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Additional Molecules Involved in Axon Guidance

Receptor tyrosine kinases, extracellular matrix molecules, and netrins, semaphorins and their receptors are also involved in axon guidance. The functions of each are explained below.

Receptor Tyrosine Kinases

Receptor tyrosine kinases can function to modulate or guide axon growth. In general these receptors are found on developing axons and allow the axon to respond to various growth factors in the local environment. For example, one type of fibroblast growth factor receptor (FGFr, a tyrosine kinase) is necessary for FGF or N-CAM-induced neurite outgrowth. The neurotrophins, NGF, BDNF, NT-3, and NT-4/5, are usually thought of as survival factors for neurons, although these growth factors can also promote axon outgrowth, allow axons to invade their target regions, and promote axon terminal arborization.

Extracellular Matrix Molecules

Extracellular matrix (ECM) molecules can act to promote or inhibit neurite outgrowth. Laminin, tenascin, collagen, fibronectin, and a number of proteoglycans have been suggested to modulate axonal outgrowth. For example, laminin can promote, while tenascin can inhibit neurite extension. Receptors for ECM molecules include the integrins as well as Ig family members.

The laminin family and its receptors are one of the best studied examples of ECM molecules with regard to neuronal development. There are at least ten different isoforms of laminins that have growth promoting or inhibiting effects on different cell types. The axonal receptors for laminins are integrins. Integrins are heterodimers whose subunit composition determines binding specificity with respect to the laminins. The integrins link ECM signals to the cytoskeleton and various signal transduction pathways. The role of laminins and other ECM molecules in neuronal development is to affect axonal guidance.

Figure 9.15 Axons are either attracted or repulsed from a netrin gradient in the developing floor plate depending on the type of netrin receptors they possess. Those axons that have DCC/UNC40 receptors will be attracted to netrin, while those that possess UNC5 receptors will be repulsed. The mechanisms by which these receptors evoke differential responses remains to be discovered.

Netrins, Semaphorins and Their Receptors

The netrins are a family of secreted proteins related to laminins in sequence They are both diffusable and cell associated. The netrins were identified using two approaches. A genetic assay in nematode uncovered a mutant gene that resulted in defects in cell migration and axon guidance and an in vitro assay using chick floor plate cell extracts identified a factor that results in outgrowth and guidance of spinal cord axons. Netrins are well conserved from nematode to human and are encoded by only one or two genes in most species. Netrin binds to two types of axonal receptors (DCC/UNC-40 and UNC-5) that determine whether the actions of netrin are attractive or repulsive.

Figure 9.16 Attraction or repulsion from a semaphorin III gradient in the ventral spinal cord. Axons of primary afferents are directed to termination zones in spinal laminae with the help of a semaphorin III gradient that attracts NT-3 responsive axons (blue) while repelling NGF responsive axons (green), and having no effect on some axons (red) that apparently receive their directional cues from other sources.

The semaphorins are a large family of cell surface or secreted proteins that mediate growth cone collapse, axonal steering, axonal branching, and axon terminal arborization. At least ten semaphorins are found in humans and they appear to function predominantly as inhibitory (repulsive) factors for axonal guidance. Neuropilins serve as receptors for secreted semaphorins, whereas members of the plexin family are receptors for other semaphorins. Secreted semaphorins do not bind directly to plexins, but plexins can associate with neuropilins. Thus, plexins are receptors for multiple classes of semaphorins, either alone or in combination with neuropilins, and trigger a signal transduction pathway controlling axon guidance.

 

 

 

 

Summary of Molecules Involved in Axon Outgrowth and Guidance
Ligand		Receptor	Ligand		Receptor
Contact Attraction			Contact Repulsion
Ig CAM		Ig CAM
Fasciclin II		Fasciclin II
NrCAM		Axonin-1	Eph Ligand		Eph Receptor
ECM		Integrin	Semaphorin I		Plexins
Laminin		Integrin	Slit		Robo
Chemoattraction			Chemorepulsion
Netrins (UNC-6)		DCC (UNC-40, Frazzled)	Netrins (UNC-6)		UNC-5
			Semaphorins (secreted)		plexins + neuropilins

Summary

The growth of developing axons depends on:

1. Physical guidance cues such as guidepost cells and pioneer axons.
2. Local and long-range molecules involved in attraction and repulsion of developing axons (the presence of which substantiates the chemoaffinity hypothesis of Sperry).
3. The presence of receptors on developing axons that allow for modulation of growth by environmental factors.

Figure 9.17 Short- and long-range cues act to guide axon growth. These guidance effects can be attractive or repulsive. Axons may use several, or all, of these forces for navigation of single segments of movement. For example, a repellent from behind will "push" an axon into a corridor marked by permissive local cues and bounded by repulsive factors, whereas an attractant at the end of the corridor may "pull" the axon toward its target (purple cell). In additional, contact attraction or axonal fasciculation (e.g., the purple axon uses the blue axon as a guidance cue) help in axonal pathfinding.