Injured nervous system Flashcards
Why can the peripheral nervous system (PNS) regenerate after injury/degeneration but the central nervous system (CNS) not?
The peripheral nervous system (PNS) can regenerate after injury due to factors such as supportive Schwann cells, a lack of strong inhibitory factors, and a more permissive environment. In contrast, the central nervous system (CNS) exhibits limited regeneration because of inhibitory factors like myelin-associated inhibitors, scar formation by reactive astrocytes, and the intrinsic lower growth potential of CNS neurons. Additionally, the differential response of support cells (Schwann cells vs. oligodendrocytes) and the proximity of target cells contribute to the contrasting regenerative abilities between the PNS and CNS. Ongoing research aims to understand these differences and develop strategies to enhance CNS regeneration.
Can we prevent degeneration and enhance regeneration?
Promoting nerve regeneration and preventing degeneration involves strategies such as neuroprotective measures (antioxidants, anti-inflammatory agents), stem cell therapy, nerve growth factors, rehabilitation, electrical stimulation, genetic approaches, drug development, and lifestyle modifications. Research in these areas aims to enhance neural function and treat neurodegenerative conditions.
Different glial cells of the CNS and PNS (histology)
Glial cells (glia, neuroglia) support neurons with structure and nutrients, maintain their environment, protection and also participate in neurotransmission/synaptic activity (tripartite synapse).
astrocytes= a type of glial cells
Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, they provide structural support, contribute to blood-brain-barrier, electrical
active (tripartite synapse; gap junctions). Radial glia serve as scaffolds for developing neurons of the cortex as they migrate to their end destinations. Microglia (specific microphages) scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions
the neurons.
Astrocytes:
Function: Provide structural support, regulate nutrient and ion concentrations, and participate in the blood-brain barrier. They are also involved in synaptic regulation and repair processes.
Microglia:
Function: Act as the immune cells of the central nervous system, defending against pathogens and removing dead or damaged cells through phagocytosis. They play a role in inflammation and the response to injury.
Oligodendrocytes (in the central nervous system) / Schwann
Cells (in the peripheral nervous system):
Function: Produce myelin, a fatty substance that wraps around axons to insulate and facilitate faster nerve impulse conduction. Oligodendrocytes myelinate multiple axons, while Schwann cells myelinate a single axon.
Ependymal Cells:
Function: Line the ventricles of the brain and the central canal of the spinal cord, contributing to the production and circulation of cerebrospinal fluid. They also provide a barrier between cerebrospinal fluid and nervous tissue.
Schwann cells, which form the myelin sheath, have
phagocytic activity and promote regrowth of axons.
Satellite cells, smalls cells which provide nutrients and
structural support to neurons in ganglia.
Enteric glial cells in digestive system ganglia support
homeostasis
Nerve damage within PNS and CNS may be generated by
different cause
Neurodegenerative diseases
(Alzheimer disease AD,
Parkinson disease PD- dopaminergic neurons neurons (helps with memory, sexual activity)
Amyotrophic lateral sclerosis ALS)
Stroke
Nutrition Neuropathies
(Diabetic/
vitamin B12 liver shortage)
Viral infection
(HIV/CMV-neuropathy,
Post Herpetic Neuralgia,
COVID-19)
Multiple Sclerosis MS
Autoimmune disease EAE
Trauma
(physical injury caused by an
external force:
cut or crushed
Regeneration of the PNS
What are some nerve guidance cues
- Self-repair of damaged peripheral nerves is good in the skin and if larger nerves are only mild/small parts affected.
- Surgical repair of damaged peripheral
nerves is good
Functional outcome is good, if small gap between nerve ends, help is needed with larger gap (>7cm)
- Nerve guidance cues
- Hydrogels
- Fibrin glue
- re-anastomosis = reuniting (as by surgery or healing)
- autologous nerve graft repair = part of other nerve of
recipient - non-autologous nerve graft repair = non-nerve part of
recipient - allograft = tissue not from recipient
Regeneration of the CNS – spinal cord injury (SCI)
no cure
* 800 people have a SCI each year in the UK
* SCI primarily affects young adults approximately 80% are males
* There is an increasing incidence in the elderly in whom rehabilitation can be more difficult
* 40,000 people live with paralysis in the UK
* Cost to the nation estimated at £1 billion per annum
Spinal cord injury:
A lesion to the spinal cord has debilitating effects, it
disrupts sensory and motor signals passing between brain and body. This results in essentially no feeling or control of function below the injury site.
Other Medical Issues
Bladder/bowel function management
Decubitus/wound infections
Assuring respiration
Treatment for spinal cord injury
Spinal stabilization
High dose steroid therapy
Until recently, treatment for spinal cord
injury has had only 2 main aims:
- Prevent further damage at the time of injury using high dose steroids
- Rehabilitation of remaining function
Wallerian degeneration (WD)
degeneration= the progressive loss of structure and/or function of neurons that culminates in cellular atrophy and death
Wallerian degeneration is an important concept that is useful in mapping the anatomic components of peripheral nerves and spinal cord segments, in recognizing peripheral or central nervous system disorders microscopically, and in understanding and predicting reinnervation by peripheral nerves.
- The active degeneration process of the axon distal to the injury site (cut or crushed) taking place in the CNS and PNS, first description by Augustus Waller in 1858 “The Case of the
Disappearing Axon”. - Neurodegeneration is a ‘Wallerian-like degeneration’ process a highly stereotyped dying back or retrograde degeneration process.
- the distal section of the axon tends to remain
electrically excitable for some time (phase I) - WD usually begins within 24–36 hours of a
lesion (lag phase II) - Anterograde degeneration of distal axon
followed by myelin sheet degradation and
macrophage infiltration (fragmentation phase III) - Regeneration phase IV only for PNS not CN
PNS – Peripheral Nerve Injury (PNI) Wallerian degeneration
In most cases, the lag phase is then followed by a rapid fragmentation phase, in which the axon breaks into many individual pieces, which are then phagocytosed by glia and immune cells. WD likely entails a cell autonomous chain of events that occur within the distal axon itself and, hence, can be considered as a “self-destruction” pathway, akin to apoptosis.
However, WD appears to involve a molecular pathway that is quite distinct from apoptosis. Entry of extracellular calcium and release of intracellular calcium stores, with consequent activation of calcium-dependent proteases
schwann cells can dedifferentiate back so they lose there myelination job.
What are the steps of wallarien degeneration
(A) intact myelinating Schwann cells enwrap an intact axon and fibroblasts are scattered between nerve fibres
(B-E) injured PNS nerve that undergoes normal Wallerian degeneration:
(B) Traumatic injury produces immediate tissue damage at the lesion site a gap may be formed between the proximal and distal nerve stumps, and Galectin-3/MAC-2+ macrophages accumulate at the lesion site within 24 hours after the injury.
(C) Destruction of distal axons is detected during normal Wallerian degeneration 36 hours after the injury.
(D) Recruitment of Galectin-3/MAC-2+ macrophages, myelin disintegration, and Galectin-3/MAC-2 expression by
dedifferentiating Schwann cells begin 48 to 72 hours after injury during normal Wallerian degeneration.
(E) Galectin-3/MAC-2+ macrophages and Schwann cells scavenge degenerated myelin during normal Wallerian degeneration, and Schwann cells proliferate further and form Büngner bands (basement membrane remains
after axons deg. & helps guiding new axons;
German doctor Otto von Büngner around 1900
Degeneration and regeneration in the PNS
In PNS
Permissive lesion environment:
- Schwann cells de-differentiate and produce neurotrophic
factors and secrete extracellular matrix (ECM) molecules for
axonal growth (integrins, interleukin-1, PDGF etc), form
skeleton of parallel-arranged microscopic tubes as basement membrane = basal lamina (Büngner bands), axons follow them = remyelination
- Scavenger cells = macrophages phagocytose peripheral
nerve debris.
- Fast removal of nerve debris
Significant intrinsic neuronal regenerative response:
- Neuronal axon tip forms growth cone, up regulation of
GAP43, CAP23 & transcription factors (c-jun, c-fos, ATF-3)
by injured neuron, regeneration-associated genes (RAGs),
Degeneration and regeneration in the CNS
Non-permissive/distructive lesion environment:
- Oligodendrocytes do not rejuvenate or regenerate, they do not secrete growth factors, inhibit axonal growth with endogenous myelin inhibitory molecules (nogo, MAG, OMgp), disordered tubes, no basement membrane to follow for axons
- Astrocytes secrete growth inhibitors including proteoglycans and form dense scar
- Slow removal of nerve debris
No significant intrinsic neuronal regenerative response:
- Neuronal axon tip forms retraction bulb disorganized
microtubules, which is associated with the accumulation of
mitochondria and anterograde transported vesicles.
Pseudounipolar dorsal root ganglia (DRG) neuron proximal
and distal axon regeneration ≈ CNS and PNS regeneration
PNS axons vs. CNS axons. (A) Proximal/central and distal/peripheral axon of pseudounipolar DRG axon.
If the peripheral branch is injured, it forms growth cone at its tip and regenerates. If the central branch is injured
in SCI, it forms dystrophic endball and cannot regenerate. Reactive astrocytes at the site of SCI produce many
factors including proteoglycans, which make a gradient and may induce dystrophic endball formation. PG,
proteoglycan; HSPG, heparan sulfate proteoglycan. (B) A comparison between CNS and PNS injuries.
Proximal/central dorsal root ganglia (DRG) axons as model
for CNS regeneration
The central collaterals of myelinated DRG cells form the ascending dorsal column projection system.
A myelinated tract, responsible for fine, discriminative, touch. It may be discretely lesioned and regenerating axons may be labelled from the periphery with a tracer.
Proximal/central dorsal root ganglia (DRG) axons as model
for CNS regeneration
Differences in regenerative ability between the central and peripheral DRG neuron axons. (A) The peripheral DRG neuron
axon (orange) is capable of growing long distances beyond the lesion site (green), possibly reinnervating its targets. (B) After injury to the dorsal column tract (left), central DRG neuron axons (orange) are not capable of regenerating beyond the lesion site.
In contrast, if the dorsal root is injured (right) the central axons can slowly grow beyond the lesion site (green) but they are not capable
of regenerating across the dorsal root entry zone. (C) Schematic drawing of the conditioning lesion effect. If an injury to
peripheral DRG neuron axons is performed some days before the central lesion (conditioning lesion), the regeneration ability of the
central axons increases (green axon): dorsal column tract fibers are able to regenerate beyond the lesion site and dorsal root axons
grow faster (injury dorsal root later, Richardson and Verge, 1987; spinal cord later, Neumann and Woolf, 1999; central DRG earlier, Ylera et al., 2009).
What are the two Main Issues
Poor intrinsic regenerative response=
*Stimulate a growth response
1. Provide neurotrophic factors
2. Enhance intrinsic growth response
3. Provide permissive bridging substrates
Inhibitory growth environment=
*Target the inhibitory lesion environment
1. Remove physical barrier (ECM/glial scar)
2. Remove/inhibit the chemical barrier (CNS
3. myelin-associated molecules)
Strategy 1: Stimulate a growth response
Tool - delivery of growth and survival factors (neurotrophic factors)
Growth and survival factors:
Neurotrophins
*Nerve growth factor (NGF)
*Brain-derived neurotrophic factor (BDNF)
*Neurotrophin-3 (NT-3
Glial cell line-derived neurotrophic factors
* GDNF
* Neurturin
* Artemin
* Persephi
Introduce with minipumps, viruses, cells…
Strategy 1: Stimulate a growth response
Neurotrophins
Tropomyosin receptor kinase or tyrosine receptor kinase A-C, homodimerization after ligand binding;
The low affinity nerve growth factor receptor p75 is not a kinase but enhances ligand binding to Trks
and reduces Trks ubiquitination, internalization and degradation.
GDNF family of ligands and the receptor tyrosine kinase Ret
The receptor tyrosine kinase Ret (“rearranged during transfection”) binds with the high affinity coreceptors GFRa1-4 (Glial cell line-derived neurotrophic factor receptor alpha 1 to 4) members of the GDNF (Glial cell line-derived neurotrophic factor) family of ligands.
Extrinsic Neurotrophin NT3 treatment promotes regeneration of dorsal column sensory axons in rat
Cholera toxin subunit B
(CTB) Alexa Fluor (AF)
Considerations:
- Neurotrophic factor effect restricted to cells expressing appropriate receptors
- Regenerating axons stay within trophic environment
Tool: OEC transplants lead to structural + functional recovery in rats
Tool: OEC transplants
They lead to structural + functional recovery in human
Tool: OEC transplants are promising for CNS repair but inconsistant
Olfactory Anatomy. With the dendrites of olfactory receptor
neurons (green) exposed in the nasal cavity for odorant
detection, the somas of neurons are entrenched in the
olfactory epithelium of the olfactory mucosa alongside
sustentacular cells (blue). As the axons of neurons
penetrate through the basal layer where globose (purple)
and horizontal basal cells (pink) are found, they are
fasciculated by olfactory ensheathing cells (OECs; red) from
the lamina propria to the olfactory bulb. Surrounding the
OECs are the olfactory nerve fibroblasts (orange), which are
thought to assist OECs in their neurosupportive
endeavours.
Olfactory Ensheathing Cell (OEC) Culture Variability.
Possible variations in OEC culture compositions. (A) OEC
(red) cultures from the olfactory mucosa or olfactory bulb
with Schwann cell (blue) contamination of various
proportions. (B) OEC cultures from the olfactory mucosa
or olfactory bulb with fibroblast (olfactory nerve fibroblast
or meningeal fibroblast; orange) contamination of various
proportions. (C) OEC cultures from the olfactory bulb with
astrocyte (yellow) contamination of various proportions.
(D) OEC cultures from the olfactory bulb with a mix of
fibroblasts (orange), Schwann cells (blue), and astrocytes
(yellow) of various respective proportions.
Tool: Classification of cells with regenerative potential
Classification of stem cells.
Stem cells can be classified according to their plasticity
in: Totipotent that give rise to all embryonic and
extraembryonic cell lines;
pluripotent that can produce all embryonic cell
types; multipotent that differentiate to a great
number of cell types;
oligopotent that have the ability to differentiate into
only a few cell lineages;
and unipotent that give rise to only one specific cell
type.
Tool: neuronal signalling cascade targets for recovery after injury
Cascade of reactions from a calcium burst and methods of activating regeneration‐ associated genes (RAGs) in damaged neurons.
MAPKKK dlk1= Mitogen‐activated protein
kinase kinase kinase dlk‐1;
pErk = Phosphorylated extracellular signal‐
regulated protein kinases;
HDAC5 = Histone Deacetylase 5;
RAGs = Regeneration associated genes;
PTEN = Phosphatase and tensin homolog;
PI3K = Phosphoinositide 3‐kinases;
AKT = Protein kinase B;
mTORC1 = Mammalian target of rapamycin
complex 1 or mechanistic target of rapamycin
complex 1;
SOCS3 = Suppressor of cytokine signalling 3;
JAK/STAT 3 = Janus kinase/signal transducer
and activator of transcription 3
Tools: Targeted neutralisation of the inhibitory growth environment.
Strategy 2
Targeted neutralisation of the inhibitory growth environment. What are the inhibitory factors?
Myelin-derived
Myelin-associated glycoprotein (MAG)
Oligodendrocyte-myelin glycoprotein (Omgp)
rtn4 (NogoA,B,C
Astrocyte-derived
Glial scar
chondroitin sulphate proteoglycans
(CSPG’s
The physical barrier/ glial scar from astrocytes
Strategy 2
Proteoglycans: Extracellular matrix glycoproteins,
consisting of a protein core with attached sugar
moieties called glycosaminoglycans (GAGs). There
are several groups depending on type of sugar
moiety (chondroitin sulphate, heparin sulphate,
keratin sulphate and dermatin sulphate
Proteoglycans
They are up-regulated by astrocytes following CNS injury
Tools: degradation of proteoglycans with chondroitinase ABC
Treatment with chondroitinase ABC - an enzyme extracted from bacteria Proteus vulgaris, selectively removes a portion of the glycosaminoglycans (GAG) side chains from the proteoglycan
Tools: degradation of proteoglycans with chondroitinase ABC in rats
Chondroitinase ABC promotes anatomical regeneration and
functional recovery in experimental models in vivo
Tools: neutralisation of the chemical inhibitory environment myelin-derived
MAG, OMgp and Nogo activate common
receptors NgR1 = Nogo-66 receptor + GPIlinked Reticulon 4 receptor = RTN4R + p75NTR or Troy, Lingo1 or Amigo3).
Regenerating neurons express receptors for these inhibitory molecules and activate RhoA
Rho protein GDP dissociation inhibitor
Guanine nucleotide exchange factor
GTPase‐activatingprotein
CRMP‐2 = Collapsin response mediator protein 2
Tools: neutralisation of the chemical inhibitory environment myelin-derived
Several neutralising agents to Nogo or the NogoR have been developed
NEP1-40 peptide antagonist, competes with NoGo-66 binding for the NgR and prevents intracellular signal
transduction/RhoA activation within regenerating neurons
Systemic NEP1-40 delivery promotes limited spinal cord regeneration
Target-specific deletion of Nogo-A&-B results in regeneration of corticospinal tract and functional recovery
Result:- significant increase in regenerating Corticospinal
tract (CST) axons. some functional recovery
ONLY demonstrated in rats.
Other authors have found NO effect
Other treatment strategies
Stem cell therapy=Only shown to work in a few patients with
acute SCI and in association with grafts
Artificial bridging grafts= No functional recovery
Bioelectronic implants= Limited functional recovery
does not address issue of regeneration
Regeneration of the CNS with
glial cell line derived neurotrophic factor (GDNF
Examples:
‐ Spinal cord injury: GDNF
‐Ret/integrin signalling in propriospinal neurons
(rat and mouse data)
‐ Parkinson disease: GDNF
‐Ret signalling in dopaminergic neurons (clinical
trials in Bristol, UK
GDNF/Ret beneficial in spinal cord injury (SCI)
- before SCI reactivated growth capacity of mature descending propriospinal neurons with
osteopontin, insulin-like growth factor 1 (IGF-1) and ciliary-derived neurotrophic factor (CDNF)
(AAV-OIC) in mice and rats - after SCI induced growth-supportive substrates with fibroblast growth factor 2 (FGF2) and
epidermal growth factor (EGF) - chemoattracted propriospinal axons with glial-derived neurotrophic factor (GDNF) delivered via
spatially and temporally controlled release from biomaterial depots placed sequentially in
depot 1 (lesion centre at lesion time) and 2 (1mm after lesion centre, 1 week after lesion)
-> increased known axon-supportive substrate molecules 11, laminin, fibronectin and collagen in
lesions + integrin-dependent axon–substrate interaction required, more astrocytes, not less
inhibitory chondroitin sulphate proteoglycans (CSPG)
-> over-ground locomotion of rats did not improve, but better electrophysiological signals across
lesions.
-> adult axon regrowth across such lesions requires 3-4 types of mechanisms/signals:
1) neuron intrinsic growth capacity
2) supportive substrate
3) Chemoattraction
-> for functional recovery in addition:
4) rehabilitation that fosters neuronal integration
GDNF/Ret beneficial in spinal cord injury
Stimulated and chemoattracted propriospinal axons
regrow robustly across anatomically complete SCI
lesions in mice receiving combined delivery of AAV-OIC
plus FGF, EGF and GDNF in two sequentially placed
hydrogel depots. a, Experimental model. D1 and D2,
hydrogel depot 1 and 2, respectively. b, Biontinylated
dextran amine (BDA)-labelled axons in composite tiled
scans of horizontal sections also stained for astrocytes
(anti-GFAP (glial fibrillary acidic protein), left) and cell
nuclei (DAPI). Dotted lines demarcate astrocyte
proximal (PB) and distal (DB) border around the lesion
core (LC). Dashed line demarcates lesion centre. 1D,
one depot; 2D, two depots; GM, grey matter. c, Top,
schematic of axon intercept. Middle, axon intercepts at
specific distances past lesion centres (colour coding
and n as in the graph below). Bottom, areas under axon
intercept curves. Dots show n mice per group. NS, not
significant versus SCI + 1D empty; . d, Surveys (top)
and details (bottom) of BDA-labelled axons. e, Threedimensional detail of BDA labelled axon and growth
cone expressing GDNFR (GFRa) in the lesion core
Substantia nigra dopaminergic neurons die in Parkinson
patients; the midbrain dopaminergic system is conserved
in humans and mic
mesostriatal pathway = motor function; Parkinson disease
dopaminergic neurons in the substantia nigra (SN)
mesocorticolimbic pathway = motivation; drug addiction
dopaminergic neurons in the ventral tegmental area (VTA
Specific nigrostriatal pathway degeneration in aged but not
young Ret deficient mice
DAT-Cre/Ret ko mice at 12-24 months:
20-35% reduction of dopaminergic neurons
40-60% reduction of striatal innervation
10-20% atrophic/loss of postsynaptic cells
100% increase of glial cells in ST
45% increase of microglia in SN
less dopamine release
Dopaminergic neurons of Ret ko mice
are not more sensitive to MPTP
Dopaminergic fibers of Ret ko mice do not resprout
efficiently 3 months after MPTP treatment
ret mediates
Ret mediates the neuroprotective and regenerative effects
of exogenous GDNF in the MPTP model of Parkinson disease
Ret mediates the neuroprotective effects of exogenous GDNF in the MPTP model of Parkinson disease.
Ret mediates the neuroprotective effects of exogenous GDNF in the MPTP model of Parkinson disease.
Ret mediates the neuroregenerative effects of
exogenous GDNF in the MPTP model of Parkinson disease
Ret mediates the neuroprotective and regenerative effects of GDNF in the MPTP model of Parkinson disease
GDNF/Ret activates Ras/MAPK, PI3K/Akt and NF-kB signalling
in dopaminergic neurons for neuroprotection and regeneration.
Beneficial GDNF/Ret signalling in the nervous system
- Ret receptor is expressed on motoneurons and dopaminergic neurons but also in the
kidney (tip cells during kidney development) - GDNF is expressed in the limb mesenchym, striatal parvalbumin-positive GABAergic
interneurons, striatal somatostatinergic or cholinergic interneurons, kidney anlage and
cultured astrocytes, oligodendrocytes, and Schwann cells. - GDNF is upregulated during spinal cord injury and peripheral nerve injury (3h-4 weeks
in sciatic nerve) and is beneficial for spinal cord neurons in combination with VEGF,
FGF, EGF, PDGF, BDNF, NT3, Chondroitinase ABC, Nogo A antibody, Schwann cell
seeded guidance channels, hydrogel, silk fibroin/alginate scaffold - GDNF/Ret expression and signalling seems unaltered in dopaminergic neurons in
Parkinson’s disease - Neuronal GDNF/Ret signalling might mediate maintenance, protection and
regeneration of substantia nigra dopaminergic neurons and motoneurons (cell survival,
axon outgrowth) via Akt, MAPK, PI3K and NF-kB signalling - Non-neuronal GDNF/Ret signalling leads also to astrocyte survival (reduced Caspase 3
activity), Schwann cell survival and proliferation, reduced oxidatic stress and microglia
alterations
Perspective of GDNF/Ret signalling in CNS regeneration
- GDNF/Ret signalling might enhance CNS regeneration in the midbrain dopaminergic
system and in spinal chord neurons
-> clinical trials using GDNF/Ret signalling in Parkinson’s disease patients (Bristol) and
spinal chord injury patients should be continued and optimize
