Axon Guidance
Growth Cones
Growth cones are the areas of axons that lead axon path-finding. They are composed of lamella and filopodia which are made up of different types of F-actin (filamentous actin). The lamellipodia actin bundles are cross-linked into a net whilst the filopodia actin bundles are polarised to form larger bundles. Both are highly mobile and are not stuck down.
Growth cones don’t turn but they reorganise. When the growth cone comes into contact with an attractive cue the tread-milling F-actin in the resting growth cone slows so it accumulates. This stabilizes the filopodia and pulls microtubules into the back of them.
Experiments have also proved that as well as attractive cues, growth cones can be affected by repellents. Images of growth cones in real time have shown how they are repelled by each other and if they come into contact with each other they can collapse. Growth cone collapse has the opposite effect to attractive cues as it destabilizes F-actin. However localised collapse contributes to growth cone reorganisation and thus guidance.
Semaphorins and the Forces of Axon Guidance
Biochemical purification of the factor from the retina responsible for causing the collapse of sensory axons has led to the identification of a family of inhibitory molecules called semaphorins. They can be membrane bound or soluble which classifies 2 of the 4 forces in axon guidance:
- Contactattraction
- Contactrepulsion
- Chemoattraction
- Chemorepulsion
Permissive Substrates
Growth cones need substrates that are both permissive for growth and to which they can attach to. Collagen is better at causing adhesion then laminin but laminin is much better at creating outgrowth then collagen. Permissive factors such as laminin promote growth in the extracellular matrix in the embryonic nervous system for example in the optic nerve and although it doesn’t dictate the direction of growth it does let the axon grow there. It works in a concentration dependent manner with only a medium expression permitting growth.
Non- Permissive Substrates
Non-permissive factors such as the semaphorins mentioned above can act to channel axon growth as experimental evidence has shown that in mice lacking Sema-3A the axons stray into the incorrect areas. Ephrins are another non-permissive factor that act by contact repulsion. These cell surface molecules are detected by receptors called Eph receptors and are involved not only in axon guidance but also has links to compartmentalising parts of the embryonic brain such as the rhombomeres. Ephrins and Ephs have a remarkable reciprocal pattern of expression in the embryo. It is the combination of both permissive and non-permissive factors that cause axon growth so they know where and where not they are allowed to grow.
Chemoattractants and Chemorepellents are Secreted from Organisers
The roof plate and the floor plate are organisers that pattern the cell types of the spinal cord but they also act as key organiser centres in relation to axon guidance as they secrete long range chemical messengers. The floor plate secretes a chemoattractant called nectrin and cells expressing this gene can turn axons. The roof plate expresses BMPs that although play a role in early brain axis formation they also act at the later stage to help guide axons. The act as chemorepellents and can cause growth cone collapse. Thus together chemoattractant and chemorepellent molecules work to guide commissural axons to their initial target of the floor plate.
It is the combination of all 4 guidance forces that are used to guide axons at different stages in their path.
Choice Points and Intermediate Targets
Most axon pathways are complex and require often sharp turns or changes along their route to their final destination. Because of this axons reach their targets in stages and encounter intermediate targets along the way. When they get to these targets they have to decide where to go next therefore they are also called choice points. To do this axons “reprogramme” at these points where although they may have previously seen permissive cues they may suddenly react as if they have seen non-permissive cues.