?
Spinal decompression
Neuroprotection (Steroid treatment, hypothermia. Hypothemia is a rare, one off treatment, used for athletes who have an injured spinal cord and this is severe cooling of the tissues which must occur within minutes/ hours)
Rehabilitation: rehabilitative training is the only certified treatment option for human patients with SCI
Assistive devices: wheelchair, walker etc.
Whats Spinal decompression and what is surgically done?
Spinal decompression
- after trauma, damage to the spinal cord causes swelling
- Surgical decompression of the cord reduces this enlargement
What hypothesis about peripheral nerve regeneration did Ramn Y Cajal come up with in 1911?
Devised hypothesis around PNS regeneration
Sliced nerve which has regrown, peripheral nerve (in drawing)
Response to injury as it did regrow
What did Cajal say about central nerve regeneration?
“Once development was ended, the founts of growth and regeneration of axons dried up irrevocably. In adult centres the nerve paths are fixed, ended, immutable. Everything may die, nothing may be regenerated. It is for science to change, if possible, this harsh decree.” (Ramon y Cajal, 1913)
What he observed: Some neurons enlarged in size (darker colour), some fibres turned around on self and avoided lesion
In a mature CNS: things are static
What organism shows successful CNA regeneration, tell me about this?
The eel-like lamprey can fully regenerate its spinal cord after transection.
Within 3 months, an injured lamprey can swim, burrow, and flip around, as normal.
Repair and regeneration occur after retransection!- nothing to do with maturation, it will continue to regrow. Better after first transection case but will continue to repair after further transections
Compare regeneration in the PNS vs CNS
Axon regeneration fails in the CNS because of the inhibitory environment, and because of the intrinsic lack of regenerative ability of CNS axons.
PNS axons regenerate because they have an intrinsically high regenerative ability, and because of the permissive environment (no myelin debris, no glial scar, Schwann cells act like astrocytes and macrophages for e.g.)
But how do you intrinsically repair an axon?
And how do you extrinsically repair an axon?
Compare PNS regeneration with cut versus crush injury…
Peripheral nerve injury and regeneration visualised in vivo in mice expressing YFP (yellow fluorescent protein) in sensory axons
Types of injury inflicted to test: Cut with scissors and crush with tweezers or forceps and molecularly break the connections but structurally still intact
Peripheral nerve repair: Crush lesions regenerate better than cut lesions due to intact extracellular matrix (ECM). Acts as guidance channel for regrowth. The cut pathway has been destroyed so not as easy to repair than the crush injury
Sufficient?
With Wallerian degeneration in the PNS, what conditions are required for successful regrowth?
What did a study by Feneley et al., observe in 1991
For regeneration to occur, Schwann cells must be present and form bands of Büngner before axons growth can occur.
The lesion gap must also be vascularised, and fibroblasts must form connective tissue.
Grafts of extracellular matrix tubes into a cut nerve are insufficient to promote regeneration. Regenerating axons are only found where there is a Schwann cell that has migrated into the graft. (Feneley et al. 1991)
Tell me about recovery after PNS injury (crush injury)
Regeneration rates vary but are usually around ~1-1.5mm/day in successful cases.
With PNS regeneration, there is a huge importance in Schwann cells and timing
Tell me about this and how it may not be perfect but its successful
What are some potential effects of this?
Schwann cells in denervated peripheral nerve only remain permissive for 2-3 months.
This can be a problem in human peripheral nerve repair as axons regenerate at around ~1mm per day. (It could take over a year for axons to regenerate to the hand from a shoulder/upper arm nerve injury.)
Result? Proximal structures (closer to brain and spinal cord) are well innervated, distal structures (away from brain and spinal cord) are poorly innervated.
In addition, muscle endplates lose their ability to become re-innervated after around a year. Muscles can become severely atrophied in the absence of innervation.
Can we implement PNS regenerative ability in the CNS?
What experiment was done with the PNS environment to try and get CNS repair, what was the outcome?
Following CNS injury, a PNS nerve graft transplanted to create a bridge for regrowing axons. Places spinal cord graft above and below the region. Result: axons grew into the graft but not beyond (back into CNS).
This is a rat model
What does (pre) conditioning lesions of the peripheral induce?
(Pre)conditioning lesions of the peripheral induce a robust regenerative response in the CNS
Crushing the peripheral nerve enhances CNS regeneration within the spinal cord of dorsal column axons (these are sensory fibres)
Not a clinical option but useful for understanding
The effect of (pre)conditioning lesions of the periphery can be mimiced with what?
What was seen?
Effect can be mimicked with injections of cAMP into the DRG
Took growth promoting molecules i.e., GAP and put into DRG
Better growth into spinal cord to overcome injury by mimicking what happens at level of cell body
Sensory fibres going though dorsal route
What are the intrinsic mechanisms to allow repair whilst also keeping the neuron viable?
Vascular supply to keep neurons alive
Diaschisis (von Monakow 1914): loss of function in a structurally intact brain region due to loss of input from an anatomically connected area that is injured. Neural/spinal shock can be due to diaschisis.
Neuronal Plasticity: can intact, nearby neurons take over the function of damaged neurons? (Developmental, synaptic)- does occur in children <5 due to plasticity as they’re still developing
Redundancy, vicariation (substitution) (Munk 1881): homologous regions from the other side or a different region assumes the function
Regeneration, replacement (certain criteria need to be met for this to occur)
What is the criteria for successful regeneration and repair in the CNS?
Neuron cell survival
Axon elongation
Axon guidance to target
Appropriate target innervation and synapse formation
Activation of target in functionally meaningful way (functional repair).
A study by Bradke et al., outlines what successful steps in growth cone formation?
Study on regenerative growth in sea slug
Outlines successful steps for growth cone formation
Axotomy= cut of axon, membrane of axon is destroyed
Influx of Ca2+ and membrane depolarisation when this happens
Leads to activation of VG-Ca2+ channels
Good thing in sea slug as Ca2+ activates ECM proteins which digest cortex which is otherwise destroyed
Actin and microtubules are depolymerised, and membrane begins to collapse
Destruction before reconstruction
Then sealing patch formed
Repolymerisation
Growth cones form and growth occurs
A study by Kamber et al., outlines what about axotomy which supported Bradke findings?
Normal calcium= fibre outgrows
Ca2+ is required for regeneration and for growth cone
With growth cone formation, where does the material needed to make a new growth cone come from?
- Recycling of axonal molecules (actin, tubulin)
- Transport vesicles on their way to the axon terminals- get proteins to growth cone
- Local translation of mRNAs- doesn’t occur as much in CNS
- Taken in from the environment
What does Axotomy lead to?
Axotomy leads to upregulation of new proteins in cell bodies, which are needed for axon growth. Growth cone regeneration may happen too fast for these molecules to arrive.
With regeneration and repair, what are some myelin inhibitors?
Tell me about each
Nogo-A, MAG (myelin-associated glycoprotein), and OMgp (oligodendrocyte myelin protein) are expressed on oligodendrocytes, where they inhibit axon regeneration.
MAG – localised to compact, mature myelin; stabilises neuronal networks; growth permissive to embryonic neurons; is released upon damage into lesion
Nogo-A acts through a receptor complex involving p75 and NgR, affecting axon growth via calcium and RhoA signalling.
OMgp – GPI-anchored protein; also expressed by neurons; mediates cell-cell interactions at nodes of Ranvier
What was seen in goldfish when experimenting with future treatments for combatting myeline debris?
What does it suggest?
Future treatments: combatting myeline debris
Goldfish neurons successfully grow over goldfish ‘oligodendrocytes’
Their oligodendrocytes don’t have inhibitory molecules which allows their neurons to grow over them
In CNS prefer to grow on artificial substrate but in PNS myeline and CNS myeline from fish it’s not effected so it’s the mammalian CNS doing something that is stopping regeneration
Future treatments: combatting myelin debris (rats and goldfish)
What do Anti-Nogo-A antibodies increase?
Axon regeneration in rat spinal cord injury
What else has been discovered about anti-NOGO-A antibodies?
What is the current future outlook against myelin-derivative inhibition?
Discrepancy with NogoA.
Several knockouts of NogoA have produced very different amounts of axon regeneration after injury (study by Lee et al., Neuron, 2010)
- One knockout regenerate vigorously
- One regenerates a bit
- One doesn’t regenerate at all.
A triple knockout of the three myelin inhibitory molecules
(NogoA, MAG, OMgp) shows no regeneration after injury
