Axon Guidance 2 Flashcards
What is the Ig superfamily and give the 3 main types
Homophilic (bind to same Ig on axon and cell surface for cell adhesion and axon growth. The three main types -
- Classical CAMs - include NCAM and L1
- Receptor protein tyrosine phosphatases – characterised by intracellular phosphatase domains activated on ligand binding
-Receptor tyrosine kinases – characterised by intracellular kinase domains activated by ligand binding
What are the cadherin subfamilies
Calcium dependent cell adhesion molecules that link cadherins to cytoskeleton (anchor-like)
The axon can grip to substrate and pull itself along. Bind homophillically
Give some examples of extracellular matrix molecules and how they bind to axons to help with axon guidance
Through integrin receptors. Secreted by schwann cells and astrocytes
Examples include - laminin (very effective) and fibronectin (supportive)
Tenascin and proteoglycans (inhibitory usually)
What are netrins
4 receptor types which attract or repel axons depending on class of axon and/or stage of development.
Forms gradients.
- DCC
(attraction), UNC5 (repulsion) and adenosine A2bR (cAMP induced attraction)
What substance is important for promotion of axon guidance in netrins
Netrin binds to DCC dimers and activates
- cAMP
- cAMP then activates Rho GTPase, cdc42 and Rac1
What happens if UNC5 binds in axon guidance
Repulsion - but causes unknown.
What happens if A2bR is blocked (type of netrin)
Prevents netrin induced growth cone guidance as its role is to activate adenylate cyclase via g protein which promotes cAMP
What are slits
Motifs (short recurring patterns of DNA) that is expressed by ECM molecules. They are inhibitory for axon growth but are important for branching.
They are the ligand for robos
What are Robos
Slit receptors (part of Ig superfamily)
What happens when slits bind to robo
May silence the function of netrin-DCC
AND/OR
Inactivate cdc42 through GAP regulation
What are semaphorins role in axon guidance
large family, mostly associated with axon repulsion
What are the families of semaphorins
2 families of plexin receptors (A and B)
How do semaphorins signal
Binding to integrins and plexins - through RhoA GTPase, causing growth cone collapse -
What are ephrins and how do the manipulate growth cone guidance
2 groups: A (GPI-anchored) and B (transmembrane).
Ephrins largely thought to cause axon repulsion, though in the dorsal side of midbrain Ephrin Bs may function by attraction
How are ephrins expressed
As ligands or recetpors - 2-way signalling.
Expressed in gradients in midline structures (optic chaism) and midbrain
What are the 3 main types of myelin proteins in axon guidance
Nogo
myelin associated glycoprotein (MAG)
oligodendrocyte myelin glycoprotein (OMgp)
How do the 3 types of myelin proteins signal
All signal through the Nogo receptor, which requires p75 co-receptor which then activates RhoA and inhibits growth cone events
Why are myelin proteins important
Thought to be major reason why mammalian CNS axons do not regenerate as inhibitory for axon guidance in the CNS
Has varied knockout results
Give an example in developing grasshopper limbs of how guidance signals combine their effects
Legg and O-connor 2003
Sema-2a REPLSION = drives neurite initiation towards the CNS by repulsion from distal limb areas, where Sema-2a is most strongly expressed.
When sema-1a removed - poor path finding within trochanter boundary but still manages to travel in general direction to make contact with Cx1 cells
Give an example in developing embryos of how guidance signals combine their effects
Commissural axons in the neural tube are thought to be repelled from the roofplate by a number of repulsive molecules -
bone morphogenic proteins (BMPs),
Semas
Ephrins
They then appear to grow along the basement membrane into the ventral half of the neural tube, where they are attracted to the floorplate by netrin. Keeping the CNS central
What happens when these axons cross the floorplate in embryo development
axons acquire sensitivity to slits expressed in the midline due to an increase in robo, and may also lose sensitivity to netrin by silencing DCC.
Thus, they do not grow back across the floorplate, but turn and grow anteriorly towards the brain and therefore keeping the CNS within the midline
What is one major difference between the PNS and CNS regarding axon growth
Unlike the PNS, the CNS in postnatal animals becomes inhibitory to the regeneration of axons after injury.
The PNS contains schwann cells which are supportive of axon growth.
Why is the CNS inhibitory postnatally to regeneration
One clear possibility is that inhibitory molecules are upregulated in the nerve environment, perhaps concomitant with a decline in supportive molecules.
Explain how the CNS differs from the PNS during their responses for cell injury.
Oligodendrocytes - CNS cells that produce myelin - are upregulated to inhibit the CNS production. Degeneration doesn’t occur.
Microglia produce inhibitory substances.
Inhibits axon growth.
In the PNS, wallerian degeneration occurs where the myelin degenerates and the schwann cell moves into the basal lamina tube and macrophages arrive. Schwann cells divide and they remove the cytoskeleton debris with the macrophages and the schwann cells form new axons.
Wallerian degeneration is CRUCIAL for regeneration. this doesn’t happen in the CNS.
What was found when embryonic neurons were transplanted into adult brains
They can grow axons and processes while their older counterparts cannot.
Can happen because the embryonic neurons are not myelinated yet.
What happens to cAMP activity in development and why
Declines with age, especially when Retinal axons become repelled by guidance cues that they found attractive earlier in development
What role does myelin associated glycoprotein (MAG) in sensory neurons
Inhibitor, coincides with a developmental decline in neuronal cAMP
Why can cAMP both cause chemorepulsion and chemoattraction
it was thought to only signal through protein kinase A (PKA). It is now known that cAMP can also signal through a distinct protein, Epac (exchange protein activated by cAMP).
What happens when cAMP levels are high regarding Epac
Murray et al 2009
Epac is more active when cAMP levels are relatively high, such as in embryonic neurons, and it mediates chemoattraction and enhances the rate of sensory axon growth.
What happens to Epac if cAMP levels decrease
Murray et al 2009
Epac activity declines if cAMP levels decrease, as happens in older neurons, and PKA signalling predominates. PKA mediates chemorepulsion and does not promote sensory axon growth.
What does the role of cAMP suggest about the CNS
a decline in endogenous cAMP levels could play a role in the transition between a CNS that supports axon growth and one where axon regeneration does not occur.
It is believed that cAMP levels decline so much in adults that they do not meet the threshold to activate Epac to promote sensory axon growth. But PKA is much more sensitive to cAMP so can be activated and therefore cause chemorepulsion and prevent sensory axon growth.
What happens to retinal axons with maturation
Campbell et al 2001
They become sensitive to semaphorin as neuropilin and plexin are unregulated in lateral development. During lateral development - this causes semaphorin and neuropillin/plexin binding, resulting in growth cone collapse.
Therefore suggesting that - such intrinsic changes to the complement of receptors expressed by growth cones, and the signalling molecules expressed within, underlie the onset of axon growth inhibition in the CNS.
What is the dorsal root entry zone (DREZ) and why is it important
The divide in the spinal cord between PNS and CNS - axon regeneration across the DREZ is not possible from a few days after birth
What causes netrin attraction
WIDER READING
UNC-40
DCC cell surface receptors
What causes netrin repulsion
WIDER READING
UNC-5
What is the phenotype of netrin deficient mice
WIDER READING
Lack commisural axon fibres therefore lacka corpus callosum
Where is netrin-3 important in malignancy
WIDER READING - Jiang et al 2021
Netrin-3 expressed in human neuroblastoma (NB) and small cell lung cancer (SCLC).
Netrin-3 silencing using NP137 showed - delayed tumor engraftment, and reduced tumor growth in animal models
What is the role of netrin-Gs
WIDER READING - Barallobre et al
Only found in vetrebrates and believed to be evolved independently of other netrins to construct the brain as they form - thalamus and mitral cells of the olfactory bulb.
Bind to NGL ligand not DCC or UNC-5
Netrin role in glial cells
WIDER READING - Von Hilchen - 2010
longitudinal glia (LG) fail to migrate medially in the early stages of neurogenesis, whereas distinct embryonic peripheral glia (ePG) do not properly migrate laterally into the periphery
Why is NTN1 (Gene coding netrin) considered to be an oncogene
WIDER READING - Arakawa 2004
The netrin-1-mediated anti-apoptotic signal that inhibits p53-induced apoptosis implies that NTN1 (which encodes netrin-1) could function as an oncogene.
NTN1-transgenic mice have reduced apoptosis of intestinal epithelial cells, leading to an increase in the spontaneous formation of hyperplasia and adenoma.
What role does slit1 and 2 have in olfactory bulb and choroid plexus development
WIDER READING -Nguyen Ba charvet - 2004
They mediate the repulsive activty of the septum and choroid plexus through migration of undifferentiated cells of the subventricular zone to the rostral migratory stream of the olfactory bulb where they differentiate to olfactory neurons
What does inhibition of Robo1 do to malignant melanoma
WIDER READING - WANG 2003
Works with von willebrand factor in tumour endothelial cells and if inhibited, reduces vessel density and tumour mass.
Mediated by slit2. Demonstrated the robo-slit binding has a angiogenic function.
How is robo1 linked to dyslexia
WIDER READING
A candidate gene for dyslexia due to it fitting into a theoretical molecular network involved in neuronal migration and neurite outgrowth that fits the understanding of dyslexia.
What is ephrin reverse signalling
WIDER READING - marquardt 2005
Forward signalling - ephrin binds to an Eph receptor which initiates signalling events in the eph receptor presenting cell .
Reverse signalling - Eph binding to ephrin initiates signal transduction events into the ephrin-expressing cell.
Erphrins and Eph receptors are expressed in many cell types.
It was showin that spinal motor neuron growth cones expressed EphA and GPI-linked ephrin A ligands which they work to develop these neurons -
EphAs direct growth cone collapse/repulsion and
ephrin-As signal motor axon growth/attraction.
What is the role of class 3 semaphorins in CNS trauma
WIDER READING - Mecollari 2014
Important role after CNS trauma.
Regulate non-neuronal and neuronal cells as semaphorins are present in scar tissue.
Class 3 semaphorins modulate axonal regrowth, re-vascularisation, re-myelination and the immune response after central nervous system trauma - some done via its ligands (VEGF and transforming growth factor)
How are cell-adhesion molecules (CAMs) classified
WIDER READING - Brackenbury 1981
Calcium-dependent - cadherins and selectins depend on Ca2+
Calcium-independent -Integrins and the Ig-superfamily CAMs do not depend on Ca2+
How else can netrin-1 responsiveness be modified
WIDER READING - Galko 2000
Proteolytic cleavage - Matalloproteases inhibitors increase netrin mediated axon growth in vitro.
DCC is a substrate for metalloprotease-dependent ctodomain shedding, and that the inhibitors block proteolytic processing of DCC and cause an increase in DCC protein levels on axons within spinal cord explants. Thus, potentiation of netrin activity by inhibitors may result from stabilization of DCC on the axons
What does netrin-1 bind to to trigger cAMP
WIDER READING - Corset 2000
the membrane-associated adenosine A2b receptor, a G-protein-coupled receptor that induces cAMP accumulation on binding adenosine
How do older growth cones respond to sema 3A
WIDER READING - Campbell 2001
Turn away rapidly from the sema 3A gradient while young growth cones are insensitive to it.
It also causes branching in 30% of repulsed axons
