Axonal Growth and Regeneration Flashcards
After the neurone has migrated to its final position what influences it’s connectivity?
Once they have migrated to their final positions, neurones begin to extend an axon towards an appropriate target tissue.
The pathway may be long and circuitous, requiring the axon to use molecular cues in the environment for navigation.
These molecular cues (trophic factors) can be short or long-range, repulsive or attractive, or promote survival . They are mainly released by glial cells.
How is the axon adapted for axonal guidance and synaptogenesis?
The distal tip of the axon (the growth cone) is morphologically specialised and enriched in receptors that detect these cues, and stimulate intracellular signalling pathways, leading to cytoskeleton rearrangement and changes in gene transcription. The cytoskeletal changes (actin filaments and microtubules) underlie growth cone dynamics, and hence control axon growth.
Depending on the molecular cue, the growth cone responds by changing direction, collapsing, stalling, forming a pre-synaptic terminal, spreading, or growing faster.
What is the main intracellular signalling pathway that controls growth cone formation and collapse?
Rho GTPases control actin assembly and disassembly. One example is RhoA, which causes growth-cone collapse
• Receptor activation leads to activation of Rho GTPases.
• This leads to controlling RhoA, which activates ROCK and LIM Kinases.
• LIMK phosphorylases to activate the Cofilin molecule, which is an essential protein for F-actin depolymerisation. Blocking LIMK prevents neurite outgrowth inhibition in embryonic chicks (Hsieh et al., 2006).
How does the history of the neurone determine axonal growth response?
A growth cone can only respond to an extracellular signal if it contains a receptor for that ligand. Each neuronal population has a specific complement of growth cone receptors. Therefore this signalling depends on the history of the neurone.
Describe the mechanism of activity-independent synaptogenesis
Axons respond to synaptogenic factors, which stimulate pre-synaptic differentiation by modulating microtubule dynamics in the growth cone.
Give examples of positive regulators of axonal growth
Long-range:
• Growth factors such as the Neurotrophins stimulate axon growth and survival of selective groups of neurones. Neurotrophins include Nerve Growth Factor (NGF), Bone Derived Neurotrophic Factor (BDNF), and NT-3, NT4/5.
• Netrin which can attract (and repel) different classes of neurones.
Short-range (Cell-associated)
• The immunoglobulin superfamily (e.g. NCAM) or
• The cadherin superfamily (e.g. N-cadherin)
Short-range (ECM-associated) include laminin and collagen, which are secreted by cells to stimulate growth cone advance down specific guidance pathways.
Give examples of negative regulators of axonal growth
Long-range:
• Netrins can attract and repel different classes of neurones
• Semaphorins such as semaphorin II (collapsin), which stimulates growth cone collapse
• Slit, which is a soluble repellent secreted by midline cells.
Short-Range (Cell-associated) are mainly ephrins which are membrane-associated repulsive ligands, which may be either GPI- anchored (A-type) or transmembrane (B-type).
Short-range (ECM-associated) such as S-laminin, tenascin, chondroitin sulphate proteoglycan (CSPGs).
How do netrins influence axonal growth?
Netrins are released by cells during spinal cord development.
Commissural neurones express a transmembrane receptor called DCC (Depleted in Colorectal Cancer) which can bind to netrins.
This allows commissural neurones which are in the dorsal half to be attracted ventrally towards the floor plate by gradients of netrin 2 (gradual) and netrin 1 (steep).
How do neurotrophins influence axonal growth?
The target for the neurotrophins are Trk tyrosine kinase receptors. Different neuronal populations have different Trk receptors - this provides a mechanism for target selection.
• Trk A has greater affinity to NGF
• Trk B has greater affinity to BDNF
• Trk C has greater affinity to NT-3/4/5
Dimerisation of the Trk receptors by the neurotrophins stimulates intracellular signalling pathways. These stimulate changes in gene expression (via CREB) that promote cell survival and axon outgrowth, and also local cytoskeleton changes via ?Ras/MAPK.
How do immunoglobulins influence axonal growth?
There is only one NCAM (Neural cell adhesion molecule) gene. However, several different forms of NCAM are achieved by alternate splicing. It is expressed on the surface of neurons, oligodendrocytes, astrocytes and Schwann cells in the nervous system.
Numerous in vitro studies have established than NCAM is a stimulator of axonal growth.
Why is alternate NCAM splicing important?
Alternative splicing is important: NCAM with PSA is abundant in early development, but decreases during later development. Itstimulates greater/faster neurite outgrowth than NCAM lacking PSA.
Whereas, VASE-NCAM is not present in early development. When it is present, stops from NCAM actin on regeneration and blocks responses to growth substratum.
This is very interesting, because nature here has used the same gene (just alternatively spliced) to change response during different times. PSA-NCAM to stimulate faster neurite outgrowth at the beginning of development, but VASE-NCAM to slow/stop this down during later development.
How do ECM components influence axonal growth?
Components of the basal lamina such as laminin and collagen provide a permissive substratum for neurite growth. Laminin in fact guides axons during development by this mechanism.
The major growth cone receptor for extracellular matrix molecules are the beta 1 integrins. Integrin receptors consistent of heterodimers composed of alpha and beta sub-units. Different combinations of alpha and beta bind different ECM molecules
• The RGD [Arg-Gly-Asp] motif in ECM molecules is a common beta 1 integrin binding motif
Repulsive cues are also present in the ECM, such as S-laminin, tenascin, chondroitin sulphate proteoglycan.
Give an example where ECM cues are important in development.
Retinal ganglion cells navigate from the retina to the optic tectum on a pathway of laminin laid down by astrocytic end-feet lining the path of the optic nerve. When they reach their target, they downregulate their integrin receptors for laminin. Essentially, astrocytes are leaving laminin bread-crumbs for the retinal ganglion cells.
Give an example where Netrins cause repulsion instead of positive axonal growth
The trochlear motor neurons in the ventral half of the spinal cord, however, are repulsed by netrin and grow dorsally away from the floor plate. They express DCC but also Unc5, and the combination changes a chemoattractive response to chemorepulsion. (Involves modulation of cAMP levels)
This is a good example of the fact that different neuronal populations can respond differently to the same ligand depending on receptor expression.
How do semaphorins influence axonal growth?
Semaphorins are soluble chemorepulsants that stimulate growth cone collapse. The receptors for semaphorins are plexins. Semaphorins are selectively repulsive and control patterning in the spinal cord. They stimulate growth-cone collapse through RhoA/ROCK/LIMK pathway.
Give an example where Semaphorins influence axonal growth.
An example of how these are used in conjunction with positive regulators of axonal growth:
• Sema III is expressed in ventral half of spinal cord
• NT3 responsive sensory neurons terminate in the ventral part
• NGF responsive sensory neurons terminate in the dorsal part.
• In vitro studies using Sema III transfected fibroblasts show that NGF responsive neurons are repulsed by Sema III, but NT3 responsive neurons are not.
Therefore NGF-responsive neurons do not enter ventral part, while NT3-responsive neurons do.
Give an example where Slit influences axonal growth.
Slit is produced by midline cells is a soluble (long-range) repulsive molecule. Neurons expressing Robo receptors for slit are repulsed.
However, they need to cross the midline, so their Robo receptors are removed from the growth cone with the help of Commissureless (Com). This allows them to cross the midline (attracted by netrin). After they have crossed, Com is down-regulated, Robo receptors are therefore again expressed in the growth cone. This prevents the neurons from re-crossing back again.
If confused look at notes ‘Axonal Growth and Guidance Lecture’.
How do ephrins influence axonal growth?
Growth cone receptors for ephrins are the Eph Receptors. There are two classes of Eph receptor in vertebrates, EphA & EphB receptors. They are tyrosine kinase receptors that activate Rho GTPases to
stimulate growth cone collapse.
Give an example where Ephrins influences axonal growth.
Gradients of ephrin expression in the optic tectum (e.g. xenopus), and differential expression levels of Eph Rs in retinal ganglion cells control the development of the topographic map of the retina in the visual cortex.
Following spinal cord damage, what determines the level of functional impairment?
Functional impairment is linked to synaptic dysfunction as a consequence of axonal/dendritic damage or degeneration. In other words, functional impairment is linked to a loss of connectivity rather than a loss of neurones.
Does the CNS regenerate spontaneously?
The central nervous system does not regenerate spontaneously. If you put a peripheral nerve graft into the CNS, it does not growth. However, it does facilitate growth of CNS neurones within the graft. This suggests the PNS has something that can facilitate growth, with the CNS can’t.
Do CNS axons sprout spontaneously?
The CNS axons and dendrites are plastic and sprout spontaneously following injury. This occurs both spared and lesioned fibres within the CNS. This is the mechanism that underpins some level of functional recovery following a brain injury such as stroke. As there is more space in the brain than spinal chord, there is more room for functional recovery following a brain lesion rather than spinal cord lesion.
Are neural progenitors involved after CNS injury?
Neural progenitor cells can proliferate, differentiate and migrate following CNS injury. This is another aspect of recovery. It’s contribution to recovery is still very unclear. Interesting research being done in it.
What are the goals of regenerative neuroscience?
- Inhibit the glial scar formation. Gliosis mainly involves astrocytes, which extend processes to the lesion. Also involves microglia which secrete cytokines etc.
- Inhibit the axon regeneration inhibitory signalling.
- Promote pro-regenerative pathways
- Replace cell loss.