Regeneration In CNS & PNS Flashcards
Current treatments for spinal cord injury
Spinal decompression
Neuro protection (steroid treatments, hypothermia - rare cases)
Rehabilitation (only certified treatment)
Assistive devices
Spinal decompression
After trauma, damage to spinal cord causes swelling
Surgical decompression of cord reduces the enlargement
Peripheral nerve regeneration
Stumps of growing axons
Central nerve regeneration
Do not regenerate - Die
Successful CNS regeneration - lamprey
Can fully regenerate its spinal cord after transection
Within 3 months- able to swim, burrow and flip around like normal
Repair and regeneration occurs after re transection
PNS vs CNS regeneration
Axon regeneration fails in CNS because of inhibitory environment and lack of regenerative ability of CNS axons
PNS axons regenerate because highly regenerative ability and permissive environment
PNS regeneration: cut vs crush
Cut: not as good as crush - larger task to accomplish
Crush: lesions regenerate better due to intact ECM. Acts as guidance channel for regrowth
Wallerian degeneration in PNS: conditions for successful regrowth
Schwann cells must be present and form bands of bungner
Lesion gap must also be vascularised and fibroblasts must for connective tissues
Grafts of extracellular matrix tubes into a cut nerve are insufficient to promote regeneration
Schwann cells needed
Recovery after PNS injury (crush) regeneration rate
Regeneration rates vary but usually around 1-1.5 mm/day in successful cases
PNS regeneration: importance of schwann cells and timing
Schwann cells in dense gated peripheral nerve only remain permissive for 2-3 months
Problem? Human rate of repair is very slow
Results? Proximal structures well innervated, distal structures poorly Innervate
Muscle end plates lose ability to become reinnervated after ~1yr
Muscles can become severely atrophied in absence of innervation
Using PNS environment within the CNS for repair
CNS injury
PNS nerve graft transplanted to create bridge for regrowing axons
Results - axons grew into the graft but not beyond (back into CNS)
Precondition lesions of the peripheral inside a robust regenerative response in CNS
Crushing the peripheral nerve enhances CNS regeneration within spinal cord of dorsal column axons
Upregulated series of genes (GAP 43)
High level of regeneration in sensory neurones
Have to do peripheral injury before CNS injury so not clinically relevant
Shown to work in optic nerve too but not motor axons
Intrinsic mechanisms to allow repair
Neuron cell survival
Axon elongation
Axon guidance to target
Appropriate target interaction and synapse formation
Activation of target in functionally meaningful way (functional repair)
Vascular supply
Regeneration (long distance?), replacement
Neuronal plasticity - nearby neurons take over the function of damaged neurons
Neuronal plasticity: developing nervous system
High potential for plasticity
Neuronal plasticity: adult
Low plasticity and low regenerative abilities
Is plasticity a more viable option for repair?
Axonal degeneration
Regeneration (we want this but not really happening)
Plasticity (can it take over? Usually more effective )
Development of the nervous system and the critical period
Critical period in nervous system: time during which reduction of neuronal numbers, remodelling of synapses and strengthening of connections occurs
Most influential time - permanent connections
Brain more plastic
Plasticity and critical period - visual system
Left and right eyes take info back to visual cortex
<5 yrs old - one eye covered, other eyes visual field expanded by plasticity
Adult - no change
How and why does the critical period close?
Perineuronal nets (PNNs)
Formed at the end of critical period
Composed of ECM including CSPGs which covers cell soma and proximal d writes of certain classes of neurones
PNNs inhibit plasticity in adult CNS
Partial PNN knockout in mice display increased plasticity following adult CNS damage
Successful steps in growth cone formation in sea snail
damage to axon membrane, entrance of calcium into axon and cell body, increase membrane depolarisation, more calcium due to calcium channels, activates calpanes, digests cortex of axon, spectrin, actin and MTs become depolymerised, vacuole internalisation, membrane begins to collapse at cut end, reseals, ca2+ return to normal, actin and MTs repolymerised, actin filaments assemble to generate force at leading edge of lamellipodium, MYS polymerise and point +ve ends towards plasma membrane, extending cell
Neurite outgrowth and axon growth cones formation
Axotomy allows calicium into axon
In absence of calcium, regeneration fails and static endbulb formed
Growth cone formation
Recycling of axonal molecules (actin,tubulin)
Transport vesicles in their way to axon terminals
Local translation of mRNAs
Take in from environment
Axotomy leads to upreg of new proteins in cell bodies. Growth cone regen may happen too fast for these molecules to arrive
Regeneration & repair - CNS myelin inhibitors
Nogo-A, MAG, OMgp are expressed on oligodendrocytes - inhibit axon regeneration
Affect signalling, block pathways if not functioning properly
Future treatments: combatting myelin debris?
Goldfish neurones successfully grow over goldfish oligodendrocytes
3T3 cells
CNS myelin - no processes
PNS myelin - small amount
SCH neurones
CNS myelin - no regrowth in mammals
Lamanin
CNS myelin from rat - avoid CNS myelin so shows inhibition of mammalian meyelin
Future treatments: combatting myelin debris
Anti-nogo A antibodies increase axon regeneration in rat spinal cord injury
T lesion in rat - knock out fibres in dorsal columns and spinal tract in rats
Anti-nogo A antibodies increase functional recovery and sprouting in non human primates SCI
Currently in phase 2 clinical trials for SCI
Phase one for Multiple Scelorisis
Very expensive and time consuming
Future outlook for myelin derived inhibition
Knockout mice don’t show as favourable repair as antibodies
Nogo A knockouts have produced variable amounts of axon regeneration after injury (regen, and no regen in different experiments)
Triple knockout if nogoA, MAG, OMgp shows no regeneration after injury
Glial scar: contribution of glial cells (normal CNS)
Microglia - Role of immune surveillance, Secrete TGFB1 and CR3
Astrocyte - neuronal support, secrete GFAP, CAMs
Oligodendrocyte precursor cell - unknown, continually cycling pop, secret PDGFaR
Glial scar: contribution of glial cells (injured CNS)
Microglia - antigen recognition, proliferation, hypertrophy, secrete also TNFa, IL-1b, IL-6 etc
Astrocytes - scar formation, hypertrophy, secretion if TNFa, IL1, IL6, tenascin
Oligodendrocyte precursor cell - rapid proliferation, hypertrophy, secretes still PDGFaR
Future treatments: combatting the glial scar with chondroitinase ABC
CABC is an enzyme that digests glycosaminoglycan side chains of CSPGs- more permissive to growth
Add nogoA antibody - makes more permissive further in theory
Following rodent SCI and digestion of glycosaminoglycan side chains with cABC, no CSPG immuno reactivity detected and fibres regrew around lesion site better than untreated (increase in plasticity rather than regeneration?)
Limitations- regrowth following treatment follows to areas where there is digestion of CSPGs
cABC enhances rehabilitation - specific forelimb reaching rehabilitation combined with cABC leads to dramatic recovery of skilled forelimb function, even with chronic injury
Combined treatments shows promise for a repair
Anti nogoA and cABC is more effective than single treatment
Improved forepaw reaching
Counter actions so have to time carefully
Astrocyte scar formation aids CNS axon regeneration
Removing scar observed worse outcome for animals
Glial scar has a protective effect of the rest CNS
Inflammatory response spreads further and don’t have control of lesion site so may get worse
Potential of cell transplants - may provide:
Bridge over/through scar
Permissive substrate
Cell replacement
Growth factors
Demyelination
Potential of cell transplants - commen cell types
Schwann cells
Olfactory ensheathing cells (olfactory neurones can regenerate)
Stem cells (induced pluripotent, embryonic, multi potent progenitors)
Regeneration & repair - PD and transplantation
Dissection of central mesencephalic tissue
Transplant preparation
Grafting procedure
Immunosuppression
Results- good results but not cures and ethical issues
Olfactory ensheathing cells (OECs)
Olfactory neurone regenerate throughout life in mammals
Relatively easy to harvest - wide range of sources and can take it from an adult
May help treat SCI
Regeneration and repair - OECs
Provide tropic support
Can phagocytise debris
Allow cells and axons to integrate through glial scar rich regions
Functional recovery and/or CNS axon regeneration has been reported when OB-derived cells were transplanted
But transplants of cells from fetal OBs or adult mucosa carried out in China and Portugal - not good procedures and outcomes
Clinical studies better
Regeneration and repair - stem cells
Embryonic
Induced pluripotent (iPSC)
Mesenchymal stem cells
Human neural stem cells (NSCs) transplanted into immune deficient mouse after T3 transection injury. Assessed for growth 7-12 weeks post injury = capacity for axon to grow around (bridge like structure)
Functional testing = better than control
Regeneration and repair - biomaterials
Hydrogels that’s mimic ECM
Provide physical or topographical cues for axonal growth
Substrate for cell delivery and survival
Important part of combined therapies (growth factors, cells etc)
Regeneration and repair - electrical stimulation and rehabilitation
Epidural stimulation uses electrical stimulation with propriospinal input derived from muscles, bones and skin that project to lower spinal cord to serve as source of neural control
CPGs - neural circuit that produces rhythmic motor pattern in the absence of descending control, potentially without sensory input
Central pattern generators - mammalian locomotion
For most quadruped mammals, assumed neural control of locomotion based on CPGs with the spinal cord
Specialised neural circuits in caudal spinal cord organise hindlimb locomotor activity
Rostoral spinal cord control forelimb movement
Coordination of both is mediated by propriospinal neurons with long axons which couple the cervical and limber enlargement of the spinal cord
Regeneration and repair - electrical stimulation and rehabilitation
Weighted supported locomotor training and epidural stimulation resulted in walking with assistive devices
Human CPGs?
Still relatively new
Critical issues in regeneration research
Acute vs chronic
Focal vs diffuse
Regeneration vs repair (plasticity?)
Rehabilitation and motivation
Evolutionary barrier to overcome
Small no. of fibres regenerate
Distance of regeneration
Relevance of regeneration
Specificity of connectivity
Adaptive and maladaptive
Efficacy of cell transplantation- preclinical vs clinical
